Design Guidelines

OVERVIEW OF PLASTICS

Most commercial plastics, also known as resins in North America, are based on the element carbon and are synthesized, or made, from simple, oil-based raw material. These starting materials are called monomers and these simple, low molecular weight materials are put together, by a process known as polymerisation (polymerisation), so as to form polymers. This term means that the final product consists of many identical , repeat units. Because the final molecular weight, or mass, is so large the material may also be referred to as a 'high polymer' or, as a 'macromolecule'. All plastics are polymers but it cannot be processed like a plastics material unless it is modified.

TYPES OF PLASTICS MATERIAL

A plastic is a polymer, which is capable of being shaped or molded under conditions of moderate temperature and pressure. Distinguished from a rubber/elastomer, by having a highern stiffness/modulus and a lack of reversible elasticity. There are two main categories of plastic and these are thermoplastics and thermosetting plastics (thermosets). Thermoplastic products, for example in injection molding or an extrudate, may be softened and reshaped whereas a thermoset product cannot. In terms of tonnage tjermoplastics are based on one monomer and are known as 'homopolymers': some are based on two monomers and are known as copolymers'.

THERMOPLASTICS (In German also Plastomere)

"A plastic that can be repeatedly softened by heating and hardened by cooling through a temperature range which is characteristic of the plastic and that in the softened state can be shaped by flow into articles by molding or extrusion." Thermoplastic scrap not damaged by thermal degradation can be reprocessed. Thermoplastic parts are weldable. When rigid, thermoplastics can be stamped or thermoformed within a given temperature range. Thermoplastics polymeric molecules are linear or branched and as a rule are soluble in specific organic solvents.

THERMOSETS (in German Duroplaste or Duromere)

"A plastic that, after having been cured by heat or by other means, is substantially infusible and insoluble." Thermosets cannot be reprocessed or welded, and only special products are thermoformable to a limited extent. They normally retain their form from low temperatures to thermal decomposition. In a thermosetting, the smaller molecules, generally in the form of a liquid or fusible preproducts, are chemically cross-linked into much larger ones. The molecules of reactive resins 1) can be cross-linked with a catalyst at room temperature as well as by elevated temperature. Cross-linked (or cured) polymers can be swelled by organic solvents but cannot be dissolved by them without decomposition.

In German " reaction resins" is the term for plastic pre-products that can be cured at room temperature as well as by thermo-setting, in the rule without evolving of velatile products, s.p. 305


AMORPHOUS AND CRYSTALLINE

Thermoplastics materials may be divided into two main categories; these are amorphous and crystalline. An amorphous, thermoplastics material is usually a hard, clear, rigid material with a low shrinkage an a low impact strength; such a material is polystyrene. A crystalline plastic also contains amorphous material and so may also be known as a semi-crystalline, thermoplastics material. Such plastics are usually tougher, softer, but can have a higher heat distortion temperature, than an amorphous, thermoplastics material: such plastics are also transulant, or opaque, have a high shrinkage and a high specific heat. The best known example of a semi-crystalline, thermoplastics material is the plastics material known as polyethylene.

ALLOYS

Alloys are blends of two or more thermoplastic polymers whose resultant properties are usually intermediates between those of each constituent. Three major alloys are discussed below:

ABS- Polycarbonate Alloy :

By alloying ABS with polycarbonate, the resulting mataerial is close in cost to ABS, but displays higher physical properties than ABS. For one such alloy, izod impact is about the same as would be expected for a rubbery, high-impact ABS. However, in contrast to hig-impact ABS, the alloy offers high-heat-distortion temperature, high modulus, and high strength. The phenomena of varying izod impact strength with thickness that is characteristic of polycarbonate is reduced in the alloy. Processability of the alloy resembles that of ABS rather than polycarbonate, which is comparatively more difficult to process than ABS.

ABS - PVC Alloy:

By alloying ABS with rigid PVC, a self-extinguishing material results that is a happy marriage of the properties fo both constituents. The alloy offers good stiffness with higher-heat-distortion temperature than is normal for rigid PVC, while retaining excellent impact and self-extinguishing properties. Applications for these alloys include: electrical plugs; power-tool housings; and television coil yokes.

Acrylic-PVC Alloy

Acrylic - PVC alloys are principally found in sheet form. Toughness, flame resistance, and impact strength are the forte of this alloy. Chemical resistance and elongation of the alloy is superior to that of acrylic, but the familiar translucency of acrylic is absent. However, opaque colors are possible in the alloy, which has found use in thermoformed materials, handling trays, and machine housings.

ELASTOMERS

An elastomer is a rubber-like material that can be stretched to at least twice its original length and return to its original length all at room temperature. A rubber compound will do the same as an elastomer but from 0°C to 150°F at any humidity. Plastics for all intent and purposes are rigid and do not stretch.
A wide range of elastomers, both thermoset and thermoplastic types, is available (Table 1 - 2). Thermoplastics elastomers can be processed by injection molding, extrusion, and blow molding.The material can be reground and reprocessed. The principal types of thermoplastics elastomers are




AUXILIARIES AND ADDITIVES FOR PLASTICS


Although they have not achieved precise definitions that are universally accepted, the terms "Auxiliaries," "additives," and "Compounding ingredients" are used with the following meanings in the ensuing pages: Auxiliaries are materials, such as catalysts, emulsifying agents, etc., which are used in the manufacture of polymers. In the final products, generally only small residues of the auxiliaries remain.
Additives include lubricants, stabilizers, flame retardants, etc. They are included for their processing functions and to influence the behavior of the compound in use. They too are added in relatively small amounts. For reasons of hygiene and better dosage, they are offered in dust free, readily dispersible, pourable formulations, or also preprocessed into master batches. In the OT microencapsulation technique additives (e.g., catalysts and flame-retardants) enclosed in a solid casing can be released when required by mechanical destruction of the microcapsules.
Compounding ingredients are materials added in relatively large proportions
(10-70% or more of the polymer) to make significant changes in the properties of the plastic compound. They include plasticizers, fillers and reinforcements.
There are no hard-and-fast lines between these categories.

Lubricants or parting agents are metal soaps, montan or paraffin waxes, waxlike polymers, higher fatty alcohols, fatty acid esters, silicons. They reduce the viscosity of molding compounds (internal lubricating action) and/or are effective as external lubricants between plastic metals and metal walls. Parting agents sprayed into moldings to be aftertreated must be silicone-free.

Stabilizers protect against thermal degradation during processing and in use and/or guard the products against oxidation and breakdown by weathering agents, particularly UV radiation. Many chemically different stabilizer systems are necessary for almost all plastics applications, and they must be appropriate not only for the polymer involved but also for the requirements dictated by each use. For stabilizer systems for PVC, manifesting also "synergistic effects" between their components. Carbon black is an almost universally applicable UV stabilizer.

Antistatic agents and conducting additives are hydrophylic substances (e.g., amino derivatives and polyethylene glycol esters) that reduce the surface resistivity of plastics by >1015 ohm, so much (³ 1019 ohm) that trouble due to friction-included electrostatic charges is prevented. Conducting additives that can, in small amounts, reduce specific resistance to 1 ohm . cm are of particular importance in the screening of high-frequency emissions from equipment in the aluminium flakes coated with coupling agent, steel microfilaments, silvered glass fibres and spheres, nickel surface coated textiles (Baymetix), special carbon blacks and carbon fibers are applied to meet the legally required EMI (electromagnetic interference) and radiofrequency damping in the 10 kHz to 140 GHz range with moderate to good screening at 60 dB. For surface-screening, conductive lacquers (Electrodag) or vacuum metallizing is used.

Flame retardants are necessary to reduce the flammability of plastics products so that they can satisfy the specifications for electrical applications, for transport vehicles and for building construction. Chlorine or - more effectively - bromine-containing organic compounds evolve products when exposed to flames that inhibit the admission of oxygen and chemically retard the flame reactions. Phosphorus-containing compounds favor carbonization and encrustation. Both types can be incorporated chemically and synergistically reinforced by the presence of antimony trioxide. Relatively large concentration of additives can have an unfavorable effect on the behavior of the product in use. Public health considerations militate against several additives. The formation of corrosive substances and the generation of smoke in the event of fires pose further fire-protection problems. Recent work is aimed at halogen-free flame retardants, e.g. 25-30% intumescent combinations of nitrogen-containing oligomers with ammonium polyphosphate (Spinflam).
Inorganic fillers proportionately reduce the flammable element within the plastic and favor encrustation during burning. Fire protection, without the development of smoke, is conferred by aluminum hydroxide, Al(OH)3, or the microfibrous Dawsonite, NaAl(OH)2CO 3, which at temperatures of about 200°C split off steam or steam and carbon dioxide. Zinc borate, Zn(BO2) 2. 2H2O, Mg(OH)2 or Mg CO3. H2o at ³ 300°C work in a similar way.

Colorants are specially prepared insoluble organic and inorganic pigments and plastics-soluble dyestuffs. Inorganic pigments provide temperature and light stability, organic the most demanding color requirements. Polymer-soluble dyestuffs are used to a limited extent for transparent products, fluorescent ones (Fuco, Lisa, DE) for luminous directional reflectance an absorption. Artificial horn and PA are surface colored with an aqueous solution. Optical-brightening agents are also soluble colorants.
Discolorations produced by reactions between sulfur-containing dyestuffs and lead or tin stabilizers, interference with the curing of resins by metal complexes, bleeding as a result of additives, the influence of pigments on crystallization and the electrical properties of plastics are all considerations that must be taken into account in the choice of colorants.
Masterbatches consisting of pigments and the plastics to be processed and concentrated pigments with higher pigment content in an inert bonding agent are used to color plastics in a dust-free manner.

Flexibilizers are chemically reactive or non reactive additives for toughening thermosetting or cold curing resins. However, some thermoplastic compounds containing small amounts of plasticizers are named flexibilized. Impact modifiers are non rigid elastomerlike polymers used for enhancing the impact of rigid thermoplastics by alloying. Most HI-modified thermoplastics contain them as spherically microdispersed phase in the rigid matrix, but there are networklike distributions of the phases too (e.g. PVC/EVA, PVC/PE-C).
Plasticizers are low molecular or oligometric additives compatible with rigid thermoplastic polymers solvating them to semirigid or leathery or rubbery plastics materials.
Random or alternating copolymerisation can result in some degree of "internal" plastification.

Coupling agents are molecular bridges between the interface of an inorganic filler and an organic polymer matrix. They contain hydrolyzable groups binding the inorganic filler and organofuctional groups in the same molecule. Semiorganic silanes and titanates have already been used for a long time for glass-fiber reinforces plastics. With specific organofuctional groups they are also very important for all kinds of polymer composites. Some of them can be added in normal additive blending instead of pretreating the fillers (e.g. "coating" by stearates) in a separate operation.

Interface layer binding is needed for multi-layer films, sheets and blow moldings combining non-compatible polymers. It can be produced by reactive gases blown between, but mostly co-extruded adhesive bonding layers are used.

Fillers, enhancers and reinforcements

From the classic Bakelite thermosetting PF molding compounds to the newest thermoplastic range of composites, fillers have been used bot only as "extenders" i.e.,to reduce the resin content of the plastic and so reduce costs, but also to improve production rates (by increased thermal conductivity) and properties of the finished products. These include modulus, impact strength, dimensional stability, heat resistance and electric properties. Such components of plastic compounds can be differentiated approximately by the aspect ratio (a.r.) of length (or length and width) to thickness as follows:

Fillers: irregularly shaped granules or spheres, a.r.³ 1
Typical property enhancers : short finers, e.g., wood flour, milled or chopped glass fibers, wollastonite, whiskers, talc or mica flakes, a.r. 10 to » 100.
Reinforcements : filaments, nonwoven or woven textile product, a.r. very large.

This list does not cover the special furface-active reinforcements used principally in rubber and elastomers, such as carbon blacks, pyrogenic highly dispered silica (Cab-O-Sil, Aerosil) or precipitated, ultrafine, carboxylated rubber-coated CaCO3 (Fortimax).

1. 1. Natural organic materials: Wood flour has traditionally been used for thermosetting compounds and recently for thermoformable PP sheets and woodlike PVC moldings (Sweden). For cellulose fibers, chips and fabrics see thermosetting molding compounds. Such materials as sisal, jute, etc. are attracting interest, especially in developing hydrophobized starch are on the market as a performance additive, in particular for biodegredable thermoplastics.

2. 2. Mineral fillers and properties enhances : Ground chalk, limestone or marble (ø ≤ 3mm, spec. surface 6-7 m2/g) and precipitated calcium carbonate (ø ≤ 0.7mm, spec. surface ≥ 30 m2/g) are generally used for valuable temperature resistance, impact strength, surface quality and stiffening agents,. In PVC, CaCo3 improves burning behavior by binding HCL. "Snow White" is anhydrous calcium sulfate approved for contact with food. Barium sulfate is a heavy filler for sound and radiation protection. Quarz flour is highly corrosion resistant, but it rather abrasive (mold wear). Calcined kaolin is used in high-voltage technology. Feldspar is of significance in transparent products on account of its refractive index.
Talc, naturally found as platelets (widely used with PP), delaminated mica flakes (up to 1mm in thickness) or mixtures of such minerals with short glass or polyester fiber, is used for well balanced improvements of modular, flexural and impact strength. Asbestos is as such as first-rate heat-resistance improver, but it is likely to be replaced by mineral-fiber mixtures, such as wollastonite or CaSO4- crystal microfiber (ø = 4-6mm, a.r. > 100; Franklin fiber, US). Foamed clay (Norpil, Tecpril, l = 0,045-0,1 W/mK) is used for high temperature thermal insulating products.
Small amounts of finely dispersed CaCO3, silica or special silicates (Sipernat) reduce sticking and improve the paperlike feel of PE films.

3. 3. Spheres : As spheres are sully isotropic, compounds very highly loaded with them flow easily in processing. Sphere-filled products are less susceptible to shrinkage and distortion. Solid glass micro-spheres ("ballotini", ø < 50 mm) improve modulus, compressive strength, hardness and surface smoothness, "Cenospheres" are mixed solid or hollow spheres manufactured from the fly ash of coal-burning power plants. For hollow borosilicate or silica spheres called microballoons (ø = 5 to 250 mm; dR = 0.28 -0.50 g/cm3) (Q-cel, US; Fillite, JP; Microcel, BE) and used for boat hulls, aircraft, and automobiles.

4. 4. Fibrous reinforcements : Fibres, fabric cuttings and fabric web are used to reinforce thermosetting molding powders and laminates . Milled or chopped short fibers < 1 mm to 0.1 mm long are incorporated in structural foams and in granulated thermoplastic compounds in disordered but homogeneous distribution. Long-fiber types are prepared by the incorporation of flowing chopped glass strands up to 6 mm in length or by the coating of rovings and then cutting into granules. Reaction resins are converted with random or oriented fibrous reinforcement in high load-bearing parts.
Textile glass reinforcements are predominantly made of continuous glass filament, which is drawn from molten glass (Type C), 5 to 25 m in diameter, mostly from alkali-poor-E-glass. Glass strand combining 100 to 200 monofilaments, and filament products such as mats varying from 250 to 900 g/cm2 (68.9 - 248 oz/in2), have been further developed. Blown-drawn short staple fibers are used for resin-bonded light mats (30-150 g/cm2) or fleece, for smooth, resin rich surfaces. For this, better acid-resistant C-glass is preferred. Textile size, which is necessary for the pretreatment of the glass filament, in combination with coupling agents becomes effective as coupling finish for reaction-resins. Noncoupling textile sizes must be burned off before further processing (e.g. finish 112 for woven glass fiber :< 0.1% residue) Mats or fleeces are heavily or lightly strengthened by means of synthetic resin binders based on PVA,EP, for GMT also on PUR.
High modul and high-strength fiber reinforcements made of R- or S- glass (trade names) are more expensive than those of E-glass fibers. Polyester, acrylic or PVC synthetic fibers are used mainly in cover fleece. Synthetic fiber reinforced plastics (SRP) have significantly lower E-moduli than GRP in the direction of the fiber, but there is a higher resistance to damage by distortion.For mechanically and thermally highly stressed parts in air and space travel, in transport vehicles and sports equipment, on the one hand, fibers made from an aromatic polyamide (Aramid) with a high proportion of 1.4 chemical bonds directly between aromatic rings (Kevlar, Twaron) are used; on the other hand, fibers made by carbonizing pitch or polyacronitrile fibers as "precursor" are employed. The commonly used carbon fibers, finished at about 1000°C, have a low modulus and high strength. Those made by progressive graphitization at 2000°C are high-modular graphite fibers (Table 1). There are IM (Intermediate Modulus)- types showing sB -values 4000 to 5000 Mpa, E 300 Gpa (Celion G40). All these fibers have lower densities and a specific strength and/or elastic modulus many times higher than that of glass fibers. They are also much more expensive. They improve the properties of the product even when present as alimited proportion of the reinforcing fibers.

Aramid-Carbon, Aramid-glass, Aramid-Carbon-Glass and carbon-glass hybrid fiber reinforcement are all commercial products. Extremely high mechanical properties and temperature resistance (>1000ºC) are manifested by inorganic fiber-forming single crystals (6whiskers) or crystalline long fibers from aluminium (Fiber FP, US) or calcium titanate silicon carbide. "Cobweb-whiskers" (Xevex, NO) containing micro-monofils result in better elongation values than single crystal whiskers. They very costly boron fibers on a tungsten-core are used for e.g. spacecraft. For high strength PE fibers.





PLASTIC MATERIAL PROPERTIES

MATERIAL RECOMMENDATIONS FOR INJECTION MOULDING

POLYSTYRENE

1. Common name : Polystyrene

2. Abbreviation : PS or GPPS, i.e.general purpose polystyrene.

3. Systematic chemical name : Poly (1 - phenylethylene)

4. Some Suppliers : 5. Trade names or trade marks :
A to Chimie Lacqrene
BASF Polystyrol
BP
CDF Chimie Gedex
Dow Styron
Hoescht Hostyren N.
Monsanto Lustrex
Montedison Edistir
Shell Carinex
Sterling Sternite

6. Material properties : A hard, rigid material which in its natural form has a high gloss, sparkle and transparency. A wide range of colours is readily available and as the mouldings can also be decorated by a large number of techniques, attractive components can be easily produced. Low water absorption and good electrical insulation properties are other desirable characteristics. However, the polymer is brittle, burns easily and has poor outdoor weathering properties. It is easy to process by injection moulding but this apparent ease can be deceptive as unless care is taken parts that stress-crack in service may result. The density, is about 1.05 gcm -3.

7. Ease of flow : Relatively easy flow; special easy flow grades available.

8. Shrinkage : Because of the low shrinkage of this amorphous material parts with good dimensional accuracy can be obtained. Shrinkage is about 0.5% (about 0.004 in/in).

9. Resistant to the following : Resistant to acids (except oxidising acids), alkalis, mineral oils, detergents and the lower molecular weight alcohols. Relatively unaffected by exposure to water and to high energy radiation.

10. Not resistant to : High temperatures and a wide range of organic solvents, e.g. aromatic and chlorinated hydrocarbons, esters and ketones. Liquids such as white spirit can cause stress-cracking and can therefore be used to detect strain. Decomposed by prolonged contact with oxidising agents such as concentrated sulphuric acid.

11. Material detection or identification : Sinks slowly in water. Burns easily with a yellow, sooty flame and produces a lot of smell, smoke and soot. Mouldings are brittle and emit a metallic ringing noise when dropped on a hard surface. Does not cut easily or cleanly with a knife. Dissolves in carbon tetrachloride.

12. Colouring : Can be readily coloured e.g. , dry colour ; masterbatch ¾tumble mixing has proved extremely successful. Normally percentage of colourant / masterbatch is 1 -4 % dependent upon base colour of styrene and tintorial power of colourant. Liquid colourant is readily accepted but can cause problems, e.g. with respect to processing and properties of the end product ¾ development taking place.

13. Material and component handling : Does not absorb water (sufficient to upset processing) normally. Store in a clean, dry area. Predrying not usually necessary but when required heat at 70°C for up to 3 hrs. GPPS has very low impact strength and low abrasion resistance and these factors should be taken into account when mouldings are handled.

14. Mould and gate considerations : Can be moulded with gates of small cross-sectional area. Pinpoint, tab, insulated runner and hot runner are widely used.

15. Flow path: wall thickness ratio : The material has a maximum flow path: wall thickness ratio which is intermediate between that of PP and ABS, e.g at 1mm wall thickness PP, 170:1; PS, 150:1; ABS, 140:1.

16. Projected area considerations : Depends on grade, temperature, etc. Generally the clamping pressure required is of the order of 1-2 tsi of projected area (15- 30 MN m -2) for GPPS. For thin walled mouldings 3 - 4 tsi (45 - 60 MN m-2) may be required. With easy flow materials only 1 - 2 tsi may be needed even for thin walled components.

17. Cylinder equipment : The cylinder is usually equipped with a shut-off nozzle when decompression is not available; a valve, to prevent back-flow, is usually fitted to the screw.

18. Screw cushion : About 3 mm.

19. Shot capacity : Because of this material's good heat stability the shot volume can be as little as 5% of the rated capacity if required; the maximum rating of the barrel can also be utilised.

20. Melt temperature ¾- as measured in the nozzle or by an air-shot technique : 220 - 260° C (428 - 500°F).

21. Barrel residence time : Good resistance to heat normally. Grades which contain ultra-violet additives may tend to yellow if left to 'cook' in the barrel.

22. Temperature settings : Please note that it is the melt temperature which is important; those in Table 1 are only suggested, initial settings. The temperature of the hydraulic oil and of the material in the hopper should not vary excessively.
TABLE 1
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone No. Location Temperatures ° C Temperatures ° F
T o From To From
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 150 180 302 356
(near the hopper)
2 Barrel middle 180 230 356 446
3 Barrel middle 210 230 410 446
4 Barrel front 210 280 410 536
5 Nozzle 210 280 410 536
Mould 10 80 50 176
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾

23. Injection Speed ¾ mould filling speed : As high as possible consistent with other requirements, e.g. surface finish. To get high speed it is not necessary to use very high pressures.

24. Injection pressure : The machine should be capable of giving the following : First stage : < 1500 bars; 150 MN m-2; 21400 psi. Second stage (dwell or follow-up pressure ) : < 750 bars; 75 MN m-2; 10700 psi.

25. Screw rotational speed (rpm) : High screw speeds possible but better to run at slow speeds.

26. Back pressure : < 150 bars; 15 MN m-2; 2140 psi.

27. Shutting down : No need to purge with another material; empty the hopper and purge the barrel clean.

28. Reprocessing : Up to 100% regrind may be run.

29. Finishing : Degating is relatively easy, e.g. by hand using a sharp knife or scalpel. Drilling or tapping, to produce a thread, may prove difficult or unsatisfactory in service, so inserts are used. Mouldings can be readily decorated, e.g. by (I) silk screening, (ii) printing, (iii) hot foiling (coloured or metallic), or (iv) painting. Components are readily joined by ultrasonic welding or solvent welding.

30. Other Comments : This material readily attracts dust and should therefore be kept covered at all times.

31. Typical Comments : Toys, containers, tape cassettes, disposable tumblers, etc. Usually parts that require a combination of colour, clarity,stiffness, low cost and good appearance but which do not require high heat, solvent and impact resistance. When suitably light stabilised the material is used in light fitting applications, e.g. diffusion louvres. The addition of glass fibres improves stiffness, impact strength and heat distortion temperature thus opening up new markets. Rubber modification and / or copolymerisation are other techniques widely employed (see Styrene Acrylonitrile and Toughened Polystyrene).


2.POLYPROPYLENE

1. Common name : Polypropylene

2. Abbreviation : PP.

3. Systematic chemical name : Poly (Polypropylene)

4. Some Suppliers : 5. Trade names or trade marks :
Amoco Chemicals Amoco PP
Anic SpA Kastilene
Ato Chimie Lacqtene P
BASF Novolen
Chemische Werke Huls Vestolen P
Chemie Linz Duplen PP
Eastman Chemical Tenite
Hercules Chemicals Profax
Hoescht Hostalen PP
ICI Propathene
Montedison Moplen
Shell Shell P
Soltex Fortilene
Solvay Eltex P

5. Maerial properties : PP usually refers to both polypropylene and to copolymers in which propylene is the major constituent. Such materials are stiffer than polyethylene and have
a lower density (0.9 g cm -3). It naturally translucent with a milky white colour ; many different colours are possible. These materials have a high softening temperature, are stiff and have very good stress-crack resistance. Electrical insulation is good and a high gloss, scratch-crack resistant surface is possible. By copolymerisation (e.g. with ethylene and / or by rubber modification) materials with improved resistance to low temperature embrittlement are produced; rubber modification improves the impact strength for both homopolymers and copolymers but reduces the Vicat softening point.

6. e of Flow : A relatively easy flow material but higher barrel and mould temperatures are needed compared to polyethylene. PP has a higher viscosity than LDPE but a than PMMA. In general, high viscosity grades give lower cavitation or void problems.

7. Srinkage : A crystalline material which shrinks by approximately 2%; it shrinks less than polyethylene (PE) but more than polystyrene. As thick sections cool more slowly than thin sections, shrinkage in thick sections is greater. Shrinkage is approximately 0.018 in/in and is nearly uniform in all directions; in this respect it is better than HDPE.

8. Resistant to the following : Acid (dilute and concentrated), alkalis, alcohols, detergents, salt solutions, fruit juices, oils, aromatic hydrocarbons, chlorinated hydrocarbons; also resistant to materials that cause environmental stress-cracking or PE . Aromatic hydrocarbons (e.g. xylene) and chlorinated hydrocarbons (e.g. chloroform) will cause swelling at room temperatures; also swollen by ethers, esters and aqueous oxidising agents.

9. Not resistant to : Outside weathering (unless stabilised), concentrated oxidising acids, e.g. strong nitric or acidic potassium dichromate. Dissolved by aromatic and chlorinated hydrocarbons at elevated temperatures, e.g. 85°C. High temperatures and contact with copper will cause serious degradation very quickly.

10. Material detection or identification : With a density of 0.9 g cm-3 this material floats in water and cuts easily with a sharp knife. At room temperature it is insoluble in all solvents but will dissolve in boiling toluene. When placed in a flame it burns readily with a yellow tipped, blue flame and burning drips are produced together with a candle-like smell; on removal from the flame it continues to burn. Little smoke is produced. Components feel hard and waxy; they are virtually unbreakable unless cut. Has a higher softening point and a lower density than HDPE. PP will float in isophorene but sink in a 80:20 mixture (by volume) of alcohol and water ¾ this comment refers to unfilled grades.




11. Colouring : A wide range of colours is possible; the accepted method is to use compounded material. Dry colouring is possible but dispersion may be difficult and streaking may occur; if possible use PP powder when dry colouring. Masterbatches are also used and in this case the granule size of the masterbatch should be similar to that of the base polymer.

12. Material and component handling : Predrying is not normally required. A great deal of PP is used to manufacture crates and with such products creep may occur due to the components being overstacked; the product may become distorted or the stack may collapse if the creep is excessive.

13. Mould and gate considerations : Gate into the thickest part of the moulding if possible and keep the wall thickness as uniform as possible so that distortion is minimised. To avoid sink marks use large full round spruse ( also full round runners if possible) and keep the size of ribs small, e.g. 50% of the adjacent wall thickness. Insulated-runner and hot-runner moulds are widely used; pinpoint gates are useful with some components but if high injection speeds are used be careful that jetting does not occur. Tab gates are widely used.
Different cooling of the mould (so as to achieve uniform cooling of the product) can help
to minimise warping. Voiding may be a problem with thick sections of homopolymer.

14. Flow path: wall thickness ratio : High ratios of flow path : wall thickness are possible, e.g. at 1mm wall thickness 175:1 may be possible. Very easy flow grades are available for use on fast cycling machines, in which the ratio may reach 350:1.

16. Projected area : Usually 2 tsi (30 MN m-2) of clamping pressure is sufficient; high MFI
Materials (easy flow grades) may be moulded at 1.5 tsi (22 MN m-2) and low MFI (stiff flow grades) may require 4tsi (60 MN m-2).

17. Cylinder equipment : Nozzle usually equipped with a sealing type nozzle and the screw
usually has a back-flow valve. Decompression may prove advantageous when high MFI materials (easy flow) are being moulded.

18. Screw cushion : About 4 mm.

19. Shot capacity : Up to approximately 85% of the cylinder's rated capacity in polystyrene may
used.

20. Melt temperature ¾ as measured in the nozzle or by an air-shot technique : Below 230° C the
melt viscosity is relatively high: process above this temperature, i.e. 230¾275° C
446¾ 525°F). Do not exceed 275° C as the viscosity changes rapidly and oxidation
may easily occur.

21. Barrel residence time : Do not leave the machine standing for long periods at high temperatures otherwise degradation will occur. PP is more susceptible to oxidation than polyethylene as it posses many tertiary carbon atoms.

22. Temperature settings: See Table 2.

23. Injection Speed ¾ mould filling speed : High speed needed, therefore set as high as possible consistent with other requirements, e.g. surface gloss and cavitation (in thick mouldings).

24. Injection pressure :The machine should be capable of giving the following : Fist stage: up to 1800 bars; 180 MNm2; 26 100 psi.Second stage (dwell or follow-up pressure): up to 1500bars; 150 MN m-2; 21 700 psi. High dwell pressure may need to be applied for long times in order to avoid voids or excessive shrinkage; however, avoid overpacking.
TABLE 2

Zone No. Location Temperatures ° C Temperatures ° F
From To From To

1 Barrel rear 170 210 338 410
(near the hopper)
2 Barrel middle 210 230 410 446
3 Barrel middle 220 250 428 482
4 Barrel front 230 260 446 500
5 Nozzle 240 270 464 518
Mould 5 80 41 176

25. Screw rotational speed (rpm) : High screw speeds are possible; it is best to set the speed so
That plasticisation suits the cooling cycle, i.e. as slow as possible.

26. Back pressure : Up to 200 bars; 20 MN m-2; 3000 psi.

27. Shutting down : No need to purge with another material when shutting down.

28. Reprocessing : A small amount of regrind (e.g. 20%) can be blended into virgin polymer.

29. Finishing : When suitably treated (e.g. etching) components may be electroplated or vacuum metallised. Hot foiling, printing and painting are also possible. Removal of thin flash may be difficult with a knife as a rough edge may be produced and the product may be scarred; such flash has been removed with a flame. Automatic degating is possible if submarine gates have been used. Large gates may be removed by hand cropping or by machine cropping.

30. Other Comments : Because of this material's resilience and flexibility some undercuts may be stripped from the mould; snap-fits are possible. Integral hinges can be moulded into a component and the hinge may have virtually indefinte flex-life.

31. Typical Components : Housings with moulded integral hinges, luggage, housewares (buckets, bowls, etc.), toys, interior parts for cars, picnic ware, washing machine drums, pump components, etc. Rubber modified PP (e.g. which contains 30% rubber) is widely used for car bumpers and rubber modified (10% rubber) copolymers are used for bottle crates
Components of high rigidity and/or high heat distortion temperature can be obtained with
Talc or glass filler. However, the addition of both these fillers raises the density, e.g. talc-filled
Polymers may have densities up to 1.3 g cm -3 and glass filled grades may reach 1.2 g cm-3
(depends on filler content)
Components made from PP can be sterilised(e.g. by steam or radiation) and so the
material is used to make hospital equipment, e.g. disposable syringes. Moulding will suffer creep if subjected to high stress and such conditions should therefore be avoided



3. ACRYLONITRILE – BUTADIENE -- STYRENE(ABS)


1. Common name : Acrylonitrile -butadiene Styrene

2. Abbreviation : ABS

3. Systematic chemical name : Not known -- one suggestion is poly(1-butenylene - g-(1-phenylethylene -co-1-cyanoethylene).

4. Some Suppliers : 5. Trade names or trade marks
Abson Abtec
Anic SpA Ravikral
BASF Terluran
Bayer Novodur
Borg Warner Cycolac
Dow Dow-ABS
DSM Ronfalin
Goodrich Abson
Monsanto Lustran
Montedison Urtal
Polymon Formid
Rhone Poulenc Afcoryl
SIR Restiran
Sterling Sternite ABS
Ugine Kulmann Ugikral

6. Material properties : A hard, tough material with good resistance to impact even at low temperatures. The material has low water absorption and this means that its electrical insulation properties, which are good, are relatively unaffected by humidity changes. It is normally available in translucent or opaque colours and the resultant mouldings can have a high gloss. The surface is resistant to scuffding and marking but the unmodified material has not got very good weathering properties. Superior heat resistance and impact strength compared to HIPS. Density about 1.05 but can vary depending on grade, i.e. whether medium impact, self-extinguishing, glass filled, etc.

7. Ease of flow : Relatively easy flowing. The ease of flow will depend on the grade selected but will, broadly speaking, be similar to PS.

8. Shrinkage : About 0.6%, i.e. about 0.005 in/in.

9. Resistant to the following : Resistant to staining and to alkalis, acids (not concentrated oxidising acids), oils and fats; the majority of alcohols and hydrocarbons.

10. Not resistant to : Acetone, carbon tetrachloride, ether, ethylene dichloride, ethyl acetate, toluene, trichloromethane, i.e. fairly low molecular weight organic materials such as ketones, esters, aldehydes and some chlorinated hydrocarbons.

11. Material detection or identification : Cuts cleanly with a knife and cuts have smooth edges; ranges from semibrittle to tough. Burns with a yellow, sooty flame and gives off an acrid odour which also smells of rubber. As the material has a density of 1.05 g cm-3 it will sink slowly in water. Resists carbon tetrachloride better than HIPS.

12. Colouring : Natural colour is an ivory shade so a wide range of colours is possible; dark colours give the best resistance to ageing. Problems may arise, however, due to the high processing temperatures employed and also due to leaching. Dry colours and masterbatches are used but may cause problems if an exact colour match is required. Where such matching is required the normal procedure is to purchase the coloured polymer from a specialist compounder or a raw material manufacturer. Base polymer colour affects the final colour and should therefore be controlled.

13. Material and component handling : Material requires moisture removal before processing in order to achieve optimum properties. Strict control of storage can help to eliminate extensive drying operations. Drying temperatures of 70 - 80°C for 2 - 4h may be used.
Many ABS mouldings are electroplated and in order to minimise surface marking the
Mouldings should be handled as little as possible and when handling is necessary clean
gloves (changed regularly) should be worn. The components should be wrapped, e.g. in
polyethylene bags.

Absorbs water more than PS or HIPS so predrying may be necessary if mouldings
are blisteredor streaked. If a vented machine is not available predry for about 4 h at
80°C. Predrying is necessary if the material has absorbed more than 0.2% moisture drying storage.

14. Mould and gate considerations : Similar to PS; pinpoint, hot runner or insulated runner may be used. Where electroplating or metallising of the components is to be performed the feed systems should be designed so that high levels of stress are avoided, e.g. by using large gates and runners.
Where unavoidable surface defects are obtained these may be masked by the use of
grained, mould surfaces, such grained surfaces also help to mask any marring which
occurs in service.

15. Flow path: wall thickness ratio : Similar to PS. At 1 mm wall thickness the flow path: wall thickness ratio will be about 150:2 for PS and 140:1 for ABS. Easy flow grades of ABS are however available and such grades will give a much higher value.

16. Projected area considerations : The clamping pressure required is usually of the order of 2 tsi of the projected area (30 m-2).

17. Cylinder equipment : Because of water absorption vented barrel can be advantageously used with this material. The cylinder is usually fitted with an open nozzle as shut-off nozzles may cause burn marks; a valve, to prevent back-flow, is usually fitted to the screw.

18. Screw cushion :About 4 mm.

19. Shot capacity : Up to 90% of the rated (PS) capacity of the machine may be used.

20. Melt temperature ¾- as measured in the nozzle or by an air-shot technique : Surface gloss is improved by high mould and melt temperatures, e.g. 220 - 260°C. Do not exceed 270°C as discoloration will occur.

21. Barrel residence time : If high melt temperatures are being used then the residence time in the barrel will be low as otherwise discoloration or changes may occur. Different barrel residence times on different machines can cause colour changes.

22. Temperature settings : Please note that it is the melt temperature which is important; those in Table 3 are only suggested, initial settings. The temperature of the hydraulic oil and of the material in the hopper should not vary excessively.

23. Injection Speed ¾ mould filling speed : Very high pressures are not needed to get high speeds; to get the best surface finish (e.g. for electroplating) use injection velocity control. Surface finish is improved by using well dried material, high melt and mould temperatures and slow filing speeds.

24. Injection pressure : The machine should be capable of giving the following : Fist stage: up to 1500 bars; 150 MN m-2; 21 400 psi. Second stage (dwell or follow-up pressure): up to 750 bars; 75 MN m-2; 10 700 psi. It is not necessary to have long dwell times or high packing pressures.

TABLE 3
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone Location Temperatures ° C Temperatures ° F
No. From To From To
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 150 180 302 356
(near the hopper)
2 Barrel middle 180 230 356 446
3 Barrel middle 210 250 410 482
4 Barrel front 210 260 410 500
5 Nozzle 210 250 410 482
Mould 60 90 140 194
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾

25. Screw rotational speed (rpm) : High screw speeds can be used with this material but it is better to run at slow speeds if possible, e.g. 100 rpm.

26. Back pressure :Up to 250 bars; 25 MN m-2; 3570 psi. At high temperatures and with insufficient back pressure air can cause scorching of the polymer.

27. Shutting down : Purging with a different material is not necessary. Note that ABS and PS are not compatible.

28. Reprocessing : When regrind is being used it is blended with virgin material in relatively small quantities, e.g. ratio of virgin to regrind is 75:25. Only good quality regrind should be used, e.g. free from oxidation, contamination, etc. Do not use regrind for electroplating applications.

29. Finishing : A wide range variety of techniques are used to degate small mouldings, e.g. cropping, cutting(scalpel or knife) or automatic systems which are operated by the mould opening action. Large gates may be removed by routing, sawing, etc. Mouldings may be decorated by printing, embossing and metallising. Assembly into large components may be made by using interference fits, gluing and ultrasonic welding. Static electricity may interfere with printing and surface charges should be removed. If the mouldings are degreased prior to electroplating then the degreasing chemical must be carefully chosen, e.g. to avoid stress cracking. Mouldings should be carefully handled and/or individually wrapped as they scratch easily, e.g. the swarf can cause scratching during machining.

30. Other Comments : A very wide range of grades of ABS is possible as, for example, the material can be made by two main methods: (i) melt mixing of nitrile rubber with SAN, or (ii) polymerising styrene and acrylonitrile in the presence of a rubber (polybutadiene or styrene-butadiene rubber). The second procedure is the commercial procedure as this gives polymers with the best balance of properties. A 'typical' composition could contain 20-30% acrylonitrile, 20 - 30% butadiene and 40 -60% styrene.

31. Typical Comments : Luggage, appliance housings, radio and TV cabinets, vacuum cleaner cases, bathroom cabinets, complete instrument panel surrounds on cars, telephone handsets, grilles, safety helmets, etc. When used in automotive applications the material is commonly electroplated as, by chemical etching techniques, good adhesion occurs between the metal and the moulding. In such applications a good surface finish on the mouldings is essential as the plating will emphasise (not hide) surface marks.



4. POLYCARBONATE (PC)


1. Common name : Polycarbonate.

2. Abbreviation : PC.

3. Systematic Chemical name : Poly(oxy-1,4-phenylene dimethylmethylene-1, 4-phenylene-oxy-carbonyl).

4. Some suppliers : 5. Trade names or trade marks:
Anic Sinvet
Bayer Makrolon
Ciba-Geigy Verafil
General Electric Lexan
Mobay Merlon
Mitsubishi Novarex

6. Material properties: Polycarbonates are a type of long-chain polyester in which aromatic
groups are linked by the carbonate ester group (OCOO) . The material is strong, stiff,
tough and transparent; it maintains its properties over a wide temperature range and is
commonly classed as an engineering plastic. The material is also virtually self-extinguishing and
has good electrical insulation characteristics but is not recommended for use in the presence of an
electric arc. Special care is required during electric arc. Special care is required during
limited resistance to notches, chemicals and ultra-violet light (see Section 30). It is fairly
expensive and susceptible to crazing when strained and this last point mars an otherwise
excellent resistance to creep. PC is use in blends with ABS as such materials are readily coloured,
strong, have good light-fastness, high heat distortion temperatures and are relatively easy to mould.

7. Ease of flow : Poor. This material has a high melt viscosity.

8. Shrinkage: An amorphous material which shrinks about 0.7% i.e. about 0.005 in/in. Shrinkage appears to be uniform, i.e. the same both with and across the direction of flow.

9. Resistant to the following : Dilute acids, oil, petrol, high molecular weight alcohols, detergents, aliphatic hydrocarbons, oxidising agents and trichloroethylene. Good resistance, for a thermoplastic, to ionising radiation.

10. Not-resistant to : Caustic Soda, amines, ammonia, strong acids, certain chlorinated hydrocarbons (e.g. chloroform and methylene chloride), dioxan, cyclohexanone, pyridine and hot phenols. Attacked by water at temperatures greater than 60°C. Frozen-in strain may be assessed by noting resistance to carbon tetrachloride.
Swollen by acetone, benzene and carbon tetrachloride; aromatic hydrocarbons,
esters and ketones may cause stress -cracking. Heavy loads, applied to PC gears, can
result in unacceptable wear resistance or high friction levels.

11. Material detection or identification: With a density of 1.2 g cm-3 this material sinks in water. When placed in a flame it burns (but not very easily) with a yellow flame and the moulding chars and blisters; the moulding will not continue to burn if the flame is removed and a faint odour of phenol is produced. Soluble in methylene chloride and ethylene dischloride. Polycarbonates have a very high carbon content and give characteristic infra-red absorption spectra. Softens at temperatures above 150°C but serviceable up to 135 °C.

12. Colouring : Due to the high processing temperatures ( and therefore degradation problems ) the choice of colours may be restricted compared to other thermoplastics, e.g. some organic colourants can be adversely affected by the high temperatures. Dry colouring is used but ensure that the colourant is dry. Compounded material is preferred by many moulders as the use of this eliminates such problems.

13. Materials and component handling : Any trace of water will cause problems, such as frothing, to occur. Therefore rigorous predrying is essential if components with superior properties (e.g. impact strength) are to be obtained. Water content must be reduced or maintained below 0.02% by heating the sealed (unopened) tins at 115°C for 3h ¾- maintain this temperature by using a heated hopper (120°C) on the machine. Wetness in the granules may be tested for by heating at 270°C for 1 min on a covered glass slide(TVI test). After cooling bubbles will be seen in damp material and the number of bubbles may be related to the water content.

14. Mould and gate considerations : Follow polystyrene (PS) practice. Pinpoint gates may be used for small articles. In general, gates should be of a reasonable depth, e.g. 70% of the maximum wall thickness and with a minimum size of 1 mm. Flash, ring and diaphragm gates are also used; use short, full round runners if possible. Hot-runner moulds are successfully used with this material.

15. Flow path : wall thickness ratio : At 1 mm wall thickness the maximum flow path:wall thickness ratio for this material is lower than that found for UPVC or PMMA ¾ this is partly due to high melt viscosity but is also due to the high set-up temperature.

16. Projected area : Usually 3 - 5 tsi (45 - 75 MN m-2) clamping pressure is used.

17. Cylinder equipment : The inside barrel surface can be damaged or etched when PC is processed. Hence replaceable barrel liners are sometimes employed as these can be changed relatively easily. Open nozzles may be employed but the screw is usually fitted with a back-flow valve. If drooling occurs with an open nozzle do not lower the nozzle temperature. If drooling is excessive fit a shut-off nozzle, e.g. a needle shut-off nozzle.

18. Screw cushion : About 4mm

19. Shot capacity: Up to 85% of the cylinders rated capacity may be used ¾ as little as 20% may be used if necessary.

20. Melt temperature : Accurate temperature control is required : 260- 310°C (500-600°F). During purging a 'good' melt will not show bubbles (moisture) or a silver lining (overheating). Periodically the injection unit should be withdrawn so that the melt can be inspected, this is because the bubbles may not show up in the moulding and 'bad' material will not make good mouldings.

21. Barrel residence time : This depends upon the temperature employed and the polymer grade ¾ some colourants may be degraded very quickly at the high melt temperatures necessary for this material.

22. Temperature settings : See Table 4. The use of high mould temperatures does not necessarily lengthen the cycle unduly as this material has a high glass transition temperature and therefore a high set-up temperature.

23. Injection speed ¾ mould filling speed: As high as possible consistent with other considerations, e.g. the pressure available, surface finish required, etc. When the flow is upset(e.g. by changes in cross-section) the surface appearance may be marred, therefore programmable injection speed is an advantage.

24. Injection pressure : The machine should be capable of giving the following: First stage: up to 2000 bars; 200 MN m-2; 29 000 psi. Second stage (Dwell or follow-up pressure): up to 1200 bars; 120 MN m-2; 17 400 psi.

TABLE 4
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone Location Temperatures ° C Temperatures ° F
No. From To From To
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 250 260 482 500
(near the hopper)
2 Barrel middle 270 280 518 536
3 Barrel middle 280 300 536 572
4 Barrel front 280 300 536 572
5 Nozzle 290 300 555 572
Mould 85 120 185 248
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾

25. Screw rotational speed (rpm) : Because of the high melt viscosity high screw speeds are not recommended. If possible a screw motor which can develop high torque should be used. Use a speed of about 45 rpm, e.g. on a 40 mm (1.57 in) screw.
26. Back Pressure : Up to 300 bars; 30 MN m-2; 4350 psi. Degradation (due to oxidation) may occur if back pressure is not used.

27. Shutting down : PC has a reputation for strong adhesion to metal and if the injection barrel is allowed to cool with PC inside then metal may be pulled out of the barrel wall. Therefore if there is a break in production(e.g. an overnight stop) empty the barrel and keep the heat on e.g. at 170°C. At the end of a run purge out with a very stiff flow PE. Do not , repeat not, put PVC, POM or celluloses into a cylinder set at PC temperatures. For thorough cleaning, it will be necessary to remove the screw from the barrel so that mechanical cleaning can be employed.

28. Reprocessing: Up to 20% of regrind can be added provided that it is dry; if more is added then the mechanical properties will suffer. Drying may take 20h at 115°C. Do not re-use material which was processed when wet or damp.

29. Finishing : Large sprue gates may be removed by milling (e.g. end-milling) and side gates may be removed by routing. For less critical applications, cropping may be employed. Take care to avoid scratching or galling ¾ PC can even be scratched with PMMA.
Components may be decorated by silk screening, spraying and hot foiling. Mouldings may be joined by or ultrasonic welding cementing, e.g. using methylene Chloride or ethylene chloride ¾ it may be better to make a cement by dissolving 8% of PC in one of these solvents. Mouldings can also be joined with expoxide resins and with hot-melt adhesives, e.g. based on polyamides.

30. Other comments : Shrinkage is reduced if glass fibre is added to the material ¾ with 30% glass fibre the shrinkage could be only 0.002 in/in but the density could be 1.43 g cm-3. When correctly stabilised the material has good outdoor weathering properties. (Weather resistance can be obtained by applying solutions containing ultra-violet absorbers.)

31. Typical components : Because of their high heat distortion temperature (HDT), low moisture absorption, low creep and high strength these materials are widely used in electronic and electrical applications. The electrical properties show little dependence on current frequency and are not substantially changed after mouldings have been immersed in water for long periods or after being heated to high temperatures, e.g. 130°C. Their clarity, impact resistance and good weathering resistance (when suitably stabilised) makes them suitable for glass replacements where breakage is a problem, e.g. due to vandalism. Therefore the material is used in outdoor lighting applications, safety helmets, filters, car lamp housings, goggles, lenses, food mixer parts, tableware, housings for computers, guard panels, terminal boxes, etc. Polycarbonates based on trimethyl bisphenol A (as opposed to bisphenol A ¾ used for conventional PC) have Vicat softening points 50°C higher, e.g. 195°C. They are not so resistant to impact, however, and they have a poorer colour.



5. POLYPHENYLENE OXIDE (MODIFIED)(PPO)

Common name : Polyphenylene oxide (modified).

Abbreviation : PPO or modified PPO.

Systematic chemical name : Modified poly(oxy-1,4-phenylene) has been suggested.

Some suppliers : 5. Trade names or trade marks :
General Electric Noryl
Engineering Polymers Noryl

5. Material properties : The term 'Noryl' covers a wide range of materials as these materials
are mixtures of PPO and other polymers, e.g. PPO and PS or PPO and TPS; they are easier to process and cost less than PPO itself. In general, modified PPO is a tough, stiff material with a wide temperature range of use (e.g. from -40 to 130°C); at elevated temperatures it maintains its good load bearing characteristics. Because of this and their low water absorption, mouldings have good dimensional stability. The material also has excellent dielectric properties, a low coefficient of expansion and may be rated as self-extinguishing and nondripping, it is an opaque material with good resistance to hydrolysis.

6. Ease of Flow : Modified PPO has a fairly high viscosity but it is lower than PPO; the viscosity is greater than PC but lower than ABS.

7. Shrinkage : This amorphous material exhibits low shrinkage, e.g. 0.007 in/in which can be considerably reduced by the addition of glass fibre (e.g. down to 0.002 in/in when 30% glass is used). The shrinkage observed varies only slightly with changes in injection pressure, melt temperature, mould temperature, wall thickness and the flow direction.

8. Resistant to the following : Outstanding resistance to aqueous environments ¾ only absorbs about 0.2% moisture from boiling water at equilibrium. Not attacked by dilute acids, alcohols, detergents, dilute alkalis.
Can withstand exposure to elevated temperatures; the heat distortion temperature
ranges from 90 to 150°C(dependent on grade).

9. Not resistant to : Halogenated and aromatic hydrocarbons, ketones and low molecular weight esters. Unless adequately stabilised, ultra-violet light resistance can be poor.

10. Material detection or identification : Has a fairly low density (1.06 g cm-3) which can be reduced (e.g. to 0.9 g cm-3) by the use of blowing agents. As the natural material is not very attractive it is usually seen as coloured opaque mouldings. Mouldings are tough and relatively difficult to cut. When burnt a yellow, sooty flame results and there is an odour of phenol.

11. Colouring : In-house colouring is not normally performed as the use of masterbatches, dry colours, etc. may lead to a serious loss of properties, delamination, etc. It is normally purchased already compounded and a wide colour range in opaque colours is available. Black or grey colours will have the best resistance to ultra-violet light.

12. Material and component handling : Will absorb 0.07% water in 24 h at room temperature (similar to PBT). Normally predrying is not necessary, if drying is necessary use 100°C and dry for up to 2 h (conditions depend on grade). Do not exceed 6h drying.

13. Mould and considerations : Tab or fan gates are preferred as these eliminate jetting, gate size should be as large as possible so that fast mould filling is obtained. Sprues and runners should also be relatively large, e.g. the small end of the sprue should be greater than 3mm diameter and the sprue should be well tapered. Generous tapers, rounded corners, etc. help to ease ejection ¾ problems may be caused through the low shrinkages experienced with this class of material. Where long flow are envisaged consideration should be given to hot runners(insulated runners are not recommended). On hot-runner moulds good temperature control is essential, use an adequate number of cartridge heaters, and in order to get long heater life under-rate the heaters, i.e. 350 W kg-1 of steel. Avoid material stagnation points, e.g. by rounding the ends of flow channels and avoiding the use of heaters in the melt stream.

14. Flow path: wall thickness ratio : Very dependent on the mixture employed . It can be tailored to suit the application within reasonable limits. It can, for example, cover the range quoted for ABS.

15. Projected area: Where venting (and therefore burning) is a problem reduce the clamping pressure to the lowest value possible so as to allow the escape of trapped volatiles. Usually 2-3 tsi (30-45 MN m-2) is sufficient.

16. Cylinder equipment : Open nozzles with an orifice diameter of 3 mm (1/8 in) or greater should be used. Drooling can be prevented by good temperature control and/ or by decompression; shut-off nozzles are not recommended. General purpose screws may be used providing that the screw length is adequate, e.g. 17:1 and that the compression ratio is not excessive, e.g. below 3.5:1.

17. Screw cushion : About 5 mm.

18. Shot capacity: 40-80% of PS shot capacity. Avoid small shots on large equipment as such severe under-utilisation will cause stagnation and therefore degradation.

19. Melt temperature : 250 - 290°C. Higher temperatures than this have been used (e.g. 310°C) with unreinforced resins when the shot size and barrel capacity have been closely matched on fast-running jobs. Some heavily filled grades are run at 300°C in order to improve flow.

20. Barrel residence time : At 280°C the barrel residence time is approximately 10 min. If a stoppage of more than 10 min is anticipated then reduce the temperature settings, e.g. so that melt is about 180°C (356°F).

21. Temperature settings : See Table 5.
TABLE 5
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone Location Temperatures ° C Temperatures ° F
No. From To From To
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 190 210 374 410
(near the hopper)
2 Barrel middle 210 240 410 464
3 Barrel middle 230 270 446 518
4 Barrel front 250 290 482 554
5 Nozzle 240 275 464 527
Mould 80 110 176 230
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
22. Injection speed¾ moulding filling speed: As high as possible consistent with surface finish, venting requirements, etc. Improved fusion of the material around the cores is achieved by the use of high speeds at high mould temperatures.

23. Injection pressure : The machine should be capable of giving the following : First stage : up to 1500 bars; 150 MN m-2; 21 400 psi. Second stage (dwell or follow-up pressure): up to 1000 bars; 100 MN m-2; 14 100 psi.

24. Screw rotational speed (rpm): Adjust speed to suit the moulding cycle, a high torque motor should be fitted and screw speed shall be kept low, e.g. below 100 rpm.

25. Back pressure : 35 bars; 3.5 MN m-2; 500 psi.

26. Shutting down : Turn off heat and empty the barrel. PS and PMMA are effective purging materials for PPO.

27. Reprocessing : Up to 100% regrind can be used; however, it is best to re-use the material as it is formed in blends with virgin material, such blends may contain up to 25% regrind. Do not use degraded or contaminated material.

28. Finishing : Because of the ductile nature of this class of materials, trimming can be performed without shaterring. Mouldings can be decorated by a wide variety of techniques, e.g. hot stamping, hot foiling, painting, printing, etc.

29. Other comments : The stiffness and creep resistance of this material is considerably enhanced by the addition of glass fibre. Flame retardant grades are available which are halogen-free.

30. Typical components : By incorporation of polystyrene into polyphenylene oxide the heat distortion temperature is lowered but it is still high enough for many engineering applications. The presence of the PS makes processing easier. The material's good resistance to detergents, and its ability to be moulded to close tolerance, has helped it to become established for washing machine, dishwasher and pump components. It has a useful combination of properties, e.g. heat resistance, impact strength and flame resistance. Most important markets are in the electrical and automotive industries, e.g. for the backplates of TV sets and for car dashboards. This material is more expensive than ABS but cheaper than PC. Structured foam components are a major market for Noryl, e.g. for machine housings. The material is reported to be under active development as a material of construction for motorcycle engines, e.g. blocks and crank cases.




6.HIGH DENSITY POLYTHYLENE (HDPE)


1. Common name :High density Polyethylene.

2. Abbreviation : HDPE.

3. Systematic chemical name : High density poly(methylene) .

4. Some suppliers : 5. Trade names or trade marks :
Anic SpA Eraclene HD
BASF Lupolen HD
BP Rigidex
Chemische Werke Huls Vestolen A
Chemplex Chemplex
Dow Dow Polyethylene
DSM Stamylan HD
Hoescht Hostalen
Montedison Moplen R O
Solvay Eltex
Union Carbide
Wacker Chemie Wacker Polyäthylen

6. Material properties : In the presence of sterospecific catalysts, and under conditions
of low temperature and pressure, ethylene will polymerise to give a substantially linear polymer. Because of its regularity this material has a higher level of crystallinity then LDPE. This increase gives plastics which have a higher density, rigidity, tensile strength, hardness, heat distortion temperature, chemical resistance, viscosity and resistance to permeability; however the impact strength is lower. For some applications the weathering resistance is adequate but is improved by compounding with carbon black. Compared to PP homopolymer, HDPE has better resistance in low temperature impact and to oxidation. PP has a higher softening point, better resistance to flexing, a higher hardness, a higher tensile strength and elongation and will also cycle faster. Moulding gloss can be similar for PP and HDPE.

7. Ease of Flow : This material is a relatively easy flow material but compared to LDPE the high density materials are usually offered as lower MFI materials in the injection moulding grades.

8. Shrinkage : Because of the linear nature of the molecule high levels of crystallinity can be obtained; high shrinkage values can therefore be obtained and these can be markedly different in two directions, i.e. across the flow and with the flow. Distortion can therefore be a problem; shrinkage values of up to 0.05 in/in are possible for thick sectioned mouldings. However, shrinkage values from 0.015 to 0.040 in/in are more usual.

9. Resistant to the following : No solvents at room temperature. Not chemically attacked by non-oxidising acids, alkalis and many aqueous solutions. Some organic materials (e.g. white spirit, carbon tetrachloride) will cause swelling; in this respect HDPE is better than PP. Theoretically, HDPE is more resistant to oxidation than LDPE; in practice they are similar (this could be due to catalyst residues).

10. Not resistant to : Ultra-violet light, oxygen at high temperatures and oxidising acids. Resists aromatic and chlorinated hydrocarbons more than LDPE. Stress-cracking can be a problem; that is cracking caused by a stress (internal or external) in the presence of an active liquid, e.g. detergent or metal soap. PP is best in this respect.

11. Material detection or identification : Similar to that described for LDPE. High density material is more chemically resistant (it dissolves in benzene at 85°C; LDPE at 60°C), more opaque and has, of course, a higher density and rigidity. It is slightly more difficult to cut than LDPE but the cut also has smooth edges. HDPE has a higher melting or softening point than LDPE but lower than PP; all three materials will float in water but can be seperated, or identified, by the use of melting point and density determinations (e.g. using alcohol/water mixtures).

12. Colouring : A wide range of pelleted, coloured compounds is available from the manufacturers although masterbatching is the main technique used at the present time (the masterbatches may be based on waxes, HDPE or LDPE). Liquid colour is used successfully; care should be taken as too much may cause screw slip. The use of certain colourants may alter crystallinity levels (and therefore shrinkage and/or distortion) no matter which way they are added, e.g. phthalocyanine pigments as used for blues and greens.



13. Material and component handling : Predrying is not normally necessary but if required it may be performed at 85°C for approximately 3h; as with many black materials (based on carbon black) predrying may be required for this colour. Some large components (e.g. crates) may need to be held on cooling jigs after mouldings for a few minutes (e.g. 10) in order to reduce warping and to maintain sizes ¾ this is necessary as crates must fit together so that they can be stacked.

14. Mould and gate considerations : Mouldings may exhibit warping caused through differential shrinkage. Warping may be minimised by differential cooling, i.e. maximum cooling in the gate region, or by proper grade selection. A grade with a narrow molecular weight distribution is required and the affect of additives on crystallinity (see Section 13) should not be forgotten. Flow path lengths should also be evened out as much as possible. For large components this may mean using multiple gates, e.g. crates commonly have four gates (one in each corner).
Hot-runner moulds are widely used and the material is good for insulated runners
because of the thermal properties (high specific heat, low thermal conductivity, etc.).

15. Flow path: wall thickness ratio : The ratio of maximum flow path to wall thickness is usually less for HDPE than for LDPE . For example at 1 mm wall thickness HDPE may be 170:1 whereas LDPE could be 200:1. The material is similar to PP in this respect.

16. Projected area: Similar to PP, i.e. 2 tsi (30 MN m-2) is usually sufficient.

17. Cylinder equipment : Barrels are usually equipped with a sealing type nozzle and the screw usually has a back-flow valve. Decompression may prove advantageous when easy flow grades (high MFI grades) are being moulded.

18. Screw cushion : About 2 to 6 mm.

19. Shot capacity: 80- 85% of cylinders rated capacity (on PS) may be used.

20. Melt temperature ¾ as measured in the nozzle or by an air-shot technique: 205 - 260°C (400 - 500 °F). Usually thin walled components use the higher temperature; crate moulding may be done at 230°C.

21. Barrel residence time : A fairly tolerant material, i.e. one is not worried about thermal decomposition provided that reasonable care is taken. It is good practice not to leave any material to 'cook' in the barrel if moulding is interrupted; turn down the temperature settings if there is a break in production. A more thermally stable material than PP.

22. Temperature settings : Please note that it is the melt temperature which is important; those in Table 6 are only suggested, initial settings. The temperature of the hydraulic oil and of the material in the hopper should not vary excessively.
Higer melt temperatures usually result in mouldings having higher gloss but, of course
Cycle times are correspondingly longer.

TABLE 6
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone Location Temperatures ° C Temperatures ° F
No. From To From To
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 160 200 320 392
(near the hopper)
2 Barrel middle 170 230 338 446
3 Barrel middle 200 260 392 500
4 Barrel front 220 280 428 536
5 Nozzle 210 270 410 516
Mould 5 50 41 122
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾

23. Injection speed¾ moulding filling speed: As high as possible consistent with other requirements, e.g. surface gloss.

24. Injection pressure : The machine should be capable of giving the following : First stage : up to 1800 bars; 180 MN m-2; 26 100 psi. Second stage (dwell or follow-up pressure): up to 1500 bars; 150 MN m-2; 21 700 psi. Avoid overpacking as this will result in mouldings which will be brittle in the gate region.

25. Screw rotational speed (rpm): Run at as low a screw speed as possible so that uniform plasticisation is obtained; this material requires a large heat input.

26. Back pressure : Up to 200 bars; 20 MN m-2; 3000 psi.

27. Shutting down : Purging with another material is not necessary when the machine is being shut down.

28. Reprocessing : The material is commonly regranulated and blended in (e.g.25 to 30%) with the virgin polymer ¾100% scrap can be run if required, provided that the material is kept clean and dry. HDPE is more tolerant than PP in this respect.

29. Finishing : HDPE is more difficult to electroplate than PP (little electroplating is done as markets do not demand such a finish), it can be decorated by vacuum metallisation, hot foiling and printing(after the surface has been made receptive, e.g. by flame treatment). Hot embossing of mouldings (with a name or legend) is common for items such as crates. Flash removal is easy (easier than for PP) ; sprues and runners are easily removed by cutting or cropping (e.g. with a knife or side cutters).

30. Typical components : This material is strong and stiff , even at low temperatures. For these reasons it is used to produce components which must have a reasonable impact strength at low temperatures, e.g. milk-bottle crates, containers used in refrigeration, fish boxes, shipping pails for paint, adhesives, etc. It is also used for containers where its rigidity and improved resistance to gas permeability is an asset compared to LDPE, e.g. food storage boxes. Widely used for housewares (buckets, bowls, etc.) but because of its higher density it is at disadvantage compared to PP. At the same price per tonne, PP would have a 10% price advantage (assuming of course that the moulding sections are the same). Because of this material's resilience, and its better resistance to stress whitening compared to PP, it is widely used to make snap-on caps, e.g. for aerosols. In storage or in use a change in colour, caused by flexing, is a disadvantage for such an application.





7. HIGH IMPACT POLYSTYRENE (HIPS)

1. Common name : Toughened Polystyrene or high impact polystyrene.

2. Abbreviation : HIPS or TPS or SB.

3. Systematic chemical name : Not known ¾ one suggestion is toughened poly(1-phenylethylene).

4. Some suppliers : 5. Trade names or trade marks :
A to Chimie Lacqrene
BASF Polystyrol
BP
Chemische Werke Huls Vestyron
Dow Styron
Hoescht Hostyren S
Monsanto Lustrex
Montedison Edistir
Union Carbide

6. Material properties : A semirigid material produced by polymerising styrene in the presence of a rubber, e.g. polybutadiene. The fine rubber particles are chemically grafted on to the PS chains and this gives materials with a high impact strength but with lower strength, stiffness, hardness and softening point (cf. PS). The transparency and surface gloss of PS is also lost by the addition of the rubber. Normally the reistance to light is poor and the material has a poorer resistance to chemicals than PS. As the rubber concentration is increased the impact strength is also increased but the stiffness, tensile strength, density and sofetning point are decreased. A very high impact grade will have a density of 1.02 g cm -3. In general, HIPS is not such a good electrical insulator as GPPS.

7. Ease of Flow : Similar to PS . Both general purpose and easy flow grades available

8. Shrinkage : Similar to PS ¾ about 0.005 in /in.

9. Resistant to the following : Can resist machining operations, e.g. drilling, punching , etc. which would shatter PS. Will resist acids, alkalis, fats, oil and salt solutions.

10. Not resistant to : A wide range of aromaatic and chlorinated hydrocarbons; poorer chemical reisitance than PS.

11. Material detection or identification : Clear mouldings not possible (cf. PS). Cuts more easily than PS and when burnt gives off an odour of burning rubber as well as styrene. Sinks very slowly in water and softens at a lower temperature than PS.

12. Colouring : Dry colouring techniques (by tumble blending) are widely used and give satisfactory results; with large colourant concentrations adhesion of the colouring system to the granules may be improved by first tumbling the polymer with butyl stearate. Masterbatches are also used, e.g. for specific purposes such as with fluorecent pigments. Liquid colours have been satisfactorily used with HIPS.

13. Material and component handling : More prone to water absorption than PS, however, predrying is not usually necessary, although heat at 70°C for up to 3 h may be used. Normal packaging procedures adopted for components.

14. Mould and gate considerations : Similar to PS . Often moulded with gates of small cross-section (e.g. for disposable cups). Pinpoint, tab, insulated runner and hot runner are widely used. Submarine-ejector type gating used, e.g. for fascia mouldings.

15. Flow path: wall thickness ratio : Available in a range of grades. At a wall thickness of 1mm standard grades can have a maximum flow parh: wall thickness ratio of approximately 130:1. Easy flow materials available but these may have a lower softening point.

16. Projected area: The clamping force requirements range from 1 to 3 tsi (15 to 45 MN m-2). Easy flow materials can usually be moulded using low clamping forces.

17. Cylinder equipment : The cylinder is usually equipped with a shut-off nozzle when decompression is not available. A valve to prevent back-flow, is usually fitted to the screw.

18. Screw cushion : Usually about 4 mm, however, on some jobs a screw cushion is not used, e.g. disposable cups. Special machines have been built to mould these items and some manufacturers recommend that the screw cushion should be eliminated; accurate and reproducible dosing is necessary on such machines.

19. Shot capacity: 9 0% of the machine rated capacity may be used.

20. Melt temperature ¾ as measured in the nozzle or by an air-shot technique: 180 - 250°C (356-482°F). Surface gloss improved by high melt and mould temperatures. To obtain the uniformly polished matt finish (associated with a satisfactory HIPS moulding) the melt temperature may need to be fairly high.

21. Barrel residence time : Not critical although it is good practice not to let the material 'cook' in the barrel.

22. Temperature settings : Please note that it is the melt temperature which is important; those in Table 7are only suggested initial settings. The temperature of the hydraulic oil and of the material in the hopper should not vary excessively.

TABLE 7
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone Location Temperatures ° C Temperatures ° F
From To From To
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 160 190 320 374
(near the hopper)
2 Barrel middle 170 210 338 410
3 Barrel middle 180 230 356 446
4 Barrel front 190 250 374 482
5 Nozzle 180 240 356 464
Mould 10 80 50 176
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
23. Injection speed¾ moulding filling speed: Should be as high as possible but consistent with other requirements, e.g. surface gloss, jetting, etc. High pressures not needed to get fast flow rates.

24. Injection pressure : First stage : up to 1500 bars; 150 MN m-2; 21 400 psi. Second stage (dwell or follow-up pressure): up to 750 bars; 75 MN m-2; 10 700 psi.

25. Screw rotational speed (rpm): Fast screw speeds are possible but not necessarily desirable as their use can result in temperature fluctuations.

26. Back pressure : Up to 150 bars; 15 MN m-2; 2140 psi.

27. Shutting down : No need to purge with another material.

28. Reprocessing : Up to 100% regrind can be run.

29. Finishing : A wide variety of techniques are used to degate mouldingds; the components may be seperated by twisting, cutting (Scalpel or knife) or by automatic systems. Cropping of edge gates is widely used. Components may be decorated by a variety of techniques (i) hot stamping (ii) painting or spraying (iii) vacuum metallising (iv) silk screening, etc.

30. Other comments : Moulding quality can be judged by surface finish: in general the better the finish, the higher is the quality for a given grade.

31. Typical components : Household appliances, toys, furniture components, disposable cups, appliance housings, etc. Applications where a certain degree of impact resistance is required and where the relatively poor chemical resistance of this material is unimportant or can be put to good use, e.g. to solvent weld components together. Scale model kits are a good example of this application.

8. NYLON

1. Common name : Nylon.

2. Abbreviation : PA6 or PA66.

3. Systematic chemical name : PA6 is poly(imino-1-oxohexamethylene).PA66 is poly[imino(1,6-dioxohexamethylene)iminohexamethylene].

4. Some suppliers : 5. Trade names or trade marks :
PA6
Aicar Naycar
Akzo Plastics Akulon
Ato Chimie Orgamide
BASF Ultramid B
Bayer Duretham B
Bergmann Bergamid B
BIP Beetle
Degusa Wolfgang Degalan 6
Emser Werke Grilon
Frisetta GMBH Frisetta Nylon B
ICI Maranyl
Rhone Poulene Technyl
Snia Viscosa Sniamid
PA66

Aicar Naycar
Akzo Plastics Akulon
BASF Ultramid A
Bergmann Bergamid A
Celanese Plastics Co. Celanese nylon 66
Ciba-Geigy Verafil
Du Pont Zytel
Dynamit Nobel Trogamid
Emser Werke GrilonT
Fabelta Fabelnyl
Frisetta GMBH Frisetta nylon A
ICI Maranyl
Rhone Poulenc Technyl
Snia Viscosa Sniamid

6. Material properties : Nylon is a generic term used to describe thermoplastics which contain the amide group
¾CONH¾;as the polymers contain many such groups they are also known as 'polyamides'. Such materials are tough, resist fatigue, impact and solvents, have low friction and absorb some water (reversibly). Many different types are possible, e.g.6; 7; 9; 11;12 and 66;610;612, etc. Where one number is used it indicates the number of carbon atoms in the raw material (e.g. a lactam); where two numbers are used (e.g. 66) then the first number indicates the number of carbon atoms in the parent diaminè and the second indicates the number of carbon atoms in parent acid. Nylon 66 and nylon 6 are the best known members of the group and as the amide groups are separated by sections based on methylene groups, they are also known as aliphatic polyamides.
Such materials compete against each other and with the acetals; nylons have superior abrasion resistance and toughness than the acetals. Nylon 66 is probably the most widely used nylon moulding material. It is slightly stronger and stiffer than nylon 6; it has a higher heat distortion temperature, absorbs less water and has a higher flexural strength. The
material is made by reacting hexamethylene diamine and adipic acid.
Nylon 6 is made from caprolactam and in many respects it is similar to type 66. It is marginally lighter in colour and with a slightly lower density (1.13g cm-3 as compared to 1.15gcm-3). The material has a lower melting point and can therefore be processed at a lower temperature; the shaping temperature range is also wider and processing can therefore be considered easier. Nylon 6 is slightly cheaper than type 66 (in UK), more flexible, has a higher impact strength and slightly better low temperature properties. In general it has better solvent, grease and detergent resistance than PA66 but its resistance to dilute mineral acids is poorer.

7. Ease of Flow : Both these materials are very easy flowing ; type 6 does not set up as sharply as PA66 and consequently is useful as a glass fibre filled grade where the ability to apply pressure, so as to avoid voiding, is an advantage.

8. Shrinkage : Polyamides are crystalline materials and so high shrinkage values may be recorded. For example, PA66 may yield values of 0.010 to 0.020 in/in; type 6 may give values of 0.010 to 0.015 in /in. The amount of shrinkage is dependent on, for example, part thickness because as part thickness increases so does the opportunity for increased crystallinity. Nylon 66 exhibits high after-moulding shrinkage, e.g. for up to 2 years after moulding.

9. Resistant to the following : In general these materials are resistant to alkalis, dilute acids(but not nitric acid), detergents, fuels, greases, oils, chlorinated hydrocarbons, aromatic hydrocarbons, ketones and esters. Some of these materials may cause polyamide mouldings to swell, e.g. chlorinated hydrocarbons.

10. Not resistant to : Strong acids, bleaching agents (e.g. hydrogen peroxide). Sunlight can cause embrittlement unless the polymer is stabilised (e.g. with carbon black). Soluble in phenol, cresol, formic acid and glacial acetic acid. Water and, for example, some alcohols may cause swelling. Water acts as a plasticiser and makes the material softer and tougher but worsens the electrical insulation properties.

11. Material detection or identification : The density of type 6 is slightly lower than type 66 (e.g. 1.13 compared to 1.15 gcm-3) therefore both will sink in water. Both are opaque, stiff, hard, tough materials which are easily cut so as to yield with smooth edges. Difficult to ignite but once alight they are not self-extinguishing. The polymer melts to a free flowing liquid (which can be pulled into strings) which drips and carries the flame with it as it falls. The flame burns blue, has a yellow edge and emits a smell of burning hair. The easiest way to distinguish 6 from 66 is to measure the melting point; 66 will melt sharply but a higher temperature, e.g. 265°C. Type 66 is also more reistant to choloroform, benzyl alcohol and trichloroethylene. When type 66 is heated in boiling N 1 N-dimethylformamide it does not dissolve ¾ PA66 does. Nylon 6 is also soluble in 4N hydrochloric acid whereas 66 is insouble.

12. Colouring : The fully compounded material is preferred; it is found that the addition o f pigment can dramatically affect properties (e.g. impact strength) and this can cause problems if trials are performed with natural polymer. A limited amount of colour masterbatch is used and liquid colout is being actively developed. Many of the grades now being moulded are fully colour compounded materials which are also lubricated and nucleated so as to get good colour, ease of flow and fast cycling.

13. Material and component handling : These materials absorb water (type 6 more than 66, e.g. 2.5% as opposed to 1.5%) and must therefore be stored in a clean, dry place. A first-in-first-out system should be adopted and the moisture-proof containers should only be opened when required and after they have reached the workshop temperature. If the moisture content is greater than about 0.2% problems will occur. Dry if possible in a vacuum drying oven at 80°C for up to 18h; the absence of air will help to prevent oxidation.

14. Mould and gate considerations : It is difficult to predict the shrinkage of nylon. Correct the production moulds after trial mouldings (made from the production compound) have been produced using production conditions. For large mouldings a short, direct spruge gate is frequently used with a taper angle of 3°. For smaller mouldings, pinpoint gates may be used ¾ for section thicknesses up to 4 mm use a gate with a diameter of about 1.5 mm. Such small gates may be used as tunnel gates so that automatic degating is obtained. Ring and diaphragm gates are used on circular components whereas film or flash gates are used for parts of large surface areas which have relatively thin sections ¾ such gates give the best filling pattern. Hot runner moulds may be used with nylons provided that (i) good sealing is obtained, (ii) accurate temperature control is possible and (iii) the melt does not come into contact with copper (this metal can cause degradation). Successful mouldings is being achieved with insulated hot-tip systems, i.e. as an alternative to conventional hot runner.

15. Flow path: wall thickness ratio : Because of the high set-up temperature of these materials the ratio of maximum flow path: wall thickness is not high as would be expected. It is better than acetals (POM) but worse than ABS. At 1mm wall thickness it is approximately 100:1.

16. Projected area: The clamping pressures required for these materials are relatively high. Type 6 would probably need slightly lower pressure but the biggest difference is between unfilled grades and glass-filled grades. Unfilled grades would need between 3 and 5 tsi (45 to 75 MN m-2) and filled matrials may need from 4 to 10 tsi (60 to 150 MN m-2). Materials may be required to flow into thin sections or the mouldings may need to have a good surface finish. Both types require high injection pressures and therefore high clamping pressures

17. Cylinder equipment : Both these matrials have a relatively sharp melting point and their melt viscosity is low. The screw is therefore fitted with a back-flow valve and the cylinder is fitted with a nozzle whose temperature can be accurately controlled. To prevent material drooling form the nozzle a shut-off nozzle (thermal seal, hydraulic seal, spring loaded needle, etc.) is usually used; drooling can also be prevented by using decompression. To prevent excess air being drawn into the cylinder (i) arrange for decompression to occur before sprue break, (ii) decompress relatively slowly and (iii) use decompression in conjunction with a thermal seal nozzle. These measures should help to prevent oxidation. The thermal seal nozzle must be designed to eliminate stagnation points and the back-flow valve must seat well (to prevent leakage).

18. Screw cushion : Between 2 and 6 mm.

19. Shot capacity: If the machine is fitted with a general purpose screw then the shot capacity will be limited, e.g. 80% of the maximum for PS. To avoid material stagnation do not use 20% . The 'standard' screw may give unacceptable melting (under-plasticised or over-plasticied melt) and in order to overcome such problems screw designed for nylon are used. If the machine has been fitted with a screw designed to suit nylon (i.e. a nylon screw) then the maximum rated capacity may be utilised¾ such a screw may be essential for some products (e.g. pharmaceuticals) where very high quality mouldings are required.

20. Melt temperature: For PA6 the melt temperature is usually in the region of 220°C but can, for some grades, be 270°C (428 -518°F). For nylon 66 the melt temperature is usually in the region of 270 °C; because of the sharp melting point and because of the risk of oxidation, the processing temperature range is much smaller for 66, e.g. 265 - 300°C (509 - 572°F). Temperature control must therefore be precise.

21. Barrel residence time : Nylon 66 will tend to degrade slightly more rapidly than PA6 at its (high) processing temperature. If there are any serious hold-ups in production turn the barrel temperature down (e.g. to 170°C) while the problem is being solved. If a grade containing a flame retardant is being processed take extra care as some grades burn up relatively quickly. Purge the barrel and turn off the heaters in the event of a serious stoppage.

22. Temperature settings : See Table 8. All temperatures must be capable of being precisely set and controlled, e.g. PID control using a deep thermocouple. If the machine is equipped with a thermal seal nozzle then the temperature of this is set slightly lower than the melting point of the polymer. Accurate mould temperature control is essential id variations in crystallinity are to be avoided.


TABLE 8
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone Location Temperatures ° C Temperatures ° F
No. PA66 PA 6 PA66 PA6

From To From To From To From To

¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 260 280 210 250 500 536 410 482
(near the hopper)
2 Barrel middle 270 290 215 260 518` 554 419 500
3 Barrel middle 270 300 220 270 518 572 426 518
4 Barrel front 270 300 225 270 518 572 435 518
5 Nozzle 265 280 220 270 509 536 428 518
Mould 60 100 60 90 140 212 140 194
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾

23. Injection speed¾ moulding filling speed: Usually fast for thin sections and slower for thicker sections.

24. Injection pressure : The machine should be capable of giving the following :First stage : up to 1500 bars; 150 MN m-2; 21 400 psi. Second stage (dwell or follow-up pressure): up to 300 bars; 30 MN m-2; 4300 psi. The screw forward time (SFT) is very important in nylon moulding and may be longer than the cooling time.

25. Screw rotational speed (rpm): To minimise temperature variations this should be set to suit the cooling cycle; it should not exceed 100 rpm. With some colours, high screw speeds can cause colour changes.

26. Back pressure : 150 bars; 15 MN m-2; 2140 psi. Excess back pressure may cause degradation due to large amounts of shear heat being generated.

27. Shutting down : Purging with different material is not normally necessary ¾when restarting moulding, allow the machine to heat soak for a few minutes after the set temperature has been reached. This should ensure that the remaining material in the barrel has melted and the risk of screw breakage will be minimised.

28. Reprocessing : Nylon is noted for its toughness; however it is only tough when it is absorbed a certain amount of water. When it comes from the mould it will be dry and will therefore be more brittle. Regranulate immediately if possible. Use up to 30% of regrind but do not regrind contaminated material or damaged material (e.g. overheated); predry regrind if required.

29. Finishing : Degating may be by drilling, milling cropping, etc.; some edge gates may be broken by tumbling and the gate angle (and length) determine the finish obtained. Normal machinery procedutes, e.g. drilling, tapping, etc., are used. Mouldings can be finished by silk screening, hot stamping etc.

30. Other comments : If mouldings of improved stability are required then they should be annealed by heating (e.g. by immersion in a nonoxidising oil) at a temperature 20°C higher than the maximum service temperature, e.g. 20 min at 150°C for PA66. The mechanical properties of polymaides are markedly dependent on cooling; slow cooling (using a hot mould) increases crystallinity and this gives harder mouldings with higher stiffness and tensile strength.

31. Typical components : Polyamides have a useful combination of properties which makes them attractive for many applications. 'Under the bonnet' applications (fuel pipes, fuel filter components, etc.) take advantage of nylon's resistance to hydrocarbons and heat. Gears, cams, levers, door furniture, etc. utilise the material's toughness, abrasion resistance, low friction and fatigue resistance. Other applications are appliance housings, fans, impellers, door handles, safety belt components, bathroom fittings, etc.
Glass-reinforced polyamides are used where the components must have a higher heat distortion temperature, greater rigidity and dimensional stability than that offered by the unreinforced material. A typical application that takes good advantage of this material's properties, is a power tool outer case or housing; a novel application is a bicycle frame.
These materials need higher injection speeds, and barrel and mould temperatures, than the unfilled grades. Cycles may be shorter as the parts can be ejected at higher temperatures; it is sometimes discoloration which limits ejection temperature. Mould shrinkage is reduced but density is increased by glass addition. For example, for PA66 with 20% glass-fibre, the shrinkage is approximately 0.005 in/in and the density is 1.28 gcm-3; with 50% glass fibre the shrinkage is 0.003 in/in and the density is 1.51 gcm-3. To avoid problems with glass fibre (warping, etc.) glass spheres are now being used.
The end uses of 6 and 66 are often duplicated; the choice between the two being governed by material availability, personal preference, price and processability.



9. ACETALS

1. Common name : Acetals, polyoxymethylene, polyformaldehyde.

2. Abbreviation : POM.

3. Systematic chemical name : The homopolymer (oxymethylene)

4. Some suppliers : 5. Trade names or trade marks :
A mcel Kematal
BASF Ultraform
Celanese Celcon
Du pont Delrin
Hoescht Hostaform

6. Material properties : Acetals are polymers based on formaldehyde and both homopolymers and copolymers
are available. Both categories of material are crystalline and they have roughly similar properties, i.e. they are stiff materials, with good fatigue endurance, resistance to creep, a low coefficient of friction and an attractive appearance. Of the two types the homopolymer has the highest tensile strength, flexural strength, resistance to fatigue and hardness. The copolymer is more thermally stable, easier to process, resists degradation by hot water better, has superior alkali resistance and higher elongation.
This type of material is commonly referred to as an engineering material and as such they compete with the nylons. The nylons have better fatigue endurance, creep and water resistance and they are stiffer.

7. Ease of Flow : Acetals flow relatively easily but because of the high temperature at which they set-up, a relatively low flow path: wall thickness ratio may be obtained.

8. Shrinkage : Being crystalline the shrinkage is high, e.g. about 2% (approximately 0.020 in/in). After-shrinkage is usually complete within 48h of moulding.

9. Resistant to the following : Most organic reagents. Acetals are unaffected by washing in solvents such as acetone, ethyl alcohol and petrol at room temperature. Resistant to weak acids and alkalis. May be swollen by long immersions in certain solvents, e.g. phenol, acetone, toluene, etc.

10. Not resistant to : Strong organic acids and oxidising agents, decomposed by dilute mineral acids; strong alkalis which attack the homopolymer do not attack copolymer so readily. Hypochlorite solutions will also attack mouldigns.

11. Material detection or identification : With density of 1.41 gcm-3 these materials sink readily in water and being crystalline they cannot be obtained in clear grades. Natural grades give hard, white mouldings which burn with a pale blue flame, giving off a pungent odour of formaldehyde but emitting little smoke. Fairly difficult to cut but the cut has smooth edges.

12. Colouring : In-house colouring is best done by means of masterbatches, e.g. at 3% concentration. Dry colouring is not so successful for colour matched jobs. Liquid colour is being developed but care should be taken to ensure that problems such as slippage, plate-out, etc. do not develop.

13. Material and component handling : These materials are less hygroscopic than the nylons but they still should be stored in a dry space. Predrying is not usually necessary but when it is, dry for abour 2 - 3h at 110°C. Acetal mouldings are generally small and are stored collectively.

14. Mould and gate considerations : To assist in rapid mould filling, and to minimise any risk of degradation caused through air compression, venting is often used when acetals are moulded. As the material has a high hot sprung off quite large undercuts. For small mouldings pinpoint gates may be used but in general gate thickness should be half the moulding's wall thickness and the gate land length should be kept low, e.g. half the gate thickness.

15. Flow path: wall thickness ratio : Copolymers are easier flowing than homopolymers. At 1mm wall thickness, the maximum flow path: wall thickness for POM would be about 100:1(PA 110:1, PMMA 95:1).

16. Projected area: For easy flow materials the clamping pressure required is approximately 3 - 4 tsi of projected area (45 - 60 MN m-2) for general purpose material it is 4 - 5 tsi (60 - 75 MN m-2).

17. Cylinder equipment : If an open nozzle is inadequate then use a nozzle valve which contains a spring loaded needle¾ in the event of degradation this type of nozzle will act as a safety valve. Use a ring type check valve on the screw.

18. Screw cushion : From 2 to 6 mm.

19. Shot capacity: From 15 to 75% of the cylinders rated capacity may be used.

20. Melt temperature: 195 - 215°C., e.g. 210°C (e.g. 410°F). Ensure that the nozzle does not become blocked because of, for example, the temperature falling.

21. Barrel residence time : As copolymers are more thermally stable than the homopolymer, longer residence times are possible for the copolymer grades. Do not allow these materials to stagnate in the barrel. If a stoppage occurs purge out every 10 to 15 min. Acetals can decompose violently.

22. Temperature settings : See table 9.
TABLE 9
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone.Location Temperatures ° C Temperatures ° F
No From To From To
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 150 170 302 338
near the hopper)
2 Barrel middle 160 180 320 356
3 Barrel middle 180 200 338 392
4 Barrel front 190 215 374 419
5 Nozzle 195 215 383 419
Mould 40 120 104 248
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
23. .Injection speed¾ moulding filling speed: A rough or porous surface may result if the mould filling speed (or material temperature) is too low.

24. Injection pressure : The machine should be capable of giving the following : First stage : < 1500 bars; 150 MN m-2; 21 400 psi. Second stage (dwell or follow-up pressure): < 750 bars; 75 MN m-2; 10 700 psi.

25. Screw rotational speed (rpm): Keep the screw speeds as low as possible so as to minimise the danger of degrading the material (by shear heating) and to give better temperature uniformity.

26. Back pressure : < 200bars; 20 MN m-2; 2300 psi.

27. Shutting down : Ten minutes before the end of the run turn off the power to the barrel heaters, withdraw the carriage away from the mould and then purge the barrel empty and leave the screw in the fully forward position. If you are changing from a halogen-containing polymer (e.g. PVC or plastic containing a flame retardant) to acetal then purge with another material (e.g. HDPE) first. If a change is being made to a high temperature plastic, e.g. PC, also purge with PR before introducing the next polymer into the barrel. To minimise fume production drop purgings into cold water.

28. Reprocessing : Providing that the material has been kept clean and dry up to 100% regrind can be used ¾ usually in lower graade mouldings. In premium grade mouldings the amount of regrind is kept low, e.g. < 25%. Lubricants may help the processing of reground material.

29. Finishing : Typical finishing operations performed on acetal components include (i) machining i.e. milling, drilling (threading no problem); (ii) sprue removal (using side cutters or scalpels); and (iii) decoration, e.g. by chrome plating and hot foiling. Components may be joined by hot-plate (300°C) welding.

30. Other comments : If glass fibre (e.g. 20%) is added to acetals then materials with higher stiffness, strength and creep resistance result . Overheating can result in the production of formaldehyde. As this is a gas then high pressures could be developed within a moulding machine (high enough to be dangerous) by the decomposing material being trapped between a sealed nozzle and the valve on the screw. Therefore do not use nozzles which seal more tightly as the pressure increases (some types of mechanical-seal nozzle do this).

31. Typical components : Acetals are fairly expensive materials which may be classed as engineering materials and they are used, for example, as metal replacements. This is because of their desirable properties, e.g. little change of impact strength with temperature, resistance to fatigue, etc. Typical parts are gears, bearings, moulded sprockets and chains, cams, fan-blades, carburettor bodies, pump impellers, aerosol components, etc.
POMs are said to be free from biological attack but as they are susceptible to ultra-violet radiation they should not be exposed to ultra-violet unless they are protected.


10. PHENOLICS

1. Common name : Phenolics or phenol-formaldehyde.

2. Abbreviation : PF.

3. Systematic chemical name : Not known ; one suggestion is phenolmethanal.

4. Some suppliers : 5. Trade names or trade marks :
Bakelite GMBH Bakelite
Bayerisches Kunstoffwerk
GMBH Chemoplast
Dynamit Nobel Troliton
Perstorp Nestorite
Raschig Resinol
Sud-West-Chemie Supraplast
N.V. Vynckier Vyncolite

6. Material properties : PFs are a class of thermosets which are based on PF resins (Novolak), fillers (Such as woodflour), lubricants, hardners, etc. They are supplied in powder form and the mouldings have a useful combination of low cost, ease and versatility of moulding, temperature resistance, solvent and chemical reistance. The electrical insulation properties are good and the material has better water resistance than MF. One major drawback of PFs is their limited colour range; black and brown are the most common colours. Mouldings are hard, stiff and exhibit low elongations. The mouldings have good creep resistance, Moulding properties are very dependent on the filler used.

7. Ease of Flow : Available in a range of flows, e.g. soft, medium and stiff. These are stiff, hard materials which require high pressures to get smooth mould filling. PFs have more stable melt rheology than aminoplastics, i.e. they are not so temperature dependent.

8. Shrinkage : About 0.010 in/in for woodflour-filled phenolics (approximately 1%).

9. Resistant to the following : Dilute acids (not all PF grades), alcohols, aromatic and chlorinated hydrocarbons, greases and oils. Mineral filled grades are also resistant to detergents..

10. Not resistant to : Strong oxidising acids and alkalis; some components (depending on grade ) may be attacked by dilute acids and alkalis, ketones and detergents. Sunlight will cause general darkening.

11. Material detection or identification : Mouldings are usually dark coloured (e.g. brown or black) and are difficult to cut. This material burns with difficulty (those filled with glass or asbestos are virtually incombustible) and cracks when burnt, giving off a phenolic odour (carbolic acid). As the material density is 1.3 ¾1.8 gcm-3 (dependent on filler type, concentration, etc.) it sinks in water. When scraped, filed or sawn the materials smell strongly of phenol and the mouldings will only soften slightly at high temperatures, e.g. above 130°C.

12. Colouring : Usually carried out at the compounding or manufacturing stage. Because of their complex chemical structure only dark colours are available ¾usually dark browns, greens and blacks.

13. Material and component handling : As long storage will affect flow it is important that a first-in-first-out storage system be adopted so as to obtain a constant turnover of materials.
Components moulded from PFs not susceptile to scuffing and so individual packaging
is not normally necessary.

14. Mould and gate considerations : Because of the abrasive nature of thermosetting materials, moulds should be hardned, polished and chrome plated. The mould should be constructed so that as wear occurs replacement is possible (e.g. the gates should be carried on pads which are replacable). Electrically heated moulds are commonly used ¾ allow approximately 1 kW of installed power for every 50Kg of mould weight and insulate the mould as much as possible. Adequate venting is essential. Circular runners give less pressure drop than trapezoidal or semi-circular runners but are more difficult to machine. Pinpoint gates of less than 0.7 mm are not usually used as the temperature rise and abrasive affect is excessive; high impact grades will require larger gates ¾ mould venting is essential. Runnerless mouldings, although difficult to perform, can show substantial material savings. Melt temperature control may need to be very accurate (e.g. ± 2°C) and accurate control over nozzle temperature is essential, e.g. using water-cooled nozzles. Live sprue moulding is a useful compromise; the runners are cured and ejected but not the sprue. This saves some material and is relatively cheap to do, the big advantage is that the mould opening movement is reduced and time is saved. Faster curing (stiffer flow) grades can also be used.

15. Flow path: wall thickness ratio : Information not available.

16. Projected area: Approximately 2 - 5 tsi (37MN m-2). Thermoset mouldings are usually of heavy cross-section and this helps to reduce clamp pressure requirements. However, the viscosity drop which occurs during mould filling off-sets this. Cavity pressure control and programmed injection speed should be of help in reducing clamp pressure requirements.

17. Cylinder equipment : A check ring assembly is not normally fitted and the compression ratio of the screw is often 1:1 or even 1:0.8. A negative compression ratio allows for ease of removal if precuring occurs. Screws may be cored for heating and cooling.

18. Screw cushion : As small as possible, e.g. 2mm. May be eliminated if a process control system is fitted. As thermosets increase in volume during curing then the increase in pressure may be sensed and used to control the shot size setting (and cure time calculations) so that the screw may bottom. By allowing for this expansion, overpacking may be reduced and material usage reduced.

19. Shot capacity: Only about 80% of the machines theoretical shot capacity is usually available as material 'flow-back' down the screw occurs during injection ¾this is minimised by fast Injection speeds and low rear zone temperatures.

20. Melt temperature ¾as measured in the nozzle or by an air-shot technique : Work done on the material by the screw will raise the material temperature by approximately 15°C., (depends upon back pressure ,screw speed,etc.) A GP phenolic will typically have a melt temperature of 105- 130°C (220 -260°F). Below 105°C it will be too viscous whereas above 130°C precure may occur. (GP means general purpose ¾such grades may be woodflour filled).

21. Barrel residence time : Keep as low as possible; do not allow the machine to stand for periods with the heat on and do not excessively under-utilise the machine. The material should be formulated so that short stoppages, e.g. 5min, can be tolerated. Use sprue-break to stop heat transfer from the mould.

22. Temperature settings : Please note that it is the melt temperature which is important; those in Table 10 are only suggested, initial settings. The temperature of the hydraulic oil and of the material in the hopper should not vary excessively.
The barrel temperature is normally maintained by circulating heated water through jackets or coils attached to the outside of the barrel. Cure time is very deprndent on temperature. For example a component with a wall thickness of 3.2 mm (1/8in) will require a cure time of 28s at a mould temperature of 175°C; at 210°C it will be 12s.

TABLE 10
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone Location. Temperatures ° C Temperatures ° F
No From To From To
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 50 70 122 158
4 Barrel front 70 85 178 183
5 Nozzle 80 110 176 230
Mould 150 210 302 410
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾

23. Injection speed¾ moulding filling speed: In general this should be as high as possible. When the material enters the cavity its temperature will be of the order of 150°C (302°F) because of the friction generated. However, very high speeds are not possible (e.g. 5 - 20 s is usual) as gas marking and /or flashing may occur. Often the machines are not powerful enough to give high speeds, i.e. the line pressures are insufficient.

24. Injection pressure : The machine should be capable of giving the following : First stage : Up to 2000 bars; 200 MN m-2; 29 000 psi. Second stage (dwell or follow-up pressure): up to1000 bars; 100 MN m-2; 14 500 psi.

25. Screw rotational speed (rpm): 60 - 90. The screw -drive motor must be capable of generating sufficient torque. Normally the torque required is not very high.

26. Back pressure : Up to 70bars; 7 MN m-2; 1000 psi.

27. Shutting down : Reduce the cylinder temperatures to 55°C (130°F) and start to purge the barrel. Stop the material feeding into the barrel and purge the barrel clean. Turn off the heaters. When restarting allow the barrel to heat soak for about 15 min at the running temperature before making a few air purging shots; if necessary remove the nozzle and scrape out cured material.

28. Reprocessing : Not normally recommended although if finely ground (e.g. below 200 mesh) from 5 to 20% of regrind can be incorporated in some grades without a serious loss of mechanical properties. GP grades are not drastically affected, fibre-filled grades may be seriously affected by regrind incorporation.

29. Finishing : PFs are only available in dark colours¾this may be changed by stoving or painting. Hot foiling can be performed but it is difficult to do well. Flash is commonly removed by finishing and/or hand filling; holes or threads can be incorporated by drilling and tapping.

30. Other comments : PFs are rated as being relatively easy to mould compared to aminoplastics.

31. Typical components : Iron, saucepan and pressure cooker handles are commonly seen components; the material is chosen for these applications because it is heat resistant and durable; it is also a low cost material. Dashboard ashtrays, coffee percolator bases and electrical connectors are other obvious applications. The maximum service temperature depends on the filler used; for the highest heat resistance, mineral-filled grades should be used. Slate, asbestos and other minerals give heat resistant materials; the best electrical resistance is obtained with ground mica. Glass or other fibrous fillers give components with the highest impact strength. If the mouldings contain organic fillers then under hot, wet conditions, fungus or moulds may grown on the surface.




11. DOUGH MOULDING COMPOUND (DMC)


1. Common name : Dough Moulding Compound.

2. Abbreviation : DMC

3. Systematic chemical name : Not known .

4. Some suppliers : 5. Trade names or trade marks :
BIP Beetle
Freeman Chemicals Freemix

6. Material properties : These materials are based on unsaturated polyester resins¾the
resins are blended with a fibrous reinforcement (usually glass), a filler, pigments, styrene and a catalyst system. The mixture has a dough-like consistency (hence the name) and in order to ease handling the composition is often supplied in a rope-like form. A wide range of formulations is possible but in general mouldings are very strong and stiff. The compounds may be specially formulated to maximise a particular property, e.g. flexural strength, resistance, low coefficient of thermal expansion, flame retardance, etc. However, in general the impact strength of injection moulded DMC is lower than that of compression moulded DMC.

7. Ease of Flow : Can be very easy flowing and fast curing, e.g. 20s for a 3-mm section (mould temperature
150°C).

8 Shrinkage : Can be very low, GP grades are only 0.2% . Low profile grades are available (obtained by adding a thermoplastic to DMC) which have little or no shrinkage, e.g. 0.0055in/in (below 0.05%). Coefficient of thermal expansion is also low, e.g. similar to that of steel.

9 Resistant to the following : Water, alcohols, aliphatic hydrocarbons, greases and oils. The mechanical properties are usually retained on weathering but the surface usually deteriorates. Mouldings can withstand exposure to high temperatures (e.g. 160°C). for long periods.

10 Not resistant to : Ketones and chlorinated hydrocarbons; the resistance to aromatic hydrocarbons, acids and alkalis is also not very good. Resistance to such chemicals is much worse at elevated temperatures.

11 Material detection or identification : These materials have a very high density, e.g. 1.7-2.1 gcm-3 (dependent on grade) and the mouldings feel stiff, hard and heavy; they are difficult to cut and will sink rapidly in water. When heated in a flame a lot of soot is formed and styrene may be smelt. Some grades burn easily whereas others are of a reduced flammability rating. Inorganic residues (e.g. glass) may be left after burning.

12 Colouring : The natural colour depends on the filler system employed. When whiting is used as the filler then beige is the natural colour. A wide colour range is available but the material must be purchased already coloured as there is no sensible way of adding colour during the moulding operation.

13 Material and component handling : Feeding of such materials may be difficult¾the material may be forced into the barrel by means of pneumatic pistons. It is normally supplied as a slightly tacky, dough-like mass in rope form ¾ avoid contact with the skin and breathing the vapour. Store in sealed containers away from heat as styrene is inflammable. Adapt a first-in-first out storagae system as the shelf-life of the material is limited, e.g.3 months at room temperature. Storage-deteriorated materials will not have the expected gloss or strength; increasing the cure time will not help.

14 Mould and gate considerations : Fibre orientation is affected by the type and position of gate used ¾ use fairly large gates to minimise fibre orientation and fibre degradation. Runners should also be as large as possible and abrupt changes in direction should be avoided. It must be accepted that there will be some fibre degradation as the material is forced through the feed system and that injection mouldings will be anisotropic. If the melt is divided by a core then poor weld strength can be a problem. Injection ¾ compression moulding reduces flow orientation; warm runner moulding reduces waste and can reduce cycle times ¾ this is because curing of the sprue is often the rate-dertermining step as sprue thickness can be large.

15 Flow path: wall thickness ratio : Not normally quoted, however mould filling is seldom a problem with DMC.

16 Projected area: DMC components are often of heavy cross-section and thus for a given shot weight, clamping requirements should be relatively low. However, because of the easy flow nature of the material, clamping pressures of the order of 2-2.5 tsi (30-37 MN m-2) are commonly employed.

17 Cylinder equipment : Screws with little or no compression ratio are employed. The material will need to be forced or 'stuffed' on the screw during screw rotation by a piston. The conventional conical hopper is replaced by a cylinder which contains the material; the ram operates inside the cylinder. To prevent material from flowing back down the screw during the injection, back-flow valves may be fitted to the screw; a valve is also fitted to the nozzle. Both the screw and the barrel should ideally be constructed from wear-resistant materials.

18 Screw cushion : Materials is so easy flowing that if a screw cushion is set then the screw may 'punch' through. This will damage the fibres and reduce material viscosity thus altering material properties.

19 Shot capacity: DMC compounds are very easy flowing and therefore require low injection pressures. Cylinders of large shot volume may thus be fitted to comparatively small machines and this allows the economic production of components with a large shot weight but of heavy cross-section.

20 Melt temperature ¾as measured in the nozzle or by an air-shot technique : DMCs are plastic at room temperature ¾ they do not, therefore, need to be heated before flow can occur. However, barrel heating is normally employed as this reduces fibre damage and speeds up cure. Temperatures of up to 60°C are used.

21 Barrel residence time : Material can reside in the barrel heating for long periods, e.g. at 60°C it is still relatively stable. It is good practice not to allow the material to stand and 'cook' in the barrel.

22 Temperature settings : Please note that it is the melt temperature which is important; those in Table 11 are only suggested, initial settings. The temperature of the hydraulic oil and of the material in the hopper should not vary excessively.


TABLE 11
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone Location Temperatures ° C Temperatures ° F
No. From To From To
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 20 40 68 104
4 Barrel front 40 60 104 140
Mould 140 160 284 320
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾

23 Injection speed¾ moulding filling speed: High injection speeds can give good surface finish; because of the ease of flow of these materials such high speeds can be obtained with relatively low pressures. The rate of filling may have to be reduced if air trapping is a problem (it can cause porosity).

24 Injection pressure : The machine should be capable of giving the following : First stage : Up to 500 bars; 50 MN m-2; 7000 psi. Second stage (dwell or follow-up pressure): up to 400 bars; 40 MN m-2; 5800 psi.

25 Screw rotational speed (rpm): 30 - 55 rpm. Screw rotational speed and back pressure should be as low as possible so as to minimise fibre damage.

26 Back pressure : 50bars; 5 MN m-2; 700 psi. High back pressures will cause fibre breakdown ¾ keep the back pressure as low as possible.

27 Shutting down : Not essential to thoroughly clean the barrel out even for overnight stops. Cool the barrel and purge clean.

28 Reprocessing : Not recommended ¾ the material be expensive to reclaim and could only be used to replace the cheap, inorganic filler.

29 Finishing : During finishing operations (e.g. machining) there is the possibility of dust generation. Adequately ventilate the area and avoid ingestion of the dust by using masks. As gates are large, and the material is tough, pnuematic side cutters may need to be employed for cropping. Tungsten tipped drills should be employed for drilling operations.
30 Can be painted without prior abrasion of the surface ¾ degreasing may be necessary. Stoving (of paints) may also be employed, e.g. 30min at 160°C.

31 Other Comments : Styrene vapour may be smelt during processing ¾ keep the moulding area well ventilated so that the styrene concentration in the working atmosphere does not exceed the threshold limit value (TLV), e.g. 100 ppm.

32 Typical components : DMC has high strength, excellent electrical properties and very atmosphere does not exceed the threshold limit value (TLV), e.g. 100 ppm. good dimensional stability; it has better heat resistance and lower creep than most thermoplastics. Grades are available which permit thick sections to be moulded without nteirnal cracks or voids. It is used to mould diesel engine head covers, drill housings guards, appliance covers, heavy duty contractors and circuit breakers. In this last application resistance to repeated arcing is important. Because DMC compounds are inherently fast curing, injection moulding does not offer very dramatic reductions in cure time (compared to compression moulding); however, injetion moulding saves weighing and dispensing time.





12. STYRENE ACRYLONITRILE (SAN)

1. Common name : Styrene acrylonitrile.

2. Abbreviation : SAN.

3. Systematic chemical name : Not known but the following has been suggested:poly(1-phenylethylene-co-1-cyanoethylene).

4. Some suppliers : 5. Trade names or trade marks :
BASF Luran
Dow Tyril
Monsanto Lustran SAN
Montedison Kostil

6. Material properties : When styrene is copolymerised with acrylonitrile (20-30%) a material with a higher sofeting point, rigidity and solvent resistance (compared to PS) results. This polymer is transparent but not water white and clear mouldings have a yellow tint (which can be offset by a blue dye). The material has better stress crack reistance than PS and a higher impact strength. Reistance to staining, e.g. by food, is good. The densities of SAN and PS are similar but SAN has a higher resistance to creep under load; maintains its impact strength from -40 to + 50°C without change and has a higher heat distortion temperature.

7. Ease of flow : In general it is a stiffer flow material than PS, therefore more pressure is needed
to get equivalent rates of flow.

8. Shrinkage : An amorphous material with low shrinkage, e.g. about 0.5% (@ 0.004 in/in).

9. Resistant to the following: Because of he polar nature of the acrylonitrile molecule these copolymers have better resistance to hydrocarbons, oils and greases than PS. It is resistant to acids, alkalis, detergents, fats and oils, petrol and salt solutions.

10. Not resistant to: Swollen by aromatics, dissolved by ketones and methylene chloride (used for for glueing). The resistance to alcohols is only fair and its resistance to chlorinated hydrocarbons is poor. Weathering resistance is not very good but is improved by stabilsers.

11. Material detection or identification : Sinks in water and cannot be cut easily with a knife. Usually found as clear mouldings which are classed as brittle. Not readily soluble in carbon tetrachloride and when burnt gives off styrene as well as a bitter or acrid odour; burns without forming drips.

12. Colouring : Widely used colours are translucent greys (smoke), brown, opal and clear. Dry colouring is not recommended except for small mouldings whose colour is relatively unimportant.. Compounded material works best because the percentage of colour added is relatively small. If this small amount is added at the moulding stage then batch to batch inconsistencies may arise with dry colour and large mouldings.

13. Material and component handling : SAN is usually used in premium mouldings and it must therefore be stored under conditions which will not result in contamination, e.g. by dust or water. Predrying is not normally required but can be performed by heating 70°C for up to 3h. To stop the surface becoming marred or marked some components are individually wrapped.

14. Mould and gate considerations : Gate sizes are larger than for PS. To get good optical propertied use tab, deep fan and edge gates. The surface finish of the tool should be of a very high standard (e.g. no imperfections and chrome plated) because of the applications of this material.

15. Flow Path : wall thickness ratio : A range of grades is available (as for most polymers) so that, for example, one grade of PS may be stiffer flow than grade of SAN. In general however SAN is stiffer flow than PS with a flow path: wall thickness ratio of approximately 140:1 at 1mm(0.04in) wall thickness, i.e. similar to ABS.

16. Projected area considerations : This material is usually moulded into components which have relatively thick sections; resistance to flow is therefore low and this can reduce the need for very high clamping pressure when moulding items such as jugs, tumblers, etc. Clamping pressures may be of the order of 2tsi(30 MNm-2) for such components. Pressures of 4 tsi (60 MNm-2) have been used for mouldings which have large surface area, e.g. music centre covers.

17. Cylinder equipment : The cylinder is usually fitted with a shut-off nozzle when decompression is not available. A valve, to prevent back-flow, is usually fitted to the screw.

18. Screw cushion : About 4 mm.

19. Shot capacity : Under-rate machine by approximately 20% (compared to polystyrene).

20. Melt temperature : About 30°C higher than for PS but do not overheat (i.e. above 270 °C) the barrel residence time may be quite low, e.g. 5 min.

21. Barrel residence time : This can be critical and can result in colour changes. At high temperatures (approaching 270°C) barrel residence time may be quite low, e.g. 5 min.

22. Temperature settings: As in table 12. High mould temperatures give parts with good surface finish and fewer moulded-in stresses therefore mouldings with improved caze reisitance and toughness result: they can be further improved by annealing, e.g. in water bath at 70°C.

TABLE 12
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone. Location Temperatures ° C Temperatures ° F
No From To From To
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 150 180 302 356
(near the hopper)
2 Barrel middle 180 230 356 446
3 Barrel middle 210 250 410 482
4 Barrel front 210 260 410 500
5 Nozzle 210 250 410 482
Mould 30 85 86 185
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾

23. Injection speed : As high as possible consistent with other requirements, e.g. surface gloss, etc.

24. Injection pressure : The machine should be capable of giving the following : First stage : up to 1500 bars; 150 MNm-2; 21 400 psi. Second stage (dwell or follow-up pressure): up to 750 bars; 75 MNm-2; 10 700 psi.

25. Screw rotational Speed(rpm) : Use slightly slower screw speeds than for PS.

26. Back Pressure : Up to 150 bars; 15 MNm-2; 2140 psi.

27. Shutting down : No need to purge with another material. Empty the hopper and purge the material clean.

28. Reprocessing : The water absorption of these materials is 10 times that of PS, i.e. sililar to that of PMMA; they should therefore be stored carefully after regranulating and will probably have to be predried.

29. Finishing : Can withstand machining operations that would crack PS. Gates may be removed by machining (milling, routing , etc.) and/or cropping ¾ gate scars are sometimes covered by self-adhesive labels. The components may be decorated by processes such as hot stamping and silk screening; the appearance may be improved by buffing or polishing.

30. Other comments : Even though this material is claimed to have twice the impact strength of PS it must not be imagined that it is very tough¾ it is a brittle, notch-sensitive material like other amorphous thermoplatics.

31. Typical Components : Cups, tumblers, dishes, picnic items, trays, disposable cutlery , houseware, tape casette storage racks, dials , cosmetic containers, etc. In general this type of material is specified when PS is not good enough. It is usually cheaper and tougher than PMMA but does not posses the weathering resistance and excellent transparency of this polymer

13. LOW DENSITY POLYETHYLENE (LDPE)



1. Common name : Low density polyethylene or low density polythene.

2. Abbreviation : LDPE.

3. Systematic chemical name : Low density poly(methylene).

4. Some suppliers : 5. Trade names or trade marks :
Anic SpA Eraclene
Anic SpA Riblene
A to Chimie Lacqtene
BASF Lupolen LD
CDF Chimie Lutrène
Chemie Linz Duplen LDPE
Dow Dow Polyathylene
DSM Stamylan LD
Du Pont Alathon
Hoescht Hostalen LD
ICI Alkathene
Montedison Fertene
Shell Carlona
Union Carbide ¾
USI Europe Petrothene

6. Material properties : LDPE is a crystalline thermoplastic and as such it is not available in transparent grades. Its natural colour is a milky white and it has a soft wax-like feel. Because of chain branching the crystallinity level is low ¾ both long-chain and short-chain branches are present. The material has a density less than water (0.92gcm-3), is tough but only has moderate tensile strength. The chemical resistance and electrical insulation properties (over a wide range of temperature) are excellent . Polyethylene may be divided into three density ranges: low density range from 0.910 to 0.925 gcm-3, medium density range from 0.926 to 0.940 and high density range from 0.940 to 0.965 gcm-3. 'Low' is also known as Type I and 'medium' is known at Type II. HDPE is sometimes known as Type III. Density is related to crystallinity; crystallinity levels are related to the number of short-chain branches and their distribution. Linear low density polyethylene (LLDPE) contains little or no long-chain branching. This affects its flow behaviour and its thermal properties. For example, it has a higher maximum service temperarure than LDPE but is lower than HDPE.

7. Ease of flow : LDPE is an easy flow material . Ease of flow is rated by the melt flow index (MFI)
the lower the number, the stiffer is the flow. However, low MFI materials have better stress-crack resistance, solvent resistance and exhibit higher impact strength (due to higher molecular weight). At 200°C a material with an MFI of 2 will have a higher viscosity than PS; one with an MFI of 0.2 would have a lower viscosity.

8. Shrinkage : When the density is 0.910 - 0.925 gcm-3 the shrinkage is approximately 0.020 - 0.050 in/in (TypeI). When the density is 0.926 - 0.940 g cm-3 (Type II) the shrinkage is about 0.015 -0.04 in/in ¾ Type III or HDPE can have very high shrinkage. The shrinkage for LDPE is about 2%.

9. Resistant to the following : No solvents at room temperature. Not chemically attacked by nonoxidising acids, alkalies and many aqueous solutions. Low water absorption even after long immersion times; addition of carbon black (to improve weathering) will increase water absorption. Absorption of other liquids (e.g. acetone and benzene) will be greater for LDPE than for HDPE.

10. Not resistant to : Ultra-violet light, oxygen at elevated temperatures and strong nitric acid. Will dissolve in benzene at 60°C. Aromatic and chlorinated hydrocarbons will cause swelling. Certain materials, e.g. detergents, may cause stress cracking of grades with a MFI greater than 0.4. HDPE is also more prone to stress cracking.

11. Material detection or identification : Floats in water and cuts easily with a knife. Insoluable in most common solvents but soluble in hot benzene or toluene. Ignites easily when placed in a flame and burns rapidly. Burning drops usually fall and there is an odour of burning wax or paraffin; the flame is blue but tipped with yellow. The melting point is approximately 105°C. It can be distinguished from HDPE and PP by density and melting point determinations. LDPE is an inert material which does not dissolve in commom chemicals.

12. Colouring : Dry colouring is used for noncritical applications but masterbatching is the preferred system where accurate and consistent colour matching is required. Compounded material is more expensive and is not so widely used as good colouring can be achived with masterbatches. Liquid colour is under active development, e.g. to overcome problems such as poor feeding.

13. Material and component handling : Predrying is not usually necessary unless hygroscopic additives are present. When necessary, 3 h at 65°C is usually sufficient.

14. Mould and gate considerations : As the MFI increases the smaller the feed system of the mould needs to be. However, the runner sizes should not be reduced too much (e.g. below 6 mm or ¼ in) as it is necessary to supply a reasonable amount of dwell pressure in order to compensate for shrinkage. A wide range of gate types is used. There is now considerable interest in the use of hot runners and insulated hot tip runners; such systems reduce scrap production and components are ejected from the mould already finished.

15. Flow Path : wall thickness ratio : This material is a very easy flow material. At 1mm wall thickness, the maximum flow path: tickness is low, e.g. at 1mm wall thickness, the maximum flow path: wall thickness LDPE, 200:1; PS 150:1. Very easy flow grades may reach 300:1 on a high injection speed machine.

16. Projected area : The clamping pressure required is of the order of 1 - 2 tsi (15-30 MN m-2).

17. Cylinder equipment : To prevent material weeping from the nozzle it is usual to fit a valve in the nozzle (e.g. spring loaded); to minimise material flowing back down the screw a valve is also fitted to the screw. Decompression (suck-back) is used as an alternative to nozzle valves, e.g. in conjunction with hot-runner moulds.

18. Screw cushion : From 2 to 6 mm.

19. Shot capacity : Because of the material's good heat stability as little as 10% of the cylinders rated capacity may be used; the maximum possible will be about 95%.

20. Melt temperature ¾ as measured in the nozzle or by an air-shot technique: 180 - 280°C

Low (Type I) 180®280°C (356 ® 536°F)
Medium (Type II) 165®220°C (330 ® 428°F)
High (Type III) 205®260°C (400 ® 500°F)

21. Barrel residence time : If the material is overheated oxidation occurs and there is a serious loss of electrical insulation ability; it is therefore necessary to incorporate antioxidants, e.g. di-b-naphtyl-p-phenylenediamine, so as to minimise change. It is good practice not to allow the material to cook in the barrel.

22. Temperature settings: Please note that it is melt temperature which is important, those in Table 17are only suggested, initial settings. The temperature of the hydraulic oil and of the material in the hopper should not vary excessively.

23. Injection speed ¾ mould filling speed : To get fast injection speeds, high pressures are not required.

24. Injection pressure : The machine should be capable of giving the following : First stage :2000 bars; 200 MNm-2 ;29 000 psi. High pressure may be required in order to overcome high viscosity. Special high pressure machines have been built to process these materials. Second stage (dwell or follow-up pressure): up to 1200 bars;120 MNm-2; 17 000 psi. Be careful not to overpack the mould.









TABLE 17
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone. Location Temperatures ° C Temperatures ° F
No From To From To
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 120 200 248 392
(near the hopper)
2 Barrel middle 160 230 320 446
3 Barrel middle 180 260 356 500
4 Barrel front 200 280 392 536
5 Nozzle 210 270 410 518
Mould 10 60 50 140
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
25. Screw rotational Speed(rpm) : Can be very high. High screw speeds can, however, result in melt temperature variations, therefore adjust the speed to suit the cooling time.

26. Back Pressure : Up to 200 bars;20 MNm-2; 2900 psi.

27. Shutting down : No need to purge with another material

28. Reprocessing : Up to 100% regrind may be used; ensure that the scrap is kept clean and dry. If the regrind is mixed with virgin polymer then the material can be re-used more than other plastics. Check that the additives in one batch are not interacting with those from another lot.

29. Finishing : Difficult to deflash because of the flexibility of the material. Hand scalpels , cropping, etc. may be used to remove the feed system. Better to design the mould so that degating is achived during mould opening. Decoration of mouldings is not very common. The surface can be made more receptive to inks or adhesives by a corona discharge or by ozone. Because of the toughness of this material , gates are removed (by cropping) when the components are still hot.

30. Other comments : Close tolerances are difficult to mould on LDPE and they are difficult to hold in services as the material has a high coefficient of thermal expansion.

31. Typical Components : Because of this material's ease of moulding and its relative low cost it has become established as a general purpose moulding material, e.g. for toys, caps and lids for containers, bowls, beakers, pipe couplings, pots, bins, etc. Transmission of gases and vapours decreases as crystallinity increases. As the MFI increases, in general, the tensile strength decreases. Exposure to high energy radiation causes crosslinking and thus a change in properties, e.g. heat resistance.
LLDPE is used in place of LDPE when improved hot or cold resistance is required, e.g. for
household appliances.


14. ACRYLIC (PMMA)

1. Common name : Polymethyl methacrylate.

2. Abbreviation : PMMA or acrylic.

3. Systematic chemical name : Poly[(1-metaoxycarbononyl1)-1-mythyetylene].

4. Some suppliers : 5. Trade names or trade marks :
Altulor Altulite
Du pont Lucite
ICI Diakon
Montedison Vedril
Resart Resarit
Rohm GMBH Plexiglas

6. Material properties : PMMA is a hard, rigid material which is reowned for its clarity (transmits about 85% of daylight) weathering resistance and depth of colour. A wide range of colours is possible and the mouldings can be readily decorated by a wide range cariety of techniques (e.g. metallising) so that attractive components can be readily produced. Low water absorption and good dimensional stability are other desirable attributes. The polymer also appears to be unaffected by contact with human tissue. Copolymertisation of methyl methacrylate (MMA) with other monomers results in copolymers whose properties are intermediate between those of PS and PMMA.

7. Ease of flow : Stiffer flow than ABS or PS and therefore pressures may be required. For thick
parts, fill slowly ¾ a variable speed injection facility would be useful.

8. Shrinkage : This amorphous material shrinks by <0.8% (0.002-0.007in/in).

9. Resistant to the following : Dilute acids and alkalis, fats, oils, aliphatic hydrocarbons (e.g. white spirit and paraffin), dilute alcohols and detergents. Swollen by alcohols, phenols, ether and carbon tetrachloride. Relatively unaffected by strong alkalis (e.g. ammonium hydroxide) and concentrated hydrochloric acid.

10. Not resistant to : Strong acids and alcoholic alkalis, chlorinated hydrocarbons, esters, low molecular weight ketones and aromatic hydrocarbons. Ethyl acetate can be used to detect strain.

11. Material detection or identification : As the density is 1.18 gcm-3 the material sinks in water. It does not cut easily with a knife and is usually clear, hard and brittle. Upon ignition the material burns with a blue flame which is tipped or edged with yellow; a sweet fruity odour is emitted but the amount of smoke evolved is low. It continues to burn when the source of ignition is removed and the flame crackles; frothing may occur (e.g. when the flame is extinguished). There is very little residue left after burning. Sofetns in the region of 100°C.

12. Colouring : Normally supplied already coloured by the manufacturer (colour matched if necessary). Masterbatches are used in preference to other systems when in-house colouring is being employed. Only a few commercial masterbatches are used, e.g. red/amber for the automotive industry and opal for the lighting industry. Liquid colouring does not appear to be widely used due to dispersion and processing problems.

13. Material and component handling : The material will absorb water if stored incorrectly, e.g. condensation may occur on the surface. Long storage is to be avoided and a first-in-first-out system should be rigorously adhered to. Components susceptible to static are normally individually wrapped, e.g. in tissue. To protest surfaces, handle carefully, e.g. by wearing gloves.
Normally suitable for moulding without any preliminary drying operations if stored correctly. The use of a vented barrel may prove very advantageous if the material is damp. Slight traces of moisture result in frothing (in the cylinder) and splash marks(in the mouldings) therefore dry
at 70°C for 4h.







14. Mould and gate considerations : Sprues and runners need to be larger than those used for PS as the material does not flow as well. If cavities can be filled quickly then small gates may be used, e.g. pingates on low wall thicknesses. However, to maintain the optical properties it is usually necessary to fill slowly and therefore short, thick gates are used. Tab gating helps to prevent problems caused by jetting.

15. Flow Path : wall thickness ratio : Depends on grade of material, part thickness, temperature, etc. At the same wall thickness, easy flow , medium flow and stiff flow material will give ratios of 150:1, 120:1 and 100:1, respectively. Compared to PS this material has much stiffer flow; the maximum flow path:wall thickness ratio for PMMA will be 100:1 whereas for PS it could be 150:1 (at 1mm wall thickness).

16. Projected area considerations : For most products approximately 3 tsi (46 MNm-2) of projected area is used; for lenses and parts with long flow section 4 tsi (62 MNm-2) may be needed to achieve satisfactory clamping.

17. Cylinder equipment : A check ring is usually fitted to the screw (which can be of general purpose design). Both valved and open nozzles may be used ¾if excessive decompression is used then splash marks may be seen in the moulding.

18. Screw cushion : 2-6 mm.

19. Shot capacity : 20-85% of the cylinders volumetric capacity may be used.

20. Melt temperature ¾ as measured in the nozzle or by an air-shot technique: The melt viscosity of acrylic moulding powders is more temperture dependent than PS or PP, therefore accurate temperature control is necessary. Melt temperatures are usually in the region of 225°C; for thin sectioned parts, melt temperature may reach 260°C.

21. Barrel residence time : If PMMA is subject to an excessive heat treatment (which is dependent on both time and temperature) then it will depolymerise to give methyl methacrylate (MMA) and therefore bubbling; 13 min at 275 °C may cause such degradation.

22. Temperature settings: Please note that it is the melt temperature which is important; those in Table 21are only suggested, initial settings. The temperature of the hydraulic oil an of the material in the hopper should not vary excessively.

23. Injection speed ¾ mould filling speed : Usually low so as to avoid jetting and to maintain optical properties.

TABLE 21
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone. Location Temperatures ° C Temperatures ° F
No From To From To
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 150 180 302 356
(near the hopper)
2 Barrel middle 180 210 356 410
3 Barrel middle 200 230 392 446
4 Barrel front 220 240 428 464
5 Nozzle 225 245 437 473
Mould 40 80 104 176
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾

24. Injection pressure : The machine should be capable of giving the following : First stage : < 1750 bars; 175 MNm-2; 25 000 psi. Second stage ( dwell or follow-up pressure): < 1050 bars; 105 MNm-2; 15 000 psi. In the case of lenses pressure may need to be applied for a long time, e.g.3min.

25. Screw rotational Speed(rpm) : Low. Usually acrylic mouldings fairly thick so the screw speed should be adjusted to suit the cooling cycle, e.g. less than 100 rpm to avoid over-heating. The screw should be driven by a high torque, low speed motor.




26. Back Pressure : <400 bars; 40 MNm-2; 5700 psi. Too low a back pressure may result in bubbles in the mouldings.

27. Shutting down : Purging with a different material is not necessary.

28. Reprocessing : Regrind must be thoroughly dried¾usually used in opaque moulding as reground material impairs optical properties. Where large amounts of scrap are being made then after regranulation, impact modifiers are blended in, and the mixture used instead of impact styrene for some applications.

29. Finishing : Threaded inserts are used instead of drilling and tapping; components may be ultrasonically welded. A high polish may be obtained on cut edges by buffing. Components decorated by silk screening and vacuum metallisation. A wide variety of degating techniques is employed, e.g (i) simple (using a scalpel), (ii) hot wire, (iii) routing,(iv) end milling and (v) peripheral deflashing using sticks cut from PF, colth laminates (Tufnol). Components can be joined by painting the edges with chloroform or with a solution made from PMMA in methylene chloride.

30. Other comments : The addition of lubricants, to improve flow, usually results in a loss of clarity. To minimise problems caused when changing from other materials some moulders keep a cylinder just for acrylic.

31. Typical Components : PMMA's excellent optical properties, depth of colour and good resistance to weathering result in its use for streetlight fixtures, lenses and light-covers on cars. Also used for instrument panels, dials and vending machine housings. Mouldings are not subject tp microbiological attack.





15. RUBBER REINFORCED POLYPROPYLENE (PP/EPDM)

1. Common name : Rubber reinforced polypropylene, or elastomer modified thermoplastic or olefin.
thermoplastic elastomer.

2. Abbreviation : EMT or PP/EP/(D)M or OTE.

3. Systematic chemical name : Blend of poly(propylene) and poly-(methylene-co-propylene).

4. Some suppliers : 5. Trade names or trade marks :
PP ¾ see recommendations for
PP
EP(D)M ¾ see recommendation for
elastomers
Compounded material
DSM Keltan
Dynamit Nobel Dynaform
Esso Vistaflex PP
Hoescht Hostalen
ICI Propathene OTE
Montedison Moplen SP
Uniroyal Uniroyal TPR

6 Material properties : When ethylene is copolymerised with propylene (e.g. the ratio of ethylene to propylene is 70:30) an amorphous rubbery material results called EPM. If a third monomer is used then the product is called EPDM ¾ an abbreviation that covers both of them is EP(D)M. Both are compatible with PP and if the ratio of rubber is less than 50% the blend is called an elastomer modified thermoplastic(EMT). Rubber contents from 10 to 40% are commonly used for injection moulding compounds. By using different grades of the base polymers and/or by altering the ratios used a wide range of properties can be offered. All of these blends can be processed on conventional thermoplastics equipment and vulcanisation is not required. The blends have good resistance to impact (even at low temepratures), can still have high Vicat softening points (e.g. > 100°C), good ageing and weathering resistance (when protected), a neutral colour (a wide colour range is therefore possible), good electrical insulation properties and a low density (0.86 - 0.9 g cm-3 when unfilled).

7 Ease of flow : Broadly speaking it is similaar to thaat found for PP; blends based on copolymers
give easier flowing grades.

8 Shrinkage : The addition of the EP(D)M will reduce shrinkage and minimise voiding compared with the base polymer; the shrinkage is reduced by about 0.2%. Because the crystallinity is reduced the difference in shrinkage between the longitudinal and transverse directions is decreased.

9 Resistant to the following : Oils , fats , alcohols and glycols. Because of the amorphous nature of the rubber, the blends have good resistance to impact at low temperatures. Commonly assessed by testing at -40°C.

10 Not resistant to : Relatively limited resistance to petrol and other hydrocarbon solvents; as the rubber content increases the solvent resistance decreases. However, as the general chemical resistance EP(D)M is good and as the chemical resistance of PP is excellent, the chemical resistance of the blends is very good.

11 Material detection or identification : When unfilled these materials float in water as they have a density of 0.9gcm-3 . Transparent mouldings are not possible and such mouldings can be cut fairly easily with a knife. If the materials are not well mixed together then on bending or flexing the mouldings may easily break. The blends burn readily and have no distinctive colour. As with most rubber-like materials, infra-red analysis provides a convenient method of identification. As the rubber concentration in the blend increases then the hardness, modulus and softening point decreases. Such blends can be easily scratched or marked, e.g. with a thumbnail.




12 Colouring : A wide range of colours is possible using masterbatches or dry colours. Masterbatches may be based on PE or PP; if possible use a masterbatch which has a higher MFI than the EMT as this will give better dispersion. Compounded material gives the best colours.
For outdoor use, use grades which contain carbon black (e.g. 2%). Light colours may be stabilised using sterically hindered amines.

13 Material and component handling : If compounded blends are being moulded then these may be treated as though they were PP, e.g. predrying is not necessary. If blends are being prepared in-house then ensure that seperation does not occur after tumble mixing the materials together. This problem may be eliminted by metering the PP and the EP(D)M into the throat of the machine ¾sometimes referred to as direct blend injection moulding (DBIM). Blends are sometimes mixed in internal mixers and in such a case the mix temperature should reach 200°C. Continuous compounding is normally preferred; the particle size of the rubber (in the compounded material) should be about 0.5mm.

14 Mould and gate considerations : These materials are softer than PP and greater care should be taken over ejecting the mouldings, e.g. a large number of large area ejectors may be needed. Softer grades may need to have a large taper on the sprue, e.g. 5°. As the follow-up pressure may need to be high (and applied for a long time so that cavitation is avoided), the gate and runner system should be designed to minimise pressure losses, e.g. short, round runners should be used. The gates and runners may need to be larger than those used for PP so that adequate follow-up pressure can be aplied; do not overpack. Ensure that the mould is adequately vented as mould filling speeds can be high ¾ see Section 23 below.

15 Flow Path : wall thickness ratio : Depends on the types of polymers selected, ratios used, etc ¾ high ratios of flow path:wall thickness are possible. As the rubber content increases with some grades, the ease of flow also increases (the rubber can function as a flow promoter).

16 Projected area : Similar to PP e.g. approximately 2 tsi (30 MN m-2).

17 Cylinder equipment : When tumble mixes, pellets or fully compounded materials are being moulded then a screw with a back-flow valve is required; the nozzle should be valved or decompression may be used. When pellets of PP and EP(D)M are being fed to the machine, or tumble mixes, special mixing screws may be used to get optimum despersions; such screws have regions of high shear and/or mixing pins along their length. Devolatilising units should function as good mixers for these materials.

18 Screw cushion : About 4mm.

19 Shot capacity : Up to 90% of the material's rated capacity (in PS) may be used.

20 Melt temperature ¾ as measured in the nozzle or by an air-shot technique: 200 - 270°C(428 -518°F). Higher temperatures can give larger flow lengths and better weld strengths (if multiple-gated moulds are being used). The use of low temperatures may, however, improve dispersion, e.g. if high screw speeds and a high back pressure is used. As too high a melt temperature can cause degradation it is best to work at approximately 250°C.

21 Barrel residence time : These materials are relatively stable ¾ it is still good practice not to let the machine stand for long periods at processing temperatures.

22 Temperature settings: : Please note that it is melt temperature which is important, those in Table 22 are only suggested, initial settings. The temperature of the hydraulic oil and of the material in the hopper should not vary excessively.

23 Injection speed ¾ mould filling speed : As high as possible consistent with other requirements. When multiple gated moulds are being used it is not necessary to use very fast filling speeds in order to get high weld strength; employ a steady filling speed through large gates and do not overpack.

24 Injection pressure : The machine should be capable of giving the following : First stage :up to 1800 bars; 180 MNm-2 ;26 100 psi. Second stage (dwell or follow-up pressure):up to 1500 bars;150 MNm-2; 21 700 psi.




TABLE 22
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone Location Temperatures ° C Temperatures ° F
No. From To From To
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 170 210 338 410
(near the hopper)
2 Barrel middle 210 230 410 446
3 Barrel middle 220 250 428 482
4 Barrel front 230 260 446 500
5 Nozzle 240 270 464 518
Mould 20 60 68 140
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾

25. Screw rotational Speed(rpm) : High screw speeds are possible but not necessarily desirable ¾ in general adjust the screw speed to suit the cooling cycle, i.e. as slow as possible.

26. Back Pressure : Up to 200 bars; 20 MNm-2; 3000 psi.

27. Shutting down : No need to purge with another material when shutting down.

28. Reprocessing : When reprocessing be careful as slight colour changes can occur if the material has been overheated. As exposure to shear at high temperatures affects dispersion then reprocessed material may have different properties.

29. Finishing : Because these materials are softer than PP, components moulded from such blends are easier to finish (e.g. desprue) than PP mouldings. As automotive components may need to be finished by painting it is best to avoid the use of silicone mould releases.

30. Other comments : Considerable interest is being expressed in extending the range of use of these materials by adding fillers and fibres, e.g. talc and glass fibre. As EP(D)M is noted for its ability to accept large quantities of fillers and oils, there is considerable scope for experiment.

31. Typical Comments : The big advantage of these materials is versatility, i.e. a wide range of grades is possible. As rubber content increases there is an increase in impact strength but a decrease in modulus, hardness, dimensional stability on heating and Vicat softening point; chemical resistance is also decreased as rubber concentration increases. The automotive industry now makes extensive use of such blends for use as car bumpers, protective strips, interior panels, dashboards, radiator grilles, etc. These materials can withstand abuse and when they break they fail in a ductile manner, i.e. they are safer for the occupants. There is now a tendency to use PP copolymers (rather than homopolymers) as by doing so the concentration of 'expensive' EP(D)M may be reduced, e.g. by 10%.
In general, products moulded from melt-compounded material exhibit higher Vicat softening points, higher elongations at break and better low temperature properties than blends based on tumble mixtures of PP and EP(D)M.
If optimum results aare expected from any blend of PP and EP(D)M then it is best to melt-compound the blend and then mould it under those conditions suggested for ABS mouldings which are to be electroplated, i.e. relatively high melt an mould temperatures, slow filling speeds and with minimum overpacking. Such conditions are particularly suitable for components of heavy wall thickness.





16. ELASTOMERS

1 INTRODUCTION

The rubber injection moulding industry now makes extensive use of the so-called two-stage injection moulding machines. These are machines in which the rubber is softened by means of a screw in one barrel and then transferred to another; at this point , the rubber has a temperature of approximately 110°C. The rubber is then forced into the closed heated mould which, for example, is held at 185°C. At these high temperatures, the setting, or vulcanisation, is extremely rapid and once it has been taken to the desired degree, the rubber moulding is ejected from the machine.
There are many different materials which can be classified as rubbers and for the purpose of this discussion, they will be divided into, general purpose rubbers, oil resistant types, and special purpose (SP) rubbers. There are approximately a dozen or so elastomers which are used widely commercially. These materials offer a very wide range of properties and it is possible to vary; the characteristics of any elastomer by use of a range of compounding ingredients. Compound flow behaviour and final product properties can therefore vary emormously, even if the same rubber is used, and for this reason only general guidelines, suitable for the more widely used materials, will be given.

2. GENERAL PURPOSE RUBBERS

2.1 Names, Abbreviations and Suppliers

2.1.1 Natural rubber

1 Common name : Natural rubber

2 Abbreviation : NR

3 Systematic chemical name : Natural poly(1-methyl-1-butenylene).

4 Some Suppliers : 5 Trade names or trade marks:

Guthrie Estates Dynat
Harrison Crossfield Harub
Hecht Heyworth Alcan Technorub
Lewis and Peat Nirub and Suprarub
Mardec Marub

This material is most commonly sold as a specified type or grade, e.g. pale crepe; ribbed smoked sheet RSS; Standard Malaysian Rubber SMR, etc.

2.1.2 Styrene-butadiene rubber

1 Common name : Styrene-butadiene rubber.

2 Abbreviation : SBR.

3 Systematic chemical name : Not known. The following has been suggested: poly (1-
butenylene-co-1-phenylethylene).

4 Some Suppliers : 5 Trade names or trade marks:

Anic Europrene Sol R
ISR Intol
Philips Solprene
Polysar Krylene
Shell Cariflex
SIR Sirel




2.1.3 Butyl rubber

1 Common name : Butyl rubber.

2 Abbreviation : IIR.

3 Systematic chemical name : Not known. The following has been suggested: poly (1,
1-dimethylene-co-1-methyl-1-butenylene).

4 Some Suppliers : 5 Trade names or trade marks:
Esso Esso butyl
Polysar Polysar butyl

2.1.4 Butadiene rubber

1 Common name : Butadiene rubber or cis-polybutadiene.

2 Abbreviation : BR.

3 Systematic chemical name : Poly(1-butenylene).

4 Some Suppliers : 5 Trade names or trade marks:
Anic Europrene
Firestone Diene
Philips Solprene
ISR Intene
Shell Cariflex BR

2.1.5 Cis-polyisoprene

1 Common name : Isoprene rubber or cis-polybutadiene.

2 Abbreviation : IR.

3 Systematic chemical name : Poly(1-methyl-1-butenylene).

4 Some Suppliers : 5 Trade names or trade marks:
Anic Europrene
Goodyear Natsyn
Shell Cariflex BR

2.1.6 Ethylene-propylene rubber

1 Common name : Ethylene propylene rubber.

2 Abbreviation : EPM or EPDM or EP(D)M.

3 Systematic chemical name : Not known. The following has been suggested for
EPM poly (methylene-co-propylene).

4 Some Suppliers : 5 Trade names or trade marks:
Du Pont Nordel
Esso Vistalon
Goodrich Epcar
Hüls Buna
ISR Intolan
Montedison Dutral

2.2 Material Properties

2.2.1 NR and SBR

The total world production of all rubbers is now of the order of 11 million tons (Mt); the two most widely used rubbers are SBR (4mt) and NR (3 Mt). These materials are therefore the work horses of the rubber industry and are used when a high level of resistance to oxidation, ozone and petroleum oil is not required. Both materials cost roughly the same and the maximum service temperature of moulded components is of the order of 70°C. Both are available in a range grades (range of molecular weights, viscosities, etc.) and all grades often used in order to get desired properties.
There are, however, differences between them; for example, if natural rubber is stretched, then crystallinity occurs and this reinforces the rubber so that it resists deformation. Because of this increased resistance to deformation the fillers used with natural rubber can be of the nonreinforcing variety (such fillers are cheap). SBR does not crystallise when stretched and so reinforcing (expensive) fillers must be used for many applications. NR has better hot tear resistance than SBR and this property is useful if there are difficulties in moulding. SBR is slower curing than natural rubber and although the cure rates can be matched, this means that more (expensive) accelerator must be used.
However, SBR has better abrasion resistance and the moulds should stay cleaner because there is less mould fouling with compounds based on this polymer; the ageing resistance must be used.
The synthetic polymer can also be made in very high molecular weight grades (much higher than those found in NR) and these grades can then be extended with oil. Oil-extended SBR is relatively cheap and this technique helps to offset the cost of expensive reinforcing fillers (e.g. carbon black).


2.2.2 Butyl rubber

Butyl rubber is a copolymer of isobutylene and isoprene and because of this it is referred to as IIR. A copolymer is made because polyisobutylene will not vulcanise on its own; the amount of isoprene is relatively small (e.g. up to 3%) and, for example, by varying the amount of this monomer, one can produce a range of grades. Increasing the isoprene content decreases the resistance to ageing but increases the rate of cure.
The isopreneis added in order to make the final polymer capable of being vulcanised by sulphur but it is found that if butyl is mixed with other unsaturated rubbers, then the sulphur will react preferentially with the added rubber and thus upset the cure reactions. For this reason, butyl rubber is incompatible with many other rubbers, e.g. NR, SBR, NBR and CR.
Butyl rubber is noted for its high impermeability to gases, its excellent ageing resistance and good chemical resistance. Despite its chemical resistance, butyl rubber is not classified as a solvent-resistant polymer, although it is better than NR and SBR in this respect. The material is also noted for being relatively slow curing and mouldings produced from this polymer exhibit low resilience (e.g. at room temperature a butyl ball will not rebound very far when dropped).

2.2.3 Butadiene rubber

This polymer is also known as cis-polybutadiene (BR) and it is the only rubber which has better resilience than natural rubber. Like other butadiene polymers, it has poor gum strengths (see SBR) and so it is essential to compound this polymer with reinforcing fillers. By using reinforcing blacks it is possible to produce mouldings which have excellent abrasion resistance. Another noteworthy feature of this polymer is its ability to accept large quantities of oil and black¾ such additions help, of course, to reduce compound costs. Both BR and SBR resist breakdown by mechanical shear. Another peculiarity of BR is that, because of its chemical structure, it requires less sulphur than other diene rubbers to achieve an optimum crosslink density. Because of this material's reasonable resistance to ozone and its good oxidation and chemical stability, it has been adopted as a replacement for natural rubber for some applications, e.g. engine mounts.

2.2.4 Polyisoprene rubber (IR)

This material, like natural rubber, is a polymer of isoprene and in order to differntiate it from that material it is sometimes known as synthetic polyisoprene. Like NR, polyisoprene has proved to be the only synthetic rubber which exhibits self-reinforcement at high elongations. However, in order to show these properties, the material must be made so that its chemical structure is very regular, i.e. it has a high cis content (e.g. 96%). Both straight and oil-extended varieties are available and by varying parameters such as cis content, molecular weight, etc., it is possible to produce a very wide range of grades. It would be expected that lowering the molecular weight and/or the cis content would produce grades which exhibit improved flow during injection mouldings.
Despite its chemical similarity to natural rubber, the material does exhibit differences. For example, it cures at a slower rate than natural rubber and lightly filled compounds have poorer hot tear strength. In general, processing is possible at lower temperatures and it is usually found that there is less heat generation when fillers are mixed with this material. As IR is compatible with NR in all proportions, it can be usefully used in blends with NR where scorch (premature vulcanisation) is a problem.

2.2.5 Ethylene propylene rubbers

These materials are sometimes referred to a spolyolefin rubbers and are produced in two main types; the saturated copolymers (which are referred to EPM or EPR) and the terpolymers (which are referred to as EPDM). Where one does not wish to differentiate between the two types of rubber, then occasionally the abbreviation EP(D)M is used.
The copolymer can only be vulcanised with an organic peroxide but the terpolymer can be vulcanised with sulphur.
As these polymers have very poor gum strengths, it is necessary to compound them with reinforcing fillers. In some ways these materials resemble butyl rubber, i.e. they have very good chemical stability and are noted for their good ageing resistance.
They also have a high resistance to breakdown during mechanical operations which generate shear. However, they are easier processing than butyl rubbers and have better resilience at ordinary temperatures. A major application for both types of ethylene - propylene rubbers is in blends with thermoplastics materials (such as polypropylene); such blends are widely used to produce items such as car bumpers.

3. OIL-RESISTANT RUBBERS

3.1 Names, Abbreviations and Suppliers

3.1.1 Nitrile rubber

1. Common name : Nitrile rubber or acrylonitrile butadine rubber or nitrile butadiene rubber.

2. Abbreviation : NBR.

3. Systematic chemical name : Not known. The following has been suggested : poly(1-butenylene-co-1-cyanoethylene).

4. Some suppliers: 5. Trade names or trade marks:
Anic Europrene
Bayer Perbunan
Goorich Hycar
Goodyear Chemigum
ISR ISR Nitrile
Polysar Kryane
SIR Sirban


3.1.2 Neoprene

1. Common name : Neoprene or polychloroprene.

2. Abbreviation : CR.

3. Systematic chemical name : poly(1-chloro-1-butenylene).

4. Some suppliers: 5. Trade names or trade marks:

Bayer Baypren
Denki Kagaku Denka
Distugil Butachlor
Du Pont Neoprene
Toyo Soda Skyprene

3.2 Material Properties

3.2.1 Acrylonitrile-butadiene rubber (nitrile rubber)

An oil-resistant rubber can be made by copolymerising butadiene with acrylonitrile, the material is known as acr ylonitrile-butadiene rubber, nitrile rubber or NBR. By varying the ratio of acrylonitrile to butadiene, it is possible to produce a range of grades in which improved oil reistance is given by those grades which have the highest acrylonitrile content (referred to as nitrile content)). Unfortunately, high acrylonitrile content usually means more difficult processing and the finished mouldings are harder and exhibit lower resilience.
It is also found that as the proportion of acrylonitrile is increased, the low-temperature flexibility of the resultant polymer is decreased progressively. This means that a compromise between oil resistance and low-temperature properties is necessary.
Unfilled vulcanisates have very low tensile strengths and it is therefore necessary to compound the material with a suitable reinforcing filler (e.g. a carbon black of a suitable type).
In general, NBR is compounded along lines similar to those practised with the other copolymer of butadiene, SBR. Like SBR, it is found that there is only a small change in plasticity with mastication and the polymer does not crystallise on stretching. It has a slower rate of cure (that cannot be so easily varied by the choice of accelerator) and mouldings exhibit better ageing than SBR. The material has good resistance to aliphatic-type oils and solvents and exhibits reasonable resistance to aromatic-type oils and solvents. (The oil resistance of a high nitrile NBR (45%) would be better than a low nitrile NBR (20%).).
Interesting properties are obtainable from blends of NBR and PVC; the major attraction of these blends is that they can be processed just like thermoplastics and the mouldings have good abrasion and oil resistance. Such mouldings do not suffer from placticiser loss in the same way that plasticised PVC does, i.e. PVC plasticised with more conventional liquid plasticisers.

3.2.2 Polychloroprene

This material (CR) was the world's first commercial synthetic rubber and is probably better known as neoprene. Mouldings made from this material are highly resistant to oxidation and can be compounded (e.g. with carbon black and antioxidants) so that they exhibit good long-term outdoor weathering properties. The material is also resistant to ozone cracking and CR mouldings have a high level of resistance to flex-cracking. Mouldings also exhibit good resistance to aliphatic (paraffinic) oils and solvents and, because the material contains a large amount of chlorine in its structure, it has very good flame resistance.
However, mouldings made from this material lose their flexibility as the temperature is lowered and it is also found that the raw polymer, and compounds based on this polymer, harden during storage due to crystallisation.
The polymer can be softened by mastication but the rate of change is slower than that found with natural rubber. The material has a tendency to stick to metal surfaces (this is characteristic of chlorinated rubbers) and in general compounds based on this polymer tend to scorchy during processing.


4 SPECIAL PURPOSE (SP) RUBBERS


Many types of special purpose rubber are possible but three have been chosen to illustrate the range of materials available.

4.1 Names, Abbreviations and Suppliers

4.1.1 Thiokol rubbers

1. Common name : Thioplasts or thiokol rubbers.

2. Abbreviation : T.

3. Systematic chemical name : Not known.

4. Some suppliers: 5. Trade name or trade marks:

Thiokol Corp. Thiokol


4.1.2 Silicones

1. Common name : Silicones.
2. Abbreviation : Depends on type¾ all contain 'Q' (which identifies the material as a silcone) and other letters. These other letters identify side group sustituents, e.g. MQ, rubbers based on polydimethyl siloxanes; VMQ, general purpose rubbers which contain vinyl groups; PMQ/PVMQ, amterials which contain phenyl groups; FVMQ, rubbers which also contain the element fluorine.

3. Systematic chemical name : Not known.

4. Some suppliers: 5. Trade name or trade marks:
Dow Corning Silastic
ICI Silcoset

4.1.3 Urethanes

1. Common name : Urethanes

2. Abbreviation : AU or EU.

3. Systematic chemical name : Not known.

4. Some suppliers: 5. Trade name or trade marks:
Bayer Urepan
Du Pont Adiprene
Thiokol Elastothane

4.2 Material Properties

4.2.1 Thioplasts or thiokol rubbers (T)

In these materials the polymer chains consist of organic sections and sections which contain more than one sulphur atom ¾the groups are referred to as polysulphide and this structure confers some unusual properties on the final mouldings. The materials have excellent resistance to oils and solvents and in this respect they are better than nitrile rubber and neoprene. The materials also have good chemical stability, resist degradation by oxygen and ozone, and are not very strong and have poor resilience; the feature which most people remember them for their disagreeable odour.

4.2.2 Silicones (e.g.MQ)

In this class of materials the main polymer chains consist of alternating atoms of silicon and oxygen; this structure is responsible for the thermal stability of the resultant polymers. The outstanding characteristic of this type of rubber is the wide temperature range of use, e.g. from -100 to +300°C. Compounds based on silicone polymers do not (i) appear to be affected by atmospheric exposure, (ii) show ozone cracking and (iii) taste or smell. They are chemically inert and have excellent electrical insulation properties which are maintained over a wide temperature range . However, the materials do not have a good oil and solvent resistance and high shrinkage occurs when the materials are set or vulcanised. Their poor strength at room temperature is offset by the fact that they maintain this strength up to very high temperatures. For good resistance to swelling, caused by solvents or fuel, specify a fluorosilicone material in preference to other types.
Reinforcing fillers that are used with silicone rubbers are not based on carbon blacks but are usually based on silica which may or may not have been treated (e.g. in order to improve the storage life of the mixed compound).
Traditionally silicone rubber is supplied already compounded; now a range of base rubbers and modifiers are available so that in-house compounding is easier. Silicons are usually cured with peroxides and the resultant decomposition products may need to be removed by postcuring, e.g. by heating the mouldings in an air-circulating oven at 200°C for 12h. Such a treatment will often improve physical properties, e.g. compression set.
Liquid silicone rubbers (LSR) are available which are very easy flowing and yet they have similar properties to conventional peroxide-cured silicons. They are supplied as two component parts which when blended will cure in a few seconds "(for small parts) at high mould temperatures, e.g. 225°C.
The components are pumped through a static mixing device and then the mix is fed to a modified moulding machine, for example, dynamic seals must be fitted to the machine so as to contain the liquid material. Low injection pressure and clamping forces are desirable characteristics of such machines (Cush and Winnan, 1981).

4.2.3 Urethanes

This group of rubbers is based on the reaction of polyols with diisocyanates; if polyols containing ether groups are used then the product will be polyether polyurethane (EU type) and if the polyol used contains ester groups then the product will be polyester polyurethane (AU type).
By varying the molecular weight it is possible to produce material which range from liquids to solids. If the molecular weight is low then the liquid can be chain extended and crosslinked by the addition of liquid polyols or diamines; such liquids may be cast into a mould and allowed to set. Very high strengths are possible, without the addition of reinforcing fillers, even for quite soft materials. Higher molecular weight polymers are soft gums which can be mixed and moulded on conventional rubber processing equipment; the crosslinking agents may be diamines, sulphur, etc. Such materials seem to possess the disadvantages of 'traditional' rubbers without any of the major advantages (e.g. cost) as they must be vulcanised and they are expensive. It is also possible to synthesise elastomers which are composed of high molecular weight linear molecules which remain thermoplastic on moulding and which can be reprocessed if required. Such materials are referred to as thermoplastic rubbers or thermoplastic elastomers and they are available in a very wide range of grades.
Polyurethane components are noted for their good ageing, resistance to oils solvents, their high strength and excellent abrasion resistance. However, their acid and alkaline resistance is not outstanding and the compression set resistance of the thermoplastic material is poor.

MATERIAL RECOMMENDATIONS ( 6-31) FOR ELASTOMERS

6. Material properties : See sections 2.2,3.2 and 4.2.

7. Ease of flow : Natural rubber is a stiffer flow material at equal filler loadings than the
synthetic rubbers and its viscosity is more temperature sensitive. If a compound's viscosity is too high then reformulate (make it safer) so that it can be run at a higher temperature, i.e. do not try and use oil extenders, etc. as these will result in lower melt temperatures.

8. Shrinkage : Because of the higher mould temperatures employed, shrinkage may be
higher than that found in compression moulding. It will probably be different because of the more complex flow patterns found in injection moulds, i.e. it will be nonuniform. If this is excessive then it may be reduced by using a slow curing compound and/or a lower mould temperature; this should give the material more time to relax or randomise. Allowances should be made in mould manufacture for final sizing of cavities and pins to be made, i.e. after initial mouldings and measurement.

9. Resistant to : See Table 24.

10. Not resistant to : See Table 24.

11. Material detection or identification : Simple tests are not so useful for rubber materials as they are for plastics. Most rubber compounds are black and for many applications a compound with the desired properties is obtained by using a blend of elastomers. Elemental analysis (i.e. to determine what elements are present) and /or infra-red spectroscopy (i.e. so as to determine what organic groups are present) is usually necessary if base polymer identification is to be successful.

12. Colouring : Although coloured compounds are possible nearly all rubber components are black; this is because of the reinforcement possible with carbon black. Reinforcing fillers, such as carbon black, cause greater temperature rises than nonreinforcing fillers during plasticisation (thermal softening) and injection.

13. Material and component handling : Compounded material is usually fed to the machine (prepared either using an internal mixer or a continuous compounder) in the form of a strip. The technique of feeding the material in the form of a powder dry blend (similar to PVC technology) is being investigated as it can give good results and saves on compounding costs.
Components are usually removed from the mould by hand, e.g. because of the flexibility of the runners and of the moulding and because of runners and of the moudling and because of part complexity.
Design the compound for scorch safety and ensure that the mixing procedure is uniform as mixing time and temperature influence compound flow.

14. Mould and gate considerations : The mould must be designed to take account of the materials characteristics and the operating procedures employed, e.g. high operating temperatures, the fluidity of many rubber compounds, the pressure absorbing nature of the material, the tendency to evolve fumes at high temperatures, etc. The steel used to make the mould must be very good quality (to withstand the pressures, erosion, corrosion, etc.) and types used include prehardened, hardened and stainless. Chrome plating has been used but may be eroded by some fillers. The steel must machine well and take a good polish, e.g. runners polished to a 16-20 m-in finish. A well balanced runner system must be used and the use of long narrow runners, excessively restricted gates and abrupt changes in direction are to be avoided as these increase the risk of scorch and lengthen mould filling times. Good mould venting is essential; ejection is usually done using pins but 'air assist' is also used. The ejector pins should be hardened and may be of the valve type to prevent flash penetration.

15. Flow path : wall thickness ratio : The mould is hotter than the compound and this means that long flow paths into thin sections are possible. This means that moulds must be well constructed; some compounds can penetrate 5mm cracks . To ensure consistent flow properties the rubber must be of consistent quality and must be available in a narrow, specified viscosity range. Viscosity and scorch measurements should be made on each batch of compound before any attempt is made to mould that batch.

16. Projected area : The clamping pressure required per square inch of projected area are relatively high (compared to compression mouldings) and are of the order of 2tsi 30 MNm-2; the figure used depends on the ease of flow of the component, depth of draw of the component, the amount of flash that can be tolerated, etc.

17. Cylinder equipment : To stop the injection pressure being transmitted to the screw's thrust bearings it is important that there is a back-flow valve between the plunger and the screw. The nozzle diameter is very important and should match a particular compound and mould ¾ it is usually about 3mm. As the diameter is decreased, heat build-up and dispersion (e.g. of filler and sulphur) increases. If the compound temperature can be reduced so as to decrease scorch tendency. If the nozzle diameter is too small then it will restrict mould filling and cause scorch; if this happens increase the diameter, e.g. in 0.5mm steps, until scorch is eliminated.

18. Plunger cushion: Usually as small as possible (e.g. 1-2 mm) so that barrel residence time is kept as low as possible.

19. Shot capacity : As little as 5% of the machine's rated capacity may be run, although it is better to match the injection unit to the mould if at all possible.

20. Melt temperature : As high as possible consistent with safe (e.g. scorch-free) working (e.g. 100°C). Higher temperatures may be employed if a screw delay facility is available. If there are signs of scorch at a high temperature (e.g. specks of overheated material in the moulding, slow or hesitant screw and/ or plunger movements, etc.), reduce barrel temperatures in 5°C steps and/or reduce the screw speed until the signs of scorch disappear.

21. Barrel residence time : All compounds will cure in the barrels if left to stand for too long ¾as the temperature increases the barrel residence time decreases. If there is any break in moulding drop the temperatures and purge the barrels clean.

22. Temperature settings : See Table 23. The mould temperature should be hot enough to give the desired speed of vulcanisation but not so hot as to cause scorch, reversion and/or nonuniformly cured mouldings. The mould is usually heated by electrical resistance elements¾ these may be located in the mould itself or in the machine platens. Both the injection unit and the plasticising unit have a heating/cooling system. Around each barrel zone there is an outer jacket through which oil can be circulated; resistance elements (cuff heaters) are clamped around this outer jacket. If the temperature exceeds what is desired or if the machine needs to be crash-cooled then the oil will remove the excess heat. The liaison block (which contains the nozzle and links the injection unit to the extruder unit) is also oil heated.




TABLE 23
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾ Location Temperatures ° C Temperatures ° F
From To From To
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Plasticising unit 80 110 176 230
Injection unit 90 125 194 257
Nozzle(liaison block) 90 120 194 248
Mould 170 200 338 392
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾

23. Injection Speed ¾ mould filing speed : Use a safe (i.e. scorch resistant) mix, adjust the machine controls (injection speed, back pressure, etc.) so as to obtain the highest possible injection temperature and mould filling speed consistent with freedom from scorch. The temperature of the material may be raised by up to 40°C if high speeds and a small nozzle are employed.

24. Injection pressure : The machine should be capable of giving the following : First stage : up to 1500 bars; 150 MNm-2; 21 400 psi. If possible do not use a compound which demands the maximum pressure as any small variation will make moulding difficult and may demand compound changes.
Second stage (dwell or follow-up pressure): up to 1200 bars; 120 MNm-2; 17 400 psi.

25. Screw rotational speed (rpm): Screw speeds up to 200 rpm are employed ¾ because of the low temperatures set on the 'extruder' barrel(up to 120°C), the use of a high screw speed (e.g. 150rpm) provides a convenient way of generating heat.

26. Back pressure : Up to 250 bars; 25 MNm-2; 3620 psi. Back pressure is used to generate a temperature rise and is also used to exclude air from the compound.

27. Shutting down: If the stoppage is going to be a long one then cool the barrel as quickly as possible (crash cool) and purge out; if the stoppage is only going to be for a short time then reduce the temperature settings (each 10°C drop will double the resistance to scorching) by about 20°C.

28. Reprocessing : For noncritical applications some companies regrind the waste and compound the cured, granulated material in with another batch. Such processing is not normally employed, however, as either the compound properties suffer or regranulation is difficult and expensive. The use of cold runner moulds can help to reduce the amount of scrap produced (manifold temperature 125°C).

29. Finishing: Because the material is soft, trimming is relatively easy and many gates may be removed by simply 'twisting'. With some mouldings, it is difficult to avoid the formation of flash and even though this is much less than that produced by compression moulding, it still must be removed, e.g. by knife or scalpel. Because it is thin and flexible it may present problems if it is removed by barrelling ¾ this is why many moulders reduce the temperature during barrelling (cryogenic tumbling).

30. Other comments : The scorch resistance (premature vulcanisation) of a mix is controlled mainly by the accelerator system used; more than one accelerator may be used to improve speed of cure. If a mix is too scorchy, a retarder may be used.

31. Typical components : Both mouldings and rubber-to-metal bonded products are produced by injection moulding. The mouldings are made in this way for a number of reasons, e.g. because they are cheaper, little or no finishing is necessary, etc. Rubber-to-metal products benefit because fresh, clean rubber is presented to the metal surface and this results in a stronger, more consistent product(e.g. a typical product would be an engine mount).
The automotive industry uses rubber injection moulding to produce windscreen wiper blades, seals, bushes, engine mounts, tyre valves, etc. In the pharmaceutical industry this process is used to produce bottle stoppers, teats, pipettes, etc.
The shoe industry uses injection machines to produce shoe sole units and to mould the shoe soles on the uppers. Another large market is the manufacture of seals, e.g. for oil and water pipes. In short it is difficult to find an industry that does not use injection moulded rubber components.




17. UNPLASTICISED POLYVINYL CHLORIDE (UPVC)


1. Common name : Unplasticised Polyvinyl chloride

2. Abbreviation : UPVC, hard PVC, or rigid PVC

3. Systematic chemical name : Poly[(1-chloroethylene ). As the term UPVC simply means that the compound is based on PVC then the compounds may be known under the trade name pf the parent polymer or, under the trade name of a proprietary compound.

4. Some suppliers : 5. Trade names or trade marks :

Anic SpA Fimat
A to Chimie Lacquyl
BASF Vinoflex
BP Geon
CdF Chimie Gedevyl
Chemische Werke Hüls Vestolit
Diamond Shamrock Dacovin
Dynamit Nobel Trosiplast
Hoescht Hostalit
ICI Corvic or Welvic
Lonza Lonza compound
Montedison Sicron
Solvay Solvic
Wacker Chemie Vinnol

6. Material properties : Unplasticised PVC has many desirable properties, e.g. it is strong, stiff, transparent, resistant to oils, etc. However it is difficult to process, can have a low impact strength and a low heat distortion temperature. These deficiencies can be minimised by additive choice, machine design and opertion. On heating, hydrochloric acid is evolved and this attacks both the machine and the operators. Without the addition of stabilisers (such as tribasic lead sulphate) processing would be impossible; it is also made easier by the use of lubricants (e.g. 2 phr of butyl stearate), processing aids (3 phr of acrylic procesing aid) and low molecular weight resins (i.e. of low k value). Impact strength may be increased significantly by mixing with other plastics (up to 15 phr of a copolymer ¾ ABS or MBS); heat distortion temperature may be raised by using fillers (e.g. 10 phr of china clay) and large concentrations of polymeric modifiers (e.g.45 parts of a modified ABS). Unless the refractive index of the additive matches that of the PVC, an opaque composition results.

7. Ease of flow : At its processing temperature this is a very stiff flow material; high temperatures
(e.g. greater than 200°C) cannot be used because of decomposition.

8. Shrinkage : This is an amorphous material so shrinkage is low, approximately 0.6%, i.e. about
0.004 in/in.

9. Resistant to the following : UPVC has excellent resistance to mineral acids (dilute and
concentrated), alkalis and detergents; its resistance to alcohols and oils is good. Chemical
and heat resistance is generally made worse by the addition of impact modifiers.

10. Not resistant to : Attacked by ketones, esters and aromatic hydrocarbons (e.g. benzene). Can have a high impact strength but is notch sensitive. Resistance to ultra-violet can be improved, e.g. by use of carbon black. Methylene chloride can be used to detect an under-gelled compound.

11. Material detection or identification : Compounds based on PVC have a density (e.g. 1.3 ¾1.6 g cm -3) and therefore they sink quickly in water. It is a self-extinguishing material which when burnt in a flame., colours the body of the flame yellow and tips the edges green. A pungent, irritating odour of hydrochloric acid results; if a piece of the material is heated on a copper wire then the flame is coloured green. It is commonly seen in the form of opaque mouldings, e.g. grey or brown pipe fittings.

12. Colouring : Pigments may be added at the high speed mixing stage; when this is done pigments of fine particle size should be used (sometimes referred to as micropigments). For some jobs (where the colour required must be held acurately) compounded material is used.
13. Material and component handling : Ingredients usually mixed together in a high speed mixer. For example: (i) PVC added to a mixer together with other solid ingredients, (ii) mixed at high speed until the melting point of the lubricant is reached, e.g. 100°C, (iii) lubricant added followed by acrylic processing and (iv) at 125 °C dumped into a cooler blender. A large ribbon mixer is used for this purpose and this cools the mix, reduces batch-to batch homogeneity and also helps reduce electrostatic charges on the powder (thus stabilising bulk density). The powder may be used directly or it may be pelleted (using an extruder). Pelleting improves dispersion, adds to the cost and reduces thermal stability. Pellets are much easier to handle than the powder.

14. Mould and gate considerations : The mould , and ideally all machine parts such as the platens, should be acid resistant. Sprue taper should be generous (e.g. 5 °C ) if ease of release is to be achieved. To minimise pressure losses the runner system should be short and of full-round cross-section, e.g. 5- 10 mm in diameter. The gates should be round and the edges of the gate should be radiused towards the component; gate thickness should be approximately three-quarters of the part thickness. Pin gates (>1mm) can sometimes be used for small components. Gate lands should be kept small. When pipe fittings are being moulded (and cores are being employed) it is important to ensure that the locking unit can generate a large opening force as the contracting material may be chrome plated so as to improve acid resistance.

15. Flow Path : wall thickness ratio : This is not an easy flow material; the flow path : wall thickness ratio ( at 1mm wall thickness) would typically be of the order of 60:1.

16. Projected area : The clamping pressure should be of the order of 2-2.5 tsi (30-34 MNm-2).

17. Cylinder equipment : To reduce the risk of stagnation and subsequent degradation, the screw should not be fitted with a nonreturn valve. A plain screw tip (streamlined or shaped to match the internal nozzle shape) or one with a 'flying spiral' formation should be used. An open nozzle is also recommended together with a screw designed for UPVC processing (e.g. compression ratio 2:1). The screw must be tough steel. The screw drive system must be adequately powered.

18. Screw cushion : This should be as small as possible (e.g. 2 mm) so as to reduce the risk of material stagnation.

19. Shot capacity : If the screw has been designed for UPVC then the full shot capacity may be used.

20. Melt temperature ¾ as measured in the nozzle or by an air-shot technique: This should be adjusted so that the material is capable of being slowly purged from the nozzle; the melt should have a smooth, glossy appearance and should be uniformly plasticised. As the processing temperature and the decomposition temperature are very close, accurate control is essential; a temperature of between 180 - 200 °C should be used. Do not exceed 210 °C.

21. Barrel residence time : Keep as short as possible as otherwise degradation will start; once it starts it will rapidly spread. In the event of a stoppage, reduce the set temperatures and purge the barrel clean. At the recommended melt temperatures the material should be capable of being heated for about 20 - 25 min without serious degradation. Do not attempt to process UPVC in a ram or plunger machine.

22. Temperature settings: See Table 13.

23. Injection speed ¾ mould filling speed : Excessive shear can cause degradation (burning) and this can be caused by forcing the material, at high speed, through tha gate. Adequate mould venting must be provided and a programmable injection speed unit is useful.

TABLE 13
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone Location Temperatures ° C Temperatures ° F
No. From To From To
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 140 160 284 320
(near the hopper)
2 Barrel middle 160 170 320 338
3 Barrel midde 170 180 338 356
4 Barrel front 180 190 356 374
5 Nozzle 190 210 374 410
Mould 20 60 60 140
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾

24. Injection pressure : The machine should be capable of giving the following : First stage : 1750 bars; 175 MNm-2; 25 000 psi. Second stage ( dwell or follow-up pressure): up to 1000 bars; 100 MNm-2; 14 500 psi.

25. Screw rotational Speed(rpm) : To minimise degradation keep the screw speed low, e.g. below 80 rpm ¾ speeds are approximately a third of those used for PS. As it is surface speed that is important, speed decreases as screw diameter increases.

26. Back Pressure : Up to 50 bars; 5 MNm-2; 700 psi. Back pressure control is very important when UPVC is being processed; it should be capable of being precisely set and must be reproducible so as to obtain an accurate heat input. Usual back pressure is 1 MNm-2; (150 psi).

27. Shutting down : Turn off the heaters and purge out slowly until the barrel is empty. Purging compounds are available which help to reduce degradation by keeping the barrel clean. Do not follow PVC with acetal (or vice versa); purge with PS first. It is better to keep a cylinder and screw just for PVC if all possible.

28. Reprocessing : Each time UPVC is heated its processing life is shortened and once degradation starts it rapidly spreads. Therefore, ensure that no degraded material is fed back into the machine. If degradation does occur, it will first be seen as a dirty patch or streak in the component. When this happens, purge until clear and try and reduce the heat input to the material.

29. Finishing : Gates are removed by end-milling, drilling, cropping, etc. Mouldings may be decorated by hot stamping and silk screening.

30. Other comments : If PVC is to be successfully moulded then the equipment must be acid resistant and accurate control (over time, speed, temperature, pressure and material ) is important.

31. Typical Components : The mechanical properties of UPVC range between those of PP and PA; because of its chlorine content it does not burn like these two materials. Pipe is manufactured from this material and injection moulding is used to make the pipe fittings. Jointing can be by solving welding or it can be achieved by using 'interference fit' couplings. The pipe system is used for rainwater, soil pipes, guttering, etc. Other parts of such plumbing systems are also made from UPVC, e.g. gullies, grilles, traps, etc.




18. PLASTICISED POLYVINYL CHLORIDE (PPVC)

1. Common name : Plasticised Polyvinyl chloride.

2. Abbreviation : Plasticised or soft PVC, or PPVC.

3. Systematic chemical name : Plasticised poly[(1-chloroethylene) has been suggested.

4. Some suppliers : 5. Trade names or trade marks :
Chemische Werke Hüls Vestolit
Dynamit Nobel Trosiplast
Lonza Lonza compound
Solvay Solvic
Wacker Chemie Vinnol
(See also unplasticised polyvinyl chloride)

6. Material properties : PVC is very versatile material as its properties can be varied over a very wide range by the use of additives, e.g. fillers, stabilisers, plasticisers, lubricants, etc. The term 'plasticised' means a plasticiser has been added in order to produce a soft, flexible product and /or in order to make processing easier. Plasticisers are usually organic liquids called esters and the type and concentration used dramatically affects the properties of the finished product ¾properties such as cost, temperature resistance (low and high), bacteria and stain resistance, fire resistance, etc. In order to get a balance of properties, blends of plasticisers are commonly used.

7. Ease of flow : Increasing the plasticiser concentration gives a more easy flowing material.

8. Shrinkage : This is an amorphous material so shrinkage varies from 0.01 to 0.05 in/in,
depending on the formulation.

9. Resistant to the following : Excellent resistance to dilute mineral acids and good resistance to alkalis. If correctly formulated resistance to aromatic hydrocarbons, detergents and greases can be reasonable. Resistance to weathering is generally good; improved by the use of ultra-violet stabilisers and/or carbon black.

10. Not resistant to : Chlorinated hydrocarbons, alcohols and ketones. Some plasticisers may be attacked by bacteria ¾ to minimise this use bactericides and/or linear esters (based on straight-chain alcohols). Plasticisers may be lost by exposure to high temperatures, water and solvents but the correct choice will help to minimise such losses.

11. Material detection or identification : Compound density is usually greater than 1g cm -3 ( up to 1® 2 1g cm -3 ) so the material sinks in water. It may be cut easily and clearly with a knife and the cuts have smooth edges. Transparent mouldings are possisble although most injection mouldings are opaque. Provided that the plasticiser level is low. PVC compounds burn slowly and tend to be self-extinguishing. When heated in a flame on a copper wire, a green coloration is produced.

12. Colouring : Masterbatching can be used but problems may arise due to nonuniformity of the final colour. Pigments of fine particles are used for dry colouring but for colour consistency a compounded material may be preferred.

13. Material and component handling : Ingredients may be blended in a high speed mixer. Suspension polymers are used ¾ when the plasticiser is added to the high speed mixer, rapid absorption occurs. The mix is then either cooled and injection moulded, or cooled, extruder pelletised and then injection moulded. It is not usually necessary to predry the material.

14. Mould and gate considerations : PVC is used to mould shoe soles and the moulds have been made from Kirksite-type alloys. These are cast to the size required and each casting contains water-cooling copper tube, each casting can then be mounted in a water-cooled steel bolster. Fast and frequent mould changing is therefore possible.
Brass and aluminium moulds have also been used. Small gates may be used, e.g. pin-gates with a diameter of 0.5mm (0.02in). Edge gates of equivalent cross-sectional area are also used on appropriate mouldings. For long running jobs moulds made from tool steel are the most popular.

15. Flow Path : wall thickness ratio : Depends upon the formulation , e.g. plasticiser type concentration, etc. but flow path: wall thickness ratio (at 1mm wall thickness) can be upto 180:1.

16. Projected area : The clamping pressure can be up to 2.5 tsi (34 MNm-2) but is dependent on formulation, product design, etc.

17. Cylinder equipment : A concentional screw and barrel arrangement may be used e.g. a back-flow valve on the screw and either decompression or a nozzle shut-off valve to prevent drooling. Small components are still produced on ram machines without thermal degradation or component property changes.

18. Screw cushion : About 4 mm.

19. Shot capacity : Up to 80% of the machines rated capacity on PS may be used.

20. Melt temperature ¾ as measured in the nozzle or by an air-shot technique: 150 ¾190°C.

21. Barrel residence time : Depends on the method of operation, formulation, etc. When using very high back pressures the critical residence time may be reduced to 5mm.

22. Temperature settings: The speed should be as high as possible and consistent with other requirements. High speeds can cause shear seperation of the components in some formulations and may therefore have to be reduced.

23. Injection speed ¾ mould filling speed : The machine should be as high as possible and consistent with other requirements. High speeds can cause shear separation of the components in some formulations and may therefore have to be reduced.

TABLE 14
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone Location Temperatures ° C Temperatures ° F
No. From To From To
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 145 155 293 311
(near the hopper)
2 Barrel middle 155 165 311 329
3 Barrel middle 165 175 329 347
4 Barrel front 175 180 347 356
5 Nozzle 180 185 356` 365
Mould 20 30 68 80
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
24. Injection pressure : The machine should be capable of giving the following : First stage : 1200 bars; 120 MNm-2; 17 400 psi. Second stage ( dwell or follow-up pressure): up to 800 bars; 80 MNm-2; 11 500 psi.

25. Screw rotational Speed(rpm) : High screw speeds are possible but not necessarily desirable. Adjust the screw speed to suit the time available, e.g. 80 - 200 rpm.

26. Back Pressure : 250 bars;25 MNm-2; 3570 psi. Back pressures higher than this have ben used, e.g. when large proportions of scrap are being run.

27. Shutting down : When a particular run has finished, turn off the heaters and purge the barrel empty. If the run has been completed purge with PS or PE until there is no trace of PVC then turn off the heaters; special purging compounds are available. Do not purge with PE may be difficult to remove and delamination may occur subsequently.

28. Reprocessing : Up to 100% regrind has been run but the amount incorporated depends upon formulation and end-use application.

29. Finishing : Hand degating (using scalpels, side cutters, etc) is common; automatic degating is used, e.g. with those compounds containing lower concentration of plasticiser.

30. Other comments : In order to get soft, flexible products which are immune to plasticiser loss, nitrile rubber (in the form of powder) may be added to the PVC up to 50 phr (based on the PVC ) may be used.

31. Typical Components : PVC can be formulated to be a soft,resilient material and as such it is
used for washer, hers, grommets, electrical flex-ends, etc. A major use for this material is in footwear and largenumber of PVC soles and heels are now manufactured by injection moulding (using ear and a large number of PVC soles and heels are now manufactured by injection moulding (using specially developed moulds and machinery.)



19. CELLULOSICS

1. Common name : Celulosics, e.g. cellulose acetate, cellulose acetate butyrate, cellulose propionoate.

2. Abbreviation : Depends on type, e.g. CA, CAB, CP.

3. Systematic chemical name : Not known ¾ the following have been suggested: cellulose ehanoate (CA) cellulose ethanoate-butanoate (CAB), cellulose propanoate (CP).

4. Some suppliers : 5. Trade names or trade marks :
CA
Bayer Cellidor
Bergmann Bergocell
British Celanese Dexel
Eastman Chemical Tenile
CAB
Bayer Cellidor
Eastman Chemical Tenile
CP
Bayer Cellidor
Eastman Chemical Tenile

6. Material properties : Cellulose is a naturally occuring high polymer which cannot be moulded unless it is modified by a combination of the following : (i) esterification, (ii) molecular weight reduction and (iii) plasticisation. CA is made by esterifying (or reacting) cellulose with acetic acid; CAB is formed when cellulose is reacted with a mixture of acetic plus butyric acid and CP is made by reacting cellulose with propionic acid. All are hard, stiff materials which have good gloss, excellent toughness and a wide colour range (e.g. from clear to black). Compared to the major synthetics CA has poorer electrical insulation properties, heat, water chemical and weathering resistance. CAB offers good weathering properties, ease of moulding (particularly for thick sections) and good appearance and toughness (specially at low temperatures). CP does not have the rancid odour associated with CAB, but is not so weather resistant; in other respects it is similar to CAB.

7. Ease of flow : By adding plasticiser (e.g. dimethyl phthalate or DMP) a wide range of grades is
available. Higher plasticiser contents give softer, tougher materials which flow more easily but which have lower heat distortion temperatures. The hard grades of these cellulose esters have similar flow properties to that of PS. Easy flow grades of all three are possible. CAB is available in softer flows than CP.

8. Shrinkage : Each ester may give the following shrinkages : CA from 0.005 to 0.008 in/in; CAB from 0.003 in/in and CP from 0.003 to 0.006 in/in (about 0.5%). The postmoulding shrinkage is negligible.

9. Resistant to the following : Aliphatic hydrocarbons, petrol, detergents, oils and greases. CA has the best resistance to aromatic and chlorinated hydrocarbons; CAB has excellent resistance to aromatic and chlorinated hydrocarbons; CAB has excellent resistance to perspiration.

10. Not resistant to : Affected by weak acids and alkalis and not resistant to strong acids and alkalis. Attacked by esters of the low molecular weight alcohols and by low molecular weight ketones and alcohols. Cellulose acetate absorbs up to 3% water (and changes its dimensions accordingly) whereas CAB and CP absorb about 1%. All are soluble in ethyl acetate, methylene chloride, acetone and o-chlorophenol.

11. Material detection or identification : Mouldings are usually fairly hard and can be clear and tough. Little smoke is produced when burnt. All three are soluble in acetone and trichloromethane. CA has the highest density at 1.26 - 1.30 g cm-3; CAB and CP are similar with densities of 1.16 - 1.22 g cm-3; CA burns slowly with a yellow flame and a vinegary odour; CAB burns with a blue flame and smells like rancid butter; CP has a blue flame tipped with yellow and the smell is quite pleasant.

12. Colouring : All three materials are available as crystal clear materials and can be purchased in a full range of compound colours (i.e.transparent, translucent and opaque). Mottled effects may be produced by adding a harder flow grade (of the same polymer) can occur with some colours. Cellulosics can, in general, be coloured by a variety of techniques.

13. Material and component handling : Care must be taken during storage to ensure that moisture contamination does not occur ¾ particularly with CA. Long storage is to be avoided and a first-in-first-out system should be adopted. The moisture will improve the flow but will also reduce moulding quality¾a high water content is shown by silver straking. Predry for 3h at 70°C but be careful not to keep the material in the oven too long as material flow and component quality will suffer due to excess volatile loss. Do not predry with other materials as they may become contaminated with plasticiser lost from the cellulosic. Predrying is not always necessary but is common.

14. Mould and gate considerations : Due to overheating (e.g. because of long residence times or frictional heating) volatiles may be evolved; these can cause burning and hinder melt fusion (necessary if the melt stream has been split). Try to avoid dividing the melt, ensure that the mould is vented and do not over-heat the material by using too small a gate at too high a speed. Pinpoint gates are now commonly used and jetting may be minimised by siting the gate (less than 3mm (1/8 in)) opposite cavity wall. To minimise component weaknesses, such gates are often used in form of tunnel gates as this may move the area of weakness to a less critical place. Box-shaped components are best gated from one side using. if possible, a flash gate. On thick components, jetting may be prevented (if small gates are used) by the use of movable or spring-loaded cores. Ribs and inserts do not usually cause problems (e.g. sinking and cracking). Due to the resilience of these materials components may be made to jump considerable undercuts. Hot-runner moulds may be used to overcome long runners and/or to minimise scrap production.

15. Flow Path : wall thickness ratio : CAB and CP will flow into thinner sections than CA. At 1 mm wall thickness the flow path: wall thickness ratio can be very high for some formulations, e.g. 300:1 is possible for easy flow formulations.

16. Projected area : Clamping pressure required is usually of the order of 1®2 tsi (15®30 MNm-2).

17. Cylinder equipment : Properly dried material at the correct temperature will not drool from the nozzle ¾ some sealing type nozzles may cause degradation. Use an open nozzle if possible and if necessary use decompression. The screw should be equipped with a back-flow valve. (These materials are still processed on ram-type machines because of their ease of flow)

18. .Screw cushion : About 3 mm.

19. Shot capacity : Can be similar to that for PS but usually about 80% of the barrel's rated capacity in PS.

20. Melt temperature ¾ as measured in the nozzle or by an air-shot technique: Depends on the type of ester and on the mount and type of plasticiser used, for example:
CA 170 - 250°C (338-480°F)
CAB 170 - 230°C (338-446°F)
CP 170 - 250°C (338-480°F)

21. Barrel residence time : On purging the melt should be smooth, glossy, free from bubbles or gassing and with a honey-like viscosity. If the material is overheated then the surface appearance may be poor (e.g. matt finish), the flow properties may change (flashing may become a problem), distortion and colour changes may occur and excessive fumes will be emitted. Therefore do not allow the material to stagnate or 'cook' in the heated barrel.

22. Temperature settings: Please note that it is melt temperature which is important, those in Table 16 are only suggested, initial settings. The temperature of the hydraulic oil and of the material in the hopper should not vary excessively.
The temperature settings are for CAB; CA will be lower by about 10°C ¾ for CP use higher temperatures (e.g. by 20°C). When moulded under the correct conditions the depth of colour and gloss of CAB and CP will be similar to that of PMMA. Be careful not to process at too low a temperature as components with poor strength may result. High mould temperatures are recommended when thick-sectioned parts (e.g. tool handles) are being moulded as such a high temperature helps to stop void formation.

23. Injection speed ¾ mould filling speed : For thin walled components high injection speeds may be possible ¾ at high speeds the viscosity of cellulosics may be increased. As this point is increased by raising the temperature (and as moulding strength is also improved by higher temperatures) work at the highest temperature possible , i.e. without colour changes, degradation, etc. Fill thick moulds slowly; a programmable injection speed facility is very useful.

TABLE 15

¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone Location Temperatures ° C Temperatures ° F
No. From To From To
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 140 160 280 320
(near the hopper)
2 Barrel middle 155 175 311 347
3 Barrel middle 170 190 338 374
4 Barrel front 185 205 365 401
5 Nozzle 180 200 356 392
Mould 40 80 104 176
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾

24. Injection pressure : The machine should be capable of giving the following : First stage : 1500 bars; 150 MNm-2 ;2140 psi. Second stage (dwell or follow-up pressure): up to 300 bars; 30 MNm-2; 4300 psi. Be careful not to overpack the mould. Adjust hold pressure and dwell time until sink marks just cease to be visible.

25. Screw rotational Speed(rpm) : Avoid overheating and get better temperature uniformity by using slow screw speeds.

26. Back Pressure : Up to 150 bars;15 MNm-2; 2140 psi. Ensure that too high a back pressure is not being used as this could cause degradation due to air trapping.

27. Shutting down : Do not purge with another material as this could cause problems (such as haziness). Switch off the heaters , withdraw the carriage away from the mould and slowly purge the barrel until empty.

28. Reprocessing : On granulation and re-use the material may have different flow characteristics; predry, blend with fresh material (up to 20% ) and keep a check on the number of times of re-use. Do not use regrind on light or transparent compounds. Ensure that the regrind is kept perfectly clean, e.g. by stripping and cleaning the grinder before use.

29. Finishing : Easy to cut, drill, tap and polish; may be joined by cementing (that is, if the two pieces are of the same type of cellulosic, e.g. both CAB). Hot stamping, vacuum metallising, etc. are used but surface cleanliness is essential if good adhesion is to be obtained.

30. Other comments : If other plastics have been processed using the same screw and barrel then before the cellulosic is used, the whole assembly must be cleaned very throughly, e.g. stripped and cleaned mechanically. CA may be counted as a 'another plastic' to CP and CAB.

31. Typical Components : CA is chosen when a reasonably tough, clear material is required and/or when special colour effects are required; it has a slight odour but can be made self-extinguishing. Examples of use are toys, appliance housings, pens, brushes, buttons, ornaments,etc.CAB has good toughness, is tough at low temperature, has good dimensional stability and excellent outdoor weathering characteristics. It is probably the most widely used cellulosic moulding material and is used in outdoor signs and letters, tool handeles, lenses, steering wheels, keys on calculators, etc.CP has higher hardness, stiffness and tensile strength than CAB and is used for brush handles, steering wheels, toys, tool handles, pen barrels, seats, etc. Both CP and CAB are more dimensionally stable and have better low temperature properties than CA. These materials are useful for signs and displays as there is little electrostatic dust attraction.



20. POLYETHERSULPHONE (PES)

1. Common name : Polyethersulphone or polysulphone or polyarylsulphone.

2. Abbreviation : PES.

3. Systematic chemical name : Not commonly used as it is very complex. For example, a suggested chemical name for Victrex is poly(oxy-1,4-phenylene-sulphonyl-1,4-phenylene).

4. Some suppliers : 5. Trade names or trade marks :
ICI Victrex
Union Carbide Udel
3 Ms Astrel
These materials differ chemically and have, for example, differing high temperature properties; the upper use temperature increases in the order Udel; Victrex; Astrel.

6. Material properties : These materials all contain aromatic groups (p-phenylene), sulphone groups (SO2) and ether groups (¾ O¾) ; other groups (diphenyl) may also be incorporated in the polymer chain. Any plastic which contains p-phenylene groups (e.g. PC) has a high heat distortion temperature, is rigid at room temperature and requires a high processing temperature. PES materials are tough and have a wide temperature range of use (e.g. from - 100 to + 150°C). Because of their chemical structure the materials are resistant to chemical changes (e.g. oxidation) on heating; electrical insulation properties are good but the tracking resistance is not outstanding. The materials have good creep resistance, are transparent, have self-extinguishing characteristics and are tough. Like PC they are notch sensitive; however, they have lower impact strengths than PC they are more expensive but possess higher heat distortion temperatures and better creep resistance.

7. Ease of flow : These materials have a high melt viscosity. In an effort to reduce the viscosity during processing, be careful that overheating does not occur as the colour of the material will change, i.e.
it will darken.

8. Shrinkage : As these are amorphous materials shrinkage is low, e.g. approximately 0.6% for the unfilled material. The presence of fillers, e.g. glass fibre, reduces the shrinkage but increase the density, e.g. from 1.37 to 1.6 g cm-3 (when 30% glass fibre is used). The unfilled material may give shrinkages of 0.007 in/in.

9. Resistant to the following : Unaffected by most inorganic reagents. Acqueous solutions do not generally attack PES although there may be some slight water absorption. Has good resistance to alkalis and to dilute acids. Aliphatic hydrocarbons, alcohols, benzene, petroleum spirit, oils and fats do not usually attack this material.

10. Not resistant to: Concentrated oxidising mineral acids. PES is soluble in highly polar organic solvents and in a number of chlorinated and aromatic hydrocarbons. A large number of organic materials will therefore attack this material, e.g. chloroform, cresols, acetone, cyclohexanone, ethyl acetate, methylethylketone, etc. If this material is to be used outside then it should be light stabilised, e.g. with carbon black.

11. Material detection or identification: This material has a high density (e.g. 1.37 gcm-3) and mouldings therefore sinks when placed in water. Being an amorphous material it can be obtained as transparent mouldings, e.g. pale-straw colour. The mouldings are tough and have a very high softening point, e.g. a Vicat sofetning point of greater than 220°C. When placed in a flame the mouldings do not burn very easily but when they burn they do so with a white flame and give off an odour of sulphur. When the moudlings are struck with a hard object they can give off a metallic sound or ring.

12. Colouring : The natural colour is a transparent amber shade and so a wide colour range is possible. It can be dyed or pigmented; dyes give transparent colours and pigments give opague colours. Dyes can be incorporated, in some grades, by the moulder but in general the addition of pigments is not recommended as agglomerates may form. Such inclusions could act as stress-raisers and cause failure during impact.

13. Material and component handling : The material contains absorbed water and should be dried at 150 °C for at least 3 h before any attempt is made to mould. Use a heated hopper if possible. A component will absorb water ¾ the rate of absorption and the amount of absorption will depend on the humidity, mouldings, shape,etc. Newly moulded specimens will increase in dimensions by about 0.2% when exposed to air or water. Stability is obtained rapidly (e.g. within two weeks) at an equilibrium water content of, about, 2%.

14. Mould and gate considerations : Because of the rigid chemical structure of these materials high set-up temperatures are obtained. Large amounts of frozen-in strain can therefore be easily incorporated into the mouldings unless care is taken. The problem is made more difficult because large runners and gates need to be used (necessary because of the high melt viscosity). The use of such large runners and gates can easily result in overpacking. Reduce such undesirable effects by keeping dwell time and pressure to the minimum, using high mould temperatures, and annealing the mouldings if necessary (e.g. by heating in glycerine at 160°C) To improve the strength of the moulded part avoid sharp corners in the mould and put a generous radius in the gate entry region. Keep pressure losses as small as possible by using short, generaously sized runners and use full-round sections where possible. Moulds are commonly heated by circulating hot oil and/or electrically powered band or cartridge heaters.

15. Flow Path : wall thickness ratio : Because these materials have high viscosities and high set-up temperatures the ratio of flow path:wall tickness is low, e.g. at 1mm wall thickness it may be of the order of 60:1.

16. Projected area : These materials can have a very high viscosity and a high set-up temperature. To get such grades to flow into relatively thin sections and/or to have a good surface finish, high pressures will be required, therefore high clamping pressures may be needed, e.g. up to 10 tsi (150 MN m-2). More easy flow grades can be handled using clamping pressures recommended for PC

17. Cylinder equipment : Because these materials have high melt viscosity it is necessary to use shut-off nozzles. An open nozzle has the added attraction that it is more streamlined and does not provide a site where stagnation can occur. Because of the high melt temperatures necessary it is very important to ensure that there are no stagnation points in the system.

18. .Screw cushion : A small screw cushion is normally employed, e.g. 4mm.

19. Shot capacity : Try to match the shot size to that of the injection unit. Do not run small shots on a barrel with a capacity as degradation may occur.

20. Melt temperature ¾ as measured in the nozzle or by an air-shot technique: The melt temperature required depends on the particular material and the mould employed. Set the machine controls so that a thermally homogenous melt is produced with a temperature of between 330- 400°C (626-752°F). Those materials that can withstand the highest end-use temperatures usually require the highest melt and mould temperature; e.g. 400°C may be required for Astrel on some jobs.

21. Barrel residence time : Because of the high temperatures employed, barrel residence times are comparatively short. If the material is left to cook in the barrel, degradation will occur; reduce the barrel temperature settings if a stoppage occurs. Periodic purging may be formed and this could give black streaks in the moulding. PC and PP have been used where extensive purging is required.

22. Temperature settings: Please note that it is melt temperature which is important, those in Table 16 are only suggested, initial settings. The temperature of the hydraulic oil and of the material in the hopper should not vary excessively.Accurate temperature control is essential when processing this material

23. Injection speed ¾ mould filling speed : As high as possible consistent with good quality mouldings ¾ high speeds may cause surface markings in the gate area and may even cause degradation (through shear).



TABLE 16
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone. Location Temperatures ° C Temperatures ° F
No From To From To
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 320 370 608 698
(near the hopper)
2 Barrel middle 330 390 626 734
3 Barrel middle 350 410 662 770
4 Barrel front 360 430 680 806
5 Nozzle 360 420 680 806
Mould 130 200 248 356
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾

24. Injection pressure : The machine should be capable of giving the following : First stage :2000 bars; 200 MN m-2 ;29 000 psi. High pressure may be required in order to overcome high viscosity. Special high pressure machines have been built to process these materials. Second stage (dwell or follow-up pressure): up to 1200 bars;120 MNm-2; 17 000 psi. Be careful not to overpack the mould.

25. Screw rotational Speed(rpm) : Use a relatively a low screw speed, e.g. less than 60 rpm, and use a screw motor that is capable of giving high torque. This material's high melt vicosity can result in high shear being developed ¾ as this will cause degradation keep the speed low.

26. Back Pressure : 400 bars;40 MNm-2; 5800 psi. Working against a small back pressure can to eliminate air, however, too high a back pressure may cause degradation. Adjust the back pressure so that when sprue break occurs the material will just weep from the nozzle ¾ then reduce slightly.

27. Shutting down : Turn off the heat and purge until clear; if restarting with PES maintain the temperatures at 200°C until start-up. If changing to another material., before turning down the temperatures, purge clear and then introduce a stiff-flow (low MFI) polyolefin. Purge this through and then drop the temperature settings gradually until (about) 260°C is reached. Purge out the polyolefin and shut down.

28. Reprocessing : Up to 30% regrind may be blended in with virgin material. As moisture does not cause chemical degradation, scrap mouldings (made from wet material) may be ground, dried and used again. Because of the material's excellent thermal stability regrind can be used several times before the physical properties change.

29. Finishing : Finishing processes include machining, welding (thermal and ultrasonic), solvent welding (accomplished with methylene chloride or dimethylformamide), painting etching, metallising, etc. Thin films of metal may be deposited by vacuum metallising or electroplating. Metal adhesion is good and copper coated components are of interest in printed circuitry.
Because of the toughness of this material , gates are removed (by cropping) when the
components are still hot.

30. Other comments : An idea of the amount of frozen-in stress can be obtained by dipping mouldings in ethyl acetate; high levels of frozen-in strain will cause cracks to develop very quickly, e.g. 2s.

31. Typical Components : Becauses these materials are expensive they are not large tonnage materials. They are used when a thermoplastic is required which has good long-term thermal stability, dimensional stability, toughness and resistance to burning. These properties together with their good electrical insulation properties have promoted their use in the microwave cooking field, e.g. for grilles, dishes, etc. These materials have extended the range of use of thermoplastics as, for certain applications, they may replace thermosets and metals, e.g. in cameras.
PES is used in dishwasher and automotive components, the electrical and electronics industry (e.g. for printed circuit boards, television components, etc.); hair dryer parts, oven, projector and fan heater components are also moulded. That is, the material is used when other cheaper thermoplastics (such as PC) are not suitable.
Within this family of materials there is a significant difference in the maximum upper end-use temperature of approximately 100°C.(the highest upper end-use temperature is approximately 250°C). Some electrical components are required to perform satisfactory after extensive oven ageing tests, e.g. 10 000h at 200°C.


21. POLYVINYLIDENE FLUORIDE (PVDF)

1. Common name : Polyvinylidene fluoride.

2. Abbreviation : PVDF or PVF2.

3. Systematic chemical name : Not known but poly(1,1-difluoroethylene) has been suggested.

4. Some suppliers : 5. Trade names or trade marks :
Dynamit Nobel Dyflor
Kay-Fries Dyflor
Kureha KF
Laporte Solef
Penwalt Kynar
Solvay Solef
Ugine Kuhlmann Foraflon

6. Material properties : There are several fluoropolymers that can be processed by injection
moulding and the group includes fluorinated ethylene-propylene(FEP), ethylene tetrafluoroethylene (ETFE), ethylene-chlorotrifluoroethylene (ECTFE), chlorotrifluoroethylene (CTFE), perfluoroalkoxy tetrafluoroethylene (PFA) and polyvinylidene fluoride (PVDF). In general these materials have excellent chemical and heat resisitance, they are good electrical insulators and have high impact strengths. PVDF has the highest dielectric constant heat deflection temperature, flexural strength and modulus; its yield strength and creep resistance is high for a fluroinated thermoplastic. The abrasion resistance of this material is similar to that of polyamides. As the thermal and light stability is very good no heat stabilisers or UV stabilisers are necessary and this means that the material can be nontoxic.
By keeping the molecular weight relatively low a PVDF material can be produced which is highly crystalline ¾ this material is used for moulding as products are stiff and strong. PVDF is relatively easy to
process for a fluoroplastic.

7. Ease of flow : Behaves similarly to PP; the melt is more viscous than a PA melt. Flows easily at 225-245°C..
The MFI depends on the grade but at 230°C with a 5kf load it could be between 1 and 18.

8. Shrinkage : As this is a highly crystalline material the shrinkage is high, e.g. approximately 2-3%. Shrinkage increases slightly as mould temperature increases. Shrinkage may be reduced to 1.5% by using reinforced grades, i.e. from 0.030 in/in to 0.020 in/in.

9. Resistant to the following : PVDF has excellent resisitance to a wide range of chemicals , e.g. halogens, salt solutions, inorganic acids and bases aliphatic and aromatic hydrocarbons, carboxylic acids and acid chlorides, mercaptanes and chlorinaated hydrocarbons. Resists degradation by ultra-violet light, a and b radiation.

10. Not resistant to : Hot concentrataed sulphuric acid, strongly basic amines, hot concentrated bases and alkali metals. Swollen by polar solvents such as acetone and ethylacetate. Dissolves with difficulty in solvents such as dimethylformamide, dimethylsulphoxide and tetramethylurea. Some stress-cracking in strong alkalis.

11. Material detection or identification : The natural colour of this material is a translucent milky white and as it is a crystalline material it is not possible to produce transparent mouldings. It has a high density (1.78gcm-3) and will therefore sink readily in water. The Vicat softening point is approximately 130°C and the crystalline melting point (as measured by differential thermal analysis (DTA)) is approximately 170°C.Relatively easy to cut and/or machine. The material has good fire resistance and will not propagate a flame. Pungent fumes of hydrofluoric acid are liberated at (about) 320°C.

12. Colouring : Some pigments (e.g. titanium dioxide) and some other inorganic materials (boron oxide) may cause rapid decomposition of PVDF. Before adding any compounding ingredients check with the supplier. Do not add glass fillers, e.g. glass fibre, as such fillers may cause rapid decomposition . Before processing PVDF in ready compounded form or only use masterbatches or dry colours specified by the supplier.

13. Material and component handling : Due to the low moisture absorbency of this material (e.g. 0.05%) predrying is not normally necessary. If the material becomes contaminated with water, e.g. due to condensation, dry at 80°C for 2-4h. Internal strains may be removed from moulded components by annealing, e.g. 2h at 160°C in an oil bath.

14. Mould and gate considerations : The use of light-alloy moulds is not normally recommended as the PVDF sticks to this type of metal. Moulds designed for PP have been successfully used with this material. As shrinkage is very high, moulding around inserts can create high stress levels¾it may be best to use a filled grade in such applications. Ensure that the mould is well vented as otherwise the air trapped may cause the PVDF to decompose in some cases. Hot-runner moulding is difficult.

15. Flow Path : wall thickness ratio : Depends on grade and type ¾can range from 25:1 up to 300:1.

16. Projected area : 2 tsi (30 MN m-2) is usually sufficient as PVDF is commonly moulded into thick sectioned components.

17. Cylinder equipment : Ensure that the screw and barrel assembly is scrupulously clean before introducing PVDF into the system¾ some contaminants can reduce the thermal stability dramatically and as hydrofluoric acid may be emitted there is an obvious hazard. Do not clean equipment by heating in an oven or a flame unless there is very good ventilation. Strip and clean the machine before moulding. Highly crystalline material can be used without a shut-off nozzle.

18. Screw cushion : As small as possible.

19. Shot capacity : Under-rate the machine, e.g. by 25%. The barrel size should be such that at most two to three times the shot weight is contained in the heating cylinder.

20. Melt temperature ¾ as measured in the nozzle or by an air-shot technique: 200 - 260°C. Higher temperatures than this, e.g. 300°C (572°F) may lead to discoloration. Low crystalline material may require 380°C and the barrel must be constructed of acid-resistant material as decomposition can occur more quickly at this temperature.

21. Barrel residence time : Temporary stoppages (e.g. up to 30 min) can be tolerated without reducing barrel temperatures. If a longer stoppage is envisaged reduce the temperatures (e.g. to 175°C ) until processing can recommence.

22. Temperature settings: See Table 18.

23. Injection speed ¾ mould filling speed : PVDF components may be of heavy cross-section and in such cases slow filling speeds should be employed. High speeds can cause a local temperature rise which may result in discoloration.

24. Injection pressure : The machine should be capable of giving the following : First stage :1500 bars; 150 MN m-2 ;21 400 psi. Second stage (dwell or follow-up pressure): 750 bars;75 MNm-2; 10 700 psi.

TABLE 18
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone Location Temperatures ° C Temperatures ° F
No. From To From To
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 160 210 320 420
(near the hopper)
2 Barrel middle 170 220 338 428
3 Barrel middle 180 240 356 464
4 Barrel front 190 260 374 500
5 Nozzle 180 250 356 482
Mould 30 120 86 248
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾

25. Screw rotational Speed(rpm) : If the screw is a tight fit in the barrel then at high speeds discoloration or even degradation , may occur. Keep the speed as low as possible, e.g. below a peripheral speed of 7.5 m min-1 (for a 30 mm screw this would be below 80 rpm).



26. Back Pressure : 300 bars;30 MNm-2; 4500 psi.

27. Shutting down : If PVDF is to be re-run then purge the barrel before switching off. If another material is to be run purge clean and then use high viscosity PE or PMMA. This material is only miscible with polyamides and acrylics. Purge with virgin polymer only; do not use purging compounds.

28. Reprocessing : Material can be reprocessed in many cases without obvious impairment of properties. If dust is generated during the regrinding operation then smoking should not be permitted as the PVDF dust falling on to the cigarette end can generate ffluorinated compounds. Keep grinders, etc. scrupulously clean.

29. Finishing : PVDF components can be welded together, e.g. by hot plate (250°C ), ultrasonic and friction welding. If components require machining then use the techniques and equipment recommended for PA.

30. Other comments : Grades with improved mechanical properties (e.g. stiffness and wear resistance) can be obtained if the material is compounded with fibrous fillers. Asbestos, and carbon fibre have been used. Glass-filled compounds have been known to cause degradation of PVDF at processing temperatures. Antistatic grades are also available.

31. Typical Components : The chemical process industry uses this material for valves, pumps, bearings, etc.; this is because of its outstanding chemical resistance, relative ease of processing, high strength, rigidity and abrasion resistance, relative ease of processing, high strength, rigidity and abrasion resistance. Because it has a high, sharp, crystalline melting point this material maintains a great deal of its chemical resistance even at a high continuous-use temperature of 130°C . It can be made to have exceptional piezo-electric properties ¾ this explains current interest in uses such as transfucers, microphones, loud-speakers, etc. Price is similar to that of PTFE but because processing is easier,finished articles tend to be cheaper.




22. POLYPHENYLENE SULPHIDE (PPS)

1. Common name : Polyphenylene Sulphide.

2. Abbreviation : PPS.

3. Systematic chemical name : Poly(thio-1,4-phenylene).

4. Some suppliers : 5. Trade names or trade marks :
Philips Ryton

6. Material properties : Crystalline, thermoplastics material based on benzene groups linked with sulphur atoms. Usually sold already compounded with glass fibres (e.g. 40%) or with mixtures of glass fibres and mineral fillers. As a result of these additions and because of its chemical structure, it is classed as an engineering polymer. It retains its strength at high temperatures and/or in aggressive environments. Some grades can have high impact strengths but most have low elongations at break. The materials are excellent electrical insulators which are unaffected by humid environments.

7. Ease of flow : Materials are rated as easy flow. At processing temperatures it flows better than PC and PPO.

8. Shrinkage : Low mould shrinkages are possible, e.g. 0.2%. Shrinkage may not be uniform as PPS is commonly filled with a fibrous filler (i.e. glass)¾ thicker sections give higher shrinkages than this sections. Exposure to temperatures above the glass transition temperature (e.g. 90°C) will cause additional shrinkage, particularly if low mould temperatures (e.g. below 130°C) were used during production.

9. Resistant to the following : Can withstand high temperatures without deformation. Combines chemical resistance with hydrolytic and dimensional stability at high temperatures. Absorbs only small amounts of water, e.g. 0.05%.

10. Not resistant to : Some chemicals at high temperatures (e.g. 90°C) have some slight affect, e.g. aromatic and chlorinated solvents. Strong oxidising acids, e.g. concentrated sulphuric acid, will attack the material.

11. Material detection or identification : With a density greater than 1 gcm-3 (e.g. 1.5 - 2.1 gcm-3 ) mouldings sink quickly in water. High density materials contain mineral fillers in addition to glass fibres. Mouldings are stiff and hard and do not cut easily with a knife. They have outstanding inherent nonflammability and can therefore only be burnt with difficulty. To set fire to a moulding a temperature of 540°C must be reached ¾ it does not drip even at this temperature. PPS chars and blisters with a smell of sulphur when heated in a strong flame.

12. Colouring : The base colour is dark brown and therefore the colour range is comparatively limited. Mouldings made from this material must be capable of performing under arduous conditions (e.g. high temperatures) and these conditions of service also restrict the colours available . Sold already compounded in pellet form.

13. Material and component handling : Grades which contain mineral fillers will require drying before moulding. This is best done in a dehumidifying drier although convection oven heating is used; drying temperatures may reach 175 °C . Try 150°C for 6h initially. Components which are required to have the best moulded in a hot mould (e.g. 140°C). Components moulded at low mould temperatures may only have heat distortion temperatures of 150°C(before annealing).

14. Mould and gate considerations : Because of the wear problems associated with heavily filled polymers the tool steel should be selected carefully; carbon steels which contain high chromium levels (also molybdenum and vanadium) should be used. Mould surfaces should be highly polished and hardened ¾ this will give a good finish to the moulding and longer life to the mould. Protective plating (e.g. hard chrome) is also useful for reducing erosion. Moulds may be heated electrically (e.g. to 140°C) and in such cases cartridge heaters are commonly employed ¾ allow 0.5 kW kg-1 of mould weight. The feed system should be designed to eliminate welds and to minimise warpage caused through fibre orientation. Diaphragm and flash gates are thus useful gate types. Because PPS does not suffer from overpacking problems to the same extent as other thermoplastics., it is not essential to use balanced runner systems; hot-runner systems have been successfully used. Because of the low mould shrinkages the sprue, cavities and cores should be adequately tapered.

15. Flow Path : wall thickness ratio : Markedly dependent on graade, melt and mould temperatures, the flow path : wall thickness ratio can be of the order of 150:1 (at a wall thickness of 1 mm). If a mould can be filled with another material then PPS can be made to fill that cavity despite the high filler loadings employed.

16. Projected area : If insufficient clamping pressure is available flashing will occur and/or the surface finish of the moulding will be poor. For thick sectioned components (e.g. greater than 3 mm) allow 2 tsi (30 MN m-2); for thinner sectional mouldings 3 tsi (45 MN m-2) may be required.

17. Cylinder equipment : Nozzles should be equipped with shut-off valves so as to prevent material drooling. Open nozzles (with accurate temperature control) have, however, been used. Screws should be fitted with back-flow valves and adequate allowance should be made for the maintenance of cylinder equipment (due to the abrasive nature of the melt).

18. Screw cushion : 4-6mm.

19. Shot capacity : Use from 50 to 85% of the machines rated shot capacity.

20. Melt temperature ¾ as measured in the nozzle or by an air-shot technique: 300 - 360°C. For a particular grade increasing the melt temperature will dramatically alter (i.e.reduce) the melt viscosity. The affect of alterations in melt temperature is mainly on viscosity as such changes do not dramatically alter strength properties. Do not exceed 370 °C as irritating gases may be produced.

21. Barrel residence time : At the high processing temperatures employed it is necessary to keep the material moving through the barrel. However, at an average moulding temperature of 310°C short stoppages can be tolerated.

22. Temperature settings: : Please note that it is melt temperature which is important, those in Table 19 are only suggested, initial settings. The temperature of the hydraulic oil and of the material in the hopper should not vary excessively. The mould temperature range may be 30 - 90°C but not 90 - 120°C. Over this second temperature range minor differences in cooling can cause major differences in crystallinity (and therefore properties).

23. Injection speed ¾ mould filling speed : Very high injection speeds can cause burning of the moulding ¾if burning occurs improve venting and/or reduce speed.

24. Injection pressure : The machine should be capable of giving the following : First stage :1500 bars; 150 MN m-2 ;21 400 psi. Second stage (dwell or follow-up pressure): 1000 bars;100 MNm-2; 14 500 psi.

TABLE 19

¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
Zone No. Location Temperatures ° C Temperatures ° F
From To From To
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾
1 Barrel rear 290 300 554 572
(near the hopper)
2 Barrel middle 300 310 572 590
3 Barrel middle 310 320 590 608
4 Barrel front 310 360 590 680
5 Nozzle 305 320 599 608
Mould 130 160 266 320
¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾¾

25. Screw rotational Speed(rpm) : As low as possible (e.g. below 100 rpm) so as to minimise fibre degradation.

26. Back Pressure : Up to 50 bars;5 MNm-2; 700 psi. Little or no back pressure is needed. Back pressure should be as low as possible consistent with good shot weight reproducibility.

27. Shutting down : For overnight stops empty the barrel, leave the screw forward and turn off the heaters. To clean out PPS use an extrusion grade of HDPE.

28. Reprocessing : Keep material that is to be reground in sealed containers as this will minimise moisture pick-up and contamination. Assume that regrind must be dried and keep level as low as possible, e.g. <25%. Many polymers will degrade at the melt temperatures required for PPS; purge before and after a PPS moulding run with a stiff flow (low MFI) HDPE.

29. Finishing : Gates readily removed, especially if a deliberate notch is designed into the gate area¾ this causes the feed to break off easily. Can be hot stamped provided that the blocking machine can reach and hold the required high temperatures.

30. Other comments : Where necessary mouldings with improved dimensional stability at elevated, temperatures (e.g. 135 - 200°C ) are obtained by using high mould temperatures (e.g. 135°C) and postmould annealing.

31. Typical Comments : The ability of this mateiral to withstand high temperatures without deformation has allowed this material to be used in connectors, terminal blocks, sockets, coil formers, bobbins and relay components. Arc resistant grades are available which are used to mould relay bases, motor housings, lamp holders and switch components. Used as a metal replacement in the automotive industry, e.g. carburettor parts, ignition plates, lamp sockets and flow control valves (for heating systems). Because of the increasing use of quartz-halogen lamps, components may reach temperatures above 200°C ¾ PPS replaces ceramics in such applications.



MANUFACTURING PROCESS OF PLASTIC

Blow molding dates back to at least 1890, when it was used to produce celluloid baby rattles. From that time forward, many companies have tried numerous means to produce blow molded parts in a variety of materials. The first polyethylene bottle was blown in December of 1942. The rest is history: the U.S. currently produces 30 to 40 billion plastic bottles per year, with the number constantly growing. For an excellent history of the plastics industry through 1972, those reading this article may wish to consult Plastics History U.S.A. by Harry Dubois, published by Cahners Books, Boston, Mass., ISBN 0-8436-1203-7.
There are basically four types of blow molding used in the production of plastic bottles, jugs and jars. These four types are: extrusion blow molding, injection blow molding, stretch blow molding and reheat and blow molding. Extrusion blow molding is perhaps the simplest type of blow molding, whereby a hot tube of plastic material is dropped from an extruder and captured in a water cooled mold. Once the molds are closed, air is injected through the top or the neck of the container; just as if one were blowing up a balloon. When the hot plastic material is blown up and touches the walls of the mold the material "freezes" and the container now maintains its rigid shape. There are various types of shuttle, reciprocating and wheel style machines for the production of extrusion blown bottles. Shuttle or reciprocating type machines can be used for small, medium and high volume production with wheel machines being the most efficient for huge volume production of certain resins.
Injection blow molding is part injection molding and part blow molding. With injection blow molding, the hot plastic material is first injected into a cavity where it encircles the blow stem, which is used to create the neck and establish the gram weight. The injected material is then carried to the next station on the machine, where it is blown up into the finished container as in the extrusion blow molding process above.
Injection blow molding is generally suitable for smaller containers and absolutely no handleware. Extrusion blow molding allows for a wide variety of container shapes, sizes and neck openings, as well as the production of handleware. Extrusion blown containers can also have their gram weights adjusted through an extremely wide range, whereas injection blown containers usually have a set gram weight which cannot be changed unless a whole new set of blow stems are built. Extrusion blow molds are generally much less expensive than injection blow molds and can be produced in a much shorter period of time.
Many people have heard about stretch blow molding in conjunction with P.E.T. bottles commonly used for water, juice and a variety of other products. There are two processes for stretch blow molded P.E.T. containers. In one process, the machinery involved injection molds a preform, which is then transferred within the machine to another station where it is blown and then ejected from the machine. This type of machinery is generally called injection stretch blow molding (ISBM) and usually requires large runs to justify the very large expense for the injection molds to create the preform and then the blow molds to finish the blowing of the container. This process is used for extremely high volume (multi-million) runs of items such as wide mouth peanut butter jars, narrow mouth water bottles, liquor bottles etc.
Another stretch blow process is commonly called reheat and blow (RHB). In this process, a preform is injection molded by an outside vendor. There are a number of companies who produce these "stock" preforms on a commercial basis. Factories buy the preforms and put them into a relatively simple machine which reheats it so that it can be blown. The value of this process is primarily that the blowing company does not have to purchase the injection molding equipment to blow a particular container, so long as a preform is available from a stock preform manufacturer. This process also allows access to a large catalog of existing preforms. Therefore, the major expense is now for the blow molds, which are much less expensive than the injection molds required for preforms.
There are, however, some drawbacks to this process. If you are unable to find a stock preform which will blow the container you want, you must either purchase injection molds and have your own private mold preforms injection molded, or you will have to forego this process. For either type of stretch blow molding, handleware is not a possibility at this stage of development. The stretch blow molding process does offer the ability to produce fairly lightweight containers with very high impact resistance and, in some cases, superior chemical resistance.
Whether using the injection stretch blow molding process or the reheat and blow process, an important part of the process is the mechanical stretching of the preform during the molding process. The preform is stretched with a "stretch rod." This stretching helps to increase the impact resistance of the container and also helps to produce a very thin walled container.
The extrusion blow molding process allows for the production of bottles in a wide variety of materials, including but not limited to: HDPE, LDPE, PP, PVC, BAREX®, P.E.T., K Resin, P.E.T.G., and Polycarbonate. As noted above, a wide variety of shapes (including handleware), sizes and necks are available. Injection blow molding allows for the production of bottles in a variety of materials, including but not limited to: HDPE, LDPE, PP, PVC, BAREX®, P.E.T., and Polycarbonate.
Besides the P.E.T. noted above for stretch blow molding, a number of other materials have been stretch blown, including polypropylene. As time goes on and technology moves forward, more materials will lend themselves to stretch blow molding as their molecular structures are altered to suit this process.
The decision as to which process will be used is based upon the desired appearance (clear or not), whether chemical or impact resistant is desired, and the desired cost/benefit relationship. The ultimate choice of materials and processes is also based upon the cost of the tooling involved and the sizes of the production runs. Some materials lend themselves to certain types of decorating better than others and some to certain types of decorating to the exclusion of others.
Listed below are representative brands of some types of the machinery we have discussed above. This list is not all-inclusive and you will find additional brands by looking through this and other packaging industry journals and magazines.
For shuttle extrusion type machines Bekum, Battenfeld/Fischer, and Hayssen are probably the best known in the United States. For injection blow molding machines JOMAR is a well known brand. For stretch blow and reheat and blow type machines there are Sidel, Nissei and other machines produced by Johnson Controls and others. For wheel machines you might wish to contact Johnson Controls or Wilmington Machinery.