Moulding materials

A. Whelan, Injection Moulding Materials, Applied Science Publishers, 1982. 

New materials also emerged. Nylon, developed brilliantly by W. H. Carothers and his team of research workers for Du Pont as a fibre in the mid-1930s, was first used as a moulding material in 1941. Also in 1941 a patent taken out by Kinetic Chemical Inc. described how R. J. Plunkett had first discovered polytetrafluoroethylene. This happened when, on one occasion, it was found that on opening the valve of a supposedly full cylinder of the gas tetrafluoroethylene no gas issued out. On subsequently cutting up the cylinder it was found that a white solid, polytetrafluoroethylene (PTFE), had been deposited on the inner walls of the cylinder. The process was developed by Du Pont and, in 1943, a pilot plant to produce their product Teflon came on stream. 

Figure 8.8. Mouldability index of some common moulding materials (after Barrie)...

Table 8.2 Some collected values for the flow path ratio of injection moulding materials...

Whilst it is inevitable that polypropylene will be compared more frequently with polyethylene than with any other polymer its use as an injection moulding material also necessitates comparison with polystyrene and related products, cellulose acetate and cellulose acetate-butyrate, each of which has a similar rigidity. When comparisons are made it is also necessary to distinguish between conventional homopolymers and the block copolymers. A somewhat crude comparison between these different polymers is attempted in Table 11.7 but further details should be sought out from the appropriate chapters dealing with the other materials. 

As a blow moulding material polypropylene has never enjoyed the success of HDPE. This is in large measure because moulding of the former requires more attention to equipment design and operating conditions. Many successful mouldings have nevertheless been made commercially including chairs, horticultural sprayers and motor car parts. 

The dried beads may be supplied as injection moulding material without further treatment or they may be compounded with additives and granulated. 

The properties of three types of poly(methyl methacrylate) (sheet based on high molecular weight polymer, lower molecular weight injection moulding material and a one-time commercial copolymer) are given in Table 15.1. 

The ease with which acrylic monomers may polymerise with each other and with other monomers has led to a host of compositions, frequently of undisclosed nature, being offered for use as moulding materials, casting resins, coating resins, finishing agents and in other applications. 

The major nylon moulding materials are each available in a number of grades. These may differ in molecular weight but they may also differ in the nature of additives which may be present. 

Table 18.15 Typical properties of polyetherimide moulding materials (Assessed by use of ASTM test methods)...

By use of a modification of the well-known Williamson synthesis it is possible to prepare a number of cellulose ethers. Of these materials ethyl cellulose has found a small limited applieation as a moulding material and somewhat greater use for surfaee eoatings. The now obsolete benzyl cellulose was used prior to World War II as a moulding material whilst methyl eellulose, hyroxyethyl eellulose and sodium earboxymethyl eellulose are useful water-soluble polymers. 

Phenolic moulding powders, which before World War II dominated the plastics moulding materials market, only consumed about 10% of the total phenolic resin production by the early 1990s. 

Recent estimates suggest that in the early 1990s the percentage breakdown of consumption of phenolic moulding materials in Western Europe was approximately ... 

Conventional U-F moulding powders originally formulated for compression moulding are often satisfactory but special injection grades have been developed. Ideally the moulding material should have good flow characteristics between 70... 

U-F moulding materials are relatively much less important than they were 30 years ago. Western European production in 1991 was approximately 70000 tonnes, slightly greater than P-F moulding material production. U-F resin used for this application is, however, probably less than 5% of total U-F resin production. 

Although occasionally in demand because of their good electrical insulation properties, aniline-formaldehyde resins are today only rarely encountered. They may be employed in two ways, either as an unfilled moulding material or in the manufacture of laminates. 

Table 25.7 Some properties of poly(ethylene terephthalate) moulding material ( Amite ) (ASTM Test Methods unless otherwise stated.)...

For an aliphatic polyester, poly(pivalolactone) has a rather high of 245°C and for such a an unexpectedly low of -10°C. It is also claimed to have good hydrolysis resistance for a polyester and this appears to be one of the reasons for its manufacture on an experimental scale by Shell with a view for use as both a fibre and as a thermoplastics moulding material. 

The composition of an epoxide moulding material will greatly depend on the specific application, and this has been discussed at length. ". The resin may be of the epoxide novolac type and there will also be present hardeners, fillers (such as silica), a silane coupling agent, pigment, flame retardant and a wax release agent. 

Allyl resins Polyester Moulding Compositions Fibre-forming and Film-forming Polyesters Polyfethylene terephthalate) Moulding Materials... 

Example 2.12 An acrylic moulding material is to have its creep behaviour simulated by a four element model of the type shown in Fig. 2.38. If the creep curve for the acrylic at 14 MN/m is as shown in Fig. 2.40, determine the values of the four constants in the model. 

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