Injection moulding polymers used

Many articles, bottles and containers in particular, are made by blow moulding techniques of which there are many variations. In one typical process a hollow tube is extruded vertically downwards on to a spigot. Two mould halves close on to the extrudate (known in this context as the parison ) and air is blown through the spigot to inflate the parison so that it takes up the shape of the mould. As in injection moulding, polymers of low, intermediate and high density each find use according to the flexibility required of the finished product. 

These include cold drawn, high pressure oriented chain-extended, solid slate extruded, die-drawn, and injection moulded polymers. Correlation of hardness to macroscopic properties is also examined. In summary, microhardness is shown to be a useful complementary technique of polymer characterization providing information on microscopic mechanical properties. 

Plasticisers are compatible and miscible with their host polymer causing them to swell and allowing them to be used for coating, moulding, spraying (when used as a suspension in a liquid solvent) or calendering, extrusion and injection moulding (when used as a polymer melt). 

Note Reaction injection moulding commonly uses two-component precursors that produce polymer networks after mixing. 

For estimation of the processability of a moulding polymer melt, the pressure to fill a standard mould has to be measured. Furches and Kachin (1989) have compared the results of several rheological tests often used to evaluate injection moulding polymer melts. 

Starch is the most commonly used agricultural raw material. Starch is inexpensive, widely available and relatively easy to handle. All-starch bioplastics are made from thermoplastic starch and formed with standard techniques for synthetic polymer films such as extmsion or injection moulding. The use of... 

Polymers owe much of their attractiveness to their ease of processing. In many important teclmiques, such as injection moulding, fibre spinning and film fonnation, polymers are processed in the melt, so that their flow behaviour is of paramount importance. Because of the viscoelastic properties of polymers, their flow behaviour is much more complex than that of Newtonian liquids for which the viscosity is the only essential parameter. In polymer melts, the recoverable shear compliance, which relates to the elastic forces, is used in addition to the viscosity in the description of flow [48]. 

When you have to estimate how a change of temperature changes the viscosity of a polymer (in calculating forces for injection moulding, for instance), this is the equation to use. 

Low molecular weight, narrow MWD polymer—used widely for injection moulding and rotational moulding. 

One unfortunate characteristic property of polypropylene is the dominating transition point which occurs at about 0°C with the result that the polymer becomes brittle as this temperature is approached. Even at room temperature the impact strength of some grades leaves something to be desired. Products of improved strength and lower brittle points may be obtained by block copolymerisation of propylene with small amounts (4-15%) of ethylene. Such materials are widely used (known variously as polyallomers or just as propylene copolymers) and are often preferred to the homopolymer in injection moulding and bottle blowing applications. 

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. 

The main application of poly (vinyl formal) is as a wire enamel in conjunction with a phenolic resin. For this purpose, polymers with low hydroxyl (5-6%) and acetate (9.5-13%) content are used. Similar grades are used in structural adhesive (e.g. Redux) which are also used in conjunction with phenolic resin. Poly(vinyl formal) finds some use as a can coating and with wash primers. Injection mouldings have no commercial significance since they have no features justifying their use at current commercial prices. 

The methacrylic polymer remains a useful glazing material. In aircraft applications it is used extensively on aircraft which fly at speeds less than Mach 1.0. They form the familar bubble body of many helicopters. On land, acrylic sheet is useful for coach roof lights, motor cycle windscreens and in do-it yourself cabins for tractors and earth-moving equipment. Injection mouldings are frequently used for plaques on the centre of steering wheels and on some fascia panelling. 

Tbe term structural foam was originally coined by Union Carbide to describe an injection moulded thermoplastic cellular material with a core of relatively low density and a high-density skin. The term has also been used to describe rigid foams that are load bearing. Today it is commonly taken to imply both of the above requirements, i.e. it should be load bearing and with a core of lower density than the skin. In this section the broader load-bearing definition will be used. Whilst structural foams are frequently made from polymers other than polystyrene, this polymer is strongly associated with such products and it is convenient to deal with the topic here. 

The polymer is not easy to process and in injection moulding melt temperatures of 300°C are employed. In order to prevent excess embrittlement by shock cooling of the melt, mould temperatures as high as 150°C may be used. The polymer may also be compression moulded at temperatures of 250-260°C. 

These polymers may be extruded and injection moulded on standard equipment used for thermoplastics. Typical melt temperatures range from about 230°C for the harder grades down to about 200°C for the softer polymers. Mould temperatures are about 25-30°C. 

Unlike other water-soluble resins the poly(ethylene oxide)s may be injection moulded, extruded and calendered without difficulty. The viscosity is highly dependent on shear rate and to a lesser extent on temperature. Processing temperatures in the range 90-130°C may be used for polymers with an intrinsic viscosity of about 2.5. (The intrinsic viscosity is used as a measure of molecular weight.)... 

By 1998, however, the Western European market had grown to over 90 000 t.p.a., that for the United States to about 140 000 t.p.a. and that for Japan to just over 60 000 t.p.a. There are also about a dozen USA and Western European manufacturers. Statistics on capacity are somewhat meaningless, as the polymer can be made using the same plant as employed for the manufacture of the much larger tonnage material PET. It is, however, quite clear that the market for injection moulded PBT is very much greater than that for injection moulded PET. 

Injection moulding The injection moulder is a machine which first melts a thermoplastic and then injects that molten polymer into a mould. Such items as baskets, bowls, bins, telephones and electronic housings are produced by this technique. It can be used for lining valves. In this case the valve would be used as part of the mould. Very thick coatings are produced which give chemical resistance to the valve. At the same time, the metal valve housing will protect the valve from mechanical damage. 

---