Materials for engineering, 3rd Edition - (Malestrom)

Organic polymeric materials 161
Polybutadiene. Styrene–Butadiene copolymer (SBR) is synthetic rubber
and again is not oil, ozone or weather resisistant. Its uses include car
tyres, gaskets and shoe heels and soles.
Polychloroprene, or neoprene (CR) is an oil-resistant rubber, which also
resists degradation from sun, ozone and weather. It has many uses, including
seals and gaskets, as well as belts and hoses, and for the lining of tanks
containing certain chemicals.
Polysiloxane, or silicone (SIL) is of low strength, but exhibits excellent
resistance to high and low temperatures coupled with good electrical
insulating properties. It is used for seals and diaphragms, tubing for food
and medical uses, as well as electrical insulation over a wide range of
temperatures.
5.2 Forming processes for polymers
The overriding consideration in the selection of a given polymer is whether
or not the material can be processed into the required article easily and
economically.
Thermosets are heated, formed and cured simultaneously, usually by
compression moulding, Fig. 5.1. The product can be removed from the mould
while still hot, so the cycle time can be relatively short.
Thermoplastics soften when heated and may be pumped, mixed and then
shaped by one of several possible methods.
5.2.1 Extruder-based processes
The simplest and most common way of achieving extrusion is to use a single
Archimedian screw in a heated barrel. Granules of polymer are fed into the
Upper mould
Moulding
Guide
pins
Lower mould
5.1 Compression moulding.