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This processing method differs from most of the
others used for the plastics. This process does not involve pressure
and the initial feedstock is liquid rather than granules or powder.
In casting, a liquid syrup, which is a monomer or modified monomer
of the plastic, is poured into a heated mould. In most cases sheets,
rods, tubes, and so on are cast for further fabrication into parts.
However, finished articles (such as fuel tanks) may be rotationally
cast in a process similar to rotational moulding of powders. The optical
properties of transparent plastics are much better if they are cast,
and there is also a general improvement in manufacturing properties.
Unfortunately not all plastics are suitable for casting. Materials
such as acetal, polycarbonate, and polypropylene cannot be made with
a sufficiently low viscosity to be poured into a mould. The most common
plastics which can be cast are acrylic, nylon, polysters, epoxies,
silicones, and urethenes. Apart from moulding useful products, casting
is also widely used for potting and encapsulation, particularly in
the electrical industry.
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