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This process can be used to process thermosets and thermoplastics,
although its use with the latter material is not all that common.
This is because with thermoplastics the mould would need to be alternatively
heated, to shape the materials, and cooled to permit ejection from
the mould cross linking materials, on the other hand, may be ejected
at the high mould tempeature. The best examples of the use of thermoplastics
in compression moulding is the production of gramophone records
from a PVC co-polymer and the moulding of contact lenses from acrylic.
However, the major advantages of compression moulding is that it
is a simple process with little waste and therefore is particularly
suitable for materials that cross-link during processing. By the
nature of process there are no parts where prematurely cured material
can get trapped and cause prolonged down-time.
Basically a compression moulding system consists of matched male
and female dies which are heated to temperatures between 125°C
-200°C. A pre-weighed charge of the material to be moulded is
placed between the two mould halves and these are then closed. Under
the heat and pressure, the polymeric material plasticizes flows
to the shape of the mould and becomes cured. The mould split lines
is usually horizontal and the mould is closed by means of a high
pressures using hydraulic press. Due to the high pressures involved
in this process, the mould must be made fully hardened tool steel.
Also, as cycle times are slow (30-60 seconds) in comparison with
injection moulding, it is a normal practice to use multicavity moulds
in order to increase production rates. This additional complexity
plus the highly polished, high precision nature of the moulds mean
that mould costs are high. The limitation of the process that insert
or complex shapes cannot be produced. The most popular application
is melamine table ware and gift articles from thermosetting resin
thrown this process.
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