Page 303 - Plastics Engineering

286 Processing of Plastics

Fig. 4.35 Details of injection mould

Finishing and polishing the mould surfaces is also extremely important
because the melt will tend to reproduce every detail on the surface of the
mould. Finally the mould will have to be hardened to make it stand up to the
treatment it receives in service. As a result of all the time and effort which
goes into mould manufacture, it is sometimes found that a very complex mould
costs more than the moulding machine on which it is used. Several features of
the mould are worthy of special mention.

(a) Gates: As mentioned earlier the gate is the small orifice which connects
the runner to the cavity. It has a number of functions. Firstly, it provides a
convenient weak link by which the moulding can be broken off from the runner
system. In some moulds the degating may be automatic when the mould opens.
The gate also acts like a valve in that it allows molten plastic to fill the mould
but being small it usually freezes off first. The cavity is thus sealed off from the
runner system which prevents material being sucked out of the cavity during
screw-back. As a general rule, small gates are preferable because no finishing