Page 341 - Plastics Engineering

324 Processing of Plastics

Pressure

Houldikg

Fig. 4.62 Principle of compression moulding
thermoset and once the crosslinking (‘curing’) is completed the article is solid
and may be ejected while still very hot. Mould temperatures are usually in the
range of 130-200°C. Cycle times may be long (possibly several minutes) so
it is desirable to have multi-cavity moulds to increase production rates. As a
result, moulds usually have a large projected area so the closing force needed
could be in the region of 100-500 tonnes to give the 7-25 MN/m2 cavity
pressure needed. It should also k noted that compression moulding is also
used for Dough Moulding Compounds (DMC) - these will be considered in
Section 4.10.2
During compression moulding, the charge of material may be put into the
mould either as a powder or a preformed ‘cake’. In both cases the material is
preheated to reduce the temperature difference between it and the mould. If
the material is at a uniform temperature in the mould then the process may be
analysed as follows.
Consider a ‘cake’ of moulding resin between the compression platens as
shown in Fig. 4.63. When a constant force, F, is applied to the upper platen the
resin flows as a result of a pressure gradient. If the flow is assumed Newtonian
then the pressure flow equation derived in Section 4.2.3 may be used
1
flow rate, Qp = - (E) TH3 (4.6)
1217 dz
For the annular element of radius, r, in Fig. 4.63 it is more convenient to
use cylindrical co-ordinates so this equation may be rewritten as

1 dP
eP = - (-) . (2nr)~3
1217 dr
Now if the top platen moves down by a distance, dH, the volume displaced
is (nr2dH) and the volume flow rate is nr2(dH/dt).

Therefore nr2 (f) 1 (E) (2nr)~3
=
1217 dr