Page 343 - Plastics Engineering

326 Processing of Plastics
- Jwdt= Jq
t
H
2nF
so
H
0 Ho

Since Ho >> H then (l/H;) may be neglected. As a result the compaction
force F, is given by

(4.45)

where H is the platen separation at time, t.
Example 4.10 A circular plate with a diameter of 0.3 m is to be compres-
sion moulded from phenol formaldehyde. If the preform is cylindrical with a
diameter of 50 mm and a depth of 36 mm estimate the platen force needed to
produce the plate in 10 seconds. The viscosity of the phenol may be taken as
lo3 Ns/m2.
Solution
x 36=n (7)’H
Volume, V=n (;)2

so H=lmm

3qV2 3 x lo3 x (n x 625 x 36)’
From (4.45) F = - = 59.6 kN
-
8ntH4 - 106 x 8n x io x (1)4
4.8 Transfer Moulding
Transfer moulding is similar to compression moulding except that instead of
the moulding material being pressurized in the cavity, it is pressurized in a
separate chamber and then forced through an opening and into a closed mould.
Transfer moulds usually have multi-cavities as shown in Fig. 4.64. The advan-
tages of transfer moulding are that the preheating of the material and injection
through a narrow orifice improves the temperature distribution in the mate-
rial and accelerates the crosslinking reaction. As a result the cycle times are
reduced and there is less distortion of the mouldings. The improved flow of
the material also means that more intricate shapes can be produced.
The success of transfer moulding prompted further developments in this area
and clearly it was only a relatively small step to an injection moulding process
for thermosets as described in Section 4.3.10.