Page 313 - Plastics Engineering

296 Processing of Plastics

The thickness of the moulding is 1 mm, hence the flow ratio = 150/1 = 150.
From Fig. 4.42 at this thickness and flow ratio, the mean effective pressure is
75 MN/m2.
Allowing an extra 15% for uncertainties and applying the viscosity factor of
1.4 for ABS, then the appropriate mean effective pressure is 75 x 1.15 x 1.4 =
120 MN/m2. For each cavity, the projected area is (~/4)(90)~ = 6360 mm2 =
6.36 x m2.
Hence, clamp force per cavity = 120 x 6.36 x = 763 kN.
The projected area of the runners is 4 x 40 x 6 = 960 mm2.
Assuming that the mean effective pressure also applies to the runner
system, then

clamp force for runners = 120 x 0.96 x = 115 kN

Hence total clamp force for 4 cavities and 1 runner system is given by

Total clamp force = (4 x 763) + 115 = 3167 kN
The required clamp force is therefore 317 tonnes.
(b) If a pin gate in the middle of the base is used instead of an edge gate,
then the flow ratio will be different. In this case

flow length = i(90) + 60 = 105

This is also the flow ratio, so from Fig. 4.42 the mean effective pressure is
50 MN/m2. Applying the viscosity factor, etc as above, then
Clamp force per cavity = 50 x 1.15 x 1.4 x (:) (90)2 x = 512 kN

In this case the runner system will be almost totally in the ‘shadow’ of the
projected area of the cavities and so they can be ignored.

Hence, total clamp force = 4 x 5 12 = 2048 kN = 205 tonnes.

Another common shape which is moulded is a thin rectangular strip. Consider
the centre gated strip as shown in Fig. 4.41(b). In the same way as before the
clamping force, F, is given by

(4.26)