Page 323 - Plastics Engineering

306 Processing of Plastics

Table 4.4
For the same part, injection moulding of thermosets can offer up to 25% production increase and
lower part-costs than compression.

Compression moulding Minutes
Open mould, unload piece 0.105
Mould cleaning 0.140
Close machine, start pressure 0.100
Moulding cycle time 2.230
Total compression cycle 2.575

Injection moulding
Unload piece, opedclose machine 0.100
Moulding cycle time 1 .goo
Total injection cycle 2.000

4.4 Thennoforming
When a thermoplastic sheet is heated it becomes soft and pliable and the
techniques for shaping this sheet are known as thermoforming. This method
of manufacturing plastic articles developed in the 1950s but limitations such
as poor wall thickness distribution and large peripheral waste restricted its use
to simple packaging applications. In recent years, however, there have been
major advances in machine design and material availability with the result that
although packaging is still the major market sector for the process, a wide
range of other products are made by thermoforming. These include aircraft
window reveals, refrigerator liners, baths, switch panels, car bumpers, motor-
bike fairings etc.
The term ‘thermoforming’ incoroporates a wide range of possibilities for
sheet forming but basically there are two sub-divisions - vacuum forming and
pressure forming.

(a) Vacuum Forming
In this processing method a sheet of thermoplastic material is heated and then
shaped by reducing the air pressure between it and a mould. The simplest type
of vacuum forming is illustrated in Fig. 4.5 l(a). This is referred to as Negative
Forming and is capable of providing a depth of draw which is 113-112 of the
maximum width. The principle is very simple. A sheet of plastic, which may
range in thickness from 0.025 mm to 6.5 mm, is clamped over the open mould.
A heater panel is then placed above the sheet and when sufficient softening
has occurred the heater is removed and the vacuum is applied. For the thicker
sheets it is essential to have heating from both sides.
In some cases Negative Forming would not be suitable because, for example,
the shape formed in Fig. 4.5 1 would have a wall thickness in the comers which
is considerably less than that close to the clamp. If this was not acceptable then