Page 349 - Plastics Engineering

332 Processing of Plastics

Chopped fibre

Mould ’ ‘ Moulding

Fig. 4.68 Spray-up technique

(c) Autoclave Moulding: In order to produce high quality, high precision
mouldings for the aerospace industries, for example, it is necessary to have
strict control over fibre alignment and consolidation of the fibres in the matrix.
To achieve this, fabric ‘pre-pregs’ (i.e. a fabric consisting of woven fibre yarns
pre-impregnated with the matrix material) are carefully arranged in layers in
an open mould. The arrangement of the layers will determine the degree of
anisotropy in the moulded article. A typical layer arrangement is shown in
Fig. 4.69(a). The pre-preg stack is then covered with a series of bleeder and
breather sheets, as shown in Fig. 4.69(b) and finally with a flexible vacuum
bag. When the air is extracted from between the flexible bag and the pre-preg
stack, the latter will be squeezed tightly on to the mould. The whole assembly
is then transferred to a very large oven (autoclave) for curing.

Pwous releose

Fig. 4.69 Diagramatic cross section of a bagged lay-up

4.10.2 Semi-Automatic Processing Methods
(a) Cold Press Moulding: The basis of this process is to utilise pressure
applied to two unheated halves of a mould to disperse resin throughout a