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Processing of Plastics 337

Resin feed
pipes

Mould clamping
screws Resin injection
\
Reinforced
material in
mould cavity

Fig. 4.74 Resin injection process

problem of voids in the residfibre laminate and offers faster cycle times with
greater uniformity of product.

4.10.3 Automatic Pr~~esses
(a) Filament Winding: In this method, continuous strands of reinforcement
are used to gain maximum benefit from the fibre strength. In a typical process
rovings or single strands are passed through a resin bath and then wound on
to a rotating mandrel. By arranging for the fibres to traverse the mandrel at a
controlled and/or programmed manner, as illustrated in Fig. 4.75, it is possible
to lay down the reinforcement in any desired fashion. This enables very high
strengths to be achieved and is particularly suited to pressure vessels where
reinforcement in the highly stressed hoop direction is important.
In the past a limitation on this process was that it tended to be restricted
to shapes which were symmetrical about an axis of rotation and from which
the mandrel could be easily extracted. However, in recent years there have
been major advances through the use of collapsible or expendable cores and in
particular through the development of computer-controlled winding equipment.
The latter has opened the door to a whole new range of products which can be
filament wound - for example, space-frame structures. Braiding machines for
complex shapes are shown in Fig. 4.76.
(b) Centrifugal Casting: This method is used for cylindrical products which
can be rotated about their longitudinal axis. Resin and fibres are introduced
into the rotating mould/mandrel and are thrown out against the mould surface.
The method is particularly suited to long tubular structures which can have a
slight taper e.g. street light columns, telegraph poles, pylons, etc.
(c) Pultrusion: This is a continuous production method similar in concept to
extrusion. Woven fibre mats and/or rovings are drawn through a resin bath
and then through a die to form some desired shape (for example a ‘plank’