Page 347 - Plastics Engineering

330 Processing of Plastics
The Semi-Automatic processes include processes such as cold pressing, hot
pressing, compression moulding of SMC and DMC, resin injection.
The Automatic processes are those such as pultrusion, filament winding,
centrifugal casting and injection moulding.
Qpical market shares for the different methods are shown in Fig. 4.66.

Contact moulding (hand lay-up
Injection moulding
--> ;pray-up)

Continuous lamination

ilament winding
r
Pultrusion
n iM
Pressure forming
Fig. 4.66 npical market shares for composite moulding methods in Europe

4.10.1 Manual Processing Methods
(a) Hand Lay-Up: This method is by far the most widely used processing
method for fibre reinforced materials. In the UK it takes up about 40% of
the FRP market. Its major advantage is that it is a very simple process so
that very little special equipment is needed and the moulds may be made
from plaster, wood, sheet metal or even FRP. The first step is to coat the
mould with a release agent to prevent the moulding sticking to it. This is
followed by a thin layer (approximately 0.3-0.4 mm) of pure resin (called a
gelcoat) which has a number of functions. Firstly it conceals the irregular mesh
pattern of the fibres and this improves the appearance of the product when it
is taken from the mould. Secondly, and probably most important, it protects
the reinforcement from attack by moisture which would tend to break down
the fibrehesin interface. A tissue mat may be used on occasions to back up the
gelcoat. This improves the impact resistance of the surface and also conceals
the coarse texture of the reinforcement. However, it is relatively expensive and
is only used if considered absolutely necessary.
When the gelcoat has been given time to partially cure the main reinforce-
ment is applied. Initially a coat of resin (unsaturated polyester is the most
common) is brushed on and this is followed by layers of tailored glass mat
positioned by hand. As shown in Fig. 4.67 a roller is then used to consolidate
the mat and remove any trapped air. The advantage of this technique is that
the strength and stiffness of the composite can be controlled by building up the
thickness with further layers of mat and resin as desired. Curing takes place
at room temperature but heat is sometimes applied to accelerate this. Ideally
any trimming should be carried out before the curing is complete because the