Page 70 - 3D Fibre Reinforced Polymer Composites

Preform Consolidation 59

Dry Area

a) Desirable flow b) Undesirable flow

Figure 3.6 Desirable and undesirable resin flow paths

3.6.4 Sealing
Adequate sealing of the mould is essential if parts of low void content are to be
produced. In RFI and SCRIMP this is accomplished with sealant tape in the normal
fashion when sealing the preform under a vacuum bag. In RTM the most common way
to produce a seal is through the use of elastomeric O-rings (materials such as silicone
rubber or VitonB). These O-rings sit in a machined groove within one half of the mould
and are compressed when the mould closes. The choice of O-ring material depends
upon the required pressure sealing capacity and the maximum temperature it will see
during the moulding cycle.
Another method that can be used is the pinch seal technique. Here the preform itself
is clamped tightly between the two mould halves to create a region of very high fibre
volume fraction. This will increase the resistance to flow of the resin in this area and
ideally create an area through which the resin cannot flow over the course of the
injection and cure. In reality pinch seals generally allow resin to leak through, which
can be a health and safety concern. The final consolidated part will also need more
extensive trimming than one produced with an O-ring seal.