Page 69 - 3D Fibre Reinforced Polymer Composites

58 30 Fibre Reinforced Polymer Composites
3.6.2 Heating and Cooling
The SCRIMP and RFI processes both operate with single-sided tooling therefore
heating is generally conducted via an external source such as an autoclave, air
convection oven or radiant heaters, or even through the use of electric heating blankets.
The selection of a heating system will be dependant upon the size of the part being
produced and the processing conditions (heating rates, cure temperature, etc). Generally
though the tools are not integrally heated as it is a less efficient, and often more costly,
way of applying thermal energy to the preform and resin with a single-sided tool.
Cooling for these processes would generally occur via natural cooling in the air.
As the RTM process uses double-sided tooling, integral heating becomes a more
likely candidate as a means to apply thermal energy. Normally the mould is heated and
cooled using temperature controlled water or oil, although electrical elements can also
be used for heating. The mould is constructed with interior channels through which the
heatingkooling fluid flows and this normally results in a very efficient, controlled
process for heating and cooling the mould. The selection of fluid temperatures will
depend upon the required heating rates and cure temperatures but also upon the size of
the mould and the thermal properties of the mould material itself. Alternate heating
techniques for the RTM process include heated platens in a press, which also has the
advantage of providing the mould clamping pressure, and external sources such as
ovens. These techniques are normally not as efficient as the integral heating process.

3.6.3 Resin Injection and Venting
This part of the mould design is one of the most critical and, although the exact details
of resin flow are different between RTM, RFI and SCRIMP, this issue is relevant to all
three of the liquid moulding techniques.
The injection ports (resin inlets) and vents (resin outlets) must be correctly
positioned so that the resin will contact all of the preform during its flow. Bypass of any
part of the preform will result in dry patches, one of the types of defects that will be
discussed in a later section. The factor common to many successful inlet/outlet designs
is that the flow path should be arranged such that the resin is flowing into a
configuration with decreasing volume. Thus the volume of air left in the preform will be
decreasing and this reduction effect helps sweep the air out of the part. Figure 3.6
illustrates examples of good and bad inlet/outlet designs with regard to this rule-of-
thumb. The reverse arrangement can be used but this generally requires a greater
understanding of the likely resin flow in order to obtain fully wet-out components. Flow
modelling can be a very important process to undertake when designing a mould,
particularly when the preform permeability is very anisotropic. There are various
commercially available software packages that can be used for this task. The details of
modelling the flow of resins in liquid moulding processes is explained in greater detail
in Parnas (2000).
Vents should be placed so as to draw the resin through preform sections that are
difficult to wet out and this is usually at the extreme end of flow paths or dead ends,
where the resin will not flow by itself. Vents must also be capable of being individually
sealed after the resin begins to bleed out as this will force the resin to flow into other
sections of the preform and, when all are sealed, will allow the final curing process to
occur under pressure. This will help reduce the possibility of voids in the finished part.