Page 68 - 3D Fibre Reinforced Polymer Composites

Preform Consolidation 57

A further issue is the potential of having preferential flow directions within the preform
that can prevent correct filling with the liquid resin. Many 3D preforms, particularly
non-crimp fabrics and those produced by weaving, can have planes of very straight
reinforcement in specific directions. This directionality can result in significant
differences in preform permeability with orientation that could lead to the resin flowing
more swiftly in certain directions (“racetracking”) and trapping off unfilled sections of
the preform. Accurate knowledge of the preform permeability with orientation and
correct design of the liquid moulding process will allow this issue to be overcome.

3.6 TOOLING

The proper design and manufacture of tooling for liquid moulding is a critical part of
successfully consolidating a 3D fibre preform. Of the three liquid moulding processes
described in this chapter (RTM, RFI and SCRIMP), both RFI and SCRIMP utilise
single-sided tools whilst the RTM process requires a closed mould system. Although
this difference does allow a greater ability for the RTM process to incorporate integral
heating and cooling systems into the tooling, many of the liquid moulding tooling issues
are common to all three process styles.

3.6.1 Tool Materials
Generally the first decision that is made in the tool design process is to choose the
material from which to manufacture the liquid moulding tool. There are many materials
which can be used, ranging from metal (steel, AI, etc) to cast resin, wood or plaster.
The choice of material will be influenced by many factors and detailed discussion of
these can be found in references such as Potter (1997) and Wadsworth (1998). Some of
the primary factors will be briefly discussed here.
The production rate is often one of the most important factors in the selection of tool
material. For low volume (100’s of parts) or prototype production, composite, cast
resin, wood or plaster tools are often used and have the advantage that they are
significantly cheaper than metal tools and thus are more suited to low production
volumes. For higher production volumes (1,000 - 10,000 + parts), metal tools (steel,
aluminium, electroformed nickel, etc) are the only possible choice due to their
durability. Although metal tools are more costly on a direct comparison with non-
metal, the higher initial tooling costs are generally outweighed by the reduced need to
repair or replace them, which is an important consideration in high volume and
production rate environments.
The processing conditions and required surface finish also affect the material choice.
Metal tools are capable of withstanding far higher service temperatures than non-metal
tools and are thus more suited for processes using resins with high cure temperatures.
Properly maintained metal tools also produce better surface finishes than non-metal,
which is particularly important in industries such as the automotive. Other issues such
as the heat transfer requirements and the need for dimensional control can also influence
the choice of tool material but generally these are secondary to the issues mentioned
above.