Page 72 - 3D Fibre Reinforced Polymer Composites

Preform Consolidation 61

often influenced by the complex geometries of the preforms that are generally
consolidated by liquid moulding. Dry spots can also be produced by a variation in
preform permeability causing unintended resin flow paths. This is particularly apparent
at the edges of preforms, where racetracking can often occur, or if the preform contains
areas of highly anisotropic permeability that can force the resin flow into unintended
directions. It is possible to repair some dry spots after manufacture by a local injection
of resin although generally not all of it is eliminated and the area is often weaker. If a
dry spot is observed to be forming during moulding a process of repeatedly blocking
and opening the outlet whilst keeping the injection pressure on can act to move the
trapped air to the outlet due to the varying pressure differentials. This process is known
as “burping” (Rackers, Howe & Kruckenberg, 1998).
Voids and porosity can be formed through a number of mechanisms. Air leaks in
vacuum assisted liquid moulding can cause large, irregular voids to form and are
generally located near the perimeter of the part or near the inletloutlet positions. They
are formed through inadequate sealing. Regular vacuum checks and replacement of
seals and fittings can eliminate these defects. Volatiles formed during the resin infusion
and cure process can also form gas-filled voids. These are generally observed as small,
isolated voids spread evenly through the component. A change of resin type to a non-
volatile producing species, or an adjustment of the vacuum pressures or cure
temperatures can help eliminate these voids. If this is not possible then an increase in
the injection pressure can help minimise the void size. The final void formation
mechanism is the mechanical entrapment of air. This is essentially a smaller version of
the dry spot formation on the scale of individual tows. The resin contains two paths
within the preform through which it can flow, between the tows and within the tows.
The driving forces for the two paths are different, capillary action within the tow and
the overall hydrodynamic pressure between the tows. If these driving forces are not
similar then the flow front can lead in one of the flow paths and lead to the entrapment
of air within the other flow path. To overcome this problem vacuum is normally applied
in the moulding process therefore any trapped voids will have an internal pressure close
to vacuum. As the hydrostatic pressure increases the voids must shrink and may
completely collapse.

3.8 SUMMARY

Liquid moulding processes are currently the only viable techniques that can be used to
successfully consolidate 30 fibre preforms. There are many different styles of liquid
moulding but they all essentially fall into the main groupings of Resin Transfer
Moulding (RTM), Resin Film Infusion (RFI) and the Seemann Composite Resin
Infusion Process (SCRIMP). A basic description of the various techniques and issues
related to the choice of equipment, resin selection, tool design and part quality have
been given in this Chapter.
Liquid moulding is in common use within a wide range of industries and is a well
established manufacturing process, but is primarily used with traditional 2D fibre
reinforcements. The consolidation of 3D preforms via liquid moulding does not appear
to hold significant challenges and examples of the use of liquid moulding 3D reinforced
composites for commercial and prototype use have been given in Chapter 1. The main