Page 56 - 3D Fibre Reinforced Polymer Composites

Manufacture of 30 Fibre Preforms 45

2.5.3 %Pinning
An alternate method to the standard stitching process was first described in late 1980's
(Evans and Boyce, 1989; Boyce et al., 1989) and subsequently has been commercially
developed by the company Aztex (a subsidiary of Foster-Miller) as Z-FiberTM
technology (Freitas et al., 1996). The technology consists of embedding previously
cured reinforcement fibres into a thermoplastic foam that is then placed on top of a
prepreg, or dry fabric, lay-up and vacuum bagged. Through judicious choice of the
material, the foam will collapse as the temperature and pressure are increased, allowing
the fibres to be slowly pushed into the lay-up (see Figure 2.36). This method can be
used during the normal autoclave cure of prepreg and for both prepreg and dry fabric
can be performed whilst the lay-up is on the tool surface itself, thus saving extra steps in
the manufacturing process. A version of this technology can be used at room
temperatures as it utilises an ultrasonic horn that heats up a local area of the z-pin foam
and preform, thus allowing a plunger to push the pre-cured reinforcement yarn into the
lay-up. Both methods have been successfully applied to carbodepoxy composites with
silicon carbide, boron and carbon reinforcement yarns. Chapter 9 contains further
details on this technology and the mechanical performance of z-pinned composites.

Heat and pressure
Foam with embedded fibres Vacuum bag

Tool

Figure 2.36 Illustration of z-pinning process

2.6 SUMMARY

The four textile processes of stitching, weaving, braiding and knitting, have the
potential to significantly reduce the cost of manufacturing many composite components
and prodhce structures that have improved mechanical performance in critical design
cases such as impact. Each of these processes has been briefly described here and their
advantages and limitations noted. The main aspects of these manufacturing techniques
have been summarised in Table 2.1 and reviews of these textile processes can be found
in the published literature (KO, 1989b; Mouritz et al., 1999; Kamiya et al., 2000).
However, one manufacturing issue that has only been only briefly mentioned here is the
potential of each manufacturing process to cause significant damage to the
reinforcement yarns and thus degrade the performance of the final composite. Although
this issue as been partly explored for the stitching process (Mouritz et al., 1997; Mouritz
and Cox, 2000) very little investigation has been done on the other techniques
mentioned here, although recent work has shown that the effects of processing damage
can be significant for 3D weaving (Lee et al., in press).