Page 43 - 3D Fibre Reinforced Polymer Composites

32 30 Fibre Ueinforced Polymer Composites

length of preform that can be braided before re-supply of the yarn is necessary is limited
by the need for the yarn to be on the moving carriers, which ideally must be small and
light for rapid braid production. Thus the production of long lengths of the preform can
be slow due to the need to re-stock the yarn carriers. One of the greatest current
disadvantages however is the fact that the 3D braiding process is still very much at the
machinery development stage. Therefore there are limitations to the type of preform
that can be made commercially and there are very few companies that have the
necessary experience and equipment to manufacture these preforms.

2.4 KNITTING

Knitting may not at first appear to be a manufacturing technique that would be suitable
for use in the production of composite components and it is arguably the least used and
understood of the four classes of textile processes described here. However, the knitted
carbon and glass fabric that can be produced on standard industrial knitting machines
has particular properties that potentially make it ideally suited for certain composite
components.

2.4.1 Warp and Weft Knitting
Two traditional knitting processes, weft knitting and warp knitting, are available to
manufacture preforms for composite structures. Both of these techniques can be
performed upon standard, industrial knitting machines with high performance yams
such as glass and carbon. One critical issue that must be considered is that the more
advanced knitting machines have electronic control systems close to the knitting region
where broken fibres can be generated. The use of carbon yarns with these machines
should be avoided as loose carbon fibres can generate electrical shorts. In warp knitting
there are multiple yams being fed into the machine in the direction of fabric production,
and each yarn forms a line of knit loops in the fabric direction. For weft knitting there is
only a single feed of yarn coming into the machine at 90" to the direction of fabric
production and this yarn forms a row of knit loops across the width of the fabric (see
Figure 2.21).

Figure 2.21 Illustration of typical a) weft and b) warp knitted fabric architectures