Page 47 - 3D Fibre Reinforced Polymer Composites

36 30 Fibre Reinforced Polymer Composites

2.4.2 Three-Dimensional Shaping
As well as producing highly conformable flat fabric, the knitting process can be used to
manufacture more complex-shaped items. Since the 1990’s significant advances in flat-
bed machine technology and design and control software has allowed the development
of commercial knitting machines that are capable of forming complex 3D shapes. The
leading knitting machine companies, Stoll (Germany) and Shima Seiki (Japan), have
lead the research and technical developments in this area and each has commercialised
their own machinery capable of producing 3D shapes. The most important
developments have been in the use of electronic controls for needle selection and knit
loop transfer, and in the sophisticated mechanisms that allow specific areas of the fabric
to be held and their movement controlled (Lo, 1999; Editor, 1996; 1997; Reider, 1996;
Stoll GmbH, 1999). These developments allow the knit architecture and the way in
which the fabric is controlled, to be designed such that as the fabric is manufactured it
will form itself into the required three-dimensional preform shape with a minimum of
material wastage, examples of which are shown in Figure 2.27. This can be
accomplished without fabric overlap or seams and with the fabric properties capable of
being designed to be uniform throughout the whole structure. This process is capable
of cutting the manufacturing costs for complex-shaped components as the time required
to form the component shape would be dramatically reduced when compared to the use
of more traditional composite manufacturing techniques (Vuure et al., 1999). In spite of
the relative infancy of this area of research a number of net-shaped components have
already been demonstrated in high performance yarns including car wheel wells (Vuure
et al., 1999), T-pipe junctions, cones, flanged pipes & domes (Epstein and Nurmi,
1991), and jet engine parts (Robinson and Ashton, 1994).

Figure 2.27 Examples of shape knitted comer fabrics designed for composite window
frames (courtesy of the Cooperative Research Centre for Advanced Composite
Structures, Ltd)