Page 48 - 3D Fibre Reinforced Polymer Composites

Manufacture of 30 Fibre Preforms 37

A speciaIised sub-group of 3D knitted preforms are sandwich fabrics, which were
developed by Verpoest et al. in the mid-1990's (Verpoest et al., 1995). They are
produced in a similar fashion to 3D woven sandwich fabrics by simultaneously knitting
top and bottom skins on a double-bed, warp knitting machine. As the two fabrics are
being formed, yams are swapped between the two faces to create the connecting pile
yarns, thus binding the two faces into an integral sandwich fabric. The density of the
pile yarns can be varied and their orientation can be aligned vertically or at an angle to
the faces in the warp direction. The two needle beds can also be programmed to
produce different knit architectures and thus produce face fabrics with different physical
characteristics.
As with 3D woven Distance Fabrics, the 3D knitted sandwich fabrics can produce
composite sandwich products with high peel and delamination resistance and although
their face fabrics will have reduced mechanical performance compared to Distance
Fabric faces, their knit architecture allows them to form far more complex shapes than
is possible with Distance Fabrics (Verpoest et al., 1995; Mouritz et al., 1999).

2.4.3 Non-Crimp Fabrics
A manufacturing technique that combines aspects of weaving and knitting is known by
either of the names; Multi-Axial Warp Knitting or stitch-bonding, but is perhaps most
commonly referred to by the style of fabric it produces, Non-Crimp Fabric (NCF). This
fabric can be produced with glass, carbon or aramid yarn (or with combinations of
these) and is unique in that fabric can contain relatively uncrimped yams orientated at
0' and at angles that can vary between +20" to -20". There are a number of generic
manufacturing processes which can be employed to produce NCF. The most commonly
used process is that developed by the LIBA Machine Company of Germany. A
schematic of this process is shown in Figure 2.28 together with an example of the type
of fabric that can be produced.
As illustrated in Figure 2.28, yarns are feed from a creel system (1) and are laid onto
a long table at the orientations required via placement heads (2), an example of which is
shown in Figure 2.29. These placement heads travel across the table and secure the
yarns at either side on a chain of needles (3) that travel along the table as the fabric is
manufactured. The lay-up of the final fabric is dictated by the control of the placement
heads motion. As well as angled fibres, if required, a chopped strand mat can be
incorporated into the fabric by the use of a chopper system (4) and further fleeces or
mats can be inserted through the use of two roll-carriers (5). The 0" fibres are the last to
be placed and can be feed from a beam (6) or a creel system and the multiple layers of
the fabric are linked together by a warp knitting machine (7). This machine has
specially designed sharp-head needles that are positioned such that the knitting process
does not penetrate and damage any yarns but instead forms the knit loop in between the
yarns (see Figure 2.30). In current, commercially available fabric the knit thread is
normally polyester, but techniques are being developed to manufacture high quality
fabric with glass or carbon knitting thread.
The process is flexible in that the variety of lay-ups is dictated only by the number
and order of the "stations" (Le. 90°, 45", chopped fibre, fleece mats, etc) that are linked
together along the length of the production table. However, due to the need to precisely
locate the angled yarns on the needle chains and to ensure the knitting needles do not
damage the yams, there are some restrictions on the size of yams used and the areal