Page 26 - 3D Fibre Reinforced Polymer Composites

Manufacture of 30 Fibre Preforms 15
The insertion of the weft yarns can be done using a number of methods. One of the
oldest techniques consists of transferring a small package of yarn in a holder (shuttle)
through the shed, the yarn being drawn out of the shuttle and laid across the warp yarns
as the shuttle moves. This is a relatively slow technique but has the advantage of
creating a closed edge to the fabric, as it is a single continuous yam that is forming the
fabric weft. More recent, high-speed techniques involve laying down separate weft
yarns across the fabric width. These weft yams are drawn through the shed
mechanically with a long slender arm (rapier) or pushed across with high-pressure
bursts of air or water. These processes are faster than shuttle looms, reaching speeds of
approximately 600 insertions/minute, but create a loose edge of cut weft yarns that
needs to be bound together so that the fabric does not fray (salvage).
The final mechanism involved in the weaving process is a comb-like device (reed)
that is used to correctly space the warp yarns across the width of the fabric and to
compact the fabric after the weft yarns have been inserted. Generally a series of
positively driven rollers are used to pull the fabric out of the loom as it is being
produced and to provide a level of fabric tension during the weaving process. It should
be noted that the resultant fabric consists only of 0" and 90" yams, conventional
weaving is incapable of producing fabrics with yarns at any other angles and this is one
of the main disadvantages of weaving over other textile processes.
Current, commercial looms generally produce fabric of widths between 1.8 - 2.5
metres at production rates of metredminute. The standard weaving process is therefore
ideally suited to the cost-effective production of large volumes of material. However,
using essentially the same equipment, the process described above can also be used to
produce more complex, multilayer fabrics that have yarns in the thickness direction
linking the layers together.

2.2.2 Multilayer or 3D Weaving
The first major difference between conventional weaving and multilayer weaving is the
requirement to have multiple layers of warp yams. The greater the number of layers
required (and thus the thickness of the preform) or the wider the fabric produced, means
a larger number of individual warp yams that have to be fed into the loom and
controlled during the lifting sequence. Therefore the source of the warp yarn for
multilayer weaving is generally from large creels in which each warp yarn comes from
its own individual yam package. Multiple warp beam systems have also been used
although this is not as common. This requirement for a large number of warp ends
raises the first disadvantage of weaving, namely that the cost of obtaining (generally)
thousands of yarns packages and the time required to set up the large number of warp
ends within the loom can be extremely expensive. This non-recurring cost becomes less
significant as the length of the fabric being woven increases but having to weave large
volumes of the same material restricts the flexibility of the process. Most multilayer
weaving is therefore currently used to produce relatively narrow width products, where
the number of warp ends is relatively small, or high value products where the cost of the
preform production is acceptable.
As most 3D composites are produced from high performance yarns (carbon, glass,
ceramic, etc) standard textile tensioning rollers are unsuitable and tension control on the
individual yarns during the weaving is critical in obtaining a consistent preform quality.
This is generally accomplished through spring-loaded or frictional tension devices on