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Most of the reinforcements used in composites are produced using textile techniques, such as weaving or braiding for high performance and nonwoven techniques for low cost. Another way, unusual in composites, is weft knitting.
(Published on April 2006 – JEC Magazine #24)
BY LAURENT VAN DE CASTEELE, RESEARCH AND DEVELOPMENT ENGINEER, IFTH / CLUB TEXTILE INTÉGRAL
Weft knitting is commonly used for garments such as socks or tee-shirts, because it fits the shapes. Its structure makes it elastic whatever the material. This characteristic can be used to produce aircraft composite reinforcements for different kinds of parts, such as 3D parts or cabin equipment. The weakness of these techniques today is that there are almost no finite element models to predict the behaviour of the knitted composites and the elasticity of the preform does not allow the manufacturing of high-performance parts. Weft knits are produced with two kinds of machine: circular machines, and flat machines where needles are moved by cams to produce loops. Almost any kind of yarn can be used – for example, aramid (Kevlar®), polyethylene (Dyneema®, Spectra®), carbon, fibreglass, basalt or metallic yarn. Note that it is possible to mix different kind of yarns in peculiar areas using the intarsia technique. For instance, it is possible to knit a Kevlar zone inside a carbon part to bring cutting resistance.
Circular machines produce a tubular product, with a given diameter and yarn density. This kind of tube can be used directly as a sleeving, or opened to become flat. Then, it can be worked – for example, cut or moulded – like a woven fabric. These machines are very productive and allow the knitting of 3D spacer fabrics, which are knitting’s equivalent to the Parabeam® in weaving. The advantage of a knitted spacer fabric is that it is very easy to shape a 3D mould due to its elasticity, and to make sandwiches for panels or pipes.
Flat machines feature two rows of needles face to face, allowing the knitting of 2D and integral preforms for many kinds of parts, such as pipes, junctions, and bodies. The main advantages of this technique is that the yarns are placed where needed, reducing the material waste to an overall 3%. This can be a considerable advantage for expensive materials and for companies engaged in an ecodesign system. As the part is 3D knitted, it does not need to be cut, and there is no more overlapping or weakness. It is also possible to have a tunnel inlay, placing a yarn straight across the part to make it very stiff.
Another interesting point is that small production batches are possible, because the machines are quickly set up and there is no minimum quantity to launch, enabling multiple attempts to achieve better design.
Today’s applications of knitted reinforcements are tubular parts, carbon disk brakes, and thermal shields, and many other parts can be successfully reengineered to include knitting technologies.