In order to increase the fabric compactness, as well as to improve the mechanical behaviour of the preform, a supplementary exterior layer is added, covering the preform. The tubular fabric is knitted perpendicularly in relation to the direction used to knit the preform (see Fig.
40) .
The fabric dimensions correspond to the ones presented in Fig. 32. The conical shape is obtained with successive, symmetrical narrowing. The tubular fabric is closed, giving a rounding effect in Ni and N2 zones. The closing can be done using the bind-off technique, though it is preferably to avoid its use for fabrics made of glass fabric. The repeated transfers lead to a significant amount of broken filaments, affecting the mechanical behaviour and even destroying the stitches during the introduction of the fabrics in/on the moulds before the RTM process.
Considering that there are no restrictions regarding the structure and its tensioning, the evolution chosen for the tubular fabric can be jersey, ixi jersey or even jersey fleece (see Fig.
41) .
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Fig. 40. Production of the supplementary layer |
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Fig. 41. Front aspect of a jersey and 1×1 miss jersey fabrics made with glass fibre count 408 tex |
Production of a glider wing made of composite material using a knitted preform
The composite material glider wing was produced using an injection process RTM (Nicolau et al., 2002). The resin used was unsaturated polyester orthophtalic resin, S226E (Neste), characterised by reduced viscosity, 150 mPa s. The pre-acceleration was obtained with a
0. 15% solution of cobalt 6%. The catalyst used was Trigonox TX 44B. This recipe formula avoids resin curing during the injection process.
