The inner walls of the preform are produced as single connecting layers of the sandwich fabric. The layer length is 80 mm, respectively 64 mm. In order to satisfy the required 5 mm thickness for the inner walls, two successive connecting layers with the same length will be knitted. After the introduction of the resin the two neighbouring connecting layers will generate an inner wall. Apart from knitting connecting layers of such high length, the production of two layers one after the other represented another problem.
During the knitting of the single connecting layer, some rows of the outer fabric are produced on the opposite bed in order to reduce the tension generated by the long period in which the needles making the outer independent fabrics miss. These supplementary rows must be taken into consideration when calculating the number of rows between the two connecting layers so that they are horizontal.
Presentation of the preform made of glass fibre
The general aspect of the preform is presented in Fig. 37. The beginning and ending zones of the outer fabrics can be observed, as well as the zones where the connecting layers were produced (see Fig. 38). One connecting zone is encircled in Fig.37.
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The successive connecting layers are exemplified in Fig. 39 where the preform is presented in cross section, showing the two connecting layers of 80 mm length.
