JEC COMPOSITES MAGAZINE - Issue #73 - June 2012 - 61

Single-step preforming
The laminate fabrics for the shell and stringers of the FRP roof can be manufactured using warp-knitting machines with multiaxial weft insertion. Enhanced fabrics with locally adjusted drapability and local reinforcements require an extended machine functionality, which is illustrated in this section. Warp-knitted non-crimp fabrics Non-crimp fabrics (NCF) are defined as drawn parallel layers of reinforcing threads or tows. They can be produced economically on warp-knitting machines with multiaxial weft insertion (Fig. 6). The loose fibre layers are connected with warp-knitting yarns by loop formation [5]. The warp-knitting elements and loop formation are described in references [6] and [7].

Fig. 7: Pattern discs (left), electro-mechanical guide bar (middle) and locally adjusted stitch type (right)

Varying stitch types during production require an adjustment of the yarn run-in from stitch course to stitch course. To adjust drapability locally, a computer-controlled beam control is also integrated [ 11]. Locally adjusted thickness Local reinforcements are placed on the loose fibre layers between the weft insertion systems and warpknitting unit. Prefabricated textiles Fig. 8: Endless (up) and finite (down) reinforcement or unidirectional fibre layers can be supplied in endless or finite form by means of feeding modules (Fig. 8). The feeding modules can lay down several parallel reinforcements with the same length at a time. Several modules can be arranged in a row to produce multi-layer textile structures [12]. Cutting and stacking unit Conventional winding of multiaxial warp-knitted NCF with high layer setup or local reinforcements can lead to deformation or damage of the textile structure. This can be avoided by cutting the NCF into pieces, stacking them and compensating the Fig. 9: Winding (left) and stacking (right) of NCF with local reinforcements local reinforcements (Fig. 9). A conveyor belt system is used to stack the individual pieces. The movement of the belt system can be divided into four phases (Fig. 10), which are described on next page. Carrying: the tailored NCF is transported at constant speed by a transport chain. It is cut out of the transport chain by diamond disc blades and subsequently separated No73 June 2012

Fig. 6: NCF (left) and warp-knitting machine with multiaxial weft insertion (right)

The drapability, thickness and width of the fabrics produced using state-of-the-art machines are constant. Drapability can be defined as the adaption of a semi-finished laminar material on curved three-dimensional surfaces [8]. In a subsequent preforming process, tailored NCF with locally adjusted properties such as drapability and local reinforcements provide benefits such as simplified handling, reduced handling steps and reduced scrap. Locally adjusted drapability Stitch length, stitch type (underlap of the guidebar lateral to process direction) and yarn run-in have an impact on the drapability of NCFs. A greater stitch length, a larger underlap and a higher yarn run-in of the warp-knitting yarns increase the formability of the material while complicating handling [ 9, 10]. A locally adjusted stitch length requires major modifications in the machine software and hardware. Therefore, this possibility was not considered. The underlap is predetermined by the movement of the guide bar, which is mainly driven by pattern discs. An electro-mechanical guide bar provides the possibility to change the underlap locally. The machine manufacturer, LIBA Maschinenfabrik GmbH, Naila, Germany, modified the machine operating system and produced the hardware (Fig. 7) [ 11]. An NCF with locally adjusted stitch type is shown in Fig. 7.

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JEC COMPOSITES MAGAZINE - Issue #73 - June 2012

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