JEC COMPOSITES MAGAZINE - Issue #76 - October/November 2012 - 78

cost effectiveness
Cutting There are several ways to substitute manual cutting operations and thus increase the productivity and reproducibility of cutting processes. Textile semi-finished parts can be cut prior to further processing. This requires an efficient intermediate storage system for these pre-cut textiles. Textiles already placed in the tool form can also be trimmed using robotically-guided cutting devices. CNC cutting tables are suitable devices to cut textiles prior to subsequent preforming processes. The cutting means used are typically rotating cutting discs or oscillating knives (see Figure 13). In both cases, the textiles are usually fixated by vacuum. Preforms for RTM processing have to fulfil certain contour tolerances as size variations can lead to race tracking and thus incomplete impregnation. Edge-trimming operations using robotically-operated ultrasonic knives are a suitable way to fulfil RTM tolerance requirements. Due to the geometrical flexibility of standard robots, even complex shaped preforms can be trimmed. An ultrasonic knife is displayed in Figure 13. Joining Along with gluing, sewing is still the most important joining technology for textile structures. Sewing technologies will play an important role in the process chain for the future manufacturing of fibre-reinforced components. Their importance in terms of cycle time reduction and cost savings can be regarded in line with those of qualified consolidation procedures [12]. In the preforming process, the purposes of sewing techniques range from the insertion of flat reinforcements and the fixation of a preform’s individual components for the subsequent processing steps to the manufacturing of load-compatible seam areas. Different sewing technologies are applied in various processing steps of multi-step preform production. Sewing technologies are chosen according to the function of the seam, the preform geometry and the future component loads [13]. In addition to assembly, sewing can be used to adjust the mechanical properties of the final FRP component. Reinforcement in the z-direction can be achieved in the seam area using high-performance sewing threads. This way, local reinforcements can be applied. Improved impregnation behaviour is another benefit of sewing technologies since the seams improve the permeability of the preform. In fibre-reinforced components, seams have to fulfil a number of requirements. Forces and torques should be transferred through the seam area without loss of mechanical properties. No significant stiffness variations should occur and different component properties such as differing thermal expansion coefficients of the materials should not cause any residual stresses. Furthermore, the seam should not cause any mass and dimension variation within the fibre-reinforced component. Not all the listed seam requirements are completely feasible, so that compromises need to be found in most cases [14]. Both conventional (e.g. double-step stitch) and 3D sewing technologies (tufting, blind stitch, ITA sewing technology) can be applied in preforming processes [15, 16]. Process chain development Efficient planning tools are required to develop economically-viable, cost-effective preforming process chains. For this purpose, ITA has developed a four-step methodology for preforming process development (Figure 15) and a software named “EcoPreform”, which makes it possible to design preforming process chains and to assess them in terms of technical feasibility and cost-effectiveness. Two performance indicators are given key importance: cycle time and unit costs. Based on the developed methodology and the database

Fig. 13: Ultrasonic and oscillating knive

Handling Handling and draping of textile fabrics, structures and sub-preforms are very important constituents of multi-step preforming processes. A number of handling devices were invented to automate the preforming process. These devices typically use needle, vacuum, Bernoulli, electrostatic or cryogenic grippers. Yet so far none of these devices are able to fulfil all the requirements of textile reinforcement structures, namely low damage, no contamination, low fibre disorientation and sufficient holding forces. Thus, most handling and draping operations are still accomplished manually while automated technologies for this part of the preforming process are currently under investigation. Figure 14 shows a modular handling device positioning a pre-cut textile into a uniaxial curved tool.

Fig. 14: Pick and place operation

Fig. 15: Four-step-methodology

78 jec composites magazine / No76 October - November 2012

JEC COMPOSITES MAGAZINE - Issue #76 - October/November 2012

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #76 - October/November 2012