Working with composites is a special challenge which requires high-tech equipment and a high level of precision. Special training of the staff, certifications like ISO 9000 and good contacts to relevant R&D institutions and partners from the targeted industries are a must. Special exhibitions and conferences worldwide, focus on composites – textiles and making-up textiles are a permanent topic.
The design process often starts from 3D, using software like CATIA, common in engineering. Lectra’s Design Concept TT software can import the files of CATIA and then flatten the 3D to 2D shapes. The preforms are then produced according to these 2D patterns and sewn. Users such as the bicycle producer Trek Bicycles, USA, confirm that they were able to cut down the development process time significantly.
Spreading & Cutting
Gerber provides automated cutting solutions for advanced composite processes (prepreg & wet lay-up, vacuum bagging, open mould/ closed mould) and for fibreglass-reinforced composite applications. All Gerber Technology systems utilize vacuum tables to ensure that materials are held securely in place for accurate, repeatable plotting and cutting. One of the users is CAM, Italy, a supplier to aeronautics. They changed from cutting the textiles by hand to cutting by Gerber DCS 2500 singly ply and were able to minimize the time for cutting very efficiently.
Dassault Systemes, known for its 3D industrial design solutions Catia and Enobia, works also for composite structures. The company recently showed a complete set of process-oriented solutions at JEC 2008, which are used to design, simulate, and manufacture composite parts on a single CATIA V5 virtual platform. Obviously, when it comes to cutting the woven, knitted or nonwoven textiles, single layer cutting is requested. High attention is drawn to precision. The large cutting tables work with air suction. Special knives or laser cutter will be necessary to cut the fibres. Among the supplying companies are Lectra, Gerber, Wastema, Expert Systemtechnik or Eurolaser.
The cuts can be laid into the moulds, like the Italian company CAM does. An alternative are made-up preforms. Here, the sewing technology comes in. EADS-CRC partnered with fellow German-based Institute for Textile Technology of the Aachen University of Technology (ITA) and KSL Keilmann Sondermaschinenbau GmbH to devise a number of key technologies that could lead to economical production of larger, 3D-reinforced composites with superior damage tolerance and structural integrity. “Preform stabilization makes it easier to handle and store composites before they go into an autoclave. At first glance this may appear to be a relatively simple task. But in practice, the technique is much trickier,” says Mr Andreas GESSLER, Manager Textile Technologies and Testing EADS Deutschland GmbH, Corporate Research Center, Ottobrunn near Munich, Germany. He continues: “Stitching must only prevent the subpreforms from relative movement. Unsuitable sewing parameters can easily destroy material properties, misalign the fibres, or ultimately cause the resin to pool in the seams.”
- Conventional lock-stitch machines are well established for preform stabilization, attaching stiffeners to flat panels, and to get fast, through-the thickness reinforcement of wide areas.
- Conventional sewing techniques, including lock stitch and chain stitch, need access to both sides of the material. So this limitation restricts these stitching techniques to flat platforms with limited size.
- Single-side chain stitching from the former company ALTIN uses a sewing needle at 45° and a 90° hook needle. This set up puts the thread chain on the upper side of the fabric and produces a simultaneous reinforcement at 900 and 450.
- Multithread chain stitch from ITA uses two sewing needles at 450 and puts the thread chain on the rear side of the fabric. The ±450 threads improve composite shear strength.
- Unchained open-yarn loops or tufts from KSL can be used to sew up to 1.57-inches (40-mm) thick materials. The open-loop structure of the threads can be kept inside the material or forced to penetrate through. This process uses thick yarns.
- Blind chain stitch also from KSL uses a curved needle [Radius = 0.98 inches (25 mm)]and puts the thread chain on the upper side of the material. With this head, the needle does not puncture through the material and is an option for sewing inside the mold.
KSL Advanced Composites is a newly established department of the KSL Keilmann Sondermaschinenbau GmbH with the ambition to offer products and services in the area of composite materials. In addition to the machines, they offer production of research samples, production of preforms, testing series on their 2D and 3D robotic sewing equipment as well as development, construction and installation of complete production systems for industrial applications.
Manufacturers typically pick a sewing yarn based on the seam’s function. Polyester yarns, for example, are sufficient for assembling preforms without the need for structural improvement. These seams just prevent the relative movement of the preform layers or subpreforms. However, the seams must not disturb the textile structure or degrade the material in-plane properties. Typically, designers can expect about a 2% drop in static compression strength. Stitch type, needle and yarn thickness, yarn tension, and fibre undulation or curvature, do the most critical parameters need tight control.
Threads for structural seams must meet more stringent criteria. Their main function is to carry and distribute out-of-plane and/or shear stress. They must be thicker, but many other properties play key roles as well. The most important is the yarn modulus in combination with the resin interface. Carbon sewing threads are the optimum choice. They provide the highest combination of stiffness and ultimate strength compared to glass and aramide yarns.
Manufacturers use special sewing yarns with high stiffness and strength to improve structural performance. Today, only threads of aramide, glass, and carbon meet the requirements. Parameters such as stitch density, seam position, and penetration angle greatly influence material properties. Markus Schneider, Toho Tenax Europe, Germany, reports on the new carbon sewing yarn: “Next to the mechanical properties further physical performances like electrical conductivity or acid/alkali and temperature resistance of carbon fibres can be utilized. In special heating systems, e.g. automotive seat heating elements or heating blankets for medical applications, carbon fibre sewing yarns can be sewed or stitched right into the base material. Carbon-based materials could enable reliability, cost efficiency and operating performance in comparison to commonly used metal yarns.”
The company Toho Tenax Co. Ltd. (Head Office: Bunkyo-ku, Tokyo,), which is engaged in the business of carbon fibres within the Teijin Group, has decided to add a new carbon fibre production line at Toho Tenax Europe GmbH with a target operation start-up date of August 2009. Demand for this product is expected to grow at a rate of about 15% between 2005 and 2010 as applications for civil aircraft and industries increase. Demand in 2010 is anticipated to exceed 40,000 tons.