JEC COMPOSITES MAGAZINE - Issue #45 - November/December 2008 - 70

KNOW-HOW Processing Hylid forming: an innovation in rubber press forming thermoplastic composites This paper describes the first development steps for a new manufacturing process to press form thermoplastic composite products. In contrast to most other press processes, this method makes use of a collection of rubber particles instead of a solid rubber stamp to form the product in the mould. Such a collection of rubber particles under pressure can be considered as a fluid, in effect making this process more related to hydro-forming than to rubber press forming. Because the method is quite costly and relies heavily on craftsmanship, many companies choose to use the matched metal forming process, involving the use of two milled metal moulds. To achieve the required reconsolidation pressure on every area of the product, the pressing forces are increased. This forming process is not optimal, but it works. Notwithstanding the fact that the start-up time is much shorter with this process, a lot of work has to be done before the actual press process can start. First, the exact shape of the metal stamp has to be calculated. As the fabric does not have a uniform thickness due to fibre shearing, the stamp cannot be drawn in a CAD program with a simple offset function with respect to the product mould drawings. The fact that fibres are a rigid material does not facilitate the engineering work, because this calls for an even more precisely shaped metal stamp. When a metal stamp is not perfectly engineered, it will either be damaged by the fabric, or it will not give a qualitatively acceptable product. For this reason the method is used to produce relatively simple thermoplastic composite products, just like with the rubber forming process. 1- DIETER DECOSTER, SIRRIS, 2- VALERIA ANTONELLI, 3- ROELOF MARISSEN, 4- ADRIAAN BEUKERS, TU DELFT, AEROSPACE ENGINEERING, DESIGN AND PRODUCTION OF COMPOSITE STRUCTURES ecause it can create the homogeneous pressure distribution that is needed during the reconsolidation phase, rubber press forming always seemed to be the most suitable process to form thermoplastic composite products. In practice, however, using this process to manufacture thermoplastic composite products involves a number of problems, prompting many companies to revert to matched metal die forming processes. The biggest difficulties with rubber press forming are the lack of knowledge regarding rubber behaviour and rubber handling problems. An example is presented in a paper by Antonelli [1]. Making a new rubber stamp is more or less a trial and error method. Usually, the metal product mould is first waxed to simulate the thickness of the composite product. Then the rubber is mixed, degassed and poured into the waxed product mould. When the rubber has hardened, the rubber stamp is attached to a press and test cycles are done. The thermoplastic products obtained during the test cycles are inspected and the rubber stamp is modified manually between each test until it presses quality products. A brief survey of companies using the rubber press forming process showed that this test phase ranges from 2 to 5 cycles for a wellknown product type, and up to 20 cycles for a totally new product. Hylid forming Despite the current problems with the rubber press forming process, it is still believed that rubber is the most suitable material for press forming thermoplastic composites, mainly due to the ability of rubber to allow some deviations in the material thickness. But before rubber dies can be used broadly in the industry, the long start-up time and subsequent high costs currently associated with the rubber press forming process must be brought back to a minimum. A new press technique was recently developed by the “Design and Production of Composite Structures” department at the Delft University of Technology’s Faculty of Aerospace Engineering [2]. This patent-pending process is a whole new approach to the way sheet material is pressed into a product mould: Instead of the solid stamp used in most press processes, jec composites magazine / No45 November-December 2008

JEC COMPOSITES MAGAZINE - Issue #45 - November/December 2008

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #45 - November/December 2008