Oct 10 2007
Among top-of-the-range cars, the Audi R8 represents supreme dynamics, cutting-edge technology and exhilarating styling. The mid-engine sports car's extravagant design features include two large air apertures at the rear. The rear bumper was designed using a plastic-based, lightweight solution that had to meet the most stringent requirements, particularly in terms of precision fit and surface quality. The material also had to be extremely tough to ensure that the bumper was resistant to minor collisions.
"In order to meet the complex requirements profile, we joined forces with our partners WAYAND, Quarzwerke and Audi to formulate a solution based on a customized grade of our polyurethane system Bayflex® 180,¡¨ explains Dieter Gaumitz, an expert in polyurethane bodywork parts at Bayer MaterialScience AG. The complete bumper module is supplied by Idar-Oberstein-based WAYAND GMBH Kunststofferzeugnisse, a well-known manufacturer of exterior and interior plastic components for the automotive industry.
The rear of the car consists of three components: the rear panel, which is connected to the retractable rear spoiler; the cover, which incorporates the apertures of the two twin tailpipes and the mounts for the transverse struts of the two air apertures; and the diffuser with integrated rear fog light that extends down to the underbody of the car. The cover and the diffuser are known as the rear bumper assembly. This assembly spans an area of approx. 2 square meters. Because of its size alone, manufacturing it in polyurethane constitutes a major feat of design and processing.
A special ingredient in the Bayflex® 180 material grade used is the finely ground reinforcing material Tremin® 939-304, which is produced for the R8 by Quarzwerke GmbH in Frechen. It helps to produce an excellent component surface and an outstanding paint finish. The material also ensures that the polyurethane system remains extremely tough over a broad temperature range from -30 ¢XC to 150 ¢XC, thus rendering the rear bumper highly resistant to stone impact.
The requirements for precision fit were particularly high in the area of the rear panel and the cover, to allow the design of optically impressive narrow gap dimensions (the "zero-gap look"). "The minimal thermal expansion of Bayflex® 180 proved to be an advantage. It is comparable with that of aluminum," says Gaumitz.
Another strength of the polyurethane system is its outstanding flow behavior, which opens up a wealth of design options. It is possible to produce large, thin-walled components with complex geometries economically using Reinforced Reaction Injection Molding (R-RIM) technology. The wall thicknesses can be varied as required. In the case of the rear bumper and the rear panel (the latter also being produced from Bayflex® 180), they are between 2 and 5 mm near the attachments, otherwise they average around 3.3 mm.
Bayflex® 180 has a heat resistance of more than 150 ¢XC. This high thermal stability allows rear panels, for example, to be fitted very close heat-emitting exhaust systems.
Bayer MaterialScience contributed an extensive range of services to the joint development work with Audi, WAYAND and Quarzwerke. For instance, it determined those polyurethane parameters that are governed by the rate of elongation by conducting high-speed tensile tests on specimens. This enabled the strength, energy absorption and dynamic load resistance of the rear bumper to be calculated using standard simulation programs and the crash behavior to be evaluated reliably and realistically. The service also included mold flow calculations for optimization of the flow behavior and the gate design.