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Panorama roof made of Makrolon

Bayer MaterialScience AG : 23 August, 2004  (Company News)
At the IAA Motor Show in September 2003, the new 'smart forfour' was hailed as an intelligent option in the compact car segment, a car that set many new standards.
It actually came on to the market in April 2004 and highlights another new application for Bayer MaterialScience's Makrolon® 2607 polycarbonate, namely the front section of the panorama roof. The transparent polycarbonate component, which adjoins the rear glass roof section, is about one square meter in size and was developed by freeglass GmbH & Co. KG of Stuttgart, Germany, in cooperation with Bayer MaterialScience and smart.

'The main arguments for making the complete front sunroof module of polycarbonate were its high stone-chip resistance and, above all, its significant weight saving. The entire sub-assembly is about 6 kg lighter than a comparable component made of normal glass. As a result, the vehicle's center of gravity is lower and the driving characteristics of the compact car are improved,' explained Dr. Martin Döbler, a Bayer MaterialScience expert. A further weight saving comes from the rear fixed side windows which, as with earlier smart models, are also made of Makrolon®.

The panorama roof actually consists of two components manufactured in a swivel-platen machine using an economical two-component injection molding process. A scratchproof coating is applied subsequently. Bonding of the polycarbonate roof to the car's steel roof frame was a major engineering challenge because the expansion coefficient of polycarbonate is more than five times that of steel.

'The trick was to apply the adhesive in such a way that it allows the plastic to expand and shrink in the heat and cold without producing any visible unevenness in the surface,' said Frank Schiemann, Key Account Manager at Bayer MaterialScience. To establish the ideal position for the adhesive bead, Bayer MaterialScience carried out extensive CAE simulations that took account, for example, of the elasticity of the adhesive under extreme temperatures.

The rear fixed side windows made of Makrolon® 2607 differ significantly from their predecessors in earlier smart models in that the guide rails for the adjacent moving side windows are now also made of this polycarbonate. The side window and integrated guide rail are injection-molded in one operation. The rubber sealing lip is also molded in place, thereby reducing the amount of work needed in subsequent assembly – another significant cost-cutting factor.

In the view of Bayer's experts, the trend towards polycarbonate glazing in cars will continue, above all because of its weight-saving potential and the fact that it offers much greater design freedom than glass. Another major benefit is the integration potential of plastic parts. Transparent panorama roofs could, for example, include complete antenna modules or sunblinds. The antennas could be concealed in the metal-free black edging of the glazing without causing any shielding problems. As development moves towards complete roof modules of plastic, Bayer MaterialScience is also considering the possibility of integrating some form of lighting or solar cells into the transparent polycarbonate roof.
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