The new "smart fortwo" epitomizes urban mobility and, with its low fuel consumption and low CO2 emission levels, is a shining example of environmental friendliness. One of the car's highlights is its lightweight roof module made of Makrolon® AG2677, a polycarbonate (PC) from Bayer MaterialScience AG that is tailor-made for automotive glazing. "With an area of around 1.2 square meters, this is to date the largest polycarbonate roof module fitted in a production-line vehicle anywhere in the world," says Dr. Sven Gestermann, Key Account Manager Automotive Glazing for Europe at Bayer MaterialScience. The plastic module is manufactured by Webasto AG, which is based in Stockdorf near Munich and is one of the world's leading manufacturers of roof modules and sunroofs for the automotive industry.
Webasto makes the roof module by the two-component injection-compression molding process on a swivel-platen injection-molding machine, followed by a coating step. In the first shot, the transparent outer skin is produced from Makrolon® AG2677, and in the second, the large-area frame is made of Bayblend® DP T95 MF. This black-pigmented blend of PC and ABS (PC+ABS) was developed by Bayer MaterialScience in conjunction with Webasto specifically as the second component for this application, and is tailor-made for large-area glazing parts made of Makrolon® AG2677. One of the blend’s particular strengths is its shrinkage behavior, which is geared to that of the PC material. "Despite its size, the roof module can be manufactured with very low internal stresses and low warpage. This is also one of the reasons for its excellent surface quality," explains Gestermann.
Because the roof element is made of plastic, the weight saving is considerable - in fact, it weighs over 40 percent less than a comparable glass module. This not only helps to save fuel, it also means that the vehicle’s center of gravity can be placed as low as possible, thereby improving its handling and agility. Reflecting on the early stages of the project, smart’s Detlev Penczek, who is responsible for the panoramic roof, said: "The weight saving, the design freedom and the possibility of integrating functions like fixing elements into the vehicle were the most important factors in our decision to make the roof of break-resistant polycarbonate."
Bayer MaterialScience and Webasto have been working together for many years on the development of panoramic polycarbonate roofs. "With this project, it was again an enormous advantage for us to be able to call on the wide-ranging expertise that Bayer MaterialScience has in the fields of materials, processing and analytics," comments Martin Pollak, head of roof module development at Webasto. The service provided by Bayer MaterialScience extended the full length of the process chain. For example, rheological calculations were used to determine an optimum gating design, and mechanical calculations served to simulate the static and dynamic behavior of the part. Experts from the plastics manufacturer were also on hand to help with mold development and the start-up of the production line.
As far as the size of polycarbonate roof modules is concerned, the full potential has still not been realized. "Tests at our company have shown that, with present-day materials and modern injection-compression molding tools and machinery, low-stress roofs with an area of up to 1.7 square meters are quite feasible. We are thus keeping up with the current trend towards large-area panoramic roofs," says Gestermann. He also expects future roof module designs to make even greater use of the shaping possibilities and integration potential of Makrolon® AG2677 and Bayblend® DP T95 MF to save costs and simplify the overall assembly. Gestermann again: "We are considering, among other things, the integration of roof spoilers, high-level lights, and components of the roof rail and water management system."