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Materials group Bayer is claiming a record with the largest ever automotive roof module made from polycarbonate.

The roof of the Smart fortwo is made from Makrolon AG2677 and has an area of around 1.2 square metres. It is manufactured by Webasto.

Bayer says the weight is 40 percent less than a comparable glass module. This not only helps to save fuel, it also means that the vehicle’s centre of gravity can be placed as low as possible, improving handling and agility, smart’s Detlev Penczek said.

Sven Gestermann of Bayer explains: “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.”

Webasto makes the roof module by the two-component injection-compression moulding process on a swivel-platen injection-moulding 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 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 behaviour, which is geared to that of the PC material.

Bayer Material Science and Webasto have been working together for many years on panoramic polycarbonate roofs.

Limits to the size of polycarbonate roof modules have still not been reached, says Gestermann: “Tests at our company have shown that, with present-day materials and modern injection-compression moulding tools and machinery, low-stress roofs with an area of up to 1.7 square metres are quite feasible. We are thus keeping up with the current trend towards large-area panoramic roofs.”