Big roof

With an area of around 1.2 square metres, Smartcar's new  'smart for two'  boasts the largest polycarbonate roof module fitted in a production-line vehicle anywhere in the world.

The lightweight roof module was manufactured by Webasto and made from Makrolon AG2677, a polycarbonate (PC) from Bayer MaterialScience that is tailor-made for automotive glazing.

Webasto makes the roof module by a 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 (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 behaviour, 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,' explained Dr. Sven Gestermann, Key Account Manager Automotive Glazing for Europe at Bayer MaterialScience.

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 centre 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.'

As far as the size of polycarbonate roof modules is concerned, the full potential has still not been realised. '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 metres are quite feasible. We are thus keeping up with the current trend towards large-area panoramic roofs,' said 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.

'We are considering, among other things, the integration of roof spoilers, high-level lights, and components of the roof rail and water management system,' Gestermann said.