A touchscreen made of Pleximid is currently used in the four-zone air conditioning unit of the BMW 5 and 7 Series.
Evonik
Preh has developed tactile surface contours for the air conditioning functions in the centre stack operating system, which BMW offers as a special feature to customers of the BMW 5 and 7 Series. "The surface is slightly indented in places, making it easier to operate", explains Hans Karl Heil, a development engineer for mechanical design at Preh. A membrane made from the thermoplastic Pleximid TT50 acts as a cover for these tactile aids.
Pleximid TT50 is a highly heat-resistant polymethylmethacrylimide (PMMI) from Evonik, which is also used, for example, for light guides in the daytime running lights of modern LED headlights. "Besides its good optical properties, the material also offers very accurate mould surface reproduction, making it ideal for three-dimensional forming, and we can simply injection-mould the component, which is very important in view of the large number of units required by the automotive industry", Heil says.
High heat resistance
Pleximid is also highly resistant to chemicals and should therefore be insensitive to traces of perspiration, cream or food on the hands of people operating the system. Mechanical stress from keys or pockets, for example, will also not pose a problem.
Owing to its very high heat deflection temperature, Pleximid should also withstand internal temperatures that can easily rise to more than 100 degrees Celsius in cars parked in the sun. "Its low rate of thermal expansion was another crucial point when it came to choosing the material, because we bond the touch screen membrane to the silicate glass display behind it. The cover must therefore show very minimal expansion due to temperature fluctuations", Heil says.
The displays on the control panel should be clearly seen even in sunshine, as Dr. Lukas Dössel from Global Product Management PMMI at Evonik explains: "No other plastic with a high heat deflection temperature shows such low birefringence. This means there are virtually no disturbing colour reflections. The display can therefore be read even by people wearing polarised sunglasses."