The housing of the map-controlled thermostat by Behr Thermot-tronik, of Kornwestheim, Germany, which is installed in BMW`s N43 series four-cylinder petrol motors, is exposed to a combination of very high mechanical, thermal and chemical stresses. To enable this complex part to withstand the application`s demanding requirements dependably over long periods, Behr decided to make the housing out of DuPont Zytel HTN high-performance polyamide.- As the thermostat is right next to the cylinder block, it works in an ambient temperature anywhere between - 40°C to +150°C - says Barbu Frunzetti, project leader at Behr Thermot-tronik. - In addition, the exterior surface is exposed to typical motor-compartment media, such as oils, greases, fuel and road salts.
Inside the thermostat housing, coolant flows at up to 140°C. The return spring`s pretensioning force against the wall of the housing comes on top of the interior pressure of up to 2,5 bar. Moreover, when in operation the whole system is constantly under vibrational stress. Taken together, these parameters exceed the capabilities of conventional polyamides used in under-the-bonnet applications.
Processing requirements further limit the range of plastics which could be used for this part. The housing`s injection-moulded halves have to be welded together in a cost-efficient and reliable manner. The resultant seams must have the strength to withstand burst pressures many times higher than the nominal pressure. In addition, the material must allow other complete plastic parts -connector plugs overmoulded in nylon 66 for the electronic control system - to be hermetically overmoulded.
- In extensive comparative tests with other thermoplastics, DuPont Zytel HTN 51G35HSLR high-performance polyamide met our demanding requirements best - Frunzetti continues. - This polyphthalamide (PPA) reinforced with 35 % glass fibre has the required high static and dynamic strength, the dimensional stability over a broad temperature range essential for this application, high chemical and hydrolysis resistance, good flowability and high compatibility with PA 66. It is also very well vibrationweldable.
