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Battery disconnect switch uses Crastin

Du Pont Engineering Polymers : 10 March, 2004  (Company News)
The injection-moulded housing of the automotive battery disconnect switch (BDS) by Tyco Electronics AMP relies on two DuPont engineering polymers, each contributing different functional characteristics.
DuPont™ Crastin® PBT thermoplastic polyester resin provides stiffness, strength and impact resistance, while DuPont™ Hytrel® thermoplastic polyester elastomer gives the required resilience and elasticity for an integrated button used for re-setting the automatic cut-out.

Both materials have good dielectric properties and they withstand aggressive media, including acids. Due to their chemical similarity, the over-moulding operation produces a tight, secure interface between the two materials.

In an accident, Tyco’s BDS automatically breaks the circuit between the battery and the car’s chassis within a few milliseconds to prevent short-circuits which could spark a fire. On account of the trend in car design to move the battery from the overcrowded motor compartment into the boot, such safety devices are more important than ever, because the connecting cables are longer, so the danger of sparks from damaged cables is greater. Tyco’s BDS are to be found on the Volkswagen Tuareg, the Audi A8 and the Saab 93 models, to mention only a few examples.

“When we were developing the housing, we had to meet a number of conflicting requirements,” says Dr. Karsten Pietsch, product developer at Tyco Electronics. “We wanted thin but strong and rigid walls for the housing; on the other hand, we needed a soft, flexible material to allow the re-set button to be used. Overall, we had to have materials with high thermal resistance and, above all, with the ability to withstand acids, because the battery disconnect switch may be installed right next to the battery’s positive pole.”

Together with Biesterfeld Plastic, distributor of DuPont’s engineering plastics in Germany, Tyco found the two-component housing with the DuPont engineering polymers a reliable and cost-effective solution. Says Pietsch: “We used to fix an EPDM roundel into the housing with adhesive: this was a relatively expensive solution. Now we first mould the flexible re-set button in Hytrel®; together with the view window we then place it into the mould, where both components are overmoulded in Crastin®, which forms the main housing. In the final assembly, the housing cover is moulded onto the base plate, so that, together with the sealed-off cable transit points, the whole unit is splash-resistant to the IP67 standard.”

For the housing Tyco uses glass-reinforced Crastin® SK605, which offers very good electrical insulation characteristics and high arc resistance. It also has substantially better resistance to battery acid than the nylon resin which Tyco originally planned to use for this application. The yellow-pigmented Hytrel® used for the button was specially blended to match the PBT resin; it also meets Tyco’s requirements for acid resistance and at the same time offers the required flexibility for this application. Both materials are easy to process, and their good flowability permits rapid moulding cycles.

Tyco Electronics is a leading manufacturer of electronic components, producing about 400 million modules and relays for all major automotive OEMs worldwide. The company’s Automotive Relays and Electronic Modules product group includes plug-in relays, PCB relays, high-current relays, modules, inductive systems as well as custom-designed solutions for all types of automotive application.
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