Automotive relay box presents a challenge

An automotive relay box with a complex design has been produced using DSM’s Arnite TV4 220, a 10% glass reinforced thermoplastic polyester (PBT) grade. The box was designed by Siemens Bordnetze for new multi-purpose vehicles.

The relay box is designed to contain all the car’s relays and fuses, such as those for the starter motor, engine management system, windscreen wipers and the central locking. Measuring 200mm by 150mm by 150mm, the box consists of five parts which are injection moulded by different moulders. Relays, fuses and cabling are fitted to these parts, before they are assembled into the complete relay box. The positions for the relays had to be accurate to within 0.2mm to provide a tight fit and prevent rattling.

The project threw up a number of design challenges, most notably the requirement to combine high stiffness and dimensional accuracy for the box with the use of elastic snap fittings for the relays themselves. In addition, time to market was critical, with the first 10,000 prototypes required in less than a year. To meet the design challenge a special grade of Arnite was developed which exhibits excellent stiffness and strength, due to the 10% GF reinforcement. This helps the box retain its shape under varying conditions, while allowing the use of elastic snap fittings for the relays. The good flow behaviour, the relatively low parallel and perpendicular shrinkage, and the elongation at break of TV4 220 are said to have resulted in a relay box with low warpage and high dimensional stability.

DSM Engineering Plastics Tel: +31 46 477 39 65