Two-Material Automotive Part Is Cored Out with Water

A three-stage injection process involving nylon, polypropylene, and water assist was demonstrated at the recent K 2004 show in Dusseldorf, Germany. The part, an engine cooling pipe, illustrated the growing sophistication of water injection and has aroused considerable interest among European auto makers. Similar parts reportedly are being evaluated at Volkswagen, Audi, and DaimlerChrysler.

Development of this 35-mm O.D. pipe involved molder and moldmaker Polytec Thermoplast GmbH of Idstein, Germany; material supplier A. Schulman; water-injection specialist PME fluidtec GmbH; and machine builder Krauss-Maffei.

A. Schulman supplied the 30% glass-filled nylon 66 for the outer skin of the pipe and the PP inner liner. Nylon provides resistance to under-hood temperatures, while PP provides a smoother inner surface and better resistance to hydrolysis by water/glycol engine coolant. Drawing on its experience in compounding nylon/PP alloys, Schulman modified both materials for better adhesion to each other.

PME fluidtec in Germany supplies a pump system that controls the timing, pressure, and volume of water injection. It’s newly available in North America from Husky and Krauss-Maffei. As shown in the schematic, this part was molded with a short-shot process that avoided the need for spillover cavities or melt pushback. Approximately 70% of the cavity was filled—first 40% with nylon and then 20% to 30% with PP—before water was injected to core out the part and cool it from the inside. The press was a 220-ton KM200-1000/3902 Multinject machine and cycle time was 48 sec.