On the television screen, it looks a lot like two giant tubes squirting out black toothpaste. A pair of reciprocating-screw extruders slides forward side by side on independently controlled platforms over horizontal mold halves. Simultaneously they deposit two 4-lb molten black charges of talc-filled PP, then slide back out of the way. The compression molds then close to form complete, fabric-lined interior door panels for the S-class Mercedes Benz.
This process was shown on a video last fall at the K'98 fair in Dusseldorf. It was developed in Germany by Kannegiesser KMH in order to reuse heavily reinforced and filled scrap. Called Extrusion Transfer Pressing (ETP), the process has been used for several years at two Johnson Controls auto-parts plants in France and Germany and at Ruettgers Automotive in Koengen, Germany. Now ETP is coming to the U.S.
Thinner, lighter parts
Last month, the technology was sold to C.A. Lawton Co., Green Bay, Wis., a maker of compression molding equipment. (Kannegiesser is quitting the plastics market.) Lawton plans to build and sell ETP equipment. Typical applications compete with compression molded GMT (glass-mat thermoplastic) and injection molded talc-filled PP. ETP parts can have thinner walls and lighter weight than injection molded parts, along with less molded-in stress, Kannegiesser says.
The process starts by plasticating glass- or talc-filled scrap from instrument panels, car-seat reinforcements, and interior trim in a reciprocating-screw extruder. Extruders come in 70- to 150-mm diameters and are specially designed for particular types of scrap--20:1 L/D for talc-filled PP, or 30:1 for long-glass-filled PP. For long-glass fiber, the extruders also use a special low-shear screw without a compression zone in order not to break up the fibers.
The proprietary die has a relatively wide opening (especially with long glass) so that material is deposited on the mold at low pressure and without damaging glass fibers. A hydraulic mechanism can change the height and width of the die opening, allowing the extruded charge to be profiled on the fly.
The patented ETP process is not limited to using scrap, but can also process virgin glass-filled pellets or it can compound long-glass fiber in line. Such in-line compounding is performed using a twin-screw extruder that feeds into the reciprocating-screw extruders.