A curious hybrid of twin-screw compounding and injection molding is proving its commercial viability for molding structural automotive parts of direct-compounded long-fiber thermoplastic (D-LFT) composites. The latest proof is the front-end carrier of the 2003 Volkswagen Golf V, injection molded of 30% long-glass-filled polypropylene by RKT Kunst­stoffe, part of Aksys Group, in Kongen, Germany. This 7.5-lb part is the third commercial application of the IMC (injection molding-compounding) process of Krauss-Maffei (U.S. office is in Florence, Ky.).
This system piggybacks a twin-screw compounding extruder on top of an injection machine with a special accumulator and a plunger injector. The accumulator allows the extruder to run continuously so as to produce more homogeneous melt quality than with conventional start-and-stop plasticating, in K-M’s view. The extruder takes in continuous glass roving and chops it while wetting it with already molten resin. As compared with conventional injection molding of long-fiber pellets, this process saves one heat history and is said to subject the glass to less shear, preserving fiber length. IMC is also said to be more suitable for heat-sensitive materials such as natural fibers. Although the machine system costs a good deal more—over $500,000 extra, including the material-handling and gravimetric feeding systems—savings in material cost are said to be 25¢ to 30¢/lb vs. LFT pellets.
Krauss-Maffei has delivered 10 of these IMC systems to European molders and two more to European university labs. Two more are on order for RKT plants in Germany and Mexico. One system is used in the U.S. for a non-automotive application. It has a plasticating capacity of more than 5000 lb/hr of PP or HDPE. The largest press in commercial operation is 2000 metric tons, though K-M has a 2700-tonner its own lab. The VW front-end carrier is molded on a 1300-ton press.
The first commercial use of IMC was to produce the front-end carrier for the Citroen C3, introduced in late 2001. The second application came early last year—a similar part for the Peugeot 307.
| 
