Among the most interesting developments is a derivative of a 30-year-old technology, transfer molding, which was originally used for multicavity molding of thermoset rubber parts. Now Injection Transfer Molding (ITM) is making its debut to process large numbers of small, equal-sized thermoplastics parts, says researcher Martin Koch.
The near-sprueless technology is seen as an alternative to conventional injection molding—and its more expensive hot runner systems—but more as a complement than a true competitor.
ITM uses a type of shooting pot (a "transfer chamber," as Koch calls it) to fill multiple cavities under identical pressure. As the diagrams show, the chamber has a heated piston fixed to the stationary platen with a melt channel bored through it. The chamber itself is attached to the mold via the insulating plate.
The plasticating screw moves forward in the normal manner to push material through the melt channel to the front of the piston. The melt is then transferred to the cavities when the chamber is pushed by the piston in the clamping unit. Melt is prevented from back-filling by a non-return valve at the front of the piston. The melt is subjected to 25% lower shear levels during mold filling than with conventional injection molding because the flow path is so short.
Holding pressure is applied by movement of the mold relative to the piston, rather than (as in conventional injection molding) by the plasticating screw. Following the holding pressure phase, the mold, still closed, moves back to its original position, thereby opening the transfer chamber for melt refilling, while the mold halves separate, allowing the parts to drop from the mold.
The IKV has developed the ITM technology so that the plastic stays molten up to a point directly in front of the cavities, without using extra heating elements or hot runners. It has been working with a modified-for-ITM mold on an Arburg Allrounder 320S to process sprueless PP cross-spacers used to center ceramic tiles.
Koch says that despite advantages, however, ITM can''t be used universally. All the cavities need to be identical, with parts no larger than 20 by 20 mm. Also, while ITM appears to be polymer-independent, a temperature difference across the mold of more than 250 deg C could limit the effective thermal insulation between the hot and cold sides.
Most important, part weight in the ITM system is less accurate than that of injection molding; therefore, the process is suitable only for production of parts whose consistency is not essential.
Similar to IKV''s reinforcement of plastics with low-melt-temperature glass (May 2002 MP/MPI), researcher Maria Holzel has found a way to produce thermoplastic compounds with defined electrical and thermal properties using blends of low-melting metals (tin, zinc, and bismuth), which melt with the polymer during molding. This technology can be used in place of metal fibers where problems are caused by their anisotropic nature. Difficulties incorporating the metals arise mainly from their very low melt viscosities compared to polymers''. By optimizing screw configuration with different mixing zones, fine metal particles were incorporated throughout the melt in a twin-screw extruder.
Along with Aircabin (Lampertheim, Germany), a division of Airbus Deutschland (Bremen), and the German Ministry of Education and Research (BMWA; Berlin), the IVK developed a method showing high potential to reduce aerospace production costs through use of foamed thermosets for such applications as overhead luggage bin lids, typically made of honeycombed aluminum. Unreinforced thermosets like phenolics generally provide high fire retardancy but poor mechanical properties. Michael Schlumm developed a way to add glass or plastic microbeads during foaming of phenolic to improve strength and rigidity.
Robert Colvin [email protected]
|Institute of Plastics Processing||www.ikv-aachen.de|