WEB EXCLUSIVE A new method developed at the University of Paderborn in Germany combines injection and blow molding techniques to produce thick-walled parts with adjacent thin-walled zones and very large hollow sections in the thick areas. This concept was presented at last year’s SPE ANTEC conference in Milwaukee by Helmut Potente, Volker Schoppner, and Martin Schafers of the University’s Institute of Plastics Technology (KTP). While the gas injection technique (GIT) can be used to hollow out thick wall sections in injection molding, the authors note that there are limits on how large a hollow section can be produced by this means. They were seeking a way to integrate hollow sections that could serve as cable channels or fluid ducts in a molded part without having to weld or glue together parts that were previously injection or blow molded. The new process, called GITBlow, was discovered by accident through a processing error in gas injection molding. Some melt unintentionally sealed the gas injection point inside the part, so the gas pressure could not be relieved prior to demolding. When the mold was opened, the still-soft plastic containing the pressurized air bubble expanded--and the idea for GITBlow was born. The authors proposed two approaches. The one-stage GITBlow process involves injection molding the part using gas assist to partially hollow out the intended channel. Initially the part is filled with melt, and then a hydraulic mold slide opens up an overflow cavity and the gas is injected. After a holding time, in which the gas applies holding pressure, the pressure is released. Then a moving core enlarges part of the cavity space around the hollow channel, and more gas is injected to expand the still-warm plastic. Gas pressure is held until the part cools and can be demolded. Alternatively, a two-stage GITBlow process uses the same initial gas-assist approach to partially hollow the part--only now the part is cooled and demolded. Then the part “preform” is transferred to a second, larger cavity--perhaps by means of a rotary disk mold. While the mold is still open, a radiant infrared heater moves into the mold area to reheat the gas-conveying channel area of the preform. Then the mold closes, and gas is injected to fully inflate the preform, followed by cooling and demolding of the finished part. +49 (0) 521/60-2451 * www.ktpweb.de
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