Gas-Assist and Blow Moulding Combined in an Injection Mould
A new processing technique combines gas-assisted injection moulding with blow moulding. The technique permits moulding of complex plastic parts that integrate a void or channel with a large cross section and thin walls.
In the February issue of Kunststoffe, Hans-Peter Heim and Helmut Ridder from the Institute for Plastics Technology in Paderborn (KTP) describe a novel processing method for plastics in which gas-assisted injection moulding is combined with blow moulding. Using this technique, large functional voids (e.g. for cable conduits or flow channels) with thin walls can be integrated into complexly shaped plastic components. This eliminates the need for involved, multi-step operations such as welding or adhesive bonding.
In the first step, a conventional gas-assist part is produced. Then, the mould cavity is enlarge significantly. In the next step, the gas pressure is increased once again, with the result that, by “inflating“ the moulded part, an extremely thin-walled functional channel forms.
For the inflation, or blow moulding, step, the temperature of the moulded part must lie in the thermoelastic or thermoplastic range for the resin being processed. The researchers at the KTP describe two methods for satisfying this condition. In the so-called direct method, the heat retained in the moulded part produced via the gas-assist process is utilised to permit the blow moulding step by immediately increasing the size of the mould cavity. As an alternative to this approach, in the two-stage method, the moulded part can be reheated in the open mould through use of a radiant heater, for instance.
A series of experiments has confirmed that, with this new processing technique, voids and channels with very thin walls can, indeed, be produced. When utilising the direct GITBlow method, a shorter cooling time is also achieved as a result of both the thinner walls and the better heat removal from the enlarged mould cavity.
The technique permits large cross sections in channnels and voids, along with a uniform wall thickness distribution. Attachment of thin-walled functional channels to delicate sections and complex geometries opens up a wide spectrum of possible applications. The elimination of secondary operations provides clear cost benefits. The University of Paderborn filed a patent application for the process in May 2004.