Case Study Hot runner system eliminates waste, increases productivity
When Weha was looking to eliminate a series of drawbacks involving the manufacture of a sealing ring, the company turned to Heitec Heisskanaltechnik. The experts developed a mould concept using hot runner nozzles for side gating. By Bastian Schreck, Heitec.
Weha-Gummiwaren-Fabrik Holzberg, a plastics processing company and manufacturer of elastomers and thermoplastics located in Berlin, Germany, was looking for a mould concept that made it possible to inject a high-precision sealing ring from the inside with six injection points to eliminate the unavoidable production of waste in the form of a cold runner rest. Heitec Heisskanaltechnik suggested using hot runner nozzles for side gating, because the nozzles are, due to their small size, least likely to interfere with the mould concept.
Optimising the mould and process parameters
In the past there were a series of drawbacks evident during the manufacture of products like the sealing ring, which could not be avoided due to technical reasons. These included the insufficient concentricity, radial run-out, cold cracks and the presence of air pockets in the plastic parts. One difficulty that arises from the use of cold runners is that it is almost impossible to influence the filling behaviour of the parts. Another problem that often emerges with similar applications is that the air remains inside the cold runner and is forced ahead of the melt until it is pushed through the bores and into the cavities.
All the problems mentioned can more or less successfully be eliminated or at least minimised by optimising the mould, the process parameters and the technical fine tuning. The biggest potential for improvement, especially if the economic aspect of the production process is taken into account, can be found in the waste of material that results in the accumulation of sprue. This large quantity of wasted material is not acceptable for both economic and ecological reasons. In the present case of the high precision sealing ring, the sprue spider, weighing 14 g, takes more than twice as much material as the ring itself, which weighs only 6.25 g.
The sealing ring has a diameter of 200 mm, a height of 8 mm and a wall thickness of 1 mm at a shot weight of 6.25 g. The tolerances demanded by the customer are very tight because the size accuracy has to guarantee the leak tightness and accuracy of location of the follow-up parts. The injection point of the plastic parts should not project beyond the contour because this would affect the functionality of the ring. The junction that cannot be eliminated due to physical reasons should not at all influence the stability of the ring by its notch effect. The material used for the production of the sealing ring is PA with 35% glass fibre.