Hot runners refine metal and ceramic injection moulding
Extremely low heat loss with two-stage shaft
Talks with feedstock and plastic parts manufacturers have shown that 80 - 90% of parts produced with MIM and CIM are manufactured by means of a cold runner with a sprue rod. While a large fraction of the sprue rod can be recycled, it would be promising to eliminate this step through the use of hot runner systems.
Both MIM and CIM require a very homogeneous temperature control in the hot runner since the materials have a very small processing window. Variations of the temperature lead to a segregation of the binder and the powder that cause shrinkage differences and the formation of cracks in the component during sintering.
For this application, Günther Heisskanaltechnik recommends its hot runner nozzles developed to meet such higher requirements. The two-stage nozzle shaft guarantees excellent isolation at the forward section of the shaft, which ensures extremely low heat loss between the hot runner nozzle and cavity and very homogeneous heat distribution within the nozzle. In addition, the frozen plastic material around the nozzle forms a "cap", thus providing a thermal separation between hot runner nozzle and cavity
However this feature produces quite the opposite effect for metal filled plastic materials because of the metal powder's conductivity. In this case, the mixture of plastic material and metal powder would draw the heat off the nozzle. That’s why Günther's nozzles used for MIM have special insulating caps made of a highly heat resistant plastics like polyetheretherketone (PEEK) or polyimide (PI) to provide thermal separation. A tubular titanium shaft located around the hot runner nozzle additionally improves the insulating effect.
This is a unique feature of the products from Günther, since most of hot runner nozzles have no two-stage shaft and achieve the sealing by direct metallic contact with the material tube in the mould insert. This leads to very high heat loss which has to be compensated by a higher temperature in the hot runner nozzle. This, in turn, causes and excessive rise of the temperature and temperature variations, so the materials cannot be processed in a reliable way.