Hardness Loss of Plastic Mold Steels: Phenomenon in Injection Molding

The standard application-oriented test for abrasive wear of polymers on plastic mold steels is the platelet wear tester method in which two steel specimens form a thin wear slit. The glass fiber-reinforced polymer melt is injected through the wear slit, and the surface of both specimens is abraded. In the injection molding process, similar conditions often occur in thin-walled parts or film gates. Drastic loss of the hardness of a powder metallurgical steel (PM steel) is discovered after approx. 200 injection cycles while performing platelet wear tests. For experimentally analyzing the reason for the loss of steel hardness during injection molding, a new platelet wear testing apparatus is developed, which can be used to measure the increase in temperature inside the steel specimen while testing. The first results show and support the hypothesis that a temperature increase above the annealing temperature occurs inside the steel due to viscous dissipation at the steel surface. In cooperation with the company voestalpine BoHLER Edelstahl & Co KG, tests on the PM steel in a dilatometer are performed to simulate the cycle-by-cycle heat pulses occurring in the injection molding process. With the results of those tests, it is possible to state a hypothesis for the reason of this hardness loss.