In injection molding along the melt conveying system several positions can be determined where just a thin melt film is flowing between steel surfaces. This happens for instance in hot runner nozzles, in between the flight land of the screw and the barrel, in film gates or even in cavities for thin-walled parts.
While testing wear of plastic mold steels with the application-oriented platelet wear tester method in which two steel platelets form a thin wear slit, which is comparable to the tribological situation in plasticizing units and molds, the phenomenon of hardness loss was first discovered. Since then, in ongoing research the hypotheses was stated that viscous dissipation leads to a severe rise in temperature that can exceed the secondary hardening temperature of the steel and therefore is responsible for the loss of hardness. With a new testing apparatus, it was possible to measure temperatures below the steel surface higher than 590°C with a PA66 reinforced with 50 wt.% glass fibers. Even for unfilled PA66 a steel temperature of 570°C was detected in a distance of 0.5 mm below the surface of the steel specimen.
With a dilatometer, temperature tests were performed on steel specimens that simulate the dissipation-dependent heat influx during an injection cycle in injection molding. These tests underline the conditions that have to be met that this “loss-of-hardness phenomenon” occurs. Furthermore, an in-depth investigation of the reasons for the hardness loss has been carried out leading to a morphological hypothesis.
Due to the high steel temperatures detected a thermal degradation of the PA66 melt is expected. Therefore, the molecular weight distribution (MWD) as a measure for the polymer degradation was analyzed for different testing conditions and compared with the MWD of the granulate.