Online analysis of melt viscosity during injection moulding with a hot runner rheometer

Abstract

Injection moulding is one of the most productive methods in plastics processing allowing the production of parts with complex geometries in a single step. To stay competitive, part quality and its reproducibility remain a major issue for injection moulding manufacturers. Since modern machine technology allows sufficient reproducibility of machine variables like axis movements and pressures within the plasticising unit, the compensation of inevitable disturbances gains importance. Differences in the residual moisture of the plastics materials, the use of reground material, batch variations or interruptions in the production process may cause fluctuations in melt viscosity. These fluctuations affect the pressure distribution within the cavity resulting in warpage or even short shots or flash formation.In this paper, a mould using pressure sensors within the hot runner manifold is used to analyse melt viscosity fluctuations during the injection moulding process. Similar to the principle of a high pressure capillary rheometer (HPCR), the viscosity can be analysed based on the pressure loss between two sensors. In contrast to steady rheometers, the transience of the injection moulding process has to be considered. Therefore, different indicators are analysed regarding their ability to distinguish polypropylene (PP) grades with different flowability. Finally, an indicator for the current melt viscosity is selected which enables an inline control of melt viscosity during injection moulding.