Test methodRheological behavior of PS polymer melt under ultra high speed injection molding

Determination of the rheological behavior of a polymer melt under high shear rate is generally considered to be crucial for accurate simulation modeling of ultra high speed injection molding, a process that has been attracting more attention from researchers. In this study, a melt viscosity measurement system under high shear rate was established using an instrumented injection molding machine combined with a slit die. Polystyrene melt was studied. From measured pressure drop and volumetric flow rate, the slit flow model was used for the calculation of viscosity at wall shear strain rates up to 105 s−1. Good agreement in shear viscosity was found between results from conventional capillary rheometry and in-line measurement made using an injection molding machine for strain rates of 7 × 103 to 104 s−1. Measured shear viscosity in the test range was found to follow shear thinning power law behavior at shear rates below 7 × 104 s−1. However, the measured viscosity values are significantly lower (28%-46%) than the database obtained from the Moldflow package software (Cross-WLF model) at melt temperatures of 200, 215, and 230 °C. Further, the reduction in the viscosity value increases as the injection speed increases. It seems that the wall-slip effect plays a dominant role, becoming more significant as the melt temperature increases. Alternatively, as shear strain rates approached or exceeded 7 × 104 s−1, shear viscosity deviations from shear thinning behavior were observed and reached a rate-independent plateau. The behavior is thought to be influenced by pressure effects on viscosity. In the present study we emphasize that the measured results are significant for the design and simulation of ultra high speed injection processes such as micromolding and thin-walled injection molding.