Study on rheological behavior of polymer melt flowing through micro-channels considering the wall-slip effect

Micro molding is attracting more attention nowadays and determination of the rheological behavior of the polymer melt within micro structured geometry is considered to be very important for the accurate simulation modeling of micro molding. The lack of commercial equipment is one of the main hurdles in the investigation of micro melt rheology. In this study, the melt viscosity measurement system for PS (polystyrene) melt flowing through a micro-channel was established using a micro-channel mold operated at a mold temperature as high as the melt temperature. From measured pressure drop and volumetric flow rate both the capillary flow model and the slit flow model were used for the calculation of viscosity utilizing Rabinowitsch and Walters corrections. It was found that the measured viscosity values in the test ranges are significantly lower (decreased by a factor of about 1.4-4.1) than those obtained from the traditional capillary rheometer at a melt temperature of 200 degrees C using both the capillary flow model and the slit flow model. As the micro-channel size decreases, the reduction in the viscosity value increases when compared with data obtained from the traditional capillary rheometer. The ratio of slip velocity relative to mean velocity was also found to increase with decreasing size of micro-channels. It seems that wall slip plays a dominant role when melt flows through micro-channels and would result in a greater percentage in apparent viscosity reduction when the size of the micro-channel decreases. In addition, the wall-slip effect becomes more significant as the melt temperature increases. In the present study we emphasize that the rheological behavior of the melt in the microscopic scale is different from that of the macroscopic scale and that current simulation packages are not suitable for micro molding simulation without considering this difference.