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 °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.

Microcellular foam injection molding provides many advantages over conventional foams and their unfoamed counterparts, but its applications are limited by visible surface quality problems such as silver streaks and swirl MICROCELLULAR INJECTION MOLDED PARTSmarks. In this study, we propose a variable mold temperature method to improve the surface quality of molded parts. Electromagnetic induction heating is used in combination with water cooling to achieve rapid mold surface temperature control during the microcellular foam injection molding process. The effect of processing parameters, such as mold temperature, melt temperature, and injection velocity on the part surface quality, was investigated using surface roughness measurements and visual inspection of the molded parts. The results show that using induction heating to increase the mold surface temperature from 100• C to 160• C can decrease surface roughness of polycarbonate moldings from 25 μm to 6.5 μm. It was also found that the flow marks formed by gas bubbles on the part surface can be removed completely at a mold temperature of 160• C. Further increases in the mold temperature show slight improvements in the surface roughness up to 180• C, at which point the surface roughness starts to level off at 5 μm. This surface roughness value reflects an 80% improvement without a significant increase in cycle time over parts molded at a mold temperature of 60• C using water heating. Higher melt temperature and faster injection speed will also improve the surface quality of microcellular injection molded parts but not as significantly. The usefulness of a variable mold temperature in improving part surface quality during microcellular foam injection molding has been successfully demonstrated.

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