Experimental and Numerical Investigation of the Flow Behavior in a Tapered Rectangular Sprue

The present study is part of a continuing effort to obtain a better understanding of the rheological behavior during the injection molding of unfilled and reinforced polymers and help improve the prediction by numerical three dimensional simulation of the process. The slightly tapered rectangular sprue of a centrally gated plaque mold was equipped with flush mounted pressure sensors to monitor the time evolution of the wall shear stress prior to entering the cavity. Two Polycarbonate with different zero-shear rate viscosities were tested at various injection speeds. Atter an initial rise, the wall shear stress in the sprue remains constant during the filling stage of the mold. The transient shear viscosity was determined from the known volumetric rate using a simplified one-dimensional flow approach and compared to the viscosity measured by traditional off-line rheometers. A finite element three-dimensional code is used to simulate the flow in the sprue with small and large width over thickness ratios. The pressures predicted are used in combination with the simplified theory to calculate the viscosity and compare the results from the experiments.