Orientation Formation in Planar Mold Filling: Experimental Results

The formation of orientation field of short fibers suspended in a highly viscous flow through a planar mold cavity is experimentally analyzed. Such flows are common in injection molding of short-fiber-reinforced composite materials. A suspension of corn syrup and nylon fibers is injected at a constant flow rate through a narrow planar inlet gate into an experimental mold cavity. The flow undergoes a sudden expansion near the inlet gate, followed by a three to one contraction downstream. Photographs of thirteen zones of interest in the vicinity of the sudden contraction are taken through transparent mold walls after the flow achieved steady conditions. Computerized image analysis is performed to obtain orientation data for all the fibers within the zones of interest. This data is used to calculate a through the thickness average of the second-order orientation tensor, which is commonly used to quantify orientation field. The experimental results are qualitatively consistent with numerical predictions based on Jeffery’s theory, but quantitative agreement is not satisfactory. Orientation distribution histograms are generated to provide a more detailed representation of the orientation field. The histograms reveal a bimodal distribution, with an alignment peak along the direction of the theoretically calculated preferred orientation, and a second peak perpendicular to the flow direction. The failure of the second-order orientation tensors to quantitatively describe the experimental data seems to be due to these bimodal distributions. Radial orientation histograms at five zones of interest are presented along with the theoretical predictions at these locations.