The orientation in the injection molding of high density polyethylene is affected by temperature, each molding cycle time and pressure. Particularly, at the filling stage, the stress results in molecular orientation. Further orientation may occur due to additional inflow of material and the inertial flow of melt elements. Therefore, injection molded articles inevitably contain frozen strains. In consequence, they possess anisotropic mechanical and optical properties.
In the present work, in order to see the temperature dependance of the orientation, the measurements of birefringence and X-ray diffraction were performed. At the same time, the packing effect was also investigated by varying the plunger forward time.
As was expected, the orientation in the gate area is much greater than the orientation in the end area. It was found that the birefringence pattern shows sharp peak at surface area, and the magnitude of the birefringence value observed for polyethylene is higher than that observed for polystyrene. Increasing the melt temperature, the magnitude of birefringence decrease slightly. On the other hand, raising the mold temperature, the magnitude of birefringence is unchanged. From X-ray diffraction results, the degree of crystallite orientation, as represented by the crystallographic a-axis, increases as the distance from the surface increases. The a-axis distribution appears to be independent of axial distance from the gate.
