Crystalline and amorphous orientations in injection molded polyethylene

The techniques of density, birefringence, and wide X‐ray diffraction were employed to characterize the microstructure of injection molded polyethylene parts. Generally, maximum crystallinity (density) occurs at the center of the molding, while the minimum crystallinity occurs near the surface. Higher densities are observed near the gate. Raising the injection temperature tends to cause a marginal increase in the crystallinity throughout the molding. Birefringence measurements suggest that the maximum orientation occurs near the surface and that the relative orientation distribution is independent of the injection temperature. X‐ray diffraction indicates that the crystallographic a ‐axis tends to orient in the flow direction while the b and c axes vary symmetrically about that direction. Increasing the injection temperature creates c ‐axis orientation near the surface, while towards the core region a ‐axis orientation is observed. Generally, near the surface it is the amorphous phase that makes the major contribution to the total orientation as measured by birefringence. Increasing the injection temperature tends to decrease the amorphous phase orientation near the surface. The crystalline phase contribution to the total orientation increases as distance from the surface increases, regardless of injection temperature.