Microstructure and the distribution of tensile properties in injection molded polyethylene

Studies conducted by many workers in recent years have indicated substantial anisotropies in injection molded thermoplastics. These anisotropies have been associated with differences in morphology (crystallinity) and orientation (shrinkage and birefringences) at various positions in the molding. They are reflected by anisotropies in tensile properties measured at different points or in different directions. However, no work has been reported to quantitatively relate the distributions in tensile properties. The present work represents an attempt to meet this challenge.Microstructure determination is achieved by making the following measurements on microtomed specimens at different positions in the polyethylene moldings: density, sonic speed, birefringence, wide-angle X-ray diffraction, dichroic ratio, and polarized light photomicrography. The above tests yield data relating to crystallinity and orientation functions for both the crystalline and amorphous phases. They also yield sufficient data to check the consistency of results obtained by the different techniques. Tensile properties are measured on microtomed samples obtained from the same regions tested for microstructure. Finally, the distributions of tensile and sonic moduli in the molding are estimated by employing some of the theoretical techniques proposed in the literature. The results show very good agreement between theoretical predictions and experimental data relating to the distributions of both sonic moduli and tensile moduli