Evaluating mechanical anisotropy of injection molded polymer products using short beam shear testing

Abstract

Thermoplastics, which are used in widely various applications from consumer goods to automobiles, are lighter than metals and ceramics and have excellent moldability. Injection molding is often used as a thermoplastic molding process because it can achieve near-net-shape molding and because it has excellent mass production capabilities. Injection molded products have a hierarchical structure, with the surface area contacting the mold designated as the skin layer and the interior of the molded product broadly identified as the core layer. The molecular orientation formed by the melt molding process is strongly dependent on the flow direction. Quantifying anisotropy is extremely important when discussing the mechanical properties of thermoplastic molded products. This study clarifies whether the mechanical anisotropy of polymer injection molded products can be evaluated quantitatively using short-beam shear testing. This study also elucidates the quantitative significance of the quantitative values.Results show that the shear stress components in each direction obtained from the short-beam shear test are components of the yield initiation shear stress under uniaxial tensile loading resolved in each direction. To quantify the degree of stress concentration attributable to the internal structure of the molded product, the ratio of the yield initiation shear stress to the shear stress components in each direction was divided by √3. Comparison of the degree of stress concentration with the shear stress components generated in the short beam shear test showed the thickness direction as weakest against shear stress and showed the width direction as the strongest.