Morphology and thermal expansion in large HDPE injection moldings

ABSTRACT Morphology and linear coefficients of thermal expansion (LCTE) within the wall of a large (10 kg) injection molded container were evaluated. The study employed polarized light microscopic birefringence techniques, differential scanning calorimetry, scanning electron microscopy (SEM), as well as thermal mechanical analysis to determine the LCTE anisotropy in the skin and core of the wall. A difference in crystallinity between skin and core was found, and a region with distinct lamellas was seen under SEM without sample etching. A large variability in anisotropy of the LCTE was found in the relatively thick (~700 μm) skin of the molding. The LCTE differences between skin and core were attributed to molecular orientation related to resin flow. LCTE anisotropy as an important source of residual stress in the transition zone between skin and core was confirmed by fractographic analysis. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47507. Fractographic marks on the crack surface: (1) rib mark, (2) hackles radiating from the crack progression front and progressing towards the surface, (3) tip of the crack in arrest and likely crack progression front, (4) fracture step, and (5) direction of the crack propagation.