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.

Distribution of Oriented Lamellar Structures in Injection-Molded High-Density Polyethylene Visualized via the Small Angle X-ray Scattering-Computed Tomography Method

“…The S1 region thickness estimated from the peak profile in the contour region was 240 μm < Δ x < 320 μm (as indicated by S1 in Figures a and a). The higher orientation layer induced by the shear force extended over distances of μm from the sample surface, which agreed with the features of the skin layers. ,− ,− ,, …”

“…The higher orientation layer induced by the shear force extended over distances of μm from the sample surface, w h i c h a g r e e d w i t h t h e f e a t u r e s o f t h e s k i n layers. 2,[5][6][7][8][9][10][11][12][13][14][15][16][18][19][20][21][22][23][24]26,27 As shown by SS1-1 and SS1-2 in Figure 3a, we found relatively high-intensity areas in the interior of the skin layer, although the intensity was lower than that of S1. Previous studies reported microscopic images of oriented molecular chains forming layers inside the skin layer.…”

“…The crystalline structures of injection-molded HDPE and their heterogeneity depend on the molding conditions. − For example, the injection speed is one of those conditions . Many studies have reported the thickness of the skin layer.…”