Relating microhardness to injection molding induced morphology: A study of microbeam synchrotron WAXD

Abstract

ABSTRACT:The relationship between microhardness and morphology of polypropylene near the surface of injection molded plaques was investigated. Crystal structures and molecular orientation from the surface to shear oriented layer were characterized by microbeam synchrotron wide angle X-ray diffraction, polarizing optical microscope and Fourier transformed infrared spectroscopy. From precise measurements, using an X-ray beam with width of 0.35 lm, the frozen layer was divided into two layers. Molecular and crystal orientations changed greatly in the surface layer but remained constant in the inner layer. These oriented morphology did not affect microhardness along the depth direction on the cross-cut surface. Microhardness was proportional to crystallinity and showed a local maximum at shear oriented layer and a minimum at the surface. Then, positional dependence of microhardness was compared at the gate and the center of the plaque. Although crystallinity near the gate was lower than that at the center position, microhardness was higher. Therefore, crystallinity mainly affects microharness but superstructure like cross-hatching lamellae which is induced in biaxially extended melt flow is also not neglected in injection molded polypropylene parts.

Viscoelastic flow analysis of surface morphology on injection‐molded polypropylene

“…Possible reasons of this difference are (a) contribution of crystal orientation and crystallinity, (b) unsuitable assumption of frozen temperature at 160°C, and (c) small Δσ by numerical calculation. From our previous study, crystallinity and crystal orientation near the surface were lower than those inside a plaque [1]. The planar extensional viscosity calculated in Fig.…”

“…Corresponding to such a change, morphology at the surface and mechanical properties should be changed in injection‐molded specimens. We have been dealing with the relationship between injection molding‐induced morphology and properties at the surface [1, 2]. The objective of this study is to understand the formation mechanism of frozen‐in orientation in injection‐molded PP.…”

Crystallization of polypropylene near the surface in injection‐molded plaques: A comparison of morphology and a numerical analysis

“…The skin morphology is different from that in other locations because moving molten PP is immediately frozen at the cold cavity wall. In our previous work [4,5], the microhardness and shear stress had their minima at the surface in the depth direction of the injection-molded plaques. Possible causes are low crystalllinity and anisotropy of morphology.…”

“…9. Although the orientation of the mesomorphic form crystal is usually ignored, as in our previous study [4], we included the diffraction of the mesomorphic and α-form crystal in the meridional maxima. The intensity distributions of the diffraction spot on β-NAPP were broad compared to the others.…”

“…Our purpose in this study was to clarify the skin morphology formation in the [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] μm depth region by a WAXD analysis of three types of nucleated PP and by an original 2-D viscoelastic flow simulation program. Birefringence with a polarizing optical microscope (POM) and micro-beam synchrotron WAXD were utilized for evaluating injection-molding-induced morphology.…”