Crystal orientation in injection molding of talc‐filled polypropylene

The crystal orientation in injection molding of talc‐filled polypropylene has been studied by means of polarizing microscopy and X‐ray diffraction measurements. The plate planes of talc particles are aligned parallel to the surface of injection molding. The c‐ and a*‐axes of polypropylene crystals are bimodally oriented to the flow direction, and the b‐axes are oriented to the thickness direction. Namely, the crystals in the injection molding of talc‐filled polypropylene show a kind of uniplanar‐axial orientation according to the classification of Heffelfinger and Burton. The degree of the bimodal orientation of c‐ and a*‐axes decreases toward the interior of the injection molding. The b‐axis orientation is strong throughout the thickness direction, although it is a little weaker at the surface skin and central regions. This peculiar crystal orientation originates from the flaky shape and crystallization nucleation ability of talc particles. In a mold cavity, talc particles are aligned parallel to the cavity wall by the action of flow, and crystallization progresses, with the (040) plane of polypropylene crystal piling on the plate planes of talc particles and the b‐axis, which is the growth direction of polypropylene crystal, being aligned to the thickness and cooling direction of injection molding. The crystal orientation in injection molding of unfilled polypropylene is basically similar to that of talc‐filled polypropylene, although the b‐axis orientation to the thickness direction is much weaker.