Injection molding of glass flake reinforced polypropylene: Flake orientation and stiffness

Abstract

Three samples of silane treated glass flakes of different diameters were dry blended with polypropylene powder and injection molded into rectangular (32 mm X 127 mm) plaques using a n edge-gated mold cavity. The thicknesses of plaques were 1.6 mm, 3.2 mm, and 6.4 mm. Tensile and flexural specimens were machined from these plaques. Average flake diameters and thicknesses were determined. It was found that aspect ratios in finished moldings are quite similar, despite the initial (before processing) differences. The flake orientation varies across the thickness; it is parallel to the plane of the molding in the outer skin layer, changing gradually to perpendicular in the core. The relative thickness of the skin where the flake orientation is parallel increases with the decreasing thickness and flake concentration. It represents about 85% of the overall thickness in 1.6 mm moldings, between 70% and 85% when the plaque thickness is 3.2 mm, and between 50% and 60% in the thickest (6.4 mm) molding. Elastic properties can be interpreted using the modified rule of mixtures. Tensile moduli depend strongly on the flake orientation in the core and on the flake concentration, whereas the influence of the core on flexural moduli is insignificant. The flake orientation coefficients determined from micrographs are in good agreement with those calculated from mechanical tests. The coefficient accounting for finite flake aspect ratio, qL, was found to be about 0.3.

Numerical prediction of flake orientation and surface color in injection molding of flake‐pigmented thermoplastics

“…Thus a 11 increases drastically as one approach to the core layer ( z < 0.2) from the wall. This result of a sandwich structure of the flake orientation is consistent with experimental observation by Bouti et al [10].…”