Glass fiber‐filled thermoplastics. II. Cavity filling and fiber orientation in injection molding

Mold filling of a rectangular cavity of three different thick nesses fed from a reservoir is studied for unfilled and glass fiber‐filled polypropylene and polystyrene. The shapes of flow fronts studied by short‐shots are affected predominantly by the thickness of the cavity with other parameters playing a less important role. Pressure drop versus volumetric flow rate inside the thinnest cavity is studied experimentally and predictions are made from a computer simulation of mold filling. The orientation of fibers in the cavity is examined using a reflect‐type microscope and the orientation is found to depend on cavity thickness, melt temperature, fiber content, and to a lesser extent, on volumetric flow rate. In the thinnest cavity, where the flow is quasi‐unidirectional, the fibers remain in the plane of flow oriented either along the flow direction or perpendicular to it, except in the region near the flow front, where they follow a “fountain” flow behavior.