Flow-induced alignment in injection molding of fiber-reinforced polymer composites

The orientation patterns observed in injection molded fiber-reinforced composites are the result of the response of the fibers to the flow in the cavity during filling and, to a lesser extent, the result of subsequent orientation during packing. Features of the filling flow affecting fiber orientation include: first, the shearing flow across the cavity thickness, second, the convective flow along the plane of the cavity, which is mainly determined by the geometry of the part, and third, the fountain flow near the advancing free surface. Among geometrical factors, the gating arrangement plays a significant role by determining the amount of extension of the fiber-loaded melt experiences upon entry into the cavity, as do inserts or contractions/expansions by altering the fiber orientation locally. In spite of the expanding usage of injection molding in fabricating short- and long-fiber-reinforced parts, experimental and computational results on this subject have not been systematically presented and discussed in recent reviews or books dealing with injection molding. The aim of this chapter is to fill this gap and presents a critical review of available experimental and computational studies in the area. This includes an overview of the injection molding process, the main characteristics of the flow patterns during filling that are of relevance to fiber orientation, and a review of experimental observations of fiber orientation in injection molded parts, including fiber orientation around weldlines. Finally, an outline of modeling techniques used in the prediction of fiber orientation in injection molding.