Some features of the injection molding of short fiber reinforced thermoplastics in center sprue‐gated cavities

Abstract This paper describes an investigation into the fiber orientation in a number of center sprue fed cavities in short glass fiber filled polypropylene and nylon. The data have been interpreted in terms of a generalized five‐layer structure resulting from the frozen skin formation and the high and low shear levels in the flowing melt. The implications for scaling up the mold size are discussed from the results obtained with different shot volumes. The fiber structure was observed to depend on location in the molding, local injection time, and injection rate. In addition the occurrence of fiber‐free layers within the moldings using the filled polypropylene increases with an increase in shot volume, which produces an inherent ‘scale‐up’ problem. Notwithstanding the mold geometry subtleties, the fiber orientation in all the moldings follows similar patterns and trends.