In fiber reinforced plastics products, the mechanical properties change by fiber orientation and distribution of fiber length. Because of that, it is important to know the fiber orientation and length distribution for design of fiber reinforced injection molded products. In this study, focusing on distribution of fiber length in glass fiber reinforced injection molded plate, the influence of plasticizing conditions and molding conditions on fiber length was investigated. In addition, we considered mechanism of fiber breakage during injection molding process. As a result of examination on the effect of plasticizing conditions on fiber breakage, when revolving speed of screw increased, the fiber length indicated the tendency to increase. This result indicates inferred that the shear stress was applied to the fiber for short time because of screw rotation decrease caused by the plasticizing time decrease. Furthermore, a lot of long fiber existed in the molded plate at the high injection rate condition and thick plate condition. In comparison of fiberlengthinmolded plate, there was more long fiber in the inner layer than the middle layer, and long fiber existed in the end part. It was considered that the change of fiber length in middle layer was caused by the growth of solidified layer. From these results, in order to suppress the fiber breakage, it was necessary to reduce the shearing force and to thin the solidified layer during injection molding.

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