The injection molding of reprocessed plastics with a preplastication plunger injection-molding machine was investigated with a focus on the processing conditions. The process of the filling of the resin into the mold is much better controlled with preplastication than with processing in a conventional injection-molding machine. Reprocessing of the resin leads to a reduction in molecular weight due to drastic changes in the resin morphology, thereby causing a reduction in melt viscosity. Direct experimental evidence for reduced viscosity was obtained from measurements of the filling pressure recorded on the machine and also with a melt-flow indexer. The results of this study provide a practical solution for reducing the resin temperature when reprocessed resin is used in the injection molding of plastics.