A modified injection molding machine with a compression mechanism, and a mold with a movable wall and shut-off mechanism, were used to investigate the effects of processing parameters on the quality of injection-compression-molded polystyrene disks. The compression start-up time, compression force, melt temperature, and part thickness were selected processing parameters. The disk moldings were evaluated based on dimensional accuracy and birefringence. It is found that the compression start-up time affects packing time, and thus greatly affects the residual orientation. If the transition from packing to compression could start before peak cavity pressure, disks with low residual orientation could be obtained. High compression force improves part quality and reduces thickness. Since both compression-induced reduction and cooling-induced shrinkage are involved, the effects of temperature and thickness are not as straightforward as the trends in conventional injection molding.