The shrinkage behavior plays an important role in the determination of the final dimensions and hence the part quality of plastic injection molded parts. In this study, the experimental design approach is used to study the effects of three processing parameters on the shrinkage behavior for along-the-flow and across-the-flow directions of injection molded parts. Multiple regression models are set up that predict the shrinkage for polystyrene (PS, amorphous thermoplastic) injection molded parts. In addition, the Taguchi method is also used for analyzing the shrinkage data. The models are verified and validated using two different materials, acrylonitrile butadiene styrene (ABS), an amorphous thermoplastic, and polypropylene (PP), a semi-crystalline thermoplastic. The study succeeded in deriving multiple regression models for PS injection molded parts having explained variances of 0.959 and 0.952 respectively in the along-the-flow and across-the-flow directions. The difference in shrinkage between the measured and predicted values of shrinkage for ABS for along-the-flow case was found to be 0.64% and for across-the-flow case was found to be 4.9%, while for PP it was found to be 9.09% and 10.1% respectively. This study was successful in finding out the effects of the processing parameters on shrinkage behavior and obtaining equations for predicting shrinkage, for along-the-flow and across-the-flow, of plastic injection molded parts.