An Analytical Study of Constrained Shrinkage of Injection Molded Semi-Crystalline Plastic Parts

This paper presents an analytical approach to predict in-mold constrained shrinkage of injection molded semi-crystalline plastic parts. Existence of mold constraints exerts a noticeable effect on the final dimensions of molded parts due to the viscoelastic characteristics of polymeric materials. Variation of in-mold cooling time introduces variations in final part dimensions in relation with the constraints. Although this inter-relationship is well applied, few investigations have been reported to analyze the effect. In the present study, an analytical approach is introduced to predict constrained shrinkage of semi-crystalline thermoplastics and experimentally examined using a case study. Essential to this analytical solution, innovative mathematical expressions of elastic modulus and relaxation time constant with respect to temperature have been introduced. A mold was constructed to produce a plate featured by holes and injected to produce experimental data. The results indicated that there is a good agreement between the prediction and the experimental data, especially at longer cooling times. It can also be shown that the material relaxation time has the most dominant influence on the final shrinkage.