Influence of process parameters and post-molding condition on shrinkage and warpage of injection-molded plastic parts with complex geometry

The quality of injection molded polymer products can be compromised due to the occurrence of residual stresses inherent to the molding process. In order to determine the parameters that influence the dimensional changes and residual stresses of injection molded polypropylene; computer simulations, chemical, thermal and mechanical characterizations were performed. Statistically, we evaluated two conditions of part geometry (with and without rib), three holding pressures (25, 60 and 96 MPa) and three post-molding conditions (without restriction, with restriction followed by water, and with restriction followed by annealing). The specimens molded with ribs and lower holding pressures showed greater warpage, which agreed with the computer simulations. The smooth specimens submitted to annealing followed by air cooling showed lower warpage in relation to the other two post-molding conditions. The thermal analysis presented a lower degree of crystallinity in the samples without post-molding restrictions. The samples submitted to the annealing heat treatment showed an increase in impact strength, and there was no significant variation in the modulus of elasticity for the post-molding conditions evaluated. The ribbed specimens exhibited higher residual stresses than the smooth specimens. Samples with greater warpage had lower residual stresses. All samples showed a tensile-compression-tensile behavior along the hole depth. The results showed that the shrinkage values were not influenced by any of the parameters evaluated. However, on the warpage the factor that exerts the greatest influence is the geometry of the part, followed by the holding pressure; and the post-molding condition presented the least influence among the analyzed factors.