Analysis of residual stresses and post molding deformation in injection molded components

This study is aimed at assessing the residual stresses and post-molding deformation in injection-molded parts as a result of processing. Molded-in residual stresses can significantly affect the functionality of injection-molded components whereas the post-molding shrinkage and warpage cause deviation from the dimensional requirements prescribed by the part design and reduce its efficacy in its operational milieu. The capability to determine a priori the processing-induced stresses and post-molding deformations would provide a means to obviate the need for prototyping in the manufacture of injection-molds and trial-and-error estimates of ideal processing windows, thereby enhancing productivity.;In order to estimate the influence of the constraint conditions on the stress development, a one-dimensional problem of polymer vitrification has been analyzed with a variety of restraint models. The stress levels were found to significantly depend upon the nature of constraints, processing conditions and the mold location. Comparisons have also been made with experimental residual-stress measurements; qualitative correlation was found despite significant disparity in the stress magnitudes. In addition, flow-induced stresses have been separately estimated and compared with the thermal-stress levels. It was found that the flow stresses were significantly smaller than their thermal counterparts, implying that they are of secondary importance in determining the stress levels and post-molding deformation.;A scheme for analyzing residual stresses and shrinkage/warpage in three-dimensional injection-molded parts has been developed. The vitrification of the polymeric component under the influence of melt pressure and thermal history has been analyzed by means of a 'localization' assumption which takes that the lateral deformations of the melt which has stopped flowing is negligible. This approximates the actual conditions extant in complex, three-dimensional injection-molded parts as a result of geometric constraints, melt-pressure and friction between the part and the mold wall. The polymer has been treated to be a transversely-isotropic viscoelastic material exhibiting thermo-rheological simplicity. The polymer melt- and mold-cooling-analyses as well as the elastic structural-response determination have been performed using commercial programs. Good correlation with experimentally available measurements of shrinkage and warpage has been observed.