Avaliação dos parâmetros que influenciam nas alterações dimensionais e nas tensões residuais do polipropileno moldado por injeção

Abstract

The final quality of the injection molded polymer products can be compromised due to the occurrence of residual stresses from the molding process, which can generate shrinkage and warpage, altering the final product dimensions. The determination of the residual stresses can be carried out by the hole-drilling method, which is not widely used for polymeric materials since it is standardized for metallic materials. Therefore, in order to evaluate the main parameters that influence in the dimensional change and in the residual stresses of the injection molded polypropylene, this report used computer simulation, thermal analysis, warpage and shrinkage measurements, tensile and impact strength tests and evaluation of the residual stresses by hole-drilling method. After molding, the specimens were submitted to three post-molding conditions (physical restraint + water, physical restraint + heating chamber, neutral). In the experimental tests, the molded specimens with lower packing pressure, as well as the ribbed molded showed greater warpage, which corroborated the computer simulations and the literature. In the post-molding conditions, it was observed that the smooth specimens submitted to the physical restraint + heating chamber showed lower warpage in relation to those that were not submitted to this condition. Regarding the shrinkage, for both geometries specimens, there were not significant variations of the averages. The thermal analysis presented lower crystallinity in the specimens with neutral post-molding condition in relation to the physical restraint + heating chamber condition. The residual stresses analysis concluded that ribbed specimens showed greater residual stresses than smooth specimens, which corroborates the literature which indicates higher concentration of residual stresses in complex geometries parts. Due to the residual stresses relief, smooth specimens with greater warpage showed lower residual stresses. It was observed that the physical restraint used to restrain warpage assisted the retention of residual stresses in the polymer structure. Therefore, considering the resources used in this study, it was showed that the warpage is related to the product geometry, to the process parameters, the post-molding conditions and the residual stress relief existing in the injection molded polypropylene structure.