Non-destructive assessment of residual stress in injection-molded polypropylene parts using THz polarization analysis

Abstract In recent years, plastic materials have increasingly been employed in the automotive and electronics industries due to their advantageous properties, such as lightweight and electrical insulation. Polypropylene (PP), in particular, is favored for its exceptional chemical resistance, low cost, and versatility, making it a popular choice in these sectors. Among various manufacturing techniques, injection molding is widely utilized for fabricating PP components. However, the complex thermo-mechanical conditions of the injection molding process, including high pressures and significant temperature gradients, often lead to the accumulation of internal residual stresses in PP components. These stresses can induce dimensional instability and mechanical degradation over time. In this study, we utilized a continuous-wave terahertz (THz) source to assess the internal residual stress in injection-molded square PP components. The polarization-dependent THz response exhibited a strong correlation with the molecular orientation of the polymer, which was associated with the internal residual stress distribution. We further validated this correlation by measuring the dimensional changes in PP specimens after precise cutting, confirming a positive relationship between residual stress and the THz polarization factor. Additionally, the influence of injection molding parameters on polymer orientation and residual stress in PP was investigated. The results demonstrated that the distance from the injection molding gate had the most significant impact on polymer chain alignment, thereby affecting both THz polarization characteristics and the residual stress distribution. This study not only confirms the utility of the previously developed THz waves-based evaluation system but also provides insights into optimizing injection molding parameters to improve residual stress management in PP materials.