Crystallinity and mechanical properties of polypropylene products foamed by microcellular injection molding process

Microcellular injection molding (MIM) is a pivotal technique for lightweight molding. Polypropylene (PP) is widely used in MIM due to light weight and easy processing. The crystallinity of foamed PP products influences the load-bearing structures and consequently mechanical properties. To address the deterioration of mechanical properties caused by internal porous structure, a novel method is proposed to regulate the crystallinity by optimizing process parameters, thereby improving mechanical properties. Wide-angle x-ray diffraction, mechanical testing, and melt flow simulation are applied to investigate the influence of process parameters on crystallinity and mechanical properties. Parameters are further optimized by establishing a multivariate nonlinear regression model for process parameters and crystallinity. The results show that increasing melt temperature, injection rate, mold temperature, and cooling time stimulate crystallization, enhancing strength and decreasing elongation at break. Conversely, high injection ratio of supercritical fluid obstructs crystallization, reducing strength, and increasing elongation at break. The crystallinity increases by 20.81%; the corresponding tensile, flexural, and shear strength and corresponding modulus increases by 5.39% and 10.28%, 7.60% and 18.50%, 5.00% and 11.37%, and elongation at break decreased by 5.19% through the optimization of regression model. The proposed method is feasible to upgrade the mechanical properties of foamed PP products. Aiming to the complexity of melt flow in microcellular injection molding caused by multiphase intercoupling, this study explores the nonlinear and strong coupling relationship between process parameters and crystallinity. By optimizing molding parameters to regulate crystallinity, the mechanical properties are enhanced. Furthermore, a multivariate nonlinear regression model is developed to further improve the macroscopic mechanical properties of foamed products. image