Optimization of polypropylene high-strength injection molding welding and interface structure analysis

Secondary injection molding is an effective method for joining polymer parts, where the weld interface plays a critical role in the overall performance of the part. This study discusses the influence of injection molding parameters on the interface strength of polypropylene and polypropylene (PP-PP) secondary injection molding and analyzes the welding interface structure. The injection molding parameters for PP-PP were optimized using the Taguchi method and the signal-to-noise ratio (S/N). The optimal injection molding parameters were determined to be a mold temperature of 120°C, injection temperature of 350°C, and injection speed of 100 mm/s. The interface tensile strength was measured at 33.42 MPa, corresponding to 98.1% of the tensile strength of the base material. Among these parameters, mold temperature has the greatest influence on the strength of the welding interface. Secondly, polarizing microscopy, scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) were employed to observe and analyze the crystal structure and fracture failure modes at the interface. The results indicate that elevated injection molding parameters promote the formation of crystal nuclei and spherulite growth at the secondary melt interface, with the spherulite size at the interface significantly affecting the interface strength. This study offers valuable reference for the welding of other polymers and fiber-reinforced materials.