Influence of injection molding parameters, melt flow rate, and reinforcing material on the weld-line characteristics of polypropylene

In this work, the relation of injection molding parameters to the mechanical properties of various polypropylene grades is investigated to find the optimal processing parameters to minimize weld-line effects like reduced mechanical and/or optical performance. Injection molded test specimens of five polypropylene grades with different melt flow rates were characterized for their mechanical behavior and compared with equal-shaped specimens with a colliding weld line in the middle of the specimen. For the production of these weld-line specimens, a special mold was used and injection molding parameters were systematically varied, tensile (ISO 527-2) and impact (ISO 179-1/1eU) properties were measured, and statistical analyses were performed to gain insight on the correlation between melt flow rate and weld-line behavior. It showed that mechanical properties of specimens with weld lines can be influenced by the processing parameters, but the effect is limited. Positive correlations were found between tool temperature and tensile modulus and strength. Polypropylene grades with low melt flow indexes seem to be more susceptible to weld-line-induced property reductions. In a second test series similar to the pure PP investigations, glass-fiber- and talc-filled PP were used to gain insight into the weld-line behavior of PP composites. In addition to mechanical characterization, optical and scanning electron micrographs were taken of the weld-line areas. It was found that the reinforcing particles align along the melt flow front in the weld-line area. As a result, the mechanical performance of weld-line specimens is poor. Injection molding parameters were found to have only a small effect.