Reinforcement of micro injection molded weld line strength with ultrasonic oscillation

Weld lines are the unfavorable defect not only in normal injection molding process but in micro injection molding process. In this study, polypropylene (PP) was chosen as the processing material and a micro dog-bone tensile test sample was selected as the objective part. The micro tensile part was prepared by the double gate injection mold. An ultrasonic generator was integrated in this mold in order to investigate the effect of ultrasonic oscillation on the micro injection molded weld line strength. The experiments were carried out for studying how the ultrasonic output power and the oscillation inducing time affect the weld line strength. Three output power levels (400, 600 and 800 W) and two inducing mode (Mode 1. the oscillation is induced from injecting moment to ejection moment; Mode 2. the oscillation is induced from injecting moment to packing procedure finishing) were set. The results show that ultrasonic oscillation has obvious influence on the weld line strength; Mode 2 always has better performance than Mode 1 for reinforcing the weld line strength; and when output power is 400 W the weld line strength is the highest. The mechanism of ultrasonic oscillation affecting the micro injection molded weld line was also analyzed by AFM (atomic force microscope) and polarized microscope.