Investigations on the weldline tensile strength of thin-wall injection molded parts

It is known that the weldline reduces the mechanical strength of the conventional injection molded parts. Systematic studies on the weldline strength of thin-wall molded parts were not yet reported. This study investigates the effect of processing conditions including melt temperature, mold temperature, injection speed and packing pressure on the weldline strength of thin-wall parts. Tensile test specimen of 2.5, 1 and 0.8 mm thick were injection molded under specified conditions. Both single gate and double gates were used to form parts with and without weldlines. Part tensile strengths were measured experimentally. From the experimental results, it was found that higher melt temperature and mold temperature as well as faster injection speed will increase weldline strength whereas higher packing pressure would decrease weldline strength. Melt temperature and mold temperature are two parameters that affect weldline strength most significantly within the current molding window. Higher melt and mold temperatures not only lower the residual stress but also help the diffusion of molecular chains leading to a higher degree of bonding at the weldline interface. On the other hand, high packing pressure leads to higher residual stress formation and reduces the molecular bonding rate. Part thickness also exhibits significant effect on weldline strength reduction. Thinner parts resulting in higher percentage of frozen layer in filling process thus limit the bonding rate at weldline interface. From the regression analysis, the KIM model with corrected terms in the fitting coefficient was found to correlate process conditions and weldline strength reduction quite well.