Effect of gate dimension on micro injection molded weld line strength with polypropylene (PP) and high-density polyethylene (HDPE)

As a defect of micro injection molding parts, weld line is unfavorable since it will influence the surface quality and mechanical properties of micro parts. Therefore, the investigation on the developing process of weld line would be a significant issue for improving the quality of micro injection molded parts. In this study, one injection mold with four micro tensile sample cavities was designed and constructed. Every cavity responses to various gate dimensions, which is marked as Gate $ {text{Nr}}.1left( {1.5 times 0.1 times 0.{text{5mm}},{text{width}} times {text{depth}} times {text{length}}} right) $ , $ {text{Nr}}.2left( {1.0 times 0.1 times 0.{text{5mm}},{text{width}} times {text{depth}} times {text{length}}} right) $ , $ {text{Nr}}.3left( {1.0 times 0.05 times 0.{text{5mm}},{text{width}} times {text{depth}} times {text{length}}} right) $ , and $ {text{Nr}}.4left( {0.5 times 0.1 times 0.{text{5mm}},{text{width}} times {text{depth}} times {text{length}}} right) $ . The effects of gate dimension of the mold on mechanical properties of weld line have been studied by experiments in different processing parameters. The tensile test was used to characterize the micro injection molded weld line strength. The results for polypropylene show that with the changing of injection pressure and mold temperature, Gate Nr.3 is corresponding to the strongest weld strength; the next is Gate Nr.2; Gates Nr.4 and Nr.1 are in the end. The difference between them is not obvious. For high-density polyethylene, Gate Nr.1 is not able to be completely filled, which is due to the blocking of stick materials and dirt based on the simulation analysis. The investigation was only carried out for the other three gate sizes; results present that Gate Nr.3 always gives the best weld line strength whatever the processing parameters are, Gate Nr.4 is next and then Gate Nr.2. There are always middle optimal values for processing parameters leading to strongest weld line strength, when injection pressure is 80 MPa, injection speed is 90 cm³/s, melt temperature is 200°C, and mold temperature is 130°C. Higher and lower processing parameters result in reduced weld line strength.