Research On Effect Of Interface Heat Transfer On Weld-line Quality During Thin-wall Injection Molding

Thin-wall injection molding technology rapidly develop in recent years as meeting the market’demand of thinner plastic parts, high productivity and more ease of integration, but the weld line defects is a serious problem difficult to solve during molding process. Due to the small part thickness, the filling melt with low heat cools quickly. As the result, different streams do not have enough time to complete fusion, the parts have a low strength and the poor surface quality. Usually used to raise the temperature, pressure and injection speed to improve weld line problems, but it always has others negative impact on thin-wall molding parts, often outweigh the benefits. Therefore, this paper basing on the heat transfer theory, searched the influence of melt and mold interface environment on the heat transfer rate, obtained a method to reduce the interfacial heat transfer rate, reached the purpose of slowing the decline of melt temperature and improving the quality of weld lines, then provided a new idea to solve thin-wall injection molding weld line defects.Based on weld-line formation mechanism and the heat transfer theory, the effectiveness of improving the weld-line quality by reducing the melt-mold interface heat transfer rate has been analyzed. And three ways to change the interface heat transfer rate has been identified:change the mold surface roughness, change mold material, coat insulation coating on mold surface.The mold of thin-wall tensile test pieces has been designed, and the mold and melt temperature measurement platform have been builded. Through injection experiments the relationship between the mold surface roughness, the mold material and the interface heat transfer rate has been established. Through the test of strength and surface V-groove depth of tensile test pieces, the influence of interface heat transfer rate on the weld-line quality under different thickness and different material molding has been studied. The analysis of that reducing the interfacial heat transfer rate can be effective in improving the thin-walled parts weld-line quality has been verificated. For0.5mm thickness part, the effectiveness of heat insulation coating application in improving weld-line quality has been studied by coating the mold cavity way. The melt temperature distribution of mold cavity when heat insulation coating was coated has been analyzed and compared with the uncoated situation using Ansys software. The results indicated that the effect of heat insulation coating on thermal hysteresis is obvious in a short period, it can effectively delay the decline of melt temperature. For thin-walled molded parts, experimental results show that the heat insulation coating can greatly enhance the tensile strength of weld-line and virtually eliminates surface V-groove, which consistent with the simulation results and the heat insulation coating applications can effectively reduce the high demand of process parameters during thin-wall molding.