Study On Weld Line Strength Of Injection Molded Parts Based On Dynamic Injection Molding

In recent years, large and complicated plastic products are increasingly used in daily life,and the appearance and the performance of the plastic products are more and more important.Weld line is a common defect of plastic products. The appearance of the products would beaffected by this kind of defect. Color difference of the products will happen during coatingand electrofacing because of the existence of weld line. Furthermore, the mechanical propertyof the plastic products would be affected too. The effect of traditional and dynamic injectionmolding on weld line strength of injection molded sample were studied in this paper.According to the former study of weld line strength, the relationship between the weldline strength and its influencing factors was summarized in this paper. Research findings:Firstly, weld line strength is decided by the structure of the weld line area. And the structureof the weld line area is the result of the action of internal and external factors. The rheologicalproperties of the melt which is decided by the properties of polymer are the internal factors.And the external factors include technological parameters, injection mold structures andinjection molding technology.The effect of traditional and dynamic injection molding on weld line strength of injectionmolded sample were studied in experiment research of this paper. The weld line studied inthis paper includes butt weld line and knit weld line. Polymer studied in this paper includesPP and GFPP.The research contents and conclusions were as below:(1) The effect of different technological parameters of traditional injection molding onstrength of butt weld line and knit weld line of PP and GFPP sample were studied. Orthogonaltest was used to arrange the experiments, and S/N evaluation and ANOVAanalysis were usedto analyze the experiment results. Research findings:1) Injection pressure was the key factorthat affected the weld line strength of PP sample and GFPP sample.2) Compared to butt weldline, strength of knit weld line was more sensitive to packing pressure.(2) The effect of amplitude and frequency of dynamic injection molding on strength of butt weld line and knit weld line of PP and GFPP sample were studied. Single factor rotationmethod was used to arrange the experiment and analyze the experiment results. Researchfindings:1) The trend of the respond of weld line strength to vibration parameters, no matterwhat kind of the weld line is, were the same when the polymer was the same. The respond ofweld line strength to vibration parameters, butt weld line or knit weld line, were differentwhen the polymer was different.2) The weld line strength of PP sample had a good respondrange of frequency, that was around6Hz.3) For GFPP sample, the effect of amplitude on theweld line strength was different when the frequency was different. And the weld line of GFPPsample got strong strength when the frequency was low and the amplitude was high.(3) The micro morphology of glass fiber in the weld line area and normal area werestudied by SEM. Research findings:1) The morphology that glass fiber lie across the PP bodyin the weld line area was found. This orientation of the glass fiber is the main reason of thereduction of strength in weld line area.2) The introduction of vibration reduced the number ofglass fibers which were lie across the PP body. It was show that the introduction of vibrationcan control the orientation of glass fiber effetely when the melts meet, and enhance the weldline strength.