Study On Forming Mechanism And Eliminating Methods Of Weld Line On Injection Molding Parts

During the process of injection molding, when there are two melt fronts encounter, the area where they conflict will be different from others both in the structures and mechanical properties, which is called weld line. Because of poor intermolecular entanglement and V-notch which may cause stress concentration, weld line is the most unsubstantial area of the part. Therefore, in recent years, the analysis on injection molded parts with weld line obtains global concerns, this is because on one hand,due to large-scale complex parts gradually manifold, on the other hand, the desires on the appearance and internal performance of parts are increasing high, how to enhance the performance on the weld line area becomes investigative emphasis. In order to increasing the quality of plastic pars and extending its using domain, it is necessary to study weld line carefully.This paper mainly researches the forming mechanism, influence factors and improving methods of weld line from theory analysis, experiments and numerical simulation. The details of this paper are as follows:(1) On the basis of researched production, this paper carefully analyzes the forming mechanism and influence factors of weld line; expounding the reasons why weld line can debase the properties of parts, and how to describe the harm of weld line for different materials(2) By experiment, that the change of melt temperature affects the Nylon66 (containing 35% fibers) parts with weld line and without weld line has been studied. Experiment shows: along with melt temperature increasing, the tensile strength of the parts with weld line and without weld line both is enhanced, whereas the stretching trains of them both fall. On the condition of the parts without weld line, the variance of tensile strength is at a very low speed, but it is intensify ten times on the parts with weld line, the difference is mostly decided by the fiber orientation structure and the molecular motion.(3) Using Taguchi method, respectively do experiments to test the influence levels of different kinds of processing conditions( melt temperature, injection pressure, packing pressure, injection speed) on Nylon66 (containing 35% glass fibers) parts with weld line. Through analysis of the variance, obtaining the optimal processing conditions of parts with weld line and the affecting level of each factor, founding out the one of them that affects maximally. On the condition of experiment, the value of influence level for melt temperature, packing pressure, injection speed and injection pressure are: 57.78, 13.95, 9.69 and 1.37, they are decreased in turn. (4) Utilizing the academic analysis and experiment results, bringing out the methods how to decrease the harms of weld line, including how to recompose processing parameters, optimize the structure of mold, actualize anneal for the parts and select proper materials.(5) Checking the improving effects of parts with weld line for burgeoning injection processes, analyzing whether they can eliminate weld line in theory.(6) Emphatically discussing the characteristics of Sequential Valve Gating (short for SVG) technique, and how to design the mold and controlling system for it. Analyzing the mechanism why SVG can eliminate weld line in theory, and utilizing Molflow to simulate the process of SVG, the molding material of which is Nylon6 (containing 35% glass fibers), validating that the analysis is right.