| Microcellular injection molding is a kind of important processing technology to forming microcellular plastics, which has multiple technical anvantages and product superiorities such as high efficiency, energy saving, high resource-using rate, non-pollution, plastic parts of lightweight and good performance, etc. Microcellular injection molding is a type of green and advanced manufacturing technology with good economical and social benefits, and has broad application prospects and huge market potential. However, because the equipment system principle and product formation mechanism in microcellular injection molding process are very complicated, leading that the current commercially available microcellular injection molding equipment system is still imperfect and very expensive, the used metering and injecting methods and technologies for supercritical fluids are not precise enough, and the building technology of microcellular injection molding equipment system is not very clear. Furthermore, in the existing studies on cell structure of microcellular injection molded parts, there also have a series of problems such as the studies on formation and elimination mechanism of surface bubble marks are un-deep, the basic researches on dynamic mold temperature assisted microcellular injection molding technology are uncomprehensive, the investigations on cell forming process and evolution law are not systematic, etc. All of these need a further in-depth study.Focusing on these above problems, this paper systematically studied the construction technology and principle of microcellular injection molding equipment system, designed and developed the supercritical N2 generating, metering and injecting system and equipment, designed and manufactured a special screw and barrel use for microcellular injection molding process and other plasticizing units, builded the microcellular injection molding equipment system and dynamic mold temperature assisted microcellular injection molding test line. And on this basis, this paper gave a systematic and in-depth study in the aspects of formation process of surface bubble marks on microcellular injection molded parts, elimination mechanism of surface bubble marks under dynamic mold temperature condition, experimental research on dynamic mold temperature assisted microcellular injection molding technology and formation and evolution mechanism of inner and outer cellular structure in microcellular injection molded parts, obtained substantial conclusions with theoretical guidance meaning and of engineering application value.By studying the process principle of microcellular injection molding with supercritical N2 as blowing agent, the technology flow procedure was designed. The generating principle of supercritical N2 was studied, and the supercritical N2 generating system and equipment were developed. Through analyzing the metering and injecting requirements for supercritical N2 in microcellular injection molding, three supercritical N2 metering and injecting methods were presented, named "bypass flow", "steady flow" and "quantitative flow". The corresponding "quantitative flow" supercritical N2 metering and injecting system for microcellular injection molding process was developed and manufactured based on programmable logic controller and touch panel techniques. The connection techniques of supercritical N2 generating, metering and injecting system with the injection machine were discussed, and achieved the well connection of self-developed supercritical N2 concerned system with the existing injection machine. Compared with the existing supercritical fluid pumping system, the developed supercritical N2 delivery system has a more accurate and more stable and easier measurement.The building technology and principle of microcellular injection molding equipment system was studied, and the construction plan of microcellular injection molding equipment system based on a normal injection machine (Haitian MA3200 energy saving servo injection molding machine) was presented. A special screw and barrel use for microcellular injection molding process were independent designed and manufactured, and showed a well exchange of the original plasticizing system in the exciting machine. Besides, a base for gas injection tip, a back pressure controlling system and equipment were independent designed and manufactured, the gas injecting valve and self-locking nozzle were selected. Finally, a whole microcellular injection molding equipment system and testing product line were constructed, the process debugging and test producing were completed. The results showed that the developed microcellular injection screw and barrel have excellent shearing and mixing performance, the constructed microcellular injection molding equipment system has reached a commercially microcellular injection molding level.The formation process of surface bubble marks on microcellular injection molded parts and their elimination mechanism under dynamic mold temperature condition were investigated. A microcellular injection mold with electric heating was designed and manufactured, the corresponding mold temperature control system and equipment were developed, and the dynamic mold temperature assisted microcellular injection molding tese line was constructed. Through analyzing the surface quality and surface bubble morphology of microcellular injection molded parts, the formation process of surface bubble marks was proposed. Base on the test line, the surface morphology of microcellular injection molded short shot part’s flow front under different mold temperatures, the surface bubble morphology of microcellular injection molded full shot parts under different mold temperatures and the surface morphology of microcellular injection molded short shot part from flow front to near gate position under high mold temperature were studied, the evolution laws of surface bubble marks under different mold temperatures was discussed, and the elimination process and mechanism of surface bubble marks with dynamic mold temperature condition was proposed. Based on the research, it was found that surface bubble marks were formed by the condensation of broken bubbles in the flow front, the influence of dynamic mold temperature on the bubble marks has experienced a "formation and elimination" process.The corresponding experimental studies on dynamic mold temperature assisted microcellular injection molding technology foundation were conducted. Using the constructed dynamic mold temperature assisted microcellular injection molding tese line, the influence of dynamic mold temperature technique on melt pressure variation in the filling and cooling stage of microcellular injection molding was investigated, the influence rule and mechanism of mold temperature on melt pressure was discussed. And accordingly, the influence of dynamic mold temperature technique on cell structure in microcellular injection molded parts was investigated, the influence rule and mechanism of mold temperature on cell average diameter, cell density and cell diameter dispersity was analyzed; the influence of dynamic mold temperature technique on mechanical properties of microcellular injection molded parts were studied, the influence rule and mechanism of mold temperature on tensile strength of tensile spline with and without weld line, flexural strength of flexural spline with and without weld line, and notched impact strength of impact spline without weld line were revealed. The study found that when mold temperature is below 90℃, the cell size, cell density, cell diameter dispersion, tensile strength and flexural strength without weld line were almost invariant, and when mold temperature is higher than 90℃, the cell size increase, cell density decrease, tensile strength and flexural strength with weld line increase along the mold temperature.In order to study the formation and evolution mechanism of cellular structure in microcellular injection molded parts, corresponding experimental methods and comprehensive characterization techniques for cell structure were developed. By investigating the cell structure on the fractured surfaces both vertical to the melt flow direction and parallel to the melt flow direction on the molded parts, two cell forming and evolution processes in microcellular injection molding were introduced in this paper:"foam in filling" process and "foam after filling" process, and the critical melt pressure which decides the two processes was analyzed. In addition, the formation of the unfoamed skin layer has "during filling" process and "after filling" process was also proposed in this paper. It was found that the unfoamed skin layer in microcellular injection molded part has two distinct regions, the outer region is a thin frozen layer which contains deformed and broken cells, and the inner region is a relatively thick solid-like layer that has no visible cells in, and the whole thickness of the unfoamed skin layer is decreased with the increase of injection speed and mold temperature, but is slightly affected by melt temperature. |
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