| Gas counter pressure (GCP) technology is a kind of dynamic gas pressure control technologies, which control parameters contain GCP pressure and GCP holding time. In recent years, GCP technology has been applied in injection molding process, which constitutes GCP assisted injection molding process. According to the classification of injection molding process, GCP assisted injection molding process contains GCP assisted conventional injection molding (CIM) process, GCP assisted chemical foaming injection molding process and GCP assisted microcellular injection molding process, meanwhile, GCP assisted injection mold contains GCP assisted CIM mold, GCP assisted chemical foaming injection mold and GCP assisted microcellular injection mold.In GCP assisted CIM process, GCP technology can revise pressure to melt during fill process, which solves pressure lacking of melt flow front in CIM process and increases dimensional accuracy of molding parts. In GCP assisted chemical foaming injection molding process and GCP assisted microcellular injection molding process, GCP technology can effectively eliminate swirl marks and sliver marks of foaming injection molding parts and improve surface quality of molding parts, which eliminate secondary machining operations, such as polishing, finishing and painting in production of conventional foaming injection molding products, it thus effectively decreases production cost, energy consumption, and environmental pollution.As a newly injection molding technology, GCP assisted injection molding process has many scientific problems and technology problems, especially in aspects of system constitution, mold structure design, product qualities control, action mechanism, etc. This paper systematically studied GCP assisted injection molding process in aspects of technological process, mold cavity gas pressure control system and control equipment, mold structure design, production line, process parameters optimization, influence mechanism, mechanical properties, etc.Based on process principle of GCP assisted injection molding, basic procedure of GCP assisted injection molding process was analyzed, calculation formulas of molding cycles of GCP assisted CIM process, GCP assisted chemical foaming injection molding process and GCP assisted microcellular injection molding process were constructed. According to requirements of GCP assisted injection molding process, mold cavity gas pressure control method and control strategy were proposed, mold cavity gas pressure control system was designed and developed, mold cavity gas pressure control equipment was developed. GCP assisted chemical foaming injection mold of a rectangular box was designed and developed. Design methods of parting line, ejection mechanism, gas channel of GCP assisted injection mold were proposed. In GCP assisted injection process, increasing pressurizing rate and unloading rate of mold cavity gas can effectively decrease molding cycle and increase production efficiency of products. Seal of parting line of GCP assisted injection mold is realized by machining seal groove and putting seal ring. Seal of ejection mechanism of GCP assisted injection mold is realized by adding seal ring pressing plate. By using the design method of main gas channel and secondary gas channel for GCP assisted injection mold, gas pressurizing/unloading rate can be increased, meanwhile, qualities of molding products can be guaranteed. GCP assisted chemical foaming injection production line was constructed by connecting self-developed mold cavity gas pressure control equipment and conventional chemical foaming injection production line. Production of GCP assisted chemical foaming injection molding products with high surface quality was realized, which proved effectiveness of developed mold cavity gas pressure control equipment and proposed mold structure design method.In GCP assisted CIM process, effects of GCP technology on filling capability of melt, "fountain effect" of melt, density of part, dimensional accuracy of part and mechanical properties of part were studied. Influence mechanism of GCP pressure and injection parameters on shrinkage ratio of GCP assisted CIM part were revealed by using orthogonal experimental design and signal to noise ratio analysing. In GCP assisted CIM process, increasing injection pressure and/or decreasing GCP pressure effectively increase fill capability of melt. Increasing GCP pressure or prolonging GCP holding time increases density and impact property of GCP assisted CIM part. GCP technology effectively decreases shrinkage ratio of GCP assisted CIM part. Compared to the shrinkage ratio of CIM sample, the shrinkage ratio of sample molded by GCP pressure=9MPa and GCP holding time=10s decreases by 17.2%. GCP technology greatly increases tensile strength and flexural strength of molding part with weld line. Compared to CIM samples with weld line, the tensile strength and flexural strength of samples with weld line molded by GCP pressure=9MPa and GCP holding time=10s increase by 30.51% and 23.69%, respectively. In GCP assisted CIM process, GCP pressure is the most important parameter to shrinkage ratio of molding part, followed by packing time, injection pressure, mold temperature, melt temperature and packing pressure, percentage contribution of above parameters are 61.232%,13.985%,7.260%,5.921%,5.918% and 1.569%, respectively.In GCP assisted chemical foaming injection process, influence mechanism of GCP technology on melt flow front, surface quality of part and inner cells were researched. Internal relationships between GCP pressure, GCP holding time, foaming agent content, melt temperature, injection pressure, injection speed and foaming layer thickness, cell diameter, cell density of GCP assisted chemical foaming injection molding sample were revealed. In GCP assisted chemical foaming injection molding process, the higher the GCP is, the higher the inhibiting effect of cell fracture behaviour of melt flow front is, the fewer the surface gas defects of molding part is, the higher the surface gloss value of part is. There are two critical GCP pressure, melt flow front cells without fracture behaviour critical GCP pressure and melt without foaming behaviour critical GCP pressure, respectively, in GCP assisted chemical foaming injection molding process. Products without any surface gas defects can be molded while GCP pressure is not lower the melt flow front cells without fracture behaviour critical GCP pressure. As a whole, low GCP pressure and GCP holding time, large foaming agent content, high melt temperature and injection pressure, reasonable injection speed can mold the GCP assisted chemical foaming injection part with large foaming layer, small cell diameter and large cell density.In GCP assisted microcellular injection molding (MIM) process, effects of GCP pressure and GCP holding time on surface quality, cell morphology and cell density of MIM part were investigated. Influence mechanism of GCP pressure on melt foaming action of MIM process was revealed. Effects of GCP pressure and injection parameters on surface sink mark depth of MIM part were studied. In GCP assisted MIM process, increasing GCP pressure and prolonging GCP holding time are all benefit to increasing surface quality and improving cell morphology of MIM part. In GCP assisted MIM production, the melt flow front cells without fracture behaviour critical GCP pressure is suggested as a reference value for GCP pressure setting to increase surface gloss and decrease surface sink mark depth of MIM products... |
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