Research Of Design And Fatigue Life For Steam Rapid Heating Cycle Injection Mold

Plastic products have been widely used in manufacture and daily life, therefore, the improvement of plastic molding technology has been continually boosted. As the demand for products of high precision, high strength, high surface quality and low cost is more and more intense, plastic molding technology which is more efficient, more economic and more environmentally friendly comes into being. Rapid heating cycle molding (RHCM) is such a kind of new molding technology. Using this technology, plastic products will have high gloss appearance without weldline, thus it can eliminated subsequent process operations such as polishing, spraying and coating. Therefore, it is also known as high gloss and no weldline injection molding.Steam heating high gloss injection mold (SRHCM mold) is studied in this thesis. Compared with conventional injection molding process, SRHCM process has added steam heating section before injection, which has effectively increased fluidity of melt when it fills the cavity, avoided the appearance of weldline and greatly improved the surface quality. In cooling stage, the cavity and the plastic part are rapidly cooled down by cooling water. The entire molding cycle of SRHCM is as long as conventional injection molding. However, SRHCM mold has a shorter lifetime than conventional one due to the rapid heating and cooling process. Simply increasing the dimensions of moulding board can strengthen the mold, but a longer lifetime is got often at the cost of molding efficiency and heating effect. Hence taking into account both molding efficiency and service lifetime, developing high efficient and long lifetime SRHCM mold is very necessary.First of all, heat exchange, thermal distortion, thermal stress and lifetime of SRHCM cavity are analyzed, which is based on the technics of SRHCM and theory of heat exchange and fatigue strength. The heat transfer coefficients of steam and cooling water have been calculated and provided as essential data for heat transfer finite element analysis of SRHCM cavity. S-N curve (fatigue strength-lifetime curve) and its fitting equation of mold steel CENA1 have been fitted based on fatigue data given by the supplier, which provides a relatively accurate material data for fatigue simulation.Factors that influence the heat transfer efficiency and lifetime of SRHCM mold are discussed in the thesis. Multi-object optimization design of channels of planar cavity model is carried through, utilizing orthogonal experiments, response surface analysis and nonlinear programming. The influence laws of the channels layout on the heat transfer efficiency, stress and lifetime are obtained and the factors are arrayed according to the impact, thus, most significant factors are found out. Fitting equations and response surfaces of experiment targets are gained for the optimization. The optimum relation is consistent with the result of finite element simulation, which is helpful to the design of channels layout of SRHCM mold for plate-shaped plastic product.Relating with the practical situation of production, three-dimensional finite element simulations in heat transfer, thermal stress and lifetime of SRHCM cavity which is used for production of 42-inch LCD TV front shell have conducted. The layout of steam channels has been altered by a new form:two channels in the side of the mold arrange in terms of one near cavity surface, and the other little far from the surface. This new arrangement has greatly improved the uniformity of heating. Demisemi analysis models have been built for simulation of entire process including stages of heating, mold closing, injection and cooling, which has more precisely reflected the rule and traits of temperature changes of cavity, plastic part and core. Heating and cooling efficiency and effect have been compared among five models with different spaces from the cavity surface to the channels. Thermal distortion and stress of cavity have been analyzed under both complete restriction and partial restriction, and it is found that a "gap-structure" in partial restriction helps to release stress and increase lifetime. During mold closing and injection stages, clamping force and injection pressure are good for reducing the stress of cavity, but high temperature of polymer melt makes cavity surface expand rapidly, and the thermal stress increase, hence its influence on thermal stress is more obvious during injection stage. Comparing stress and lifetime among five models, a good correspondence between result of stress simulation and lifetime estimation has been obtained. According to analysis above, optimization design of SRHCM cavity has been carried out, aiming at longer lifetime and better heating effect. At present, SRHCM cavity of 42 inches LCD TV front shell has adopted the partial restriction method, diameters of channels are 8 mm, spaces between channels are 18 mm, spaces from channels to the cavity surface are 12 mm, and channels in the side of the mold have been arranged in the new form. In this way, SRHCM mold will have shorter heating time within 30 seconds, better heating uniformity and own a longer lifetime at the same time. This kind of SRCHM mold has been on stream, and brought better benefit.