Numerical Simulation Based On A Viscoelastic Constitutive Model And Expeirmental Study On The Filling Stage Of Overlfow Water-assisted Co-injection Molding Process

Water-assisted co-injection molding (WACIM), an innovative fluid assistedinjection molding, combines the water-assisted injection molding (WAIM) andco-injection molding (CIM). It has both the advantages of WAIM and CIM, andovercomes their technical limitations. So it has a broad application prospects. Theflow behaviors of each fluid during the WACIM are complex for the differencerheological properties between the skin and inner melts and the cool water penetrationin the inner melt. These factors will affect the morphology, micro-structure andmechanical performance of products. There are few research reports about WACIMavailable. In order to offer the technical support and theoretical guidance, theinfluence factors and their laws of the molding process on the final produces qualitiesshould be explored essentially for providing optimum method for the WACIM. In thisthesis, the theoretical and experimental study is carried out for the filling process ofoverflow WACIM (O-WACIM). The main research contents are listed as follows:1) With reasonable assumptions and simplicities for the sequentially injections ofmelts and water during the WACIM process, based on the three governing equationsof fluid mechanics and Giesekus viscoelastic constitutive model, the mathematicalmodel was developed to describe the flow behaviors of melts and water during thefilling process of O-WACIM. The finite volume method (FVM) was adopted to solvethe mathematical model. The volume of fluid (VOF) method was used to track thefree interfaces, and an additional artificial compression item was introduced to dealwith the discontinuous change of volume fraction in the free interface area.2) A new solver which can be used to describe the complex multi-phasemulti-layers flow behaviors of viscoelastic melts during the filling process ofO-WACIM was developed. The numerical simulation for a case was performed.Compared with the simulation results which based on a viscous constitutive model, itcan be found that the viscoelastic constitutive model is more reasonable to describethe velocity field and material distribution during the filling process of O-WACIM.3) Numerical simulations and experimental study of WAIM, a special case ofO-WACIM, was carried out. The simulation result showed that increasing waterpressure decreased the residual wall thickness of melt, which was verified by experiment. Orthogonal experimental test, which took melt temperature, waterpressure and water delay time into account, was conducted and it was discovered thatwater pressure was the main factor on the products quality.4) The numerical simulations of the filling stage of O-WACIM were performedto investigate the influences of processing parameters on the products qualities andthe first normal stress difference (FNSD). The results showed that:①With the waterinjection pressure increasing, the water injection time, the residual wall thickness ofthe skin melt and the inner melt decreased, while the hollow ratio and the FNSDincreased;②With the pre-filled volume of inner melt increasing, the residual wallthickness of the inner melt increased and the skin melt changed insignificantly, whilethe hollow ratio, the water injection time and the FNSD decreased;③With the innermelt injection velocity increasing, the residual wall thickness of the skin meltdecreased and the inner melt changed insignificantly, while the hollow ratio, the waterinjection time and the FNSD increased;④With the sequence of melts changing, thefill effects became worse and water injection time increase obviously;⑤With theelastic component(p) increasing, the residual wall thickness of the inner meltdecreased and the skin melt changed insignificantly, the hollow ratio, the waterinjection time and the FNSD increased;⑥With the relaxation time () of the innermelt increasing, the residual wall thickness and the FNSD of the inner melt increased,the residual wall thickness and the FNSD of the skin melt changed insignificantly,while the hollow ratio, water injection time increased.5) Experimental study on O-WACIM is carried out based on the experimentplatform developed independently. The research studied the effects of differentprocess conditions on the quality of the product, such as water injection pressure,water injection delay time, pre-filled volume of inner melt, inner melt injection speed,inner melt delay time and melt injection sequence. Results showed that the simulationconclusions are consistent with the experiment, the mathematic model, numericalmethods and the new solver are valid and reliable. Experiments proved theO-WACIM is the best forming method compared with short-shot WACIM, andoverflow GACIM. Some reasonable advices about process optimization ofO-WACIM are proposed based on the simulation and experiment conclusions.