The Numerical Simulation On Water-assisted Co-injection Molding Process

A new advanced polymer molding technology of water-assisted co-injection molding (shortened as WACIM) was put forward in this article for the first time. The process of WACIM is that the skin melt and core melt are sequential partly injected into mold by a co-injection molding mechine, then the high pressure water is injected into the core melt in mold by water assisted control equipment after some delay time which makes the skin melt and core melt fill fully mold and holds the packing pressure of a mold, the hallow multilayer composite plastic goods with smooth inner surfaces is finally produced. WACIM has the advantages of both co-injection molding technique and water-assisted injection molding technique, and it is the envirmental friendly tecneque by which high performance, low cost goods are produced. So the technique should hope to becomel one of the most advanced polymer molding techniques by which the future important problems of engineering material will be resolved. Because of the 3D, transient, non-isothermal multiphase and multilayer molding process and the interaction of stresses on the interfaces between adjacent melts of WACIM, therefore, the WACIM has its special transport rule of flow and dynamic characters, which makes its molding mechanism very complicated. Up to the present, few of researches on WACIM technology, molding mechanism and simulation have been reported at home and abroad. The researches on the theorey and simulation on polymer WACIM filling process are conducted in this paper, which yields the following main achievements:On the basis of rheology, fluid dynamics, thermodynamics and so on, reasonable assumptions were put forward, the 3D, transient, pure viscous, non-isothermal mechanism model is established to describe the polymer melt filling process of WACIM by using the idea that variable fields of one element can be determined by the sum of each phase's variable field of the element weighted by each phase's volume fraction, Moreover, with the techniques of CAE, the efficient and stable numerical algorithm and the simulation system of WACIM process were developed, which established the scientific foundation of theory to guide the design of process technology and mold for WACIM process.Aim at the problem which the existing theory of water assisted injection couldn't reflect the water penetrating dynamics, based on the assumption of a fictitious fluid,the controling equation describing water penetrating dynamics with easy numercal simulation is established for the first time whch could reflect the pressure field, velocity field , temperature field in filling water area and water penetrating dynamics,at the same time , avoids the connatural non-linear problem of primary controling equation of describing water penetrating dynamicsBased on the 3D, transient, non-isothemal WACIM process mechanism mode, the corresponding mixed stable numerical algorithm based on the low-order interpolation with fast convergence was established by the stable finite element methods of Mini-Element method ,SUPG method (Streamline upwind /petrov -Galerkin method), penalty function method and uncoupling solution of velocity field,temperature field and fraction of fluid volume, and the pure finite element front tracking method and interfacal reconstruction technique of moving front interface and stratified interface were put forward , which effectively decreases the needs of the computer CPU and memory capacity, the three dimentional large-scale, high nonlinear finite element simulation on multiphase and multilayer WACIM process was implemented in common personal computer.Based on simulation of WACIM process, the influences of process parameters and polymer rheological parameters on WACIM process were systemcally studied, and the influencing mechanisms were disclosured by theoretical analysis. Research results showed that with increasing of the consistency factor, rheological index, the pre-filling volume fracton and injection pressure of core mel, the penetrating length of core melt and water decrease ,but the penetrating width of core melt and water increase, and with the increasing of delay time of water and the injection temperature of core melt, the penetrating length of core melt and water increase, In addition, water injecting temperature affects penetrating length and penetrating width of core melt hardly,but water penetrating length increases with the increasing of the water injecting temperature.Research results showed that in WACIM molding process of polymer bifurcation goods, the unbalance filling phenomena of core melt and water occures in different branch easily. when the viscosity ratio of core and skin melt is higher than 1, the core melt and water filling process always are in balance filling state which core melt and water penetrating length of different branch are equal. But when the viscosity ratio of core and skin melt is lower than 1, the core melt and water filling process always are in unbalance filling state which core melt and water penetrating length of different branch are not equal. Core melt and water unbalance filling trend increased with the increasing of water delay time.In order to avoid the unbalance filling phenomena of core melt and water, the viscosity ratio of core and skin melt of WACIM molding process must be higher than 1.