The Numerical Simulation On Three Dimension Viscoelastic Multilayer Coextrusion Molding Process

In coextrusion molding process, the interaction between the polymer melt viscoelastic nature and unavoidable fluctuations of the operating conditions results in fluctuating die swell which makes the producing of the multilayer coextrusion profile with the precise size and complex shape a technology challenge.Because of the interaction of stresses on the interfaces between adjacent melts of the multiphase-multilayer flow molding process of the polymer coextrusion molding, therefore, the multiphase-multilayer flow molding process of the polymer coextrusion has its special transport rule of flow and dynamic characters, which makes its coextrusion swell mechanism very complicated. Up to the present, few of researches on the coextrusion swell have been reported at home and abroad. Based on theories of rheology and viscoelastic hydrokinetics etc., the full three dimension isothermal viscoelastic theory model describing coextrusion process and the corresponding stable numerical algorithm with fast convergent were established in this paper, and the influencing rules and mechanism of process parameters and polymer melt viscoelastic rheological parameters on the coextrusion die swell were studied by means of numerical simulation in this pape. Based on above study, a new advanced gas-assisted coextrusion molding technology was putt forward in which the coextrusion die swell can be basically eliminated , and the mechanism of eliminating the coextrusion die swell was disclosed by theoretical analysis. The following main achievements were yielded:Based on process characteristics of coextrusion molding and gas-assisted coextrusion molding and the theories of rheology and hydrokinetics etc., under reasonable assumption, the full 3-D steady isothermal viscoelastic theoretical model of describing coextrusion and gas-assisted extrusion molding process was established. In order to establish the finite element model of the theoretical model easily, the expansions of PTT constructive equation along x, y, z coordinates were derived.Considering the non-convergence and unstable problem of solving the equations due to the coupling of the viscoelasticity, the tracing of the swell free surface and control equation, the corresponding high efficient stable finite element numerical algorithm with fast convergence was established by the mixed stable finite element discrete techniques of Galerkin method, penalty function method, EVSS method, EVSS/SU method and Mini-Element method etc.. And based on this research, the finite element numerical simulation of coextrusion molding and gas-assisted coextrusion molding process were implemented.The influencing rules of the viscoelastic property parameters and process parameters on the coextrusion die swell were systematically studied by means of direct finite element numerical simulation, and the mechanism of coextrusion die swell was disclosed by theoretical analysis. The research results showed that the coextrusion die swell was caused by the melt secondary flow in the multilayer coextrusion molding process, and the die swell of core melt, skin melt and whole die mainly depent on the intensity of the melt secondary flow in the nearby exit of die, which is proportioned to the first normal stress difference.Based on the mechanism research of a traditional coextrusion molding die swell, a new advanced gas-assisted coextrusion molding technology was putt forward in which the coextrusion die swell can be basically eliminated, and the mechanism of eliminating the coextrusion die swell was disclosed by theoretical analysis. The research results showed that the gas-assisted coextrusion molding die swell is proportioned to the intensity of the melt sedcondary flow in the nearby inlet of gas assisted segment, and the gas-assisted coextrusion molding die swell is hardly impacted by the melt viscoelastic rheological properties, which makes one die to be suitable for different polymer molding.The gas-assisted coextrusion molding process can not only implemente the automatic and accurate control of production dimension, but also can save energy. At the same time, the technological difficulty that is caused by the traditional coextrusion die swell was solved in the design of a coextrusion die.