The Numerical Simulation Of Interfacial Instability For Gas-assisted Co-extrusion Molding

Gas-assisted co-extrusion molding (shorten for GACEM) is a new polymer multiphase-multilayer flow molding technique, and compared with conventional co-extrusion, GACEM has very obvious advantage. Because of layered interfacial instability of co-extrusion always existing in multiphase-multilayer flow molding, the influencing rules of rheological parameters and process parameters of polymer melt on interfacial instability for GACEM were studied by means of numerical simulation in this paper. At the same time, based on theories of rheology and hydrokinetics etc., the mechanism of interfacial instability was disclosed. The following main achievements were yielded:(1) Based on technique characteristics of GACEM and theories of polymer rheology, hydrokinetics and thermodynamics etc. , both the full 3D, unsteady, non-isothermal pure viscous and 2D, unsteady, isothermal viscoelastic theoretical models of describing GACEM process were established by means of reasonable assumption.(2) The tracking of polymer melt front interface and layered interface of non-Newtonian pure viscous and viscoelastic GACEM process were implemented by means of VOF method and moving Lagrange interfacial technique; On the basis of the mixed finite element stable discrete techniques of Galerkin method, penalty function method, SUPG method and DEVSS/SU method etc., corresponding stable finite element numerical models with fast convergence were established for high non-linearity 3D pure viscous and 2D viscoelastic theoretical models of GACEM. Finally, with above numerical model systems, direct finite element numerical simulations of interfacial instability for GACEM were implemented.(3) The influencing rules of polymer melt viscosity, relaxation time, extrusion velocity, polymer melt temperature and depth ratio of melt etc. on interfacial instability of GACEM were studied by means of direct finite element numerical simulation, at the same time, distributed rules of variable fields for velocity, pressure, temperature, shear rate ,first normal stress difference and stress etc. and layered interfacial morphology were given, and based on above research and analysis, interfacial instability of GACEM was caused by the secondary flow which was arose by normal stress difference, and the trend of layered interfacial instability increased as relative jumping degree of normal stress difference on layered interface increasing.