Numerical Simulation Of Three-dimensional Viscoelastic Melt Filling And Packing Processes In Injection Molding

Injection molding is one of the most important methods in plastic molding processing field because of its high production efficiency,high precision of products,and easy to process complex products.During the filling and packing processes of injection molding,the material performance,mold structure and processing parameters which have great influences on the melt flow state,thermal state and stress state,are directly related to the appearance quality and mechanical properties of the final products.However,it is difficult to solve the complicated engineering problem of injection molding by using traditional analytical and experimental methods.With the gradual maturity of computational rheology theory,numerical simulation supplies a powerful tool for studying the polymer melt filling and packing processes.At present,most of the numerical simulations of filling and packing processes are performed based on the 2.5-dimensional(2.5D)viscous flow approximation.However,these2.5D models can not accurately predict the complex rheological and thermodynamic behaviors of visocelastic polymer melt in thick wall or complex cavities.In this dissertation,several mathematical models which can reasonably describe 3D non-isothermal viscoelastic polymer melt filling and packing processes are firstly given based on the characteristics of melt flow and heat transfer in injection molding process.Subsequently,the numerical methods which can effectively solve the corresponding governing equations are developed.Finally,the numerical methods combing the domain extension technique are successfully extended to simulate the filling and packing processes of viscoelastic polymer melt in 3D complex cavities.The main works of this dissertation are as follows:(1)In order to break through the limitations of 2.5D model,a 3D non-isothermal Newtonian-viscous two-phase flow model is applied to describe the viscous melt filling process.The governing equations for gas and viscous melt flow in the cavity are unified into a generalized Navier-Stokes equations based on the Level Set function,which greatly reduces the complexity of 3D multi-field coupling problem.The high-resolution finite difference method is employed to solve the Level Set equations,and the finite volume SIMPLE method on collocated grid is adopted to solve the governing equations of 3D melt flow and heat transfer because of its good stability and easy of implementation.(2)A full 3D non-isothermal Newtonian-viscoelastic two-phase flow model is presented to simulate the multi-variable,strong-coupling and non-linear viscoelastic melt filling process.In this model,the non-linear rheological behavior of branched polymer melt is described by the XPP constitutive equation,and the temperature dependence of polymeric viscosity andrelaxation times are described using the Arrhenius equation.To accurately simulate 3D non-isothermal viscoelastic melt filling process,the energy source term and solid stress boundary conditions are deduced based on the XPP constitutive equation.(3)Based on the 3D Newtonian-viscous two-phase flow model,the 3D viscous melts filling processes are simulated.Firstly,the Cross melt filling process in a thin plate cavity is simulated,and the 3D viscous two-phase flow model is verified by comparing the results with those obtained by MOLDFLOW.Then,the domain extension technique is introduced to handle the complex cavities,and the dynamic simulations for Cross melts filling processes in3 D complex cavities are successfully realized.In particular,the evolution process of melt front interface,fountain effect as well as solidification phenomenon are predicted accurately.(4)Using the 3D Newtonian-viscoelastic two-phase flow model,the 3D dynamic numerical simulations for viscoelastic melts filling processes are performed.The isothermal XPP melt filling process in a plate cavity is simulated firstly,then the non-isothermal XPP melt filling process in a plate cavity with an insert is simulated.The distributions of physical quantities,such as pressure,temperature and flow-induced stresses in 3D cavities are predicted.The effects of model parameters of XPP and processing parameters on the normal stress differences are discussed.Numerical results show that the present 3D model can effectively simulate the real-world viscoelastic melt filling process.(5)A 3D non-isothermal weakly compressible viscoelastic flow model is presented for predicting the non-isothermal XPP melt packing process.The compressibility of melt is described by the dual-domain Tait state equation in this model,and the finite volume CLEAR method on collocated grid is firstly used to solve the governing equations of 3D packing process.In order to improve the accuracy of the simulation of 3D viscoelastic melt packing process,the high resolution AVLSMART schemes for the convective fluxes of momentum and constitutive equations are constructed based on the deferred correction method.(6)The 3D numerical simulation technique for melt packing process is applied to simulate 3D viscoelastic melt packing process.Firstly,the non-isothermal XPP melt packing process in a plate cavity is simulated.The distributions of melt density,pressure,temperature and flow-induced stresses in packing process are studied.The effects of melt temperature and holding pressure on the first normal stress difference and density are analyzed.The predicted flow-induced birefringence is consistent with the experimental result.Then the simulation of XPP melt packing process in 3D hemispherical shell cavity is successfully carried out.