Model And Simulation Of Viscoelastic Polymer Melts Injection Molding

Injection molding is one of the most widely employed method of polymer processing. The flow of polymer melts in a cold cavity is a typical example of an unsteady, non-isothermal flow of viscoelastic fluids. Every particle in the material experiences a complex thermo-mechanic history which is important since the viscoelastic nature of the polymer results in development of shear stress and normal stress and large elastic deformation during flow with subsequent incomplete relaxation during the cooling stage. The resultant residual stresses, which determine the orientation in the final molded part, the orientation is important since it influences the mechanical and optical performance of the molded part. Because of the complexity of the process, numerical simulation is essential to understand the mechanism behind orientation. The objective of this thesis is to develop a numerical simulation model for the buildup and relaxation of stresses and molecular orientation in injection molding process of amorphous polymer. The following works have been finished:1. In terms of continuum mechanics and nonequilibrium irreversible thermodynamics theories, compressible Leonov constitutive model is derived to describe large recoverable elastic strain development in non-isothermal flow of compressible viscoelastic melts. It is tempting to specify the model and then compare it to basic rheological tests in the hope that such a model will yield a good description of the nonlinear viscoelasticity of polymer melts. This sort of preliminary attempt was made for steady and small-amplitude oscillatory shear flow and stress relaxation following cessation of steady shear flow.2. A unified method for handing the temperature dependent of both dynamic and steady rheological data is presented. Based on assumption that relaxation spectra derived from data at different temperature can be made to superimpose by vertical and horizontal shifts, vertical and horizontal activation energies are estimated independently from dynamic rheological data, then using the estimated activation energies, shift the raw data to reference temperature for extracting the temperature dependence.3. An efficient numerical simulation has been developed for predicting 3D temperature profile of injection mold. In the simulation, the fluctuating component of mold wall temperature is considered to be negligibly small and mold heat transfer is reduced as three-dimensional static heat conduction; heat transfer within the molded-part is treated as transient one-dimensional heatconduction. An modified three-dimensional boundary element method is used for solving cyclic-averaged temperature of the mold containing complex and thin cavity and circular cooling channels.4. To simulate buildup and relaxation of flow-induced stresses and molecular orientation in injection molding process, A mathematical model is derived that describes the unsteady and non-isothermal flow of viscoelastic polymer melts in the thin wall mold cavity on the base of thin film approximation. By means of numerical simulation, the results are given in terms of residual stresses and associated birefringence in the molded part, as influenced by the processing conditions. The result indicates that, for a given polymer, the main factors affecting residual stresses and associated birefringence are flow rate and melt temperature.This work is supported by the project of Research on the Polymer Processing volution of Micro structureart Quality Control Eased on Numerical Simulation, supported by The National High Technology Research and Development Program (863 Program,2002AA336120).