Research For The Key Technology Of 3D Numerical Simulation Of Plastic Injection Molding And Crystallization

Injection molding is one of the most important manufacturing processes for plastic products. The quality and performance of injection molded parts depend not only on the material, but also on how the material is processed. With computer-aided engineering(CAE) tools, better understanding of parts during process can be achieved to help engineers to improve part design and optimize processing conditions. However the traditional CAE techniques for the simulation of injection molding, called by the middle-plane technique and dual domain technique based on Hele-Shaw approximation, pose inherent limitations when the part has complex geometrical configuration or thick walls. Three dimensional (3D) simulation based on the solid element can give deeper insight into molding process by providing more detailed information than traditional techniques.Injection molding process and crystallization of semicrystalline polymers were modeled systematically using the finite element method, and programs were developed to simulate the injection molding and crystallization. The following work was included:(1) The GLS(Galerkin/Least-squares) method is employed to prevent the potential numerical instabilities by adding to the weighting functions with their derivatives, resulting in the integrated symmetric and stabilized finite element formulations using arbitrary interpolation functions for velocity and pressure. The similar segregated stabilized finite element formulation using equal-order interpolation functions for velocity and pressure can be obtained based on the PSPG(Pressure-Stabilizing/ Petrov-Galerkin) method. Because of the convection term and small heat conduction coefficient in the governing equation of energy, spurious oscillation associated with the classic Galerkin finite element method is induced. And GLS/GGLS(Galerkin gradient least-squares) methods are applied to avoid these oscillations. The expanded 3D FAN scheme is applied to capture the advanced melt front.(2) During the packing stage, the polymer melt solidifies partly with the decrease of temperature. Molten and solid polymers interact, and act as a whole entity by the packing pressure. A 3D two-phase coupling model combining the flow of compressible melt with the deflection of viscoelastic solid is established. During the cooling stage, a 3D viscoelastic constitutive model is presented to show the evolvement of residual thermal stresses while part deflects confined to the mold cavity. To simulate the warpage of plastic parts ejected from cavity, a 3D elastic constitutive model is given. And so far, the complete 3D approximation of postfilling is made.(3) In the numerical simulation of filling stage, advanced melt front, fountain flow, effect of runway and temperature field of the part were studied. Fine and coarse finite element mesh were used to show the stability and accuracy of the proposed algorithms. Different processing conditions were adopted to investigate the effect of processing parameters on the filling stage. Results were compared to show that the hypothesis of compressible flow leads to more reasonable injection pressure than incompressible flow. Examples showed that the proposed algorithms could be accurate and practical for parts with complex geometry.(4) Numerical simulations were carried out to study the effect of processing parameters on the residual thermal stresses and shrinkage of parts. The elastic and viscoelastic constitutive models were adopted seperately to computer residual thermal stresses and deflection for the solid during the packing and cooling stages.(5) The stress-induced induction time index and crystallization models for semicrystalline plastics were proposed based on the theory that stress-induced orientation of polymer chains increases the equilibrium melting temperature. The effect of crystallinity on viscosity of the polymer melt and temperature due to latent heat of fusion is described. To investigate the effect of processing parameters on the crystallinity, the injection molding process of semicrystalline plastics was simulated under different processing conditions.In addition, lots of numerical examples showed the results of presented 3D approaches developed herein were in good agreement with the experimental results or results of the well-known commercial software Moldflow. It was suggested that the presented 3D approaches could present accurate, stable and practical results.