Research Of Residual Stresses And Warpage Of Plastic Injection Molding Based On Surface Model

In this thesis, concerning the characteristic of injection molding, appropriate assumptions and boundary conditions are introduced, and the mathematical models and numerical methods of residual stresses and warpage are built. Based on the presented models, commercialized simulation software of residual stresses and warpage analysis of injection molding are developed.Based on the theories of viscoelastic and time-temperature superposition, linear thermal viscoelastic model and thermal viscous elastic model are presented. By introducing appropriate assumptions, incremental formulation of linear thermo viscoelastic model and linear thermo viscous elastic model are built. In these models, the effects of the packing pressure and temperature history are taken into account adequately. The process of residual stresses computation is divided into three steps, appropriate boundary conditions are employed according to the feature of the each steps. In this model, finite difference method is used to discrete the mathematical models spatially and temporally. The simulation results show that the linear thermal viscoelastic model can be replaced by linear thermo viscous elastic model from the view of engineering, since the results of the two models are close.During warpage analysis, flat shell element is employed, which is consist of membrane element and plate bending element. By comparing several typical membrane elements and plate bending elements, the Optimal Triangle Membrane Element (OPT element) and Refined Discrete Kirchhoff Mindlin bending element (RDKTM element) are selected. The flat shell element which is consists of the selected OPT element and RDKTM element exhibits excellent performance, since it has accurate displace formulation and is insensitive to aspect ration of elements.The conventional shell finite element analysis is based on the middle surface, but present injection molding CAE is based on the surface. In order to solve this disaccord, a novel approach of warpage analysis based on surface model is presented. In this approach, the thickness of mesh is set as the half thickness of local thickness of part since meshes exist at both of the upper and bottom of the part. Multi-point Constraint (MPC) is used to handle the relationship of two corresponding nodes which distribute at upper and bottom meshes respectively. The typical algorithms for dealing with MPC equations are the master-slave elimination method, penalty function and Lagrange multiplier methods. Concerning many injection molded parts exhibit complex geometry, each of these three methods is not competent. In this thesis, the Lagrange multiplier based elimination method is presented. This method can assure the structural response of parts is correct, and it has the performance of high computational efficiency and simple implementation process.By introducing Design of Experiment (DOE), a large number of parts are manufactured. The effects of melt temperature, injection rate, mold temperature, packing pressure, packing time and cooling time on the final shrinkage and warpage of parts are analyzed using Analysis of Variance (ANOVA). The simulations based on the models presented in this thesis under the same process conditions with that of DOE is carried out, the simulation results match that of experiment basically, thereby the simulations are validated.