Numerical Simulation On Microcellular Foam Injection Molding

Microcellular foam plastics containing a high concentration of bubbles on the order of 5 to 100 microns in size can be made by microcellular injection molding (shortened as MIM). It offers several advantages such as lower density, higher dimensional stability, higher impact strength and stiffness, better thermal and electrical insulation properties than unfoamed material, and fewer loss of tensile strength than macrocellular foams. The process of MIM also have some advantages, such as shorter cycle time, lower injection pressure and clamp force etc.. There are four stages in MIM: 1) inject the supercritical CO₂ or N₂ into the barrel, create and maintain a single-phase solution in molten polymer. 2) inject the single-phase into the mold and nucleation happens because of a sudden pressure or temperature drop. 3) The bubble grows and finally fill the mold. 4) solidification and molding. The existence of supercritical gas and the cells make the MIM process much more complicated and the studies on MIM are still few at home and abroad so far, so it seems important to do some research work about microcellular injection molding mechanism. Numerical researches on MIM are implemented in the dissertation, which yield the following main achievements:Employing the "cell unit"model, the cell growth model is established. On the basis of the model, the transferring of mass and momentum during the process of cell growth are described by the theories of hydrokinetics and diffusion law. Considering the coupling interaction of the influence of the cells on the density and viscosity of the macroscopic single-phace fluid with the influence of cell growth on driving of flow, and coupling with the equation of the cell growth, the 2.5D, unsteady isothermal theoretical model describing the microcellular injection molding process is established.By means of the stabled finite element techniques of Discontinuous Galerkin method, upwind method and weighted average method in which the volume contribution of stratified control volume is used as the weighted function, the corresponding effective stable finite element model of MIM process is established. Through numerical simulation on MIM, the relationships of the quality of the molded part with the process parameters and rheological parameters are studied and the influencing laws of those parameters on the final bubble radius, volumetric shrinkage, warpage and residual stress are yielded.In addition, the influence of N₂ as the foaming agent on the process of MIM has been compared with the influence of CO₂. Based on the theory of rheology, the influencing mechanisms are disclosed.