Numerical Simulation Of Microcellular Injection Molding

Microcellular foams, characterized by cell size about 1μm～50μm and cell density about 10⁹ cells/cm³ï½ž10¹² cells/cm³, are drawing increased attention. This concept was based on the theory if a cellular structure could be develop where the bubble size was smaller than the flaws inherent in the polymer structure, the cell could not affect the mechanical properties, however, the cell will reduce the effect of the flaws inherent and stop the further development of the flaws under stress factor. Microcellular injection molding (also commercially known as the Mucell process) was invented by Massachusetts Institute of Technology (M.I.T.) and commercialized by Trexel. The first stage of the MuCell process is to inject supercritical carbon dioxide (CO₂) or nitrogen (N₂) into the barrel, create and maintain a single-phase solution in molten polymer. The second stage is to inject the single-phase into the mold quickly and then a large number of nucleation sites are formed by the sudden pressure drop. The third stage is bubble growth take place during the injection and solidification and the final shape of the part with uniform cell size and density is provided by the mold through solidification.Microcellular plastic has many excellent performances. Compared to conventional injection molding, microcellular injection molding has many benefits as following: Power cost saving 36%; Significant cycle timer reductions 20-50%; Material reductions 5-30%. Microcellular injection molding will enlarge the field of developing new plastic product, optimizing the injection molding process and saving the cost. Microcellular plastic has been widely used in many fields such as avigation and spaceflight, motorcar, medicine, electron, packaging and so on. To improve the efficiency of manufacture and reduce the cost, more and more enterprises have took interest and attention to modify the injection machine for Mucell process.This study presents the theory and process of microcellular injection molding and investigates the process of cell nucleation, cell growth and shaping. We introduce reasonable assumption and present a simulation model for microcellular injection molding based on the numerical algorithms. The main work of this paper include following parts:1. Discuss the process of forming polymer/gas single-phase with supercritical fluid (SCF) mixing, dissolving and diffusing in polymer. Discuss and analyze the theory of bubble nucleation, the development of the model of bubble growth and solidification with the final shape.2. Based on Hydrokinetics and Thermodynamics, a mathematic model was introduced that includes the energy equation, the continuity equation, momentum equation, and a group of equations that describe the mass diffusion of dissolved gas and growth of micro-cells in the microcellular injection molding.3. Taking account of the effect of cell growth on melt flow and viscosity, we treat the microscopic multiphase fluid as a macroscopic single-phase fluid. The equations of continuity and momentum, as well as energy equation are employed to describe the flow and heat transfer in the simulation. The microscopic model of the cell is coupled with the macroscopic flow model to describe the microcellular injection molding.4. Galerkin weighted residual method is employed to discretize the governing equation of pressure field and finite difference method is adopted to discretize the energy equations and the cell growth model. Numerical scheme is configured to solve those discrete equations to simulate the microcellular injection molding.