Research On Surface Quality Improvement Of Microcellular Foam Injection Molding Parts

With oil prices rising in recent years, the polymer resin prices is also rising. Based on the plastics parts quality, how to use new process to save materials is a research hotspot now. Microcellular foam processing technology was born in this background. The microcellular foam technology can reduce parts weight, shrinkage and dimensional changes, eliminate the products sink marks, reduce the melting viscosity, process cycle times and process holding pressure. Thus, this technology has been widely used in home application, aerospace, automotive and other industries. The effect of process parameters on surface roughness of microcellular injection parts is studied in this paper.Based on the improved models of the classical nucleation, the cell growth and surface roughness, the surface roughness values of microcellular foam injection molding parts is calculated. MOLDFLOW software is used to simulate the polymer melt flilling process and flow front temperature distribution. Equations are solved by MATLAB software The effect proportion of process parameters on part's surface roughness is gotten by Taguchi experiment methods. The relationship between main process parameters and part surface roughness is obtained by multiple regression analysis method.On the base of classical nucleation theory and effect of the supercritical gas on the free energy, the duality system model of polymer melt and supercritical gas is built. Thus refer to the constant of mass quantity in unit mole system and the calculation model of thermodynamic chemical potential, the free energy change mathematic model is founded. At the same time, considering the surface energy difference between the mixer of polymer melt and supercritical gas with only polymer melt, the mixer free energy calculation model is setup through the weight fraction of gas in the mixer. The free energy and surface energy of mixer is used to edit the classic nucleation theory. And the new nucleation model is built. The new nucleation theory is applied into a foam system. Use this new nucleation theory to get the nucleation velocity and density in the PC/N2 foam system.After the simulation model built in the MOLDFLOW software, above achieved nucleation velocity and density are set as boundary conditions. Then, filling result and flow front temperature distribution at the reference point are gotten.The bubble growth model offered by Amon and Denson is used to simulate the bubble growth. Expansion of the bubble occurs due to two mechanisms: hydrodynamical controlled growth and diffusion-controlled growth. The two control equations are transformed into the simplified equations. The two equations are solved to obtain the final radius of the parts surface bubble. The filling time and temperature at the reference point, the nucleation velocity and density are used as the boundary conditions to calculate the bubble radius at the reference point.The surface roughness calculation model is established. Then the radius of the surface bubble can be transformed into surface roughness values. The simulate result is 23.61μm and the experiment result is 26μm. So the research method is appropriate for predicting the part surface roughness.Finally, the Taguchi experimental methods, multiple regression techniques are used to study the relationship between microcellular injection molding parts surface roughness and process parameters. The process parameters adjustment method to reduce microcellular injection parts surface roughness is provided. A mathematical model between the processing parameters and microcellular parts surface roughness are gotten by the linear regression analysis. And the regression model statistical analysis shows the relationship between process parameters and surface roughness can be expressed well by the mathematical model.