Research On Key Technologies Of Microcellular Foam Injection Moulding Process

In recent years, the polymer resin price is rising due to the petroleum shortage. How to save plastics on the premise to ensure the plastics part quality is one of the research hotspots. Microcellular foam injection moulding process is developed in this background. There are large dense microcells in the injection plastics part because of the supercritical CO2 or N2 mixed into the plastics melt. It can save a lot of resin due to the microcells, which is about 5~100μm , in the part. At the same time, the parts also have good mechanical properties. So the technology is widely used in the homework appliance, aerospace and auto industy etc. But recent study shows that big cell size causes the injection part quality problems. Thus one of research directions in this domain is how to control the cell size to ensure the part quality. In this study, the effect of process parameters on the cell size and final part quality is deeply reserached.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 calulation 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. The relationship between nucleation velocity and density with the saturation pressure and melt temperature is obtained. At last the simulation result is compared with experiment results. The results by the new nucleation theory are consistent with experiment results by Mr. Colton and Vipin kumar well. So it shows that the new nucleation model is more accurate to present the whole nucleation process than the classical one.Considering the former work that large difference between the academic results with experiment, the classical cell growth model is edited from below two sides. One is the new nucleation theory is considered as the base of cell growth model. The other is that the material properties model of polymer-gas mixer is built by the improvement of diffusivity, melt viscosity, surface tension and the properties of polymer and gas. Thus the new cell growth theory is founded. The new model is applied into a flat part moulding process and the effect trend of melt temperature, pre-filled volume, injection time, cooling time and SCF percentage on the cell growth is studied.Based on the present nucleation and cell growth model, the effect proportion of process parameters on cell morphology is gotten by using of the numerical simulation and Taguchi experiment methods. The process conditions adjustment programme is proposed to avoid the shortcomings of microcellular foam injection molding parts. Thus the relationship model between main process parameters and cell morphology is gotten by multiple regression analysis method. The regression model is used as objective function in the optimization process. The optimization method is simulated annealing. The total process condition design method is used into the flat model and good result is obtianed. Thus the precision of microcellular foam injection molding process parameters design is guaranteed.The process conditions design system model , which is based on the numerical simulation technology, is founded. And then the whole computer aided design software system of microcellular foam injection molding process design system is programed by visual basic language. The software system includes Taguchi experimet design system and process condition design sysetm. In the latter system, the improved nucleation and cell growth models are integrated. With Taguchi experiment design method, multiple regression analysis method and simulated annealing, the accuracy of computer aided design system of process conditions has been ensured.At last the microcellular foam injection moulding process design system is used into the realitical production of complicated injection part. The process parameters are optimised through the process design methods and software system that are presented in this study. The optimized process conditions reduce the part warpage form 1.8mm to 0.3mm. Thus the part quality is improved and satisfied with the part design requirement. All these show the theories and deisgn methods in the study is accurate and effective on the actual production.