Simulation Prediction On Bubble Growth Mechanism And Microcellular Injection Molding In Microcellular Foaming

The bubbles size of microcellular plastics is small.A large number of bubbles can reduce the consumption of materials,improve the mechanical properties of products and the thermodynamics of injection molding.The foaming process of microcellular plastics is complicate.It mainly includes cell nucleation,growth and stability.The bubble growth and stability determine the final properties of the microcellular foam plastics.Thus,it is necessary to study bubble growth and stability mechanism in the microcellular molding process,which can promote the development of microcellular foaming technology.The mathematical model of the bubble growth was established by combining the cell model with the momentum equation,the mass equation,the constitutive equation and the diffusion equation.In order to simplify the numerical calculation,the Lagrange coordinate transformation and the finite volume method were used to transform and disperse the control equations,respectively.Based on the mathematical model of bubble growth,bubble stability model was established.The simulation program was written using MATLAB to simulate the bubble growth and stability process.The numerical algorithm is validated by compared the simulated data with the experimental results.A polystyrene?PS?/supercritical CO₂ system was used as an example to study and analyze the bubble growth process and stability under different process conditions.The results show that when temperature up,gas concentration increase and pressure decay,the radius and early growth rate of bubble will increase.The greatest impact on the bubble growth process is CO₂ concentration,followed by pressure,while the effect of temperature is smaller.Increasing the pressure and reducing the gas concentration will increase the bubble stability.The effect of temperature on bubble stability is complicate.lower or higher temperature will shorten bubble stability.The greatest effect of the bubble stability is CO₂ concentration,and the temperature is least.The Moldflow software was used to simulate the filling and pressure maintenance process of microcellular injection molding.The results show that the filling process mainly includes the filling and foaming stage.In the filling stage,the pressure gradually increases and the amount of melt that is ejected also rises linearly with time.In the foaming stage,the pressure suddenly decreases,the bubble nucleus grows quickly,and the amount of melt also decreases accordingly.The pressure-maintaining process is mainly to add a small amount of rubber when the melt is cooled and contracted,and the amount of the ejected melt is less.In addition,based on the molding process,the final microcellular structure under different parameters was further discussed.The results show that the increase of injection time and volume at the start of the foaming will result in the reduction of the final micropore size.Melt temperature has a great effect on the final bubble structure.As the melt temperature gradually increases,the final radius also increases significantly.The bubble radius increases linearly with the mold temperature,in general,the mold temperature has less influence.The filling of the mold cavity will be incomplete when the gas concentration is small.The final radius will increase with the increase of the bubble concentration when the gas concentration is large.The Moldflow software was used to simulate the possible defects in the microcellular injection molding process.The results show that a weld line and a large number of cavitations appear at the boundary of the product.In addition,the value of shrinkage index and residual stress are also higher than the required values.The defects were optimized by orthogonal test to find the best process parameters.The mean and range analysis were used to investigate the effect of mold temperature,melt temperature and injection time on the weld marks,cavitations,sink marks and residual stresses.And the optimal level of the defects was determined respectively.A simulation study based on the best process parameters found that the adjustment of the process parameters has obvious improvement on weld lines,sink marks and residual stresses,but has little effect on cavitations.