Population Balance Model For Foaming Process In Extrusion Die

The preparation of lightweight materials has always been the focus of the development of new materials in China in which high-performance foaming materials are the most representative lightweight materials.Physical foaming with supercritical carbon dioxide(scCO2)is a green and safe manufacturing technology.Studying the foaming process of polymer/CO2 melt in the extrusion die has important theoretical research and application value for the preparation of high-performance foaming materials.In this paper,a polystyrene(PS)/CO2 foaming system was used as the research object.The experimental results show that processing conditions,including the screw speed and the foaming temperature,can affect the behavior of the cells crushing and coalescence,providing theoretical guidance for the preparation of excellent foaming products.The influencing factors of foaming process include:solubility,surface tension,glass transition temperature,and the rheological property of the polymer under scCO2.In this paper,SL equation was used to obtain the solubility of CO2 in PS.The glass transition temperature was determined by the Chow model.The effects of temperature,pressure,and CO2 concentration on the zero shear viscosity were predicted by the WLF model and the WLF-Chow model.A finite volume method was used to construct a three-dimensional numerical model of the foaming process of PS in the extrusion die.Coupling with rheokinetics model,continuity equation,momentum equation,laminar diffusion equation of species concentration,population balance model,cell stretch model and bubble growth model,the mathematic descriptions of PS extrusion foaming process was accomplished.In order to ensure the reliability of the numerical model and method,the results of experiment was provided for the simulation calculation,and the influence of shear on the dynamic evolution of bubble was clarified.On this basis,the effect of die structure on foaming process was studied,and the effect of shearing and stretching on bubble nucleation and growth were quantitatively characterized.The results show that the die shrinkage can affect the distribution of shear flow field,the convergence angle of can affect the distribution of shear and tension stress,and the effect of shear on nucleation is greater than tensile.