Rheological Properties Of Polymer Micro-Foaming Fluid

The micro-foamed polymer constitutive model is used to describe the rheological properties of micro-foamed plastics during processing.Factors such as shear rate,temperature and pressure,affect the pure polymer constitutive model,moreover,gas concentration,nucleation rate and cell diameter also affect the rheological properties of gas-rich melts.And the interaction of these factors increases the difficulty of constructing the constitutive model.Until now there is no more accurate constitutive model to describe the rheological properties of the gas-rich melt.Therefore,it is significant to research the rheological properties of rich gas melt and construct a more accurate and applicable constitutive model,which not only develops the theoretical system of micro-foamed injection molding,but also provides important guiding significance for process design and production products in actual production processing.Based on the pure polymer constitutive model,compared and analyzed the intrinsic constitutive model of the gas-rich melt,a new constitutive model is proposed in this paper.Experiments were carried out on several important factors which affected the constitutive model which parameters are fitted by the least square method,and the model was optimized and improved.At last,a constitutive model describing the rheological properties of gas-rich melts was successfully constructed by analyzing the effects of temperature,gas content and shear rate on the viscosity.Besides,the reference range of process parameters in production processing was proposed.The main research results were as follows:(1)The ability to describe the gas-rich melt and construction method of Moldflow model is analyzed,based on the Cross-WLF viscosity model of pure polymer melt.It is found that the Moldflow model does not consider the shear rate for the gas-rich melt.The Moldflow model was improved according to the effect of temperature,pressure,shear rate and gas content on the viscosity of rich gas melt,and the rich gas viscosity model based on Moldflow model was proposed.(2)Experimental research was carried out on several important factors that have a great influence on viscosity,such as temperature,gas content and shear rate.The pressure capillary rheometer was used as the experimental instrument.Polypropylene was used as the polymer matrix and the foaming agent was azodicarbonamide(AC).The viscosity was tested at different temperatures and different gas contents,respectively.The result shows that viscosity value was decreasing with the increasing of temperature,foaming agent and the shear rate.(3)The parameters of Cross-WLF equation,Moldflow model equation and the newly established equation were fitted by the least squares method according to the experimental data,and the residuals are analyzed to optimize the Moldflow model.Comparing the fitting results with the experimental data,it was founded that the Moldflow model can predict the change trend of the rich gas viscosity,and the predicted viscosity value is lower than the test value,However,the improved model prediction value is in good agreement with the test value,which proves the advantage of the model.(4)According to the theoretical and experimental results,the optimum temperature should be between 190℃ and 210℃,the optimum foaming agent content should be between 3%and 4%,and the shear rate should be greater than 1000 s-1,when PP is used by microcellular foaming process.