Numerical Simulation Of PET Supercritical CO₂ Microcellular Foaming Extrusion Process

With the increasing demand for plastic products,the manufacture of low-cost,low-density and high-strength plastic materials has received extensive attention.Under the background of’carbon neutralization’,the research on polyethylene terephthalate（PET）and supercritical CO₂ microcellular foaming materials has become a new hot spot.Microcellular foaming is a foaming material with a cell diameter of 10μm and a cell density of more than10⁸ cells/cm³.PET material has the advantages of recyclability and CO₂ gas has the advantages of low manufacturing cost and high safety.The PET microcellular foamed sheet manufactured by the two has the advantages of good mechanical properties and fatigue resistance,low water absorption,high thermal stability and safety,which can be used in high temperature environment.In this thesis,a PET microcellular foaming extrusion die suitable for a single-stage co-rotating twin-screw extruder system was designed.Through the Computational Fluid Dynamics（CFD）software Fluent,the mold flow channel was simplified to establish a physical model,and the numerical simulation method of the extrusion process of polymer microcellular foaming was studied.In order to accurately simulate the extrusion process of polymer homogeneous solution,the theoretical model of foaming process was analyzed,and the numerical simulation method of gas-liquid two-phase flow high pressure extrusion process of viscoelastic non-Newtonian fluid in Fluent software was studied.The established model was used to study the stability of single bubble extrusion in shear flow field from a microscopic point of view,and the uniformity of bubble distribution in polymer homogeneous solution extrusion process from a macroscopic point of view.Finally,experimental verification was carried out.The specific research work is as follows:1.The extrusion system of a single-stage co-rotating twin-screw PET supercritical CO₂microcellular foaming sheet was introduced,and a PET microcellular foaming extrusion die was designed.The theoretical model of bubble nucleation and growth in the extrusion channel was studied.The flow state of PET homogeneous solution in the extrusion process was analyzed,and the fully developed inlet flow model was used to describe the dynamic extrusion flow of PET.2.The User-Defined Functions（UDF）program which can accurately describe the extrusion flow of PET melt was written in Fluent software.In the circular tube shear flow field,the extrusion state of supercritical CO₂ bubbles in the flow channel was simulated,and the stability of bubble extrusion in the extrusion process was studied.Comparing the simulation results,it is found that the increase of shear rate is the main factor causing the increase of bubble aspect ratio.The extrusion speed has little effect on the bubble aspect ratio.The increase of polymer rheological index and surface tension is beneficial to the stability of supercritical CO₂ bubbles during extrusion.3.Based on the Computational Fluid Dynamics-Population Balance Model（CFD-PBM）,the bubble distribution phenomenon in the extrusion process was simulated by using the extrusion die designed in this thesis,and the influence of process conditions on the uniformity of bubble distribution in microcellular foam sheet was studied.Comparing the simulation results,it is found that the bubble size distribution in the whole outlet area is more uniform under the condition of 60°template convergence angle.By increasing the extrusion speed and reducing the initial volume fraction of supercritical CO₂ under the premise of satisfying the foaming ratio,the uniformity of bubble distribution can be improved.4.The extrusion experiment was carried out by using a single-stage twin-screw PET microcellular foaming sheet extrusion system.The bubble distribution was observed by SEM.The experimental results were compared with the numerical simulation results to verify that the established simulation model can accurately simulate the PET microcellular foaming extrusion process.The optimization scheme of PET supercritical CO₂ microcellular foaming extrusion process was proposed.