Numerical Simulation Of Microcellular Plastics Extrusion Molding Process

Micocellular plastics,characterized by cell size smaller than10μm and cell density larger than 10⁹cells/cm³,are drawing increased attention due to their unique properties and potential applications.In this study,based on theories of polymer rheology,hydrokinetics and thermodynamics etc.,the governing equations of a continuous extrusion foaming process in the capillary nozzle are presented.The equations are solved by using finite element software POLYFLOW and postprocess software FIELDVIEW.Supercritical carbon dioxide(CO₂),a potential replacement of the traditional foaming agents(chlorofluorocarbons),is applied to produce microcellular foams of polystyrene.The design of foams with a desired cell structure mainly depends on the selection of proper operating conditions.The contraction flow in the extrusion die is simulated with POLYFLOW to predict profiles of pressure,velocity,shear—rate and stress.The results show that:1.The viscosity of PS/CO₂ melt is decreased with the increase of CO₂ concentration,the impact of melt viscosity in high—concentration areas small than low—concentration areas.In the areas of CO₂ dissolved,the increasing of the CO₂ concentration and flow rate can increase the percentage of the nuclear cell number, but the cell size is also increased.Lower die temperature can generate high density and smaller cell size.Higher pressure drop rate is benefit to a higher nucleation and smaller size of the cells.2.PS/CO₂ melt gets stronger shear and stretch stress with the increasing of the convergent angle,this will benefit to a smaller size of a cell.with the reduce of capillary nozzle's diameter,pressure drop and pressure drop rate in the die get larger, which will benefit to a higher density and smaller size cell.The Supercritical fluid injection system was designed by ourselves.Extruder barrel and gas injection inlet was ameliorated and improved,a new extruder screw was optimized and designed through the numerical simulation.Integrated with the result of simulation,die molding for forming was designed,and Micocellular foams extrusion molding device was constructed preliminary.