Numerical Simulation Of Flow Within The Die And Cell Nucleation In The Microcellular Plastics Extrusion Molding Process

Microcellular foams, characterized by cell size smaller than 10μm and celldensity larger than 10⁹cells/cm³, are drawing increased attention due to their uniqueproperties and potential applications. In this study, based on theories of polymerrheology, hydrokinetics and thermodynamics etc., the governing equations of acontinuous extrusion foaming process in the capillary nozzle are presented. Theequations are solved by a finite element method using FLUENT (CFD) andTECPLOT software is applied to deal with a great deal of data.Supercritical carbon dioxide (CO₂), a potential replacement of the traditionalfoaming agents (chlorofluorocarbons), is applied to produce microcellular foams ofpolystyrene. The design of foams with a desired cell structure mainly depends on theselection of proper operating conditions. Among them, foaming temperature, pressuredrop or pressure drop rate, and CO₂ concentration are three key variables that need tobe addressed. These three variables determine the changes in the viscosity, solubility,surface tension, diffusivity, and other physical properties, as well as playing importantroles in mechanisms of cell nucleation and cell growth. The state of the melt flowduring an extrusion foaming process including the distribution of pressure field,velocity field etc. are computed and analyzed in details, and the effects of operatingconditions on the cell structure are studied qualitatively.It is shown that the viscosity reduction in the high CO₂ concentration or a highCO₂ content region is not as dramatic as the low CO₂ content region, a high CO₂concentration or a high mass flow rates induces a higher nucleation density but alarger cell size. Above the foaming temperature, cell size decreases and cell densityincreases with a decrease of the temperature. What's more, a large pressure drop orpressure drop rate is favorable for our purposes (i.e., a small cell size and a large celldensity).The conclusion of this dissertation can optimize technics of a continuousextrusion foaming process.