The production process for expanded polypropylene foam beads (EPP),which consists of steaming,depressurizing,cooling,and ageing operations,was consistently simulated by a set of mathematical models.The models,developed from Yang and Lee's ageing models (Yang and Lee,J.Cell.Plast,39,59 (2003)) for extrusion foam products,were extended in terms of the fundamental aspects of mass and heat transport phenomena.Evaporation and condensation of blowing agents and heat conduction during the steam chest molding and ageing processes were modeled.The governing equations were established by integrating the mass transfer equations of steam and air at the intercellular walls with the constitutive equations of evaporation and condensation in each cell,the equation of heat conduction from the mold to the foam,and the mechanical force balance equation on the cell walls.The models simulate the stress exerted on the steaming chest mold and predict the expansion behavior of cells in the EPP bead-foam board throughout the ageing process.A sensitivity analysis was also performed by the models to find the key factors which might allow the theoretical determination of the ejection time and shortening of the length of the ageing process.
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