Viscoelastic Simulation of Deformation-Induced Bubble Coalescence in Foaming Process

Viscoelastic simulations of deformation-induced bubble coalescence in forming process under isothermal condition were carried out. Multi mode PTT model was used in a simplified model for deformation of Polypropylene(PP) melt between bubbles in plane strain. 2-Dimentional Lagrangian FEM was adopted in the unsteady creeping flow simulation. The numerical results demonstrates that the thinnest part between neighboring gas bubbles is easily deformable and consequently coalescence occurs. Still more, creep recovery by elastic residual stress promotes the coalescence. The strain-hardening property is very effective to the coalescence prevention. Furthermore, the relaxation time distribution of polymer melts has significant influence on the behavior. Composition of the relaxation time that is a little longer than the elongational time makes a deformed bubble unstable and causes bubbles to coalesce. The numerical simulation could explain the deference in appearances of foamed sheets made of various PP.