Numerical Study of Die Design for PVC Foam Extrusion

Bubble growth inside the die is one of the critical issues in the PVC foam extrusion process. Burning and clogging might have occurred when bubble growth exceeds the critical size, leading to non-uniform outlet velocity and pressure profile inside the die. Therefore, a design modification for die geometry is necessary. This study aims to suppress the bubble generation by making the pressure inside the die greater than the bubble nucleation pressure and to make the velocity distribution uniform at the die exit. A new concept of adding choker bars inside different dies (namely, T-die, fishtail die, and coat-hanger die) is proposed. Three choker bars consecutively placed one after another inside the dies slit sections were numerically simulated to observe and discuss the effects of the proposed geometric shape on the outlet velocity profile in the non-isothermal generalized Newtonian flow. The results show that the velocity distributions at the exit of the die were nearly uniform in all three dies. Among the dies, the fishtail die shows the best outlet velocity uniformity. The pressure values inside all dies are found to be higher than the nucleation pressure except at the last 0.5 mm near the die outlet. This confirmed that the bubble growth could only begin near the 0.5 mm die outlet region in all dies.