Flow‐Regulated Bubble Evolution in Supercritical N 2 Extrusion Foaming of FEP in a Multi‐Wire Die

ABSTRACT Supercritical N 2 extrusion foaming of fluorinated ethylene propylene (FEP) within a geometrically constrained multi‐wire die involves coupled melt flow, gas transport, bubble nucleation, and bubble growth. In this study, a coupled finite‐element and population‐balance framework was developed to describe these processes. The rheological properties of the FEP melt and the solubility and diffusivity of N 2 in FEP were determined experimentally at 300°C and incorporated into the model. Dissolved‐N 2 transport was coupled with classical‐nucleation‐theory‐based bubble nucleation and diffusion‐controlled bubble growth. The effective nucleation parameter was calibrated using the 19 MPa extrusion foaming experiment and then kept unchanged for assessment at 21 and 23 MPa. The simulations reproduced the characteristic cell‐size scale and the experimentally observed decrease in mean cell diameter with increasing pressure. The simulations show that the four‐wire configuration produces persistent cross‐sectional variations in velocity and convective transport, while the major pressure release occurs in the converging and downstream shaping sections. The resulting pressure decrease drives supersaturation ratio development and bubble nucleation. Increasing the inlet pressure enhances the predicted nucleation response and shifts the outlet bubble‐size distribution toward smaller diameters, whereas increasing the downstream die length extends the effective growth path and promotes bubble expansion. Orthogonal range analysis further indicates that the downstream die length has the strongest influence on bubble diameter and gas volume fraction, while the upstream die length has the largest influence on bubble number density within the investigated design space. These results provide a flow‐based framework for understanding and regulating bubble‐structure development within a geometrically constrained multi‐wire die.