Experimental and numerical investigation of shape asymmetry in slit die foam extrusion

Foam extrusion through slit dies often suffers from shape asymmetry that compromise product quality and increase waste. This work combines experiments and computational fluid dynamics simulations to investigate the influence of die geometry and pulling velocity on foam board morphology. Two slit dies with different central heights are tested at varying pulling speeds using low-density polyethylene with isobutane as a blowing agent. Experiments show that dies with a large central height promote nearly rectangular boards with uniform bubble morphology, whereas those with a small gap produce pronounced N-shaped distortions and surface irregularities. Higher pulling velocity mitigates asymmetry of the extrudate shape but reduces expansion and leads to smaller, more elongated bubbles due to rapid quenching. Despite employing a simplified Newtonian rheology and a reduced computational domain, simulations qualitatively reproduce the experimental trends, elucidating the contribution of the velocity and pressure gradients at the die exit that drive asymmetric expansion.