Extrusion Blog

Die swell (extrudate swell) is the expansion of molten polymer beyond the die opening’s dimensions as it exits the die. It happens because the melt stores elastic strain while flowing through the die and releases it once confining wall pressure disappears at the exit. Die designers compensate by undersizing the die opening relative to the target part dimension, adjusting land length, and streamlining the entrance to control how much elastic strain builds up.

Melt fracture is surface or bulk distortion in extruded polymer—from fine sharkskin roughness to severe chaotic waviness—that appears once shear stress at the die wall exceeds the polymer’s critical shear rate. Causes include excessive output rate, sharp die geometry, and low melt elasticity. Fixes reduce shear stress, redesign the die land and entrance, or add processing aids.