Extrusion Die Swell Calculator

Die swell (also called extrudate swell or the Barus effect) is what happens when a polymer melt exits an extrusion die and expands beyond the die's own opening — the melt has been storing elastic energy from being sheared and stretched on its way through the die, and it recovers that energy once it's free of the die walls, causing the cross-section to bulge. This calculator estimates the swell ratio B (extrudate diameter ÷ die diameter) by starting from a base swell value tied to the resin's inherent melt elasticity (PVC barely swells at 5%, while LDPE — known for high melt elasticity — can swell 60% or more), then adjusting for three process variables: a longer die land length relative to diameter (L/D ratio) gives the melt more time to relax before exiting, cutting swell roughly as the inverse square root of L/D; higher shear rate pumps in more stored elastic energy and increases swell (a weak power-law relationship here); and lower melt temperature relative to the material's reference processing temperature increases swell because cooler melt is more elastic. Because swell inflates the extrudate beyond the die opening, achieving a specific target diameter means under-sizing the die: requiredDieDiameter = targetDiameter / swellRatio, which is the number a toolmaker actually needs to cut the die correctly.

The calculator also back-solves for the L/D ratio that would keep swell under a 10% target for the given material and conditions. Keep in mind this uses simplified empirical correlations rather than full viscoelastic modeling (like the Tanner equation from first normal stress differences) — real die swell also depends on flow history, draw-down after the die, and cooling rate, so validate against a trial run before cutting production tooling.