Extrusion

Monofilament Die Swell (Barus Effect) Calculator

The filament leaving the die is fatter than the hole it came from. Cut the die for the swell, not for the product.

Die Capillary
mm

First normal stress difference over twice the wall shear stress.

Downstream Draw-down
×

Take-up speed over extrusion speed at the die.

Die Swell Ratio

— ×

Extrudate diameter over die diameter

Extrudate Geometry

Extrudate Diameter
— mm
Diameter After Draw-Down
— mm
Cross-Section Swell
— ×
Extrudate Cross-Section
— mm²
Diameter Increase
— %

Tanner's relation assumes a long capillary with fully developed flow; short dies swell more because the entry strain has not relaxed. Swell also falls as die temperature rises and as residence time in the land lengthens, so treat this as the starting point for a die trial rather than a final dimension.

Monofilament Die Swell (Barus Effect) Calculator — free, with the formula and a worked example, at Textile School.