Extrudate Swell

Elastic Storage

Molecular entanglements act like springs that are loaded under the high pressure of the extruder. When the melt exits the orifice, the extrudate swell magnitude depends on how much of that energy was able to relax during the transit through the die land.

Tooling Compensation

Die designers must size the opening smaller than the desired final part dimensions to account for this predictable expansion. Because extrudate swell is influenced by temperature and shear rate, a die that works for one resin may produce incorrect dimensions if the material is swapped.

Dimensional Control

Wall thickness in complex window profiles is maintained by balancing the flow rates and the resulting expansion across different segments of the die. High levels of extrudate swell can lead to distorted shapes or internal stresses that cause the part to warp after cooling. Moulders use rheological additives to reduce this effect when high precision is required.