Meaning
Elastic recovery causes a polymer melt to expand in cross-section as it emerges from an extrusion die. Known as extrusate swell, this phenomenon arises because the polymer molecules decompress and return to their random coil state after being squeezed through the narrow die channel. It governs the final dimensions of extruded profiles, tubes, and blow-molded parisons.
The effect stops being noticeable at very low shear rates or high temperatures.
Polymer Elasticity
Melt elasticity is linked to the molecular weight distribution of the resin. Highly branched polymers exhibit considerable elastic memory and thus undergo greater extrusate swell when exiting the die. This behavior requires molders to adjust the die dimensions to compensate for the expected expansion.
Die Geometry
Long land lengths in a die allow the polymer molecules more time to relax before they exit, which reduces the swell. Increasing the shear rate through the die conversely increases the degree of expansion. Proper design of the die helps to control the size of the extruded profile.
Defect Control
Uncontrolled swell leads to thick walls and dimensional non-compliance in finished profiles. It is managed by adjusting the draw-down ratio and cooling line speed. Correct puller speed ensures the product reaches its target diameter.
