In 2D extrusion flows, swelling at the die exit is caused by both the velocity rearrangement and the relaxation of the normal stresses. The velocity profile in the channel results mainly from viscous forces, while the normal stress is a viscoelastic effect connected to the shear rate. The material may also exhibit properties such as strain thinning and strain hardening, but these effects in extrusion are negligible.

In 3D extrusion flows, the normal-stress difference also plays a role in the swelling, but velocity rearrangements in 3D generate much more dramatic effects than in 2D. Indeed, a 3D cross-section may be such that the resulting velocity distribution is strongly non-uniform. Typically, low velocities are encountered in narrow cross-sections and tiny details, while high velocities are encountered in wide-open regions. At the die exit, significant deformations may occur, which usually lead to a further reduction of the previously narrow cross-sections.

Therefore, more so in 3D than in 2D, an appropriate flow balancing inside the die (based on stresses, velocity, pressure, and so on) may improve the flow. Finally, as in 2D, the effects of the elongational properties are negligible, compared to those resulting from velocity rearrangement and normal-stress difference, and they can therefore be neglected.