5. Single-channel

The regularity of layer thickness depends to a large extent on the die: as it passes through, the interfaces continue to evolve under the action of viscous and elastic forces. As the trajectories of the particles vary considerably, depending on whether they pass through the center or the edges, the resulting deformations are not homogeneous.

Each polymer, in its layer, has its own viscosity characteristics, and each layer is subject to different shear rates depending on its position in the structure; in addition to shear forces, there are elastic forces (due to normal stress differences) acting perpendicular to the interfaces, resulting in a very complex dynamic force field that deforms the interfaces as the flow spreads out in the die, stretches and becomes thinner.

The impact of elastic forces, with certain polymers, is far from negligible...