This process causes defects in the final product. To
this purpose, conical nozzles are generally used to allow a gradual decrease
of the section,which avoids the formation of a dead zone. Finally,
in the capillary tube, away from the variation of section, the material
moves according to the piston flow. The movement occurs on a liquid
thin layer with low concentration in solid, in direct contact with the
die wall, in which is concentrated the shear deformation