The areas of greatest interest for the purposes of the rheological
characterization are represented by the junction between the cylinder
and the capillary tube, inwhich occurs section of reduction (die entry region)
and the capillary tube. In the case of extrusion of a plasticmaterial,
the material in the extrusion chamber is subjected only to compressive
stresses and moves with the piston. The material can adhere to the
extruder wall or slide without friction, thus generating either higher
or lower compressive stresses. In the die entry region, thematerial is instead
subjected to shear stresses.