Abstract: A longitudinal stream of cooling air is caused to flow around the cylinder (10) and over the length of each of one or more heating-and-cooling zones. This longitudinal flow is facilitated by specially designed cooling fins (22; 25). The performance of the extruder is improved in that the temperature over the cross-section of the cylinder is more uniform. Moreover, wear and tear on the extrusion screw and on the cylinder are greatly reduced.

Abstract: A device for feeding an extrusion head for plastic material includes a main extruder connected to a main die for extruding a continuous main flow of material, and an auxiliary feed mechanism associated with the main die and selectively controlled to cover the outer exit flow of the main die with a surface layer. The auxiliary feed mechanism continuously conducts two auxiliary flows, a first flow forming the surface layer and the second flow being mixed with the main flow. An invertor valve permits selective exchanging of the two auxiliary flows such that the second flow can cover the main flow and the first flow can be mixed with the main flow.