Abstract: An extrusion head for producing hollow profiles such as thick-wall large diameter PVC pipe has its flow channel divided by a hollow mandrel into two coaxial portions of a flow cross section greater than that of the common portion of the flow channel downstream of these coaxial portions and running through the mouth of the head. According to the invention, the flow channel progressively diminishes the flow section over the length thereof at least from the plate at which the coaxial portions start to eliminate structural defects caused by failure of the streams of synthetic resin to adequately merge.

Abstract: An extruder for polymeric materials, comprising a housing forming a mastication stage with one or more feedscrews, includes a preheating and premixing stage with a cylindrical chamber centered on a vertical axis which is eccentrically traversed by a supply passage for the feeding of polymeric material to the mastication stage. The chamber, which is heated from the outside, contains two mixing wheels jointly rotatable about the axis, each wheel having a plurality of raker arms projecting from a hub to the chamber periphery in an off-radial direction and with a tilt causing the arms of the upper wheel to draw incoming fresh material toward the axis and down into the orbit of the arms of the lower wheels which in turn drive the material toward the periphery and, to the extent that an outlet port leading to the mastication stage is obstructed, back into the orbit of the arms of the upper wheel for recirculation.

Abstract: A tube-making die has an outer tube defining a flow axis and having an inner wall centered on the flow axis. A mandrel is supported within the tube by a plurality of radially extending webs and has an outer wall centered on the die axis and turned toward the inner wall. The walls define an annular axially extending flow chamber that is subdivided by the webs into a plurality of passages. At the juncture between the side walls of the webs and the inner and outer walls of the tube and mandrel, respectively, grooves are formed which extend tangentially of the inner and outer walls and/or radially of the side walls of the web.