Double screw extruder working thermoplastics especially PVC, avoiding overheating, inhomogeneity and excessive wear

摘要

Over 10 % - 70 % of the screw length starting from the inlet, indentations or pockets are machined into the projecting thread sections of the screw(s). Additionally or instead, they are made into the screw core, in the so-called orphan gaps and/or in outer regions of the threads. The gaps vary in size with rotation of the screw. An Independent claim is included for the corresponding processing method. Preferred features: Threads of both screws are similarly dressed. The configuration assures material passes through regions of intense shearing, alternating with regions of comparative calm and mixing. Recesses are introduced radially, or at an angle to the radial, of no more than ! 45 deg . Their depth is 0.25-2.5 times that of the gap, preferably 0.0 % - 0.8 % of the gap width. Width of the threads, measured at the location of the recesses, is 65 % - 95 % of the original width. Semicircular recesses are made into the core. This ensures corresponding roller gap width variation twice per rotation or more. In the semicircular depressions, the melt is worked, as though in the nip of a calendar. Depressions are machined into the core, causing the roller gap (17) to alter during each revolution. The depth of the gap varies about a mean value, by up to ! 25%, preferably by 15%. In the running surfaces of the threads, semicircular depressions are introduced. This ensures that melt experiences both axial conveying and localized rotation, the mixing and working caused, resembles that occurring in a roller gap.