Screw-type machine
A single screw extruder comprising a casing, at least one bore which is formed in the casing and has a diameter Da, at least one screw which is arranged in each bore in a rotatably drivable manner, which is provided with single-threaded screw elements with an external diameter Da, and which has a screw tip arranged at a downstream end, in relation to a direction of flow, said screw tip in turn being provided with a screw stem having a stem diameter Di, a nozzle plate which is arranged downstream of and adjacent to the screw tip so as to close the at least one bore, which is provided with a plurality of nozzle holes, and which is formed on an annular surface of the nozzle plate, wherein the screw tip is double-threaded with two screw flights which taper in the direction of flow down to the stem diameter Di, wherein the last half screw flight of each screw flight increases in each case in the direction of flow until the external diameter Da is reached, wherein a shearing tool is arranged at the screw tip directly in front of the nozzle plate, wherein the shearing tool has two arms which are in each case arranged superimposed with the downstream end of the respective screw flight of the screw tip and are attached to a hub, wherein to a diameter Dmin of the hub applies.
A stripper device which is elastically supported in the direction of the axis is attached to the down-stream end of thetwin screw extruder, said stripper device bearing against the inner side of the nozzle plate facing the bore and stripping off material to be conveyed. This embodiment is to be used for conveying polymer pellets containing active ingredients which may be pharmaceutical products, plant protection products, feed additives and food additives.
These embodiments are not suitable for the extrusion of materials which are capable of flowing only if a substantial amount of shearing action is provided. Such materials are in particular Bingham media where the material does not start to flow until the flow limit, i.e. a particular shear stress, is exceeded. Such materials are for example plastic ceramic compounds, colloidal dispersions, slurries of glass beads, etc.
A sheet production extruder has a nozzle plate with knives bearing against the inner side thereof, said knives being pressed against the nozzle plate by the material pressure. The function of these knives, which are pressed against the nozzle plate, is to create predetermined breaking points in the individual strands of material which are created in each nozzle hole, ensuring that the individual strand of material breaks off after leaving the nozzle plate.