The screw element
The screw portions of sheet production extruder element between the ring portions first cause a pressure buildup which then urges the substance through the annular gap between extruder housing and the ring portions with shearing action and elongation, and the pressure is reduced again. The recurring sequence of shear gap passage, pressure buildup, shear gap passage, etc., on the screw elements causes a defined stress on the substance and thus a uniform stress, without unduly stressing the substance or the screw.
The annular and/or shear gap between the ring portions and the concave circular segments of the twin screw extruder housing can have a different height to produce a sufficient mixing and/or kneading effect in accordance with the particular substance. The ring portion might for this purpose correspond only to the core diameter of the screw shaft. However, the annular gap preferably has a height of from 10 percent to 90 percent of the flight depth of the screw. Especially preferred is a diameter of the ring portions that corresponds approximately to the center distance of two adjacent shafts.Before the substance is stressed during passage through the annular or shear gap, it must be transported a certain conveying distance by a screw portion to build up the required pressure. For this purpose the screw portions located between two adjacent ring portions generally have a length of at least, preferably at least, of the screw diameter. The ring portions are preferably formed by grooves turned into the screw element. The turned grooves preferably have a depth of for example ? or less of the flight depth. The angle of the flanks of the turned grooves can be for example 30 to 90 degrees. Preferably, oblique grooves are turned, in particular with an angle of about 60 degrees to the shaft axis.
By stock removal on the screw crest and flanks, the screw element can be provided with further portions. Thus, in particular a mixing section with substantially neutral conveying action can be provided by stock removal.Thus, the invention provides in particular for multi-shaft single screw extruder a screw element which can meet the most diverse requirements of process engineering from defined longitudinal mixing of large volumes in the deciliter range to intensive and lasting action in the centi- to milliliter range.After the annular gaps the screw flight can continue at the same pitch angle. That is, the screw portions of the screw element can form a continuous screw flight, apart from the turned interruptions in the area of the ring portions.The ring portions permit additional dispersing surfaces to be gained. A substantial enlargement of dispersing surface can moreover be obtained if the screw portions between the ring portions are disposed at a progressive angular offset from each other with the same sense of rotation, for example at an angular offset by half the flight angle. The angularly offset screw portions form faces angularly offset in step-like fashion as additional dispersing surfaces.