Twin screw extruder for continuous working
The twin screw extruder for continuous working and/or processing of flow - able substances has at least two parallel, corotating shafts equipped with intermeshing screw elements. The face of each screw element is limited by circular arcs corresponding to the outside screw diameter, the screw core diameter and at most the center distance of the screw elements. The screw elements are guided on circular segments of the extruder housing that are parallel to the shafts.
A screw core diameter d and at most a center distance Ax of the screw elements, at least one of said screw elements including at least three axially spaced concentric ring portions and respective screw portions positioned axially along the entire distance between said concentric ring portions for providing a defined stress on the flow-able substance and uniform stress without unduly stressing the substance or the screw element as the flow-able substance flows in an axial direction with respect to the length of the shafts; and a sheet production extruder housing having concave circular segments that receive said shafts having said screw elements, wherein each said screw element is a solid, monolithic, one-piece element into which two or more grooves are cut to form said ring portions.
The screw portions of the screw element between the ring portions first cause a pressure buildup which then urges the substance through the annular gap between single screw 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 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.
The inventive twin screw extruder can have only two screw shafts. However, it can also have at least three shafts disposed along a circle or circular arc with the same central-angle distance within a cavity of the extruder housing, the extruder housing being provided on the radially inner and outer sides of the cavity with concave circular segments parallel to the extruder axis, on which the screw elements are tightly guided, as described for example in EP-B-0788867. The inventive screw elements thus permit substance to pass through the ring of shafts from one side of the cavity to the other.