In a screw machine, to extend the material to be prepared with the latter, at least one processing element is configured as an extension-kneading element, the crest region of which has a reduced thickness compared to a flank region and, with adjacent processing elements, forms an extension channel for the material in the region between the triangles, which tapers in cross section in the flow direction of the material. Because of the tapering of the extension channel, the material is accelerated in the flow direction thereof and extended, so that high-molecular particles in the material are divided and the latter is therefore homogenized.