A
screw extruder for processing melts of synthetic plastic materials or synthetic plastic material in powder or granulated form to effect mixing, kneading and homogenizing of the material and causing chemical reactions thereof comprises a long main conveyor screw and a short auxiliary screw parallel thereto and disposed in the feed zone of the casing of the extruder. The screws are unidirectionally rotated in tight engagement with each other. Single- screw screw extruders if suitably dimensioned are generally quite satisfactory for carrying out the hereinbefore referred to procedural steps. In fact, the kneading action of single- screw extruders in many instances is superior to that of
twin screw extruders the screw elements of which mesh though not with a tight profile. However, single- screw screw extruders have inherently certain disadvantages. The dwell time of the material within the extruder cannot be kept constant within a narrow range and such constancy of the dwell time is very important for most of the processing operations which are to be carried out in the extruders. Moreover, a positive and uniform flow of the material to be processed through treatment zones of the extruder cannot be reliably obtained with single- screw screw extruders, especially if quantity processing is to be carried out.
Twin- screw extruders, the screws of which are in tight engagement and which are unidirectionally rotated produce high quality dispersion and satisfy to a high degree the other aforelisted condition. They also permit adequate constancy of the dwell time and produce due to the manner in which they are constructed positive and substantially uniform flow of the material and substantially uniform and predictable processing of the material fed into the extruders in the zones thereof to which are assigned the respective stages of the processing. However, twin- screw extruders are extremely expensive due to the complexity of the structure thereof and as a result use of twin- screw extruders is often not economically acceptable as it is well known to experts in the art.
Various other means such as feed-in rollers and conical feed-in screws have been proposed for single- screw extruders so as to improve the feed-in and the flow of the material, and also to increase the quantity of the material that can be processed. As it is well known in the art, overfeeding and underfeeding, especially in the first few turns of the conveyor screw, that is, in the turns adjacent to the feed port of the extruder, have a particularly strong effect on the over-all performance of the extruders.
It has been proposed to provide in the feed zone of a single- screw extruder an auxiliary screw which is parallel and rotated in opposition to the main conveyor screw of the extruder and which is in mesh with the turns of the main screw though not in tight engagement therewith. Such an arrangement serves primarily to improve the kneading action in the extruder, for which purpose the main screw and the auxiliary screw usually have different diameters.
Tests have shown that the provision of a parallel and meshing auxiliary screw rotated in opposition to the main screw and disposed in the feed zone of a one- screw screw conveyor does not provide a better solution of the aforepointed out disadvantages than the aforediscussed extruders without an auxiliary screw.
