Screw segment with a greater pitch
In order to simplify the equipping of the shafts, the face surfaces of the combined elements in the invention are delimited by circular arcs, which correspond to the feed screw diameter, the feed screw core diameter and are no larger than the centre distance of the axes of sheet production extruder. The combined elements can then simply be placed on the shafts in such a way that the face surfaces delimited by the circular arcs align with each other. Thus fault-free mounting of the combined elements that interlock with the combined elements of the adjacent shaft is made significantly easier.
So that the benefits of the combined elements take effect with regard to bending strength and wear, the combined element has a length greater than the diameter of the screw, and specifically, more than double the diameter of the screw.The feed screw segment of the combined element is preferably designed with a double lead, as a double-lead screw leads to a larger feed quantity than a triple-lead screw, but compared to a single-lead screw has a greater bending strength.
If a working segment of the combined element also has a screw-shaped surface, i.e. c or counter pitch direction, this is also preferably designed with a double lead. The surface of the working segment can also simply have a spiral shaped progression similar to a single screw extruder, for example in the case of kneading blocks. So in accordance with the invention, a kneading block segment with double-lead cam plates arranged in a spiral that have a pitch direction in accordance with the feed screws, for example, can be combined with a double-lead kneading block segment with an opposing pitch direction to form one element.
With the high temperatures to which the material being processed is subjected, the shafts and the feed and working elements sitting on them expand differently. This results in the formation of a gap between the shaft and the elements sitting on them, into which the molten material, such as plastic, can penetrate and possibly combust, and in any case can bind the shaft with the elements so firmly that the elements can only be withdrawn from the shaft after cooling with the greatest of difficulty. This problem is accentuated in the long combined elements used in the invention to such an extent that at times the elements cannot be withdrawn at all and the whole shaft has to be discarded.
Each shaft is preferably divided into several shorter shaft sections, whereby the shaft sections are formed to be able to be axially distorted in the shaft core. For the axial distortion ability, an axially fixed rotating element is preferably pivoted at one end of the shaft section in the shaft core, which is equipped in a torque-proof manner with an external thread, which engages in the inner thread in the core of the twin screw extruder. For this, the shaft section can have at one end a rotating element with an external thread and on the other end the inner thread in the core. There can also be shaft sections that at both ends have such rotating elements with external threads and which are connected to shaft sections that have internal threads in the core at both ends.