The technology of mixing extruders
Since, apart from improvements in details, the technology of mixing extruders has not changed in the last fifteen years, the present invention was based on the object of presenting a mixing and homogenizing extruder which, in comparison with the known apparatuses, permits an increased output rate with reduced investment costs and at least equally good mixing effect, and allows a distinctly shorter overall length and a broadening of the previous application areas of mixing and homogenizing extruders. The plasticizing work which can be accomplished by the new extruder should be freely adjustable in dependence on the properties of the extrudate. By combining the two known designs of mixing sections, it was possible to create an extruder having considerable advantages over the mixing and homogenizing extruders of the described prior art. For instance, with an extruder having a pin-lined barrel zone and a transfer zone having additional pins, it has been found that, with the same mixing quality and the same screw speed, the drive power of the extruder could be reduced by up to 50% and the melt throughput increased by up to 60% to 100%.
These excellent results bring about, furthermore, a 50% reduction in the drive torque, which results in a great reduction in the gear transmission costs in the production of the extruder. In addition, by combining the pin-lined barrel technology and the transfer section technology, the component length of the mixing zone necessary for the same mixing quality can be reduced by about 50% with respect to an extruder which operates only on the pin-lined barrel principle. Due to the arrangement of radially adjustable restricting pins which protrude radially in the part of the transfer zone of the extruder barrel in which the barrel channels have approximately their greatest channel volume, the proposed extruder can be set for the processing of different rubber compounds. Consequently, the plasticizing capacity or the frictional energy converted in the transfer section for the extrudate can be preselected as desired, and specifically for the particular compound, by the depth of penetration of the restricting pins into the barrel channels and into the processing space of the extruder. Consequently, along with the screw speed and the processing section temperature, recourse can be made to a further freely selectable process parameter in comparison with the previously known generic extruders.
The design of the extruder screw and of the sheet production extruder in the transfer zone also allows inexpensive manufacture while maintaining the self-cleaning capability and mixing quality of the extruder, inasmuch as the number of channels and consequently the number of flights is independent of the channel cross-sectional area. Finally, the use of barrel grooves, known per se, in the feed zone of the extruder, in combination with the pin-lined barrel section and transfer section, allows a further surprising improvement in the mixing quality and output rate of the extruder.In particular, in the case of rubber-based extrudates which are difficult to process, an extruder feed zone, in which a helical groove was made in the extruder barrel inner wall, had its starting point below the feed opening of the extruder, and was formed downstream along a length of about 3 screw diameters, proved to be particularly advantageous.