An extruder for viscous masses
Twin screw extruder conveys by virtue of internal friction and generates a drag flow along the extruder screw. If, in addition to conveyance, a pressure gradient has to be built up, a pressure volume flow acting in the opposite direction is superposed on the drag volume flow. This leads to increased dissipative heating and to a lowering of the actually conveyable mass flow. By contrast, a positive-displacement gear pump is a volumetric conveyor, and therefore, here, the conveying rate depends on the counterpressure to a substantially lesser extent. The solution according to the invention therefore makes it possible to convey relatively independently of pressure, while a low mass temperature can be achieved by virtue of the positive-displacement gear pump designed according to the invention.
The conveying behavior of elastomeric masses, such as rubber mixtures, depends greatly on various parameters. If, on account of the prevailing viscosity of the mass, the positive-displacement gear pump conveys to a higher extent than the extruder gear, an underpressure occurs in the transitional region. This may lead to the formation of small bubbles, so that the quality of the conveyed mass does not conform to requirements. In order to rule this out, although the conveying rate of the extruder screw can be increased, this then gives rise, however, for a mixture of, for example, low viscosity, to a corresponding overpressure in the transitional region, which subjects the structure to load and causes the mass temperature to rise more highly than necessary.
For the filling pressure into the pump to be designed optimally in the case of different mixtures, according to a further feature of the invention the positive-displacement gear can have a drive that is independent of the single screw extruder screw. The solution according to the invention therefore makes it possible by simple means to keep the quality of the extrudate constant, even when different masses, such as, on the one hand, synthetic rubber mixtures and, on the other hand, mixtures of natural rubber, are employed. If viscous masses only within a relatively limited viscosity range are to be processed and construction is to be simplified, according to a further feature of the invention it is possible for the positive-displacement gear and the extruder screw to be connected frictionally or nonpositively. This avoids the need for a separate drive for the positive-displacement gear and/or the conveying gear.
Alternatively, the conveying gear may have the drive. A higher conveying volume by the positive-displacement gear pump can be achieved in that, on that side of the sheet production extruder screw which lies opposite the conveying gear, a further conveying gear is arranged which likewise meshes with the positive-displacement gear. Alternatively, on the periphery of the extruder screw, a plurality of conveying gears may be arranged which are driven by one or more positive-displacement gears.