User:Thilo117/Ringextruder

Ring extruders are machines used in the plastics and food industry and belong to the upper group of extruders. Ring extruders are mainly used in plastics technology, where they are used for the plasticization and for compounding of plastics.

Structure and principle of operation
Ring extruders work according to the principle of tightly combing, equally rotating twin screw extruders. However, with the ring extruder, 12 fixed screws are arranged coaxially in a circle. All adjacent shaft centers have the same center distance and the screw profiles interlock tightly. The screw shafts are arranged around a central, fixed core and are usually equipped with two-course screw elements. During the process, the shafts rotate at identical speed around their own center axis. Thus, the motion sequences of two neighboring screws are analogous to the equally rotating twin screw. In principle, the ring extruder can therefore fulfil all the tasks of the twin-screw extruders in at least the same quality. In the process unit, screw and housing elements can be selected modularly. This makes it possible to combine individual elements into a screw configuration and thus define functional zones along the process unit. These functional zones are then enclosed by suitably selected housing elements and are designed towards certain basic operations, like for example conveying, mixing or kneading. The subdivision of the housing cylinder results in separately controllable temperature zones that can be specifically adapted to the respective process steps. The central core of the process unit is also built up from individual segments and can be tempered via the core wave with liquid media. Apart from the process unit in which the screws are located, the main components of the ring extruder differ from other extruders only in the built-in gearbox. The reducing and branching gears use a central shaft driven by the motor, which in turn drives the drive shafts of the screws. The resulting, very symmetrical design results in a balanced distribution of forces and is very similar to the proven planetary gearboxes.

Process engineering advantages over twin-screw extruders
Two adjacent screws have an area in which their profiles interlock. This area is called intermeshing area and provides some process-relevant properties. In the case of the twin screw extruder, only one intermeshing area results from the two screws, whereas in the case of the ring extruder with 12 screws arranged in a circle, there are also 12 intermeshing areas. Furthermore, 12 material flows are formed in the ring extruder when using the already mentioned two-course screw elements, whereas only three material flows result in the twin screw.

Degassing
Due to the increased number of melt flows, the ring extruder offers the plasticificate a significantly larger, non-wetted surface area with a comparable melt volume in cross-section, provided that the throughput level is equivalent. Due to four times the number of melt flows, the melt pools have lower layer thicknesses and thus shorter diffusion paths. In addition, due to the higher number of intermeshing areas per screw, the product is transferred more frequently from screw to screw and the product surface is renewed extremely frequently due to the complex flow processes in the intermeshing area. In summary, it can be said that the increased surface/volume ratio, the division of the melt into smaller portions, as well as the high surface regeneration rate in combination with the large-volume process space significantly increase the degassing performance compared to the twin screw.

Dispersive and Distributive Mixing
In the screw channels outside of the intermeshing areas, mainly simple shear flows can be found, while in the intermeshing areas increased strain flow components result from very complex flow mechanisms. These are extremely efficient and energy-saving in terms of dispersion. Since the ring extruder has a higher number of intermeshing areas per shaft, the melt passes through such intermeshing areas relatively more often when transported through the process unit. In each cross-section of the extruder, there is also more melt in intermeshing areas in terms of volume. This advantage means that the cutting and mixing processes in the ring extruder are carried out much more efficiently and yet a reduced, specific energy input into the plasticificate is achieved. This also results in higher product quality and reduced energy consumption.

Wear
Due to the high strains in the intermeshing area, shafts of twin-screw extruders must be particularly resistant to bending. In detailed investigations, several hundred bar mass pressure was measured for conventional kneading elements. Spreading forces caused by this can cause extreme signs of wear or, in the worst case, even lead to contact with the housing. Because each screw in the ring extruder has two neighboring screws, the screws are stored on both sides by the extruded product. As a result, the spreading forces partially cancel each other out and a tarnishing of the screws on the extruder housing can be almost ruled out.

Tempering
Another advantage resulting from the geometry of the ring extruder is the larger specific heat transfer area. This insight was gained from comparable extrusion tests with twin-screw extruders and a ring extruder and is clearly evident for all sizes with regard to the respective comparable twin screw in terms of throughput.

Main areas of application

 * Recycling of PET beverage bottles
 * Compounding of rubber mass

Literature
Klemens Kohlgrüber, Michael Bierdel, Harald Rust: Polymer-Aufbereitung und Kunststoff-Compoundierung. Hanser-Verlag, München 2019, ISBN 978-3-446-45832-1