Extrusion International 5-2022

83 Extrusion International 5/2022 Maag gear pumps operate with very high precision. Depending on the vis- cosity of the product, they guarantee a tolerance range of a few bar pressure variation. This is ensured by the spe- cial gearing and the special geometry of the gear wheels. The precise and stable volume flow rate reduces the extruder's pressure demand, deliver - ing greater efficiency and extended service life, which are highly valued at- tributes in the recycling industry. The Maag melt pump was devel- oped one step further for this indus- try. The extrex6 recycling version looks just like the standard model. This recycling version, however, fea- tures special tooth clearances, larger toothing gaps, and new types of bearings protected to prevent large- sized particles penetrating into their lubrication gaps, all of which allows tolerance for particles up to 4 mm in size, depending on the pump rating. In such cases there is no need even for a protective filter upstream of the pump, as a study at a European cus- tomer demonstrated over a period of several months. The pump doubled its service life, even in post-consumer recycling – the pinnacle of plastics re- cycling, as it is where the highest con- tamination levels are found. To remove impurities and contami- nants from the recycling melt stream and so achieve the desired product quality, Maag supplies the ERF and ECO Series of continuous-running high-performance melt filters. Melt flows from the outside to the inside through a permanently rotating filter drum with a large number of conical holes. The impurities contained in the melt are held back on the filter surface where they are immediately removed by a stripper, fed to the discharge sys- tem and discharged from the process. A clean filter surface is therefore avail - able at every rotation. Uniform pellets as a finished product During underwater pelletizing that combines a number of advantages and innovations in the current Pearlo model, the melt reaches the die plate via a process-controlled, hydraulically actuated start-up valve, where it is forced through die holes into the cut- ting chamber through which process water flows. The result is a highly uni - form spherical pellet. In the downline dryer, 95 per cent of the water is al- ready separated via a pre-dewatering system, with the remaining water then being removed by means of cen- trifugal separation. The residual heat of the pellets ensures thermal residual drying and thus results in very low residual moisture values. The process water is then filtered and its tempera - ture regulated in the water treatment unit before being returned to the cut- ting chamber. During strand pelletizing, the plas- tic melt is routed to the die head. Here polymer strands are extruded via the die plate and conveyed to the cutting rotor. The exact dwell time in the cool - ing water is determined according to the polymer, before cylindrical pellets of the highest quality are cut. From floor covering to engine compartment The systems supplied by Maag are suitable for small pilot lines right up to processes for large throughputs of 12 to 13 tonnes per hour. The latter often run around the clock and are switched off at most once a year for revision. It is therefore all the more important that the systems work very robustly and that the need for spare parts is mini- mized. In the meantime, many industries are rethinking the use of recycling materials. For example, floor cover - ings are usually made of compressed wood fibers, but increasingly they are also being produced with recycled plastics. By contrast with MDF floor coverings, plastic flooring can also be installed in damp rooms. Furthermore it is possible to recycle the laminate at the end of its life and return it to the material cycle. Another example is the production of pallets. They have to be particularly impact-resistant and strong in every- day use in order to bear the heavy loads. Recycled materials now make a valuable contribution here, too. Pallets containing recycled material have the same properties as pallets produced from virgin material. One more example for which Maag supplied a line for compounding of vir- ginmaterial: This application involves a polymer compound for the production of films. Here, the quality of the later filmsurfacewas particularly important. No specks were to be visible caused by gels or agglomerates as residues from the compounding process. The constant-pressure filtration of elastic particles from the virgin material, with enormous, uninterrupted service lives, in combination with a melt pump and underwater pelletizing, is the opti- mum solution for this customer. Conclusion The examples show that there is now nothing to prevent the use of recycled material even for high-quality prod- ucts. However, compounding process- es require a great deal of know-how about materials, surfaces and the inter- action of the individual systems. Only with carefully matched components, such as melt conveying, filtration, pel - letizing and drying, can high-quality pellets and micropellets be produced. Author Christian Makert / Andreas Liebing Maag Pump Systems AG Aspstr. 12, 8154 Oberglatt, Switzerland www.maag.com Comparison extrex standard gear pump and extrex recycling gear pump