PETpla.net Insider 04 / 2011

MATERIAL / RECYCLING 10 PET planet insider Vol. 12 No. 04/11 www.petpla.net New conical technology boosts rPET processing* Conical and co-rotating The use of co-rotating twin-screw extruders is common in compounding. The conical design offers a very wide per- formance spectrum, especially in PET scrap processing. The range of applications varies, from production of high quality pellets or direct extrusion to sheet and fibres made from PET re-grind or pellets, A-PET, C-PET, G-PET, washed PET flakes or URRC-PET flakes, PET flakes or pellets with pigments, fillers etc. Approved products comply with FDA standards for food applications. *based on an article by Dipl.-Ing Gunther Klammer, Managing Director of M-A-S Maschinen und Anlagenbau Schulz GmbH. Pucking, Austria, with: Prof Dr Mont. Clemens Holzer; Dipl.-Ing Stephan Schuschnigg; and Dipl.-Ing Ramesh Kumar Selvasankar, all of the Chair of Plastics Processing, Montanuni- versitat Leoben. Twin-screw extruders with intermeshing, co-rotating screws are the leading processing technology in compounding and fulfil an essential function in plastic production and pro- cessing. The New Conical Technology (NCT) developed by MAS combines the benefits of conical extruders with those of parallel co-rotating twin- screw extruders. Compared with con- ventional extruders, the conical design offers substantially higher filling vol- umes and material intake. This results in a higher screw filling level, which gives the highest possible output with comparatively high pressure ratios and low melt temperatures. The co- rotating design provides total homog- enization. Generously dimensioned screw shafts, paired with strong back- pressure bearings, lead to a robust design, long service life, and thus pro- vide efficient PET compounding and processing. The machine also has low energy consumption. A project with the Chair of Plastics Processing at the Montanuniversität Leoben, Austria, was initiated in order to put the experiences already gained with conical, co-rotating twin-screw extruders on a firm theoretical founda- tion and establish the framework for further development. In the first part of the project, the discharge zone was closely analyzed. Beginning with conservation laws and geometric con- ditions, appropriate equations were developed for different screw channel geometries to describe the volumetric flow in relation to back pressure. The changing geometric data in the case of the conical twin-screw extruder meant that calculations had to be per- formed iteratively, using a computer program. With the slightly modified Erd- menger profile, the dimensions of the outer screw and root diameters, tip width, number of flights, and shape of the channel formed by the flights are geometrically dependent on each another. These dependen- cies are a result of the ‘co-rotating’ and ‘intermeshing’ aspects of the two screws, and are thus attributable to the principle of motion. These relation- ships have a key influence, not only on the screw profiles that can be cre-