Extrusion International 2-2019

43 Extrusion International 2/2019 Recycled PET from post-consumer bottle flake is highly sought-after in the industry as a raw material for thermoforming packaging, fibers or packaging tapes. Against the back- ground of increased environmental awareness, such products are also widely accepted – and in consider- able demand – among consumers. Depending on the application, the recycled materials often have to meet high requirements in terms of appearance and / or mechani- cal properties. For instance, even food packaging made entirely from bottle flake is not allowed to con- tain any contaminants. Additionally, tapes or yarn which are stretched must not exhibit any defects that could cause them to tear and disrupt production. The purity of the recycled material is central to its usability. Therefore, in the PET world, the efficiency of the melt filtration process is far more critical than usual on the recycled material production line. The screen changers and screens normally em- ployed for this purpose have limita- tions here, whereas Ettlinger’s self- cleaning ECO filter systems permit compliance with even the toughest specifications. The core component of the ECO filter is a rotating, cylin- drical steel screen with millions of conical holes drilled by laser. When melt flows through this screen from the outside to the inside, any con- taminants are retained on the sur- face and continuously removed by a scraper. The decisive breakthrough as far as PET recycling is concerned was the recent market launch of a microperforation with an unprec- edented filtration fineness of 60 µm. Limitation of woven steel mesh screens Woven steel mesh screens are made of fine metal wires. A square mesh is the simplest and most commonly encountered type. The side length of the openings corresponds to the screen’s nominal mesh size. Howev- er, in practice, a square mesh allows much larger particles (factor 1.41) to pass owing to the diagonal. Further- more, the individual wires are only linked together loosely. As the filter cake grows and the pressure rises, some meshes may be enlarged, so that particles bigger than the nomi- nal mesh size slip through. Contaminants contained in the PET bottle flake such as aluminum or pa- per as well as “black specks” – black, degraded plastic particles that occur during the process –regularly find their way into the recycled material. The resulting scrap pushes up the cost. This also has negative impacts on the downstream extrusion process because the build-up of a filter cake on woven steel mesh screens ac- companied by strong pressure fluc- tuations and the need for frequent screen changes creates extra work, as does cleaning with a reverse melt flow (backflush). Lasered microperforation is the answer These traditional woven steel mesh screens contrast with Ettlinger’s rig- id filter screens, whose laser-drilled microperforation impresses with much better separation efficiency. The woven steel mesh screens have a mesh size corresponding to the cross section of the conical holes in the filter screen, which are ta- pered in the direction of the melt flow. When melt flows through the screens, all contaminants larger than the hole diameter are retained on the filter surface. Thanks to the new 60 µm filtration fineness, the few residual contaminants are no longer visible to the human eye. The PET recycling process also has the basic ability to convert recycled material into new products with vi- sual, mechanical and organoleptic properties that are indistinguish- able from virgin material. The sig- nificantly reduced occurrence of black specks and other particles helps make the line more efficient because there are virtually no torn tapes or fibers; ultimately leading to substantial savings. Added to this is the system-specific advantage of all Ettlinger-built melt filters, namely the continuous, con- stant filtration efficiency over sev- eral weeks or months without any process interruptions. They work ac- cording to the principle that contam- inants are removed from the micro- perforated filter screen by a scraper, and immediately discharged, with every rotation. The filtration surface remains absolutely clean as a result and the melt filter works with a constant pressure for exceptionally long periods. Contaminants such as small elastic particles like silicone etc. which could otherwise be forced through the screen after a while, are unable to collect on the surface. Previously unused material flows acquire commercial value Ettlinger’s continuous melt filters regularly open up new possibilities for materials that have traditionally been considered too heavily con- taminated or that contain extremely problematic contaminants. The new 60 µm filter screen makes the use of PET fines from recycled bottles an even more commercially attrac- tive option. These fines occur during the sorting, grinding and washing processes and tend to be contami- nated with large amounts of paper and aluminum. Although up to 20 tons of fines per month are gener- ated even in small PET bottle recy- cling plants, this material is sold at a low price for want of an alternative. Ettlinger’s continuous, high speed ECO melt filter will let customers convert their PET material flows into an even higher quality – and hence more profitable – end product. Ettlinger Kunststoffmaschinen GmbH www.ettlinger.com