Extrusion International 6-2019

30 Extrusion International 6/2019 EXTRUSION INTERNATIONAL from the feed zone to the granula- tion station. Due to the following in-line melt filtration this way is addi- tionally extended. The melt tempera- ture increases with the path length. On the one hand by the heat transfer from the cylinder heater, on the oth- er hand by the shearing heat gener- ated by the transverse and longitudi- nal flow friction in the screw flights. The latter may well be affected by the speed of rotation of the screw, however, to a large extent depends on the specific properties of the poly- mer. With the usual throughput, the melt arrives at the granulation sta- tion with around 40 to 60 °C above and extruded sheet products, such as thermoformed refrigerated box housings (Picture 2). The next step was the launch of the processing of foils and plastic bags of different origin (Picture 3). A trivial finding was that the achievable recy- cling qualities are directly dependent on the cleanliness and purity of the input materials. In order not to have to compromise on material quality, Kaskada invested from the begin- ning on in combinations of shredders and washing units. Today, all film and fabric flakes are processed in a washed state, leaving the washing lines with an adherent residual mois- ture of between 6 and 8 percent (Pic- ture 4). This moisture content needs to be further reduced before the ac- tual extrusion processing. In an usual recycling-line consisting of a cutter-compactor followed by single-screw extruder this is possible to some extent in the cutter compac- tor. There the frictional heat generat- ed between the plastic flakes forces a major share of the moisture content to evaporate. Experience has shown that these system produces good re- sults, but not in all cases. Kaskada CEO Daniel Nedev said: “Since the single-screw extruders are usually run with very long L/D-ratios in the range of 45 to 52:1, the regrind has to travel a very long distance the usual process temperature. Too high to qualify it as gentle process- ing. As the consequence we end up with material characteristics worse than expected due to a presumable degradation. Although we are able to compensate the degradation ef- fects within certain limits by mixing input material qualities, it is not a satisfactory solution. That’s why we started looking for alternatives.” Material protection and quality increase through MAS recycling concept Daniel Nedev continues: “This alter- native was offered by the Austrian Picture 6: Compared to the cutter compactor / single-screw extruder systems, also operated by Kaskada, the MAS recycling line feeds the plastic melt with a 60 °C lower melt temperature into the granulator, resulting in a less thermally degraded re- granulate Picture 7: Kaskada owner and CEO Daniel Nedev demonstrates the high tear resistance and transparency of HDPE films made from Kaskada granules