INNOVATION | FILM


non-homogenous feedstock and additives used in products that must be homogenised in the recycling process. In a normal extruder, material is heated thermally and by mechanical friction. When the melt is at the correct temperature, it is homogenised. However, at that point temperature-sensitive polymers crosslink and unmelted lumps are formed in the melt. Our extruder is short, conical and co-rotating, meaning that the screw geometry can be optimised. By working with Caroda, we have found a solution to minimise the size of the gels. Normally, gels appear as crystal-like granules on the film surface. We have been able to optimise the process so that there are a minimum number of visible micro gels that feel like powder in the film. This provides much better printing possibilities and improved sustainability in the circular economy.” The special design of the conical, co-rotating


MAS extruder permits a twin-screw extruder to be used for economical film recycling without pre- densification or agglomeration prior to the extru- sion process. “The conical design provides a big feed opening and a large intake volume as a result,” says Lehner. “For example, with the MAS 93-K unit, the feed opening has a size of 200 × 313 mm, with screw diameters of 2 × 93 mm at the melt outlet and 2 × 186 mm at the material intake. As a result of the large intake volume, it is also possible to feed film fluff without pre-densification or agglomeration prior to the extrusion process. Therefore, it is now possible to have the benefits of a co-rotating


twin-screw to produce pellets of the maximum possible quality. Flexible, exchangeable kneading elements ensure the right amount of shear, needed to get the best possible homogenisation. In comparison to conventional single-screw extruders used in film recycling, a co-rotating twin-screw provides significantly better homogenisation, which is essential in cases where different polymers are combined to achieve a high-quality pellet.” He says: “There are also challenges involved in


the recycling and extrusion of LLDPE film flakes. After the melt has passed through the CDF melt filter it is well prepared for final venting because it is well homogenised and free of contaminates. A short single-screw extruder, with a standard L/D of around 13.5, is installed downstream of the CDF filter and is equipped with a highly efficient degassing zone, ensuring the highest venting quality. Three large venting ports with relevant vacuum pump performance, provide high venting efficiency making it suitable for the most difficult degassing applications.” According to MAS, the key for this new film


recycling technology is the conical co-rotating twin-screw extruder. The screws are segmented and adjusted with exchangeable mixing and kneading elements. The tailored screw geometry, in combination with a suitable screw speed, allows the shear rate to be adjusted to the exact level required to prevent gels and crosslinking occur- ring. The short processing unit of the MAS extruder means that power and general production costs related to output quality are low. Based on data from existing customers, MAS says that the energy consumption with PE is approximately 0.3-0.35 kw/ kg for the extrusion process and melt filtration. The company adds that disassembling the


PrintCYC members have tested alternative ink formulations for printing on BOPP and LDPE films Images: PrintCYC


18 PLASTICS RECYCLING WORLD | January/February 2021


screws is straightforward and achieved at the rear of the extruder, meaning that it is only necessary to remove the adapter between MAS and the melt filter or melt pump. It is easy to move the cylinder forward and to swivel it to the side. The screws can then be removed from the cylinder at the rear. Time consuming removal of the downstream equipment is not necessary. Disassembling the screws takes between 30 minutes and around two hours depending on the extruder size, says MAS. PrintCYC is an initiative focussed on recycling printed plastic films and processing of recyclate. Its current members are the machine suppliers Brückner Maschinenbau, Kiefel and PackSys Global, CPP film specialist Profol, ink manufacturer Huber- group, converter Constantia Flexibles and the recycling technology group Erema. The initiative is coordinated by Annett Kaeding-Koppers as inde-


www.plasticsrecyclingworld.com


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