Extrusion International 3-2021

48 Extrusion International 3/2021 EXTRUSION INTERNATIONAL thoroughly these extraneous ma- terials can be separated, the easier is the further processing of the old lawn. The next recycling step is the shredding down to particles 10 to 40 mm in length with a bulk density (uncompacted) of approx. 50 to 100 g/l (Picture 3). The shredded material consists of: approx. 40% PE (the actual grass- blades) approx. 20% PP (base fabric) approx. 40% latex/natural rubber or PU (backing on the base fabric) It contains up to 15 percent by vol- ume of impurities, consisting of: > up to 8% extraneous materials (dust, sand, soil) > up to 1% rubber (old tire granulate) or EPDM from approx. 2 mm down to dust, > 3 - 5%moisture Material tests have shown that the MAS-system-technology is capable to process this PE/PP- fi ber-mixture into a high-quality plastic granulate. Their basic processing component is the DRD (Double Rotary Disc) dry cleaner-unit. Processing stage one: dry cleaning – simple but effective The shredded material is fed to the DRD (Double Rotary Disc) dry clean- er. In it the majority of all granular contaminants and moisture adher- ing to the fi ber material is ef fi ciently separated. It is basically a centrifuge with a double rotor system running inside. Double because the rotor disc has differently shaped rotor blades on the bottom and top (= patented double rotor disc system). The system is operated in batches. The under- side of the rotor sucks the shredded material out of a buffer container and transports it into the centrifuge. Gravity ensures that larger and heavy contaminants, such as metal parts, stones, etc. are not transported into the process chamber at all, but are separated into an upstream heavy material separator (Picture 4). Sub- sequent, the pre-cleaned material is forced into a turbulent hot air circuit by the rotor. The warm air absorbs the moisture and transfers it off in exchange with the outside air. The warm air is generated via a central heating element register. In addi- tion, the declining moisture content and the friction between the plastic particles created by the turbulence cause residual dirt to be separated off tangentially by centrifugal force via lateral separation sieves (Picture 5). The cleaned plastic batch is then discharged at periodic intervals by centrifugal force after opening a pneumatic fl ap and fed to a material silo. Processing stage two: plasticizing, fi ltering and degassing The plastic fraction fed from the DRD-dry-cleaner via the buffer Picture 5: Detail view through a tangential sieve into the centrifuge chamber of the DRD 26 system. Below the double rotor system with 12 rotor blades, above the heating register housing (Photo: Author) Picture 6: MAS cascade extrusion line, consisting of a conical MAS twin-screw extruder, a CDF-disk-melt- fi lter and aMAS single-screw-extruder with degassing unit (Picture: MAS)