Extrusion International 4-2018

27 Extrusion International 4/2018 EXTRUDER, RECYCLING “Do not throw waste away, it can be used in another way!” Regranulation is the easiestway to recycle apolymeric material and, for that purpose, single-screwextruders are employed. However, when the aim is to provide an added value to the material or to preserve its fragile stability, the use of a co-rotating twin-screw extruder is mandatory. Thanks to this type of extruder – which is a continuous dynamic mixer – fillers, reinforcingmaterials, colors andother components can be added to a less noble product thus enabling it to acquire further value Through a careful selection of mate- rials and an accurate analysis of their composition, the co-rotating twin- screw extruder guarantees products of constant quality, even when using recycling materials. In order to reprocess the materials we intend to recycle, they firstly need to be ground. If for the single-screw ex- truder, which works ‘full-mouthed’ (the screw is always covered in mate- rial), accurate feeding is not essential, the twin-screw extruder, which works ‘hungry-mouthed’ (the screw is never covered in material), a constant and meticulous feeding is strictly required, especially if other ingredients are add- ed to the polymers in the recipe. Ob- taining a free-flowing material from the grinding process is therefore nec- essary and, whereas this is not pos- sible, tape or vibrating-channel feed- ers can be used so as to guarantee a smoother feeding process. Moreover, installing a forced feeder on the main feeding may be useful as well. This system will accordingly facilitate the introduction of the material into the extruder. Recyclable polymeric materials may derive from two different sources: industrial wastes or part of the huge amount of Municipal Sol- id Waste (MSW). The for- mer is of known origin and composition and – once sorted – they can be more easily reprocessed, while the latter must be separated first – using appropriate machin- ery – to be then treated and finally processed. If the former is unlikely contaminated by dirt or foreign materials, the latter require to be washed and are often subject to various types of contamination. It is precisely during the sorting and the washing phases that some prob- lems, which could affect the process- ing of materials from MSW, may arise. For instance, this may happen when the following elements are present: • water, which must absolutely be re- duced, since high quantities can limit compounding and granulation pro- cesses, as well as decrease the produc- tivity; • other polymers, metals or pollut- ants such as wood or paper. In this last case, the filtration system – with the help of a special device positioned in the final part of the extruder, can be a valid solution to these inconve- niences, thus guaranteeing the qual- ity of the material. Wastes are washed with special soaps whose residues may release smells through the processing phase. If it were to occur, a potential addition of anti-odor additives has to be consid- ered depending on the final applica- tion of the product. A recycling process is advantageous only when the material is clearly iden-