Extrusion 5-2018

sary and, whereas this is not possible, tape- or vibrating-channel feeders 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 accord- ingly facilitate the introduction of the material into the extruder. Recyclable polymeric materials may derive from two different sources: indus- trial wastes or part of the huge amount of Municipal Solid Waste (MSW). The former is of known origin and composi- tion and – once sorted – they can be more easily reprocessed, while the latter must be separated first – using appro- priate machinery – to be then treated and finally processed. If the former is un- likely 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 pro- blems, which could affect the processing of materials from MSW, may arise. For instance, this may happen when the following ele- ments are present: T hrough a careful selection of materi- als and an accurate analysis of their composition, the co-rotating twin-screw extruder guarantees products of cons- tant quality, even when using recycling materials. In order to reprocess the materials we in- tend to recycle, they firstly need to be ground. If for the single-screw extruder, which works ‘full-mouthed’ (the screw is always covered in material), 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 strict- ly required, especially if other ingredients are added to the polymers in the recipe. Obtaining a free-flowing material from the grinding process is therefore neces- 32 Extruder, Recycling Extrusion 5/2018 “Do not throw waste away, it can be used in another way!” Regranulation is the easiest way to recycle a polymeric material and, for that purpose, single- screw extruders 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, reinforcing materials, colors and other components can be added to a less noble product thus enabling it to acquire further value. • water, which must absolutely be re- duced, since high quantities can limit compounding and granulation proces- ses, as well as decrease the productivity; • other polymers, metals or pollutants 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 inconveniences, thus guaran- teeing the quality 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 considered de- pending on the final application of the product. A recycling process is advantageous only when the material is clearly identifiable, volumes are substantial and the market proves to be receptive concerning its reuse. Polymers as polyethylene (PE) and poly- propylene (PP) can be recycled showing satisfactory results. Polyethylene tereph- thalate (PET) from bottles, which is an example of MSW polymers, has a rather clear composition, it can be easily selec- ted and its reuse for the production of bottles in the food sector has recently been authorized.