SCREWS & BARRELS | MACHINERY
Fundamental research is helping to push the design of screws – and other extruder components – to the next level. Lou Reade reports
Innovations in screw design
Screw design is critical to extruder performance – such as throughput and product quality – yet the process is often far from perfect, and can create screws that perform poorly. While more designers are using the latest
software tools to perfect their designs, academic researchers are working hard to create new concepts that could lead to better-performing screws in future.
Fractal design David Kazmer, professor of plastics enginering at the University of Massachusetts Lowell, has designed a screw using ‘fractal’ design that helps control the plastication path of materials in single screw extrusion. He told delegates at last year’s Antec conference in the USA that the design uses multiple channels in the transition and feed sections to improve control of extrusion scale up. “Screw design guidelines are based on rules of thumb – and often inaccurate analysis – that leads
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to sub-optimal designs,” he said. He cited several examples of research that
improve on these general rules – including work by Covas which “found the results from experimental validation with the implemented screw designs to be inferior, due to the poor accuracy of the underlying models of simulations”. Scale-up rules are notoriously unreliable, he said. While some follow the rules, others deviate from them: for instance, flow rate increases with the cube of the extruder scale, while head conduction decreases according to the square of the scale. Other factors – such as shear rate and residence time – should be independent of scale, but are much more complex due to conveyance and recirculation dynamics of the materials being processed, he said. “The conveyance and melting of thermoplastic feedstock remains neither efficient nor consistent,” he said. “It seems that improved extrusion process- es could be more reliably developed with lower equipment and processing costs.”
� March 2018 | FILM & SHEET EXTRUSION 39
Main image: David Kozmer used the principle of fractals to design a screw with better control of plastication
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