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processing feature | Kneaders


and the required torque transmission,” he says. New screw elements have also been developed to improve material transport effi cacy and dispersing action. Buss introduced four-fl ight kneader technology in 2001, when it launched is Quantec kneader for the production of PVC compounds. “The four fl ight design resulted in more intensive mixing and signifi cantly higher throughputs, without increasing screw rpm,” says Senoner. Four-fl ight technology results in an improved conveying stability because one screw fl ight always covers the gap between the next two fl ights downstream, creating more labyrinthine fl ow paths that improve transport. If required, the fl ights can also be arranged with no offset to provide better distributive mixing for compounds containing additives such as conductive carbon blacks.


Comparison of output rates from current and previous Buss Quantec generations


applications, halogen-free fl ame retardant (HFFR) formulations, and cross-linkable polyethylene (XLPE). The fundamentals of the kneader, based on a


three-fl ight concept, remained largely unchanged for over half a century after its invention. The biggest improvements were principally in torque and modularity, just as with regular twin-screw compounding extruders. It was only in the 1990s that increased computer


design power enabled new designs to be thoroughly investigated. “Buss evaluated designs with fi ve and even six fl ights, and in the end we chose four,” says Hans- Ulrich Siegenthaler, an independent consultant who left Buss at the end of 2014 after some 20 years with the company. “Technical and procedural characteristics have been refi ned and optimized in recent years to comply better with specifi c user requirements,” says Marco Senoner, head of marketing and strategy at Buss. “This involved optimizing the geometric relationships of the kneading zone (outer to inner diameter, and outer diameter to stroke ratios) to further improve the input material feed


The four-fl ight working principle used in Buss Quantec kneaders


There are also larger volumes in the process zone, thanks to an increased ratio between the outside and inside screw diameters. In addition, Buss raised the screw pitch, screw speed and drive power.


“Four-fl ight technology opens up far more extensive design modifi cation and optimization possibilities than before,” says Senoner. The longer fl anks of the screw fl ights not only improve the conveying stability, but also have a better conveying effect. Buss does still make three-fl ight MKS models occasionally, for special applications such as powder coatings.


Senoner says that the original core purpose of the new design, to double throughput while enhancing product quality, is normally exceeded in practice. Buss now uses four-fl ight technology on its MX type ma- chines as well, which are for compounding more temperature- and shear-sensitive plastics.


The Quantec line features a very compact cascade set-up of compounder and discharge extruder/pelletizer. “It does other products too, like XLPE, but this is basically our PVC series,” says Senoner. The Quantec is now in its third generation, the G3, which was unveiled at K 2013. He says the design has been further standard- ized, but it can be adapted to different applications more


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COMPOUNDING WORLD | August 2015


www.compoundingworld.com


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