SCREWS AND BARRELS | MACHINERY
Facing the challenges of wear and abrasion
Screws and barrels must stand up to the full rigours of the compounding process and today’s demanding formulations are intensifying those challenges. However, there are solutions at hand, writes Mark Holmes
As formulations move to ever more demanding polymers and additives, twin-screw compounding extruders are being called on to withstand increas- ingly extreme processing conditions, abrasive fillers and corrosive volatiles, as well as accommo- dating the unpredictable nature of some recycled plastics. The brunt of the challenge is borne by the extruder screws and barrels, which are expected to handle these new demands without compromising on performance or process flexibility. In terms of development, compounding
extruder manufacturers and suppliers of replace- ments parts alike are fine tuning their screws and barrels to perform at their peak in any processing applications. At the same time, they are hoping to improve processing characteristics while extending serviceable lifetimes and simplifying changeover and maintenance times. In terms of wear, the very high screw-to-barrel loadings found in twin screw compounding machin- ery can cause both adhesive and abrasive wear at the interface, according to Nordson Corporation. “The potential for wear and corrosion is especially great in process-
ing rigid PVC and other halogenated materials, plastic/wood composites, and highly filled com- pounds,” says Walter Smith, Senior Extrusion Application Engineer, who adds that one key approach to meeting these challenges and extend- ing the working life of the plasticating system is to use screws and barrels with super-hard surfaces. “Compatibility of screw and barrel is especially critical in counter rotating twin-screw extrusion,” Smith says. “Nordson Corporation’s bimetallic barrels with the Xaloy X-800 lining last up to four times as long as barrels with standard nitrided alloys (Nitralloy), providing greater resistance to abrasive fillers and corrosive volatiles. As a com- panion to this lining, Nordson recommends use of Xaloy 830 screw surfacing.” According to Nordson, both of these Xaloy
materials are composites consisting of super-hard tungsten carbide particles uniformly dispersed in a nickel alloy matrix. In abrasion tests with 20,000 wear cycles, a Nitralloy barrel combined with a molybdenum screw hard-surfacing exhibited nearly double the wear of an Xaloy X-800/Xaloy X-830 combination, as measured by volume loss of barrel material. In addition, the wear resistance of the Xaloy X-800 barrel was said to be consistent throughout its depth, whereas hardness
Main image: Improving wear resistance of screws and barrels is critical to ensure viable compounding of often highly abrasive high performance fillers
www.compoundingworld.com
August 2019 | COMPOUNDING WORLD
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PHOTO: COPERION
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