MACHINERY | SCREWS AND BARRELS
and wear resistance progressively decreased with depth into the Nitralloy material. An even greater difference was appar- ent in tests of corrosion in 20% HCl, where the Nitralloy barrel suffered more than four times the corrosive wear. Of course, replacing a barrel is
costly so, in addition to supplying completely new Xaloy X-800 twin barrels,
Above: Nordson’s X-800 barrel lining technol- ogy can extend lifetime by up to four times over standard nitride alloys
Nordson says it can also reline existing worn
barrels. Using a proprietary process, Nordson can produce new twin liners that can be fitted into a bored-out casing. A perfect custom fit is achieved, ensuring optimal heating and cooling of the polymer during processing.
Right: CPM is using assymetric screw profile know-how developed for the Extricom line of ring extruders in its T-Profile conveying element designs
54
Screw evolution The evolution of twin-screw extruders for com- pounding applications has resulted in today’s machines operating at ever increasing screw speeds and torque density in order to achieve the higher productivity levels required, according to Adam Dreiblatt, Director of Process Technology for CPM Extrusion Group. “While the machine design has advanced over recent decades, the co-rotating and self-wiping conveying screws and kneading elements developed by Rudolph Erdmenger have remained essentially intact as described in the original patent from the 1950s,” he says. “When operated at current screw speeds of up to 1200 rpm – the first generation twin-screw extruders were limited to screw speeds below 300 rpm - conventional kneading elements used for melting and dispersive mixing create localised pressure and shear peaks that result in over-heat- ing of the polymer and degradation of properties. This phenomena is a direct result of the self-wiping Erdmenger profile,” Dreiblatt explains. “The current focus is to design elements that minimise or eliminate these shear and pressure peaks, allowing machines to operate at high screw speed without adversely affecting the performance of the compounded material. One such develop- ment by Extricom Extrusion, now part of CPM Extrusion Group, utilises an asymmetric screw profile that was originally developed for the RE RingExtruder. Replacing the traditional Erd- menger self-wiping elements with the new patented ‘T-Profile’ elements has been demon- strated to dramatically reduce specific energy and localised overheating of the polymer.” Dreiblatt says these new element designs retain the self-wiping characteristic in the intermesh and
COMPOUNDING WORLD | August 2019
barrel surface by manipulating the tip clearances in a non-symmetric arrangement and have been successfully scaled-up from Extricom’s laboratory to machines running at up to 6 tonnes per hour in RE RingExtruder applications such as PET recycling (bottle-to-bottle). In such applications, the T-Profile kneading elements are said to be able to achieve a lower IV drop than is possible with traditional kneading elements. In rubber compounding, where traditional kneading elements can generate excessive temperatures, a reduction in specific energy up to 35% has been documented using these new asymmetric element designs. “T-Profile kneading elements are now being deployed on twin-screw extruders with equally successful results. In one example, compounding of high viscosity polymer – HDPE pipe compound with fractional melt flow – on a 92 mm twin-screw is limited in screw speed to avoid shear degradation of the compound. Direct replacement of traditional kneading elements with T-Profile elements allows the machine to operate at higher screw speed without increasing melt flow,” he says. “In this case, the production rate was increased by 20% at the higher operating speed while maintaining the compound specifications.” The T-Profile design can also be applied to conveying elements, where Dreiblatt says the asymmetric profile provides channel-to-channel material exchange (effectively mixing) within the varying tip clearances. “Application of T-Profile conveying elements for devolatilisation improves degassing efficiency by varying the melt film thickness with each revolution of the screw while under vacuum,” he says. “Reduction in VOC can be achieved by replacing traditional conveying elements with these new designs that provide a high degree of surface renewal.” The T-Profile elements represent one of several new High Performance Element (HPE) designs developed at Extricom for the RE RingExtruder.
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PHOTO: NORDSON
PHOTO: CPM EXTRUSION
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