PROCESSING | SCREWS AND BARRELS
Right: Xaloy 800 inlay is claimed to provide the highest level of barrel wear and corrosion resistance
system,” says Walter Smith, Senior Extrusion Process Engineer. “In order to achieve complete and uniform dispersion of ingredients, Nordson recommends screws equipped with its StrataBlend I and II mixers.”
Smith says the most common screw materials
are heat treated 4140 and 4340 alloy steels. “For compounding high loadings of fillers and rein- forcements or corrosive resins and additives, these alloys provide poor to fair resistance to wear and corrosion. Nitrided steel provides better wear resistance, and stainless steel offers better corro- sion resistance. For performance superior to these materials, Nordson offers several alternatives,” he says.
Thermal spraying For extreme wear resistance, Xaloy X-8000 nickel alloy with tungsten carbide is a thermal spray coating for screws processing highly filled or corrosive materials. The company says that it can be applied to the full length of the screw or to specific areas susceptible to wear. It forms a metallurgical bond to the base steel, with an estimated bond strength of 280 MPa, as compared with 72 MPa for standard thermal spray coatings. Xaloy X-830 is a nickel-based alloy with tungsten carbide for high-hardness flight welding. This is plasma-transfer arc welded onto the flight lands. The alloy is an anti-corrosive, anti-abrasive material best used in combination with Xaloy X-800 barrel liners. For root protection against abrasives, an
alternative to Xaloy X-8000 encapsulation is a J-shaped weld applied to the “pushing” side of the screw flight, particularly in the feed section to avoid premature wear from abrasive additives metered into the barrel from the hopper (Figure 2). The alloy used is Stellite 6, a high-hardness cobalt-based metal. For more extensive protection from abra- sion, Nordson can also encapsulate the entire root
with a weldment of Stellite 6 in the area of the screw where high loadings of abrasive filler are fed into the system. For the lining of extruder barrels, Nordson says
its Xaloy X-800 inlay provides the highest level of wear and corrosion resistance, including resistance to attack by aggressive volatiles. This inlay consists of tungsten carbide particles dispersed in a nickel alloy matrix. It can be used with filler loadings up to 50% and with corrosive substances such as flame retardants and fluorocarbons. These extrusion barrels can be manufactured up to a 600 mm ID, depending on the length and weight. China’s Nanjing KY Chemical Machinery says it has developed new Hot Isotactic Pressing (HIP) materials to improve the service life of screw elements. According to the company, its HIP coatings can be directly powder-formed and treated materials can achieve 100% densification, which improves overall mechanical properties. KY Chemical says its PMHIP coating offers a number of favourable characteristics including high alloy content, uniform carbide formation by heat treatment and long service life. Compared with domestic Chinese high-speed tool steel, service life is increased by 5-10 times.
Other claimed benefits for its PMHIP steel include the ability to meet individual requirements for wear or corrosion resistance and more consistent product processing. The company says the coating meets the requirements of machinery with torque ratings of up to 18Nm/cm³ as the screw elements can adopt a soft core structure, which provides improved toughness.
Figure 2: J-weld application of Nordson’s Xaloy X-8000 can provide root protection on screw flights Image: Nordson Corporation
64 COMPOUNDING WORLD | August 2020
CLICK ON THE LINKS FOR MORE INFORMATION: �
www.centuryextrusion.com �
www.comacplast.com �
www.jsw.co.jp �
www.feddem.com �
www.coperion.com �
www.extremecoatings.net �
www.nordson.com �
www.keyatwinscrew.com
www.compoundingworld.com
IMAGE: NORDSON CORPORATION
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