machinery | Screw protection
parent metal of the screw and improved resistance to wear,” says Nordson. “Compared with standard HVOF tungsten carbide
coatings, the Xaloy MPX technology exhibits 61% less mass loss in ASTM G65 abrasion testing, 18% less mass loss in ASTM G77 sliding wear testing, and 8.5% greater bond strength in ASTM C633 bond testing,” the company says. As an example of corrosion resistance, it says the Xaloy MPX coating survived more than 1,000 hours of salt fog testing, in part attributed to its near-zero porosity. “Nordson’s Xaloy MPX screw coating provides
Above: Application of thermal spray powder coating to a screw at Nordson
Experts developed a method to weld a powder mix of very small, round tungsten carbides in a nickel-layer to the surface of the screw bore. With 60-70% of tungsten carbides, we reach a very high level of wear resistance,” he says. The company claims that, using the round carbides, the adhesive wear partnership with the screw segments becomes very favorable compared to well-known brazing systems. On a general level, Zimmermann says today’s higher
torque ratings and longer processing sections are making adhesive wear more important than in the past. “It is a fact that smaller shafts at higher speeds get more torque bending and tend to [require] support towards the barrel wall, especially in the kneading sections and also at the die end. Screw segments are pushed towards the barrel wall with great forces, which creates a huge adhesive wear component. A very well prepared combination of screw material and barrel material needs to be found to get this wear component under control.”
Right: Nordson claims the fine particles in its
new Xaloy MPX allow it to outperform
current HVOF alternatives
Abrasion resistant Tungsten carbide also features in a new development from Nordson Corp, which the company introduced at K 2016. “A technology that uses exceedingly tiny tungsten carbide spheres barely larger than the particles in cigarette smoke has yielded a coating for single and twin plasticating screws that resists abrasive and corrosive wear better than standard coatings applied by high-velocity oxygen fuel (HVOF) thermal spray,” the company claims in a press release. The new Xaloy MPX thermal spray coating is made up of particles only 0.005 mm in diameter. Consisting primarily of tungsten carbide, the particles are said to be six to seven times smaller than those in standard HVOF coatings and more densely packed owing to a uniform spherical shape produced in a controlled plasma process. “The combination of ultra-fine particle size, higher coating density, and a 2.5 times greater application velocity results in a stronger bond with the
38 PIPE & PROFILE EXTRUSION | November/December 2016
best-in-class wear protection compared with standard HVOF coatings, and its higher bond strength eliminates potential for chipping,” says Mark Colella, global product manager for the Xaloy brand. Nordson recommends the Xaloy MPX technology for
resistance to abrasion from compounds with medium (15 to 35%) filler loadings and for resistance to corrosion from halogenated materials, including flame retardants and PVC. For filler contents above 35%, Nordson supplies the high-hardness coating Xaloy X-8000. The Xaloy MPX technology can be employed across
the full range of screw diameters and may be applied to the entire screw or to specific areas. The standard layer thickness is 300 microns, with variations utilised depending on the area of the screw. Nordson says the new screw coating should be used with a barrel lined with its Xaloy X-800 nickel/tungsten carbide alloy or a comparable inlay. Treatment specialist Extreme Coatings offers CarbideX, a blend of tungsten carbide in a nickel or cobalt matrix engineered to resist wear on screws in equipment producing abrasive compounds such as glass-reinforced polyamides or calcium carbonate-filled PVC. According to the company, feed screws coated with CarbideX are two to three times more wear resistant than standard bimetallic screws. CarbideX is also very useful where corrosion is the
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