Smaller spherical tungsten carbide particles provide substantially better wear resistance.

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Spherical tungsten carbide particles in a proprietary matrix.

Plasma spray screw coating process.

Coated screw flight.

At last month’s K 2016 show in Germany, Nordson Polymer Processing Systems, New Castle, Pa., introduced a new coating for single and twin injection and extrusion screws that is said to resist abrasive and corrosive wear significantly better than standard coatings applied by high-velocity oxygen fuel (HVOF) thermal spray. The new Xaloy MPX thermal-spray coating is made up of tiny tungsten carbide spheres only 5 microns in diameter, one-sixth to one-seventh the size in standard HVOF coatings. The small size and uniform shape of the particles, produced in a plasma process, permits them to be more densely packed in the proprietary matrix. These factors, along with 2.5 times greater application velocity, reportedly results in a stronger bond with the parent metal of the screw and improved wear resistance.

Compared with standard HVOF tungsten carbide coatings, 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, Nordson reports. As an example of corrosion resistance, the new coating survived more than 1000 hr of salt fog testing, due in part to its near-zero porosity, the company says.

Nordson recommends the Xaloy MPX coating for resisting abrasion from plastic compounds with medium filler loadings (15-35%) and for resisting corrosion from halogenated materials such as flame retardants and PVC. For filler contents above 35%, Nordson supplies the Xaloy X-8000 high-hardness coating.

New Xaloy MPX coating can be applied to any screw diameter and to the entire screw or just specific areas. Standard layer thickness is 300 microns. Nordson says the MPX coating should be used with a barrel lined with its Xaloy X-800 nickel/tungsten carbide alloy or a comparable inlay.