Augers and Screws
HVAF and HVOF Thick Wear Resistant WC-Co-Cr Coatings for Augers and Screws
Kermetico HVAF equipment provides an effective way to protect augers and screws from corrosion and abrasive wear. Kermetico HVAF thermal spray systems deposit ductile, high-bond, thick tungsten carbide coatings harder than 1,600 HV300. The gas impermeable HVAF screw coatings are superior to any other thermal spray technology in wear protection of injection and extrusion screws and transport augers.
The Abrasive Wear of Screws and Augers
Transport screws are efficient systems for material handling. In addition to conveying of solid, semi-solid, wet and sticky materials, they are also used in production processes for drying, cooling and mixing. These processes cause extensive wear damage to the screw/auger flights and shafts.
Unprotected equipment could fail, causing a loss in productivity and expensive repairs.
Abrasive wear is probably the most common type of wear in the plasticating unit. If abrasion is allowed to continue unchecked, it will result in a gradual decrease in machine performance and, eventually, complete failure. In abrasive wear, the most common culprits are the fillers and additives in the resin.
Corrosion attacks the metal surface even when the machine is not in operation.
It breaks down the metal surface and renders it more susceptible to mechanical wear. Resins that have chlorinated and fluorinated components, such as PVC and PTFE, are very corrosive. Even a material that normally is not corrosive by nature can degrade under inappropriate screw design and a corrosive environment.
The best defense against corrosion and wear is the proper component material selection and the use of wear and corrosion resistant coatings.
A Surface Treatment to Protect a Screw from Corrosion and Wear
Carbide coatings deposited by using different high velocity technologies (HVOF, HVAF and Detonation), are extremely wear-resistant. The industry uses tungsten carbides in combination with cobalt, nickel or cobalt-chrome matrixes preferably. The erosion and corrosion behavior of thermal spray coatings is a major factor for the assessment of the coating application.
Kermetico HVAF equipment is used to deposit tungsten carbide coatings that are wear-resistant and tough enough to be used at all angles of impingement.
We recommend WC-CoCr-bound powders because they exhibit the highest wear resistance coupled with an adequate corrosion resistance in any environment except very high pH environments or hydrofluoric acid. Even 100 microns (0.004″) of our WCCoCr coating is gas-tight, protecting a base metal of a screw from corrosive agents as a barrier coating.
Since 2006 numerous different screws and augers have been sprayed for both new and repair work in our job shop, and Kermetico have never seen them again. They are still in service.
Blast and Spray Augers and Screws with Kermetico High Velocity Equipment
Usually, auger coatings are deposited using robotic blast and spray operations. Kermetico blast an auger surface with a Kermetico HVAF gun (it is extremely fast and uniform) and spray with the same gun after switching the powder feed hose and perhaps changing the nozzle. It is much faster, more accurate and consumes much less grit than conventional air blasting. Hot grit blasting provides very even surface preparation and induces less stress into the base metal.
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