machinery | Screw protection
Above: A hard coated feed screw with mir- ror finish by Extreme Coatings
uniform layer of chrome on complex geometries,” claims Kvinge. “This makes chrome susceptible to premature wear as the thickness and inherent porosity cannot handle an aggressive process.” Service life with CarbideX CPR is claimed to be up to five times longer than with HCP, and screws can be recoated many times. For now, the company does not coat barrels. “We
provide consumable alloys to the manufacturers of barrels,” Kvinge says. “We focus on line-of-sight outside diameter (OD) surfaces for our thermal spray process; however, we do have an experimental inside diameter (ID) thermal spray gun that we are evaluating.”
Wear advances Bill Novak, product manager at extruder producer Leistritz, says there have been significant advances in just about all types of materials for improving wear resistance in equipment. “The most prevalent use is PM steels, which offer the most cost effective alternative. All extruder suppliers have access to the same metallurgies. Where Leistritz excels is in choosing the best suited metallurgy to protect against the wear type, or combination of wear types,” he says.“While the mechanisms of abrasive, adhesive, and corrosive wear may be known by manufacturers, there are only a few who harden/heat treat the PM steel consistently enough and provide guidance on matching barrel and screw element materials,” Novak claims. A recent advance in PM steels at Leistritz yielded
VSA4 materials with excellent wear resistant as well as corrosive resistant properties, “but it is in the heat treatment and finishing that allows the parts to be crafted to such tight tolerances and long-lasting effect,” according to Novak. This is accomplished with an outer shell of VSA4 material surrounding an inner core of more resilient yet tough steel. This screw element metallurgy, in conjunction with the VSA4 barrel metallurgy, is claimed to provide an excellent combina- tion to protect in most processing tasks. Century Extrusion has also developed a number of material options designed to maximise part life in both
40 PIPE & PROFILE EXTRUSION | November/December 2016
abrasive and corrosive wear applications, according to twin screw systems engineering manager Janik Bessinger. “It is important to note that part life is affected by the material of construction and the geometry or mechanical design of the part – which differs significantly among twin screw extruder OEMs,” Bessinger adds. Century’s proprietary wear material 10V-12, used to manufacture bi-metallic metal screw segments, was developed for high wear applications with moderate corrosion. The material is produced from powdered steel and uses the HIP process. It also features a soft core (made from carbon steel) to eliminate localised spline deformation at screw interfaces and crack propagation in the root of the screw spline resulting from unexpected torque spikes. “As the requirement for higher corrosion resistance
has increased, we have developed a number of other PM steels loaded with vanadium carbide for wear resistance and higher levels of chromium or nickel for corrosion resistance,” says Century’s components engineering manager Bob Stratz. “In addition, we now offer PM bi-metal screw elements with cores made from stainless steel to protect the inner part of the element from corrosion.” Barrel wear solutions create a greater engineering
challenge, according to Stratz. Wear surfaces can be applied via surface hardening, through-hardening, cladding and also the application of materials via welding and numerous other processes. “Through hardening and surface hardening have limited wear resistance because of hardness limitations and wear surface thickness limitations. Cladding processes are inherently plagued by hair line cracks. These cracks typically do not hurt abrasive wear performance, but when corrosive attack is present, compounds will penetrate the cracks and attack less corrosive base materials causing failures,” he explains.Century says it has approached thes problems from two directions. First, it has developed CX 77 with high levels of tungsten carbide for abrasive wear resistance and nickel for corrosion resistance. “For highly corrosive applications, we have introduced CX 77 with stainless steel backing,” Bessinger says. “We have also developed numerous hipped barrel wear solutions and continue to expand into new materials. The hipping process allows for highly tailored metallurgy and does not produce surface cracks. We offer hipped materials containing up to 65% tungsten carbide and high levels of chromium or nickel.” Advanced tool steel and surface treatment R&D is a
major area of focus at Steer and one that the company claims can significantly improve process efficiency, durability and life of its compounding platforms, elements and components. Thiru Arumugam, head of
www.pipeandprofile.com
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