machinery | Screw protection
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
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. At K 2016, Extreme Coatings may be ready to
showcase some new formulations, Kvinge says. These involve variations on size and composition of the constituent carbides to tailor a coating to a specific wear mode. “This can be towards the high end of the market but also our CPR is truly a product to replace highly toxic chrome plating,” he says. Kvinge adds that the company has been very
successful with using its CPR for treating twin co–rotat- ing compound mixing rotors, but less so with twin screw elements. “Our standard thickness of 0.75 mm per side is not sufficient to extend element life signifi- cantly,” he admits. “Most processors will allow screw elements to wear up to 3-4 mm. If corrosion is present then we can add triple or more life to screw elements.” 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.” Bill Novak, Product Manager at extruder producer
Right: Krauss- Maffei Ber-
storff says its oval liners are
a cost-effective solution for
wear-intensive compounding processes
50
Leistritz, says there have been significant advances in just about all types of materials for improving wear resistance in compounding 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,”
COMPOUNDING WORLD | August 2016
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 abrasive and corrosive wear applications, according to Twin Screw Systems Engineering Manager Janik Bessinger. “As engineered plastics evolved, compound- ing applications that generated both high abrasive and corrosive wear became common. Century Extrusion recognised this trend 20 years ago and began to develop wear materials to improve part life in these applica- tions,” he says.“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 Engineer- ing 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
www.compoundingworld.com
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