Screws & barrels | machinery feature
Flight insurance: screw protection technologies
Correct design and production – along with innovative coatings techniques – help to ensure life for extruder screws
Wear protection is an important element of screw design – and there are many ways to achieve it. EJS Industry of China recently improved the
production of bimetallic screws at its facility in Zhoush- an. It says that it receives many requests for ‘hardfac- ing’ screws, to ensure that they have sufficient wear resistance for processing filled plastics. “Most bimetallic screws from China cannot avoid
small cracks on screw flights, especially for large screws with wide screw threads,” said the company. However, EJS managed to resolve the problem by using a plasma transferred arc (PTA) welding machine, while developing a special way of cooling down the screw. The company makes the screws from a variety of
alloys. The most wear-resistant screws incorporate nickel, chromium, tungsten, cobalt and boron into the steel.
The company recently took delivery of a centrifugal
casting machine from Taiwan – which provided a stable temperature and a superior performance to traditional furnace casting, says EJS.
Extreme performance Extreme Coatings says that its CarbideX CPR technol- ogy – for encapsulation of conical and parallel twin feedscrews – was developed in response to environ- mental concerns over chrome plating, and the need for better protection against wear and corrosion. At last year’s K2016, it showed the new super-wear
resistant tungsten carbide coating technology for plasticising feedscrews. Its CPR technology (which stands for Chrome Plating
Replacement) offers several advantages over chrome, including very low porosity – meaning that corrosive gases from PVC cannot attack the base of the screw. In applications where abrasive fillers cause premature wear, tungsten carbide encapsulation provides superior wear resistance combined with a high level of corrosion
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resistance – which minimises wear and maximises profit, says the company. “Advancements in polymer technology demand
parallel advancements in feedscrew and processing technology,” said Curt Kadau, president of Extreme Coatings. “We have kept pace with these advancements and will continue to develop new products to meet the demands of our customers.” CarbideX CPR is a blend of carbon and chromium in
a nickel matrix. According to the company, it is applied two to three times thicker and more uniformly than standard hard chrome plating, and fully encapsulates the entire working surface of the screw.
Added resistance Leistritz says that power metallurgy (PM) steels have proved an effective way of boosting the wear resistance of screws. One recent advance in this area led to Leistritz’s
VSA4 materials, which have high resistance to both wear and corrosion. “The heat treatment and finishing allows the parts to
be crafted to tight tolerances and long-lasting effect,” said Bill Novak, product manager at Leistritz.
March 2017 | PIPE & PROFILE EXTRUSION 39
EJS avoided cracks in its hardfaced
screw flights by developing a
new method of cooling
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