Understanding what causes wear in extruder screws and barrels – and choosing the correct treatment – is key to extending lifetimes and reducing replacement costs

Damage limitation: how to prevent extruder wear

Plastics processing can take its toll on a machine, with many materials capable of causing damage to the screw and barrel of an extruder over time. For instance, many pipe and profile operations

process highly filled PVC compounds in twin-screw extruders because they combine superior mixing with high output. However, the high screw-to-barrel loading can cause adhesive and abrasive wear at the interface – as can the corrosion generated by the chlorinated material. One way to offset this, and extend the working

life of the system, is to use screws and barrels with super-hard surfaces. “Compatibility of screw and barrel is especially critical in counter-rotating twin-screw extrusion,” said Jeff Huskey, director of OEM sales at Nordson. He said that his company’s bimetallic barrels

with the Xaloy X-800 lining last up to four times longer than those with standard nitrided alloys (Nitralloy), because they provide greater resistance to abrasive fillers and corrosive volatiles. In addi- tion, Nordson recommends use of a tungsten carbide screw surfacing. Both the X-800 barrel lining and a tungsten carbide screw surfacing (such as Xaloy X-830) are composites consisting of tungsten carbide parti- cles – uniformly dispersed in a nickel alloy matrix. In abrasion tests with 20,000 wear cycles, a Nitralloy barrel with molybdenum screw hard-surfacing had nearly double the wear of a Xaloy X-800/Xaloy

X-830 combination. The wear resistance of the Xaloy X-800 barrel was consistent throughout its depth, whereas hardness and wear resistance progressively decreased with depth into the Nitralloy material. An even greater difference was seen in tests of

corrosion in 20% HCl – with the Nitralloy barrel exhibiting more than four times the corrosive wear. As well as supplying new Xaloy X-800 twin

barrels, Nordson can reline worn barrels. Relining protects the original investment in the barrel by extending its working life, and costs much less than a new barrel. “We produce new twin liners that fit into a

bored-out casing,” said Huskey. “A perfect custom fit ensures proper heating and cooling of the polymer during processing.”

Screw protection Davis-Standard recently published a white paper explaining some of the factors behind screw wear. The document, written by the company’s vice president of extrusion technology, John Christiano, says there are three main causes of screw wear: abrasive, adhesive and corrosive. “The type and rate of wear depends on many variables that include mechanical alignment of the equipment, the operating conditions used, screw design, polymer chemistry, type of abrasive addi- tives and the materials of construction,” he said.

Main image: Nordson says that its bimetal- lic barrels with Xaloy X-800 lining last up to four times longer than those with standard alloys

� November/December 2019 | PIPE & PROFILE EXTRUSION 25

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