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Extruder Screw Materials
Extruder screws are mostly made up of alloy steel and then undergo further
secondary treatment operations depending upon the nature of steel. Some of these
treatments include chrome plating, nitriding, flight surface hardening etc. some of
screw materials and the treatment options to make it fit for use as extruder screw are:
Screw Material Treatment
Alloy steel 4140 Flame hardened
Nitralloy 135-M Nitrided
Tool steel CPM 9V Heat treatment
Special alloy Nickel 718 Age hardened
Nitriding: The Screw is under constant friction and abrasion with the material and
the walls of barrel and operating at high temperatures. To withstand this continuous
condition the feed screw is given special treatment called Nitriding. Nitriding is a heat
treating process that diffuses nitrogen into the surface of a metal to create a case
hardened surface. It is predominantly used on steel, but also titanium, aluminium
and molybdenum.
Other parts of machine like gears, crankshafts, camshafts, valve parts, extruder
screws, extrusion dies, injectors and plastic-mould tools are also nitrided for long life
and performance.
The three main methods used are: gas nitriding, salt bath nitriding, and plasma
nitriding.

SYMPTOMS OF WEAR: SCREWS & BARRELS
An understanding of nature and causes of wear is essential to the selection and use
of Screw, Barrel and other components. Three types of wear that occur, often in
combination are:
Abrasive Wear: mostly occurs due to scouring (rubbing) effect of hard foreign
abrasive particles coming into contact with the screw or barrel, which wears
(erodes) away the metal in the screw or barrel, most often in the melting and mixing
sections of the screw. Foreign particles such as chrome plating, particles in
reinforced resins like glass fibers/spheres, calcium carbonate, powdered metals or
ceramics and other objects can gouge the barrel or screw and even break
segments out of the screw flights. Other possible causes are excessive heat
generated due to shearing, likewise excessive friction due to Cold pellets moving
into the transition section of the screw causing a scrubbing action that result in
abrasive wear. All this will cause abrasive wear, especially if the components are
not made from high quality wear resistant materials.
Typical Causes of Abrasive Wear are Improper Component Materials, Inadequate
Screw Design, Incorrect Heat Profile or Heater Band Failure, Excessive Back
Pressure, Failure to Use Magnets
Adhesive Wear: Adhesive wear occurs due to Rubbing of metal surfaces against
each other with sufficient force to cause the removal of material from the less wear
resistant surface. For surfaces having comparable chemical analysis and

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