Most extrusion screws have hard-surfacing materials welded on the flights to provide longer wear life. When extreme wear occurs in the absence of abrasive fillers, a burr on the flights provides a clue as to what happens in the process.
Hard-surfacing failures without evidence of burrs indicate possible weld bond issues. A burr on the trailing side indicates a very high side force is causing the flights to gall or even weld to the barrel material, literally pulling the two surfaces apart.
The burr is usually due to wedging caused by the screw momentarily plugging with solid polymer at a radial location. If a similar force does not balance pressure from the plugged channel on the other side, the screw is pushed with tremendous force against the barrel.
Wedging is a screw design issue where the melting rate of the screw is inadequate to match the compression rate.
The pressure necessary to bend the screw so it can contact the barrel over relatively short lengths can be as high as 25,000 psi. This force may only last for a fraction of a second.
The screw in the compression section is a double-spiral wedge driven by the full torque of the extruder drive, giving it enormous compressive force.
A burr on both sides of the flights usually indicates a barrel alignment issue whereby the barrel is causing the screw to bend with each revolution to conform to the barrel.
The power of the extruder drive crushes the flights and develops galling, resulting in a burr on both sides. It is also good practice to grind the old weld off and check for cracking before rebuilding the screws.
Today almost all screw manufacturers use a technique called plasma transferred arc (PTA) welding to apply the hard-surfacing material to the screw flights.
Find out more about weld failures in extrusion in this Plastics Technology column.
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