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Feed Screw Wear-American Kuhne
Screw and Barrel Wear Sabotage Output Quality
Now you can afford to do something about it.
Wear of screws and barrels can significantly effect throughput and quality of extrusion products. As wear occurs, the natural reaction of process engineers and operators is to simply increase screw speed to compensate for the evolving loss in output. While this approach is successful in accomplishing the short-term goal to regain throughput, the increase in screw speed can significantly increase melt temperature causing a variety of related process problems.
How much is it costing you now?
In high output and/or low melt temperature applications, any wear will reduce usable rate, since the extruder is operated in a maximized condition. Certainly wear of 0.020" on the screw diameter will reduce rate measurably (perhaps 10%), and rebuilding would be easily justified. Most manufacturers will allow screws to wear to .050-.070" or more, living with reduced rates or higher than desired melt temperatures (and the resulting decrease in productivity and quality).
When should you replace?
Depending upon the application, screw life can range from 6 months to 5 years. As a general industry-wide rule of thumb, it is believed that typical barrel life is, on average, 2 to 3 times that of the screw life (with compatible wear resistant materials). Tungsten Carbide based materials (Colmonoy 83 on screws and Xaloy 800 or equivalent on barrels) typically yield 30% to 100% longer life compared to the "standard" materials (Colmonoy 56 and Xaloy 102 or equivalent).
What about hi-tech treatments?
Fortunately, the cost "adders" to upgrade from conventional treatments to more wear resistant materials have dropped significantly in recent years and, as a result, many extrusion processors have decided on the Tungsten Carbide pairing of the screws and barrel linings to maximize life. In some cases, wear guarantees can be included to provide added insurance of the value of your added investment.
The Bottom Line
While it may be difficult to accurately ascertain the wear rate and resulting impact on productivity with your existing screw and barrel, it's never too late to inspect them as any significant wear (over .020" to .040") will certainly indicate a measurable loss in output and/or quality. The added benefit of a new screw and barrel, is that base line data can be documented, to be compared at regular intervals, for routine evaluation of output loss and melt temperature increase. Given today's production pressures and/or requirements for maximum profitability, new/rebuilt screws and new barrels are often easily justified with fast pay back, often in just a few months.
