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).

In extrusion or injection molding equipment, adhesive wear is the principal mode of wear when the resin being processed contains little or no abrasive filler. The rotating feed screw contacting the barrel liner will, over time, result in micro-welding of metal between surfaces and a loss of material.

Factors that contribute to this type of wear include:

Straightness of screw and barrel
Alignment of drive, barrel, feed housing and screw
Feed screw design
Non-uniform barrel heating
Improper barrel support
Unsupported downstream components (dies)
High head pressure

Insufficient depth in the metering zone for a given material can reduce plasticating rates by 28% or more, but for some resins, too deep of a metering depth will not apply the proper amount of shear on the resin to allow for a uniform melt to be delivered to the mold. That’s why a metering depth that’s just right really pays off.