Just like filament being grinded, filament being crushed isn’t ideal. The issues are often related but can be resolved.
Signs the extruder is crushing filament:
A tell-tale sign for filament being crushed is that is appears deformed. This piece of filament in the picture for example, is clearly deformed and is no longer round. It was crushed between the idler and feeder drive gear.
Explanation:
This problem is related to [Extruder is grinding filament] and is usually a precursor of that. This happens a lot when:
The feeder gear idler pressure is way too high, deforming the filament while it passes through the feeder.
The same part of filament is getting retracted, pushed forward, retracted multiple times. Printing something with a lot of retractions in short order will wreak havoc on the filament, especially if the feeder idler pressure is squeezing the filament a bit too tight.
How to fix:
Idler Tension
If your extruder is equipped with some form of idler tension adjustment, decrease the tension. Lacking that you might be able to modify the feeder in some other manner (for example shortening the idler spring or replacing it with a softer spring). Or try a different, harder filament type or brand.
Too many retractions
If the problem is caused by too many retractions, use a slicer software that allows better control and reduce the amount of retractions occurring.