I don't print any abrasive materials at all. Pretty much only normal PLA and PETG.
I noticed, that my print quality gradually went down quite a bit, especially in the last few prints. I had a lot of stringing, weird blobs, and scarred surfaces.
Now, the print quality is as good as it should be!
They are dirt cheap. You can get a set of 10-15 generic ones, in different sizes, for only a few bucks. Don't forget that they are consumables.
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Wouldn’t it be better to replace them regularly by some metric, uses, time, etc?
Yes, but that is also going to require a ton of extra effort to track and assumes the nozzles themselves wear consistently. There would probably also need to be modifiers based on materials used, and even brands. For a professional print farm or a business trying to squeeze every last dollar out of the operation, this might be worth it. For a hobby where we expect a lot of waste? Eh, fuck it, just replace it when prints start going south.
If you're a Klipper user, odds are you'll have easy access to both print hours and filament length extruded. Some marlin printers also track this information as well.
Granted, nozzle wear is also filament specific, but it's better than nothing.
My point was that "sporadically" could mean after one use or three weeks. There is no rhyme or reason to it and it wouldn’t be helpful.