I had contaminated spools. Identify them and bin them.
The main thing I learned dealing with this is the internet is borderline worthless when you have a slightly obscure problem. You will get told, repeatedly, to do cold pulls, use cleaning filament, or “hAvE yOu TrIeD dRyInG yOuR fIlAmEnT”. You will continue to be told this even when you explain that you replaced all the parts with brand new ones, or even when you get a brand new printer that exhibits the exact same issue straight out the box.
If you have stuff in your prints that shouldn’t be there, work inside-out and eliminate parts as you go. Whatever it is, it’s either already in the printer, or it’s getting in there via the filament.
Clean the printer fully. Scrub the leadscrews down with iso and relube. Make sure your plates are thoroughly cleaned with soap and hot water. Replace your PTFE tubing. Remove the extruder, take apart, and clean with iso alcohol. Clean the nozzle sock, nozzle, and hot end - if you have a full replacement hotend, swap that in instead. If you have an AMS, disconnect it. Feed the filament directly to the printer so you rule the AMS out - you don’t want to open the AMS to clean that if you don’t have to (Trust me, that sucks). Get some light in there and clean anything suspicious looking with iso. Carefully but thoroughly vacuum the whole machine.
If the printer is definitely 100% clean and the issue persists, then the junk is getting in there afterwards. Fully clean the room the printer is in, though unless your printer is in an uninsulated garage or some place that is just naturally dirty/grimy, it’s probably not that.
If the printer is clean, and the environment is clean, then it’s getting in there on or in the filament. Sorting this out is going to be terrible. Make sure everything is clean and then put a brand new spool in - one you unseal right there and then (Note here - it is not 100% that this new spool is not contaminated. I learned this the hard way. If this new spool does not print clean this does not mean the printer is not clean. Reclean everything and test another new spool until it prints correctly.) This should, hopefully, print clean. Now test your other spools. If you get a clean print, then test a new spool and it’s got junk in it, that spool is an issue. Each time you get junk in a print, clean the printer and retest with a known clean spool.
Hopefully you are now in a position where you are printing clean, and you have a separate pile of bad spools. If you want to try and rescue those bad spools, you can put them on a filament respooling machine with a filament wiper attached to the tubing. This will wipe the whole reel of filament down as you respool it. If the contaminate is only on the surface of the filament, this may fix the spool.
If this doesn’t work, put the filament under a bright light and see if you can detect anything baked into the spool. If you find that, you can send it to BBL support. Don’t expect anything.
