I have a Creality K2 Plus with a CFS and have used about 15 0.4mm nozzles so far. I usually try other things when I have a problem but if I switch nozzles and it solves the issues, then I assume the nozzle was the problem. I save these nozzles so I can clean them up to able to reuse them. The process I use is as follow: Soak the nozzles in MEK and Ethyl Acetate (because PLA needs one and PETG needs the other). I cover them (so the alcohol doesn't evaporate) and I leave them for a day or two before I start the next step. I use a brass/copper brush and get everything off the outside that I can and make them look good. The same brush that I use after every print to clean the nozzle up. Sometimes the nozzles are still dark but I use pliers or other tools to scrape off all filament. Sometimes I have to clean out threads as well. I then heat up the nozzle temperature on the printer to 240 (covers PLA and PETG) then use the push poke I bought to push all filament out that I can-from the top side. Next, I use a small nozzle needle (made for this purpose) and push up through, from the bottom, till it moves easily up and down. Lastly, I extrude cleaning filament through till it no longer shows color. So I finally got around to soaking the nozzles in MEK and Ethyl Acetate to loosen them up and then did the above cleaning. My results were good except for three nozzles. Two of the nozzles were so clogged I could not push filament out at all (tried higher temps as well). I attached a picture where you can see the filament on the inside of the nozzle, but I really leaned on them and nothing moved. One nozzle came apart when I pulled the poke back up and out. Also a screenshot attached. The last one was the most interesting where the sharp end of the nozzle came off and the rest fell apart as well. As you can see I was using a purple filament. The last photograph is after an unusual attach from some inter-dimensional black demon that was some how extruded through one of the nozzles. :)