Help, my MK2 jams randomly
Please help.
I have a Prusa Mk2 in good condition and it jams randomly.
It lives in a spare room, with no cold drafts or other heat problems.
I'm using a new spool of gray PLA from MakerGeeks.com. It states the nozzle temp should be 235 but I've had problems running that hot so I ran at 230 instead. It will run for hours and then the nozzle will jam, PLA will stop flowing, and prints will fail.
I'm using the original nozzle, which I did clean with a brass wire brush before starting. Before running the most recent print, which ran for over 7 hours before jamming, I fully disassembled the hot end, carefully, and removed all the old PLA stuck to it. I had to burn the PLA out of the nozzle with flame, then wire brush the threads and tip clean with the brass wire brush. All the other PLA was scraped away while hot from the outside of the throat.
My working theory is that the prints seem to fail in areas where printing stops and starts a lot and that PLA is over heating when retracted and hardening in the nozzle. Either that, or the nozzle is somehow worn out or damaged on the interior and causing jams.
I've done all the self-checks and calibrations and prints will start OK but fail some random hours later due to nozzle jam.
Since I'll have to disassemble and burn out the nozzle again to fix this, I'd like some real help on how to fix this before I do that again.
Thanks.
-John C
Re: Help, my MK2 jams randomly
It's not uncommon for filament to have contaminants that plug the nozzle. Basically a chip of something that fits through the 1.75mm extruder making the spool; but not the 0.4mm nozzle. To cure this you need to do a cold pull to remove the crud that is plugging this up. It's also possible you have collected enough dust and debris down the PFTE/Heatbreak it is jamming. You need to clean the filament path by removing the nozzle and blowing air up through the filament path; or do a full disassembly on the hot end to clean out everything.
Re: Help, my MK2 jams randomly
I used to have this issue with my MK2 before upgrading to 2.5. Even then there were occasional filaments that caused issues. It would print fine for an hour or so, then suddenly jam. In my cases, I kept finding that the removed filament had a larger than normal blob on the end when I pulled it back out. Clipping the blob and re-inserting would start printing again immediately. I believe the air in the MK2 model meant to cool the top part of the nozzle isn't sufficient for long-term prints. As you mentioned, it's likely that heat builds up enough that the filament softens above the heat break, or in a slight gap between the PTFE tube and the heat break. When you have many retractions happen all at once (usually at the top/end of a print, where it moves between several remaining points to print), that allows the blob to move higher in the tube and cool in the wrong place, and then can't be pushed back down.
One test you might try is to remove all retraction on a print that seems to fail early on you (so you don't waste too much plastic) and see if it keeps happening. There were a couple updates in the 2.5 upgrade that increased fan cooling that seems to have fixed the issues for me. At least, the one filament that always caused jams with my 2 setup is now running find with 2.5 and 2.5S. I was never able to completely eliminate it in my MK2 setup with specific filaments, while other filaments printed just fine. Something about the melting temp was just right to cause that jam with retraction over time.