Creality CR-10 Mini “Cold Pull” and Bowden Fitting Cleanout

Had a bit of a filament jam on the Creality this week, and today I tried a “cold pull” procedure with the intent of cleaning any foreign material and scorched filament out of the nozzle. I think that was an issue needing to be solved, but I also created another problem along the way.

3D printer filament blobs from nozzle cleanout

In the background, you can see that I’m not as good at cutting glass as I am at … well, let’s not make any hasty characterizations I’d regret later.

A “cold pull” is a procedure to remove anything stuck inside a 3D printer’s nozzle. You heat the nozzle to extrusion temperature, push in filament and get it stuck to the reservoir of filament inside the nozzle, and let it cool. You then heat it back up and as it approaches extrusion temperature, pull hard on the filament. The idea is that the gunk inside the nozzle will be heated from the outside in; so it’ll be melted and mobile where it’s in contact with the nozzle but the center will still be attached to the filament in a big plug, allowing you to pull out more of the gunk than during a normal all-melted retraction.

On the far left is the “original” end of the Bowden tube. I wasn’t even able to retract the filament out of it, nor was I able to remove the tube without bringing the assembly up to extrusion temperatures. Given the coloration on this and subsequent filament blobs and the fact that I’ve run many meters of white filament through the printer since I last used black, I’m pretty sure that does represent scorched filament that would no longer flow forward and did need to be pulled back out.

But … the next filament plugs in the left half of the photo are a combination of the appearance of success at pulling out charred PLA and likely the beginning of trouble. For you see, the filament being pushed in started to pool up past the 1.75-mm ID of the nozzle into the 4.0-mm ID of the heat break where the PTFE Bowden tube was supposed to seat, or so I reconstruct from subsequent events.

After hitting diminishing returns on the cold pulls but still not being able to push filament through, I used the back end of a finish nail to give a good shove on the blockage. It both managed to squooge a bunch of trapped filament out the nozzle (exhibits “S trap” and “baby squid”) and also brought a nice chunk of char back out with the nail head.

I believe I tried another filament cold pull, which failed to bring much out but probably refilled the bottom of the heat break. I think I then reassembled the Bowden tube and tried pushing filament through the nozzle using the extruder motor. Evidence suggests the tube didn’t seat, so the molten filament pooled, cooled, and created a plug at the bottom of the heat break below the end of the Bowden tube, preventing further filament flow.

Finally I removed the Bowden tube again, then carefully used another nail head to scrape down the heat break walls, apparently getting them clean and pulling out another filament plug. A third nail provided final scraping and came out clean (the cake was fully baked!) and the printer once again works after reassembly.

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