Filament seems to fuse into PTFE tube when print finishes
MK3S, Anycubic PLA. 205/60 , 0.4 Brass nozzle, both fans working correctly. Printer works flawlessly, but on longer prints, once they finish, the problem raises its ugly head: when I attempt to start the next print, the extruder stepper starts hammering and no filament is extruded. I have tried setting the nozzle temp to 250 with no success, no ooze at all. I have tried removing the nozzle and using the extruder to feed and retract filament through the PTFE, but it will not budge. There is no way to get the filament to unload either, and will usually break where the drive gears have weakened it. At this point, I must take the extruder apart and when I pull the PTFE tube out, the filament is stuck solid in the hot end of the tube. Background: I have used a full spool of this filament before with the generic PLA presets without issue, however this new spool was oozing and producing stringy prints. I reduced the nozzle temp to 205, and the oozing and stringing stopped, prints came out fine, but then this problem showed up. I think that somehow, when a long print finishes, and the hot-end cooling fan stops, there is enough residual heat to melt the filament that is still inside the PTFE tube. It then cools and sticks to the tube. When the next print job calls for the pre-heating of the nozzle, the cooling fan prevents the filament from melting where it is stuck, resulting in a jam. I may be receiving my reward for using cheap filament, right? As I mentioned, this spool of filament seems to have a lower melting temperature than normal. I will disassemble and clear the jam, and then try the same print with Prusament filament at presets and see if the problem is solved. Any thoughts or similar experience would be appreciated. I will update as I explore possible solutions.
RE: Filament seems to fuse into PTFE tube when print finishes
Update: Well all cleared and a new nozzle, ready to go with Prusament this time. When I disassembled the hot end it was nearly impossible to pull out the PTFE tube, since short segment of the filament was naturally welded to the hotend below the heatbreak portion and actually broke there with part of it still stuck in the PTFE tube. It took quite an effort to get it out. Yes, I am using genuine Prusa PTFE tubes, supplied with my MK3S Upgrade, yes I am inserting them fully into the body of the hotend, tapered end first with the 'funnel' end up, and firmly seated when I pull up the locking collar. I did some flexibility comparisons between the two filaments. They both seemed similar, snapping cleanly off when bent quickly, and thinning and becoming flexible when bent slowly. Using Prusaslicer 2.0 to generate new G-code for the part using the 0.2mm Quality setting for Prusament. Lets see what happens.
RE: Filament seems to fuse into PTFE tube when print finishes
This is why we unload filament when the hot end is heated. But the heat break is 2.2 mm from the PTFE seat down to the neck, then the bore drops to 2.0 mm. This step is one reason why filament jams in the heat break. In most cases, when unload fails, the stub can be pulled up through the PTFE manually similar to a cold pull. Open the door, preheat, and pull the filament out. Way back when I was having these jams - before I replaced my heat break with the stock E3D version - I had to do that manual unload many times. And I never seriously damaged the PTFE. When I did replace the heat break, I replaced PTFE as a matter of caution, not because I needed to.
RE: Filament seems to fuse into PTFE tube when print finishes
I think I understand what you are saying, but this is not my first rodeo, :-), I have gone through 10 or so spools of filament, unloading and reloading different colors of PLA and often switching from PLA to PETG and back without problems. I have always preheated and only pulled the filament out when directed to by the info screen. But this circumstance is different for some reason. Preheating the nozzle, even to 250 does not release the filament for an unload and any attempt at a cold pull results in the filament breaking right at the extruder drive gears, requiring a full dismantle of the hot end. Even on a preheat prior to unloading the filament, the cooling fan runs to keep the filament between the extruder gears and the heat break solid. My suspicion is that somehow, when my print job finishes and the cooling fan stops, residual heat from the heat block makes its way up past the heatbreak causing the this filament (with a lower melting temperature) just above it in the 2mm bore and the bottom end to soften/melt enough to lock into place as it cools. Then I think when the heater fires up again, either in preparation for a print, or in preparation for a filament unload, the cooling fan also starts, preventing the filament in the area of the heatbreak from re-melting resulting in a jam. Please offer your thoughts on this idea.
RE: Filament seems to fuse into PTFE tube when print finishes
Update 2. As I mentioned above, I tried a print using Prusament with the 0.2MM Quality settings. After 5 hours of printing, I suddenly heard the extruder stepper clicking. I have placed a mark on the shaft so that it is easy to see whether or not the shaft is turning, and indeed, it had stopped. Another freaking Jam! There goes the cheap filament argument, right? Up to that point, the job went flawlessly. SO annoying! Anyone who can help me sort this problem out would earn a place in my will (LOL) but seriously. HELP. I have spare PTFE Tubes.
I stopped the print, and set the nozzle temp to 250. as the heat went up, filament started to ooze from the nozzle, proving that it wasn't plugged. That means that the problem is somewhere between the Nozzle and the drive gears. I attempted an unload with no success, so I undid the tension screw fully and tried a manual pull. The filament actually stretched out but did not pull, weird! I wonder if I have a cooling fan issue. The selftest reports that the cooling fan is OK.
RE: Filament seems to fuse into PTFE tube when print finishes
Well, maybe this IS my first rodeo. I think with this warm weather I am(for the first time) experiencing heat creep. But it's a sad day when one doesn't learn something, right? I will attempt to keep the temp under control, It seems like a real PITA to need to be right there at the end of a print to quickly unload the filament. Often I will set 6-9 hour prints to run overnight but I will not be there right at the finish. Is there some g-code that I can incorporate to do this automatically. If not, and Prusa can do it with the MMU, why don't they offer that option in the firmware for their flagship MK3S?