So this is a bit of an odd problem, but basically trying to describe the situation, the gear on the extruder seems to be grinding and tearing the filament as it passes up into the bowden tube, for lack of better words. I "think" (still very much a newbie, only been printing for about a month) it only seems to occur on large prints where multiple retractions happens it looks like; the first time it occurred it happened on an overnight print, so I didn't catch anything. But tonight I was able to catch it mid way through a print that was failing when I checked it, and the filament was moving up and down as if it were retracting, but wasn't advancing at all since it had almost tore through the filament so nothing was coming out on the hot end.
Looking at my spreadsheet of printer stats, this has only happened on larger prints where I disable combing in Cura (so no retractions occur), as last night I had a long print go through fine with combing on, and thinking about it both times it has failed was on longer prints with combing off so it could retract (due to flat surfaces on the print that I didn't want z scaring to show on due to leaked filament).
So I guess the question is, is this a slicer setting I can change in Cura to prevent this, or is this a mechanical problem with the extruder that I can fix? I've just been using the default Cura retraction settings (6.5mm retraction distance, 25mm/s retraction speed, 0.8mm minimum retraction distance, etc), and on small prints I've never had a problem with combing off, so that's probably why I haven't noticed this before, it seems to be the larger prints that have problems. I guess the obvious solution would be to leave combing on, but again I hate z-scaring, especially on prints I don't plan on painting, so would rather fix the problem and let it be able to retract when I want it to.
Any and all suggestions welcomed. Thanks!
Try limiting retraction length to 4.5 max, and increase retraction speed to 60mm/s. If your filament keeps getting grinded, limit retractions and increase retraction window. Increase the temperature a bit (5-10º) may also help (the higher the temp the lower the force needed to push the plastic through the nozzle). You could also try an extruder with tension control, as the one I designed, and lower it a bit, so the filament does not deform that much.
I wound up taking the extruder off. Taking the gear off the extruder motor (you have a small wrench for that), working two drops of oil into each end of the motor shaft. The ball bearings are very close to there. This made the extruder 300% better and I've hd not grinding or tearing of filament.
I've had this happening to me too. Sometimes because of too many retractions per mm of filament. This caused the filament to become flatter and eventually get stuck either right after it exited the extruder or in the e3d hotend at the heat break. My solution to this problem is to limit the "Maximum Retraction Count" and "Maximum Extrusion Distance Window" in Cura to around 30 times/3 mm and it solved my problem.
Another possible source of the problem is over extrusion but that's probably not your case since you only experience the problem when you print with lots of retractions.
Yet another source of the problem is excessive retraction distance which causes the retracted filament to cool down and harden above the the heat break and thus blocking the hot end. This happens in e3d v6 hot ends especially as the heat break becomes narrower. The filament looks like a spiral with a thicker end when you pull it out of the hot end.
Printing too close to the previous layer can also create too much back pressure in the hot end and cause the extruder to skip steps and flatten the filament in the process.
Ok I found the first option, but I don't see a maximum extrusion distance window option; I do see a minimum extrusion distance window though, is that what you meant?
Yes. That's it.
When I got my I3, after a while I started noticing all kinds of extruder-related problems. It drove me nuts trying to fix it. Basically, as it shipped, the screws that held the gripping gear into place were just not set in right. I managed to take the spring off, fiddle around with the orientation of the gear, and then really tighten the set screws down, and haven't had a problem since.
I didn't quite have the same experience you did, but I did notice that when the gear was slipping a bit, it would not actually move the filament on retractions, so it would just grind a little on the filament during retractions, and then move forward, but kind of skip. I suspect on a print with more retractions it might have chewed it apart like you describe? Dunno.. Take the gear off and really tighten it up.
I flipped mine 180' so I could get to the screws easier actually.. That seemed to help.
Have you noticed if it makes a difference what filament the issue happens on? You could try a cold pull and see if you can un-stick anything stuck in the nozzle. You can youtube how to do that. I guess it is best to use nylon, but the only time I did it I just used white PLA. I didn't even take my boden tube off the hot end because the connector was really stuck and I didnt feel like messing with it. (more risky though because then you can get junk stuck in the tube.) My problem was different though...the extruder would skip every once in a while during extrusion, which made it more obvious the nozzle was partially clogged. Also, What temps are you using?
Have you checked if your nozzle might be partly clogged? I have bought acupuncture needles with 0.3mm for cleaning the hotend every few hours to avoid this problem.
I'm not familiar with combing, so I'm newer to 3DP than you, but my Cura retraction settings for my Anycubic i3 Mega (full metal) are default and different than yours: 6mm, 40mm/s, 1.5mm. For what that's worth.
I expect you have checked for binding on the filament support and spool increasing tension on the filament.