Extrusion problems with rough-surface filament

[Hank Dietz]

I have an M2 which is about 4 months old and has had a problem I never saw on my 1.5-year-old M2: pretty much from day one, it has been picky about smoothness of the filament.   I originally had assumed this touchy behavior was simply an unfortunate side-effect of the cartridge heater (since my other M2 doesn't have that). Although I use a variety of mostly PLA filaments (with minor, but successful, test prints of T-Glase and ABS), the MakerGear Black PLA that came with the newer M2 happens to have a rough surface (unlike the previous spool of Black PLA from MakerGear), and it has been one of the most problematic filaments. A similarly rough-surfaced glow-in-the-dark PLA and soft PLA were also problematic. The problem comes in the form of what seemed to be a hot clog, with extrusion sometimes stopping in the middle of a print and always coming out of the head curly unless I turned-up the temperature. I usually was able to temporarily clear the situation by bumping the heat a little and forcing PLA through by hand without disassembling the hot end. Smooth PLA filaments didn't stop extruding, but over months came out of the head increasingly curly... printing ok despite that.

On Wednesday, one of my students and I took the hot end apart to clean it and finally resolve the issue. It wasn't a hot clog at all. However, inside the PEEK tube there was essentially a flap of material partially blocking the path. Smooth filament moved past it with minimal pressure, sliding the flap out of the way, but sometimes, depending on how it hit the flap, rough filament would wedge against it and could barely be pushed through. After a few minutes using an unbent paperclip to work and pick-at the surprisingly well-attached flap, I was able to get it to break free. I'm not sure what the flap was, although it was black in color and I'm assuming it was burnt filament?

In any case, I don't quite know how this flap got there.

Is this a problem others have seen? Unfortunately, I didn't photograph it because it is hard to get a camera to see inside the tube, so you'll have to go by my description.

I suspect that it might be caused by a minor rough spot in the end of the inner tube giving hot filament someplace to stick upon retraction. In fact, my best guess is that it started life as a tiny shaving on, and still attached to, the inner thread. With filament changes, the crud could quickly build-up to create the largish flap I saw. I change filament much more often on the newer M2 than on the old M2, so that probably amplified the problem.

[Jin Choi]

jimc has a lot of experience with using different filaments. He has reported here, and on the new forum, that using the same nozzle for different filaments tends to build up strata on the walls. Maybe that is what you encountered.

[Hank Dietz]

On Friday, June 6, 2014 3:31:17 PM UTC-4, Jin Choi wrote:

jimc has a lot of experience with using different filaments. He has reported here, and on the new forum, that using the same nozzle for different filaments tends to build up strata on the walls. Maybe that is what you encountered.

Not exactly. I'm not kidding when I say this was a flap -- like a hinged little blob inside the tube and pivoting off one edge of the interior wall. We've had nothing like this on our other M2, which has been used with as many different filaments (just not swapping as frequently) and in 1.5 years has only had a couple of hot clogs.

However, I definitely do agree that frequent filament swaps worsen these sorts of problems. For example, what temp should one be using to retract one filament before putting in another? The ideal temps seem to vary a lot more for retracting the filament than they do for feeding it; perhaps there is more variation in the "stretch" you get from different filaments. In any case, variations in temps mean a mix of material remnants in the head can have some goey when others are still semi-rigid, thus spreading mess inside the tube. Incidentally, the vast majority of filaments I'm using are simply PLA from different vendors in different colors -- but there does seem to be as much as a 15-degree difference in optimal operating temp, and soft PLA seems to get particularly goey.

[Jin Choi]

I've read recommendations that you retract out the filament just above the glass transition temperature, so that any remaining plastic comes out all together as a blob. I never remember to do this, so I haven't tried it.

[Tim]

I've read about this, too;  it's called a "cold pull" (although you don't really want it to be truly cold).  But, can you pull against the extruder gear?  Or just disable the prohibition on extruding cold (the M103 command), and then run the extruder in reverse?

I've found that PET+ (at least the white colored type) pulls out very well, almost always comes out clean.