Strange "Peeling" of PLA in Extruder
I've been having the strange occassional problem with prints following a print with black filament, where at some point during the first layer, there will be a smear of black filament in the color filament, as shown on the top of the two prints in the picture below.
I initially thought it was left over filament on the end of the nozzle, but I've been cleaning that before just about every print. I also noticed "peels" or curls of filament gathering in the extruder compartment that I've had to clear out on occasion.
The other day I was changing out green filament and when I hit the unload button, the green filament came out, but also a thin peel of filament came out of the PTFE tube as well, shown in the picture below how it came out.
Green peel along with filament
The filament didn't appear to have been "peeled" so I'm assuming this was build up, and I'm thinking that my sporadic problem with black was the same situation, although in that case, the peel works its way down into the extrusion as opposed to being ejected on an unload.
Anybody have experience with this? Are my extruder screws too tight?
Thanks.
RE: Strange "Peeling" of PLA in Extruder
Two possible causes for the discolored prints:
- Filament accumulating in the hotend. This will be common if you switch from a higher temp filament (e.g. PETG) to lower temp (e.g. PLA) if some of the higher temp stuff gets stuck. This can also contribute to partial clogs. Read up on cold pulls.
- Filament accumulating on the nozzle and heater block. A fair amount of old filament can build up and eventually drop off into prints at inopportune times. Clean the nozzle and block at printing temp with a soft brass brush, being careful around the delicate wires. If using a coated nozzle, a strip of cardboard is useful for cleaning the nozzle. Consider a coated nozzle and/or silicone sock for the heater block to reduce build-up.
The peeling is a bit unusual. Do check the extruder gear tightness. ~1mm protrusion from the extruder housing is what works for me. Also check the Bondtech gear idler in the door of the extruder housing to make sure it rotates freely.
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RE: Strange "Peeling" of PLA in Extruder
A nozzle not tightened against the heat break can also cause some of this. Ooze from around the heat break and nozzle threads cooks and browns on the way out and drips. But won't generally cause a nice uniform transition as in the photo.
There are cleaning filaments that help remove old filament from the hot end. But I've also experienced debris building up in the heat break below the PTFE and above the step that has caused jams; but can be cleared by removing the nozzle and blowing air up and down through the filament path; the dust going everywhere. It follows that under some conditions the dust (from a prior part, and usually from chewed filament) can settle and melt with the new filament to create a different color than expected. But this should be fairly short lived and happen only once per filament change.
ps: Is this on an MMU setup?
RE: Strange "Peeling" of PLA in Extruder
Thanks, this is just a straight MK3 set up. Haven't updated to MK3s yet. I don't have any oozing around the heatbreak, and I am running with a silicone sock. I typically wipe it down, especially when I'm switching from black to something else. This printer has only been printing PLA, so haven't had to work with the challenges of major temp deltas.
I at first thought it might be burnt filament, but you are right, the spread is much too uniform. The best I can come up with is that the peel remained in the hotend when switching filament and then worked its way out.
It was the strangest thing, since when I hit the unload button, the peel came up alongside the regular filament extraction. That was the first time I've seen that, but I have seen much smaller "peels" accumulate in the cavity around the bondtech gears.
I'm going to look at loosening the extruder door screws and see if that helps.
RE: Strange "Peeling" of PLA in Extruder
I am theorizing, and only have cursory plastic extrusion experience (worked next to our plastics people and was fascinated by what they did), but when manufactured, filament sliding through a colder than expected forming die could cause filament to cool and remelt, forming a second skin on the final product. This would pass through an extruder without issue, but then easily slough off farther down the path where it meets PTFE friction. I wouldn't expect it to be common place, but within the realm of what might explain what you are seeing. If it happens with any other spool of filament, then ignore me (or you have a reason to stop buying that brand), but your experience is just unique enough I wouldn't rule out outlandish explanations.