Prusa-firmware: [BUG] Extra extrusion after filament change

[espr14]

i3 MK3S FW 3.8.1

1. Filament runs out during print
2. Printer pauses and asks for filament change.
3. Filament is pulled out.
4. Filament is pulled in.
5. Answer for question is YES (everything OK).
6. Now it extrudes extra filament which is immediately attached to the printed object !!!

Should not extrude extra filament but do retraction. Since there's no default retraction length, there should be Marlin instruction added which defines default retraction length. There could be retraction instruction as well.

[DRracer]

This extra purge is a feature and the user shall count with that. It is there to prime the extruder and to reduce (avoid) the gap in the print caused by missing filament. Removing excess filament from the print is less a problem than adding back missing filament.

[espr14]

Missing filament is bad for external perimeter but not for the rest (in most cases). But this excess filament is attached to the print always. So, at least, it would be nice to turn fan to 100% while extruding this excess filament not to attach to the print well.

[wavexx]

It was weird to understand this for me at first, but it works the following way: leave the excess attached to the nozzle while swapping the filament. As soon as the extruder starts moving, grab it with a quick motion to make a nice break at the nozzle. If you do that, the result is almost perfect (next to zero gap or blob).

You cannot "grab" the filament unless you have some excess being extruded.

[Area5142]

I think this is related to the unfixed issue "M600 does unwanted extrude after 'is color clear?' confirmation" #197

I print almost only in PETG and this behaviour often makes the extra filament stick to the nozzle instead of the perimeter. The new lump on the nozzle is then dragged along making more harm than a tiny hole in the perimeter.

I have found no way to turn this "feature" off...

I understand that this extra purge is to build up the pressure in the nozzle - maybe there is another way to do that or reduce the problem:
1) Move nozzle to the spot where printing start before extruding (avoiding extrusion in the air).
2) No extra extrude, but let the new LA1.5 (Linear Advance) handle all pressure control in the nozzle.
3) Making a way to turn this feature off in the firmware.

[Area5142]

Another relevant reference: "Blobs upon resume after M600" #904

[wavexx]

The problem has already been described, it boils down to the fact that between "is color clear?" and the extruder moving some material has oozed already. We could retract just before the "is color clear?" prompt, but stuff like TPU would probably still leak a bit.

LA cannot do magic either. It has to assume the nozzle is fully primed in order to work.

Is it really such a big issue? When swapping filament some excess will always be there. I just pull it at the very end instead of doing it right after the prompt.

I also print a ton of PETG, so I know really well the effect you describe.

[Area5142]

@wavexx
I try to quickly grab the extra extruded filament, but when printing things like flat signs the room from the build plate to the nozzle is very small (only about 10mm). It would help a lot if the nozzle had a larger distance over the build plate - that would make room for multiple purges and cleaning the nozzle.
A short retraction (2mm) after "is color clear" would also help and properly prevent filament sticking to the nozzle.

[espr14]

The amount of extra extrusion is very much. It's like 10 cm with 0.4 nozzle. This can lead to collision during printing. Even with 1 mm nozzle it is more than enough. Moreover, only 0.6 mm nozzle is supported.

[GunSmoker]

This is definitely not a bug. I do remember instructions on changing filament, which explicitly says that you must grab extruded filament and hold it in place until hotend will move. Moving hotend will break filament from nozzle, leaving clean primed nozzle, and filament blob on your tweezers.

[gogglespisano]

"I do remember instructions on changing filament, which explicitly says that you must grab extruded filament and hold it in place until hotend will move. Moving hotend will break filament from nozzle, leaving clean primed nozzle, and filament blob on your tweezers."

This doesn't work very well for me. When I grab hold of the filament change blob if often breaks loose from the nozzle.

How does this help with priming and pressurizing? The extra extrusion is into open air just like the filament change extrusion 2 seconds earlier. The only need for this is if you wait a long time after the filament change to continue printing. How about a timer? Less than a few seconds - no extrusion; Longer - do the extra extrusion.

[sledjunk]

Sorry I posted #2801 as it is a double post. However, I will add that after the filament change is done, and while the printer is moving to the start position, the extruder is turning, so it really is almost impossible to prevent a blob, especially with PETG.

[wavexx]

Forgot about this until now.. I had a few filament swaps yesterday, and I might have to agree.

As @sledjunk said the extrusion keeps going until XY are in-line with the print. Sometimes grabbing the filament as late as possible is not always easy on large prints.

Thinking again, I'd still do an extrusion before starting to move (the filament just oozes out even if you're fast with the button), followed by a quick retract and then move. This way if you grab the last bit of filament before (or during) the extruder moves there's less ooze going on.

I tried this by simulating a change with some gcode and seems to work "good enough", certainly not worse than before even if you click "ok" after ~30s or so.

Some ooze is still present even with a retract though. The move back to the print is slowish.. I'm not sure if this speed is fully intentional?

[sledjunk]

I tried again yesterday and I used a strip of plastic (actually a ruler that came with overture PETG) and held it to the nozzle when it landed on the job so that it caught most of the blob. It helped somewhat, but you almost have to ruin one print so that you know where the nozzle is going to land on the job.

[gogglespisano]

If there was a beep and 1 second pause just before moving back to print you could easily grab it.

This would be good at the start of prints too. I've gotten used to grabbing the ooze just before a print starts by watching the temps and knowing when it's about to start. If I don't grab the ooze, it's hit and miss if it will fall off during leveling and be laying in the middle of the print area.

[sledjunk]

I haven't really had an issue with start of print because of the purge done after bed leveling and before the print begins

[gogglespisano]

I don't have a problem after the purge starts, but once in a while the preheat ooze will fall off while leveling and then get into the print.

It happens often enough that I got in the habit of grabbing in just before the leveling starts.

[benr916]

Yea, this is still an issue even on the latest version firmware and PrusaSlicer. Ive gotten pretty good at grabbing that extra filament that leaks out but it would be nice if I didnt have to babysit the change over so much.