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[Solved] MMU2S and SpoollJoin failures  

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Wheart
(@wheart)
New Member
MMU2S and SpoollJoin failures

Hi!

"Ordinary" setup: MK3S + MMU2S + PrusaPrint on RPi3 (given up with PiZeroW... it was way too weak).

For some longer prints (and remains of filament spools) I've decided to use spool-join feature.

At first I did it with PLA, spools 1&2 - worked like a charm, in the morning big print was ready, few days later I've found a bit of old filament in bowden tube #1 - is there any procedure to unload it gracefully (i.e. move the carriage to sixth position (far right) and eject filament remains in "forward" direction?

Now I'm trying to do same thing with PETG, once again positions 1 & 2 and it fails in strange way (three different prints):

  1. printer loads filament into hotend (some extrusion on front right corner), than unloads filament and "park" hotend (back, left).
  2. on display still "Loading filament 2"
  3. "temperature graph in OctoPrint freezes, no error on MMU2... (!?) after pressing the knob MMU cuts the tip of the filament, and tries to load the filament once again... just to start whole procedure once again from step one. There is no change on display (!).
  4. After reaching three attempts, MMU signals a filament loading error, pressing the buttons resolves problem, now there is an indication of MMU back to OK, resuming printing - and keep loop of steps 1 and 3 - once again 3 times...
  5. And right now, trying to catch as many details as possible: After another manual "recovery" from loading error printer did one more loop 1, 3, after pressing printer knob once again cut the filament tip and loaded filament successfully, resuming printing...

One more observation - when bondtech gears of extruder catch filament, there is a moment of grinding - while extruder is pulling filament towards the hot end, MMU tries to REMOVE the filament from extruder - sometimes extruder motor reverses direction in this moment for a while, and filament is then unloaded...

Did you ever noticed such behaviour? I've placed printer in an enclosure, and for PETG I kept it closed (about 40C inside, power supply is moved outside) - so maybe an electronics is overheating - how to diagnose this?

Regards,
Wojciech

Posted : 31/08/2020 10:39 pm
Wheart
(@wheart)
New Member
Topic starter answered:
RE: MMU2S and SpoollJoin failures

OK, another three months, some frustrations, but things just started to match each other and problem is solved... just one thing: IR sensor.

"Chimney" was a bit deformed from tension of the screws. "Stiff" PLA still triggered the sensor, as well as blobs on "usual" PETG ends. But "soft" PETG had problems - if "blob" on the tip was large enough, it triggered IR sensor for a while (and printer has no problem with printing when FINDA "sees" filament, but IR sensor doesn't!).

TL; DR unless you want to take trouble-shooting journey with me 🙂

Problem recently escalated around new "cursed" spool of PETG from different vendor than usual. It almost always failed to load, but when unloading, its tip was just like from manual - bullet shaped, no wider part. Little touch on IR sensor housing when filament reached bondtech wheels made printer running, just same trick worked with failing spool join (or filament loading on hot printer) and filament changes (PLA worked nearly flawlessly, PETG often failed to load or reload)... It should give me something to think about, but I was doing it with no intention - just forced filament into printing head, being happy when it worked.

In meantime printer underwent though cleaning and lubricating, rewriting firmware, filament feed line mods (new spool stands to eliminate original buffer), and I've finally disassembled top part of extruder...

The moment of enlightenment come, when for some unknown reason printer started extruding on load ("as usual" - small amounts were common), but this time kept running for longer time with BOTH extruder and MMU motors working! After some time it unloaded filament (again - "as usual"), and retried loading, but there was much more extruded filament under the nozzle...

Do you remember nice, bullet-shaped, smooth filament tips? Yeah, now having connected all the dots whole problem is obvious - steppers worked just like on earlier loading stage, when MMU feeds filament till extruder catches its end. But haven't received signal from IR sensor, after some time it gives up (timeout), takes filament back, and retries (3 times as I remember). After third attempt "MMU loading error" appears on display, heater is switched off and printer awaits for operator intervention.

What about MMU trying to remove filament? It looks, that if the blob at the end of filament was big enough to trigger IR sensor, two things could happen: it may stop at bondtech gears, keeping sensor triggered (usually at cold start...) - printer starts printing as soon as hotend reaches its working temperature (Do you remember? No IR sensor checking, just FINDA!), or... it may be pushed past gears into into hot hotend - in that situation IR sensor indicates loss of filament and whole procedure of loading is aborted... Rolling a dice (or better pushing filament tip into hot zone back and forth) enough times malformed it enough to trigger the sensor... but with new firmware I've activated cutter...

So it is my mistake.

Few suggestions, which would make troubleshooting such cases much easier:

  • please, add indication of filament sensors (IR sensor and FINDA) state on display (when loading/unloading only) or as simple LEDs (there was even tutorial how to solder LED and resistor on IR sensor - maybe it could be soldered there when leaving the factory?)
  • add possibility to cancel loading procedure
  • add header to connect "external keyboard" to MMU (or ability "push" buttons from main unit) - when printer is kept in enclosure, it is hard to reach MMU's buttons;

Best regards,
Wojciech

Posted : 19/11/2020 12:44 am
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