[BUG] Extensive heat creep into Nextruder causes filament grinding

[ilka-schulz]

This is a hardware bug but I could not find any repo for the MK4 hardware so I post it here.

Printer model

MK4 (pre-assembled) with original Prusa enclosure

Firmware version

6.0.0+14794

Upgrades and modifications

1. removed plastic cover from the LoveBoard
2. using original Prusa 0.6 mm brass nozzle
3. filament spool is held outside enclosure – issue is even worse with filament on original spool holder inside enclosure

Printing from...

PrusaLink

Describe the bug

Observations

1. After printing one piece, the nextruder gets significantly warm. Using my thermal imaging camera, I was able to identify 51 °C roughly one minute after the print finished inside the print head close to the big gear wheel. The temperature in there seems to be more or less homogeneous so it is fair to assume that the big gear wheel pushing the filament is roughly 51 °C, too.
2. When printing several parts directly after each other, the printer will eventually struggle to push the filament through the nozzle.
   • The gear wheel grinds the filament and thus looses traction.
   • The nozzle does not clog.
   • For a couple layers, one can visually see how extrusion rates go down, then at some point the filament stops moving completely and the gear wheel grinds a chunk out of it.

How to reproduce

1. print some ColorFabb HT @ 270 °C with any nozzle (0.4 mm / 0.6 mm original Prusa brass nozzle)
2. use the original Prusa enclosure
3. use Input Shaper "Structural" mode
4. print for at least 3 h without cooling down in between

Conditions

These are my specific conditions. You might be able to reproduce the issue with slightly different settings, too. Especially if your ambient temperature is higher than 21 °C.

condition	value
temperature inside enclosure	≈ 31 °C
temperature outside enclosure	21 °C (controlled by AC)
humidity outside enclosure	usually 60 % RH
Filament	ColorFabb HT (some cristalline PET)
Filament feed	spool hangs outside enclosure to reduce filament temperature
nozzle diameter	0.6 mm
nozzle temperature	270 °C
bed temperature	105 °C
print time	121 min per print + heating, etc., I am printing many pieces in a row

Expected behavior

• printer prints part without grinding filament
• print head / nextruder gear wheel does not get that warm, especially not 51 °C

Files

No response