Skip to content

Prusa MK3

A great printer with a great lifespan.

Older pages:

Stuck filament issue

As posted here: https://forum.prusa3d.com/forum/english-forum-general-discussion-announcements-and-releases/help-filament-jams-slips-on-print-start-filament-change-needed-to-continue/

Symptoms

  1. Filament gets stuck, accompanied by a "clicking" noise from the extruder slipping on the filament.
  2. It happens more often on the first layer, and in prints requiring lots of retractions.
  3. Can't be fixed by cleaning the nozzle, swapping filaments, replacing the PTFE tube, etc.
  4. It can be "fixed" temporarily by removing the filament during the print (selecting "change filament" on the front LCD), cutting the tip, and reinserting it.
  5. The tips of the stuck filament look like this

alt text

Final diagnostic:

  • A blob of melted plastic is observed around the heatbreak, most visible when the hotend is removed from the extruder and inspected from the side.

alt text

Cause

As it turns out, the problem was due to heat creep.

Heat creep caused in turn by a nasty blob of filament, that leaked at some point, hardened, and then got stuck around the heatbreak.

Heat creep causes the filament to soften above the heatbreak. Unfortunately, the MK3's heatbreak is slightly wider on the upper end (2.2 mm) than the lower end (2.0 mm). Thus, the filament's tip expands, filling the upper part of the heatbreak.

Once this happens, the tip can no longer fit get past the heatbreak, resulting in a filament jam.

It also explains the peculiar shape of the filament tips once removed.

heat creep

Fix

  • Remove the heater block from the hotend (follow Prusa's guides).
  • Carefully remove the melted plastic from around the heatbreak, and clean it.
  • Reassemble the hotend.

History

The issue started during 2020. I ended up changing the PTFE tube.

It came back in 2022 when I changed to a new filament. Raising the extruder temperature to 230ºC for the entire print helped.

Relevant searches:

I had dealt with this issue for years, and finally found a solution. Searching the internet for "heatbreak jams prusa" immediately returns a bunch of posts with the exact same symptoms.

There is even a video about this by 3D Printing Nerd:

I am surprised that prusa support never replied to my emails with a solution, even though it is a well known problem.

Also, I'll add how I got to the right search terms. After a serious jam I was forced to disassemble the hotend entirely, and by mere conincidence found hardened plastic gunk between the heater block and the heat sink.

The material completely surrounded the heatbreak, which in turn may have helped heat creep up to the heat sink, causing these jams with charachteristic shapes in the filament end (pics in the first post).

I'm happy I found the issue. I hope I get many more years out of this fantastic printer.

Upgrades