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Filament suddenly gets stuck during printing  

Thomas Trummer
New Member
Filament suddenly gets stuck during printing

Hi, I've had my MK2 with multi-material upgrade for a few years.

I mostly used PLA or ABS as filament, but I constantly had a variety of problems with ABS (wrapping, stuck filament, ...).

Now I bought a roll of ASA from FormFutura called Apollox. The wrapping problem is now gone, but the stuck filament still isn't.

With about every second part (5-6 hours of printing time per part) the filament starts to get stuck and hardly anything comes out of the nozzle.

After stopping, it will automatically discharge. This doesn't work at all anymore. It just rattles and crashes and the filament no longer moves.

I can only remove the filament by dismantling the MMU and pulling the filament out with pliers and force.

The tip of the filament may indicate the problem. The tip is always thickened and you can lift this thick area with your fingernail. So this is a crescent-shaped structure that sticks to the filament but is not actually fused to the filament.

Could this thickening be the problem?


These are the settings I use for the Apollox ASA Filament:


Does anyone know what can be done about it?


This topic was modified 2 months ago by Thomas Trummer
Posted : 06/02/2024 3:06 pm
Veteran Member Moderator
RE: Filament suddenly gets stuck during printing

As far as I can gather from your message, 

You have a printer, like this

this is a Mk2 prusa with MMU1 
When you say the filament keeps jamming... 
can you describe where the jams are occurring?
the most likely places are shown in this picture

the green area comprises the steel 'Cooling Tubes' and the 4 into 1 filament guide area. 
The Orange area comprises the single feed tube from the 4 into 1 filament guide area, into the e3dV6 Heatsink, where it is lined with PTFE Tube
And the Red Area comprises the stainless steel heatbreak and nozzle area. 

When you initially load filaments, the MMU1 drives the filaments forward, until they are resting inside the steel cooling tubes, in the Green area. 

then when a print starts, one of the 4 available filaments is fed down to the waiting (Fully heated) nozzle
This typically causes a bit of extrusion, which helps clear the hot end of any previous filament. the waste is wiped off on the waste pile (Purge Block)
then the printer begins printing the part of the model that represents that filament, on that layer
then when finished, the printer drives the extruder back to the purge block, to wipe off the nozzle, whilst the filament is withdrawn and rammed, to re form the tip of the filament.  the filament is then moved back and forth inside the PTFE tubing to re-form the diameter of the end of the filament, the filament is then drawn back into the steel cooling tube, and the process continues with the next filament. 
if the filament is too thick, it can jam in the steel tubes.   (This is usually a Ramming / cooling moves issue)

If a filament jams going into the Orange area, the problem is usually a tip forming issue (Often because the filament is a bit hot, during the ramming phase)

If the filament jams in the red area of the extruder, the issue is often, due to excess retraction,  or heat creep. (or a combination of both)

Typical  default retraction is 4mm  I reduce this to 2mm...   
If there are a lot of retractions, the hot filament is drawn back into the cold side of the heatbreak, transferring heat into where it should be cold.  the longer retractions cause more  heat transfer...  which can cause blockages low down...   especially with PLA... 

If the printer is in an enclosure, or the ambient temperature is higher than average, or the side fan, is clogged,  you can get poor cooling of the 'Cold End' (the heat sink and the top part of the heatbreak)  this can cause the filament to soften and compress into the heatbreak area where it can Jam
Over and above this...    the extruder PTFE Liner, should be a narrower than normal ptfe diameter  (1.85 to 1.9mm ID instead of 2mm ID)
PTFE, does wear during use. so it's worth replacing after a lot of use 
(PTFE Trimming guide)
Capricorn PTFE, comes in various sizes,  the 1.9mm size is OK
The left hand fan, is very small, and noisy. there is a grill, on the right hand side of the  cooling duct, this partly blocks the airflow, later fan ducts were produced without this grill, you may wish to consider removing, the grill!   to improve flow. 
I would clean out the Fan, Fan duct and Heatsink  of all fluff, 
I would consider peeling the label off the hub of the fan, and put a drop or two of oil into the bearings, wipe off any excess and replace the label. 

the Bowden tubes between the extruders and the 4 into 1 concentrator, should have an ID of 2mm to constrain the filament without causing too much friction, 

the filament feed tubes are best with a larger internal diameter, 2.5mm or 3mm, to reduce friction.

If you clean up the air paths, replace the hot end PTFE , reduce the retraction and oil the fan, 
you should find the printer returns to working, 
If not, concentrate on the filament forming  options above. 

Good luck, 


I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK

Posted : 06/02/2024 6:35 pm
Thomas Trummer
New Member
Topic starter answered:
RE: Filament suddenly gets stuck during printing

Hi Joan,

thanks for your answer.

The problem now appears to be resolved. I had built a case to reduce wrapping problems with ABS. If I leave the housing open I no longer have the problem with the thickened filament.



Posted : 13/02/2024 11:05 am