Hot End Replacement - Covered in Filament Blob  

New Member

After assembling the original i3 MK3S going through all the calibration steps multiple times I was able to print 3 simple objects.  On the 4th longer print (after re-calibrating) I came to find that a massive filament blob had formed all around the hot end.  I heated up, but still could not get it all off without having to cut the wires to the hot end....

First, very surprised this could have happened and why the Prusa didn't stop the print?  Is this covered under warrantee? 

Secondly, at this point I assume I need to replace the hot end.  Are instructions on how to best do this?

How do I prevent this from happening again?

Posted : 28/03/2021 6:44 pm
Illustrious Member

If a massive blob of filament was able to form, it sounds like you walked away and quit paying attention. Not a good idea until you've mastered the printer. If this was after reassembling the hotend, there are several likely culprits:

  • Bad bed adhesion after working on the printer due to bed contamination. The part might have popped loose and caught around the nozzle and trapped filament as it continued to extrude.
  • Improper Live-Z adjustment. If you didn't recalibrate your Live-Z after working on the extruder (you didn't mention this specifically), there might not have been sufficient "squish" to get the filament to adhere properly. Again, the part might have popped loose. 
  • Improper hotend assembly. If the nozzle is not snug up against the heat break inside the heater block, filament can leak out the top of the block and ooze out over the block, nozzle, and eventually the print. 

The printer can detect filament breaks and crashes with the on-board sensors. None of these are really things a sub-$1,500 printer will detect as everything is moving and extruding. It's just extruding into a blob that grew out of control. 

I wouldn't expect this to be covered by warranty as the manual does stress the need to watch prints, particularly for the entire 1st layer, to prevent exactly this sort of problem. It sounds like it ran for quite some time unobserved if it got so bad as to damage the hardware.

You will definitely need to replace:

  • Thermistor.
  • Heater cartridge.

Depending on how gummed up things are, you may need to replace:

  • Heater block (if thermistor and cartridge can't be removed).
  • Heatbreak (if block needs to be replaced and you can't get it out).

About the only hotend part that is guaranteed to be OK is the heatsink. The good news is that the parts are readily available and not overly expensive. It's a bit of annoying work, but takes about an hour. It's a good opportunity to upgrade your heatbreak, heater block, and nozzle if you're so inclined.

The best prevention steps include:

  • making sure your Live-Z is correct
  • giving the bed a good clean before long prints
  • ensure nozzle is reassembled correctly
  • observing that first layer in its entirety before leaving the printer unattended
  • monitor the print using something like OctoPrint if you must be away
  • possibly use an OctoPrint plugin like Spaghetti Detector
  • consider using a silicone sock on the heater block to reduce the chance of errant filament adhering.

It's a bummer, but it happens. Hope you get it all working again quickly. I would keep spare hotend parts on hand since this is more-or-less inevitable.

My notes and disclaimers on 3D printing and miscellaneous other tech projects
He is intelligent, but not experienced. His pattern indicates two dimensional thinking. -- Spock in Star Trek: The Wrath of Khan...
Posted : 28/03/2021 8:59 pm
Eminent Member

Just for reference - this happened to me as well - a heat gun may work. I did not have one at the time I had to replace the hot end assembly. The reason this occurred on mine is that I was using PETG on a textured bed that was not calibrated properly - poor adhesion of PETG - lifted and gathered etc - this was completely my fault for not testing the adhesion properly. Sometimes you need to raise temps on hot end and bed with different materials - there is some trial and error. I hope you get it fixed - the replacement was not too bad. For me - waiting for the parts from Prusa Research was the worst of it. BUT I see you can get parts on Amazon now - but buyer beware - I am not sure if they are original. Good luck.

Posted : 29/03/2021 2:17 am
Illustrious Member

FYI - in the US, PrintedSolid, MatterHackers, and Filastruder are are reputable resellers of the Prusa versions of the E3D parts.

My notes and disclaimers on 3D printing and miscellaneous other tech projects
He is intelligent, but not experienced. His pattern indicates two dimensional thinking. -- Spock in Star Trek: The Wrath of Khan...
Posted : 29/03/2021 2:37 am

Please Login or Register