Could cold pulls destroy the 1.85mm PTFE tube in the hotend?

Could cold pulls destroy the 1.85mm PTFE tube in the hotend?  

Active Member

Hi all, 


Would like to start a discussion around possible effects of cold pulls on the PTFE tube in the hotend and how it impacts MMU2S operation. My MK2S and MMU2S are stock, zero mods at this moment. Here is what I found after I started having jams with the MMU2S: 


The diameter of unloaded tips was ~2.15 mm and this prevented filament from unloading through the ID 2mm PTFE tube btw extruder and MMU. At this point it was obvious to me that the 1.85mm was no longer 1.85mm anymore, so I disassembled the extruder and found that it became at least 2mm inside. After I replaced it with the spare one, my tips again became thin and since then I had zero issues with MMU2S.


This situation led me to consider possible causes for this widening of internal diameter of the PTFE tube.


Could it be that cold pulls destroy the 1.85mm PTFE tube by forcing through it 2.2mm wide filament (which could have expanded in the heartbreak) during cold pulls?

Prusa Bear MK3S + MMU2S. Hotend: TriangleLab Dragon Hotend + Rubber Sock.
TheZeroBeast MMU2S firmware...
Posted : 06/09/2020 10:13 am
New Member



I'm wondering this myself.  I've got a ~2-month old MK3S that's 100% stock, and has been printing great.  I just finished assembling my new MMU2S yesterday and tried my first multi-color print.  It jammed 3 times in about 2 hours of printing while retracting the filament back through the PTFE tube between the hot-end and the MMU2S.  I measured the tips that got jammed and they're coming out in the range of around 1.95mm-2.05mm, which is a bit baffling since the PTFE tube in the extruder (which should set the tip size) should be 1.85mm.


I had to do my first (and only) cold pull about a week ago to clear a clogging issue.  It took a *lot* of force to get the tip out of the PTFE tube in the hot-end once it started moving.  I was also wondering if I permanently deformed that tube, which is setting me up for disaster with the MMU2S retractions.  I guess the only way to know is to disassemble everything again and swap out the tube.  This thing is pretty-much unusable right now as it is, so I guess I don't really have a choice.

Posted : 14/09/2020 1:44 pm

Please Login or Register