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Filament backflow issue with AA .4mm cores?


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Posted · Filament backflow issue with AA .4mm cores?

Hello everyone,


I have been having a blast with my new UM3 in the last couple of weeks but the relationship has been love & hate so far.

My main unresolved issue so far, which has happened 3x, is the backflow of filament during a print. I have had to clean the excess goo pileup above the nozzle on the brass plate which also stops me from easily removing the core in the printhead. The two clips to release the printcores need a bit of travel and if excess filament gook gets in there - it's almost impossible to remove the core without pliers or force. The weird thing is that I get proper adhesion and the prints come out fine until I open the printhead tray to find residue on the silicone white rubber and of course a stuck core with excess goo on the brass plate. So frustrating. Am I doing anything wrong? Is it possible that the white silicone isn't doing its job and some filament flows back up during print?


Since cleaning my cores with all the excess gook, the little red ring gets cut and broken. Is it important to go forward and print again without it? Does it need a replacement asap? This has only happened on my AA .4mm cores and I also own the Hard Core with no issues so far. No issues with the BB core either.

FYI - I was printing Proto Pasta magnetic iron on multiple prints with Hard Core and no backflow issues. It remains sportless and what I would expect. Is this a bad design from UM? User error somehow? Am I missing a procedure or setting in Cura?  


I have added a picture of my core once removed with 'force' from my UM3. It feels awful every time and the cleaning is no easy task. I actually burned myself cleaning the core with a hair dryer only to blister a couple days later and forcing me to go to ER.


Hoping someone has a clue on how to avoid this issue. Everything else seems under control for me printing wise. The UM3 is awesome and I feel honored to be part of this community.


Cheers everyone,




A new UM3 owner.




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    Posted · Filament backflow issue with AA .4mm cores?

    It looks like you have a leak just above the block.  Or did that come from below the print head?  I can't tell without seeing if your print failed before this happened.  But I'm leaning towards the fact that you have a leak.


    You should contact your reseller and get a new one.  Or if you are too shy do do such a thing you could (as a last resort?) try to fix it yourself.  It's VERY delicate but this video will show you where it can break easily and how to take it apart and tighten it without breaking anything:




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    Posted · Filament backflow issue with AA .4mm cores?

    THX so much GR5 for your response!

    I did see this video before and since I just purchased the UM3 a couple weeks ago, I would like to avoid voiding my warranty if possible. The crazy thing about my printer and my two AA .4mm cores is that they have now both had this issue. I had the backflow problem 1st couple of days and MatterHackers asked UM to replace a defective core with a new one. Now a second one has failed? Over extrusion issues? In other words, does my printcore 1 over extrude filament? it would explain why my prints don't fail but the extra filament has to go somewhere? I am absolutely torn on the subject and wish I could find the answer to this problem to eradicate once and for all. Is my extruder 1 the culprit? No idea....  





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