Problem with filament getting left in print head on extraction
 
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Bill Lugg
(@bill-lugg)
Eminent Member
Problem with filament getting left in print head on extraction

I just built and installed my new MMU2S last week and all the initial tests seemed to go fine including the first layer calibration using PLA.  I wanted to print a spacer for my Repwinders in PETG and found the MMU2S wouldn't feed it into the print head.  It also couldn't extract it.  I ended up having to disassemble the head to get the filament out and just to be safe cleared the nozzle too (though I didn't find a problem there).

 

I put it all back together and went through all the calibration steps and did several first layer calibrations with PLA again with no problem and resliced the spacer to use PLA instead of PETG as I figured it didn't really matter for this part.  I attempted a couple of prints and was having adhesion issues so I washed the build plate with soap and water as suggested in some comments I read on another Prusa article.  Then ran another first layer calibration to dial it in a little better.  Finally, I went to print the PLA version of the spacer again and it appears I've got filament stuck in the hot end again.  It didn't get stuck coming out after the calibration, but the MMU2S can't feed it in to the print head and it's behaving the same as before. 

 

I'm frankly getting tired of disassembling the print head to get these fragments out.  Is there something I'm doing wrong that's causing this problem?  How can I avoid it in the future?  I can see that I've I'm printing a multi color part this could be a serious problem.

 

Thanks for the help.

Bill Lugg

Bill Lugg...
Posted : 20/06/2021 11:19 pm
Baklin
(@baklin)
Reputable Member
RE: Problem with filament getting left in print head on extraction
Posted by: @bill-lugg

 and found the MMU2S wouldn't feed it into the print head.  It also couldn't extract it. 

With my own MMU I can figure out most problems as I can see what is happening. Going by the descriptions of others I find difficult.

But the MMU doesn't feed the filament in to the print head. It just feeds it to the bondtech gears in the extruder and that triggers the filament sensor in the extruder. That is when the MMU stops feeding and the extruder takes over.

If the filament sensor isn't triggered the MMU will keep on  feeding the filament and the extruder will try to pull it too. That can grind away the filament and make it impossible to extract. The extruder should extract the filament and when the filament is passed (above) the bondtech gears the MMU should take over again and pull the filament all the way back.

 

But again this is something that will get messed up when the filament sensor isn't trigger at the right time.

For example when the bondtech gears grind through the filament, the filament gets thinner at that spot and can have the filament sensor show there is no filament loaded. And that is when the MMU could start pulling and grinding on the filament.

 

Here is a video that has really helped me understand what the MMU does during all the steps of loading, printing and unloading.

 

Posted : 21/06/2021 5:28 pm
gglockner liked
Bill Lugg
(@bill-lugg)
Eminent Member
Topic starter answered:
RE: Problem with filament getting left in print head on extraction

Thanks for the link to the video, it helped a lot.  I also found a link to another video by the same individual covering how to fix a blockage when the filament breaks just below the Bondtech gears, which looked like it was going to be my silver bullet.  I heated the hot end and tried pushing the blockage out with a 0.061" brass rod and pliers (as my hands weren't cutting it) and it wouldn't budge.

 

I've attached a photo of the bit of PLA stuck in the PTFE tube in the hot end.  Once I toot the tube out of the hot end I was able to pop the fragment out with the rod and my hand...makes me wonder if the rod was stuck on the edge of the tube, though I tried inserting it several times.

 

My concern is that I've fiddled with the MK3S+ and the MMU2S for about 1-2 hours (print time) and this has happened twice.  Clearing the problem is rather labor intensive.  Prior to this the printer has worked flawlessly for over a year.  I wonder if there is a larger underlying problem I'm not seeing that's causing this.

 

Any thoughts?

 

Thanks

Bill Lugg

Bill Lugg...
Posted : 22/06/2021 10:32 pm
Bill Lugg
(@bill-lugg)
Eminent Member
Topic starter answered:
RE: Problem with filament getting left in print head on extraction

Here's another photo of the plug...

Bill Lugg...
Posted : 22/06/2021 10:32 pm
ssill2
(@ssill2)
Noble Member
RE: Problem with filament getting left in print head on extraction

Do make sure you've fully calibrated both the FINDA and the IR sensor.  This can't be stressed enough.  if the IR sensor isn't calibrated properly it won't tell the MMU to stop sending filament down.

Make sure your extruder's idler bolt isn't too tight.  according to support the tip of the bolt should be flush with the edge of the nut under the hex window or at must 1mm protruding.  This was causing quite a few issues for me and loosening the bolt solved a lot of my issues.

The MMU is pretty fickle beast but recently I've been able to tame it for the most part.

Posted : 25/06/2021 4:37 pm
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