MK3S stops extruding mid print and extruder is "knocking"  

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rene2
(@rene2)
Active Member

Hi there,

I'm a complete starter with 3 printing and assembled my new MK3S myself. After a few good prints (Benchy,3D all In One test with Overhang) the printer is starting to make trouble and is stopping to extrude the filament in the mid of the print. The start is always ok. I'm printing with the shipped silver PLA. Haven't tried anything else, yet.

I tried to test some ideas from this thread: MK3S stops extruding mid print but nothing worked and I'm kind of tapping in the dark on how to analyze the problem correctly. 

When I had his problem the first time, the filament was broken directly before the PTFE tube. As I could neither get the filament out of the wheels nor the further into the PTFE-tube I had to disassembly the extruder and I switched the PTFE tube as I have read that that is sometimes the problem.

I have attached you a few photos from my last prints. Hope this points you towards the problem:

Could unload the filament of my last print, looked like this (click on the image to view the HighRes pictures):

The printer had trouble at this point:

Another print stopped extruding at the same point:

This last part is mainly just making lines and create a flat level that grows into the z-axis. However I also got 2 prints that were successful.

 

Here the nozzle stopped extruding for a while and then extruded again as you can see that there are missing a few levels of filament on the z-axis:

 

Except for the first time this happened, I was always able to pull out the filament. However, sometimes after a failed print during the pre-heat (because I wanted to unload and re-load the filament) the extruder was already working again without any help from me which confuses me as I stopped the print on purpose as there was nothin extruded anymore and then after a few minutes it was working again. Maybe the cool down made it somehow working again? The extruder-motor and also the extruder itself is very hot after a few 10 minutes. 

 

I tried:

- Increse/decrease nozzle-temperature (210 -> 215; 210 -> 205)

- adjusting the Screw with the spring (tightening it and loosening it)

- decrease the speed of the print by 5%

 

So far I could not see that something of this worked. Maybe you can give me more insights what the actual problem is so that I can tailor the modifications better instead of making just trial and error 😉 

 

Best,

René 

 

Posted : 23/07/2019 9:31 pm
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 -2
(@-2)
Illustrious Member

Hot to the touch, 55c, is a nominal operating temp for the extruder motor. 

Measure the stub at the end of the filament unloaded after a jam.  If any part of the unloaded stub is 2.20 mm, you are probably a victim of the Prusa heat break.

 

It is always wise to get more than one opinion... as for trusting Prusa? No way man....
Posted : 24/07/2019 12:49 am
rene2
(@rene2)
Active Member

I measured it right now. It looks similar to your picture although it is not that extreme. It is around 2mm:

As I understand correctly, the heat break is a component of the extruder. How am I a victim of that? How can I make it better? What I'm confused about is that it prints normal, then there are a few z levels missing and then it starts extruding again without any support from my side. How is this possible? Had another such print yesterday ... .

 

However, I switched to 15mm instead of 10mm and got a complete print today. Might be by chance so I'm trying a second time with 15mm. 

 

 Best,

René 

Posted : 24/07/2019 6:40 pm
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 -2
(@-2)
Illustrious Member

In the Prusa heat break, the inner diameter begins at 2.2 mm, then steps down to 2.0 mm.  Melt will retract and flow up into the 2.2 mm zone and cool, creating a stub that no longer fits in the 2.0 mm section. If the stub is only 2.0 mm, then the jam may not be related to the heat break step. 

If you do pull a stub that is 2.2 mm, the heat break on non-MMU printers can be replaced with a standard E3D-V6 heat break that is 2.0 mm throughout, no step to cause jams.

 

It is always wise to get more than one opinion... as for trusting Prusa? No way man....
Posted : 24/07/2019 9:49 pm
lindharin
(@lindharin)
Eminent Member

If you want more info about the heat break issue, you can take a look at this thread started by antimix.  For a discussion about trouble that (semi-)consistently happens at the same Z level, you may find something of use in this thread started by rgshah01.  I don't know that either of those will help with your situation, but it may give you some info or things to test that will help narrow down your problem.  

Posted : 25/07/2019 2:48 am
rene2
(@rene2)
Active Member

Thanks a lot for the additional infos! That what I was looking for as I want to understand what is happening. 

I got now several successful 0.15mm PLA and several 0.15mm PETG prints so the issue occurs at least with PLA only at 0.10mm. Will read your links, maybe that helps me to understand what's going wrong at 0.10mm 🙂 

Posted : 25/07/2019 9:30 pm
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 -2
(@-2)
Illustrious Member

Theory says smaller layers provide less overall flow to move the melt out before it cools and jams.  The melt that is pumped higher has a better chance to cool and then jam in the 2.2 zone. When printing thicker layers, the overall flow is higher, and less melt accumulates in the 2.2 mm zone.

Similar to why simple parts don't jam, but complex parts with a high retraction count are more prone. 

It is always wise to get more than one opinion... as for trusting Prusa? No way man....
Posted : 25/07/2019 9:59 pm
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