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New Prusa MK3S, extruder jammed  

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Illustrious Member
RE: New Prusa MK3S, extruder jammed

No arguments Bob - more airflow - cooler air - can help reduce temperatures of things it is blowing across.  But I was referring to increasing flow in the heat sink helping the temps in the shaft area. The spaces are isolated by about an inch of PTFE tubing, a lousy conductor of heat.

I've never experienced plastic softening above the heat break transition zone.  Not to say it's impossible, but unless in an enclosure that's being heated or allowed to warm to print ABS or other plastics where ambient hits 50c+ ... I am dubious it happens.  I am also skeptical of melted PLA due to heat creeping out from the motor to the gears unless also compromised by user applied conditions like an enclosure.

Voj - as for hacking the printer to accommodate some trivial mod like a fan change that isn't tested and verified to actually do anything useful?  Come on.  There are 5v fans that move more air; after all, the stock fan is 0.05a / 0.25w.  A Sunon ME40100V1-000U-F99 moves about 10x the air for $8 and looks like a direct replacement. I haven't installed one because I don't need more air across my hot end, but it'd far easier upgrade than adding inverters and new wiring. I'm going to start calling you Rube.

I read an article or blog, I think it was on the E3D or MicroSwiss, could have been elsewhere, where they talked about the heat break transition zone, and how plastic can't really melt above the neck because of the design: 40w simply can't push that many calories up the thin stainless neck material, especially with a heat sink attached, as long as there is minimal air flowing over the heat sink.  And both companies now make titanium heat breaks for people needing a bit better performance over stainless.  With one of the known jamming mechanisms being cold melt extruding above the diameter change in the Prusa modifed heat break, and the geometry of that shelf being at the lower edge of the cold-hot transition zone, provides evidence the heat break is working as expected. Matters change a bit with ABS or higher print temps, but only marginally. 40w is 40 watts.

Voj  - you're a guy who likes math/engineering challenges: what is the thermal conduction and temperature gradient cross-section of the E3D-V6 1.75mm heat break with the heat sink attached with the stock Noctua fan blowing across it when the heater block is held at 250c ??   Physics should give a good answer .... plenty of freeware cad solutions out there to assist.  From there it'd be easy to say what effect doubling or tripling or even decuple air flow will have.

Back to the OP: Nick, I know we have more or less destroyed the thread with hypotheticals, a photo will help point to the real source of the problem. And why are you thinking the is a blob somewhere in the hot end?

Posted : 08/06/2019 11:05 am
Nick-i3mk3s
(@nick-i3mk3s)
Eminent Member
Topic starter answered:
RE: New Prusa MK3S, extruder jammed

Thanks for the info everyone, as a newbie in 3D printer troubleshooting and maintenance, this is really valuable!

I've indeed observed filament grinding, to the point the filament is eroded to a thickness of 0.5 mm and there is PLA dust everywhere. I believe it's a secondary issue, caused by the extruder motor trying to push the jammed filament, therefore eroding it until it's too thin to maintain grip.

I've never noticed excessive heating of the extruder motor, but will keep an eye on it as soon as the PTFE tube replacement arrives and I can resume printing.

Posted : 08/06/2019 11:11 am
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(@)
Illustrious Member
RE: New Prusa MK3S, extruder jammed

Reread - you damaged the PTFE ... makes sense now.  

When you get things back together, and if it happens again. Snap a couple good photos and post them.

Also - when you install the new PTFE, be sure to install a PTFE Collect Clip to lock it in place.

 

This post was modified 2 years ago by --
Posted : 08/06/2019 11:20 am
Nick-i3mk3s
(@nick-i3mk3s)
Eminent Member
Topic starter answered:
RE: New Prusa MK3S, extruder jammed

Tim, here is a picture of the lower piece of filament that I recovered when disassembling the hot end to remove the jam:

As shown, the diameter of the blob is ca. double of the regular filament diameter, which is what makes me believe got it stuck on the hot side of the PTFE tube since the blob is too wide to allow retraction of the filament through the PTFE tube. When trying to unload the filament, it simply snapped off just above the PTFE tube, leaving ~30 mm stuck inside the hot end.

Posted : 08/06/2019 11:21 am
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(@)
Illustrious Member
RE: New Prusa MK3S, extruder jammed

The heat break only has one area where it's that large a diameter, and that's the area where the PTFE tube fits.  Looks like the PTFE was not being locked in place, more reason to ensure you use a collect clip. 

Which end of that photo is the down side?

What the heat break looks like inside:

 

This post was modified 2 years ago by --
Posted : 08/06/2019 11:27 am
Vojtěch
(@vojtech-3)
Honorable Member
RE: New Prusa MK3S, extruder jammed
Posted by: Tim

Voj - as for hacking the printer to accommodate some trivial mod like a fan change that isn't tested and verified to actually do anything useful?  Come on.  There are 5v fans that move more air; after all, the stock fan is 0.05a / 0.25w.  A Sunon ME40100V1-000U-F99 moves about 10x the air for $8 and looks like a direct replacement. I haven't installed one because I don't need more air across my hot end, but it'd far easier upgrade than adding inverters and new wiring. I'm going to start calling you Rube.

You may have noticed that I also said that for me, the Noctua has been enough so far as an extruder fan. I live in a moderate climate and ambient temperatures rarely get hot. I only have a 24V fan for part cooling, because I couldn't find any 5V blower that'd deliver more airflow and would still have a tacho output. The additional air volume does make a difference for part cooling when printing PLA.

Voj  - you're a guy who likes math/engineering challenges: what is the thermal conduction and temperature gradient cross-section of the E3D-V6 1.75mm heat break with the heat sink attached with the stock Noctua fan blowing across it when the heater block is held at 250c ??   Physics should give a good answer .... plenty of freeware cad solutions out there to assist.  From there it'd be easy to say what effect doubling or tripling or even decuple air flow will have.

I might. But I think that in this case, I'll simply slap a thermocouple on the heatbreak and take a reading.

Posted : 08/06/2019 11:29 am
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Nick-i3mk3s
(@nick-i3mk3s)
Eminent Member
Topic starter answered:
RE: New Prusa MK3S, extruder jammed

Yes the PTFE tube got deformed because the extruder stepper tries to pull the hot end side blob through the tube, eventually jamming it somewhere half way before snapping off the filament, and I have to pull it out with pliers manually. See picture of the PTFE tube below.

It's now deformed to the point I can't insert it into the hot end assembly any more, so I've ordered spare PTFE tubing from Ebay to replace it.

Posted : 08/06/2019 11:31 am
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(@)
Illustrious Member
RE: New Prusa MK3S, extruder jammed

Voj - I was just going to measure the air temp coming off the lower end of the heat sink... to difficult to get a good thermal contact to use a TC with any accuracy: I ain't gonna thermal epoxy one on my printer... lol..

Posted : 08/06/2019 11:32 am
Nick-i3mk3s
(@nick-i3mk3s)
Eminent Member
Topic starter answered:
RE: New Prusa MK3S, extruder jammed

Tim, in the filament picture, the side with the blob (lower side) is the side towards the extruder nozzle.

Posted : 08/06/2019 11:34 am
Vojtěch
(@vojtech-3)
Honorable Member
RE: New Prusa MK3S, extruder jammed
Posted by: Nick-i3mk3s

Tim, here is a picture of the lower piece of filament that I recovered when disassembling the hot end to remove the jam:

Oh god, what is this monster? 🙂 This shouldn't be possible with a correctly assembled and tightened hot-end. The nozzle and the heatbreak must be touching and tightened against each other inside the heater block, so that there is a smooth transition for the filament. It seems that in your case there is quite a bit of space between the nozzle and the heatbreak which gets filled with plastic and results in a clog.

Posted : 08/06/2019 11:36 am
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(@)
Illustrious Member
RE: New Prusa MK3S, extruder jammed

Nick - this is a new one.  Never seen anything like it before.   Whilst this was happening, can you confirm your hot end fan was running?  They are so quiet I know I wouldn't notice it not spinning.

 

Posted : 08/06/2019 11:39 am
Nick-i3mk3s
(@nick-i3mk3s)
Eminent Member
Topic starter answered:
RE: New Prusa MK3S, extruder jammed

I don't think the PTFE itself ever melted, the melting point of PTFE is 600 K and I've never printed above 215°C for PLA. The deformation of the PTFE tube is purely mechanical I believe, caused by the stepper extruder trying to pull the thickened PLA filament through the PTFE tube when trying to unload the filament. Sometimes unloading works if the blobs aren't too large (inner diameter of the PTFE tube: 2 mm), but if it's too big then it jams and the filament snaps off where it has been grinded down by the stepper wheels.

Posted : 08/06/2019 11:39 am
Nick-i3mk3s
(@nick-i3mk3s)
Eminent Member
Topic starter answered:
RE: New Prusa MK3S, extruder jammed

Yes I do have the black clip installed to keep the PTFE tube in place, and the tube always gets pushed all the way in, otherwise it wouldn't line up with the extruder wheels, if my logic is correct?

Posted : 08/06/2019 11:42 am
Vojtěch
(@vojtech-3)
Honorable Member
RE: New Prusa MK3S, extruder jammed
Posted by: Tim

Voj - I was just going to measure the air temp coming off the lower end of the heat sink... to difficult to get a good thermal contact to use a TC with any accuracy: I ain't gonna thermal epoxy one on my printer... lol..

My plan is a blob of thermally conductive paste. 🙂 But it'll have to wait till the evening (Europe) again.

Posted : 08/06/2019 11:42 am
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(@)
Illustrious Member
RE: New Prusa MK3S, extruder jammed

Voj - that blob would have broken off if it was created in the heater block. Plus there'd be all sorts of additional symptoms: like plastic oozing out everywhere.   This had to have formed in the PTFE channel in the heat break.  And for the top of the heat sink to get that hot, the hot end fan must have been off or blocked and not spinning. 

Posted : 08/06/2019 11:44 am
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(@)
Illustrious Member
RE: New Prusa MK3S, extruder jammed
Posted by: Nick-i3mk3s

Yes I do have the black clip installed to keep the PTFE tube in place, and the tube always gets pushed all the way in, otherwise it wouldn't line up with the extruder wheels, if my logic is correct?

There are now two lengths of PTFE, one for the MK3, and a different length for the MK3S ...

Posted : 08/06/2019 11:45 am
Nick-i3mk3s
(@nick-i3mk3s)
Eminent Member
Topic starter answered:
RE: New Prusa MK3S, extruder jammed

As far as I could tell, both fans were running normally, so the heat sink should have been cooled enough to prevent the PTFE tube from getting soft.

Voj, thanks for the link! I read through it and noticed that in the assembly guide, thermal paste is applied on the heat break before screwing on the heat sink. However, I don't think thermal paste was applied to my assembly when I received it in the kit, and I never applied thermal paste when reassembling the hot end after unjamming the filament every time. Are there cons to applying thermal paste on the threaded part of the heat break?

Posted : 08/06/2019 11:49 am
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(@)
Illustrious Member
RE: New Prusa MK3S, extruder jammed

The kit Prusa ships has paste applied and comes assembled and properly torqued.  If you didn't disassemble it - I'm comfortable with the assumption paste isn't the root issue.

After unjamming, and taking it all apart, there is still thermal grease in the threads unless you actively removed it. Enough that it's probably not a major issue. 

As for cons, no. Better contact is only good.

Posted : 08/06/2019 11:53 am
Nick-i3mk3s
(@nick-i3mk3s)
Eminent Member
Topic starter answered:
RE: New Prusa MK3S, extruder jammed

I wasn't aware of the existence of PTFE tubes with different lengths for the MK3 and MK3S, so it's possible that the one I received in my kit was the wrong length.

I'll check how far the PTFE tube is inserted in the heat break before screwing the heat sink back on.

Posted : 08/06/2019 11:53 am
Vojtěch
(@vojtech-3)
Honorable Member
RE: New Prusa MK3S, extruder jammed
Posted by: Tim

Voj - that blob would have broken off if it was created in the heater block. Plus there'd be all sorts of additional symptoms: like plastic oozing out everywhere. 

Well yes, unless the whole hot-end was diassembled first.

 This had to have formed in the PTFE channel in the heat break.  And for the top of the heat sink to get that hot, the hot end fan must have been off or blocked and not spinning. 

What I'm wondering about is the perfectly conical imprint on the blob. What confuses me is Nick's:

Posted by: Nick

Tim, in the filament picture, the side with the blob (lower side) is the side towards the extruder nozzle

I could imagine that conical part being the imprint of the cone where the PFTE tube meets the heatbreak, but then Nick's description needs to be reversed. The long narrow part would need to be towards the nozzle. But it'd all make sense, the remaining question being how it is possible for the filament to melt in that place, which shouldn't be hot.

Posted : 08/06/2019 11:56 am
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