PTFE coupler - Theory and practical details

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Anders Olsson
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PTFE coupler - Theory and practical details

Post by Anders Olsson »

So, as many of you probably know the PTFE (teflon) coupler is kind of the Achilles heel of the Ultimaker 2.
However there are several things about this little troublemaker that are not that well documented which are very useful to know:

Different version of the coupler:
Ultimaker has provided three different versions of the PTFE-coupler:
V1: Made from pure PTFE, factory default on early UM2, approximately the first half year or so.
V2: Made from glass filled PTFE, factory default on most UM2.
V3: Made from TFM PTFE, introduced in the autumn 2015 and factory default on UM2+
On the UM2+, the spring that pushes the PTFE towards the heater block is replaced by a solid spacer which is part of the solution to overcome the problems, more about that later.
The solid spacer that replaces the spring was possible because Ultimaker canceled the dual extrusion, which would have needed the springs for individual adjustment of the nozzle heights.
You are most likely to come across V2 and V3 of these. The glass filled one can be identified on its matte surface compared to the very white glossy surface of pure PTFE: (Pure PTFE vs. Glass filled PTFE)
2016-02-14-6753.jpg
Common failure mode:
After a few hundred hours the printer is starting to underextrude. Most common is that it underextrudes a few minutes into the print and in particular after retractions.
This is because the inside of the PTFE coupler has deformed in a way that the filament expands inside the PTFE coupler forming a plug which stops it from reacting the heater block.
The problems normally only occur when printing PLA.
2015-01-23-4900.jpg
(You can see some sort of deformation here, even if it is not so clear)

Why is the PTFE coupler needed?
When you try to push the filament into the heater block and the plastic melts, the molten plastic will try to get out the same way as you push it in.
The PTFE coupler prevents this by only allowing the molten plastic to flow in the tiny gap between the coupler and the filament. The molten plastic there gets in contact with the colder filament and becomes solid again.
This way you have kind of a plastic seal that continuously regenerated while printing.

Why is it made from PTFE?
PTFE has some very interesting properties.
First of all it can withstand high temperatures, it is not uncommon that it is has a continuous service temperature of 260 C.
Then it is the low thermal conductivity of PTFE (and other polymers) which means even when pressed against the heater block, the temperature of the PTFE quickly drops a few millimeter away from the heater block.
The most important property of PTFE though is it's exceptionally low friction and the inability of other materials to stick to PTFE. This is particularly important when dealing with hot plastic, which tends to stick to things and to cause friction.
So it is basically hard, or even impossible, to find a replacement for PTFE for this particular application.

Why does it fail?
Well, there are many theories here, the common one is that it is worn and eventually needs to be replaced.
I have a slightly different opinion, which seems to be confirmed by latest revision of the coupler/spacer.
My experience is that you can print hundreds of hours with no problem and then have a sudden deformation, which often seems to be related to the feeder skipping due blocked nozzle or similar.
We have a few things that needs to be explained here. First, it is important to understand the pressures involved in the extrusion process.
The PTFE has to form a perfect seal towards the heater block. If not, the pressure from molten plastic that leaks in between the heater bock and the spaces is enough to overcome the pressure from the spring that pushes the ptfe towards the heater block.
This might not be immediately obvious, but basic hydraulics and the fact that the area of the end of the PTFE is about four times the are of the filament means you theoretically can get up to 20 kg lifting force on the PTFE. That is way much more than the force of the spring.
ultimaker_2_nozzle_teflon_space.jpg
Now, here is my theory:
What happens when the PTFE seals, the plastic is hot and the extrusion pressure increases from for example a blocked nozzle is simply that the molten filament pushes into the PTFE so hard that it deforms and is pushed upwards.
PTFE_hydraulic_deformation.jpg
That would explain why the deformation sometimes seems instant and why the deformation gets that strange shape and position.

How to solve it
The first attempt to solve the problem with a glass filled PTFE spacer is not that great. While glass filled PTFE has higher modulus (is more stiff) it has lower strength. You can compare the data here, the compressive and tensile strength is particularly interesting for this case: http://catalog.wshampshire.com/Asset/ps ... n_ptfe.pdf
glass_filled_vs_normal_ptfe.jpg
(picture from the document in the link)

The third TFM-PTFE version though should be able to handle the hydraulic pressure much better. I think this alone is not enough to solve the problem and what Ultimaker did was to also put a solid spacer instead of the spring.
This means that if the PTFE has much less chance of deforming, since it has nowhere to go.
The reports from the testers does suggest that the TFM-PTFE more or less solves the problem once and for all.
UM2plus_spacer.png
(UM2+ solid spacer instead of the spring, yes I might have stolen that picture somewhere :) )

Alternative solutions
I have used an alternative solution, since long before the TFM.
It is a thin washer of Polyimide, "Tecasint 2011", that sits in between the heater block and the PTFE. It is sold as the I2K washer at 3dSolex: http://3dsolex.com/i2k-insulator
http://se.rs-online.com/web/p/plastic-rods/6889391
2015-03-04-5352.jpg
The theory is that the polyimide should be hot enough for the plastic not to stick to it, while it lowers the temperature of the PTFE enough for it to be strong enough to resist the hydraulic forces. The main thing for me though was to be able to print Polycarbonate at 295 C and then go back to PLA at 210 C without having to rebuild the printer.
I have also tested an PTFE lined aluminum spacer together with the I2K which seems to work. More about that later.
Last edited by Anders Olsson on February 14th, 2016, 5:38 pm, edited 6 times in total.
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Neotko
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Re: PTFE coupler - Theory and practical details

Post by Neotko »

Wow it actually it's able to cut that much heat than the ptfe don't reachs 255+?

I'm starting to think about changing my hotend to um2! Umo with the peek won't ever be able to go that high.

It's an amazing peace of info btw!
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Re: PTFE coupler - Theory and practical details

Post by Anders Olsson »

Neotko wrote:Wow it actually it's able to cut that much heat than the ptfe don't reachs 255+?

I'm starting to think about changing my hotend to um2! Umo with the peek won't ever be able to go that high.

It's an amazing peace of info btw!
Well, I was printing Polycarbonate at 295 C for about five hours a few days ago, and the PTFE spacer looked like new when I removed it :)

I think my latest version with the PTFE lined aluminum in combination with the I2K is something I will look into more though.
2016-02-13-6744.jpg
2016-02-13-6743.jpg
It just "feels" better somehow.
I had been thinking about making a spacer like this for a long time, but it was not until I got bored of making ruby nozzles the other day that I finally made one of these :-)
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Re: PTFE coupler - Theory and practical details

Post by Neotko »

Holy! That's awesome. With that you could change the ptfe with a cheaper non-cnc ptfe tube section? Ofc I2K isn't cheap but looks like a long lasting more easy to renew version. It would need very little skills to make the inner liner.
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Re: PTFE coupler - Theory and practical details

Post by Anders Olsson »

The PTFE tube is this one: https://www.elfa.se/sv/isolerslang-ptfe ... &simi=99.5
I think the price is per meter, so it is about 5 Euro per meter then, which I guess will last about a life time :-)

We used this PTFE tube at the Delta printer at work for quite some time now, since I rebuilt the E3D V5 to have PTFE lining.
It seems to work well, but we only print PLA there so we haven't reached dangerous temperatures yet.

Here is some more "nerd porn" for you, measuring the temperature of the aluminum spacer. (or at least trying to measure it)
FLIR0377.jpg
FLIR0377.jpg (151.4 KiB) Viewed 21011 times
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Re: PTFE coupler - Theory and practical details

Post by Titus »

Everyone keeps talking about UM2 PTFE couplers, but no one ever mentions UMO PTFE. I wonder why that is? Are these more robust? Or simply not an issue. Or could some peoples(including mine atm) problems be related to this as well?
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Re: PTFE coupler - Theory and practical details

Post by Neotko »

I think umo users just don't care much XD. The feeder motor ratio it's way overpowered, so even when the coupler starts to deform the gear can keep up with the extra pressure. Ofc when the coupler dies there's filament leak between the peek/hotbarrel (black goo). But also on umo at normal temps with just a 25x25 fan poited to the peek/ptfe coupler, the life spam of the coupler it's a lot. And also the peek does a bit like the Tecasint, cutting the heat that arrives to the peek. But also...

But for high temps on umo the problem it's the peek, even if you use a really good ptfe like the TFM on the new um2+, the problem remains there. Peek can go 260C but it will slowly melt (been there done that). Specially since your target temp sometimes go beyond 260. Peek at 343 becomes a mess. So in order to have 255+ on umo, you need active cooling and something different than peek. For example a full tecasint replace of the peek, but that could cost more than a full um2 hotend.

I'm starting to save to change two of my umo+ hotends to um2.
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Re: PTFE coupler - Theory and practical details

Post by ivan.akapulko »

295?!!! Jaw's dropped pn a floor.
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Re: PTFE coupler - Theory and practical details

Post by LePaul »

I'd be curious to see how you change your UMO hotend to UM2
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Re: PTFE coupler - Theory and practical details

Post by Anders Olsson »

ivan.akapulko wrote:295?!!! Jaw's dropped pn a floor.
Yes, I have been printing up to 300 C with the I2K washer and it works fine.
You probably don't want to go higher than that because the heater and the temperature sensor might not like it.

Another thing to keep in mind is that some plastics can generate toxic fumes if overheated badly.

I think it is for that reason, and the simplicity of having only one component, that Ultimaker found 260 C to be a suitable upper temperature limit for the Ultimaker 2.

The only filament I have own that actually needs more than 260 C is the polycarbonate, and that is a bit tricky to print for other reasons, so you probably would not sell it as an official Ultimaker filament even if the printer reached 300 C by default.
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Re: PTFE coupler - Theory and practical details

Post by Blizz »

Very nice article, very useful information. Thanks. Now where's the +1 button!
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Re: PTFE coupler - Theory and practical details

Post by ivan.akapulko »

Anders Olsson wrote:
ivan.akapulko wrote:295?!!! Jaw's dropped pn a floor.
Yes, I have been printing up to 300 C with the I2K washer and it works fine.
You probably don't want to go higher than that because the heater and the temperature sensor might not like it.

Another thing to keep in mind is that some plastics can generate toxic fumes if overheated badly.

I think it is for that reason, and the simplicity of having only one component, that Ultimaker found 260 C to be a suitable upper temperature limit for the Ultimaker 2.

The only filament I have own that actually needs more than 260 C is the polycarbonate, and that is a bit tricky to print for other reasons, so you probably would not sell it as an official Ultimaker filament even if the printer reached 300 C by default.
The main question is, is there a need to go above 300 degrees? The fact that there are plastics that need the temperature of this level, and what the UM2 can do that already in itself is amazing
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Re: PTFE coupler - Theory and practical details

Post by Neotko »

Maybe to print peek? But that would need another bed I suppose...
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Re: PTFE coupler - Theory and practical details

Post by Anders Olsson »

Yes, PEEK is the only "interesting" plastic I know of that would need more than 300 C.
I think you have to go closer to 400 C though, which probably would need more extensive rebuilding of the hotend.

PEEK is an very interesting material though, I would love to be able to print it!
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Re: PTFE coupler - Theory and practical details

Post by Meduza »

Yes PEEK is more like 370 degrees, it would take a all new hotend design. I would also like to print PEI, that is also in the 350+ degree range.

Both are quite expensive as filament, but in some applications it could be worth it.
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