PTFE coupler - Theory and practical details
Posted: February 14th, 2016, 4:34 pm
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)
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.
(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.
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.
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
(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.
(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
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.
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)
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.
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.
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.
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
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.
)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
I have also tested an PTFE lined aluminum spacer together with the I2K which seems to work. More about that later.