UM2 PTFE fit in stainless steel part

[drayson]

Hi guys,

currently I'm in a project to update my beloved UMO with a (chinese) UM2 head. Decided to go this cheaper way as it is just a proof of concept for a dual nozzle setup with some specialities... Dont want to machine and play around on original parts in several iterations and destroy money...

So here's the question:
How tight should the PTFE part fit in the stainless steel part?
A bit loose so it turns freely or tight so it cant....?

Any hints appreciated...

Thanks!

[nilrog]

Been a while since I put an UM2+ head on my UMO and put in a new TPE...but from what I remember you can turn it...after all you need to be able to easily replace it

[Anders Olsson]

It is a quite tight fit on genuine Ultimaker parts.
If you turn the stainless part upside down the PTFE certainly should not fall out, but you should be able to push the PTFE all the way down using your fingers without it being painful. That is the best way I can describe the required force

[drayson]

Damn, my current parts fall apart easily when put upside down...

[Anders Olsson]

A tight fit helps keeping the PTFE in shape when the extruder applies pressure, but it is probably not the end of the world if it is a slightly loose fit.

The important thing is that the PTFE is pressed towards the block so there is a tight seal between the block and the PTFE, or you will have a leak there which also tends to cause extrusion problems.
PTFE is not that rigid though, so you don't want to push it too hard towards the block, just enough to make a good seal.

The UM2 originally used a spring to apply a suitable pressure on the PTFE, but it turned out that the spring was too weak.
If there was a slight leak between PTFE and block the hydraulic force from the molten filament could simply lift the PTFE, creating a really large leak/plug of filament between PTFE and the block.

A rigid spacer was preferable compared to a stronger spring.
Combined with the TFM-PTFE this has made the UM2-series much more reliable.