Post
by Anders Olsson » March 11th, 2018, 3:55 pm
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.