How to fix a jamming and oozing Merlin Hotend

UPDATE: Post title update + Well I managed to transfer the Merlin hotend in it’s final state. The dead state. The tiny thread where the nozzles go is damaged. I am not able to tighten them. This is an definite kaputt-state. Nevertheless this guide below should be helpful.

For my current WIP printers I was looking for a nice hotend. The E3D is omnipresent if you search for a reliable and solid hotend. It prints nearly every plastic including flexible plastics like TPE. Albeit I didn’t want to spend 70€ for it so I found the Merlin hotend.

The special thing about this hotend is that it does not have an ordinary brass M8 sized nozzle but an airbrush sized tiny nozzle. Included in the package are three nozzles. 0.5mm, 0.3mm and 0.2mm. This promises very fine and accurate prints as the nozzle does not iron over the print surface. You get a fan and a heater catridge with it, too. Although this hotend is designed to be passive cooled. But we all know what to do with promises advertisment makes. Means, cool it anyways !

My first prints went really really nice as seen here but soon the Doom Hammer™ hit me hard. No really. This is actually a real world thing. Kind of… my godness. I deliberately left out the second ‘o’.

The hotend suffered from constant jams or cloggs. Call it what ever you like. Nasty events of FAIL.

Problem:

After heating up and cooling down the hotend jammed / clogged. This could be reproducible. After thorougly cleaning the inside it instantly clogged again.

Cause:

I contacted the reprap forums and came across more fellows in misery. It turned out that the PTFE inline in the PEEK barrel did not close up on the brass thread. Therefore molten plastic could slip past it, cool down at the brim and clog the rest of the filament. This gap can be caused by several things.

1. The PTFE inline gets to hot and melts
2. While assembling the hotend, the PTFE inline is not screwed it correctly and therefore does not line up precisely with the brass thread

Merlin hotend PTFE inliner partly unscrewed. The groove is to better screw the PTFE inliner thread

PTFE inline inside the PEEK barrel of Merlin hotend

PTFE inline inside the PEEK barrel of Merlin hotend

Solution:

The PTFE inliner needs to be screwed further down the PEEK barrel. Just 1-2 mm deeper so that when screwing the brass threat in, the brass pushes against the PTFE and shoves the PTFE back until it is nicely lined up with the end of the PEEK barrel (the end where the  aluminium cap is put on). After having all the parts BUT the airbrush nozzle attached take a fine drill (1.9-2.0mm) and VERY VERY CAREFULLY drill through the whole PTFE inliner cause it tends swage and therefore reduce the diameter for the filament. Carefully free the bore from chippings to not jam the fine hotend.
This *SHOULD* guarantee a good fit of the PTFE and the brass piece.

BUT. Big fat: BUT !!!

After finishing this up and having the first successful *layers* printed I thought I got over it but then Doom Hammer™ stroke again.

Problem 2:

After 3-5mm of layers printed, the hotend stops extruding enough filament and eventually stops extruding plastic at all and plastic oozes out of the bottom of the PEEK barrel .

Printed the first 40 layers fine (@0.1mm). Then started to ooze and loosing preasure

Printed the first 40 layers fine (@0.1mm). Then started to ooze and loosing preasure

Cause:

Either the thread in the PEEK barrel got worn out by the great many cleansings or by …yeah what ever.

Solution:

Disassembe the whole shit thing again and clean all the threads. Next take some PTFE/Teflon tape or better plumbers hemp because it is more resistant to heat than PTFE and wrap maybe 2 layers of it around the brass thread. The number of layers might differ for you. The PEEK barrel should fit very tight onto the brass thread. If you use PTFE tape you need to retighten the PEEK after some heating phases as it tends to melt and leave enough space for plastic to slip again.

Solution 2: (not yet tried out myself)

Replace the PTFE inliner with a PTFE tube. Drill yourself a PEEK barrel.
A member on the reprap forum pointed this out and offered me his service.

After fiddling around for like EVER I greatly appreciate such an offer.

…to be continued.