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Under extrusion on UM2

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Hey all,

We're having an issue with our UM2. It prints fine, except that it seems to be under extruding. I've tried quite a few solutions, but haven't managed to fix the problem. So, hopefully someone here has an idea on how to tackle this issue.

A list of the solutions I've tried so far:


  • Used a different feeder setup.
  • Changed the PTFE coupler.
  • Swapped out the spring for a spacer.
  • Attached a different fan shroud.
  • Checked bowden tube for narrowing.
  • Changed the nozzle.
  • Checked the steppermotor steps.


Any help would be greatly appreciated.

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If hard filament like PLA is wound too strong on the spool, especially near the end, then this creates a huge amount of friction in the bowden tube, coupler and nozzle. Also, this narrow bending radius acts as a strong spring, that resists unwinding. The UM2 feeder motor has enough power, but seems to have difficulty translating this power into filament movement. I guess that the pits that the knurled wheel bites into the filament get stretched out (=sort of partial slipping). If they get stretched by 20%, then that would amount to 20% underextrusion.

For PLA, try this: with the PLA spool installed on the printer, unwind a bit of filament. Be careful that the spool does not keep unwinding all by itself, thus have someone or something braking it !!!

Now manually straighten this piece of filament by winding it in the opposite direction around a skater board wheel or any other cylinder of about 7cm diameter. Then wind the next bit of filament in the opposite direction. And so on, until you have straightened about 2m of filament.

Then gently roll it up again on the spool. Now it will be sitting very, very loose on the spool, with a radius of about 30 to 50cm. Now it will have almost no friction anymore in the bowden tube and nozzle, and no unwinding resistance.

Try if this works.

There could be a lot of other causes for underextrusion: dirty nozzle (partially blocked, narrowed), worn out teflon coupler, bowden tube clips clamping too much and narrowing the tube, too low printing temp, etc... But since you already changed these or verified them, I would guess too much filament friction and unwinding resistance due to a too narrow bending radius is most probable. Especially for hard materials like PLA, that do not bend easily.

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Thanks for the answers! Sorry for the late reply, been quite busy.

That could very well have been the issue. I'm currently not near the printers, or I would give it a go. What I can say is that we upgraded one of our two UM2 to a UM2+, and this seems to have fixed the issue completely. It prints perfectly again, leading me to think that the geared feeder fixed the issue.

When I'm in tomorrow I'll give your idea a try Geert.

And thanks again for the help!

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The alternative is to print off the roll as I have been doing for the past 2 years, a bit wastful if you dont plan things right, but this solved it for me. However as Geert states, as you get to the end of the roll you will need to unwind it a bit, especially on the old UM2 feeder!

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