When the extruder on my 3D printer goes back and forward, it may pull some filament, unrolling the spool, and then return to a position that causes the filament to drop from the spool, which later causes a tangle.

A perfect solution for this would be to pass the filament through something that only lets it go one way, unrolling the spool, but doesn't let it back through.

I have two ideas of how to achieve this, but they both seem overengineered:

  1. An extra stepper motor that is synced with the extruder
  2. A ratchet bearing with a couple of 3D printed rolls

I suppose my problem is commonly encountered, so maybe there's a simpler solution that I'm missing?

  • $\begingroup$ There are lots of solutions to this problem. The key word to look for is "filament buffer". $\endgroup$ May 22 at 16:07
  • $\begingroup$ Or a DC motor that is always trying to run the spool backwards? Strong enough to roll the spool back but not so strong as to overcome the extruder motor? How much engineering skill do you have to implement a solution? $\endgroup$
    – user77232
    May 23 at 18:37

1 Answer 1


For the Prusa line of printers, there is a device known as the MMU in versions 2 and 3 to the best of my understanding. It operates by pulling in the desired color/material to the extruder/hot-end and then retracting it to provide for a color/material change. Operators of this device experience problems similar to yours.

There are third-party creators who have designed models known as buffers. Using the search terms "mmu buffer" results in a number of options, most of which are overkill for your concerns.

Thingiverse search does present what is described as Prusa MMU Mini Buffer MK2 and shows a single filament device providing for "take-up" of the retracted filament.

Mini Buffer Image from linked site.

Printables has a similar collection, most of which are also overkill, in my opinion.

I have experienced similar problems, especially with a new, full spool. I have also seen attachments to the axle of the spool holder that places a bar across the flanges of the spool, but am currently unable to locate such a model. My print attempt failed due to modifications I've made to the axle and the model has been binned.

Additionally, I redirected the filament vertically downward on the back of the (enclosed) printer, through a loop of PTFE tubing and upward again to the filament entry point. When the retraction action took place, the filament was pushed rearward but not downward, nor upward, causing no effect on the spool. It's a hack job using cable ties to hold the PTFE tubing to the fan grilles but it works. Unfortunately no photos and it would have to be custom fit to your configuration.

Some of the MMU solutions incorporate a ramped axle holder on which the spool is pulled upward while filament is being extruded, then allowed to roll downward, returning the excess filament to the spool. Depending on your level of irritation, this may not be overkill, as it is a mechanically simple design and appropriately reliable.


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