Prusa MK4 Nextruder MMU3 Parts

April 22, 2024 UPDATE:

Finally, Prusa published printable parts for the MMU Nextruder so these parts can be considered obsolete but if you don't have the small 3x1mm magnet and want to use my parts go for it.

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I upgraded my MMU2s to MMU3 in order to work with the Prusa MK3s+ and everything was wonderful. Then I purchased the MK4 and wanted to install the MMU3 on it but it was not compatible yet. I purchased the parts I thought were needed and then waited for the firmware release. 

Since April 2024 the MK4 has worked with the MMU3 but I was missing some extruder parts that were not published because “they are not printable”. At this moment I decided to make it work anyway and started the design of the missing parts of the Nextruder. Here they are.

This upgrade is still experimental but works great for me. Things to consider before the instructions:

• Parts need to be printed in a strong material, stronger than PETG; I printed mine in ASA because they have to withstand spring flexion and high temperature so they can't bend under stress or creep. ASA shrinks a bit so if you have tolerance issues try scaling up the parts (0,1 or 0,2 % should be enough)
• Use at least 4 perimeters and avoid elephant foot or warping
• Follow the Prusa manual for the correct installation of the parts and planetary gears; then perform the gears calibration as shown by the printer

Here you can find the official manual:

https://help.prusa3d.com/guide/9a-mk4-nextruder-mod_683106

https://help.prusa3d.com/guide/10a-mk4-setup-and-calibration_426826

Differences with the original instructions and parts:

• I reused all the original Nextruder parts like screws, they work fine with my mod
• I did not modify the Idler geometry but you have to reprint it in ASA anyway; maybe I will optimize it in the future if it does not work as expected
• For the magnet I used the spare 3x3mm or you can use the one you take out from the filament sensor. It slides inside the Idler Nut sideways.
• In the Swivel part, the screw will thread in the printed parts. At the moment I think it is the best way to maintain a certain width in the part so you will not use the nuts
• The tension screws work great without any modification
• They way the Main Plate will not last 3d printed so take care of it; at the moment mine is perfectly fine. I also modelled a place for the O-ring but I did not experiment with a TPU-printed one yet

Filament sensor calibration: this is the hardest part of making it work. I think the magnet is too large for the use Prusa intended here so there will be some issues. What you have to achieve is that the filament sensor senses the filament only when inserted. Follow the calibration procedure in the printer and do this:

• First, open the swivel and loose as much as possible the spring screws

• Push the slider as far as possible at the right side

• When there is no filament in try to secure the swivel on the idler but not all the way in

• Now select “continue”: the printer will set the state with no filament while you stand still
• If performed correctly, now you can latch the swivel completely on the idler and insert the filament: the printer should sense it. If not, push the slider all the way to the left

• Complete the calibration by removing the filament and pushing the slider to the right

• Finally, tighten the spring screws as before

Let me know if there are any problems, and I will try to improve it.

Nextruder printed parts for MMU3 compatibility.