With the rising popularity of 3d printers, more and more people are finding the limitations of the average printer only being able to use one material at a time. I am quite fond of Prusa’s solution to this.

MMU2S stands for Multi Material Unit, this is their improved version 2. This device allows Prusa printers to use 5 different filaments with one nozzle and extruder assembly. This means that you only need one nozzle and your X axis only needs to carry one extruder.
With that out the way, this review will be separated into a few parts.

  1. Modifications I made to the unit and printer.
  2. Multiple extruders in Prusaslicer.
  3. Final thoughts.

Modifications

There are three primary modifications I have made to the printer. All parts are printable luckily, I recommend using PETG or better. The first two improve the filament path to reduce friction, the last one improves visual checking and maintenance. I will include links to the .stl files where applicable.
The first part is an improved back plate to hold the PTFE tubes more securely as the stock plate can potentially crush the tubes and increase friction. This new plate takes M10 passthrough fittings. This is available in thingiverse here.

The second part is more to personal preference on how far you wanted to go, the core of the improvement is to let the PTFE tube go straight from the MMU2S to the gears. This reduces friction and potential sticking points, also helps flexible filaments. I decided to just replace the entire extruder assembly with a modified pitstop extruder designed by Mihaidesigns but there are other options that you can find online if you don’t want to go this far.

The last upgrade helps to visually check what the unit is doing by allowing you to see the condition of the sensor as well as easy access to clean out the sensor if it does get fouled. You can find this part on thingiverse here.

Slicing

The MMU2S only currently works with Prusaslicer to my knowledge. You can get Prusaslicer from here.

Slicing your prints for the MMU2S is quite easy. You use the slicer like normal but now you can select 5 filaments from the right menu. The top filament correlates to the left tool on the MMU2S. each tool is assigned a colour. For a multi material print, you need multiple .stl’s as only one tool can be assigned per .stl. When you have your model, assign the tools you want, and it is ready to slice.

When a multi material model is sliced a purge block is created, this varies in size based on how many tool changes are needed. This highlights the problem with multi material single nozzle systems, this purge block weighs more than the actual model. This problem is not unique to the Prusa MMU2S, this happens to nearly every standard single extruder multi material setup. There are ways to mitigate this but we won’t get into it now. No purge block is generated when a single tool print is sliced.

Final Thoughts

There are lots of problems, there is no denying it. There is also no denying that it delivers a unique ability. Being able to print TPU and PLA at the same time to make flexible joints or print BVOH or PVA for water soluble support material. There are so many applications including the convenience of having 5 filaments ready. There is an option in the settings for spool join where as soon as one filament runs out it will start feeding from the next tool. When you have the mechanism dialled in, you get equal quality to the stock printer.
If you have a Prusa MK2.5 to MK3S printer, are willing to tinker and like interesting mechanical systems then I can’t recommend it enough.

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