MMU2S Super Upgrade (MSU) - Selector

The MSU Selector has been designed from scratch and it's the result of many weeks of design iterations, printing and testing. Other than the sleek design, it has many other benefits:

• It lowers the selector failure rate to less than 5% (0% to 2% in my tests).
• A mechanical “door” that controls the filament inside the sensor, not allowing it to get into the PINDA ball channel.
• It provides a much larger space for the filament to get into the selector and to freely move inside of it.
• The PTFE tube goes much deeper inside the selector, eliminating filament jamming points.
• Precise control of the PINDA sensor height with the integrated PINDA screw.
• Two-way cutting blades to help reducing filament jams when the selector moves.

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Why?

There is a hate-and-love relationship with the great MMU2S in our Prusas. It makes it possible to create amazing things but it is unfortunately flawed and fails way too often, killing the user experience. This is even worst in a professional environment where one would like to count on their tools to perform their work.

Making the MMU2S reliable is a lot of work. There are dozens of mods out there, each one contributing a bit towards the same goal. The MSU Selector is a great first step to take as it solves many pieces of the equation.

Keep in mind that the MSU Selector will not solve all the MMU2S problems magically. You still have to think about other optimizations (filament roller, buffer, extruder) and ensure that the slicer settings are optimal to avoid issues with stringing, for example.

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Printed Parts

There are 5 parts to be printed, all sharing the following:

• Prusament PETG Jet Black is ideal, to match the printer design.
• The 0.20mm QUALITY slicer profile does a good job.
• Parts can be printed together and sequential printing works well.
• The STL files are already in the right printing orientation.
• Important: apply “Variable layer height” + “Adaptive” for every part individually.

Printing “Selector Right”

The trick part to print is “Selector Right.stl”. Due to the design of the original selector, it requires support for printing and since the support has to cover a large area with very little height it becomes very difficult to remove.

After many-many tests the following worked well to print it:

• Support material: Prusament PLA Galaxy Black (other PLA may work). For the whole support not only then interface.
• Support settings:
  • Style: Snug
  • Support on build plate only: yes
  • Top contact Z distance: 0
  • Top interface layers: 3
  • Pattern angle: 135
  • Interface pattern spacing: 0
  • XY separation: 0.4mm

Bill of Materials (BOM)

Apart from the above printed parts, the following is needed to assemble the MSU Selector:

• 1x Steel Ball 9/32in (maybe the existing 7mm one works)
• 1x PTFE Pneumatic Fitting PC4-M10
• 2x Blades 9mm
• 2x Brass Tubes 5x6x25bt (can be taken from the original selector)
• 1x Trapezoid nut (can be taken from the original selector)
• 2x Screws M3-12

And a very important part of the upgrade:

• 1x PTFE Tube 3mm ID x 4mm OD, 373mm long.

Assembly Instructions

1. Lay “Selector Left” on a table.
2. Following the comparison graphic you'll find among the project images, insert the following parts in their respective places / slots:
   • “Front” in the front slot.
   • “Door” in the middle.
   • “Pinda Screw” into its slot
   • The steel ball into its channel.
   • One of the blades into its back slot. Use a tool to push it into place.
3. Now hold the “Selector Right” part and patiently do the following:
   • Insert the other blade into the back slot. Use a tool to push it into place and to hold it while doing the next point (patience required here).
   • Insert “Selector Right” onto “Selector Left” using the back coupling slot until the selector fully closes (with all parts inside).
   • At this stage, the selector should look like ready. You should be able to turn the Pinda Screw and hear the sound of the door and steel ball moving inside when spinining it upside-down.
4. Put the trapezoid nut into place and fix it to the selector using the two M3-12 screws. This will also hold all the pieces of the selector together as a single unit.
5. Insert the two brass tubes into place. Light use of a hammer may help.
6. Screw the PC4-M10 fitting into the front of the selector.

Installation Instructions

Installing the MSU Selector is pretty straightforward. It involves the following:

1. Disconnect the PTFE tube from the front of the old selector and from the extruder.
2. Disconnect the PINDA sensor from the top of the old selector.
3. Remove the old selector (see bellow).
4. Install the MSU selector (see bellow).
5. Connect the new 3x4 PTFE tube to the MSU Selector and to the extruder.
6. Screw the PINDA sensor into the selector by inserting it into the hole at the top of the selector and turning the screw handle at the side. You should screw it to the end and calibrate it (see bellow).

To do the above steps 3 and 4, simply perform the tasks described in the original MMU2S assembly manual, specifically the Step 22 and Step 23 related to “Selector motor assembly”. To remove the old selector follow the manual in inverse order. To install the MSU Selector do it just like the manual explains.

PINDA Sensor Calibration

The screw integrated into the MSU Selector allows for precise positioning of the PINDA sensor. The following steps should help finding the perfect spot for it:

1. Turn the printer on. This will make the PINDA led (visible from the top of it) work.
2. Insert the small (1.5mm) allen key that came with your Prusa into the small hole you'll find in the side of the MSU Selector. This will push up the steel ball inside of it, simulating the presence of filament.
3. Insert the PINDA sensor into the selector and screw it down until the red led inside of it turns off.
4. Turn back three moves of the screw. This should satisfy the tolerance needed to not catch the filament too early.

Now to test it, simply load filament into any of the MMU2S slots then press and hold its middle button. The MMU2S will insert the filament into the selector and will remove it as soon as the PINDA sensor feels it. If the sensor doesn't catch the filament, screw the sensor down accordingly until it works.

PTFE Tube Calibration

Since the PTFE tube has been replaced and the length that the MMU2S must push the filament in to reach the extruder may have changed, as a final optimization is is also recommended to perform a Individual Filament Calibration.

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Have fun!