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MMU2S Setup and Inspection

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Last updated 17 days ago
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After successfully finishing the build of your Original Prusa MMU2S upgrade and going through 8. Preflight check & Calibration of the assembly manual you should be good to go. However, if you are encountering persistant issues, we have compiled a checklist of what you want to double-check.

For more details about the Multi-Material Upgrade components and their place in the assembly, you can consult our 360 viewer

Necessary calibrations

With the hundreds of filament changes that can go into a single MMU print, the system needs to know where each filament is at all times. Therefore, there are two sensors, that both have to be set up properly. You can see if they trigger correctly on the LCD menu under Support -> Sensor info. Each should trigger "1" of filament is present and "0" when it's not.

IR filament sensor calibration

This is one of the two sensors on the MMU2S, located on the extruder of the printer. Before you start printing, make sure your IR filament sensor mechanism is properly calibrated.

SuperFINDA sensor calibration

The SuperFINDA is the second sensor on the MMU2S. Its position is crucial to ensure smooth operation. Its assembly can also collect debris or strings from filament changes. To set it up and inspect it, please see FINDA setup and troubleshooting.

Hardware configurations

Idler screws tension

Just like the extruder idler screw, its MMU2S counterparts need to be tightened just right. If the idler springs are over-tightened, it may cause the idler motor to skip. Over-tightening can also cause the pulleys to be "choked" by the filament.

To find the perfect tension, tighten the screw to about 0.5 mm (0.02 in) below the edge of the hole, on both sides of the idler body, and follow these steps:

  1. All 5 filaments have to be loaded to the hobbed pulley's position.
  2. Move the selector to the far right (5th filament position) and back, by long-pressing the right and left control buttons.
  3. If you hear clicking, loosen the screws a little bit and try to move the selector again.
  4. Repeat until there is no more clicking. 
If only 2-3 filaments are loaded, you may need to adjust the side without filaments a little tighter than the other. For example, if only filament 1, 2, and 3 is loaded, tighten the screw on the right, next to filament 5. You can use a permanent marker and make a thin line on the inner edge of the hole to mark the correct screw position for future reference.

Spools and buffer positioning

The spool holders shouldn't be too close to each other. Ideally, the spools should be positioned as described and shown in the handbook (picture below).

If you decide for any reason not to use the long PTFE tubes, or not to use the filament buffer, you have to make sure the filaments will not be tangling one into another. Also, make sure the spools are stable and won't tip over. Spools which are almost empty is more prone to cause problems with tangling or falling over.

Assembly inspection

Idler Bearings

The five bearings that are protruding from the idler cylinder must be spinning freely on their shafts. If that is not the case, you can always remove and inspect the bearing by pushing out the shaft from the opposite side they are inserted. Use a rod or Allen key at least 100 mm (~4 in) long.

Securing the idler on the shaft

The idler cylinder has to be secured by two M3x10 screws which act as grub set screws on the MMU2S extruder motor shaft.

Be extremely careful during the tightening! If these screws are too loose, you will have problems with loading and unloading the filament, and if you overtighten these two screws, the idler will crack.

Keep in mind that these two M3x10 screws might also get loose over time, so check the tightening every few dozen print hours. Another potential issue for you to verify is whether the two screws are tightened against the flat part of the extruder motor shaft. See 4. Idler body assembly of the assembly manual.

Idler rotation

The idler must have some space on the left or right side and around the circumference of the idler barrel (see the light blue spacers in the picture below). The solution is to move the idler barrel a little bit away from the idler body and extend the gap. Around 1 mm (0.04 in) gap on both sides is more than enough. Test by rotating the idler by hand when the printer is off. 

Align the pulleys

The filaments are being pushed through the MMU2S unit using 5 hobbed pulleys, mounted on the long shaft of the pulley motor. The grub screws must be:

  • Oriented on the left side of the teeth, away from the Pulley motor.
  • Perfectly aligned with the filament holes and filament, as shown in the photo below.
  • Secured by the small black grub screw against the flat side of the Pulley motor shaft.

Over time, filament dust may accumulate in the teeth of the pulleys. The best practice is regularly to clean them with a mini-brush or a similar tool.

Short PTFE tubes orientation

After the filament is grabbed by the hobbed pulley, it is pushed into one of the five 19 mm (0.75 inches) PTFE tubes. These tubes are chamfered on one end, which provides some extra space for the unloaded filament, as the tip might be slightly thicker than the rest of the filament. Therefore, the chamfered end must be facing out, away from the pulleys, as explained in 5. Pulley body assembly of the Assembly manual.

Rear PTFE holder overtightened

This holds the long PTFE tubes where you feed in filament to the MMU2S. Before tightening the four M3x18 screws, make sure that:

  • The half-circular grooves are perfectly aligned with the half-circular grooves of the pulley body.
  • The screws are not tightened too much, deforming the long PTFE tubes, changing their inner diameter.
  • The long PTFE tubes have been inserted all the way in, past the holder, touching the Pulley body. The corresponding steps in the assembly manual are Step 13 and 14.

Selector blade

Make sure the little blade that is supposed to cut filament strings is secured in place with two M3x10 screws. For more info please see MMU2S Selector not moving.

Slicer settings

The shape of the filament tip is very important for a successful MMU2S print. The tip should be pointy but without any lump or string. The diameter of the tip can be slightly bigger than the rest of the filament, but not by much.

First of all, try to adjust the hotend temperature (increase or decrease it by increments of 2°C) which in most cases will solve the problem. You can do so during the print in the Tune Menu or when slicing the model.

If the problem persists, head back to PrusaSlicer and increase the number of cooling moves by 2 or 3 from Filament settings -> Advanced -> Number of cooling moves). Furthermore, you can try to increase the unloading speed by increments of 10 mm/s from Filament settings -> Advanced -> Unloading speed.

Test prints

If you are sure that all potential issues mentioned above have been avoided, feel free to try these test G-codes (sliced for your MK3S/MK3S+ + MMU2S upgrade):


G-code download

2 filaments, 12 color changes, 25 minutes


3 filaments, 16 color changes, 25 minutes


4 filaments, 16 color changes, 25 minutes


5 filaments, 16 color changes, 25 minutes


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