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MMU2S Setup and Inspection

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Last updated 3 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 manual, there are still a few things you might want to check before putting your newly upgraded printer to use. This article focuses on the most common mistakes of the assembly and mechanical calibration of the Multi-Material Upgrade unit, mounted on the frame.

Any usage and troubleshooting of the MMU2S upgrade require the base printer (MK3S or MK2.5S) to be in perfect shape and working order and print without any problem when the filament is fed directly into the extruder.


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


Idler body assembly

Idler bearings rotating

The five bearings that are protruding from the idler cylinder must be centered and spinning freely on their shafts. If that is not the case, you can always remove and inspect the bearing after pushing the shaft out using a 1,5 mm thick metal rod/wire or an Allen key that is at least 100 mm long.

If one of the bearings does not rotate or even appears to be damaged, this guide will help you replace it.

Idler rotating

The idler may sometimes be incorrectly centered, which means that it is hitting the idler body either on the left or on the right side - the gap between these two parts isn’t big enough (see the light blue spacers in the picture below). You can check that by rotating the idler by hand when the printer is off. The solution is to move the idler barrel a little bit away from the idler body and thus extend the gap. Around 1 mm of space on both sides is more than enough.


Securing the idler on the shaft

The idler cylinder has to be properly tightened by two M3x10 screws (that act as grub screws/set screws) on the MMU extruder motor shaft.

Be extremely careful during the tightening! If these screws are too loose, you will have problems with loading and unloading of 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 in the assembly manual.

Pulley body assembly

Hobbed pulleys alignment

The filaments are being pushed through the MMU unit using 5 hobbed pulleys mounted on a long pulley motor axis. The grub screws of these pulleys must be facing to the left, away from the Pulley motor. The pulleys must be perfectly aligned with the filament holes, as shown in the photo below, and well tightened by the small black grub screw against the flat part of the Pulley motor axis. If any of these 3 conditions are not met, the filament will be loaded or unloaded slower than expected.

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

19 mm PTFE tubes positioning

After the filament is grabbed by the hobbed pulley, it is pushed into one of the five 19 mm (0.75 inch) long 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.

That is why 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

Rear PTFE holder plate is the part shown in the photo below, you'll find it from behind where it holds the long PTFE tubes coming from the MMU. Before tightening the four M3x18 screws, make sure the half-circular grooves are perfectly aligned with the half-circular grooves of the pulley body. Also, make sure the screws are not tightened too much. The idea here is to avoid any deformation of the long PTFE tubes, which would cause unwanted friction of the filament.

If necessary, clean the little grooves with pliers or a small file.

Also please note that all of the long PTFE tubes have to be inserted all the way in. There is a circular hole for each PTFE tube in the pulley body, you should be able to push at least 0.5 to 1 mm (0.02 - 0.04 inch) of the tube inside the hole and thus the PTFE tube should hold in the MMU unit even before the rear PTFE holder is installed.

The corresponding steps in the assembly manual are these: Step 13 and Step 14.

Selector blade assembly

Make sure the little blade that is supposed to cut filament strings is secured in place with two M3x10 screws that are tightened enough. The blade also has to be placed exactly as shown on the photo below. Otherwise, the blade can slip out of its socket and block the movement of the selector. See this step in the manual.

If the blade keeps slipping out, a small drop of glue can help as well.

Spools and buffer positioning

As mentioned above, in case the PTFE tubes are squished or bent too much, it might cause excessive resistance to the filament moving inside. The spool holders shouldn't be too close to each other. Ideally, the spools should be positioned as described and shown in the handbook:

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.


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.

F.I.N.D.A. sensor calibration

See if the FINDA sensor is correctly calibrated as described in FINDA setup and troubleshooting. Always keep in mind the selector must be free of any leftover filament and dust.

During the loading and unloading procedures, the F.I.N.D.A. sensor will display "N/A" within LCD menu -> Support -> Sensor Info.

IR sensor calibration

Check that the sensor is working properly. Open LCD menu -> Support -> Sensor info and you will see the IR value. If the filament is not present, the value should be “0”. If the filament is loaded in between the Bondtech gears, you should see “1”. If this is not the case, perform the IR filament sensor calibration (MMU2S).

During the loading and unloading procedures, the IR Filament sensor will display "N/A" within LCD menu -> Support -> Sensor Info.


If you are sure that all potential issues mentioned above have been avoided, feel free to try these testing g-codes (sliced for your MK3S + MMU2S upgrade and PLA filament):

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


The MMU2S upgrade is only available as a kit because, in order to operate it successfully, you need the experience from the building procedure.


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