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.
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.
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.
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.
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.
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.
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 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.
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.
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.
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:
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:
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.
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).
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
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