MMU2 Troubleshooting  

MMU2 Troubleshooting

MMU2 is here!: https://www.prusaprinters.org/multi-material-upgrade-2-0-is-here/

Manual: https://manual.prusa3d.com/c/Original_Prusa_i3_MK3_to_Multi_Material_2_upgrade

NOTE: A lot of this content is duplicated from the manual but it's here as "common mistakes". This post only covers the hardware aspect; make sure that you test with the pre-sliced models. Other people will assist you with Slic3r usage; it is not my slicer of choice.

If you are still waiting for your MMU2 kit, there are still a couple of things you can do while you are waiting. You can print your own R3 extruder parts and you can check that your nozzle is properly fitted. See below for both these items.

Note that the printed extruder body parts are included in the kit, along with textile sleeve for extruder cabling and "upgraded" PTFE tubing, but If you want to print your own extruder body, PLA works well; just print the parts with the worst possible colours and we can have a competition to find the baddest-looking Mk3+MMU2 🙂

R3 parts are described and downloadable from this post: https://shop.prusa3d.com/forum/general-discussion-announcements-and-releases-f61/new-printed-parts-extruder-version-r3-b7--t23017.html

If you haven't done so already, please upgrade your extruder to the R3 parts:

MK3: https://manual.prusa3d.com/c/Upgrading_extruder_R2_to_R3_on_MK3

MK2.5: https://manual.prusa3d.com/c/Upgrading_extruder_R2_to_R3_on_MK2.5

Before you start, please note that the MMU2 should be installed on a completely stock Mk2.5 or Mk3 printer and that the printer should be printing well. If this is not the case, then get these issues sorted first and the MMU2 will work so much better! Once you get the MMU2 working you can then modify your printer as much as you like.

First thing you need to know is that it can work very well. There's plenty of photos around showing you it works (some of mine here: http://www.kisslicertalk.com/viewtopic.php?f=15&t=2371 ). So your problem may be human error and you now need to accept that it may just be possible that you made a mistake somewhere. The only issue is where, and that's what this post will attempt to address. There is also a possibility that you have a faulty part, but if there's nothing obvious, you need to deal with build errors first.

Just a quick note about the Festos. After a (very) short period of time, they will not release the PTFE tubing. I am now in the habit of just hand-tightening them, so they are easy to release in case of any problem (usually caused by me doing something wrong).

So, let's get started, from the bottom up:

Extruder body. You have received or printed new R3 extruder body printed parts. Make sure that you use them, taking the time to install them properly. They ensure correct alignment of the filament through the extruder. Also, make sure that the tension springs seat properly in the new body; there may be some excess filament in the holes which needs to be cleaned out.

Nozzle. The "melt zone" needs to be as short as possible, so there needs to be only a very small gap between the nozzle and the heater block. This gap should be less than 0.5 mm, but there must be a very small gap so that the nozzle can seal properly against the heat break. Refer to steps 3, 4, 5 and 26 of the E3D V6 assembly manual: https://e3d-online.dozuki.com/Guide/V6+Assembly/6

PTFE tubing. You do need to use the tubing supplied in the kit, replacing the existing tube. There should be a taper at the top end and both taper and chamfer at the bottom and it must be inserted properly. I always unscrew the heat break from the heat sink by half a turn, push in the PTFE tube and lift the black ring to lock it in place. Then tighten up the heat break into the heat sink.

Bondtech gears. Now you have installed the replacement extruder body, you need to ensure that the Bondtech drive gears are aligned properly: https://manual.prusa3d.com/Guide/5.+E-axis+assembly/510?lang=en#s8192 (Step 15) You also need to ensure that the Bondtech idler meshes nicely with the drive gear and that the door holding the idler moves freely. Ensure that the idler axle is fully inserted and tight.

Bondtech gear tension. If you have any filament loaded, remove it, turn the printer off and unscrew the festo at the top of the extruder. Tighten the tension screws so that the top of the screw head is above the the extruder body by 0.5mm. Then insert some filament from the top of the extruder through the Bondtech gears. Make sure that the idler door opens a little (and smoothly) as you do this. The tension screw heads should now be 0.5mm below the extruder body.

Filament Sensor Cover. You have installed a replacement cover and short PTFE tubing. Ensure the PTFE is tapered at the upper end. Getting this properly aligned seems to have caused some users (including myself) difficulties. Best to push a straight piece of filament into the extruder so it is held in place by the Bondtech gears insert tube into cover, and then slide the tube and cover over the filament. Screw the cover into place before removing the filament.

Bowden tube. The supplied tubing is high-precision and has tapers at both ends, but if you have problems with filament getting stuck in this tube, you can replace with regular PTFE tubing (360mm long) with a taper at the top end only.

Selector cover. This is the small part to which the top end of the Bowden tube is attached. Remove the Festo and ensure that the hole in the cover is concentric with the outlet hole of the selector.

MMU drive gears. Ensure they are lined up properly. The spacing of these should be exactly 14mm.

MMU Idlers. Check again to ensure that the idler rollers are centrally positioned and that they run freely and smoothly.

MMU Filament Inlet. Make sure that the PTFE tubing really is pushed all the way into the MMU (it can be a bit tight). The filament feed tube is essential for proper operation, but you may want to try with shorter tubes and different spool holder outlets.

MMU Tension. Ensure that the idlers are not in a "filament locked" position. The tension screw heads should be about 0.5mm below the surface of the idler cover. Ensure that you can lift the idler lid at least 2mm without too much force. If you are getting filament shards in the MMU, you probably need to tighten the tension screws by half a turn and keep doing this until it stops happening. Make sure you don't tighten the screws too much so the idler lid can't lift up.

MMU. Make sure the unit is properly attached to the printer frame; it should not be able to slide sideways.

Spool Holders. Ensure that filament can pass freely through the filament guides. When unloading, there are large filament loops created at the spool holder guides; you will have to ensure that the loop of one filament does not loop over that of its neighbour. Correct positioning of the spool holders really is a matter of trial and error. I have a shelf and run 2 down 3 up, with the central upper one further back than the other two, and with the lower two spools under the outside top two.

Filament feed. With filament loaded in all 5 positions, remove the Festo from the selector outlet and feed each filament through in turn. Pull and push the filament and ensure that there is similar, but not excessive friction when the filament is moved.

MMU Calibration. Follow the manual and do this at least 3 times in a row. And check/adjust each time. Keep checking until you have 3 calibrations where no adjustment is necessary.

A few other things to consider:

If you have to cancel a print and use the firmware "Unload" feature, it may not unload the filament cleanly and can leave a very long string from the extruder to the MMU. If this happens, make sure you remove the string.

If you do get a string , then it can be pushed down into the hot end and block subsequent loads. This can be cleared by heating the extruder removing the extruder Festo, pushing in some filament and doing a couple of "cold pulls" (see notes below about motor current).

The extruder motor current was set to a rather high value (30) in earlier firmware. The new R3 printed extruder body allows you to print with lower E-motor power and therefore a cooler E-motor. Motor current is set as follows:

M911 Enn ; Set the E motor holding current to nn
M912 Enn ; Set the E motor printing current to nn
M913 ; View the motor current settings

Be aware that motor current may be altered during tool changes (I have not checked the firmware for this as yet) so if you want to change it, it may be that you will need to reset your value after every tool change. Personally, I have currents set at 20 for printing, 30 during a tool change (unloading) and 20 for holding. When working on the printer (cold pull etc) I simply set holding current to zero.

Octoprint. Yes, it works fine - just check (and amend if necessary) the G-code script "After a job is cancelled" and if you cancel a print, don't forget to unload the filament (remembering the notes above)!

Do not get frustrated. Take your time, work through your problem and maybe even have a good night's sleep. If you do get a blockage in the hot end, you should not have to dismantle the extruder. An occasional "cold pull" at 110 degrees (PLA) keeps the hot end under control.

If you have managed to get through all of the above, congratulations, you should now have a working MMU2! Just make sure that you don't have your print temperature set too high (about 205 degrees for PLA) and start printing.

If there's something not covered above please post your query below and I will try to work through a solution with you. But please understand that I have a day job and a family, so please don't duplicate queries and also accept that you won't get an immediate response.

Don't forget to post photos of your prints in the gallery!

Peter