Notes on the MK4 and MMU3

My interpretation of the MK4 and MMU3

Last Update July 10, 2024

I have built 2 MK4 kits and have upgraded an MK3 to MK3.9.  All three passed setup easily and work well.  I have upgraded my MMU2 to MMU3 to use on the MK4.  Overall I like Prusa Research reliability with good support and a large community of users.   My MK4 prints well out of the box.  Saving a few dollars on cheaper systems may cost you thousands of dollars in time.  The 3D printing experience for me is vastly improved with the MK4 series printers.

First impressions of the MK4 Firmware 6.0x

I am very happy with the MK4.   It is about 20% faster overall and up to 50% with the Input Shaper enabled.  My wife notes it is quieter than my MK3S+.   Vase mode printed parts on the MK4 are stronger vs. the MK3s+ 

The MK4 can put reliable 3d printing into more hands.   Load filament, keep sheets clean and run, run, run.  That is what I have been doing.  

There are still some minor problems with seeping of filament from the nozzle when not running and heating.  Extra retraction may be needed at the end of a print.  This is eliminated with the MMU3 setup.

USB thumb drive warning

The cheap USB sticks that come with the MK4 printer tend to have problems and are often replaced.  It is easy to get a good, quality replacement.   Make sure the new USB thumb drive is USB2, NOT USB3, 3+ etc. Through harsh experience I learned that a USB3 drive may cause severe problems with WIFI based file transfers.   Prusalink, Pusaslicer and Prusaconnect all had problems with file transfer failures when using a USB3 thumb drive.   I have used multiple USB2 drives with no WIFI based file transfer problems.   Recommendations for the MK4 USB thumb drive:

Significant Details

Mods I have done for the MK4

These mods may not be a big change, but there is no downside to these mods that I see.  Making parts a little stronger/better than the default settings also helps.  Adding the parts listed to the nextruder should not effect  input shaper settings.  The added percentage of weight to the nextruder is very small. 
  1. Important, Put a sock on it.  You must tell the printer it has a sock,  Enable the sock in Settings-> Hardware-> Nextruder-> Silicone Sock.
  2. Important Deflector for Heat Break fan inflow.  A good thing to have based on initial reports that state the heat break fan may interfere with the cooling flow on the part. ASA and ABS noted as having a problem. 
  3. Important  Improved filament cooling fan.   Cooling air all around, no dead spot as the original.  Need ASA or other High temp filament for printing.  The original was ASA. 
  4. Important Make the Y axis belt mount 1 piece.   For better strength and reliability of Y axis (my mod).   A new Tensioner is also needed with this mod.
  5. Medium  Nextruder fan deflector for Heat Break fan outflow. Used the V2  deflector. Works well at deflecting the air up away from the bed. Used PETG, 0.2mm Using Organic supports on the one wall only. To install, remove the heatbreak fan, loosen the 3 Nextruder mount bolts just a little to allow it to snap in. Then tightened the nextruder bolts and remounted the fan.
  6. As needed. Squash Ball Feet x 6Shorter ones for use with MMU3 that is horizontal.    Squash ball feet help reduce noise/vibration.
  7. Housekeeping Z axis Dust covers, the 2 piece version.   Keeps filament junk from accumulating.
  8. Housekeeping X axis Cable guide.
  9. High Heat enclosure.   Buddy Board Cooling.   See also Remixes for this, to use in a Prusa Enclosure.   If you intend to use a high temp in the enclosure. 55C or above in my estimate.  Not needed for PLA/PETG or similar temperature ranges.
  10. Convenience  PC4-M5 Festo Fitting and PTFE tube anchored at the other end added.   See explanation and picture below.  MMU3 has NEW festo fitting that have a different internal diameter.
  11. NON-MMU3 printers Nextruder mods for flex filaments. 
    1.  Improved Idler for flex filaments.  This may be needed based on what I would describe as tolerance buildup.   I suggest 0.2mm layers,  4 profiles and 30% infill.  PETG or better.  High heat tends to cause this part to bend.   MMU3 version is stronger, higher heat resistance.
    2. For flex filaments.  Nextruder main plate mod.   Helps contain the filament in the Nextruder. 0.2mm layer, 4 profiles, 25% infill. 

This is significantly fewer important mods than I had with the the MK3s+ and MMU2  to get it to my standards.

MK4 vs MK3S+

What stayed the same on the MK4 vs. the MK3s+?   The frame assembly, Y and X axis rods, Y carriage, heat bed and the power supply.  The Full 4K upgrade offered by Prusa makes it a full upgrade.  All except the new lighter frame. Not a structural issue. 

You could buy a full MK4 kit and have 2 printers when done.  It could be a better value, depending on space available and your needs.  Be warned, you may not want to use the MK3 series anymore after using the MK4. 

If you are on a budget and want nearly 100% functional upgrade, I recommend the MK3.9. Gets everything but the finer servos.  I have built one and ran it before giving it to my grand kids.   It runs the input shaper.   Smoother Servos still can be added later if desired.  For most the upgrade would be great.  You print your own printable parts so don't tear the MK3x apart early if you do an upgrade.  

Some things Prusa told you about the MK4

Some nice things that may not be mentioned

Dimension differences of the MK4 vs MK3s+  From measuring the MK3s vs MK4 parts.

Notes on the MK3.9/4.0 upgrade: The v1 Mk3x to MK4 (full) assembly instructions show the frame assembly, power supply, Y and X axis rods, Y axis carriage and and heat bed are re-used when in good condition.  These parts do not change in the MK4  The Y heatbed thermistor is changed for the new Buddy board connection.  The instructions then links you into the MK4 build at the point the buddy board is installed.   

MK4 V1 vs. V2/3 printed parts

Look  on, in the comments on the MK4 printable parts. A user compared parts and listed the ones that did not change and the ones that did change.   Those that did not change were still marked V2 instead of V1.  Most changes appear to be driven by the Prusa assembly group, to make assembling a printer easier and faster.

Some things I think should still improve, minor overall.  See Software rule #16

Changes to the MMU3 vs MMU2  Updated 5/2/2024

The MMU3 vs the MMU2 works much better.   MK4 and MMU3 work together as a single machine. 

The changes to the MMU3 vs. MMU2 improve reliability,  filament flow, sensor function and software interface.   It seems to cover the majority of pain points I had with the original MMU2.  All the printed parts changed.  The 2 mount feet for the MMU2 could be used on the MK3S+, but the MK3.9/4 will need the new mount feet.  I upgrade the MMU2 to the MMU3 using version 4 of the MMU3 parts.

The MMU option must be enabled in the MK4/MK3.9 settings.  What isn't mentioned  currently there is also a "Cutter" setting to enable.  This enables "Forming" the tip.  I turned it on at the start.  Hopefully Prusa will document that somewhere.

  Buffer changes

MMU3 Nextruder changes.

I think all these changes should be in All Nextruder assemblies. 

What makes the best buffer and MMU3 setup?

I have tried multiple styles of spool holders,  buffers, re-winders and orientations.   Most re-winders take up too much room or have problems if they are not designed for the exact retract length needed.  The "printed" clutch systems I tried were unreliable.    A good feeder to the MMU3 must follows these rules.  

  1. Minimize the length of the PTFE tubes.   Length = friction.  
  2. Rollers for the filament spools, minimize friction
  3. Minimize the joints.   Every joint can snag or add friction.
  4. Minimize the curves.   Don't reverse the curve of the filament coming off the spool! Smooth direction changes with little binding. 
  5. Easy access.

In the end a dry box with rollers, the Prusa MMU3 buffer and a high, rear entry into the enclosure to feed the MMU3 is the best combination I have had.  It meets all the rules.  I used a slot for the PTFE tubes going into the enclosure so the tubes would flex and not have a tight turn.  The only thing that could make this better would be to have a dry box big enough to put the buffer in.

My dry box with the buffer on the side.  Keeps tubes to the buffer short.  Direction thru the buffer follows the curve of the filament coming off the spool.

Tube entry into the enclosure.  High to reduce the bends and a slot to allow flex and adjustment in the tubes. My need to access the back of the MMU has been reduced with the MMU3 changes.

The MMU3 on a horizontal mount.   I like it horizontal for access in the back, but you must also consider the ptfe tube clearance on tall prints.  



The "MMU2 Underslung control Panel Mount" mod should NOT be re-used.   Prusa notes show they accounted for EMI problems. 

Printing a nameplate.  The nameplate in the is bare, no text.  You can add the lettering using the TEXT function in PrusaSlicer.   Use the settings shown below, 2 lines are typed.  | double space 1 double-space | and on through 5.  Note the XYZ offset from the object coordinates.  Depending on how you have the part rotated, the XY sign may change.   I have the side with the rev marking DOWN against the plate.   After slicing, but before exporting, select the 2.2 height and add a color change.  The MK4 will stop and ask to unload/load filament.   An extra purge before finalizing the filament loading is recommended to assure a good color change.

My print of the name plate.

Improvements I would like to see 

Firmware:  Build progress picture should show progress in Z, or have the option to show it that way.

Making it a better machine tool.   That is a very long discussion. 

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