Notes on the MK4 and MMU3
My interpretation of the MK4 and MMU3
Last Update Sept 8, 2023
I built an MK4 and plan to upgrade 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 vs MK3 series printers.
First impressions of the MK4 Firmware 4.7.0-4.7.2 and 5.0 RC1
I am very happy with the MK4. I carefully built
it from a kit. Everything checked out and initial setup worked
well. WiFi, PrusaLink, and ease of use have greatly improved with
4.7.x firmware. It is about 20% faster overall and more speed coming
with Firmware upgrades in the future. My wife notes it is quieter
than my MK3S+. Many parts on the MK4 are stronger vs. the
The MK4 is a big improvement that 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. I have found extra purging at
filament change may help. To get all air bubbles out of the melt
area. Hopefully software updates can minimize this in the
- The Nextruder has 2 points of contact on the filament and is
stronger. It has a quick release for filament removal.
The extrude gear on the MK3s+ often did not grab the filament enough to
pull it from a remote storage, through a buffer and keep it flowing
without skipping. Happened when the filament rolls were wound
tight and "stuck" at the edges. The Nextruder will pick up a
roll when this happens and keep going until it comes free.
Happened about 5 times on one print and there were no discontinuities on
- 32 bit hardware. Hopefully it is enough to handle all the
processing that will be needed for a "fast" machine.
- Easier to use menus.
- The Z height Auto. No more qualifying and recording each
sheet. Just put the sheet on and the machine adjusts.
It only measures the print area also. First layers are
perfect. Great coverage and no gaps. 1 layer silhouettes are
- Love board shortens all the connections on the Nextruder. No
more re-threading and re-wrapping wires when a thermistor has to be
replaced. It is much faster to change out the tip, almost as fast
as the demos make it seem.
- Cold swap of the MK4 style nozzle. No more heating and
re-tightening when swapping a nozzle. If using the V6 nozzle
adapter, the hot torquing is still needed.
- PrusaLink WiFi. 2.4G max WiFi. Great for monitoring the
machine, transferring files and starting the print. Still
listed as Beta software.
- Faster printing by about 20%, Much faster with Input Shaper
software. It appears Prusa-Research is beginning to understand the
compensation needs for the hardware and filament done with
software. Things the NC machining industry has already learned and
implemented in modern high speed machines.
- Better integration promised with the MMU3. 100% reliable
filament breaks are needed. For me this is the major failure of
the MK3S+ with MMU2. Some filament colors could cause the filament
break to fail, so often it was unusable for color changing in a
Mods I have done for the MK4
- Put a sock on it. Partsbuild.com
You must tell the printer it has a sock, Enable the sock in
Settings; Hardware, Nextruder Silicone Sock.
- Deflector for the Heat sink Fan
A good thing to have based on initial reports that state the hot-end fan
may interfere with the cooling flow on the part.
Ball Feet x 6. I have a remote power supply in the
Prusa Enclosure. I wanted the unit a little higher to help keep it
off the power cables for the remote power supply. Squash ball feet
may also help reduce noise/vibration.
- PC4-M6 and PTFE tube anchored at the other end added. See
explanation and picture below.
This is significantly fewer mods than I did with the the MK3s+ to get it
to my standards. All minor, a big improvement.
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
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. 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
- 10mm Rods for Z axis. Since X rides on Z, Z needs to be
- New frame is same thickness as MK3, but honeycomb for strength and
- 10mm more Z travel.
- Nextruder for feeding filament. Double contact, gear reduction
- New hot-end design and faster switch outs.
- Auto Z set, Change the sheet and no re-calibrating needed.
- New servos for faster performance and smoother printing
- New electronics. 32 bit software
- Love board to help eliminate re-doing the wiring from the hot-end to
the motherboard for any hot-end changes. Changing tips, thermistors or
heaters should only take a few minutes.
- More sensors.
- Faster and much faster once they get the Input Shaper firmware into
- WiFi, with web browser support.
- USB memory card.
- Firmware upgrades from the USB.
- 400 pulse per rev servos.
Some nice things that may not be mentioned
- Touch screen could be activated in a future firmware update.
- Firmware options with 32 bit system have expanded greatly.
More improvements may come beyond the Input Shaper.
- X axis tension adjustment uses 2 screws end to end, so the head of a
screw is against the X axis motor. This makes adjusting the
X axis tension work well. The screw head doesn't notch the motor
housing and slide out as it did on my MK3s+ originally.
- X axis mounts have 4 long screws in the body. These screws act
as the stops for the X axis rods. Better holding the distance
needed to align X axis on the Z axis screws.
- Linear bearings are fully captured on both ends for Z axis. Also
bolted covers, no press fits. Z axis is a single bearing not
2. The 2 bearings in the MK3 Z axis had a gap in between allowing
the bearings to move some.
- X, Y and Z linear bearings have a rubber mount in the bolt on
covers. I think this helps prevent squashing the bearing out of
- Z axis servos have a rubber mount between the servo motor and the Z
axis mount. Some vibration control, but mainly helps you be able
to align the Z axis lead screws.
- X and Y axis idlers have a V mount for the pin. As you tension
the belt, the idler pin is pulled into the V. Once pulled in the
pin is captured on both ends. No worry about it ever coming out.
- X and Y axis belts are cut to the correct length.
- The belt tension tool you can print for the MK3S+ works on the MK4.
- Z and X axis mounts are bigger, stiffer.
- X and Y mounts have reference surfaces to aid in positioning the drive
- X and Y axis drive gears have 2 set screws.
- Y axis motor uses a thermal pad to dissipate heat through the printer
- Access to the motherboard is from the outside of the printer.
Not inside the frame where it is hard to get at and see what you are
doing. Very helpful with my big hands.
- Wire guide clips added to help guide some wires. These are extruded
plastic parts, not 3D printed ones.
- The Nextruder top is already threaded for a mount. It is an M6
thread. Use a PC4-M6 fitting. See the setup in my enclosure
until the MMU3 is ready. I suggest using PTFE tube with one end
held in the Nextruder and the other end held in a mount before the
filament is fed from the roll. To minimize or eliminate Z
deflection of the X axis. The default with the roll hanging above
the Nextruder and only the filament fed into the Nextruder may cause a
problem. When the filament sticks on the spool, it will cause a Z
deflection as the Nextruder pulls the filament. With a PTFE tube
anchored at both ends, the pull is against the tube and Z deflection is
minimized or eliminated.
- Bolts are packaged based on size, not assembly operation. A
little easier to find bolts I think.
- Peep holes to assure X and Z axis rods are fully seated.
- Bent ends on the hot-end plastic stiffener cable that helps hold the
wires out of the way. Places for the bent piece in the hot end and
cabinet mount to assure proper twist and lift.
- You can easily dis-engage the idler of the Nextruder for removing
filament as needed.
- Better instructions for lubricating the linear bearings, you may still
need to wipe off excess from the rails the first few prints.
- Hot-end Wire channels are big enough to easily route the wires.
- The Enclosure remote power supply conversion for the MK3S+ will work,
but the quick dis-connect will not. 2 wires change.
- WiFi is 2.4g max. Plenty fast. File transfer speed
is relatively slow, but it is not the WiFi that is the problem. It
supports real time monitoring of the MK4.
- Screen preview of the part in an ISO view. Copy of the slicer G
code display routine.
- 1 wire for the power panic connection, the MK3S+ had 2.
- Extra ground wire connecting the LCD, Buddy Board and power supply.
- PrusaLink to monitor, load and run from a browser on the
- 5 Foot notes can be shown on the screen and may be selected in setup
by the user. Nozzle temp, speed, Z, X, Y and others.
Dimension differences of the MK4 vs MK3s+ From measuring
the MK3s vs MK4 parts.
- The Nextruder tip is 10mm higher than the MK3s tip. This
gives the extra 10mm of Z travel. 220mm total.
- The Nextruder is 64mm wide vs 50mm wide for the MK3S
hotend. The MK4 servo mounts for the Z axis lead screws are
7mm wider on both sides, 14mm total. The 10mm rod is 9mm wider on
each side, 18mm total.
- The 10mm Z axis rod is 325mm long vs 320mm for the MK3s+ 8mm Z axis
rod The rod is held deeper in both bottom and top mounts.
MK4 13.5mm depth for rod on bottom mount and 12.8mm for the top
mount. Compared to the MK3s with 5.2mm on the bottom and
6.9mm on the top. MK4 10mm rod mounts 2x deeper overall. The
Prusa mini 10mm rod is 341mm long.
- The top Z axis mount is 4-5mm above the top of the frame.
- The MK4 Z axis servo mount is 2mm lower when the 2mm pad is
- The X rods engage a little shallower on the MK4. 52mm estimated
vs 62mm on the MK3s. They are still 3 diameters deep on each end on the
MK4, this is good.
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 axis 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.
I haven't seen the kit, but from this I assume the following about
an MK3.9 and full MK4 upgrade.
- The linear bearings are all replaced
- The 10mm Z axis rods are included.
- All the MK4 printed parts are used.
- The newer heat bed mounting hardware is used.
- The MK3.9 kit does not include new X,Y and Z axis servos.
- The aftermarket "Bear" frame should work as long as you have updated
MK4 parts for the bear frame.
MK4 V1 vs. V2/3 printed parts
Look on printables.com, 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
Some things I think should still improve, minor overall. See Software rule #41
- Many are marked as a fastener change. A relief in the bolt hole so the
bolt goes in easier. Relief in some of the square nut holes and taper in
the hex/locking nut holes to make it easier to insert the nuts.
- Some of the axis mounts are a little thicker.
- Some mating parts now have a relief area around the bolts, My guess
this helps clamp better.
- Some cable guide changes in the Nextruder assembly.
- 2 versions of the LCD covers and mounts, must be a 2nd style LCD board
now. V3 has differences compared to V2.
- There are 2 versions of the nextruder tops. Have to wait for the kits
to see if there is a note that specifies one or the other.
- Support: Hotend parts supply, thermistors, heaters, heat block, nozzle
adapters. Supplies are becoming more readily available 9/2023.
- Support: Enclosure quick disconnect not yet available. 2 wire
needs to change, I can hack my own fix. Many are waiting on the
- Software/hardware. G code upload speed is limited by
something in the hardware/software. The 2.4G WiFi works well, it
is not related to WiFi speed. You can upload while the printer is
- Build Instructions: Y axis adjuster with a 45mm vs. 40 mm screw. To
assure bolt end is always in the "captured" part of the nut.
- Build Instructions: Tighten the screws. How
tight? Especially when there are areas in the instructions
that say "Don't over tighten". I am 6' 3"/188cm with hands
that cover a soccer ball. Can you tell me a way to measure what
Prusa thinks tight is? I see in the instructions about
changing a nozzle in the MK4 heater block they now recommend using a
- Build Instructions: What is a "small amount" of lubricant for the
- Build Instructions: You aren't told why X and Y axis are plugged in
last. It is to prevent damage to the board from the servos
generating power as you move the axis around during assembly.
- Build: One 90 degree torx wrench has a different size on each
end. It confuses people and they don't find the correct end,
even though the instructions warn you.
- Build: Thermal pad for the Y axis motor. I had the double blue
sided peel off cover. One would not peel off.
- Build: Y axis servo uses the bed for a heat sink. X axis servo has
nothing for additional cooling. I know it gets better air
flow than the Y axis motor, but not much. I will add heat sinks to
- Build: The cooling air nozzle for the part. With all the
work done on the MK4, more work should have been done here.
- Build:Using the frame to cool the Y axis motor. Hot spots on the
frame can twist the printer out of square.
- Build: Nextruder fan needs a deflector and guard. Some have
found the fan influx draft interferes with the filament cooling during
printing. It was my first print.
- Build: Anti-backlash for the Z axis. Currently the mount is too
short to add it. With the new Z axis bearings I may not be
able to modify it to add the longer anti-backlash nuts as I did for the
MK3S+. I have read all the arguments why Z anti-backlash is not
needed. My 40 years experience with NC machines tells me they are
beneficial. It may not matter that much, but everything matters.
MMU3 with MK4 integration testing Oct-Nov
2023 for me.
- The MMU2 will not work with the MK4/MK3.5/MK3.9. The new buddy
board has a different connector. It appears the MK4 firmware is written
for the MMU3 firmware.
- The MK3s+ will work with the MMU3. Now available.
- MK4/MK3.5/MK3.9 will work with the MMU3, with the correct cable.
- A new part for the Nextruder with the MMU3, need more details.
Changes to the MMU3 vs MMU2
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 change. The 2 mount feet for the MMU2 could be
used on the MK3S+, but the MK3.9/4 will need the new mount feet.
- Rewrite of the software. Better messages, functions, reliability
and integration with the printer.
- PD board to assure no power drop outs to the electronics board.
- Main Idler has a metal connector for the mount to the servo. The
previous printed mount was the primary problem with Idler reliability.
- SuperFinda now has a magnet and a magnetically active ball. The
magnet will pull down the ball for better accuracy in filament
- SuperFinda has a hole to sight initial alignment.
- The blade to cut strings of filament now cuts both ways.
- The PTFE feeder tube from the buffer goes all the way to the feed
rollers. Eliminating drag and joints that would snag filament
- E3D collets hold the 5 PTFE feeder tubes in place.
- Sight holes to assure the PTFE feeder tubes from the buffer are pushed
in all the way.
- Serial number and safety stickers!
- Label plate added to show location of the selector, 1-5.
- If sliced without an MMU filament selection, printer will ask which
filament to use.
- New mount feet. The old mount feet are too thick and will interfere
with the Nextruder on the MK3.9/4
- Standard cover for the electronics board also keeps the tension screws
from falling out.
- Loading of filament can be done with 3 buttons, L/R for position,
middle starts the load.
- Roller wheels in 5 cassettes. Each can be removed, loaded and
re-inserted. This follows the design of the majority of
buffers. Much easier to load.
- Buffer has feet to stand on the side.
- The PTFE tubes enter/exit from the same side through the
- A warning that some 3rd party re-winding spools may not work with MMU3
and reduced friction?
- For those that like the Electronics board mounted on the bottom, the
"MMU2 Underslung control Panel Mount" mod should still work.
- Printing a nameplate. The nameplate in the printables.com is
bare, no text. You can add the lettering using the TEXT function
in PrusaSlicder. Use the settings shown below, 2 lines are
typed. | double space 1 doublespace | 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
A better cooling nozzle that better covers the printed filament.
Z anti-backlash screws.
- Input Shaper needs constant improvement to include more nozzle sizes
and a wide range of filaments.
- Build progress picture should show progress in Z, or have the option
to show it that way.
- First purge by the part, not the same place for each print. This
was shown in the MK4/XL previews, but it is not part of the Prusa 2.6.0
slicer. I believe that the software must progress past the
"techie" user only with a slicer option for modifying start
location. There is room at the top for at least 3 locations.
- Fixed Eject and unload from the MMU3 without needing to heat the
hotend when there is no filament in the hotend. July2023
report states it has been implemented, in firmware that supports the
MMU3. If the hotend shows no filament loaded, it will not be
- Fiixed Organic supports fail to generate when there is a skirt with
more than 3 layers. Reported fixed for PrusaSlicer
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