Introduction: Prusa MMU2S Symmetric Filament Buffer

This is a modified version of the Prusa MMU2S Filament Buffer. The clear version is my mine. The black one is Prusa's.


The Prusa i3 design is a single extruder design. Along with the MMU unit, you can print multi material/color through a single extruder! However, there is a problem. When switching filaments the loaded filament must be returned so that a new one can be loaded. While we can easily pull more filament from a spool, we cannot wind it back on! The buffer gives the filament a place to go when switching between filaments.


The original buffer was just 6 plates of plastic that were held apart with 3D printed pieces that were screwed together. It needs to be large enough to allow a fair size loop, but other than that there aren't any constraints on shape or size. A very smart solution to the filament rewind problem. Good job Prusa!


The original buffer is the right idea, but not quite the right execution. The problem is the buffer shape (see black buffer picture). The inlet side is the fat side. Once you feed in filament using the PTFE tubes, you cannot grab the filament to guide it to the other side. Filament spends its life on a spool and wants to bend. When loading the filament, more often than not the filament would shoot out the wrong spot. The original buffer is also black so you cannot see inside to adjust the filament as you feed it. It is very difficult to use.


The fix is simple. Make the buffer symmetric! If we had a skinny end on both the inlet and the outlet, then it is easy to grab the filament at either end of the buffer. At that point, you can guide it through easily with a spare piece of 12" PTFE tubing (see final section for video demonstration).

In addition to the new buffer plate design, this instructable uses clear plastic for the plates. This allows you to see into the buffer and diagnose printing issues!

Step 1: What Is Involved?


I spent somewhere around $50 US in materials. You can expect to spend somewhere between 10 - 20 hours making the plates and assembling the buffer depending on your skill level. It will likely take up your weekend.


Cardboard (or Chipboard) for the stencil. It needs to be a bit larger than the original buffer plate.

6 sheets of 0.050" 11" x 14" non-glare plastic glazing by Plaskolite. Cost was $6.68 a sheet x 6 = $40.08

You might consider buying a few of the 0.050" 8" x 10" sheets to practice on.

3D printed pieces. You can use the pieces from your original buffer, but you'll need to print it again to get another of the long parts that holds the tubes. STL. GCODE.

10 ptfe tubes from original buffer.

Nuts/Screws from original buffer.

A spare 12" length of ptfe tube for 1.75 mm filament. This allows us to easily pass filament from one side to another.


Safety glasses to keep plastic bits out of your eyes.

Scissors for cardboard stencil.

Sharpie for stenciling on plastic sheets.

Plastic cutter to score plastic sheets.

A straight edge (scrap wood, ruler, etc) to assist in scoring straight lines.

Dremel for cutting and sanding.

Drill with 1/8" and 5/32" bits. 1/8th is for screw holes. 5/32 is for cleaning up printed parts.

2.5 mm hex bit for drill, or Allen wrench from Prusa Kits to operate the screws.

At least two C clamps for scoring/breaking plastic sheets.

Pieces of wood and a workbench to clamp to.

A rubber mallet for breaking the scored plastic sheets.

Step 2: Make the Stencil


We will now make a buffer plate stencil out of cardboard so we can easily mark the shape onto the plastic sheets.


While Prusa has released the design for the 3D printed buffer pieces, I couldn't find the design files for the buffer plates. I took my best effort at measuring and created a sketch using Fusion 360. The exact dimensions aren't important as you'll be dremeling/drilling to fit. Not shown in the dimensions is the thickness, which is 2 mm.

Fusion 360 files are located here:


The dimension diagram is academic. The easiest way to make a stencil is to take the existing plate, and trace the side with the skinny neck onto some cardboard or chipboard. Then flip the plate over, and trace the skinny side again. When you're done, cut the stencil out of the cardboard with scissors.

You can ignore the screw holes in the cardboard stencil. Those were from an earlier version of the buffer that used chipboard instead of plastic. The chipboard version of the buffer was not strong enough, so we're using plastic instead.

Step 3: Apply Stencil on to Plastic Sheets


Get 6 sheets with the buffer plate design on them.


We're going to copy the buffer plate design into the plastic sheets. When doing so, try to align the stencil to the edges of the sheet to minimize the amount of total cuts.

Put the cardboard stencil on each sheet and draw the pattern onto the plastic using the sharpie. If you make a mistake, use rubbing alcohol to clean up the marker and try again.

Don't remove the protective cover from the plastic sheets. First, you'll get cleaner breaks with the plastic on. It also helps protect against dirt and scratches during the construction process.

Step 4: Score All the Plastic Sheets


Get 6 sheets with the cuts scored.


Use the plastic cutting tool to score the sheets.

For each sheet, there should be two score marks per corner, plus two long score marks for the straight edges. Take the plastic cutter and score the spots shown using a straight edge.

See picture for details. I scored on top of the black lines. In person the score marks will be easy enough to see, but I added red lines next to the score marks to show where the are. The red arrows are just to call attention to the red lines.

Score the same spot 4 or 5 times to ensure a clean break.

Step 5: Break the Score Marks


Break the score marks for each plate using the C-Clamps and the rubber mallet.

We're now going to break away the majority of the plastic that we don't want. This will give us space to cleanup with the dremel. While you could technically cut the entire thing with the dremel, scoring is probably faster and will create cleaner cuts.


Let's start with one of the long score marks.

Take the plastic sheet and arrange it so that the waste material is suspended over air. The part of the buffer that we want to keep should be supported on both top and bottom with wood or your work bench. The score mark should be close to the supported section so that the break occurs at the score mark.

Secure the wood on top/bottom using C-Clamps.

Use the rubber mallet to break the plastic at the score mark. If the clamps are securing the plastic, and you have the score mark just exposed past the wood, you should have clean breaks as in the pictures above.

You can trim away the excess film.

Repeat for the other long score mark, and for each buffer plate.


The square score marks are similar to the straight score marks, but you need to position your wood scraps so that they hang over your work bench. The goal is to expose a square area of waste material.

Secure with C-Clamps and break with the mallet like before.

Repeat for all corners on all plates.


The cuts on the corners will likely be rounded, and even the side cuts might not be perfect. It's okay. We'll clean it up in the next step.

The only caveat is that you need a clean edge where the screws will end up. See picture above for critical areas. If these are damaged, you should probably throw the sheet away and make a new one.

Step 6: Cleanup With Dremel


Cleanup the buffer plate cuts using the dremel cutting wheel. Sand using the sanding wheel. Adjust as necessary.


At this point your straight cuts are probably pretty good, but your corner cuts are likely a little round.

Before proceeding, put your safety glasses on!

Use the dremel cutting wheel to make your cuts more exact. Use the sanding wheel to smooth out any imperfections. It also helps to sand the top/bottom of the cuts (briefly!) in order to make them finger smooth.


Try to make the buffer plates approximately the same size on all sides. We will be using printed parts on each of the four sides to hold the plates apart. If the buffers aren't the same, then they won't fit in the printed parts.

The actual size of the plates doesn't matter too much as long as they are all the same and they fit in the printed parts.

Use the sanding wheel to ensure they all have the same dimensions and use the printed pieces as a guide.

Step 7: Drill Some Holes


Insert the buffer pieces into the printed parts and drill the screw holes for all 6 plates.


Peel back the protective film from all the sheets, as you won't be able to get it off once you stick the plates into the printed pieces.

Insert the neck portion of the plates into the plastic parts (the ends that hold the ptfe tubes) shown with the magenta arrow. There are a few things to note here.

First, we'll need all the printed parts from the original buffer kit except the one shown with the blue arrow. We need a duplicate of the part shown with the magenta arrow because we have two of them with this plate design. You'll need to print out another of this part.

Second, for this duplicate part, note the orientation will be upside down of the other side (meaning screws will be on top for one part, and nuts will be on top for the duplicate). On one side of the printed part, 3 tubes must go in one end and pass straight through to the other. Similarly for the 2 tubes on the other side. See pictures above for clarification.

Insert the plates into the printed pieces noted by the magenta arrow. You're going to be drilling into the screw holes in the printed part. Ensure that the plate inserts enough into the printed part so that the hole you drill isn't at the edge of the plate. You need a few millimeters or the edge will crack when you drill it.

I used clamps to push at the printed pieces to ensure they didn't move as I drilled. This is optional, but helpful.


Once you are happy the plates are aligned you can start drilling.

Use the screw holes as a guide and use the 1/8th " drill bit. Don't push too hard on the drill. You don't want to slam into the plate below as you exit the plate you're currently drilling.

Drill through all six layers for the 4 holes.

Secure the plates using screws and nuts. Remember the orientation of the printed pieces!

Step 8: Drill More Holes


Attach printed pieces for the buffer sides.


Slide the printed pieces that hold the buffer sides onto the buffer plates. Note that some of these printed pieces have a hook. This is meant to attach to the printer frame. I used the same picture as the last step (which shows the duplicate part flipped), so you can compare to the previous step. See pictures for orientation.


Drill the holes into these pieces and screw them into place.

Step 9: Attach Tubes and Attach to Printer


Attach ptfe tubes. Attach buffer to printer. Enjoy a functional buffer!


We now want to attach the ptfe tubes to the printed parts. Attach the tubes from the MMU to the buffer outlet, and add the short ptfe tubes to the buffer inlet. Ensure that you have the tubes connected correctly so that filament could go in tube 1 inlet, through the buffer to tube 1 outlet, up to the MMU.

You want the tubes to protrude out the printed part by about 1 mm. However, you don't want the tube to be squished at all. Any resistance in the tubes will lead to jams and failed prints. If you have any trouble inserting the tube, use the 5/32 drill bit to widen the hole.

Once the hole is wide enough for the tube, you can secure it using the screw. The screw will bite a little bit into the tube. Slowly tighten the screw while moving the tube in/out of the printed part. When you can no longer move the tube, you've found the minimum tightness in order to hold the tube. Position the tube at the right distance (1 mm) and secure it at the minimum tightness.

Next take some filament and slide it in/out of the tube. Continue to tighten the screw as you move the filament. When the filament gets harder to move in the tube you have found your maximum tightness. Loosen the screw until the filament is free to move again.

Repeat for all ptfe tubes.


Using the hooks on the printed pieces, hook the buffer onto the back of your printer.

Load some filament and enjoy the buffer! :)


To load the buffer, load filament into the inlet side until it exits the tube. Then take a spare 12" ptfe tube and feed the filament into it. You can now easily guide both to the other side of the buffer. From there remove the filament from the spare pfte tube and load it into the tube that feeds the MMU2.


I had problems with the MMU2 when I first got it.

My ptfe tubes were too tight (see above), and filament tips were too stringy during retractions (which causes jams). I solved the string issues by increasing cooling moves for that filament.

Once I solved these issues, I found that I could print multi-material prints without jams or failures, time and time again. It's magic! Praise be to Prusa!

See these other resources for more in depth help:

1. Read the handbook thoroughly. It will save you time in the long run.

2. MMU2 Tips Chris' Basement (a 20 minute primer which will likely solve your problems)

3. MMU2 Troubleshooting 101: The Easy Fixes (long but very worthwhile)

4. How to Improve the quality of your 3D prints on the Original Prusa i3 MK3

5. Extruder Motor Visualizer (if you print these you can see what your motors are doing)

6. Prusa blog (lots of useful articles here)