MCS 2 - Space Saving Spool and Buffer System for MMU3 for Core One and MK4S

Intro

Changelog:

• Aug 29, 2026 - Added a hole guide part, a small optional part to glue onto the open adjustment coupler to aid in filament insertion

• March 1, 2026 - Added several Core One Buffer parts, created new universal spool hub part, reduced overall height by 1.5 inches, built new prototype to confirm

• Feb 20, 2026 - Initially shared with MK4S focus and tested all PTFE tube specific lengths for MK4S

I made this to fit two printers and the MMU3 in a small space.

This spool, PTFE tube, and buffer multicolor system (MCS) sits in a small footprint.  It works with the MK4S and Core One.  A strong simple wood frame and metal brackets for strength combine with minimal custom printed parts to position PTFE tubes and spool. 

This is similar to the quad spool layout used on the INDX Core One for spools #2-5. Spool #1 has easy access in front.

When placed to the right of your MK4S or Core One with MMU3, the total distance from the left side of the to the right side is only 25 inches of table space! (23 inches if you let the MK4S X motor dangle off the table). The buffer and spools all fit in a compact space that is only 7 inches wide, 20 inches deep, and 18.5 inches high.

Key Features:

1. COMPACT - Everything sits to the right of the printer, not behind or above it in a compact space

2. CARDBOARD SPOOLS! - Cardboard spools are easy to use with center ball bearing spool holder exact fit hubs

3. QUICK LOAD BUFFER - Leverages the buffer you already have, the Prusa Buffer, with the ribbon cassett remix, and secures the buffer firmly to the base.

4. OPEN ADJUSTMENT - Allows a fine manual touch open coupler to make it easy to first roughly load all the filament into the PTFE tubes without having to insert them all the way to the MMU back, and then gently push each one into the MMU3 as a more controlled loading process.

5. CONTROLLED PTFE TUBING - All filament path PTFE tubing is controlled and secured with no sharp turns for a low friction filament path.

6. LOW COST/TIME TO BUILD/PRINT - The parts needed here are simplified and print easily and wood is used in place of plastic to make a stronger structure. All five spools, buffer, and PTFE tubing can be picked up with one hand using a built in handle as well.

Parts Needed

Wood, Metal, PTFE Tubing

Wood - Home Depot $47

• $24 - 1x  hardwood oak board 2.5 inches wide and .5 inch thick (you will cut this into four pieces 1x 9.5 inch (front vertical), 2x 18.5 inch (the two middle verticals),  and one at 1x 11 inch  (the handle) so get a total lengths that allow those parts to be sectioned out. These are sold as hobby wood at Home Depot for $12 each for a 48 inch lengths so you need two of those. 

• $23 - 1x  board .75 inch oak board seven inches wide and 20 inches long. It probably will be 7.25 inches wide as the boards naturally come in that width. Look for Red Oak Stair Risers which are the perfect thickness and width and come in Oak!  These cost $23 each for a 48 inch piece

All the wood should be ideally be solid oak or high quality finishing plywood. Do not use pine or MDF or even cheap plywood as the wood screws will come out. After drilling the holes you might consider staining or priming/painting it before attaching the hardware. 

Hardware - Home Depot $19

• 5x Four inch long metal 5/16 bolts - $5

• 5x 5/16 washers $1 (you could get 10 and place washers on both sides) 

• 10x 5/16 hex nuts (normal nuts not locking nuts) $1

• 6x 3x3 inch metal flat brackets with .5 inch wood screws included $9

• 4x adhesive .75 inch square rubber or felt nubs/feet/pads $3

• (and drill and saw, sandpaper/files you already have)

Printer Hardware $25

• A mix of m3 18mm, 20mm, 30mm, and nuts that you probably already have

• 15 PC4-M10 fittings $10 https://www.amazon.com/StLiSuYmx-Pneumatic-Connector-Fittings-Compatible/dp/B0F62FZK4R

• 25 feet of 4 outer/3 inner mm diameter PTFE tubing $14 - After cutting all the lengths (8, 8, 8, 8, 8, MK4S - 21.75, 20, 18.5, 17, 15.5, 16, 16, 21, 24, 33, Core One - measure yourself) here you will have a little over four feet left over over  https://www.amazon.com/dp/B07B8CT1YH

Common Printed Parts - Core One and MK4S

Most of these parts are small connectors that print very quickly. 

1.  ALL PTFE Brackets (3x right and 2x left)

2.  ALL PTFE Adjuster Parts (The Adjustment Coupler and its bracket)

3.  PTFE Spacer https://www.printables.com/model/460874-short-ptfe-spacer-bar-for-mmu   and clips   https://www.printables.com/model/889415-tight-ptfe-tube-clips-2-8-chamfered

4. 5 pairs of 51-55mm Universal Hubs https://www.printables.com/model/1609248-universal-spool-hub-51-55mm

5. The Prusa buffer altered R4wheels and loading aid - see tips in buffer section!!

   1. Get just loading strips here from Ticat - https://www.printables.com/@CherryHillTech/collections/3163295

   2. Get the R4 wheels remix for the strips here from Bluex -  https://www.printables.com/model/953671-loading-aid-for-the-mmu3-buffer-segment-r4

6. PC4-M10 MMU3 Rear Connector https://www.printables.com/model/542409-mmu3-pc4m10-rear-connector

Buffer Printed Parts - Core One

The Core One needs a larger buffer capacity than the MK4S. The tube going between the Nextruder and the MMU3 selector is 390mm for the enclosed version and 450mm for the lite version, vs the 360mm tube for the MK4. The buffer needs to comfortably hold this amount, plus a little more just to be on the safe side, so ideally the Core One Buffer needs to hold 550mm or 22 inches of looped filament.  

There are several parts here I designed that add this extra capacity to the buffer. I was able to test and it comfortably holds 23 inches of filament using all three options to increase the capacity, namely on the top, bottom and back. If you were to remove one of these below you could get by without the back expansion (BufferPlate Larger) and still have a buffer room of 21-22 inches. In that case use the same part there as the MK4, the Back Buffer Plate.

Print and use these for the Core One buffer parts:

• All Buffer Brackets Core One  - includes:

  • 1x PrusaBufferBracket_CoreOne

  • 2x BufferPlateSecuredSpanner_CoreOne

  • 2x BufferPlate Larger  https://www.printables.com/model/1615597-mmu-buffer-plate-core-one-plate-holder-larger 

• Top Expansion - Taller Segmenter - https://www.printables.com/model/1312663-segmenter-increased-height

All of these changes above increase the buffer volume by about 10mm in the upwards, downwards, and rear directions. The taller segmenter will replace the top part, the spanners the lower, and the bufferplate the rear. To fit into the higher hole on the buffer triangular part we need a longer prusabufferbracket. 

Buffer Printed Parts - MK4S

The MK4S Buffer needs these parts. On the bottom we need some plate holders to span the metal brackets and attach the base.  On the top we need  a bracket to attach to the front post, and on the rear a plate to ensure filament doesn't get pushed out incorrectly.

1. All Buffer Brackets MK4S includes:

   1. 1x Main bracket

   2. 2x buffer secured brackets

2. Back Buffer Plate https://www.printables.com/model/1616722-buffer-back-plate-stronger

Assembly Instructions

Step 1 - Prepare the Boards and Hardware

First cut the wood to these dimensions below. All the wood is a half inch thick except the base board is 3/4 inch thick.

With a 5/16 drill bit, drill the five spool rod holes.  The offset for the holes from the bottom is 4.5 inches and from the top is 5 inches as shown in the photo below.

Then layout all the other hardware parts:

• 6x 3x3 inch brackets/screws

• 5x 4 inch 5/16 bolts

• 10x 5/16 nuts

• 5x  5/16 washers

• 4x rubber adhesive pads (optional)

This is the tag on the 1/2 inch thick wood I got at Home Depot.

Step 2 - Build the Front Spool Holder

Take the 9.5 inch board and position it in the front left corner, using the three inch brackets to position it. Make sure you have it oriented so the spool hole is 4.5 inches off the base. Offset the brackets a half inch from each other so their black screws don't collide with each other. Insert the spool bolt wash and one nut.

Screw a PC4_M10 jack into a  PTFE_RIght_Bracket part. Position it at the top of the wood support. (Note that the included files have a improved version of this part, both the right and left side versions, that has been improved with a small chamfer under the long arm, so the one in the files doesnt quite look like the brackets  pictured.)

Screw two holes and  secure the right one with one M3 18mm bolts and a hex nut.

Place the Buffer Bracket on the back side and secure the top hole with a m3 20mm, inserting it from the buffer side, and secure with a hex nut on the PTFE side.

With the buffer bracket in place, drill a hole through for the lower hole, and insert a m3 18mm from the buffer side, and secure with a hex nut.

Step 3 - Build the Tall Spool Holders

The middle support wood beam for spools #2 and #3 is 6.5 inches from the front. The 3 inches hinges (positioned at the top of the photo below) offset it  from the side of the base. Make sure the bottom spool hole is 4.5 inches off the base (the top hole is 5 inches from the top).

Use the 11 inch wood part to positon the other 18.5 inch beam for spool #4 and #5. Use the 11 inch board as a measure. Place it against the base of each beam to ensure they are in a line a span the entire 11 inches.

Make sure the vertical spool beam is positioned so the bottom spool holes are 4.5 inches off the base and the top hole is 5 inches from the top.  Once in position, attach the brackets, and spool rods, washers, and nuts.

Place the eleven inch long bar and clamp it at the top so you can use the left and right PTFE Brackets to place and mark and drill the one hole on the left and one on the right, place the PTFE holder and drill the final two holes all the way through both vertical beams and the horizontal 111 inch beam. 

Secure all four holes with m3 30mm hex bolts and m3 hex nuts with the brackets. Screw in the two PC4-M10 jacks.

Place two spools with their spool hubs onto a vertical support. We want to ensure the PTFE bracket for the lower spool holder is in the correct place just for sanity.

The top of the bracket should be exactly nine inches above the base.  Mark the wood at this location with a pencil line and double check it  Below is shows nine inches exactly.

Secure the left and right lower PTFE brackets with m3 18mm screws and hex nuts, and screw in the PC4-m10 jacks.

This is what it will look like with all five spools mounted. Since they are center hub exact fit, any spool will work, have almost no friction, nor will it become dislodged, or fall off.

Step 4 - Attach the Buffer

Note the buffer is upgraded with a remix of the Prusa R4 wheels that has a channel where a strip (orange here) can be inserted.  However, putting these together correctly is a pain and takes a while. This strip allows us to load the buffer entirely from the top without removing cartridges. It allows us to place the buffer with the slots at the top, and to anchor it down since we don't need to use a loading tool anymore in the bottom.  This is a key remix that is linked at the top and makes this entire design possible. A few tips that I ran into so you can avoid all these mistakes:

• When you assemble these cassettes you need to insert the strips before screwing them together.

• The strips insert so the flat smooth side goes around the wheel.

• Sand the wheels  in two places!! First, at the point on the wheels where the seam line appears on the in the groove of the wheel. Second, sand the seam on the rim of the wheel, as this seam can interfere with the ribbon moving smoothly.

• Sand the entrance to the entrance the exit channels as well as it is overly narrow and any printing error or bump there will cause problems with pushing the filament out when loading

• After assembly, be careful as the strips are fragile. Don't place the buffer upside down as it is easy to crush and deform the ends of the strips. If that happens you can use a heat gun to repair the weakened bend. It is really damaged you need to print a new strip and dissemble the cassette.

• Prep your filament tips careefully when loading and cut at two angles so the tip forms a symmetrical pointed tip

On the bottom of the buffer remove the two Prusa plate holders (I am not sure why mine had three at the bottom, as only two should be there) and attach two Plate holder Secured (the one with the hole) secured. If you have the Core One use the Core One version of these instead.

Here are the larger Core One buffer plates on the bottom:

Position the buffer over the buffer bracket.  The buffer should rest its weight entirely on the two buffer plate secured parts that we just attached (shown at the top in the pic above as it is upside down). 

If you have the Core One use the Buffer bracket with the longer knob on top.

Here is the back buffer plate on the back.

But if you are using the Core One consider using two of the BufferPlate_CoreOne parts to increase the buffer volume. They are on the left side below. Also, for the Core One use the larger segmenter shown on top.

It helps to have a helper! The large peg in the buffer bracket inserts into one top hole in the buffer and slightly protrudes. The two side rails of the bracket securely position it.

The two Plate Holder Secured custom parts raise the buffer up so it doesn't hit any of the metal brackets, and there is a hole that overlaps the side of the plate to secure each one in a direction counter to any pulling force.

Make sure to get a drill bit that is not too large or too small to first predrill the two holes.   Use a m3 10mm bolt to bolt it into the side of the base plate wood.

Step 5 - Install the PTFE Open Adjustment Coupler and Bracket

The open adjustment coupler sits between the buffer and MMU3 and allows you to feel the filament and do the final insertion into the MMU3, rather than pushing the entire length from back at the spool. You install a small bracket that holds this is the correct position. This section is entirely optional but gives you much needed access to hold and manage the filament between the buffer and MMU3.

Print in PETG, ideally on a textured bed, with supports inside the screw holes. Use snug supports for the threads and push them out from the center area with a thin hex tool - they push out as one piece and the tool wont damage the threads.

Carefully screw in all ten of the PC4_M10 inserts. If it isn't aligned as you insert one the thread it back out and start again. Set aside 15 minutes or so to get this right. Use the wrench tool that comes with the Prusa official tools as it fits these perfectly to tighten them.

The orange part below is a new optional addition I added in Aug 2026, which is highly recommended. This small nine minute print can be superglued onto the adjustment coupler on the buffer end.  I printed it in Atomic Orange PLA. The enlarged holes will help guide filament into the far hole as you initially load it from the buffer.  Only use one of these on one end, the end closer to the MMU3 as the other end doesn't need it and it will shorten the open area to hold the filament. No supports are needed and print with the enlarged holes face up on the print bed.

Attach the coupler bracket just below the top bar in the center of the vertical wood beam.  The bracket screws into the wood with two m3 20mm screws. Position it as shown in the picture, right between the top bar and spool hex bolt.

Now we will cut the PTFE tubing to these exact lengths:

• Five lengths, each one eight inches (same for Core One)

(ideally 3mm inner diameter, 4mm outer diameter), and place in the Adjustment coupler and the front slots of the buffer. Place the Adjustment Coupler in the bracket.

Another view is below:

You then cut the filament to the preferred lengths below to have it bend like this into your MMU3.  For the PTFE tube lengths,  measure twice cut once, and insert them into the MMU3. 

Slide the PTFE Spacer Bar onto them. The bumps on the spacer go on the outer (back) side of the curve, as this spacer has a slight internal curve to it and matches the spacing in the the MMU as well.

Here are the PTFE lengths between the Buffer and MMU for the MK4S. For the Core One you will need to figure the PTFE tube lengths below yourself.

• MMU 1  - 21.75 inches (goes from the far left hole on the back of the MMU to the to the far right hole of the adjuster couple

• MMU 2 - 20 inches

• MMU 3 - 18.5 inches

• MMU 4 - 17 inches 

• MMU 5 - 15.5 (goes from the far right hole on the back of the MMU to the far right hole of the adjuster couple)

Here is what they look like going from the adjuster to the MMU3 on the MK4S.

From underneath:

From the front:

From above:

Step 6 - Route PTFE Tubes to the Spools

If you are facing the front spool, spool #1, that PTFE tube goes into the back far right collet holes in the buffer. Spool #5 is the far back left collet hole.  Measure and cut the PTFE tubes as you like to get the bends desired to the spools and the numbering you want.  Ideally Spool 2 and 3 are on the middle and spool 4 and 5 are in the back.

Here are the lengths I used:

• Front Spool  #1 (blue hub in photo below) - 16 inches - connect to far right in buffer

• Spool #2 (yellow) - also 16 inches - connects second from the right in buffer

• Spool #3 Top Front Position (blue) 21 inches  - goes into 3rd hole from right on buffer

• Spool #4 Top Rear Position (yellow) 24 inches - goes into 4th hole from right on buffer

To prevent the PTFE tubes from spool 1 and 2 from rubbing against the edge spool 3, install a small 2 tube spacer clip as shown below.  Print two of these. Link here - https://www.printables.com/model/889415-tight-ptfe-tube-clips-2-8-chamfered

The PTFE tube length for Spool #5 in the lower rear is 33 inches. Use another 2 tube spacer clip to push it out away from contact with spool #4 as shown below.

Now print five pairs of the 51-55 Universal Spool Hubs - two for each spool.

Use PETG and 3 perimeters, organic supports, .25 structural settings, and 25% infill. Remove the supports.

Print in this orientation on the printer bed.

Insert your (2mm outer, 8mm inner, 7mm thick) bearings that came with your original spools. They will protrude about 1mm from the side.

Place the hubs  with on either side of a spool and secure the spool with a hex nut on the end of the bolt.

Most spools fit the size of the universal hub I designed above rom 51 to 55mm, but If you have spool sizes outside this range see the other linked sizes for finding my older design of hubs with a 50, 56, 58, and 60mm hub size here. These work fine but you need to also print out some spacers with them. You can see I was using several of them even in the photos here. I used a different color for each size to keep them straight. 

For wider spools you should have on hand a longer six inch 5/16 inch bolt that you could use. For 2 kg spools and longer rods also put a washer on the bolt head side as well to help lock it in place better.

Front view:

Left side view:

Right side view:

Optionally, also install the four silicon bumper adhesives on the underside corners of the main wood board.

Conclusion

I made this because I needed  to fit two MK4S/MMU3 setups on my printer rack which is only 58 inches wide, which leaves only 29 inches for each complete multicolor setup.  After tweaking the design I am happy that I was able to get the total width down to 25 inches for the printer, MMU3, buffer, and spools. 

I also wanted to reduce the time to make this, so I made all the parts as simple as possible to print and fully documented it. 

The spooIs  are stacked vertically to save space in a quad formation, with the fifth spool on the front, and the total height is still less than the height of a spool above the printer on the normal singlecolor spoolholder.

The Prusa R4 Buffer with the R4 buffer cassette/ribbon loading remix makes it now possible to not only orient the buffer vertically, but also screw it down, as we don't need to remove the cassettes or access the back to load it anymore. This small change really makes the design much smaller as the PTFE tubes go up into the air vertically, matching the spool arrangement.

I really like using the open adjustment coupler as it helps loading and troubleshooting, as well as positioning the curve of PTFE tubes correctly. However, it is optional and saves time and cost to not use it.

The exact fit spool hub center ball bearing design is perfect as it lower friction and works with all spools equally well, even cardboard!

This MCS2 design is an evolution from my earlier MCS design, where the printer faces to the right and the entire spool buffer system sits behind it. The previous design works well as it makes the spools more accessible, and the slot buffer is easier to load, but it takes up a lot of space, so I feel this MCS2 design is better overall as space is the most important factor.