Bufferless MMU3 on Core One w/ Rewinder Spools

Small footprint, bufferless setup for MMU3 on Core One with Rewinder Spools.

BEWARE: The instructions and parts below are a work in progress, some of the BOM numbers might be inaccurate in quantity. I have yet to extensively test this setup, so builder beware, but I have printed a number of models with no interventions so far.

I didn't see anything that was an optimal setup for MMU on the Core One, so I came up with this solution.

Objectives:

• Small footprint (all or most, spools + MMU3, on top of the Core One)

• No buffers, I didn't have the space for a buffer setup, which leads to 

• Use of Rewinders

• No modification to existing hardware (no drilling holes or cutting parts from Core One), I wanted to go back to stock if needed

• Low cost (Preferably with mostly printed parts)

• Mostly or all printed parts (minus screws, nuts, PTFE tubes, bearings, collets, magnets, etc...)

• MMU easy to access and maintain if needed

• Easy to assemble

Instructions:

• Remove rivets on top panel.

• Replace vent with "Core One - MMU - Vent - Cut.stl"

• Turn top panel counterclockwise, "Core One - MMU - Vent - Cut.stl", short side should be towards the back of the printer, since this is where the PTFE from MMU enters.

• If you have a CORE One+, remove venting "C1+_UPG_VENT_BLOCK_R1-A.stl", this will cause the "c1_printhead_cover_right_lever_r1.stl" to hit the vent block while printing, so this is not compatible.

• Put m3ns nuts into the following:

  • "Core One - Rewinder Rails - V4 - Rail.stl"

  • "Core One - Rewinder Rails - V4 - Anchor to 2020.stl"

• "Core One - Rewinder Rails - V4 - Rail.stl" has two holes with a sacrificial layer for m3 screws, use an m3 allen key to break it before screwing screws in.

• Attach "Core One - Rewinder Rails - V4 - Anchor to 2020.stl" to top panel, use m3 hexnut and m3x18 screw for this.

• (Optional) Insert 10x 6x3 magnets into "Core One - Rewinder Rails - V4 - Rail.stl", this helps to align the rails, but is not needed. This setup was initially going to use only the magnets instead of screws + magnets to secure the rail holders to the top, but I wanted more peace of mind, so I added screws, but I left the magnets in tact as an optional feature.

• Attach "Core One - Rewinder Rails - V4 - Handle.stl" to "Core One - Rewinder Rails - V4 - Rail.stl"

• Insert tnuts into extrusions

  • extrusion closest to the front needs 16x tnuts on top, 2x tnuts on bottom

  • back extrusion needs 8x tnuts on top, 2x tnuts on bottom

• Slide in the Handle+Rail roughly 24mm into each of the 2020 extrusions, secure with 2x m3x10 screws from the bottom. Tip for securing is to lightly screw in the tnut into Handle+Rail combo and then slide them into the 2020 extrusions, tighten when at the proper length.

• Blind fit the 2020 extrusion+handle+rail combo on top of the Core One, after getting fit properly, screw in the handle+rail combo to the extrusion.

• Take off the rail holders, it is easier to continue with installation when not on top of the Core One.

• Then continue with installation of Core One MMU3 community version - rewind spool remix, alternative for "Core One MMU3 rewind ptfe.stl" in this remix made for generic E3D bowden collets under "Rewinder Spool Holders" folder.

Settings and Tips:

• Recommended to print with less creeping material, no PLA. I printed outside mounts and spool holders with PETG and the parts inside the chamber with higher temperature ASA or PCCF.

• Use Spool Adapters for cardboard spools.

• Keep the shortest and straightest path for each PTFE tube, any sharp bends will cause issues during color change, shorten or lengthen PTFE tube where needed, especially with the PTFE tube going into the extruder. When cutting PTFE to length, err on the side of cutting it on the longer side, so you can cut it down as needed, you don't want to undercut and need more length.

• Set MMU Length under Settings -> Hardware -> MMU: 850mm

• Edit 3/27/2026: I made a PC4-M5 fitting compatible with 66mm width collet as well as a compatible PTFE Fitting. Original: Use the "PTFE_FITTING_R1.stl" from the MMU3 printable parts, under Models -> CORE ONE (ENCLOSED VERSION) - SPECIFIC PARTS or the modified one I made for E3D bowden collects ("Core One - MMU - PTFE Holder for Extruder.stl"). This helps tremendously to align the filamen loading to extruder. Without this, I was troubleshooting loading issues for far longer than I wanted to admit.

• Optional: Y-Splitter, I used this one, I might modify this, but has worked ok for me so far while using the MMU, side spool PTFE sometimes will pop out while I am inserting filament into the extruder.

• When using the "Core One - MMU - Rewind PTFE.stl", spools can be loaded from above or below. I am using it where I am loading spools from below, but it is harder to load the filament into the MMU this way, so I might change this, but both directions are options here.

• When buying 608 bearings for rewinders, make sure to clean, repack with grease, and test each one. These are crucial to getting the rewinders to work. 

• Keep the length of the PTFE from MMU to Extruder short, if it is too long it might hit the limit for the rewinders, as well as, hit the max configurable length of 1000mm.

• Printed standard vslot 2020 extrusions either 2 x 230mm extrusions (that fit on my buildplate) or you can buy a full 460mm 2020 from retailer of choice. Amazon had a few last I checked.

• This is the one I printed, included in model files are the ones I lengthened and printed.

• Around 1kg of filament to print (With Prusa Slicer 30% grid infill with 4 vertical walls), this estimate excludes the printed 2020 extrusions, but includes Spool Rewinder mounts and Rewinders from original models.

• 2020 m3 T-nuts or you can print them with inserted hex nuts, there's a few models floating around. If buying, I recommend the spring loaded ones in case you forget to insert a few into the extrusions after putting things together.

BOM:

Total BOM:

• 28x m3 tnuts (printed or metal ones)

• 12x m3 square nuts

• 4x m3 hex nut

• 20x 6x3 round magnets (optional)

• 8x m3x10

• 4x m3x at least 25 for handle to 2020 holder

• 2x m3x at least 18 for top plex to 2020 anchor

• 2x m3x30 for 2020 holder to 2020 anchor

• PTFE: 1x 380mm for Extruder to Single End Y-Spliiter

• PTFE: 1x 490mm for Double End Y-Splitter to MMU

• PTFE: 1x 278mm for Double End Y-Splitter to Side Spool Filament Sensor

BOM Breakdown per part/assembly:

• 16x tnuts for front of spool holder plus "Core One - MMU - Rewind PTFE.stl"

• 8x tnuts for back of spool holder

• 6x tnuts for rail joiner

• 4x tnuts for handles to 2020

• 4x m3nuts for handles to rail holder

• 4x m3nuts for Anchor to 2020

• 2x m3nuts for top plexi to 2020 anchor

• 2x m3nuts for MMU to Core One Mount

• 6-11x bowden ptfe clips, depending how you want to direct the PTFE from the rewinder to MMU of spools

• 20x 6x3 magnets (optional)

• PTFE: 1x 380mm for Extruder to Single End Y-Splitter

• PTFE: 1x 490mm for Double End Y-Splitter to MMU

• PTFE: 1x 278mm for Double End Y-Splitter to Side Spool Filament Sensor

• 4x m3x10 for 2020 holder to 2020 from bottom

• 4x m3x10 for rewinder holder to rail holder from top

• 4x m3x at least 25 for "Core One - Rewinder Rails - V4 - Handle.stl" to "Core One - Rewinder Rails - V4 - Rail.stl"

• 2x m3x at least 18 for top panel to "Core One - Rewinder Rails - V4 - Anchor to 2020.stl"

• 2x m3x30 for 2020 holder to 2020 anchor

• 1x hexnut for anti-sag solution

• 1x hexnut for MMU holder

• 5x E3D collets (66mm width) for Spool PTFE holders.

• 1x E3D collet for "Core One - MMU - PTFE Holder for Extruder.stl"

Other Models:

• Required - MMU3 PTFE tube guide - To keep PTFE tube into MMU straight

• Required - Prusa MMU3 - PTFE Fitting for E3D Bowden Coupler - Alternative to MMU3 PTFE tube guide compatible with generic E3D bowden collets

• Required - Rewinder disks remixed - Easier printing than other rewinder disks, increases reliability of rewinders

• Prusa CORE One - Modified Vent For PTFE Passthrough For MMU3 - Modified to allow PTFE to passthrough from MMU to extruder via the top panel

• Inner Diameter Adapter for Cardboard Spools - Increases reliability of rewinders

• Instant Y-Splitter for Bambu AMS X1C P1S P1P - To allow for easy swapping between side spool and MMU use 

• Spool Auto Rewinder Body Solid - Cosmetic, I liked the cleaner look of this.