MMU BOX – Rewind Buffer System for Prusa MMU3 (Modular Multi-Material Filament Management)

MMU BOX is a mechanically engineered, mostly 3D-printable rewind buffer system designed specifically for the Prusa MMU3.

It replaces the stock filament buffer with a compact, modular system featuring independent active rewind per spool.

This project has been successfully used in long real-world multi-material prints — including a 10-hour TPU / PETG / PLA combined print and extended high-swap testing.

This is not a concept. It works.

It is, however, a mechanical DIY system — and that distinction matters.

Why it exists

The stock MMU buffer works, but it introduces loose filament loops and space inefficiency.

MMU BOX approaches the problem differently:

• Each spool has its own controlled rewind

• No loose filament loops

• Clean, organized filament routing

• Full visual control of all spools

The result is a compact and mechanically predictable filament management system.

Important: Read Before Building

• This is a mechanical system.

• It requires understanding, assembly, and calibration.

• Manual pre-tensioning is required before prints

(You rotate the rewind rollers until the clutch begins to slip. Fully documented in the manual.)

• 3D printed flat springs can creep over time

(Performance depends on material choice and clutch setup)

V2 PETG springs are included. Some users report long-term stability. A few have experienced reduced rewind force over time.

• This is not plug & play

• If you expect AMS-level automation out of the box, this is not that.

• If you enjoy tuning mechanical systems and understanding how they work, this will make sense to you.

What MMU BOX Provides

• Active rewind per spool

• Independent ON/OFF rewind control

• Modular design (1–5–10+ spools possible)

• Compact arranged spool layout

• Front-accessible filament path

• Acrylic front panel for visual control

• Optional wall mount

• Flexible PTFE routing (rear or front exit)

Demonstrated Capability

• 10h TPU + PETG + PLA multi-material print: CLICK HERE

• ~400 swap reliability test: CLICK HERE

These are full, uncut operation recordings.

 Who this project is for

• Tinkerers

• Users who read manuals

• People who enjoy optimizing mechanical systems

Who this is not for

• people who expect AMS-like experience out of the box

• people who don’t like fine-tuning or testing

Files included

• all STL parts

• Detailed Printing Guide

• Full assembly manual

• User Manual

• Advanced Multi-Material Printing notes

Bill of material (BOM)

You can buy the full hardware kit from Blurolls: CLICK HERE

Or buy the required parts separately - BOM (Hardware & Materials for MMU BOX):

• Filament for printing: 

  • Approximately 5,3 kg of PLA 

  • ASA / ABS / PC-CF or other high temp material (The bottom line is PETG): Aproximately 180 g

  • PETG: Approximately 200 g

• Bearings: 30x 625ZZ (Example: https://a.aliexpress.com/_ExzgMcW )

• Magnets: 10x Neodymium Rectangular Magnets (20mm x 10mm x 5mm)
  (Example: https://a.aliexpress.com/_EHF00PM )

• PTFE Tubing (2.5mm Inner Diameter): Approximately 5 meters (Example: https://a.aliexpress.com/_EHWb3We )

• Acrylic Panel (Clear Recommended): 1x piece, 550mm x 280mm x 3mm

• Zip Ties: Around 50 pieces (or 1 standard pack).
  200 mm x 3.5 mm x1 mm 

• Compression Springs (Choose ONE option):

1. Option A: 3D-Printable Clutch Springs (STL files provided with the MMU BOX design)

2. Option B: Metal Clutch Springs (5 pieces). (Example: https://a.aliexpress.com/_ExVb6HC )
   Specifications: Length: 15mm; Inner Diameter (ID): ≥10mm (minimum); Outer Diameter (OD): ≤14mm (maximum); Wire Diameter: 0.8-1.2mm

• Heat Shrink Tubing Highly Recommended: (approx. 60mm Inner Diameter), ~0.5 meters 
  (Example: https://a.aliexpress.com/_EyFYqRU )
  
  Optional Enhancement: 

• LED Lighting: LED Strip (e.g., 5V White), minimum 0.5 meters
  (Example: https://a.aliexpress.com/_EGZqloK )

Printer compatibility

• The MMU BOX is designed to work with your Prusa MMU3 unit on the following printers:

  • Original Prusa i3 MK3S / MK3S+

  • Original Prusa MK4 / MK4S / MK3.9 / MK3.9S / MK3.5 / MK3.5S

• Flexible Positioning: back and downward facing front PTFE outlets allows for placement on top of a printer enclosure (e.g., IKEA Lack, Original Prusa Enclosure), on a nearby shelf, or even wall-mounting.

• MMU BOX dimensions (width, depth, height): 550x370x405 mm.

• The clutch calibration process:

Project Status

MMU BOX was originally released as a paid Printables Store model.

As of February 2026, it is released for free.

The design has proven itself. Many users run it successfully. It deserves to be accessible.

I keep supporting users with design improvements and answering their questions / help with troubleshooting, but I am no longer actively developing this project.

Final Notes

MMU BOX exists because multi-material printing can be better organized, more controlled, and mechanically satisfying.

If you enjoy building systems — this is one of the most interesting MMU3 upgrades you can print.

Changelog

Note: As of February 2026, MMU BOX is offered as a FREE model.

2025/07/21 | Clutch Calibration Tool V2

The clutch calibration tool is replaced with an improved design.

2025/06/13 | V2 Rewind Springs & Clutch Calibration Tool

Released V2 PETG flat springs.

Added clutch calibration tool.

Updated clutch housing with calibration marks.

Manuals and BOM updated.

2025/06/06 | Revised Pretensioner Bridges (V2)

1 magnet per bridge for gentler pretensioning.

2025/06/06 | Flywheel & Clutch Components V2

Improved tolerances for smoother assembly.

2025/06/06 | Improved Filament Path Tolerances

Smoother filament movement with V2 parts.

2025/05/24 | Wall Mount Released

2025/05/18 | Ratchet V3 Released

2025/05/15 | Ratchet V2 & Shelf/Wall Mount Support

Thank You

Thank you for supporting the MMU BOX and being part of its journey. This project was born out of a personal desire to improve multi-material printing on the MMU3, and your feedback has shaped it into what it is now. 😊

Happy printing.