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RMU MK3 Filament Buffer Troubleshooting

The RMU MK3 is a third-party tower-style filament buffer by the Wedge Group that replaces the stock Prusa MMU2S buffer. It stores filament in the tower body when not in use and uses rotating wheels and a latch mechanism to provide low-friction feeding for the MMU2S system.

Warning

The RMU MK3 is designed for use with the MMU2S only. It is not compatible with the MMU3. If you have upgraded to the MMU3, this guide does not apply.

Note

The MMU2S is extremely friction-sensitive. Even small increases in filament resistance can cause load/unload failures, missed tool changes, or under-extrusion mid-print. When troubleshooting MMU2S problems, always rule out the buffer first.

Table of Contents

Filament Not Feeding Through the RMU

Symptoms: The MMU2S attempts to load a filament but it never reaches the extruder, the selector reports a load failure, or you hear clicking/grinding at the MMU2S drive gears.

Likely Causes

  • Bad tip shape. The RMU's rollers and channel slots require a clean, sharp-angled tip to guide filament through without snagging. A blunt, bulbous, or melted tip will catch on every edge.
  • Kinked or bent filament. Filament that has a set curve or kink from the spool will resist entering the channel and may miss the roller entirely.
  • Filament not seated in the channel. Filament that is sitting against the channel wall rather than in the groove will not engage the roller correctly.

Solutions

  1. Unload the filament completely and cut a fresh tip at a sharp 45-degree angle with flush cutters. The tip should be cleanly tapered — no whiskers, no blob.
  2. Pull approximately 200–300 mm of filament off the spool and inspect it. Straighten out any visible curves by running it between your fingers or gently bending it in the opposite direction.
  3. Manually guide the filament into the correct channel slot before triggering a load. Hold it in place and confirm it is sitting in the groove before releasing.
  4. Trigger the load and watch the filament enter the tower. If it misses the channel, the issue is tip shape or alignment — repeat steps 1–3.

Tip

If load failures are recurring across multiple filaments, check that your MMU2S unload tip-shaping settings (in PrusaSlicer or the printer EEPROM) are producing clean tips consistently. A poor unload routine will cause repeated RMU entry failures.

Excess Friction or Under-Extrusion During Prints

Symptoms: Under-extrusion that only occurs during MMU2S multi-material prints, inconsistent extrusion on tool changes, or the MMU2S shows increased load current / IR sensor anomalies.

Likely Causes

  • Dirty or worn rollers. Filament dust and debris accumulate on the rollers over time, reducing their ability to rotate freely and adding drag instead of reducing it.
  • Debris inside the tower. Filament shavings from the MMU2S drive gears can travel up the PTFE path and settle inside the tower channels.
  • Filament binding in the channel. A kink, a twist, or an oversized loop inside the tower creates a friction point every time the filament moves.
  • Loose or misaligned PTFE connections. If the PTFE tubes at the entry or exit of the RMU are not fully seated, filament catches on the gap at every load and unload cycle.

Solutions

  1. Remove all filament from the tower.
  2. Use compressed air to blow out each channel from both ends. Direct a short burst into the PTFE tube entry point to clear debris from the channel slot.
  3. Wipe each roller/wheel with a lint-free cloth or cotton swab lightly dampened with isopropyl alcohol (IPA, 90% or higher). Spin the roller by hand and wipe again until no discoloration transfers to the cloth.
  4. Allow the rollers to dry completely before reloading filament.
  5. Check each PTFE tube connection at the RMU entry and exit. Press firmly to seat, and confirm there is no visible gap between the tube end and the channel inlet.
  6. Reload filament into each channel and manually pull it back and forth to feel for any binding point. Smooth movement with minimal resistance is the target.

Warning

Do not use acetone, strong solvents, or lubricants on the rollers. These can degrade the roller material or leave a residue that attracts more debris.

Tip

Run a single-filament print after cleaning. If the friction issue disappears during single-material printing but returns during MMU2S multi-material printing, the RMU is almost certainly the source — the repeated load/unload cycles are where a dirty or worn buffer reveals itself.

Filament Tangling Inside the Tower

Symptoms: Filament visibly bunched or knotted inside the tower, audible rattling during prints, load/unload failures that are inconsistent (sometimes works, sometimes does not).

Likely Causes

  • Retract winds filament back too loosely. If the MMU2S retracts faster than the buffer wheel can accommodate, filament can pile up in a loose loop and tangle around itself.
  • Spool core size mismatch. Filament from a very large diameter spool core retains a wide coil shape that does not sit neatly inside the tower channel. Filament from a very small core may have tight curls that resist lying flat.
  • Latch not fully closed. If the latch or door does not engage fully, filament loops can escape the tower body during retract and re-enter tangled.

Solutions

  1. Open the latch and visually inspect the interior of the tower. Carefully work out any tangles by hand before attempting another load — do not try to power through a tangle with the MMU2S.
  2. Check that the latch snaps fully into the closed position. It should hold firmly without any play. If the latch is loose or worn, inspect it for cracks or deformation.
  3. Test with a different spool from a different manufacturer, ideally a 1 kg spool with a standard 53–55 mm inner core diameter. If the tangling stops, the original spool geometry is the cause.
  4. If you can observe the retract cycle, watch the filament entering the tower. It should coil smoothly and lie against the tower wall. If it is bouncing or looping erratically, the retract is happening faster than the buffer can manage — check if your slicer or firmware allows adjusting MMU retract speed.

Note

The RMU tower has a finite filament capacity. If you are loading very long lengths of filament into the buffer before starting a print, reduce the amount of slack inside the tower — enough to fill the expected retract without excess.

Filament Escaping or Falling Out of the Tower

Symptoms: Filament found outside the tower body after a print, visible loops hanging out of the tower opening, or the buffer door/latch found open when it should be closed.

Likely Causes

  • Latch not fully engaged. The most common cause. A partially closed latch allows filament to push past it during retract.
  • Stiff filament springing back. Materials like PETG, ABS, and ASA have higher stiffness and spring-back than PLA. A loaded length of stiff filament can generate enough lateral force to push through an improperly latched door.
  • Overfilled channel. Too much filament inside the tower creates pressure on the latch.

Solutions

  1. Close the latch firmly and confirm it is positively engaged — you should feel or hear a click, and it should not move when you press on it lightly.
  2. If the latch does not hold reliably, inspect it for wear, warping, or print layer delamination if it is a printed part. The Wedge Group provides replacement hardware; contact them if the latch is damaged beyond use.
  3. When loading stiff filaments (PETG, ABS, ASA, PA), load less slack into the tower than you would for PLA. Just enough to accommodate the expected retract distance is sufficient — do not pre-load excessive coils.
  4. Load stiff filaments slowly and ensure each loop lies flat inside the tower before closing the latch.

Tip

PLA is the easiest material to run through the RMU. If you are new to the buffer or have been having persistent latch issues, validate the setup with PLA first before moving to stiffer materials.

One Channel Fails to Load Consistently

Symptoms: One specific channel (e.g., channel 3) fails to load reliably while the others work correctly. Load failures are repeatable on that channel across multiple filaments.

Likely Causes

  • Worn roller on that channel. Individual rollers wear at different rates depending on use. A frequently used channel will wear faster.
  • Debris lodged in that channel slot. A fragment of filament tip or a clump of dust can partially block a single channel without affecting the others.
  • PTFE tube misaligned at that channel. If the PTFE tube serving that channel has shifted, there is a gap or angle change that catches filament tips.

Solutions

  1. Remove filament from the affected channel and compare the roller visually to the rollers on channels that work correctly. Look for flat spots, scoring, or surface degradation.
  2. Use compressed air to clear the specific channel slot. Follow with a thin brush or folded piece of paper to dislodge any debris sitting in the groove.
  3. Inspect the PTFE tube at both ends of the affected channel. Press it firmly to seat, confirm there is no visible gap, and check that it runs straight without a sharp bend near the entry point.
  4. Manually feed a fresh-cut filament tip through the affected channel by hand and feel for the exact point where it catches. This will tell you whether the problem is at the channel entry, along the roller, or at the PTFE exit.
  5. If the roller is visibly worn and other steps do not resolve the issue, contact the Wedge Group for a replacement roller for that channel position.

Warning

Do not assume a channel-specific failure is caused by the filament or the spool. Test the same spool in a working channel. If it loads cleanly there, the problem is isolated to the hardware of the failing channel.

General Maintenance

Regular maintenance keeps the RMU running with the low friction the MMU2S requires. The following schedule works well for moderate use (several multi-material prints per week).

Cleaning Schedule

Interval Task
Every 5–10 prints Blow out all channels with compressed air
Every 10–20 prints IPA wipe on all rollers; check latch engagement
Monthly Full inspection — PTFE tube seating, roller condition, latch mechanism, interior of tower for debris accumulation

What to Use

  • Compressed air: Use short bursts directed into the channel openings and PTFE tube entry points. Hold the can upright to avoid propellant spray.
  • Isopropyl alcohol (IPA): 90% concentration or higher. Apply to a lint-free cloth or cotton swab, not directly to the rollers.
  • Lint-free cloths or cotton swabs: For roller cleaning.

What to Avoid

  • Do not use petroleum-based lubricants, PTFE spray, or silicone oil inside the tower or on the rollers. These attract filament dust and create a grinding paste over time.
  • Do not use acetone or aggressive solvents near any printed parts of the RMU.

Inspecting for Wear

Check the following during monthly inspections:

  • Rollers: Should spin freely by hand with no rough spots. Surface should be uniform with no flat spots or scoring.
  • Latch mechanism: Should close positively and hold under light lateral pressure. No cracking or deformation in the latch arm or catch.
  • PTFE tubes: No kinks, no yellowing or clouding that indicates heat exposure, fully seated at all connection points.
  • Tower body (if printed): No layer separation, no cracks at mounting points or latch attachment.

Tip

Keep a short log of which channel each filament slot uses most often. Heavily used channels will wear faster and need more frequent cleaning. Rotating which physical spool is in which channel can distribute wear more evenly over time.