Feature: Tip Forming and Purging¶

Concept¶

Every toolchange needs two things done well: a clean filament tip (no blobs, no long hairs - a good tip looks like a tiny spear) so the next load doesn't jam, and a purge big enough to clear the old color/material out of the nozzle without wasting excessive filament.

Each has the same three-way choice, and they're independent of each other:

• Tip forming: Happy Hare's own standalone macro (_MMU_FORM_TIP), a toolhead-mounted filament cutter (_MMU_CUT_TIP) instead of forming a tip at all, or leave it to the slicer's own in-print tip-forming.
• Purging: Happy Hare's own standalone macro (_MMU_PURGE), an add-on purge system like Blobifier, or leave it to the slicer's wipe tower.

There's also a separate, additive cutting option: a servo-driven cutter mounted at the MMU itself (EREC-style) rather than at the toolhead - see Servo Cutter below. It trims the tip after unload; you still form a decent tip beforehand the normal way.

Hardware Setup¶

Two independent menus, both unconditional (every MMU type gets them):

These produce, in mmu.cfg's general parameters (not the file literally named mmu_parameters.cfg):

force_form_tip_standalone : 1              # 1 = always standalone tip forming (turn the slicer's off)
form_tip_macro             : _MMU_FORM_TIP  # or _MMU_CUT_TIP, BLOBIFIER, or your own
extruder_form_tip_current  : 100
slicer_tip_park_pos        : 0              # Only matters if the slicer forms tips instead

force_purge_standalone   : 0                # 0 = slicer's wipe tower in-print, standalone otherwise
purge_macro               : ''               # Blank = no standalone purge; set to _MMU_PURGE, BLOBIFIER, ...
extruder_purge_current    : 100

Servo Cutter (MMU-mounted)¶

An MMU-end cutter (the classic EREC design, or similar) trims the tip after unload, rather than replacing tip forming - you still want form_tip_macro: _MMU_FORM_TIP so a decent tip exists going into the cut. Enabling it under Tip Forming / Cutting adds a servo pin prompt and generates [mmu_servo cut_servo] in mmu.cfg; its own tuning (open/close angles, feed/cut length, cut attempts) lives in mmu_macro_vars.cfg's _MMU_SERVO_CUTTER_VARS. See Macro: Servo Cutter for the build/wiring side.

Parameter Setup¶

Tuning tip forming¶

The default _MMU_FORM_TIP macro (mmu_form_tip.cfg) works similarly to PrusaSlicer/SuperSlicer's own tip-forming, so their multi-material filament profiles are a reasonable starting point even if you're not using slicer tip forming at all - add a Prusa printer + MMU preset from the system presets, then pick the MMU printer specifically (not Single); its filament presets tagged @MMU in the name are the multi-material ones with real tip-forming settings behind them:

Pick the filament type you're tuning for (PLA, ABS, PETG, ...) and use its Multimaterial section settings as your own starting point:

Its tuning variables live in mmu_macro_vars.cfg's _MMU_FORM_TIP_VARS section. Tuning by hand:

1. Remove the bowden tube at the toolhead so you can feed filament directly.

2. Cut a ~400mm fragment of the filament you're tuning, heat the extruder, and hold the fragment to the gears:

   MMU_LOAD EXTRUDER_ONLY=1
   

3. Extrude a few mm by hand to prime, then:

   MMU_TEST_FORM_TIP
   

   This runs the tip-forming macro and ejects the result for inspection.

4. Adjust one variable at a time on the command line - changes are sticky for the session:

   MMU_TEST_FORM_TIP cooling_moves=5 unloading_speed=15
   

5. Re-insert the fragment (MMU_LOAD EXTRUDER_ONLY=1 again) and repeat. Expect this to take a few dozen attempts to converge.

Full parameter reference: MMU_TEST_FORM_TIP.

Toolhead cutter (_MMU_CUT_TIP)¶

Instead of tuning a tip-forming macro at all, a toolhead-mounted filament cutter can just cut the tip cleanly and skip the forming step entirely - switch to it with form_tip_macro: _MMU_CUT_TIP in mmu.cfg.

• Advantages: a clean, hair-free tip every time, with none of the fiddly per-filament tuning tip forming needs; works the same regardless of filament type.
• Trade-offs: it's extra hardware to build and wire in the first place, the blade itself is a consumable that eventually needs replacing, and if the cutter lives in the toolhead, cutting there wastes a little more filament and time during the purge than a clean-formed tip would.

Mechanically, a toolhead cutter is normally triggered by moving the toolhead itself into a fixed position that presses a depressor or servo arm against the cutter's own activation mechanism - rather than the cutter having its own motor. The routine Happy Hare drives around that move looks roughly like this: on seeing a toolchange, the toolhead moves to the prime tower (or wherever purging happens) and runs a few cooling moves to firm up the filament so the cut doesn't leave it stringy; the filament is then positioned for the cut - tuned to leave the minimum practical length behind while still getting a clean cut; the toolhead moves to a staging position just outside the actual cut point, then makes a slow approach into the cut, a quick cutting move, a quick retract, and a slow return to staging. Unload, load of the next filament, purge, and resume follow as normal.

Purge volumes¶

Slicers offer purge volumes three ways - a simple per-tool unload/load value, a full N×N matrix for precise per-pair control, or an automatic pigment-percentage calculation - each a tradeoff between setup effort and waste:

The manual and matrix options are both set directly in the slicer, with no Happy Hare involvement. PrusaSlicer/SuperSlicer also offer a fourth, automatic option: enabling Advanced wiping volume (under Printer Settings → Single Extruder MM Setup) lets the slicer calculate purge volume from each filament's pigment percentage (0-1) instead of a fixed number - tune the percentage per filament to fine-tune the result. If you use the same base filament profile for several colors, you'll need to duplicate that profile per color so each can carry its own pigment percentage, and make sure the right profile is selected for each tool.

Happy Hare can also calculate its own matrix from filament colors, as an alternative to any of the slicer's own methods - typically run from your print-start macro:

MMU_CALC_PURGE_VOLUMES SOURCE=gatemap MULTIPLIER=1.1     # From the gate map's (or Spoolman's) colors
MMU_CALC_PURGE_VOLUMES SOURCE=slicer MIN=50              # From the slicer's own tool colors, floored at 50mm3

MMU_CALC_PURGE_VOLUMES SOURCE=gatemap MULTIPLIER=1.1

Purge map updated. Use 'MMU_SLICER_TOOL_MAP PURGE_MAP=1' to view

Full parameter reference: MMU_CALC_PURGE_VOLUMES. Volumes default to a 0-800mm³ range unless overridden with MIN=/MAX=. View the result with MMU_SLICER_TOOL_MAP PURGE_MAP=1 (or SPARSE_PURGE_MAP=1 to drop toolchanges that can't actually occur in the loaded print) - each cell is the calculated purge volume, in mm³, for that row-to-column transition:

MMU_SLICER_TOOL_MAP PURGE_MAP=1

Purge Volume Map (mm^3):
To -> T0   T1   T2   T3   T4   T5   T6   T7   T8
T0    -   129  230  192  221  221  223  480  223
T1   316   -   210  155  393  393  325  606  325
T2   383  179   -   189  354  354  195  551  195
T3   322   78  143   -   359  359  264  570  264
T4   133  155  124  165   -    -   165  354  165
T5   133  155  124  165   -    -   165  354  165
T6   314  224   98  212  352  352   -   541   -
T7   102  122  120  116   65   65  111   -   111
T8   314  224   98  212  352  352   -   541   -

printer.mmu.toolchange_purge_volume is the number your own purge logic should actually read during a toolchange - it's the sum of the matrix value above, toolhead residual filament, and any filament left behind by a cut tip, not just the matrix on its own.

Purging without a wipe tower¶

A dedicated purge system (like Blobifier) lets you disable the slicer's wipe tower entirely - it still needs a purge volume matrix to work from, which can come from any of: Happy Hare's own g-code preprocessing of the slicer's matrix, MMU_SLICER_TOOL_MAP PURGE_VOLUMES=... directly, or MMU_CALC_PURGE_VOLUMES above. All three end up in printer.mmu.slicer_tool_map.purge_volumes for your macros to read:

MMU_SLICER_TOOL_MAP PURGE_VOLUMES=70                                    # One value for every transition
MMU_SLICER_TOOL_MAP PURGE_VOLUMES=70,70,70,70,70,70,70,70,70            # N values - one per tool's "to" column
MMU_SLICER_TOOL_MAP PURGE_VOLUMES=70,70,70,...                         # NxN (or 2xN, see Command Reference) - full matrix

A flat PURGE_VOLUMES=70 fills every transition with the same value:

MMU_SLICER_TOOL_MAP PURGE_MAP=1

Purge Volume Map:
To -> T0   T1   T2   T3   T4   T5   T6   T7   T8
T0    -   140  140  140  140  140  140  140  140
T1   140   -   140  140  140  140  140  140  140
T2   140  140   -   140  140  140  140  140  140
T3   140  140  140   -   140  140  140  140  140
T4   140  140  140  140   -   140  140  140  140
T5   140  140  140  140  140   -   140  140  140
T6   140  140  140  140  140  140   -   140  140
T7   140  140  140  140  140  140  140   -   140
T8   140  140  140  140  140  140  140  140   -

(A single input value of 70 still shows as 140 per cell above - the displayed total is the sum of both tools' contribution to that transition, consistent with how the manual per-tool unload/load volumes combine.)

Toolhead calibration and toolhead_ooze_reduction¶

Correct purge volumes still assume the toolhead's own geometry (toolhead_extruder_to_nozzle, toolhead_sensor_to_nozzle, toolhead_residual_filament - all in mmu.cfg's toolhead settings) is calibrated accurately. toolhead_residual_filament in particular - how much filament is naturally left behind in the extruder after ejection - is often bigger than expected (2-3mm on a low-flow extruder, 12-15mm on a high-flow one), and is worth calibrating properly before reaching for the setting below as a band-aid.

toolhead_ooze_reduction is a small final adjustment on top of that, tunable live mid-print:

MMU_TEST_CONFIG toolhead_ooze_reduction=1

Blobs on the purge tower after a load mean the value is too low (more reduction needed); a big gap after a load means it's too high. A working value is usually small, ±3mm at most, once the toolhead geometry and toolhead_residual_filament are actually correct - needing a large or negative value is itself a sign one of those two is off, not that this setting needs to compensate further. Persist the final value in mmu.cfg once you're happy with it.

Commands¶

MMU_TEST_FORM_TIP                        # Run the tip-forming macro standalone, for tuning
MMU_TEST_PURGE LAST_TOOL=0 NEXT_TOOL=3   # Run the standalone purge macro for a specific tool transition
MMU_CALC_PURGE_VOLUMES SOURCE=gatemap    # Recalculate the purge matrix from current filament colors
MMU_SLICER_TOOL_MAP PURGE_MAP=1          # View the current purge volume matrix
MMU_TEST_CONFIG toolhead_ooze_reduction=1

Full parameter reference: MMU_TEST_FORM_TIP, MMU_TEST_PURGE, MMU_CALC_PURGE_VOLUMES, MMU_SLICER_TOOL_MAP.

Printer variables exposed¶

printer.mmu.toolchange_purge_volume, .slicer_purge, .slicer_tool_map.purge_volumes, .extruder_filament_remaining, and .filament_remaining/.filament_remaining_color (residual/cut-fragment tracking) - see Printer Variables: extruder / filament remaining. printer.mmu.action also includes Forming Tip/Cutting Tip/Cutting Filament/Purging states while each of these runs.

Troubleshooting¶

• Tip forming needs 40-50 attempts and still isn't right - normal; tuning by feel is genuinely slow, which is exactly why a filament cutter exists as an alternative. Change one variable at a time.
• Blobs on the purge tower after a load - lower toolhead_ooze_reduction, but check toolhead_residual_filament and the toolhead's own geometry settings first if the needed change is large.
• Big gaps on the purge tower after a load - raise toolhead_ooze_reduction instead.
• Wasting a lot of filament on purging - review whether you're using the slicer's simple per-tool volumes (least efficient) versus the full matrix or Happy Hare's own color-based calculation.
• Switched to a toolhead cutter but tips are still being formed the old way - form_tip_macro needs to actually be changed to _MMU_CUT_TIP; simply having MMU_HAS_TOOLHEAD_CUTTER fitted doesn't switch it automatically unless that was the default chosen in menuconfig.

See also¶

• Command Reference: MMU_TEST_FORM_TIP
• Command Reference: MMU_TEST_PURGE
• Command Reference: MMU_CALC_PURGE_VOLUMES
• Command Reference: MMU_SLICER_TOOL_MAP
• Macro: Blobifier / Macro: Servo Cutter - Blobifier and the servo cutter build/wiring
• Feature: G-code Preprocessing - !purge_volumes! and friends
• Feature: Gate/TTG Maps - the slicer tool map this all reads from
• Feature: Filament Bypass - the tip-forming tuning timesaver
• Feature: Sync-Feedback Buffer - sync_form_tip/sync_purge