Flushing Volumes: The Complete Settings Guide
Every color change on an AMS or multi-material printer purges a slug of molten filament to clear the old color out of the melt zone — that slug is the flushing volume, and it's where most multi-color waste comes from. Set it per pair instead of accepting one blanket number and you can cut the pile of purge waste dramatically without ever seeing a red streak in your white parts.
What flushing volumes actually are
When the printer swaps from filament A to filament B, the nozzle and melt zone are still full of A. The slicer commands a purge — measured in cubic millimeters (mm³) of filament — to push the old material out before B starts printing anything visible. Flushing volume (you'll also see it called purge volume) is simply the size of that purge.
Crucially, it is not one number. Both Bambu Studio and OrcaSlicer store a matrix of values, one for each from-color → to-color pair. Switching from white to black needs almost nothing — a trace of white inside black is invisible. Switching from black to white needs a lot, because even a whisper of black pigment tints white gray. On top of the matrix sits a single multiplier that scales every cell at once, which makes it the quickest lever when a whole print looks slightly contaminated or wildly wasteful.
Where to find the setting
In Bambu Studio, open a project in the Prepare tab and look at the filament list in the left panel. Next to it sits a “Flushing volumes” button that opens the matrix dialog: a grid of from/to values in mm³, a Recalculate action that derives values from the filament colors, and the global multiplier field. OrcaSlicer inherits the same system — the flushing volumes button lives beside the filament selection in the Prepare panel and opens an equivalent matrix-plus-multiplier dialog. In both slicers the values are saved with the project, so a matrix tuned for one color combination doesn't silently follow you into the next project with different filaments.
How the auto-calculated matrix works — and where it fails
The auto-calculation looks at the color you've assigned to each filament slot and estimates how hard the transition is: similar colors and light-into-dark pairs get small values, dark-into-light pairs get large ones. As a starting point it's genuinely good, and you should always hit recalculate after changing any filament color rather than editing a stale matrix.
Its blind spot is pigment strength. The algorithm only sees the RGB swatch, not how aggressive the actual pigment is. Reds, blacks and heavily saturated blues cling to the nozzle walls far longer than their swatch suggests, and whites, naturals and transparents show contamination far more readily than their swatch suggests. So the pairs that need manual raises are predictable: anything → white, anything → clear/natural, and red or black → any light color. Raise those individual cells; don't punish the whole matrix with a huge multiplier for two bad pairs.
Numbers that actually work
Auto-calculated cells typically land in the low-to-mid hundreds of mm³. Sensible working ranges once you start tuning:
| Transition | Typical flushing volume |
|---|---|
| Light → dark (white → black, yellow → blue) | 100–200 mm³ |
| Similar mid-tones | 200–400 mm³ |
| Dark → light (black → white, red → natural) | 500–800 mm³ |
| Anything → transparent/clear | High end, often 700+ mm³ |
| Global multiplier | 1.0 default; 1.2–2.0 when colors bleed everywhere |
The trade-off is blunt: every mm³ you flush is filament in the waste bin (the “poop” pile behind a Bambu printer), and every mm³ you skimp is a risk of muddy color for the first centimeters after a swap. Tune toward the smallest values that still give clean transitions on your filaments — pigment loading varies between brands, so a matrix that's clean with one brand's black may streak with another's.
Settings that interact with flushing
- Prime tower. The tower exists to stabilize pressure after a swap; part of the flush can be directed into it. A wider tower gives more purge real estate per layer — if transitions look dirty only on short layers, widen the tower before raising the matrix.
- Flush into objects' infill / flush into objects. These options recycle purge material into sparse infill or entire sacrificial objects instead of the waste chute. Safe when the infill is fully hidden, the materials are compatible (same polymer, similar temperatures), and the object isn't light or transparent — contaminated infill shows straight through thin light-colored walls and clear parts. Never enable it for a white or clear model.
- Filament change G-code. The machine's change sequence performs its own ramming and cut before the matrix flush runs, so the matrix is additional purging on top. If you've customized change G-code, remember the two purge together — don't double-count.
- Printing order. Where you control color assignment, order swaps light → dark within a layer when possible. A layer that goes white → yellow → red → black flushes a fraction of what the reverse order costs.
Material caveats
Flushing assumes the two filaments melt and shear the same way. Mixing TPU with PETG (or other chemically unlike pairs) in one print is risky regardless of flush size — residue of one can weaken or jam the other, which is why we treat multi-material TPU as its own topic in the TPU on AMS guide. And wet filament purges badly: moisture foams the melt, leaving bubbles of old color clinging in the nozzle so transitions stay dirty at volumes that were previously fine. If a proven matrix suddenly needs 50% more, dry the filament before touching the numbers.
Start here: the tuning ladder
- Assign accurate colors to every slot and hit recalculate. The auto matrix is only as good as the swatches it's given.
- Print once at defaults and note which specific transitions look dirty — almost always dark → light pairs.
- Raise only the dirty cells toward 500–800 mm³ (dark → light) and lower clean light → dark cells toward 100–200 mm³.
- Use the multiplier (1.2–2.0) only when everything bleeds — e.g. after switching to a heavily pigmented brand — and drop it back to 1.0 once per-pair values are dialed.
- Recover waste last: once colors are clean, enable flush into infill on dark, opaque, same-material objects to claw back filament.
Troubleshooting: symptom to setting
| Symptom | Setting to change |
|---|---|
| Old color streaks after a swap, one pair only | Raise that single matrix cell 100–150 mm³ |
| Every transition slightly muddy | Multiplier to 1.2–1.5; suspect wet filament first |
| Huge purge pile, colors already clean | Lower light → dark cells; multiplier back to 1.0 |
| Gray tinge inside white/clear parts | Disable flush into objects for those parts |
| Dirty transitions only on small layers | Widen the prime tower |
| Proven matrix suddenly insufficient | Dry the filament; check for a brand/pigment change |
Frequently asked questions
How much flushing volume do I need per color change?
It depends on the pair. Light-to-dark transitions like white to black need only 100–200 mm³, similar mid-tones 200–400 mm³, dark-to-light like black to white 500–800 mm³, and anything into transparent or clear often 700+ mm³. The global multiplier stays at 1.0 by default; raise it to 1.2–2.0 only when every transition bleeds, then drop it back once per-pair values are dialed in.
Why are my color transitions dirty even after recalculating flushing volumes?
The auto-calculation only sees the RGB swatch, not pigment strength. Reds, blacks and saturated blues cling to the nozzle far longer than their swatch suggests, while whites, naturals and transparents show contamination more readily — so raise those individual matrix cells manually. And wet filament purges badly: moisture foams the melt, so if a proven matrix suddenly needs about 50 % more, dry the filament before touching the numbers.
Should I enable flush into objects’ infill?
Only as the last step, after transitions are already clean. It’s safe when the infill is fully hidden, the materials are compatible (same polymer, similar temperatures) and the object is dark and opaque. Never enable it for white or clear models — contaminated infill shows straight through thin light-colored walls and transparent parts. Used on dark, same-material objects it claws back real filament from the purge pile.
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