Multi-Material Purge Volume Calculator
Calculate purge volume needed for clean color transitions on MMU and tool-changer printers.
Covers bowden path length, nozzle volume, and transition count.
Every time a multi-material system switches colors, it needs to push enough new filament through the hotend to clear out the previous color completely. The volume required depends on how much of the old filament is still in the system.
The two contributors
The melt zone in the hotend holds a small but significant volume. For a standard 0.4mm nozzle with a typical melt zone, this is roughly 15-30 mm3. Clearing just this zone takes about 60-80mm of filament pushed through.
The bowden tube is the bigger factor on Prusa MMU2, Bambu AMS with PTFE path, and similar systems. The entire tube from the selector or buffer to the hotend inlet contains filament that has already touched the previous color. You do not need to push all of it out, but you need enough new filament to create a clean melt zone. In practice, about 40-60% of the bowden length worth of new filament is needed for an acceptable color change.
The formula
The purge length in filament millimeters is:
purge_length = 80 + bowden_length x 0.5
For a 600mm bowden path: 80 + 300 = 380mm of filament. Volume: purge_length x pi x (1.75/2)^2 = purge_length x 2.405 mm3
Light to dark vs dark to light. Dark-to-light transitions (black to white) need significantly more purge volume than light-to-dark. The calculator gives a standard estimate; add 50-80% for dark-to-light changes.
The total waste filament for the job is: volume_per_change x number_of_changes, converted to grams using the filament density.