Multi-colour printing: colours mix, purging wastes a lot of filament or the prime tower fails

Symptom

On multi-colour parts the light colour comes out stained by the dark one (for example, white turns grey next to black), purging wastes a huge amount of filament, or the prime tower falls over or comes out with blobs.

Likely causes, from most to least likely

1. Purge volume too low: after a filament change some of the previous plastic remains in the nozzle, and going from dark to light needs much more purging than light to dark.
2. The colours set in the slicer do not match the real filaments, so the automatic purge is calculated for other colours.
3. Special materials: transparent, soluble or additive-filled ones (very glossy, glitter) need more purging; matte ones a little less.
4. Purging into infill with few walls: the dark purged colour shows through light walls.
5. A tall, narrow prime tower, or one far from the part: it wobbles or falls.

Checks to tell which one it is

• Open the slicer's purge volume table and look at the dark-to-light changes.
• Check that the colours set in the slicer resemble the loaded filaments.
• Print a small sample with the problem combinations before the big part.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Match the slicer colours

   Assign slicer colours that match the real filaments so the purge is calculated correctly. Creality gives the example of slicing red and white but printing black and white: the result comes out mixed.

   Check: The purge values change when you adjust the colours.

   If that does not fix it: Increase purging for the problem change (next step).

2. Increase purging where colours mix

   Increase only the cell for the staining change in the purge table, or the overall multiplier. In Creality's example, the black-to-white purge goes from 755 to 850; for the K2 Plus it suggests a multiplier of 1 to 1.3. With transparent, soluble or glitter filaments raise it; with matte ones you can lower it a little.

   Check: The light colour comes out clean in the sample.

   If that does not fix it: Also make sure the purge does not end up visible (next step).

3. Purge into infill or another object

   To waste less, use the purge in the infill or in an auxiliary object; in PrusaSlicer these are «Wipe into this object's infill» and «Wipe into this object». If the purged colour shows through the walls, add perimeters.

   Check: Less filament is purged with no visible stains.

   If that does not fix it: Check the prime tower (next step).

4. Look after the prime tower

   Place the tower close to the part and, if it is tall and narrow, give it a brim or the stabilisation cone. If blobs appear in the centre of the tower, Prusa attributes it to the previous filament's unload; on the edge, to incomplete purging: update the slicer and its profiles.

   Check: The tower stays intact up to the last change.

   If that does not fix it: Contact your printer's support with photos of the part and tower and the G-code.

5. Empty the purge in time

   On printers that eject purge down a chute, check before starting that it is clear, do not cover it with accessories and empty the container often on long prints with many changes (Bambu Lab).

   Check: Purge drops without piling up for the whole print.

   If that does not fix it: If the chute clogs, check the nozzle wiper and the chute itself.

Prevention

• Keep slicer colours the same as the real filaments.
• Run a small test for every new dark-to-light combination.
• Use purging into infill or auxiliary objects to waste less.

Still not fixed?

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Printers it applies to

Any FDM (filament) 3D printer, from any brand.

Official sources

This guide summarises the official documentation linked here:

• Purging volumes (MMU) · Prusa Research
• Wipe tower · Prusa Research
• Flush Volume Setup Guide (Creality Print) · Creality
• Troubleshooting for K2 Plus Color Mixing Issue · Creality
• Handling the waste generated from the purging process · Bambu Lab
• Prime tower (Bambu Studio) · Bambu Lab