Key takeaways
- Purge scales almost linearly with the number of colour changes. Cutting changes by 27 % cut purge by about 26 % in Bambu Lab's example.
- On small multicolour parts, purge can weigh many times the part: Bambu Lab has an example saving nearly 180 g of purge on a part of about 35 g just by laying it down.
- The quickest way to estimate cost: grams of purge reported by the slicer × filament price per gram. At 300,000 VND/kg, 83 g of purge is about 25,000 VND — for a cube.
- Orientation can cut purge by 80 % — or multiply it 22 times. Always slice both orientations before printing.
- Multi-toolhead or multi-nozzle printers eliminate most purge; makers publish reductions of 85–90 %.
A multicolour print on a single-nozzle printer has two weights: the part, and the plastic extruded and thrown away to change colour. The slicer reports both, but people usually only look at the first. This article is about the second — it's bigger than you think, and most of it can be cut without changing printers.
Where the plastic goes when you change colour
- 01
Flush
After retracting the old filament and loading the new one, the printer extrudes the old plastic left in the nozzle until what comes out is cleanly the new colour. This falls into the waste chute beside the bed.
- 02
Wipe
The nozzle sweeps across a wiper to shed what it just flushed, so it doesn't carry flecks of the old colour into the part.
- 03
Prime the tower
The printer prints a layer onto the prime tower — a sacrificial block beside the part — so flow stabilises before it returns to the part.
The flush volume isn't fixed. According to Bambu Lab, colour is the main factor: going from dark to light needs far more flushing than light to dark, because even a trace of dark colour leaves a streak. Material properties count too — when switching to a support material, for example, the printer flushes extra so support doesn't stick to the part.
How many grams, how much money
| Case | Colour changes | Purge | ≈ Purge cost |
|---|---|---|---|
| 4-colour cube, 0.2 mm layers | 153 | 83 g | ≈ 25,000 VND |
| Same part, 0.28 mm layers | 111 | 61 g | ≈ 18,000 VND |
| Tall ~35 g part, standing vs lying down | nearly 80 % fewer | nearly 180 g less | ≈ 54,000 VND saved |
| Cube, flushing into a sacrificial object | — | about 70 g → under 1 g | ≈ 21,000 VND put into another object |
Two things stand out. First, purge tracks the number of changes almost exactly: 27 % fewer changes, 26 % less purge. Second, on small parts purge easily weighs many times the part — in the tall-part example, the saving weighs five times the part itself. For a full cost including electricity and machine time, use the print cost calculator with the total grams (part + purge + prime tower), not just the part.
Six ways to cut it, easiest first
- Raise the layer height. Fewer layers, fewer changes. From 0.2 to 0.28 mm, Bambu Lab's example cut purge by 26 %. The trade is more visible layer lines.
- Reorient the part. Changes per layer are roughly the number of colours on that layer minus one. A tall, thin part whose colours change with height should lie down if the surface allows — Bambu Lab's example cut purge by nearly 80 % and time by 70 %. But if the colours are already stacked in bands, lying down can multiply purge 22 times. Always slice both orientations.
- Put several parts on one plate. Each layer still changes the same number of times, but the purge is shared across more parts.
- Flush into support and into sacrificial objects. Flushing into support is on by default in Bambu Studio. Add parts that don't need to look good — fixtures, parts you'll paint — with the flush-into-this-object option: Bambu Lab's example takes about 70 g of purge down to under 1 g.
- Flush into infill. Effective, but mixed colour can show through thin walls, especially with light colours and translucent plastic. Use at least a few walls.
- Lower the flush multiplier. Slicer defaults err on the safe side. A factor of 0.8–0.9 saves quickly, but test-print: under-flushing shows up as layers of mixed colour.
When it's worth moving to a multi-toolhead printer
Multi-toolhead or multi-nozzle printers keep each colour in its own nozzle, so purge almost disappears: Snapmaker publishes up to 85 % less, Co Print up to 90 %, Creality up to 85 % for the K3 — all from the makers' own tests. If most of your work is multicolour and the slicer keeps reporting purge as heavy as the part or heavier, the filament saving is a real line in the sums. If you mostly print single-colour parts, it's close to zero. The machines are compared in multi-toolhead 3D printers.
Purge isn't the cost of colour. It's the cost of colour changes.
One thing doesn't change whatever the printer: multicolour printing means several spools sitting in the open at once. In a humid climate, that's why multicolour prints string more than single-colour ones — see how to dry filament.
Frequently asked
How much filament does multicolour printing with an AMS use?
It depends on the number of colour changes, not the number of colours. Bambu Lab's example: a four-colour cube at 0.2 mm layers needs 153 changes and 83 g of purge. The slicer reports the exact figure for your part before printing — read it alongside the part weight.
How do I reduce purge when printing multicolour?
Reduce changes: raise the layer height, orient the part so the fewest layers carry multiple colours, put several parts on one plate. Then make use of the purge: flush into support, infill or a sacrificial object. Only last, lower the flush multiplier to 0.8–0.9 and test-print.
Can purge be recycled?
In principle purge is clean plastic, but it's usually mixed in colour and material, so it's hard to recycle into quality filament. The more practical approach is to stop it becoming waste in the first place — flush into infill, support or a sacrificial object.
Does flushing into infill weaken the part?
With the same material, not meaningfully — only the colour differs. The main risk is appearance: mixed colour can show through thin walls, especially with light colours or translucent plastic. Flushing between materials of different families is where adhesion between them becomes the problem.