Multi-colour printing: AMS, tool changers and more explained

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At its most basic, an FDM printer has one nozzle and processes one filament. For several colours in one part there are therefore different solutions, from the simple pause to a printer with several complete print heads. They differ above all in how much time and material each colour change costs.

Colour change by pause: getting started

The simplest method needs no additional hardware. In the slicer you define at which layer height the colour should change. The printer stops there, you swap the filament by hand and resume the print. Technically this is usually the M600 command, which many slicers insert with a right-click on the layer slider.

This works well for signs, lettering or logos where the colour only changes from layer to layer. Several colours within the same layer are not possible this way. You also have to be at the printer at the right moment.

Filament changers with one nozzle

This is the most widespread system. An add-on unit holds several spools, usually four per unit, and feeds the matching filament to the nozzle as needed. Examples are the AMS (Automatic Material System) from Bambu Lab, the CFS (Creality Filament System), the ACE (Anycubic Color Engine), the QIDI Box or the MMU3 (Multi Material Unit) from Prusa. AMS is therefore a Bambu Lab product name, but it is often used for all of these systems. On some printers such a system is built in.

A colour change roughly runs like this:

1. The current filament is cut off or retracted.
2. The unit pulls it back out of the print head.
3. The new filament is pushed up to the nozzle.
4. The nozzle is purged until the old colour is completely gone.

The last step is the problem: hotend and nozzle still contain molten material of the old colour. It has to be pushed out before the new colour looks clean. This purge waste ends up either as small sausages behind the printer, affectionately called “poop” in the scene, or in a purge tower (purge tower or prime tower), which the slicer places next to the part.

Many of these systems also store the filament protected from moisture, and some actively dry it. Every change costs time. With hundreds of changes in one print, hours add up quickly.

Reducing purge waste

For parts with many colour changes, the waste can weigh more than the part itself. These levers help:

• Purge into the infill: The old colour is printed into the part’s filling, which nobody sees later. In Bambu Studio and OrcaSlicer the option is called “Flush into objects’ infill”, in PrusaSlicer “Wipe into this object’s infill”.
• Purge into supports: Support structures also end up in the bin, so the mixed colour can go there.
• Purge into a sacrificial object: An additional part on the plate, printed in mixed colours, for example a cube or filler material.
• Adjust purge volumes: Going from light to dark needs less purge material than going from dark to light. The slicers calculate the volumes per colour pair, and they can often be reduced with a factor. Lower them carefully and test, otherwise you get colour streaks.
• Save colour changes: Several parts on the plate at the same time share the changes per layer. Arrange colours layer by layer rather than mixed where possible.
• Choose the colour order so that light colours come before dark ones.

More on supports in Support structures, on the topic of filling in Infill.

Two nozzles

Some printers have two nozzles on one print head, of which one is active at a time. Each material has its own hotend. Switching between the two nozzles creates hardly any waste. A typical combination is part material and support material, for example PLA with water-soluble PVA. With an additional filament changer, more colours are also possible, but then each nozzle purges separately again.

With two nozzles, the height difference between them must be exactly right, otherwise the inactive nozzle drags across the print. Current machines calibrate this automatically.

Tool changers

A tool changer has several complete print heads, parked at the edge. The printer picks up the matching head, prints with it and puts it back. Each material has its own nozzle, so almost no purge waste is produced. The change takes only a few seconds.

This also makes it possible to combine materials that do not work through the same nozzle, such as PLA with TPU or part material with soluble support material. The drawback: tool changers are more expensive, mechanically more complex and the number of colours is limited to the number of heads.

Pros and cons at a glance

What not every system can do

• TPU is too soft for most filament changers. It can often only be printed through a separate input.
• Cardboard spools can cause abrasion in some units or slip. Adapter rings or rewinding onto plastic spools help.
• Fibre-reinforced filaments wear out feeders and are not approved in every system. Check the manufacturer’s information.

Do you need this?

If you only occasionally print a sign with two-colour lettering, the pause is enough. If you regularly print figures, logos or parts with soluble supports, a system is worthwhile. Which printers have which system is shown in the list Printers for multi-colour printing. How to colour models quickly in the slicer is explained in Colouring in multi-colour in the slicer.

Continue with: What does a 3D print cost?, to factor purge waste into your costs.