Glossary · Function

3D printing

Poop and prime tower (purge)

When a printer changes colour in the middle of a print, it first has to push out the old colour. This waste ends up as small plastic worms behind the printer or in a small tower printed alongside the actual part.

Explained in more detail

When changing colour or material, the hotend and nozzle still hold molten material of the old colour. It is pushed out until the new colour comes through cleanly. Bambu Lab printers purge at a spot at the back of the build volume and eject the material through a chute, where it lands as flat "sausages". Alternatively or additionally, the slicer places a prime tower (purge tower or wipe tower) next to the part. It purges at every change and steadies the material flow after the change. Printers without a chute, such as those with the Prusa MMU3, purge entirely into the tower. Bambu Studio and OrcaSlicer calculate the purge volume per colour pair in mm³, and going from dark to light takes much more than the reverse. For parts with many changes, the waste can weigh more than the part itself.

Where does the term come from?

In English, "to purge" means to clean or cleanse (from Latin purgare), and "to prime" means to prepare, like filling a pump before it delivers. "Poop" is colloquial English for excrement, a jokey nickname from the community because of the shape of the ejected sausages.

In practice

• In Bambu Studio and OrcaSlicer enable "Flush into objects' infill", in PrusaSlicer "Wipe into this object's infill": the mixed colour ends up invisibly in the infill.
• Supports or an additional sacrificial object on the plate can also take up purge material instead of throwing it away.
• Reduce the purge volume only carefully by factor and test: too little gives colour streaks, especially when changing from dark to light.
• Printing several parts at once spreads the changes per layer and noticeably reduces the waste per part.
• Arrange colours layer by layer rather than mixed, and print light colours before dark ones.

Not to be confused

Not to be confused with the skirt or the cleaning line at the start of a print: these only serve to prime the nozzle. A prime tower is also not a support structure; it stands free beside the part.

Frequently asked questions

How can I save filament in multi-colour printing?

Purge into infill, supports or a sacrificial object, print several parts at once and arrange colours layer by layer. You can carefully reduce the purge volume as long as no colour streaks appear.

Can I switch off the prime tower?

On printers with a chute, such as those from Bambu Lab, this is often possible. There the tower mainly serves an even material flow after the change. Without it, the first few millimetres after a change can turn out untidy. Printers without a chute need the tower.