It occurred to me that one way to potentially eliminate all filament swap waste from purge towers or Bambu-style filament “poops” is to instead do something similar to a “purge object” or a “wipe object” but where the object is… filament.

The idea was somewhat inspired by Stefan’s video from a few months ago, which first introduced me to the idea of printing filament: https://m.youtube.com/watch?v=CQ-N1fr4N0w

If the purge object is new filament, then you don’t need to figure out what to use it for immediately; you can store the mixed-color filament for later use until you have a genuine need for a color-agnostic or structural object. No more piles of fidget spinners you didn’t really want.

A few other thoughts:

• The filament probably needs a minimum bend radius so it can feed into an extruder without breaking. One option would be to print a circle or a rounded square / squircle type of shape toward the perimeter of the build volume (potentially multiple concentric ones, or spiralized shapes). Another option would be to print disconnected straight segments.
• The filament doesn’t need to be printed flat. It could potentially spiral upwards (likely with supports). That could help avoid print head collisions for taller models (in which case you might not want the head to have to get back down to <1.75mm from the print bed). In the most general case, the filament could be printed at arbitrary angles, even vertically, and could start at some height above the build plate (potentially supported by the object itself, for example in an object where color changes don’t start at the very bottom). Maybe the angle could even be optimized to provide the best match between purge volume per layer and volume of new filament printed on each layer.
• A good solution probably requires a good way to connect multiple segments of printed filament together
• Mixing materials rather than just colors seems like a bad idea to me (e.g. soluble supports) but I’m going to mention it as a possibility anyway; you could conceive of something inspired by Stefan’s composite material and intelligently organize the different materials within your new filament.
• You could potentially control the mixes / transition colors that go into your purge filament, or even choose multiple different new filaments for different color transitions. For example, you might have separate new filaments for red-green, green-blue, and blue-red transitions. Maybe you want it create particularly pretty new filament, or avoid particularly ugly combinations.

I don’t actually have a multi-color / multi-filament printer, and I don’t have time to experiment with this (even though the first prototype could be as trivial using a filament shape as the wipe object in PrusaSlicer). I’m mostly sharing this to establish prior art in case someone nefarious seeks to patent something similar in the future (which is also why I added some half-baked thoughts that might make other things become obvious to someone skilled in the art), though it’d be great if the idea is actually good and someone could implement it well. Or for all I know this does already exist.

Thanks for taking the time to read this! Feel free to repost/share/steal the idea if you like it.