It's already a thing in orca and prusaslicer at least

No idea if if ever took off, but e3d had a hotend that could mix 2 filaments. I've never seen one in person, but messed around with the chimera a bit when it was first launched.

https://www.filastruder.com/products/chimera-cyclops?variant=1035850192

It's not here yet that I know of. But I feel like it's going to get figured out in a couple of years. I daydream I had was not to blend but to have a filament that would change color with different temperatures.

Binder jet printing is an older 3D printing technology in which adhesive is sprayed on layers of powder to build an object. A mix of ink can be included in the spray to produce detailed color combinations.

https://en.wikipedia.org/wiki/Powder_bed_and_inkjet_head_3D_printing

I feel like I definitellyyy saw a CMYK printer in a VOD of some printing convention a couple years ago? It was presented by some company and looked huge and expensive, wish I could remember who they were.

What I'd love to see even more than that is some kind of pigment cartridge system. You'd use natural, unpigmented filament and the printer would inject pigment in different mixtures as it prints. Just seems... I dunno. More efficient?

I remember seeing a YouTube video (that I might search for and find again if I remember) where some company had developed (and probably patented) an approach that involved inkjet-coloring as it printed. I remember the colors looked pretty pale and washed-out looking. Not vibrant the way I would have hoped. But the approach might be one that could be improved on. 🤷

It kinda depends on what your goal is. Is it making a custom filament? or are you thinking of blending colors on the go? And are you looking for commercially available solutions?

If you're looking for commercial solutions, then your guess is as good as mine. I know you can get filament recycling systems, where you crush failed prints and supports before then extruding usable filament. But I think that's not what you're looking for.

Going the tinker route instead, I think it was CNC kitchen that experimented with extruding their own filament by printing spiral of thick filament and then running it through a 1.75mm nozzle. They even got their logo embedded, and it was detectable in the finished prints layers.

But if you're looking to blend on the go then maybe some manual g-code manipulation and custom hotends could work. But given that I asked about experiences with dual input hotend like the one below, and all the answers were along the lines of "don't even bother", I guess that you having to do a triple or quad input hotend won't make it more likely to succeed.

You would need some mixing chamber. That holds liquid plastic and churns it to achieve the results.

Not to mention having all the filament the same plastic composition.

Injection molds sometimes get stuck plastic in the hearing unit, but it doesn’t mix well and is streaky. Without some staging in a chamber it likely wouldn’t work.

Even with clear filament and dyes, it would taking mixing to incorporate the dyes.