Multi-color printing isn’t one invention — it’s at least four different answers to the same problem. The color is in the strand of plastic the printer melts, commonly called filament. Changing colors means swapping what filament is being printed, mid-print, without making a mess. And there are four main types of machines, each handling that swap its own way.
The most commonly available type of machine uses one nozzle, with a feeder that swaps filaments in and out. Every swap leaves old color sitting in the nozzle, so first it flushes: the old color gets pushed out through a chute in little pellets — makers call it printer poop — and that’s where most of it goes. How much depends on the colors: dark into light flushes the most, because dark pigment lingers — swap black for white and you get the biggest flush; swap light for dark and it barely needs any flushing at all. Then it lays a few lines on a prime tower, which stabilizes the nozzle pressure so the filament layers go down even every time. The tower grows layer by layer right alongside the print, and it ends up exactly the same height as the finished print. Add up the flush and the tower, and every swap costs roughly half a gram of plastic. Sounds tiny — but a busy print swaps hundreds of times, and on some prints the waste pile ends up weighing more than the print itself.
A tool changer skips the purge entirely. It parks the whole print head and grabs a different one, already loaded with the next color. No shared nozzle means no old color to flush out — but the fresh nozzle still needs its pressure stabilized, so it lays a few lines on a small prime tower first. Same tower, a fraction of the waste — and a machine that costs serious money.
Dual-nozzle printers split the difference: two nozzles take turns while the idle one waits off to the side — each keeps its own filament, so there’s nothing to flush between swaps.
And the oldest method needs no hardware at all — pause the print, swap the filament by hand, push ten or twenty centimeters through until the new color runs clean, and your colors change layer by layer — each color starts exactly at the layer where you make the swap.
That’s the swap, four ways — every machine just trades something different to make it happen.