A DIY 3D printer that’s upside-down, on purpose? The Positron introduction.

Here's the Hackaday article.

Here are comments from people who tried it before:

Bridging works a bit differently upside down, sometimes better, sometimes worse. But usually the same.

Turns out all of the forces related to viscosity, nozzle pressure, stretching of the semi-molten filament, etc., are all much greater than gravity, so all of the aspects of FDM printing (oozing, bridging, curling, etc.) are virtually unchanged when you print upside-down.

I’ve been waiting for him to finish his design for quite a while. But it looks like he just put a video out a few days ago where he showed he rebuilt it twice since he made that video and it looks like there are many improvements.