Here's an overview of 3D printed molds for injection molding.[1] These aren't the first people to do this. The mold has to have a much higher melting point than the material being molded, of course.

Formlabs has a "high temp resin", melting point around 289C.[2] US$200/liter. This is for their stereolithography machine. Filament-based 3D printers, not going to work for this.

This makes sense for production shops that need to make a few hundred of something.

[1] https://www.3dhubs.com/knowledge-base/3d-printing-low-run-in...

[2] https://archive-media.formlabs.com/upload/HighTemp-DataSheet...