Why? CNC machining provides better quality and is faster. You can CNC a simple mold in minutes with precision that exceeds any 3D printing technology.

At one point aluminum tools were a good trade off in time vs quality, but these days they only save maybe 10-20% of the tooling schedule and cost over steel. Tool life and robustness isn’t great so they’re not so popular any more. Usually when someone says “soft tooling” these days, they just mean softer steel which is slightly cheaper and faster to machine, at some trade off in tool life.

There are already 3D printed steel core and cavity inserts for injection molds. They laser sinter powdered metal and then run a very tiny ball end mill around each layer to clean up the surface finish. Traditional CNC and EDM is still how most molds are made, but occasionally it makes sense to 3D print. You can get cooling lines in patterns that are impossible to machine and you can make tools faster sometimes. The costs for the machines are very high, and the size of the parts are limited.

Why? You just 3D print steel ones.

You use the metal sintering printer to deposit the metal. Then you use an in-envelope milling head to polish it.

The cooling can be conformal since the shape isn't limited by milling technology. And, since molds have a lead time of at least 6 weeks (and generally worse because something always goes wrong), speed isn't the issue.

There was a good talk at Molding 2018 by the CEO of a Silicon Valley medical prototyping company about how his folks simply won't deal with conventional molds anymore and he has bought 3 of the 3D metal printers with in-envelope milling heads.

Unfortunately, the Molding conference website is a dumpster fire for useful technical information. If I can remember the name of the company, I'll reply.

High temperature resins can be 3d-printed and used for molds. These can be used for low-run injection molding or vacuumforming. Formlabs have some content on the topic: https://formlabs.com/blog/braskem-3d-printing-injection-mold...

Is there actual 3D printing of metals? Last time I checked there was a very difficult process of 3d printing resin-on-dust, then pre-baking the dust-resin form, then soaking it in molten metal. Also it wasn't aluminium.