The Rapid Prototyping Center at MSOE has several patents around creating mold casts with 3D printers. IIRC, one of their techniques allows building a tetra lattice coolant path around the part cavity, allowing more optimal cooling than traditional methods (increasing water turbulence, getting closer to the cavity face). They weren't printing the tools directly, but the tools could be made from the parts they generated.

That was about a decade ago, so I'm sure they've moved onto more exciting things.