Raw material for 3D printed injection mold
P20 mold steel is one standard. Hardened parts are required for long life, depending on the service and material (some materials are quite abrasive). 

You can get a small number of relatively poor quality shots out of epoxy if it is properly supported by a metal box. Your best bet if you want to include 3D printing in the equation is probably to use epoxy or lost plastic casting from a 3D-printed master. Aside from the requirements of core and cavity with sufficient strength, there are requirements for vents, cooling tubes (bubblers and such like) ejector pins and slides that complicate most real molds. Productivity demands a high heat transfer rate, for very small quantities of expensive parts, thermal design may be less important. 

Temperatures and pressures are very high in injection injection molding- high enough to melt the materials you mention, and the pressures are in the 10K PSI range, so a 4" x 4" projected cavity will have a pressure in the 80 ton region.

If you have sintered and filled metal 3D parts they may be suitable, but from the prices I've seen you'd be better off to use conventional machining. Finish is also very important if you want the part to come out of the mold, with hours of semi-manual polishing not uncommon. If you don't have fine surface finish you will need extreme draft angles.

The requirements can be considerably relaxed if you are molding soft parts such as the PVC or TPE overmolds on cables. The pressure is less and the finish less important because the plastic is soft, but the temperatures are still quite high. This is sort of a sub-category of injection molding and specialized machines are used.