This is already doable. look here for an affordable way of getting injection molding up and running. the life time for the molds are quite limited, but it is fairly cheap to print more than one. it will only cost the resin cost. The reason why resins are the right choice here is that thermoplastics will always have to be extruded at a higher temperature than what the castings are done at. resin on the other hand is not hardened by heating and cooling and so can have very different heat resistance