Injection molding is something hobbyists do once in a while. Plans for the machines are not hard to find (Vince gingery wrote the book most use). Unlike 3d.printing though, you can't go from CAD to a part directly, instead you need to build the mold and that is not easy makes the process a lot more difficult and it needs more tools (a milling machine) and skill.
Still the results are a lot better than 3d printing and it isn't as hard as you might think. It is worth looking into, to see if maybe it is for you.