I can fulfill your request. A friend of mine machines his own multi-part molds and has two injection molding presses. I have lunch with him every Friday, so after lunch, I will stop by his shop and take a couple of pics of some of his mold masters (he make most of them out of aluminum) and his presses and post them for you. He does mostly On3 and a little bit of On30. Lots of detail parts. Last week he was machining driver wheel center masters for a little Hawaiian Rwy 2-4-2. Once the mold is complete, he puts the mold in one of the presses and makes plastic parts. He then takes the injection molded plastic wheel centers and builds a "tree", bonding the sprues together. Perhaps as many as 30 or more wheel centers. Then he puts the "tree" into a slurry. When that sets, he burns out the plastic and spin casts the part "tree" in brass. This is sort of a condensed version of the process and I may have forgot a step or three.
Thank you Tim, this is sort of what I have in mind. I am realizing I need to be more specific. What I would love to see out pure curiosity is a picture of the cavity in a tool or mold into which the styrene is injected. Ideally it would be a RR subject or something similarly recognizable and not for example a tail light or bottle cap.
Bill Welch, who may be sorry he posted the request.