You think Lego are incredible, check out Gundam Plastic Models. They have runner's made of multiple colours, and ready assembled hinges that are cast in a single injection mold.
There's a comment on YouTube explaining how the ready assembled hinges are made:
> Michael Karnesis
> 1 month ago
> I'm coming here way after the fact even though I missed the boat on the first wave of comments because nobody has explained the leg inner frame correctly.
> The leg inner frame is molded in sequence in ABS plastic, which is semi-crystaline and only bonds to itself. The second plastic is in Polypropylene which is fully crystalline and also only bonds to itself. This creates working joints without having to have radically different thermoset temperatures because the two plastics chosen simply don't cling to each other. No additional coating of release agent is required at all, it just uses the plastics' innate properties.
They have to transfer the plastic between the two molds though? Any differences at all would cause problems. It's not like they can use release pins like the coloured runners?
I know nearly nothing about injection molding, but my understanding was that the part stays in the same mold but they inject a different plastic to fill a different cavity.
But I don't know enough about this process, by a large margin. I didn't even know co-molding was a thing.
The RG models in particular are insane!
Check out this video of Adam Savage making one: https://www.youtube.com/watch?v=xfmD1yYqP6k