There’s an episode of How It’s Made that shows the Heller factory producing a few different kits, including showing moulds and how they’re actually used. Of course, finding that particular episode may be a bit difficult 
Looking for it now, I can’t find it, but there is a good YouTube video about how Bandai makes gundam kits:
Not the greatest quality, but you can see the whole process. Or much the same at Zvezda:

Looking for it now, I can’t find it, but there is a good YouTube video about how Bandai makes gundam kits:
Not the greatest quality, but you can see the whole process. Or much the same at Zvezda:
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