Hiya Wotto,
I've done a lot of hobby-plastic casting 10+yrs ago, this was an offshoot form my Aluminium casting hobby, making bike parts and engineering prototype fittings etc.
The plastics i made were mainly for my (broken) slotcars, windows, bodies, wheels etc etc...
There's many ways to make the moulds, and these days there are "hobby plastic moulding kits" available (something i never had the privelage of).
Check out "injection moulding" on Google for a vast array of building your own injection system (or regular moulding too).
I've seriously been thinking about revisiting my skills to now make pinball playfield parts & ramps at affordable prices ;-)
Good Luck
MM.
