There is a quite simple technic that you can use. With electro plating you can create a metallic layer of zinc or copper or nickel on you part. If you make the deposition thick enough it can make a mold. You will need to put some conductive paint on your plastic part first as it is not conductive.
Conductive paint : ~14€/400mL.
Copper Sulfate : ~20€/1kg
Car battery, a pair of crocodile pincers.
And then you remove the plastic by heating. It's easier if it is wax, but perhaps it would work as well if you do it with a soluble material like the ones for support.
However, it only works if the end material is supple enough to be extracted. Otherwise you need to cut the design to make separate mold parts.
CATIA has excellent molding design tools.

On a different note, for the release agent. If your part is big enough, most composite manufacturers cover their big (read costly) molds with teflon adhesive or a teflon coating. I never saw it in Europe but in China you can find it in spray cans.