Instructables

How would I make a mold for Casting Plastic parts?

I want to make a scratch built custom transformer but I want to make a mold of some existing parts to make it so how would I go about doing that?

Uhm. I'd say Plaster of Paris. I have this book from the library on how to make your own shoes. The author made a mold of her foot by putting her foot in a shoebox (Ew. why does this sound awkward and a little disturbing?) and pouring Plaster of Paris over it. (By the way she does this with two pairs of stockings on. When she pulls her foot out one of the stockings stay in the mold. I think that's worth noting.) This makes a negative cast. Once this dries you oil the insides of the molds with cooking oil then pour more Plaster in. After this dries then you chisel away the negative mold. (In the book she uses a hammer and a flathead screwdriver... which makes me twitch. In middle school shop class my teacher always emphasized the importance of using tools for their intended purposes. Use a real chisel if you can.) After you do then you should end up with a copy of the parts you have. Though... I realize now that you'll probably end up with a plaster Transformer. o.0 I guess you could stop with the plaster at the negative mold and try to find other material for the positive mold. Actually... You know what, I complicate things! Try looking on the sidebar under "Related". Right now I'm seeing a bunch of mold making instructables.
Immersing body parts in plaster of paris is a *bad* idea. Thin layers to reenforce a mold or as used in bone setting are fine, but immersion is not.

When it sets, heat is generated. With enough plaster it is possible to cause burns serious enough to lead to surgery and and loss of affected body parts.

http://news.bbc.co.uk/1/hi/education/6485481.stm
http://www.independent.co.uk/news/education/education-news/schoolgirl-lost-fingers-in-plaster-of-paris-1801591.html

I know yours was an old comment, but I've just seen it and thought the safety aspect was worth a mention.