These links may help:

http://en.wikipedia.org/wiki/Moulding_(process)
http://en.wikipedia.org/wiki/Injection_moulding
http://tfwiki.net/wiki/Mould

I tried Googling "DIY Moulding" and "DIY Moulding toys" but just the search results I got were highly inappropriate and irrelevant to toys... at least, the toys we're talking about (or at least I freakin' hope so!). Erm... nobody try Googling that if you're at work or in front of company. Holy Nexus Maximus...