A lot of small volume tooling is made from Kirksite, an alloy that can be cast. The airplane guys use Kirksite as do the small run auto folks for their stamping dies. Kirksite also works for plastic molding,

http://www.armstrongmold.com/pages/rapidarticle.html

The nice thing about Kirksite is that you can melt it down after the part run. The bad thing about Kirksite is that people DID melt it down and the tooling is lost!!

mike in tucson