HotRod wrote:
I want to cast some basic copies of parts and then make some additions and
changes to the plastic mold. I will then re-cast a negative mold to create
the metal part. Right now I'm thinking of aluminum parts but right now this
is more hobby than anything. How would I tell if a product is made of
styrene or polyethelene and what do they melt like? Making an injection
system will be simple if thats whats needed.

Polyurethane of the two-part type are commonly used for casting limited
quantities of plastic parts. Rigid and flexible are available for
making both molds and casts. Some have fillers, some are fast to cure,
some are slow to cure, etc. Here is a link to one of a multitude of
suppliers: http://www.smooth-on.com. They have an informative site with
sufficient information to get you started.

As to injection molding, thermoplastic resins like polystyrene, need to
be heated uniformly, and not overheated or held to long at their
processing temperature, which might resemble cold honey, not very
pourable. Most recylable plastic products have a recycle symbol
somewhere with a number or abriviation like PS for polystyrene. Good
luck with your project.
--
Billy Hiebert
HIEBERT SCULPTURE WORKS
Small Part Injection Molding
http://www.hieberts.com