On Mar 22, 5:23*pm, AK wrote:
I want to make a small mold I would later use to create a fiberglass
container to hold lead weight. It would be nice if *mold material can
withstand 500 degrees Fahrenheit so I can pour lead into it and then
cover it by fiberglass. Alternatively I would use the mold to create a
fiberglass container first and fill it up with lead pellets. So my
question is what do I use to make a mold, gypsum if so what kind,
anything else?

Andrzej

High temp RTV rubber can withstand up to 1000deg F. It's not cheap
stuff, but it will work quite nicely...

If you use gypsum or plaster products, just make two molds, (after
your first mold is made make a male model from it to cast new females
off of...) as it would be difficult to get out your lead w/out
breaking it, assuming it survives the thermal shock of the pour. Heat
up your plaster mold in an oven to within 100deg of your pour first to
avoid that pitfall, but you must very slowly heat the plaster up since
it may have micro water in pores that will superheat and make it crack
as the steam exits. Take at least 4 hrs to bring your mold up to temp
(after 24 hrs minimum cure) , ramping up 100deg/hr until at
400ish... plaster is cheap and easy to work with, so it will
probably be most practical. note-plaster expands 1-3% when cured, so
take this into account for allowances if you have critical tolerances.

If you need precision plaster/gypsum there is a product called
Ultracal (in 30 or 60 minute cure versions) made by US gypsum that
does not expand or shrink. it's cheap enough to use too at like
$30/50# . It feels and mixes just like plaster of paris but is
gypsum based so it cures like cement. Phenomenal product actually...

There's always the option to just trap your lead shot in epoxy
thickened with cabosil or microballoons etc, which seems much easier
but will obviously not provide the same density as a solid block...

-Paul