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Ed Huntress
 
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Default Strongest / Toughest material moldable in a Silicone RTV Mold?

"Roger Head" wrote in message
...
Hi Ed,

I know nothing about casting (and probably won't be doing any in the
forseeable future), but I've followed this thread and been intrigued by
your last comment "If you're using plaster, it chills too slowly for

ZA-8."

Why did you suppose he might be using plaster? And what does "chills too
slowly for ZA-8" mean/imply?


Tim is doing a lot of experimenting with melting metal and casting, and one
of the mold materials experimenters often try is plaster of Paris. Since he
was experiencing grain growth, I thought he might be using it.

Plaster insulates the melted metal and chills it relatively slowly. This
promotes crystal growth and weakness in the finished part, as well as a
coarse crystal structure where it fractures. To get fine grain and the
resulting ductility and, to varying degrees, higher strength, you have to
chill the molten metal more quickly than plaster allows.

Grain growth varies by metal type. Some metals are immune to the problem.
Zinc alloys vary widely in this regard, as do aluminum alloys. Generally
speaking, aluminum casting alloys, such as A356, have similar properties
whether sand-cast or plaster-cast in sections up to around 1/4". Anything
thicker than that will benefit noticeably from being cast in green sand,
because the water in the sand helps to chill the casting more quickly.

There is a hierarchy of chill rates for different mold types. Metal molds
generally chill quickest. Graphite molds are next. Green sand is next.
Petrobond and baked sand are next. Plaster and plaster-based investments are
the slowest (most insulating) of common mold materials.

Ed Huntress