Hi all,
yesterday there was some discussion about casting. An old master
goldsmith told me that in making models for sand castings, he had to
provide for expansion of the castings (making the models smaller) as
opposed to the models for vacuum or centrifugal casting. But another one
said it were not true, but sand castings would shrink also. Is anybody out
there able to verify the one or other opinion from his/her experience?
Markus
In a message dated 97-04-13 05:43:16 EDT, you write:
but sand castings would shrink also. Is anybody out there able to verify the one or other opinion from his/her experience?
Markus
Almost all types of metals will shrink when cast. It actually has nothing
to do with the casting process but more to do with the melting process.
When you increase the level of energy in a piece of metal with heat it
causes the outer layers of the elements electron shells to become unstable
and expand. This causes them to bond more easily with other elements. A
prime example is silver will bond with copper to form sterling silver when
heated. It will also bond with anything else with a few extra electrons to
spare such as sulpher and oxygen. This gives you porosity after repeated
heatings. But back to my point. This energy causes the metal to expand and
it will stay that way until it is allowed to cool. When you pour the metal
into a mold it is going to shrink. The rate of shrinkage is more pronounced
the thicker the piece is. For instance your shrinkage rate might be as much
as 8%. If the section in question is 1 mm thick the shrinkage will only be
8% of 1mm or .08mm. If the area is 4mm thick then the shrinkage could be as
much as .32mm. The ways to combat this situation is to ensure enough of a
button to feed the piece and heep the button molten while the casting
itself cools. This allows the casting to draw upon the molten metal above
to fill any void created by shrinkage. This works best with vacuum casting.
If the button cools to rapidly it can actually draw some of the metal back
out of your mold in rare cases. All metal will shrink as it cools with very
few exceptions for some of the rare earth elements, and one of the
radioactive isotopes. RED
Well, I did a lot of sand castings for signet rings which we hammered into
shape and sold to crest engravers. My experience is that the ring casts
much thicker, probably due to some sand burning. The gold also shrinks as
usual. The rings used to come out pretty thick, a size 10 model would cast
at about a size 6 or larger. Geeze, that was, well, probably about 22
years ago, and they say the first thing to go is your memory… 
So the
ring may have cast at a six or seven, but it cast really thick. After an
hour or so of hammering to shape, then another hour or more of filing, we
probably produced about the best signet styles in the world. The hammered
gold engraved like butter, the engravers preferred our more expensive hand
worked rings over the centrifugally cast patterns because of all the
hammering I guess. The certainly don’t make 'em like they used to, ya
know?
Jeffrey Everett
Almost all types of metals will shrink when cast. <
Thank you for your explanation. The shrinking of metals is well known by
me (it should, as I actually teach that stuff);-). Perhaps I didn’t
succeed in making clear what I wanted to know. Try again: The properties
and the behaviour of metals stand out of question, they are the same
whatever casting method is used. What the old man told me gave me some
thought on the casting process. He showed me some models, and I could see
that wires were a bit on the thin side, for example. I had no reason to
doubt the experience of someone with 50 years in the trade. Perhaps the
behaviour of the sand, from any dampness or something could cause this
expansion. As I wrote in an earlier message, there are casters who can
actually enlarge a model by injecting wax into a rubber mold that is
treated so it will “give”, whereas wax by itself contracts in cooling like
metals. So I came to the impression that there was something similar with
sand casting, i.e. not the metal expands, but the sand shrinks, this way
“enlarging” the void that fills with metal. I’d now like to know if
someone has made similar observations in sand casting (Thanks Jeffrey, on
this occasion). Hope this will make clear my question. Markus