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No shrinkage silver casting


#1

i got molds made of wax carvings a few years ago, is there any way
to have no shrinkage of the silver casting that is made, i guess i
made my model too small, didn’t account for shrinkage, and the small
amount of size makes a difference to me, also, and worse, the #1
thing not to do, i gave away the original silver model, or do i
just ask daniel grandi?? dave


#2

I think Rio sells something called ‘no shrink pink’ vulcanizing
rubber…that is supposed to have zero shrinkage…but I’m not aware
of any way other than making a new mold from the original or a new
original to get back the lost size.

Jeanne


#3
i got molds made of wax carvings a few years ago,,, is there any
way to have no shrinkage of the silver casting that is made 

it’s a bit tricky since to start with, even if the mold itself had
no shrinkage, the wax you inject into it will shrink on cooling, so
it will tend to be slightly smaller than the original.

There are methods you can use to reduce the shrinkage, though.
Casting into a hotter mold will give a little less shrinkage, since
the investment expands as it gets hotter, so the metal is cast into a
slightly larger mold. But as that cools, there is then shrinkage
again of the mold, which can slightly compress the softer metals like
silver. But it’s still less shrinkage than you get with a cooler mold
temperature. However, the hotter mold temp on casting will then also
increase the risk of some types of porosity or other casting
problems.

Another, perhaps more effective method, is to use a dental
crystobalite investment. These investments have a higher thermal
expansion rate than the usual jewelry investments, since they’re
intended to compensate for metal shrinkage in casting in order for
cast dental inlays to still fit the cavity prepared in the tooth.
This still doesn’t help you with the shrinkage of the wax when
injected, but it can make up much of the difference. The dental
investments, because of the high thermal expansion, work better with
small casting flasks and a flask liner to cushion some of that
expansion.

Peter


#4

Dave,

As you have noticed there is shrinkage when making a mold. The
rubber shrinks, the wax shrinks, and the metal shrinks. Probably the
best solution is to re carve the models larger.

There are bismuth alloys which expand when freezing but not by much,
you might have to go through a multi step process. Way out side
thought is water really expands when frozen, a fast cold rtv might
be usable for a new mold but would take a while to cure below
freezing. (Just ignore me, I have spent too much time in a Vermont
winter, brain cells tend to ice up :slight_smile:

Jeff
Demand Designs
Analog/Digital Modelling & Goldsmithing
http://www.gmavt.net/~jdemand


#5
i guess i made my model too small, didn't account for shrinkage,
and the small amount of size makes a difference to me 

As a couple have said - you’re sunk. There’s ways to squeeze a half a
percent on it, maybe - Peter said it well. However, there was once a
model build-up machine, that was a glorified electroformer. You could
try electroforming, but it’s tricky - your ring size will shrink,
your windows and piercing will shrink in size, but in between will
grow in dimension. It’s about the only way to get significant growth
or account for shrinkage in a real way. No shrink molds are only
valuable when you have the model, obviously.

http://www.donivanandmaggiora.com


#6

On several occasions I used a contract caster the offered same day
service for platinum. These castings came out bigger because of the
type of investment that apparently expands just slightly. I didn’t
like this service because not only was it more expensive because of
the $50 up charge for same day, the castings were heavier, which in
platinum made a big cost difference and also since I had already
accounted for the usual shrinkage in my molds so now my rings were
over sized just a bit. But this material might be just what you need.
I have never used it myself, but I don’t see why an investment
designed for platinum wouldn’t work for silver. Sorry, I don’t know
what the product is called but perhaps an Orchadian who casts
platinum knows.

Stephen Walker


#7

This is a long shot but, if the design will allow it you could
section the thing in half then rejoin with a spacer in between. This
will only gain you in one dimension however and only in the location
you add the spacer. Really depends on what the item is and just
where/why you need the extra size.


#8

Hi all,

I wanted to use electroformede pieces with my designs, so I contacted
a company that does that. I was planning on starting with natural
materials, such as nuts and leaves and wanted to have hollow, empy
forms at the end. The guy I spoke on the phone with told me that’s
impossible, and the nut will stay inside the gold coating, trapped
there, and “why is this a problem? nuts are light!”…I read you can
burn the organic material from inside the final piece and it seems
only logical. Has anybody got experience with this stuff?

Thanks a lot,
Dana