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Increase casting shrinkage

Everything about lost wax casting seems to be about keeping the
shrinkage to as little as possible. I have a situation on my bench
that would work out a lot better if I could increase the shrinkage.
A ring that a customer wants narrower and smaller. Is there an
investment or some other trick that increases shrinkage?

Stephen Walker

One thing that we’ve used is to make a mold, cast the piece in
sterling, then make a mold of that. By using different kinds of
rubber or RTV, which have different shrink rates, you can alter the
shrinkage to suit.

Is there an investment or some other trick that increases

Rio sells (sold?) stuff called Reducit that you can either make a
mold of or inject into a mold, but it shrinks a LOT, and the
surface, after shrinkage, is somewhat rough.


Make a silicon mold of the ring, take wax impression, cast it, does
not have to be gold, clean the casting and if you still want more
shrinkage repeat the process. For every cycle you should reduce 10 to
20% depending on the caster, rubber used for mold and etc.

You could try to reduce it with Reducit


1). Make a vulcanized rubber mold of the piece you want to “shrink”.
2) Shoot a wax copy from that mold. 3). Clean up wax model, weigh and
sprue; invest, burn out and cast in the metal desired. 4) After
casting, do whatever cleanup to the casting as is necessary. By now,
it will have shrunk approximately 15%. If you need more, do 1-4

Chris van Laer

I don’t have the original post but… Make a mold of the piece then
use that to form a piece in original PMC which shrinks a great deal.
Then make a mold of the result after you’ve baked the PMC and you’ve
got a much smaller piece.


Thanks Chris and Neil for your suggestions. Apparently my casting
technique is too good, because I am not getting much shrinkage at
all. I was hoping that someone can say that a certain brand of
rubber vulcanized at a specific temperature will shrink the most or
that x brand of investment mixed at a certain way will shrink the
more than the others do. Yes, I could make a new model from scratch
or I could expremiment with different materials and figure it out
for myself, but I suspect that someone has a slick trick to save me
the trouble.

The Reduc-it compond sounds very useful for some things and I will
probably try it, but it does not help much for this particular
situation because all I want is a little more shrinkage than usual.
I am curious if anyone has tried Reduc-it in vaulcaniszed rubber
molds or if it is at all useful for rings?

Steve Walker

When I’ve wanted lots of shrinkage from a metal model I use a rubber
such as Castaldo Gold, vulcanize and then quench the mold in water
until it’s cool. I’ve found that this increases shrinkage. Repeat if


Dear Stephen,

If all you want is a little more shrinkage than usual, you could try
some of the “tricks” that most people use everyday to create
shrinkage without knowing it.

Some simple ways, for example, include increasing the injection
temperature of your wax or using a long, thin injection sprue (and
thus reducing wax flow during injection). Attaching your sprue to the
thinnest part of your model will help a lot, too. Those three work
every time.

Michael Knight

Thanks to all of you who contibuted to this thread. Here is what I

Vulcanised Yellow Label rubber at 345 F (308 F is recomended)
Quenched the mold in cold water.

Heated the wax 15 F hotter than usual.

Results: ring shrunk from finished model to finished 14K ring by
about 7%. Good enough! (10% would have been my target)

BTW Castraldo products are very consistent. The wax made from a
white label mold, vulcanised at 308 F and air cooled was only 0.1 mm
wider than the mold I pushed to get maximum shrinkage. The model was
6.9 mm wide, the waxes 6.8 and 6.7, the casting 6.65 and the
finished ring 6.5.

Stephen Walker