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Silver shrinkage


#1

I need to cast some plastic parts. I was planning on doing this in
silver. However, it is imperative that the castings be the same size
as the original plastic parts; there are three pieces that must fit
together when finished. These three pieces house another metal part
which is not being cast. Thus, if there is too much shrinkage, the
existing metal part, which must fit inside the three cast parts, will
not fit!

Evidently, this means that the silver parts shrink after they are
cast and removed from the investment, right? I have never considered
this before because the things I have made did not require that they
remain a specific size after casting. You know, when I made a rig to
a certain size, it usually came out about right. That is, it didn’t
require a great deal of filing to get the ring to fit. What, am I
being stupid here or something?

Anyway, how do I take shrinkage into account for casting these parts?
I have been told that Brass or Bronze shrinks less than silver;
around 2%. How much does silver shrink? Any suggestions out there
or is this a hopeless venture? Thanks for any suggestions that come
my way.

Mike


#2

Hi Mike, shrinkage depends upon many factors, investment temperature,
metal temp, shape, volume and size of part etc, shrinkage reservoirs
(feeds to part during cooling)…

You can get close to original sizes using a dental trick. Take
fibrefax tape (used to be asbestos but that is unnaceptable now) and
line the flask with this. As the investment expands during burnout it
crushes the tape to walls of the slower expanding steel flask and so
makes the hole where the wax model was bigger than it started out.
When your silver casts in place and shrinks it ends up at the original
size of the wax. Or so the theory goes…

Charles

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