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Approximating casting shrinkage in Rhino


#1

Hi

I am designing a piece using Rhino.

It will go through printing then casting then rubber mould then
casting again. Talking about sterling silver. Generally the question
is : If I want for example wire to come out 1mm in diameter what
would I do it in Rhino? 1.1? 1.2?

Or if I want round bezel final wall thickness of 0.5mm what do I
make it in Rhino?

After printingit will come out with Cad slices. Some parts cant’t be
touched up while in wax. And after each process the surface needs
some work as well. Anyone has got experience to give me approximate
guidence.

Thanks


#2

Hi,

There is an article at MJSA by the owner of Casting House in Chicago
who did an extensive study on this subject. Contact Peggy Jo Donahue
through Facebook to get the link.

In my professional opinion, it is best to do your own research using
your own equipment. I have made several objects in Matrix/Rhino and
there are varying factors. After talking to the end users, clients
that is, I have seen results vary from casting heavier objects say a
25x30mm plate, 2mm thick, in Rhino, get cast and after finishing for
molding it dropped to 1.15mm thick. This happened because of two
reasons heavier object and casting was bad or over polished. The
lighter objects seem to fair better with less shrinkage. However,
you would have to go back to your original print job. Print it on the
highest resolution you can. Take an RTV mold of the printed model.
Now, cast the piece, finish, and do another RTV mold or use
non-shrink pink rubber. Compare the two models after shooting the wax
in both molds. Do this for a heavier object and a lighter one.
Determine the percentage of loss or shrinkage and make your models
in Rhino larger to compensate.

The printing machines only make what they are fed…your polygonal
stl or slc files.

Now that I have said all that…I would give yourself 10% larger on
the heavier items and 5% on the smaller ones. This will establish a
baseline for you. Knowing casting like I do, and have taught it at
GIA, you also get more shrinkage depending on when you cool the item
down. Follow protocol when doing small, large, and what metal you are
casting. There are countless articles on that alone.

I hope this helps.

Russ Hyder
The Jewelry CAD Institute


#3

Hey Joseph,

There are a lot of variables and really it depends more on what
machine you are growing on. Different machines give different
shrinkage rates. If you are having the model grown on a wax based
machine then you will experience the least amount of shrinkage as
they usually use a cocoon growing method and there isn’t any curing
involved. You will get a product that will come out almost identical
to what you made in CAD (and by identical I mean like .001 or less
difference and you may even see an expansion from what you
designed!) and extremely little shrinkage in casting over what you
would experience with a hand wax. If you are growing in a
resin/plastic however you will experience between.01 and.02 mm
difference because of the stretching and curing processes. They also
will shrink up to 3% more in casting depending on what investments
you use. If you are using this growing method I would recommend
increasing your overall design by 5% to accommodate for this. I am
sure you are aware of the normal casting and molding shrinkages. I
hope this helps! If you still have questions let me know what
specific machine you are using and I might have more specific data
that would be helpful.


#4

Thanks everybody for advice.

I talked to the company who will do printing and casting for me
their advice is to scale it up 3%. That is entering 1.03 into command
line.

Joseph