I’d like to cast the same style ring in several different sizes
using the lost wax process. I understand that I should expect 8-10%
shrinkage (according to the Caster I like to use) which would be an
average of 9% shrinkage. I’m casting in sterling.
For example, if I wanted to make a size 6.5 ring should I calculate
it in this manner?: 16.8 (internal diameter) x 1.09 = 18.3. 18.3
would be the increased internal diameter of a wax model since it
would shrink about 9%.
You should not worry about the shrinkage, smith the cast ring up to
the size you want after finishing the inside.
this will take out inconsistencies in the ring shank shape as well
as slightly work harden the casting.
any shrinkage you get will not decrease the size of the ring by much
more than a 1/4 of a size.
if there is an issue with more than 1/4 of a size, you simply anneal
the ring after sizing.
any wax that i carve or take from a rubber i try to cast slightly
smaller than needed and custom fit after pre-finishing. You end up
with a better fit and have not lost time doing complex calculations
that will never be correct. Thermal expansion and contraction can
only be accurately predicted if you are very consistent with the
amount of water that you put in your investment and then very carful
with the heat you use when burning out. Even doing this you would
need to keep a log of every casting you did and measure weights and
diameters that will match the equipment you are using.
In short don’t worry about it. 5 min with a smithing hammer and a
polished mandrel and you’ll have it perfect as long as it started
smaller than you needed.
On a separate but related matter, if I wanted castings that included
a bezel for a rub over setting, how much bigger should I make the
bezel to allow for shrinkage?
Eg if I want to set a 5 mm dia cabochon into the finished casting,
what should the internal diameter of the bezel be on the model.
I should not have asked about casting shrinkage in the original
post. I should have asked about mold shrinkage. This is my intended
question.
Something I didn’t add to the original post is that I’ll be making a
wax model I’ll use to make a mold that has 8-10% advertised
shrinkage.
Finally, using the mold to cast in silver.
I will not be doing the mold making or casting. Leaving this up to
my chosen professional casting company.
Ring shrinkage from original model to mold: A size 6.5 model ring
with an internal diameter of 16.8mm would be shrunk to a mold with an
ID of 15.3mm which is a bit smaller than a size 5.
What I usually do is put a narrow piece of tape around the stone
when I’m forming the wax bezel around it. It allows just enough
extra room that the bezel fits perfectly after casting. I find that
white artist’s tape has just enough adhesion to stick to the stone
but be easy to remove.
If you are only going to make 1-2 it is ez enough to do in clean up
If you are asking about production proto type on original model.
There is my 2 cents.
You are 2-3 stages from where you want to get. Your model will get
cast or molded then mold will get injected with wax and cast again.
Each step here will shrink in size from your original.
The shrinkage of 8-10% for molding sounds a bit much. For a
vulcanized (heat & preasure) molds using silicone rubber should be
about 4-5%. RTV (cold molds) are about 1-2%.
Then after casting there will be clean up which will add shrinkage.
Here is what I am doing now for anything that will get molded.
Print out 3 models all scaled in increasing sizes from 1.0125% to
1.05% and pick one that will give best result. Then cast and clean
up. Then mold.
contraction of this magnitude is typical for all matter (except
water) during transition from liquid to solid state. The only remedy
for this is to place the right size of sprue(s) on the right place(s)
in order to make the sprues solidify last and allowing liquid metal
to compensate the contraction of metal in the mold.
Another effect is the contraction caused by the physical property of
solid metals to expand when heated up or to contract when cooled
down. This is characterized by the coefficient of thermal expansion
(CTE) for all matter.
Your metal supplier can provide you with the CTE for the metal you
use.
Solutions for this are achieved with special investments which
exhibit expansion by themselves during setting time - in addition to
the thermal expansion like any other matter. This extra expansion
will result in a larger cast piece which will contract to the desired
size. The only drawback is a stronger mold hard to break.
Phosphate bonded investments like Invest C/P or Invest RP of
Ti-Research are made for this purpose and are available from your
preferred supplier.
Orchidians interested in this topic are welcome at our booth # B6369
at JCK Las Vegas next week. May all your molds have a complete fill.