Shrinkage in casting

Jewelry Discussion
1

Would anyone like to comment on the amounts of shrinkage involved in
the following process: An original model to a rubber mold(using pink
Castaldo rubber) to an injected wax to a casting in (a) 14k yellow
gold and (b) 950 Platinum. We are trying to prefit a cabochon or
calibrated stone so that there is the minimum amount of trimming
before setting. We use heavy prongs or half(split) bezels in these
settings to allow for variations in the “calibrated” stones. Does
anyone have a formula that they find relatively accurate for
determining the size of the opening in the original?

Jim Malone
Diamond Point Metalsmiths
http//www.adirondackjewelers.com

2

Jim:

All shrinkage rates are not alike. Shrinkage depends on many factors,
one of these being the mold rubber used. The type of wax used and the
wax temperature, pressure, and cool down time, the type of metal
used, melt temperature, and cool down time, all affect shrinkage.
There is also the casting surface, that you will abrade away or
polish, that you must consider.

If you really want to know EXACTLY what the shrinkage rate is, try
this: take four equal lengths of square wire (I used silver). One
piece is 1 mm X 1 mm, one piece is 2 mm X 2 mm, one piece is 3 mm X 3
mm, and one is 4 mm X 4 mm solder these to a bar and sprue them. What
you now have looks like a fork, with four different sized tines.

Now, make a mold. Inject a wax, and measure the four square rods. How
close are they to your master? Now, cast the wax, and measure the
casting. How much have you lost?

You can now experiment with various mold rubbers, waxes, wax
temperatures and pressures, casting alloys, melt temperatures, etc.
Record your results. You may be surprised by how much variation there
is. You will also find that the four square rods do not shrink at
the same rate.

With this simple tool, you can now accurately predict shrinkage rates
for a variety of castings, and adjust the size of your master models
so that stones will fit perfectly every time.

Now, if I could only find perfectly sized stones…

Doug Zaruba

3

i have a formula that worked pretty good but it was more seat of the
pants than the figures might imply. for a rough figure assume 10%
shrinkage overall but the shrinkage on the bezel will be more on one
plane than the other. so to start you need to make the model about 2%
bigger all around in the bezel. i know that’s not ten percent but it
works on many models. this is a good starting point, however it is
more seat of the pants than the figures might imply. the best way to
determine the base shrinkage with your mold materials is to make a
test model run. to do this make a round bezel model and an oval and
any standard shape you use. don’t get fancy just a bezel and backing
plate. weigh the model without the spru attached and record. now
measure that width length and height of the bezels with a micrometer.
it has been my experience that the shrinkage on one plane is greater
than the other so if possible mold the bezels with the long side
towards the top of the mold then remold the models with the bezel 90%
rotated to the side of the mold. often times putting the model at an
angle rather than straight up and down will distribute the wax evenly
and remove this variable. you might want to try this too. do these
three molds to determine the best shrinkage pattern for your use.
mold up. now move the test run and move to the wax department. make
several wax runs carefully recording the temperature and injection
pressure of the wax machine. this is very important, i often found
that the biggest problem with model wt. or sizes changing after test
runs was due to the wax dept. increasing wax injection pressure to
speed up production. it is also true that a hot mold will tend to
"give" more than a cold one also adding wt and/or mass to a given
model. it is best to have a lot of molds for precision castings so
they don’t get too hot and stretch. mark each wax or spru tree so you
know which wax is which. It is best to keep each injection run
separate if possible and make at least ten of each. (P.S. if you make
the model out of sterling i can provide the shrinkage formula for 14k
wt.) now cast the models - take the castings and weigh - now measure
the width length and height of the cast bezels with a micrometer.
check all the castings of each run and add the results together then
divide by the # of castings measured. if you cast at least ten good
samples this is simplified cause you just have to divide by ten. the
difference between the two (model and casting) will tell you the
shrinkage that your process produces. there should be a difference in
the castings molded up /sideways and angled pick the one that is
closest to the shrinkage desired in the material that you are using.
this will give you a rough idea on your shrinkage table. variables
include model style overall, materials used, wax pressures casting
temps and many others. as i said before this is more a “feel” for the
shrinkage than a science these tests will give you a baseline to work
with that as time passes will become more accurate as you add
experience to your data. if you are able to use sterling as your
model material you can accurately determine wt. shrinkage in the
casting also. but to do this you need to make the castings out of the
finished material and weigh them as samples. that’s a whole other
world that i can post later if you like. (if you’re doing gypsy
settings or backless ones the shrinkage will be radically different).
as always this advice is what worked for me. if there’s a better way
I’m sure someone will post and i look forward to reading about new
methods that i don’t know. if you want more info I’ll be glad to do
this as a direct E-mail or a general post. but i will warn you of two
things. one i don’t have info on platinum but could advise how to set
up figures and two on account of “things” after next week i won’t be
around for at least two and maybe three weeks. kinda sick and might
not be able to post for a while.

Talk to you later Dave Otto

4

Dear J. Malone,

This is Michael Knight at CASTALDO.

Casting shrinkage is a perplexing subject that has probably driven
more people nuts than any other casting problem, including porosity.

The problem is that there is not one cause of casting shrinkage, or
two or three but about 10,000 causes.

Everyone thinks in terms of rubber shrinkage. Some people go further
and include wax shrinkage, investment shrinkage and some brave souls
even broaden their thinking to include metal shrinkage.

But now lets delve deeper : how is the piece sprued? A short fat
sprue will result in less wax shrinkage than a long thin one. Sprueing
to the thickest part of the piece will result in less shrinkage than
sprueing to the thinnest part. Multiple sprues will reduce shrinkage.
Gentle curves where the sprue meets the model will reduce shrinkage
and sharp right angle joints will increase shrinkage. Wax temperature,
wax pressure, injection dwell time, wax type, rubber vulcanizing
temperature, vulcanizing time . . . . . . and about 9,980 other
factors.

Under the circumstances, most people allow 5% to 7 % for shrinkage
and just leave it at that. Good Luck!