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Ring shrinkage to the right size


#1

Hi everyone,

It sure seems to me like I am making all possible mistakes on my way
to offer my jewelry to retail stores. This time the problem is
SHRINKAGE.

I don’t have a lot of stones in my jewelry, and I already long given
up on the chance to create the perfectly sized bezel in wax, but I
seem to have an unsolvable problem with ring sizes and shrinkage.

I would like to offer my rings in two sizes (6 and 7). I’ve received
advice that I should create the model in whatever size, and then
size the ring each and every time in wax.

This doesn’t seem economical - assuming I’ll be able to sell more
than a few, resizing every time and then fixing the textures in gold
will take a lot of precious time.

So I would like to create a mold for each size, and be able to cast
the ring in gold in directly the right size.

Problem is - how do you make the mold the right size, if you get
unpredictable shrinkage that changes the size? Can Orchid members
share their method?

I have tried different casters using different kinds of rubber,
promising empty promises about the amount of shrinkage to expect.

My latest attempt was to get someone to use a rubber called “Zero”,
which is supposed to deliver zero shrinkage. And it actually did
produce a wax with very little shrinkage. Problem is: when the wax
was cast in gold, the final ring shrunk by about 2% (between the wax
and metal stages)!

Does this make sense to anyone? Is this the sort of percentage to
expect between these two stages?

Thank god for Orchid!
Jonathan Silo


#2

Hi Jonathon:

You need to calculate and allow for shrink in designing your model.

Keep in mind that even though 4X ZeroShrink mold rubber does not
shrink, the wax you will inject does shrink, no matter what you do.
Each successive step in the process provides additional shrink.

Shrink is more predictable if you use mold rubbers that offer
consistent hardness and shrink from batch to batch. Want me to tell
you who makes such consistent rubbers?

You need consistent wax as well. Select a Ferris wax and try to use
the same wax long term. I suggest Ferris because I’ve found it to be
most consistent in shrink and other attributes over the years. Keep
in mind that many distributors will private label Ferris waxes, so
you need to ask questions about sources. It is a good idea to buy
from a dealer who actually offers the Ferris brand straight up. Keep
in mind color has nothing to do with properties, unless the color is
being used to identify a specific grade.

Regards,

Bill Mull
Zero-D Products, Inc.
precision engineered materials solutions
www.zerodproducts.com


#3

Hello Jonathan ; Yes it makes sense this is what we do in large
volume production companies, and that is what I did for the last
twenty years.I ran the model and mold making Dept’ and here is what
we did;

For size 6 make your master alittle over 6 and a half,
for size 7 make your master about 7 and 2/3

Then make your molds, cast, and they should come out very close.

Frankenstein


#4

Jonathan,

Are you casting your own pieces? I ask because I ask my caster what
size to make the original in for shrinkage to a 6, 7, 8… etc… I
bring him the original and ask for his advice.

I’d rather the ring be a tad bit small, as I find it easier to
stretch the ring a little bit to make it the correct size,
especially if there is texture and you don’t want to have to cut and
size it in the metal.

I had to laugh when I read that someone told you to size each one
individually! Oh my, that’s all I would ever do and even then my
orders would be late.

I mold sizes 5-8 in almost all of my rings. I make the original wax a
size 10- mold it, get numerous waxes. I put that original and the
original mold aside for save keeping. I then size the waxes for their
correct size and make a whole other set of molds from these. So, I’m
working on generation #2 by the time I’m done.

Amery Carriere Designs
Romantic Jewelry with an Edge
www.amerycarriere.com


#5

Dear Jonathan,

Any good zero or low-shrinkage rubber mold material will do what it
promises to do – produce a mold with zero or little shrinkage.

But that’s not what you are really interested in. What you really
want is a zero or low-shrinkage CASTING.

And that’s not produced automatically by the simple choice of a mold
material.

Casting shrinkage (as opposed to mold shrinkage) is the result of
100’s of factors introduced by the mold maker and the caster. Most
of them are not even aware of the factors and just chalk it all up to
"variability". Others know the factors involved but feel they cannot
cope with them and still run a commercial operation.

The only answer I know of is to find a good caster, figure out his
shrinkage rates and size your waxes to give the results you want
using his services. At least that way you’ll have eliminated many of
the variables caused by using different casters and their different
procedures.

Michael Knight
Castaldo Products Mfg. Corp.


#6

Hi Jonathan and other interested parties,

Designing ring models for molding and casting can be done so that
you get exact size rings out of finishing.

It’s the entire process that needs to be taken into consideration
when designing ring models. My first advice is to talk to your
caster. That caster should not only do casting, but should also have
a full understanding of molding, model making and finishing to be
able to answer your questions.

As a manufacturer, we don’t use low shrink/ no shrink rubber for
production purposes so calculating shrinkage is an important part of
dayly life at our Factory.

We do use those rubbers for other purposes depending on the
requirements and the production volumes.

When we produce rings, we usually have to produce a multitude of
sized production models for the customer.

This can be done by hand or through the Cad Cam process.

By Hand…

We start by producing a Master Model.

The only use this master has is to be molded and then we make as
many Production Model rings from that Master Mold as needed and we
size them either 1/4 size or 1/2 size larger than the required
finished piece of jewelry. The 1/4 or 1/2 size will depend on the
type of mold rubber we will be using for production.

To accomplish this correctly, the Master Model needs to be approx
15% oversize in every dimension… this includes band thickness,
setting and prong thickness, bezel thickness etc.

You will loose approx 7% going from the master model to the finished
production model and the rest of the shrinkage will go from the
Production model to the final finished piece.

Assume you are going to make production models for size 5 to size 8
which are the most popular sizes (in the US) you should make your
Master Model a size 9 so that you simply need to cut out a section of
the shank on each production model to achieve the correct production
model size.

The other advantage to making a Master is that you can use the Head
or Top of the ring to make earrings, pendants and possibly bracelet
links that match the ring design by simply injecting another wax from
the master mold and doing your modifications.

Now, other areas that you need to know about will be the shrinkage
for stone settings for prongs and Bezels.specially if you want the
stones to fit the castings nearly perfectly without having to have a
master stone setter !!! As a casting company that does finished
products, we do this all the time and the model making side of this
takes time and a decent amount of experience.

Now that Production models and Molds have been made for the correct
shrinkage, we have to look at the finishing side of things.

In picking the shrinkage factors of the production model, you have
to understand how much metal you will remove from the inside of the
ring when you are doing your finishing. We take our raw castings clip
the sprue very close then put the ring on a mandrel and lightly tap
it with a leather hammer to make sure the shank is perfectly round.
The roundness is important as in the next step, we sand the interior
of the ring to make sure there are no partying lines or defects on
the inside. If the ring was not rounded as I just described, you
will have some trouble in the later part of finishing that will
require more time to rectify. After sanding the inside, we grind the
remaining nib of the sprue off the outside of the ring. Now, your
ring is ready for other forms of finishing… perhaps you may use a
combination of vibratory finishing and hand polishing. The
rounding,sanding and polishing of the inside of the ring can easily
take 1/8 th of a size.

Hope this is helpful.

Daniel Grandi
http://www.racecarjewelry.com
We do casting in a multitude of metals,finishing, model making for
designers, jewelers and people in the trade


#7

Hello Daniel Grandi w/racecarjewelry.com

Excellent discussion on what everyone should know about sending out
a casting! Thanks for sharing.

Hope to see you in Tucson at the Orchid Dinner, Judy in Kansas, who
has had good experience with Racecar Jewelry’s work.

Judy M. Willingham, R.S.


#8
Designing ring models for molding and casting can be done so that
you get exact size rings out of finishing. 

Oooh, good description, Daniel! If and when I ever retire from doing
casting in my own studio for models, this is just the kind of
service I’d like to have. Like, if I moved into a sailboat and became
simply a wax carver, for instance… Just dreaming…

M’lou Brubaker
Minnesota, USA
http://www.craftswomen.com/M’louBrubaker