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.
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
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.
We do casting in a multitude of metals,finishing, model making for
designers, jewelers and people in the trade