Making GOOD molds

I’ve been making jewelry for over 30 years, but something that has
always driven me nuts is mold making.

I have vulcanized molds for small runs for ever.

Bruce D. Holmgrain
JA Certified Master Benchjeweler, CAD/CAM Services

   I've been making jewelry for over 30 years, but something that
has always driven me nuts is mold making. 
   I have vulcanized molds for small runs for ever. 

As a response to my inane intro to a problem that I am having, let
me continue with a further intro to my problem and a question or two
for response.

As I was saying, I have worked with model makers and casters that
would make molds of everything with little or no care as to the
results of a quick mold production. As a result, crappy models
result. This is not a big problem with very small runs of rings that
may be touched up, but in the case of say, precision diamond links,
this can be disastrous.

I am currently making a diamond necklace with about 80 links
totalling 14 1/2 inches. These are little more than bezels with some
engraved features. I no longer have the patience to try to repair
each and every link. I want a good clean shiftless and full model. I
would like to be able to cast, clip, grind down the sprue and tumble
with a minimum of labor. Normally, this requires me to make numerous
molds with my fate being relegated to the forces that be. In my case
this really is an art. I would like to turn this into something more
of a science.

Currently, I use silicone or other RTV molds. I have made vulcanized
rubber molds for about 30 years. I get equal or better results with
RTVs and am currently using I currently use a hand pumped injector,
but just invested in a pressurized mold injector to eliminate
variable pressures. Mold slippage seems to be a problem with a lot
of molds. I cut or form locks in the molds with formers. Incomplete
injection is a problem that I hate to deal with. Increased pressure
and relief cuts tend to help the mold to breath, but can result in
more flashing.

I don’t think that I can name all of the problems associated with
attempts to make decent models from rubber molds. I would really
appreciate hearing how some of you have overcome some or even all of
the problems associated with precision models.


I would really appreciate hearing how some of you have overcome
some or even all of the problems associated with precision models. 

Bruce, perhaps the most annoying problems I find with rubber molds
are when the two halves of the mold shift laterally, so that parting
line is a step, rather than a tiny line on the surface. I’ve found
that the best way to deal with this is that in cutting the mold, one
tries to place the parting line at an edge or corner. Then, in
addition to this, the direction of the cut at that line is NOT
parallel to the plane of the mold, but rather, at a 45 degree to it.
If you imagine a simple flat band ring cut this way, the outer mold
parting line would form a ridge in the mold extending up and out
from the plane of the mold. Then an opposing cut farther from the
model takes the parting plane back to the centerline of the mold.
The result is that the mold has what I think of as a “dam” around the
models main “cavity” in one half of the mold, while the other side
has a sort of “moat” surrounding a portion of the mold forming a
convex plug that fits into a depression of the other side of the
mold with the model cavity in the center. What this does is to ensure
that there can be no lateral shifting of the mold halves to each
other. Only if the wax pressure is so high as to force the two
halves of the mold apart, making a wider parting line, do you then
get distortion at the line. Any relief cuts needed in the mold are
similarly planned and placed, so that their various fins or whatever
lock into place and cannot shift in the mold cavity. I often use at
least some relief cuts, in all but the simplest models. The mold is
then powdered with baby powder, getting the powder into the relief
cuts. Then holding the mold open to expose the cuts, I blow as much
of the powder back out again with compressed air. This will leave
just the faintest bits of powder in the cuts to allow air to escape.
I don’t powder the molds again, but if lubricant is needed, use a
small amount of silicone spray, which doesn’t rough up the surface.
Even with the compressed air, I generally expect the first couple
injections to have visible parting lines from remaining talc, but
after a couple shots, usually the mold works pretty well. Then the
main thing seems to be the patience to wait long enough for the wax
to fully set up, so one doesn’t distort or bend it when removing it
from the mold. Sprues attached to the model before molding should
ideally be the same that you will want for casting. Also, don’t omit
the step of examining each model before using it. With even the best
molds, including metal molds, some waxes don’t quite make it.
Easiest to catch them before you go any further. And with pieces
needing lots of multiple links, I find it easiest to inject and cast
a fair number of extras. That will allow you to easily discard any
that are sub par after casting.

Hope that helps.

Peter Rowe

Progress Machine and tool corp. offers a mold rubber. This is a grey
compound, and it has produced the finest finish on a injected wax.
The brand name is Akron jewelry rubber. Vey consistent waxes, less
mold release and powder. Waxes come out with a glassy finish. I
recommend using a wax called Plast-o-wax(Blue). I can’t give away too
many of my grandpa’s secrets if ya know what I mean, but he was a
great mold maker and came up with a way to solve all of those
iritating waxing problems. One day I will just sell the idea to a
tool company and nobody would have problems with getting detail or
over flashing, and nasty parting lines.

The best of waxes only lies in the best of molds…

Brad B

Hi Bruce,

I have had much better locks in my moulds these days, I cannot claim
the cedit for it as I was sent some moulds cut by someone else. what
you do is this, get a length of brass tube about 8-10mm in diameter
from a model shop, cut off 30mm and fileone end to a sharp edge
inside and out.Very important to do both. then put a handle on the
other end, a bit of dowel will do.

When you have vulcanised your mould you cut in at the corners
vertically , ie from a side that you put the plates you hold the
mould with when you inject , but only just over half way. when you
cut open the mould you go carefully at the corners until you get to
the plug you have cut . DO NOT CUT IT Go around ti and pull back the
bit with the hole and keep going as normal.

Sometimes I will put in an extra key like this in the middle for
extra location I find it very easy to do.

This gives moulds with very little flash , I reccomend arrowroot
powder for air release, and low temperature vulcanising silicone, I
dont know if it is available in the US

I hope this makes sense, just think of an apple corer.

Tim Blades.
Glos UK

Hi Bruce,

Rio Grande has a two tape video set Mold Making for Jewelry, that is
quite good.

A few things that may help:

Draw on your master model (metal or wax model) with an extra fine
red Sharpie Marker, exactly where you want your parting line to be.
This ink transfers onto the rubber. Now you can start cutting your
mold up from the sprue, and then cut the rubber along the inked
line. Now you have excellent control of where your mold separates.

For precise hinges: I make the mold with a steel pin (fit snug) in
the hinges. (The pin should be longer than the hinge.) When you cut
apart the mold, make sure the ends of the pin are embedded in one
half of the mold. After you cut the mold, remove the metal model,
put the pins back into the rubber and inject the wax around them
into the mold. Take the wax out of the mold, slide out the pins and

I hope this helps!

Kate Wolf, in Portland, Maine, offering wicked good workshops by the


Just thought I’d mention that if you don’t want to (or don’t have the
facilities to) make the tube mentioned below, then check with a
scientific Lab supply house for cork borers – the tools used for
cutting holes in rubber and cork stoppers (so you can stick glass
tubing, thermometers, etc. through them). You’ll probably have to
buy a set of graduated sizes, but who knows, you might find some use
for the other sizes too, once you get them!


Progress Machine and tool corp. offers a mold rubber. This is a
grey compound, and it has produced the finest finish on a injected
wax. The brand name is Akron jewelry rubber. 

Thanks for the kind words, Brad.

The premise that consistent product yields consistent results is
underlined by your kind remarks.

We devote all of our energy to perfect results, whether it be in our
natural rubbers or our silicones. Our products are all unique in
this sense. They never vary in shrink or hardness.

Our goal has always been Zero Defects, hence the name ZERO-D
Products, makers of Akron Jewelry Rubber.


Bill Mull

Zero-D Products, Inc.
Precision engineered materials solutions
Toll free 800-382-3271 in USA, Mexico and Canada
Phone 440-942-1150
Fax 440-942-2130


I’ll be trying out the dam/mote method. After reading your
description, I think that I may have seen this in the past, and may
even have a mold or two done by commercerial casters using this
method. Makes good sense.

I’ll be trying your idea as well. Sounds like some extreme locks.

For all. Thanks for all the thoughts and suggestions. I even got a
call from Zero-D products with some suggestions.