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Mold frame holes

I have always wondered why my mold frames have holes in the sides.
Some frames have one hole on one end and a couple of them have holes
on two sides.

All I can see is rubber leaking out when I make a mold. What good
does that do?

Your advice is appreciated.



We tend to put extra rubber when we vulcanize and this actually
saves the frame from breaking and also provides a gate for the excess
to ooze out.

Years ago these frames were cast and some of them had bad source
holes and weak spots where they cracked.

I seen customers put holes on ones that did not have.

Kenneth Singh
46 jewelry Supply


You know it is necessary to pack the mold frame with extra rubber to
insure a complete fill. Maybe even half a layer. After the air
pockets are filled, the extra rubber has to go somehwere. When the
rubber is forced out between the plattens and the frame, the
resulting mold can be uneven or somewhat distorted. Practically, it
is just easier to pull off the ‘worms’ that leak out the sides, than
to give it a trim with the scissors.

Thank you for the heads up on the Gold Label.

Best, Jon

Dear Lori,

The subject of vented vs. unvented mold frames has some similarities
to passionate discussions about religion: each side is immovably
convinced of the correctness of its position and unwilling or unable
to listen to the opposite point of view.

Molding technology throughout the rubber industry (non-jewelry, that
is) holds that molds should be closed, without vents, in order to
build up internal pressure in the mold cavity in order to force the
rubber into all the nooks and crannies of the design.

Believers in vented jewelry mold frames seem comforted by the sight
of rubber flowing out of the vent holes: to them this means that all
is well inside the dark and unseen cavity of the frame.

Thousands upon thousands of molds have been made with vented mold
frames and even more without them. Both styles seem to work.

There is a belief in some quarters that rubber shrinkage is reduced
by the use of vented mold frames, but there is no empirical proof of
this. I remember attending an early Santa Fe conference in the 1980’s
at which Louis Sanchez, then at Rio Grande, presented research on
this subject. He compared shrinkage rates for molds made from
unvented mold frames with those made and mold frames with various
size holes in various positions.

I remember well his conclusion: “In conclusion, I have no

So – whatever mold vent hole religion you belong to, you might as
well continue as you are.

Michael Knight

I have always wondered why my mold frames have holes in the sides. 

The holes are used to locate the sprue of the metal model being
molded. Different sides give you different placement options for the
model in the rectangular frame.

There are two common approaches to packing molds. In one, where
there are no holes in the frame except for the one occupied by the
model, a high level of skill and experience is required. The packing
of the rubber is in carefully fitted pieces and the resulting
flexibility or stiffness of the mold is the result of the makers

In another approach, a hole is made in each side of the frame. When
the model is inserted in one this leaves three. The holes are
normally about 3mm across. This allows a less skilled person to pack
a mold successfully. It may sometimes have speed advantages. The
rubber can simply be laid in in sheets, pushed together tightly
around the model so it sticks together, but without filling empty or
hollow spaces. Instead extra rubber (2 sheets or so) is packed on
top of the mold, and the vulcanizer tightened repeatedly for the
first five minutes of the vulcanizing, until the platens of the
vulcanizer touch the mold frame all around. The softening rubber then
flows throughout the model and excess can flow out the holes, like
little worms emerging… The mold will have a repeatable degree of
hardness, or flexibility as excess material leaves the mold instead
of contributing to its eventual hardness.

It is important when the mold has cooled enough to cut in the frame
to slide a knife in the sides all around the inside rim of the rubber
mold to cut off the ‘worms’ from the inside. Then you can grasp them
and pull them out of the mold leaving an empty hole for next time. If
you just pull at them you will have a plugged hole and have to
redrill it, not a pleasant expereince.


And I thought the hole in a mold frame was so you can insert the
sprue rod connected to your model so the piece does not float around
as a result of the pressure applied to vulcanized molds.

Richard Hart


From an engineering standpoint, the holes through the sides of mold
frames are of no benefit at all, except perhaps to the manufacturer
of the mold rubber, as we get to make a little more rubber.

I wish to caution you about one of the responses to your question.
If you are using separate sprue formers and buttons, the holes cannot
be used to locate the sprue, as once the sprue former is in the hole,
and particularly if it moves outward, deeper into the frame, you will
be unable to remove the mold from the frame. If you have sprue
formers in which the button and sprue are one piece, so that the
sprue former cannot move deeper into the hole, you will not have a

Hardness, flexibility and shrink cannot be affected by the jeweler’s
handling of the vulcanizer.

When we make the rubber, the chemistry is what determines the shrink
and hardness. No man has the strength to place so much pressure on
the rubber that he might affect the hardness or the shrink. Our flex
grade is 35 shore A hardness, and the shrink is 2.3%–period!

If you feel comfortable with the movement of rubber through the
frame, it will generally do no harm. Keep in mind though, that
reducing the pressure within the mold frame, by permitting rubber to
leak through holes, can create non-fill situations.


Bill Mull
Zero-D Products, Inc.