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Rubber Mold Cutting

Does anyone know of any good learning resources on cutting rubber

Thank You , George

Swest has great work shops in Atlanta . in fact Stuller now owns
Swest and had a great demo on mold cutting. If you would like to ask
me direct questions e- mail me direct and I will answer you. Leona

Does anyone know of any good learning resources on cutting rubber

Don’t know of any good books on it but I do know practice, practice
practice does help A LOT. If I REALLY wanted to learn the how to’s,
I think I would pay a good mold cutter to give me some lessons. To
fine one of these (really good mold cutters) I would contact mold
rubber magr/reseller companies and ask them who they might recommend.
They might have some info on it as well. I have watched
"professional mold cutters" and it is amazing what they were able to

John Dach
MidLife Crisis Enterprises
Cynthia Thomas Designs
Cynthia’s sculptures are at:

Hallo everybody I am a new member of orchid from Antwerp the city of
diamonds. I am an independent jewelry designer and goldsmith,
silversmith, moldmaker etc. My name is Nicolas and I will like to
reply directly to you George to your question about cutting rubber
molds. The best way I believe to learn how to cut rubber molds is
just do it. Keep in mind that the rubber has to be used for more then
one time so you have to cut it in a way that it will hold firmly the
same position all the time. Personally I read first the book Jewelry
concepts and technology by Oppi Untracht, the chapter about mold
making and then I started by myself experimenting with the molds. If
you use Castaldo rubber inside the box there is also some useful
of how you should cut the rubber open. if you want I can
give you some tips more on how you start cutting one mold and with
what you have to take care. With friendly greetings Nicolas

Nicholas, I for one would like very much for your Hints on Cutting
Molds. Please do continue to enlighten us - I am sure there are more
like me.


Practice, practice, practice! And go buy a box of castaldo white or
gold mold rubber, it has a lot of info in the box besides the rubber.

Matt the Catt
Find a good book #1

Your timing is perfect. I have my first dozen molds sitting here
waiting to be cut open. And while I’ve seen the pictures and read the
instructions, any advce is greatly appreciated. Things as basic as
how do I hold it – in a vice? Cathy Icardo, France

Hi Lorrie, Thanks for encourage me to give some hints for the subject.
So here we go: First of all the most important tools that you need is
a good surgical knife with sharp blades. I use nr 11 surgical blades
those that are straight and come to a sharp point. There are a lot of
different shapes with different numbers but these are always did my
job so far. Do not forget that every time that you cut one rubber open
you have to change the blade in order to have a sharp point otherwise
you can cut yourself real bad. Trust me I did the mistake once and I
paid the price. Rubber cuts much easier when is warm so once you
vulcanize it let it rest for 5 minutes and when you feel that it does
not burn yor fingers cut with scissors the access rubber that is being
formed on the upper and under level in order to make compete contact
with the metal plates so you don’t have a problem when you shoot it
with wax. You can also let it cool down till is completely cold but
then you should use some lubricant in order to cut easier. The
lubricant that i use is common detergent for dishes with a little bit
of water. That will do fine. Then you start cutting the rubber from
the cone, this the brass cone that you can buy in any jewelry tools
store and they exist in different sizes depending how many layers of
rubber you use and gives the shape that fits the nose of the injector,
a straight line all around the rubber mold till you reach the opposite
side of the cone.This cut is made in the middle of the thickness of
the rubber mold and is should be no more then 1mm deep. Use mostly the
sharp point of the knife and if you are a beginer go slowly all
around. If you go a little deeper don’t worry you still doing ok.
Then you start cutting the four corners. You move your knife from
this straight line that you just made upwards and cut each corner in
order to create four higher points that will keep the mold in position
once is being cutted totally open. Hereby I will send also a couple of
scans with this in order to understand what I mean. After your corners
are free you cut towards the model with small strokes each time and
cut one time left, next time right, in order to create a kind of a
wavy pattern that will hold the mold in position. The under part of
the mold should slightly thicker then the above side. You should take
care where you cut and you should try to cut the mold open on a way
that you don’t come in the middle of the model or in a place that it
will be very hard to clean afterwards when the piece is casted. Keep
in your mind a visual picture of the model and before you cut the mold
totally open and you are near the model take a deep breath and go
easy with your knife and try to free the model on places that are easy
to be cleaned. This is a begin I hope that I can be of any help. One
advice, start with easy models and thenonce you have the techniek
under control you can explore more possibilities. Hereby I send also
two examples of two different ways of cutting a rubber mold. The one
that I explain that is the more proffesional one and the one that is
much easier to begin with. For more hints let me know something
Always on your service Nicolas

** Attachment Removed **

Hallo Cathy, The best way to hold the mold in the beginning is with
your hands, once you open the above side then you can hold the one
side of the rubber with a good pair of pliers and the best for rubber
is the ones that are used for welding that have little teeth that can
grab the rubber good but do not destroy it. I mostly use my hands, in
the begin I try to use pliers but I found out that it goes better
with my two hands because I have better control of the mold.

This pliers you can buy them from any hardware store that has small
welding machines for steel. With friendly greetings Nicolas

I have been cutting rubber molds for 29 years and maby I can help you
get oner the lumps of parting your molds.First Hold the mold in your
left hand if you are right handed and vice verda. 2Nd- find a old beer
can opener that has a pointed hook on one end. 3rd.- Identify the
center line of the mold or half the mold thickness by marking that
line all around the mold. 4th.- Start your first cut at the sprue
openind of the mold… 5th.- buy two scapel knives, one with a straight
handle and one with a bent handle preferdly with a plastic handle.
6th.- buy straight and hooked surgical knives from your jewelry
supply house,buy atleast 25 each kind. These knives are extremly
shark and will cut your fingers so easy if not careful in the process.
7th start with the hook knife first and partthe mold starting at the
sprue and cut in about a 1/4" deep to the first corner and around the
corner for at least a 1’ to 1 1/2". At the corner back track about
3/4" and rotate your knife blade so it will cut a half vee in the
rubber. Then start on the other side of the corner you have just cut
and cut tha other half of the vee that you have just done. This vee
cutting makes a kee lock in the corner of the mild .Now start at the
sprue opening and cut to the other corner ans around it just like you
did on the oposite corner. Cut your vee in this corner. 8th.- now that
this is done you are ready to part the mold. By remembering where
part is sure helps out when starting to make the parting cut on the
mold… Cut into the mold until you see the part. One thing I didn.t
mention at the begining is secure the beer can opener in a secure
bench vice with the hook on the opener sticking out and the hook
pointed down. I have learned not to cut the mold completly into two
parts. By not doing this you assure that the mold is aligned up
perfictly . 9th.- when you have cut the mold at the critical points
you can now use the straight blade to do the rest of the cutting as
most of it is straight cutting. After the mold is finished dust it
with pwrting powder and then it is ready to be used… If this doesn’t
help you as maby I haven’t explained it good enough if you will esnd
me a blank video I would be glad to do a cutting sequence for you .
Yours :::Billy S. Bates Royal Wildlife Miniatures

Nicolas, By and large I agree with what you’ve written, I use a few
different techniques, described later, but I have to question some of
your advise.

First, I sometimes leave the thick excess flap of rubber attached
while cutting molds; it can be quite useful as a “handle” that can
also be clamped in vise. Another tip regarding this is the use of
spreading devices such as medical hemostats that latch while holding
parts away from each other. Rubber molds can be very tiring to cut,
and tired hands are more likely to slip (with a scalpel involved!) so
any means of making it physically easier is something I’ll consider –
even jamming bits of rubber eraser or other stuff into the partly cut
mold to help keep it spread open.

   ,,,   Rubber cuts much easier when is warm so once you vulcanize
it let it rest for 5 minutes and when you feel that it does not burn
yor fingers cut with scissors the access rubber that is being formed
on the upper and under level in order to make compete contact with
the metal plates so you don't have a problem when you shoot it with
wax. You can also let it cool down till is completely cold. 

Although it is true that still hot, or warm molds are easier to cut,
it’s also the cause of warped molds, bending away from their center
mass so that the edges of the molds don’t naturally meet unless the
parts are clamped together. This happens more with thick molds that
have more temperature difference between the outside and the inside.
This warping often doesn’t matter if the parts fit under clamping
pressure, but sometimes it can also distort the parting lines enough
to cause major problems. Allowing the mold to cool uniformly and
slowly is the solution to this, and in fact the best way I’ve found to
do it is to keep the frame clamped in contact with the vulcanizer and
turn off the vulcanizer so it cools slowly. This assures that the
process of vulcanizing and the stresses of internal shrinking will be
spread evenly throughout the mold. After you’ve cut a mold this way
the parts will naturally fit everywhere without undue clamping

  ... Then you start cutting the rubber from the cone, ... should be
no more then 1mm deep. 

Why such a tiny 1mm cut? Why straight? There isn’t any apparent harm
to this, but I can’t see an advantage either. I begin the zig-zag cuts
immediately, and usually about 5mm (3/16") deep. I would advise a
first straight cut away from the sprue cone so that part doesn’t zig
and zag immediately. and also I use a straight cut down the sprue
itself. Zig-zag cuts align the parts everywhere else, with smaller
waviness from shorter cuts as they approache the model’s surface. This
allows extremely accurate fitting - non sliding - mold sections.

My choice of scalpel blade is a #12 Bard Parker, Rib Back Carbon
Steel. This is a hooked blade with the cutting edge on the inside of
the curve. It allows me to cut in angles that a straight blade can’t
do, and I can slide the back of the blade along the model while
cutting AWAY (sometimes pulling the blade backwards while cutting)
from the model instead of toward it, resulting in fewer injuries to
the original model.

Silicon spray helps in releasing wax models from the mold, and it
also helps in the lubrication while cutting, but it also can trap air
resulting in incomplete waxes. Many mold makers use extra cuts around
such trouble spots, to let the air out, but they then have extra
parting lines. A better solution is to puncture the mold wherever air
gets trapped with a polished needle sharp ice pick, and then dust
the resulting holes with french chalk (coarse talc - a concern here is
that some talc can contain asbestos)

A common error even among those who have been making an using molds
for a lifetime is to keep the wax too hot. This results in MORE, not
less bubbles and incomplete injections. Injection wax works best when
it just barely stays liquid. Slower smoothly filling molds allow the
air to escape in front of the infilling wax.

Sometimes a molded in detail can be added with an additional item
vulcanized together in the mold such as hinge pins for forming hinge
holes, or a screw to cast internal threads. for an example of that see
my watch winding knob at: The 6th and 7th photos

A final point, beginners ought to know that they don’t actually need
an expensive vulcanizing press to accomplish quality rubber molds. An
ordinary kitchen oven (bake at 300 for an hour) and couple stout metal
plates with matching holes for clamping through-bolts can accomplish
the same thing. Be sure to test the oven temperature with a good
thermometer, they can be WAY off.

Alan Heugh

The tool that I use to spread the mold is a simple one. Buy yourself a
bottle opener and bend the pointed end in a little. You then screw the
bottle opener to your bench and leave enough of an over hang to allow
room for the mold to be held comfortably. You hold the mold against
this point so that it finds its way into the cuts already made. By
utilizing the bottle opener and your bench as a stable platform of
purchase, you can easily open the mold apart one handed and continue
cutting with very little effort with your other hand.

Best Regards.
Neil George

For those designers that do not want to “practice, practice,
practice” or cut their fingers using the surgical knives : go the
"self releasing baby powder way". No skill is required; thus you can
spend your precious time making jewelry. After cutting the rubber
strips to fit the mold, divide the layers in equal parts. You now
have 2 packs to make a 2 piece mold. Peel the protective cloth and
adhere the strips from one pack together, press or pound to get a
tight fit; do the same with the other half except the last inner
piece ( the one that will be facing the opposite side); on this piece
cut the 4 corners, approx. 1/2" square or smaller depending on the
size of your model, transfer these corners on to the first stack.
Then slap this last piece onto the latter half. You have now created
a locking devise to prevent the 2 halves from sliding off each other.
Be sure to cut out the center bottom with a slight cone cavity to
accommodate injector nozzle cone…this will be sandwiched along with
the model between the 2 packs of rubber. Apply baby or talcum powder
on each half with a soft brush. Slap both sides together to blow off
as much powder as possible, only a thin unnoticeable film is
sufficient. Place one half of the rubber side into the aluminum mold,
center your model and the nozzle cone, place the other half on top of
it. Slap the top and bottom mold plates and insert into the hot
vulcanizer. After the required time (read the makers time chart), let
it cool off and pry open the two part mold…no cutting. For
projects that are more than 3/8" thick, you will have to cut out the
center two slices , more or less in the shape of the model, to
accommodate the project. Then powder and proceed with the
vulcanizing. For really intricate models; the “spaghetti” method can
be used. Cut a long thin strip,like a spaghetti; apply powder ,roll
it up and shove it into the cavity of the model. Sandwich everything
together as above and vulcanize. When done; part the halves, pull out
the spaghetti and when you release it , it will automatically ball up
in the cavity shape. Assemble the parts together and inject wax.
Enjoy and save your fingers for making jewelry. Min Azama in humid Tokyo.

For the novice mold cutters:

We use two small pairs of “ViseGrip” brand pliers to help hold molds
while cutting. They are the 5 1/2" size. The difference is - that we
silver soldered a 3 inch “C” clamp to what would normally be the
upper side. One of the “C” clamps is soldered to hold a pair of
ViseGrips vertically to the bench top - the other soldered to hold
the second pair horizontally. They work amazingly well. You can
adjust them to grab any thickness of rubber, as gently or firmly as
you may desire, and being able to use one vertically, the other
horizontally - pretty much covers all you’d need…

Another tool we made up is similar to what you see in some of the
jewelers catalogs. Using a heavy duty spring clamp available from the
local jewelers supply (with the strange name - Harbor Freight! - or
any other hardware store) and a 24" length of 3/8" “S” link chain,
you can make one that works as well or better than those sold in the
catalogs. (For 90% less!) The only thing we found we had to do was to
bend the tips 45 degrees and file some “teeth” into 'em. Triangle
file did it in a couple minutes. Don’t make 'em too sharp - leave a
little space 'tween the notches… (The metal clamps look a little
like giant clothes pins.)

To use this contraption, you can screw one end of the chain into the
back of your bench top at any distance that allows the loose end to
be handy to your bench pin. Then hook an open link at the other end
of the chain into the hole thoughtfully provided in the clamp. Clamp
onto the mold where you need to cut, and pull against the anchored
chain to separate it as you cut…

I rarely use this - but there are times (molds) that are awkward to
get into & sometimes make it handy.

One other thing… I have found that a “post” made of a 3/8" dowel or
metal rod, about 2 1/2" inches long, and stuck into a hole drilled
halfway through the back of my wax bench pin is used for almost every
ring mold I make. My shanks are generally heavy and rectangular in
cross section. This means that in order to get a crisp edge on them,
I need to cut all the way around inside and outside the shank, to let
the air escape. (I know this is not the best way to describe it - but
I’m falling asleep…)

In order to do this, I need to be able to see. Putting the mold half

  • working side up - on top of the post, I can press down and
    manipulate the mold so that I can easily see to cut it…

Hope this last part wasn’t too obscure. If anyone has a problem with
it, let me know and I will rephrase it… Time for bed.

Brian P. Marshall
Stockton Jewelry Arts School
708 W. Swain Rd.
Stockton, CA 95207
209-477-6731 Office/Fax
209-477-6535 Workshop/Classrooms

Hello Allen, You do not actually have to cut rubber molds at all. I
have made about two hundred molds since I stopped cutting them. (about
three thousand before that I did cut) If you are making a mold of
eight pieces of rubber, make up two pacs of four sheets each. Of the
two sheets that face one another in the center of what will soon be
the mold, remove the corners of one with scissors and stick them to
the other. This will create locks in the mold so the two halves will
go back together. Coat everything liberally with plain baby powder,
put the model in the middle, put it all in a mold frame and vulcanize
it. When you have cooked and cooled the mold, grab the two sides with
two pair of pliars and pull. The mold will separate where the powder
was, the model will fall out and you are ready to squirt wax. You can
indeed do the vulcanizing in your home oven. I wrote an article about
how to do this in the early eighties and Castaldo rubber co. was kind
enough to put a copy in each five pound box of rubber they sold for
the next fifteen or so years. So put away that scalpel before you hurt
yourself! Have fun. Tom Arnold

Dear Arnold, Your post on rubber molds without cutting is good advice
and I agree with all your basic contentions. However, I would add
that one should be careful about what is used as a parting powder.
You have suggested that baby powder will do the job nicely, but I
would like to point out that baby powder is seldom any longer talcum
powder ( a mineral substance ) but, rather, it is cornstarch ( a
vegetable substance) I haven’t experimented with using cornstarch,
but theoretically, it bodes disaster…you might end up with a
gooey mess. Moreover, the consensus of opinion is that talcum powder
should not contain anything other than talcum…no aromatics or
other additives. As for talcum powder, you might be well advised to
keep in mind that one of the probable causes for the substituion of
cornstarch for talcum may lie in the fact that steatite, the rock
from which talc is derived, invariably contains microscopic particles
of asbestos…draw your own conclusions ! I still use the stuff, but
I would recommend using it up wind. Ron at Mills Gem, Los Osos, CA.

Tom, I’m sure this powdered method is fine for simple shapes; If
that’s what I was doing I’d consider it, but I have a more baroque
artistic soul - that requires more thoughtful planning. I’ve seen and
used powdered molds, and I don’t like them. Keys in the corners only,
as you suggest, will allow other areas of the mold to slip, distorting
details, while the zig-zag cut method I prefer interlocks everywhere.
Sometimes I make three or four or even five piece molds, with tiny
zig-zags and air vents to solve very intricate designs.

I design molds with sections that come out of the middle in areas so
the rubber can be gently nudged away inward from delicate INTERNAL
wax areas. How can you determine – except in a vague general way –
where the parting lines occur with the powder method? How can you mold
complicated internal shapes? I don’t think you can do it as well if
it’s not a controlled parting line location.

You’ll indeed make the MOLD faster – no argument there – but then
every item you produce from it will take longer than it would if you’d
spent a few minutes more time and care making a better-designed mold
in the first place. With a quickly made and haphazard mold, parting
lines will often cross through important details. That’s poor design
resulting in lower quality, wasted trimming and finishing time, and
false economy.

I haven’t cut myself, seriously, while cutting molds for over 30
years and I think I’ll risk it again. The first thing I do before
cutting an important mold is to expend a minute or two sharpening a
NEW scalpel blade while watching what I’m doing with a microscope; I
lap-strop off the manufacturing irregularities so the blade cuts where
I want it to with less force and more control. Alan Heugh

Thank you all for your help.

I’ve now cut open my first 18 molds and am getting better at it. A
few need to be redone – underpacked, the piece moving, less than
perfect cutting. But it’s going much smoother than I expected. The
hook method works great. I’ve done some molds in vulcanised silicone
and some in rubber (Castaldo orange). The rubber comes out of the
frame easier, but both take a bit of muscle to release. The ease of
cutting open seems to be about the same, and I haven’t sliced my
fingers yet! I take breaks when my fingers get tired. Today I’ll try
the ‘baby powder’ method. Next step, wax injecting. Thankyou all, Cathy
in Anjou, France, where the sun isn’t shining this summer.

I haven’t used powder of any kind for separating molds so the
following is supposition at this point.

Most baby powders & powders meant to be used by us humans usually
has a fragrance added to whatever the powder is, be it good old talcum
powder, cornstarch or some other material. Many times the fragrance
contains an oil of some kind that can react with the rubber used for
the mold. In my book, it’s always best to err on the side of
conservatism, if I don’t know what’s in something & how it’ll affect
what I’m doing, I don’t use it.

Back when all tires used inner tubes, most shops working with any
kind of tire used a product called ‘Tire Talc’ to dust the inside of a
tire before putting the tube in. The idea was that it’d keep the tube
from vulcanizing to the tire over time. It worked! From what I
remember, the ingredient was just powder talc; nothing else added.

The last time I got a new set of tires, I saw a can of ‘Tire Talc’ in
the tire shop; so evidently it’s still available. You might check with
the guys that actually do the work at you nearest tire store to see
where they get theirs. There might be other names for it, so you may
have to describe what job you want to do with it, basically keep 2
pieces of rubber from sticking together.


As a reply to Dave Arens comments about talc; talc is a naturally
occurring mineral which is widely utilized in the ceramic industry to
formulate glazes and clay-bodies. If there are any ceramicists
(potters) in your area, university clay departments, etc. you should
be able to locate a source for the minute amount a jeweler will need.
Otherwise any ceramic supply house will have it available in bulk.
(depending upon minimum size package and shipping requirements, many
ceramic components can be purchased by the pound or the fraction