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Writing ID on silicon RTV molds

Bit of an off-beat question. How do you write on silicon RTV molds?
We need to put the mold number on them, but even Sharpie just rubs


Dear Noel,

We follow a simple method, once the mold is ready, we cut a fine
pocket on the thicker part of the mold towards the top, using the
surgical blade and insert a paper slip bearing the number or any
identification. This can only be done with clear silicon molds,. As
for other colors there are special silicone marking inks (used for
printing on hoses) if you can get a sample from a manufacturer.

Khushroo from Mumbai


It is a problem. I use RTV silicon for my foundry molds and nothing
I have found will “stay” on the rubber, BUT - - my molds are big
enough that they all need some some sort of mother mold to give a
foundation to the rubber and I use either fiberglass or a plaster
like material. SO, I imbed a piece of paper in the last glass coat
in the five glass or just write on the plaster/hydra-stone. If you
are using either vulcanizing or catalyzed molds without any type of
mother, you might try putting a string tag into the mold material
before vulcanizing, or some sort of thin metal tag that can be
written on/embossed and this material be imbedded into the
vulcanized rubber. Just a couple of thoughts…

john dach

How about using a ball burr to write/scuff the letters onto the side
of the mold, then rub sharpie ink into the roughed up silicon? Might
not be pretty, but should work.

Not a solution to your problem, but a similar one. I wanted to label
silicone vulcanised moulds. In my youth at the seaside you could
stamp a message into aluminium strip, you turned the dial and pulled
the lever. Well this is how a Dymo label machine works. You can get
heavy duty imprinters and although the plastic imprinted tape
doesn’t work, (it just flattens out again) there is metal tape that
does. I had to find some on E bay but it does exist. You have to put
it in right side out, on the surface, you get a great impression.

Perhaps you could use the plastic Dymo tape for RTV, double sided
tape it to the mould. Failing that write in mirror writing with a
permanent marker on the inside of the mould.


How do you write on silicon RTV molds? 

Two methods I’ve used. With clear silicon RTV, I write/type/print
the number on a slip of paper and put it into the rubber facing a
side, near the end where it will be imbedded, but out of the way. You
can also cut a slit into the rubber after it cures and imbed a slip,
but then it can also come out, or get dirty, etc. With non-clear
types including vulcanized rubbers, I took a sheet of thin aluminum
and some number/letter stamps and made a bunch of small bits of
aluminum with letters and numbers stamped into them. I used "reverse"
stamps, so they read “wrong” looking at the tags themselves. Now,
pack the mold or pour it, with the intended number or label spelled
out with those tags. For a vulcanized mold, just insert them between
the mold frame or mold plate (clear stamped image facing the rubber)
and the rubber before vulcanizing. With poured molds stick the tags
to the frame or mold plate with a bit of sticky wax so they don’t
float away.

After the mold is cured or vulcanized, those tags can be removed
from the rubber and saved for reuse with the next mold, and they
leave an impression in the rubber that now reads correctly. Not
written, so no ink to come off, but permanently “embossed” into the

Or use the impression in the tape as an “imprinting” unit on the
mold while the RTV is still liquid. It would make either a raised or
depressed lettering/numbering impression on that mold surface. Once
the mold is cured, remove the tape and you have the image in the

As to metal (aluminum most likely) tape, nursery supplies may be a
source. Also, Aluminum nursery tags, that are made to “write on with
one or pencil or stylus” on a soft surface so the metal is allowed
to deform under the point of the writing instrument, and use this
"tag"to"impress" the data into the still liquid mold material. I
have to try this myself.

Happy Turkey day.
john dach

Would it be possible for one to impress the mold on the outside
before it sets with one of those steel numerical stamps. I know they
come in sets with the alphabet, but pehaps they also make them with
numbers. Alma

How do you write on silicon RTV molds? We need to put the mold
number on them, 

One way is the Dymo labeling tool, you know the one with the raised
letters? You buy the one that uses aluminum strips instead of
plastic ones. Type out what you want and impress that into the RTV
before it cures, leave it there.

There’s a thread about that somewhere on Orchid.

Otherwise, they sell little squares that you can add to a mold and
then that spot you can write on.


Try a hand held metal labeled

Those are available in letters and numbers in a number of different
sizes. Problem with trying to use them on the mold is they would only
make an “impression” while the rubber is liquid, but the "impression"
would smooth over in the liquid material. If one tired to use the
stamps on cured rubber, there will be NO impression made as the
rubber is now cured and in it’s final state and will not take any
sort of impression.

Does this make sense?

john dach

I once got a pro mold made. They wrote my name on a small slip of
paper thencut a slit in the mold and put that inside. Brilliant and


If one tired to use the stamps on cured rubber, there will be NO
impression made as the rubber is now cured 

If you were commenting on my suggestoin, John, you missed a bit. The
stamps aren’t for the rubber. You use them on thin metal, like shim
stock, or thin aluminum (or anything) sheet metal, to make small
stamped tags that get inserted in the mold frame, and stay there till
the mold is cured or vulcanized. Then they can be removed for reuse.
They leave a permanent impression because they are there for the
whole curing/vulcanizing cycle. One can make entire stamped strips
with a whole stock/style number, or as I do, a “font” of individual
letters and numbers of thin sheet. They end up around 1/4 inch
square, and are lined up as desired in the mold frame. One mold maker
I know routinely does much the same with a little photoetched or
milled tag with the company logo, so the molds themselves are marked
with the logo and copyright mark…

Thanks John for the explanation. It would be handy to be able to use
the metal numbers, but as you explain it is not feasible. Alma

silly suggestion perhaps about labeling molds but would heating
metal punch numbers for impression work?

Barbara who is still digging out after the snowfall Today, the
generator gets freed.

I love all the responses and ideas! So fun to practically hear
everyones wheels whirring!

The first thing I will try when I get back to work is the piece of
paper in a slit for the molds already made.

The second thing I will try is an amalgam of a few suggestions. I
will try embossing the numbers into heavy duty aluminum foil with a
rounded point like a dull pencil (I actually did this as an art
medium for drawing when I was a munchkin) and try putting that on
the mold frame.


silly suggestion perhaps about labeling molds but would heating
metal punch numbers for impression work? 

With traditional vulcanized rubber molds, you can burn/melt markings
into the rubber with a heated punch, or just any heated tool. Wax
pens turned up high work fine. But the vulcanized rubber molds also
can be marked with ballpoint pens and sharpies, so these molds aren’t
a problem. With silicone rubber molds, both vulcanized and RTV types,
the rubber is quite heat resistant. You can even cast lower melting
metals, like pewter, directly into such rubber molds (think of spin
casting of cheap white metals, for example). So hot tools do nothing
useful with these rubbers. And they don’t permanently accept markers
and pens. Thus this entire thread…

We could all take a giant leap into the 21st century and use RFID
chips embedded in our silicon molds.

I found this company in a random Google search:

I made little copper tags with stamped numbers and placed them on
top of rubber molds to be vulcanized. I removed the copper after
vulcanization and there were little letters standing up in the space
where the copper used to be sitting. They looked cool, but my caster
told me she had to cut a piece of thin cardboard to plug into the
square impression, because the dip in the outside mold surface
caused her problems when she was injecting some molds. So I went
back to using a ballpoint or sharpie pen to mark molds. The pens
work best if I clean the surface with some alcohol first. If I
notice the marking getting faint or smudged after a time, I re-clean
and mark again. It is not an elegant method, but it is working well
enough for me.

Mlou Brubaker

This might sound a little crazy but then again I am that way. I take
a pieceof paper and place it on the top of the silicone rubber mold
just a strip large enough to write on and then do my vulcanizing of
the piece. the paper sticks to the rubber so I can then write on it.
put the paper on the siliconerubber before you put your talc or mica
powder… Also the backing on the mold material that comes with it
will work… Does make the mold stiffer where the paper is located…