Back to Ganoksin | FAQ | Contact

Silicone RTV problems

Little is more frustrating than trying to use the telephone to
figure out why a customer is having trouble with a product. I hear “I
read the instructions”, “I weighed it carefully”, “I cleaned it
thoroughly”, and so on.

Please do not misunderstand, it does not happen all that often to
begin with, but I do hear some really odd-ball goofs.


Read the instructions thoroughly. Silicone RTV is an engineering
material with specific properties that are designed for your
application. It is not Campbell’s soup.

Do not dip the model in light machine oil before making the mold.
(Do not create your own release agent). Release is not needed for
most materials, including carved waxes and RP resins.

Do not use silicone spray on glass frames. Use TFE spray. Call me if
you can’t find any.

Do not try to de-air in a tall narrow vessel, use the largest
diameter, shallow bowl you can fit under the bell jar. This exposes
more surface area to the vacuum and reduces rise and speeds de-air.

Do not use a postal scale, baby scale, fish scale, bathroom scale.
Yes, I have heard each of those. Use a scale capable of reading to
1/10 gram. You will never have a problem, other than maybe
arithmetic. There is not a lot of tolerance in 10 to 1 ratio, unless
you intend to change the properties of the mold. Call me about that.
I may have an alternate catalyst for you.

Resist placing your uncured mold in a freezer over-night to de-air.
It works, but moisture can cause problems from time to time.

Resist trying to fast cure in a kitchen oven or toaster oven.
Controlled 120F cures our silicones in one hour.

Make sure your vacuum pump has been maintained properly and is
pulling sufficient vacuum.

Test spot a material you are thinking of using as a model. When you
make a normal mold, drip a little of the leftover on the material you
plan or hope to use as a model. If the little drip cures, go for it.
I am not being insensitive. The “little drip” is the droplet of RTV,
not the craftsman.

Realize that if the mold you made last year has become gummy, it is
not silicone. Some companies do not make it very clear that they are
selling polyurethane RTV.

Making a silicone mold is very simple if you do not try to discover
new ways to do it. Our instructions are based on 17 years of
experience with silicone RTV and may not always be interchangeable
with another brand.

If I have left anything out, please chime in with more suggestions.
If you know of any other craziness, please let’s hear it. Together we
may be able to set aside some of the HOOKIE-BOO.

Please call me with questions.

Bill Mull
Zero-D Products, Inc.

Bill, I’m not sure if yours is the product I use, but it’s similar.
I love this stuff! It’s so easy to use. If the mold is small, my
suggestion is to forget degassing for bubbles. Just brush a thin
layer on the model and check for bubbles, then pour the rest in.

Anyway, it is easy to use if you follow the simple instructions and
don’t monkey with it. I second your advice.


I am very surprised to read that people are having problems with
silicone RTV molds. I have been using them for over thirty years and
can think of only two instances that I had any problems. One of them
I was trying to make a mold of a rattlesnake head. The organic oils
from the specimen retarded the cure on the RTV. I solved the problem
by going to a freeze dried specimen. No oils no water just empty dry
specimen. The only other problems I have run into are the durometers
or harness of varying materials from different manufacturers. The
solution to that is just try them and select the one that best suits
your needs. I usually make RTV molds from waxes that I wish to put
into production pieces. If I have metal masters I use silicone
vulcanized rubber from Contenti Company called Moldex. Again any oils
on the master and you are out of luck.

One solution for bubbles, if you do not have a good vacuum machine,
is to apply a contact coat on the master with a soft haired brush.
Work the material in all the crevices and corners making sure no air
bubbles are trapped. Then just pour in the rest of the mold material.
The body of the mold may have a lot of small bubbles but the contact
surface will still be clean and clear. I have a mold the was made
this way and is still serviceable after 30 years. I also use this
technique for making molds with leftover mold material. If I want a
mold of say a piece of wood or texture of a surface I just brush on
the leftover material and let it harden over nite. It is very viscous
stuff and does not flow easily on flat or level surfaces. Don’t know
how else to be of help except when debubbling I pull a vacuum until
the material start to rise to the top of the mold frame and then
release the vac. I do this 10-15 times until the rise is not so
pronounced and will not exceed the confinement of the mold frame. I
then go ahead and pull a full vacuum. This also works if you have a
poor vacuum and can’t pull 28". That is about all I can think of. I
do weigh my materials on a gram scale to the 10th of a gram.

The final thing I have learned about silicone rubber RTV molds is
that if your was is too hot it can cause out gassing and leave small
bubbles in the wax. I found this doing layup molds as opposed to
injected molds. Hope some of this helps someone.

Frank Goss

Hello Bill;

I’ve not used the Zero-D products, but I know many who do. I thought
I’d add another consideration to your discussion.

When mixing the compound with the catalyst, make sure you get the
catalyst scraped off the sides of the mixing bowl and down into the
compound. The catalyst and the compound have different consistencies.
If the catalyst stays un-mixed and adhered to the side of the mixing
bowl, you may not get enough catalyst into the mix, and some catalyst
may slide down when you fill the mold and remain, unmixed, in the
mold, which will result in un-cured areas, usually on the top or
along the plates.

David L. Huffman

Hi Bill, again.

One more thing, I’ve found with some compounds, if you are using
super-glue to afix a wax model to a sprue rod, the glue affects the
cure of the compound. Better to attach the model with wax. I’v also
found that epoxy will work without affecting the compound, but it
must be completely cured.

David L. Huffman

I have been using RTV from last 5 years and found that, if mixed
correctly as per the manufacturer’s instrucions, the RTV mould will
give a definite positive result, however if someone needs cure the
mold faster, place it near to a light source (bulb & not a florescent
tube) this way the mold gets cured within 1~2hrs. as compared to a
12hrs curing time…

Best Regards,