RTV mold making

I have done some molds using RTV 421 from General Electric. It is
very soft it has a 12-18 shore, and when I inject wax it deforms
a little. I tried to use some vertical plaques, in order to use
them as a skeleton and prevent deformation. Do any one out there
has any experience in RTV mold making, opening and wax injecting



Call Tekcast, they have a wide variety of RTV from 2 main
manufacturers, Rhone-poulenc and Corning. They can send you a
catalog with enough explanation for you to choose which RTV is
best suited to your needs. I personally have used RTV-585 with
catalyst 60R and was very satisfied with the wax injection

Tekcast Industries
phone: 914-5760222   800-usa-4tek

Fady Sawaya

Dear Mr. Estrada,

This is Michael Knight of CASTALDO. We make a easy to use,
tough, strong, long lasting RTV rubber that is perfect for
jewelry injection and is a small fraction of the price you may be
accustomed to paying. The durometer is Shore 40, about the same
as our Gold Label.

I'd be happy to send you a free sample if you let me have your

shipping address.

Anyone else out there interested?? 

		Michael Knight

F.E. Knight, Inc.
120 Constitution Blvd.
Franklin, MA 02038
United States of America
Voice: 508-520-1666
Fax: 508-520-2402
E-mail: FEKnight@ziplink.net

This may help you…I use GI300 from Silicones Inc. in High
Point, NC. They ship C.O.D. or will take a check if you don’t
want to set up an account. Usually I get a ten pound set which
includes 1 lb liquid activator to the 9 lbs of base. you mix it
8 to 1 by scale and it sets up beautifully in small paper boxes.
(no heat necessary) and can be poured and cured in sections by
using a little vaseline on the sections after they are cured and
before pouring the next section. Cures in about 4 hours unless
the air is very warm then it’s quicker. I’ve been using these
molds for about 8 years and they shoot beautifully. I do hold
the molds between two pieces of plywood when injecting.
Hope this helps you…Margot

Gus, I have been using RTV to make molds for about 25 years. I
have never used the GE products. As a rule I use the Silastic RTV
from Dow Corning. I use the E, a white material, for softer molds
and the L < a green material, for the more rigid molds. Both work
just fine unless you introduce organic oils in some form. The
oils will retard the hardening process. Both are avoilable in 1
pound cans or 5 lb. Good shelf life and I have some molds that
are 25 years old and just like new. Check out the specs
available from Dow. Also Castaldo makes a RTV ,not a silicone
rubber, that I have used a couple of times. It is more available
than the Dow product but is sensative to humidity and must be
stored seperate from vulcanized molds. Both cost about the same.
My choice is still the Dow Corning products. they have an 800
number you can call for suppliers. Frank

Gus: I’ve said this before but I’ll repeat myself for your
bennefit. Castaldo has atwo products that are great one is a
clear two-part RTV which requires vacuming. The other is my
favorite. It’s called “Quick Silv” and requires no vacum chamber
and cures in as little as 15 minuites. Call 1-508-520-1666 for
info and a free sample.

Steve Klepinger

Hi Gus, You might try Silastic J RTV mold rubber. It’s a two
part silicone based product. There’s the white base material
which is mixed 10 parts to one part green catalyst. When the
stuff is mixed to a homogenous green, it’s ready to pour. You
need to buy or make frames. I use about 14 gauge brass strips
that are about 3/4 inches (can use other widths) wide by 12
inches long. I bend them into a flat bottomed “U” frame that can
stand up by itself. Then I solder a sprue base to the inside
bottom of the “U” frame. I make sure that the sides are
absolutely flat and then I attach my sprued model or item to the
sprue base with wax or glue. Then I rubberband two pieces of 1/8"
plexiglass sheet, one on each side. I carefully pour the super
viscous gunk into the frame. Then I vacuum it to debubble it. I
have to periodically release the vacuum to let the bubbles pop
and have the gunk keep from overflowing. This goes on for at
least 4 minutes. Then I set the frame on paper or in a pan just
in case there’s a leak, and I leave it to cure for 12 to 15
hours. If the frame is flush and the rubberbands or clamps are
tight, you shouldn’t have leaks.

After the mold is cured, take the frame apart and cut as you
would any other rubber mold. The big difference is that this
stuff cuts like butter so you have to be very careful. You can’t
pull hard and cut as you would with regular rubber.It doesn’t
offer the same kind of resistance so be careful that you don’t
cut toward yourself. Since it’s opaque, you need to remember how
you oriented the model in the frame. I mark one plexiglass plate
as front and as soon as I disassemble it, I felt pen mark that
side of the mold. Also, this stuff doesn’t have the same tear
strength as rubber so you can’t cut corkscrew pullouts. They will
rip apart.

When you’re cutting the mold, anything fragile used as a model
will probably be destroyed by cutting, bending or cracking. The
big advantage of this stuff is that you can mold things that
can’t take heat, like a wax model you don’t want to cast first.
Remember, though, that if you’re not confident in cutting it
right the first time, your wax, especially an original carving,
is history.

Vent and shoot the mold using two mold plates. This mold
self-releases, giving a slick surface, and there’s very little
shrinkage. If you need to vent more air, a trick that sometimes
works is to carefully powder your venting cuts, not the mold
cavity, with a fine brush.

Hope this helps you out.

Can’t resist … so here’s a blatant plug for good ol’ Gesswein

Gesswein carries both the Dow Corning Silicone E-Type RTV and
also the Castaldo RTV product. Available for same day shipment.

Best Regards,

Elaine Corwin
Gesswein Co.
Bridgeport CT USA
Orders: 1-800-544-2043
Phone: 203-366-5400
Fax: 203-335-0300

Dear Donna,

Congrats on JA Certification.

Some additional comments on J RTV.

First of all it is the BEST Silicone for jewelry available. And
I do mean the very best! I have benn comaparring various
silicones since 1976 when I first used E RTV. I get lots of
samples at school for comparrison. Some of my molds from back
then are still as good as the day they where made.

One note you shouldn’t forget to mention is that you can
contaminate the stuff by touching anything rubber including
rubber bands. I use either “C” clamps or spring welders clamps to
hold the frames together. Anything containing Sulfer will also
prevent the silicone from drying. It’s best to wash your hands if
you think you have touched anything rubber or sulfer. Even
touching a gum rubber mold for injecting can keep the silicone
from drying by touching your model or mold frame.

1/4 inch plexiglass is more rigid and won’t bow like thinner
materials. It’s also cheap.

Sometimes I will superglue a spru onto a model. Make sure the
glue is not a contaminant.

If you are in a time crunch the silicone will also cure in a
warm oven in about 15 minutes.

I would half to ask you if your molds have any bubbles. Usually
you vacuum in the bowl first so that the material rises and falls
and THEN pour and revacuum about 20 times up and down.

One additional tip for everyone is to use Magnisium Carbonate
for a mold powder. It is non toxic and Has been used for years in
other molding industries. Unlike baby powder it doesn’t have any
moisturizers or skin softners in it. Baby powder is good for baby
butts but will build up on molds.

All the Best & Congrats!

TR the Teacher & Student

Hi again Gus,

Some corrections/updates on Silastic J RTV mold material. I
think I said it sets up in 12 to 15 hours. It’s more like 15 to
18 hours even though the product info says 24 hours. I’ve poured
at 4pm and then cut at 8am the next day.

Other info:

  1. Clean mold frame and plates with denatured alcohol.

  2. Mix in translucent/transparent flexible plastic bowl,
    something without sharp corners to trap catalyst and prevent easy
    mixing. Translucency permits seeing proper mixing since judged
    by color homogeneity.

  3. Use rubber spatula to mix. Can clean bowl and spatula by
    letting residual gunk cure. It can be peeled off.

  4. Make different size frames, but always with extra 1 1/2" to
    2" extra height for vacuuming. I make different thickness frames
    and different width frames so I use only the minimum amount of
    RTV compound. The stuff is not cheap!

5.I mark my mold plate with felt pen to designate pouring depth
so I don’t waste.

  1. I estimate weight by weighing an existing mold that’s about
    the desired size and adding about 10 grams to it for waste
    material stuck to utensils. I divide that weight by 11. This
    amount (1/11) is the weight of the catalyst. Ten times this
    amount is the weight of the base. I tare the bowl on my scale and
    then pour in the base material. I then carefully drip the
    catalyst to reach the final total weight. I usually transfer
    some catalyst into a small squeeze bottle so I can add a drop at
    a time when I’m almost up to weight.

    1" x 2" x 1 5/8" mold is approximately 72 grams
    2" x 2 1/2" x 3/4" mold is approximately 81 grams

  2. Be sure you don’t have Silastic J come in contact with
    regular rubber. It won’t cure where there is contact. For
    instance, if you mold a model in regular rubber and then try to
    mold it in Silastic J, you’ll have uncured, sticky slime on your
    model. You would have to absolutely clean your model, be it by
    refinishing, heating and pickling, whatever.

Happy molding, Donna

I’ve used the Green RTV, Rio Grande sells and it works “GREAT”,
I love the stuff I was alittle hard at first but once you get it
figured out you’ll love it!! no parting line problems and shoots
great everytime. Good luck and try it out! By the way I do’nt
work or get paid for telling you that, LOL, it’s just a good
product, not cheap, but does the job,

                                          Matt the Catt

Dear TR, Thank you for your email. I did a follow-up posting on
Silastic J that mentioned rubber contaminant problems.

My 1/8" plexiglass plates are no more than a 1/4" wider than my
frames so they don’t warp. I use two strong rubber bands, each in
a spread “x” formation. If any silicone compound gets on the
rubber bands, I throw the rubberbands away after the mold has set

I don’t bother vacuuming the stuff in a bowl first because so
much air gets trapped during the pouring anyway. I scrape every
last bit into my molds that I can. I haven’t counted the number
of up and downs I do, but about four minutes of
vacuuming/releasing,etc seems to do the trick. I have never had
any bubbles.

The next time I do a mold, I’ll try the 15 minutes in a warm
oven. By the way, exactly what temperature are you using? 200
degrees Fahrenheit?

I have used Silastic E, the white product, in the past but I
find it a little too soft. J is much better.

Thanks again,

Dear Donna,

If you haven’t used “L RTV” try it as soon as you can. You’ll
think you died and went to heaven. It is harder than the “E” and
softer than the “J”. I use it at least twice a week. All the
silicone samples I get I compare to the “L RTV”.

I used rubber bands when I started but would flip the plexi on
occasion and the silicone wouldn’t cure on the sides that touched
the rubber bands. Your hands are also touching the rubber banmds
extensively and could give you some bad molds sooner or later.
Now I don’t take the chance and use the clamps. Carpenters clamps
also work well and they have those neat rubber coated ends.

You can cure the mold in 10 minutes if you want, (at the 200
degree temp) but your mold will expand at the top slightly. I use
about 150 degrees and leave the mold in the for about 30 to 45
minutes or untill it is dry to the touch. I cure all my molds
this way. The total dry time for air drying is abour 48 hours so
by oven baking you can change this to the shorter time. This does
not harm or damage the mold in any way. And you reach the
materials final hardness right away.

I usually mix seven to ten molds at a time so pre-vacuuming
before pouring is a must. If I poured into the various size mold
frames I use, the stuff would rise out of the frames and make a
mess. Once it falls in the bowl (during the first vacuuming) it
does not rise to that height a second time. So pouring the molds
is a breese. The “L RTV” is also great because after the pouring
and multiple secondary vacuuming any bubbles left come to the top
and pop themselves.

Happy Molding! & nice talking,

TR the Teacher & Student

Dear TR,

Thanks for the great info. Is L RTV also a Dow Corning product?
I haven’t seen it in catalogs. How is it sold(kit sizes) and
priced and do you know any West coast suppliers?

I usually make only two or three Silastic molds at a time and my
frame heights are consistent so the risings of gunk coincide
pretty well. I don’t have to worry about random overflows. I also
religiously clean my frames and plexi plates with denatured
alcohol every time I set up so there is no possiblity of flipping
the plexi and introducing contact with rubber from the bands or
dirty fingerprints. Also, any notations or height marks done in
magic marker are removed so I know every mark is current when I
set up.

You’re definitely right to vacuum the bowl if you’re doing that
many pours (7 to 10)at one time.

Thank you again very much!

Dear Donna,

“L RTV” is Silastic and as far as I’m concerned the very best
silicone available. Some Dow Corning venders may carry it but no
larger jewelry suppliers as of yet.

We used to get it from a Dow supplier for our college but
discontinued that type of purchasing. We turned the purchasing
over to Stebgo Metals in So. St. Paul MN. They carry one pound
as well as ten pound kits. Their price is also very good. A ten
pound kit is under $200.00.

They have been advertising in AJM lately and are great people to
work with. They can ship anywhere and can be reached at

Best Regards,

TR the Teacher & Student

Too many years ago to remember I made RTV molds of flat items. I had
a liquid that when painted on a cured layer of RTV a second layer of
fresh RTV would no stick to the original layer. Is there still such a
liquid and if so what is it and where can I buy it.

The process I used was: I laid down a layer of modeling clay and
build a wood frame (before I knew there were aluminum frames) around
it. I poked depressions in the clay to act as locking lugs. I laid
the flat wax model with sprue on top of the clay so that it stuck to
the clay. Poured RTV on top of the clay and wax model. When the RTV
cured I turned the whole thing over, coated the cured layer of RTV
with the liquid and poured another fresh layer of RTV on the
original. The cured RTV mold could be separated at the painted joint.

I don’t get around much any more and my knowledge of RTV mold making
is somewhat limited. I am probably describing a process many of you
use. Its sort of like telling everyone how to make a wheel.

Sure could use some help

Hi Lee,

I am not sure what substance you used to coat the RTV, but I do know
who would. There is a store in the Village in NYC that is great for
stuff like this. I attended a workshop there and was nothing but
pleased with them. If you call them they will point you to the right
substance and you can even buy it from them online as well. They are
very helpful. The store is The Compleat Sculptor. Here’s their info:


By Phone:

Outside NY City: 800-9-SCULPT
Inside NY City: 212-243-6074 By Fax:
212-243-6374 By Email:
Technical Support Hotline:

Augest Derenthal
Cry Baby Designs


All RTV mold materials mfgrs. have one or more releases/parting
agents like you are asking about. I use silicon brush or spray for
any RTV’s except silicon and for silicon RTV’s is use Teflon. Either
can usually be bought at a good hardware store or auto parts store,
or from your mold materials supplier. I find that release materials
from the auto parts store is a bit cheaper that the mold supplier. I
use a lot of silicon RTV (Smoothon) and I get a Teflon spray and
bulk liquid from a company called Miller-Stephens. Teflon "should"
work for other types of RTV than silicons, but I usually use silicon
releases as they are a bit cheaper. Be sure to do a test no matter
what you choose to use.

John Dach

PS If using silicon or polysulfide RTV materials, be sure the clay
you use does NOT have any sulphur in it as sulphur will inhibit the
RTV from curing where ever it in contact with the clay.