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Vulcanizing Vs. RTV

To John Dach:

I would be interested in your more detailed explanation as to why you
quit vulcanizing and went to RTV.


An added RTV vs. Vulcanized question. According to ads and catalog
descriptions RTV has a shorter self life (cured) and/or less number
of copies before deterioration. How about some real world experience?
And are the RTVs improving in recent years?

Thank in advance, Ed


Molds can be made directly from wax masters therefore zero shrink
factor (rather than going from wax to metal [shrink and possible loss
from casting failure] to rubber), multitude of durometers
(hardnesses) of mold materials are available, long life of molds,
ability to easily make 2 (or more) part molds with no cutting, better
wax release than from vulcanized materials, no vulcanizer needed,
easier to show a new person how to make RTV molds than vulcanized
molds (no frame packing needed), no special frames are needed to make
molds in and I use RTV’s in the foundry mold room most every day so I
am VERY familiar with them.

If I think of more reasons, I will let you know.

John Dach
MidLife Crisis Enterprises
Cynthia Thomas Designs
Cynthia’s sculptures are at:
Maiden Metals,
A small bronze foundry, no web site yet!!

Hello Ed, RTV descrbes a wide variety of products. A lot of folks
call them all “Silicone”. A true silicone mold has a very long shelf
life, but can tend to get brittle and parts of it can break off. The
same is true with a poly based RTV. The most popular compound (and
most often misrepresented as “silicone”) is an isocyanate two-part
compound.This compound has good flexibility and is easier to cut than
most RTV’s but, this is the one that has a reputation for melting like
the wicked witch of the west, usually after a couple of years but,
it’s still a useful tool. All of the RTV’s are a two part system. The
true silicone compound is quite a bit more expensive than the others.
One should not think of the choice not as “either-or” but as another
tool in a moldmakers library of knowledge. Natural rubber is by far
the best choice for long term production, in my opinion. Don’t settle
for anything less than Castaldo. I have seen and used natural rubber
molds that are over fifty years old that still work great! However,
concerns such as shrinkage for a modelmaker working on a series of
production molds, say for a set of rings of the same design in every
size, can benefit from the nonshrink property of an RTV … then make
your final production molds in a natural rubber. Some folks just dont
want to buy a vulcanizer, which is cool but, make sure you keep your
model on file so you can remake the mold when necessary. Not mentioned
yet in this discussion are the Vulcanized Silicone rubbers. This is a
true silicone compound that require vulcanization to cure. Castaldo
makes them in a variety of beautiful colors! They are a little
slippery to cut but, once you get the knack it’s fun and the surface
of the wax injections can be the best! Natural rubber is still the
best to make intricate molds with plugs, spirals, pins, etc. I use an
RTV when molding large pieces that would be difficult to cut
otherwise.I set up my model in modeling clay that contains no sulfer
(it will prevent the curing of an RTV).(“Klean-Klay” available from
Art Chemical Products, Huntington, Indiana, 46750) Put the clay in a
small box and shape the clay right where you want the parting line to
be. Make sure to put some small holes in the clay that will become
your locks, have high walls on your box and seal all the corners with
tape. Some folks use furniture spray wax but, I use a parafin based
spray mold release. Mix the RTV, pour half your mold, let it cure,
remove the clay, turn the cured half over in the same box, spray with
mold release again withe model in place, and pour the second half. The
mold will separate easily right where you chose your parting line
earlier, in the clay. Oh yeah, don’t forget to mold in place your
sprue former and sprue(s). If any body is interested I will be
offering a four day intensive moldmaking workshop,later this summer.
Please e-mail me off-line. John, J.A.Henkel Co.,Inc., Moldmaking,
Casting & Finishing

I have used the RVT a lot and I always hoped to get a vulcanizer
thinking this would give me more detail in texture. So have others
found it is equal? Thanks Coit

Mold life of many of the RTV rubbers is nearly indefinite (silicon
and the urthanes while the poly sulfides {black tuffy without the
stink} seem to be good for at least 20+ years). Silicon and poly
sulfide molds are self releasing too, especially the silicon! As far
as mold life, casting hundreds waxes in the molds, to date, has not
caused any noticeable degradation. Even if there is a problem with
degradation for any reason, you still “could have” the wax master and
thus could make a new mold if needed (haven’t needed to do this to
date, but we do keep the masters just in case).

There are many ways to skin a cat, this is just one. If you REALLY
like the vulcanized molds, they work fine and have been doing so for
many decades. The RTV’s are newer, offer some advantages (to me
anyway) over the vulcanized materials so, now, I use the RTV’s
exclusively. I have not had but limited experience with vulcanized
molds, but my wife has been using them for 25+ years and now I make
all of her molds with RTV’s. We still shoot in some of the "old"
vulcanized molds, but I prefer making waxes in the RTV’s.

RTV’s are easy to TRY. No equipment just some “container” to hold
the liquid rubber around the piece until the rubber goes off. A fast
and easy “test” mold frame is a PVC pipe coupler cut in two, length
wise with a scroll saw or jewelers saw (make an area of the cut on at
lease one side “odd” so you know how to put it back together in the
proper order). Attach your piece as you would with wax or metal and
injector button to a piece of glass or plastic, put the cut/assembled
pipe over the piece, seal the pipe to the glass or plastic with tape
or for me, oil clay, fill part way with vacuumed (if possible) RTV
and vacuum the piece/pipe assembly, fill assembly full with RTV and
let is sit til “hard”. Now you can test the RTV against the
vulcanized mold and see what you think.

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

        I have used the RVT a lot and I always hoped to get a
vulcanizer thinking this would give me more detail in texture.  So
have others found it is equal?  

All I can say is if the detail is in the piece/master, it will be in
the RTV mold. What type of RTV were/are you using? Might be the
type of rubber… not suited for what you are doing. We get
fingerprints in our RTV’s .

MidLife Crisis Enterprises
Cynthia Thomas Designs
Cynthia’s sculptures are at:
Maiden Metals,
A small bronze foundry, no web site yet!!

This is Michael Knight at CASTALDO.

We have a new RTV rubber similar to Silastic but not thick as tar,
not frightfully expensive, not easily torn and lacking in strength,
not hard to mix, hard to de-bubblize and not hard to use. And not a
silicone – a newer technology.

It’s called our CASTALDO� LiquaCast 0% Shrinkage RTV Liquid Jewelry
Molding rubber. You can read more about it at our web site at:

You can make a mold of absolutely anything. I’ve made molds of
strawberries and chocolate cherry bon-bons.( you can see them on the
web site). And it’s perfect for making molds of wax carvings-- no risk
of losing your carving in a bad casting!

I’d be happy to send anyone interested a small free sample. Please
let me have your shipping address – no P. O. boxes, please.


		Orchid Notice: 
		Please contact the author off list!

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

End of forwarded message

...ability to easily make 2 (or more) part molds with no cutting... 

How can you make two or more part RTV molds with no cutting? Thanks

Hi Noel, Please re-read the original post with the detailed
explanation for making a two(or more) part mold. Hint…the model is
set up in modeling clay and poured in two separte operations.
John, J.A.Henkel co.,Inc., Modlmaking, Casting & Finishing

Noel, It is easy to make two or more part molds with RTV. I have
been doing it for years. The simplest way is to coat your model with
a release…I use Vasoline, then spru your model into a mold box (any
neoprene release plastic will do…also coat the inside of the box
with vasoline) about half way down the width of one end. Since you
are then going to lay the box on its back, you might have to support
the model if the spru base is not heavy enough. You can do this with a
toothpick between the back of the model and the box. During injection
this can act as a pressure release as well. Mix the RTV and pour in
enough to fill the box about halfway up the side of the model. Put it
aside and let set overnight. Next day, cut two or three short (about
1/2 to 1 inch) shallow (1/8 inch) “trenches” into the RTV to the
around the model to act as alignment keys. Coat the surface of the
RTV including the “trenches” with vasoline, mix and pour enough RTV to
fill the box. The next day, pry the mold out of the box and the two
halves will come apart in your fingers.

This works best on models with no serious undercuts. There are other
techniques for making more sophisticated molds of rings and complex
items that have hollows, undercuts, etc. Cheers from Don at The
Charles Belle Studio where simple elegance IS fine jewelry!

    How can you make two or more part RTV molds with no cutting?
Thanks --Noel 

There are a couple of ways. What I do is release the master with a
good release agent (silicon, teflon) so the clay in the next step
doesn’t stick to the master. Then build a layer of soft oil clay (no
sulfur clay only) up on the master to the place where you want the
mold to to separate and make it larger than the master to give you an
alignment/sealing edge around the piece and make some rounded indents
in this field to act a registration buttons. Build a dam of clay on
the outer area of the edge, or place a mold frame over the master,
resting it on/pressing it slightly into, the clay to make the first
1/2 of the mold. Pour in the RTV to fill. When the RTV is hardened,
turn over the “mold”, master,clay assembly and remove the clay that
was used to create the separation barrier for the first half of the
mold. Now add the second half of the mold frame or make another clay
half with RTV. When hard remove the master and there you have your 2
piece mold without cutting.

Upon re-reading this, it is not as clear in works as I would like,
but I hope it at least gives you an idea as to what is to be done.
This is the method I use for most all of the molds I make in the
foundry. If there are other questions, feel free to contact me.
There are a number of books available showing this and other

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

I have been making two part RTV molds for quite some time now. Lately
I have started using the aluminium mould frames designed for
vulcanised molds to contain the silicon rubber. I place the frame on a
piece of glass. A little vacuum grease between the two surfaces
prevents any of the liquid RTV from leaking out. I then pour in a thin
base layer and cure this quickly by placing the mold in the oven at
about 60 degree celcius. Once this is hard I attach the sprued
original to the mold frame. Most of these frames have holes drilled
through them for this purpose. I then stick two tefflon pieces to the
base layer, using a little RTV as adhesive.These will form holes in
the next layer, which will act as lock lugs for the two parts of the
mold. You can use almost any plastic or metal shape for this purpose.
The only requirements are that the RTV does not stick to it and that
the shape is not wider at the top than at the base (I know - kind of

Once the tefflon pieces have adhered to the base layer, I pour in the
next layer. At this point you have to be sure where you want your
dividing line to be. With simple pieces this would just be a
horizontal line. With complicated pieces you may have to incline the
mold at various angles, allowing successive layers to set before
continuing. You can also block off areas that you do not want filled
at this stage with plasticine.If your original is made out of wax,
this part is quite time consuming. Otherwise curing can again be sped
up in the oven. This can be messy if you did use plasticine, as it
releases oil when heated.

Once the second layer has set, you can remove the spacers (lock
lugs). I then spray the surface of the second layer with a tefflon
spray to act as a barrier coat. You can also buy special fluid, from
the RTV suppliers, that can be painted on. (Check with your supplier
what can be used as some RTV’s will not set if in contact with the
wrong materials.) Once this has dried, I pour on the final layer. Once
this has set you can remove the mold from the frame, and your 2 part
RTV mold is ready.

If you have originals with a lot of surface detail, it is best to
paint a bit of RTV onto the bottom surface before pouring the lower
layers. Otherwise you are likely to trap some small air bubbles.

Hope this helps.

Now, does anyone have some detailed instructions for making 2 part
vulcanised molds? I have not had much success with them.

All of the best from a chilly South Africa.

Nils Schwarz
Johannesburg, South Africa


2 part vulcanized molds can be made by dusting the “bottom” layer of
the 1/2 filled mold with talc, or some other barrier material. Then
add the second layer and vulcanize it. Also, the newish putty type
vulcanizing materials are easier to do a 2 part vs the “older” rigid,
cut and fit rubbers.

Hope this is enough info to answer your question.

John Dach

One way is to make a flat framed base (PVC pipe section) of
non-sulfur modeling clay, embed model halfway into clay, put in
indents to form positioning pins. Cover exposed half with RTV.
After it sets, remove clay, a little seperating dust, and fill newly
exposed side with RTV. Mold requiring no cutting. Don’t forget the
to use a sprue former in the clay to allow for a way to fill mold.

Castaldo now has a new silicone mold material that, if it’s like the
"Belicone" brand I use, is a great product. (I’ve always loved
Castaldo’s consistant quality and price). It’s soft and moldable,
like modeling clay. You vulcanize it like the old rubber molds,
using the same mold frames, except, you fill half the mold, using
inexpensive metal locators in the corners, sink your model into the
material, dust with a parting powder, and pack it the rest of the way
up. After you cure it in the vulcanizer, you just pull the two
halves of the mold apart. No cutting! It’s really fast, and as you
get to understand the geometry of the way molds should part along a
model, you can “program” a good division, and it actually, since it
has to flow in to the model, sort of helps find it’s own best, sort
of “fluid logic” point of separation. If the Castaldo product is
priced competitively, which I would expect, it’ll get my vote.

David L. Huffman

Each Particular rubber has it’s own uses depending on the situation.
RTV’s are usefull when a customer has carved a wax and is afraid that
it may be lost in casting by the caster( this can happen ). This is
used as a backup mold in case the casting fails.( i would suggest
that you ask your caster what he thinks before spending the extra
money) In our case, we have not lost a carved wax (casting) in 10
years due to the extra pecautions we take in our investing procedures
. This has allowed us to reduce the cost to our customers. RTV molds
usually cost more as they do take more time to setup properly for a
manufacturer. For an individual jeweler/ designer, the RTV molds
allow you to have less equipment reducing your initial investment in
machinery and molds.

Using heat cured (vulcanized molds) ,multiple vulcanizers and a large
assortment of mold frames allows the manufacturer to set up the molds
quickly and get production moving faster. As an example, We have in
some cases had as many as 80 different styles to produce for our
customers in a short period of time (24 hours ) and this was
accomplished in 10 hours of mold making time and 1 person doing all
the work. We used Vulcanized Silicone rubber which is easy to setup
and with a experience allows the mold maker to follow the items
contours (reducing/eliminating parting lines)and powder seperating
instead of hand cutting. Both methods work well (handcutting and
powder seperating)if the mold maker is good at whatever method he

There is shrinkage in the injection wax process as well as in the
casting process so using an RTV only eliminates one of the
shinkages.Also, A very real problem with making Multiple RTV molds on
some items is the possibility that the wax may eventually be broken
and you no longer have a usable model. Some casters will tell you that
is not a problem as the RTV has no shrinkage and they can simply take
a casting and make a new mold…This is not true ! There is the
injection and casting shinkage and then the casting needs to be
cleaned up (which removes metal)and so the next mold and castings
will have a different thickness and a lower weight than the original.
For most manufacturers, it is quicker if the model is made of metal
and has had the proper shrinkage calculated into it. Any caster who
has a lot of experience should be able to advise you and make either
type of mold for their customers. Just my $ worth . Daniel Grandi mold making, casting, finishing in
gold, silver, bronze and pewter for designers and jewelers in the