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Cold pour silicone VS vulcanizing rubber


Just a quick word on making vulcanised mould - I make mine without a
vulcaniser. Instead I place two aluminium plates either side of the
mould frame which are then clamped with 3 or 4 “G” clamps and placed
in my kiln for an hour with a retighten after 8 minutes of the
initial 1 hour period - works fine for me.

Alan Lewis


Since the differences have been discussed and are clear we make both
types. Some customers have preferences but, try making thirty molds
in eight hours in a vulcanizer. We do this overnight.

Larry Paul Casting Co. Inc.


Hi Folks, Here is some info regarding cold molds versus vulcanized
molds . some of this info has been mentioned by others and some has
not. Some benefits of a cold mold (RTV, etc) are reduced shrinkage of
the mold only . It is not “O” % shrinkage as many would like you to
believe… Some claim 1/2 % some are actually 1 1/2 % and this can
change depending on mix, and heat if applied.All the other shrinkage
through the casting process are the same. The benefit of a cold mold
is that you can mold carved waxes and an assortment of non metallic
as well as metalic designs and objects. OK… Now, lets look at it
from the manufacturing side… I don’t mean making a few pieces every
other day… I mean manufacturing… which is what we do and on some
days in excess of 5000 pcs per day. In the instance of reasonable
volume, these are the pit falls of a cold mold and the benefits of a
vulcanized silicone based mold. In our case, many customers send us
carved waxes to do limited production. We reccommned using a
vulcanized silicone mold in all cases and this is why. First, the wax
is cast into metal and then the wax is perfected . You won’t Believe
how many little file marks and small problems have been left in a
carved wax model until you do this.Now that the model is perfect in
metal, you won’t have the problems you would have to deal with in a
cold Rtv mold . So, Now, we are dealing with a much better model as
it has been perfected and a mold that can be cured in under an hour.
In a production situation, you can make as many vulcanized molds as
you need depending on the volume of the order.

Now, if you had made an RTV mold, suppose the wax model was broken
when it was removed from the mold… Now, you cannot replace that
mold if it was made incorrectly as the wax model is now Toast !!! Wax
models are fragile in most cases . You never know what the next big
selling item in your line is going to be so, you also may be faced
with making multiple molds and not have a model or a usable mold .So,
This is one of many reasons why we recommend casting your wax model,
perfecting the model in metal and then making a vulcanized silicone
based mold.

Best Wishes,

Daniel Grandi We do casting, finishing and a whole lot more for
designers, jewelers, students , catalogs and people in the trade.


Dear Dan, It’s too bad (in the spirit of great competition) we all
couldn’t have a great “Mold Off!”

I have made rubber molds for production since 1977. Hundreds of
them. The Castaldo gum rubber indeed are a fine mold. I also do quite
a lot of production.

I have never worn out one of my silicone molds. I have on the other
hand wore out the gum rubber molds. And I am talking about hundreds
and hundreds of waxes from one mold. This goes for school molds also.
The silicone does not deteriorate like the natural rubbers do. The
waxes release much better also.

I will put the Silastic L RTV Silicone up against any mold and any
mold material. Great stuff. Again I compare all molds to this product
as my standard.

Happy Molding

Todd Hawkinson
T.R. the Teacher


We make molds of almost every custom project . We have the rubber
pre-cut and stacked into the 3 or 4 layer half mold with only a cut
to fit the sprue former .We keep a box full on hand of these . Using
pre made mold locks and powder seperating technique a mold is
assembled and placed in the vulcanizer in about a minute .the powder
separation allows the mold to be pulled apart with no cutting ,
total time involved is less than 5 minutes a mold .It is rarely that
the 45- 60 minutes to vulcanize affect our work flow enough to
matter This has saved our b**ts many times .Many customers can
understand an idea better from a wax than a sketch or even computer
aided modeling . My point being that it is very easy and takes little
time or money [after buying the equiptment of course ] to keep a
permanent record of your work , and of course if you resell a
"custom" or “original” piece from a pre made mold it increases your
efficiency [profit ] considerably . Powder seperation has its
limitations but works well for archival purposes and 80 - 90- % of
all our work in general .With multi mold frames you can easily do 10

  • molds a day if necissary .

Mark Clodius


Hello mold makers - old and new. I heartily commend Daniel Grandi’s
explanation of the reasons for using the vulcanized silicone rubber.
When my hands became arthritic, nothing was more painful than
packing and cutting the old rubber molds. Tried that lovely, soft,
brick-red silicone rubber and never looked back. It works like
modeling clay and can be used with a separator powder (or creme) to
eliminate cutting at all, using the purchased (or invented)
registers. Do be mindful, however, that when you switch, you cannot
use your old rubber- contaminated frames and plates as is. I had to
buy new ones, since there was a stock of rubber to give away and
frames with it. Presumably there would be a way to “sterilize” the
old equipment, but I’ve no expertise on that. I also know from
other’s experience that using the kiln or an oven to vulcanize molds
using C clamps works just fine till you can afford a vulcanizer and
timer. Try it, you’ll like it. My silicon came from Contenti, by
the way. Rio and others have it as well now - just read the
catalog carefully and look for that tell-tale color. And, if
anyone has found a secure way to clean the old frames, let us know
how please.

Pat Hicks


Pat, I have used the Contenti silicone rubber for my mold=
s since
about 1980 and love it; yet I have found the occason to use the
natural rubber from time to time (usually because a customer insists
on it) and do not keep special frames for those rare instances. So
before and after using my frames for the natural rubber I thoroughly
scrub my frames with steel wool and xylene (Xylenol); this is a
powerful organic solvent (with a powerful odor) and has thus far
proven to remove the offending chemical traces on the frames that
would deter vulcanization. It is available in the paint section of
most hardware stores.We used this stuff as a wax solvent in preparing
histology specimens back in my lab tech days.

I make sure that I wear rubber gloves and do the cleaning
outdoors because of its strong and lingering odor, its toxicity, its
flammabilty, and probable carcinogenicity; but it works very well for
cleaning the mold frames.

Paul D. Reilly
The Paul Reilly Company
Colorado Springs, Colorado

And, if anyone has found a secure way to clean the old frames, let
us know how please. 

'46irst naptha, or equivalent solvent, till visible traces of the
old ’ natural rubber are gone, then to be sure, i sandblast them.
Works fine. I ’ don’t bother cleaning the mold plates, though.
they’re just bits of sheet ’ aluminum after all, and easier and
cheaper to just use new ones.



Hi Todd, I would certainly do a “mold off” competition just for the
fun of it… My experience in mold making dates back to 1968 ,in
Thailand when I was learning mold making from a 60year old
professional… I was 13 years old.My first hand cut mold was a ring
that is called a " princess ring" It had 28 + stones in the head in
3 levels with wire basket work underneath… no hole to remove the
rubber… all rubber had to be removed from inbetween each wire…
This ring was normally made in sections… by other companies… we
successfully made it in one piece… The first production run
required us to make production models and molds from size 4 to size
9… 10 molds of each … all hand cut. Suffice it to say that on
average from 1968 to 1978 I made daily between 15 to 20 molds of a
similar nature…This was because we made molds of each style ring
we produced in sizes 4-9 for ladies and 8 - 12 for men My family had
the first casting company in Thailand.

In late 1979, I came to the US ( I am An American) , went to work
for a Jewelry Supply house as an engineer designing machinery for
all forms of casting and mold making. Within 3 months of working
there… I developed the heat cured silicone mold rubber and a variety
of molding techniques to deal with the fact that the first heat
cured silicone mold material did not have the tear strength that
castaldo Organic rubber had. Then, I was sent to every jewelry show
in the US with vulcanizers and an assortment of models to show the
mold making Technique. This was the first time anyone had seen this
done in public … for free !!! People were going back to their shops
all over New york and the various other places that I was selling and
showing the techniques to get models of every nature you can
imagine… you can also imagine that these were always the " problem
pieces" they had and could not get good molds from their own
moldmakers… At the show, I made every one of these molds … and
they worked very well . The owner Of the company I worked for did
not believe that this material would sell in the very beginning as
all mold makers we met had no clue how to use it and I had to
basically teach many people how to do complex powder seperated
molds. After the first show… the material sold 1600 lbs… the next
show , it doubled etc… On the 4 th year of doing shows, the owner
of Castaldo came to me at the show and said " look, Daniel, I have
copied your rubber ! " … I promptly took out the new material we
were selling which was the improved version that had much more tear
strength and showed this to him. Castaldo later on came out with a
number of versions as even they could see that the heat cured
silicone was the wave of the future… Now, when you go to shows,
You See the Castaldo rubber company showing mold making techniques
and getting sales that way.

I quit being a salesman back in 1986 … I did not work for Castaldo
… And I never got dime one for being the original developer of this
for the lost wax trade… nor was I ever mentioned . It is a thing of
the past.

So , when it comes to doing a " mold Off" competition… It would
not scare me at all… At my shop, I run 4 vulcanizers and when I
sit down to make molds, I make between 25 and 50 molds in an 8 hour
period… I use tripple and quadruple frames double stacked in all
the vulcanizers…Figure this one out… 4 vulcanizers x 4 cavity
moldframe x 2 in each mold and a curing time of 2 hours as they are
double stacked = 32 molds every 2 hours . The most molds I have made
in a single day with an assistant was 128 molds . No one believes
this until they actually come and see it being done. This is what I
call " cooking" ! How many molds have I made in up to now… Probably
upwards of 80,000 molds … I guess you might say that I have enough
mold making experience :slight_smile: . How do I accomplish this many molds…
Well, first , the silicone rubber I use comes in 25" square sheets
1/4 " thick. I built my own “cookie cutters” So that all I have to do
is press the cutter into the rubber and pull out a perfectly cut
piece of rubber that fits perfectly into the mold frames that I use.
I make my own frames with a cnc machine So I have about 60 sizes of
mold frames to accomodat almost anything that comes into my shop…
I aslo have coresponding cookie cutters for all of them. This means
that I can ste up all the rubber for 4 molds in under 1 minute. I can
shape the silicone rubber to follow the countours of the models very
quickly so that the partying lines are exactly were I want them…
This takes me under a minute to do as the silicone is soft like
clay. Then, with powder seperation, I don’t have to hand cut the mold
when it is finally cured.It will seperate exactly where I want it
to. I use small locators in the molds … 7 in each mold … in a
specific patern that helps to eliminate parting lines . This was a
development that I had to do to use automatic clamp injection systems
that I built for high speed waxing. ( 1000 + waxes per day/employee
) Also, Have you ever heard of the spinwax process… this is where
youhave up to 30 models molded into one round mold 12" in diameter by
1" or more high… these are used in a spinning wax injection system
for extrem high volume of simple components such as charms . (
complex components can also be done, but the mold making becomes much
more difficult). I bring this up as I am also the original developer
of the spinwax system which was sold in the early 80"s by the
company I designd the machines for…We stopped selling these
systems after 4 years as it required me to spend at least 2 to 3
weeks with each customer that bought a system … to teach them how
to make these type of molds…It took up wayyy to much of my time. I
have one of these in my shop Incase I should need to produce 10,000
waxes per day with one employee… It has been used on rare
occasions after the 1990’s as the volume in this state has dropped
quite a bit.

I know that much of what I say is unbelievable to many people in the
casting field … ( Ihave been told by many that I am a b.s.
artist) , but , when people see what we can do in this company of
mine, they change their minds quickly!

Some of the other accomplishments that I have built … Many of
these concepts and products were stolen from me by various “
trustworthy” individuals … An automatic " tree cutter" … It
removes 300 castings off of a single cast tree , puts them in a box
, undamaged … in 17 seconds. 5 of these were built … 4 are /were
beign used by the biggest US manufacurers … I have the original .

The automatic clamp /wax injector… Design ws stolen from me and
reproduced overseas before I could get/ afford a patent in 1981

Tivac vacuum enhancers for casting flasks

Spinwax system. The list is much longer, but I gotta get some sleep

In any case, these are just some of the reasons why we are
considered by many as one of the best casting/finishing companies…
We build just about every machine in house for our own use. And I
have a lot of manufacturing techniques/tricks that really help make a
product . Best wishes, Daniel Grandi We do casting , finishing and a
whole lot more for designers, stores, jewelers , students and people
in the trade. Contact:


I’ve found a quick fix is to wrap aluminum foil around the frame
and, a sheet between the silicone and the plates. Larry