I have been casting silver for about 5 years and I think I have a
good handle on the process of lost wax casting. I need to advance on
to making my own molds and then wax injection. What I need to know
from all of you knowledgeable folks is what is the best process to
use. I see many different methods of the mold making process, but I
need good advice on which way to go. Any help will be appreciated.
Thank you in advance,
Ken Moore
Ken
I am experimenting with certain items, I find that "Silicone Rubber"
molds are totally non-shrinking. Vulcanizing rubber tends to have a
shrinkage factor or about 8-10%. Silicone is a liquid that is poured
into a metal frame, it hardens in just 1-2 hours without any heating
or compression. You can still inject this medium with any heated wax
you desire.
This medium is still soft enough to cut afterwards, similar to the
vulcanizing rubber… try it and you might even like it…
Gerry Lewy
It depends on what you are going to be casting…I cast silver
filigree and I use silicone based bellicone red for my molds. It is
a putty like rubber that you put in the form, then you can actually
put your original model in place and, to an extent, choose how you
want the mold to split by building up the shape of the bottom layer
to where you want it to divide, then dust the entire lower layer with
talcum powder. Try to avoid getting it on your original. Make sure
you put extra in a corner or two. Put the second layer of silicone
putty on top, carefully, then vulcanize. When you are done, find the
corner that you put extra talcum in…you should be able to
separate those corners, then pull the mold apart along the layer the
talcum is without major cutting with an exacto knife. This is really
a life saver if you are doing thin designs like I do, which are only
about a millimeter thick. They would be really hard to hit correctly
if you had to cut them apart! In addition, silicone based rubber
needs no release agent and tends to be sturdier and give a shinier
finish on your waxes.
You also need to look at what kind of work you are casting when you
choose which injection wax to use. While there are some that are
meant for detail work, I still prefer blue plast-o-wax or the
equivalent because it’s tough enough to get it loose from the molds
without it breaking apart.
Jeanne
Ken,
Molds… buy a vulcanizer, not cheap and 100 pound machine to deal
with. Industry standard, cheap and tough molds. Slight odour factor.
RTV, silicone or polyurethane. Casts more than regular vulcunizied
rubber and is not as durable. About all you need is a vac.
Ya gota learn to cut any type of mold (nasty sharp blade and
possibly some blood) Any method or rubber will allow fancy work to
make a two + piece mold, cutting is usually less work and effort.
Wax injection… air powered injector is the only sane way to go.
Industry standard practice is a good starting point, those old grey
guys in the back office have managed to learn a few things. Special
tasks call for special techniques and tools. No way am I going to
vulcanize a marshmallow unless life gets very boring 
jeffD
Hi Gerry,
I spoke to my teacher about this, in that I like to use RTV for my
wax moulds, and he preferrs vulcanized rubbers.
I asked about his choice, and he stated that is was a cost matter.
The RTV’s are more expensive.
Regards Charles A.
HI Jeff,
RTV, silicone or polyurethane. Casts more than regular vulcunizied rubber and is not as durable. About all you need is a vac.
I think it depends on the RTV, I have some moulds that have been in
use for 10 years, and they’re still good, they’ve taken white metal
as well as wax.
The RTV I used was a Dowe Corning product, and I used the catalyst
that contains anti-corrosives.
They were more expensive compared to vulcanised rubber.
I’m using Barnes RTVs these days, some more flexible, some skin safe,
even two part RTV putties, mainly for wax models, sometimes resins.
I’ll get a vacuum one day, but the bombs away method has worked fine
so far.
Regards Charles A.
Hi Ken,
Welcome to another learning curve. Like you I started with lost wax
casting. More recently I acquired a wax injector along with a
collection of tools and equipment I bought from a jeweller going out
of business. I decided that for my first foray into mould making I
would use RTV rubber. The only additional piece of equipment I
required for this was a relatively inexpensive moulding frame. You
also need a vacuum chamber for debubblising the mixture. I had that
as part of my vacuum casting machine. I decided to use Castalo
LiquaGlass RTV compound. To me this has three advantages, firstly
that it sets clear so it is easier to cut open, secondly since it
works at room temperature I can make patterns out of relatively
fragile and heat sensitive materials and third, since it does not
shrink during setting the waxes I produce are the same size as the
original pattern. For instance I can make a mould from a wax pattern
and produce waxes that duplicate that pattern. The disadvantages are
that the material is relatively expensive and less durable that heat
vulcanised rubber. However since I am only interested in making
short runs this is not an issue. As for the expense, it would take a
lot of tins of RTV to pay for a $1000+ vulcaniser.
With the RTV your mixing has to be very accurate I weigh out the
mixture components to an accuracy of 0.1g.
DO some reading and then just try it out. My first mould was a
learning experience but works just fine, I have already made three
pieces using waxes from it.
Enjoy
Jenny
Dear All,
It’s discouraging to hear the same old misconceptions repeated.
I’m referring to Gerry Lewy’s comment that silicone rubber molds
shrink not at all and “vulcanizing rubber tends to have a shrinkage
factor of about 8-10%.”
There is rubber shrinkage and there is casting shrinkage and the two
are totally different.
Liquid silicone rubber compounds do indeed, as Mr. Lewy states, have
as shrinkage rate of 0%. But that’s for the rubber itself – less
than perfect casting techniques can easily generate shrinkage rates
of 8-10% in the final casting even with liquid silicone rubber molds.
Vulcanizing silicones (not all silicones are liquid RTVs and not all
liquid RTVs are silicones) and natural rubbers have varying rates of
shrinkage, from 2.3% for our Castaldo White Label to our Castaldo
Very Low Temperature (vulcanizing) silicone molding rubber, with a
shrinkage rate of 0.1%.
Once again, that’s the rubber shrinkage.
Length, thickness and placement of the injection sprue, injection
time,temperature and pressure, how soon after injection the wax was
cast and 100s of other factors create shrinkage (as well as
porosity).
I hope all this was of help.
Michael Knight
Castaldo