Hi all, I need some thoughts on mold making.
At this point— (given the expense of getting my propane torch
safe) I won’t be investing in a vulcanizer for some time.
There are some products out there such as Quick-sil and RTV that
apparently do not require heat for curing and make reasonably sturdy
jewelry type molds.
The question is which product to use-- and whether the non-heat
method nets a good detailed mold-- and faithful reproduction of the
original.
Thanks in advance…
CS
Carol- We use both rubber and RTV for molding. We use RTV for
replica and wax molds. I really like it. It makes wonderfully
detailed molds. Waxes are really easy to get out. No spray or parting
powder needed. They are really easy to cut apart. You can actually
see where you are cutting.
However…
It’s expensive. It doesn’t seem to be as sturdy as a rubber mold. It
has to be really well mixed. You have to mix until you get tired.
You’ll have to have a vacumn jar to get the air out before you pour.
Use a huge mixing bowl. We use an old tupperware container. it
really bubbles up big time. When you pour into the little glass plate
mold frame it looks like there are a million air bubbles in it.
Relax. Wait an hour or so, and it’ll clear. It also takes over night
to cure unless you add some heat from a lamp or something.
Hope this helps.
Have fun setting up your studio and make lots of stuff.
Jo
At this point--- (given the expense of getting my propane torch safe) I won't be investing in a vulcanizer for some time.
A vulcanizer is a handy tool, but not the only way to cure that type
of mold. Your kitchen oven, along with a decent oven thermometer, can
be used to get a heated environment at the right temperature for
curing the rubber. Then a large heavy duty C clamp, along with some
steel plates heavier than the usual thin aluminum mold plates used in
vulcanizers, can be used to compress the packed mold frame. Less
convenient, sure, but doesn’t hurt your oven at all (for food use or
in any other way) and makes just as good a mold.
As to the room temp molds, The silicone liquids work well,
especially if you’re able to vacuum them, (at least, with the
versions that need that) The clear silicones also offer an easier
mold cutting experience especially for beginners, since it’s easy to
see where you’re cutting in the transparent rubber. And when you
inject it, you can see how it’s filling too. Akron has some nice
liquid silicone rubbers, as well as vulcanizable and traditional
natural rubbers. Castaldo too has some good rubbers, especially their
traditional natural rubbers (need heat). And their vulcanizable
silicones are nice. I can’t say I’m a fan of their non-silicone
liquids, or their new very low temp rubber. These aren’t, I believe,
actually silicones, but something else, and though they work, they’re
not as nice to work with. Lime green, for one thing, is a color
that’s just way too close to a dentist experience for me (not that
this really matters). More, these compounds seem to cure to something
that somehow never seems quite finished. A bit oily feeling, a bit
still like clay, or gummy, and not as strong as true silicones. They
work on some things, not so well on others, but I don’t like them at
all as much as the true silicones. But they do offer the advantage of
being very fast. A working cold mold in an hour or less. There’s
something to be said for being able to start with a hand carved or
CAD/CAM wax at ten in the morning, and from there, molded and a dozen
duplicate injections invested and in the burn out oven by 3… The
traditional liquid cold silicones would need 24 hours or so to cure.
But then the molds are better.
cheers
Peter
Hi CS;
Quick Sil will still require that you clamp the mold, putting the
compound under pressure. For any and all RTV’s, it’s necessary to
vacuum them. Rio sells a relatively inexpensive system (around $125)
that consists of heavy aluminum plates and a couple bolts. You
pre-heat the unit, then, with your mold frame packed, you can clamp
in the frame and vulcanize it in a toaster oven. Going with this
system will allow you to use the more economic silicone rubber
compound. I use one called “econosil”. RTV’s are relatively expensive
and, as I said, require a vacuum pump. I also like Quck Sil, but I
only use it if I have a model that won’t take heat and I don’t have
time to wait for an RTV compound to set.
David L. Huffman
You could use air pressure (put mold frame in a pressure pot) to get
rid of the air bubbles instead of vacuuming the material, or better
yet, use both. As to the mixing, try a small Jiffy Mixer to mix the
material. MUCH faster than by hand. To clean the mixer head, use it
a couple of times (to build up a decent layer of rubber) then just
pull it off. A friend of mine had a good number of mixer heads and
just burned the stuff off (had a bronze foundry).
John Dach
Carol, my preference is clear silicone RTV from Zero-D products. You
will get great results from silicone RTV as well as the less costly
Polyurethane RTVs. The polyurethanes tend to get sticky in humid
climates, but silicone RTV molds are permanent. You will need a way
to remove air from the mix.
Craig
The traditional liquid cold silicones would need 24 hours or so to cure. But then the molds are better.
the silicone I have can be fast cured in a 210 degree oven for about
an hour. though it does shrink about 1 to 2 % when you do this.
Robert L. Martin
Goldsmith/Platinumsmith
Diamond Setter
since 1976
The traditional liquid cold silicones would need 24 hours or so to cure. But then the molds are better.
Thanks for the kind mention of Akron Jewelry Rubber in your post
Peter.
I wish to point out that our silicone RTVs can be cured in one hour
if the model can withstand about 120F, as most can. Silicone RTV is a
very good choice with grown resin models, as many of the resins react
with the heat cure silicones, retarding the cure.
The less costly RTVs are generally Polyurethane, and although less
costly, are not necessarily permanent molds, especially in humid
conditions. Polyurethane tends to revert toward liquid in certain
conditions. The mold can become quite gummy.
Silicone chemistry changes rapidly, permitting us to continue to
improve these mold rubbers. If you haven’t tried the heat cure
silicones in a few years, it may be worth a call to us.
Regards,
Bill Mull
Zero-D Products, Inc.
http://www.zerodproducts.com
You could use air pressure (put mold frame in a pressure pot) to get rid of the air bubbles instead of vacuuming the material
John, What is a pressure pot? Is that a pressure cooker?
CS
I have to jump in here. It is not necessary to vacuum an RTV mold. I
made quite a lot of them back in the 70’s without a vacuum chamber.
I also regularly make molds of interesting surfaces with excess
material when making a regular injection mold. It is really quite
simple. Using a fine camel hair brush, brush a contact coat of RTV on
the model. Then carefully pour in the rest of the material to fill
the mold frame. The resulting mold will not be a strong as one that
has been vacuumed, but it will be quite serviceable for years. I have
a mold of a rattlesnake head that is close to 30 years old and still
works great it was done this way and the detail is incredible. I have
molds of wood grain, leather, metal and mineral materials.As long as
you work in a good contact coat of the catalyzed RTV with the brush
and work out all the air bubbles in the corners and crevices you
should have no problems. I usually use one of the more rigid
materials for this process as the air bubbles trapped in the body of
the mold do cause it to be more flexible and less durable… I love
RTV materials for making molds of carved waxes and other materials
that will not take the heat or pressure of a vulcanizer. One caution.
Any sort of oil including body oils if you are making biological
models, or oil as in oil clays will cause the RTV curing process to
be retarded…
Frank Goss
It is a sort of pressure cooker but most will take higher pressures.
We use a high pressure paint pot (will operate at 150 psi but
certified for much higher). Generally only a few dozen pounds
pressure (40 - 60) is needed. Most pressure cookers are rated at 15 -
20 psi and if a home cooker, they are pretty small. Also they often
leak a bit. The pot we use is 7.5 gallons and is set up with pressure
gauge to keep a set pressure in the pot, as well ad a safety relief
gauge. We use it for very clear (no bubbles) clear resin castings,
about the only way to get clear bubble free castings. Got it on ebay
for $200.00. This large size is $1000.00 ++ new. For small jewelry
molds (we got the big one to do life sized faces) you shod be able to
get a pot for a LOT LESS.
John Dach
i like RTV’s particularly LiquaGlass. What do all of you electricity
dependent jewelry makers do when the power is out? One more reason i
like RTV products…with LiquaGlass (Castaldo) it is very clear,
vulcanizes rapidly and if you do detailed work is very appropriate
and eliminates any guessing where your cuts are to go…rer
Okay…this made me laugh. The 2 days ago I was about to work on
some filigree work with the torch when the power went out. Lightening
struck behind our house and knocked a power line down/out. True, I
don’t need the power for the torch work…the lights, however, are a
very important part of my work. 
So…IMHO, I may not be electricity-dependent in some areas, but am
in others. I took a break.
Kim Paluch
http://of-the-earth.org