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RTV a large object

Hello Orchidians!

First of all I would like to say how much I really appreciate this
site. It has really been an invaluable source over the months.

Now my question…I am carving a wax model of a badge that measures
roughly 3" by 3.5" and is roughly a half inch deep (eventually it
will be cast into a brass badge). I would like to make a back-up mold
of it as it is my first time casting with wax and somehow I am sure
that it’s probably not going to go as smooth as I plan, grin… I was
looking at using Castaldo’s RTV Liquacast but noticed that all of the
frames only go to 6 inches in height - nothing near the 3.5 in width
that I need. The nice lady at Gesswein said she would look into this
for me but it got me thinking how do you folks handle things like
this? I didn’t find any larger frames in my Rio or AJS catalogs
either. So what gives? Anyone have any work arounds? Feel free to
contact me off line.



I use cardboard and hot glue for an open sided mold but any strip of
sheet metal bent to a U shape and clamped between two pieces of
glass. RTV is an excellent idea.


 ...I am carving a wax model of a badge that measures roughly 3" by
3.5" and is roughly a half inch deep (eventually it will be cast
into a brass badge). I would like to make a back-up mold of it 


You can make your own RTV mold frame. Get a strip of 12 gauge sheet
metal (brass, copper or aluminum) that is as wide as the thickness
of the mold you wish to make. The length of the strip should be long
enough to provide for the two side walls and floor of the mold plus
an extra six inches or so. The extra six inches will allow for
expansion during the debubblizing phase. You will also need two
Plexiglas plates to clamp to your metal frame. You probably want
Plexiglas that is thick enough not to flex - 1/8" or thicker. You
will need clamps, clips or heavy duty rubber bands to hold your side
plates against your metal frame.

  1. You are going to make a flat bottomed “U” with your metal strip.
    The bottom of the “U” dictates the width of your mold. The two
    vertical legs provide the side walls plus extra height to allow for
    expansion of the mold compound during the vacuuming process.

  2. After you bend the “U”, flat lap the faces of the “U” so a
    perfect contact between the metal frame and the Plexiglas plates is

  3. You can solder or glue a sprue former on the floor of the “U”.

  4. The Plexiglas plates should extend slightly beyond the metal
    frame but not too much. You want to minimize chance for flexing when
    you clamp or rubber band the plates against the metal frame.

  5. You might want to smooth the edges of the Plexiglas to prevent
    sharp edges from cutting the rubber bands or you.

NOTE: If you are using opaque RTV compound, use a marking pen and
mark the height of you mold on one of the plates. Also, mark which
side is front. When you fill your mold frame after having inserted
your model, use the marker line to determine how much to fill. If
your mold frame isn’t stable enough to withstand repeated vacuuming
and some jiggling, attach it with tape or a rubber band to a block or
something to increase it’s stability. Debubblelize with a vacuum
machine. If the compound rises to the point of overflowing,
temporarily release the vacuum to shrink back the compound. You may
have to restore and release the vacuum several times. After your mold
is cured, remember to run the marking pen along the front edge of the
exposed top of your mold BEFORE you remove it from the frame or
you’ll have to find out the hard way which side is front.

You can make inexpensive jewelry model sized RTV frames by buying
scrap Plexiglas from plastics dealers (like Cadillac or Tap Plastics)
and strips of brass from from hobby or hardware stores (K&E Metals).

Hope this helps you out.
Donna Shimazu

Dear Kennedi,

Go to a hobby or hardware store. They should have a brass metal
selection. Find brass strips about a foot long. They are machine cut
so they are a true width. You should need between 3/4 of an inch to
an inch in width. These are usually no more than a dollar in cost.
Measure your piece and bend the frame to fit around the model. You
can hammer a bend by holding the brass in a vise. Leave about an inch
around the model. Buy a brass spru former and tin solder it into the
bottom of the frame. The frame should look like one of the Ferris
mold frame available everywhere.

Next find some 1/4 inch Plexiglas. This will cut with a wood workers
coping saw or band saw if you have access to one. Cut so there is
space around the frame by about 3/4 of an inch. This is to replace
the glass ones provided from a jewelers supply. Safer and cheaper.

Finally at a hardware store or woodworkers store there are large
spring clamps that should fit and hold everything together. The
Plexiglas should not bend being 1/4 inch thick.

Attach your model with a 6 gage wax wire held on with sticky wax.
Make sure it doesn’t fall off. The silicone has to pour over and
around this set up.

Mix your silicone like you usually do pour and wait for it to cure.
After curing it should just pop out of the frame.

I have made dozens and dozens of these frames for a fraction of what
the purchased ones cost.

Best Regards,
Todd Hawkinson

Hi Kennedi,

I bought a sheet of acrylic plastic from Home Depot. I cut it into
pieces to match the size of mold frame I wanted. Make sure the edges
are square for a better fit.

Epoxy a sprue former to the base of the frame. Clean the model with
denatured alcohol. Some RTV compounds are very forgiving. Others
will not cure over fingerprints and leave gooey marks where the oils
from your fingers clung to the wax. Make sure the model is dry. The
alcohol can mess up the cure too. Attach the model to the sprue

Next I set up the sides by taping (I used masking tape) them
together. I’ve made some huge bracelet molds this way. Just be sure
that all the cracks are sealed so that the RTV compound does not leak
out during vacuuming. If you push the mold by warming it a bit, be
sure not to soften the sprues too soon or the object will fall over
and touch the side of your mold frame. You won’t know this happened
until you cut it!

Remember, you’re going to have to cut this big mold. Not so bad a
thing if you’re using soft material like Liqui-Cast. It can become
difficult if you use something nice and hard like J-Silastic RTV

Chuck in Asheville

Making a mold frame for RTV:

Aluminum bar stock, 2 glass plates, 2 spring clamps, and a sprue

You can find aluminum stock in almost any thickness and width - I
beleive we use 1/16th" to 1/8" in thickness - and a width to
accomodate the mold master. You’ll need a vise and a hacksaw.
Figure out how much material you will need, cut it to length. Bend
it into a “I_I” or “U” shape that will contain the object you are
molding. Allow a little space so it won’t touch the sides. Make
sure that the width of the stock you’ve chosen also allows a little
wiggle room. Clamp it straight and true in the vise before bending.

Clamp the 2 glass plates to the “U” with the spring clamps, and you
are ready to test pour. (The spring clamps we use resemble huge
clothespins and can be found in most hardware stores)

Test first with syrup or molasses to see if there will be leaks.
Usually the two corners expand a bit when bent sharply and need to
be sanded flush with the rest of the frame. Sometimes you haven’t
made your bends square and true, and you may have to do some
tweaking and further sanding.

Glue a plastic, wax, brass or aluminum sprue former at the bottom
of your “U”. Use a plastic, brass, or aluminum rod as the sprue to
your piece. Glue everthing in position. Some plastics will actually
bond to some RTVs, so check/test first on small samples of whatever
you have available.

Not clear enough? Let me know.

Brian P. Marshall
Stockton Jewelry Arts School
Stockton, CA USA

Otto frei has 4" x 6" inside molds both in 1" thick and 1-1/2"

items number 122.54 and 122.547 in 2004 catalog page 188.

Also you might check into the methods used for larger sculpture
molds You can get some help from:


You might want to consider sending it out to be molded, you will get
the results you are looking for without having to buy extra
equipment and tools that you will rarely use.

If you build a frame it will work.

the local hardware store should have some aluminum strips about 3/4"
wide all you have to do is make a frame as big as you need in the
shape of a big U and find some glass or aluminum plates that fit,
like from a vulcanizer and some large rubber bands, a spru base and
away you can make them as large as you need and that will fit
in your bell jar. I like silastic rtv it is more forgiving and has a
longer archival shelf life with out special care. that’s my. 2 cents
worth and reference to a baseball movie.

Robert L. Martin
diamond setter

    You can make your own RTV mold frame. Get a strip of 12 gauge
sheet metal (brass, copper or aluminum) that is as wide as the
thickness of the mold you wish to make. 

The easiest way of making mould boxes for RTV is to use Lego bricks.

The only thing you have to worry about is the wrath of the small
person you have ‘borrowed’ the bricks from.

Bill Bedford

The easiest way of making mould boxes for RTV is to use Lego bricks

Bill you are a genius, I have been mangling wood and aluminium for
ages for odd size mould boxes.

Thank you

Andy Parker, Agate House Lapidary
Ulverston, Cumbria, England
Tel: 01229 584023

Continue from:

I just wanted to thank everyone for all wonderful help given to me
with the RTV mold problems I was having. You guys are really a
creative bunch who know how to get the job done. It reminded me that
who ever coined the old cliche "jack of all trades, master of none"
really never knew a jeweler - and it needs to be changed…to
something more like “jack of all trades, master of adaptation”.
Thanks again for all your help!