Making RTV Molds
By: Shep Paine
(From notes compiled by Judy Brown on a seminar given by Shep Paine at the 1987 "Chicago Show"...)
The molds are made with RTV (Room Temperature Vulcanizing) rubber. RTV rubber vulcanizes at room temperature. It
comes in two parts that you mix together. Store the RTV rubber mixes in a cool place. This will extend their shelf life
There is a wide variety of RTV rubber on the market. It is usually manufactured and sold for industrial use only. Look
under Pattern Maker's Supplies in the Business section of the Yellow Pages. It is not inexpensive. A small can will cost
about $20, a large can will be about $100. Eager plastics in Bridgeport sells "RTV 325" with a fast cure catalyst (1 - 2
hours) or with a slow cure catalyst (cures overnight). They sell this only in the $100 gallon container.
Characteristics of RTV to look for:
1. RTV should be easy to mix. Look for mixing instructions that specify less precise rations - 10 parts to 1 drop for
example rather than gram weights.
2.Tear strength is important. Look for a high tear strength on the directions. A higher tear strength tends to be more
3. Cure inhibitions. You have to be careful what the rubber comes in contact with. Plasticene (modeling clay) is a
common substance that inhibits the cure. A thin, smooth coat of Vaseline over the modeling clay will eliminate it.
4.Be sure your RTV rubber will handle hot metal at 400-500°. Eager RTV mentioned above lists a lower temperature,
but Shep has used it with hot metal with no problems.
One Piece Mold
The simplest mold is a single one piece mold. The pattern piece to be molded can be made out of brass, plastic, epoxy
putty, wood, etc. Try a simple piece to begin with. A base for a figure is a good start. You can mold from another base
or carve one yourself.
1.Take of piece of Plasticene (modeling clay) and flatten It out. Brush with Vaseline. Push pattern piece into Plasticene.
Build a dam of Plasticene around the edge, 1" higher than the pattern piece.
2.Mix RTV and catalyst in a cup with a wooden stick. Be careful about directions. It is better to add too little catalyst to
start. Too much catalyst will harden the RTV before it can be poured. Too little catalyst will merely lengthen the
hardening time. You will soon get the hang of exactly how much to add.
3.Pour RTV/catalyst mixture into mold. You want to avoid air bubbles. There are several ways to pour into the mold.
Dump all in at once, pour in a thin ribbon, or Shep's preference, pour into one corner and let liquid drift across the
NOTE: Leave the stirring stick in the cup to find out when the mold has set up.
4.When cured (check curing time on RTV package), carefully pull mold apart. Let mold cure a couple of days before
Two Part Mold
A two-part mold is a bit more complicated to make. As with the one piece mold, the pattern piece to he molded can be
made out of brass, plastic, epoxy putty, wood, etc. Try a simple piece to begin with. Shep demonstrated with an ax he
had molded from scrap lead for one of his figures.
1.Use Plasticene to create a false first half of the mold. Coat with Vaseline. Embed pattern piece half way in Plasticene.
You can control the parting line by how you place the piece in the mold.
2.Build sides of mold box from styrene plastic with corner slits so that the pieces may be assembled for more than one
mold. Press styrene plastic box around pattern piece, embedding sides in the Plasticene.
3.Make a sprue to pour the lead into the mold with a lump of modeling clay at the top of the pattern piece, not quite
touching the piece but touching the side of the styrene plastic box. Poke dimples in the clay, around the pattern piece to
keep the two halves of the mold in alignment. Any dull instrument will do. Not too many, four or five on a side will do.
4.Mix RTV and catalyst in a cup with a wooden stick. Be careful about directions. Add too little catalyst to start. Too
much catalyst will harden the RTV before it can be poured. Too little catalyst will merely lengthen the hardening time.
5.Brush RTV mixture over pattern part to insure a good copy, then pour at least 1" over the Plasticene.
6.When cured (well) push the first half of the mold to the bottom of the box, so you can pour the second half. Gently pull
mold and pattern piece away from Plasticene. You now have the first half of the mold.
7.Cover first half of the mold with a thin coat of Vaseline so second half won't stick. Make sure the pattern piece is fully
seated in the first half of the mold and hasn't popped loose, otherwise, rubber will seep underneath and ruin the mold.
8.Build sides of styrene box around the first half of the mold. Add second piece of modeling clay as a plug, touching the
top of the styrene box but not quite the top of the pattern piece.
9.Mix RTV with catalyst as before. Brush RTV mixture over pattern part to insure a good copy, then pour at least 1" over
the first half of the mold. Cure.
10.When cured (well) pull styrene plastic box sides off and gently pull 2nd half of mold away from the first half. Take out
pattern piece. Let mold cure a couple of days before casting. Cut the short distance from the plug to the pattern for
access for the melted metal.
Air vents in your mold are necessary. In order to get the metal in, you have to get the air out. You will cut one or two
vents in the mold to use as an air passage. Look at the open mold carefully and follow the line that the molten metal will
take with your eye to see where the vents should be placed. Remember to take in consideration the position the mold
will be in when the metal is poured. The vent should always go up eventually so the metal doesn't go up the vent. Keep
vents fairly narrow to avoid this. Make a casting with no air vents the first time, then you can tell from the missing parts
of the casting where the air vents should be cut. This method is fine with simple pieces, but when you mold a head, the
nose creates an additional problem. For this you drill a small hold all the way through the nose to the back of the mold
and vent the hole in the back of the mold.
Casting your two-piece mold:
The source of metal for your casting. Most figures are made of an alloy of tin and lead. Higher tin content is better for
small parts. The best metal to use is pieces from other metal figures and their bases. Preferable are Post Militaire,
Almond, Barton and Le Cimier. Shep keeps two scrap boxes —one box for the larger parts with a higher lead content
and one box for the smaller scrap pieces with a higher tin content.
1.Begin by warming the mold to 200° in your oven. This keeps the molten metal from cooling too quickly in the mold and
will give you a better casting.
2.Melt your scrap metal. Shep uses a bent soup ladle to hold the metal to be cast. You will need a block of wood for the
handle to rest on while the metal is melting. Turn your gas burner on your stove on high (400-500°). Rest the soup ladle
directly on the burner using the block of wood to keep it in place. It should melt within 5 minutes. When melted, turn the
gas down, just keep the metal liquid. The higher the temperature of the lead, the more detailed the parts. Occasionally
scrape the crud off the top of the melted lead. You don't want this in your castings.
3.Lightly powder both halves of the mold with talcum powder before casting. The quality of the castings will be better.
4.To clamp the mold, use 2 pieces of wood and a C-clamp. The C-clamp is placed on the side of the mold so it will rest
on a block of wood. The tension is tricky here. If the mold doesn't fall out and you can still pull the C-clamp off, the
tension is usually correct.
5.Set mold upright with block of wood under C-clamp. Pour molten metal. Jiggle C-clamp against block to settle metal.
When the metal in the plug turns from shiny (molten) to dull, it is solidified.
6.When metal is solidified, wait a few moments, and take off C-clamp. Set mold aside for a few minutes before opening.
One final note, there are laws against piracy. You can mold copies of figures you have purchased for your own use, but
not for your friends, even to give away.