HOW TO MAKE A TWO-PART RUBBER MOLD
OF A COMPLICATED MODEL, PAGE 2
4) Use clay working tools, and refine the
level of the clay, so it comes to exactly halfway on
each part of the model.
the clay. The best tool for this is your fingertips.
a groove in the clay, about ½ inch away from the
model. This will make a ridge in one half of the mold,
and a matching groove in the other half of the mold,
which will help keep everything in alignment.
5) Decide where you want to put the fill
hole for the mold. In this example, we could fill the
mold from the top, where the saddle is, or the bottom,
where the horse’s belly is. The reasons we do NOT want
to fill the mold from the top are:
• The fill hole creates a connection of
casting material to the cast object that must be
cleaned off every casting. We do not want this to be
in a detailed or obvious place, such as the saddle of
• If the fill hole was in the saddle area, casting
material would have to flow down into the legs, and
air would have to escape from the legs at the same
time. This would probably not work, with the result
being air bubbles trapped in the legs, and voids in
the cast object.
We therefore choose to fill the mold from the
belly of the horse. We construct a funnel that will
allow the casting material to flow into the belly of
the horse. In this case, we use a small plastic mixing
cup, and the plastic cap from a cheap pen hot-melt
glued to the cup. The plastic cap which will make our
filler channel isn’t quite long enough, so we make the
final connection to the model with clay.
Filling the mold from the belly of the horse
means the casting material will flow into the body of
the horse, then rise up into the legs. The legs will
have air in them that needs a way to escape. We
therefore add air vents to each leg, and also add air
vents to the high points on the horses head and snout.
The horse’s tail does not need a vent because the
highest point on the tail is connected to the rear leg
- any air in the tail will be able to escape through
The air vents are made from 10 gauge plastic
coated electrical wire. This is a handy material for
making air vents because:
• It is the right diameter, about 1/8 inch.
• Most moldmaking materials won’t stick to the vinyl
• You can bend the wire to any compound curve and it
will hold the shape.
The electrical wire and the filling funnel
are treated as if they were part of the model - each
is embedded in the clay, so one half is in clay, and
the other half is exposed.
a PDF source list
Includes 26 suppliers of all mold making and
casting materials mentioned in ALL the demonstrations
on this web site, including silicone rubber,
polyurethane rubber, latex rubber, polyurethane
plastic, epoxy resin, polyester resin, molding clay,
foam board, release agents, pottery plaster, Hydrocal,
metal powders, other plastic resin fillers, and
gelatin molding compound. With company names,
addresses, phone numbers, and web sites. Downloads to
your computer as a PDF file.