How To Make Flexible
Basically, there are five different types of flexible
mold compounds in significant use today: polyurethanes, polysulfides,
silicones, latex and hot melts. Each type has some advantages and
disadvantages over the other types.
The "Hot-Melt" (polyvinyl chloride) systems
are inexpensive, since the worn molds can be melted and recast.
However, they are hazardous to handle because of the high temperatures
required for melting. This also limits their use to model and pattern
materials that are heat resistant.
has the advantage of producing a very elastic, thin-walled, strong
mold, but shrinkage is high and molds must be supported to prevent
distortion. Because as many as 10 to 20 brush coats must be applied
for adequate thickness, with time allowed to "dry" in
between each coat, making the mold is slow and time consuming.
rubbers make excellent molds for casting resins and foams, but
material cost is extremely high.
rubbers (Smooth-On FMC Series) are quite versatile and are especially
useful for casting plaster, Hydrocal, and other water mixed cements.
They also find use for casting wax candles and some limited use
for resin casting. They are easy to handle, non-adhesive and can
be poured over clay or practically any type of model or pattern,
using very simple release agents such as soap or Vaseline. They
can be used to take impressions directly from a waterclay or plastelene
model, so that permanent master models can then be cast in plaster
flexible mold compounds have limited shelf stability and are
"moisture conscious", but they are easy to handle with
simple mixing ratios and good fluidity. They can be used for pouring
practically any type of cementitious material, casting resin or
foam formulation. Cost of material is far below that of the silicones
and appreciably lower than polysulfides.
From the standpoint of general utility and economy,
the polyurethanes (Smooth-On PMC Series) surpass all other types.
For this reason, the mold making technology that follows deals primarily
with urethanes though much of it is applicable to both polysulfides
EQUIPMENT AND ACCESSORIES
The workshop and all materials should be maintained
at, or near, room temperature. Cleanliness is most important and
good ventilation is essential. DO NOT INHALE FUMES
of the rubber compounds or the release agents.
The workbench should be accessible from at least
two sides and should have a hard stone or composition top. It must
be level in all directions, should be waxed and covered with brown
wrapping paper to protect against spillage.
containers and stirring paddles should be of metal or plastic
and always spotlessly clean, warm and dry. Use 1-inch fiber brushes
cut down to 1-inch length for application of wax release agent.
A regular short fiber scrubbing brush should be used for buffing
wax from deep grain or fine design of model or pattern.
Free Plastalina Clay (The Compleat Sculptor, New York City)
is excellent for modeling, laying up and sealing models and dams.
1/4"-3/8" plywood should be used for mounting models.
Scrap pieces can be sawed to rectangular shapes and used for straight
side retaining dams.
Lightweight sheet metal, linoleum and heavy cardboard
can be used for circular or irregular shaped retaining walls. Sash
cord and masking tape are used to bind the walls to the model baseboard.
1/2" X 2" furring and "2 x 4's" cut to short
lengths make good wedges. For plaster casting, use regular Molding
or Casting Plaster (U.S. Gypsum Co.)
fiber (hemp or sisal) for reinforcing plaster. If not available,
burlap or orange and onion bags, cut into small pieces, will make
a fair substitute. If the model cannot be fastened to the baseboard
with nails or screws, use a paste glue (Elmers Glue) or an epoxy
adhesive. (Smooth-On Super-lnstant Epoxy bonds in a few minutes
and very little is needed to do the holding job.)
An accurate balance
or scale is necessary, preferably one that will weigh in grams
and will handle up to 5 or 10 kg.
The model may be made of practically any material.
Non-porous surfaces - glass, glazed ceramics, metals,
and plastics - require only the application of a suitable release
agent to prevent adhesion. Porous surfaces - wood, plaster, modeling
clay - require a sealer first, then a release agent. Specific recommendations
are given in each of the Technical Bulletins for various Smooth-On
The model must be securely fastened to a baseboard,
cut to such dimensions so that when the side walls or retaining
dam is set up against it, there will be proper clearance all around
the model equal to the desired wall thickness of the finished mold.
The line where the model meets the baseboard should be sealed with
clay to prevent the liquid rubber from running under the model and
forcing air bubbles out into the rubber.
MAKING THE MOLD
There are five basic types of molds:
Simple, flat-back, one-piece mold.
B. Split, one-piece mold, unshelled.
C. One-piece mold, shelled.
D. Split, one-piece mold,
E. Multi-piece mold, unshelled,
The primary purpose of flexible molds is to be able
to remove a cast piece that has undercuts or "backdraft".
The more complex the model (such as extended arms and legs, and
many reverse curves), the greater the number of mold pieces required.
Fortunately, nearly any shape or configuration can be cast in either
an open face, split one-piece or a twopiece flexible mold.