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How To Make Flexible
Molds - Page 2
Click on the illustrations
below to see full size
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A.
Simple Flat-Back One-Piece Mold:
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B.
Split, One-Piece Mold, Unshelled:
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 1.
The model should be mounted securely (to prevent it from floating
in liquid rubber) on a piece of 1/4"-1/2" plywood
that extends beyond all points of the model by a distance equal
to the desired wall thickness of the cast mold. (See Fig. 1.)
 2.
Seal the joint between the model and backboard with clay.
3. Apply sealer (if necessary) and release
agent to model and backboard.
4. Treat the side wall in similar fashion and set it in place.
Be sure to seal bottom edge and outside corners with clay.
Put on binder cord and pull up tight with wedges. (See Fig.
2.)
 5.
For casting irregular shaped molds, the baseboard should be
cut to the desired contour. Use light gauge sheet metal, linoleum
or cardboard for the side wall. Overlap and tape pieces together.
Hold in position with cord and wedges. (See Fig. 3.)
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 Candle
molds, molds having one long axis and one short axis or those
that are somewhat conical in shape without excessive reverse
draft and deep undercuts can be made in one piece and split
with a razor blade, sharp knife or mold
key knife. The dam may be any shape from round to rectangular
or square (See Fig. 4.), providing the baseboard is shaped to
allow proper thickness of rubber between the model and sides.
The model, of course, must be fastened and sealed securely to
the baseboard, and treated for proper release. Be sure the dam
is leak-proof and held firmly with clay or masking tape to the
baseboard. When pouring the rubber, pause to tilt the whole
assembly in all directions to move out trapped air.
 After
the rubber has completely cured and the dam has been removed,
split the mold down one side and half way across the bottom,
on a line that will facilitate ease of removal. (See Fig.
5.)
Rubber bands or cord can be used to hold the mold tightly
together and, if it will not support its own weight, the original
dam can be used as a shell.
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Shelling
A Mold
The real purpose
for shelling a mold is to conserve rubber. The greater the
difference between the peaks and valleys on the surface of
the model, the greater becomes the saving realized by shelling
rather than by pouring around a model, using only perpendicular
flat or round retaining walls. More labor is required to make
a shelled mold and more care must be exercised in getting
perfect rubber flow around the model and eliminating air from
reverse draft, overhang and very fine detail. The extra materials
required are soft
modeling clay, a roller, foil or cloth, plaster,
hemp
fiber or cut burlap, trowels
or spatulas, tubing for vents and furring strips for foundations.
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C.
One-Piece Mold, Shelled:
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 The
simplest piece to "shell" is the flat-back cast in
an open mold. The model must be firmly mounted and a low frame
placed around it to contain the clay and plaster. Aluminum foil
or thin cotton cloth is then laid over the model. Plastelene
is next, rolled out to the desired thickness (3/8" - 1/2")
between 2 dowels or pieces of pipe (Fig. 6.), cut up to small
sections and laid over the model.
 Close
and smooth clay joints, and apply release agent. Add a clay
plug to the highest point, to form the pour hole for the rubber.
Set pieces of tubing or small removable dowels in all high
points of the clay, to allow air to escape while the rubber
is being poured. Mix plaster and chopped or cut fibers (hemp,
fiberglass or burlap) to a creamy consistency and trowel
onto the clay to the desired thickness. (Fig. 7.)
After the plaster has set and cooled,
remove the shell and clay and thoroughly clean the model.
Apply sealer (if required) and release
agent to the model and inside of shell, and reassemble.
Make sure the pour hole and vents are all clean. Replace and
lock shell exactly in same position in which it was formed.
It should be weighted down and sealed with clay around the
edges. Pour the rubber until it fills the space and remains
level at the top of the pour hole. Rubber should flow from
each vent hole. When it does, stop the hole with clay.
 When
the rubber has set, build a level foundation frame (Fig.
8) on the shell with pieces of small size wood furring.
Bond the pieces of wood to the shell with strips of burlap
or cloth dipped in a plaster mix. Using a carpenter's level,
build the framework so that the mold will be perfectly level
when set up to pour a casting.
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| D.
Split One-Piece Mold - Shelled |
 The
split one-piece shelled mold is made in order to economize on
rubber. It is used to good advantage when the model has a relatively
long vertical axis and varies greatly in cross diameters. (See
Fig. 9.) The usual construction procedures as already outlined
are followed, and after the model has been covered with the
desired thickness of clay blanket (and the clay plug to form
the pour hole placed on top) a vertical center line is inscribed
in the clay. Cardboard templates, of width equal to the thickness
of the plaster shell, are cut to conform roughly to the contour
of the center line (see Fig. 10). The first onehalf of the plaster
shell is then built up, working away from the templates. When
the plaster has set, remove the templates, cut in the "notches"
(Fig. 11) for the station "knobs" on the second half
of the mold (Fig. 12). The top of the shells should be level
and of large enough area to act as a base when the mold is set
up for pouring. If necessary, a frame of 1" x 2" furring
can be built as a base on each shell section. (See
Fig. 8). Don't forget to apply sealer (if required) and
release agent to insides of the shells before pouring the elastomer.
Let stand for 6-8 hours after pouring, remove shells and split
the rubber mold with a razor blade. |
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two-piece mold can be easily shelled; however, a three-piece
shell requires careful planning to insure proper joining of
the sections and parting of the shell from the rubber when it
contains a casting. Fortunately, a two-piece rubber mold can
be so designed that it will handle practically any shaped model. |
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E.
Multi-Piece Molds, Shelled or Unshelled:
 Two
piece molds must be used for pieces that are not of continuous
solid construction, such as figurines with spaces between
the arms and body or between the legs (see Fig. 13-open space
between tail and body), and for closed molds when the model
has many complex curves and too deep reverse draft or cross
sections so that it will not draw from a one-piece split mold.
It might be necessary to go to a three-piece mold if the model
is extremely intricate. The procedure is the same as for a
two-piece mold, except that parting lines have to be determined
very accurately and dams have to be set up so that one section
of the mold at a time can be poured and allowed to set. As
soon as the first section has cured, move the dam to the proper
position to pour the second section, and so forth, until the
mold is complete.
Making
The Two-Piece Open End Mold
First of all, study the model very carefully
to determine where the parting line must be inscribed to handle
the deepest overhang or reverse draft.  The
parting should come somewhere near bisecting the model along
one axis. Remember when there are openings through the model,
they must be "cored" and this is done by laying
out the center line about midway between the top and bottom
side of the opening and laying the clay up to the line. In
Fig. 13, Section A, and Fig. 14, the center line is marked
on two sides (center) of the tail, and the clay is built up
to the line and straight over to center line of the body.
Another example using the same procedure would be to form
the space between the spread legs of a standing figure.
 After
the model has been completely circumscribed by the parting
line, it is placed lengthwise on the backboard cut to size
to allow for the mold wall thickness. (The long axis of model
should be as nearly parallel to the baseboard as possible.)
Now build plastelene from the backboard up to the parting
lines. Pieces of wood may be laid on the board first (close
to the model) to cut down on the amount of clay used. Smooth
the top surface of the clay, cut in a stationing groove and
square the outside wall of the clay with the baseboard (Fig.
14-15).
Apply sealer (if required) and release
agent on the exposed surfaces of the model and the clay supporting
it. Set up the side boards and pour elastomer to proper height,
at least 1/2" above the high point of the model. Allow
to set overnight, then remove side walls and all clay without
separating rubber from the model. Thoroughly clean side of
the model that was embedded in the clay.
 Apply
release agent to all surfaces that will be exposed during
the pouring of the second half of the mold. Mix and pour the
elastomer and let stand overnight.
 Remove
both halves of mold (Fig. 17.) from model and use heavy rubber
bands or cord to hold it together. If mold is not self-supporting,
make a 1/4" plywood shell and slip it over the mold.
Recommended
release agent must be applied before use.
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