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PART 4 - Other Pourable Mold Configurations And Techniques
The next few pages offer instructions on making other types of molds using pourable mold rubber. These molds include:

A) Cut, one-piece mold, unshelled (without a support shell).

B) One-piece mold, shelled (with a support shell).

C) Split one-piece mold, shelled (with a support shell).

D) Multi-piece mold, unshelled or shelled.

Split, One-Piece Mold, Unshelled (Without A Support Shell)

Models that are three-dimensional, having one long axis and one short axis, or those that are somewhat conical in shape without deep undercuts (candles, for example) can be molded in one piece and cut with a razor blade or sharp knife.

The containment field can be a paint can or bucket. If you need to construct, you can use sheet metal or cardboard and contour to any shape model provided that the baseboard is shaped to allow proper thickness of rubber between the model and sides.

Again, make sure that the model is fastened and sealed securely to the base board with clay or a glue gun. Apply sealing agent and/or release agent to model as necessary, and secure containment field walls with tape or a glue gun.

When pouring the rubber, pause to tilt the whole assembly in all directions to move out entrapped air.

 After the rubber has completely cured and the containment field has been removed, use a razor knife to carefully cut the mold down one side and half way across the bottom, on a line that will facilitate ease of removal.

Rubber bands or mold straps can be used to hold the mold tightly together and, if it will not support its own weight, the containment field can be used as a support.

Shelling A Mold

The biggest advantage to "shelling" a mold is that it minimizes the amount of rubber used. This saves you money.

"Shelling a mold" refers to the use of plaster and clay to fill space that would otherwise be occupied by rubber. The greater the difference between the peaks and valleys on the surface of the model, the greater becomes the savings realized by shelling rather than by pouring around a model, using only perpendicular flat or round retaining walls. Had we made a "shelled" mold of our first cameo model, rather than pour rubber around it, we would have used much less rubber.

Additional Materials Needed for Shelling A Mold:
• Molding plaster and water
• Water clay or other soft modeling clay
• Aluminum foil or cellophane plastic wrap
• Plywood or acrylic sections for foundation

One-Piece Mold, Shelled (With A Support Shell)

 A mold box is constructed and our model is centered and secured to the platform. Aluminum foil or cellophane plastic wrap is then laid over the model.

Roll clay to the desired thickness (3/8"/ .95 cm) between 2 dowels or pieces of pipe and cut up into small sections. Then lay the clay sections over the model. Smooth the clay seams, and apply release agent.


 Add a clay plug to the highest point, to form the pour hole for the rubber. Set pieces of soda straws or small removable dowels in all high points of the clay. These will serve as air vents and allow air to escape while rubber is being poured. Mix and apply Plasti-Paste™ from Smooth-On to form the shell.

 Once the Plasti-Paste™ has cured, remove the shell and clay strips from the model surface. Apply release agent to inside of Plasti-Paste™ shell, and fit exactly over model. The next step is to mix and pour rubber. Pour rubber slowly into pour plug. Air will vent out through the bleeder straws. The liquid rubber will fill the space previously occupied by the clay.

When the rubber has cured, build a level foundation frame on the shell. Mix Plasti-Paste™ and build a suitable support for the foundation frame. Using a carpenter's level, build the framework so that the mold will be perfectly level when casting into it.

Split, One-Piece Mold, Shelled (With A Support Shell)

The split, one-piece shelled mold is another technique that minimizes the amount of rubber used. To illustrate, we select a three-dimensional figurine that has a relatively long vertical axis, is narrow at the top and larger in circumference at the base. The model is secured to the base with clay or a glue gun.

As was done with our one piece-mold (shelled) that was illustrated earlier, modeling clay is rolled into thin sections. A clay blanket is layered over the model, and a "pour plug" is formed at the top. (As you will see, once our support shell is formed and the clay blanket removed, the liquid rubber will be poured through this pour plug).

 The next step is to identify or "map out" where the segments of the support shell will be applied. The number of shell segments depends on the complexity of the model (angles and undercuts). This model is relatively simple and therefore requires a two-piece support shell.

A vertical center line is lightly inscribed up one side and down the other. This line identifies the two halves of the shell. Cardboard templates, of width equal to the thickness of the support shell, are cut to conform to the contour of the center lines. The templates are gently affixed (pressed into the clay along the center lines).

Applying The Support Shell
The purpose of a support shell is to maintain the shape of the cured rubber mold and prevent distortion when casting (pouring) into it (making a reproduction). There are a variety of support shell materials to choose from.

The most common is straight plaster or plaster mixed with chopped fiber. It is inexpensive, easy to work with and versatile.

A disadvantage to using plaster is that it is heavy. Large support shells are cumbersome to handle.

 Alternatives include fiberglass and polyester resin--also cheap and very lightweight, but noxious fumes are a problem. Smooth-On makes a mother mold material called "Plasti-Paste™," a fiber-filled, two-component plastic that is easy to use, lightweight and has no odor. It is more expensive, however, than both plaster or polyester.

For this illustration, we mix plaster and water. While waiting for the plaster to thicken to a "workable" consistency, vaseline is applied to the cardboard templates to release the plaster once it is set. Chopped fiber is mixed with the plaster to thicken and the first half of the plaster shell is then built up, working away from the templates.

When the plaster has set, remove the cardboard templates. What remains is the first half of the support shell. Using a knife or screwdriver, carve out circular notches-- or registration keys--at regular intervals around the inside perimeter of the shell. As we will see later, these "keys" provide a locking mechanism when both halves of the shell are complete and everything is assembled for casting.

Petroleum jelly will separate the second half of the shell from the first half, and is applied to the inside perimeter. Make sure the key notches are well-coated.

Plaster and water are again mixed with chopped fiber and built up to form the second half of the shell. Make sure that the negative key notches are filled with the plaster mix.

Once the plaster is dry, it should separate easily from the first half of the shell. Note that the positive keys fit exactly into the negative notches, providing the desired registration effect.

Next, both shell halves should be removed, and the clay removed from the model surface and perimeter.

Seal the inside of the plaster support shells with Smooth-On's SuperSeal™ or shellac and let dry. Follow with a thorough application of Universal Mold Release™. If pouring silicone rubber, use acrylic spray only as the release.

 Apply sealer and release agent to the model surface as previously directed, and assemble the shell halves over the model. Be careful to position the shell halves exactly as they were when the clay was present. Use the keys to align and secure the shell tightly together with mold straps, elastic bands or tape.

Mix mold rubber as previously directed and pour slowly into the pour plug. If necessary, seal any leaks along the shell seams with modeling clay. Let the rubber cure as directed on the mold rubber's technical bulletin.

After the rubber has fully cured:
Demold-- Separate the two halves of the plaster shell and remove from the cured rubber. Using a razor knife, cut the rubber vertically from top to bottom at a single point (preferably on the back of the model so that any seam that might be reflected in the casting will be on the back). Cut slowly and carefully. Don't cut yourself!


For More Complicated Models. . .
The techniques covered thus far address making molds of relatively simple models. These techniques fall short, however, if the model is moderately complex. These include:

The main issue in considering these three examples is ease of demold. You must develop a plan of attack for making a mold of your piece, and consider how much of a challenge demolding it will be--either in removing the original model from the cured mold, or removing a casting from the finished mold.

Plan Ahead . . . Avoid "Mechanical Lock"
The risk in not properly "engineering" your mold for easy demold: the model can become "mechanically locked" inside the mold structure. The only remedy you have if your piece becomes mechanically locked is to destroy the mold to remove it.

 Two-Piece, Open-End Mold
To illustrate making a two-piece, open-end mold, we will use a model ("Jungle Cat") that has some reverse draft, a difficult undercut (under the mouth) and a space between the tail and the back left foot (Section A).

The first step is to visually divide the model into two halves.

The parting line is inscribed (as illustrated) and the model is laid horizontally into a mold box. Modeling clay is then built up from the platform to the parting line. The model should be as level as possible and parallel to the base.

Mold box side walls are then adjusted, giving enough space around the model to allow for suitable rubber mold wall thickness.

When there are openings through the model (Section A), 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 building the clay up to that line.

 After the clay is fully built up to the parting line, the top surface is smoothed with fingers (use alcohol or other solvent). Keys and/or a registration line can then be inscribed around the perimeter of the model.

Apply a sealing agent and release agent as directed (depending on the mold rubber you are using) over the model, clay and sidewalls of the mold box. Mix and pour rubber as directed, making sure to have at least a half-inch (1.27cm) of rubber over the highest point of the model. Let the rubber cure overnight.

After the rubber has fully cured, remove the side walls and all clay without separating the model from the rubber. Thoroughly clean the side of the model that was embedded in the clay (acetone works well to remove clay), and remove any residual clay from the cured rubber.

Place the cured rubber with model face up into the mold box. Level and secure sidewalls on all four sides, again making sure there is at least a half-inch (1.27cm) clearance above the highest point of the model. Apply a sealer (SuperSeal™) to the model, if necessary, and let dry.

 Apply a release agent over the model and mold rubber. If using polysulfide or urethane rubber, use Universal Mold Release™. If using silicone rubber, use Mann Ease Release™ 800 or Vaseline thinned with mineral spirits for separating silicone from silicone. Do not use silicone-based release agents, as this will cause the silicone to stick to itself. Mix, pour rubber, and let cure as directed.

Demold-- After the rubber has fully cured, remove the model from both halves. Assemble the mold by aligning the positive and negative keys. Use heavy rubber bands or mold straps to hold both halves together when casting into the mold.

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