Guide to Working with Plastics: A Primer on Poured Shapes -- Flexible Molds to Hold Detail

When you want to cast an object whose contours contain intricate details or an irregular shape, a flexible mold is the best way to ensure faithful reproduction of the original. Similar in principle to a hollow plaster mold but made with different materials, a flexible mold will bend when removed from the master, so all the eccentric forms and fine details will be preserved intact.

A simple flexible mold for one-time use can be made from plasticine, an oil-base modeling compound. Molds that are reusable and suited to more complex shapes are made of synthetic rubber, poured in liquid form around the master—the object being replicated—and allowed to cure, or harden. Of the three available types of synthetic rubber—silicone, polysulfide and polyurethane—the least expensive is polyurethane, which can be purchased from plastics suppliers and stores that sell sculptor’s materials.

Polyurethane molding compound is prepared by combining two components; the resulting chemical reaction makes the mixture set in six hours. Another 10 hours at room temperature is required for the compound to cure completely. Careful weighing of the components with an accurate scale is essential: An error of 5 per cent in either component can change the consistency of the mold. If the mold is too soft, it may tear; if too hard, it may crack when pulled from the casting.

The pouring consistency of properly mixed molding compound resembles that of thick syrup. It remains pourable for only 20 minutes; therefore, it should be mixed just before you are ready to pour your mold.

To protect the molding compound from moisture, which can keep it from curing fully and make the mold susceptible to tearing, use metal, glass or plastic utensils and containers for mixing. Wooden utensils hold moisture, and so do disposable paper containers unless they are plastic-lined. To sculpt an original master instead of making a copy, use plasticine. When the mold is finished, coat it with petroleum jelly, wrap it in a waterproof covering and store it in a cool, dark, dry place.

The four common types of flexible molds that are shown here and on the following pages allow you to cast almost any shape. Three are of synthetic rubber; one is of plasticine.

The simplest of the rubber molds, ideal for casting flat reliefs, is the one-piece surface mold. The two-piece shell mold is the best choice for fragile masters with relatively long vertical axes and varying horizontal diameters. Its external plaster shell lends support to the mold during casting and also reduces cost, since it occupies mold space that would otherwise have to be filled with the more expensive molding compound. The box mold is excellent for oddly shaped masters, but in making this mold the master is frequently handled; it is not recommended for delicate, easily broken masters.

The plasticine pressed mold, handy for duplicating small, flat reliefs, can be used immediately, without curing. But the mold yields only one casting because, in being removed from the casing, it must be stretched out of shape.

A One-Piece Surface Mold

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1---Preparing the master. Rest the master—in this case, a ceiling medallion—face up on a wood en block, leaving at least a 1-inch border, then secure the master to this wooden base with a smooth seal of plasticine. Fashion an open box around the master by building up four walls of plasticine at the edge of the wooden base. Make the walls about ½ inch thick and at least 1 inch higher than the highest point of the master. Coat the master, the base and all four interior plasticine walls with petroleum jelly, applying it as described.

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2---Pouring the mold. Mix the molding com pound, and pour it into the box until it covers the master by 1/2 inch at its highest point. Allow the compound to cure for 16 hours at room temperature; then remove the plasticine walls, invert the mold and lift off the wooden base.

3---Removing the master. To loosen the master, grasp the mold on opposite sides and gently bend and twist it. Then place the mold on a flat surface and remove the master by wedging your fingers between the master and the rim of the mold.

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A Shell Mold in Two Pieces

1--- Covering the master with plasticine. Seal the master—here, a towel-bar bracket—to a wooden base (Step 1), leaving a border at least 2 inches wide, then cover the entire surface of the master with a blanket of plasticine 3/4 inch thick. Construct a solid column of plasticine at the top of this blanket, about 1 inch high and 1 inch thick, to shape the sprue hole through which the molding compound will be poured.

2---Implanting shims. Inscribe a vertical parting line down the center of both sides of the plasticine blanket, and press metal shims into the plasticine along the two lines. Cut the shims from disposable aluminum cake or pie tins (aluminum foil is not stiff enough), making them about 2 inches long and 1 inch wide. Angle a pair of shims on each side to make a key in the plaster shell that will eventually encase them (inset). Overlap the shims so that there is no space be tween them, but do not carry the line of shims over the top of the sprue column.

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3---Building the plaster shell. Cover the plasticine blanket with an inch-deep layer of molding plaster, bringing it level with the top of the shims and the top of the sprue column. Al low the plaster to dry—this usually takes between three and five days.

Mark the outline of the plaster shell on the wood en base, then separate the two halves of the shell along the shim lines. Remove the plasticine blanket and the shims. Coat the master and the inside of the plaster shell with petroleum jelly. Then reassemble the shell halves around the master, using the outline on the wood en base as a guide. Fasten the shell halves together with reinforced packaging tape, and seal the shell to the base with a bead of plasticine.

4---Pouring the mold. Mix a batch of molding compound, and pour it into the plaster shell until it rises to the lip of the sprue hole (inset). Allow 16 hours for the mold to cure at room tempera ture, then separate the halves of the shell and remove it. Lift the mold away from the base.

5---Removing the master. Use a utility knife to cut through the finished mold, slicing down one side and halfway across the bottom. Gently peel the mold back and remove the master. When casting with the mold, seal the seam with plasticine to prevent leakage.

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A Box Mold for Odd Shapes

1--- Building a plasticine bed. Cut a wooden base large enough to leave a 2-inch border around the master—in this case, an ornamental door knob. Place a layer of plasticine on the base and rest the master in it so that the parting line, where the two parts of the mold will separate, runs parallel to the base. Build up a bed of plasticine around the master to the level of the parting line. Then erect a 0.5” plasticine wall around the bed and base, carrying the wall about 1 inch higher than the highest point on the master. Press the bed against the wall.

Fashion a roll of plasticine about ½ inch in diameter, and wedge it between the master and one wall. Press the roll into the bed to form a half- round berm, defining half the circular sprue hole—through which the liquid resin will be poured (inset). Then circle the master with a V groove, carved into the plasticine bed, to serve as a key for joining the mold parts. Apply a coat of petroleum jelly to the exposed master, the plasticine bed and the inside of the plasticine walls.

2---Pouring the first piece. Mix a batch of molding compound, and pour it slowly over the master and the plasticine bed until the level is ½ inch above the highest point of the master. When the molding compound has set, in about six hours, invert the entire assembly in preparation for pouring the second half of the mold.

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3---Removing the plasticine bed. Lift out the wood base and the plasticine bed, along with the plasticine berm shaping the sprue hole. Repair the wall if necessary. Fit a plasticine roll, ½ inch across, into the half-round sprue-hole depression, to complete the sprue. Coat the master, the mold half and the walls with petroleum jelly.

4---Pouring the second piece. Mix another batch of molding compound, and pour the second half of the mold to a level 1/2 inch above the highest point of the master. Allow the compound to cure for 16 hours; then remove the plasticine walls and separate the two halves of the mold. Lift out the roll of plasticine forming the sprue.

5---Removing the master. Twist the finished mold to loosen the master. Then remove it from the mold. If this mold will not stand on its own during casting, brace it with two squares of wood, cut to size and taped to the sides of the mold.

A Simple Pressed Mold for Replacing a Part

Making the mold. Coat the master—in this case, a decorative corner molding—with petroleum jelly or a light-grade oil, then push an inch-thick slab of plasticine onto it, pressing the plasticine firmly into the detail of the master. Remove the plasticine carefully and check the impression to be sure that all details have been picked up. Lay the mold flat, and cast a duplicate of the master immediately.