Guide to Working with Plastics -- A Primer on Poured Shapes: Rigid Molds to Shape Liquid Plastic Resins

Although rigid plastics have many useful applications, it is the liquid plastics that truly capitalize on the material’s major characteristic—its ability to be molded into any shape desired. Indeed, most of the plastic objects found in the home are formed by a variety of molding processes that turn liquefied plastic into everything from contoured tool handles to the appliance housings that neatly follow the functions of the working parts within.

In factories these processes are handled by sophisticated machines that heat and shape the plastic in one continuous operation. Phono graph records, for example, are made by compression molding: The halves of a 200-ton press melt the plastic and also form the grooves. Camera parts are formed in an injection-molding machine, which liquefies plastic pellets and squirts them into a mold under enormous pressure, as much as 40,000 pounds per square inch—all in a cycle that may consume no more than 10 seconds. And the familiar squeeze bottle is made by a blow-molding machine, which forces - the liquid plastic against the walls of a mold in a repetitive process that is much like blowing up an endless succession of balloons.

None of these processes can be duplicated in the home workshop, but a surprising number of the objects they create can be produced by simpler means. Liquid resins can be cast in handmade molds—of clay, plastic or rubber—that have been shaped around a master, or model; the master may itself be handmade. Another technique is to layer liquid plastic and fabric over a homemade wooden form. Though neither approach requires machinery, they both call for patience, dexterity, and the ability to think in three dimensions. In short, modeling objects in plastic is very like creating a work of art.

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Line-for-line copy. A duplicate doorknob cast from liquid plastic resin has hardened and is ready to be removed from its synthetic-rubber mold. Made in two parts, the mold reproduces the con tours of the original doorknob, or master. The mold, too, is a casting: The rubber compound is poured, in two stages, into a box containing the master. Once the compound hardens, the master is removed and the cavity becomes the mold for the new item. In lieu of the wooden box, built here of scrap lumber, the mold frame could be made of plasticine.

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Working Safely amidst the Vapors

Casting or laminating plastics releases toxic vapors that require more filtering than a paper dust mask can provide. The answer is a charcoal-cartridge respirator. This piece of equipment consists of a rubber face mask that can be fitted with a replaceable cartridge containing charcoal granules to filter out harmful vapors. On some models the respirator can also be fitted with a fabric filter for dust. Use of both dust filter and cartridge is strongly advised for sanding newly cast fiberglass laminate or for sawing finished laminate.

Respirators, filters and cartridge refills are available at large hardware stores, auto-paint stores and dealers listed under “Safety Equipment” in the classified directory. Some professionals recommend replacement of a cartridge after about 10 hours of use; certainly it should be replaced as soon as any smell or taste is detected. Dust filters should be replaced whenever breathing becomes difficult or after a full day of exposure to dust.

Fit is important and should be checked according to the manufacturer’s instructions. Clean the mask after use, and store it in a sealed plastic bag.

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Rigid Molds to Shape Liquid Plastic Resins

Many plastic objects found in the home—from doorknobs to bathtubs— began as liquid plastic resins that were shaped and hardened in molds. In factories these molds are made of hardened steel, to enable them to withstand the wear and tear of mass production. But at home, where the operation is on a much smaller scale, liquid plastics can be shaped in molds made of such common place, easily worked materials as plaster, wood or synthetic rubber. The latter is a molding compound available at plastics-supply houses.

Molds used for forming liquid plastics fall into two categories. Either they are cavities into which liquid epoxy or poly ester is poured, or they are contoured forms over which successive layers of plastic are laid, usually in conjunction with a reinforcing material such as fiber glass. In a variation on this latter technique, shallow reliefs, such as ceiling medallions, are cast in surface molds.

The first step in casting anything in a hollow mold is to examine the object you will duplicate, called the master. De pending on its shape and surface details, you may choose either to make a rigid plaster mold or to use a flexible mold of synthetic rubber.

If the master has a complex shape, with reverse curves, undercuts and many incised details, a flexible rubber mold will be easier to pull cleanly from its surface. More durable than plaster, a synthetic rubber mold is also preferable if you plan to make many castings of the same shape. Under any other circumstances, however, a rigid plaster mold is better because it is so much cheaper. Make it of molding plaster, which hardens more slowly than plaster of Paris, giving you more time to work. Molding plaster, commonly used for wall patching, is available at most building-supply stores.

In some instances, a plaster mold is poured in two parts in a frame built around the master; in other cases, it is made by applying plaster to the surface of a master in multiple sections. This choice, too, depends on the complexity of the master. To determine how many mold parts you will need and where their separations will fall, you must analyze the shape of the master. As a general rule the mold will separate along the widest part of the master. But you must also look for reverse curves or undercuts that would interfere with the removal of the mold unless it subdivides into additional pieces. When you have located the dividing lines between mold parts—called the parting planes—draw them on the master with chalk or a marking pen.

The frame used to contain the liquid plaster for a two-part mold may be a plastic container or a cardboard box. To seal the seams of the box, use masking tape or the pliable rope caulking often used for sealing around windows. If you do not have a ready-made container of the right size, you can build the frame with scrap lumber or shape it with potter’s clay.

In preparation for pouring, you will also need to plan how to immobilize the master within, the frame. If you have made the frame of clay, you can simply add a pedestal of clay for the master or use the technique shown, Step 1. A wooden, plastic or cardboard frame requires more ingenuity. Depending on the location of the parting planes, you may be able to suspend the master from a wooden crosspiece nailed to the top of the frame. Otherwise, anchor it to the side of the frame with a wood spacer, fastening the spacer to the master with existing hardware on the master, such as screws or bolts, or with glue; then attach the spacer to the frame with screws.

Whatever anchoring arrangement you use, keep in mind that the anchor also creates the sprue hole for the mold— the hole through which the casting material is poured.

Before the master is secured in the frame, it must be coated with a parting agent to prevent plaster from adhering to it. The best all-purpose parting agent is petroleum jelly, heated to the melting point in a double boiler, then evenly applied to the surface of the master with an artist’s brush. For a final smoothing, the coat of jelly is then reheated with a heat gun or a hair dryer while any excess jelly is blotted up with the brush. Another parting agent—one that is simpler to apply but leaves a less polished surface—is liquid dishwashing detergent. Also applied in a thin coat with a small brush, the detergent must dry for about five minutes before the plaster comes into contact with it.

Timing is important in mixing and using plaster to make a rigid mold. The prepared plaster will remain thin enough to pour for three to five minutes. After that, it will be about as thick as cake frosting and can be laid over the master by hand for up to another five minutes. Beyond five minutes, it will usually be come cakey, suitable only for patching; in five minutes more, it will become too granular to use at all.

To mix plaster and water, use a flat-bottomed plastic mixing pan and put into it roughly four fifths as much water as the liquid plaster you will need. Sift plaster dust into the pan through your fingers, sprinkling it evenly over the surface of the water until it begins to form small islands of dry plaster on the surface. When these islands take about 10 seconds to become moist, blend the plaster solution with a large metal mixing spoon, stirring it for no more than 30 seconds. It is now ready for pouring.

Although the plaster mold will be hard enough to remove from the master in 15 to 30 minutes, it must dry, or cure, for five to seven days before use. Otherwise, the moisture remaining in the plaster might ruin the casting. When all moisture has evaporated from the plaster, the mold will feel much lighter and its surface powdery rather than clammy. You can accelerate the curing process by put ting the mold in a 150° F. oven for 12 to 24 hours, depending on its size.

The last step in making the mold is to check the mold’s interior surface for flaws. Use paint thinner to remove any petroleum jelly left by the master, then fill or rebuild any pockmarks or chipped edges with spackling compound. Smooth away any surface roughness with a fine grade of steel wool. If there are cracks or breaks in the cured mold, they can be repaired with white glue.

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Suiting the Mold Type to the Shape of the Master

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Finding the parting plane. The sections of a mold meet each other along a flat surface, called the parting plane, which is positioned so that each section can be removed from the master without interference. The number as well as the location of the planes is determined by the shape of the master—as illustrated by the three differently shaped doorknobs shown.

With its symmetrical shape, the top doorknob can have either a vertical or a horizontal parting plane—a two-part mold would separate cleanly in either direction. The undercut sides of the middle doorknob require a mold that separates vertically, to prevent interference from the base of the knob.

The bottom doorknob requires a mold with two parting planes. Because the modeling at the top of the master prevents mold sections from separating vertically, one parting plane cuts horizontally through the knob’s widest part, sectioning off the top. However, the reverse curve in the base of the knob would prevent the removal of a single mold section there, so a second parting plane runs vertically through the knob, from its base to the horizontal parting plane.

A Two-Part Mold Made of Poured Plaster

1---Constructing a fall-away frame. Using scrap lumber, build a box with four sides and a base whose inferior length, width, and depth are roughly 1½ inches greater than the dimensions of the master—in this example, a decorative coat hook. Connect the sides at the corners with wood screws, then nail the base to the sides with common nails or brads.

Draw the parting plane on the master with a marking pen. On the inside of the box, draw a cross mark at the point where the master will be anchored. Extend the horizontal line of the cross mark to use as a guideline in pouring the plaster. Then draw a matching cross mark on the out side of the box.

Seal the inside joints with masking tape, pressing strips of tape into the corners and over the seams between the bottom and the sides (inset). Coat the interior of the box thoroughly with a thin layer of petroleum jelly.

----Handling Molding Plaster----

--Never use stored plaster without first mixing a test batch; if it has absorbed moisture from the air, it will not harden properly.

--Control the rate at which liquid plaster thickens by altering water temperature, adding salt or vinegar, or adjusting stirring time. Warm water speeds hardening; small amounts of salt accelerate hardening, and up to a cup of vinegar retards it; lengthy stirring makes plaster harden faster.

--Cover the work surface with a sheet of polyethylene plastic; plaster does not adhere to this material.

--Simplify cleanup by coating the mixing pan and mixing spoon with a light film of kerosene before you start the mixing process.

--Rinse your tools in a bucket of water before plaster hardens on them. Let the plaster settle, then dispose of solids and liquids separately. Never pour plaster into your plumbing.

2---Anchoring the master in the frame. Cut a wood spacer to fit the surface on the master to which it will be attached. Fasten the spacer to the master with glue or with any existing hard- ware—in this example, the coat hook screw.

Then coat both the master and the spacer with petroleum jelly, taking care not to disturb the parting lines drawn on the master. Position the master in the frame, lining up the horizontal parting line with the cross-mark extension on the box, and drive a screw through the box into the spacer. To avoid marring the jelly-coated master, hold the work only by the spacer throughout this operation.

3---Pouring the first mold section. In a shallow pail, mix enough plaster to fill the frame up to the horizontal parting line, marked on the side of the box. Immediately pour it in a steady stream into the frame until it reaches the parting line. Tap the sides of the box several times with your hand to release any bubbles from the plaster.

4---Keying the parts. When the plaster is firm but not yet hard (in about 15 minutes), press the tip of a spoon into its surface to carve out a semi circular indentation—called a key—that will be used in aligning the two parts of the completed mold. Turn the spoon 90° from the direction of the first key and make a second indentation, at right angles to the first.

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When the plaster has set (usually in another 15 minutes), coat its surface with a thin layer of petroleum jelly. Mix a second batch of plaster and pour it into the frame, filling the frame completely. Again, gently tap the sides of the frame to remove any air bubbles.

5---Dismantling the frame. When the second batch of plaster has set, invert the frame and re move all its fasteners, including the screw that holds the spacer to the box. Set aside the parts of the frame and gently separate the two halves of the mold, If the parts stick, insert small wooden wedges between the halves to pry them apart; tap the wedges gently with a mallet. When the parts are separated, remove the master (inset).

A Three-Piece Mold for Copying a Complex Shape

1---Creating a clay dam to hold the plaster. Ring the top of the master with a shallow, saucer-like dam by pressing a 1/2-inch-wide strip of potter’s clay against the horizontal parting line, aligning the top of the strip with the parting line. Cut the strip to size before you apply it, using clay rolled about 1/8 inch thick. If the dam fails to stay in place, buttress it with vertical strips of clay braced against the work surface.

Pinch two ridges in the top of the dam, on opposite sides of the master, to serve as keys in assembling the finished mold. Then coat the top of the master, above the dam, with petroleum jelly, using the technique described.

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2---Applying the plaster basecoat. Fill a wide-nozzled squeeze bottle with plaster mixed to pouring consistency, as described. Squirt a steady stream of plaster over the top of the master, holding the bottle about 6 inches above the surface and covering the entire area circumscribed by the dam with a thin layer of plaster. When this layer has set, in about 15 minutes, repeat the procedure to build up a double layer of plaster about 1/8” thick.

3---Applying the top coat. Mix a batch of plaster and allow it to thicken to molding consistency, as described. Using your finger tips as a scoop, daub the plaster over the base coat until the combined thickness of the two coats equals about 1/2 inch, just slightly less than the width of the dam.

When the top coat has set, in about 15 minutes, peel off the clay dam and coat the flat edge of the mold with petroleum jelly.

4 ---Forming vertical dams. Make two strips of clay ½ inch wide and 1/8” thick, as in Step 1. Press them against the master, aligning them with opposite sides of the vertical parting lines; the master will be divided in two sections, one slightly larger. Pinch a ridge at the midpoint of each strip, for keys in assembling the mold. Put a coat of petroleum jelly on one section of the master between the two dams

Apply two coats of plaster to the dammed area as in Steps 2 and 3. When the plaster has set, peel off the two clay dams.

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5---Completing the mold. Spread petroleum jelly on the flat edges of the second mold piece and on the remaining exposed surface of the master. Build up layers of plaster over this surface, as de scribed in Steps 2 and 3, until the third piece of mold is as thick as the other two pieces.

6---Removing the mold. When the plaster of the third mold piece has set, separate the mold pieces by driving small wooden wedges into the seams with a rubber mallet. Insert the wedges into the horizontal seam first, placing them at regular intervals around the top mold piece. Gently tap the wedges in succession, driving them to the same depth to equalize their pressure. When the top piece is free, repeat the procedure to separate the sides, alternately driving wedges into both side seams. When the side pieces separate, remove the master.

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A Wooden Mold for a Fiberglass Shower Base

1---Assembling the sides. To construct the sides of a mold for a shower base, cut four pieces of smooth 1-by-6 lumber, making two pieces as long as the sides of the planned base and two pieces 1½” shorter. Miter the ends and bevel the edges of the pieces so that the sides slant about 15° inward from bottom to top. Screw the pieces together to make a four-sided frame, countersinking the screws so that they are below the surface of the wood.

2---Attaching the top. Nail four wood cleats in side the small end of the frame, positioning them flush with the edge of the frame. Cut a piece of smooth ¾-inch plywood to match the interior dimensions of the small end of the frame. Invert the frame, center it over the plywood and, using the cleats as nailing blocks, drive nails through the cleats into the plywood.

Turn the box over and cut strips of quarter-round molding to fit into the angles between the sides and the plywood top. Nail the molding to the sides with finishing nails, then recess the nails with a nail set.

Use a putty knife and wood putty to cover the nail- heads and screwheads, fill the seams, and smooth any imperfections in the mold’s surface. Sand the entire surface smooth, starting with a medium-grade sandpaper and finishing with a fine grade. Round the corners of the frame, and sand the edges of the molding flush with the top and sides, creating a smooth curved surface over the entire outside of the mold.

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3---Attaching the base. Make a base for the mold with a layer of ½-inch plywood topped with 1/2-inch hardboard, 4 inches larger in each dimension than the open end of the frame. Rest the mold on its top, center the base smooth side down on the mold, then screw the base to the frame.

4---Smoothing the inside angles. Use potters clay to round out the interior angles of the mold. Fashion the clay into ropes about ½ inch in diameter by rolling it between the flats of your hands and a sheet of polyethylene. Press the clay ropes into the angles between the sides and the base of the mold, simultaneously shaping them to a smooth arc with the bowl of a spoon.

Making Casts of Original Sculpture:

Molds can be used for casting original designs as well as for duplicating existing objects. Once you have mastered the fundamentals of mold making, it is a short step to the creation of new shapes for any use you desire.

Plastic castings are particularly well suited for outdoor use, since the finished products resist the elements. You can fashion fanciful adornments—a fountain, a weather vane, a set of out door lanterns, or a jardiniere—or re place architectural ornaments such as a finial on a Victorian gable.

Whatever project you choose, the first step is to sculpt or model a master identical in size and shape to the finished product. The simplest, most popular medium for making a master is potter’s clay. Small objects can be formed in solid clay; larger objects, and those with projecting parts, require a reinforcing substructure, called an armature.

The armature is built of wood, galvanized pipe or concrete-reinforcing bar roughly in the shape of the object, then covered with wire mesh for the clay to adhere to. For refining and smoothing the master, use the cutting tools, shapers and scrapers sold by art-supply stores for use with potter’s clay.

Plan your mold to suit your master as you would for any object— with one exception. If you will be making only one casting from an elaborate master and do not care about preserving the master, you can use plaster to make a disposable mold, called a waste mold, instead of using a more expensive flexible mold.

In construction, a waste mold is similar to a hollow plaster mold; it is made of plaster layered over a master. But the master is always made of potter’s clay, and instead of being separated by clay dams, the sections of the mold are walled off from each other by rows of metal shims pressed into the master along the parting planes. The shims are crimped at intervals to form keys in the edge of the plaster (Step 2).

The clay master of a waste mold is considered expendable; pieces of it may stick to the mold during dismantling— especially in undercut areas. These pieces of clay are flushed from the mold surface with a stream of water.

Also expendable are the mold sections themselves. When the casting is complete, the plaster is broken away with a chisel and a mallet. To avoid accidentally damaging the casting, work gently and gradually, with shallow cuts. Or do as professionals do: When you construct the mold, add vegetable dye to the first layer of plaster. Then, when you chip off the mold, a change in the color of the plaster will warn you that the casting is not far below.