Guide to Working with Plastics: A Primer on Poured Shapes--Casting in a Mold: Using Resin in Liquid Form

Although a casting acquires shape and texture from the contours of its mold, its weight, color and strength are deter mined by the casting method and the materials used. Plastic objects can be cast from three liquid resins—epoxy, polyester or acrylic. Each is combined with a hardener that stiffens, or cures, during casting. But the resins differ in cost, in ease of use and in adaptability to the job.

Acrylics have the greatest optical clarity but cost substantially more than other resins. Epoxy resins normally cure to a translucent amber, although they can also be purchased in clearer formulas. They are best used when you want the casting to look like a filler material such as stone dust, powdered wood or powdered metal. Because of their greater adhesive properties, epoxies can hold more filler than other resins can; they can also be combined with a diluent, or thinner, that increases this capacity. Several formulations of epoxy resins and hardeners are available; to be sure of getting the best combination, discuss your needs with the plastics supplier.

Polyester resins are the least expensive, most versatile liquid plastics. Almost as clear as the acrylics, they allow the greatest margin of error in mixing. However, pure polyester resins shrink more than other casting resins as they cure.

When size is critical, you can compensate for shrinkage in either of two ways. For solid castings, you can introduce more resin into the mold by extending the sprue with a collar of clay; as the resin in the mold shrinks, more resin seeps in from the reservoir in the sprue. With hollow castings, in which the resin is painted onto mold sections before assembly, shrinkage along the seams is controlled by the use of a resin containing polyester putty, a dimensionally stable filler that clings to the mold edges.

The casting process begins with mold preparation. A plaster mold must be sealed with shellac to make it smooth and nonabsorbent. Flexible molds need no finishing but, like plaster molds, they must be coated with a mold release or a parting agent, so that the cured casting will slip easily from the mold. The most effective parting agents are polyvinyl alcohol, silicone spray or grease.

To stabilize an irregularly shaped mold during the curing process, add wads of clay or place the mold in sand. Once the cured casting is removed from the mold, it can be finished like any plastic. You can cut off any unwanted projections with a dovetail saw or a hacksaw, and file and sand any rough spots left on the casting, then polish as desired.

During the casting process, work in a well-ventilated area and always use a charcoal-cartridge respirator. Wear rubber gloves and an old shirt with long sleeves. If you get any resin on your skin, use acetone to remove it, then wash with soap and water and apply a skin lotion.

Everything you will need for casting plastic resins is available through plastics suppliers.

Techniques for Mixing and Pouring Liquid Resins

The appearance and performance of a casting made with liquid plastic resins depend on careful measuring and mixing; Mix the ingredients thoroughly, but take care not to introduce moisture or air bubbles.

Ingredients for epoxy and acrylic resins should be measured by weight with an accurate scale. Mix hardener for polyester resins by the drop. The pro portion of resin to hardener depends on the characteristics you want. In general, the higher the proportion of hardener to resin, the shorter the curing time; but humidity, temperature and the thickness of the casting also affect how fast or slow the resin cures. To adjust the proportions, mix several test batches under the actual conditions of use, then pick the one that produces the best combination of curing time and physical properties. Keep in mind that the faster the curing, the more brittle the casting—but a casting that cures too slowly may remain rubbery.

To avoid air bubbles in the finished resin, handle the liquid ingredients with care. When preparing to mix them, pour them slowly down the side of the mixing container or mold. Mix gently but thoroughly; if bubbles form, paddle them to the surface. If you are using the mixing attachment of a power drill to combine the ingredients, start or stop the drill with the attachment completely immersed in the resin and run the drill slowly to avoid bubbles.

Make sure that all mixing containers are clean and dry and don’t have a wax lining, which could contaminate the resin with wax or moisture. For stirring, use a narrow metal kitchen spatula or a clean, flat wooden stick.

The following guidelines give the correct sequence for mixing most resins, hardeners, pigments and fillers:

RESINS AND HARDENERS. Weigh the resin and hardener separately. Always be sure to pour the hardener into the resin. For polyester resin, add the hardener to the resin one drop at a time.

RESIN AND PUTTY. Pour polyester resin into the polyester putty, then add the cream hardener supplied with the poly ester putty. Measure the hardener by squeezing specified lengths from the tube onto your stirrer.

ADDING PIGMENTS. To color acrylic and epoxy resins, add pigments a little at a time to both resin and hardener until each is the desired color, then pour the hardener into the resin. To color a polyester resin, add the pigment to the resin only, then add hardener. To color a polyester resin-and-putty mix ture, first combine the putty and resin, then add the pigment and finally the cream hardener.

ADDING FILLER MATERIAL. Add powdered wood, stone dust, or powdered metal to polyester resins and putty res ins following the same sequences used for adding pigments. With epoxies, thin the resin first with a diluent, add hardener, then add filler.

Pouring a Solid Casting in a Flexible Mold

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1. Applying the mold release. Using a small nylon- or natural-bristled brush, cover the entire inside of both halves of the synthetic-rubber mold—in this case, for an oval doorknob—with a coating of polyvinyl-alcohol mold release, including the face and key as well as the mold impression and sprue. When the alcohol in the mold release has dried, leaving only a slick surface, apply a second coating, If there are undercut areas of the mold that are difficult to reach with a brush, pour a small amount of the mold release directly into the mold and swish it around; then pour out the excess. When the second coat of polyvinyl alcohol has dried, join the two parts of the mold by fitting the keys together.

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2. Sealing the mold. With the two halves of the mold held securely together, press a strand of plasticine modeling clay, about ¾-inch in diameter, along the seam line (left), sealing off all but the sprue hole through which the resin will be poured. For convenience, work with small lengths of clay, overlapping their ends by about 1 inch. Then snap wide, flat rubber bands around the two halves of the mold at intervals of about 1 inch (above, right). Don’t let the rubber-bands cover the sprue hole, and don’t make them so tight that they warp the mold or press it out of alignment. Snap a second set of rubber bands over the mold, perpendicular to the first set and at the same intervals.

Mix resin, hardener, pigments and fillers ac cording to the manufacturer’s instructions and the guidelines opposite. Be sure, at this point, to put on a charcoal-cartridge respirator.

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3. Filling the mold with resin mixture. Hold the mold, slightly tilted, sprue hole up, and pour the resin mixture slowly into the sprue. Let the res in slide gently down into the mold, following the contours of the casting impression; this will re duce the number of air bubbles that form.

4. Removing air bubbles. Hold the filled mold at an angle, sprue up, and tap it lightly several times against the work surface. Turn the mold one quarter revolution and, with the sprue still angled up, tap it against the surface again. Continue turning and tapping the mold until you return to the original position. Set the mold aside, sprue up, and allow the casting to cure.

Use the resin left in your mixing cup to determine when the casting has cured. When the surface of the resin in the cup is no longer tacky to the touch, press your finger firmly against it. The casting has cured when your finger no longer leaves an impression. Remove the casting from the mold by taking the rubber bands and plasticine off and pulling the two halves of the rubber mold apart. One half will still be clutching the casting. Simply peel the mold from the casting. Remove any defects and finish the casting as described.

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Making a Hollow Casting in a Rigid Mold

1. Preparing the mold interior. To cast the hollow knob used in this example, coat the inside face of each part of the plaster mold with shellac, using a small brush with nylon or natural bristles. Handle the mold pieces gently to prevent chipping. When the shellac is dry, in six to eight hours, spread mold release over the impression, as in Step 1, but stop ½ inch short of the edge of the impression (inset). The resulting border allows the resin to grip the edges of the mold pieces while curing, and thus reduces shrinkage along the seams.

Mix a casting compound consisting of three parts polyester putty to one part polyester resin; use a cream hardener and add any pigments desired. Be sure to wear a charcoal-cartridge respirator during the mixing process.

2. Coating the mold with resin. Use a small brush to spread the polyester putty-resin mixture evenly into the impression on each piece of the mold. Cover the impressions completely, including the gripping border, with a coating about 1/16-inch thick. When this coat has partially cured to a gelatinous consistency (usually after about 10 minutes), mix another batch of poly ester putty and resin, then brush on another coat. Mix and add a third layer when the second has partially cured, building up a combined thickness of about 1/4 inch.

At this point, if your casting is large and demands reinforcing, lay resin-soaked pieces of fiber glass mat into the impressions of the mold. Be careful to keep the mat pieces well within the borders of the impressions.

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3. Assembling the resin-coated mold. Fit the parts of the mold together in proper alignment and secure them temporarily with rubber bands or light-gauge wire. Then drill holes sideways through the seams of the mold with a high-speed twist drill bit, spacing the holes about 4 inches apart. Drive wood screws— use a size slightly larger in diameter than the drill bit—through the holes and then take out the temporary fasteners.

If the mold seams are not thick enough to support screw holes, fasten the parts of the mold together with light-gauge wire, twisting the ends of the wire to tighten it against the mold (inset).

4. Adding a bonding coat of resin. Mix a second batch of resin and pour it into the mold through the sprue hole. Swirl it around so that it coats the inside of the casting and fills any hairline cracks at the seam lines; pour out the excess. When the first bonding coat has hardened, add further coats if desired, increasing the thickness of the casting by about 1 6 inch each time.

5. Reinforcing the seams. While one of the internal bonding coats is still gelatinous, push a piece of resin-impregnated fiberglass mat through the sprue hole (left). Spread the fiberglass and press it against the inside of the casting with a wooden dowel . If the casting you are working with is a large one, insert additional pieces of fiberglass for extra reinforcement. Lap the fiberglass over the seams of the separate pieces of the mold, thoroughly bonding them together.

When the casting has hardened, remove the screws or the wires holding the mold together. Gently separate the pieces of the mold, pulling them away from the casting. If the mold does not separate easily, hammer lightly on wooden wedges to gently pry it apart along the seams.

Refining the Details of the Finished Casting

--Removing surface imperfections. Use the rasplike faces and edges of small woodworking tools called rifflers (inset) to carve or shave away blemishes acquired during the curing process or while the casting was being removed from the mold. If a defect is extensive, restore it with a resin patch, and use rifflers to sculpt detail into the patch. A small chisel or utility knife can also be used for this purpose.