How To Make Flexible Molds

FOREWORD

Basically, there are five different types of flexible mold compounds in significant use today: polyurethanes, polysulfides, silicones, latex and hot melts. Each type has some advantages and disadvantages over the other types.

The "Hot-Melt" (polyvinyl chloride) systems are inexpensive, since the worn molds can be melted and recast. However, they are hazardous to handle because of the high temperatures required for melting. This also limits their use to model and pattern materials that are heat resistant.

Latex has the advantage of producing a very elastic, thin-walled, strong mold, but shrinkage is high and molds must be supported to prevent distortion. Because as many as 10 to 20 brush coats must be applied for adequate thickness, with time allowed to "dry" in between each coat, making the mold is slow and time consuming.

Silicone rubbers make excellent molds for casting resins and foams, but material cost is extremely high.

Polysulfide rubbers (Smooth-On FMC Series) are quite versatile and are especially useful for casting plaster, Hydrocal, and other water mixed cements. They also find use for casting wax candles and some limited use for resin casting. They are easy to handle, non-adhesive and can be poured over clay or practically any type of model or pattern, using very simple release agents such as soap or Vaseline. They can be used to take impressions directly from a waterclay or plastelene model, so that permanent master models can then be cast in plaster or Hydrocal.

Polyurethane flexible mold compounds have limited shelf stability and are "moisture conscious", but they are easy to handle with simple mixing ratios and good fluidity. They can be used for pouring practically any type of cementitious material, casting resin or foam formulation. Cost of material is far below that of the silicones and appreciably lower than polysulfides.

From the standpoint of general utility and economy, the polyurethanes (Smooth-On PMC Series) surpass all other types. For this reason, the mold making technology that follows deals primarily with urethanes though much of it is applicable to both polysulfides and silicones.

EQUIPMENT AND ACCESSORIES

The workshop and all materials should be maintained at, or near, room temperature. Cleanliness is most important and good ventilation is essential. DO NOT INHALE FUMES of the rubber compounds or the release agents.

The workbench should be accessible from at least two sides and should have a hard stone or composition top. It must be level in all directions, should be waxed and covered with brown wrapping paper to protect against spillage.

Mixing containers and stirring paddles should be of metal or plastic and always spotlessly clean, warm and dry. Use 1-inch fiber brushes cut down to 1-inch length for application of wax release agent. A regular short fiber scrubbing brush should be used for buffing wax from deep grain or fine design of model or pattern.

Sulphur Free Plastalina Clay (The Compleat Sculptor, New York City) is excellent for modeling, laying up and sealing models and dams. 1/4"-3/8" plywood should be used for mounting models. Scrap pieces can be sawed to rectangular shapes and used for straight side retaining dams.

Lightweight sheet metal, linoleum and heavy cardboard can be used for circular or irregular shaped retaining walls. Sash cord and masking tape are used to bind the walls to the model baseboard. 1/2" X 2" furring and "2 x 4's" cut to short lengths make good wedges. For plaster casting, use regular Molding or Casting Plaster (U.S. Gypsum Co.)

Use chopped fiber (hemp or sisal) for reinforcing plaster. If not available, burlap or orange and onion bags, cut into small pieces, will make a fair substitute. If the model cannot be fastened to the baseboard with nails or screws, use a paste glue (Elmers Glue) or an epoxy adhesive. (Smooth-On Super-lnstant Epoxy bonds in a few minutes and very little is needed to do the holding job.)

An accurate balance or scale is necessary, preferably one that will weigh in grams and will handle up to 5 or 10 kg.

THE MODEL

The model may be made of practically any material.

Non-porous surfaces - glass, glazed ceramics, metals, and plastics - require only the application of a suitable release agent to prevent adhesion. Porous surfaces - wood, plaster, modeling clay - require a sealer first, then a release agent. Specific recommendations are given in each of the Technical Bulletins for various Smooth-On elastomers.

The model must be securely fastened to a baseboard, cut to such dimensions so that when the side walls or retaining dam is set up against it, there will be proper clearance all around the model equal to the desired wall thickness of the finished mold. The line where the model meets the baseboard should be sealed with clay to prevent the liquid rubber from running under the model and forcing air bubbles out into the rubber.

MAKING THE MOLD

There are five basic types of molds:

A. Simple, flat-back, one-piece mold.
B. Split, one-piece mold, unshelled.
C. One-piece mold, shelled.
D. Split, one-piece mold, shelled.
E. Multi-piece mold, unshelled, or shelled.

The primary purpose of flexible molds is to be able to remove a cast piece that has undercuts or "backdraft". The more complex the model (such as extended arms and legs, and many reverse curves), the greater the number of mold pieces required. Fortunately, nearly any shape or configuration can be cast in either an open face, split one-piece or a twopiece flexible mold.

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