A mold is a hollow shape which exactly encloses the shape of a desired object. The object is usually created by pouring a liquid into the mold and allowing it to solidify: typical liquids include molten metal or plastic, plaster of Paris, epoxy resin.

Molds generally divide into two classes: solid or flexible.

There are five different types of flexible mold compounds in significant use today.

Hot-Melt

polyvinyl chloride

Polyvinyl chloride

Polyvinyl chloride, commonly abbreviated PVC, is a thermoplastic polymer. It is a vinyl polymer constructed of repeating vinyl groups having one hydrogen replaced by chloride. Polyvinyl chloride is the third most widely produced plastic, after polyethylene and polypropylene. PVC is widely used in...

• inexpensive, worn molds can be melted and recast.
• hazardous to handle due to high temperatures required for melting.
• use limited to model and pattern materials that are heat resistant.

Latex

• produces a very elastic, thin-walled, strong mold
• shrinkage is high
• molds must be supported to prevent distortion.
• 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

• excellent molds for casting resins and foams
• material cost is extremely high.

Polysulfide rubbers

• versatile and are especially useful for casting plaster, Hydrocal, and other water mixed cements.
• can be used for casting wax candles and some limited use for resin casting
  Resin casting
  Resin casting is a method of plastic casting where a mold is filled with a liquid synthetic resin, which then hardens. It is primarily used for small scale production like industrial prototypes and dentistry...
  
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• 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
  Vaseline
  Vaseline is a brand of petroleum jelly based products owned by Anglo-Dutch company Unilever. Products include plain petroleum jelly and a selection of skin creams, soaps, lotions, cleansers, deodorants and personal lubricants....
  
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• 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

• limited shelf stability
• "moisture conscious"
• easy to handle with simple mixing ratios and good fluidity.
• 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 surpass all other types.

Equipment and accessories

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