When using plaster to make molds, it's important to match the plaster to the job. Plaster formulations differ in terms of how porous they are when they set, how strong they whet as they cure and how much they shrink during curing. All of these factors impact how each type of plaster is used. Understanding the nature of mold-making plasters is important as you plan your molding project.
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Pottery Plaster represents a good compromise between cost and durability. It produces a very smooth mold and cures with a compressive strength of 2,400 PSI. It has enough porosity to work well with casting materials than require an absorbent mold in order to cure properly.
Plaster of Paris
Plaster of Paris is a common, very inexpensive plaster that is good for use in children's craft projects. It is also frequently used for molds that will only be used once or twice; it is very soft and cures to a compressive strength of just 2,000 PSI. It is fairly porous and must be sealed before it is used with liquid casting compounds like latex.
Gypsum cement is a compound that handles like plaster but reaches a much greater hardness and density. It attains a compressive strength of 1,000 PSI just 1 hour after mixing, and a full 5,000 PSI when fully cured. Gypsum cement is more expensive than pottery plaster or plaster of Paris, but its greater strength reduces the chance of breakage and produces a mold that can be used many times without loss of detail.
High-Strength Gypsum Cement
Some gypsum cement applications are formulated to quickly reach very high strengths -- 4,000 PSI in just an hour, which is twice as strong as plaster of Paris reaches at full cure. These products are typically used as casting materials, but they are also sometimes used in industrial settings where molds must survive for hundreds of castings.