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Mixing Plaster of Paris for Mold Making


Weigh both the plaster and water for each mix. The water-to-plaster ratio is critical because it governs both the strength and the absorptive capacity of the mold. Use Consistency is a measure of this ratio and represents parts of water by weight per 100 parts plaster. You can vary the consistency successfully between 74 to 82 with the lower numbers creating higher strength molds and the higher numbers creating higher absorption.


Use potable water at temperatures between 70° and 100°F. Variations in temperature produce variations in setting time. The higher the temperature of the water, the shorter the set time.


Sift the plaster by hand into water slowly and evenly. Do not drop handfuls of plaster directly into the water. Allow soaking for 1-2 minutes. The plaster should be fully dispersed in the water before mixing. Small batches require less soaking than large batches.


Mixing effects strength, absorption, hardness, and other properties. Mix consistantly from batch to batch to develop uniform molds with optimal strength. Longer mixing times give higher strength and shorter setting times.


Carefully pour the mixture into the case to prevent air entrainment and provide a uniform, smooth surface. Bump or agitate the case during pouring and right after pouring to prevent air bubbles at or near the surface. When possible pour the plaster into the deepest area so the slurry flows evenly across the surface of the case. Don't pour a large amount of slurry directly on one spot of the case mold or you may get densification of the plaster at the point where it strikes the surface of the case. This produces a hard spot, giving uneven absorption.


Dry quickly to a constant weight. Provide uniform and rapid circulation, equal temperatures throughout the area, and provisions for exhausting a portion of the air while replacing it with fresh air. The maximum temperature at which plaster molds are safe from calcination is 120°F. With substantial free water in the mold, higher drying temperatures can be used without difficulty. As drying progresses, the temperature must be reduced to prevent calcination. Before removing molds from a dryer, the temperature should approach that of the area around the dryer to prevent thermal shock.


Keep molds in a dry, stable environment indoors. Do not stack more than 2 pallets high. Keep from drafts. Rotate stock.