Ceramic Industry

Drying Plaster Molds

March 1, 2002
Forced-hot-air drying can extend the lifetime of plaster molds while saving users time and money.

Those in the ceramic industry often complain about the too-short life of plaster molds, along with their uneven absorption rates and long drying times at ambient temperatures. However, extensive testing* has shown that there’s a fairly simple, long-term and economical solution to these problems: forced-hot-air drying of molds. Properly designed forced-hot-air dryers save time, extend the life of molds and reduce the cost of each mold and finished piece.

Figure 1. Mold strength begins to increase after the mold is 92% dry, reaching 30% gains in strength when the mold is 100% dry.
When forced-hot-air drying is done following the proper guidelines, users can achieve significant strength gains after the mold is 92% dry. In fact, these gains can be as high as 30% when the mold is 100% dry (see Figure 1). While the tests were conducted on several specific pottery plasters,** the advantages of forced-hot-air drying can be obtained with any plaster mold.

How a Plaster Mold Dries

To attain uniform results and achieve the maximum mold strength, plaster molds must be dried properly. But wherever a mold is being dried—in the workroom, outdoors or in a dryer—it will rarely become 100% dry without the use of forced-hot-air drying methods.

Drying involves transferring excess water from the mold to the surrounding air. Large amounts of water must be used to obtain a mixable slurry. After the plaster has been mixed, poured and has set, the excess, or “free,” water must be removed. When evaporating this free water using forced-hot-air drying, the following process takes place. As the mold sets or hardens, a chemical reaction causes the piece to heat slightly. Because the mold contains more water than what is required for the chemical reaction, this excess water starts evaporating from the mold.

When a new, wet mold is placed in a forced-hot-air dryer, rapid evaporation begins, which keeps the mold cooler than the air in the dryer. Water from the mold’s interior then moves to the surface to replace the evaporating moisture. But as evaporation continues, the water moving to the mold’s surface isn’t sufficient to keep the mold cool, so its surface temperature rises, even though the mold’s center is still moist.

As evaporation decreases, the mold’s surface temperature approaches the air temperature in the dryer. Once the mold’s surface temperature is close to the oven air temperature, the rest of the free water evaporates slowly—that’s where the 30% strength gain is reached—and the entire mold, from surface to center, warms to the approximate oven air temperature. When this occurs, the forced-hot-air drying process is complete.

The Best Dryer Temperature

All plaster molds should be dried as quickly as is safely possible so that maximum strength and uniformity can be reached. Molds also need to be dried to a constant weight. Given these factors, maximum temperature ranges when drying plaster molds are crucial. While the recommended surface temperatures for pottery plaster products can vary, the oven air temperature should typically range from 140 to 150?F. Operating above these temperatures can result in surface calcinations—that is, the surfaces of the molds, especially those in front of hot-air ducts, will become soft and powdery. (Your plaster supplier should be able to provide you with the recommended surface temperature for your particular plaster.)

Figure 2. The right airflow pattern is necessary for proper circulation in the drying chamber.

Air Circulation Within the Dryer

Spacing of the molds in the dryer is important so that air can move freely around and between them to remove moisture. The best drying rooms or ovens provide uniform and rapid air circulation with no “dead spots” of little or no air movement, equal temperatures throughout the area, and the ability to exhaust a portion of the air while replacing it with fresh air (see Figure 2).

High humidity surrounding the drying room or oven inhibits the drying process because the air pulled into the room can’t pick up much moisture from the molds as the air is heated. Molds should be placed on racks or separated by runners so that the water vapor can escape.

Following are some important questions to ask when designing and using drying equipment:

  • Is the burner large enough to supply the necessary heat energy?
  • Is sufficient humid air being exhausted from the dryer’s system?
  • Is the outlet of the exhaust duct well away from the fresh air intake?
  • Is hot air entering the dryer chamber at 150?F or below?
  • Is air circulation within the dryer fast enough? Are fans within the dryer designed to operate at the dryer’s temperature?
  • Are applicable building and safety codes being met?
  • Can the flame or heating element be seen from inside the drying chamber? These should not be visible to avoid potential radiant heat damage.
  • Has production increased since the dryer was designed? If so, larger equipment might be needed.
  • Are the spaces between the plaster molds large enough to allow adequate air circulation? There should be approximately 2 in. between molds. Consider contacting a heating and ventilating engineer to check the system for proper airflow and temperature.


Figure 3. Controlling the temperature and humidity results in faster drying.

Faster Drying

To optimize the benefits of forced-hot-air drying and really take advantage of this technology, a dryer with humidity and temperature controls should be used. These allow for higher drying temperatures and adjustable relative humidity settings, which shorten drying time.

For example, a temperature of 140∞F with 40% relative humidity will result in faster drying than 140?F with 50% relative humidity. If the relative humidity is 50%, the temperature should be increased to 150?F (see Figure 3).

Forced-hot-air drying of gypsum molds, following the above guidelines, increases the longevity of molds and improves their performance. Pottery plaster gains strength, decreasing the money spent on plaster and increasing the quality and value of each finished piece. In addition, uniform molds produce uniform absorption.

These factors together increase production while also saving time and money. While everyone’s operations are different, forced-hot-air drying is the best way to optimize the drying of plaster molds.

For More Information

For more information about forced-hot-air drying of plaster molds, contact the Industrial Products Division of United States Gypsum Co., 125 S. Franklin St., Chicago, IL 60606-4678; (800) 487-4431; or visit the company's website at www.gypsumsolutions.com.