Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
1999-11-30
2002-06-04
Marcantoni, Paul (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
C106S778000
Reexamination Certificate
active
06398864
ABSTRACT:
BACKGROUND ART
Plaster molds are particularly useful in the production of ceramic articles such as dishes, vases, toilets and the like. They are used for casting ceramic articles because of gypsum's favorable water absorption properties. Plaster is also known as calcium sulfate hemihydrate, calcium sulfate semihydrate, calcium sulfate half-hydrate, stucco, or Plaster of Paris. When making a mold, Plaster of Paris is mixed with water to form a slurry. The slurry is then poured over a form, and allowed to hydrate until the setting reaction is complete. As the plaster sets, the hemihydrate form is converted to the dihydrate form through chemical bonding of water to form an interlocking matrix of calcium sulfate dihydrate crystals. As the dihydrate is formed, capillaries are formed in the plaster, giving it the ability to absorb water after the gypsum mold is dry. For use in molds, this ability to absorb water allows the mold to “wick away” water from the clay slip that is poured into the mold to produce “greenware” which must be fired in a kiln to make a ceramic article.
The use of plaster for preparing molds for the production of shaped articles is well known in the art. For example, U.S. Pat. No. 2,391,855 discloses making permeable plaster bodies and the addition of additives to control expansion of the plaster after the initial set. The addition of potassium salts to plaster that is formed into boards or molds is taught in U.S. Pat. No.4,443,261. Both of these references are hereby incorporated by reference.
In molding pieces of ceramicware, most of the time is used in the process of creating the greenware and then drying the gypsum mold. When the clay slip is poured into the gypsum mold, water is absorbed by the gypsum, allowing the clay particles to deposit on the inner surface of the gypsum mold. Greenware is formed when enough water has been absorbed to allow the cast article to hold its shape and be removed from the mold, although the article is still very fragile at this stage.
The forming process may be accelerated by a process known as medium pressure casting. It includes applying pressure to the clay body while it is still in the mold. Calcined gypsum in the alpha form, which is made by heating gypsum under pressure, produces a mold that is stronger than one formed with beta calcined gypsum and is more suitable for pressure casting. Molds made from alpha gypsum, however, have a disadvantage because the lower porosity of set gypsum formed from alpha calcined gypsum results in longer times to purge and dry the molds.
Gypsum mold materials that absorb water from the clay slip reduce the time required to form greenware. This improves the efficiency of the ceramicware making process by freeing up the mold to be used in casting another article. This is known as “turn over”. However, before the gypsum mold can be reused, it must be dried. The water that has been absorbed into the capillaries must be purged from the gypsum mold so that the capillaries will have the capacity to draw sufficient moisture from the next ceramic article. Generally, the gypsum mold is treated with heat or pressure to drive off the absorbed water. Heating or pressurizing the molds to accelerate the drying process also requires costly expenditures of energy. Additionally, the molds begin to deteriorate after many uses because drawing of the water into and out of the capillaries causes erosion. Erosion of the gypsum mold over time, due to gypsum's natural solubility in water, leads to a loss of detail in the gypsum mold which is then transferred to the cast greenware article. Normally a plaster mold can be used for an average of 80 cycles before erosion makes it unusable.
Many additives are known to change the properties of the plaster used in the mold. Molds made from alpha calcined gypsum are stronger, more dense and less porous compared to those manufactured from beta calcined gypsum. In general, additives that improve the hardness of the gypsum mold usually reduce the porosity of the mold. Strength may also be increased by casting a denser gypsum mold; however, this typically has a negative effect on porosity. Loss of porosity reduces both the ability of the mold to wick moisture away from the clay slip, and also reduces the purge rate of the gypsum mold. Thus, although stronger molds may last longer, they may slow down the production of the greenware articles by increasing both the absorption time and the purge time of the gypsum molds. Molds which are difficult to purge also add to the expense of the process by requiring higher energy usage in the drying process.
Some of these problems have been solved by the use of resin molds in place of gypsum molds for casting of ceramic articles. These molds provide fast turnover of the mold, but the resins used therein are considerably more expensive than gypsum—based compositions. Resin molds must also be made from special machinery. This adds to the cost of the process as the design of the mold needs to be outsourced. In addition, resin molds are only cost effective for large production volumes as the molds are costly to make, store, and maintain.
It is therefore an object of this invention to provide improved gypsum molds for casting of ceramic articles that have enhanced absorption and purge rates.
It is another object of this invention to provide an inexpensive mold that retains strength for long life in addition to an increase in absorption and purge rate of absorbed water.
It is yet another object of this invention to provide an energy efficient mold for producing ceramic articles that does not require large amounts of energy to be expended to purge and dry the mold.
It is still another object of this invention to improve efficiency of production of ceramic articles by reducing the turn over time required to prepare molds for casting of another article.
It is another object of this invention to provide an improved mold formed from alpha calcined gypsum that absorbs water more quickly during pressure casting and reduces the time required to form the greenware.
DISCLOSURE OF INVENTION
The above-listed objects are met or exceeded by the present invention, which features a gypsum composition that produces an improved mold for casting ceramic articles. The mold has the improved ability to draw moisture from the article being cast, allowing it be produced faster, especially during pressure casting. Drying of the gypsum mold after use is also more efficient as it can be purged of the absorbed moisture more readily than a conventional gypsum mold. These improvements in absorbing and purging of water are done in a manner that does not compromise the strength of the mold or the efficiency of the casting process.
More specifically, the present invention provides a plaster composition for making gypsum molds for the manufacture of ceramicware. The plaster composition is comprised of a calcium sulfate hemihydrate, one or more of ammonium sulfate and potassium sulfate and one or more of ammonium L tartrate and potassium sodium tartrate. A weight ratio of from about 1:1 to about 10:1 of one or more of ammonium sulfate and potassium sulfate to one or more of ammonium L tartrate and potassium sodium tartrate must be maintained. The one or more of potassium sodium tartrate and ammonium L tartrate is present in an amount of from about 1 to about 10 pounds per ton of dry calcium sulfate hemihydrate, and the total of the ammonium sulfate, potassium sulfate, ammonium L tartrate and potassium sodium tartrate is from about 2 to about 35 pounds per ton of the dry hemihydrate.
Applicants have found that the combination of potassium sodium tartrate and potassium sulfate in a specified ratio produces a gypsum mold with a pore structure that facilitates enhanced purging of the water from the mold in preparation for its subsequent use. An improved mold made of alpha calcined gypsum also facilitates absorption of water during pressure casting. Ease in drying of the mold means that less energy is required in terms of pressure or heat to drive off the water that was abso
Grussing Jeffrey F.
Immordino Salvatore C.
Przybysz Andrzej A.
Greer Burns & Crain Ltd.
Janci David F.
Lorenzen John M.
Marcantoni Paul
United States Gypsum Company
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