The use of air-release press molding has been widely adopted in the ware forming industry. The process involves pressing a quantity of plastic clay between cooperating male and female molds or dies formed of porous, fluid-permeable material. In addition to shaping the ware, the pressing operation also substantially dewaters the clay by forcing excess water into the pores of the molds. Release of the shaped ware, which adheres to the mold faces, is accomplished without distorting or damaging the ware by applying fluid pressure to a conduit embedded in one of the mold bodies so that the fluid passes from the conduit, diffuses throughout the porous mold body and exits through the mold face as a uniform blanket shortly before the male and female die members are separated. The shaped clay ware adheres to the second mold or die which is transferred to a ware depositing station where fluid pressure is applied in a like manner to the second mold member to complete the release of the formed article. This basic process is disclosed in U.S. Pat. Nos. 2,584,109 and 2,584,110.
The original air-release mold bodies were formed of high grade gypsum plaster or gypsum cement which was found to have nearly ideal porosity for proper fluid permeability. Perforated metal tubing or permeable woven tubing was cast in the plaster molds to provide the required fluid conduits. The gypsum materials, however, were of limited hardness and consequently over the course of repeated pressing operations the faces of the molds would gradually wear away until the molds became unusable and had to be replaced. Although the service life of gypsum molds varied depending on the characteristics of the plastic clay being pressed, the configuration of the molds, the applied force and other factors, the practical service life of gypsum molds was generally no more than about 1000 pressing operations. This necessitated relatively frequent replacement of the molds with the attendant disadvantages of expense for replacement molds, loss of production time, and non-uniformity of the produced ware due to slight differences between molds. Consequently, the ware forming industry searched for a substitute material for forming the molds which would have the required porosity characteristics closely approximating those of gypsum plaster and which would have a greater hardness enabling it to resist wear.
A crystalline bonded ceramic comprising at least 70% alumina, up to 15% ball clay and up to 15% talc fired to a point short of the theoretical density for the ceramic has been found to be a virtually ideal material. This material is disclosed in U.S. Pat. No. 3,384,499. The alumina, ball clay and talc composition with or without additives, such as manganese dioxide or carbon black, is formed into a slurry, cast and subsequently fired at a temperature ranging from 2,000.degree. F to 2,350.degree. F. The exact firing conditions are controlled to prevent the ceramic mold body from reaching its theoretical density, i.e. the maximum density that the material would achieve if fired to an essentially solid nonporous state. Control of the firing conditions will be discussed in greater detail hereinafter.
The necessary fluid conduit means were formed in the interior of the mold bodies by casting a combustible tubing in the interior of the alumina, ball clay, and talc mold body which was later consumed during the firing operation to leave an open conduit running through the mold body. It was generally considered necessary to form the fluid conduit in the interior of the mold body in order to provide for maximum transfer of fluid from the conduit to the mold body by utilizing the entire circumference of the conduit.
Porous fluid-release molds and dies formed from the new alumina, ball clay and talc material have vastly superior tensile strength, hardness, and wear resisting properties. Whereas a fluid-release, porous mold of gypsum material had a useful service life of approximately 1000 pressing operations, molds and dies formed of the new material were capable of resisting wear and breakage and had a potential useful service life of literally tens of thousands of pressing operations.
In actual operation it was found, however, that the fluid permeability of molds or dies formed of the new alumina, ball clay and talc material progressively decreased so that after two or three thousand pressing operations, the molds were useless because they would no longer conduct sufficient fluid to effect a smooth release of the ware from the mold faces. After just a few thousand pressing operations, the new molds had to be replaced even though the mold faces were not appreciably worn or broken. The explanation given by the art for this gradual decrease in fluid permeability was that silicates from the plastic clay being formed into ware were carried by the water pressed out of the clay and absorbed in the pores of the mold, and were deposited in the pores, thereby progressively closing off the passageways for the fluid and eventually rendering the mold unusable. It was postulated that such a deposition of silicates had also occurred in the gypsum plaster molds and dies but that the service life of the gypsum molds was so short that the problem of decreased fluid permeability had not arisen.
Extensive research was done attempting to eliminate the clogging problem by attacking the deposition of silicates in the interior pores of the mold body. The composition of the plastic clay being pressed was varied in an attempt to prevent the deposition of silicates. Numerous types of acid and alkaline washes were attempted for the purpose of dissolving and removing deposited silicates. All the efforts of the art were unavailing. Despite strenuous efforts over a period of years to solve the problem, molds and dies formed of the new alumina, ball clay, and talc composition still became unusable after approximately two or three thousand pressings. Since the expense of preparing the new alumina, ball clay and talc molds is substantially greater than the cost of preparing mold bodies of gypsum plaster, the competitive advantage of the new material over gypsum was retarded by the clogging problem.