Gypsum composition for mold production

Abstract

The purpose of the present invention is to provide, for example, a gypsum composition for mold production capable of contributing to improved workability in a production step for gypsum forms (molds) which, in production of a denture model, are necessary for production of a model of a resin-made artificial gingiva (gums) part, which each differ from each other, and for which an elaborate shape is required. Specifically, the purpose of the present invention is to provide a gypsum composition for mold production which exhibits favorable fluidity as a mixture in slurry form containing water, which thus can improve workability by being used in tertiary embedment for filling gaps in a polymerization flask when producing a mold, and which, when cured, can form a favorable mold in which the occurrence of cracks in the gypsum is suppressed. The present invention is a gypsum composition for mold production characterized by containing hemihydrate gypsum, wherein the hemihydrate gypsum contains α-type hemihydrate gypsum and β-type hemihydrate gypsum at a ratio of 25:75 to 65:35.

Description

Gypsum composition for making molds

[0001] The present invention relates to a gypsum composition for producing a gypsum mold used in producing a model, for example, a gypsum composition for producing a mold suitable for producing a model of a denture (false tooth), etc. More specifically, the present invention relates to a gypsum composition for producing a mold, which exhibits good fluidity when kneaded in a slurry form by adding water, thereby improving workability when producing a mold (plaster mold), and which can form a mold in a good condition in which the occurrence of cracks in the gypsum is suppressed when hardened.

[0002] Gypsum compositions are used as plaster molds (casting molds) when making various models, and have the property of being able to faithfully invert the fine patterns of the model and obtain an extremely smooth surface. An example of a model that requires such properties is a so-called denture. In response to a society with an increasing longevity, better-fitting complete and partial dentures are required, and various materials used for artificial teeth and artificial gums (see Patent Document 1) and manufacturing devices have been developed. Among these, gypsum compositions are useful materials for making molds required for making models such as dentures, which have different shapes for each model and require precision, and they are also required to have the property of being able to more easily and stably produce good-quality molds, so-called plaster molds. Hereinafter, molds obtained from gypsum compositions will also be referred to as plaster molds.

[0003] The following describes the fabrication of "dentures" as an example of a model having the above-mentioned characteristics. According to the inventors' research, the "problem of improving workability in the mold formation stage," which is necessary when fabricating artificial gums (gums) formed from resin, depends on the properties of the gypsum composition used to form the mold. In other words, the workability in filling the gypsum composition and the hardening time of the gypsum composition significantly affect the efficiency of the mold formation process. In addition, the surface condition of the gums (gums) formed by pouring and hardening the resin into the cavity (space) of the resulting mold depends on the inner surface condition of the cavity of the gypsum mold used. To achieve a better-fitting "denture," the following basic performance is required. Specifically, when the denture is placed in the mouth, the artificial teeth must be fixed and held to the artificial gums (gums) so as to provide a good occlusion. Furthermore, the surface condition of the artificial gums (gums) formed from the resin must not cause discomfort to the user wearing the denture. Therefore, in the manufacture of dentures, a good mold capable of forming individually different and precisely shaped artificial gums (gums) made of resin, and a resin material for forming good artificial gums (gums) using the mold are required.

[0004] Thus, just like artificial teeth, the quality of the resin artificial gums (gums) that support and integrate the artificial teeth is also an important factor, and therefore, research into gypsum compositions used in the production of resin artificial gums (gums) is desired. Patent Document 1 proposes a resin material for dentures formed using such a mold. Furthermore, what is important for denture manufacturers is to supply dentures with excellent fit as quickly as possible. In this regard, it is particularly desirable to improve the work efficiency of dental technicians when producing dentures (models), and achieving improved work efficiency would also be beneficial for denture users.

[0005] Japanese Patent Application Laid-Open No. 2020-204011

[0006] For example, in the series of complicated denture (model) manufacturing processes, the following steps can be cited as causes of impaired workability for dental technicians. That is, workability is impaired in the "process of manufacturing a mold (plaster mold) for forming a resin artificial gum (gum) portion," which is necessary when manufacturing a "denture" in which artificial teeth and resin artificial gum (gum) are integrally formed by pouring resin into a mold cavity to form a resin artificial gum (gum) portion. According to the inventors' investigations, as will be described later, dental technicians' workability is impaired particularly at the stage of the mold manufacturing process, which requires a precise shape that is individually different and can reproduce the shape of the gums. It would be extremely useful if this issue could be resolved by improving the gypsum composition for mold manufacturing.

[0007] Therefore, an object of the present invention is to provide a gypsum composition for mold production that can contribute to improving workability in the process of producing molds that are required to have individually different and precise shapes, which are necessary, for example, when producing artificial gingiva (tooth gum) parts made of resin.

[0008] The above object is achieved by the present invention described below. That is, the present invention provides the following gypsum composition for mold production: [1] A gypsum composition for mold production, comprising gypsum hemihydrate, wherein the gypsum hemihydrate contains alpha-type gypsum hemihydrate and beta-type gypsum hemihydrate in a ratio of 25:75 to 65:35.

[0009] Preferred embodiments of the gypsum composition for mold production of the present invention include the following inventions. [2] The gypsum composition for mold production according to the above item [1], wherein the ratio of the α-type hemihydrate gypsum to the β-type hemihydrate gypsum is 25:75 to 60:40. [3] The gypsum composition for mold production according to the above item [1], wherein the ratio of the α-type hemihydrate gypsum to the β-type hemihydrate gypsum is 30:70 to 60:40. [4] The gypsum composition for mold production according to any one of items [1] to [3], wherein the gypsum hemihydrate accounts for 95 parts by mass or more of 100 parts by mass of the gypsum composition for mold production. [5] The gypsum composition for mold production according to any one of items [1] to [4], further comprising a water-reducing agent, wherein the amount of the water-reducing agent added is 0.02 to 0.2 parts by mass relative to 100 parts by mass of the gypsum hemihydrate. [6] The gypsum composition for mold production according to any one of [1] to [5] above, wherein the pot life of a slurry-like kneaded product obtained by adding water so that the mixed water amount is 45% is 8 minutes or more, and the hardening time is adjusted to 30 minutes or less.

[0010] According to the present invention, it is possible to provide a useful gypsum composition for mold production (hereinafter referred to as a "gypsum composition for molds") that can effectively contribute to improving workability in the process of producing molds that are required to have individually different and precise shapes, as exemplified above, which are necessary when producing models.

[0011] 2A is a schematic cross-sectional view showing the state inside a polymerization flask 10 to illustrate the use of the casting gypsum composition of the present invention. The polymerization flask must be supported by a gypsum model (hereinafter also referred to as a "working model") 3 that simulates the patient's oral cavity, which is made by pouring gypsum into an impression of the patient taken at a dental clinic or the like. A "wax denture having artificial teeth 2b and a wax ridge 2a" 2 is embedded without gaps in the gypsum composition. This is a schematic perspective view illustrating the state in which the "wax denture having artificial teeth and a wax ridge" 2 is placed in a gypsum composition 4 for primary investment filled in a container 5a, which is a lower component of the polymerization flask, during the process of making a mold used to make a resin artificial gum. This is a schematic perspective view illustrating the state in which the "wax denture having artificial teeth and a wax ridge" 2 placed in the lower container 5a constituting the polymerization flask shown in FIG. 2A is being embedded in a gypsum composition 6 for secondary investment. 1 is a schematic perspective view of an example of a polymerization flask used in the mold preparation process, and its interior must be in the state shown in Fig. 1 during the mold preparation process. That is, when an upper member 5b serving as a lid is placed on a lower container 5a of the polymerization flask, the inside of the polymerization flask 10 must be filled with the gypsum composition without any voids. It is a schematic perspective view for illustrating the state in which a slurry-like kneaded product 1 obtained by adding water to the foundry gypsum composition of the present invention is poured into voids that are not filled with the gypsum composition and are formed in the polymerization flask in the state of Fig. 3.

[0012] The present invention will be described in more detail below with reference to preferred embodiments. First, the process of fabricating a mold required for fabricating artificial gingiva, which the inventors have identified as a problem that impairs the workability of dental technicians in the series of cumbersome denture fabrication processes, will be described. For the explanation, the fabrication of complete dentures (hereinafter referred to as "dentures"), commonly known as "full dentures," will be used as a representative example. The process of fabricating artificial gingiva (gums) made of resin that fixes and supports artificial teeth in the series of cumbersome "denture" fabrication processes is outlined below.

[0013] First, a "wax denture having artificial teeth and a wax ridge" is prepared as described below. Next, a plaster mold is prepared using the wax denture to prepare a resin artificial gingiva (gum). The gypsum composition for a mold of the present invention is used to fill the unfilled plaster portion that occurs in the polymerization flask when preparing the mold, thereby effectively contributing to improving the workability of the "denture" preparation process. The following explanation is an example using a polymerization flask 10 with a structure divided into two parts, upper and lower, as shown in Figures 1 to 3. That is, as shown in the figures, a polymerization flask was used that is composed of a lower container member 5a and an upper lid member 5b (sometimes referred to as lid 5b or cap).

[0014] First, a wax ridge 2a is provided on a working model 3, and artificial teeth 2b are positioned on the wax ridge at a location that will allow for good occlusion for the denture user (see Figure 1). The wax ridge is made of a material such as wax that can be melted in a microwave oven or with hot water. During this process, with the artificial teeth 2b fixed to the wax ridge 2a, carving is performed on the surface of the wax ridge 2a, which will serve as a base for supporting the artificial teeth 2b, to visually reproduce the gingiva (gum) area. In this way, a "wax denture having artificial teeth 2b and wax ridge 2a" 2 is produced.

[0015] Next, as shown in Figure 2A, the "wax denture having an artificial tooth and a wax ridge" 2 obtained as described above is fixed in the lower container 5a of the polymerization flask 10, which has a structure divided into upper and lower halves, using a gypsum composition for primary investment 4. This operation is called "primary investment."

[0016] Next, as shown in Figure 2B, a gypsum composition 6 for secondary investing is applied to the "wax denture having an artificial tooth and a wax bank" 2 obtained after the primary investing as described above, to cover the wax bank 2a and the artificial tooth 2b. This operation is called "secondary investing."

[0017] Next, the upper lid 5b is placed on the polymerization flask, and then a slurry-like kneaded mixture of the casting gypsum composition of the present invention and water is poured into the polymerization flask 10 to fill any voids that have formed inside, thereby embedding the "wax denture having artificial teeth and a wax bank" 2 in the gypsum composition. This operation is called "tertiary embedding." Hereinafter, the "wax denture having artificial teeth 2b and a wax bank 2a" 2 may be simply referred to as the "wax denture 2."

[0018] The above-described series of embedding steps can provide a mold for forming a resin artificial gingiva (gums) portion, in which the artificial teeth 2b are supported and fixed with the occlusion adjusted. As described below, in the present invention, the tertiary embedding operation for obtaining this artificial gingiva formation mold is performed using the casting gypsum composition of the present invention, which exhibits good fluidity as a slurry kneaded product when water is added, thereby enabling the easy production of a good-quality resin artificial gingiva (gums). The procedure for producing the artificial gingiva (gums) portion of the wax denture 2 from resin using the casting mold (plaster mold) for artificial gingiva formation is as follows.

[0019] First, the plaster filled in a polymerization flask containing the slurry kneaded material used for the tertiary investing is allowed to harden sufficiently, and then the wax (wax bank 2a) that constitutes the artificial gingiva (gum) portion of the wax denture 2 is thoroughly melted using a microwave oven or hot water to form a hollow mold (plaster mold) in which the wax portion has melted. The resulting mold has the artificial teeth 2b that were held in place by the wax before melting, now held in place by the hardened plaster. In addition, the state of the carved gingiva (gum) is transferred to the inner surface of the cavity formed by the melted wax flowing out and where the wax was removed.

[0020] Next, the cavity in the plaster mold is filled with resin for forming artificial gums, and the resin is allowed to harden. Finally, the mold is broken, and the artificial gums (gums) made of resin and the artificial teeth that are firmly fixed and supported by the artificial gums are removed. The artificial teeth and artificial gums are then carefully polished. This results in a "denture" in which the artificial teeth are properly positioned in the artificial gums (gums) made of resin, with the occlusion adjusted accordingly.

[0021] As a result of extensive research into improving the workability of the above series of steps, the inventors have found that improved workability can be achieved by improving a foundry gypsum composition and using it in the following operations, which have the following problems. In the process of filling a mold in which a cavity has been formed by melting the wax with resin, the resin is typically injected under pressure into the cavity left by the wax removal, in order to fill it with a dense resin. This can lead to cracks and breaks in the hardened gypsum composition used in the secondary investment, which can cause burrs and impair the smoothness of the surface of the formed resin gums. Furthermore, conventional gypsum compositions used in the tertiary investment have poor fluidity when mixed with water. Furthermore, in order to develop the strength of the hardened gypsum composition, it is generally necessary to apply vibrations with a vibrator and carefully fill the gypsum composition for the tertiary investment into the polymerization flask to prevent voids from forming.

[0022] In contrast to the above, when the casting gypsum composition of the present invention is used for the tertiary investing, as shown in Figures 1 and 2A, the "wax denture having artificial teeth 2b and wax ridge 2a" 2 is quickly embedded in the gypsum composition 6 used for the secondary investing and the gypsum composition 1 used for the tertiary investing, which is made by pouring the casting gypsum composition of the present invention in a slurry form. According to the studies of the present inventors, as a result, the hardened product of the casting gypsum composition of the present invention used for the tertiary investing cooperates with the hardened product of the gypsum composition used for the secondary investing, thereby achieving the surprising effect of developing sufficient strength to suppress cracks and breaks caused by the pressure during resin injection.

[0023] The polymerization flask 10 used in the above process comes in various shapes, including not only the two-divisible type exemplified above, but also three-divisible types. However, regardless of the structure, as described above, the hardened plaster must ultimately be removed from the polymerization flask filled with plaster and then broken to remove the dentures. Therefore, as shown in FIGS. 1 to 3, the polymerization flask 10 is divided into upper and lower sections. As shown in FIGS. 2A and 2B, a high-viscosity gypsum composition for secondary investment 6 is placed from the top of the open, divided container 5a on top of the "wax denture having artificial teeth and artificial gums (gum)" 2, which is placed in the lower container 5a that serves as the polymerization flask and fixed with a gypsum composition for primary investment 4, so as to cover the "wax denture having artificial teeth 2b and wax ridge 2a."

[0024] Thereafter, an upper member 5b, which functions as a lid for the lower container 5a of the polymerization flask 10, is placed on the lower container 5a and integrated as shown in FIG. 3 , after which the gypsum composition is allowed to harden. Here, when the upper member 5b, which serves as a lid, is placed on the container 5a of the polymerization flask after the secondary burying and integrated, an unfilled void portion that is not filled with gypsum is formed in the polymerization flask 10 after integration, although the size may vary. Therefore, a tertiary burying operation is required to fill this void portion with the gypsum composition. Specifically, for example, as shown in FIG. 3 , a tertiary burying operation is performed in which the gypsum composition is filled through a hole 7 provided on the side of the polymerization flask 10, which leads to the unfilled void portion in the polymerization flask that is not filled with gypsum. In this case, in the conventional technology, the gypsum composition having a high viscosity, which has been adjusted for the tertiary burying, is gradually and carefully filled into the polymerization flask over a period of time using a vibrator or the like so as to prevent the formation of unfilled voids.

[0025] In the prior art, as described above, the "wax denture having artificial teeth 2b and wax bank 2a" 2 is carefully embedded in the gypsum composition over a long period of time so that there are no voids, and then the gypsum is hardened, and after hardening, a hardened gypsum body for producing a mold in which the wax denture 2 is embedded in the gypsum composition is removed from the polymerization flask 10. The present inventors recognized that what impairs the workability of dental technicians in the denture mold production process described above is the step of filling with gypsum composition "voids that are not filled with gypsum and occur in the polymerization flask," which is unavoidable due to the structure of the polymerization flask (tertiary embedding).

[0026] To address the above-mentioned technical challenges, it would be extremely useful in terms of work if a gypsum composition having the following properties could be developed. Specifically, it is desired to develop a gypsum composition for mold production that allows the gypsum composition to be quickly and easily filled in a good condition into voids that occur in the polymerization flask during tertiary embedding, that gives a slurry-like kneaded product with excellent fluidity, and that does not produce burrs in the resin portion that will become the artificial gingiva (gums) formed using a mold (plaster mold) having a cavity left behind after the wax bank 2a has been removed.

[0027] The present inventors conducted extensive research to develop a gypsum composition that can exhibit the above-mentioned properties in order to address the technical issues that arise when producing a mold in the denture production process. As a result, they arrived at the present invention. Specifically, the gypsum composition for mold production of the present invention is characterized by containing gypsum hemihydrate, and the gypsum hemihydrate contains alpha-type gypsum hemihydrate and beta-type gypsum hemihydrate in a ratio of 25:75 to 65:35.

[0028] When water is added to the above-described gypsum casting composition to form a slurry, and the gypsum composition is then filled into the voids in the polymerization flask that have not yet been filled with gypsum, the gypsum composition has a moderate fluidity, allowing for fast and easy filling of the voids in the polymerization flask with a dense gypsum composition. This significantly improves the work efficiency when producing a casting mold (gypsum mold) for producing a resin artificial gingiva (dental gum).

[0029] Furthermore, by filling the polymerization flask with the foundry gypsum composition of the present invention in the tertiary investing process as described above, the following excellent effects can be obtained. As described above, the process for preparing a mold (plaster mold) for producing artificial gingiva (gums) involves the following operations: After filling the polymerization flask with the primary, secondary, and tertiary gypsum compositions, the gypsum is allowed to harden sufficiently, and the wax (wax bank) is dissolved to form a cavity, completing the mold (plaster mold), which is then filled with a resin to form the artificial gingiva (gums). The polymerization flask is then disassembled, the mold is removed from the flask, the mold is broken, and the model (denture) is removed from the mold. Here, the mold containing the model (denture) obtained as described above is in a state where the foundry gypsum composition of the present invention used in the tertiary investing process and the high-viscosity gypsum composition previously filled into the polymerization flask in the secondary investing process for embedding the wax denture, which is performed before the tertiary investing process, are seamlessly integrated. Therefore, when the cavity of the mold is filled with the resin that will become the artificial gums (gums), the casting gypsum composition of the present invention used in the tertiary investment and the gypsum composition used in the secondary investment work together to prevent cracks or other defects caused by the filling of the resin that will become the artificial gums (gums). When the mold is broken and removed, the resin artificial gums (gums) that make up the "denture" are in good condition, free of burrs that may result from cracks. This reduces the need for post-processing, such as burr removal. Furthermore, the mold can be broken without any problems after the resin has hardened. Finally, by carefully removing the gypsum adhering to the artificial teeth, etc., a "denture" in good condition can be obtained.

[0030] Below, we will explain the materials for forming the casting gypsum composition of the present invention, which can achieve the above-mentioned excellent effects. First, the casting gypsum composition of the present invention is used for the applications exemplified above, and is not used for manufacturing casting molds (gypsum molds) for casting metal. Therefore, fire resistance or high heat resistance is not required, and therefore it does not contain refractory materials such as cristobalite or quartz. Furthermore, this specification has been described using "dentures" as an example of a model requiring a mold, but its use is not limited to "dentures." The casting gypsum composition of the present invention can also be widely used in the production process of casting molds used when producing resin models and the like for various other purposes. By using the casting gypsum composition of the present invention, various models with precise shapes and reduced burrs can be efficiently obtained.

[0031] The foundry gypsum composition of the present invention is provided as a powder, and is prepared into a slurry by adding water when used. The term "powder" as used herein means a powder having an average particle diameter of 200 μm or less. The average particle diameter in this specification is the median diameter (d ５０ The average particle size of the foundry gypsum composition in the present invention can be measured using a laser diffraction particle size distribution analyzer (manufactured by Nikkiso Co., Ltd., trade name: Microtrac HRA).

[0032] [Hemihydrate Gypsum] The foundry gypsum composition of the present invention contains gypsum hemihydrate. Specifically, calcium sulfate hemihydrate (CaSO4·½H2O) is called gypsum hemihydrate. Since gypsum hemihydrate reacts with water to convert to gypsum dihydrate, a gypsum slurry prepared by kneading gypsum hemihydrate with water quickly hardens when poured into a polymerization flask. For this reason, it is widely used in the production of molds. Gypsum hemihydrate is obtained by calcining gypsum dihydrate (calcium sulfate dihydrate). Examples of the gypsum dihydrate used as a raw material include natural gypsum, by-product gypsum, and recycled gypsum obtained through a gypsum product recycling process. The gypsum hemihydrate constituting the foundry gypsum composition of the present invention may be obtained from any of the above-mentioned raw gypsum dihydrate materials. Here, the amount of gypsum hemihydrate in the foundry gypsum composition of the present invention can be, for example, 95 parts by mass or more per 100 parts by mass of the foundry gypsum composition. More preferably, it can be 98 parts by mass or more.

[0033] The foundry gypsum composition of the present invention is characterized by containing the above-described gypsum hemihydrate, and the hemihydrate is composed of alpha-type gypsum hemihydrate and beta-type gypsum hemihydrate in a ratio of 25:75 to 65:35. A preferred embodiment of the foundry gypsum composition of the present invention is one in which the ratio of alpha-type gypsum hemihydrate to beta-type gypsum hemihydrate is 25:75 to 60:40. A more preferred embodiment is one in which the ratio of alpha-type gypsum hemihydrate to beta-type gypsum hemihydrate is 30:70 to 60:40. The average particle size of the gypsum hemihydrate is preferably about 10 to 80 μm, and more preferably about 20 to 50 μm. The alpha-type gypsum hemihydrate and beta-type gypsum hemihydrate, which are essential for the foundry gypsum composition of the present invention, are described below. As described below, it is believed that the foundry gypsum composition of the present invention achieves the effects of the present invention that have not been achieved in the past by using the gypsum hemihydrate in a composition defined as described above.

[0034] (α-Hemihydrate Gypsum) α-Hemihydrate Gypsum can be obtained by calcining the above-mentioned raw material gypsum dihydrate in water or steam under pressure. Compared to β-hemihydrate gypsum, which will be described later, α-hemihydrate gypsum can be mixed with a smaller amount of mixed water (i.e., low mixed water). Therefore, the gypsum hardened body obtained using α-hemihydrate gypsum has high strength (compressive strength). As mentioned above, the α-hemihydrate gypsum used in the present invention preferably has an average particle size of about 10 to 80 μm, and more preferably about 20 to 50 μm.

[0035] (β-type hemihydrate) β-type hemihydrate is obtained by calcining the raw material gypsum dihydrate as described above in the atmosphere. Compared to the above-mentioned α-type hemihydrate, a larger amount of mixed water is required for kneading. Therefore, the gypsum hardened body obtained using β-type hemihydrate has low strength (compressive strength). According to the studies of the present inventors, if the amount of mixed water is reduced to the same amount as that used for α-type hemihydrate, the gypsum hardened body obtained using β-type hemihydrate will have higher strength than the gypsum hardened body obtained using α-type hemihydrate. As mentioned above, the β-type hemihydrate used in the present invention preferably has an average particle size of about 10 to 80 μm, more preferably about 20 to 50 μm.

[0036] Conventionally, the main raw material used in gypsum compositions for making molds used in producing "dentures" is β-type hemihydrate gypsum. Alpha-type hemihydrate gypsum has not been used as a raw material for the following reasons. To obtain a denture embedded in a mold, it is ultimately necessary to break the mold (gypsum hardened body) and remove the denture. In contrast, if alpha-type hemihydrate gypsum is used in the gypsum composition for making a mold, the strength of the gypsum hardened body is too high, making it difficult to break, and there is a concern that forcible breaking could damage the denture embedded in the mold. Dentures are intricately crafted individually taking into consideration the user's bite and appearance, and therefore damage is unacceptable. For this reason, even if alpha-type hemihydrate gypsum is used as a raw material for gypsum compositions for molds, it has only been in relatively small amounts. For the reasons mentioned above, this has become common technical knowledge regarding gypsum compositions for molds used in molds (gypsum molds) used in producing "dentures" and the like.

[0037] In response to the above-mentioned current situation, the present inventors have recognized that, as described above, it is important to be able to quickly and easily fill voids that occur in a polymerization flask due to its structure, etc. with a gypsum composition in order to improve the workability in producing "dentures." As a means for filling unfilled voids with the gypsum composition that occurs in a polymerization flask using a gypsum composition for a mold, which has the above-mentioned technical common sense, it would be extremely useful to provide a gypsum composition that allows the gypsum composition to be quickly and easily filled into voids without using special equipment such as pressure fitting or a vibrator, and even if a vibrator is used, without requiring careful and long time as is the case currently.

[0038] Based on this understanding, the present inventors conducted extensive research and found that it is possible to provide a foundry gypsum composition that can achieve the above-mentioned objectives by a very simple means of configuring the hemihydrate gypsum raw material constituting the foundry gypsum composition to contain α-type hemihydrate gypsum and β-type hemihydrate gypsum in a ratio of 25:75 to 65:35, as specified in the present invention. That is, by using α-type hemihydrate gypsum and β-type hemihydrate gypsum within the ratio range specified in the present invention, the fluidity of the gypsum composition can be improved by adding water to the powder material to form a slurry with, for example, a 45% water content. This allows the gypsum composition to be quickly and easily filled into voids (spaces) in the polymerization flask that are not filled with gypsum due to the structure of the polymerization flask. Furthermore, because the slurry obtained using the foundry gypsum composition of the present invention exhibits good fluidity, the risk of air bubbles being mixed in during kneading is extremely low, eliminating the need for an antifoaming agent. Alternatively, even if an antifoaming agent is added, the amount of the antifoaming agent can be significantly reduced. Therefore, the foundry gypsum composition of the present invention has another effect of being industrially useful in terms of raw material costs.

[0039] In addition to the above, the kneaded product of the foundry gypsum composition of the present invention and water, even when kneaded with a small amount of water, has the following useful properties. The kneaded product has low viscosity and excellent fluidity. Moreover, although the resulting gypsum hardened body uses a high proportion of β-type hemihydrate gypsum, which results in low strength (compressive strength), the mold, which is a gypsum hardened body obtained using the foundry gypsum composition, has sufficient compressive strength to prevent cracking when resin to form artificial gums (gums) for resin dentures is injected into the cavity. Furthermore, after the resin hardens, the mold can be broken without any problems, allowing the denture (model) inside to be removed.

[0040] Furthermore, the casting gypsum composition of the present invention is well integrated with the high-viscosity gypsum composition used for secondary investing, preventing cracks and other defects from occurring in the hardened gypsum composition used for secondary investing. Therefore, the resin artificial gums (gums) obtained by supplying and hardening a resin for forming the gum portion into the resulting mold cavity are in good condition and free of burrs. This facilitates secondary processing after the resin hardens, resulting in excellent cost performance.

[0041] Furthermore, according to the studies of the present inventors, when the foundry gypsum composition of the present invention contains a water-reducing agent (dispersant), the amount of the water-reducing agent added can be 0.02 to 0.2 parts by mass per 100 parts by mass of the hemihydrate gypsum having the above-described configuration. In other words, with the foundry gypsum composition of the present invention, even if the amount of the water-reducing agent (dispersant) added is relatively small, such as 0.2 parts by mass or less, the effects of adding a water-reducing agent can be obtained. Therefore, the foundry gypsum composition of the present invention can reduce raw material costs and is also economical in this respect.

[0042] Examples of the water-reducing agent that can be used include naphthalene sulfonic acid-based dispersants, melamine-based dispersants, polycarboxylic acid-based dispersants, and phosphonic acid-based dispersants. However, the water-reducing agent is not limited to these, and any of the conventionally known water-reducing agents (dispersants) can be used.

[0043] The casting gypsum composition of the present invention may contain conventionally known additives as long as they do not impair the intended purpose. Specifically, set adjusters such as set accelerators or set retarders can be added as appropriate. Set accelerators include, for example, gypsum dihydrate and potassium sulfate, and other common set accelerators can be used. Set retarders can include, for example, sodium citrate and other common set retarders.

[0044] The present invention will now be described in more detail with reference to examples and comparative examples. Hereinafter, parts and percentages are by mass unless otherwise specified.

[0045] Examples 1 to 5 and Comparative Examples 1 and 2: Foundry gypsum compositions of Examples 1 to 5 and Comparative Examples 1 and 2 were prepared using the compositions shown in Table 1. During preparation, 0.03 parts of gypsum dihydrate and 0.2 parts of potassium sulfate were added as a set accelerator, 0.025 parts of sodium citrate as a set retarder, and 0.025 parts of potassium tartrate as an expansion inhibitor, per 100 parts of the foundry gypsum composition, under the same conditions. Furthermore, when the prepared composition was used for the tertiary embedding of "dentures" to ensure good fluidity when poured into a polymerization flask, the composition was adjusted to achieve the following flow values. That is, the powdered gypsum composition having the above formulation was mixed with water to form a slurry-like kneaded mixture with a water content of 45%, and the amount of water-reducing agent shown in Table 1 was added so that the flow value was 115 mm to 135 mm.

[0046] [Evaluation Method and Results] The powdered foundry gypsum compositions of the Examples and Comparative Examples, each having the formulation shown in Table 1, were mixed with water to a water content of 45% to prepare a slurry-like kneaded product, or a gypsum hardened product obtained by hardening the kneaded product, and the evaluations were carried out as follows. The evaluation results are shown in Table 1.

[0047] (Usable time) Water (23±2°C) was measured into a dental rubber bowl to obtain a predetermined mixing amount, and the powdered gypsum mold composition of the test example and comparative example was poured into the bowl over 10 seconds. After leaving the mixture for 20 seconds, the mixture was mixed with a spatula for 60 seconds to obtain a kneaded product. The stirring speed during mixing was 120 rpm in all cases. Hereinafter, the slurry-like kneaded product obtained as described above will be referred to as "gypsum slurry."

[0048] After mixing, a small amount of gypsum slurry was poured every 30 seconds. The time 30 seconds before the viscosity of the gypsum slurry increased and it stopped flowing continuously (dropping in small, sharp drops) was defined as the time during which the gypsum slurry could be poured, or the so-called "work time." To ensure the working time targeted by the present invention, the work time must be 8 minutes or more.

[0049] (Setting Time) The setting time of the foundry gypsum compositions of the Examples and Comparative Examples was measured in accordance with JIS T6604 Dental Plaster of Paris - 5.5 Setting Time Test. Specifically, the measurement was carried out according to the following procedure.

[0050] Place the ring mold for measuring the setting time on the center of the glass plate. Prepare a gypsum slurry in the same manner as when measuring the "working time" described above, and pour the gypsum slurry into the ring mold for measuring the setting time until it rises slightly above the top surface of the ring mold. Align the height with the top edge of the ring mold for measuring the setting time and scrape the gypsum slurry evenly with a spatula. Using a Vicat needle device (1 mm needle, 300 g), gently drop the needle from the surface of the gypsum slurry under its own weight one or two minutes before the expected setting time. Move the mold along with the glass plate so that the next needle can be gently dropped under its own weight from the surface of the sample onto a new part of the ring mold for measuring the setting time, at least 5 mm away from the wall and other needle penetration marks. Wipe the needle clean, then touch the tip of the needle to the surface of the gypsum slurry and secure the rod with the set screw. Read the scale, loosen the set screw at 15-second intervals, and release the rod. The time from when the powdered gypsum composition comes into contact with water until the penetration depth reaches 2 mm or less is defined as the "setting time." In the present invention, the target setting time is set to 30 minutes or less, because a slow setting time makes it impossible to move on to the next step.

[0051] (Water floating during kneading) As shown below, the test procedure for "water floating during kneading" was the same as that for measuring the setting time of the above-mentioned foundry gypsum composition until halfway through, so it was confirmed when measuring the previous "setting time." - The ring mold for measuring setting time was placed in the center of a glass plate. - The gypsum mold compositions of the examples and comparative examples to be measured were mixed, and gypsum slurry was poured into the ring mold for measuring setting time until it rose slightly above the top surface. - The height was adjusted to the top edge of the ring mold for measuring setting time, and the gypsum slurry was scraped off flat with a spatula. - The appearance of water floating on the top surface of the ring mold was visually observed and evaluated by the time the Vickers needle penetration depth reached 1 mm from the bottom surface of the ring mold.

[0052] (Flow Measurement) The flow values ​​of the foundry gypsum compositions of the Examples and Comparative Examples were measured in accordance with JIS T6604 Dental Plaster of Paris-5.4 Fluidity Test. Specifically, the measurements were performed according to the following procedure.

[0053] - Place a glass plate on a vibration-free surface and place the ring mold in the center of the glass plate. - Pour the gypsum slurry into the ring mold until it reaches the top surface of the ring. - 2 minutes and 15 seconds after the start of mixing, the cylindrical ring mold, 50 mm high and 35 mm in inner diameter, is lifted vertically from the glass plate at a speed of approximately 10 mm / s, so that the gypsum slurry poured into the ring mold spreads onto the glass plate. - One minute after the ring mold is lifted, the maximum and minimum diameters of the spread of the gypsum slurry at the bottom are measured in mm, and the arithmetic average value calculated from these measurements is used as the flow value.

[0054] (Compressive Strength) The compressive strength of the gypsum hardened body obtained using each of the gypsum mold compositions to be measured was measured in accordance with JIS T6604 Dental Plaster of Paris - 5.8 Compressive Strength Test. Specifically, the measurement was performed using the following procedure. To measure compressive strength, a cylindrical compressive strength mold with a height of 40 mm and an inner diameter of 20 mm, consisting of five sections, was used. The compressive strength mold coated with a release agent was placed in the center of a glass plate. The gypsum mold composition to be measured was mixed, and the gypsum slurry was poured into the mold until it rose slightly above the edge of the compressive strength mold on the glass plate. During this process, the mold was gently vibrated while pouring to minimize the inclusion of air bubbles. Before the mixture hardened and lost its luster from the surface, a glass plate coated with a release agent was pressed against the top surface of the mold. Forty-five minutes after the start of mixing, the gypsum hardened body sample for compressive strength measurement was removed from the mold. Sixty minutes after the start of mixing, a compressive strength tester was used to destructively test five samples of the gypsum hardened body for compressive strength measurement, and the maximum force (F) applied was recorded. The compressive strength (S) was then calculated using the following formula: S = F / 314. In the present invention, a compressive strength of "10 MPa or more" was targeted as the strength of the gypsum hardened body so that the hardened body of the gypsum composition used in secondary burying would not crack when a resin for forming artificial gums (gums) for a resin denture was injected into a mold obtained using the gypsum composition for casting.

[0055]

[0056] As shown in Table 1, when the gypsum composition of Comparative Example 1, in which the amount of α-hemihydrate gypsum was 20 parts, was used, the setting time of the gypsum took 42 minutes or more, whereas when the gypsum composition of the Example was used, it was confirmed that the setting time could be shortened to about 20 minutes. Furthermore, as in Comparative Example 2, when the amount of α-hemihydrate gypsum was increased to 70 parts, water floatation occurred during setting time measurement, and it was confirmed that the composition was not suitable for use in producing a gypsum hardened body. Note that, since water floatation occurred with the gypsum composition of Comparative Example 2, evaluation was not performed regarding anything other than water floatation during kneading.

[0057] 1: Gypsum composition for mold production of the present invention (for tertiary investing) 2: Wax denture having artificial teeth and wax bank 2a: Wax bank 2b: Artificial teeth 3: Working model 4: Gypsum composition for primary investing 5a: Lower container of polymerization flask 5b: Upper lid of polymerization flask 6: Gypsum composition for secondary investing 7: Hole for filling with gypsum composition for tertiary investing 10: Polymerization flask

Claims

1. A gypsum composition for producing a mold, comprising gypsum hemihydrate, the gypsum hemihydrate containing α-type gypsum hemihydrate and β-type gypsum hemihydrate in a ratio of 25:75 to 65:

35.

2. The gypsum composition for producing a mold according to claim 1, wherein the ratio of α-type hemihydrate gypsum to β-type hemihydrate gypsum is 25:75 to 60:

40.

3. The gypsum composition for producing a mold according to claim 1, wherein the ratio of α-type hemihydrate gypsum to β-type hemihydrate gypsum is 30:70 to 60:

40.

4. The gypsum composition for producing a mold according to any one of claims 1 to 3, wherein the hemihydrate gypsum accounts for 95 parts by mass or more of 100 parts by mass of the gypsum composition for producing a mold.

5. The gypsum composition for producing a mold according to any one of claims 1 to 3, further comprising a water reducing agent, and the amount of the water reducing agent added is 0.02 to 0.2 parts by mass per 100 parts by mass of the hemihydrate gypsum.

6. The gypsum composition for mold preparation according to any one of claims 1 to 3, wherein the pot life of a slurry-like kneaded product obtained by adding water so that the mixed water amount is 45% is 8 minutes or more, and the hardening time is adjusted to 30 minutes or less.