Plaster composition for mold making and method for making dentures

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

[Technical Field]

[0001] The present invention relates to a gypsum composition for producing a gypsum mold used in the production of models, and particularly to a gypsum composition for mold production suitable for producing models such as dentures (false teeth). More specifically, the present invention relates to a gypsum composition for mold production, wherein a slurry-like kneaded product added with water exhibits good fluidity, so that workability in producing a mold (gypsum mold) can be improved, and a good mold with suppressed occurrence of cracks in the gypsum after hardening can be formed. [Background Art]

[0002] Gypsum compositions are used as gypsum molds (molds) for producing various models, and have the property of being able to faithfully reverse fine patterns of a model and obtain an extremely smooth flat surface. A so-called false tooth is an example of a model that requires such properties. In response to an aging society, there is a demand for full dentures and partial dentures with better fit, and various materials for artificial teeth and artificial gingiva (gums) (see Patent Document 1) and various production apparatuses have been developed. Among these, gypsum compositions, which are useful materials for producing molds required for manufacturing models such as dentures that have different shapes for each model and require high precision, are also required to have the property of enabling production of good molds, so-called gypsum molds, with better workability and higher stability. Hereinafter, a mold obtained from a gypsum composition is also referred to as a gypsum mold.

[0003] The following explanation uses the fabrication of dentures as an example of a model possessing the above characteristics. According to the inventors' research, the "problem of improving workability in the mold formation stage" necessary when fabricating artificial gums (gingiva) made of resin depends on the characteristics of the gypsum composition used to form the mold. That is, the workability in filling the gypsum composition and the hardening time of the gypsum composition greatly affect the efficiency of the mold formation work. In addition, the surface condition of the gum portion formed by pouring resin into the resulting mold cavity (space) and hardening it depends on the internal condition of the cavity of the gypsum mold used. In order to make dentures that fit better, the following basic performance is required. That is, when the dentures are placed in the mouth, the artificial teeth are fixed and held in place by the artificial gums (gingiva) so that they are in a state of good occlusion, and furthermore, the surface condition of the artificial gum portion formed of resin does not cause discomfort to the wearer of the dentures. Therefore, in the manufacture of dentures, a good mold capable of forming resin artificial gums (gingiva) that are individually different and have precise shapes, and a resin material for forming good artificial gums (gingiva) using the mold are required.

[0004] Thus, just like with artificial teeth, the quality of the resin-based artificial gum (gingiva) portion that supports and integrates with the artificial teeth is also an important factor, and therefore, it is desirable to investigate the gypsum composition used in the production of the resin-based artificial gum (gingiva) portion. Patent Document 1 proposes a resin material for dentures that is formed using such a mold. Furthermore, it is important for denture manufacturers 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 making dentures (models), and if this improvement in work efficiency is achieved, it will also be beneficial for denture users. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-204011 [Overview of the project] [Problems that the invention aims to solve]

[0006] For example, in the complex process of manufacturing dentures (models), the following steps can be cited as causes of impaired work efficiency for dental technicians. Specifically, work efficiency is impaired in the "process of manufacturing a mold (plaster mold) for forming the resin artificial gum portion," which is necessary when manufacturing the final "denture" in which the artificial teeth and resin artificial gum portion are integrally formed by pouring resin into the cavity of the mold to form the resin artificial gum portion. According to the inventors' research, as will be described later, the work efficiency of dental technicians is impaired in particular at the stage of manufacturing the above-mentioned mold, which is individually different and requires a precise shape to reproduce the shape of the gums. If this can be resolved by improving the plaster composition for mold manufacturing, it would be extremely useful.

[0007] Therefore, the object of the present invention is to provide a gypsum composition for mold making that can contribute to improving workability in the mold making process, which requires molds with individually different and precise shapes, such as when making artificial gum (gingival) parts made of resin. [Means for solving the problem]

[0008] The above objective is achieved by the present invention as described below. That is, the present invention provides the following gypsum composition for mold making. [1] A gypsum composition for mold making, characterized in that it contains hemihydrate gypsum, wherein the hemihydrate gypsum contains α-type hemihydrate gypsum and β-type hemihydrate gypsum in a ratio of 25:75 to 65:35.

[0009] The following invention is a preferred embodiment of the gypsum composition for mold making of the present invention. [2] The mold-making gypsum composition according to [1] above, wherein the ratio of α-type hemihydrate gypsum to β-type hemihydrate gypsum is 25:75 to 60:40. [3] The mold-making gypsum composition according to [1] above, wherein the ratio of α-type hemihydrate gypsum to β-type hemihydrate gypsum is 30:70 to 60:40. [4] The hemihydrate gypsum is the mold-making gypsum composition according to any one of [1] to [3] above, comprising 95 parts by mass or more of 100 parts by mass of the mold-making gypsum composition. [5] The gypsum composition for mold making according to any of [1] to [4] above, further comprising a water-reducing agent, wherein 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] A gypsum composition for mold making according to any of [1] to [5] above, wherein the slurry-like mixture obtained by adding water to a water content of 45% has a pot life of 8 minutes or more and is adjusted to have a hardening time of 30 minutes or less. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a useful mold-making gypsum composition (hereinafter referred to as "mold-making gypsum composition") that can effectively contribute to improving workability in the mold-making process, which is necessary when making models and requires individual differences and precise shapes, as illustrated above. [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic cross-sectional view showing the inside of a polymerization flask 10 to illustrate the circumstances under which the mold gypsum composition of the present invention is used. Inside the polymerization flask, it is necessary that the "wax denture" 2 having an artificial tooth 2b and a wax rim 2a, which is supported by a gypsum model 3 (hereinafter also called the "working model") that mimics the inside of a patient's mouth, made by pouring gypsum into an impression of the patient taken at a dental clinic or the like, is embedded without any gaps in the gypsum composition. [Figure 2A] This is a schematic perspective view illustrating the process of creating a mold for producing resin-based artificial gums, in which a "wax denture" 2, which has an artificial tooth and a wax rim, is placed in a gypsum composition 4 for primary investment, which is filled into a container 5a, a component of the lower part of the polymerization flask. [Figure 2B]Figure 2A is a schematic perspective view illustrating the process of embedding a "wax denture" 2, which has an artificial tooth and a wax rim, in a gypsum composition 6 for secondary investment, which is placed in the lower container 5a of the polymerization flask shown in Figure 2A. [Figure 3] This is a schematic perspective view of an example of a polymerization flask used in the mold-making process. The interior of the flask must be in the state shown in Figure 1 during the mold-making process. That is, when the upper lid component 5b is placed on top of the lower container 5a of the polymerization flask, the inside of the polymerization flask 10 must be filled with the gypsum composition without any voids. [Figure 4] This is a schematic perspective view illustrating how a slurry-like mixture 1, which is made by adding water to the mold-making gypsum composition of the present invention, is poured into the voids in the polymerization flask that have been formed in the state shown in Figure 3. [Modes for carrying out the invention]

[0012] The present invention will be described in more detail below with reference to preferred embodiments. First, the present inventors will explain the process of creating a mold necessary for creating artificial gums, which is a problem that impairs the work efficiency of dental technicians in the series of complicated denture manufacturing processes. For the explanation, the case of creating a complete denture (hereinafter referred to as "denture"), commonly called a "complete denture," will be used as a representative example. The outline of the process of creating resin artificial gums (gums) that fix and support the artificial teeth in the series of complicated "denture" manufacturing processes is as follows.

[0013] First, a wax denture with artificial teeth and a wax rim is fabricated as described below. Next, a plaster mold is made using the wax denture to fabricate artificial gums (gingiva) made of resin. The plaster composition for the mold of the present invention is used in the areas where plaster is not filled in the polymerization flask when fabricating this mold, and as a result, it effectively contributes to improving the workability of the denture fabrication process. The following explanation is an example using a polymerization flask 10 that is divided into upper and lower parts, as shown in Figures 1 to 3. That is, as shown in the figures, a polymerization flask consisting of a lower container member 5a and an upper lid member 5b (sometimes called a lid 5b or simply a lid) is used.

[0014] First, a wax rim 2a is placed on the working model 3, and the artificial tooth 2b is positioned on the wax rim at a location that will provide a good bite for the denture wearer (see Figure 1). The wax rim is formed from a material such as wax that can be melted in a microwave oven or hot water. Along with this process, with the artificial tooth 2b fixed to the wax rim 2a, the surface of the wax rim 2a, which serves as the base for supporting the artificial tooth 2b, is sculpted to realistically reproduce the gum area. In this way, a "wax denture" 2 having an artificial tooth 2b and a wax rim 2a is fabricated.

[0015] Next, as shown in Figure 2A, the "wax denture with artificial teeth and wax rim" 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 the gypsum composition 4 for primary investment. This operation is called "primary investment".

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

[0017] Next, cover the upper part of the polymerization flask with the upper lid 5b, then pour the slurry-like kneaded mixture of the mold gypsum composition of the present invention and water into 10 inside the polymerization flask, and fill the voids generated inside, whereby the "wax denture having artificial teeth and a wax rim" 2 is embedded in the gypsum composition. This operation is referred to as "third embedding". Hereinafter, the "wax denture" 2 having "artificial teeth 2b and a wax rim 2a" may be simply referred to as "wax denture 2".

[0018] Through the series of embedding steps described above, a mold for forming a resin artificial gingiva (gum) portion, in which the artificial teeth 2b are supported and fixed with their occlusion adjusted, can be obtained. As will be described later, in the present invention, as described above, by using the mold gypsum composition of the present invention, in which the slurry-like kneaded mixture added with water exhibits good fluidity, in the third embedding operation for obtaining this mold for artificial gingiva formation, it is achieved that a resin artificial gingiva (gum) in a good condition can be obtained with good workability. The procedure for producing the artificial gingiva (gum) portion of the wax denture 2 from resin using the mold (gypsum mold) for artificial gingiva formation is as follows.

[0019] First, after sufficiently curing the gypsum filled in the polymerization flask including the slurry-like kneaded mixture used for the third embedding, the wax (wax rim 2a) constituting the artificial gingiva (gum) portion of the wax denture 2 is sufficiently dissolved out using a microwave oven or boiling water, thereby forming a mold (gypsum mold) in which the wax portion is melted to form a cavity. In the obtained mold, the artificial teeth 2b, which were held at predetermined positions by the wax before melting, are held at the predetermined positions by the cured gypsum. In addition, the shape of the sculpted gingiva (gum) is transferred to the inner surface of the cavity left after the wax is removed, which is formed by the outflow of the melted wax.

[0020] Next, resin for forming artificial gums is filled into the cavity of this plaster mold, and the resin is allowed to harden. Finally, the mold is broken open, and the resin artificial gums and the artificial teeth, which are firmly fixed and supported by the artificial gums, are removed. After that, the artificial teeth and artificial gum area are carefully polished. This results in a denture in which the artificial teeth are properly positioned on the resin artificial gums according to the adjusted bite.

[0021] The inventors of this invention have diligently studied how to improve the workability in the above series of processes and have found that by improving the mold gypsum composition and using it in the following operations where the following problems occur, improved workability can be achieved. In the process of filling the mold into a cavity formed by the melting of the wax with resin, in order to achieve a dense resin filling, the resin is usually injected under pressure into the cavity left after the wax has been removed. As a result, cracks and fissures may occur in the hardened gypsum composition used in secondary investment, and these parts become burrs, which may impair the smoothness of the surface of the formed resin gingiva (gums). Furthermore, in this case, the conventional gypsum composition used in tertiary investment has poor fluidity when mixed with water, and in order to achieve the strength of the hardened gypsum composition, it was generally necessary to carefully fill the polymerization flask with the gypsum composition for tertiary investment by vibrating it with a vibrator, so as not to create any voids.

[0022] In contrast to the above, when the mold gypsum composition of the present invention is used for tertiary investment, as shown in Figures 1 and 2A, the "wax denture" 2 having an artificial tooth 2b and a wax rim 2a is quickly embedded in the gypsum composition 6 used for secondary investment and the gypsum composition 1 used for tertiary investment, which is made by pouring the mold gypsum composition of the present invention in slurry form. According to the inventors' studies, as a result, the hardened body of the mold gypsum composition of the present invention used for tertiary investment works in cooperation with the hardened body of the gypsum composition used for secondary investment to exhibit a remarkable effect: sufficient strength is achieved to suppress cracks and fractures caused by the pressure during resin injection.

[0023] The polymerization flask 10 used in the above process can come in various shapes, and is not limited to the two-part structure exemplified earlier; for example, it can also have a three-part structure. However, regardless of the structure, as mentioned above, it is necessary to remove the hardened plaster from the polymerization flask filled with plaster, and then break the plaster to remove the denture. Therefore, as schematically shown in Figures 1 to 3, it has a structure that separates into upper and lower halves. Then, as shown in Figures 2A and 2B, a high-viscosity secondary investment plaster composition 6 is placed on top of the "wax denture 2 having artificial teeth and artificial gums (gums)" which is fixed by the primary investment plaster composition 4 and placed in the lower container 5a that serves as the polymer flask, so as to cover the "wax denture 2 having artificial teeth 2b and wax rim 2a" 2 from the upper part of the divided and open container 5a.

[0024] Next, the upper member 5b, which functions as a lid for the lower container 5a of the polymerization flask 10, is placed on top of the lower container 5a and integrated as shown in Figure 3, after which the gypsum composition is hardened. Here, after the secondary embedding, when the upper member 5b, which acts as a lid, is placed on top of the container 5a of the polymerization flask and integrated, unfilled voids are formed inside the polymerization flask 10, although they differ in size, because the gypsum has not been filled into them after integration. Therefore, it becomes necessary to perform a tertiary embedding operation to fill these voids with the gypsum composition. Specifically, for example, as shown in Figure 3, the tertiary embedding operation is performed by filling the gypsum composition through holes 7 provided on the side of the polymerization flask 10, which lead to the unfilled voids inside the polymerization flask that have not been filled with gypsum. In this process, conventional techniques involve gradually and carefully filling the polymerization flask with a high-viscosity gypsum composition, prepared for tertiary embedding, using a vibrator or the like over time to ensure that no unfilled voids remain.

[0025] In conventional techniques, the "wax denture" 2 having an artificial tooth 2b and a wax rim 2a is carefully embedded in the gypsum composition over time as described above, ensuring there are no voids. After the gypsum is hardened, the hardened gypsum body is removed from the polymerization flask 10 to create a mold in which the wax denture 2 is embedded in the gypsum composition. The inventors of the present invention recognized that the process of creating the denture mold described above impairs the work efficiency of the dental technician due to the filling process of the gypsum composition into the "voids that are not filled with gypsum and occur inside the polymerization flask" (tertiary embedding), which is unavoidable due to the structure of the polymerization flask.

[0026] To address the technical challenges described above, the development of a gypsum composition with the following characteristics would be extremely useful in practice. Specifically, it is desirable to develop a gypsum composition for mold manufacturing that allows for quick and easy filling of the voids in the polymerization flask during tertiary embedding with the gypsum composition, has excellent fluidity as a slurry-like mixture, and does not produce burrs on the resin portion that forms the artificial gingiva (gums) when the mold (gypsum mold) has a cavity left after the wax dike 2a has been removed.

[0027] The present inventors diligently studied to develop a gypsum composition that exhibits the above-described properties in order to address the technical challenges in the process of creating molds used in the denture manufacturing process described above, and as a result arrived at the present invention. Specifically, the gypsum composition for mold manufacturing of the present invention contains hemihydrate gypsum, and the hemihydrate gypsum is characterized by containing α-type hemihydrate gypsum and β-type hemihydrate gypsum in a ratio of 25:75 to 65:35.

[0028] By adding water to the above-described mold gypsum composition to create a slurry, and performing a tertiary embedding to fill the voids in the polymerization flask with the gypsum composition, the appropriate fluidity allows for quick and easy filling of the voids in the polymerization flask with a dense gypsum composition. This significantly improves the work efficiency when creating molds (gypsum molds) for producing resin artificial gums.

[0029] Furthermore, by filling the polymerization flask with the mold gypsum composition of the present invention during the tertiary investment as described above, the following excellent effects can be obtained. As previously stated, the following operations are performed in the process of creating a mold (gypsum mold) for making artificial gums (gums). After filling the polymerization flask with the primary, secondary, and tertiary gypsum compositions, the gypsum is allowed to harden sufficiently, the wax (wax rim) is melted out to form a cavity and complete the mold (gypsum mold), and the resin that will become the artificial gums (gums) is filled into the cavity. After that, the polymerization flask is disassembled and the mold is removed from the flask, the mold is broken and the model (denture) is removed from the mold. Here, the mold having the model (denture) obtained as described above is in a state in which the mold gypsum composition of the present invention used in the tertiary investment and the high-viscosity gypsum composition that was previously filled into the polymerization flask during the secondary investment for embedding the wax denture, which is performed before the tertiary investment, are integrated without any problems. Therefore, when the resin that will become the artificial gums is filled into the cavity of the mold, the mold gypsum composition of the present invention used for tertiary investment and the gypsum composition used for secondary investment work together to prevent cracks or other defects caused by the filling of the resin that will become the artificial gums. As a result, the resin artificial gum portion that makes up the denture, when the mold is broken and removed, is in good condition without any burrs or other defects caused by cracks. Therefore, the need for post-processing such as burr removal can be reduced. In addition, 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] The forming materials for the mold gypsum composition of the present invention, which provides the excellent effects described above, are described below. First, the mold gypsum composition of the present invention is used for the applications exemplified above and is not used for the manufacture of molds (gypsum molds) for casting metal. For this reason, fire resistance and high heat resistance are not required, and therefore it does not contain refractory materials such as cristobalite or quartz. Furthermore, although this specification uses "dentures" as an example of a model requiring a mold, its use is not limited to "dentures". The mold gypsum composition of the present invention can also be widely used in the mold manufacturing process when making resin models and the like for various other purposes. By using the mold gypsum composition of the present invention, various models with precise shapes and reduced burr generation can be efficiently obtained.

[0031] Furthermore, the gypsum composition for molds of the present invention is provided in powder form and is mixed with water to form a slurry-like mixture before use. In this invention, "powder" means a particle with an average particle diameter of 200 μm or less. In this specification, the average particle diameter is the median diameter (d) in the particle size distribution measurement determined by laser diffraction-scattering. 50 This refers to the particle size at ). The average particle size of the mold gypsum composition in this invention can be measured using a laser diffraction particle size distribution analyzer (Nikkiso Co., Ltd., product name: Microtrac HRA).

[0032] [Gypsum hemihydrate] The mold gypsum composition of the present invention contains hemihydrate gypsum. Specifically, hemihydrate gypsum refers to calcium sulfate half-hydrate (CaSO4·1 / 2H2O). Since hemihydrate gypsum reacts with water to change into dihydrate gypsum, a gypsum slurry prepared by kneading hemihydrate gypsum and water hardens rapidly when poured into a polymerization flask. For this reason, it is widely used in the production of molds. Hemihydrate gypsum is obtained by calcining dihydrate gypsum (calcium sulfate dihydrate). Examples of dihydrate gypsum used as raw materials in this case include natural gypsum, by-product gypsum, and recycled gypsum obtained through a gypsum product recycling process. The hemihydrate gypsum constituting the mold gypsum composition of the present invention may be obtained from any of the above-mentioned raw materials. Here, the amount of hemihydrate gypsum in the mold gypsum composition of the present invention can be, for example, 95 parts by mass or more out of 100 parts by mass of the mold gypsum composition. More preferably, it can be 98 parts by mass or more.

[0033] The mold gypsum composition of the present invention contains the above-mentioned hemihydrate gypsum, and is characterized in that the hemihydrate gypsum is composed of α-type hemihydrate gypsum and β-type hemihydrate gypsum in a ratio of 25:75 to 65:35. A preferred form of the mold gypsum composition of the present invention is one in which the ratio of α-type hemihydrate gypsum to β-type hemihydrate gypsum is 25:75 to 60:40. A more preferred form is one in which the ratio of α-type hemihydrate gypsum to β-type hemihydrate gypsum is 30:70 to 60:40. Furthermore, the average particle size of these hemihydrate gypsums is preferably about 10 to 80 μm, and more preferably about 20 to 50 μm. The α-type hemihydrate gypsum and β-type hemihydrate gypsum, which are essential for the mold gypsum composition of the present invention, will be described below. As will be described later, we believe that the unprecedented effects of the present invention have been obtained by configuring the mold gypsum composition of the present invention as described above.

[0034] (α-type hemihydrate gypsum) α-type hemihydrate gypsum can be obtained by pressurizing and calcining the dihydrate gypsum raw materials mentioned above in water or steam. Compared to β-type hemihydrate gypsum, which will be described later, α-type hemihydrate gypsum can be mixed with a smaller amount of water (i.e., low water content). Therefore, the hardened gypsum obtained using α-type hemihydrate gypsum has greater strength (compressive strength). As mentioned above, the α-type hemihydrate gypsum used in this 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 gypsum) Beta-type hemihydrate gypsum is obtained by calcining dihydrate gypsum, the raw material mentioned above, in the atmosphere. Compared to the α-type hemihydrate gypsum described above, a larger amount of water is required for mixing. Therefore, the hardened gypsum obtained using beta-type hemihydrate gypsum has lower strength (compressive strength). According to the inventors' studies, if the amount of water used is reduced to the same amount as that used for α-type hemihydrate gypsum, the hardened gypsum obtained using beta-type hemihydrate gypsum will have higher strength than the hardened gypsum obtained using α-type hemihydrate gypsum. As mentioned above, the beta-type 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.

[0036] Traditionally, the main raw material used in gypsum compositions for mold making when fabricating dentures has been β-type hemihydrate gypsum. α-type hemihydrate gypsum is not used as a raw material for the following reasons: To obtain dentures embedded in the mold, it is ultimately necessary to break the mold (hardened gypsum body) to remove the dentures. However, if α-type hemihydrate gypsum is used in the gypsum composition for mold making, the hardened gypsum body becomes too strong, making it difficult to break, and there is a concern that the dentures embedded in the mold may be damaged if it is forced to break. Dentures are intricately made, individually tailored to the user's bite and appearance, so damage is unacceptable. For this reason, even if α-type hemihydrate gypsum was used as a raw material in gypsum compositions for molds, it was only in relatively small amounts. And, for the reasons mentioned above, this has become common technical knowledge regarding gypsum compositions for molds (gypsum molds) used in the fabrication of dentures and the like.

[0037] In response to the above-mentioned situation, the inventors recognized that, as previously stated, enabling the quick and easy filling of gypsum composition into voids in the polymerization flask due to its structure, etc., is important for improving the workability in the manufacture of dentures. Therefore, it would be extremely useful to provide a gypsum composition that allows for quick and easy filling of voids in the polymerization flask without using special devices such as press-fitting or vibrators, or even if a vibrator is used, without requiring the careful and time-consuming process currently in place.

[0038] Based on the above understanding, the inventors diligently studied and found that it is possible to provide a mold gypsum composition that can achieve the above objectives by an extremely simple means: the composition of the hemihydrate gypsum raw material constituting the mold gypsum composition contains α-type hemihydrate gypsum and β-type hemihydrate gypsum in a ratio of 25:75 to 65:35, as defined in the present invention. That is, by using α-type hemihydrate gypsum and β-type hemihydrate gypsum within the ratio range defined in the present invention, the fluidity of the gypsum composition can be improved by adding water to the powder material, for example, by mixing it into a slurry with a water content of 45%. This makes it possible to quickly and easily fill the gypsum composition into the voids (spaces) in the polymerization flask that are not filled with gypsum due to the structure of the polymerization flask, etc. Furthermore, since the slurry obtained using the mold gypsum composition of the present invention exhibits good fluidity, the risk of air bubbles being mixed in during kneading is extremely low, and an antifoaming agent is not required. Alternatively, even if an antifoaming agent is added, the amount required can be significantly reduced. Therefore, the gypsum composition for molds of the present invention also has another advantage: it is industrially useful in terms of raw material costs.

[0039] In addition to the above, the mixture of the mold-making gypsum composition of the present invention and water, as described above, is useful even when mixed with a small amount of water, possessing the following properties. The mixture has low viscosity and excellent fluidity, and although it uses a high proportion of β-type hemihydrate gypsum, which results in a hardened gypsum body with low strength (compressive strength), the mold obtained using this mold-making gypsum composition has sufficient compressive strength so that it does not crack when resin is injected into its cavity to form the artificial gums (gums) of a resin denture. Furthermore, after the resin hardens, the mold can be broken without any problems, and the denture (model) inside can be removed.

[0040] Furthermore, the mold gypsum composition of the present invention integrates well with the high-viscosity gypsum composition used for secondary investment, preventing cracks and other defects from forming in the hardened gypsum composition used for secondary investment. As a result, the resin artificial gingiva (gums) obtained by supplying resin for forming the gum portion into the resulting mold cavity and allowing it to harden is in a good condition without burrs. This makes secondary processing after the resin hardens easier and is economically advantageous.

[0041] Furthermore, according to the inventors' studies, when the mold gypsum composition of the present invention contains a water-reducing agent (dispersant), the amount of the water-reducing agent added can be set to 0.02 to 0.2 parts by mass per 100 parts by mass of hemihydrate gypsum of the above-described configuration. In other words, with the mold gypsum composition of the present invention, the effect of adding a water-reducing agent can be obtained even with a relatively small amount of water-reducing agent (dispersant) of 0.2 parts by mass or less. For this reason, the mold gypsum composition of the present invention can reduce raw material costs and is also economically superior.

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

[0043] The mold gypsum composition of the present invention may contain conventionally known additives, provided that they do not impair the intended purpose. Specifically, curing modifiers such as curing accelerators or curing retarders can be added as appropriate. Examples of curing accelerators include gypsum dihydrate and potassium sulfate, and other common curing accelerators can be used. Examples of curing retarders include sodium citrate and other common curing retarders. [Examples]

[0044] Next, the present invention will be described in more detail with reference to examples and comparative examples. Hereinafter, parts or % refer to mass unless otherwise specified.

[0045] <Examples 1-5 and Comparative Examples 1 and 2> The mold gypsum compositions for Examples 1-5 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 hardening accelerators, 0.025 parts of sodium citrate as a hardening retarder, and 0.025 parts of potassium tartrate as an expansion inhibitor were added under the same conditions to 100 parts of the mold gypsum composition. Furthermore, in order to ensure good fluidity when pouring the prepared composition into the polymerization flask for tertiary investment in the fabrication of dentures, the composition was adjusted to achieve the following flow values. Specifically, when water was added to the powdered gypsum composition with the above formulation and mixed to form a slurry with a water content of 45%, 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 Evaluation Results] For each of the powder gypsum mold compositions in the examples and comparative examples shown in Table 1, evaluations were conducted using a slurry-like mixture prepared by adding water to a ratio of 45% or a hardened gypsum body obtained by curing the mixture, as described below. The evaluation results are shown in Table 1.

[0047] (Pot life) Water (23±2℃) was measured into a dental rubber bowl to the specified mixing volume, and the powdered mold-making gypsum composition of the test example and comparative example was added to it over 10 seconds. After standing for 20 seconds, the mixture was kneaded with a spatula for 60 seconds to obtain the respective mixtures. The stirring speed during kneading was 120 rpm in all cases. Hereinafter, the slurry-like mixture obtained as described above will be referred to as "gypsum slurry".

[0048] After mixing was complete, the process of pouring small amounts of gypsum slurry was repeated every 30 seconds. The pot life, or "working time," was defined as the point 30 seconds before the viscosity of the gypsum slurry increased and it stopped flowing continuously (it began to fall in clumps). In order to secure the working time that is the objective of this invention, a working time of 8 minutes or more must be ensured.

[0049] (Curing time) The curing time of the mold gypsum compositions in the examples and comparative examples was measured in accordance with JIS T6604 Dental calcined gypsum - 5.5 curing time test. Specifically, the measurement was performed using the following procedure.

[0050] Place the ring-shaped device for measuring curing time in the center of the glass plate. Obtain a gypsum slurry using the same method as when measuring the "pot life" as explained earlier, and pour the gypsum slurry into the ring mold for measuring the hardening time until it slightly rises above the top surface of the ring mold. Align the top edge of the ring-shaped mold used for measuring curing time with the plaster slurry and use a spatula to smooth it out. Using a Vicar needle device (1mm needle, 300g), the needle is gently dropped by its own weight from the surface of the plaster slurry one or two minutes before the expected curing time. Move the mold, along with the glass plate, to a new section of the ring-shaped curing time measuring ring that is at least 5 mm away from the walls and other needle marks, so that the next needle can be gently dropped from the sample surface by its own weight. • After wiping the needle clean, touch the tip of the needle to the surface of the plaster 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 mold gypsum composition comes into contact with water until the penetration depth reaches 2 mm or less is defined as the "curing time". In this invention, since a slow curing time would prevent the next step from being completed, the target curing time was set to within 30 minutes.

[0051] (Water floating during mixing) Regarding "water separation during mixing," the test procedure is similar to the measurement of the hardening time performed on the mold gypsum composition described above, as shown below, and was therefore confirmed when measuring the "hardening time" mentioned earlier. Place the ring-shaped device for measuring curing time in the center of the glass plate. Mix the gypsum mold compositions of the examples and comparative examples to be measured, and pour the gypsum slurry into the ring mold for measuring the hardening time until it slightly rises above the top surface. Align the top edge of the ring-shaped mold used for measuring curing time with the plaster slurry and use a spatula to smooth it out. The Vicker needle device was evaluated by visually observing whether water would surface on the top of the ring-shaped mold before the needle insertion depth reached 1 mm from the bottom of the ring-shaped mold.

[0052] (Flow measurement) The flow values ​​for the mold gypsum compositions of the examples and comparative examples were measured in accordance with JIS T6604 Dental calcined gypsum - 5.4 Flowability test. Specifically, the measurements were performed using the following procedure.

[0053] Place a glass plate on a surface that does not vibrate, and place the ring-shaped object in the center of this glass plate. Pour the plaster slurry into the ring mold until it reaches the top surface of the ring. • A cylindrical ring mold, 50 mm in height and 35 mm in inner diameter, is lifted vertically from the glass plate at a speed of approximately 10 mm / s 2 minutes and 15 seconds after the start of mixing. This causes the gypsum slurry that had been poured into the ring mold to spread onto the glass plate. One minute after the ring-shaped mold is removed, the maximum and minimum diameters of the gypsum slurry bottom are measured in millimeters, and the arithmetic mean of these measurements is taken as the flow value.

[0054] (Compression strength) The compressive strength of the hardened gypsum bodies obtained using the gypsum mold composition to be measured was determined in accordance with JIS T6604 Dental calcined gypsum - 5.8 Compressive strength test. Specifically, the measurement was performed using the following procedure. • For measuring compressive strength, a cylindrical mold with a height of 40 mm and an inner diameter of 20 mm, consisting of five sections, is used. Place the mold for compression strength, coated with a release agent, in the center of the glass plate. Mix the gypsum composition for the mold to be measured, and pour the gypsum slurry into the glass plate until it slightly rises above the edge of the mold used for compressive strength testing. To minimize the incorporation of air bubbles, gently vibrate the mold while pouring the slurry. Before the mixture hardens and loses its gloss, press the glass plate coated with a release agent onto the top surface of the mold. 45 minutes after mixing begins, remove the hardened gypsum sample from the mold for compressive strength measurement. Sixty minutes after mixing begins, five samples of hardened gypsum for compressive strength measurement are subjected to fracture testing using a compressive strength testing machine, and the maximum applied force (F) is recorded. The compressive strength (S) was then calculated using the following formula. S=F / 314 In this invention, the target compressive strength of the hardened gypsum composition is "10 MPa or more" to prevent the hardened gypsum composition used in secondary embedding from cracking when resin is injected into a mold obtained using a mold-making gypsum composition to form the artificial gums (gingiva) of a resin denture.

[0055] TIFF0007917168000001.tif127170

[0056] As shown in Table 1, when the gypsum composition of Comparative Example 1, which contained 20 parts α-type hemihydrate gypsum, was used, the hardening time of the gypsum took 42 minutes or more, whereas when the gypsum composition of the example was used, it was confirmed that the hardening time could be shortened to about 20 minutes. Furthermore, as shown in Comparative Example 2, when the amount of α-type hemihydrate gypsum was 70 parts, water separation occurred during the hardening time measurement, confirming that it is not suitable for the application of producing hardened gypsum. Note that since water separation occurred in the gypsum composition of Comparative Example 2, evaluations other than water separation during mixing were not performed. [Explanation of Symbols]

[0057] 1: The present invention provides a gypsum composition for mold manufacturing (for tertiary embedding). 2: Wax dentures with artificial teeth and wax rims 2a: Wax Embankment 2b: Artificial teeth 3: Working model 4: Gypsum composition for primary investment 5a: Lower container of polymerization flask 5b: Top lid of polymerization flask 6: Gypsum composition for secondary investment 7: Holes for filling with gypsum composition for tertiary implantation. 10: Polymerization flask

Claims

1. A mold-making gypsum composition used in the process of making a mold for forming a resin model, which is a slurry-like mixture of water added to a gypsum composition for secondary embedding that has been applied for the purpose of obtaining a mold for forming a resin part, and which is used in the operation of tertiary embedding, A gypsum composition for mold making, characterized in that it contains hemihydrate gypsum, wherein the hemihydrate gypsum comprises α-type hemihydrate gypsum and β-type hemihydrate gypsum in a ratio of 25:75 to 65:35, and further, water is added to the composition to a water content of 45% and mixed to form a slurry-like mixture which has a pot life of 8 minutes or more and a hardening time of 30 minutes or less.

2. The gypsum composition for mold making 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 mold making 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 hemihydrate gypsum is the gypsum composition for mold making 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 mold making.

5. Furthermore, the gypsum composition for mold making according to any one of claims 1 to 3, wherein 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. A method for manufacturing a denture in which an artificial tooth and a resin artificial gum are integrally formed, comprising a mold manufacturing step in which a primary embedding gypsum composition is filled into a polymerization flask, a wax denture having an artificial tooth and a wax rim is placed in the filled primary embedding gypsum composition, a secondary embedding gypsum composition is applied to the placed artificial tooth and wax rim and secondary embedding is performed, and then a tertiary embedding operation is performed in which a slurry-like mixture of a tertiary embedding gypsum composition, obtained by adding water and mixing it, is filled into the voids in the polymerization flask that are not filled with the gypsum composition, wherein the tertiary embedding gypsum composition is the mold manufacturing gypsum composition described in any one of Claims 1 to 3.

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