Coated sand, method for producing coated sand, kit for making casting mold, and method for producing casting mold

Abstract

This coated sand is obtained by depositing a calcium salt on the surface of a refractory granular material by means of water. This method for producing coated sand comprises mixing a refractory granular material, water, and a calcium salt. This kit for making a casting mold contains each of the coated sand and water glass independently. This method for producing a casting mold comprises filling a mixture containing the coated sand and water glass into a form for producing a casting mold and then curing the water glass.

Description

Coated sand, method for producing coated sand, mold making kit, and method for producing mold

[0001] The present invention relates to coated sand, a method for manufacturing coated sand, a mold-making kit, and a method for manufacturing a mold. This application claims priority to Japanese Patent Application No. 2023-148903, filed on September 14, 2023, the contents of which are incorporated herein by reference.

[0002] One type of mold-making process is the water glass process. Various types of this water glass process are known. One example of a method for manufacturing a mold using the water glass process is to add water glass, a binder, to a refractory granular material such as silica sand and knead it to form mixed sand, fill the mixed sand into a mold for mold production, and heat the water glass to harden the sand to bind and solidify it, thereby manufacturing a mold (thermosetting mold). In the case of a thermosetting mold, when the water glass is dehydrated by heating, the silanol groups of sand such as silica sand react with the water glass, and a condensation reaction proceeds, forming a mesh-like hardened network.

[0003] However, in the case of thermosetting molds, the hardened network is reversible, so thermosetting molds are prone to strength loss due to moisture absorption. Furthermore, the presence of sodium from water glass in the mold can reduce heat resistance (reduced hot strength). Because molds are exposed to high temperatures when molten metals such as iron, copper, and aluminum are poured into them, poor heat resistance can result in casting defects such as seizure and glare, deformation, or mold wall movement, resulting in reduced dimensional accuracy of the casting. Therefore, molds are required to have sufficient heat resistance to maintain sufficient strength even when exposed to high temperatures.

[0004] Therefore, so-called dicalcium silicate-based molds have been proposed, which use calcium salts as hardeners and promote hardening through a substitution reaction between sodium ions in water glass and calcium ions. For example, Patent Document 1 discloses a self-hardening molding sand for dicalcium silicate molds that contains 100 parts by mass of silica sand, 0.5 to 2 parts by mass of dicalcium silicate, 6 parts by mass of water glass, and 5 to 20 parts by mass of porous siliceous particles.

[0005] Japanese Unexamined Patent Publication No. 50-75117

[0006] However, with conventional die-casting molds, dust from calcium salts and dust from crushed silica sand are likely to be generated during mold production and dismantling, which can easily deteriorate the working environment.The present invention aims to provide coated sand, a method for producing coated sand, a mold-making kit, and a mold-making method that can produce molds that are excellent in moisture resistance and heat resistance while maintaining strength, and that are less likely to generate dust during mold production and dismantling.

[0007] The present invention has the following aspects. [1] Coated sand, in which a calcium salt is adhered to the surface of a refractory granular material via water. [1-1] The coated sand of [1] above, in which the refractory granular material is foundry sand having a refractoriness of SK-32 or higher as measured in accordance with JIS R 2204. [2] The coated sand of [1] or [1-1] above, in which the refractory granular material is artificial sand obtained by a fusion method. [3] The coated sand of [2] above, in which the ratio of the water is 0.01 to 0.15 parts by mass per 100 parts by mass of the artificial sand. [4] A method for producing coated sand, in which a refractory granular material, water, and a calcium salt are mixed. [4-1] A method for producing coated sand of [4] above, in which the refractory granular material is foundry sand having a refractoriness of SK-32 or higher as measured in accordance with JIS R 2204. [5] A method for producing coated sand according to [4] or [4-1] above, wherein the refractory granular material is artificial sand obtained by a fusion method. [6] A method for producing coated sand according to [5] above, wherein the ratio of water is 0.01 to 0.15 parts by mass per 100 parts by mass of the artificial sand. [7] A foundry mold-making kit independently comprising the coated sand according to any one of [1] to [3] above and water glass. [8] A method for producing a foundry mold, comprising filling a mold for producing a foundry mold with a mixture containing the coated sand according to any one of [1] to [3] above and water glass, and hardening the water glass. [9] A method for producing a foundry mold according to [8] above, wherein the mixture is filled into the mold, heated, and hardened by heat curing.

[0008] According to the present invention, it is possible to provide coated sand that can produce molds that have excellent moisture resistance and heat resistance while maintaining strength, and that are less likely to generate dust during mold production or mold dismantling, a method for producing coated sand, a mold-making kit, and a method for producing a mold.

[0009] The following describes in detail the embodiments for carrying out the present invention, but the present invention is not limited to the embodiments described below, and various modifications are possible without departing from the gist of the present invention. In this specification and claims, a numerical range expressed as "to" means a numerical range including the numbers before and after "to" as the lower and upper limits. In the following specification, a "mold" refers to a mold formed using the coated sand or mold-making kit of the present invention. Furthermore, "strength" refers to the strength of the mold at room temperature.

[0010] [Coated Sand] One embodiment of the coated sand of the present invention will be described below. The coated sand of this embodiment is a refractory granular material having a calcium salt attached to the surface thereof via water. At least a portion of the surface of the refractory granular material is coated with the calcium salt via water. Note that components other than calcium salt may be attached to the surface of the refractory granular material as needed, as long as the effects of the present invention are not impaired.

[0011] <Refractory Granular Material> Refractory granular materials can be widely used as foundry sands, including conventionally known natural sands such as silica sand, chromite sand, zircon sand, olivine sand, amorphous silica, alumina sand, and mullite sand, as well as artificial sand. Used refractory granular materials recovered (reclaimed sand) or reclaimed (recycled sand) can also be used. These refractory granular materials may be used alone or in combination of two or more. Furthermore, the refractory granular material is preferably foundry sand having a refractoriness of SK-32 or higher as measured in accordance with JIS R 2204, "Test Method for Refractoriness of Refractories and Refractory Raw Materials." The refractoriness may be SK-34 or higher, SK-35 or higher, SK-36 or higher, or SK-38 or higher.

[0012] As a refractory granular material, artificial sand is preferred because it is less likely to break during mold dismantling, can further reduce dust generation, and can increase recovery rates. Examples of artificial sand include artificial sand obtained by a melting method (melting artificial sand), artificial sand obtained by a flame fusion method (flame fusion artificial sand), and artificial sand obtained by a sintering method (sintered artificial sand). Among these, melting artificial sand and flame fusion artificial sand are preferred because of their excellent compatibility with water glass, as described below. Of these, melting artificial sand is more preferred because of its ability to reduce production costs. The specific conditions for the melting artificial method, flame fusion artificial method, and sintering artificial method are not particularly limited. Artificial sand may be produced using known conditions, such as those described in JP-A-5-169184, JP-A-2003-251434, and JP-A-2004-202577. As an example, artificial sand obtained by a melting method can be obtained by using starting materials containing, for example, aluminum oxide and silica, melting them with heat, and granulating them. The artificial sand preferably has a sphericity of 0.8 or more. The sphericity can be determined, for example, by the method described in JP 2009-119469 A.

[0013] The average particle size of the refractory granular material is preferably 50 to 600 μm, more preferably 75 to 500 μm, and even more preferably 150 to 300 μm. In another aspect of the present invention, the average particle size of the refractory granular material is preferably 50 to 350 μm, more preferably 75 to 250 μm, and even more preferably 100 to 200 μm. When the average particle size of the refractory granular material is equal to or greater than the above-mentioned lower limit, a mold having higher strength and superior moisture resistance can be obtained. When the average particle size of the refractory granular material is equal to or less than the above-mentioned upper limit, the surface quality of the castings cast using the mold is excellent. The average particle size of the refractory granular material is the median diameter of 50% of the cumulative volume of the refractory granular material measured by dynamic light scattering. Furthermore, the average particle size of the coating sand is substantially the same as the average particle size of the refractory granular material.

[0014] <Calcium Salt> Calcium salts play a role as hardeners for water glass, which will be described later. Examples of calcium salts include dicalcium silicate (Ca ２ SiO ４), tricalcium silicate (Ca ３ SiO ５ ), dicalcium silicate (2CaO-SiO ２ ), tricalcium silicate (3CaO-SiO ２ ), 3CaO-2SiO ２ , Ca ３ Si ２ O ７ , CaO—SiO ２ , CaSiO ３ Examples of the calcium salt include calcium silicates such as calcium carbonate, calcium hydroxide, and calcium silicate. Among these, calcium silicate and calcium carbonate are preferred, and dicalcium silicate, dicalcium silicate, and calcium carbonate are more preferred. These calcium salts may be used alone or in combination of two or more.

[0015] When artificial sand such as fusion-process artificial sand, flame-fusion-process artificial sand, or sintered-process artificial sand is used as the refractory granular material, the proportion of calcium salt in the coated sand is preferably 0.05 to 0.7 parts by mass, more preferably 0.1 to 0.6 parts by mass, and even more preferably 0.2 to 0.5 parts by mass, per 100 parts by mass of the artificial sand. When the proportion of calcium salt is equal to or greater than the lower limit, the moisture resistance and heat resistance of the mold are further improved. When the proportion of calcium salt is equal to or less than the upper limit, the generation of calcium salt dust can be further suppressed when manufacturing a mold using the coated sand of this embodiment or when dismantling the resulting mold after use. Additionally, the strength of the mold is further enhanced. Furthermore, the amount of water required to adhere the calcium salt to the surface of the refractory granular material is less likely to be excessive, thereby maintaining good fluidity of the coated sand.

[0016] When natural sand such as silica sand is used as the refractory granular material, the proportion of calcium salt in the coated sand is preferably 0.2 to 2.8 parts by mass, more preferably 0.4 to 2.4 parts by mass, and even more preferably 0.8 to 2 parts by mass, per 100 parts by mass of natural sand. When the proportion of calcium salt is equal to or greater than the lower limit, the moisture resistance and heat resistance of the mold are further improved. When the proportion of calcium salt is equal to or less than the upper limit, the generation of calcium salt dust can be further suppressed when manufacturing a mold using the coated sand of this embodiment or when dismantling the resulting mold after use. In addition, the strength of the mold is further increased. Furthermore, the amount of water required to adhere the calcium salt to the surface of the refractory granular material is less likely to be excessive, and the fluidity of the coated sand can be maintained well.

[0017] <Water> Water acts as an adhesive to bond the refractory granular material and the calcium salt, and the presence of water allows the calcium salt to remain on the surface of the refractory granular material.

[0018] When artificial sand such as fused sand, flame-fused sand, or sintered sand is used as the refractory granular material, the proportion of water in the coated sand is preferably 0.01 to 0.15 parts by mass, more preferably 0.03 to 0.13 parts by mass, and even more preferably 0.05 to 0.12 parts by mass, per 100 parts by mass of the artificial sand. When the proportion of water is equal to or greater than the lower limit, calcium salts adhere sufficiently to the surface of the refractory granular material, further suppressing dust generation. When the proportion of water is equal to or less than the upper limit, the artificial sand is less likely to agglomerate, and the fluidity of the coated sand can be maintained at a good level. Typically, the coated sand is stored in a tank such as a hopper, and is supplied from the hopper to a mixer where it is mixed with the water glass described below. Good fluidity of the coated sand facilitates its supply from the hopper to the mixer. From the viewpoint of more reliably obtaining the effects of the present invention, when artificial sand such as fused sand, flame-fused sand, or sintered sand is used as the refractory granular material, the mass ratio of calcium salt to water (calcium salt / water) is preferably 0.3 to 70, more preferably 0.76 to 20, and even more preferably 1.67 to 10.

[0019] When natural sand such as silica sand is used as the refractory granular material, the proportion of water in the coated sand is preferably 0.04 to 0.6 parts by mass, more preferably 0.12 to 0.52 parts by mass, and even more preferably 0.2 to 0.48 parts by mass per 100 parts by mass of natural sand. When the water proportion is equal to or greater than the lower limit, calcium salt adheres sufficiently to the surface of the refractory granular material, thereby further suppressing dust generation. When the water proportion is equal to or less than the upper limit, the natural sand is less likely to aggregate, and the fluidity of the coated sand can be maintained. To ensure the effects of the present invention, when natural sand such as silica sand is used as the refractory granular material, the mass ratio of calcium salt to water (calcium salt / water) is preferably 0.3 to 70, more preferably 0.76 to 20, and even more preferably 1.67 to 10. In the present invention, the proportion of water in the coated sand can be measured by known methods, for example, as follows. 20 g of coated sand is placed in a crucible and dried at 105°C for 60 minutes. Thereafter, the dried coated sand is allowed to cool for 10 minutes and then allowed to cool in a desiccator for 30 minutes, and the moisture content (proportion of water in the coated sand) is calculated from the change in weight.

[0020] <Method for Producing Coated Sand> The coated sand is obtained by mixing the above-described refractory granular material, water, and a calcium salt. By mixing the refractory granular material, water, and calcium salt, the calcium salt adheres to the surface of the refractory granular material via the water. When mixing these components, additional components other than the refractory granular material, water, and calcium salt may be added as needed, as long as the effects of the present invention are not impaired. If the refractory granular material and / or calcium salt already contain a sufficient amount of water, additional water may not be added. In this case, the moisture content of the refractory granular material and / or calcium salt is appropriately adjusted before mixing the refractory granular material and the calcium salt so that the coated sand contains a predetermined amount of water. The mixing method is not particularly limited. The mixing temperature is not limited, but a temperature of 10 to 40°C is preferred.

[0021] <Effects> The coated sand of this embodiment described above has calcium salts attached to the surface of the refractory granular material via water, so calcium salt dust is less likely to be generated when manufacturing a mold using the coated sand of this embodiment or when dismantling the resulting mold after use.In addition, the refractory granular material is less likely to shatter when dismantling the mold, resulting in a good working environment.

[0022] Furthermore, since the calcium salt attached to the surface of the refractory granular material acts as a hardener for the water glass, the use of the coated sand of this embodiment allows for the production of molds with superior moisture resistance compared to conventional thermosetting molds. Additionally, since the water glass hardens through a substitution reaction between the sodium ions in the water glass and calcium ions, a mold with excellent heat resistance, i.e., high hot strength, can be obtained. Therefore, molds obtained using the coated sand of this embodiment can suppress casting defects, deformation, and mold wall movement, even when pouring molten metals with high melting temperatures, such as iron and copper, and can produce castings with high dimensional accuracy. Thus, the coated sand of this embodiment is also suitable for producing molds that require high precision, such as complex-shaped molds for cores.

[0023] [Mold-making kit] One embodiment of the mold-making kit of the present invention will now be described. The mold-making kit of this embodiment contains the coated sand of the present invention and water glass, each of which is independent of the other. Here, "independently contained" means that the components are present in a state where they are not mixed or in contact with each other (i.e., the components are isolated from each other). The coated sand and water glass are first mixed and brought into contact when the mold-making kit is used. The mold-making kit may be, for example, a collection of containers in which each component is separately stored.

[0024] The water glass contained in the mold making kit is not particularly limited, and conventionally known water glass can be used. For example, sodium silicate (specifically, No. 1, No. 2, No. 3 and sodium metasilicate (Type 1 and Type 2) as described in JIS K 1408:1966), potassium silicate, and mixtures thereof can be used. In addition, the SiO ２ and M (M = K ２ O or Na ２O) molar ratio (SiO ２ / M) is preferably 1.6 to 4.0, more preferably 2.0 to 3.5, even more preferably 2.0 to 3.2, particularly preferably 2.0 or more but less than 3.1, and most preferably 2.1 to 3.0. When the molar ratio of water glass is equal to or greater than the lower limit, a sufficient curing rate can be obtained. When the molar ratio of water glass is equal to or less than the upper limit, a mold with excellent storage stability can be obtained.

[0025] The Baume degree of the water glass at 20°C is preferably 30 to 60, more preferably 40 to 50. When the Baume degree of the water glass is equal to or higher than the lower limit, the mold strength is improved. When the Baume degree of the water glass is equal to or lower than the upper limit, a good mixed sand can be prepared.

[0026] [Method for Manufacturing a Casting Mold] One embodiment of the method for manufacturing a casting mold of the present invention will be described below. In the method for manufacturing a casting mold of this embodiment, a mixture containing the coated sand of the present invention and water glass (hereinafter also referred to as "mixture (M)" or "mixed sand (M)") is filled into a mold for mold production (hereinafter also referred to as a "casting mold"), and the water glass in the mixture (M) is hardened to produce a casting mold. The casting mold may also be manufactured using the casting mold-making kit of the present invention described above. In this case, the coated sand and water glass constituting the casting mold-making kit are mixed to prepare the mixture (M), and the resulting mixture (M) is filled into a casting mold. The mixture (M) may contain components other than the coated sand and water glass of the present invention as needed, as long as the effects of the present invention are not impaired.

[0027] The proportion of water glass is preferably 1 part by mass or more but less than 6 parts by mass, more preferably 1.2 to 3 parts by mass, and even more preferably 1.5 to 2 parts by mass, per 100 parts by mass of the refractory granular material constituting the coating sand. If the proportion of water glass is equal to or greater than the above-mentioned lower limit, a mold of sufficient strength can be obtained. The higher the proportion of water glass, the greater the proportion of calcium salt that must be adhered to the surface of the refractory granular material in the coating sand, which makes it more likely that dust will be generated during mold production and dismantling. If the proportion of water glass is equal to or less than the above-mentioned upper limit, there is no need to adhere more calcium salt than necessary to the surface of the refractory granular material, and dust generation can be further suppressed.

[0028] Furthermore, as the proportion of water glass decreases, the strength of the mold tends to decrease. However, when the proportion of water glass is low, for example, when the proportion of water glass is equal to or less than the above upper limit, it is preferable to harden the water glass by thermal hardening. Specifically, it is preferable to heat the mixture (M) filled into a mold for casting. Heating promotes dehydration of the water glass, which promotes reaction and condensation reaction with the silanol groups of the refractory granular material, making it easier to form a hardened network, and further improving the strength of the mold.

[0029] The heating temperature (curing temperature) is preferably 100 to 300°C, more preferably 120 to 250°C, and even more preferably 130 to 200°C. If the heating temperature is equal to or higher than the lower limit, the water glass will be sufficiently cured in a short time, and a mold with sufficient strength will be obtained. If the heating temperature is equal to or lower than the upper limit, sufficient strength for removal will be obtained. The heating time (curing time) is not particularly limited and may be determined appropriately depending on the shape and size of the mold; for example, it is preferably 1 to 120 minutes, more preferably 3 to 90 minutes, and even more preferably 5 to 60 minutes.

[0030] The mold for casting may be heated in advance. The temperature of the mold for casting is preferably 100 to 300° C., more preferably 120 to 250° C., and even more preferably 130 to 200° C. Furthermore, the mixture (M) may be filled into the heated mold for casting and then further heated.

[0031] According to the mold manufacturing method of this embodiment described above, coated sand with calcium salt attached to the surface of the refractory granular material via water is used, so calcium salt dust is less likely to be generated when kneading with water glass or when filling the mold for molding. Furthermore, calcium salt dust is less likely to be generated or the refractory granular material is less likely to be crushed when the mold is dismantled, resulting in a good working environment. In particular, if the water glass is hardened by thermal curing, a mold of sufficient strength can be obtained even if the proportion of water glass is reduced. Furthermore, by reducing the proportion of water glass, the proportion of calcium salt attached to the surface of the refractory granular material can be reduced accordingly, further suppressing dust generation.

[0032] Furthermore, the mold manufacturing method of this embodiment produces a mold with excellent moisture resistance and heat resistance. The mold thus produced can suppress casting defects, deformation, and mold wall movement, even when pouring molten metals with high melting temperatures, such as iron and copper, and can produce castings with high dimensional accuracy. The mold manufacturing method of this embodiment is also suitable for producing molds that require high precision, such as complex-shaped molds for cores.

[0033] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. The materials used in each example are shown below. Various measurement methods are also shown below.

[0034] [Measurement and Evaluation Methods] <Measurement of Flexural Strength> The flexural strength of the test pieces obtained in each of the Examples and Comparative Examples was measured using the measurement method described in JACT Test Method SM-1.

[0035] Example 1 Preparation of Coated Sand Artificial sand obtained by a fusion process ("Alsand #650" manufactured by Ito Kiko Co., Ltd., average particle size 212 μm) was used as the refractory granular material. 100 parts by mass of the refractory granular material, 0.5 parts by mass of calcium silicate (dicalcium silicate) as a calcium salt, and 0.01 parts by mass of water were mixed to obtain coated sand in which the calcium salt adhered to the surface of the refractory granular material via the water.

[0036] <Preparation of Test Pieces> As water glass, a sodium silicate aqueous solution (molar ratio (SiO ２ / Na ２The coated sand and water glass were mixed to obtain a mixture (M). The ratio of the coated sand to the water glass was 1.5 parts by mass per 100 parts by mass of the refractory granular material constituting the coated sand. A resin mold was prepared, containing six rectangular parallelepiped shapes measuring 10 mm in length, 60 mm in width, and 10 mm in height. The resulting mixture was immediately filled into the mold at 25°C and 50% humidity and heat-treated at 150°C for 1 hour to obtain test pieces. The test pieces were removed from the mold and cooled to room temperature (25°C). The bending strength (room temperature strength) of three test pieces was measured and the average value was calculated. The results are shown in Table 1. The remaining three test pieces were left at 25°C and 60% humidity for one week, and a moisture resistance test was performed. The bending strength of the test pieces after leaving them was measured and the average value was calculated. The results are shown in Table 1.

[0037] [Examples 2 to 9] Coated sand was prepared in the same manner as in Example 1, except that the amounts of calcium salt and water were changed to the values ​​shown in Tables 1 to 3. Test pieces were made using the resulting coated sand, and their bending strengths were measured. The results are shown in Tables 1 to 3. The results of Example 2 are also shown in Table 4.

[0038] For Examples 2, 5, and 8, nine test pieces were produced using a resin mold having nine rectangular parallelepiped shapes measuring 10 mm in length, 60 mm in width, and 10 mm in height. Of the nine test pieces, room temperature strength was measured for three test pieces, and a moisture resistance test was performed on the remaining three test pieces, and bending strength was measured. The remaining three test pieces were heat-treated at 800°C for 15 minutes and subjected to a heat resistance test. The test pieces were then cooled to room temperature (25°C), and the bending strength (hot strength) of the cooled test pieces was measured and the average value was calculated. The results are shown in Tables 1 to 3.

[0039] [Examples 10 and 11] As the refractory granular material, artificial sand obtained by a fusion process ("Alsand #450" manufactured by Ito Kiko Co., Ltd., average particle size 300 μm) or artificial sand obtained by a fusion process ("Alsand #750" manufactured by Ito Kiko Co., Ltd., average particle size 150 μm) was used, and coated sand was prepared in the same manner as in Example 1, except that the amount of water was changed to the values ​​shown in Table 4. Test pieces were made using the obtained coated sand, and bending strength was measured. The results are shown in Table 4.

[0040] Example 12 Preparation of Coated Sand Artificial sand obtained by a fusion process ("Alsand #650" manufactured by Ito Kiko Co., Ltd., average particle size 212 μm) was used as the refractory granular material. 100 parts by mass of the refractory granular material, 0.5 parts by mass of calcium carbonate as a calcium salt, and 0.01 parts by mass of water were mixed to obtain coated sand in which the calcium salt adhered to the surface of the refractory granular material via the water.

[0041] <Preparation of Test Pieces> As water glass, a sodium silicate aqueous solution (molar ratio (SiO ２ / Na ２ The coated sand and water glass were mixed to obtain a mixture (M). The ratio of the coated sand to the water glass was 1.5 parts by mass per 100 parts by mass of the refractory granular material constituting the coated sand. A resin mold was prepared, containing six rectangular parallelepiped shapes measuring 10 mm in length, 60 mm in width, and 10 mm in height. The resulting mixture was immediately filled into the mold at 25°C and 50% humidity and heat-treated at 150°C for 1 hour to obtain test pieces. The test pieces were removed from the mold and cooled to room temperature (25°C). The bending strength (room temperature strength) of three test pieces was measured and the average value was calculated. The results are shown in Table 1. The remaining three test pieces were left at 25°C and 60% humidity for one week, and a moisture resistance test was performed. The bending strength of the test pieces after leaving them was measured and the average value was calculated. The results are shown in Table 5.

[0042] [Examples 13 to 20] Coated sand was prepared in the same manner as in Example 12, except that the amounts of calcium salt and water were changed to the values ​​shown in Tables 5 to 7. Test pieces were made using the resulting coated sand, and their bending strengths were measured. The results are shown in Tables 5 to 7. The results of Example 13 are also shown in Table 8.

[0043] For Examples 13, 16, and 19, nine test pieces were prepared using a resin mold containing nine rectangular parallelepiped molds measuring 10 mm in length, 60 mm in width, and 10 mm in height. Of the nine test pieces, three were subjected to room temperature strength measurement, and the remaining three were subjected to a moisture resistance test and bending strength measurement. The remaining three test pieces were subjected to a heat treatment at 800°C for 15 minutes and a heat resistance test. The test pieces were then cooled to room temperature (25°C), and the bending strength (hot strength) of the cooled test pieces was measured and the average value was calculated. The results are shown in Tables 5 to 7.

[0044] [Examples 21 and 22] As the refractory granular material, artificial sand obtained by a fusion process (manufactured by Ito Kiko Co., Ltd., "Alsand #450", average particle size 300 μm) or artificial sand obtained by a fusion process (manufactured by Ito Kiko Co., Ltd., "Alsand #750", average particle size 150 μm) was used, and coated sand was prepared in the same manner as in Example 12, except that the amount of water was changed to the values ​​shown in Table 8. Test pieces were made using the obtained coated sand, and bending strength was measured. The results are shown in Table 8.

[0045] Comparative Examples 1 to 3 Test pieces were prepared in the same manner as in Example 1, except that artificial sand obtained by a melting method ("Alsand #650" manufactured by Ito Kiko Co., Ltd., average particle size 212 μm), artificial sand obtained by a melting method ("Alsand #450" manufactured by Ito Kiko Co., Ltd., average particle size 300 μm), or artificial sand obtained by a melting method ("Alsand #750" manufactured by Ito Kiko Co., Ltd., average particle size 150 μm) was used instead of the coated sand, and the bending strength was measured. The results are shown in Table 9.

[0046] For Comparative Example 1, nine test pieces were produced using a resin molding die having nine rectangular parallelepiped shapes measuring 10 mm in length, 60 mm in width, and 10 mm in height. Of the nine test pieces, room temperature strength was measured for three test pieces, and a moisture resistance test was performed on the remaining three test pieces to measure their bending strength. The remaining three test pieces were heat-treated at 800°C for 15 minutes and subjected to a heat resistance test. The test pieces were then cooled to room temperature (25°C), and the bending strength (hot strength) of the cooled test pieces was measured and the average value was calculated. The results are shown in Table 9.

[0047]

[0048]

[0049]

[0050]

[0051]

[0052]

[0053]

[0054]

[0055]

[0056] As is clear from Tables 1 to 8, the test pieces made using the coated sand obtained in Examples 1 to 22 had high strength and excellent moisture resistance. The test pieces obtained in Examples 2, 5, and 8 also had excellent heat resistance. Furthermore, in Examples 1 to 22, dust was not easily generated when preparing the coated sand and when manufacturing the test pieces. Furthermore, dust was not easily generated when dismantling the obtained test pieces. On the other hand, as is clear from the results in Table 9, the test pieces obtained in Comparative Examples 1 to 3 had poor moisture resistance. Furthermore, the test piece obtained in Comparative Example 1 also had poor heat resistance.

Claims

1. Coated sand, in which calcium salts are adhered to the surface of refractory granular material via water.

2. The coated sand according to claim 1, wherein the refractory granular material is artificial sand obtained by a fusion process.

3. The coated sand according to claim 2, wherein the ratio of the water to 100 parts by mass of the artificial sand is 0.01 to 0.15 parts by mass.

4. A method for producing coated sand by mixing a refractory granular material, water and a calcium salt.

5. The method for producing coated sand according to claim 4, wherein the refractory granular material is artificial sand obtained by a fusion process.

6. The method for producing coated sand according to claim 5, wherein the ratio of the water to 100 parts by mass of the artificial sand is 0.01 to 0.15 parts by mass.

7. A kit for mold making, comprising the coated sand according to any one of claims 1 to 3 and water glass, each independently.

8. A method for manufacturing a mold, comprising filling a mixture containing the coated sand according to any one of claims 1 to 3 and water glass into a mold for mold manufacturing, and allowing the water glass to harden.

9. The method for producing a mold according to claim 8, wherein the mixture is filled into the mold and heated to harden the water glass by thermal hardening.

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