WO2025058051A1 - Coated sand, method for producing coated sand, kit for making casting mold, and method for producing casting mold - Google Patents

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, kit for casting and method for producing casting

The present invention relates to coated sand, a method for producing coated sand, a kit for molding a foundry mold, and a method for producing a foundry mold.
This application claims priority to Japanese Patent Application No. 2023-148903, filed in Japan on September 14, 2023, the contents of which are incorporated herein by reference.

One type of mold-making process is the water glass process, of which various types are known.
One known example of a method for manufacturing a mold using a water glass process is to add water glass, which is a binder, to a refractory granular material such as silica sand and mix them to form mixed sand, which is then filled into a mold for mold production to form a mold, and the water glass is heated to harden the sand to bind and solidify it to produce 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.

However, in the case of a thermosetting mold, the hardened network is reversible, so the strength of the thermosetting mold is easily reduced by moisture absorption.
In addition, if sodium derived from water glass is present in the mold, the heat resistance may decrease (reduction in hot strength). Since the mold is exposed to high temperatures when a liquid (molten metal) made by melting metals such as iron, copper, and aluminum at high temperatures is poured into it, if the mold has low heat resistance, casting defects such as seizure and glare may occur, deformation may occur, or the mold wall may move, resulting in a decrease in the dimensional accuracy of the casting. Therefore, the mold is required to have heat resistance that allows it to exhibit sufficient strength even when exposed to high temperatures.

Therefore, a so-called dicalcium silicate-based mold has been proposed, which uses a calcium salt as a hardener and promotes hardening through a substitution reaction between the sodium ions in the water glass and the calcium ions. For example, Patent Document 1 discloses a self-hardening casting 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.

Japanese Unexamined Patent Publication No. 50-75117

However, in the case of conventional die-casting molds, dust from calcium salts is generated during mold manufacturing and dismantling, and dust is easily generated due to the crushing of silica sand, which can easily deteriorate the working environment.
The present invention aims 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 when the molds are manufactured or dismantled, a method for manufacturing the coated sand, a mold making kit, and a method for manufacturing the molds.

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

The present invention provides coated sand that can produce molds that are excellent in moisture resistance and heat resistance while maintaining strength, and that generate little dust during mold production or dismantling, a method for producing coated sand, a mold-making kit, and a method for producing molds.

The following describes in detail the forms for implementing 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 numerical values before and after "to" as the lower and upper limits.
In the following description, the term "mold" refers to a mold produced using the coated sand or mold-making kit of the present invention. Furthermore, the term "strength" refers to the strength of the mold at room temperature.

[Coated sand]
Hereinafter, one embodiment of the coated sand of the present invention will be described.
The coated sand of this embodiment is a refractory granular material having a surface to which a calcium salt is attached via water. At least a portion of the surface of the refractory granular material is coated with the calcium salt via water.
If necessary, components other than calcium salts may be adhered to the surface of the refractory granular material within the limits that do not impair the effects of the present invention.

<Fire-resistant granular material>
As the refractory granular material, a wide variety of materials generally used as casting sand can be used, including conventionally known materials such as natural sands such as silica sand, chromite sand, zircon sand, olivine sand, amorphous silica, alumina sand, and mullite sand, and artificial sand. In addition, used refractory granular materials that have been recovered (recovered sand) or regenerated (regenerated sand) can also be used.
These refractory granular materials may be used alone or in combination of two or more.
The refractory granular material is preferably foundry sand having a refractoriness of SK-32 or more as measured in accordance with JIS R 2204 "Testing method for refractoriness of refractories and refractory raw materials". The refractoriness may be SK-34 or more, SK-35 or more, SK-36 or more, or SK-38 or more.

As the refractory granular material, artificial sand is preferred from the viewpoints that it is less likely to be crushed when the mold is dismantled, the generation of dust can be further suppressed, and the recovery rate can be further increased.
Examples of the artificial sand include artificial sand obtained by a melting method (melting method artificial sand), artificial sand obtained by a flame melting method (flame melting method artificial sand), and artificial sand obtained by a sintering method (sintering method artificial sand). Among these, the melting method artificial sand and the flame melting method artificial sand are preferred from the viewpoint of particularly excellent compatibility with water glass described later, and among these, the melting method artificial sand is more preferred from the viewpoint of suppressing the production cost.
The specific conditions for the melting method, the flame melting method, and the sintering method are not particularly limited, and the artificial sand may be produced under known conditions described in, for example, JP-A-5-169184, JP-A-2003-251434, JP-A-2004-202577, etc. As an example, artificial sand obtained by the melting method is obtained by using starting materials containing, for example, aluminum oxide and silica, melting them by heat or the like, 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.

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 embodiment 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. If the average particle size of the refractory granular material is equal to or greater than the above lower limit, a mold having higher strength and more excellent moisture resistance can be obtained. If the average particle size of the refractory granular material is equal to or less than the above upper limit, the surface of the casting cast using the mold is excellent.
The average particle size of the refractory granular material is the median size of 50% of the total volume of the refractory granular material measured by dynamic light scattering.
Additionally, the average particle size of the covering sand is substantially the same as the average particle size of the refractory granular material.

<Calcium salts>
The calcium salt acts as a hardener for the water glass, which will be described later.
Examples of calcium salts include calcium silicates such as 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 , and CaSiO ₃ ; calcium carbonate; and calcium hydroxide. 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.

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 ratio of calcium salt in the coated sand is preferably 0.05 to 0.7 parts by mass, more preferably 0.1 to less than 0.6 parts by mass, and even more preferably 0.2 to 0.5 parts by mass, relative to 100 parts by mass of the artificial sand. If the ratio of calcium salt is equal to or greater than the lower limit, the moisture resistance and heat resistance of the mold are further improved. If the ratio 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 obtained mold after use. In addition, the strength of the mold is further increased. Furthermore, the amount of water required to adhere calcium salt to the surface of the refractory granular material is unlikely to be excessive, and the fluidity of the coated sand can be well maintained.

When natural sand such as silica sand is used as the refractory granular material, the ratio of calcium salt in the coating 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. If the ratio of calcium salt is equal to or greater than the above lower limit, the moisture resistance and heat resistance of the mold are further improved. If the ratio of calcium salt is equal to or less than the above upper limit, the generation of calcium salt dust can be further suppressed when manufacturing a mold using the coating sand of this embodiment and when dismantling the obtained mold after use. In addition, the strength of the mold is further increased. Furthermore, the amount of water required to adhere calcium salt to the surface of the refractory granular material is unlikely to be excessive, and the fluidity of the coating sand can be well maintained.

<Water>
The water acts as an adhesive between the refractory granular material and the calcium salts, and its presence keeps the calcium salts on the surface of the refractory granular material.

When artificial sand such as molten sand, flame fused sand, or sintered sand is used as the refractory granular material, the proportion of water in the covering 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, relative to 100 parts by mass of the artificial sand. If the proportion of water is equal to or greater than the lower limit, calcium salts are sufficiently attached to the surface of the refractory granular material, and dust generation can be further suppressed. If the proportion of water is equal to or less than the upper limit, the artificial sand is less likely to aggregate, and the fluidity of the covering sand can be maintained well. Usually, the covering sand is stored in a tank such as a hopper, and is supplied from the hopper to a mixer and mixed with water glass, which will be described later. If the fluidity of the covering sand is good, it is easier to supply the covering sand from the hopper to the mixer.
From the viewpoint of obtaining the effects of the present invention more reliably, when artificial sand such as molten sand, flame-fused sand, sintered sand or the like 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.

When natural sand such as silica sand is used as the refractory granular material, the proportion of water in the covering 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 the natural sand. If the proportion of water is equal to or greater than the lower limit, calcium salt adheres sufficiently to the surface of the refractory granular material, and dust generation can be further suppressed. If the proportion of water is equal to or less than the upper limit, the natural sand is less likely to aggregate, and the fluidity of the covering sand can be well maintained.
In order to obtain the effects of the present invention more reliably, 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 a known method, for example, the following method: 20 g of the coated sand is placed in a crucible and dried at 105° C. for 60 minutes. The dried coated sand is then 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.

<Method of manufacturing coated sand>
The coated sand is obtained by mixing the above-mentioned 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 other than the refractory granular material, water, and calcium salt may be further mixed as necessary within a range that does not impair the effects of the present invention. Note that if the refractory granular material and/or the calcium salt already contain a sufficient amount of moisture, it is not necessary to add water separately. In this case, before mixing the refractory granular material and the calcium salt, the moisture content of the refractory granular material and/or the calcium salt is appropriately adjusted so that the covering sand contains a predetermined amount of water.
The method for mixing these is not particularly limited.
The mixing temperature is not limited, but is preferably, for example, 10 to 40°C.

<Action and effect>
The coated sand of the present embodiment described above has calcium salt adhered to the surface of the refractory granular material via water, so that calcium salt dust is less likely to be generated when the coated sand of this embodiment is used to manufacture a mold or when the resulting mold is dismantled after use, and the refractory granular material is less likely to shatter when the mold is dismantled, resulting in a good working environment.

In addition, since the calcium salt adhered 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 makes it possible to obtain a mold that is more moisture resistant than conventional thermosetting molds.
In addition, since the water glass hardens by a substitution reaction between sodium ions and calcium ions in the water glass, a mold with excellent heat resistance, i.e., high hot strength, can be obtained. Therefore, the mold obtained by using the coated sand of this embodiment can suppress casting defects, deformation, and movement of the mold wall, even when a metal with a high melting temperature such as iron or copper is melted and poured, and can cast a casting with high dimensional accuracy. Thus, the coated sand of this embodiment is also suitable for manufacturing molds that require high accuracy, such as molds with complex shapes such as cores.

[Mold making kit]
Hereinafter, one embodiment of the mold making kit of the present invention will be described.
The mold making kit of this embodiment independently contains the coated sand of the present invention and water glass.
Here, "independently possessed" means that each component is present in a state where it is not mixed or in contact with each other (i.e., each component is isolated from each other). The coated sand and the water glass are first mixed and contacted when the mold making kit is used.
The mold making kit may be, for example, a collection of containers each containing a separate component.

The water glass contained in the mold making kit is not particularly limited, and any conventionally known water glass can be used, such as sodium silicate (specifically, No. 1, No. 2, No. 3 described in JIS K 1408:1966 and sodium metasilicate (Type 1 and Type 2)), potassium silicate, or a mixture thereof.
The molar ratio ( _SiO2 /M) of SiO2 _to M (M = _K2O or _Na2O ) in the water glass 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 and less than 3.1, and most preferably 2.1 to 3.0. If the molar ratio of water glass is equal to or more than the lower limit, a sufficient hardening speed can be obtained. If the molar ratio of water glass is equal to or less than the upper limit, a mold with excellent storage stability can be obtained.

The Baume degree of the water glass at 20°C is preferably 30 to 60, more preferably 40 to 50. If the Baume degree of the water glass is equal to or higher than the lower limit above, the mold strength will be well developed. If the Baume degree of the water glass is equal to or lower than the upper limit above, good mixed sand can be prepared.

[Method of manufacturing the mold]
Hereinafter, one embodiment of the method for producing a mold of the present invention will be described.
In the method for manufacturing a mold of this embodiment, a mixture containing the coated sand of the present invention described above 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 "mold for mold making"), and the water glass in the mixture (M) is hardened to manufacture the mold.
A mold may be produced using the above-mentioned mold-making kit of the present invention. In this case, the coated sand and water glass constituting the above-mentioned mold-making kit are mixed to prepare a mixture (M), and the mixture (M) obtained is filled into a mold for mold-making.
The mixture (M) may contain components other than the coated sand and water glass of the present invention, if necessary, within a range that does not impair the effects of the present invention.

The proportion of water glass is preferably 1 part by mass or more and 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 more than the above lower limit, a mold with 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 covering sand, and dust is more likely to be generated during mold production and dismantling. If the proportion of water glass is equal to or less than the above 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.

In addition, as the proportion of water glass decreases, the strength of the mold tends to decrease, but when the proportion of water glass is low, for example when the proportion of water glass is below the upper limit value mentioned above, it is preferable to harden the water glass by thermal curing. Specifically, it is preferable to heat the mixture (M) filled in the casting mold. Heating promotes the dehydration of the water glass, and the reaction and condensation reaction with the silanol groups of the refractory granular material progresses, making it easier to form a hardened network, and further improving the strength of the mold.

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 mold strength will be obtained.
The heating time (curing time) is not particularly limited and may be appropriately determined depending on the shape and size of the mold, but is preferably 1 to 120 minutes, more preferably 3 to 90 minutes, and even more preferably 5 to 60 minutes.

The temperature of the mold for casting may be preferably 100 to 300°C, more preferably 120 to 250°C, and even more preferably 130 to 200°C.
In addition, the mixture (M) may be filled into the heated casting mold and then further heated.

According to the mold manufacturing method of the present embodiment described above, the coated sand, in which calcium salt is attached to the surface of the refractory granular material via water, is used, so that calcium salt dust is unlikely to be generated when kneading with water glass, when filling the mold for casting, etc. Furthermore, when the mold is dismantled, calcium salt dust is unlikely to be generated and the refractory granular material is unlikely to be crushed, and the working environment is good.
In particular, if the water glass is hardened by heat curing, a mold with sufficient strength can be obtained even if the ratio of water glass is reduced. Also, if the ratio of water glass is reduced, the ratio of calcium salt to be attached to the surface of the refractory granular material can be reduced accordingly, and dust generation can be further suppressed.

Moreover, the mold manufacturing method of the present embodiment can provide a mold with excellent moisture resistance and heat resistance. The mold thus obtained can suppress casting defects, deformation, and mold wall movement even when pouring molten metal with a high melting temperature, such as iron or copper, and can cast products with high dimensional accuracy.
The mold manufacturing method of this embodiment is also suitable for manufacturing molds that require high precision, such as molds with complex shapes such as cores.

The present invention will be explained in detail below with reference to examples, but the present invention is not limited to these. The materials used in each example are shown below. Various measurement methods are also as follows.

[Measurement and evaluation method]
<Measurement of bending strength>
The bending 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.

[Example 1]
<Preparation of coated sand>
As the refractory granular material, artificial sand obtained by a fusion method (manufactured by Ito Kiko Co., Ltd., "Alsand #650", average particle size 212 μm) was used.
100 parts by mass of 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.

<Preparation of test pieces>
As the water glass, an aqueous solution of sodium silicate (molar ratio (SiO ₂ /Na ₂ O) 2.5, Baume degree 48) was used.
The coated sand and water glass were mixed to obtain a mixture (M). The ratio of the coated sand to the water glass was set to 1.5 parts by mass of water glass per 100 parts by mass of the refractory granular material constituting the coated sand.
A resin molding die having six rectangular parallelepiped patterns each having a length of 10 mm, a width of 60 mm and a height of 10 mm was prepared.
The mixture thus obtained was immediately filled into a prepared mold under conditions of a temperature of 25° C. and a humidity of 50%, and heat-treated at 150° C. for 1 hour to obtain a test piece.
The test pieces were removed from the mold and cooled to room temperature (25°C), after which the bending strength (normal 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 for one week under conditions of a temperature of 25°C and a humidity of 60%, and a moisture resistance test was performed. After leaving the test pieces, the bending strength was measured and the average value was calculated. The results are shown in Table 1.

[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 bending strength was measured. The results are shown in Tables 1 to 3. The results of Example 2 are also shown in Table 4.

In addition, for Examples 2, 5, and 8, nine test pieces were produced using a resin molding die in which nine rectangular parallelepiped patterns measuring 10 mm in length, 60 mm in width, and 10 mm in height were formed.
Of the nine test pieces, three test pieces were subjected to the measurement of room temperature strength, and the remaining three test pieces were subjected to a moisture resistance test and the bending strength was measured.
The remaining three test pieces were subjected to a heat treatment at a temperature of 800°C for 15 minutes, and a heat resistance test was performed. After that, the test pieces were cooled to room temperature (25°C), and the bending strength (hot strength) of the test pieces after cooling was measured, and the average value was calculated. The results are shown in Tables 1 to 3.

[Examples 10 and 11]
As the fire-resistant granular material, artificial sand obtained by a fusion method (manufactured by Ito Kiko Co., Ltd., "Alsand #450", average particle size 300 μm) or artificial sand obtained by a fusion method (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 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.

[Example 12]
<Preparation of coated sand>
As the refractory granular material, artificial sand obtained by a fusion method (manufactured by Ito Kiko Co., Ltd., "Alsand #650", average particle size 212 μm) was used.
100 parts by mass of 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 was adhered to the surface of the refractory granular material via the water.

<Preparation of test pieces>
As the water glass, an aqueous solution of sodium silicate (molar ratio (SiO ₂ /Na ₂ O) 2.5, Baume degree 48) was used.
The coated sand and water glass were mixed to obtain a mixture (M). The ratio of the coated sand to the water glass was set to 1.5 parts by mass of water glass per 100 parts by mass of the refractory granular material constituting the coated sand.
A resin molding die having six rectangular parallelepiped patterns each having a length of 10 mm, a width of 60 mm and a height of 10 mm was prepared.
The mixture thus obtained was immediately filled into a prepared mold under conditions of a temperature of 25° C. and a humidity of 50%, and heat-treated at 150° C. for 1 hour to obtain a test piece.
The test pieces were removed from the mold and cooled to room temperature (25°C), after which the bending strength (normal 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 for one week under conditions of a temperature of 25°C and a humidity of 60%, and a moisture resistance test was performed. After leaving the test pieces, the bending strength was measured and the average value was calculated. The results are shown in Table 5.

[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 bending strength was measured. The results are shown in Tables 5 to 7. The results of Example 13 are also shown in Table 8.

For Examples 13, 16, and 19, nine test pieces were produced using a resin mold in which nine rectangular parallelepiped dies measuring 10 mm in length, 60 mm in width, and 10 mm in height were formed. Of the nine test pieces, three test pieces were subjected to room temperature strength measurement, and the remaining three test pieces were subjected to a moisture resistance test to measure bending strength.
The remaining three test pieces were subjected to a heat treatment at a temperature of 800°C for 15 minutes, and a heat resistance test was performed. After that, the test pieces were cooled to room temperature (25°C), and the bending strength (hot strength) of the test pieces after cooling was measured and the average value was calculated. The results are shown in Tables 5 to 7.

[Examples 21 and 22]
As the fire-resistant granular material, artificial sand obtained by a melting method (manufactured by Ito Kiko Co., Ltd., "Alsand #450", average particle size 300 μm) or artificial sand obtained by a melting method (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 value 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.

[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 (manufactured by Ito Kiko Co., Ltd., "Alsand #650", average particle size 212 μm), artificial sand obtained by a melting method (manufactured by Ito Kiko Co., Ltd., "Alsand #450", average particle size 300 μm), or artificial sand obtained by a melting method (manufactured by Ito Kiko Co., Ltd., "Alsand #750", 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.

For Comparative Example 1, nine test pieces were produced using a resin molding die in which nine rectangular parallelepiped shapes measuring 10 mm in length, 60 mm in width, and 10 mm in height were formed.
Of the nine test pieces, three test pieces were subjected to the measurement of room temperature strength, and the remaining three test pieces were subjected to a moisture resistance test and the bending strength was measured.
The remaining three test pieces were subjected to a heat treatment at a temperature of 800° C. for 15 minutes, and a heat resistance test was performed. After that, the test pieces were cooled to room temperature (25° C.), and the bending strength (hot strength) of the test pieces after cooling was measured and the average value was calculated. The results are shown in Table 9.

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 were also excellent in heat resistance.
In addition, in Examples 1 to 22, dust was not easily generated when preparing the coated sand and when manufacturing the test pieces. In addition, 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 were poor in moisture resistance. In addition, the test piece obtained in Comparative Example 1 was also poor in heat resistance.

Claims (9)

Coated sand, in which calcium salts are attached to the surface of a fire-resistant granular material via water. The coated sand according to claim 1, wherein the refractory granular material is artificial sand obtained by a fusion process. The coated sand according to claim 2, wherein the ratio of the water is 0.01 to 0.15 parts by mass per 100 parts by mass of the artificial sand. A method for producing coated sand by mixing refractory granular material, water, and calcium salt. The method for producing coated sand according to claim 4, wherein the refractory granular material is artificial sand obtained by a fusion method. The method for producing coated sand according to claim 5, wherein the ratio of the water is 0.01 to 0.15 parts by mass per 100 parts by mass of the artificial sand. A mold-making kit comprising the coated sand according to any one of claims 1 to 3 and water glass, each of which is independent of the other. 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 hardening the water glass. The method for manufacturing a mold according to claim 8, wherein the mixture is filled into the mold and heated to harden the water glass by thermal curing.