KR102281945B1 - Water-soluble core material and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a water-soluble core material and a method for manufacturing the same. According to the present invention, the method comprises the steps of: (a) preparing a water-soluble inorganic binder; (b) preparing a mixture by mixing the water-soluble inorganic binder with the inorganic metal oxide; (c) preparing a core material by injection or pressure molding the mixture; (d) preparing a sealant, and (e) coating the sealant on the surface of the core material may be provided.
It is possible to provide a water-soluble core material manufactured through this.

Description

Water-soluble core material and manufacturing method thereof

The present invention relates to a water-soluble core material and a method for manufacturing the same, and more particularly, serves as a mold for molding a product or a core material for molding a hollow shape inside a product, and is easily separated from the product after molding and It relates to a removable water-soluble core material and a method for manufacturing the same.

Since there are very many hollow structures among fiber-reinforced composite parts used in aviation, automobiles, and leisure/sports fields, in order to realize this shape, a core material must be inserted inside and then molded by pressing from the outside, or It should be molded by pressing from the inside out using a blade.

As described above, the water-soluble core material is installed inside the mold and used to form a space matching the shape of the water-soluble core in the mold. That is, after installing the water-soluble core in the mold, molten metal is injected into the mold to form a space that matches the shape of the water-soluble core.

Conventional core materials do not satisfy all of the characteristics of the material in terms of required characteristics such as heat resistance, dimensional stability, collapsibility and strength, but only partially satisfy them.

Among them, the CO ₂ type has excellent heat resistance and strength, but has very poor disintegrability, the green sand type lacks heat resistance and strength, and the resin type has problems with heat resistance, causing problems such as burning.

In addition, a core material manufactured using NaCl is used for die casting and gravity casting. However, if the thickness of the core is thin or the structure is complicated, the pores of the core are contracted and cracks occur during casting. In particular, during pressure die casting, the core may be destroyed.

In order to compensate for this, burning was prevented by forming a coating film on the surface with a heat-resistant coating material, and disintegration property after casting was improved by molding by mixing pitch or sawdust.

However, when heat resistance is improved by coating, the air permeability is often lowered, and if the coating film is insufficient, sintering occurs, and when pitch or sawdust is mixed and molded, the amount of gas generated from the core rapidly increases.

Accordingly, there is a need to develop a core material in which problems such as sintering are improved without reducing air permeability, and strength is improved.

In order to solve the above problems, the present invention serves as a core material so that a product with a hollow inside or a product that cannot be molded with a general mold among the field of fiber-reinforced plastic composites can be molded, and the product is easily formed after product molding. An object of the present invention is to provide a water-soluble core material that can be separated and removed from and a method for manufacturing the same.

In order to solve the above problems, the method for manufacturing a water-soluble core material according to an embodiment of the present invention comprises the steps of: (a) manufacturing a water-soluble inorganic binder; (b) preparing a mixture by mixing the water-soluble inorganic binder with the inorganic metal oxide; (c) preparing a core material by injection or pressure molding the mixture; (d) preparing a sealant; and (e) coating the sealant on the surface of the core material may be provided.

Here, the step (a) is characterized in that the water-soluble inorganic binder is prepared by mixing at least two of sodium phosphate, sodium tetraborate and magnesium sulfate with water.

In addition, the step (a) is characterized in that 340 to 360 parts by weight of sodium phosphate monobasic, 18 to 28 parts by weight of sodium tetraborate, and 138 to 158 parts by weight of magnesium sulfate are mixed with 400 parts by weight of water to prepare a water-soluble inorganic binder. do.

In addition, in step (a), the temperature of the water is 45 to 55 ℃, characterized in that it is prepared by mixing for 8 to 12 minutes.

In addition, in step (b), when injection molding in step (c), 30 to 35 parts by weight of the water-soluble inorganic binder are mixed with 100 parts by weight of the inorganic metal oxide to prepare a mixture.

In addition, the step (b) is characterized in that when the pressure molding in the step (c), 10 to 15 parts by weight of the water-soluble inorganic binder is mixed with 100 parts by weight of the inorganic metal oxide to prepare a mixture.

In addition, the step (c) is characterized in that the mixture is press-molded to prepare a core material ^{having a molding density of 1.8 to 2.2 g/cm 3 .}

In addition, the step (d) comprises the steps of slowly adding and dissolving polyvinyl alcohol (PVA) in water to prepare a solution; It may include cooling the solution and mixing ethyl alcohol with the solution to prepare a sealant.

In addition, it is possible to provide a water-soluble core material manufactured by the method for manufacturing a water-soluble core material according to an embodiment of the present invention.

As described above, the water-soluble core material and its manufacturing method according to the embodiment of the present invention use an inorganic binder and a ceramic-based powder to prepare a water-soluble core material, and to prevent the resin from penetrating into the material during molding, the surface is coated with a water-soluble sealant or the like. By being manufactured by coating, it serves as a mold for molding a product or a core material for molding a hollow shape inside the product, and can be easily separated and removed from the product after molding the product.

In the case of manufacturing a composite material using such a water-soluble core material, the manufacturing cost is relatively low compared to the conventional molding methods such as RIM, RTM, and Autoclave, which are commercialized for manufacturing a composite material, so that it is possible to secure prototype manufacturing competitiveness.

In addition, it is possible to implement a desired shape and maximize the weight reduction because the manufacturing and shape can be changed in various ways compared to the conventional core material and mandrel.

In addition, since it is water-soluble, it is an eco-friendly material, and there may be no additional cost for removing the core material.

Furthermore, the water-soluble core material manufactured according to the present invention is expected to have an effect that it can be usefully used in the manufacture of hollow composite products having a shape difficult to remove the fuel tank and the inner mandrel.

1 is a flowchart sequentially showing a method of manufacturing a water-soluble core material according to an embodiment of the present invention.
2 is a flowchart sequentially illustrating steps S40 of FIG. 1;
3 is a graph showing the average compressive strength of Examples 1 to 5 and Comparative Example 1 of the present invention.
4 is a graph showing the average compressive strength of Examples 6 to 10 and Comparative Example 2 of the present invention.
5 is a graph of average results of flexural strength of Examples 1 to 5 and Comparative Example 1 of the present invention.
6 is a graph of average results of flexural strength of Examples 6 to 10 and Comparative Example 2 of the present invention.

Since the present invention may have various changes and may have various forms, specific embodiments will be described in detail in the text. However, this is not intended to limit the present invention to the specific disclosed form, it should be understood to include all modifications, equivalents and substitutes included in the spirit and scope of the present invention.

The terms used in the present application are only used to describe specific embodiments, and are not intended to limit the present invention. The singular expression includes the plural expression unless the context clearly dictates otherwise. In the present application, terms such as "comprise" or "have" are intended to designate that a combination of features, numbers, steps, elements, etc. described in the specification exists, and includes one or more other features or numbers, It should be understood that the possibility of the presence or addition of combinations of steps, elements, etc. is not precluded in advance.

Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Terms such as those defined in commonly used dictionaries should be interpreted as having a meaning consistent with the meaning in the context of the related art, and should not be interpreted in an ideal or excessively formal meaning unless explicitly defined in the present application. does not

Here, repeated descriptions, well-known functions that may unnecessarily obscure the gist of the present invention, and detailed descriptions of configurations will be omitted. The embodiments of the present invention are provided in order to more completely explain the present invention to those of ordinary skill in the art.

The present invention relates to a water-soluble core material and a method for manufacturing the same, and more particularly, a water-soluble core material is manufactured using an inorganic binder and ceramic-based powder, and the surface is coated with a water-soluble sealant, etc. to prevent resin from penetrating into the material during molding The main content is a water-soluble core material that can be used as a mold for molding a product or a core material for forming a hollow shape inside a product, and can be easily separated and removed from the product after product molding and a manufacturing method thereof.

Hereinafter, an embodiment of the present invention will be described in detail with reference to FIGS. 1 to 6 attached thereto.

1 is a flowchart sequentially illustrating a method of manufacturing a water-soluble core material according to an embodiment of the present invention, and FIG. 2 is a flowchart sequentially illustrating steps S40 of FIG. 1 .

1, the method for manufacturing a water-soluble core material according to an embodiment of the present invention comprises the steps of preparing a water-soluble inorganic binder (S10), mixing an inorganic metal oxide with a water-soluble inorganic binder to prepare a mixture (S20), a mixture It may include the step of manufacturing the core material by injection or pressure molding (S30), the step of manufacturing the sealant (S40), and the step of coating the sealant on the surface of the core material (S50).

First, the step of preparing a water-soluble inorganic binder (S10) is a step of preparing a water-soluble inorganic binder by mixing a water-soluble metal salt with water, and mixing two or more of sodium phosphate, sodium tetraborate and magnesium sulfate with water to obtain a water-soluble inorganic binder. A binder can be prepared.

Here, the monobasic sodium phosphate is sodium hydrogen phosphate, and has the formula NaH ₂ PO ₄ ·nH ₂ O (n = 0, 1, 2), the aqueous solution (1→20) is slightly acidic, and the pH is 4.1. From the aqueous solution, it is possible to obtain a double beard at 41°C or lower, a single beard at 41-58°C, and an anhydrous salt at 100°C. In addition, it is easily soluble in water, but insoluble in alcohol and ether, and becomes metaphosphate when heated. It has slightly deliquescent properties and also has a buffering action.

This monobasic sodium phosphate may be used in 340 to 360 parts by weight based on 400 parts by weight of water in step S10, and 350 parts by weight is preferably used.

This is because when the monobasic sodium phosphate is less than 340 parts by weight, the physical properties of the core material may be deteriorated, and when it exceeds 360 parts by weight, it may be eluted insoluble in water.

Sodium tetraborate has the formula Na ₂ B ₄ O ₇ and is sometimes called sodium diborate (in this case, sodium tetraborate refers to Na ₂ O 4B ₂ O ₃ ), and decahydrate is called borax. , one of the components of the buffer solution.

18 to 28 parts by weight of this sodium tetraborate may be used based on 400 parts by weight of water in step S10, and 23 parts by weight is preferably used.

This is because, when sodium tetraborate is less than 18 parts by weight, the physical properties of the core material may be deteriorated, and when it exceeds 28 parts by weight, it may not be completely dissolved in water and may be eluted.

Magnesium sulfate has many hydrochloric salts other than anhydrous salts. Usually, magnesium sulfate refers to heptahydrate salts, and these are called Fsom salts, and natural ones are especially called saly salts (瀉利鹽). The chemical formula is MgSO ₄ , and the anhydrous salt is a white crystalline powder with a melting point of 1,185° C. and a specific gravity of 2.66. It is soluble in 100g of water at 26.9g at 0℃ and 68.3g at 100℃, slightly soluble in alcohol. The state of the hydrous salt changes depending on the temperature at which the crystals are precipitated from the saturated aqueous solution.

138 to 158 parts by weight of this magnesium sulfate may be used based on 400 parts by weight of water in step S10, and 148 parts by weight is preferably used.

This is because when magnesium sulfate is less than 138 parts by weight, physical properties may be deteriorated, and when it exceeds 158 parts by weight, it may not be completely dissolved in water and may be eluted.

In addition, in step S10, it is preferable to prepare a water-soluble inorganic binder by mixing monobasic sodium phosphate, sodium tetraborate, magnesium sulfate and water, that is, 350 parts by weight of sodium monobasic phosphate, 23 parts by weight of sodium tetraborate in 400 parts by weight of water. It is more preferable to prepare a water-soluble inorganic binder by mixing 148 parts by weight of magnesium sulfate.

In addition, the temperature of the water in step S10 may be 45 to 55 ℃, it is more preferable to use 50 ℃ water. This is because sodium phosphate, sodium tetraborate, and magnesium sulfate are most efficiently dissolved within the temperature range of the water.

In addition, in step S10, each composition may be mixed in the same mixing ratio as above and mechanically stirred for 8 to 12 minutes, preferably stirring for 10 minutes.

At this time, if the stirring time is less than 8 minutes, the monobasic sodium phosphate, sodium tetraborate, and magnesium sulfate may not all be dissolved, and if it exceeds 12 minutes, further stirring is inefficient in the already dissolved state.

The step of preparing a mixture by mixing an inorganic metal oxide with a water-soluble inorganic binder (S20) is a step of preparing a mixture by mixing a water-soluble inorganic binder with a ceramic-based inorganic metal oxide according to the method of forming the core material in step S30.

Here, the inorganic metal oxide preferably includes at least one of aluminum oxide powder, magnesium oxide powder, and titanium dioxide powder, but is not limited thereto, and may further include various inorganic metal oxides.

First, when the core material is injection molded in step S30, in step S20, 30 to 35 parts by weight of a water-soluble inorganic binder may be mixed with 100 parts by weight of an inorganic metal oxide to prepare a mixture, and a water-soluble inorganic binder 35 to 100 parts by weight of an inorganic metal oxide It is more preferable to mix parts by weight.

At this time, when the content of the water-soluble inorganic binder is less than 30 parts by weight, the curability may be reduced during molding, and if it exceeds 35 parts by weight, the water-soluble inorganic binder may be eluted during molding.

In addition, when the core material is press-molded in step S30, in step S20, 10 to 15 parts by weight of a water-soluble inorganic binder may be mixed with 100 parts by weight of an inorganic metal oxide to prepare a mixture, and a water-soluble inorganic binder 15 to 100 parts by weight of an inorganic metal oxide. It is more preferable to mix parts by weight.

At this time, when the content of the water-soluble inorganic binder is less than 10 parts by weight, the curability may be reduced during molding, and if it exceeds 15 parts by weight, the water-soluble inorganic binder may be eluted during molding.

The step of manufacturing the core material by injection or pressure molding of the mixture (S30) is a step of manufacturing the core material by molding by injecting the mixture into an injection molding mold, or by injecting the mixture into an injection molding mold and pressing with a press.

When the pressure molding is performed in step S30, a core material having a molding density of 1.8 to 2.2 g/cm ³ may be manufactured, and it is preferable to prepare a core material having a molding density of 2.0 g/cm ^{3 .}

This means that when the molding density of the core material is ^{less than 1.8 g/cm 3} , the shape of the specimen is not maintained well, and ^{when it exceeds 2.2 g/cm 3} , the molding mold may be rather deformed due to high pressure.

In addition, in step S30, after molding, the manufactured core material may be dried.

At this time, drying may be performed by primary drying at room temperature (20±5° C.) for 24 hours, followed by secondary hot air drying at 60° C. for 48 hours.

As described above, in the hot air drying after drying at room temperature, it is preferable to divide the drying into primary and secondary as described above, because only the surface of the core material is dried and the inside is not dried when the hot air drying is performed immediately.

The step of preparing the sealant (S40) is a step of preparing a sealant for coating the surface of the core material. Referring to FIG. 2, polyvinyl alcohol (PVA) is slowly added and dissolved in water to prepare a solution ( S41), cooling the solution (S42), and mixing ethyl alcohol with the solution may include a step (S43) of preparing a sealant.

First, in step S41, polyvinyl alcohol (PVA) may be slowly added and dissolved to prepare a solution. Here, 10 to 12 parts by weight of polyvinyl alcohol may be dissolved in 100 parts by weight of water, and 12 parts by weight is preferably dissolved.

At this time, when the polyvinyl alcohol is dissolved in less than 10 parts by weight, the sealant function may be deteriorated, and when it is dissolved in more than 12 parts by weight, the viscosity of the sealant increases, making it difficult to apply for coating on the core material.

Here, the temperature of water is preferably 80 to 85°C, more preferably 80°C, in order to dissolve polyvinyl alcohol.

In step S42, the solution prepared in step S41 may be cooled to room temperature. This is to prevent vaporization of ethyl alcohol to be mixed later.

In step S43, ethyl alcohol may be mixed with the solution cooled in step S42 to prepare a sealant. The mixing of ethyl alcohol in this way allows the sealant to dry quickly when the sealant is coated on the core material.

In step S43, 25 to 35 parts by weight of ethyl alcohol may be further added to the solution and mixed to prepare a sealant, and 30 parts by weight is preferably added to prepare the sealant.

In this case, when the amount of ethyl alcohol is less than 25 parts by weight, the effect of shortening the drying time of the sealant is insignificant, and when it exceeds 35 parts by weight, it may react with PVA to cause gelation of the sealant.

In the step (S50) of coating the sealant on the surface of the core material, the final water-soluble core material may be manufactured by coating the surface of the core material prepared in step S30 with the sealant prepared in step S40.

In step S50, a sealant may be coated on the surface of the core material by using spraying and brushing as a coating method.

Specifically, in step S50, the sealant may be coated by first applying the sealant to the surface of the core material, drying it for 3 hours or more, and then applying the sealant a second time and drying the sealant.

It is possible to provide a water-soluble core material according to an embodiment of the present invention through the manufacturing method as described above.

The water-soluble core material thus prepared has a compressive strength of 4 MPa or more, a water solubility of 50 g/min·m ² or more, and a coefficient of thermal expansion of 5x10 ^-6 mm/mm·℃ or less.

As described above, the water-soluble core material and its manufacturing method according to an embodiment of the present invention prepare the water-soluble core material using an inorganic binder and ceramic-based powder, and make the surface water-soluble so that the resin does not penetrate the material during molding. By being manufactured by coating with a sealant, it serves as a mold for molding a product or a core material for molding a hollow shape inside the product, and can be easily separated and removed from the product after molding the product.

In the case of manufacturing a composite material using such a water-soluble core material, the manufacturing cost is relatively low compared to the conventional molding methods such as RIM, RTM, and Autoclave, which are commercialized for manufacturing a composite material, so that it is possible to secure prototype manufacturing competitiveness.

In addition, it is possible to implement a desired shape and maximize the weight reduction because the manufacturing and shape can be changed in various ways compared to the conventional core material and mandrel.

In addition, since it is water-soluble, it is an eco-friendly material, and there may be no additional cost for removing the core material.

Furthermore, the water-soluble core material manufactured according to the present invention is expected to have an effect that it can be usefully used in the manufacture of hollow composite products having a shape difficult to remove the fuel tank and the inner mandrel.

Hereinafter, the present invention will be described in more detail by way of examples.

However, the following examples are only illustrative of the present invention, and the content of the present invention is not limited by the following examples.

In addition, the embodiments of the present invention described below can be implemented by various substitutions, modifications, and changes within the scope without departing from the technical spirit of the present invention, and these implementations are of the technical field to which the present invention belongs from the description of the above-described embodiments. If you are an expert, you can easily implement it.

[ Example ]

[Example 1]

A water-soluble inorganic binder was prepared by mixing 350 parts by weight of sodium monobasic phosphate and 23 parts by weight of sodium tetraborate with 400 parts by weight of water at 50° C., and 15 parts by weight of a water-soluble inorganic binder was mixed with 100 parts by weight of an inorganic metal oxide to prepare a mixture. , The mixture was injected into an injection molding mold and press-molded to prepare a core material having ^{a molding density of 2.0 g/cm 3 .}

[Example 2]

It was prepared in the same manner as in Example 1, except that a water-soluble inorganic binder was prepared by mixing 350 parts by weight of sodium phosphate and 148 parts by weight of magnesium sulfate in 400 parts by weight of water.

[Example 3]

It was prepared in the same manner as in Example 1, except that a water-soluble inorganic binder was prepared by mixing 148 parts by weight of magnesium sulfate and 23 parts by weight of sodium tetraborate in 400 parts by weight of water.

[Example 4]

It was prepared in the same manner as in Example 1, except that a water-soluble inorganic binder was prepared by mixing 350 parts by weight of sodium phosphate monobasic, 148 parts by weight of magnesium sulfate, and 23 parts by weight of sodium tetraborate to 400 parts by weight of water.

[Example 5]

It was prepared in the same manner as in Example 1, except that the mixture was molded by injecting it into an injection molding mold.

[Example 6]

It was prepared in the same manner as in Example 2, except that the mixture was molded by injection into an injection molding mold.

[Example 7]

It was prepared in the same manner as in Example 3, except that the mixture was molded by injecting it into an injection molding mold.

[Example 8]

It was prepared in the same manner as in Example 4, except that the mixture was molded by injection into an injection molding mold.

[Comparative Example 1]

A water-soluble inorganic binder was prepared by mixing 350 parts by weight of monobasic sodium phosphate with 400 parts by weight of water at 50° C., and 15 parts by weight of a water-soluble inorganic binder was mixed with 100 parts by weight of an inorganic metal oxide to prepare a mixture, and then the mixture was placed in an injection mold. Injection and press molding were performed to prepare a core material having a molding density of 2.0 g/cm ^{3 .}

[Comparative Example 2]

It was prepared in the same manner as in Comparative Example 1, except that a water-soluble inorganic binder was prepared by mixing 148 parts by weight of magnesium sulfate in 400 parts by weight of water.

[Comparative Example 3]

It was prepared in the same manner as in Comparative Example 1, except for molding by injecting the mixture into an injection molding mold.

[Comparative Example 4]

It was prepared in the same manner as in Comparative Example 2, except for molding by injecting the mixture into an injection molding mold.

[ Experimental example One] of core material Compressive strength analysis

In order to compare the core materials of Examples 1 to 4 and Comparative Examples 1 and 2 and Examples 5 to 8 and Comparative Examples 3 and 4, compressive strength was measured.

At this time, the compressive strength (MPa) was measured twice, first and second, and the average was derived.

The results are shown in Table 1, Figure 3, Table 2, and Figure 4 below.

Comparative Example 1 Comparative Example 2 Example 1 Example 2 Example 3 Example 4 One 1.27 2.23 3.50 7.64 2.87 8.91 2 1.91 2.23 3.18 7.96 3.18 8.91 Average 1.59 2.23 3.34 7.80 3.03 8.91 Standard Deviation 0.45 0.00 0.23 0.23 0.22 0.00

Comparative Example 3 Comparative Example 4 Example 5 Example 6 Example 7 Example 8 One 1.74 2.24 5.66 7.26 2.70 13.88 2 1.58 2.05 5.30 8.22 3.01 11.97 Average 1.64 2.15 5.48 7.73 2.86 12.93 Standard Deviation 0.09 0.13 0.25 0.69 0.22 1.35

As can be seen from Table 1 and FIG. 3, it was confirmed that the compressive strength of Examples 1 to 4 was higher than that of Comparative Examples 1 and 2, and the compressive strength of Example 4 was the highest. In addition, as can be seen from Table 2 and FIG. 4 , it was confirmed that the compressive strength of Examples 5 to 8 was higher than that of Comparative Examples 3 and 4, and it was confirmed that the compressive strength of Example 8 was the highest.

[ Experimental example 2] of core material Flexural strength analysis

In order to compare the core materials of Examples 1 to 4 and Comparative Examples 1 and 2 and Examples 5 to 8 and Comparative Examples 3 and 4, flexural strength was measured.

At this time, the flexural strength (MPa) was measured twice in the first and second rounds in the case of press molding, and in the case of injection molding, in the first, second, and third measurements, and the average was derived.

The results are shown in Table 3, Figure 5, Table 4, Figure 6 below.

At this time, - means the intensity value is too low to measure.

1 in comparison Comparative Example 2 Example 1 Example 2 Example 3 Example 4 One 4.19 - 5.27 10.15 5.54 15.96 2 4.58 3.69 5.27 8.74 5.60 14.51 Average 4.39 3.69 5.27 9.45 5.57 15.24 Standard Deviation 0.28 0.00 1.00 0.04 1.03

Comparative Example 3 Comparative Example 4 Example 5 Example 6 Example 7 Example 8 One 2.06 3.52 6.02 9.39 7.51 11.87 2 1.86 3.84 6.20 7.91 7.77 11.87 3 1.80 3.73 5.93 7.77 7.77 12.27 Average 1.91 3.70 6.05 8.36 7.68 12.00 Standard Deviation 0.14 0.16 0.14 0.90 0.15 0.23

As can be seen from Table 3 and FIG. 5, it was confirmed that the flexural strength of Examples 1 to 4 was higher than that of Comparative Examples 1 and 2, and the flexural strength of Example 4 was the highest. In addition, as can be seen from Table 4 and Figure 6, it was confirmed that the flexural strength of Examples 5 to 8 was higher than that of Comparative Examples 3 and 4, and it was confirmed that the flexural strength of Example 8 was the highest. Therefore, The water-soluble core material of the present invention has excellent compressive strength and flexural strength, and it is considered that the strength is improved.

Above, a specific part of the content of the present invention has been described in detail, for those of ordinary skill in the art, this specific description is only a preferred embodiment, and the scope of the present invention is not limited thereby It will be obvious. Accordingly, the substantial scope of the present invention will be defined by the appended claims and their equivalents.

Claims (4)

(a) preparing a water-soluble inorganic binder;
(b) preparing a mixture by mixing the water-soluble inorganic binder with the inorganic metal oxide;
(c) preparing a core material by injection or pressure molding the mixture;
(d) preparing the sealant; and
(e) coating the sealant on the surface of the core material;
The step (a) is,
A water-soluble inorganic binder is prepared by mixing 340 to 360 parts by weight of monobasic sodium phosphate, 18 to 28 parts by weight of sodium tetraborate and 138 to 158 parts by weight of magnesium sulfate to 400 parts by weight of water,
The step (a) is,
The temperature of the water is 45 to 55 ℃, it is prepared by mixing for 8 to 12 minutes,
The step (c) is,
Dry the manufactured core material, primary drying at room temperature, and secondary drying by hot air drying at 60°C,
The step (c) is,
When the mixture is press-molded, it is prepared as a core material having a molding density of 1.8 to 2.2 g/cm ^{3 ,}
Step (d) is,
Preparing a solution by slowly adding and dissolving 10 to 12 parts by weight of polyvinyl alcohol (PVA) in 100 parts by weight of water;
cooling the solution and
and mixing 25 to 35 parts by weight of ethyl alcohol with the solution to prepare a sealant.
According to claim 1,
Step (b) is,
When injection molding in step (c), 30 to 35 parts by weight of the water-soluble inorganic binder is mixed with 100 parts by weight of the inorganic metal oxide to prepare a mixture.
According to claim 1,
Step (b) is,
When press-molding in step (c), 10 to 15 parts by weight of the water-soluble inorganic binder is mixed with 100 parts by weight of the inorganic metal oxide to prepare a mixture.
A water-soluble core material manufactured by the manufacturing method of any one of claims 1 to 3.

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