JP2930158B2 - Method for producing collapsible sand core - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to disintegration having good pressure resistance and good coating properties and easy disintegration for use in high pressure die casting of a casting having an undercut portion, such as a closed deck type automobile engine. The present invention relates to a method for manufacturing a sand core. More specifically, a sand core prototype formed using sand for shell mold is treated with a mineral acid, then washed with water, and further treated with an inorganic salt, so that the first coating agent is thickened only once. A sand core that is suitable for high-pressure casting, which can be hardened even if it is coated with a second coating agent containing aluminum powder and has excellent collapsibility after casting. It relates to a manufacturing method. Here, good coating property means that when coating the coating material on the sand core prototype, the coating agent does not penetrate deeply in a thin liquid state while spreading inside the sand core prototype, or Without being repelled from the surface of the sand core prototype, only on the entire surface layer of the sand core prototype,
It is to be formed firmly and easily with a predetermined thickness and easily and reliably, and to be coated so as not to peel off, and to sufficiently withstand the high casting pressure during casting.

[0002]

2. Description of the Related Art Conventionally, for example, when a closed deck type automobile engine block or other castings such as aluminum alloys and magnesium alloys having an undercut portion are manufactured by die casting, a collapsible sand core is used. Die-casting is performed using the same. In order to obtain a collapsible sand core, first, the sand is hardened to a desired shape, and then a coating agent is applied to the surface of the hardened sand core prototype. To prevent the breakage of the core or the molten metal from entering the sand core.
After casting, attempts have been made to disintegrate the sand core with little force so that it can be easily taken out and that the sand can be taken out to every corner. Of course, in this case, various attempts have been made on the components of the sand core prototype, the method of hardening the sand, the components of the coating agent, the coating method, etc., but no satisfactory one has been obtained. It is. As the coating agent, for example, as described in JP-A-63-40639, a first coating agent composed of a powdery refractory and a metal oxide, and a A second coating agent containing scale-like aluminum powder, mica, or the like, which is applied to the substrate, is used.

[0003] Among them, the hardening method, warm box method,
There are a shell mold method, a cold box method and the like.
As the Hardox method, for example, Japanese Patent Publication No. 64-989
The technique described in Japanese Patent Publication No. 8 is known. In this method, the sand core prototype is composed of a binder mainly composed of sand, an acid-curable resin and an oxidizing agent.
Cured by sulfur dioxide.

[0004]

In the above-mentioned Hardox method, when sand formed into a desired shape is hardened to obtain a sand core prototype, it is hardened using sulfur dioxide, that is, sulfurous acid gas. Therefore, because sulfur dioxide is used,
The working environment is poor, and Japanese factories do not like the use of gases that can harm the human body. Also, even if sulfurous acid gas is used, it is very difficult to install ancillary equipment to prevent adverse effects on the human body and the working environment from deteriorating. Are also subject to laws and regulations.

[0005] Therefore, the present inventor has reviewed the merits of the shell mold method using a binder instead of the oxidizing agent and the sulfurous acid gas. In the shell mold method, instead of using a sulfur dioxide gas to solidify a mixture of sand and a binder to obtain a sand core prototype, a resin coated sand (RCS) previously coated with a phenolic synthetic resin such as phenolic resin is used. Then, it is blown with compressed air into a mold for sand core prototype molding, and is heated and cured to form a mold. However, in this case,
Even if the same coating agent as the coating agent that had been performed fairly well in the above-mentioned Hardox method was applied to the sand core prototype, the coating agent was repelled without getting wet,
It did not work.

On the other hand, the inventor of the present invention uses RCS to mold a sand core prototype using a shell mold method, and treats this sand core prototype with a mineral acid such as dilute sulfuric acid or phosphoric acid. Then, the surface of the dried sand core prototype is coated with a slurry-like coating agent consisting of a neutral aqueous dispersion mainly composed of powdered refractory, and then dried to form a collapsible sand core. He invented a method of obtaining it and filed a patent application. Although the collapsible sand core obtained by this method is effective as such, the coating is firstly applied in order to more reliably prevent the molten metal from being inserted into the collapsible sand core during high-pressure quiquist. An attempt was made to coat a second coating agent containing aluminum powder as described in JP-A-63-40639 on the surface of the coating layer. However, no good coating was obtained with this combination. That is, the acid remaining near the surface of the sand core prototype permeates through the first coating layer and oozes out to the second coating layer, and reacts with the aluminum powder which is a component in the second coating agent. . As a result, the second coating layer is bald or peeled.

[0007]

According to the present invention, RC is used.
A step of molding a sand core prototype using S, a step of treating the sand core prototype with an acid, a step of drying the sand core prototype treated with the acid, and a step of drying the dried sand core prototype. A step of washing with water, a step of treating the washed sand core prototype with an inorganic salt, a step of drying the sand core prototype treated with the inorganic salt, and a step of powdering the surface of the dried sand core prototype. A step of coating a slurry-like first coating agent comprising a neutral aqueous dispersion containing a refractory as a main component, a step of drying a sand core obtained by coating, and a step of drying the coated sand core. A collapsible sand core is obtained by a step of coating a slurry-like second coating agent containing aluminum powder and a step of drying the sand core obtained by coating.

As an acid for treating the sand core prototype, for example, a mineral acid such as sulfuric acid or phosphoric acid is used. When treating the sand core prototype with an acid, for example, the sand core prototype is immersed in an acidic solution such as dilute sulfuric acid, or the acidic solution is brushed or sprayed on the surface of the sand core prototype. Next, the sand core prototype treated with this acid is dried. Thereafter, the dried sand core prototype is washed with water, which simply involves immersing the sand core prototype in water or spraying water on the sand core prototype. Then, the washed sand core prototype is treated with an inorganic salt. In this case, the inorganic salts are Li ⁺ , Na ⁺ , K ⁺ , CS ⁺ , Cu ⁺ , Cu
²⁺ , Mg ²⁺ , Ca ²⁺ , Ba ²⁺ , Zn ²⁺ , A1
³⁺ , Mn ²⁺ , Fe ²⁺ , Fe ³⁺ , Co ²⁺ , Ni
Cations such as ²⁺ , NH ₄ ⁺ and F ⁻ , Cl ⁻ , Br
⁻ , I ⁻ , NO ₃ ⁻ , CO ₃ ²⁻ , SO ₄ ²⁻ , PO ₄
An anion such as ^3- is generated in a form that neutralizes the charge. For example, Na ₂ SO ₄ , K ₂ CO ₃ , MgCl
₂ , Ba ₃ (PO ₄ ) ₂ , Al ₂ (SO ₄ ) ₃ , MnC
l ₂ , FeSO ₄ , NH ₄ NO _{3 and the} like. When the sand core prototype obtained as described above is treated with an inorganic salt, for example, it is immersed in an inorganic salt solution such as an aqueous solution of BaCl ₂ , Al ₂ (SO ₄ ) ₃ , Is applied to the surface of the sand core prototype by brushing, spraying, or thinly applying a fine powder of inorganic salt.

[0009]

In the present invention, first, for example, a sand core prototype is formed using RCS as described above, and then the sand core prototype is immersed in a mineral acid solution such as dilute sulfuric acid. Mineral acid is impregnated inside the sand core prototype, and then the sand core prototype is dried. In this case, if the molded sand core prototype (ocher) is immersed in a mineral acid solution such as a dilute sulfuric acid solution, the mineral acid adheres to or penetrates into the surface layer of the sand core prototype. Swear. When this sand core prototype is pulled out of the immersion tank and dried at 80 to 200 ° C. for several minutes to 2 hours, the sand core prototype turns black-brown. This is because the phenolic synthetic resin such as hardened phenolic resin that bound the sand was dehydrated and carbonized by the mineral acid that had adhered to or penetrated the sand core prototype. In other words, the hardened calcinate synthetic resin that connects the sand to each other promotes the oxidative decomposition reaction by the heat during drying. As a result, the color becomes dark brown and the strength decreases. This decrease in strength is sufficient to withstand the pressing force during casting. At the time of casting, due to the heat of the molten metal, the hardened carboxylate synthetic resin naturally deteriorates by heat.
If the strength is reduced in advance as described above, the sand core can be extremely easily and easily removed from the casting after the casting.

[0010] By applying a slurry-like first coating agent comprising a neutral aqueous dispersion mainly composed of a powdered refractory to the sand core prototype obtained through such acid treatment and drying, Due to the mineral acid present on and near the surface of the sand core prototype, the first coating agent is instantaneously aggregated and is not repelled on the surface of the sand core prototype, but is thickened on the surface of the sand core prototype. If you can coat it and then dry it,
A homogeneous first coating layer of the desired thickness is formed. By the way, as described above, when the slurry-like second coating containing aluminum powder is applied on the first coating layer, the acid remaining near the surface of the sand core prototype becomes the first coating. It permeates through the layer and oozes out to the second coating layer and reacts with aluminum powder which is a component in the second coating agent. As a result, the second coating layer is bald or peeled. Therefore, the sand core prototype obtained through the above-described acid treatment and drying is immersed in water for several seconds to several minutes, for example, to remove the acid existing near the surface of the sand core.

By the way, if the first coating agent in the form of a slurry composed of a neutral aqueous dispersion mainly composed of a powdery refractory is to be applied in a state where the acid near the surface is removed, the acid treatment is not performed. Similarly to this, the first coating agent is repelled on the surface of the sand core prototype and cannot be coated thickly on the surface of the sand core prototype. Therefore, the sand core prototype washed with water as described above was replaced with BaCl ₂ ,
The inorganic salt is impregnated near the surface of the sand core prototype by immersing in an aqueous solution of an inorganic salt such as an aqueous solution of Al ₂ (SO ₄ ) ₃ .
Next, the sand core prototype is dried. Next, a slurry-like first coating agent composed of a neutral aqueous dispersion mainly composed of powdered refractory is coated on the surface of the sand core prototype obtained by drying after performing such treatment. I do. In this case, if the sand core prototype washed with water as described above is immersed in an inorganic salt aqueous solution such as a BaCl ₂ aqueous solution before coating, the inorganic salt adheres to the surface layer of the sand core prototype. Or soak. After the sand core prototype is pulled out of the dipping tank and dried at 80 to 200 ° C. for several minutes to 2 hours, the inorganic salt powder uniformly adheres to the surface layer of the sand core prototype.

The sand core prototype obtained through such acid treatment, drying, washing with water, inorganic salt treatment and drying is added to a first slurry-like neutral water dispersion mainly composed of a powdery refractory. By applying the coating agent of the above, the coating agent is instantaneously agglomerated by the inorganic salt present on and near the surface of the sand core prototype, and is not repelled on the surface of the sand core prototype.
The surface of the sand core prototype can be coated thickly. Thereafter, when this is dried, a uniform first coating layer having a desired thickness is formed.

[0013] In this way, the sand core prototype having the first coating layer formed thereon is coated with the slurry-like second coating agent containing aluminum powder. Next, when dried at room temperature to 200 ° C. for several minutes to 2 hours, a second coating layer containing aluminum powder is uniformly formed on the first coating layer. The second coating layer thus formed is stable for a long period of time without leaving the aluminum powder of the component no longer attacked by the acid even when left in the high humidity atmosphere as well as in the air. is there.

In this way, even in the shell mold method, the sand core prototype can be solidified, and the surface of the sand core prototype is made of a slurry made of a neutral aqueous dispersion mainly composed of a powdered refractory. The first coating agent can be easily and reliably coated in a desired state, and the slurry-like second coating agent containing aluminum powder can be reliably and easily applied on the first coating layer in a desired state. The second coating layer can be stably maintained for a long period of time.

When the disintegratable sand core obtained by the present invention is used, the sand core is not broken or cracked at the time of casting the molten metal under high pressure as in the case of high-pressure gquiast. The molten metal does not enter the sand core. In addition, when the molten metal solidifies after casting and the cast product is removed from the mold, the sand core is collapsed and removed, and the sand core can be easily collapsed and removed with little force. The sand can be sufficiently and reliably taken out to all corners without leaving sand at the corners of the casting surface. In other words, the acid treatment step and the subsequent drying step contribute to the dehydration and carbonization of the phenolic synthetic resin such as hardened phenolic resin, etc., in which the acid permeated inside the sand core has bound the sand, The washing step, the inorganic salt treatment step, and the subsequent drying step include a slurry-like first coating made of a neutral aqueous dispersion mainly composed of a powdery refractory and a slurry-like second coating containing aluminum powder. It contributes to ensuring that the coating is firmly formed.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS To manufacture a sand core prototype, first, a resin-coated sand (RCS) in which phenolic resin such as phenolic resin is coated on sand is prepared. RC
S is produced by a cold method, a semi-hot method, or a dry hot method based on the kneading temperature and properties of a phenolic resin such as a phenol resin, but the dry hot method is preferred in terms of productivity, stability, and cost. That is, after a solid resin is melt-coated on sand heated to 130 to 160 ° C. by a mixer, a hexamine aqueous solution as a hardening agent is used for a novolak resin, and only water is added for a resol resin. Also, when the adhesion of the sand particles is reduced while rapidly cooling by aeration, a wax such as calcium stearate is dispersed to obtain a dry free-flowing RCS. As the sand, silica sand, zircon sand, chromite sand, cerabeads, or the like, or their regenerated sand is used.

The RCS was blown together with pressurized air into a mold having a predetermined sand core-shaped cavity, and a sand core prototype was molded by a so-called shell mold method. In this case, the heating temperature of the core molding die is, for example, 200 to 300 ° C., preferably 230 to 270 ° C.
C. and heated for about 30 seconds to about 2 minutes to harden the sand core prototype to a predetermined strength. For example, bending strength 20-50k
g of sand core prototype was obtained.

Next, the sand core prototype thus formed is treated with an aqueous acid solution. Examples of the acid include mineral acids such as sulfuric acid and phosphoric acid. The sand core prototype is immersed in an aqueous solution of these acids, absorbed by the sand core prototype, and then dried by heating. The concentration of the aqueous solution is within 200 times the dilution ratio (98% concentrated sulfuric acid, 89% phosphoric acid dilution ratio). 20 dilution ratio
If it exceeds 0 times, the disintegration of the sand core after casting is reduced and the treatment effect is lost. The immersion time varies depending on the concentration of the treatment solution and the affinity between the sand core prototype and the treatment solution.
From a short time of seconds to about 5 minutes.

If the sand core prototype is difficult to get wet with the processing solution, the sand core prototype is immersed in a hydrophilic organic solvent such as methanol for a short time and then immersed in the processing solution, or the above-mentioned hydrophilicity is added to the processing solution. The organic solvent is mixed until the sand core prototype becomes wet with the processing solution before processing. Heat drying of the immersion-treated sand core prototype requires a shorter time as the temperature is higher, and is approximately 30 minutes at 120 ° C. as a guide. The acid may be used as it is without diluting, and the acid is concentrated sulfuric acid (98%).
In the case of a liquid such as phosphoric acid or phosphoric acid (89%), it is only necessary to crush and remove the sand core prototype. As described above, when a solution with a low concentration such as dilute sulfuric acid is used, drying is necessary to evaporate water. However, when not diluted with water, such as concentrated sulfuric acid, drying is not necessary. . The treated sand core prototype turned blackish brown, and its transverse rupture strength decreased in proportion to the treatment concentration. However, the decrease in strength is considered to be due to the progress of carbonization degradation by acid in the case of sulfuric acid or phosphoric acid.

Next, the sand core prototype that has been acid-treated and dried as described above is washed with water. In the water washing, the sand core prototype may be immersed in water or water may be sprayed on the sand core prototype, but the former is simple and reliable. That is, the sand core prototype is immersed in water such as tap water for several seconds to several minutes and then taken out. The washed sand core prototype may or may not be dried, but is then treated with an aqueous solution of an inorganic salt. As the inorganic salt, BaCl ₂ , Al ₂ (SO
₄ ) inorganic salts such as ₃ ;

The washed sand core prototype is immersed in an aqueous solution of these inorganic salts, absorbed near the surface of the sand core prototype, and then dried by heating. The concentration of the aqueous solution is within 200 times the dilution ratio. When the dilution ratio exceeds 200 times, the coating thickness of the first coating agent is too small, and the treatment effect is lost. The immersion time varies from a short time of 0.5 seconds to about 5 minutes, depending on the concentration of the treatment liquid and the affinity between the washed sand core prototype and the treatment liquid. Drying of the immersion-treated sand core prototype requires a shorter time as the temperature is higher, and is approximately 30 minutes at 120 ° C. as a guide. The inorganic salt may be used as it is without diluting, and the fine powder of the inorganic salt is applied to the washed sand core prototype, and excess inorganic salt fine powder is wiped off.
As described above, when a solution having a low concentration such as an aqueous solution of BaCl ₂ is used, drying is required to evaporate water. However, when the solution is not diluted with water, drying is not necessary.

Next, the surface of the sand core prototype treated as described above is coated with a first coating agent.
In this case, the sand core prototype may be immersed in the first coating agent, or the surface of the sand core prototype may be brushed or sprayed with the first coating agent. The first coating agent comprises fine powder silica and fine powder alumina as main components, and has a solid content of 5 with a small amount of colloidal silica added.
A slurry of 0 to 90% by weight was obtained. When the solid content is 50% by weight or less, the thickness of the first coating layer becomes thin, and when it is 90% by weight or more, it becomes extremely difficult to stir the slurry. The pH of the first coating agent was adjusted to 7.0 ±
If it is not maintained at 1.0, precipitation and solidification may occur even under stirring.

As the first coating agent, another coating agent can be used. For example, about 30 to 80% by weight of an inorganic refractory material such as graphite, mica, fused silica, alumina, magnesia, carbon black and zircon powder, and about 1 inorganic binder such as colloidal silica, alumina sol, clay and amine-treated bentonite. It is also possible to use water consisting of up to 25% by weight and water. In this case, more preferred are fused silica and colloidal silica. In addition, about 10% by volume of methanol and kaolin may be added.

In the first coating agent, acid treatment,
Heat drying, water washing, inorganic salt treatment, and heat drying are sequentially performed, and the treated sand core prototype is immersed for several seconds, and then heated and dried. Drying conditions are about 120 ° C. and about 10 minutes.
When the inorganic salt treatment is not performed, the first coating is repelled from the surface of the sand core prototype and can hardly be painted, but is sufficiently thick and penetrates into the interior of the sand core prototype. And the coating film is firm.

After finishing the first coating, a slurry-like second coating agent containing aluminum powder is coated. As the second coating agent, for example, 500 g of flaky aluminum powder, 10 g of sodium dodecylbenzenesulfonate as a wetting agent, and 1 g of octyl alcohol as a defoaming agent per 1 liter of a 3% water-soluble phenol resin solution. A well-stirred mixture, or a mixture thereof with mica, carbon black, zircon powder or the like can be used.
The second coating is performed by dipping the sand core prototype after the first coating in a second coating agent, brushing or spraying the surface of the sand core prototype with the second coating agent. After drying, dry to form. Drying conditions are about 120 ° C. and about 10 minutes.

An experimental example will be described below as a more detailed example. (Experimental Examples 1-4, and Comparative Examples 1-4) Flat Sand 10
Using RCS coated with 2 parts of phenolic resin (including hexamine curing agent) for 0 parts, weight of about 2 parts
A plurality of Kg sand core prototypes for engine blocks were formed by a shell mold method. Molding conditions were a mold temperature of 250 ° C., a blowing pressure of 0.8 kg / cm ² , and a heating time of 90 seconds.
The bending strength of the sand core prototype after standing for one day was 38 kg.

Next, 49 parts of water was added to 1 part of 98% concentrated sulfuric acid.
And 99 parts were mixed to prepare aqueous solutions having dilution ratios of 50 and 100 times, respectively. After the sand core prototype was immersed in this treatment liquid for 1 minute, it was dried in a circulating hot air heating furnace at 120 ° C. for 30 minutes. At this stage, the same first and second coatings as in the present experimental example were performed without performing any subsequent washing and inorganic salt treatment, and the results are shown in Table 1 below as Comparative Examples 1 and 3. Then, the diluted sand core prototype which had been treated with the diluted sulfuric acid was immersed in tap water for 10 seconds and washed with water. Similarly, at this stage, the same first and second coatings as those in this experimental example were performed without any subsequent inorganic salt treatment, and the results are shown in Table 1 below as Comparative Examples 2 and 4.

Thereafter, the water-washed sand core prototype was taken out, and 9 parts, 49 parts and 199 parts of water were mixed with 1 part of MgCl ₂ to obtain an aqueous solution having a dilution ratio of 10, 50 and 200 parts, which was obtained by mixing 1 to 2 parts. After immersion for 2 seconds, it was dried in a circulating hot air heating furnace at 120 ° C. for 30 minutes. (Experimental Examples 1-4)

The samples not subjected to the water washing and subsequent treatments (Comparative Examples 1 and 3), the samples not subjected to the treatment after the inorganic salt treatment (Comparative Examples 2 and 4), and the sand core prototypes subjected to the inorganic salt treatment and drying To the same first coating agent, respectively.
After immersion for 2 seconds, 10 minutes in a circulating hot air heating furnace at 120 ° C.
Dried for minutes. The composition of the first coating agent was prepared by suspending 3 parts of colloidal silica in 20 parts of water in 50 parts of fine powder silica and 30 parts of fine powder alumina, and was adjusted to pH 7.2. . Table 1 shows the bending strength of the sand core after drying.
Shown in

[0030]

[Table 1]

After finishing the first layer coating, a second layer coating was next performed. As the coating agent for the second layer, 500 g of scaly aluminum powder, 10 g of sodium dodecylbenzenesulfonate as a wetting agent, and 1 g of octyl alcohol as a defoaming agent were thoroughly mixed with 1 liter of a 3% aqueous phenol resin solution. A mixture was used. That is, the sand core prototype coated with the first layer was immersed in the second layer coating agent for 1 to 2 seconds, and then dried in a circulating hot air heating furnace at 120 ° C. for 10 minutes. The sand core prototype obtained as described above was left for 7 days in a thermo-hygrostat maintained at a temperature of 33 ° C. and a relative humidity of 90%. Table 1 shows the state of the second coating layer after 7 days.

The sand core obtained as described above was set in a mold, and the aluminum alloy ADC10 was cast at a casting pressure of 600 k.
g / cm ² , Gate speed 200 mm / Sec, Pouring temperature 7
High pressure casting was performed at 60 ° C. After casting, sand was removed using a conventional core knockout machine.
In cases 2 and 4, the core sand was completely removed, and an excellent cast product was obtained. In the case of Experimental Example 3, the removal of the core sand was slightly good, but in the case of Comparative Examples 1 to 4, the removal of the core sand was poor. The results are summarized in Table 1.

[0033]

As described above, according to the present invention, a step of molding a sand core prototype using a resin-coated sand coated with a carbonated synthetic resin, a step of treating the sand core prototype with an acid, Drying the sand core prototype treated with the acid, washing the dried sand core prototype with water, treating the washed sand core prototype with an inorganic salt, and treating with the inorganic salt A step of drying the sand core prototype, and a step of coating the surface of the dried sand core prototype with a slurry-like first coating agent comprising a neutral aqueous dispersion mainly composed of a powdered refractory. Drying the sand core obtained by the coating, coating the coated sand core with a slurry-like second coating agent containing aluminum powder, Since the collapsible sand core is manufactured by the step of drying the sand core, the sand core prototype is reduced to a desired strength in advance, and when the sand core is coated with the first coating agent, the first core material is removed. The coating agent forms a first coating layer of uniform and appropriate thickness without being repelled on the surface of the sand core prototype. When the first coating layer is coated with a second coating agent containing aluminum powder, the second coating layer maintains a stable state for a long time even under high humidity. Therefore, the sand core withstands high casting pressure during casting and has good disintegration properties after casting.

That is, when high-pressure casting such as die casting is performed using the collapsible sand core obtained in the present invention, the molten metal is not inserted into the sand core, and after casting, the product is When discharging sand, because of the good disintegration of the sand core,
Sand can be discharged easily, reliably and completely. Of course, no sand remains on the casting surface of the product after the sand is discharged, and the product is very smooth. Therefore, even if such a sand core is used for casting a product having a very complicated shape, for example, a cooling jacket portion of a closed-deck type engine block, a sufficiently satisfactory working condition is obtained. And the product can be obtained easily and reliably.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-52-125416 (JP, A) JP-B 3-14539 (JP, B2) JP-B-49-46449 (JP, B1) (58) Field (Int. Cl. ⁶ , DB name) B22C 9/10, 3/00, 13/08

Claims (1)

(57) [Claims] 1. A step of molding a sand core prototype using a resin-coated sand coated with a phenolic synthetic resin, a step of treating the sand core prototype with an acid, and a step of treating the acid with the acid Drying the sand core prototype with water, treating the washed sand core prototype with an inorganic salt, and drying the sand core prototype treated with the inorganic salt. Coating the surface of the dried sand core prototype with a slurry-like first coating agent composed of a neutral aqueous dispersion containing a powdered refractory as a main component; A step of drying the core, a step of coating the coated sand core with a slurry-like second coating agent containing aluminum powder, and a step of drying the coated sand core. A method for producing a disintegratable sand core.