JP2935393B2 - 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, by treating a sand core prototype molded with sand for shell mold with alkali, then washing with water, and further treating with an organic acid salt, the coating agent can be thickly coated only once. The present invention relates to a method for producing a sand core which is excellent in collapse property after casting and is suitable for high pressure casting. 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 It is formed to a uniform thickness with a predetermined thickness, uniformly, reliably and easily without being repelled from the surface of the sand core prototype, and over the entire surface layer of the sand core prototype. That is, it can 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 form a sand core prototype using a shell mold method, treats the sand core prototype with an alkaline aqueous solution such as an aqueous caustic soda solution, and then prepares the sand core prototype. To obtain a collapsible sand core by coating the surface of a dried sand core prototype with a slurry-like coating agent consisting of a neutral aqueous dispersion mainly composed of powdered refractory and drying it And applied for a patent. Although the collapsible sand core obtained by this method is effective to some extent, in order to more surely prevent the molten metal from being inserted into the collapsible sand core during high-pressure die-casting, the first coating is used. As a coating layer,
An attempt was made to coat the surface with a second coating agent containing aluminum powder as disclosed in the above-mentioned JP-A-63-40639. However, no good coating was obtained with this combination. That is, the alkali 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 aluminum powder as 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.
Forming a sand core prototype using S; treating the sand core prototype with an alkali; drying the sand core prototype treated with the alkali; and drying the dried sand core prototype. A step of washing, a step of treating the washed sand core prototype with an organic acid salt, a step of drying the sand core prototype treated with the organic acid salt, and a step of applying a powder to the surface of the dried sand core prototype. A collapsible sand core is obtained by a step of coating a slurry-like coating agent comprising a neutral aqueous dispersion mainly comprising a refractory in the form of a slurry, and a step of drying the sand core obtained by coating.

[0008] As an alkali for treating the sand core prototype,
For example, an alkali such as caustic soda or caustic potash is used. When the sand core prototype is treated with an alkali, for example, the sand core prototype is immersed in an inorganic alkali solution such as caustic soda aqueous solution, or an alkaline solution is brushed or sprayed on the surface of the sand core prototype. Next, the sand core prototype treated with the alkali 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 organic acid salt.

[0009] The organic acid salt used for treating the sand core prototype is as follows.
Salts formed between organic acids and inorganic ions. Formic acid,
Acetic acid, propionic acid, acrylic acid, methacrylic acid, oxalic acid, fumaric acid, maleic acid, succinic acid, tartaric acid, citric acid, butanetetracarboxylic acid, benzoic acid, phthalic acid, terephthalic acid, trimellitic acid, benzenesulfonic acid Anions of carboxylic acids such as sulfonic acid, toluenesulfonic acid and the like and organic acids such as sulfonic acid and Li ⁺ , Na ⁺ , K ⁺ , C
s ⁺ , Cu ⁺ , Cu ²⁺ , Mg ²⁺ , Ca ²⁺ , Ba
²⁺ , Zn ²⁺ , Al ³⁺ , Mn ²⁺ , Fe ²⁺ , Fe
An inorganic cation such as ³⁺ , Co ²⁺ , Ni ²⁺ , and NH ₄ ⁺ is generated in a form that neutralizes electric charge. Examples include potassium acetate, sodium acetate, magnesium acetate, calcium acetate, zinc acetate, barium acetate, magnesium acrylate, calcium acrylate, zinc acrylate, aluminum acrylate, sodium benzoate and the like. When treating the sand core prototype with an organic acid salt, for example, immersing it in an organic acid salt solution such as an aqueous solution of calcium acetate, brushing the organic salt solution on the surface of the sand core prototype, , Spraying or dusting with fine powder of organic acid salt.

[0010]

In the present invention, first, for example, a sand core prototype is formed by using RCS as described above, and then the sand core prototype is immersed in an alkaline solution such as aqueous caustic soda. Alkaline is impregnated into the inside of the child prototype, and then the sand core prototype is dried. In this case, if the molded sand core prototype (ocher) is immersed in an alkaline solution such as caustic soda aqueous solution, the alkali adheres to or penetrates into the surface layer of the sand core prototype.
After lifting the sand core prototype from the immersion tank,
After drying at 0 ° C for several minutes to 2 hours, the sand core prototype turns reddish brown. This is because the phenolic synthetic resin such as hardened phenolic resin that bound the sand was oxidized and decomposed by the alkali adhering to or infiltrating 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. This thermal oxidative decomposition reaction is thought to be the formation of a hydroperoxide structure at the methylene group and the methine group of the cured phenolic resin, followed by the decomposition of the unstable hydroperoxide structure. This also reduces the strength. This decrease in strength is sufficient to withstand the pressing force during casting. Naturally, the hardened carboxylate-based synthetic resin is thermally deteriorated by the heat of the molten metal during casting. However, if the strength is reduced in advance as described above, sand from the casting removed from the mold after casting can be obtained. When removing the core, the sand core can be removed very easily and easily.

[0011] By applying a slurry-like first coating agent composed of a neutral aqueous dispersion mainly composed of a powdery refractory to the sand core prototype obtained through such alkali treatment and drying, The first coating agent is instantaneously agglomerated by the alkali present on the surface of the sand core prototype and near the surface, and is not thickly repelled on the surface of the sand core prototype. After that, if it is dried, a uniform first coating layer having a 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 alkali 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 alkali treatment and drying is immersed in water for several seconds to several minutes, for example, to remove the alkali present near the surface of the sand core.

By the way, even if it is attempted to apply a slurry-like first coating agent comprising a neutral aqueous dispersion mainly composed of a powdery refractory with the alkali near the surface removed, the alkali treatment As in the case where no sand core is used, 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. Thus, the sand core prototype washed with water as described above is immersed in an organic acid salt aqueous solution such as an aqueous solution of calcium acetate to infiltrate the organic acid salt into the sand core prototype. Dry the prototype. Next, a slurry-like coating agent consisting of a neutral aqueous dispersion mainly composed of a powdery refractory is coated on the surface of the sand core prototype obtained by treating with an organic acid salt and drying. -Run. In this case, if the sand core prototype washed with water as described above is immersed in an aqueous solution of an organic acid salt such as an aqueous solution of calcium acetate before coating, the organic acid is present on the surface layer and inside the sand core prototype. Salt adheres or soaks. When the sand core prototype is pulled out of the immersion tank and then dried at 80 to 200 ° C. for several minutes to 2 hours, the organic acid salt powder uniformly adheres to the surface layer of the sand core prototype.

A slurry coating made of a neutral aqueous dispersion mainly composed of a powdery refractory is applied to the sand core prototype obtained through such alkali treatment, drying, water washing, organic acid salt treatment and drying. When the agent is applied, the coating agent is instantaneously aggregated by the organic acid salt present on and near the surface of the sand core prototype, and is not repelled on the surface of the sand core prototype. Can be coated thickly on the surface. Thereafter, when this is dried, a homogeneous coating layer having a desired thickness is formed.

[0014] In this manner, the slurry-like second coating agent containing aluminum powder can be coated on the sand core prototype having the coating layer formed thereon. In that case, then at room temperature to 200 ° C for several minutes to
After drying for 2 hours, a second coating layer containing aluminum powder is uniformly formed on the first coating layer. The second coating layer formed in this way is stable for a long period of time, even when left in the air or in a high-humidity atmosphere, so that its aluminum powder is no longer attacked by alkali. is there.

In this way, even in the shell mold method, the sand core prototype can be solidified, and a slurry formed of a neutral water dispersion mainly composed of powdered refractory is formed on the surface of the sand core prototype. Can easily and reliably be coated in a desired state. When the collapsible sand core obtained according to 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 die casting, and the molten metal can be sanded. There is no intrusion into the 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 alkali treatment step and the subsequent drying step contribute to the oxidative decomposition of the phenolic synthetic resin, such as hardened phenolic resin, that bound the sand by the alkali infiltrated into the sand core. The organic acid salt treatment step and the subsequent drying step contribute to assuredly and firmly forming a slurry-like coating composed of a neutral aqueous dispersion mainly composed of a powdery refractory.

[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 curing agent for novolak resin, and only water for resol resin, are charged to evaporate latent heat of water and A wax such as calcium stearate is dispersed at the time when the adhesion between the sand particles is reduced while rapidly cooling by aeration, to obtain a dry-state 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 alkali solution. Examples of the alkali include inorganic alkalis such as sodium hydroxide and sodium hydroxide. The sand core prototype is immersed in an aqueous solution of these alkalis, absorbed by the sand core prototype, and then dried by heating. The concentration of the aqueous solution is within 200 times the dilution ratio. If the dilution ratio exceeds 200 times, the disintegration of the sand core after casting decreases,
Processing effect is lost. The immersion time varies depending on the concentration of the processing solution and the affinity between the sand core prototype and the processing solution.
It is from a short time of 0.5 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 alkali may be used as it is without diluting. If the alkali is a fine powder such as caustic soda or causal, spray the powder on the sand core prototype and wipe off the excess powder. As described above, when a solution having a low concentration such as an aqueous solution of sodium hydroxide is used, drying is required to evaporate water. However, when the solution is not diluted with water, drying is not necessary. The bending strength of the treated sand core prototype decreases in proportion to the treatment concentration.

Next, the sand core prototype which has been subjected to the alkali treatment 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 organic acid salt.

As the organic acid salt, calcium acetate,
Examples include zinc acetate and aluminum acrylate. The washed sand core prototype is immersed in an aqueous solution of these organic acid salts, absorbed by the sand core prototype, and then dried by heating. The concentration of the aqueous solution is within 200 times the dilution ratio. Dilution ratio is 2
If it exceeds 00 times, the coating thickness is too thin and the processing effect is lost. The immersion time varies depending on the concentration of the treatment solution and the affinity between the washed sand core prototype and the treatment solution.
From a short time of 5 seconds to about 5 minutes.

If the sand core prototype is difficult to 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. The hydrophilic organic solvent is mixed and processed until the sand core prototype becomes wet with the processing solution. The heating and 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 organic acid salt may be used as it is without diluting. The fine powder of the organic acid salt is applied to the washed sand core prototype, and excess organic acid salt fine powder is wiped off. As described above, when a solution having a low concentration such as an aqueous solution of calcium acetate 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 coating agent. In this case, the sand core prototype may be dipped in the coating agent, or the surface of the sand core prototype may be brushed or sprayed with the coating agent. The coating agent was a slurry having a solid content of 50 to 90% by weight containing fine powder silica and fine powder alumina as main components and a small amount of colloidal silica. If the solid content is less than 50% by weight, the thickness of the coating layer becomes thin, and if it exceeds 90% by weight, it becomes extremely difficult to stir the slurry. If the pH of the coating agent is not maintained at 7.0 ± 1.0, precipitation and solidification may occur even under stirring.

It should be noted that other coating agents can be used as the coating agent. 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.

An alkali treatment in the coating agent,
Heat drying, washing with water, treatment with an organic acid salt, and then immersing the heat-dried sand core prototype for a few seconds, followed by heat drying. Drying conditions are about 120 ° C. for about 10 minutes. In the case where the coating with an organic acid salt is not performed, the coating is repelled from the sand core prototype and is hardly coated. On the other hand, the coating is sufficiently thick and has little penetration into the sand core prototype. The membrane is firm. The coating layer may be a single layer, but the product and the coating
In order to improve the releasability from the tinting layer, two layers are more preferable. As a coating agent for forming the second coating layer, for example, 500 g of mica powder per liter of a 3% aqueous phenol resin solution, and sodium dodecylbenzenesulfonate 10 g as a lubricant As an antifoaming agent, octyl alcohol (1 g) obtained by well stirring and mixing can be used. This coating is performed by immersing the sand core prototype after finishing the first layer coating in the second layer coating agent, or brushing the surface of the sand core prototype with the second layer coating agent. After painting or spraying, it is dried to form.

As another example of the coating agent for the second layer, for example, 500 g of flaky aluminum powder, 10 g of sodium dodecylbenzenesulfonate as a wetting agent per liter of a 3% aqueous phenol resin solution, 1 g of octyl alcohol as a defoaming agent, mixed well, mixed with mica, carbon black,
What mixed zircon powder etc. can be used. This second layer coating is performed by immersing the sand core prototype after the first layer coating in the second layer coating agent, or brushing the surface of the sand core prototype with the second layer coating agent. After dripping or spraying, 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 the curing agent hexamine) per 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, 1 part of caustic soda was filled with water for 49 hours each.
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 aqueous solution 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 water washing and organic acid salt treatment, and are shown in Table 1 below as Comparative Examples 1 and 3. . Next, the sand core prototype that had been treated with an aqueous solution of sodium hydroxide and dried was dipped in tap water for 10 seconds and washed with water. Similarly, at this stage, the same first and second coatings as those of the present experimental example were performed without any subsequent treatment with an organic acid salt, and the results are shown in Table 1 below as Comparative Examples 2 and 4. .

Thereafter, the washed sand core prototype was taken out, and 9 parts, 49 parts, and 19 parts of water were added to 1 part of calcium acetate.
The mixture was immersed in an aqueous solution having a dilution ratio of 10, 50, or 200 parts obtained by mixing 9 parts for 1 to 2 seconds, and then 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 treatment or later (Comparative Examples 1 and 3), the samples not subjected to the treatment after calcium acetate (Comparative Examples 2 and 4), and the sand core prototypes treated with calcium acetate and dried. With the same first code
After being immersed for 1 to 2 seconds in a heating agent, it was dried in a circulating hot air heating furnace at 120 ° C. for 10 minutes. The composition of the first coating agent is obtained 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.
The pH was adjusted to 7.2. The bending strength of the dried sand core is shown in Table 1.

[0031]

[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. When aluminum acrylate was used as the organic acid salt instead of calcium acetate, and the other treatment and operation were performed in exactly the same manner, excellent results were obtained as in the case of calcium acetate.

[0034]

As described above, in the present invention, a step of molding a sand core prototype using a resin-coated sand coated with a carboxylate synthetic resin, and treating the sand core prototype with an alkali. A step of drying the sand core prototype treated with the alkali; a step of washing the dried sand core prototype with water; a step of treating the washed sand core prototype with an organic acid salt; A step of drying the sand core prototype treated with an acid salt, and applying a slurry-like coating agent comprising a neutral aqueous dispersion mainly composed of a powdered refractory to the surface of the dried sand core prototype. Since the coating step and the sand core obtained by the coating are dried to produce a collapsible sand core by the step, the sand core prototype is previously reduced to a desired strength. And coating with the first coating agent. To time, co - coating agent without being repelled by the surface of the sand core prototype, co uniform and appropriate thickness - forming the coating layer. When the first coating layer is coated with a second coating agent containing aluminum powder, the second coating layer maintains a stable state even under high humidity for a long time. maintain. Therefore, the sand core withstands a 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, no molten metal is inserted into the sand core, and after the casting, the product is removed from the product. 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.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-63-171245 (JP, A) JP-A-56-139256 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) B22C 9 / 10,1 / 22 B22C 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 alkali, and a step of treating the alkali with the alkali Drying, washing the dried sand core prototype with water, treating the washed sand core prototype with an organic acid salt, and drying the sand core prototype treated with the organic acid salt And a step of coating the surface of the dried sand core prototype with a slurry-like coating agent comprising a neutral aqueous dispersion mainly composed of a powdered refractory, and a step of coating the sand core obtained by the coating. Drying the collapsible sand core.