JP2929582B2 - 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 the sand core prototype formed by the warm box method with a mineral acid, then washing with water, and further treating with an inorganic salt, the first coating agent can be coated thickly only once. The present invention relates to a method for producing a sand core suitable for high-pressure casting, in which the coating layer can be firmly formed even when a second coating agent containing aluminum powder is coated thereon, and has excellent disintegration properties after casting. It is. 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 warm box method in which a curing agent is used instead of the oxidizing agent and the sulfurous acid gas. In the warm box method, sulfur dioxide gas is not used to solidify a mixture of sand and a binder to obtain a sand core prototype, but, for example, in a mold for a sand core prototype heated to 90 to 240 ° C. Then, a mixture of sand and a binder is blown with compressed air to be heated and cured to form a mold. However,
In this case, even if the same coating agent as the coating agent, which has been performed fairly well in the above-mentioned Hardox method, is applied to the sand core, the coating agent penetrates into the sand core prototype and has a sufficient thickness. No coating layer was obtained.

On the other hand, the inventor of the present invention formed a sand core prototype by using a warm box method using sand core aggregate such as silica sand or zircon sand and an organic binder such as furan resin. After treating the core prototype with a mineral acid such as dilute sulfuric acid, phosphoric acid, etc., it is dried on a sand core prototype to form a slurry-like slurry consisting of a neutral aqueous dispersion mainly composed of powdered refractory. A method for obtaining a collapsible sand core by coating a coating agent and drying it was invented 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. 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, sand,
A step of molding a sand core prototype using a furan-based resin and a curing agent for heat curing the resin, a step of treating the sand core prototype with an acid, and a step of drying the sand core prototype treated with the acid Washing the dried sand core prototype with an inorganic salt, treating the washed sand core prototype with an inorganic salt, drying the sand core prototype treated with the inorganic salt, and drying the dried sand core prototype. A step of coating the surface of the sand core prototype with a slurry-like first coating agent comprising a neutral aqueous dispersion mainly composed of a powdered refractory, and drying the sand core obtained by this coating. A collapsible sand core is obtained by 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. .

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⁺ , Al
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, as described above, a sand core prototype is formed using sand, a furan-based resin, and a curing agent for heat curing the resin, and then the sand core prototype is diluted with sulfuric acid or the like. Then, the mineral acid is impregnated into the sand core prototype by immersing it in a mineral acid solution, and then the sand core prototype is dried.
In this case, if the molded sand core prototype (ocher, brown to dark green) is immersed in a mineral acid solution such as a dilute sulfuric acid solution, the mineral acid adheres to the surface layer and inside of the sand core prototype. Or soak. When this sand core prototype is pulled out of the dipping tank and dried at 80 to 200 ° C. for several minutes to 2 hours, the sand core prototype turns black. This is because the hardened furan resin that bound the sand was dehydrated and carbonized by the mineral acid that had adhered to or penetrated the sand core prototype. That is, the cured furan resin that connects the sand to each other accelerates the carbonolysis reaction by the heat during drying. This gives
Blackening and strength decrease. This decrease in strength is sufficient to withstand the pressing force during casting. Naturally, the hardened furan resin is thermally degraded by the heat of the molten metal during casting. However, if the strength is reduced in advance as described above, the sand core can be removed from the cast product removed from the mold after casting. The sand core can be removed very easily and easily.

[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, The surface of the sand core prototype is instantaneously agglomerated by the mineral acid present on and near the surface of the sand core prototype, and does not penetrate deep inside the sand core prototype. When the first coating layer having a desired thickness is formed, a uniform first coating layer having a desired thickness can be formed. By the way, as described above, the slurry-like second slurry containing aluminum powder is formed on the first coating layer.
When the coating is applied, 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 aluminum as a component in the second coating agent is removed. Reacts with powder. as a result,
The second coating layer balds or peels. 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 penetrates into the inside 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 BaC
The inorganic salt is impregnated near the surface of the sand core prototype by immersing it in an inorganic salt aqueous solution such as l ₂ , Al ₂ (SO ₄ ) ₃ aqueous solution, and then the sand core prototype is dried. Next, on the surface of the sand core prototype obtained by drying after such treatment,
A slurry-like first coating agent comprising a neutral aqueous dispersion mainly composed of a powdery refractory is coated. 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. When the coating agent is applied, the coating agent instantaneously agglomerates due to the inorganic salts existing on and near the surface of the sand core prototype, and does not penetrate deep inside the sand core prototype. Thick coating can be applied to the surface of the core prototype. 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 warm box 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 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 collapsible 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, unlike high pressure die casting. 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 hardened furan resin that has bound the sand by the acid soaked in the sand core. In the subsequent drying step, a first slurry-like coating composed of a neutral aqueous dispersion mainly composed of a powdery refractory and a second slurry-like coating containing aluminum powder are firmly formed. Contribute to make.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS When a sand core prototype is manufactured, first, a furan resin and a curing agent for heat curing the resin are mixed with sand core aggregate such as core sand. Sand core aggregate is silica sand, zircon sand, chromite sand, cerabeads, etc. Furfuryl-based resin is furfuryl alcohol / formaldehyde resin, furfuryl alcohol / urea / formaldehyde resin, furfuryl alcohol / phenol / formaldehyde Resin, furfuryl alcohol / phenol /
A so-called warm box furan resin such as urea / formaldehyde resin is used. In addition, as a curing agent for the resin heat curing, benzenesulfonic acid, phenolsulfonic acid, toluenesulfonic acid, xylenesulfonic acid,
Use is made of a salt of at least one of lower alkyl sulfonic acids and at least one of aluminum, copper, zinc, iron and ammonium. Further, a small amount of copper chloride, zinc chloride, iron chloride or the like may be used in combination as a curing accelerator.

A mixture of these components is blown together with pressurized air into a mold having a predetermined sand core-shaped cavity, for example, by a method called a warm box method. Was molded. In this case, the heating temperature of the core molding die is, for example, 90 to 240 ° C., preferably about 90 to 200 ° C., and is heated for about 1 minute to harden the sand core prototype to a predetermined strength. . For example, bending force 2
A sand core prototype of 0 to 50 kg 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. 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 it. If the acid is a liquid such as concentrated sulfuric acid (98%) or phosphoric acid (89%), it is only necessary to immerse 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 turns black, and its bending strength decreases in proportion to the treatment concentration.
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 ). And the like.

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.
The thickness of the first coating is almost 0 mm when the inorganic salt treatment is not performed and almost penetrates into the sand core prototype. On the other hand, the first coating is sufficiently thick and has little penetration into the sand core prototype. Moreover, the coating 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
0 parts, 1.5 parts of furan resin, 0.6% of a commercially available curing agent mainly containing p-toluenesulfonic acid salt as a curing agent for furan resin (FC-100 manufactured by Kao Quaker, copper salt of p-toluenesulfonic acid as main component) Parts and commercial additives (Kao Quaker J-20, main component silane compound)
0.06 parts were mixed, and a plurality of sand core prototypes for an engine block having a weight of about 2 kg were formed by a warm box method.
Molding conditions: mold temperature 120 ° C, blowing pressure 4.5kg / c
m ² , and the heating time was 90 seconds. After standing for one day, the sand core prototype had a transverse rupture strength of 35 kg.

Next, 39 parts of water were added to 1 part of 98% concentrated sulfuric acid.
And 79 parts were mixed to prepare aqueous solutions having a dilution ratio of 40 and 80 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, in the present invention, the step of molding a sand core prototype using sand, a furan-based resin, and a curing agent for heat curing the resin, and treating the sand core prototype with an acid. And drying the sand core prototype treated with the acid.
A step of washing the dried sand core prototype 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 drying the dried sand core prototype. A step of coating the surface of the child prototype with a slurry-like first coating agent comprising a neutral aqueous dispersion mainly composed of a powdery refractory, and a step of drying the sand core obtained by this coating; 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 to produce a collapsible sand core. As a result, the sand core prototype is reduced to a desired strength in advance, and when coating with the first coating agent, the first coating agent does not penetrate into the sand core prototype and is evenly distributed. In forming the first coating layer of a suitable thickness. 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 at a high temperature. 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 operation can be performed. The condition and product can be obtained easily and reliably.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁶ , DB name) B22C 9/10, 3/00

Claims (1)

(57) [Claims] 1. A step of molding a sand core prototype using sand, a furan-based resin, and a curing agent for heat curing the resin, a step of treating the sand core prototype with an acid, and a step of treating the sand with the acid. Drying the core prototype, washing the dried 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. And a step of 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. Drying the coated sand core, coating the coated sand core with a slurry-like second coating agent containing aluminum powder, and drying the coated sand core. Made of collapsible sand core Construction method.