JP2000102841A - Resin coated sand grain for shell mold - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a resin coated sand grain for a shell mold high in mold strength and further less in formaldehyde emission. SOLUTION: This resin coated sand grain for a shell mold comprises a sand grain and a resin coated layer which is composed of a thermosetting resin being coated on the surface of the sand grain. This resin coated layer includes a carboxylic acid hydrazide compound or additionally a nitrogen-containing compound such as urea, etc. By containing the carboxylic acid hydrazide compound, a mold strength is increased, and thus the sand grain having a high mold strength and less formaldehyde emission is obtained by containing both carboxylic acid hydrazide compound and nitrogen containing compound.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resin-coated sand particle used in a shell mold method, and more particularly to a resin-coated sand particle for a shell mold which has improved mold strength.

[0002]

2. Description of the Related Art A shell molding method conventionally known as a kind of a molding method is a method of forming resin-coated sand particles (Resin resin).
Coated Sand (hereinafter abbreviated as RCS) is filled in a heated mold, the resin coated on the sand particles is melted to bond the sand particles, and the resin is further cured by heating. Is released from the mold. The resin-coated sand particles used in the shell molding method are composed of sand particles made of silica sand or the like, and a resin coating layer as a binding material made of a thermosetting resin such as a novolac phenol resin coated on the surface of the sand particles. ing.

As means for improving the strength of the RCS template, there is a disclosure of mixing a phenolic resin with a silane coupling agent and an amide having a relatively low molecular weight such as formamide, dimethylformamide, and acetamide (Japanese Patent Publication No. 3-77018). No.). However, the improvement in strength in this case is limited to about 10%, and there is no further improvement in strength even if the added amount is increased. Further, the effect of improving the strength is due to the synergistic effect of the silane coupling agent and the amide compound, and the amount of the expensive silane coupling agent cannot be reduced.

As another method for improving the strength of a mold, a method of adding an aromatic carboxylic acid such as benzoic acid or salicylic acid is known. However, aromatic carboxylic acids generate a strong pungent odor at high temperatures, and thus have a problem of deteriorating the working environment of mold making. Further, in this case, there is a problem that the melting point of the coating resin layer and the fusion point of RCS are lowered. Further, when the mold is formed by the shell mold method, the mold temperature is about 250 to 350 ° C. At this time, an odor is generated,
This is a cause of the deterioration of the working environment in the mold making factory. The cause of this bad smell is a novolak-type phenol resin used as a binder for resin-coated sand particles. That is, it is considered that a bad odor is generated due to volatilization of low molecular components contained in the novolak type phenol resin or generation of formaldehyde and ammonia due to thermal decomposition of hexamethylenetetramine as a curing agent. In particular, low molecular components of phenols such as free phenol are easily perceived by smell even at a low concentration, and are a major cause of bad smell.

[0005] For this reason, in recent years, in consideration of the improvement of the working environment in mold molding, a novolak type phenol resin in which the content of free phenol is reduced as much as possible has been used as a binder for resin-coated sand particles (particularly). Fairness 7-6
No. 4905). On the other hand, as a formaldehyde reduction technique, the addition of a nitrogen-containing compound such as urea or aminophenol has an effect of reducing free aldehyde generated from RCS. However, in this case RC
The decrease in the warm strength of S is large and not practical.

Although it is possible to reduce the amount of formaldehyde generated by reducing the amount of hexamine to be added, the warm strength of the resulting mold is significantly reduced also in this case. Further, in the RCS using a resol type phenol resin not using hexamine as a curing agent, the amount of formaldehyde can be greatly reduced, but the use at room temperature is greatly reduced, so that the application is greatly restricted.

JP-A-10-36681 discloses a deodorant resin composition containing a hydrazide compound as a deodorant for formaldehyde odor. This resin composition has been shown to be effective as a deodorant substance,
When a hydrazide compound is added to S, the effect of suppressing formaldehyde is somewhat recognized, but the effect is not sufficient.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and a first object is to provide an RC
An object of the present invention is to provide shell-molded resin-coated sand particles that improve the mold strength of S. And the further challenge is RCS
It is an object of the present invention to provide shell-molded resin-coated sand particles having improved mold strength and reduced formaldehyde generation.

[0009]

Means for Solving the Problems The present inventor first studied an additive effective for improving the strength of the mold, and discovered that a carboxylic acid hydrazide compound was effective as this additive, and completed the first invention of the present invention. It was done. In addition, the present inventor has proposed that at least one type of nitrogen selected from urea and thioureas, aromatic compounds such as aminophenol, phenylhydrazine and aniline, amides such as acetamide and acetanilide, and semicarbazone together with the carboxylic acid hydrazide compound. The present inventors have found that the addition of the containing compound can improve the strength of the mold and also suppress the generation of formaldehyde, and have completed the second invention of the present invention.

[0010] That is, the resin-coated sand particles for shell mold of the first invention is a resin-coated sand particle comprising a sand particle and a resin coating layer made of a thermosetting resin coated on the surface of the sand particle. Contains a carboxylic acid hydrazide compound. The resin-coated sand particles for shell molding of the first invention can produce a mold having high strength. The amount of the carboxylic acid hydrazide compound to be compounded in the resin coating layer of the shell-coated resin-coated sand particles is as follows:
0.1 to 10 parts by weight is preferable for 100 parts by weight of the thermosetting resin.

[0011] The resin-coated sand particles for a shell mold of the second invention is a resin-coated sand particle comprising a sand particle and a resin coating layer made of a thermosetting resin coated on the surface of the sand particle.
The resin coating layer, carboxylic acid hydrazide compound and urea and thioureas, aminophenol, phenylhydrazine, aromatic compounds such as aniline, acetamide, amides such as acetanilide, semicarbazone,
And at least one nitrogen-containing compound selected from the group consisting of: The resin-coated sand particles for shell molding of the second invention can produce a mold having high strength and can effectively suppress the generation of formaldehyde generated during molding of the mold. The amounts of the carboxylic acid hydrazide compound and the nitrogen-containing compound incorporated in the resin coating layer of the shell-coated resin-coated sand particles are 0.01 to 0.5% by weight and 0.01 to 0% by weight, respectively, based on 100 parts by weight of the sand particles. It is preferred to be 0.5% by weight.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The resin-coated sand particles for shell molding of the first and second inventions comprise sand particles and a resin coating layer made of a thermosetting resin coated on the surface of the sand particles. This resin coating layer is formed by blending a carboxylic acid hydrazide compound or a carboxylic acid hydrazide compound and a specific nitrogen-containing compound at the time of producing a thermosetting resin, or when coating sand particles with the thermosetting resin. I have. As a result, the shell-coated resin-coated sand particles can increase the strength of the mold, or can suppress the amount of formaldehyde generated during the molding of the mold as well as the strength of the mold.

The mechanism by which the resin-coated sand particles of the present invention increase the mold strength is not clear, but it is assumed that the --CONHNH ₂ bond of the carboxylic hydrazide compound promotes the curing of a thermosetting resin, for example, a phenol resin. Is done. The sand particles used for the resin-coated sand particles for shell molding of the present invention are not particularly limited, but silica sand (Si) is used similarly to the conventional resin-coated sand particles for shell molding.
O ₂ ), chromite sand, zircon sand and the like can be used.

Although the thermosetting resin forming the resin coating layer of the present invention is not particularly limited, it is preferable to use a phenol resin. In consideration of the improvement of the working environment in mold making, it is preferable to use a novolak-type phenol resin in which the content of free phenol is reduced as much as possible (a phenol resin having a free phenol content of 1% by weight or less). In addition, a resol type phenol resin can be used. Even in this case, improvement in mold strength can be expected. Although the resin-coated sand particles for shell mold of the present invention increase the strength of the resulting mold, the melting point and R
This has the advantage of not lowering the fusion point of CS.

Therefore, the resin-coated sand particles for shell molding of the present invention can reduce the amount of expensive silane coupling agents usually added in order to increase the mold strength of the resin-coated sand particles for conventional shell molding. Become. Further, by increasing the amount of the carboxylic acid hydrazide compound added, there is an advantage that the strength of the mold can be expected to be improved by about 30% or more as compared with the conventional product. By improving the mold strength, the resin-coated sand particles for a shell mold of the present invention can be used as a mold having a complicated shape, a thin mold, and a non-ferrous metal mold.

[0016] The resin-coated sand particles for a hot box can also improve the warm strength. The addition amount of the carboxylic acid hydrazide compound contained in the resin-coated sand particles for the shell mold is desirably in the range of 0.1 to 10% by weight based on the resin content, and when less than 0.1% by weight, the effect of addition is not exhibited. On the other hand, if it exceeds 10% by weight, the amount of gas generated during the molding of the mold is increased and the cost is increased.

Examples of the carboxylic acid hydrazide compound include formic hydrazide, acetic hydrazide, propionic hydrazide, salicylic hydrazide, butyric hydrazide,
Monobasic acid hydrazides such as benzoic acid hydrazide and hydroxybenzoic acid hydrazide are oxalic acid dihydrazide, malonic acid dihydrazide, succinic dihydrazide, adipic dihydrazide, sebacic dihydrazide, maleic dihydrazide, fumaric dihydrazide, malic acid dihydrazide, tartaric acid dihydrazide, and malate dihydrazide. Phthalic dihydrazide, isophthalic dihydrazide, terephthalic dihydrazide,
And dihydrazide of dibasic acid such as dimer acid dihydrazide.

The carboxylic acid hydrazide compound and urea and thiourea,
Addition of aromatic compounds such as aminophenol, phenylhydrazine, aniline, etc., amides such as acetamide, acetanilide, and at least one nitrogen-containing compound selected from semicarbazones in combination with mold molding without impairing mold strength. The amount of formaldehyde generated at the time can be reduced.

As described above, the carboxylic acid hydrazide-based compound has a slightly lower formaldehyde reduction efficiency, but has the advantage of improving the mold strength. The nitrogen-containing compound described above has a high formaldehyde reduction efficiency, but has a disadvantage of reducing the strength of the mold. Therefore, when the carboxylic acid hydrazide compound and the nitrogen-containing compound are added together in a specific amount range, the respective disadvantages can be compensated and an excellent effect can be exhibited.

The addition of the carboxylic acid hydrazide and the nitrogen-containing compound can be carried out when producing the resin for the resin coating layer or when forming the resin coating layer on sand particles. In particular, by adding carboxylic acid hydrazide and a nitrogen-containing compound when forming a resin coating layer on sand grains to form RCS, the amount of formaldehyde generated during mold making is reduced, thereby improving the working environment.

Examples of the nitrogen-containing compound include urea, thiourea, phenylurea, O-aminophenol, m-aminophenol, p-aminophenol, phenylhydrazine, p-nitrophenylhydrazine, 2,4-dinitrophenylhydrazine, aniline , Toluidine, acetanilide, semicarbazone, acetamide, formamide N-methylformamide, N, N-dimethylformamide, and the like. In particular, urea, aminophenol and the like enhance the effect of suppressing formaldehyde and have a small effect on the reduction in mold strength, and are preferred. .

The amount of the carboxylic acid hydrazide compound is preferably in the range of 0.01 to 0.5% by weight based on 100 parts by weight of the sand particles. If it is less than this range, the effect cannot be exhibited, and if it is too large, a problem due to gas generation at the time of casting occurs, which is not preferable. The nitrogen-containing compound is 0.01 to 0.5
A range of weight% is preferred. If it is less than this range, the effect is not exhibited, and if it is more than this range, the strength of the mold is undesirably reduced.

In the present invention, it is preferable to add calcium stearate which has been conventionally used for improving the lubricity of resin-coated sand particles. In order to produce the resin-coated sand particles for a shell mold of the present invention, carboxylic acid hydrazide or a mixture of carboxylic acid hydrazide and a nitrogen-containing compound in a resin may be used together with a usual additive. Alternatively, carboxylic acid hydrazide or a carboxylic acid hydrazide and a nitrogen-containing compound may be added together with a lubricant such as calcium stearate when forming the resin-coated sand particles. At this time, it can be manufactured without complicating the operation by mixing and adding to ordinary additives.

[0024]

EXAMPLES Specific examples of the resin-coated sand particles for shell molding of the present invention will be described below. (Resin Production Example 1) [Production of Novolak-Type Phenolic Resin] First, a phenol resin was prepared as a binder by the following method. 100 parts by weight of phenol, 52 parts by weight of 37% formalin, and 0.5 parts by weight of oxalic acid were charged into a reaction vessel equipped with a stirrer, a heating device, and a cooling tube, and reacted under stirring for 3 hours under reflux. Next, the reaction solution is heated and dehydrated under reduced pressure,
When the temperature of the reaction solution reaches 160 ° C., 2 parts by weight of ethylenebisstearamide, 3 parts by weight of adipic dihydrazide and 1 part by weight of aminotriethoxysilane are added, mixed and cooled, and the carboxylic acid hydrazide compound according to the first invention is cooled. Was obtained.

(Resin Production Example 2) The carboxylic acid hydrazide compound according to the first invention was compounded by the same operation as in Resin Production Example 1 except that 3 parts by weight of sebacic acid dihydrazide was added instead of adipic acid dihydrazide. A novolak resin was obtained. (Resin Production Example 3) A novolak resin containing no carboxylic acid hydrazide compound according to the present invention was obtained in the same manner as in Resin Production Example 1 except that adipic acid dihydrazide was not added.

(Resin Production Example 4) (Production of resol type phenolic resin) 100 parts by weight of phenol, 50 parts by weight of 86% paraformaldehyde, and 2 parts by weight in a reactor equipped with a stirrer, a heating device, and a cooling pipe.
10 parts by weight of 5% ammonia water was charged and a condensation reaction was carried out at 50 ° C. for 4 hours. After adding and mixing 3 parts by weight of adipic acid dihydrazide to the reaction solution, the solution was heated and dehydrated under reduced pressure, and when the reaction solution reached 80 ° C., cooled, and the resole-type resin containing the carboxylic acid hydrazide compound according to the first invention was cooled. Obtained.

(Resin Production Example 5) A resole resin containing no carboxylic acid hydrazide compound according to the present invention was obtained in the same manner as in Resin Production Example 4 except that adipic acid dihydrazide was not added. (Example 1) 100 parts by weight of dried fuka sand heated to 160 ° C was charged into a speed mixer, 2 parts by weight of a novolak-type resin containing the carboxylic acid hydrazide compound was added, and the mixture was stirred for 25 seconds. Part 1.
It was dissolved in 5 parts of water and mixed and added. After stirring until the sand particles collapsed, 0.1 part of calcium stearate was added, and the mixture was further stirred for 20 seconds. Then, sand was taken out from the mixer and cooled to obtain RCS1 of the first invention of the present invention.

Example 2 RCS2 of the first invention of the present invention was produced in the same manner as in Example 1 except that the novolak resin of Example 1 was changed to a novolak resin containing the carboxylic acid hydrazide compound. (Example 3) The novolak type resin of Example 1 was replaced with a novolak type resin containing no carboxylic acid hydrazide compound, and a mixture of 0.1 part of calcium stearate and 0.06 part of adipic acid dihydrazide was added after the collapse of the sand particles. RCS3 according to the first invention of the present invention was obtained in the same manner as in Example 1 except for the above.

(Comparative Example 1) RCS6 containing no carboxylic acid hydrazide compound was obtained in the same manner as in Example 1 except that two parts of novolak type resin were used. (Example 4) The adipic dihydrazide of Example 3 was added to 0.1%.
Except having changed to 18 parts, RCS4 of this 1st invention was obtained by the same operation as Example 3.

Example 5 100 parts by weight of dried fuka sand heated to 150 ° C. was charged into a speed mixer, and 2 parts by weight of a resole type resin containing a carboxylic acid hydrazide compound was added. Parts of water were mixed.
After stirring until the sand particles collapsed, 0.1 part of calcium stearate was added, the mixture was further stirred for 20 seconds, and then the sand was cooled to obtain RCS5 according to the first invention.

Comparative Example 2 RCS7 containing no carboxylic acid hydrazide compound was obtained in the same manner as in Example 4, except that the resol type resin containing no carboxylic acid hydrazide compound was used instead of the resol type resin of Example 4. Table 1 shows the characteristics of RCS1 to RCS7 thus obtained.

[0032]

[Table 1]

The mark * was added to the sand when the resin was mixed, and the others were added to the resin production. Fusing Point: Measured in accordance with JACT test method C-1. Room temperature strength: Measured in accordance with JIS K-6910. As shown in Table 1, the RCS of each Example shows that the normal temperature strength of the mold is improved as compared with the Comparative Example, and shows the effect of adding the carboxylic acid hydrazide compound. The same effect is obtained even when the addition of the hydrazide compound is carried out at the time of resin production and at the time of sand particle coating.

(Preparation of resin for hot box) 70 parts by weight of 37% formalin, 10 parts by weight of urea, 10 parts by weight of furfuryl alcohol, and 2 parts by weight of potassium hydroxide as a catalyst are charged into a reaction vessel with respect to 10 parts by weight of phenol. The mixture was heated and reacted at 80 ° C. for 3 hours, and after removing condensed water from the condensation reaction product under reduced pressure, 15 parts by weight of methanol was added and mixed to prepare a hot box resin.

Next, 100 parts of zinc chloride, 40 parts of ammonium chloride and 200 parts of water were mixed and stirred to prepare a curing agent liquid. (Example 6) Fratary sand 1 with a sand mixer
After adding 0.4 parts by weight of the above curing agent to 00 parts by weight and mixing and stirring for 2 minutes, the hot box resin 2 obtained above was added.
Parts and 0.06 parts by weight of adipic dihydrazide were added and mixed for 2 minutes to obtain RCS8 containing the carboxylic acid hydrazide compound of the first invention.

(Comparative Example 3) RCS9 containing no carboxylic acid hydrazide compound was prepared in the same manner as in Example 6 except that no adipic dihydrazide was added in the production process of RCS8. (Evaluation) RCS8 and RCS9 produced above were respectively blown into a core blowing machine manufactured by Shinto Kogyo Co., Ltd.
The test piece was prepared by baking at 250 ° C. for a predetermined time of 5 seconds to 60 seconds, punched out and cut for 15 seconds, and the bending strength of the test piece was measured to examine the warm strength. The results are shown in Table 2.

As shown in Table 2, the RCS 8 according to the first embodiment of the present invention to which adipic acid dihydrazide was added exhibited an improvement in the warm strength at each time as compared to the RCS 9 to which no adipic acid dihydrazide was added. . Therefore,
The warm strength of RCS can be improved by adding a carboxylic acid hydrazide compound such as adipic acid dihydrazide.

[0038]

[Table 2] Example 7 100 parts by weight of dried fuka sand heated to 160 ° C. was charged into a speed mixer, 2 parts by weight of a novolak type shell molding resin (composition novolak type resin) was added, and the mixture was stirred for 25 seconds. Parts by weight were dissolved in 1.5 parts by weight of water and added. After stirring until the sand particles collapse, adipic dihydrazide 0.1 part by weight,
0.1 part by weight of urea and 0.1 part of calcium stearate
The mixture was stirred for further 20 seconds, and then the sand was cooled to obtain RCS10 containing both the carboxylic acid hydrazide compound of the second invention and a predetermined nitrogen-containing compound.

Example 8 RCS11 containing both the carboxylic acid hydrazide compound of the second invention and a predetermined nitrogen-containing compound was prepared in the same manner as in Example 7 except that sebacic acid dihydrazide was used instead of adipic acid dihydrazide. Obtained. Example 9 m-aminophenol instead of urea
RCS12 containing both the carboxylic acid hydrazide compound of the second invention and a predetermined nitrogen-containing compound was obtained in the same manner as in Example 7 except that 1 part by weight was used.

Example 10 The carboxylic acid hydrazide compound of the second invention and the carboxylic acid hydrazide of the second invention were prepared in the same manner as in Example 7 except that adipic acid dihydrazide and urea were added at the same time as the resin, and after disintegration of sand particles, calcium stearate was added. RCS13 containing both nitrogen-containing compounds of (Comparative Example 4) Both a carboxylic acid hydrazide compound and a predetermined nitrogen-containing compound were contained in the same manner as in Example 7, except that 0.4 parts by weight of hexamine was used, and after the disintegration of sand particles, only calcium stearate 0.1 was added. Got no RCS14.

(Comparative Example 5) The same procedure as in Example 7 was repeated except that a mixture of 0.1 part by weight of calcium stearate and adipic acid dihydrazide was added after the disintegration of the sand particles, and the mixture contained 0.1 part by weight of the carboxylic acid hydrazide compound. RCS15 containing no nitrogen-containing compound was obtained. (Comparative Example 6) After the disintegration of the sand particles, calcium stearate was added to the mixture.
RCS16 was obtained in the same manner as in Example 7, except that a mixture of 1 part by weight and 0.1 part by weight of urea was added, but not containing a carboxylic acid hydrazide compound but containing a predetermined nitrogen-containing compound.

Comparative Example 7 A carboxylic acid hydrazide compound and a predetermined nitrogen content were prepared in the same manner as in Example 7 except that 0.2 parts by weight of hexamine was used, and only 0.1 part by weight of calcium stearate was added after the disintegration of the sand particles. RCS17 free of both compounds was obtained. (Comparative Example 8) RCS18 containing neither a carboxylic acid hydrazide compound nor a predetermined nitrogen-containing compound was obtained in the same manner as in Comparative Example 4 except that hexamine was added using a resole type resin.

Table 3 shows the formulations of RCS10 to RCS18 and the results of evaluation of the characteristics.

[0044]

[Table 3]

Fusing point: Performed in accordance with JACT test method C-1. Room temperature strength: Performed in accordance with JIS K-6910. Warm strength: 22.5 × 22.5 × 2 heated to 250 ° C.
Each RCS was poured into a 00 mm mold, baked for 20 seconds, and the die strength was measured 15 seconds after the die was removed. Formaldehyde generation: Each RCS was baked in a 300 ° C. tube furnace for 5 minutes, the generated gas was absorbed in water, and measured by absorption spectroscopy. As shown in Table 3, Examples 7-1
The RCS of 0 is based on the fact that the strength at room temperature and the warm strength are improved and the amount of generated formaldehyde is reduced and the hydrazine compound and the nitrogen-containing compound are contained, as compared with Comparative Examples 4 and 7 which do not contain the hydrazide compound and the nitrogen-containing compound. The effect is shown.

When only the hydrazide compound of Comparative Example 5 was used, the strength was improved, but the effect of suppressing formaldehyde was poor. When only the nitrogen-containing compound of Comparative Example 6 did not contain the hydrazide compound, the strength was reduced. In Comparative Example 7, the strength was reduced because the amount of hexamine, which is the curing agent of Comparative Example 4, was reduced. In this case, since the amount of formaldehyde generated from hexamine is small, the generated amount is also small. Further, in the case of the resol type phenol resin of Comparative Example 8, the amount of formaldehyde generated is small, but the strength is lower than in the case of the novolak type phenol resin.

[0047]

As described in detail above, the resin-coated sand particles for shell molding of the first invention can increase the strength of the mold by including the carboxylic acid hydrazide compound in the resin coating layer. By improving the mold strength, it can be used as a mold having a complicated shape, a thin-walled mold, or a non-ferrous metal mold. In addition, the amount of resin added can be reduced by improving the mold strength, so that the amount of gas generated during mold molding and casting can be reduced, and gas defects can be reduced.

Further, by using the carboxylic acid hydrazide of the second invention and the nitrogen-containing compound in combination, the amount of formaldehyde generated can be reduced without lowering the mold strength, and the working environment can be improved. Further, the carboxylic acid hydrazide compound does not lower the melting point of the range or the fusion point of RCS even if it is mixed with the resin or added during the production of RCS. Strength improvement effect can be maintained without lowering.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Eiichi Nagasaka 1141-1 Okawagahara, Iino, Fujioka-cho, Nishikamo-gun, Aichi Prefecture Inside Aisin Chemical Co., Ltd. 1141-1 Ojigahara, Aisin Kako Co., Ltd. (72) Inventor Hiromi Tomishige 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation F term (reference) 4E092 AA21 AA28 AA31 AA45 BA04 CA03 4F202 AJ01 AJ03 CD01

Claims (4)

[Claims] 1. A resin-coated sand particle comprising a sand particle and a resin coating layer made of a thermosetting resin coated on the surface of the sand particle, wherein the resin coating layer contains a carboxylic acid hydrazide compound. Resin coated sand grains for shell molding. 2. The resin-coated sand particles for a shell mold according to claim 1, wherein the carboxylic acid hydrazide compound is 0.1 to 10 parts by weight based on 100 parts by weight of the thermosetting resin. 3. A resin-coated sand particle comprising a sand particle and a resin coating layer made of a thermosetting resin coated on the surface of the sand particle, wherein the resin coating layer comprises a carboxylic acid hydrazide compound, urea and thiourea. , Aminophenols, phenylhydrazine, aniline, and other aromatic compounds, acetamide, amides such as acetanilide, semicarbazone,
Resin-coated sand particles for shell molds, comprising at least one nitrogen-containing compound selected from the group consisting of: 4. The method according to claim 1, wherein the carboxylic acid hydrazide compound accounts for 0.01 to 0.5% by weight and the nitrogen-containing compound accounts for 0.01 to 0.5% by weight based on 100 parts by weight of the sand grains. The resin-coated sand particles for shell molding according to claim 3.