JP2003062639A - Resin-coated sand for shell mold - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide resin-coated sand which, at the time of making a shell mold, solidifies phenolic resin rapidly and gives a high cold strength of a mold made thereby, and also has a good collapsibleness and does not corrode a die at the time of casting a light metal. SOLUTION: This is a resin-coated sand for a shell mold that the casting sand particles are coated by using phenolic resin components, featuring that the phenolic resin components comprise (A) novolak phenolic resin, (B) resor phenolic resin with a melting point being more than 60 deg.C, (C) polyethylene glycol with a numerical average molecular weight being more than 1000 and (D) alkaline metal nitrate, the mix proportion by weight of the phenolic resins (A), (B) (A/B) is 20/80 to 70/30, and in the 100 weight proportion as the total amount of phenolic resins (A+B), 2 to 20 weight proportion as (C) polyethylene glycol and 0.5 to 2 weight proportion as (D) alkaline metal nitrate are contained.

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a resin-coated sand for shell molding obtained by coating an aggregate such as heated silica sand with a phenolic resin composition. Particularly, it is suitably used for casting a metal having a lower melting point than iron, such as aluminum and magnesium. 2. Description of the Related Art Resin coated sand for shell molding is obtained by adding a phenol resin to an aggregate such as heated silica sand.
It is obtained by cooling after coating and adding a lubricant such as calcium stearate. The shell mold molding method is a method in which a mold can be molded in an extremely short time by firing resin-coated sand in a mold at 250 to 300 ° C. Shell molds are widely used as economical mold manufacturing methods for casting automotive parts and the like because of their mold properties such as high strength and heat resistance and good dimensional stability. In recent years, the use of light metals such as aluminum and magnesium has been increasing in cast automobile parts in order to reduce weight.
In the casting of these low-melting metals, since the internal temperature of the shell mold during pouring remains at a low temperature of 300 to 400 ° C., the thermal decomposition of the phenolic resin in the shell mold becomes insufficient, and the shell mold after casting is Since sufficient strength is maintained, it is sometimes extremely difficult to efficiently take out the shell core with a complicated mold shape. In such a case, in order to take out the cast-in shell core, it is inevitable to take a method in which the core is heated for a long time through a firing furnace and then subjected to impact to collapse. This is a major obstacle to improving productivity and saving energy. [0003] Such a resin-coated sand for light alloy includes a compound which promotes thermal decomposition of a phenolic resin of a shell mold at the time of casting and promotes disintegration of the mold (hereinafter, referred to as a compound).
Various methods have been reported for adding and mixing a disintegration accelerator during the production of a resin-coated sand. For example,
JP-A-58-84636 discloses a method of adding ammonium chloride, zinc chloride, and the like to a phenol resin. According to this method, although the shell mold has high cold strength and good collapsibility, it is difficult to mold. At the time of casting or casting. See also
8-77738 discloses Ia, Ib, IIa, II
b, IIIa, IVa, Va, VIb, VIIa, VI
There are examples of carboxylate salts of elements selected from each group of Ib and VIII. According to this method, although the cold strength of the shell mold is high, the collapsibility is good, and the mold does not corrode at the time of casting, there is an advantage that the curing speed of the phenol resin is reduced. In addition, Tokiko 31-72
Japanese Patent Publication No. 56 has examples of potassium nitrate and sodium nitrate. However, in order to obtain sufficient disintegration, it is necessary to use a large amount of potassium nitrate and sodium nitrate, and there is a drawback that the mold strength is reduced. DISCLOSURE OF THE INVENTION [0004] The present invention relates to a method for producing a shell mold, in which the phenol resin cures rapidly, the obtained mold has a high cold strength, and the mold disintegration of the mold during light metal casting. And a resin-coated sand that does not corrode the mold. [0005] The present invention is a resin-coated sand in which sand for casting is coated with a phenolic resin composition, wherein the phenolic resin composition comprises (A) a novolak-type phenolic resin, ) Melting point is 60
Phenolic resin having a number average molecular weight of 1000 or more; (D) an alkali metal nitrate; and a weight mixing ratio (A / B) of the phenolic resins (A) and (B). ) Is 20/80 ~
70/30, the total amount of phenolic resin (A + B)
A resin-coated sand for shell molding, comprising 2 to 20 parts by weight of the polyethylene glycol (C) and 0.5 to 2 parts by weight of the alkali metal nitrate (D) based on 100 parts by weight. is there. The resin-coated sand for shell molding of the present invention (hereinafter referred to as “resin-coated sand”)
Will be described. First, the components blended in the phenolic resin composition (hereinafter, referred to as “composition”) used in the resin-coated sand of the present invention will be described in detail. The phenolic resin blended in the composition used in the present invention is a novolak-type phenolic resin (A) obtained by reacting a phenol and an aldehyde with an acid catalyst.
And a resol type phenol resin (B) obtained by reacting phenols and aldehydes with a basic catalyst. In the production of such a phenol resin, commonly used raw materials, catalysts and the like can be used, and the production conditions thereof are not particularly limited. Phenols used as raw materials include phenol, cresol, bisphenol A and the like. Aldehydes include formalin, paraformaldehyde and the like. The reaction catalyst used in the production of the novolak type phenol resin is an acidic substance such as sulfuric acid, hydrochloric acid, paratoluenesulfonic acid, oxalic acid, or an organic metal salt such as zinc acetate. In addition, a basic substance such as aqueous ammonia, triethylamine, sodium hydroxide, potassium hydroxide, barium hydroxide or the like is used as a reaction catalyst when producing a resol-type phenol resin. [0007] Among the phenolic resins blended in the composition used in the present invention, the novolak type phenolic resin (A) is not particularly limited, and any one such as a liquid or a solid at room temperature can be used. In some cases, these may be used in combination. On the other hand, the resol type phenol resin (B) preferably has a melting point of 60 ° C. or higher, more preferably 65 ° C. or higher.
By using a resin having such a melting point, a resin-coated sand having good fluidity can be obtained without blocking. Such a resol-type phenol resin can be obtained by sufficiently removing condensed water while preventing gelation during production. The novolak type phenol resin reacts with the hexamethylenetetramine or resole type phenol resin and cures. Since it cures with a high crosslinking density, it can provide high strength, but has the property of difficulty in thermal decomposition. On the other hand, the resol-type phenol resin also acts as a curing agent for the novolak-type phenol resin, and cures without requiring hexamethylenetetramine. The crosslinked density of the obtained cured product is low and tends to be thermally decomposed. In the composition used in the present invention, the weight ratio (A / B) of the novolak type phenolic resin (A) to the resol type phenolic resin (B) is 2
It is preferably 0/80 to 70/30, and more preferably 25/75 to 65/35. With such a mixing ratio, the balance between the strength and the thermal decomposition property of the shell mold can be achieved. When the proportion of the novolak-type phenolic resin (A) is larger than the upper limit, the disintegration is reduced, and when the proportion of the resol-type phenolic resin (B) is larger than the upper limit, the strength of the mold may be reduced. . These phenol resins may be used by mixing them in advance, or may be added separately at the time of producing a coated sand. Also, depending on the required mold properties,
Hexamethylenetetramine may be used in combination. [0009] In the present invention, polyethylene glycol (C) is blended to impart thermal decomposability to the composition.
The polyethylene glycol used in the present invention has a lower decomposition temperature than cured phenolic resin and therefore has good thermal decomposability, and has good compatibility with phenolic resin and does not cause a reduction in mold strength. The polyethylene glycol preferably has a number average molecular weight of 1,000 or more, more preferably 2,000 or more. By using a resin having such a molecular weight, good fluidity can be imparted to the resin-coated sand. When the molecular weight is the lower limit described above, the melting point of the obtained coated sand decreases, the occurrence of consolidation, the decrease in fluidity occurs, and the storage density and the packing density in the mold decrease, which may hinder use. is there. The amount of polyethylene glycol is 2 to 20 parts by weight, more preferably 2 to 1 part by weight, per 100 parts by weight of the total amount of phenolic resin (A + B).
5 parts by weight. By blending such an amount of polyethylene glycol, the thermal decomposability of the composition and the fluidity of the resin-coated sand can be imparted. If the compounding amount is less than the lower limit, the effect of improving the thermal decomposability is poor, and if it exceeds the upper limit, the melting point of the obtained coated sand is lowered, the occurrence of consolidation, the flowability is reduced and the storage stability is reduced. The packing density in the mold may be reduced, which may hinder use. In addition, the blend of polyethylene glycol,
It may be added during the synthesis of the phenolic resin or added during the production of the resin-coated sand. In the present invention, an alkali metal nitrate is further blended as a disintegration accelerator. Examples of the alkali metal nitrate here include sodium nitrate, potassium nitrate, and lithium nitrate, and these may be used alone or in combination. The mixing amount of the alkali metal nitrate is 0.1 to 100 parts by weight of the total amount of the phenol resin (A + B).
5 to 2.0 parts by weight, more preferably 0.6 to 2.0 parts by weight.
It is 1.8 parts by weight. By blending the alkali metal nitrate in such an amount, good disintegrability can be imparted to the resin-coated sand. If the amount of the alkali metal nitrate is less than the lower limit, the effect of accelerating the disintegration is poor, and if the amount is more than the upper limit, the strength of the mold is lowered, which is not practical. The alkali metal nitrate is preferably added as an aqueous solution at the time of compounding the composition from the viewpoint of uniformity. In addition, other disintegration promoters such as carboxylate and carbonate may be used in combination as long as they conform to the gist of the present invention. In the composition used for the resin-coated sand of the present invention, in addition to the above-mentioned materials, a lubricant such as ethylenebisstearylamide and a modifier such as an aminosilane coupling agent may be blended, if necessary. . In addition, silica sand is generally used as an aggregate used for resin coated sand for shell mold, but zircon sand,
Special sand such as artificial mullite sand or the like may be mixed or used alone. The resin coated sand for shell mold of the present invention can be produced by a usual method. That is, the novolak-type phenol resin, the resol-type phenol resin, and polyethylene glycol are mixed and kneaded with an aggregate such as silica sand heated to about 130 to 150 ° C., and further, an alkali metal nitrate is added as an aqueous solution, and then air-cooled. While the mixing is continued, when sufficient fluidity is generated in the sand, a lubricant may be blended as necessary to obtain a resin-coated sand. The amount of the composition added to the aggregate can be arbitrarily selected within a range known in the art. The resin-coated sand of the present invention, together with a commonly used novolak-type phenolic resin, a resol-type phenolic resin having a low crosslinking density at the time of curing and a good thermal decomposability, and a good thermal decomposability without lowering the mold strength It is considered that by containing a predetermined amount of solid polyethylene glycol and further blending an alkali metal nitrate as a disintegration promoter, an extremely excellent thermal decomposition accelerating action is provided as a synergistic effect. As a result, the strength of the phenol resin after casting is lost, and the phenol resin can be easily discharged from the casting. Also, blended polyethylene glycol,
Since the alkali metal nitrate has extremely low corrosiveness to the mold, it is considered that no rust or the like is generated. Hereinafter, the present invention will be described specifically with reference to examples. However, the present invention is not limited to the embodiments. All “%” described in the text indicate “% by weight”. Production Example 1 of Phenol Resin (Novolak Type Phenol Resin) Phenol 1000 was placed in a reaction vessel equipped with a cooler and a stirrer.
g, 10 g of oxalic acid, and 561 g of 37% formalin. After the temperature was gradually raised and the reaction was carried out at 95 to 100 ° C. for 3 hours from the time when the temperature reached 95 ° C., heating was continued and normal pressure dehydration was performed until the internal temperature reached 130 ° C. The heating was further continued, and the pressure was gradually reduced to 6.7 kPa. After the temperature in the reaction system reaches 180 ° C., ethylene bisstearyl amide 1
0 g was added and mixed, and the mixture was discharged from the reaction vessel to a cooling vessel to obtain 960 g of a novolak type phenol resin. Production Example 2 of Phenol Resin (Novolak Type Phenol Resin) Phenol 1000 was placed in a reaction vessel equipped with a cooler and a stirrer.
g, 10 g of oxalic acid, and 561 g of 37% formalin. After the temperature was gradually raised and the reaction was carried out at 95 to 100 ° C. for 3 hours from the time when the temperature reached 95 ° C., heating was continued and normal pressure dehydration was performed until the internal temperature reached 130 ° C. The heating was further continued, and the pressure was gradually reduced to 6.7 kPa. After the temperature in the reaction system reaches 180 ° C., ethylene bisstearyl amide 10
g and polyethylene glycol 220 having a molecular weight of 21,000
g was added, mixed and discharged from the reaction vessel to a cooling vessel to obtain 1160 g of a novolak type phenol resin. The content of polyethylene glycol in the resin was 19%. Preparation Example 3 of Phenol Resin (Resole Type Phenol Resin) Using the reaction vessel of Preparation Example 1, 1000 g of phenol,
1840 g of 7% formalin, followed by 120 g of 28% aqueous ammonia and 50 g of a 50% aqueous sodium hydroxide solution were added. After the temperature was gradually raised and the temperature reached 96 ° C., a reflux reaction was performed for 25 minutes, and then ethylenebisstearyl amide 5
After adding 0 g, the pressure was gradually reduced to 6.7 kPa. 1
After reaching 05 ° C, discharge from the reaction vessel and quench to 70 ° C.
1150 g of a resol type phenol resin was obtained. Example 1 After 8 kg of flattery sand heated to 130 ° C. was put into a mixer, the novolak type phenol resin 80 of Production Example 1 was added.
g, 80 g of the resole-type phenolic resin of Production Example 3 and 16 g of polyethylene glycol having a molecular weight of 12,000, and after kneading for 45 seconds, an aqueous solution in which 1 g of potassium nitrate was dissolved in 100 g of water was added. At the time when the fluidity was extremely high, 8 g of calcium stearate was added and mixed to prepare a resin-coated sand. Examples 2 to 3 Resin-coated sands were prepared according to the procedure for preparing resin-coated sands of Example 1 by changing the blending of phenolic resin, polyethylene glycol and alkali metal nitrate as shown in Table 1. Comparative Examples 1 and 2 Resin-coated sands were prepared according to the procedure for preparing resin-coated sands of Example 1 by changing the blending of phenolic resin, polyethylene glycol and alkali metal nitrate as shown in Table 1. The obtained resin-coated sand was subjected to bending strength, fusion point, bend (bending amount), disintegration, and mold corrosion tests. Table 1 shows the composition of the composition and the results of measuring the properties of the resin-coated sand. [Table 1] (Measurement method) (1) Flexural strength: According to JIS-K6910. The firing was performed at 250 ° C. for 30 seconds. (2) Fusing point: A resin-coated sand is placed on a copper rod having a temperature gradient, and after 60 seconds, air of 0.5 kg / cm ² is blown off to blow off the resin-coated sand, and the resin-coated sand is fused to the copper rod. The lowest temperature of the portion where the heating was performed was taken as the fusion point. (3) Bend (bending amount): Japan Casting Technology Association Test Method, S
M-3. A plate-like test piece having a thickness of 5 x 40 x 180 and a thickness of 5 x 40 x 180 is fired at 250 ° C for 30 seconds, and after 10 seconds, a load is applied and the amount of deflection is read. The larger the value, the greater the deformation after molding and the slower the curing. (4) Disintegration: Resin coated sand having an inner diameter of 25 m
m, filled into a 150 mm long iron pipe, pre-fired at 250 ° C. for 30 minutes, covered with aluminum foil,
Heat treatment in reducing atmosphere for hours. After cooling, take out the iron pipe from the aluminum foil and add 50 to the iron pipe by the method shown in FIG.
An impact is given with a 0 g hammer, and the number of impacts until the residual sand in the iron pipe disappears is measured. In FIG. 1, A indicates a sample, and B indicates a hammer. The hammer rotates about the fulcrum C. The height of the fulcrum C is 3 with respect to the sample A.
0 cm. The hammer was lifted to a horizontal position and allowed to fall naturally, impacting and impacting the sample.
The smaller the number of impacts until the residual sand in the iron pipe disappears, the better the collapsibility. (5) Mold corrosion test:
The iron pipe used for the collapsibility test was taken indoors (2
(0 ° C., 65%), and two weeks later, the inner surface of the iron pipe was observed for rust. From the results shown in Table 1, Examples 1 to 3 are resin-coated sands using a composition in which novolak-type phenolic resin, resol-type phenolic resin, polyethylene glycol and alkali metal nitrate were mixed in appropriate amounts, respectively. Disintegration is improved without impairing properties such as strength. On the other hand, in Comparative Example 1, the resol type phenol resin was not used as the phenol resin component, and polyethylene glycol for imparting thermal decomposability was not added, so that the disintegration of the mold was reduced. In Comparative Example 2, zinc chloride was used in place of the alkali metal nitrate as the disintegration promoter of the present invention, but the disintegration and bending strength were reduced, and rust was observed on the iron pipe. According to the present invention, there is provided a shell using a composition comprising a novolak type phenol resin, a resol type phenol resin having a melting point of 60 ° C. or higher, polyethylene glycol having a number average molecular weight of 1000 or higher, and an alkali metal nitrate. It is a resin-coated sand for a mold, and can produce a mold excellent in disintegration without substantially reducing the strength, the fusion point, the curing speed, and the like of the mold. By using a mold having such excellent disintegration properties, the sand baking step after casting can be omitted, so that energy saving and the step shortening can be achieved. Further, since no rust is generated in the mold, it can be suitably used for economical casting of light metals such as aluminum.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic side view of an impact tester for measuring disintegration [Description of symbols] A Sample B Hammer C Hammer fulcrum

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Claims (1)

Claims: 1. A resin-coated sand in which sand for casting is coated with a phenolic resin composition, wherein the phenolic resin composition comprises (A) a novolak-type phenolic resin, and (B) a melting point of 60%.
Phenolic resin having a number average molecular weight of 1000 or more; (D) an alkali metal nitrate; and a weight mixing ratio (A / B) of the phenolic resins (A) and (B). ) Is 20/80 ~
70/30, the total amount of phenolic resin (A + B)
A resin coated sand for shell molding, comprising 2 to 20 parts by weight of the polyethylene glycol (C) and 0.5 to 2 parts by weight of the alkali metal nitrate (D) based on 100 parts by weight.