Foundry sand for 3D additive manufacturing and its manufacturing method

Abstract

To provide foundry sand for layered manufacturing, which can provide a sand mold that is difficult to deform even when a water-based mold wash is used. [Solution] The present invention relates to foundry sand for use in three-dimensional additive manufacturing, characterized in that (1) the particles constituting the foundry sand include (a) sand grains and (b) an organic layer formed on the surface of the sand grains, and (2) the organic layer includes a furan resin precursor or a fatty acid with a melting point of 40 to 75°C, and a heteropolyacid.

Description

[Technical Field]

[0001] The present invention relates to a new foundry sand for three-dimensional additive manufacturing and a method for producing the same, and more particularly to foundry sand to be supplied to a three-dimensional additive manufacturing machine, which is a sand mold machine for casting, and a method for producing the same. [Background technology]

[0002] Additive manufacturing is a technology that creates molded objects by stacking sliced ​​2D layers based on 3D data such as 3D-CAD. In practice, objects with desired shapes can be created by stacking 3D additive manufacturing materials using a 3D printer. Various materials are used for 3D additive manufacturing, including metals, resins, sand, plaster, and ceramics.

[0003] Among these, sand is suitable for use as a casting mold, and sand molds are also manufactured using additive manufacturing. In particular, 3D additive manufacturing technology, which can directly realize 3D shapes created using 3D-CAD or other tools as sand molds, has been attracting attention in recent years. In such sand mold manufacturing methods, various manufacturing methods and 3D additive manufacturing materials have been proposed with the aim of improving defects in the sand molds and layer flow.

[0004] Patent Document 1 proposes a technology for efficiently forming sand molds by using foundry sand containing sand and a solid acid catalyst mixed with the sand, which allows the binder to harden quickly when added to the foundry sand.

[0005] However, in the case of the above-mentioned foundry sand, the solid acid catalyst is not uniformly dispersed throughout the sand, and when a binder is added, the curing of the furan resin precursor becomes uneven (distributed), making it difficult to increase the strength of the sand mold. Furthermore, the technique of increasing the amount of furan resin, curing agent, etc., which is often adopted for increasing strength in self-hardening furan sand molds, is not applicable to this technology.

[0006] In addition, when a 3D additive manufacturing process is carried out using the foundry sand produced by this method, if a binder is applied to the layered foundry sand, the applied binder comes into contact with the solid acid catalyst, which acts as a hardener on the surface of the foundry sand, and a hardening reaction immediately occurs. As a result, although the sand mold develops a certain amount of initial strength, if the amount of binder is small, the binder gradually hardens before covering all of the foundry sand grains, and no further strength increase can be expected.

[0007] Furthermore, in the manufacturing method of the foundry sand, two types of solid acid catalysts with different particle size distributions are prepared and mixed with sand to produce the foundry sand. This is to improve the filling properties when producing 3D additive models, but the need to pay attention to the particle size distribution of the solid acid catalysts makes the manufacturing method of the foundry sand complicated.

[0008] In addition to the above documents, for example, a method for manufacturing a sand mold is known, which includes a step of mixing a solid carboxylic acid with refractory particles, a refractory particle layer forming step of forming a refractory particle layer containing refractory particles mixed with the solid carboxylic acid, and a binder composition supplying step of supplying a binder composition containing a resin in which the solid carboxylic acid acts as a hardener to a desired region of the refractory particle layer, and in which the refractory particle layer forming step and the binder composition supplying step are sequentially repeated (Patent Document 2).

[0009] In particular, the document describes a method in which, in the process of adding a solid carboxylic acid to refractory particles, the solid carboxylic acid is dissolved in a solvent, and then the refractory particles are mixed with the solid carboxylic acid, and the solvent is removed by volatilization to obtain the foundry sand.

[0010] However, solid carboxylic acids have low affinity for solvents and cannot be uniformly dispersed in the solvent, and when a solution in such a state is mixed with refractory particles, it is impossible to uniformly disperse the solid carboxylic acids on the surfaces of the refractory particles.

[0011] Furthermore, since the solid carboxylic acid is only weakly adhered to the surface of the refractory particles, the solid carboxylic acid peels off and falls off from the refractory particles due to physical friction between the refractory particles that occurs inside equipment such as the recoater of a 3D additive manufacturing machine, making it impossible to expect stable strength development.

[0012] A manufacturing method is known in which a material in which a polycarboxylic acid is mixed with a refractory granular material is laid out in layers, and then a sugar binder is injected into desired areas of the laid-out material in layers, and this process is repeated until a three-dimensional additive manufacturing object is formed. After the molding is completed, the three-dimensional additive manufacturing object is heated to harden the sugar binder (Patent Document 3).

[0013] However, in this method, the 3D additive manufacturing object is made only of polycarboxylic acid and sugar binder, so it has lower heat resistance than sand molds made with furan resin precursors, making it impossible to use it as a sand mold for casting iron-based materials, which have a relatively high melting temperature.

[0014] Furthermore, Patent Document 3 describes a method of accelerating the hardening reaction between the sugar binder and polycarboxylic acid by heating to 150°C or higher in order to develop practical handling strength (i.e., for removal).

[0015] However, this method requires the application of heat from the outside, as a hardening reaction over time cannot be expected. Heating the sand mold for removal is not necessary in the self-hardening sand mold process, which leads to reduced work efficiency.

[0016] On the other hand, a method for manufacturing a press die has been disclosed in which a furan resin, which is an acid-curing organic self-hardening resin, is used as a binder (Patent Document 4).

[0017] However, when a furan resin is applied to sand grains as a binder, if a water-based mold wash is used, the problem of deformation of the sand mold occurs, as in the case shown in Patent Document 6 below.

[0018] In addition, a technique is known in which water glass is applied to quartz sand particles as a granular material, and the resulting molded body is hardened by heating, and then the molded body is further exposed to an atmosphere enriched with gaseous water to improve its strength (Patent Document 5).

[0019] However, in the above technology, water glass is used as a binder, and enriched water vapor is required to increase strength. It is also stated that heating cannot increase strength, and the purpose of heating is clearly stated as a means to achieve enrichment of water vapor. In other words, the above technology solely utilizes the principle of water glass solidification. In addition, in the present invention, not only is enrichment of water vapor not required, but there is also no need to consider the firing atmosphere; conversely, enrichment of water vapor has a negative effect on the curing of furan resin.

[0020] In response to this, the applicant of the present application has proposed and filed a patent application for foundry sand for additive manufacturing, which is capable of providing a sand mold having high strength and high dimensional accuracy. The foundry sand is used in a method for manufacturing a sand mold by additive manufacturing, which includes sequentially repeating the steps of forming a layer containing foundry sand and solidifying a predetermined region of the layer by adding a binder to the region, and which is characterized in that (1) a furan resin organic layer containing a furan resin precursor and an acid component is formed on the surface of the sand grains constituting the foundry sand, and (2) the furan resin organic layer has a solubility in methanol (25°C) of 32% or more (Patent Document 6).

[0021] The furan resin organic film formed on the surface of the foundry sand grains produced by this method is in a semi-cured state, making it possible to cure the furan resin precursor evenly throughout the foundry sand when a binder is added. In addition, because the foundry sand appears dry, insufficient filling of the foundry sand does not occur even in layering methods that do not allow sand tamping, making it possible to produce sand molds that combine high strength with high dimensional accuracy.

[0022] However, in Patent Document 6, deformation may occur when the water-based mold wash is applied and dried.

[0023] Generally, when casting iron, a technique is known in which an inorganic material called a mold wash is applied to the sand mold to prevent defects such as burn-in. In this case, a water-based mold wash is typically used, which is a slurry in which inorganic powder is dispersed in an aqueous solvent. The water-based mold wash is then applied to the sand mold by immersing the sand mold in the water-based mold wash (dipping method). The sand mold is then subjected to a process of dehydration and drying by heating or other methods.

[0024] When a sand mold made from conventional foundry sand, such as that described in Patent Document 6, is dipped into a slurry of an aqueous mold wash, the sand mold absorbs the water contained in the aqueous mold wash, causing problems such as deformation and cracking during the subsequent drying process. This is thought to be due to the fact that the curing reaction of the self-hardening furan resin has not progressed sufficiently in the sand mold immediately after additive manufacturing, and the furan composition in the middle of curing is presumably in a state where it is susceptible to absorbing water. In other words, if there are any parts in the sand mold that are not completely hardened, and they absorb a large amount of water due to the application of the aqueous mold wash, these parts will soften due to the heat, and the sand mold will be unable to support its own weight and will deform.

[0025] This problem (phenomenon) can occur not only with the technology of Patent Document 6, but also with any additive manufacturing material that uses an acid-curing furan resin for additive manufacturing. [Prior art documents] [Patent documents]

[0026] [Patent Document 1] Patent No. 6880302 [Patent Document 2] Patent No. 6595327 [Patent Document 3] Patent Publication No. 2023-142648 [Patent Document 4] Patent Publication No. 2002-316299 [Patent Document 5] Special Table 2020-520808 [Patent Document 6] Patent Publication No. 2022-71870 Summary of the Invention [Problem to be solved by the invention]

[0027] Therefore, a main object of the present invention is to provide foundry sand for three-dimensional additive manufacturing, which can provide a sand mold that is resistant to deformation even when a water-based mold wash is used.

[0028] The present inventors have conducted extensive research in light of the problems of the prior art and have found that particles (particle groups) having a specific structure can achieve the above object, thereby completing the present invention.

[0029] That is, the present invention relates to the following foundry sand for three-dimensional additive manufacturing and a method for producing the same. 1. Molding sand used for three-dimensional additive manufacturing, (1) The particles constituting the foundry sand include (a) sand grains and (b) an organic layer formed on the surface of the sand grains, (2) The organic layer contains a furan resin precursor or a fatty acid having a melting point of 40 to 75°C and a heteropolyacid. Foundry sand for 3D additive manufacturing characterized by: 2. The foundry sand for three-dimensional additive manufacturing according to Item 1, wherein the heteropolyacid contains at least one of silicotungstic acid and phosphotungstic acid. 3. The foundry sand for three-dimensional additive manufacturing according to item 1, containing 0.5 to 2.5 parts by mass of heteropolyacid per 100 parts by mass of sand grains. 4. The foundry sand for 3D additive manufacturing according to Item 1, wherein the organic layer comprises solid particles of a heteropolyacid supported on a carrier containing a furan resin precursor or a fatty acid having a melting point of 40 to 75°C. 5. The foundry sand for 3D additive manufacturing according to Item 1, wherein the organic layer comprises an aqueous wetting layer containing a heteropolyacid formed on the surface of the sand grains and a hydrophobic layer containing a furan resin precursor or a fatty acid with a melting point of 40 to 75°C, and the surface of the aqueous wetting layer is covered with the hydrophobic layer. 6. A three-dimensional additive manufacturing kit comprising the molding sand for three-dimensional additive manufacturing according to any one of items 1 to 5 above and a coating binder. 7. A method for producing molding sand for three-dimensional additive manufacturing, comprising: (1) a step of mixing at least sand particles with a coating agent containing a furan resin precursor or a fatty acid having a melting point of 40 to 75°C at 100°C or less to obtain precursor particles whose surfaces are coated with a coating layer containing the furan resin precursor or the fatty acid; and (2) mixing precursor particles and solid particles of heteropolyacid at 100°C or less to obtain composite particles in which the solid particles are supported on a coating layer on the surface of the precursor particles; A method for producing foundry sand, comprising: 8. A method for producing molding sand for three-dimensional additive manufacturing, comprising: (1) a step of mixing at least sand particles with an aqueous solution of a heteropolyacid at 100°C or less to obtain precursor particles having surfaces of the sand particles coated with an aqueous wetting layer containing the aqueous solution; (2) mixing precursor particles with a furan resin precursor or a coating agent containing a fatty acid having a melting point of 40 to 75°C at 100°C or less to obtain composite particles in which the aqueous wet layer is covered with a hydrophobic layer containing the fatty acid; A method for producing foundry sand, comprising: 9. The manufacturing method according to item 7 or 8, further comprising the step of adding 0.5 parts by mass or less of a silane coupling agent per 100 parts by mass of sand grains. 10. A method for manufacturing a sand mold using the molding sand for 3D additive manufacturing according to any one of items 1 to 5, comprising: (1) a step of forming a layer containing the foundry sand and a step of adding a coating binder to a predetermined area of ​​the layer to solidify the area, thereby obtaining a molded body; and (2) A step of obtaining a sand mold by heat-treating the green body not containing an aqueous mold wash at 40 to 250°C in air or an inert gas atmosphere. A method for manufacturing a sand mold, comprising: 11. A sand mold for casting iron-based materials, comprising the molding sand for 3D additive manufacturing according to any one of items 1 to 5. 12. The sand mold for casting iron-based materials according to item 11, further comprising a water-based mold wash. [Effects of the Invention]

[0030] The present invention provides foundry sand for 3D additive manufacturing, which can provide a sand mold that is resistant to deformation even when a water-based mold wash is used. In particular, it is possible to provide foundry sand that achieves relatively high sand mold strength, enables the use of a water-based mold wash, and is also favorable for the working environment. The present invention is particularly suitable for use as foundry sand for 3D additive manufacturing.

[0031] The foundry sand of the present invention has an organic layer present on the surface of the sand grains, which contains a furan resin precursor or saturated fatty acid and a heteropolyacid as a curing agent. This organic layer is usually in a semi-cured state. By forming such an organic layer on the surface of the sand grains, when a coating binder is sprayed onto the foundry sand of the present invention, it can be cured evenly across the entire sand grain, resulting in a sand mold with high strength. In particular, sufficiently high strength can be obtained even a few minutes (e.g., 30 minutes) after the sand mold is made.

[0032] In particular, the present invention uses a heteropolyacid as a curing agent, which has a higher solubility in water than sulfonic acid. This allows the heteropolyacid to reach all the way to the center of the bridges between the sand grains that form the cured product of the furan resin precursor, thereby achieving higher strength and contributing to the prevention of deformation of the sand mold.

[0033] Thus, even when a sand mold is manufactured by the additive manufacturing method using the foundry sand of the present invention, the sand mold can be given high strength.

[0034] Furthermore, when the foundry sand of the present invention is in a dry state, even in a layered manufacturing method (additive molding method) in which sand compaction is difficult or impossible, there is no risk of insufficient filling of the foundry sand, and it is possible to increase the strength of the sand mold.

[0035] Furthermore, the heteropolyacid used as a hardener in the foundry sand of the present invention is substantially free of sulfur components, and therefore, the effects of sulfur components as described below can be avoided in advance.

[0036] Generally, a liquid (molten metal) made by melting metals such as iron or aluminum at high temperatures is poured into a sand mold formed from foundry sand to produce a casting. However, during the pouring process, the sulfur components contained in the foundry sand decompose thermally, generating sulfur dioxide gas and other substances, which have a negative impact on the environment.

[0037] Furthermore, when casting iron-based materials, the presence of sulfur in the casting sand (or sand mold) can cause sulfurization of the casting, which can lead to poor graphite spheroidization (metal structure), particularly in spheroidal graphite cast iron, and can result in reduced strength or toughness of the casting.

[0038] In contrast, as described above, the foundry sand of the present invention and the sand molds produced therefrom have an extremely low or 0% sulfur content, which prevents the sulfur from affecting the working environment and iron-based materials. Therefore, the foundry sand of the present invention can be suitably used for casting iron-based materials with high melting points, which are difficult to achieve with conventional sand molds, and can be widely used in the production (casting) of cast iron machine parts, for example.

[0039] The foundry sand of the present invention having such characteristics and a kit comprising a combination of the foundry sand and a coating binder can be suitably used for molding a sand mold by the layered molding method.

[0040] The foundry sand of the present invention has been developed to be particularly suitable for additive manufacturing. However, it can also be used as a so-called self-hardening sand mold material, and in particular, when combined with artificial sand and furan resin, it can be used as a countermeasure against casting defects such as veining.

[0041] Furthermore, the foundry sand manufacturing method of the present invention allows for more reliable and efficient production of the foundry sand of the present invention. The inventors have discovered that controlling the solubility of the organic layer and the amount of reactive water is necessary to achieve high sand mold strength. In particular, controlling the amount of water in the organic layer is important as a method for controlling the solubility of the organic layer. Furthermore, by devising a method for dispersing the hardener in the organic layer, more desirable effects can be achieved. By incorporating these into the manufacturing method of the present invention, stable sand mold strength can be achieved with relatively small amounts of hardener and resin. On the other hand, even in foundry sand that has a higher intergranular moisture content than dry sand but an external appearance that is between a dry and wet state (hereinafter also referred to as an "intermediate state"), the presence of the organic layer allows for uniform dispersion of the hardener, thereby enabling the sand to exhibit sufficiently high strength. [Brief explanation of the drawings]

[0042] [Figure 1] 1 shows a schematic diagram of the particles of the foundry sand of the present invention. [Figure 2] 1 is a schematic diagram of a particle of foundry sand according to a first embodiment of the present invention. [Figure 3] 1 is a schematic diagram of a particle of foundry sand according to a second embodiment of the present invention. [Figure 4] 1 shows the appearance of Example 23 after the water-based mold wash has dried and the appearance of Comparative Example 11 after the water-based mold wash has dried. [Figure 5] 1 shows the appearances of the molding sands of Example 3, Example 21 and Comparative Example 9, respectively. [Figure 6] The results of the flowability test for Example 21 and Comparative Example 9 are shown below. DETAILED DESCRIPTION OF THE INVENTION

[0043] 1. Casting sand for 3D additive manufacturing The foundry sand for three-dimensional additive manufacturing of the present invention (the foundry sand of the present invention) is foundry sand used for three-dimensional additive manufacturing, (1) The particles constituting the foundry sand include (a) sand grains and (b) an organic layer formed on the surface of the sand grains, (2) The organic layer contains a furan resin precursor or a fatty acid having a melting point of 40 to 75°C and a heteropolyacid. It is characterized by:

[0044] A schematic diagram of a particle of the foundry sand of the present invention is shown in Fig. 1. The foundry sand (particle) 10 shown in Fig. 1 has an organic layer 12 formed on the surface of a core sand grain 11. It is particularly desirable that substantially the entire surface of the sand grain 11 be covered with the organic layer 12, but as long as the effects of the present invention are not impaired, some portions may be left uncovered by the organic layer.

[0045] The organic layer 12 contains a furan resin precursor or a fatty acid having a melting point of 40 to 75° C., and a heteropolyacid. That is, the present invention encompasses a case in which the organic layer contains a furan resin precursor and a heteropolyacid, a case in which the organic layer contains a fatty acid having a melting point of 40 to 75° C., and a heteropolyacid.

[0046] The structure of the organic layer 12 is not particularly limited, but preferred embodiments include (a) a structure in which solid particles of a heteropolyacid are supported on a carrier containing a furan resin precursor or a fatty acid having a melting point of 40 to 75°C (Embodiment 1), and a structure in which an aqueous wetting layer containing a heteropolyacid and a hydrophobic layer containing a furan resin precursor or a fatty acid having a melting point of 40 to 75°C are formed on the surface of sand grains, and the surface of the aqueous wetting layer is covered with the hydrophobic layer (Embodiment 2).

[0047] FIG. 2 is a schematic diagram of the first embodiment. The foundry sand 10 shown in FIG. 2 has a carrier (carrier layer) 12a containing a furan resin precursor or a fatty acid with a melting point of 40 to 75°C formed around sand grains 11, and solid particles 12b of heteropolyacid supported on the carrier. The solid particles 12b of heteropolyacid need only be fixed to the carrier, and may be contained within the carrier or adhered to the surface of the carrier. The properties of the foundry sand according to the first embodiment are not limited, but due to the structure described above, it is usually in a dry state (dry powder).

[0048] FIG. 3 is a schematic diagram of the second embodiment. The foundry sand 10 shown in FIG. 3 has an aqueous wetting layer 22a containing a heteropolyacid formed around each sand grain 11, and a hydrophobic layer 22b containing a furan resin precursor or a fatty acid with a melting point of 40 to 75°C formed on the aqueous wetting layer 22a. Since the surface of the aqueous wetting layer 22a is substantially covered by the hydrophobic layer 22b, evaporation of the water contained in the aqueous wetting layer 22a is prevented, thereby maintaining the wet state. The properties of the foundry sand according to the second embodiment are not limited, but due to the structure described above, it is usually in a wet state (wet mixture). In this case, the heteropolyacid is contained in the aqueous wetting layer in the form of a solution (particularly an aqueous solution).

[0049] (A) Materials Constituting the Foundry Sand of the Present Invention (A-1) Grains of sand Sand grains (i.e., sand grains before being coated with an organic layer) are used as the core particles that make up the foundry sand of the present invention. Either natural silica sand or artificial sand can be used as the sand, which is an aggregate of sand grains. Examples of artificial sand include mullite, spinel, and alumina. In addition, a mixture of new natural silica sand and roasted recycled sand may also be used. In the present invention, it is preferable to use new artificial sand from the viewpoint of suppressing igloss as much as possible. There is no limitation on whether the artificial sand is produced by the melting method or the sintering method.

[0050] The particle size of the sand particles used as bone sand is not particularly limited, but generally, a particle size index AFS of 35 to 120 is preferred, and more preferably an AFS of 60 to 100. The particle size can be adjusted as needed using known classification methods.

[0051] (A-2) Organic layer The organic layer is a coating that covers part or all of the surface of the sand grain, and preferably covers the entire sand grain.

[0052] In the foundry sand of the present invention, the organic layer does not directly participate in bonding the sand grains that make up the foundry sand before the foundry sand of the present invention is layered. When the humidity between the sand grains is low, the foundry sand product of the present invention appears dry and has excellent fluidity. On the other hand, even in an intermediate state where the humidity between the sand grains is high, the presence of the organic layer ensures that the hardener is uniformly dispersed, and the intermediate state provides sufficient packing and practical layering properties, allowing the product to exhibit relatively high strength.

[0053] The organic layer contains (a1) a furan resin precursor or (a2) a fatty acid having a melting point of 40 to 75°C, and (b) a heteropolyacid.

[0054] (a1) Furan resin precursor The furan resin precursor is not limited as long as it can form a furan resin by condensation polymerization or the like, and examples thereof include furfuryl alcohol, furan resin prepolymer, etc. In particular, it is desirable to use furfuryl alcohol and a furan resin prepolymer in combination in order to suppress reaction heat and reduce the viscosity of the resin.

[0055] Examples of furan resin prepolymers include a polymer of furfuryl alcohol alone, a copolymer of furfuryl alcohol and an aldehyde compound, a copolymer of furfuryl alcohol, urea, and an aldehyde compound (urea-modified furan resin prepolymer), a copolymer of furfuryl alcohol and furfural, etc. These can be used alone or in combination of two or more.

[0056] In the present invention, a copolymer of furfuryl alcohol, urea, and an aldehyde compound (urea-modified furan resin prepolymer) is particularly preferred because it can be easily strengthened. Examples of the aldehyde polymer compound include formaldehyde, acetaldehyde, glyoxal, and furfural. In the present invention, formaldehyde is particularly preferred.

[0057] When furfuryl alcohol and a furan resin prepolymer are used in combination as the furan resin precursor, the content of furfuryl alcohol is preferably in the range of 35 to 60 parts by mass, assuming that the total of both is 100 parts by mass, from the viewpoint of viscosity.

[0058] The content of the furan resin precursor is generally 0.025 to less than 2.2 parts by mass, more preferably 0.03 to 2.1 parts by mass, and most preferably 0.1 to 0.3 parts by mass, per 100 parts by mass of sand. By setting the content within this range, the strength of the resulting sand mold can be more reliably increased.

[0059] The content (solid content) of the furan resin precursor in the organic layer is not particularly limited as long as it is blended to achieve the above-mentioned content, but it is usually about 1 to 90 mass %, and can be in the range of about 20 to 90 mass %. Therefore, it can be set to, for example, 30 to 60 mass %. It can also be set to, for example, 1 to 80 mass %, 1.5 to 60 mass %, or even 5 to 25 mass %.

[0060] (a2) Fatty acids with a melting point of 40 to 75°C In the foundry sand of the present invention, the fatty acid having a melting point of 40 to 75°C (hereinafter simply referred to as "fatty acid") is mainly used to coat the surfaces of the sand grains in order to homogeneously disperse the acid component.

[0061] In the present invention, saturated fatty acids having 12 or more carbon atoms (C12) are preferred, and particularly preferred are saturated fatty acids that are solid at room temperature (40° C. or less) and have a melting point of 40 to 75° C. Among these, it is desirable to use at least one of lauric acid, palmitic acid, and stearic acid.

[0062] The content of the fatty acid is usually 0.015 to less than 1.2 parts by mass, preferably 0.02 to 1.1 parts by mass, and most preferably 0.1 to 0.2 parts by mass, per 100 parts by mass of sand. By setting the content within this range, the bending strength of the resulting sand mold can be more reliably increased.

[0063] The content (solid content) of the fatty acid in the organic layer is not particularly limited as long as it is blended to achieve the above-mentioned content, and therefore can be, for example, in the range of about 1 to 60 mass %, or can be set to, for example, 5 to 50 mass %, or even 6 to 25 mass %.

[0064] (b) Heteropolyacid In the foundry sand of the present invention, the heteropolyacid mainly functions as an acid catalyst (hereinafter also referred to as "acid curing agent") for curing the applied binder used in the lamination step, and by using this, it is possible to provide a sand mold that can exhibit higher strength.

[0065] Heteropolyacids are condensed acids containing two or more elements and oxygen, and the present invention can use one or more of various heteropolyacids. In particular, heteropolyacids that are substantially free of elemental sulfur are preferred. Note that the term "substantially free" in the present invention does not exclude cases where elemental sulfur is present at the level of unavoidable impurities. Therefore, the content of elemental sulfur contained as an unavoidable impurity in commercially available heteropolyacids is acceptable. For example, if the content of elemental sulfur is approximately 50 ppm by mass or less (particularly 0 to 10 ppm by mass), there is almost no problem due to elemental sulfur, and this falls under the category of "substantially free."

[0066] Furthermore, the heteropolyacid is preferably water-soluble. This allows the heteropolyacid to be suitably present in the aqueous wet layer in the form of an aqueous solution. Specifically, the solubility in 100 g of water (at 27°C) is preferably 100 to 1000 g / 100 g of water, and more preferably 300 to 900 g / 100 g of water. By using a compound with such high solubility, the heteropolyacid can be incorporated at a higher concentration than a sulfonic acid-based curing agent (the solubility is approximately 50 g / 100 g of water) at the same temperature, making it possible to achieve higher strength more quickly.

[0067] Furthermore, in the present invention, from the viewpoint of making the heteropolyacid function effectively as an acid curing agent, a heteropolyacid having an acid dissociation constant pKa (25° C.) indicating acid strength of 5 or less can be suitably used.

[0068] Examples of such heteropolyacids include at least one of phosphotungstic acid, silicotungstic acid, phosphomolybdic acid, and silicomolybdic acid, with at least one of silicotungstic acid and phosphotungstic acid being particularly preferred. These heteropolyacids may be known or commercially available. Heteropolyacids obtained by known production methods may also be used.

[0069] The content of the heteropolyacid component in the foundry sand of the present invention is not limited, but is usually about 0.5 to 2.5 parts by mass, more preferably 0.6 to 2.0 parts by mass, and most preferably 1.0 to 1.5 parts by mass, per 100 parts by mass of sand grains. By setting the content within this range, higher sand mold strength can be obtained.

[0070] The content (solid content) of the heteropolyacid in the organic layer is not particularly limited as long as it is blended to achieve the above-mentioned content (ratio to sand grains), but it can usually be set within a range of about 10 to 99% by mass, and can also be set within a range of about 10 to 80% by mass. Therefore, it can be set, for example, to 30 to 60% by mass, or it can be set, for example, to 40 to 99% by mass, or it can be set to 50 to 95% by mass, or even it can be set to 55 to 93% by mass.

[0071] In order to disperse the heteropolyacid efficiently and uniformly among the sand particles, if the heteropolyacid is in the form of a powder, the average primary particle size is usually about 0.6 to 30 μm, preferably 1 to 20 μm.

[0072] A heteropolyacid having such a particle size can be suitably obtained, for example, by using a spray dryer.

[0073] The spraying method of the spray dryer is not particularly limited, and examples thereof include a four-fluid nozzle method, an atomizer disk method, etc. In particular, in the present invention, it is preferable to employ a four-fluid nozzle method, since it can improve dispersibility and produce finer particles.

[0074] The inlet temperature of the spray dryer is not limited, but is usually set to about 100 to 200°C, and preferably set to 150 to 180°C.

[0075] The temperature of the exhaust gas from the spray dryer is usually set to about 50 to 150°C, and it is particularly preferable to set it to 80 to 120°C.

[0076] The pressure inside the spray dryer is not particularly limited, but is usually set to about 0.1 to 1.0 MPa, and is particularly preferably set to 0.3 to 0.8 MPa.

[0077] Furthermore, in order to disperse the heteropolyacid more uniformly in the sand particles, it is also possible to use a solution prepared by dissolving the heteropolyacid in at least one solvent selected from ethanol and water.

[0078] The content of the heteropolyacid in the solution can be appropriately set depending on the type of heteropolyacid and solvent used, but is usually about 10 to 60 mass %, and preferably about 30 to 50 mass %, and therefore can be, for example, 15 to 35 mass %.

[0079] (c) Other ingredients The organic layer may contain other components within the range that does not impair the effects of the present invention, such as at least one of a solvent, a crosslinking agent, and the like.

[0080] As the solvent, for example, water can be preferably used, from the viewpoint that it has a relatively high boiling point and it is easy to control the solubility of the organic layer made of the furan resin precursor.

[0081] As the crosslinking agent, a silane coupling agent can be preferably used. Known or commercially available silane coupling agents can be used. In the present invention, at least one of aminopropylmethyldimethoxysilane and 3-methacryloxypropyltrimethoxysilane can be preferably used because they can react with the organic layer and are less likely to produce silane-derived precipitates.

[0082] When a crosslinking agent is used, its content is not limited, but can generally be, for example, 0.002 to 0.5 parts by mass, and particularly 0.001 to 0.5 parts by mass, per 100 parts by mass of sand grains. Therefore, it can be set to, for example, 0.02 to 0.5 parts by mass. Setting it within this range increases the bonding strength between the organic layer and the sand grains, and also increases the adhesion between the coating binder and the organic layer, resulting in increased sand mold strength. In this case, the content of the crosslinking agent in the organic layer should be within the above-mentioned range, and can be, for example, about 1 to 50% by mass (particularly 20 to 40% by mass), but is not limited thereto.

[0083] 2. Method for producing molding sand of the present invention The method for producing the foundry sand of the present invention is not limited as long as it is a method that can form an organic layer on the sand grains. However, the foundry sand of the first and second embodiments can be suitably produced by the following method.

[0084] (1) Manufacturing Method of the First Embodiment The foundry sand according to the manufacturing method of the first embodiment is a method for manufacturing foundry sand for three-dimensional additive manufacturing, (1) a step of mixing at least sand particles with a coating agent containing a furan resin precursor or a fatty acid having a melting point of 40 to 75°C at 100°C or less to obtain precursor particles whose surfaces are coated with a coating layer containing the furan resin precursor or the fatty acid (precursor particle preparation step); and (2) a step of mixing precursor particles and solid particles of heteropolyacid at 100°C or less to obtain composite particles in which the solid particles are supported on a coating layer on the surface of the precursor particles (composite particle preparation step); The foundry sand can be suitably produced by a method for producing foundry sand characterized by comprising the steps of:

[0085] Precursor particle preparation process In the precursor particle preparation step, at least sand particles and a coating agent containing a furan resin precursor or a fatty acid with a melting point of 40 to 75°C are mixed at 100°C or less to obtain precursor particles in which the surfaces of the sand particles are coated with a coating layer containing the furan resin precursor or the fatty acid.

[0086] The sand grains, furan resin precursor, fatty acid with a melting point of 40 to 75°C, etc. may be the same as those explained in "1. Foundry sand for laminated sand molds" above.

[0087] As the coating agent, (a1) a furan resin precursor or (a2) a coating agent containing a fatty acid having a melting point of 40 to 75°C is used, as described above.

[0088] When the above-mentioned (a1) furan resin precursor is used, the furan resin precursor is not particularly limited. However, it is preferable to use a combination of furfuryl alcohol and a furan resin prepolymer (particularly, a urea-modified furan resin prepolymer), as shown in the examples, in that a semi-cured furan resin precursor can be more reliably formed.

[0089] In this case, the ratio of the two may be the ratio described above. The ratios in the coating agent can be adjusted to, for example, about 35 to 85 mass% of furfuryl alcohol and about 15 to 65 mass% of furan resin prepolymer, but are not limited thereto. The content of the furan resin precursor in the coating agent is usually about 85 to 100 mass%, and can be set to, in particular, about 90 to 98 mass%, but is not limited thereto.

[0090] The mixing conditions are not limited as long as a coating layer can be formed on the surface of the sand particles with the coating agent, but the temperature is preferably less than 100°C, particularly 70°C or less, with 50 to 60°C being particularly preferred. Note that the temperature here refers to the sand temperature (the same applies hereinafter), i.e., it refers to the temperature of the material itself, not the ambient temperature.

[0091] The coating agent preferably contains a solvent (preferably water), which more reliably brings the organic layer into a semi-cured state. The solvent content can be appropriately set depending on the type of furan resin precursor or solvent used, but is usually about 1 to 10% by mass, and preferably 3 to 7% by mass, of the coating agent.

[0092] When using the fatty acid (a2) above, it is desirable to heat the sand particles before mixing the fatty acid. This allows the fatty acid particles to dissolve and coat the sand particles more uniformly. Specific examples of fatty acids that can be used include those described above.

[0093] The sand temperature is preferably 100°C or less, more preferably 70°C or less, and even more preferably 50 to 60°C.

[0094] In order to disperse the fatty acids efficiently and more uniformly among the sand particles, it is preferable to dissolve the fatty acids in at least one organic solvent having a boiling point of less than 100°C that can dissolve the fatty acids, such as diethyl ether, benzene, or ethanol, and among these, it is more preferable to use ethanol.

[0095] The content of the fatty acid in the solution can be appropriately set depending on the type of saturated fatty acid or solvent used, but is usually preferably 2 to 10% by mass, and more preferably 3 to 5% by mass.

[0096] In this step, the coating agent as described above is used, and this coating agent and sand grains are mixed at 100°C or lower (preferably 60°C or lower, more preferably 40 to 50°C).

[0097] The mixing means is not limited, and for example, a temperature-controllable stirring device (kneader, mixer, etc.) may be used for mixing. These devices may be commercially available products.

[0098] In this way, precursor particles can be obtained in which the surfaces of the sand grains are coated with a coating layer containing the furan resin precursor or fatty acid.

[0099] Composite particle preparation process In the composite particle preparation step, the precursor particles and heteropolyacid powder are mixed at 100° C. or less to obtain composite particles in which the solid particles are supported on the coating layer on the surface of the precursor particles.

[0100] The heteropolyacid may be any of those described above, and for example, at least one of silicotungstic acid and phosphotungstic acid can be suitably used.

[0101] The solid particles of heteropolyacid can be the heteropolyacid powder described above, and therefore, for example, powdered heteropolyacid having an average primary particle size of 0.6 to 30 μm (preferably 1 to 20 μm) can be suitably used.

[0102] Such solid particles of heteropolyacid and precursor particles are mixed at 100°C or lower (preferably 80°C or lower, more preferably 60 to 80°C).

[0103] The mixing means is not limited, and for example, a temperature-controllable stirring device (kneader, mixer, etc.) may be used for mixing. These devices may be commercially available products.

[0104] In this way, composite particles can be obtained in which the solid particles are supported on the coating layer on the surface of the precursor particles. That is, foundry sand can be obtained as composite particles in which solid particles of heteropolyacid are supported on a carrier in the organic layer containing a furan resin precursor or a fatty acid having a melting point of 40 to 75°C.

[0105] If the obtained foundry sand is not immediately subjected to the additive manufacturing process, it is generally desirable to store it in an environment controlled at a temperature of 15°C to 25°C and a humidity of 45% or less, and particularly in an environment controlled at a temperature of 20°C to 25°C and a humidity of 40% or less. When storing it outdoors, it is preferable to store it in a sealed container such as a drum in a sealed state.

[0106] (2) Manufacturing Method of the Second Embodiment The foundry sand according to the manufacturing method of the second embodiment is a method for manufacturing foundry sand for three-dimensional additive manufacturing, (1) a step of mixing at least sand particles with an aqueous solution of a heteropolyacid at 100°C or less to obtain precursor particles in which the surfaces of the sand particles are coated with an aqueous wetting layer containing the aqueous solution (precursor particle preparation step); (2) a step of mixing precursor particles with a furan resin precursor or a coating agent containing a fatty acid having a melting point of 40 to 75°C at 100°C or less to obtain composite particles in which the aqueous wet layer is covered with a hydrophobic layer containing the fatty acid (composite particle preparation step); The foundry sand can be suitably produced by a method for producing foundry sand characterized by comprising the steps of:

[0107] Precursor particle preparation process In the precursor particle preparation step, at least sand particles and an aqueous solution of a heteropolyacid are mixed at 100° C. or less to obtain precursor particles whose surfaces are coated with an aqueous wetting layer containing the aqueous solution.

[0108] As explained above, the heteropolyacid may suitably be at least one of silicotungstic acid and phosphotungstic acid.

[0109] The aqueous solution of heteropolyacid can be prepared by dissolving the heteropolyacid in a solvent. The solvent can be a) water, b) a water-soluble organic solvent, or c) a mixture thereof. In the present invention, at least one of water and ethanol is particularly preferred.

[0110] The content of the heteropolyacid present in the solution is not particularly limited, but is usually about 10 to 80 mass %, and more preferably 10 to 60 mass %.

[0111] As described above, the amount of the aqueous solution of heteropolyacid to be mixed may usually be set so that the amount of heteropolyacid is 0.5 to 2.5 parts by mass per 100 parts by mass of sand grains.

[0112] In particular, in the present invention, it is preferable to adjust the water content so that the mixture of sand particles and the aqueous solution becomes a wet mixture (in the form of a kneaded material or soybean pulp). Therefore, a wet mixture can be suitably obtained by appropriately adjusting the concentration of the aqueous solution. The water content can be adjusted not only by the concentration of the aqueous heteropolyacid solution, but also by the temperature during mixing (volatilization of water), the particle size distribution of the sand particles, the amount used, etc.

[0113] As mentioned above, the mixing temperature may be 100°C or lower, but it is particularly preferably 60°C or lower, and more preferably 40 to 50°C.

[0114] The mixing means is not limited, and for example, a temperature-controllable stirring device (kneader, mixer, etc.) may be used for mixing. These devices may be commercially available products.

[0115] In this way, precursor particles can be obtained in which the surfaces of sand grains are coated with an aqueous wetting layer containing the aqueous solution. The precursor particles having such an aqueous wetting layer on their surfaces constitute the wet mixture. The heteropolyacid is present in the aqueous wetting layer in the form of a solution (particularly an aqueous solution). In this respect, it differs from the first embodiment in which solid particles of heteropolyacid are present.

[0116] Composite particle preparation process In the composite particle preparation step, the precursor particles are mixed with a furan resin precursor or a coating agent containing a fatty acid having a melting point of 40 to 75°C at 100°C or less to obtain composite particles in which the aqueous wet layer is covered with a hydrophobic layer containing the fatty acid.

[0117] As the coating agent containing a furan resin precursor or a fatty acid with a melting point of 40 to 75°C, for example, the same coating agent as that used in the method for producing foundry sand of the first embodiment can be suitably used.

[0118] Such a coating agent and the precursor particles are mixed at 100°C or lower (preferably 70°C or lower, more preferably 50 to 60°C).

[0119] The mixing means is not limited, and for example, a temperature-controllable stirring device (kneader, mixer, etc.) may be used for mixing. These devices may be commercially available products.

[0120] In this way, composite particles can be obtained in which the aqueous wetting layer is covered with a hydrophobic layer containing the fatty acid. That is, foundry sand can be obtained as composite particles in which the organic layer has an aqueous wetting layer containing a heteropolyacid formed on the surface of the sand grain and a hydrophobic layer containing a furan resin precursor or a fatty acid with a melting point of 40 to 75°C, and the surface of the aqueous wetting layer is covered with the hydrophobic layer.

[0121] 3. 3D additive manufacturing kit The present invention encompasses a 3D additive manufacturing kit comprising the foundry sand for 3D additive manufacturing of the present invention and a binder for application. That is, the foundry sand of the present invention and the binder are stored separately before use, and are provided as a two-component kit in which the two are mixed at the time of use.

[0122] The foundry sand of the present invention, when it has low intergranular humidity as in the first embodiment, is in the form of loose, dry sand, and can be directly spread (sprinkled) by filling it into a 3D additive manufacturing machine. In other words, it is possible to allow the individual grains of foundry sand to fall naturally. On the other hand, when it has intermediate intergranular humidity as in the second embodiment, the filling properties may be lower than those of dry sand, but it can still exhibit practical strength.

[0123] The foundry sand of the present invention may contain additives other than the foundry sand of the present invention, provided that the effects of the present invention are not impaired. For example, various additives such as anti-slip agents and thickeners, as described below, may be mixed with the foundry sand beforehand.

[0124] The coating binder is a binder that is spread onto the formed foundry sand layer during 3D additive manufacturing. In particular, when manufacturing a sand mold using a 3D additive manufacturing machine, the viscosity (25°C) of the coating binder is preferably 1 to 15 mPa s so that it can be stably discharged from the print head.

[0125] The type of coating binder is not particularly limited, and any binder that can be used in a publicly known or commercially available three-dimensional additive manufacturing machine may be used. In particular, in the present invention, it is preferable to use a coating binder containing a furan resin precursor. The content of the furan resin precursor in the binder can be, for example, 80 to 100 mass %, particularly 90 to 100 mass %, but is not limited thereto.

[0126] The furan resin precursor is not limited as long as it can form a furan resin by condensation polymerization or the like, and examples thereof include furfuryl alcohol, furan resin prepolymer, etc. In particular, it is desirable to use furfuryl alcohol and furan resin prepolymer in combination for the reasons of suppressing reaction heat and reducing the viscosity of the resin.

[0127] Examples of furan resin prepolymers include a polymer of furfuryl alcohol alone, a copolymer of furfuryl alcohol and an aldehyde compound, a copolymer of furfuryl alcohol, urea and an aldehyde compound (urea-modified furan resin prepolymer), a copolymer of furfuryl alcohol and furfural, etc. These can be used alone or in combination of two or more.

[0128] In the present invention, a copolymer of furfuryl alcohol, urea, and an aldehyde compound (urea-modified furan resin prepolymer) is particularly preferred because it can be easily strengthened. Examples of the aldehyde compound include formaldehyde, acetaldehyde, glyoxal, and furfural. In the present invention, formaldehyde is particularly preferred.

[0129] The binder for application may contain other components as long as they do not impair the effects of the present invention. For example, additives such as a solvent, a crosslinking agent, and a curing accelerator may be added as needed. In particular, water is preferred as the solvent in the present invention because it slows the reaction rate and tends to increase the final sand mold strength.

[0130] Examples of the curing accelerator include at least one of resorcinol, cresol, hydroquinone, phloroglucinol, methylene bisphenol, bishydroxymethylfuran, etc. Among these, resorcinol is particularly preferred because it has a fast reaction rate and is likely to increase the strength of the sand mold.

[0131] The amount of hardening accelerator added is adjusted as needed in consideration of the amount of reaction water generated, etc., and therefore it is not an essential component and does not necessarily have to be included.

[0132] Furthermore, since the foundry sand, the other component of the kit, has an organic layer containing an acid component, the coating binder can strongly bond the foundry sands together without the need for a silane coupling agent. Therefore, the content of the silane coupling agent in the coating binder is usually about 0 to 1% by mass, and preferably 0 to 0.1% by mass. Therefore, the composition may not contain a silane coupling agent. Alternatively, the content may be set to, for example, 0.05 to 0.15% by mass. By reducing the content of the silane coupling agent to a low level or 0% by mass, the coating binder can be stored stably for a longer period of time.

[0133] Furthermore, an amine compound can be added to the coating binder as needed. By adding an amine compound, the change in viscosity of the furan resin precursor over time can be effectively suppressed. From this perspective, alkylamines having 10 or less carbon atoms are preferred as the amine compound, and butylamine is particularly preferred.

[0134] The amount of the amine compound added is not limited, but if it is added in excess, problems such as a slower curing rate and the induction of gas defects due to nitrogen may occur. Therefore, the content of the amine compound in the coating binder is preferably about 0.001 to 1 mass%, more preferably 0.001 to 0.5 mass%, and even more preferably 0.005 to 0.1 mass%, and most preferably 0.01 to 0.05 mass%. Therefore, it is possible to set it within the range of 0 to 0.5 mass%, for example.

[0135] Regarding the amount of coating binder used, due to the mechanical settings of the printer head, the upper limit that can be applied is preferably 3 parts by mass or less per 100 parts by mass of sand. Usually, it is in the range of about 0.5 to 3 parts by mass per 100 parts by mass of the foundry sand of the present invention, and 1 to 3 parts by mass is particularly preferred. If the amount of coating binder is too small, the sand mold may not have sufficient strength. Therefore, as long as the amounts of foundry sand and binder are in the above ratio at the time of use, it is not necessary for the kit of the present invention to be packaged in the above ratio.

[0136] In the present invention, the type of furan resin precursor contained in the coating binder and the type of furan resin precursor contained in the organic layer of the foundry sand of the present invention may be the same or different. Therefore, for example, if the organic layer of the foundry sand of the present invention contains a combination of furfuryl alcohol and a furan resin prepolymer (especially a urea-modified furan resin prepolymer), a coating binder containing a combination of furfuryl alcohol and a furan resin prepolymer (especially a urea-modified furan resin prepolymer) can be used.

[0137] In this case, particularly when the same types of organic layers are used, the organic layer of the foundry sand and the coating binder will blend well together, resulting in higher sand mold strength. For example, the foundry sand of the present invention, whose organic layer contains a combination of furfuryl alcohol and a furan resin prepolymer (particularly a urea-modified furan resin prepolymer), can be used as foundry sand suitable for combination with a coating binder containing furfuryl alcohol and a furan resin prepolymer as the coating binder used in additive manufacturing.

[0138] However, even when the same types are used, it is not necessarily necessary to completely match, for example, the composition ratio, additives, etc., and it is possible to fine-tune the compounding ratio of furfuryl alcohol, furan resin polymer, etc., the type of additive, etc., while taking into consideration the balance between the strength of the casting and the curing time, and also taking into account the characteristics of the 3D additive manufacturing machine to be used, etc.

[0139] 4. Use of molding sand for 3D additive manufacturing The foundry sand of the present invention can be used for lamination in the same manner as known foundry sands. More specifically, the foundry sand of the present invention can be suitably used as the foundry sand in a method for producing a sand mold by three-dimensional additive manufacturing, which includes sequentially repeating the steps of forming a layer containing foundry sand (layer formation step) and adding a coating binder to a predetermined region of the layer and solidifying that region (coating binder addition step). As explained above, a sand mold having a desired shape can be produced by supplying the foundry sand of the present invention to a known or commercially available three-dimensional additive manufacturing machine. In other words, the foundry sand of the present invention can be suitably used as the foundry sand for use in a three-dimensional additive manufacturing machine.

[0140] Such a 3D additive manufacturing machine may, for example, have a unit, a foundry sand supply unit, a coating binder supply unit, and an operation unit, and may be capable of producing a sand mold from 3D-CAD data. The foundry sand supply unit supplies foundry sand to the unit, and includes a foundry sand tank that stores the foundry sand and a recoater that can eject the foundry sand while moving horizontally. The coating binder supply unit includes a print head that ejects the coating binder to the unit. Such a device itself may be a publicly known or commercially available device.

[0141] When manufacturing a sand mold using 3D additive manufacturing, an additive manufacturing method can be used that includes sequentially repeating the steps of forming a layer containing foundry sand (layer formation step) and adding a coating binder to a predetermined area of ​​the layer to solidify that area (coating binder addition step).In this process, the hardener contained in the organic layer of the foundry sand, in which the surfaces of the sand grains are made of furan resin, contributes to the solidification of both the organic layer and the coating binder, thereby obtaining the desired sand mold.

[0142] Layer formation process In the layer formation process, a layer containing foundry sand is formed. More specifically, a layer containing foundry sand can be formed by sprinkling the foundry sand from above. The foundry sand of the present invention is essentially a loose, dry sand, so it can be allowed to fall naturally (spread). Even when the humidity between the sand grains is high, when using a 3D additive manufacturing machine such as the one described above, the layer can be reliably formed by discharging the foundry sand onto a flat surface from a horizontally moving recoater.

[0143] In the layer forming step, the layer may contain components other than the foundry sand, as long as they do not interfere with smooth layer formation. Such components may include components added to known foundry sands.

[0144] In conventional AM processes (using a two-liquid mixing process involving wet sand, a hardener, and a binder resin for application), when artificial sand produced by a melting process or other method is used as the foundry sand, its surface smoothness and roundness can cause liquid bridging (agglomeration) between the foundry sand particles. This impairs the fluidity of the sand, which is important for AM processes, making it difficult to use small grain sizes. For this reason, a drying process has been proposed to dry the sand and improve its fluidity, but this limits the usable sand to sintered artificial sand. In contrast, the foundry sand of the present invention can be more reliably laminated regardless of sand type or grain size by adding a fatty acid such as linoleic acid to the organic layer as an anti-slip agent, or by using foundry sand in a state where the intergranular humidity is high (i.e., a so-called intermediate state). This prevents the foundry sand layer of the present invention from collapsing, even when using melting artificial sand, which has high smoothness and roundness, inside the build box (job box) of a 3D AM machine.

[0145] The thickness of the layer depends on the particle size of the foundry sand, the desired shape of the sand mold, etc., but is usually about 100 to 400 μm, and preferably 200 to 300 μm. When the layer formation step is repeated, the thickness of the layers formed in each layer formation step may be the same or different.

[0146] Coating binder addition process A coating binder is added (sprayed) to a predetermined area of ​​the layer, causing that area to solidify. A single layer containing molding sand is formed in the layer formation process, and the coating binder is added to an area based on the data for the cross-sectional shape of the desired sand mold. For example, when using a 3D additive manufacturing machine such as the one described above, the predetermined area can be solidified by spraying the coating binder onto the layer from a horizontally moving print head (nozzle). The coating binder, etc., can also be one described in "3. 3D additive manufacturing kit" above.

[0147] After the series of steps consisting of the layer forming step and the coating binder adding step are repeated, the portions to which the coating binder has not been added are removed, thereby obtaining a sand mold having the desired shape.

[0148] The sand mold obtained as described above can be aged as needed. This accelerates the curing of the resin component, resulting in a sand mold with greater strength. The aging conditions can be, for example, a temperature of 15 to 25°C and a humidity of 45% or less, particularly a temperature of 20 to 25°C and a humidity of 40% or less, but are not limited thereto. For example, a temperature of 20 to 25°C and a humidity of 30 to 45% can also be used. The aging time can also be, for example, about 1 to 48 hours, but is not limited thereto.

[0149] Furthermore, a step of applying a mold wash to the obtained sand mold can be carried out as needed. The method for applying the mold wash to the sand mold is not particularly limited, and examples thereof include brushing, dipping, etc. Brushing is also acceptable, but dipping is preferable if the mold wash is to be applied more uniformly over the entire sand mold, but is not limited to this. In this case, either an aqueous or non-aqueous mold wash can be used as the mold wash, but it is preferable to use an aqueous mold wash from the standpoint of safety, etc.

[0150] However, dipping sand molds made from conventional additive manufacturing molding sand into a water-based mold wash is not recommended. This is because the furan resin precursor is not sufficiently polymerized in the sand mold immediately after lamination. In addition, dipping the sand mold into a water-based mold wash also allows the water contained in the water-based mold wash to penetrate into the mold, reducing the strength of the sand mold or causing deformation of the sand mold when the water-based mold wash dries.

[0151] From this perspective, in the present invention, as described below, it is desirable to heat the sand mold after the aging process or instead of the aging process before applying the aqueous mold wash, thereby promoting the curing reaction between the moisture contained in the sand mold and the unreacted furan resin precursor and bringing it closer to complete curing. In this case, the heat treatment temperature is preferably, for example, about 40 to 250°C, more preferably 50 to 180°C, and most preferably 50 to 100°C, but is not limited thereto. The heat treatment time is also preferably, for example, 10 minutes to 2 hours, and more preferably 10 minutes to 1 hour, but is not limited thereto.

[0152] The foundry sand of the present invention controls the reactive water generated during the polymerization of the furan resin precursor (suppressing poor internal curing). Additionally, by performing a heat treatment (pre-drying) before applying the aqueous mold wash, the curing reaction of the furan resin can be accelerated, leading to near complete curing. This prevents a decrease in bending strength even when the aqueous mold wash is applied to the sand mold (e.g., by dipping), and effectively prevents deformation that may occur in the sand mold when the aqueous mold wash dries.

[0153] Sand molds obtained using suitable casting sand in this way can realize high strength and a high degree of design reproducibility. They are also suitable for mass production, with the sand mold usually being removed in less than three hours, and in as fast as one hour. This means that additive manufacturing can be widely applied to the manufacture of casting products that require mass production.

[0154] 5.Sand mold The present invention includes a sand mold (particularly a sand mold for casting iron-based materials) containing the molding sand for 3D additive manufacturing of the present invention. The sand mold of the present invention is constructed by bonding and adhering individual pieces of the molding sand of the present invention to each other, and can exhibit high strength. In particular, the present invention also includes a sand mold (particularly a sand mold for casting iron-based materials) containing a water-based mold wash.

[0155] The sand mold of the present invention can be used as a casting sand mold for casting virtually any material (especially metals), but is particularly suitable for casting using iron-based materials. Generally, when casting iron-based materials, if the sand mold (or the molding sand that constitutes it) contains sulfur, sulfurization of the casting occurs, which can cause poor graphite spheroidization (metal structure), which affects the quality of the casting, and can lead to a decrease in the strength of the casting. In contrast, the molding sand that constitutes the sand mold of the present invention is substantially free of sulfur, thereby avoiding the problems associated with sulfur.

[0156] The sand mold of the present invention can be manufactured by the above-mentioned three-dimensional additive manufacturing method, but the sand mold can be particularly suitably manufactured by the following method.

[0157] That is, a method for manufacturing a sand mold using the molding sand for three-dimensional additive manufacturing of the present invention, (1) a step of forming a layer containing the molding sand and a step of adding a coating binder to a predetermined area of ​​the layer to solidify the area, and repeating these steps in sequence to obtain a molded body (lamination step); (2) A step of obtaining a sand mold by heat-treating the green body not containing an aqueous mold wash at 40 to 250°C in air or an inert gas atmosphere (heat-treatment step). The sand mold can be suitably manufactured by a method for manufacturing a sand mold, which is characterized by comprising the steps of:

[0158] Lamination process In the lamination step, a step of forming a layer containing the foundry sand and a step of adding a coating binder to a predetermined area of ​​the layer to solidify the area are repeated in sequence to obtain a molded body.

[0159] The lamination process may be carried out in the same manner as the 3D additive manufacturing method described above in "4. Use of molding sand for 3D additive manufacturing."

[0160] Heat treatment process In the heat treatment step, the green body not containing the aqueous mold wash is heat treated at 40 to 250°C in the air or in an inert gas atmosphere to obtain a sand mold. That is, the heat treatment is performed before the aqueous mold wash is added (applied) to the green body. This makes it possible to more reliably suppress deformation of the sand mold that may occur when the aqueous mold wash is applied to the sand mold.

[0161] The heat treatment step may be carried out in the same manner as the heat treatment described above in "4. Use of molding sand for 3D additive manufacturing."

[0162] Therefore, the heat treatment temperature is usually 40 to 250° C., and preferably 50 to 180° C., and most preferably 50 to 100° C., but is not limited to this. The heat treatment time can also usually be about 10 minutes to 2 hours, and more preferably 10 minutes to 1 hour, but is not limited to this.

[0163] The heating atmosphere may be atmospheric air or an inert gas atmosphere. The inert gas is not limited, and any of nitrogen gas, argon gas, helium gas, etc. may be used.

[0164] Generally, when producing an iron-based molded body by casting an iron-based material in a sand mold, an aqueous mold wash (especially an aqueous dispersion of an inorganic material) is applied to at least the area of ​​the sand mold that comes into contact with the iron-based material (molten metal). When an aqueous mold wash is applied to a conventional sand mold, the sand mold may deform or crack. In contrast, the sand mold of the present invention uses a heteropolyacid (which has a higher solubility in water than sulfonic acid) as an acid curing agent, which effectively disperses (penetrates) to the center of the bridges between sand grains that form the cured body of the furan resin precursor, thereby achieving high strength.

[0165] In addition, by carrying out the heat treatment as described above, the furan resin precursor can be brought closer to complete curing more reliably. Moreover, since the heteropolyacid, which is an acid curing agent, is thoroughly dispersed as described above, the heat treatment can further promote three-dimensional polymerization of the furan resin precursor, which can effectively contribute to preventing deformation (deformation of the sand mold, generation of cracks, etc.).

[0166] The sand mold thus obtained can be used as a casting sand mold for casting various materials (particularly metals), as described above, but is particularly suitable for casting iron-based materials.

[0167] When an iron-based molded product is produced by casting an iron-based material using the sand mold of the present invention, for example, a production method including the steps of: a) applying a water-based mold wash to the sand mold of the present invention; and b) pouring molten iron-based material into the sand mold can be suitably used.

[0168] The above step a) can be carried out in the same manner as the above-mentioned coating step, but if the sand mold has been coated with a water-based mold wash in advance, the above step a) can be omitted.In this case, therefore, an iron-based molded product can be suitably produced by a method including a step of pouring a molten iron-based material into the sand mold of the present invention containing the water-based mold wash.

[0169] A water-based mold wash in which an inorganic component is dispersed in an aqueous solvent can be preferably used. As the inorganic component, for example, a water-based mold wash containing at least one oxide such as silica, alumina, magnesia, or zirconia can be preferably used. These may be publicly known or commercially available products. Therefore, various additives (dispersants, antifoaming agents, thickeners, etc.) contained in commercially available products may be contained in the water-based mold wash within a range that does not impair the effects of the present invention.

[0170] The aqueous solvent may be (a) water, (b) a water-soluble organic solvent, or (c) a mixture of water and a water-soluble organic solvent (aqueous solution).The water-soluble organic solvent is not limited, but may be at least one alcohol such as methanol, ethanol, or isopropyl alcohol.

[0171] The method for applying the water-based mold wash is not limited, and any of various application methods such as dipping, spraying, brushing, roller, etc. can be used depending on the size of the sand mold, etc. The water-based mold wash may be applied to at least the area of ​​the sand mold surface that comes into contact with the molten metal, for example, but the entire sand mold may also be coated.

[0172] The coating step may be carried out immediately after the heat treatment step in the sand mold manufacturing method described above, but it may also be carried out immediately before use (casting) of the sand mold after the heat treatment step. For example, after manufacturing a sand mold that has undergone the heat treatment step, a water-based mold wash can be applied to the sand mold when the sand mold is to be used after a certain period of time has elapsed (more specifically, before pouring molten metal into the sand mold).

[0173] After the aqueous mold wash has been applied to the surface of the sand mold, a drying step may be carried out. Drying may be either natural drying or heat drying, but in the case of heat drying, it is usually carried out at a temperature in the range of 50 to 180°C, more preferably 80 to 120°C. The heat drying time can be appropriately changed depending on the heating temperature, etc., but can usually be appropriately determined within a range of about 30 to 120 minutes.

[0174] Next, in the above step b), the molten iron-based material is poured into the sand mold. More specifically, the molten iron-based material is poured into the sand mold so as to contact the surface of the sand mold on which the coating film made of the water-based mold wash has been formed. The iron-based material may be any material containing iron as the main component (especially with an iron content of 50% by mass or more), and may have a composition suitable for various iron-based products.

[0175] After the molten iron-based material is poured and casting is completed, the sand mold is broken and the iron-based molded body is taken out according to a known method. [Example]

[0176] Examples and comparative examples are shown below to explain the characteristics of this study in more detail. However, the scope of the present invention is not limited to the examples. In each table, "%" indicating the content of the composition means "% by mass."

[0177] Example 1 300 g of new sand (ARS fused new sand, AFS65 mullite-based) was prepared as sand particles (Alsand #650, manufactured by Ito Kiko Co., Ltd., angle of repose 25°) (ARS fused new sand). Palmitic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as a coating agent, and tungstosilicic acid (manufactured by Nippon Inorganic Chemical Industry Co., Ltd., D50 = 1.0 μm) was used as an acid hardener. While heating the sand, 0.2 parts by weight of palmitic acid was added per 100 parts by weight of the artificial sand when the sand temperature reached 50°C, and the mixture was stirred and mixed for 60 seconds. The sand temperature was 70°C. Next, 1.0 part by weight of tungstosilicic acid per 100 parts by weight of the artificial sand was added to the resulting mixture, and the mixture was stirred and mixed for 120 seconds to obtain dry foundry sand.

[0178] Example 2 The same dry molding sand as in Example 1 was prepared.

[0179] Example 3 The sand grains were replaced with 300 g of new sand (CBX sintered new sand) made with AFS64 mullite-based sintered artificial sand (Cerabeads X♯650, manufactured by Itochu Ceratec Co., Ltd., angle of repose 28°), palmitic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and the acid hardener tungstosilicic acid (manufactured by Nippon Inorganic Chemical Industry Co., Ltd., D50 = 1.0 μm). While heating the sand, 0.1 parts by weight of palmitic acid was added per 100 parts by weight of the artificial sand when the sand temperature reached 50°C, and the mixture was stirred and mixed for 60 seconds. The sand temperature was 70°C. Next, 1.0 parts by weight of tungstosilicic acid per 100 parts by weight of the artificial sand was added to the resulting mixture, and the mixture was stirred and mixed for 120 seconds to obtain dry foundry sand.

[0180] Example 4 The sand grains were replaced with 300 g of new AFS78 mullite-based fusion-process artificial sand (Yamakawa Sangyo Co., Ltd., Espearl #75L, angle of repose 24°) (EP fusion new sand). Palmitic acid (Fujifilm Wako Pure Chemical Industries, Ltd.) and tungstosilicic acid (Nippon Inorganic Chemical Industry Co., Ltd., D50 = 1.0 μm) were also added as an acid hardener. While heating the sand, 0.1 parts by weight of palmitic acid was added per 100 parts by weight of the artificial sand when the sand temperature reached 50°C, and the mixture was stirred and mixed for 60 seconds. The sand temperature was 67°C. Next, 1.0 parts by weight of tungstosilicic acid per 100 parts by weight of the artificial sand was added to the resulting mixture, and the mixture was stirred and mixed for 120 seconds to obtain dry foundry sand.

[0181] Example 5 The sand grains were changed to 300 g of new AFS96 natural silica sand (Albany #90, angle of repose 36°, manufactured by Tochu Corporation) (ALB silica sand new sand), palmitic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and tungstosilicic acid (D50 = 1.0 μm, manufactured by Nippon Inorganic Chemical Industry Co., Ltd.) as an acid hardener. While heating the sand, 0.2 parts by mass of palmitic acid was added per 100 parts by mass of silica sand when the sand temperature reached 50°C, and the mixture was stirred and mixed for 60 seconds. The sand temperature was 70°C. Next, 1.0 part by mass of tungstosilicic acid per 100 parts by mass of silica sand was added to the resulting mixture, and the mixture was stirred and mixed for 120 seconds to obtain dry foundry sand.

[0182] Example 6 The sand grains were replaced with AFS56 roasted reclaimed sand (Tochu Corporation, reclaimed sand No. 6, angle of repose 34°) (roasted reclaimed sand) and AFS96 virgin natural silica sand in a 7:3 ratio by mass. Palmitic acid (Fujifilm Wako Pure Chemical Industries, Ltd.) and tungstosilicic acid (Nippon Inorganic Chemical Industry Co., Ltd., D50 = 1.0 μm) were also added as an acid hardener. While heating the sand, 0.2 parts by mass of palmitic acid was added per 100 parts by mass of the mixed sand when the temperature of the mixed sand reached 50°C, and the mixture was stirred and mixed for 60 seconds. The sand temperature was then 70°C. Next, 1.0 part by mass of tungstosilicic acid per 100 parts by mass of the mixed sand was added to the resulting mixture, and the mixture was stirred and mixed for 120 seconds to obtain dry molding sand.

[0183] Example 7 Molding sand was prepared in the same manner as in Example 1, except for the composition and conditions shown in Table 4. In particular, the following changes were made to Example 1: 1.1 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand, and the mixture was stirred and mixed for 60 seconds. The sand temperature was 75°C. Next, 1.5 parts by mass of tungstosilicic acid was added to the resulting mixture, and the mixture was stirred and mixed for 120 seconds to obtain dry molding sand.

[0184] Example 8 Molding sand was prepared in the same manner as in Example 1, except for the composition and conditions shown in Table 4. In particular, the following changes were made to Example 1: 0.02 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand, and the mixture was stirred and mixed for 60 seconds. The sand temperature was 70°C. Next, 1.5 parts by mass of tungstosilicic acid was added to the resulting mixture, and the mixture was stirred and mixed for 120 seconds to obtain dry molding sand.

[0185] Example 9 Foundry sand was prepared in the same manner as in Example 3, except for the composition and conditions shown in Table 4. In particular, the following changes were made to Example 3: 0.1 parts by mass of stearic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added as a coating agent to 100 parts by mass of the artificial sand, and the mixture was stirred and mixed for 60 seconds. The sand temperature was 70°C. Next, 1.0 part by mass of tungstosilicic acid was added to the resulting mixture to 100 parts by mass of the artificial sand, and the mixture was stirred and mixed for 120 seconds to obtain dry foundry sand.

[0186] Example 10 Foundry sand was prepared in the same manner as in Example 3, except for the composition and conditions shown in Table 4. In particular, the following changes were made to Example 3: 0.1 parts by mass of lauric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added as a coating agent to 100 parts by mass of the artificial sand, and the mixture was stirred and mixed for 60 seconds. The sand temperature was 69°C. Next, 1.0 part by mass of tungstosilicic acid was added to the resulting mixture to 100 parts by mass of the artificial sand, and the mixture was stirred and mixed for 120 seconds to obtain dry foundry sand.

[0187] Example 11 Molding sand was prepared in the same manner as in Example 3, except for the composition and conditions shown in Table 5. In particular, the following changes were made to Example 3: 0.1 parts by mass of the furan resin composition "Configuration 1" in Table 1 was added as a coating agent to 100 parts by mass of artificial sand, and the mixture was stirred and mixed for 60 seconds. The sand temperature was 60°C. Next, 1.0 part by mass of tungstosilicic acid was added to the resulting mixture to 100 parts by mass of artificial sand, and the mixture was stirred and mixed for 120 seconds to obtain dry molding sand.

[0188] [Table 1]

[0189] Example 12 Foundry sand was prepared in the same manner as in Example 1, except for the composition and conditions shown in Table 5. In particular, the following changes were made to Example 1: 0.2 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand, and the mixture was stirred and mixed for 60 seconds. The sand temperature was 66°C. Next, a solution prepared by mixing 1.0 part by mass of phosphotungstic acid (manufactured by Nippon Inorganic Chemical Industry Co., Ltd., D50 = 55 μm) as an acid hardener and 1.0 part by mass of ethanol as a solvent was added to the resulting mixture, and the mixture was stirred and mixed for 120 seconds to obtain dry foundry sand.

[0190] Example 13 Foundry sand was prepared in the same manner as in Example 3, except for the composition and conditions shown in Table 5. In particular, the following changes were made to Example 3: 0.1 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand, and the mixture was stirred and mixed for 60 seconds. The sand temperature was 70°C. Next, 1.0 part by mass of tungstosilicic acid (manufactured by Nippon Inorganic Chemical Industry Co., Ltd., D50 = 20 μm) was added to the resulting mixture as a hardener, and the mixture was stirred and mixed for 120 seconds to obtain dry foundry sand.

[0191] Example 14 Foundry sand was prepared in the same manner as in Example 3, except for the composition and conditions shown in Table 5. In particular, the following changes were made to Example 3: 0.1 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand, and the mixture was stirred and mixed for 60 seconds. The sand temperature was 80°C. Next, a solution prepared by mixing 1.0 parts by mass of tungstosilicic acid as an acid hardener and 1.0 parts by mass of ethanol as a solvent per 100 parts by mass of artificial sand was added to the resulting mixture, and the mixture was stirred and mixed for 120 seconds to obtain dry foundry sand.

[0192] Example 15 Molding sand was prepared in the same manner as in Example 1, except for the composition and conditions shown in Table 5. In particular, the following changes were made to Example 1: 0.1 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand, and the mixture was stirred and mixed for 60 seconds. The sand temperature was 76°C. Next, 2.0 parts by mass of tungstosilicic acid was added to the resulting mixture, and the mixture was stirred and mixed for 120 seconds to obtain dry molding sand.

[0193] Example 16 Molding sand was prepared in the same manner as in Example 1, except for the composition and conditions shown in Table 5. In particular, the following changes were made to Example 1: 0.1 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand, and the mixture was stirred and mixed for 60 seconds. The sand temperature was 70°C. Next, 0.5 parts by mass of tungstosilicic acid was added to the resulting mixture, and the mixture was stirred and mixed for 120 seconds to obtain dry molding sand.

[0194] Example 17 The sand grains were replaced with 300 g of new sand (AFS64 mullite-based sintered artificial sand) (Cerabeads X♯650, manufactured by Itochu Ceratec Co., Ltd., angle of repose 28°) (CBX sintered new sand). The additives included the silane coupling agent "Configuration 2" listed in Table 2, palmitic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and tungstosilicic acid (manufactured by Nippon Inorganic Chemical Industry Co., Ltd., D50 = 1.0 μm) as an acid curing agent. While heating the sand, 0.5 parts by mass of the silane coupling agent (Configuration 2) was added to 100 parts by mass of the artificial sand when the sand temperature reached 50°C, and the mixture was stirred and mixed for 60 seconds. The sand temperature was 66°C. Next, 0.1 parts by mass of palmitic acid was added to 100 parts by mass of the artificial sand, and the mixture was stirred and mixed for 60 seconds. The sand temperature was 77°C. Finally, 1.0 part by mass of tungstosilicic acid was added to the resulting mixture relative to 100 parts by mass of artificial sand, and the mixture was stirred and mixed for 120 seconds to obtain dry foundry sand.

[0195] [Table 2]

[0196] Example 18 Molding sand was prepared in the same manner as in Example 1, except for the composition and conditions shown in Table 5. In particular, the following changes were made to Example 1: 2.1 parts by mass of the furan resin composition "Configuration 1" in Table 1 was added as a coating agent to 100 parts by mass of artificial sand, and the mixture was stirred and mixed for 60 seconds. The sand temperature was 74°C. Next, 1.5 parts by mass of tungstosilicic acid was added to the resulting mixture to the artificial sand, and the mixture was stirred and mixed for 120 seconds to obtain dry molding sand.

[0197] Example 19 Molding sand was prepared in the same manner as in Example 1, except for the composition and conditions shown in Table 5. In particular, the following changes were made to Example 1: 0.03 parts by mass of the furan resin composition "Configuration 1" in Table 1 was added as a coating agent to 100 parts by mass of artificial sand, and the mixture was stirred and mixed for 60 seconds. The sand temperature was 66°C. Next, 1.5 parts by mass of tungstosilicic acid was added to the resulting mixture to 100 parts by mass of artificial sand, and the mixture was stirred and mixed for 120 seconds to obtain dry molding sand.

[0198] Example 20 Molding sand was prepared in the same manner as in Example 1, except for the composition and conditions shown in Table 5. In particular, the following changes were made to Example 1: 0.2 parts by mass of the furan resin composition "Configuration 1" in Table 1 was added as a coating agent to 100 parts by mass of artificial sand, and the mixture was stirred and mixed for 60 seconds. The sand temperature was 60°C. Next, 0.8 parts by mass of tungstosilicic acid was added to the resulting mixture to 100 parts by mass of artificial sand, and the mixture was stirred and mixed for 120 seconds to obtain dry molding sand.

[0199] Example 21 The sand grains were replaced with 300 g of new sand (CBX sintered new sand) made with AFS64 mullite-based sintered artificial sand (Cerabeads X♯650, manufactured by Itochu Ceratec Co., Ltd., angle of repose 28°), palmitic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and silicotungstic acid (manufactured by Nippon Inorganic Chemical Industry Co., Ltd., D50 = 1.0 μm) as an acid hardener. While heating the sand, an aqueous solution of 1.0 parts silicotungstic acid and 3.3 parts water as a solvent was added to 100 parts of the artificial sand when the sand temperature reached 40°C, and the mixture was stirred and mixed for 90 seconds. The sand temperature was 65°C. Next, 0.1 parts palmitic acid per 100 parts of the artificial sand was added to the resulting mixture, and the mixture was stirred and mixed for 60 seconds to obtain intermediate-state foundry sand.

[0200] Comparative Example 1 300 g of new AFS65 mullite-based fusion-process artificial sand (Alsand #650, manufactured by Itoh Kiko Co., Ltd., angle of repose 25°) and tungstosilicic acid (manufactured by Nippon Inorganic Chemical Industry Co., Ltd., D50 = 1.0 μm) as a hardener were prepared. The sand was not heated. When the temperature of the artificial sand was 25°C, 0.4 parts by mass of tungstosilicic acid was added to 100 parts by mass of the artificial sand, and the mixture was mixed and stirred for 120 seconds to obtain dry foundry sand.

[0201] Comparative Example 2 Foundry sand was prepared in the same manner as in Comparative Example 1, except for the composition and conditions shown in Table 6. In particular, the following changes were made to Comparative Example 1: silicotungstic acid was prepared as an acid hardener. The sand was not heated. When the temperature of the artificial sand was 25°C, 1.0 part by mass of silicotungstic acid was added to 100 parts by mass of the artificial sand, and the mixture was mixed and stirred for 120 seconds to obtain dry foundry sand.

[0202] Comparative Example 3 Molding sand was prepared in the same manner as in Example 1, except for the composition and conditions shown in Table 6. In particular, the following changes were made to Example 1: 1.2 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand, and the mixture was stirred for 60 seconds. The sand temperature was 70°C. Next, 1.5 parts by mass of tungstosilicic acid was added to 100 parts by mass of artificial sand, and the mixture was mixed and stirred for 120 seconds to obtain dry molding sand.

[0203] Comparative Example 4 Molding sand was prepared in the same manner as in Example 1, except for the composition and conditions shown in Table 6. In particular, the following changes were made to Example 1: 0.01 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand, and the mixture was stirred for 60 seconds. The sand temperature was 75°C. Next, 1.5 parts by mass of tungstosilicic acid was added to 100 parts by mass of artificial sand, and the mixture was mixed and stirred for 120 seconds to obtain dry molding sand.

[0204] Comparative Example 5 Foundry sand was prepared in the same manner as in Example 1, except for the composition and conditions shown in Table 6. In particular, the following changes were made to Example 1: 0.1 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand, and the mixture was stirred for 60 seconds. The sand temperature was 70°C. Next, 3.0 parts by mass of tungstosilicic acid was added to 100 parts by mass of artificial sand, and the mixture was mixed and stirred for 120 seconds to obtain dry foundry sand.

[0205] Comparative Example 6 Foundry sand was prepared in the same manner as in Example 1, except for the composition and conditions shown in Table 6. In particular, the following changes were made to Example 1: 0.1 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand, and the mixture was stirred for 60 seconds. The sand temperature was 72°C. Next, 0.4 parts by mass of tungstosilicic acid was added to 100 parts by mass of artificial sand, and the mixture was mixed and stirred for 120 seconds to obtain dry foundry sand.

[0206] Comparative Example 7 Molding sand was prepared in the same manner as in Example 11, except for the composition and conditions shown in Table 6. In particular, the following changes were made to Example 11: 2.2 parts by mass of the furan resin composition of "Configuration 1" was added to 100 parts by mass of artificial sand, and the mixture was stirred for 60 seconds. The sand temperature was 64°C. Next, 1.5 parts by mass of tungstosilicic acid was added to 100 parts by mass of the artificial sand, and the mixture was mixed and stirred for 120 seconds to obtain dry molding sand.

[0207] Comparative Example 8 Molding sand was prepared in the same manner as in Example 11, except for the composition and conditions shown in Table 6. In particular, the following changes were made to Example 11: 0.02 parts by mass of the furan resin composition of "Configuration 1" was added to 100 parts by mass of artificial sand, and the mixture was stirred for 60 seconds. The sand temperature was 70°C. Next, 1.5 parts by mass of tungstosilicic acid was added to 100 parts by mass of the artificial sand, and the mixture was mixed and stirred for 120 seconds to obtain dry molding sand.

[0208] Comparative Example 9 The sand grains were changed to 300 g of new sand (AFS64 mullite-based sintered artificial sand) (Cerabeads X♯650, manufactured by Itochu Ceratec Co., Ltd., angle of repose 28°) (CBX sintered new sand) and tungstosilicic acid (manufactured by Nippon Inorganic Chemical Industry Co., Ltd., D50 = 1.0 μm) as an acid hardener. The sand was not heated. When the temperature of the artificial sand was 25°C, an aqueous solution of 1.0 part tungstosilicic acid and 3.3 parts water as a solvent was added to 100 parts of the artificial sand, and the mixture was stirred for 60 seconds to obtain wet foundry sand.

[0209] Test Example 1 (1) Transverse strength (test piece strength (TP strength)) 200 g of each sample was weighed into a metal container. The coating binders listed in Tables 4 to 6 were added to all molding sand samples. Four types of coating binders were used, each with a different furfuryl alcohol content. The compositions are shown in Table 3. The mixture was prepared by stirring for 5 seconds using a stirrer. The resulting mixture was packed into a 10 mm x 10 mm x 60 mm mold and left for 30 minutes under conditions of 23 to 25°C and 40 to 45% humidity, after which it was demolded. In this way, test pieces for measuring bending strength were prepared.

[0210] Furthermore, prior research by the inventors has confirmed that a mixing time of 5 seconds is sufficient to reproduce the strength after 3D lamination in a strength reproduction test conducted in a laboratory without using a 3D printer. To reproduce the conditions of 3D lamination, where it is not possible to forcibly mix the molding sand and coating binder, it is important to shorten the mixing time extremely.

[0211] Then, using the test pieces, the three-point bending strength was measured (a) immediately after punching (after 30 minutes), (b) after 3 hours, and (c) after 24 hours using a force gauge "ZTS-1000N" manufactured by IMADA Co., Ltd. The results are shown in Tables 4 to 6. The strength after 3 hours was 25 kg / cm 2 Those that met the above criteria were deemed to have passed.

[0212] [Table 3]

[0213] (2) Sand grain pore humidity measurement The humidity of the molding sand was measured using a Testo digital thermo-hygrometer "605-H1" in a room regulated to a temperature of 23-25°C and a humidity of 40-45%. Approximately 300g of molding sand sample was placed in a glass beaker, and the sensor of the digital thermo-hygrometer was inserted into the sample until it was completely submerged, after which it was left to stand for 5 minutes. The value (humidity) was read after 5 minutes. The results are shown in Tables 4-6.

[0214] (3) Fluidity test The fluidity test was conducted in a room with a temperature of 23-25°C and humidity of 40-45%. Various types of molding sand were prepared, and 100g of molding sand was weighed into a paper cup and lightly compacted. This process was repeated until the weight of the molding sand in the paper cup reached 300g. A metal bat was then placed over the paper cup containing the molding sand, and the cup and bat were turned upside down together, ensuring that they were not misaligned, and placed on a level surface. The paper cup was gently pulled out, and the shape of the sand pile after it was completely removed was visually confirmed. Those that collapsed were marked with an "O" and those that maintained their shape were marked with an "X." The results are shown in Tables 4-6.

[0215] [Table 4]

[0216] [Table 5]

[0217] [Table 6]

[0218] As is clear from the results in Tables 4 to 6 and Figure 6, the foundry sands of the Examples exhibit the desired strength and fluidity. It can be seen that the presence of an organic layer that dissolves in furfuryl alcohol makes it possible to uniformly support the acid hardener on the surface of the sand grains. Regardless of the type of sand, the grain size, the type of coating agent, or the type of acid hardener, the average strength was approximately 40 kg / cm3 within 3 hours after molding. 2 It was also confirmed that high strength could be achieved.

[0219] In contrast, the target bending strength was not achieved in Comparative Examples 1 and 2. The reason for this is presumably that the acid curing agent was not uniformly dispersed due to the absence of an organic layer. In Comparative Example 2, although the amount of curing agent or coating binder added was increased compared to Comparative Example 1, no improvement in bending strength was observed. This demonstrates that the organic layer of the present invention plays a role in improving the strength of the sand mold.

[0220] In Comparative Example 3, it is presumed that the addition of an excessive amount of palmitic acid as a coating agent inhibited the curing reaction, preventing the reaction with the coating binder from progressing so much that strength did not increase.

[0221] In Comparative Example 4, the amount of palmitic acid added as a coating agent was insufficient, which prevented the acid hardener from being uniformly supported on the surface of the sand particles, and therefore the desired bending strength was not obtained.

[0222] In Comparative Example 5, an excessive amount of acid curing agent was added, which resulted in the sudden production of a large amount of reaction water during the curing of the coating binder, which stopped the polymerization of the furan resin precursor and caused poor internal curing.

[0223] In Comparative Example 6, it is believed that the amount of acid curing agent added was insufficient, so the curing reaction did not proceed sufficiently, and the target bending strength was not obtained.

[0224] In Comparative Example 7, the furan resin composition "Configuration 1" serving as the coating agent was added in excess, which resulted in the rapid production of a large amount of reaction water during the curing of the furan, resulting in an insufficient reaction heat for curing, and thus the target bending strength could not be obtained.

[0225] In Comparative Example 8, the amount of the furan resin composition "Constitution 1" added as a coating agent was insufficient, so Constitution 1 and the acid curing agent completely cured on the surface of the sand grains. This is thought to have resulted in the lack of strength, as the coating agent was unable to fulfill its primary role of assisting in the development of strength.

[0226] In Comparative Example 9, the amount of water contained in the aqueous acid curing agent solution was excessive, which suppressed the reaction heat generated during furan curing, and this stopped the polymerization of the furan resin precursor, presumably resulting in poor internal curing.

[0227] Test Example 2 (1) Pre-drying the test piece The molding sand sample prepared under the conditions of Example 11 described in Table 5 was used to create a 10mm x 10mm x 200mm test piece using a 3D printer VX500 (manufactured by Voxeljet) in an environment of a temperature of 23-25°C and a humidity of 40-45%.

[0228] The thickness of the molding sand discharged from the recoater was set to 300 μm, forming a layer of molding sand sample. The coating binder of Preparation Example 1 listed in Table 3 was sprayed from the printer head into a predetermined area on the layered molding sand sample. These steps were repeated until the test piece reached the predetermined thickness.

[0229] Thirty minutes after the completion of molding, the test pieces were removed from the molding box, and the unhardened molding sand adhering to the test pieces was removed using a brush and then a blower, after which they were pre-dried in firing furnaces set at 50°C (Example 22), 100°C (Example 23), and 180°C (Example 24).

[0230] A molding sand sample was also prepared under the conditions of "Preparation Example 5" shown in Table 7. The molding sand sample was prepared as follows: 5 kg of new mullite-based sintered artificial sand AFS108 (Cerabeads #1450 manufactured by Itochu Ceratec Co., Ltd., angle of repose 31°) (CBX sintered new sand), a furan resin composition "Configuration 1" shown in Table 1, and a curing agent "Configuration 3" shown in Table 8 were prepared. Table 9 shows the water solubilities (measured values) of the heteropolyacids used in the examples, as well as the water solubilities (measured values) of the curing agents shown in Table 8.

[0231] When the temperature of the artificial sand reached 35°C, 0.3 parts by mass of the furan resin composition of Feature 1 was added to 100 parts by mass of the artificial sand, and the mixture was stirred and mixed in a kneader for 30 seconds. The sand temperature was 41°C. Next, 0.3 parts by mass of the curing agent of Feature 3 and 0.3 parts by mass of water as a solvent were added to the resulting mixture, and the mixture was mixed in the kneader for 30 seconds. Finally, in a drying step, the mixture was stirred for 240 seconds while blowing cold air into the kneader until the sand temperature reached 60°C, and then discharged from the kneader to obtain foundry sand.

[0232] The binder to be applied in Preparation Example 3 shown in Table 3 was prepared in a printer head, and test pieces were prepared in the same environment and by the same method as above. The test pieces were pre-dried in a baking oven set at 100°C (Comparative Example 11). Test pieces were also prepared without pre-drying (Comparative Example 10).

[0233] [Table 7]

[0234] [Table 8]

[0235] [Table 9]

[0236] (2) Drying of water-based coating agents The pre-dried test pieces were dipped into the aqueous wash slurry of "Preparation Example 6" listed in Table 10, and immediately after removing the test pieces from the slurry, the coating of Preparation Example 6 was dried in a baking oven at 100°C for 60 minutes. The test pieces before pre-drying and after pre-drying and drying of the coating of Preparation Example 6 were stacked and observed visually. If gaps were found between the test pieces, they were considered to have been deformed and the width of the gap (mm) was measured. If no gaps were found, they were considered to have been deformed and scored "none." The results are shown in Table 11.

[0237] [Table 10]

[0238] [Table 11]

[0239] As is clear from the results of Examples 22 to 24 shown in Table 11, the test pieces of the examples showed no deformation even after the water-based mold wash had dried.

[0240] In contrast, in Comparative Examples 10 and 11, heteropolyacid was not used as a curing agent, and therefore deformation of the test pieces became significant if they were not pre-dried (Comparative Example 10), and it was found that deformation of the test pieces was unavoidable even if they were pre-dried.

[0241] As described above, by introducing a heteropolyacid as an acid hardener into foundry sand having an organic layer, it is possible to effectively suppress or prevent deformation of a sand mold formed from that foundry sand, even when a water-based mold wash is applied to the sand mold.

Claims

1. Foundry sand used for three-dimensional additive manufacturing, (1) The particles constituting the foundry sand include (a) sand grains and (b) organic layers formed on the surfaces of the sand grains, (2) The organic layer contains a furan resin precursor or a fatty acid having a melting point of 40 to 75°C, and a heteropolyacid. Foundry sand for three-dimensional additive manufacturing.

2. 2. The foundry sand for three-dimensional additive manufacturing according to claim 1, wherein the heteropolyacid comprises at least one of silicotungstic acid and phosphotungstic acid.

3. 2. The foundry sand for three-dimensional additive manufacturing according to claim 1, wherein the heteropolyacid is contained in an amount of 0.5 to 2.5 parts by mass per 100 parts by mass of sand grains.

4. 2. The foundry sand for three-dimensional additive manufacturing according to claim 1, wherein the organic layer comprises solid particles of a heteropolyacid supported on a carrier containing a furan resin precursor or a fatty acid having a melting point of 40 to 75°C.

5. 2. The foundry sand for three-dimensional additive manufacturing according to claim 1, wherein the organic layer comprises an aqueous wetting layer containing a heteropolyacid formed on the surface of the sand grains, and a hydrophobic layer containing a furan resin precursor or a fatty acid with a melting point of 40 to 75°C, and the surface of the aqueous wetting layer is covered with the hydrophobic layer.

6. A three-dimensional additive manufacturing kit comprising the molding sand for three-dimensional additive manufacturing according to any one of claims 1 to 5 and a coating binder.

7. A method for producing foundry sand for three-dimensional additive manufacturing, comprising: (1) a step of mixing at least sand particles with a coating agent containing a furan resin precursor or a fatty acid having a melting point of 40 to 75°C at 100°C or less to obtain precursor particles in which the surfaces of the sand particles are coated with a coating layer containing the furan resin precursor or the fatty acid; and (2) mixing precursor particles and solid particles of heteropolyacid at 100°C or less to obtain composite particles in which the solid particles are supported on a coating layer on the surface of the precursor particles; A method for producing foundry sand, comprising:

8. A method for producing foundry sand for three-dimensional additive manufacturing, comprising: (1) a step of mixing at least sand particles with an aqueous solution of a heteropolyacid at 100°C or less to obtain precursor particles having surfaces of the sand particles coated with an aqueous wetting layer containing the aqueous solution; (2) A step of mixing precursor particles with a furan resin precursor or a coating agent containing a fatty acid having a melting point of 40 to 75°C at 100°C or less to obtain composite particles in which the aqueous wet layer is covered with a hydrophobic layer containing the fatty acid; A method for producing foundry sand, comprising:

9. The method according to claim 7 or 8, further comprising the step of adding 0.5 parts by mass or less of a silane coupling agent per 100 parts by mass of sand grains.

10. A method for manufacturing a sand mold using the molding sand for three-dimensional additive manufacturing according to any one of claims 1 to 5, (1) a step of forming a layer containing the foundry sand and a step of adding a coating binder to a predetermined region of the layer to solidify the region, and repeating these steps to obtain a molded body; and (2) A step of obtaining a sand mold by heat-treating the molded body not containing an aqueous mold wash at 40 to 250°C in air or an inert gas atmosphere. A method for manufacturing a sand mold, comprising:

11. A sand mold for casting iron-based materials, comprising the molding sand for three-dimensional additive manufacturing according to any one of claims 1 to 5.

12. 12. The sand mold for casting ferrous materials according to claim 11, further comprising a water-based mold wash.

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