Foundry sand for 3D additive manufacturing and method for manufacturing the same

Abstract

To provide a foundry sand for additive manufacturing that can provide a sand mold that is resistant to deformation even when water-based mold coating agents are used. [Solution] The present invention relates to a 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 heteropoly acid.

Description

Technical Field

[0001] The present invention relates to a novel casting sand for three-dimensional laminated molding and a method for manufacturing the same. In particular, it relates to casting sand supplied to a three-dimensional laminated molding machine, which is a sand mold machine for casting, and a method for manufacturing the same.

Background Art

[0002] The laminated molding method is a technique for producing a molded body by stacking sliced two-dimensional layers based on three-dimensional data such as 3D-CAD. In practice, an object having a desired shape can be created by laminating a three-dimensional laminated molding material with a 3D printer. As the three-dimensional laminated molding material, various materials such as metals, resins, sands, gypsums, ceramics, etc. are used.

[0003] Among these, sand can be suitably used as a sand mold as a mold, and thus the sand mold is also manufactured by the laminated molding method. In particular, in recent years, three-dimensional laminated molding technology that can directly realize a three-dimensional shape created by 3D-CAD or the like as a sand mold has attracted attention. In such a method for manufacturing a sand mold, various manufacturing methods or three-dimensional laminated molding materials have been proposed for the purpose of improving defects in the sand mold, improving laminated fluidity, and the like.

[0004] In Patent Document 1, a casting sand including sand and a solid acid catalyst mixed with the sand has been proposed, which can cause the binder to be cured early when the binder is added to the casting sand, and can efficiently mold a sand mold.

[0005] However, in the above casting sand, the solid acid catalyst is not uniformly dispersed throughout the sand, and when the binder is added, unevenness (distribution) occurs in the curing of the furan resin precursor, making it difficult to increase the strength of the sand mold. Further, in order to increase the strength, a method of increasing the addition amounts of furan resin, hardener, etc., which is often adopted in self-hardening furan sand molds, is not applicable to this technology.

[0006] In addition, when manufacturing a three-dimensional additively fabricated object using the foundry sand produced by the above method, when a binder is applied to the layered foundry sand, the applied binder comes into contact with the solid acid catalyst, which is a hardening agent on the surface of the foundry sand, and a hardening reaction occurs immediately. Therefore, although the initial strength of the sand mold is achieved to some extent, if only a small amount of binder is used, the binder solidifies sequentially before it can cover all the foundry sand grains, so no further increase in strength can be expected.

[0007] Furthermore, regarding the method for producing 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 done to improve the packing properties when manufacturing three-dimensional additively manufactured objects, but the need to pay attention to the particle size distribution of the solid acid catalysts complicates the method for producing the foundry sand.

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

[0009] In particular, the invention describes a method for producing foundry sand in which, in the process of adding solid carboxylic acid to refractory particles, the solid carboxylic acid is dissolved in a solvent, the refractory particles are mixed, and the solvent is volatilized and removed to produce foundry sand.

[0010] However, solid carboxylic acids have low affinity for solvents and therefore cannot exist uniformly in the solvent. Even if a solution in this state is mixed with refractory particles, it is impossible to uniformly disperse the solid carboxylic acids on the surface 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 can peel off due to physical friction between the refractory particles inside the recoater or other equipment of a 3D additive manufacturing machine, making it impossible to expect stable strength development.

[0012] A manufacturing method is known in which a material containing polycarboxylic acids mixed with refractory granular material is laid in layers, and then a sugar binder is injected into desired areas of the layered material, and this process is repeated until a three-dimensional additively manufactured object is formed. After the molding is complete, the sugar binder is hardened by heating the three-dimensional additively manufactured object (Patent Document 3).

[0013] However, in this method, since the three-dimensional additively manufactured object consists only of polycarboxylic acid and sugar binder, it has lower heat resistance than sand molds using furan resin precursors. For this reason, it cannot be used as a sand mold for iron-based casting, which has a relatively high melting point.

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

[0015] However, this method does not rely on hardening reactions over time, so external heat must be applied. Heating for sand mold removal is unnecessary in the self-hardening sand mold process, leading to a decrease in work efficiency.

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

[0017] However, when furan resin is applied to sand grains as a binder, using a water-based mold coating agent results in the problem of sand mold deformation, similar to the case shown in Patent Document 6 below.

[0018] Furthermore, a technique is known in which the strength of a molded body obtained by coating quartz sand particles with water glass and hardening it by heating is further improved by exposing it to an atmosphere enriched with gaseous water (Patent Document 5).

[0019] However, the above technology uses water glass as a binder, and enriched water vapor is required to increase its strength. It is also stated that heating cannot increase the strength, and that the purpose of heating is to achieve water vapor enrichment. In other words, the above technology relies solely on the principle of water glass solidification. Furthermore, in the present invention, not only is water vapor enrichment unnecessary, but there is also no need to consider the firing atmosphere, and on the contrary, water vapor enrichment has an adverse effect on the curing of the furan resin.

[0020] In response to this, the applicant has proposed and filed a patent application for a foundry sand for additive manufacturing that can provide a sand mold having high strength and high dimensional accuracy, and is used in a method of manufacturing a sand mold by additive manufacturing which includes the steps of 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, 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.

[0021] Because the furan resin organic film formed on the surface of the foundry sand grains manufactured by this method is in a semi-cured state, it is possible to cure the furan resin precursor evenly throughout the foundry sand when the binder is added. In addition, since the foundry sand is seemingly dry, it does not cause insufficient filling of the foundry sand even in lamination methods where compaction of the sand is not possible, thus enabling the manufacture of sand molds that possess both high strength and high dimensional accuracy.

[0022] However, in Patent Document 6, it may be deformed when applying and drying an aqueous coating agent.

[0023] Generally, when performing iron-based casting, a technique of applying an inorganic material called a coating agent to a sand mold for the purpose of suppressing welding defects and the like is known. In this case, as the coating agent, an aqueous coating agent in the form of a slurry in which powder of an inorganic material is dispersed in an aqueous solvent is generally used. Then, by a method of dipping the sand mold in such an aqueous coating agent (dipping method), the aqueous coating agent is applied to the sand mold. Thereafter, it is subjected to a step of dehydration and drying by heating or the like.

[0024] When dipping a sand mold made of conventional casting sand as in Patent Document 6 in a slurry of an aqueous coating agent, since the sand mold absorbs moisture contained in the aqueous coating agent, problems such as deformation or cracking of the sand mold occur during the subsequent drying process. This is presumably because in the sand mold immediately after laminated molding, the curing reaction of the self-hardening furan resin has not sufficiently progressed, and it is presumed that the furan composition during curing is in a state where it easily absorbs water. That is, if a portion that is not completely cured exists in the sand mold, when a large amount of water is absorbed by the application of the aqueous coating agent, that portion becomes soft due to heat, and it is considered that the sand mold cannot support its own weight and is deformed.

[0025] In addition, such problems (phenomena) can occur similarly not only in the technology of Patent Document 6 but also in a laminated molding material laminated by an acid-curing type furan resin.

Prior Art Documents

Patent Documents

[0026]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Summary of the Invention

Problems to be Solved by the Invention

[0027] Therefore, the main object of the present invention is to provide casting sand for three-dimensional laminated modeling that can provide a sand mold that is difficult to deform even when using an aqueous coating agent. Means for Solving the Problems

[0028] As a result of intensive research in view of the problems of the prior art, the present inventor has found that particles (particle groups) having a specific configuration can achieve the above object, and has completed the present invention.

[0029] That is, the present invention relates to the following casting sand for three-dimensional laminated modeling and a method for producing the same. 1. Casting sand used for three-dimensional laminated modeling, (1) The particles constituting the casting 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, Casting sand for three-dimensional laminated modeling, characterized in that. 2. The casting sand for three-dimensional laminated modeling according to item 1 above, wherein the heteropolyacid contains at least one of silicotungstic acid and phosphotungstic acid. 3. The casting sand for three-dimensional laminated modeling according to item 1 above, which contains 0.5 to 2.5 parts by mass of heteropolyacid with respect to 100 parts by mass of sand grains. 4. The casting sand for three-dimensional laminated modeling according to item 1 above, wherein solid particles of heteropolyacid are supported on a carrier containing a furan resin precursor or a fatty acid having a melting point of 40 to 75°C. 5. The three-dimensional additive manufacturing casting sand according to item 1, wherein the organic layer comprises an aqueous wet layer containing a heteropoly acid 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 wet layer is covered by the hydrophobic layer. 6. A kit for three-dimensional additive manufacturing, comprising the casting sand for three-dimensional additive manufacturing described in any one of items 1 to 5 above, and a binder for coating. 7. A method for producing foundry sand for three-dimensional additive manufacturing, (1) A step of obtaining precursor particles in which the surface of the sand particles is coated with a coating layer containing the furan resin precursor or fatty acid by mixing at least sand particles with a furan resin precursor or a coating agent containing a fatty acid with a melting point of 40 to 75°C at a temperature of 100°C or less, and (2) A step of obtaining composite particles in which the solid particles are supported on a coating layer on the surface of the precursor particles by mixing the precursor particles and the solid particles of the heteropoly acid at a temperature of 100°C or less. A method for producing foundry sand, characterized by containing the following: 8. A method for producing foundry sand for three-dimensional additive manufacturing, (1) A step of obtaining precursor particles in which the surface of the sand particles is coated with an aqueous wet layer containing the aqueous solution by mixing at least sand particles and an aqueous solution of heteropoly acid at a temperature of 100°C or less. (2) A step of obtaining composite particles in which the aqueous wet layer is covered with a hydrophobic layer containing the fatty acid by mixing precursor particles with a furan resin precursor or a coating agent containing a fatty acid with a melting point of 40 to 75°C at a temperature of 100°C or less. A method for producing foundry sand, characterized by containing the following: 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 to 100 parts by mass of sand grains. 10. A method for manufacturing a sand mold using the three-dimensional additive manufacturing sand described in any one of paragraphs 1 to 5 above, (1) A step of obtaining a molded body by sequentially repeating the steps of forming a layer containing the foundry sand and solidifying a predetermined area of ​​the layer by adding a coating binder to that area, and (2) A process to obtain a sand mold by heating the molded body, which does not contain an aqueous mold coating agent, in air or an inert gas atmosphere at 40 to 250°C. A method for manufacturing sand molds, characterized by including the following: 11. A sand mold for casting iron-based materials, comprising the three-dimensional additive manufacturing sand described in any one of items 1 to 5 above. 12. A sand mold for casting iron-based materials according to item 11, further comprising a water-based mold coating agent. [Effects of the Invention]

[0030] According to the present invention, it is possible to provide a foundry sand for three-dimensional additive manufacturing that can provide a sand mold that is resistant to deformation even when water-based mold coating agents are used. In particular, it is possible to provide a foundry sand that enables relatively high sand mold strength, allows the use of water-based mold coating agents, and provides a good working environment. The present invention can be suitably used as foundry sand for three-dimensional additive manufacturing.

[0031] The foundry sand in this invention has an organic layer on the surface of the sand grains that contains a furan resin precursor or saturated fatty acid and a heteropoly acid 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 applied to the foundry sand of this invention, the entire sand grain can be cured uniformly, resulting in a sand mold with high strength. In particular, sufficiently high strength can be obtained even within a few minutes after the sand mold is made (for example, after 30 minutes).

[0032] In particular, since the present invention uses a heteropoly acid, which has higher solubility in water than sulfonic acid, as a curing agent, the heteropoly acid can penetrate thoroughly to the central part of the intergranular bridge that forms the cured furan resin precursor. This allows for higher strength to be obtained and contributes to preventing deformation of the sand mold.

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

[0034] Furthermore, if the foundry sand of the present invention is in a dry state, even in lamination methods (lamination molding methods) where compaction of the sand is difficult or impossible, there is no risk of insufficient filling of the foundry sand, and it becomes possible to increase the strength of the sand mold.

[0035] Furthermore, since the heteropoly acid used as a hardening agent in the foundry sand of the present invention is substantially free of sulfur components, the effects of sulfur components as described below can be avoided.

[0036] Generally, castings are made by pouring molten metal (iron, aluminum, etc.) into sand molds formed from foundry sand. However, during the pouring process, sulfur components contained in the foundry sand decompose due to the heat, generating sulfur dioxide gas and other substances that have adverse effects on the environment.

[0037] Furthermore, especially when casting iron-based materials, the presence of sulfur components in the casting sand (or sand mold) can cause sulfurization of the casting. In this case, particularly in spheroidal graphite cast iron, poor spheroidization (metal structure) of graphite can occur, potentially leading to a decrease in the strength or toughness of the casting.

[0038] In contrast, as described above, the foundry sand and sand molds produced using the present invention have a sulfur content that is controlled to be extremely low or 0%, thus preventing the impact of sulfur on the working environment and on iron-based materials. Therefore, it can be suitably used for casting iron-based materials with high melting temperatures, which is difficult to handle with conventional sand molds, and can be widely used, for example, in the manufacture (casting) of cast iron machine parts.

[0039] The kit comprising the foundry sand and a coating binder of the present invention, having these characteristics, can be suitably used to create sand molds by additive manufacturing.

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

[0041] Furthermore, the method for producing foundry sand according to the present invention allows for more reliable and efficient production of the foundry sand. The inventors have found that controlling the solubility of the organic layer and the reaction water is necessary to obtain 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 hardening agent in the organic layer, more desirable effects can be obtained. By introducing these into the production method of the present invention, it is possible to achieve stable sand mold strength with relatively small amounts of hardening agent and resin. On the other hand, even if the foundry sand has high intergranular moisture compared to dry sand, but its external form is in a state between dry and wet (hereinafter also referred to as "intermediate state"), the presence of the organic layer allows for uniform dispersion of the hardening agent, making it possible to achieve sufficiently high strength. [Brief explanation of the drawing]

[0042] [Figure 1] This shows a schematic diagram of the particles of the foundry sand of the present invention. [Figure 2] A schematic diagram of the particles of the foundry sand according to Embodiment 1 of the present invention is shown. [Figure 3] A schematic diagram of the particles of the foundry sand according to Embodiment 2 of the present invention is shown. [Figure 4] The appearance of the aqueous coating agent in Example 23 after drying and the appearance of the aqueous coating agent in Comparative Example 11 after drying are shown, respectively. [Figure 5] The appearance of the foundry sand from Example 3, Example 21, and Comparative Example 9 is shown below. [Figure 6] The results of the fluidity tests for Example 21 and Comparative Example 9 are shown. [Modes for carrying out the invention]

[0043] 1. 3D additive manufacturing sand The foundry sand for three-dimensional additive manufacturing of the present invention (foundry sand of the present invention) is a 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 comprises a furan resin precursor or a fatty acid with a melting point of 40-75°C and a heteropoly acid. It is characterized by the following:

[0044] Figure 1 shows a schematic diagram of the foundry sand particles of the present invention. In the foundry sand (particles) 10 shown in Figure 1, an organic layer 12 is formed on the surface of the core sand grain 11. In particular, it is desirable that substantially the entire surface of the sand grain 11 is covered with the organic layer 12, but there may be parts that are not covered with the organic layer as long as they do not hinder the effects of the present invention.

[0045] The organic layer 12 contains a furan resin precursor or a fatty acid with a melting point of 40-75°C and a heteropoly acid. In other words, the present invention includes cases in which the organic layer contains a furan resin precursor and a heteropoly acid, or cases in which it contains a fatty acid with a melting point of 40-75°C and a heteropoly acid, etc.

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

[0047] Figure 2 shows a schematic diagram of Embodiment 1. In the foundry sand 10 shown in Figure 2, a carrier (carrier layer) 12a containing a furan resin precursor or a fatty acid with a melting point of 40-75°C is formed around the sand grains 11, and solid particles 12b of heteropoly acid are supported on the carrier. The solid particles 12b of heteropoly acid only need to be fixed to the carrier, and may be contained in the carrier or fixed to the surface of the carrier. The properties of the foundry sand according to Embodiment 1 are not limited, but due to the structure described above, it is usually in a dry state (dry powder state).

[0048] Figure 3 shows a schematic diagram of Embodiment 2. In the foundry sand 10 shown in Figure 3, an aqueous moist layer 22a containing heteropoly acid is formed around the sand grains 11, and a hydrophobic layer 22b containing a furan resin precursor or a fatty acid with a melting point of 40-75°C is formed on the aqueous moist layer 22a. In this case, since the surface of the aqueous moist layer 22a is substantially covered by the hydrophobic layer 22b, the evaporation of water contained in the aqueous moist layer 22a is prevented, and the moist state is maintained. The properties of the foundry sand according to Embodiment 2 are not limited, but due to the structure described above, it is usually in a wet state (moist kneaded state). In this case, the heteropoly acid is contained in the aqueous moist layer in a solution (especially an aqueous solution).

[0049] (A) Substances constituting the foundry sand of the present invention (A-1) Sand grains In this invention, sand grains (i.e., sand grains before they are coated with an organic layer) are used as the core of the particles constituting the foundry sand. The sand, which is an aggregate of sand grains, can be either natural silica sand or artificial sand. 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 this invention, it is preferable to use new artificial sand from the viewpoint of keeping the igloss lower. The artificial sand is not limited to being produced by a melting method or a sintering method.

[0050] The particle size of the sand used as aggregate is not particularly limited, but generally, a particle size index of AFS 35 to 120 is preferred, and AFS 60 to 100 is more preferred. The particle size can be adjusted as needed by 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 grains, preferably covering the entire sand grain.

[0052] In the foundry sand of the present invention, the organic layer does not directly participate in the bonding of the sand grains constituting the foundry sand before the sand is laminated. When the intergranular humidity is low, the foundry sand of the present invention appears dry and has excellent fluidity. On the other hand, even in an intermediate state with high intergranular humidity, the hardening agent is uniformly dispersed due to the presence of the organic layer, and because it is an intermediate state, the packing ability is sufficiently high, and practical lamination properties can be obtained, thus enabling the development of relatively high strength.

[0053] The organic layer comprises (a1) a furan resin precursor or (a2) a fatty acid with a melting point of 40-75°C, and (b) a heteropoly acid.

[0054] (a1) Furan resin precursor The furan resin precursor is not limited to any material capable of forming a furan resin by condensation polymerization or the like, and examples include furfuryl alcohol and furan resin prepolymers. In particular, it is desirable to use furfuryl alcohol and furan resin prepolymer in combination for the reasons of suppressing reaction heat and lowering the viscosity of the resin.

[0055] Examples of furan resin prepolymers include polymers of furfuryl alcohol alone, copolymers of furfuryl alcohol and aldehyde compounds, copolymers of furfuryl alcohol, urea and aldehyde compounds (urea-modified furan resin prepolymers), and copolymers of furfuryl alcohol and furfural. These can be used individually or in combination of two or more.

[0056] In this invention, a copolymer of furfuryl alcohol, urea, and an aldehyde compound (urea-modified furan resin prepolymer) is particularly preferred because it is easy to increase its strength. Examples of the aldehyde polymer compound include formaldehyde, acetaldehyde, glyoxal, and furfural. In this invention, formaldehyde is particularly preferred.

[0057] When using furfuryl alcohol and furan resin prepolymer in combination as furan resin precursors, it is desirable that the content of furfuryl alcohol be in the range of 35 to 60 parts by mass, assuming a total of 100 parts by mass of both, from the viewpoint of viscosity.

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

[0059] The content ratio (solid content) of the furan resin precursor in the organic layer is not particularly limited as long as it is formulated to have the content shown above, but it is usually around 1 to 90% by mass, and can be further set to around 20 to 90% by mass. Therefore, for example, it can be set to 30 to 60% by mass. Alternatively, for example, it can be 1 to 80% by mass, or 1.5 to 60% by mass, or even 5 to 25% by mass.

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

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

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

[0063] The proportion of fatty acids (solid content) in the organic layer is not particularly limited, as long as it is formulated to have the content shown above. Therefore, it can be in the range of approximately 1 to 60% by mass, or it can be set to, for example, 5 to 50% by mass, or even 6 to 25% by mass.

[0064] (b) Heteropoly acids In the foundry sand of the present invention, the heteropoly acid primarily functions as an acid catalyst (hereinafter also referred to as "acid hardening agent") for curing the coating binder used in the lamination process. By using this, it is possible to provide a sand mold that can exhibit higher strength.

[0065] A heteropoly acid is a condensed acid containing two or more elements and oxygen, and in this invention, one or more heteropoly acids can be used. In particular, in this invention, a heteropoly acid that is substantially free of sulfur is preferred. In this invention, "substantially free" does not mean that sulfur is excluded even if it is present at an unavoidable impurity level. Therefore, the amount of sulfur contained as an unavoidable impurity in commercially available heteropoly acids is acceptable. For example, if the sulfur content is around 50 ppm by mass or less (especially 0 to 10 ppm by mass), there is virtually no problem caused by sulfur, and therefore it falls under the category of "substantially free".

[0066] Furthermore, it is preferable that the heteropoly acid is water-soluble. This allows the heteropoly acid to exist suitably in the aqueous wet layer in the aforementioned aqueous wet layer. Specifically, the solubility per 100g of water (at 27°C) is preferably 100 to 1000g / 100g water, and more preferably 300 to 900g / 100g water. By using a material with such high solubility, it is possible to include heteropoly acid at a higher concentration than sulfonic acid-based curing agents (which have a solubility of approximately 50g / 100g water) at the same temperature, thereby enabling the achievement of higher strength more quickly.

[0067] Furthermore, in the present invention, from the viewpoint of effectively utilizing heteropoly acids as acid curing agents, heteropoly acids having an acid dissociation constant pKa (25°C) of 5 or less, which indicates acid strength, can be suitably used.

[0068] Examples of such heteropoly acids include at least one of phosphotungstic acid, silicatungstic acid, phosphomolybdic acid, and silicamolybdic acid, with silicatungstic acid and at least one of phosphotungstic acid being particularly preferred. These heteropoly acids can be known or commercially available. Heteropoly acids obtained by known manufacturing methods can also be used.

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

[0070] The proportion of heteropoly acid in the organic layer (solid content) is not particularly limited as long as it is formulated to match the content (ratio to sand grains) as described above, but it can usually be set within a range of 10 to 99% by mass, or within a range of 10 to 80% by mass. Therefore, it can be set, for example, to 30 to 60% by mass, or to 40 to 99% by mass, or to 50 to 95% by mass, or even to 55 to 93% by mass.

[0071] In order to efficiently and uniformly disperse heteropoly acids among sand grains, if the heteropoly acid is in powder form, its average primary particle diameter is usually around 0.6 to 30 μm, and particularly preferably 1 to 20 μm.

[0072] Heteropoly acids having such particle size can be suitably obtained, for example, by a spray dryer.

[0073] The spraying method of the spray dryer is not particularly limited, and examples include a four-fluid nozzle system and an atomizer disc system. In particular, in the present invention, it is preferable to adopt a four-fluid nozzle system in order to improve dispersibility and obtain finer particles.

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

[0075] Furthermore, the exhaust temperature of the spray dryer should normally be set to around 50-150°C, and it is particularly preferable to set it to 80-120°C.

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

[0077] Furthermore, to more homogeneously disperse the heteropoly acid among the sand grains, it is also possible to use a solution obtained by dissolving the heteropoly acid in at least one solvent, such as ethanol and water.

[0078] The amount of heteropoly acid contained in the solution can be appropriately set depending on the type of heteropoly acid and solvent used, but it is usually around 10 to 60% by mass, and particularly preferably around 30 to 50% by mass. Therefore, for example, it can also be 15 to 35% by mass.

[0079] (c) Other ingredients The organic layer may contain other components, as long as they do not interfere with the effects of the present invention. For example, it may contain at least one of the following: a solvent, a crosslinking agent, etc.

[0080] Water, for example, can be suitably used as a solvent. This is because it has a relatively high boiling point and allows for easy control of the solubility of the organic layer consisting of the furan resin precursor.

[0081] As a crosslinking agent, a silane coupling agent can be suitably used. This can be a known or commercially available agent. In the present invention, at least one of aminopropylmethyldimethoxysilane and 3-methacryloxypropyltrimethoxysilane can be suitably used because it can react with the organic layer and does not easily form silane-derived precipitates.

[0082] When using a crosslinking agent, its content ratio is not limited, but generally it can be, for example, 0.002 to 0.5 parts by mass per 100 parts by mass of sand grains, and more particularly 0.001 to 0.5 parts by mass. Therefore, it can also be set to, for example, 0.02 to 0.5 parts by mass. By setting it within this range, the bonding strength between the organic layer and the sand grains can be further enhanced, and the adhesion between the coating binder and the organic layer can be further enhanced, resulting in a higher sand mold strength. In this case, the content of the crosslinking agent in the organic layer can be as described above, for example, 1 to 50% by mass (particularly 20 to 40% by mass), but is not limited to this.

[0083] 2. Method for producing the foundry sand of the present invention The method for producing the foundry sand of the present invention is not limited to any method that can form an organic layer on sand grains, but 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 to obtain precursor particles in which the surface of the sand grains is coated with a coating layer containing the furan resin precursor or fatty acid by mixing at least sand grains with a furan resin precursor or a coating agent containing a fatty acid with a melting point of 40 to 75°C at a temperature of 100°C or less (precursor particle preparation step), and (2) A step to obtain composite particles in which the solid particles are supported on a coating layer on the surface of the precursor particles by mixing the precursor particles and the solid particles of the heteropoly acid at a temperature of 100°C or less (composite particle preparation step), It can be suitably produced by a method for producing foundry sand that is characterized by containing [a certain substance].

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

[0086] The sand particles, furan resin precursor, fatty acids with a melting point of 40-75°C, etc., can be the same as those described in "1. Foundry sand for layered sand molds" above.

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

[0088] When using the furan resin precursor described in (a1) above, the furan resin precursor is not particularly limited, but it is preferable to use a combination of furfuryl alcohol and a furan resin prepolymer (especially a urea-modified furan resin prepolymer), as shown in the example, in that it is possible to more reliably form a semi-cured furan resin precursor.

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

[0090] The mixing conditions are not limited as long as a coating layer can be formed on the surface of the sand grains with the coating agent, but the temperature is preferably less than 100°C, particularly 70°C or lower, and especially preferably 50-60°C. Note that the temperature referred to here is the sand temperature (the same applies below), that is, the temperature of the material itself, not the ambient temperature.

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

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

[0093] The heating should be such that the sand temperature is 100°C or lower, preferably 70°C or lower, and more preferably 50-60°C.

[0094] To efficiently and more homogeneously disperse fatty acids in sand grains, it is preferable to dissolve them using at least one organic solvent that is soluble in fatty acids and has a boiling point of less than 100°C, such as diethyl ether, benzene, or ethanol, with ethanol being more preferable.

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

[0096] In this process, the above-mentioned coating agent is used, and this coating agent and sand grains are mixed at a temperature of 100°C or lower (preferably 60°C or lower, more preferably 40-50°C).

[0097] The mixing method is not limited to any particular method; for example, mixing can be done using a temperature-controlled stirring device (kneader, mixer, etc.). Commercially available devices may be used for this purpose.

[0098] In this way, precursor particles can be obtained in which the surface of the sand grain is 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 above-mentioned precursor particles and heteropoly acid powder are mixed at a temperature of 100°C or lower to obtain composite particles in which the solid particles are supported on a coating layer on the surface of the precursor particles.

[0100] Furthermore, any of the heteropoly acids described above may be used, and for example, at least one of silicic acid and phosphotungstic acid can be suitably used.

[0101] The solid particles of the heteropoly acid can be the heteropoly acid powder described above. Therefore, for example, a powdered heteropoly acid with an average primary particle diameter of 0.6 to 30 μm (preferably 1 to 20 μm) can be suitably used.

[0102] The solid particles of such heteropoly acid and the precursor particles are mixed at a temperature of 100°C or lower (preferably 80°C or lower, more preferably 60-80°C).

[0103] The mixing method is not limited to any particular method; for example, mixing can be done using a temperature-controlled stirring device (kneader, mixer, etc.). Commercially available devices may be used for this purpose.

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

[0105] Furthermore, if the obtained foundry sand is not immediately used in the additive manufacturing process, it is desirable to store it in an environment where the temperature is controlled to 15°C to 25°C and the humidity to 45% or less, and especially in an environment where the temperature is controlled to 20°C to 25°C and the humidity to 40% or less. If stored outdoors, it is preferable to store it in a sealed container such as a drum.

[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 to obtain precursor particles in which the surface of the sand particles is coated with an aqueous wet layer containing the aqueous solution by mixing at least sand particles and an aqueous solution of heteropoly acid at a temperature of 100°C or less (precursor particle preparation step), (2) A step to obtain composite particles in which the aqueous wet layer is covered with a hydrophobic layer containing the fatty acid by mixing the precursor particles with a furan resin precursor or a coating agent containing a fatty acid with a melting point of 40 to 75°C at a temperature of 100°C or less (composite particle preparation step), It can be suitably produced by a method for producing foundry sand that is characterized by containing [a certain substance].

[0107] Precursor particle preparation process In the precursor particle preparation step, precursor particles are obtained in which the surface of the sand particles is coated with an aqueous moist layer containing the aqueous solution by mixing at least sand particles and an aqueous solution of heteropoly acid at a temperature of 100°C or below.

[0108] As described above, the heteropoly acid can preferably be at least one of, for example, silicic acid and phosphotungstic acid.

[0109] Aqueous solutions of heteropoly acids can be prepared by dissolving the heteropoly acid in a solvent. Suitable solvents include a) water, b) water-soluble organic solvents, or c) mixtures thereof. In this invention, at least one of water and ethanol is particularly preferred.

[0110] The amount of heteropoly acid present in the solution is not particularly limited, but is usually around 10 to 80% by mass, and more preferably 10 to 60% by mass.

[0111] As mentioned above, the amount of heteropoly acid aqueous solution should typically be set so that it is 0.5 to 2.5 parts by mass of heteropoly acid per 100 parts by mass of sand grains.

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

[0113] As mentioned above, the mixing temperature should be 100°C or lower, but it is particularly preferable to keep it below 60°C, and even more preferable to keep it between 40°C and 50°C.

[0114] Furthermore, the mixing method is not limited; for example, mixing can be done using a temperature-controlled stirring device (kneader, mixer, etc.). Commercially available devices may also be used.

[0115] In this way, precursor particles can be obtained in which the surface of the sand grains is coated with an aqueous moist layer containing the aqueous solution. Such precursor particles having an aqueous moist layer on their surface constitute the aforementioned wet mixture. In the aqueous moist layer, heteropoly acid exists in a solution (particularly an aqueous solution) state. In this respect, it differs from the first embodiment in which solid particles of heteropoly acid are present.

[0116] Composite particle preparation process In the composite particle preparation step, the above-mentioned precursor particles are mixed with a furan resin precursor or a coating agent containing a fatty acid with a melting point of 40-75°C at a temperature of 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 a coating agent containing a furan resin precursor or a fatty acid with a melting point of 40 to 75°C, for example, one similar to the coating agent used in the method for producing foundry sand in the first embodiment can be suitably used.

[0118] The coating agent and precursor particles are mixed at a temperature of 100°C or lower (preferably 70°C or lower, more preferably 50-60°C).

[0119] The mixing method is not limited to any particular method; for example, mixing can be done using a temperature-controlled stirring device (kneader, mixer, etc.). Commercially available devices may be used for this purpose.

[0120] In this way, composite particles can be obtained in which an aqueous moist layer is covered with a hydrophobic layer containing the fatty acid. That is, as composite particles, foundry sand can be obtained in which the organic layer has an aqueous moist layer containing a heteropoly acid 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 moist layer is covered with the hydrophobic layer.

[0121] 3.3D additive manufacturing kit The present invention encompasses a kit for three-dimensional additive manufacturing, comprising the foundry sand for three-dimensional additive manufacturing and a binder for coating. Specifically, the foundry sand and binder are stored separately before use and mixed together at the time of use, and the kit is provided as a two-component kit.

[0122] As in the first embodiment, the foundry sand of the present invention is a loose, dry sand if the humidity between the sand grains is low, so it can be directly scattered (sprinkled) when filled into a 3D additive manufacturing machine. In other words, it is also possible to allow each particle of the foundry sand to fall naturally. On the other hand, as in the second embodiment, if the humidity between the sand grains is high, which is an intermediate state, the packing ability may be lower compared to dry sand, but a practical strength can still be achieved.

[0123] The foundry sand of the present invention may contain additives other than the foundry sand of the present invention, as long as they do not interfere with the effects of the present invention. For example, as described below, it is also possible to provide the foundry sand pre-mixed with various additives such as anti-slip agents and thickeners.

[0124] The coating binder is a binder that is sprayed onto the formed casting sand layer during 3D additive manufacturing. In particular, when manufacturing sand molds using a 3D additive manufacturing machine, it is preferable that the viscosity (at 25°C) of the coating binder is 1 to 15 mPa·s in order to stably discharge the coating binder from the print head.

[0125] The type of coating binder is not particularly limited, and any known or commercially available binder usable with a 3D 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 can be, for example, 80 to 100% by mass in the binder, and particularly 90 to 100% by 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 include furfuryl alcohol and furan resin prepolymers. In particular, it is desirable to use furfuryl alcohol and furan resin prepolymer in combination for reasons of suppressing reaction heat and lowering the viscosity of the resin.

[0127] Examples of furan resin prepolymers include polymers of furfuryl alcohol alone, copolymers of furfuryl alcohol and aldehyde compounds, copolymers of furfuryl alcohol, urea and aldehyde compounds (urea-modified furan resin prepolymers), and copolymers of furfuryl alcohol and furfural. These can be used individually or in combination of two or more.

[0128] In this invention, a copolymer of furfuryl alcohol, urea, and an aldehyde compound (urea-modified furan resin prepolymer) is particularly preferred because it is easy to increase its strength. Examples of the aldehyde compound include formaldehyde, acetaldehyde, glyoxal, and furfural. In this invention, formaldehyde is particularly preferred.

[0129] Other components may be included in the coating binder as long as they do not hinder the effects of the present invention. For example, additives such as solvents, crosslinking agents, and curing accelerators may be included as needed. In particular, water is preferred as the solvent in the present invention because it slows down the reaction rate and makes it easier to achieve high final sand mold strength.

[0130] Examples of hardening accelerators include at least one of resorcinol, cresol, hydroquinone, phloroglucinol, methylenebisphenol, and bishydroxymethylfuran. Among these, resorcinol is particularly preferred because it reacts quickly and makes it easy to obtain high strength in sand molds.

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

[0132] Furthermore, because the foundry sand, which is the other material in the kit, has an organic layer containing acidic components, the coating binder can strongly bond the foundry sands together even without a silane coupling agent. Therefore, the silane coupling agent content in the coating binder is usually about 0 to 1% by mass, preferably 0 to 0.1% by mass. Consequently, a composition without a silane coupling agent is also acceptable. For example, it can also be set to 0.05 to 0.15% by mass. By setting the silane coupling agent content to a small amount or 0% by mass in this way, the coating binder can be stored stably for a longer period of time.

[0133] Furthermore, amine compounds can be added to the coating binder as needed. By adding amine compounds, the change in viscosity of the furan resin precursor over time can be effectively suppressed. From this viewpoint, alkylamines with 10 or fewer carbon atoms are preferred as the amine compound, and butylamines are more preferred.

[0134] While there is no limit to the amount of amine compound added, excessive amounts can lead to problems such as slower curing speed and the induction of gas defects due to nitrogen. Therefore, the amine compound content in the coating binder should preferably be around 0.001 to 1% by mass, more preferably 0.001 to 0.5% by mass, even more preferably 0.005 to 0.1% by mass, and most preferably 0.01 to 0.05% by mass. Consequently, it is also possible to set the content within the range of, for example, 0 to 0.5% by mass.

[0135] Regarding the amount of binder used for coating, due to the mechanical settings of the printer head, it is preferable that the upper limit that can be applied is 3 parts by mass or less per 100 parts by mass of sand. Normally, 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 it is particularly preferable to be 1 to 3 parts by mass. If there is too little binder used for coating, the strength of the sand mold may not be obtained. Therefore, it is sufficient that the amount of foundry sand and binder is in the above ratio when used, so it is not necessary for the kit of the present invention to be kitted in the above ratio.

[0136] Furthermore, 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 (particularly a urea-modified furan resin prepolymer), a coating binder containing a combination of furfuryl alcohol and a furan resin prepolymer (particularly a urea-modified furan resin prepolymer) can be used.

[0137] In this case, especially when the same type of material is used for both, the compatibility between the organic layer of the foundry sand and the coating binder improves, resulting in higher sand mold strength. For example, the foundry sand of the present invention, which includes a combination of furfuryl alcohol and furan resin prepolymer (particularly urea-modified furan resin prepolymer) in its organic layer, can also be used as a foundry sand suitable for a coating binder containing furfuryl alcohol and furan resin prepolymer when used in additive manufacturing.

[0138] However, even when using the same type of material, it is not always necessary to completely match the composition ratio, additives, etc. It is possible to fine-tune the blending ratio of furfuryl alcohol, furan resin polymer, etc., and the types of additives, taking into account the balance between casting strength and curing time, as well as the characteristics of the 3D additive manufacturing machine being used.

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

[0140] Such a 3D additive manufacturing machine can be, for example, one that includes a unit, a foundry sand supply unit, a binder supply unit for coating, and an operating unit, and that can create sand molds from 3D-CAD data. The foundry sand supply unit is a unit that supplies foundry sand to the unit, and includes a foundry sand tank for containing the foundry sand, a recoater that can discharge the foundry sand while moving horizontally, etc. The binder supply unit includes a print head that discharges the binder for coating to the unit. Such devices themselves can be publicly known or commercially available.

[0141] When manufacturing a sand mold using a three-dimensional additive manufacturing method, an additive manufacturing method can be adopted that includes, as described above, a step of forming a layer containing foundry sand (layer formation step) and a step of solidifying a predetermined area of ​​the layer by adding a coating binder (coating binder addition step), repeating these steps in sequence. At this time, the hardening agent contained in the organic layer of foundry sand, where the surface of the sand grains is made of furan resin, contributes to the solidification of both the organic layer and the coating binder, thereby enabling the production of 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 it from above. The foundry sand of the present invention is basically a loose, dry sand, so it can also be smoothly allowed to fall naturally (scattered). Even when the humidity between sand grains is high, when using a 3D additive manufacturing machine as described above, the layer can be reliably formed by discharging the foundry sand onto a flat surface from a horizontally moving recoater.

[0143] Furthermore, in the layer formation process, components other than foundry sand may be included in the layer, as long as they do not hinder smooth layer formation. Such components may include those already known to be added to foundry sand.

[0144] In conventional additive manufacturing methods (a two-component mixing process of wet sand, a hardener, and a binder resin for coating), when artificial sand produced by a melting method is used as the sand grain, the surface smoothness and roundness of the sand cause liquid crosslinking (aggregation) when a liquid agent is added to the sand grain. This impairs the sand's fluidity, which is crucial in additive manufacturing, making it difficult to use small-grained sand grains. For this reason, a drying process has been proposed to dry the sand and improve its fluidity, but this limits the usable sand grains to sintered artificial sand. In contrast, with the sand of the present invention, if a fatty acid such as linoleic acid is added to its organic layer as an anti-slip agent, or if the sand is in a state of high inter-grain pore humidity (i.e., a so-called intermediate state), stable layering can be more reliably obtained regardless of the sand type, grain size, etc. This prevents the collapse of the sand layer of the present invention within the build box (job box) in a 3D additive manufacturing machine, for example, even when using melted artificial sand with high smoothness and roundness as the sand grain.

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

[0146] Binder addition process for coating The layer is solidified by adding (spraying) a coating binder to a predetermined area of ​​the layer. Although a single layer containing foundry sand is formed by the layer formation process, the coating binder is added to an area based on the data of the cross-sectional shape of the target sand mold. For example, when using a 3D additive manufacturing machine as 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 be the one described in "3.3D Additive Manufacturing Kit" above, for example.

[0147] After a series of steps consisting of a layer formation step and a coating binder addition step are repeated, a sand mold with a predetermined shape can be obtained by removing the portion where the coating binder has not been added.

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

[0149] Furthermore, a process of applying a mold coating agent to the obtained sand mold can be carried out as needed. The method of applying the mold coating agent to the sand mold is not particularly limited and includes, for example, brush application and dipping. Brush application is acceptable, but if you want to apply the mold coating agent more uniformly to the entire sand mold, dipping is preferable, but is not limited to this. In this case, either a water-based mold coating agent or a non-water-based mold coating agent can be used, but from the standpoint of safety, it is desirable to use a water-based mold coating agent.

[0150] However, dipping sand molds made from conventional additive manufacturing sand into a water-based mold coating is not recommended. This is because the furan resin precursor has not yet fully polymerized in the sand mold immediately after layering. In addition, dipping the sand mold into a water-based mold coating causes the water contained in the coating to penetrate the mold, leading to a decrease in the strength of the sand mold or deformation of the sand mold when the water-based mold coating dries.

[0151] From this viewpoint, as will be shown later, in the present invention, it is desirable to heat-treat the sand mold beforehand, either after or in lieu of the above-mentioned aging, prior to the application of the water-based mold coating agent, in order to promote the curing reaction between the water contained inside the sand mold and the unreacted furan resin precursor, thereby bringing it closer to complete curing. In this case, the heat treatment temperature is preferably, for example, around 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] In the foundry sand of this invention, the reaction water generated when the furan resin precursor polymerizes is controlled (suppression of internal hardening defects). In addition, by performing heat treatment (pre-drying) before applying the water-based mold coating agent, the hardening reaction of the furan resin is accelerated, bringing it closer to complete hardening. As a result, the flexural strength does not decrease even when the water-based mold coating agent is applied to the sand mold (dipping, etc.), and deformation that may occur in the sand mold during the drying of the water-based mold coating agent can be effectively prevented.

[0153] Sand molds obtained using suitable foundry sand in this way can achieve high strength and high design reproducibility. Furthermore, they offer excellent mass-producibility, as sand molds can usually be removed in less than 3 hours, and sometimes as quickly as 1 hour. This allows additive manufacturing to be widely applied to the production of cast products where mass production is required.

[0154] 5. Sand mold The present invention encompasses sand molds (particularly sand molds for casting iron-based materials) containing the three-dimensional additive manufacturing sand of the present invention. The sand mold of the present invention is constructed by the bonding and joining of individual pieces of the sand of the present invention to one another, thereby achieving high strength. In particular, the present invention also encompasses sand molds (particularly sand molds for casting iron-based materials) containing a water-based mold coating agent.

[0155] The sand mold of the present invention can be used as a casting sand mold for casting substantially any material (especially metals), but is particularly suitable for casting iron-based materials. Generally, when casting iron-based materials, if sulfur components are present in the sand mold (or the foundry sand that constitutes it), sulfurization of the casting occurs, causing poor spheroidization (metal structure) of graphite, which affects the quality of the casting and may lead to a decrease in the strength of the casting. In contrast, the foundry sand that constitutes the sand mold of the present invention substantially does not contain sulfur components, thus avoiding problems caused by sulfur components.

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

[0157] In other words, a method for manufacturing a sand mold using the three-dimensional additive manufacturing sand of the present invention, (1) A step of obtaining a molded body by sequentially repeating the steps of forming a layer containing the foundry sand and solidifying a predetermined area of ​​the layer by adding a coating binder to that area (lamination step), and (2) A step to obtain a sand mold by heating the molded body, which does not contain an aqueous mold coating agent, in air or an inert gas atmosphere at 40 to 250°C (heat treatment step). It can be suitably manufactured by a sand mold manufacturing method characterized by including [a specific element].

[0158] Lamination process In the lamination process, a molded body is obtained by sequentially repeating the steps of forming a layer containing the foundry sand and solidifying a predetermined area of ​​the layer by adding a coating binder to that area.

[0159] The layering process can be carried out in the same manner as the 3D additive manufacturing method described in "4. Use of Foundry Sand for 3D Additive Manufacturing" above.

[0160] Heat treatment process In the heat treatment process, a sand mold is obtained by heat-treating the molded body, which does not contain the aqueous mold coating agent, in air or an inert gas atmosphere at 40 to 250°C. That is, the heat treatment is performed prior to adding (applying) the aqueous mold coating agent to the molded body. This makes it possible to more reliably suppress deformation of the sand mold that may occur when the aqueous mold coating agent is applied to the sand mold.

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

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

[0163] The heating atmosphere can be either air or an inert gas atmosphere. The inert gas is not limited to any specific type; for example, nitrogen, argon, or helium can all be used.

[0164] Generally, when manufacturing iron-based molded bodies by casting iron-based materials in a sand mold, a water-based mold coating agent (especially an aqueous dispersion of inorganic materials) is applied to at least the area of ​​the sand mold that comes into contact with the iron-based material (molten metal). In conventional sand molds, when a water-based mold coating agent is applied, deformation or cracking of the sand mold can occur. In contrast, the sand mold of the present invention uses heteropoly acid as an acid curing agent, which has a higher solubility in water than sulfonic acid. As a result, the acid curing agent is effectively dispersed (penetrated) to the center of the intergranular bridges that form the cured furan resin precursor, making it possible to obtain high strength.

[0165] Furthermore, by applying the heat treatment described above, the furan resin precursor can be brought closer to complete curing with greater certainty. Moreover, as mentioned above, since the heteropoly acid, which is an acid curing agent, is thoroughly dispersed, the heat treatment can further promote the three-dimensional polymerization of the furan resin precursor, thereby effectively contributing to the prevention of deformation (deformation of the sand mold, occurrence of cracks, etc.).

[0166] As described above, the sand mold obtained in this manner can be used as a casting sand mold for casting various materials (especially metals), but it is particularly suitable for casting iron-based materials.

[0167] When manufacturing an iron-based molded product by casting an iron-based material using the sand mold of the present invention, for example, a manufacturing method including a) applying a water-based mold coating agent 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 step in (a) above can be carried out in the same manner as the coating step described above, but if a water-based mold coating agent has been applied to the sand mold in advance, the step in (a) above can be omitted. In this case, an iron-based molded product can be suitably manufactured by a method that includes the step of pouring molten iron-based material into the sand mold of the present invention containing a water-based mold coating agent.

[0169] As the aqueous coating agent, a coating agent in which an inorganic component is dispersed in an aqueous solvent can be suitably used. As the inorganic component, an aqueous coating agent containing at least one oxide such as silica, alumina, magnesia, or zirconia can be suitably used. These can be known or commercially available. Therefore, within the limits that do not hinder the effects of the present invention, various additives (dispersants, defoamers, thickeners, etc.) found in commercially available products may be contained in the aqueous coating agent.

[0170] As the aqueous solvent, (a) water, (b) a water-soluble organic solvent, or (c) a mixture (aqueous solution) of water and a water-soluble organic solvent can be used. As the water-soluble organic solvent, although not limited, at least one of alcohols such as methanol, ethanol, and isopropyl alcohol can be suitably used.

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

[0172] The coating process may be carried out immediately after the heat treatment process of the sand mold manufacturing method described above, but it is also possible to carry out the coating process immediately before use (casting) of the sand mold after the heat treatment process, rather than immediately after use. For example, a water-based mold coating agent can be applied to a sand mold when it is to be used after a certain period of time has elapsed since the heat treatment process was manufactured (more specifically, before molten metal is poured into the sand mold).

[0173] After applying the water-based mold coating agent to the sand mold surface, a drying process should be carried out. Drying can be done by natural drying or heat drying, but if heat drying is used, it is usually done at a temperature in the range of 50 to 180°C, and more preferably at 80 to 120°C. The heat drying time can be appropriately changed depending on the heating temperature, etc., but it can usually be determined within the range of 30 to 120 minutes.

[0174] Next, in step b) above, 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 come into contact with the surface of the sand mold coated with a water-based mold coating agent. The iron-based material can be any material that contains iron as its main component (especially with an iron content of 50% by mass or more), and an iron-based material with a composition appropriate for various iron-based products may be used.

[0175] After pouring in molten iron-based material and completing the casting, the sand mold can be broken and the iron-based molded body removed according to known methods. [Examples]

[0176] Examples and comparative examples are shown below to provide a more detailed explanation of the features of this study. However, the scope of the present invention is not limited to these examples. In each table, the percentages indicating the compositional content all represent "mass percent".

[0177] Example 1 300 g of new AFS65 mullite-based molten artificial sand (Alsand #650, manufactured by Ito Kiko Co., Ltd., angle of repose 25°) (ARS molten new sand) was prepared as sand grains, along with palmitic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as a coating agent and silitangustic acid (manufactured by Nippon Muki Kagaku Kogyo Co., Ltd., D50=1.0 μm) as an acid hardening agent. While heating the sand, when the sand temperature of the artificial sand reached 50°C, 0.2 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand and stirred and mixed for 60 seconds. The sand temperature was 70°C. Next, 1.0 part by mass of silitangustic acid was added to 100 parts by mass of artificial sand in the resulting mixture and stirred and mixed for 120 seconds to obtain dry foundry sand.

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

[0179] Example 3 A mixture was prepared by replacing the sand grains with 300g of new AFS64 mullite-type calcined artificial sand (Cerabeads X#650, manufactured by Itochu Ceratec Co., Ltd., angle of repose 28°) (CBX sintered new sand), along with palmitic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and silitangustic acid (manufactured by Nippon Inorganic Chemical Industry Co., Ltd., D50=1.0μm), which is an acid hardening agent. While heating the sand, when the sand temperature of the artificial sand reached 50°C, 0.1 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand and stirred and mixed for 60 seconds. The sand temperature was 70°C. Next, 1.0 part by mass of silitangustic acid was added to 100 parts by mass of artificial sand in the resulting mixture and stirred and mixed for 120 seconds to obtain dry foundry sand.

[0180] Example 4 A mixture was prepared by replacing the sand grains with 300g of new AFS78 mullite-based molten artificial sand (Espearl #75L, manufactured by Yamakawa Sangyo Co., Ltd., angle of repose 24°) (EP molten new sand), along with palmitic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and silitangustic acid (manufactured by Nippon Inorganic Chemical Industry Co., Ltd., D50=1.0μm) as an acid hardening agent. While heating the sand, when the sand temperature of the above artificial sand reached 50°C, 0.1 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand and stirred and mixed for 60 seconds. The sand temperature was 67°C. Next, 1.0 part by mass of silitangustic acid was added to 100 parts by mass of artificial sand in the resulting mixture and stirred and mixed for 120 seconds to obtain dry foundry sand.

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

[0182] Example 6 The sand grains were replaced with a mixed sand prepared by stirring and mixing AFS56 roasted recycled sand (Tochu Co., Ltd. recycled sand No. 6, angle of repose 34°) and AFS96 new natural silica sand in a ratio of 7:3 parts by mass. Palmitic acid (Fujifilm Wako Pure Chemical Industries, Ltd.) and silitangustic acid (Nippon Inorganic Chemical Industry Co., Ltd., D50=1.0μm), an acid hardening agent, were also prepared. While heating the sand, when the sand temperature of the mixed sand reached 50°C, 0.2 parts by mass of palmitic acid was added to 100 parts by mass of the mixed sand and stirred and mixed for 60 seconds. The sand temperature was 70°C. Next, 1.0 part by mass of silitangustic acid was added to 100 parts by mass of the mixed sand to obtain dry foundry sand.

[0183] Example 7 Foundry 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 compared to Example 1: 1.1 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand and stirred for 60 seconds. The sand temperature was 75°C. Next, 1.5 parts by mass of silicatungstic acid was added to the resulting mixture per 100 parts by mass of artificial sand and stirred for 120 seconds to obtain dry foundry sand.

[0184] Example 8 Foundry 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 compared to Example 1: 0.02 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand and stirred for 60 seconds. The sand temperature was 70°C. Next, 1.5 parts by mass of silicatungstic acid was added to the resulting mixture per 100 parts by mass of artificial sand and stirred for 120 seconds to obtain dry foundry 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 compared to Example 3: Stearic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added as a coating agent at a ratio of 0.1 parts by mass per 100 parts by mass of artificial sand and stirred and mixed for 60 seconds. The sand temperature was 70°C. Next, 1.0 part by mass of silicatungstic acid was added to the resulting mixture at a ratio of 1.0 parts by mass per 100 parts by mass of artificial sand and 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 compared to Example 3: As a coating agent, 0.1 parts by mass of lauric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added per 100 parts by mass of artificial sand and stirred and mixed for 60 seconds. The sand temperature was 69°C. Next, 1.0 part by mass of silicatungstic acid was added to the obtained mixture per 100 parts by mass of artificial sand and stirred and mixed for 120 seconds to obtain dry foundry sand.

[0187] Example 11 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 compared to Example 3. As a coating agent, 0.1 parts by mass of the furan resin composition "Composition 1" from Table 1 was added to 100 parts by mass of artificial sand and stirred and mixed for 60 seconds. The sand temperature was 60°C. Next, 1.0 part by mass of silicatungstic acid was added to the obtained mixture to 100 parts by mass of artificial sand and stirred and mixed for 120 seconds to obtain dry foundry 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 compared to Example 1. 0.2 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand and stirred for 60 seconds. The sand temperature was 66°C. Next, a solution of phosphotungstic acid (manufactured by Nippon Inorganic Chemical Industry Co., Ltd., D50=55μm) as an acid hardening agent (1.0 part by mass) and ethanol as a solvent (1.0 part by mass) was added to the obtained mixture and stirred 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 compared to Example 3: 0.1 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand and stirred for 60 seconds. The sand temperature was 70°C. Next, 1.0 part by mass of silicatungstic acid (manufactured by Nippon Inorganic Chemical Industry Co., Ltd., D50=20μm) was added to the resulting mixture as a hardening agent per 100 parts by mass of artificial sand and stirred 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 compared to Example 3. 0.1 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand and stirred for 60 seconds. The sand temperature was 80°C. Next, a solution of 1.0 part by mass of silicatungstic acid as an acid hardening agent and 1.0 part by mass of ethanol as a solvent was added to the obtained mixture and stirred for 120 seconds to obtain dry foundry sand.

[0192] Example 15 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 compared to Example 1: 0.1 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand and stirred for 60 seconds. The sand temperature was 76°C. Next, 2.0 parts by mass of silicatungstic acid was added to the resulting mixture to 100 parts by mass of artificial sand and stirred for 120 seconds to obtain dry foundry sand.

[0193] Example 16 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 compared to Example 1: 0.1 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand and stirred for 60 seconds. The sand temperature was 70°C. Next, 0.5 parts by mass of silicatungstic acid was added to the resulting mixture per 100 parts by mass of artificial sand and stirred for 120 seconds to obtain dry foundry sand.

[0194] Example 17 A mixture was prepared in which the sand grains were replaced with 300g of new AFS64 mullite-type calcined artificial sand (Cerabeads X#650, manufactured by Itochu Ceratec Co., Ltd., angle of repose 28°) (CBX sintered new sand), and as additives, the silane coupling agent "Composition 2" shown in Table 2, palmitic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and the acid hardening agent silate tungstic acid (manufactured by Nippon Inorganic Chemical Industry Co., Ltd., D50=1.0μm) were prepared. While heating the sand, when the sand temperature of the above artificial sand reached 50°C, 0.5 parts by mass of the silane coupling agent of Composition 2 was added to 100 parts by mass of the above artificial sand and 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 artificial sand in the obtained mixture and stirred and mixed for 60 seconds. The sand temperature was 77°C. Finally, 1.0 part by mass of silicatungstic acid was added to the resulting mixture per 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 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 compared to Example 1. As a coating agent, 2.1 parts by mass of the furan resin composition "Composition 1" from Table 1 was added to 100 parts by mass of artificial sand and stirred and mixed for 60 seconds. The sand temperature was 74°C. Next, 1.5 parts by mass of silicatungstic acid was added to the obtained mixture to 100 parts by mass of artificial sand and stirred and mixed for 120 seconds to obtain dry foundry sand.

[0197] Example 19 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 compared to Example 1. As a coating agent, 0.03 parts by mass of the furan resin composition "Composition 1" from Table 1 was added to 100 parts by mass of artificial sand and stirred and mixed for 60 seconds. The sand temperature was 66°C. Next, 1.5 parts by mass of silicatungstic acid was added to the obtained mixture to 100 parts by mass of artificial sand and stirred and mixed for 120 seconds to obtain dry foundry sand.

[0198] Example 20 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 compared to Example 1. As a coating agent, 0.2 parts by mass of the furan resin composition "Composition 1" from Table 1 was added to 100 parts by mass of artificial sand and stirred for 60 seconds. The sand temperature was 60°C. Next, 0.8 parts by mass of silicatungstic acid was added to the obtained mixture to 100 parts by mass of artificial sand and stirred for 120 seconds to obtain dry foundry sand.

[0199] Example 21 A mixture was prepared by replacing the sand grains with 300g of new AFS64 mullite-type calcined artificial sand (Cerabeads X#650, manufactured by Itochu Ceratec Co., Ltd., angle of repose 28°) (CBX sintered new sand), along with palmitic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and silitangustic acid (manufactured by Nippon Inorganic Chemical Industry Co., Ltd., D50=1.0μm), which is an acid hardening agent. While heating the sand, when the sand temperature of the artificial sand reached 40°C, an aqueous solution of 1.0 part by mass of silitangustic acid and 3.3 parts by mass of water as a solvent was added to 100 parts by mass of the artificial sand, and the mixture was stirred for 90 seconds. The sand temperature was 65°C. Next, 0.1 parts by mass of palmitic acid was added to the obtained mixture per 100 parts by mass of artificial sand, and the mixture was stirred for 60 seconds to obtain intermediate-state foundry sand.

[0200] Comparative Example 1 300g of new AFS65 mullite-based molten artificial sand (Alsand #650, manufactured by Ito Kiko Co., Ltd., angle of repose 25°) and silitangustic acid (manufactured by Nippon Muki Kagaku Kogyo Co., Ltd., D50=1.0μm), a hardening agent, were prepared. The sand was not heated. With the artificial sand at a temperature of 25°C, 0.4 parts by mass of silitangustic acid were added to 100 parts by mass of artificial sand, and the mixture was 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 compared to Comparative Example 1: Silicate tungstic acid, which is an acid hardening agent, was prepared. The sand was not heated. With the artificial sand temperature at 25°C, 1.0 part by mass of silicate tungstic acid was added to 100 parts by mass of artificial sand, and the mixture was stirred for 120 seconds to obtain dry foundry sand.

[0202] Comparative Example 3 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 compared to Example 1: 1.2 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand and stirred for 60 seconds. The sand temperature was 70°C. Next, 1.5 parts by mass of silicatungstic acid was added to 100 parts by mass of artificial sand and mixed and stirred for 120 seconds to obtain dry foundry sand.

[0203] Comparative Example 4 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 compared to Example 1: 0.01 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand and stirred for 60 seconds. The sand temperature was 75°C. Next, 1.5 parts by mass of silicatungstic acid was added to 100 parts by mass of artificial sand and mixed and stirred for 120 seconds to obtain dry foundry 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 compared to Example 1: 0.1 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand and stirred for 60 seconds. The sand temperature was 70°C. Next, 3.0 parts by mass of silicatungstic acid was added to 100 parts by mass of artificial sand and 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 compared to Example 1: 0.1 parts by mass of palmitic acid was added to 100 parts by mass of artificial sand and stirred for 60 seconds. The sand temperature was 72°C. Next, 0.4 parts by mass of silicatungstic acid was added to 100 parts by mass of artificial sand and mixed and stirred for 120 seconds to obtain dry foundry sand.

[0206] Comparative Example 7 Foundry 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 compared to Example 11. 2.2 parts by mass of the furan resin composition of "Constitution 1" was added to 100 parts by mass of artificial sand and stirred for 60 seconds. The sand temperature was 64°C. Next, 1.5 parts by mass of silicatungstic acid was added to 100 parts by mass of artificial sand and mixed and stirred for 120 seconds to obtain dry foundry sand.

[0207] Comparative Example 8 Foundry 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 compared to Example 11. 0.02 parts by mass of the furan resin composition of "Constitution 1" was added to 100 parts by mass of artificial sand and stirred for 60 seconds. The sand temperature was 70°C. Next, 1.5 parts by mass of silicatungstic acid was added to 100 parts by mass of artificial sand and mixed and stirred for 120 seconds to obtain dry foundry sand.

[0208] Comparative Example 9 A mixture of 300g of new AFS64 mullite-type calcined artificial sand (Cerabeads X#650, manufactured by Itochu Ceratec Co., Ltd., angle of repose 28°) (CBX sintered new sand) and silitangustic acid (manufactured by Nippon Inorganic Chemical Industry Co., Ltd., D50=1.0μm), which is an acid hardening agent, was prepared. The sand was not heated. With the artificial sand at a temperature of 25°C, an aqueous solution of 1.0 part by mass of silitangustic acid and 3.3 parts by mass of water as a solvent was added to 100 parts by mass of the artificial sand, and the mixture was stirred for 60 seconds to obtain wet foundry sand.

[0209] Test Example 1 (1) Flexural strength (test piece strength (TP strength)) 200g of each sample was weighed into a metal container. The coating binders listed in Tables 4-6 were added to all the foundry sand samples. Four types of coating binders with different furfuryl alcohol content were prepared. Their compositions are shown in Table 3. The mixtures were stirred for 5 seconds using a stirrer to prepare the mixtures. The resulting mixtures were packed into 10mm x 10mm x 60mm molds and left for 30 minutes under conditions of 23-25°C and 40-45% humidity before being removed from the molds. Test pieces for measuring flexural strength were prepared in this manner.

[0210] Furthermore, prior studies by the inventors have confirmed that setting the stirring time to 5 seconds is sufficient to reproduce the strength after 3D layering in strength reproduction tests conducted in a laboratory without using a 3D printer. To simulate the conditions of 3D layering, where forcible mixing of the foundry sand and coating binder is not possible, in a laboratory setting, it is crucial to drastically shorten the stirring time.

[0211] Subsequently, the three-point bending strength of the test piece was measured using an IMADA ZTS-1000N force gauge at (a) immediately after die-cutting (30 minutes later), (b) 3 hours later, and (c) 24 hours later. The results are shown in Tables 4-6. The strength at 3 hours was 25 kg / cm². 2 Those meeting the above criteria were deemed to have passed.

[0212] [Table 3]

[0213] (2) Sand grain pore humidity measurement The humidity of the foundry sand was measured in a room adjusted to a temperature of 23-25°C and a humidity of 40-45% using a Testo 605-H1 digital thermometer and hygrometer. Approximately 300g of foundry sand was placed in a glass beaker, and the sensor of the digital thermometer and hygrometer was inserted into the sample until it was completely submerged. The sample was then 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 adjusted to a temperature of 23-25°C and a humidity of 40-45%. Various types of foundry sand were prepared, and 100g of each was measured into a paper cup and lightly compacted. This process was repeated until the weight of the foundry sand in the paper cup reached 300g. After that, a metal tray was placed over the paper cup containing the foundry sand, and the cup and tray were inverted together, ensuring they did not shift, and placed on a level surface. The paper cup was carefully removed, and the shape of the sand pile after complete removal was visually inspected. A "○" indicated that the sand pile had collapsed, and a "×" indicated that the sand pile maintained its shape. 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 4-6 and Figure 6, the foundry sand in the examples can exhibit the desired strength and fluidity. The presence of an organic layer that dissolves in furfuryl alcohol enables the uniform loading of the acid hardening agent onto the surface of the sand grains. Regardless of the type of sand, grain size, coating agent, or acid hardening agent, an average load of approximately 40 kg / cm³ was achieved 3 hours after molding. 2 We were also able to confirm that such a high intensity expression can be obtained.

[0219] In contrast, the target flexural strength was not achieved in Comparative Examples 1 and 2. This is presumed to be because the absence of an organic layer prevented the acid curing agent from being uniformly dispersed. In Comparative Example 2, despite increasing the amount of curing agent or coating binder added compared to Comparative Example 1, no improvement in flexural strength was observed. From this, it can be seen 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, a coating agent, inhibited the curing reaction, preventing sufficient reaction with the coating binder and thus resulting in a lack of strength improvement.

[0221] In Comparative Example 4, it is believed that the desired flexural strength could not be obtained because the acid hardening agent could not be uniformly supported on the surface of the sand grains due to insufficient addition of palmitic acid, which is a coating agent.

[0222] In Comparative Example 5, it is believed that the addition of an excessive amount of acid curing agent resulted in a sudden and large generation of reaction water during the curing of the coating binder, which halted the polymerization of the furan resin precursor and caused internal curing failure.

[0223] In Comparative Example 6, it is believed that the curing reaction did not proceed sufficiently due to insufficient addition of the acid curing agent, and therefore the target flexural strength could not be obtained.

[0224] In Comparative Example 7, an excessive amount of the furan resin composition "Composition 1," which is a coating agent, was added. As a result, a large amount of reaction water was generated all at once during the curing of the furan, and there was insufficient reaction heat for curing, so the target flexural strength could not be obtained.

[0225] In Comparative Example 8, the amount of furan resin composition "Composition 1," which is the coating agent, was insufficient, causing Composition 1 and the acid curing agent to completely harden on the surface of the sand grains. As a result, it was unable to fulfill its intended role as a coating agent, which is to assist in strength development, and thus the resulting strength was insufficient.

[0226] In Comparative Example 9, the amount of water contained in the acid curing agent aqueous solution was excessive, which suppressed the reaction heat generated during furan curing. As a result, the polymerization of the furan resin precursor stopped, leading to poor internal curing.

[0227] Test Example 2 (1) Pre-drying of test pieces Using a VX500 3D printer (manufactured by Voxeljet), test pieces measuring 10mm x 10mm x 200mm were fabricated from a casting sand sample prepared under the conditions of Example 11 described in Table 5. The sample was printed at a temperature of 23-25°C and a humidity of 40-45%.

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

[0229] Thirty minutes after the molding process was complete, the test pieces were removed from the build box, and any uncured casting sand adhering to them was removed using a brush and then a blower. Pre-drying was then performed in firing ovens set to 50°C (Example 22), 100°C (Example 23), and 180°C (Example 24).

[0230] In addition, a foundry sand sample prepared under the conditions of "Preparation Example 5" described in Table 7 was prepared. The method for preparing the foundry sand sample is as follows: 5 kg of new AFS108 mullite-based sintered artificial sand (Cerabeads #1450, manufactured by Itochu Ceratec Co., Ltd., angle of repose 31°) (CBX sintered new sand), the furan resin composition "Composition 1" from Table 1, and the curing agent "Composition 3" from Table 8 were prepared. Table 9 shows the solubility in water of the heteropoly acid used in the example (measured value), along with the solubility in water of the curing agent from Table 8 (measured value).

[0231] With the sand temperature of the artificial sand reaching 35°C, 0.3 parts by mass of the furan resin composition of Configuration 1 was added to 100 parts by mass of the artificial sand and 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 "Configuration 3" and 0.3 parts by mass of water as a solvent were added to the obtained mixture per 100 parts by mass of the artificial sand and mixed in the kneader for 30 seconds. Finally, as a drying process, stirring was continued for 240 seconds while blowing cold air (air) into the kneader until the sand temperature reached 60°C, and then it was discharged from the kneader to obtain casting sand.

[0232] The coating binder of Preparation Example 3 described in Table 3 was prepared on the printer head, and test pieces were produced in the same environment and by the same method as above. The test pieces were pre-dried in a firing furnace set at 100°C (Comparative Example 11). Also, test pieces without pre-drying (Comparative Example 10) were produced.

[0233]

Table 7

[0234]

Table 8

[0235]

Table 9

[0236] (2) Drying of the aqueous coating agent The pre-dried test pieces were dipped in the slurry of "Preparation Example 6" described in Table 10, which is an aqueous coating agent. After pulling the test pieces out of the slurry, the coating film of Preparation Example 6 was dried in a firing furnace at 100°C for 60 minutes immediately. The test pieces before pre-drying and after the coating film drying of Preparation Example 6 were overlapped and arranged side by side, and visually observed. As a result, for those with a gap observed between the test pieces, the width (mm) of the gap was measured as those with deformation, and those without a gap observed were regarded as those without deformation and marked as "none". The results are shown in Table 11.

[0237] [Table 10]

[0238] [Table 11]

[0239] As is clear from the results of Examples 22-24 described in Table 11, the test pieces in the examples showed no deformation even after the water-based coating agent dried.

[0240] In contrast, in Comparative Examples 10 and 11, since heteropoly acid was not used as a curing agent, deformation of the test pieces became significant unless pre-drying was performed (Comparative Example 10), and it was found that deformation of the test pieces was unavoidable even with pre-drying.

[0241] As described above, by introducing heteropoly acid as an acid hardening agent in foundry sand having an organic layer, it becomes possible to effectively suppress or prevent deformation of sand molds formed from that foundry sand even when a water-based mold coating agent is applied.

Claims

1. Foundry sand used for 3D 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 comprises a furan resin precursor or a fatty acid with a melting point of 40 to 75°C and a heteropoly acid. A casting sand for three-dimensional additive manufacturing characterized by the following features.

2. The three-dimensional additive manufacturing casting sand according to claim 1, wherein the heteropoly acid comprises at least one of silicic acid and phosphotungstic acid.

3. The three-dimensional additive manufacturing casting sand according to claim 1, wherein the sand contains 0.5 to 2.5 parts by mass of heteropoly acid per 100 parts by mass of sand grains.

4. The three-dimensional additive manufacturing casting sand according to claim 1, wherein the organic layer is a carrier containing a furan resin precursor or a fatty acid with a melting point of 40 to 75°C on which solid particles of heteropoly acid are supported.

5. The three-dimensional additive manufacturing casting sand according to claim 1, wherein the organic layer comprises an aqueous wet layer containing a heteropoly acid 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 wet layer is covered by the hydrophobic layer.

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

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

8. A method for manufacturing foundry sand for three-dimensional additive manufacturing, (1) A step of obtaining precursor particles in which the surface of the sand particles is coated with an aqueous wet layer containing the aqueous solution by mixing at least sand particles and an aqueous solution of heteropoly acid at a temperature of 100°C or less. (2) A step of obtaining composite particles in which the aqueous wet layer is covered with a hydrophobic layer containing the fatty acid by mixing the precursor particles with a furan resin precursor or a coating agent containing a fatty acid with a melting point of 40 to 75°C at a temperature of 100°C or less. A method for producing foundry sand, characterized by containing the following:

9. The manufacturing 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 three-dimensional additive manufacturing sand as described in any one of claims 1 to 5, (1) A step of obtaining a molded body by sequentially repeating the steps of forming a layer containing the foundry sand and solidifying a predetermined area of ​​the layer by adding a coating binder to that area, and (2) A step of obtaining a sand mold by heating the molded body, which does not contain an aqueous mold coating agent, in air or an inert gas atmosphere at 40 to 250°C. A method for manufacturing sand molds, characterized by including the following:

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

12. Furthermore, the sand mold for casting iron-based materials according to claim 11, further comprising a water-based mold coating agent.

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