Two-layer casting mold and production method for same

Abstract

Provided is a two-layer casting mold that makes it possible to suppress gas defects, casting surface deficiencies, and deformation. Provided is a two-layer casting mold that includes: an inner layer that includes an inorganic binder–coated sand that includes a metasilicate or a hydrate of a metasilicate as an inorganic binder; and an outer layer that includes a coated sand selected from the group that consists of resin binder–coated sands and inorganic binder–coated sands.

Description

Two-layer mold and its manufacturing method

[0001] The present invention relates to a two-layer mold and a method for making the same.

[0002] A mold having a two-layer structure consisting of a high-strength outer layer and an inner layer with excellent collapsibility is known, with the aim of ensuring strength during pouring and improving collapsibility after pouring.

[0003] Patent Document 1 (JP Patent Publication No. 10-166106) describes a two-layer shell mold characterized in that at least a part of the shell mold has an integral structure of an outer layer and an inner layer formed of thermosetting resin-coated sand with different grain size indexes, and the outer layer is formed with a larger grain size index than the inner layer.

[0004] Patent Document 2 (JP 2009-178758 A) describes a core manufacturing device for manufacturing a core comprising an outer layer and an inner layer formed of thermosetting resin-coated sand with different particle size indexes, the outer layer being formed with a larger particle size index than the inner layer.

[0005] Japanese Patent Laid-Open No. 10-166106 Japanese Patent Laid-Open No. 2009-178758

[0006] In the manufacture of two-layer molds using thermosetting resin-coated sand, when a first layer (e.g., an outer layer) is formed and then a second layer (e.g., an inner layer) is formed on top of the first layer, the first layer may act as an insulating layer and prevent heat conduction to the second layer, which may result in poor hardening of the second layer or an extended cycle time.

[0007] Thermosetting resin-coated sand is widely used because it offers high productivity during mold making and can produce high-quality castings. However, gas can be generated during pouring due to the thermal decomposition of the resin, which can lead to gas defects in the casting. On the other hand, the use of inorganic binder-coated sand can suppress gas defects in the casting. However, single-layer molds made using inorganic binder-coated sand lack heat resistance and are at risk of producing poor casting surfaces and deformation.

[0008] The present invention aims to provide a mold that can suppress the occurrence of gas defects and can suppress casting surface defects and deformation. Another object of the present invention is to provide a method for manufacturing a mold that can suppress hardening defects, shorten the cycle time, and suppress the occurrence of gas defects.

[0009] The inventors discovered that by forming the inner layer of a two-layer mold using a specific inorganic binder-coated sand, it is possible to suppress the occurrence of gas defects, resulting in a two-layer mold with reduced casting surface defects and deformation, and that in the manufacture of such a two-layer mold, poor hardening can be suppressed and the cycle time can be shortened, leading to the completion of the present invention.

[0010] The present invention includes the following aspects. [Aspect 1] A two-layer mold comprising an inner layer containing inorganic binder-coated sand and an outer layer containing coated sand selected from the group consisting of resin binder-coated sand and inorganic binder-coated sand, wherein the inorganic binder-coated sand contains metasilicate or a hydrate thereof as the inorganic binder. [Aspect 2] The two-layer mold according to Aspect 1, wherein the outer layer contains resin binder-coated sand. [Aspect 3] The two-layer mold according to Aspect 1 or 2, wherein the resin binder-coated sand contains at least one resin binder selected from the group consisting of cross-linked curable resins and their cured products, thermoplastic resins, and carbohydrates. [Aspect 4] The two-layer mold according to Aspect 3, wherein the resin binder-coated sand contains a phenolic resin. [Aspect 5] The two-layer mold according to any one of Aspects 1 to 4, wherein the gradation index of the refractory aggregate contained in the coated sand of the outer layer is greater than the gradation index of the refractory aggregate contained in the inorganic binder-coated sand of the inner layer. [Aspect 6] The two-layer mold according to any one of Aspects 1 to 5, wherein the refractory aggregate contained in the coating sand of the outer layer has a gradation index of at least 80. [Aspect 7] The two-layer mold according to any one of Aspects 1 to 6, wherein the thickness of the outer layer is 0.3 mm to 100 mm. [Embodiment 8] A method for manufacturing a two-layer mold, comprising: preparing a molding die having a model surface and a model space; filling the model space of the molding die with coated sand selected from the group consisting of resin binder-coated sand and inorganic binder-coated sand; hardening the coated sand to form an outer layer on the model surface; discharging the unhardened coated sand from the molding die; filling the model space of the molding die with inorganic binder-coated sand; hardening the inorganic binder-coated sand to form an inner layer on the outer layer; and, if necessary, discharging the unhardened inorganic binder-coated sand from the molding die, wherein the inorganic binder-coated sand contains metasilicate or a hydrate thereof as the inorganic binder.

[0011] The present invention provides a two-layer mold that can suppress the generation of gas defects, suppress deformation, and obtain a good casting surface. Furthermore, the present invention also makes it possible to suppress insufficient hardening and shorten the cycle time when manufacturing such a two-layer mold.

[0012] The above description should not be considered as a disclosure of all embodiments of the present invention and all advantages associated with the present invention.

[0013] 1A and 1B are schematic cross-sectional views of a two-layer mold according to an embodiment of the present invention;

[0014] Representative embodiments of the present invention will now be described in more detail with reference to the drawings for illustrative purposes, but the present invention is not limited to these embodiments.

[0015] In this disclosure, the bending strength of inorganic binder-coated sand is a value measured when tested according to the following procedure. The inorganic binder-coated sand is filled into a molding die heated to 150°C, and then held in the molding die for 120 seconds to solidify, thereby producing a test piece (2.54 cm x 2.54 cm x 20 cm). The breaking load of the test piece is measured using a strength tester (a digital molding sand strength tester manufactured by Takachiho Seiki Co., Ltd.). The bending strength is calculated from the breaking strength using the following formula. A higher bending strength indicates a stronger mold. Bending strength (N / cm ２ ) = 1.5 × (L × W) / (a ​​× b ２ ) L: distance between supports (cm), W: breaking load (N), a: width of test piece (cm), b: thickness of test piece (cm)

[0016] In this disclosure, the bending strength of resin binder-coated sand is a value measured when tested according to the following procedure. In accordance with JIS K 6910:1995, a JIS-type test piece (10 mm x 10 mm x 60 mm, firing conditions: 250°C, 60 seconds) is prepared using the resin binder-coated sand. The bending strength (kgf / cm) of the JIS-type test piece is measured in accordance with JACT test method SM-1 (Bending strength test method, Japan Foundry Association). ２ ) and measured in SI units (N / cm ２The higher the bending strength, the stronger the mold.

[0017] [Two-Layer Mold] In one embodiment, the two-layer mold comprises an inner layer containing inorganic binder-coated sand and an outer layer containing coated sand selected from the group consisting of resin binder-coated sand and inorganic binder-coated sand. The inorganic binder-coated sand contains metasilicate or its hydrate as the inorganic binder. The outer layer is the layer that comes into contact with the molten metal during pouring.

[0018] The two-layer mold, whose inner layer is formed from the inorganic binder-coated sand, can reduce the amount of gas generated and suppress the occurrence of gas defects. Furthermore, the inorganic binder-coated sand has a low fusion point and can be hardened at low temperatures, so that poor hardening during the production of the two-layer mold can be suppressed and the cycle time can be shortened.

[0019] A schematic cross-sectional view of one embodiment of a two-layer mold is shown in Figure 1. The two-layer mold 10 includes an inner layer 12 containing inorganic binder-coated sand and an outer layer 14 containing coated sand selected from the group consisting of resin binder-coated sand and inorganic binder-coated sand. In Figure 1, the two-layer mold 10 is shown as a solid core.

[0020] The inorganic binder-coated sand contains refractory aggregate and an inorganic binder attached to the surface of the refractory aggregate.

[0021] The refractory aggregate is not particularly limited as long as it is a granular material that can be used for molds. Examples of refractory aggregates include general sands such as silica sand and recycled silica sand; specialty sands such as alumina sand, olivine sand, zircon sand, and chromite sand; slag particles such as ferrochrome slag, ferronickel slag, and converter slag; artificial particles such as alumina particles and mullite particles, and recycled particles thereof; alumina balls; and magnesia clinker. The refractory aggregates can be used alone or in combination. The refractory aggregate may be new, recycled, or recovered, having been used once or multiple times as foundry sand to form molds, or a mixture of new and recycled or recovered aggregates.

[0022] Refractory aggregates generally have a gradation index of 20 to 160. In this disclosure, the gradation index of the refractory aggregate and the coating sand is determined in accordance with the AFS coefficient standard defined in JACT Test Method S-1 (Grain Size Test Method for Molding Sand, Japan Foundry Association). A large gradation index indicates fine particles, and a small gradation index indicates coarse particles.

[0023] The refractory aggregate is preferably spherical. Specifically, the particle shape coefficient of the refractory aggregate is preferably 1.2 or less, more preferably 1.0 to 1.1. The particle shape coefficient (also called particle size index) is a measure of the external shape of a particle, and the closer the value is to 1, the closer the particle is to a spherical shape (true sphere). The particle shape coefficient is calculated by measuring the actual surface area of ​​aggregate particles per 1 g using a sand surface area measuring device (manufactured by George Fischer) and dividing the result by the theoretical surface area of ​​the aggregate. The theoretical surface area of ​​aggregate is the surface area when all aggregate particles are assumed to be spherical.

[0024] The inorganic binder includes metasilicate and its hydrate. Examples of metasilicate include sodium metasilicate and potassium metasilicate. The metasilicate hydrate is preferably at least one selected from the group consisting of sodium metasilicate pentahydrate, sodium metasilicate nonahydrate, and potassium metasilicate hydrate, and more preferably at least one selected from sodium metasilicate pentahydrate and sodium metasilicate nonahydrate. The mixing ratio of sodium metasilicate pentahydrate to sodium metasilicate nonahydrate is preferably 10:90 to 90:10, and more preferably 20:80 to 80:20.

[0025] Metasilicates and their hydrates, which are silicate-based binders, are advantageous in terms of mold strength and flexibility. Furthermore, metasilicates and their hydrates can dry inorganic binder-coated sand despite their relatively low melting points. This improves the room-temperature fluidity of the inorganic binder-coated sand, allowing it to be effectively hardened at low temperatures using a conventional molding machine.

[0026] The resin binder-coated sand contains refractory aggregate and a resin binder attached to the surface of the refractory aggregate. The refractory aggregate may be the same as that described for the inorganic binder-coated sand.

[0027] The resin binder may be, for example, at least one selected from the group consisting of crosslinked curable resins and their cured products, thermoplastic resins, and carbohydrates.

[0028] The crosslinking curable resin is a resin that crosslinks and cures under heating or without heating (room temperature) in the presence or absence of a curing agent or curing catalyst such as hexamethylenetetramine, organic ester, organic acid, carbon dioxide gas, peroxide, metal ion, or amine. Examples of the crosslinking curable resin include phenolic resins, phenolic urethane resins, epoxy resins, urea resins, melamine resins, unsaturated polyester resins, polyfunctional acrylamide resins, unsaturated alkyd resins, unsaturated fatty acid-modified alkyd resins, diallyl phthalate resins, and combinations of two or more of these. The crosslinking curable resin is preferably at least one selected from the group consisting of phenolic resins and phenolic urethane resins.

[0029] Examples of thermoplastic resins include polyvinyl alcohol, polyvinyl acetate, polystyrene, styrene-acrylonitrile copolymer, styrene-butadiene-acrylonitrile copolymer, ethylene-vinyl acetate copolymer, polyvinyl chloride, polymethyl methacrylate, cellulose acetate, polycarbonate, and paraffin. From the viewpoint of film-forming properties, the thermoplastic resin is preferably at least one selected from the group consisting of polyvinyl alcohol, polyvinyl acetate, polystyrene, ethylene-vinyl acetate copolymer, polymethyl methacrylate, cellulose acetate, and polycarbonate.

[0030] Examples of carbohydrates include glucose, fructose, galactose, lactose, sucrose, maltose, trehalose, starch, glycogen, and cellulose. From the viewpoint of film-forming properties, the carbohydrate is preferably at least one selected from the group consisting of trehalose, starch, and glycogen.

[0031] The resin binder preferably contains a phenolic resin, which facilitates the production and storage of the resin binder-coated sand and can provide the resin binder-coated sand with excellent mold properties (e.g., heat resistance and strength).

[0032] The inorganic binder-coated sand and the resin binder-coated sand may contain additives such as mold disintegrants, hardening accelerators, anti-caking agents, coupling agents, lubricants, mold release agents, and deodorizers, and may also contain particles such as silica, red iron oxide, iron sand, and graphite.

[0033] Inorganic binder-coated sand and resin binder-coated sand can be produced by coating the surface of refractory aggregate with an inorganic binder or a resin binder using a kneading coating method (e.g., a dry hot coating method, a semi-hot coating method, or a cold coating method) or a powder solvent method, and then heating the resulting mixture as necessary.

[0034] The inner layer containing the inorganic binder-coated sand can suppress the occurrence of gas defects and the resulting casting defects, and can reinforce the outer layer while ensuring the collapsibility of the mold.

[0035] The inorganic binder-coated sand in the inner layer is preferably relatively lower in strength than the coated sand used in the outer layer in order to provide good disintegrability to the mold. The lower limit of the bending strength of the inorganic binder-coated sand in the inner layer is not particularly limited as long as it is strong enough to maintain the shape of the mold. From the viewpoint of disintegrability, the bending strength of the inorganic binder-coated sand in the inner layer is preferably 294 N / cm ２ (approximately 30 kgf / cm ２ ) or less, more preferably 196 N / cm ２ (approx. 20 kgf / cm ２ ) below.

[0036] The binder content of the inorganic binder-coated sand in the inner layer is determined depending on the disintegrability and other properties (e.g., strength) required for the mold, the type and particle size of the refractory aggregate, and the type of inorganic binder. The binder content of the inorganic binder-coated sand in the inner layer is generally 0.2% by mass to 5% by mass based on the mass of the inorganic binder-coated sand, and preferably 0.5% by mass to 4% by mass from the viewpoint of the reinforcing effect of the outer layer.

[0037] The grain size index of the refractory aggregate and coated sand contained in the inorganic binder-coated sand in the inner layer is preferably 20 to 80, more preferably 20 to 50, from the viewpoint of suppressing the generation of gas defects and improving the disintegration property of the mold.

[0038] The inner layer may be solid or hollow. The thickness of the inner layer can be determined appropriately depending on the shape of the mold and the strength and collapsibility required for the mold. The thickness of the inner layer is not particularly limited, but it is advantageous for the inner layer to occupy a larger proportion of the total volume of the mold than the outer layer. The inner layer of this embodiment has excellent gas permeability, which is particularly advantageous for suppressing the occurrence of gas defects and can also improve the collapsibility of the mold.

[0039] The outer layer containing coated sand selected from the group consisting of resin binder-coated sand and inorganic binder-coated sand is the main layer that provides the mold with the strength necessary to maintain its shape during pouring, and directly affects the quality of the casting surface.

[0040] The coated sand in the outer layer preferably has a strength equal to or higher than that of the inorganic binder coated sand used in the inner layer in order to provide the mold with strength sufficient to withstand pouring of molten metal. The bending strength of the coated sand in the outer layer is preferably 294 N / cm ２ (approximately 30 kgf / cm ２ ) or more, more preferably 392 N / cm ２ (approx. 40 kgf / cm ２ ) That's all.

[0041] The binder content of the outer layer of covering sand is determined depending on the strength and other properties (e.g., low thermal expansion) required for the mold, the type and particle size of the refractory aggregate, and the type of binder. The binder content of the outer layer of covering sand is generally 1% by mass to 10% by mass based on the mass of the covering sand, and is preferably 1.5% by mass to 6% by mass from the viewpoints of ease of production of the covering sand and stability of quality.

[0042] The grain size index of the refractory aggregate and the covering sand contained in the outer layer is preferably 80 or more, more preferably 90 to 160, from the viewpoint of improving the quality of the casting surface.

[0043] It is preferable that the gradation index of the refractory aggregate contained in the outer layer of coated sand is larger than that of the refractory aggregate contained in the inner layer of inorganic binder-coated sand, or that the gradation index of the outer layer of coated sand is larger than that of the inner layer of inorganic binder-coated sand, thereby effectively suppressing the occurrence of gas defects while improving the surface strength of the mold and the quality of the casting surface.

[0044] The thickness of the outer layer can be appropriately determined depending on the size and shape of the mold, the type, strength, and granularity index of the covering sand, the type of molten metal used in casting, the pouring method, and the strength and collapsibility required for the mold. The thickness of the outer layer is preferably 0.3 mm to 100 mm, more preferably 0.3 mm to 50 mm, and even more preferably 0.5 mm to 10 mm. The thickness of the outer layer may be the same throughout the outer layer, or may vary. By setting the thickness of the outer layer within the above range, it is possible to reduce the occurrence of gas defects and improve the collapsibility of the mold while ensuring the mold strength required for casting.

[0045] The outer layer preferably contains resin binder-coated sand. Resin binder-coated sand has good hot fluidity, which increases the surface strength of the two-layer mold and further improves the quality of the casting surface. In addition, the outer layer containing resin binder-coated sand has high hot heat resistance, which can suppress seizure during casting.

[0046] The two-layer mold can be used as a master mold or a core, and can be used particularly advantageously as a core.

[0047] [Method for manufacturing a two-layer mold] In one embodiment, a method for manufacturing a two-layer mold includes: preparing a molding die having a model surface and a model space; filling the model space of the molding die with coated sand selected from the group consisting of resin binder-coated sand and inorganic binder-coated sand; hardening the coated sand to form an outer layer on the model surface; ejecting the unhardened coated sand from the molding die; filling the model space of the molding die with inorganic binder-coated sand; hardening the inorganic binder-coated sand to form an inner layer on the outer layer; and, if necessary, ejecting the unhardened inorganic binder-coated sand from the molding die.

[0048] A method for manufacturing a two-layer mold according to one embodiment will be described with reference to Figure 2. A molding die 20 having a pattern surface 22 and a pattern space 24 is prepared (Figure 2(a)). The pattern space 24 of the molding die 20 has an opening (opening toward the bottom in Figure 2(a)), through which coated sand can be introduced into the pattern space 24 and unhardened coated sand can be discharged from the pattern space 24. The molding die 20 consists of two mold sections, which can be moved horizontally. Although not shown in Figure 2, the molding die 20 is equipped with a mechanism for turning the molding die 20 up and down, a heating device such as an electric or gas heater, and a vibration device such as a vibrator.

[0049] The model space 24 of the molding die 20 is filled with coated sand 15 selected from the group consisting of resin binder coated sand and inorganic binder coated sand (FIG. 2(b)). The coated sand 15 can be filled by an under-blow method using a pressurized supply mechanism that supplies the coated sand 15 with compressed air. Specifically, the molding die 20 is heated to 250°C to 300°C, and the coated sand 15 is injected from the opening of the molding die 20 at a pressure of 2 kg / cm. ２ ~6 kg / cm ２ Blow filling with.

[0050] By hardening the coated sand 15, the outer layer 14 is formed on the model surface 22. Thereafter, the unhardened coated sand 15 is discharged from the opening of the molding die 20 (FIG. 2(c)).

[0051] When filling or discharging the coated sand 15, it is desirable to vibrate the molding die 20 with a vibrator or the like in order to pack the coated sand 15 densely and to make the outer layer 14 thin and uniform.

[0052] Next, the molding die 20 is inverted, and the inorganic binder-coated sand 13 is blown into the model space 24 from the opening of the molding die 20 using a top blow method at a pressure of 2 kg / cm. ２ ~6 kg / cm ２ The mixture is then blown into the mold (FIG. 2(d)).

[0053] As the inorganic binder-coated sand 13 hardens, the inner layer 12 is formed on the outer layer 14 (FIG. 2(e)). The inorganic binder-coated sand 13 is preferably filled and hardened when the outer layer 14 is in a softened or semi-hardened state. This further promotes integration of the outer layer 14 and the inner layer 12.

[0054] Finally, the two-layer mold 10 can be released from the molding die 20 by horizontally moving the mold portions of the molding die 20 away from each other (FIG. 2(f)).

[0055] After the inner layer 12 is formed, the uncured inorganic binder-coated sand 13 may be discharged from the molding die 20, for example, by inverting the molding die 20. This allows the formation of a hollow two-layer mold that is excellent in disintegration properties and in which the generation of gas defects is particularly suppressed.

[0056] The combination of the under-blow method (outer layer 14) and the top-blow method (inner layer 12) described in Figure 2 is suitable for producing cores with simple shapes, and is advantageous in terms of thinning the outer layer and the molding cycle. Other production methods that can be used include forming the outer layer 14 by the under-blow method and the inner layer 12 by the under-blow method; forming the outer layer 14 by the top-blow method and the inner layer 12 by the under-blow method; and forming the outer layer 14 by the top-blow method and the inner layer 12 by the top-blow method. When producing cores with complex shapes, it is preferable to form the outer layer 14 by the top-blow method.

[0057] In addition to the above-described embodiment, for example, a dump-type main mold making machine equipped with two dump boxes can be used to form an outer layer and then form an inner layer on top of the outer layer, thereby manufacturing a two-layer mold that serves as the main mold.

[0058] Although the embodiments of the present invention have been described above, the present invention is not limited thereto. The technical scope of the present invention is defined only by the claims. The present invention can be modified, for example, by adding, deleting, or substituting constituent elements of the present invention, provided that the modifications do not depart from the spirit of the present invention. For example, the materials used, various conditions, etc. can be modified as appropriate within the scope of the present invention.

[0059] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0060] The following inorganic binder-coated sand was used: A. Refractory aggregate: fused artificial sand #40 (Espearl #40, manufactured by Yamakawa Sangyo Co., Ltd., particle size index 37), inorganic binder: sodium metasilicate nonahydrate (sodium metasilicate nonahydrate, manufactured by Sanbo Chemical Industry Co., Ltd.) 2% by mass, particle size index 36. B. Refractory aggregate: fused artificial sand #100 (Espearl #100, manufactured by Yamakawa Sangyo Co., Ltd., particle size index 108), inorganic binder: sodium metasilicate nonahydrate (sodium metasilicate nonahydrate, manufactured by Sanbo Chemical Industry Co., Ltd.) 3% by mass, particle size index 107.

[0061] The following resin binder-coated sand was used: C. Refractory aggregate: fused artificial sand #100 (Espearl #100, manufactured by Yamakawa Sangyo Co., Ltd., particle size index 108), resin binder: phenolic resin (SP1007, manufactured by Asahi Organic Chemicals Co., Ltd.) 2% by mass, hardener: hexamethylenetetramine (manufactured by Mitsubishi Gas Chemical Company, Inc.) 0.30% by mass, particle size index 106; D. Refractory aggregate: silica sand #40 (Fremantle Sand, manufactured by Mitsubishi Corporation Building Materials Co., Ltd., particle size index 38), resin binder: phenolic resin (SP6905U, manufactured by Asahi Organic Chemicals Co., Ltd.) 3% by mass, hardener: hexamethylenetetramine (manufactured by Mitsubishi Gas Chemical Company, Inc.) 0.45% by mass, particle size index 36

[0062] [Casting test] The cylindrical mold core was set in a main mold (hollow portion Φ80 × 80H) made by the alkali resol ester hardening process, and molten aluminum alloy (700±10°C) was poured into it. After cooling to room temperature, the main mold was disassembled and the casting and core were taken out.

[0063] [Crumble] The core was removed from the casting using an air hammer, and the condition of the core was evaluated by touching it with the hand. A: The core removed after casting crumbled easily with the hand. B: The core removed after casting crumbled with the hand, but small lumps remained. C: The surface of the core removed after casting crumbled, but a large lump remained in the center. D: The core removed after casting was resistant to crumble on both the surface and the center, and a large lump remained.

[0064] [Scratch Hardness] The molded two-layer or cylindrical mold core was held in an environment of 23°C and 60% RH for one hour after molding. The scratch hardness of the surface of the two-layer or cylindrical mold core was then measured using a scratch hardness tester (GF type), and the average of three measurements was recorded as the scratch hardness (mm). Specifically, the scratch hardness was measured by pressing the tip of the scratch hardness tester against the surface of the two-layer or cylindrical mold core and rotating the black lever on the top clockwise once and then counterclockwise once, five times. The depth to which the tip of the scratch hardness tester had penetrated into the surface of the two-layer or cylindrical mold core was then measured by reading the scale (mm) on the side of the scratch hardness tester. A smaller measured value indicates a higher scratch hardness.

[0065] [Cast surface] After the core was removed, the surface condition of the core side of the casting was evaluated visually and by touch. Ratings A and B were considered acceptable. A: No irregularities, and no resistance when touched with the hand. B: Slight resistance when touched with the hand. C: Some irregularities were observed. D: Roughness was observed over the entire surface.

[0066] [Gas Generation Amount] The molded two-layer or cylindrical mold core was cut in half, and the entire cross section, including the inner and outer layers, was uniformly filed to obtain a 1-g sample. The sample was placed in a cylindrical copper sample tube (approximately 0.7 cm diameter x 7.7 cm) and covered with a commercially available insulating material, Kaowool. The furnace temperature of a PGD-type gas pressure measuring instrument (manufactured by George Fischer) was then raised to 700°C, and the sample tube was placed at the end of the furnace and adjusted under a nitrogen atmosphere. The sample tube was then placed in a sealed furnace, and the pressure of the gas generated at the measurement temperature of 700°C was measured with a pressure sensor. Pressure data was collected using a signal converter until the pressure value became constant, i.e., gas generation ceased. The amount of gas generated per 1 g of sample (mL / g) was calculated from the obtained pressure data using a pressure-volume conversion calibration curve prepared using the decomposition of potassium bicarbonate.

[0067] [Gas defects] After the core was removed, the casting was cut and visually evaluated for gas defects. Ratings A and B were considered acceptable. A: No gas defects were observed on the surface or cut surface. B: Gas defects (pinholes) were observed on part of the contact surface of the mold. C: Gas defects (pinholes) were observed on the entire contact surface of the mold. D: Large gas defects such as blowholes were observed.

[0068] [Example 1] Resin binder-coated sand C was poured into a cylindrical casting mold heated to 250°C under a pressure of 3 kgf / cm ２ The mold was then inverted and vibrated for 5 seconds to discharge the unhardened sand, followed by 15 seconds of firing to form an outer layer. The thickness of the outer layer was in the range of 2.0 mm to 4.0 mm. The mold was then rotated forward and filled with inorganic binder-coated sand A using the top blow method, and then held for 60 seconds to form an inner layer. A cylindrical two-layer mold core with an integrated outer and inner layer was obtained. The aforementioned casting test was then carried out to evaluate the collapsibility, casting surface, and gas defects.

[0069] Example 2 A two-layer mold core was obtained in the same manner as in Example 1, except that the resin binder-coated sand C was replaced with inorganic binder-coated sand B and the molding die temperature was changed to 150°C. The thickness of the outer layer was in the range of 3.0 mm to 5.0 mm. A casting test was then conducted to evaluate the disintegration property, casting surface, and gas defects.

[0070] Comparative Example 1 A two-layer mold core was obtained in the same manner as in Example 1, except that the inorganic binder-coated sand A was changed to resin binder-coated sand D. The thickness of the outer layer was in the range of 2.0 mm to 4.0 mm. A casting test was then conducted to evaluate the disintegration property, casting surface, and gas defects.

[0071] Comparative Example 2 In a cylindrical casting mold heated to 250°C, inorganic binder-coated sand A was poured under a pressure of 3 kgf / cm. ２ The mixture was filled by a top blow method for 3 seconds, and then fired for 90 seconds to obtain a cylindrical mold core. Thereafter, the above-mentioned casting test was carried out to evaluate the collapsibility, casting surface, and gas defects.

[0072] Comparative Example 3 A cylindrical mold core was obtained in the same manner as in Comparative Example 2, except that inorganic binder-coated sand A was changed to inorganic binder-coated sand B. Thereafter, a casting test was carried out to evaluate the disintegration property, casting surface, and gas defects.

[0073] The configuration of the mold used and the evaluation results of the castings are shown in Table 1.

[0074]

[0075] By forming the inner layer from inorganic binder-coated sand containing metasilicate or its hydrate as the inorganic binder, a two-layer mold capable of producing castings with few gas defects can be obtained.

[0076] The two-layer mold of the present disclosure can be used as a master mold and a core in producing high quality castings, and can be particularly advantageously used as a core.

[0077] 10 Two-layer mold 12 Inner layer 13 Inner layer inorganic binder coated sand 14 Outer layer 15 Outer layer coated sand 20 Forming mold 22 Model surface 24 Model space

Claims

1. A two-layer mold comprising an inner layer containing inorganic binder-coated sand and an outer layer containing coated sand selected from the group consisting of resin binder-coated sand and inorganic binder-coated sand, wherein the inorganic binder-coated sand contains metasilicate or its hydrate as the inorganic binder.

2. The two-layer mold of claim 1, wherein said outer layer comprises resin binder coated sand.

3. The two-layer mold according to claim 1 or 2, wherein the resin binder-coated sand contains at least one resin binder selected from the group consisting of cross-linked curable resins and their cured products, thermoplastic resins, and carbohydrates.

4. The two-layer mold of claim 3, wherein said resin binder coated sand comprises a phenolic resin.

5. A two-layer mold according to claim 1 or 2, wherein the granularity index of the refractory aggregate contained in the coated sand of the outer layer is greater than the granularity index of the refractory aggregate contained in the inorganic binder-coated sand of the inner layer.

6. The two-layer mold according to claim 1 or 2, wherein the refractory aggregate contained in the covering sand of the outer layer has a grain size index of 80 or more.

7. The two-layer mold of claim 1 or 2, wherein the thickness of the outer layer is between 0.3 mm and 100 mm.

8. A method for manufacturing a two-layer mold, comprising: preparing a molding die having a model surface and a model space; filling the model space of the molding die with coated sand selected from the group consisting of resin binder-coated sand and inorganic binder-coated sand; hardening the coated sand to form an outer layer on the model surface; discharging the unhardened coated sand from the molding die; filling the model space of the molding die with inorganic binder-coated sand; hardening the inorganic binder-coated sand to form an inner layer on the outer layer; and, if necessary, discharging the unhardened inorganic binder-coated sand from the molding die, wherein the inorganic binder-coated sand contains metasilicate or a hydrate thereof as the inorganic binder.

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