Easily demoulded anti-cracking coated sand and preparation method thereof

Abstract

The present application belongs to the technical field of molding materials for casting, and particularly relates to an easy-to-demold anti-cracking coated sand and a preparation method thereof. The coated sand formula comprises silica sand, phenolic resin, urotropine, calcium stearate, chromite sand, zircon sand, anti-cracking agent, demolding agent and silane coupling agent. The anti-cracking agent is prepared by sol-gel and inert gas calcination of hydroxylated carbon nanotubes and tetraethyl orthosilicate after chelation of acetylacetone with lithium acetate dihydrate, magnesium acetate tetrahydrate and aluminum isopropoxide. The demolding agent is prepared by wet impregnation of polyvinylpyrrolidone and glucose, and staged carbonization under nitrogen after hydrothermal reaction of hexagonal boron nitride and molybdenum disulfide. The preparation of the coated sand comprises preheating the silica sand, adding the chromite sand, zircon sand, phenolic resin, calcium stearate, anti-cracking agent and demolding agent in sequence, spraying the silane coupling agent alcohol-water solution, adding the urotropine aqueous solution, cooling and drying with cold air, and screening. The coated sand has good anti-cracking performance, low demolding resistance and excellent surface quality of the castings.

Description

Technical Field

[0001] This invention belongs to the technical field of molding materials for casting, specifically relating to an easy-to-demold, crack-resistant coated sand and its preparation method. Background Technology

[0002] Coated sand refers to molding sand or core sand with a solid resin film coated on the surface of the sand grains before molding. It is used in the precision casting production of cast steel and cast iron parts and is hailed as one of the best molding materials in the automotive, tractor, and hydraulic component industries. Compared with traditional molding materials such as clay sand, water glass sand, and resin self-hardening sand, coated sand has a series of advantages, including excellent fluidity, good core surface quality, good collapsibility, low moisture absorption, and the ability to achieve sand mold additive manufacturing through selective laser sintering. Therefore, it occupies an irreplaceable position in the production of high-end precision castings. The coated sand process was invented by Dr. Kronen of Germany during World War II. Initially, powdered thermosetting phenolic resin was mechanically mixed with raw sand and cured upon heating. Later, it was developed into a coating process system using thermoplastic phenolic resin as a binder, hexamethylenetetramine as a latent curing agent, and supplemented with lubricants. When the coated sand is heated, the resin melts, and under the action of the methylene groups decomposed by hexamethylenetetramine, the linear structure is transformed into a non-melting three-dimensional structure, thereby achieving curing and molding. Since the 1980s, due to the rapid development of the automobile industry and the increasing demands on casting quality for machinery exports, coated sand technology has developed rapidly in my country. The amount of resin added has been greatly reduced, and new products and technologies have emerged continuously. The annual output has reached more than 900,000 tons and is widely used in the production of complex castings such as automobile cylinder blocks, cylinder heads, intake manifolds, and various pipe fittings and pump bodies.

[0003] However, traditional coated sand faces two major technical challenges during high-temperature casting. The first is the problem of sand mold cracking. The most commonly used aggregate in coated sand is silica sand, which undergoes a crystal transformation from α-quartz to β-quartz when heated to about 573 degrees Celsius, accompanied by a volume expansion of about 2.4%. At the same time, there is a significant temperature gradient between the surface and interior of the sand mold during casting, resulting in uneven thermal stress. When the tensile stress on the surface of the sand mold exceeds its high-temperature strength, cracking occurs, and molten metal seeps into the cracks, forming vein defects. These are prominent defects on the surface of the casting, appearing as irregular slits, stripes, or networks, often occurring at the inner corners and hot spots of the casting, seriously affecting the casting quality and yield. The industry has tried various solutions to address this problem, including adjusting the performance indicators of silica sand, selecting special sands with low expansion rates such as zircon sand and chromite sand, adding anti-veining additives, and using resins with better thermoplasticity. However, all of these solutions have limitations to varying degrees. For example, zircon sand is scarce and expensive, and chromite sand has too high a density and sharp particle shape, which consumes a lot of resin binder and easily causes the sand core to be poorly filled. Existing anti-veining additives mostly use simple physical compounding of organic and inorganic materials, which has a single function and cannot simultaneously meet the dual requirements of stress relief and structural reinforcement at high temperatures.

[0004] The second problem is the difficulty in demolding. After the coated sand solidifies in the mold, the sand mold or core often adheres to the mold to varying degrees, resulting in high demolding resistance, mold damage, and even casting scrap. The main reasons for demolding difficulties include insufficient mold draft angle, improper control of curing temperature, and poor performance of the release agent. Existing coated sand formulations usually add calcium stearate as a lubricant, but its lubrication effect is limited under high temperature conditions, making it difficult to meet the demolding performance requirements of complex castings. Commonly used coated sand release agents on the market mostly use graphite or organic oil-based materials, but these materials are prone to failure and carbonization at high temperatures, forming carbon deposits, which not only affect the demolding effect but also contaminate the mold and shorten its life. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide an easy-to-demold, crack-resistant coated sand and its preparation method.

[0006] In a first aspect, the present invention provides a method for preparing easily demolded, crack-resistant coated sand, comprising the following steps: S1. By weight, place 90-110 parts of silica sand in a sand mixer and preheat it at 180-220℃ to obtain preheated silica sand; adjust the temperature of the preheated silica sand to 140-160℃, add 2-8 parts of chromite sand and 1-5 parts of zircon sand, and stir to mix; add 1.5-3.5 parts of phenolic resin and 0.05-0.15 parts of calcium stearate, and continue stirring to mix; add 0.5-2.5 parts of lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent and 0.3-1.5 parts of layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent, and stir; add a mixture containing 0.1-0.4 parts of γ-aminopropyltriethoxysilane and 0.5-2.0 parts of ethanol and water, and continue stirring; S2. Add a solution containing 0.2-0.6 parts hexamethylenetetramine and 0.46-1.40 parts deionized water, and stir; cool to 60-80℃ and continue stirring, dry, and vibrate to sieve.

[0007] In this invention, the overall mixing mechanism of easily demolded and crack-resistant coated sand is a complex reconstruction process integrating aggregate thermodynamic activation, polymer melt rheology, and multiphase interface chemical coupling. Initially, silica sand undergoes high-temperature preheating treatment in a forced mixing device. This process completely evaporates the physically adsorbed moisture on the surface of the silica sand and stimulates the chemical reactivity of the particle surface. After adding chromite sand and zircon sand, these refractory micropowders are tightly filled into the micropores of the silica sand under high-speed mechanical shearing, significantly improving the thermal conductivity of the skeleton and the overall refractoriness. After adding phenolic resin and calcium stearate, the phenolic resin rapidly melts into a low-viscosity fluid under the thermal conduction of the high-temperature silica sand, fully wetting the silica sand, chromite sand, and zircon sand and spreading into a continuous and dense liquid film on their surface. Calcium stearate plays a synergistic role in lubrication and assisting demolding. Following this, a pre-prepared lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent and a layered boron nitride-molybdenum disulfide composite high-temperature lubricant and release agent were introduced, uniformly adhering to the surface of the liquid phenolic resin film and partially embedding into the interior of the resin film under strong shearing. To address the extremely poor compatibility between inorganic mineral powders such as lithium magnesium aluminum silicate and organic phenolic resin, a mixture containing γ-aminopropyltriethoxysilane, ethanol, and water was introduced into the process. This γ-aminopropyltriethoxysilane underwent moderate hydrolysis in a strictly controlled mixture of ethanol and water, generating sufficient highly active silanol groups while inhibiting ineffective intermolecular self-condensation. These highly active silanol groups dehydrated and condensed with the hydroxyl groups on the surfaces of silica sand, chromite sand, zircon sand, and the anti-cracking agent to form covalent bonds, while the amino groups at the other end chemically crosslinked with the macromolecular chains of the liquid phenolic resin. In the final stage, after the solution containing hexamethylenetetramine and deionized water is atomized and sprayed, cold air must be immediately forced in for rapid cooling. The cold air not only removes a large amount of sensible heat but also utilizes the enormous latent heat from the instantaneous evaporation of deionized water to rapidly lower the system temperature below the critical temperature at which irreversible cross-linking of the phenolic resin and hexamethylenetetramine occurs. This highly efficient transient forced heat transfer and rapid heat removal temperature control mechanism prevents premature curing of the phenolic resin, keeping the easily demolded and crack-resistant coated sand particles loose and highly fluid. Only in the subsequent heating and molding process will hexamethylenetetramine induce complete three-dimensional network cross-linking and curing of the phenolic resin, resulting in ideal high-strength molding performance.

[0008] According to a preferred embodiment of the present invention, in step S1, the preheating treatment at 180-220°C takes 15-20 minutes.

[0009] According to a preferred embodiment of the present invention, in step S2, the time for cooling to 60-80°C and continuing to stir is 5-10 minutes.

[0010] According to a preferred embodiment of the present invention, the preparation method of the lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent includes: A1, dissolving 6-10 parts by weight of lithium acetate dihydrate and 12-18 parts by weight of magnesium acetate tetrahydrate in 30-50 parts by weight of deionized water, stirring in a water bath at 38-42°C to obtain a metal salt solution; dissolving 15-25 parts by weight of aluminum isopropoxide in 70-90 parts by weight of anhydrous ethanol, first adding 3-6 parts by weight of acetylacetone, stirring and reacting at room temperature, then adding 15-25 parts by weight of tetraethyl orthosilicate and 0.8-1.5 parts by weight of glacial acetic acid, stirring, adjusting the pH to 4-5, to obtain... A. Aluminum silicate sol precursor: Add metal salt solution dropwise to aluminum silicate sol precursor, and continue stirring after the addition is complete; Add a mixture containing 1.5-3.0 parts of hydroxylated multi-walled carbon nanotubes and 35-50 parts of anhydrous ethanol, disperse by ultrasonication, and reflux at 84-86℃ to obtain gel; A2. Dry the gel at 118-122℃ to obtain dry gel; Place the dry gel in a tube furnace, calcine at 545-555℃ under a nitrogen or argon protective atmosphere, then calcine at 895-905℃, and cool to room temperature with the furnace to obtain calcined product; Grind the calcined product and sieve it.

[0011] In this invention, the synthesis mechanism of the lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent is based on a sterically hindered sol-gel process and in-situ grain boundary anchoring effect. Since highly reactive aluminum isopropoxide readily undergoes uncontrolled hydrolysis and precipitation upon contact with water, the preparation process first utilizes the bidentate structure of acetylacetone to replace some of the ligands in aluminum isopropoxide, forming a stable sterically hindered structure, thereby significantly reducing the hydrolysis rate of aluminum isopropoxide. Subsequently, glacial acetic acid is added to adjust the pH. In this controlled system, the modified aluminum isopropoxide undergoes a stable co-hydrolysis and condensation reaction with tetraethyl orthosilicate, and is mixed with lithium acetate dihydrate and magnesium acetate tetrahydrate dissolved in deionized water to construct a uniform three-dimensional aluminum silicate inorganic network, encapsulating lithium and magnesium ions within it. Simultaneously, multi-walled carbon nanotubes (MWCs) with pre-treated surfaces through hydroxylation via mixed acid oxidation are added to the system in anhydrous ethanol dispersion. In a refluxed liquid environment, the active hydroxyl groups on the surface of the carbon nanotubes undergo dehydration condensation with the silanol groups in the inorganic gel network, forming strong covalent bonds that anchor the hydroxylated MWCs in situ at the molecular level. The reflux mechanism effectively ensures that low-boiling-point solvents such as anhydrous ethanol do not evaporate and dry out, maintaining a homogeneous reaction environment. The dried gel is then subjected to stepwise high-temperature calcination under an absolutely oxygen-free nitrogen or argon atmosphere. This prevents the hydroxylated MWCs from being oxidized and burned at high temperatures and promotes dehydration and structural rearrangement of the inorganic gel, ultimately crystallizing into a lithium magnesium aluminum silicate phase with negative thermal expansion characteristics. During this phase transition, the covalently anchored hydroxylated MWCs are well-distributed at the grain boundaries of the lithium magnesium aluminum silicate, constructing a rigid-flexible three-dimensional hybrid network. When the easily demolded and crack-resistant coated sand is heated, the negative expansion characteristics of lithium magnesium aluminum silicate offset the volume thermal expansion of the silica sand, while hydroxylated multi-walled carbon nanotubes absorb and dissipate heat dissipation stress in large quantities through fiber bridging and pull-out mechanisms, giving the easily demolded and crack-resistant coated sand extremely excellent thermal shock stability and crack resistance.

[0012] According to a preferred embodiment of the present invention, in step A1, the reflux reaction time at 84-86°C is 4-6 hours.

[0013] According to a preferred embodiment of the present invention, in step A2, the calcination time at 895-905°C is 2-4 hours.

[0014] According to a preferred embodiment of the present invention, the preparation method of the layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent includes: B1, mixing 6-10 parts by weight of hexagonal boron nitride and 3-6 parts by weight of molybdenum disulfide, adding 150-250 parts by weight of deionized water, and stirring; transferring to a reaction vessel, adding 0.1-0.2 parts by weight of sodium dodecyl sulfate, and hydrothermally reacting at 178-182°C, followed by natural cooling to obtain a mixture; centrifuging the mixture to obtain a precipitate; washing the precipitate sequentially with deionized water and anhydrous ethanol to obtain a washed precipitate; and then... The precipitate was dried at 78-82℃ to obtain the dried product; the dried product was redispersed in 30-50 parts of deionized water, mixed with 0.3-0.6 parts of polyvinylpyrrolidone and 1.0-2.0 parts of anhydrous glucose, stirred, and continuously stirred at 80-90℃ until it was evaporated to dryness to obtain the coated mixture; B2, the coated mixture was placed in a tube furnace, heated to 200-250℃ under a nitrogen protective atmosphere and held, then heated to 500-550℃ under a nitrogen protective atmosphere and held, and then naturally cooled to room temperature under a nitrogen atmosphere, ground and sieved.

[0015] In this invention, the preparation reaction mechanism of the layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent encompasses three dimensions of physicochemical transformation: hydrothermal intercalation and exfoliation, supramolecular self-assembly, and controlled pyrolysis carbonization. Both hexagonal boron nitride and molybdenum disulfide are two-dimensional crystalline materials whose interlayers are bound by weak van der Waals forces. In a high-temperature, high-pressure, deionized water hydrothermal closed environment, sodium dodecyl sulfate molecules, driven by a strong thermodynamic force, spontaneously intercalate into the interlayer space between the hexagonal boron nitride and molybdenum disulfide crystals, opening them up and exfoliating them into ultrathin two-dimensional nanosheets with extremely high specific surface area and surface activity. In the subsequent wet impregnation stage, the product, after washing and drying and redispersing in deionized water, is mixed with polyvinylpyrrolidone and anhydrous glucose. During the dynamic process of continuous stirring and slow evaporation of deionized water, as the concentrations of polyvinylpyrrolidone (PVP) and anhydrous glucose in the system gradually increase, PPVP and anhydrous glucose, under the combined induction of capillary contraction and intermolecular hydrogen bonds, uniformly and densely precipitate and coat the surface of hexagonal boron nitride and molybdenum disulfide nanosheets, constructing a uniformly structured coated mixture precursor. In the tube furnace heat treatment stage, strictly controlling the extremely slow programmed heating rate is the core mechanism determining success or failure. PPVP and anhydrous glucose release a large amount of volatile gases during pyrolysis; the extremely slow heating process allows the gaseous products to escape calmly, avoiding the bursting of the coating structure caused by the instantaneous and violent expansion of internal gas pressure. Under the strict isolation of a nitrogen protective atmosphere, the outer layers of PPVP and anhydrous glucose eventually dehydrate and carbonize, forming an amorphous, high-temperature resistant carbon film tightly attached to the core surface of hexagonal boron nitride and molybdenum disulfide. This layered boron nitride and molybdenum disulfide composite high-temperature lubricant release agent exhibits a triple synergistic lubrication mechanism: molybdenum disulfide provides initial interlayer slip at lower temperatures, the amorphous carbon film plays an isolating and synergistic lubricating role in the medium to high temperature range, and the extremely high-temperature resistant hexagonal boron nitride acts as the ultimate robust slip skeleton at the extreme high temperatures of metal casting, ensuring perfect demolding of metal castings.

[0016] According to a preferred embodiment of the present invention, in step B1, the hydrothermal reaction time at 178-182°C is 24-30 hours.

[0017] According to a preferred embodiment of the present invention, in step B2, the time for holding the temperature at 500-550°C is 1-2 hours.

[0018] In a second aspect, the present invention provides an easy-to-release, crack-resistant coated sand prepared according to the preparation method of the easy-to-release, crack-resistant coated sand.

[0019] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention addresses the technical challenge of high-temperature cracking in coated sand by introducing a lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent, synergistically solving the problem from two dimensions: stress relief and structural enhancement. The lithium magnesium aluminum silicate crystal phase in this anti-cracking agent exhibits negative thermal expansion characteristics, generating a shrinkage effect at high temperatures, effectively offsetting the volume expansion caused by the phase transformation of silica sand and alleviating the accumulation of thermal stress inside the sand mold. Simultaneously, carbon nanotubes dissipate fracture energy through fiber bridging and crack deflection effects, delaying crack propagation. Compared with the simple physical mixing of anti-veining agents in the prior art, this invention uses a sol-gel co-condensation reaction to chemically bond carbon nanotubes to the silicate network, forming a three-dimensional hybrid network structure. In the formulation, chromite sand acts as a chilling material, accelerating local cooling and reducing temperature gradients. Zircon sand has a low coefficient of thermal expansion, working synergistically with the anti-cracking agent to effectively suppress vein defects during casting and improve the casting yield.

[0020] (2) This invention achieves a stable low-friction coefficient lubrication effect over a wide temperature range by designing a layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent, significantly reducing demolding resistance. In this release agent, molybdenum disulfide provides initial interlayer slip at lower temperatures, the amorphous high-temperature resistant carbon film formed by wet impregnation and segmented carbonization plays an isolating and synergistic lubricating role in the medium-to-high temperature range, and the extremely high-temperature resistant hexagonal boron nitride acts as the ultimate robust slip skeleton at the extreme high temperatures of metal casting. The temperature connection of the three-layer lubrication mechanism ensures that the release agent maintains a stable lubrication state across the entire temperature range. Compared with traditional coated sand, the coated sand of this invention provides smoother demolding, leaves the sand mold surface intact, and significantly extends the mold's service life.

[0021] (3) The coating sand preparation method of the present invention ensures the uniform dispersion of each component and the full play of the synergistic effect through strict process parameter control and sequential operation. In the preparation of the anti-cracking agent, the use of acetylacetone pre-chelation modification ensures the uniformity of the silica-alumina sol, and inert gas protection calcination avoids the oxidation of carbon nanotubes; in the preparation of the release agent, hydrothermal reaction promotes two-dimensional heterogeneous self-assembly, wet impregnation achieves uniform coating, and nitrogen segmented carbonization ensures the integrity of the molybdenum disulfide layered structure; in the mixing of the coating sand, rapid cooling with cold air avoids premature cross-linking of the resin, and the alcohol-water dilution of the silane coupling agent prevents boiling over and ensures hydrolysis activation. The coating sand of the present invention has excellent comprehensive performance and has extremely high industrial application value. Detailed Implementation

[0022] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention. Example

[0023] This embodiment provides a method for preparing easily demolded, crack-resistant coated sand, the steps of which include: S1. Place 100g of silica sand (AFS value 60) in a forced sand mixer and preheat it at 200℃ for 17.5min to obtain preheated silica sand; adjust the temperature of the preheated silica sand to 150℃, add 5g of chromite sand and 3g of zircon sand, and stir at 350rpm for 2.5min; add 2.5g of thermoplastic phenolic resin and 0.1g of calcium stearate, and continue stirring at 350rpm for 1.5min; add 1.5g of lithium magnesium aluminum silicate-carbon nanotubes. The hybrid network anti-cracking agent and 0.9g of layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent were stirred at 450rpm for 2.5min. 0.25g of γ-aminopropyltriethoxysilane (KH-550) and 1.25g of alcohol-water mixed diluent (in which the volume ratio of anhydrous ethanol to deionized water was 9:1) were mixed and stirred at room temperature for 1min to pre-hydrolyze it. Then, it was added to the sand mixer by spraying and stirred at 450rpm for 1min.

[0024] S2. Dissolve 0.4g of hexamethylenetetramine in 0.93g of deionized water to prepare a 30% aqueous solution. Add the solution to the sand mixer by spraying and keep stirring at 400rpm for 2.5min. Then, introduce cold air to cool down to 70℃ at a rate of 15℃ / min and continue to stir and dry at 400rpm for 7.5min. When the material is loose and has no moisture, stop the machine and discharge the material. Classify the material through a double-layer vibrating screen of 40 mesh and 140 mesh to obtain the finished product of easy-to-demold and crack-resistant coated sand.

[0025] Preparation of lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent: A1. Dissolve 8g of lithium acetate dihydrate and 15g of magnesium acetate tetrahydrate in 40g of deionized water. Stir magnetically at 300rpm for 30min in a 40℃ water bath to obtain a clear metal salt solution. In another beaker, dissolve 20g of aluminum isopropoxide in 80g of anhydrous ethanol, add 4.5g of acetylacetone, and stir magnetically at 400rpm for 30min at room temperature to completely chelate the aluminum isopropoxide into a stable aluminum-acetylacetone complex. Then add 20g of tetraethyl orthosilicate and 1.15g of glacial acetic acid to the chelated solution, and continue stirring for 20min. Measure and adjust the pH to 4.5 using a precision pH meter to obtain a clear aluminosilicate sol precursor. Slowly add the above metal salt solution dropwise to the aluminosilicate sol precursor at a rate of 2mL / min, while maintaining vigorous stirring (500rpm) during the addition. After the addition is complete, continue stirring for 1h. Separately, 2.25 g of hydroxylated multi-walled carbon nanotubes were dispersed in 42.5 g of anhydrous ethanol and ultrasonically dispersed at 200 W power and 40 kHz frequency for 30 min to obtain a uniform carbon nanotube dispersion. This dispersion was slowly added to the above mixed sol and ultrasonically dispersed for another 20 min. Then, it was transferred to a round-bottom flask and refluxed in an oil bath at 85 °C with stirring (400 rpm) for 5 h to obtain a gel.

[0026] A2. The obtained gel was transferred to an evaporating dish and dried in a 120℃ forced-air drying oven for 12 hours to obtain a dry gel. The dry gel was placed in a tube furnace, and high-purity nitrogen gas (purity ≥99.999%) was introduced at a flow rate of 0.5 L / min. The temperature was increased from room temperature to 550℃ at a rate of 2℃ / min, and calcined at 550℃ for 3 hours. Then, the temperature was increased to 900℃ at a rate of 2℃ / min, and calcined at 900℃ for 3 hours. After calcination, the furnace was allowed to cool naturally to room temperature in a nitrogen atmosphere to obtain a black calcined product. The calcined product was placed in a planetary ball mill and lightly ground at 250 rpm for 45 minutes using zirconia balls (ball-to-material ratio 5:1). The product was then passed through a 200-mesh standard sieve to obtain a lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent.

[0027] Preparation of layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent: B1. Mix 8g of hexagonal boron nitride and 4.5g of molybdenum disulfide, add 200g of deionized water, and magnetically stir at 300rpm for 15min to form a homogeneous suspension. Transfer the suspension to a 250mL high-pressure hydrothermal reactor lined with polytetrafluoroethylene, add 0.15g of sodium dodecyl sulfate, seal the reactor, and place it in an oven for hydrothermal reaction at 180℃ for 27h. After the reaction, allow it to cool naturally to room temperature. Open the reactor, transfer the product to a centrifuge tube, and centrifuge at 8000rpm for 10min. Discard the supernatant, and wash the precipitate three times with deionized water (50mL each time) and twice with anhydrous ethanol (30mL each time), centrifuging at 8000rpm for 10min after each wash. Transfer the washed precipitate to a vacuum drying oven and dry at 80℃ and a vacuum of -0.09MPa for 12h to obtain the dried product. The dried product was redispersed in 40g of deionized water, and 0.45g of polyvinylpyrrolidone and 1.5g of anhydrous glucose were added. The mixture was ultrasonically stirred at 200W power and 40kHz frequency for 30min until completely dissolved. Then it was transferred to a round-bottom flask and stirred continuously at 300rpm in an 85℃ water bath to evaporate the water (about 3h) to obtain a coated mixture with a uniform organic precursor coating.

[0028] B2. The coating mixture was placed in a ceramic boat and then placed in a tube furnace. High-purity nitrogen was introduced at a flow rate of 0.5 L / min, and the temperature was raised to 225°C at a rate of 3°C / min. The mixture was held at 225°C for 1.5 h for dehydration pretreatment. Subsequently, the temperature was raised to 525°C at a rate of 5°C / min and held at 525°C for 1.5 h for carbonization treatment. After treatment, the mixture was allowed to cool naturally to room temperature in a nitrogen atmosphere. The carbonized product was lightly ground using a planetary ball mill (200 rpm, 30 min) and passed through a 300-mesh standard sieve to obtain a layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent. Example

[0029] The difference between this embodiment and Embodiment 1 is that this embodiment provides a method for preparing easily demolded, crack-resistant coated sand, the steps of which include: S1. Place 90g of silica sand in a sand mixer and preheat it at 180℃ for 15min to obtain preheated silica sand. Adjust the temperature of the preheated silica sand to 140℃, add 2g of chromite sand and 1g of zircon sand, and stir at 300rpm for 2min. Add 1.5g of phenolic resin and 0.05g of calcium stearate, and continue stirring for 1min. Add 0.5g of lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent and 0.3g of layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent, and stir at 400rpm for 2min. Add a mixture containing 0.1g of γ-aminopropyltriethoxysilane and 0.5g of a mixture of ethanol and deionized water (ethanol:deionized water volume ratio 19:1), and continue stirring for 1min.

[0030] S2. Add a solution containing 0.2g hexamethylenetetramine and 0.46g deionized water, stir for 2 minutes; circulate cold air to cool to 60℃ and continue stirring for 5 minutes, dry, and vibrate to sieve.

[0031] Preparation of lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent: A1. Dissolve 6g of lithium acetate dihydrate and 12g of magnesium acetate tetrahydrate in 30g of deionized water and stir in a 38℃ water bath to obtain a metal salt solution; dissolve 15g of aluminum isopropoxide in 70g of anhydrous ethanol, add 3g of acetylacetone first, stir and react at room temperature for 20min, then add 15g of tetraethyl orthosilicate and 0.8g of glacial acetic acid, stir for 15min to adjust the pH to 4, to obtain a silica-alumina sol precursor; add the metal salt solution dropwise to the silica-alumina sol precursor, and continue stirring for 1h after the addition is complete; add a mixture containing 1.5g of hydroxylated multi-walled carbon nanotubes and 35g of anhydrous ethanol, sonicate for 30min (power 200W, frequency 40kHz), and reflux at 84℃ for 4h to obtain a gel.

[0032] A2. Dry the gel at 118℃ for 12 hours to obtain a dry gel; place the dry gel in a tube furnace and calcine it at 545℃ for 3 hours under an argon protective atmosphere by heating at 2℃ / min, then calcine it at 895℃ for 2 hours by heating at 2℃ / min, and then cool it to room temperature with the furnace to obtain the calcined product; lightly grind the calcined product using a planetary ball mill (200 rpm for 30 minutes) and pass it through a 200-mesh sieve.

[0033] Preparation of layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent: B1. Mix 6g of hexagonal boron nitride and 3g of molybdenum disulfide, add to 150g of deionized water, and stir for 15min. Transfer to a reaction vessel, add 0.1g of sodium dodecyl sulfate, and hydrothermally react at 178℃ for 24h. After natural cooling, obtain a mixture. Centrifuge the mixture (8000rpm, 10min) to obtain a precipitate. Wash the precipitate three times with deionized water and twice with anhydrous ethanol to obtain a washed precipitate. Dry the washed precipitate at 78℃ and a vacuum of -0.09MPa for 12h to obtain a dried product. Redisperse the dried product in 30g of deionized water, mix with 0.3g of polyvinylpyrrolidone and 1.0g of anhydrous glucose, sonicate until completely dissolved, and evaporate to dryness with continuous stirring at 80℃ to obtain a coated mixture.

[0034] B2. Place the coating mixture in a tube furnace and heat it to 200°C at 2°C / min under a nitrogen protective atmosphere and hold it for 1 hour. Then heat it to 500°C at 5°C / min under a nitrogen protective atmosphere and hold it for 1 hour. Allow it to cool naturally to room temperature under a nitrogen atmosphere, lightly grind it (200 rpm, 30 min), and pass it through a 300-mesh sieve. Example

[0035] The difference between this embodiment and Embodiment 1 is that this embodiment provides a method for preparing easily demolded, crack-resistant coated sand, the steps of which include: S1. Place 110g of silica sand in a sand mixer and preheat it at 220℃ for 20min to obtain preheated silica sand; adjust the temperature of the preheated silica sand to 160℃, add 8g of chromite sand and 5g of zircon sand, and stir at 400rpm for 3min; add 3.5g of phenolic resin and 0.15g of calcium stearate, and continue stirring for 2min; add 2.5g of lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent and 1.5g of layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent, and stir at 500rpm for 3min; add a mixture containing 0.4g of γ-aminopropyltriethoxysilane and 2.0g of ethanol and deionized water (ethanol:deionized water volume ratio 14:1), and continue stirring for 1min.

[0036] S2. Add a solution containing 0.6g hexamethylenetetramine and 1.40g deionized water, stir for 3 minutes; circulate cold air to cool to 80℃ and continue stirring for 10 minutes, dry, and vibrate to sieve.

[0037] Preparation of lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent: A1. Dissolve 10g lithium acetate dihydrate and 18g magnesium acetate tetrahydrate in 50g deionized water and stir in a 42℃ water bath to obtain a metal salt solution; dissolve 25g aluminum isopropoxide in 90g anhydrous ethanol, add 6g acetylacetone first, stir and react at room temperature for 40min, then add 25g tetraethyl orthosilicate and 1.5g glacial acetic acid, stir for 30min and adjust the pH to 5 to obtain a silica-alumina sol precursor; add the metal salt solution dropwise to the silica-alumina sol precursor, and continue stirring for 1h after the addition is complete; add a mixture containing 3.0g hydroxylated multi-walled carbon nanotubes and 50g anhydrous ethanol, sonicate for 30min (power 200W, frequency 40kHz), and reflux at 86℃ for 6h to obtain a gel.

[0038] A2. Dry the gel at 122℃ for 12 hours to obtain a dry gel; place the dry gel in a tube furnace and calcine it at 555℃ for 3 hours under a nitrogen protective atmosphere by heating at 2℃ / min, then calcine it at 905℃ for 4 hours by heating at 2℃ / min, and then cool it to room temperature with the furnace to obtain the calcined product; lightly grind the calcined product using a planetary ball mill (300 rpm, 60 min) and pass it through a 200-mesh sieve.

[0039] Preparation of layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent: B1. Mix 10g of hexagonal boron nitride and 6g of molybdenum disulfide, add to 250g of deionized water, and stir for 15min. Transfer to a reaction vessel, add 0.2g of sodium dodecyl sulfate, and hydrothermally react at 182℃ for 30h. After natural cooling, obtain a mixture. Centrifuge the mixture (8000rpm, 10min) to obtain a precipitate. Wash the precipitate three times with deionized water and twice with anhydrous ethanol to obtain a washed precipitate. Dry the washed precipitate at 82℃ and a vacuum of -0.09MPa for 12h to obtain a dried product. Redisperse the dried product in 50g of deionized water, mix with 0.6g of polyvinylpyrrolidone and 2.0g of anhydrous glucose, sonicate until completely dissolved, and then evaporate to dryness with continuous stirring at 90℃ to obtain a coated mixture.

[0040] B2. Place the coating mixture in a tube furnace and heat it to 250°C at 5°C / min under a nitrogen protective atmosphere and hold it for 2 hours. Then heat it to 550°C at 5°C / min under a nitrogen protective atmosphere and hold it for 2 hours. Allow it to cool naturally to room temperature under a nitrogen atmosphere, lightly grind it (200 rpm, 30 min), and pass it through a 300-mesh sieve.

[0041] Comparative Example 1 The difference between this comparative example and Example 1 is that the lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent is not added; the other components and steps are exactly the same.

[0042] Comparative Example 2 The difference between this comparative example and Example 1 is that the layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent is not added; the other components and steps are exactly the same.

[0043] Comparative Example 3 The difference between this comparative example and Example 1 is that the lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent and the layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent are not added at the same time, while the other components and steps are exactly the same.

[0044] The performance of the easy-to-demold, crack-resistant coated sand obtained in Examples 1-3 and Comparative Examples 1-3 was tested in accordance with national and industry standard testing specifications.

[0045] Cracking temperature test: The coated sand prepared in Examples 1-3 and Comparative Examples 1-3 was cured at 180℃ for 2 min to prepare standard test blocks with dimensions of 70mm×70mm×20mm. Five test blocks were prepared for each example and comparative example. The test blocks were placed in a high-temperature furnace and heated from room temperature to 1200℃ at a heating rate of 10℃ / min. During the heating and holding process, the surface of the test blocks was continuously observed using a high-definition camera, and the temperature at which the first crack appeared was recorded. The average value of the five test blocks was taken as the cracking temperature (unit: ℃).

[0046] Demolding resistance test: The coated sand prepared in Examples 1-3 and Comparative Examples 1-3 was filled into a standard metal mold. The mold cavity size was 50mm×50mm×20mm and the draft angle was 1°. After curing at 180℃ for 3 minutes, demolding was performed at a constant speed of 50mm / min. The maximum demolding resistance was recorded using a digital push-pull force gauge (range 0N~500N, accuracy ±1N). The average value (unit: N) was taken for 5 tests for each example and comparative example.

[0047] Vein grading test: The coated sand prepared in Examples 1-3 and Comparative Examples 1-3 were made into standard sand cores with a size of 200mm×50mm×30mm. Molten cast iron was poured into the cores at 1420℃. After pouring, the cores were allowed to cool naturally to room temperature. After demolding, the degree of vein defects on the surface of the casting was visually observed and graded into four levels: no veins, slight veins, obvious veins, and severe veins.

[0048] Sand Adhesion Area Test: The castings obtained from the vein rating test were used to calculate the proportion of the area of ​​sand adhering to the casting surface to the total surface area of ​​the casting using image analysis software (ImageJ). The average value (unit: %) of three castings cast in each example and comparative example was taken.

[0049] Surface roughness test: The casting obtained from the vein rating test is used to measure the arithmetic mean roughness Ra value of the casting surface using a surface roughness meter. The sampling length is 2.5 mm and the evaluation length is 12.5 mm. The average value is taken at 5 different locations for each casting (unit: μm).

[0050] The performance test data above are shown in Table 1.

[0051] Table 1 Performance Test Results Cracking temperature (°C) 1020 1050 1000 820 980 750 Demolding resistance (N) 85 78 92 88 156 160 Pulse rating No veins No veins slight veins Obvious veins slight veins Severe veins Sand adhesion area (%) 1.8 1.5 3.5 8.6 4.2 15.2 Surface roughness Ra (μm) 8.5 7.9 10.2 18.3 11.5 25.6 The test results in Table 1 clearly show that Examples 1-3 are significantly better than Comparative Examples 1-3 in key performance indicators such as cracking temperature, demolding resistance, vein rating, sand adhesion area, and surface roughness. This fully demonstrates that the present invention has successfully solved the technical problems of easy cracking and difficult demolding of existing coated sand at high temperatures by introducing a lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent and a layered boron nitride-molybdenum disulfide composite high-temperature lubricating demolding agent.

[0052] Specifically, Comparative Example 3 lacked both modifiers (lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent and layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent), its cracking temperature was only 750℃, its release resistance was as high as 160N, its vein pattern rating was severe veining, its sand adhesion area reached 15.2%, and its surface roughness Ra was 25.6μm, making it the worst performer. Comparative Example 1 lacked only the anti-cracking agent, its cracking temperature was only 820℃, its vein pattern rating was obvious veining, its sand adhesion area was 8.6%, and its roughness was 18.3μm. Although its release resistance was similar to that of Example 1 (88N vs 85N), due to the lack of stress relief and fiber bridging effect of the anti-cracking agent, the sand mold cracked significantly at high temperatures, resulting in severe veining and sand adhesion.

[0053] Comparative Example 2, lacking only the layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent, exhibited a release resistance as high as 156N, significantly higher than Examples 1-3 (78-92N). This indicates that without the addition of the layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent, the sand mold and the die were severely adhered to each other, making demolding difficult. However, its cracking temperature of 980℃ and its slight vein rating were slightly lower than those of the Examples, suggesting that the lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent played a role.

[0054] In contrast, Examples 1-3 contain two modifiers (lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent and layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent), and the cracking temperature of all of them reaches above 1000℃ (up to 1050℃ in Example 2), the release resistance of all of them is below 92N (only 78N in Example 2), the vein rating of all of them is no vein or slight vein, the sand adhesion area of ​​all of them is below 3.5%, and the surface roughness of all of them is below 10.2μm, showing excellent overall performance.

[0055] The above data show that the lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent and the layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent work synergistically: the anti-cracking agent counteracts the phase transformation expansion of silica sand through negative thermal expansion, and carbon nanotubes bridge and delay crack propagation, significantly improving the cracking temperature and anti-veining ability; the layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent reduces the friction coefficient in the entire temperature range and greatly reduces the release resistance through the three-layer lubrication mechanism of hexagonal boron nitride, molybdenum disulfide and carbon film.

[0056] Therefore, the present invention effectively solves the technical problems of high-temperature cracking and difficult demolding of existing coated sand, and achieves the dual goals of easy demolding and crack prevention.

Claims

1. A method for preparing easily demolded, crack-resistant coated sand, characterized in that the steps include... include: S1. By weight, place 90-110 parts of silica sand in a sand mixer and preheat it at 180-220℃ to obtain preheated silica sand; adjust the temperature of the preheated silica sand to 140-160℃, add 2-8 parts of chromite sand and 1-5 parts of zircon sand, and stir to mix; add 1.5-3.5 parts of phenolic resin and 0.05-0.15 parts of calcium stearate, and continue stirring to mix; add 0.5-2.5 parts of lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent and 0.3-1.5 parts of layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent, and stir; add a mixture containing 0.1-0.4 parts of γ-aminopropyltriethoxysilane and 0.5-2.0 parts of ethanol and water, and continue stirring; S2. Add a solution containing 0.2-0.6 parts hexamethylenetetramine and 0.46-1.40 parts deionized water, and stir; cool to 60-80℃ and continue stirring, dry, and vibrate to sieve.

2. The method for preparing the easy-to-demold, crack-resistant coated sand according to claim 1, characterized in that, In step S1, the preheating treatment at 180-220℃ takes 15-20 minutes.

3. The method for preparing the easy-to-demold, crack-resistant coated sand according to claim 1, characterized in that, In step S2, the temperature is lowered to 60-80℃ and stirring is continued for 5-10 minutes.

4. The method for preparing the easy-to-demold, crack-resistant coated sand according to claim 1, characterized in that, The preparation method of the lithium magnesium aluminum silicate-carbon nanotube hybrid network anti-cracking agent includes: A1, dissolving 6-10 parts by weight of lithium acetate dihydrate and 12-18 parts by weight of magnesium acetate tetrahydrate in 30-50 parts by weight of deionized water, stirring in a water bath at 38-42℃ to obtain a metal salt solution; dissolving 15-25 parts by weight of aluminum isopropoxide in 70-90 parts by weight of anhydrous ethanol, first adding 3-6 parts by weight of acetylacetone, stirring and reacting at room temperature, then adding 15-25 parts by weight of tetraethyl orthosilicate and 0.8-1.5 parts by weight of glacial acetic acid, stirring, and adjusting the pH to 4-5 to obtain a silica-alumina sol precursor. A1. Add the metal salt solution dropwise to the silica-alumina sol precursor. After the addition is complete, continue stirring. Add a mixture containing 1.5-3.0 parts of hydroxylated multi-walled carbon nanotubes and 35-50 parts of anhydrous ethanol, disperse by ultrasonication, and reflux at 84-86℃ to obtain a gel. A2. Dry the gel at 118-122℃ to obtain a dry gel. Place the dry gel in a tube furnace and calcine it at 545-555℃ under a nitrogen or argon protective atmosphere, then calcine it at 895-905℃, and cool it to room temperature with the furnace to obtain the calcined product. Grind the calcined product and sieve it.

5. The method for preparing the easy-to-demold, crack-resistant coated sand according to claim 4, characterized in that, In step A1, the reflux reaction is carried out at 84-86℃ for 4-6 hours.

6. The method for preparing the easy-to-demold, crack-resistant coated sand according to claim 4, characterized in that, In step A2, the calcination time at 895-905℃ is 2-4 hours.

7. The method for preparing the easy-to-demold, crack-resistant coated sand according to claim 1, characterized in that, The preparation method of the layered boron nitride-molybdenum disulfide composite high-temperature lubricating release agent includes: B1, mixing 6-10 parts by weight of hexagonal boron nitride and 3-6 parts by weight of molybdenum disulfide, adding 150-250 parts by weight of deionized water, and stirring; transferring to a reaction vessel, adding 0.1-0.2 parts by weight of sodium dodecyl sulfate, and hydrothermally reacting at 178-182℃, followed by natural cooling to obtain a mixture; centrifuging the mixture to obtain a precipitate; washing the precipitate sequentially with deionized water and anhydrous ethanol to obtain a washed precipitate; and then... Dry at 8-82℃ to obtain the dried product; redisperse the dried product in 30-50 parts of deionized water, mix with 0.3-0.6 parts of polyvinylpyrrolidone and 1.0-2.0 parts of anhydrous glucose, stir, and continue stirring at 80-90℃ until dry to obtain the coated mixture; B2, place the coated mixture in a tube furnace, heat to 200-250℃ under a nitrogen protective atmosphere and hold, then heat to 500-550℃ under a nitrogen protective atmosphere and hold, then cool naturally to room temperature under a nitrogen atmosphere, grind and sieve.

8. The method for preparing the easy-to-demold, crack-resistant coated sand according to claim 7, characterized in that, In step B1, the hydrothermal reaction time at 178-182℃ is 24-30 hours.

9. The method for preparing the easy-to-demold, crack-resistant coated sand according to claim 7, characterized in that, In step B2, the temperature is raised to 500-550℃ and held for 1-2 hours.

10. A type of easily demolded, crack-resistant coated sand, characterized in that, The easy-release, crack-resistant coated sand is prepared by the method described in any one of claims 1-9.