Powdered solidified inorganic self-hardening sand and core making method

Abstract

The application relates to a sand and core making method, in particular to a powder solidification inorganic self-hardening sand and core making method, and belongs to the casting technical field. A powder solidification inorganic self-hardening sand comprises the following components in percentage by weight: 0.8-2.0% of a powder solidification agent based on the weight of natural silica sand, 1.8-3.0% of an inorganic binder based on the weight of the natural silica sand and the rest of the natural silica sand. Compared with the prior art, the application has the following remarkable advantages: the powder self-hardening mode is realized through a modified water glass and a powder hardening agent, so that the sand mold reaches the strength, collapse, moisture resistance of the resin sand and the low gas emission, yielding property of the traditional water glass sand are comprehensively combined.

Description

Technical Field

[0001] The invention relates to sand and a core making method, in particular to powder solidified inorganic hard sand and a core making method, belonging to the technical field of casting. Background Art

[0002] The foundry industry is energy-intensive and highly polluting, particularly in terms of its environmental impact, particularly during the production process, due to its generation of solid waste and air pollution. In recent years, with increasing national environmental protection requirements, foundries have faced increasing production pressures. The government has established a growing number of new standards and tightened existing ones. Stringent requirements have been imposed on pollutant emissions, and scientific research to reduce these emissions has been actively promoted. The largest source of pollution during the metal smelting and casting process is the molding material. This pollution is generated by subsequent processing in the production workshop, including sand mixing, mold making, core fabrication, pouring, solidification and cooling, shakeout, cleaning, and polishing. The true solution to casting pollution is to eliminate the generation of harmful gases and solid waste at the source—the molding material itself.

[0003] Organic resin sand molding materials. Furan resin, phenolic resin, triethylamine and other organic resins are used in casting. They are the category of molding materials with the best comprehensive performance in modern times. They have significant advantages in five aspects, including sand mold strength, moisture absorption resistance, surface roughness, collapsibility and regeneration performance. Its shortcomings mainly lie in three aspects, which need further improvement: 1. High gas emission and maximum gas emission rate. When using resin sand to produce castings, intrusive pores are one of the most common defects. At present, the gas emission of coated sand has generally been reduced to about 12ml / g, but compared with the gas emission of new inorganic binders of less than 5ml / g, it still needs to be improved. In addition, the maximum gas emission rate of resin sand is also a difficult point that needs further improvement. Due to the poor air permeability of resin sand, the maximum gas emission rate is also one of the important factors that cannot be ignored compared with the gas emission. This may require starting from improving the flame retardant properties of the resin. 2. The mold sand has poor yieldability. It's generally accepted that resin sand is typically used to produce iron castings, but not steel castings. However, with increasing surface requirements for castings, an increasing number of steel castings have been produced in recent years using either directly or in combination with coated sand molds / cores. However, due to the long-lasting high temperatures and poor yield properties of resin sand, castings are prone to cracking. In the automotive industry, where coated sand is widely used for stainless steel castings, cracking remains one of the most significant and common problems. 3. Hazardous gas emissions. Incomplete combustion of resin easily produces various harmful gases and small organic volatiles, which is currently the most significant problem facing resin sand. Current measures include using aromatic resins such as wood-scented and bonnie resins, adding exhaust hoods during core making and pouring, and using spray neutralization. However, these measures are only temporary solutions and require significant equipment investment, energy consumption, and maintenance costs, adding to the burden on foundries and further complicating the already low-profit foundry industry.

[0004] Inorganic binder sand molding materials. Traditional inorganic binder sands mainly include silicate sand and phosphate sand. The binder for silicate sand is water glass, which includes sodium water glass, potassium water glass, and lithium water glass. These are aqueous solutions of alkali metal silicates, commercially known as soda ash. Sodium water glass is primarily used in casting. Water glass sand can be hardened using three methods: hot hardening, gas hardening, and self-hardening. The hot hardening method uses heating equipment to dehydrate and harden the sand mold. This method offers high strength and requires a water glass addition level of 3% to 4%. However, it is inefficient and energy-intensive. When the sand mold is heated, the surface solidifies first, making it difficult to remove the internal water vapor. Consequently, thick and large sand molds lack strength. The conventional CO2 gas hardening method uses water glass as a binder. Due to its simple equipment, easy operation, flexible use, and low cost, it has been widely used in most steel casting production both domestically and internationally. It offers rapid hardening and high strength. After hardening, demolding is performed, resulting in high casting precision. However, the molding sand has low strength and requires the addition of as much as 7% to 8% or more water glass. Its high water content makes it susceptible to moisture, resulting in poor hardness and permeability in winter. Its poor disintegration makes regeneration of used sand difficult, and the waste gas from large amounts of used sand contributes to alkaline environmental pollution. The self-hardening method with a powder hardener utilizes small, water-absorbing powders with large surface areas to dehydrate the water glass and increase its viscosity, thereby generating adhesion. This type of self-hardening powder sand is simple to operate and low-cost, producing high-strength sand molds in a very short time. However, the large amount of powder hardener added results in significant binder loss, making it impossible to reduce the binder dosage, typically to 6% to 7% or more. This, in turn, increases dust pollution, limiting its application.

[0005] The organic ester self-hardening method uses organic esters instead of CO2 gas as a hardener for water glass. The resulting molding sand exhibits high strength, allowing the addition of water glass to be reduced to below 3.5%. It also exhibits excellent hardness and permeability in winter, with an adjustable hardening rate. It also exhibits excellent collapsibility, and allows for dry sand regeneration, with a recycling rate of ≥70%, reducing environmental pollution from alkaline water glass waste sand. The molding sand exhibits excellent thermoplasticity and low gassing, which can overcome defects such as cracks and pores that can occur in furan resin sand-based steel castings. It also overcomes process challenges associated with CO2 water glass sand, such as poor mold surface stability and overblowing. However, the sand hardens slowly and exhibits poor fluidity.

[0006] In summary, inorganic sands with water glass as the primary binder offer numerous advantages and are widely used in the foundry industry. However, they suffer from three prominent issues: 1. Low strength. Whether carbon dioxide-hardened or ester-hardened, water glass sand is significantly weaker than resin sand. Furthermore, its surface readily absorbs moisture, causing it to lose strength. This results in pinholes and surface adhesion in castings. Therefore, water glass sand cannot meet the surface quality requirements for high-end castings. 2. Poor moisture absorption resistance. Phosphate-based inorganic binders exhibit some characteristics of organic resins, particularly good collapsibility and resistance to high temperatures, thermal shock, and corrosion. Silicate binders are slightly better than phosphates in terms of hygroscopicity, but they also have unresolved issues. During storage of molds or sand cores, they absorb moisture from the air, reducing their strength, especially during periods of high humidity. 3. Poor collapsibility. Water glass sand undergoes secondary sintering and hardening at temperatures around 800°C. As a result, its residual strength after casting is often much higher than that of the original sand, resulting in poor collapsibility. Controlling the impurity content, increasing the purity of water glass, and reducing the amount of water glass added are possible ways to improve its disintegration properties.

[0007] Purpose of the Invention

[0008] The purpose of the present invention is to propose a powder-cured inorganic self-hardening sand and a core making method in response to the shortcomings of the above-mentioned prior art, so as to solve the problems of large gas emission, poor yielding property and harmful gas emission of the existing resin sand and low strength, poor moisture absorption resistance and poor disintegration property of traditional water glass sand.

[0009] The present invention first proposes a powder-cured inorganic self-hardening sand, which includes the following components in weight percentage: a powder curing agent accounting for 0.8% to 2.0% of the weight of natural silica sand, an inorganic binder accounting for 1.8% to 3.0% of the weight of natural silica sand, and the balance being natural silica sand.

[0010] The powder curing agent in the above ratio includes the following components in percentage by weight:

[0011] 10% to 23% aluminum powder,

[0012] 70% to 80% amorphous silica powder,

[0013] 5% to 10% aluminum carbide powder,

[0014] 0.3% to 1.5% graphite powder,

[0015] 0.5% to 1.5% talcum powder.

[0016] The particle size of the three main powders involved in the reaction—aluminum powder, amorphous silica powder, and aluminum carbide powder—should be controlled between 15 and 75 μm. The advantage is that when the powder particle size is less than 15 μm, the powder size is small, the specific surface area ratio is large, and the reaction is violent, which is not conducive to stable control of the curing reaction process. When the powder particle size is greater than 75 μm, the powder reaction speed is slow, which will reduce the curing speed and thus affect production efficiency.

[0017] The inorganic binder in the above ratio is a silicate binder. Furthermore, the weight percentage composition of the silicate binder is:

[0018] Sodium silicate 30%~40%

[0019] Potassium hydroxide 5%~10%

[0020] Disodium hydrogen phosphate 1% to 3%

[0021] Alumina 0.1%~1%

[0022] Iron oxide 0.1%~1%

[0023] Magnesium oxide 0.1%~1%

[0024] Zinc oxide 0.1%~1%

[0025] Sodium hydroxide 5%~10%

[0026] The rest is water.

[0027] Water glass sand is one of the three most widely used sand molds in foundry production. Hardening methods include CO2 hardening method, self-hardening method and heat hardening. The three hardening methods all involve dehydrating the water glass binder to form a gel and solidify it. The inventors have found that improving the water glass modulus within a certain range can improve its strength, but its fluidity can be reduced. A small amount of phosphate system is used to increase the strength to more than 2Mpa, thus reaching the strength of resin sand. Disodium hydrogen phosphate is added to the water glass as a reinforcing agent to modify it. Resin binders can burn and volatilize at high temperatures, so they have good collapsibility, while water glass can melt at high temperatures. This is the main reason for its poor collapsibility. Research has shown that adding a soluble medium to the water glass prevents it from melting at high temperatures. Block glass crystals are formed after cooling. High-melting-point oxides such as aluminum oxide, iron oxide, zinc oxide and magnesium oxide destroy the glass crystals formed by melting the water glass at high temperatures. Therefore, aluminum oxide, iron oxide, zinc oxide and magnesium oxide are added to the water glass as a collapsing agent to modify it. In order to further enhance its disintegration properties, potassium hydroxide is added. After high-temperature sintering, water glass will form foamy potassium feldspar, which will destroy the high-temperature melting to form glass crystals and improve the disintegration properties.

[0028] The research is to use the powder self-hardening method to combine the advantages of CO2 hardening method and heat hardening for curing. Traditionally, some powders that can absorb water, have small particles and large surface areas are used to increase the viscosity of water glass by dehydration, thereby generating bonding force. Such as dicalcium silicate, ferrosilicon powder, red mud, chromium slag, sodium fluorosilicate, etc. The powder curing agent of the present invention adopts sodium hydroxide added to water glass as a generating agent, and uses aluminum powder to release heat and thus thermally cure. Aluminum carbide and water glass are dehydrated to accelerate curing and can be used as a disintegrating agent to increase its disintegrating properties. Amorphous silicon dioxide is added to be soluble in water glass to increase the modulus and strength, and a dense hardened film is formed on the periphery of the water glass gel to increase its anti-hygroscopic properties. The fluidity of water glass sand is increased by adding graphite powder and talcum powder.

[0029] A heat-curing powder self-hardening method is achieved by the above-mentioned ratio. The present invention further provides a powder-curing inorganic hard sand core making method, comprising the following steps:

[0030] Step 1: Prepare the curing agent - Mix the following weight percentages of powder to make inorganic self-hardening sand self-heating powder curing agent

[0031] 10%~23% aluminum powder

[0032] 70%~80% amorphous silica powder

[0033] 5%~10% aluminum carbide powder

[0034] 0.3%~1.5% graphite powder

[0035] 0.5% to 1.5% talc;

[0036] Step 2: Add inorganic binder - add 1.8% to 3.0% of the total weight of inorganic binder to natural silica sand and mix evenly;

[0037] Step 3: Add curing agent - After the mixing is completed in step 2, add the inorganic self-hardening sand self-heating powder curing agent prepared in step 1 according to 0.8% to 2.0% of the total weight of natural silica sand, mix evenly, and make mixed sand;

[0038] Step 4: Sand filling and molding - fill the mixed sand made in step 3 into the molding mold;

[0039] Step 5: Solidification and shaping - After 1-5 minutes of reaction, heat release and dehydration, the mixed sand is solidified and shaped in the mold to obtain the required powder solidified inorganic self-hardening sand core.

[0040] Compared with the prior art, the present invention has the following significant advantages: by modifying water glass and powder hardener to achieve powder self-hardening, the sand mold can achieve the strength, collapse and moisture absorption resistance of resin sand while combining the low gas evolution and yieldability of traditional water glass sand. DETAILED DESCRIPTION

[0041] Example 1

[0042] A method for making cores from powder-cured inorganic hard sand comprises adding 1.8% to 3.0% of the total weight of an inorganic binder to raw sand (natural silica sand) to mix the sand, then adding 0.8% to 2.0% of the total weight of the mixed sand to the powder curing agent, filling the sand into a mold using a continuous sand mixer, and curing the inorganic binder through an exothermic reaction between the powder curing agent and the inorganic binder, thereby obtaining inorganic self-hardening sand.

[0043] The composition of natural silica sand is as follows:

[0044] Table 1

[0045]

[0046] The composition of the powder curing agent is as follows:

[0047] Table 2

[0048]

[0049] The composition of the inorganic binder is as follows:

[0050] Table 3

[0051]

[0052] Preparation of sand cores by powder-cured inorganic self-hardening sand includes the following steps:

[0053] (1) Add 2.4% of the total weight of inorganic binder to the raw sand and continue mixing for 30s to 90s;

[0054] (2) After mixing, add powder curing agent according to 1.5% of the total weight of the original sand;

[0055] (3) Manually fill the inorganic self-hardening sand mixed in step (2) into the mold;

[0056] (4) After 5 minutes of reaction, heat release, dehydration and solidification, the inorganic self-hardening sand core is obtained.

[0057] The performance is as follows:

[0058] Table 4

[0059]

[0060] Example 2

[0061] A method for making cores from powder-cured inorganic hard sand comprises adding 1.8% to 3.0% of the total weight of an inorganic binder to raw sand (natural silica sand) to mix the sand, then adding 0.8% to 2.0% of the total weight of the mixed sand to mix the sand, and using a continuous sand mixer to fill the sand into a mold. The powder curing agent and the inorganic binder undergo an exothermic reaction to cure the inorganic binder, thereby obtaining inorganic self-hardening sand.

[0062] The composition of raw sand is as follows:

[0063] Table 5

[0064]

[0065] The composition of the powder curing agent is as follows:

[0066] Table 6

[0067]

[0068] The inorganic binder is a silicate binder, and the composition is as follows:

[0069] Table 7

[0070]

[0071] The preparation of sand molds by powder solidification of inorganic self-hardening sand includes the following steps:

[0072] (1) Add 2.4% of the total weight of inorganic binder to the raw sand and continue mixing for 30s to 90s;

[0073] (2) After mixing, add powder curing agent according to 1.5% of the total weight of the original sand;

[0074] (3) Manually fill the inorganic self-hardening sand mixed in step (2) into the mold;

[0075] (4) After 2 minutes of reaction, heat release, dehydration and solidification, the inorganic self-hardening sand core is obtained.

[0076] The performance is as follows:

[0077] Table 8

[0078]

[0079] Comparative Example 1

[0080] This comparative example uses the same raw sand and binder as in the examples, and the composition of the raw sand is shown in the following table. The difference is that the content of aluminum powder and amorphous silica powder in the powder curing agent is different from that of the present invention.

[0081] Table 9

[0082]

[0083] The composition of the powder curing agent is as follows:

[0084] Table 10

[0085]

[0086] The inorganic binder is a silicate binder, and the composition is as follows:

[0087] Table 11

[0088]

[0089] Preparation of sand cores by powder-cured inorganic self-hardening sand includes the following steps:

[0090] (1) Add 2.4% of the total weight of inorganic binder to the raw sand and continue mixing for 30s to 90s;

[0091] (2) After mixing, add powder curing agent according to 1.5% of the total weight of the original sand;

[0092] (3) Manually fill the mold with the non-self-hardening sand mixed in step (2);

[0093] (4) The curing speed is too fast and the sand mold cannot be formed.

[0094] The performance test is as follows:

[0095] Table 12

[0096]

[0097] Comparative Example 2

[0098] The sand core was made of ethylene glycol diacetate, a conventional commercial organic curing agent, and water glass as a binder. The performance indicators are shown in Table 13. The performance indicators of the prepared sand core such as immediate strength, 24-hour strength and 1000°C gas evolution were all lower than those of the preparation method in the embodiment.

[0099] Table 13

[0100]

[0101] In addition to the above embodiments, the present invention may also have other implementations. Any technical solution formed by equivalent replacement or equivalent transformation falls within the scope of protection required by the present invention.

Claims

1. A powder-cured inorganic self-hardening sand comprising the following components in percentage by weight: A powder curing agent accounting for 0.8% to 2.0% of the weight of natural silica sand, an inorganic binder accounting for 1.8% to 3.0% of the weight of natural silica sand, and the balance natural silica sand; the powder curing agent includes the following weights: Percentage of components: 10% to 23% aluminum powder, 70% to 80% amorphous silica powder, 5% to 10% aluminum carbide powder, 0.3% to 1.5% graphite powder, 0.5% to 1.5% talc; the inorganic binder is a silicate binder, and the silicate binder includes the following components in percentage by weight: 30% to 40% sodium silicate, 5% to 10% potassium hydroxide, 1% to 3% disodium hydrogen phosphate, 0.1% to 1% alumina, 0.1% to 1% iron oxide, 0.1% to 1% magnesium oxide, 0.1% to 1% zinc oxide, 5% to 10% sodium hydroxide, The rest is water.

2. The powder-cured inorganic self-hardening sand according to claim 1, characterized in that: The particle size of the aluminum powder, amorphous silicon dioxide powder and aluminum carbide powder is 15 μm to 74 μm.

3. A method for making a core from powder-cured inorganic self-hardening sand, comprising the following steps: Step 1: Prepare a powder curing agent - Mix the following components in weight percentage: 10% to 23% aluminum powder, 70% to 80% amorphous silicon dioxide powder, 5% to 10% aluminum carbide powder, 0.3% to 1.5% graphite powder, and 0.5% to 1.5% talc powder to obtain a powder curing agent; The second step is to add an inorganic binder - add 1.8% to 3.0% of the total weight of the inorganic binder to the natural silica sand and mix them evenly; the inorganic binder is a silicate binder, and the silicate binder includes the following components in percentage by weight: 30% to 40% sodium silicate, 5% to 10% potassium hydroxide, 1% to 3% disodium hydrogen phosphate, 0.1% to 1% alumina, 0.1% to 1% iron oxide, 0.1% to 1% magnesium oxide, 0.1% to 1% zinc oxide, 5% to 10% sodium hydroxide, The rest is water; Step 3: Add curing agent - After the mixing in step 2 is completed, add the powder curing agent prepared in step 1 at a rate of 0.8% to 2.0% of the total weight of the natural silica sand, mix evenly, and make mixed sand; Step 4: Sand filling and molding - fill the mixed sand made in step 3 into the molding mold; The fifth step is solidification and shaping - through exothermic reaction and dehydration, the mixed sand is solidified and shaped in the mold to obtain the required powder solidified inorganic self-hardening sand core.

4. The method for making cores from powder-cured inorganic self-hardening sand according to claim 3, characterized in that: The second step mixing time is 30s-90s.

5. The method for making cores from powder-cured inorganic self-hardening sand according to claim 3, characterized in that: The exothermic reaction time in the fifth step is 1 min to 5 min.

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