Silica sol investment precision casting process

Abstract

The invention relates to the technical field of silica sol fired molds, and discloses a silica sol fired mold precision casting process which comprises the following steps: preparing alkaline silica sol, refractory powder, polyethylene glycol-400, an ammonium zirconium carbonate aqueous solution and propylene carbonate, mixing the alkaline silica sol with the polyethylene glycol-400 and the ammonium zirconium carbonate aqueous solution to form high-steady-state cross-linked precursor silica sol; propylene carbonate is hydrolyzed to produce acid to trigger ammonium zirconium carbonate to release zirconium hydroxyl, and the zirconium hydroxyl and silica sol are subjected to dehydration polycondensation, so that the coating is converted from external physical dehydration to internal chemical homogeneous crosslinking; and in the shell making process, a two-stage dynamic temperature and humidity induction curing process is matched to complete shell drying. According to the method, the problem that the coating surface crust seals internal moisture is solved, the peeling risk in the dewaxing stage is reduced, zirconium oxide crystals generated through zirconium element phase change participate in solid-phase sintering in the high-temperature roasting stage, the defect of sintering activity of a pure silica sol system is overcome, and the normal-temperature wet strength and high-temperature roasting density of the shell are improved.

Description

Technical Field

[0001] This invention relates to the field of silica sol investment casting technology, specifically a silica sol investment casting precision casting process. Background Technology

[0002] Silica sol investment casting is commonly used to manufacture metal castings with high dimensional accuracy requirements. The shell preparation process has a direct impact on the quality of the final casting. In existing shell-making processes, the curing of silica sol shells mainly relies on the physical evaporation of moisture from the outside to the inside. This physical dehydration method causes the coating surface to dry preferentially and form a skin. The resulting surface colloidal structure hinders the further drying of internal moisture. This phenomenon makes the overall drying cycle of the shell longer. Furthermore, in the subsequent high-pressure steam dewaxing stage, the moisture retained inside the shell is rapidly vaporized and expanded by heat, which can easily damage the coating structure and cause defects such as interlayer peeling or delamination.

[0003] Meanwhile, in order to meet the stress requirements of the module during dewaxing and handling, and to improve the room temperature and wet strength of the shell, traditional shell-making processes usually add some organic polymers as reinforcing components to the silica sol slurry system. However, these organic components will undergo pyrolysis and volatilization in the subsequent high-temperature baking stage of the shell, thus leaving microscopic pores in the inorganic silicon-oxygen skeleton. The generation of these pores destroys the compactness and continuity of the shell matrix structure, resulting in a decrease in the high-temperature baking strength of the shell and easy peeling in the subsequent dewaxing stage. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a silica sol investment casting process that solves the problem of long drying cycles for existing silica sol shells, which leads to peeling during the subsequent dewaxing stage.

[0005] To achieve the above objectives, the present invention is implemented through the following technical solution: a silica sol investment casting process, comprising the following raw materials in parts by weight: 1000 parts of alkaline silica sol; 1500 to 2500 parts of refractory powder; 2 to 8 parts of polyethylene glycol-400; 5 to 15 parts of zirconium ammonium carbonate aqueous solution; and 10 to 20 parts of propylene carbonate.

[0006] By adopting the above technical solution, and by using propylene carbonate in combination with ammonium zirconium carbonate and polyethylene glycol-400 to establish an internal crosslinking and tension regulation mechanism within the system, the effects of improved room temperature wet strength and high temperature calcination strength are achieved. The specific mechanism is as follows:

[0007] Propylene carbonate undergoes hydrolysis in the aqueous phase of alkaline silica sol, consuming hydroxide ions in the system and generating propylene glycol and carbon dioxide, which promotes a homogeneous decrease in the pH of the system.

[0008] When the pH of the system drops to a certain threshold, the ammonium zirconium carbonate distributed inside undergoes a decomplexation reaction, releasing zirconium hydroxyl groups;

[0009] The free zirconium hydroxyl groups undergo a dehydration condensation reaction with the silanol hydroxyl groups on the surface of the silica sol particles, and simultaneously crosslink and solidify within the coating space to construct a zirconium silicate inorganic network. This process changes the drying method of the coating from external dehydration to internal chemical gelation.

[0010] Propylene glycol, generated from the hydrolysis of propylene carbonate, is mixed with pre-placed polyethylene glycol-400 to form a binary tension-reducing system. This system, through spatial arrangement within the pore fluid, interferes with the hydrogen bonds of water molecules, thereby reducing the surface tension of the coating pore fluid and mitigating the destructive effects of capillary contraction stress on the inorganic framework.

[0011] Preferably, the highly stable crosslinking precursor silica sol is formed by mixing the alkaline silica sol with the polyethylene glycol-400 and an aqueous solution of zirconium ammonium carbonate.

[0012] By adopting the above technical solution, the components are uniformly dispersed and rheologically stable by premixing polyethylene glycol-400 and zirconium ammonium carbonate aqueous solution with alkaline silica sol.

[0013] Preferably, the refractory powder is divided into surface layer powder and back layer powder; the surface layer powder is zircon powder or corundum powder with a particle size of 320 mesh; the back layer powder is mullite powder or corundum powder with a particle size of 80 mesh.

[0014] By adopting the above technical solution, and using refractory powders of different particle sizes as the surface and back layers respectively, the inner surface of the shell is made dense and the outer structure is breathable.

[0015] Preferably, the method for preparing the highly stable crosslinking precursor silica sol includes the following:

[0016] The alkaline silica sol is added to a reaction vessel, and the refractory powder is dispersed at a high shear speed of 800 to 1200 rpm at 20 to 26 degrees Celsius for 15 to 20 minutes. Then the speed is reduced to 100 to 300 rpm, and the polyethylene glycol-400 and zirconium ammonium carbonate aqueous solution are slowly added dropwise in sequence. The mixture is stirred continuously for 20 to 30 minutes to obtain a highly stable crosslinking precursor silica sol.

[0017] 15 to 25 minutes before the planned module coating operation, the propylene carbonate is added dropwise at a low shear speed of 30 to 60 revolutions per minute to the highly stable crosslinking precursor silica sol, and mixed for 10 to 15 minutes to obtain the final product.

[0018] By adopting the above technical solution, the mixing process of step-deceleration dispersion and low-shear dripping is used to obtain uniform suspension of powder and avoid premature gelation of propylene carbonate due to excessively high local concentration.

[0019] A precision casting process for silica sol investment casting, employing the aforementioned method for shell fabrication using silica sol investment casting, includes the following:

[0020] S1. Immerse the wax mold assembly into the surface working silica sol for single-layer coating and slurry control, and then evenly sprinkle the surface sand material on the silica sol surface.

[0021] S2. Send the module into an environmentally controlled drying chamber and perform two-stage dynamic temperature and humidity induced curing to dry and cure the single layer.

[0022] S3. Immerse the module after the single-layer curing into the back layer working silica sol for coating and slurry control, sprinkle back layer sanding material, and perform the same two-stage dynamic temperature and humidity induced curing as S2. Repeat this process several times to build the back layer layer by layer. The last time, only coating is done without sanding to obtain the shell.

[0023] S4. The completely dried shell is sent into a high-pressure steam dewaxing kettle for dewaxing treatment;

[0024] S5. The dewaxed shell is transferred into a baking furnace for high-temperature constant-temperature baking, and after cooling in the furnace, the final silica sol investment casting shell is obtained.

[0025] By adopting the above technical solution, the process of internal cross-linking combined with external temperature and humidity dynamic control ensures that the coating is in a high-humidity environment in the early stage of cross-linking to avoid surface dehydration and skin formation. At the same time, the shell undergoes phase transformation and reconstruction during the later high-temperature sintering. Therefore, the shell peeling defect is reduced and the density is improved. The specific process is as follows: In the high-temperature sintering stage, the zirconium element in the silicon-oxygen zirconium network constructed in the early stage undergoes phase transformation and decomposition to generate zirconium oxide crystals. Zirconium oxide particles fill the pores of the silicon-oxygen framework and participate in solid-state sintering, making up for the lack of sintering activity of the pure silica sol system.

[0026] Preferably, the specific implementation method of the dynamic temperature and humidity induced curing is as follows:

[0027] The first stage involves maintaining a high humidity and low wind speed environment in the drying chamber to inhibit the evaporation and crusting of moisture on the coating surface, and to facilitate homogeneous cross-linking by the hydrolysis of propylene carbonate inside the coating to produce acid.

[0028] The second stage involves maintaining a constant ambient temperature in the drying chamber, gradually reducing the relative humidity at a linear rate, and simultaneously increasing the ambient wind speed to remove moisture from deep pores.

[0029] By adopting the above technical solution, the setting of first inhibiting moisture evaporation to promote internal cross-linking and then reducing humidity to promote moisture discharge results in the effect of consistent internal and external curing rates and elimination of surface skin formation.

[0030] Preferably, the process parameters for the dynamic temperature and humidity induced curing are as follows:

[0031] In the first stage, the ambient temperature is controlled at 22 to 26 degrees Celsius, the relative humidity is controlled at 80% to 90%, the ambient wind speed is controlled at 0.3 to 0.8 meters per second, and the duration is 25 to 40 minutes.

[0032] The second stage involves maintaining the ambient temperature at 22 to 26 degrees Celsius, reducing the relative humidity to 35 to 45 percent at a linear rate over a period of 50 to 70 minutes, while simultaneously increasing the ambient wind speed to 2.0 to 3.0 meters per second.

[0033] By adopting the above technical solution, and by using temperature and humidity boundary values ​​that match the hydrolysis kinetic cycle, the process parameters are executed stably and the molding is stable.

[0034] Preferably, the surface sand material mentioned in S1 is 200-mesh mullite sand or corundum sand; the back sand material mentioned in S3 is 60-mesh mullite sand or corundum sand.

[0035] By adopting the above technical solution, and using inorganic sand material that corresponds to the particle size of the powder, a tight bond between the coating and the sand layer is achieved.

[0036] Preferably, in S4, the process parameters for the dewaxing treatment are: steam pressure set to 0.5 to 0.8 MPa, dewaxing temperature controlled at 150 to 170 degrees Celsius, and pressure holding time of 10 to 20 minutes.

[0037] By adopting the above technical solution, and by using appropriate steam pressure and dewaxing temperature parameters, the effect of complete dewaxing and maintenance of the shell structure is achieved.

[0038] Preferably, in S5, the process parameters for the high-temperature constant-temperature calcination are: heating to 850 to 1050 degrees Celsius at a heating rate of 5 to 10 degrees Celsius per minute, and calcining at a constant temperature for 1 to 3 hours.

[0039] By adopting the above technical solution, and using solid-phase heat treatment parameters of uniform heating and high-temperature constant temperature, the effect of sintering the matrix silica and completing ceramization is achieved.

[0040] This invention provides a silica sol investment casting process for precision casting. It offers the following advantages:

[0041] 1. This invention employs a two-stage dynamic temperature and humidity-induced curing process. In the first stage, high humidity and low wind speed are maintained to inhibit preferential dehydration and skin formation on the coating surface, ensuring sufficient internal cross-linking. In the second stage, dehumidification and increased wind speed are used to expel deep-seated moisture. This process, which first inhibits evaporation to promote cross-linking and then reduces humidity to promote drying, maintains the consistency of curing rates inside and outside the coating, shortens the drying cycle of the shell, and reduces the risk of peeling and delamination during the subsequent dewaxing stage.

[0042] 2. In the subsequent high-temperature sintering stage of the shell, the zirconium element within the silicon-oxygen-zirconium inorganic network constructed in this invention undergoes a phase transformation and decomposes, generating zirconium oxide crystals in situ. Zirconium oxide particles can directly fill the pores of the silicon-oxygen framework and participate in solid-state sintering, compensating for the insufficient sintering activity of the pure silica sol system. This mechanism avoids the problem of residual micropores after pyrolysis of organic polymers added in traditional processes, thus improving the high-temperature sintering strength and structural density of the shell.

[0043] 3. This invention introduces propylene carbonate, an aqueous solution of zirconium ammonium carbonate, and polyethylene glycol-400 into an alkaline silica sol system. The hydrolysis of propylene carbonate lowers the system's pH, triggering the release of zirconium hydroxyl groups from the zirconium ammonium carbonate, which then undergoes dehydration and polycondensation with the silica sol. This transforms the coating process from external physical dehydration to internal chemical homogeneous cross-linking. Simultaneously, the propylene glycol generated from the hydrolysis forms a binary tension-reducing system with polyethylene glycol-400, lowering the surface tension of the pore fluid and mitigating the destructive effects of capillary shrinkage stress on the inorganic framework. This improves the room-temperature wet strength of the shell and reduces the formation of microcracks. Attached Figure Description

[0044] Figure 1 This is a step diagram of the present invention. Detailed Implementation

[0045] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0046] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0047] The alkaline silica sol is industrial grade alkaline silica sol with a silica mass fraction of 30%, a pH value of 9.5, an average particle size of 14 nm, and a kinematic viscosity of 4.5 mm² / s.

[0048] Zircon powder, made of industrial-grade zirconium silicate powder, CAS number 10101-52-7, with a particle size of 320 mesh for the topcoat and 80 mesh for the backcoat.

[0049] Mullite powder, industrial grade mullite powder, CAS number 1302-93-8, with a particle size of 200 mesh for the topcoat and 60 mesh for the backcoat.

[0050] The zirconium carbonate ammonium aqueous solution, industrial grade, CAS number 22829-98-7, has a zirconium dioxide equivalent mass fraction of 20%, a pH value of 9.0, and is a colorless to pale yellow transparent liquid.

[0051] Polyethylene glycol-400 is made of analytical grade polyethylene glycol, CAS number 25322-68-3, with an average molecular weight of 400 and a kinematic viscosity of 37~45 mm² / s at 20℃.

[0052] Propylene carbonate, specifically analytical grade propylene carbonate, CAS number 108-32-7, with a purity greater than or equal to 99.0% and a moisture content less than or equal to 0.1%.

[0053] Styrene-butadiene latex is selected from industrial-grade carboxylated styrene-butadiene latex, with CAS number 9003-55-8, solid content of 50%, pH value of 8.0, and glass transition temperature of -5℃.

[0054] Propylene glycol, specifically analytical grade 1,2-propanediol, CAS number 57-55-6, with a purity greater than or equal to 99.5%.

[0055] Preparation Example 1: This preparation example provides a method for preparing a surface working silica sol, including the following steps:

[0056] 1000g of alkaline silica sol was pumped into a reactor equipped with a high-speed dispersion paddle. At an ambient temperature of 22°C, stirring was started and the speed was set to 1000 rpm. 2500g of zircon powder with a particle size of 320 mesh was slowly added and dispersed for 20 minutes until no obvious powder lumps were observed. Then, the stirring speed was reduced to 200 rpm, and 5g of polyethylene glycol-400 and 10g of zirconium ammonium carbonate aqueous solution were slowly added dropwise. After the addition was completed, the temperature inside the reactor was maintained at 22°C and stirring was continued for 25 minutes to obtain a highly stable crosslinking precursor silica sol. 20 minutes before the planned module coating operation, 15g of propylene carbonate was added dropwise at a low shear speed of 50 rpm to the above highly stable crosslinking precursor silica sol. After the addition was completed, the mixture was stirred at a low speed for 15 minutes to obtain the surface working silica sol.

[0057] Preparation Example 2: This preparation example provides a method for preparing a surface working silica sol, including the following steps:

[0058] 1000g of alkaline silica sol was pumped into a reactor equipped with a high-speed dispersion paddle. At an ambient temperature of 20°C, stirring was started and the speed was set to 800 rpm. 2500g of zircon powder with a particle size of 320 mesh was slowly added and dispersed for 15 minutes until no obvious powder lumps were observed. Then, the stirring speed was reduced to 100 rpm, and 2g of polyethylene glycol-400 and 5g of zirconium ammonium carbonate aqueous solution were slowly added dropwise. After the addition was completed, the temperature inside the reactor was maintained at 20°C and stirring was continued for 20 minutes to obtain a highly stable crosslinking precursor silica sol. 15 minutes before the planned module coating operation, 10g of propylene carbonate was added dropwise at a low shear speed of 30 rpm to the above highly stable crosslinking precursor silica sol. After the addition was completed, the mixture was stirred at a low speed for 10 minutes to obtain the surface working silica sol.

[0059] Preparation Example 3: This preparation example provides a method for preparing a surface working silica sol, including the following steps:

[0060] 1000g of alkaline silica sol was pumped into a reactor equipped with a high-speed dispersion paddle. At an ambient temperature of 25°C, stirring was started and the speed was set to 1200 rpm. 2500g of zircon powder with a particle size of 320 mesh was slowly added and dispersed for 20 minutes until no obvious powder lumps were observed. Then, the stirring speed was reduced to 300 rpm, and 8g of polyethylene glycol-400 and 15g of zirconium ammonium carbonate aqueous solution were slowly added dropwise. After the addition was completed, the temperature inside the reactor was maintained at 25°C and stirring was continued for 30 minutes to obtain a highly stable crosslinking precursor silica sol. 25 minutes before the planned module coating operation, 20g of propylene carbonate was added dropwise at a low shear speed of 60 rpm to the above highly stable crosslinking precursor silica sol. After the addition was completed, the mixture was stirred at a low speed for 15 minutes to obtain the surface working silica sol.

[0061] Preparation Example 4: This preparation example provides a method for preparing a back-layer working silica sol, including the following steps:

[0062] 1000g of alkaline silica sol was pumped into a reactor equipped with a high-speed dispersion paddle. At an ambient temperature of 22°C, stirring was started and the stirring speed was set to 1000 rpm. 1500g of mullite powder with a particle size of 80 mesh was slowly added and dispersed for 20 minutes until no obvious powder lumps were observed. Then, the stirring speed was reduced to 200 rpm, and 5g of polyethylene glycol-400 and 10g of zirconium ammonium carbonate aqueous solution were slowly added dropwise. After the addition was completed, the temperature inside the reactor was maintained at 22°C and stirring was continued for 25 minutes to obtain a highly stable crosslinking precursor silica sol. 20 minutes before the planned module coating operation, 15g of propylene carbonate was added dropwise at a low shear speed of 50 rpm to the above highly stable crosslinking precursor silica sol. After the addition was completed, the mixture was stirred at a low speed for 15 minutes to obtain the back layer working silica sol.

[0063] Example 1: This example provides a silica sol investment casting process. Please refer to the appendix. Figure 1 This includes the following steps:

[0064] In the first stage, take the cleaned wax mold component and immerse it in the surface working silica sol prepared in Example 1. After staying for 8 seconds, lift it out and rotate it to control the coating thickness so that the thickness of a single wet film is uniform. The thickness of a single wet film is controlled at 0.5 mm. Then, 200-mesh mullite sand is evenly spread on the surface of the silica sol in a rain-type sander.

[0065] In the second stage, the module is placed in a controlled drying chamber and subjected to two-stage dynamic temperature and humidity induced curing. In the first stage, the ambient temperature of the drying chamber is set at 24°C, the relative humidity is controlled at 85%, and the ambient wind speed is controlled at 0.5 m / s for 30 minutes. In the second stage, the ambient temperature is kept constant at 24°C, and the relative humidity is reduced to 40% at a linear rate within 60 minutes, while the ambient wind speed is smoothly increased to 2.5 m / s.

[0066] In the third stage, after the single-layer drying and curing is completed, the module is immersed in the back layer working silica sol prepared in Preparation Example 4 for coating, covered with 60-mesh mullite sand, and subjected to the same two-stage dynamic temperature and humidity induced curing parameters as in the second stage.

[0067] In the fourth stage, the third stage was repeated four times in total, and the back layer was constructed layer by layer. In the last stage, only the back layer silica sol was coated and no sand was sprinkled for sealing treatment, resulting in a shell with a total of 6 layers.

[0068] The completely dried mold shell is sent into a high-pressure steam dewaxing kettle. The steam pressure is set to 0.6 MPa, the dewaxing temperature to 160℃, and the holding time to 15 minutes, so that the internal wax mold is completely melted and discharged.

[0069] The dewaxed shell is transferred into a baking furnace and heated to 1000°C at a heating rate of 8°C / min. It is then baked at a constant temperature for 2 hours and then cooled to room temperature in the furnace to obtain the final silica sol investment casting shell.

[0070] Example 2: This example provides a silica sol investment casting process, including the following steps:

[0071] The surface layer working silica sol of Preparation Example 2 was used for surface coating. The coating and slurry control operations were the same as in Example 1. Then, 200-mesh mullite sand was evenly sprinkled on. The preparation method of the back layer working silica sol was exactly the same as that of Preparation Example 2, except that 2500g of zircon powder with a particle size of 320 mesh was replaced with 1500g of mullite powder with a particle size of 80 mesh.

[0072] The module was placed in a controlled drying chamber. In the first stage, the ambient temperature was set at 22°C, the relative humidity was controlled at 80%, and the ambient wind speed was controlled at 0.3 m / s for 25 minutes. In the second stage, the temperature was kept constant at 22°C, and the relative humidity was reduced to 45% at a linear rate within 50 minutes, while the ambient wind speed was smoothly increased to 2.0 m / s.

[0073] The backing layer is coated using the aforementioned working silica sol, covered with 60-mesh mullite sand, and dried using the same drying parameters as in the second stage. The backing layer is then repeated to a total of 6 layers, with the last layer not covered with sand.

[0074] The shell is fed into the dewaxing kettle, and the steam pressure is set to 0.5 MPa, the dewaxing temperature to 150℃, and the holding time to 10 minutes.

[0075] After dewaxing, the shell is transferred into a baking furnace and heated to 850°C at a heating rate of 5°C / min. It is then baked at a constant temperature for 3 hours and cooled to room temperature with the furnace.

[0076] Example 3: This example provides a silica sol investment casting process, including the following steps:

[0077] The surface layer working silica sol of Preparation Example 3 was used for surface coating. The coating and slurry control operations were the same as in Example 1. Then, 200-mesh mullite sand was evenly sprinkled on. The preparation method of the back layer working silica sol was exactly the same as that of Preparation Example 3, except that 2500g of zircon powder with a particle size of 320 mesh was replaced with 1500g of mullite powder with a particle size of 80 mesh.

[0078] The module was placed in a controlled drying chamber. In the first stage, the ambient temperature was set at 26°C, the relative humidity was controlled at 90%, and the ambient wind speed was controlled at 0.8 m / s for 40 minutes. In the second stage, the temperature was kept constant at 26°C, and the relative humidity was reduced to 35% at a linear rate within 70 minutes, while the ambient wind speed was smoothly increased to 3.0 m / s.

[0079] The backing layer is coated using the aforementioned working silica sol, covered with 60-mesh mullite sand, and dried using the same drying parameters as in the second stage. The backing layer is then repeated to a total of 6 layers, with the last layer not covered with sand.

[0080] The shell is fed into the dewaxing kettle, and the steam pressure is set to 0.8 MPa, the dewaxing temperature to 170℃, and the holding time to 20 minutes.

[0081] After dewaxing, the shell is transferred into a baking furnace and heated to 1050°C at a heating rate of 10°C / min. It is then baked at a constant temperature for 1 hour and cooled to room temperature in the furnace.

[0082] Example 4: This example provides a silica sol investment casting process, including the following steps:

[0083] The preparation method of the working silica sol for the surface layer in this embodiment is the same as that in Preparation Example 1, except that 2500g of zircon powder with a particle size of 320 mesh is replaced with 2500g of corundum powder with a particle size of 320 mesh. The preparation method of the working silica sol for the back layer in this embodiment is the same as that in Preparation Example 4, except that 1500g of mullite powder with a particle size of 80 mesh is replaced with 1500g of corundum powder with a particle size of 80 mesh. After the surface layer is coated, 200-mesh corundum sand is sprinkled on top, and after the back layer is coated, 60-mesh corundum sand is sprinkled on top. The remaining coating control parameters, two-stage dynamic drying parameters, number of cyclic shell layers, dewaxing parameters, and high-temperature calcination parameters are exactly the same as those in Example 1.

[0084] Example 5: This example provides a silica sol investment casting process, including the following steps:

[0085] 1000g of alkaline silica sol was pumped into a reactor equipped with a high-speed dispersion paddle. At an ambient temperature of 22°C, stirring was started and the speed was set to 1000 rpm. 2500g of zircon powder with a particle size of 320 mesh was slowly added and dispersed for 20 minutes. Then, the speed was reduced to 200 rpm, 15g of propylene carbonate was added, and the mixture was stirred for 5 minutes. Finally, 5g of polyethylene glycol-400 and 10g of zirconium ammonium carbonate aqueous solution were slowly added dropwise, and stirring was continued for 10 minutes to obtain the surface working silica sol, which was used immediately. The preparation method of the back working silica sol was the same, except that the zircon powder was replaced with 1500g of mullite powder with a particle size of 80 mesh.

[0086] The above-mentioned surface working silica sol and back working silica sol were used for coating and sanding operations. The specifications of the sanding material, the two-stage dynamic drying parameters, the number of shell layers in the cycle, the dewaxing parameters, and the high-temperature calcination parameters were all exactly the same as those in Example 1.

[0087] Test Example 1: Take 500 mL of the working silica sol obtained after adding propylene carbonate and mixing at low speed for 15 minutes in Example 1, and quickly transfer it to a glass reactor with a water bath constant temperature jacket and a sealed lid. Set the water bath temperature and maintain it at 24°C.

[0088] The calibrated industrial online pH meter electrode and the test rotor of the rotational viscometer are inserted into the silica sol through the reserved holes on the sealing cap, ensuring that the measuring probe is completely immersed in the silica sol and does not contact the bottom and side walls of the reactor. The reactor is kept sealed throughout the test process to simulate the state where moisture and ammonia do not volatilize in a high-humidity, windless environment.

[0089] Turn on the rotational viscometer and set a constant low shear rate to eliminate the interference of shear thinning effect on the test results.

[0090] From the moment the silica sol is transferred into the reactor, the pH value and apparent dynamic viscosity of the system are read and recorded synchronously every 5 minutes, and this monitoring continues until the viscometer reading exceeds the range or the silica sol loses its macroscopic fluidity.

[0091] Table 1: Relationship between pH value and apparent dynamic viscosity of silica sol over time under sealed and constant temperature conditions

[0092] time pH value Apparent dynamic viscosity 0 9.42 412.3 5 9.38 415.7 10 9.31 421.1 15 9.24 430.5 20 9.15 448.2 25 9.02 473.8 30 8.87 512.6 35 8.68 584.1 40 8.41 753.9 45 8.16 1342.4 50 7.92 3581.7 55 7.74 8945.2 60 7.58 Beyond the range

[0093] According to the data in Table 1, propylene carbonate can control the decrease in acidity in a closed, weakly alkaline silica sol system. During the initial 0-20 minute test, the pH of the system slowly decreased from 9.42 to 9.15, and the corresponding apparent dynamic viscosity increased slightly from 412.3 mPa·s to 448.2 mPa·s. The alkaline hydrolysis reaction of propylene carbonate proceeds gradually, with the existing free ammonia and bicarbonate ions providing a buffering effect. This buffering effect ensures that, in actual production, the silica sol will not undergo rapid thickening or localized aggregation in the mixing tank or coating tank before being coated onto the wax mold surface.

[0094] As the monitoring time was extended to 30 to 40 minutes, the pH value decreased to the range of 8.87 to 8.41. Within this pH range, ammonium zirconium carbonate began to undergo a decomplexing reaction. As free hydroxide ions were continuously consumed by the hydrolysis of propylene carbonate, the zirconium carbonate complex structure became unstable, releasing highly reactive zirconium hydroxyl groups. Table 1 shows that the viscosity accelerated from 512.6 mPa·s to 753.9 mPa·s at this point, indicating that the inorganic three-dimensional network of zirconium silicate began to construct synchronously throughout the entire silica sol volume. Since the test was conducted under sealed conditions, the increase in concentration due to solvent evaporation was excluded; therefore, the increase in viscosity is entirely attributed to the development of the internal chemical cross-linked network.

[0095] After 45 minutes, the pH value dropped to 8.16 or below, and the apparent dynamic viscosity increased significantly, from 1342.4 mPa·s to 8945.2 mPa·s, and lost its fluidity at 60 minutes. This process confirms that the silica sol framework can be cured solely through endogenous chemical reactions without the removal of macroscopic moisture. This data trend supports the present invention's approach of avoiding preferential skin formation on the surface and achieving uniform internal cross-linking. By introducing propylene carbonate, the microscopic defects caused by excessively rapid drying in traditional processes are overcome.

[0096] Test Example 2: Using deionized water as solvent, four groups of simulated pore liquid samples were prepared under constant temperature conditions of 24℃. The first group was a blank control group, which was pure deionized water. The second group had polyethylene glycol-400 added to make its mass fraction 0.5%. The third group had propylene glycol added to make its mass fraction 1.5%. This concentration corresponds to the theoretical content of propylene glycol generated in the system after the complete hydrolysis of propylene carbonate in Example 1. The fourth group had both polyethylene glycol-400 and propylene glycol added, with mass fractions of 0.5% and 1.5%, respectively.

[0097] Take 50 mL of each of the above four groups of solution samples and place them in glass sample dishes that have been soaked in hot chromic acid cleaning solution and rinsed with deionized water. Transfer the glass dishes containing the samples into the constant temperature test chamber of the fully automatic surface tension meter and let them stand for 20 minutes to allow the internal temperature of the system to be uniform and the surface to reach a thermodynamically stable state.

[0098] Surface tension was measured using the platinum ring method. Before the test, the instrument sensor was calibrated using standard weights. Then, a cleaned platinum ring was immersed below the sample liquid surface, and the test platform was slowly lowered at a rate of 0.1 mm / s. The maximum tensile force at the moment the liquid film ruptured was recorded and converted into surface tension.

[0099] Each sample group was measured three times independently. After each measurement, the platinum rings were cleaned alternately with anhydrous ethanol and deionized water and heated over an alcohol lamp flame until red-hot to eliminate residual interference from surfactants.

[0100] Table 2: Surface tension test results of simulated pore fluids with different components at 24℃

[0101] Test group Components Measured value 1 (mN / m) Measured value 2 (mN / m) Measured value 3 (mN / m) Average value (mN / m) Group 1 Deionized water 71.86 72.11 71.94 71.97 Group 2 0.5% Polyethylene Glycol-400 63.42 62.87 63.15 63.14 Group 3 1.5% Propylene Glycol 65.81 66.05 65.73 65.86 Group 4 0.5% polyethylene glycol-400 + 1.5% propylene glycol 52.34 51.78 52.19 52.10

[0102] According to the data in Table 2, polyethylene glycol-400 and propylene glycol exhibit a clear synergistic tension-reducing effect in the aqueous system. The average surface tension of the first group of deionized water was 71.97 mN / m, which is consistent with the physical properties of pure water at room temperature. In the second group, the surface tension decreased to 63.14 mN / m after adding 0.5% polyethylene glycol-400 alone. In the third group, the surface tension decreased to 65.86 mN / m after adding 1.5% propylene glycol alone. Although these two groups caused some interference with the cohesion between water molecules, the reduction was limited, indicating that a single high-molecular-weight polyether or a small-molecular-weight alcohol cannot form a high-density molecular arrangement at the gas-liquid interface.

[0103] The fourth group, containing both polyethylene glycol-400 and propylene glycol, showed a significant decrease in average surface tension to 52.10 mN / m. This value was not only lower than that of the second and third groups, but its decrease also exceeded the combined tension-reducing effects of the two individual components. This confirms the existence of an interfacial synergistic effect of asymmetric molecular configurations in the binary system. At the microscopic interface, small-volume propylene glycol molecules penetrate and disrupt the short-range hydrogen bond network between surface water molecules, while long-chain polyethylene glycol-400 molecules occupy a larger physical space at the gas-liquid interface and form steric hindrance.

[0104] During the second stage of the silica sol-type shell curing process, the evaporation of moisture inside the pores causes the liquid surface to exhibit a meniscus-like depression. According to the Laplace equation, the capillary negative pressure is proportional to the surface tension of the liquid. The system represented by the fourth group of simulated liquids can spontaneously assemble into a low surface tension environment within the cross-linked network. At this time, the propylene glycol generated in situ and the pre-placed polyethylene glycol-400 work together to limit the surface tension of the solid-liquid-gas three-phase interface. The lower surface tension controls the capillary contraction stress generated when the moisture is extracted within the tensile strength range that the rigid zirconia network can withstand, thus preventing the capillary pore walls from collapsing under pressure. This chemical and rheological test result proves from a physicochemical perspective that, without using traditional polymer organic emulsions that easily cause high-temperature pores, it is still possible to eliminate brittle microcrack defects generated during the coating drying process.

[0105] Test Example 3: Take 10 sets of standard flat medium-temperature wax molds with dimensions of 100mm×50mm×10mm, clean the residual release agent on the surface with anhydrous ethanol, and dry at room temperature for later use.

[0106] According to the formulations and process parameters set in Examples 1 to 5 and Comparative Examples 1 to 5, the flat wax molds were coated and sanded. During the drying and curing process, a water-based follow-up moisture metering probe was locally coated on the sample surface, and the coating state was determined by mechanical scratch method. When the probe showed that the moisture content dropped to below 5% and there was no sand adhering or deep plastic deformation on the surface scratch, the layer was determined to be dry. The actual time taken for the surface layer and each back layer to reach this state was recorded, and the average drying time of a single surface layer and the average drying time of a single back layer were calculated.

[0107] After completing all shell-making cycles according to the set number of layers and drying thoroughly, the sample is sent to a high-pressure dewaxing kettle for steam dewaxing.

[0108] Clean the surface of the dewaxed shell sample with low-pressure compressed air, apply dye penetrant evenly to the surface, let it stand for 10 minutes, wipe off excess liquid, spray developer, cover the shell surface with a transparent grid plate with an accuracy of 0.1 mm, count the number of dyed microcracks in a 10 cm² standard area, and measure the projected area of ​​the interlayer peeling or delamination areas on the shell cross-section, and calculate its percentage of the total surface area.

[0109] Table 3: Statistical data on drying time and surface defects after dewaxing for each group of shells

[0110] Test group Average drying time (h) for a single layer Average drying time (h) for a single backing layer Number of surface microcracks (cracks / 10cm²) Percentage of delamination / peeling area (%) Example 1 1.15 1.32 0 0.0 Example 2 1.42 1.78 1 0.4 Example 3 1.08 1.25 0 0.2 Example 4 1.18 1.35 0 0.0 Example 5 1.31 1.54 2 1.1 Comparative Example 1 4.65 6.82 4 2.5 Comparative Example 2 2.84 3.51 6 17.6 Comparative Example 3 1.12 1.28 21 0.8 Comparative Example 4 1.58 1.95 9 4.2 Comparative Example 5 0.85 1.15 14 24.3

[0111] According to the data in Table 3, the average drying time of the single-layer and back layer of Examples 1 to 5 was controlled between 1.08h and 1.78h. The drying time of the single-layer and back layer of Comparative Example 1 reached 4.65h and 6.82h, respectively. Comparative Example 1 relied on traditional latex and physical natural air drying. The moisture had to overcome capillary resistance to diffuse and evaporate gradually from the inside to the outside. The mass transfer efficiency was limited by the concentration gradient of the environment. The examples introduced a pH decrease induced by the hydrolysis of propylene carbonate, which caused the zirconium ammonium carbonate to decomplex and trigger the SiOZr network to crosslink and solidify synchronously in the three-dimensional space of the coating. This chemical phase change process reduced the amount of free water discharged to meet the requirements of coating the next layer. It changed from simple physical dehydration to chemical gelation, which shortened the process time.

[0112] Comparative Example 2 did not add propylene carbonate, and Comparative Example 5 eliminated the water retention period with phased temperature and humidity control. The interlayer peeling area of ​​these two groups reached 17.6% and 24.3%, respectively. Under the condition of lack of internal homogeneous triggering or external high humidity protection, the moisture on the coating surface evaporated rapidly, and the surface silica sol particles underwent irreversible physical aggregation due to the sharp increase in concentration, forming a dense silica gel skin. This dense skin blocked the drainage channels of the internal residual moisture, resulting in the coating interior being in a wet state of incomplete cross-linking. During subsequent high-pressure steam dewaxing and heating, the residual liquid water inside rapidly vaporized and expanded, and the generated vapor pressure broke through the local structural strength, causing macroscopic peeling between the shell layers. The example suppressed the initial surface water loss through a high humidity environment, and with the continuous acid production of internal propylene carbonate, maintained a consistent cross-linking rate inside and outside, eliminating this skin effect.

[0113] Comparative Example 3, without the addition of polyethylene glycol-400, had 21 microcracks per 10 cm² after dewaxing. The inorganic network constructed by zirconium silicate and silica sol has high rigidity. When the pore water was drained later, the Laplace negative pressure generated by the meniscus of the pore liquid surface acted on the rigid skeleton. Comparative Example 3 relied solely on propylene glycol generated by the hydrolysis of propylene carbonate, which failed to form sufficient steric hindrance at the gas-liquid interface. The surface tension of the pore liquid did not drop below the safe threshold, resulting in microfracture of the solid skeleton under the action of drying shrinkage stress. The example retained the pre-placed long-chain polyether and the in-situ generated small molecule alcohol, which constituted a binary asymmetric tension reduction and alleviated the capillary shrinkage stress. Comparative Example 4 used propylene glycol directly to replace propylene carbonate, which lacked the kinetic process of slow release triggering and in-situ generation. The number of microcracks was 9 per 10 cm², and the area of ​​interlayer peeling accounted for 4.2%, indicating that the tension reduction and cross-linking effect of direct mixing was not as stable as that of reaction coupling in-situ generation.

[0114] Test Example 4: A standard rectangular wax mold with dimensions of 120mm×20mm×6mm was selected. According to the corresponding process parameters of Examples 1 to 5 and Comparative Examples 1 to 5, the alternating coating, sanding and curing operations of the surface layer and the back layer were completed to prepare a shell sample with a total of 6 layers. After the dewaxing stage, the sides of the shell sample were ground flat with a grinding wheel to remove edge burrs and irregular protrusions, and a sheet-like test strip for the standard three-point bending test was obtained. 20 parallel samples were prepared for each group.

[0115] Each group of samples was randomly divided into two batches. The first batch of samples, after dewaxing and without high-temperature calcination, was placed directly on a universal testing machine for room temperature rupture modulus testing. The test adopted the three-point bending method, with a span of 50 mm and a loading rate of 1.5 mm / min. The maximum load at which the sample broke was recorded, and the bending strength value was calculated according to the standard formula.

[0116] The second batch of samples was transferred into a box-type resistance furnace and heated to 1000℃ at a rate of 8℃ / min and kept at that temperature for 2 hours. After cooling to room temperature with the furnace, the high-temperature calcination flexural strength of each group of samples was tested and recorded using the same equipment parameters and calculation formulas as in the second stage.

[0117] Discrete values ​​with a deviation exceeding 15% in each data set are removed, and the arithmetic mean of the remaining valid data is taken as the final intensity characterization result for that group.

[0118] Table 4: Flexural strength test data of each shell group in the dewaxed state at room temperature and the calcined state at high temperature.

[0119] Test group Moisture strength at room temperature (MPa) High-temperature roasting strength (MPa) Example 1 4.82 8.15 Example 2 4.31 7.64 Example 3 4.97 8.36 Example 4 5.12 8.82 Example 5 4.65 7.89 Comparative Example 1 3.94 4.21 Comparative Example 2 2.15 3.58 Comparative Example 3 3.12 4.86 Comparative Example 4 3.65 5.34 Comparative Example 5 1.83 2.94

[0120] According to the data in Table 4, the shells in the example groups exhibited higher structural mechanical properties than the comparative examples in both the room temperature curing stage and the high temperature calcination stage. Comparative Example 1 used styrene-butadiene latex as a reinforcing modifier, and its room temperature wet strength was 3.94 MPa, mainly relying on the flexible organic liquid film formed between the polymer particles to provide initial adhesion. The room temperature wet strength of Example 1 reached 4.82 MPa, without the addition of any organic polymer macromolecules. The establishment of this strength was entirely attributed to the decomplexation of zirconium ammonium carbonate induced by the hydrolysis of propylene carbonate, releasing activated zirconium hydroxyl groups. The silanol on the surface of the silica sol undergoes dehydration and condensation, constructing an isotropic SiOZr inorganic three-dimensional cross-linked network inside the coating. In Comparative Example 2, due to the absence of propylene carbonate, the cross-linking reaction is limited to the epidermal layer, and there is a lack of effective chemical bonding between the internal solid particles. This is macroscopically manifested as a sharp decrease in room temperature wet strength to 2.15 MPa. Comparative Example 5 eliminated the initial high-humidity water retention stage, and the rapid skinning of the coating surface led to incomplete development of the internal matrix network. Its room temperature strength was only 1.83 MPa, confirming the decisive role of the homogeneous cross-linking mechanism in the overall rigid molding of the shell.

[0121] After calcination at 1000℃, the microstructure of the silica sol shell underwent thermodynamic reconstruction. The high-temperature calcination strength of Comparative Example 1 was only 4.21 MPa, showing a minimal increase compared to its room-temperature state. Styrene-butadiene latex underwent oxidative pyrolysis and carbonization volatilization at temperatures above 400℃, transforming the space originally occupied by the polymer into micropores, disrupting the continuity of the inorganic framework and offsetting the strength increase brought about by the sintering of silica itself. The high-temperature calcination strengths of Examples 1 to 5 generally jumped to the range of 7.64 MPa to 8.82 MPa. The introduced ammonium zirconium carbonate underwent phase transformation decomposition during high-temperature calcination, generating nanoscale zirconium oxide crystals in situ. These high-surface-energy zirconium oxide particles were dispersed in the amorphous silica... In the silicon matrix, the inorganic precursor network plays a dual role of physical filling and phase transformation strengthening under high-temperature solid-state reaction, promoting further densification of the silicon-oxygen skeleton. Comparative Example 3 did not add polyethylene glycol-400, while Comparative Example 4 used the method of directly adding propylene glycol. During the room temperature drying stage, both failed to effectively establish binary synergistic tension reduction, and micro-stress cracks had formed inside the rigid skeleton. These pre-existing micro-defects became stress concentration sources during high-temperature testing, resulting in calcination strengths of only 4.86 MPa and 5.34 MPa, respectively, which were far lower than the levels of the examples. The change trajectory of the mechanical data proves that the dispersion strengthening mechanism of the inorganic precursor network in this scheme at high temperature can effectively replace the traditional organic latex and avoid the pyrolysis pore-forming effect.

[0122] Test Example 5: Collect shell fragments without cracks from Examples 1, 3, 4, and Comparative Examples 1, 2, and 4, which were calcined at 1000°C for 2 hours and cooled to room temperature. Take a test block with a mass of about 50g from each group, clean it in anhydrous ethanol for 15 minutes using an ultrasonic cleaner to remove dust adhering to the surface, and dry it in a drying oven at 105°C until constant weight. Record the dry weight.

[0123] The apparent porosity of the shell samples was determined using the Archimedes vacuum drainage method. The dried samples were placed in a vacuum desiccator and evacuated until the residual pressure was below 2.5 kPa and maintained for 30 minutes. Distilled water was slowly poured in until the samples were completely submerged, and the vacuum was maintained for another 20 minutes. After the vacuum was released, the samples were allowed to stand in the air for 1 hour. The suspended weight of the samples in water and the wet weight in the air after wiping off the surface water with multiple layers of moist gauze were weighed using a hydrostatic balance. The apparent porosity of each group of shell samples was calculated based on the test data.

[0124] Using the shell-making process parameters of the above embodiments and comparative examples, standard medium-temperature wax mold components for stepped test blocks were made. The coating, sanding, dewaxing and 1000℃ baking processes were completed to prepare a complete shell. 304 stainless steel alloy was melted in a medium-frequency induction melting furnace. When the temperature of the molten steel reached 1580℃, it was poured into the shell which was in a red-hot state of 850℃.

[0125] After the casting cools naturally to room temperature with the mold shell, it undergoes mechanical vibration to remove the shell. The casting is then placed in a shot blasting machine, and 0.3mm stainless steel shot is used to clean the residual shell on the surface using the same process parameters. Using a stylus-type surface roughness measuring instrument, five non-overlapping measurement areas are selected on the bottom surface of the thickest section of the stepped test block of the casting. The sampling length is set to 2.5mm, and the surface roughness Ra value is measured. After removing abnormal fluctuation data, the arithmetic mean is taken as the surface roughness characterization data of this group of castings.

[0126] Table 5: Test data on apparent porosity of shell in sintered state and surface roughness of 304 stainless steel castings

[0127] Test group Shell porosity (%) Surface roughness Ra (μm) of casting Example 1 18.42 2.15 Example 3 19.17 2.32 Example 4 17.85 1.98 Comparative Example 1 26.73 4.86 Comparative Example 2 28.31 5.42 Comparative Example 4 22.68 3.71

[0128] According to the data in Table 5, the inorganic crosslinking and in-situ sintering system of the present invention forms a shell inner surface layer with higher density after high-temperature calcination. Comparative Example 1 uses conventional styrene-butadiene latex as a binder modifier, and its shell porosity reaches 26.73%, with a corresponding casting surface roughness Ra value of 4.86 μm. The macromolecular organic polymer undergoes oxidative decomposition during the high-temperature calcination stage from 400°C to 600°C. After the carbon chain breaks and volatilizes, it leaves foaming pores in the silica skeleton matrix. These through pores reduce the overall density of the shell. During high-temperature molten steel pouring, the capillary effect promotes the penetration of liquid metal and metal oxides into the shell interior, resulting in microscopic mechanical and chemical sand adhesion reactions, which leads to a higher surface roughness of the casting after cleaning.

[0129] In Example 1, the porosity of the mold shell was reduced to 18.42%, and the surface roughness Ra of the casting was 2.15 μm. The polyethylene glycol-400 and propylene glycol generated by the hydrolysis of propylene carbonate in the system are both low molecular weight carbon oxides. They vaporize and volatilize in the early stage of heating and calcination, without carbon residue or large-volume pyrolysis pore formation. The zirconium ammonium carbonate component combined in the silicon-oxygen framework undergoes thermal decomposition above 800℃, generating nanoscale zirconium oxide grains in situ. The nano-zirconia in the high-energy state fills the nanoscale pores generated when silica and silica condense with each other in a physical dimension. On the other hand, it undergoes solid-phase sintering with the surrounding amorphous silica at a high temperature of 1000℃, promoting material migration and densification at the interface. This inorganic in-situ strengthening mechanism blocks the penetration path of high-temperature molten steel into the depth of the mold shell pore network.

[0130] Comparative Example 2, lacking homogeneous cross-linking triggering from propylene carbonate, relied solely on surface dehydration for molding. Its internal skeleton was loosely arranged, exhibiting a porosity as high as 28.31% and a roughness Ra value increasing to 5.42 μm. This reflects the more severe pore enlargement phenomenon caused by the incompletely cross-linked silica-oxygen network during high-temperature shrinkage. Comparative Example 4, using direct addition of propylene glycol to replace propylene carbonate hydrolysis, had a porosity of 22.68% and an Ra value of 3.71 μm. While the direct introduction of small-molecule alcohol partially reduced tension, it failed to participate in constructing the three-dimensional co-directional cross-linked network induced by the slow release of propylene carbonate. Lacking an endogenously triggered, the well-structured skeleton exhibited inferior resistance to sintering shrinkage and densification compared to the coupled reaction system of this invention during high-temperature reconstruction. The data confirms that this invention, through in-situ chemical evolution of inorganic precursors, possesses engineering practicality in controlling shell porosity and improving the surface finish of the final casting.

[0131] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A casting mold for silica sol, characterized in that, It is made from raw materials comprising the following parts by weight: alkaline silica sol, 1000 parts; refractory powder, 1500-2500 parts; polyethylene glycol-400, 2-8 parts; Aqueous solution of zirconium carbonate, 5-15 parts; propylene carbonate, 10-20 parts.

2. A silica sol casting mold according to claim 1, characterized in that, The alkaline silica sol is mixed with polyethylene glycol-400 and an aqueous solution of zirconium ammonium carbonate to form a highly stable crosslinking precursor silica sol.

3. A silica sol casting mold according to claim 1, characterized in that, The refractory powder is divided into surface layer powder and back layer powder; the surface layer powder is zircon powder or corundum powder with a particle size of 320 mesh; the back layer powder is mullite powder or corundum powder with a particle size of 80 mesh.

4. A silica sol casting mold according to claim 2, characterized in that, The preparation method of the highly stable crosslinking precursor silica sol includes the following steps: A. Add the alkaline silica sol to a reaction vessel and disperse the refractory powder at a high shear speed of 800-1200 rpm at 20-26°C for 15-20 minutes. Then reduce the speed to 100-300 rpm and slowly add the polyethylene glycol-400 and zirconium ammonium carbonate aqueous solution dropwise in sequence. Continue stirring for 20-30 minutes to obtain a highly stable crosslinking precursor silica sol. B. 15-25 minutes before the planned module coating operation, add the propylene carbonate to the highly stable crosslinking precursor silica sol at a low shear speed of 30-60 rpm, and mix for 10-15 minutes to obtain the final product.

5. A silica sol investment casting process, characterized in that, The method for shell fabrication using silica sol casting as described in any one of claims 1-4 includes the following steps: S1. Immerse the wax mold assembly into the surface working silica sol for single-layer coating and slurry control, and then evenly sprinkle the surface sand material on the silica sol surface. S2. Send the module into an environmentally controlled drying chamber and perform two-stage dynamic temperature and humidity induced curing to dry and cure the single layer. S3. Immerse the module after the single-layer curing into the back layer working silica sol for coating and slurry control, sprinkle back layer sanding material, and perform the same two-stage dynamic temperature and humidity induced curing as S2. Repeat this process several times to build the back layer layer by layer. The last time, only coating is done without sanding to obtain the shell. S4. The completely dried shell is sent into a high-pressure steam dewaxing kettle for dewaxing treatment; S5. The dewaxed shell is transferred into a baking furnace for high-temperature constant-temperature baking, and after cooling in the furnace, the final silica sol investment casting shell is obtained.

6. The silica sol investment casting process according to claim 5, characterized in that, The specific implementation method of the two-stage dynamic temperature and humidity induced curing described in S2 and S3 is as follows: The first stage involves maintaining a high humidity and low wind speed environment in the drying chamber to inhibit the evaporation and crusting of moisture on the coating surface, and to facilitate homogeneous cross-linking by the hydrolysis of propylene carbonate inside the coating to produce acid. The second stage involves maintaining a constant ambient temperature in the drying chamber, gradually reducing the relative humidity at a linear rate, and simultaneously increasing the ambient wind speed to remove moisture from deep pores.

7. The silica sol investment casting process according to claim 6, characterized in that, The process parameters for the two-stage dynamic temperature and humidity induced curing are as follows: In the first stage, the ambient temperature is controlled at 22-26℃, the relative humidity is controlled at 80%-90%, the ambient wind speed is controlled at 0.3-0.8m / s, and the duration is 25-40 minutes. In the second stage, the ambient temperature is kept constant at 22-26℃, and the relative humidity is reduced to 35%-45% at a linear rate over a period of 50-70 minutes, while the ambient wind speed is smoothly increased to 2.0-3.0 m / s.

8. The silica sol investment casting process according to claim 5, characterized in that, The surface sand material mentioned in S1 is 200-mesh mullite sand or corundum sand; the back sand material mentioned in S3 is 60-mesh mullite sand or corundum sand.

9. The silica sol investment casting process according to claim 5, characterized in that, In S4, the process parameters for the dewaxing process are: The steam pressure is set to 0.5-0.8 MPa, the dewaxing temperature is controlled at 150-170℃, and the pressure holding time is 10-20 minutes.

10. The silica sol investment casting process according to claim 5, characterized in that, In S5, the process parameters for the high-temperature constant-temperature calcination are as follows: Heat to 850-1050℃ at a heating rate of 5-10℃ / min, and calcine at a constant temperature for 1-3 hours.

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