Lost foam casting process

Abstract

The invention relates to the technical field of evanescent mode casting, and discloses an evanescent mode casting process which comprises the following steps: adopting a closed pouring system; setting a pouring temperature; a bottom pouring type pouring system is adopted; a furan resin self-hardening sand core is embedded in the part difficult to fill sand, after a process hole with the diameter smaller than or equal to 50 mm is formed, filling is conducted through an EPS block, sealing is conducted through a high-temperature-resistant adhesive tape, and compaction is assisted through vibration compaction equipment during sand filling; through multi-process collaborative optimization, a full-process controllable lost foam casting system is constructed, the casting density is improved compared with a traditional process, and the tensile strength is improved; the rejection rate is greatly reduced, and the production cycle is obviously shortened; and the dry sand recycling rate is high, and the comprehensive cost is reduced. According to the technical scheme, the problems of discontinuous mold filling, gas retention, dry sand collapse and the like in lost foam casting are solved, and a standardized solution is provided for industrial production of high-end castings.

Description

Technical Field

[0001] This invention relates to the field of lost foam casting technology, specifically a lost foam casting process. Background Technology

[0002] Casting, as one of the core processes in metal forming, is widely used in machinery manufacturing, aerospace, and other fields. Traditional sand casting relies on parting line molding and core assembly, which has inherent drawbacks such as complex molding processes, high mold wear, and low dimensional accuracy of castings. Especially for complex structural parts, multiple core assembly is required, leading to a 30%-50% increase in production cycle time, and defects such as sand inclusions and porosity are easily generated due to sand mold collapse. While metal mold casting can improve efficiency, the high cost of molds makes it difficult to meet the needs of small-batch, multi-variety production. With the upgrading of high-end manufacturing industries' demands for lightweight and complex castings, the limitations of traditional casting processes in terms of precision, efficiency, and cost control are becoming increasingly prominent.

[0003] Lost foam casting (EPC) achieves "moldless casting" through the vaporization of a foam model, offering significant advantages such as eliminating the need for mold removal, reducing tooling, and suitability for forming complex structural parts. Its process principle involves using a negative pressure sand mold to fix the foam model; as molten metal fills the mold, the foam vaporizes simultaneously, forming a cavity in the casting. Compared to traditional processes, this reduces molding steps by 40%, and improves the dimensional accuracy of castings to CT6-CT7 level. However, existing lost foam casting still faces several challenges in industrial applications: Conflict between filling and gas control: Open gating systems are prone to discontinuous filling of molten metal. If the gas generated by EPS vaporization (about 1000-1500L / kg) cannot be discharged in time, it will form a gas film at the front of the molten metal, causing cold shut defects.

[0004] Insufficient high-temperature performance of dry sand: When conventional dry sand is washed by molten metal at temperatures above 1300℃, the high-temperature strength retention rate is less than 50%, and the sand particles collapse, resulting in sand inclusion defects.

[0005] Low feeding efficiency: Traditional riser design does not take into account the vacuum effect under negative pressure environment. A negative pressure zone is easily formed at the top of the riser, reducing the feeding efficiency by 30%-40%, and causing frequent shrinkage defects at hot joints.

[0006] Existing technologies attempt to improve defects by adjusting negative pressure parameters or optimizing riser shape, but no systematic solution has been formed. For example, simply increasing the negative pressure to above 600 mmHg can accelerate gas discharge, but it will exacerbate model deformation; adding ordinary silica sol can improve the strength of dry sand, but due to the large particle size (D50>5μm), the adhesive film is prone to cracking at high temperatures, and the improvement in anti-collapse is limited.

[0007] To address the aforementioned issues, this invention proposes a solution: optimizing the gating system ratio through fluid dynamics modeling, enhancing dry sand performance using nanocomposite technology, and combining negative pressure-riser synergistic control to reconstruct the lost foam casting process system from three dimensions: filling stability, gas discharge efficiency, and dry sand strength, thereby achieving a systematic improvement in casting quality. Summary of the Invention

[0008] To address the problems in the prior art, this invention provides a lost foam casting process.

[0009] The technical solution adopted by this invention to solve its technical problem is: a lost foam casting process, comprising: A closed gating system is adopted. For cast iron parts, the cross-sectional area ratio of the sprue, runner, and ingate is 1.1:1.2:1.4; for cast steel parts, the cross-sectional area ratio is 1:1.1:1.2-1.4. Control negative pressure range: 400-500 mmHg for cast steel, 300-400 mmHg for cast iron, and 50-100 mmHg for cast aluminum, maintain for 5 minutes; Set the pouring temperature: Cast steel 1450-1700℃, ductile iron 1380-1450℃, gray cast iron 1360-1420℃, copper alloy 1150-1200℃, aluminum alloy 700-750℃. A cylindrical or spherical hidden riser is installed above or beside the hot spot of the casting. The riser is connected to the atmosphere through a ceramic tube, and the riser feeding distance covers the hot spot and the end area. A bottom-pouring gating system is adopted, with the ingate set at the bottom of the casting. It is combined with a hollow sprue with an inner diameter of 30-80mm. The pressure head of the molten metal is controlled to exceed the gas pressure at the EPS interface by the formula Hp>P_gas / (ρg). Where Hp is the pressure head height, P_gas is the EPS interface gas pressure, ρ is the density of the molten metal, and g is the acceleration due to gravity. Furan resin self-hardening sand cores are embedded in areas where sand is difficult to fill. After opening process holes with a diameter of ≤50mm, EPS blocks are used to fill the holes and high-temperature resistant tape is used to seal them. During sand filling, a vibration compaction device is used to assist in compaction.

[0010] As a further technical solution, the formula for calculating the cross-sectional area of ​​the ingate is: Finner = g / (0.31ut√Hp); Where Finner is the sum of the cross-sectional areas of the ingate (mm²), g is the total weight of the liquid metal (kg), u is the flow coefficient (0.3-0.4), t is the pouring time (s, t=0.85×√g for negative pressure pouring, g is the weight of the casting), and Hp is the pressure head height (mm).

[0011] As a further technical solution, the formula for calculating the distance between the sprue and the mold body is: S=K+αg+βH; Where S is the distance (mm), K is a constant (120mm for cast iron, 140mm for cast steel), α=0.08mm / kg, g is the weight of the casting (kg), β=0.06, and H is the height of the casting (mm).

[0012] As a further technical solution, the molding dry sand is as follows: for ferrous metal parts, AFS40-60 grit sand, round or semi-polygonal sand, temperature <50℃, silica content >85%, and loss on ignition <0.3%; for cast aluminum parts, AFS50-100 grit sand is used.

[0013] As a further technical solution, the pretreatment method of the molding dry sand includes: drying the dry sand, with the drying temperature controlled at 60-80℃ and the drying time at 1-2 hours.

[0014] As a further technical solution, the drying process ensures that the moisture content of the dried sand is ≤0.1%.

[0015] As a further technical solution, the sieved dry sand is treated with magnetic separation equipment to remove iron, with a magnetic field strength of 160-200mT, to remove ferromagnetic impurities.

[0016] As a further technical solution, 0.8-1.0% by mass of modified silica sol is added as an additive to the dry sand after magnetic separation, and the mixture is stirred evenly and then left to stand for 10-15 minutes.

[0017] As a further technical solution, the modified silica sol is prepared by mixing silica sol with a solid content of 25-30% with methyltrimethoxysilane at a mass ratio of 10-15:1, adding nano-titanium dioxide particles accounting for 0.4-0.6% of the silica sol mass, reacting at 55-70℃ with a stirring speed of 220-300 rpm for 2-3 hours, adjusting the pH value to 8-9, and removing methanol by vacuum distillation to obtain the modified silica sol.

[0018] As a further technical solution, ultrasonic-assisted treatment is introduced during the preparation of the modified silica sol. The ultrasonic power is 400-500W and the frequency is 40-60kHz. The ultrasonic treatment continues for 30-45 minutes after the reaction begins. After the entire reaction is completed, the mixture is filtered through a 0.22μm membrane and spray-dried to produce a powdered additive. The inlet air temperature for spray drying is 180-200℃, and the atomization pressure is 0.3-0.5MPa.

[0019] The beneficial effects of this invention are: This invention provides a lost foam casting process, which has the following multiple advantages: I. The role of closed-loop gating system in optimizing mold filling By employing a closed gating system and optimizing the runner ratio (sprue:gate:ingate = 1.1:1.2:1.4 for cast iron, and 1:1.1:1.2 for cast steel), continuous flow of molten metal during the molding process is ensured, avoiding the flow interruption problems caused by flow rate fluctuations in traditional open systems. Through scientific design of the ingate cross-sectional area and the distance between the sprue and the mold body, the molten metal pressure head can effectively exceed the EPS interface gas pressure, suppressing arcing defects.

[0020] II. Negative pressure and riser synergistic feeding mechanism Precise control of the negative pressure range (400-500 mmHg for cast steel, 300-400 mmHg for cast iron) and maintenance for 5 minutes, combined with a cylindrical / spherical concealed riser at the hot spot (connected to the atmosphere via a ceramic tube), creates a directional temperature field. The negative pressure environment accelerates the discharge of EPS vaporized gases, while the riser eliminates shrinkage porosity at the hot spot through feeding. Together, these two factors increase the casting density to over 99.6%. Compared to traditional risers, this design improves feeding efficiency and reduces shrinkage porosity in the hot spot area, showing particularly significant effects on materials with high shrinkage rates, such as cast steel.

[0021] III. Performance Enhancement of Dry Sand Pretreatment Process After drying (moisture content ≤0.1%), magnetic separation for iron removal, and treatment with modified silica sol, the high-temperature resistance to collapse of the molding sand is significantly improved. The modified silica sol, through silane coupling and nanocomposite technology, forms a high-temperature resistant adhesive film on the sand grain surface, enabling the dry sand to retain 82% of its strength at 1300℃, effectively preventing sand grain detachment caused by molten metal erosion. Simultaneously, optimized air permeability of the dry sand improves gas discharge efficiency, reduces porosity defects, and lowers the surface roughness Ra of the casting.

[0022] IV. Forming Guarantee of Sand Filling and Compaction Process Self-hardening sand cores are embedded in difficult-to-fill sand areas, and process holes are opened to fill EPS. Combined with vibration compaction technology (amplitude 0.5-1mm, frequency 50-60Hz), the sand filling problem of complex structural parts is solved. This process ensures that the overall compactness of the sand mold is ≥90%, and the top sand intake is <50mm, avoiding model deformation and sand mold collapse. It significantly improves the molding accuracy, especially for long rods and thin-walled parts, and the dimensional tolerance can be controlled within ±0.5mm.

[0023] This invention constructs a fully controllable lost foam casting system through multi-process synergistic optimization: the density of castings is improved compared to traditional processes, and the tensile strength is increased; the scrap rate is significantly reduced, and the production cycle is significantly shortened; the dry sand recycling rate is high, and the overall cost is reduced. The technical solution of this invention overcomes the problems of discontinuous filling, gas retention, and dry sand collapse in lost foam casting, providing a standardized solution for the industrial production of high-end castings. Detailed Implementation

[0024] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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.

[0025] This invention provides a lost foam casting process, comprising: (a) Standardized design of the gating system For closed-loop gating systems, the gating cross-sectional area of ​​cast iron parts is controlled using a ratio of sprue:gating:ingate = 1.1:1.2:1.4. This ratio, derived through fluid dynamics calculations, ensures continuous flow of molten metal during the casting process. Taking HT300 castings as an example, this ratio stabilizes the molten metal filling speed at 0.8-1.2 m / s. Actual measurements show that this ratio reduces the incidence of cold shut defects in castings by over 80%. For cast steel parts, the gating cross-sectional area ratio is 1:1.1:1.2. Through a gradient reduction design, the temperature loss during molten steel filling can be controlled within 50-80℃. In a ZG35CrMo bearing housing casting, using this ratio, the temperature difference at the end of the gating is ≤30℃, effectively preventing incomplete filling caused by uneven temperature.

[0026] The precise calculation of the ingate cross-sectional area uses a hydraulic formula: Finner = g / (0.31ut√Hp); where G is the total weight of the liquid metal (kg), including the weight of the casting and the gating system; u is the flow coefficient, taken as 0.35; t is the pouring time (s), and for negative pressure pouring, t = 0.85 × √G; Hp is the pressure head height (mm), which must satisfy Hp > Pgas / (ρg), where Pgas is taken as 2000 Pa, and ρ is the density of the liquid metal (7.3 g / cm³ for cast iron). Taking a 50kg gray cast iron part as an example: t = 6.01s, Hp = 200mm, Finner = 32.5mm 2 The design incorporates four ingates, each with a cross-sectional area of ​​8.125 mm². During filling, the flow rate of the molten metal fluctuates by ≤5%, ensuring smooth filling.

[0027] The distance between the sprue and the mold body is calculated using the formula: S = K + αg + βH; for cast iron, K = 120mm, for cast steel, K = 140mm, α = 0.08mm / kg, β = 0.06. For a 50kg gray cast iron part (300mm high), the calculated S = 120 + 0.08 × 50 + 0.06 × 300 = 142mm. Actual measurements at this distance show that the model surface temperature is ≤60℃, with no softening or deformation, ensuring the model's dimensional accuracy.

[0028] (ii) Synergistic control of negative pressure and temperature Table 1 Negative Pressure Parameter System

[0029] When the cast steel parts are subjected to a negative pressure of 450 mmHg, the exhaust speed of EPS vaporization gas can reach 0.5 m / s. According to actual measurements by a gas flow meter, this speed can ensure that the gas pressure in the cavity is always lower than the molten metal pressure, effectively avoiding the defect of flameout.

[0030] The pouring temperature specifications for different materials are as follows: Cast steel: 1450-1700℃, commonly used 1550℃, heated by medium frequency induction furnace, heating rate 10℃ / min, holding for 30min to ensure uniform temperature, temperature measuring instrument accuracy ±5℃; Gray cast iron: 1360-1420℃, commonly 1380℃, add 0.5% ferrosilicon inoculant before tapping, inoculation time 5min; Aluminum alloy: 700-750℃, commonly 720℃, heated by resistance furnace, covered with chloride salt to prevent oxidation, slag removal ≥ 3 times.

[0031] (III) Integrated Design of Riser and Gating System Riser Function and Structure: The riser adopts a cylindrical or spherical concealed riser with a diameter of 50-100mm, connected to the atmosphere through an 8-12mm ceramic tube. Taking a Φ50mm hot section as an example, it is equipped with a Φ80mm cylindrical riser, with the feeding distance covering a 100mm range around the hot section. Infrared thermometer monitoring shows that the temperature in the riser area is 30-50℃ higher than that of the hot section, achieving directional solidification and reducing the shrinkage porosity defect rate by more than 90%.

[0032] The bottom-pouring gating system features an ingate located at the bottom of the casting, complemented by a 30-80mm inner diameter hollow sprue. For an 80kg steel casting using a Φ60mm hollow sprue, the filling time is 7.6s, the molten metal flow velocity at the front edge is 1.1m / s, and observation using a high-speed camera shows no air entrapment during the filling process. The slag inclusion defect rate is reduced by 60% compared to the top-pouring system.

[0033] (iv) Pretreatment process of dry sand for molding Table 2 Performance Requirements for Dry Sand Foundation

[0034] The dry sand needs to be washed and screened, with round sand particles accounting for ≥80%. The air permeability index of AFS40-60 sand is ≥150, which is measured by an air permeability tester and meets the requirements of lost foam casting.

[0035] Preprocessing process control Drying treatment: A hot air circulating oven was used to dry the sand particles at 60-80℃ for 1.5 hours, with a heating rate of 5℃ / min. The moisture content was ≤0.1%. After drying, the water film thickness on the surface of the sand particles was <1μm, and the moisture content was measured by a Karl Fischer moisture analyzer with an error of ≤0.01%. Magnetic separation for iron removal: Using a drum-type magnetic separator with a magnetic field strength of 180mT and a drum speed of 20r / min, the iron removal efficiency reaches over 95%, and the content of ferromagnetic impurities is <0.05%; Preparation of modified silica sol: Raw material ratio: 12kg of silica sol with 25% solid content, 1kg of methyltrimethoxysilane, and 60g of nano titanium dioxide; Reaction conditions: 60℃, stirring at 250 rpm for 2.5 h, pH adjusted to 8.5 with 25% ammonia; Ultrasonic treatment: 450W power, 50kHz frequency, 35min treatment, ultrasonic amplitude 5μm; Post-processing: methanol removal by vacuum distillation (vacuum degree -0.09MPa, temperature 65℃), filtration through a 0.22μm polyvinylidene fluoride membrane, and powdering by spray drying (inlet air 190℃, atomization pressure 0.4MPa), yield approximately 85%; Addition process: 0.9% modified silica sol by weight of dry sand, stir at 60 r / min for 15 min in a drum mixer, let stand for 12 min, and the uniformity error of the sand mixing is ≤3%.

[0036] (v) Standardized operation of sand filling and compaction Sand bed preparation and sand filling process: The bottom of the sand box is pre-filled with 100-150mm of dry sand, which is then pre-compacted using a ZP-100 type plate vibrator at a vibration frequency of 50Hz and an amplitude of 1mm, achieving a compaction degree of over 85%. When filling sand for long rod models, a symmetrical sand filling method is used, with a filling speed ≤50mm / s, and the deformation of a 500mm long rod model <0.5mm.

[0037] For areas difficult to fill with sand, complex internal cavities are treated by embedding furan resin self-hardening sand cores. The resin dosage is 1.5% of the sand weight, and the curing agent is 3% of the resin weight. The curing time is 20 minutes. Process holes ≤50mm are made, filled with EPS blocks, and then sealed with 3M high-temperature resistant tape (temperature resistance 300℃). The tensile strength of the tape is ≥20N / cm.

[0038] The vibration compaction parameters were determined using a Z30 three-dimensional vibration table with an amplitude of 0.8 mm and a frequency of 55 Hz. Vibration was performed for 3-5 minutes for every 100 mm of sand added. The top sand intake was controlled at 40-50 mm. The overall compaction of the sand mold was tested using a compaction tester. The overall compaction was ≥90%, and the difference in compaction between upper and lower layers was ≤5%.

[0039] The following are specific examples: Example 1: Gray cast iron part (HT300 gearbox, G=50kg, H=300mm) 1. Casting and gating system parameters The hot spot is located at the flange in the middle of the gearbox and has a diameter of 50mm. The total area of ​​the ingate is 32.5 mm², divided into 4 runners, each with an area of ​​8.125 mm², and they are symmetrically distributed. The distance between the sprue and the mold body is 142mm. The sprue is Φ50mm hollow and 500mm high.

[0040] 2. Process parameters Negative pressure 350 mmHg, maintained for 5 minutes, air pump flow rate 150 m³ / h; The pouring temperature was 1380℃, the pouring time was 6.01s, and the filling speed was 1.0m / s. The riser is a Φ80mm cylindrical blind riser, the ceramic tube is Φ10mm and the height is 100mm.

[0041] 3. Dry sand treatment AFS40-60 dry sand, dried at 70℃ for 1.5 hours, with a moisture content of 0.08%; After magnetic separation, add 1.8 kg of modified silica sol, mix for 15 min, and let stand for 12 min. Vibrate the sand three times during filling, each time for 5 minutes, with a top sand intake of 40mm.

[0042] 4. Molding effect The casting has a surface roughness Ra=6.3μm, X-ray inspection shows no cold shuts or porosity, a density of 99.7%, a tensile strength of 320MPa, and a hardness of HB200, meeting the requirements of GB / T9439-2010.

[0043] Example 2: Cast steel part (ZG35CrMo bearing housing, G=80kg, H=400mm) 1. Key parameters The gating system ratio is 1:1.1:1.2, and the total area of ​​the ingate is 45.2 mm². 2 Divided into 5 strips, each strip is 9.04mm thick. 2 ; The distance between the sprue and the mold body is 170.4mm; the sprue is Φ60mm hollow and 600mm high. The hot spot has a diameter of 60mm and is located at the bearing mounting position.

[0044] 2. Process Control Negative pressure 450 mmHg, maintained for 5 minutes, vacuum degree -0.06 MPa; The pouring temperature was 1550℃. Before unloading, the slag was removed twice and the vessel was allowed to stand for 10 minutes. The riser is a Φ100mm spherical dark riser, the ceramic tube is Φ12mm, and the feeding distance is 120mm.

[0045] 3. Dry sand pretreatment Silica sol: methyltrimethoxysilane = 15:1, nano titanium dioxide 90g, reaction at 70℃ for 2h; Ultrasonic power 500W, frequency 40kHz, treatment time 45min; Spray drying with an inlet air temperature of 200℃ and an atomization pressure of 0.5MPa yielded 2.7kg of modified silica sol.

[0046] 4. Performance Indicators The casting was tested by UT and found to be defect-free. It has a tensile strength of 780MPa, a yield strength of 480MPa, and an elongation of 1.8%, which meets the GB / T11352-2009 standard.

[0047] Example 3: Aluminum alloy part (ZL102 housing, G=20kg, H=150mm) 1. Gating system design The sprue ratio is 1:1.1:1.2, the total area of ​​the sprue is 18.7 mm², and it is a single-pass sprue. The distance between the sprue and the mold body is 130.6mm; the sprue is Φ30mm hollow and 300mm high. The hot spot has a diameter of 30mm and is located at the boss on the shell.

[0048] 2. Process parameters Negative pressure 80 mmHg, maintained for 5 minutes, air pump flow rate 50 m³ / h; The pouring temperature is 720℃, a funnel-type pouring cup is used, and the slag-blocking screen has a mesh size of 40. The riser is a Φ50mm cylindrical blind riser, the ceramic tube is Φ8mm and the height is 80mm.

[0049] 3. Dry sand treatment AFS50-100 dry sand, dried at 60℃ for 2 hours, with a moisture content of 0.05%; The modified silica sol was added at a rate of 0.8% (1.2 kg), stirred at a speed of 50 r / min, and allowed to stand for 15 min. Vibrate twice during sand filling, 3 minutes each time, with a top sand intake of 30mm.

[0050] 4. Molding Results The casting has a density of 2.68 g / cm³, a density of 99.3%, and no leakage was found during the airtightness test (0.2 MPa water pressure), meeting the requirements of GB / T1173-2013.

[0051] Example 4: Ductile iron part (QT600-3 crankshaft, G=60kg, H=350mm) 1. Key Calculations The total area of ​​the ingate is 36.8 mm², divided into 4 runners, each run having an area of ​​9.2 mm². The distance between the sprue and the mold body is 145.8mm, and the sprue is a Φ55mm hollow structure. The hot spot is 45mm in diameter and is located at the crank pin.

[0052] 2. Process Control Negative pressure 380 mmHg, casting temperature 1420℃, inoculant addition 0.8%; The modified silica sol had a silane ratio of 13:1 and a nano titanium dioxide content of 0.5%, and was reacted at 65°C for 2.5 hours. Ultrasonic power 480W, frequency 55kHz, processing time 38min.

[0053] 3. Performance Testing The spheroidization grade is 3, the number of graphite spheres is 150 / mm², the tensile strength is 620MPa, and the elongation is 3%, which meets the requirements of GB / T1348-2013.

[0054] Example 5: Copper alloy part (ZCuSn10Pb1 bushing, G=40kg, H=250mm) 1. Technological characteristics The gating ratio is 1.1:1.2:1.4, similar to that of cast iron parts, with a total gating area of ​​25.6 mm². The distance between the sprue and the mold body is 138.2mm, and the sprue is a Φ40mm hollow structure. The modified silica sol had a silane ratio of 14:1 and 0.55% nano-titanium dioxide, and was reacted at 68℃ for 2.2h.

[0055] 2. Molding effect The casting has a hardness of HB170, a pearlite content of ≥70% in its metallographic structure, no shrinkage defects, and a wear resistance test (load 500N, friction speed 2m / s) with a wear amount ≤0.05mm / h.

[0056] The following are comparative examples. Comparative Example 1: Open gating system (cast iron part, comparative example 1) 1. Differences in process The gating ratio is 1.4:1.2:1 (open type). The cross-sectional area of ​​the ingate is empirically taken as 50 mm², but not calculated. Atmospheric pressure casting, without a hollow sprue, the distance between the sprue and the mold body is 100mm; The dry sand was not dried, with a moisture content of 0.5%, and was not magnetically separated or modified with silica sol. No vibration was used during sand filling, and the top sand intake was 80mm.

[0057] 2. Defect Results There are 5 cold shuts on the upper surface of the casting, the longest being 8 mm, and the depth of the cold shut is 2 - 3 mm; 8 gas holes are detected by X-ray, the maximum diameter is 3 mm, and the density is 96.6%; The tensile strength is 250 MPa, a 21.9% decrease compared to Example 1, and obvious gas hole defects can be seen on the fracture surface.

[0058] 3. Cause analysis Open pouring leads to the interruption of the filling of the molten metal, the flow rate fluctuates by up to \30%, and the temperature of the front molten metal drops by more than 50 °C, forming cold shuts; under normal pressure, the gasified gas of EPS stays, forming gas holes; the high moisture content of the dry sand results in poor air permeability, and the gas discharge efficiency is reduced by 40%, further exacerbating the defects.

[0059] Comparative Example 2: Insufficient negative pressure (cast steel part, compared with Example 2) 1. Process differences The negative pressure is 200 mmHg, maintained for 3 min, and the distance between the sprue and the mold body is 100 mm; No ceramic tube is set in the riser, and the feeding distance is less than 80 mm; The dry sand is not subjected to magnetic separation, the addition amount of modified silica sol is 0.5%, and no ultrasonic treatment is carried out; The sand filling compactness is ≤ 80%, and the top sand allowance is 30 mm. [[ID=​​​​​​​​​​​​​​​​​​​​​​​​​​​

[0063] As can be seen from Table 3, in the embodiment, due to the closed casting and negative pressure maintenance, EPS gas was continuously extracted during the filling of the molten metal mold, and the modified silica sol improved the air permeability of the dry sand, resulting in a defect rate of 0. Comparative Example 1: Open casting caused the molten metal to stop flowing, and the gas formed a high-pressure zone in the cavity under normal pressure, resulting in a significant increase in the number of cold shuts and pores. In Comparative Example 2, insufficient negative pressure resulted in incomplete gas discharge, and the riser was not connected to the atmosphere to form a vacuum, leading to shrinkage defects.

[0064] Test 2: Dry Sand Collapse Resistance Test (Refer to GB / T2684-2014) Prepare a Φ50mm×50mm dry sand sample and hold it in a 1300℃ high-temperature furnace for 30 minutes. The compressive strength retention rate of the sample was determined after cooling. Table 4

[0065] As can be seen from Table 4, the dry sand in the example, due to the formation of a nanocomposite network structure by the modified silica sol, still maintains more than 80% of its strength at high temperatures, effectively resisting the erosion of molten metal. The comparative example without the addition of modified silica sol showed that the strength retention rate of the dry sand after high temperature was less than 50%, and sand particles were prone to falling off, resulting in sand inclusion defects. Nano-titanium dioxide in modified silica sol enhances the high-temperature oxidation resistance of dry sand.

[0066] Experiment 3: Mechanical Properties Testing of Castings (GB / T228.1-2010) Prepare a standard tensile specimen with a diameter of 10 mm and a length of 50 mm at a tensile speed of 5 mm / min. Three samples were tested in each group, and the average value was taken. Table 5

[0067] As can be seen from Table 5, the castings in the examples have a tensile strength that is 5-10% higher than the standard value and an elongation that meets the standard due to their dense and defect-free internal structure. In Comparative Example 1, cold shuts and pores create stress concentration points, resulting in a 21.9% decrease in tensile strength and a 52% decrease in elongation. The shrinkage porosity defect in Comparative Example 2 resulted in coarse grains in the cast steel parts, and a simultaneous decrease in strength and plasticity, verifying the decisive influence of insufficient negative pressure on the feeding effect.

[0068] 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 lost foam casting process, characterized in that, include: A closed gating system is adopted. For cast iron parts, the cross-sectional area ratio of the sprue, runner, and ingate is 1.1:1.2:1.

4. For cast steel parts, the cross-sectional area ratio is 1:1.1:1.2-1.4; Control negative pressure range: 400-500 mmHg for cast steel, 300-400 mmHg for cast iron, and 50-100 mmHg for cast aluminum, maintain for 5 minutes; Set the pouring temperature: Cast steel 1450-1700℃, ductile iron 1380-1450℃, gray cast iron 1360-1420℃, copper alloy 1150-1200℃, aluminum alloy 700-750℃. A cylindrical or spherical hidden riser is installed above or beside the hot spot of the casting. The riser is connected to the atmosphere through a ceramic tube, and the riser feeding distance covers the hot spot and the end area. A bottom-pouring gating system is adopted, with the ingate set at the bottom of the casting. It is combined with a hollow sprue with an inner diameter of 30-80mm. The pressure head of the molten metal is controlled to exceed the gas pressure at the EPS interface by the formula Hp>P_gas / (ρg). Where Hp is the pressure head height, P_gas is the EPS interface gas pressure, ρ is the density of the molten metal, and g is the acceleration due to gravity. Furan resin self-hardening sand cores are embedded in areas where sand is difficult to fill. After opening process holes with a diameter of ≤50mm, EPS blocks are used to fill the holes and high-temperature resistant tape is used to seal them. During sand filling, a vibration compaction device is used to assist in compaction.

2. The process according to claim 1, characterized in that, The formula for calculating the cross-sectional area of ​​the ingate is: Fingate = g / (0.31ut√Hp); Where Finner is the sum of the cross-sectional areas of the ingate (mm²), g is the total weight of the liquid metal (kg), u is the flow coefficient (0.3-0.4), t is the pouring time (s, t=0.85×√g for negative pressure pouring, g is the weight of the casting), and Hp is the pressure head height (mm).

3. The process according to claim 1, characterized in that, The formula for calculating the distance between the sprue and the mold body is: S = K + αg + βH; Where S is the distance (mm), K is a constant (120mm for cast iron, 140mm for cast steel), α=0.08mm / kg, g is the weight of the casting (kg), β=0.06, and H is the height of the casting (mm).

4. The process according to claim 1, characterized in that, The molding dry sand is as follows: for ferrous metal parts, use AFS40-60 grit sand, round or semi-polygonal, temperature <50℃, silica content >85%, and loss on ignition <0.3%; for cast aluminum parts, use AFS50-100 grit sand.

5. The process according to claim 4, characterized in that, The pretreatment methods for the molding dry sand include: drying the dry sand at a temperature of 60-80℃ for 1-2 hours.

6. The process according to claim 5, characterized in that, The drying process ensures that the moisture content of the dried sand is ≤0.1%.

7. The process according to claim 6, characterized in that, The sieved dry sand is treated with magnetic separation equipment to remove iron, with a magnetic field strength of 160-200mT, to remove ferromagnetic impurities.

8. The process according to claim 7, characterized in that, Add 0.8-1.0% by mass of modified silica sol as an additive to the dry sand after magnetic separation, mix and stir evenly, and let stand for 10-15 minutes.

9. The process according to claim 8, characterized in that, The modified silica sol is prepared by mixing silica sol with a solid content of 25-30% with methyltrimethoxysilane at a mass ratio of 10-15:1, adding nano-titanium dioxide particles accounting for 0.4-0.6% of the silica sol mass, reacting at 55-70℃ with a stirring speed of 220-300 rpm for 2-3 hours, adjusting the pH value to 8-9, and removing methanol by vacuum distillation.

10. The process according to claim 9, characterized in that, The modified silica sol is prepared by introducing ultrasonic-assisted treatment with an ultrasonic power of 400-500W and a frequency of 40-60kHz. The ultrasonic treatment is carried out for 30-45 minutes after the reaction begins. After the entire reaction is completed, the sol is filtered through a 0.22μm membrane and spray-dried to produce a powdered additive. The inlet air temperature for spray drying is 180-200℃, and the atomization pressure is 0.3-0.5MPa.

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