Liquid nitrogen seepage air impact molding method and device for freeze casting

Abstract

The present invention belongs to the field of casting molding technology, and specifically relates to a liquid nitrogen infiltration air-blast molding method and device for freeze casting. Liquid nitrogen is injected into the top of the sand mold before air blasting. Pre-compaction is achieved through a small air blast combined with compaction through a large air blast, and full use is made of the primary air blast and the secondary air flow rebound to uniformly infiltrate the liquid nitrogen into the sand mold. This is conducive to obtaining a casting mold with uniform freezing density and excellent freezing effect after the air blast. The molding time is short and the quality is high. The molding time is shortened by half compared with the existing technology, which can meet the needs of large-scale and mass production and is more conducive to improving the quality of castings.

Description

Technical Field

[0001] The invention relates to the technical field of casting molding, in particular to a liquid nitrogen seepage gas impact molding method and device for freeze casting. Background Art

[0002] In recent years, with the rapid development of the world's industrialization, the demand for the foundry industry has become increasingly greater. Traditional casting uses resin or bentonite, coal powder, etc. as molding sand binders. The working environment is filled with dust, exhaust gas caused by incomplete combustion of resin, etc., and a large amount of industrial waste is generated. It not only pollutes the environment and harms the health of workers, but the cost of subsequent recycling and treatment cannot be underestimated.

[0003] Traditional freeze casting uses liquid nitrogen to freeze a mixture of sand and water. The resulting mold consists of only two materials: sand and water. Automatic sand dropout occurs after pouring, simplifying post-pouring cleanup. This method is simple, environmentally friendly, and cost-effective. While meeting the needs of green development, the method suffers from slow freezing rates and uneven sand mold density, impacting casting quality and making it unsuitable for large-scale, mass production. Summary of the Invention

[0004] In order to solve the problems existing in the prior art, the main purpose of the present invention is to provide a liquid nitrogen infiltration air impact molding method and device for freeze casting.

[0005] To solve the above technical problems, according to one aspect of the present invention, the present invention provides the following technical solutions:

[0006] A liquid nitrogen infiltration air impact molding method for freeze casting comprises the following steps:

[0007] S1. Sand filling;

[0008] S2. Liquid nitrogen is flushed into the sand box;

[0009] S3. Small air pre-compacting; open the small air valve, the compressed air in the compressed air storage chamber enters the sand box, completing the small air pre-compacting;

[0010] S4. Atmospheric pressure compaction: After closing the small pressure valve, open the atmospheric pressure valve, and the high-pressure air in the air tank enters the sand box to complete the atmospheric pressure compaction;

[0011] S5. Take out the pattern and obtain the casting mold.

[0012] As a preferred embodiment of the liquid nitrogen infiltration air blast molding method for freeze casting described in the present invention, in which: in the step S1, a model is placed in a sand box on the mold bottom plate, and molding sand is spread in the sand box. The amount of molding sand depends on the size of the mold. Generally, the filling height of the sand is preferably above 20 cm, and simple vibration is performed to ensure that the molding sand is filled in the deep concave parts of the template and the sand box.

[0013] As a preferred embodiment of the liquid nitrogen infiltration air impact molding method for freeze casting described in the present invention, in step S1, excess molding sand above the auxiliary frame is scraped off by a scraper.

[0014] As a preferred embodiment of the liquid nitrogen infiltration air impact molding method for freeze casting described in the present invention, in step S1, silica sand is used as molding sand, clay is added to increase the adhesion of the molding sand, the moisture content in the molding sand should be maintained at 3-5wt%, and the clay content should be 3-10wt%.

[0015] As a preferred embodiment of the liquid nitrogen infiltration air-blast molding method for freeze casting described in the present invention, in step S2, the small air-blast valve is opened, and liquid nitrogen is flushed into the sand box through the liquid nitrogen flushing inlet, and a heat preservation device is used to ensure that the temperature of the flushed liquid nitrogen is ≤-130°C; the small air-blast valve is closed, and the mixture is left to stand for 2-3 minutes to allow the liquid nitrogen to fully penetrate into the gaps in the molding sand.

[0016] As a preferred embodiment of the liquid nitrogen infiltration air-blast molding method for freeze casting described in the present invention, in which: in the step S2, the mass ratio of liquid nitrogen to molding sand is 0.5-1.0, the liquid nitrogen flow rate is controlled at 20-30 mL / s, and the liquid nitrogen may leave residue in the small air-blast valve, which can be completely discharged through pre-compacting with the small air-blast.

[0017] As a preferred embodiment of the liquid nitrogen infiltration air impact molding method for freeze casting described in the present invention, wherein: in the step S3, before the small air impact pre-compacting, the compressed air with a pressure of 3-15MPa is stored in the compressed air storage chamber; during the small air impact pre-compacting, the pressure increase rate of the compressed air entering the top of the sand box and contacting the molding sand is 100-180MPa / s, and the small air impact pre-compacting is completed in 3-5s of air impact.

[0018] As a preferred embodiment of the liquid nitrogen infiltration air impact molding method for freeze casting described in the present invention, in which: in the step S4, before atmospheric impact compaction, high-pressure air with a pressure of 15-30 MPa is stored in a gas tank; when atmospheric impact compaction is performed, the pressure increase rate of the high-pressure air entering the top of the sand box and contacting the molding sand is 150-230 MPa / s, and the atmospheric impact compaction is completed in 5-10 seconds.

[0019] To solve the above technical problems, according to another aspect of the present invention, the present invention provides the following technical solutions:

[0020] A liquid nitrogen seepage air impact molding device for freeze casting, comprising:

[0021] Gas tank, large air flush valve, small air flush valve, sand box, compressed air storage chamber;

[0022] The gas tank is located on the upper part of the sand box and is used to store high-pressure gas. The high-pressure air enters the sand box through the opening and closing of the large air punch valve to complete the large air punch compaction; the compressed air storage chamber is located on the upper part of the sand box and is not connected to the gas tank. It is used to store compressed gas. The compressed air enters the sand box through the opening and closing of the small air punch valve to complete the small air punch pre-compacting.

[0023] As a preferred embodiment of the liquid nitrogen seepage air impact molding device for freeze casting described in the present invention, a small hole is provided on the small air impact valve, and the ratio of the area of the small hole to the area of the small air impact valve is 1:20-50.

[0024] As a preferred embodiment of the liquid nitrogen seepage air impact molding device for freeze casting described in the present invention, the area ratio of the small air impact valve to the large air impact valve is 1:8-20.

[0025] As a preferred embodiment of the liquid nitrogen infiltration air-blast molding device for freeze casting described in the present invention, the device further comprises an auxiliary frame, a mold base, a workbench, and a liquid nitrogen inlet. The auxiliary frame is located above the sand box and below the gas tank, the mold base is located below the sand box, the workbench is located between the mold base and the sand box, and the liquid nitrogen inlet is connected to the compressed air storage chamber. Liquid nitrogen is introduced into the sand box by opening and closing a small air-blast valve.

[0026] The beneficial effects of the present invention are as follows:

[0027] The present invention proposes a liquid nitrogen infiltration air-blast molding method and device for freeze casting. Liquid nitrogen is injected into the top of the sand mold before air blasting. Pre-compaction is achieved through a small air blast combined with compaction through a large air blast. Fully utilizing the primary air blast and the rebound of the secondary air flow allows the liquid nitrogen to evenly penetrate the sand mold, which is beneficial for obtaining a casting mold with uniform freezing density and excellent freezing effect after the air blast. The molding time is shortened by half compared with the existing technology, and the performance of the sand mold, such as tensile strength, compressive strength, and air permeability, is improved by more than 30% compared with the existing technology. This can meet the needs of large-scale and mass production and is more conducive to improving the quality of castings. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0029] Figure 1 This is a schematic diagram of the liquid nitrogen infiltration and air impact molding device for freeze casting of the present invention.

[0030] Description of Figure Numbers:

[0031] 1-gas tank, 2-liquid nitrogen flush inlet, 3-large air flush valve, 4-auxiliary frame, 5-sand box, 6-workbench, 7-compressed air storage chamber, 8-small air flush valve, 9-mold bottom plate.

[0032] The purpose, features and advantages of the present invention will be further described with reference to the accompanying drawings and in conjunction with the embodiments. DETAILED DESCRIPTION

[0033] The following will be a clear and complete description of the technical solutions in the embodiments. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0034] The present invention provides a liquid nitrogen infiltration air-blast molding method and device for freeze casting, which has short molding time and high quality, can meet the needs of large-scale and mass production, and is more conducive to improving the quality of castings. Liquid nitrogen is injected into the upper part of the sand mold before air-blasting, and pre-compaction is achieved through a small air-blast combined with compaction through a large air-blast, and the liquid nitrogen is fully utilized by the primary air-blast and the secondary air-flow rebound to uniformly penetrate the sand mold, which is conducive to obtaining a casting mold with uniform freezing density and excellent freezing effect after the air-blasting.

[0035] According to one aspect of the present invention, the present invention provides the following technical solutions:

[0036] A liquid nitrogen infiltration air impact molding method for freeze casting comprises the following steps:

[0037] S1. Sand filling;

[0038] S2. Liquid nitrogen is flushed into the sand box 5;

[0039] S3 small air pre-compacting; open the small air valve 8, the compressed air in the compressed air storage chamber 7 enters the sand box 5, completing the small air pre-compacting;

[0040] S4 atmospheric punch compaction; after closing the small air punch valve 8, open the atmospheric punch valve 3, the high-pressure air in the gas tank 1 enters the sand box 5, completing the atmospheric punch compaction;

[0041] S5. Take out the pattern and obtain the casting mold.

[0042] In step S1, a mold is placed in a sand box 5 on a mold base 9. Molding sand is then spread in the sand box 5. The amount of sand depends on the size of the mold. Generally, the filling height is preferably at least 20 cm. Simple jarring is performed to ensure that the sand fills the deep recesses of the mold and the sand box. Excess sand above the auxiliary frame 4 is scraped off with a scraper. Silica sand is used for the molding sand, and clay is added to increase its adhesiveness. The moisture content of the molding sand should be maintained at 3-5wt%, and the clay content should be 3-10wt%. Specifically, the moisture content of the molding sand should be maintained within a range of, for example, but not limited to, 3wt%, 3.5wt%, 4wt%, 4.5wt%, or 5wt%, or any two thereof; and the clay content should be within a range of, for example, but not limited to, 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, or 10wt%. Because the molding sand does not use resin, bentonite, or coal powder as a sand binder, no waste gas is generated after pouring, and the amount of waste sand is greatly reduced, making it environmentally friendly.

[0043] In step S2, the small air flush valve 8 is opened, and liquid nitrogen is flushed into the sand box through the liquid nitrogen flushing inlet 2. A heat preservation device is used to ensure that the temperature of the liquid nitrogen is ≤-130°C. The small air flush valve 8 is closed and the sand is left to stand for 2-3 minutes to allow the liquid nitrogen to fully penetrate the gaps in the molding sand. The mass ratio of liquid nitrogen to molding sand is 0.5-1.0, and the liquid nitrogen flow rate is controlled at 20-30mL / s. The liquid nitrogen may remain in the small air flush valve 8 after being flushed. Pre-compacting with the small air flush can expel as much of the residual liquid nitrogen in the small air flush valve 8 as possible. Specifically, the standing time is, for example, but not limited to, any one of 2 min, 2 min 10 s, 2 min 20 s, 2 min 30 s, 2 min 40 s, 2 min 50 s, and 3 min, or a range between any two of them; the mass ratio of liquid nitrogen to molding sand is, for example, but not limited to, any one of 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, and 1.0, or a range between any two of them; the liquid nitrogen flow rate is controlled to be, for example, but not limited to, any one of 20 mL / s, 22 mL / s, 24 mL / s, 26 mL / s, 28 mL / s, and 30 mL / s, or a range between any two of them.

[0044] In step S3, before the small air punch pre-compacting, the compressed air with a pressure of 3-15 MPa is stored in the compressed air storage chamber; during the small air punch pre-compacting, the pressure increase rate of the compressed air entering the top of the sand box and contacting the molding sand is 100-180 MPa / s, and the small air punch pre-compacting is completed in 3-5 seconds. Specifically, the air pressure of the compressed air is, for example, but not limited to, any one of 3MPa, 4MPa, 5MPa, 6MPa, 7MPa, 8MPa, 9MPa, 10MPa, 11MPa, 12MPa, 13MPa, 14MPa, and 15MPa, or a range between any two of them; the pressure increase rate is, for example, but not limited to, any one of 100MPa / s, 110MPa / s, 120MPa / s, 130MPa / s, 140MPa / s, 150MPa / s, 160MPa / s, 170MPa / s, and 180MPa / s, or a range between any two of them; the air impulse time is, for example, but not limited to, any one of 3s, 3.5s, 4s, 4.5s, and 5s, or a range between any two of them.

[0045] In the step S4, before atmospheric compaction, high-pressure air with a pressure of 15-30MPa is stored in the gas tank; when atmospheric compaction is performed, the pressure increase rate of the high-pressure air entering the top of the sand box and contacting the molding sand is 150-230MPa / s, and the atmospheric compaction is completed in 5-10 seconds. Specifically, the pressure of the high-pressure air is, for example, but not limited to, any one of 15MPa, 16MPa, 17MPa, 18MPa, 19MPa, 20MPa, 21MPa, 22MPa, 23MPa, 24MPa, 25MPa, 26MPa, 27MPa, 28MPa, 29MPa, 30MPa or the range between any two thereof; the pressure increase rate is, for example, but not limited to, 150MPa / s, 160 The air pressure is any one of MPa / s, 170 MPa / s, 180 MPa / s, 190 MPa / s, 200 MPa / s, 210 MPa / s, 220 MPa / s, and 230 MPa / s, or a range between any two of them; the air impulse time is, for example but not limited to, any one of 5s, 5.5s, 6s, 6.5s, 7s, 7.5s, 8s, 8.5s, 9s, 9.5s, and 10s, or a range between any two of them.

[0046] According to another aspect of the present invention, the present invention provides the following technical solutions:

[0047] A liquid nitrogen seepage air impingement molding device for freeze casting, such as Figure 1 Shown, including:

[0048] Gas tank 1, large air flush valve 3, small air flush valve 8, sand box 5, compressed air storage chamber 7;

[0049] The gas tank 1 is located above the flask 5 and is used to store high-pressure gas. The high-pressure air is allowed to enter the flask 5 by opening and closing the large air impact valve 3 to complete the large air impact compaction. The compressed air storage chamber 7 is located within the gas tank 1 and is not connected to the gas tank 1. It is used to store compressed gas. The small air impact valve 8 is opened and closed to allow compressed air to enter the flask 5 to complete the small air impact pre-compacting. The small air impact valve 8 has a small hole, and the area ratio of the small hole to the area of the small air impact valve 8 is 1:20-50. The area ratio of the small air impact valve 8 to the large air impact valve 3 is 1:8-20. Specifically, the area ratio of the small hole to the small air shock valve 8 is, for example but not limited to, any one of 1:20, 1:22, 1:24, 1:26, 1:28, 1:30, 1:32, 1:34, 1:36, 1:38, 1:40, 1:42, 1:44, 1:46, 1:48, 1:50, or a range between any two of them; the area ratio of the small air shock valve 8 to the large air shock valve 3 is, for example but not limited to, any one of 1:8, 1:9, 1:10, 1:11, 1:12, 1:13, 1:14, 1:15, 1:16, 1:17, 1:18, 1:19, 1:20, or a range between any two of them.

[0050] The device also includes an auxiliary frame 4, a mold base plate 9, a workbench 6, and a liquid nitrogen flushing inlet 2. The auxiliary frame 4 is located above the sand box 5 and below the gas tank 1, the mold base plate 9 is located below the sand box 5, and the workbench 6 is located below the mold base plate 9 and the sand box 5. The liquid nitrogen flushing inlet 2 is connected to the compressed air storage chamber 7, and the liquid nitrogen is introduced into the sand box by opening and closing the small air flushing valve 8.

[0051] The technical solution of the present invention is further described below with reference to specific embodiments. Example 1

[0052] A liquid nitrogen infiltration air impact molding method for freeze casting comprises the following steps:

[0053] S1. Sand Filling: Place the pattern in the sand box 5 on the mold base 9. Spread the molding sand inside the sand box 5. The amount of molding sand depends on the mold size. The filling height is 50 cm. Simple jarring is performed to ensure that the sand is fully filled into the template and deep recesses of the sand box. The jarring time is approximately 20 seconds at a frequency of 60 Hz. Use a scraper to remove excess sand above the auxiliary frame 4. Ensure that the amount of molding sand filled per box is ≤ 3-5% of the total sand filling volume. Silica sand is used, with clay added to enhance adhesion. The water content of the sand should be maintained at 3wt% and the clay content at 5wt%.

[0054] S2. Flushing liquid nitrogen into the flask 5; open the small air flush valve 8 and flush liquid nitrogen into the flask through the liquid nitrogen flush inlet 2. Use a heat preservation device to ensure that the incoming liquid nitrogen temperature is ≤ -130°C. Close the small air flush valve 8 and wait for 2 minutes to allow the liquid nitrogen to fully penetrate the gaps in the molding sand. The mass ratio of liquid nitrogen to molding sand is 0.75, and the liquid nitrogen flow rate is controlled at 20 mL / s.

[0055] S3. Small air punch pre-compacting; before the small air punch pre-compacting, the compressed air with a pressure of 10MPa is stored in the compressed air storage chamber 7; the small air punch valve 8 is opened, and the compressed air in the compressed air storage chamber 7 enters the sand box 5 to complete the small air punch pre-compacting; the pressure increase rate of the compressed air entering the top of the sand box and contacting the molding sand is 150MPa / s, and the small air punch pre-compacting is completed in 3s.

[0056] S4. Atmospheric impact compaction; before atmospheric impact compaction, high-pressure air with a pressure of 30MPa is stored in the gas tank 1; when atmospheric impact compaction is performed, the small air impact valve 8 is closed and the atmospheric impact valve 3 is opened, and the high-pressure air in the gas tank 1 enters the sand box 5 to complete atmospheric impact compaction; the pressure increase rate of the high-pressure air entering the top of the sand box and contacting the molding sand is 200MPa / s, and the atmospheric impact compaction is completed in 5s of air impact.

[0057] S5. Take out the pattern and obtain the casting mold.

[0058] The casting of aluminum alloy castings with variable wall thickness is achieved by using the casting mold described in Example 1. A spraying method is used in the mold cavity to evenly spray the insulating coating. The molten metal is poured into the mold. The molten metal solidifies into a shell in the mold. After the shell is formed on the outside, a coolant is quickly sprayed on the thick part of the frozen mold wall to quickly dissolve the thick part of the frozen mold wall and disperse the sand. Example 2

[0059] A liquid nitrogen infiltration air impact molding method for freeze casting comprises the following steps:

[0060] S1. Sand Filling: Place the pattern in the sand box 5 on the mold base 9. Spread molding sand inside the sand box 5. The amount of sand depends on the mold size. The filling height is 40 cm. Simple jarring is performed to ensure that the sand is fully filled into the template and deep recesses of the sand box. The jarring time is approximately 30 seconds at a frequency of 100 Hz. Excess sand above the auxiliary frame 4 is scraped off with a scraper. Ensure that the amount of sand filled per box is ≤ 3-5% of the total sand filling volume. Silica sand is used, with clay added to enhance adhesion. The water content of the sand should be maintained at 5wt% and the clay content at 6wt%.

[0061] S2. Flushing liquid nitrogen into the flask 5; open the small air flush valve 8 and flush liquid nitrogen into the flask through the liquid nitrogen flush inlet 2. Use a heat preservation device to ensure that the incoming liquid nitrogen temperature is ≤ -130°C. Close the small air flush valve 8 and wait for 2 minutes to allow the liquid nitrogen to fully penetrate the gaps in the molding sand. The mass ratio of liquid nitrogen to molding sand is 0.7, and the liquid nitrogen flow rate is controlled at 25 mL / s.

[0062] S3. Small air punch pre-compacting; before the small air punch pre-compacting, the compressed air with a pressure of 15MPa is stored in the compressed air storage chamber 7; the small air punch valve 8 is opened, and the compressed air in the compressed air storage chamber 7 enters the sand box 5 to complete the small air punch pre-compacting; the pressure increase rate of the compressed air entering the top of the sand box and contacting the molding sand is 160MPa / s, and the small air punch pre-compacting is completed in 5s.

[0063] S4. Atmospheric impact compaction; before atmospheric impact compaction, high-pressure air with a pressure of 30MPa is stored in the gas tank 1; when atmospheric impact compaction is performed, the small air impact valve 8 is closed and the atmospheric impact valve 3 is opened, and the high-pressure air in the gas tank 1 enters the sand box 5 to complete atmospheric impact compaction; the pressure increase rate of the high-pressure air entering the top of the sand box and contacting the molding sand is 200MPa / s, and the atmospheric impact compaction is completed in 8s.

[0064] S5. Take out the pattern and obtain the casting mold.

[0065] The casting of ductile iron is achieved by using the casting mold described in Example 2. Molten iron is poured into the casting mold by centrifugal casting, and the ductile iron solidifies into a shell in the casting mold.

[0066] The above description is only a preferred embodiment of the present invention and does not limit the patent scope of the present invention. All equivalent structural transformations made by using the contents of the present invention specification under the inventive concept of the present invention, or direct / indirect application in other related technical fields are included in the patent protection scope of the present invention.

Claims

1. A liquid nitrogen infiltration gas impact molding method for freeze casting, characterized in that: The steps include: S1. Sand filling; Silica sand is used as molding sand, and clay is added to increase the bonding strength of the molding sand. The moisture content in the molding sand is maintained at 3-5wt%, and the clay content is 3-10wt%; S2. Flushing liquid nitrogen into the flask; open the small air flush valve, flush liquid nitrogen into the flask through the liquid nitrogen flush inlet, then close the small air flush valve and let it sit for 2-3 minutes; control the liquid nitrogen flow rate at 20-30 mL / s; the mass ratio of liquid nitrogen to molding sand is 0.5-1.0; and provide a small hole in the small air flush valve, with the ratio of the hole area to the small air flush valve area being 1:20-50. S3. Small air pre-compacting: Open the small air valve, and the compressed air in the compressed air storage chamber enters the flask, completing the small air pre-compacting. During the small air pre-compacting, the compressed air enters the top of the flask and contacts the molding sand at a pressure increase rate of 100-180 MPa / s. The air pre-compacting is completed in 3-5 seconds. The small air pre-compacting can expel as much residual liquid nitrogen as possible from the small air valve. S4. Atmospheric pressure compaction: After closing the small pressure valve, open the atmospheric pressure valve. The area ratio of the small pressure valve to the atmospheric pressure valve is 1:8-20. The high-pressure air in the air tank enters the sand box to complete the atmospheric pressure compaction. During the atmospheric pressure compaction, the high-pressure air enters the top of the sand box and contacts the molding sand at a pressure increase rate of 150-230MPa / s. The atmospheric pressure compaction is completed in 5-10 seconds. S5. Take out the pattern and obtain the casting mold.

2. The molding method according to claim 1, characterized in that: In step S3, before the small air punch pre-compacting, the compressed air with a pressure of 3-15 MPa is stored in the compressed air storage chamber.

3. The molding method according to claim 1, characterized in that: In step S4, before atmospheric compaction, high-pressure air with a pressure of 15-30 MPa is stored in a gas tank.

4. A liquid nitrogen seepage gas impingement molding device for freeze casting, used to implement the molding method according to claim 1, characterized in that: include: Gas tank, large air flush valve, small air flush valve, sand box, compressed air storage chamber; The gas tank is located on the upper part of the sand box and is used to store high-pressure gas. The opening and closing of the atmospheric flush valve allows high-pressure air to enter the sand box to complete the atmospheric flush compaction; the compressed air storage chamber is located on the upper part of the sand box and is not connected to the gas tank. It is used to store compressed gas. The opening and closing of the small air flush valve allows compressed air to enter the sand box to complete the small air flush pre-compacting.

5. The molding device according to claim 4, characterized in that The device also includes an auxiliary frame, a mold base, a workbench, and a liquid nitrogen flushing inlet; the auxiliary frame is located above the sand box and below the gas tank, the mold base is located below the sand box, the workbench is located below the mold base and the sand box, the liquid nitrogen flushing inlet is connected to the compressed air storage chamber, and liquid nitrogen is introduced into the sand box by opening and closing a small air flushing valve.

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