CN115815535A - Liquid nitrogen seepage air-blast molding method and device for freeze casting - Google Patents

Abstract

The invention belongs to the technical field of casting molding, and specifically relates to a liquid nitrogen percolation air-impact molding method and device for frozen casting. Before air-impact, liquid nitrogen is injected into the top of the sand mold, pre-compacted by small air-impact combined with atmospheric-impact compaction, and fully utilized The first air blast and the second air flow rebound make the liquid nitrogen evenly infiltrate into the sand mold, which is beneficial to obtain a mold with uniform freezing density and excellent freezing effect after air blasting. The molding time is short and the quality is high. Compared with the existing technology, the molding time is shortened Double, can meet the needs of large-scale mass production, and is more conducive to improving the quality of castings.

Description

Method and device for liquid nitrogen percolation air-impact molding for frozen casting

technical field

The invention relates to the technical field of casting molding, in particular to a liquid nitrogen percolation air-impact molding method and device for frozen casting.

Background technique

In recent years, with the rapid development of the world's industrialization level, the demand for the foundry industry is increasing. Traditional casting uses resin or bentonite, coal powder, etc. as molding sand binders, and the working environment is full of dust. Exhaust gas, etc., also produces a large amount of industrial waste, which not only pollutes the environment, but also damages the health of workers, and the cost of subsequent recycling and treatment cannot be underestimated.

The traditional freezing casting method adopts liquid nitrogen cooling method to realize the freezing molding of the mixture of molding sand and water. The casting mold obtained by the freezing casting method only includes molding sand and water. After pouring, it can realize automatic sand falling, which simplifies pouring The final cleaning process is simple, environmentally friendly, and cost-saving. Although it meets the needs of green development, the freezing rate is slow, and the compactness of the sand mold is uneven, which affects the quality of castings, and is not suitable for large-scale mass production.

Contents of the invention

In order to solve the problems existing in the prior art, the main purpose of the present invention is to provide a liquid nitrogen percolation air-impact molding method and device for frozen casting.

In order to solve the above technical problems, according to one aspect of the present invention, the present invention provides the following technical solutions:

A liquid nitrogen percolation air-impact molding method for frozen casting, comprising the following steps:

S1. Fill sand;

S2. Liquid nitrogen is flushed into the sand box;

S3. Small air punch pre-tightening; open the small air punch valve, the compressed air in the compressed air storage chamber enters the sand box, and completes the small air punch pre-tightening;

S4. Atmospheric flushing and compacting; after closing the small air flushing valve, open the atmospheric flushing valve, and the high-pressure air in the air tank enters the sand box to complete the atmospheric compaction;

S5. Take out the pattern and obtain the casting mold.

As a preferred solution of the liquid nitrogen percolation air-impact molding method for frozen casting according to the present invention, wherein: in the step S1, the model is placed in the sand box on the mold bottom plate, and the molding sand is laid in the sand box. The amount of molding sand depends on Depending on the size of the mold, the height of the sand filling is generally better than 20cm, and it is simply shaken to ensure the filling of the molding sand in the deep recessed parts of the formwork and the flask.

As a preferred solution of the liquid nitrogen percolation air-impact molding method for frozen casting according to the present invention, wherein: in the step S1, the excess molding sand above the auxiliary frame is scraped off by a scraper.

As a preferred solution of the liquid nitrogen percolation air-impact molding method for frozen casting according to the present invention, wherein: in the step S1, silica sand is used for the molding sand, clay is added to increase the adhesion of the molding sand, and the moisture in the molding sand should be kept at 3-5wt%, clay content 3-10wt%.

As a preferred solution of the liquid nitrogen percolation air-impact molding method for frozen casting according to the present invention, wherein: in the step S2, the small air-impact valve is opened, and liquid nitrogen is injected into the sand box through the liquid nitrogen inflow port, using The heat preservation device ensures that the temperature of the liquid nitrogen being flushed is ≤ -130°C; close the small air flushing valve and let it stand for 2-3 minutes to allow the liquid nitrogen to fully penetrate into the gaps in the molding sand.

As a preferred solution of the liquid nitrogen percolation air-impact molding method for frozen casting according to the present invention, wherein: in the step S2, the mass ratio of liquid nitrogen to molding sand is 0.5-1.0, and the flow rate of liquid nitrogen is controlled at 20-30mL /s, there may be residues in the small air flush valve when the liquid nitrogen is flushed in, and it can be completely discharged through the small air flush pre-tightening.

As a preferred solution of the liquid nitrogen percolation air impact molding method for frozen casting according to the present invention, wherein: in the step S3, before the small air impact pre-compacting, the compressed air with a pressure of 3-15 MPa is stored in the compressed air. In the air storage chamber; when the small air punch is preloaded, the compressed air enters the top of the flask and contacts the molding sand at a pressure increase rate of 100-180MPa/s, and the small air punch preload is completed in 3-5s.

As a preferred solution of the liquid nitrogen percolation air-impact molding method for frozen casting according to the present invention, wherein: in the step S4, the high-pressure air with a pressure of 15-30 MPa is stored in the air tank before the air is compacted. Inside; in the real time of atmospheric flushing, the pressure increase rate of high-pressure air entering the top of the flask and contacting the molding sand is 150-230MPa/s, and the air compaction is completed within 5-10s of air flushing.

In order to solve the above technical problems, according to another aspect of the present invention, the present invention provides the following technical solutions:

A liquid nitrogen percolation air-impact molding device for frozen casting, comprising:

Air tank, large air flushing valve, small air flushing valve, sand box, compressed air storage chamber;

The air tank is located on the upper part of the sand box and is used to store high-pressure gas. Through the opening and closing of the atmospheric flushing valve, the high-pressure air enters the sand box to complete the compacting of the atmospheric flushing; the compressed air storage chamber is located on the upper part of the sand box and is not connected to the air tank. , used to store compressed gas, through the opening and closing of the small air flushing valve, the compressed air enters the sand box to complete the small air flushing pre-tightening.

As a preferred solution of the liquid nitrogen percolation air-impact molding device for frozen casting according to the present invention, small holes are 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.

As a preferred solution of the liquid nitrogen percolation air flush molding device for frozen casting according to the present invention, the area ratio of the small air flush valve to the large air flush valve is 1:8-20.

As a preferred solution of a liquid nitrogen percolation air-impact molding device for frozen casting according to the present invention, the device also includes an auxiliary frame, a mold bottom plate, a workbench, and a liquid nitrogen flushing inlet, and the auxiliary frame is located in the sand box. The upper part is located at the lower part of the gas tank, the mold bottom plate is located at the lower part of the sand box, the workbench is located at the mold bottom plate and the lower part of the sand box, the liquid nitrogen flushing inlet is connected with the compressed air storage chamber, and the small air flushing valve is opened and closed to realize Introduce liquid nitrogen into the sandbox.

The beneficial effects of the present invention are as follows:

The present invention proposes a method and device for liquid nitrogen percolation air-impact molding for frozen casting. Liquid nitrogen is injected into the top of the sand mold before air-impact, pre-compacted by small air-impact combined with atmospheric-impact compaction, making full use of primary air-impact and secondary airflow The rebound makes the liquid nitrogen penetrate into the sand mold evenly, which is beneficial to obtain a mold with uniform freezing density and excellent freezing effect after air blasting. Compared with the existing technology, the molding time is doubled, and the properties of the sand mold such as tensile strength and compressive strength , gas permeability, etc. are improved by more than 30% compared with the existing technology, which can meet the needs of large-scale mass production, and is more conducive to improving the quality of castings.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only These are some embodiments of the present invention. For those skilled in the art, other drawings can also be obtained according to the structures shown in these drawings without creative effort.

Fig. 1 is a schematic diagram of a liquid nitrogen percolation air-impact molding device for frozen casting according to the present invention.

Explanation of reference numbers:

1-air tank, 2-liquid nitrogen flushing inlet, 3-atmospheric flushing valve, 4-auxiliary frame, 5-sand box, 6-workbench, 7-compressed air storage chamber, 8-small air flushing valve, 9-mold bottom plate .

The realization of the purpose of the present invention, functional characteristics and advantages will be further described in conjunction with the embodiments and with reference to the accompanying drawings.

Detailed ways

A clear and complete description will be made below in conjunction with the technical solutions in the embodiments. Apparently, the described embodiments are only some of the embodiments of the present invention, but not all of them. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without creative efforts fall within the protection scope of the present invention.

The invention provides a liquid nitrogen seepage air-impact molding method and device for frozen casting, which has short molding time and high quality, can meet the needs of large-scale mass production, and is more conducive to improving the quality of castings. Above the sand mold, pre-compacting with small air impact combined with air impact compaction, making full use of the first air impact and the rebound of the secondary air flow to allow liquid nitrogen to penetrate into the sand mold evenly, which is conducive to obtaining a mold with uniform freezing density and excellent freezing effect after air impact .

According to one aspect of the present invention, the present invention provides following technical scheme:

A liquid nitrogen percolation air-impact molding method for frozen casting, comprising the following steps:

S1. Fill sand;

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

S3. Small air punch pre-compacting; open the small air punch valve 8, the compressed air in the compressed air storage chamber 7 enters the sand box 5, and completes the small air punch pre-compacting;

S4. Atmospheric flushing and compacting; after closing the small air flushing valve 8, open the atmospheric flushing valve 3, and the high-pressure air in the air tank 1 enters the sand box 5 to complete the atmospheric flushing;

S5. Take out the pattern and obtain the casting mold.

In the step S1, the model is placed in the sand box 5 on the mold bottom plate 9, and the molding sand is spread in the sand box 5. The amount of molding sand depends on the size of the mold. Generally, the height of the sand filling is preferably more than 20 cm. The filling of molding sand in the deep concave part of the sand box. Scrape off the redundant molding sand above the auxiliary frame 4 by a scraper. Silica sand is used for the molding sand, and clay is added to increase the cohesiveness. The moisture in the molding sand should be kept at 3-5wt%, and the clay content should be 3-10wt%. Specifically, the moisture content in the molding sand should be maintained at, for example, but not limited to, 3wt%, 3.5wt%, 4wt%, 4.5wt%, 5wt%, or any one of both; the clay content is, for example, but Not limited to any one of 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, or any range between any two. Since the molding sand does not use resin, bentonite, coal powder, etc. as the molding sand binder, no waste gas will be generated after pouring, and the amount of waste sand is also greatly reduced, which is environmentally friendly.

In the step S2, open the small air flushing valve 8, pour liquid nitrogen into the sand box through the liquid nitrogen flushing inlet 2, and use a heat preservation device to ensure that the temperature of the liquid nitrogen flushed into it is ≤ -130°C; close the small air flushing valve 8, and let it stand for 2 -3min, so that liquid nitrogen can fully penetrate into the gaps in the molding sand. The mass ratio of liquid nitrogen to molding sand is 0.5-1.0, and the flow rate of liquid nitrogen is controlled at 20-30mL/s. There may be residues in the small air flush valve 8 when the liquid nitrogen rushes in, and as much air can be discharged as much as possible through the small air flush pre-tightening Flush valve 8 with residual liquid nitrogen. Specifically, the standing time is, for example, but not limited to, 2min, 2min10s, 2min20s, 2min30s, 2min40s, 2min50s, 3min, or a range between any two; 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, 1.0 or the range between any two; the flow rate of liquid nitrogen is controlled at, for example but not limited to, 20mL/s, 22mL/s s, 24mL/s, 26mL/s, 28mL/s, 30mL/s any one or the range between any two.

In the step S3, before the small air punch is pre-compacted, the compressed air with a pressure of 3-15 MPa is stored in the compressed air storage chamber; when the small air punch is pre-compacted, the compressed air enters the top of the sand box to contact the molding sand to increase the pressure. The speed is 100-180MPa/s, and the air impact is 3-5s to complete the small air impact pre-tightening. Specifically, the 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, 15MPa or a range between any two The pressurization 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, 180MPa/s or any range between the two; the air burst time is, for example but not limited to, any one of 3s, 3.5s, 4s, 4.5s, 5s or any range between the two.

In the step S4, before the atmosphere is compacted, the high-pressure air with an air pressure of 15-30 MPa is stored in the air tank; when the atmosphere is compacted, the pressure increase rate of the high-pressure air entering the top of the flask and contacting the molding sand is 150-30 MPa. 230MPa/s, air punching 5-10s to complete the air punching compaction. Specifically, the air 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 any The range between the two; the boost rate is for example but not limited to 150MPa/s, 160MPa/s, 170MPa/s, 180MPa/s, 190MPa/s, 200MPa/s, 210MPa/s, 220MPa/s, 230MPa Any one of /s or the range between any two; air burst time is for example but not limited to 5s, 5.5s, 6s, 6.5s, 7s, 7.5s, 8s, 8.5s, 9s, 9.5s, 10s Any one of them or the range between any two.

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

A liquid nitrogen percolation air-impact molding device for frozen casting, as shown in Figure 1, comprising:

Air tank 1, large air flushing valve 3, small air flushing valve 8, sand box 5, compressed air storage chamber 7;

The air tank 1 is located on the upper part of the sand box 5 and is used to store high-pressure gas. The high-pressure air enters the sand box 5 through the opening and closing of the atmospheric flushing valve 3 to complete the compacting of the atmospheric flushing; the compressed air storage chamber 7 is located in the air tank 1 And it is not connected with the gas tank 1, it is used to store the compressed gas, and the opening and closing of the small air flushing valve 8 realizes that the compressed air enters the sand box 5 to complete the small air flushing pre-tightening; the small air flushing valve 8 offers small holes, and the area of the small holes is the same The area ratio of small air flushing valve 8 is 1:20-50. The area ratio of the small air flushing valve 8 to the large air flushing valve 3 is 1:8-20. Specifically, the ratio of the area of the small hole to the area of the small air flushing valve 8 is, for example but not limited to, 1:20, 1:22, 1:24, 1:26, 1:28, 1:30, 1:32, 1:34, Any one of 1:36, 1:38, 1:40, 1:42, 1:44, 1:46, 1:48, 1:50 or the range between any two; small air flush valve 8 and Atmospheric valve 3 area ratio is 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 any range between the two.

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 at the upper part of the sand box 5 and the lower part of the gas tank 1, and the mold base plate 9 is located at the lower part of the sand box 5. , the workbench 6 is located at the lower part of the mold base plate 9 and the sand box 5, the liquid nitrogen flushing inlet 2 communicates with the compressed air storage chamber 7, and the introduction of liquid nitrogen into the sand box is realized through the opening and closing of the small air flushing valve 8.

The technical solutions of the present invention will be further described below in conjunction with specific embodiments.

Example 1

A liquid nitrogen percolation air-impact molding method for frozen casting, comprising the following steps:

S1. Sand filling; place the model in the sand box 5 on the mold bottom plate 9, and spread the molding sand in the sand box 5. The amount of molding sand depends on the size of the mold, and the sand filling height is 50cm. Simply shake to ensure the deep recessed parts of the formwork and the sand box For filling of molding sand, the compaction time is about 20s, and the compaction frequency is 60Hz. Use the scraper to scrape off the redundant molding sand above the auxiliary frame 4 to ensure that the deviation of the amount of molding sand filled in each box is ≤ 3-5% of the total sand filling volume of the sand box. The molding sand adopts silica sand, and clay is added to increase the cohesiveness. The moisture in the molding sand should be kept at 3wt%, and the clay content should be 5wt%.

S2. Liquid nitrogen rushes into the sand box 5; open the small air flush valve 8, and pour liquid nitrogen into the sand box through the liquid nitrogen flush inlet 2, and use a heat preservation device to ensure that the temperature of the liquid nitrogen ≤ -130°C; close the small air flush valve 8 , let it stand for 2 minutes, so that the liquid nitrogen can fully penetrate into the gaps in the molding sand. The mass ratio of liquid nitrogen to molding sand is 0.75, and the flow rate of liquid nitrogen is controlled at 20mL/s.

S3. Small air punch pre-compacting; before the small air punch pre-compacting, store compressed air with a pressure of 10MPa in the compressed air storage chamber 7; open the small air flush valve 8, and the compressed air in the compressed air storage chamber 7 enters the sand box 5. Complete small air impact pre-compacting; the pressure increase rate of the compressed air entering the top of the flask and contacting the molding sand is 150MPa/s, and the air impact is 3s to complete the small air impact pre-compacting.

S4. Atmospheric flushing; before the atmospheric flushing, store the high-pressure air with a pressure of 30MPa in the air tank 1; when the atmospheric flushing is real-time, close the small air flushing valve 8 and open the atmospheric flushing valve 3, and the gas tank 1 The high-pressure air enters the sand box 5 to complete the atmospheric compaction; the pressure increase rate of the high-pressure air entering the top of the flask and contacting the molding sand is 200MPa/s, and the air flushing takes 5s to complete the atmospheric compaction.

S5. Take out the pattern and obtain the casting mold.

Using the casting mold described in Example 1 to realize the casting of aluminum alloy castings with variable wall thickness, the spraying method is adopted in the mold cavity to evenly spray the heat-insulating coating, and the molten metal liquid is poured into the casting mold, and the molten metal solidifies in the casting mold Shelling, after the outer shell is formed, quickly spray coolant on the thick wall part of the frozen casting mold, and quickly dissolve the thick wall part of the frozen casting mold by dissolving the sand.

Example 2

A liquid nitrogen percolation air-impact molding method for frozen casting, comprising the following steps:

S1. Sand filling; place the model in the sand box 5 on the mold bottom plate 9, and spread the molding sand in the sand box 5. The amount of molding sand depends on the size of the mold, and the sand filling height is 40cm. Simply shake to ensure the deep recessed parts of the formwork and the sand box For molding sand filling, the compaction time is about 30s, and the compaction frequency is 100Hz. Use the scraper to scrape off the redundant molding sand above the auxiliary frame 4 to ensure that the deviation of the amount of molding sand filled in each box is ≤ 3-5% of the total sand filling volume of the sand box. The molding sand adopts silica sand, and clay is added to increase the cohesiveness. The moisture in the molding sand should be kept at 5wt%, and the clay content should be 6wt%.

S2. Liquid nitrogen rushes into the sand box 5; open the small air flush valve 8, and pour liquid nitrogen into the sand box through the liquid nitrogen flush inlet 2, and use a heat preservation device to ensure that the temperature of the liquid nitrogen ≤ -130°C; close the small air flush valve 8 , let it stand for 2 minutes, so that the liquid nitrogen can fully penetrate into the gaps in the molding sand. The mass ratio of liquid nitrogen to molding sand was 0.7, and the flow rate of liquid nitrogen was controlled at 25mL/s.

S3. Small air punch pre-compacting; before the small air punch pre-compacting, store compressed air with a pressure of 15MPa in the compressed air storage chamber 7; open the small air flush valve 8, and the compressed air in the compressed air storage chamber 7 enters the sand box 5. Complete 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 160MPa/s, and the air impact is 5s to complete the small air impact pre-compacting.

S4. Atmospheric flushing; before the atmospheric flushing, store the high-pressure air with a pressure of 30MPa in the air tank 1; when the atmospheric flushing is real-time, close the small air flushing valve 8 and open the atmospheric flushing valve 3, and the gas tank 1 The high-pressure air enters the sand box 5 to complete the atmospheric compaction; the pressure increase rate of the high-pressure air entering the top of the flask and contacting the molding sand is 200MPa/s, and the air flushing is 8s to complete the atmospheric compaction.

S5. Take out the pattern and obtain the casting mold.

The casting mold described in Example 2 is used to cast the ductile iron, and the molten iron is poured into the mold by centrifugal casting, and the ductile iron is solidified into a shell in the mold.

The above description is only a preferred embodiment of the present invention, and does not limit the patent scope of the present invention. Under the inventive concept of the present invention, the equivalent structural transformation made by using the content of the description of the present invention, or directly/indirectly used in other related All technical fields are included in the patent protection scope of the present invention.

Claims (10)

1. A liquid nitrogen percolation air-impact molding method for freezing casting, is characterized in that, comprises the steps: S1. Fill sand; S2. Liquid nitrogen is flushed into the sand box; S3. Small air punch pre-tightening; open the small air punch valve, the compressed air in the compressed air storage chamber enters the sand box, and completes the small air punch pre-tightening; S4. Atmospheric flushing and compacting; after closing the small air flushing valve, open the atmospheric flushing valve, and the high-pressure air in the air tank enters the sand box to complete the atmospheric compaction; S5. Take out the pattern and obtain the casting mold. 2. The molding method according to claim 1, characterized in that, in the step S1, silica sand is used for the molding sand, clay is added to increase the adhesiveness of the molding sand, the moisture in the molding sand is kept at 3-5wt%, and the clay content is 3 -10wt%. 3. The molding method according to claim 1, characterized in that, in the step S2, open the small air flush valve, pour liquid nitrogen into the sand box through the liquid nitrogen flush inlet, close the small air flush valve, and let stand for 2- 3min. 4. The molding method according to claim 1, characterized in that, in the step S2, the mass ratio of liquid nitrogen to molding sand is 0.5-1.0, and the flow rate of liquid nitrogen is controlled at 20-30mL/s. 5. The molding method according to claim 1, characterized in that, in the step S3, before the pre-tightening of the small air punch, the compressed air with a pressure of 3-15 MPa is stored in the compressed air storage cavity; In real-time pre-tightening, the pressurization rate of the compressed air entering the top of the flask and contacting the molding sand is 100-180MPa/s, and the small air-blow pre-tightening is completed in 3-5s. 6. The molding method according to claim 1, characterized in that, in the step S4, before the atmosphere is compacted, the high-pressure air with an air pressure of 15-30 MPa is stored in the air tank; when the atmosphere is compacted, When the high-pressure air enters the top of the flask and contacts with the molding sand, the pressurization rate is 150-230MPa/s, and the air punching takes 5-10s to complete the air punching compaction. 7. A liquid nitrogen seepage air-impact molding device for frozen casting, used to realize the molding method according to claim 1, characterized in that it comprises: Air tank, large air flushing valve, small air flushing valve, sand box, compressed air storage chamber; The air tank is located on the upper part of the sand box and is used to store high-pressure gas. Through the opening and closing of the atmospheric flushing valve, the high-pressure air enters the sand box to complete the compacting of the atmospheric flushing; the compressed air storage chamber is located on the upper part of the sand box and is not connected with the air tank for storage. Compressed gas, through the opening and closing of the small air flushing valve, the compressed air enters the sand box to complete the small air flushing pre-compaction. 8. The molding device according to claim 7, wherein a small hole is opened on the small air flush valve, and the ratio of the area of the small hole to the area of the small air flush valve is 1:20-50. 9. The molding device according to claim 7, characterized in that the area ratio of the small air flush valve to the large air flush valve is 1:8-20. 10. The molding device according to claim 7, characterized in that, the device also includes an auxiliary frame, a mold bottom plate, a workbench, and a liquid nitrogen flushing inlet; the auxiliary frame is located at the upper part of the sand box and the lower part of the gas tank, and the mold bottom plate is located at The lower part of the sand box, the workbench is located at the bottom of the mold and the lower part of the sand box, the liquid nitrogen flushing inlet is connected with the compressed air storage chamber, and the liquid nitrogen is introduced into the sand box through the opening and closing of the small air flushing valve.

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