Frozen sand mold hollowing and following design and low temperature medium transmission frozen forming method

Abstract

The application provides a hollowed-out follow-up design of a frozen sand mold and a low-temperature medium transmission freezing forming method, and the hollowed-out sand mold with follow-up cooling channels is integrally formed through additive manufacturing technology, and heat is taken away in the form of effective heat exchange mainly in the form of heat conduction and heat convection between the low-temperature medium and the normal-temperature sand mold in the cooling pipeline, so that strong cooling of the frozen sand mold is formed to realize rapid freezing. The integrally designed sand mold effectively prevents the precision loss problem caused by sand block assembly, the follow-up cooling channels are arranged, the offline freezing of the water adhesive sand mold is carried out, the service life of the printing equipment is improved while the freezing efficiency is improved, the characteristics and advantages of the sand mold additive manufacturing are fully utilized, and the rapid cooling of the sand mold is realized through the hollowed-out design and the transmission of the low-temperature medium. The application is simple in operation, low in cost, and suitable for the rapid production of the frozen sand mold and the effective utilization of the low-temperature energy.

Description

Technical Field

[0001] This invention relates to the field of cryogenic sand mold additive manufacturing, and in particular to a cryogenic sand mold hollow conformal design and cryogenic medium transport freezing forming method. Background Technology

[0002] Casting is one of the main methods for obtaining blanks for mechanical products and is an important basic process in the machinery and equipment industry. More than 80% of products in the casting industry are completed through sand casting. Cryogenic sand casting additive manufacturing technology addresses the problems of poor production flexibility, long manufacturing cycles, and excessive resource waste in small-batch production of traditional sand casting methods. It proposes to use water freezing to replace the binder in the molding sand, creating product prototypes by layering materials. This process offers high flexibility, short production cycles, and high material utilization.

[0003] Traditional sand molds are generally dense, which not only makes it difficult to control the cooling and forming process of the casting during subsequent pouring, but also prevents the dense frozen sand mold from freezing quickly at low temperatures during the cryogenic sand casting stage, affecting sand mold production efficiency and increasing energy consumption in the refrigeration and freezing processes. In CNC machining processes, due to limitations of machine tools, ambient temperature, and processing time, frozen sand molds are often cut into sections, making integrated molding difficult and resulting in assembly errors. In cryogenic sand additive manufacturing, traditional methods require pre-mixing the raw sand with refrigerant, and both sand spreading and printing processes must be carried out at low temperatures. This significantly increases the burden on the sand mold printer, especially the print head assembly, which struggles to operate normally for extended periods at low temperatures, thus greatly affecting the printing quality and forming efficiency of the frozen sand mold. Since the entire cryogenic sand additive manufacturing process must be completed at low temperatures, unnecessary freezing occurs, increasing energy consumption and hindering the realization of green sand mold production. Summary of the Invention

[0004] To address the aforementioned issues, this invention discloses an integrated forming method for cryogenic sand mold additive manufacturing with hollow conformal design and quantitative low-temperature medium transfer. This method compensates for the loss of precision in the segmented assembly of cryogenic sand molds, prevents the cryogenic sand molds from collapsing prematurely due to insufficient strength during the forming process, reduces the use of raw sand in the cryogenic sand mold additive manufacturing process, further reduces energy consumption, extends the service life of sand mold printing equipment, and improves the operability and flexibility of sand mold freezing.

[0005] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0006] A method for integrating cryogenic sand mold additive manufacturing with hollow conformal design and quantitative transport of cryogenic media, characterized by the following steps:

[0007] Step 1: Determine the geometric features of the casting and create a model of the casting using 3D modeling software;

[0008] Step 2: Design a corresponding freezing sand mold model based on the three-dimensional model of the casting, and set cooling pipes inside the freezing sand mold following the contour of the cavity wall;

[0009] Step 3: Design the hollowed-out shape of the freezing sand mold for setting the cooling pipes, and establish a hollowed-out conformal model of the freezing sand mold;

[0010] Step 4: Use finite element analysis software to perform strength verification based on the established sand mold model, and optimize the design of the cooling pipes and hollow structure of the frozen sand mold based on the overall stress field distribution of the sand mold.

[0011] Specifically, finite element analysis software is used to check the strength of the established sand mold model. Based on the overall stress field distribution of the sand mold, the cooling pipes and hollow structure of the frozen sand mold are optimized. For example, additional reinforcing ribs or supporting trusses are added to the orthogonal truss structure where the strength is insufficient. Secondary topology optimization is performed on the areas with insufficient strength in the hollowing software. The hollow structure at the bottom of the model is appropriately trimmed to strengthen the hollow structure by increasing the contact area of ​​the substrate at the bottom of the model, so as to prevent it from collapsing during the printing process.

[0012] Step 5: The 3D slicing software generates the contour information of each printed section based on the simulated and optimized frozen sand mold hollow conformal model;

[0013] Step 6: Under normal temperature conditions, use a digital sand mold 3D printer to import the printing outline information and print the designed room temperature sand mold.

[0014] Step 7: Take out the room temperature sand mold, pass copper mold cooling pipes through the pre-set complex cooling pipeline to form a cooling channel, and introduce a certain flow of low temperature medium into the cooling channel for timed and temperature-controlled heat exchange flow to freeze and solidify the frozen sand mold.

[0015] Step 8: After verifying that the strength of the frozen sand mold meets the standard, clean up the excess dry sand and proceed with the subsequent casting.

[0016] Furthermore, the cooling pipeline structure can be an S-shaped pipeline, a grid pipeline, or a heat exchange pipeline using both in combination, designed to conform to the shape of the casting structure.

[0017] Furthermore, the diameter of the cooling pipe is 2mm to 20mm, the wall thickness of the cooling pipe is 1mm to 10mm, and the amount of sand the cooling pipe absorbs between the mold and the cavity wall is 10mm to 50mm.

[0018] Furthermore, by using a hollow design to reserve a sand discharge port in the cooling pipes, the dry sand remaining in the cooling pipes can be discharged after the printed room temperature sand mold is removed, which facilitates the installation of the cooling pipes.

[0019] Furthermore, when designing the perforation, the thickness of the inner and outer shells of the sand mold and the perforation structure are taken into account. The thickness of the inner and outer shells of the sand mold is 20mm to 50mm; the basic perforation structure includes orthogonal truss structure, honeycomb structure, topological lattice structure and reinforcing rib structure; the perforation structure can be applied to the entire sand mold or to a part of the sand mold.

[0020] For cooling pipes with a relatively long total length, orthogonal truss or topological lattice structures are recommended; reinforcing rib structures are suitable for placement on thicker outer shells; honeycomb structures are suitable for castings with relatively simple shapes.

[0021] Furthermore, the strength of the optimized frozen sand mold is not less than 0.8 MPa, and the overall weight reduction of the sand mold is 30% to 60%.

[0022] Furthermore, both the sand-laying and printing processes are carried out at room temperature. The nozzle sprays pure water adhesive as needed based on the current cross-sectional information of the sand mold, and the room temperature sand mold is initially bonded through the capillary force and fine adhesive force between the water films.

[0023] Furthermore, the cryogenic medium can be a liquid such as liquid nitrogen, with a temperature reaching -196℃ for rapid freezing; the cryogenic medium can also be a gas such as cold air or cryogenic nitrogen, with a temperature controlled between -2℃ and -50℃. When introducing the cryogenic gaseous medium, a cooling pipe with heat dissipation holes is used. A portion of the cryogenic medium can pass through the heat dissipation holes and enter the hollowed-out sand mold through the sand discharge port for efficient heat exchange, achieving rapid freezing.

[0024] The beneficial effects of this invention are:

[0025] 1. This invention incorporates a contour-following cooling pipe system inside the sand mold. During the freezing process, the input of the low-temperature medium can be precisely controlled at a controlled temperature and time, achieving accurate control of the sand mold freezing process and reducing unnecessary freezing. Circulating extremely low-temperature fluids such as liquid nitrogen within the cooling pipes facilitates rapid freezing of the sand mold. Furthermore, introducing low-temperature cold air into the cooling pipes allows the cold air to enter the perforated interior of the sand mold, carrying away internal heat through highly efficient convection heat exchange, thus achieving rapid freezing.

[0026] 2. This invention lays and prints sand molds at room temperature. The sand mold is bonded by capillary force between water films, and the laid dry sand acts as a printing support, ensuring the sand mold has sufficient strength and preventing defects such as collapse. After the sand mold is removed from the printing equipment, it is frozen by a low-temperature medium. This eliminates the need for pre-cooling the molding sand and avoids reliance on a low-temperature environment, saving energy, extending the lifespan of the printing equipment, and protecting expensive and fragile components such as the printhead.

[0027] 3. This invention enables offline low-temperature freezing of sand molds, allowing for flexible pre-design of cooling pipe structures based on the casting structure. Before production, the cooling pipes and hollowed-out structures can be optimized through simulation and other means to ensure the strength of the sand mold in actual production. During the subsequent freezing process, the cooling medium and corresponding flow rate, temperature, and other parameters can be selected according to actual needs, improving the operability and flexibility of freezing. Attached Figure Description

[0028] Figure 1 This is a flowchart illustrating the specific process of the present invention.

[0029] Figure 2 This is a schematic diagram of the structure of the frozen sand mold using the present invention.

[0030] Figure 3 This is a schematic diagram of the grid-shaped cooling pipe structure in this invention.

[0031] Figure 4 This is a schematic diagram of a honeycomb-shaped hollow structure used in this invention.

[0032] Reference numerals in the attached drawings: 1. Outer shell; 2. Cooling pipes; 3. Sand drain port; 4. Inner shell; 5. Hollowed-out structure; 6. Cavity; 7. Cooling pipes; 8. Heat dissipation holes. Detailed Implementation

[0033] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. It should be noted that the terms "front," "rear," "left," "right," "up," and "down" used in the following description refer to directions in the accompanying drawings, and the terms "inner" and "outer" refer to directions toward or away from the geometric center of a specific component, respectively.

[0034] Specifically, the following steps are included:

[0035] Step 1: Determine the geometric features of the casting and establish a casting model using 3D modeling software. In this embodiment, the frozen sand mold for manufacturing high neck flanges is used as an example.

[0036] Step 2: Design the corresponding frozen sand mold model based on the three-dimensional model of the high neck flange casting, and set the cooling pipe 2 inside the frozen sand mold along the contour of the cavity 6 wall. The structure of the cooling pipe 2 can be an S-shaped pipe, a grid pipe, or a heat exchange pipe that uses both in combination. Taking into account the structural characteristics of the high neck flange, the efficiency of the hollow structure 5 and the installation of the cooling pipe 7 will be optimized in the future.

[0037] This embodiment uses a crisscross pipe design, i.e., a grid-like pipe structure, such as... Figure 3 As shown. The diameter of cooling pipe 2 is 10mm, the wall thickness of cooling pipe 7 is 5mm, and the sand intake between cooling pipe 2 and the wall of cavity 6 in the sand mold is 20mm.

[0038] Step 3: The cryogenic sand mold with cooling pipes 2 is designed with a perforated structure, taking into account the thickness of the inner and outer shells and the perforated structure 5. Based on the dimensional characteristics of the high-neck flange, the thickness of the inner shell 4 of the cryogenic sand mold is set to 30mm, and the thickness of the outer shell 1 is set to 20mm. The basic perforated structure includes orthogonal truss structure, honeycomb structure, topological lattice structure, and reinforcing rib structure. In this embodiment, a honeycomb structure is used. Figure 4 The sand mold features a hollow design inside; this hollow design allows for the pre-reserved sand discharge port 3 in the cooling pipes, which is used to drain the dry sand remaining in the cooling pipes 2 after the room-temperature sand mold is removed, facilitating the installation of the cooling pipes 7. The inner and outer shells of the hollow sand mold, along with the honeycomb structure in the middle, provide support to meet the requirements of high-temperature thermodynamics and hydrostatic pressure of the molten iron.

[0039] Step 4: Using Procast casting simulation software, simulate the thermal stress experienced by the hollow sand mold during the pouring process. Perform strength verification on the sand mold model using finite element analysis software such as Ansys. Optimize the cooling pipes and hollow structure 5 of the frozen sand mold based on the overall stress field distribution. The final frozen sand mold strength is not less than 0.8 MPa, and the overall weight reduction of the sand mold is approximately 30%–60%. Figure 2 As shown.

[0040] Step 5: The 3D slicing software generates the contour information of each printed section based on the simulated and optimized frozen sand mold hollow conformal model;

[0041] Step 6: Use a digital sand mold 3D printer to import the printing outline information and print the designed room temperature sand mold; the sand laying process and the printing process are carried out at room temperature. The nozzle sprays pure water adhesive as needed according to the current layer cross-sectional information of the sand mold, and the room temperature sand mold is initially bonded by the capillary force and fine adhesive force between the water films.

[0042] Step 7: Remove the room temperature sand mold and insert a cooling pipe 7 with heat dissipation holes through the pre-designed complex cooling pipeline 2 to form a cooling channel. Only the inlet and outlet of the cooling channel are open; the remaining ports are sealed with heat-insulating soft materials such as asbestos gaskets. In this embodiment, liquid nitrogen is introduced from a self-pressurized liquid nitrogen tank into the cooling channel in the form of gas and micro-droplets using a vaporizer. The temperature of the cryogenic medium is controlled at approximately -40℃, and the pressure is 0.4MPa. The cryogenic medium is continuously introduced for 0.5 hours for freezing and solidification. A portion of the cryogenic medium can pass through the heat dissipation holes 8 and the sand discharge port 3 into the hollowed-out sand mold for efficient heat exchange via thermal convection, achieving rapid freezing of the frozen sand mold.

[0043] Step 8: After verifying that the strength of the frozen sand mold meets the standard, clean up the excess dry sand and proceed with the subsequent casting.

[0044] The technical means disclosed in this invention are not limited to those disclosed in the above embodiments, but also include technical solutions composed of any combination of the above technical features.

[0045] The technical means disclosed in this invention are not limited to those disclosed in the above embodiments, but also include technical solutions composed of any combination of the above technical features.

Claims

1. A method for freezing and shaping sand molds with hollowed-out conformal design and low-temperature medium transfer freezing, characterized in that, The method includes the following steps: Step 1: Determine the geometric features of the casting and create a model of the casting using 3D modeling software; Step 2: Design a corresponding freezing sand mold model based on the three-dimensional model of the casting, and set cooling pipes inside the freezing sand mold following the contour of the cavity wall; Step 3: Design the hollowed-out shape of the freezing sand mold for setting the cooling pipes, and establish a hollowed-out conformal model of the freezing sand mold; Step 4: Use finite element analysis software to perform strength verification on the established sand mold model, and optimize the design of the cooling pipes and hollow structure of the frozen sand mold according to the overall stress field distribution of the sand mold; the cooling pipe structure adopts S-shaped pipes, grid pipes, or a combination of both heat exchange pipes, and is designed according to the structural characteristics of the casting. Step 5: The 3D slicing software generates the contour information of each printed section based on the simulated and optimized frozen sand mold hollow conformal model; Step 6: Under normal temperature conditions, use a digital sand mold 3D printer to import the printing outline information and print the designed room temperature sand mold. Step 7: Take out the room temperature sand mold, pass copper mold cooling pipes through the pre-set complex cooling pipeline to form a cooling channel, and introduce low temperature medium into the cooling channel for timed and temperature-controlled heat exchange flow to freeze and solidify the frozen sand mold at low temperature. Step 8: After verifying that the strength of the frozen sand mold meets the standard, clean up the excess dry sand and proceed with the subsequent casting.

2. The cryogenic sand mold hollowing conformal design and low-temperature medium transport freezing forming method according to claim 1, characterized in that: In step 4: the strength is checked using finite element method, and additional stiffeners or supporting trusses are added to the orthogonal truss structure where the strength is insufficient; secondary topology optimization is performed on the areas with insufficient strength in the hollowing software; the hollowing structure at the bottom of the model is appropriately trimmed, and the hollowing structure is reinforced by increasing the contact area of ​​the substrate at the bottom of the model to prevent it from collapsing during the printing process.

3. The cryogenic sand mold hollowing conformal design and low-temperature medium transport freezing forming method according to claim 1, characterized in that, The diameter of the cooling pipe is 2mm to 20mm, the wall thickness of the cooling pipe is 5mm to 10mm, and the amount of sand absorbed by the cooling pipe between the cooling pipe and the cavity wall in the sand mold is 10mm to 30mm.

4. The cryogenic sand mold hollow conformal design and low-temperature medium transport freezing forming method according to claim 1, characterized in that, The hollow design allows for the pre-reserved sand drain ports in the cooling pipes, which are used to drain any unfrozen dry sand remaining in the cooling pipes, facilitating the connection and installation of the cooling pipes.

5. The cryogenic sand mold hollowing conformal design and low-temperature medium transport freezing forming method according to claim 1, characterized in that, When designing the hollow structure, the thickness of the inner and outer shells of the sand mold and the hollow structure should be considered as a whole. The thickness of the inner and outer shells of the sand mold is 20mm to 50mm. The basic hollow structure includes orthogonal truss structure, honeycomb structure, topological lattice structure or reinforcing rib structure. The hollow structure can be applied to the entire sand mold or to a part of the sand mold.

6. The cryogenic sand mold hollow conformal design and low-temperature medium transport freezing forming method according to claim 1, characterized in that, The strength of the optimized frozen sand mold is not less than 0.8 MPa, and the overall weight of the sand mold is reduced by 30% to 60%.

7. The cryogenic sand mold hollow conformal design and low-temperature medium transport freezing forming method according to claim 1, characterized in that, Both the sand-laying and printing processes are carried out at room temperature. The nozzle sprays pure water adhesive as needed based on the current cross-sectional information of the sand mold. The sand mold is initially bonded at room temperature through capillary force and fine adhesive force between the water films.

8. The cryogenic sand mold hollow conformal design and low-temperature medium transport freezing forming method according to claim 1, characterized in that, The cryogenic medium can be a liquid or a gas; the liquid temperature reaches -196℃ for rapid freezing, or the gas temperature is controlled at -2℃. Between -50℃; when introducing a low-temperature gas medium, a cooling pipe with heat dissipation holes is used. A portion of the low-temperature medium can pass through the heat dissipation holes and enter the hollowed-out sand mold through the sand discharge port for efficient heat exchange and rapid freezing.

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