Method for improving strength of 3D printed sand core

Abstract

The invention belongs to the technical field of part manufacturing, and particularly relates to a method for improving the strength of a 3D printed sand core, which comprises the following steps: 3D printing the sand core, reserving a placing hole and a sand blowing hole when the sand core is printed, and prefabricating a plugging block; blowing off the dry sand in the placing hole and the sand blowing holes, plugging two ends of the placing hole and the sand blowing holes by using plugging blocks, reserving 1-5 sand blowing holes as pouring holes, and fixing and sealing the plugging blocks and the sand core through core adhesive; liquid metal is poured into the placing hole through the pouring hole; after the liquid metal is cooled and formed, a gap is naturally formed between the metal core bar and the sand core based on the principle of thermal expansion and cold contraction, and casting glue is squeezed into the gap between the sand core and the metal core bar to fill the gap; and removing the blocking blocks at two ends of the placing hole. According to the invention, the placement efficiency of the core bar of the complex sand core is effectively improved, the stability of the core bar is higher, and the situation that the core bar is difficult to insert due to hole position deviation or size mismatching when the core bar is manually inserted is avoided.

Description

Technical Field

[0001] The present invention belongs to the technical field of component manufacturing, and in particular relates to a method for improving the strength of a sand core for 3D printing. Background Art

[0002] The 3D printing sand core technology has significantly improved the flexible production capacity of the casting industry through digital manufacturing and material innovation, and is particularly suitable for the development of complex castings in fields such as aerospace and automotive. With the maturity of technologies such as thermal regenerated sand, the large-scale application cost has approached that of traditional processes, becoming the core driving force for the upgrading of modern casting industry.

[0003] In order to improve the strength of the sand core, a core frame usually needs to be placed in the sand core and a pre-tightening force is applied to the core frame. For example, the patent with the publication number CN108480566B discloses a method for placing a core frame in an additive manufacturing core. The core frame is pushed in by an external force, so that the core frame is in close contact and combination with the filled resin sand. The filled resin sand around the core frame is more tightly and firmly combined with the sand core, avoiding delamination, separation, and falling off between the core frame and the filled resin sand, and between the filled resin sand and the sand core.

[0004] For example, the patent with the publication number CN102248131B discloses a sand core core frame pre-tightening device, which can continuously enhance the stiffness of the sand core core frame during use by adjusting the deformation degree of the compression spring to generate different pre-tightening forces and continuously acting on the sand core core frame.

[0005] The existing devices still have the following deficiencies:

[0006] 1. The above-mentioned existing core frame placement methods are only applicable to the placement of relatively simple sand core core frames, and are unable to place core frames with complex shapes or in three-dimensional directions. Moreover, the above methods have extremely high precision requirements for hole design. If the hole positions deviate or the sizes do not match, it will cause difficulties in inserting the core frame and insecure fixation. In addition, manually inserting the core frame easily damages the sand core structure.

[0007] 2. For sand cores with complex shapes, the existing core frame placement method is to place multiple core frames, and the multiple core frames are connected by connecting rings. This method requires manual assembly of multiple core frames, resulting in low core frame placement efficiency and poor connection stability between the core frames.

[0008] 3. When the existing technology fixes the sand core fixed end and pre-tightens the core frame, the sand core is first fixed in the sand box through the core head, and then the core frame is pre-tightened by a separate pre-tightening device. The fixation of the sand core and the pre-tightening of the core head are realized through different components and operations respectively, and the operation is relatively cumbersome. Moreover, when the core frame is pre-tightened, the force on the sand box is relatively concentrated, which is likely to cause deformation or damage to the sand box. Summary of the Invention

[0009] The object of the present invention is to provide a method for improving the strength of a sand core for 3D printing in view of the problems raised in the above background art.

[0010] To achieve the above object, the present invention adopts the following technical solutions: A method for improving the strength of a sand core for 3D printing, comprising the following steps:

[0011] S1. 3D print the sand core, and reserve placement holes and sand blowing holes during the printing process, and prefabricate blocking blocks.

[0012] S2. Blow out the dry sand in the placement holes and sand blowing holes, use the blocking blocks to seal both ends of the placement holes and the sand blowing holes, and reserve 1-5 sand blowing holes as pouring holes. The blocking blocks are fixed and sealed with the sand core through core sticking glue.

[0013] S3. Pour liquid metal into the placement holes through the pouring holes.

[0014] S4. After the liquid metal cools and forms, based on the principle of thermal expansion and contraction, a gap is naturally formed between the metal core and the sand core. Squeeze casting glue into the gap between the sand core and the metal core to fill the gap.

[0015] S5. Remove the blocking blocks at both ends of the placement holes, open threaded holes at both ends of the metal core, and then repair the defects on the surface of the sand core.

[0016] S6. Fix the sand core in the sand box, and apply a pre-tightening force to the core through the core pre-tightening device to complete the fixation and strengthening of the sand core.

[0017] Further, the diameter of the placement hole is calculated based on the volume of the sand core. The value range of the diameter of the placement hole is [10, 30] mm. When the volume of the sand core < 1L, the diameter of the placement hole is taken as 10 mm. When the volume of the sand core > 20L, the diameter of the placement hole is taken as 30 mm. The specific calculation formula for the diameter of the placement hole is as follows:

[0018]

[0019] In the formula, D is the diameter of the placement hole, and V is the volume of the sand core.

[0020] Further, the diameter of the sand blowing hole is calculated based on the diameter of the placement hole. The calculation formula for the diameter of the sand blowing hole is:

[0021] d = 0.5D

[0022] In the formula, d is the diameter of the sand blowing hole.

[0023] Further, the dry sand in the placement holes and sand blowing holes is blown in the reverse direction by compressed gas, and the blowing time is 15 s.

[0024] Further, the melting point of the liquid metal needs to be at least 100 °C lower than the melting point of the core material to ensure that the core is not affected during casting.

[0025] Further, squeezing casting glue into the gap between the core and the metal core bone to fill the gap includes the following steps:

[0026] S41. Calculate the size of the gap between the core and the metal core bone and inject the corresponding volume of casting glue. The calculation formula is as follows:

[0027] ΔV = α×V0

[0028] In the formula, α is the solidification shrinkage rate of the liquid metal, and V0 is the initial volume of the metal liquid;

[0029] S42. Heat the core to 80 °C and keep it at a constant temperature for 2 hours to preliminarily cure the casting glue colloid and release the shrinkage stress;

[0030] S43. Raise the temperature of the core to 150 °C at a rate of 2 °C per minute and keep it at a constant temperature for 2 hours to achieve diffusion bonding at the metal-casting glue interface.

[0031] Further, the pre-tightening device includes a slide rail fixed on the outer side wall of the sand box. The slide rail is made of heat-insulating material. Two symmetric sliders are slidably arranged on the slide rail. Two symmetric limit blocks are fixed at both ends of the slide rail. A spring is arranged between the slider and the adjacent limit block. A connecting rod is rotatably connected to the slider. The other end of the connecting rod is rotatably connected to a rotating seat. A pre-tightening screw is rotatably connected to the rotating seat. In the natural state of the spring, the pre-tightening screw is completely outside the sand box.

[0032] Further, the pre-tightening method of the pre-tightening device includes the following steps:

[0033] S61. Place the core at the installation position in the sand box;

[0034] S62. Screw the end of the threaded section of the pre-tightening screw into the threaded hole at the end of the core bone to position the core;

[0035] S63. Continue to screw the pre-tightening screw into the core bone according to the required pre-tightening force to control the compression deformation amount of the spring, thereby controlling the magnitude of the spring force to achieve the control of the pre-tightening force magnitude. The pre-tightening effect is achieved by continuously applying the pre-tightening force to the core bone through the spring force.

[0036] Compared with the existing technology, the advantages of the present invention are as follows:

[0037] 1. The present invention places the core bone by pouring, utilizes the fluidity of the liquid metal to make the metal cover the complex internal structure of the sand core, enhances the overall rigidity, reduces the risk of the sand core breaking or deforming during the casting process, is applicable to the placement of core bones for various complex sand cores, and avoids the situation of difficult core bone insertion caused by hole position deviation or size mismatch when manually inserting the core bone. In addition, the present invention also avoids the situation of secondary damage to the sand core when manually inserting the core bone.

[0038] 2. When placing the core bone in the present invention, it is formed by one-time pouring, effectively improving the efficiency of placing the core bone of the complex sand core, and the stability of the core bone is higher.

[0039] 3. The pre-tightening of the core bone and the fixation of the sand core in the present invention are achieved through the same operation, integrating the originally step-by-step positioning, clamping, and pre-tightening processes into a single adjustment action, significantly simplifying the process flow.

[0040] 4. The present invention increases the contact area between the pre-tightening device and the sand box through the slide rail, avoids local stress on the sand box during pre-tightening, and thus reduces the possibility of sand box deformation or damage. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 is the overall structural schematic diagram during the 3D printing and forming of the sand core in a method for improving the strength of a 3D printed sand core provided by the present invention;

[0042] Figure 2 is the structural schematic diagram of the sand core when filling the casting adhesive in a method for improving the strength of a 3D printed sand core provided by the present invention;

[0043] Figure 3 is the structural schematic diagram of the sand core after the core bone is placed in a method for improving the strength of a 3D printed sand core provided by the present invention;

[0044] Figure 4 is the overall structural schematic diagram during the pre-tightening of the core bone in a method for improving the strength of a 3D printed sand core provided by the present invention;

[0045] Figure 5 is the structural schematic diagram of the pre-tightening device in a method for improving the strength of a 3D printed sand core provided by the present invention;

[0046] Figure 6 is the overall flow chart of a method for improving the strength of a 3D printed sand core provided by the present invention;

[0047] Figure 7 is the flow chart of the filling gap of the casting adhesive in a method for improving the strength of a 3D printed sand core provided by the present invention;

[0048] Figure 8It is a flowchart of core fixing and core bone pre-tightening in a method for improving the strength of a 3D-printed sand core provided by the present invention.

[0049] In the figure, 1 is a sand box, 2 is a sand core, 21 is a placement hole, 22 is a sand blowing hole, 23 is a plug, 24 is a metal core bone, 25 is a casting adhesive, 26 is a threaded hole, 3 is a slide rail, 31 is a slider, 32 is a limit block, 33 is a spring, 34 is a connecting rod, 35 is a rotating seat, and 36 is a pre-tightening screw. Specific embodiments

[0050] The following embodiments are for illustrative purposes only and are not intended to limit the scope of the present invention.

[0051] As Figures 1-8 shown, a method for improving the strength of a 3D-printed sand core includes the following steps:

[0052] S1. 3D-print the sand core 2, and reserve a placement hole 21 and a sand blowing hole 22 when printing the sand core 2, and prefabricate a plug 23;

[0053] Among them, the placement hole 21 refers to a prefabricated metal casting channel inside the sand core 2, and its path is determined according to the force analysis of the sand core 2. Specifically, three-dimensional modeling software can be used for topological optimization design to achieve the matching of the core bone 24 structure and the stress distribution of the sand core 2. The sand blowing hole 22 refers to an auxiliary channel communicated with the placement hole 21, which is convenient for removing residual uncured sand grains during the printing process. The plug 23 refers to a prefabricated module for closing the hole opening, which can be specifically made of a resin composite material compatible with the material of the sand core 2, and airtight sealing is achieved through a core bonding adhesive. The diameter of the placement hole 21 is calculated based on the volume of the sand core 2. The value range of the diameter of the placement hole 21 is [10, 30] mm. When the volume of the sand core 2 < 1 L, the diameter of the placement hole 21 is 10 mm. When the volume of the sand core 2 > 20 L, the diameter of the placement hole 21 is 30 mm. The specific calculation formula for the diameter of the placement hole 21 is as follows:

[0054]

[0055] In the formula, D is the diameter of the placement hole, and V is the volume of the sand core

[0056] The diameter of the sand blowing hole 22 is calculated based on the diameter of the placement hole 21. The calculation formula for the diameter of the sand blowing hole 22 is:

[0057] d = 0.5D

[0058] In the formula, d is the diameter of the sand blowing hole;

[0059] Specifically, the volume of the core 2 is used as the core calculation benchmark. The three-dimensional volume is converted into a one-dimensional aperture parameter through a cube root function, so that the diameter of the placement hole 21 for different-sized cores 2 forms a geometric proportional relationship with its volume. When the volume of the core 2 is in the middle range, the aperture value is accurately calculated through a mathematical formula to avoid size deviations caused by simple linear ratios. When the volume exceeds the critical range, the extreme aperture is directly adopted, which not only ensures the structural integrity of the small-volume core 2 but also meets the load-bearing requirements of the large-volume core 2, effectively avoiding problems such as the fixing failure of the core bone 24 or the structural damage of the core 2 caused by improper aperture design, and improving the success rate and structural reliability of the placement of the core bone 24 in the complex-shaped core 2;

[0060] Furthermore, based on the linear relationship between the gas flow rate and the channel cross-sectional area in fluid mechanics, the diameter of the sand-blowing hole 22 is set to 50% of the diameter of the placement hole 21, so that the compressed air can form a sufficient flow rate to remove sand grains during the purging operation and can also avoid the local wall thickness of the core 2 being thinned due to too large an aperture. By establishing an accurate mathematical constraint relationship, it can be ensured that the sizes of the sand-blowing holes 22 and the placement holes 21 of different specifications of the core 2 are automatically matched, eliminating the size deviations caused by manual empirical values;

[0061] S2. Blow out the dry sand in the placement hole 21 and the sand-blowing hole 22. Use the plug 23 to block both ends of the placement hole 21 and the sand-blowing hole 22 and reserve 1 - 5 sand-blowing holes 22 as pouring holes. The plug 23 is fixed and sealed with the core 2 through core-bonding glue. The dry sand in the placement hole 21 and the sand-blowing hole 22 is purged in the reverse direction by compressed gas. The purging time is 15 s. Among them, the reverse purging of compressed gas means that the blowing direction forms a reverse flow with the sand grain accumulation direction when the core 2 is formed. Specifically, it can be realized by using a gas gun device with a direction control valve. The reverse air flow can effectively peel off the loose sand grains attached to the inner wall of the hole. The purging time of 15 seconds can be achieved by controlling the opening and closing of the air valve through a timer;

[0062] S3. Pour the liquid metal into the placement hole through the pouring hole. Specifically, the melting point of the liquid metal needs to be at least 100 °C lower than the melting point of the core material to ensure that the core is not affected during pouring;

[0063] Among them, the melting point of the liquid metal refers to the critical temperature value at which the metal material changes from a solid state to a liquid state. Specifically, low-melting-point metal materials such as aluminum alloy and zinc alloy can be used to achieve this. For example, the melting point of aluminum alloy is about 660°C. The melting point of the core sand 2 material refers to the melting temperature threshold of the main components of the core sand 2. Specifically, a composite system of silica sand and resin binder can be used to achieve this. For example, the melting point of silica sand is 1710°C. The setting of a melting point difference of at least 100°C is based on the balance relationship between the solidification shrinkage of the metal and the thermal stability of the core sand 2. A natural thermal barrier is formed through the melting temperature difference between the metal and the core sand 2 material. During pouring, a vacuum pump is connected to the pouring hole, and the vacuum degree is ≤ -0.08 MPa to avoid porosity defects;

[0064] Specifically, during the pouring process, when the liquid metal is injected into the placement hole, the temperature of the metal liquid is higher than the instantaneous temperature of the contact surface of the core sand 2. Since the melting point of the core sand 2 material is more than 100°C higher than that of the metal liquid, the heat transferred from the metal liquid to the surface of the core sand 2 is not sufficient to reach the phase change temperature of the core sand material. This temperature gradient control enables only limited heat conduction to occur on the surface of the core sand, without causing material melting or structural deformation. The heat released during the solidification process of the metal is absorbed and dissipated through the high thermal stability of the core sand 2 material, and the internal pore structure of the core sand 2 is maintained intact;

[0065] S4. After the liquid metal cools and forms, based on the principle of thermal expansion and contraction, a gap naturally forms between the metal core frame 24 and the core sand 2. Casting glue 25 is squeezed into the gap between the core sand 2 and the metal core frame 24 to fill the gap. In this embodiment, the casting glue 25 is a two-component polyurethane glue, and after curing, the tensile strength is ≥ 30 MPa, and the injection pressure is 0.1 - 0.3 MPa. Specifically, squeezing the casting glue 25 into the gap between the core sand 2 and the metal core frame 24 to fill the gap includes the following steps:

[0066] S41. Calculate the size of the gap between the core sand 2 and the metal core frame 24 and inject the corresponding volume of casting glue 25. The calculation formula is as follows:

[0067] ΔV = α × V0

[0068] In the formula, α is the solidification shrinkage rate of the liquid metal, and V0 is the initial volume of the metal liquid;

[0069] S42. Heat the core sand 2 to 80°C and keep it at a constant temperature for 2 hours to preliminarily cure the casting glue 25 colloid and release the shrinkage stress;

[0070] S43. Raise the temperature of the core sand 2 to 150°C at a rate of 2°C per minute and keep it at a constant temperature for 2 hours to achieve diffusion bonding at the metal-casting glue interface;

[0071] Among them, the solidification shrinkage rate α refers to the ratio of the volume change during the transformation of liquid metal from the liquid state to the solid state. Specifically, it can be obtained from the thermophysical property parameter table of the metal material and is used to quantify the shrinkage amount after metal solidification and then determine the gap size. The gradient temperature control process includes an 80°C constant temperature stage and a 150°C heating stage, which is specifically realized by a segmented temperature control device. By controlling the phase change rate and molecular diffusion behavior during the curing of the casting adhesive 25, the dual goals of stress release and interfacial bonding are achieved;

[0072] Specifically, the gap size is calculated through the mathematical relationship between the solidification shrinkage rate and the initial volume of the metal, ensuring that the filling amount of the casting adhesive 25 precisely matches the gap space to avoid insufficient or excessive filling. During the 80°C constant temperature stage, the casting adhesive 25 completes the crosslinking and curing of the colloid and releases the shrinkage stress, eliminating potential interfacial bonding hazards. Subsequently, when the temperature is raised to 150°C at a controlled rate, the molecules on the metal surface and the casting adhesive 25 undergo diffusion migration through thermal activation to form a transition layer of metallurgical bonding, enhancing the interfacial bonding strength;

[0073] S5. Remove the plugs 24 at both ends of the placement holes 21, drill threaded holes 26 at both ends of the metal core bone 24, and then repair the surface defects of the sand core 2;

[0074] S6. Fix the sand core 2 in the sand box 1 and apply a pre-tightening force to the core bone through the core bone pre-tightening device to complete the fixation and strengthening of the sand core 2. Among them, the pre-tightening device includes a slide rail 3 fixed on the outer side wall of the sand box 1. The slide rail 3 is made of heat-insulating material. Two symmetric sliders 31 are slidably arranged on the slide rail 3. Two symmetric limit blocks 32 are fixed at both ends of the slide rail 3. A spring 33 is arranged between the slider 31 and the adjacent limit block 32. A connecting rod 34 is rotatably connected to the slider 31. The other end of the connecting rod 34 is rotatably connected to a rotating seat 35. A pre-tightening screw 36 is rotatably connected to the rotating seat 35. In the natural state of the spring 33, the pre-tightening screw 36 is completely outside the sand box 1. The pre-tightening method of the pre-tightening device includes the following steps:

[0075] S61. Place the sand core 2 at the installation position in the sand box 1;

[0076] S62. Screw the threaded end of the pre-tightening screw 36 into the threaded hole 26 at the end of the core bone to position the sand core 2;

[0077] S63. Continue to screw the pre-tightening screw 36 into the core bone according to the required magnitude of the pre-tightening force to control the compression deformation amount of the spring 33, thereby controlling the magnitude of the spring force of the spring 33 to control the magnitude of the pre-tightening force. The pre-tightening effect is achieved by continuously applying a pre-tightening force to the core bone 2 through the spring force of the spring 33;

[0078] Specifically, during the process of screwing the pre-tightening screw 36 into the threaded hole 26 of the core bone, an axial displacement is generated. This displacement is converted into a controllable pre-tightening force through the compression deformation of the spring 33. When the pre-tightening screw 36 continues to be screwed in, the slider 31 on the slide rail 3 is pushed by the reaction force of the spring 33, so that the pre-tightening force is transmitted to both ends of the core bone through the rotating seat 35 and the connecting rod 34. The change in the compression amount of the spring 44 has a linear relationship with the screw-in depth of the thread, realizing continuous adjustment of the pre-tightening force. The limiting blocks 32 on both sides of the slide rail 3 and the spring 33 together form a distributed support structure, which converts the concentrated load into a stress field distributed along the length direction of the slide rail 3, avoiding the deformation or damage of the sand box 1 caused by the local stress exceeding the bearing threshold of the sand box 1;

[0079] The present invention places the core bone by pouring, eliminates assembly errors through in-situ metal forming, adapts to the requirements of core bones with any complex shape, and effectively improves the placement efficiency and stability of the core bone of the complex sand core 2. The present invention also avoids secondary damage to the sand core 2 caused by mechanically inserting the core bone, and significantly improves the interface bonding strength through the metal-colloid composite core bone structure. In addition, the present invention also realizes the synchronous completion of the positioning and pre-tightening operations of the sand core 2, reducing the process time-consuming of the step-by-step operations in the traditional process. The distributed support structure effectively reduces the stress peak in the local area of the sand box 1, avoiding the deformation or cracking problem of the sand box 1 caused by stress concentration. The composite structure of threaded connection and elastic support provides the ability to maintain a continuous and stable pre-tightening force while ensuring the positioning accuracy.

[0080] The above are only the preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for improving the strength of a sand core for 3D printing, characterized in that, It includes the following steps: S1. 3D print the sand core, and reserve placement holes and sand blowing holes when printing the sand core, and prefabricate plug blocks. S2. Blow out the dry sand in the placement holes and sand blowing holes, use the plug blocks to seal both ends of the placement holes and the sand blowing holes, and reserve 1 - 5 sand blowing holes as pouring holes. The plug blocks are fixed and sealed with the sand core through core - sticking glue. S3. Pour liquid metal into the placement holes through the pouring holes. S4. After the liquid metal cools and forms, based on the principle of thermal expansion and contraction, a gap is naturally formed between the metal core frame and the sand core. Squeeze casting glue into the gap between the sand core and the metal core frame to fill the gap. S5. Remove the plug blocks at both ends of the placement holes, open threaded holes at both ends of the metal core frame, and then repair the defects on the surface of the sand core. S6. Fix the sand core in the sand box, and apply a pre - tightening force to the core frame through the core frame pre - tightening device to complete the fixation and strengthening of the sand core.

2. The method for improving the strength of a sand core for 3D printing according to claim 1, characterized in that The diameter of the placement hole is calculated based on the volume of the sand core. The value range of the diameter of the placement hole is [10, 30] mm. When the volume of the sand core < 1 L, the diameter of the placement hole is taken as 10 mm. When the volume of the sand core > 20 L, the diameter of the placement hole is taken as 30 mm. The specific calculation formula for the diameter of the placement hole is as follows: In the formula, D is the diameter of the placement hole, and V is the volume of the sand core.

3. A method for improving the strength of a sand core for 3D printing according to claim 2, characterized in that, The diameter of the sand blowing hole is calculated based on the diameter of the placement hole. The calculation formula for the diameter of the sand blowing hole is: d = 0.5D In the formula, d is the diameter of the sand blowing hole.

4. A method for improving the strength of a 3D printed sand core according to claim 1, characterized in that, The dry sand in the placement holes and sand blowing holes is reverse - purged by compressed gas, and the purging time is 15 s.

5. A method for improving the strength of a sand core for 3D printing according to claim 1, characterized in that, The melting point of the liquid metal needs to be at least 100 °C lower than the melting point of the sand core material to ensure that the sand core will not be affected during pouring.

6. A method for improving the strength of a sand core for 3D printing according to claim 1, wherein Squeezing casting glue into the gap between the sand core and the metal core frame to fill the gap includes the following steps: S41. Calculate the size of the gap between the sand core and the metal core frame and inject the corresponding volume of casting glue. The calculation formula is as follows: ΔV = α×V0 In the formula, α is the solidification shrinkage rate of the liquid metal, and V0 is the initial volume of the metal liquid. S42. Heat the sand core to 80 °C and keep it at a constant temperature for 2 hours to make the casting glue colloid preliminarily solidify and release the shrinkage stress. S43. Raise the temperature of the sand core to 150 °C at a speed of 2 °C per minute and keep it at a constant temperature for 2 hours to make the metal - casting glue interface diffuse and bond.

7. A method for improving the strength of a 3D printed sand core according to claim 6, characterized in that, The pre - tightening device includes a slide rail (3) fixed on the outer side wall of the sand box. The slide rail (3) is made of heat - insulating material. Two symmetric sliders (31) are slidably arranged on the slide rail (3). Two symmetric limit blocks (32) are fixed at both ends of the slide rail (3). A spring (33) is arranged between the slider (31) and the adjacent limit block (32). A connecting rod (34) is rotatably connected to the slider. The other end of the connecting rod (34) is rotatably connected to a rotating seat (35). A pre - tightening screw (36) is rotatably connected to the rotating seat (35). In the natural state of the spring (33), the pre - tightening screw (36) is completely outside the sand box.

8. A method for improving the strength of a 3D printed sand core according to claim 7, characterized in that, The pre - tightening method of the pre - tightening device includes the following steps: S61. Place the sand core at the installation position in the sand box. S62. Screw the threaded end of the pre - tightening screw into the threaded hole at the end of the core frame to achieve the positioning of the sand core. S63. Continue to screw the pre-tightening screw into the core bone according to the magnitude of the required pre-tightening force to control the compression deformation of the spring, thereby controlling the magnitude of the spring force to achieve the control of the pre-tightening force magnitude. The pre-tightening effect is achieved by continuously applying the pre-tightening force to the core bone through the spring force.

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