A method suitable for detecting the densification of a powder bed in binder jet additive manufacturing

Abstract

The present invention discloses a method applicable to detecting the densification of a powder bed in binder jet additive manufacturing, belonging to the technical field of binder jet forming, and comprising the following steps: Step 1: According to the effective forming cavity of the forming cylinder, design the number of detection points in the effective forming cavity and the positions of each detection point; Step 2: Determine the size of the model box according to the number and positions of the detection points, and use a binder jet 3D printing device to print a model box at each detection point while spreading the powder; Step 3: Take out all the model boxes one by one; Step 4: Pour out the powder in each model box one by one and weigh it to obtain the mass of the powder in each model box, and measure the inner wall size of the corresponding model box one by one to obtain the volume of the model box, then calculate the density of the powder in each model box, and further calculate the standard deviation of the density of the powder in all the model boxes, so as to judge the accuracy during the printing of the powder bed.

Description

Technical Field

[0001] The present invention relates to the technical field of binder jetting forming, and more specifically, to a method suitable for detecting the density of a powder bed in binder jetting additive manufacturing. Background Art

[0002] In recent years, as an advanced additive manufacturing technology, the binder jetting forming technology of metal materials has attracted wide attention in the industrial field due to its advantages such as no heat source input, wide material applicability, high forming rate, and low production cost. Its basic process is as follows: First, a forming powder material is laid to form a powder bed by using a doctor blade or a powder spreading roller, and a two-dimensional pattern required for the binder is evenly and quantitatively jetted through a nozzle group to complete the single-layer powder bonding forming. The powder bed descends by a layer thickness distance and is repeated layer by layer until the forming is completed. Secondly, a green body without residual powder is obtained through drying and curing and powder cleaning. Finally, a product part with the required mechanical properties is formed through debinding and sintering.

[0003] However, in existing production application cases, there are still many technical bottlenecks in the binder jetting forming technology, mainly the structural deformation and dimensional control of product parts after sintering. The invention patent with the publication number CN113015896A discloses a device and method for measuring the density of a powder bed in 3D printing / additive manufacturing operations. The device includes a device for determining the powder mass and a device for determining the volume of the powder during the spreading process, thereby allowing the determination of the powder bed density (the powder density of the layer). The device for determining the volume of the powder includes a laser assembly, which is adapted to move the laser in two directions, left and right, on the surface of the powder layer for scanning. The device also allows the determination of the layer surface profile of the process involved in the AM operation and the static and dynamic powder properties. Although it can dynamically measure the powder bed performance layer by layer, the existing statement shows that the binder jetting forming is an isotropic forming method different from selective laser melting. In fact, the shrinkage rate in the forming direction Z is significantly greater than that in the horizontal directions X and Y. The reason is that the binder jetting process is a process of laying and bonding powder beds layer by layer. As the number of forming layers increases, the powder in the powder bed continuously increases. Under the action of the powder gravity, the Z-direction forces received at each position are different, resulting in different powder densities at each position unit, that is, different densifications at each position unit. Especially when preparing product parts with larger dimensions in the height direction, structural deformation and uncontrollable dimensions are more likely to occur after sintering, affecting the product consistency and qualification rate.

[0004] Therefore, a method suitable for detecting the density of a powder bed in binder jetting additive manufacturing is provided to detect the discrete situation of the density of each position unit in the powder bed. Summary of the Invention

[0005] In view of this, the present invention aims to provide a method suitable for detecting the density of a powder bed in binder jet additive manufacturing, for detecting the discreteness of the density of each position unit of the powder bed, providing a judgment basis for the laying quality and stability of the powder bed, and at the same time providing a theoretical basis for the adjustment of the model data of the preformed product parts and the sintering process of the postformed green body.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A method suitable for detecting the density of a powder bed in binder jet additive manufacturing, comprising the following steps:

[0008] Step 1: According to the effective forming cavity of the forming cylinder, design the number of detection points in the effective forming cavity and the positions of each detection point, ensuring that all detection points are evenly distributed in the effective forming cavity;

[0009] Step 2: Determine the size of the model box according to the number and positions of the detection points, and use a binder jet 3D printing device to print a model box at each detection point;

[0010] Step 3: After the model boxes at all detection points are printed and cured, clean the powder around the model boxes and take out all the model boxes one by one;

[0011] Step 4: Pour out the powder in each model box one by one and weigh it to obtain the mass of the powder in each model box, and measure the inner wall dimensions of the corresponding model box one by one to obtain the volume of the model box, and then calculate the density of the powder in each model box, and further calculate the standard deviation of the density of the powder in all model boxes, so as to judge the accuracy when printing this powder bed;

[0012] Among them, the density calculation formula of the powder in each model box is: ρ = m / v;

[0013] In the formula, ρ is the density of the powder in this model box, m is the mass of the powder in this model box, and v is the volume of this model box;

[0014] The standard deviation of the density of the powder in all model boxes is calculated by the following formula:

[0015] ρ 均 = 1 / N (ρ1 + ρ2 + ρ3 + …… ρ N )

[0016] σ = {1 / N [(ρ1 - ρ 均 ) 2 + (ρ2 - ρ 均 ) 2 + (ρ3 - ρ 均 ) 2 …… + (ρN -ρ 均 ) 2} 1 / 2

[0017] In the formula, N represents the number of all model boxes, ρ 均 represents the average density of the powder in all model boxes, and σ represents the standard deviation of the density of the powder in all model boxes.

[0018] The beneficial effects that the present invention can achieve: By arranging a plurality of uniformly arranged monitoring points in the forming cylinder, printing corresponding model boxes at each detection point, and then obtaining the mass of the powder laid inside the model box during the printing process, and combining with the volume of the model box, the powder density in the model box can be determined. By calculating the standard deviation of the powder density in all model boxes, the accuracy during the powder bed printing can be judged, providing a basis for evaluating the laying quality and stability of the powder bed, and at the same time providing a theoretical basis for adjusting the model data of the preformed product parts and the sintering process of the post-formed green body.

[0019] Preferably, in step one, the forming cylinder is a cube or cuboid structure with an open top. The effective forming cavity in the forming cylinder includes multiple forming surfaces arranged in sequence along its height direction. At least 9 detection points are evenly arranged in each forming surface, and the 9 detection points respectively correspond to the center position, four corner positions, and midpoint positions of the four sides of the forming surface of this layer.

[0020] Preferably, in step two, there is a first gap h1 between the model box corresponding to the bottom forming surface and the bottom wall of the forming cylinder, and the first gap h1 is not less than 5 mm.

[0021] Preferably, in step two, there is a second gap h2 between the inner side wall of the forming cylinder and the adjacent model box, and the second gap h2 is not less than 5 mm.

[0022] Preferably, in step two, there is a third gap h3 between adjacent model boxes in the same forming surface, and the third gap h3 is not less than 5 mm.

[0023] Preferably, in step two, there is a fourth gap h4 between adjacent model boxes in adjacent forming surfaces, and the fourth gap h4 is not less than 5 mm.

[0024] Preferably, in step two, the model box includes a box body and a box cover. The box body has an open top, and the box cover is provided on the open top of the box body.

[0025] Preferably, the model box further includes a locking assembly for locking the box body and the box cover. The locking assembly includes two L-shaped clamping plates and two clamping protrusions. The box body is of a cuboid structure. One ends of the two L-shaped clamping plates are respectively fixed on two opposite side walls of the box cover, and the other ends thereof are arranged oppositely, so as to form a chute penetrating along the direction perpendicular to the two opposite side walls of the box cover between the L-shaped clamping plates and the lower cover surface of the box cover; the two clamping protrusions are respectively fixed on the upper ends of the outer wall surfaces of two opposite side walls of the box body and are slidably connected in the chute.

[0026] Preferably, there is a fifth gap h5 between the end of the L-shaped clamping plate far from the box cover and the outer wall of the box body, and the range of the fifth gap h5 is 0.15 mm - 0.24 mm.

[0027] Preferably, the side length of the box body is 10 - 50 mm, the height is 5 - 8 mm, and the wall thickness is 2 - 5 mm.

[0028] Through the above technical solutions, compared with the prior art, the present invention discloses a method applicable to detecting the density of a powder bed in binder jet additive manufacturing. By setting a plurality of detection points in the forming cylinder, printing corresponding model boxes at each detection point, and then obtaining the mass of the powder laid inside the model box during the printing process, and combining with the volume of the model box, the powder density inside the model box can be determined. By calculating the standard deviation of the powder density in all model boxes, the accuracy during the powder bed printing can be judged, providing a basis for evaluating the laying quality and stability of the powder bed, and at the same time providing a theoretical basis for adjusting the model data of the pre-formed product parts and the sintering process of the post-formed green body. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] 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 required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained according to the provided drawings without creative efforts.

[0030] Figure 1 It is a schematic structural diagram of the model box of the present invention.

[0031] Figure 2 It is a schematic structural diagram of the distribution of the model box in the forming cylinder in Embodiment 1 of the present invention.

[0032] Figure 3 It is a schematic structural diagram of the placement state of the first layer of models on the xoy plane in Embodiment 1.

[0033] Figure 4Schematic diagram of the placement state structure of the first layer model in the yoz plane in Embodiment 1.

[0034] In the figure: 1. Forming cylinder, 2. Model box, 21. Box body, 22. Box cover, 23. L-shaped clamping plate, 24. Clamping protrusion. Detailed implementation manners

[0035] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0036] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the accompanying drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present invention.

[0037] In the present invention, unless otherwise clearly defined and limited, the terms "installation", "connection", "connection", "fixation", etc. should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or integrated; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the internal communication of two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0038] Please refer to Figures 1 - 4 , the embodiments of the present invention disclose a method suitable for detecting the density of a powder bed in binder jet additive manufacturing, including the following steps:

[0039] Step 1: According to the effective forming cavity of the forming cylinder 1, design the number of detection points in the effective forming cavity and the positions of each detection point to ensure that all detection points are evenly distributed in the effective forming cavity;

[0040] Step 2: Determine the size of the model box 2 according to the number and positions of the detection points, import the printing data into the computer system, and use a binder jet 3D printing device to print a model box 2 at each detection point;

[0041] Step 3: After the printing and curing of the model boxes 2 at all the detection points are completed, clean the powder around the model boxes 2 and take out all the model boxes 2 one by one;

[0042] Step 4: Pour out the powder in each model box 2 one by one and weigh it to obtain the mass of the powder in each model box 2. Measure the inner wall dimensions of the corresponding model box 2 one by one to obtain the volume of the model box 2. Then calculate the density of the powder in each model box 2, and further calculate the standard deviation of the density of the powder in all the model boxes 2, so as to judge the accuracy during the powder bed printing;

[0043] Among them, the density calculation formula of the powder in each model box 2 is: ρ = m / v;

[0044] In the formula, ρ is the density of the powder in the model box 2, m is the mass of the powder in the model box 2, and v is the volume of the model box 2;

[0045] The standard deviation of the density of the powder in all the model boxes 2 is calculated by the following formula:

[0046] ρ 均 = 1 / N (ρ1 + ρ2 + ρ3 + …… ρ N )

[0047] σ = {1 / N[(ρ1 - ρ 均 ) 2 + (ρ2 - ρ 均 ) 2 + (ρ3 - ρ 均 ) 2 …… + (ρ N - ρ 均 ) 2} 1 / 2

[0048] In the formula, N represents the number of all the model boxes 2, ρ 均 represents the average density of the powder in all the model boxes 2, and σ represents the standard deviation of the density of the powder in all the model boxes 2.

[0049] Specifically, in Step 1, since the bottom wall of the forming cylinder 1 can slide along its height direction, the effective forming cavity refers to the inner cavity of the forming cylinder 1 corresponding to the upper part of the bottom wall of the forming cylinder 1 that meets the printing requirements.

[0050] Specifically, in step one, the forming cylinder 1 has a cube or cuboid structure with an open top. The effective forming cavity in the forming cylinder 1 includes multiple forming surfaces arranged in sequence along its height direction. The detection points in each forming surface can be set to 9 points, 25 points, or 81 points. In the actual operation process, the more detection positions there are, the more detailed the detection data will be, and the more truly the quality of the powder bed can be reflected. The detection points are at least evenly set to 9, and the 9 detection points respectively correspond to the center position, the four corner positions, and the midpoint positions of the four side edges of this layer of forming surface.

[0051] Specifically, in step two, there is a first gap h1 between the model box 2 corresponding to the bottom layer forming surface and the bottom wall of the forming cylinder 1, and the first gap h1 is not less than 5 mm. Because in the initial stage of powder spreading, during the powder spreading process, the powder is compacted by relying on the self-gravity of the powder and the interaction force between the scraper and the laid powder. If the laid powder is too thin, the overall density of the powder bed will be poor, which may cause the problem of "part detachment" or affect the accuracy of the formed part. However, if the laid powder is too thick, it will waste the existing powder and affect the production efficiency. Therefore, the first gap h1 is preferably 5 mm.

[0052] Specifically, in step two, there is a second gap h2 between the inner side wall of the forming cylinder 1 and the adjacent model box 2, and the second gap h2 is not less than 5 mm. Because all the bottom plates of the forming cylinders 1 of the current binder jetting forming equipment on the market realize the powder bed laying between layers through up and down movement. If the green body of the model box 2 is placed closely against the inner wall of the forming cylinder 1, it will inevitably cause damage to the green body due to the mutual friction between the powder bed and the cylinder wall during this process. Therefore, a certain gap is reserved between the box close to the inner wall of the forming cylinder 1 and the forming cylinder 1, and the optimal value of this gap is 5 mm.

[0053] Specifically, there is a third gap h3 between adjacent model boxes 2 within the same forming surface, and the third gap h3 is not less than 5 mm.

[0054] Specifically, there is a fourth gap h4 between adjacent model boxes 2 in adjacent forming surfaces, and the fourth gap h4 is not less than 5 mm. That is to say, the interval between any model box 2 and its adjacent surrounding model boxes 2 is not less than 5 mm, so as to ensure the convenience of taking out the box during the powder cleaning process. It should be noted that the gaps h1 - h4 are set according to the test needs and are not necessarily the same.

[0055] Specifically, for a forming cylinder with special dimensions or shapes, the model box proposed by the present invention can also be used during detection. It can determine the distribution of detection points according to its own shape and size, and there is no need to use the 9-point, 25-point or 81-point detection method for detection, but it still needs to satisfy that h1 - h4 are all not less than 5 mm.

[0056] Specifically, such as Figure 1As shown in the figure, in step two, the model box 2 includes a box body 21 and a box cover 22. The top of the box body 21 is open, and the box cover 22 is set on the open top of the box body 21. Due to the setting of the box cover 22, a sealed storage space is formed inside the box, which can ensure that the quality of the powder in the model box 2 does not change during the processes of powder spreading - inkjet printing - drying - baking - powder cleaning, thereby improving the detection accuracy; after the powder cleaning is completed, by opening the box cover 22, it is easy to take out the powder to be tested; at the same time, the model box 2 with a cover can be placed in multiple layers to detect the density of the powder bed at different heights, that is, to obtain the influence of the powder spreading process on the density of the powder bed.

[0057] Specifically, the model box 2 further includes a locking assembly for locking the box body 21 and the box cover 22. The box body 21 adopts a regular cube structure, such as a cuboid, a cube, a cylinder, etc., which is convenient for measuring and calculating its volume. Usually, the box body 21 is in the structure of a cuboid or a cube, and the box cover 22 adopts a pull - out box cover 22. That is, the locking assembly includes two L - shaped clamping plates 23 and two clamping protrusions 24. One ends of the two L - shaped clamping plates 23 are respectively fixed on the opposite side walls of the box cover 22, and their other ends are arranged oppositely, so that a chute is formed between the L - shaped clamping plate 23 and the lower cover surface of the box cover 22 and runs through in the direction perpendicular to the opposite side walls of the box cover 22; the two clamping protrusions 24 are respectively fixed on the upper ends of the outer wall surfaces of the opposite side walls of the box body 21 and are slidably connected in the chute. During the process of taking parts and powder cleaning, the box body 21 and the box cover 22 remain fastened to prevent the powder in the box from escaping and affecting the accuracy of the detection result. After the model box 2 is taken out, by pulling the box cover 22, the clamping protrusions 24 can be disengaged from the inner cavity of the U - shaped buckle, so that the box cover 22 is separated from the box body 21, and thus the powder in the box can be poured out.

[0058] Specifically, due to the characteristics of the forming process, during the binder jetting forming process, the action of the binder and the powder is diffusion and penetration. There is a fifth gap h5 between the end of the L - shaped clamping plate 23 far from the box cover 22 and the outer wall of the box body 21, and the range of the fifth gap h5 is 0.15mm - 0.24mm, which prevents the box cover 22 and the box body 21 from bonding to form the same part under this action during the forming process, resulting in the box cover 22 being unable to be opened.

[0059] Since the size of the model box 2 is too small, the measurement error is large, and if the size is too large, the representativeness of the measurement result is reduced; therefore, the optimal length and width of the box body 21 are 10 - 50mm, the optimal height is 5 - 8mm, and the optimal wall thickness is 2 - 5mm.

[0060] In Step 4, the judgment method is as follows: when σ ≥ 0.05, it indicates that the powder bed is not suitable for manufacturing parts by binder jet additive manufacturing; when 0.01 ≤ σ < 0.05, it indicates that the powder bed is suitable for manufacturing parts with lower precision requirements; when σ < 0.01, it indicates that the powder bed has good quality and can be used for manufacturing parts with higher precision requirements.

[0061] Example 1:

[0062] As Figure 2 , taking the effective forming cavity size of the forming cylinder 1 as an example, with a length of 100 mm * width of 100 mm * height of 80 mm, a 3*3 model structure arrangement is formed according to the 9-point detection method for each layer. A total of four layers are set (the model box 2 in the figure is not fully shown). The size specification of the powder bed density detection model box 2 is set as a length of 20 mm * width of 20 mm * height of 10 mm (designed wall thickness of 2 mm). As Figure 3 in the first layer of the model box placement state on the xoy plane (i.e., the horizontal plane), h2 = h3 = 10 mm, Figure 4 in the first layer of the model box placement state on the yoz plane (i.e., the vertical plane), h2 = h3 = 10 mm, h1 = h4 = 5 mm.

[0063] The model box 2 is subjected to binder forming, curing, and powder cleaning according to the 3*3 model structure of the 9-point detection method. Finally, 36 test specimens can be obtained. Taking the first-layer specimen on the xoy plane as an example, in the order of (-1, -1, 0), (0, -1, 0), (1, -1, 0), (-1, 0, 0), (0, 0, 0), (1, 0, 0), (-1, 1, 0), (0, 1, 0), (1, 1, 0), the model box 2 is taken out one by one, and the powder inside each model box 2 is completely taken out. The mass m of the powder inside the model box 2 is measured using a precision balance and calibrated as m1~m9 in this order. At the same time, the internal cavity dimensions x mm, y mm, and z mm of the model are measured using a vernier caliper to obtain the volume v of the model box 2, which is calibrated as v1~v9 in this order. According to the aforementioned density formula, the densities ρ1~ρ9 are calculated. Similarly, the second layer starts from (-1, -1, 1), and the densities ρ 10~ ρ 18 are continuously calculated according to the rules of the first layer; the third layer starts from (-1, -1, 2), and the densities ρ 19 ~ρ 27 are continuously calculated according to the rules of the first and second layers; the fourth layer starts from (-1, -1, 3), and the densities ρ 28 ~ρ 36 are continuously calculated according to the rules of the first, second, and third layers.

[0064] According to the calculation formula for the standard deviation of the density of the powder in all the aforementioned model boxes 2, the standard deviation of the density of the powder in all the model boxes 2 in this test is calculated as follows:

[0065] ρ 均 = 1 / 36 (ρ1 + ρ2 + ρ3 + …… ρ 36 )

[0066] σ = {1 / 36 [(ρ1 - ρ 均 ) 2 + (ρ2 - ρ 均 ) 2 + (ρ3 - ρ 均 ) 2 …… + (ρ 36 - ρ 均 ) 2} 1 / 2

[0067] In the formula: ρ represents the average density of the powder in all the model boxes in this test, and σ represents the standard deviation of the density of the powder in all the model boxes in this test.

[0068] The quality of the density of the powder bed in binder jet additive manufacturing is judged by the degree of dispersion among the actual densities at each detection point, and the degree of dispersion can be known from the standard deviation of the density of the powder in all the model boxes 2; when σ ≥ 0.05, it indicates that the powder bed is not suitable for producing parts by binder jet additive manufacturing; when 0.01 ≤ σ < 0.05, it indicates that the powder bed is suitable for producing parts with lower precision requirements; when σ < 0.01, it indicates that the quality of the powder bed is good and can be used for producing parts with higher precision requirements.

[0069] The various embodiments in this specification are described in a progressive manner. Each embodiment focuses on the differences from other embodiments. The same or similar parts among the various embodiments can be referred to each other. For the devices disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple, and the relevant parts can be referred to the description in the method part.

[0070] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but will be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method suitable for detecting the density of a powder bed in binder jet additive manufacturing, characterized in that, It includes the following steps: Step 1: According to the effective forming cavity of the forming cylinder (1), design the number of detection points in the effective forming cavity and the positions of each detection point to ensure that all detection points are evenly distributed in the effective forming cavity; Step 2: Determine the size of the model box (2) according to the number and positions of the detection points, and use a binder jet 3D printing device to print a model box (2) at each detection point; Step 3: After the model boxes (2) at all detection points are printed and cured, clean the powder around the model boxes (2) and take out all the model boxes (2) one by one; Step 4: Pour out and weigh the powder in each model box (2) one by one to obtain the mass of the powder in each model box (2), measure the inner wall size of the corresponding model box (2) one by one to obtain the volume of the model box (2), then calculate the density of the powder in each model box (2), and further calculate the standard deviation of the density of the powder in all model boxes (2); Among them, the density calculation formula of the powder in each model box (2) is: ρ = m / v; In the formula, ρ is the density of the powder in the model box (2), m is the mass of the powder in the model box (2), and v is the volume of the model box (2); The standard deviation of the density of the powder in all model boxes (2) is calculated using the following formula: ρ 均 = 1 / N (ρ1 + ρ2 + ρ3 + …… ρ N ) σ = {1 / N[(ρ1 - ρ 均 ) 2 + (ρ2 - ρ 均 ) 2 + (ρ3 - ρ 均 ) 2 …… + (ρ N - ρ 均 ) 2} 1 / 2 Where N represents the number of all model boxes (2), ρ 均 represents the average density of the powder in all model boxes (2), and σ represents the standard deviation of the density of the powder in all model boxes (2).

2. The method for detecting the density of a powder bed in binder jet additive manufacturing according to claim 1, characterized in that In Step 1, the forming cylinder (1) is a cube or cuboid structure with an open top. The effective forming cavity in the forming cylinder (1) includes multiple forming surfaces arranged in sequence along its height direction. At least 9 detection points are evenly arranged in each forming surface, and the 9 detection points respectively correspond to the center position, four corner positions, and midpoint positions of the four sides of the forming surface of this layer.

3. A method for detecting the densification of a powder bed in binder jet additive manufacturing according to claim 2, characterized in that, In Step 2, there is a first gap h1 between the model box (2) corresponding to the bottom forming surface and the bottom wall of the forming cylinder (1), and the first gap h1 is not less than 5 mm.

4. A method for detecting the densification of a powder bed in binder jet additive manufacturing according to claim 3, characterized in that, In Step 2, there is a second gap h2 between the inner side wall of the forming cylinder (1) and the adjacent model box (2), and the second gap h2 is not less than 5 mm.

5. A method for detecting the density of a powder bed in binder jet additive manufacturing according to claim 4, characterized in that, In Step 2, there is a third gap h3 between adjacent model boxes (2) in the same forming surface, and the third gap h3 is not less than 5 mm.

6. A method for detecting the density of a powder bed in binder jet additive manufacturing according to claim 5, characterized in that, In Step 2, there is a fourth gap h4 between adjacent model boxes (2) in adjacent forming surfaces, and the fourth gap h4 is not less than 5 mm.

7. A method for detecting the densification of a powder bed in binder jet additive manufacturing according to any one of claims 1-6, characterized in that In Step 2, the model box (2) includes a box body (21) and a box cover (22). The top of the box body (21) is open, and the box cover (22) is covered at the open top of the box body (21).

8. A method for detecting the densification of a powder bed in binder jet additive manufacturing according to claim 7, characterized in that The model box (2) further includes a locking assembly for locking the box body (21) and the box cover (22). The locking assembly includes two L-shaped clamping plates (23) and two clamping protrusions (24). The box body (21) has a cuboid structure. One ends of the two L-shaped clamping plates (23) are respectively fixed on two opposite side walls of the box cover (22), and their other ends are arranged oppositely, so as to form a chute penetrating along the direction perpendicular to the two opposite side walls of the box cover (22) between the L-shaped clamping plates (23) and the lower cover surface of the box cover (22); the two clamping protrusions (24) are respectively fixed on the upper ends of the outer wall surfaces of two opposite side walls of the box body (21), and are slidably connected in the chute.

9. A method for detecting the densification of a powder bed in binder jet additive manufacturing according to claim 8, characterized in that, There is a fifth gap h5 between the end of the L-shaped clamping plate (23) far from the box cover (22) and the outer wall of the box body (21), and the range of the fifth gap h5 is 0.15 mm - 0.24 mm.

10. A method for detecting the density of a powder bed in binder jet additive manufacturing according to claim 8, characterized in that, The length and width of the box body (21) are 10 - 50 mm, the height is 5 - 8 mm, and the wall thickness is 2 - 5 mm.

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