Method suitable for detecting compactness of binder spraying additive manufacturing powder bed

Abstract

The invention discloses a method suitable for detecting the compactness of a binder injection additive manufacturing powder bed, and belongs to the technical field of binder injection molding, and the method comprises the following steps: 1, designing the number of detection points in an effective molding cavity and the position of each detection point according to the effective molding cavity of a molding cylinder; 2, the sizes of the model boxes are determined according to the number and the positions of the detection point positions, and one model box is printed at each detection point position while powder is laid through an adhesive spraying 3D printing device; 3, taking out all the model boxes one by one; 4, the powder in each model box is poured out one by one and weighed, the mass of the powder in each model box is obtained, the size of the inner wall of the corresponding model box is measured one by one to obtain the volume of the model box, then the density of the powder in each model box is calculated, and the density of the powder in each model box is calculated. And the density standard deviation of the powder in all the model boxes is calculated, so that the printing accuracy of the powder bed is judged.

Description

Technical Field

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

[0002] In recent years, binder jetting technology for metal materials, as an advanced additive manufacturing technology, has attracted widespread attention from the industry due to its advantages of no heat source input, wide material applicability, fast molding rate, and low production cost. The basic process is as follows: First, a scraper or powder roller is used to lay the molding powder material to form a powder bed, and the two-dimensional pattern required by the binder is evenly and quantitatively sprayed through the nozzle group to complete the single-layer powder bonding molding. The powder bed drops a layer thickness distance and is repeated layer by layer until the molding is completed; secondly, a green blank without residual powder is obtained after drying, curing and powder cleaning; finally, product parts with the required mechanical properties are formed through degreasing and sintering.

[0003] However, in the existing production application cases, binder injection molding technology still has many technical bottlenecks, mainly the structural deformation and dimensional control of product parts after sintering. The invention patent with publication number CN113015896A discloses a device and method for measuring the density of a powder bed in a 3D printing / additive manufacturing operation, the device comprising a device for determining the mass of the powder and a device for determining the volume of the powder during the spreading process, thereby allowing the powder bed density (the powder density of the layer) to be determined. The device for determining the volume of the powder includes a laser assembly, which is suitable for moving the laser in both left and right directions 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 performance of the powder bed layer by layer, existing statements indicate that binder jetting is different from selective laser melting in that it is an isotropic forming method. 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 the powder bed layer by layer. As the number of forming layers increases, the powder in the powder bed continues to increase. Under the action of powder gravity, the Z-direction force at each position is different, resulting in differences in the powder density of each position unit, that is, the density of each position unit is different. In particular, when preparing product parts with larger dimensions in the height direction, structural deformation and uncontrollable dimensions are more likely to occur after sintering, affecting product consistency and qualified rate.

[0004] Therefore, a method suitable for detecting the density of a powder bed for binder jetting additive manufacturing is provided to detect the discreteness of the density of units at each position of 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 binder jet additive manufacturing powder bed, which is used to detect the discreteness of the density of each position unit of the powder bed, provide a basis for judging the laying quality and stability of the powder bed, and provide a theoretical basis for the model data adjustment of the early stage molding product parts and the sintering process of the later stage molding green billet.

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

[0007] A method for detecting the compactness of a powder bed for binder jetting additive manufacturing, comprising the following steps:

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

[0009] Step 2: Determine the size of the model box according to the number and location 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 the inspection points are printed and solidified, 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 then calculate the density standard deviation of the powder in all model boxes, so as to determine the accuracy of the powder bed printing;

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

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

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

[0015] ρ 均 =1 / N(ρ 1 +ρ 2 +ρ 3 +……ρ N )

[0016] σ = {1 / N[(ρ 1 -ρ 均 ) 2 + (ρ 2 -ρ 均) 2 + (ρ 3 -ρ 均 ) 2 ……+(ρ N -ρ 均 ) 2 ]} 1 / 2

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

[0018] The beneficial effects that can be achieved by the present invention are as follows: by setting a plurality of evenly arranged monitoring points in the forming cylinder, a corresponding model box is printed at each detection point, and then the mass of the powder laid inside the model box during the printing process is obtained, and then combined with the volume of the model box, the powder density in the model box is determined, and the density standard deviation of the powder in all model boxes is obtained by calculation, so as to judge the accuracy of the powder bed during printing, and provide a basis for judging the laying quality and stability of the powder bed, and at the same time, provide a theoretical basis for the model data adjustment of the early molding product parts and the sintering process of the later molding green blank.

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

[0020] Preferably, in step 2, there is a first gap h1 between the model box corresponding to the molding surface of the bottom layer and the bottom wall of the molding cylinder, and the first gap h1 is not less than 5 mm.

[0021] Preferably, in step 2, 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 2, there is a third gap h3 between adjacent model boxes on the same molding surface, and the third gap h3 is not less than 5 mm.

[0023] Preferably, in step 2, there is a fourth gap h4 between two adjacent model boxes in adjacent molding surfaces, and the fourth gap h4 is not less than 5 mm.

[0024] Preferably, in step 2, the model box comprises a box body and a box cover, the top of the box body is open, and the box cover is arranged at the open top of the box body.

[0025] Preferably, the model box also includes a locking assembly for locking the box body and the box cover, the locking assembly includes two L-shaped card plates and two locking protrusions, the box body is a rectangular structure, one end of the two L-shaped card plates are respectively fixed on the two side walls of the box cover that are relatively arranged, and the other ends thereof are relatively arranged so that a slide groove that passes through the two side walls of the box cover that are relatively arranged vertically is formed between the L-shaped card plates and the lower cover surface of the box cover; the two locking protrusions are respectively fixed to the upper ends of the outer wall surfaces of the two side walls of the box body that are relatively arranged, and can be slidably connected in the slide groove.

[0026] Preferably, a fifth gap h5 is provided between the end of the L-shaped cardboard away from one end of 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 box body has a side length of 10-50 mm, a height of 5-8 mm, and a wall thickness of 2-5 mm.

[0028] It can be seen from the above technical scheme that compared with the prior art, the present invention discloses a method suitable for detecting the density of the powder bed of binder jet additive manufacturing, by setting multiple detection points in the forming cylinder, printing a corresponding model box at each detection point, and then obtaining the mass of the powder laid inside the model box during the printing process, and then combining the volume of the model box to determine the powder density in the model box, and obtaining the density standard deviation of the powder in all model boxes by calculation, so as to judge the accuracy of the powder bed during printing, and provide a basis for judging the laying quality and stability of the powder bed, and at the same time, provide a theoretical basis for the model data adjustment of the early molding product parts and the sintering process of the later molding green blank. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

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

[0031] Figure 2 This is a schematic structural diagram of the distribution of the model boxes in the forming cylinder in Example 1 of the present invention.

[0032] Figure 3 This is a schematic diagram of the structure of the first layer model placement state of the xoy surface in Example 1.

[0033] Figure 4 This is a schematic diagram of the structure of the first layer model placement state of the yoz surface in Example 1.

[0034] In the figure: 1, forming cylinder, 2, model box, 21, box body, 22, box cover, 23, L-shaped clamping plate, 24, clamping convexity. DETAILED DESCRIPTION

[0035] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0036] In the description of the present invention, it is necessary to understand that the terms "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship are based on the orientation or position relationship shown in the drawings, and are 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 cannot be understood as a limitation on the present invention.

[0037] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0038] See also Figure 1-Figure 4 The embodiment of the present invention discloses a method for detecting the density of a powder bed in binder jetting additive manufacturing, comprising the following steps:

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

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

[0041] Step 3: After the model boxes 2 at all the inspection points are printed and solidified, 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, and measure the inner wall dimensions of the corresponding model box 2 one by one to obtain the volume of the model box 2, and then calculate the density of the powder in each model box 2, and then calculate the density standard deviation of the powder in all model boxes 2, so as to determine the accuracy of the powder bed printing;

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

[0044] Wherein, 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 all powders in the model box 2 was calculated using the following formula:

[0046] ρ 均 =1 / N(ρ 1 +ρ 2 +ρ 3 +……ρ N )

[0047] σ = {1 / N[(ρ 1 -ρ 均 ) 2 + (ρ 2 -ρ 均 ) 2 + (ρ 3 -ρ 均 ) 2 ……+(ρ N -ρ 均 ) 2 ]} 1 / 2

[0048] 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.

[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 top of the bottom wall of the forming cylinder 1 that meets the printing requirements.

[0050] Specifically, in step 1, the molding cylinder 1 is a cube or rectangular structure with an open top. The effective molding cavity in the molding cylinder 1 includes multiple molding surfaces arranged in sequence along its height direction. The detection points in each molding surface can be set to 9 points, 25 points or 81 points. In actual operation, the more detection positions there are, the more detailed the detection data is, and the more it can truly reflect the quality of the powder bed. At least 9 detection points are evenly set, and the 9 detection points correspond to the center position, four corner positions and the midpoint positions of the four sides of the molding surface of this layer.

[0051] Specifically, in step 2, there is a first gap h1 between the model box 2 corresponding to the molding surface of the bottom layer and the bottom wall of the molding cylinder 1, and the first gap h1 is not less than 5mm. Because in the early stage of powder laying, the powder is compacted by its own gravity and the interaction force between the scraper and the laid powder during the laying process. If the powder is laid too thin, the overall density of the powder bed is poor, which may cause the problem of "parts falling off" or affect the accuracy of the molded parts. However, if the powder is laid too thick, the existing powder will be wasted, affecting the production efficiency. Therefore, the first gap h1 is preferably 5mm.

[0052] Specifically, in step 2, there is a second gap h2 between the inner wall of the molding cylinder 1 and the adjacent model box 2, and the second gap h2 is not less than 5 mm. Because the bottom plate of the molding cylinder 1 of the current binder injection molding equipment on the market all realizes the interlayer powder bed laying by up and down movement, if the green blank of the model box 2 is placed close to the inner wall of the molding cylinder 1, the green blank will inevitably be damaged 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 molding cylinder 1 and the molding cylinder 1, and the optimal gap is 5 mm.

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

[0054] Specifically, there is a fourth gap h4 between two adjacent model boxes 2 in adjacent molding 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 surrounding adjacent model boxes 2 is not less than 5 mm, thereby ensuring 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 test needs and are not necessarily the same.

[0055] Specifically, for forming cylinders with special sizes or shapes, the model box proposed in the present invention can also be used for inspection. The distribution of detection points can be determined according to its own shape and size, and there is no need to use 9-point, 25-point or 81-point detection methods for inspection. However, h1-h4 must still be no less than 5mm.

[0056] Specifically, Figure 1As shown, 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 set at the top of the box body 21. Due to the provision of the box cover 22, a closed 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 process of powder laying-inkjet-drying-drying-powder cleaning, thereby improving the accuracy of the detection; after the powder cleaning is completed, the powder to be tested can be easily taken out by opening the box cover 22; 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 laying process on the density of the powder bed.

[0057] Specifically, the model box 2 also includes a locking assembly for locking the box body 21 and the box cover 22. The box body 21 adopts a regular cubic structure, such as a rectangular parallelepiped, a cube, a cylinder, etc., so as to facilitate measurement and calculation of its volume. Usually, the box body 21 is a rectangular parallelepiped or a cube structure, and the box cover 22 adopts a pull-out box cover 22, that is, the locking assembly includes two L-shaped card plates 23 and two card protrusions 24, one end of the two L-shaped card plates 23 is respectively fixed on the two side walls of the box cover 22 that are arranged oppositely, and the other ends thereof are arranged oppositely, so that a slide groove that passes through the two side walls of the box cover 22 that are arranged oppositely is formed between the L-shaped card plates 23 and the lower cover surface of the box cover 22 in a direction perpendicular to the two side walls of the box cover 22 that are arranged oppositely; the two card protrusions 24 are respectively fixed on the upper ends of the outer wall surfaces of the two side walls of the box body 21 that are arranged oppositely, and can be slidably connected in the slide groove. During the process of taking out the items and cleaning the powder, the box body 21 and the box cover 22 remain buckled to prevent the powder in the box from falling out and affecting the accuracy of the test results. After the model box 2 is taken out, the box cover 22 can be pulled out to make the protrusion 24 fall out of the U-shaped buckle cavity, so that the box cover 22 is separated from the box body 21, thereby pouring out the powder in the box.

[0058] Specifically, due to the characteristics of the molding process, during the binder injection molding process, the role of the binder and the powder is diffusion and penetration, and a fifth gap h5 is provided between the end of the L-shaped card plate 23 away from one end of 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, so as to prevent the box cover 22 and the box body 21 from bonding to form the same part under this action during the molding process, so that the box cover 22 cannot be opened.

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

[0060] In step 4, the judgment method is: when σ≥0.05, it indicates that the powder bed is not suitable for producing parts using binder jetting 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 powder bed has good quality and can be used to produce parts with higher precision requirements.

[0061] Embodiment 1:

[0062] like Figure 2 , taking the effective molding cavity size of molding cylinder 1 as 100mm long * 100mm wide * 80mm high as an example, a 3*3 model structure is arranged according to the 9-point detection method for each layer, and a total of four layers are set (the model box 2 is not fully shown in the figure). The size specification of the powder bed density detection model box 2 is set to 20mm long * 20mm wide * 10mm high (design wall thickness 2mm), such as Figure 3 When the first layer of the model box is placed on the middle xoy surface (i.e. horizontal surface), h2=h3=10mm, Figure 4 When the first layer of the model box is placed on the middle yoz surface (i.e. the vertical surface), h2=h3=10mm and h1=h4=5mm.

[0063] The model box 2 is subjected to binder molding, curing, and powder cleaning according to the 9-point detection method 3*3 model structure, and finally 36 test samples are obtained. Taking the first layer sample of the xoy surface as an example, the model boxes 2 are taken out one by one 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), and the powder inside each model box 2 is completely taken out. The mass m of the powder inside the model box 2 is tested by a precision balance, and it is calibrated as m in this order. 1 ~m 9 At the same time, use a vernier caliper to measure the inner dimensions of the model, x mm, y mm, and z mm, and obtain the volume v of the model box 2, and calibrate it in this order as v 1 ~v 9 , the density ρ is calculated according to the above density formula 1 ~ρ 9 Similarly, the second layer starts with (-1,-1,1) and continues to calculate the density ρ according to the first layer rules 10~ ρ 18 ; The third layer starts with (-1,-1,2) and the density ρ is obtained by continuing to calculate according to the first and second layer rules 19 ~ρ 27 ; The fourth layer starts with (-1,-1,3) and the density ρ is obtained by continuing to calculate according to the rules of the first, second and third layers 28 ~ρ 36 .

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

[0065] ρ 均 =1 / 36(ρ 1 +ρ 2 +ρ 3 +……ρ 36 )

[0066] σ = {1 / 36[(ρ 1 -ρ 均 ) 2 + (ρ 2 -ρ 均 ) 2 + (ρ 3 -ρ 均 ) 2 ……+(ρ 36 -ρ 均 ) 2 ]} 1 / 2

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

[0068] The density of the powder bed for binder jetting additive manufacturing is judged by the degree of dispersion between the actual density of each detection point. The degree of dispersion can be known from the density standard deviation of all powders in the model box 2. When σ≥0.05, it indicates that the powder bed is not suitable for the production of parts using binder jetting additive manufacturing. When 0.01≤σ<0.05, it indicates that the powder bed is suitable for the production of parts with lower precision requirements. When σ<0.01, it indicates that the powder bed is of good quality and can be used to produce parts with higher precision requirements.

[0069] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to 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 apparent to those skilled in the art, and the general principles defined herein may 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 the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for detecting the compactness of a powder bed in binder jetting additive manufacturing, characterized in that: The following steps are involved: Step 1: According to the effective molding cavity of the molding cylinder (1), the number of detection points in the effective molding cavity and the position of each detection point are designed to ensure that all detection points are evenly distributed in the effective molding cavity; Step 2: Determine the size of the model box (2) according to the number and position 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 the inspection points are printed and solidified, clean the powder around the model boxes (2) and take out all the model boxes (2) one by one; Step 4: Pour out the powder in each model box (2) one by one and weigh them to obtain the mass of the powder in each model box (2), and measure the inner wall dimensions of the corresponding model box (2) one by one to obtain the volume of the model box (2), and then calculate the density of the powder in each model box (2), and then calculate the standard deviation of the density of the powder in all model boxes (2); The density calculation formula of the powder in each model box (2) is: ρ=m / v; Wherein, 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) was calculated using the following formula: r 均 =1 / N(ρ1+ρ2+ρ3+……ρ N ) σ={1 / N[(ρ1-ρ 均 ) 2 +(ρ2-ρ 均 ) 2 +(ρ3-ρ 均 ) 2 ……+(r N -r 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. A method for detecting the density of a powder bed in binder jetting additive manufacturing according to claim 1, characterized in that: In step one, the molding cylinder (1) is a cube or rectangular parallelepiped structure with an open top, and the effective molding cavity in the molding cylinder (1) includes a plurality of molding surfaces arranged in sequence along its height direction, and at least 9 detection points are evenly arranged in each molding surface, and the 9 detection points correspond to the center position, four corner positions and the midpoint positions of the four sides of the molding surface of the layer, respectively.

3. A method for detecting the density of a powder bed in binder jetting additive manufacturing according to claim 2, characterized in that: In step 2, a first gap h1 is provided between the model box (2) corresponding to the molding surface of the bottom layer and the bottom wall of the molding cylinder (1), and the first gap h1 is not less than 5 mm.

4. A method for detecting the density of a powder bed in binder jetting additive manufacturing according to claim 3, characterized in that: In step 2, a second gap h2 is provided between the inner side wall of the molding 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 jetting additive manufacturing according to claim 4, characterized in that: In step 2, there is a third gap h3 between adjacent model boxes (2) on the same molding surface, and the third gap h3 is not less than 5 mm.

6. A method for detecting the compactness of a powder bed in binder jetting additive manufacturing according to claim 5, characterized in that: In step 2, a fourth gap h4 is provided between two adjacent model boxes (2) in adjacent molding surfaces, and the fourth gap h4 is not less than 5 mm.

7. A method for detecting the density of a powder bed in binder jetting additive manufacturing according to any one of claims 1 to 6, characterized in that: In step 2, the model box (2) comprises a box body (21) and a box cover (22), wherein the box body (21) has an open top, and the box cover (22) is arranged to cover the open top of the box body (21).

8. A method for detecting the density of a powder bed in binder jetting additive manufacturing according to claim 7, characterized in that: The model box (2) further comprises a locking assembly for locking the box body (21) and the box cover (22), the locking assembly comprising two L-shaped clamping plates (23) and two clamping protrusions (24); the box body (21) is a rectangular parallelepiped structure; one end of the two L-shaped clamping plates (23) are respectively fixed to opposite side walls of the box cover (22), and the other ends thereof are arranged opposite to each other, so that a slide groove penetrating in a direction perpendicular to the opposite side walls of the box cover (22) is formed 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 to the upper ends of the outer wall surfaces of the opposite side walls of the box body (21), and are slidably connected in the slide groove.

9. A method for detecting the density of a powder bed in binder jetting additive manufacturing according to claim 8, characterized in that: A fifth gap h5 is provided between the end of the L-shaped card plate (23) away from one end of 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 jetting additive manufacturing according to claim 8, characterized in that: The box body (21) has a length and width of 10-50 mm, a height of 5-8 mm, and a wall thickness of 2-5 mm.

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