A control method of a droplet on-demand ejection process based on water hammer effect

Abstract

The application discloses a control method of a droplet on-demand spraying process based on a water hammer effect, and the method is characterized in that: a series of images of the droplet are captured by using an industrial camera, droplet features are calculated through a threshold segmentation, an open operation and a connected domain extraction method, and the moving-up completion time of a spray pipe in a spraying device or a spray pipe with a different spray hole diameter is adjusted accordingly, so that the droplets with required quantity, size and initial velocity are obtained. The application can effectively control the droplet on-demand spraying process based on the water hammer effect, and further improve the quality of the droplets and manufactured components.

Description

Technical Field

[0001] This invention belongs to the field of additive manufacturing, specifically relating to a control method for the on-demand spraying process of molten droplets based on the water hammer effect. Background Technology

[0002] Metal additive manufacturing, which uses a three-dimensional digital model to create components through layer-by-layer fabrication, offers a significant opportunity for the development and transformation of the manufacturing industry. Currently, laser-based metal additive manufacturing technology, especially laser powder bed sintering technology using powder bed materials, can achieve near-net-shape forming of complex components and is the most mature commercial additive manufacturing technology. However, the extremely high cooling rate experienced by materials in laser additive manufacturing can lead to deformation and cracking in the printed components. Furthermore, laser additive manufacturing incurs high equipment and raw material costs, which to some extent limits its development and application. In contrast, microdroplet jetting metal additive manufacturing technology offers advantages such as relatively low cooling rates, lower equipment and raw material costs, and better printed component performance, making it another mainstream direction in metal additive manufacturing.

[0003] Microdroplet jetting metal additive manufacturing can be divided into two categories: continuous jetting and on-demand jetting. Continuous jetting produces continuous droplets, but due to the extremely high frequency of droplet generation, it is difficult to precisely control the number of droplets and the landing point of each droplet. On-demand jetting, on the other hand, allows for the jetting of one droplet per trigger signal, enabling better control over the number and landing point of droplets and facilitating high-precision manufacturing. On-demand jetting metal additive manufacturing mainly has three approaches: binder jetting, metal nanoparticle jetting, and metal droplet jetting. Binder jetting uses an inkjet printhead to jet binder layer by layer onto a powder bed, bonding the printed preform, followed by debinding and sintering to improve its density and mechanical properties. Metal nanoparticle jetting suspends nanoscale metal particles in inkjet printable ink, prints the ink layer by layer, and heats it to evaporate the solvent, thus obtaining a preform, which is then debinded and sintered. Both of these methods require debinding and sintering of the printed preform, increasing process complexity and introducing problems such as sintering shrinkage and deformation. Metal droplet jetting, which uses liquid metal as the jetting material, avoids the aforementioned problems and has therefore attracted increasing attention.

[0004] Currently, researchers have proposed various driving methods for metal droplet ejection, including piezoelectric, pneumatic, and electromagnetic methods. The main principle behind these methods is to generate a pressure pulse within a relatively sealed liquid reservoir to drive the ejection of molten droplets. Piezoelectric methods utilize the contraction of a piezoelectric element surrounding the reservoir to generate the pressure pulse. However, piezoelectric elements cannot operate at high temperatures; therefore, to eject high-melting-point metals, a piston or similar device is needed to separate the piezoelectric element from the reservoir, increasing the overall size and complexity of the equipment. Pneumatic methods use high-pressure gas to generate the pressure pulse; however, due to the compressibility of gas, the response speed of pneumatic drive systems is typically low, limiting the volumetric accuracy and frequency of droplet ejection. Electromagnetic methods require an external magnetic or electric field to generate the pressure pulse using electromagnetic force, resulting in high equipment complexity. In summary, the above-mentioned metal droplet ejection driving methods struggle to simultaneously address the melting point of the ejected metal, equipment size and complexity, ejection accuracy, and frequency.

[0005] To address the above issues, CN116786838A discloses a device and method for on-demand injection of high-melting-point metal droplets utilizing the water hammer effect. This method offers advantages such as the ability to inject high-melting-point metals, ease of integration of multiple nozzles, and high injection accuracy and frequency. The method involves reciprocating a nozzle containing high-melting-point liquid metal along its axis. As the nozzle moves upward, the liquid metal collides with the conical surface of the nozzle's bottom cavity. Due to the water hammer effect, a large pressure pulse is generated at the nozzle, driving the injection of molten metal droplets. However, this method does not provide specific methods for determining the nozzle orifice diameter and the upward velocity of the nozzle. An inappropriate combination of nozzle orifice diameter and nozzle upward velocity can cause several problems: the droplet size may deviate from the required range, affecting the dimensional accuracy and precision of the printed component; satellite droplets may be generated, failing to achieve the goal of generating one droplet per nozzle reciprocating motion; the initial velocity of the droplet may be unsuitable—an excessively high initial velocity can cause splashing after droplet deposition, while an excessively low initial velocity may result in a low liquid phase fraction during deposition, hindering the formation of good adhesion between the droplet and the substrate, as well as among the droplets themselves. Therefore, it is necessary to control the droplet ejection process of this method, adjusting the nozzle upward velocity and, if necessary, replacing the nozzle with one of appropriate orifice diameter to obtain droplets that meet the requirements. Summary of the Invention

[0006] To address the aforementioned problems in existing technologies, this invention proposes a control method for on-demand injection of molten droplets based on the water hammer effect. The method is rationally designed, overcomes the shortcomings of existing technologies, and has excellent results.

[0007] A control method for on-demand droplet injection based on water hammer effect includes the following steps:

[0008] Step 1: Illuminate the droplet on-demand spraying device based on water hammer effect by arranging an illumination source on the side below the illumination source, and place an industrial camera on the opposite side of the illumination source to capture images of the droplets.

[0009] Step 2: Set the target range of the area of ​​the molten droplet in the pixel coordinate system of the image [S]. min ,S max and the target range of initial velocity [v] min ,v max ];

[0010] Step 3: Select initial process parameters for the droplet on-demand spraying device, including nozzle orifice diameter r, displacement-time waveform of the nozzle during the upward movement stage, and upward movement completion time ΔT;

[0011] Step 4: Send a jet trigger signal to cause the nozzle to reciprocate axially once, performing one droplet jetting operation. Use an industrial camera to continuously capture images of the droplets, obtaining a series of images with a bright background and dark droplets. i , where i represents the image number;

[0012] Step 5: Perform image processing on each image to extract the number N of molten droplets in the image. i If there exists i such that N i >1, or for any i, N i =0, then proceed directly to step 6; if there exists i such that N i If =1, then calculate the area S and initial velocity v of the molten droplet in the pixel coordinate system, and then execute step 6;

[0013] Step 6: Based on the results of Step 5, adjust ΔT or r according to the different situations, and then repeat Steps 4 to 6.

[0014] Furthermore, in step 6, based on the results of step 5, the following cases are handled:

[0015] If there exists i such that N i If the value is greater than 1, then increase ΔT and return to step 4;

[0016] If for any i, N i If ΔT = 0, then decrease ΔT and return to step 4;

[0017] If S>S max And v>v min , or S>S min And v>v max If so, increase ΔT and return to step 4;

[0018] If S max And v <v min , or S min And v <v​​max If so, decrease ΔT and return to step 4;

[0019] If S min And v>v max If so, replace the nozzle with one larger r and return to step 4;

[0020] If S>S max And v <v min If so, replace the nozzle with one of smaller r and return to step 4;

[0021] If S∈[S min ,S max And v∈[v min ,v max If the condition is met, then return directly to step 4.

[0022] Furthermore, step 5, image processing for each image includes the following sub-steps:

[0023] Step 5.1: Perform threshold segmentation on the image to extract the region where the molten droplets are located in the image;

[0024] Step 5.2: Perform an opening operation on the image after threshold segmentation to remove interference regions from the image;

[0025] Step 5.3: Extract connected components from the image obtained in Step 5.2 to obtain the number of connected components in the image, which is considered as the number of molten droplets N. i .

[0026] Further, in step 5, the area of ​​the connected region in image I1 is calculated and regarded as the area S of the droplet in the pixel coordinate system. The pixel coordinates y1 and y2 of the centroid of the connected region in I1 and I2 in the vertical direction are calculated. The initial velocity of the droplet in the pixel coordinate system is calculated as v = (y2-y1) / Δt, where Δt is the time interval between the capture of two adjacent frames.

[0027] Furthermore, in step 4, the reciprocating motion of the nozzle can be either down-to-up or up-to-down.

[0028] The beneficial technical effects of this invention are as follows:

[0029] Compared with the prior art, the present invention uses machine vision for feedback and adjusts the nozzle's upward movement completion time accordingly, and replaces nozzles with different nozzle orifice diameters when necessary, in order to avoid the generation of satellite droplets and obtain liquid with appropriate volume and initial velocity, which is beneficial to improving the quality of the manufactured components. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the droplet-on-demand injection device based on the water hammer effect in this invention;​

[0031] Among them, 1-on-demand molten droplet ejection device based on water hammer effect; 2-industrial camera; 3-lighting source; 4-substrate; 5-printed component; 6-molten droplet; 11-nozzle; 12-nozzle drive device; 13-crucible; 14-heating device; 15-molten metal; 111-vent hole; 112-liquid inlet hole; 113-nozzle.

[0032] Figure 2 This is a flowchart of a control method for a droplet-on-demand injection process based on the water hammer effect in this invention; Detailed Implementation

[0033] The specific implementation methods and effects of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0034] A control method for a water hammer-based on-demand droplet ejection process, wherein the water hammer-based on-demand droplet ejection device 1 is as follows: Figure 1 As shown, the system includes a nozzle 11, a nozzle drive device 12, a crucible 13, and a heating device 14. The crucible 13 contains molten metal 15. The heating device 14 is located outside the crucible 13 and maintains the temperature of the molten metal 15 above its melting point. The upper end of the nozzle 11 is connected to the nozzle drive device 12, and the lower end passes through a small hole at the bottom of the crucible 13. A vent hole 111 is provided at the upper part of the nozzle 11 to connect its internal cavity with the external environment. A liquid inlet hole 112 is provided on the lower side of the nozzle to allow the molten metal 15 in the crucible 13 to flow into the nozzle 11. A spray hole 113 is provided at the bottom of the nozzle 11. When a spray trigger signal is received, the nozzle drive device 12 drives the nozzle 11 to perform one reciprocating motion along its axial direction. During the upward movement of the nozzle 11, a pressure pulse is generated inside the nozzle 11 due to the water hammer effect, driving molten droplets 6 to be generated from the spray hole 113. The stroke of the spray drive device 12 is fixed. Below the droplet-on-demand ejection device 1 is a substrate 4, and the droplets 6 fall onto the substrate 4 to form the printed component 5.

[0035] When controlling the droplet ejection process, the steps of the control method are as follows: Figure 2 As shown, the details are as follows:

[0036] Step 1: Illumination is provided by a light source 3 positioned below the side of the water hammer-based droplet on-demand spraying device 1. An industrial camera 2 is positioned opposite the light source 3 so that it can capture images of the molten droplets 6 ejected from the nozzle 113. Figure 1 As shown;

[0037] Step 2: Set the target area [S] of droplet 6 in the pixel coordinate system of the image captured by the industrial camera. min ,S max and the target range of initial velocity [v] min ,vmax These two target ranges are calculated based on the actual required volume range and initial velocity range of the molten droplet 6 in the spatial coordinate system;

[0038] Step 3: Select initial process parameters for the droplet on-demand spraying device, including the nozzle orifice diameter r, the displacement-time waveform of the nozzle 11 during the upward movement, and the upward movement completion time ΔT. The displacement-time waveform can be any monotonically increasing continuous function, such as a proportional function or a sine function between 0 and π / 2 phases. Since the reciprocating motion of the nozzle drive device 12 has a fixed stroke, given the displacement-time waveform, ΔT is inversely proportional to both the average speed and the maximum speed of the nozzle 11 during the upward movement. Therefore, the upward movement speed of the nozzle 11 can be adjusted by adjusting ΔT.

[0039] Step 4: A jet trigger signal is emitted, causing the nozzle 11 to reciprocate axially once, performing one droplet jetting operation. The industrial camera 2 continuously captures images of the droplet 6. Due to the presence of the illumination source 3, a series of images are captured. i (i represents the image number) all have the characteristics of a brighter background area and a darker droplet area;

[0040] Step 5: Perform image processing on each image to extract the number N of molten droplets 6 in the image. i If there exists i such that N i >1, or for any i, N i =0, then proceed directly to step 6; if there exists i such that Ni = 1, then calculate the area S and initial velocity v of the droplet in the pixel coordinate system, and then proceed to step 6.

[0041] Specifically, image processing for each image includes the following sub-steps:

[0042] Step 5.1: The grayscale difference between the background and droplet 6 in the image is significant. Therefore, threshold segmentation is performed first to extract the region where droplet 6 is located in the image. Threshold segmentation can be performed using Otsu's thresholding method or by setting a fixed threshold based on experience.

[0043] Step 5.2: Since there may be noise or other interference in the image, the binary image obtained in step 5.1 is opened to remove the interference area in the image.

[0044] Step 5.3: Extract connected components from the image obtained in Step 5.2 to obtain the number of connected components in the image, which is considered as the number N of molten droplets 6. i ;

[0045] When calculating the area S and initial velocity v of the molten droplet 6 in the pixel coordinate system, the area of ​​the connected region in image I1 is calculated and regarded as the area S of the molten droplet in the pixel coordinate system. The vertical pixel coordinates y1 and y2 of the centroid of the connected region in I1 and I2 are calculated. The initial velocity v of the molten droplet in the pixel coordinate system is calculated as v = (y2-y1) / Δt, where Δt is the time interval between the capture of two adjacent frames, and the vertical pixel coordinates are positively downward.

[0046] Step 6: Based on the results of Step 5, adjust ΔT or r according to the different situations, and then repeat Steps 4 to 6.

[0047] Based on the results of step five, proceed according to the following situations:

[0048] a) If there exists i such that N i If the value is >1, it means that at least two molten droplets 6 have appeared in some images, that is, satellite droplets have appeared. This means that the upward movement speed of nozzle 11 is too fast. Therefore, ΔT is increased to reduce the upward movement speed of nozzle 11, and then the process returns to step 4 to perform the next injection.

[0049] b) If for any i, N i =0, which means that no droplets 6 are generated. This means that the upward movement speed of nozzle 11 is too slow. Therefore, ΔT is reduced, and then the process returns to step 4.

[0050] c) If S>S max And v>v min This indicates that the droplet volume exceeds the upper limit of the target range, requiring a reduction in the upward velocity of nozzle 11. This also reduces the initial velocity of the droplet. However, since the initial velocity of the droplet is not below the lower limit of the target range, there is still room for reduction. Therefore, ΔT can be increased to reduce the upward velocity of nozzle 11, where S>S min And v>v max The situation is the same as above, so in both cases, ΔT is increased, and then we return to step 4;

[0051] d) If S max And v <v min , or S min And v <v max If , then it represents the opposite of c), so in both cases ΔT is reduced and we return to step 4;

[0052] e) If S min And v>v max If either ΔT is increased or decreased, it is impossible to simultaneously satisfy the range of droplet volume and initial droplet velocity. Therefore, it is necessary to replace the nozzle 11 with one with a larger orifice diameter r to increase the droplet volume and decrease the initial droplet velocity. Then return to step 4.

[0053] ​​​f) If S>S max And v <v min In the opposite case to e), it is necessary to replace the nozzle with one of smaller r and return to step 4.

[0054] g) If it does not belong to any of the above cases, then it means S∈[S min ,S max And v∈[v min ,v max At this point, the droplet volume and initial velocity are both within the target range, so no adjustment is needed. Simply return to step 4 and continuously monitor whether the number, volume, and initial velocity of the droplets are within acceptable limits.

[0055] Furthermore, it should be noted that for the droplet-on-demand injection device 1 based on the water hammer effect, the reciprocating motion of its nozzle 11 can be either down-to-up or up-to-down. The generation of droplets 6 occurs during the upward movement of the nozzle 11, and the above control methods are all aimed at the upward movement of the nozzle 11.

[0056] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A control method for on-demand droplet injection process based on water hammer effect, characterized in that, Includes the following steps: Step 1: Illuminate the droplet on-demand spraying device based on water hammer effect by arranging an illumination source on the side below the illumination source, and place an industrial camera on the opposite side of the illumination source to capture images of the droplets. Step 2: Set the target range of the area of ​​the molten droplet in the pixel coordinate system of the image [S]. min , S max and the target range of initial velocity [v] min , v max ]; Step 3: Select initial process parameters for the droplet on-demand spraying device, including nozzle orifice diameter r, displacement-time waveform of the nozzle during the upward movement stage, and upward movement completion time ∆T; Step 4: Send a jet trigger signal to cause the nozzle to reciprocate axially once, performing one droplet jetting operation. Use an industrial camera to continuously capture images of the droplets, obtaining a series of images with a bright background and dark droplets. i , where i represents the image number; Step 5: Perform image processing on each image to extract the number N of molten droplets in the image. i If there exists i such that N i >1, or for any i, N i =0, then proceed directly to step 6; if there exists i such that N i =1, then calculate the area S and initial velocity v of the droplet in the pixel coordinate system, and then execute step 6; Step 6: Based on the results of Step 5, adjust ∆T or r according to the different situations, and then repeat Steps 4 to 6. In step 6, based on the results of step 5, the following cases are handled: If there exists i such that N i If ΔT > 1, then increase ΔT and return to step 4; If for any i, N i = 0, then decrease ∆T and return to step 4; If S > S max And v > v min , or S > S min And v > v max If so, increase ∆T and return to step 4; If S < S max And v < v min , or S < S min And v < v max If so, decrease ∆T and return to step 4; If S < S min And v > v max If so, replace the nozzle with one larger r and return to step 4; If S > S max And v < v min If so, replace the nozzle with one of smaller r and return to step 4; If S∈[S min , S max And v∈[v min , v max If the condition is met, then return directly to step 4.

2. The control method for the on-demand injection process of molten droplets based on the water hammer effect according to claim 1, characterized in that, Step 5, which involves image processing for each image, includes the following sub-steps: Step 5.1: Perform threshold segmentation on the image to extract the region where the molten droplets are located in the image; Step 5.2: Perform an opening operation on the image after threshold segmentation to remove interference regions from the image; Step 5.3: Extract connected components from the image obtained in Step 5.2 to obtain the number of connected components in the image, which is considered as the number of molten droplets N. i .

3. The control method for the on-demand injection process of molten droplets based on the water hammer effect according to claim 1, characterized in that, In step 5, the area of ​​the connected region in image I1 is calculated and regarded as the area S of the droplet in the pixel coordinate system. The pixel coordinates y1 and y2 of the centroid of the connected region in I1 and I2 in the vertical direction are calculated. The initial velocity of the droplet in the pixel coordinate system is calculated as v = (y2 - y1) / ∆t, where ∆t is the time interval between the capture of two adjacent frames.

4. The control method for the on-demand injection process of molten droplets based on the water hammer effect according to claim 1, characterized in that, In step 4, the reciprocating motion of the nozzle can be either down then up, or up then down.

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