Finite element analysis method for mold surface change

Abstract

The invention discloses a finite element analysis method for die profile change, which comprises the following steps of: firstly, establishing a finite element model based on an ideal die profile, and acquiring profile error data of an actual stamping die and a product by using a high-precision measuring instrument; secondly, reading an ideal model inp file by using a python language, updating coordinates of a node where a fine change position of a mold profile is located, setting a series of different types of fine changes based on a measurement change result, and performing batch modeling in a combined manner; and finally, performing finite element post-processing analysis, and comparing the influence of different types of micro-changes on the forming quality of the high-precision stamping product to obtain the size and distribution of better profile micro-changes so as to guide subsequent die repair and numerical control machining. According to the method, the efficiency and accuracy of establishing the finite element model can be improved, and die repair and design of the stamping die profile can be better guided.

Description

Technical Field

[0001] The present invention relates to the technical field of the influence of die precision on precision metal plastic forming in finite element simulation, and particularly relates to a finite element analysis method for the change of die surface. Background Art

[0002] Stamping products with complex shape structures and high precision requirements have strict requirements for various quality indicators such as standard deviation, range, flatness, etc. In the process of metal plastic forming, for stamping products with large sizes and low precision requirements, when analyzing the factors affecting the forming quality of the products, the influence of die errors and deformations is generally not considered. However, for stamping products with micro-scale features and high precision, the influence of die surface errors and micro-changes cannot be ignored.

[0003] In the process of machining dies by CNC milling cutters, manufacturing errors of the dies will be caused by tool wear, vibration, heat transfer, and improper setting of parameters such as tool path and machining sequence. At the same time, during the stamping use process, problems such as surface wear, deformation, and misalignment of the dies will also occur. Therefore, for precision forming, it is very necessary to analyze the influence of die errors on the forming results and summarize the rules, and then guide the production. In engineering experience, machining allowances need to be reserved for stamping dies so as to feedback and repair the dies according to the measurement results of the forming quality of the products, and continuously cycle until the machining requirements are finally met. The cost of manufacturing and opening a stamping die is relatively expensive. If the influence of the micro-changes of the die surface is analyzed by the method of frequent repair of the dies, it will not only affect the operation of the production line, but also rely too much on the experience of die engineers and cannot perform the analysis of multiple experimental variables. Finite element simulation is needed to save time and economic costs.

[0004] When analyzing the influence of the micro-changes of the die surface, several models need to be established based on the finite element model of the ideal die surface according to different numerical values, intervals, and types of the micro-changes of the errors. Due to the complex structural characteristics of precision dies themselves, if the method of re-modifying the three-dimensional die model is adopted, a lot of time will be spent in the process of reconstructing the model, especially in the re-meshing, because the mesh size of precision forming dies is often measured in microns, and the number of meshes of a set of dies can reach hundreds of thousands or even millions. In addition, re-establishing the geometric model of the die surface means changing the topological relationship between the original mesh nodes and elements, which is not conducive to controlling a single variable. ABAQUS has great advantages in secondary development. Through the ABAQUS scripting interface, the coordinate information of the die nodes can be modified in Python language, and it is easy to achieve batch modification of the die surface for the size and distribution of die surface errors. (Song Shiguang. Research and Prediction on Wear of Hot Stamping Dies for Ultra-High Strength Steel Sheets [D]. South China University of Technology, 2022.). Summary of the Invention

[0005] Aiming at the above deficiencies, the purpose of the present invention is to provide a finite element analysis method for the change of the die surface. Through the ABAQUS script interface and secondary development using Python, the coordinate information of the nodes at the micro-variation positions of the die surface is modified, replacing the traditional method of re-establishing a batch of geometric models and generating corresponding finite element models. Finally, in the finite element post-processing stage, for the finite element models with different micro-variations of the die surface, the control variable method is used to evaluate the influence of their distribution characteristics on the product forming quality respectively, obtain the influence degree and law of the micro-variation of the die surface on the product quality, and then guide the die repair and design, or put forward appropriate requirements for the die numerical control machining.

[0006] The present invention is realized by at least one of the following technical solutions.

[0007] A finite element analysis method for the change of the die surface includes the following steps:

[0008] S1. Establish a finite element model of the ideal die surface in the ABAQUS software, divide the initial mesh, and refine the mesh for the potential change interval, and set the contact relationship and boundary conditions of the die surface according to the actual working conditions.

[0009] S2. Measure the errors of the actual upper and lower die surfaces, and measure the forming quality indexes of the product surface after stamping.

[0010] S3. Based on the ABAQUS script interface, use the Python language to read the inp file of the ideal model, and update the coordinates of the nodes at the die surface variation positions based on the measured errors of the die surface, and generate several new inp files.

[0011] S4. Establish multiple finite element models according to different changes of the die surface, and perform finite element post-processing analysis on the multiple finite element models, summarize the influence law of different variables on the product forming quality indexes, obtain the optimal micro-variation distribution result of the die surface, and repair the die.

[0012] S5. Then use the repaired die to produce products again, and measure the forming quality of the product surface after repair according to step S2.

[0013] Preferably, in step S2, when measuring the errors of the actual die surface, a high-precision measuring instrument is selected. The high-precision measuring instrument includes a coordinate measuring machine to ensure that the measurement error is less than 5μm.

[0014] Preferably, in step S2, when measuring the forming quality indexes of the product surface after stamping, the standard deviation, range, and flatness error at different positions should be included.

[0015] Preferably, in step S4, multiple finite element models are established according to different changes in the die surface, including establishing multiple finite element models according to the different sizes, ranges, and distribution characteristics of the changes in the die surface;

[0016] When establishing multiple finite element models, according to the change characteristics of the measured die, multiple finite element models including random error, uniform error, and stepped error forms are generated.

[0017] Preferably, for step S3, after reading the ideal model inp file using the python language, the node numbers of the corresponding die are traversed through a for loop statement.

[0018] Preferably, in ABAQUS, for the area where the die surface changes, record the initial two-dimensional (x, y) coordinate range of the changed area, and use an if statement to perform interval judgment on the traversed node numbers to determine whether the two-dimensional (x, y) coordinates of the traversed nodes are within the coordinate range of the changed area. If the node coordinates are within the area range, the coordinates of the node need to be modified.

[0019] Preferably, the linear interpolation method is used to modify the coordinates. The linear interpolation method is used to update the coordinates of the nodes where the die surface changes. Taking the intersection point of the changed interval and the unchanged interval as the reference invariant, the new coordinates of the nodes at each point within the micro-change interval are calculated according to the linear interpolation method.

[0020] Preferably, the formula for the linear interpolation method is:

[0021]

[0022] where z 0 is the invariant reference z coordinate, z 1 is the z coordinate of the moving plane, z represents the z coordinate of the node to be updated within the moving interval, e is the change amount, e ≤ 100 μm, and the (x, y) coordinates of the nodes within the interval remain unchanged. Preferably, in step S4, before performing finite element post-processing analysis on multiple finite element models, the boundary conditions of the original finite element model need to be modified accordingly. The boundary conditions include the moving stroke of the stamping die, that is, the displacement size.

[0023] Preferably, in step S4, the results of finite element post-processing analysis on multiple finite element models include outputting coordinates, thickness, stress, and strain, and obtaining the influence results of coordinate changes on the product forming quality.

[0024] Compared with the existing technology, the beneficial effects of the present invention are:

[0025] For complex-structured models, such as cases with excessive fillets and a large number of surface features, it is simple and feasible to modify their three-dimensional coordinate method based on certain variation rules by reading the original model inp file, effectively improving the efficiency of model establishment. At the same time, for models with minor changes in the surface, the original topological relationship between mesh nodes and elements will not be changed, nor will the original contact and material flow modes of the model, so that a single variable can be controlled to analyze the influence of different micro-changes on the forming quality of the product and avoid causing disturbance errors. Description of the Drawings

[0026] Figure 1 It is a flowchart of a finite element analysis method for the change of the die surface of the present invention;

[0027] Figure 2 It is a finite element model diagram of the stamping die in this example;

[0028] Figure 3 It is a schematic diagram of local micro-changes in the die surface of this example. Detailed Implementation Modes

[0029] The present invention will be further described below in conjunction with the drawings and specific examples. The preferred embodiments of the present invention are shown in the drawings. The function of the drawings is to supplement the description of the text part of the specification, enabling people to intuitively and vividly understand each technical feature and the overall technical solution of the present invention, but it cannot be understood as a limitation on the protection scope of the present invention.

[0030] It should be noted that certain terms are used in the specification and claims to refer to specific objects. Those skilled in the art should understand that technicians may use different nouns to refer to the same object. For example, the terms "comprising" or "including" mentioned throughout the specification and claims are open-ended terms, so they should be interpreted as "including but not limited to". The subsequent description in the specification is for the purpose of describing the preferred embodiments of implementing the present invention, but the description is based on the general principles of the specification and is not intended to limit the scope of the present invention. The protection scope of the present invention shall be defined by the appended claims.

[0031] As Figure 1 shown, a finite element analysis method for the change of the die surface in this embodiment includes the following steps:

[0032] (1) For the stamping die to be analyzed, convert the geometric model of the stamping die into the stp format, import it into the ABAQUS software, divide the initial mesh, and arrange denser seeds in the region of interest with potential fine changes for mesh refinement. For stamping forming, select the quasi-static analysis method. In the analysis step module of the ABAQUS software, appropriately reduce the analysis step time required for stamping while ensuring the convergence of the results; in the field variable output module of the ABAQUS software, select variables including coordinates, thickness, stress, and strain to generate post-processing contour plots; in the interaction module of the ABAQUS software, select an appropriate friction coefficient and choose the face-to-face contact setting; in the load module of the ABAQUS software, set the appropriate forces and boundary conditions for the model. The finite element model of one example of the present invention is as shown in Figure 2 shown.

[0033] (2) Measure the errors of the actual upper and lower die surfaces and measure the forming quality indexes of the product surface after stamping forming;

[0034] When measuring the errors of the actual die surfaces, use a high-precision measuring instrument (measurement error ≤ 5μm) to collect and measure the upper and lower die surfaces of the analyzed die and the forming quality indexes of the product, especially the data at the key feature positions of the product that need to be focused on.

[0035] In this example, the positions that have a greater impact on the product forming quality are the top surfaces of the runners and the back surfaces of the ridges of the upper and lower dies. Considering the characteristics of the large number of runners in the bipolar plate die to be measured, multiple parallel scanning path cross-section lines are respectively set for the upper and lower die surfaces, and a coordinate measuring machine is used to collect the data of the midpoints of the runner tops and the ridge ends of the die surfaces. It should be noted that the geometric dimensions of the measurement object in this example are relatively narrow. Considering the size of the measurement probe, only one point is tested on the upper and lower planes of each runner to replace the data of all points on the entire plane. In other embodiments, as many coordinates of points in the plane of interest should be measured as possible to reduce the measurement error. It should be pointed out that the subtle changes in the die surfaces described in the present invention include manufacturing errors caused by numerically controlled machine tools, surface wear and deformation during the reciprocating stamping use of the die, and centering deviations caused by die positioning. This example only explains the manufacturing errors.

[0036] In this example, the product forming quality indexes include both parameters such as height and flatness between different regions of the surface and statistical quantities such as standard deviation, range, and variance. This example has the characteristics of a large number, consistent shape, and array-type runner structure, and quality indexes such as runner height deviation and flatness are the key evaluation objects.

[0037] In this example, the measured die surface data has two functions: First, measuring the die surface data can obtain the magnitude and distribution of minute changes. Based on the above data, it can be fed back to the CNC machining process to optimize machining parameters such as the feed rate of the die surface in CNC machining and the tool path. This is because when the CNC milling cutter cuts the die surface, phenomena such as systematic machining errors and tool breakage may occur. Second, by measuring the die surface data, the positions with larger machining errors can be obtained, and at the same time, the maximum machining error amount can be obtained. Based on this, a series of coordinate change combinations with different distribution rules and minute change amounts ranging from 0 to the maximum error value can be generated. Multiple stamping dies machined by the same CNC machine tool have similar but inconsistent error distribution characteristics. Based on the coordinate values of one die measured, a batch of minute change models with similar distribution rules can be generated and promoted.

[0038] The minute changes specifically include factors such as the magnitude of change, range, and distribution characteristics. To find the more effective minute changes in the die surface for guiding die repair and producing products with higher yield rates and better die surface quality indicators, a list of die surface changes including different forms such as random errors, uniform errors, and stepped errors can be generated.

[0039] (3) After generating a series of die surface change lists, based on the script interface provided by ABAQUS, secondary development is carried out using the Python language. First, read the inp file of the original die surface finite element model, and based on the minute changes generated above, modify the node coordinates at the corresponding positions of the die surface. Finally, re - establish an inp file.

[0040] When carrying out secondary development using the Python language, first, the corresponding module embedded in ABAQUS in Python should be imported. Second, use the open() function to read the original inp file, and use the readlines() function to read all the lines of the entire file and save them in a list.

[0041] Use a for loop statement to traverse the list generated above. Inside the loop, use an if judgment statement, and the expression is the content at the beginning of the line where the die information to be modified is located. Then, a write flag writeflag needs to be added. When the corresponding judgment statement is true, the flag is true, and the continue statement is run inside the for loop. When the for loop traverses to the end content of the line where the die information to be modified is located, the break statement is run to jump out of the for loop. The purpose of this step is to screen out the lines where the die node coordinate information to be modified is located.

[0042] As an example, after screening out all rows of the coordinate information of the mold nodes to be modified, the if judgment statement is used to screen out the screening of the area where the surface node coordinates change. First, use the split(",") statement to divide each row where the node information is located into four parts, generating a list with 4 elements: node number, x coordinate, y coordinate, and z coordinate. And make corresponding conversions to the original str types of the elements: convert the node number to int type, and convert the three coordinates to float type; secondly, in the finite element model of ABAQUS, the position range of the mold nodes to be modified needs to be obtained through query. In this example, the area where the mold changes slightly is mainly the bottom or top surface of the runner-like structure, as shown in Figure 3 shown in C, which is defined as the area of interest, while the side wall part connected to it through a fillet is regarded as an invariant position, as shown in Figure 3 shown in A, and the transition fillet is as shown in Figure 3 shown in B. Since the bottom or top surface is a plane parallel to the xOy plane, transitions to the side wall through a fillet structure, and the runner has the same geometric characteristics in the y direction. Therefore, when screening the coordinates, query the two ends of the x values at the connection between the transition fillet of the bottom / top surface of each runner and the side wall in turn, that is, the x value shown at point A, which is the position range of the mold nodes to be modified.

[0043] As another example, after screening out the position range of the mold nodes to be modified, the linear interpolation method is used to change the coordinates of all nodes within the interval, as shown in Figure 3 shown. The linear interpolation method ensures the natural connection of the nodes in the transition area and avoids mesh distortion. The formula of the linear interpolation method is:

[0044]

[0045] This formula is the assignment formula for the change of the z coordinate of the nodes within the interval. Among them, z 0 is the invariant reference z coordinate, z 1 is the z coordinate of the small change plane, z represents the z coordinate of the transition point and the point on the change plane within the change interval, and e is the small change amount. The coordinates of the nodes (x, y) within the interval remain unchanged.

[0046] In this example, as shown in Figure 3 shown, H 2 is the modified runner height, while the adjacent runner height H 1 is the unmodified height. H 2 is numerically equal to H 1 plus the surface change amount e.

[0047] (4) Establish multiple finite element models according to different changes in the die surface, and perform finite element post-processing analysis on the multiple finite element models in ABAQUS software. Summarize the influence laws of different variables on the product forming quality index, obtain the distribution result of the optimal fine changes in the surface, and repair the die.

[0048] Before establishing multiple finite element models according to different changes in the die surface, it is necessary to modify some boundary conditions of the inp file, such as the movement stroke, load force, etc. In this example, since the z coordinates of some nodes on the die surface are translated compared to the ideal plane, the gap between the upper and lower dies and the blank changes. When performing finite element analysis on the influence of fine changes, it is necessary to control a single variable. Therefore, it is necessary to modify the z-direction movement amount of the upper die in the load module. At the same time, if necessary in the actual situation, the coordinates of the parts in the assembly need to be translated.

[0049] (5) Use the repaired die to produce products again, and measure the forming quality of the die surface of the repaired products according to step S2.

[0050] After finite element post-processing analysis, in the field variable output module, select variables including coordinates, thickness, stress, strain, etc. to generate post-processing contour maps. Obtain the influence results of the coordinate changes of different fine changes on the product forming quality, and analyze the reasons for the results from the mechanism. In this example, for the coordinate parameters, extract the cross-sectional line data of multiple products, calculate the height difference between the top and bottom surfaces of different runners, and then use statistical methods to summarize the range and standard deviation between multiple features. For stress and strain data, from the perspective of material plastic deformation and flow, analyze how different fine changes in the product surface affect the product quality. Based on batch data, obtain the optimal type of fine changes in the surface, and then put forward more reliable and theoretically based opinions on die repair rework and numerical control machining. It should be noted that it is necessary to use the repaired die to produce products again, and measure the forming quality of the die surface of the repaired products according to step (2) as a cyclic analysis method.

[0051] The preferred embodiments of the present invention disclosed above are only used to help illustrate the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to the specific embodiments described. Obviously, many modifications and changes can be made according to the content of this specification. This specification selects and specifically describes these embodiments to better explain the principle and practical application of the present invention, so that those skilled in the relevant technical field can understand and utilize the present invention well. The present invention is only limited by the claims and their full scope and equivalents.

Claims

1. A finite element analysis method for mold surface changes, characterized in that: The following steps are involved: S1. Establish a finite element model of the ideal mold surface in ABAQUS software, divide the initial mesh, refine the mesh for the potential change interval, and set the contact relationship and boundary conditions of the mold surface according to the actual working conditions; S2. Measure the actual error between the upper and lower mold surfaces, and measure the forming quality index of the product surface after stamping; S3. Based on the ABAQUS script interface, use Python language to read the ideal model inp file, update the coordinates of the nodes where the mold surface changes based on the measured error of the mold surface, and generate several new inp files; S4. Establish multiple finite element models according to different changes in the mold surface, and perform finite element post-processing analysis on multiple finite element models, summarize the influence of different variables on product forming quality indicators, obtain the distribution results of fine changes in the better surface, and repair the mold; S5. Re-use the repaired mold to produce the product, and measure the forming quality of the product surface after the repair according to step S2.

2. The finite element analysis method for mold surface changes according to claim 1, characterized in that: In step S2, when measuring the actual mold surface error, a high-precision measuring instrument is selected, and the high-precision measuring instrument includes a three-coordinate measuring machine to ensure that the measurement error is less than 5 μm.

3. The finite element analysis method for mold surface changes according to claim 2, characterized in that: In step S2, when measuring the forming quality index of the product surface after stamping, the standard deviation, range, and flatness error at different positions should be included.

4. The finite element analysis method for mold surface changes according to claim 1, characterized in that: In step S4, multiple finite element models are established according to different changes of the mold surface, including establishing multiple finite element models according to different change sizes, ranges, and distribution characteristics of the mold surface; When establishing multiple finite element models, multiple finite element models including random errors, uniform errors, and step errors are generated according to the changing characteristics of the measured mold.

5. The finite element analysis method for mold surface changes according to claim 1, characterized in that: For step S3, after reading the ideal model inp file using the Python language, the node numbers of the corresponding mold are traversed through the for loop statement.

6. The finite element analysis method for mold surface changes according to claim 5, characterized in that: In ABAQUS, for the area where the mold surface changes, the initial two-dimensional (x, y) coordinate range contained in the changed area is recorded, and the traversed node numbers are judged using an if statement to determine whether the traversed node two-dimensional (x, y) coordinates are within the coordinate range of the changed area. If the node coordinates are within the area range, the coordinates of the node need to be modified.

7. The finite element analysis method for mold surface changes according to claim 6, characterized in that: The linear interpolation method is used to modify the coordinates. The linear interpolation method is used to update the coordinates of the nodes where the mold surface changes. The intersection of the change interval and the unchanged interval is taken as the reference invariant. The new coordinates of the nodes of each point in the subtle change interval are calculated according to the linear interpolation method.

8. The finite element analysis method for mold surface changes according to claim 7, characterized in that: The formula for linear interpolation is: Among them, z0 is the invariant reference z coordinate, z1 is the variable plane z coordinate, z represents the z coordinate of the node to be updated in the variable interval, e is the change amount, e≤100μm, and the node (x, y) coordinates in the interval remain unchanged.

9. The finite element analysis method for mold surface changes according to claim 1, characterized in that: In step S4, before performing finite element post-processing analysis on multiple finite element models, the boundary conditions of the original finite element models need to be modified accordingly. The boundary conditions include the movement stroke of the stamping die, that is, the displacement size.

10. The finite element analysis method for mold surface changes according to claim 1, characterized in that: In step S4, finite element post-processing analysis results are performed on multiple finite element models, including output coordinates, thickness, stress, and strain, to obtain the results of the influence of coordinate changes on product forming quality.

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