Method for manufacturing stamping tool with structure part

Abstract

To provide a method and a system for manufacturing a stamping tool having a structure part of which both of surface quality and accuracy for producing a bipolar plate for a fuel battery are high.SOLUTION: In a method of manufacturing a stamping tool, after a target geometry and an intermediate geometry of a structure part of the stamping tool are regulated, a work piece is assembled to a machine tool, and is processed by processing of a first type. A geometry of an intermediate member to be processed is measured by a measuring device, a processing pass is calculated while calculating a geometry deviation expressing a difference from the target geometry. In addition, the structure part of the intermediate member is selectively processed by processing of a second type that is different from the processing of a first type. By removing the difference between the geometry of the intermediate member to be processed and the target geometry by removing a material in the intermediate member, a final member having the structure part with the target geometry is obtained.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a stamping tool having a structure and further to a system for manufacturing a stamping tool having a structure. [Background technology]

[0002] In recent years, new energy sources have become increasingly important because of their potential contribution to ameliorating climate change. One of these new energy sources is based on proton exchange membrane (PEM) fuel cells. This technology is considered a promising technology for power generation for transportation systems such as e-mobility, as it can provide high energy density while protecting the environment.

[0003] The main component of such a fuel cell is considered to be the bipolar plate. This bipolar plate serves as the support plate for the two poles required for the fuel cell, i.e. the negatively charged anode plate carrying H2 and the positively charged cathode carrying O2. Various materials can be used for the bipolar plate, e.g. graphite, composites and metals. Metals have the advantage over the other two materials that they are characterized by good electrical conductivity, high mechanical strength and low production costs. So far, many methods have been developed to manufacture metallic bipolar plates. Known methods are roll forming, hydroforming, rubber pad forming and stamping.

[0004] WO 2008 / 034818 discloses an apparatus for producing bipolar plates for fuel cells. The apparatus comprises a reactor in which a workpiece forms the first electrode and a tool forms the second electrode. The workpiece and the tool are electrically conductively connected by an electrolyte, and a potential difference is at least temporarily created between the workpiece and the tool. The poles of the workpiece and the tool can be interchanged as cathode or anode with respect to both operating modes, in order to carry out a removal process on the workpiece for forming structures on its surface and a coating process for applying a coating to the workpiece directly in succession in the reactor.

[0005] However, developing a method suitable for mass production remains a challenge. Among these methods, the stamping process is the favored process for many manufacturers due to its processing efficiency. For mass production, it is important to find an efficient method to produce bipolar plates with high quality. For the stamping process, it is important to have a stamping tool with very high accuracy and good surface finish.

[0006] Typically, stamping tools are machined by machine tools. In particular, milling is applied. This process has the advantages of high surface quality and rapid processing. However, the processing accuracy is limited. In particular, stamping tools have a number of fine structures. The accuracy of these fine structures must be very high. It is very difficult to achieve such accuracy by milling. Summary of the Invention [Problem to be solved by the invention]

[0007] The object of the present invention is to provide a method for manufacturing a stamping tool having structures with high surface quality and high precision, in particular with improved processing efficiency. [Means for solving the problem]

[0008] According to the invention, these objects are achieved by means of the features of the independent claims, further advantageous embodiments emerge from the dependent claims and the description.

[0009] The present invention is directed to a method for manufacturing a stamping tool with a structure, in particular a stamping tool for producing bipolar plates for fuel cells, comprising the steps of: defining a target geometry and an intermediate geometry for the structure, the intermediate geometry defining a geometry of an intermediate part that requires further processing to achieve the target geometry, mounting a workpiece on a machine tool, processing the workpiece by a first type of processing based on the defined intermediate geometry to obtain an intermediate part, measuring the geometry of the processed intermediate part by a measuring device to obtain geometry data of the intermediate part, determining by a processing unit geometry deviation data representing a difference between the geometry of the processed intermediate part and the target geometry based on the measured geometry data and the defined target geometry data, calculating by a processing unit a processing path based on the determined geometry deviation data, and selectively processing the structure of the intermediate part by a second type of processing different from the first type of processing in response to the calculated processing path to obtain a final part with a structure having the target geometry by removing material in the intermediate part to eliminate the difference between the geometry of the processed intermediate part and the target geometry.

[0010] Preferably, the first type of process is a milling process and the second type of process is a laser ablation process.

[0011] Alternatively, the first type of machining is electrical discharge machining, electrical discharge chemical etching or electrochemical machining.

[0012] The accuracy of the stamping tool, and in particular the accuracy of the structures formed on the stamping tool, directly affects the accuracy of the parts produced by stamping. It is therefore essential to produce the structures with high accuracy. However, when high accuracy is required, the processing time is long. By combining different types of processing processes, it is possible to benefit from the advantages of both processing processes and optimize the processing of such stamping tools. While the milling process has the advantage of high surface quality and fast processing, the laser processing process is a very precise processing process. Furthermore, laser processing has certain limitations regarding the geometry to be achieved. For example, grooves, grooves with straight side walls or cavities are difficult to process by a laser beam. The present invention provides a method that allows different parts on a workpiece to be selectively processed by different processing processes to obtain a final part with a target geometry.

[0013] An intermediate part is not a final part, but a part that requires further processing. An intermediate geometry is therefore defined to be applied to a first type of processing. The purpose of the first type of processing is to achieve this intermediate geometry. In particular, this intermediate geometry is designed to provide sufficient material in the intermediate part to be removed by the second type of processing.

[0014] In one variant, the milling process and the laser machining process are carried out by means of the same machine tool, which in particular may have a measuring device integrated therein.

[0015] In another variant, the first type of processing, for example a milling process, is performed by a first machine tool, and the second type of processing, for example laser processing, is performed by a second machine tool. In particular, a 5-axis laser machine tool can be applied. EP 2 301 706 A1 and EP 3 047 932 A1 disclose such a laser processing process using a 5-axis laser machine tool.

[0016] In an advantageous variant, in order to save processing time, only a part of the structure is processed by the second machine tool for laser processing. In some cases, the structure is not completely processed, but only a certain part of the structure requires extremely high accuracy. Therefore, only this certain part is processed subsequently by the second machine tool in order to achieve high accuracy. This second machine tool is therefore applied to selectively process the structure. By selectively processing the structure by the second machine tool, the quality of the produced parts can be further improved and at the same time processing time can be saved.

[0017] In particular, the method further comprises inputting the measured geometry data into a processing unit and calculating a machining path by the processing unit based on the measured geometry data and a target geometry of the structure. In addition, the measuring device may measure the surface roughness to determine the surface quality.

[0018] The measuring device is configured to be able to measure the entire part of the machined in-process part, and in particular the measuring device measures all positions of the entire part.

[0019] The processing unit is configured to determine differences between the geometry of the machined intermediate machined part and a target geometry, the determined differences allowing to calculate a machining path for the second type of machining.

[0020] The processing unit may be an external computer or a control unit of the first machine tool or of the second machine tool.

[0021] In some embodiments, the structure includes a plurality of recesses, each of which has two side walls and a bottom surface. In particular, the recesses may have a high aspect ratio or a low aspect ratio. Thus, the recesses are one or more of holes, grooves, and pockets. Furthermore, the grooves are one or more of square grooves, semicircular grooves, and T-shaped grooves. The grooves may have a straight path or a curved path. In some embodiments, the stamping tool is applied to manufacture bipolar plates for fuel cells. The structure of the stamping tool includes a plurality of grooves. The grooves may be divided into different groups according to their shapes. The grooves in each group have the same geometry, and the grooves in different groups have different geometries, for example, different cross-sections or different dimensions. Each groove has two side walls and a bottom surface. The cross-sections of the grooves may have various shapes, such as square, semicircular, and trapezoidal.

[0022] In some variations, the cross section of the groove may vary along the length of the groove. In some variations, the depth of the groove may vary within a groove. In particular, the depth of the groove may vary along the length. The measuring device is configured to measure all positions of the intermediate part, so that the difference between the target geometry and the geometry of the intermediate part at all positions of the part can be precisely determined.

[0023] The method includes the steps of: defining at least one target depth and at least one intermediate depth shallower than the target depth for each groove; machining the workpiece by a milling process performed by a first machine tool to obtain an intermediate part having a plurality of grooves with intermediate depths; measuring the machined intermediate part by a measuring device to obtain measured geometry data of the intermediate part; determining by a processing unit depth deviation data representing a difference between the depth of the machined intermediate part and the target depth based on the measured geometry data and the defined target geometry; and machining a bottom surface of the groove of the intermediate part by a laser machining process based on the determined depth deviation data to remove material at the bottom surface of the groove to eliminate the difference between the depth of the machined intermediate part and the target depth, thereby obtaining a final part having a structure having the target depth.

[0024] Bipolar plates made of metal for fuel cells have the advantage of very good conductivity. However, the depth of the grooves provided in the stamping tool must be machined with high accuracy and precision, since deviations in the depth of the grooves would cause corrosion in the bipolar plate. The depth of the grooves must be precisely machined, since the cross-section of all the grooves must not vary too much, in particular all the grooves must have the same depth. Furthermore, the depth of each groove must be precisely machined to achieve the defined depth. To achieve the required accuracy, for example + / - 1 μm, the milling process has its limitations. This drawback can be compensated for by the laser machining process, which is known as a high-precision and high-precision machining process. However, in order to maintain the machining efficiency, only the grooves are machined by the laser machine tool for correction purposes. Preferably, only the bottom surface of the groove is machined by the laser machining process, and the groove depth of the machined intermediate part is corrected to achieve the target depth. In this way, the machining time can be further optimized.

[0025] The measured depth data can be compared with the target depth and a deviation is determined. This deviation defines the thickness of material to be removed from the bottom surface. For laser ablation, the remaining material can be divided into a number of layers and ablation can be performed layer by layer. If the bottom surface is too large to be processed in one position of the laser head, it is divided into several so-called patches. To ablate a defined area in one patch, the laser head remains in the same position and an optical scanner moves the laser beam so that all positions within the range of one patch are ablated. To process the next patch, the laser head is moved to another position.

[0026] If necessary, the laser head may rotate during processing of a patch to allow for the ablation of locations on the workpiece that are difficult to reach without the laser head rotating.

[0027] Furthermore, the intermediate depth is shallower than the target depth to ensure that there is enough material left at the bottom of the groove for laser machining.The stamping tool manufacturing method of the present invention allows for the production of cost-effective, corrosion-resistant stamping tools for bipolar plates by rough milling and laser finishing.

[0028] After milling, measurements are made mechanically or optically. In particular, measurements are made by CMMs, touch probes, laser distance sensors, laser ToF (time of flight) sensors, confocal sensors, interferometers or optical coherence tomographs to determine the geometric data of the machined intermediate part. In particular, the groove depths of all grooves in the intermediate part are measured and stored.

[0029] The geometry data of the intermediate part is obtained by measuring the intermediate part at all positions.

[0030] Based on the measured geometry data, a difference between the geometry of the machined intermediate part and the target geometry is calculated, which difference is corrected by applying a second type of machining to remove further material. A processing unit, e.g. a computer, can compute efficient machining paths and apply them to control the machine tool to selectively remove material from the machined intermediate part.

[0031] In one variant, the measurement is performed during a machining pause during machining by the first machine tool. When the measured depth reaches a defined intermediate depth, the machining process can be stopped. If the measured depth is less than the defined intermediate depth, the milling process can be continued. If the measurement is performed during a machining pause, it is preferred to incorporate the measuring device into the first machine tool, or the tool changer can remove the cutting tool and incorporate a machining device into the first machine tool for the measurement.

[0032] In another variation, the measurement is performed at the end of the milling process.

[0033] Consideration should be given to locating the measuring device outside the machine tool or to integrating it into the first or second machine tool.

[0034] In one variant, a number of measuring devices are provided and are integrated into the first and second machine tools, for example a first measuring device is integrated into the first machine tool and a second measuring device is integrated into the second machine tool, which increases the flexibility for performing measurements and optimizes the machining process.

[0035] Typically, the workpiece is clamped by a clamping system provided on the machine table. This clamping system comprises a pallet and a chuck. The chuck is mounted on the machine table and the workpiece is mounted on the pallet. To facilitate handling, after processing by the first machine tool, the pallet with the intermediate part is automatically transferred from the first machine tool to a measuring device or to a second machine tool. This saves time for mounting and dismounting the workpiece and ensures accurate positioning of the workpiece on the machine table.

[0036] One drawback of laser processing is that the laser beam has a certain width, so that when it hits the bottom surface of the groove, it cannot reach all positions on the bottom surface, especially positions at the connecting edge of the side wall and the bottom surface. To avoid this drawback and ensure that the laser beam can reach the entire bottom surface of the groove, the machine table of the laser machine tool can be tilted or rotated slightly during processing. Usually, the laser beam scanning is performed by an optical scanner integrated in the machine head, so that the machine table does not move during laser scanning. However, by tilting and / or rotating the machine table during processing, the laser beam can reach more positions. Thus, the second machine tool is provided with a machine table for mounting the intermediate part, which is tilted or rotated during processing. In a preferred variant, if the part is too large or heavy, the laser head is tilted or rotated instead of the part.

[0037] The invention further relates to a system for machining a stamping tool with a structure, comprising a first machine tool configured to machine a workpiece by a milling process to obtain an intermediate part with a structure with a defined intermediate geometry, a measuring device configured to measure the intermediate part to obtain geometry data of said intermediate part, and a second machine tool configured to machine the intermediate part with a structure with a defined target geometry by a laser machining process based on the measurement data to obtain a final part with a structure with a defined target geometry, in particular the laser machining being a laser ablation process.

[0038] In one variant, the system comprises a tool changer configured to exchange a machine tool for milling mounted on the first machine tool with the measuring device.

[0039] The present invention provides a system for manufacturing stamping tools for producing polar plates, which allows for better accuracy in the production of groove geometries and thus significantly improves the corrosion resistance of metallic polar plates produced by applying such stamping tools, and in addition, the system allows both the feasibility of fuel cell engines and the cost-effective production of bipolar plates.

[0040] In one variant, femtosecond lasers are applied for laser ablation. The short pulse width of this type of laser allows to reduce the thermal effects of the laser beam, thus optimizing the surface quality achieved by ablation.

[0041] By applying laser ablation, a highly accurate target geometry, in particular a target depth, can be achieved. The laser ablation process is substantially less sensitive to machine axis errors than other machining processes that require machine tools. For example, in a milling process, an error in the axis will immediately affect the tool end. This error in the tool end will result in a geometric error in the machined workpiece, in particular a depth error in the machined structure provided in the workpiece. The laser ablation process does not require machine tools. The laser beam is controlled by an optical axis to reach a defined position in the workpiece. If the laser beam reaches the surface of the workpiece within the so-called focus tolerance, i.e., within the range of, for example, + / - 0.1 mm, the ablation is performed precisely as specified, without any geometric errors caused by errors in the machine axis. This means that errors in the laser machine axis do not affect the overall accuracy of the ablation process. With the laser ablation process, a target geometry can be achieved more precisely than with other machining processes, for example milling. The laser beam therefore ablates exactly what needs to be ablated with respect to the geometric measurement.

[0042] Therefore, in a preferred variant, a milling process is applied as a roughing process and laser ablation is applied as a finishing process to obtain the target geometry.

[0043] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A more particular description of the present invention will now be described in detail with reference to the accompanying drawings, in which: FIG. [Brief description of the drawings]

[0044] [Figure 1] FIG. 2 is a three-dimensional view of a stamping tool including multiple grooves. [Diagram 2] FIG. 2 is an enlarged view of a portion D shown in FIG. [Diagram 3] FIG. 2 is a cross-sectional view of multiple grooves. [Figure 4]FIG. 2 is a plan view of multiple grooves. [Diagram 5] FIG. [Figure 6] FIG. 2 is a partial cross-sectional view of the final component. [Figure 7] 1 is a diagram of the steps of a method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] In Fig. 1 a three-dimensional view of a stamping tool 1 for manufacturing bipolar plates for fuel cells is shown. The stamping tool is a metal plate having dimensions, for example, 450 mm length, 150 mm width and 30 mm height. The stamping tool has a number of grooves 10 formed thereon. There are many groups of grooves, each with different shapes and dimensions. In the following description only one group of grooves will be described in order to show the application of the method of the invention. However, the method is also applicable to other groups of grooves, even if these have different shapes and dimensions.

[0046] The grooves of this group extend horizontally parallel to one another and are equally spaced from one another, as shown in FIG. 2, which is an enlarged view of portion D shown in FIG.

[0047] FIG. 3 shows a cross-section of a number of grooves of this group, and FIG. 4 shows its plan view. The grooves have a rectangular cross-section with two side walls 12 and a bottom surface 11. As shown in FIG. 3, the grooves have a defined groove width W1, for example 1.25 mm. The surface between two grooves is called a rib 20, and therefore the distance between two grooves is called the rib width, shown as W2 in FIG. 4. The grooves also have a defined groove depth d, which is the distance between the top surface of the rib and the bottom surface of the groove. FIG. 3 shows a simplified example. However, the grooves may have different depths. The depth within one groove may vary. The ribs may have curved surfaces. The bottom surface of the groove may have a hardened surface.

[0048] The examples given herein should not be construed as limiting the application of the method, which can be applied to manufacture other metal parts having structures that require high accuracy and precision. Such metal parts may be produced by milling combined with laser ablation.

[0049] 5 and 6 show cross-sectional views of one groove during different machining stages. The workpiece is prepared and assembled on a first machine tool for the milling process. It is also possible to first perform surface machining and / or grinding of the workpiece before milling it. The first machine tool can be any machine tool capable of performing the milling process. The groove and the rib are milled to achieve a defined intermediate depth d1. Alternatively, the first machine tool is a machine tool for grinding and milling.

[0050] Milling not only allows the productivity of producing intermediate geometries, but also a very good surface quality, resulting in ribs that cannot be machined by laser processing. In particular, milling allows the machining of straight walls and very good surfaces at the rounded upper edges, which are crucial for the stamping process. Thus, the ribs, the upper corners and the side walls of the grooves are only machined by milling, and not by laser processing.

[0051] FIG. 5 shows the machined intermediate part. The groove has two upper corners 13, two lower corners 14 and a bottom surface 11. The groove depth of the machined intermediate part is not milled to reach the target depth, but only to reach an intermediate groove depth of d1. The intermediate groove depth is defined before machining and is smaller than the depth of the final part, which is defined as the target groove depth. The milling is performed in such a way that the final surface roughness of the ribs and the upper corners of the groove is achieved.

[0052] A measuring device is applied to measure all positions of the intermediate part and calculate the difference between the intermediate depth d1 and the target depth df. The intermediate depth is calculated by comparing the position of the rib with the position of the bottom surface.

[0053] Figure 6 shows the final part. In comparison with the intermediate part, the grooves of the final part are machined by a second machine tool, namely a laser machine tool. A 5-axis laser machine tool can be used. During this step, the bottom surface of the groove is ablated by the laser beam and further material is removed to achieve the defined target depth of the groove. As shown in Figure 6, the final part has a greater depth than the intermediate part and the lower corners have a different shape caused by the laser machining. However, the shape of the lower corners is not critical for this application.

[0054] FIG. 7 shows a flow chart of the method of the invention. In a first step S1, the workpiece is milled by a first machine tool to obtain an intermediate part with a groove having an intermediate depth. In a second step S2, the intermediate part is measured by a measuring device. This measuring device can be arranged outside the machine tool or can be integrated into one of the first and second machine tools. It is also possible to integrate two measuring devices into both machine tools so that measurements can be made during processing. In a third step S3, the remaining depth is calculated by comparing the measured intermediate depth with the final depth. In a fourth step S4, the intermediate part is subsequently processed by a laser machine tool to obtain a groove having a final depth. [Explanation of symbols]

[0055] 1 Stamping tools 2 Workpiece 10 grooves 11 Bottom of the groove 12 Groove sidewall 13 Lower corner of groove 14 Upper corner of groove 20 groove rib

Claims

1. A method for manufacturing a stamping tool (1) having a structure (10), in particular a stamping tool for producing bipolar plates for fuel cells, comprising the steps of: a. defining a target geometry and an intermediate geometry for the structure, the intermediate geometry defining an intermediate part geometry that requires further processing to achieve the target geometry; b. mounting the workpiece on a machine tool; c. machining the workpiece by a first type of machining based on the defined intermediate geometry to obtain the intermediate part; d. measuring the geometry of the machined intermediate part with a measuring device to obtain geometry data of the machined intermediate part; e. determining, by a processing unit, geometry deviation data representing a difference between the geometry of the machined intermediate part and the target geometry based on the measured geometry data and the defined target geometry; f. calculating, by the processing unit, a machining path based on the determined geometric deviation data; g. selectively machining the structure of the intermediate part by a second type of machining different from the first type of machining according to the calculated machining path, and removing material in the intermediate part to eliminate the difference between the geometry of the machined intermediate part and the target geometry, thereby obtaining a final part with the structure having the target geometry; The method includes:

2. The method of claim 1 , wherein the first type of process is a milling process and the second type of process is a laser ablation process.

3. 3. The method of claim 1 or 2, wherein the first type of machining is performed by a first machine tool and the second type of machining is performed by a second machine tool.

4. The method of any one of claims 1 to 3, wherein machining the structures comprises machining a plurality of recesses (10) each having two side walls and a bottom surface.

5. The method of claim 4 , wherein only the bottom surface of the recess is machined by the second type of machining.

6. The method of claim 4 or 5, wherein machining the structure comprises machining a plurality of grooves (10).

7. a. defining for each groove at least one target depth (df) and at least one intermediate depth (d1) less than the target depth; b. machining the workpiece by a milling process performed by a first of the machine tools to obtain the intermediate part having the plurality of grooves having the intermediate depth; c. measuring the in-process part with the measuring device to obtain measured geometry data of the in-process part; d. determining, by the processing unit, depth deviation data representing a difference between the depth of the machined intermediate part and the target depth based on the measured geometry data and the defined target geometry; e) machining the bottom surface of the groove of the intermediate part by laser machining based on the identified depth deviation data, and removing material at the bottom surface of the groove to remove the difference between the depth of the machined intermediate part and the target depth, thereby obtaining a final part having the structure having the target depth; The method of claim 6, further comprising:

8. 8. The method according to claim 1, wherein the measuring is performed mechanically or optically, in particular the measuring is performed by a coordinate measuring machine.

9. 9. The method according to claim 1, wherein the measurement is performed during a processing pause during the first type of processing or at the end of the first type of processing.

10. 10. The method according to claim 2, wherein the measuring device is arranged outside the machine tool or is integrated into the first machine tool or the second machine tool, in particular a plurality of measuring devices are integrated into the first machine tool and the second machine tool.

11. 11. The method according to claim 2, further comprising clamping the workpiece to a pallet and automatically transporting the pallet with the intermediate part from the first machine tool to the measuring device or to the second machine tool after the milling process.

12. 12. The method according to claim 2, wherein the second machine tool has a machine table for mounting the intermediate part, and the machine table is tilted or rotated during machining.

13. 13. A method according to any one of claims 3 to 12, wherein the second machine tool comprises a machining head including a galvanometer for emitting a laser beam, and rotates the machining head during machining.

14. 1. A system for manufacturing a stamping tool having a structure, comprising: a. a first machine tool configured to machine a workpiece by a milling process to obtain an intermediate part having a structure with a defined intermediate geometry; b. a measuring device configured to measure the in-process part to obtain geometry data of the in-process part; c) a second machine tool configured to process the structure of the intermediate part by a laser machining process based on the measurement data to obtain a final part with the structure having a defined target geometry, in particular the laser machining is a laser ablation process; A system comprising:

15. The system of claim 14 , wherein the measurement device is removably mounted to the first machine tool or the second machine tool.

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