METHOD FOR MANUFACTURING A POLAR PLATE OF A FUEL CELL AND ASSOCIATED MOLDING PLATE

Description

[0001] The present invention relates to a method for forming a polar plate for a bipolar fuel cell separator, as well as a forming installation implementing such a method.

[0002] A fuel cell is a device that generates electricity through an electrochemical reaction between a fuel, such as hydrogen, and an oxidant, such as oxygen from the air. This discussion focuses on proton exchange membrane fuel cells with a solid electrolyte – also known as PEMFCs – which typically consist of a stack of individual cells, each acting as an electrochemical generator.

[0003] Schematically, each cell comprises two polar plates, between which are arranged a cathode element and an anodic element, separated by a solid electrolyte in the form of an ion-exchange membrane, made, for example, of a perfluorinated sulfide polymer material. Each polar plate has a front face, with a central portion containing channels for the flow of hydrogen and oxygen, and a back face, opposite the front face. In a cell, the front face of each polar plate is oriented towards the membrane.

[0004] For two adjacent cells, a pole plate from one cell is positioned back-to-back with a pole plate from the other cell. These two pole plates together form a bipolar separator, also called a bipolar plate. A cooling circuit, through which a coolant such as glycol water circulates, is generally located between the two pole plates of the bipolar separator. Hydrogen, air, and the coolant are fluids that are continuously supplied to the fuel cell during its operation. Openings are provided in each pole plate around the central portion to allow the fluids to flow between two adjacent cells.Thus, each bipolar separator ensures on one side the fuel supply to the cell adjacent to that side and on the other side the oxidizer supply to the cell adjacent to that other side, the supplies provided by the bipolar separators being in parallel.

[0005] This section focuses on metallic pole plates, which are made from sheet metal. The openings are generally created by perforation, while the channels are formed by stamping. For optimal fuel cell performance, the pole plates must be very thin and have very tight manufacturing tolerances. As a general guideline, a pole plate is typically made from 0.1 mm thick sheet metal, while each channel has a depth of 0.2 mm to 0.3 mm, with a tolerance of less than 0.01 mm.

[0006] To reduce production costs, the polar plates are mass-produced from a strip wound into a coil, the strip being perforated and stamped and then finally cut to form each polar plate.

[0007] It is known to perform these three operations, possibly subdivided into sub-steps, with a high-capacity press comprising several tools, which together form the tooling of this press. Such a press generally includes a frame, a slide that is movable relative to the frame, and a work area, called a table, which is fixed relative to the frame and positioned opposite the slide. The tools are distributed along the table. Between each press operation, the strip is advanced a fixed distance along the table so as to progressively shape it. As an order of magnitude, a pole plate, for example, has a width of 180 mm, while the table has a length of between 1500 mm and 2000 mm, with eight to ten tools typically distributed along the table.

[0008] To ensure each operation is performed satisfactorily, centering holes are made in the strip at regular intervals, while the press includes pins configured to fit into the centering holes in order to align the strip with the tools mounted on the press. The centering holes are located around the periphery of the central portion.

[0009] However, the stamping process significantly deforms the strip towards the center of each central section, resulting in a misalignment of the centering holes. This random misalignment can reach 0.15 mm or even more, leading to a reduction in the quality of subsequent operations, particularly the cutting operation.

[0010] US-2018 / 223408-A1, US-2015 / 280252-A1, EP-3 951 9645-A1 and US-2021 / 305614-A1 each describe prior art forming processes.

[0011] It is these problems that the invention intends to address in particular, by proposing a polar plate forming process offering better precision.

[0012] To this end, the invention relates to a method for forming a pole plate for electrochemical cells of a fuel cell, the method being implemented by means of a forming installation comprising at least one stamping press, which is configured to form the pole plates in series from a metal strip, each pole plate being formed from an elementary section of the strip, the stamping press comprising: a movable slide arranged opposite a fixed table, the slide being moved by an actuation device between a high position and a low position, the slide and the table together defining a working volume of the press, and a stamping tool, which is connected to the slide and which is configured to emboss the strip, when the slide moves from its high position to its low position, a network of fluid circulation channels, in which: The forming process includes a so-called stamping stage and a so-called downstream stage, subsequent to the stamping stage and implemented by means of a so-called downstream press, belonging to the forming installation and separate from the stamping press, and during the stamping stage, while the strip is received in the working volume of the stamping press, the slide moves from its upper position to its lower position and stamps a network of fluid circulation channels onto the strip.

[0013] According to the invention, Once the channel network is stamped onto the strip, while the strip is held tight in the stamping tool, a mark is formed in the strip, using a locating tool carried by the slide, and in the downstream step, the strip is positioned relative to the downstream press by means of positioning devices mounted on the downstream press and which cooperate with the mark.

[0014] Thanks to the invention, the reference mark formed during the stamping step is positioned after the strip has been deformed by the stamping tool. During subsequent steps, the strip is positioned using this reference mark, thus the shaping performed during these steps is positioned with greater precision relative to the channel network. The overall quality of the polar plate, particularly in terms of dimensional accuracy, is therefore improved.

[0015] According to advantageous but not mandatory aspects of the invention, such a forming process may incorporate one or more of the following features taken individually or in any technically permissible combination: The mark is formed in the strip while the slide is held in the lowered position for a predetermined time interval, while the stamping tool exerts a predetermined force on the strip. The predetermined time interval is greater than 0.2 s, preferably greater than 0.3 s, and preferably greater than 0.4 s. The predetermined force is between 150 kN and 300 kN, preferably between 170 and 250 kN, and preferably between 180 and 200 kN. The forming process includes an upstream step, which is prior to the stamping step and during which a primary marker is formed in the strip, using a primary marker belonging to the forming installation, while in the stamping step, the strip is positioned relative to the stamping press by means of positioning devices mounted on the stamping press and which cooperate with the primary marker.The primary locating tool is mounted on the slide of an upstream press, which is part of the forming installation and is separate from the stamping press.

[0016] The invention also relates to a bipolar plate forming installation, the forming installation being configured to implement the forming process as described above and comprising several presses, the presses including at least one stamping press, which is configured to implement a stamping step, and a downstream press, which is configured to implement a downstream step, subsequent to the stamping step, in which: Each press includes a movable slide arranged opposite a fixed table, the slide being moved by an actuation device between an upper and a lower position, the slide and the table together defining a working volume of the corresponding press. The stamping press includes: a stamping tool, which is connected to the corresponding slide and is configured to emboss the strip when the corresponding slide moves from its upper to its lower position; a network of fluid circulation channels; and a registration tool, which is carried by the slide and is configured to form a registration mark on the strip once the channel network is imprinted on the strip, while the strip is held tightly in the stamping tool. The downstream press includes positioning devices.which are mounted on the downstream press and configured to cooperate with the marks formed on the strip, so as to position the strip relative to the downstream press.

[0017] The invention will be better understood, and other advantages thereof will become more apparent, in the light of the following description of an embodiment of a forming process and a forming installation, in accordance with its principle, given solely by way of example and with reference to the accompanying drawings, in which: [ Fig 1 ] there figure 1 is a perspective view of a polar plate forming facility; [ Fig 2 ] there figure 2 is a perspective view of a polar plate, represented schematically; [ Fig 3 ] there figure 3 is a schematic representation of the steps in a forming process implemented with the forming installation of the figure 1 ; Fig 4 ] there figure 4 represents on a larger scale, on three inserts a) to c), frames IVa, IVb and IVc to the figure 3 , And [ Fig 5 ] there figure 5 is a perspective cross-section of a press in the forming plant of the figure 1 .

[0018] A forming plant 10 is shown at the figure 1 The forming installation 10 is configured to form pole plates for electrochemical cells of a fuel cell. A pole plate 100 is shown in the figure 2 .

[0019] The pole plate 100 is made of sheet metal, for example, stainless steel. The pole plate 100 has a generally rectangular shape and extends along a plate plane P100. The pole plate 100 includes a central portion 102, in which a network of channels 104 is formed for circulating a fluid necessary for the operation of the fuel cell. This fluid is, for example, hydrogen, air, and glycol water. The network of channels 104 is schematically represented by three lines. A center 105 of the network of channels 104 is defined as a geometric centroid of the network of channels 104. In the illustrated example, the pole plate 100 has a rectangular shape, while the center 105 is located, schematically, at the intersection of the diagonals of this rectangle.

[0020] The polar plate 100 also includes perforations 106, which are provided around the periphery of the central portion 102 and are intended for the passage of fluids from one side to the other of the polar plate 100. In the illustrated example, the perforations 106 are distributed in two groups of three perforations, the shape and arrangement of the perforations 106 not being limiting.

[0021] The forming installation 10 is configured to form the polar plates 100 in series from a strip 12. The strip 12 is a metal band, which is generally transported coiled, in the form of a roll 14. The roll 14 is uncoiled at the inlet of the forming installation 10, the strip 12 being formed in the forming installation 10, that is to say, shaped and cut in the presses of the forming installation 10, to form the polar plates 100. Each polar plate 100 is thus formed from an elementary section 13 of the strip 12 and corresponds, apart from the scraps and losses of material generated during the forming, to an elementary section 13 of the strip 12.

[0022] The forming installation 10 comprises three separate presses 20. Each press 20 includes a frame 22, which is generally in the shape of an elongated parallelepiped extending along a height axis Z20. When the press 20 is in its operating configuration, the frame 22 rests on the ground, with the height axis Z20 perpendicular to the ground. The ground is assumed to be horizontal, and therefore the height axis Z20 is assumed to be vertical. The frame 22 comprises four peripheral faces 12, including a front face 23A, a rear face 23B opposite the front face 23A, an upstream face 23C, and a downstream face 23D, opposite the upstream face 23C and perpendicular to the front face 23A and rear face 23B. On the figures 1 And 5 , the presses 20 are represented in perspective, the front face 23A of each press being oriented towards the left of the figures, while the downstream face 23D is oriented towards the right.

[0023] For each press 20, the front faces 23A and rear faces 23B are orthogonal to a depth axis Y20 of the press, while the upstream faces 23C and downstream faces 23D are orthogonal to a transverse axis X20 of the press, the three transverse axes X20, depth Y20 and height Z20 being oriented to form a direct reference frame.

[0024] Each press 20 includes a slide 24, which is movable relative to the frame 22, the slide 24 being guided in translation relative to the frame 22 along the height axis Z20, here by means of slides 25 which are visible at the figure 5 Each press 20 also includes a table 26, which is fixed relative to the frame 22 and which is arranged opposite the corresponding slide 24, the slide 24 and the table 26 together delimiting a working volume V20 of the press 20. In the operating configuration of the press 20, the table 26 is located under the slide 24.

[0025] Each press 20 also includes an actuation device 28, which moves the slide 24 between its upper and lower positions. The slide 24 is closer to the table 26 in the lower position than in the upper position. By extension, each press 20 is in a lower configuration, or in an upper configuration, when the corresponding slide 24 is in its lower position, or in its upper position, respectively. Each press 20 moves from its upper configuration to its lower configuration when it is said to be triggered, and then returns to its upper configuration after a predetermined time interval.

[0026] Each press 20 is equipped with a tooling 30 for shaping the strip 12. As illustrated in figures 3 And 4The shaping of the strip 12 is carried out in several successive stages, each of these stages being performed using a specific tool mounted on one of the presses 20. All the shaping tools mounted on a single press 20 constitute a tooling set 30 for that press 20. The tooling set 30 for each press 20 therefore includes, depending on the case, one or more shaping tools. "Shaping" refers to an operation that deforms the strip, for example, plastic deformation, cutting, drilling, etc. Simple elastic deformation, inspection, or cleaning operations are therefore not considered shaping.

[0027] The three presses 20 are aligned with each other; more precisely, the transverse axes X20 of the three presses 30 are aligned. The press 20 located between the other two is called the "intermediate press 40". The press 20 positioned opposite the upstream face 23C of the intermediate press 40 is called the "upstream press 50", while the third press 20, positioned opposite the downstream face 23D of the intermediate press 40, is called the "downstream press 60".

[0028] In figure 5 The intermediate press 40 is shown in cross-section along a plane orthogonal to the depth axis Y20, revealing the interior of the intermediate press 40. For each press 20, each tool—and by extension each tooling 30—of this press comprises a movable portion 34, which is carried by the corresponding slide 24, and a fixed portion 36, fixed opposite the movable portion 32 on the corresponding table 26. Each movable portion 34, together with the associated fixed portion 36, defines a working area of ​​the corresponding press 20, each working area being configured to receive an elementary section 13 of the strip 12.

[0029] We will now detail the steps in the strip forming process 12. On the figure 3 , at the input of the forming process, there is a roller 14, which is unrolled, the strip 12 advancing progressively, the advance of the strip 12 being represented from left to right and from top to bottom.

[0030] In a first step 200, referred to as the "primary registration" step, a primary registration mark 202 is formed in the strip 12 using a primary registration tool 204 belonging to the forming installation 10. The primary registration tool 204 here comprises two punches, while the primary registration mark 202 is formed by two holes, each arranged along a respective edge of the strip. According to an alternative (not shown), the primary registration mark 204 is obtained by plastic deformation of the strip 12, for example by punching. However, the primary registration mark 202 is preferably formed by one or more holes.

[0031] The primary locating tool 204 is mounted here on the slide 24 of the upstream press 50. The primary marker 202 is thus formed on the strip 12 each time the slide 24 of the upstream press 50 moves from its upper to its lower position, in other words, each time the upstream press 50 is activated and moves from its upper to its lower configuration. Preferably, a primary marker 202 is thus formed for each elementary section 13 of the strip 12.

[0032] Between each activation of the upstream press 50, once the upstream press 50 has returned to its raised position, the strip 12 is moved relative to the upstream press 50 along the table 26, parallel to the X20 axis. The strip 12 advances sequentially, with an advance increment equal to the length of each segment 13 measured along the strip 12, parallel to the X20 axis, and a predetermined advance rate. The strip 12 advance defines an upstream-downstream direction of the forming unit 10. Generally, the advance rate is equal to the activation rate of the press.

[0033] The upstream press 50, and more generally each press 20, includes a strip feeder 12, configured to control the advance of the strip 12 along the corresponding table 26. The feeder is not shown. The advance of the strip 12 is preferably synchronized for each press 20, each press 20 being configured to move the corresponding slide 24 to the lower position after each advance.

[0034] The upstream press 50 also includes positioning elements 38, which are configured to cooperate with the primary marker 202 formed in the strip 12 so as to position the strip 12 relative to the upstream press 50 after each advance movement of the strip 12. The positioning elements 38 are implemented by positioning fingers, which are inserted into the holes of the primary markers 202. The positioning fingers are preferably conical. This results in precise and repeatable positioning of the strip 12 relative to the tools 30 of the upstream press 50. The positioning elements 38 can be movable, particularly along the direction of the Z20 axis, with a reciprocating motion having the same frequency as the press's triggering frequency.

[0035] Next, after the primary registration step 200, during a perforation step 210, perforations 106 are made through the strip 12 by means of a perforation tool 121. The perforation step 210 is therefore a shaping step. The perforation tool 121 comprises six punches, each configured to make a respective perforation 106. The punches are mounted on the slide 24 of the upstream press 50 and are configured to cooperate with a die fixed to the table 26 of the upstream press 26. The die is not shown.

[0036] During the perforation step 210, the positioning devices 38 thus allow good alignment of the strip 12 with respect to the punches used to form the perforations 106, and by extension with respect to the perforation tool 121.

[0037] The primary registration step 200 and the perforation step 210 are, for example, two steps which each correspond to a distinct triggering of the upstream press 50. The primary registration step 200 and the perforation step 210 are, for example, two steps which each correspond to one of two immediately successive distinct triggerings of the upstream press 50.

[0038] Next, after the perforation step 210, during a so-called stamping step 220, the strip 12 is stamped, in other words, the strip 12 is plastically deformed, so as to imprint in relief the network of channels 104 for fluid circulation, by means of a stamping tool fixed to the slide 24 of the corresponding press. The stamping step 220 is therefore a shaping step. Typically, the stamping tool comprises two complementary shape dies, which are positioned on either side of the part to be stamped, here the strip 12. The stamping step 220 is implemented here by the intermediate press 40, which is therefore a stamping press, whose tools 30 include a stamping tool 42, which includes a movable die 43A, which is fixed to the slide 24 by a fixing device, and a complementary die 43B carried by the table 26. The fixing device is not shown.Typically, the fixing device provides a bearing area between the mobile matrix 43A and the slider 24, this bearing area being flat.

[0039] During the stamping step 220, the strip 12 tends to deform, so the primary marker 202, formed on the strip 12, moves relative to its original position on the strip 12 and can no longer fulfill its role as a reference. To overcome this problem, during the stamping step 220, once the channel network 104 is formed by stamping on the strip 12, while the strip 12 is held tightly in the stamping tool 42, which corresponds to the fact that the corresponding slide 24 is in the lower position or close to its lower position, a so-called secondary marker 222 is formed in the strip, using a secondary locating tool 224 carried by this slide 24. In the illustrated example, the secondary locating tool 224 comprises two perforation punches, while the secondary marker 222 is formed by two holes, each arranged along a respective edge of the strip 12.In general, the secondary marker 222 is preferably formed of one or more holes in the strip 12. Preferably, a secondary marker 222 is formed for each elementary section 13 of the strip 12.

[0040] The secondary registration tool 224 includes another actuating device, called the secondary actuating device, which is carried by the corresponding slide 24 and which moves the perforation punches while the strip is held tightly in the stamping tool 42, so as to form the secondary registration mark 222 in the strip 12. The secondary actuating device is not shown. Thus, the secondary registration tool 224 is placed in a working position when the slide 24 is in its lowered position or close to its lowered position, while the strip 12 is held tightly in the stamping tool 42, the secondary actuating device then being activated to form the secondary registration mark 222. The secondary registration mark 222 can thus be formed on the strip 12 at the time chosen by the operator, as long as the strip 12 is held tightly in the stamping tool 42.

[0041] The actuation device 28 of the intermediate press 40 includes a pressing actuator 46, which moves the corresponding slide 24 between its upper and lower positions and is configured to exert a pressing force on this slide 24 when the slide 24 is in its lower position and the strip 12 is being pressed by the pressing tool 42. The pressing actuator 46 here includes a connecting rod, which is mounted at its upper end on an eccentric crank shaft 47 pivoting eccentrically about an axis parallel to the depth axis Y20. The pressing actuator 46 is connected to the slide 24 by a connection point 49. In the example, the pressing actuator 46 includes a connecting rod, which is connected, at its lower end, to the slide 24 by a pivot or ball joint forming the connection point 49 through which the pressing force is transmitted.The actuation device 28 also includes a servomotor 48, represented here by a cylinder protruding from the corresponding rear face 23B, which is configured to control the eccentric rotational movements of the crank eccentric shaft 47. In other words, the servomotor 48 is configured to drive the pressing actuator 46 so that it drives the slide 24 in a reciprocating translational movement along the vertical axis Z20, between its upper and lower positions, at a frequency that is the press's trigger frequency. In simplified terms, the actuation device 28 functions like a crankshaft whose rotation is controlled by the servomotor, while the crank eccentric shaft 47 drives the slide 24 in a reciprocating motion between its upper and lower positions.Schematically, the pressing force is directed along a pressing axis A49, which is parallel to the height axis Z20 and passes through the connection point 49 between the pressing actuator 46 and the slide 24. Advantageously, the pressing axis A49 is arranged so as to pass through the channel network 104 formed on the strip during the stamping step 220. Preferably, the pressing axis A49 is aligned with the center 105 of the channel network 104. In the illustrated example, the stamping tool 42 is positioned vertically below the pressing actuator 46, specifically vertically below the connection point 49 between the pressing actuator 46 and the slide 24.

[0042] According to examples, the stamping tool 42 is placed in the center of the working volume of the intermediate press 40. Thus, any deformations of the frame 22 during the stamping step are distributed symmetrically around the pressing axis A49, which contributes to the homogeneity of the pressing force during the formation of the channel network 104, and therefore contributes to the quality of the stamping.

[0043] For this purpose, the intermediate press 40 advantageously comprises an odd number of pressing actuators 46. In particular, the intermediate press 40 preferably comprises a single pressing actuator 46. When the intermediate press 40 comprises a single pressing actuator 46, this pressing actuator 46 is thus arranged above the working area, aligned with the center of the working area along the height axis Z20. When the intermediate press 40 comprises several pressing actuators 46, for example three, the pressing actuators 46 are distributed along the shaft 47, one of the pressing actuators 46 being substantially located in the center of the shaft 47 and aligned with the center of the working area along the height axis Z20.

[0044] In general, in presses of the prior art, the table has an opening in its center, designed to evacuate the material chips generated during the shaping operations. However, this opening tends to reduce the rigidity of the table, which tends to flex during the operation of the press, this flexing reducing the precision of the stamping operation.

[0045] Preferably, the table 26 of the intermediate press 40 is a solid table, without a central chip evacuation opening, for example, made from a solid block of metal. Of course, if necessary, tapped holes or equivalent are made in the table for attaching the shaping tools.

[0046] During the back-and-forth movement of the slide 24 between its upper and lower positions, the slide 24 reaches extreme positions, specifically a lower position and an upper position. In the case of the stamping step 220, as the slide 24 moves from the upper position to the lower position, it is understood that the strip 12, held between the two movable dies 43A and fixed dies 43B of the stamping tool 42, is clamped between these two dies 43A and 43B before the slide 24 reaches the lower position. As the slide 24 approaches its lowered position, the clamping force of the dies 43A and 43B—and by extension the pressing force of the pressing actuator 46—gradually increases, plastically deforming the strip 12 to emboss the channel network 104. The clamping force reaches its maximum when the slide 24 reaches its lowered position. Then, the slide 24 begins to move upwards.The strip 12, clamped between the two dies 43A and 43B, initially relaxes elastically as the two dies 43A and 43B move away from each other. The clamping force gradually decreases until it becomes zero. Thus, the clamping force on the strip 12 by the stamping tool 42 is applied not only when the slide 24 is in its lowered position, but also for a range of positions around this lowered position, which are referred to as near the lowered position.

[0047] Since the movements of the slide 24 are controlled by the servomotor 48, it is understood that the servomotor 48 allows control of, in particular, the speed of descent of the slide, the speed of ascent of the slide, as well as the time interval during which the strip 12 is held tight in the stamping tool 42 or even the clamping force - or pressing force - exerted on the strip 12 during this time interval.

[0048] Preferably, during the stamping step 220, the pressing force is maintained, by the servomotor 48, for a predetermined time interval, and at a predetermined value, when the slide 24 is in the lower position or close to its lower position, the secondary locating tool 224 being triggered during this time interval, so as to form the secondary marker 222 in the strip 12. The predetermined time interval during which the pressing force is maintained is called the "holding time", while the predetermined value of the pressing force is called the "holding force".

[0049] This ensures that the transient effects of stamping, in particular the vibrations of the intermediate press 40 and the elastic return of the strip 12, are finished before forming the secondary mark 222 on the strip 12. The secondary mark 222 is thus placed more precisely on the strip 12.

[0050] The holding time is chosen to be greater than 0.2 s (second), preferably greater than 0.3 s, preferably even greater than 0.4 s, while the holding force is between 150 kN (kilo Newton) and 300 kN, preferably between 170 and 250 kN, preferably even between 180 and 200 kN.

[0051] It is advantageous to retract, during the holding time, the positioning elements 38 which cooperate with the primary locating, in order to avoid abnormal wear and pollution generated by the deformation of the strip 12 during the stamping operation 220. The positioning elements 38 can be re-engaged after the expiry of the holding time in particular for the end of the transfer of the strip 12 in the intermediate press 40.

[0052] According to some embodiments, the servomotor 48 is slowed down as the slide 24 approaches its lower position, so as to keep the stamping tool 42 tight on the strip 12. Of course, the control method of the intermediate press 40 depends on the technology used for this press, and the specialist will be able to transpose the example described here to presses of other technologies.

[0053] For comparison, the pressing force required to stamp the strip 12, in other words the stamping force, is on the order of 200 tonnes, or approximately 2000 kN. This ensures proper positioning of the secondary marker 222, while avoiding excessive stress on the servomotor 48.

[0054] Next, once the secondary mark 222 is formed on the strip 12, the intermediate press 40 returns to its high configuration, and the strip 12 is moved according to the forward movement.

[0055] The forming process includes a step 230 of cutting the strip 12, which is subsequent to the stamping step 220 and during which the strip 12 is finally cut, thus forming the polar plate 100. The cutting is therefore a shaping step, which is done here using a cutting tool 232, which separates each elementary section 13 of the strip 12.

[0056] Where applicable, the forming process includes other shaping steps subsequent to the stamping step 220, for example, pre-cutting steps, reworking of perforations 106, etc. The shaping steps subsequent to the stamping step are called "downstream steps" of the forming process, with the stamping step 230 being a specific example of a downstream step. Thus, the forming process includes at least one downstream step.

[0057] The downstream step(s) are preferably carried out in the downstream press 60. Advantageously, during at least one downstream step, the strip 12 is positioned relative to the downstream press 60 by means of secondary positioning elements 238, which are mounted on the downstream press 60 and which cooperate with the secondary marker 222, in particular by complementary shape, so as to position the strip 12 relative to the downstream press 60. The secondary positioning elements 238 are here represented by positioning fingers, which are received in the holes of the secondary marker.

[0058] Preferably, the secondary marker 222 is used to position the strip 12 during each of the downstream steps. Consequently, only the stamping step 220 is carried out in the intermediate press 40, the stamping step 220 preferably including the forming of the secondary marker 222. In other words, the tooling of the intermediate press 40 includes, in addition to the stamping tool 42, the secondary marker 224.

[0059] Generally, when designing the forming installation 10, each press 20 is designed to exert a maximum pressing force, called the nominal force, which is a function in particular of the maximum force required to perform the task for which this press 20 is intended and a safety factor.

[0060] In prior art forming installations, the stamping press generally performs other shaping steps, particularly punching and cutting operations. These prior art presses are thus designed to exert a nominal pressing force ranging from 800 to 1000 tonnes, or 8 to 10 MN (Meganewons).

[0061] In the forming installation 10, the downstream steps are carried out by the downstream press 60, while the steps prior to the stamping step 220, called upstream steps, are carried out by the upstream press 50.

[0062] In particular, the primary locating step 200 is carried out by the upstream press 50, in which the primary locating tool 204 is mounted on the slide 24 of the upstream press 50. Similarly, in the illustrated example, the perforation step 210 is also carried out by the upstream press 50.

[0063] Thus, in the forming plant 10, the pressing force of the downstream press 40 is used only for the stamping step 220. The intermediate press 40 is designed to generate a nominal pressing force of less than 4 MN, or approximately 400 tonnes. Preferably, the nominal pressing force is less than 3 MN, and even more preferably less than 2 MN. The intermediate press 40 is significantly less expensive than a press used in a prior art forming plant.

[0064] An internal portion 16A is a portion of the strip 12 that is received into the working volume of a press 30, while a portion of the strip 12 located between two adjacent presses 30 is an external portion 16B of the strip 12. On the figure 1The strip 12 therefore comprises three internal portions 16A and two external portions 16B. The forming installation 10 includes tensioning devices, which are arranged between two adjacent presses 30 and are configured to maintain tension on the internal portions 16A of the strip 12, while keeping the external portions 16B of the strip 12 slack. The tensioning devices are not shown. In practice, they may be gripper transfers, known as "digital" transfers, moving along the X20 axis by the length of the elementary section 13 with a movement along the Z20 axis of approximately 5 mm.

[0065] According to examples, the tensioning elements are combined with the advancement elements.

[0066] With the external portions 16B relaxed, the transmission of mechanical stresses along the strip 12 between two consecutive presses 30 is avoided. In particular, since the stamping step 220 tends to "pull" on the strip 12, the relaxed external portions 16B eliminate the risk of misalignment of the strip 12 relative to the tools of the upstream and downstream steps.

[0067] The embodiments and variants mentioned above can be combined to generate new embodiments of the invention.

Claims

1. A method of forming a polar plate (100) for electrochemical cells of a fuel cell, the method being implemented by means of a forming plant (10) comprising at least one stamping press (40), which is configured to form the polar plates in series from a metal strip (12), each polar plate being provided in an elementary section (13) of the strip, the stamping press comprising: - a movable slider (24) arranged opposite a stationary table (26), the slider being moved by an actuation device (28) between an upper position and a lower position, the slider and the table together delimiting a working volume of the press, - a stamping tool (42), which is connected to the slider and is configured to stamp a network of channels (104) for the circulation of fluids in relief on the strip, when the slider moves from the upper position thereof to the lower position thereof, wherein: - the forming method comprises a so-called stamping step (220) and a so-called downstream step (230), subsequent to the stamping step and implemented by means of a so-called downstream press (60), belonging to the forming plant (10) and distinct from the stamping press (40), - during the stamping step, while the strip (12) is received in the working volume of the stamping press, the slider (24) moves from upper position thereof to the lower position thereof and stamps on the strip a network of circulation channels (104) for fluids, the forming method being characterized in that: - once the channel network (104) is stamped on the strip (12), while the strip (12) is held clamped in the stamping tool (42), a reference mark (222) is formed on the strip by means of a marking tool (224) carried by the slider, - during the downstream step (230), the strip is positioned with respect to the downstream press (60) by means of positioning members (238) mounted on the downstream press and which cooperate with the reference mark.

2. The forming method according to claim 1, wherein the reference mark (222) is formed on the strip (12) while the slider (24) is held in the lowered position for a predetermined period of time while the stamping tool (42) exerts a predetermined force on the strip.

3. The forming method according to claim 2, wherein the predetermined time interval is greater than 0.2 seconds, preferably greater than 0.3 seconds, else preferably greater than 0.4 seconds.

4. The forming method according to any one of claims 2 or 3, wherein the predetermined force is comprised between 150 kN and 300 kN, preferably between 170 and 250 kN, else preferably between 180 and 200 kN.

5. The forming method according to any one of claims 1 to 4, wherein: - the forming method comprises a so-called upstream step (200) which precedes the stamping step (220) and during which a primary reference mark (202) is formed on the strip (12) using a primary marking tool (204) belonging to the forming plant (10). - during the stamping step (220), the strip (12) is positioned relative to the stamping press (40) by means of positioning members (38) mounted on the stamping press and which cooperate with the primary reference mark.

6. The forming method according to claim 5, wherein the primary marking tool (204) is mounted on the slider (24) of an upstream press (50), which is part of the forming plant (10) and is distinct from the stamping press (40).

7. A forming plant (10) for bipolar plates (100), the forming plant being configured to implement the forming method according to any one of claims 1 to 6 and comprising a plurality of presses (30), the presses including at least one stamping press (40), which is configured to implement a stamping step (220), and a downstream press (60), which is configured to perform a downstream step, subsequent to the stamping step, wherein: - each press comprises a movable slider (24) arranged opposite a stationary table (26), the slider being moved by an actuation device (28) between an upper position and a lower position, the slider and the table together delimiting a working volume of the corresponding press, - the stamping press (40) comprises: • a stamping tool (42), which is connected to the corresponding slider and which is configured to stamp a network of channels (104) for the circulation of fluids in relief on the strip (12), when the slider moves from the upper position thereof to the lower position thereof, and • a marking tool (224), which is carried by the slider (24) and is configured to form a reference mark (222) on the strip after the network channel is printed on the strip, while the strip (12) is held clamped in the stamping tool (42), - the downstream press (60) comprises positioning members (238) mounted on the downstream press and configured to cooperate with reference marks (222) formed on the strip, so as to position the strip relative to the downstream press.