Bipolar plates and methods for embossing channel structures

Abstract

The present invention relates to a method for embossing a channel structure (3) with a plurality of parallel channel portions (5) in a planar metal sheet (11), in particular to form half plates (2, 2') for a bipolar plate (1) of an electrochemical cell, the method comprising the following steps: Providing a planar metal sheet (11) having a uniform initial thickness (d5); inserting the metal sheet (11) into a forming tool (12), wherein a base surface (BE) of the sheet (11) defined by the undeformed planar metal sheet (11) is provided to rest on a tool plane defined by a tool portion (13) of the forming tool (12); and forming a plurality of channel portions (5), wherein each channel portion (5) is designed to have two non-parallel sides (7, 8) so as to allow material from embossed portions (9, 10) of the metal sheet (11), located outside the sides and remaining in a plane parallel to the base surface (BE) and / or the base surface (BE) throughout the forming process, to be displaced to the sides (7, 8), each side (7, 8) extending from the base surface (BE) to an adjacent parallel plane.

Description

[Technical Field]

[0001] The present invention relates to a method for embossing a channel structure with a plurality of parallel channel portions in a planar metal sheet to form a half-plate or metal sheet, and further to a bipolar plate for an electrochemical cell, in particular a fuel cell, manufactured using such a method. [Background technology]

[0002] A method for forming a plate-shaped blank is known, for example, from DE 10 200 04 133 A1. Products that can be produced using this known forming method have walls with reduced thickness. Before forming, the blank from which the product is produced is provided with weld channels in the areas where the blank material flows during the forming process, which results in a reduced wall thickness compared to the wall thickness of the blank, the weld channels extending in the direction of the material flow that occurs during the forming process.

[0003] The forming method disclosed in Patent Document 2 is designed to produce pot-shaped metal parts from flat material. This forming method combines deep drawing and pressing, and is said to be particularly suitable for producing parts with very complex shapes that have eccentric parts or areas with small material thickness. In particular, the forming method according to Patent Document 2 should be able to eliminate weld seams.

[0004] Patent Document 3 describes a molding method for forming a predetermined overpressure break point in a battery cover, where material from the battery cover needs to flow into a mold recess by moving a stamp portion of a molding tool closer to the mold recess so that the remaining distance corresponds to the minimum wall thickness at the predetermined overpressure break point.

[0005] Patent Document 4 discloses a compression tool for producing half shells with high dimensional stability. When forming the half shells, the side walls of the tool are moved perpendicular to the direction of movement of the stamp. Thus, during forming, several tool parts undergo adjustment movements in mutually perpendicular directions.

[0006] For example, US Pat. Nos. 5,629,999, 5,799,122, 5,799,133 and 5,799,143 describe various forming methods for forming metal sheets under the influence of temperature.

[0007] US Pat. Nos. 5,699,949 and 5,729,296 disclose various devices for electrohydraulic or electromagnetic forming of metal sheets.

[0008] Patent document 10 describes a press forming method for forming a flat metal sheet by press forming, in which a shear deformation process is carried out in which the material flows towards the curved section.

[0009] Patent document 11 discloses a forming tool and a method for producing deep-drawn metal sheet parts. In addition to the deep-drawing, a second manufacturing step is carried out, particularly in the form of extrusion.

[0010] Patent document 12 describes a method for the continuous production of vehicle parts from metal sheets, for which the metal sheets are heated before they are separated and / or shaped. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] German Patent No. 19755964 [Patent Document 2] German Patent Application Publication No. 102008031421 [Patent Document 3] German Patent Application Publication No. 102019103606 [Patent Document 4] German Patent Application Publication No. 102013103612 [Patent Document 5] European Patent No. 3485992 [Patent Document 6] European Patent No. 0946311 [Patent Document 7] German Patent No. 19529429 [Patent Document 8] European Patent Application Publication No. 2292343 [Patent Document 9] German Patent Invention No. 102007013017 [Patent Document 10] US Patent Application Publication No. 2016 / 158821 [Patent Document 11] German Patent Application Publication No. 102010044788 [Patent Document 12] German Patent Application Publication No. 102017124724 Summary of the Invention [Problem to be solved by the invention]

[0012] The object of the present invention is to specify a metal sheet forming option, further developed with respect to the aforementioned prior art, that is particularly suitable for the manufacture of channel structures of bipolar plates of electrochemical cells, in particular fuel cells, and that allows for wall thickness variations within the final product. [Means for solving the problem]

[0013] This object is achieved according to the present invention by a method for embossing a channel structure, comprising a step according to claim 1. The embossing method according to claim 1 is particularly suitable for producing at least one metal sheet or half plate for a bipolar plate according to claim 7. The embodiments and advantages of the invention described below in relation to a device, i.e. a bipolar plate, or a bipolar plate component that is part of a bipolar plate, in particular in the form of a half plate, also apply mutatis mutandis to the embossing method, i.e. the forming method, and vice versa.

[0014] A method for embossing a channel structure with a plurality of parallel channel portions in a planar metal sheet, in particular to form half plates or metal sheets for bipolar plates, comprising: providing a planar metal sheet having a uniform initial thickness; Inserting a metal sheet into a forming tool, the metal sheet being provided such that a base surface of the metal sheet defined by an undeformed planar metal sheet rests on a tool plane defined by a tool portion of the forming tool; and forming a plurality of channel portions, each of the channel portions being formed to have two non-parallel sides, each extending from the base surface to an adjacent parallel surface, such that material of the embossed portion of the metal sheet that is located outside the sides and that remains in the base surface and / or in a plane parallel to the base surface throughout the forming process is displaced to the side.

[0015] The invention is based on the idea that during deep drawing of a metal sheet the flat structure of the starting product, i.e. the initially planar metal sheet, is essentially preserved, and the wall thickness of the metal sheet in different metal sheet sections remains unchanged or is reduced by forming.

[0016] The reduction in wall thickness occurs during deep drawing, particularly in areas where material is displaced from the base surface of the metal sheet used as the starting product, in order to produce walls that are inclined relative to the base surface, or in extreme cases perpendicular to the base surface. Provided that the parts of the starting product located outside the inclined part remain in an unchanged position throughout the forming process, only the material that was originally in the surface part is available to form the inclined part, which corresponds to the perpendicular projection of the inclined part on the base surface.

[0017] This means that the more pronounced the slope of the corresponding part of the final product that protrudes from the base surface, the less material is available to shape it using deep drawing. It should also be kept in mind that, depending on the material used, a high degree of forming can lead to work hardening, making the product more susceptible to cracking. While this risk can be countered in conventional methods by avoiding steep sides in the embossed structure, this can impair fluid flow optimization if the embossed structure restricts the channels for liquid and / or gaseous fluids.

[0018] The embossing method according to the present application effectively addresses the conflict of objectives between the forming surface and the flow surface in that the material flows to a large extent in planes located between areas of different inclination of the metal sheet relative to the base surface as the reference surface in each case. Compared to conventional deep drawing methods, the embossing method has many features of extrusion. The parts where the material flows to the sides, i.e., the embossed parts, remain in their original position during the embossing process. That is, the entire part is not pulled to the sides during forming.

[0019] In particular, the embossed portions can be located on the base surface. Similarly, there can be embossed portions, each constituting the bottom of a channel portion. Material from at least one embossed portion located outside the channel portion and on the base surface is preferably pressed into at least one of its adjacent side surfaces. Furthermore, material from at least one embossed portion located in a plane parallel to the base surface, which is located between two side surfaces and constitutes the bottom of the channel portion parallel to the base surface, is preferably displaced into one of its adjacent side surfaces. The channel portion, whose sides are reinforced with material from the base surface and / or a plane parallel to the base surface, can have a cross section, in particular, having a trapezoidal basic shape. The side surfaces of the channel portion are inclined, for example, at an angle of at least 45° and at most 78° relative to the base surface, although both sides do not necessarily have to have the same inclination angle. Embodiments in which the side surfaces are completely or partially arcuate, for example, arcuate or elliptical, are also possible. Similarly, a completely planar shape of the channel bottom formed between the side surfaces is not required. It is also possible to eliminate the extensive channel bottom, in which case the channel can in particular have a U- or V-shape.

[0020] According to various possible variations of the method, the thickness of an originally flat metal sheet can be reduced, for example, to a minimum of 70% of the initial thickness by forming the metal sheet. Another value that influences the material distribution occurs when the projection is considered relative to the base surface. In this respect, material may accumulate in the side regions, which can be expressed as a projected thickness of 105% to 150% of the initial thickness.

[0021] When embossing of the flat area displaces a particularly large amount of material, the flow of material from the flat metal sheet used as the starting product to the sides of the channel structure can even be so significant that the maximum wall thickness of the final product is found in the side areas rather than in the flat area.

[0022] Embossing the flat area to accumulate material in the side area allows the thickness of the embossed area to be, for example, 50% to 95%, particularly 60% to 90%, of the initial thickness of the metal sheet. Embossing also has the advantage that a final product with a relatively narrow tolerance in thickness at different surface portions can be produced using metal sheets whose thickness varies greatly due to quality variations within the same metal sheet.

[0023] The embossing depth of the channel structure created by forming a metal sheet is, for example, two to ten times the initial thickness of the metal sheet. If all planar areas of the metal sheet were processed in the same way as the embossed areas, the wall thickness of the final product could theoretically be uniform. On the other hand, if the forming process results in varying wall thicknesses for different surface areas, and these areas may have a linear shape in the plan view of the structured plate, i.e., the formed metal sheet, the minimum wall thickness of the half plate may be, for example, at least two-thirds of the maximum wall thickness of the final half plate. The minimum wall thickness of the half plate may be present, in particular, in the planar areas outside the lateral sides of the channel structure, i.e., in the embossed areas. The maximum wall thickness may also be present in the planar areas of the structured half plate, but, as already mentioned, it may also be present in the lateral areas of the half plate.

[0024] This forming method can be carried out on both coated and uncoated metal sheets. The metal sheets are particularly steel sheets. The formed metal sheets with channel structures can be used, in particular, for bipolar plates in fuel cell systems or electrolysis systems for producing hydrogen. The bipolar plates can comprise at least one metal sheet embossed according to the present invention. In particular, however, the bipolar plates comprise two metal sheets embossed according to the present invention, one for the anode side of the bipolar plate and one for the cathode side of the bipolar plate. The two embossed half plates are typically joined together by welding to form the bipolar plate.

[0025] Exemplary embodiments of the invention will now be described in more detail with reference to the drawings, which are presented in simplified and partly exaggerated form. [Brief explanation of the drawings]

[0026] [Figure 1] Shows embossed structures on metal sheets or half plates. [Figure 2] 1 shows the features of a tool for embossing a metal sheet or half plate. [Figure 3] 1 shows a bipolar plate. [Figure 4] 1 shows a fuel cell system with multiple fuel cells. DETAILED DESCRIPTION OF THE INVENTION

[0027] Figure 1 shows a half-plate 2, designated by the reference number 2. Two such half-plates 2, 2' can be joined together, for example by welding, to form a bipolar plate 1 (see Figure 3). In a fuel cell system 100 (see Figure 4), the bipolar plate 1 separates half-cells of a first fuel cell from half-cells of a further fuel cell. For the basic function of the bipolar plate 1, please refer to the prior art cited at the beginning.

[0028] The half-plates 2 have an embossed structure 3, which in this case is a channel structure with a plurality of parallel channel portions 5, which are located in the active field of the subsequent fuel cell system 100. The channels for the fluid flowing through the fuel cell system 100 can be formed on the outer surface of the bipolar plate 1 between the two overlapping half-plates 2, 2'. The fluid is the coolant and working material of the fuel cell system 100.

[0029] The channel portion 5 formed by the embossed structure 3 has a wall generally designated 4. The wall 4 is constituted by two side surfaces 7, 8 and a bottom 9. A planar main area of ​​the half-plate 2 located on the base surface BE, designated 6, is located outside the channel portion 5. The wall thickness of the planar main area 6 is designated d1 unless otherwise reduced, as will be explained in more detail below.

[0030] The channel portion 5 has a trapezoidal cross-sectional shape, the inclination angle of the lateral surfaces 7, 8 relative to the main region 6 and thus relative to the base surface BE is indicated by α. Deviating from the exemplary embodiment outlined, it is also possible for the various lateral surfaces 7, 8 to be inclined at different angles relative to the base surface BE. In any case, the bottom 9 is located here in an embossing surface PE that is parallel to the base surface BE. The distance between the base surface BE and the embossing surface PE constitutes the embossing depth, indicated by PT, of the embossed structure 3.

[0031] When forming the embossed structure 3, first an undeformed planar metal sheet 11 is inserted into a forming tool 12. The forming tool 12 has a lower tool part designated 13 and an upper tool part designated 14. The lower tool part 13 provides a tool plane WE on which rests the base surface BE of the half-plate 2 to be formed from the metal sheet 11. The basic shape of the embossed structure 3 of the half-plate 2 is predetermined by the cross-sectional shape of the tool parts 13, 14 which have tool contours 16, 17 for this purpose.

[0032] Furthermore, in the example shown in FIG. 2, an embossing contour 15 of the tool top 14 can be seen, which is located directly next to the area where the channel structure 3 is to be formed. This embossing contour 15 ensures a reduced wall thickness d2 in the corresponding area of ​​the half-plate 2, referred to as the embossed area 10, resulting in material displacement. The reduced wall thickness d2 is the minimum wall thickness that can be achieved by forming. The material displacement means that the material flows laterally through the half-plate 2, particularly towards the side surfaces 7 and 8. This results in a wall thickness d3 in the area of ​​the side surfaces 7 and 8, which is greater than the imaginary wall thickness that would be achieved without the embossing process. PW denotes the projected wall thickness of the side surfaces 7 and 8, which is the projection relative to the base surface BE.

[0033] Furthermore, in this case, embossing also occurs in the region of the bottom 9, where material from the bottom 9, which constitutes the embossed portion, flows onto the side surfaces 7, 8. The thickness of the bottom 9 is indicated by d4. Material flow during forming occurs both from the main region 6 to the side surfaces 7, 8 and from the bottom 9 to the side surfaces 7, 8, ensuring complete filling of the cavities formed between the tool contours 16, 17 in the closed forming tool 12. Material flows not only into the curved regions adjacent to the flat regions, but also along the entire length of the formed side surfaces 7, 8. The initial thickness, indicated by d5, of the undeformed metal sheet 11 is in the range of 50 μm to 100 μm in the exemplary embodiment.

[0034] 3 shows a bipolar plate 1 comprising two half plates 2, 2' made of stainless steel, formed by the method according to the invention and joined together by welding. The bipolar plate 1 has an inlet area 30a for the fluid with an opening 40 and an outlet area 30b for the fluid with an opening 40'. A gas distribution structure 50 with a channel structure 3 is arranged between them and comprises a channel portion 5 (see FIG. 1).

[0035] Figure 4 shows a fuel cell system 100, as an example of an electrochemical cell, that includes a plurality of fuel cells 20. The same reference numerals as in Figure 3 indicate the same elements. The fuel cell 20 includes two bipolar plates 1, 1', between which is disposed a polymer electrolyte membrane 70 or membrane electrode unit (MEA), typically covered on both sides with gas diffusion layers. [Explanation of symbols]

[0036] 1,1´ Bipolar Plate 2, 2´ Half Plate 3 Embossed structure, channel structure 4. Wall 5 channel part 6 Planar main area 7 Side 8 Side 9 Bottom 10 Embossed area 11 Undeformed metal sheet 12 molding tools 13 Tool bottom 14 Tool top 15 Embossed Contours 16 Tool contour at the top of the tool 17 Tool contour at bottom of tool 20 Fuel Cell 30a Inflow area 30b Exit area 40, 40´ opening 50 Gas distribution structure 70 Polymer electrolyte membrane 100 Fuel Cell System α angle BE base surface d1, d2, d3, d4, d5 wall thickness PE embossed surface PT Embossing Depth PW Projected wall thickness WE Tool Plane

Claims

1. A method for embossing a channel structure (3) with a plurality of parallel channel portions (5) in a planar metal sheet (11) to form half plates (2, 2') of a bipolar plate of an electrochemical cell, comprising: providing said flat metal sheet (11) having a uniform initial thickness (d5); Inserting the metal sheet (11) into a forming tool (12), the base surface (BE) of the sheet (11) defined by the undeformed planar metal sheet (11) being provided to rest on a tool plane (WE) defined by a tool portion (13) of the forming tool (12); forming the plurality of channel portions (5) to have a cross section having a trapezoidal basic shape, each channel portion (5) being formed to have two side surfaces (7, 8) that approach each other as they move away from a base surface (BE) and a bottom (9) located therebetween, each side surface (7, 8) extending from the base surface (BE) to an adjacent parallel surface, the side surfaces (7, 8) of each channel portion (5) being at an angle of 45° or more and 78° or less with respect to the base surface (BE), and the embossing depth (PT) of the channel portion (5) being selected to be 2 times or more and 10 times or less the initial thickness (d5) of the metal sheet (11), the material of the embossed portions (9, 10) of the metal sheet (11), which are located outside the side surfaces (7, 8) and which remain in the base surface (BE) and / or in a plane parallel to the base surface (BE) throughout the forming process, is displaced at the side surfaces (7, 8), whereby the projected wall thickness (PW), which is the dimension of the side surfaces (7, 8) measured in the normal direction to the base surface (BE), increases by at least 105% and up to 150% of the initial wall thickness (d5).

2. 2. The method according to claim 1, characterized in that the material of at least one embossed portion (10) located outside the channel portion (5) and in the base surface (BE) is pressed into at least one of the adjacent side surfaces (7, 8).

3. A method as described in claim 1 or 2, characterized in that the bottom (9) is parallel to the base surface (BE) and the material of at least one embossed portion (9) in the plane constituting the bottom (9) is displaced to one of the side surfaces (7, 8) adjacent to the at least one embossed portion (9).

4. 2. A method according to claim 1, characterized in that as a result of the forming step, the thickness (d3) of the side surfaces (7, 8) is reduced to 70% or more of the initial thickness (d5).

5. The method described in claim 1, characterized in that both side surfaces (7, 8) constituting the trapezoid are inclined at different angles.

6. A bipolar plate (1, 1') of an electrochemical cell, comprising at least one half-plate (2, 2') having a channel structure (3) with a plurality of parallel channel portions (5) for a medium to flow through said electrochemical cell, a bipolar plate (1, 1') in which the plurality of channel portions (5) have a cross section having a trapezoidal basic shape, each channel portion (5) having two side surfaces (7, 8) that approach each other as they move away from a base surface (BE) of the half plates (2, 2') and a bottom (9) located therebetween, each side surface (7, 8) extending from the base surface (BE) to an adjacent parallel surface, the side surfaces (7, 8) of each channel portion (5) being at an angle of greater than or equal to 45° and less than or equal to 78° with respect to the base surface (BE), the half plates (2, 2') having a minimum wall thickness (d2) that is greater than or equal to two-thirds of the maximum wall thickness of the half plates (2, 2'), the minimum wall thickness (d2) being defined outside the side surfaces (7, 8) of the channel portions (5), and the maximum wall thickness (d3) being defined within the area of ​​the side surfaces (7, 8) of the channel portions (5).

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