Abstract:
A bipolar plate having a surface structure for a flow field, inner forms such as holes and slots in the plate, and a reference geometry on the outer form thereof is produced by means of a hydraulic fine-blanking press having an at least three-stage progressive die, in the first stage of which a blank is cut partially free from a metal-foil strip, subsequently in a second stage the surface structure is formed in the connected blank via compression-forming using a forming force of at least 2500 to 3500 kN, the inner form, the slots and the reference geometry are formed by way of cutting in, and on, the blank and then, in a third stage, cutting of the outer contour and removal of the finished blank in the form of the bipolar plate are carried out successively in the stroke cycle.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method for producing a bipolar plate comprising a metal-foil strip having a a flow field, inner forms such as holes and slots in the plate, and at least one reference geometry on the outer form thereof, in which the strip is fed to a tool, which opens at the lower dead-center position of the ram of a press, and closes at the upper dead-center position of the ram of the press, is clamped between an upper tool part, which comprises at least a forming punch, a cutting- and piercing punch and a guide and is mounted on the press table, and at least one lower tool part, which performs a stroke between the upper and lower dead-center positions upon closing, and includes a cutting die, the strip finally being subjected to a surface structure for cutting and forming operation. 
     The invention further relates to an apparatus (in the specification hereafter referred to as a “device”), for producing a bipolar plate comprising a metal-foil strip having a surface structure for a flow field, inner forms such as holes and slots in the plate, and a reference geometry on the outer form thereof, the device comprising a progressive die driven by a ram of a press, the upper tool part thereof, which is mounted on the press table, including at least one piercing and cutting punch and a guide holder, and the lower tool part thereof, which performs a stroke between the upper and lower dead-center positions of the piston, including at least one cutting die. 
     As is known, bipolar plates for fuel cells comprise an anode side and a cathode side, which are optionally separated from one another by an intermediate wall (interconnection plate). The anode and cathode sides are formed as structured flat plates, the function of which in a fuel cell is to provide an electric connection for the fuel cell and to supply this with hydrogen and air/oxygen as the oxidizing agent. In order to ensure that the fuel cell supply is uniform across the cross-sectional area, the bipolar plate has a flow field formed of a plurality of adjacently disposed channels or grooves, through which the process gases flow along the grooves to the reaction zones, and through which the resultant product water drains off. The bipolar plate comprises holes, which are separated from one another, via which the hydrogen that serves as the fuel, air/oxygen that serves as the oxidizing agent and water that serves as the coolant are simultaneously fed into the respective flow channels. Complex technical requirements are placed on the bipolar plate, namely: that it conduct the gases and current with minimal electrical resistance; have high resistance to aggressive chemical conditions; dissipate the resultant reaction heat; be corrosion-resistant to fuel, oxygen and water; separate the gases from one another; and have high temperature resistance, strong mechanical strength, and dimensional stability, at a low weight and with a small volume. 
     It is known to use metal foils for such bipolar plates, which are brought into the desired shape via stamping, deep drawing, punching or forming operations based on active means, such as hydroforming (DE 10 2005 021 487 A1, DE 10 2009 059 769, DE 10 2004 016 318 A1, DE 10 2009 036 039 A1). All these known solutions have the disadvantage that the required geometrical trueness cannot be ensured. As a result, an offset arises in the channel geometries and the holes in the bipolar plates, which can lead to safety-relevant malfunctions of the fuel cell. Thermal processes (see DE 10 2010022094A1) regularly necessitate cost-intensive remachining due to the resultant burr. 
     Furthermore, DE 10 2010 020 178 A1 makes known a method for producing a metallic bipolar plate for a fuel cell stack, which has a gas distribution structure on each of the two sides thereof. The gas distribution structures are created simultaneously on both sides by way of shearing. 
     This known method is limited to a thickness range of 0.5 to 5 mm, and thus the bipolar plates attain considerable weight and very large dimensions. 
     SUMMARY OF THE INVENTION 
     Proceeding from the state of the art, the object of the invention is to markedly increase the geometrical trueness and dimensional consistency of the bipolar plates via forming on a fine-blanking press while eliminating all finishing on the contours and ensuring economic advantages. 
     The gist of the solution according to the invention is to carry out the forming operation on the metal foil using the greatest possible forming force in the center of a fine-blanking press using an at least three-stage progressive die, in the first stage of which a blank is cut partially free in the strip; subsequently, in a second stage, the surface structure is formed in the blank via compression-forming using a forming force of at least 2500 to 3500 kN, the inner form, the slots and the reference geometry are formed by way of cutting in, and on, the blank; and then, in a third stage, cutting of the outer contour and removal of the finished blank for the bipolar plate are carried out successively in the stroke cycle. 
     It is advantageous that the surface structure is created by a forging die in the lower tool part and a suitable chasing form in the upper tool part, and the inner form (holes) and slots in the bipolar plate are created via a cutting and piercing punch in the upper part and female die openings disposed in the punch plate of the lower part, successively but within the same operation. This ensures that all important part geometries attain the best possible dimensional consistency with respect to one another and that locating pins are not needed in order to ensure the dimensional consistency of the contour from forming to cutting. 
     A further advantage is that, by way of compression-forming, different surface structures, and preferably a field of grooves, meander-shaped or spiral channels or a knob field can be formed in the blank, and grooves or channels having different cross-sectional shapes, such as u-, oval-, semicircular-, parabolic-, or v-shaped cross sections can be formed, thereby ensuring that the method according to the invention is highly flexible with respect to the market requirements of the plate manufacturer. 
     In a preferred embodiment of the method according to the invention, the following further measures are carried out: 
     a) free-cutting the end-face edges and, optionally, the lateral edges of the blank for a bipolar plate, wherein the blank remains connected to the strip, 
     b) opening the tool upon the return stroke of the forming, piercing and cutting punch, and advancing the strip with the free-cut blank for compression-forming, piercing and cutting, wherein, after the strip has been clamped once more, the compression-forming of the surface structure is carried out first, followed by piercing within the blank and cutting the reference geometry on the outer form of the blank in a vertical sequence within the second working stage while retaining the connection of the blank to the strip, 
     c) re-opening the tool upon retraction of the forming, cutting and piercing punch to the top dead-center position and advancing the free-cut, compression-formed, pierced and cut blank to the third working stage for cutting the outer contour of the bipolar plate, wherein the blank is separated from the previously clamped strip and, after a fall-through motion, is removed from the closed tool vertically below the cutting plane by way of a conveyor belt. 
     For electric contact to the bipolar plates, it has proven expedient for the end-face edge of the blank to be formed, during the return stroke of the forming, piercing and cutting punch within the second working stage, into a leg extending vertically in the direction of the upper part, by way of a bending punch provided in the upper part, wherein the geometrical trueness and dimensional consistency are not affected because the blank remains in the clamped state. 
     It is particularly advantageous that a further working stage, in the form of an optional no-load stroke, is carried out between the second and third working stages. This ensures that the states of stresses associated with the considerable forming forces do not affect the subsequent blank, because consecutive blanks in the strip have sufficient separation. 
     In a further embodiment of the method according to the invention, foils made of stainless steel, preferably of the quality 1.4404, X2CrNiMo18-14-3, steel, aluminum, aluminum alloys, titanium, titanium alloys and HASTELLOY™ alloys having a thickness of 0.1 to 0.5 mm are suitable as the metal-foil strip. 
     According to the invention, it is, of course, also possible to use metal compound alloys. 
     In terms of the assembly of the blanks produced according to the invention, it is particularly advantageous that a plurality of references, which ensure exact orientation and positioning of the individual blanks during joining and assembly, are cut into the outer form of the blank. 
     The device according to the invention is characterized in that the progressive die comprises at least one first stage for partially free-cutting the end-face edges and, optionally, the lateral edges of a blank for the bipolar plate, a second stage for forming the surface structure in the blank, piercing the inner forms and cutting the reference geometries on the free-cut but connected blank and a third stage for cutting the outer contour and removing the bipolar plate from the tool, wherein a fine-blanking press, having a knife-edged ring piston and a counterholder piston acting hydraulically in opposition on the upper tool part and the lower tool part, is provided as the press, the knife-edged ring piston and counterholder piston being coordinated with one another such that, in the second stage, at least one chasing form for forming precedes the piercing and cutting punches during the reciprocating motion thereof. 
     In a further advantageous embodiment of the device according to the invention, the lower tool part comprises at least one forging die inserted into a holder, the forging die having a structure adapted to the shape, configuration and position of the surface structure and holes in the bipolar plate; and the upper tool part has at least one chasing form, having a suitably formed surface structure, which is correspondingly adapted to the forging die and is inserted into a punch-guiding insert. 
     In a further embodiment of the device according to the invention, the first stage comprises, in the lower tool part, at least first and second cutting openings disposed in the cutting die for partially free-cutting the blank, and a free-cutting punch, wherein a waste channel is dedicated to the cutting opening that leads to a conveyor belt disposed underneath the lower tool part for removing waste from the tool. 
     According to a further advantageous embodiment of the device according to the invention, the second stage in the lower tool part comprises: at least one forging die inserted into a holder that has a surface structure adapted to the shape, configuration and position of the surface structure in the bipolar plate; at least one third and fourth cutting opening disposed in the cutting die for the inner-form punch and the cutting punch; and at least one chasing form having an suitably formed surface structure, which is correspondingly adapted to the forging die and is inserted into a punch-guiding insert, and comprises at least the inner-form and cutting punch in the upper tool part, wherein a waste channel is dedicated to each cutting opening, and leads to a conveyor belt disposed underneath the lower tool part for removing waste from the tool. 
     According to a further embodiment of the device according to the invention, the third stage in the upper tool part comprises at least one outer-contour cutting punch for cutting out the blank completely and at least one fifth cutting opening disposed in the cutting die, wherein a removal channel is dedicated to the cutting opening, which leads to a conveyor belt disposed underneath the lower tool part for removing the finished blank from the closed tool. 
     According to a preferred variant of the device according to the invention, a further stage in the form of a no-load stage can be inserted between the second and third stages. 
     It is particularly advantageous, in terms of electrical contact, that at least one bending punch for bending the end-face edges of the blank be integrated in the upper tool part in the second stage. This bending punch moves vertically during the return stroke of the stamping, piercing and cutting punch, in the direction of the lower part, and thereby forms the end-face edge of the blank into a projecting leg. 
     The forging die and the chasing form have a surface structure comprising grooves, meander-shaped spiral channels or knobs, which facilitates retrofitting the device according to the invention for different surface structures. The grooves or channels of the surface structures can therefore also be adjusted for various desired cross-sectional shapes, for example u-, oval-, semicircular-, parabolic- or v-shaped cross sections. The device according to the invention also makes it possible to use metal-foil strips made of stainless steel, preferably of the quality 1.4404, X2CrNiMo18-14-3, steel, aluminum, aluminum alloys, titanium, titanium alloys and HASTELLOY™ alloys having a thickness of 0.1 to 0.5 mm, and preferably 0.1 mm. 
     The solution according to the invention is characterized by allowing for the provision of anode- and cathode-side bipolar plates having high geometric trueness and dimensional consistency while eliminating all finishing on the contours and ensuring economic advantages. 
     Further advantages and details will become apparent from the description that follows, with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view of a bipolar plate having a surface structure, inner forms and reference geometries according to the prior art, 
         FIG. 2  is a cross section along section line II-II of  FIG. 1 , 
         FIG. 3  is a sectional view of the device according to the invention in the opened state, 
         FIG. 4  is a schematic depiction of the sequence of the method according to the invention, 
         FIG. 5  is a perspective view of the forging die for forming the surface structure, 
         FIG. 6  is a perspective view of the chasing form for forming the surface structure, and 
         FIG. 7A-E  are views of various surface structures of the chasing form and the forging die that can be formed in the blank for a bipolar plate with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 and 2  show an example of a metallic, multilayer bipolar plate  1  according to the prior art. This known bipolar plate comprises a first plate  2 , a second plate  3  and an intermediate plate  4  inserted between the first and second plates  2  and  3 . A surface structures, for example in the form of channels or ridges, is formed in the plates  2  and  3  in the longitudinal direction of the plates, and therefore the side of the intermediate plate  4  having the first plate  2  forms a so-called flow field for a cooling medium and the other side of the intermediate plate  4  having the second plate  3  forms a further flow field for a cooling medium. The flow fields  102  and  103  make it possible to distribute the reactants (hydrogen, air/atmospheric oxygen) evenly to the left and right of a proton-conducting membrane  104 . Weld seams  6   a  hold the bipolar plate  1  together and minimize electrical resistance. The plates  2 ,  3  and  4  are circumferentially connected in a water-tight manner at the end-face and lateral edges S and SR thereof by way of a welding seam  6 . 
     In the end-face region S and partially at the lateral edge SR, the plates  2  and  3  have an inlet opening  7  for hydrogen, an inlet opening  8  for air/oxygen and an inlet opening  9  for the cooling medium and corresponding outlet openings  10 ,  11  and  12  for the cooling medium and the reaction products. Reference geometries  13 ,  14  and  15  are formed in both end-face regions S of the bipolar plate  1 . 
     Reference geometries  13 ,  14  and  15  serve to compensate for forming-induced geometrical tolerances in subsequent assembly and, in particular, to minimize the offset of the individual layers of a bipolar plate. 
     The device according to the invention is depicted schematically in  FIG. 3 . The metal-foil strip  16 , which is wound onto a non-illustrated coil and is made of the material X2CrNiMo 1.4404 or X2CrNiMo 18-14-3 having a thickness of 0.1 mm, is fed between the lower tool part  17  and the upper tool part  18  into the opened tool  19 . The metal-foil strip  16  lies on a cutting die  20  of the lower tool part  17 . 
     The upper tool part  18  is immovably mounted on the upper press table  23 , which is held by the press stand  21  of a fine-blanking press  22 . The knife-edged ring piston  24 , which is actuatable by the knife-edged ring hydraulics of the fine-blanking press  22 , is guided in the press table  23  and acts on the chasing form  26  of a forging die via the thrust bolt  25 . The thrust bolt  25  is axially displaceable in a holding plate  27 , which is accommodated by an upper tool-change plate  28 , and in an upper base  29 , thereby ensuring that forming forces can be transmitted to the chasing form  26  without tilting moment. 
     The further active elements, such as the free-cutting punch  30  for cutting a blank Z ( FIG. 4 ) partially free of the metal-foil strip  16 , the inner-form punch  31  for piercing the inner forms (inlet openings  7 ,  8 ,  9  and outlet openings  10 ,  11 ,  12 ), the cutting punch  32  for cutting the reference geometries  13 ,  14  and  15  and the outer-contour cutting punch  33 , are mounted on the upper base  29 . The free-cutting punch  30 , the inner-form punch  31 , the cutting punch  32  and the outer-contour cutting punch  33  are guided in a punch-guiding insert  34 , which lies in a guide holder  34   a.    
     Spring elements  35  for clamping the metal-foil strip  16  and making cutting travel possible for the active elements  30 ,  31 ,  32  and  33  are located between the punch-guiding insert  34  and the upper base  29 . 
     The lower tool part  17  has a lower interchangeable plate  36 , which is directly connected to the press ram  37  of the fine-blanking press  22 , the press ram performing a reciprocating motion from bottom dead center UT and top dead center OT. The lower base  38  is mounted on the lower interchangeable plate  36 , and a lower pressure pad  39 , against which the counterholder piston  40  of the counterholder hydraulics works, is accommodated in the lower base. 
     The cutting die  20  is located on the lower base  38 , the cutting die comprising a first and second cutting opening  41  and  43  for the free-cutting punch  30 , a third cutting opening  42  for the inner-form punch  31 , a fourth cutting opening  44  for the reference geometry and a fifth cutting opening  45  for the outer-contour cutting punch  33 . Individual removal channels  46  are dedicated to each of the cutting openings  41  and  42 , which extend through the lower base  38  and lead to a common conveyor belt  47  disposed underneath the cutting openings  41  and  42 . It is therefore possible to easily remove cutting waste even when the tool is closed. 
     A removal channel  48  is also dedicated to each of the cutting openings  43  and  44 , through which the cutting waste is discharged onto a common conveyor belt  49 . A removal channel  50  extending vertically through the lower base  38  for discharging the finished blanks Z onto a further conveyor belt  51  is located underneath the cutting opening  45 . Furthermore, a forging die  53  is disposed on the lower base  38  in a holder  52 , the forging die being matched to the chasing form  26  belonging to the upper tool part  18  for forming. 
     The sequence of the method according to the invention is described in greater detail in the following with reference to  FIG. 4 , in combination with  FIG. 3 . For clarity,  FIG. 4  only shows the lower tool part  17  having a metal-foil strip  16  fed in the moving direction L. The device according to the invention for carrying out the method according to the invention comprises four stages in this example, namely: a first stage I, for partially free-cutting the end-face and lateral edges S and SR of the blank Z for the bipolar plate  1 ; a second stage II, for forming the surface structure  5  in the blank Z, piercing the inlet openings  7 ,  8 ,  9  and outlet openings  10 ,  11 ,  12  and cutting the reference geometries  13 ,  14  and  15  into the free-cut blank Z; an additional stage IIIa as the no-load stage; and a stage III for cutting the outer contour and removing the finished blank Z from the tool. A fine-blanking press  22  having a knife-edged ring piston  24  and a counterholder piston  40  acting hydraulically in opposition on the upper tool part  18  and the lower tool part  17  is used as the press. 
     Before the operations are started, the metal-foil strip  16  is clamped between the punch-guiding insert  34  of the upper tool part  18  and the cutting die  20  of the lower tool part  17 . By way of the free-cutting punch  30 , the blank Z is cut partially free at the end-face edges S and the lateral edges SR at the corresponding cutting openings  41  and  42  in the cutting die  20  in the first stage I. 
     The remaining regions of the blank Z remain connected to the metal-foil strip  16 . 
     The tool opens upon the return stroke of the active elements and releases the clamped metal-foil strip  16 . A non-illustrated feed moves the metal-foil strip  16  comprising the connected blank Z into the stage II, where the metal-foil strip  16  is clamped once more when the tool is closed. 
     The knife-edged ring hydraulics are coordinated such that the chasing form  26  precedes the piercing and cutting punches  31  and  32 , respectively, during the reciprocating motion thereof. This makes it possible to carry out the forming and cutting successively in a vertical sequence within stage II, wherein the blank Z remains connected to the metal-foil strip  16 . 
     The chasing form  26  and the forging die  53  are provided with a matching surface structure, which corresponds to the desired surface structure  5  in the flow field of the bipolar plate  1  (see  FIGS. 7A-E ). 
     The forming force for forming the surface structure in the connected blank Z is at least 2500 to 3500 kN, thereby ensuring that the entire flow field attains a uniform structure. 
     Once the forming, piercing and cutting in stage II is completed, the active elements perform a return stroke once more and the tool opens. 
     The metal-foil strip  16  comprising the connected, formed and cut blank Z can be pushed via the feed into stage IIIA, in which only a no-load stroke is performed. This ensures that adequate separation arises between the individual blanks Z and prevents an overlap of unwanted states of stresses in the subsequent blank resulting from the considerable forming forces. 
     After the tool opens once more, the feed moves the metal-foil strip  16  comprising the connected blank Z into stage IIIa. The strip and the blank Z are clamped. The outer-form cutting punch  33  completely severs the blank Z from the metal strip film  16  at the cutting opening  45 . The finished blank Z then drops through the removal channel  50  onto the conveyor belt  51 , which conveys the blank out of the tool. 
       FIGS. 5 and 6  show the forging die  53 , which is mounted on the lower interchangeable plate  36  of the lower tool part, and the chasing form  26 , which is accommodated in the punch-guiding insert  34  of the upper tool part. 
       FIGS. 7A-E  show possible surface structures for the chasing form and forging die  26  and  53 , respectively, which can be stockpiled as exchangeable units for different surface structures. 
     The method according to the invention makes it possible to easily and cost effectively produce anode- and cathode-side plates and intermediate plates for bipolar plates with high dimensional consistency and effectiveness.