mold for polymerization of seal on metal plate

Abstract

The invention relates to a double-shell mold (50) for polymerizing a seal (36, 40) on a metal plate (20), in particular a bipolar plate for a fuel cell, the mold (50) comprising at least a first shell (51) having a first inner surface (54) and a second shell (52) having a second inner face (56), the first shell (51) and the second shell (52) being configured to be able to be joined to each other by positioning the first inner surface (54) and the second inner surface (56) opposite each other so as to delimit a cavity (58) capable of receiving the metal plate (20), the first shell (51) and the second shell (52) being formed from a main material and, at least one of the first shell (51) and the second shell (52) comprising, on at least a portion of its inner surface (54, 56), a coating formed from a damping material having a hardness lower than the hardness of the main material.Figure for abstract: Fig. 4.

Description

Title of the invention: mold for polymerization of seal on metal plate Field of invention

[0001] The present invention relates to a mold for polymerizing a seal on a metal plate, in particular a bipolar plate for a fuel cell, as well as a method for polymerizing the seal on said metal plate using said mold. Technological background

[0002] Fuel cells are electrochemical reactors which produce an electrical voltage by oxidizing a reducing fuel, such as dihydrogen, on one electrode, and reducing an oxidizing oxidant, such as oxygen in the air, on the other electrode.

[0003] In particular, there are proton exchange membrane fuel cells (known by the Anglicism "Exchange Membrane Fuel Cell" or PEMFC). These fuel cells consist of a stack of several cells each comprising an ion-conducting electrolyte surrounded by two electrodes. In the case of PEMFC type cells, the ion-conducting electrolyte is a proton-conducting polymer membrane and the electrodes consist of a porous medium carrying a catalyst such as platinum, the electrolyte and electrode assembly being called a membrane electrode assembly (MEA). Each MEA is brought into contact with reactive gases on its two opposite faces (for example hydrogen and air) by means of interconnection plates also called bipolar plates.

[0004] A bipolar plate is thus interposed between the cathode of a first MEA and the anode of a second MEA adjacent to the first MEA. Thus, a bipolar plate supplies the first MEA with oxidant (air) at its cathode and the second MEA with reducing fuel (in particular hydrogen) at its anode. In addition, a cooling circuit can be arranged inside the bipolar plate in which a heat transfer fluid circulates, which can be liquid or gaseous and serves to provide heat to enable an endothermic reaction or, on the contrary, to extract heat produced by an exothermic reaction. The bipolar plate / MEA pair constitutes a cell and a fuel cell is formed from a stack of several cells.

[0005] The stack of cells is held between two end plates which apply a uniform pressure on all of the cells. Furthermore, the stack of cells has chimneys which extend between the two end plates and pass through each bipolar plate and each MEA of the stack by means openings made in the bipolar plates and the AMEs. These chimneys allow the transport of reactive fluids (for example hydrogen and air), the fluids to be evacuated (for example water) and the heat transfer fluid.

[0006] The stack of cells also includes sealing gaskets positioned on the surface of the bipolar plates, in particular at the periphery of said plates and around the chimneys in order to ensure sealing so that the fluids do not leak and do not mix. The sealing must therefore be effective to allow proper operation of the fuel cell.

[0007] Several methods exist in the prior art for the installation and polymerization of seals on bipolar plates. One method consists of depositing a seal composition on the bipolar plate and then placing the bipolar plate in an oven so as to polymerize the composition and form the seal. A disadvantage of this method is that it is difficult to obtain seals having the desired shape and thickness. Furthermore, the seals on one side and the other side cannot be made simultaneously.

[0008] Another method is to use a double-shell mold to form the seal on the bipolar plate. According to a first variant of this method, a seal composition is deposited on the bipolar plate as in the method previously described and then the bipolar plate is positioned in a double-shell mold. The bipolar plate is then compressed between the two shells of the mold while heating so as to polymerize the composition and form the seal. According to a second variant of this method, the seal composition is deposited directly in the mold and then the bipolar plate is positioned in the mold. This method allows the seal to have the desired shape and thickness. However, the pressure exerted on the bipolar plate can be significant and since certain areas of the bipolar plate are thin, this method can damage the bipolar plate. Statement of the invention

[0009] An objective of the invention is therefore to design a mold making it possible to improve the way of forming the joints on the bipolar plate so that said joints have an optimized shape and / or thickness.

[0010] Another objective of the invention is that the seals formed provide effective sealing to prevent leaks of the various fluids circulating through the stack of cells.

[0011] The invention also aims to prevent the plate from being damaged, in particular deformed or cut, during the polymerization of the seal using the mold.

[0012] To this end, the invention relates to a double-shell mold for polymerization of a seal on a metal plate, in particular a bipolar plate for a fuel cell, the mold comprising at least a first shell having a first inner surface and a second shell having a second inner face, the first shell and the second shell being configured to be able to be joined to each other by positioning the first inner surface and the second inner surface opposite each other so as to delimit a cavity capable of receiving the metal plate, the first shell and the second shell being formed from a main material and, at least one of the first shell and the second shell comprising, on at least part of its inner surface, a coating formed from a damping material having a hardness lower than the hardness of the main material.

[0013] The coating ensures that the pressure exerted on the first and / or second shell(s) during polymerization of the seal does not damage the metal plate and does not deform the seal, in particular before it is polymerized. It is thus possible to deposit a seal composition on the metal plate and to polymerize this composition so as to form a seal having the desired shape and / or thickness.

[0014] According to a preferred embodiment, the metal plate is a bipolar plate and the seal(s), once polymerized, provide optimized sealing when the bipolar plate is used in a fuel cell.

[0015] The coating is preferably positioned at least on the areas of the first and second interior surfaces which are in contact with the seal(s) to be polymerized and / or with fragile parts of the metal plate, in particular the thinnest portions. According to a possible variant, the coating can cover larger areas, such as for example the entire first interior surface and / or the entire second interior surface.

[0016] Furthermore, the main material has sufficient rigidity so that the first and / or second shell(s) retain(s) their shape despite the pressure exerted during polymerization by means of the mold and that the pressure is transmitted homogeneously over a large part, or even all, of the first interior surface and / or the second interior surface.

[0017] Preferably, the damping material does not have any adhesion property with the material making up the seal, whether before, during or after polymerization. Thus, the damping material does not risk sticking to the seal or fusing with the latter. Similarly, the seal does not risk remaining stuck to the damping material or fusing with the latter. Thus, once the polymerization is complete, the first and second shells can be separated from the metal plate and the seal(s), leaving the damping coating and the seal(s) intact.

[0018] According to particular embodiments of the invention which can be taken alone or in combination: - the damping material comprises at least one elastomer; such a material allows the damping coating to absorb part of the pressure exerted by the first and / or second shell(s) on the metal plate while maintaining a homogeneous shape allowing the seal to be given the desired shape; - the damping material may comprise at least one elastomer chosen from silicones, fluorosilicones, Teflon, polyurethanes, ethylene-propylene-diene monomer rubber (or EPDM), nitriles, neoprenes, or one of their mixtures; - the damping material may comprise at least one silicone chosen from liquid silicone rubber (LSR) or high consistency silicone rubber (HCR); - the damping material has a hardness ranging from 35 to 65 shore A; such hardness allows the damping material to be sufficiently rigid to give the seal the desired shape and sufficiently flexible to absorb part of the pressure exerted by the first and / or second shell(s) on the metal plate so that the metal plate is not damaged; the hardness can in particular be measured according to the ISO7619-1 standard; - the coating has a thickness ranging from 0.3 to 10 mm; such a thickness allows the coating to have sufficient damping capacity; the thickness can be chosen in particular according to the properties of the damping material, in particular its hardness; - the coating comprises a first portion having a first thickness, the first portion being configured to be in contact with the metal plate when the metal plate is received in the cavity, and a second portion having a second thickness, the second portion being configured to be in contact with the seal to be polymerized when the metal plate present in the cavity comprises the seal to be polymerized, the first thickness being greater than the second thickness; the thickness is thus greater when the coating is intended to be placed directly against the metal plate and less important when the seal is positioned between the coating and the metal plate; in fact, when the coating is in direct contact with the seal, the latter contributes to absorbing part of the pressure exerted by the first and / or the second shell on the metal plate metal and the coating thickness may be less; - both of the first shell and the second shell(s) have the coating of a damping material; this allows the seal(s) to be formed simultaneously on each of the two faces of the damping plate; and - the coating of the first hull is identical to the coating of the second hull.

[0019] The invention also relates to a method for forming and polymerizing a seal on a metal plate using the mold described above, the metal plate comprising a first face and a second face opposite the first face, the method comprising the following steps: - place the seal on at least one of the first face and the second face of the metal plate, - position the first shell in contact with the first face and the second shell in contact with the second face so that the metal plate is positioned in the cavity delimited by the first shell and the second shell, and - apply pressure so as to simultaneously press the first shell against the first face and the second shell against the second face to allow the seal to polymerize.

[0020] The method makes it possible to polymerize the seal(s) deposited on the metal plate, giving it (them) the desired shape and / or thickness, while avoiding damaging the metal plate.

[0021] According to one embodiment, the cavity receiving the metal plate can be subjected to a temperature of at least 100°C when the pressure is exerted for the polymerization of the seal. Such a temperature allows optimized polymerization of the seal(s).

[0022] Advantageously, the damping material withstands temperatures ranging from 100 to 180°C and is not damaged during the process. The first and second shells can be reused a large number of times.

[0023] Furthermore, the pressure applied to each of the first shell and / or the second shell(s) has a value ranging from 500 kg to 1000 kg, preferably from 1000 kg to 3000 kg. Such pressure allows optimized polymerization of the seal(s) while giving it the desired shape and thickness, without damaging the metal plate. Brief description of the Figures

[0024] Other characteristics and advantages of the invention will appear on reading the description which follows, given solely by way of example and with reference to to the attached drawings, in which:

[0025] [Fig.l] [Fig.l] is a schematic and perspective view of a fuel cell according to one embodiment;

[0026] [Fig.2] [Fig.2] is a schematic and perspective view of a stack of fuel cell of [Fig.l];

[0027] [Fig.3] [Fig.3] is a schematic and perspective view of the assembly of two metal plates for forming a bipolar plate of a battery according to figures 1 and 2;

[0028] [Fig.4] [Fig.4] is a schematic cross-sectional view of a plate bipolar inserted into a mold according to one embodiment of the invention.

[0029] [Fig.5a] [Fig.5a] is a schematic sectional view of a mold shell according to a first embodiment of the invention;

[0030] [Fig.5b] [Fig.5b] is a schematic sectional view of a mold shell according to a second embodiment of the invention;

[0031] [Fig.5c] [Fig.5c] is a schematic cross-sectional view of a hull of mold according to a third embodiment of the invention; and Detailed description of an exemplary embodiment

[0032] With reference to Figures 1 and 2, a fuel cell 10 according to one embodiment of the invention comprises a stack 2 of cells 12, said assembly being held between a first end plate 4 and a second end plate 5. The first and second end plates 4 and 5 apply uniform pressure to the assembly of cells by means of a holding structure 6.

[0033] The stack 2 of cells 12 is crossed by a first chimney 14, a second chimney 16 and a third chimney 18. In this embodiment, the first chimney 14 transports hydrogen to a first outlet 14', the second chimney transports a heat transfer fluid to a second outlet 16', and the third chimney transports air to a third outlet 18'.

[0034] Furthermore, a first collector 8 is arranged between the stack 2 and the first terminal plate 4 and a second collector 9 is arranged between the stack 2 and the second terminal plate 5. The first and second collectors 8 and 9 make it possible to recover the electric current generated by the stack 2 of cells 12.

[0035] [Fig. 2] represents a set 11 of three cells 12, namely cells 12a, 12b, and 12c, which forms a part of the stack 2, one cell being formed by a membrane electrode assembly (MEA) 19 and a bipolar plate 20.

[0036] The set 11 of cells 12 comprises three AMEs 19a, 19b and 19c each comprising an ion-conducting electrolyte (not shown) surrounded by two electrodes (not shown), the electrodes being an anode and a cathode.

[0037] The assembly 11 also comprises three bipolar plates 20a, 20b and 20c each formed by a first metal plate 22a (b,c) and a second metal plate 24a (b,c).

[0038] With reference to Figures 2 and 3, the first and second metal plates 22a and 24a comprise alternating grooves and ribs which extend parallel to each other along an axis X. The first and second plates 22a and 24a are positioned opposite each other by placing the ribs of one plate opposite the grooves of the other plate while leaving a space between the two plates. The first and second metal plates 22a and 24a are then welded to each other to form the bipolar plate 20a. The welding is carried out along a weld line (not shown) of each metal plate 22a and 24a. In this way, a circuit in which a cooling liquid can circulate is formed between the two metal plates 22a and 24a of the bipolar plate 20a.

[0039] The positioning of the first and second metal plates 22a and 24a relative to each other also makes it possible to form first fluid conveying tunnels 26a between each bipolar plate 20a (b,c) and the FAME which is superior to it 19a (b) and also second fluid conveying tunnels 28a between each bipolar plate 20a (b,c) and the FAME which is inferior to it 19b (c).

[0040] In this embodiment, the first fluid conveying tunnels 26a allow the circulation of hydrogen which is thus in contact with the FAME anode 19a, and the second fluid conveying tunnels 28a allow the circulation of air which is thus in contact with the FAME cathode 19b.

[0041] Furthermore, as illustrated in [Fig. 3], each plate comprises orifices forming the first chimney 14, the second chimney 16 and the third chimney 18. Each metal plate 22 thus comprises four orifices 30 for circulation of the cooling fluid, two orifices 34 for circulation of hydrogen and two orifices 32 for circulation of air.

[0042] The bipolar plate 20 comprises a peripheral seal 36. The four orifices 30 for circulation of the cooling fluid are positioned inside the peripheral seal 36 so that the fluid can circulate inside the bipolar plate 20.

[0043] The two hydrogen circulation orifices 34 and two air circulation orifices 32 are positioned outside the peripheral seal 36 and make it possible to supply fluid to the first and second circulation conduits 26 and 28.

[0044] The peripheral seal on one face of the bipolar plate 20 makes it possible to prevent any leakage of hydrogen from the first routing tunnels and the peripheral seal on the other face makes it possible to prevent any leakage of air from the second routing tunnels.

[0045] Furthermore, in order to avoid any leakage of fluid into unwanted areas, the bipolar plates 20 also include oval seals positioned around the orifices 30, 32 and 34 for fluid circulation.

[0046] With reference to Figure 4, a mold 50 for polymerization of the peripheral seals 36 and oval seals 40 on the bipolar plate 20 is shown in a section in which only the peripheral seal is visible in cross-section. According to other possible sections not shown, the peripheral seal and the oval seals 40 would also be visible.

[0047] The mold 50 is double-shelled and thus comprises a first shell 51 and a second shell 52. The first shell 51 has a first interior surface 54 and the second shell has a second interior surface 56. The first and second shells 51 and 52 can be assembled to each other by positioning the first interior surface 54 opposite the second interior surface 56 so as to delimit a cavity 58 capable of receiving the bipolar plate 20a.

[0048] The first and second shells 51 and 52 are formed from a main material which is covered, on at least a portion of the first and / or second inner surfaces 54 and 56, by a coating 62 formed from a damping material. The damping material has a hardness lower than the hardness of the main material.

[0049] Thus, when the bipolar plate 20 is positioned in the cavity 58, pressure can be exerted on the first and / or second shells 51 and 52 without risking damaging the bipolar plate 20. Indeed, the pressure exerted on the first and second shells 51 and 52 during the polymerization of the joints is partially damped by the coating 62 and the pressure exerted by the first and / or second shells 51 and 52 on the bipolar plate 20 is thus reduced.

[0050] Furthermore, the coating 62 ensures that the joints to be polymerized are not deformed and / or crushed by the first and / or second shell(s). It is thus possible to deposit a joint composition on the bipolar plate and to polymerize it while allowing the joint to have the desired shape and / or thickness.

[0051] The main material has sufficient rigidity so that the first and / or second shell(s) 51 and 52 retain their shape despite the pressure exerted during polymerization by means of the mold and that the pressure is transmitted homogeneously over a large part, or even all, of the first interior surface 54 and / or the second interior surface 56.

[0052] Preferably, the damping material does not have any adhesion property with the material making up the peripheral seals 36 and oval seals 40, whether before, during or after polymerization. Preferably, the damping material is different from the material making up the seals. The seals 36 and 40 may for example be made of fluorosilicone and the damping material is then preferably a material other than fluorosilicone. Thus, the damping material is not at risk to stick to the seals 36 and 40 or to fuse with them. Similarly, the seals 36 and 40 are not likely to remain stuck to the damping material or to fuse with it. Thus, once the polymerization is complete, the first and second shells 51 and 52 can be separated from the bipolar plate 20 and the seals 36 and 40, leaving the damping coating 62 and the seals 36 and 40 intact.

[0053] Preferably, the damping material forming the coating 62 has a hardness ranging from 35 to 65 shore A. Such a hardness allows the damping material to be sufficiently rigid to give the seal the desired shape and sufficiently flexible to absorb part of the pressure exerted by the first and second shells 51 and 52 on the bipolar plate 20 so that the bipolar plate is not damaged.

[0054] Hardness can in particular be measured according to the ISO7619-1 standard.

[0055] The damping material may comprise at least one elastomer, and in particular an elastomer chosen from silicones, fluorosilicones, Teflon, polyurethanes, ethylene-propylene-diene monomer rubber (or EPDM), nitriles, neoprenes, or a mixture thereof.

[0056] When the damping material comprises at least one silicone, said silicone may be chosen from liquid silicone rubber (known by the Anglicism “liquid silicone rubber”, LS R) or high consistency silicone rubber (known by the Anglicism “high consistency rubber”, HCR).

[0057] The coating is preferably positioned at least on the areas of the first and second interior surfaces 52 and 54 which are in contact with the seal(s) to be polymerized and / or with fragile parts of the bipolar plate 20. According to a possible variant, the coating 62 can cover larger areas, such as for example the entire first interior surface 54 and the entire second interior surface 56.

[0058] As illustrated in Figures 5a, 5b and 5c, the coating can be arranged according to different embodiment variants.

[0059] According to a first variant illustrated in [Fig.5a], the coating can be positioned on the surface of the ribs 70 which protrude from the inner surface 54 of the first shell 51 and / or the second shell 52. These first parts are those which are in contact with thin and fragile portions of the bipolar plate and the coating 62 thus ensures that these thin and fragile portions of the plate are not damaged.

[0060] According to a second variant illustrated in [Fig.5b], the coating can be positioned over the entire interior surface 54 (56) of the first shell 51 and / or of the second shell 52, and in particular on the horizontal and vertical surfaces, the horizontal surfaces being those which are parallel to the plane formed by the plate bipolar plate 20 when the latter is placed in the cavity 58 and the vertical surfaces being those which are perpendicular to the plane formed by the bipolar plate 20. Thus, all the zones of the peripheral joints 36 and of the bipolar plate 20 are protected from excessive pressure by the coating 62.

[0061] According to a third variant embodiment illustrated in [Fig.5c], the coating 62 can be positioned only on the horizontal surfaces of the ribs 70 and the groove 72. Thus, the coating 62 protects the areas which exert the greatest pressure on the bipolar plate 20 and the peripheral seal 36.

[0062] Figures 5a, 5b and 5c refer to a section of the first shell 51 in which only the groove 72 for the peripheral seal 36 is visible. However, the description of the variants and their advantages also apply to the grooves corresponding to the oval seals 40 and which are not shown in the figures.

[0063] Preferably, the first and second shells 51 and 52 both have the coating 62, the latter preferably being identical on the first and second shells 51 and 52.

[0064] Furthermore, the thickness of the coating 62 may range from 0.3 to 10 mm. Such a thickness allows the coating 62 to have sufficient damping capacity. The thickness may in particular be chosen in particular as a function of the properties of the damping material, in particular its hardness. The thickness may in particular be greater for low hardness and less for higher hardness.

[0065] According to a possible embodiment, the coating 62 may comprise a first thickness on one or more portions which are in contact with the bipolar plate 20 when the latter is positioned in the cavity 58, and a second thickness on one or more portions which are in contact with the peripheral seals 36 and oval seals 40 to be polymerized. According to this embodiment, the first thickness is greater than the second thickness. The thickness is thus greater when the coating 62 is intended to be placed directly against the bipolar plate 20 and less greater when the peripheral seals 36 and oval seals 40 are positioned between the coating 62 and the bipolar plate 20 and the seals 36 and 40 then contribute to absorbing part of the pressure exerted by the first and second shells 51 and 52 on the bipolar plate 20.

[0066] The method of forming and polymerizing the peripheral seals 36 and oval seals 40 on the bipolar plate 20 by means of the mold 50 comprises the following steps: - depositing the peripheral seals 36 and oval seals 40 on at least one of the first face 64 and the second face 66 of the bipolar plate 20, preferably on the first and second faces 64 and 66, - position the first shell 51 in contact with the first face 64 and the second shell 52 in contact with the second face 66 so that the bipolar plate is po located in the cavity 58 delimited by the first shell 51 and the second shell 52, and - apply a pressure P, preferably on the first and second shells 51 and 52 so as to simultaneously press the first shell 51 against the first face 64 and the second shell 52 against the second face 66 to allow the polymerization of the peripheral seals 36 and oval seals 40.

[0067] The method makes it possible to polymerize the peripheral 36 and oval 40 seals deposited on the bipolar plate 20, giving them the desired shape and / or thickness, while avoiding damaging the bipolar plate.

[0068] According to one embodiment, the cavity 58 receiving the bipolar plate 20 can be subjected to a temperature of at least 100°C when the pressure P is exerted for the polymerization of the peripheral seals 36 and oval seals 40. Such a temperature allows optimized polymerization of the seals).

[0069] Advantageously, the damping material withstands temperatures ranging from 100 to 180°C and is not damaged during the process. The first and second shells 51 and 52 can thus be reused a large number of times.

[0070] Furthermore, the pressure P applied to each of the first shell 51 and the second shell 52 has a value ranging from 500 kg to 10,000 kg, preferably from 1,000 kg to 3,000 kg. Such pressure is permitted by the hardness of the main material and allows optimized polymerization of the peripheral seals 36 and oval seals 40 while giving them the desired shape and thickness, without damaging the bipolar plate 20.

[0071] In the embodiment just described, the mold is described for polymerizing a seal on a bipolar plate. However, the mold 50 can also be used for polymerizing a seal on any type of metal plate.

Claims

Claims

1. Double-shell mold (50) for polymerizing a seal (36, 40) on a metal plate (20), in particular a bipolar plate for a fuel cell, the mold (50) comprising at least a first shell (51) having a first inner surface (54) and a second shell (52) having a second inner face (56), the first shell (51) and the second shell (52) being configured to be able to be joined to each other by positioning the first inner surface (54) and the second inner surface (56) opposite each other so as to delimit a cavity (58) capable of receiving the metal plate (20), the first shell (51) and the second shell (52) being formed from a main material and, at least one of the first shell (51) and the second shell (52) comprising, on at least a portion of its inner surface (54, 56), a coating (62) formed from a damping material having a hardness lower than the hardness of the main material.

2. The mold (50) of claim 1, wherein the damping material comprises at least one elastomer.

3. The mold (50) of claim 1 or 2, wherein the damping material has a hardness ranging from 35 to 65 shore A.

4. A mold (50) according to any preceding claim, wherein the coating has a thickness ranging from 0.3 to 10 mm.

5. Mold (50) according to any one of the preceding claims, wherein the coating (62) comprises a first portion having a first thickness, the first portion being configured to be in contact with the metal plate (20) when the metal plate is received in the cavity (58), and a second portion having a second thickness, the second portion being configured to be in contact with the seal to be polymerized (36, 40) when the metal plate (20) present in the cavity (58) comprises the seal (36, 40) to be polymerized, the first thickness being greater than the second thickness.

6. A mold according to any preceding claim, wherein both the first shell (51) and the second shell (52) have the coating (62) of a damping material.

7. A mold according to claim 6, wherein the coating of the first shell (51) is identical to the coating of the second shell (52).

8. A method of forming and polymerizing a seal on a metal plate metal by means of the mold (50) according to any one of claims 1 to 7, the metal plate (20) comprising a first face (64) and a second face (66) opposite the first face, the method comprising the following steps: - deposit the seal (36, 40) on at least one of the first face (64) and the second face (66) of the metal plate (20), - positioning the first shell (51) in contact with the first face (64) and the second shell (52) in contact with the second face (66) so that the metal plate (20) is positioned in the cavity (58) delimited by the first shell (51) and the second shell (52), and - applying pressure so as to simultaneously press the first shell (51) against the first face (64) and the second shell (52) against the second face (66) to allow the polymerization of the seal (36, 40).

9. A method according to claim 8, wherein the cavity (58) receiving the metal plate (20) is subjected to a temperature of at least 100°C when the pressure is exerted for the polymerization of the seal (36, 40).

10. A method according to claim 8 or 9, wherein the pressure applied to each of the first shell (51) and the second shell (52) has a value ranging from 500 kg to 1000 kg, preferably from 1000 kg to 3000 kg.

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