A suitable element for forming, between two plates of a heat exchanger, fins with thinned leading or trailing edges, corresponding heat exchanger and manufacturing process.

Abstract

Element adapted to form, between two plates of a heat exchanger, fins with thinned leading or trailing edges, corresponding heat exchanger and manufacturing process. Element (14) adapted to extend between two plates of a heat exchanger, intended to be traversed by a fluid (16) in a general flow direction (E), comprising a metal sheet (22) forming groups of successive corrugations in a transverse direction (T1) perpendicular to the general flow direction, the groups being successive in the general flow direction, the corrugations of one group being offset transversely with respect to the neighboring corrugations of another group, the corrugations forming fins (28), the metal sheet defining openings, each of the openings being delimited by a leading edge (36) of one of the fins and a trailing edge (38) of another of the fins.Each fin comprises a main portion (40) extending between its leading edge and trailing edge in the general flow direction. Either the leading edge and trailing edge of each fin form a progressive thinning (42) in the general flow direction. Figure for the abbreviation: Figure 4,

Description

Title of the invention: Adapted element for forming, between two plates of a heat exchanger, fins with thinned leading or trailing edges, corresponding heat exchanger and manufacturing process

[0001] DOMAIN

[0002] The present invention relates to an element adapted to extend between two plates of a heat exchanger, in particular a plate heat exchanger, the element being intended to be traversed by a fluid in a general flow direction, the element comprising a metal sheet forming groups of corrugations offset from one another, the corrugations comprising portions forming fins along which the fluid is intended to flow after dividing, the sheet defining openings intended to be used by the fluid.

[0003] The invention also relates to a heat exchanger comprising a plurality of such elements, as well as a method for obtaining at least one such element. PRIOR ART

[0004] Such elements are also called "serrated waves." They have been used since the advent of brazed aluminum plate heat exchangers, both in the automotive industry and for heat exchangers intended for cryogenic gas treatment. These elements provide a large exchange surface area between two plates of the heat exchanger.

[0005] These elements are obtained from a strip.

[0006] In the automotive industry, equipment manufacturers use roller presses which allow both the shearing of the strip to obtain slots, and the formation of corrugations, or waves, the slots being deformed into said openings during the formation of the corrugations.

[0007] For cryogenic exchangers, the formation of the undulations is generally carried out on wide straight mechanical presses, using tools that ensure both the bending into waves, the shearing, and the offsetting of the undulations, and therefore of the fins, relative to each other.

[0008] In order to improve the quality of the shearing, this operation is normally carried out on the strip, while it is still a horizontal strip upstream of the press which produces the formation of the waves.

[0009] Such heat exchangers are satisfactory from the point of view of heat exchange, allowing efficient exchange between two or more fluids, each of these fluids passing through the flap waves located in its path. However, the circulation Fluids in such exchangers require compressing or pressurizing the fluids to predetermined pressure levels, using pumps or compressors, which consume energy.

[0010] An object of the invention is to provide elements for heat exchangers that preserve the efficiency of heat exchange and allow the cost of heat exchange to be reduced. Summary of the invention

[0011] The invention relates to an element adapted to extend between two plates of a heat exchanger, in particular a plate heat exchanger, the element being intended to be traversed by a fluid in a general flow direction, the element comprising a metal sheet forming corrugation groups, each of the corrugation groups comprising successive corrugations in a first transverse direction perpendicular to the general flow direction and intended to be parallel to the two plates, the corrugation groups being successive in the general flow direction, the corrugations of any one of the corrugation groups being offset in the first transverse direction relative to the neighboring corrugations of another of the corrugation groups, the corrugations comprising portions forming fins along which the fluid is intended to flow after having divided,the metal sheet defining openings intended to be used by the fluid, each opening being delimited by an edge, said edge forming a leading edge of one of the fins and a trailing edge of another fin located upstream with respect to said one of the fins, each fin comprising a principal portion extending between its leading edge and its trailing edge in the general flow direction, wherein one and / or the other of the leading edge and trailing edge of each of the fins forms a progressive thinning in the general flow direction from the principal portion.

[0012] According to other advantageous aspects of the invention, the element comprises one or more of the following features, taken individually or in all technically possible combinations:

[0013] - the thinning forms a point in section along a plane parallel to the direction general flow and the first transverse direction;

[0014] - the thinning comprises two sections; and

[0015] - each of the two sides defines, in section along one or the plane parallel to the direction general flow and to the first transverse direction, an angle with the main part, the angle having a tangent between 0.15 and 1.0, preferably between 0.20 and 0.80.

[0016] The invention also relates to a heat exchanger comprising a plurality of parallel plates, and a plurality of elements as described above, each of the elements extending between two of said plates, each of the elements being intended to be traversed by a fluid in a general flow direction.

[0017] The invention also relates to a method for manufacturing an element as described above, comprising the following steps:

[0018] - supply of a metal strip, preferably made of aluminium,

[0019] - perforation of the strip to obtain groups of slots, the slots of each of the groups of slits being aligned along a slit direction, each of the slits being delimited by a first lip and a second lip facing each other in a principal direction perpendicular to the slit direction, and

[0020] - forming the strip to obtain the corrugation groups, the slots being deformed to form said openings, the first lip being intended to form the leading edge of one of the fins and the second lip being intended to form the trailing edge of another of the fins,

[0021] the process comprising, prior to forming, a thinning step of at least a part of the first lip and / or at least a part of the second lip to obtain said progressive thinning of one and / or the other of the leading edge and trailing edge of each of the fins.

[0022] According to other advantageous aspects of the invention, the method comprises one or more of the following features, taken individually or in all technically possible combinations:

[0023] - the thinning step is carried out at a constant volume of material of the strip, by mechanical pressure from at least one press tool or rolling tool;

[0024] - the thinning step is carried out using a first tool and a second tools arranged on either side of the strip opposite at least one of the slots, and adapted to be brought closer together in a pressing direction until a working position, in which the strip is gripped between the first tool and the second tool, and in which a surface of the first tool and a surface of the second tool form a mold around at least a part of the first lip and / or at least a part of the second lip;

[0025] - each of the surface of the first tool and the surface of the second tool forms a protrusion in the pressing direction, the protrusion comprising two facets, each of the two facets defining an angle with the principal direction, the angle having a tangent between 0.15 and 1.0, preferably between 0.20 and 0.80; and

[0026] - the thinning step is carried out by removing material, the material being removed from the strapping on at least part of the first lip and / or at least part of the second lip. Brief description of the drawings

[0027] The invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the accompanying drawings, in which:

[0028] [Fig-1] [Fig.1] is a schematic view of a heat exchanger according to the invention,

[0029] [Fig.2] [Fig.2] is a schematic, partial, perspective view of an element of a prior art heat exchanger, having parts structurally analogous to those of an element of the heat exchanger shown in [Fig. 1],

[0030] [Fig.3] [Fig.3] is a schematic, partial, sectional view of the element of the prior art exchanger shown in [Fig.2],

[0031] [Fig.4] [Fig.4] is a schematic, partial, sectional view of the element represented in [Fig. 1], showing the differences with the prior art heat exchanger element shown in Figures 2 and 3,

[0032] [Fig. 5] [Fig. 5] is a schematic, cross-sectional view allowing comparison the prior art heat exchanger element shown in Figures 2 and 3, and the element shown in Figures 1 and 4, and

[0033] [Fig.6] [Fig.6] is a schematic, cross-sectional view of a step in a process according to the invention, the process for obtaining the element shown in figures 1, 4 and 5. DETAILED DESCRIPTION Heat exchanger

[0034] With reference to [Fig.1], a heat exchanger 10 according to the invention is described.

[0035] The heat exchanger 10 is advantageously a plate heat exchanger, for example made of aluminium.

[0036] The heat exchanger 10 comprises a plurality of parallel plates 12 and a plurality of elements 14, each of the elements being intended to be traversed by a fluid 16, or a fluid 18 in a general flow direction E.

[0037] The heat exchanger 10 is advantageously brazed.

[0038] Fluid 16 is for example a hot fluid, and fluid 18 is a cold fluid, intended to receive heat from fluid 16 in the heat exchanger 10.

[0039] In an alternative (not shown), the heat exchanger 10 is adapted to admit more than two fluids.

[0040] The plates 12 extend in the example along the general flow direction E and along a first transverse direction Tl perpendicular to the general flow direction E.

[0041] A second transverse direction T2 is also defined, perpendicular to the general flow direction E and to the first transverse direction T1. For each of the fluids 16, 18, an upstream direction and a downstream direction are also defined along the general flow direction E.

[0042] In the very schematic example, five plates 12 are shown, which define four passages 20A, 20B for the fluids 16, 18, which advantageously flow in counter-current to each other. Of course, according to an alternative not shown, the number of plates 12 and passages 20 is higher.

[0043] Manifolds (not shown) allow the fluid 16 to be brought into the passages 20A and distributed in these passages, then the fluid 16 to be recovered after heat exchange with the fluid 18. Similarly, manifolds (not shown) allow the fluid 18 to be brought into the passages 20B and distributed in these passages, then the fluid 18 to be recovered after heat exchange with the fluid.

[0044] Advantageously, each of the passages 20A, 20B comprises several of the elements 14 so as to be completely filled by the elements 14 in order to minimize the appearance of preferential passages for the fluids 16, 18 and to ensure good distribution of the fluids 16, 18 in the heat exchanger 10.

[0045] According to a particular embodiment, the passages 20A, 20B are filled by only one of the elements 14.

[0046] According to an unrepresented variant, passages 20A, 20B comprise several layers of elements 14 superimposed in a second transverse direction T2. ​​Elements

[0047] Each of the elements 14 extends between two of the plates 12.

[0048] Advantageously, the elements 14 are structurally similar to each other. However, the elements 14 may have different dimensions in order to adapt to the shape of the passages and to fill the passages 20A, 20B. Elements in prior art

[0049] We will first describe an element 14A of the prior art with reference to figures 2 and 3, then we will describe by difference an element 14 according to the invention.

[0050] The element 14A comprises a metal sheet 22, for example of aluminium, forming groups of corrugations 24A, 24B, 24C..., the corrugations 24 of each of the groups 24A, 24B, 24C... being successive in the first transverse direction Tl and comprising portions 26 forming fins 28 along which the fluid 16 or 18 is intended to flow after having divided.

[0051] The metal sheet 22 has, for example, a thickness T between 0.20 mm and 0.60 mm.

[0052] The undulations 24 form gutters 30, above and below the metal sheet 22, which the fluid 16 or 18 can use.

[0053] In figures 1 to 4, few undulations 24 are shown for the sake of clarity, but in practice the number of undulations is advantageously much higher.

[0054] Advantageously, the surface area of ​​the fins 28 is more than one hundred times greater than the surface area of ​​the plates 12 adjacent to the element 14A.

[0055] The metal sheet 22 defines openings 32 intended to be used by the fluid 16 or 18, each of the openings 32 being delimited by an edge 34.

[0056] As will be seen below, the opening 32 is advantageously obtained by deformation of a strip having a slot.

[0057] The corrugation groups 24A, 24B, 24C... are successive in the general flow direction E, and the corrugations 24 of any one of the corrugation groups are offset in the first transverse direction T1 relative to the neighboring corrugations 24 of another of the corrugation groups. This offset creates a kind of staggered pattern and forces the fluid 16 or 18 to pass through the openings 32. The passage of the fluid 16 or 18 through the openings 32 causes it to pass from one side of the metal sheet 22 to the other.

[0058] The edge 34 of the openings 32 forms a leading edge 36 of one of the fins 28 and a trailing edge 38 of another of the fins 28 located upstream with respect to said one of the fins.

[0059] Each of the fins 28 comprises a main part 40 extending between its leading edge 36 and its trailing edge 38 in the general flow direction E.

[0060] The main part 40 advantageously extends perpendicularly to the first transverse direction T1, the leading edge 36 and the trailing edge 38 extending for example along the second transverse direction T2.

[0061] In the prior art, the leading edge 36 and the trailing edge 38 have, in view along the second transverse direction T2, a rectangular shape.

[0062] The fluid 16 or 18 in question strikes the leading edge 36 head-on, creating turbulence and energy dissipation. The fluid 16 or 18 then recombines behind the trailing edge 38, creating the same problems and possibly a cavitation phenomenon.

[0063] As can be seen in [Fig. 5], the trailing edge 36 of one of the fins 28 and the leading edge 38 of the nearest downstream fin 28 create a baffle of width a. Elements according to the invention

[0064] With reference to figures 3 and 4, we will now describe one of the elements 14 according to the invention.

[0065] The element 14 has a general shape (undulation groups 24A, 24B, 24C... forming fins 28 offset from one group to the other) analogous to that of the element 14A according to the prior art shown in Figures 2 and 3. Similar parts bear the same numerical references and will not be described again. Only the differences will be described in detail below.

[0066] Advantageously, each of the leading edge 36 and the trailing edge 38 of each of the fins 28 forms a progressive thinning 42 in the general flow direction E from the main part 40. In the example, the leading edge 36 and the trailing edge 38 of each of the fins 28 are symmetrical to each other with respect to a plane P perpendicular to the general flow direction E.

[0067] By "thinning," we mean that the edge in question becomes progressively thinner, the thickness T being measured in the first transverse direction Tl, as one moves away from the main part 40 in the general flow direction E. The leading edge 36 thins upstream and / or the trailing edge 38 thins downstream. In the example, the main part 40 has a constant thickness T along the general flow direction E.

[0068] According to an unrepresented variant, the leading edge 36 and the trailing edge 38 have different shapes from each other.

[0069] According to other variants (not shown), only the leading edge 36 or only the fluid edge 38 is thinned relative to the main part 40, and the edge which is not thinned has, for example, the rectangular shape of the prior art ([Fig.3]).

[0070] Advantageously, the thinning 42 forms a point 44 in section along a plane P' parallel to the general flow direction E and to the first transverse direction TL. For example, the thinning 42 is symmetric with respect to a median plane M of the main part 40.

[0071] In the example, the thinning 32 is delimited by two sides 42A, 42B.

[0072] In an alternative (not shown), the thinning 42 forms a bevel.

[0073] According to yet another variant (not shown), the thinning 42 has a convex shape, preferably rounded, for example ogive-shaped.

[0074] For example, each of the panels 42A, 42B defines, in section along the plane P', an angle a with the main part 40 (or with the median plane M), the angle a having a tangent between 0.15 and 1.0, preferably between 0.20 and 0.80.

[0075] As can be seen in [Fig.5], the trailing edge 38 of one of the fins 28 and the leading edge 36 of the nearest downstream fin 28 creates a baffle of width b greater than the width a of the prior art. Manufacturing process

[0076] We will now describe a manufacturing process according to the invention with reference to [Fig.6], allowing the element 14 to be obtained.

[0077] First, a 44 metal strip is obtained, preferably made of aluminum.

[0078] Then the strip 44 is perforated to obtain groups of slots 46A (only one of which is shown in [Fig. 6]), the slots 46 (only one of which is shown) of each of the slot groups 46A being aligned along a slot direction D1. Each of the slots 46 is delimited by a first lip 48 and a second lip 50 facing each other in a principal direction D2 perpendicular to the slot direction D1.

[0079] Next, the strip 44 is deformed to obtain the corrugation groups 24A, 24B, 24C..., the slots 46 being deformed to form the openings 32. The first lip 48 is intended to form the leading edge 36 of one of the fins 28 and the second lip 50 is intended to form the trailing edge 38 of another of the fins 28.

[0080] These steps are known in themselves and are not illustrated.

[0081] The windowing operation by perforation of the strip 44 is for example carried out using punches (not shown) substantially rectangular, assembled in parallel, and of a thickness typically equal to that of the strip 44. Their movement occurs in a downward vertical manner, and gives rise to a front section 52 of the fins 28, which is rectangular.

[0082] Wave forming is then carried out on vertical presses forming the fins 28 by a downward vertical bend, and the next fin 28 by an upward vertical movement.

[0083] Prior to forming, at least a part of the first lip 48 and / or at least a part of the second lip 50 is thinned to obtain said progressive thinning 42 of one and / or the other of the leading edge 36 and the trailing edge 38 of each of the fins 28.

[0084] Advantageously, the thinning step is carried out at constant material volume of the strip 44, by mechanical pressure from at least one press tool 54, or, according to an unrepresented variant, a rolling tool.

[0085] According to an unrepresented variant, the thinning step is carried out by material removal, the material being removed from the strip 44 on at least part of the first lip 48 and / or at least part of the second lip 50. Micromachining is carried out, for example micro-milling, electro-erosion, or material removal by application of a laser, a water jet, or ultrasound.

[0086] For example, the thinning step is carried out, as shown in [Fig. 6], using a first tool 54A and a second tool 54B arranged on either side of the strip 44 opposite at least one of the slots 46, and adapted to be brought closer together in a pressing direction D3 until a working position is reached. In the working position, the strip 44 is gripped between the first tool 54A and the second tool 54B, and a surface 56 of the first tool 54A and a surface 58 of the second tool 54B form a mold around at least part of the first lip 48 and / or at least part of the second lip 50.

[0087] In the example, each of the surface 58 of the first tool 54A and of the surface 58 of the second tool 54B forms a protrusion 60 in the pressing direction D3. The protrusion 60 comprises, for example, two faces 60A, 60B, each of the two faces 60A, 60B defining an angle [3] with the principal direction D2. The angle [3] has, for example, a tangent between 0.15 and 1.0, preferably between 0.20 and 0.80. The angle [3] is, for example, substantially equal to the angle a. Benefits

[0088] Thanks to the characteristics described above, the elements 14 preserve the efficiency of the heat exchanger 10 and make it possible to reduce the cost of heat exchange.

[0089] Indeed, the use of elements 14 results in a reduction of pressure losses, commonly referred to as head losses. The reduction of head losses of the fluids 16, 18 moving in the heat exchanger 10 leads to an overall saving of the energy used in conducting the chemical engineering processes in which the heat exchanger 10 is integrated.

[0090] In the prior art, the fluid 16, 18 flowing in one of the channels 30 delimited by two adjacent fins 28 in the first transverse direction T1 strikes the leading edge 36 of the next fin 28 head-on, generating turbulence, which is a loss of energy. This phenomenon is then repeated at the trailing edge 38 with the appearance of eddies, which also dissipate energy. In a typical configuration of a brazed aluminum plate heat exchanger, these phenomena are repeated several million times.

[0091] In the elements 14 according to the invention, the leading edge 36 and / or the trailing edge 38 of the fins 28 are no longer frontally perpendicular to the general flow direction E, which reduces the aforementioned problems.

[0092] Another advantage of element 14 according to the invention lies in the fact that the width b of the baffle is significantly increased compared to the width a of the prior art, which allows the production of denser waves, without significant prejudice on the pressure loss, leading to an increase in exchange performance and ultimately to a reduction in the volume of the heat exchanger 10.

[0093] Since the pressure losses are lower, it becomes possible to use elements 14 in which the undulations 24 are denser (more numerous per unit volume), without significant prejudice to the pressure loss, leading to an increase in exchange performance and ultimately to a reduction in the volume of the heat exchanger 10.

[0094] Thus, the elements 14 according to the invention make it possible to reduce costs, thanks to a reduction in pressure losses and / or a reduction in the volume of the heat exchanger 10.

[0095] Thanks to the invention, it becomes possible for example to produce elements 14 whose sheet 22 has a thickness of approximately 0.20 mm and which form 40 fins per inch (2.54 cm) in the first transverse direction Tl, i.e. approximately 16 fins per cm.

[0096] In the manufacturing process, the thinning step is easy to perform because it takes place on one or both lips 48, 50 of the slots 46 while the strip 44 is still flat. Then, the strip 44 is deformed, and the slots 46 become the openings 32. Comparison with the prior art - Simulations

[0097] In order to quantify the difference in performance, in terms of pressure losses and heat exchange capacity, the prior art elements 14A and the elements 14 according to the invention were subjected to a CFD (Computational Fluid Dynamics) dynamic flow simulation using a fine mesh of the passages 20A, 20B under strictly identical flow conditions. The calculation is performed using methods known per se.

[0098] In addition to visualizing the fluid streams and velocity fields, the calculation provided a numerical result for energy dissipation (pressure losses) and heat exchange. The same calculation, using the same input conditions, was performed on elements 14 and 14A, allowing for a direct comparison of their performance.

[0099] These calculation results showed a significant reduction in pressure losses for a substantially constant heat exchange performance.

[0100] In a more macroscopic way, denoting by G the mass flow rate in kg / m2 / second, the pressure loss is globally proportional to the square of G.

[0101] The amount of heat exchanged is proportional to G.

[0102] If the use of the elements 14 according to the invention allows for a reduction of, for example, 25% in the pressure drop, all other things being equal, it becomes possible to reduce the flow area by approximately 12%, since 1.122 is approximately equal to 1.25. With the flow area reduced by approximately 12%, the mass flow rate increases in the same proportion, and the amount of heat exchanged remains the same. This results in a significant cost reduction for the manufacturer of the heat exchanger 10.

Claims

Demands

1. An element (14) adapted to extend between two plates (12) of a heat exchanger (10), in particular a plate heat exchanger, the element (14) being intended to be traversed by a fluid (16, 18) in a general flow direction (E), the element (14) comprising a metal sheet (22) forming corrugation groups (24A, 24B, 24C, ...), each of the corrugation groups (24A, 24B, 24C, ...) comprising successive corrugations (24) in a first transverse direction (T1) perpendicular to the general flow direction (E) and intended to be parallel to the two plates (12), the corrugation groups (24A, 24B, 24C, ...) being successive in the general flow direction (E), the corrugations (24) of any one of the groups of undulations (24A, 24B, 24C, ...) being offset in the first transverse direction (Tl) with respect to the undulations (24) neighboring one of the other groups of undulations (24A, 24B, 24C, ...), the undulations (24) comprising portions (26) forming fins (28) along which the fluid (16, 18) is intended to flow after dividing, the metal sheet (22) defining openings (32) intended to be used by the fluid (16, 18), each of the openings (32) being delimited by an edge (34), said edge (34) forming a leading edge (36) of one of the fins (28) and a trailing edge (38) of another fin (28) located upstream with respect to said one of the fins (28), each of the fins (28) comprising a principal portion (40) extending between its leading edge (36) and its trailing edge (38) in the general flow direction (E), characterized in that one and / or the other of the leading edge (36) and the trailing edge (38) of each of the fins (28) forms a progressive thinning (42) in the general flow direction (E) from the main part (40).

2. Element (14) according to claim 1, wherein the thinning (42) forms a point (44) in section along a plane (P') parallel to the general flow direction (E) and to the first transverse direction (Tl).

3. Element (14) according to claim 1 or 2, wherein the thinning (42) comprises two sides (42A, 42B).

4. Element (14) according to claim 3, wherein each of the two sides (42A, 42B) defines, in section along a plane (P') parallel to the general flow direction (E) and the first transverse direction (Tl), an angle (a) with the main part (40), the angle (a) having a tangent between 0.15 and 1.0, preferably between 0.20 and 0.

80.

5. Heat exchanger (10) comprising a plurality of parallel plates (12), and a plurality of elements (14) according to any one of claims 1 to 4, each of the elements (14) extending between two of said plates (12), each of the elements (14) being intended to be traversed by a fluid (16, 18) in a general flow direction (E).

6. A method for manufacturing an element (14) according to any one of claims 1 to 4, comprising the following steps: - supplying a metal strip (44), preferably made of aluminum, - perforating the strip (44) to obtain slot groups (46A), the slots (46) of each slot group (46A) being aligned along a slot direction (D1), each slot (46) being delimited by a first lip (48) and a second lip (50) facing each other in a principal direction (D2) perpendicular to the slot direction (D1), and - forming the strip (44) to obtain the corrugation groups (24A, 24B, 24C, ...), the slots (46) being deformed to form said openings (32), the first lip (48) being intended to form the leading edge (36) of one of the fins (28) and the second lip (50) being intended to form the trailing edge (38) of another of the fins (28), characterized in that the process includes, prior to forming, a thinning step of at least a part of the first lip (48) and / or at least a part of the second lip (50) to obtain said progressive thinning (42) of one and / or the other of the leading edge (36) and the trailing edge (38) of each of the fins (28).

7. A method according to claim 6, wherein the thinning step is carried out at a constant volume of material of the strip (44), by mechanical pressure from at least one press tool (54) or a rolling tool.

8. A method according to claim 6 or 7, wherein the thinning step is carried out using a first tool (54A) and a second tool (54B) arranged on either side of the strip (44) opposite at least one of the slots (46), and adapted to be brought closer together in a pressing direction (D3) to a working position, wherein the strip (44) is gripped between the first tool (54A) and the second tool (54B), and wherein a surface (56) of the first tool (54A) and a surface (58) of the second tool (54B) form a mold around at least a part of the first lip (48) and / or at least a part of the second lip (50).

9. A method according to claim 8, wherein each of the surface (56) of the first tool (54A) and of the surface (58) of the second tool (54B) forms a protrusion (60) in the pressing direction (D3), the protrusion (60) comprising two flats (60A, 60B), each of the two flats (60A, 60B) defining an angle (|3) with the principal direction (D2), the angle (|3) having a tangent between 0.15 and 1.0, preferably between 0.20 and 0.

80.

10. A method according to claim 6, wherein the thinning step is carried out by material removal, the material being removed from the strip (44) on at least a part of the first lip (48) and / or at least a part of the second lip (50).

Images