METHOD FOR REFORMING STIFFENING SECTIONS IN BATTERY CELL HOUSINGS

Abstract

A housing body and a method for manufacturing a housing comprise the extrusion or deep drawing of a housing with a plurality of side walls defining a first cavity. The plurality of side walls are formed without stiffening sections. After extrusion and deep drawing of the housing, a plurality of stiffening sections are formed on the housing using an expandable die, a hydraulic die, and / or hydroforming.

Description

INTRODUCTION

[0001] The present invention relates to a method for producing a housing for battery cells.

[0002] A method for producing a housing according to the preamble of claim 1 is essentially disclosed in DE 199 49 923 A1. A method of essentially comparable type is also disclosed in DE 10 2017 120 544 A1.

[0003] Further prior art can also be found in the documents DE 10 2019 106 154 A1, US 2006 / 0 063 067 A1, US 2011 / 0 250 492 A1, US 4 339 049 A and CN 1 03 227 297 A.

[0004] Electric vehicles (EVs), such as battery electric vehicles (BEVs), hybrid vehicles, and / or fuel cell vehicles, comprise one or more electric motors and a battery system with one or more battery cells, modules, and / or packs. A power control system is used to control the charging and / or discharging of the battery system during charging and / or driving. SUMMARY

[0005] A method for manufacturing a housing body includes extruding or deep drawing a housing body including a plurality of sidewalls defining a first cavity, wherein the plurality of sidewalls are formed without stiffening portions, and forming a plurality of stiffening portions on the housing body after extruding or deep drawing the housing body. Forming the plurality of stiffening portions includes enclosing the plurality of sidewalls of the housing body using a die, and inserting an outer punch of an expandable physical punch into the first cavity of the housing body. The outer punch includes a plurality of punches and a plurality of sidewalls defining a second cavity and a plurality of sidewall cavities configured to receive the plurality of punches.The method includes inserting an inner punch into the second cavity to bias the plurality of punches from a retracted position to a punching position to punch a plurality of stiffening portions into at least one of the plurality of side walls of the housing body.

[0006] In other features, the plurality of stiffening sections includes at least one recess or stiffening bead, and the housing body is made of stainless steel, aluminum, or alloys thereof.

[0007] In other features, the plurality of stiffening portions extend outwardly from an outer side surface of the housing body. The plurality of punches includes a body and a projection extending transversely from the body. The method includes biasing the projection of the plurality of punches toward the retracted position of the plurality of punches using a spring. The plurality of sidewall cavities have first inclined surfaces, and the plurality of punches include bodies with second inclined surfaces.

[0008] In other features, the plurality of punches are inclined at an acute angle to the direction of movement of the inner punch. The plurality of punches move in a direction transverse to a direction of movement of the inner punch. Edges of the inner ends of the plurality of punches are inclined at an acute angle to the direction of movement of the inner punch.

[0009] In other features, a first pattern of stiffening portions on one of the plurality of side walls of the housing body is complementary to a second pattern of stiffening portions on another of the plurality of side walls of the housing body to enable nesting of adjacent side walls of the housing body.

[0010] In other features, forming the plurality of stiffening portions comprises enclosing the plurality of sidewalls of the housing body using a die, and inserting an outer punch of an expandable physical punch into the first cavity of the housing body. The outer punch includes a plurality of punches and a plurality of sidewalls defining a second cavity and a plurality of sidewall cavities configured to receive the plurality of punches. The method includes sealing the second cavity, supplying fluid into the second cavity, and biasing the plurality of punches into the plurality of sidewalls of the housing body using the fluid to punch a plurality of stiffening portions into the plurality of sidewalls of the housing body.

[0011] In other features, the method includes sealing the plurality of punches from the sidewall cavities. Sealing the plurality of punches from the plurality of sidewall cavities includes disposing an "O" ring between the plurality of punches and the plurality of sidewall cavities.

[0012] In other features, forming the plurality of stiffening portions includes enclosing the plurality of sidewalls of the housing body using a die including a plurality of second cavities, and using fluid in the first cavity of the housing body to drive the sidewalls into the plurality of second cavities to form a plurality of stiffening portions on the sidewalls of the housing body.

[0013] A battery case includes a case body formed by extrusion or deep drawing and having a plurality of sidewalls defining a first cavity. A plurality of stiffening portions are formed in at least one of the plurality of sidewalls of the case body.

[0014] In other features, the plurality of stiffening sections includes at least one recess or stiffening bead. The housing body is made of stainless steel, aluminum, or alloys thereof. The plurality of stiffening sections extend outwardly from an outer side surface of the housing body. A first pattern of stiffening sections on one of the plurality of sidewalls of the housing body is complementary to a second pattern of stiffening sections on another of the plurality of sidewalls of the housing body to enable nesting of adjacent sidewalls of the housing body.

[0015] Further areas of applicability of the present invention will become apparent from the detailed description, claims, and drawings. The detailed description and specific examples are for illustrative purposes only. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will be better understood from the detailed description and the accompanying drawings, in which: Fig. 1 is a side cross-sectional view of an exemplary battery cell having cathode electrodes, anode electrodes, and separators disposed within a battery cell casing in accordance with the present invention; Fig. 2A is a perspective view of an exemplary battery cell casing; Fig. 2B is a perspective view of an exemplary battery cell housing with a pressure-responsive vent cap; Fig. 3A to 3C are perspective views of exemplary battery cell housings with post-formed stiffening sections according to the present invention; Fig. 4A to 4C are perspective and side views illustrating an exemplary nesting of battery cells with post-formed stiffening beads according to the present invention; Fig. 5A to 5C are perspective and side views showing an exemplary nesting of the battery cell with post-formed stiffening recesses according to the present invention; Fig. 6A to 6C are perspective views of exemplary housings with post-formed stiffening sections for battery cells according to the present invention; Fig. 7A to 7B are functional block diagrams showing a tool for punching stiffening sections in housings using a physical or hydraulic punch or by hydroforming according to the present invention; Fig. 8A to 8D are side cross-sectional views of exemplary expandable physical punches according to the present invention; Fig. 9A to 9B are side cross-sectional views of an exemplary hydroforming process according to the present invention; Fig. 10A to 10B are side cross-sectional views of an exemplary expandable hydraulic punch according to the present invention; and Fig. 11A through 11C are side cross-sectional views of exemplary expandable physical punches according to the present invention. Reference numerals may be reused throughout the drawings to identify similar and / or identical elements. DETAILED DESCRIPTION

[0017] Although the housings according to the present invention are shown in connection with battery cells for electric vehicles, the housings can also be used in stationary applications and / or other applications.

[0018] Battery cells are arranged in casings, such as prismatic and cylindrical casings, made of metals such as stainless steel, aluminum, or other metals. During cycling, the sidewalls of the battery cells can bulge due to internal pressure. Conventional prismatic casings are manufactured from aluminum using extrusion or deep-drawing processes. However, the casings manufactured in this way are not rigid enough to prevent bulging due to internal pressure.

[0019] Simple vertical stiffening beads extending to an open edge of the enclosure (to facilitate removal from a die) have been used to stiffen flat surfaces of metal enclosures manufactured by extrusion or deep drawing. However, more complex stiffening beads and / or recesses are not possible using extrusion or deep drawing processes.

[0020] The present invention relates to methods for post-forming stiffening sections (e.g., beads, indentations, manufacturer logos, and / or other shapes) after the housing body has been manufactured (e.g., by extrusion or deep drawing processes). The method involves punching outwardly directed stiffening sections (e.g., shallow beads, indentations, and / or other shapes) into the sidewalls of the prefabricated housing using an expandable physical or hydraulic punch or by hydroforming. In some examples, the housing body (without the stiffening sections) is manufactured in a conventional 2- or 3-part process (e.g., extrusion process, deep drawing process, or other process).

[0021] After its manufacture, the housing is placed in a die including cavities corresponding to the locations of the stiffening beads and / or recesses on the housing body. In some examples, an expandable physical or hydraulic punch is inserted into the housing body with one or more punches in a retracted position. The one or more punches are physically or hydraulically extended into one or more sidewalls of the housing body to punch stiffening portions into the one or more sidewalls. In other examples, hydroforming is used.

[0022] The stiffening sections can take a variety of shapes, such as stiffening ridges, recesses, or other geometric shapes such as manufacturer logos. The stiffening sections can be arranged at any location or in any direction on wide and / or narrow sidewalls of the enclosure. Once formed, the stiffening sections geometrically stiffen the sidewalls (and / or around the edges) of the enclosure. The stiffening sections reinforce the enclosure body at a given sidewall thickness. Alternatively, the stiffening sections allow the use of thinner metal material for the enclosure body sidewalls, reducing weight without compromising strength and / or stiffness. The stiffening sections can also be used to define gas flow channels, which serve to direct the flow of hot vent gases to the vent during thermal events.

[0023] In some examples, the stiffening sections have a depth in the range of 0.2 to 1.0 times the thickness of the package walls. In some examples, a width of the bead is in a range of 2 mm to 6 mm. In some examples, the stiffening sections include stiffening beads that extend horizontally, vertically, and / or diagonally. In some examples, a first pattern of stiffening sections on one side of the package body is complementary to a second pattern of stiffening sections on another side of the package body to enable nesting of adjacent packages, saving packaging space.

[0024] With reference to Fig. 1, a battery cell 10 comprises C cathode electrodes 20, A anode electrodes 40, and S separators 32 arranged in a predetermined order in a stack 12 within a housing 50, where A, C, and S are integers greater than one. The C cathode electrodes 20-1, 20-2, ..., and 20-C comprise cathode active layers 24 arranged on one or both sides of the cathode current collectors 26. The anode electrodes 40-1, 40-2, ..., and 40-A comprise anode active layers 42 arranged on one or both sides of the anode current collectors 46.

[0025] In some examples, the anode active layers 42 and / or the cathode active layers 24 are freestanding electrodes disposed adjacent to (or attached to) the current collectors. In some examples, the anode active layers 42 and / or the cathode active layers 24 comprise coatings including one or more active materials, one or more conductive fillers / additives, and / or one or more binders. In some examples, the cathode current collectors 26 and / or the anode current collectors 46 comprise a foil, a mesh, and / or an expanded metal layer.

[0026] In some examples, the current collectors are made of one or more materials selected from a group consisting of copper, stainless steel, brass, bronze, zinc, aluminum, and their alloys. In some examples, the outer tabs 28 and 48 connected to the current collectors of the cathode and anode electrodes are located on opposite sides of the battery stack (as in Fig. 1), on different sides or on the same side of the battery stack.

[0027] With reference to Fig. 2A and Fig. 2B, a battery cell 58 includes a housing 60. In some examples, the housing 60 has a prismatic shape with rectangular cross-sections in the x-, y-, and z-axis planes. In some examples, the housing 60 includes a housing body 61 having sides 80 and 82 defining an open rectangular prism. In some examples, the sides 80 have a lesser width than the sides 82. In some examples, the housing 60 includes the bottom portion 86. In other examples, the bottom portion 86 is attached after the housing 60 is formed. In some examples, the housing body is manufactured by extrusion processes, deep drawing processes, or other processes.

[0028] A lid portion 84 and optionally a bottom portion 86 are attached to the housing body 61 to close the upper and lower openings of the housing body 61, respectively. The battery cell 58 includes outer terminals 62 and 64 that extend through the lid portion 84. The stack 12 comprising the C cathode electrodes 20, the A anode electrodes 40, and the S separators 32 is arranged in the housing 60.

[0029] The outer poles 62 and 64 are connected to the outer tabs 28 and 48 of the C cathode electrodes 20 and A anode electrodes 40, respectively. Fig. 2A, the cover portion 84 does not include a pressure-dependent vent cap. In Fig. 2B, the lid portion 84 (and / or the bottom portion 86) includes a pressure-dependent vent cap 66. The pressure-dependent vent cap 66 is configured to release vent gases when the pressure in the inner housing is greater than a predetermined pressure.

[0030] With reference to Fig. 3A to 3C, a battery cell 94 includes a housing 96 having one or more side surfaces 128 with a plurality of stiffening sections 122. While the stiffening sections 122 have recesses, they may also have other shapes, such as ridges or other shapes. In some examples, the recesses have a rectangular, hemispherical, and / or arcuate shape. It is understood that the stiffening sections 122 may be disposed on the other side surface 129, the lid, and / or the bottom.

[0031] In some examples, the stiffening sections 122 are arranged in a predetermined pattern 120 on the side surface 128 of the battery cell 94. In some examples, the stiffening sections 122 are arranged on the surfaces of the housing 96 that have the largest area. In some examples, two or more side surfaces of the battery cell 94 have the same pattern of stiffening sections 122. In other examples, opposite side surfaces of the battery cell 94 have different patterns of stiffening sections 122.

[0032] In Fig. 3A, the stiffening portions 122 include recesses extending outwardly from the side surface 128 of the housing 96. When two or more of the battery cells 94 are arranged side by side, in some examples, the stiffening portions 122 of the facing side surfaces of adjacent battery cells are offset to facilitate nesting. In some examples, a thermal barrier layer or thermal interface material and a cooling plate are disposed between adjacent battery cells.

[0033] In Fig. 3B and Fig. 3C, the stiffening sections 122 comprise stiffening beads or ribs with a rectangular cross-section. The stiffening sections 122 extend vertically, horizontally, and / or diagonally along one or more of the side surfaces. The stiffening sections 122 are arranged in a first predetermined pattern on one side of the battery cell 94 and in a second predetermined pattern on an opposite side surface of the battery cell 94.

[0034] In some examples, the first predetermined pattern and the second predetermined pattern are identical. In other examples, the stiffening sections of the first predetermined pattern and the second predetermined pattern are different. For example, the stiffening sections on one side are complementary to allow for nesting or overlapping to minimize space requirements (e.g., offset from the stiffening sections on the opposite side). In other words, adjacent battery cells can be arranged in close proximity (e.g., the stiffening sections are nested when aligned and / or positioned adjacent to each other).

[0035] In some examples, a back volume created by the stiffening sections is used to direct vent gases within the housing to the vent. In other examples, the back volume is filled with a material to create a smooth surface. In other examples, the material includes an insulating material or a thermally conductive material. In other examples, the material includes a first inhibiting material such as sodium bicarbonate.

[0036] With reference to Fig. 4A to 4C, an exemplary battery cell 400 with stiffening beads 416 and 418 is shown. In Fig. 4A and Fig. 4B, the stiffening beads 416 and 418 are arranged offset from one another on the opposite sides 410 and 412 in order to avoid nesting of the stiffening beads of adjacent battery cells 400-1, 400-2, ..., and 400-C (see Fig. 4C) and thus reduce the space required.

[0037] With reference to Fig. 5A to 5C, an exemplary battery cell 450 with stiffening recesses 466 and 468 is shown. In Fig. 5A and Fig. 5B, the stiffening recesses 466 and 468 on the opposite sides 460 and 462 are arranged offset from one another in order to allow nesting of the stiffening recesses of adjacent battery cells 450-1, 450-2, ..., and 450-C (see Fig. 5C) and thus reduce the space required.

[0038] With reference to Fig. 6A, a battery cell 500 includes a housing 504 having one or more sides 510 with a first or peripheral region 516 and a second or central region 530 (e.g., located within the peripheral region 516). The central region 530 of the sides 510 of the battery housing is typically the hottest spot when thermal runaway occurs. In some examples, stiffening portions are omitted in areas of the housing that tend to be hotter, such as the central region 530 of the sides 510. The stiffening portions 518 and 520 are located in the peripheral region 516 (while the central region 530 does not include any stiffening portions).

[0039] The battery case in Fig. 6A helps limit heat transfer or spread between adjacent battery cells. There are no ridges or depressions in the central region 530 where one or both adjacent battery cells are hottest. In some examples, a thermal insulation material or thermal interface material and a cold plate (see below) are disposed between the central regions 530 of the adjacent battery cells.

[0040] In Fig. 6B, a battery cell 540 includes a housing 504 having one or more sides 510 with a pattern 546 of stiffening beads 548 extending parallel in the horizontal direction. In Fig. 6C, a battery cell 560 includes a housing 504 having one or more sides 510 with a pattern 566 of stiffening ridges 568 extending parallel in both horizontal and vertical directions. For example, a rectangular pattern with interconnected horizontal and vertical ridges may be used.

[0041] With reference to Fig. 7A, a tool 700 includes one or more dies 710 and an expandable punch 720. In some examples, the one or more dies 710 include split dies 710-1 and 710-2 disposed around the housing above a lower die surface 710-3. A positioning device 712 can be used to position a housing body 724 on the lower die surface 710-3 and to open and close the dies 710-1 and 710-2 to the side surfaces of the housing body 724.

[0042] With the housing body 724 positioned within the tool 700, the expandable punch 720 is inserted into an internal cavity of the housing body 724. The expandable punch 720 includes one or more punches 728 that are physically or hydraulically driven outwardly or laterally into an interior surface of the housing body 724 by an actuator 730. In some examples, the actuator 730 includes a physical punch that moves in a first direction and contacts and physically biases one or more punches 728 in a second direction. In some examples, the second direction is transverse to or at an acute angle to the first direction.

[0043] In other examples, the actuator 730 is a hydraulic actuator that supplies high-pressure fluid to the expandable punch 720 or pressurizes the hydraulic fluid within the punch. The high pressure of the hydraulic fluid biases the punches 728 outward into the inner surface of the housing body 724. A controller 732 is configured to control the positioning device 712 and / or the actuator 730.

[0044] With reference to Fig. 7B, a tool 748 is similar to the tool in Fig. 7A. The tool 748 includes an actuator 750 and a punch / die 754. After the housing body 724 is positioned in the tool 748, the positioning device 712 places the punch / die 754 over an open end of the housing body 724 to create a fluid seal. Fluid is supplied to the cavity of the housing body 724. The pressure of the fluid is increased by applying pressure to the punch / die 754. The high-pressure fluid forces the sidewalls of the housing body 724 outward into the cavities (see below) defined on inward-facing surfaces of the dies 710 to form the stiffening sections.

[0045] With reference to Fig. 8A to 8D show exemplary expandable physical punches. In Fig. 8A and Fig. 8B, an expandable physical punch 800 includes an inner punch 810 and an outer punch 828. The expandable physical punch 800 is inserted into the housing body 724. The housing body 724 is enclosed by the die (as shown above). The outer punch 828 includes side walls 829 that define an internal cavity 830. The outer surfaces of the side walls 829 are disposed adjacent to the inner surfaces of the side walls of the housing body 724. During formation of the stiffening features, the inner punch 810 is inserted into the internal cavity 830 to bias the punches 834 disposed in the side walls 829 of the outer punch 828 outward into the side walls of the housing body 724. The punches 834 move from a retracted position ( Fig. 8A) into a punching position ( Fig. 8B) to punch one or more side walls of the housing body 724 when biased outwardly by the inner punch 810.

[0046] The side walls 829 and the removable wall portions 841 define a cavity 842 for receiving the punches 834. In some examples, the removable wall portions 841 can be removed to allow replacement of the punches 834. The punches 834 include a body 833 having an inner end 835 and an outer end 836. The inner end 835 extends into the inner cavity 830 when retracted. During punching, the outer end 836 is biased into the inner surface of the side walls of the housing body 724. In some examples, the punches 834 include the body 833 and a projection 837 (e.g., an annular projection extending transversely to a longitudinal direction of the body). In some examples, a spring 844 is disposed between the removable wall portions 841 and the projection 837 of the punches 834 to bias the punches 834 to the retracted position.In some examples, the outer ends 836 of the inner punch 810 and edges 840 of the punches 834 are inclined at an acute angle to a direction in which the inner punch reciprocates.

[0047] In Fig. 8C and Fig. 8D, another exemplary expandable physical punch 900 is shown. The side walls 910 of an outer punch 912 define an inner cavity 916 for receiving the inner punch 810. The side walls 910 further define cavities 920 with an inclined surface 921. The cavities 920 receive punches 924 having a body 926 with an inclined surface 928. Due to gravity, the inclined surfaces 928 of the punches 924 move along the inclined surfaces 921 of the cavities to bias the punches 924 to the retracted position. The inner punch 810 biases inner ends 930 of the punches outwardly so that the outer ends 932 punch the stiffening sections onto the housing body 724 (see Fig. 8D).

[0048] With reference to Fig. 9A and Fig. 9B, a hydroforming tool 934 may be used to form the stiffening sections on the housing body 724. In Fig. 9A, the hydroforming tool 934 includes dies 935 with cavities 936 at locations corresponding to the stiffening sections. The housing body 724 is inserted into a cavity defined between the dies 935. The cavity of the housing body 724 is at least partially filled with liquid. A punch / die 939 is inserted into a cavity of the housing body 724 to compress the liquid and thus increase the pressure in the cavity. When the liquid pressure is sufficiently increased, the liquid pushes the side walls into the cavities 936 to form the stiffening sections ( Fig. 9B).

[0049] In Fig. 10A and Fig. 10B, an exemplary expandable hydraulic punch 950 is shown. The side walls 960 of an outer punch 962 define an interior cavity 966. The side walls 960 further define cavities 968 for mutually receiving punches 972. The punches 972 include inner ends 976 and outer ends 978. An actuator 987 applies pressure to a die / punch 986, which in turn applies pressure to the fluid in the housing body 724 to increase the pressure and cause the punches 972 to move laterally into the housing body 724 and punch the stiffening features in the housing body 724. The fluid transfers the pressure to the punches 972 without direct contact, which reduces wear. After the fluid pressure is released, the punches 972 return to the retracted position.

[0050] In some examples, pressures in the range of 50 to 300 MPa are used. In some examples, the stiffening sections have a width in the range of 2 to 6 mm and a depth in the range of 0.2 mm to 0.5 mm. In some examples, the stiffening sections extend around the corners of the housing.

[0051] The fluid exerts sufficient pressure to bias the inner ends 976 of the punches 972 outward. The outer ends 978 are biased into the inner surface of the side walls of the housing body 724 to form the stiffening sections. In some examples, a seal 983 (e.g., an O-ring) provides a seal between the punches 972 and the cavities 968.

[0052] With reference to Fig. 11A to 11C, another exemplary expandable physical punch 1050 is shown. In Fig. 11A, side walls 1060 of an outer punch 1062 define cavities 1067. The cavities 1067 include one or more inclined surfaces 1068, 1069, etc., inclined at an acute angle to a direction in which an inner punch (not shown) reciprocates (e.g., vertically). The cavities 1067 receive punches 1072 having a projection 1078 with an inclined surface 1080. The punches 1072 may also have an inclined surface 1076 at an inner end 1077. A split die 1090 includes cavities 1092 that are aligned with the punches 1072 when the punches 1072 are in a punching position.

[0053] In Fig. 11A, the punches 1072 are biased by gravity along the inclined surfaces into the retracted position, with inner ends 1077 extending into an inner cavity 1094 of the outer punch 1062. Outer ends 1079 of the punches are spaced from the side walls of the housing body 724. In Fig. 11B, an inner punch 1098 moves into an inner cavity 1094 and biases the inner ends 1077 of the punches 1072 outwardly and upwardly into the inner surface of the side walls of the housing body 724 to form stiffening sections in the housing body 724 at 1100. In Fig. 11C, the inner punch 1098 is removed and the punches 1072 move to the retracted position due to gravity.

Claims

[1] A method for producing a housing (724) for battery cells, comprising: Extruding or deep drawing a housing body (724), wherein the housing body (724) comprises a plurality of side walls defining a first cavity, and wherein the plurality of side walls are formed without stiffening sections; and Forming a plurality of stiffening sections (518, 520) on the housing body (724) after extruding or deep drawing the housing body (724); characterized by , that forming the plurality of stiffening sections (518, 520) comprises: Enclosing the plurality of side walls of the housing body (724) using a die; Inserting an outer punch (828) of an expandable physical punch (800) into the first cavity of the housing body (724), wherein the outer punch (828) comprises a plurality of punches (834) and a plurality of sidewalls (829) defining a second cavity (830) and a plurality of sidewall cavities (842) configured to receive the plurality of punches (834); and Inserting an inner punch (810) into the second cavity (830) to bias the plurality of punches (834) from a retracted position to a punching position to punch a plurality of stiffening sections (518, 520) into at least one of the plurality of side walls of the housing body (724). [2] The method of claim 1, wherein: the plurality of stiffening sections (518, 520) comprises at least one recess or a stiffening bead and the housing body (724) is made of stainless steel, aluminum or their alloys. [3] The method of claim 1, wherein the plurality of stiffening portions (518, 520) extend outwardly from an outer side surface of the housing body (724). [4] The method of claim 1, wherein the plurality of punches (834) comprises a body (833) and a projection (837) extending transversely from the body (833). [5] The method of claim 4, further comprising biasing the projection (837) of the plurality of punches (834) toward the retracted position of the plurality of punches (834) by means of a spring (844). [6] The method of claim 1, wherein the plurality of sidewall cavities (842) have first inclined surfaces (921) and the plurality of punches (924) comprise bodies (926) having second inclined surfaces (928). [7] The method of claim 1, wherein the plurality of punches (1072) are inclined at an acute angle to the direction of movement of the inner punch (810). [8] The method of claim 1, wherein the plurality of punches move in a direction transverse to a direction of movement of the inner punch (810). [9] The method of claim 2, wherein edges of the inner ends of the plurality of punches (834) are inclined at an acute angle to the direction of movement of the inner punch (810).

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