Battery module

Abstract

A retaining element comprises: a body extending in the predetermined direction; and fixed ends extending in a direction intersecting the predetermined direction from both ends of the body, each end facing a corresponding face of opposite end plates, each of the opposite end plates being a face of a corresponding end plate in the predetermined direction. A fixing recess configured to receive the fixed end is provided at one end of the face on a side closer to the body. At least a portion of an edge of the fixing recess on a side opposite the body and at least a portion of an edge of the fixed end on the side opposite the body have a projection and recess shape. The edge of the fixing recess and the edge of the fixed end are welded together.

Description

BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to a battery module. 2. Description of the related prior art JP 7 348 930 B discloses a battery module comprising a plurality of battery cells stacked in a predetermined direction, a pair of end plates enclosing all the battery cells between them, and a retaining element attached to the end plates. The retaining element pulls the end plates towards each other, thereby holding the battery cell group in place. SUMMARY OF THE INVENTION It is conceivable to attach the end plates and the retaining element of the JP 7 348 930 B by welding. However, achieving a high bond strength between the end plate and the retaining element is not easy when welding. The present invention was made in view of the above circumstances, and one objective of it is to provide a battery module in which end plates can be attached to a retaining element by welding, while achieving a high fastening strength between the end plates and the retaining element. A battery module of a first aspect comprises: a pair of end plates spaced apart from each other in a predetermined direction; a plurality of battery cells stacked in the predetermined direction and enclosed between the end plates; and a retaining element connected to the end plates to pull the end plates toward each other. The retaining element comprises: a body extending in the predetermined direction; and fixed ends extending in a direction intersecting the predetermined direction from both ends of the body, each facing a corresponding end face of the end plates. Each of the opposing end faces is an end face corresponding to one of the end plates in the predetermined direction. A fixing recess for receiving the fixed end is provided at one end of the end face on a side closer to the body.At least part of the edge of the mounting recess on a side opposite the body and at least part of the edge of the fixed end on the same side comprise a projection and recess shape. The edge of the mounting recess and the edge of the fixed end are welded together. In the battery module of the first aspect, the retaining element comprises: the body, which extends in the predetermined direction; and the fixed ends, which extend in a direction that intersects the predetermined direction from both ends of the body and each faces a corresponding end face of the end plates. Each of the opposing end faces is an end face of a corresponding end plate in the predetermined direction. The fixing recess for receiving the fixed end is provided at the end of the end face of the end plate on the side facing the body. At least a portion of the edge of the fixing recess on the side opposite the body and at least a portion of the edge of the fixed end on the side opposite the body have a protrusion and recess shape.Therefore, when the fixed end of the retaining element is fitted into the mounting recess of the end plate, the end plate and the retaining element (i.e., the fixed end) are positioned in the up-down direction. Compared to a case where the entire edge of the mounting recess on the side opposite the body and the entire edge of the fixed end on the side opposite the body have a straight profile, the overall length of the edge of the mounting recess on the side opposite the body and the overall length of the edge of the fixed end on the side opposite the body is also longer. This configuration increases the weld length resulting from laser welding the edges of the mounting recess and the fixed end on the side opposite the body, making it easier to achieve a high bond strength between the end plate and the retaining element. According to a battery module of a second aspect in the first aspect, the protrusion and depression shape is a wave-shaped section. In the case of the battery module of the second aspect, it is less likely that the end plate will be damaged if, for example, the edge of the fixed end on the side opposite the body comes into contact with a part of the end plate other than the mounting recess. According to a battery module of a third aspect of the first aspect, the protrusion and depression shape comprises a plurality of rectangular protrusions and a plurality of rectangular depressions. In the case of the battery module of the third aspect, the total length of the edge of the mounting recess on the side opposite the body and the total length of the edge of the fixed end on the side opposite the body are rather long. As described above, the battery module according to the invention enables the attachment of the end plate and the retaining element by welding, ensuring a high attachment strength between the end plate and the retaining element. BRIEF DESCRIPTION OF THE FIGURES Features, advantages, and technical and industrial significance of exemplary embodiments of the invention are described below with reference to the accompanying figures, in which identical symbols denote identical elements, and wherein: Fig. 1 is a perspective view of a battery module according to a first embodiment; Fig. 2 is a perspective view showing a state in which an end plate and retaining elements are separated from each other; Fig. 3 is a front view of the battery module; Fig. 4 is a perspective view similar to Fig. 2, showing a battery module according to a second embodiment; and Fig. 5 is a front view of the battery module according to the second embodiment. DETAILED DESCRIPTION OF THE EXECUTION FORMS A battery module according to a first embodiment is described below with reference to Figures 1, 2 to 3. The arrows OB, VO and LI in the figures denote the upward direction in the upward-downward direction of the vehicle, the forward direction in the front-rear direction of the vehicle, and the left direction in the left-right direction of the vehicle, respectively. A battery module 10 according to the first embodiment is attached to a vehicle (electrified vehicle). In the present embodiment, the vehicle is a battery-electric vehicle. The battery module 10 according to the first embodiment is housed in a battery casing (not shown) which is attached to a vehicle body. The electrical energy of the battery module 10 (battery cells 17) is supplied, for example, to an electric motor (not shown), which transmits the driving force to the drive wheels of the vehicle. The battery module 10 comprises a battery pack 15, end plates 20 and retaining elements 30. As shown in Fig. 1, the battery pack 15, which extends from front to back, comprises a plurality of battery cells 17 and a plurality of insulating elements (spacers) (not shown), each arranged between adjacent battery cells 17. For simplicity, only nine battery cells 17 are shown in Fig. 1. However, the battery pack 15 actually comprises more than nine battery cells 17. Each battery cell 17 is a lithium-ion cell, and a battery casing 18, which defines the external shape of the battery cell 17, has a rectangular parallelepiped shape. The battery casing 18 is made of metal. The battery cells 17 are electrically interconnected. The battery module 10 comprises a pair of front and rear end plates 20. In the present embodiment, the end plates 20 are manufactured by aluminum die casting. The end plates 20 can be manufactured, for example, by vacuum die casting or laminar flow die casting. Alternatively, the end plates 20 can also be manufactured from aluminum by extrusion. The front end plate 20 is located immediately in front of the foremost battery cell 17, and the rear end plate 20 is located immediately behind the rearmost battery cell 17. The front and rear end plates 20 are symmetrical with respect to the front and rear directions. As shown in Fig. 2, each end plate 20 has a substantially rectangular parallelepiped shape. Each of the opposing end faces 22 of the front and rear end plates 20, i.e., the front end face of the front end plate 20 and the rear end face of the rear end plate 20, is provided with mounting recesses 24 at its right and left edges. The mounting recesses 24 are symmetrical with respect to the right-left direction. The upper end of each mounting recess 24 is located below the upper edge of the end face 22, and the lower end of each mounting recess 24 is located above the lower edge of the end face 22. The left end of the left mounting recess 24 and the right end of the right mounting recess 24 are both open.Furthermore, the right end of the left mounting recess 24 and the left end of the right mounting recess 24 each have a projection and recess form, instead of a straight profile extending in an up-down direction when viewed from the front. More precisely, the right end of the left mounting recess 24 and the left end of the right mounting recess 24 are each formed as a wave-shaped section (projection and recess form) 26. The pair of right and left retaining elements 30 is symmetrical with respect to the left-right direction. In the present embodiment, each retaining element 30 is an aluminum extrusion. Each retaining element 30 comprises a body 32, which has a flat, plate-like shape and extends in a direction perpendicular to the left-right direction. The body 32 has a substantially rectangular shape, with its front-to-back dimension being larger than its top-to-bottom dimension when viewed from the side. The front and rear ends of the body 32 are configured as end structures 34, the top-to-bottom dimension of which is shorter than that of the rest of the body 32 in the top-to-bottom direction. Each retaining element 30 comprises a pair of upper and lower flanges 36. The upper and lower flanges 36 are each connected to the upper and lower edges of the body 32, respectively, except for the end structures 34. Each flange 36 extends substantially perpendicular to the body 32. The retaining element 30 further comprises a pair of front and rear fixed ends 38, each connected to the front and rear end structures 34. Each end structure 34 intersects a corresponding fixed end 38. The inner edge of the right and left fixed ends 38, i.e., the edge on the side opposite the corresponding end structure 34, has a projection and recess shape rather than a straight profile extending downwards when viewed from the front. More precisely, the inner edge of the right fixed end 38 and the inner edge of the left fixed end 38 are each formed as a wave-shaped section (projection and recess shape) 40. Each fixed end 38 has essentially the same shape as the corresponding mounting recess 24 when viewed from the front.Each wavy section 40 has the same shape as the wavy section 26 of a corresponding fastening recess 24, viewed from the front. The right and left retaining elements 30 are connected to the battery pack 15. That is, as shown in Fig. 1, the left retaining element 30 is brought towards the battery pack 15 from the left side, the front fixed end 38 is fitted from the front into the left mounting recess 24 of the front end plate 20, and the rear fixed end 38 is fitted from the rear into the left mounting recess 24 of the rear end plate 20. Similarly, the right retaining element 30 is brought towards the battery pack 15 from the right side, the front fixed end 38 is fitted from the front into the right mounting recess 24 of the front end plate 20, and the rear fixed end 38 is fitted from the rear into the right mounting recess 24 of the rear end plate 20.The corrugated section 40 of the fixed end 38 can be in contact with the corrugated section 26 of the fastening recess 24, or the corrugated section 40 and the corrugated section 26 can be opposite each other with a small gap between them. Consequently, the upper and lower flanges 36 of the left retaining element 30 are each in contact with the upper and lower surfaces of each battery cell 17, respectively, with a small gap between them. Furthermore, the inner surface of the body 32 of the left retaining element 30 is in contact with the left side surface of each battery cell 17, or with a small gap between them. Similarly, the upper and lower flanges 36 of the right retaining element 30 are in contact with the upper and lower surfaces of each battery cell 17, respectively, or with a small gap between them.Furthermore, the inner surface of the body 32 of the right retaining element 30 is in contact with the right side surface of each battery cell 17 or faces the right side surface of each battery cell 17 with a small gap between them. When the right and left retaining elements 30 are attached to the battery pack 15 in this way, the length of each retaining element 30 in the front-to-back direction is longer than in its free state. Therefore, the retaining elements 30 pull the front and rear end plates 20 towards each other. The battery pack 15 is thus clamped by the front and rear end plates 20 in a front-to-back direction. Furthermore, the front surfaces of the front fixed ends 38 of the right and left retaining elements 30 are flush with the end face 22 of the front end plate 20, and the rear surfaces of the rear fixed ends 38 of the right and left retaining elements 30 are flush with the end face 22 of the rear end plate 20.The corrugated section 40 of the front fixed end 38 of the right and left retaining elements 30 is joined to the corrugated section 26 of a corresponding mounting recess 24 of the front end plate 20 by laser welding. That is, a weld section 45 is formed along the entire corrugated section 40 of the front fixed end 38 of the right retaining element 30 and the entire corrugated section 26 of the right mounting recess 24 of the front end plate 20, and another weld section 45 is formed along the entire corrugated section 40 of the front fixed end 38 of the left retaining element 30 and the entire corrugated section 26 of the left mounting recess 24 of the front end plate 20.Therefore, the wave-shaped section 40 of the front fixed end 38 of each of the right and left retaining elements 30 is attached to the wave-shaped section 26 of a corresponding mounting recess 24 of the front end plate 20. Although not shown in the figures, the corrugated section 40 of the rear fixed end 38 of the right and left retaining element 30 is joined to the corrugated section 26 of a corresponding mounting recess 24 of the rear end plate 20 by laser welding. Accordingly, a weld section 45 is formed along the entire corrugated section 40 of the rear fixed end 38 of the right retaining element 30 and the entire corrugated section 26 of the right mounting recess 24 of the rear end plate 20, and another weld section 45 is formed along the entire corrugated section 40 of the rear fixed end 38 of the left retaining element 30 and the entire corrugated section 26 of the left mounting recess 24 of the rear end plate 20.Therefore, the wave-shaped section 40 of the rear fixed end 38 of the right and left retaining element 30 is each attached to the wave-shaped section 26 of a corresponding mounting recess 24 of the rear end plate 20. Functions and effects The functions and effects of the first embodiment are described below. In the battery module 10 of the first embodiment, each retaining element 30 comprises the body 32, which extends in the front-back direction, and the fixed ends 38, which each extend in the left-right direction from both ends of the body 32 (i.e., from both ends of the end structures 34) and each face the front face 22 of a corresponding end plate 20 in the front-back direction. The fastening recesses 24, into which the fixed ends 38 are fitted, are formed at both ends of the front face 22 of each end plate 20, each located on the side closer to a corresponding body 32.Furthermore, the edge of each fastening recess 24 on the side opposite the body 32 is formed as a corrugated section 26, and the edge of each fixed end 38 on the side opposite the body 32 is formed as a corrugated section 40, the shape of which corresponds to that of the corrugated section 26. Thus, when the fixed ends 38 of the retaining element 30 are each fitted into the fastening recesses 24 of the end plates 20, the end plates 20 and the retaining element 30 (i.e., the fixed ends 38) are positioned in the up-down direction.Furthermore, compared to a case where the entire edge of each fastening recess 24 on the side opposite the body 32 and the entire edge of each fixed end 38 on the side opposite the body 32 have a straight profile extending in the upward-downward direction, the overall length of the edge of each fastening recess 24 on the side opposite the body 32 and the overall length of the edge of each fixed end 38 on the side opposite the body 32 is longer. This configuration increases the weld length resulting from laser welding the edges of the fastening recess 24 and the fixed end 38 on the side opposite the body 32, thus making it easier to achieve a high fastening strength between the end plate 20 and the retaining element 30. Furthermore, the end face of each corrugated section 40 of the retaining element 30 has a curved shape. Therefore, for example, even if the corrugated section 40 of the retaining element 30 comes into contact with a part of the end plate 20 other than the mounting recess 24 during the connection of the retaining element 30 to the end plate 20, it is less likely that the end plate 20 will be damaged by the retaining element 30. Furthermore, the front surfaces of the front fixed ends 38 of the right and left retaining elements 30 are flush with the end face 22 of the front end plate 20, and the rear surfaces of the rear fixed ends 38 of the right and left retaining elements 30 are flush with the end face 22 of the rear end plate 20. Therefore, the front-to-rear dimension of the battery module 10 can be reduced compared to a case where, for example, the front surfaces of the front fixed ends 38 of the right and left retaining elements 30 are in front of the end face 22 of the front end plate 20, or the rear surfaces of the rear fixed ends 38 of the right and left retaining elements 30 are behind the end face 22 of the rear end plate 20. Next, a battery module according to a second embodiment is described with reference to Figures 4 and 5. The same components as in the first embodiment are identified by the same symbols, and a detailed description thereof is omitted. A battery module 50 according to the second embodiment differs from the first embodiment in the shapes of the end plates 60 and the retaining elements 70. The material and the manufacturing process of the end plates 60 are essentially the same as in the first embodiment, and the material and the manufacturing process of the retaining elements 70 are also essentially the same as in the first embodiment. The front and rear end plates 60 are symmetrical with respect to the front and rear directions. As shown in Fig. 4, each end plate 60 has a substantially rectangular parallelepiped shape. Mounting recesses 64, which are symmetrical with respect to the left-right direction, are formed on the right and left edges of the opposing end faces 62 of the end plates 60. The upper end of each mounting recess 64 is located below the upper edge of the end face 22, and the lower end of each mounting recess 64 is located above the lower edge of the end face 22. The left end of the left mounting recess 64 and the right end of the right mounting recess 64 are both open.Furthermore, the right end of the left mounting recess 64 and the left end of the right mounting recess 64 each have a projection and recess form, instead of a straight profile extending in an up-down direction when viewed from the front. More precisely, the right end of the left mounting recess 64 and the left end of the right mounting recess 64 are each formed as a rectangular projection and recess structure (projection and recess form) 66. The rectangular projection and recess structure 66 has a shape in which essentially square recesses 67 and essentially square projections 68 are arranged alternately in the up-down direction. The pair of right and left retaining elements 70 is symmetrical with respect to the left-right direction. Each retaining element 70 comprises a pair of front and rear fixed ends 72, each connected to the front and rear end structures 34. Each end structure 34 overlaps with a corresponding fixed end 72. The inner edge of the right and left fixed ends 72, i.e., the edge on the side opposite the corresponding end structure 34, has a projection and recess shape rather than a straight profile extending upwards when viewed from the front. More precisely, the inner edge of the right fixed end 72 and the inner edge of the left fixed end 72 are each formed as a rectangular projection and recess structure (projection and recess shape) 74.The rectangular projection and recess structure 74 has essentially the same shape as the rectangular projection and recess structure 66 when viewed from the front. The rectangular projection and recess structure 66 has a shape in which essentially square recesses 75 and essentially square projections 76 are arranged alternately in a top-to-bottom direction. The right and left retaining elements 70 are connected to the battery pack 15. That is, the left retaining element 70 is brought towards the battery pack 15 from the left side, the front fixed end 72 is fitted from the front into the left mounting recess 64 of the front end plate 60, and the rear fixed end 72 is fitted from the rear into the left mounting recess 64 of the rear end plate 60. Similarly, the right retaining element 70 is brought towards the battery pack 15 from the right side, the front fixed end 72 is fitted from the front into the right mounting recess 64 of the front end plate 60, and the rear fixed end 72 is fitted from the rear into the right mounting recess 64 of the rear end plate 60. Consequently, the upper and lower flanges 36 of the left retaining element 70 are each in contact with the upper and lower surfaces of each battery cell 17, respectively, with a small gap between them. Furthermore, the inner surface of the body 32 of the left retaining element 70 is in contact with the left side surface of each battery cell 17, or with a small gap between them. Similarly, the upper and lower flanges 36 of the right retaining element 70 are in contact with the upper and lower surfaces of each battery cell 17, respectively, or with a small gap between them.Furthermore, the inner surface of the body 32 of the right retaining element 70 is in contact with the right side surface of each battery cell 17 or faces the right side surface of each battery cell 17 with a small gap between them. When the right and left retaining elements 70 are attached to the battery pack 15 in this way, the length of each retaining element 70 in the front-to-back direction is longer than in its free state. Therefore, the retaining elements 70 pull the front and rear end plates 60 towards each other. The battery pack 15 is thus clamped by the front and rear end plates 60 in the front-to-back direction. Furthermore, the front surfaces of the front fixed ends 72 of the right and left retaining elements 70 are flush with the end face 62 of the front end plate 60, and the rear surfaces of the rear fixed ends 72 of the right and left retaining elements 70 are flush with the end face 62 of the rear end plate 60. The rectangular projection and recess structure 74 of the front fixed end 72 of the right and left retaining element 70 is connected to the rectangular projection and recess structure 66 of a corresponding mounting recess 64 of the front end plate 60 by laser welding. That is, a weld section 77 is formed along the entire inner edge of the rectangular projection and recess structure 74 of the front fixed end 72 of the right retaining element 70 and the entire inner edge of the rectangular projection and recess structure 66 of the right mounting recess 64 of the front end plate 60, and another weld section 77 is formed along the entire inner edge of the rectangular projection and recess structure 74 of the front fixed end 72 of the left retaining element 70 and the entire inner edge of the rectangular projection and recess structure 66 of the left mounting recess 64 of the front end plate 60.At this point, the inner edge of the rectangular projection and recess structure 74 of the fixed end 72 can be in contact with the inner edge of the rectangular projection and recess structure 66 of the mounting recess 64, or the inner edge of the rectangular projection and recess structure 74 and the inner edge of the rectangular projection and recess structure 66 can be opposite each other with a small gap between them. Therefore, the rectangular projection and recess structure 74 of the front fixed end 72 of each of the right and left retaining elements 70 is attached to the rectangular projection and recess structure 66 of a corresponding mounting recess 64 of the front end plate 60. Although not shown in the figures, the rectangular projection and recess structure 74 of the rear fixed end 72 of each of the right and left retaining elements 70 is connected by laser welding to the rectangular projection and recess structure 66 of a corresponding mounting recess 64 of the rear end plate 60.That is, a weld section 77 is formed along the entire inner edge of the rectangular projection and recess structure 74 of the rear fixed end 72 of the right retaining element 70 and the entire inner edge of the rectangular projection and recess structure 66 of the right mounting recess 64 of the rear end plate 60, and another weld section 77 is formed along the entire inner edge of the rectangular projection and recess structure 74 of the rear fixed end 72 of the left retaining element 70 and the entire inner edge of the rectangular projection and recess structure 66 of the left mounting recess 64 of the rear end plate 60. Therefore, the rectangular projection and recess structure 74 of the rear fixed end 72 of the right and left retaining elements 70 is each attached to the rectangular projection and recess structure 66 of a corresponding mounting recess 64 of the rear end plate 60. Functions and effects The functions and effects of the second embodiment are described below. In the battery module 50 of the second embodiment, each retaining element 70 comprises the body 32, which extends in the front-back direction, and the fixed ends 72, which each extend in the left-right direction from both ends of the body 32 (i.e., from both ends of the end structures 34) and each face the front face 62 of a corresponding end plate 60 in the front-back direction. The fastening recesses 64, into which the fixed ends 72 are fitted, are formed at both ends of the front face 62 of each end plate 60, each located on the side closer to a corresponding body 32.Furthermore, the edge of each fastening recess 64 on the side opposite the body 32 is formed as a rectangular projection and recess structure 66, and the edge of each fixed end 72 on the side opposite the body 32 is formed as a rectangular projection and recess structure 74, the shape of which corresponds to that of the rectangular projection and recess structure 66. Thus, when the fixed ends 72 of the retaining element 70 are each fitted into the opposite end faces 62 of the end plates 60, the end plates 60 and the retaining element 70 (i.e., the fixed ends 72) are positioned in the up-down direction.Furthermore, compared to a case where the entire edge of each fastening recess 64 on the side opposite the body 32 and the entire edge of each fixed end 72 on the side opposite the body 32 have a straight profile extending in the upward-downward direction, the overall length of the edge of each fastening recess 64 on the side opposite the body 32 and the overall length of the edge of each fixed end 72 on the side opposite the body 32 are longer. More precisely, the overall length of the edge of each fastening recess 64 on the side opposite the body 32 and the overall length of the edge of each fixed end 72 on the side opposite the body 32 are longer than the overall length of the edge of each fastening recess 24 on the side opposite the body 32, respectively.The total length of the edge of each fixed end 38 on the side opposite the body 32 of the first embodiment. This configuration increases the weld length resulting from laser welding the edges of the mounting recess 64 and the fixed end 72 on the side opposite the body 32, thus making it easier to achieve a high fastening strength between the end plate 60 and the retaining element 70. Although the battery module has been described according to the embodiments above, various design changes can be made without departing from the spirit and scope of the present invention. For example, the corrugated section 26 can be formed on part of the end (edge) of the fastening recess 24 on the side opposite the body 32 of the retaining element 30. In this case, a corrugated section 40 corresponding to the corrugated section 26 is formed at the fixed end 38. The rectangular projection and recess structure 66 can, for example, be formed at the entire end (edge) of the fastening recess 64 on the side opposite the body 32 of the retaining element 70. In this case, a rectangular projection and recess structure 74 is formed at the fixed end 72, corresponding to the rectangular projection and recess structure 66. The projection and recess shapes formed at the fastening recesses 24, 64 and the fixed ends 38, 72 may differ from the shapes of the wave-shaped section 26, 40 and the rectangular projection and recess structures 66, 74. For example, the projection and recess shape may have a single projection 68 or 76. The vehicle can be an electrified vehicle, which differs from a battery electric vehicle and includes an electric motor that uses electrical energy from battery module 10. For example, the vehicle can be a hybrid electric vehicle or a plug-in hybrid electric vehicle. QUOTES INCLUDED IN THE DESCRIPTION This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature JP 7 348 930 B [0002, 0003]

Claims

Battery module comprising: a pair of end plates spaced apart from each other in a predetermined direction; a plurality of battery cells stacked in the predetermined direction and arranged between the end plates; and a retaining element connected to the end plates to pull the end plates towards each other, the retaining element comprising: a body extending in the predetermined direction, and fixed ends extending in a direction intersecting the predetermined direction from both ends of the body, each facing a corresponding opposite end face of the end plates, each of the opposite end faces being an end face of a corresponding end plate in the predetermined direction; a mounting recess configured to receive the fixed end, provided at one end of the end face on a side closer to the body;at least part of an edge of the mounting recess on a side opposite the body and at least part of an edge of the fixed end on the side opposite the body have a protrusion and recess shape; and the edge of the mounting recess and the edge of the fixed end are welded together. Battery module according to claim 1, wherein the projection and recess shape is a wave-shaped section. Battery module according to claim 1, wherein the projection and recess shape comprises several rectangular projections and several rectangular recesses.

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