Battery module

Abstract

According to the battery module, an end plate comprises a first part, a second part and a cavity, the first part and the second part are oppositely arranged, the cavity is defined by the first part and the second part, a reinforcing structure is arranged in the cavity and connected with the first part and the second part, and the thickness of the first part is larger than that of the second part. When the battery modules are assembled, the thicker first parts of the end plates are arranged close to the battery modules, so that the rigidity of the sides, close to the battery modules, of the end plates is higher, the deformation of the end plates when the end plates bear the charge-discharge cycle expansion force of the battery cells can be reduced, on one hand, the battery cells can be effectively protected, and on the other hand, the weld joints of the outer frames of the modules can be prevented from being fractured; the reinforcing structure is arranged in the cavity and can provide certain buffering and absorbing effects on lateral extrusion force of the end plate, the extrusion force borne by the first part of the end plate is reduced, and then the battery cell can be better protected.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery, in particular to a battery module. BACKGROUND

[0002] With the continuous development of power battery technology, large capacity and large cell are continuously applied to battery module, and the mechanical structure level of the battery module is an important influencing factor of battery grouping and battery safety. Among them, the module end plate used for battery grouping not only bears the expansion force of the cell during charging and discharging cycle, but also meets the mechanical properties such as vibration, impact and extrusion to ensure the safety of the cell. CONTENT OF THE UTILITY MODEL

[0003] Therefore, the purpose of the present application is to provide a battery module to solve or partially solve the problems in the background art.

[0004] To achieve the above purpose, the first aspect of the present application provides an end plate for a battery module, comprising a first part, a second part arranged oppositely and a cavity formed by the first part and the second part, a reinforcing structure is arranged in the cavity, the reinforcing structure is connected with the first part and the second part, and the thickness of the first part is greater than the thickness of the second part.

[0005] Optionally, the difference between the thickness of the first part and the thickness of the second part is 1-3mm.

[0006] Optionally, a plurality of reinforcing structures are arranged in the cavity, and the plurality of reinforcing structures are arranged in parallel.

[0007] Optionally, the reinforcing structure is arranged obliquely, the angle between the oblique arrangement direction and the first plane is 10°-80°, and the first plane is perpendicular to the plane where the first part is located.

[0008] Optionally, the angle between the oblique arrangement direction and the first plane is 30°-60°.

[0009] Optionally, the reinforcing structure is an arc-shaped structure.

[0010] Optionally, the reinforcing structure comprises a first reinforcing part and a second reinforcing part, the first reinforcing part is an arc-shaped structure protruding towards the second part, and the second reinforcing part is an arc-shaped structure protruding towards the first part.

[0011] Optionally, the reinforcing structure comprises a first reinforcing part and a second reinforcing part, and the first reinforcing part and the second reinforcing part are both L-shaped structures or V-shaped structures.

[0012] Optionally, the first part has a thickness of 3-5 mm, and / or the second part has a thickness of 2-3 mm, and / or the reinforcing structure has a thickness of 1-3 mm.

[0013] The second aspect of the present application provides a battery module, comprising two oppositely arranged end plates according to any one of the first aspect and two oppositely arranged side plates, the two end plates and the two side plates jointly form a containing space, and a cell stack is contained in the containing space, and the first part of each end plate is arranged close to the cell stack.

[0014] As can be seen from the above, the battery module provided by the present application has the first part with a thickness greater than that of the second part, so that the rigidity of the first part is greater than that of the second part. When the battery module is assembled, the first part of the end plate with a relatively large thickness is arranged close to the battery module, so that the rigidity of the side of the end plate close to the battery module is relatively large, thereby reducing the deformation of the end plate when it bears the cell expansion force during charging and discharging cycles. On the one hand, the cell can be effectively protected, and on the other hand, the welding seam of the module outer frame can be avoided from breaking; the reinforcing structure is arranged in the cavity, which can provide a certain buffering and absorption effect for the lateral extrusion force of the end plate, thereby reducing the extrusion force on the first part of the end plate, and further better protecting the cell and effectively solving the mechanical safety problem caused by insufficient extrusion resistance of the end plate. BRIEF DESCRIPTION OF DRAWINGS

[0015] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0016] Figure 1 It is a first structure schematic diagram of the end plate of the embodiment of the present application;

[0017] Figure 2 It is a first cross-sectional schematic diagram of the end plate of the embodiment of the present application;

[0018] Figure 3 It is a second cross-sectional local enlarged schematic diagram of the end plate of the embodiment of the present application;

[0019] Figure 4 It is a third cross-sectional local enlarged schematic diagram of the end plate of the embodiment of the present application;

[0020] Figure 5 It is a fourth cross-sectional local enlarged schematic diagram of the end plate of the embodiment of the present application;

[0021] Figure 6A second structure diagram of an end plate of an embodiment of the present application;

[0022] Figure 7 A structure diagram of a battery module of an embodiment of the present application.

[0023] In the figure: 1, end plate; 11, first part; 12, second part; 13, cavity; 14, reinforcing structure; 141, first reinforcing part; 142, second reinforcing part; 143, first interval; 144, second interval; 145, transition part; 146, third interval; 147, fourth interval; 15, avoiding hole; 16, hoisting hole; 2, side plate; 3, cell stack. DETAILED DESCRIPTION

[0024] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application is further described in detail below in combination with specific embodiments and with reference to the accompanying drawings.

[0025] It should be noted that, unless otherwise defined, the technical terms or scientific terms used in the embodiments of the present application should be understood as the usual meaning understood by those skilled in the art to which the present application belongs. The terms "first", "second" and the like used in the present application do not represent any order, number or importance, but are only used to distinguish different components. The terms "include" or "contain" and the like mean that the elements or objects before the terms cover the elements or objects listed after the terms and their equivalents, without excluding other elements or objects. The terms "connect" or "connected" and the like are not limited to physical or mechanical connection, but can include electrical connection, whether direct or indirect. The terms "up", "down", "left", "right" and the like are only used to represent relative positional relationship, which may change accordingly when the absolute position of the described object changes.

[0026] With the continuous development of power battery technology, large-capacity and large-cell are continuously applied to battery modules, and the mechanical structure level of the battery module is an important influencing factor affecting the battery grouping and the safety of the battery. Among them, the module end plate used for battery grouping not only bears the expansion force of the cell during charging and discharging cycle, but also meets the mechanical properties such as vibration, impact and extrusion to ensure the safety of the cell.

[0027] In the related art, when the module end plate bears the expansion force of the cell during charging and discharging cycle, it is easy to cause excessive deformation of the module end plate due to insufficient rigidity, so that the welding seam of the module outer frame is broken. Moreover, when the end plate is extruded, the end plate is cracked at a small extrusion displacement due to insufficient bending resistance, which cannot provide better protection for the cell.

[0028] Therefore, how to provide a module end plate with rigidity and bending resistance is a problem to be solved.

[0029] Based on this, the application provides an end plate for a battery module. Figure 1 A first structural diagram of the end plate is shown. As shown in the figure, Figure 1 The end plate 1 includes oppositely arranged first and second parts 11 and 12, and a cavity 13 formed by the first and second parts 11 and 12, and a reinforcing structure 14 is arranged in the cavity 13. The reinforcing structure 14 is connected to the first and second parts 11 and 12. The thickness of the first part 11 is greater than that of the second part 12.

[0030] Specifically, the thickness of the first part 11 is greater than that of the second part 12, so that the rigidity of the first part 11 is greater than that of the second part 12. When the battery module is assembled, the end plate 1 covers both sides of the battery module, and the first part 11 of the end plate 1 with a greater thickness is arranged close to the battery module, so that the rigidity of the side of the end plate 1 close to the battery module is greater, thereby reducing the deformation of the end plate 1 when bearing the expansion force of the battery cell during charging and discharging cycles. On the one hand, it can effectively protect the battery cell, and on the other hand, it can avoid the rupture of the welding seam of the module outer frame.

[0031] In specific implementation, the first part 11 can be a side plate on the side of the end plate 1 close to the battery module, and the second part 12 can be a side plate on the side of the end plate 1 away from the battery module. The first and second parts 11 and 12 are connected by the top and bottom plates of the end plate 1, so that the first and second parts 11 and 12 form the cavity 13.

[0032] When the end plate 1 bears lateral extrusion, the cavity 13 can provide a certain buffer space for lateral extrusion, thereby improving the bending resistance and extrusion resistance of the end plate 1, increasing the extrusion failure displacement of the end plate 1, and thereby making the end plate 1 better protect the battery cell.

[0033] The reinforcing structure 14 is connected to the first and second parts 11 and 12. The reinforcing structure 14 can be one or more, which is not limited here.

[0034] The reinforcing structure 14 can increase the cross-sectional rigidity of the end plate 1 on the one hand, so as to avoid too low cross-sectional rigidity caused by the arrangement of the cavity 13. On the other hand, the reinforcing structure 14 can provide a certain buffer and absorption effect for the lateral extrusion force of the end plate 1, so as to reduce the extrusion force on the inside of the end plate 1, thereby better protecting the battery cell and effectively solving the mechanical safety problem caused by insufficient extrusion resistance of the end plate 1.

[0035] In some embodiments, the difference between the thickness of the first part 11 and the thickness of the second part 12 is 1-3 mm.

[0036] Specifically, when the difference between the thickness of the first part 11 and the thickness of the second part 12 is 1-3 mm, the thicknesses of the first part 11 and the second part 12 are moderate, the rigidity of the first part 11 is large enough, the deformation of the end plate 1 when bearing the expansion force of the battery during charging and discharging cycles can be effectively reduced, and the battery can be better protected, and the rigidity and thickness of the second part 12 can also meet the requirements of the actual welding process.

[0037] If the difference between the thickness of the first part 11 and the thickness of the second part 12 is too large, the rigidity of the first part 11 is large enough, but the rigidity and thickness of the second part 12 may not meet the requirements of the actual welding process, so it cannot be welded; if the difference between the thickness of the first part 11 and the thickness of the second part 12 is too small, the rigidity of the first part 11 is not improved enough, and the deformation of the end plate 1 when bearing the expansion force of the battery during charging and discharging cycles cannot be effectively reduced, so that the end plate 1 is deformed too much when bearing the expansion force of the battery during charging and discharging cycles, which leads to the fracture of the welding seam of the module outer frame, and thus the battery cannot be effectively protected.

[0038] Exemplarily, the difference between the thickness of the first part 11 and the thickness of the second part 12 can be 1 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm, etc.

[0039] The thickness of the first part 11 can be 3-5 mm, and exemplarily, the thickness of the first part 11 can be 3 mm, 3.5 mm, 4 mm, 4.5 mm, 5 mm, etc.

[0040] The thickness of the second part 12 can be 2-3 mm, and exemplarily, the thickness of the second part 12 can be 2 mm, 2.5 mm, 3 mm, etc.

[0041] The thickness direction of the first part 11 and the second part 12 is the horizontal direction, that is, the direction perpendicular to the plane where the first part 11 or the second part 12 is located.

[0042] In this application, by limiting the thickness of the first part 11 and the second part 12, the rigidity of the first part 11 is ensured to effectively reduce the deformation of the end plate 1 when bearing the expansion force of the battery during charging and discharging cycles, and the battery is better protected, and the rigidity and thickness of the second part 12 can also meet the requirements of the actual welding process.

[0043] Figure 2 A first cross-sectional view of the end plate 1 is shown. As shown in Figure 2 In some embodiments, a plurality of reinforcing structures 14 are arranged in the cavity 13, and the plurality of reinforcing structures 14 are arranged in parallel.

[0044] Specifically, multiple reinforcing structures 14 are arranged in parallel, so that the multiple reinforcing structures 14 can buffer and absorb the lateral compressive force of the end plate 1 along the same direction, thereby improving the degree of buffering and absorption of the lateral compressive force, and further improving the compressive and bending resistance of the end plate 1.

[0045] In some embodiments, see continue to see Figure 1 and Figure 2 The reinforcing structure 14 is inclined, and the angle between the inclined direction and the first plane is 10° to 80°. The first plane (e.g.) Figure 1 The plane shown in B) and the plane where the first part 11 is located (as shown in B) Figure 1 The plane shown in A is perpendicular to the plane shown in the middle.

[0046] Specifically, the reinforcing structure 14 can be an inclined plate-like structure.

[0047] Compared to the reinforcing ribs that are perpendicular to the first part 11 or cross-shaped in the prior art, the inclined reinforcing structure 14 can deform and displace to a certain extent when subjected to lateral compression, thereby increasing the buffering effect on the extrusion force and effectively improving the extrusion resistance of the end plate 1 without reducing the elastic stiffness of the end plate 1.

[0048] The angle between the tilted direction and the first plane is 10° to 80°. For example, the angle between the tilted direction and the first plane can be 10°, 20°, 30°, 40°, 50°, 60°, 70°, 80°, etc.

[0049] Experiments have shown that when the included angle is between 10° and 80°, it can increase the displacement of the end plate 1 in the event of compression failure, thereby improving the compression resistance of the end plate 1.

[0050] Furthermore, the angle between the inclined setting direction and the first plane is 30° to 60°. Experiments have verified that when the angle is between 30° and 60°, it can significantly increase the compression failure displacement of the end plate 1, thereby significantly improving the compression resistance of the end plate 1.

[0051] In some embodiments, the thickness of the reinforcing structure 14 is 1 to 3 mm. Exemplarily, the thickness of the reinforcing structure 14 can be 1 mm, 2 mm, 3 mm, etc.

[0052] Specifically, when the thickness of the reinforcing structure 14 is 1-3 mm, the rigidity of the entire end plate 1 can not be reduced, and the extrusion resistance of the end plate 1 can be effectively improved. When the thickness of the reinforcing structure 14 is too thin, the cross-sectional rigidity of the end plate 1 is poor. When the thickness of the reinforcing structure 14 is too thick, the rigidity of the reinforcing structure 14 is good, but the bending resistance is poor, the extrusion failure displacement is small, and the end plate 1 is prone to fracture when subjected to lateral extrusion.

[0053] The thickness direction of the reinforcing structure 14 is the vertical direction (i.e., the direction parallel to the plane in which the first part 11 or the second part 11 is located).

[0054] Figure 3 A local enlarged schematic view of a second cross section of the end plate 1 is shown. As shown in Figure 3 In some embodiments, the reinforcing structure 14 is an arc-shaped structure. Compared with a plate-shaped structure arranged obliquely, the arc-shaped structure has better bending resistance, and can better improve the bending resistance of the end plate 1.

[0055] In some embodiments, continuing to refer to Figure 3 As shown in, the reinforcing structure 14 includes a first reinforcing part 141 and a second reinforcing part 142, the first reinforcing part 141 is connected with the first part 11 and has a first interval 143 with the second part 12, and the second reinforcing part 142 is connected with the second part 12 and has a second interval 144 with the first part 11. In this way, when the end plate 1 is subjected to lateral extrusion, the first interval 143 can provide a buffer space for the movement and deformation of the first reinforcing part 141, and the second interval 144 can provide a buffer space for the movement and deformation of the second reinforcing part 142, thereby increasing the buffering and absorbing capacity of the reinforcing structure 14 to lateral extrusion force, and further improving the extrusion resistance and bending resistance of the end plate 1.

[0056] In some embodiments, continuing to refer to Figure 3 As shown in, the first reinforcing part 141 is an arc-shaped structure protruding in the direction of the second part 12, and the second reinforcing part 142 is an arc-shaped structure protruding in the direction of the first part 11.

[0057] Specifically, the first reinforcing part 141 and the second reinforcing part 142 are both arc-shaped structures, and the arc-shaped structures have better bending resistance and deformation capacity, which can better improve the bending resistance of the end plate 1. In addition, the protruding directions of the first reinforcing part 141 and the second reinforcing part 142 are opposite, and the staggered arrangement of the first reinforcing part 141 and the second reinforcing part 142 can provide more buffer space for the lateral extrusion force, thereby increasing the extrusion failure displacement of the end plate 1.

[0058] Figure 4 A local enlarged schematic view of a third cross section of the end plate 1 is shown. As shown in Figure 4As shown, in some embodiments, the reinforcing structure 14 comprises a first reinforcing portion 141 and a second reinforcing portion 142, both of which are L-shaped structures, and the first reinforcing portion 141 and the second reinforcing portion 142 of the two L-shaped structures are connected by a transition portion 145.

[0059] Specifically, one end of the first reinforcing portion 141 of the L-shaped structure is connected with the first portion 11, and the other end is connected with the transition portion 145. Similarly, one end of the second reinforcing portion 142 of the L-shaped structure is connected with the second portion 12, and the other end is connected with the transition portion 145. In this way, the first reinforcing portion 141 and the transition portion 145 form a third spacing 146, and the second reinforcing portion 142 and the transition portion 145 form a fourth spacing 147. The existence of the third spacing 146 and the fourth spacing 147 further provides more space for the movement and deformation of the first reinforcing portion 141 and the second reinforcing portion 142, and thus can better buffer and absorb the lateral extrusion force, so as to further increase the bending resistance and extrusion resistance of the end plate 1.

[0060] Figure 5 A partial enlarged schematic view of a fourth cross section of the end plate 1 is shown. As Figure 5 As shown, in some embodiments, the reinforcing structure 14 comprises a first reinforcing portion 141 and a second reinforcing portion 142, both of which are V-shaped structures.

[0061] Specifically, the first reinforcing portion 141 and the second reinforcing portion 142 of the two V-shaped structures are connected to form a reinforcing structure 14 of Z-shaped structure, so that the first reinforcing portion 141 and the second reinforcing portion 142 are staggered, which can provide more space for the movement and deformation of the first reinforcing portion 141 and the second reinforcing portion 142, and thus can better buffer and absorb the lateral extrusion force, thereby increasing the extrusion resistance of the end plate 1.

[0062] Further, the included angle of the V-shaped structure can be 45°-135°. For example, the included angle of the V-shaped structure can be 45°, 55°, 65°, 75°, 85°, 95°, 105°, 115°, 125°, 135°, etc.

[0063] When the included angle of the V-shaped structure is within the range, the bending angle of the V-shaped structure is moderate, which can provide effective buffer space to increase the extrusion resistance of the end plate 1. When the included angle of the V-shaped structure is too small, the distance between the first reinforcing part 141 and the second reinforcing part 142 is too close, so that when extruded, the deformable space between the first reinforcing part 141 and the second reinforcing part 142 is small, which may interfere and reduce the buffering effect; when the included angle of the V-shaped structure is too large, the first interval 143 between the first reinforcing part 141 and the second part 12 and the second interval 144 between the second reinforcing part 142 and the first part 11 are too small, so that when extruded laterally, the buffering space of the first reinforcing part 141 and the second reinforcing part 142 is too small, which cannot effectively buffer the lateral pressure, and thus cannot effectively improve the extrusion resistance and bending resistance of the end plate 1.

[0064] Figure 6 A second structural diagram of the end plate 1 is shown. As shown in Figure 6 The end plate 1 also includes a clearance hole 15 and a lifting hole 16 to facilitate the assembly process of the actual battery module.

[0065] In this application, in order to better test the extrusion resistance of the end plate 1, a series of tests are conducted on end plates 1 with different parameters, and the test data is shown in Table 1 below.

[0066] Table 1 Test Data List

[0067]

[0068]

[0069] In Table 1 above, A is the thickness of the first part 11, B is the thickness of the second part 12, C is the thickness of the reinforcing structure 14, D is the included angle of the reinforcing structure 14 and the horizontal plane, E is the bending resistance coefficient of the cross section of the end plate 1, and F is the three-point bending extrusion failure displacement of the end plate 1.

[0070] From the comparison of Test Examples 1-3 and Test Examples 4-5, it can be seen that under the condition that other parameters remain unchanged, increasing the thickness of the first part 11 can increase the bending resistance coefficient of the cross section of the end plate 1 and significantly increase the bending extrusion failure displacement of the end plate 1, which indicates that increasing the thickness of the first part 11 can improve the stiffness and bending resistance of the end plate 1.

[0071] From the comparison of Test Examples 6-7, it can be seen that under the condition that other parameters remain unchanged, increasing the thickness of the reinforcing structure 14 will increase the stiffness of the end plate 1, but the bending extrusion failure displacement will decrease, which indicates that increasing the thickness of the reinforcing structure 14 will reduce the bending resistance of the end plate 1.

[0072] As can be known from the comparison of test examples 8-9, under the condition that other parameters are unchanged, reducing the angle between the reinforcing structure 14 and the horizontal plane can increase the bending extrusion failure displacement, indicating that reducing the angle between the reinforcing structure 14 and the horizontal plane can improve the bending resistance of the end plate 1.

[0073] As can be known from the above data, compared with the end plate 1 whose first part 11 and second part 12 have the same thickness, the extrusion resistance of the end plate 1 whose first part 11 has a greater thickness than the second part 12 is improved by 50%-200%, ensuring that the end plate 1 does not break during extrusion and reducing the deformation of the module expansion force, effectively protecting the battery cell.

[0074] The end plate 1 of the present application can greatly improve the extrusion resistance of the end plate 1 without reducing the stiffness of the end plate 1 by improving the structure of the end plate 1, which can effectively solve the mechanical safety problem caused by the insufficient structural strength of the module battery cell cycle expansion force and the module extrusion.

[0075] The second aspect of the present application provides a battery module, Figure 7 The structure of the battery module is shown. As shown in the figure, Figure 7 The battery module includes two oppositely arranged end plates 1 and two oppositely arranged side plates 2, and the two end plates 1 and the two side plates 2 form a containing space, and the containing space contains a battery cell stack 3. The first part 11 of each end plate 1 is arranged close to the battery cell stack 3.

[0076] Specifically, since the thickness of the first part 11 is greater than the thickness of the second part 12, the stiffness of the first part 11 is greater than the stiffness of the second part 12. The first part 11 of each end plate 1 with a greater thickness is arranged close to the battery cell stack 3, so that the stiffness of the end plate 1 close to one side of the battery module is greater, thereby reducing the deformation of the end plate 1 when bearing the battery cell charge and discharge cycle expansion force, which can effectively protect the battery cell on the one hand and avoid the rupture of the weld of the module outer frame on the other hand.

[0077] It should be understood by those skilled in the art that the discussion of any of the above embodiments is only exemplary and is not intended to limit the scope of the present application (including the claims) to these examples; the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other changes of the different aspects of the present application as described above. In order to be brief, they are not provided in detail.

[0078] The embodiments of the present application are intended to cover all such alternatives, modifications and variations falling within the broad scope of the appended claims. Therefore, any omission, modification, equivalent replacement, improvement, etc. made in the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A battery module, characterized by, The application relates to a battery cell stack container, which comprises two oppositely arranged end plates and two oppositely arranged side plates, the two end plates and the two side plates jointly enclosing a containing space, and the containing space contains a battery cell stack, the end plate comprises a first part, a second part and a cavity formed by the first part and the second part, the cavity is provided with a reinforcing structure, the reinforcing structure is connected with the first part and the second part, the thickness of the first part is greater than the thickness of the second part, and the first part of each end plate is arranged close to the battery cell stack.

2. The battery module of claim 1, wherein, The difference between the thickness of the first part and the thickness of the second part is 1-3 mm.

3. The battery module of claim 1, wherein, A plurality of reinforcing structures are arranged in the cavity in a spaced mode, and the reinforcing structures are arranged in parallel.

4. The battery module of claim 3, wherein, The reinforcing structure is arranged in an inclined mode, the included angle between the direction of the inclined arrangement and a first plane is 10-80 degrees, and the first plane is perpendicular to the plane where the first part is located.

5. The battery module of claim 4, wherein, The included angle between the direction of the inclined arrangement and the first plane is 30-60 degrees.

6. The battery module of claim 1, wherein, The reinforcing structure is an arc-shaped structure.

7. The battery module of claim 1, wherein, The reinforcing structure comprises a first reinforcing part and a second reinforcing part, the first reinforcing part is an arc-shaped structure which protrudes towards the second part, and the second reinforcing part is an arc-shaped structure which protrudes towards the first part.

8. The battery module of claim 1, wherein, The reinforcing structure comprises a first reinforcing part and a second reinforcing part, and the first reinforcing part and the second reinforcing part are both L-shaped structures or V-shaped structures.

9. The battery module of claim 1, wherein, The thickness of the first part is 3-5 mm, the thickness of the second part is 2-3 mm, and / or the thickness of the reinforcing structure is 1-3 mm.

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