Manufacturing method for rectangular battery cans

Abstract

To efficiently manufacture a prismatic battery can without impairing product precision.SOLUTION: A manufacturing method of a prismatic battery includes a first step of molding an intermediate cup 2 including a temporary bottom surface 2b formed by drawing and having an oval or rectangular shape and a wider area than the bottom surface of the prismatic battery can, an inclined wall portion 2a that continues obliquely to both end sides in the long axis direction of the temporary bottom surface 2b, a short side wall portion 2c continuous via the inclined wall portion 2a, and a long-side side wall portion 2d continuous to both ends of the temporary bottom surface 2b in the short-axis direction via a curved portion 2e, a second step of deforming the intermediate cup such that a boundary portion between the temporary bottom surface 2b and the inclined wall portion 2a gradually approaches the end of the bottom surface of the prismatic battery can in the long side direction, and the width of the temporary bottom surface in the direction of the short axis is close to the width of the bottom surface of the prismatic battery can in the direction of the short side, and a third step of performing ironing or drawing / ironing to reduce the plate thickness of the short side wall portion 2c and the long side wall portion 2d.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a thin rectangular can used as an outer case for a battery such as a lithium ion secondary battery. [Background technology]

[0002] This type of rectangular can is widely used as a battery case because of its high loading efficiency, excellent rigidity, and excellent heat dissipation, but due to the demand for low cost and high productivity, it is usually manufactured by drawing and ironing using metal plates such as aluminum. However, since the shape is a rectangle with a large difference between the lengths of the short side and the long side and a deep shape, it is difficult to uniformly change the wall thickness around the entire circumference compared to the case of processing a can with a uniform shape such as a cylindrical shape. In particular, the bent part, which is the boundary between the wide long side wall part and the narrow short side wall part, becomes a resistance to the circumferential plastic flow accompanying the drawing and ironing processes, and because of this, wrinkles, cracks, breakage, etc. are likely to occur when the amount of drawing or ironing is increased. Therefore, in general, the amount of processing per time must be reduced, which increases the number of steps and slows down the forming speed, resulting in a problem of impaired productivity.

[0003] Conventionally, attempts have been made to solve such problems, and examples of such attempts are described in Patent Document 1 and Patent Document 2. The invention described in Patent Document 1 is a method for achieving high energy density and improved pressure resistance by thinning and reducing weight, in which a metal plate battery can material is deep-drawn to form a first intermediate cup body having a nearly elliptical shape, and the first intermediate cup body is continuously redrawn in multiple stages to form a second intermediate cup body having a nearly elliptical shape with a small minor axis / major axis ratio, and then a DI process is performed in which drawing and ironing are continuously performed in one go to form a shape having a nearly rectangular cross section and a short side plate part having a thicker thickness than a long side plate part. When forming a rectangular can having a nearly rectangular cross section, the long side plate part and the short side plate part have a significantly different shape or area, and the corners of the long side plate part and the short side plate part are bent at almost right angles, which causes the flow of the material to be non-uniform. As a result, cracks, breaks, and the like are easily generated in the short side plate part having a small area, and stable processing is difficult. In contrast to this, in the method described in Patent Document 1, the short side plate portions are formed to be thick.

[0004] The invention described in Patent Document 2 is a method of forming a slanted wall portion that is narrowed (reduced in diameter) from the side wall portion toward the bottom surface on the entire circumference (four sides) of a rectangular bottom surface in each of the first step of drawing and the second step of redrawing. That is, in the first step, a drawing process is performed on an aluminum alloy plate as a raw material to form a first rectangular cylinder having a slanted wall portion that is narrowed in diameter from the side wall portion toward the bottom surface on all four sides, and in the second step, the first rectangular cylinder is redrawn to form a second rectangular cylinder having a smaller outer shape and a higher height while leaving the slanted wall portion. The second rectangular cylinder is then ironed to reduce the thickness of the side wall portion to 60% or less of the original plate thickness, and the slanted wall portion is processed into a side wall portion that is perpendicular to the bottom surface to form a rectangular can. In the method described in Patent Document 2, during the drawing processes in the first and second steps, inclined wall portions are provided between the four sides of the bottom portion and each of the side wall portions, so that the deformation angle between the bottom portion and the side wall portions is smaller than a right angle. As a result, the deformation resistance of the material is reduced, resulting in high formability and ensuring plate thickness accuracy and shape accuracy. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 4119612 [Patent Document 2] Patent No. 6193794 Summary of the Invention [Problem to be solved by the invention]

[0006] When a cylindrical body is formed from a flat blank by drawing, the portion that will become the peripheral wall or side wall is shrink-formed. In the drawing of the first and second steps of the method described in Patent Document 1, material flows in the peripheral direction (horizontal direction perpendicular to the drawing direction) due to such shrink-formation. However, since there is a large difference in area between the side wall on the long side and the side wall on the short side, and the boundary between these side walls is bent, material flow beyond the bent portion is difficult to occur, and therefore not only does the side wall on the short side become thicker, but the side wall on the long side also gathers material on the short side (near the bent portion) and becomes thicker. As the drawing progresses and the bent portion becomes more distinct (the bending angle approaches a right angle), the material flow is further restricted, so the material near the bent portion increases and becomes thicker. The resistance (deformation resistance) during redrawing and ironing increases as the thickness of the processed portion increases, so that in the portion where the material gathers and becomes thick, such as near the bent portion and the side wall on the short side, as described above, a large deformation resistance occurs. In addition, the deformation resistance is the highest immediately before the material flows or deformation occurs. Therefore, in the method described in Patent Document 1, even if the side wall portion on the short side can be formed thick to improve the pressure resistance, the forming load during redrawing and ironing increases, and the corner portion between the bottom and the side wall portion and the side wall portion may be pulled strongly and become thin, or cracks or breaks may occur. In other words, in the method described in Patent Document 1, in order to improve productivity by increasing the amount of drawing or ironing to reduce the number of steps, it is necessary to reduce the deformation resistance of the bent portion and the side wall portion on the short side, and there is still room for improvement.

[0007] On the other hand, in the method described in Patent Document 2, since the inclined wall portion is formed on the entire circumference (four sides) of the bottom surface portion during the drawing process, the deformation resistance of that portion can be reduced, but since the drawing process is performed on four sides with clear bends (edges) at the boundaries, wrinkles may occur on those inclined wall portions, or traces of these may remain on the side walls of the product, which may cause the quality to deteriorate. In addition, the bottom surface portion surrounded by the inclined wall portion in the method described in Patent Document 2 is the portion that will become the bottom surface portion of the product, and the square tube during the forming process has a shape that spreads upward from the bottom surface portion with a narrow area. The square tube, which is the intermediate product, is successively held by fingers and sent to the next process, and is gradually processed into the shape of the product. In that case, it is necessary for the square tube to stand independently stably when the fingers are removed, but the square tube formed by the method described in Patent Document 2 has a narrow area of ​​the bottom surface portion and a shape that spreads upward from the bottom surface portion, which is an unstable shape for standing independently. Therefore, there is a problem that jamming is likely to occur, such as falling over, tilting, or shifting from the processing position during the continuous processing process.

[0008] The present invention has been made in response to the above-mentioned conventional technical problems, and has an object to provide a method for efficiently manufacturing rectangular battery cans without compromising product precision. [Means for solving the problem]

[0009] In order to achieve the above object, the present invention relates to a method for manufacturing a prismatic battery can in which an aluminum alloy blank is subjected to drawing and ironing processes, and the cross section of the prismatic battery can when cut in a plane parallel to a bottom surface has a rectangular shape, the method comprising the steps of drawing the blank to form a temporary bottom surface that is oval or rectangular and has a larger area than the bottom surface of the prismatic battery can, inclined wall portions that are continuous with and inclined obliquely to the temporary bottom surface on both ends in the major axis direction of the temporary bottom surface, short side wall portions that are continuous with the inclined wall portions, and long side wall portions that are continuous with both ends in the minor axis direction of the temporary bottom surface via curved portions. the first step of forming an intermediate cup having a temporary bottom surface and an inclined wall portion; a second step of deforming the intermediate cup by drawing or drawing and ironing so that a boundary portion between the temporary bottom surface and the inclined wall portion gradually approaches an end portion in the long side direction of the bottom surface of the rectangular battery can and a width of the temporary bottom surface in the short axis direction approaches a width of the bottom surface of the rectangular battery can in the short side direction; and a third step of performing ironing or drawing and ironing after the second step to reduce the plate thickness of the short side side wall portions and the long side side wall portions.

[0010] Furthermore, in the present invention, the inclined wall portion of the intermediate cup is a surface that is inclined at a predetermined angle starting from a position closer to the short side wall portion than the end portion in the long side direction of the bottom surface of the rectangular battery can, the position of the starting point being a position closer to the short side wall portion than the end portion in the long side direction of the bottom surface of the rectangular battery can and closer to the end portion in the long side direction of the bottom surface of the rectangular battery can than the central portion between the end portion in the long side direction of the bottom surface of the rectangular battery can and the short side wall portion, the predetermined angle being 25° or more and 45° or less, and the inclined wall portion may be continuous with the short side wall portion via a smoothly curved surface. Effect of the Invention

[0011] According to the present invention, when the blank is drawn in the first step to form the intermediate cup, an inclined wall portion is formed at both ends in the long axis direction of the elliptical or oval cross-sectional shape, and a curved portion is formed at both ends (both sides) in the short axis direction to connect to the side wall portion on the long side. Therefore, since there is almost no part that is bent at a right angle or an acute angle smaller than that, that is, a part that inhibits the material flow or increases the deformation resistance, on the entire circumference of the temporary bottom surface, which has an area larger than the bottom surface of the product, processing can be performed without causing wrinkles or cracks. Since the inclined wall portion is only formed on both ends in the long side direction, and the temporary bottom surface has an area larger than the bottom surface of the product, it is possible to stably keep the intermediate cup standing even when it is sent to each process in succession when the intermediate cup is processed continuously, and therefore jamming during continuous processing can be avoided and productivity can be improved.

[0012] The inclined wall portion in the present invention is a surface that inclines obliquely upward from both ends of the long side of the temporary bottom surface, and its starting point is set at a position somewhat closer to the short side wall portion than the ends of the long side of the bottom surface of the rectangular battery can in the long side direction, so that when the intermediate cup is drawn or drawn and ironed in the second step, processing or deformation begins from the long side wall portion that is continuous with the temporary bottom surface via the curved surface portion, and the inclined wall portion and short side wall portion begin to be processed and deformed later. Therefore, at the beginning of processing or deformation of the long side wall portion, which has a larger area than the short side wall portion, the inclined wall portion and short side wall portion located on the short side are not actively constrained, and the constraining force is weak. As a result, material flow is likely to occur at the corners (or bends) between the long and short side walls and at the short side walls, making it possible to reduce the load required for molding (initial molding force).In addition, even if the molding speed is increased, molding can be performed without causing cracks or breaks, and the amount of processing per run can be increased.

[0013] In addition, a die for drawing or drawing and ironing has an entry section for gradually introducing a workpiece, followed by a land section for defining the outer dimensions or outer shape of the workpiece. In the present invention, the inclination angle of the inclined wall section is set to 25° or more, so that when the temporary bottom surface or the curved section on the short side starts to contact the entry section of the die, most of the inclined wall section (the upper section) does not contact the die. Therefore, the processing of the inclined wall surface and the short side wall section can be reliably delayed with respect to the processing of the long side wall section and its curved section. In addition, by providing the inclined wall section, the height of the short side wall section is increased by the amount of the inclined wall section, but in the present invention, the inclination angle is set to 45° or less, so that the height of the short side wall section can be approximated to the height of the long side wall section. As a result, the overall shape of the intermediate cup is a well-balanced shape that is stable and self-supporting, and jamming caused by the intermediate cup falling over or becoming tilted during the process of transportation between successive processing steps can be suppressed. In addition, the difference in height between the long side wall portion and the short side wall portion of the intermediate cup (ear height difference) is reduced, and the portion that protrudes significantly upward can be eliminated. Therefore, the installation height of the strip that pulls the intermediate cup out of the punch does not need to be particularly high so that the intermediate cup does not get caught during the conveyance process. In other words, since the intermediate cup can be pulled out of the punch at a low position and dropped, the cause of jamming, such as the intermediate cup falling over, can be eliminated or suppressed. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a perspective view showing an example of a prismatic battery can to which the present invention is applied. [Diagram 2] FIG. 2 is a process explanatory diagram illustrating an example of the method of the present invention, showing the shape of an intermediate cup at each process. [Diagram 3] FIG. 2 is a perspective view showing an example of an intermediate cup formed in a first step. [Figure 4] FIG. 1A is a longitudinal sectional view of the intermediate cup, and FIG. 1B is a longitudinal sectional view showing an example of an intermediate cup in which no inclined wall portion is formed. [Diagram 5]5A and 5B are partial cross-sectional views showing a schematic diagram of a state in which the intermediate cup is drawn and ironed in the second step, where (a) is a plan cross-sectional view and (b) is a longitudinal cross-sectional view. [Figure 6] 5A and 5B are partial cross-sectional views showing a state in which an intermediate cup not provided with an inclined wall portion is drawn and ironed in a second step, where (a) is a plan cross-sectional view and (b) is a longitudinal cross-sectional view. [Figure 7] FIG. 4 is a diagram illustrating a change in an inclined wall portion due to drawing and ironing. [Figure 8] FIG. 11 is a partial explanatory diagram showing the change in shape of the intermediate cup in steps f1 to f6 as viewed from the bottom side. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] First, a rectangular battery can manufactured in accordance with an embodiment of the present invention will be described, with an example shown in Figure 1. As shown in Figure 1, a rectangular battery can (hereinafter simply referred to as a rectangular can) 1 has a rectangular cross section when cut along a plane parallel to the bottom surface 1a, with the width of the side wall 1b on the long side being significantly greater than the width of the side wall 1c on the short side, and is a can with a bottom in which the bottom surface 1a and the side walls 1b, 1c are integrally formed. This rectangular can 1 is manufactured by drawing, ironing, etc., using a metal plate (specifically an aluminum alloy plate) as the material for the purpose of overall robustness, airtightness, and low cost.

[0016] An example of the change in shape of the rectangular can 1 shown in Fig. 1 during the process of manufacturing it by the method of the present invention is shown in Fig. 2. First, in the first step, a blank is drawn to form an intermediate cup 2. Since the rectangular can 1 to be manufactured has a rectangular cross section as shown in Fig. 1, the blank is generally made elliptical in order to minimize the amount of scrap. The drawing in this first step may be a so-called punching process in which the intermediate cup 2 is dropped below a die (not shown), or it may be a process in which the intermediate cup 2 is raised together with a punch (not shown) and returned to the pass line for the next process.

[0017] An example of a specific configuration of the intermediate cup 2 is shown in Fig. 3 and Fig. 4. The part corresponding to the bottom surface has an elliptical or oval shape in a plan view seen from above, and as shown in the longitudinal cross section of Fig. 4(a), both ends in the long axis direction are formed with inclined wall parts 2a inclined obliquely upward at a predetermined angle. The part between the inclined wall parts 2a is the temporary bottom surface 2b in the embodiment of the present invention, and therefore the temporary bottom surface 2b has an oval or rectangular shape. The length l of the temporary bottom surface 2b in the long axis direction (length in the long side direction) is longer than the length L of the long side direction of the rectangular can 1, which is the product, and the width w of the temporary bottom surface in the short axis direction (length in the short side direction) is larger than the width W of the rectangular can 1. In other words, the area of ​​the temporary bottom surface 2b is wider than the bottom surface 1a of the rectangular can 1, and is wide enough for the intermediate cup 2 to stand stably by itself. In addition, the peripheral edge (contour) of the tip side (opposite the temporary bottom surface 2b) of the inclined wall portion 2a is curved in an elliptical shape, and a side wall portion rises from that portion almost perpendicular to the temporary bottom surface 2b, and this side wall portion becomes the short side wall portion 2c. Therefore, the short side wall portion 2c is a wall-like portion curved so as to surround the tip side of the temporary bottom surface 2b by approximately 180° in the circumferential direction, and since the inclined wall portion 2a is interposed between this short side wall portion 2c and the temporary bottom surface 2b, the bending angle (or curvature angle) between these three is an angle (obtuse angle) that is gentler than a right angle. The tip peripheral portion of the inclined wall portion 2a is continuous with the short side wall portion 2c via a smooth curved surface.

[0018] On the other hand, the side wall along the long side of the temporary bottom 2b rises almost perpendicularly to the temporary bottom 2b, and a smoothly curved portion 2e is formed between the temporary bottom 2b and the side wall 2d on the long side (both ends in the short axis direction). The curved portion 2e extends somewhat to the side of the inclined wall 2a. The radius of curvature of the curved portion 2e is larger than the radius of curvature of the portion between the bottom 1a of the rectangular can 1 as a product and the side wall 1b on the long side, and the long side wall 2d is gently and smoothly connected to the temporary bottom 2b.

[0019] Here, the inclined wall portion 2a will be further explained. The inclined wall portion 2a is a portion that is formed so as to be inclined contiguous with the temporary bottom surface 2b, at a portion that will become the short side wall portion 1c of the rectangular can 1 as a finished product, and at a portion that will become both ends in the long side direction of the long side wall portion 1b. This inclined wall portion 2a extends obliquely upward toward the short side wall portion 2c, starting from the boundary portion with the temporary bottom surface 2b. As shown in FIG. 4(a), the starting point 3 is closer to the short side wall portion 2c than the end portion in the long side direction of the rectangular can 1 as a finished product, and is closer to the end portion than the center between said end portion and the short side wall portion 2c. Expressing this in terms of the dimensions of each portion, if the distance between the short side wall portions 2c (the major axis of the ellipse that is the shape of the intermediate cup 2) is D1, then (D1-L) / 2 is the distance between the end of the rectangular can 1 as a product in the long side direction and the short side wall portion 2c, and the dimension Δl to the center of the distance is Δl={(D1-L) / 2} / 2 The starting point 3 of the inclined wall portion 2a is set at a position equal to or less than Δl (>0) from the end in the long side direction of the finished rectangular can 1. Since the area between the temporary bottom surface 2b and the inclined wall portion 2a is smoothly curved so that there is no edge, the starting point 3 may be the end of the smoothly curved portion on the temporary bottom surface 2b side, or may be the center of the smoothly curved portion.

[0020] As shown in Fig. 4(a), the inclined wall portion 2a has a surface (plane) that can be expressed as a straight line in a vertical cross section in the intermediate portion between the starting point 3 and the short side wall portion 2c, but this is not limited to this, and the intermediate portion may be curved to some extent. Therefore, the inclined wall portion 2a can be said to be a wall portion in which the radius of curvature near the starting point 3 and the radius of curvature near the so-called terminal end side of the short side wall portion 2c are small, and the radius of curvature in the intermediate portion is large compared to these radii of curvature, or the radius of curvature is close to infinity. Note that the "radius of curvature" here refers to the radius of curvature in the vertical cross section of Fig. 4(a).

[0021] Furthermore, the inclination angle θ of the inclined wall portion 2a (angle measured upward from an extension line of the temporary bottom surface 2b) is set to 25° or more and 45° or less. This is to reliably delay the start of processing of the inclined wall portion 2a and the short side wall portion 2c compared to the processing of the long side wall portion 2d and the curved portion 2e described above when the intermediate cup 2 is redrawn or drawn and ironed, as described later. Also, this is to make the intermediate cup 2 stable and self-supporting. Note that, when the inclined wall portion 2a is slightly curved, the inclination angle θ may be the angle of a tangent at the center when viewed in the vertical cross section of FIG. 4(a). The inclined wall portion 2a of the above-mentioned shape can be formed by providing an inclined portion corresponding to the above-mentioned shape in a punch (not shown) for performing drawing.

[0022] When drawing the intermediate cup 2 of the above-mentioned shape from an aluminum alloy blank, the material tends to concentrate on the short side (both end sides in the long side direction), and the height of the short side wall 2c (height from the temporary bottom surface 2b: ear height) tends to be lower than the long side wall 2d. However, in the embodiment of the present invention, the inclined wall 2a is provided, so that the short side wall 2c is raised and the height (ear height) of its upper end from the temporary bottom surface 2b is increased. This state is shown in FIG. 4(a), and the height of the long side wall 2d and the height of the short side wall 2c are almost equal. For comparison, FIG. 4(b) shows a vertical cross-sectional shape when the inclined wall 2a is not provided. The material flows in the circumferential direction, so that the material gathers on both end sides in the long side direction, making it difficult to proceed with the processing, and as a result, the ear height does not increase, and a certain large difference Δh occurs with respect to the height of the long side wall 2d.

[0023] When performing multiple stages of drawing or drawing and ironing in succession, the intermediate cup 2, which is fitted to the punch by the process, is removed from the punch (not shown) by the stripper fingers. The stripper fingers are fixed at a position where the upper end of the thick short side wall portion 2c is abutted against the punch, and as the punch rises, the short side wall portion 2c is hooked on the stripper fingers, and the punch further rises to push the intermediate cup 2 down from the punch and remove it. As shown in FIG. 4(b), if the height of the short side wall portion 2c is low and the stripper fingers are installed at a low position accordingly, the upper end of the long side wall portion 2d of the intermediate cup 2 moving on the pass line will be caught by the stripper fingers. If the installation height of the stripper fingers is increased to avoid this, the intermediate cup 2 will be removed from the punch at a high position and dropped, which increases the possibility that the dropped intermediate cup 2 will roll or shift position. 4(a), if the long side wall portion 2d and the short side wall portion 2c are approximately equal in height, there is no need to increase the installation height of the stripper fingers, and therefore the drop height of the intermediate cup 2 can be lowered to prevent the intermediate cup 2 from falling over or becoming displaced. Also, in the embodiment of the present invention, such a height difference Δh is reduced and the shape as a whole becomes close to a perfect rectangular parallelepiped, so that the intermediate cup 2 has a stable, self-supporting shape, which makes it easier to avoid or suppress jamming due to falling over during transportation between multiple processing steps.

[0024] The intermediate cup 2 obtained in the first step is subjected to a plurality of drawing processes and drawing and ironing processes in the second step, and finally, the intermediate cup 2 is formed into the rectangular can 1 shown in FIG. 1. In FIG. 2, the first step is indicated by the reference symbol f1, the subsequent plurality of second steps are indicated by the reference symbols f2 to f8, and the shape of the intermediate cup 2 in each step is also shown. The processing in each step is to gradually make each corner part (corner part) of the intermediate cup 2 a right angle, to deform each part (draw) so that the width and length of the temporary bottom surface 2b become the bottom surface 1a of the rectangular can 1, and to iron each side wall part 2c, 2d to make it thinner. The processing is performed in the same way as the normal drawing process or drawing and ironing process, by setting the intermediate cup 2 on a die D and lowering a punch P toward the inside of the die D to apply a deforming force and ironing to the intermediate cup 2.

[0025] 5(a) and (b) show schematic diagrams of the processing state in the second step. The die D has an opening shape that reduces the length and width of the intermediate cup 2 processed in the preceding step by a predetermined amount. In contrast, the length and width of the temporary bottom surface 2b in the intermediate cup 2 are smaller than the interval between the short side wall portions 2c (the length of the intermediate cup 2) and the interval between the long side wall portions 2d (the width of the intermediate cup 2). Therefore, when the intermediate cup 2 is set on the die D, the temporary bottom surface 2b falls slightly into the die D, and the left and right curved portions 2e are caught on the open end of the die D. In the embodiment of the present invention, the aforementioned inclined wall portion 2a is inclined upward from the temporary bottom surface 2b side at an angle of 25° or more, so that the inclined wall portion 2a and the short side side wall portion 2c continuous with the upper side thereof are separated from the die D. This state is maintained even immediately after the punch P descends and starts processing the intermediate cup 2. That is, the processing of the long side wall portion 2d is performed prior to the processing of the short side wall portion 2c. Therefore, the processing of the long side wall portion 2d and the curved portion 2e below it is started without particularly restraining the inclined wall portion 2a and the short side wall portion 2c, so that immediately after the processing starts, the material flow in the direction shown by the arrows in Fig. 5(a) and (b) is possible or promoted. In particular, in the long side wall portion 2d, the material flow in the circumferential direction toward the short side wall portion 2c becomes easy, so the molding resistance is reduced. In other words, by reducing the forming load at the start of forming (initial movement), abnormalities such as cracks and breaks can be avoided in advance, or the amount of forming (processing amount) per one time can be increased to improve the processing efficiency and reduce the number of processing steps.

[0026] For comparison, the processing state in the second step when the inclined wall portion 2a is not provided is shown in Fig. 6(a) and (b). The arrows in Fig. 6(a) and (b) indicate the direction of material flow. When the inclined wall portion 2a is not provided, drawing and ironing are started simultaneously on the entire circumference of the bottom side of the intermediate cup 2. That is, as shown in Fig. 6(b), at the same time as the processing starts, the short side portion of the temporary bottom surface 2b is sandwiched between the punch P and the die D, so that not only is the flow of material toward this portion restricted, but the material also tries to flow from the short side wall portion 2c toward the long side wall portion 2d as shown in Fig. 6(a). Therefore, the material flow in the circumferential direction is restricted as a whole in the intermediate cup 2, so that the initial forming resistance increases, and the forming force must be increased.

[0027] The intermediate cup 2 obtained in the first step is processed by drawing and ironing so that the length and width of the temporary bottom surface 2b are reduced to form a long and narrow rectangle, and the side walls 2c, 2d are gradually thinned to form a taller height. An example of the shape change in this case is shown in Figure 2. Specifically, the tip end portion of the inclined wall portion 2a is stretched and raised perpendicular to the temporary bottom surface 2b so as to become part of the short side wall portion 2c, and the inclination angle is gradually increased. In addition, the starting point 3 is gradually moved to a position that becomes the short side of the bottom surface 1a of the rectangular can 1. Such a change in the inclined wall portion 2a is shown in the diagram of Figure 7. In Figure 7, the position of the starting point 3 is indicated by a short straight leader line.

[0028] The inclined wall portion 2a described above changes as shown in FIG. 7, and is finally resolved into a corner bent at a nearly right angle between the bottom surface 1a and the short side wall portion 1c of the rectangular can 1 (or a curved portion with a large curvature and a short curvature). The inclination angle is in the range of 25° to 45° in the process up to that point, but when the overall shape approaches a rectangular can 1 to a certain extent, the inclination angle of the inclined wall portion 2a exceeds 45°. When the inclination angle becomes this large, the radius of curvature of the curved portion 2e between the long side wall portion 2d and the temporary bottom surface 2b becomes small, and the length (arc length) of the curved portion 2e becomes short, so that the difference in the delay in the start of processing due to the inclined wall portion 2a becomes small, and the processing of the short side wall portion 2c and the processing of the long side wall portion 2d start almost at the same time. However, when the processing progresses to such a shape, the processing thereafter mainly becomes a squeezing process, so the flow of the material in the circumferential direction becomes small. Therefore, even if the inclined wall portion 2a is almost completely eliminated, the molding resistance or the forming force of the initial movement can be reduced.

[0029] By drawing and ironing the intermediate cup 2, the inclined wall 2a is reduced and the curved portion 2e on the long side is also gradually reduced, and the curved portion 2e is finally eliminated by becoming a corner bent at a nearly right angle between the bottom surface 1a and the long side wall 1b of the rectangular can 1 (or a curved portion with a large curvature and a short curvature). Such changes in the shape of the inclined wall 2a and the curved portion 2e are shown in FIG. 8 as a view from the bottom side. In the example shown here, the inclined wall 2a is almost eliminated in the step indicated by the symbol f6. At this stage, the processing is mainly performed by ironing, and the material is generated in the direction of the punch movement (vertical direction), and there is almost no plastic flow in the circumferential direction, so there is no need to distribute the forming force in the circumferential direction.

[0030] 2, in the steps indicated by the symbols f7 and f8, the can is ironed to form the final shape of the rectangular can 1, and, if necessary, the upper ends of the side walls 2c, 2d are cut off to make the heights uniform, i.e., trimming. The ironing at this stage corresponds to the third step of the present invention.

[0031] As described above, by providing the inclined wall portion 2a, circumferential plastic flow is likely to occur during the process in which a large amount of processing is performed by drawing. Therefore, the forming force (particularly the initial processing load) can be reduced, in other words, the amount of processing can be increased while avoiding abnormalities such as cracks and breakage, and thus the processing efficiency or productivity can be improved. In addition, in the method of the present invention, by providing the above-mentioned inclined wall portion 2a, factors that restrict or restrain the material flow on the short side during drawing are reduced, so that efficient forming can be performed without causing wrinkles, cracks, or breakage, and a rectangular can 1 with good accuracy or quality can be obtained. Furthermore, when processing is performed in a plurality of consecutive steps, the intermediate cup 2 can be transported between each processing step and set in the die in a stable manner, so that jamming can be avoided and processing can be performed efficiently, and in this respect productivity can also be improved. [Explanation of symbols]

[0032] 1 Square can 1a Bottom 1b Long side wall 1c Short side wall 2 medium cups 2a Slanted wall section 2b Temporary bottom 2c Short side wall 2d Long side wall 2e Curved section 3 Starting point

Claims

1. A method for manufacturing a prismatic battery can in which an aluminum alloy blank is subjected to drawing and ironing processes to produce a battery can having a rectangular cross section when cut along a plane parallel to the bottom surface, comprising the steps of: a first step of forming an intermediate cup by drawing the blank to have a temporary bottom surface that is oval or rectangular and has a larger area than the bottom surface of the rectangular battery can, an inclined wall portion that is continuous with and inclined obliquely to both ends of the temporary bottom surface in the major axis direction, a short side wall portion that is continuous via the inclined wall portion, and a long side wall portion that is continuous with both ends of the temporary bottom surface in the minor axis direction via a curved portion; a second step of deforming the intermediate cup by drawing or drawing and ironing such that a boundary portion between the temporary bottom surface and the inclined wall portion gradually approaches an end portion in a long side direction of the bottom surface of the rectangular battery can, and a width of the temporary bottom surface in a short axis direction approaches a width of the bottom surface of the rectangular battery can in a short side direction; a third step of performing an ironing process or a drawing and ironing process to reduce the plate thickness of the short side wall portion and the long side wall portion after the second step; A method for producing a rectangular battery can, comprising:

2. The method for producing a rectangular battery can according to claim 1, the inclined wall portion of the intermediate cup is a surface that is inclined at a predetermined angle from a position closer to the short-side wall portion than an end portion in a long-side direction of the bottom surface of the rectangular battery can, the position of the starting point is at a position closer to the short side wall portion than the end portion in the long side direction of the bottom surface of the rectangular battery can, and at a position closer to the end portion in the long side direction of the bottom surface of the rectangular battery can than a central portion between the end portion in the long side direction of the bottom surface of the rectangular battery can and the short side wall portion; The predetermined angle is equal to or greater than 25° and equal to or less than 45°, The inclined wall portion is continuous with the short side wall portion via a smooth curved surface. A method for manufacturing a rectangular battery can comprising the steps of:

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