Patent Publication Number: US-2010116014-A1

Title: Blank for metal can and method for producing metal can

Description:
TECHNICAL FIELD 
     The invention relates to a blank for a metal can and a method for producing metal cans. More particularly, the invention mainly relates to an improvement in a blank for a metal can. 
     BACKGROUND ART 
     Cylindrical metal cans with bottoms (hereinafter simply referred to as “metal cans”) have been widely used, for example, in the fields of food, beverages, quasi-drug, transport devices, and batteries, and there is an increasing demand for metal cans. With respect to the production of metal cans, various methods have been proposed depending on the material, structure, use, etc. of metal cans. 
       FIGS. 8A  to  FIG. 8C  are drawings showing a method for producing battery cans, which are representative metal cans.  FIG. 8A  is a plan view showing a blanking step of punching out circular blanks  51  (hereinafter “blanks  51 ”) from a hoop material  50 . In  FIG. 8A , the loss material left after the blanks  51  have been punched out from the hoop material  50  is illustrated as hatching of parallel lines for convenience sake.  FIG. 8B  is a perspective view showing a drawing step for obtaining a cylindrical intermediate cup-shaped product  52  with a bottom.  FIG. 8C  is a perspective view showing a DI (Drawing and Ironing) step for obtaining a battery can  53 . This production method includes the step illustrated in  FIG. 8A , the step illustrated in  FIG. 8B , the step illustrated in  FIG. 8C , and a finishing step. 
     In the step of  FIG. 8A , the hoop material  50  is punched into circular shapes to obtain the blanks  51 . The hoop material  50  is, for example, a nickel-plated steel plate that is shaped like a long belt. From each of the blanks  51 , the battery can  53  is produced. In the step of  FIG. 8B , the blank  51  is drawn to obtain the intermediate cup-shaped product  52 . In the step of  FIG. 8C , the intermediate cup-shaped product  52  is subjected to a DI process to obtain the battery can  53 . That is, DI process is a process in which a drawing operation and an ironing operation are performed continuously and simultaneously. In the finishing step, the open edge of the battery can  53  is trimmed to make the open edge flat and adjust the dimensions in the longitudinal direction, thereby obtaining a finished product of the battery can  53 . 
     According to this production method, the circular blank  51  is transformed into a similar shape, i.e., transformed into the intermediate cup-shaped product  52  and the battery can  53  which are also circular in cross-sectional shape. There is thus an advantage in that the open edge of the battery can  53  obtained by the DI process of the intermediate cup-shaped product  52  has only a small amount of wavy distortion due to the process, which is to be cut in the finishing step. However, there is a problem in that the step illustrated in  FIG. 8A  leaves a relatively large amount of loss material. 
     That is, when the blanks  51  are punched out from the hoop material  50 , the hoop material  50  needs to be held somewhere so that it does not become displaced. Thus, when the blanks  51  are arranged so as to be in contact with one another, they cannot be punched out. Hence, as illustrated in  FIG. 8A , the blanks  51  to be punched out are staggered so that they are spaced apart from one another. However, in such an arrangement, even if the adjacent blanks  51  are arranged so as to be as close to one another as possible, a relatively large amount of loss material is left, as illustrated in  FIG. 8A . As a result, the yield of the produced battery cans  53  relative to the amount of the hoop material  50  used becomes significantly low. 
     Also, regular hexagonal blanks have also been used (for example, see Patent Document 1). In the case of a regular hexagonal blank, the perimeter of the blank is composed of six sides (straight lines) of the same length. Thus, when regular hexagonal blanks are punched out, they can be arranged so that one blank is adjacent to six other blanks, with adjacent two blanks sharing one side. By cutting the six sides of each blank in two stages, it is possible to secure an area to be held during the cutting. Blanks can thus be produced only by the cutting step. In this case, no loss material is left between the blanks except for a slight amount of triangular loss material left at both ends of the hoop material in the width direction thereof. 
     However, when a regular hexagonal blank is subjected to a drawing process and a DI process, the open edge of the resultant battery can becomes distorted relatively largely due to the process at the positions corresponding to the six vertices of the hexagon. The wavy distortion due to the process is cut as loss material in the finishing step, but the cut amount becomes relatively large. It is therefore not possible to sufficiently enhance the yield of the produced battery cans  53  relative to the amount of the hoop material  50  used, compared with the use of circular blanks. That is, no matter which of circular blanks and regular hexagonal blanks is used, there is little difference in the overall amount of loss material in all the steps, although the step that leaves a large amount of loss material is different. 
     To solve such problems associated with conventional techniques, a different type of method for producing battery cans has been proposed.  FIG. 9A  to  FIG. 9C  are drawings showing a different type of method for producing battery cans (for example, see Patent Document 2).  FIG. 9A  is a plan view showing a blanking step of punching out almost regular hexagonal blanks  54  (hereinafter “blanks  54 ”) from a hoop material  50 .  FIG. 9B  is a perspective view showing a drawing step for obtaining a cylindrical intermediate cup-shaped product  57  with a bottom.  FIG. 9C  is a perspective view showing a DI step for obtaining a battery can  58 . 
       FIG. 10  is an enlarged plan view of the main part of  FIG. 9A .  FIG. 11  is an enlarged plan view of a part of an inter-blank loss part  59 . In  FIGS. 9A to 11 , the loss material left after the blanks  54  have been punched out from the hoop material  50  is illustrated as hatching of horizontal parallel lines for convenience sake. 
     This production method includes the step illustrated in  FIG. 9A , the step illustrated in  FIG. 9B , the step illustrated in  FIG. 9C , and a finishing step. 
     In the step of  FIG. 9A , the hoop material  50  is punched into circular shapes to obtain the blanks  54 . Each of the blanks  54  is in the shape of a regular hexagon having six arc-shaped vertices  54   a  between the sides of the regular hexagon. In the step of  FIG. 9B , the blank  54  is drawn to obtain an intermediate cup-shaped product  57 . At the open edge of the intermediate cup-shaped product  57 , the vertices  54   a  of the blank  54  become deformed due to the stress of the drawing process, thereby resulting in the formation of protrusions extending in the longitudinal direction of the intermediate cup-shaped product  57 . Also, formed between the adjacent protrusions are depressed parts  57   b.    
     In the step of  FIG. 9C , the intermediate cup-shaped product  57  is drawn and ironed to obtain a battery can  58 . The size of the battery can  58  becomes slightly smaller than that of the drawn and ironed battery can produced by using a regular hexagonal blank. Also, at the open edge of the battery can  58 , the wavy process distortion of the intermediate cup-shaped product  57  becomes more severe due to the stress of the drawing and ironing process. As a result, relatively large wavy protrusions  58   a  are formed as process distortion. At the same time, depressed parts  58   b  deeper than the depressed parts  57   b  are formed between the adjacent protrusions  58   a . The depressed parts  58   b  are susceptible to cracking. It is thought that these protrusions  58   a  and depressed parts  58   b  are formed because the blank  54  is basically in the shape of a regular hexagon although it has the six arc-shaped vertices  54   a.    
     In the finishing step, the protrusions  58   a  and the depressed parts  58   b  on the open edge are cut. In consideration of cracking of the depressed parts  58   b , there is a need to cut the open edge slightly more than necessary. For example, it is desirable to cut along the cutting line shown by the two-dot chain line in  FIG. 9C . If the cracks of the depressed parts  58   b  are not fully cut away, the opening of the battery can after the finishing step may become cracked, thereby resulting in a high product defective rate. Cutting along the cutting line shown by the two-dot chain line increases the amount of loss material. Therefore, the production method of Patent Document 2 fails to sufficiently enhance the yield of the produced battery cans  58  relative to the amount of the hoop material  50  used. 
     Also, the use of the blank  54  having such a shape causes various problems in the production process. For example, when the shaping of the intermediate cup-shaped product  57  is performed by a transfer process, the six protrusions  58   a  formed on the open edge may interfere with the detachment of the intermediate cup-shaped product  57  from the punch by the stripping tool. Also, when the intermediate cup-shaped product  57  is transported to the next step, the protrusions  58   a  may hit and damage the transport equipment. Also, in the next DI step, the protrusions  58   a  may come into contact with the drawing tool, thereby leading to misoperation. As a result, mass production becomes difficult. 
     The protrusions  58   a  formed on the open edge of the battery can  58  in the step of  FIG. 9C  tend to have unequal lengths since the material ductility is not sufficiently even. Thus, when the produced battery can  58  is detached from the punch, the distal ends of the unequal-length protrusions  58   a  unevenly come into contact with the stripper, so that the battery can  58  cannot be detached properly. Even if the battery can  58  can be detached, part of the battery can  58  may become damaged. Also, at least a part of the protrusions  58   a  may become broken, and the broken pieces may remain in the die, thereby damaging the battery can  58  that will be subsequently subjected to this process. 
     Also, Patent Document 2 fails to provide detailed description and illustration of dies and equipment for punching out blanks except for the following brief statement: “Punches and dies are prepared to punch out blanks that are basically in the shape of a regular hexagon with arc-shaped rounded corners. The punches having a punching section and dies are disposed in a punching press so that the adjacent blanks are arranged at equal intervals and at equal margins at both ends of a long rolled steel plate (hoop material) in the width direction thereof. At this time, the punches and dies are staggered in the same number in the width direction of the plate, and in two parallel rows in the longitudinal direction. The long rolled steel plate is punched while being continuously fed to the punching press”. 
     It is assumed from this statement that the technique of Patent Document 2 employs a punching means including punches and dies for punching out blanks at one time. When such a punching means is employed, a plurality of blanks need to be arranged with a space, called “feeding bridge”, provided between a blank and adjacent two blanks. This arrangement is similar to that of the blanks  51  illustrated in  FIG. 8A , and a large amount of loss material is thus left after the blanks  51  have been punched out. This can also become an obstacle to enhancing the yield of the produced battery cans relative to the amount of the hoop material used. 
     Meanwhile, there has been proposed a battery can that is produced using a steel plate with an aluminum layer formed on the surface (for example, see Patent Document 3). Patent Document 3 states that the use of this steel plate significantly enhances the service life of the ironing tool in the battery can production process. However, it is silent as to any technique to enhance the yield of the produced battery cans relative to the amount of the steel plate used. 
     Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-263974 
     Patent Document 2: Japanese Laid-Open Patent Publication No. Hei 11-309517 
     Patent Document 3: Japanese Laid-Open Patent Publication No. 2001-313008 
     DISCLOSURE OF THE INVENTION 
     Problem to be Solved by the Invention 
     Further, based on  FIG. 9A  to  FIG. 11 , the shape of the blanks  54  is discussed with respect to the application of the punching step of the invention which will be described later. As illustrated in  FIG. 9A , the arrangement of the blanks  54  in the hoop material  50  is similar to that of the regular hexagonal blanks. That is, one blank  54  is adjacent to six other blanks  54 , and adjacent two blanks  54  share one side without feeding bridge therebetween. The one side shared by the adjacent two blanks  54  corresponds to a cutting line  61  shown by the two-dot chain line in  FIG. 10 . 
     Also, the triangular part surrounded by three adjacent vertices  54   a  is the inter-blank loss part  59 . Also, at both ends of the hoop material  50  in the width direction, the triangular part surrounded by adjacent two blanks  54  and the side edge of the hoop material  50  in the width direction is a side-edge loss part  60 . 
     To produce the blanks  54 , first, predetermined areas of the hoop material  50  are punched out by using die tools  62  composed of a punch and a die having blades of the shape corresponding to the inter-blank loss part  59  and the side-edge loss part  60 . The punched hoop material is then cut along the cutting lines  61  shared by the adjacent two blanks  54 . Since one blank  54  has six cutting lines  61 , performing this cutting in at least two stages ensures that there are some areas to be held during cutting. In this way, the individual blanks  54  are obtained. 
     However, in order to punch out the inter-blank loss part  59 , the die tool  62  needs to have a triangular blade whose three vertices are pointed very sharply. When such a die tool  62  is used, the punch or the hoop material  50  comes off the punched position at the moment of punching. As a result, the sharp edge of the blade is susceptible to chipping or abrasion, which shortens the service life of the die tool  62 . Also, since the punched parts tend to be left with jagged edges, mass production is difficult and the quality of the produced battery can deteriorates. 
     Also, in order to take as many blanks  54  as possible from one hoop material  50 , as illustrated in  FIG. 10 , the blanks  54  are arranged so that the vertices  54   a  of the blanks  54  positioned at the end of the hoop material  50  in the width direction are aligned with the side edge of the hoop material. However, in such an arrangement, it is necessary to punch out the side-edge loss part  60  that is in the shape of a substantially isosceles triangle having two vertices A and one vertex B. The vertex A is a part where the vertex  54   a  of the blank  54  contacts the side edge of the hoop material  50 , and its angle is an acute angle of approximately 30°. Also, the vertex B is a part where the vertices  54   a  of the blanks  54  contact one another, and its shape is as sharp-pointed as one of the vertices of the inter-blank loss part  59 . 
     In order to punch out the side-edge loss part  60  with the vertices A and B, the die tool  62  needs to have a sharp blade similar to the one necessary to punch out the inter-blank loss part  59 . However, the use of such a die tool  62  causes problems similar to the ones associated with punching out the inter-blank loss part  59 . 
     Also, after the inter-blank loss parts  59  and the side-edge loss parts  60  are punched out, cutting is made along the cutting lines  61 . The cutting needs to be made so that the obtained blanks  54  have the same shape. To do this, as illustrated in  FIG. 11 , the end of the cutting line  61  must be accurately aligned with the tip of the sharp-pointed vertex of the inter-blank loss part  59 . However, even if the movement of the hoop material  50  is controlled with very high accuracy, making no feeding error is almost impossible, and it is very difficult to cut so that the ends of the cutting line  61  are aligned with the tips of the vertices of the inter-blank loss parts  59 . 
     In actual operation, the punched hoop material is often thinly sliced, while being scraped, by the die tool along the cut section from which the inter-blank loss part  59  has been punched out, i.e., the same area of the hoop material is undesirable cut twice. If such cutting twice occurs, the die tool wears out severely and the life of the die tool is greatly shortened, which makes mass production difficult. In addition, the blank  54  obtained by cutting tends to be left with jagged edges. Also, metal pieces produced by cutting twice tend to adhere to the blank  54 . When these blanks  54  are used to make cans, the resulting battery cans have flaws, scratch marks, deformation, etc. As a result, the quality of the produced battery cans deteriorates. 
     It is therefore an object of the invention to provide blanks for metal cans capable of easy mass production, wherein more blanks can be taken from a hoop material of the same area with less loss material than conventional blanks without shortening the tool life, and almost no jagged edges are left on the cut section or almost no metal pieces are produced in the cutting and other steps. 
     It is another object of the invention to provide a method for producing metal cans by using the metal can blanks of the invention. 
     Means for Solving the Problem 
     The invention provides a blank for a metal can. The blank has a shape that is surrounded by: 
     n external tangents to a circle about which a regular n-gon is circumscribed, the midpoint of each of the n external tangents being a point of contact between each of the n sides of the regular n-gon and the circle, each of the n external tangents shorter than one side of the regular n-gon; 
     at least one selected from: n partial arcs of the circle each positioned between the adjacent external tangents in the circumferential direction of the circle; and n partial lines each connecting both ends of each of the partial arcs; and 
       2   n  linear or arc-shaped connect lines each connecting an end of each of the external tangents with the most adjacent end of each of the partial arcs or partial lines. 
     Preferably, the n external tangents have the same length, the n partial arcs or partial lines have the same length, and the  2   n  connect lines have the same length. 
     In one embodiment of the invention, the length of the external tangents is preferably equal to the length of the partial arcs or partial lines or shorter than the length of the partial arcs or partial lines. 
     In another embodiment of the invention, the length of each of the external tangents is preferably 16% to 60% of the length of one side of the regular n-gon. 
     Preferably, each of the partial arcs or partial lines has, at its midpoint, an outwardly projecting part. 
     The projecting part is preferably triangular, quadrangular, trapezoidal, semicircular, or sectoral. 
     The regular n-gon is preferably a regular hexagon or regular octagon. 
     The metal can is preferably a battery can. 
     The invention also provides a method for producing metal cans, including: 
     a blanking step of punching and cutting a hoop material to obtain the metal can blanks of the invention; 
     a first drawing step of drawing each of the metal can blanks to obtain a cylindrical intermediate cup-shaped product with a bottom; 
     a second drawing step of drawing the intermediate cup-shaped product to obtain a cylindrical metal can with a bottom; and 
     a finishing step of trimming an open edge of the metal can to obtain a finished product of the metal can with a flat open end face. 
     The blanking step preferably includes: 
     a blank positioning step of setting imaginary lines for staggering the metal can blanks of the invention on a surface of the hoop material in such a manner that the adjacent metal can blanks share one of the external tangents; 
     a loss part punching step including a step of punching inter-blank loss parts and a step of punching side-edge loss parts to form punched holes and openings in the hoop material; and 
     a cutting step of cutting the hoop material between the opposing vertices of the adjacent punched holes and between the opposing vertices of the punched holes and the openings along the imaginary lines of the external tangents. 
     The blank positioning step is preferably a step of setting the imaginary lines for arranging the metal can blanks in a zigzag in such a manner that in each of the metal can blanks arranged at the ends of the hoop material in the width direction thereof, the external tangent facing the external tangent that forms the inter-blank loss part is positioned near the side edge of the hoop material in the width direction thereof. 
     The step of punching inter-blank loss parts is preferably a step of punching inter-blank loss parts to form punched holes in the hoop material, each of the inter-blank loss parts being surrounded by the imaginary lines of three partial arcs of three adjacent metal can blanks each facing the other two metal can blanks and the imaginary lines of six connect lines connecting the imaginary lines of the three partial arcs. 
     The step of punching side-edge loss parts is preferably a step of punching side-edge loss parts to form openings in the hoop material, each of the side-edge loss parts being surrounded by the imaginary lines of two metal can blanks adjacent at the ends of the hoop material in the width direction thereof and the side edge of the hoop material in the width direction thereof. 
     The metal cans are preferably battery cans. 
     EFFECT OF THE INVENTION 
     The blank for a metal can of the invention has a particular shape, which is a basically circular shape composed of n external tangents, n partial arcs, and  2   n  connect lines connecting these. When such metal can blanks are imaginarily staggered on the surface of a hoop material, the amount of loss material cut from the hoop material by punching decreases significantly. Also, when this blank is formed into an intermediate cup-shaped product and a battery can, the amount of cut loss material decreases since relatively small protrusions grow on the open edge thereof. This allows a significant improvement in the yield of produced metal cans (product yield) relative to the amount of the hoop material used. 
     Also, since the vertices of the loss material are not pointed sharply, the tool life is not shortened. Also, in the cutting and other steps, jagged edges are not left on the cut section and no metal pieces are produced. Further, since the protrusions having grown on the open edge of the intermediate cup-shaped product and metal can have an almost uniform length, no problem occurs in such steps as detachment from the machine and transportation. At this time, the protrusions do not become bent, broken, cracked, or the like, so that metal pieces, metal powders, or the like do not attach to the intermediate cup-shaped product and metal can. Hence, the ratio of damage of finally obtained metal cans lowers significantly, so that high quality battery cans can be produced with high yields and high productivity. 
     Also, in the production method of metal cans of the invention, the blanking step and the finishing step leave loss material. In the blanking step, inter-blank loss parts and side-edge loss parts are left as loss material, but their amounts are minimized. Also, in the finishing step, protrusions extending from the open edge of the metal can in the longitudinal direction thereof are left as loss material. Although the number of these protrusions is greater than conventional one, their length is significantly shorter than that of conventional protrusions, and the depressed parts between the protrusions are almost free from cracks. Thus, by merely cutting the protrusions and the vicinity thereof, high quality metal cans can be obtained, and the amount of loss material can be reduced. This permits an improvement in material yield. 
     Also, by performing two steps of punching the inter-blank loss parts and the side-edge loss parts from the hoop material and cutting along the external tangents after the punching, the blanks can be punched out with high efficiency, and metal cans can be produced with high productivity. Also, the production method of the invention is applicable, but not particularly limited, to the production of cylindrical metal cans with bottoms for various applications. Such examples include battery cans and beverage cans. According to the production method of the invention, it is advantageously possible to produce high quality metal cans with high productivity while permitting a reduction in material costs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a blank for a metal can in one embodiment of the invention. 
         FIG. 2  is a plan view showing a part of a process in a production method of metal cans in another embodiment of the invention. 
         FIG. 3A  is a longitudinal sectional view schematically showing the structure of a first press and a step of positioning a blank set part  1 V in a production process of an intermediate cup-shaped product. 
         FIG. 3B  is a longitudinal sectional view schematically showing the structure of the first press and a step of punching a blank  1  in the production process of an intermediate cup-shaped product. 
         FIG. 3C  is a longitudinal sectional view schematically showing the structure of the first press and a step of drawing the blank  1  in the production process of an intermediate cup-shaped product. 
         FIG. 4  is an enlarged plan view showing a part of the imaginary arrangement of blank set parts on a hoop material. 
         FIG. 5  is a perspective view showing the appearance of the intermediate cup-shaped product. 
         FIG. 6A  is a longitudinal sectional view schematically showing the structure of a drawing and ironing machine used in the production method of metal cans in another embodiment of the invention. 
         FIG. 6B  is a perspective view showing the appearance of a battery can obtained by a second drawing step in the production method of metal cans in another embodiment of the invention. 
         FIG. 7A  is a plan view showing the shape of an inter-blank loss part in another mode. 
         FIG. 7B  is a plan view showing the shape of an inter-blank loss part in another mode. 
         FIG. 7C  is a plan view showing the shape of an inter-blank loss part in another mode. 
         FIG. 7D  is a plan view showing the shape of an inter-blank loss part in another mode. 
         FIG. 7E  is a plan view showing the shape of an inter-blank loss part in another mode. 
         FIG. 7F  is a plan view showing the shape of an inter-blank loss part in another mode. 
         FIG. 7G  is a plan view showing the shape of an inter-blank loss part in another mode. 
         FIG. 7H  is a plan view showing the shape of an inter-blank loss part in another mode. 
         FIG. 7I  is a plan view showing the shape of an inter-blank loss part in another mode. 
         FIG. 8A  is a plan view showing a blanking step in a production method of battery cans. 
         FIG. 8B  is a perspective view showing a drawing step in the production method of battery cans. 
         FIG. 8C  is a perspective view showing a DI step in the production method of battery cans. 
         FIG. 9A  is a plan view showing a blanking step in another production method of battery cans. 
         FIG. 9B  is a perspective view showing a drawing step in another production method of battery cans. 
         FIG. 9C  is a perspective view showing a DI step in another production method of battery cans. 
         FIG. 10  is an enlarged plan view showing the main part of  FIG. 9A . 
         FIG. 11  is an enlarged plan view showing a part of the inter-blank loss part illustrated in  FIG. 9A  to  FIG. 9C  and  FIG. 10 . 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
       FIG. 1  is a plan view of a blank  1  for a metal can (hereinafter referred to as simply a “blank  1 ”) in one embodiment of the invention. The blank  1  has a basically circular flat shape that is surrounded by six external tangents  3 , six partial arcs  2 , and twelve connect lines  4 . The shape of the blank  1  is clearly different from that of the regular hexagonal blank  54  illustrated in  FIG. 9A  to  FIG. 9C . The six external tangents  3  are spaced from the six partial arcs  2 . More specifically, an end of an external tangent  3  is connected to the end of the partial arc  2  that is closest to the end of the external tangent  3  by a connect line  4 . There are twelve such connections between the end of the external tangent  3  and the end of the partial arc  2  by the connect line  4 . 
     When the blank  1  is drawn to obtain an intermediate cup-shaped product that is described later, these connected parts between the ends of the external tangents  3  and the ends of the partial arcs  2  by the connect lines  4  become twelve protrusions extending from the open edge of the intermediate cup-shaped product in the longitudinal direction thereof. These protrusions are short, and the number of these protrusions is twelve, which is twice six that is the number of protrusions formed when a regular hexagonal blank is used. That is, smaller protrusions than conventional ones are formed in greater number. Therefore, the influence by the uneven material ductility is reduced, and the lengths of the twelve protrusions become almost the same. 
     As a result, in a molding apparatus, the twelve protrusions evenly come into contact with the stripper, so the intermediate cup-shaped product or metal can can be smoothly detached without slanting. Also, since the protrusions do not become bent or partially or totally broken, the occurrence of flaws, scratch marks, deformation, etc. is significantly suppressed in the finally obtained battery can. Also, a can jam does not occur. Hence, the product yield improves and high quality metal cans can be obtained. 
     In  FIG. 1 , an imaginary regular hexagon  8  (hereinafter referred to as simply a “regular hexagon  8 ”) shown by the chain double-dashed line is circumscribed about an imaginary circle  7  (hereinafter referred to as simply “circle  7 ”) shown by the chain double-dashed line. The regular hexagon  8  has six sides, and each of the six sides has a point of contact  9  with the circle  7 . That is, there are six points of contact  9  between the regular hexagon  8  and the circle  7 . When a regular n-gon is circumscribed about the circle  7 , there are n points of contact between the circle  7  and the regular n-gon. 
     Each of the external tangents  3  is a tangent to the circle  7 . It is a straight line shorter than the length of one side of the regular hexagon  8 , with the mid point being the point of contact  9 . These external tangents  3  are spaced from one another and the number of the external tangents  3  is six. It is preferable that the six external tangents  3  have the same length. In this case, the imaginary arrangement of the blanks  1  on the surface of the hoop material  50  becomes easy. Also, since the shapes of the loss materials become almost the same, the loss materials can be easily punched out. When a regular n-gon is used instead of the regular hexagon  8 , there are n external tangents. 
     It is also preferable that the length of the external tangent  3  be 16% to 60% of the length of one side of the regular hexagon  8 . If it is less than 16%, the amount of loss material is large, and the service life of the die tool may become shortened due to cutting twice, blade chipping or abrasion in the punching step. Also, if it exceeds 60%, the shape of the blank  1  becomes almost hexagonal, which may cause problems associated with the formation of a regular hexagonal blank. Also, in the step of making a battery can, large wavy process distortion may appear, thereby damaging the molding machine or lowering the product yield. The length of the external tangent  3  is more preferably set in the range of 20 to 30% of the length of one side of the regular hexagon  8 . 
     Each of the partial arcs  2  is a part of the circle  7  positioned between an external tangent  3  and another adjacent external tangent  3 . These partial arcs  2  are spaced apart from one another, and the number of the partial arcs  2  is six. When a regular n-gon is used instead of the regular hexagon  8 , there are n partial arcs  2 . 
     It is preferable that the six partial arcs  2  have the same length. It is also preferable that the partial arcs  2  have the same length as the external tangents  3 . As used herein, “the same length” encompasses substantially the same length. In this case, the length between the end of the partial arc  2  and the end of the external tangent  3  becomes relatively large, and each vertex of an inter-blank loss part  13  which will be described later has the shape of a semicircle with a relatively large diameter, or has a linear shape. 
     Also, when the length of the partial arc  2  is not the same as that of the external tangent  3 , it is preferable that the length of the partial arc  2  be longer than that of the external tangent  3 . When the partial arc  2  is longer than the external tangent  3 , the vertex of the inter-blank loss part  13  has the shape of a semicircle with a small diameter. Hence, problems such as shortened life of the die tool and cutting twice are more likely to occur. Also, since the area of the inter-blank loss part  13  becomes relatively large, the material yield up to this step may slightly decrease. 
     However, an intermediate cup-shaped product  10  which will be described later will hardly get caught on the tool etc. during the process, so the process is hardly suspended. Further, since protrusions  10   a  of the intermediate cup-shaped product  10  become small, the area cut in the finishing step becomes very small. As a result, the overall material yield improves compared with the case where the partial arc  2  and the external tangent  3  have the same length. 
     If the external tangent  3  is longer than the partial arc  2 , the shape of the blank becomes close to a regular hexagon rather than the intended basically circular shape. Thus, the same problems as those associated with the use of a regular hexagonal blank to produce a metal can are likely to occur. It is therefore preferable that the length of the external tangent  3  be equal to that of the partial arc  2  or shorter than the partial arc  2 . 
     The lengths of the partial arc  2  and the external tangent  3  are more specifically described with reference to  FIG. 11  for convenience sake. In the blank  1 , the partial arc  2  and the external tangent  3  have almost the same length. In this case, the distance between the adjacent ends of the partial arc  2  and the external tangent  3  becomes relatively large and the connect line  4  thus becomes long. Hence, the inter-blank loss part  13  and the punched hole thereof have a shape including a vertex  31  that is shaped like a semicircle having a relatively large diameter.
         Contrary to this, if the partial arc  2  is longer than the external tangent  3 , as the difference in length increases, vertices  31 A and  31 B shaped like semicircles having smaller diameters are formed, as shown by the two-dot chain line in  FIG. 11 . In this case, problems such as cutting twice and shortened service life of the die tool are likely to occur. In addition, the area of the inter-blank loss part increases and the material yield also decreases. However, since the area of the intermediate cup-shaped product  10  cut in the finishing step decreases, the overall material yield improves, as described above. There is also an advantage in that the intermediate cup-shaped product does not get caught on the tool etc. during the process and thus the process is not suspended.       

     Conversely, if the external tangent  3  is longer than the partial arc  2 , there is an advantage in that an almost triangular inter-blank loss part having large-diameter semicircular vertices is formed. However, the shape of such a blank becomes close to a regular hexagon rather than the intended basically circular shape of the blank  1  of the invention. As a result, as illustrated in  FIG. 9B  and  FIG. 9C , the intermediate cup-shaped product  57  having large and deep depressed parts  57   a  on the open edge and the battery can  58  having high protrusions  58   a  on the open edge are undesirably formed. It is therefore preferable to design the shape of the blank  1  such that the partial arc  2  and the external tangent  3  have almost the same length or the external tangent  3  is shorter than the partial arc  2 . 
     It is also preferable that the partial arc  2  have, at the midpoint, an outwardly projecting part. While the shape of the projecting part is not particularly limited, it is preferably triangular, quadrangular, trapezoidal, semicircular, sectoral, or the like. In this embodiment, since there are six partial arcs  2 , there are also six projecting parts. These six projecting parts become the protrusions longitudinally extending from the open edge of the intermediate cup-shaped product and battery can obtained by working the blank  1 . 
     By providing the projecting parts, eighteen protrusions extending from the open edge of the intermediate cup-shaped product in the longitudinal direction of the intermediate cup-shaped product are formed. Among them, twelve protrusions are derived from the connecting parts between the external tangents  3  and the partial arcs  2 , and six protrusions are derived from the projecting parts positioned at the center of the partial arcs  2 . As the number of the protrusions increases, the lengths of the protrusions decrease commensurately and become uniform. As a result, the strippability of the intermediate cup-shaped product and battery can by the stripper is improved. This prevents the occurrence of problems during the transport to the next step and prevents such problems as chipping, breakage, and cracking of the opening of the intermediate cup-shaped product and the battery can. 
     In this embodiment, the partial arc  2  is an arc-shaped curve, but it is not limited thereto. For example, it may be a partial line connecting both ends of the partial arc  2 . The partial line may also have a projecting part at the midpoint in the same manner as described above. 
     Each of the connect lines  4  connects an end of one external tangent  3  to the end of the partial arc  2  or partial line that is closest to the end of the one external tangent  3 . These connect lines  4  are spaced from one another, and the number of the connect lines  4  is twelve. When a regular n-gon is used instead of the regular hexagon  8 , there are  2   n  connect lines  4 . The  2   n  connect lines  4  preferably have the same length. Also, the connect lines  4  are linear or arc-shaped. 
     When the connect lines  4  are linear or arc-shaped, the edge of each vertex of the almost triangular inter-blank loss part  13  becomes linear or arc-shaped without becoming sharply pointed. As a result, the vertex of the inter-blank loss part  13  has some width, although slight, in the direction perpendicular to the cutting direction. There is thus no need to provide the die tool for punching out the inter-blank loss part  13  with a sharp-pointed part. Hence, the service life of the die tool does not become shortened due to blade chipping or abrasion. In addition, the inter-blank loss part  13  can be formed so as to have an accurate shape without leaving jagged edges. 
     Also, even if the hoop material is fed with a slight feeding error and is cut at a position deviating from the external tangent  3  that is the correct cutting line, no problem occurs because the vertex of the inter-blank loss part  13  has some width in the direction perpendicular to the cutting direction. That is, cutting twice, i.e., being thinly sliced while being scraped by the die tool along the cut section of the punched hole of the inter-blank loss part  13 , does not occur. Hence, the die tool can have sufficient life. Therefore, the productivity can be dramatically improved. 
       FIG. 2  is a plan view showing a part of a process in the method of producing metal cans according to another embodiment of the invention. As used herein, “a part of a process” refers to a blanking step and a first drawing step. In  FIG. 2 , from the upstream side toward the downstream side in the transport direction (direction of the arrow) of the hoop material  11 , there are provided positions at which the blanks  1  are imaginarily arranged, a loss-part punching mechanism  12 , a first blank-punching-drawing mechanism  17 , and a second blank-punching-drawing mechanism  19  in this order. Also, in  FIG. 2 , the loss material parts cut from the hoop material  11  by punching are illustrated as hatching of parallel horizontal lines for convenience sake. 
     The method of producing metal cans according to the invention includes a blanking step, a first drawing step, a second drawing step, and a finishing step. 
     The blanking step includes, for example, a blank positioning step, a loss part punching step, and a cutting step. In this step, for example, a long belt-like nickel-plated steel plate of predetermined width is used as the hoop material  11 , which is the battery can material. However, this is not to be construed as limiting, and it is possible to use metal plates made of various metal materials for use as metal can materials. The hoop material  11  is intermittently transported at a given feeding pitch P in the direction shown by the arrow. 
     In the blank positioning step, first, as illustrated in  FIG. 2 , blank set parts  1 V, which are the positions at which the blanks  1  are to be punched out, are imaginarily arranged on the surface of the upstream-side end of the hoop material  11  in the transport direction thereof. The blank set parts  1 V are shown by the two-dot chain line in  FIG. 2 . The blank set parts  1 V are staggered, for example, in five parallel rows along the longitudinal direction of the hoop material  11 , in such a manner that one blank set part  1 V and an adjacent blank set part  1 V share one external tangent imaginary line  3 V. Thus, one blank set part  1 V is adjacent to each of six other blank set parts  1 V while sharing one external tangent imaginary line  3 V. 
     Also, at the ends of the hoop material  11  in the width direction thereof, the blank set parts  1 V are arranged so that the imaginary line  3 V of the external tangent  3  facing the imaginary line  3 V of the external tangent  3  that extends in the longitudinal direction of the hoop material to form one side of the inter-blank loss part  13  is positioned near the side edge of the hoop material  11  in the width direction thereof. The feeding pitch P of the hoop material  11  is set to the distance between the centers of two blank set parts  1 V that are adjacent in the longitudinal direction of the hoop material  1 . The center of the blank set part  1 V refers to the center of the circle  7  illustrated in  FIG. 1 . 
     In this step, first, based on the imaginary arrangement of the blank set parts  1 V, the inter-blank loss parts  13  and the side-edge loss parts  14  are punched out from the hoop material  11  by using the loss-part punching mechanism  12 . That is, this step includes an inter-blank loss part punching step and a side-edge loss part punching step. Punching out the inter-blank loss parts  13  leaves punched holes, and punching out the side-edge loss parts  14  leaves openings. In  FIG. 2 , in order to facilitate understanding, the punched inter-blank loss parts  13  and the punched side-edge loss parts  14  are shown by the horizontal parallel lines. In this way, the loss-part punching mechanism  12  produces the hoop material  11  in which the blank set parts  1 V are jointed together with only the external tangent imaginary lines  3 V. 
     In the transport direction of the hoop material  11 , the loss-part punching mechanism  12  is disposed downstream of the area where the blank set parts  1 V are imaginarily arranged. 
     The inter-blank loss part is an almost triangular part surrounded by the imaginary lines of three partial arcs of three adjacent metal can blanks each of which faces the other two metal can blanks and the imaginary lines of six connect lines connecting those imaginary lines of three partial arcs. More specifically, the inter-blank loss part  13  is an almost triangular part surrounded by the partial arc imaginary lines  2 V of three adjacent blank set parts  1 V and the connect line imaginary lines  4 V at both ends of the partial arc imaginary lines  2 V. 
     Also, the side-edge loss part  14  is an almost triangular part surrounded by the imaginary lines of two metal can blanks that are adjacent at the end of the hoop material  11  in the width direction thereof and the side edge of the hoop material  11  in the width direction. More specifically, it is an almost triangular part surrounded by the side edge of the hoop material  11 , external tangent imaginary lines  3 V of two blank set parts  1 V that are adjacent in the longitudinal direction of the hoop material  11 , partial arc imaginary line  2 V, half of another partial arc imaginary lines  2 V, and the connect line imaginary lines  4 V connecting the above-mentioned external tangent imaginary lines  3 V to the above-mentioned partial arc imaginary lines  2 V. 
     It is clear that the area of the loss parts  13  and  14  for obtaining the blanks  1  is significantly reduced compared with the area of the loss parts for obtaining the conventional circular blanks  51  illustrated in  FIG. 8A . It is also clear that the area of the loss parts  13  and  14  is equivalent to the area of the loss parts for obtaining the conventional blanks  54  illustrated in  FIG. 9A . The blanks  54  are in the shape of an almost regular hexagon having six arc-shaped vertices  54   a . This indicates that by designing the shape of the blanks  1 , the material yield relative to the hoop material  11  becomes high. 
     The hoop material  11  with the punched holes corresponding to the inter-blank loss parts  13  and openings corresponding to the side-edge loss parts  14  is transported to the first blank-punching-drawing mechanism  17  and the second blank-punching-drawing mechanism  19 , where the remaining operation (cutting) of the loss part punching step and the first drawing step are successively performed. 
     The first blank-punching-drawing mechanism  17  is equipped with first presses  18 . The first presses  18  cut and deep draw the blank set parts  1 V in the second and fourth rows in the width direction from one end of the hoop material  11  in the width direction thereof in the imaginary arrangement of the blank set parts  1 V illustrated in  FIG. 2 . The first presses  18  is equipped with a punching die (not shown) for simultaneously cutting five of the six external tangent imaginary lines  3 V of one blank set part  1 V. In  FIG. 2 , the positions cut by the punching die are shown by the thick line. 
     In the blank set part  1 V newly fed to the first press  18 , since the preceding blank set part  1 V in the same row in the transport direction has been already punched out, the external tangent imaginary line  3 V shared by the preceding blank set part  1 V and the newly fed blank set part  1 V has been cut. The newly fed blank set part  1 V is thus connected to five blank set parts  1 V, excluding the preceding blank set part  1 V, with the external tangent imaginary lines  3 V. Hence, the blank set part  1 V whose position has been adjusted in the first press  18  is punched into the blank  1  when the remaining five external tangent imaginary lines  3 V are simultaneously cut. Immediately after the punching, the blank  1  is drawn to form the intermediate cup-shaped product  10 . 
     After the blank  1  is punched out by the first blank-punching-drawing mechanism  17 , the hoop material  11  is fed to the second blank-punching-drawing mechanism  19 . The second blank-punching-drawing mechanism  19  is equipped with second presses  20 . The second presses  20  successively perform the remaining operation of the loss part punching step (cutting) and the first drawing step (deep drawing) on the blank set parts  1 V in the first, third and fifth rows in the width direction from one end of the hoop material  11  in the width direction thereof in the imaginary arrangement of the blank set parts  1 V illustrated in  FIG. 2 . That is, they punch and deep draw the blank set parts  1 V remaining on the transported hoop material  11 . 
     The second presses  20  is equipped with a punching die (not shown) for cutting the external tangent imaginary lines  3 V remaining on the transported blank set parts  1 V. The positions cut by the punching die are shown by the thick line. The blank set part  1 V whose position has been adjusted in the second press  20  is punched into the blank  1  when the remaining one external tangent imaginary line  3 V is cut. Immediately after the punching, the blank  1  is drawn to form the intermediate cup-shaped product  10 . 
       FIG. 3A  to  FIG. 3C  are longitudinal sectional views schematically showing the structure of the first press  18 .  FIG. 3A  illustrates the positioning step of the blank set part  1 V.  FIG. 3B  illustrates the punching step of the blank  1 .  FIG. 3C  illustrates the drawing step of the blank  1 . 
     The first press  18 , which includes a die holder  21 , a drawing die  22  (drawing die), a blanking die  23 , first and second punch holders  24  and  27 , a blanking punch  28 , a drawing punch  29  (drawing punch), and a stripper  30 , punches and draws the blank  1 . 
     The die holder  21  supports the drawing die  22 . The blanking die  23  is fixed around the outer open edge of the drawing die  22  in a protruded state. The blanking die  23  has a blade for cutting the external tangent imaginary line  3 V of the blank set part  1 V. This blade is provided at five locations along the edge of the regular hexagonal hole shape in the blanking die  23 . The blanking punch  28  also has a blade for cutting the external tangent imaginary line  3 V of the blank set part  1 V. This blade is provided at five locations along the edge of the tip of the blanking punch  28  having a regular hexagonal cross-sectional shape. 
     In the step illustrated in  FIG. 3A , the hoop material  11  is intermittently transported onto the upper end face of the blanking die  23 , and the position of the blank set part  1 V is set to a predetermined position. 
     In the step illustrated in  FIG. 3B , after the positioning of the blank set part  1 V, the blanking punch  28  and the drawing punch  29  (drawing punch) held in the first and second punch holders  24  and  27 , respectively, move toward the dies  22  and  23 . Then, the blades (not shown) of the blanking die  23  and the blades (not shown) of the blanking punch  28  simultaneously cut the five external tangent imaginary lines  3 V remaining in the hoop material  11 , so that the blank  1  is punched out from the blank set part  1 V. The punched blank  1  is temporarily held between the blanking punch  28  and the drawing die  22 . 
     In the step illustrated in  FIG. 3C , the blank  1  sandwiched between the blanking punch  28  and the drawing die  22  is rammed by the cylindrical drawing punch  29 . This causes the blank  1  to be pushed into the drawing die  22  and drawn to a shape conforming to the circular cross-sectional shape of the drawing punch  29 . As a result, the cylindrical intermediate cup-shaped product  10  with a bottom illustrated in  FIG. 5  can be obtained.  FIG. 5  is a perspective view showing the appearance of the intermediate cup-shaped product  10 . 
     When the drawing punch  29  and the blanking punch  28  move upward, the intermediate cup-shaped product  10  is seized and stopped by the stripper  30  with springs, so that only the drawing punch  29  and the blanking punch  28  return to the original positions illustrated in  FIG. 3A . This operation is repeated. The second presses  20  in  FIG. 2  have the same structure as that of the first presses  18  except that the blade is provided only at one location of each of the blanking die  23  and the blanking punch  28 . Also, it is preferable to form the intermediate cup-shaped product  10  by performing a two-stage drawing process to obtain a first intermediate cup-shaped product  10  and a second intermediate cup-shaped product  10  in this order, since the distortion due to the process is further reduced. 
       FIG. 4  is an enlarged plan view of a part of the imaginary arrangement of the blank set parts  1 V on the hoop material  11 . Specifically,  FIG. 4  illustrates the inter-blank loss parts  13  and the vicinity thereof. In  FIG. 4 , in order to clearly show the inter-blank loss parts  13 , they are illustrated as hatching of horizontal parallel lines for convenience sake. Based on  FIG. 4 , the step of  FIG. 3A , i.e., the step of punching the blank  1  by using the blanking die  23  and the blanking punch  28  is more specifically described. 
     The inter-blank loss part  13  and the punched hole thereof have an almost triangular outer shape. Also, the three vertices  31  of the punched hole are formed by opposing partial arc imaginary lines  4 V of adjacent two blank set parts  1 V, and their shape is almost semicircular. Thus, the inter-blank loss part  13  can be punched out by using die tools that are equipped with blades having no sharp-pointed part. As a result, blade chipping or abrasion is unlikely to occur, and the service life of the die tools is not shortened. Also, the inter-blank loss part  13  can be punched into an accurate shape in a reliable manner, and the punched hole is not left with jagged edges or the like. 
     Contrary to this, the die tools for punching the blank  54  illustrated in  FIG. 9A  need blades having sharp-pointed parts. Since sharp-pointed parts are susceptible to chipping or abrasion, the service life of the die tools is shortened and the punched hole tends to be left with jagged edges. 
     Also, after these inter-blank loss parts  13  are punched out, cutting is made along the external tangent imaginary lines  3 V. At each end of the external tangent imaginary line  3 V is the vertex  31  of punched hole of the inter-blank loss part  13 . The vertex  31  is in the shape of a semicircle having a relatively large diameter, and the length (width) in the direction perpendicular to the external tangent imaginary line  3 V increases as the distance from the end of the external tangent imaginary line  3 V increases. Thus, even if the hoop material  11  is fed with a slight feeding error and is cut along a cutting line  3 Va or  3 Vb deviating from the correct external tangent imaginary line  3 V, for example, cutting twice is highly unlikely to occur, i.e., the cut section of the punched hole is highly unlikely to be sliced thinly by the die tool. 
     Cutting twice damages the blank  1 , adversely affects the quality of the finally obtained battery can, and shortens the service life of the die tools such as the blanking punch  28  and the blanking die  23  illustrated in  FIG. 3 . Also, cutting twice produces metal pieces, which adhere to the blank, so that the finally obtained metal can may have flaws, scratch marks, deformation, etc. That is, a large advantage of the invention is that cutting twice does not occur, and it is therefore possible to prevent the service life of the die tools from being shortened, improve productivity, and heighten the quality of the metal can. Further, as shown in  FIG. 11 , the punched blank  1  is not left with jagged edges. 
     Also, the intermediate cup-shaped product  10  obtained by the above-described forming process is formed by drawing the blank  1  that is almost circular. Thus, as illustrated in  FIG. 5 , the open edge has wavy protrusions  10   a . These protrusions  10   a  have grown from the connecting parts between the partial arcs  2  and the external tangents  3  of the blank  1  by the connect lines  4 . In the intermediate cup-shaped product  10 , the number of the protrusions formed is 12, which is twice six, which is the number of the protrusions of the intermediate cup-shaped product formed from a regular hexagonal blank. Since the protrusions  10   a  are relatively small, they are less influenced by uneven material ductility. The twelve protrusions  10   a  thus become small and have almost the same length. 
     Thus, when this intermediate cup-shaped product  10  is detached from the drawing punch  29  of the press  18  illustrated in  FIG. 3 , the twelve protrusions  10   a  can evenly contact the stripper  30 . Hence, the intermediate cup-shaped product  10  can be smoothly detached without slanting. Further, when the intermediate cup-shaped product  10  is transported to the next step, namely, the second drawing step, which will be described later, the protrusions  10   a  do not damage such tools as the feeding pawl and drawing die due to a contact with these tools. 
     In the second drawing step, the intermediate cup-shaped product  10  obtained by the first drawing step is drawn, preferably drawn and ironed, to obtain a battery can  33 .  FIG. 6A  and  FIG. 6B  are drawings showing a part of a process in the method for producing metal cans in another embodiment of the invention.  FIG. 6A  is a longitudinal sectional view schematically showing the structure of a drawing and ironing machine  32 .  FIG. 6B  is a perspective view showing the appearance of the battery can  33  obtained by the second drawing step.  FIG. 6  specifically illustrates the second drawing step. 
     The intermediate cup-shaped product  10  is drawn and ironed by the drawing and ironing machine  32 . In this DI process, one drawing operation and three ironing operations are continuously performed at one time. This produces the cylindrical battery can  33  with a bottom illustrated in  FIG. 6B . 
     The drawing and ironing machine  32  includes an intermediate product feeder  34 , a DI punch  37 , a die mechanism  38 , and a stripper  39 . 
     The intermediate product feeder  34  successively transports the intermediate cup-shaped product  10  to a forming area. The DI punch  37  is driven by a flywheel (not shown), and pushes the intermediate cup-shaped product  10  whose position has been adjusted in the forming area toward a drawing die  38 A of the die mechanism  38 . The die mechanism  38  is sequentially provided with the drawing die  38 A, and first to third ironing dies  38 B,  38 C, and  38 D. These dies  38 A to  38 D are arranged in series so as to be concentric with the axis of the DI punch  37 . The second ironing die C has an ironing hole whose the diameter is smaller than that of the ironing hole of the first ironing die  38 B. The third ironing die  38 D has an ironing hole whose the diameter is smaller than that of the ironing hole of the second ironing die C. The stripper  39  detaches the battery can  33  from the drawing and ironing machine  32 . 
     The intermediate cup-shaped product  10  is transported to the forming area by the intermediate product feeder  34 , positioned, and then punched by the DI punch  37 . As a result, the inner face thereof is drawn to a shape corresponding to the shape of the outer face of the DI punch  37  by the drawing die  38 A. That is, the intermediate cup-shaped product  10  is drawn to an internal diameter almost the same as the internal diameter of the battery can  33  that is the final product. 
     After the passage through the drawing die  38 A, due to the punching of the DI punch  37 , the intermediate cup-shaped product  10  is subjected to a first ironing operation by the first ironing die  38 B. As a result, the circumferential wall is stretched and its thickness is reduced, and in addition, its hardness is enhanced due to work hardening. After the passage through the first ironing die  38 B, due to further punching of the DI punch  37 , the cup-shaped product  10  is sequentially subjected to a second ironing operation and a third ironing operation by the second ironing die  38 C and then the third ironing die  38 D. As a result, the circumferential wall is gradually stretched and its thickness is reduced, and in addition, its hardness is enhanced due to work hardening. Upon completion of the passage of the cup-shaped product  10  through the third ironing die  38 D, the cylindrical battery can  33  of the desired shape having a bottom can be obtained. The battery can  33  is detached from the drawing and ironing machine  32  by the stripper  39 . 
     The open edge of the battery can  33  has protrusions  33   a , which are the protrusions  10   a  of the intermediate cup-shaped product  10  slightly stretched due to the ironing. However, the protrusions  33   a  have smaller dimensions (length×width) than those of the protrusions  58   a  of the battery can  58  illustrated in  FIG. 9C . This is because the dimensions (length×width) of the protrusions  10   a  from which the protrusions  33   a  are formed are smaller than those of the protrusions of the intermediate cup-shaped product of the battery can  58 , and the number of the protrusions  33   a  formed is twelve, which is twice six in the case of the battery can  58 . 
     Thus, when the open edge of the battery can  33  is trimmed by cutting, less material is lost. Due to the less loss in the finishing step, the material cost of the battery can  33  according to the production method of the invention is reduced compared with the can produced from the blank  54 , although the material yield of the blank  1  is almost the same as that of the conventional blank  54  of  FIG. 9A  to  FIG. 9C . 
     Also, the number of the protrusions  33   a  is as many as twelve, and their lengths (length in the longitudinal direction of the battery can  33 ) become almost the same without large variation. Hence, when the battery can  33  is detached from the DI punch  37  by the stripper  39 , the protrusions  33   a  evenly contact the stripper  39  with good balance. As a result, the battery can  33  can be smoothly detached from the DI punch  37  without slanting or the like. 
     Contrary to this, according to the conventional production method illustrated in  FIG. 9A  to  FIG. 9C , the number of the protrusions  58   a  formed on the open edge of the battery can  58  is as few as six, their heights are large, and their heights are varied. Thus, when the battery can  58  is detached from the DI punch, the protrusions  58   a  contact the stripper in an unbalanced manner, thereby resulting in bending or breakage of the protrusions  58   a , cracking or breakage of the depressed parts  58   b , etc. The broken metal pieces can cause damage to the battery can  58  or cause a can jam, resulting in a poor product yield. 
     In this embodiment, a part of the punching step is performed by the loss-part punching mechanism  12 , the remaining operation (cutting) of the punching step and the first drawing step are performed by the first blank-punching-drawing mechanism  17  and the second blank-punching-drawing mechanism  19 , and the second drawing step is performed by the drawing and ironing machine  32 . 
     [Finishing Step] 
     In this step, the twelve protrusions  33   a  formed on the open edge of the battery can  33  obtained in the previous step are cut and removed such that the battery can  33  have a predetermined flat open end face and predetermined dimensions, in order to obtain a finished product of the battery can. 
       FIG. 7A  to  FIG. 7I  are plan views showing the shapes of inter-blank loss parts  13 A to  13 I in different embodiments. The inter-blank loss parts  13 A to  13 I have an almost triangular outer shape in the same manner as the inter-blank loss part  13 , except that the shapes of partial arcs or partial lines  2 A to  2 I and connect lines  4  and  4 A are different. Each of the external tangents  3  is a straight line of predetermined length. 
     The inter-blank loss part  13 A illustrated in  FIG. 7A  has the same structure as the inter-blank loss part  13  except that it includes the partial lines  2 A instead of the partial arcs  2 . The inter-blank loss part  13 A has three vertices that are almost triangular, and each vertex has a semicircular shape formed by two partial lines  2 A and two connect lines  4 . Each partial line  2 A is a bent line consisting of two straight lines that are connected at the central part of the bent line, with the connected part protruding toward the inner side of the inter-blank loss part  13 A. That is, the partial line  2 A has a projecting part at the central part thereof. 
     The inter-blank loss part  13 B illustrated in  FIG. 7B  has the same structure as the inter-blank loss part  13  except that it includes the partial lines  2 B instead of the partial arcs  2 . The inter-blank loss part  13 B has three vertices that are almost triangular, and each vertex has a semicircular shape formed by two partial lines  2 B and two connect lines  4 . Each partial line  2 B is also bent in the same manner as the partial line  2 A, but has a semicircular projecting part  2   b  at the central part thereof. In other words, the shape of the partial line  2 B is such that two straight lines are connected by the small semicircular projecting part  2   b  at the central part thereof. The projecting part  2   b  protrudes toward the inner side of the inter-blank loss part  13 B. 
     The inter-blank loss part  13 C illustrated in  FIG. 7C  has the same structure as the inter-blank loss part  13  except that it includes the partial lines  2 C instead of the partial arcs  2 . The inter-blank loss part  13 C has three vertices that are almost triangular, and each vertex has a semicircular shape formed by two partial lines  2 C and two connect lines  4 A. Each partial line  2 C is also bent in the same manner as the partial line  2 A, but has a trapezoidal projecting part  2   c  at the central part thereof. In other words, the shape of the partial line  2 C is such that two straight lines are connected by the small semi-trapezoidal projecting part  2   c  at the central part thereof. The projecting part  2   c  protrudes toward the inner side of the inter-blank loss part  13 C. 
     The inter-blank loss part  13 D illustrated in  FIG. 7D  has the same structure as the inter-blank loss part  13  except that it includes the partial lines  2   d  instead of the partial arcs  2 . The inter-blank loss part  13 D has three vertices that are almost triangular, and each vertex has a semicircular shape formed by two partial lines  2   d  and two connect lines  4 . The shape of each partial line  2   d  is such that the central part is a straight line and both ends of the straight line are connected to the connect lines  4  by arc-shaped curves. In other words, the partial line  2   d  is composed of the central straight line both ends of which are extended to the connect lines  4  via the arc-shaped parts. The partial line  2   d  as a whole protrudes toward the inner side of the inter-blank loss part  13 D. 
     The inter-blank loss part  13 E illustrated in  FIG. 7E  has the same structure as the inter-blank loss part  13  except that it includes the partial lines  2   e  instead of the partial arcs  2 . The inter-blank loss part  13 E has three vertices that are almost triangular, and each vertex has a semicircular shape formed by two partial lines  2   e  and two connect lines  4 . The shape of each partial line  2   e  is such that the central part is an arc-shaped curve and both ends of this curve are connected to the connect lines  4  by straight lines. In other words, the partial line  2   e  is composed of the central arc-shaped part both ends of which are extended to the connect line  4  via the straight line parts. The partial line  2   e  as a whole protrudes toward the inner side of the inter-blank loss part  13 E. 
     The inter-blank loss part  13 F illustrated in  FIG. 1F  has the same structure as the inter-blank loss part  13  except that it includes the partial lines  2 F instead of the partial arcs  2  and includes the connect lines  4 A instead of the connect lines  4 . The inter-blank loss part  13 F has three vertices that are almost triangular, and each vertex has a trapezoidal shape formed by two partial lines  2 F and two connect lines  4 A. Each of the partial lines  2 F and the connect lines  4 A is a straight line. 
     The inter-blank loss part  13 G illustrated in  FIG. 7G  has the same structure as the inter-blank loss part  13  except that it includes the partial lines  2 G instead of the partial arcs  2  and includes the connect lines  4 A instead of the connect lines  4 . The inter-blank loss part  13 G has three vertices that are almost triangular, and each vertex has a trapezoidal shape formed by two partial lines  2 G and two connect lines  4 A. Each partial line  2 G is a bent line in the same manner as the partial line  2 A of the inter-blank loss part  13 A in  FIG. 7A . The partial line  2 G as a whole protrudes toward the inner side of the inter-blank loss part  13 G. Each connect line  4 A is a straight line. 
     The inter-blank loss part  13 H illustrated in  FIG. 7H  has the same structure as the inter-blank loss part  13  except that it includes the partial lines  2 H instead of the partial arcs  2  and includes the connect lines  4 A instead of the connect lines  4 . The inter-blank loss part  13 H has three vertices that are almost triangular, and each vertex has a trapezoidal shape formed by two partial lines  2 H and two connect lines  4 A. Each partial line H is a line similar to the partial line  2 D of the inter-blank loss part  13 D in  FIG. 7D . The partial line  2 H as a whole protrudes toward the inner side of the inter-blank loss part  13 H. Each connect line  4 A is a straight line. 
     The inter-blank loss part  13 I illustrated in  FIG. 7I  has the same structure as the inter-blank loss part  13  except that it includes the partial lines  2 I instead of the partial arcs  2  and includes the connect lines  4 A instead of the connect lines  4 . The inter-blank loss part  13 I has three vertices that are almost triangular, and each vertex has a trapezoidal shape formed by two partial lines  2 I and two connect lines  4 A. Each partial line  2 I is a line similar to the partial line  2 E of the inter-blank loss part  13 E in  FIG. 7E . The partial line  2 I as a whole protrudes toward the inner side of the inter-blank loss part  131 . Each connect line  4 A is a straight line. 
     The inter-blank loss parts  13 A to  13 I have an almost triangular shape, and the tip of each of the three vertices thereof is semicircular or linear. Thus, unlike the case where the tip of the vertex has a sharp-pointed shape, the die tools for punching the inter-blank loss parts  13 A to  13 I are not susceptible to chipping or abrasion. The service life of the die tools is thus not shortened, and the die tools can be used without any trouble during the service life. Also, when the inter-blank loss parts  13 A to  13 I are punched out and the part between the opposing vertices of two adjacent punched holes is cut, problems such as cutting twice do not occur in the cutting step, since the tips of the vertices are semicircular or linear. The die tools are thus not susceptible to chipping, abrasion or the like, and the predetermined service life can be secured. 
     In the inter-blank loss part  13 B or  13 C, the partial line  2 B or  2 C has the projecting part  2   b  or  2   c  at the central part thereof. In the inter-blank loss part  13 D,  13 E,  13 G,  13 H, or  13 I, the partial line  2 D,  2 E,  2 G,  2 H, or  2 I as a whole forms the projecting part  2   d ,  2   e ,  2   g ,  2   h , or  2   i . By providing these projecting parts, the number of the protrusions formed on the open edge of the intermediate cup-shaped product  10  becomes  18 , the lengths of the protrusions are further shortened, and the lengths of the protrusions become more uniform. This permits a further improvement in the strippability of the intermediate cup-shaped product  10  and the battery can  33  by the strippers  30  and  39 . Also, during the transport to the next step, it is also possible to further prevent the transport or other tools from having problems. It is also possible to further prevent the open edge of the intermediate cup-shaped product  10  and the battery can  33  from having problems such as chipping, breakage, or cracking. 
     In this embodiment, the production of the battery cans  33  has been described, but this is not to be construed as limiting. The production method of the invention is applicable to the production of, for example, cylindrical metal cans with bottoms for beverages. In this case, it is also possible to produce high quality metal cans with high productivity and in a preferable manner while reducing the material costs, in the same manner as described in this embodiment. 
     INDUSTRIAL APPLICABILITY 
     The blank for a metal can according to the invention and the production method of metal cans using the same are applicable to the production of cylindrical metal cans with bottoms. Cylindrical metal cans with bottoms are used, for example, in the fields of food, beverages, quasi-drug, transport devices, batteries or the like.