JP2025040727A - Sheet metal forming method - Google Patents

Abstract

To provide a plate material molding method capable of increasing plate thickness of an annular projection in a wide range at a simple manufacturing step while preventing unintended deformation of a material by relative push-in.SOLUTION: A plate material molding method has: a step which molds a central projection composed of a central plate part and a side wall part on an outer peripheral side in a plate-like part of a plate material; a step for molding an outer peripheral curved part on the outer periphery of the side wall part; and a step for deforming at least the outer peripheral curved part and molding an annular projection where plate thickness is selectively increased. In the step for molding the annular projection part, a pressing force is given to the central projection in the direction opposite the projection direction while the outer peripheral curved part is supported by a support member relatively movable in a projection direction of the central projection, and at least a part of what composes the side wall part on a support surface of the support member is moved while compression stress in the projection direction of the side wall part is generated, so that the annular projection whose plate thickness selectively increases is molded.SELECTED DRAWING: Figure 3

Description

The present invention relates to a method for forming sheet metal that will become, for example, can lids and can bottoms.

Metal can containers (metal cans) are required to have as thin a plate thickness as possible in order to conserve material resources and reduce weight, while still maintaining pressure resistance. For example, in the case of can lids and can bottoms, a phenomenon known as buckling can occur due to an increase in internal pressure, so there is a strong demand for ingenuity in maintaining pressure resistance.

For example, in order to reduce the thickness of the plate material that will become the can lid or bottom while maintaining its pressure resistance, it is known to form an annular protrusion consisting of an inner peripheral wall portion, a curved convex portion, and a part of the outer peripheral wall portion on the outer side of the central plate portion that will become the lid surface or bottom surface, and to increase the plate thickness of the curved convex portion.

For example, Patent Document 1 describes how, in a can lid having a center panel portion (central plate portion), a panel wall portion (inner peripheral wall portion), a chuck wall radius portion (curved convex portion), a chuck wall portion (outer peripheral wall portion), and a curled portion, the plate thickness t2 at the lower end of the panel wall portion is made greater than the plate thickness t1 of the center panel portion (t2>t1), thereby improving the pressure resistance while reducing the plate thickness of the plate material. Patent Document 1 also describes how the plate thickness t3 of the chuck wall radius portion is made greater than the plate thickness t1 of the center panel portion (t3>t1), indicating the necessary range for improving the pressure resistance.

JP 2022-16093 A

However, when forming an annular protrusion on a plate material using a conventional method, such as the can lid manufacturing method described in Patent Document 1, if the annular protrusion is formed in a single stroke using upper and lower dies, the plate thickness on the inner circumference side of the annular protrusion increases significantly, and it is not possible to increase the plate thickness of the annular protrusion over a wide range. In order to increase the plate thickness of the annular protrusion over a wide range using the conventional method, the central plate portion must be pressed relative to the plate portion in multiple stages, and a die of a different shape must be used each time, making the manufacturing process complicated.

Furthermore, by pressing (pushing) the plate material relatively, not only does it increase in thickness but it also tries to release stress by deformation and material flow. However, with conventional methods, the plate material is prone to flow toward the outer wall, making it difficult to maintain the required shape for, for example, a can lid or bottom. In other words, the range in which relative pressing can be performed while maintaining the shape is narrow, and depending on the required performance, there are cases where a sufficient increase in plate thickness cannot be achieved.

The present invention aims to address these circumstances. In other words, the objective of the present invention is to provide a method for forming a sheet material that can increase the thickness of the annular protrusion over a wide range using a simple manufacturing process while preventing unintended deformation of the material due to relative pressing (pressing).

In order to solve such problems, the method for forming a plate material of the present invention has the following features.
The method includes the steps of: forming a central protrusion consisting of a central plate portion and a side wall portion on its outer periphery in a plate-shaped portion of a plate material; forming an outer periphery curved portion on the outer periphery of the side wall portion; and deforming at least the outer periphery curved portion to form an annular protrusion having a selectively increased plate thickness, wherein in the step of forming the annular protrusion, the outer periphery curved portion is supported by a support member that is movable relatively in the protruding direction of the central protrusion, and a pressing force is applied to the central protrusion in the opposite direction to the protruding direction, thereby generating a compressive stress in the protruding direction in the side wall portion, while moving at least a portion that constituted the side wall portion on the support surface of the support member, thereby forming the annular protrusion having a selectively increased plate thickness.

The sheet material forming method of the present invention, which has these characteristics, can increase the sheet thickness of the annular protrusion over a wide range using a simple manufacturing process while preventing unintended deformation of the material due to relative pressing (pressing).

FIG. 1 is a side view of a plate having an annular protrusion in schematic form. FIG. 1 is a flow diagram showing a process for forming a plate material having an annular protrusion in a schematic form. 1A to 1C are explanatory diagrams of a forming process of a plate material having an annular protrusion in a schematic form. FIG. 2 is a side view of the can lid according to the first embodiment. FIG. 2 is a flow chart showing a can lid forming process in the first embodiment. FIG. 4 is a first explanatory diagram of a can lid forming process in the first embodiment. FIG. 4 is a second explanatory diagram of the can lid forming process in the first embodiment. 1 is a side cross-sectional view showing a schematic configuration of a forming device used in a can lid forming process in the first embodiment. FIG. 5 is an explanatory diagram of a ring-shaped protrusion forming process (plate thickness increasing forming) process and a can lid shape forming process in the can lid forming process in the first embodiment. FIG. FIG. 11 is a first explanatory diagram of an annular protrusion forming (plate thickness increasing forming) step in the can lid forming process in the first embodiment. FIG. 11 is a second explanatory diagram of the annular protrusion forming (plate thickness increasing forming) step in the can lid forming process in the first embodiment. FIG. 2 is a partial end view partially showing a side end surface of the can end preform in the first embodiment. FIG. 2 is a partial end view partially showing a side end surface of the can end preform after the annular protrusion forming (plate thickness increasing forming) step in the first embodiment. FIG. 11 is an explanatory diagram of an annular protrusion forming (plate thickness increasing) process in an example of the first embodiment. FIG. 11 is a diagram showing the lip height [mm] versus the support force [kN] in an example of the first embodiment. FIG. 2 is a partial end view partially showing a side end surface of a can end preform after an annular protrusion forming (plate thickness increasing) process in an example of the first embodiment. FIG. 14B is a partially enlarged view showing a part of FIG. 14A. FIG. 11 is a diagram showing the plate thickness [mm] at measurement points A to F of the annular protrusion relative to the support force [kN] during the annular protrusion molding (plate thickness increasing molding) process in an example of the first embodiment. 10 is an explanatory diagram of a can bottom forming process in the second embodiment. FIG.

Hereinafter, an embodiment of the present invention (the present embodiment) will be described with reference to the drawings. In the following description, the same reference numerals in different drawings indicate parts with the same function, and duplicate descriptions in each drawing will be omitted as appropriate.

[Overview]
A method for forming a plate material, which is a general embodiment (summary form) of the present invention, forms a formed material 1 shown in Fig. 1. The formed material 1 has a center panel portion (center plate portion) 12 which is a generally circular, generally flat surface located approximately at the center of a generally disk-shaped plate material 11, an annular protruding portion 13 located on the outer periphery of the center panel portion, and an outer panel portion (outer plate portion) 14 located further on the outer periphery of the center panel portion.

The plate material 11 is made of a material mainly made of metal. The material mainly made of metal is not particularly limited, but may be, for example, a single metal material such as aluminum, an aluminum alloy, or steel, or a clad material of these metals, or a composite metal material in which one or both sides of these metals are subjected to a surface treatment, painting, lamination, or the like.

A curved portion 121 is formed on the outer edge of the center panel portion 12, and an inner wall portion 131 is formed so as to extend downward in a substantially straight line from the end of the curved portion 121. The curved convex portion 132 is a curved portion formed so that its inner end is connected to the lower end of the inner wall portion 131 and its outer end is connected to the outer wall portion 133. The outer wall portion 133 is formed so as to extend upward in a substantially straight line from the upper end of the outer periphery side of the curved convex portion 132 and to be connected to the curved portion 141 of the outer panel portion 14. The annular protrusion 13 is composed of the inner wall portion 131, the curved convex portion 132, and the outer wall portion 133.

As shown in Figure 2, the forming process of this formed material 1 includes a blank punching process S1, a central protrusion forming process S2, a peripheral curved portion forming process S3, and an annular protrusion forming process (plate thickness increase forming) S4. In these forming processes (S1 to S4), the shape of the plate material 11 is changed as shown in states a1 to a5 in Figure 3 to form the formed material 1 in state a6.

In the blank punching process S1, the shape of the plate material 11 to be processed is formed into a roughly disk shape (state a1 in Figure 3), although the shape varies depending on the desired product (for example, it may be elliptical or polygonal). For example, upper and lower punching tools (not shown) are used to punch the plate material 11 to be processed into a roughly disk shape.

In the central protrusion forming process S2, for example, using upper and lower central drawing tools (not shown) for drawing the approximately central portion of the approximately disc-shaped plate material 11, the outer panel portion (outer plate portion) 25 of the approximately disc-shaped plate material 11 is fixed while drawing the inner periphery, thereby forming the central protrusion 22 consisting of the center panel portion (central plate portion) 23 and the side wall portion 24 on the outer periphery, as shown in state a2 in Figure 3.

In the outer peripheral curved portion forming process S3 and the annular protrusion forming (plate thickness increase forming) process S4, for example, the plate material 11 on which the central protrusion 22 has been formed is formed using an upper tool U1 and a lower tool L1. The upper tool U1 comprises an annular upper outer tool U11, an upper central tool U12, and an annular side wall guide tool U13 located between them. The lower tool L1 comprises an annular lower outer tool L11, a lower central tool L12, and an annular curved portion support tool (support member) L13 located between them.

In the outer periphery curved portion forming process S3, as shown in state a3 in FIG. 3, the plate material 11 on which the central protrusion 22 consisting of the center panel portion 23 and the side wall portion 24 has been formed is placed on the lower tool L1, and then the upper outer tool U11 is lowered. As a result, the outer panel portion (outer plate portion) 25 of the plate material 11 is sandwiched and fixed between the abutment surface U11a of the upper outer tool U11 and the abutment surface L11a of the lower outer tool L11. This fixation of the outer panel portion continues until the annular protrusion forming (plate thickness increase forming) process S4 is completed.

In this state, in the outer periphery curved portion forming process S3, the upper central tool U12 and the side wall guide tool U13 are lowered as shown in states a3 and a4 in FIG. 3. At this time, the lowering of the upper central tool U12 pushes down the lower central tool L12 so that the contact surface U12a of the upper central tool U12 pinches the center panel portion 23 between the contact surface L12a of the lower central tool L12 (state a4 in FIG. 3). In addition, the lowering of the side wall guide tool U13 positions the side wall portion 24, which is inclined downward and outward, between the inner guide surface U13a of the side wall guide tool U13 and the outer guide surface L12b of the lower central tool L12. By positioning the side wall guide tool U13 and the lower central tool L12 around the side wall portion 24, when compressive stress is applied to the side wall portion 24 in the subsequent annular protrusion forming (plate thickness increasing forming) process S4, buckling of the side wall portion 24 due to the compressive stress can be prevented.

The upper central tool U12 descends so that the height position of its contact surface U12a approaches the height position of the contact surface (support surface) L13a of the curved portion support tool (support member) L13. That is, the curved portion support tool (support member) L13 is relatively movable in the protruding direction (upward direction) of the central protrusion 22. This curved portion support tool L13 applies a support force toward the protruding direction to the plate material 11 on its contact surface (support surface) L13a. Therefore, as the upper central tool U12 and the side wall guide tool U13 descend, the outer periphery of the side wall portion 24 is supported by the curved portion support tool L13 that applies this support force, and a curved outer periphery curved portion 26 is formed in a curved shape that follows the shape of the contact surface L13a. In this way, a preform 11a having a peripheral curved portion 26 is formed from the plate material 11 (state a4 in FIG. 3).

In the annular protrusion molding (plate thickness increase molding) process S4, for example, as shown in states a4 and a5 in FIG. 3, the outer peripheral curved portion 26 of the molded preform 11a is supported by the contact surface (support surface) L13a of the curved portion support tool (support member) L13, while a pressing force is applied to the central protrusion 22 in the direction opposite to the protruding direction.

At this time, the upper central tool U12 continuously descends so as to press down the lower central tool L12 with its contact surface U12a. This applies a pressing force to the center panel portion 23 sandwiched between the contact surface U12a and the contact surface L12a of the lower central tool L12, and presses the center panel portion 23 downward. At the same time, the side wall portion guide tool U13 positions the side wall portion 24 between its inner guide surface U13a and the contact surface L12b of the lower central tool L12, thereby preventing the side wall portion 24 from buckling due to the compressive stress applied to the side wall portion 24. As a result, the upper central tool U12 applies a pressing force greater than the supporting force of the curved portion support tool L13 to the outer circumferential curved portion 26 via the side wall portion 24.

By forming an annular protrusion (thickening formation) by successively lowering the upper central tool U12 and the side wall guide tool U13 in this manner, the outer circumferential curved portion 26 is deformed, and at least a portion that constituted the side wall portion 24 on the contact surface L13a of the curved portion support tool L13 is moved outwardly from the central protrusion 22 while compressive stress is generated in the protruding direction in the side wall portion 24. As a result, an annular protrusion 13 is formed in which the plate thickness is selectively increased based on the moved portion (state a5 in FIG. 3).

After forming the formed material 1 having this annular protrusion 13, the formed material 1 is removed from the upper tool U1 and the lower tool L1 to obtain the formed material 1 in state a6 in FIG. 3. In the formed material 1, the annular protrusion 13 has an increased thickness over a wide area, including the curved convex portion 132 between the inner wall portion 131 and the outer wall portion 133.

In this method of forming the sheet material 11 in the outline form, the center panel portion 23 is pressed (pressed) relative to one another continuously (in one stroke) using the same device (upper and lower dies) having an upper tool U1 and a lower tool L1. In other words, this method of forming the outline form does not perform this pressing in multiple stages using dies of different shapes each time, but rather can form the annular protrusion 13 with a wide range of increased sheet thickness in this simple manufacturing process.

In addition, instead of the example described here, the forming process of the formed material 1 may use the same device (upper and lower dies) consistently from the blank punching process S1 to the annular protrusion forming (thickness increasing) process S4. Alternatively, the forming process of the formed material 1 may use a device (upper and lower dies) different from the device (device having the upper tool U1 and lower tool L1) used in the annular protrusion forming (thickness increasing) process S4 from the blank punching process S1 to the outer periphery curved portion forming process S3. In this case, the preform 11a is prepared in advance in this different device, and the preform 11a is sequentially placed between the upper tool U1 and lower tool L1 described above.

In addition, in this overview, an example has been described in which the upper central tool U12 is lowered relative to the curved portion support tool (support member) L13 so as to approach the curved portion support tool (support member) L13, but this is not limiting. The curved portion support tool (support member) L13 is movable relatively in the protruding direction (upward direction) of the central protrusion 22. Therefore, instead of the example described here, the curved portion support tool (support member) L13 may be raised relative to the upper central tool U12 so as to approach the upper central tool U12, thereby applying a pressing force in this protruding direction to the plate material 11 on the abutment surface (support surface) L13a.

In this case, the center panel portion 23 is sandwiched and fixed between the upper central tool U12 and the lower central tool L12, and in this state, the lower outer tool L11 and the curved portion support tool (support member) L13 rise so as to push up the upper outer tool U11 and the side wall guide tool U13. This forms an outer peripheral curved portion 26 having a curved shape that follows the shape of the contact surface L13a of the curved portion support tool L13 on the outer periphery of the side wall portion 24, and then the annular protrusion 13 is formed.

The following describes first and second embodiments, which are specific embodiments of the present invention. In the first and second embodiments, descriptions of configurations that are the same as those in the above-mentioned general form or configurations that are the same as each other will be omitted.

[First embodiment]
The method for forming a plate material in the first embodiment is for forming a can lid 3, as shown in Fig. 4, as a formed material. The can lid 3 has a center panel portion (central plate portion) 31, a panel wall portion (inner peripheral wall portion) 321, a chuck wall portion consisting of a first chuck wall portion (first outer peripheral wall portion) 323 and a second chuck wall portion (second outer peripheral wall portion) 33, and a curl portion 34. Here, the panel wall portion 321, the chuck wall radius portion 322, and the first chuck wall portion 323 form an annular protruding portion 32.

The center panel portion (central plate portion) 31 is located approximately in the center of the can lid 3 and consists of a generally circular, generally flat surface. If the can lid 3 is a stay-on-tab type, the center panel portion 31 is provided with a tab or score for opening.

A curved portion 311 is formed on the outer edge of the center panel portion 31, and a panel wall portion (inner peripheral wall portion) 321 is formed so as to extend downward in a substantially straight line from the end of the curved portion 311. The chuck wall radius portion (curved convex portion) 322 is a curved portion formed so that its inner peripheral end is connected to the lower end of the panel wall portion 321 and its outer peripheral end is connected to the first chuck wall portion (first outer peripheral wall portion) 323.

The first chuck wall portion (first outer peripheral wall portion) 323 extends upward in a substantially straight line from the upper end of the outer peripheral side of the chuck wall radius portion (curved convex portion) 322. The second chuck wall portion (second outer peripheral wall portion) 33 extends in a substantially straight line from the upper end of the outer peripheral side of the first chuck wall portion 323 so as to be inclined upward and outward, and is formed so that its upper end is connected to the curved portion of the curl portion 34.

This can lid 3 forming method includes the method of forming the sheet material 11 in the above-mentioned outline form as part of the entire process. That is, as shown in FIG. 5, this can lid 3 forming method includes a blank punching process S11, an outer periphery drawing process S12, a central protrusion forming process S13, an outer periphery curved portion forming process S14, an annular protrusion forming (sheet thickness increasing forming) process S15, and a can lid shape forming process S16. In these forming processes (S11 to S16), the shape of the sheet material 41 is changed as shown in states a11 to a18 in FIG. 6 and FIG. 7 to form the can lid 3 in state a19 in FIG. 7.

In the blank punching process S11, for example, upper and lower punching tools (not shown) are used to punch the workpiece plate material 41 into a substantially disk-like shape, and the plate material 41 is formed into a substantially disk-like shape (state a11 in FIG. 6).

The plate material 41 is made of a material mainly composed of metal. The metal material is not particularly limited, but may be, for example, a single metal material such as aluminum, an aluminum alloy, or steel, or a clad material of these metals, or a composite metal material in which one or both sides of these metals have been subjected to a surface treatment, painting, lamination, or the like.

In the outer peripheral drawing process S12, for example, an upper and lower outer peripheral drawing tool (not shown) for drawing the outer peripheral portion of the approximately disk-shaped plate material 41 is used to draw the outer peripheral portion while fixing the plate-shaped portion on the inner side of the outer peripheral portion of the approximately disk-shaped plate material 41 shown in state a11 in FIG. 6. This forms an outer peripheral portion (side surface) 43 that extends approximately perpendicularly (downward) to the plate-shaped portion 42 on the outer peripheral side of the plate-shaped portion 42 of the plate material 41, as shown in state a12 in FIG. 6.

In the central protrusion forming process S13, for example, for the plate material 41 in state a12 of FIG. 6, a central drawing upper and lower tool (not shown) for drawing the approximate center of the plate-shaped portion 42 is used to draw the inner periphery of the plate-shaped portion 42 while fixing the outer panel portion (outer plate portion) 420, which is the outer peripheral portion of the plate-shaped portion 42. As a result, as shown in state a13 of FIG. 6, a central protrusion 44 consisting of a center panel portion (center plate portion) 45 and a side wall portion 46 on the outer peripheral side is formed on the inner periphery side of the outer panel portion (outer plate portion) 421 of the plate material 41. The shape of the center panel portion (center plate portion) 45 is not particularly limited, but may have a curved (dome) shape that protrudes gently in the protruding direction of the central protrusion 44, as exemplified in FIG. 6.

In the outer peripheral curved portion forming process S14 and the annular protrusion forming (plate thickness increase forming) process S15, the plate material 41 with the central protrusion 44 formed therein (state a13 in FIG. 6) is formed using a forming device M1 having an upper tool U2 and a lower tool L2, for example, as shown in FIG. 8. As shown in FIG. 8 and state a14 in FIG. 6, the upper tool U2 includes an annular upper outer tool U21, an upper central tool U22, and an annular side wall guide tool U23 located therebetween. The lower tool L2 includes an annular lower outer tool L21, a lower central tool L22, and an annular curved portion support tool (support member) L23 located therebetween.

As shown in FIG. 8, the molding device M1 has an upper tool U2 equipped with cushion pins Ua1 and Ua2 connected to an air cylinder, and a lower tool L2 equipped with cushion pins La1 and La2 connected to an air cylinder. In the upper tool U2, the upper central tool U22 and the side wall guide tool U23 can be moved up and down by the cushion pin Ua1 connected to the air cylinder, and the upper outer tool U21 can be moved up and down by the cushion pin Ua2 connected to the air cylinder.

In the lower tool L2, the lower center tool L22 and the curved section support tool (support member) L23 can be moved up and down by cushion pin La1 connected to an air cylinder, and the lower outer tool L21 can be moved up and down by cushion pin La2 connected to an air cylinder. Note that the molding device M1 may be equipped with an elastic member such as a spring member instead of the cushion pins Ua1, Ua2, La1, and La2 connected to such air cylinders.

In the outer peripheral curved portion forming process S14, as shown in state a14 in FIG. 6, the plate material 41 with the central protrusion 44 formed therein is placed on the lower tool L2, and then the upper outer tool U21 is lowered. This causes the outer panel portion 421 of the plate material 41 to be sandwiched and fixed between the abutment surface U21a of the upper outer tool U21 and the abutment surface L21a of the lower outer tool L21. This fixation of the outer panel portion 421 continues until the annular protrusion forming (plate thickness increasing forming) process S15 is completed.

In this state, in the outer periphery curved portion forming process S14, the upper central tool U22 and the side wall guide tool U23 are lowered. At this time, as shown in states a14 and a15 in FIG. 6, the lowering of the upper central tool U22 presses down the lower central tool L22 so that the abutment surface U22a sandwiches the center panel portion 45 between the abutment surface L22a and the upper central tool U22.

The upper central tool U22 descends so that the height position of its contact surface U22a approaches the height position of the contact surface (support surface) L23a of the curved portion support tool (support member) L23. That is, the curved portion support tool (support member) L23 is relatively movable in the protruding direction (upward direction) of the central protrusion 44. This curved portion support tool L23 applies a support force toward the protruding direction to the plate material 41 on its contact surface (support surface) L23a. Therefore, as the upper central tool U22 and the side wall guide tool U23 descend, the outer periphery of the side wall portion 46 is supported by the curved portion support tool L23 that applies this support force, and a curved outer periphery curved portion 47 is formed in a curved shape that follows the shape of the contact surface L23a. In this way, a preform 41a having a peripheral curved portion 47 is formed from the plate material 41 (state a15 in FIG. 6).

In the annular protrusion molding (plate thickness increasing process) process S15 (state a15 and state a16 in FIG. 6), for example, the outer peripheral curved portion 47 of the molded preform 41a is supported by the contact surface (support surface) L23a of the curved portion support tool (support member) L23, while applying a pressing force to the central protrusion 44 in the direction opposite to the protruding direction.

At this time, the upper central tool U22 continuously descends so as to press down the lower central tool L22 with its contact surface U22a. This applies a pressing force to the center panel portion 45 sandwiched between the contact surface U22a and the contact surface L22a of the lower central tool L22, and presses the center panel portion 45 downward. At the same time, the side wall portion guide tool U23 positions the side wall portion 46 between its inner guide surface U23a and the outer guide surface L22b of the lower central tool L22, thereby preventing the side wall portion 24 from buckling due to the compressive stress applied to the side wall portion 46. This causes the upper central tool U22 to apply a pressing force to the outer peripheral curved portion 47 via the side wall portion 46.

This pressing (thickening forming) caused by the continuous descent of the upper central tool U22 and the side wall guide tool U23 pushes the center panel portion 45 downward and deforms the outer circumferential curved portion 47. This thickening forming also causes compressive stress in the side wall portion 46 in the protruding direction of the central protrusion 44, while at least a portion that constituted the side wall portion 46 is moved outwardly of the central protrusion 44 on the contact surface L23a of the curved portion support tool L23.

By this annular protrusion forming (thickness increasing forming) process S15, an intermediate formed can lid 30 (state a16 in FIG. 6) is formed (before the can lid is shaped) having an annular protrusion 32a in which the thickness is selectively increased based on the moved portion. This annular protrusion 32a has an increased thickness over a wide area, including the chuck wall radius portion (curved convex portion) 322a.

In this way, in the molding method of the plate material 41 of the first embodiment, the center panel portion 45 is pressed (pressed) relative to one another continuously (in one stroke) using the same device (upper and lower dies) having the upper tool U2 and the lower tool L2. In other words, the molding method of the first embodiment does not perform this pressing in multiple stages using dies of different shapes each time, but rather can mold the annular protrusion 32a with an increased plate thickness over a wide range in this simple manufacturing process.

In this manner, the intermediate can lid formed body 30 (before the can lid is shaped) having the annular protrusion 32a is formed, and then the intermediate can lid formed body 30 is removed from the upper tool U2 and the lower tool L2.

The can lid shape forming process S16 is a process for reforming the shape of the intermediate molded body 30 of the can lid to form it into the shape of a can lid. This can lid shape forming process S16 includes a process for forming a chuck wall portion consisting of a first chuck wall portion (first outer peripheral wall portion) 323 and a second chuck wall portion (second outer peripheral wall portion) 33 shown in the latter state a19.

The can lid shape forming process S16 may further include at least one or both of the following steps, in any order or simultaneously with the above-mentioned process for forming the chuck wall portion: a known process (not shown) for forming the curled portion 34 on the outer periphery side of the annular protrusion 32a (or the annular protrusion 32 after forming) and a process for forming the center panel portion (central plate portion) 45 into a desired shape, for example, into a substantially flat surface.

In the can lid shape forming process S16, as shown in state a17 of FIG. 7, for example, the shape of the intermediate formed body 30 of the can lid is reformed using a can lid shape forming device M2 having an upper tool U3 and a lower tool L3. The upper tool U3 has an upper outer tool U31, an upper central tool U32, and an annular processing tool U33 located between them. The lower tool L3 has an annular lower outer tool L31 and a lower central tool L32. The lower tool L3 also has an annular annular protrusion support tool L33 located between the lower outer tool L31 and the lower central tool L32.

In this can lid shape forming process S16, as shown in state a17 in FIG. 7, after placing the intermediate formed body 30 of the can lid (before the can lid is shaped) on the lower tool L3, the upper outer tool U31, the upper central tool U32, and the processing tool U33 may be lowered, for example, simultaneously to perform the can lid shape forming process S16. Alternatively, depending on the shape of the can lid, the timing of the descent of the upper outer tool U31, the upper central tool U32, and the processing tool U33 may be changed as appropriate.

At this time, as the upper central tool U32 descends, the annular protrusion 32a is supported by the contact surface (support surface) L33a of the annular protrusion support tool L33, and the annular protrusion 32a between the lower central tool L32 and the lower outer tool L31 is formed into an annular protrusion 32 (state a18 in FIG. 7) shaped along the contact surface (support surface) L33a of the annular protrusion support tool L33. That is, at this time, the first chuck wall portion (first outer peripheral wall portion) 323 (see state a19) in the chuck wall portion is formed.

At this time, the upper outer tool U31 descends, so that its contact surface U31a comes into contact with the outer panel portion 421 on the contact surface (support surface) L31b of the lower outer tool L31, fixing the outer panel portion 421. In this state, the processing tool U33 descends, so that the outer panel portion 421 is sandwiched between the contact surface U33a and the contact surface L31a, thereby forming a second chuck wall portion (second outer peripheral wall portion) 33 (see state a19) that conforms to the shapes of the lower outer tool L31 and the processing tool U33. In this way, a chuck wall portion (see state a19) consisting of the first chuck wall portion (first outer peripheral wall portion) 323 and the second chuck wall portion (second outer peripheral wall portion) 33 on the outer periphery side of the first chuck wall portion (first outer peripheral wall portion) 323 is formed.

At this time, the curled portion 34 can also be formed on the outer periphery of the lid. In this case, the lower tool L3 may be provided with a curling tool (not shown) on the outer periphery of the lower outer tool L31. As described above, when the processing tool U33 descends with the outer panel portion 421 fixed, the outer panel portion 421 sandwiched between the abutment surface U33a and the abutment surface L31a flows toward the outer periphery. When the curling tool (not shown) descends in this state, the curled portion 34 is formed in a shape that conforms to the abutment surface U31a of the upper outer tool U31 and the abutment surface (not shown) of the curling tool (not shown) (state a18 in FIG. 7).

At this time, the center panel portion (central plate portion) 45 can also be formed into a desired shape, for example, into a substantially flat surface. That is, the upper central tool U32 descends so that its contact surface U32a presses down on the lower central tool L32, thereby forming the center panel portion (central plate portion) 45 sandwiched between the contact surface U32a and the contact surface L32a of the lower central tool L32 into a substantially flat center panel portion (central plate portion) 31 (state a18 in FIG. 7).

The can lid 3 reformed in this manner is removed from the can lid shape forming device M2 to obtain the can lid 3 shown in state a19 in FIG. 7. The reformed can lid 3 in state a19 has a shape in which the annular protruding portion 32, consisting of the panel wall portion (inner peripheral wall portion) 321, the chuck wall radius portion (curved convex portion) 322, and the first chuck wall portion (first outer peripheral wall portion) 323, protrudes approximately perpendicularly to the extension direction of the center panel portion 31, which has been shaped into a substantially flat surface.

Note that, here, the can lid shape forming process S16 in FIG. 7 has been described as simultaneously including a process for forming the chuck wall portion and a process for forming the center panel portion (center plate portion) 45 into a substantially flat surface, but these processes may be included in any order as separate steps.

Here, the annular protrusion forming (thickness increase forming) process S15 and the can lid shape forming process S16 will be further explained. In FIG. 9, which is drawn without the upper tool, state b1 shows the shape of a portion of the preform 41a formed in the outer periphery curved portion forming process S14. In this state b1, the center panel portion 45 is located above the uppermost position H, which is the highest vertical height of the outer panel portion 421.

In this annular protrusion forming (thickness increase forming) process S15, as shown in FIG. 10A, the outer panel portion 421 of the preform 41a is sandwiched between the upper outer tool U21 and the lower outer tool L21 to create a restraint portion K1 and fix the outer panel portion 421. Then, in this state, the abutment surface (support surface) L23a of the curved portion support tool L23 applies an upward support force, as shown by the arrow Y101, to the outer peripheral curved portion 47. At this point, the preform 41a having this outer peripheral curved portion 47 has the shape shown in FIG. 11A.

Then, in this state, the upper center tool U22 and the side wall guide tool U23 are lowered. At this time, as shown by arrow Y91 in state b2 of FIG. 9, the outer periphery curved portion 47 of the preform 41a is supported by a supporting force from below, and as shown by arrow Y92, a pressing force is applied from above the center panel portion 45 of the preform 41a. This pressing force from above and below generates a compressive stress in the outer periphery curved portion 47.

Then, in this annular protrusion forming (plate thickness increase forming) process S15, the upper central tool U22 and the side wall guide tool U23 are further lowered. With this continuous lowering, as shown in states b2 and b3 in FIG. 9, the downward pressing force (by the upper central tool U22) indicated by arrow Y92 becomes greater than the upward supporting force (by the curved portion support tool L23) indicated by arrow Y91. As a result, the curved portion support tool L23 is pressed down.

At this time, the upper central tool U22 applies a further downward pressure, as indicated by the arrow Y102, to the center panel portion 45 of the preform 41a, so that the center panel portion 45 is pressed further downward as shown in FIG. 10B. This causes the outer peripheral curved portion 47 shown in FIG. 10A to deform.

As shown in Figure 10B, a compressive stress is applied to the side wall 46 (Figure 10A) in the protruding direction of the central protrusion 44 by the downward pressing force indicated by arrow Y103 from the upper central tool U22 and the upward supporting force indicated by arrow Y104 from the curved portion support tool L23. As a result, as shown in Figure 10B, at least a portion (Figure 10A) that constituted the side wall 46 on the contact surface L23a of the curved portion support tool L23 moves outward from the central protrusion 44 as indicated by arrow Y105, while generating a compressed portion K2 due to this compressive stress.

At this point, the formed body 30a-1 shown in FIG. 11B is formed. This formed body 30a-1 has an annular protruding portion 32a consisting of a panel wall portion (inner peripheral wall portion) 321a, a chuck wall radius portion (curved convex portion) 322a, and a first chuck wall portion (first outer peripheral wall portion) 323a, and the plate thickness is increased over a wide range including the chuck wall radius portion 322a.

In this way, the intermediate can lid formed body 30 (before the can lid is shaped) shown in state b3 in Figure 9 is formed. As shown in state b3, in the intermediate can lid formed body 30, the uppermost position of the center panel portion 45 is located below the uppermost position H of the outer panel portion 421.

Then, in the can lid shape forming process S16, as shown in state b4 in FIG. 9, the intermediate formed body 30 of the can lid is reformed by applying downward pressing force as indicated by arrows Y93 and Y94. As a result, as shown in state b4, a can lid 3 is obtained that has a formed annular protrusion 32, a chuck wall portion, and a center panel portion (central plate portion) 31 consisting of a substantially flat surface. In the can lid 3, the formed annular protrusion 32 has an increased plate thickness within the required range, including the chuck wall radius portion 322.

In addition, instead of the example described here, the can lid 3 forming process may use the same device (upper and lower dies) from the blank punching process S11 to the annular protrusion forming (thickness increasing) process S15. Alternatively, the can lid 3 forming process may use a device (upper and lower dies) different from the device (device having the above-mentioned upper tool U2 and lower tool L2) used in the annular protrusion forming (thickness increasing) process S15 from the blank punching process S11 to the outer periphery curved portion forming process S14. In this case, the preform 41a is prepared in advance in this different device, and the preform 41a is placed between the upper tool U2 and the lower tool L2.

[Example of the first embodiment]
Next, an example of the first embodiment will be described. In this example, as shown in Fig. 12, a preform (not shown) was used to form an intermediate molded body 30A for a can lid using a molding device M3. The preform was formed from a plate material made of an aluminum alloy (A5182P-H19, material plate thickness 0.235 mm) with a coating formed on both sides.

The molding device M3 in FIG. 12 has the same configuration as the molding device M1 described above. That is, in the molding device M3, the upper tool U2A corresponds to the upper tool U2 described above, and the lower tool L2A corresponds to the lower tool L2 described above. As a result, the upper outer tool U21A, the upper central tool U22A, and the side wall guide tool U23A correspond to the upper outer tool U21, the upper central tool U22, and the side wall guide tool U23 described above. In addition, the lower outer tool L21A, the lower central tool L22A, and the curved portion support tool (support member) L23A correspond to the lower outer tool L21, the lower central tool L22, and the curved portion support tool (support member) L23 described above.

Using this forming device M3, the same process as the annular protrusion forming (plate thickness increase forming) process S15 described above was performed. That is, as shown in FIG. 12, a downward pressing force (8.0 kN) indicated by the arrow Y202 was applied by the upper outer tool U21A to the lower outer tool L21A, thereby fixing the outer panel portion 421A sandwiched between the lower outer tool L21A and the upper outer tool U21A.

In this state, the lower center tool L22A and the curved portion support tool (support member) L23A supported the portion that would become the annular protrusion 32A together with the center panel portion 45A with the upward supporting forces indicated by arrows Y203 and Y204, while the lower center tool L22A and the side wall guide tool U23A were continuously lowered to continuously apply a downward pressing force indicated by arrow Y201 (plate thickness increasing molding). Through this plate thickness increasing molding, the center panel portion 45A and the annular protrusion 32A were formed.

In this way, an intermediate can lid molded body 30A (before the can lid is shaped) was obtained, which includes a center panel portion 45A and an annular protruding portion 32A having a chuck wall radius portion (curved convex portion) 322A on its outer periphery.

In FIG. 12, LH (Lip Height) is the vertical length [mm] from the top position of the outer panel portion 421A to the outer tip.

In this example, the plate thickness was increased by varying the upward support force [kN] indicated by the arrow Y204 by the curved section support tool (support member) L23A. The measured values of the lip height (LH in FIG. 12) [mm] for each value of support force [kN] are shown in FIG. 13.

In Figure 13, the black circle plots indicate the value of lip height (mm). As shown in Figure 13, even if the support force (kN) was increased, there was no significant change in the value of lip height (mm). This result means that the plate thickness increase molding does not affect the shape of the outer panel portion 421A.

Figure 14A shows a portion of a can lid formed in this embodiment. Figure 14B shows an enlarged view of the annular protrusion 32A in Figure 14A, and shows the panel wall portion (inner peripheral wall portion) 321A, chuck wall radius portion (curved convex portion) 322A, and first chuck wall portion (first outer peripheral wall portion) 323A that make up the annular protrusion 32A. Also, in Figure 14B, A to F indicate the measurement points for the plate thickness [mm] of the annular protrusion 32A.

In this example, the plate thickness [mm] was measured at measurement points A to F (Fig. 14B) of the annular protrusion 32A in the can lid formed as shown in Fig. 14 for each value of the upward support force [kN] indicated by the arrow Y204 (Fig. 12) by the curved portion support tool (support member) L23A during plate thickness increasing forming. The measurement results are shown in Fig. 15.

According to this embodiment, as shown in FIG. 15, by increasing the support force [kN], the plate thickness [mm] could be increased from the original plate thickness of 0.235 mm over a wide range of measurement points A to F on the annular protrusion 32A. In this embodiment, when the support force was the largest, 8.9 kN, the plate thickness increased significantly compared to the original plate thickness over a wide range of measurement points A to F. Also, as shown in FIG. 15, the plate thickness could be increased sufficiently at measurement points B to E within the range of the chuck wall radius portion (curved convex portion) 322A. Thus, in this embodiment, the plate thickness could be increased in a required range including not only the plate thickness on the inner periphery of the annular protrusion but also the plate thickness on the outer periphery of the annular protrusion in a single plate thickness increasing forming process.

[Second embodiment]
The method for forming a sheet material in the second embodiment is for forming a can bottom 6 shown in state a27 of Fig. 16 as a formed material. This method for forming a can bottom 6 includes the method for forming the sheet material 11 in the above-mentioned outline form as a part of the entire process.

This method of forming the can bottom 6 includes the same steps as those shown in Figure 5 above, namely the blank punching step S11, the outer periphery drawing step S12, the central protrusion forming step S13, the outer periphery curved portion forming step S14, the annular protrusion forming (plate thickness increasing forming) step S15, and the can bottom shape forming step S16, all of which are shown in Figure 5. In these forming steps (S11 to S16), the shape of the plate material 51 is changed as shown in states a21 to a26 in Figure 16, and then reformed to form the can bottom 6 in state a27.

In the blank punching step S11 in the process of forming the can bottom 6, for example, upper and lower punching tools (not shown) are used to punch the plate material 51 to be processed into a roughly disk-like shape, and the plate material 51 is formed into a roughly disk-like shape (state a21 in Figure 16).

In the peripheral drawing process S12 in the can bottom 6 forming process, for example, using upper and lower peripheral drawing tools (not shown) for drawing the peripheral portion of the approximately disk-shaped plate material 51, the plate-shaped portion inside the peripheral portion of the approximately disk-shaped plate material 51 shown in state a21 in FIG. 16 is fixed and the peripheral portion is drawn. As a result, as shown in state a22 in FIG. 16, a peripheral portion (side surface) 53 is formed on the outer periphery of the plate-shaped portion 52 of the plate material 51, extending approximately perpendicularly (upward) to the plate-shaped portion 52.

In the central protrusion forming step S13 in the can bottom 6 forming process, for example, for the plate material 51 in state a22 of FIG. 16, a central drawing upper and lower tool (not shown) is used to draw the approximately central portion of the plate-shaped portion 52, and the outer panel portion (outer plate portion) 521 of the plate-shaped portion 52 is fixed while being formed into an annular curved portion 522 that protrudes gently in the approximately vertical direction (downward), and in this state, drawing is performed on the inner circumference side. As a result, as shown in state a23 of FIG. 16, the central protrusion 54 consisting of the dome panel portion (central plate portion) 55 and the side wall portion 56 on the outer circumference side is formed. The dome panel portion (central plate portion) 55 has a dome shape that protrudes gently in the protruding direction of the central protrusion 54.

In the outer peripheral curved portion forming process S14 and the annular protrusion forming (plate thickness increase forming) process S15, for example, the plate material 51 (state a23 in FIG. 16) on which the central protrusion 54 has been formed is formed using a forming device having an upper tool U4 and a lower tool L4. As shown in state a24 in FIG. 16, for example, the upper tool U4 comprises an annular upper outer tool U41, an upper central tool U42, and an annular side wall guide tool U43 located therebetween. The lower tool L4 comprises an annular lower outer tool L41 and an annular curved portion support tool (support member) L42 located further inward than the annular lower outer tool L41.

In the outer periphery curved portion forming process S14, as shown in state a24 of FIG. 16, the plate material 51 with the central protrusion 54 formed therein is placed on the lower tool L4, and then the upper outer tool U41 is lowered. This causes the annular curved portion 522 of the plate material 51 to be sandwiched and fixed between the abutment surface U41a of the upper outer tool U41 and the abutment surface L41a of the lower outer tool L41. The fixing of the annular curved portion 522 continues thereafter until the annular protrusion forming (plate thickness increasing forming) process S15 is completed.

At the same time, in the outer periphery curved portion forming process S14, the upper central tool U42 and the side wall guide tool U43 are lowered. At this time, as shown in state a24 in FIG. 16, the upper central tool U42 is lowered so that its contact surface U42a contacts the dome panel portion 55.

The upper central tool U42 descends so that the height position of its contact surface U42a approaches the height position of the contact surface (support surface) L42a of the curved portion support tool (support member) L42. That is, the curved portion support tool (support member) L42 is relatively movable in the protruding direction (upward) of the central protrusion 54. When the plate material 51 is positioned on the contact surface L42a, the curved portion support tool L42 applies a support force to the plate material 51 in the protruding direction. Therefore, as the upper central tool U42 and the side wall guide tool U43 descend, the outer periphery of the side wall 56 is supported by the curved portion support tool L42 that applies this support force, and a curved outer periphery curved portion 57 is formed in a curved shape that follows the shape of the contact surface L42a. In this way, a preform 51a having an outer periphery curved portion 57 is formed from the plate material 51 (state a25 in FIG. 16).

In the annular protrusion molding (plate thickness increase molding) process S15 (states a25 and a26 in FIG. 16), for example, while the outer peripheral curved portion 57 of the molded preform 51a is supported by the contact surface (support surface) L42a of the curved portion support tool (support member) L42, a pressing force is applied to the central protrusion 54 consisting of the dome panel portion 55 and the side wall portion 56 in the direction opposite to the protruding direction.

At this time, the upper central tool U42 continuously descends so as to press down the dome panel portion 55 with its contact surface U42a, thereby applying a pressing force to the dome panel portion 55, and by lowering this dome panel portion 55, a pressing force is applied to the outer peripheral curved portion 57 located between the tip contact portion U43a and the contact surface L42a.

This pressing (thickening forming) caused by the continuous descent of the upper central tool U42 and the side wall guide tool U43 pushes the dome panel section 55 downward and deforms the outer circumferential curved section 57. This thickening forming also causes compressive stress in the side wall section 56 in the protruding direction of the central protrusion 54, while at least a portion of the side wall section 56 that constituted the side wall section 56 on the contact surface L42a of the curved section support tool L42 is moved outward from the central protrusion 54.

By this annular protrusion forming (thickness increasing forming) process S15, an intermediate can bottom formed body 60 (state a26 in FIG. 16) is formed (before the can bottom is shaped) having an annular protrusion 62a whose thickness is selectively increased based on the moved portion. This annular protrusion 62a has an increased thickness over a wide range, including the so-called bottom radius portion 622a. The formed intermediate can bottom formed body 60 (before the can bottom is shaped) is then removed from the upper tool U4 and the lower tool L4.

In this way, even in the molding method of the plate material 51 of the second embodiment, the relative pressing (pressing) of the dome panel portion 55 is performed continuously (in one stroke) using the same device (upper and lower dies) having an upper tool U4 and a lower tool L4. In other words, the molding method of the second embodiment also does not perform this pressing in multiple stages using dies of different shapes each time, but rather can mold the annular protrusion portion 62a with an increased plate thickness to the required range in this simple manufacturing process.

The can bottom shape forming process S16 is a process in which the shape of the intermediate can bottom formed body 60 is reformed as necessary to form it into the shape of the can bottom, and this process includes a process of forming the shape of the can bottom annular protrusion 62. After reforming in this can bottom shape forming process S16, the can bottom 6 is removed from the reforming device (not shown) to obtain the can bottom 6 (state a27 in Figure 16).

The can bottom shaping step S16 can be carried out by a conventionally known method, for example, a rotating roll method or a press molding method.

In the second embodiment, if an inner surface painting process is required, it is preferable to perform this process prior to the can lid shape forming process S16.

The can bottom 6 in state a27 has a dome panel portion (central plate portion) 61, a dome panel wall portion (inner peripheral wall portion) 621, a bottom radius portion (curved convex portion) 622, a chime lower portion (first outer peripheral wall portion) 623, and a chime portion (second outer peripheral wall portion) 63. Here, the dome panel wall portion 621, the bottom radius portion 622, and the chime lower portion 623 form the can bottom annular protrusion portion 62. The can bottom annular protrusion portion 62 has a shape that is slightly inwardly inclined due to reforming, and the plate thickness has been increased within the necessary range including the bottom radius portion 622.

The can bottom shape forming process S16 may further include a process of forming the dome panel portion (central plate portion) 55 into a desired shape, such as a substantially spheroidal surface, a substantially conical surface, a substantially flat surface, or other shapes other than a sphere, as necessary. In this case, the can bottom shape forming process S16 may include a process of forming the shape of the can bottom annular protrusion portion 62 and a process of forming the dome panel portion (central plate portion) 55 into, for example, a substantially flat surface, simultaneously or in any order, as a process of reforming the shape of the intermediate molded body 60 of the can bottom in state a26 of FIG. 16.

Although the embodiments of the present invention have been described above in detail with reference to the drawings, the specific configuration is not limited to these embodiments, and the present invention includes design changes and the like that do not deviate from the gist of the present invention. In addition, the above-mentioned examples can be combined by reusing each other's technologies as long as there are no particular contradictions or problems in their purpose and configuration, etc.

1: Molded material, 3: Can lid, 6: Can bottom, 11, 41, 51: Plate material, 11a, 41a, 51a: Preform, 12, 23, 31, 45, 45A: Center panel portion (central plate portion), 13, 32, 32a, 32A, 62a: Annular protrusion, 14, 25, 420, 421, 421A, 521: Outer panel portion (outer plate portion), 22, 44, 54: Central protrusion, 24, 46, 56: Side wall portion, 26, 47, 57: Outer curved portion, 30, 30A: Intermediate molded body of can lid (before the shape of the can lid is molded), 30a-1: Molded body, 33: Second chuck wall portion (second outer peripheral wall portion), 34: Curl portion, 42, 52: Plate-shaped portion, 43, 53: Outer peripheral portion (side surface), 60: (Can bottom intermediate molded body of the can bottom (before the shape is formed), 121, 141, 311: curved portion, 131: inner peripheral wall portion, 132: curved convex portion, 133: outer peripheral wall portion, 321, 321a, 321A: panel wall portion (inner peripheral wall portion), 322, 322a, 322A: chuck wall radius portion (curved convex portion), 323, 323a, 323A: first chuck wall portion (first outer peripheral wall portion), 522: annular curved portion, 55, 61: dome panel portion (center plate portion), 62: can bottom annular protrusion portion, 63: chime portion (second outer peripheral wall portion), 621: dome panel wall portion (inner peripheral wall portion), 622, 622a: bottom radius portion (curved convex portion), 623: chime lower portion (first outer peripheral wall portion)

Claims (7)

a step of forming a central protrusion portion, which is composed of a central plate portion and a side wall portion on an outer periphery of the central protrusion portion, on a plate-shaped portion of the plate material;
forming a curved outer periphery portion on an outer periphery of the side wall portion;
and forming an annular protruding portion having an increased thickness by deforming at least the outer circumferential curved portion,
In the step of forming the annular protrusion,
The outer circumferential curved portion is supported by a support member movable relative to the protruding direction of the central protruding portion, and a pressing force is applied to the central protruding portion in a direction opposite to the protruding direction, thereby generating a compressive stress in the protruding direction in the side wall portion, and at least a portion constituting the side wall portion is moved on the support surface of the support member, thereby forming the annular protruding portion having a selectively increased plate thickness.
A molding method comprising the steps of: The molding method according to claim 1, characterized in that in the step of forming the annular protrusion, at least a portion that constitutes the side wall portion on the support surface of the support member is moved outwardly of the central protrusion. The molding method according to claim 1 or 2, characterized in that the plate material is made of a material mainly composed of metal. A method for forming can ends, comprising the forming method described in claim 1 as part of the entire process. After the step of forming the annular protrusion,
The process of forming the can lid shape is as follows:
5. The method for forming a can end according to claim 4, further comprising a step of forming a chuck wall portion. A method for forming a can bottom, comprising the forming method described in claim 1 as part of the entire process. After the step of forming the annular protrusion,
The process of forming the bottom of the can is as follows:
7. The method for forming a can bottom according to claim 6, further comprising at least a step of forming a shape of the can bottom annular protrusion.