Patent Description:
A rectangular can has been used as a container in a secondary battery (power storage battery) or the like in view of accommodation capability and the like. Such a rectangular can includes a rectangular can body and a sealing lid for closing an opening of the rectangular can body. In the rectangular can, a step on which the sealing lid is to be placed is provided on an inner wall surface of the rectangular can. The sealing performance can be increased the closer an angle formed by a surface of the step and the inner wall surface is to <NUM> degrees. However, in a conventional manufacturing method, it has been difficult to bring the angle close to <NUM> degrees. PTL <NUM> discloses a manufacturing method for a rectangular can having the features of the preamble of claim <NUM>.

An object of the present disclosure is to provide a manufacturing method for a rectangular can according to which brings an angle formed by a surface of a step provided on a rectangular can body and an inner wall surface of the rectangular can body closer to <NUM> degrees.

The present disclosure employed the following means in order to achieve the object.

That is, the manufacturing method for a rectangular can of the present disclosure is a manufacturing method for a rectangular can including a rectangular can body that has a rectangular tube and a bottom, and a sealing lid for closing an opening of the rectangular can body, the manufacturing method including:.

the step is formed by performing a plurality of instances of a pressing operation performed by the punch.

According to the present disclosure, it is possible to bring the angle formed by a surface of the step provided on the rectangular can body and the inner wall surface of the rectangular can body closer to <NUM> degrees by forming the step through multiple instances of the pressing operation performed by the punch.

The outer wall surface of the sidewall formed in the step of manufacturing the intermediate molded article may be constituted by a level plane,.

This makes the forming of the step less difficult compared to the case where the step is formed by simply performing multiple instances of the pressing operation performed by the punch on a plate-shaped portion.

In the pressing operation performed by the punch, a movement amount of the punch moving toward the die may increase each instance of the pressing operation, which is performed repeatedly. In addition, in the pressing operation performed by the punch, a movement amount of the punch moving toward the die may increase in stages as the number of instances of the pressing operation, which is performed repeatedly, increases. Furthermore, in the pressing operation performed by the punch, a movement amount of the punch moving toward the die may also be the same for all pressing operations, which are performed repeatedly.

The connection surface formed in the step of manufacturing the intermediate molded article may be provided at positions reaching four corners of the rectangular tube, and the step may be formed also at the four corners in the step-forming step.

The step of manufacturing the intermediate molded article may include a drawing and ironing step of molding the rectangular tube, and
a step surface that is level with the step surface of the step may be formed in advance at the four corners of the rectangular tube through the drawing and ironing step.

A trimming step of removing a portion, which is unneeded for a final product, on a side opposite to the bottom side of the intermediate molded article may be included after the step-forming step, and
at least a part of the sidewall on which the step is to be formed in the portion to be removed in the trimming step may be removed in advance prior to the step-forming step.

As described above, the present disclosure brings the angle formed by the surface of the step provided on the rectangular can body and the inner wall surface of the rectangular can body closer to <NUM> degrees.

Hereinafter, a mode of carrying out the disclosure will be described illustratively and in detail based on an embodiment with reference to the drawings.

A manufacturing method for a rectangular can according to an embodiment of the present disclosure will be described with reference to <FIG>. A rectangular can obtained through the manufacturing method according to the present embodiment will be described before the manufacturing method for the rectangular can is described.

A rectangular can that is manufactured using the manufacturing method according to the present embodiment will be described with reference to <FIG> is a perspective view showing an overview of the rectangular can. For the sake of convenience in the description, <FIG> shows a state prior to a sealing lid <NUM> being attached to a rectangular can body <NUM>. The rectangular can manufactured through the manufacturing method includes the rectangular can body <NUM>, and the sealing lid <NUM> that closes an opening of the rectangular can body <NUM>. The rectangular can body <NUM> and the sealing lid <NUM> are obtained by machining a metal plate such as an aluminum alloy, a cold rolled steel plate, or stainless steel. The rectangular can body <NUM> is constituted by a rectangular tube having two pairs of sidewalls (short sidewalls <NUM> and long sidewalls <NUM>) that have inner wall surfaces that face each other, and a bottom <NUM>. Note that the pair of sidewalls with a shorter width among the two pairs of sidewalls are called the short sidewalls <NUM> and the pair of sidewalls with a longer width are called the long sidewalls <NUM>.

Steps 11a are respectively provided on the inner wall surfaces of the pair of short sidewalls <NUM>. In a state in which the sealing lid <NUM> is placed on surfaces of the steps 11a, the sealing lid <NUM> is fixed to the rectangular can body <NUM> through laser welding, arc welding or the like. The rectangular can according to the present embodiment can be suitably used as a container for a secondary battery (power storage battery). In this case, the sealing lid <NUM> is fixed after power generation elements constituting the battery, such as electrodes and separators, have been installed in the rectangular can body <NUM>.

An overview of a manufacturing method for a rectangular can according to the present embodiment will be described below. The manufacturing method for the rectangular can includes a step of manufacturing the rectangular can body <NUM> and a step of fixing the pre-manufactured sealing lid <NUM> to the rectangular can body <NUM> as described above. The step of manufacturing the rectangular can body <NUM> is composed of a step of molding an intermediate molded article <NUM>, a step of forming the above-described steps 11a in the intermediate molded article <NUM>, and a trimming step of removing unnecessary portions.

In addition, the step of manufacturing the intermediate molded article includes at least a step of carrying out punching on a metal plate in order to obtain a raw material (blank) with a predetermined outer shape, a step of carrying out drawing and ironing multiple times on the blank, and a pressing step of forming the above-described bottom <NUM>. Since a known technique (e.g., see <CIT>) may be employed as appropriate for these steps, description thereof is omitted. As for the trimming step, a known technique may be employed as well, thus description thereof is omitted.

The intermediate molded article <NUM> manufactured through the step of manufacturing the intermediate molded article will be described with reference to <FIG> is an overall view of the intermediate molded article of the rectangular can. <FIG> shows a plan view of the intermediate molded article, and schematic cross-sectional views of two locations (a cross-sectional view taken along line AA and a cross-sectional view taken along line BB in the plan view). The intermediate molded article <NUM> includes a pair of portions <NUM> that are to be the short sidewalls <NUM> in a final product, a pair of portions <NUM> that are to be the long sidewalls <NUM> in the final product, a portion <NUM> that is to be the bottom <NUM> in the final product, and a portion <NUM> that is to be removed during the trimming step. Note that a dotted line 150a in <FIG> shows a cutting surface that is to be cut in the trimming step. The portion <NUM> to be removed is on a side near the opening with respect to this dotted line 150a. A section constituted by the pair of short-sidewall-forming portions <NUM> and the pair of long-sidewall-forming portions <NUM> corresponds to the rectangular tube in the intermediate molded article <NUM>, and the bottom-forming portion <NUM> corresponds to the bottom in the intermediate molded article <NUM>.

In the next step, that is, the step-forming step, the steps 11a are formed on the inner wall surfaces of the short-sidewall-forming portions <NUM> serving as the sidewalls. The outer wall surface <NUM> of the short-sidewall-forming portion <NUM> is constituted by a level plane. The inner wall surface of the short-sidewall-forming portion <NUM> includes a first parallel surface <NUM> that is parallel to the outer wall surface <NUM>, a second parallel surface <NUM> that is parallel to the outer wall surface <NUM> as well, and an inclined surface <NUM> serving as a connection surface that connects the first parallel surface <NUM> and the second parallel surface <NUM>. The first parallel surface <NUM> is formed on a side near the bottom <NUM> (bottom-forming portion <NUM>) with respect to the portion where the step 11a is to be formed. The second parallel surface <NUM> is formed on a side opposite to the bottom <NUM> side (i.e., on a side near the opening) with respect to the portion where the step 11a is to be formed. The second parallel surface <NUM> is constituted such that the distance to the outer wall surface <NUM> is shorter than the distance between the first parallel surface <NUM> and the outer wall surface <NUM>. Accordingly, the short-sidewall-forming portion <NUM> is constituted such that the thickness of the portion at which the second parallel surface <NUM> is formed is less than the thickness of the portion at which the first parallel surface <NUM> is formed.

In the step-forming step described hereinafter, a portion of the inclined surface <NUM> is compressed. Accordingly, a step surface 11b of the step 11a is formed at a position of a boundary between the first parallel surface <NUM> and the inclined surface <NUM>.

In the intermediate molded article <NUM>, step surfaces <NUM> that are level with the step surfaces 11b of the step 11a are provided at four corners of the rectangular tube formed by the pair of short-sidewall-forming portions <NUM> and the pair of long-sidewall-forming portions <NUM>. The step surfaces <NUM> are formed through the drawing and ironing step in the step of manufacturing the intermediate molded article.

The step-forming step will be described with reference to <FIG>. <FIG> is a schematic cross-sectional view of a machining apparatus to be used in the step-forming step in the manufacturing method for the rectangular can. <FIG> is an illustrative diagram illustrating a mechanism of the machining apparatus to be used in the step-forming step in the manufacturing method for the rectangular can, and more specifically, is a diagram illustrating a mechanism for driving a punch. <FIG> is a process diagram of the step-forming step in the manufacturing method for the rectangular can. <FIG> shows a cross section for each member. <FIG> are diagrams illustrating examples of a manner of driving the punch.

An example of the machining apparatus that can be used in the step-forming step will be described with reference to <FIG> and <FIG>. The machining apparatus is constituted by an upper mold <NUM> and a lower mold <NUM>. The upper mold <NUM> includes a die <NUM> and a bottom pad <NUM>. The lower mold <NUM> includes a mandrel <NUM>, a punch <NUM>, a drive mechanism <NUM> that moves the punch <NUM> relative to the mandrel <NUM>, and an air vacuum hole <NUM> for vacuuming when the mandrel <NUM> is to be inserted into the intermediate molded article <NUM> and sending air when the mandrel <NUM> is to be pulled out. The lower mold <NUM> also includes an upper platform <NUM> and a lower platform <NUM> that support the mandrel <NUM> and the punch <NUM>.

The upper mold <NUM> and the lower mold <NUM> are formed to move toward each other and away from each other in the vertical direction. The intermediate molded article <NUM> is placed on the mandrel <NUM> of the lower mold <NUM> with the portion <NUM> to be removed on a lower side.

The bottom pad <NUM> is pressed to a lower surface of the bottom-forming portion <NUM> of the intermediate molded article <NUM> while an up-down stroke of pins <NUM> is guided by guides <NUM> provided on a frame of the upper mold <NUM> and pressing force of the bottom pad <NUM> is adjusted by an air cylinder <NUM>. Accordingly, the intermediate molded article <NUM> is prevented from being misaligned in the vertical direction during machining. After the intermediate molded article <NUM> is positioned by the bottom pad <NUM>, the die <NUM> is lowered while being guided by a guide <NUM>, which is provided to move along pins <NUM>. The die <NUM> lowers to a position facing both the punch <NUM> and a support portion <NUM> of the mandrel <NUM>. The die <NUM> is lowered until a block <NUM> provided below the die <NUM> abuts on a block <NUM> provided on the lower platform <NUM>, and thus the <NUM> is positioned.

The mandrel <NUM> includes a guiding portion <NUM> that guides an inner peripheral surface of the intermediate molded article <NUM> when the intermediate molded article <NUM> is to be placed on the mandrel <NUM>, and the support portion <NUM> that supports a part on a side near the bottom-forming portion <NUM> with respect to the step-forming portion. A lower surface of the support portion <NUM> of the mandrel <NUM> and an upper surface of the punch <NUM> are slidably in contact with each other and a height of a boundary line therebetween matches that of the step surface 11b of the step 11a formed in the intermediate molded article <NUM>. The movement of the mandrel <NUM> is regulated by the die <NUM> across the intermediate molded article <NUM> when the punch <NUM> is moved in the horizontal direction by the drive mechanism <NUM> to be described.

A positioning pin structure <NUM> is provided on the sliding surface between the mandrel <NUM> and the punch <NUM>. When the regulation of the movement of the mandrel <NUM> by the die <NUM> is removed, the mandrel <NUM> and the punch <NUM> are automatically mutually positioned so that the mandrel <NUM> and the punch <NUM> return to a home position (origin) where the mandrel <NUM> and the punch <NUM> overlap at approximately the same position in a plan view. The sliding surface of the punch <NUM> is provided with a pin <NUM> that is pressed toward the sliding surface of the mandrel <NUM> by a pressing means <NUM> such as a spring and the sliding surface of the mandrel <NUM> is provided with a plate-shaped recess at a position facing a tip of the pin <NUM> when in the home position, the recess having a cross-section with a curvature greater than that of a spherical head of the tip of the pin and a diameter greater than that of the pin. In a state in which no external force in the horizontal direction is applied, the mandrel <NUM> and the punch <NUM> move relative to each other so as to enter a state in which the tip of the pin <NUM> comes into contact with the deepest portion (central portion) of the recess due to the pressing force of the pressing means <NUM>, and thus the centers of the mandrel <NUM> and the punch <NUM> match.

Next, the drive mechanism <NUM> for driving the punch <NUM> will be described. The drive mechanism <NUM> is included in order to move the punch <NUM> in the horizontal direction relative to the fixed die <NUM> and the mandrel <NUM>. More specifically, the drive mechanism <NUM> reciprocally moves multiple members in an integrated manner, the multiple members being surrounded by a thick dotted line U in <FIG> and including the punch <NUM>, the upper platform <NUM>, and the lower platform <NUM>.

The drive mechanism <NUM> is a linear motion mechanism using a so-called link mechanism (crank mechanism). The drive mechanism <NUM> includes a servomotor <NUM>, an eccentric pin (crank shaft) <NUM> that is fixed to a rotation shaft 471a of the servomotor <NUM>, and a connection member <NUM> and a link portion <NUM> that convert an axial rotational motion of the eccentric pin <NUM> into a linear reciprocal motion of the lower platform <NUM>. The lower platform <NUM> is allowed to move only in a linear reciprocal direction.

<FIG> is a diagram illustrating a mechanism for converting the rotational motion caused by the drive mechanism <NUM> into the linear reciprocal motion and main constituent members viewed from above. The central axis of the eccentric pin <NUM> in a portion fixed to the connection member <NUM> is formed to turn about a central axis of a rotation shaft 471a of the servomotor <NUM>. Thick lines in <FIG> illustrate positions of the eccentric pin 472a, the connection member 473a, and the link portion 474a when the central axis of the eccentric pin 472a in the fixed portion described above is located directly upward in the drawing with respect to the central axis of the rotation shaft 471a of the servomotor <NUM>. On the other hand, thin lines illustrate positions of the eccentric pin 472b, the connection member 473b, and the link portion 474b when the central axis of the eccentric pin 472b in the fixed portion described above is located rightmost in the drawing with respect to the central axis of the rotation shaft 471a of the servomotor <NUM>.

The fixed portion in the eccentric pin <NUM> turns in a direction illustrated by an arrow R in <FIG> according to the rotation position of the rotation shaft 471a of the servomotor <NUM>. Thus, the connection member <NUM> moves, whereby the lower platform <NUM>, which is allowed to move only in the linear reciprocal movement direction, moves reciprocally in a direction illustrated by an arrow L in <FIG>. The servomotor <NUM> can be controlled so that the rotation shaft 471a is rotated clockwise or counterclockwise at a desired angle. Accordingly, an amount of the reciprocal movement of the lower platform <NUM> can be controlled by rotating (swinging) the rotation shaft 471a at a desired angle. This enables a movement amount of the punch <NUM> to be controlled.

A method for forming the step 11a in the intermediate molded article <NUM> using the machining apparatus configured as described above will be described with reference to <FIG> shows a state in which each member is located at the home position by the function of the positioning pin structure <NUM>. At the home position, a minute gap is formed between the outer wall surface <NUM> of the short-sidewall-forming portion <NUM> of the intermediate molded product <NUM> and the die <NUM>.

When the movement of the punch <NUM> is started by the drive mechanism <NUM>, the punch <NUM> moves toward the die <NUM>. At this time, the support portion <NUM> of the mandrel <NUM> is moved toward the die <NUM> due to frictional resistance caused by the punch <NUM>. Accordingly, the short-sidewall-forming portion <NUM> is also pressed by the support portion <NUM> and moves toward the die <NUM>, and thus the short-sidewall-forming portion <NUM> enters a state of being sandwiched by the die <NUM> and the support portion <NUM> (see <FIG>). Thus, the die <NUM> is provided on a side near the outer wall surface of the short-sidewall-forming portion <NUM>, which is the sidewall on which the step 11a is to be formed, with respect to the intermediate molded article <NUM>. The support portion <NUM> of the mandrel <NUM> and the die <NUM> sandwiches the sidewall (short-sidewall-forming portion <NUM>) therebetween on the bottom <NUM> side (bottom-forming portion <NUM> side) relative to the portion where the step 11a is formed in the inner wall surface on which the step 11a is to be formed.

Thereafter, by further moving toward the die <NUM>, the punch <NUM> presses a portion on a side opposite to the bottom <NUM> with respect to the portion where the step 11a is formed in the inner wall surface on which the step 11a is to be formed, and thus the sidewall (short-sidewall-forming portion <NUM>) is compressed between the punch <NUM> and the die <NUM> (see <FIG>).

The step 11a is not formed through one instance of a compressing operation performed by the punch <NUM>. That is, the punch <NUM> is controlled so as to compress the short-sidewall-forming portion <NUM> by a predetermined amount, and thereafter return in the direction of moving away from the die <NUM> (see <FIG>). At this time, the punch <NUM> may be moved to such a degree that the state in which the support portion <NUM> of the mandrel <NUM> sandwiches the short-sidewall-forming portion <NUM> between the support portion <NUM> and the die <NUM> be maintained.

Thereafter, once again, the punch <NUM> is controlled to move toward the die <NUM>, and the short-sidewall-forming portion <NUM> is further compressed by the punch <NUM> and the die <NUM>. In this manner, the compression amount of the short-sidewall-forming portion <NUM> is increased little by little due to multiple instances of the repeatedly performed pressing operation by the punch <NUM>. As shown in <FIG>, the portion of the inclined surface <NUM> in the short-sidewall-forming portion <NUM> is compressed in the step-forming step.

<FIG> shows a state in which a final pressing operation has been performed by the punch <NUM>, and <FIG> shows a state in which the final pressing operation by the punch <NUM> has ended and the step 11a has been formed on the short-sidewall-forming portion <NUM>. Note that the step surface 11b of the step 11a is level with the step surfaces <NUM> that have already been formed at the four corners of the rectangular tube of the intermediate molded article <NUM>. As described above, after the step 11a is formed, the intermediate molded article <NUM> is removed from the machining apparatus, and in the trimming step, the portion <NUM> to be removed is removed and the final product is obtained.

Here, several examples of methods for controlling the movement amount of the punch <NUM> in the pressing operation performed by the punch <NUM> will be described with reference to <FIG>. Note that as described above, the movement amount of the punch <NUM> can be controlled by controlling the rotational angle of the rotation shaft 471a of the servomotor <NUM>.

The movement amount by which the punch <NUM> is moved toward the die <NUM> may be controlled by being increased each instance of the repeatedly performed pressing operations. <FIG> shows, using a graph, a relationship between an elapsed time and the movement amount of the punch <NUM> according to the example <NUM> of the control. By this control, the compression amount of the short sidewall formation portion <NUM> gradually increases due to the movement amount of the punch <NUM> being gradually increased each instance of the pressing operation. This makes it possible to form a step 11a having a desired shape without difficulty.

The movement amount by which the punch <NUM> is moved toward the die <NUM> may be controlled by being increased in stages as a number of instances of the repeatedly performed pressing operations increases. <FIG> shows, using a graph, a relationship between an elapsed time and the movement amount of the punch <NUM> according to the example <NUM> of the control. The movement amount of the punch <NUM> is gradually increased every three instances of the pressing operation. By this control as well, the compression amount of the short-sidewall-forming portion <NUM> gradually increases. This makes it possible to form a step 11a having a desired shape without difficulty.

The movement amount by which the punch <NUM> is moved toward the die <NUM> may be controlled such that it is identical for all pressing operations that are performed repeatedly. <FIG> shows, using a graph, a relationship between an elapsed time and the movement amount of the punch <NUM> according to the example <NUM> of the control. Depending on material, thickness, or the like of the product, it is possible to form a step 11a having a desired shape with this kind of control as well.

With the manufacturing method according to the present embodiment, due to the step 11a being formed by multiple instances of a pressing operation performed by the punch <NUM>, it is possible to bring the angle formed by the step surface 11b of the step 11a provided in the rectangular can body and the inner wall surface of the rectangular can body closer to <NUM> degrees. This makes it possible to increase the sealing performance when the opening is closed by the sealing lid <NUM>.

Since the inclined surface <NUM> is provided in the intermediate molded article <NUM> in advance and the portion of the inclined surface <NUM> is compressed in the step-forming step, it is possible to form the step 11a with even less difficulty compared to a case of forming a step by simply performing multiple instances of a pressing operation performed by the punch <NUM> on a plate-shaped portion.

Among the portion <NUM> to be removed in the trimming step, at least a part of the sidewalls (short-sidewall-forming portions <NUM>) on which the steps 11a are to be formed may be removed in advance prior to the step-forming step. This will be described with reference to <FIG> is an external view of an intermediate molded article 100X in the case where a part of the portion <NUM> to be removed of the intermediate molded article <NUM> shown in <FIG> has been removed in advance. In this intermediate molded article 100X, a part of the portion <NUM> to be removed, which is surrounded by a thick line 150X in <FIG>, has been removed in advance. This makes it easier for the material to move out when the short-sidewall-forming portion <NUM> is compressed by the punch <NUM> in the step-forming step, and therefore it is possible to form the step 11a with even less difficulty.

Note that the step of removing the part may be performed at any time, as long as it is before the step-forming step. For example, by removing the part in advance when punching is carried out on a metal plate in order to obtain a raw material (blank), the part can be removed without increasing a number of steps.

Although the steps 11a are formed on the pair of short sidewalls <NUM> in the embodiment described above, steps may also be formed on the pair of long sidewalls <NUM>. The steps may be formed on both the pair of short sidewalls <NUM> and the pair of long sidewalls <NUM>.

The inclined surface <NUM> serving as the connection surface provided in the intermediate molded article <NUM> is constituted by a planar surface in the above-described embodiment. That is, the inclined surface <NUM> is represented by a straight line when viewed in a cross-sectional view taken along line AA in <FIG>. However, the connection surface is not limited to this. For example, the connection surface may be constituted by an inclined surface with a curved surface shape. In this case, the inclined surface is indicated by a circular arc or an elliptical arc when viewed in a cross-sectional view taken along line AA in <FIG>. The connection surface may be constituted by a step-shaped surface.

The inclined surface <NUM> is provided in advance in the intermediate molded article <NUM> and the portion of the inclined surface <NUM> is compressed in the step-forming step in the above-described embodiment. However, the step may be formed through only multiple instances of the pressing operation performed by the punch <NUM> without providing the inclined surface <NUM> depending on raw material, thickness, or the like of the product. In this case, as shown in <FIG>, at least a part of the portion <NUM> to be removed may be removed in advance.

The step surface <NUM> that is level with the step surface 11b of the step 11a is formed at the four corners of the rectangular tube through the drawing and ironing step in the step of manufacturing the intermediate molded article in the above-described embodiment. However, the step surfaces provided at the four corners of the rectangular tube may also be formed through the above-described step-forming step. In this case, the inclined surface <NUM> serving as the connection surface formed in the step of manufacturing the intermediate molded article may be provided up to positions reaching the four corners of the rectangular tube in the intermediate molded article <NUM>. Note that the four corners of the rectangular tube may be constituted by curved surfaces (so-called "R surfaces"). In the step-forming step, the step 11a is formed on not only the short-sidewall-forming portions <NUM>, but also positions reaching the four corners of the rectangular tube.

Claim 1:
A manufacturing method for a rectangular can including a rectangular can body (<NUM>) that has a rectangular tube and a bottom (<NUM>), and a sealing lid (<NUM>) for closing an opening of the rectangular can body (<NUM>), the manufacturing method comprising:
a step of manufacturing an intermediate molded article (<NUM>; 100X) that has the rectangular tube and the bottom (<NUM>); and
a step of forming respective steps (11a) on which the sealing lid (<NUM>) is to be placed on at least a pair of facing inner wall surfaces (<NUM>, <NUM>, <NUM>) among two pairs of facing inner wall surfaces (<NUM>, <NUM>, <NUM>) in the rectangular tube,
characterized in that
in the step-forming step, using
a die (<NUM>) that is provided on an outer wall surface side of a sidewall (<NUM>) on which the step (11a) is to be formed,
a mandrel (<NUM>) for sandwiching the sidewall (<NUM>) between the mandrel (<NUM>) and the die (<NUM>) at a portion on a bottom side with respect to a portion where the step is to be formed in the inner wall surface (<NUM>, <NUM>, <NUM>) on which the step is to be formed, and
a punch (<NUM>) for pressing a portion on a side opposite to the bottom side with respect to the portion where the step (11a) is to be formed in the inner wall surface (<NUM>, <NUM>, <NUM>) on which the step (11a) is to be formed and compressing the sidewall (<NUM>) between the punch (<NUM>) and the die (<NUM>),
the step (11a) is formed by performing a plurality of instances of a pressing operation performed by the punch (<NUM>).