EP3919200A1

EP3919200A1 - Seamless can body and method for producing seamless can body

Info

Publication number: EP3919200A1
Application number: EP20749070.7A
Authority: EP
Inventors: Tomomi Kobayashi
Original assignee: Toyo Seikan Group Holdings Ltd
Current assignee: Toyo Seikan Group Holdings Ltd
Priority date: 2019-01-30
Filing date: 2020-01-13
Publication date: 2021-12-08
Also published as: CN113507993A; CN118768448A; US20220097896A1; CN118768449A; CN113507993B; TWI840492B; TW202041299A; EP3919200A4; WO2020158355A1

Abstract

[Object]To provide a seamless can body in which the sheet thickness of a raw sheet (blank) is reduced, pressure resistance of a can bottom is enhanced, buckling is restrained, and problems with blacking and cleaning are solved, and a manufacturing method of the seamless can body.[Solving Means]A seamless can body 1 having a tubular body section 10 and a can bottom section 20. The can bottom section 20 includes an outer circumferential bottom section 202a extending from a lower end of the tubular body section 10 such as to decrease in diameter toward the inside and an annular grounding section 202b positioned further inside than the outer circumferential bottom section 202a. In a case where t1 is the sheet thickness of the outer circumferential bottom section 202a and where t2 is the sheet thickness of the annular grounding section 202b, the relation of t2 > t1 is satisfied.

Description

[Technical Field]

The present invention relates to a seamless can body and a method of manufacturing a seamless can body.

[Background Art]

Conventionally, what is called a seamless can body in which a can body section is molded by drawing and ironing has been known. Such a seamless can body has the can body section thinned by ironing and is thus excellent in lightness in weight. On the other hand, at a bottom section of the seamless can body, it is difficult to adopt a processing method for forcibly performing the thinning, such as ironing, so that the sheet thickness of the can body bottom section is not largely varied from the raw material thickness. Since the bottom section is required to have strength (pressure resistance) for resisting deformation due to internal pressure, various proposals for thinning the raw material thickness in order to contrive weight reduction even at the can body bottom section and for maintaining or enhancing pressure resistance have hitherto been made, inclusive of pieces of patent literature set forth below.
For example, PTL 1 and PTL 2 disclose what is called bottom reforming performed for the purpose of preventing a phenomenon (buckling) in which a dome section at the can bottom is inverted and which is generated when the internal pressure of the can exceeds a pressure resisting strength. Specifically, PTL 1 and PTL 2 disclose bottom reforming in which an inner circumferential wall of a grounding section of the can bottom that is located inside in a radial direction orthogonal to the can axis is pressed to thereby mold a recess.

[Citation List]

[Patent Literature]

[PTL 1]
JP 2018-103227A
[PTL 2]
JP 2016-47541A
[PTL 3]
JP 2000-176575A
[PTL 4]
JP Hei 9-285832A
[PTL 5]
WO 2018/070542
[PTL 6]
JP 2016-43991A

[Summary]

[Technical Problems]

However, the bottom reforming has the following problems.
Specifically, the bottom reforming step presses an inner circumferential wall at the can bottom by use of a molding roller or the like, to mold a recess. When pressing by use of the molding roller or the like is conducted, as described in PTL 3, there have been problems that blacking is liable to occur at the pressed part and that agglutination of a metallic material to the molding roller or the like is liable to occur.
In addition, at the time of pressing, a lubricant is applied to perform processing smoothly, and a step of cleaning the lubricant is thus necessary after the bottom reforming. Therefore, further improvement has been demanded from the viewpoint of cost required for cleaning and environmental load.
Besides, in recent years, for contriving weight reduction of seamless can bodies, further thinning of the sheet thickness of a raw sheet (blank) that is yet to be drawn and ironed has been demanded. However, when the bottom reforming described above is performed, the raw metal material at the pressing part is extended and thinned through the processing, so that there has been a limitation with respect to thinning of the sheet thickness of the raw sheet (blank).
In addition, as indicated by PTL 4, the present inventor has disclosed a technology for enhancing pressure resistance of the seamless can body. According to the technology, pressure resistance is enhanced, however, the sheet thickness distribution of each part of the can body (particularly the can bottom section) is not optimized sufficiently. Therefore, the technology does not sufficiently satisfy the demand for weight reduction of the can body.
Further, PTL 5 discloses a two-piece can body characterized in that the sheet thickness of a grounding section at the can bottom is larger than the sheet thickness of the raw material that is yet to be processed. However, a device used in the technology is complicated, and thus, there is a problem that it is difficult to realize such a two-piece can body on an industrial level or that equipment cost is raised.
The present inventor repeated extensive and intensive investigations in consideration of the problems exemplified above. As a result, the present invention has been made which makes possible to provide, by a simple manufacturing device, a seamless can body and a method of manufacturing the seamless can body in which the sheet thickness of the raw sheet (blank) is reduced, and at the same time, pressure resistance of the can bottom is enhanced to restrain buckling, and the problems with blacking and cleaning are also solved.

[Solution to Problems]

In order to achieve the above object, according to one embodiment of the present invention, (1) there is provided a seamless can body having a tubular body section and a can bottom section. The can bottom section includes an outer circumferential bottom section extending from a lower end of the tubular body section such as to decrease in diameter toward the inside, and an annular grounding section located further inside than the outer circumferential bottom section. In a case where t1 is a sheet thickness of the outer circumferential bottom section and where t2 is a sheet thickness of the annular grounding section, the relation of t2 > t1 is satisfied.
In addition, in order to achieve the above object, according to one embodiment of the present invention, (2) there is provided a seamless can body includes a tubular body section and a can bottom section including at least an outer circumferential bottom section extending from a lower end of the tubular body section through a boundary part such as to decrease in diameter toward the inside. A sheet thickness of the lower end of the tubular body section is substantially equal to a sheet thickness of an intermediate part of the tubular body section in an axial direction.
Note that, according to (1) or (2) above, (3) it is preferable that the can bottom section further includes an inside end section 202c located further inside than the annular grounding section, and in a case where t3 is a sheet thickness of the inside end section, the relation of t3 > t1 is satisfied.
Besides, according to (3) above, (4) it is preferable that a sheet thickness gradually increases from the outer circumferential bottom section to the inside end section such that t3 > t2 is satisfied.
In addition, according to any one of (1) to (4) above, (5) it is preferable that the can bottom section further includes a rising section 202d rising upward from the inside end section, and in a case where t4 is a sheet thickness of an upper end of the rising section, the relation of t4 > t1 is satisfied.
Besides, according to (5) above, (6) it is preferable that the can bottom section further includes a can dome section that is connected to the rising section and bulges to protrude upward, and a sheet thickness gradually increases from the can dome section to the inside end section such that t3 > t4 > t5 is satisfied in a case where t5 is a sheet thickness of a center of the can dome section.
In addition, according to (6) above, (7) it is preferable that t5 < t1 is further satisfied.
Besides, in any one of (5) to (7) above, (8) it is preferable that a ring groove in which a connection section between the rising section and the dome section protrudes toward the outside with respect to a can body axis is formed.
In addition, according to (2) above, (9) it is preferable that a sheet thickness of the boundary part is substantially equal to the sheet thickness of the intermediate part.
Besides, according to (2) or (9) above, (10) it is preferable that, in a case where t_WL is the sheet thickness of the lower end of the tubular body section and where twc is the sheet thickness of the intermediate part of the tubular body section in the axial direction, the relation of twc ≤ t_WL < 1.09×t_WC is satisfied.
In addition, according to (10) above, (11) it is preferable that the relation of twc ≤ t0 < 1.09×t_WC (where t0 is the sheet thickness of the boundary part) is satisfied in the tubular body section.
Besides, according to (1) to (11) above, (12) it is preferable that 60 degrees specular glossiness from the lower end of the tubular body section to the vicinity of the boundary part is equal to or more than 300%.
In order to achieve the above object, according to one embodiment of the present invention, (13) there is provided a method of manufacturing a seamless can body having a tubular body section and a can bottom section. The method includes a first molding step of molding a raw metal material into a cup body including the tubular body section, a cup outer circumferential bottom section extending from a lower end of the tubular body section such as to decrease in diameter, an inclined section extending upward toward the inside from the cup outer circumferential bottom section, and a cup dome section bulging upward from an end portion of the inclined section at a first height; and a second molding step of applying a pressing force to the cup dome section toward an outside of a can by using an upper molding member while the cup outer circumferential bottom section of the cup body is brought into contact with a lower molding member, to press down the cup dome section such as to have a second height lower than the first height, and to apply compressive stresses in a meridian direction and a circumferential direction, and then pressing the inclined section into the lower molding member while a thickness of the inclined section is increased.
In addition, according to (13) above, (14) it is preferable that, in the second molding step, the inclined section is pressed into the lower molding member, to thereby form an annular grounding section 202b located further inside than an outer circumferential bottom section, an inside end section 202c located further inside than the annular grounding section, and a rising section 202d rising upward from the inside end section and connected to a can dome section, and a ring groove in which a connection section (outermost end 201e) between the rising section 202d and the can dome section 201d protrudes toward the outside with respect to a can body axis is formed such that the inside diameter (dx) of the connection section becomes greater than the inside diameter (dy) of the inside end section 202c.
Further, in order to achieve the above object, according to one embodiment of the present invention, (15) there is provided a method of manufacturing a seamless can body. The method includes a first molding step of molding a raw metal material into a cup body having a tubular body section thinned by ironing, an outer circumferential bottom section extending from a lower end of the tubular body section, and a bulging section bulging from the outer circumferential bottom section toward an opening at a first height; and a second molding step of pressing down the bulging section such as to have a second height lower than the first height. In the first molding step, the lower end of the tubular body section is drawn to form the outer circumferential bottom section extending from the lower end of the tubular body section through a boundary part such as to decrease in diameter toward the inside, such that a sheet thickness of the lower end of the tubular body section becomes substantially equal to a sheet thickness of an intermediate part of the tubular body section in an axial direction.

[Advantageous Effects of Invention]

According to the seamless can body of the present invention, a can bottom higher in pressure resistance than the can bottom obtained by conventional bottom reforming can be obtained even in the case where the sheet thickness of the raw sheet (blank) is reduced. Therefore, a seamless can body can be manufactured by use of a raw sheet (blank) thinner than that used in the prior art, and the amount of metallic material to be used can be reduced, which is advantageous on a cost basis. Further, weight reduction in the seamless can body leads to reductions in recycle cost, transportation cost, and the like.
Further, according to the method of manufacturing a seamless can body of the present invention, it is possible to enhance the pressure resistance of the can bottom and restrain buckling by a simple manufacturing device even in the case where the sheet thickness of the raw sheet (blank) is reduced. At the same time, the problem with blacking as encountered in bottom reforming can be solved. Further, since the conventional bottom reforming step and the step of cleaning the lubricant after the bottom reforming are not needed, there are large merits on a cost basis and an environmental basis.

[Brief Description of Drawings]

[FIG. 1]
FIG. 1 depicts schematic views of a seamless can body 1A according to a first embodiment.
[FIG. 2]
FIG. 2 is an enlarged view depicting a can bottom of the seamless can body 1A according to the first embodiment.
[FIG. 3]
FIG. 3 depicts graphs indicating a sheet thickness at each point of the seamless can body 1A according to the first embodiment.
[FIG. 4]
FIG. 4 depicts diagrams of a first molding step in a method of manufacturing a seamless can body according to the first embodiment.
[FIG. 5]
FIG. 5 depicts diagrams of a second molding step in the method of manufacturing a seamless can body according to the first embodiment.
[FIG. 6]
FIG. 6 is a schematic view depicting a compressive stress exerted on a rising section in the first embodiment.
[FIG. 7]
FIG. 7 is a partial enlarged view of a can bottom of a seamless can body used in a comparative example 1.
[FIG. 8]
FIG. 8 is a schematic view depicting a longitudinal section of a whole seamless can body 1B according to a second embodiment.
[FIG. 9]
FIG. 9 depicts comparative diagrams of the seamless can body 1B according to the second embodiment and a seamless can body having a conventional structure, in the vicinity of a lower end 10e of a tubular body section 10.
[FIG. 10]
FIG. 10 depicts diagrams of a first molding step in a method of manufacturing the seamless can body 1B.
[FIG. 11]
FIG. 11 depicts schematic views of an α part and a β part of FIG. 10 in a partially enlarged manner.
[FIG. 12]
FIG. 12 depicts diagrams of a second molding step in the method of manufacturing the seamless can body 1B.
[FIG. 13]
FIG. 13 is an enlarged view depicting a can bottom of the seamless can body 1A according to the first embodiment to which a boundary part BP depicted in the second embodiment is applied.
[FIG. 14]
FIG. 14 depicts schematic views for comparison in structure between a seamless can body according to a conventional technique and the seamless can body according to the present embodiment.
[FIG. 15]
FIG. 15 depicts schematic views of another example (first example) applicable to the first molding step in the method of manufacturing the seamless can body 1B depicted in FIG. 10.
[FIG. 16]
FIG. 16 depicts schematic views of yet another example (second example) applicable to the first molding step in the method of manufacturing the seamless can body 1B depicted in FIG. 10.
[FIG. 17]
FIG. 17 is a schematic view for explaining the position of the boundary part BP in the present embodiment.

[Description of Embodiments]

A seamless can body and a method of manufacturing the seamless can body according to the present invention will specifically be described below with referring to the drawings, as needed. Note that the following embodiments illustrate an example of the present invention to explain the details thereof and do not limit the present invention intentionally.

[First Embodiment]

As illustrated in FIG. 1, a seamless can body 1A according to the present embodiment is a seamless can body having a tubular body section 10 and a can bottom section 20. As depicted in FIG. 1(a) and FIG. 1(b), in the present embodiment, the can bottom section 20 preferably includes a can bottom central part 201 which does not make contact with a horizontal surface when the seamless can body is mounted on the horizontal surface, and a foot part 202 located on the outside of the can bottom central part 201.
The can bottom central part 201 of the seamless can body 1A according to the present embodiment may have a horizontal shape or may have a dome shape bulging toward the inner surface side of the can (bulging such as to protrude upward) as depicted in FIG. 1(a).
As depicted in FIG. 1(b), in the present embodiment, the foot part 202 of the can bottom section 20 is defined as a part extending from a lower end 10e of the tubular body section 10 and then extending along a can body axis RA direction up to an outermost end 201e of the can bottom central part 201.
Note that, as depicted in an enlarged sectional view of the foot part 202 in FIG. 2, "the outermost end 201e of the can bottom central part 201" is a part where, in the case where the can bottom central part 201 is in the dome shape, the diameter of the dome is maximum.
In the present embodiment, the foot part 202 has an annular grounding section 202b which is located in the lowest part in a Z axis direction. Specifically, it can be said that the annular grounding section 202b is a part that makes contact with a horizontal surface in the case where the seamless can body 1A according to the present embodiment is mounted on the horizontal surface.
Besides, a part ranging from the lower end 10e of the tubular body section 10 to the annular grounding section 202b is defined as an outer circumferential bottom section 202a.
Specifically, in the present embodiment, the foot part 202 includes the outer circumferential bottom section 202a extending from the lower end 10e of the tubular body section 10 such as to decrease in diameter toward the inside and the annular grounding section 202b located further inside than the outer circumferential bottom section 202a.
In other words, the outer circumferential bottom section 202a of the seamless can body 1A according to the present embodiment has a ring shape and is located to the lower end 10e of the tubular body section 10 further outside than the annular grounding section 202b.
In the present embodiment, the width, area, and the like of the ring of the outer circumferential bottom section 202a are not particularly limited, and known shapes are applicable to the inclination angle and curved state of the ring. In other words, the outer circumferential bottom section 202a may be rectilinear in section, may be in an arcuate shape curved toward the inside of the can body, or may be in an arcuate shape curved toward the outside. In addition, the outer circumferential bottom section 202a may have a part curved toward the inside and the remaining part curved toward the outside, and these parts may be connected consecutively.
As depicted in FIG. 2, in the present embodiment, it is preferable that the outer circumferential bottom section 202a has an inflection point IP in its sectional view, from the viewpoint that the seamless can body 1A can easily be mounted on a lid of a can of the same kind.
As depicted in FIG. 2, the seamless can body 1A according to the present embodiment further includes an inside end section 202c located further inside than the annular grounding section 202b. The inside end section 202c is defined as a part of the abovementioned foot part 202 which is the closest to the can body axis RA side in a sectional view.
In addition, the seamless can body 1A according to the present embodiment further includes a rising section 202d extending upward (in a + direction of the Z axis) from the inside end section 202c. In the sectional view depicted in FIG. 1(a) or FIG. 2, the rising section 202d is defined as a part ranging from the inside end section 202c to the outermost end 201e in the can bottom central part 201 direction.
The seamless can body 1A according to the present embodiment is characterized in that the relation of "t2 > t1" is satisfied in the case where t1 is the sheet thickness of the outer circumferential bottom section 202a and t2 is the sheet thickness of the annular grounding section 202b. By satisfying such a relation, a favorable pressure resistance can be imparted while the weight of the can body is reduced in the seamless can body 1A according to the present embodiment. In addition, by satisfying the relation of t2 > t1, strength against deformation can be imparted in the case where the seamless can body 1A is dropped with the can bottom section 20 directed downward, and thus, this is preferable.
Note that the sheet thickness (t1) of the outer circumferential bottom section 202a is the sheet thickness at an intermediate point of the length (the length along the shape) from the lower end 10e to the annular grounding section 202b.
In the seamless can body 1A according to the present embodiment, it is also preferable that the relation of "t3 > t1" is satisfied in the case where t3 is the sheet thickness of the inside end section 202c. By satisfying such a relation, a favorable pressure resistance can be imparted while the weight of the can body is reduced in the seamless can body 1A according to the present embodiment. Besides, by satisfying the relation of t3 > t1, strength against deformation can be imparted in the case where the seamless can body 1A is dropped with the can bottom section 20 directed downward, and thus, this is preferable.
The abovementioned designation of thickness in the present invention is for the following reasons.
Specifically, in the case where the liquid accommodated in the seamless can body is beer or a carbonated drink, an internal pressure is always exerted on the can bottom. In the case where a shock is exerted on the can bottom in a state in which the internal pressure is exerted or in the case where the internal pressure applied to the can bottom is abruptly increased for some reason, the internal pressure of the can exceeds the pressure resisting strength of the can bottom, and a phenomenon (buckling) in which the dome section of the can bottom is inverted is generated.
To restrain the buckling phenomenon, the pressure resisting strength of the can bottom should be increased. To achieve this, a method of thickening the sheet thickness of the can bottom part is contemplated.
However, due to the demands for weight reduction and resource saving in recent years, the sheet thickness of the raw sheet (blank) has been becoming thinner, so that simply thickening the sheet thickness of the raw sheet (blank) for enhancing the pressure resisting strength of the can bottom is against the demands.
In view of the foregoing circumstances, the present inventor made extensive and intensive investigations in order to realize a seamless can body that simultaneously satisfies the demands for weight reduction of cans and desired pressure resisting strength of the can bottom. As a result, the present inventor has conceived the present invention which achieves, while setting the sheet thickness of the raw sheet (blank) to be comparable to or smaller than that in the prior art, enhancement of pressure resisting strength of the can bottom by thickening only the part of the can bottom that is liable to contribute to enhancement of pressure resisting strength.
According to the present invention, since a raw sheet (blank) thinner than that in the prior art can be adopted for the can body section, the sheet thickness of the can body section which is comparable to or smaller than that in the prior art can be provided by rigorous drawing and ironing similar to those in the prior art. Therefore, it can be said that demands for weight reduction and enhancement of pressure resisting strength of the can bottom can both be satisfied at a higher dimension.
As depicted in FIG. 1(a) and FIG. 2, in the seamless can body 1A according to the present embodiment, the foot part 202 of the can bottom section 20 is connected to the can bottom central part 201 (can dome section 201d) at the part of the outermost end 201e, from the inside end section 202c through the rising section 202d.
In the present embodiment, the rising section 202d may be a straight line or a curved line in section that extends in a vertical direction (+ direction of the Z axis) from the inside end section 202c.
In addition, as depicted in FIG. 1(a) and FIG. 2, the rising section 202d may be a straight line or a curved line in section that extends along a straight line of Z = -aX (Z > 0).
As illustrated in FIG. 1(a), the rising section 202d is connected to the can bottom central part 201 (can dome section 201d) such that an inside diameter (dx) of the abovementioned outermost end 201e is greater than an inside diameter (dy) of the inside end section 202c.
In other words, as depicted in FIG. 1(a) and FIG. 2, in the vicinity of the outermost end 201e, the shape in a sectional view is a substantially U-shape (⊂ or ⊃) .
In addition, referring to FIG. 1(a), a ring groove in which the outermost end 201e protrudes toward the outside with respect to the can body axis RA is preferably provided between the inside end section 202c and the can dome section 201d such as to extend toward the + direction of the Z axis.
By adopting the abovementioned shape, the pressure resistance of the seamless can body 1A according to the present embodiment can be enhanced.
Note that, in the present embodiment, the outer circumferential bottom section 202a preferably has the inflection point IP in its sectional view as described above. This inflection point IP may be located further in the + direction of the Z axis than the outermost end 201e as depicted in FIG. 2, or may be located in the - direction of the Z axis.
In the present embodiment, it is also preferable, from the viewpoint of weight reduction and pressure resistance of the can body, that the relation of "t4 > t1" is satisfied in the case where t4 is the sheet thickness at a part of the outermost end 201e that connects the rising section 202d and the can bottom central part 201.
As depicted in FIG. 1(a), it is preferable that the seamless can body 1A according to the present embodiment further includes the can dome section 201d that is connected to the rising section 202d and bulges to protrude upward, at the can bottom section 20. In other words, in the present embodiment, the shape of the can bottom central part 201 is preferably a dome shape as depicted in FIG. 1(a).
In the case where t5 is the sheet thickness at the center of the can dome section 201d, it is preferable that, with respect to the relation between the sheet thickness (t3) of the inside end section 202c and the sheet thickness (t4) of the rising section 202d, the sheet thickness t5 satisfies the following relation. $t3 > t4 > t5$
Specifically, it means that, in the continuous metal sheet provided from a central part of the can dome section 201d toward the outside up to the inside end section 202c, the sheet thickness gradually increases.
Further, in the present embodiment, in the case where tz is the sheet thickness of the raw sheet (blank), it is preferable that the relations of "t1 > tz," "t2 > tz," "t3 > tz," and "t4 > tz" are all satisfied as depicted in FIG. 3, from the viewpoint of desired pressure resistance of the seamless can body.
On the other hand, in the present embodiment, there is no problem if the sheet thickness (t5) at the center of the can dome section 201d is equal to or less than the sheet thickness (tz) of the raw sheet (blank) (t5 ≤ tz).
Note that, in the present embodiment, as depicted in FIG. 3(a), it is preferable that the sheet thicknesses satisfy the relation of "t3 > t2 > t1." In other words, it is preferable that the sheet thickness gradually increases in the order of the outer circumferential bottom section 202a, the annular grounding section 202b, and the inside end section 202c.
By satisfying such a relation, preferable pressure resistance can be imparted to the seamless can body 1A according to the present embodiment.
In addition, by satisfying the abovementioned relation of "t3 > t2 > t1," it is possible to suppress an increase in weight of the can even in the case where the sheet thickness at a part of the t3 is increased, and thus, this is preferable. The reason lies in that, since the positions of the parts at the t1, t2, and t3 are closer to the can body axis RA in the order of t1 → t2 → t3, the volumes occupied by the respective parts decrease in the order.
As a result, the pressure resistance can be enhanced while an increase in weight of the can is suppressed, and thus, this is preferable.
However, this is not limitative of the present embodiment, and the thicknesses of t2 and t3 may be the same as depicted in FIG. 3(b), or the thickness of t2 may be the largest as depicted in FIG. 3(c).
Note that the sheet thickness tz of the raw sheet (blank) is only required to have a sheet thickness ordinarily adopted in the case of manufacturing a seamless can body, and a metal sheet with the thickness tz of approximately 0.15 to 0.4 mm is blanked to be used as a raw sheet (blank), but this thickness is not limitative.
As described above, in the seamless can body 1A according to the present embodiment, it is preferable that, from the viewpoint of the desired pressure resistance, the sheet thickness of the can bottom section 20 has the abovementioned relation.
In other words, in the seamless can body 1A according to the present embodiment, the average sheet thickness of the can bottom section 20, particularly, the foot part 202 is thicker than the can bottom central part 201.
Further, it is preferable that the thickness of the can dome section 201d is smaller than the thickness of the outer circumferential bottom section 202a. In other words, it is preferable that "t5 < t1."
In regard of the enhancement of pressure resistance by having the abovementioned relation of the sheet thicknesses, the following reasons are considered.
The buckling pressure is a numerical value indicating the pressure resistance. In other words, the buckling pressure is a peak value of pressure until occurrence of the phenomenon in which the dome section protruding toward the inside of the can bottom is deformed to be inverted to the outside by the internal pressure.
The process of occurrence of the phenomenon of buckling can be explained as follows.
First, when the dome section having a substantially spherical shape starts receiving the internal pressure, the dome section itself is not deformed immediately, but the product of the projection area of the dome section and the internal pressure becomes a force for pushing out the dome section to the outside of the can, thereby acting on the annular grounding section 202b, the inside end section 202c, and the rising section 202d to exert a load and deform them.
In other words, the outer circumference of the dome section is supported by the narrow region ranging from the annular grounding section 202b to the rising section 202d.
When the deformation of the region ranging from the annular grounding section 202b to the rising section 202d proceeds due to a further rise in the internal pressure, the function for supporting the outer circumference of the dome section is lost. In other words, the annular grounding section 202b, the inside end section 202c, and the rising section 202d becomes unable to keep an annular shape with the can body axis RA as the center, the outermost end 201e located at the outer circumference of the dome section and connected to the rising section 202d loses a circular shape, and the can dome section 201d connected to the outermost end 201e becomes unable to keep a spherical shape, so that the strength of the dome section is rapidly lowered and the dome section is then inverted (buckled) to the outside of the can.
Therefore, in order to enhance the pressure resistance, it is considered to be effective to increase the sheet thickness of the outer circumference of the dome section compared with the case of increasing the sheet thickness of the dome section itself. Accordingly, in the case where the thickness of the outer circumferential bottom section 202a is larger than the sheet thickness at the center of the can dome section 201d, that is, in the case where "t5 < t1," the pressure resistance desirable in the present embodiment can be obtained.
Note that a second height Hp of the can dome section 201d of the seamless can body 1A is not particularly limited and can be a height comparable to that of a known seamless can body having a dome section.
Note that, in the present embodiment, the kind of the raw metal material used for the seamless can body 1A is not particularly limited. In other words, known metal sheets ordinarily used for seamless can bodies, such as an aluminum alloy sheet and a surface-treated steel sheet, can be used. In addition, the metal sheet may have a known film laminated thereon or may be subjected to surface-treatment such as organic resin coating or chemical conversion treatment, as required.
The seamless can body 1A according to the present embodiment is subjected to known necking, flange forming, or screw forming, and after beer, a carbonated drink, or the like is filled in the seamless can body 1A as a content, a lid is attached to an opening by a known method.

Next, the method of manufacturing the seamless can body 1A according to the present embodiment will be described.
The method of manufacturing a seamless can body according to the present embodiment is a method of manufacturing the seamless can body 1A having the tubular body section 10 and the can bottom section 20 as depicted in FIG. 1(a) and is characterized by including at least a first molding step and a second molding step which will be described in detail below.
Note that, in the method of manufacturing a seamless can body according to the present embodiment, as a method of molding the tubular body section 10, for example, a known method such as a method described in PTL 4 can be adopted.
On the other hand, a method of molding the can bottom section 20 is particularly characterized by including at least the first molding step and the second molding step described in detail below.
The method of manufacturing the seamless can body according to the present embodiment will be described below.
First, by use of the abovementioned raw sheet (blank), the can body section is formed by a known method to prepare a precursor 3 having a cup shape.
Note that, as depicted in FIG. 4, the raw metal material (precursor 3) may have a cup shape which does not have a dome and which is obtained by known drawing and ironing or the like. In addition, the raw metal material (precursor 3) may have a cup shape having a dome, insofar as the first molding step and the second molding step described below can be realized.
By applying the first molding step and the second molding step described below to this precursor 3, the seamless can body 1A according to the present embodiment can be obtained.
First, in the method of manufacturing the seamless can body 1A according to the present embodiment, in the first molding step depicted in FIG. 4, the raw metal material (precursor 3) is molded into a cup body 2 having the tubular body section 10, a cup outer circumferential bottom section A extending from the lower end 10e of the tubular body section 10 such as to decrease in diameter, an inclined section S extending upward toward the inside from the cup outer circumferential bottom section A, and a cup dome section D bulging upward from an end portion Se of the inclined section S at a first height Ho.
Here, the end portion Se of the inclined section S can be said to be a connection point with the cup dome section D.
The first molding step depicted in FIG. 4 can be performed on the precursor 3 including the tubular body section 10 molded by a known pressing step or the like, either as separate steps by use of an upper mold and a lower mold or a step performed at a stroke final stage subsequent to an ironing step.
As a specific example, as depicted in FIG. 4, the first molding step is carried out by using a tubular punch 401 that is located in the precursor 3 having the cup shape and that supports the precursor 3, a hold down ring 501 that supports the outer circumferential bottom section of the precursor 3 in cooperation with the punch 401, and a doming die 502.
First, the outer circumferential bottom section of the precursor 3 is held by a circumferential wall part 402 (taper section) of the punch 401 and a tapered support section 503 of the hold down ring 501, and the punch 401 and the doming die 502 are driven such as to engage with each other and are relatively moved close to each other, so that the cup body 2 having the cup dome section D at the bottom thereof at the first height Ho can be obtained.
Here, the shape of the cup body 2 obtained in the first molding step will be described. Specifically, the inclined section S of the cup body 2 extends upward toward the inside from the cup outer circumferential bottom section A.
In other words, as depicted in FIG. 4, the inclined section S of the cup body 2 includes a curved line part and a straight line part provided between the lowest part of the cup body 2 in the Z-axis direction and the connection point (end section Se ) with the cup dome section D.
As depicted in FIG. 4(c), the inclined section S is preferably inclined at a predetermined angle θ₁, without being perpendicular.
Specifically, it is preferable that the angle θ₁ formed between the inclined section S and the Z axis is 5° to 30° from the viewpoint of favorably controlling the sheet thicknesses of the respective parts in the second molding step described below.
In addition, it is more preferable that the angle θ₁ between the inclined section S and the Z axis is 10° to 30° since spray coating is easily performed in the case of forming a coating film on an inner surface by a spray coating method after the first molding step.
In addition, it is preferable that the radius of curvature R at a angle θ₂ formed between the cup outer circumferential bottom section A and the inclined section S is set such that R = 5×t0 to 15×t0, from the viewpoint of favorably controlling the sheet thicknesses of the respective parts in the second molding step described below.
Further, it is preferable that the first height Ho of the cup dome section D of the cup body 2 is greater than a second height Hp of a can dome section 201d of the seamless can body 1A obtained in the second molding step described later. As described below, the reason for this is to apply a compressive stress to the inclined section S while the cup dome section D of the cup body 2 is pressed down in the second molding section described later. In other words, the first height Ho of the cup dome section D of the cup body 2 is preliminarily set to be large to finally obtain a preferable second height Hp of the can dome section 201d of the seamless can body 1A.
Subsequently, the second molding step will be described.
After the cup body 2 having the cup outer circumferential bottom section A and the inclined section S is molded in the first molding step, the second molding step is carried out as follows.
Note that, for example, a known cleaning step, surface-treatment step, printing step, coating step, or shaping step for the tubular body section, or necking-in (decreasing diameter) in such a range as not to hamper the second molding step may be carried out on the cup body 2, as required, between the first molding step and the second molding step.
Further, if necessary, for the purpose of securing conveyability and corrosion resistance after the first molding step, an outer surface coating can be applied to the part ranging from the cup outer circumferential bottom section A to the inclined section S, with a lowest end curvature section of the cup body 2 set as a center.
In the second molding step, processing is performed on the cup body 2 by using a mold different from a mold used in the abovementioned first molding step, to mold the seamless can body 1A. Specifically, while the cup body 2 is brought into contact with a cup outer circumferential side holder 60 as a lower molding member, a pressing force is applied to the cup dome section D of the cup body 2 in the outside direction of the can (-Z axis direction) by use of a dome pressing-down tool 70 as an upper molding member.
Alternatively, while the cup body 2 is brought into contact with the lower molding member and the upper molding member, a pressing force may be applied in the +Z axis direction by use of the lower molding member.
More specifically, as depicted in FIG. 5, the cup outer circumferential bottom section A of the cup body 2 is mounted on the cup outer circumferential side holder 60. The dome pressing-down tool 70 is relatively lowered, and a support section 701 of the dome pressing-down tool 70 makes contact with the cup dome section D. Here, the cup outer circumferential side holder 60 has a tapered surface 601 and a groove 602. With the dome pressing-down tool 70 further pressed down after the cup outer circumferential bottom section A of the cup body 2 makes contact with the tapered surface 601, the metal of the inclined section S of the cup body 2 is guided into the groove 602 while receiving a compressive stress and is pushed into the groove 602.
Then, the cup dome section D is pressed down such that the second height Hp is lower than the first height Ho. Simultaneously, by use of the upper molding member (dome pressing-down tool) and the lower molding member (cup outer circumferential side holder), a compressive stress σ_Φ in the meridian direction and a compressive stress σ_θ in the circumferential direction are applied to the inclined section S.
Note that FIG. 6 is a schematic view depicting a compressive stress applied when the inclined section S is formed at the rising section 202d in the present embodiment.
In other words, when the inclined section S is pressed into the groove 602 of the lower molding member, the compressive stress σ_Φ produced in the meridian direction by the pressing force of the dome pressing-down tool 70 and the compressive stress σ_θ produced in the circumferential direction due to movement toward the radial-directionally inner side to follow the lower molding member are simultaneously applied to the inclined section S, and the thickness of the raw metal material of the inclined section S is increased (arrow direction σ_ψ in FIG. 6) .
In this way, the seamless can body 1A is obtained after the second molding step is conducted.
After the molding is finished, it is sufficient to relatively raise the dome pressing-down tool and take out the seamless can body 1A from the cup outer circumferential side holder.
Here, the seamless can body 1A obtained after the second molding step is preferably the abovementioned seamless can body 1A according to the present embodiment.
In other words, the seamless can body 1A obtained after the second molding step preferably has the outer circumferential bottom section 202a and the annular grounding section 202b as depicted in FIG. 1 and satisfies the relation of "t2 > t1" in the case where t1 is the sheet thickness of the outer circumferential bottom section 202a and where t2 is the sheet thickness of the annular grounding section 202b.
Note that it is more preferable that the second molding step has the following characteristics.
Specifically, in the second molding step, by pressing the abovementioned cup body 2 into the cup outer circumferential side holder 60 in the second molding step, the inclined section S is formed into the annular grounding section 202b located further inside than the outer circumferential bottom section 202a, the inside end section 202c located further inside than the annular grounding section 202b, and the rising section 202d rising upward from the inside end section 202c and connected to the can dome section 201d.
In the second molding step, it is preferable that the ring groove in which the outermost end 201e protrudes toward the outside with respect to the can body axis RA is formed such that the inside diameter (dx) of the connection point (outermost end 201e) between the rising section 202d and the can dome section 201d of the seamless can body 1A is greater than the inside diameter (dy) of the inside end section 202c.
Conventionally, there has been a reform molding method (bottom reforming) of forming the abovementioned groove by use of a rotating roller or a split mold. However, in the conventional method, the processed part is liable to be thin, and it has been difficult to form a sufficiently deep groove.
According to the method of the present invention, the sheet thickness of the ring groove part does not become thin and tends to be thick, so that a deep groove can be reasonably formed.
In the method of manufacturing the seamless can body according to the present embodiment, between the first molding step and the second molding step, the shape and length of an upper part of the cup outer circumferential bottom section A of the cup body 2 do not change.
Specifically, when the cup body 2 is mounted on the cup outer circumferential side holder 60, the lowest point in the Z-axis direction of a surface where the cup outer circumferential bottom section A of the cup body 2 and the tapered surface 601 of the cup outer circumferential side holder 60 make contact is set to a point T. The position of the point T does not change according to lowering of the dome pressing-down tool 70 and pressing-down of the cup dome section D. (see FIG. 5)
On the other hand, in the second molding step, the part of the inclined section S of the cup body 2 is formed into a part of the outer circumferential bottom section 202a, the annular grounding section 202b, the inside end section 202c, and the rising section 202d of the seamless can body 1A. In other words, the whole inclined section S of the cup body 2 is finally put into the groove 602 of the cup outer circumferential side holder 60.
Note that, in the second molding step, the contact between the cup body 2 and the upper and lower molds does not undergo conspicuous sliding. Therefore, damage is not generated in the metal surface of the cup body 2, and it is thus unnecessary to use a lubricant.
As illustrated in FIG. 5, the abovementioned point T becomes the inflection point IP in the seamless can body 1A. Because of a compressive stress applied in the second molding step, the metal length is shortened as follows.
Specifically, the metal length from the inflection point IP to the outermost end 201e in FIG. 5(f) is shortened to 0.85 to 0.99 times the metal length from the point T to the end portion Se in FIG. 5(b).
On the other hand, the thickness of the raw metal material of the part is increased in the second molding step such that the part most increased in thickness is increased to 1.1 to 1.3 times the raw sheet thickness (t0) .

EXAMPLES

The details of the first embodiment of the present invention will be described below by illustrating examples and comparative examples. However, the present invention is not limited at all to the following examples.

(Example 1)

A drawn and ironed can (DI can) with an internal volume of 350 mL was manufactured by the following method.
First, an aluminum alloy sheet (JIS H 4000 A3104-H19 material, 0.28 mm) was prepared as a raw sheet. Next, a predetermined amount of a known cupping oil was applied to both sides of the aluminum alloy sheet, as a lubricant at the time of drawing.
Next, immediately after the aluminum alloy sheet was blanked into a disk shape with a diameter of 160 mm by a drawing machine, the resultant sheet was drawn to be a drawn cup (not illustrated) with a diameter of 90 mm.
The drawn cup thus obtained was conveyed to a body maker (can body manufacturing machine) and was re-drawn into a shape with a diameter of 66 mm. Then, by using a coolant, the drawn cup was subjected to ironing to obtain a drawn and ironed precursor 3 with a shape of 66 mm in diameter, 130 mm in height, and 0.105 mm in side wall minimum thickness.
Subsequently, for molding of the can bottom, the precursor 3 obtained as above was subjected to the first molding step and the second molding step as follows.
First, the first molding step was conducted at the stroke final stage of the step subsequent to the ironing by the body maker, and a cup body 2 having a cup outer circumferential bottom section A and an inclined section S was obtained by use of the punch 401, the hold down ring 501, and the doming die 502 depicted in FIG. 4. The lengths and sheet thicknesses of the cup outer circumferential bottom section A and the inclined section S in this instance are as set forth in Table 1.
Next, in the second molding step, by use of the dome pressing-down tool 70 as the upper molding member and the cup outer circumferential side holder 60 as the lower molding member which are depicted in FIG. 5, a cup dome section D was pressed down, and the thickness of the raw metal material of the inclined section S was increased, thereby molding a seamless can body 1A.
Subsequently, the sheet thicknesses t1 to t5 of the respective parts were measured. Note that the position of each of the parts at t1 to t5 is as described in the above embodiment and FIG. 2. In addition, the method of measuring the sheet thickness is as follows. Specifically, the molded seamless can body 1A was embedded in an epoxy resin and was then cut together with the epoxy resin along a longitudinal axis (Z axis) of the seamless can body 1A. After a central section was exposed by cutting and careful polishing, the thicknesses t1 to t5 of the respective parts were measured with a measuring microscope. The sheet thicknesses of the parts are set forth in Table 1.

(Example 2)

The operation of an example 2 was conducted similarly to the example 1 except that the raw sheet thickness was 0.225 mm and the side wall minimum thickness of the precursor 3 was 0.093 mm. The sheet thicknesses and the like of the parts of the seamless can body thus obtained are set forth in Table 1.

(Comparative Example 1)

The molding of the can bottom was conducted in one step according to a known can bottom molding method by use of a known can bottom molding mold. Except this, the operation of a comparative example 1 was conducted similarly to the example 1.
Note that a partial enlarged view of the can bottom of the seamless can body used in the comparative example 1 is depicted in FIG. 7.
The sheet thicknesses and the like of the parts of the seamless can body thus obtained are set forth in Table 1. It is to be noted that the numerical value of t3 in Table 1 was obtained by measuring a lower end ((1) in FIG. 7) of the inclined section and that the numerical value of t4 was obtained by measuring an upper end ((2) in FIG. 7) of the inclined section.

(Comparative Example 2)

The seamless can body obtained in the comparative example 1 was subjected to bottom reforming. Specifically, an inner circumferential wall of the grounding section of the can bottom that was located inside in a radial direction orthogonal to the can body axis was pressed by a rotating roller, to mold a recess in an annular shape. Except this, the operation of a comparative example 2 was conducted similarly to the comparative example 1. The sheet thicknesses and the like of the parts of the seamless can body thus obtained are set forth in Table 1.

(Comparative Example 3)

The operation of a comparative example 3 was conducted similarly to the comparative example 2 except that the raw sheet thickness was 0.225 mm and the side wall minimum thickness was 0.093 mm. The sheet thicknesses and the like of the parts of the seamless can body thus obtained are set forth in Table 1.

[Evaluation]

The DI cans obtained by the above methods were evaluated by the following method. The results are illustrated in Table 1.

[Pressure Resistance Test Method]

In a state in which a cup is filled with water, an opening end is sealed with a plug provided with a water feeding pipe. Next, pressurized water is fed from a water feeding pump into the cup through the water feeding pipe. The internal pressure of the cup then rises, and at a certain time point, the dome section is instantaneously deformed (buckled) to be inverted to the outside. Normally, simultaneously with this deformation, the internal pressure of the can is suddenly lowered. A maximum value of the internal pressure of the can during this process is set as a withstanding pressure (MPa).

[Table 1]

		Each size of cup body			Each size of can bottom section								Evaluation
	Thickness of original sheet	Height of cup dome Ho (mm)	Metal length of inclined section S (mm)	Thickness of inclined section S (mm)	Height of can dome Hp (mm)	Inside diameter dx (mm)	Inside diameter dy (mm)	t1 (mm)	t2 (mm)	t3 (mm)	t4 (mm)	t5 (mm)	Weight of can (g) [corresponding to 350 mL can]	Withstanding pressure (MPa)
Example 1	0.280	16.0	4.6	0.287	12.3	48.86	46.00	0.287	0.299	0.310	0.289	0.261	11.2	1.043
Example 2	0.225	16.0	4.6	0.231	12.3	48.86	46.00	0.231	0.240	0.249	0.231	0.211	9.6	0.760
Com. Ex. 1	0.280	-	-	-	12.3	45.66	46.00	0.285	0.265	0.242	0.258	0.265	11.2	0.620
Com. Ex. 2	0.280	-	-	-	12.3	46.64	46.00	0.284	0.265	0.241	0.250	0.265	11.2	0.781
Com. Ex. 3	0.225	-	-	-	12.3	46.65	46.00	0.228	0.213	0.192	0.200	0.210	9.6	0.588

The results of examples and comparative examples indicate that, by controlling the thicknesses of specific parts of the can bottom, a favorable pressure resistance (equal to or more than 0.618 MPa demanded for use for carbonated drinks) can be obtained even in the case where the sheet thickness of the raw sheet (blank) is thin.

[Second Embodiment]

While conventional seamless can bodies are excellent in lightness in weight as described above, there is still a point to be improved in their can body sections constituting the side surfaces. Specifically, in recent years, competitive power of products is secured by applying various designs to the can body section, and from such a viewpoint, image clarity as uniform as possible is demanded of the can body section.
However, in the conventional method of manufacturing a seamless can body, the surface condition of the molded can body section has not been aligned in the axial direction, and high metallic luster could not be obtained particularly in the vicinity of the lower end of the can body section or at a reduced diameter part (outer circumferential bottom section) provided between the can body section and the can bottom section.
This point will be described in detail by using FIG. 14.
FIG. 14(a) schematically illustrate a partial state of a can body section and a tip end part of an ironing punch immediately after completion of ironing. As illustrated, a cylindrical section of the punch has, near the tip end thereof, a tapered shape ranging from a point A to a point B. This tapered shape is provided for ensuring gradual rising of the ironing ratio at the time of starting ironing. Therefore, the can body part corresponding to the tapered part is a region having a wedge-shaped sheet thickness distribution. Note that, as depicted in FIG. 9 and the like, this region may also be referred to as a "body wall step (BWS)." In addition, on the lower side of the BWS, a part which is comparatively largely reduced in diameter toward the inside of the can and which may also be referred to as a body wall radius (BWR) is formed.
In the case where the ironing is conducted, the glossiness of the ironed surface at the point B located at the lower end of the BWS is substantially comparable to the glossiness of the original raw material surface, the glossiness of the ironed surface increases toward the point A located at the upper end of the BWS, and a maximum glossiness is exhibited at and beyond the point A.
FIG. 14(b) is a diagram depicting a partial state of the can body section and the tip end part of the ironing punch at the time point when the dome section is formed in the can bottom by relatively moving the doming die into the inside of the tip end of the ironing punch after completion of ironing. A bottom surface part of the can bottom is drawn into the inside to become a dome section, so that a part located at the point A in FIG. 14(a) is deviated to the point A', and a part located at the point B is deviated to a point B'. Note that the moving amount (deviation amount) of each of these points is, for example, approximately 2 to 5 mm. In the vicinity of a lowermost part of a cylindrical part of the can body section, there is still a part in which glossiness is low and image clarity of print is also poor, and it has hitherto been demanded to provide a seamless can body in which this part has a high metallic luster and which is high in design property.
Note that, while it is possible to increase the deviation amount by simply increasing the amount by which the doming die enters, this approach has a problem in that the internal volume of the can to be molded is conspicuously reduced and, at the same time, the amount of material of the can to be used is increased.
In view of this, in the second embodiment described later, as a result of repetition of extensive and intensive investigations in consideration of the above-exemplified problem, it is made possible to provide a seamless can body capable of imparting excellent image clarity to the drawn and ironed can body section and a method of manufacturing the seamless can body. In addition, in the second embodiment, it is made possible to provide a seamless can body having a high metallic luster at a reduced diameter part (outer circumferential bottom section) provided between the can body section and the can bottom section and a method of manufacturing the seamless can body.
Note that, in the following description, elements similar in configuration and function to those of the seamless can body 1A of the first embodiment above will be denoted by the same reference numbers, and descriptions thereof will be omitted appropriately.

As depicted in FIG. 8, a seamless can body 1B according to the present embodiment is a seamless can body that includes a tubular body section 10 and a can bottom section 20 including at least an outer circumferential bottom section 20a extending from a lower end of the tubular body section 10 through a boundary part BP such as to decrease in diameter toward the inside. Note that, while a part on the upper side beyond the tubular body section 10 in FIG. 8 has a neck flange shape as an example, the structure of a known seamless can body having an opening 10a is also applicable to the part on the upper side beyond the tubular body section 10. Here, "the lower end 10e of the tubular body section 10" can be defined as a part which is located substantially at the lower end of the cylindrical surface and as a lower end of a region in which curved surface printing can be performed by, for example, a known dry offset system in the case where printing is applied to the outer surface of the seamless can body.
The tubular body section 10 is a part constituting a side surface of the seamless can body 1B and is formed by drawing and ironing a known metal sheet of aluminum, steel, or the like which will be described later. The tubular body section 10 has an appropriate thickness depending on the use and has, for example, a thickness of approximately 0.07 to 0.40 mm.
In the present embodiment, it is defined that the tubular body section 10 is provided between the lower end 10e, which is described later, as a lower end part and the boundary with a neck shoulder (the part decreased in diameter toward the upper side in the axial direction) as an upper end part, as depicted in FIG. 8.
The can bottom section 20 includes at least the outer circumferential bottom section 20a extending from the abovementioned lower end 10e of the tubular body section 10 such as to decrease in diameter toward the inside, as depicted in FIG. 8, and a bulging section 20b bulging from the outer circumferential bottom section 20a toward the opening 10a.
Note that, as is clear from the FIG. 8, the outer circumferential bottom section 20a and the bulging section 20b in the present embodiment are defined as different sections with an annular grounding section 20c as a boundary. The annular grounding section 20c is a part that, when the seamless can body 1B is mounted on a flat surface such as a table, grounds on the surface. Therefore, the outer circumferential bottom section 20a, the annular grounding section 20c, and the bulging section 20b in the present embodiment can be said to correspond to the foot part 202 and the can bottom central part 201 in the first embodiment described above. In this instance, particularly the annular grounding section 20c in the present embodiment corresponds to the annular grounding section 202b in the first embodiment.
In addition, "the boundary part BP" in the present embodiment can be defined as a boundary with a region concerning the external appearance on the can bottom side (that is, a region which can normally be observed from the outside of the can), defined as a part extending from the lower end 10e of the tubular body section 10 to the outer circumferential bottom section 20a through inflection as depicted in FIG. 17, and defined as a point where the angle γ formed between a tangential line of the outer surface at the boundary part BP and a grounding surface P is 45°.
The reason why the point where the angle γ is 45° is defined as the boundary part BP in the present embodiment is as follows. Specifically, at a position where the γ is less than 45°, the normal line to the outer surface directs excessively downward. Then, for example, in a state in which the can to which the present invention is applied is placed normally (upright) on a showcase or the like, the reflected light is less likely to enter the visual field, and therefore, it is difficult to exhibit the excellent luster property of the can outer surface which is the gist of the present invention.
Note that, as depicted in FIG. 9(a), in a conventional structure, the amount by which a thin wall part of the tubular body section is drawn downward when forming the dome section is extremely small, and therefore, a part in the vicinity of the boundary part BP is relatively thick.
On the other hand, in the seamless can body 1B according to the present embodiment, as depicted in FIG. 9(b), a part on the lower end side of the tubular body section 10 including the lower end 10e of the tubular body section 10 having undergone ironing is drawn into the side of the outer circumferential bottom section 20a, and therefore, a part of the outer circumferential bottom section 20a beyond the boundary part BP and at least in the vicinity of the boundary part BP includes a metal sheet having undergone ironing.
In other words, in the seamless can body 1B according to the present embodiment, it can be said that the sheet thickness t0 at at least the boundary part BP is substantially equal to the sheet thickness twc (see FIG. 8) at an intermediate part of the tubular body section 10.
Therefore, the tubular body section 10 of the present embodiment has a high glossiness and can exhibit uniform image clarity from the upper end to the lower end in regard of the axial direction (Z direction in FIG. 8) and further up to the position of the boundary part BP, as compared to the conventional structure. Note that the ironing ratio required for the ironed tubular body section 10 to exhibit a high glossiness varies depending on the characteristics of the raw material and processing conditions; as a non-limitative example, a total ironing ratio is preferably at least equal to or more than 60%.
Note that, in the present embodiment, it is desirable that the relation of t_WC ≤ t_WL < 1.09×t_WC is satisfied, and more preferably the relation of t_WC ≤ t_WL < 1.05×t_WC is satisfied, in the case where t_WL is the sheet thickness in the vicinity of the boundary part BP of the tubular body section 10 (for example, the lower end of the tubular body section 10) and where twc is the sheet thickness of an intermediate part of the tubular body section 10 in the axial direction (Z direction), as depicted in FIG. 8. In such a manner, it is possible to maintain pressure resistance of the seamless can body 1B while image clarity of the can side surface is enhanced. Note that "the intermediate part of the tubular body section 10 in the axial direction" in the present embodiment may not necessarily be strictly the sheet thickness at a midpoint in the axial direction and can be defined as inclusive of the vicinity of the midpoint.
Further, in the present embodiment, it is also desirable that the relation of t_WC ≤ t0 < 1.09×t_WC is satisfied, more preferably the relation of t_WC ≤ t0 < 1.05×t_WC is satisfied, in the case where twc is the sheet thickness at an intermediate part of the tubular body section 10 in the axial direction as depicted in FIG. 8. If t0 is less than twc, there is a possibility that axial load strength at this part may be lowered, and if t0 is equal to or more than 1.09 times twc, the glossiness at a lower end part of the tubular body section is lowered, and thus, it becomes difficult to obtain the effects of the present invention.
As a result, it is possible to maintain pressure resistance of the seamless can body 1B while image clarity of the can side surface is enhanced.
In addition, since the ironed metal sheet extends beyond the boundary part BP to reach at least a part of the outer circumferential bottom section 20a, it is desirable that the 60 degrees specular glossiness from the lower end 10e of the tubular body section 10 to the vicinity of the boundary part BP is equal to or more than 300%. If the 60 degrees specular glossiness in the vicinity of the boundary part BP is less than 300%, surface roughness, dullness, or the like is noticeable on an external appearance basis at the relevant part, and therefore, appeal as a product is lowered.
Note that the specular glossiness in the present embodiment is measured according to the measuring method defined in JIS Z 8741-1997.
Note that, in the present embodiment, the kind of the raw metal material used for the seamless can body 1B is not particularly limited. In other words, known metal sheets ordinarily used for seamless can bodies, such as an aluminum alloy sheet or a steel sheet (for example, tinplate or the like), can be used. In addition, the metal sheet may have a known film laminated on the inner surface side thereof or may be subjected to surface-treatment such as organic resin coating or chemical conversion treatment, as required.
Besides, the seamless can body 1B according to the present embodiment has a lid attached to the opening 10a by a known method after the seamless can body 1B is subjected to, for example, known flange processing, necking, screw processing, or the like, and beer, a carbonated drink, coffee, juice, liquid food, or the like is accommodated in the seamless can body 1B.

Next, a method of manufacturing the seamless can body 1B according to the present embodiment will be described referring to FIGS. 10 to 12, as required.
The method of manufacturing the seamless can body 1B according to the present embodiment is a method of manufacturing a seamless can body having the tubular body section 10 and the can bottom section 20 depicted in FIG. 8 and is characterized by including a first molding step and a second molding step which will be described in detail below.

[First Molding Step]

In the method of manufacturing the seamless can body 1B according to the present embodiment, in the first molding step depicted in FIG. 10, a raw metal material (precursor 3) is molded into a cup body 2 including the tubular body section 10, an inclined section S extending upward toward the inside from a boundary part BP provided at the lower end of the tubular body section 10, and a cup dome section D bulging upward from an end portion Se of the inclined section S at a first height Ho. Here, the end portion Se of the inclined section S can be said to be a connection point with the cup dome section D.
The first molding step of the present embodiment is performed, by use of an upper mold and a lower mold, on a precursor 3 including the tubular body section 10 thinned through ironing and molded by a known pressing step or the like. Specifically, the first molding step of the present embodiment can be conducted at a final end position (the vicinity of a bottom dead center) of a punch stroke of a molding machine for ironing or can be performed in a machine different from the machine used for ironing.
As a specific example, as depicted in FIG. 10, the first molding step is carried out by a tubular punch 401 that is located in the precursor 3 having a cup shape and supports the precursor 3, and a doming die 502 that supports an outer circumferential bottom section of the precursor 3 by cooperating with the punch 401. Of these components, a lower end of the punch 401 is recessed toward the upper side so as to correspond to the doming die 502, and a circumferential wall part 402 is formed along the circumferential direction. Note that, while a single arcuate shape is exemplified as a sectional shape of the circumferential wall part 402 in FIG. 10 in the present embodiment, this shape is not limitative. For example, a combined shape of a plurality of arcs and tapered surfaces as depicted in FIG. 15 or FIG. 16 may be adopted.
First, when the precursor 3 is pressed such as to be interposed between the punch 401 and the doming die 502, a bottom surface of the precursor 3 bulges toward the opening by the doming die 502, and a lower end circumferential edge comes into a state of being pulled by the circumferential wall part 402. In other words, in the first molding step, the outer circumference of the precursor 3 is supported by the circumferential wall part 402 of the punch 401, and the punch 401 and the doming die 502 are driven such as to engage with each other, so that the cup body 2 having the cup dome section D with a first height Ho at the bottom thereof can be obtained.
Note that, in the case where wrinkles are generated at the circumferential wall part 402 and its vicinity when the cup dome section D is formed in the first molding step, a wrinkle pressing member 80 (also referred to as a hold down ring) exemplified in FIG. 16 may be disposed if necessary, and the molding may be conducted by applying a wrinkle pressing force by the circumferential wall part 402 and the wrinkle pressing member 80.
In this instance, it is necessary to set the first height Ho of the cup dome section by matching the raw material amounts such that the respective materials constituting the cup dome section D, the end portion Se, and the inclined section S can constitute the can bottom section 20 in FIG. 8 in the second molding step to be conducted later. As a result, the first height Ho in the present embodiment is higher than the dome height in the conventional structure, and therefore, the amount by which the tubular body section 10 is drawn into the side of the outer circumferential bottom section 20a is also increased in association with the first height Ho.
As a result, as depicted in FIG. 11, the site originally constituting the lower end of the tubular body section 10 at the time of ironing is drawn into the side of the outer circumferential bottom section 20a beyond the boundary part BP provided between the tubular body section 10 and the outer circumferential bottom section 20a (more specifically, the point A and the point B located in the tubular body section in the example depicted in FIG. 11 are drawn into the side beyond the boundary part BP). In other words, in the first molding step, the lower end 10e of the tubular body section 10 is drawn to form a part of the outer circumferential bottom section 20a extending from the lower end 10e of the tubular body section 10 such as to decrease in diameter (substantially the part is still in the state of a curved surface in the vicinity of the boundary part BP and is referred to as a first outer circumferential bottom section 20a').
Here, the shape of the cup body 2 obtained in the first molding step will be described.
The inclined section S of the cup body 2 extends upward toward the inside from the first outer circumferential bottom section 20a'. In other words, as depicted in FIG. 10(c) or the like, the inclined section S of the cup body 2 includes a curved line part and a straight line part interposed between the lower most part of the cup body 2 in the Z-axis direction and the connection point (end portion Se) with the cup dome section D.
Note that, in the first molding step, the part including the inclined section S and the cup dome section D is also referred to as a bulging section. Therefore, the cup body 2 of the present embodiment includes the tubular body section 10 and the bulging section formed at a bottom surface of the tubular body section 10.
The shape of the cup dome section D is an example, and the top of the dome may have, for example, a horizontal surface shape instead of a curved surface shape.
Further, the first height Ho of the cup dome section D of the cup body 2 is preferably greater than a second height Hp of the can dome section 201d in the seamless can body 1B obtained in the second molding step. One of the reasons for this is to apply a compressive stress to the inclined section S while the cup dome section D of the cup body 2 is pressed down in the second molding step described later. In other words, the first height Ho of the cup dome section D of the cup body 2 is preliminarily set to be large to finally obtain a favorable second height Hp of the can dome section 201d of the seamless can body 1B.
In other words, in the first molding step, the bulging section bulging at the first height Ho is first formed such as to extend from the first outer circumferential bottom section 20a' in the vicinity of the boundary part BP toward the opening 10a. Then, in the second molding section described later, the bulging section is pressed down such as to have a second height lower than the first height Ho.

[Second Molding Step]

Next, referring to FIG. 12, the second molding step of the method of manufacturing the seamless can body 1B according to the present embodiment will be described.
After the cup body 2 having the first outer circumferential bottom section 20a' and the inclined section S is molded in the first molding step, the second molding step is conducted as follows.
Note that, for example, a known cleaning step, surface-treatment step, printing step, coating step, or shaping step for the tubular body section, or necking-in (squeezing) in such a range as not to hamper the second molding step may be carried out on the cup body 2, as required, between the first molding step and the second molding step. Further, if necessary, for the purpose of securing conveyability and corrosion resistance after the first molding step, an outer surface coating can be applied to the part ranging from the grounding section at the lowermost end of the cup body 2 to the inclined section S.
In the second molding step, processing is performed on the cup body 2 by using a mold different from a mold used in the abovementioned first molding step, to mold the seamless can body 1B. Specifically, as depicted in FIG. 12, while the cup body 2 is brought into contact with the lower molding member, a pressing force is applied to the cup dome section D of the cup body 2 in the can outside direction (-Z axis direction) by use of the upper molding member.
More specifically, as depicted in FIG. 12(a), the vicinity of the boundary part BP of the cup body 2 is mounted on a cup outer circumferential side holder 60. Next, a dome pressing-down tool 70 is relatively lowered, so that a support section 701 of the dome pressing-down tool 70 is brought into contact with the cup dome section D, as depicted in FIG. 12(b). Note that, while the shape of the support section 701 is substantially coincident with the shape of the cup dome section D in FIG. 12, the shape may not necessarily be coincident. For example, a difference in curvature may be provided such that a high pressure is exerted on an outer circumferential part of the cup dome section D.
Here, the cup outer circumferential side holder 60 has a tapered surface 601 and a groove 602. After the boundary part BP and the first outer circumferential bottom section 20a' of the cup body 2 make contact with the tapered surface 601, the dome pressing-down tool 70 is further lowered. In such a manner, as depicted in FIG. 12(c), the metal of the inclined section S of the cup body 2 is molded such as to follow the tapered surface 601 while receiving a compressive stress.
Next, as depicted in FIG. 12(d), the dome pressing-down tool 70 is further pressed down, so that the remaining part (a part of the metal other than the part corresponding to the tapered surface 601) of the inclined section S of the cup body 2 is guided into the groove 602. In this instance, the cup dome section D is pressed down such as to have a second height Hp lower than the first height Ho. Simultaneously, similarly to the case described in the first embodiment, a compressive stress σ_Φ in the meridian direction and a compressive stress σ_θ in the circumferential direction are applied to the inclined section S by use of the upper molding member (dome pressing-down tool) and the lower molding member (cup outer circumferential side holder). (see FIG. 6)
As a result, the metal corresponding to the tapered surface 601 of the cup body 2 constitutes the outer circumferential bottom section 20a, and the metal guided into the groove 602 constitutes the abovementioned annular grounding section 20c. Further, the part on the upper side from the annular grounding section 20c constitutes the bulging section 20b. (see FIG. 12(e))
In this way, the can bottom section 20 of the seamless can body 1B is obtained through the second molding step.
After the above molding is finished, it is sufficient to relatively raise the dome pressing-down tool and take out the seamless can body 1B from the cup outer circumferential side holder, as depicted in FIG. 12(e).
The seamless can body 1B molded by the manufacturing method according to the present embodiment described above has the tubular body section 10 serving as the can side surface and having a substantially uniform surface condition from the upper end to the lower end in the axial direction, and can thus exhibit excellent appearance and image clarity.
Note that the second embodiment described above can be said to be substantially common with the first embodiment except for the can bottom section (mainly the shape of the dome section at the can bottom). Therefore, it is needless to say that the technical thought concerning the relation of the sheet thicknesses in the present embodiment and the technical thought concerning the metallic luster at the can body section and can bottom section are also applicable to the first embodiment similarly to the second embodiment, unless contradiction is generated.
Conversely speaking, the seamless can body 1B molded by the manufacturing method according to the present embodiment, by incorporating therein the characteristics of the abovementioned first embodiment (the inside end section 202c, the rising section 202d, the outermost end 201e, and the can dome section 201d), can further exhibit effects similar to those of the first embodiment.
Thus, when the shape of the dome section of the seamless can body 1A according to the first embodiment is applied to the seamless can body 1B according to the second embodiment and FIG. 1 and description thereof and FIG. 13 are put together, in the seamless can body according to the present invention, a seamless can body capable of imparting excellent image clarity to the drawn and ironed can body section while imparting high pressure resistance and a manufacturing method of the seamless can body can be realized.
Note that, since the foot part 202 in FIG. 13 varies in sheet thickness from the boundary part BP (see FIG. 8) toward the annular grounding section 202b, as is clear from FIGS. 1, 8, and 11 and the like, the part where the thickness is t1 is shifted to the side of the annular grounding section 202b with respect to the position of the sheet thickness t0 at the boundary part BP. In this instance, it is preferable to have the characteristic that the sheet thickness t0 < the sheet thickness t1, the characteristic that t_WC ≤ t_WL < 1.09×t_WC indicated in the second embodiment, the characteristic that t_WC ≤ t0 < 1.09×t_WC, and the characteristic that the 60 degrees specular glossiness from the lower end 10e of the tubular body section 10 to the vicinity of the boundary part BP is equal to or more than 300%.
In this way, the seamless can body according to the present invention can have both the characteristics of the first embodiment and the second embodiment mentioned above, the tubular body section 10 serving as the can side surface has a substantially uniform surface condition from the upper end to the lower end in the axial direction thereof and exhibits excellent appearance and image clarity, and simultaneously, excellent pressure resistance can be secured at the can bottom section.
Here, it will be described again, by using FIG. 14, that the seamless can body 1B described in the second embodiment is advantageous over the conventional structure at least in glossiness.
Here, FIG. 14(a) depicts, on an extraction basis, the structure in the vicinity of the lower end of the can body section of a seamless can body obtained immediately after ironing is performed by a conventional manufacturing method, and FIG. 14(b) depicts, on an extraction basis, the structure in the vicinity of the lower end of the can body section obtained after doming is further performed. On the other hand, FIG. 14(c) depicts, on an extraction basis, the structure in the vicinity of the lower end of the can body section of the seamless can body 1B according to the present embodiment.
Note that, while the sheet thickness t_WL of the lower end of the tubular body section 10 is equal to the sheet thickness twc of an intermediate part of the tubular body section in the axial direction in FIG. 14(c), the present invention is not limited to this form. As described above, t_WL < 1.09×t_WC may be satisfied.
In other words, in the case where ironing is conducted for molding a seamless can body, as described above, in FIG. 14(a) illustrating the conventional technique, ironing is first started at the point B with an ironing ratio of 0, the ironing ratio then increases toward the point A, and the ironing ratio reaches the maximum at and beyond the point A. Therefore, regarding the glossiness of the ironed surface of the can body section, for example, the glossiness at the point B is substantially comparable to the glossiness of the original raw sheet surface, the glossiness increases toward the point A, and a maximum glossiness is exhibited at and beyond the point A.
Then, FIG. 14(b) depicts that the doming die is relatively moved toward the inside of the tip end of the ironing punch after completion of ironing, resulting in a state in which the dome section is formed at the can bottom section. As a result, a part of the can bottom section is drawn into the dome section, the site originally located at the point A is deviated to a point A', and the site originally located at the point B is deviated to the point B'. Note that the deviation amount of each site in the conventional technique is, for example, approximately 2 to 5 mm. Therefore, a part that is low in glossiness and poor in image clarity of print is still left at the lowermost portion of the tubular section of the can body.
On the other hand, in the seamless can body 1B according to the present embodiment, as is clear from FIG. 14(c) or the like, the ironed metal sheet extends beyond the boundary part BP to reach at least a part of the outer circumferential bottom section, so that the glossiness in the vicinity of the boundary part BP is comparable to that of the can body section. As a result, the can body section can have a high glossiness ranging from the upper end to the lower end in the axial direction thereof.
The first embodiment and the second embodiment described above are one example in which the gist of the present invention is embodied, and modifications may be made, as required, in such ranges as not to depart from the gist of the invention. Further, known structures may be added to the seamless can bodies described in the first embodiment and the second embodiment, in such ranges as not to depart from the gist of the present invention.

[Industrial Applicability]

According to the present invention, it becomes possible to enhance pressure resistance and restrain the phenomenon of buckling while the sheet thickness of the raw sheet (blank) of the seamless can body is reduced. Therefore, it becomes possible to reduce the manufacturing cost, transportation cost, and the like of the seamless can bodies. In addition, since fuel and the like necessary for the manufacture or transportation is also reduced, it is possible to realize manufacture of seamless can bodies with consideration to environment.
Besides, the present invention is applicable to containers required to enhance appearance and image clarity and is particularly applicable to can bodies capable of storing liquid such as drinks and drugs.

[Reference Sign List]

1A, 1B:: Seamless can body
2:: Cup body
3:: Precursor
10:: Tubular body section
10e:: Lower end
20:: Can bottom section
20a:: Outer circumferential bottom section
20a':: First outer circumferential bottom section
20b:: Bulging section
201:: Can bottom central part
201d:: Can dome section
201e:: Outermost end
202:: Foot part
202a:: Outer circumferential bottom section
202b:: Annular grounding section
202c:: Inside end section
202d:: Rising section
A:: Cup outer circumferential bottom section
D:: Cup dome section
S:: Inclined section
Se:: End portion
Hp:: Height of can dome section (second height)
Ho:: Height of cup dome section (first height)
60:: Lower molding member (cup outer circumferential side holder)
70:: Upper molding member (dome pressing-down tool)

Claims

A seamless can body including a tubular body section and a can bottom section,
the can bottom section including
an outer circumferential bottom section extending from a lower end of the tubular body section such as to decrease in diameter toward an inside, and

an annular grounding section located further inside than the outer circumferential bottom section,

wherein, in a case where t1 is a sheet thickness of the outer circumferential bottom section and where t2 is a sheet thickness of the annular grounding section, the following relation is satisfied. $t 2 > t 1$
A seamless can body comprising:
a tubular body section; and

a can bottom section including at least an outer circumferential bottom section extending from a lower end of the tubular body section through a boundary part such as to decrease in diameter toward an inside,

wherein a sheet thickness of the lower end of the tubular body section is substantially equal to a sheet thickness of an intermediate part of the tubular body section in an axial direction.
The seamless can body according to claim 1 or 2,
wherein the can bottom section further includes an inside end section located further inside than the annular grounding section, and

in a case where t3 is a sheet thickness of the inside end section, the following relation is satisfied. $t3 > t 1$
The seamless can body according to claim 3, wherein a sheet thickness gradually increases from the outer circumferential bottom section to the inside end section such that t3 > t2 is satisfied.
The seamless can body according to any one of claims 1 to 4,
wherein the can bottom section further includes a rising section rising upward from the inside end section, and

in a case where t4 is a sheet thickness of an upper end of the rising section, the following relation is satisfied. $t4 > t 1$
The seamless can body according to claim 5, wherein the can bottom section further includes a dome section that is connected to the rising section and bulges to protrude upward, and a sheet thickness gradually increases from the dome section to the inside end section such that t3 > t4 > t5 is satisfied in a case where t5 is a sheet thickness of a center of the dome section.
The seamless can body according to claim 6, wherein t5 < t1 is further satisfied.
The seamless can body according to any one of claims 5 to 7, wherein a ring groove in which a connection section between the rising section and the dome section protrudes toward an outside with respect to a can body axis is formed.
The seamless can body according to claim 2, wherein a sheet thickness of the boundary part is substantially equal to the sheet thickness of the intermediate part.
The seamless can body according to claim 2 or 9,
wherein, in a case where t_WL is the sheet thickness of the lower end of the tubular body section and where twc is the sheet thickness of the intermediate part of the tubular body section in the axial direction, the following relation is satisfied. $t_{WC} \leq t_{WL} < 1.09 \times t_{WC}$
The seamless can body according to claim 10,
wherein the following relation (where t0 is the sheet thickness of the boundary part) is satisfied in the tubular body section. $t_{WC} \leq t 0 < 1.09 \times t_{WC}$
The seamless can body according to any one of claims 1 to 11, wherein 60 degrees specular glossiness from the lower end of the tubular body section to the vicinity of the boundary part is equal to or more than 300%.
A method of manufacturing a seamless can body including a tubular body section and a can bottom section, the method comprising:
a first molding step of molding a raw metal material into a cup body including the tubular body section, a cup outer circumferential bottom section extending from a lower end of the tubular body section such as to decrease in diameter, an inclined section extending upward toward an inside from the cup outer circumferential bottom section, and a cup dome section bulging upward from an end portion of the inclined section at a first height; and

a second molding step of applying a pressing force to the cup dome section toward an outside of a can by using an upper molding member while the cup outer circumferential bottom section of the cup body is brought into contact with a lower molding member, to press down the cup dome section such as to have a second height lower than the first height, and to apply compressive stresses in a meridian direction and a circumferential direction, and then pressing the inclined section into the lower molding member while a thickness of the inclined section is increased.
The method of manufacturing a seamless can body according to claim 13,
wherein, in the second molding step,
the inclined section is pressed into the lower molding member, to thereby form an annular grounding section located further inside than an outer circumferential bottom section, an inside end section located further inside than the annular grounding section, and a rising section rising upward from the inside end section and connected to a can dome section, and

a ring groove in which a connection section between the rising section and the can dome section protrudes toward an outside with respect to a can body axis is formed such that an inside diameter of the connection section becomes greater than an inside diameter of the inside end section.
A method of manufacturing a seamless can body, the method comprising:
a first molding step of molding a raw metal material into a cup body including a tubular body section thinned by ironing, an outer circumferential bottom section extending from a lower end of the tubular body section, and a bulging section bulging from the outer circumferential bottom section toward an opening at a first height; and

a second molding step of pressing down the bulging section such as to have a second height lower than the first height,

wherein, in the first molding step, the lower end of the tubular body section is drawn to form the outer circumferential bottom section extending from the lower end of the tubular body section through a boundary part such as to decrease in diameter toward an inside, such that a sheet thickness of the lower end of the tubular body section becomes substantially equal to a sheet thickness of an intermediate part of the tubular body section in an axial direction.