Seamless can body and method for manufacturing seamless can body

Abstract

To provide a seamless can body capable of suppressing buckling by simultaneously thinning the plate thickness of a blank and increasing the pressure resistance of a can bottom and solving the problem of black discoloration or cleaning, and a method for manufacturing the seamless can body.SOLUTION: A seamless can body 1 comprises a cylindrical barrel part 10 and a can bottom 20. The can bottom 20 includes an outer peripheral bottom part 202a continuous so as to be reduced in diameter from the bottom end of the cylindrical barrel part 10 to the inside, and a peripheral ground part 202b located inside the outer peripheral bottom part 202a, and t2>t1 is set, where t1 denotes the plate thickness of the outer peripheral bottom part 202a and t2 denotes the plate thickness of the peripheral ground part 202b.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a seamless can body and a method for manufacturing a seamless can body. [Background technology]

[0002] Conventionally, so-called seamless can bodies have been known, in which the can body and other parts are formed by drawing and ironing. These seamless can bodies have excellent light weight because the can body is thinned by ironing. On the other hand, it is difficult to employ a processing method that forcibly thins the wall, such as ironing, in the bottom of these seamless can bodies, and the thickness of the can body bottom does not vary significantly from the material thickness. Because the bottom is required to have strength (pressure resistance) to resist deformation due to internal can pressure, various proposals have been made to reduce the material thickness of the can body bottom to reduce its weight while maintaining or improving pressure resistance.

[0003] For example, Patent Documents 1 and 2 disclose a so-called bottom reforming process that is performed to prevent the phenomenon of the dome portion of the can bottom inverting (buckling), which occurs when the internal pressure of the can exceeds the pressure resistance strength. Specifically, the bottom reforming process is disclosed in which a recess is formed by pressing the inner peripheral wall located radially inward of the ground contact portion of the can bottom, perpendicular to the can axis. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-103227 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-47541 [Patent Document 3] Japanese Patent Application Laid-Open No. 2000-176575 [Patent Document 4] Japanese Patent Application Publication No. 9-285832 [Patent Document 5] WO2018 / 070542 publication DISCLOSURE OF THE INVENTION [Problem to be solved by the invention]

[0005] However, the bottom reforming process has the following problems. That is, in this bottom reforming process, the recesses are formed by pressing the inner peripheral wall of the can bottom using a forming roller or the like. When pressing using this forming roller or the like, there are problems such as blackening occurring at the pressed area and adhesion of metal material to the forming roller or the like, as described in Patent Document 3.

[0006] Furthermore, during pressing, lubricating oil is applied to ensure smooth processing, but a process is required to wash off this lubricating oil after the bottom reforming process. Therefore, further improvements were required from the perspective of the cost and environmental impact of cleaning.

[0007] Furthermore, in recent years, in order to reduce the weight of seamless can bodies, there has been a demand for thinner and thinner blanks before the drawing and ironing process. However, when the bottom reforming process is performed, the thickness of the metal material in the pressing portion is stretched and thinned by the process, which limits the thickness of the blank.

[0008] Furthermore, as shown in Patent Document 4, the present inventors have disclosed a technology for improving the pressure resistance of seamless can bodies. However, although this technology improves pressure resistance, it does not sufficiently optimize the thickness distribution of each part of the can body (particularly the can bottom). Therefore, it does not fully satisfy the demand for lighter can bodies.

[0009] Furthermore, Patent Document 5 discloses a two-piece can body characterized in that the thickness of the contact portion of the can bottom is thicker than the thickness of the material before processing. However, this technology has problems such as the complicated equipment required, making it difficult to realize on an industrial level, or incurring high equipment costs.

[0010] The present inventors have conducted extensive research in light of the problems exemplified above, and as a result have been able to provide a seamless can body that can reduce the thickness of the raw sheet (blank), increase the pressure resistance of the can bottom, suppress buckling, and solve the problems of blackening and cleaning, as well as a manufacturing method thereof using a simpler manufacturing apparatus, thereby arriving at the present invention. [Means for solving the problem]

[0011] In order to achieve the above object, one embodiment of the seamless can body is characterized in that (1) it is a seamless can body having a cylindrical body portion and a can bottom portion, wherein the can bottom portion includes an outer peripheral bottom portion that continues from the lower end of the cylindrical body portion so as to taper inward, and a circumferential ground contact portion located inside the outer peripheral bottom portion, and wherein, when the thickness of the outer peripheral bottom portion is t1 and the thickness of the circumferential ground contact portion is t2, t2 > t1.

[0012] Furthermore, in the above (1), (2) it is preferable that the can bottom further includes an inner end portion 202c located inside the circumferential ground contact portion, and when the plate thickness of the inner end portion is t3, t3 > t1.

[0013] In addition, in the above (2), it is preferable that (3) the plate thickness gradually increases from the outer circumferential bottom portion to the inner end portion so that t3>t2.

[0014] In addition, in any of the above (1) to (3), (4) it is preferable that the can bottom further includes a rising portion 202d rising upward from the inner end portion, and when the thickness of the upper end of the rising portion is t4, t4>t1.

[0015] In addition, in the above (4), (5) it is preferable that the can bottom further includes a can dome portion that is continuous with the rising portion and bulges upward to form a convex shape, and when the plate thickness at the center of the can dome portion is t5, the plate thickness gradually increases from the can dome portion to the inner end portion so that t3 > t4 > t5.

[0016] Also, in the above (5), (6) furthermore, it is preferable that t5 < t1.

[0017] Also, in any of the above (4) to (6), (7) it is preferable that a ring groove is formed such that the connection portion between the rising portion and the dome portion is convex toward the outside of the can body axis.

[0018] To achieve the above object, a method for manufacturing a seamless can body according to an embodiment of the present invention is (8) a method for manufacturing a seamless can body having a cylindrical body portion and a can bottom portion, the method comprising: forming a metal material into a cup body having a cylindrical body portion, a cup outer peripheral bottom portion that continues from the lower end of the cylindrical body portion and has a reduced diameter, an inclined portion that extends upward from the cup outer peripheral bottom portion toward the inside and is inclined toward the center of the dome, and a cup dome portion that bulges upward at a first height from an end portion of the inclined portion; and a second forming step of applying a pressing force outward of the can from the cup dome portion with an upper die forming member while abutting the cup outer peripheral bottom portion of the cup body against a lower die forming member, thereby pushing down the cup dome portion to a second height lower than the first height and applying a meridional and circumferential compressive stress to the inclined portion, pushing it into the lower die forming member while increasing the thickness of the inclined portion, forming a circumferential contact portion located inside the outer peripheral bottom portion, and satisfying t2 > t1 when the plate thickness of the outer peripheral bottom portion is t1 and the plate thickness of the circumferential contact portion is t2.

[0019] <​​​​

[0020] According to the seamless can body of the present invention, even when the thickness of the blank is reduced, a can bottom with higher pressure resistance than that obtained by conventional bottom reforming processing can be obtained. Therefore, seamless can bodies can be manufactured using thinner blanks than conventional ones, which is advantageous in terms of cost because it reduces the amount of metal material used. Furthermore, the weight reduction of seamless can bodies can lead to reductions in recycling costs, transportation costs, etc.

[0021] Furthermore, according to the seamless can body manufacturing method of the present invention, even when the thickness of the raw plate (blank) is thin, it is possible to increase the pressure resistance of the can bottom and suppress buckling using a simple manufacturing device. It is also possible to solve the problem of blackening that is a problem in bottom reforming. Furthermore, since the conventional bottom reforming process and the subsequent process of washing away lubricating oil are not required, there are significant cost and environmental benefits. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a schematic diagram showing a seamless can body 1 according to the present embodiment. [Figure 2] 2 is an enlarged view showing the can bottom of a seamless can body 1 in this embodiment. FIG. [Figure 3] 2 is a graph showing the thickness at each point in the seamless can body 1 of the present embodiment. [Figure 4] 3A to 3C are diagrams showing a first forming step in the method for manufacturing a seamless can body according to the present embodiment. [Figure 5] 4A to 4C are diagrams showing a second forming step in the method for manufacturing a seamless can body according to the present embodiment. [Figure 6] 5A and 5B are schematic diagrams illustrating compressive stress applied to a rising portion in the present embodiment. [Figure 7] FIG. 2 is a partially enlarged view of the bottom of a seamless can body used in Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0023] The seamless can body and the method for manufacturing the same of the present invention will be specifically described below with reference to the accompanying drawings. Note that the following embodiment is intended to illustrate the present invention by way of example, and is not intended to intentionally limit the present invention.

[0024] <Seamless can body> As shown in Fig. 1, the seamless can body 1 of this embodiment is a seamless can body having a cylindrical body portion 10 and a can bottom portion 20. In this embodiment, as shown in Figs. 1(a) and 1(b), the can bottom portion 20 preferably includes a can bottom center portion 201 that does not contact a horizontal surface when the seamless can body is placed on the horizontal surface, and foot portions 202 located outside the can bottom center portion 201. The can bottom center portion 201 of the seamless can body 1 in this embodiment may be horizontal, or may be dome-shaped, rising toward the inner surface of the can (bulging upward to form a convex shape), as shown in FIG. 1(a).

[0025] In this embodiment, as shown in Figure 1(b), the foot portion 202 of the can bottom 20 is defined as the portion from the lower end 10e of the cylindrical body 10 to the outermost end 201e of the can bottom center portion 201 in the direction of the can body axis RA. As shown in the enlarged cross-sectional view of the foot portion 202 in Figure 2, the "outermost end 201e of the can bottom center portion 201" is the part of the dome shape where the diameter of the dome is maximum, if the can bottom center portion 201 is dome-shaped.

[0026] In this embodiment, the circumferential ground contact portion 202b is the lowest part of the foot portion 202 in the Z-axis direction. In other words, when the seamless can body 1 of this embodiment is placed on a horizontal surface, the circumferential ground contact portion 202b can be said to be the part that comes into contact with the horizontal surface. The area from the lower end 10e of the cylindrical body 10 to the circumferential ground contact portion 202b is defined as the outer circumferential bottom portion 202a.

[0027] That is, in this embodiment, the foot portion 202 includes an outer peripheral bottom portion 202a that continues from the lower end 10e of the cylindrical body portion 10 so as to narrow inward, and a circumferential ground contact portion 202b that is located more inward than the outer peripheral bottom portion 202a. In other words, in the seamless can body of this embodiment, the outer circumferential bottom portion 202a is located in a ring shape extending from the circumferential ground contact portion 202b to the lower end 10e of the cylindrical body portion 10.

[0028] In this embodiment, there are no particular limitations on the ring width or area of ​​the outer peripheral bottom portion 202a, and any known shape can be applied to its inclination angle and curvature. That is, the cross section may be linear, or may be an arc-shaped shape that curves toward the inside of the can body, or conversely, an arc-shaped shape that curves outward. It may also be a shape in which part of the bottom portion curves inward and the rest curves outward, with these shapes connected continuously. In this embodiment, as shown in FIG. 2, it is preferable that the outer circumferential bottom portion 202a has an inflection point IP in its cross section, which makes it easier to place the can on top of the lid of the same kind of can.

[0029] 2, the seamless can body 1 of this embodiment further includes an inner end portion 202c located more inward than the circumferential ground contact portion 202b. This inner end portion 202c is defined as the portion of the foot portion 202 that is closest to the can body axis RA in the cross-sectional view. Furthermore, the seamless can body 1 of this embodiment includes a rising portion 202d extending upward (in the positive direction of the Z axis) from the inner end portion 202c. In the cross-sectional view shown in FIG. 1(a) or FIG. 2, the rising portion 202d is defined as the portion from the inner end portion 202c to the outermost end 201e toward the can bottom center portion 201.

[0030] The seamless can body of this embodiment is characterized in that, when the thickness of the outer peripheral bottom portion 202a is t1 and the thickness of the circumferential ground contact portion 202b is t2, the relationship "t2 > t1" holds. By satisfying this relationship, the seamless can body 1 of this embodiment can be provided with preferable pressure resistance while achieving a reduction in the weight of the can body. Furthermore, by satisfying t2 > t1, strength against deformation when the seamless can body 1 is dropped with the can bottom 20 facing downwards can be provided, which is preferable. The thickness (t1) of the outer circumferential bottom portion 202a is the thickness at the midpoint of the length (length along the shape) from the lower end 10e to the circumferential ground contact portion 202b.

[0031] Furthermore, in the seamless can body of this embodiment, when the thickness of the inner end portion 202c is t3, it is preferable that the relationship "t3 > t1" holds. By satisfying this relationship, the seamless can body 1 of this embodiment can be provided with preferable pressure resistance while achieving a reduction in the weight of the can body. Furthermore, by satisfying t3 > t1, it is possible to provide strength against deformation when the seamless can body 1 is dropped with the can bottom 20 facing downward, which is preferable.

[0032] The thickness is specified in the present invention for the following reasons. That is, when the liquid contained in a seamless can is beer or a carbonated drink, the bottom of the can is constantly under internal pressure. If an impact is applied to the bottom of the can while this internal pressure is still applied, or if the internal pressure applied to the bottom of the can suddenly increases for some reason, the internal pressure of the can exceeds the pressure resistance strength of the bottom of the can, causing the dome of the bottom of the can to invert (buckling).

[0033] In order to suppress this buckling phenomenon, it is necessary to increase the pressure resistance strength of the can bottom, and one possible method for achieving this is to increase the thickness of the can bottom. However, due to recent demands for weight reduction, the thickness of raw sheets (blanks) is becoming thinner. Therefore, simply increasing the thickness of the raw sheet (blank) in order to increase the pressure resistance strength of the can bottom would go against the above demand.

[0034] Therefore, the present inventors conducted extensive research to realize a seamless can body that simultaneously satisfies the requirements for a lighter can and for a high pressure-resistant can bottom. As a result, they succeeded in increasing the pressure-resistant strength of the can bottom by thickening only those portions of the can bottom that are likely to contribute to improving the pressure-resistant strength, while keeping the thickness of the raw sheet (blank) the same as or thinner than conventional ones, and arrived at the present invention.

[0035] According to the present invention, a thinner blank than conventional can bodies can be used, and therefore, by performing the same rigorous drawing and ironing process as conventionally, it is possible to achieve a body plate thickness that is the same as or thinner than conventional can body thicknesses. As a result, it is possible to achieve both the requirements for weight reduction and the pressure resistance strength of the can bottom at a high level.

[0036] As shown in Figures 1(a) and 2, in the seamless can body of this embodiment, the foot portion 202 of the can bottom 20 is connected to the can bottom center portion 201 (can dome portion 201d) at the outermost end 201e from the inner end portion 202c via the rising portion 202d.

[0037] In this embodiment, the rising portion 202d may be a straight line or a curve extending vertically (in the positive direction of the Z axis) from the inner end portion 202c in the cross section. As shown in FIGS. 1(a) and 2, the rising portion 202d may be a straight line or a curve extending along a line Z=-aX (Z>0) in the cross section.

[0038] As shown in FIG. 1(a), the rising portion 202d is connected to the can bottom center portion 201 (can dome portion 201d) so that the inner diameter (dx) of the outermost end 201e is larger than the inner diameter (dy) of the inner end portion 202c.

[0039] In other words, as shown in FIGS. 1(a) and 2, the area near the outermost end 201e is roughly shaped like a "⊂" or "⊃" in the cross section. Referring to Figure 1(a), it is preferable that, in the positive direction of the Z axis, between the inner end 202c and the can dome portion 201d, there is a ring groove whose outermost end 201e is convex toward the outside of the can body axis RA.

[0040] By adopting the above-described shape, it is possible to improve the pressure resistance of the seamless can body 1 of this embodiment.

[0041] As described above, in this embodiment, the outer circumferential bottom portion 202a preferably has an inflection point IP in its cross section. This inflection point IP may be located in the positive direction of the Z axis relative to the outermost end 201e, or conversely, in the negative direction of the Z axis, as shown in FIG.

[0042] In this embodiment, if the thickness of the outermost end 201e where the rising portion 202d and the can bottom center portion 201 connect is t4, it is also preferable from the viewpoint of reducing the weight and pressure resistance of the can body that the relationship "t4>t1" holds.

[0043] As shown in Fig. 1(a), the seamless can body 1 of this embodiment preferably further includes a can dome portion 201d that bulges upwardly and is continuous with the rising portion 202d in the can bottom 20. That is, in this embodiment, it is preferable that the shape of the can bottom central portion 201 is a dome shape as shown in Fig. 1(a).

[0044] When the thickness of the center of the can dome portion 201d is t5, it is preferable that the thickness (t3) of the inner end portion 202c and the thickness (t4) of the rising portion 202d satisfy the following relationship. t3>t4>t5 That is, this means that the thickness of the metal plate that continues from the center portion of the can dome portion 201d outward to the inner end portion 202c gradually increases.

[0045] Furthermore, in this embodiment, as shown in FIG. 3, when the thickness of the raw plate (blank) is t0, it is preferable to satisfy the relationships "t1>t0", "t2>t0", "t3>t0", and "t4>t0" from the viewpoint of the pressure resistance desired for the seamless can body 1. On the other hand, in this embodiment, there is no problem if the thickness (t5) of the center of the can dome portion 201d is equal to or less than the thickness (t0) of the raw plate (blank) (t5≦t0).

[0046] In this embodiment, it is preferable that the thicknesses of the respective parts have the relationship "t3>t2>t1" as shown in Fig. 3(a). In other words, it is preferable that the thickness gradually increases in the order of the outer circumferential bottom part 202a, the circumferential ground contact part 202b, and the inner end part 202c. By satisfying this relationship, it is possible to impart preferable pressure resistance to the seamless can body 1 of this embodiment.

[0047] Furthermore, satisfying the above-mentioned relationship of "t3>t2>t1" is preferable because it makes it possible to suppress an increase in the weight of the can even if the thickness of the t3 portion is increased. The reason for this is that the positions of t1, t2, and t3 become closer to the can body axis RA in this order, and therefore the volume occupied by each of them becomes smaller in this order. As a result, the pressure resistance of the can can be improved while suppressing an increase in weight, which is preferable.

[0048] However, this embodiment is not limited to this, and the thicknesses of t2 and t3 may be the same as shown in FIG. 3(b), or the thickness of t2 may be the largest as shown in FIG. 3(c).

[0049] The thickness of the raw sheet (blank) may be any thickness that is normally used to manufacture seamless can bodies, and a metal sheet having a thickness of approximately t0 = 0.15 mm to 0.32 mm can be punched out and used as the raw sheet (blank), but is not limited to the above thickness.

[0050] As described above, in the seamless can body 1 of the present embodiment, it has been stated that it is preferable from the viewpoint of desired pressure resistance that the plate thickness of the can bottom portion 20 has the above-described relationship. That is, in the seamless can body 1 in the present embodiment, it is preferable that the average plate thickness of the leg portion 202 of the can bottom portion 20 is thicker than that of the can bottom center portion 201.

[0051] Furthermore, it is preferable that the thickness of the can dome portion 201d is smaller than the thickness of the outer peripheral bottom portion 202a. That is, it is preferable that "t5 < t1".

[0052] Although the reason for the improvement in pressure resistance due to having the above-described relationship of plate thickness is not yet clear in detail, the following reasons can be considered. The buckling pressure is the one that numerically shows the pressure resistance. That is, the peak value of the pressure until the phenomenon occurs in which the dome portion convex inside the can bottom is deformed so as to invert outward by the internal pressure is called the buckling pressure.

[0053] The process in which the buckling phenomenon occurs can be explained as follows. [[ID=ID=18]]First, when the dome portion having a substantially spherical shape starts to receive the internal pressure, it does not deform immediately by itself, and the product of the projected area of the dome portion and the internal pressure becomes a force that pushes the dome portion outward of the can, and acts to apply a load and deformation to the circumferential grounding portion 202b, the inner end portion 202c, and the rising portion 202d. In other words, the outer periphery of the dome portion is supported by the members in the narrow region from the circumferential grounding portion 202b to the rising portion 202d.

[0054] Furthermore, when the deformation of the region from the circumferential grounding portion 202b to the rising portion 202d progresses due to the increase in the internal pressure, the function of supporting the outer periphery of the dome portion is lost. That is, the circumferential grounding portion 202b, the inner end portion 202c, and the rising portion 202d cannot maintain an annular shape centered on the can axis RA, and the outermost end 201e located on the outer periphery of the dome portion connected thereto also takes a shape that breaks the circle. Furthermore, since the can dome portion 201d connected thereto cannot maintain a spherical shape, the strength of the dome portion rapidly decreases and the dome portion reverses (backs up) outward of the can.

[0055] Therefore, in order to improve the pressure resistance, rather than increasing the thickness of the dome portion itself, it is considered more effective to increase the thickness of the plate at the outer periphery of the dome portion. Thus, when the thickness of the outer peripheral bottom portion 202a is greater than the thickness of the center of the can dome portion 201d, that is, when "t5 < t1", desirable pressure resistance can be obtained in the present embodiment.

[0056] Regarding the height Hp of the can dome portion 201d in the seamless can body 1, there is no particular limitation, and it can be set to the same height as a known seamless can body having a dome portion.

[0057] In the present embodiment, the type of metal material used for the seamless can body 1 is not particularly limited. That is, a known metal plate usually used for seamless cans, such as an aluminum alloy plate or a surface-treated steel plate, can be used. Further, the metal plate may be appropriately surface-treated, such as laminated with a known film, coated with an organic resin, or subjected to chemical conversion treatment.

[0058] The seamless can body 1 of the present embodiment is subjected to known necking, flanging, or threading processes, and after beer, carbonated beverages, etc. are contained as contents, a lid is attached to the opening by a known method.

[0059] <Method for manufacturing a seamless can body> Next, the method for manufacturing the seamless can body in the present embodiment will be described. The method for manufacturing a seamless can body in this embodiment is a method for manufacturing a seamless can body 1 having a cylindrical body portion 10 and a can bottom portion 20 as shown in FIG. 1(a), and is characterized by including at least a first molding step and a second molding step as described in detail below.

[0060] In the method for manufacturing a seamless can body according to the present embodiment, a known method such as that described in Patent Document 4 can be used to form the cylindrical body portion 10. On the other hand, the method for forming the can bottom 20 is characterized by including at least a first forming step and a second forming step, which will be described in detail below.

[0061] A method for manufacturing a seamless can body according to this embodiment will be described below. First, the above-mentioned metal material (blank) is used to form a can body by a known method, thereby preparing a precursor 3 having a cup shape. As shown in Fig. 4, the metal material (precursor 3) may have a cup shape without a dome obtained by a known drawing and ironing method, etc. Alternatively, the metal material may have a cup shape with a dome as long as the following first and second forming steps can be realized.

[0062] The precursor 3 is subjected to the following first and second forming steps to obtain the seamless can body 1 of this embodiment.

[0063] First, in the first forming step of the method for manufacturing the seamless can body 1 of this embodiment, as shown in FIG. 4 , a metal material (precursor 3) is formed into a cup body 2 having a cylindrical body portion 10, a cup outer periphery bottom portion A that continues from the lower end 10e of the cylindrical body portion 10 so as to decrease in diameter, a sloped portion S that extends inward and upward from the cup outer periphery bottom portion A, and a cup dome portion D that bulges upward from an end portion Se of the sloped portion S at a first height Ho. Here, the end Se of the inclined portion S can also be said to be a connection point with the cup dome portion D.

[0064] The first forming step shown in FIG. 4 can be carried out as a separate step using an upper mold and a lower mold on a precursor 3 in which a cylindrical body portion 10 has been formed by a known pressing process or the like, or can be carried out at the final stage of the stroke following the ironing process. As a specific example, as shown in FIG. 4, the first molding step is carried out using a cylindrical punch 401 positioned within and supporting a cup-shaped precursor 3, a hold-down ring 501 supporting the outer bottom of the precursor 3 in cooperation with the punch 401, and a doming die 502. First, the tapered portion 402 of the punch 401 and the tapered support portion 503 of the hold-down ring 501 hold the outer bottom portion of the precursor 3, and the punch 401 and the doming die 502 are driven so as to engage with each other and move relatively close to each other, thereby obtaining a cup body 2 having a cup dome portion D of Ho at the bottom.

[0065] Here, the shape of the cup body 2 obtained by the first molding step will be described. That is, the inclined portion S of the cup body 2 extends inward and upward from the outer circumferential bottom portion A of the cup. That is, the inclined portion S of the cup body 2 refers to the curved and straight portions sandwiched between the lowest part of the cup body 2 in the Z-axis direction and the connection point Se with the cup dome portion D, as shown in FIG.

[0066] As shown in FIG. 4(c), it is preferable that the inclined portion S is not vertical, but is inclined at a predetermined angle θ1. That is, it is preferable that the angle θ1 formed by the inclined portion S and the Z axis is 5° to 30° from the viewpoint of suitably controlling the plate thickness of each portion in the second forming step described below. Furthermore, it is more preferable that the angle θ1 between the inclined portion S and the Z axis is 10° to 30°, since this makes it easier to spray paint when forming a coating film on the inner surface by spray painting after the first molding step.

[0067] Furthermore, it is preferable that the radius of curvature R at the angle θ2 formed by the cup outer circumferential bottom A and the inclined portion S be R=5×t0 to 15×t0 from the viewpoint of suitably controlling the plate thickness of each portion in the second forming process described below.

[0068] Furthermore, the height Ho of the cup dome portion D in the cup body 2 is preferably larger than the height Hp of the can dome portion 201d in the seamless can body 1 obtained in the second forming step described below. The reason for this is that, as will be described later, compressive stress is applied to the sloped portion S while the cup dome portion D in the cup body 2 is pressed down in the second forming step described below. In other words, the height Ho of the cup dome portion D in the cup body 2 is increased in advance, so that a preferred height Hp of the can dome portion 201d can be finally obtained in the seamless can body 1.

[0069] Next, the second molding step will be described. After the cup body 2 having the cup outer circumferential bottom portion A and the inclined portion S is formed by the first forming step, the following second forming step is carried out.

[0070] Between the first and second molding steps, the cup body 2 may be subjected to any of the known cleaning, surface treatment, printing, painting, shaping of the cylindrical body, or necking (narrowing) processing as long as it does not interfere with the second molding step. Furthermore, if necessary, in order to ensure transportability and corrosion resistance after the first molding process, an outer surface coating can be applied to the area ranging from the cup outer periphery bottom A to the inclined portion S, centered on the lowest curvature portion of the cup body 2.

[0071] In the second molding step, the cup body 2 is processed using a mold different from the molding mold used in the first molding step described above, to form a seamless can body 1. That is, while the cup body 2 is brought into contact with the lower molding member, a pressing force is applied to the cup dome portion D of the cup body 2 in the outward direction of the can (in the −Z-axis direction) using the upper molding member. Alternatively, while the cup body 2 is in contact with the lower molding member and the upper molding member, a pressing force may be applied in the +Z-axis direction using the lower molding member.

[0072] 5, the cup outer periphery bottom portion A of the cup body 2 is placed on the cup outer periphery side holder 60. The dome pressing down tool 70 is lowered relatively, and the support portion 701 of the dome pressing down tool 70 comes into contact with the cup dome portion D. Here, the cup outer periphery side holder 60 has a tapered surface 601 and a groove 602, and after the cup outer periphery bottom portion A of the cup body 2 comes into contact with the tapered surface 601, the dome pressing down tool 70 is pressed down further, and the metal of the sloped portion S of the cup body 2 is guided and pressed into the groove 602 while being subjected to compressive stress.

[0073] Then, the cup dome portion D is pressed down to a second height Hp that is lower than the first height Ho. At the same time, a compressive stress σ in the meridian direction is applied to the inclined portion S using an upper mold forming member (dome pressing down tool) and a lower mold forming member (cup outer peripheral holder). φ and circumferential compressive stress σ θ is applied.

[0074] FIG. 6 is a schematic diagram showing the compressive stress applied when the inclined portion S is formed in the rising portion 202d in this embodiment. That is, when the inclined portion S is pressed into the groove 602 of the lower mold forming member, the pressing force of the dome pressing tool 70 causes a compressive stress σ φ and the circumferential compressive stress σ due to the movement inward in the radial direction in an attempt to follow the lower mold forming part. θ At the same time, the thickness of the metal material at the inclined portion S increases (in the direction of the arrow σ in FIG. 6). ψ ). In this way, the seamless can body 1 is obtained after the second forming step. After the molding is completed, the dome pressing tool is relatively raised, and the seamless can body 1 is removed from the cup outer peripheral holder.

[0075] Here, the seamless can body 1 obtained after the second molding step is preferably the seamless can body 1 of the present embodiment described above. That is, the seamless can body 1 obtained after the second molding step preferably has an outer peripheral bottom portion 202a and a circumferential ground contact portion 202b as shown in FIG. 1, and further preferably has a thickness of t1 of the outer peripheral bottom portion 202a and a thickness of the circumferential ground contact portion 202b, such that the relationship "t2>t1" holds.

[0076] It is more preferable that the second molding step has the following characteristics. That is, in the second molding step, it is preferable to press the above-mentioned cup body 2 into the lower mold molding member 60 of the second molding step, thereby forming the inclined portion S into a circumferential ground portion 202b located inside the outer peripheral bottom portion 202a, an inner end portion 202c located inside the circumferential ground portion 202b, and a rising portion 202d that rises upward from the inner end portion 202c and connects to the can dome portion 201d.

[0077] In the second molding process, it is preferable that a ring groove is formed in which the outermost end 201e is convex outward from the can body axis RA so that the inner diameter (dx) of the connection point (outermost end 201e) between the rising portion 202d and the can dome portion 201d of the seamless can body 1 is larger than the inner diameter (dy) of the inner end 202c. Conventionally, there has been a reform molding method (bottom reform processing) that uses a rotating roll or a split mold to form the ring groove as described above. However, with this conventional method, the processed area tends to become thin, making it difficult to form a sufficiently deep groove. According to the method of the present invention, the thickness of the ring groove portion tends to increase rather than decrease, and a deep groove can be formed without difficulty.

[0078] In the method for manufacturing a seamless can body of this embodiment, the shape and length of the upper part of the cup outer circumferential bottom part A of the cup body 2 are not changed between the first and second forming steps. That is, when the cup body 2 is placed on the outer peripheral holder 60, the lowest point in the Z axis direction of the surface where the cup outer peripheral bottom A of the cup body 2 contacts the tapered surface 601 of the cup outer peripheral holder 60 is defined as point T. The position of point T does not change as the dome pressing tool 70 descends and the cup dome portion D is pressed down (see FIG. 5).

[0079] Meanwhile, in the second forming step, the portion that was the inclined portion S of the cup body 2 is formed into a part of the outer circumferential bottom portion 202a, the circumferential ground contact portion 202b, the inner edge portion 202c, and the rising portion 202d of the seamless can body 1. In other words, the inclined portion S of the cup body 2 is eventually entirely inserted into the groove 602 of the cup outer circumferential holder 60. In this second molding step, there is no significant sliding between the cup body 2 and the upper and lower dies, so the metal surface of the cup body 2 is not damaged and there is no need to use a lubricant.

[0080] As shown in Fig. 5, the point T is an inflection point IP in the seamless can body 1. The compressive stress imparted by the second forming step shortens the metal length as described below. That is, the metal length from the inflection point IP to the outermost end 201e in FIG. 5(f) is about 0.85 to 0.99 times shorter than the metal length from point T to Se in FIG. 5(b).

[0081] On the other hand, the thickness of the metal material in that portion is increased by the second forming step to 1.1 to 1.3 times the thickness of the base plate (t0) at the portion where the thickness increases most. [Example]

[0082] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0083] Example 1 A drawn and ironed can (DI can) with an internal volume of 350 mL was produced by the method described below. First, an aluminum alloy plate (JIS H 4000 A3104-H19 material, 0.28 mm) was prepared as a raw plate. Next, a predetermined amount of known cupping oil was applied to both sides of the aluminum alloy plate as a lubricant during drawing.

[0084] Next, the aluminum alloy plate was punched out into a disk shape with a diameter of 160 mm using a drawing machine, and immediately thereafter, the aluminum alloy plate was subjected to drawing to form a drawn cup (not shown) with a diameter of 90 mm. The obtained drawn cup was transported to a body maker (can body manufacturing machine) and re-drawn to a shape of 66 mm in diameter, and then ironed using coolant to produce precursor 3, which was made by drawing and ironing to a shape of 66 mm in diameter, 130 mm in height, and a minimum side wall thickness of 0.105 mm.

[0085] Next, in order to perform the forming process of the can bottom, the precursor 3 obtained above was subjected to the following first forming step and second forming step. First, the first forming step was performed in the final stroke stage of the process following ironing by the body maker, and a cup body 2 having a cup outer periphery bottom portion A and a sloped portion S was formed using a punch 401, a hold-down ring 501, and a doming die 502 shown in Fig. 4. The lengths and thicknesses of the cup outer periphery bottom portion A and the sloped portion S at this time are as shown in Table 1.

[0086] Next, in the second forming step, the cup dome portion D was pressed down and the thickness of the metal material at the inclined portion S was increased using the upper forming member 70 and the lower forming member 60 shown in FIG. 5 to form the seamless can body 1.

[0087] Next, the thickness of each of the portions t1 to t5 was measured. The locations of the portions t1 to t5 were as described in the above embodiment and shown in FIG. 2. The thickness was measured as follows. That is, the molded seamless can body 1 was embedded in epoxy resin, and then cut along the longitudinal axis (Z-axis) of the seamless can body 1 together with the epoxy resin. After exposing the central cross section by cutting and careful polishing, the thickness of each of the portions t1 to t5 was measured using a measuring microscope. The thickness of each portion is shown in Table 1.

[0088] Example 2 The same procedure as in Example 1 was carried out except that the thickness of the raw plate was 0.225 mm and the minimum thickness of the side wall of the precursor 3 was 0.093 mm. The plate thicknesses of each part of the obtained seamless can body are shown in Table 1.

[0089] (Comparative Example 1) The can bottom was formed in one step using a known can bottom forming mold and a known can bottom forming method. FIG. 7 shows an enlarged view of a part of the bottom of the seamless can body used in Comparative Example 1. The thickness of each part of the obtained seamless can body is shown in Table 1. In Table 1, the value of t3 was obtained by measuring the bottom end of the inclined part ((1) in Figure 7), and the value of t4 was obtained by measuring the top end of the inclined part ((2) in Figure 7).

[0090] (Comparative Example 2) The seamless can body obtained in Comparative Example 1 was subjected to bottom reforming. That is, a rotating roll was used to press the inner peripheral wall located radially inward of the ground contact portion of the can bottom in a direction perpendicular to the can body axis, thereby forming a circumferential recess. The rest of the process was the same as in Comparative Example. The thickness of each part of the obtained seamless can body, etc., are shown in Table 1.

[0091] (Comparative Example 3) The same procedure as in Comparative Example 2 was carried out except that the thickness of the blank was 0.225 mm and the minimum thickness of the side wall was 0.093 mm. The thicknesses of each part of the obtained seamless can body are shown in Table 1.

[0092] [evaluation] The DI cans obtained by the above method were evaluated by the following methods. The results are shown in Table 1.

[0093] [Pressure resistance test method] With the cup filled with water, the open end is sealed with a plug equipped with a water pipe. Then, pressurized water is sent into the cup from a water pump through the water pipe. The internal pressure of the cup rises, and at a certain point, the dome part instantly deforms, inverting outward (buckling). Usually, at the same time as this deformation, the internal pressure of the can drops suddenly. The maximum internal pressure of the can during this period is the pressure resistance (MPa).

[0094] [Table 1]

[0095] The results of the examples and comparative examples showed that by controlling the thickness of specific parts of the can bottom, desirable pressure resistance (0.618 MPa or more, required for use in carbonated beverages) can be obtained even when the thickness of the base plate (blank) is thin. [Industrial Applicability]

[0096] According to the present invention, it is possible to reduce the thickness of the raw plate (blank) of a seamless can body while improving the pressure resistance and suppressing the buckling phenomenon. This makes it possible to reduce the manufacturing costs and transportation costs of seamless can bodies. Furthermore, it is possible to reduce the fuel required for manufacturing and transportation, making it possible to manufacture seamless can bodies in an environmentally friendly manner. [Explanation of symbols]

[0097] 1 Seamless can body 2 cup body 3. Precursor 10 Cylindrical body 10e bottom edge 20 Can bottom 201 Center of can bottom 201d Can Dome Section 201e outermost edge 202 Foot 202a Bottom of outer periphery 202b Circumferential grounding part 202c inner end 202d Rising section A Cup outer bottom D cup dome S slope part Se end Hp Height of the can dome (second height) Ho Cup dome height (first height) 70 Upper mold forming member 60 Lower mold forming member

Claims

1. A seamless can body having a cylindrical body portion and a can bottom portion, the can bottom portion includes a can bottom center portion on the can body axis side of the seamless can body, and a foot portion located outside the can bottom center portion and connected to a lower end of the cylindrical body portion, the foot portion includes an outer circumferential bottom portion that continues from the lower end of the cylindrical body portion inwardly so as to decrease in diameter, a circumferential ground contact portion located more inward than the outer circumferential bottom portion, an inner end portion located more inward than the circumferential ground contact portion and vertically above it, and a rising portion that rises upward from the inner end portion, When the plate thickness of the circumferential ground contact portion is t2 and the plate thickness of the inner end portion is t3, "t3 ≧ t2" is satisfied. A seamless can body characterized by:

2. 2. The seamless can body according to claim 1, wherein the inner diameter of the outermost end of the raised portion is larger than the inner diameter of the inner end portion.

3. When the plate thickness of the outer periphery bottom is t1, The seamless can body according to claim 1 or 2, wherein t2>t1.

4. The seamless can body according to claim 3, wherein t3>t1.

5. When the plate thickness at the upper end of the rising portion is t4, The seamless can body according to claim 3 or 4, wherein t4>t1.

6. the can bottom further includes a dome portion that is continuous with the rising portion and bulges upward to be convex, 6. The seamless can body according to claim 5, wherein the thickness of the dome portion at the center thereof is t5, and the thickness gradually increases from the dome portion to the inner end portion so that t3 > t4 > t5.

7. The seamless can body according to claim 6, wherein t5<t1.

8. 7. The seamless can body according to claim 6, wherein a ring groove is formed outward from the can body axis, the ring groove being convex at the connection portion between the rising portion and the dome portion.

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