Method for manufacturing shell for can lid and apparatus for manufacturing shell for can lid

Abstract

To suppress the generation of cracks on a panel wall.SOLUTION: While a metal plate is held between a die core ring that is a lower mold and a pressure sleeve that is an upper mold, a chuck wall portion is formed in a shape in which a first curving portion and a second curving portion continue. When a center portion of the metal plate is held between a panel punch that is a lower mold and a die center that is an upper mole, the metal plate is bent by a first bending surface and a second bending surface so as to be pushed in toward a clearance formed between the die core ring and the first bending surface of the panel punch by the first bending surface of the panel punch and an annular projecting portion having the second bending surface of the die center, thereby forming a recession on the metal plate. While keeping the holding state, it relatively rises with respect to the die core ring and the pressure sleeve, a recessed portion is dropped in the clearance to gradually increase the depth of the recession to be an annular recessed portion, thereby forming a counter sink portion. A panel portion is formed inside the counter sink portion.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a shell for a can lid that becomes a can lid wound around an open end of a can body and a shell manufacturing apparatus for a can lid.

Background Art

[0002] Conventional beverage can lids include a central disc-shaped panel portion, a countersink portion on the outer peripheral portion thereof, and a curl portion continuously provided via a chuck wall portion on the outer periphery thereof. A score for forming an opening is formed in the panel portion, and a tab for opening operation is provided. In recent years, due to the demand for thinning, instead of a chuck wall portion having a substantially straight taper shape, a chuck wall portion having a continuous shape of two concave and convex curved portions in the longitudinal section is adopted, and the shape with a smaller diameter of the panel portion is increasing.

[0003] For example, in Patent Document 1, the chuck wall is formed in a shape in which a curved portion convex inward in the radial direction in the lower portion and a curved portion concave inward in the radial direction (convex outward in the radial direction) in the upper portion are continuous. In this case, a can lid having an overall diameter of (D4) = about 56.896 mm and a can lid having (D8) = 59.360 mm are disclosed. In terms of the nominal diameter of those skilled in the art, the former has a nominal diameter of 200, and the latter has a nominal diameter of 202. Also, the inner diameter of the outer wall of the countersink portion is (D2) = about 49.708 mm for a 200 diameter and (D6) = 48.514 mm for a 202 diameter, and the radius of curvature of the inner bottom surface of the countersink portion is about 0.254 mm for a 200 diameter and about 0.229 to 0.279 mm for a 202 diameter. The plate thickness is about 0.2159 mm for the blank before forming the can lid. The symbols in parentheses are the symbols used in the publication (the same applies to Patent Document 2).

[0004] On the other hand, as a method for manufacturing such a can lid, Patent Document 2 discloses. In this Patent Document 2, while sandwiching the cup portion (C) between the annular outer pressure sleeve (55) and the upper portion (99) of the die core ring (98), the inclined or frustum-shaped surface (156) and the curved annular surface (158) formed on this upper portion (99), A chuck wall is formed between the corresponding surfaces (143, 144) of the inner pressure sleeve (80). Further, by moving the central portion of the cup portion (C) up and down while sandwiching it between the central punch (65) of the die and the circular panel punch (118), a concave surface (116, 168) formed on the outer peripheral portion of the panel punch (118) forms a countersink.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] By the way, in the can lid described in Patent Document 1, the chuck wall is formed in a shape in which a curved portion that protrudes radially inward at its lower portion and a curved portion that is concave radially inward (convex radially outward) at its upper portion are continuous. Therefore, the planar diameter of the panel portion is limited to a small size. In this case, the longitudinal length of the score processed on the panel portion is limited to a short length, the pourability deteriorates, and the liquid residue in the bottle can increases. In this regard, if the planar diameter of the panel portion is simply increased, not only will minute cracks occur in the wall portion on the panel portion side (hereinafter referred to as the panel wall) of the countersink portion continuous with the outer peripheral portion of the panel portion, but also when the internal pressure of the can rises and a buckling phenomenon occurs in which the countersink portion is inverted, a part of the panel portion will be lifted and protrude angularly, and cracks will occur at the tip of the corner portion, resulting in a problem that the beverage sprays out.

[0007] The present invention has been made in view of such circumstances, and an object thereof is to provide a method for manufacturing a shell for a can lid and a manufacturing apparatus for a shell for a can lid that can suppress the occurrence of cracks in a panel wall.

Means for Solving the Problems

[0008] A method for manufacturing a shell for a can lid according to the present invention is a method for manufacturing a shell for a can lid including a central disk-shaped panel portion, an annular recessed portion that is continuous with the outer periphery thereof and is directed upward, a countersink portion that forms the annular recessed portion, a chuck wall portion that gradually increases in diameter upward from the outer peripheral edge of the countersink portion, a shoulder portion that extends radially outward from the outer peripheral edge of the chuck wall portion, and a curl-shaped portion that is continuous with the shoulder portion and has an outer peripheral edge portion folded back radially inward, by press-forming so as to sandwich a metal plate between a set of upper die and lower die. A panel punch formed on a first curved surface in which a corner connecting an upper surface and a side surface is convex outward is used for the lower die, and a die core ring provided with a gap on the radially outer side of the panel punch is used. At the same time, for the upper die, a die center having an annular convex portion formed on a second curved surface that protrudes from the outer peripheral portion on the lower end side toward the gap of the lower die and has a tip portion convex toward the lower die, and a pressure sleeve on the radially outer side of the die center are used. While sandwiching the metal plate between the die core ring and the pressure sleeve, the chuck wall portion is formed in a shape in which a first curved portion convex radially inward and a second curved portion concave radially inward (convex radially outward) above the first curved portion are continuous. When clamping the central portion of the metal plate between the panel punch and the die center inside the chuck wall portion in the radial inner direction, the first bending surface and the second bending surface of the annular convex portion are used to bend the metal plate so as to push it into the gap, thereby forming a dent in the metal plate. Next, while maintaining the clamping state by the panel punch and the die center, the panel punch and the die center are relatively moved upward with respect to the die core ring and the pressure sleeve that clamp the metal plate outside thereof, so that the dented portion of the metal plate is gradually dropped into the gap to gradually increase the depth of the dent and form the annular recess, thereby forming the countersink portion continuous with the inner peripheral edge of the second curved portion, and forming the panel portion inside the countersink portion.

[0009] Here, in the conventional method for manufacturing a shell for a can lid, when changing from the bottom dead center shown in FIG. 9(a) to the upward stroke shown in FIG. 9(b) during the forming process, on the mold circumscribing the portion that will become the countersink portion, the metal plate 9J is extremely bent to form a bending point P1. Thereafter, when the forming of the countersink portion proceeds as shown in FIGS. 9(c) and 9(d), the bending point P1 on the metal plate 9J returns linearly, and as shown in FIG. 9(d), the bending point P2 moves to the position (panel wall) on the panel side of the countersink portion. As a result of such excessive work hardening accumulating in the panel wall, minute cracks are generated. This phenomenon is more prominent as the thickness of the metal plate used is thinner, and is also more prominent as the gap between the inner diameter surface of the die core ring 36J and the outer diameter surface of the panel punch 33J is smaller.

[0010] In the present invention, a chuck wall portion is formed by sandwiching a metal plate, and the metal plate is bent by the first bending surface of the panel punch and the second bending surface of the annular convex portion, so that a recess that becomes the annular recess of the countersink portion is formed in advance along the second bending surface. By doing so, it is possible to eliminate a portion where bending stress is concentrated without causing a bending point in the metal plate. And since the recess is formed so as to gradually deepen, excessive work hardening does not occur in the panel wall of the countersink. As a result, it is possible to suppress the occurrence of cracks in the panel wall and suppress the ejection of the beverage even if a buckling phenomenon occurs.

[0011] The apparatus for manufacturing a shell for a can lid of the present invention is an apparatus for manufacturing a shell for a can lid, which is press-formed so as to sandwich a metal plate between a set of upper die and lower die, and includes a central disk-shaped panel portion, a countersink portion that is continuous with the outer peripheral portion thereof and forms an upward annular recess, a chuck wall portion that gradually increases in diameter upward from the outer peripheral edge of the countersink portion, a shoulder portion that extends radially outward from the outer peripheral edge of the chuck wall portion, and a curl portion that is continuous with the shoulder portion and has an outer peripheral edge portion folded back radially inward. At the central portions of the upper die and the lower die, a die center and a panel punch for forming the panel portion are provided in a facing state, and outside these die center and panel punch, a pressure sleeve and a die core ring for forming the chuck wall portion having a shape in which a first curved portion that protrudes radially inward and a second curved portion that is concave (protrudes radially outward) above it are continuous are provided in a facing state. A first curved surface where a corner connecting the upper surface and the side surface of the panel punch is convex outward is formed, and on the outer peripheral portion of the lower end side of the die center, an annular convex portion is formed that protrudes toward the lower die, has a tip end convex toward the lower die, and is located radially outward of the first curved surface. The height from the lower end surface of the die center to the apex of the second curved surface of the annular convex portion is set to be 0.3 mm or more and 1.2 mm or less. A gap is formed between the lower die core ring and the panel punch, and the annular convex portion is disposed to face the gap.

Advantages of the Invention

[0012] According to the present invention, it is possible to suppress the occurrence of cracks in the panel wall. As a result, even if a buckling phenomenon occurs due to an increase in internal pressure, the occurrence of cracks can be suppressed and liquid leakage can be prevented.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Mode for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present invention will be described with reference to the drawings. First, the shell 1 for a can lid before it becomes the can lid 10 will be described. This shell 1 for a can lid is made of an aluminum alloy and, as shown in FIGS. 1 and 2, has a disk-shaped panel portion 2 disposed at its central portion, a countersink portion 5 that forms an annular recess 4 that is continuous from the outer peripheral edge of the panel portion 2 via a corner portion 3 and extends downward, a chuck wall portion 6 that extends upward from the outer peripheral edge of the countersink portion 5 while substantially increasing in diameter, a shoulder portion 7 that extends radially outward from the outer peripheral edge of the chuck wall portion 6, and a curl-shaped portion 8a that is continuous with the shoulder portion 7 and has its outer peripheral edge portion bent downward.

[0015] The panel portion 2 is formed in a disk shape as a whole, a tab 50 is attached to its center, and a score (not shown) or the like that becomes an opening for drinking by raising this tab 50 is formed on the upper surface. The corner portion 3 has a shape that connects between the outer peripheral edge of the panel portion 2 and the inner peripheral edge of the countersink portion 5 by a convex curved surface.

[0016] In the countersink portion 5, the inner wall portion 52 (corresponding to the panel wall of the present invention) that is continuous with the corner portion 3 extends so as to slightly increase in diameter as it goes vertically downward, and an inner bent portion 55 and an outer bent portion 56 outside thereof are continuous from the lower edge of the inner wall portion 52 directly or via a flat surface or an arc surface with a slight width. These bent portions 55 and 56 together form the bottom of the countersink portion 5. And an outer wall portion 57 extends upward from the outer bent portion 56 while slightly increasing in diameter, and the inner wall portion 52, the inner bent portion 55, the outer bent portion 56, and the outer wall portion 57 constitute a countersink portion 5 having a substantially U-shaped cross section in the longitudinal section. Note that, since the inner wall portion 52 and the outer wall portion 57 are inclined so as to gradually approach each other as they go downward, the annular recess 4 gradually narrows in width as it goes downward.

[0017] The chuck wall portion 6 is formed in a shape including a first curved portion 18 that is continuous with the upper edge of the outer wall portion 57 of the countersink portion 5 and is curved so as to protrude inward in the radial direction, and a second curved portion 19 that is continuous with the upper edge of the first curved portion 18 directly or via a flat surface or an arcuate surface having a slight width and is curved so as to protrude outward in the radial direction. Due to the continuous shape of these first curved portion 18 and second curved portion 19, the overall shape is such that the diameter expands upward.

[0018] The shoulder portion 7 is continuous with the upper edge (outer peripheral edge) of the second curved portion 19 of the chuck wall portion 6, and extends outward in the radial direction while expanding in diameter while being curved so as to protrude upward. In this case, since the second curved portion 19 of the chuck wall portion 6 is curved with its inner peripheral surface (the outer surface as the can lid 1) facing obliquely upward, the overall shape is such that the diameter gradually expands from bottom to top, and the shoulder portion 7 continuous with this second curved portion 19 also expands in diameter from bottom to top. The curl-shaped portion 8a is continuous with the outer peripheral edge of the shoulder portion 7 and spreads outward in the radial direction, and its outer peripheral edge portion is bent downward. This curl-shaped portion 8a is then formed into a curl portion 8 having a shape in which the downward-facing outer peripheral edge portion is folded back inward in the radial direction through a curling process. In addition, a score or the like serving as a drinking opening is formed in the panel portion 2, a tab 50 for an opening is attached to the center of the panel portion 2, and a lining material 51 is provided on the back surface of the curl portion 8, whereby a can lid 10 is formed.

[0019] On the other hand, the can body 40 around which this can lid 10 is wound is formed in a shape in which an outward flange portion 43 is continuously formed via a bent portion 42 at the upper end of a neck portion 41 having a reduced diameter from a bottomed cylindrical body portion (not shown). When the can lid 10 is placed on the can body 40, it is arranged such that the concave surface on the outer peripheral surface (inner surface in the case of the can lid 10) side of the shoulder portion 7 of the can lid 10 faces the bent portion 42 of the can body 40, and the back surface from the shoulder portion 7 to the curled portion 8 is arranged to face the upper surface of the flange portion 43. Then, in that state, when the curled portion 8 of the can lid 10 is wound and tightened along the circumferential direction while being integrally wound with the flange portion 43 of the can body 40, the flange portion 43 in a folded state enters into the wound-in portion of the curled portion 8, and a double-wound and tightened portion (not shown) in which the can lid 10 and the can body 40 sandwich each other is formed.

[0020] In the can lid 10 configured as described above, when the nominal diameter is 202 (the outermost diameter of the wound and tightened portion is 53.9 mm to 54.5 mm, and the outermost diameter D1 of the curled portion 8 of the can lid 1 before winding and tightening is 59.1 mm to 59.5 mm), the diameter D2 of the lowermost end of the countersink portion 5 is 46.5 mm to 47.0 mm, the diameter D3 of the panel portion 2 is 42.5 mm to 44.5 mm, the depth P from the topmost surface of the shoulder portion 7 or the curled portion 8 to the inner bottom surface of the countersink portion 5 is 6.1 mm to 6.6 mm, and the height H1 from the lowermost end of the countersink portion 5 to the lower surface of the panel portion 2 is 1.8 mm to 2.3 mm. The plate thickness T is 0.203 mm to 0.213 mm. Also, the radius of curvature R3 of the outer surface of the first bent portion 18 is set to 0.9 mm to 3.0 mm, and the radius of curvature R4 of the outer surface of the second bent portion 19 is set to 0.6 mm to 3.0 mm. Further, the radius of curvature R5 of the outer surface of the shoulder portion 7 is set to 1.2 mm to 3.0 mm. And the angle θ1 formed by the common tangent line A between the outer surface of the shoulder portion 7 and the outer surface of the first bent portion 18 and the outer surface of the panel portion 2 is 45° to 70°.

[0021] When the nominal diameter is 200 (the outermost diameter of the tightening part is 51.9 mm to 52.5 mm, and the outermost diameter D1 of the curl part 8 of the can lid 1 before tightening is 56.8 mm to 57.2 mm), the diameter D2 of the lowermost end of the countersink part 5 is 44.1 mm to 44.5 mm, the diameter D3 of the panel part 2 is 40.7 mm to 42.5 mm, the depth P from the topmost surface of the shoulder part 7 or the curl part 8 to the inner bottom surface of the countersink part 5 is 6.1 mm to 6.6 mm, and the height H1 from the lowermost end of the countersink part 5 to the lower surface of the panel part 2 is 1.8 mm to 2.3 mm. The plate thickness T is 0.203 mm to 0.213 mm. In addition, the radius of curvature R3 of the outer surface of the first bending part 18 is set to 0.9 mm to 3.0 mm, the radius of curvature R4 of the outer surface of the second bending part 19 is set to 0.6 mm to 3.0 mm. Also, the radius of curvature R5 of the outer surface of the shoulder part 7 is set to 1.2 mm to 3.0 mm. And the angle θ1 formed by the common tangent line A between the outer surface of the shoulder part 7 and the outer surface of the first bending part 18 with the outer surface of the panel part 2 is 45° to 70°.

[0022] Also, when the diameter D3 of the panel part 2 is less than 93% of the diameter D2 of the lowermost end of the countersink part 5, the pourability deteriorates and the liquid residue further increases. For this reason, in this embodiment, the diameter D3 of the panel part 2 is 93% or more of the diameter D2 of the lowermost end of the countersink part 5.

[0023] A score or the like serving as a drinking mouth is formed on the panel part 2 of the can lid shell 1, a tab 50 for opening is attached to the center of the panel part 2, and a lining material 51 (not shown in FIG. 1) is provided on the back surface of the curl part 8, thereby forming the can lid 10 shown in FIG. 2(b).

[0024] As shown in FIG. 3, a can lid shell forming device 11 for manufacturing the can lid shell 1 of this embodiment is formed by attaching an upper die 12 and a lower die 13 to a press machine (not shown), arranging a metal plate 9 between the upper die 12 and the lower die 13, and forming the can lid shell 1 while punching it into a circular shape. The main part thereof is shown in FIGS. 4 to 8.

[0025] In the upper mold 12, a die center 23 for forming the surface of the panel portion 2 of the shell 1 and an annular blank draw die 24 surrounding the die center 23 are provided. An inner pressure sleeve 25 and an outer pressure sleeve 26 are provided between the blank draw die 24 and the die center 23. In this case, the tip portion 25a of the inner pressure sleeve 25 is disposed on the die center 23 side, and the tip portion 26a of the outer pressure sleeve 26 is disposed on the blank draw die 24 side. The tip portion 25a of the inner pressure sleeve 25 and the tip portion 26a of the outer pressure sleeve 26 are arranged in a double sleeve shape concentric with the axis of the die center 23.

[0026] Further, on the outer peripheral portion of the lower end side of the die center 23, an annular convex portion 231 is formed which protrudes toward the lower mold 13 and has a second bending surface 232 whose tip portion is convex toward the lower mold 13, a third bending surface 233 formed on the radially inner base end side of the second bending surface 232, and a fourth bending surface 234 formed on the radially outer base end side of the second bending surface 232. The height of this annular convex portion 231, that is, the height w1 from the lower end surface of the die center 23 to the apex of the second bending surface 232, is set to be 0.3 mm or more and 1.2 mm or less. The radius of curvature of the second bending surface 232 is 0.3 mm to 0.8 mm, the radius of curvature of the third bending surface 233 is 0.3 mm to 1.2 mm, and the radius of curvature of the fourth bending surface 234 is 0.8 mm to 1.8 mm.

[0027] On one side, in the lower die 13, an annular cutting edge die 34 for punching a metal plate 9 between a circular panel punch 33 for forming the back surface of the panel portion 2 of the shell 1 and a blank drawing die 24 facing the die center 23 of the upper die is provided. Between this cutting edge die 34 and the panel punch 33, a lower pressure sleeve 35 facing the blank drawing die 24 of the upper die and a die core ring 36 facing the inner pressure sleeve 25 and the outer pressure sleeve 26 are provided. In this case, the tip 35a of the lower pressure sleeve 35 is arranged on the cutting edge die 34 side, and the tip 36a of the die core ring 36 is arranged on the panel punch 33 side. The tip 35a of the lower pressure sleeve 35 and the tip 36a of the die core ring 36 are arranged in a double ring shape concentric with the axis of the panel punch 33 (coaxial with the axis of the aforementioned die center 23).

[0028] A gap 60 for forming a countersink portion 5 is formed between the panel punch 33 and the die core ring 36. The width of this gap 60 is set to 1.1 mm to 1.6 mm and is in a facing state with the annular convex portion 231 of the aforementioned die center 23. Also, the corner connecting the upper surface and the side surface of the panel punch 33 is formed into a first bending surface 331 that protrudes outward, and the radius of curvature of the first bending surface 331 is set to 0.3 mm to 1.1 mm. This first bending surface 331 is arranged radially inward of the second bending surface 232 and is provided together with the second bending surface 232 for bending the metal plate 9 to form a recess 91.

[0029] Next, a method for forming the shell 1 for the can lid by the can lid shell forming device 11 configured as described above will be described. When a metal plate 9 made of an aluminum alloy or the like is placed between the upper die 12 and the lower die 13 of the can lid shell forming device 11 and the upper die 12 is lowered to close both dies 12 and 13, first, as shown in FIG. 4, the metal plate 9 is clamped between the blank drawing die 24 of the upper die 12 and the lower pressure sleeve 35 of the lower die, and the metal plate 9 is punched into a circular shape between the blank drawing die 24 and the cutting edge die 34. When the upper die 12 is further lowered, as shown in FIG. 5, the metal plate 9 is annularly clamped between the outer pressure sleeve 26 and the die core ring 36 inside the outer peripheral edge portion clamped between the blank draw die 24 and the lower pressure sleeve 35. When the upper die 12 is further lowered in this clamped state, as shown in FIG. 6, the outer peripheral edge portion of the metal plate 9 is clamped between the blank draw die 24 and the lower pressure sleeve 35 and drawn, and the curled portion 8a is formed so as to bend at the tip outer peripheral portion of the die core ring 36. On the other hand, inside the outer pressure sleeve 26, the inner pressure sleeve 25 is lowered to clamp the metal plate 9 between the die core ring 36, forming the chuck wall portion 6 having the upper curved portion 19 and the lower curved portion 18, while clamping the central portion of the metal plate 9 between the die center 23 and the panel punch 33. Next, as shown in FIG. 7, while relatively moving with respect to the inner pressure sleeve 25 and the die core ring 36 so as to slightly raise the die center 23 and the panel punch 33, the panel portion 2 and the countersink portion 5 are formed.

[0030] The forming process of this countersink portion 5 will be described in more detail with reference to Fig. 8. As described above, since a gap 60 is formed between the panel punch 33 and the die collar 36, when the die center 23 and the panel punch 33 sandwich the metal plate 9, as shown in Fig. 8(a), the metal plate 9 is pressed by the annular convex portion 231 toward the gap 60, and the metal plate 9 is bent by the first bending surface 331 of the panel punch 33 and the second bending surface 232 of the annular convex portion 231 to form a recess 91. By previously forming the recess 91 along the second bending surface 232 in this way, the site where the bending stress concentrates is eliminated without causing an extreme bending point on the metal plate 9. While maintaining this clamped state, when moving upward relative to the inner pressure sleeve 25 and the die collar 36 from the bottom dead center shown in Fig. 8(a) as shown in Fig. 8(b), since the portion of the metal plate 9 disposed above the gap 60 has no support from below, it enters into the gap 60 along the first bending surface 331. At this time, the recess 91 extends downward in the gap 60 as the die center 23 and the panel punch 33 move upward, and deforms so that the depth of the recess gradually increases. As a result, as shown in Fig. 8(c), the recess 91 is formed into the annular recess 4, which becomes the countersink portion 5.

[0031] When the forming is completed in this way, as shown in Fig. 8(d), the die center 23 is separated from the panel portion 2 and raised, and the inner pressure sleeve 25, the outer pressure sleeve 26, and the blank draw die 24 are raised to separate them from the shell 1 for the can lid. As described above, the shell 1 for the can lid has a score or the like serving as a drinking opening formed in the panel portion 2, a tab 50 for an opening attached to the center of the panel portion 2, and a lining material 51 provided on the back surface of the curl portion 8, thereby becoming the can lid 10.

[0032] The lid 10 of the can manufactured in this way forms the chuck wall portion 6 by sandwiching the metal plate 9, and bends the metal plate 9 by the first bending surface 331 of the panel punch 33 and the second bending surface 232 of the annular convex portion 231. By previously forming a recess 91 that becomes the annular recess 4 of the countersink portion 5 along the second bending surface 232, it is possible to eliminate the site where the bending stress concentrates without causing an extreme bending point on the metal plate 9. And since the recess 91 is formed so as to gradually deepen, excessive work hardening does not occur on the panel wall (inner wall portion 52) of the countersink portion 5. Thereby, it is possible to suppress the occurrence of cracks in the panel wall (inner wall portion 52), and even if a buckling phenomenon occurs, it is possible to suppress the ejection of the beverage.

[0033] Note that the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. For example, in order to set the outer diameter of the panel punch flat portion larger, the first bending surface 331 of the panel punch is made a combination of two different R values, and the radius of curvature of the R connected from the panel punch flat surface is set smaller than the radius of curvature of the R connected to the panel punch outer diameter. In this case, the diameter D3 of the panel portion 2 can be surely made 93% or more of the diameter D2 at the lowermost end of the countersink portion 5.

Example

[0034] A can lid was produced with the dimensions of each part shown in FIG. 1 being a plate thickness T = 0.208 mm, an outer diameter D1 = 59.3 mm, P = 6.35 mm, and H1 = 2.0 mm. At this time, in the example, an annular convex portion having a radius of curvature R = 0.4 mm for the second bending surface, a radius of curvature R = 1.02 mm for the third bending surface, a radius of curvature R = 1.55 mm for the fourth bending surface, and a height of 0.45 mm was provided at the die center. In the comparative example, the radius of curvature R of the corner portion of the die center was set to 1.55 mm without providing an annular convex portion. Also, in both the example and the comparative example, the radius of curvature R of the first bending surface of the panel punch was set to 0.8 mm to form a countersink portion. These can lids were measured for the winding strength on the can body, the pressure resistance strength, and the number of lids that cracked during buckling. As the pressure resistance strength, the can with the lid tightened was fixed at a position 95 mm from the grounding part to a hydraulic back buckling tester (manufactured by Ace Tech Co., Ltd.), and the internal pressure of the can was increased at a pressure increasing speed of 33 kPa / s by hydraulic pressure, and the maximum pressure reached until the dome-shaped central part was inverted (back buckled) was evaluated. Also, the presence or absence of cracks at the time of this back buckling was visually observed. Measurement was performed on 120 cans each, and the results are shown in Table 1.

[0035]

Table 1

[0036] In both the examples and the comparative examples, there is no significant difference in the average pressure resistance strength. However, in the comparative examples, cracks occurred in 100% of the cases at the time of back buckling, while in the examples, no cans with cracks occurred.

Explanation of symbols

[0037] 1 Shell for can lid 2 Panel part 3 Corner part 4 Annular concave part 5 Countersink part 52 Inner wall part (panel wall) 6 Chuck wall part 7 Shoulder part 8 Curl part 8a Curl-shaped part 9 Metal plate 10 Can lid 11 Shell forming device 12 Upper die 13 Lower die 23 Die center 231 Annular convex part 232 Second bending surface 233 Third bending surface 234 Fourth bending surface 24 Blank draw die 25 Inner pressure sleeve 26 Outer pressure sleeve 33 Panel punch 331 First Curved Surface 34 Cutting Edge Die 35 Lower Pressure Sleeve 36 Die Core Ring 50 Tab 51 Lining Material 60 Gap 61 Convex Curved Surface 62 Vertical Wall Surface 63 Concave Curved Surface 64 Flange Surface 64a Arc Surface

Claims

1. A method for manufacturing a shell for a can lid, which comprises press-forming a metal sheet so as to sandwich it between a set of upper and lower dies, and forming a central disc-shaped panel portion, a countersink portion that is continuous with the outer periphery thereof and forms an annular recess directed upward, a chuck wall portion whose diameter gradually increases upward from the outer peripheral edge of the countersink portion, a shoulder portion that extends radially outward from the outer peripheral edge of the chuck wall portion, and a curl-shaped portion that is continuous with the shoulder portion and whose outer peripheral edge portion is folded back radially inward, wherein: a panel punch formed on a first curved surface in which a corner connecting an upper surface and a side surface is convex outward is used for the lower die, and a die core ring provided with a gap radially outside the panel punch is used, and a die center having an annular convex portion formed on a second curved surface that protrudes from the outer peripheral portion on the lower end side toward the gap of the lower die and whose tip portion is convex toward the lower die is used for the upper die, and a pressure sleeve outside the die center in the radial direction is used; while sandwiching the metal sheet between the die core ring and the pressure sleeve, the chuck wall portion is formed in a shape in which a first curved portion convex radially inward and a second curved portion concave radially inward (convex radially outward) above the first curved portion are continuous; when sandwiching the central portion of the metal sheet between the panel punch and the die center inside the chuck wall portion in the radial direction, the metal sheet is bent so as to be pushed toward the gap by the first curved surface and the second curved surface of the annular convex portion, thereby forming a dent in the metal sheet. Then, while maintaining the clamping state by the panel punch and the die center, the panel punch and the die center are relatively moved upward with respect to the die core ring and the pressure sleeve that sandwich the metal sheet outside thereof, so that the dented portion of the metal sheet is gradually dropped into the gap to gradually increase the depth of the dent and form the countersink portion that is continuous with the inner peripheral edge of the second curved portion, and the panel portion is formed inside the countersink portion.

2. An apparatus for manufacturing a shell for a can lid, which presses and forms a metal plate between a set of upper and lower dies to form a central disc-shaped panel portion, a countersink portion that is continuous with the outer peripheral portion thereof and forms an upwardly directed annular recess, a chuck wall portion that gradually increases in diameter upward from the outer peripheral edge of the countersink portion, a shoulder portion that extends radially outward from the outer peripheral edge of the chuck wall portion, and a curl portion that is continuous with the shoulder portion and has an outer peripheral edge portion folded back radially inward. A die center and a panel punch for forming the panel portion are provided opposite each other at the central portions of the upper die and the lower die. Outside these die center and panel punch, a pressure sleeve and a die core ring are provided opposite each other, which form the chuck wall portion having a shape in which a first curved portion that protrudes radially inward and a second curved portion that is concave radially inward (convex radially outward) above it are continuous. A corner portion connecting the upper surface and the side surface of the panel punch is formed on a first curved surface that protrudes outward. An annular convex portion is formed on the outer peripheral portion of the lower end side of the die center, which protrudes toward the lower die, has a tip portion that protrudes toward the lower die and is convex, and is located radially outward of the first curved surface. The height from the lower end surface of the die center to the apex of the second curved surface of the annular convex portion is set to be smaller than the height from the lowermost end of the countersink to the lower surface of the panel portion. A gap is formed between the lower die core ring and the panel punch, and the annular convex portion is disposed opposite the gap. The panel punch and the die center can move relatively upward with respect to the die core ring and the pressure sleeve. An apparatus for manufacturing a shell for a can lid, characterized by the above.

3. The apparatus for manufacturing a shell for a can lid according to claim 2, wherein the height from the lower end surface of the die center to the apex of the second curved surface of the annular convex portion is set to be 0.3 mm or more and 1.2 mm or less, and the height from the lowermost end of the countersink portion to the lower surface of the panel portion is 1.8 mm to 2.3 mm.

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