TWO-PIECE METAL CAN AND MANUFACTURING METHOD THEREOF

Abstract

A two-piece metal can includes a domed section, a first arcuate portion having a first curvature radius is joined smoothly to a second arcuate portion having a second curvature radius that is formed between the first arcuate portion and a counter section. A ratio between the first curvature radius and the second curvature radius (the first curvature radius/the second curvature radius) falls within a range between a lower limit value and an upper limit value expressed as: Lower limit value=−(0.148·Y)+(0.025·DP)+1.92; and Upper limit value=(0.140·Y)−(0.054·DP)+3.97, given that a height of the domed section 5 from an outer circumferential end of the second arcuate portion to the top portion is Y, and that an outer diameter of the domed section is DP.

Description

TECHNICAL FIELD

This invention relates to a two-piece metal can in which a trunk section and a bottom section are formed integrally using an aluminum or aluminum alloy sheet material and a manufacturing method thereof, and more specifically, to a manufacturing method for shaping the bottom section into a desired shape or structure.

BACKGROUND ART

One example of a structure of the bottom section of the metal can of this kind is disclosed in the publication of Japanese Patent No. 6448217. The can described in the publication of Japanese Patent No. 6448217 has been developed for the purpose of improving the strength of a bottom section, and in the can described therein, a dome part having a specific configuration is formed inside of an annular grounding part. According to the teachings of Japanese Patent No. 6448217, a counter part is continuously formed on the inner periphery of the annular grounding part, and the dome part is formed on the counter part to protrude upwardly (toward the inner side of the can, or depressed from the outer side of the can). The grounding part is an annular section having an arcuate cross-section protruding downwardly. The counter part is a so-called inclined wall portion that is slightly inclined toward the center axis of the can. The dome part is a curved section formed in the inner peripheral side of the counter part, and smoothly curved toward the center of the can. In the curved section, a curvature radius of a central portion falls within a range from 55 mm to 62 mm, and a curvature radius of a portion extending radially outwardly from the central portion falls within a range from 33 mm to 37 mm. According to the teachings of Japanese Patent No. 6448217, therefore, the central portion and the portion extending radially outwardly from the central portion are formed continuously without forming a sharp bend therebetween. For this reason, the curved section will not be subjected to a stress concentration so that the strength of the can against the load applied from an internal space of the can may be enhanced.

SUMMARY OF INVENTION

Technical Problem to be Solved by the Invention

Thus, the two curved portions (having arcuate cross-sections) of the dome part in the bottom of the can may be formed smoothly and continuously without forming a sharp bend therebetween by setting the curvature radii of the curved portions within the ranges described in Japanese Patent No. 6448217. Consequently, the strength of the dome part may be enhanced, and a fragile portion as a starting point of so-called buckling may be eliminated. However, as a result of various experimentations conducted by the inventors of the present invention, the inventors have found a fact that the strength of the dome part is affected not only by the local stress concentration but also by configurations of the dome part itself and a portion in the vicinity thereof, and also found an optimum configuration of the dome part for preventing an occurrence of the buckling.

The dome part is deformed or buckled by a rise in an internal pressure or a large impact load applied thereto when the can is dropped to the ground. Consequently, the central portion of the dome part protruding upwardly is flattened and eventually inverted to protrude downwardly, or the portion of the dome part inside of the periphery thereof (as a boundary between the dome part and the counter part) is entirely reversed to protrude downwardly. For example, if the dome part is formed to significantly protrude upwardly, a resistance against the load reversing the dome part to protrude downwardly may be increased thereby enhancing the strength of the dome part. However, if the dome part protrudes significantly upwardly, an internal content of the can is reduced. Thus, an allowable height of the dome part is limited, but it is necessary to set the height of the dome part within the range of such limitation so as to ensure the strength.

In general, as described in the publication of Japanese Patent No. 6448217, the dome part is shaped to have a cross-section in which a plurality of arcs are joined smoothly and continuously. Thus, those arcs have to be joined smoothly and continuously. To this end, curvature radii of the arcs are restricted, and in addition, the curvature radius of the respective arcs are preferably set taking account of the load applied thereto. Specifically, when the can is dropped, an impact load is applied to the central portion of the dome part substantially perpendicularly, and to the peripheral portion of the central portion obliquely. Therefore, the curvature radius of the arc in the central portion of the dome part is set greater than that of the arc in the peripheral portion. However, in order to enhance the strength of the dome part, it is not sufficient to merely increase the curvature radius of the arc in the peripheral portion, and it is necessary to satisfy a predetermined relation between the curvature radius of the arc in the central portion and the curvature radius of the arc in the peripheral portion.

Further, the dome part is protruded more significantly and the strength thereof is increased by reducing a diameter thereof or a diameter of the grounding part (or a rim part). However, in order to allow the can to erect stably, it is necessary to maintain a diameter of the grounding part to a somewhat large diameter. That is, it is necessary to form the dome part to protrude upwardly so as to ensure the strength while maintaining the diameter of the grounding part to a somewhat large diameter. Thus, the strength of the dome part is greatly affected by the diameter of the dome part or the grounding part.

In the can described in the publication of Japanese Patent No. 6448217, the curvature radii of the two arcuate surfaces forming the dome part are set to the specific values to eliminate a sharp bend at which the stress is concentrated. However, the curvature radii of the two arcuate surfaces are not set to those specific values without taking account of how the deformation load acts, and without taking account of other factors affecting the strength. Therefore, the metal can has to be improved to enhance the strength of the bottom of the metal can against deformation such as buckling of the dome part, in other words, to reduce a thickness while maintaining the deformation strength. In view of the foregoing, the inventors have studied the factors affecting the strength of the metal can and developed the present invention.

An object of the present invention is to provide a two-piece metal can in which the strength of a domed bottom section formed integrally with a trunk section is enhanced, and a method for manufacturing the two-piece metal can in such a manner as to reduce a thickness of the two-piece metal can so as to use the material efficiently.

Means for Solving the Problem

According to one aspect of the present invention, there is provided a two-piece metal can in which a cylindrical trunk section is formed integrally with a bottom section closing a lower end of the trunk section. The bottom section includes: a rim section whose diameter is smaller than a diameter of the trunk section, and which is shaped entirely into a circular shape protruding downwardly; a counter section extending upwardly from a lower end of the rim section toward an inner circumferential side to serve as an inner circumferential wall of the rim section; and a domed section formed continuously from an upper end of the counter section in which a top portion is formed at a central portion thereof. In order to achieve the above-explained objective, according to one aspect of the present invention, the domed section includes: a first arcuate portion that is formed around a center of the domed section whose cross-section is an arcuate cross-section having a predetermined first curvature radius; and a second arcuate portion that is formed between the first arcuate portion and the counter section while being joined smoothly to the first arcuate portion, and whose cross-section is an arcuate cross-section having a predetermined second curvature radius. In addition, a ratio between the first curvature radius and the second curvature radius (the first curvature radius/the second curvature radius) falls within a range between a lower limit value and an upper limit value expressed as:

$\mathrm{Lower}\mathrm{limit}\mathrm{value}=-\left(0.148·Y\right)+\left(0.025·\mathrm{DP}\right)+1.92;\mathrm{and}$ $\mathrm{Upper}\mathrm{limit}\mathrm{value}=\left(0.140·Y\right)-\left(0.054·\mathrm{DP}\right)+3.97,$

given that a height of the domed section from an outer circumferential end of the second arcuate portion to the top portion is Y, and that an outer diameter of the domed section is DP.

In the metal can according to the present invention, a third arcuate portion may be formed between the second arcuate portion and the counter section while being joined smoothly to the second arcuate portion and the counter section. A cross-section of the third arcuate portion may also be an arcuate cross-section, and a third curvature radius of the third arcuate portion may be 1.5 mm or larger but 3.5 mm or smaller.

In the metal can according to the present invention, a diameter of the rim section may be 46 mm or larger but 48 mm or smaller.

According to another aspect of the present invention, there is provided a manufacturing method of a two-piece metal can in which a bottom section is formed integrally with a trunk section by drawing and ironing a metal sheet. The bottom section includes: a rim section whose diameter is smaller than a diameter of the trunk section, and which is shaped entirely into a circular shape protruding downwardly; a counter section extending upwardly from a lower end of the rim section toward an inner circumferential side to serve as an inner circumferential wall of the rim section; and a domed section formed continuously from an upper end of the counter section in which a top portion is formed at a central portion thereof. In order to achieve the above-explained objective, according to another aspect of the present invention, the domed section includes a first arcuate portion that is formed around a center of the domed section whose cross-section is an arcuate cross-section having a predetermined first curvature radius; and a second arcuate portion that is formed between the first arcuate portion and the counter section while being joined smoothly to the first arcuate portion, and whose cross-section is an arcuate cross-section having a predetermined second curvature radius. In addition, the bottom section is formed by a doming punch such that a ratio between the first curvature radius and the second curvature radius (the first curvature radius/the second curvature radius) falls within a range between a lower limit value and an upper limit value expressed as:

$\mathrm{Lower}\mathrm{limit}\mathrm{value}=-\left(0.148·Y\right)+\left(0.025·\mathrm{DP}\right)+1.92;\mathrm{and}$ $\mathrm{Upper}\mathrm{limit}\mathrm{value}=\left(0.140·Y\right)-\left(0.054·\mathrm{DP}\right)+3.97,$

given that a height of the domed section from an outer circumferential end of the second arcuate portion to the top portion is Y, and that an outer diameter of the domed section is DP.

According to the manufacturing method of the present invention, the outer diameter of the domed section may correspond to an outer diameter of the doming punch.

According to the manufacturing method of the present invention, a third arcuate portion may be formed between the second arcuate portion and the counter section while being joined smoothly to the second arcuate portion and the counter section. Across-section of the third arcuate portion may also be an arcuate cross-section, and a third curvature radius of the third arcuate portion may be 1.5 mm or larger but 3.5 mm or smaller.

According to the manufacturing method of the present invention, the rim section may be formed into an annular shape in which a diameter thereof is 46 mm or larger but 48 mm or smaller.

Advantageous Effects of Invention

In the metal can according to the present invention, and according to the manufacturing method of the present invention, the height Y of the domed section is determined based on an internal volume of the metal can, and the diameter DP of the domed section is determined based on the diameter of the rim section set to a value possible to allow the metal can to stand in a stable manner. Further, the upper limit value and the lower limit value of the ratio between the first curvature radius and the second curvature radius is determined based on the height Y and the diameter DP of the domed section. Given that the above-mentioned ratio is set to the upper limit value, a strength of the domed section is enhanced to the maximum so that a maximum internal pressure not to cause a buckling of the domed section may be increased. In addition, a drop impact strength of the domed section may be enhanced to the maximum. That is, a height at which the domed section will not be buckled even if the metal can is dropped therefrom may be heightened to the maximum. However, the strength of the domed section decreases significantly given that the aforementioned ratio exceeds the upper limit value. That is, the upper limit value of the aforementioned ratio is a critical value to increase the strength of the domed section. According to the present invention, the domed section is shaped such that the aforementioned ratio between the curvature radii is set to the upper limit value at which the strength of the domed section is enhanced to the maximum or falls within a predetermined range lower than the upper limit value. Therefore, a thickness of the metal can may be reduced without reducing or while maintaining the strength of the domed section. For this reason, it is possible to reduce the cost of the metal can, and the material of the metal can may be saved effectively.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view schematically showing an intermediate product of the metal can according to the present invention formed in-process.

FIG. 2 is an enlarged cross-sectional view showing a cross-section of a half of the bottom section.

FIG. 3 is a graph showing a relation between a curvature radius ratio and the pressure resistance measured while fixing the rim section according to Example 1.

FIG. 4 is a graph showing a relation between the curvature radius ratio and the pressure resistance measured while fixing the rim section according to Example 2.

DESCRIPTION OF EMBODIMENT(S)

Here will be explained the exemplary embodiments of the metal can and the manufacturing method thereof according to the present invention. Note that the embodiments shown below are merely examples of the present disclosure which should not limit a scope of the present invention.

An example of an intermediate product of a metal can 1 according to the present invention formed during a forming (manufacturing) process is schematically shown in FIG. 1 and FIG. 2. The example shown therein is a two-piece can formed by drawing and ironing e.g., an aluminum sheet, in which a trunk section 2 and a bottom section 3 are formed integrally. The trunk section 2 has a simple cylindrical shape, and the bottom section 3 is formed integrally with a lower end portion of the trunk section 2. As illustrated in an enlarged scale in FIG. 2, in the bottom section 3, the lower end portion of the trunk section 2 is processed to gradually reduce a diameter thereof (i.e., shrunk) to form a rim section 4. The rim section 4 is also referred to as a ground section, and a portion in an inner circumferential side of the rim section 4 is subjected to the so-called doming process to be shaped into a domed section 5 that is curved to project toward an internal space of the trunk section 2 (i.e., upwardly in FIG. 1). Thus, the metal can 1 has a structure possible to withstand the internal pressure exerted e.g., by sparkling beverage held therein, and to withstand an impact load applied thereto when it is dropped or handled roughly.

The rim section 4 serves as a contact surface or a contact ring of the metal can 1 being placed. In the rim section 4, an outer circumferential portion is shrunk and an inner circumferential portion is bent upwardly so that the rim section is shaped entirely into a circular shape protruding downwardly. In order to allow the metal can to 1 stand stably, a diameter D4 of the rim section 4 is set to 46 mm or larger but 48 mm or smaller. A tip (i.e., a lower end) of the rim section 4 is curved smoothly to project downwardly and to have a predetermined curvature radius.

An inner circumferential wall of the rim section 4 serves as a counter section 6 that is a cylindrical wall erecting parallel to the central axis of the metal can 1, or a tapered wall in which a diameter of an upper portion thereof is slightly reduced. The domed section 5 is formed continuously from an upper end portion of the counter section 6, and the domed section 5 including a boundary with the counter section 6 is curved entirely to form a smooth curved surface (protruding upwardly).

In the example shown in FIG. 2, the domed section 5 is formed of a plurality of arcuate portions smoothly joined to one another. In the domed section 5, a first arcuate portion 5A is formed within a predetermined area around a center of the domed section 5, and a cross-section of the first arcuate portion 5A along the center axis is an arcuate cross-section having a predetermined curvature radius Ra. A second arcuate portion 5B is formed within a predetermined area on radially outer side of the first arcuate portion 5A, and a cross-section of the second arcuate portion 5B along the center axis of the metal can 1 is also an arcuate cross-section having a predetermined curvature radius Rb. A radial distance from the center axis to a boundary between the first arcuate portion 5A and the second arcuate portion 5B (that is, a radius r measured from the central axis) is about half of a radial distance from the center axis to a radially outer end of the second arcuate portion 5B. A position of the above-mentioned boundary varies according to the curvature radii R and Rb.

The radially outer end of the second arcuate portion 5B comes close to an upper end of the counter section 6, and the second arcuate portion 5B is joined to the counter section 6 through a third arcuate portion 5C. In other words, the third arcuate portion 5C is formed between the radially outer end of the second arcuate portion 5B and the upper end of the counter section 6. The third arcuate portion 5C is formed to connect the second arcuate portion 5B smoothly to the counter section 6, and a curvature radius thereof (i.e., a curvature radius of an arcuate cross-section along the central axis) is set to e.g., 1.5 mm or larger but 3.5 mm or smaller. Thus, the arcuate portions 5A, 5B, 5C and the counter section 6 are “smoothly connected” to one another to have a common tangent line at each connection edge, or to reduce an angular difference between tangent lines at each connection edge as much as possible even if the tangent lines are not completely overlap each other.

As illustrated in FIG. 2, the central portion of the domed section 5 is a top portion 7 as a highest portion, and the first arcuate portion 5A extends within the above-mentioned radius r while being flattened gradually toward the top portion 7. On the other hand, the second arcuate portion 5B extends within a predetermined area of radially outer side of the first arcuate portion 5A, and an outer peripheral portion thereof is curved gradually downwardly. Therefore, when the metal can 1 filled with the content is dropped and an impact force is applied thereto, the first arcuate portion 5A is supported by the second arcuate portion 5B from below. It is considered that a greater vertical deformation load derived from the impact force is applied to the first arcuate portion 5A that is almost horizontal than that applied to the inclined second arcuate portion 5B. Taking account of such deformation load that buckles the domed section 5, in order to enhance the strength of the domed section 5, it is preferable to reduce the diameter of the rim section 4 or the domed section 5 and to form the domed section 5 to protrude significantly upwardly. Here, the strength of the domed section 5 is enhanced to the extent not to be buckled by the internal pressure of the metal can 1 or the downward impact load applied thereto. The strength of the domed section 5 is governed not only by curvature radii of the arcuate portions 5A, 5B, and 5C, but also by a diameter and a height thereof. In addition, the diameter D4 of the rim section 4 which affects the standing stability of the metal can 1 is associated with the diameter of the domed section 5, and hence the diameter of the domed section 5 is restricted. Further, the internal volume of the metal can 1 is associated with the height of the domed section 5, and hence the height of the domed section 5 is also restricted.

The inventors of the present invention have intensively examined the preferred shape of the domed section 5 taking account of the way the load is applied to the domed section 5 to buckle the domed section 5, and the above-mentioned restrictions. Furthermore, the inventors of the present invention have also examined the effects of the arcuate portions 5A, 5B, and 5C, and a diameter DP and a height Y of the domed section 5 to the strength of the domed section 5. Details and results of the examination will be described hereinafter.

First of all, test pieces individually having the shape shown in FIG. 1 were prepared. According to the manufacturing method of the present invention, each of the test pieces was formed by drawing and ironing a blank punched out of a metal sheet to shape the blank into a cup shape, and pressing the bottom section thereof by an after-mentioned doming punch to shape the bottom section into a predetermined shape. That is, the test pieces were formed by conventionally known procedures.

Example 1

In order to conduct the example 1 of the examination, variety of can bodies respectively having the shape shown in FIG. 1 were prepared using a thin metal sheet (aluminum-alloy sheet) whose thickness was 0.24 mm. In each of the can bodies, the diameter D4 (i.e., a grounding diameter) of the rim section 4 was set commonly to 48 mm, but a bottom depth DD was varied to 11.7 mm, 11.4 mm, 11.1 mm, and 10.8 mm. Specifically, the bottom depth DD is a distance from the lower end of the rim section 4 (or the metal can 1) to the top portion 7 of the domed section 5. The doming punch 8 (shown in FIG. 2) having a convex surface congruent with the domed section 5 on its upper end, and whose outer diameter DP was 45.14 mm was used to manufacture the can bodies. In the embodiment of the present invention, the outer diameter DP corresponds to a diameter DP of the domed section 5. Further, a substantial height Y of the domed section 5 corresponds to a height from the boundary between the second arcuate portion 5B and the third arcuate portion 5C to the top portion 7 of the domed section 5. Specifically, in the can body in which the bottom depth DD was 11.7 mm, the height Y of the domed section 5 was 6.95 mm. In the can body in which the bottom depth DD was 11.4 mm, the height Y of the domed section 5 was 6.61 mm. In the can body in which the bottom depth DD was 11.1 mm, the height Y of the domed section 5 was 6.26 mm. In the can body in which the bottom depth DD was 10.8 mm, the height Y of the domed section 5 was 5.93 mm.

In the above-mentioned can bodies (i.e., the metal cans), the curvature radius Ra of the first arcuate portion 5A and the curvature radius Rb of the second arcuate portion 5B were varied to the extent that the arcuate portions 5A to 5C are joined smoothly to form a curved surface protruding upwardly. The examination was conducted to measure pressure resistance of each of the can bodies by pressurizing the liquid (i.e., water) held in the can bodies while fixing the rim section 4 from below and from radially outer side. Specifically, the pressure resistance corresponds to a pressure at which the domed section was buckled. That is, the pressure resistance corresponds to the strength of the domed section.

Measured values of the pressure resistance are indicated in a diagram shown in FIG. 3. In FIG. 3, the horizontal axis represents a ratio (Ra/Rb) between the curvature radius Ra of the first arcuate portion 5A and a curvature radius Rb of the second arcuate portion 5B (hereinafter, also referred to as the curvature radius ratio), the vertical axis represents the pressure resistance (MPa) with respect to the curvature radius ratio, the curve L1 represents measured values of the metal can 1 in which the bottom depth was 11.7 mm, the curve L2 represents measured values of the metal can 1 in which the bottom depth was 11.4 mm, the curve L3 represents measured values of the metal can 1 in which the bottom depth was 11.1 mm, and the curve L4 represents measured values of the metal can 1 in which the bottom depth was 10.8 mm. The measured values are also shown in Table 1.

	TABLE 1
	0.24t Grounding diameter ø48 Pressure resistance (Mpa)
	DD = 11.7 mm	DD = 11.4 mm	DD = 11.1 mm	DD = 10.8 mm
	Measured	Ratio to	Measured	Ratio to	Measured	Ratio to	Measured	Ratio to
Ra/Rb	Value	max	Value	max	Value	max	Value	max
1.00	0.843	80.3%	0.784	77.1%	0.739	75.8%	0.701	77.0%
1.20	0.888	84.6%	0.830	81.7%	0.777	79.7%	0.729	80.1%
1.40	0.930	88.6%	0.876	86.2%	0.807	82.8%	0.760	83.5%
1.60	0.969	92.3%	0.913	89.8%	0.852	87.3%	0.794	87.3%
1.80	0.988	94.1%	0.943	92.8%	0.886	90.8%	0.823	90.4%
1.90	1.000	95.2%	0.966	95.0%	0.902	92.5%	0.839	92.2%
2.00	1.011	96.3%	0.975	95.9%	0.914	93.7%	0.855	94.0%
2.05	1.018	97.0%	0.984	96.7%	0.922	94.5%	0.867	95.2%
2.10	1.023	97.5%	0.988	97.2%	0.929	95.2%	0.874	96.1%
2.15	1.026	97.7%	0.988	97.1%	0.934	95.7%	0.879	96.6%
2.20	1.031	98.2%	0.995	97.9%	0.941	96.4%	0.885	97.2%
2.25	1.038	98.9%	0.998	98.1%	0.951	97.5%	0.890	97.8%
2.30	1.040	99.0%	1.002	98.5%	0.956	98.0%	0.895	98.3%
2.35	1.044	99.5%	1.005	98.9%	0.963	98.8%	0.900	98.8%
2.40	1.043	99.3%	1.012	99.5%	0.968	99.2%	0.906	99.6%
2.45	1.046	99.6%	1.013	99.6%	0.975	100.0%	0.910	100.0%
2.50	1.050	100.0%	1.017	100.0%	0.974	99.9%	0.890	97.8%
2.55	1.050	100.0%	1.016	99.9%	0.951	97.5%	0.868	95.4%
2.60	1.049	99.9%	0.993	97.7%	0.922	94.6%	0.845	92.8%
2.65	1.036	98.6%	0.965	94.9%	0.905	92.8%	0.822	90.3%
2.70	1.006	95.8%	0.941	92.5%	0.874	89.6%	0.800	87.9%
2.80	0.963	91.7%	0.887	87.3%	0.821	84.2%	0.758	83.3%
2.90	0.920	87.6%	0.864	85.0%	0.778	79.7%	0.709	77.9%

As can be seen from FIG. 3 and Table 1, the pressure resistance increases with an increase in the aforementioned ratio, and the pressure resistance increases to the maximum value within a specific range of the aforementioned ratio. However, the pressure resistance decreases significantly given that the aforementioned ratio exceeds the value at which the pressure resistance increases to the maximum value (i.e., an upper limit value). That is, the upper limit value of the aforementioned ratio is a critical value to increase the pressure resistance, and the upper limit value decreases gradually with a reduction in the bottom depth DD.

In Table 1, ratios of the pressure resistance to the maximum value thereof (ratio to max) are also listed. As can be seen therefrom, the curvature radius ratio (Ra/Rb) at which the ratio of the pressure resistance is about 3% less than the maximum value, that is, the ratio of the pressure resistance to the maximum value is about 97%, increases with a reduction in the bottom depth DD. According to the present invention, the curvature radius ratio at which the pressure resistance is about 97% is employed as the lower limit value. Here, given that the ratio of the pressure resistance to the maximum value is greater than about 97%, the pressure resistance is considered to be substantially equal to the maximum value taking account of slight variations in material and processing. Therefore, the lower limit value of the curvature radius ratio is set to the value at which the ratio of the pressure resistance to the maximum value is “about 97%”.

Relation among the upper limit value and the lower limit value of the curvature radius ratio (Ra/Rb), the height Y of the domed section 5 governed by the bottom depth DD, and a diameter DP of the doming punch 8 (diameter of the domed section) associated with the grounding diameter D4 are shown in Table 2.

	TABLE 2
	Bottom depth DD
	11.7	11.4	11.1	10.8
Grounding	Diameter of		Height Y
diameter	doming punch		6.95	6.61	6.26	5.93
ø48	ø45.14	Lower limit of Ra/Rb	2.02	2.07	2.12	2.17
		Upper limit of Ra/Rb	2.51	2.46	2.41	2.36

Values of the curvature radii Ra and Rb at each curvature radius ratio are listed in Table 3.

	TABLE 3
	0.24t Grounding diameter ø48
	DD = 11.7 mm	DD = 11.4 mm	DD = 11.1 mm	DD = 10.8 mm
Ra/Rb	Ra	Rb	Ra	Rb	Ra	Rb	Ra	Rb
1.00	38.257	39.732	41.376	43.216
1.20	40.36	33.641	41.89	34.905	43.59	36.326	45.50	37.906
1.40	42.50	30.365	44.08	31.482	45.84	32.734	47.81	34.142
1.60	44.67	27.927	46.30	28.932	48.11	30.067	50.14	31.343
1.80	46.87	26.043	48.54	26.968	50.41	28.002	52.50	29.172
1.90	47.98	25.254	49.67	26.144	51.57	27.136	53.69	28.261
2.00	49.10	24.545	50.81	25.403	52.73	26.362	54.89	27.440
2.05	49.65	24.224	51.38	25.062	53.31	26.005	55.48	27.068
2.10	50.21	23.914	51.95	24.738	53.90	25.661	56.08	26.709
2.15	50.77	23.619	52.52	24.430	54.48	25.340	56.68	26.368
2.20	51.34	23.335	53.10	24.132	55.07	25.029	57.29	26.038
2.25	51.90	23.068	53.67	23.854	55.66	24.733	57.89	25.728
2.30	52.47	22.810	54.25	23.584	56.24	24.455	58.49	25.433
2.35	53.03	22.568	54.82	23.330	56.83	24.186	59.10	25.147
2.40	53.60	22.333	55.40	23.084	57.42	23.928	59.71	24.874
2.45	54.17	22.108	55.98	22.849	58.01	23.682	60.31	24.617
2.50	54.74	21.894	56.56	22.624	58.61	23.443	60.92	24.367
2.55	55.31	21.688	57.14	22.409	59.20	23.217	61.53	24.127
2.60	55.88	21.491	57.72	22.202	59.80	22.997	62.14	23.897
2.65	56.45	21.302	58.30	22.004	60.39	22.790	62.75	23.677
2.70	57.02	21.120	58.89	21.811	60.99	22.588	63.36	23.465
2.80	58.17	20.776	60.06	21.449	62.18	22.209	64.58	23.066
2.90	59.32	20.457	61.23	21.115	63.38	21.856	65.81	22.693

Example 2

In order to conduct the example 2 of the examination, can bodies having a shape shown in FIG. 1 were prepared using a thin metal sheet (aluminum-alloy sheet) whose thickness was 0.215 mm. In each of the can bodies, the diameter D4 (i.e., a grounding diameter) of the rim section 4 was set commonly to 46 mm, but the bottom depth DD was also varied to 11.7 mm, 11.4 mm, 11.1 mm, and 10.8 mm. The doming punch 8 (shown in FIG. 2) whose outer diameter DP was 43.14 mm was used to manufacture the can bodies. As described, the outer diameter DP corresponds to the diameter DP of the domed section 5. In the can body in which the bottom depth DD was 11.7 mm, the height Y of the domed section 5 was 7.01 mm. In the can body in which the bottom depth DD was 11.4 mm, the height Y of the domed section 5 was 6.66 mm. In the can body in which the bottom depth DD was 11.1 mm, the height Y of the domed section 5 was 6.31 mm. In the can body in which the bottom depth DD was 10.8 mm, the height Y of the domed section 5 was 5.97 mm.

In the above-mentioned can bodies (i.e., the metal cans), the curvature radius Ra of the first arcuate portion 5A and the curvature radius Rb of the second arcuate portion 5B were varied to the extent that the arcuate portions 5A to 5C are joined smoothly to form a curved surface protruding upwardly. The examination was conducted to measure pressure resistance of each of the can bodies by pressurizing the liquid (i.e., water) held in the can bodies while fixing the rim section 4 from below and from radially outer side. Specifically, the pressure resistance corresponds to the pressure at which the domed section was buckled. That is, the pressure resistance corresponds to the strength of the domed section.

Measured values of the pressure resistance are indicated in a diagram shown in FIG. 4. In FIG. 4, the horizontal axis represents the curvature radius ratio (Ra/Rb) between the first arcuate portion 5A and the second arcuate portion 5B, the vertical axis represents the pressure resistance (MPa) with respect to the curvature radius ratio, the curve L5 represents measured values of the metal can 1 in which the bottom depth was 11.7 mm, the curve L6 represents measured values of the metal can 1 in which the bottom depth was 11.4 mm, the curve L7 represents measured values of the metal can 1 in which the bottom depth was 11.1 mm, and the curve L8 represents measured values of the metal can 1 in which the bottom depth was 10.8 mm. The measured values are also shown in Table 4.

	TABLE 4
	0.215t Grounding diameter ø46 Pressure resistance (Mpa)
	DD = 11.7 mm	DD = 11.4 mm	DD = 11.1 mm	DD = 10.8 mm
	Measured	Ratio to	Measured	Ratio to	Measured	Ratio to	Measured	Ratio to
Ra/Rb	Value	max	Value	max	Value	max	Value	max
1.00	0.795	83.6%	0.739	79.6%	0.693	76.9%	0.663	78.3%
1.20	0.834	87.7%	0.780	84.0%	0.728	80.9%	0.684	80.7%
1.40	0.861	90.5%	0.812	87.5%	0.770	85.5%	0.715	84.4%
1.60	0.889	93.4%	0.847	91.2%	0.801	88.9%	0.744	87.8%
1.80	0.908	95.4%	0.873	94.0%	0.825	91.7%	0.772	91.1%
1.90	0.914	96.1%	0.885	95.4%	0.841	93.3%	0.786	92.8%
2.00	0.923	97.0%	0.897	96.7%	0.853	94.7%	0.803	94.7%
2.05	0.926	97.3%	0.903	97.3%	0.858	95.3%	0.807	95.2%
2.10	0.928	97.5%	0.907	97.7%	0.867	96.2%	0.816	96.3%
2.15	0.934	98.2%	0.908	97.8%	0.872	96.8%	0.822	97.0%
2.20	0.937	98.5%	0.913	98.3%	0.877	97.4%	0.830	98.0%
2.25	0.941	98.9%	0.918	98.8%	0.882	98.0%	0.832	98.2%
2.30	0.941	98.9%	0.921	99.3%	0.884	98.1%	0.838	98.9%
2.35	0.944	99.3%	0.923	99.4%	0.891	98.9%	0.841	99.2%
2.40	0.944	99.2%	0.922	99.3%	0.894	99.2%	0.845	99.7%
2.45	0.945	99.4%	0.926	99.7%	0.899	99.8%	0.845	99.8%
2.50	0.949	99.8%	0.927	99.9%	0.901	100.0%	0.847	100.0%
2.55	0.950	99.8%	0.927	99.9%	0.901	100.0%	0.838	98.9%
2.60	0.951	100.0%	0.928	100.0%	0.883	98.1%	0.822	97.0%
2.65	0.951	100.0%	0.923	99.5%	0.866	96.2%	0.799	94.2%
2.70	0.948	99.7%	0.914	98.5%	0.844	93.8%	0.781	92.1%
2.80	0.932	98.0%	0.870	93.8%	0.811	90.1%	0.739	87.2%
2.90	0.894	94.0%	0.830	89.4%	0.768	85.3%	0.704	83.0%

As can be seen from FIG. 4 and Table 4, the pressure resistance increases with an increase in the curvature radius ratio, and the pressure resistance increases to the maximum value within a specific range of the curvature radius ratio. However, the pressure resistance decreases significantly given that the aforementioned ratio exceeds the value at which the pressure resistance increases to the maximum value (i.e., an upper limit value). That is, the upper limit value of the curvature radius ratio is a critical value to increase the pressure resistance, and the upper limit value decreases gradually with a reduction in the bottom depth DD.

In Table 4, ratios of the pressure resistance to the maximum value thereof (ratio to max) are also listed. As can be seen therefrom, the curvature radius ratio (Ra/Rb) at which the ratio of the pressure resistance is about 3% less than the maximum value, that is, the ratio of the pressure resistance to the maximum value is about 97%, increases with a reduction in the bottom depth DD. According to the present invention, the curvature radius ratio at which the pressure resistance is about 97% is employed as the lower limit value for the reason explained in the Example 1.

Relation among the upper limit value and the lower limit value of the curvature radius ratio (Ra/Rb), the height Y of the domed section 5 governed by the bottom depth DD, and the diameter DP of the doming punch 8 (diameter of the domed section) associated with the grounding diameter D4 are shown in Table 5.

	TABLE 5
	Bottom depth DD
	11.7	11.4	11.1	10.8
Grounding	Diameter of		Height Y
diameter	doming punch		7.01	6.66	6.31	5.97
ø46	ø43.14	Lower limit of Ra/Rb	1.96	2.01	2.06	2.11
		Upper limit of Ra/Rb	2.62	2.57	2.52	2.48

Values of the curvature radii Ra and Rb at each curvature radius ratio are listed in Table 6.

	TABLE 6
	0.215t Grounding diameter ø46
	DD = 11.7 mm	DD = 11.4 mm	DD = 11.1 mm	DD = 10.8 mm
Ra/Rb	Ra	Rb	Ra	Rb	Ra	Rb	Ra	Rb
1.00	35.121	36.442	37.916	39.567
1.20	36.91	30.764	38.27	31.892	39.79	33.148	41.49	34.567
1.40	38.74	27.667	40.13	28.667	41.69	29.776	43.44	31.024
1.60	40.60	25.367	42.02	26.267	43.62	27.263	45.42	28.381
1.80	42.48	23.601	43.94	24.412	45.58	25.318	47.42	26.343
1.90	43.43	22.862	44.91	23.635	46.56	24.510	48.43	25.488
2.00	44.39	22.196	45.88	22.943	47.56	23.776	49.44	24.724
2.05	44.87	21.890	46.37	22.621	48.05	23.444	49.95	24.369
2.10	45.36	21.595	46.86	22.316	48.55	23.124	50.46	24.032
2.15	45.84	21.320	47.35	22.026	49.05	22.819	50.97	23.712
2.20	46.32	21.058	47.85	21.746	49.56	22.524	51.49	23.402
2.25	46.81	20.805	48.34	21.484	50.06	22.249	52.00	23.113
2.30	47.30	20.563	48.83	21.235	50.56	21.986	52.52	22.832
2.35	47.79	20.333	49.33	20.992	51.07	21.731	53.03	22.569
2.40	48.28	20.114	49.83	20.761	51.58	21.488	53.55	22.313
2.45	48.77	19.904	50.33	20.540	52.08	21.260	54.07	22.068
2.50	49.26	19.704	50.83	20.329	52.59	21.037	54.59	21.834
2.55	49.75	19.512	51.33	20.127	53.10	20.824	55.11	21.610
2.60	50.25	19.325	51.83	19.933	53.61	20.620	55.63	21.395
2.65	50.74	19.149	52.33	19.748	54.12	20.425	56.15	21.189
2.70	51.24	18.977	52.83	19.570	54.63	20.237	56.67	20.991
2.80	52.23	18.657	53.84	19.232	55.66	19.880	57.72	20.615
2.90	53.23	18.357	54.86	18.916	56.69	19.550	58.77	20.268

As evidenced by the first and the second examples, there are certain relations between: the height Y and the diameter DP of the domed section; and the curvature radius ratio at which the pressure resistance increases to the maximum value (i.e., an upper limit value), and between: the height Y and the diameter DP of the domed section; and the curvature radius ratio at which the pressure resistance is about 97% with respect to the maximum value. In FIGS. 3 and 4, the upper limit values and the lower limit values are defined by the oblique sides of the inverted trapezoids. Each range between the oblique sides of the inverted trapezoid individually corresponds to each region enclosed by the thick line in Table 1 and Table 4. Specifically, the upper limit value and the lower limit value may be expressed as:

$\mathrm{Lower}\mathrm{limit}\mathrm{value}\left(\mathrm{Ra}/\mathrm{Rb}\right)=-\left(0.148·Y\right)+\left(0.025·\mathrm{DP}\right)+1.92;\mathrm{and}$ $\mathrm{Upper}\mathrm{limit}\mathrm{value}\left(\mathrm{Ra}/\mathrm{Rb}\right)=\left(0.140·Y\right)-\left(0.054·\mathrm{DP}\right)+3.97.$

These expressions are so-called experimental formulas developed based on the results of the foregoing examples taking account of errors caused during a normal manufacturing process. Given that the manufacturing conditions of the metal cans are altered, an actual measurement result may be slightly different from a calculation result. However, such slight difference is tolerable in practical use. Accordingly, the above-listed formulas are applicable for a practical method and process for manufacturing the metal can according to the present invention.

That is, the ratio between the curvature radius Ra of the first arcuate portion 5A and the curvature radius Rb of the second arcuate portion 5B may be determined arbitrarily using the above-explained formulas. Therefore, the height Y and the diameter DP of the domed section 5 may be varied arbitrarily at the design phase as long as the curvature radius ratio Ra/Rb falls within the range between the lower limit and the upper limit thereof. For this reason, the domed section 5 may be designed geometrically with reference to e.g., the above-mentioned Table 3 and Table 6 such that the first arcuate portion 5A and the second arcuate portion 5B are joined smoothly to each other.

Thus, according to the exemplary embodiment of present invention, the shape of the domed section 5 possible to enhance the strength thereof to the maximum may be determined. In the foregoing examples, the pressure resistance of the can body was measured while fixing the rim section 4. This is because such pressure resistance of the two-piece metal can to be exerted in the situation where the rim section is fixed correlates with the strength against the impact force applied from the content (or the drop impact strength). That is, the drop impact strength of the metal can may be increased by increasing the pressure resistance to be exerted in the situation where the rim section is fixed. According to the exemplary embodiment of the present invention, therefore, the domed section 5 may be designed to enhance the strength thereof to the maximum. For this reason, a thickness of the metal can or the material of the metal can may be reduced thinner than that of the conventional metal can. As a result, it is possible to reduce the cost of the metal can, and the material of the metal can may be saved effectively.

Lastly, the metal can and the manufacturing method thereof according to the present invention should not be limited to the foregoing examples. For example, a diameter and a height of the domed section, a wall thickness, curvature radii of the arcuate portions may be altered within the scope of the present invention.

Claims

1-7. (canceled)

8. A two-piece metal can, Lower ⁢ limit ⁢ value = - ( 0. 1 ⁢ 48 · Y ) + ( 0. 0 ⁢ 25 · DP ) + 1.92; and Upper ⁢ limit ⁢ value = ( 0. 1 ⁢ 40 · Y ) - ( 0. 0 ⁢ 54 · DP ) + 3.97, given that a height of the domed section from an outer circumferential end of the second arcuate portion to the top portion is Y, and that an outer diameter of the domed section is DP.

wherein a cylindrical trunk section is formed integrally with a bottom section closing a lower end of the trunk section,

the bottom section includes a rim section whose diameter is smaller than a diameter of the trunk section, and which is shaped entirely into a circular shape protruding downwardly, a counter section extending upwardly from a lower end of the rim section toward an inner circumferential side to serve as an inner circumferential wall of the rim section, and a domed section formed continuously from an upper end of the counter section in which a top portion is formed at a central portion thereof,

the domed section includes a first arcuate portion that is formed around a center of the domed section whose cross-section is an arcuate cross-section having a predetermined first curvature radius, and a second arcuate portion that is formed between the first arcuate portion and the counter section while being joined smoothly to the first arcuate portion, and whose cross-section is an arcuate cross-section having a predetermined second curvature radius, and

a ratio between the first curvature radius and the second curvature radius (the first curvature radius/the second curvature radius) falls within a range between a lower limit value and an upper limit value expressed as:

9. The two-piece metal can as claimed in claim 8,

wherein a third arcuate portion whose cross-section is an arcuate cross-section having a third curvature radius is formed between the second arcuate portion and the counter section while being joined smoothly to the second arcuate portion and the counter section, and

the third curvature radius is 1.5 mm or larger but 3.5 mm or smaller.

10. The two-piece metal can as claimed in claim 8, wherein a diameter of the rim section is 46 mm or larger but 48 mm or smaller.

11. The two-piece metal can as claimed in claim 9, wherein a diameter of the rim section is 46 mm or larger but 48 mm or smaller.

12. A manufacturing method of a two-piece metal can, Lower ⁢ limit ⁢ value = - ( 0. 1 ⁢ 48 · Y ) + ( 0. 0 ⁢ 25 · DP ) + 1.92; and Upper ⁢ limit ⁢ value = ( 0. 1 ⁢ 40 · Y ) - ( 0. 0 ⁢ 54 · DP ) + 3.97, given that a height of the domed section from an outer circumferential end of the second arcuate portion to the top portion is Y, and that an outer diameter of the domed section is DP.

wherein a bottom section is formed integrally with a trunk section by drawing and ironing a metal sheet,

the bottom section includes a rim section whose diameter is smaller than a diameter of the trunk section, and which is shaped entirely into a circular shape protruding downwardly, a counter section extending upwardly from a lower end of the rim section toward an inner circumferential side to serve as an inner circumferential wall of the rim section, and a domed section formed continuously from an upper end of the counter section in which a top portion is formed at a central portion thereof,

the domed section includes a first arcuate portion that is formed around a center of the domed section whose cross-section is an arcuate cross-section having a predetermined first curvature radius, and a second arcuate portion that is formed between the first arcuate portion and the counter section while being joined smoothly to the first arcuate portion, and whose cross-section is an arcuate cross-section having a predetermined second curvature radius, and

the bottom section is formed by a doming punch such that a ratio between the first curvature radius and the second curvature radius (the first curvature radius/the second curvature radius) falls within a range between a lower limit value and an upper limit value expressed as:

13. The manufacturing method of the two-piece metal can as claimed in claim 12, wherein the outer diameter of the domed section corresponds to an outer diameter of the doming punch.

14. The manufacturing method of the two-piece metal can as claimed in claim 12,

wherein a third arcuate portion is formed between the second arcuate portion and the counter section while being joined smoothly to the second arcuate portion and the counter section, and

a cross-section of the third arcuate portion is an arcuate cross-section in which a curvature radius thereof is 1.5 mm or larger but 3.5 mm or smaller.

15. The manufacturing method of the two-piece metal can as claimed in claim 13,

wherein a third arcuate portion is formed between the second arcuate portion and the counter section while being joined smoothly to the second arcuate portion and the counter section, and

a cross-section of the third arcuate portion is an arcuate cross-section in which a curvature radius thereof is 1.5 mm or larger but 3.5 mm or smaller.

16. The manufacturing method of the two-piece metal can as claimed in claim 12, wherein the rim section is formed into an annular shape in which a diameter thereof is 46 mm or larger but 48 mm or smaller.

17. The manufacturing method of the two-piece metal can as claimed in claim 13, wherein the rim section is formed into an annular shape in which a diameter thereof is 46 mm or larger but 48 mm or smaller.