U.S. patent application number 09/795236 was filed with the patent office on 2001-07-26 for can bottom having improved strength and apparatus for making same.
This patent application is currently assigned to Crown Cork & Seal Technologies Corporation. Invention is credited to Cheng, Gin-Fung, Jones, Floyd A..
Application Number | 20010009107 09/795236 |
Document ID | / |
Family ID | 22220611 |
Filed Date | 2001-07-26 |
United States Patent
Application |
20010009107 |
Kind Code |
A1 |
Cheng, Gin-Fung ; et
al. |
July 26, 2001 |
Can bottom having improved strength and apparatus for making
same
Abstract
A can bottom having an approximately frustoconical portion
extending downwardly and inwardly from the can side wall, an
annular nose portion extending downwardly from the approximately
frustoconical portion, and a central portion extending upwardly and
inwardly from the nose. The nose is formed by inner and outer
circumferentially extending frustoconical walls that are joined by
a downwardly convex arcuate portion. The inner surface of the
arcuate portion of the nose has a radius of curvature adjacent the
nose inner wall of at least 0.060 inch. The central portion of the
can bottom has a substantially flat disc-shaped central section,
having a diameter of at least about 1.40 inches, and an
approximately dome-shaped and downwardly concave having a radius of
curvature no greater than 1.475 inches. In a preferred embodiment
of the invention, the inner surface of the arcuate portion of the
nose is formed by a sector of a circle and has radius of curvature
is no greater than about 0.070 inch. An apparatus for making the
can bottom comprises a nose punch whose distal end has a radius of
curvature that is equal to the radius of curvature of the can
bottom nose and a die whose radius of curvature equals that of the
dome.
Inventors: |
Cheng, Gin-Fung; (Downers
Grove, IL) ; Jones, Floyd A.; (Wheaton, IL) |
Correspondence
Address: |
WOODCOCK WASHBURN KURTZ
MACKIEWICZ & NORRIS LLP
One Liberty Place - 46th Floor
Philadephia
PA
19103
US
|
Assignee: |
Crown Cork & Seal Technologies
Corporation
|
Family ID: |
22220611 |
Appl. No.: |
09/795236 |
Filed: |
February 28, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09795236 |
Feb 28, 2001 |
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09557522 |
Apr 25, 2000 |
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6220073 |
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09557522 |
Apr 25, 2000 |
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09325591 |
Jun 3, 1999 |
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6131761 |
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09325591 |
Jun 3, 1999 |
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09090000 |
Jun 3, 1998 |
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Current U.S.
Class: |
72/348 |
Current CPC
Class: |
B21D 22/30 20130101;
B65D 1/165 20130101 |
Class at
Publication: |
72/348 |
International
Class: |
B21D 051/26 |
Claims
What is claimed:
1. A can comprising: a) a side wall portion having a diameter of
about 2.6 inches; and b) a bottom portion formed integrally with
said side wall portion, said bottom portion comprising: (i) an
approximately frustoconical portion extending downwardly and
inwardly from said side wall portion; (ii) an annular nose portion
extending downwardly from said approximately frustoconical portion,
(iii) a substantially flat disc-shaped central section, and (iv) an
annular dome section disposed between said substantially flat
central section and said nose, said annular dome section being
arcuate in transverse cross-section and downwardly concave, said
annular dome section having a radius of curvature no greater than
about 1.475 inches.
2. The can according to claim 1, wherein said radius of curvature
of said annular dome section is about 1.45 inches.
3. The can according to claim 1, wherein said substantially flat
disc-shaped central section has a diameter of at least about 0.14
inches.
4. The can according to claim 1, wherein said nose has a base
portion, and wherein said substantially flat disc-shaped central
section is displaced from said nose base portion by a height that
is at least about 0.41 inches.
5. The can according to claim 1, wherein said nose portion is
formed by inner and outer circumferentially extending walls joined
by a downwardly convex arcuate portion, said arcuate portion having
inner and outer surfaces, said inner surface of said arcuate
portion having a radius of curvature adjacent said nose inner wall
of at least 0.060 inch.
6. The can according to claim 5, wherein said radius of curvature
of said inner surface of said arcuate portion of said nose is no
greater than about 0.070 inch.
7. The can according to claim 5, wherein said radius of curvature
of said inner surface of said arcuate portion of said nose is about
0.060 inch.
8. The can according to claim 5, wherein said radius of curvature
of said inner surface of said arcuate portion of said nose is about
0.065 inch.
9. The can according to claim 5, wherein said radius of curvature
of said inner surface of said arcuate portion of said nose is about
0.070 inch.
10. The can according to claim 5, wherein in transverse
cross-section said arcuate portion of said nose is a sector of a
circle.
11. The can according to claim 1, wherein said side wall and bottom
portions are formed of aluminum.
12. The can according to claim 1, wherein said aluminum forming
said nose has a thickness, said thickness being less than 0.011
inch.
13. A can comprising: a) a side wall portion having a diameter of
about 2.6 inches; and b) a bottom portion formed integrally with
said side wall portion, said bottom portion comprising: (i) an
approximately frustoconical portion extending downwardly and
inwardly from said side wall portion; (ii) an annular nose portion
extending downwardly from said approximately frustoconical portion
and forming inner and outer walls, (iii) a substantially flat
disc-shaped central section having a diameter of at least about
0.14 inches, and (iv) an annular section connecting said
substantially flat central section to said inner wall of said nose,
said annular section being arcuate in transverse cross-section and
downwardly concave, said annular section having a radius of
curvature no greater than about 1.475 inches.
14. The can according to claim 13, wherein said radius of curvature
of said annular section has a radius of curvature of about 1.45
inches.
15. The can according to claim 13, wherein said substantially flat
disc-shaped central section has a diameter of 0.139 inches.
16. The can according to claim 13, wherein said nose has a base
portion, and wherein said substantially flat disc-shaped central
section is displaced from said nose base by a height that is at
least about 0.41 inches.
17. The can according to claim 13, wherein said nose portion is
formed by inner and outer circumferentially extending walls joined
by a downwardly convex arcuate portion, said arcuate portion having
inner and outer surfaces, said inner surface of said arcuate
portion having a radius of curvature adjacent said nose inner wall
of at least 0.060 inch.
18. The can according to claim 17, wherein said radius of curvature
of said inner surface of said arcuate portion of said nose is no
greater than about 0.070 inch.
19. The can according to claim 17, wherein said radius of curvature
of said inner surface of said arcuate portion of said nose is about
0.060 inch.
20. An apparatus for forming the bottom of a can, said can bottom
having an annular nose formed therein, comprising: a) a centrally
disposed die having a forming surface that is approximately
dome-shaped and upwardly convex, said forming surface having a
radius of curvature no greater than about 1.475 inches; b) a nose
punch movable relative to said die, said nose punch having a distal
end, said distal end formed by inner and outer circumferentially
extending walls joined by a downwardly convex arcuate portion, said
arcuate portion having a radius of curvature adjacent said inner
wall that is within the range of 0.060 to 0.070 inches; and c) a
ram for causing relative motion between said nose punch and said
die.
21. The apparatus according to claim 20, wherein said forming
surface has a radius of curvature no greater than about 1.45
inches.
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S.
application Ser. No. 09/090,000, filed Jun. 3, 1998, entitled Can
Bottom Having Improved Pressure Resistance and Apparatus for Making
Same, hereby incorporated by reference in its entirety.
FIELD OF THE INVENTION
[0002] The current invention is directed to a can, such as a metal
can used to package carbonated beverages. More specifically, the
current invention is directed to a can bottom having improved
strength.
BACKGROUND OF THE INVENTION
[0003] In the past, cans for packaging carbonated beverages, such
as soft drinks or beer, have been formed from metal, typically
aluminum. Such cans are conventionally made by attaching a can end,
or lid, to a drawn and ironed can body that has an integrally
formed bottom.
[0004] Certain parameters relating to the geometry of the can
bottom play an important role in the performance of the can. In can
bottoms employing an annular nose, discussed further below, the
diameter of the nose affects the ability to stack or nest the
bottom of one can into the top end of another can. Nose diameter
also affects the resistance of the can to tipping over, such as
might occur during filling.
[0005] In addition to stacking ability and anti-tipping stability,
strength is also an important aspect of the performance of the can
bottom. For example, since its contents are under pressure, which
may be as high as 90 psi, the can must be sufficiently strong to
resist excessive deformation due to internal pressurization.
Therefore, ad important strength parameter for the can bottom is
buckle strength, which is commonly defined as the minimum value of
the internal pressure required to cause reversal, or inversion, of
the domed portion of the can bottom--that is, the minimum pressure
at which the center portion of the can bottom flips from being
concave downward to convex downward. Another important parameter is
drop resistance, which is defined as the minimum height required to
cause dome inversion when a can filled with water and pressurized
to 60 psi is dropped onto a hard surface.
[0006] In addition to satisfying performance requirements, there is
tremendous economic incentive for can makers to reduce the amount
of metal used. Since billions of such cans are sold each year, even
slight reductions in metal usage are desirable. The overall size
and general shape of the can is specified to the can maker by the
beverage industry. Consequently, can makers are constantly striving
to reduce the thickness of the metal by refining the details of the
can geometry to obtain a stronger structure. Only a few years ago,
aluminum cans were formed from metal having a thickness of about
0.0112 inch. However, aluminum cans having thicknesses as low as
0.0108 inch are now available.
[0007] One technique for increasing the strength of the can bottom
that has enjoyed considerable success is the forming of a outwardly
concave dome in the can bottom. Beverage cans, such as those for
soft drinks and beer, typically have a side wall diameter of about
2.6 inches. Conventionally, the radius of curvature of the bottom
dome is at least 1.550 inch. For example, U.S. Pat. No. 4,685,582
(Pulciani et al.), assigned at issue to National Can Corporation,
discloses a can having a side wall diameter of 2.597 inches and a
dome radius of curvature of 2.120 inches. Similarly, U.S. Pat. No.
4,885,924 (Claydon et al.), assigned at issue to Metal Box plc,
discloses a can having a side wall diameter of 2.59 inches and a
dome radius of curvature of 2.0 inches, while U.S. Pat. No.
4,412,627 (Houghton et al.), assigned at issue to Metal Container
Corp, discloses a can having a side wall diameter of 2.600 inches
and a dome radius of curvature of 1.750 inches.
[0008] The strength of a domed can bottom is further increased by
forming a downwardly and inwardly extending frustoconical wall on
the periphery of the bottom that terminates in an annular bead, or
nose. The nose has circumferentially extending inner and outer
walls, which may also be frustoconical. The inner and outer walls
are joined by an outwardly convex arcuate portion, which may be
formed by a sector of a circle. The base of the arcuate portion
forms the surface on which the can rests when in the upright
orientation
[0009] According to conventional can making technology, the radius
of curvature of the inner surface of the arcuate portion of the
nose in such domed, conically walled can bottoms was generally
0.050 inch or less. For example, prior to the development of the
current invention, the parent of the assignee of the instant
application, Crown Cork & Seal Company, sold aluminum cans with
202 ends (i.e., the diameter of the can end opposite the bottom is
2{fraction (2/16)} inch) in which the radius of curvature of the
inside surface of the nose was 0.050 inch. Similarly, U.S. Pat. No.
3,730,383 (Dunn et al.), assigned at issue to Aluminum Company of
America, and U.S. Pat. No. 4,685,582 (Pulciani et al.), assigned at
issue to National Can Corporation, disclose a nose having a radius
of curvature of 0.040 inch.
[0010] Moreover, it was heretofore generally thought that the
smaller the radius of curvature of the nose, the greater the
pressure resistance of the can bottom, as discussed, for example,
in the aforementioned U.S. Pat. No. 3,730,383. Consequently, U.S.
Pat. No. 4,885,924 (discussed above), U.S. Pat. No. 5,069,052
(Porucznik et al.), assigned at issue to CMB Foodcan plc, and U.S.
Pat. No. 5,351,852 (Trageser et al.), assigned at issue to Aluminum
Company of America, all disclose methods for reducing the radius of
curvature of the nose in order to increase the strength of the can
bottom. U.S. Pat. No. 5,351,852 suggests reworking the nose so as
to reduce its radius of curvature to 0.015 inch, while U.S. Pat.
No. 5,069,052 suggests reworking the nose so as to reduce its
radius of curvature on the inside surface to zero and on the
outside surface to 0.040 inch or less.
[0011] In addition to its geometry, the manufacturing apparatus and
techniques employed in forming the can bottom can affect its
strength. For example, small surface cracks can be created in the
chime area of the can bottom if the metal is stretched excessively
when the nose is formed. If, as sometimes occurs, these cracks do
not initially extend all the way through the metal wall, they may
go undetected during inspection by the can maker. This can result
in failure of the can after it has been filled and closed, which is
very undesirable from the standpoint of the beverage seller or the
ultimate customer. The smaller the radius of curvature of the nose,
the more likely that such cracking will occur. Since the radius of
curvature of the nose adjacent its inner wall is thought to have a
greater impact on buckle strength than the radius adjacent the
outer wall, some can manufacturers have utilized a nose shape that
is more complex than a simple circle sector by employing two radii
of curvature--a first inside surface radius of curvature adjacent
the outer wall that is above 0.060 inch and a second inside surface
radius of curvature adjacent the inner wall that is below 0.060
inch. For example, U.S. Pat. No. 4,431,112 (Yamaguchi), assigned at
issue to Daiwa Can Company, discloses a domed can bottom, although
one that does not have a conical peripheral wall, with a nose
having a first radius of curvature adjacent its inner wall of about
0.035 inch (0.9 mm) and a second radius of curvature adjacent its
outer wall of about 0.091 inch (2.3 mm). Another can manufacturer
has employed a domed, conically walled bottom in a 204 end can in
which the inner surface of the nose, whose outer wall is inclined
at an angle of about 26.5.degree. with respect to the can axis, has
a first radius of curvature adjacent the nose inner wall of about
0.054 inch and a second radius of curvature adjacent the outer wall
of about 0.064 inch.
[0012] Notwithstanding the improvements heretofore achieved in the
art, it would be desirable to provide a can bottom having a
geometry that optimized performance, especially with respect to
buckle resistance, drop resistence, and stackability and
manufacturability.
SUMMARY OF THE INVENTION
[0013] It is an object of the current invention to provide a can
bottom having a geometry that optimized performance, especially
with respect to buckle resistance, stackability and
manufacturability. This and other objects is accomplished in a can
comprising a side wall portion and a bottom portion formed
integrally with the side wall portion. The bottom portion comprises
(i) an approximately frustoconical portion that extends downwardly
and inwardly from the side wall portion, (ii) an annular nose
portion that extends downwardly from the approximately
frustoconical portion, (iii) a substantially flat disc-shaped
central section, and (iv) an annular dome section disposed between
the substantially flat central section and the nose, the annular
dome section being arcuate in transverse cross-section and
downwardly concave, the annular dome section having a radius of
curvature no greater than about 1.475 inches.
[0014] In one embodiment of the invention, the can side wall has a
diameter of about 2.6 inches, the radius of curvature of the
annular dome section is about 1.45 inches, the substantially flat
disc-shaped central section has a diameter of at least about 0.14
inches, and the substantially flat disc-shaped central section is
displaced from a base portion of the nose by a height that is at
least about 0.41 inches. In this embodiment, the nose portion is
formed by inner and outer circumferentially extending walls joined
by a downwardly convex arcuate portion that has inner and outer
surfaces, and the inner surface of the arcuate portion has a radius
of curvature adjacent the nose inner wall of at least 0.060
inch.
[0015] The invention also encompasses an apparatus for forming can
bottom that has an annular nose formed therein. The apparatus
comprises (i) a centrally disposed die having a forming surface
that is approximately dome-shaped and upwardly convex, the forming
surface having a radius of curvature no greater than about 1.475
inches, (ii) a nose punch movable relative to the die, the nose
punch having a distal end, the distal end formed by inner and outer
circumferentally extending walls joined by a downwardly convex
arcuate portion, the arcuate portion having a radius of curvature
adjacent the inner wall that is within the range of 0.060 to 0.070
inches, and (iii) a ram for causing relative motion between the
nose punch and the die.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is an isometric view of a can having a bottom
according to the current invention.
[0017] FIG. 2 is a cross-section taken through line II-II shown in
FIG. 1, showing the can bottom according to the current
invention.
[0018] FIG. 3 is a cross-section through the can bottom of the
current invention nested into the end of a similar can.
[0019] FIG. 4 is a graph showing the effect of varying the radius
of curvature of the inner surface of the nose on the buckle
strength of a can bottom.
[0020] FIG. 5 is a graph showing the effect of varying the radius
of curvature of the inner surface of the nose on the buckle
strength of a can bottom when the diameter of the nose is varied so
as to maintain approximately constant depth of penetration at
nesting.
[0021] FIG. 6 is a longitudinal cross-section taken through a
bottom forming station according to the current invention.
[0022] FIG. 7 is a longitudinal cross-section taken through the
nose punch according to the current invention shown in FIG. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0023] A can 1 according to the current invention is shown in FIG.
1. As is conventional, the can comprises an end 3, in which an
opening is formed, and a can body. The can body is formed by a
cylindrical side wall 4 and a bottom 6 that is integrally formed
with the side wall. The side wall 4 has a diameter D.sub.1. As is
also convention, the can body is made from a metal, such as steel
or, more preferably, aluminum, such as type 3204, 3302 or 3004
aluminum plate having an H-19 temper.
[0024] As shown in FIG. 2, the can bottom 6 comprises an
approximately frustoconical portion 8 that extends downwardly and
inwardly from the side wall 4. The frustoconical portion 8 includes
an arcuate section 10, having a radius of curvature R.sub.1, that
forms a smooth transition into the side wall 4. The frustoconical
portion 8 also preferably includes a straight section that forms an
angle .alpha. with respect to the axis 7 of the side wall 4.
[0025] As also shown in FIG. 2, an annular nose 16 extends
downwardly from the frustoconical portion 8. The nose 16 preferably
comprises inner and outer approximately frustoconical walls 12 and
13, respectively. It should be noted that the inner wall 12 is
sometimes referred to in the art as the "chime." Preferably, the
inner wall 12 has a straight section that forms an angle .gamma.
with respect to the axis 7 of the side wall 4, while the outer wall
13 has a straight section that forms an angle .beta. with respect
to the axis. The inner and outer walls 12 and 13 are joined by a
circumferentially extending arcuate section 18. The inner wall 12
includes an arcuate section 22, having a radius of curvature
R.sub.5, that forms a smooth transition into a center portion 24 of
the bottom 6. The outer wall 13 includes an arcuate section 14,
having a radius of curvature R.sub.2, that forms a smooth
transition into the frustoconical portion 8.
[0026] In transverse cross-section, the portion of the inner
surface 29 of the arcuate section 18 of the nose 16 adjacent the
inner wall 12 has a radius of curvature R.sub.3. Similarly, the
portion of the inner surface 29 of the arcuate section 18 adjacent
the outer wall 13 has a radius of curvature R.sub.4. The radii of
curvature of the outer surface 30 of the nose 16 will be equal to
the radii of curvature of the inner surface 29 plus the thickness
of the metal in the arcuate portion 18 of the nose, which is
generally essentially the same as the starting metal plate.
Preferably, R.sub.3 equals R.sub.4. Most preferably, the inner
surface 29 of the arcuate portion 18 is entirely formed by a sector
of a circle so that only one radius of curvature forms the entirety
of the arcuate portion 18 of inner surface of the nose 16, as shown
in FIG. 2. The center 19 of the radius of curvature R.sub.3 forms a
circle of diameter D.sub.2 as it extends around the circumference
of the bottom 6. The base 27 of the nose 16, on which the can 1
rests when in the upright orientation, is also formed around
diameter D.sub.2. The center 21 of radius of curvature R.sub.1 of
the arcuate section 10 is displaced from the center 19 of radius of
curvature R.sub.3 in the axial direction by a distance Y.
Preferably, as the value of R.sub.3 is increased, as discussed
below, the value of Y is decreased so that the sum of
Y+R.sub.3remains constant.
[0027] An approximately dome-shaped center portion 24 extends
upwardly and inwardly from the nose 16. The most central section 26
of the center portion 24 is disc-shaped, having a diameter D.sub.3
and being substantially flat. An annular portion 25 of the center
portion 24 is arcuate in transverse cross-section, having a radius
of curvature R.sub.6, and connects the central section 26 to the
inner wall 12 of the nose 16. The can bottom 6 has a dome height H
that extends from the base 27 of the nose 16 to the top of the
center portion 24.
[0028] As shown in FIG. 3, when two similarly constructed cans are
stacked one atop the other, the bottom 6 of the upper can will
penetrate into the end 3 of the lower can so that the base 27 of
the nose 16 of the upper can extends a distance d below the lip
formed on the seaming panel 40 of the lower can.
[0029] FIG. 4 shows the results of a finite element analysis, or
FEA, aimed at showing how the buckle strength, defined as discussed
above, varies with the radius of curvature of the nose 16 in the
bottom of a can having a 202 end and employing the geometry defined
in Table I and shown in FIG. 2:
1TABLE I Can Bottom Geometric Parameters For FEA Diameter D.sub.1
2.608 inches (66.24 mm) Diameter D.sub.2 1.904 inches (48.36 mm)
Diameter D.sub.3 0.100 inch (2.54 mm) Radius R.sub.1 0.170 inch
(4.32 mm) Radius R.sub.2 0.080 inch (2.03 mm) Radius R.sub.3
Variable Radius R.sub.4 Equals R.sub.3 Radius R.sub.5 0.060 inch
(1.52 mm) Radius R.sub.6 1.550 inch (39.37 mm) Distance Y + R.sub.3
0.361 inch (9.17 mm) Dome Height H 0.405 inch (10.29 mm) Angle
.alpha. 60.degree. Angle .beta. 25.degree. Angle .gamma.
8.degree.
[0030] A 202 end can having a bottom defined by the geometry
specified in Table I and with a nose 16 having an inner surface 29
with a radius of curvature R.sub.3 of 0.050 inch is known in the
prior art. As shown in FIG. 4, increasing the radius of curvature
R.sub.3 of the nose inner surface 29 to 0.060 inch results in a
dramatic increase in buckle strength. Specifically, the finite
element analysis predicted that, contrary to the conventional
wisdom in the can making art, increasing the nose inner surface
radius from 0.050 inch to 0.060 inch in such a can bottom would
increase the buckle strength by almost 10%, from 95 psi to 104
psi.
[0031] Unfortunately, increases in the nose inner surface radius of
curvature beyond 0.060 inch did not yield continued increases m
buckle strength, but actually reduced buckle strength, although the
buckle strength remained above that obtained with the 0.050 inch
radius of curvature previously employed for such a can bottom.
[0032] In order to check these theoretical predictions, twelve
ounce beverage cans having 202 ends were made using bottom
geometries specified in Table I and shown in FIG. 2 with three
different radii of curvature R.sub.3 for the inner surface 29 of
the nose arcuate portion 18--0.050, 0.055 and 0.060 inch. Cans with
each size radius of curvature were made using two different dome
heights H and from two different types of 0.0108 inch (0.27 mm)
thick aluminum plate--type 3204 H-19 and type 3304C5 H-19 so that,
altogether, there were twelve different types of cans. The cans
were tested for four strength related parameters--(i) buckle
strength, defined as discussed above, (ii) bottom strength,
obtained by measuring the minimum axial load required to collapse
the can bottom when the side wall is supported, (iii) drop
resistance, obtained by dropping water-filled cans pressurized to
60 psi from varying heights, and (iv) axial load, obtained by
measuring the minimum axial load required to collapse the
unsupported can side wall. The results of these tests, which are
averaged for at least six cans of each type, are shown in Table II.
In addition, the penetration depth d at stacking was measured and
is shown in Table III.
2TABLE II Comparative Test Results -- Variable Nose Radius Of
Curvature Bottom Drop Buckle Strength Strength Resistance Axial
Load (psi) (lbs) (inches) (lbs) Type 3204 H-19 Aluminum H = 0.0405
R.sub.3 = 0.050 96.7 273.7 6.7 232.8 R.sub.3 = 0.055 98.3 274.7 6.9
229.6 R.sub.3 = 0.060 103.8 284.7 7.6 205.1 H = 0.0415 R.sub.3 =
0.050 97.7 273.0 6.7 227.6 R.sub.3 = 0.055 99.5 276.7 6.8 231.2
R.sub.3 = 0.060 105.0 283.7 6.8 220.9 Type 3304C5 H-19 Aluminum H =
0.0405 R.sub.3 = 0.050 95.7 268.7 5.9 245.3 R.sub.3 = 0.055 99.5
278.0 5.9 237.8 R.sub.3 = 0.060 100.5 268.3 6.8 245.7 H = 0.0415
R.sub.3 = 0.050 96.7 269.3 6.0 238.8 R.sub.3 = 0.055 99.5 275.7 6.1
242.7 R.sub.3 = 0.060 100.8 272.0 6.3 237.0
[0033]
3TABLE Ill Comparative Test Results - Nose Radius vs. Stacking
Depth Radius of Curvature, R.sub.3 Stacking Depth, d 0.050 inch
0.083 inch 0.055 inch 0.069 inch 0.060 inch 0.062 inch
[0034] The comparative strength test results shown in Table II
confirm the fact that, contrary to the conventional wisdom,
increasing the radius of curvature R.sub.3 of the inner surface 29
of the arcuate portion 18 of the nose 16 on can bottoms of the type
specified in Table I and shown in FIG. 2, at least up to 0.060
inch, increases, rather than decreases, the buckle resistance.
[0035] Unfortunately, as shown in Table III, it was found that
although increasing the radius of curvature R.sub.3 of the nose 16
at its inner surface 29 from 0.050 inch to 0.060 inch dramatically
increased buckle strength, it reduced the depth of penetration at
stacking from 0.083 inch to 0.062 inch. This undesirable aspect,
which compromises the stackability of the can, occurred because
increasing the radius R.sub.3 of the nose inner surface 29 pushes
the nose outer wall 13 radially outward.
[0036] FIG. 5 shows the results of a finite element analysis of a
can bottom having the geometry specified in Table I and shown in
FIG. 2 except that the diameter D.sub.2 of the nose 16 was
decreased as its radius of curvature R.sub.3 at the nose inner
surface increased in the manner shown in Table IV:
4TABLE IV Variation of Nose Diameter With Nose Radius of Curvature
Nose Radius, R.sub.3 (inches) Nose Diameter, D.sub.2 (inches) 0.050
1.904 0.060 1.890 0.065 1.884 0.070 1.877
[0037] As can be seen in FIG. 5, coupling increases in the nose
radius of curvature R.sub.3 with appropriate decreases in the nose
diameter D.sub.2 theoretically results in constantly increasing
buckle strength within the 0.050 inch to 0.070 inch nose radius
range. In fact, the most dramatic increase occurs as the radius of
curvature of the inside surface of the nose is increased from 0.065
inch to 0.070 inch.
[0038] In order to test the theoretical predictions from the finite
element analysis discussed above, twelve ounce cans having 202
ends, and bottoms as shown in FIG. 2, were made from Alcoa 3004
H-19 aluminum plate having an initial thickness of 0.0108 inch
(0.27 mm). Half of the cans were made using a bottom geometry that
is known in the prior art, which is designated A in Table V, and
the other half were made using one embodiment of the geometry of
the current invention, which is designated B. Consistent with the
theoretical analysis discussed above, the two can bottom geometries
differed in two respects. First, contrary to conventional thinking,
the radius of curvature R.sub.3 of the nose 16 at its inner surface
29 was increased to 0.060 inch. Second, the diameter D.sub.2 of the
nose was decreased to 1.890 inch.
5TABLE V Can Bottom Geometric Parameters For Comparative Testing -
Nose Dim. Can Bottom A Can Bottom B Diameter D.sub.1 2.608 inches
(66.24 mm) 2.608 inches (66.24 mm) Diameter D.sub.2 1.904 inches
(48.36 mm) 1.890 inches (45.95 mm Diameter D.sub.3 0.100 inch (2.54
mm) 0.100 inches (2.54 mm) Radius R.sub.1 0.170 inch (4.32 mm)
0.170 inch (4.32 mm) Radius R.sub.2 0.080 inch (2.03 mm) 0.080 inch
(2.03 mm) Radius R.sub.3 0.050 inch (1.27 mm) 0.060 inch (1.52 mm)
Radius R.sub.4 0.050 inch (1.27 mm) 0.060 inch (1.52 mm) Radius
R.sub.5 0.060 inch (1.52 mm) 0.060 inch (1.52 mm) Radius R.sub.6
1.550 inch (39.37 mm) 1.550 inch (39.37 mm) Distance Y + 0.361 inch
(9.17 mm) 0.361 inch (9.17 mm) R.sub.3 Height H 0.405 inch (10.29
mm) 0.405 inch (10.29 mm) Angle .alpha. 60.degree. 60.degree. Angle
.beta. 24.degree. 25.degree. Angle .gamma. 8.degree. 8.degree.
[0039] Comparative testing was again preformed on the two groups of
cans and the results, which are reported as the average for at
least six cans, are shown in Table VI.
6TABLE VI Comparative Test Results - Varying Nose Radius And Nose
Diameter Can Bottom A Can Bottom B Buckle Strength 93.7 psi 100.1
psi Bottom Strength 267.2 lbs 269.7 lbs Drop Resistance 7.3 inches
6.8 inches Axial Load 224.1 lbs 236.8 lbs Penetration Depth d 0.085
inch (2.16 mm) 0.086 inch (2.18 mm)
[0040] As can be seen, the buckle strength of the cans made
according to the current invention was almost 7% greater than that
of the prior art cans (i.e., 100.1 psi versus 93.7 psi). Such an
increase is very significant. For example, it is expected that this
increase in buckle strength will allow the 90 psi buckle strength
requirement commonly imposed by carbonated beverage bottlers to be
satisfied even if the thickness of the initial metal plate is
reduced from 0.0108 inch to 0.0104 inch--a reduction of almost 4%.
Such a reduction in plate thickness will yield a significant cost
savings. The slight reduction in drop resistance is not thought to
be statistically significant.
[0041] The thickness of the metal in the inner chime wall 12 was
also measured for the two types of cans. These measurements showed
that the chime wall thickness for the can bottom according to the
current invention (type B) was 0.0003 inch greater than that for
the can bottom of the prior art (type A)--i.e., 0.0098 inch (0.249
mm) versus 0.0095 (0.241 mm). The increase in chime wall thickness
is also significant because it shows that the current invention
results in less stretching of the metal in the critical chime area
(the more the metal is stretched, the thinner it becomes).
Manufacturing trials have shown that this reduction in metal
stretching reduces the incidence of can failure due to chime
surface cracking.
[0042] Finally, by decreasing the nose diameter D.sub.2, the depth
of penetration d was maintained, thereby ensuring that the increase
in nose radius of curvature did not compromise stackability even in
a can having a relatively small end (i.e., size 202). In this
regard, the relatively small angle .beta. of the nose outer wall 13
(i.e., 25.degree.) also aids in obtaining good penetration. Thus,
according to the current invention, if good stackability is a
requirement, (i) the radius of curvature R.sub.3 of the inner
surface 29 of the arcuate portion 18 of the nose 16 should be
maintained within the 0.060 inch to 0.070 inch range, (ii) the
angle .beta. of the outer wall 13 of the nose should be no greater
than about 25.degree., and (iii) the diameter D.sub.2 of the nose
should be no greater than 1.89 inch for cans having ends of size
202 or smaller.
[0043] Unfortunately, decreasing the nose diameter D.sub.2 will
reduce the tipping stability of the can when oriented in the
upright position. Tipping stability is important since a wobbly can
may not fill properly during processing and may cause an annoyance
to the ultimate consumer. Therefore, it may be undesirable to
increase the nose radius of curvature to values beyond 0.070 inch
in cans having 202 ends, since that would result in nose diameters
less than 1.877 inch if the stacking penetration is maintained
constant. Moreover, although the greatest increase in buckle
strength was obtained with a 0.070 inch value for the nose inner
surface radius R.sub.3, this value also results in the smallest
nose diameter D.sub.2. Therefore, depending on the relative
importance of the stackability versus the tipping stability
requirements, the optimum value of the radius of curvature R.sub.3
of the inner surface 29 of the arcuate portion 18 of the nose 16
may be less than 0.070 inch, such as about 0.060 inch or about
0.065 inch.
[0044] According to another aspect of the invention, the strength
of the bottom 6 can also be increased by careful adjustment of the
radius R.sub.6 of the center portion 24. Specifically, it has been
found that a surprising increase in the drop resistance can be
achieved by reducing the radius R.sub.6. This reduction in R.sub.6
is preferably accompanied by an increase in the diameter D.sub.3 of
the substantially flat central section 26 and an increase in the
dome height H.
[0045] Table VII shows the results of drop resistance and buckle
strength testing for 12 ounce 202 cans having three different
bottom geometries. The bottom geometries were the same as those of
Can Bottom B shown in Table V unless otherwise indicated. Each can
bottom was formed from aluminum (Alcoa 3104) of three different
initial thicknesses on a pilot line. Twelve cans were tested in
each geometry/thickness. The results of tests on these cans are
shown in Tables VI and VII below.
7TABLE VI Comparative Test Results - Varying Dome Dimensions -
Pilot Line Can Bottom B Can Bottom C Can Bottom D Radius R.sub.6
1.550 in 1.475 in 1.450 in (39.37 mm) (37.47 mm) (36.83 mm)
Diameter D.sub.3 0.100 in 0.140 in 0.139 in (2.54 mm) (3.56 mm)
(3.53 mm) Height H 0.405 in 0.405 in 0.410 in (10.29 mm) (10.29 mm)
(10.41 mm) Remaining parameters the same as Table I 0.0108 inch
Thickness Drop Resistance Average 6.07 inches 6.64 inches 8.00
inches Maximum 7 inches 8 inches 9 inches Minimum 5 inches 6 inches
7 inches Buckle Strength Average 99.8 psi 98.2 psi 98.7 psi Maximum
100.4 psi 99.0 psi 99.5 psi Minimum 99.2 psi 97.6 psi 97.5 psi
0.0106 inch Thickness Drop Resistance Average 5.50 inches 6.07
inches 7.29 inches Maximum 6 inches 7 inches 8 inches Minimum 5
inches 5 inches 6 inches Buckle Strength Average 95.2 psi 94.0 psi
94.6 psi Maximum 95.7 psi 95.6 psi 95.8 psi Minimum 94.2 psi 93.2
psi 93.7 psi 0.0104 inch Thickness Drop Resistance Average 4.79
inches 5.79 inches 6.36 inches Maximum 5 inches 7 inches 7 inches
Minimum 4 inches 4 inches 6 inches Buckle Strength Average 94.1 psi
92.3 psi 93.3 psi Maximum 95.9 psi 93.4 psi 93.8 psi Minimum 93.7
psi 91.6 psi 92.3 psi
[0046]
8TABLE VII % Chance In Drop Resistance and Buckle Strength Over
Bottom B Bottom C Bottom D Metal Thickness Drop Buckle Drop Buckle
0.0108 inch +8.6% -1.6% +31.8% -1.1% 0.0106 inch +10.4% -1.2%
+32.5% -0.6% 0.0104 inch +20.9% -1.9% +32.8% -0.8%
[0047] As can be readily seen, by reducing the dome radius R.sub.6
to values no greater than 1.475 inches results in increased drop
resistance. Specifically, reducing the dome radius R.sub.6by 0.075
inches from 1.550 inches to 1.475 inches, while simultaneously
increasing the diameter D.sub.3 of the substantially flat central
dome section 26 by 0.040 inches from 0.10 inches to about 0.14
inches (bottom C), results in an increase in drop resistance of
about 10 to 20% depending on the metal thickness and a reduction in
buckle strength of only about 1 to 2%. Further reducing the dome
radius R.sub.6 another 0.025 inches to about 1.45 inches, while
maintaining D.sub.3 at about 0.14 inches and simultaneously
increasing the dome height H by 0.005 inches to about 0.41 inches
(bottom D) increases the improvement in drop resistance to over 30%
for all three metal thickness without further decreases in buckle
strength.
[0048] In order to confirm these results, 12 ounce 202 cans were
made having bottom geometries B and D, as above, as well as
geometries E and F, defined generally in Table VIII below, at two
different commercial can manufacturing plants from 3004 aluminum
having an initial thickness of 0.0106 inches.
9TABLE VIII Bottom Geometries - Varying Dome Dimensions -
Manufacturing Plants Can Bottom E Can Bottom F Radius R.sub.6 1.55
in (39.37 mm) 1.50 in (38.1 mm) Diameter D.sub.3 0.100 in (2.54 mm)
0.110 in (2.79 mm) Height H 0.41 in (10.41 mm) 0.41 in (10.41 mm)
Remaining parameters the same as Table I
[0049] Twelve can were made in each of the four geometries. The
results of testing on these cans is shown in Table IX below.
10TABLE IX Comparative Tests Results - Varying Dome Dimensions
Bottom B Bottom E Bottom F Bottom D Plant #1 Avg. Height H 0.406
0.411 0.410 0.411 in in in in Drop Resistance Average 5.5 5.3 6.0
6.9 inches inches inches inches Maximum 6 6 7 8 inches inches
inches inches Minimum 5 5 5 6 inches inches inches inches Buckle
Strength Average 96.9 97.5 96.2 96.4 psi psi psi psi Maximum 97.6
98.2 96.0 97.0 psi psi psi psi Minimum 96.0 96.2 94.5 96.0 psi psi
psi psi Axial Load Average 215.7 235.4 239.8 209.1 lbs lbs lbs lbs
Maximum 249 250 257 246 lbs lbs lbs lbs Minimum 192 192 220 184 lbs
lbs lbs lbs Plant #2 Avg. Height H 0.405 0.411 0.411 0.411 in in in
in Drop Resistance Average 6.3 5.75 6.4 6.6 inches inches inches
inches Maximum 7 6 7 8 inches inches inches inches Minimum 5 5 6 6
inches inches inches inches Buckle Strength Average 96.7 96.7 96.7
96.2 psi psi psi psi Maximum 97.6 97.6 97.8 96.9 psi psi psi psi
Minimum 96.0 95.8 95.9 94.9 psi psi psi psi Axial Load Average
224.5 235.4 232.5 223.6 lbs lbs lbs lbs Maximum 238 245 246 232 lbs
lbs lbs lbs Minimum 218 227 180 209 lbs lbs lbs lbs
[0050] Since plant #1 had been running 0.0108 inch thick metal just
prior to the test, it was suspected that the reduction in axial
load for bottom geometry D may have been due to insufficient time
to stabilize the process. Consequently, a second batch of geometry
D cans were run and found to have about the same drop resistance
(6.8 inches average) and buckle strength (95 psi average) but
significantly higher axial load (244 lbs average).
[0051] As can be seen by comparing the test results for bottom
geometry D with those for bottom geometry B, reducing the dome
radius R.sub.6 to 1.450 inches, along with simultaneously
increasing the substantially flat central section diameter D.sub.3
to 0.140 inches and increasing the dome height H to 0.410 inches,
resulted in a 25.5% increase in drop resistance at plant #1,
although only a 4.8% increase at plant #2, with minimal effect on
buckle strength (less than 1%). Also, comparing the results for
bottom geometry E to bottom geometry B shows that increasing the
dome height H without reducing the dome radius R.sub.6 actually
decreases drop resistance.
[0052] Therefore, according to the current invention, in order to
optimize the strength of the bottom of a can, such as a can having
a sidewall diameter of about 2.6 inches (66 mm). the radius R.sub.6
of the dome should be no greater than about 1.475 inches (37.47 mm)
and, more preferably, should be about 1.45 inches (36.8 mm). In
addition, the diameter D.sub.3 of the substantially flat central
section should be at least about 0.14 inches (3.6 mm), and
preferably should equal about 0.14 inches, and the dome height H
should be at least about 0.41 inches (10.4 mm), and preferably
should be equal to about 0.41 inches.
[0053] A preferred apparatus and method for forming the can bottom
6 disclosed above is discussed below.
[0054] In conventional can forming processes, metal stock is placed
into a press in which it is deformed into the shape of a cup. The
cup is then conveyed to a wall ironing machine and redrawn into the
general shape of the side wall and bottom of the finished can.
Next, the redrawn cup is passed through ironing stations that
eventually form the side wall into the final shape of the finished
can. In addition, a bottom forming station is employed to shape the
bottom of the can. A can bottom forming station is disclosed in
aforementioned U.S. Pat. No. 4,685,582 (Pulciani et al.), hereby
incorporated by reference.
[0055] As shown in FIG. 6, an apparatus 41 for making the can
bottom 6 of the current invention comprises (i) a ram 42, (ii) a
nose punch 52, discussed further below, (iii) a substantially
cylindrical punch sleeve 44 encircling the nose punch, (iv) a
centrally disposed doming die 50 having an upwardly convex forming
surface, (v) a support surface 48, (vi) an extractor 46, and (vii)
a central retaining bolt 54.
[0056] In operation, the unformed bottom metal stock is placed over
the punch sleeve 44 and nose punch 52. The travel of the ram 42
then moves the punch sleeve 44 and nose punch 52 toward the doming
die 50 so that the metal stock is eventually pressed against the
doming die forming surface and drawn over the distal surfaces of
the punch sleeve and the nose punch, as shown in FIG. 6, thereby
forming the can bottom 6.
[0057] As shown in FIG. 6, the doming die 50 has a radius of
curvature R.sub.6' that approximates the radius R.sub.6 of
curvature of the dome section 24. The radius of curvature R.sub.6'
is displaced from the axial centerline by a distance X that
approximates one half the diameter D.sub.3 of the substantially
flat central section 26. Thus, in a preferred embodiment of the
invention, the radius of curvature R.sub.6' of the doming die 50
should be no greater than about 1.475 inches (37.47 mm), and more
preferably about 1.45 inches (36.8 mm). In addition, the center of
R.sub.6' should be displaced from the axial centerline by at least
about 0.07 inches (1.8 mm) and the dome height H should be at least
about 0.41 inches (10.4 mm).
[0058] As shown in FIG. 7, according to the current invention, the
distal end 61 of the nose punch 52 has (i) a radius of curvature
R.sub.3' adjacent its inner wall 62, (ii) a radius of curvature
R.sub.4' adjacent its outer wall 63, and (iii) a diameter D.sub.2'.
According to the current invention, (i) the radii of curvature
R.sub.3' and R.sub.4' of the nose punch 52 are equal to the radii
of curvature R.sub.3 and R.sub.4 of the inner surface 29 of the
nose 16 of the can bottom 16 discussed above, and (ii) the diameter
D.sub.2' of the nose punch is equal to the diameter D.sub.2 of the
nose of the can bottom discussed above. Thus, preferably, the
radius of curvature R.sub.3' of the distal end 61 of the nose punch
52 adjacent its inner wall 62 is greater than 0.060 inch. Most
preferably, (i) the distal end 61 of the nose punch 52 is formed by
a sector of a circle so that the radius of curvature R.sub.4'
adjacent the outer wall 64 is equal to R.sub.3', (ii) the radius of
curvature R.sub.3' is also less than 0.070 inch, and (iii) the
diameter D.sub.2' is no greater than 1.89 inch when making a can
having a size 202 end or smaller.
[0059] The present invention may be embodied in other specific
forms without departing from the spirit or essential attributes
thereof and, accordingly, reference should be made to the appended
claims, rather than to the foregoing specification, as indicating
the scope of the invention.
* * * * *