Note: Descriptions are shown in the official language in which they were submitted.
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METAL CAN BODY
The invention relates to a can body comprising a beaded, essentially round
cylindrical circumferential metal wall.
Such a can body is known from e.g. EP0780314 disclosing a can-wall with beads
wherein various parameters defining the bead geometry are disclosed. In
particular in
EP0780314 it is proposed to vary the bead length depending on local
susceptibility to
collapsing.
According to the present invention it is an objective to improve can
performance
by improving axial load and panelling properties and to thereby create
opportunities to
reduce consumption of packaging material.
Axial load is to be understood as a load on the can wall caused by forcing the
top
towards the bottom of the can.
Panelling is to be understood as a phenomenon caused by forces acting on the
can wall where the forces are essentially not parallel to the wall, such as
forces
exerted on the can wall if the assembled en closed can is put in a pressurised
vessel.
Although it is generally possible to increase either panelling strength or
axial load
strength, in the known can to a certain extent the one goes at the expense of
the other.
According to the invention it is now possible to produce a can body in which a
suitable combination of axial load strength and local panelling strength can
be
achieved if the wall comprises purposively selected thick and thin walled
annular
sections and the wall is at least partly provided with beads.
Until now, the extent to which one could use the effect of providing beads to
increase the can wall's strength in one aspect was limited by the fact that
the beads
reduce the strength of the wall in another aspect. The inventors have realised
that
increasing the thickness of the material in the region where beading would be
desirable could (more than) compensate for the relevant strength reduction.
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It was found that surprisingly, if a can body comprises a thicker annular wall
portion in combination with beads according to a suitable beading profile,
technically
feasible and improved combinations of axial and panelling strength as required
in
certain packaging cans such as food cans, e.g. made by drawing and wall
ironing
(DWI), can be achieved, and consequently better can performance and new down-
gauging possibilities come into reach.
It should be noted that depending on the dimensions and properties of the
packaging in question, the beads may be horizontal beads, i.e. beads that form
an
"endless groove", the extreme of the "valley" lying in a plane perpendicular
to the
centre line of the can body, one or more spiral beads, or vertical beads.
As the bead profile in particular also the orientation of the beads, will
influence
the increase and/or reduction of the local axial strength and panelling
strength
respectively, the effects of beading and "vary-thickness" can be varied and
optimised
taking also this factor of "shape and orientation of beading" into account.
It is remarked that providing thick and thin walled annular sections in a can
wall
of a drawn and wall ironed can body is known e.g. from EP1294622 and
US3951296.
These publications concern cans with walls having at least one integrated
reinforcing
rib to achieve increased resistance against buckling of the wall and improved
resistance against vacuum.
In a preferred embodiment the can body according to the invention is one
wherein the metal can wall is wall ironed, such as in a DWI can. As it is well
established to mass produce wall ironed cans and as it is possible to perform
wall
ironing in such a way that a thicker annular wall portion is realised and
considering that
beading is a well established method step in can-making, such a can body
according
to the invention represents a very cost effective and reliable new packaging
product.
In a preferred embodiment of the invention the can body is provided with at
least
an annular portion of a relatively small wall thickness provided with
relatively shallow
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beads and an annular portion of a relatively large wall thickness provided
with
relatively deep beads so as to increase the ratio mechanical performance /
metal
consumption.
Mechanical performance should be understood as a combination of both
adequate axial load strength and adequate panelling strength meeting the
applicable
requirements. Metal consumption should be understood as a term that may be
expressed in the form of a volume, thickness or weight of the sheet metal used
for the
making the can body in question and/or of the material forming part of the
resulting can
body.
By providing each annular portion of a can wall with locally optimised
combinations of wall thickness and bead depth, it is possible to achieve a
lower
packaging metal consumption for a certain "mechanical" performance, or
conversely
better performance for the same metal consumption. This renders direct
advantages
of smaller material consumption, and further advantages regarding logistical
and
environmental aspects, e.g. in the form of reduction of weight to be
transported and
recycled in the distribution chain.
In an embodiment the annular portion of relatively large wall thickness is
positioned in the middle region of the wall. In essentially symmetrical
packagings, e.g.
food cans, the middle region of the wall will generally be most susceptible to
panelling.
Providing the annular portion of relatively large wall thickness in the middle
region of
the wall, enables provision of (heavier) beading to the extent required with a
view to
optimising axial load strength and panelling strength locally.
In an embodiment there is an annular portion of relatively small wall
thickness on
either side of the annular portion of relatively large wall thickness. In a
usual
packaging, e.g. a known food can, the top and bottom region of the wall
situated on
either side of the middle region are supported by the lid and the bottom of
the closed
can respectively. In this embodiment, according to the invention, the top and
bottom
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region are additionally supported by the annular portion of relatively large
wall
thickness positioned in the middle region of the wall. As a consequence the
top and
bottom region become less critical regions for the can performance aspects
discussed
here, and a smaller wall thickness can be realised there.
The invention will now be explained in more detail in non-limitative examples
describing experiments that were conducted and results that were obtained.
Reference will be made also to the drawings showing in
Fig. 1 a schematic representation of the wall thickness at various locations
of a
known can body in unbeaded condition;
Fig. 2 a schematic representation of the wall thickness at various location of
a
can body according to the invention in unbeaded condition;
Fig. 3 a known can bead profile and a can wall thickness profile;
Fig. 4 a can bead profile and a can wall thickness profile according to the
invention.
EXAMPLES
In order to establish the effects of the invention, two types of drawn and
wall
ironed cans were used in a set of trials.
One can type is a known standard 0 73 mm 2 piece hO = 110 mm drawn and
wall ironed (DWI) beaded food can and the other can type is a can that is very
similar
to the standard can in appearance and dimensions, but has the features of the
invention.
All cans were made from T57CA standard tinplate, using conventional can-
making processes including drawing and wall ironing (DWI) and beading.
The can body according to the invention was drawn and wall ironed according to
EP1294622 using a stepped punch to realise a "drawn and wall ironed vary wall
thickness can". The wall of the can body was subsequently provided with beads
in a
standard beading machine.
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In order to carry out investigations regarding different bead profiles, the
beading
tools were built up using assemblies of individual beading rings, allowing
variation of
the bead profile and bead groupings.
The key issue was to seek improved can concepts in view of standard food can
5 requirements, namely regarding panelling: the closed can should be able to
withstand
a certain prescribed minimum pressure difference over the can wall (external
pressure
less internal pressure) of e.g. 1.00, as well as axial load strength: the
closed can
should be able to withstand a minimum axial load as defined hereinabove of
e.g. 1500
N, by combining a"vary wall thickness" and a "wall bead concept". The
following
research programme of test runs was carried out:
Thick- Bead profile
Test ness Thickness Nrbeads Nr beads top
run Blank Can wall middle group bottom groups
(mm)
A 0.27 Vary 9 5
B 0.27 Vary 7 6
C 0.27 Vary 5 7
D 0.27 Vary 7 6
E 0.27 Vary 7 6
F1 0.27 Uniform 19
Ga 0.27 Vary 19
H 0.26 Vary 7 6
TABLE 1 Test runs
' F represents a can body according to the state of the art that is
manufactured in an industrial
can-making plant
2
G is a copy of F with vary wall thickness, manufactured using lab can-making
equipment
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In the cans according to the invention, the thicker annular portion of the
wall
produced coincided with the middle bead group, the beads running along the can
wall
being divided into 3 groups located in the top, middle and bottom region of
the can wall
respectively, see e.g. Fig. 4.
Each beading group had a specific bead depth, e.g. for test run B the top and
bottom group (6 beads) 0.28 mm and 0.26 mm respectively, and the middle group
(7
beads) 0.37 mm, see Fig. 4
The resulting panelling and axial load properties follow from table 2 below.
Test Axial Panelling Bead groups
run load Average (Nr. of beads) av. depth
average (mm)
(kN) (bar) Top-middle-bottom
A 1.96 1.25 (5) 0.24 (9) 0.37 (5) 0.25
B 2.04 1.21 (6) 0.28 (7) 0.37 (6) 0.26
c 2.08 1.16 (7) 0.20 (5) 0.35 (7) 0.24
D 1.99 1.23 (6) 0.24 (7) 0.40 (6) 0.24
E 2.21 1.22 (6) 0.22 (7) 0.37 (6) 0.22
F 1.83 1.41 (19) 0.45
G 1.60 1.32 (19) 0.42
H 2.22 1.34 (6) 0.16 (7) 0.42 (6) 0.25
TABLE 2 Results
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Large series of more than 10000 units were produced during trial runs. The
results proved that the new and inventive can bodies allow a packaging steel
gauge
reduction from 0.27 mm to 0.26 mm, the resulting can body according to the
invention
still complying with the applicable can strength requirements, this
representing an
impressive improvement.
The axial load and panelling strength properties of the can body manufactured
from down gauged packaging steel (test run H), viz. axial load 2.22 kN and
panelling
strength 1.34 bar, easily meet even the strictest requirements. Results were
fully
reproducible and consistent throughout the whole production in the trial run.
Test runs B, D and E produced satisfactory results and showed that a marked
increase in axial and panelling performance is achieved in a vary wall
thickness and
vary bead depth food can according to the invention.
Thanks to this, further down gauging of the packaging steel in question to as
thin
as e.g. 0.255 mm and even thinner now becomes possible.
Fig. I in particular schematically shows the left half of a known can body in
unbeaded condition in longitudinal section. For the average can body according
to test
run F, the wall at location h = 1 mm had a thickness of 159 m, at h= 5 mm a
thickness of 160 m, near the middle of the wall at h = 50 mm the material had
a
thickness of 122 m, the bottom of the can being located at hO = 110 mm.
Fig. 2 in particular schematically shows the left half of a can body according
to
the invention in unbeaded condition in longitudinal section. The average can
body
according to test runs A, B, C, D, E, G, at h = 1 mm had a thickness of 140
m, in the
top region at h1 = 28 mm a thickness of 113 m, in the middle region at h2 =
54 mm a
thickness of 138 m, in the bottom region at h3 = 76 mm a thickness of 106 m,
the
bottom of the can being located at hO = 110 mm.
In Fig. 3 the wavy line (black) most remote from the horizontal axis
mentioning h,
represents the outer contour of a can body according to test run F. The
horizontal axis
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represents h (in mm) as shown in Fig. 1. The vertical axis (dimension and
scale not
shown) represents the local can radius: An area above the wavy line lies
outside the
can body.
In Fig. 3 the other line (grey) represents the thickness profile. Again the
horizontal axis represents h (in mm) as shown in Fig. 1. Here, the vertical
axis
(dimension and scale not shown) represents the local material thickness.
As is shown graphically in Fig. 3, the known beaded can has a constant
material
thickness over most of its can wall height, and a constant bead profile over
its beaded
region, running from h = approx. 18 mm to h = approx. 87 mm.
In Fig. 4 the lines represent the same aspects as in Fig. 3, but now for a
typical
can body according to the invention, e.g. a can body according to test run E.
As can be seen in Fig. 4, in a preferred embodiment the can body according to
the invention has in combination a stepped wall thickness ("vary wall
thickness") and
bead groups. The wall thickness e.g. is in the order of 110 m from h = 15 mm
to h=
35 mm, in the order of 140 m from h = 45 mm to h = 60 mm, and in the order of
110
m from h = 70 mm to h < 110 mm. The respective annular thinner and thicker
wall
portions coincide with bead groups having beads with a smaller or larger bead
depth.
For the beads located in the top and bottom regions where h is from 18 mm to
38
mm and where h is from 68 mm to 88 mm, bead depth may be in the order of e.g.
0.25
mm, and for those located in the middle region where h is from 38 to 68 mm,
bead
depth may be in the order of e.g. 0.40 mm.
As a result of the invention, it is possible to increase panelling strength
where
this is most desired, namely in the kind of cans under consideration in the
mid-height
region, but without unacceptably impairing the axial strength, because
additional axial
strength is provided by greater local material thickness.
It will be understood that for different can configurations with regard to
bottom
construction, manufacturing process and lid attachment different thickness and
bead
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profiles may apply and that the resulting effect of the combination of "vary
thickness"
and "vary bead profife" (such as in particular "vary bead depth") may be
optimised on a
case by case basis to find the ideal balance for product performance and
manufacturing effort in view of the specific packaging material to be used,
e.g.
packaging steel (tinplate) or aluminium sheet, polymer coated steel or
aluminium
sheet, the specific packaging variety, e.g. a two-piece DWI packaging, and the
purpose e.g. to realise a heat treatable packaging for preserved foods.
