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 paneling 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.

Paneling 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 paneling 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 paneling 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.

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 paneling 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 paneling 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 U.S. Pat. No. 3,951,296. 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 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 paneling 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 paneling. 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 paneling 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 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 Ø 73 mm 2 piece h0=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.

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 requirements, namely regarding paneling: 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:

TABLE 1

Test runs

Thickness

Bead profile

Test
Blank
Thickness
Nr beads
Nr beads top

run
(mm)
Can wall
middle group
bottom groups

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

G²
0.27
Vary
19

H
0.26
Vary
7
6

¹F represents a can body according to the state of the art that is manufactured in an industrial can-making plant

²G is a copy of F with vary wall thickness, manufactured using lab can-making equipment

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 paneling and axial load properties follow from table 2 below.

TABLE 2

Results

Axial

load
Panelling
Bead groups

Test
average
Average
(Nr. of beads) av. depth (mm)

run
(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

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 paneling strength properties of the can body manufactured from down gauged packaging steel (test run H), viz. axial load 2.22 kN and paneling 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 paneling 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. 1 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 μm, at h=5 mm a thickness of 160 μm, near the middle of the wall at h=50 mm the material had a thickness of 122 μm, the bottom of the can being located at h0=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 μm, in the top region at h1=28 mm a thickness of 113 μm, in the middle region at h2=54 mm a thickness of 138 μm, in the bottom region at h3=76 mm a thickness of 106 μm, the bottom of the can being located at h0=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 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 μm from h=15 mm to h=35 mm, in the order of 140 μm from h=45 mm to h=60 mm, and in the order of 110 μm 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 paneling 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 profiles may apply and that the resulting effect of the combination of “vary thickness” and “vary bead profile” (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.

Metal Can Body

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