Note: Descriptions are shown in the official language in which they were submitted.
CA 02850244 2014-03-27
WO 2013/045263 PCT/EP2012/067620
1
Description
PROCESS FOR CLOSING METAL CANS
Technical Field
[0001] The present invention relates to a process for closing a metal can
body,
suitable for containing an edible product, with a can lid.
Background Art
[0002] Canning is the process of preserving an edible product by processing
and
sealing it in an airtight metal can. Cans are typically either two-piece or
three-piece cans. In the case of a two-piece can, a can body is formed by
punching a metal plate to form a cylinder closed at one end. The can is
then filled and the open end closed by seaming a lid to the can body
during the canning process. In the case of a three-piece can, a can body,
open at both ends, is formed by rolling and seaming a metal plate. A first
end is closed by seaming a lid to the can body. The can is then filled and
the second end closed by seaming a lid to the can body during the
canning process.
[0003] Once a metal can has been filled and sealed, it is typically heated in
order
to cook and/or sterilise the edible product and interior of the can. Heating
the can in this way increases the internal pressure of the can. Cooking
generally takes place within a cooker referred to as a "retort". The retort
pressure is set in an attempt to balance the internal and external
pressures to which the can is subjected, i.e. to minimise the pressure
differential and the stresses to which the can is subjected. This pressure
balancing allows the metal thickness, and therefore costs, of the cans to
be reduced because it minimises the possibility that the cans will fail
during the cooking process. Nonetheless, in some retorts this pressure
balancing is not precise, and cans are still subjected to large pressure
differentials during the cooking process, requiring that the cans have some
minimum level of structural integrity.
[0004] It is noted that, in some canning processes, cans are filled with a hot
product prior to seaming the end closure onto the can body. When the
product subsequently cools, the internal pressure is reduced relative to the
external pressure. It is sometimes desirable to minimise this negative
2
pressure, again as a way of reducing the metal thickness. FR 1,119,542
and in FR 2,753,684 describe can lid structures and canning processes
with the aim of mitigating this problem. Specifically, lids are provided which
deform inwardly, in a concave manner, following seaming and cooling. As
well as reducing the negative pressure, the lids are able to temporarily
return to their original, flat, position when the can and its contents are
subsequently heated (e.g. during a cooking and/or sterilisation process) in
order to minimise the increase in internal pressure. W001/19683,
EP0127155 and US2003/021920 describe methods of sealing containers.
Disclosure of Invention
[0005] It is an object of the present invention to mitigate the problems that
arise
from the extreme positive and negative pressure differentials to which
metal cans are subjected during a cooking/sterilisation process. This
object is achieved by providing an improved process for sealing a metal
can such that the can is able to better withstand extreme pressure
differentials.
[00061 According to a first aspect of the invention there is provided a
process for
closing a metal can body, suitable for containing an edible product, with a
can lid, to provide a metal can suitable for heating in a retort. The process
comprises: placing a metal lid over and in contact with an open end of a
metal can body; applying an inwardly directed mechanical force to the lid
such that at least a central portion of the lid is deformed into the interior
space of the metal can body; and seaming the lid to the can body to form
an airtight seal between the lid and the can body.
[0007] The metal lids of the invention are typically at least semi-rigid and
have a
thickness greater than 100um of metal such as single-reduced steel. Thus
the lids of the invention may be conventional can ends which are fixed to
the can body by double seaming.
[0008] Embodiments of the present invention provide an improved process for
closing metal cans with lids that are able to withstand an increase in
internal pressure without having to increase thickness of the metal used to
form the can body and/or the lid. Particularly preferred embodiments may
provide an improved process for closing metal cans with lids that are able
to withstand an increase in internal pressure even when the thickness of
CA 2850244 2018-07-06
'
3
the metal used to form the can body and/or the lid is reduced close to the
minimum thickness of 100pm. The improved process forms a concave
deformation in the lid which is independent of the filling temperature of the
metal can, and which enables the metal can to better withstand extreme
positive and negative pressure differentials. In a particularly preferred
embodiment, the process can be carried out without requiring existing
canning machines to be modified in order to accommodate a lid with a side
profile of greater thickness.
[0009] The method typically includes providing a seaming apparatus which
comprises a seaming chuck (or "punch") and seaming rolls. The step of
applying an inwardly directed mechanical force to the lid is provided by
moving the seaming chuck into contact with the lid and maintaining that
contact throughout the remainder of the step of seaming the lid to the can
body. Thus the process of the present invention preferably takes place in a
seaming apparatus and the single change part required is that of the
seaming chuck. The lid requires handling independently of the can body
using conventional machinery and prior to being deformed in the seaming
apparatus.
[0010] The central portion of the lid may be deformed inwardly to the extent
that
the headspace volume inside the metal can is reduced by between 50%
and 100%.
[0011] The metal can may be cylindrical and the lid may be circular.
[0012] The lid may be deformed by the inwardly directed force to a generally
more dished shape.
[0013] The lid may comprise one or more beads extending across the surface.
[0014] Prior to being deformed, the lid may be capable of passing horizontally
through a slot having an opening of less than 6mm for a lid of nominally
153mm diameter.
In one aspect, there is provided a process for closing a metal can body,
suitable for containing an edible product, with a can lid to provide a metal
can suitable for heating in a retort, the process comprising:
placing a metal lid over and in contact with an open end of a metal
can body;
CA 2850244 2018-07-06
3a
applying an inwardly directed mechanical force to the lid such that
at least a central portion of the lid is deformed into an interior space of
the
metal can body;
seaming the lid to the can body to form an airtight seal between the
lid and the can body; and
prior to seaming of the lid to the can body, holding a seaming panel
of the can lid off of a flange at the upper edge of the can body using a
plurality of dimples disposed around the seaming panel of the can lid.
Brief Description of Drawings
[0015] Figure 1 is a flow diagram outlining steps of a process according to an
embodiment of the present invention;
Figures 2 and 3 are cross-sections through a lid showing application of a
mechanical forming force in a seaming apparatus;
CA 2850244 2018-07-06
CA 02850244 2014-03-27
WO 2013/045263 PCT/EP2012/067620
4
Figure 4 is the lid of Figures 2 and 3 on completion of seaming;
Figure 5 is the lid of Figure 4 deformed outwardly during a cooking
process;
Figure 6 is a lid that can be seamed to a metal can body and suitable for
use with the process of Figures 1 to 5; and
Figure 7 is a cross-section through the lid of figure 6, prior to it being
seamed to a metal can.
Mode(s) for Carrying Out the Invention
[0016] As previously discussed, the extreme positive and negative pressure
differentials to which metal cans are subjected during a
cooking/sterilisation process can cause the structural integrity of the can to
fail. A new process for seaming a lid onto a can will now be described, with
reference to the figures, that mitigates these extreme positive and negative
pressure differentials and the problems arising from them. The sealed can
resulting from the process is capable of withstanding both negative and
positive internal pressure with respect to an ambient external pressure.
This is facilitated by deforming a substantially flat lid in a seaming
apparatus immediately before seaming the lid onto the can body.
Substantially flat in this context is taken to mean that the overall profile
of
the lid is flat, such that any corrugations in the centre panel profile (i.e.
the
central portion of the lid) do not project above the seaming panel or below
the bottom of the countersink.
[0017] Figure 1 is a flow diagram which describes the steps of a canning
process
which includes a process for closing the metal can according to an
embodiment of the present invention. The steps of the process are as
follows:
1. Transfer the edible product into an empty open metal can body.
2. Place a substantially flat lid over and in contact with the open end of the
can.
3. Apply downwards pressure on central region of the lid to deform the lid
into the interior space of the can and allow displaced air to escape.
4. Seam the lid to the can body.
5. Heat the can in order to cook and/or sterilise the contents.
CA 02850244 2014-03-27
WO 2013/045263 PCT/EP2012/067620
[0018] In Step Al, as there is no fill temperature requirement, the edible
product
can be hot or cold as it is transferred into the can body. The can body may
be intended to form a "two-piece" can, whereby the first piece is the body
of the can which is formed by punching the body from a sheet of metal,
and the second piece is the lid that is used to seal close the open end of
the metal can. Alternatively the can body may be intended to form a
"three-piece" can, whereby the body of the can has two open ends, and is
closed at each open end by a lid. In the context of this process, an
"open can" may be a two-piece can with no lid, or a three-piece can which
is closed by a lid at only one of the two open ends.
[0019] It is noted that for a three-piece can, the first lid may be seamed to
the can
body during application of an inwardly directed force so as to form an
airtight seal between the lid and the can body prior to filling the can body.
This results in a three-piece can with is already closed by a deformed lid at
one end. For this embodiment, steps Al and A5 of figure 1 are omitted.
[0020] Once the open metal can has been filled to the desired level with the
edible product, a substantially flat lid is placed over and in contact with
the
open end of the can. The substantially flat lid may be a lid such as that
described in FR 1,119,542.
[0021] The lid is substantially flat such that it can pass through existing
size
restrictions within standard machines used in a manufacturing and/or
canning factory without any substantial adjustments being required to the
machinery. Typically, size restrictions within such machinery are such that
the lids of nominally 153mm diameter would be able to pass horizontally
through a slot with a height of less than 6 mm.
[0022] The substantially flat lid may include an array of circumferentially
spaced
recesses, known in the canmaking trade as "Swedish dimples" and as
described below with reference to figures 6 and 7.
[0023] The lid is made deformable by circular beads that form an area of
circular
corrugation 2 extending radially inwards from the outside edge of the lid 3
towards a flat central part of the lid 4 and shown in the cross sectional
view of a lid at step A2 of the process in figure 2. At this point, the lid is
loosely placed onto the open end of a metal can body. The dimples 12
CA 02850244 2014-03-27
WO 2013/045263 PCT/EP2012/067620
6
hold the seaming panel 14 up off a flange at the upper edge of the can
body 16. The corrugated portion is shown in area 2 of the lid, and the flat
central portion is shown in area 4 of the lid. Figure 2 also shows a forming
block, seaming chuck, or "punch" 5, which may be part of a can seaming
tool.
[0024] The punch 5 may have a lower surface that conforms to the shaped
profile
of the lid. The punch 5 (which is also referred to as a seaming chuck) is
used to apply a downward force F on the lid 1 in Step A3 and as shown in
Figure 3. The side wall of the metal can body provides an opposing force
to hold the outside edge of the lid in place. By applying a downwards
force, a central region of the lid is deformed part way into the interior
space of the metal can. This deformation gives the lid a generally more
inwardly dished shape.
[0025] Some of the air from the top of the can between the edible product and
the
lid (known as the "headspace") is displaced. At this stage, the lid is only
held in place on top of the metal can body by the punch 5 that is applying
the force F, and is not yet fully seamed to the can body, although the
seaming process may be considered to have been initiated by the contact
of the seaming chuck or punch with the can end. Therefore, the air
displaced from the headspace is able to escape the metal can through the
unsealed gap between the lid and the side wall of the can body.
[0026] The presence of dimples (figure 6, reference 12) prevents build-up of
pressure in the headspace of the can body by venting while the centre
panel of the lid is pushed down during application of the punch. Once the
lid has been deformed by the desired amount, the lid is then fully seamed
to the metal can by folding metal of the lid and the can body together in
step A4 (figure 1) to create an airtight seal between the lid and the can
body. During formation of a double seam, any dimples are hidden within
the seam and serve no further purpose. The force F may still be applied to
the lid while the seaming process is carried out.
[0027] Figure 4 is a cross section through the lid after step A4. The dotted
line 8
in Figure 4 shows the original position and shape of the lid in Step A2, and
the arrow A shows the extent of the concave deformation. The lid is
CA 02850244 2014-03-27
WO 2013/045263 PCT/EP2012/067620
7
deformed to the extent that the headspace volume of the metal can (i.e.
the volume of air in the headspace of the can) is reduced by between 50%
and 100%.
[0028] The can is then heated in a retort in order to cook and/or sterilise
the
edible product inside in Step A5. During the heating process, the lid can be
"pushed out" by the rise in internal pressure within the metal can to form a
convex deformation, or dome, such as that shown in Figure 5. In figure 5,
the dotted line 10 shows the position and shape of the lid after seaming
and prior to the can being heated. The arrow B shows the extent to which
the lid is deformed by the increase in internal pressure.
[0029] Figure 6 shows an example of a lid 1 that may typically be used in the
process according to an embodiment of the present invention as shown in
figure 1. The lid 1 includes dimples 12 in the panel 14 of the lid which will
form part of a seam when the lid is fixed to the can body.
[0030] A partial side section of a lid with Swedish dimples is shown in figure
7. In
the section of figure 7, a dimple 12 is clearly shown in the seaming panel
14.
[0031] The embodiments described herein refer to a metal can body of
circularly
cylindrical shape and a circular lid. However, the can body and lid are not
restricted to these shapes, and may, for example, be generally square
cylindrical and square respectively. Further examples may include elliptical
or rectangular lids, and suitably shaped respective can bodies.
[0032] It will be appreciated by the person of skill in the art that various
modifications may be made to the above described process without
departing from the scope of the present invention. For example, the
process may also comprise in Step A3 applying an inwardly directed force
on a central region of a second lid that is seamed to the bottom of a three
piece metal can to elastically deform the bottom lid into the can to allow a
larger volume of air to be displaced from inside the can prior to seaming
the top lid onto the open end of the can body.
[0033] A further alternative may be that the can body does not contain the
edible
product and is not sealed at the bottom at the time the lid is deformed and
seamed to the top end of the can. In this alternative, as there is no can
CA 02850244 2014-03-27
WO 2013/045263 PCT/EP2012/067620
8
bottom on the can body, once the substantially flat lid has been placed
over and in contact with the top open end of the can body, it can either be
deformed first by a downwards pressure and then seamed to the can
body, or it can be seamed first and then deformed by the downwards
pressure. Even if the lid is seamed to the can body first, the air displaced
when deforming the lid is able to escape the can through the open bottom.
The can body with the deformed top can then be filled through the opening
in the bottom prior to it being sealed with a can bottom.
