Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a method of
manufacture of tubular bodies, particularly for prcserved
food cans, by rolling up a thin metal sheet (having a
thickness typically not in excess of 0.5 mm) into a tube
and welding the adjacent edges of the rolled sheet.
A conventional process for manufacturing tubular
bodies for preserved food cans consists in rolling up
a rectangular metal sheet to bring the two edges together,
seaming the edges to one another and sealing the junc-
tions by brazing with a tin-lead alloy.
This process has drawbacks: the overlapping
of the opposite sides of the metal sheet constitutes
a loss of material and creates a bead whose edges hinder
the crimping o~ the bottoms of the can and which increases
the number of rejects. The lead contained in the brazing
material contaminate the food products.
Numerous attempts have been made to find a more
satisfactory solution, particularly by electrically
welding overlapping edge regions of the rolled up sheet.
The use of brazing material is avoided and the amount of
overlap may be decreased when elaborate continuous electric
welding with wire-electrode on automatic machines is used.
There remains however an overlap which constitutes a
]ongitudinal bead.
Such a bead is avoided when use is made of the
process described in U.S. Patent Specification No.
4,152,573 (SAURIN et al). According to that process,
a metal sheet is rolled up,until the opposite sides of
the sheet are brought edge to edge without overlapping
and said sides are welded edge to edge by means o~ a
laser beam. There is no extra thickness and no brazing
material. The weld which can be obtained with laser
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welding and with high quality electric welding is of
- such strength that the metal sheet rather than the weld
fails when the can is subjected -to excessive expansion.
It is an object of the invention to make use of that
feature. ~t is another object to reduce the overall
cost of a tubular body for preserved good cans and the
like.
According to the invention, there is provided a
process for manufacturing a tubular body, particularly
for a preserved food can, comprising the steps of
rolling up a metal sheet of small thickness until the
opposite edge portions of the sheet contact, welding
said edge portions by a method which insure a resistance
to tensile forces equivalent to that of the metal sheet
and subjecting the tubular body to radial expansion.
The expansion step provides numerous advantages.
If the tubular bodies must be transported before use,
they may be subjected to the expansion operation on
arrival, which reduces considerably the volume to be
transported. The expansion allows starting with sheet
metal having a thickness greater than that which it is
desired to attain for the can. In particular, when a
thickness from 0.15 to 0.16 mm can be accepted (cans
which have not to withstand successively a high internal
pressure and vacuum during a further sterilizing oper-
ation), one may start with conventional sheet metal from
0.2 to 0.3 mm thick, less costly than the o.i5 mm metal
sheet because of the economy of a second rolling, during
manufacture thereof. The expansion step may be carried
on the same apparatus as welding or on a separate appa-
ratus.
Welded seams, whatever -their method of manufacture,
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may present micro-leaks which, in the case of cans or
- preserved foods, cause impairment of the products. Such
micro-leaks are difficult to detect. sut, when the body
is subjected to expansion, the micro-leak results into
tear easily discernible on inspection and this is an
additional advantage.
For obtaining a high quality weld, the confronting
edge portions of the metal sheet should be accurately
positioned during laser welding, for avoiding overlap,
~0 radial offset and distortion which would result in small
gaps between the edges.
For that purpose, the rolling and welding steps
may be carried out by circulating a metal sheet having a
length much in excess o the length of said tubular body
into an external guide which rolls up said sheet until
its opposite edges are in contact and said edges are
pressed against a stationary internal core, the gap between
the core and guide being selected for preventing over-
lapping of the edges and then through the location where
'O the laser beam is focussed.
The core may occupy the whole cross section of the
tubular body. ~owever, a core may also be provided on
which only the edge regions of the sheet will bear.
The laser beam is advantageously focussed over a diameter
less than 0.05 mm and in the thickness of the edge-to-
edge joint.
This extremely fine focussing allows the heat
required for melting the me al sheet to be released only
there where it is required. The laser energy is used
~0 under the best conditions and the amount of heat in the
metal is kept to a minimum. When tin-plate is used,
focussing has the advantage of only volatilizing the tin
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over an extremely small width, and reducing the risks of
subsequent corrosion. This process also makes possible
to use black steel slleet, which is subsequently coated
with varnish, chrome iron sheet or iron sheet slightly
alloyed with tin on the surface. In this respect, it may
be reminded that varnish does not adhere firmly to the
edge of the sheet iron when there is an overlap, with
the risk of corrosion, particularly from inside in the
case of cans for preserved foods,
The laser will be advantageously a continuous
emission laser, although the use of a pulsed laser may be
contemplated having a sufficiently high frequency for the
welding to be continuous. Typically, the laser used
will emit in the infrared. When it is desired to weld
bodies of cans for preserved foods or drinks, a sheet
0.2 to 0.3 mm thick may be used and a minimum power
density of 5000 kW/cm2 will be generally required in order
to have an acceptable speed of advance. This result may
particularly be reached with a CO2 laser.
The sides of the sheet must have a straightness
tolerance compatible with the dimensions of the focal
spot of the laser, i.e. in practice less than 0.05 mm,
to avoid gaps between the contacting edges. The radial
offset (in the direction of the thickness of the sheet)
shall not exceed 20% of the thickness.
In another embodiment of the invention, the
edges of`an individual metal sheet used for manufacturing
a tubular body are retained in contact during welding by
claws against which the regions of the sheet close to
the edges are retained by vacuum forces, said claws being
located to retain the edges without exerting substantial
abutting forces in the circumferential direction of the
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tubular body~
The invention will be better understood from
the following description of particular
embodiments of the invention. The description refers to
the accompanying drawings, in which:
Figure 1 is a general diagram, in elevation,
of an apparatus for carrying out the process of the
invention;
Figures 2 and 3 are views, on an enlarged scale,
:0 respectively in vertical section and in horizontal section,
of the rolling unit of the apparatus;
Figure 4 is a simplified diagram showing the
relative location of the edges of the metal sheet
during welding;
L5 Figures 5a and 5b are schematic sectional views
of claws for maintaining the edges of the metal sheet in
contact during welding;
Figure 6 is a schematic view of a device for
radially expanding a tubular body.
The apparatus shown schematically in figure 1
is intended to supply tubular bodies from a reel of
strip iron. It will be assumed that the tubular bodies
are to be used as bodies of cans for preserved foods.
The apparatus may be regarded as comprising
successively in the travelling direction of the sheet
iron, a feed unit 10, a shaping unit 11, a welding
generator 12, an expansion unit 13 and a sectioning
station 14.
The feeding unit 10 may comprise a reel of -
:;
strip-iron sheared in width with an accuracy in straight-
ness of approximately 0.01 mm. This strip-iron may be
annealed sheet iron, which may be cut with the required
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accuracy.
- Rolling unit 11 (Figures 2 and 3) comprises
an external guide 15 having for example a slope of
about 14~ on which the sheet iron slides towards the
gap between an outer ring 16! advantageously made from
ceramic material, and an internal core 17, advantage-
ously made from graphitic steel. The internal diameter
of ring 16 and the external diameter of core 17 are
slightly less than the external and internal diameters
of tube 18 to be formed, the difference being the same
over the two diameters. In practice, for a diameter
between 5 and 10 cm, the difference between the diameter
of the ccre and the internal diameter of the tube to be
formed will be less than 10 ~, typically of 8 ~ for she`et
;15 iron 0.2 mm thick.
Due to its conicity, the guide progressively
rolls up the strip-iron until the edges are in contact,
from a point situated in the immediate vicinity of ring
16.
The welding must be effected in the immediate
vicinity of ring 16, to avoid gaping of the edges of
the tube 18, delivered by the rolling unit 11. In
practice, the maximum distance between the point where the
welding is effected and the outlet of ring 16 is about
`~25 100 mm. This distance will however be given as low a
value as possible.
The welding laser generator should concentrate
the energy on a very small size focal spot, so as to
obtain a power density of at leas~ 5kW/cm2. In practice,
the laser power should be 1 kW, at least. Then a welding
speed of the order of 20 m/mn can be obtained and must
be maintained with an accuracy of the order of - 5~.
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The speed may be substantially increased with a more
- powerful laser, of 2 kW for example.
Generator 12 may comprise a CO2 laser supplying
an infrared beam at 10.6 ~. The cylindrical laser beam
will be focussed by a lens whose focal length will
be as short as possible, since reduction of this focal
.,
- length results in an increased tolerance on positioning
in peripheral direction. With a continuous emission
laser of lkW and lens having a focal length of 2.5
~- 10 inches, deviations of up to 0.08 mm in the width direction
are allowed. In all cases, the tolerance h for displacement
in height of the two edges is of the order of 0.2 P.,
` H being the thickness of the sheet iron, and the focal spot
must be formed within the thickness of the weld, between
the faces of the sheet iron.
,~
- The laser need not be placed in the immediate
vicinity of the welding station. In practice, it will
often be advantageous to place it at a distance (for
instance greater than 1 m) and to reflect its cylindrical
output beam towards the welding point where it is
concentrated by the lens. To prevent projections of
material from damaging the focussing lens, the beam is
advantageously directed obliquely in relation to the tube
18 rather than perpendicularly.
The device shown in figure 1 comprises, after
the welding station, a zone in which the welded tube is
guided between driving caterpillar tracks 25 towards the
expansion unit. The essential element of this unit is
an internal mandrel of low conicity (a few percent).
Mandrel 19 is secured to core 17 and remains stationary.
Lubrication of mandrël 19 may be ensured by oil-
feed from the rolling unit. This oil-feed may be effected
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by means of a groove 20 (figure 2) provided in the upper
- part of core 17, finishing in oil-ways ~or distributing
oil on mandrel 19. Groove 20 is located in front of
the laser beam and avoids local heating up of the core.
Tube 18 is removed from mandrel 19 by a constant
speed drive mechanism which may be of any type usually
used, for example in cable making. The mechanism may
comprise several caterpillar tracks, such as those shown
at 21 and 22 in figure 1.
Finally, the apparatus comprises a sectioning
station formed by a shearing machine only the blade 23 of
which is shown, which intervenes when tube 18 arrives
against a stop 24. The tube may be sheared at the station
at a length which is several times the height of an
individual can body. Th~n the sectlons are subsequently
- divided at unit height on a high output machine.
In practice, the expansion carried out on the
device will be generally of approximately 20% of the
diameter.
By way of numerical example, it may be indicated
that an apparatus for manufacturing bodies of cans for
preserved food having a volume of 0,75 1 comprises a
shaping unit about 1 m long, a free zone of 1 m provided
with pulling caterpillar tracks 25, an expansion zone
of 0.60 m, a pulling zone of 2 m equipped with caterpillar
tracks 21 and 22.
The laser generator may be replaced with a
focussed electron beam generator of sufficient power and
energy since the electrons must traverse air between the
output of the ~enerator and the weld location.
The successive work stations may be provided for
supplying tubular bodies whose cross-section is not
circular but flattened (which facilitates transport),
g
11361~
polygonal with rounded angles or even oval or elliptic
; shape (which facilitates guiding of the edge regions).
Rather than a continuous manufacturing process
from a reel of metal sheet, a process for manufacturing
~5 each body individually may be used. Then, each body
- in turn will be expanded on a machine which ma~l be
located immediately after the welding unit.
- Re~erring to figures 5a and 5b, there is shown ;
two successive steps of the welding process. First, a
~,Q metal sheet is rolled up on a conventional rolling
~ machine (not shown) which delivers an open cylinder which
- is biased toward closure by its own resiliency. There
is no need to describe such rolling units since they
can be of the type currently used ~or manufacturing
tubular bodies on electric welding units which use an
electrode wire. The rolling unit is located to deliver
the metal sheet on a mandrel 27 (figure Sa) provided
with a radially directed longitudinal rib of width 1.
The resiliency of the sheet retains the two edges in
!O contact with the rib.
During the welding operation, the edges should
be retained against each other precisely, but without
exerting circumferentially directed forces which would
result in radial distortions of an extent inconsistent
~5 with the low axial depth of the zone in which energy
focussing occurs.
For that result, claws 29 and 30 are used
against which the portions of the sheet close to the
edges are retained by vacuum forces. The claws 29 and 30
O are first applied against the sheet as indicated on figure
5a. Solenoid valves 31 are then energised to connect
rubber cups 32 located in chambers of the claws to a
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``
-
vacuum reservoir 33 provided with vacuum pump 34.
`- Several cups should be provided in each claw and connected
- to the same solenoid valve. The solenoid valves 31 are
of the three_way type and connect the cups to atmosphere
when de-energised. They are located as close as possible
- to the cups ~or reducing the volume which is alternately
under Vacuum and under atmospheric pressure at a minimum.
When the claws have a~hered to the portions of
the sheet close to the edges, both ciaws are moved
together (transversally to the direction of the drawing
when referring to figure 5a). When the claws have moved
beyond the rib, one of the claws is moved toward the
other by distance 1. Then, welding is carried out by the
same process as described in U.S. Specification 4,152,573,
as illustrated on figure -5b. As soon as the tubular
body has moved beyond the welding beam, it is released by
de-energising the solenoid valves 31. The claws are then
brought bac~ to their starting position and grip a
new rolled sheet. Several sets of claws will generally
be provided and moved along a closed path in a way similar
to a merry-go-round.
The expansion step may be carried out on a machine
located immediately after the welding unit. Referring to
figure 6, there is shown a machine which comprises two
pressing rollers 36 and 37 and an expansion roller 35.
The tubular body is inserted between rollers 36 and 37
which are then rotated as indicated by arrows f and roller
35 is moved in the direction of arrow F until the required
expansion rate has been attained. Then the tubular body is
ejected. In practice, several rollers 35 carried by the
same carriage will generally be proviaed.
Such an expanding machine may ~e located in a plant
.
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. ~ ~
-
- separate from the welding apparatus, situated for example
in the ~lant where the bottoms are welded. It allows a
reduction in transport costs, the individual bodies
: being manufactured from sheet metal of a thickness
S greater than the thickness of use (for example close
to 0 50 mm), :hipped and expanded on arrival.
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