Method and apparatus for the seam welding of containers

For butt welding or lap welding container bodies, each body is guided between an outer current-carrying electrode roller and an inner roller. According to the invention the inner roller is movably mounted and exerts an opposing pressure to the rigidly mounted outer roller. This results in improved weld quality.

BACKGROUND OF THE INVENTION 
The invention relates to a method for the seam welding of container bodies 
of thin, preferably tinned, sheet metal. The invention also relates to an 
apparatus for carrying out the method. 
"Thin" or "very thin" sheets are metal sheets with a thickness of 
approximately 0.1 to 0.5 mm. These may be tinned (so-called tinplate), or 
given other coatings, or may be uncoated (blackplate). Welding of 
containers especially of tinplate, and in particular can bodies, with an 
overlapping welded seam using a copper wire intermediate electrode is 
known. Until now this has proved to be the only technically and 
commercially successful method, as the coating of tin on the sheet causes 
severe contamination of the electrode, which in industrial use can only be 
overcome with the constantly self-renewing copper wire electrode. 
Investigations have also been made into the possibility of roller seam 
welding an overlapping seam without wire, with tungsten or molybdenum 
welding rollers (Paul Schindele, Investigations into the welding behaviour 
of tungsten and molybdenum alloys in the resistance seam welding of 
electrolytically tinned very thin sheet, Thesis at Munich Institute of 
Science and Technology, 1983; summary published in "neue verpackung" May 
1984). Such a technique has not found acceptance in practice. 
In the production of welded steel tubes, it is a known process to weld 
without overlap, by means of roller seam welding. In this case, copper 
electrode rollers can be used, since no tinning is present. The steel 
tubes welded by this process have a minimum wall thickness of 0.4 mm, and 
a high force has to be exerted on the tube through pressure rollers at the 
sides of the seam (Deutscher Verband fur Schweisstechnik e.V., Code of 
Practice DVS 2911). Steel tube welding technology has so far not provided 
any stimulus for the welding of cylindrical bodies of very thin tinned 
sheet (with a thickness of eg. approximately 0.19 mm), in which the 
conditions are essentially different and in which--by contrast with tube 
manufacture--no continuous seam is produced. 
However, it has been found that it is also possible in principle to weld 
container bodies of thin sheet without overlap, although it is not easy to 
obtain a uniformly tight joint. Conventionally, when welding a lapped 
joint a spring mounted outer welding roller and a rigidly mounted inner 
welding roller are used. In the case of butt welding--which may be 
performed with or without an intermediate wire electrode--problems occur, 
resulting in a lack of sealing in the welded seam. 
SUMMARY OF THE INVENTION 
The object, therefore, is to provide a welding method and an apparatus 
which enable butt welding of container bodies to be performed with more 
consistent quality of weld. 
According to the invention, this is achieved in a method of the 
abovementioned kind by welding the longitudinal margins of the bodies 
edges to edge, guiding the body between a current-carrying outer roller 
and a movably mounted, non-current-carrying inner roller, and pressing the 
inner roller against the outer roller with a predetermined force. 
Surprisingly, it has been found that much better weld quality can be 
obtained in the case of butt welding if, instead of giving the outer 
welding roller a movable mounting--as in lap welding--the inner welding 
roller is movably mounted. The explanation for this surprising effect 
probably lies in the fact that butt welding is particularly sensitive to 
the forces which are produced by the rapid insertion of the moving body 
between the outer roller and the inner roller. As the inner roller is not 
current-carrying in the butt welding process, and can therefore be made 
with very low mass, the movable mounting of this roller brings about a 
particularly large reduction in these insertion forces. Although an expert 
would hitherto have considered it unnecessary, and undesirable in view of 
the small amount of space available, to provide a movable mounting for the 
inner roller inside the body, the present invention does adopt this course 
and thereby obtains improved weld quality. 
The new method can also be used to advantage in the welding of a lapped 
seam. In particular, this can improve weld quality at the start of each 
welded seam. 
The apparatus for carrying out the method is comprised of an outer welding 
roller with two electrodes and a movably mounted, non-current carrying 
inner roller pressed against the outer roller. 
Making the inner roller of lightweight ceramic material is particularly 
preferred. One particular configuration has an inner roller made in two 
parts, providing a uniform contact pressure on either side of the butt 
joint. It is also preferable for the inner roller to be shaped as a sphere 
.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows schematically part of a can body 1 which is being butt welded 
by the roller seam welding process. This is performed by means of two 
electrodes 2 and 3 which are separated from one another by insulation 4, 
and form a welding roller 5 located outside the body. An inner roller 8 is 
rotatably mounted on a supporting arm 7 on the lower arm of the seam 
welding machine, inside the body 1. This roller 8 is not current-carrying. 
In the illustrated example the roller 8 comprises two disks 9 and 10 of a 
hard metallic material and an insulating centre layer 11. Furthermore, 
calibrating rollers 12 are arranged outside the body in a known manner, 
ensuring the roundness of the body. In addition to the calibrating rollers 
12 shown in the drawing, other calibrating rollers, not visible in the 
drawing, are usually provided around the circumference of the body 1. 
The body is guided between the welding roller 5 and the opposite roller 8 
so that the edges of the body are butted together, without overlapping. 
This can be effected by suitable guide rails (so-called Z-rails), which 
are known in themselves from the forming of overlapping seams, and are 
therefore not described or illustrated here, but which are in this case 
set so that no overlapping occurs, and the two longitudinal margins of the 
body are pressed together edge to edge. This is achieved with an H-shaped 
rail. The guide rail may bring the edges of the body together so that they 
are level with each other but are still at a slight gap of eg. 0.1 mm from 
each other. 
The inner roller 8 is movably mounted so that it is able to execute a 
movement in the vertical direction, as indicated in FIG. 1 by the arrow A, 
when the body 1 is pushed between the roller 8 and the roller 5. The 
roller 8 is urged towards the rigidly mounted roller 5 with an adjustable 
force, which may be applied eg. by a spring or by pneumatic means. The 
outer roller 5 is normally rigidly mounted in this arrangement, thus 
reversing the situation which applies in lap welding, where this roller 
would be spring-mounted. However, it is also possible in principle for the 
roller 5 to have a movable, sprung mounting, so that both rollers are 
movably mounted. 
FIG. 2 shows in highly schematic form a first embodiment of the movable 
mounting of the inner roller 8. The outer roller 5 has a rotatable 
mounting which is fixed with respect to the machine; this mounting is not 
shown here. For the movable mounting of the roller 8, a rocker 15 is 
linked by a pivot 16 to the conventional lower arm 14 of the seam welding 
machine. The rocker 15 is therefore movable, and it carries the roller 8. 
To urge the roller 8 towards the roller 5 under spring loading, a 
compression spring 17 is provided, as shown in FIG. 2. This spring bears 
at one end on the rocker 15, and is acted on at its other end by the head 
of a screw 18 the other end of which has a screw thread 20 and is screwed 
into the lower arm 14. By varying the depth of insertion of the screw 18 
into the lower arm 14, it is possible to adjust the spring force acting on 
the rocker 15 and hence on the inner roller 8 for the welding of the body 
1. 
FIG. 3 shows another embodiment in a perspective view. In this case only 
the lower arm 24 is shown, with the inner roller 8, which consists of two 
parts 8' and 8". In this example the movable mounting and springing of the 
roller are obtained pneumatically by the provision of a chamber, supplied 
with compressed air at a pressure of eg. 6 bar, under the two roller 
halves 8' and 8". The two roller halves are able to move independently of 
one another. The force exerted by the inner roller 8 can be adjusted by 
varying the air pressure. The air pressure may be varied while the seam 
welding machine is in operation, and may eg. be lower at the start (and if 
need be at the end) of each body than in the remainder of the body 1. This 
provides a means of controlling the pressure of the inner roller 8 to 
adapt the force which it opposes to the outer roller to the changing 
conditions during welding. Control may of course also be electrical or 
electromagnetic, in which case an arrangement similar to that of FIG. 2 
may be used, the force of the spring being varied eg. by the force of an 
electrical actuator or of an electromagnet, or the spring itself being 
replaced with an electrical actuator or electromagnet. In this way, a 
constant force opposing the current-carrying outer roller can be produced. 
This force control also makes it possible to reduce wear in no-load 
running (during run-up without can bodies) by reducing the pressure of the 
inner roller. 
FIG. 4 shows a further embodiment, in which the same reference symbols have 
been used to refer to the same parts as before. FIG. 4 shows, in highly 
schematic form, that a pressure roller 25 can additionally be provided 
outside the body 1, opposite the rollers 5 and 8. This roller 25 is 
spring-mounted, and its spring force assures a symmetrical loading of the 
butt joint. 
FIGS. 5a and 5b show the inner roller arrangement in a further embodiment, 
viewed in elevation and in section along the line 5b--5b, respectively. A 
rocker 35, which is acted on by the compression spring 27, is mounted on 
the arm 34 so that it is movable about the pivot 26. The rocker 35 carries 
the shaft 30, on which are mounted bearings 29 and the inner roller 28 
which opposes the welding roller 5. 
FIGS. 6a and 6b show in a similar way to FIGS. 5a and 5b, a further 
embodiment with the same reference symbols denoting the same parts. The 
spring force is exerted by a pneumatic cylinder arrangement 33. 
FIGS. 7a and 7b show a pneumatically sprung inner roller 38 to oppose the 
welding roller 5, with a compressed air feed 36, which forms a cushion 37 
of compressed air under the roller 38, provided on the arm 34. 
FIGS. 8a and 8b show another embodiment similar to that of FIGS. 7a, 7b, 
provided with an inner roller which is in two parts 28' and 28". 
FIGS. 9a and 9b show an embodiment with a pneumatically sprung spherical 
inner roller 39 for opposing the welding roller 5. This embodiment results 
in less wear of the inner roller. The embodiment shown in FIGS. 10a,10b 
additionally has a slip ring 40 for the ball 39. 
The embodiments shown may also be used for welding an overlapping seam, 
with the inner roller functioning as a current-carrying welding roller, 
with intermediate wire electrode, in a known manner.