Method of and Apparatus for Producing Can Bodies Welded Along the Longitudinal Seam

The invention relates generally to a method of producing can bodies, and more particularly to a method of manufacturing rounded can bodies, welded along a longitudinal seam on a resistance welding apparatus in which overlapping portions of the can bodies are passed through a pair of opposed pressure-roller electrodes and are continuously welded together, whereby the electrodes are connected to a source of electric energy by way of connecting means and the electric energy source generates a substantially rectangular welding current waveform in the welding-electrode rollers.

Sources of current are employed for fabricating rounded can bodies welded along the longitudinal seam and which generate an intermittent or periodic, nearly rectangular current in the pressure-roller electrodes are known in the prior art, as exemplified by West German Offenlegungsschrift No. 16 13 730 (U.S. Pat. No. 3,553,567) and Swiss Pat. No. 483 292 (British Pat. No. 1,180,979). To obtain a weld of the highest quality, alternating current pulses are utilized which have a higher frequency than the standard line or power network frequency. By using an alternating current, the danger of arcing which occurs in d.c. welding and the resultant destruction of the seam to be welded can largly be avoided. However, the use of a higher-frequency alternating current requires a considerable expenditure in circuitry, e.g., for the control of the start and end of the welding operation and for the control of the welding power to be used. However, another important disadvantage of the prior art welding processes which use a higher-frequency welding current is to be seen in the heating of the resistance welding apparatus which is caused by eddy currents. In addition to the electrical losses resulting therefrom, a non-negligible expenditure and technical effort is needed to cool the resistance welding apparatus, with the result that its efficiency is still further reduced. Additionally, in most resistance welding apparatus, special antimagnetic materials must be employed for many components in order to keep the eddy-current induced heating within controllable bounds.

Accordingly, it is an object of the present invention to provide a process for manufacturing rounded can bodies welded along a longitudinal seam, while avoiding the drawbacks of the prior art welding processes, by means of which a continuous welding seam free of interruptions can be achieved whereby a synchronization of the start and end of the welding operation is possible.

These objects are achieved in accordance with the present invention by matching the length or duration of a half wave of the nearly rectangular welding current to the time a can body is passed between the pressure-roller electrodes.

Utilizing the process of the present invention, heating of the resistance welding apparatus caused by inductive eddy currents is largly avoided. A continuous weld is obtained so that one make speak of a quasi d.c. welding in the a.c. welding process according to the present invention since the length or duration of a half wave of the welding current corresponds exactly to the welding time of a can body.

Since it is possible to superimpose a higher-frequency current component upon the nearly rectangular welding current, the energy which is required during the welding operation, which is a function of various factors such as the delivery rate of the can bodies, the thickness of the plates to be welded, etc., can be controlled directly.

The invention will now be described with reference to the accompanying drawings, in which:

FIG. 1a shows a number of rounded can bodies which are conveyed at a predetermined rate;

FIG. 1b shows the waveform of a welding current synchronous with FIG. 1a;

FIG. 1c shows the waveform of a welding current synchronous with FIG. 1a and having a superimposed higher-frequency current component;

FIG. 1d shows the waveform of a welding current synchronous with FIG. 1a without polarity reversal; and

FIG. 2 shows a characteristic curve of a welding transformer in a B/H diagram.

FIG. 1a shows rounded can bodies 10, e.g., can bodies which are passed at a predetermined rate between pressure-roller electrodes of a resistance welding apparatus (not shown) and are welded together along their longitudinal seam. As soon as a can body 10 is passed between the pressure-roller electrodes, the welding current i is started. It reaches its maximum amplitude in a short time, that is to say, with a small time constant, and now remains nearly constant during the time the can body is passed between the welding electrodes. The welding current i is cut off precisely at the end of the welded can body 10. The beginning of the next can body 10 causes the welding current i to be turned on again, but with reversed polarity. The welding operation, as described takes place hereinabove, again as a result of a nearly constant current pulse, the duration of which corresponds exactly to the time during which the can bodies 10 are passed between the pressure-roller electrodes. Thus, with the method of the present invention, an a.c. resistance welding results with a welding-current frequency which is proportional to the delivery rate of the can bodies and to the length of the can body.

The welding current i in FIG. 1b shows a rectangular shape. By controlling the time constant of the welding circuit respectively the magnitude and form of the voltage, the slope (which is to be understood as the change of current per unit of time) can be determined.

To illustrate this with an example, if the deliverly rate is v.sub.1 =50 m/min and the height of the can body is h=125 mm (corresponding to a 1-kilogram can) and if the distance between the can bodies is disregarded, then 6.67 can bodies per second are passed between the pressure-roller electrodes. Thus, the frequency of the welding current i which a welding transformer must transfer from a source of electric enegy at full welding power output is f.sub.s =3.3 Hz.

Since, normally, on a resistance welding apparatus, different heights of can body must be processed, another example will be given. If a delivery speed is v.sub.2 =50 m/min and a can-body height is h=210 mm, then 3.97 can bodies per second are passed between the pressure-roller electrodes. Hence, it follows that under the same conditions as in the previous example the frequency of the welding current i is f.sub.s =2 Hz.

By superimposing a higher-frequency current component on the low frequency a.c. welding current, as shown in FIG. 1c, the regulation of the energy fed to the welding spot which, as is known, is proportional to the square of the amplitude of the welding current i, is possible.

FIG. 1d shows a welding current having no polarity reversal, so that half waves with the same polarity succeed one another.

A corresponding design of a welding transformer respectively measures for the de-energization during the welding pauses, i.e., during the non-welding periods, permit a welding current i as discussed above. The modifications described above for controlling the current shape and the modulation with higher-frequency current components are likewise possible using known modulation and rectification techniques.

In order to make use of this method, an important precondition lies in designing an associated welding transformer such that the lower transferable limit frequency can be selected to be very low without changing the power transfer. In the above example, the lower limiting frequency of the welding current amounts, on the one hand, to f.sub.s =3.3 Hz and, on the other hand to f.sub.s =2 Hz. However, as shown in FIG. 2, these extremely low frequencies can be obtained in a welding transformer only when the working point A.sub.1 on the magnetic characteristic curve is chosen so low that operation outside the saturation range is assured (e.g., in FIG. 2 this is shown as a value below A.sub.2).

The rapid rise of the welding current i mentioned above is achieved by reducing the time constants or the magnitude and form of the voltage, which causes the rise of the welding current according to an e-function in conformity with known principles. As is known, the time constant results from the ratio of the sum of the inductive reactances to the sum of the ohmic resistances in an equivalent circuit diagram of a welding circuit. By reducing the effective inductances, e.g., by arranging the current-carrying conductors, etc. as concentrically as possible, the time constant is effectively reduced.

In the majority of applications, it is a great advantage if the rate of rise and decay of the welding current can be controlled by means of the steps described above. In this way, during the welding of a can body 10 and at the end of the welding operation the welding of a faulty can body 10 can effectively be avoided, since imperfections are prevented by too rapid a rise (which can cause spraying of the material) or by a belated decay (which can cause the creation of a protruding weld bead) of the welding current. By strictly controlling the start and end of the welding operation and the rate of rise and decay of the welding current i, it is possible to reduce the can-body spacing in the region of the pressure-roller electrodes to almost zero.

Thus, on the basis of the foregoing discussion, it can be stated that the method according to the invention permits a favorable combination of the advantages of a.c. resistance welding with those of d.c. resistance welding.

It is to be understood that the above-described arrangements are simply illustrative of the application of the principles of this invention. Numerous other arrangements may be readily devised by those skilled in the art which embody the principles of the invention and fall within its spirit and scope.