METHOD FOR WELDING

Abstract: There is disclosed a method for welding plastic-coated metal comprising approximating the pieces to be welded, removing the coating from the welding site using a laser to heat it and then laser-welding the metal-to-metal contact at the welding site in the absence of the coating.

Description: METHODFORWELDING This invention relates to welding, more particularly to welding plastic-coated metal. Plastic-coated steel is nowadays available in which the plastic has strong adherence to the steel. This would be a good material from which to make canisters, such as food cans, in place of the conventional tin and chromium oxide coated steel on to which natural and/or synthetic lacquers are adhered. This conventional material is lap or butt welded using electrical resistance techniques, after first removing the lacquer layer. The plastic coating of the new steel is, however, much more difficult to remove. If it is incompletely removed, the weld may fail or the electrical welder can become contaminated by the plastic, and this will break the welding electrodes and halt the welding machine, with substantial uneconomic downtime and lost production. The present invention provides a method for welding plastic-coated metal that is not prone to such failure. The invention comprises a method for welding plastic-coated metal comprising approximating the pieces to be welded, removing the coating from the welding site using a laser to heat it and then laser-welding the metal-to-metal contact at the welding site in the absence of the coating. The coating may be evaporated by the laser. The pieces may be butt- or lap-welded. A pulsed laser may be used, of which the mark/ space ratio and/or the pulse shape may be selected to produce a weld of optimum quality. The method may be applied to the welding of metal, particularly steel, canisters. The weld site, after welding, may be coated with plastic. After the weld site has cooled some plastic may be removed from adjacent thereto and redeposited on the metal - this may be done by laser also. Embodiments of welding plastic-coated metal according to the invention will now be described with reference to the accompanying drawings, in which :- Figure 1 is a series of cross-sections of a butt weld being formed; and Figure 2 is a series like Figure 1 of a lap weld being formed. The drawings illustrate a method for welding plastic 11 coated metal 12, e.g. steel, comprising approximating (Figures 1A,2A) the pieces to be welded, removing (Figures 1B,2B) the coating 11 from the welding site 13 using a laser 14 to heat it and then laser welding (Figures 1C,2C) the metal-to-metal contact at the welding site 13 in the absence of the coating 11. The coating 11 may be evaporated by the laser 14 - evaporated plastic may recondense on to cooler material adjacent the weld site, as may droplets of molten plastic. A pulsed laser may be used. Control of the mark/ space ratio of the pulse and/or of pulse shape may allow optimum weld quality to be achieved for different types and thicknesses of plastic and substrate. It may be that removal of plastic is incomplete and that some plastic inclusions contaminate the weld - the degree to which this happens may be controlled by control over the pulsing in order to produce an acceptable weld quality. Another method for controlling the process is to control the laser beam geometry so that different beam energy densities could be used for coating removal and for welding. For certain applications, notably for food cans, it may be important to replace the plastic coating over the weld. Whilst this may be effected in different ways, and whilst it may be satisfactory to use a different coating (a prior-art lacquer, for example) it may well be possible in some circumstances to remove some plastic from adjacent the weld site and redeposit it on the exposed metal after the weld has been effected and the site cooled. A laser - perhaps the same laser - may be used to remove the plastic for this purpose.

Claims: CiAIMS 1. A method for welding plastic-coated metal comprising approximating the pieces to be welded, removing the coating from the welding site using a laser to heat it and then laser-welding the metal-to-metal contact at the welding site in the absence of the coating. 2. A method according to claim 1, in which the coating is evaporated by the laser. 3. A method according to claim 1 or claim 2, in which the pieces are butt-welded. 4. A method according to claim 1 or claim 2, in which the pieces are lap-welded. 5. A method according to any one of claims 1 to 4, in which a pulsed laser is used. 6. A method according to claim 5, in which the mark/space ratio of the pulse is selected to produce a weld of optimum quality. 7. A method according to claim 5 or claim 6, in which the pulse shape is selected to produce a weld of optimum quality. 8. A method according to any one of claims 1 to 7, applied in the welding of metal canisters. 9. A method according to any one of claims 1 to 8, in which the weld site, after welding, is coated with plastic. 10. A method according to claim 9, in which after the weld site has cooled some plastic is removed from adjacent thereto and redeposited on the metal. 11. A method according to claim 10, in which the plastic is removed from adjacent the weld site by a laser.

Inventors: TYRER JOHN, TYRER, JOHN
Applicants: TYRER JOHN, UNIV LOUGHBOROUGH, LOUGHBOROUGH UNIVERSITY OF TECHNOLOGY, TYRER, JOHN
Application year: 1995
Main classification: B23K26
IPC codes: B23K26