Abstract:
A coating apparatus with a spray head is provided for coating can bodies internally. The spray head is supplied with coating material through a feed line which passes through the seam welding machine for the container bodies. The combined spray and extraction head is also connected to a suction line which extracts excess coating material which has not been deposited on the internal walls of the container bodies, and returns it through the welding machine. In this way, the escape of coating material between the individual can bodies can largely be avoided, thus reducing contamination of equipment.

Description:
This is a divisional of co-pending application Ser. No. 08/629,974 filed on Apr. 9,1996 now U.S. Pat. No. 6,110,526. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a method for coating container bodies internally as they emerge in succession from a welding machine. The invention further relates to an apparatus for coating container bodies internally as they emerge in succession from a welding machine. 
     It is known to coat containers or container bodies internally with liquid or powder lacquers. Special problems arise in the coating of container bodies, particularly can bodies, emerging in succession from a welding machine in which the longitudinal seam of the bodies has been welded. If only the welded seam is to be covered, it is known to convey coating material through the welding machine and to apply it to the welded seam. However, if the whole of the internal wall of the container is to be coated, problems arise, as the deposition rate is only approximately 60%, which means that approx. 40% of the coating material, which is in the form of a mist of powder or liquid particles, does not remain, but escapes from the body, contaminating the conveyor line and its surroundings. 
     To give an indication of the level of this contamination, it can be stated as an example that in order to coat the weld seam only on the internal wall of the body of a ½ kg can 114 mm long, approximately 100 mg of powder coating material must be sprayed, of which, assuming the deposition rate to be as stated, 40 mg passes into the environment. With a daily production of 500,000 cans, this results in a considerable quantity of powder being discharged into the environment, and which, despite recycling by an external extraction system, may cause damage eg. to the bearings of the conveyor line, and even to those of the welding machine. If the container body is to be coated over its whole internal circumference, ie. over 360°, the quantity of powder involved will be increased several times more. 
     SUMMARY OF THE INVENTION 
     Accordingly the object which lies at the basis of the invention is to provide a method, which does not possess or which mitigates the drawbacks which have been mentioned, for coating the whole of the internal walls of container bodies over the entire circumference. 
     This is achieved, in a method of the abovementioned kind, by feeding the coating material from the welding machine end and spraying it inside the can body more or less uniformly onto the internal wall, and by extracting sprayed coating material inside the can body for return and returning of the said material via the welding machine end of the spray process. 
     Because extraction of sprayed material takes place inside the can body itself, contamination of the environment with the coating material can be effectively prevented, as only a small amount of material remains to be removed by the external extraction system. The quantity of material which the external extraction system fails to remove is likewise reduced, resulting in a significant diminution in external contamination. The material in question is returned and can be disposed of or reused. 
     Furthermore the gap between adjacent can bodies in the coating area is preferably reduced with respect to the normal conveying gap, eg. by braking the bodies in front. This additional step enables a further reduction to be made in the coating material escaping through the gaps between successive bodies. 
     A further object of the invention is to provide an apparatus for coating container bodies internally in which the disadvantages which have been described can be avoided or reduced. This is accomplished in the abovementioned apparatus by providing at least one coating material feed line through the welding machine and at least one coating material return line through the welding machine, and by providing a spray and suction head connected to the feed and return lines and positioned so as to act on the interior of the can body. With a more uniform application over the circumference, there is a reduction in the amount of powder to be returned. This can be produced by a rotating spray head. Also, it is possible to provide for slightly more powder to be applied at the weld seam, to ensure that the step in level is satisfactorily covered. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described in detail by way of example, with reference to the drawings, in which: 
     FIG. 1 shows an apparatus for coating can bodies internally, in highly schematic form, together with the corresponding equipment for forming and welding the can bodies; 
     FIG. 2 shows a first embodiment of the combined spray and suction head; and 
     FIG. 3 shows a further embodiment of a spray and suction head. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A can body production line, which also includes an internal coating apparatus according to the invention, is shown in FIG. 1 in highly schematic form. This production line comprises a welding machine  1  in which the longitudinal seams of the can bodies are welded in a known manner. Can bodies are supplied to the welding machine in a known manner. Flat metal blanks  4  are removed singly from a stack  3  and are passed to a conveyor  5 . The conveyor  5  conveys the blanks into a rounding unit  2  in which the flat blank is rounded into a can body  6 . From the rounding unit  2  the can body is guided into the welding machine. Inside the welding machine, the can body is guided by a corresponding rail  7  (the so-called “Z-rail”) so that a welded seam, usually a lap weld, can be formed by means of the welding rollers  10  and  9  in a known manner. The lower welding roller  9  is arranged on a welding arm  8  which is located inside the can body, a is the Z-rail  7 . Wire electrodes (not shown) are normally used for the welding. The welded can bodies exit from the welding machine on the conveyor  5 . 
     In accordance with the invention, an apparatus is provided for coating the internal walls of the welded can bodies, not only in the region of the welded seam, but the whole of the internal walls. For this purpose, a preparation and pumping unit  11  is provided for the coating material, which can be stored in a tank  21 . The coating material itself is a known, commercially available coating powder which after being sprayed on to the internal wall of the can body fuses thereon and forms a coating after cooling. Fusion may occur through the residual heat following the welding of the can body, and/or separate heating may be provided. This is known in itself, and will not be described further. Instead of a coating material in powder form, a liquid coating material could be provided. 
     The coating material is fed into the can body from the welding machine end of the apparatus, that is to say, the feed line for the coating material comes from the welding machine. This means that it is necessary for the feed line to be introduced at the can body rounding stage and that it has to be made to pass through the welding machine inside the rounded and welded bodies. Both a feed line  12  for the coating material, and a return line  13  through which a part of the sprayed coating material is extracted back through the welding machine, are provided. In the example shown, extraction is performed by the same unit  11  as that which provides the feed; but other arrangements are also possible. The excess coating material extracted is fed back to the tank  21 , and reused. A combined spray and suction head  14 , connected to the lines  12  and  13 , is provided within the can body itself. 
     According to a preferred arrangement, elements  22  are also provided on the conveyor to reduce the gap between bodies in the coating area, ie. after they.have been transported away from the welding machine, by reducing the speed of the can in front so that the next can catches up and the gap between cans is reduced. In FIG. 1 this is represented schematically, the gap between can bodies measuring dl before the coating area is reached and the smaller amount d 2  in the coating area. At the welding point, the gap may be smaller, eg. 0.8 mm or less. This may be brought about eg. by the braking elements  22 , which are shown only schematically, and which slightly brake the can bodies  6  before and after they leave the coating area so that the ensuing can body  6  (just emerging from the welding machine) closes up. The smaller gap d 2  between bodies results in less excess coating material not being caught for extraction and escaping. This results in a further improvement in the method and apparatus. 
     The reduction in the gap can of course be brought about in other ways, eg. by providing two different conveyors, running at different speeds, one for the can bodies in the coating area and the other for the can bodies before the coating area. Other modifications of the installation shown in FIG. 1 can of course be made within the scope of the invention. Other welding machines, other conveyors, and other alternative means of forming and feeding the blanks  4 , may be adopted. 
     FIG. 2 shows, likewise in a schematic and partly sectioned view, a first example of an embodiment of the combined spray and suction head  14 . In this embodiment the head  14  has a first spray element  15  which cooperates with a second spray element  16  to form a spray nozzle  18 . This spray nozzle may be a single annular nozzle  18 , or several orifices arranged in a ring may be provided. The nozzle  18  is supplied with coating material by the feed line  12 , which, in the example shown, has a connection  20  angled with respect to the conveying direction into a chamber  19  in the spray element  15 . The resulting configuration, with the line  12  connected at an angle into the chamber, allows a particularly good swirl effect on the powder coating material, and thus a homogenous discharge of this coating material from the nozzle  18 , to be obtained. 
     The embodiment shown is, of course, intended only as an example in this regard, and there are many possible alterations of the exemplified spray parts  15 , 16 , 19  and  20  which would also produce a highly uniform discharge of coating material over the entire internal wall of the can body  6 : for example, the conical spray part  16  can be rotated. The spray and suction head  14  also has a suction opening  17  connected to the suction line  13 . Coating material which has not been deposited on the internal wall of the can body, and which is freely suspended inside the body in the form of spray mist, is sucked into this funnel-shaped suction opening  17 . Extraction via the funnel  17  and the line  13  needs to prevent, so far as possible, the spray mist  23  (the presence of which is merely suggested in the drawing) from escaping through the gaps between the successive can bodies  6 , and from contaminating the equipment. 
     The illustrated configuration of the suction funnel  17  is also intended merely as an example. This funnel may be made larger or smaller, or the funnel-shaped configuration of the suction opening may be omitted altogether. The leading edge of the suction opening may also be located at a smaller or greater distance from the spray nozzle  18 . The optimal design for each individual case can be relatively easily determined by experiment. In this example, the extraction always takes place after spraying, that is to say, downstream thereof in the can body conveying direction. 
     FIG. 3 shows a further embodiment of a spray and suction head  14 , in which extraction occurs before spraying, that is to say, upstream thereof in the conveying direction. A spray element  25  is again provided, which, together with an element  26 , forms a spray nozzle  28 . This spray nozzle may again be annular, or consist of individual orifices. The feed line  12  has in this case a straight connection to the spray element  25 , 26 . As shown in the example, a chamber  29  may again be formed in the spray element  25 , and may cause the coating material to be distributed more evenly. Further baffles or swirl elements could also be provided instead of or in addition to the chamber to produce a desired flow and distribution of coating material. 
     In the example shown, the suction element  27  is arranged as a ring surrounding the spray element  25  and forms a suction funnel which is connected to the suction line  13 . The precise configuration of the suction element  27  and its distance from the nozzle  28  may be varied in many ways in this example also. This suction arrangement also serves to extract free, non-deposited coating material back through the line  13 . Also the line  13  could be made annular and coaxial with the line  12 .