Abstract:
A method for multilayered coating, whereby hurdles for the item that is to be treated are guided for galvanic dip coating ( 2 ) through in a separate circuit. Downstream from the tank, the hurdles are changed over ( 12 ) and the item that is to be treated is guided in another hurdle for electrophoretic coating through a tank ( 4 ) in another separate circuit.

Description:
RELATED APPLICATIONS 
     This application is an application under 35 U.S.C. §371 of PCT/EP99/05410 filed on Jul. 28, 1999, which claims priority to German application 198 36 245.4 filed on Jul. 29, 1998. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a process and an installation for multiple coating, in which the material to be treated is passed through baths in which it is treated chemically, by electrodeposition and/or by electrophoresis. 
     BACKGROUND OF THE INVENTION 
     Processes and devices of this type are used to provide objects which at least partially comprise a conductive material with a surface whose technical or optical properties are adapted to the specific intended use. This can be achieved by chemical or electrochemical treatment of the surface, for example chromating of the surface of a metallic workpiece, and by electrolytic or electrophoretic application of a topcoat. 
     The known wet processes involve a very high level of outlay, since the material to be treated has to be passed through a series of vessels which are arranged in succession and contain a very wide variety of treatment liquids, for example cleaning agents, electrolytes and electrodip coatings. This takes place in continuous throughput installations with conveyors which move the material being treated from dip tank to dip tank through a treatment line. 
     Since large pieces of material to be treated can be guided individually and small pieces, with a view to achieving a homogeneous surface, can only be guided with a limited bed height through the individual baths, conventional installations are extraordinarily complex and have a relatively low throughput. 
     A particular problem is that of achieving a surface treatment which is uniform in all parts, for example a full-area electrodeposition and/or coating without flaws. Flaws of this type result in particular at the contact points between the material being treated and the conveying means for transporting the material being treated through the individual baths. 
     SUMMARY OF THE INVENTION 
     The invention is based on the problem of improving the economics of the known processes and devices for multiple coating without impairing the surface quality of the material being treated. 
     The solution to this problem consists in a process in which trays holding the material being treated are guided, with the aid of a plurality of conveyors, along a circuit from treatment station to treatment station and through the individual bath vessels for coating by electrodeposition or electrophoresis as well as through any pretreatment and subsequent treatment vessels. This takes place in two separate circuits which the material being treated passes through in succession, while the trays in each case only pass through one circuit. This means that a tray change is required when the material being treated is transferred from one circuit into the other. 
     The material being treated is initially pretreated on an electrodeposition tray and is then provided with an undercoat, for example of a zinc-nickel alloy, by electrodeposition, followed by subsequent treatment, for example chromating, conversion coating or passivation. It then leaves the first circuit and is transferred to another tray or into the second circuit, in which a topcoat is applied by electrophoresis, and after the coating operation the material being treated is usually dried. After the drying, the coating trays can be unloaded, cleaned and then again supplied with the material being treated from the first circuit. 
     During the coating, the trays are preferably moved horizontally between the upper and lower anodes or cathodes; this may take place continuously. Preferably, however, the electrodeposition and coating trays are guided in steps at least through the individual bath vessels. 
     The process according to the invention can particularly advantageously be carried out in an installation which comprises a plurality of tray conveyors. These conveyors are arranged in two groups, in each case in such a way that they form a closed circuit for the trays. The two circuits are connected to one another via a tray changer in which the material being treated passes from the electrodeposition circuit into the coating circuit or is transferred from an electrodeposition tray to a coating tray which guides the material being treated through a bath vessel, a dryer, if appropriate with a cooling zone, and preferably through an unloading station and/or a cleaning station. 
     The basis of the tray conveyors in the bath vessels is preferably formed by a lifting and displacement frame which moves the trays in steps through the vessels. The drive required for this purpose may comprise a pushing cylinder and a lifting cylinder which, by means of a finger, engages in a slot in the lifting and displacement frame. In this way, it is possible for a plurality of trays which have been placed in each case on a support, for example bearing rails, to be picked up simultaneously and moved a distance forwards and then put back down again. 
     The trays preferably have lateral bearing arms, by means of which they are supported on bearing rails which simultaneously supply the current to the trays and therefore to the material being treated. Grippers which are connected to the lifting and displacement frame and with the aid of which the trays on the bearing rails are moved, for example pushed, from one position to the next position, are able to act on these bearing arms. To ensure a coating which is as uniform and continuous as possible, the trays preferably move between anodes or cathodes arranged above and below them during the coating by electrodeposition and/or electrophoresis. 
     To prevent the formation of arcs and resultant burning during contacting, the installation may be equipped with two groups of contacts which are each closed alternately and in this way ensure uninterrupted supply of current to the trays. To make this possible, the vessel may be equipped with two groups of contacts. One group is fixedly connected to the lifting and displacement frame and therefore moves with the frame from position to position, while the other group of movable contacts is arranged in a stationary position, for example fixed to the vessel. 
     A tray from a frame with cross-bars and an electrically conductive head wire with an exposed contact strip on which the material being treated rests is particularly suitable for the electrodeposition. The tray may consist entirely of an electrically conductive material; in this case, with the exception of the contact strip, it has an electrically insulating coating. 
     On the other hand, however, the trays may also have a sawtooth bearing edge for the material being treated. A tray of this type is described in German patent 41 42 997, the content of which is deemed to form part of the present description. 
     The pallet changer preferably has linear bearing means, which can be displaced along a bridge and are preferably arranged in a carriage, above a lifting bench or a tray conveyor. 
     The conveyor may also have a lifting and displacement frame, on which in each case one arm of a plurality of two-armed angled levers engages, the other arms of which levers are connected to one another via a linkage. The lifting and displacement frame as well as one of the angled levers may each be provided with a dedicated drive, for example may be connected to in each case one hydraulic cylinder. Gripper arms, which are connected to the lifting and displacement frame, for the trays and bearing rails, which are arranged in the vessel, for the trays may be provided with contact pins for receiving the trays, so that the trays are always energized either via the gripper arm pins or via the bearing rail pins. 
     In order for the trays to be introduced and removed, the bath vessels or their lifting and displacement frames are preferably equipped with a tray manipulator, which may be equipped with a displaceable tilting table and a holding-down device for the material to be treated. 
     Finally, the coating conveyor may also run through an unloading station with linear bearing means, which can be moved in between the bearing elements or cross-bars of the trays, and an unloading station rake which can be displaced along the linear bearing means, and/or a cleaning station with driven cleaning spatulas or displaceable cleaning heads. 
    
    
     The invention is explained in more detail below with reference to exemplary embodiments which are illustrated in the drawing, in which: 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a plan view of an installation for applying an undercoat and a topcoat to the material being treated, 
     FIG. 2 shows a side view of a lifting and displacement frame in a bath vessel, 
     FIG. 3 shows a cross section through the bath vessel shown in FIG. 2, on line III—III, 
     FIG. 4 shows an enlarged view of a part of the lifting and displacement frame with a contact which is fixed to the frame, 
     FIG. 5 shows an enlarged view of a part of the bath vessel with a contact fixed to the vessel, 
     FIG. 6 shows a vertical section through a tray, 
     FIG. 7 shows a section through the tray shown in FIG. 6, on line VII—VII, 
     FIG. 8 shows an enlarged view of part of the tray shown in FIGS. 6 and 7, 
     FIG. 9 shows a diagrammatic illustration of a bath vessel for applying a topcoat by electrophoresis, 
     FIG. 10 shows an enlarged view of the lifting and displacement frame arranged in the bath vessel shown in FIG. 9, 
     FIG. 11 shows a section on line XI—XI in FIG. 10, 
     FIG. 12 shows an enlarged view of the tray support in the bath vessel, 
     FIG. 13 shows a tray with a lifting table beneath it, 
     FIG. 14 shows the tray from FIG. 13 with the lifting table raised, 
     FIG. 15 shows a manipulator for introducing and removing the trays, 
     FIG. 16 shows a diagrammatic view of a tray changer, 
     FIG. 17 shows a diagrammatic view of an unloading station, 
     FIGS. 18 to  20  show diagrammatic views of various cleaning stations. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The installation comprises two circuits, namely an electrodeposition circuit with a bath vessel  1  for pretreatment, a bath vessel  2  for coating by electrodeposition and a bath vessel  3  for subsequent treatment of the material being treated, which is moved through the bath vessels  1 ,  2 ,  3  on trays. The other circuit comprises a bath vessel  4  for coating by electrophoresis and a dryer  5  with a cooling zone  6 , an unloading station  7  and a cleaning station  8 . 
     The material to be treated is moved through the two circuits on trays  9 , for one circuit, and  10 , for the other circuit; this is achieved with the aid of tray conveyors  11  which connect the individual stations  1 ,  2 ,  3  and  4 ,  5 ,  6 ,  7 ,  8  of the installation to one another. The conveyors may be conventional chain, belt or plate conveyors, while special tray conveyors are arranged in the bath vessels. 
     The two circuits are connected to one another via a tray changer  12  which transfers the material being treated from the pallets  9  of the electrodeposition circuit to the pallets  10  of the coating circuit. 
     In the vessel  2  there are an upper anode  13  and a lower anode  14 , between which the trays  9  are guided. This takes place with the aid of a lifting and displacement frame  15 , on which a horizontal pushing cylinder  16  which is fixed to the vessel acts directly and a lifting cylinder  17 , which is likewise fixed to the vessel, acts indirectly via a pivot lever fixed to the vessel. The pivot lever  18  is connected in an articulated manner to a further pivot lever  20 , fixed to the vessel, via a connecting rod  19 . The two pivot levers  18 ,  20  each engage, by means of a finger  21 ,  22 , in a longitudinal slot  23 ,  24  in the lifting and displacement frame  15 . The frame  15  is provided with gripper arms  25 , which are arranged in pairs, and in each case one contact pieces  26  which is situated between the gripper arms, is vertically movable and is spring-mounted. 
     The upper anodes  13  are supplied with current via a bridge structure  27 , while the lower anodes  14  have a power supply which is not shown. 
     The trays  9  have vertical bearing arms  29  with S-shaped holding pieces  30  made from copper, by means of which they are supported hanging from bearing rails  31  of the vessel. The bearing arms  29  serve as supply conductors and are in contact either with the vertically movable contact pieces  26  on one side of the bath vessel  2  or pivotable contact pieces  32  on the other side of the bath vessel. The pivotable contact pieces  32  are situated at that end of an approximately U-shaped angled lever  33  which is pivotably mounted at  34  and are actuated by a pivoting cylinder  35 . 
     In their lowered position, the gripper arms  25  take hold of the contact pieces  30  of the holding arms  29  and, during the pushing cylinder stroke, push them along the conductor rails  31  from one position to another. When a position has been reached, the pivoting cylinders  35  are actuated and the contact pieces  32  come into contact with the oppositely located contact pieces  30  of the holding arms  29 . As soon as this has happened, the piston rod of the pushing cylinder  16  is extended and, at the same time, the lifting cylinder  17  is actuated, so that the gripper arms  15  come off the contact pieces  30  of the holding arms  29 . At the same time, the contact pieces  26  cease to be in contact with the oppositely located contact pieces  30  of the holding arms  29 . However, this does not interrupt the supply of current to the trays, since, during the movement of the lifting and displacement frame to the right in FIG. 2, the stationary contact pieces  32  are in contact with the oppositely located contact pieces  30  of the holding arms  29 . 
     The tray  9  illustrated in FIGS. 6 to  8  comprises a frame  36 , from which the holding arms  29  with the contact pieces  30  extend upward. Cross-bars  37 , the upwardly facing free ends of which are provided with a contact wire  38 , are arranged in the frame. The frame  36  and the cross-bars, and also the contact wire, like the holding arms  29 , have a rubber electrical insulation  39 , which, however, leaves a contact strip  40  exposed on the wire  38 . The material being treated rests on the contact wires  38  and is thus supplied with current via the contact strips  40 . 
     The vessel  4  for the electrophoretic application of a topcoat which is illustrated in FIG. 9 has an associated overflow vessel  41 , a collection vessel  42  and a run-off slope  43 , which leads to the collection vessel  2  and above which two blowers  44 ,  45  equipped with nozzles are arranged. 
     In the vessel  4  there is a tray conveyor with a lifting and displacement frame  46 . This frame is connected to the ends of a plurality of angled levers  47 , the running rollers  48  of which engage in longitudinal slots  49 ; it can be moved to and fro with the aid of an advancing cylinder  50 . The angled levers  47  are, at  51 , articulatedly mounted on the bath vessel  4  and are articulatedly connected to one another by means of a linkage  52 . Moreover, a guide rod  53  and the piston rod of a lifting cylinder  54  engage on the lower lever arm of an angled lever  47 . The lifting and displacement frame  46  can be simultaneously lifted and moved to and fro with the aid of the advancing cylinder  50  and, if appropriate, also the lifting cylinder  54 . 
     Furthermore, the lifting frame is equipped with a plurality of tray manipulators  54 , which are used to introduce the trays into the bath and to remove the trays from the bath. 
     The lifting frame  46  is provided with bearing arms  55 , at the lower ends of which bearing pins  56  are arranged. Furthermore, bearing rails  57  with contact pins  58  on which the trays  10  rest while they are at a set position, as shown in FIG. 11, are situated in the bath vessel  4 . When the trays  10  are being conveyed from one position to the next, the contact pins  56  of the bearing arms  55  pick up the trays, which are thus always energized via the stationary contact pins  58  or the contact pins  56  which are fixed to the frame. Anodes  14  are arranged beneath the trays  10  and tubular anodes  59 , the tubes  60  of which are provided with outlet openings (not shown) for the electrolyte, are arranged above the trays  10 . 
     To enable the material being treated which is situated on the trays to be transferred, a lifting table  61 , which is equipped with contact pins  62  and linear bearing means  63 , is arranged beneath the movement path of the trays  10 . In the event of an upward movement of the lifting table  62 , the linear bearing means  83  engage between the cross-bars of the tray  10  and, in this way, lift the material being treated  64  off the cross-bars, as illustrated in FIG.  14 . In the process, the contact pins  62  of the lifting table  61  pick up the tray  10  from the stationary contact pins  58 , so that the tray is energized without interruption. The material being treated  64  on the tray  10  can be shifted with the aid of a slight relative movement between the lifting table  61  and the tray  10 , so that different contact points are created. 
     The tray manipulators  54  substantially comprise a column  65 , which can be raised and lowered with the aid of a lifting drive  66 . Furthermore, a cable drive  67 , of which a cable  68  is guided over a guide roller  69  at the top end of the column  65  to a holding-down plate  70 , is arranged on the column. At the lower end, the column  65  is connected to a tilting bench  72  which can pivot about a pin  71  and with the aid of which a tray  9 ,  10  can be inclined, so that it can be drip-dried or the material being treated can be immersed in the bath without any bubbles being formed. The tilting bench  72  is actuated with the aid of a driven pivot arm  73  via a Bowden cable  24 . 
     The column  65  can be moved vertically with the aid of the lifting drive  66 , and in this way the tray  9 ,  10  can either be lowered into the bath vessel  1 ,  2 ,  3 ,  4  or moved upward to such an extent that it passes into the region of influence of a removal device  75 . 
     The material being treated can be transferred onto the tray  9 ,  10  of a lifting table  77  with the aid of a lifting table  76 . This takes place with the aid of linear bearing means  78 , which, via a bearing arm  79 , are connected to a carriage  80  on a driven spindle  81 . The linear bearing means  78  move horizontally into the tray situated on the lifting table  77  and, when the lifting table  76  is lowered, pick up the material being treated, which, as a result of further displacement of the linear bearing means  78 , is then deposited on the tray  9 ,  10  of the lifting table  77  of a conveyor arranged in parallel. 
     As shown in FIG. 17, the trays are likewise unloaded with the aid of a lifting table  76  and linear lifting means  78 , of which a rake  83  arranged on a carriage  82  pushes down the material being treated, which, via a chute  84 , passes into a collection vessel  85 . 
     The trays  10  are cleaned with the aid of pivotable spatulas  86  made from thin sheet metal which are arranged on a turning ring  87 , on which the piston rod of a turning cylinder  88  engages. The turning ring is mounted displaceably on a frame  89  and can be moved to and fro with the aid of a pushing cylinder  90 . 
     As shown in FIG. 19, it is also possible for a plurality of spatulas  91  to be arranged in a common frame  92  which can be raised and lowered and for its part is mounted in a framework  94  via a turning ring  93  and is turned by means of an adjustment cylinder  95 . 
     Alternatively, the framework  94  may be provided with rails  95  on which a carriage  96  with rails  97  is displaceable, on which cleaning heads  98  are displaceably arranged. The cleaning heads  98  make it possible to clean the trays  10  with the aid of high-pressure water, a mechanically acting blasting means, liquid nitrogen, laser beams or brushes.