EP0270408A1 - Electrostatic painting installation for water paint - Google Patents

Abstract

1. An installation for electrostatic spraying of low-resistivity paint comprising a plurality of primary circuits for circulating paint in a closed loop, each circuit (28, 30) which is connected to earth containing a paint of a different colour, conveying means bearing like objects to be painted which are spaced from each other and displacing them past a plurality of paint sprayers, the latter being positioned to paint respective specific zones of said objects, each sprayer (A, B) being provided with a spray member connected to a high voltage (74) and being supplied with paint by an assembly of valves and intermediate tanks (32, 34) which are insulated from the earth and which can be selectively connected to said primary circuits, characterised in that, said objects being regularly spaced in such a way that the fronts of two consecutive objects are separated by a substantially constant distance along the conveying means : - the number of intermediate tanks (32, 34) is equal to the number of primary circuits, each tank corresponding to such a circuit, - each intermediate tank is connected to the primary circuit corresponding thereto by a controlled connecting means (28, 30), - each intermediate tank is connected to all the sprayers by means of respective controlled selector valves (52A, 54A, 52B, 54B), - the sprayers (A, B) are all substantially disposed in the same plane perpendicular to the path of movement of said objects or in such planes which are approximately separated from each other by said substantially constant distance, and - control means (76) are arranged to provide pilot control in respect of the or some connecting means only when no sprayer is raised to a high voltage.

Description

The invention relates to an electrostatic paint spraying installation and more particularly to a new type of paint called "water", usable on automobile bodies and having a low resistivity. These new paints arouse the interest of the users because the pollution of the environment during their implementation is particularly reduced. The invention aims more particularly to solve the problems posed by the low resistivity of these new paints, when they are used with electrostatic projectors brought to a high voltage and when the paint is supplied via long circuits of paint circulation in closed loop, earthed.

A conventional electrostatic paint spraying installation, in a production unit as large as an automobile manufacturing plant, for example, generally comprises several very long closed-loop paint circulation circuits (these circuits can cross a whole part of the factory) which establish the link between large paint tanks and the different projection booths. It is therefore necessary to provide at least one such circuit per color and another circuit of the same kind, for the solvent or diluent. For obvious safety reasons, these circuits are connected to earth. They will be called for convenience "primary circuits" throughout the description.

Furthermore, in a paint spray booth, the objects to be painted, in this case the bodies in the example envisaged, are carried by conveying means passing through said booth and thus pass opposite several electrostatic projectors positioned at different levels and oriented in different directions, often articulated in relation to their own supports, so as to paint respective areas of said objects. The dimensions and shape of the objects (and especially in the case of bodywork) are such that at least nine projectors are generally required per paint station. It is usual to offset the sprayers longitudinally along the conveying means. However, those skilled in the art have always sought to space the projectors as little as possible because the organic solvent paints used until now dry relatively quickly. However, in order for the layers of paint applied by different and offset projectors to be able to connect satisfactorily, the layer or layers successively applied must still be sufficiently liquid to merge with each other.

Another problem whose data changes with the appearance of new paints is that of color changes. Indeed, in the automobile industry in particular, there is no question of painting the bodies in series according to color. The most frequent case, on the contrary, is that where the color must be changed practically at each bodywork. This implies being able to implement extremely rapid rinsing and drying cycles of the electrostatic projection means. This is generally achieved without too much difficulty with organic solvent paints. Thus, if the installation is designed in such a way that the color change involves an emptying-rinsing-drying operation of a tank capable of receiving paints of different colors, the sequence of color change is necessarily slow and constitutes a serious obstacle to the functioning of the system.

The most important problem to be solved, however, is that which results from the low resistivity of the water-based paint itself. Indeed, when the high voltage is directly applied to the paint spraying member, a spray bowl rotating at high speed for example, the high voltage generator is then connected to earth via the paint itself, since it has a resistivity so low that it establishes an electrical connection of negligible resistance up to the primary circuit.

Various solutions have been devised in an attempt to resolve or circumvent the problems stated above.

It has thus been proposed to connect the projector to earth and to bring the objects to be covered to a high voltage. This is what, for example, the document DEOS 3344703 describes. The invention does not relate to this type of solution which has significant drawbacks and in particular that of having to maintain the object (a bodywork) at a high potential throughout the spray. In addition, dust tends to settle on the object brought to high voltage.

Other solutions consist in not bringing to high voltage, the spraying member in contact with the paint. Ionizing electrodes are then provided near said spraying member. This is the case for example the system described in French Patent ^No. 1574988. The disadvantage of such a system is that the paint particles sprayed leaves the spraying element (the rotary bowl) with an electric charge which the leads to the ionizing electrodes. The ions emitted by these electrodes must then be fixed on the particles and change the electric sign of their charge so that they can then travel towards the object. It has been observed that certain particles do not have time to change their electrical sign during their journey towards the electrodes and that they reach and pollute the latter, which has the effect of making the insulating support of the electrodes electrically conductive and therefore to establish after a certain time a conductive path between high voltage and earth. These anomalies also sometimes result in splashes of paint on the bodies, formed by an agglomeration of paint particles having reached the electrodes.

The invention also does not relate to this category of solutions but seeks to retain the advantages of the electrostatic charge of the paint by the spraying member directly brought to high voltage, that is to say in particular by the bowl of spraying rotating at high speed.

With this in mind, it has previously been proposed to interpose, between the primary circuit and the headlight, "sectioning" reservoirs of the electrical connection, that is to say to carry out an intermediate storage of the paint, capable of be brought to high voltage and isolated from earth. For example, U.S. Patent ^No. 4544570 performs the isolation by spraying the paint from the primary circuit into an intermediate receptacle. This solution introduces the risk of seeing the paint soiled or oxidized by the ambient air or thickened by partial evaporation of the water and co-solvents.

French Patent ^No. 2572662 proposes to fill the intermediate storage of the amount of paint needed just to each application. This operation is carried out in the air. However, each change of color requires, as mentioned above, to clean the intermediate tank and all the pipes connected to it. In addition, the reestablishment of the insulation after filling is done by draining and drying a sufficient portion of conduit, upstream of the intermediate tank. All of this requires prohibitive time each time the color changes. French Patent ^No. 2454846 proposes a similar arrangement to the above but requires a projector, as many intermediate reservoirs as there are colors. In an installation comprising several projectors, teaching it would therefore indiscriminately multiply the number of intermediate tanks, and therefore the cost of the installation.

The invention aims to eliminate all the problems set out above and proposes a new arrangement, not requiring in particular that an intermediate reservoir per color, whatever the number of projectors and also making it possible to minimize the time of color change thanks to a new arrangement of the projectors, relative to each other.

In this spirit, the invention therefore relates to an installation for electrostatic spraying of paint with low resistivity comprising several primary circuits for circulating paint in a closed loop, each circuit, connected to the ground, containing a paint of different color, conveyor means carrying substantially similar objects to paint spaced from each other and moving opposite several paint projectors, the latter being positioned to paint respective specific areas of said objects, each projector being provided with a projection member connected to a high voltage and being supplied with paint by a set of valves and intermediate reservoirs isolated from the ground and selectively connectable to the aforementioned primary circuits, characterized in that said objects being regularly spaced so that the fronts of two consecutive objects are separated by a substantially constant distance along the means of conveying:
the number of intermediate tanks is equal to the number of primary circuits, each tank corresponding to such a circuit,
each intermediate tank is connected to the primary circuit which corresponds to it by a controlled connection means,
- each intermediate tank is connected to all the sprayers by respective controlled selection valves,
the projectors are all substantially situated in the same plane perpendicular to the path of said objects or in such planes approximately separated from each other by said substantially constant distance, and
- control means are arranged so as not to control the or certain connection means only when no projector is brought to a high voltage.

This last characteristic means that the projectors generally have no objects to paint in front of them. The front of the two consecutive objects is defined relative to the direction of movement.

Of course, the case where all the projectors can be grouped in the vicinity of the same plane corresponds to the case of relatively small objects to be painted. For the more difficult case of bulky objects and in particular automobile bodies requiring a large number of headlamps, these will be grouped in the vicinity of several planes, generally two or three, separated from each other by the "substantially constant distance" defined here. -above. In fact, water-based paints remain liquid longer than organic solvent paints, so that the spacing of the projectors can be greater without damaging the connection of the paint layers.

It is understood that with this arrangement, all the projectors are at the same time in a state of painting and at the same time in a state of possibly undergoing a cleaning and drying cycle (when they are between the objects in movement) without waste of time. The intermediate tanks no longer need to be isolated from each other and are brought up to level "on demand", each time the projectors are not facing the objects to be painted, the high voltage being then temporarily lowered or disconnected from all the projectors. Therefore, it should be noted that the invention lends itself well to a simplification of the paint supply means, since only one intermediate tank per color can be common to all the projectors.

• - la figure 1 illustre schématiquement, en élévation, une cabine de projection électrostatique de peinture conforme à l'invention, associée à des moyens de convoyage transportant des carrosseries d'automobiles;
• - la figure 2 est une vue de dessus selon la coupe II-II de la figure 1; et
• - la figure 3 est un schéma de principe de l'installation alimentant en peinture la cabine de la figure 1, simplifiée au cas de deux projecteurs (symbolisant par exemple les deux plans) et de deux couleurs différentes seulement.

The invention will be better understood and other advantages thereof will appear more clearly in the light of the description which follows, of an example of an electrostatic paint spraying installation, in accordance with its principle, given only by way of example and made with reference to the appended drawings in which:

• - Figure 1 schematically illustrates, in elevation, an electrostatic paint spray booth according to the invention, associated with conveying means transporting automobile bodies;
• - Figure 2 is a top view along section II-II of Figure 1; and
• - Figure 3 is a block diagram of the installation supplying paint to the cabin of Figure 1, simplified in the case of two projectors (symbolizing for example the two planes) and only two different colors.

With particular reference to FIGS. 1 and 2, there is shown an electrostatic paint projection booth 11, traversed longitudinally at ground level, by conventional conveyor means 12, carrying objects to be painted spaced apart from one another. other. In this case, the objects 13 are here car bodies. They are therefore, quite bulky to the point of requiring nine paint projectors 15 to 23. Each projector comprises a projection member 26 such as for example a projection bowl rotating at high speed, brought to high voltage.

The projectors 15 to 17 are oriented downwards to paint the upper parts of the body, the projectors 18 and 19 are oriented horizontally and located on either side of the conveying means for painting the lower lateral parts, while the projectors 20 to 23 are oriented and located analogously to the projectors 18 and 19 but to paint the upper lateral parts. Conventionally, the bodies 13 follow one another on the conveying means 12 and are regularly spaced so that the fronts of two consecutive bodies are separated by a distance d substantially constant. This feature is taken advantage of in the context of the invention.

In fact, according to an important characteristic of the invention, the headlights 15 to 23 are divided into two groups A and B, the headlights of these two groups all being substantially located in two respective planes at least approximately perpendicular to the path of the bodies 13 and separated from each other, by the distance d defined above, approximately. Of course, if the number of searchlights justified it, provision could be made to separate them into three or more groups, the searchlights being each time distributed in separate planes of this same distance. On the other hand, if the dimension of the objects to be painted made it possible to have all the projectors in the vicinity of the same plane, this arrangement, completed as indicated below, would be within the scope of the invention. The goal as it clearly appears in FIGS. 1 and 2 is, in fact, to ensure that all the projectors can be at all times in a state, either to paint, or not to paint or possibly to undergo a color change cycle.

Referring to Figure 2, there is shown schematically the entire system for supplying paint to two projectors A and B which symbolize, for example, all the projectors 15 to 19 of group A and all the projectors 20 to 23 of group B, as shown in FIGS. 1 and 2. Installation is also simplified in the case of only two colors. The elements or sub-assemblies whose number depends on the number of projectors or groups of projectors and the elements or sub-assemblies whose number depends on the number of colors will be indicated below. In the example described, there are therefore only two primary circuits for circulating paint in a closed loop 28, 30 (of the type defined above, that is to say connected to the ground) and a primary circulation circuit for solvent 33, analogous to circuits 28, 30. This solvent circuit is unique. Of course, there are as many primary paint circuits as there are different colors.

Projectors A, B can be connected by a set of valves to intermediate tanks 32, 34 isolated from the ground. There is an intermediate tank per primary paint circuit, each tank corresponding to a circuit. Each reservoir 32, 34 is also connected to the primary circuit which corresponds to it by a controlled connection means 38, 40 respectively. In the example, this is a known device forming a quick coupling, such as one of those sold under the brand "STAUBLI". Such a device, when it is disconnected under the action of an automatic control, has its male and female ends sufficiently distant from each other so that there is a perfect fluidic and especially electrical disconnection, without risk of creation of electric arc.

Constant level filling means, comprising a level detector 44 and a valve 46 controlled by the latter, are associated with each intermediate tank. The valve 46 is interposed between the intermediate tank 32 or 34 and the corresponding primary circuit 28 or 30, here downstream of the connection means 38 or 40. More specifically, the level detector 44, sensitive to the level of paint in the intermediate tank corresponding, comprises a pneumatic part supplied by a source of compressed air 48. The valve 46 is controlled by a pneumatic circuit 42 controlled by the detector. Refilling in the intermediate tank is therefore linked to the simultaneous existence of two conditions:
- the connection means 38 or 40 must be connected, and
- valve 46 must be open.

Each reservoir 32, 34 is hermetically closed but can nevertheless be placed either in the atmosphere or under pressure by an electrically controlled valve 50. One of the inputs of this valve is to the atmosphere and the other is connected to the air source 48. Thus, the reservoir is normally kept under overpressure except at times when paint is taken from the primary circuit to perfect the level in the intermediate tank. In this case, the valve 50 is controlled so that the tank is vented.

The paint is taken from the tank by a pump 51. Each tank 32, 34 is connected to all the sprayers A, B by respective controlled selection valves 52A, 52B and 54A, 54B. More specifically, the output of the pump 51 from each reservoir 32, 34 flows into a secondary closed-loop paint circulation circuit 56 or 58, respectively, returning to the same reservoir and connected to the inputs of the selection valves 52A, 52B or 54A, 54B, respectively, establishing the connection with the various projectors A, B.

Thus, the valves 52A, 54A corresponding to the projector A are grouped in a color changing unit 60A while the valves 52B, 54B corresponding to the projector B are grouped in another color changing unit 60B. The outputs of the valves grouped in a color changing unit are connected to a collector 62A or 62B connected to the corresponding projector A or B. Between the output of each color changing unit and the projector, there are inserted conventional sub-assemblies such as a flow regulator 66, a cut-off valve 67 and a discharge valve 68, the output of which is connected to a reservoir. recovery 69, isolated. The valves 67 and 68 are actuated in the usual way during the rinsing and color change cycles.

The primary solvent circulation circuit 32, also connected to earth, is connected to the inputs of as many flushing valves 70A, 70B as there are projectors, by means of a controlled connection means 72 analogous to means 38, 40 described above. Each color change unit thus contains a flushing valve, the outlet of which is connected to the corresponding collector 62A or 62B. If it was necessary to anticipate a color change cycle on certain projectors before the end of the spraying period for other projectors, we could consider adding an intermediate solvent tank connected exactly like the tanks 32 or 34, thereby defining a secondary circuit for circulating the solvent in a closed loop.

Furthermore, the compressed air source 48 is connected to as many push-dry valves 72A, 72B as there are projectors, the outputs of these valves being respectively connected to the collectors 62A, 62B. Each color change unit thus includes such a push-dry valve.

It should be noted that the invention perfectly allows the use of only one adjustable high voltage source 74, common to all sprayers. This is possible because all the sprayers are in use at the same time. This is the solution that has been shown. The high voltage source 74 is conventional, its value is adjustable at any time between zero volts and a nominal voltage of the order of 120kV.

The control of the high voltage source 74 as well as of all the valves described above (with the exception of valves 44) as well as connection means 38, 40 and 72, is operated and synchronized by control means 76 , centralized for example at a control desk. Such control means can have very different structures, electromechanical or electronic; they are associated or not with a computer ... Their design is (once the painting sequences defined) within the reach of those skilled in the art and that is why they have not been described in more detail. A feature, according to the invention, of these control means is that they are arranged (programmed) to control the or certain connection means 38, 40 or 72 only when the high voltage source 74 has previously been brought to zero or at an acceptable residual voltage.

The operation is as follows.

The intermediate tanks 32, 34 are filled with paints of different colors coming respectively primary circuits 28, 30 and the pumps 51 maintain a continuous circulation of paint in the secondary circuits 56, 58. In each color change unit 60A, 60B, paints of different colors are therefore presented at the fluid inlets of the valves selection 52A, 52B, 54A, 54B. It should be noted that, upstream of the projectors, the paint is never sprayed or projected by jet or by gravity, in the open air, from one tank to another. This prevents it from evaporation or degradation and allows it to retain its original qualities. Solvent from the primary circuit 32 is also presented to the fluid inlets of the valves 70A, 70B, while pressurized air is presented to the fluid inlets of the valves 72A, 72B.

When one or two colors are selected and two consecutive bodies pass opposite the headlights A and B, the control means 76 open the corresponding valves 52 or 54 and the high voltage is brought to its nominal value. During all the time which corresponds to the passage of the bodies in front of the headlights or groups of headlights, A and B, the connection means 38, 40 and 72 are electrically and hydraulically "open", that is to say that the connection with the primary circuits is cut.

As soon as the bodies leave the field of the headlights A and B, the control means 76 emit a succession of control signals for:
- quickly reduce the high voltage to zero
- engage the selection valves 52 and / or 54 which were open
- close the connection means 38, 40 and possibly 72 if a color change cycle is necessary. From this moment, the intermediate tanks 32, 34 which were kept under pressure by air coming from the compressed air source 48, via the valves 50, are put into the atmosphere by the change of condition of the latter and the tank (s) whose valves 46 have been preselected by the level detector 44, are leveled during the time interval when the headlights are not facing the bodies. It is also during this time interval that color change cycles are carried out when necessary, that is to say when the bodies which follow one another are not programmed to be of the same color. Such a color change cycle which includes the sequential control of the valves 67, 68, 70 and 72 concerned, is conventional and will not be described in detail. The color change units 60A or 60B are then programmed with different colors when the headlights are returned to service, by actuating the corresponding valves 52 or 54. Before the projectors are put back into service, the connection means 38, 40 and possibly 72 are again opened (disconnected) before the high-voltage source 74 rises again.

Claims (8)

1- Installation of electrostatic spraying of paint with low resistivity comprising several primary circuits for circulation of paint in closed loop, each circuit (28, 30), connected to the ground, containing a paint of different color, conveyor means carrying objects similar to painting spaced from each other and moving them facing several paint projectors, the latter being positioned to paint respective specific areas of said objects, each projector (A, B) being provided with a projection member connected to a high voltage (74) and being supplied with paint by a set of valves and intermediate tanks (32, 34) isolated from the ground and selectively connectable to the aforementioned primary circuits, characterized in that, said objects being regularly spaced so that the fronts of two consecutive objects are separated by a substantially constant distance along the conveying means:
the number of intermediate tanks (32, 34) is equal to the number of primary circuits, each tank corresponding to such a circuit,
each intermediate tank is connected to the primary circuit which corresponds to it by a controlled connection means (28, 30),
- each intermediate tank is connected to all the sprayers by controlled selection valves (52A, 54A, 52B, 54B) respectively,
the projectors (A, B) are all substantially situated in the same plane perpendicular to the path of said objects or in such planes approximately separated from each other by said substantially constant distance, and
- Control means (76) are arranged so as to control the or certain connection means only when no headlamp is brought to a high voltage. 2- Installation according to claim 1, characterized in that each intermediate tank (32,34) comprises constant level filling means comprising a level detector (44) and a valve controlled (46) by the latter, said valve being interconnected between said intermediate tank and the circuit corresponding primary. 3- Installation according to claim 1 or 2, characterized in that a secondary circuit for circulation of paint in closed loop (56, 58) is established between each intermediate tank (32, 34) and the aforementioned selection valve inputs connected to different projectors. 4- Installation according to claim 3, characterized in that each projector (A, B) is supplied with fluids by a color changing unit comprising as many selection valves (52, 54) as there are intermediate tanks, the outputs of these selection valves being connected to a collector (62A, 62B) connected to said projector. 5- Installation according to one of claims 1 to 4, characterized in that it further comprises a primary circuit for circulation of solvent in closed loop (32), connected to the ground and a controlled connection means (72) connecting this solvent circuit has as many flushing valves (70A, 70B) as there are projectors. 6- Installation according to all of claims 4 and 5, characterized in that each color change unit (60A, 60B) mentioned above comprises such a flushing valve whose outlet is connected to the corresponding collector. 7- Installation according to one of the preceding claims, characterized in that it comprises air supply means (48) connected to as many controlled push-dry valves (72A, 72B) as there are projectors and whose outputs are respectively connected to said projectors. 8- Installation according to all of claims 4 and 7, characterized in that each color change unit comprises such a push-dry valve whose outlet is connected to the collector (62) corresponding.

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