Patent Publication Number: US-9901941-B2

Title: Electrostatic spray device for spraying a liquid coating product, and spray facility comprising such a spray device

Description:
This is a National Stage application of PCT international application PCT/EP2014/057995, filed on Apr. 18, 2014 which claims the priority of French Patent Application No. 1353485 entitled “ELECTROSTATIC SPRAY DEVICE FOR SPRAYING A LIQUID COATING PRODUCT, AND SPRAY FACILITY COMPRISING SUCH A SPRAY DEVICE”, filed with the French Patent Office on Apr. 22, 2013, both of which are incorporated herein by reference in their entirety. 
     The present invention relates to an electrostatic spray device for spraying a liquid coating product that comprises, inter alia, means for bringing the liquid coating product to a zone for spraying that product in the form of droplets. The invention also relates to a spray facility for spraying coating product that in turn comprises at least one such sprayer. 
     In the field of the electrostatic spraying of coating products, it is known to use an electrostatic field to improve the deposition performance during the spraying of coating product in the form of sprayed droplets. 
     In the case of a so-called “internal” or “contact” charge, the coating product comes into contact with an electrode brought to a non-zero electric potential, such that each droplet of coating product sprayed is assigned an electrostatic charge q when it detaches from the rim of a rotating bowl. When such a droplet thus charged is subjected to an electrostatic field with intensity E, that droplet undergoes a force F with intensity q*E when it detaches from a film of coating product. Such a charge mode causes little dirtying on the sprayer because the electrostatic and aeraulic forces that are applied on the droplets are all oriented in the same direction, i.e., toward the object to be coated. One drawback of this charge mode lies in the fact that, if the coating product is conductive, which is in particular the case for hydrosoluble coating products, it is necessary to isolate the sprayer brought to the high voltage from the supply system for supplying coating product at the earth potential. To do that, it is known, for example from EP-A-0,274,322, to use one or more reservoirs onboard a multiaxial robot, which is globally satisfactory, but makes a facility for spraying coating product that incorporates such a system more complex. 
     In the case of the so-called “external” or “Corona” charge, the droplets of coating product that leave the rotating bowl pass in the vicinity of electrodes brought to a non-zero electric potential, such that they encounter ions bombarded by the electrodes and end up being electrostatically charged and attracted by the object to be coated, which is at the earth potential. This charging mode makes it possible to keep the coating product at the earth potential for spraying, without risk of short-circuiting the generator. It is, however, very sensitive to dirtying of the electrodes and the deposition performance depends on outside conditions such as humidity, outside temperature, spraying speed, etc. 
     It is known from JP-A-11,276,937 to equip an outer surface of a bowl of a sprayer with electrodes made from a semi-conductive material and that are charged without contact with a point-shaped electrode. 
     It is also known from EP-A-2,213,378 to use two series of electrodes mounted on a stationary body of a rotating sprayer, those two series of electrodes being respectively supplied by two voltage sources. 
     In the known materials, lines of an electrostatic field used to transport droplets of coating product can reform on a charge electrode of that product, which decreases the effectiveness of the charge and the transport phenomenon. 
     The present invention aims to offset the drawbacks of the “internal” and “external” charge modes considered above, while being applicable to electrically conductive coating products and avoiding closing of the transport field lines on a charge electrode. 
     To that end, the invention relates to an electrostatic sprayer for spraying a liquid coating product, that sprayer comprising a rotating bowl and means for driving that bowl around a rotation axis, the bowl defining a concave surface for distributing the coating product and a rim that delimits a spraying zone for the coating product. The sprayer is equipped with at least one first ionizing charge electrode of droplets of coating product, the ionizing charge electrode being positioned, relative to the rim and along the rotation axis, opposite the spraying zone, between that edge and the means for driving the bowl, and at least one second electrode for creating an electrostatic field to transport droplets toward an object to be coated, that second electrode being mounted on a fixed body of the sprayer. According to the invention, the sprayer comprises a third electrode, also mounted on the stationary body and that is brought to an intermediate electric potential between those of the first and second electrodes during the operation of the sprayer. 
     Owing to the invention, the droplets of coating product that leave the rim of the bowl can be electrostatically charged effectively, which makes it possible to next use an electrostatic phenomenon to steer those droplets toward an object to be coated, within a facility comprising such a sprayer. 
     According to advantageous but optional aspects of the invention, such a sprayer may incorporate one or more of the following features, considered in any technically allowable combination:
         The electrode is mounted on the bowl, radially around the inner part of the bowl that defines the concave distributing surface.   A ring made from an insulating or semi-conductive material is inserted, axially along the rotation axis, between the electrode and the spraying zone.   The ring defines the spraying rim.   The ring is inserted between the electrode and an inner part of the bowl defines the spraying rim.   The ring defines a portion of the outer radial surface of the bowl, between an edge of the electrode turned toward the spraying zone and the spraying rim.   The inner part of the bowl is made from metal.   The electrode is provided with at least one ionizing raised portion, in particular ionizing points.   The electrode is positioned in an air outlet skirt orifice toward the spraying zone.   The sprayer comprises differentiated control and supply means of the ionizing charge electrode, the second electrode and/or the third electrode.   A ring made from an insulating or semi-conductive material is inserted, along the rotation axis, between the second electrode and the third electrode.   A cap made from an insulating or semi-conductive material is inserted, along the rotation axis, between the first and second electrodes, in particular between the first and third electrodes.   The third electrode is inserted, along the rotation axis, between the first electrode and the second electrode.   During operation, the electric potentials applied to the second and third electrodes have the same sign.       

     The invention also relates to a facility for spraying a liquid coating product that comprises at least one sprayer as mentioned above. 
    
    
     
       The invention will be better understood and other advantages thereof will appear more clearly in light of the following description of three embodiments of a sprayer according to its principle, provided solely as an example and done in reference to the appended drawings, in which: 
         FIG. 1  is a diagrammatic block diagram of an electrostatic facility for spraying a coating product according to the invention comprising a rotating sprayer according to the invention, 
         FIG. 2  is a partial and enlarged sectional block diagram, along line II-II in  FIG. 1 , 
         FIG. 3  is an enlarged view of detail III in  FIG. 2 , 
         FIG. 4  is a sectional view similar to  FIG. 2  for a sprayer according to a second embodiment of the invention, 
         FIG. 5  is a sectional view similar to  FIG. 2  for a sprayer according to a third embodiment of the invention. 
     
    
    
     The facility  1  shown in  FIG. 1  comprises a conveyor  2  able to move objects O to be coated along an axis X 2  perpendicular to the plane of  FIG. 1 . In the example of the figures, the object O moved by the conveyor  2  is a motor vehicle body. 
     The facility  1  also comprises an electrostatic sprayer  10  of the rotary type, and which comprises a bowl  20  forming a spraying member and supported by a body  30  inside which a turbine  40  is mounted for rotating the bowl, around an axis X 30  defined by the body  30 . The turbine  40  comprises a stator  41  and a rotor  42 . Reference X 20  denotes the central axis of the bowl, which is combined with the axis X 30  in the configuration with the bowl mounted on the turbine  40 . The body  30  is considered to be stationary because it does not rotate around the axis X 30  when the sprayer  10  is operating. 
     In the present description and irrespective of the embodiment, the front of the sprayer  10  is oriented toward the object O to be coated. Thus, for example, a front part of the sprayer is closer to the object O than a rear part. 
     The body  30  also contains a high-voltage unit  50  connected to the rotor  42  by a high-voltage cable  51  and supplied by a high-voltage generator that is not shown, but is known in itself. A supply line  60  for supplying the bowl with liquid coating product is also provided in the body  30 . This line is connected to a supply source for supplying coating product at the earth potential. 
     The body  30  is optionally vertically movable, as shown by the double arrow F 1 , which allows it to perform a sweeping movement. It can also be mounted at the end of the arm of a multiaxial robot. 
     The sprayer is used to create a cloud N of droplets of coating product and to steer that cloud toward the object O, while depositing a layer C of coating product on that object, the thickness of that layer being exaggerated in  FIG. 1  to make it easier to see. 
     The structure of the bowl  20  can be seen in  FIG. 2 . It includes a body  21  that defines a surface  212  for distributing the liquid coating product up to a spraying rim  214 . The body  21  is made from an electrically insulating material, for example polyether ether ketone (PEEK). The bowl  20  also comprises an outer frame  22  made from metal. A ring  24  is inserted, along the axis X 20 , between the front of the bowl  20  and an edge  225  of frame  22  that is oriented toward the front of the bowl  20 . The ring  24  defines a rim  242  for spraying the coating product. 
     The material of the ring  24  is described as semi-conductive and has a resistivity that allows the flow of electric charges. This resistivity is such that, when a part made from that material is subject to a potential difference U, it is traveled by a current I that is sufficient to slow the electrostatic surface charges. That current I is lower than the maximum current that can be delivered by the generator. 
     Within the meaning of the present invention, a semi-conductive material has a resistivity comprised between 10 6  and 10 14  ohm·cm. According to a more restrictive definition, it may be considered that the resistivity of a semi-conductive material is comprised between 10 7  and 10 13  ohm·cm, or even between 10 9  and 10 11  ohm·cm. Thus, the electrical properties of a semi-conductive material are clearly different between a conductive material, the resistivity of which is traditionally considered to be less than 10 3  ohm·cm, and an insulating material, the resistivity of which is traditionally considered to be greater than 10 15  ohm·cm. 
     As an example, the ring  24  may be made from polyamide filled with carbon fibers, polytetrafluoroethylene (PTFE) filled with conductive particles, or polyether ether ketone (PEEK) filled with conductive particles. 
     A metal deflector  26  is mounted in a central part of the bowl  20  and makes it possible to deflect the flow of coating product, coming from the line  60  through an injector  32  and centered on the axis X 30 , toward the surface  212 . 
     Alternatively, the deflector  26  may not be made from metal. 
     Reference Z 2  denotes a zone bordered by the spraying rim  244  and which extends, from that rim and along the combined axes X 20  and X 30 , moving away from the deflector  26 , over an axial distance smaller than 10 mm, preferably approximately 5 mm. This zone Z 2  constitutes a spraying zone for the liquid coating product, in which droplets  3000  of coating product form, as explained below. 
     The rotor  42  of the turbine  40  is made from metal and connected to the cable  51 , which makes it possible to bring it to the high voltage when the high-voltage unit  50  is active. Since the frame  22  is made from metal, and is therefore electrically conductive, and in contact with the rotor  42 , it is also brought to the high voltage in that case. As an example, it is considered that, during operation of the sprayer  10 , the frame  22  is brought to a negative high voltage of −20 kV. It then forms a first negative electrode. 
     The coating product, which is hydrosoluble in this example, flows from the line  60 , through the injector  32  that is grounded, then through the deflector  26 . This product then forms a film  1000  that is distributed over the surface  212  up to the spraying rim  214 , where it forms filaments  2000  that tear into droplets  3000 , under the effect of the centrifugal force in particular, in the zone Z 2 . These droplets then form the cloud N that extends to the object O, along the axis X 30 . 
     During operation of the sprayer  10 , the liquid coating product flows from the supply source at the earth potential toward the outlet orifice of the injector  32  through a line  60  and in the injector  32 , where it is kept grounded. It then flows along the surface  212 , which is isolated from the electrode  22  by the body  21 . After traveling the surface  212 , the film  1000  of coating product licks an inner surface  242  of the ring  24  that is formed by a frustoconical segment  2422  and an annular segment  2424  and perpendicular to the axis X 30 . The spraying rim  214  is formed at the junction between the segments  2422  and  2424 . 
     The ring  24  made from a semi-conductive material sufficiently isolates the film  1000  from the frame  22  brought to −20 kV to avoid a short-circuit between that frame and the coating product supply circuit, the line  60  of which is grounded. Thus, the relatively insulating nature of the ring  24  avoids a short-circuit between the means for placing the frame  22  at the high voltage and the ground. The coating product in liquid form thus flows from the supply source at the earth potential to the spraying rim, in the vicinity of which a high-voltage electrode is implanted. 
     Furthermore, the frame  22  is provided, on its outer peripheral surface  226 , with points  227  regularly distributed around the axis X 20 . 
     During operation, the electrode  22  is brought to a negative high voltage of −20 kV, through the rotor  42 . Due to this negative high voltage, negative ions are created in the vicinity of the points  227  toward the spraying rim  214 . These ions, represented by “−” signs in  FIG. 3 , result in negatively charging the filaments of coating product  2000  and droplets of coating product  3000  being formed in the zone Z 2 . Thus, the electrode  22  constitutes a charge electrode for the droplets  3000  by ionization, or Corona effect, when they form in the zone Z 2 . 
     In other words, the use of the ring  24  made from semi-conductive material makes it possible, owing to its relatively insulating nature, to charge the paint droplets  3000  by ionization in the zone Z 2 . 
     Alternatively, the ring  24  can be made from an electrically insulating material. 
     It will be noted that the electrode formed by the frame  22  is positioned on the outside of the bowl and that it radially surrounds the body  21 . 
     The sprayer  10  comprises lines  33  arranged in the body  30  to create an air skirt for shaping the cloud N of droplets  3000  toward the object O. This air skirt flows from the body  30  and toward the front of the sprayer  10 , as shown by the arrows F 2 . The lines  33  are regularly distributed around the axis X 30  and supplied from an annular distributing chamber  35 , which in turn is supplied by a hose  37  connected to an air source (not shown). The skirt air F 2  in particular licks the outer radial surface  245  of the ring  24 . This results in continuously drying that surface and prevents the generation of electrostatically charged droplets  3000  on that surface, which limits the risks of short-circuit. 
     The skirt air F 2  also licks the outer radial surface  226  of the frame  22 , which also results in drying it. 
     The skirt air also drives the negative ions from the points  227  forward, i.e., toward the zone Z 2 , where they encounter the droplets  3000 , which they then negatively charge. 
     The sprayer  10  is also equipped with a second annular electrode  70  that is mounted on the body  30 , behind the rim  214 , i.e., opposite the object O relative to that rim, in the usage configuration of the facility  1 . The electrode  70  is supplied with high voltage from a high-voltage unit  80  to which it is connected by a cable  81 . 
     During the operation of the sprayer  10 , the electrode  70  is brought to the high voltage, with the same sign as that of the potential of the frame  22 . In the example, the electrode  70  is brought to a potential of −80 kV, such that an electrostatic field E is created between the object O and the electrode, that field in particular applying in the zone Z 2  where the droplets  3000  leave the spraying rim  214  of the bowl  20 . The droplets  3000 , which are charged, are then subjected to an aeraulic force due to the skirt air and an electrostatic force whose intensity is equal to their charge q multiplied by the intensity of the electrostatic field E, that force tending to drive the droplets  3000  toward the object O. In that sense, the electrode  70  pushes the droplets  3000  back toward the object O and can be described as a repelling electrode, while the field E can be described as a transport field. 
     The sprayer is also equipped with a third electrode  90 , inserted along the axis X 30 , between the electrodes  22  and  70  and brought to an intermediate potential between the potentials of those electrodes. The function of this third electrode is explained below, in reference to the third embodiment. 
     In the second and third embodiments of the invention shown in  FIG. 4  and following, the elements similar to those of the first embodiment bear the same references. In the following, we essentially describe what distinguishes each of these embodiments from the first embodiment. 
     In the second embodiment of the invention shown in  FIG. 4 , a bowl  20  is used that comprises an insulating body  21  defining a surface  212  for distributing a film  1000  of liquid coating product as well as a spraying rim  214  that borders a zone Z 2  defined as above and in which droplets  3000  of coating product form from webs  2000  pulled from the film  1000 . 
     Electrodes  122  are positioned in lines  33  by which an air skirt shown by the arrows F 2  emerges and that is designed to shape the cloud N of droplets of coating product  3000 . The electrodes  122  are made from metal, in the shape of a finger and each having a slender front tip  122 A, which favors the ionization phenomenon of the air in the vicinity of those electrodes. The electrodes  122  are electrically connected to one another and to a high-voltage unit by a cable  51 . These electrodes  122  therefore make it possible, by ionization, to charge the droplets that form and cross through the zone Z 2 . 
     As in the first embodiment, a repelling electrode  70  is provided in the body  30  of the sprayer, which makes it possible to create an electrostatic field E for transporting droplets  3000  of coating product that are negatively charged, toward an object O to be coated. 
     In this embodiment, the electrodes  122  may be brought to a potential of −20 kV, while the repelling electrode  70  is brought to electric potential of −80 kV during operation of the sprayer  10 . 
     Alternatively, the electrodes  122  may not protrude relative to the front face  35  of the body  30  of the sprayer  10 , which is oriented toward the object to be coated. 
     According to another alternative, the electrodes  122  can be positioned outside the lines  33 , radially to the inside or outside of a geometric circle centered on the axis X 30  and along which those lines are positioned. 
     In all cases, the electrodes  122  are situated, along the axis X 30 , between the electrode  70  and the spraying rim  214 . 
     Also in this embodiment, the sprayer is equipped with a third electrode  90  inserted, along the axis X 30 , between the electrodes  122  and  70  and brought to an intermediate potential between the potentials of those electrodes. The function of this third electrode is also explained below. 
     In the first and second embodiments, the body  30  is equipped with a cap  31  made from an electrically insulating material, that cap being provided with a passage opening for the repelling electrode  70 . The cap  31  extends in particular, along the axis X 30 , between the electrode  70  and the front of the body  30  by which the skirt air F 2  exits. It is therefore inserted, along that axis, between the ionizing charge electrode  22  or  122  and the repelling electrode  70 . 
     In the third embodiment of  FIG. 5 , the bowl  20  comprises an outer frame  22  made from an electrically conductive material, in particular metal, as well as a distributor  23 , also made from metal and the inner radial surface  232  of which constitutes a distributing surface of the film  1000  of coating product up to a spraying rim  234  defined at the outer radial edge of the distributor  23 . A ring  24  made from an electrically insulating material or semi-conductive material is inserted, on the outside of the bowl  20 , between the frame  22  and the outer radial part of the distributor  23 . An annular volume V 20  is defined between the outer radial surface  235  of the distributor  23  and the inner radial surface  225  of the frame  22 . 
     As in the first embodiment, the rotor  42  of the turbine  40  is brought to the high voltage. This rotor is in contact with the frame  22 , which is therefore also brought to the high voltage and forms an electrode. 
     Furthermore, the bowl  20  comprises a hub  29  made from an electrically insulating material and that serves as an interface with the rotor  42 , that hub extending by a collar  292  inserted radially between the frame  22  and the distributor  23 , on the side of the volume V 20  oriented toward the rotor  42 . Thus, the ring  24 , the volume V 20  and the collar  292  ensure galvanic ionization between the electrode  22  and the distributor  23 , which can be brought to different electric potentials. 
     Alternatively, the ring  24  and/or the hub  29  may be made from a semi-conductive material. 
     On its outer peripheral surface  226 , the electrode  22  is provided with a series of points  227  that extends radially outward relative to the axis X 30 . Alternatively, the series of points  227  may be replaced by a sharp circular rim. 
     The bowl  20  also comprises a metal deflector  26  comparable to that of the first embodiment. 
     During operation, the electrode  22  is brought to a negative high voltage of −20 kV, through the rotor  42 . Due to that negative high voltage and as in the first embodiment, negative ions are created in the vicinity of the points  227 , by ionization of the ambient air. Thus, the electrode  22  constitutes a charge electrode for the droplets  3000  by ionization, or Corona effect, when they form in the zone Z 2 . 
     It will be noted that the electric potential of the distributor  23  and deflector  26  may be floating, since the distributor  23  and the deflector are electrically isolated from the electrode  22 . The elements  23  and  26  are made from metal in order to have a good abrasion resistance with respect to the coating product. 
     In the case where the ring  24  is electrically insulated, it makes it possible to maintain a potential difference between the distributor  23  and the electrode  22 , on either side of the ring  24 . 
     Furthermore, a second electrode  70 , called repelling electrode, is mounted on the body  30  and brought to −80 kV during operation of the sprayer. It creates an electrostatic field E for transporting the droplets  3000  toward an object O to be coated. The droplets  3000 , which are negatively charged, move up the electrostatic field while being “repelled” by the electrode  70 . 
     As before, a flow of skirt air shown by the arrows F 2  is used to shape the cloud N of droplets that forms in the zone Z 2 . The skirt air jet makes it possible to continuously dry the outer radial surface  226  and the points  227  of the electrode  22  as well as the outer radial surface  245  of the ring  24 , which prevents the accumulation of droplets and limits the risks of short-circuit. Means like those  33 ,  35  and  37  of the first embodiment are used to create the skirt air flow. 
     As in the first embodiment, the body  30  is equipped with a third stabilizing electrode  90  that is positioned, along the axis X 30 , between the electrode  70  and the rim  234  of the bowl  20 . In other words, the stabilizing electrode  90  is inserted, along the axis X 30 , between the electrodes  22  and  70 , therefore closer to the electrode  22  than the repelling electrode  70 . A ring  92  made from an insulating or semi-conductive material is inserted, along the axis X 30 , between the electrodes  70  and  90 . 
     The electrodes  70  and  90  are respectively connected to high voltage sources by cables  81  and  83 . One such high voltage source is visible in  FIG. 1 , in the form of a generator  80  connected to the cable  81 . The cable  83  is, in turn, either connected to the generator  80  by means of a voltage divider bridge  85 , or connected to a generator specific to it. Other methods of powering the electrode  90  can be considered. 
     During the operation of the sprayer  10 , the electrode  90  is brought to an intermediate potential between that of the ionizing charge electrode  22  and that of the repelling electrode  70 . As an example, this intermediate potential may be set at approximately half of the potential of the second electrode, i.e., −40 kV in the example. The stabilizing electrode  90  makes it possible to form a screen with respect to the field lines resulting from the repelling electrode  70 , which thus do not tend to close on the charge electrode  22 . This prevents the electrostatic field created by the repelling electrode  70  from disrupting the ionization phenomenon of the droplets  3000  in the zone Z 2 . 
     In practice, the potentials of the second and third electrodes have the same sign and the potential of the third electrode  90  may be chosen to be between 0% and 90% of that of the second electrode  70 . 
     The preceding observations are also valid for the first and second embodiments. 
     The cap  31  of this embodiment extends between the electrode  90  and the front of the body  30 . It is therefore axially inserted between the electrodes  22  and  70 , more particularly between the electrodes  22  and  90 . This cap is, in turn, made from an insulating material. 
     In the embodiment of  FIG. 5 , the means for bringing the skirt air F 2  behind the bowl are not shown. They may be identical to or different from those of the first and second embodiments. 
     Irrespective of the embodiment, the ionizing charge electrode, i.e., the frame  22  in the first embodiment, the electrodes  122  in the second embodiment and the electrode  22  in the third embodiment, extend(s) at least partially between the spraying rim  214  and the rotor  42  of the turbine  40 , along the axis X 30 . Thus, this ionizing electrode is correctly positioned to effectively charge the droplets  3000  of coating product that pass through the spraying zone Z 2 . 
     Irrespective of the embodiment, the first charge electrode on the one hand, and the second and third electrodes for creating the field E for transporting and stabilizing on the other hand, can be supplied with electric voltage at different moments. In other words, they can be activated separately. For example, to penetrate a part forming a Faraday cage, such as an optical beacon housing, it is possible to reduce the transport field by decreasing or zeroing out the absolute value of the supply voltages of the second and third electrodes, favoring the charge by the first electrode, so that the droplets  3000  penetrate the Faraday cage. The principle consists of activating the first, second and third electrodes independently of one another, based on the geometry of the object O to be coated. It is also possible to activate the second electrode or modify its potential on the fly, during spraying, since the response times of the charge means are approximately 200 ms, which is compatible with the typical movement speeds of a sprayer relative to an object to be coated. 
     To that end, the high voltage units  50  and generators  50  and  80  of the first embodiment and their control means, or the similar means used in the other embodiments, constitute control and supply means that are differentiated and independent from the charge electrodes  22  or  122  and electrodes  70  for creating the transport field E. Likewise, the generators  80  and equivalent devices of the first embodiment and their control means, as well is the similar means used in the other embodiments, constitute control and supply means that are differentiated and independent of the second and third electrodes  70  and  90 . 
     However, alternatively, the first, second and third electrodes may be supplied by outlets with different levels of the same electrostatic generator  80 . 
     The invention is particularly applicable in the case where the sprayer is of the gun type, i.e., designed to be held in an operator&#39;s hand. It is also applicable to automatic sprayers. 
     The invention has been described in reference to embodiments in the case where the potential of the electrodes is negative. It can, however, be implemented in the case where that potential is positive. 
     The technical features of the embodiments and alternatives considered above may be combined with one another to create new embodiments.