Electrostatic repulsion device for deflecting electrostatically charged spray particles

Abstract: An electrostatic repulsion device (10) is provided in an electrostatic spray paint booth having a grounded ceiling and conveyor (26) to improve transfer efficiency in applying the paint to a workpiece (22) and maintain a safe working environment without requiring protective fencing between an operator and the application equipment (10,12). The device (10) includes a length of electrical cable (96) having a central resistive conductor (98) and an insulation layer (100). The cable (96) is designed to produce low electrical capacitance in operation when connected to a high voltage source (46). Conductive members or pins (114) are mounted along the cable (96) through the insulation (100,104) to contact the conductor (98). The conductive members or pins (114) are arranged parallel to the spray path (30) in two longitudinal groups, with the first set being diametrically opposed to the second set. Each conductive member or pin (114) has a discharge end that is substantially flush with the outside surface of the insulation (104) of the electrical cable (96). The device (10) is transversely mounted with respect to a spray path (30) of charged, atomized paint particles dispensed from a non-conductive, rotary atomizer or low capacitance spray gun (12), so that an electrostatic field from the first set of conductive members repels the atomized, charged paint particles away from the grounded ceiling and conveyor (26) of the paint booth towards a workpiece (22). The electrostatic field produced by the second set of conductive members (114) bleeds accumulated charge from the low electrical capacitance repulsion device (10) before a high energy spark or discharge can occur whenever a grounded operator or object comes into proximity with the device (10). This safety feature eliminates the need for protective fencing and interlocks within the spray booth.

Description: This invention relates to electrostatic spray coating. Devices for electrostatically charging coating material that is atomized by a rotary atomizer or spray gun and sprayed towards a workpiece are generally known. Such coating devices typically include a conduit for bringing pressurized coating material within the device to a spray head or rotating cup to atomize the coating material. In the case of rotary atomizers, in the past the entire atomizer including the rotating cup have been constructed from metal and charged to a high voltage to charge the coating material as it is being atomized and sprayed from the edge of the spinning cup. The charged, atomized coating material particulate is dispensed forwardly towards a workpiece that is typically hung from a conveyor by a suspending member such as a hook or the like. The workpiece is typically connected through the hook and a conductive member associated with the conveyor to an electrical ground. The charged atomized coating is attracted to the electrically grounded workpiece to completely coat it due to electrostatic effect. US-A-4872626 discloses another type of apparatus in which an annular electrode charging element is mounted about a non-charging, electrically grounded spray head for the purpose of charging the particulate emitted from the spray head to the opposite polarity of the workpiece to be coated. The charging electrode includes an electrical conductor mounted within an insulated sleeve with conductive pins mounted so that one end contacts the electrical conductor and the other end terminates in a depression on the face of the insulative sleeve directed towards the workpiece to be coated. The electrostatic field generated extends from the electrode to the electrically grounded spray head to charge the particulate moving through the electrostatic field. The spray heads or rotary cups which atomize the coating material, spray or throw the atomized coating in a tangential direction relative to the particulate flow path towards the workpiece producing a spray pattern that is larger than the workpiece to ensure that the surface of the workpiece is completely coated. Atomized particles which are not deposited onto the workpiece are called overspray. The greater the amount of overspray from the dispensing device the less the transfer efficiency of the coating material (i.e., the proportion of the paint sprayed which is actually applied to the workpiece). Moreover, overspray will either fall by gravity to the floor, be carried by air currents to the overspray control system, or be attracted to other grounded objects in the vicinity of the overspray such as the booth ceiling, and conveyor. Where overspray is allowed to accumulate on conveyor hooks, the electrical grounding connection between the books and the workpieces can be reduced or lost in which case even more overspray is produced because the charged particles are no longer attracted to the workpieces. This problem has been solved with the metal rotary atomizers of the prior art described above by including a metal rod, charged to the same potential as the atomizing cup, located above the rotary atomizer and below the ceiling to repel the charged particles away from the ceiling and conveyor and towards the workpiece. Since the entire rotary atomizer has in the past been a highly charged, sizeable piece of metal equipment, it has been necessary to enclose it in a protective fence with interlocks which cut power to the atomizer when any access doors in the fencing are opened to protect operators. The metal repulsion rods described above, normally equipped with a row of sharp pins projecting from them, have also been highly charged and sizeable pieces of metal equipment and so they also have been located within the protective fencing. Recently, a low capacitance electrostatic rotary atomizer, constructed substantially entirely from non-metallic components, has been made commercially available which does not present shock hazards to operators, and consequently, has not required protective fencing or electrical interlocks. With this type of rotary atomizer, the placement of a metal repulsion rod above the atomizer to control overspray by repelling the paint particles towards the workpiece and away from the ceiling and conveyor, would create a safety hazard requiring protective fencing and interlocks which deprives the user of the advantage of having a low capacitance, safe rotary atomizer. An electrostatic repulsion device for deflecting electrostatically charged particles in accordance with the invention comprises an electrical conductor characterised in that the conductor has a high electrical resistance and is surrounded by a low capacitance insulating layer, and in that a plurality of electrically conductive members is provided, each member having one end in contact with the conductor and extending outwardly through the insulating layer. The electrically conductive members may be arranged in diametrically opposed pairs, each pair being aligned with the direction of the particulate spray. In such an arrangement the forward facing members repel the atomized particles towards the workpiece while the rearward facing members produce a fringing field that bleeds charge from the device as a grounded object is brought in proximity thereto, or as an operator approaches from behind, to prevent a rapid discharge from the device and the attendant ignition or shock hazards. The electrically conductive members may be in the form of pins, and the outer or discharge ends of the members or pins may be flush with the outer surface of the insulating layer. One embodiment of a device in accordance with the invention is constructed from a length of electrical cable having an insulated covering and an electrical conductor therein. The cable is connected to a high voltage power cable through a connector having a conductive member therein which electrically connects the high voltage power cable to the conductor within the electrical cable. Concentrically mounted about the insulated covering of the electrical cable is an electrically insulative cover to provide further electrical insulation. The insulative cover has diametrically opposed holes at predetermined intervals along its length. Electrically conductive pins are mounted within the holes and through the insulation of the electrical cable so that they electrically connect with the conductor of the cable. The distance between adjacent members, or between adjacent diametrically opposed pairs of members, may be such that electrostatic field lines emanating from the outer or discharge ends of the members form lines of equal potential at a constant distance from the conductor. The conductor may have two segments extending symmetrically outwardly from a high voltage power supply connection. In an alternative embodiment in accordance with the invention the distal ends of the segments may be joined so that the conductor is in the form of a closed loop positioned concentrically with respect to the atomising cup or spray head of the electrostatic rotary atomiser or spray gun. Such a structure provides a continuous electrostatic field which repels the charged atomized particles away from the atomizer or spray head and towards the workpiece. Moreover, the further towards the workpiece such a circular repulsion device is located, the smaller the spray pattern produced by the atomizer or spray head and vice versa. Thus, this embodiment of the repulsion device can be used to symmetrically collapse or expand the spray pattern produced by the atomizer or spray head. The advantage provided with either embodiment whether it is used to repel the pattern away from the ceiling or conveyor, or to symmetrically collapse or expand the pattern, or even to shape the pattern in other ways, is to provide pattern shaping in a system employing a low capacitance rotary atomizer, or low capacitance (i.e. resistive) spray gun, without creating a safety hazard which would require fencing and interlocks to maintain operator safety. A device constructed in accordance with the invention may be used with other low capacitance electrostatic coating devices such as, for example, resistive electrostatic spray guns for both powder and liquid coating applications. Such a device can be used with these spray guns to achieve advantages similar to those obtained with the low capacitance rotary atomizer although the invention is preferably used with a low capacitance rotary atomizer. The invention will now be described by way of example only and with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of one embodiment of an electrostatic repulsion device in accordance with the invention used with an electrostatic spray dispensing device to coat a workpiece. Fig. 2A is a partial cross-section view of the device of Fig. 1. Fig. 2B is a cross-sectional view of a section of the device of Fig. 1. Fig. 3 is a schematic drawing of the electrostatic field produced along the surface of the repulsion device of Fig. 1. Fig. 4 is a perspective view of an alternative embodiment of a device in accordance with the present invention. Fig. 4A is a cross-sectional detail view along lines 4A-4A shown in Fig. 4. An electrostatic repulsion device constructed according to the principles of the present invention is shown in use with a coating material dispensing device in Fig. 1. The repulsion device 10 and dispensing device 12 are shown mounted to a support stand 14. Support stand 14 has a base 16 and a vertical rod 18 that may telescope vertically above joint 19 to alter the distance between repulsion device 10 and dispensing device 12. Dispensing device 12 is shown connected to a high voltage supply 20 via a high voltage cable 21. Dispensing device 12 is a low capacitance rotary atomizer of the type described in U.S. Patent No. 4,887,770, which is available from Nordson Corporation of Amherst, Ohio, as a Model RA 12 rotary atomizer. This rotary atomizer is constructed from non-metallic components, and has a charging system which is not a part of the present invention. The rotary atomizer has a low electrical capacitance so no protective fencing is required to protect the operator from electrical shocks, and thus, none is shown. While the invention will be described with respect to this rotary atomizer for simplicity, the invention is also applicable to low capacitance (i.e., resistive) powder or liquid electrostatic spray guns. A conduit (not shown) brings coating material to device 12 and eventually to the interior of the rotating cup 12a to produce electrostatically charged atomized coating material which is sprayed from the forward end of device 12. The charged, atomized coating material sprayed from the cup 12a of device 12 is directed towards a workpiece 22 that is suspended by a member 24 such as a hook or the like from a conveyor 26. Conveyor 26 brings workpiece 22 near the cup 12a of device 12 to coat the workpiece 22. The spray pattern 30 shown in Fig. 1 includes an overspray portion 32. Workpiece 22 is electrically grounded through hook 24 and a conductive element (not shown) within conveyor 26. Workpiece 22 remains electrically grounded to electrically attract the charged atomized particles within spray pattern 30 as long as electrical continuity is maintained between the hook 24 and workpiece 22. To prevent overspray 32 from being attracted to and coating the ceiling (not shown), conveyor 26 and especially hook 24, which would deteriorate the ground connection to workpiece 22, the low capacitance electrostatic repulsion device 10 is used as shown in Fig. 1. This device includes a repulsion rod or member 38 transversely oriented to the flow of spray 30 which emits an electrostatic field 40 forwardly of rod 38. This forward electrostatic field 40 is at the same electrostatic potential as the particulate within spray pattern 30 and consequently repels the particulate downwardly from the repulsion rod and the electrically grounded ceiling and conveyor 26 towards workpiece 22. Electrostatic field 40 causes that portion of overspray 32 which would otherwise be available to coat hook 24 to drop close enough to workpiece 22 to be attracted to it instead. Thus, the transfer efficiency of the coating material dispensed by atomizer 12 to paint workpiece 22 is improved by electrostatic field 40. Moreover, as will later be disclosed in more detail, this advantage is produced without introducing a dangerously capacitively charged piece of equipment into the spray environment. Repulsion rod 38 also provides an electrostatic field 42 which extends rearwardly to reduce the risk of an ignition or shock hazard to an operator approaching from the rear of the device. A grounded operator who moves into proximity to the rear of repulsion rod 38 contacts field 42 and bleeds the electrostatic charge from rod 38 to avoid a shock or a high energy spark which could ignite the volatile paint spray booth environment. Electrostatic repulsion device 10 is connected to stand 14 by means of an extension 43 which is connected to joint 44, both of which have a passage therethrough. Extending from high voltage power supply 46 is a high voltage cable 48 which passes through joint 44 and retaining nut 45. Cable 48 has a contact (not shown) at its end which engages jumper 74 (Fig. 2A) within extension 43. As described in detail hereinafter, jumper 74 is electrically connected through a resistor 78 and a conductive member or pin 52 (Fig. 2B) to a conductor 98 (Fig. 2B) within repulsion rod 38 to supply electrostatic fields 40, 42 through conductive members which are mounted within holes in repulsion rod 38 as is described below. The details of the connections between extension rod 43 and high voltage cable 48 are shown in Fig. 2A. Extension 43 is preferably a cylinder constructed of non-deformable material such as nylon stock or the like having a longitudinal passage 64 therethrough. At lower end 66, extension 43 is tap drilled to receive threaded adaptor 68. Threaded end 69 of adapter 68 receives high voltage electrical cable 48 and retaining nut 45 is tightened about end 69 to abut the contact on the end of cable 98 to a like contact on the end of jumper 74 to electrically connect them within extension 42. Jumper 74 is installed within central passage 64 and has a washer and pin contact 73 on its top end in Fig. 2A which is identical to the contact and pin of the opposite end. A conductor 76 within jumper 74 connects the pin contacts at each end to give electrical continuity through the length of jumper 74. End 70 of extension 43 has a jam nut 72 threaded thereon and a telescoping male extension 80 which is received within connector 50. Mounted through jam nut 72 at end 70 is a resistor 78 which contacts the washer and pin contact 73 of jumper 74 at one end and extends beyond the telescoping end 80 of extension 42 within connector 50 at the other end. An O-ring seal 82 is provided at the uppermost portion of end 80. Resistor 78 is well known within the art and is a 275 megohm resistor with a power rating of 5 watts. Jumper 74 is deliberately oversized in length so that when device 10 is assembled, as described in detail below, conductive member 52 (Fig. 2B), resistor 78, jumper 74, and the end of cable 48 are mechanically compressed together to ensure good electrical contact. The details of the connection between repulsion rod 38 and connector 50 are shown in Fig. 2B. Mounted at end 86 of connector 50 is a cap 88 which is securely held within connector 50 by threads which are sealed by O-rings 90. Cap 88 has a bore or passage 92 therethrough with openings 94 at opposed ends, which are all transversely oriented with respect to extension 43 and connector 50. A length of electrical cable 96, having an electrical conductor 98 surrounded by insulation layer 100, is mounted with passage 92 so that repulsion rod 38 is divided into two equal length segments extending from opposite sides of connector 50. Conductive member or pin 52 mounted within cap 88 engages resistor 78 at one end and conductor 98 at its other end to establish electrical continuity from the high voltage cable 48 through jumper 74 and resistor 78 to conductor 98. Cable 96 as well as cable 48 and jumper 74 are of the type disclosed in U.S. Patent No. 4,576,827. The cable disclosed in U.S. 4,576,827 has a continuous resistive core so that at any point along its length the rate and amount of any capacitive discharge is limited. Secured within openings 94 of cap 88 and mounted about the cable segments extending from the opposite sides of connector 50 are protective and electrically insulating tubes or sheaths 104. These tubes are primarily constructed from a material having a very low electrical capacitance so little charge accumulates on their outside surface even when present in electrostatic fields 40 and 42. Such sheaths are well known within the art, and are constructed from a phenolic resin having a NEMA G-10 rating. Such a sheath permits cleaning with solvent without degrading its electrical or structural characteristics. Protective tubes 104 are epoxied to cap 88 by filling grooves 108 with an epoxy glue prior to their insertion into openings 94. Pairs of diametrically opposed holes 112, as shown in Fig. 2B, are provided along the longitudinal length of tubes 104. Within these holes there are mounted conductive members or pins 114. Each conductive member 114 is inserted in hole 112 and through insulative layer 96 until one end electrically contacts conductor 98. Where a cable of the type disclosed in US 4,576,827 is used, conductor 98 is a bundle of continuous silicon carbide fibers which conduct the charge to the spray device but also have a resistivity which limits the rate of capacitive discharge. The other end of conductive members 114 are cut flush with the outer surface of tube 104 to eliminate discharge ends beyond the surface of the tube 104 for electrostatic fields 40, 42. Holes 112 may be approximately .30-.035 inches (0.76-0.89 mm) long and have a diameter of approximately .030 inches (0.76 mm). Holes 112 are separated by a distance to maintain the edge of electrostatic field 42 at a location where a grounded object begins to bleed charge from the surface of tube 104 without creating an ignition hazard. Where a cable of the type disclosed in US 4,576,827 is used for the cable length in repulsion rod 38 as well as for jumper 74 and cable 48 and which is connected to a Nordson Corporation Model EPU9 Power Supply, the distance between holes 112 is approximately 2 inches (51 mm) and each sheath 104 has 16 pairs of the diametrically opposed holes separated by this distance. Fig. 3 shows a row of holes 112 within tube 104 to illustrate the electrostatic fields produced by conductive members 114 mounted therein. The phantom lines shown in Fig. 3 represent equipotential lines of the electrostatic field emanating from pins 114 that extend from the surface of tube 104. Additionally, these field lines extend out of the plane of the diagram and represent the repulsive force that directs the particulate in overspray 32 towards workpiece 22. Adjacent holes arranged in the row are separated by a distance that prevents the equipotential lines extending from a discharge end from terminating on the surface of tube 104 at a point between adjacent holes. This permits the electrostatic field to present equipotential lines at a substantially constant distance from tube 104 so the field does not have gaps. Thus, a grounded object cannot be moved into the electrostatic field without crossing an equipotential line that represents an electrical potential great enough to bleed charge from rod 38 to the grounded object without a high energy discharge. Electrostatic field 40 improves the transfer efficiency of spray 30 onto workpiece 22 in two ways. First, the like charge of field 40 and the atomized particles within spray pattern 30 repels the particles downwardly toward workpiece 22. Second, since more of the particles are directed to workpiece 22 and away from hook 24, workpiece 22 remains in good electrical contact with the electrically grounded conveyor 26 to attract the charged particles and improve transfer efficiency. This improvement in transfer efficiency is moreover obtained while maintaining the safe painting environment provided by a low capacitance rotary atomizer. These effects may be maximised when repulsion rod 38 is even with or up to approximately 2 inches (51 mm) behind, the discharge end of rotary atomizer cup 12a of device 12 and approximately 18 to 24 inches (46 to 61 cm) above device 12. In a similar fashion, an electrostatic field having the same configuration and orientation about tube 104 is provided along the rearward surface of tube 104 by conductive members 114 which are mounted within holes 112 diametrically opposed to the holes 112 along the forward edge. When an operator or grounded object moves into contact with electrostatic field 42, the capacitance available for discharge immediately begins to migrate to the operator or grounded object through a low energy, safe discharge since the surface of tube 104 and the associated cable and conductive pins have a very low electrical capacitance. As mentioned above, the cable used in repulsion rod 38 has a resistive core. This means that the core of the cable which is conducting the electrical charge to the pins 114 is not adding appreciably to the capacitance which is available for discharge. Moreover, there is very limited capacitance available for discharge in the electric circuit between the rod 38 and the power supply 46 due to the use of a resistive cable for cable 48 and jumper 74, as well as the placement of resistor 78 just before the rod 38. Thus, the conductive mass of the pins 114 themselves possibly provide the largest capacitance available for immediate discharge to an operator or grounded object. By providing these pins 114 in opposed pairs diametrically opposite one another, it is not possible to "sneak up" on a pin from behind to draw a sudden and substantially complete discharge of all available capacitance since the rearward pin will tend to discharge the capacitance of its associated forward pin as it is being slowly discharged from an approaching operator or object. An alternative embodiment of the present invention is shown in Fig. 4. In this embodiment, tube 104 has been fabricated in a circular configuration with cable 96 mounted therein to form a charged closed loop or perimeter for generating a forward electrostatic field about spray 30. Dispensing device 12 is concentrically mounted within the area surrounded by tube 104 to focus spray 30 towards work surface 22. As with the previous embodiment, as shown in Detail 4A, ring 104 includes pins 112a mounted within diametrically opposed holes 114a which serve the same purpose as described above. With this embodiment, as the ring 104 is moved along atomizer 12 towards the forward discharge edge of cup 12a, the field produced by forward pins 114a repels the atomized particles of the conical or bell-shaped spray pattern more strongly towards the workpiece being painted, with the result that forward movement of ring 104 symmetrically collapses or closes down the diameter of the spray pattern. Conversely moving the ring 104 rearwardly along atomizer 12 allows the pattern to open up towards the normal size it would assume due to the centrifugal forces of cup 12a. Thus, to prevent overspray from coating the conveyor hooks 24, the ring 104 is moved forwardly enough to adequately collapse the spray pattern. Likewise, ring 104 could be moved axially along the body of fixed, or mounted, low capacitance electrostatic spray gun to produce the same effect. One or two electrostatic repulsion devices could be vertically oriented about dispensing device 12 to create vertical planes to direct spray 30 towards workpiece 22. Alternatively, a pair of repulsion devices could be employed horizontally above and below the atomizer to repel overspray away from both the ceiling and the floor. The distances between dispensing device 12 and repulsion rods 38 could be varied to produce symmetrical or asymmetrical spray patterns towards workpiece 22. In a method of making electrostatic repulsion device 10, cable 96 is centered within passage 92 of connector cap 88 (Fig. 2B). Conductive member 52 is inserted through cap 88 to electrically contact conductor 96. Grooves 108 of protective sheath 104 are filled with epoxy and inserted into openings 94 to secure them about cable 96. Conductive members 114 are then inserted into holes 112 so they electrically engage conductor 96 and the electrical continuity is checked between each pin and member 52 with a multimeter as is well known within the art. Pins 114 are then cut flush with the surface of sheath 104. Cap 88 is threaded into connector 50 and the threads are sealed by O-rings 90. With reference to Fig. 2A, cable adaptor 66 is threaded onto the end of extension 43 opposite connector 50 and extension 43 is filled with dielectric grease as well as the end of connector 50. This dielectric grease is used to completely fill the cavities where the high voltage components are mounted to prevent a dielectric breakdown and its attendant spark if air were permitted in these cavities. Jumper 74 is inserted into passage 64 within extension 43. Resistor 78 is then inserted into extension 43 to engage jumper 74 and jam nut 72 is threaded onto the upper end of extension 43. This end of extension 43 is threaded into connector 50 so resistor 78 electrically contacts conductive member 52 to complete the assembly of repulsion device 10. Adapter 68 is filled with dielectric grease for the securement of cable 48 to extension 43. The end portion of high voltage power cable 48 is inserted into extension 42 to electrically contact jumper 74 and is threadably secured to the threads 69 by a suitable threaded coupling (not shown) which is mounted a few inches from the end of cable 48. High voltage cable 48 may now be connected to a high voltage power supply to provide the electrostatic potential needed to create electrostatic fields 40, 42. The invention is also applicable to low capacitance electrostatic liquid and powder spray guns. In addition, for example, an electrical cable having an insulative layer whose surface has a low electrical capacitance and a conductor with a high enough resistance could be used without sheaths 104.

Claims: An electrostatic repulsion device for deflecting electrostatically charged particles comprising an electrical conductor (98) characterised in that the conductor (98) has a high electrical resistance and is surrounded by a low capacitance insulating layer (104), and in that a plurality of electrically conductive members (114) is provided, each member (114) having one end in contact with the conductor (98) and extending outwardly through the insulating layer (104). A device according to Claim 1 characterised in that the members (114) are arranged in diametrically opposed pairs, each pair being aligned with the direction of the particulate spray (30). A device according to Claim 1 or 2 characterised in that the members are in the form of pins, and in that the outer or discharge end of each pin (114) is flush with the outer surface of the insulating layer (104). A device according to Claim 1, 2 or 3 characterised in that the distance between adjacent members (114), or between adjacent diametrically opposed pairs of members (114), is such that electrostatic field lines (42) emanating from the outer or discharge ends of the members (114) form lines of equal potential at a constant distance from the conductor (98). A device according to any preceding Claim characterised in that the conductor (98) has two segments extending symmetrically outwardly from a high voltage power supply connection (50). A device according to Claim 5 characterised in that the distal ends of the segments are joined so that the conductor is in the form of a closed loop positioned concentrically with respect to the atomising cup or spray head (12a) of the electrostatic rotary atomiser or spray gun (12). Apparatus for electrostatic spray coating comprising an electrostatic rotary atomiser or spray gun (12) and an electrostatic repulsion device according to any preceding Claim. A method of electrostatic spray coating comprising producing a spray of electrostatically charged coating particles directed towards a workpiece held at a different electrical potential, characterised by generating an electrostatic field adjacent the charged particle spray by means of an electrostatic repulsion device according to any one of Claims 1 to 6, so as to deflect the spray of particles towards the workpiece. The method according to Claim 8 wherein the electrically conductive members are arranged in diametrically opposed pairs comprising the aligning of the pairs of members with the direction of the particulate spray. The method according to Claim 8 or 9 wherein the electrical conductor is in the form of a closed loop comprising positioning the conductor concentrically with the axis of spray of charged particles and moving the conductor along the axis so as either to constrict the size of the spray pattern or to allow the spray pattern to expand towards its normal size.

Inventors: HARTLE RONALD J, HARTLE, RONALD J.
Applicants: NORDSON CORP, NORDSON CORPORATION
Application year: 1992
Main classification: B05B5
IPC codes: B05B5, B05D1