Electrostatic rotary atomizing spray device with improved atomized cup

Abstract: An electrostatic, liquid spray, rotary atomizer has an atomizer housing, a power supply within the housing and an atomizer cup at a front end of the housing. The atomizer cup, which is formed of a non-conductive material, has several elongate conductive pathways embedded in the body of the cup. Each conductive pathway has one end exiting an outer surface at a rear end of the cup for receiving the charge from the power supply and another end exiting an inner surface at a front end of the cup for conveying an electrical charge from the power supply to the liquid (paint) particles passing through the atomizer cup. A number of conductive extensions are embedded in a frustroconical front portion of the cup. Each conductive extension has one end which is contiguous with the inner surface exiting end of selected ones of the conductive pathways, a first opposite end portion exiting an outer surface of the frustroconical front portion of the cup and a second opposite end portion exiting an inner surface of the frustroconical front portion of the cup. An annular charge ring is mounted to the front of the atomizer housing and is configured to accommodate the atomizer cup with conductive pathways and extensions and has an access hole for facilitating insertion of a tool for quickly demounting the atomizer cup for cleaning or replacement. An electrode is provided for maintaining a small voltage at the access hole. The rotary atomizer with the improved charge ring and rotary cup can be mounted to a robot and connected to the liquid supply by an elongated spiral passageway to increase the electrical resistance between the atomizer and the liquid supply so that a very small electrical charge, if any, will be present in the passageway carrying paint to the rotary cup.

Description: This invention relates to a rotary atomizer device for spraying a liquid coating material. Rotary atomizers are a type of liquid spray coating device which includes an atomizer head rotatable at high speed (typically 10,000-45,000 revolutions per minute) by an air turbine motor to apply liquid coating material, such as paint, in atomized form onto the surface of a workpiece. The atomizer head is usually in the form of a disc or cup which includes an interior wall that defines a cavity and terminates in an atomizing edge. Liquid coating material delivered to the interior of the cup flows outwardly under centrifugal force along the interior wall of the cup and is expelled radially outward from the peripheral edge of the cup to form a spray pattern of atomized droplets of coating material. To improve the transfer efficiency of the coating process, an electrostatic charge is imparted to the coating material so that the pattern of atomized coating material is attracted to an electrically grounded workpiece. An example of an electrostatically charged rotary atomizer is disclosed in commonly assigned U.S. Patent No. 4,887,770 ('770) to Wacker et al., which is expressly incorporated herein in its entirety by reference. Prior to the '770 patent, one of the hazards associated with the use of the conductive atomizing cup was the possibility of operator shock or ignition of combustible coatings because of the high voltage at which the cups were maintained. For example, as disclosed in U.S. Patent No. 4,369,924, a charge is transferred through a turbine shaft from a power supply to the rotary atomizer cup. Since, both the cup and the entire rotary atomizing housing were metal and were charged to a high voltage, there is a significant safety hazard since the atomizer carries sufficient charge to severely shock an operator. Therefore, protective fences and interlocks have to be installed around the atomizer. The '770 patent, listed before, discloses a low capacitance, rotary atomizer which, while electro-statically charging the coating paint at the rotary atomizer cup, does not store sufficient charge to present a shock hazard and therefore does not have to be protected by fences and safety interlocks. To charge the atomizer in the '770 patent, external electrode probes (462) direct the charge into the cup (20). Still, under certain strict testing conditions, a charge on the equipment could still cause some safety concerns. Another problem associated with prior art rotary atomizers is that the rotary atomizer cups have not been easy to disassemble and clean. For example, in U.S. Patent No. 4,838,487, a deflecting member (28) is held in place against atomizing bell (10) by spacers (36). However, in operation, dried paint can collect on the front surface (30) of the deflector member. Then, the flow of paint across the front surface with the dried paint has a tendency to form an irregular coating on the part being sprayed. Still another problem associated with the prior art electrostatic spray guns was related to the isolation of the spray gun with the liquid supply. One solution, as described in U.S. 4,139,155 to Hastings was to provide a spiral conduit in the flow passage. It is an object of the present invention to provide an improved electrostatic rotary atomizing spray device It is another object of the present invention to provide a rotary atomizer device for spraying a liquid coating and method of operating same wherein an improved rotary cup has a plurality of conductive pathways for transferring electrostatic energy to the paint without igniting the propane used in the FM 7260 test. Still another object of the present invention is to provide a rotary atomizer device for spraying a liquid coating and method for assembling the device the atomizing head or cup can be easily removed from the atomizer device for cleaning. It is still a further object of the present invention to provide an apparatus and method for transferring charge to a high speed atomizer head or cup through a charge ring mounted to the front of the rotary atomizer housing so that the charge is dissipated to prevent the need for protecting an operator from being shocked. Still another object of the present invention is to provide an access hole in the charge ring for insertion of a tool to restrain free rotation of the turbine shaft to which the atomizer cup is secured to facilitate quick demounting of the atomizer cup for cleaning or replacement. Yet another object of the present invention is to provide an additional electrode in the electrical circuit for transferring charge from the power supply to the atomizer cup through a charge ring, wherein the additional electrode is disposed in the access hole of the charge ring for presenting a low voltage in the vicinity of the access hole that provides a small spark which is insufficient to ignite the propane used in the FM 7260 test. Still another object of the present invention is to mount the rotary atomizer with the improved charge ring and rotary cup to a robot and connected to the liquid supply control valve by an elongated spiral passageway to increase the electrical resistance between the atomizer and the liquid supply so that a very small electrical charge, if any, will be present in the passageway carrying paint to the rotary cup. US Patent 5474236 discloses a rotary atomizer according to preamble of claim 1, having a low capacitance, cup-shaped, atomizing head mounted for rotation about an axis of rotation has a coating material flow surface forming a forward cavity. A rotary drive coupled to the atomizing head rotates the atomizing head about the axis of rotation. High voltage electrostatic energy is conducted through the rotary drive directly into the atomizing head whereby changed coating materials flows outwardly across the flow surface. In another embodiment, a semi-conductive ring is mounted on the front end of the atomizer to transfer the high voltage electrostatic energy into the atomizing head and/or to dissipate the electrostatic energy in the rotary drive or in the atomizing head. The present invention provides an atomizer cup for an electrostatic rotary atomizing spray device for spraying a liquid coating material, the atomiser cup comprising a body is formed primarily of a non-conductive material having a one end, another opposite end, a bore extending therethrough from the one end to the other end, an outer surface, a longitudinal axis, a rear section symmetrically disposed about the longitudinal axis, a generally frustoconically shaped forward section contiguous with and forward of the rear section and symmetrically disposed about the longitudinal axis, the rear section having an outer surface and the frustroconically shaped forward section including a front edge, an outer surface and an inner surface; and a plurality of elongate conductive or semi-conductive pathways embedded within the body for carrying an electrical charge, the pathways extending from the rear section to the frustoconically shaped forward section of the atomizer cup, characterised in that at least one pathway has an end which exits at the inner surface of the frustoconically shaped forward section and an end which exits on the outer surface of the frustoconically shaped forward section. The atomizing cup has conductive pathway for transferring electrostatic energy to the coating material preferably without igniting the propane used in the FM 7260 test. In one embodiment, each pathway has one end exiting an outer surface at a rear end of the cup for receiving the electrical charge from the power supply. Each conductive pathway has another end exiting an inner surface at a front end of the cup. The conductive pathways convey an electrical charge from the power supply to liquid (paint) particles passing through the atomizer cup. A number of conductive extensions are embedded in a frustroconical front portion of the cup. Each conductive extension has a first end portion contiguous with the inner surface, exiting end of selected ones of the conductive pathway and a second opposite end portion exiting an outer surface of the frustroconical front portion of the cup. The conductive extensions also have a second opposite end portion exiting an inner surface of the frustroconical front portion of the cup. In a preferred embodiment, an annular charge ring mounted to the front portion of the atomizer is configured to accommodate the atomizer cup with the conductive pathways and extensions. The charge ring has an access hole for facilitating insertion of a tool for restraining free rotation of the turbine shaft to which the atomizer cup is secured to facilitate quickly demounting the atomizer cup for cleaning or replacement. An electrode disposed within the access hole and electrically connected to the electrical circuit presents a low voltage in the vicinity of the access hole for providing a small spark that is insufficient to ignite the propane used in the FM 7260 test. In order that the access hole does not compromise the air-tightness of the atomizer housing, a duckbill valve is provided in the access hole. In the preferred embodiment there is provided a rotary atomizer device wherein high electrostatic charge is transferred from a power supply to the improved high speed atomizer cup secured to a shaft driven by an air turbine motor. The cup is secured to the turbine shaft to facilitate quick demounting of the cup for cleaning or replacement. The rotary atomizer can be mounted to a robot and connected to a liquid supply by an elongated spiral passageway. The invention will now be described by way of example and with reference to the accompanying drawings in which: Fig. 1 is a cross sectional side view of an embodiment of a rotary atomizer; Fig. 2 is an enlarged partial sectional view of the rotary drive shaft assembled together with the atomizer cup; Fig. 3 is a side view, in cross section, of a charge ring disposed at the front end of the atomizer housing shown in Fig. 1, both for transferring high electrostatic charge to the atomizer head and for directing a flow of vectored air onto the atomizer head to prevent paint wrap back onto the atomizer housing and for shaping the spray of paint ; Fig. 4 is a rear view of the charge ring of Fig. 3 showing the resistors of a charging circuit embedded in the ring; Fig. 5 is a cross-sectional side view, along line 5-5 of Fig. 7, of an improved rotary atomizer head having a plurality of conductive pathways embedded therein, in accordance with the invention; Fig. 6 is a side view of the improved atomizer cup of Fig. 5, showing the conductive pathways exiting an outer surface of the atomizer cup, in accordance with the invention; Fig. 7 is a front view of the improved atomizer cup showing ends of the conductive pathways exiting an inner surface of the atomizer cup, in accordance with the invention; Fig. 8 is a side view, in cross-section, of a charge ring component of the atomizer housing which is configured to accommodate the atomizer cup of Figs. 5, 6 and 7 and which has an access hole for facilitating insertion of a tool for quickly demounting the atomizer cup from a turbine shaft to which it is secured, in accordance with the invention; Fig. 8A is a side view, in cross-section of a duckbill valve disposed in the access hole in the charge ring of Fig. 8, in accordance with the invention; Fig. 9 is a rear view of the charge ring of Fig. 8 showing the charge ring and placement of resistors, in accordance with the invention; Fig. 10 is block diagram of a charge ring circuit particularly adapted to convey charge from the power supply to the atomizer cup, in accordance with the invention; Fig. 11 is a side view, in cross section, of a charge ring disposed at the front end of the atomizer housing shown in Fig. 1 and to which is mounted the atomizer cup of Figs. 5, 6 and 7; Fig. 12 is a three-dimensional view of a rotary atomizer for transferring high electrostatic charge to an improved rotary atomizer cup mounted onto a robot, in accordance with the invention; and Fig. 13 is a side view of the rotary atomizer as shown in Fig. 12. ROTARY ATOMIZER Referring to Fig. 1, there is illustrated an electrostatic, liquid spray, rotary atomizer 10, which is very similar to the construction of rotary atomizers, described in detail in US Patents 6056215 and 5697559. but with certain modifications in accordance with an additional embodiment of the invention.- The rotary atomizer 10 includes an atomizer housing 12 having a forward section 14, an intermediate section 16, and a rear section 18 which collectively define an interior chamber 20. An air control element 22, incorporating an annular charge ring 24, as shown in detail in Fig. 1, is detachably mounted to the forward section 14. Annular charge ring 24 has a front wall 26 provided with a circular bore 28 that is coincident with a longitudinal axis of rotation 30 that extends through atomizer housing 10. An internal power supply 32, located within interior chamber 20, generates high voltage electrostatic energy in the range of from about 30,000 volts DC to about 100,000 volts DC. Power supply 32 is electrically connected to air control element 22 by electrical voltage transfer structure, as previously described in detail in US Patents 6056215 and 5697559. Rotary drive mechanism 34, located within the interior chamber 20 of rotary atomizer 10, is preferably an air driven type turbine motor 36 which includes internal air bearings (not shown), a driving air inlet (not shown), and a braking air inlet (not shown) for controlling the rotational speed of a turbine wheel 38, all of which components are well known in the art. Turbine motor 36 includes a rotary drive shaft 40 that extends through and is rotatably supported within a turbine housing 42. Rotary drive shaft 40 extends through circular bore 28 of annular ring 24 and has an atomizer cup or head 44 mounted at one end. Drive shaft 40 further extends into a turbine drive wheel housing 46 at the opposite end and is connected to turbine wheel 38. A stationary, liquid flow tube 48 extends completely through rotary drive mechanism 34, and is in fluid communication with an air operated valve 50 at one end and atomizing cup 44 at the opposite end for transferring a liquid coating from the valve 50 to the atomizing cup. Referring to air turbine motor 36, a source of pressurized turbine drive air is connected by a passageway (not shown) through manifold plate 52 and valve plate 54 to the turbine wheel housing 46 to spin air turbine drive wheel 38 according to conventional practice. That is, the stream of turbine drive air is directed against the outer perimeter of drive wheel 38 to rotate the wheel about the longitudinal axis 30 extending through rotary atomizer 10. A source of brake air is also connected by a passageway (not shown) through manifold plate 52 and valve plate 54 to the turbine wheel housing 46 for application against upstanding brake buckets (not shown) projecting from the side face of turbine wheel 38. The atomizer housing 10, as shown in Fig. 1, includes an outer casing 56 with a larger diameter rear end section 58 enclosing manifold plate 52, valve plate 54, and interface plate 60. Outer casing 56 also includes a tapered front end section 62 which has a cylindrical, rear end portion 64 received within the open front end 66 of the rear end section 58 of outer casing 56. An air gap 68, as shown in Fig. 1, formed by the spacing between the large diameter front end 66 of rear end section 58 and the smaller diameter cylindrical rear end portion 64 of front end section 62, provides an exhaust path for a portion of the air exhausted from the turbine wheel housing 46, as discussed in more detail below. DRIVE SHAFT AND FEED TUBE The hollow motor drive shaft 40, connected at a first end 70 to turbine wheel 38 disposed in the turbine wheel housing 46 of rotary drive mechanism 34, extends forward along axis of rotation 30 to traverse the entire length of rotary drive mechanism 34 so that the opposite second end 72 of drive shaft 40 projects outward through circular bore 28 of annular charge ring 24. The second end 72 of drive shaft 40 has a threaded section (not shown) and a frustroconically shaped end adapted to securely attach rotary atomizer head 44. Motor drive shaft 40 has a throughbore 74 which is aligned with axis of rotation 30 and extends the length of the drive shaft. A device for supplying coating material, typically paint, includes a removable coating material feed tube 48 which extends the length of throughbore 74. Tube 48 has a first end 76 which communicates with the interior of atomizer cup 44 and which preferably carries a removable nozzle 78. An opposite second end 80 of feed tube 48 is removably mounted to valve 50, as generally shown in Fig. 1. When disposed in throughbore 74 of drive shaft 40, feed tube 48 is supported in cantilever fashion free of contact from the interior wall of bore 74, as disclosed in the 5,100,057 patent, to form the cylindrically shaped air passage 82. EXHAUST AIR An air exhaust passageway 84 is connected at one end to the interior of turbine wheel housing 46 and has a restrictor plug 86 at the opposite end. While a single air exhaust passageway 84 is illustrated, it is within the scope of the invention to provide a plurality of spaced exhaust passageways, each containing a restrictor plug 86, as desired. A discussion of the flow of exhaust air is described with respect to Figs. 22 and 23 of US Patent 6056215. ATOMIZER HEAD An aspect of the embodiment of the invention, relating to the provision of exhaust air to the atomizer head or cup 44, relates to the assembly of the head or cup 44 onto the end of rotary drive shaft 40, as illustrated in Figs. 1 and 2. The atomizer cup 44, as illustrated in Figs. 1 and 2, has an hour glass-like shape and maybe uniformly constructed of the composite material including a low capacitance insulating material, an electrically conducting material, and a binder material as previously described in detail in US Patents 6056215 and 5697559. Alternatively, the cup may be molded from insulative and conductive materials as shown in prior U.S. Patent 4,887,770. . As seen in Figs. 1 and 2, rotary atomizing cup 44 for atomizing coating material is constructed of a rotatable cup body 88 having a hour glass-like shape and a longitudinal axis 90 extending therethrough which coincides with the axis of rotation 30 through the rotary atomizer 10 when cup 44 is mounted onto rotary drive shaft 40 so as to project outward from annular ring 24. Cup body 88 has an inner flow surface 92 adapted to direct flow of the liquid coating material through cup body 88 and an outer surface 94, which in turn, is adapted to direct flow of shaping and vectored air, as previously described in detail in US Patent 6056215 and 5697559. Turning now to the construction of the inner flow surface 92 of rotatable cup body 88, the base section 96 is adapted for mounting the cup body onto the free end of rotary drive shaft 40, by conventional means such as with a threaded connection. A nozzle receiving portion 98 located in an intermediate section 100 is adapted to receive nozzle 78 extending outward from feed tube 38 which in turn is projecting outward from rotary shaft 40. A distribution receiving portion 102 having a conical inner surface 104 is symmetrically disposed about longitudinal axis 90 and is adjoined to the nozzle receiving portion 98 at its inner smaller diameter end and to a forward flow surface 106 at its outer larger diameter end. The forward flow surface 106 is located in the frustroconically shaped end section 108 and terminates at an atomizing lip 110. The forward flow surface 106 forms a forward cavity across which charged coating material flows and is propelled radially outward across atomizing lip 110 to form atomized droplets of coating material adapted for application to a workpiece. Since the cup 44 is semi-conductive or has conductive portions, the coating material becomes charged as it flows in contact with the cup. Therefore, an atomized pattern of charged coating material is produced. The manner in which the paint is atomized by cup 44 is generally described in detail in US Patents 6056215 and 5697559. The hour glass-like shape of rotary atomizing cup 44 in combination with the vectored air supply greatly reduces air usage and paint wrap back problems because of a low, i.e., substantially zero, differential pressure condition across atomizing lip 110. This is beneficial because it provides for improved flow pattern control and clean operation, and there is less tendency for paint wrapback, especially when the system is used in combination with the vectored air, as previously described. The rotary atomizing cup 44 further includes a distributor 112 with a conical insert 114, as seen in Figs. 1 and 2, mounted in the inner flow surface 104. The end of the conical insert 114 is disposed in the outlet end of the nozzle 78 and in spaced relation thereto to allow the coating material to flow into the flow passage 116 between the conical surface 104 and the end 118 of the distributor so that the coating material is forced to flow across flow surface 106 and then across the atomizer lip 110. The distributor 112 also directs the air flowing from air passageway 82 into chamber 120 between the inner flow surface 92 and the nozzle 78 into the flow passage 116 where the air mixes with the coating material before flow across flow surface 106 and then across the atomizer lip 110. In the operation of the electrostatic spray device, a flow of the liquid coating material is directed through a fluid tube 48 extending through and disposed within the rotary drive shaft 40. The rotary drive shaft is rotated by the air turbine motor 34 which simultaneously rotates the atomizer head 44. A first portion of the exhaust air from the air turbine motor 34 is directed through the cylindrically shaped air passage 82 and into the atomizer head 44 to create an air barrier within air passage 82 that prevents the liquid coating material being dispensed by the atomizer head from flowing back into air passage 82. The first portion of the air also serves to mix with the coating material within the atomizer head to improve the delivery of the atomized coating material. A second portion of the exhaust air from the air turbine motor flows through the plug 86 from the atomizer housing along an outer surface 62 of the front end section 14 of the atomizer housing 12. ADDITIONAL EMBODIMENT OF ATOMIZER CUP There follows a discussion of another embodiment of an atomizer housing and cup 200, shown in Figs. 3, 4 and 5, which is generally similar in size and shape to the previously-described atomizer cup 44, yet markedly different in certain respects from that and other atomizer cups as previously described in detail in US Patents 6056215 and 5697559. As will become apparent, as a result of the differences in the atomizer cup 200, certain changes are also made to other components of the previously-described electrostatic, liquid spray, rotary atomizer 10. It is required that spray devices, such as those described herein, must pass a FM Standard 7260 test. The 7260 test involves placing an electrostatic charge on the atomizer cup 44, 200 and placing the cup into a bag filled with propane. Then a ground ball, in other words a metal ball on the end of a rod, is brought near the cup. If a spark jumps from the cup to the ground ball and the propane ignites, the spray device fails the 7260 test. It is an object of the invention to provide an atomizer cup that facilitates passing the 7260 test. Generally, whereas the previously-described atomizer cup 44 is constructed of a semi-conductive composite material including a low capacitance insulating material and an electrically conducting material and a binder material, the atomizer cup 200 of this embodiment is constructed primarily of a non-conductive material such as PEEK or PPS-Rayton (polyphenylene sulfide), with a plurality of conductive pathways embedded therein for carrying the electrical charge supplied by the power supply 32 from one end of the atomizer cup 200 to the other end of the atomizer cup 200. Generally, the atomizer cup 200 is sized and shaped for use with an electrostatic, liquid spray, rotary atomizer 10, such as is illustrated in Fig. 1, which has an atomizer housing 12 and an air control element 22 incorporating an annular charge ring 24 with a circular bore 28 which is on an axis 150 which is coincident with a longitudinal axis of rotation 34 that extends through the atomizer housing 12. Such a rotary atomizer 10 would also have an internal power supply 32, located within and interior chamber 20 for generating high voltage electrostatic energy in the range of from about 30,000 volts DC to about 100,000 volts DC. The atomizer cup 200 is comparable in many respects to the atomizer head 30 of Figs. 1 and 2. The atomizer cup 200 is annular, has a one (rearward) end 201, another opposite (forward) end 203, an hour glass-like shape, a longitudinal axis 202 and a bore 204 extending therethrough. The longitudinal axis 202 coincides with the axis of rotation 30 through the rotary atomizer 10 when the atomizer cup 200 is mounted onto the rotary drive shaft 42 so as to project from annular ring 24. The atomizer cup 200 has an inner flow surface 204 (bore) that is adapted to direct flow of the coating material through the atomizer head 200 and an outer surface 206 that is adapted to direct the flow of shaping and vectored air. The atomizer cup 200 includes a base section 208 symmetrically disposed about the longitudinal axis 202. The outer surface 206, in the vicinity of base section 208, has a cylindrical bottom surface portion 210. An intermediate section 214 of the atomizer cup 200, symmetrically disposed about the longitudinal axis 202, includes an outer surface formed of a first surface portion 216 which is adjoined to (contiguous with) the body surface portion 210 and tapers inward, a second surface portion 218 which tapers outward, and a concave intermediate surface portion 220 which extends between the first and second surface portions 216 and 218, respectively. A generally frustroconically shaped end section 222 is symmetrically disposed about the longitudinal axis 202 and has an outer surface 224 which intersects (is contiguous with) the second surface portion 218 of intermediate section 214 and terminates with a front edge surface 226 at the front end 203 of the atomizer cup 200. The bottom portion 210 is rearward of the intermediate section 214 which is rearward of the frustroconically shaped end section 222. Conversely, the frustroconically shaped end section 222 is forward of the intermediate section 214 which is forward of the bottom portion 210. Turning now to the construction of the inner flow surface 204 of the atomizer cup 200, a mounting portion 228 in the base section 208 is at least partially threaded (not shown) and adapted for mounting the atomizer cup 200 onto the free end of the rotary drive shaft 40. A nozzle receiving portion 230 in the intermediate section 214 adjoins (is contiguous with) the mounting portion 228 and is adapted to receive the nozzle 78 extending outward from the feed tube 76 which is projecting outward from the rotary shaft 40. The distribution receiving portion 231 of the atomizer head or cup 200 differs somewhat from the distribution receiving portion 104 of the cup 44. The distribution receiving portion 231 of cup 200 has a conical surface 232 which is symmetrically disposed about the longitudinal axis 202 and is adjoined to (contiguous with) the nozzle receiving portion 230 at its inner smaller diameter end and has a forward flow surface 234 which is of somewhat lesser radial extent than the forward flow surface 106. The forward flow surface 234 is similarly located in the frustroconically shaped end section 222 and terminates at an atomizing lip 226 which is the front edge 203. The forward flow surface 234 forms a forward cavity across which charged coating material flows and is propelled radially outward across atomizing lip 226 to form atomized droplets of coating material adapted for application to a workpiece. The hour glass-like shape of the atomizing cup 200 in combination with the vectored air supply, as described herein, greatly reduces air usage and paint wrap back problems because of a low, i.e., substantially zero, differential pressure condition across the atomizing lip 226. This is beneficial because it provides for improved flow pattern control and clean operation, and there is less tendency for paint wrapback. While the improved pattern control results in a more uniform circular cloud of paint, there is still a slight tendency for the paint to wrapback because of the vacuum behind the atomizer cup 200. The vectored air works together with atomizer 200 to break up the vacuum and prevent paint wrapback and to shape the paint pattern, by reducing the diameter of the paint cloud. COMPOSITION OF THE ATOMIZER CUP The atomizer cups described hereinabove are constructed of a semi-conductive composite material including a low capacitance insulating material and an electrically conducting material and a binder material. The atomizer cup 200 of this embodiment is constructed primarily of a non-conductive material such as PEEK or PPS-Rayton (polyphenylene sulfide), with a plurality of conductive pathways 240,242 embedded therein for carrying the electrical charge supplied by the power supply 32 along the length of the atomizer cup 200 from a position which is rearward of the intermediate section 214 to the frustroconically shaped end section 222 of the atomizer cup 200. The conductive pathways 240,242 are made of an electrically conducting material which is preferably a carbon containing material, and more particularly a carbon fiber. Other electrically conducting materials such as carbon black or particulate graphite can be used for the conductive pathways. As shown in Figs. 5, 6 and 7, a plurality (ten are shown) of conductive pathways 240,242 are formed in the body of the atomizer cup 200 and are of two types (sets). A first type (set) of conductive pathway 240 is elongate and traverses the length of the atomizer cup 200 from its cylindrical bottom surface portion 210 to its frustroconically shaped end section 222, within the body of the atomizer cup 200. Each elongate conductive pathway 240 has a first end 240a which exits from within the body of the atomizer cup 200 so as to be exposed at an external surface of the cylindrical bottom surface portion 210, and has a second end 240b which exits from within the body of the atomizer cup 200 so as to be exposed at an internal surface 232 of the frustroconically shaped end section 222. There are preferably five such elongate conductive pathways 240 disposed within the body of the atomizer cup at evenly-spaced intervals about the axis 202 at a first distance (radius) "R1" from the axis 202. The five first ends 240a (only two of these first ends 240a are visible in the view of Fig. 6) of the five conductive pathways 240 exit the outer surface 206 of the bottom section 210 at evenly-spaced intervals at a second distance (radius) "R2" from the axis 202 which is the external radius of the bottom section 210. The five second ends of 240b (all of which are visible in the view of Fig. 7) of the five conductive pathways 240 exit the inner surface 232 of the frustroconically shaped end section 222 at evenly-spaced intervals at a third distance (radius) "R3" from the axis 202 which is approximately equal to the first distance "R1" from the axis and which is typically less than the second distance "R2". As best viewed in Fig. 5, the second ends 240b, or end portions, of the first elongate conductive pathways 240 are preferably enlarged (increased) in diameter(cross-dimension) as contrasted with the main body portion of the first elongate conductive pathways 240. A second type (set) of conductive pathway 242 is elongate and traverses the length of the atomizer cup 200 from its cylindrical bottom surface portion 210 to its frustroconically shaped end section 222, within the body of the atomizer cup 200. Each elongate conductive pathway 242 has a first end 242a which exits from within the body of the atomizer cup 200 so as to be exposed at an external surface of the cylindrical bottom surface portion 210, and has a second end 240b which exits from within the body of the atomizer cup 200 so as to be exposed at an internal surface 232 of the frustroconically shaped end section 222. There are preferably five such elongate conductive pathways 242 disposed within the body of the atomizer cup at evenly-spaced intervals about the axis 202, preferably at the same first distance (radius) "R1" from the axis 202, preferably between adjacent ones of the first type of conductive pathways 240. The distances (R1) of the conductive pathways 240 and 242 from the axis 202 are preferably the same as one another, and are limited only by the thickness of the body portion of the atomizer housing 200. The five first ends 242a (only two of these first ends 242a are visible in the view of Fig. 6) of the five conductive pathways 242 exit the outer surface 206 of the bottom section 210 at evenly-spaced intervals at the second distance (radius) "R2" from the axis 202 which is the external radius of the bottom section 210. The five second ends of 242b (all of which are visible in the view of Fig. 7) of the five conductive pathways 242 exit the inner surface 232 of the frustroconically shaped end section 222 at evenly-spaced intervals, preferably at the same third distance (radius) "R3" from the axis 202. However, it is within the scope of this invention that the distance from the axis 202 for the second ends 240b of the first elongate conductive pathways 240 need not all be the same as one another, that the distance from the axis 202 for the second ends 242b of the second elongate conductive pathways 242 need not all be the same as one another, and that the distance from the axis 202 for the second ends 242b of the second elongate conductive pathways 242 need not all be at the same distance from the axis 202 as the second ends 240b of the first elongate conductive pathways 240. As best viewed in Fig. 5, the second ends 242b, or end portions, of the second elongate conductive pathways 242 are preferably enlarged (increased) in diameter(cross-dimension) as contrasted with the main body portion of the second elongate conductive pathways 242. As described thus far, the second elongate conductive pathways 242 are suitably identical to the first elongate conductive pathways 240. The second elongate conductive pathways 242 differ from the first elongate conductive pathways 240 in the following manner. The second conductive pathways 242 have extension portions 244 which extend from the second ends 242b of the second conductive pathways 242, within the body of the frustroconically shaped end section 222 and branch off to exit both the outer and inner surfaces of the frustroconically shaped end section 222 near the front edge 226 of the atomizer cup 200. The extension portions 244 of the second elongate conductive pathways 242 are suitably of the same material as the second elongate conductive pathways 242, are elongate, and have first ends 244a which are adjoined to(contiguous with) the second ends 242b of the second elongate conductive pathways 242. At opposite ends of the extension portions 244, each extension portion 244 branches off so as to have a first opposite end portion 244b which exits the outer surface portion 218 of the frustroconically shaped end section 222 near the front edge 226 of the atomizer cup 200 at a fourth distance (radius) "R4" from the axis 202, and a second opposite end portion 244c which exits the forward flow surface 234 of the frustroconically shaped end section 222 near the front edge 226 of the atomizer cup 200 at a fifth distance (radius) "R5" from the axis 202. .Suitable dimensions for the distances "R1", "R2", "R3" and "R4" and "R5" are: the distance "R1" is approximately 0.390-0.395 inches (0.991 to 1.003 cm); the distance "R2" is approximately 0.6115-0.6130 inches (1.5532 to 1.5570 cm); the distance "R3" is approximately 0.390-0.395 inches (0.991 to 1.003 cm); the distance "R4" is approximately 0.900 inches (2.286 cm); and the distance "R5" is approximately 0.700 inches (1.778 cm). The atomizer cup 200 made of a non-conductive material with conductive passageways embedded therein provides a noticeable improvement over the semiconductive atomizer cup 44 with respect to passing the FM Standard 7260 test. The conductive pathways 240 and 242 (including extension 244) can be conductive or semi-conductive and have a resistivity measured in ohm-centimeters (ohms times centimeters). Analytically, each cup 200 and the particles (e.g., paint particles) being charged (i.e., the charging process) have an impedance, and it important to "match" these impedances for maximum transfer efficiency and, consequently, to minimize the amount of power dissipated (heat generated) in the cup 200. As the resistivity decreases, the cup becomes more conductive and more current flows at a given potential difference, thereby increasing power dissipated in the cup. It has been found that a workable resistivity range is between 104 and 106 ohm-centimeters. The cup is preferably produced to be closest to the higher end of this resistivity range to ensure that the cup passes the 7260 test. . The number and cross-sectional dimensions of the conductive pathways 240 and 242 also affects the transfer efficiency. As fewer (e.g., than 10) pathways are used, the transfer efficiency goes down. There is, of course, a design limitation on making the diameter of the pathways 240 and 242 much smaller than 0.065 inches (0.1651 cm), because of manufacturing (e.g., injection molding) limitations, and they must be small enough to fit (be embedded) within the body of the cup. A suitable diameter (cross-dimension) for the first and second pathways 240 and 242 themselves, and their first ends 240a and 242a, respectively is 0.095 inches. The larger (increased cross-dimension) second ends 240b and 242b are limited in size so that they don't touch each other and form a conductive ring (annulus) on the inside surface of the cup. It is believed that having such a conductive ring on the front inside surface of the cup would cause too much charge to accumulate on its front surface, thereby causing the cup to fail the 7260 test. The circular cross-section of the pathways and their ends is somewhat arbitrary, and is limited only by manufacturing processes. CONVEYING CHARGE TO THE CONDUCTIVE PATHWAYS As mentioned above, as a result of the differences between the atomizer cup 200 and the atomizer cup 44, certain changes are also appropriately made to other components of the electrostatic, liquid spray, rotary atomizer 10 to which the atomizer cup 200 is mounted, as shown in Fig. 11. The previously-described electrostatic, liquid spray, rotary atomizer 10 incorporates an annular charge ring 22, shown in detail in Figs. 3 and 4, as previously described in detail in Application Nos. 08/834,290 and 08/404,355, which is detachably mounted to the front surface 23 of the forward section 14 of the atomizer housing. The annular ring 22 has a front wall 26 provided with a circular bore 28 about an axis 150 which is coincident with a longitudinal axis of rotation 30 that extends through the atomizer housing 12. There follows a discussion of another embodiment of an annular charge ring 250, as shown in Figs. 8 and 11 which is generally similar in size and shape to the previously described annular ring 22, yet markedly different in certain respects from the previously-described annular ring 22. The annular charge ring 250 has an outer surface 258 which is tapered inward from the forward section 14 of the atomizer housing 12 to the front wall 252 which has a circular throughbore 254 about an axis 256 which is coincident with a longitudinal axis of rotation 30 that extends through the atomizer housing 12 and coincident with the longitudinal axis 202 of the atomizer cup 200 when the cup 200 is mounted within the charge ring 250. The inner chamber 260 of the annular ring 250 has a flow directing section formed of a generally cylindrical wall 264 which is symmetrically disposed about the longitudinal axis 256. When the annular ring 250 is mounted onto the rotary atomizer housing 14, the longitudinal axis 256 coincides with the axis of rotation 30 through the rotary atomizer 10. These features are similar to the corresponding features of the annular ring 22. Preferably, but not necessarily, a plurality of ribs 262 are evenly spaced and disposed in parallel relation with the axis 256 along the inner surface 264 of the cylindrical wall 262. The ribs 262 are sized to engage the outer surface of turbine housing 42 when the annular ring 250 is assembled with conventional means, such as screws, to the front surface 23 of forward section 14. The open passageways between the ribs 262 and the turbine housing 42 provide a flow path for the vectored air to flow in the forward direction through the circular wall 264. The annular ring 250 includes air control members 266 formed in circular bore 254 for directing the flow of vectored air around the atomizing head 44, as discussed in more detail hereinabove. The air control members 266 include a plurality of slots 268 extending outward from the airflow surface 270 of the circular bore 254 which suitably are angled and spaced from one another in the manner of the annular ring 22, as previously described in detail in Application Nos. 08/834,290 and 08/404,355 to direct flow of vectored air against the surface of atomizer cup 200. These features, and the advantages accruing to same, are similar to the corresponding features of the annular ring 22. The design of the annular charging ring 250 differs from that of the annular ring 22 in the following respects. As shown in Figs. 8, 9 and 10, high voltage electrostatic energy is transferred from the power supply 32 via an electrical circuit including a conductor 280 (compare 319 of ring 22). A resistor "A" 282 (compare 164) which has one end connected to the conductor 280 is mounted within the annular ring 250 in a cylindrical housing 281. Another end of the resistor "A" 282 is connected with a conductor 283. Three resistors 284a, 284b and 284c (compare resistors 168a, 168b, 168c), also labeled "B", "D" and "C", respectively, each having one end connected to the conductor 283 and each having a cup-charging electrode 286a, 286b and 286c (compare electrodes 174a, 174b, 174c) extending from another end thereof, are potted with an epoxy material into a channel 288 (compare 170) between the cylindrical wall 264 (compare 148) and the inner surface 290 (compare 172) of the annular ring 250 (compare 22). Another resistor 292, labeled "5" is connected at one end to the conductor 280 and has an electrode 294 extending from its other end. The electrodes 286a, 286b and 286c ("B", "D" and "C") are electrostatic charging and field electrodes projecting from the front surface of wall 26 of the annular charging ring 250. The resistors 284a, 284b and 284c lower the spark potential at the electrodes 286a, 286b and 286c, respectively. Although there are only three of these electrodes 286a, 286b and 286c, it must be remembered that the atomizer cup 200 is rotating so that the first ends of all ten first ends of conductive pathways 240 and 242 will pass in close proximity to each of the three electrodes 286a, 286b and 286c to receive electrostatic charge therefrom. A non-conducting end cap or ferrule 296 is disposed over the electrode end of each of the charging resistors 284a, 284b and 284c so that, as best viewed in Fig. 8 which shows one charging resistor 284c and one electrode 286c, the charging electrode projects through the end cap 296. When the atomizer cup 200 is fitted within the front opening 254 of the charge ring 250, the first ends 240a and 242a of the conductive pathways 240 and 242, respectively, are positioned to be as close as possible to the ends of the charging electrodes (286a, 286b and 286c). Preferably, the first ends 240a and 242a are no greater than 0.200 inches (0.508 cm) away (longitudinally) from the ends of the charging electrodes 286a, 286b and 286c when the atomizer cup 200 is mounted to the annular charging ring 250 to keep the gap therebetween to a minimum. A minimal amount of clearance, for example 0.020 inches (0.0508 cm) is generally desirable to avoid mechanical wear between the rotating atomizer cup 200 and ends of the charging electrodes 286a, 286b and 286c. Referring to Fig. 8A, an access hole 300 is provided in the outer wall to extend through the annular charging ring 250 to create an approach way to reach the drive shaft within the interior of the atomizer housing. This access hole 300 permits the insertion of a rigid, elongate tool (not shown) such as an Allen wrench into the outer wall of the annular charging ring which will engage a corresponding recess (hole) in the surface of the rotary drive shaft 40 so that the rotary drive shaft may be prevented from freely rotating. This permits the operator to quickly demount the atomizer cup 200 for cleaning or replacement, without requiring dismantling the charge ring 250. In order to ensure the airtight integrity of the annular charge ring 250, a check valve such as a "duckbill" valve 302 is disposed in the access hole 300 between an inlet orifice 304 and an outlet orifice 306 thereof. In this manner, the airtightness of the charge ring 250 is maintained to ensure the desired flow of vectored air therethrough. The access hole 300 forms an air gap through which electrostatic charges can leak, thereby compromising the ability to pass the FM 7269 test. Therefore, a small electrode 294 from the resistor 292 ("E") is inserted into the charge ring 250 near the access hole 300. The resistor 292 is preferably of high resistance, such as 50 megohms. In this manner, a small (low) voltage will be present in the vicinity of the access hole 300, and a small spark that is insufficient to ignite the propane used in the FM 7260 test may be generated at the access hole 300. If the electrode 294 were not provided near the access hole 300, the ground ball used in the FM 7260 test would pull voltage off the rotary drive shaft 40. This is because the rotary drive shaft is not grounded and, in use, will eventually charge to the full voltage potential of the power supply 38. In this manner the atomizer cup 200 can quickly be changed, without the need to first remove the annular ring 250. The addition of a fourth resistor 292 and a fourth probe electrode 294 which are different in function than the three charging resistors 284a, 284b and 284c and corresponding charging electrodes 286a, 286b and 286c, respectively, is a key difference between the charging ring 22 and the charging ring 250. The result of incorporating a charging ring 250 in combination with a rotary cup 200 is a low capacitance, rotary atomizer that is able to pass the FM 7260 test. ROTARY ATOMIZER MOUNTED ON ROBOT Referring to Fig. 12, there is illustrated a low capacitance, rotary atomizer 300 having a annular charge ring properties and because it has been found to be impermeable to solvents used in many of the paints being sprayed with the rotary atomizer 300. The tube 332, being in the shape of a spiral, extends the length of the fluid path from the supply device 330 to the rotary cup 200 and forms a voltage isolator that is long enough to increase the total electrical resistance of the paint column between the rotary atomizer 300 and the supply valve 330 to reduce the electrical current through the paint column to an extent that the paint being charged at the rotary cup 200 does not build up a charge in the rotary atomizer which could cause a potential hazard to an operator or cause the rotary atomizer 300 to fail the FM7260 test. In the preferred embodiment, the spiral tube 332 is about four feet in length and the control valve 330 is grounded. The invention in the preferred embodiments provides an improved electrostatic rotary atomizing spray device has an improved rotary, high speed, atomizing cup with a plurality of conductive pathways for transferring electrostatic energy from the electrodes of a charge ring to the paint without igniting the propane used in the FM 7260 test. An access hole in the charge ring for insertion of a tool to restrain free rotation of the turbine shaft to which the atomizer cup is secured facilitates 250 mounted to the forward section 14 of the atomizer housing 12. As shown in Fig. 11, a atomizer cup 200 is mounted to the electrostatic liquid spray, rotary atomizer 300, as shown in detail in Fig. 11. The rotary atomizer 300 is mounted to a robot arm 310 which in turn is secured to a conventional industrial robot, the details of which are not part of the present invention. The rear of the manifold plate of rotary atomizer 300, such as plate 52 shown in Fig. 1, is mounted to three support structures 312, 314 and 316. Each of the support structures 312, 314 and 316 has a first support rod 318, 320 and 322, respectively, which are secured at one end to the robot arm 310. The support structures 312, 314, 316 also have second support rods 324, 326 and 328, respectively, which extend at a transverse angle (typically about 90 degrees to the vertical support 312, 314 and 316 rods 318, 320 and 322, respectively, and which are attached the rear end of the rotary atomizer 300, preferably to the manifold plate, such as manifold plate 52. A supply or control valve device 330 provides a supply of the coating material to the air operated valve 50 by a passageway (not shown). The supply valve 330 is connected to the flow passageway in the manifold plate leading to the air operated valve 50 through a tube 332 formed into an elongated spiral for the majority of its length. The spiral tube is preferably made of Teflon because of its good insulation or dielectric quick demounting of the atomizer cup for cleaning or replacement. An additional electrode in the electrical circuit and disposed in the access hole of the charge ring presents a low voltage in the vicinity of the access hole for providing a small spark that is insufficient to ignite the propane used in the FM 7260 test. The rotary atomizer with the improved charge ring and rotary cup can be mounted to a robot and provided with an elongated spiral passageway connecting the supply pump to the rotary atomizer to increase the electrical resistance between the atomizer and the supply pump so that insufficient electrical charge will be present in the passageway carrying paint to the rotary cup to ignite the propane used in the FM 7260 test.

Claims: An atomizer cup (200) for an electrostatic rotary atomizing spray device (10) for spraying a liquid coating material, the atomiser cup comprising a body (88) is formed primarily of a non-conductive material having a one end (201), another opposite end (203), a bore (204) extending therethrough from the one end to the other end, an outer surface (206), a longitudinal axis (202), a rear section (201) symmetrically disposed about the longitudinal axis (202), a generally frustoconically shaped forward section (222) contiguous with and forward of the rear section (201) and symmetrically disposed about the longitudinal axis (202), the rear section (201) having an outer surface (206) and the frustroconically shaped forward section (222) including a front edge (226), an outer surface (218) and an inner surface (232,234); and a plurality of elongate conductive or semi-conductive pathways (240, 242) embedded within the body (88) for carrying an electrical charge, the pathways (240,242) extending from the rear section (201) to the frustoconically shaped forward section (222) of the atomizer cup, characterised in that at least one pathway (242) has an end (242b,244c) which exits at the inner surface (232,234) of the frustaconically shaped forward section (222) and an end (244b) which exits on the outer surface (218) of the frustoconically shaped forward section (222). An atomizer cup as claimed in Claim 1 wherein one or more of the pathways (240, 242) have ends (240b,244c) exiting at the inner surface (232,234) of the frustroconically shaped forward section (222). An atomizer cup as claimed in Claim 1 or 2 wherein one or more of the pathways (240, 242) have ends (244b) exiting at the outer surface (232,234) of the frustroconically shaped forward section (222). An atomizer cup according to any preceding claim wherein plural pathways (240, 242) have ends exiting at the inner and/or outer surface (232, 234) of the frustroconically shaped forward section (222) and wherein the ends (240b, 242b, 244c) are circumferentially disposed on the inner and/or outer surface(232,234) of the frustoconically shaped forward section (222). An atomizer cup as claimed in any preceding Claim wherein the pathways (240, 242) have opposite ends (240a,242a) exiting at the outer surface(206) of the rear section (201). An atomizer cup as claimed in any preceding Claim wherein the pathways (240, 242) have opposite ends (240a,242a) proximate a source of electrical energy (286c). An atomiser cup according to any preceding claim wherein the inner surface of the body (88) has a nozzle receiving portion (228). An atomizer cup according to any preceding claim, wherein the body (88) has an hour glass like shape. An atomizer cup according to any preceding claim, wherein the pathways (240,242) have a resistivity which provide the atomizer cup and the particles being charged with an impedance which is matched so as to minimise the amount of power dissipated in the cup (200). An electrostatic rotary atomizing spray device for spraying a liquid coating materials comprising an atomiser housing (12,14) which defines an interior chamber; a charging ring (250) having a front wall (252) with a circular bore (254), the charging ring (250) being mounted on the atomiser housing (12,14), a drive shaft (40) within the interior of the atomiser housing (12,14) and extending through the circular bore (254), the drive shaft (40) being attached at a first end to a motor within the atomiser housing (12,14) and at a second end to the atomizer cup (200) according to any preceding claim and further including an annular charge ring (250), a power supply (32), and a cup-charging electrode (286a,286b,286c) for transferring electrostatic energy from the power supply (32) to the atomizer cup (200) via the pathways (240, 242). A device according to claim 10, wherein the charge ring (250) has an access hole (300) extending through the charge ring (250) to create an approach way for reaching the drive shaft (40) within the atomiser housing (12,14). A device according to claim 11, wherein an electrode (294) is disposed near the access hole (300) for providing a low voltage at the access hole (300). A device according to claim 11 or 12, further comprising a check valve disposed in the access hole (300). A rotary atomizer system comprising the atomiser cup (200) of any of claims 1 to 10, wherein a fluid tube (48) directs a flow of liquid coating material to the atomizing cup (200) whereby the material is electrically charged as it passes through the atomizing cup (200), and further including a robot arm (310) having an electrically grounded liquid supply control device (330) attached thereto, a voltage isolator (332) connecting the fluid tube (48) to the electrically grounded liquid supply control device (330) to prevent electrical current from being transferred through the liquid coating material, which is being charged at the atomizing cup (200), to the control device (330). A system according to claim 14 wherein the atomizing cup (200) has a low capacitance and the voltage isolator (332) is a spiral tube.

Inventors: BEAM HAROLD, CONRAD JEFFREY T, FRANCIS WOODIE, HANSINGER MICHAEL P, KARBOWNICZEK JOSEPH JEROME, SCHROEDER RONALD, BEAM, HAROLD, CONRAD, JEFFREY T., FRANCIS, WOODIE, HANSINGER, MICHAEL P., KARBOWNICZEK, JOSEPH JEROME, SCHROEDER, RONALD
Applicants: NORDSON CORP, NORDSON CORPORATION
Application year: 1998
Main classification: B05B5
IPC codes: B05B7, B05B15, B05B5, B05B13, B05B3