US3613993A

US3613993A - Electrostatic painting method and apparatus

Info

Publication number: US3613993A
Application number: US771135A
Authority: US
Inventors: Meredith C Gourdine; Edward L Collier; Gerald P Lewis; Harold Mccrae; Donald H Porter
Original assignee: Gourdine Systems Inc
Current assignee: Energy Innovations Inc; Gourdine Systems Inc
Priority date: 1968-10-28
Filing date: 1968-10-28
Publication date: 1971-10-19
Anticipated expiration: 1988-10-19
Also published as: SE361830B; GB1245335A; JPS4938693B1; DE1953989A1; DE1953989C3; NL147651B; DE1953989B2; BR6913681D0; FR2021672A1; NL6915251A

Abstract

Apparatus and method for electrostatically charging a stream of atomized paint particles discharged from a spray gun. An emitter electrode is positioned in spaced relation to the end of the spray gun to establish a corona discharge from the electrode tip to a conductive portion of the gun end so that the direction of the discharge is opposite to the direction of flow of the particle stream. The apparatus may comprise an attachment unit for converting conventional spray guns to electrostatic spray guns, including a dielectric flow path-defining member downstream of the spray gun.

Description

Ilnited States Patent Inventors Meredith C. Gourdine West Orange; Edward L. Collier, Morris Plains; Gerald P. Lewis, West Orange; Harold McCrae, Upper Montclair; Donald H. Porter, Carlstadt, all of NJ.

Appl. No. 771,135

Filed Oct. 28, 1968 Patented Oct. 19, 1971 Assignee Gourdine Systems, Inc.

Livingston, NJ.

ELECTROSTATIC PAINTING METHOD AND APPARATUS 22 Claims, 5 Drawing Figs.

US. Cl 239/3, 239/15 Int. Cl B05b 5/02 Field of Search 239/3, 15;

[56] References Cited UNITED STATES PATENTS 2,959,353 11/1960 Croskey et al. 239/15 2,995,393 8/1961 Charp 239/15 X 3,009,441 11/1961 Juvinall 239/15 X 3,195,264 7/1965 Ward,.lr 239/3 X 3,292,860 12/1966 Williams et al. 239/15 Primary Examiner-Lloyd L. King Assistant Examiner-Gene A. Church Att0rneyBrumbaugh, Graves, Donohue & Raymond ABSTRACT: Apparatus and method for electrostatically charging a stream of atomized paint particles discharged from a spray gun. An emitter electrode is positioned in spaced relation to the end of the spray gun to establish a corona discharge from the electrode tip to a conductive portion of the gun end so that the direction of the discharge is opposite to the direction of flow of the particle stream. The apparatus may comprise an attachment unit for converting conventional spray guns to electrostatic spray guns, including a dielectric flow path-defining member downstream of the spray gun.

PATENTEDBU 19 I97! IOX 78 Fla 5 ///v/',/-J'/-/:5 MEREDITH C. GOURDINE,

I DONALD H. PORTER h4/(M1'l\lll7/)/ /14 ,41) "2 ,1 f 1mm their ATTORNEYS ELECTROSTATIC PAINTING METHOD AND APPARATUS BACKGROUND OF THE INVENTION The present invention relates to electrostatic spray painting, and particularly to an electrostatic spray painting apparatus and method wherein atomized particles of paint, or other coating material, are projected from a spray gun through a high-density corona discharge zone toward the article to be coated.

Electrostatic spray guns for discharging a stream of charged atomized paint particles have been widely used in the paint industry. Such guns usually include means for atomizing and discharging a stream of a paint into the atmosphere either by impinging jets of compressed air on the paint stream (air type), or by discharging the paint through a fine aperture under high pressure on the order 1,000 pounds per square inch (airless type). Normally, an electrostatic charge is imparted to the individual paint particles concurrently with, or immediately after, their atomization. The charge transfer is typically accomplished by contacting the atomized particles with an electrode at high potential or by passing them through a corona discharge extending transversely of the stream of particles. Usually the workpiece to be painted is maintained at or near ground potential so that an electric field between the gun and the workpiece promotes attraction of the charged particles to the workpiece.

Most known devices impress a direct current voltage between the emitter (corona) and attractor electrodes ranging between 40,000 to 100,000 volts. High voltages of this order present several serious practical problems; for example, flashover sparking may occur between the emitter and attractor electrodes or between the emitter electrode and an adjacent conducting surface. Such sparking can result in fires and injury to operating personnel. Moreover, high-potential apparatus require the use of substantial insulation and cumbersome resistors to render them electrically safe. Heavy conductors and large power packs are also needed to meet power consumption requirements of such systems.

High voltages have heretofore been considered essential, however, for the reason that known electrifying apparatus, when operated at low voltages, did not produce a charge transfer to the individual particles sufficient to create the required charge density needed to produce high-quality paint finishes. Additionally, most typical apparatus tend to bulkiness, lack of versatility and complexity. These and other objectionable features of the prior art are overcome by the present invention.

SUMMARY OF THE INVENTION In accordance with the present invention, electrostatic charging of the stream of coating particles is accomplished at low-operating potentials. An emitter, or corona, electrode is positioned downstream from an attractor electrode, which may be the spray gun head, and a corona discharge having a substantial rearward component is established between the emitter and attractor electrodes.

The emitter electrode preferably comprises a necdlelike member, the tip of which is disposed on or near the longitudinal axis of the particle stream. With the emitter electrode so located, it has been found that a high charge is transferred to the atomized paint particles, even when the discharge potential is as low as 8 to 12 kilovolts. With the corona discharge being directed rearwardly, a high concentration of atmospheric ions is produced which traverse the stream of atomized paint particles in a direction opposite to the travel of the particles. This relationship between the ionizing discharge field and the paint particle stream greatly increases the surface charge attainable by the individual atomized particles over that which occurs when the electric field extends either primarily transversely to or parallel with the stream. Since the atomized paint particles must travel longitudinally through the ion stream rather than perpendicularly across it, the effective residence time of each particle in the ion stream is increased. In addition, some improvement in charging attributable to friction produced by ion-particle collision is also thought to exist. Both of these factors, the more friction-producing collisions and the longer exposure time of the particle to the ionized region, tend to increase the charge transferred to a given paint particle.

To minimize the turbulence introduced into the particle stream by the emitter electrode, and also to prevent the agglomeration of paint particles on the electrode, the electrode may be mounted to extend into the particle stream in the upstream direction at an angle with the longitudinal axis of the stream of from 30 to 60. It also may be bent along a predetermined radius of curvature to further reduce the formation of agglomerates on the electrode shank.

The electrical discharge from the needle tip can terminate at a separate attractor electrode, the air horns, spray nozzle, or other portion of the spray head. Downstream confinement of the charged particle stream for the development of a higher space charge field, when required, is accomplished through the use of a dielectric channel member receiving the charged particle flow.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of these and other features of the invention, as well as the objects and, advantages thereof, reference may be made to the following detailed description and to the drawing, in which:

FIG. 1A is a side elevation view, partly in section, of a basic electrostatic charging apparatus in accordance with the present invention showing schematically the electrical circuit of the charging apparatus;

FIG. 18 is an end view of the charging apparatus of FIG. IA;

FIG. 2 is a side elevation view, partly in section, of an exemplary embodiment of the charging apparatus showing the charging apparatus mounted on an air-type spray gun;

FIG. 3 is a side elevation view, partly in section, of another embodiment of the charging apparatus showing the charging apparatus mounted on an airless-type spray gun; and

FIG. 4 is a schematic illustration, partly in cross section, showing the application of coating particles to workpiece surfaces by the charging apparatus in accordance with the inventron.

DESCRIPTION OF AN EXEMPLARY EMBODIMENT FIGS. IA and IB illustrate schematically an electrostatic charging apparatus in accordance with the present invention mounted on a conventional air-type spray gun 10 in surrounding relation to the spray head I2, spray nozzle I4 and air horns 16. The air-type gun is shown for illustrative purposes only, the charging apparatus being usable with equal facility with airless-type guns as well.

The charging apparatus includes a high-voltage power supply 118, an emitter electrode 20 and an attractor electrode, comprising in FIGS. IA and 1B the sprayhead 12, including its component parts, the spray nozzle 14 and'the air horns 16. As will be more fully described below,'the particular component serving as the terminus for the electrical discharge depends on the geometry of the gun end andits location relative to the emitter electrode 20. Since all or part of the spray head may function as the attractor electrode, and since it is desirable for safety purposes to maintain the attractor electrode at ground potential, the spray head 12 is grounded by a suitable conductor 21.

Although the charging apparatus is depicted in the drawing in conjunction with spray guns having spray heads which are electrically conductive, and thus capable of functioning as attractor electrodes, it is to be understood that it is not essential that the gun spray head be composed of a metallic or other conductive material. For example, the spray head can be made of a dielectric material and have a'conductive element either imbedded in the material or mounted on the head to function as the attractor electrode. Alternatively, the attractor electrode can be carried as a separate element by a housing surrounding the spray head. At least a portion of the attractor electrode, whatever its form, should be positioned rearwardly of the emitter electrode so that a component of the corona discharge is directly in opposition to the flow of the paint particle stream.

Voltages in the range of from 8 to 12 kilovolts are provided by the power supply 18, which may also be grounded. A current limiting resistance 22 of suitable value is preferably inserted in the line between the power supply 18 and the emitter electrode 20 to reduce the possibility of sparking and to establish correct loading of the circuit at the operating voltage.

A housing 24 comprised of a dielectric material, such as nylon, mounts the emitter electrode 20 in proper relation to the spray head 12 and air horns 16. The emitter electrode 20 may be either curved or straight, for reasons later set forth, and is in the form of an elongated needlelike wire. Preferably, the housing 24 is arranged on the gun so that the tip 26 of the electrode is positioned substantially on the longitudinal axis of the atomized paint spray as it is discharged from the nozzle 14. The electrode tip 26 is spaced downstream of the nozzle 14 and horns 16 a predetermined distance which may be varied to position the tip 26 at the optimum point as required by the magnitude of the voltage applied, the structural geometry of the spray gun and the pattern of the atomized paint particle stream.

The tip 26 may be positioned even with or rearwardly of the ends of the air horns. Ordinarily, however, the tip 26 will be downstream of the horns 16 and the nozzle 14. When an airless-type gun is used, the emitter tip is correspondingly spaced with respect to the nozzle 14 and spray head 12.

With the tip 26 in the position shown in FIG. 1A, and current supplied to the emitter electrode, a continuous corona discharge is created, causing a high-density stream of atmospheric ions to flow in a rearwardly and outwardly direction from the vicinity of the emitter tip 26 to the spray head 12, the nozzle 14 and the horns 16. At close spacings of the tip 26 from the spray nozzle 14, the corona discharge may extend principally between the tip and the nozzle 14, at least when no paint is being discharged from the nozzle. Under normal operating circumstances, however, the corona discharge extending from the tip to the nozzle is quenched by the flow of atomized paint particles so that the principal path followed by the ion stream is from emitter tip 26 to the horns 16. In airlesstype guns, the ion stream normally extends between the emitter tip and the spray nozzle when no paint is being discharged, and between the tip and both the spray nozzle and spray head when paint is being discharged.

In the representative embodiment shown in FIG. 2, the charging apparatus comprises a converter unit, shown generally at 30, which attaches to a conventional, nonelectrostatic spray gun 32 to convert it to an electrostatic spray painting device. The spray gun is similar to that described in connection with FIGS. 1A and 1B and, in addition, has a knurled collar 34 for removably mounting the spray head 36. Also, the conventional air horns 38 have inwardly directed openings 40 in their adjacent faces through which air jets are passed to impinge against the atomized paint particles. The air jets prevent dispersed paint particles from depositing on the horns and thus fouling the spray gun and also shape the paint particle stream into the long, narrow elliptical pattern generally preferred for most spray painting operations.

The converter unit includes in general a housing 42, an annular mounting ring 44, a channel member 46, all of which are composed of a suitable dielectric material, and an emitter electrode 48. Apertures 52 are circumferentially spaced around the mounting ring 44, thereby forming breather passages between the ring and housing. The converter unit 30 is attached to the gun by means of one or more setscrews 54 threaded into the housing 42 and the ring 44 to engage the collar 34.

A lead wire 56 from the power supply 18 (and resistor 22) is received in an opening 60 in the housing 42 and is connected to the electrode 48 by a setscrew 62. The electrode shank 480 extends through the inner surface of the housing 42 and into the opening 60 for this purpose.

A counterbore 64 of a diameter substantially greater than that of the gun spray head 36 is provided in the housing 42 concentrically of the paint stream axis. Immediately adjacent the point where it protrudes into the counterbore 64, the electrode leg 48a, or shank, is bent at a right angle to extend in a direction downstream of and generally parallel to the direction of particle flow. At a point downstream of the horns 38, the electrode is reversely bent along a radius of curvature to form a leg 48b. The angle subtended by the

legs

48a and 48b is preferably within the range of from 30 to 60, so that the electrode enters the spray pattern at about the same angle.

The radius of curvature of the bend 48:: is preferably within the range of from one-sixteenth inch to three-eighths inch to avoid the undesirable accumulation, or agglomeration, of large paint droplets on the emitter electrode. This is an important factor in spray painting systems, inasmuch as the droplets of paint found to agglomerate on a sharply bent electrode are, under certain circumstances, reentrained in the paint stream and deposited on the workpiece, thereby resulting in a poor quality finish. Also, the diameter of the counterbore 64 should be sufficiently large so that the curved portion 48c is well spaced from the paint stream axis. This not only reduces the turbulence introduced into the paint particle stream by the emitter electrode, but also further reduces the possibility of paint droplets accumulating on the electrode.

The tip of the electrode 48 is preferably positioned on the axis of the paint stream downstream from the spray nozzle (not shown) and the ends of the horns 38. But as noted above in connection with FIGS. 1A and 1B and as shown in FIG. 4, the electrode tip may actually be located evenly with or upstream of the downstream ends of the horns. Although the spacing between the tip and the inner surfaces of the horns 38 will vary among commercial guns, it is contemplated that the spacing ranges from one-fourth inch to one-half inch to maintain a corona discharge both during and absent discharge of the atomized paint. Such spacing will produce, for example, a particle charge mass ratio of 2-8 microcoulombs per gram at a typical voltage of 10 kilovolts, a ratio that has been found to be highly effective in coating workpieces with a high-quality finish.

The outwardly flared dielectric channel member 46 extends downstream from the housing 42 concentrically of the atomized particle stream axis. In addition to guarding against sparking, it shields the gun spray head 36, horns 38 and electrode 48 against the deposition of atomized paint particles which have become dispersed and drift back towards the gun.

It also acts as a guide channel to shape the spray pattern and direct the stream of charged paint particles toward the workpiece. The guiding effect of the channel member 46 results from the buildup of an electric potential on the inner surface of the channel member caused by the deposition of charged particles on the inner surface of the member. This buildup creates an electric field which opposes the movement of other like-charged particles from the particle stream toward the channel member so that, after a period of time, an equilibrium is established whereby essentially all of the charged particles pass through the channel member and are directed toward the workpiece. As a consequence, the charge carried by the particles is enhanced and a stronger space charge immediately downstream of the gun is produced than is attained without a channel member. This further results in the conversion of more kinetic energy of each particle into potential energy in overcoming the space charge field. Since the efficiency of an electrostatic coating system increases with an increase in the charge carried by the coating particles, the efficiency of the system can be increased by use of the channel member 46.

A second embodiment of the attachment unit, shown generally at 70, is shown in FIG. 3 in combination with an airless-type spray gun 72. Since an airless-type spray gun has no air horns, the corona discharge is initially established between the tip of the electrode 74 and the spray nozzle 76. Under certain operating conditions, the ion stream may also extend from the tip to the endmost surfaces of the spray head 7%. This latter path normally is followed only when paint particles are being discharged from the gun. The spacing of the electrode tip from the nozzle 76 is set at approximately one-fourth to one-half inch, preferably on or near the paint stream axis. The channel member 30 functions in a manner similar to the channel member 46 and is likewise preferably provided with an outwardly flared downstream end (not shown).

in this embodiment, the emitter electrode 74 is shown as a straight needlelike member. Although the curved electrode and the straight electrode are disclosed with an air-type gun and an airless-type gun, respectively, it is to be understood that the disclosure is made in this manner purely for illustrative purposes and that the electrodes may be interchanged as desired. Although the straight electrode interferes with the atomized particle stream to a greater extent than does the curved electrode, a corona discharge is established directly to the gun end and highly efficient coating of workpieces is obtained with a far simpler electrode arrangement than is presently known in the prior art. Moreover, the straight electrode is particularly useful for applications where the channel member cannot be used because of space or other limitations and there is danger of the curved electrode being brought near enough to the workpiece for sparking to occur.

in the schematic illustration of FIG. 4, a conventional, nonelect'rostatic gun is shown with an electrostatic converter unit 90 mounted on it to provide an electrostatic painting system for painting solid workpieces positioned within a chamber 92. Upon supplying current to the emitter electrode and discharging atomized paint particles from the spray nozzle, a stream of charged atomized paint particles moves through the channel member 94 and is exhausted into the dielectric chamber 92, creating a space charge field. For purposes of explanation, the chamber 92 is shown to contain a solid workpiece 96 of arbitrary shape suitably supported within the chamber.

As the charged paint particles are emitted from the gun into the chamber 92, the workpiece 96 receives an even coating over its entire surface, since the space charge field in the chamber is effective to induce the charged particles to seek a potential lower than the space charge potential. As portions of the surface attain a higher charge, and thereby a greater potential, the attractive forces between other charged particles and the workpiece 96 will decrease and, assuming a dielectric workpiece surface, the charged particles will tend to deposit on those surface portions at a lower potential. Of course, the gun may be used without the chamber 92 to paint workpieces located in open areas, although the rapidity with which the workpieces will be coated will be reduced somewhat.

From the foregoing it is apparent that the invention provides an improved, far simpler electrostatic coating apparatus. Because of its low operating potential capability, owing to short electrode spacing, the apparatus does not require the use of bulky insulation, heavy lead lines, large power packs, etc., and therefore is particularly adapted for converting conventional hand-held spray guns.

It will be understood by those skilled in the art that the embodiments described are intended to be merely exemplary, in that they are susceptible of modification and variation without departing from the spirit and scope of the invention. For example, although the charging apparatus has been disclosed in the form of a converter unit for converting conventional nonelectrostatic spray guns to electrostatic painting systems, it may also be incorporated into a spray-painting installation as an integral part of the spray gun or its associated equipment. All such modifications and variations, therefore, are intended to be included within the scope and spirit of the invention as defined in the appended claims.

1. Apparatus for charging particles of coating material comprising: a spray gun having an electrically conductive spray head for issuing a stream of particles, an emitter electrode spaced from the spray head, means for establishing a corona discharge in the space between the emitter electrode and the spray head and terminating at the spray head, and means for positioning the emitter electrode within the particle stream downstream of and in spaced relation to the spray head such that the principal component of the corona discharge extends in the upstream direction, whereby the particles pass through the corona discharge in opposed relation thereto.

2. Apparatus as defined in claim 1 wherein the spray head includes a spray nozzle for issuing the spray particles and wherein the corona discharge terminates at least in part at the spray nozzle and at least in part at the portion of the spray head immediately surrounding the spray nozzle.

3. Apparatus for charging particles of coating material comprising: a spray gun having an electrically conductive spray head for issuing a stream of particles, the spray head including at least one air horn for directing a jet of air toward the particle stream, an emitter electrode disposed downstream of and in spaced relation to the spray head, and means for establishing a corona discharge having its principal component extending upstream in the space between the emitter electrode and the spray head so that the corona discharge terminates at least in part at the air horn, whereby the particles pass through the corona discharge in opposed relation thereto.

4. Apparatus for charging particles of coating material comprising: a spray gun having an electrically conductive spray head for issuing a stream of particles, an elongate needlelike emitter electrode positioned downstream of and in spaced relation to the spray head, the emitter electrode having a pointed tip disposed substantially on the longitudinal axis of the particle stream, and means for establishing a corona discharge having its principal component extending upstream in the space between the emitter electrode tip and the spray head, whereby the particles pass through the corona discharge in opposed relation thereto.

5. Apparatus in accordance with claim 4 wherein the spray head includes at least one air horn for directing a jet of air toward the particle stream and wherein the corona discharge is established at least in part between the emitter electrode and air horn.

6. Apparatus as defined in claim 4 wherein the spray head includes a spray nozzle for issuing the spray particles and wherein the corona discharge is established at least in part between the emitter electrode and the spray nozzle.

7. Apparatus in accordance with claim 4 wherein the emitter electrode is so disposed as to extend into the particle stream in the upstream direction at an angle of from about 30 to 60 with respect to the longitudinal axis of the particle stream so that the tip of the emitter electrode is directed at least partly toward the spray head.

8. Apparatus in accordance with claim 7, wherein the shank of the emitter electrode is bent along a predetermined radius of curvature to extend back toward the spray head.

9. Apparatus as defined in claim 8 wherein said predetermined radius of curvature is within the range of from one-sixteenth inch to three-eighths inch.

lit). Apparatus as defined in claim 8 wherein the tip of the emitter electrode is spaced from the spray nozzle within the range of from one-fourth inch to one-half inch.

111. Apparatus in accordance with claim 6 wherein the emitter electrode is so disposed as to extend into the particle stream in the upstream direction at an acute angle with respect to the longitudinal axis of the particle stream so that the tip of the emitter electrode is directed at least partly toward the spray head.

12. Apparatus in accordance with claim 4 wherein the tip of the emitter electrode is directed at least partly toward the spray head, whereby the stream of particles issuing from the spray head is directed in opposition to the direction of the emitter electrode tip.

13. Apparatus for charging particles of coating material comprising: a spray gun having an electrically conductive spray head for issuing a stream of particles, an emitter electrode disposed downstream of and in spaced relation to the spray head, and means, including a high-potential power supply for maintaining the emitter electrode at a potential within the range of from 8 to 12 kilovolts and a conductor connecting the spray head to ground, for establishing a corona discharge having its principal component extending upstream in the space between the emitter electrode and the spray head, whereby the particles pass through the corona discharge in opposed relation thereto.

14. An electrostatic spray charging apparatus for attachment to a nonelectrostatic spray gun having an electrically conductive spray head for discharging a stream of particles comprising: a housing member composed of dielectric material, an elongate emitter electrode having a pointed tip carried by the housing member, means on the housing member for adjustably attaching the housing member to the nonelectrostatic gun to position the emitter electrode tip at a point spaced downstream from the spray head and substantially on the longitudinal axis of the spray stream, and means for maintaining a corona discharge between the tip of the emitter electrode and the spray head in a zone occupied by the particle stream, the corona discharge having its principal component extending in the upstream direction, whereby the spray particles pass through the corona discharge in opposed relation thereto.

15. An electrostatic charging apparatus as defined in claim 14 wherein the corona discharge terminates in part at the portion of the spray head immediately surrounding the discharged stream of particles, and the means for maintaining the corona discharge between the emitter electrode tip and the spray head includes a high-potential power supply and a conductor attachable to the spray head for connecting the spray head to a reference potential.

16. Apparatus as defined in claim 15 wherein the power supply provides a potential within the range of from 8 to 12 kilovolts.

17. Apparatus as defined in claim 14 wherein the emitter electrode is bent along a predetermined radius of curvature to form first and second leg portions, the first leg portion being secured at its free end to the housing and extending therefrom in the downstream direction; the second leg portion extending in a direction having a substantial upstream component and having said pointed tip at its free end, whereby said pointed tip is directed generally in opposition to the particle flow direction when the housing is attached to the spray gun.

18. Apparatus as defined in claim 17 wherein the first leg portion of the emitter electrode is generally parallel with the longitudinal axis of the particle stream and wherein the angle subtended by the first and second leg portions is within the range of from 30 to 60.

19. Apparatus as defined in claim 18 wherein said predetermined radius of curvature is within the range of from one-sixteenth inch to three-eighths inch.

20. Apparatus according to claim 14 wherein the emitter electrode includes an elongated member forming an angle of between about 30 and 60 with the longitudinal axis of the particle stream and having a tip portion located closest to the spray head.

21. Apparatus for charging particles of coating material comprising: a spray gun having an electrically conductive spray head for issuing a stream of particles, an elongated needlelike emitter electrode having a pointed tip, the tip being disposed substantially on the longitudinal axis of the particle stream in downstream spaced relation to the spray head, the emitter electrode further being disposed so as to extend into the particle stream in the upstream direction at an angle of from about 30 to 60 with respect to the longitudinal axis of the particle stream so that the tip of the emitter electrode is directed at least partly toward the spray head, and means for establishing a corona discharge having its princi al component extending in the upstream direction in e space between the emitter electrode and the spray head, whereby the particles pass through the corona discharge in opposed relation thereto.

22. In the method of electrostatically painting an object by means of a charged cloud of particles wherein paint is atomized upon being discharged from a spray head and formed into a particle spray stream in the presence of a high concentration of atmospheric ions, the improvement comprising the steps of:

positioning the tip of an emitter electrode at a point substantially on the longitudinal axis of the particle spray stream and spaced downstream from the spray head, and

establishing a potential difference between the emitter electrode tip and the spray head sufiicient to maintain a corona discharge therebetween whose principal component is directed upstream through the portion of the particle spray stream between the electrode tip and the spray head, whereby the particles pass through the corona discharge in opposition to the principal component thereof.

Claims

2. Apparatus as defined in claim 1 wherein the spray head includes a spray nozzle for issuing the spray particles and wherein the corona discharge terminates at least in part at the spray nozzle and at least in part at the portion of the spray head immediately surrounding the spray nozzle.
3. Apparatus for charging particles of coating material comprising: a spray gun having an electrically conductive spray head for issuing a stream of particles, the spray head including at least one air horn for directing a jet of air toward the particle stream, an emitter electrode disposed downstream of and in spaced relation to the spray head, and means for establishing a corona discharge having its principal component extending upstream in the space between the emitter electrode and the spray head so that the corona discharge terminates at least in part at the air horn, whereby the particles pass through the corona discharge in opposed relation thereto.
4. Apparatus for charging particles of coating material comprising: a spray gun having an electrically conductive spray head for issuing a stream of particles, an elongate needlelike emItter electrode positioned downstream of and in spaced relation to the spray head, the emitter electrode having a pointed tip disposed substantially on the longitudinal axis of the particle stream, and means for establishing a corona discharge having its principal component extending upstream in the space between the emitter electrode tip and the spray head, whereby the particles pass through the corona discharge in opposed relation thereto.
5. Apparatus in accordance with claim 4 wherein the spray head includes at least one air horn for directing a jet of air toward the particle stream and wherein the corona discharge is established at least in part between the emitter electrode and air horn.
6. Apparatus as defined in claim 4 wherein the spray head includes a spray nozzle for issuing the spray particles and wherein the corona discharge is established at least in part between the emitter electrode and the spray nozzle.
7. Apparatus in accordance with claim 4 wherein the emitter electrode is so disposed as to extend into the particle stream in the upstream direction at an angle of from about 30* to 60* with respect to the longitudinal axis of the particle stream so that the tip of the emitter electrode is directed at least partly toward the spray head.
8. Apparatus in accordance with claim 7, wherein the shank of the emitter electrode is bent along a predetermined radius of curvature to extend back toward the spray head.
9. Apparatus as defined in claim 8 wherein said predetermined radius of curvature is within the range of from one-sixteenth inch to three-eighths inch.
10. Apparatus as defined in claim 8 wherein the tip of the emitter electrode is spaced from the spray nozzle within the range of from one-fourth inch to one-half inch.
11. Apparatus in accordance with claim 4 wherein the emitter electrode is so disposed as to extend into the particle stream in the upstream direction at an acute angle with respect to the longitudinal axis of the particle stream so that the tip of the emitter electrode is directed at least partly toward the spray head.
12. Apparatus in accordance with claim 4 wherein the tip of the emitter electrode is directed at least partly toward the spray head, whereby the stream of particles issuing from the spray head is directed in opposition to the direction of the emitter electrode tip.
13. Apparatus for charging particles of coating material comprising: a spray gun having an electrically conductive spray head for issuing a stream of particles, an emitter electrode disposed downstream of and in spaced relation to the spray head, and means, including a high-potential power supply for maintaining the emitter electrode at a potential within the range of from 8 to 12 kilovolts and a conductor connecting the spray head to ground, for establishing a corona discharge having its principal component extending upstream in the space between the emitter electrode and the spray head, whereby the particles pass through the corona discharge in opposed relation thereto.
14. An electrostatic spray charging apparatus for attachment to a nonelectrostatic spray gun having an electrically conductive spray head for discharging a stream of particles comprising: a housing member composed of dielectric material, an elongate emitter electrode having a pointed tip carried by the housing member, means on the housing member for adjustably attaching the housing member to the nonelectrostatic gun to position the emitter electrode tip at a point spaced downstream from the spray head and substantially on the longitudinal axis of the spray stream, and means for maintaining a corona discharge between the tip of the emitter electrode and the spray head in a zone occupied by the particle stream, the corona discharge having its principal component extending in the upstream direction, whereby the spray particles pass through the corona discharge in opposed relation thereto.
15. An electrostatic charging apparaTus as defined in claim 14 wherein the corona discharge terminates in part at the portion of the spray head immediately surrounding the discharged stream of particles, and the means for maintaining the corona discharge between the emitter electrode tip and the spray head includes a high-potential power supply and a conductor attachable to the spray head for connecting the spray head to a reference potential.
16. Apparatus as defined in claim 15 wherein the power supply provides a potential within the range of from 8 to 12 kilovolts.
17. Apparatus as defined in claim 14 wherein the emitter electrode is bent along a predetermined radius of curvature to form first and second leg portions, the first leg portion being secured at its free end to the housing and extending therefrom in the downstream direction; the second leg portion extending in a direction having a substantial upstream component and having said pointed tip at its free end, whereby said pointed tip is directed generally in opposition to the particle flow direction when the housing is attached to the spray gun.
18. Apparatus as defined in claim 17 wherein the first leg portion of the emitter electrode is generally parallel with the longitudinal axis of the particle stream and wherein the angle subtended by the first and second leg portions is within the range of from 30* to 60*.
19. Apparatus as defined in claim 18 wherein said predetermined radius of curvature is within the range of from one-sixteenth inch to three-eighths inch.
20. Apparatus according to claim 14 wherein the emitter electrode includes an elongated member forming an angle of between about 30* and 60* with the longitudinal axis of the particle stream and having a tip portion located closest to the spray head.
21. Apparatus for charging particles of coating material comprising: a spray gun having an electrically conductive spray head for issuing a stream of particles, an elongated needlelike emitter electrode having a pointed tip, the tip being disposed substantially on the longitudinal axis of the particle stream in downstream spaced relation to the spray head, the emitter electrode further being disposed so as to extend into the particle stream in the upstream direction at an angle of from about 30* to 60* with respect to the longitudinal axis of the particle stream so that the tip of the emitter electrode is directed at least partly toward the spray head, and means for establishing a corona discharge having its principal component extending in the upstream direction in the space between the emitter electrode and the spray head, whereby the particles pass through the corona discharge in opposed relation thereto.
22. In the method of electrostatically painting an object by means of a charged cloud of particles wherein paint is atomized upon being discharged from a spray head and formed into a particle spray stream in the presence of a high concentration of atmospheric ions, the improvement comprising the steps of: positioning the tip of an emitter electrode at a point substantially on the longitudinal axis of the particle spray stream and spaced downstream from the spray head, and establishing a potential difference between the emitter electrode tip and the spray head sufficient to maintain a corona discharge therebetween whose principal component is directed upstream through the portion of the particle spray stream between the electrode tip and the spray head, whereby the particles pass through the corona discharge in opposition to the principal component thereof.