US3680779A

US3680779A - Method and apparatus for electrostatic spraying

Info

Publication number: US3680779A
Application number: US78025A
Authority: US
Inventors: Dennis A Reilly
Original assignee: Oxy Dry Sprayer Corp
Current assignee: Oxy Dry Sprayer Corp
Priority date: 1970-10-05
Filing date: 1970-10-05
Publication date: 1972-08-01
Anticipated expiration: 1989-08-01

Abstract

One or more auxiliary electric fields are established for an electrostatic powder sprayer to further control the movement of the powder particles from the area of the usual primary electric field to the surface to be coated, so that a more uniform powder profile is laid down on the surface to be coated and the amount of powder fly around is reduced.

Description

United States Patent Reilly [54] A METHOD AND APPARATUS FOR ELECTROSTATIC SPRAYING [72] Inventor: Dennis A. Reilly, Berkeley, Calif.

[73] Assignee: Oxy-Dry Chicago, Ill.

[22] Filed: Oct. 5, 1970 [21] Appl. No.: 78,025

Sprayer Corporation,

[52] US. Cl. ..239/3, 117/17, 117/93.4, 118/623, 239/15, 317/3 [51] 1.1. CI. ..B05b 5/00 [58] Field of Search ..239/3, 15; 117/93.4, 17; 118/621, 623, 629, 630, 638; 317/3 [56] References Cited UNITED STATES PATENTS 2,608,176 8/1952 Jenkins et al. ..117/93.4 X 2,754,225 7/1956 Gfeller ..239/3 X 2,082,182 6/1937 Schacht ..1l7/93.4 X

[151 3,680,779 [451 Aug. 1,1972

Primary Examiner-M. Henson Wood, Jr.

Assistant Examiner-Edwin D. Grant Attomey-Wolfe, Hubbard, Leydig, Voit & Osann [57] ABSTRACT One or more auxiliary electric fields are established for an electrostatic powder sprayer to further control the movement of the powder particles from the area of the usual primary electric field to the surface to be coated, so that a more uniform powder profile is laid down on the surface to be coated and the amount of powder fly around is reduced.

19 Claims, 9 Drawing Figures momma a M a. f; .4 h

METHOD AND APPARATUS FOR ELECTROSTATIC SPRAYING BACKGROUND OF THE INVENTION This invention relates to electrostatic sprayers, and more particularly to an improved method and apparatus for laying down a uniform coating on a moving surface. The advantages of the instant invention are realized to a high degree in the application thereof to l another or from one part of the web to another when the sheets are stacked or the web is folded. Various forms of powders, commonly referred to as anti-offsetting agents, and powder spraying mechanisms have been used. However, for modern high speed printing operations, electrostatic powder sprayers have been most widely employed in connection with an anti-offsetting agent, such as highly particulate starch or the like.

The known electrostatic sprayers are characterized by including a metering roller for dispensing the powdered anti-offsetting agent at a substantially uniform rate and one or more dielectric sheathed electrodes or glass housed gas discharge tubes (hereinafter sometimes collectively referred to for convenience as insulated conductors) which are spaced from the roller up to 1 cm. or so. The metering roller dispenses the powder with a geometry generally corresponding to the cross-sectional geometry of the desired powder lay down profile (e.g., a line geometry). The insulated conductors are axially aligned with the metering roller and an alternating current field is established between them and the metering roller. The field is of sufficient strength, i.e., has a sufficient rms voltage gradient, to ensure ionization of each of the air spaces between the metering roller and the insulated conductors. Consequently, the individual particles of powder presented by the metering roller assume electrical charges that correspond in polarity to the existing polarity of the ac. field. Further, it has been found that due to the convergence of the ac. field toward the insulated conductor or conductors there is a dielectric force tending to strip powder particles from the metering roller and accelerate them towards the insulated conductor or conductors. During the existing or first half cycle of the field, the particles are, therefore attracted toward the insulated conductors. However, during the next or succeeding half cycle of the field, when the field polarity reverses, the charged particles are repelled from the insulated conductors, and most of them are accelerated out of the influence of the field to fall under the force of gravity. Ideally, the free falling particles of powder settle to form a coating with a uniform profile on a freshly printed sheet or web passing beneath the sprayer at the time.

Unfortunately, a great deal of difficulty has been experienced with the air turbulence and drafts that are inevitably present during the operation of electrostatic sprayers, particularly when such sprayers are used in conjunction with high speed printing operations. Such air currents have a very substantial effect on the free falling powder and cause a significant departure from the ideal uniform powder laydown, since the powder particles are very lightweight and extremely small, say on the order of to 80 microns. Elaborate precautions, such as the inclusion of air curtains and the like, have been taken to eliminate the so-called powder flyaround caused by the air movement, but these are quite costly and have not been altogether successful, since many of the sources of disturbing air currents are 0 in the immediate vicinity of the sprayer itself.

SUMMARY OF THE INVENTION A general object of the present invention is to provide methods and apparatuses for electrostatic spraying which provide increased control over the laydown of a sprayed coating.

Another object of the present invention is to provide a method and apparatus for electrostatic powder spraying which minimizes powder fly-around. A related object is to provide an electrostatic powder spraying method and apparatus which minimizes the amount of powder required to lay down a powder coating with a desired profile. A more specific related object is to provide a method and apparatus of the foregoing type wherein the powder laydown is substantially undisturbed by normal ambient air currents.

Still another object of this invention is to provide a method and apparatus for electrostatic powder spraying wherein there is increased control over the powder motion. A related and more specific object is to provide a method and apparatus of the foregoing type which is suitable for electrostatic powder spraying of articles traveling above or below the sprayer, or vertically, or with any other orientation relative thereto that may be desired. A further object of the instant invention is to provide an improved method and apparatus for electrostatic powder spraying which may be readily incorporated into existing equipment, as well as new equipment, at a relatively low cost.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the present invention will become apparent as the following detailed description is read in conjunction with the attached drawings, in which:

FIG. 1 is a simplified perspective view of a typical electrostatic powder sprayer with which the present invention may be advantageously employed;

FIG. 2 is a sectional view of the sprayer taken along the line 2-2 in FIG. 1 and incorporates an electrical schematic diagram to illustrate one embodiment of the invention;

FIG. 3 is partly in sectional and partly in schematic form to show another embodiment of the invention;

FIG. 4 illustrates the advantageous combination of the embodiments of the invention depicted in FIGS. 2 and 3;

FIG. 5 generally corresponds to FIG. 2, but shows that the orientation of the article to be coated relative to the electrostatic powder sprayer of the instant invention is immaterial;

FIG. 6 is partly in sectional and partly in schematic form to illustrate an embodiment of the invention wherein the article to be coated is used as an electrode;

FIG. 7 shows the sprayer in sectional form and includes a simplified electrical diagram of one circuit for operating the sprayer with a potential difference between its metering roller and ground;

FIG. 8 generally corresponds to FIG. 6 but the electrical diagram is modified to illustrate another and more economical circuit for operating the sprayer with a potential difference between its metering roller and ground;

FIG. 9 is a sectional view of the sprayer, together with an electrical diagram of a circuit for deriving the voltages required by any one of the various embodiments of the instant invention from a single source.

I DETAILEDDESCRIPTION While the invention is described hereinafter in connection with certain illustrated embodiments, it is to be understood that the intent is not to limit it to those embodirnents. To the contrary, the intent is to cover all modifications, equivalents and alternatives as are within the spirit and scope of the invention as defined by the appended claims.

Turning to the drawings, and particularly to FIGS. 1 and 2, the electrostatic powder sprayer shown includes a powder box or reservoir 11 which has an open lower side within which a metering roller 12 is disposed. Mounting brackets 13 and 14 are secured to the powder box 11 at its opposite ends. The metering roller 12 is journalled on the mounting brackets 13 and 14 and driven by a variable speed motor 15 through a speed reduction box 16 and a shaft coupling 17.

In keeping with accepted practices, the circumference of the metering roller 12 is spaced from the inner surfaces of the powder box 11 and prepared, by etching, grinding or the like, to comprise numerous closely spaced pockets or grooves in which the powder 21 to be dispensed collects. Further, for ensuring that the powder 21 is dispensed from the powder box 11 at a substantially uniform rate and with a distribution or geometry corresponding to the cross-sectional line geometry desiredfor powder laydown profile, there are a pair of spring-

like wiper blades

22 and 23 secured (by means not shown) to the inner surfaces of the powder box 11. The

wiper blades

22 and 23 tangentially engage the circumferenceof the metering roller 12 along lines that are slightly below and respectively fore and aft of the axis of revolution of the metering roller 12. Of course, the rate at which the powder 21 is dispensed from the powder box 11 depends on the speed at which the metering roller 12 is driven, together with the pressure exerted against the metering roller 12 by the

wiper blades

22 and 23.

In the particular embodiment shown in FIGS. 1 and 2, a primary electric field is established by an ac. voltage source 25 which is connected at one side to the roller 12 and at its other side to a pair of dielectric sheathed electrodes or glass housed gas discharge tubes (i.e., insulated conductors) 26 and 27. However, it is to be understood that while it is preferable and advantageous that one or the other of the metering roller 12 or

conductors

26 and 27 be insulated to prevent arc discharging therebetween, it is not absolutely essential to the broader aspects of the invention. Typically, the metering roller 12 is grounded as indicated at 28. The

conductors

26 and 27 are respectively located fore and aft of the axis of rotation of the metering roller 12, and each of them is supported on insulated straps 29 or the like by the powder box 11. Suitably the distance between the circumference of the metering roller 12 and each of the

insulated conductors

26 and 27 is on the order of about 1 cm.

As is generally the case in electrostatic powder spraying, the ac. .voltage source 25 has an rms output voltage on the order of approximately 10 kv., so that the air spaces between the metering roller 12 and the

insulated conductors

26 and 27 are ionized (i.e., areas of corona discharge activity). Further, the ac. field between the metering roller 12 and

conductors

26 and 27 converges or intensifies in the area of the

conductors

26 and 27 so that there is a dielectric force acting on the particles which tends to accelerate them towards the

conductors

26 and 27 independently of the polarity of the field or the presence of a charge on the particles. Thus, the electrical forces are alone sufficient to ensure a flow of particles from the metering roller 12, and additional accelerators such as air jets or the like, which might disturb the uniformity of the powder distribution, are not required. Hence, as will be seen, during one half cycle of the ac. voltage supplied by the source 25, the particles of powder 21 presented to the ionized areas by the metering roller 12 and under the influence of the dielectric force assume changes that correspond in polarity to the polarity of the existing half cycle of the field, and the particles are, therefore, attracted toward the

insulated conductors

26 and 27. During the next or opposite polarity half cycle, the charged powder particles are repelled by the

insulated conductors

26 and 27, with the result that most of them are accelerated out of the influence of the primary electric field. As previously noted, in conventional electrostatic powder spraying, the powder particles that are accelerated out of the influence of the electric field established by the ac. voltage source 25 fall freely under the force of gravity toward the surface 31 to be coated, such as the freshly inked surface of a sheet or web, as it move under the sprayer, such as on a conveyor or the like generally indicated at 32. However, due to the inevitably air currents, substantial quantities of powder are blown off course to create the aforementioned powder fly-around problem.

Thus, in accordance with the present invention, one or more auxiliary electric fields are established to further control the movement of the powder particles from the area of the metering roller 12 to the surface 31 to be coated. As will be appreciated, particularly after consideration of the various embodiments here disclosed, the auxiliary field or fields may be established in a variety of different ways to afford various degrees of control. However, in each instance, the further control provided aids in achieving a more uniform powder laydown profile on the surface 31 to be coated, while at the same time substantially reducing the quantity of powder involved in whatever powder fly-around may still exist.

For example, as shown in FIG. 2, to provide a secondary electric field there may be a voltage source 34 that is connected between the metering roller 12 and an electrode 35 positioned beneath the surface 31 to be coated. The electrode 35 may be in a variety of different configurations, such as a bar, sheet, or mesh. But

whatever its form may be, its longitudinal axis preferably lies parallel to the axis of rotation of the metering roller 12 to ensure that the auxiliary field is substantially uniformly distributed. The voltage source 34 may be either a d.c. source or an a.c. source. In either event, however, the effective voltage level of the source 34, i.e., d.c. voltage level in the case of a d.c. source and rms voltage level in the case of an a.c. source, is low enough to ensure that the, primary field predominates, but high enough to ensure that a substantial majority of the powder particles accelerated out of the primary field are captured by and come under the influence of the secondary field. For a typical electrostatic powder spraying set-up, wherein there is a distance of a few inches between the metering roller 12 and the surface 31 to be coated, a suitable effective voltage level for the voltage source 34 is on the order of, say, 18-20 kv.

The powder particles that come under the influence of the secondary field are constrained thereby and accelerated toward either the surface 31 to be coated or the metering roller 12 in dependence on the polarity of the particle charge relative to the existing polarity of the secondary field. Preferably the secondary field has a frequency that is low relative to that of the primary field to afford sufficient time for a quantity of powder particles to travel from the area of the

insulated conductors

26 and 27 to the surface 31 to be coated without an intervening reversal of the polarity of the secondary field. Due to the constraining influence of the secondary field, ambient air currents have little, if any, effect on the distribution of the powder particles. Thus, the powder particles that are accelerated toward the surface 31 form a statistically uniform profile coating on it, whereas the particles that are accelerated toward the metering roller 12 are recycled. There is, therefore, very little powder involved in any powder fly-around.

The flow of powder 21 from the metering roller 12 is literally pulsed on and off at a frequency that is determined by the rate at which the relative polarities of the primary and secondary fields change. In the event that the voltage source 34 is a d.c. source, the rate at which the relative polarities of the primary and secondary fields change is determined by the frequency of the a.c. source 25. On the other hand, in the event that the voltage source 34 is an a.c. source, the same rate is determined by the frequencies of the

sources

25 and 34. In either case, the determinative frequency or frequencies can be readily selected so that the relative polarities of the primary and secondary fields change at a rate selected to turn the powder flow on and off in timed synchronism with the rate at which successive articles 31 to be coated are moved past the sprayer. Thus, the instant invention affords a way to obtain an extremely high degree of effective utilization of the available powder supply, even when the articles 31 to be coated are separated on the conveyor 32.

In the embodiment of FIG. 2 the secondary field extends from the metering roller 12 to below the surface 31 to be coated. However, the disturbing effects of ambient air currents can also be substantially reduced by a secondary field which extends only part of the way between the metering roller 12 and the surface 31. Accordingly, in keeping with the broader aspects of the present invention, in the embodiment of FIG. 3 a voltage source 36 is connected between the metering roller 12 and what, in this instance, are shown as being a set of parallel, equidistantly spaced, bar-type electrodes 37a-37. The primary advantage of this embodiment is that there is an increased uniformity of the secondary electric field due to the decreased distance between the metering roller 12 and the electrodes 37a-37d which, in turn, increases the uniformity of the distribution of the powder particles. However, there is not the same degree of constraint for the powder particles as in the embodiment of FIG. 2, since they leave the influence of the secondary field before being deposited on the sur face 31.

Again, the source 36 may be either a d.c. or an a.c. source. However, if it is a d.c. source, the powder particles that are traveling toward the surface 31 decelerate once they pass the plane of the electrodes 37a-37d, and are, therefore, increasingly subject to the disturbing effects of ambient air currents. This drawback can be alleviated to an extent, generally at the expense of sacrificing the capability of synchronizing the powder flow with the movement of successive articles past the sprayer, by using an a.c. source with characteristics as described below for the voltage source 36. More particularly, with an a.c. source 36 to create the secondary field, its frequency can be set relative to the frequency of the voltage source 25 so that the rate at which the polarity of the secondary field is reversed is synchronized with the rate at which the polarity of the primary field is reversed, and its phase angle may be set relative to the phase angle of the a.c. source 25 so that the secondary field polarity reversal lags the primary field polarity reversal by a period that is substantially equal to the transit time for the powder particles from the metering roller 12 to the plane of the electrodes 37a-37. As will be appreciated, the result of the foregoing is that the powder particles 21 are repelled as they pass the electrodes 37a-37d and, therefore, accelerated toward the surface 31 to be coated, rather than being retarded. Of course, the additional acceleration makes the powder particles less susceptible to the disturbing efiects of the ambient air currents.

The advantageous features of the total constraint afforded by the embodiment of FIG. 2 and the more uniform field distribution of FIG. 3 may be readily combined, as shown in FIG. 4. Thus, the voltage source 34 and the electrode 35 may be used to create a secondary field extending from the metering roller 12 to below the surface 31 to be coated, and the voltage source 36 may be used to create a tertiary field in the proximity of the metering roller 12. Both of the

voltage sources

34 and 36 may be either a.c. or d.c. sources, or one of them may be an a.c. source and the other a d.c. source. If both

sources

34 and 36 are d.c. sources, they are selected so that the electrodes 37a-37d are held at a potential intermediate the potentials at which the metering roller 12 and the electrode 35 are held. On the other hand, if either or both of the

sources

34 and 36 are a.c. sources, they are selected so that the potential of the electrodes 37a-37d is at least periodically, if not always, intermediate the potentials of the metering roller 12 and the electrode 35. Of course, when the potential of the electrodes 37a-37d is intermediate the potentials of the metering roller 12 and the electrode are identical or complimentary. Furthermore, the

powder particles that are traveling toward the surface 31 to be coated do not decelerate as they pass the plane of the electrodes 37a-37d. Rather, they then come under the constraining influence of the secondary field and are further accelerated thereby toward the surface 31 to be coated.

As will be appreciated in the embodiment of FIG. 4 the flow of powder particles 21 from the metering roller 12 may be turned on and off in synchronism with the movement of successive articles to be coated past the powder sprayer. Such synchronism can be affected most efficiently by using an a.c. source 34 to establish the secondary field and by selecting its frequency so that the relative polarities of the primary and secondary fields change in synchronism with and at the rate at which successive articles are moved past the sprayer. Substantially the same result may, of course, also be obtained by using an a.c. source 36 to create the tertiary field and by selecting its frequency so that the relative polarities of the primary and tertiary fields change at the rate with which successive articles are moved past the sprayer, but with a leading phase angle sufficient to compensate for the transit time of the powder particles from the plane of the electrodes 37a-37d to the plane in which the articles are moved past the sprayer. The last mentioned method for synchronously pulsing the powder flow on and off is less efficient than the first mentioned simply because some of the powder particles that are repelled toward the metering roller 12 by the change of polarity of the tertiary field relative to the primary field may nevertheless be captured by the secondary field and accelerated thereby away from the metering roller.

Another of the more unusual features of the present invention is that the orientation of the surface 31 to be coated relative to the powder sprayer is not critical. As previously noted, prior an electrostatic powder sprayers have relied on gravity to carry the powder particles and have, therefore, been limited to coating articles that are passed below them. However, with a secondary field to constrain the particles of powder and accelerate them toward the surface 31 to be coated, the orientation of the surface 31 relative to the sprayer is not critical, and may be such that the gravitational forces actually tend to deflect the powder particles away therefrom (i.e., the secondary or auxiliary field forces may be angularly displaced from the gravitational forces). For example, as shown in FIG. 5, the surface 31 may be located above the sprayer, since the secondary field between the metering roller 12 and the electrode 35 permits the powder particles to be accelerated toward the surface 31 against the gravitational force. For the particular embodiment shown, the primary field is preferably created between the metering roller 12 and a single insulated conductor 38 that is disposed intermediate the metering roller 12 and the surface 31. Otherwise, the embodiment of FIG. generally corresponds to and has the same features as the embodiment of FIG. 2.

If the article to be coated is sufficiently effective as a dielectric, say a plastic material or the like, the article 31 may itself be used as an electrode for the secondary field. To this end, as shown in FIG. 6, a charge is deposited on the article 31 at the input side or upstream of the powder sprayer, such as by exposing it to a corona discharge from an electrode 41 that is fed by a voltage source 42. As the charged article 31 is moved beneath the powder sprayer, the voltage gradient between the metering roller 12 and the article 31 to be coated creates a secondary field in much the same manner and with the same characteristics as previously described in connection with the embodiment of FIG. 2. A conventional static eliminator 43 may be disposed at the output side or downstream of the powder sprayer for discharging the coated article 31, so that the article 31 may be more readily stacked or folded.

Generally, the components of the conveyor 32, such as the gripper bars 44 and drive chain 45 (FIG. 2), are conductive and, therefore, assert a distorting influence on the secondary field. However, in keeping with one of the more detailed aspects of the present invention, the distortion of the secondary field may be minimized by referencing the metering roller 12 and the electrode 35 to potentials that are of opposed polarities relative to ground potential. To accomplish this-end, while still referencing the a.c. potential used to create the primary field to the potential of the metering roller 12, the a.c. voltage source 25 may be connected as in FIG. 7 across the primary winding 46 of a d.c. isolating transformer 47 which, in turn, has its secondary winding 48 connected between the metering roller 12 and one or more insulated conductors 49. Alternatively and more economically, the same effect may be achieved as in FIG. 8 by connecting the a.c. voltage source 25 across the metering roller 12 and the insulated conductor 48 through respective d.c. isolating capacitors 51 and 52. In that event the capacitors 51 and 52 preferably have values on the order of ten or more times the capacitance between the metering roller 12 and insulated conductor 49.

In passing it is noted that the single insulated conductor 49 shown in the embodiments of FIGS. 7 and 8 is located rearward and slightly below the axis of revolution of the metering roller 12. Likewise, in FIG. 9 the single insulated conductor is located below but forward of the axis of revolution of the metering roller 12. These showings are made simply to graphically indicate known possible alternatives for the basic electrostatic powder sprayer.

Finally, it will be seen that the various voltages required for electrostatic powder spraying in accordance with the present invention may be derived from a single a.c. voltage source 53. For example, FIG. 9 illustrates a simplified single source counterpart for the embodiment of FIG. 3 wherein the secondary and tertiary fields are d.c. fields. More particularly, in the embodiment of FIG. 9, the a.c. source 53 is connected across the primary winding 54 of a transformer 55. The metering roller 12 and the insulated conductor 49 are connected to opposite ends or across the secondary winding 56 of the transformer 55 by respective d.c. isolating capacitors 57 and 58. To derive the d.c. potentials for the secondary and tertiary fields, there is a full wave rectifier, here shown as being a voltage doubler 59, connected across the secondary winding 56 of the transformer 54. Further a voltage dividing potentiometer 60 with a pair of sliders 61 and 62 is connected across the full wave rectifier 59. The full d.c.

voltage developed by the full wave rectifier 59 is applied between the metering roller 12 and the electrode 35 to create the secondary field, but only a part of it, as determined by the setting of the voltage dividing slider 61, is applied across the metering roller 12 and the electrodes 37a-37d to create the tertiary field. Thus, the electrodes 37a-37d are held at a potential intermediate the potentials of the metering roller 12 and the electrode 35, so that the powder particles leaving the tertiary field are captured by the secondary field and accelerated thereby toward the surface 31 to be coated. The dc. potentials relative to ground at which the metering roller 12, electrode. 35, and electrodes 37a-37 and held are established by the setting of the grounded slider 62.

CONCLUSION In view of the foregoing, it will now be understood that the present invention provides a method and apparatus for electrostatic powder spraying which ensures increased uniformity of the powder profile deposited on the surface to be coated, while at the same time minimizing the amount of powder flyaround. It will also be appreciated that the present invention may be used to advantage for spraying articles that have any desired orientation relative to the sprayer. Finally, it will be seen that the present invention is entirely compatible with existing electrostatic powder sprayers and can, therefore, be used with them, as well as with new equipment.

I claim as my invention:

1. In an electrostatic sprayer for depositing a material on an article spaced from and moving relative to said sprayer along a predetermined path, said sprayer including a metering means for supplying particles of said material, a conductor means spaced at a predetermined distance from said metering means intermediate said metering means and said path, and first circuit means connected to said metering means and said conductor means for establishing a primary a.c. electric field therebetween whereby said particles are accelerated away from said metering means; the combination of at least one electrode means spaced at a distance from said metering means greater than said predetermined distance, and second circuit means connected between said metering means and said electrode means for establishing an auxiliary electric field therebetween, said electrode means being positioned relative to said metering means and said path such that said auxiliary field further controls movement of said particles between said metering means and said article.

2. The electrostatic sprayer of claim 1 wherein successive articles are moved relative to said sprayer along said path at a predetermined rate, said auxiliary field is a dc. field, and said primary a.c. field has a frequency selected to synchronize the movement of said particles from said metering means to said path with the rate at which said articles are moved relative to said sprayer.

3. The electrostatic sprayer of claim 1 wherein successive articles are moved relative to said sprayer along said path at a predetermined rate, said auxiliary electric field is an ac. field, and said primary and auxiliary a.c. fields have relative frequencies and phases selected to synchronize the movement of said particles from said metering means to said path with the rate at which said articles are moved relative to said sprayer.

4. The electrostatic sprayer of claim 1 wherein said path is disposed between said metering means and said electrode means, whereby the auxiliary field constrains said particles after they are accelerated out of said primary field and until they are deposited on said article.

5. The electrostatic sprayer of claim 4 wherein successive articles are moved relative to said sprayer along said path at a predetermined rate, the auxiliary field is an ac. field, and the primary and auxiliary fields have relative frequencies and phases that are selected to cause said particles to move toward and away from said metering means in timed synchronism with the movement of said articles relative to said sprayer.

6. The electrostatic sprayer of claim 1 wherein said path is oriented relative to said sprayer means such that any gravitational force acting on said particles tends to deflect such particles away from said path.

7. The electrostatic sprayer of claim 1 wherein said electrode means is positioned between said metering means and said path, whereby the auxiliary electric field constrains said particles after they are accelerated out of said primary field for only a part of the distance between said metering means and said path.

8. The electrostatic sprayer of claim 7 wherein successive articles are moved relative to said sprayer along said path at a predetermined rate, the auxiliary electric field is an ac. field, and said primary and auxiliary a.c. fields have relative frequencies and phases such that said particles are moved toward and away from said path in timed synchronism with the movement of said articles relative to said sprayer.

9. The electrostatic sprayer of claim 7 wherein the auxiliary electric field created is an ac. field, and said primary and auxiliary a.c. fields have relative frequencies and phases such that said particles are accelerated toward said path by said auxiliary field after passing through the distance between said metering means and said first electrode means.

10. The electrostatic sprayer of claim 4 wherein said metering means and said electrode means are referenced to respective potentials which are of opposite polarities relative to a ground potential.

11. The electrostatic sprayer of claim 10 wherein said electrode means is a first electrode means, and further including a second electrode means which is referenced to a potential which is intermediate the potentials to which said metering means and said first electrode means are referenced.

12. The electrostatic sprayer of claim 4 wherein said electrode means is a first electrode means, and further including a second electrode means disposed between said metering means and said path, whereby a tertiary electric field of increased uniformity relative to said auxiliary field is established to provide an increased uniformity of distribution of said particles relative to the article to be coated.

13. The electrostatic sprayer of claim 12 further including a single ac. voltage source which is connected by said first and second circuit means to said metering means and said first and second electrode means for establishing said primary, auxiliary, and tertiary electric field.

14. The electrostatic sprayer of claim 1 wherein said article is a dielectric, and further including a means positioned at an input side of said sprayer for depositing a predetermined charge on said article whereby said auxiliary electric field is established between said metering means and said article, and a means positioned at an output side of said sprayer for discharging said article.

15. The electrostatic sprayer of claim 1 wherein one of said metering means and said conductor means is insulated.

16. A method for electrostatically spraying a powder coating on an article comprising the steps of:

continuously supplying particles of powder, exposing said particles in a predetermined area at a distance from said article to an ac. primary electric field so that successive groups of particles are oppositely charged relative to a predetermined reference potential for said primary field, and

creating an auxiliary electric field to control movement of said particles for at least a portion of said distance.

17. The method of claim 15 wherein said auxiliary electric field extends for at least the entirety of said distance, whereby it controls movement of said particles from the area of said primary field to said article.

18. The method of claim 16 wherein said auxiliary electric field asserts a force on said particles in a direction that is angularly, displaced from the direction of any gravitational forces acting thereon.

19. The method of claim 15 further including the step of moving successive articles to be coated past a predetermined spraying station at a predetermined rate; and wherein said auxiliary electric field is an ac. field, and said primary and auxiliary fields have relative frequencies and phases that are selected to cause said powder particles to move towardand away from said station in timed synchronism with the movement of successive articles thereby.

Claims

2. The electrostatic sprayer of claim 1 wherein successive articles are moved relative to said sprayer along said path at a predetermined rate, said auxiliary field is a d.c. field, and said primary a.c. field has a frequency selected to synchronize the movement of said particles from said metering means to said path with the rate at which said articles are moved relative to said sprayer.

3. The electrostatic sprayer of claim 1 wherein successive articles are moved relative to said sprayer along said path at a predetermined rate, said auxiliary electric field is an a.c. field, and said primary and auxiliary a.c. fields have relative frequencies and phases selected to synchronize the movement of said particles from said metering means to said path with the rate at which said articles are moved relative to said sprayer.

5. The electrostatic sprayer of claim 4 wherein successive articles aRe moved relative to said sprayer along said path at a predetermined rate, the auxiliary field is an a.c. field, and the primary and auxiliary fields have relative frequencies and phases that are selected to cause said particles to move toward and away from said metering means in timed synchronism with the movement of said articles relative to said sprayer.

8. The electrostatic sprayer of claim 7 wherein successive articles are moved relative to said sprayer along said path at a predetermined rate, the auxiliary electric field is an a.c. field, and said primary and auxiliary a.c. fields have relative frequencies and phases such that said particles are moved toward and away from said path in timed synchronism with the movement of said articles relative to said sprayer.

9. The electrostatic sprayer of claim 7 wherein the auxiliary electric field created is an a.c. field, and said primary and auxiliary a.c. fields have relative frequencies and phases such that said particles are accelerated toward said path by said auxiliary field after passing through the distance between said metering means and said first electrode means.

13. The electrostatic sprayer of claim 12 further including a single a.c. voltage source which is connected by said first and second circuit means to said metering means and said first and second electrode means for establishing said primary, auxiliary, and tertiary electric field.

16. A method for electrostatically spraying a powder coating on an article comprising the steps of: continuously supplying particles of powder, exposing said particles in a predetermined area at a distance from said article to an a.c. primary electric field so that successive groups of particles are oppositely charged relative to a predetermined reference potential for said primary field, and creating an auxiliary electric field to control movement of said particles for at least a portion of said distance.

18. The method of claim 16 wherein said auxiliary electric field asserts a force on said particles in a direction that is angularly displaced from the direction of any gravitational forces acting thereon.

19. The method of claim 15 further including the step of moving successive articles to be coated past a predetermined spraying station at a predetermined rate; and wherein said auxiliary electric field is an a.c. field, and said primary and auxiliary fields have relative frequencies and phases that are selected to cause said powder particles to move toward and away from said station in timed synchronism with the movement of successive articles thereby.