Electrostatic Spray Gun Having An Adjustable Spray Material Orifice

Abstract

An electrostatic spray gun in which coating or other material is metered through an orifice adjustable in size to accommodate materials of various viscosities and consistencies and is thereafter atomized upon being injected as a conical fluid sheet into a central stream of high velocity air. Adjustment of the orifice and thus of the flow rate of the material is accomplished by rotating the spray head relative to the gun body. Further atomization of the material and development of the spray are produced by fluid streams exiting through an annular opening and other fluid orifices spaced around the spray outlet opening. The atomized particles are charged electrically by a rearwardly directed corona discharge established between a corona electrode supported on the spray head and an attractor ring positioned on the spray head in surrounding relation to the spray outlet opening.

Description

BACKGROUND OF THE INVENTION

This invention relates to electrostatic spray guns of the air-type and, more particularly, to air-type spray guns in which a single, adjustable spray head is used to spray coating or other material of various viscosities and consistencies.

Currently available air-type, electrostatic spray guns typically atomize the material being sprayed by discharging it through a central orifice of fixed size into the path of high velocity, converging air streams in the form of jets or vortices. Normally, electrical charges are imparted to the atomized spray material either by contact charging of the material prior to atomization, that is to say, by charging the material while it is within the spray gun itself, or by passing the atomized particles through an electric field maintained between the spray gun and an object to be coated or transversely to the spray pattern. Upon issuing from the gun, the particles produce a space charge cloud and associated electrical field gradients that attract the charged particles to the object to be coated.

Such devices, however, often to not achieve optimum atomization of the spray material, or fail to impart adequate electrical charges to the atomized particles, or both, particularly where the same spray head is sought to be used to spray materials of diverse viscosities and consistencies. As a direct result, high potentials and large power supplies are required and excessive quantities of materials are used.

Attempts directed toward improving the performance of fixed orifice guns have included the provision of a multiplicity of interchangeable spray heads or spray orifice assemblies, each of which is suitable for use with specific types or viscosity ranges of sprayable materials. Although giving greater flexibility of operation to spray guns, the use of multiple spray heads increases the cost of the guns and requires considerable labor to determine and locate the correct head for the material to be sprayed and thereafter to mount it in position on the spray gun. Moreover, the inherent limitations associated with the central discharge of spray material tending to impair complete atomization and charging are not avoided, nor is it possible to make fine adjustments to the flow rate of the material to achieve optimum spray conditions.

SUMMARY OF THE INVENTION

The spray apparatus of the present invention eliminates the foregoing and other disadvantages of present spray devices by utilizing a novel spray head of which the orifice governing the flow of the spray material is readily adjustable to accommodate materials of diverse viscosities and consistencies and which assures, by a combination of injecting the spray material as a fluid sheet into a central stream of high velocity, atomizing gas and directing additional gas streams against the resulting spray stream, a thorough admixing of the spray material and the atomizing gases more fully to atomize and disperse the spray material in the spray stream. Accordingly, electrical charges are more efficiently imparted to the atomized particles, with reduced voltage and current requirements.

The spray material, which may be a liquid, powder, slurry or other sprayable material, under pressure is supplied through appropriate passages in the gun body to a narrow annular chamber defined between the spray head and the gun body in surrounding relation to the adjustable spray material orifice and the spray outlet opening.

The adjustable orifice through which the material flows connects the annular chamber with the path of the central atomizing stream. It includes a valve seat on the spray head and a hollow poppet valve positioned in a bore in the gun body, the valve member normally engaging the valve seat to prevent the flow of spray material and being axially retractable from the valve seat when the gun trigger is pulled. Rotation of the spray head relative to the gun body adjusts the clearance between the valve seat and the poppet valve when the valve is retracted to control the thickness of the fluid sheet of spray material flowing from the annular chamber. The clearance appropriate for any particular application is selected according to the viscosity of consistency of the material being sprayed and to the degree of atomization and electrical charging desired.

Atomizing fluid, normally air, under pressure is supplied to the spray head for primary and secondary atomization of the spray material, with the primary air passing through the hollow poppet valve and exiting there from as a narrow high velocity stream into which the spray material sheet is directed, producing atomization. The resulting spray stream is carried away from the spray head by an annulus of air issuing through an annular opening formed around the outlet opening of the spray head. In addition, secondary atomization and dispersion of the spray particles is produced by air jets passing through an annular array of small forwardly tapering orifices spaced around the annular air opening and through other pattern-controlling orifices in the spray head.

The atomized particles are charged electrically by a corona discharge field established between a corona electrode supported in an insulator piece on the spray head and an attractor ring positioned at the forward end of the spray head in surrounding relation to the spray outlet opening. The corona electrode extends transversely into the spray stream and curves rearwardly to terminate at a point in front of the opening, thus creating an intense ion concentration at the immediate vicinity where the particles issue from the gun, and preferably is constructed of wire of a size that permits self cleaning due to vibration of the electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference may be made to the following description of an exemplary embodiment, taken in conjunction with the figures of the accompanying drawings, in which

FIG. 1 is a side elevation of the electrostatic spray gun of the present invention, with the operating parts in the normal closed position and with parts broken away for clarity;

FIG. 2 is a front end view of the spray gun of FIG. 1; and

FIG. 3 is an expanded detail view of the spray head, showing the poppet valve in the open position.

DESCRIPTION OF A REPRESENTATIVE EMBODIMENT

Generally, an exemplary embodiment of the spray gun of the present invention includes a handle section 10 and a body 12 of the same general organization and operation as those disclosed in the expired U.S. Pat. No. 1,962,911 to Roselund, a detachable spray head 14 and an electrostatic, particle charging unit 16 supported on the spray head. Inasmuch as the handle section 10 and the body 12 are generally conventional, they are described here only to the extent necessary for an understanding of the operation of the spray gun.

The handle section 10 as shown in FIG. 1 includes a pistol grip portion 18 which encloses an air passage 20 adapted to be connected through a fitting 21 to an external supply of air under pressure and a passage 22 containing an electrical conductor 24 leading to a trigger-operated switch 26 for energizing a direct current high-voltage power supply (not shown) connected to the particle charging unit 16.

A trigger-operated, main air supply valve 28 allows the flow of air to the spray head 14 when the trigger is actuated and, as shown in FIG. 1, completely shuts off the flow of air to the gun when the trigger is released.

An air passage 30 connects the main air valve 28 with a hollow tube 32, which receives at its rearward end a fine adjustment valve 34 and a retainer nut 36 for the valve. This valve regulates the flow of pattern-shaping air to the spray head and is adjusted to pass the desired quantity by manipulation of the nut 36 and an operator knob 38. The tube 32 projects beyond the rear surface of the gun handle 10 and is enlarged to form a nut 40 that bears against the facing surface of the handle section to draw the body 12 firmly against the handle section upon engagement of cooperating threads 41 carried by the forward end of the tube and the body 12. Air for primary and secondary atomization of the spray material passes from the air passage 30 to the gun body 12, and hence to the spray head 14, through an annular clearance defined between the tube 32 and a bore 42 in the handle section 10.

The flow of pattern-shaping air is adjusted by manipulation of the knob 38 so as to vary the clearance between the valve 34 and a cooperating valve seat 44 formed on the interior of the tube 32. A central bore 46 in the tube 32 communicates with passages in the forward portion of the gun to convey the air passed by the valve 34 to the spray head. Although not shown, the tube 32 has openings formed therein adjacent the upper end of the air passage 30 so that the air is admitted to the bore 46 when the valve 34 is open.

While the airflow through the bore 42 is referred to generally as the atomizing air, including both primary and secondary air, and that regulated by the valve 34 is termed the pattern-shaping air, the denomination of these airflows as such is in no way intended to mean that they function solely to atomize the spray material or to shape the pattern. Rather, these terms are used merely for convenience of description and to identify the principal function of the respective airflows. For example, when a pulverant material, or powder is sprayed, the primary and secondary air passing through the bore 42 serves to disperse or suspend, the particles of pulverant material in the spray stream and, thus, "atomizes" the material within the meaning of that term as used here and in the claims.

Also contained in the handle section 10 is a counterbore 48 into which is threaded a mechanism 50 for adjusting the stroke of the stem 52 of the spray material control valve and for returning the valve to the closed position upon release of the trigger.

The gun body 12 is generally cylindrical in configuration, and includes a bore 56 and a central bore 58 which are in axial alignment, when the body 12 is attached to the handle 10, with the bores 42 and 48, respectively, of the handle section. As noted, threads 41 are provided on the forward end of the bore 56 for receiving the corresponding threaded forward end of the tube 32, whereby the tube may be rotated to secure the handle section and body together. The valve stem 52 extends from the mechanism 50 and is slidably received within the central bore 58, with a seal being established between the stem and the bore by the gasket 62 and the gland nut 64.

Atomizing air entering the bore 56 from the bore 42 is conveyed to the central bore 58 through a passage 66 and is thereafter conveyed forwardly and outwardly through a longitudinally extending passage 68 surrounding a narrow diameter portion of the valve stem 52 and through one or more passages 70 in the body 12, this air then constituting the air used for dispersion of the spray and for secondary atomization of the material. It is also conveyed inwardly through the slots 72 in the valve stem 52 to a longitudinally extending bore 74 in the stem 52 (see FIG. 3) and exists from the bore 74 through an opening 75 in the forward end of the valve stem 52 as a high velocity stream that produces primary atomization of the spray material. The pattern-shaping air leaves the bore 46 and is directed through communicating passages 76 and 78 to the spray head.

The material to be sprayed, for example, paint, is delivered under pressure to the gun body 12 through a spigot 80 and is conveyed through a bore 82 to the spray head. Flow of the paint to the spray head is controlled by operation of the trigger in a manner more fully set forth below.

Essentially, the spray head includes a cap-shaped nozzle 84, a fan ring 86 encircling the forward portion of the nozzle, and having a pair of opposed air horns 88 extending forwardly therefrom, and a disc-shaped attractor ring 90 positioned over the front end of the nozzle 84. The attractor ring 90, nozzle 84, body 12 and the handle section 10 are preferably metallic and may conveniently be constructed of aluminum or other suitable metal.

The spray head is screwed on the forward portion of the gun body 12 through cooperating threads 92 formed on the gun body and on a ring member 94 that is force fitted or otherwise secured to the nozzle 84. Although the ring member 94 may be integrally formed with the nozzle 84, it preferably is a separate member to facilitate fabrication of the spray head. The pitch and type of the threads 92 are selected so that the spray head is held firmly in position when threaded on the gun body but not so firmly as to impair easy manual rotation of the spray head for purposes of adjustment of the flow of spray material. To facilitate such adjustment, a knurled hand grip 96 is formed on the rearward portion of the nozzle 84, although other convenient means may of course be provided.

When in position on the gun body, the nozzle 84 defines with facing surfaces of the gun body 12, a series of annular chambers 98, 100 and 102 (see FIG. 3) for receiving air from the passages 78 and 70 and paint from the bore 82, respectively. Pattern-shaping air entering the chamber 98 is conveyed through passages 104 to a further annular chamber 106 formed between the fan ring 86 and the nozzle 84, from where it is conveyed through passages 108 to the air horns 88 and is discharged through small orifices 110 to develop and confine the spray pattern as the spray issues from the gun.

Secondary air entering the chamber 110 from the passage 70 flows through passages 112 to an annular chamber 114 defined between the front end of the nozzle 84 and the disc-shaped attractor ring 90. From there, the air is directed through an annular opening 116 encircling the spray as it issues from the gun to carry the atomized particles from the region of the spray head. Additional openings in the form of small, forwardly tapering orifices 118 (see FIG. 2) are spaced uniformly around the annular opening 116, through which air from the chamber 114 is directed against the issuing spray to further atomize and disperse the paint particles. Any suitable number of orifices 118 may be provided in the attractor ring 90 and with any suitable spacing between the individual orifices, the number and location of the orifices shown in FIG. 2 merely being illustrative of one possible arrangement.

It is an important function of the annulus of air emerging from the opening 116 and the jets discharged from the orifices 118 to prevent the finely atomized and electrically charged particles from being attracted to the spray head. This is necessary in order to keep the gun from fouling and is of particular importance in the present gun because the rearwardly directed corona discharge sets up an "electric wind" tending to drive the particles toward the spray head and because the method of atomization, together with the efficient manner of charging the spray, produces finely atomized particles of high electrostatic mobility. While such high mobility enhances the overall efficiency of the painting operation, there is also an increased tendency for the particles to migrate toward the grounded spray head.

When the trigger is actuated to supply air to the spray head, the valve stem 52 is retracted to cause a conically shaped poppet valve 122 at the end of the stem to be withdrawn from engagement with a mating, forwardly tapering valve seat 124 formed on the nozzle 84 and extending radially inwardly from the chamber 102 to terminate in a spray outlet opening 126 formed in the nozzle 84. That is, the poppet valve is moved from the position shown in FIG. 1 to the position shown in FIG. 3.

The paint thereupon flows radially from the chamber 102 through the clearance between the valve 122 and the valve seat 124 and is injected into the primary airstream as a conical fluid sheet of which the thickness is controllable by the spray head adjustment. A conical configuration is preferred for the valve 122 and the valve seat 124 in that a positive seal is readily made upon release of the trigger to achieve complete stoppage of the paint flow, notwithstanding that the valve seat 124 is movable relative to the gun body 12, and hence to valve 122. Moreover, it is an important feature of the invention that the injection of the paint into the central airstream as a conical sheet produces substantially improved atomization of the paint over that obtained in conventional spray guns, resulting in a fine mist of atomized particles which are easily given large electrostatic charges at low ionization voltage and current levels. Because of their fine size and large electrical charges, the particles have a high electrostatic mobility. Accordingly, the electrical field gradients to the workpiece set up by the space charge cloud of the charged particles tend to be of high value. The fine particle size and high electrostatic mobility also enhance the quality of the finish given to the workpiece inasmuch as the particles are readily attracted to all surfaces of the workpiece to form a uniform coating.

To prevent throttling of the paint flow by the high velocity airstream, the internal diameter of the spray outlet opening 126 should be larger than the internal diameter of the opening 75 in the valve stem 52. For example, successful operation has been achieved with liquids having viscosities up to 150 centipoise with the opening 126 being 0.005 inches greater in diameter than the opening 75.

As shown in FIG. 3, the openings 75 and 126 are in axial alignment, with the primary air issuing from the opening 75 as a cylindrical mass positioned concentrically within the surrounding valve seat 124 and outlet opening 126. Ideally, the valve seat 124 and the mating surface of the poppet valve 122 are inclined at approximately 45.degree. to the longitudinal axis of the bore 74 in the valve stem 52. Nonetheless, other angles of incidence may be used, including 90.degree.. Superior atomization of the paint is obtained, however, when the paint is injected into the airstream at about 45.degree. angle.

The poppet valve 122 (see FIG. 3) includes a metallic or nonmetallic, resilient valve member 128 mounted securely on a reduced diameter portion of the valve stem 52 and is shaped, together with the front surface of the valve stem 52, to match the conical configuration of the valve seat 124. A gasket 130 is positioned between the rearward end of the valve member 128 and the valve stem 52 to prevent the leakage of paint along the length of the valve stem. Preferably, the valve member 128 and the gasket 130 are constructed of a low friction material, such as silicone rubber, polytetrafluoroethylene or the like, to facilitate sliding movement of the poppet valve within the bore 58 of the gun body.

If desired, the whole forward part of the poppet valve 122 may be constructed of resilient material, so that upon engagement with the valve seat 124 the resilient material will expand radially to effect a seal between the gun body 12 and the valve stem 52. In this case, the annular gasket 130 may be omitted.

To reduce wear on the valve member 128 and the gasket 130 and to simplify fabrication of the spray gun, a sleeve 132 constructed of suitable valving material, such as brass, for example, may be inserted in a counterbore 134 in the front end of the gun body 12. The forward end of the sleeve 132 is conically shaped to correspond with the front portion of the poppet valve. Thus when the poppet valve is retracted, the clearance between the valve seat 124 and the poppet valve 122 is identical to that between the valve seat 124 and the front surface of the sleeve 132. A uniform thickness flow path for the paint is established, therefore, from the annular chamber 102 to the point of intersection with the primary airstream.

Adjustment of the clearance between the valve seat 124 and the poppet valve 122 and the sleeve 132, is accomplished simply by grasping the hand grip 96 on the nozzle 84 and rotating the spray head 14 clockwise or counterclockwise, as required, to decrease or increase, respectively, the clearance. The poppet valve, when the trigger is fully depressed, is always retracted to a position in alignment with the front surface of the sleeve 132. Should the stroke of the valve stem 52 be either too long or too short to position the poppet valve correctly, an adjustment can be made through manipulation of the knob 54 to provide for the proper length of stroke.

As the atomized particles emerge from the outlet opening 126, they are charged electrically by a corona discharge field established in opposition to the particle flow between the spray head and a corona electrode 136 supported in an insulator piece 138 which, in the embodiment shown in the drawings, is positioned on one of the air horns 88 of the fan ring 86. The corona electrode 136 and the insulator piece 138, together with the lead wire 140 and a spring connector 142, form the spray charging unit 16.

Agglomeration of the paint on electrode 136 is avoided by mounting it to first extend transversely into the spray pattern and then curve rearwardly to terminate at a point in front and preferably on the longitudinal axis of the outlet opening 126. Suitably, the electrode is constructed of wire, for example, piano wire, of a diameter within the range of from 0.015 to 0.025 inches so that the electrode will be self cleaning due to the vibrations induced by the impinging spray and airstreams.

The insulator piece 138 is shown in FIG. 2 as being attached to the air horn 88 by screws 144, but it may be mounted on the air horn in any suitable manner. Alternatively, the insulator piece 138 may be eliminated and the electrode 136 supported directly by an air horn 88. In either case, both the insulator piece and the fan ring preferably are constructed of dielectric material.

As previously mentioned, when the trigger is actuated the switch 26 connects a high voltage source (not shown) to the corona electrode to establish a corona discharge between the electrode tip and the portion of the nozzle 84 surrounding the spray outlet opening. Preferably, the attractor ring 90 is recessed (see FIG. 3) in the vicinity of the opening 126 so that the corona discharge terminates principally at the nozzle and only to a lesser extent at the attractor ring 90. The voltage needed to set up a corona discharge field of the required magnitude between the electrode and the nozzle will be determined by such factors as the spacing of the electrode tip and the spray conditions. Normally a voltage within the range of from 5 to 10 kilovolts is sufficient. Ideally, the entire gun, including the attractor ring 90 and the nozzle 84, are connected to electrical ground.

With a corona discharge thus set up in opposition to the spray as it issues from the spray head, the paint material is atomized and charged electrically nearly simultaneously, and the position of the emitter electrode tip within the spray pattern insures that all of the paint particles will be highly and efficiently charged as they pass through the corona discharge field. The advantages and improved particle charging characteristics of a rearwardly directed corona discharge field of the configuration shown in the drawings are set forth fully in the copending application Ser. No. 771,135, filed Oct. 28, 1968 and assigned to the assignee of the present application.

OPERATION

with the supplies of air and paint, both under pressure, attached to the gun, and, with the particle charging unit 16 disconnected from the power supply, the trigger is depressed to open the main air valve 28 and retract the poppet valve 122 so that air and paint, respectively, are delivered to and discharged from the spray head. Any necessary adjustment of the clearance between the valve seat 124 and the poppet valve 122 is then made by rotating the spray head in the appropriate direction to control the flow of paint from the annular chamber 102. Also, the degree of atomization of the dispersion of the particles throughout the spray pattern and the shape of the pattern itself can then be regulated by adjustment of the fine adjustment air valve 34. In this way, optimum flow rates for the supply of paint and air to the spray head are obtained and resulting economies in operation of the gun are realized.

Should it develop during the spraying operation that further adjustments of the paint flow rate or of the primary or secondary airflow rates are needed, or if a new material is to be sprayed which requires a different setting of the spray head and air valves, the necessary adjustments can be quickly and easily made with a minimal loss of time. When test spraying has been completed and final adjustments made to the spray gun, the particle charging unit 16 is reconnected to the power supply and the gun is ready for operation. The trigger is so arranged that upon being depressed it actuates the power switch 26 and opens the main air valve 28 prior to retracting the poppet valve 122 so that the corona discharge is established between the electrode 136 and the spray head, and the primary and secondary air is supplied to the spray head, before the flow of paint from the annular chamber 102 begins. Thus, when the trigger is fully depressed, and the poppet valve thereby retracted to a position of alignment with the front surface of the sleeve 132, a fully atomized and charged spray will be discharged from the gun with virtually no waste of the material being sprayed.

It will be understood by those skilled in the art that the above-described embodiment is intended to be merely exemplary, in that it is susceptible of modification and variation with departing from the spirit and scope of the invention. For example, although the spray gun has been illustrated and described as being useful in spraying paint, it is not limited to such use and, if fact, may be used with a wide variety of sprayable materials, including liquids, slurries, free-flowing powders, most types of coating materials and preservatives; and may also be used for spraying insecticides and fungicides, or the like, in agricultural applications. In that it is capable of easy adjustment to accommodate such diverse materials, whether of high or low viscosity or of uniform or nonuniform consistency, it will be apparent that the spray gun according to the present invention has general application and is not restricted to any particular use or types of materials. All such variations and modifications, therefore, are included within the scope of the invention as set forth in the appended claims.

Claims

We claim:

1. Apparatus for discharging a spray of finely divided material, comprising

a body having a front end surface and a longitudinally extending bore opening into the front end surface,

a nozzle member movably mounted on the body and defining with the front and surface thereof a first annular chamber for receiving material to be sprayed,

an annular valve seat on the nozzle member radially inwardly of the annular chamber and facing the front end surface of the body and terminating in a spray opening at the front surface of the nozzle member,

a valve disposed in the longitudinal bore in the body, the valve having a bore terminating in an opening in the front surface of the valve and in longitudinal alignment with the spray outlet opening, the valve being movable between a forward position in which it sealingly engages the valve seat and a retracted position in which it is withdrawn a predetermined distance from the valve seat.

a ring carried by the nozzle member and defining with the front end surface of the nozzle member a second annular chamber for receiving an atomizing fluid and further defining an annular opening communicating with the second annular chamber, the annular opening being disposed in surrounding relation to the spray outlet opening in the nozzle member,

means for moving the nozzle member longitudinally of the body to adjust the clearance between the valve seat and the valve to allow material of various consistencies to be fed from the first annular chamber to the spray outlet opening,

means for supplying material to be sprayed under pressure to the first annular chamber,

means for supplying atomizing fluid under pressure to the bore in the valve and to the second annular chamber so as to discharge streams of atomizing fluid through the opening in the valve and through the annular opening surrounding the spray outlet opening, and

means for selectively moving the valve member to the retracted position to cause the spray material to flow from the first annular chamber radially inwardly into the stream of atomizing fluid exiting from the valve so as to produce atomization of the spray material, the atomized material being discharged as a spray through the spray outlet opening in the nozzle member and being further atomized and dispersed by the fluid stream exiting from the annular opening surrounding the spray outlet opening.

2. Apparatus according to claim 1, in which the opening in the front surface of the valve and the spray outlet opening in the nozzle member are circular and coaxially, the diameter of the opening in the valve being smaller that the diameter of the outlet opening to allow free flow of the spray material from the first annular chamber into the atomizing fluid stream.

3. Apparatus according to claim 2 in which a plurality of forwardly and inwardly directed orifices are provided in the ring, the orifices being spaced circumferentially around the annular opening surrounding the spray outlet opening and communicating with the second annular chamber to discharge a plurality of atomizing fluid streams against the spray to further atomize and develop the spray.

4. Apparatus according to claim 3, further comprising means for establishing a corona discharge in opposition to the spray flow between a corona electrode disposed forwardly of the spray outlet opening and the spray head.

5. Apparatus according to claim 4 in which the corona electrode is supported on the spray head and extends transversely into spray and thereafter curves rearwardly to terminate at a point in front of the spray outlet opening, and in which the portion of the ring adjacent the spray outlet opening is recessed from the foremost surface of the ring so that the corona discharge is established principally between the corona electrode and the portions of the nozzle member surrounding the spray outlet opening.