US3587967A

US3587967A - Spray coating apparatus

Info

Publication number: US3587967A
Application number: US3582A
Authority: US
Inventors: David H Badger
Original assignee: Ransburg Corp
Current assignee: Ransburg Corp
Priority date: 1970-01-08
Filing date: 1970-01-08
Publication date: 1971-06-28
Anticipated expiration: 1988-06-28

Abstract

AN ELECTROSTATIC SPRAY GUN FOR FORMING A SPARY OF AIRATOMIZED, ELECTRICALLY CHARGED PAINT PARTICLES COMPRISES A GROUNDED METAL BARREL HAVING AT ITS FORWARD END A PASSAGE WITHIN WHICH THE SPRAY IS FORMED. PAINT IS SUPPLIED TO SUCH PASSAGE THROUGH A NARROW ANNULAR PAINT ORIFICE IN THE PASSAGE WALL AND IS ATOMIZED BY AN AIRSTREAM DIRECTED INWARDLY AND FORWARDLY OF THE PASSAGE FROM AN ANNULAR AIR ORIFICE LOCATED IMMEDIATELY REARWARD OF THE PAINT ORIFICE. AN IONIZING ELECTRODE SUPPORTED IN REAR OF THE PAINT AND AIR ORIFICES EXTENDS FORWARDLY ON THE AXIS OF THE PASSAGE PAST THE PAINT ORIFICE AND TERMINATES AT A POINT SPACED REARWARDLY FROM THE FORWARD END OF THE PASSAGE. AN APPROPRIATE VOLTAGE SOURCE MAINTAINS THE ELECTRODE AT ELEVATED POTENTIAL TO CREATE BETWEEN THE ELECTRODE AND THE WALL OF THE PASSAGE AN ELECTROSTATIC FIELD THROUGH WHICH THE SPRAY PASSES IMMEDIATELY AFTER IT IS FORMED.

Description

United States Patent [72] Inventor DavidH.Badger [54] SPRAY COATING APPARATUS 12 Claims, 2 Drawing Figs.

52 user 239/15, 239/427.3,11s/121 511 1111. or B05b 5/02 50] Field oiSearch :1: 239/3,15, 427.3; 118/121 [56] 1 References Cited UNlTED STATES PATENTS 3,169,882 2/1965 .luvinall et al 9/1967 Beach,Jr

3.339.841 239/3(X) FOREIGN PATENTS l,038,865 8/l966 Great Britain 239/15 ABSTRACT: An electrostatic spray gun for forming a spray of air-atomized, electrically charged paint particles comprises a grounded metal barrel having at its forward end a passage within which the spray is formed. Paint is supplied to such passage through a narrow annular paint orifice in'the passage wall and is atomized by an airstream directed inwardly and forwardly of the passage from an annular air orifice located immediately rearward of the paint orifice, An ionizing electrode supported in rear of the paint and air orifices extends forwardly on the axis of the passage past the paint orifice and terminates at a point spaced rearwardly from the forward end of the passage. An appropriate voltage source maintains the electrode at elevated potential to create between the electrode and the wall of the passage an electrostatic field through which the spray passes immediately after it is formed.

SPRAY COATING APPARATUS This application is a continuation of our prior application, Ser. No. 741,374, filed July 1, 1968, now abandoned.

BACKGROUND OF THE INVENTION While most electrostatic spray coating systems provide an electrostatic field extending to a grounded article being coated and promoting the deposition of charged spray particles on the article, it has heretofore been proposed to employ a spraying device which would charge the spray before its discharge and to rely for the electrostatic depositing effect solely on the electrostatic attraction of the grounded article for the charged spray particles. Such a device commonly embodies an atomizer adapted to discharge a spray of atomized particles through an electrostatic field maintained between an ionizing electrode and a counterelectrode. The electrode and counterelectrode can be located near the point of spray formation and hence can be rather closely spaced with theresult that a field of adequate strength to charge the spray can be maintained at relatively low voltages. Further, it is possible to employ as the counterelectrode a grounded housing which encloses the charged ionizing electrode and, thereby protecting the electrode against close approach to or by a grounded object or personnel, greatly lessens the possibility of dangerous sparks or shocks. One type of device employing an ionizing electrode enclosed in a grounded housing is shown in US. Pat. No. 2,302,289.

While prior devices of the type just mentioned possess the advantages of requiring only relatively low voltages and providing comparative safety, they possess a disadvantage, when used in spraypainting, in that paint collecting on the counterelectrode under the influence of the field is thrown therefrom in the form of large drops which are objectionable both because they mar the finish and because, if charged at all, they are charged oppositely to the spray proper.

SUMMARY OF THE INVENTION In a preferred form of deviceembodying the invention, the grounded barrel is formed of concentric, inner, intermediate, and outer metal sleeves. The inner and intermediate sleeves are constructed and arranged to define between them a passage and the annular orifice for the atomizing air, while the intermediate and outer sleeves define a paint passage and the annular paint orifice. Insulating means located in the inner sleeve rearwardly of the air orifice supports the ionizing electrode, which extends forwardly past the paint orifice and terminates within the barrel. The electrode is maintained at high voltage by connection to the ungrounded terminal of a highvoltage source in order to maintain between it and the grounded barrel an electrostatic field through which the spray passes. Air ions formed at the electrode and attracted to the grounded barrel charge the spray particles by ion bombardment with the result that when the discharged spray is directed toward a grounded article to be coated the charged particles will be electrostatically attracted to the article. At its forward end the outer sleeve may be provided with diametrically opposite orifices for the discharge of spray-shaping air, such orifices communicating with one or more supply passages extending longitudinally in the wall of the outer sleeve. If the gun is to be handheld, the barrel may be provided with a grounded handle and with a trigger for controlling the discharge of paint and of atomizing and spray-shaping air.

DESCRIPTION OF THE DRAWING FIG. 1 is an illustration, largely diagrammatic in character, illustratingan electrostatic paint-spraying system embodying a handheld spray gun; and

FIG. 2 is a fragmental axial section, on an enlarged scale, showing details of the construction at the front end of the gun barrel.

DESCRIPTION OF THE PREFERRED EMBODIMENT The system shown in FIG. 1 comprises a handheld spray gun designated in its entirety by the reference numeral 10 and including a barrel 11. and a handle 12.

Conduits

13 and 14 extending from the lower end of the handle 12 respectively connect the gun to a source 15 of air under pressure and a source 16 of liquid to be sprayed. A voltage lead 17 extends into the handle 12 from one terminal of a voltage source 18, the other terminal of which is grounded as indicated at 19. A trigger 20- mounted on the gun controls the supply of air, liquid, and voltage in any convenient manner. Exterior portions of the gun 10, including the barrel 11, handle 12, and trigger 20 are of metal andare grounded as through a conductive sheave surrounding, but insulated from, the voltage lead 17. FIG. I also shows a grounded article 21 in position to receive spray from the gun.

The barrel ll of the gun, as shown in FIG. 2, comprises an inner sleeve 22, an intermediate sleeve 23, and an outer sleeve 24, all of metal and concentrically arranged. Opposed faces of the inner and

intermediate sleeves

22 and 23 are spaced apart radially to define an annular, longitudinally extending air passage 25 terminating in an annular air orifice 26 communicating with the interior 27 of the inner sleeve. The opposed surfaces of the intermediate and

outer sleeves

23 and 24 similarly define an annular, longitudinally extending paint passage 28 terminating in an annular paint orifice 29 communicating with the passage 27 immediately forwardly of the air orifice 26. The air passage 25 and paint passage 28 communicate interiorlyof the gun with the air conduit 14 and the paint conduit 14 respectively. Paint supplied to the gun flows through the paint passage 28, is formed into an annular film at the end of the intermediate sleeve 23, and emerges as the inner edge of such film into the interior 27 through the paint orifice 29. As it thus emerges, it is impinged upon by the annular airstream discharged through the air orifice 26. Such air orifice, as shown in the drawing, is formed by opposed frustoconical surfaces at the ends of the inner and intermediate sleeves to direct the annular airstream inwardly and forwardly of the gun, with the result that the paint particles formed by atomization at the paint orifice 29 are directed forwardly and toward the barrel axis for emission from the barrel through an opening 30 at the front end of the sleeve 24.

Mounted within the sleeve 22 well in rear of the air and paint orifices is a plug 32 of insulating material which supports a slim elongated spray-charging electrode 33 connected interiorly of the gun to the high-voltage lead 17. The charging electrode 33 is of fine wire and extends forwardly to the vicinity of the paint orifice 29, but desirably not far enough forwardly to receive any atomized paint.

If desired, the barrel 11 may include means for shaping the paint spray discharged through the opening 30. Such means may, as shown in FIG. 2, take the form ofa metal ring 35 having a frustoconical, forwardly flaring inner face 36. Diametrically disposed, oblique passages 37 in the ring 35 open into the face 36 and communicate at their rear ends with an annular groove 38 formed in the front face of the outer sleeve 24. The groove 38 in turn communicates with one or more air passages 39 extending rearwardly through the wall of the outer sleeve 24 and communicating at their rear ends, through appropriate flow-regulating means (not shown), with the air conduit 14. Air discharged from the passage 37 converts the discharged spray from one circular cross section to one of flat or oval cross section, as is frequently desired in spray-painting operations.

The orientation of the flat spray just-referred to relative to the handle 12 can be controlled by making the ring 35 angularly adjustable about the axisof the barrel 11. To that end, it may be formed with an outwardly projecting annular flange 41 secured against the front face of the outer sleeve 24 by a screw-threaded retaining ring 42.

With the electrode 33 connected to the voltage source 18, an electrostatic field will exist between the electrode and the inner surfaces of the metal sleeves. As that field is concentrated at the pointlike tip of the electrode, a zone of high ion concentration is created on the axis of the barrel and for wardly of the paint orifice 29. As the paint emerges from the orifice 29 it is impinged upon and atomized by the airstream emerging from the air orifice 26, and the atomized particles are propelled through the zone of high ion concentration to be charged by ion bombardment. With the gun directed at a grounded article to be coated, the airstream discharged through the opening 30 will carry the atomized spray particles to the vicinity of such article, and the charges on the particles will cause them to be attracted to and deposited thereon.

The radially inward component of velocity of the air emerging from the air orifice 26 initially propels the atomized paint particles toward the axis of the passage 27 and opening 30, thus providing a dual benefit. The particles, while still in a state of relatively high concentration, are forced through the zone of high ion concentration at and adjacent the centrally disposed tip of the electrode 33, thereby promoting effective charging. In addition, the initial inward inclination of the airstream opposes the tendency of particles charged by ion bombardment to deposit electrostatically on the inner surfaces of the grounded sleeves and ring 35, thus minimizing the deposit of particles on surfaces at the forward end of the gun.

Since a substantial potential gradient exists at and immediately adjacent the paint orifice 29, the particles as originally formed may tend to possess an undesirable charge opposite to that which they later acquire by ion bombardment. Accordingly, it is desirable to reduce as far as possible the gradient of the field at and immediately adjacent the paint orifice. This result can be attained by making the inwardly presented surfaces of the barrel as nearly flush with each other as is practical, thereby providing mutual shielding of the edges of those surfaces and reducing field strength and the undesirable'charging effect at the inner edge of the paint film emerging from the orifice 29.

Since the spray passage formed by the opening 30 and the forward portion of the interior 27 of sleeve 22 may have a relatively small diameter, say substantially less than 1 inch, an intense particle-charting field can be produced between the electrode and the conductive walls of discharge passage 27 with only a few kilovolts applied to the electrode 33. As the entire exterior of the gun is grounded, danger of undesirable electrical discharges between it and other grounded objects is entirely eliminated. By locating the tip of the electrode far enough rearwardly from the front end of the gun, the danger that it might approach a grounded object closely enough to cause an undesirable discharge is rendered practically negligible. While it is desirable, for the reason just noted, to locate the electrode tip well back from the front end of the gun, a location too far back is to be avoided since the spray, after its initial convergence, will expand; and undue expansion of the spray before its emergence from the gun would cause paint to be deposited in the interior gun surfaces. Preferably, the axial distance between the electrode tip and the front end of the gun should be at least equal to the radius of the passage 27 and most desirably about equal to the diameter of such passage.

While the gun shown has been described, for convenience, as one for use in the application of paint, it will be understood that it may be used with other coating materials or, in general, for the creation of a charged spray of any liquid.

1 claim:

1. An electrostatic spray gun, comprising a barrel having a forwardly opening discharge passage,

an annular liquid orifice in the wall of said passage, and

an annular air orifice in the wall of the passage adjacent to and rearwardly of said liquid orifice,

said air orifice being directed forwardly and inwardly of said passage whereby air under pressure emerging from the air orifice will atomize liquid emerging from the liquid orifice and propel the atomized liquid particles forwardly and toward the center of said passage.

insulating means mounted in said barrel in the rear of said air o rifice,and an ionizing electrode supported by said insulating means and extending in said passage to a tip which is located substantially on the passage axis and forwardly of the liquid orifice.

2. A spray gun according to claim 1 wherein the discharge passage of said barrelincluding said annular orifices to atomize liquid is conductive and comprises a terminus for an electrostatic field from the ionizing electrode.

3. A spray gun according to claim 1 wherein the electrodetip is spaced rearwardly from the front end of the barrelby a distance substantially equal at least to the diameter o'f-said passage. 1

4. A spray gun according to claim 1 wherein the barrel in eludes, at its front end, an annular ring having a forwardly flaring, frustoconieal inner surface coaxial with said discharge passage, said ring being adjustable about the barrel axis and being provided with passages for sprayshaping air opening in said inner surface and communicating with a longitudinal air passage in the barrel.

5. A spray gun according to claim I wherein surface portions of the wall of said passage adjacent said orifices are substantially flush with each other.

6. A spray gun according to claim 1 wherein said barrel comprises inner, intermediate, and outer nested, coaxial sleeves, the opposed surfaces of said inner and intermediate sleeves being spaced apart to define an air passage terminating in said air orifice, and the opposed surfaces of said intermediate and outer sleeves being spaced apart to define a liquid passage terminating in said liquid orifice.

7. A spray gun according to claim I wherein the electrodetip is spaced rearwardly from the front end of the barrel by a distance equal at least to the radius of said passage.

8. An electrostatic spray gun comprising a barrel having a spray discharge passage formed by a wall with at least a conductive forward portion, and with a forwardly facing opening,

an ionizing electrode including a tip in said passage and rearwardly of said opening and electrically insulated from said conductive portion at the passage walls, said electrode and said conductive portion of the passage wall being connected to a source of voltage to create a highly ionized zone in said passage,

an atomizer at the passage walland rearwardly of said tip and arranged to form and direct a spray of coating material forwardly and inwardly of said passage wall to pass through the highly ionized zone and outwardly through said opening.

9. An electrostatic spray gun, comprising a barrel having a forwardly opening discharge passage,

a liquid orifice in said passage, and

an air orifice in said passage adjacent to said liquid orifice,

said air orifice being directed forwardly and inwardly of said passage whereby air under pressure emerging from the air orifice atomizes liquid emerging from the liquid orifice and propels the atomized liquid particles forwardly and toward the center of said passage,

insulating means in said barrel, and

an ionizing electrode supported by said insulating means and extending in said passage to a tip, at least part of said discharge passage being conductive and comprising a terminus for an electrostatic field from the ionizing electrode.

10. A spray gun according to claim 9 wherein at least the tip of said ionizing electrode is located substantially on the axis of said passage.

11. A spray gun according to claim 10 wherein said barrel is grounded.

12. A spray gun according to claim 10 wherein said liquid orifice forms the liquid into a thin film as the liquid emerges from said liquid orifice.