US3031145A - Electrostatic spray painting apparatus - Google Patents

Description

April-24, 1962 Filed Sept- F. A. CROSKEY 3,031,145

ELECTROSTATIC SPRAY PAINTING APPARATUS 4 Sheets-Sheet 1 All? ,w fl SUPPLY if i pp/Zg/UPE PAM/7' F RESERVO/R INVENTOR.

A TTOPNEY April 24, 1962 F. A. CROSKEY 3,031,145

ELECTROSTATIC SPRAY PAINTING APPARATUS I Filed Sept. 21. 1959 4 Sheets-Sheet 2 April 24, 1962 F. A CROSKEY I 3,031,145

ELECTROSTATIC SPRAY PAINTING APPARATUS Filed Sept. 21. 1959 4 Sheets-Sheet 3 fli W Xi ///4W/ .1.

IN VENTOR. fier /f [ray/72y BY Filed Sept. 21, 1959 April 24, 1962 F. A. CROSKEY 3,031,145

ELECTROSTATIC SPRAY PAINTING APPARATUS 4 Sheets-Sheet 4 ATT OP/VE Y United States Patent C) 3,031,145 ELECTRGSTATEC SPRAY PAKNTING APPARATUS Frank A. Croskey, New Baltimore, Mich, assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware Filed Sept. 21, 1959, Ser. No. 841,140 10 Claims. (Cl. 23915) This invention relates to electrostatic coating apparatus, and more particularly to improved electrostatic spray painting apparatus.

One feature of the invention is that it provides improved electrostatic spray painting apparatus; another feature of the invention is that it provides an electrostatic spray painting system utilizing a pneumatic spray gun wherein the gun is encased in a housing of dielectric material, only the nozzle of the gun projecting out of said housing, whereby electron leakage from the spray gun is substantially entirely confined to the vicinity of the nozzle; another feature of the invention is that it provides electrostatic spray painting apparatus wherein paint is charged with a high voltage as it flows through a dielectric passage associated with the gun, thereby increasing the charging efficiency; and still another feature of the invention is that means are provided for varyin the length of the dielectric passage through which the paint moves toward the nozzle so that the apparatus may be readily adjusted for optimum charging efiiciency regardless of the type or viscosity of paint or other coating material which is used.

Other features and advantages of the invention will be apparent from the following specification and from the drawings, in which:

FIG. 1 is a diagrammatic representation of an electrostatic painting system utilizing the invention;

FIG. 2 is an enlarged View, partly in vertical section, of the spray gun and associated apparatus of FIG. 1;

FIG. 3 is a front view of the spray gun and associated apparatus taken along the line 3-3 of FIG. 2;

FIG. 4 is a further enlarged vertical sectional view of the lower portion of the apparatus of FIGS. 2 and 3, being taken along the line 4-4 of FIG. 3; and

FIG. 5 is a continuation of FIG. 4, showing the upper portion of the apparatus and being taken along the line 5-5 of FIG. 3.

Electrostatic spray painting systems comprise arrangements wherein atomized paint particles carry an electrostatic charge which, by virtue of well-known physical laws, causes the particles to be drawn toward and deposited upon a grounded workpiece which passes along a conveyor adjacent the spray emitting means. Two general types of electrostatic spray painting apparatus have been in use commercially. In one of these types, paint is sprayed from a conventional type of pneumatic gun through a high voltage electrostatic field toward a workpiece. During their passage through the electrostatic field, the atomized paint particles acquire a charge which causes them to be drawn toward and deposited on the workpiece. While this type of electrostatic coating system provides a much higher efficiency than is achieved by a conventional pneumatic spray gun without the use of electrostatic forces, it has been found that the charging efliciency is relatively low as the atomized particlespass through the electrostatic field so that there is a certain amount of overspray, that is, a certain percentage of the atomized paint particles are not deposited upon the work but by-pass the work and must be collected in a water curtain or other conventional overspray collecting means.

Another form of electrostatic spray painting utilizes a spinning disk or bell. Paint is fed to the disk or bell and spreads as a film toward the periphery thereof from which it is dispersed into the atmosphere as a charged 3,031,145 Patented Apr. 24, 1962 cloud, generally because of a combination of centrifugal and electrostatic forces. This type of electrostatic spray painting apparatus has been found to be disadvantageous because of uncontrolled electric feed back through the paint feed system which lowers the charging efliciency. Furthermore, this type of spray painting apparatus has a serious disadvantage in that the quantity of paint which can be dispersed is relatively small. Both types of apparatus described above are disadvantageous in that they will only handle certain kinds of paint and different mechanical arrangements must be provided to handle different paints having different chemical constituencies and viscosities.

This invention provides an improved electrostatic spray painting arrangement which will increase the charging efficiency of the paint over either of the systems described above, which will handle a much greater quantity of paint than the spinning disk or spinning bell type and which is readily adaptable to various types of paint having difierent chemical constituencies and different viscosities.

According to the invention, a conventional pneumatic type spray gun is utilized. The entire body of the gun, which is of metal, is charged to a high voltage, preferably of the order of 45 kv. The gun is encased in a housing of dielectric material (preferably epoxy resin), the housing having an opening through which the nozzle of the gun projects whereby electron leakage from the gun is substantially entirely confined to the vicinity of the nozzle. Paint is fed to the gun toward the nozzle through a dielectric passage and means are provided for charging the paint as it flows through this passage. This results in a vastly improved charging efficiency over other electrostatic spray painting arrangements. Furthermore, means are provided for varying the effective dielectric length of the passage so that the apparatus can be readily adjusted for different types of paints having different chemical constituencies and different viscosities.

Referring to the drawings, and particularly to FIG. 1, the improved electrostatic painting system is shown diagrammatically. The reference character 10 represents a conventional conveyor which is grounded and which carries a plurality of workpieces, as the workpiece 12, past a painting zone represented diagrammatically by the fanshapcd spray pattern 14 which is emitted by the spray emitting means. The spray emitting means comprises a gun assembly designated generally as 16 suspended from an overhead support 1%. The gun is charged with a high voltage, preferably a negative voltage of the order of 45 kv., which is generated by a power supply 20 connected to the gun by a lead 22. The other side of the power supply is grounded. Paint is fed to the gun assembly from a paint reservoir 24 which is connected by a feed tube 26 incorporating a shut-off valve 28 to a control device designated generally as 30. This control device is also grounded through a circuit including a microammeter 32. High pressure air for controlling operation of the spray gun is obtained from a high pressure air supply 34 which is connected through a line 36 incorporating a shut-off valve 38 to the spray gun assembly, and atomizing air for the spray gun is obtained from an atomizing air supply 40 which is connected to a line 42 incorporating a shut-off valve 44 to the spray gun assembly.

FIGS. 2 and 5 show the construction of the spray gun. The gun, which is designated generally as 50, is a conventional commercially obtainable pneumatic spray gun having a metallic body 52 which, according to the invention, is encased in a dielectric housing, preferably of epoxy resin. This housing, which is indicated on the drawings at 54, is molded around the spray gun in tight sealing engagement with the voltage supply lead 22, a paint supply tube and the atomizing and high pressure air supply leads 42 and 36, respectively. The housing has an opening 56 through which the nozzle of the spray gun projects.

During construction of the spray gun and housing assembly, I find it preferable to utilize modeling clay to space certain of the spray gun parts from the epoxy resin during the molding operation. Certain of these protective clay arrangements are illustrated in the drawings,

60 in FIGS. 2 and representing a seal consisting of modcling clay which surrounds the side port control of the spray gun. This control is adjusted for proper spray fan width before embedding the gun in the resin. Other details of a similar type are illustrated in the drawings where 62 in FIG. 2 shows modeling clay which is packed in the mounting hole of the spray gun to prevent resin from leaking into the high voltage jack during the molding operation. A similar seal is obtained at 64 where a modeling clay seal is utilized during the molding operation to prevent the epoxy resin from working into the inside of the gun and jamming the operating pin in the gun. Similarly, a modeling clay seal 66 surrounds the control pin adjustment of the spray gun to prevent this pin from being frozen into the epoxy resin housing.

Since the gun is of conventional commercially obtain- :able type, only a brief description will be given as to its construction. A metal high voltage jack connector for the high voltage supply is shown at 65 and this connector receives a conventional banana plug 67 which is connected to the end of the high voltage lead. The lead projects into the gun through a nylon high voltage cable retainer 68 which, as shown in FIG. 2, is in tight sealing relation with the epoxy resin housing 54. The cable itself is a conventional type of coaxial high voltage cable having a ground shield 70 which terminates at the outer end of the nylon cable retainer 68 and is secured thereto.

Inside the ground shield 70, there is a conventional insulated high voltage lead 72. A high voltage, preferably a negative voltage of the order of 45 kilovolts developed by the power supply 20, is fed through the cable arrangement above described to the metallic body 52 of the pneumatic spray gun 50 so that the entire gun is charged with a high voltage.

The-gun includes a piston operated fluid control pin '74 which controls the paint feed opening to the gun nozzle 76. As shown in FIGS. 2, 3 and 5, the nozzle projects out of the opening 56 in the epoxy resin housing 54. Since all the other leads and connections into the epoxy resin housing include insulating material between the gun and the external surface of the housing, electron leakage from the highly charged spray gun is substantially entirely confined to the vicinity of the nozzle 76. When the pin is withdrawn as shown in FIG. 5 by high pressure air from the supply 34 and atomizing air (preferably at about pounds pressure) is supplied to the gun from the atomizing air supply 40 while paint or other coating material is supplied to the nozzle 76 from the reservoir 24, the gun will emit an atomized spray of paint or other coating material in a fan-shaped pattern as illustrated diagrammatically in FIG. 1. Should the high pressure air supply 34 be shut off, an internal spring (not shown) forces the pin 74 to the left in FIG. 5, sealing off the nozzle in conventional manner.

In order to permit return movement of the piston which drives the control pin 74, there is an exhaust tube 80 which extends out of the epoxy resin housing and is in tight sealing engagement therewith. This tube preferably is made of high dielectric material to avoid electron leakage.

Atomizing air is supplied to an air nozzle 78 which is mounted on the fluid emitting nozzle 76 and which projects'through the opening 56 in the epoxy resin housing. By using an air nozzle made of high dielectric material, preferably micarta, electron leakage is restricted to the vicinity of the metallic fluid nozzle 76 and spark hazard from'the gun is reduced.

The atomizing air lead 42 is connected by a coupler 82 to a nylon tube 84 which is molded within the housing 54 and which is connected to the atomizing air inlet of the spray gun body. Similarly, the high pressure air feed tube 36 is connected by a coupler 86 to a nylon tube 88 which is molded in the epoxy resin housing 54 and which is connected to the high pressure inlet of the spray gun. These connections provide a tight sealing arrangernent coupled with a dielectric connection to prevent electron leakage.

A nylon tube 90 is connected to the paint inlet nipple 92 and projects out of the epoxy resin housing. This tube is molded into the housing in a manner similar to the tubes 84 and 88 so that it also is in tight sealing relation to the housing. A metallic coupler 94 is threaded into a counterbore 90a formed in the free end of the tube 90 and mounts a metallic tube 96 having a bore 98 which is quite restricted when compared with the bore 100 of the nylon tube 90. A second nylon tube 102 has a threaded counterbore 102a into which the coupler 94 is threadably received, and the tube 102 has a relatively small bore 104 extending entirely therethrough which is somewhat larger in diameter than the restricted bore 98 of the member 96. At its lower end, the tube 102 is formed with a threaded counterbore 102b which receives another coupler 106 which in turn is threadably connected to the metallic housing 108 of the control device 30. The paint feed tube 26 is also connected to this metallic housing and a passage 110 is formed in the housing to provide fluid communication between the paint feed tube 26 and the coupler 106 so that paint from the reservoir 24 may feed through the tube 26, the passage 110 in the control member housing 108, and the bores of the coupling member 106, the tube 102, the coupling member 94, the restrictive member 96, and the tube 90 to the paint feed inlet of the spray gun.

The control housing mounts a rotatable metallic drum 112 which may be turned by a control knob 114 connected to the drum by means of a shaft 116. A wire conductor 118 is mounted on the periphery of the drum, being connected to the drum by means of a slot 120 and an anchor pin 122 as shown in FIG. 4. The wire 1'18 extends through a fluid packing gland 124 in the control housing and projects up through the bore of the coupler 106 into the bore 104 of the nylon tube 102. By turning the control knob 114, the extent which the wire projects up into the bore of the tube 102 can be adjusted, and therefore, the effective dielectric length of the bore 104 can be controlled. If the wire 118 is moved upwardly in FIG. 5 until it contacts or comes very closely adjacent the metallic coupler 94, the dielectric effect of the bore 104 in the nylon tube 102 is substantially destroyed. As shown in FIG. 1, the metallic housing of the control device 30 is connected to ground through a microammeter 32.

I have found that most effective electrostatic spray painting can be obtained if the paint is charged while flowing in a pool in the gun and if the feed back or current flowing through this paint pool is of the order of 100 microamperes when a charging voltage of the order of 45 kv. is used. With my arrangement, the control knob 114 can be manipulated to provide a feed back current of the order of 100 microamperes through the paint pool regardless of variations in chemical constituency, viscosity, and the like of the paint. Since the paint feed tube 26 is of dielectric material and since the paint itself has a relatively high resistance, the ground connection at the control housing effectively prevents any feed back through the tube 26 to the paint reservoir itself.

With my improved apparatus, it is possible to obtain effective spray painting with metallic and non-metallic paints as well as with water soluble coating materials. As one example, if a synthetic enamel is reduced with a solvent made of 40% normal butyl alcohol and 60% SC-100 solvent in the ratio of approximately 1% quarts of solvent to one gallon of paint, a current leakage to ground of approximately 100 microamperes will be maintained with a spacing of 1 /8 inches between the top of the restricted member 96 and the fluid inlet of the spray gun to which the tube 90* is connected when the conductor 118 is substantially shorted out against the coupler 94. I have found that current as low as 40 microamperes will produce good results with the paint mixture described above. In the event current leakage rises to more than 150 microamperes, the nylon tube chamber will become heated, and after several hours of operation, the paint will form a pigment layer around the internal feed chamber in the spray gun. If metallic paints are used, it is merely necessary to back off the conductor 118 until a current reading of approximately 100 microamperes is obtained.

I have found that I can obtain the desired controlled feed back as described above with an arrangement wherein the bore 104 of the nylon tube 102 has a diameter of the order of .052 inch. A smaller bore than this may tend to clog when metallic paints are used. With a feed passage of this size, the apparatus will spray between five and six ounces of paint per minute which will give proper coverage of paint on the workpiece at a thickness of about one mil.

While I have shown and described one embodiment of my invention, it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. Electrostatic coating apparatus of the character described, including: spray emitting means having a nozzle; a circuit including a lead directly connecting said spray emitting means with a source of high voltage; means for supplying fluid coating material to said spray emitting means, said supplying means providing a dielectric passage through which said fluid passes towards said nozzle; circuit means connecting the downstream end of said passage to said source of high voltage; and means for connecting the upstream end of said passage to a source of electrical potential which difiers from the potential of said source of high voltage whereby liquid coating material in said passage is charged to a high voltage.

2. Electrostatic coating apparatus of the character described, including: spray emitting means having a nozzle; means for charging said spray emitting means with a high voltage; means for supplying fluid coating material to said spray emitting means, including a tube providing a dielectric passage through which liquid passes toward said nozzle; means connecting the downstream end of said passage to said charging means; and means for connecting the upstream portion of said passage to ground, said last-mentioned means including a conductor in the passage and a control device for selectively varying the distance which said conductor extends into the passage to vary the effective length of said dielectric passage.

3. Apparatus of the character claimed in claim 2, wherein said last-mentioned means includes a grounded metallic drum having said conductor mounted thereon and a control knob for turning the drum to vary the distance which the conductor extends into the passage.

4. Electrostatic coating apparatus of the character described, including: spray emitting means having a nozzle; means for charging said spray emitting means with a high voltage; means for supplying fluid coating material to said spray emitting means, said supplying means including a first tube connected to said spray emitting means and providing a dielectric passage through which said fluid passes toward said nozzle, a conductor in said tube having a restricted fluid passageway therethrough, and a second tube connected to said conductor and having a restricted dielectric fluid passageway therethrough; means connecting the downstream end of said first tube to said 6 charging means; and means for connecting the upstream end of said second tube to ground, said last-mentioned means comprising a wire in the passage of said second tube and means for varying the extent which said wire projects into said passage to vary the effective dielectric length thereof.

5. Apparatus of the character claimed in claim 4, wherein said last-mentioned means includes a grounded metallic drum having said conductor mounted thereon and a control knob for turning the drum to vary the length of wire which extends into the passage.

6. Electrostatic coating apparatus of the character described, including: spray emitting means comprising a spray gun having a fluid emitting nozzle; a housing of dielectric material encasing said spray gun and having an opening through which said nozzle projects; means for changing said spray gun with a high voltage; means for supplying liquid coating material to said spray gun, said supplying means including a first tube connected to said spray gun and providing a dielectric passage through which said fluid passes toward said nozzle, a conductor in said tube having a restricted fluid passageway therethrough, and a second tube connected to said conductor and having a restricted dielectric fluid passageway therethrough; means connecting the downstream end of said first tube to said charging means; and means for connecting the upstream end of said second tube to ground, said last-mentioned means comprising a wire in the passage of said second tube and means for varying the extent which said wire projects into said passage to vary the effective dielectric length thereof.

7. Electrostatic coating apparatus of the character described, including: spray emitting means comprising a pneumatic spray gun of electrically conductive material, said spray gun having a fluid emitting nozzle; a housing of epoxy resin molded around and tightly encasing said spray gun and having an opening through which said nozzle projects; an air nozzle mounted on said fluid emitting nozzle and projecting through said opening outside said housing; means for charging said spray gun with a high voltage; means for supplying atomizing air to said spray gun for said air nozzle; means for charging said spray gun with a high voltage, means for supplying fluid coating material to said spray gun, said supplying means including a first tube connected to said spray gun and providing a dielectric passage through which said fluid passes toward said nozzle, a conductor in said tube having a restricted fluid passageway therethrough, and a second tube connected to said conductor and having a restricted dielectric fluid passageway therethrough; means connecting the downstream end of said first tube to said charging means; and means for connecting the upstream end of said second tube to ground, said last-mentioned means comprising a wire in the passage of said second tube and means for varying the extent which said wire projects into said passage to vary the eflective dielectric length thereof.

8. Electrostatic coating apparatus of the character described, including: spray emitting means having a nozzle; a solid housing of dielectric material solidly encasing said spray emitting means in enveloping contact therewith and having an opening through which only said nozzle projects; and a circuit directly connecting said spray emitting means with a source of high voltage, the lead of said circuit entering said housing in tight insulated sealing relation therewith, whereby electron leakage from said spray emitting means is substantially entirely confined to the vicinity of said nozzle.

9. Electrostatic coating apparatus of the character described, including: spray emitting means comprising a spray gun having a fluid emitting nozzle; a solid housing of high dielectric material solidly encasing said spray gun in enveloping contact therewith and having an opening through which only said nozzle projects; an air nozzle mounted on said fluid emitting nozzle and projecting through said opening outside said housing; a circuit directly connecting said spray gun with a source of high voltage; means for supplying liquid coating material to said spray gun; means for supplying atomizing air to said spray gun for said air nozzle, the lead of said circuit and said supplying means entering said housing in tight insulated sealing relation therewith and being connected to said spray gun through dielectric means whereby electron leakage from said spray gun is substantially entirely confined to the vicinity of said nozzles.

10. Electrostatic coating apparatus of the character described, including: spray emitting means comprising a spray gun of electrically conductive material, said spray gun having a fiuid emitting nozzle; a solid housing of high dielectric synthetic resin material solidly encasing said spray gun in enveloping contact therewith and having an opening through which only said nozzle projects;

a circuitdirectly connecting said spray gun with a source of high voltage; means for supplying liquid coating material to said spray gun; and means for supplying atomizing air to said spray gun, the lead of said circuit and said supplying means entering said housing through a sheath of insulating material which is in tight insulated sealing relation with said housing whereby electron leakage from said spray gun is substantially entirely confined to the vicinity of said nozzle.

References Cited in the file of this patent UNITED STATES PATENTS 2,302,185 Campbell Nov. 17, 1942 2,913,186 Sedlacsik Nov. 17, 1959 FOREIGN PATENTS 813,388 Great Britain May 13, 1959

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