US3448925A - Air spray gun for electrostatic coating systems - Google Patents

Description

June 10, 1969 E vG, CROSS AIR SPRAY GUN FOR ELECTROSTATIC COATING SYSTEMS Filed Oct. 2l, 1966 ATTORNEY United States Patent O 3,448,925 AIR SPRAY GUN FOR ELECTROSTA'II'C COATIN G SYSTEMS Earl G. Cross, Du Quoin, Ill., assiguor to Turco Manufacturing Company, Du Quoin, Ill., a corporation of Missouri Filed oet. 21, 1966, ser. No. 588,603 Int. Cl. Bb 5/04, 1/34, 7/08 U.S. Cl. 239--15 6 Claims ABSTRACT 0F THE DISCLOSURE An air and liquid type electrostatic spray gun with concentric passages for air in the inner passage and liquid in the outer passage. The inner passage has a longitudinally adjustable conically tapered plug terminating an a flange actinlg to deflect air radially. There are whirler means upstream of the plug.

This invention relates to improvements in electrostatic spray cooling systems in which liquid coating material is atomized into fine particles, and the atomized particles are electrostatically charged and attracted to and deposited on an article to be coated. The invention relates particularly to a novel and improved air spray gun for atomizing the liquid coating material and controlling the pattern of the atomized spray produced.

In present day electrostatic spray coating systems, liquid coating materials such as paints or enamels are atomized into a spray finely-divided droplets, and an electrostatic iield is provided for attracting the atomized spray to the article to be coated so that the coating is deposited on said article with high efficiency. These electrostatic coating systems differ from each other chiefly in the method employed for atomizing the coating material, the coatinlg eiiiciency varying with the type of atomization employed.

In the earliest of the electrostatic coating systems, an electrostatic field was created between a grid electrode and the article to be coated, and a standard air spray gun was located within or proximate to the field for atomizing the paint. The atomized spray would become charged by the electrostatic field and would be attracted to the article to be coated, which was maintained at the opposite polarity. While the deposition eiiiciency of such systems was appreciably higher than that of a spray gun used without electrostatic deposition, the eliiciency was nevertheless lower than desired. This loss of deposition eiciency resulted chiefly in the manner in which the air was employed for atomizing the liquid, such air being necessarily directed in a high velocity stream over the liquid so that the atomized particles and the stream of air left the air gun in a forward direction at high velocity. As a result, some of the atomized paint would be propelled entirely through the field without becoming charged or being attracted to the articles to be coated. In addition some atomized particles, striking the article at high velocity, would rebound and settle to the floor. Further, conventional air guns have a relatively small pattern of dispersion, which when coupled with the high velocity of the air stream would cause the coating to be concentrated unevenly on the coated object.

The air spray systems described above were succeeded by more recent systems employing other means for atomizing the liquid in attempting to remedy the aforementioned difficulties. These recent systems included employing rotating centrifugal atomizers and those utilizing the electrostatic force of the iield for atomizing the liquid. The centrifugal atomizers produced a spray of atomized paint without great forward velocity, but with some radial velocity, so that deposition efficiency was increased. Howice ever, the size of the spray particles were not readily controllable, and the cost of the centrifugal atomizer apparatus was relatively high. The systems which employed electrostatic force for atomizing the liquid proved highly etlicient since the atomized paint was subject to no physical forces for imparting to it a forward velocity. However, the paint spray became so highly charged that it was difficult, if not impossible for the field to deposit it within recesses and cavities or upon sharp edges of articles to be coated.

It is an object of the present invention to provide in an electrostatic coating system, a spray gun which atomizes liquid coating material by the force of air passing therethrough, and yet is capable of producing an atomized spray with little forward velocity so that the deposition efficiency is appreciably greater than that of the air spray guns previously employed.

Another object of the invention is the provision of an air spray gun of the character described which is capable of ready adjustment for varying the spray pattern over a wide range, the gun being adapted, where desired, to produce a spray pattern of maximum area which is substantially larger than the patterns heretofore achievable with standard spray guns.

Additional objects and advantages of the invention will become apparent during the course of the following speciiication when taken in connection with the accompanying drawings, in which:

FIG. 1 is a side elevational view of an air spray gun made in accordance with the invention;

FIG. 2 is an exploded view of the front portion of the air spray gun shown in FIG. l;

FIG. 3 is a central longitudinal section through the front portion of the air spray gun;

FIG. 4 is a rear perspective view on an enlarged scale, of the air distribution member forming part of the air spray gun; and

FIG. 5 is a section taken along line 5-5 of FIG. 1.

Referring in detail to the drawings, there is shown in FIG. 1 an air spray gun made in accordance with the present invention and especially constructed for use in an electrostatic coating system, as will be presently explained. The gun generally includes a forward head 12 made of conductive metal, and an insulated tubtular casing 14 connected to and carrying the head 12, and serving as the rear body portion of the gun.

The head 12 is formed of a main body member 16, a hollow threaded cap 18 and an air regulating member 20. The body member 16 is preferably formed of a single block of sturdy, conductive metal, such as bronze, machined to provide a cylindrical end portion 22, an intermediate portion 24 of smaller diameter and a tubular front end portion 26 of still smaller diameter. The intermediate portion 22 is provided with external screw threading 28.

The head body member 16 is provided with a central longitudinal bore 30 of relatively large diameter, extending continuously entirely through the tubular front end portion 26, the intermediate portion 24 and a portion of the cylindrical rear end portion 22. At its rear end, the bore 30 communicates with a co-axial, internallythreaded bore 32 of smaller diameter, the bore 312 extending centrally through the rear wall of end portion 22.

A longitudinal bore 34 is also drilled inwardly and partially through the head rear end portion 22, said bore 34 being parallel to and spaced transversely from the bore 30, and serving as an air inlet passage. A transverse bore 36 drilled inwardly from the side wall of end portion 22, provides communication between bores 34 and 30, as shown in FIG. 3. The outer end of bore 36 is internally threaded and receives screw 38 which acts as a plug to close oil the end of bore 36.

Another longitudinal bore 40, spaced from the central bore 30 and diametrically opposed to the bore 34, extends from the rear wall of head end portion 22 to the front wall of intermediate portion 24. This bore 40, which opens immediately behind the front end portion 26, serves as the paint inlet passage of the head. An air inlet tube 42 is secured within the mouth of bore 34, in communication therewith, and a similar paint inlet tube 44 is secured within the mouth of bore 40 and communciates therewith. The tube 42 is coupled to a flexible air pipe 46 leading to a source of air under pressure, while the tube 44 is coupled to a exible pipe 48 leading to and fed by a reservoir containing paint or other liquid coating material.

The cap 18 is provided at its rear end with internal threading 50 sized to mesh lwith the external threading 28 on the intermediate portion 24 of main body member y16 so that said cap may be securely mounted on the body member in the manner shown in FIG. 3. The threading 50 terminates in an inwardly-inclined peripheral shoulder 52 which abuts a corresponding bevelled surface 53 at the front end of the intermediate portion 24, these mating surfaces S2 and 53 abutting in the fully-mounted position of cap 18 to prevent further inward movement of the latter. The cap also has a shallow circular Iwell 54 adjacent to and extending inwardly from shoulder 52. In the mounted position of the cap 18 shown in FIG. 3, the end wall of said well 54 is spaced from the front wall of the intermediate body member portion 24 to form an annular paint passage 56 communicating with the paint inlet bore 40.

The front end portion of cap 18 is provided with a longitudinal bore 58 which extends from the front wall thereof to the circular well 56, and communicates with the latter. This bore 58 is of lesser diameter than the Well 54 and of slightly larger diameter than the tubular front end portion 26 of the main body member 16. In the mounted position of the cap 18, said tubular front end portion 26 extends through the bore 58, projecting a slight distance forwardly of the cap front wall, as shown in FIG. 3. The circumferential wall of the tubular portion 26 is also spaced inwardly of the wall of bore 58, dening a narrow annular paint outlet passage 60 which communicates with the annular chamber 56.

The air regulating member has a cylindrical rear end section 62 having an external diameter slightly smaller than the diameter of longitudinal bore 30, so that the cylindrical section 62 has a precise, snug sliding lit within said bore 30. Integral with and extending forwardly of said section 62 is a tapered section 64 having a rear end of small diameter and a peripheral wall which increases progressively in diameter toward the front end of said section 64, as shown in FIG. 2. At its front end, the section 64 has an integral annular ilange 66, the external diameter of which is slightly larger than the external diameter of the tubular front end portion 26 of body member 16. As shown in FIG. 3, a central longitudinal bore 68 extends entirely through the air regulating member 20.

The cylindrical section 62 of air regulating member 20 has an annular shoulder 70` bordering its area of junction with the smaller diameter tapered section 64. At its rear end, the cylindrical section 62 has a recessed portion 72, as best seen in FIG. 4, at the base of which a plurality of small bores 74 are drilled. The bores 74 are arranged in a circular row surrounding the central longitudinal bore 68, and extend to and open through the shoulder 70 at the front of cylindrical section 62. As shown in FIGS. 3 and 4, the bores are inclined relative to the longitudinal axis of the air regulating member 20, this inclination imparting a substantially spiral path to the air passing through said member 20, as will be presently described.

The air regulating member 20 is mounted upon an adjusting member indicated generally by reference numeral 80. The adjusting member includes a shank 82 made of electrically-conductive material, and having a rear section of relatively large diameter bearing screw threading 84 sized to mesh with the internal threading of bore 32. At its forward end, the shank 82 has an axially-aligned extension 86 of smaller diameter, which is not threaded and is sized to extend through the longitudinal bore 68 of air regulating member 20. The portion of shank extension 86 which projects forwardly of the air regulating member 20 bears external threading 88 and terminates in a sharp pointed tip `90. The threading 88 is sized to receive a nut 92.

The adjusting member 80 also includes a shank 94 made of electrically insulating material such as a rigid plastic and coupled -co-axially to the rear end of the shank 82, as a continuation of the latter, by a sleeve 96. In assembled condition, the shank `94 projects rearlwardly from the rear end of the insulated tubular casing 14, and secured thereto is an insulating iinger piece 98 by means of which the entire adjusting member 80 may be rotated.

Fine adjustment of the member 80 is aorded by a tensioning plate 100 having a central threaded aperture 102 receiving the threaded shank 82. As shown in FIG. 5, the plate 100 is bifurcated, having depending legs l104 which straddle the air inlet pipe 44 and prevent the plate 100 from turning as the shank 82 is screwed therethrough. A compression spring 106 is mounted on the shank 82 between the plate 100 and the rear wall of cylindrical end portion 22 of body member v16. The spring 106 exerts a rearward biasing force upon plate 100 to provide screw tension upon the adjusting member 180, permitting fine adjustment and preventing creeping.

In assembling the air gun, the adjusting member [80 is inserted through the rear open end of tubular casing 14 and the threaded metal shank 82 is screwed through the plate 100 and the threaded bore 32 until the pointed tip projects from the front of the tubular front portion 26. The air regulating member 20 is then inserted on the l'front end of the shank extension 86 and slid rearwardly until it engages and is stopped by the larger diameter of shank 82. The nut 92 is then tightened on the threaded portion 88, clamping the air regulating member 20 securely on the front end of the adjusting member 80.

As previously indicated, the air gun of the invention is intended to be employed in an electrostatic coating system and to operate in conjunction with an electrostatic eld which will charge the atomized coating particles leaving the gun and attract these particles to an object to be coated. The conductive metal parts of the gun head may therefore be connected to one terminal of an electrical power source of high voltage so that the gun is the charging electrode of the electrostatic field, while the opposite terminal of the power source is grounded and connected through ground to the articles to be coated, which constitute the receiving electrode of the lield. There is thus maintained a high potential variant between the air gun and the articles to be coated which establishes the electrostatic field, as is well known in the art. For this purpose, there is shown in the drawings an insulated electrical lead 108 which is soldered or otherwise secured to the conductive body member 16, the lead 108 being connected to the high voltage terminal of the usual electrical power supply (not shown).

In practice, articles to be coated (not shown), are normally suspended from a conveyor which is maintained at ground potential and so arranged that the grounded articles pass successively in front of the air gun head. Because of the electrostatic eld established, the atomized paint emitted from the gun head is attracted to the grounded article or articles passing in front of the gun, so that the spray is deposited on the articles with high efiiciency. In such an arrangement, the gun is mounted spaced from and facing the articles transported by the conveyor, the casing 14 being clamped upon a supporting stand.

In operation of the air gunfor atomizing liquid, paint or other liquid coating material is fed to the body of the air gun through the feed pipe 48 and the coupled inlet tube 44, the paint flowing through bore 40, circulating through annular chamber 56 and then passing through the narrow annular passage 60, leaving the gun head in the form of a fine annular forwardly directed stream emitted from the front open end of the annular passage 60.

At the same time, air under pressure is fed through pipe 46 and its coupled inlet tube 42, the air travelling through bores 34 and 36 into the large central longitudinal bore 30, the forward end of which is closed off by the closely fitting rear cylindrical section 62 of air regulating member 20. The air then passes through the plurality of small bores 74 and enters the space between the tapered section 64 and the inner surface of the tubular front portion 26 of body member 16. Because of the tapered conical shape of the section 64, the air stream is subjected to a venturi action, and flows through this area at low pressure, exiting through the annular opening 110 defined between the mouth of the tubular portion 26 and the wide diameter portion of the air regulating member front section 64. Since the flange 66 is located immediately forwardly of this opening, the annular air stream is diverted by flange 66 from longitudinal forward movement to a flow in a radial direction, as indicated by the arrow in FIG. 3. The air stream, flowing in this radial direction, thus impinges upon the stream of lquid coating material issuing from the mouth of passage 60, atomizing the liquid stream to produce a spray of finely divided liquid particles which leave the front end of the air gun head in radial directions.

When high voltage is supplied to the air gun head through lead 108, the electrical charge concentrates at the pointed tip 90, so that said tip becomes the charging electrode of an electrostatic field eminating therefrom, and the potential gradient of the field is highly concentrated at the point of tip 90. Because of the sharp pointed configuration of the tip 90, and the consequent accumulation of high voltage charge thereon, an ionic discharge occurs at said tip. On the other hand, the location of the other metallic parts of the atomizer head behind the pointed tip 90, effectively shields these parts from the electrostatic field created between the tip 90 and the articles to be coated, so that the liquid coating material, at its zone of atomiztaion is remote from the point of highest potential concentration of the field, and the atomization itself is unaffected by electrostatic forces.

Thus, as the liquid is atomized, it is carried from the gun in a radial direction by the air stream, and the atomized spray enters the aforesaid electrostatic field. The spray particles are subjected to the ionization discharge emitted from the pointed tip 90 and receive a so-called space charge of a polarity opposite to that of the grounded article or articles spaced forwardly of the tip 90. The ionized paint particles are therefore attracted to the grounded articles and carried thereto by the electrostatic field for deposition thereon.

It will be appreciated that the air stream within the gun head in passing through the inclined bores 74 of air regulating member is imparted with a rotary, spiral path of travel indicated by the arrows in FIG. 4. At the same time, the air stream travels through the venturi-shaped passage defined by the air regulating member front section 64 so that it exits from the air gun head at low pressure. The atomized particles and carrier air stream thus provide a wide radial spray pattern with little movement or velocity so that the spray parti-cles tend to hang within the electrostatic field and have ample opportunity to receive the aforementioned space charge. The air gun of the present invention is therefore capable of supplying a fine atomized spray in such a manner that the atomized paint is deposited with high efficiency, with negligable loss of paint resulting from overspray or fall-out. In addition, the radial 4dispersion of the atomized liquid from the gun provides a wide spray pattern effecting an appreciably greater area of coverage than has been possible with conventional air guns. Further, it has been observed that even with the larger spray patterns provided, the normal tendency for the paint to feather or thin out in the marginal zones of the pattern, is materially reduced. While with conventional air spray guns, the density of the paint spray deposited is substantially decreased in wide marginal areas of the spray, resulting in uneven coating, the gun of the present invention may be adjusted to decrease this low density fringe zone to a remarkable degree. This benefit can be attributed to the coaction of the circular atomizer air movement and the creation of the low pressure area at the precise point of atomization.

The area ofthe spray pattern may be varied over a wide range by means of the adjusting member 80. This may be accomplished during the actual painting operation by manually grasping and turning the finger piece 98. The threaded shan-k l82 is thus selectively turned in either direction to move the air regulating member 20V forwardly or rearwardly within the tubular portion 26. Movement of the member 20 in a forward direction brings a smaller ldia-meter portion of the tapered section 64 into registry with the mouth of the tubular portion 26 to increase the size of the annular air outlet opening and also reduces the venturi effect of said tapered section 64, so that the spray pattern is increased in size. Movement of memiber 20 in a rearward direction has the opposite effect and reduces the area of the atomized spray pattern. In this manner, the paint pattern may be selectively adjusted from a diameter of approximately two inches to a pattern of thirty-six inch diameter.

It is to be understood that while the electrostatic system described herein contemplated the connection of the high voltage power supply to the metal |body of the air gun so that the gun itself served as one electrode of the electrostatic field, the gun may be employed in conjunction with an external field. In this instance an external charging electrode in the nature of a wire grid or the like may be mounted in the vicinity of the gun head to create an electrostatic field between such electrode and the articles to be coated, and the gun itself will be free of electrical connections. The atomized spray emitted by the gun will enter the external field and become charged in the same manner as previously indicated.

While a preferred embodiment of the invention has been shown and described herein, it is obvious that numerous omissions, changes and additions may be made in such embodiment without departing from the spirit and scope of the invention.

What is claimed is:

1. For use in an electrostatic spray coating system employing an electrostatic field for depositing liquid coating material upon articles, an air spray gun for atomizing said liquid coating material and discharging the atomized material into said electrostatic field, said air spray gun comprising a body having a central longitudinal bore opening at the forward end of said body, means for connecting the rear portion of said central longitudinal bore to a source of air under pressure, said body also including a second longitudinally extending bore spaced from said central bore, -means for connecting said second bore to a source of liquid coating material, an air regulating member slidably located in the forward end of said central longitudinal bore for longitudinal movement therein, said air regulating member having a cylindrical rear portion of sufficient diameter to fit snugly within said central -bore and having air-diverting inclined channels for causing air passing therethrough to travel in a rotary path, and a tapered substantially conical front portion downstream of said channels and terminating in a front peripheral flange, said conical front portion having a rear end of small diameter and a peripheral wall increasing progressively in diameter toward said front flange to provide a venturi-shaped air passage to decrease the pressure of air passing therethrough, said air regulating member being located within said central bore with a portion of said conical front portion located in the mouth of said bore and deiining an annular air outlet opening and said peripheral flange located forwardly of said air outlet opening in position to direct the air radially outward from said air spray gun, said second bore communicating with an annular liquid outlet passage having a -mouth surrounding and concentric with said air outlet opening, whereby the liquid leaving said mouth is engaged and atomize-d by the air owing radially from said air outlet opening, and adjusting means for selectively moving said air regulating member longitudinally within said central bore, whereby to adjust the size of said air outlet opening and the size of said venturi air passage.

2. An air spray gun according to claim l1 in which said air-diverting inclined channels comprise a plurality of inclined through bores extending from the rear surface of said air regulating member rear portion to the front surface thereof and communicating with said venturi air passage, whereby the air leaving said bores travels in a rotary path through said venturi air passage and exits at low pressure therefrom 3. An air spray gun according to claim 1 in which said adjusting means comprises a longitudinally-extending screw shank secured to said air regulating member and threadedly engaging a portion of said `body member, said shank having a rear portion projecting from the rear end of said gun body and exposed for manual turning of said screw shank to adjust the position of said air regulating member within said central bore.

4. An air spray gun according to claim 1 in which said ygun body comprises a body member including a cylindrical section having external screw threading and a tubular section of lesser diameter projecting forwardly of said cylindrical section, said central longitudinal bore extending through said sections and opening at the front end of said tubular section, and a threaded cap engageable with the external threading of said cylindrical section for mounting said cap thereon.

5. An air spray gun according to claim 4 in which said cap has a longitudinal bore of slightly greater diameter than the external diameter of said body member tubular section, said cap -bore receiving said tubular section in the mounted position of said cap and for-ming with the exterior surface of said tubular section an annular liquid passage communicating with the second bore of said body member and opening adjacent the front end of said tubular section.

6. An air spray gun according to claim 5 in which the cylindrical section of said body member joins said tubular section at a lateral shoulder, said second longitudinal bore extending through said cylindrical section and opening through said shoulder, said cap in its mounted position having an internal surface spaced from said shoulder and forming therewith an annular liquid chamber connecting said second longitudinal bore with said annular liquid passage.

References Cited UNITED STATES PATENTS 758,206 4/ 1904 Grundell et al 239-4025 1,002,594 9/1911 Mason et al. 239-417 X 1,714,654 5/ 1929 Ballgoe Z39-402.5 3,000,574 9/1961 Sedlacsik 239-15 3,049,092 8/1962 Sedlacsik et al. 239-15 X 3,059,613 10/1962 Nakaya 239-15 X 3,219,276 11/1965 Norris 239-15 X 3,296,015 1/1967 Juvinall et al. Z39-15X 3,317,138 5/1967 Fraser 239-15 FOREIGN PATENTS 998,124 9/1951 France. 693,997 7/ 1953 Great Britain.

EVERETI W. KIRBY, Primary Examiner U.S. C1. X.R.

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