US 3169882 A
Description (OCR text may contain errors)
1965 J. w. JUVINALL ETAL 3,
ELECTROSTATIC COATING METHODS AND APPARATUS Filed Oct. 5, 1960 4 Sheets-Sheet 1 INVENTORQ JAMES w. JUVINALL ERHARD KOCK BY JAMES c MARSH b d/v Him 5:29 I I J. w. JUVINALL ETAL 3,169,882
ELECTROSTATIC COATING METHODS AND APPARATUS 4 Sheets-Sheet 2 Feb. 16, 1965 Filed Oct. 5, 1960 INVENTOR. JAMES w. JUVINALL ERHARD KOCK BY JAMES c.
Feb. 16, 1965 ,1. w. JUVINALL ETAL 3,169,882
ELECTROSTATIC COATING MBI'HQDS AND APPARATUS 4 Sheets-Sheet 3 Filed Oct. 5, 1960 mm Nb Om on I I I I i I I I l I I I I I I I I I I I l I lllffnfl n (Ill/Il/l/l/Al/fM Illllllllllil I v n lllllllllll I I i I I I I I I l a I ll lll lll I I I I l l I I v I! INVENTOR.
JAMES W. JUVINALL ERHARD. KOCK BY JAMES C. ARSH AH eys 4 Sheets-Sheet 4 16, 1965 J. w. JUVINALL ETAL ELECTROSTATIC COATING METHODS AND APPARATUS Filed Oct. 5, 1960 3,159,882 Patented Feb. 1965 3,169,882 ELECTROSTATIC COATING METHODS AND APPARATUS I James W. Juvinall,Erhard Kmk, and James C. lliarsh,
Indianapolis, Ind., assignors to Ransburg Electro- Coating Corp., Indianapolis, Ind a corporation of Indiana Filed Oct. 5, 1960, Ser. N 60,657 13 Claims. (Cl. 11793. t)
This invention relates to apparatus and methods for electrostatic spray coating, and particularly to an electrostatic coating system using mechanical atomization of liquid coating material. This application is a continuation-in-part of our prior application Ser. No. 703,122, filed December16, 1957, now abandoned.
The efficiency of deposition of liquid spray particles on an article being coated is materially improved by utilization of electrostatic forces, and for more than fifteen years electrostatic spray painting has been widely utilized commercially. During the first several years of use of electrostatic spray painting, all commercially available systems used conventional compressed air spray guns which atomized and directed the spray of liquid material into an electrostatic charging and depositing field set up between the article and a plurality of grid wires or electrode points spaced a distance of about a foot and up to several feet from the tip of the spray gun, and well outside the path of the spray. Hundreds of such systems are still in commercial use. More recently, improved efliciencies have been attained by a method which eliminates the compressed air previously relied on for atomization, and effects atomization electrostatically by the same' field which effects deposition of the atomized particles.
The method last mentioned has attained remarkably high efiiciencies in the utilization of the atomized coating material; but its commercial use requires highly specialized equipment and presents problems in satisfactorily atomizing certain types of coating materials, in obtaining spray patterns of desired shape, in starting and stopping atomization, and frequently in atomizing a sufficient volume of coating material. Most of such problems are absent, or less acute, in the method using a spray gun in which the atomization is effected by compressed air; but with such a spray gun the deposition efiiciencies as measured by the percentage of atomized paint deposited on the article have been considerably lower, and in many instances only about one-half of that obtained with electrostatic atomization.
The relatively large loss of coating material heretofore characterizing electrostatic painting systems employing the air spray gun has generally been attributed to the effect of the air in carrying a considerable portion of the atomized particles away from the article despite the electrostatic attractive forces present, and this loss has been generally regarded as inevitable. Attempts have been made to increase the depositing efficiency of the air gun method by improving the power pack and by reducing the amount of air used to atomize the coating material and project the atomized particles into the field. While these expedients have effected some improvement in depositing efficiency, the efliciency remained low even when the air was reduced to the minimum necessary to properly atomize the coating material.
In addition to decreasing the air, efiorts have been made to more fully utilize the electrostatic forces so as to effect electrostatic deposition of a greater portion of the spray particles on the articles. These efforts have been generally to provide a plurality of highly charged wires or points about the spray and/or about the article to be coated. A cluster of six or more electrode points have been mounted on the spray gun in positions spaced well away from, but surrounding the spray; a series of electrode points has been arranged on each side of the spray gun in lines having an extent equal to or greater than the extent of the article to be' coated; and a number of electrode wires have been conformed to the shape of the article being coated. None of these expedients has provided any significant increase in the. depositing efficiencies of electrostatic coating systems using-an air 'spray gun. a v
In even the most eflicient prior electrostatic coating systems using air spray guns, the quantity of paint lost in overspray has been great enough to present substantial problems in protecting the health of operating person nel, in preventing property damage from oversprayed paint, and in reducing fire hazards. In industrial production, the operation has had to be carried out in spray booths equipped with exhaust systems capable of providing air velocities of at least 100 feet per minute, and such air velocities themselves increased overspray losses. Overspray collecting equipment such as cleanable baffles, and in many cases even water curtains, were required. No prior electrostatic systems of which we areaware made it possible to use a hand-held air spray gun in they painting of buildings, installed equipment, piping, and the like without the production of substantial overspray creating both a health hazard for the operator and the possibility of damage to surfaces not intended to be painted.
We have found that we can employ electrostatic forces to achieve very high deposition efliciencies and still retain the advantages of the compressed-air spray gun in respect to volume of paint atomized, pattern shape, in-
stantaneous initiation and termination of atomization,
and commercially satisfactory atomization of difficulty atomizable materials; and we have further found that we can achieve those advantages in a compressed-air spray gun which may be manually manipulated with safety in spite of its employment of high voltage. With deposition efiiciencies readily attainable by our invention, the problems of exhaust and overspray recovery are greatly simplified, reduced exhaust air velocities with consequent reductions in power consumption and heat losses are made possible, simple filters can replace water curtains, and the field for practical use of a hand-held gun spraying without benefit of a booth is greatly extended. We accomplish these results by providing at the gun spray-charging electrode means which, when charged to high electric potential, creates a high concentration of atmospheric ions in a zone so disposed that all, or a substantial majority of, the spray particles pass through it promptly following their formation. Preferably the electrode means is a single fine wire located within the spray, conveniently a wire supported within the gun to project forwardly through and beyond the paint-emitting orifice of the spray gun; but the electrode means may be mounted beside the paint orifice so that it will be outside of but close to the spray. In any event, the ionized zone created by the charging electrode means is so situated that it is traversed by the spray particles while both the spray particles in the zone and the atmospheric ions in the zone are in a state of high spatial concentration. An example of such a zone is one which surrounds a point of local potential gradient of the order of that existing about the electrically isolated-Le, unshieldedtip of a I s i t l i l 1 t i I i U I operating electrode means may consist solely of the articlev being coatedrbut preferably it comprises both such electrode we locate such counter-electrode well outside the path of the spray and preferably rearwardly of the atomization zone. So located, the counter-electrode will riotinhibit theexistence of a substantial spray-depositing field between the charging electrode and the work. However, a substantial field will also exist between the charging electrode and counter-electrode; and to prevent the counter-electrode from becoming coated with spray by the effect of that field, we project spray particles from the gun toward thearticles with sufiicient momentum to overcome their tendency to deposit on the counter-electrode and to carry them into a region where they will be deposited on the grounded articles by virtue of their own electrical charges with the assistance of such field as exists between the articles and the charging electrode. This latter field is preferably a field of a strength which would in the absence of both paint and compressed air provide a current flow-from the charging electrode to the article being coated of at least about one microampere and most desirably five microamperes or more.
' Contrary to what; might be expected, we find that a properly positioned counter-electrode can have a measurable eifect increasing deposition efiiciencies even though the average potential gradient between it and the charging electrode may be less than that between the charging electrode and the article being coated. Thus, if a hand-held spray gun provided with an appropriate charging electrode at its front end and a grounded handle near its rear end is used to spray a grounded article, the grounded handle, serving as a counter-electrode, can effectan increase in deposition efficiency even if the distance between the charging electrode and the handle is greater than that between the charging electrode and the article. However, we prefer an arrangement in which the distance between the charging electrode and counterelectrode will be less than the distance between the charging electrode and the article being coated in an ordinary spray-coating operation. In such an arrangement, the presence of the counter-electrode will eifect a marked improvement in deposition etliciency. This arrangement makes possible the use of power packs of lower output voltage, produces a compact gun which may be readily manipulated by hand, and simplifies the problem of making a gun safe against fire hazards and shocks to personnel, as well as providing other advantages.
A charging electrode located, as our preferred electrode'is, within the path of the spray can adversely affect the quality of the atomization and hence the quality of the finish produced. Thus, the electrode may collect coating material and return it to the spray in the form of particles so large that they degrade the quality of the finish. Wehave found that this effect can be reduced by reducing the diameter of the electrode and/or by directing one or more air jets laterally against the electrode. When the electrode projects forwardly through the paint orifice, the atomizing air tends to prevent it from collecting coating material; but in order for the atomizing air to keep the electrode free of paint, it may be necessary to discharge it in such'quantity that the deposition efficiency would be adversely affected. We therefore prefer to use an electrode of no more than about 0.050 inch in diameter, most desirably one about 0.010 inch in diameter, and to provide in the gun one or more orifices which discharge air jets directed transversely of the spray and against the electrode it being understood that the importance of the transverse air increases as the diameter of the electrode increases. Transverse air jets may also be used to shape the spray and provide a spray pattern more desirable than the generally circular pattern produced in the absence of transverse air jets.
Other features and advantages Will be apparent from the following specificationand the drawings, in which:
FIG. 1 illustrates a novel air spray gun constructed in accordance with our invention;
FIG. 2 is a front end view of the spray gun shown in FIG. 1;
FIG. 3 is a detailed cross-sectional view of the center portion of the spray gun shown in FIGS. 1 and 2;
FIG. 4 illustrates another form of air spray gun constructed in accordance with our invention and adapted for hand operation;
FIG. 5 shows the spray gun of FIG. 4 in use painting bicycle frames;
FIG. 6 illustrates another form of hand air spray gun constructed in accordance with the invention;
FIG. 7 is a sectional View taken along line 7-7 of FIG. 6;
FIG. 8 is a sectional view taken along line 8-8 of FIG. 6 showing details of the valving mechanism; and
FIG. 9 is a partial sectional view taken along line 9-9 of FIG. 6.
In FIG. 1 the spray gun shown generally as 10 is mounted by a bracket 11 on the end of a shaft 12 constructed of highly insulating material. Gun 10 consists essentially of a main gun body 14, a liquid cap 15 and an air cap 16, all of metal, with a metal rod 17 projecting axially through thebody and caps. Means are provided for supplying liquid coating material, atomizing air, air for turning the gun on and off, and high voltage to the spray gun. A passageway 20 is designed to be connected to a liquid coating material supply hose; passageway 21 to an atomizing air hose; passageway 22 to a source of compressed air for controlling the operation of the spray gun; and stud 23 is connected to a source of high voltage.
Liquid cap 15 is threadedly connected to gun body 14 and contains an axial passageway 15a. Air cap 16 is mounted concentrically about liquid cap 15 by means of a threaded ring 25. Cap 16 may contain one or more atomizing air holes, the most important being an axially located circular hole designed to surround the forward portion of liquid cap 15 so as to form an annular air hole 16a about liquid passageway 15a. Additional air holes may be provided such as those best shown in FIGS. 2 and 3.
Turning now to FIGS. 2 and 3, in addition to annular hole 16a, cap 16 is provided with four pairs of additional atomizing air holes. Four identical air holes 16b are aligned vertically and the axes of these air holes 16b are generally parallel to that of liquid passageway 15a and annular air hole 16a. The four air holes 16c are located generally horizontally of the axis of the gun and the axis of each air hole 16c is arranged to project a stream of air transversely acrom the major axis of the gun as is best shown in FIG. 3.
Air cap 16 also includes a pair of spray pattern-shaping air horns 27 and 28 which project forwardly beyond cap 16. Air horns 27 and 28 are constructed of a highly insulating material such as nylon and contain one or more air holes which will direct streams of air transversely across the projected axis of gun 10.
Rod 17 performs three functions, namely, control of the supply of both liquid coating material and air to the forward end of gun 10 and also creation of a highly ionized zone forward of the end of the spray gun as will hereinafter be more fully described. Rod 17 is arranged to be movable forth and back along the axis of gun and contains a cylindrical valve 17a which may be arranged to block compressed air passageway 21 and a'conical valve 17b which is designed to seat in a recess in liquid cap to control the flow of liquid into passageway 15a. The forward end of rod 17 is of extremely small diameter and may be a wire 17c of about 0.020 inch diameter or preferably even smaller which, when the rod is'in its rearward position and spray gun 10 is operating, will project approximately V2 inch beyond the forward face of air cap 16.
Rod 17 is moved from its forward (inoperative) position to its rearward (operative) position by a conventional compressed air mechanism. A coil spring 30 compressed against a piston 31 mounted on the rear end of rod 17 urges rod 17 forwardly so that valve 17b is seated in liquid cap 15 and valve 17a blocks air passage way 21. When air under pressure is supplied through passageway 22 to air chamber 32, piston 31 will be retracted against spring 30 and drive rod 17 rearwardly to open liquid passageway 15:: and air passageway 21.
Liquid coating material is normally supplied through passageway 20 under a slight pressure, normally two or three pounds per square inch, and compressed air is supplied through passageway 21 under pressures of about 12 to 15 pounds per square inch and preferably not over 20 pounds per square inch. When rod 17 is retracted the liquid coating material will flow through passageway 15a around wire 170. At the same time, air under pressure will flow through annular air hole 16a and through air holes 16b and 16c to atomize the stream of liquid projecting into the open atmosphere from the end of passageway 15a into a spray of finely divided liquid coating material particles. This spray would, if not further modified, be deposited onto an article in a generally circular pattern which for most commercial spray painting operations is not acceptable. Accordingly, streams of air are projected transversely across the atomized spray from air horns 27 and28, as is best shown in FIG. 1, so as to an the spray outwardly into a long narrow elliptical pattern which is desired for most commercial spray coating operations.
The entire gun 10 is maintained at a high negative potential of the order of 100 kilovolts by virtue of stud 23 being connected to a source of high voltage 34. Gun 10 is arranged facing generally toward an article to be coated and about 12 inches from the article which may be moving past the gun on a conveyor line. The gun is preferably arranged so as to project the spray of liquid coating material directly at the article to be coated; however, in some instances it may be desirable to arrange the gun at an angle to the article or to the line of a series of articles carried past the gun on a conveyor.
The articles to be coated are usually grounded through their conveyor and when the tip of electrode wire 170 is at about 100 kilovolts, a highly ionized zone will be created in the center of the spray which will provide an extremely effective charging and depositing arrangement for the spray particles which are atomized in a zone behind the tip of wire 17c. The particles will be projected by the air stream issuing from the air orifices (air holes 16a, 16b and 16c) generally along the lines of force extending from the tip of wire 17c and terminating on the article to be coated. Since air horns 2.7 and 28 are of insulating material and the balance of gun 10, though of metal, is considerably rearward of the tip of wire 17c and contains no sharp edges or projecting portions, the lines of force from the article will be concentrated on the wire 17c near its tip. If air horns 27 and 28 are sufficiently rounded and located somewhat rearwardly of the wire tip, as one-half inch, the horns may be made of metal without seriously detracting from the high efficiencies of our present invention.
For maximum effect, the tip of Wire 170 should be located forwardly of the zone of atomization of the liquid so that the spray particles will be projected through the highly ionized zone created about the electrode tip. In any event, the tip of wire 170 should projectat least one-eighth of an inch beyond the forward face of the air cap with the atomization phenomenon as herein described. Wire 170 may be extended an inch or even several inches axially forward of gun 10 beyond the face of cap 16 and still retain remarkably high deposition efilciencies. However, radial displacement of the electrode tip away from the axis of the spray results in rapidly decreasing the etiiciency of deposition, and the tip of the electrode should be closer to the axis of the spray pattern than to the end of the liquid orifice or the face of the air cap.
As an example of one arrangement giving a very high deposition efliciency, the gun constructed as shown in FIGS. -1-3 was arranged 12 inches from an article and kilovolts (negative) was applied to the wire tip 17:: from a high voltage pack having an AC. ripple of 5%. 100 cc. per minute of a commonly used industrial baking enamel was fed to passageway 20 and 12 p.s.i. of atom izing air was fed to passageway 21. The wire 17c extended /2 inch beyond the face of air cap 16 and was of 0.010 inch diameter. Under these conditions, the zone of atomization of the liquid was at about an eighth of an inch or slightly less forwardly of the face of air cap =16. The forward end of liquid cap 15 surrounding wire 170 is a thin-walled cylindrical tube having an inside diameter of 0.040 inch and an outside diameter of 0.100 inch and its forward end i flush with the forward face of air cap 16.
Under these conditions the air velocity created near the surface of the article, which was 12 inches from the forward end of the spray gun, was approximately 1,200 feet per minute. We have found that with the charging and depositing arrangement of our invention itis desirable that air velocity at the article surface be not more than about 2,000 feet per minute, and air velocities at the article surface much higher than this result in marked decrease in depositing efliciency.
Voltages near or in excess of 100 kilovolts are desirable in use of the gun of FIGS. 1-3 but present a danger of sparking or arcing in case one of the articles (or some other grounded object) approaches too closely to the charged gun. To lessen this possibility and to permit the highly charged gun to be placed close to the article, a high resistance 36 in the orderof 100 megohms may be inserted in the line from voltage source 34 to stud 23 and preferably immediately adjacent to the stud. While the value of this resistance may vary, depending upon the voltage of gun 10, a resistance of about one megohm for each kilovolt applied to the gun will provide a substantial lessening of the danger of arcing or sparking.
Turning now to FIGS. 4 and 5, there is shown a sprayv gun 40 adapted for manual operation. The gun is connected by supply lines contained within a conduit 41 to a source of liquid coating material 42, a source of atomizing air 43 and a high voltage source 44. So that the gun may be operated without the danger of sparking or arcing which might result in a fire or discomfort to the operator, the gun embodies certain principles set forth in the application of Juvinall and Marsh, Serial No. 572,752, filedMarch 20, 1956. Gun 40 has a maintubular body 45 made of suitable highly insulating material such as nylon which is approxirnately 12 inches long. Body 45 is bored axially to provide passageway 45a for liquid coating material, passageway 45b for atomizing air, and a central passageway adapted for an insulating tube 46 which contains means for transmitting high voltage to the forward end of the gun.
The rear end of tube 46 contains a polyethylene covered wire 48 from high voltage source 44. The forward end of wire 48 is connected to a resistor 49 of high value, say in the order of 450 megohms when the output volt age of high voltage pack 44 is kilovolts.
' ated needle valve -52," and finally through passageway 45a. 'Atomizing' air is supplied to the forward end of the gun from compressed air source 43, a compressed air supply line 54, a valve 56, and finally through passageway 45b.' Liquid valve 52 and air valve 55 are jointly controlled by a trigger 58 which is pivotally mounted adjacent a metal handle 60. Gun 40 is adapted to be held in the hand of an operator by handle 69 'which is grounded as indicated in FIG. and the operator is able to control the supply of liquid and compressed air to the forward end of the gun by operation of trigger 58.
The forward end of gun 40 includes a liquid cap 61 containing an axial liquid passageway 61a, and an air cap 62 having one or more atomizing air holes 62a located closely adjacent to liquid passageway 61a. Caps 61 and 62 are made of highly insulating material such as nylon, although when highly abrasive materials are to be sprayed a cylindrical metal, sapphire, or other liner may be inserted in cap 61 about passageway 61a. Th atomizing air holes in air cap 62 may be arranged substantially as shown in FIGS. -2 and 3 previously described. Air cap 62 also contains a pair of pattern shaping air horns 62b which contain air holes designed to project streams .of air transversely across the axis of the atomized spray so as to shape the spray into the commercially desirable fan pattern.
A slim metal needle electrode 64 projects axially through liquid passageway 61a to a point somewhat forwardly of the furthest projection of air horns 62b. Electrode 64 is mounted in the forward end of gun body 45 and the rear end of the electrode is electrically connected by a spring 65 with the -forward end of high resistor 49 to provide a highly ionized zone adjacent the tip of electrode 64. Due to the very small electrical capacity of electrode 64 and spring 65, together with the current limiting effect of high resistor 49, the presence of a grounded article (or a person) closely adjacent or even touching the highly charged needle -'electrode 64 will not result in any arcing or undesirable electrical discharge which might cause a the or discomfort to a person. The effective electrical capacity of conductive elements, such as the electrode 64 and spring 63, which lie beyond the resistor 49, as measured by their discharge characteristics, is desirably'kept a low as possible, and in the case of a handheld gun, and preferably even in the case of an automatic gun,'should not be greater than that of a metal sphere of about 3 cm. radius and is preferably less than that of a metal sphere of 1 cm. radius.
In FIG. 5, spray gun 40 is shown in use painting a series of metal bicycle frames 66 moving past the operator of the gun on hangers 67 suspended from a conveyor 68, which i grounded. Bicycle frames must be coated with a high quality finish such as is obtainable by spray coating and yet previous spray coating operations with mechanical atomizers on frames have proved extremely wasteful due to their irregular shape and the wide, open spaces within the frame. However, with our invention is is possible to combine the advantages of compressed air in atomizing coating material difficult to atomize by other means, instantaneous starting and stopping of the spray operation and desirable pattern shapes with the high deposition efficiencies of electrostatic coating and still retain a system entirely safe from fires and personnel shock.
In an operation as illustrated in FIG. 5, with .150 kilovolts applied to a resistor 49 of 450 megohrns resistance and with the gun positioned at customary spraying distances (six to twelve inches) from grounded articles to be coated, the voltage drop across resistor 49 will approximate 50 kilovolts, thus causing the potential of the electrode 64 to be about 100 kilovolts. Hence, there will exist in the vicinity of the electrode the same highly efiicient particle-charging condition as described above in connection with the gun 10. V q
. In such an arrangement, the presence of the grounded handle 60 may or may not have any significant effect on deposition efficiency, depending upon the distance between the forward end of the gun and the grounded work. At the shorter spraying distances occasionally occurring in the operation of a hand-held spray guni.e., distances less than six inchesthe grounded handle of the gun 40 above described would have substantially no effect; for with a gun body twelve inches in length, the handle would be considerably further than the work from the charging electrode and the average potential gradient of the field between the charging electrode and the work would be so .much greater than that of the field between the charging electrode and the handle that the former field would effectively determine the atmosphere-ionizing conditions adjacent the charging electrode. However, as the spraying distance increases beyond about six inches the handle 60 begins to exert on deposition efiiciency a measurable effect which increases appreciably as the spraying distance increases. When the spraying distance equals or exceeds the distance between the electrode 64 and the grounded handle, the increase in deposition etficiency attributable to the presence of the handle is very marked.
Using a handle or some other element mounted at a fixed distance from the charging electrode means as a counterelectrode raises the possibility of using a potential source of lower output voltage than those previously referred to; for, by reducing the separation of those electrodes, the same average potential gradient could be obtained with a corresponding reduction in the voltage difference maintained. Further, the reduction in voltage difference facilitates the incorporation of desirable safety features. The gun shown in FIGS. 6-9 is one which takes advantage of those possibilities.
The gun 70 shown in FIGS. 6-9 comprises a body in the form of a tubular barrel 71 of insulating material receiving at its front and rear ends plugs 72 and 73, also of insulating material and conveniently cemented in place. Carried by the front plug 72 are a liquid cap 74 and an air cap 75 having a pair of oppositely disposed air horns 76. Functionally, the liquid cap '74 and air cap 75 are similar to the corresponding elements in the guns previously described, liquid discharged from the tip of the liquid cap being atomized by air delivered through an annular orifice surrounding such tip and, if desired, through adjacent auxiliary openings in the face of the air cap. In their inwardly directed faces, the air horns 76 have forwardly and inwardly inclined orifices 77 for the delivery of spray-shaping jets. Disposed within the cenh'al passage of the liquid cap 74, rearwardly of the captip, is an electrode support 78 formed of insulating material and having a cruciform shape in cross-section, and extending through such support and projecting forwardly through and beyond the liquid orifice is a charging electrode 79.
The front plug 72 is provided with a plurality of axially extending passages and with a central, forwardly opening recess 80 which communicates with the interior of the liquid cap 74. The passages in the plug 72 include a passage 81 through which spray-shaping is delivered to the horns 76, a passage 82 (FIG. 7) for atomizing air, and a passage 83 through which liquid is supplied to the central recess 80. Atomizing air delivered from the passage 82 enters an annular groove in the exterior of liquid cap 74 and flows through oif axis passages 84 therein to an annular space which surrounds the tip of the liquid cap and communicates with the atomizing-air orifices in the face of the air cap 75.
The rear plug 73, which is hollow, projects rearwardly beyond the barrel 71 for attachment of a handle conveniently comprising two similar, complementary parts 85 having portions 86 which surround and are secured to the projecting rear end of the plug 73. Secured within the handle at its lower end is a block 87 extending through which is a first passage for a high-voltage lead 88 and two additional passages adapted to be connected respectively to air and liquid supply lines 89 and 90 (FIG. 9). Also secured within the handle, in rear of the plug 73, is a valve housing 92. The handle members 85 and, conveniently, the block 87 and valve housing 92 are of metal and are connected to ground in a manner to be described below.
Within the barrel 71 and handle 85 there is a sheath 93 of insulating material, conveniently polyethylene, the front end of which is received within a central recess in the rear face of the plug 72. For a distance re-arwardly from its front end, the bore of the sheath 93 is enlarged for the reception of a resistor 94 the front end of which is connected with the rear end of the electrode 79, through a connector 95 that extends through the material of the plug 72 into the recess 80. The connector 95 is desirably of fine wire so that the combined capacity of it and the electrode 79 will be as small as possible and at least no greater than that indicated as a maximum in the above description of the gun 40.
The high voltage lead designated in its entirety by the reference numeral 88 comprises a central conductor 97, a covering 98 of insulation thereon, a braided wire sheath 99 surrounding the covering 98, and, if desired, an outer covering 100. The outer covering 100 of the high voltage lead 88 terminates within the block 87 to expose the braided wire sheath 99, which continues inwardly into a counter-bored recess in the upper face of the block where it receives a tapered expander ring 101 which expands the sheath-end into firm engagement with the block 87, thus both anchoring the lead 88 to the gun and establishing an effective electrical connection between the braided sheath and the block 87, and through the latter, with the handle 85. Grounding of the sheath 99 at the power pack thus grounds the handle. The conductor 97 with its insulating covering 98 continues upwardly through the insulating sheath 93 into association with the rear end of the resistor 94 to which the conductor 97 may be electrically connected in any convenient manner.
etails of the valve housing 92 are illustrated in FIG. 8. As there shown, the housing has an air inlet port 105 and a liquid inlet port 106 communicating respectively through tubes 107 and 108 with the air and liquid passages in the block 87 at the lower end of the handle. In its front face (FIGS. 6 and 9), the valve housing 92 has an atomizing-air outlet port 110 connected through a tube 111 with the atomiz-ing-air passage 82 in the front plug 72, a shaping-air outlet port 112 connected through a tube 113 to the passage 81 in the front plug, and a liquid outlet port 114 connected through a tube 115 to the liquid passage 83 of the front plug. The tubes 111, 113 and 115 are all of insulating material. The shaping-air outlet port 112 is connected to the atomizing-air outlet port 110 through a passage 112' (FIG. 9) which is formed in the valve housing 92 and which includes a valve 116 regulating the supply of spray-shaping air to the outlet port 112. Communication of the air inlet port with the air outlet ports 110 and 112 is controlled by a valve 117, while communication of the liquid inlet port 106 with the liquid outlet port 114 is controlled by a valve 118. The valves 117 and 118 have stems 120 that extend through legs 121 which are formed integrally with the housing 92 and project forwardly and downwardly on opposite sides of the insulating sheath 93 within the handle 85. At their forward ends the stems 120 have heads 122 guided for sliding movement within the legs 121 and acted upon by compression springs 123 which bias the valves 117 and 118 toward closed position.
Pivotally mounted within the angle between the barrel 71 and the grip portion of the handle 85 is a trigger 125 carrying adjustable screws I26 adapted to engage the heads 122 as the trigger is moved toward the grip portion of the handle, whereby to open the valves 117 and 118. An adjustable stop screw 127 is carried by the trigger 125 in position to engage the grip portion of the handle 85 and limit opening movement of the valves 117 and 118. The screws 126 permit control of the sequence in which the valves 117 and 118 are opened when the trigger 125 is actuated.
A gun which We have constructed in accordance with FIGS. 6-9 had a barrel of such a length that the axial distance between the front end of the handle portion 86 and the face of the air cap 75 was about 5% inches. To provide a satisfactory spraycharging zone adjacent the electrode 79 of such gun, we have employed a power pack having a rated voltage (negative) of 60 kilovolts. To render the gun safe, we have used a resistor 94 of megohms resistance and have incorporated an additional resistance of 100 megohms in the output circuit of the power pack.
When a gun as just described is so positioned that the distance between the tip of the electrode 78 and the grounded handle is no greater than that between the electrode tip and some other grounded object, the electrode tip will have a negative potential of about 50 kilovolts and the field between the electrode 79 and the handle 85-86 will have an average potential gradient in air of somewhat over eight kilovolts per inch, which is adequate to maintain a zone of the desired high atmospheric-ion concentration adjacent the electrode 79.
Because of the resistor 94 (together with the supplemental resistance in the power pack) and the low electrical capacitance (about that of a metal sphere of 0.2 cm. radius) of the conductive elements located forwardly of the resistor, the gun of FIGS. 6-9 possesses fully satisfactory safety features. passage in the gun 70 includes a portion at the rear of the gun electrically connected to the grounded handle. This feature makes it possible to spray with safety paints or other liquids so electrically conductive that, in the absence of the electrical connection between the liquid column and the grounded handle, high voltage would feed back through the paint line to the liquid source and perhaps build up therein a dangerous store of electrical energy. At the same time, the connection of the liquid column to the grounded handle provides from the elec trode 78 to ground a leakage path which, depending on the conductivity of the liquid, -might lower the voltage of the electrode. For this reason, we prefer to employ paints of such resistivity that the resistance of the paint column between the charging electrode and the point at which it is grounded will be great enough to prevent the potential of the charging electrode from dropping below a value which will provide an average potential gradient of at least five, and preferably nearer ten, kilovolts per inch in the field extending from the charging electrode. It may be noted here that, while high conductivity of the liquid may lower deposition etiiciency it does not otherwise atfect the spray and does not lower the safety factor.
Compared with the gun 40, the shorter distance between the charging electrode and counter-electrode in the gun 70 makes it possible to maintain an adequately effective charging zone at the electrode 79 with a lower applied voltage; and the possibility of employing a lower applied voltage in turn makes possible the use of a smaller and less expensive power pack and a lighter and more flexible high voltage lead 88. The latter feature is of importance in a hand-held gun, since a heavy, relatively stiff lead such as required by higher voltage would handicap the operator in moving the gun.
It might be though that reduction of the electrode spacing beyond that existing in the gun 70 just described would make possible the use of a power pack of still lower output voltage, and such is indeed the case. However, we have found that if the gun is to remain safe against sparking and shocks to personnel, the spacing and applied voltage cannot be reduced much beyond those As in the gun 40, the liquid provided in the gun 70 of H68. 69 without causing a decrease in deposition efficiency. The reason for this appears tobe the relation between such factors as the resistance which the resistor must possess to render the gun safe and effect on total field current of changes in the electrode spacing. If the applied voltage and the spacing between the charging and counter-electrodes are both progressively reduced while maintaining the resistance of the resistor at the minimum consistent with the desired degree of safety, a condition is reached where the deposition efficiency decreases. Improvement in the deposition efliciency could be effected by lowering the resistance or raising the applied voltage; but either of those expedients would lower the safety characteristics.
Of course, if some sacrifice in respect to safety characteristics or deposition efficiency is permissible, the electrode spacing can be reduced considerably below that provided in the gun 70 as above described. For a gun intended for manual manipulation, we regard it advisable to maintain a high safety factor; and that can be done at relatively low electrode spacings and voltages with deposition efiiciencics which, while below those provided by the specific gun 70, are still well above those obtainable with non-electrostatic spray guns. For example, a gun generally similar to the gun 70 but constructed to space the front of the handle or other counter-electrode only one and onehalf inches from the face of the air cap will possess satisfactory safety characteristics and give fair deposition efficiency if provided with a resistor 94 of about 40 megohms resistance to which a potential of kilovolts is applied through an additional resistance of about megohrns in the voltage pack.
In all the guns shown and specifically described above, the charging electrode means is a single fine wire projecting forwardly through and beyond the liquid orifice of the gun. While that form and disposition give the best results and are preferred, they are not essential. As for form, the base of the electrode is less important than the outer portion, and the base of the electrode may have a diameter greater than any mentioned above if the remainder is adequately attenuated, as by tapering. With respect to disposition, the charging electrode might be disposed at one side of the liquid orifice; but, as its lateral spacing from the emerging liquid increases, its ability to charge the liquid particles and to promote high deposition efficiency falls off rapidly. Hence, we would not ordinarily employ an offset electrode disposed outside the spray path unless it was located not more than about one-half inch from the axis of the liquid orifice, and would prefer that such an electrode be spaced no more than one-quarter inch from such axis, since the proportion of atmospheric particles charged by the electrode and projected into the spray is reduced as the distance of the electrode from the spray increases. As previously noted, we prefer that if the electrode is offset its tip be closer to the axis of the liquid orifice than to the plane of the orifice. The effectiveness of an offset electrode in charging the spray particles can be increased to some extent by extending the electrode forwardly into the spray. We have obtained fair deposition efficiencies with a wire of 0.010 inch diameter extending forwardly six inches from the paint orifice, offset three-quarters of an inch from the axis, and maintained at a potential of 90 kilovolts, but the deposition efficiency may be significantly improved by reducing the offsetting of the electrode to one-quarter inch. We prefer that the charging effect be produced by only a single charging electrode as any additional electrode detracts from the charging effect desired by our invention. Under most commercial conditions not only a single axial electrode but even a single electrode radiallyoffset a little from the axis of the spray produces better charging effects than a pair of electrodes spaced the same distance from the axis. For example, a single electrode protruding /2" from the face of the air cap, offset /z" from the liquid axis and maintained at 100 kilovolts gave a deposition efiiciency such that essentially comparable deposition efficiency would be obtained with a pair of oppositely located electrodes protruding /6" from the air cap only when they were kept within from the axis of the liquid orifice and energized to a level f kilovolts. We believe that any additional electrode or electrodes exercise a mutual shielding effect which tends to lower the field intensity between them, and which exerts a repellant effect on the atmospheric ions generated by each. In any event, the use of additional electrodes clearly reduces the ion concentration in the ionized zone which is desired by this invention. In some circumstances more than one electrode can be used with satisfactory results, but only under restricted conditions.
A pair of short electrodes, as electrodes projecting /2" forwardly of the air cap, will give improved charging over prior commercial air gun systems only if they are spaced not more than about A" from each other. Even very long electrodes do not obviate this reduction in charging efiiciencies by a second electrode, as electrodes which project 6" forwardly of the face of the cap in the spray will give such improved charging only if they are no more than 1" from each other. Increasing the number of electrodes materially further detracts from the desired charging effect as even three electrodes spaced symmetrically about the orifice and a half inch therefrom fail to attain the high charging effect that is obtained by this invention, and an annular corona generating electrode surrounding the liquid orifice will not produce the high charging obtained by the preferred form of this invention. What has just been set forth relative to the length and disposition of charging electrodes applies with substantially equal force to guns, like those of .FIGS. 4-9 which are provided with counter-electrodes and to guns, like that of FIG. 1, which do not have counter-electrodes.
It is, of course, important thatthe tip of the electrode be located in a region which is not unduly shielded electrically. For this reason, if the gun has a metal air cap, the electrode should extend forwardly beyond the air horns of such cap.
The diameter of the electrode is of importance in respect to its capacity to generate charged atmospheric particles, smaller diameter wires being more effective than large-diameter wires in this respect. In the case of a gun used in a painting operation and having an electrode located in or extending into the spray, the diameter is important from another aspect. Such an electrode will tend to collect paint, which will be removed from it both by electrostatic action and'by the wiping effect of the air stream in which the paint particles are suspended, and the larger the diameter of the electrode the greater will be the size of the paint particles so removed from it. The presence of large particles in a paint spray can adversely affect the quality of the finish produced, and for finishes of the better quality required in industry it is desirable that the spray produce a maximum spot size of no more than 0.015 inch. Maximum spot size, as used herein, is defined as the average diameter of the ten largest spots measured on a 4 x 6' flat grounded target passed sufficiently rapidly through the spray zone in a plane perpendicular to the axis of the atomizer and 12" from its forwardmost portion so that substantially all of the spray particles deposited form non-overlapping spots.
The effect of electrodes in the spray on the quality of atomization produced is not dependent solely on their diameter but is also influenced by other factors such as the extent to which they project into the spray and the effect of air jets to which they are exposed. Thus, a wire electrode of 0.010 inch diameter projecting through and one-quarter inch beyond the liquid orifice has little or no effect on the quality of atomization even at relatively low atomizing-air pressures inthe neighborhood of 12 p.s.i.; but if it projects much farther, or into the region where the jet or jets of atomizing air have spread appreciably and lost velocity, deterioration of atomization can be expected. The tendency of any electrode projecting through the liquid orifice to degrade the quality of atomization can in large measure be ofiset by increasing the pressure of the atomizing air and hence the velocity of such air over the electrode surface; but the necessity for increasing the atomizing air pressure is to some extent a disadvantage. With liquids readily susceptible to electrostatic atomization, electrodes several inches in length and up to about 0.050 inch in diameter exhibit less tendency to degrade atomization than do electrodes of the same diameter and only an inch or so in length. Unless the electrode or its support is inordinately large, any deleterious effect on atomization of an electrode projecting well into the spray can be effectively eliminated by directing one or more cross jets of air on the electrode. In the guns shown in the drawing the spray-shaping air jets discharged from the air horns can serve this purpose, and their function in this respect is regarded as a feature of our invention.
In view of the above considerations, we prefer to use electrodes of no more than about 0.010 inch in diameter, although electrodes up to 0.050 inch in diameter may be used if the quality of finish is not critical or if the electrode is exposed to air jets of adequate velocity.
While we have referred above to the air jets delivered from the orifices of the air horns as functioning to shape the spray and to discourage the accumulation of paint on the charging electrode, these air jets have another important effect which is especially noticeable where, as we prefer, the air from the orifice or orifices in the face of the air cap is delivered under relatively low pressure, say in the neighborhood of 12 psi. Under such circum stances, the air jets from the air horns have an appreciable effect in reducing the size of the larger particles in the atomized spray. Moreover, since the jets from the air horns are so directed that the major component of velocity is perpendicular to the spray axis they have very little effect in increasing the momentum of the air which is projected from the gun toward the work and which tends to reduce deposition efficiency by carrying spray particles beyond the work where they would be lost as overspray. In practicing our invention, we have noticed that a substantial improvement in deposition elficiency results from maintaining the pressure of air supplied to the orifices in the face of the air cap low enough that air jets from the air horns contribute substantially to the quality of atomization as measured by the size of the largest spots produced by the spray on a flat surface. References to air pressure herein apply to pressures as measured immediately behind the orifice.
What has been said above regarding the tendency of charging electrodes located within the spray to increase the size of the larger particles in the spray applies with equal, if not greater, force to counter-electrodes. Of course, the counter-electrodes in a spray painting system are not usually disposed directly in the path of the spray; but the prior art counter-electrodes of which we are aware have nevertheless been located close enough to the spray to collect spray particles. It may be noted in this connection that counter-electrodes always possess a spray-attracting potential. Paint accumulating on a counter-electrode is attracted back toward the spray and leaves the counterelectrode in the form of large particles which become incorporated in the spray and are deposited with the normal spray particles on the work, where they degrade the finish. We employ the term spray-traversed region herein to denote the region in and adjacent the spray which should be kept free of counter-electrodes or other objects possessing a particle-attracting potential. Such region includes not only the path actually occupied by the spray when not subjected to lateral electrostatic deflecting forces but also the surrounding space within which a counter-electrode or other grounded object would become coated as a result of its attraction for the charged spray particles.
We have found that the disposition of the counterelectrode well beyond any position in which it would receive spray particles is not incompatible with effective spray-charging. Preferably, the counter elcctrode is located well rearwardly of the forward end of the gun; but with the voltages and atomizing-air pressures we prefer to use, it may be located approximately in the plane of such gun-end if spaced far enough, say two inches or more, from the spray-axis. Increasing the voltage increases the tendency of the counter-electrode to become coated, while increasing the pressure of the atomizing air has the opposite effect.
As previously indicated, we have obtained somewhat higher deposition efficiencies with guns having counterelectrodes than with guns which do not have counterelectrodes. We believe this difference to be due to the relation between the paths respectively followed by the spray particles and by the charged atmospheric particles. The latter particles, because of their relatively low mass, are much more susceptible to electrostatic forces than are the paint particles. Accordingly, while the initial portions of the paths followed by the paint particles are almost entirely determined by the blast of atomizing air, the paths of the charged atmospheric particles are substantially influenced by the field. The charge on the paint particles is built up by collisions with charged atmospheric particles, and such collisions are believed to be more frequent if the two types of particles are following intersecting paths than if they are following generally parallel paths, especially since any paint particle possessing even an incipient charge tends to repel the similarly charged atmospheric particles. Where an effective field from the charging electrode extends to a counter-electrode located well away from the spray, charged atmospheric particles within the spray and attracted toward the countor-electrode follow paths extending transversely, and even perhaps partially rearwardly, of the spray, thus promoting the frequency of the collisions by which the charges on spray particles are built up. This may explain the fact, previously noted, that a grounded counter-electrode can provide a measurable increase in deposition efiiciency even though its distance from the charging electrode is greater than that of the grounded work.
While highly efiicient charging of the spray particles can be produced by a field between a charging electrode and a counter-electrode, and while such efiicient particlecharging in itself will result in high deposition efficiencies, we still regard it as of some importance that a substantial field exist between the charging electrode and the work. In any given arrangement, at least a rough measure of the effective strength of the field extending from the charging electrode to the Work is provided by the magnitude of the field current flowing to the work when the gun is not being supplied with liquid or compressed air. We prefer arrangements in which, under such conditions, the field current flowing to the work is at least about one microampere and most desirably at least five microamperes. This factor has an influence in determining the minimum practical spacing between the charging and counterelectrodes.
In order that our invention may attain improved deposition etficiencies over prior commercial air gun electrostatic systems, we find it necessary to charge the spray particles to at least a certain minimum ratio of chargeto-mass. Additional substantial advantages accrue from charging to a certain higher ratio. One method of de termining the ratio of charge-to-mass is by depositing charged particles on a suitable target and measuring the current carried to the target by the particles and measuring the weight of particles deposited on the target per unit of time. For such a test to give dependable results, it is necessary to take into account any current borne by atmospheric-ions discharging on the target. It is also necessary to standardize the rate of paint delivery from the gun, the air velocity, and the paint atomized.
We have found that the following test procedure gives acceptably accurate results as a measure of the relative effectiveness of various spray-charging arrangements:
We employ a grounded-target in the form of a fiat sheet, conveniently of aluminum foil, about inches long and 20 inches high, arranged in a vertical plane. The spray gun is disposed at a distance of 24 inches from such target and directed to discharge perpendicularly against the center thereof. If the spray pattern is not circular, the major axis should be horizontal. Midway between the target and the gun and centered with respect to both, we dispose a grounded grid in the form of 24 vertical metal rods 1 inch. in diameter and 42 inches in length, disposed on 3 inch centers and in a plane parallel to the target. First and second microammeters are inserted respectively in the ground leads of the grid and target. The air discharge from the gun is adjusted to provide a maximum velocity of 2,400 feet per minute at a distance of 12 inches from the gun. Almost all of the ionized air particles will be attracted to the grid and discharged thereon, but a small portion of the atmospheric ions will passthrough the grid to be discharged on the target.
NVith the above described apparatus set up in still air, and with the spray charging-means energized to operating voltage, paint is delivered to the gun at 100 cc. per minute, and the readings of both rnicroammeters recorded. The paint is then turned off, which generally results in an increase in the reading of the first microammeter. When such an increase occurs the voltage is reduced until the reading of the first microammeter attains its original value, and the reading of the second microammeter is noted. The difference between the latter reading and the original reading of the second microammeter satisfactorily represents the current carried to the target by the paint particles alone under .test conditions. If the reading of the first microammeter does not increase when the paint is turned oil, the current carried by the paint is obtained by subtracting the current through the second microammeter, at operating voltage, without paint, from the reading of that meter with paint.
The ratio (R) of electrical charge to the weight of the atomized test paint, which can conveniently be expressed in microcoulombs per gram, is determined by the formula:
I represents the current in microamperes carried by the charged paint to the target, as determined in the preceding paragraph;
W represents the weight of the target in grams prior to test;
W represents the weight of the target in grams after exposing the target, under test conditions, to the paint in which for an accurately determined time interval of approxi-,
mately 10 seconds, and then baking the painted target for 20 minutes at 300" F.;
t represents the exact time in seconds during which the target is exposed to the spray; and
S is the fraction representing by weight the solids content of the test paint.
l 5 a solution made by diluting the alkyd to 50% non-volatile content with mineral spirits shall be in the range Z; to Z on the Gardner-Holdt scale at 77 F.
Pigment and resin shall be ground to a fineness of 7 NS on Hegman gage. After grinding, the paint is re duced with mineral spirits to a viscosity of 16 seconds as measured in a Ford #4 Cup at 77 F.
The mineral spirits referred to hereinabove shall have an aromatic content of 18%, a KB value of 38,,distillation range of 314 to 385 F., and fiash'point of F. as tested in the Tag Closed Cup.
As measured by the above test procedure, the charging system of our invention is able to obtain much higher cha=rge-to-mass ratios than those heretofore obtained by commercial air gun electrostatic systems. We have determined that to achieve such higher deposition efficiencies, the charge-to-mass ratio must be at least microcoulomb per gram; and further substantial advantages are attained by utilizing charging means which provides a eharge-to-mass ratio of at least three microcoulombs per gram. The guns of FIGS. l-9 as above described provide a charge-to-mass ratio of three rnicrocoulombs per gram or even higher, while the modified manual gun referred to above as employing a 20 kilovolt applied voltage is capable of providing a ratio of microcoulomb per gram. The latter gun, while somewhat less cfficient than the others, has the advantage that the cost and weight of its power pack are each but a small fraction of that of the other guns. When we hereinafter refer to a charging means which is capable of charging a spray to at least a certain number of microcoulombs per gram, we are referring to the operation of that means under the above test procedure.
As has been indicated above, the de ree to which the spray gun is to be made safe against the danger of sparking and shocks to personnel may influence the design of the gun and the voltages used. In a mechanically supported, remotely controlled gun, like that of FIG. 1,
protection against shocks to personnel may not be important, and it is for that reason that it may be permissible to construct the entire gun of metal and thus increase its electrical capacitanw to a value which, upon discharge of the stored energy, could cause a disagreeable shock. It is desirable, however, to protect the gun of FIG. 1 against the possibility that a spark might be drawn from the charging electrode 17:: to an article whose distance from such electrode might be reduced b yond normal spraying distance because of swinging movement of the article or its improper location on the conveyor. With a gun like that of FIG. 1, where the only effective spray-charging field is that between the electrode 17c and the work, optimum deposition efiiciency is obtained when the voltage of the gun is maintained at the maximum consistent with freedom from sparking. At a spraying distance of 12 inches, the gun might be maintained at a potential as high as kilovolts. Under such conditions, the field current would be about 0.25 milliampere; and if the resistanoe 36 had a value of 200 megohms, it would be necessary to apply to it a voltage of 200 kilovolts in order to maintain the gun at the stated 150 kilovolts. If the spraying distance was reduced to six inches, the field curmnt would be increased to around 0.55 milliampere, and the gun potential lowered to about 90 kilovolts, to create an average potential gradient of about 15 kilovolts per inch, which is substantially below the gradient that would involve any danger of sparking.
In a hand-held gun, the use of voltages which would provide potential gradients as high as those just mentioned would complicate the problem of incorporating safety characteristics required where the gun is to be held in the hand. Further, where an effective particlecharging field exists between the charging electrode and a counter-electrode, as is the case with the hand-held guns herein described, we find as a general rule that increasing the average potential gradient between the chargring and counter-electrodes beyond about eight kilovolts per inch provides comparatively little increase in deposition etiiciency and may, in some cases, even reduce deposition efiiciency somewhat.
In referring herein to paint, we intended that term to include lacquers, enamels, porcelain frits, and in general any type of settable liquid material which, after application to an article surface and as a result of either its own inherent characteristics or of subsequent processing, will set to form a dry film on the surface.
The term electrostatic field is used herein to apply to a field which in normal operation does not are or spark.
In the commercial electrostatic coating of many nonconductive articles, it is common to provide within or behind the article a backing electrode maintained at the same potential (ground, in practice) which would be applied to the article if it were conductive. As far as it affects the field, field current, and depositing etficienciss, such an arrangement is the equivalent of one in which the article is conductive. References herein to the article as forming a part of the cooperating electrode means and to the intensity of the field between the article and the charging electrode are therefore to be taken as applicable irrespective of whether the article is a conductive article or a non-conductive article provided with a backing electrode.
Earlier herein we have referred to the possibility that electrostatic atomization may occur from charging electrodes disposed within the spray. That reference is not to be taken as indicating that our invention relies on electrostatic forces to effect or significantly promote atomization. Rather, the rate at which we mechanically atomize by using the interaction between the paint and air-a rate of at least cc. per minute (and perhaps as high as 500 cc. per minute or more)- is many times the rate at which paint could be electrostatically atomized to a comparable degree of fineness from our charging electrode means as embodied in our invention.
The advantage in the way of overspray reduction provided by our invention is greatest in the painting of articles which present toward the gun surfaces whose aggregate area is small relative to the area of the spray pattern. For example, in painting a row of 1 inch diameter metal broom handles spaced on 3 inch centers with an air spray gun directed perpendicularly to the plane occupied by the rods, the most efficient previously commercially used particle charging spray painting arrangement utilizing air guns of which we are aware produces about four to six times the overspray produced with a gun and spray painting system embodying a preferred form of our invention.
It is to be understood that the specific spray guns illustrated and above described are set forth merely by way of example and that our invention is not limited to them. We note particularly that our invention is not limited to spray guns in which the liquid is atomized with the use of compressed air; for our invention includes features which can be used advantageously in hydrostatic spray guns in which liquid is ejected under high pressure through a small orifice and is atomized by reaction with the atmosphere.
1. The method of electrostatically spray coating a grounded article surface portion of substantial area which comprises creating an electrostatic depositing field having said surface portion as one terminus with the lines of force distributed thereover, the depositing field at its other terminus having substantially all of the lines of force from said surface portion concentrated at a single sharp electrode tip facing toward the article, said electrode being negative with respect to said article and the electrostatic field at said electrode tip having an average potential gradient at least of the order of 5,000 volts per inch providing a highly ionized zone immediately adjacent said 18 electrode tip, atomizing coating material issued as a solid liquid stream from a small orifice by interaction with air into a spray of fine coating material particles projected toward the article from a location more remote from the article than said electrode tip and with the axis of the spray pattern near said tip, and shaping the spray pattern by air issuing from another orifice in a member so constructed and located relative to the electrode tip as not to detract from the high field concentration thereat, the field creating connection to said electrode being cornplcted through a resistor of at least about one megohm per kilovolt of a high potential source, the zone of atomization being so located relative to the electrode tip that the particles are projected in paths generally coin ciding with those of the lines of force extending from said electrode tip to said surface portion, said spray particles having velocities not in excess of about 3,000 feet per minute adjacent said surface, the distance of the article from the electrode tip being sutiicient to permit dispersion of the spray particles and the average potential gradient of the charging and depositing field extending from said electrode tip to the article surface portion being materially greater than the average potential gradient of any other electrostatic field extending to said surface portion from any other point within said depositing field.
2. In a method of electrostatically painting an article wherein paint is atomized by interaction with air at an atomizing zone adjacent an atmosphere-ionizing electrode and is formed into a spray, the steps of positioning a counter-electrode in spaced relation to said first men tioned electrode and far enough from the spray that it does not receive any appreciable number of spray particles, maintaining between said electrodes an electrical potential difference sufficient to be capable of charging a spray to a charge of at least 2 microcoulomb per gram, and maintaining the article within the spray and at such a position and electrical potential relative to said atmosphereionizing electrode that, in the absence of the sprayed paint, the field current flowing between the article and such electrode would be at least 1 microampcre.
3. In an electrostatic spray coating system wherein articles are coated by having spray particles electrostatically attracted toward and deposited thereon while still in liquid state, means for atomizing and charging liquid coating material, comprising: an atomizing device composed substantially entirely of insulating material for atomizing liquid coating material into line spray particles by interaction with air into an elongated spray pattern; and a particle charging portion comprising slim elongated electrode means connected to a source of high voltage and having a tip located close to the axis of the spray pattern to provide forwardly of the zone of atomization and near such axis a single highly ionized zone through which the atomized spray particles pass, said tip being suificiently forward of and widely spaced from all other conductive elements near it to constitute a concentrated terminus of substantially all the lines of electrostatic force emanating from the article surface portion being coated.
4. In an electrostatic spray coating system wherein grounded articles are coated by having spray particles electrostatically attracted toward and deposited thereon while still in liquid state, means for atcrnizing and charging liquid coating material, "comprising: an atomizing' dcvice having a small orifice adapted to have liquid coating material issued therefrom as a solid liquid stream, an adjacent orifice adapted to have air issued therefrom toward said liquid stream to atomize it into fine spray particles, and forwardly projecting means of insulating material having an orifice adapted to have air issued therefrom to affect the shape of the spray pattern; and a particle charging portion comprising the tip of a slim elongated electrode means connected to a source of high voltage and providing a single concentrated highly ionized zone forwardly of the first mentioned orifice near the axis of the spray pattern, said tip being sulficiently forward of and widely spaced from all other conductive elements to constitute a concentrated terminus of substantially all the lines of electrostatic force emanating from the article surface portion being coated, the tip of said electrode means being spaced from any grounded object by a distance equal to at least several times the distance between said tip and said orifice.
5. In an electrostatic spray coating system wherein articles are coated by having spray particles electrostatieally attracted toward and deposited thereon while still in liquid state, means for atomizing and charging said particles, comprising: an atomizing device having a small liquid orifice, an atomizing air orifice, and a pattern shaping air orifice; means for supplying liquid coating material to said liquid orifice; means for supplying air to said air orifices at a pressure insufiicient to create air velocities at the article surface in excess of about 2,000 feet per minute, but sufficient to atomize the liquid stream into a spray of fine coating material particles and to shape said spray into an elongated pattern; slim elongated electrode means having sharp end means facing toward the article; and a source of high potential having its negative terminal connected to said electrode and its positive terminal connected to the article being coated, said high potential means applying to said electrode a potential sufficient to establish a field between said end means and the article with an average potential gradient at least of the order of about 5,000 volts per inch, the average potential gradient of the charging and depositing field extending from said electrode end means to the article =being materially greater than the average potential gradient of any other electrostatic field extending to said surface portion from any other point within said depositing field, said electrode end means being sufiiciently forward of and widely spaced from all other conductive elements near it to provide a concentrated terminus for substan tially all the lines of electrostatic force emanating from the article surface portion being coated, being located farther from said liquid orifice than the zone of atomization and closer to the spray axis than to the liquid orifice, and providing adjacent itself a single concentrated highly ionized zone through which the spray passes.
6. A compressed air spray gun for the electrostatic spray coating of an article, comprising means forming a conduit for liquid coating material, means forming a conduit for compressed air, the terminal portions thereof being in juxtaposition, means forming an electrode means for connecting said electrode to a multi-kilovolt source, the gun portions in the vicinity of the terminal ends of said conduits being formed of non-conductive material except for said electrode, said air and liquid conduits being disposed and arranged to cause the compressed air to atomize the liquid at the terminal end of said liquid conduit and direct the atomized liquid and air as a spray, said electrode means having an ionizing portion located adjacent the discharge ends of said conduits for providing forwardly of said discharge ends a single highly ionized zone through which said spray is projected before it expands appreciably, whereby to cause prompt charging of the atomized liquid particles in the spray and facilitate their movement onto the article.
7. A compressed air spray gun as defined in claim 6 wherein said electrode includes an elongated wire member of less than 0.05 inch in diameter disposed within the terminal end of the liquid conduit and projecting forwardly therefrom.
8. An atomizer for forming and projecting a spray of finely divided electrically charged particles of paint, comprising a spray gun body having at its front end a liquid orifice for the emission of paint to be atomized and projected as a spray, at least the front portion of said body being substantially entirely of insulating material, means for supplying paint to the liquid orifice, a thin elongated electrode supported from said body, and means including a multi-megohm resistor for connecting said electrode to a multi-kilovolt source of high voltage, said elongated electrode projecting forwardly from said bodyand terminating in an ionizing portion providing a single highly ionized atmospheric zone through which said spray is projected to highly charge the spray particles.
9. In an electrostatic spray coating system wherein grounded articles are coated by having paint spray particles electrostatically attracted toward and deposited thereon while still in liquid state, means for atomizing and charging said particles comprising: an atomizing device having a small liquid orifice; means for supplying paint to said liquid orifice and projecting it therefrom into the open as a solid stream and for mechanically atomizing the stream by interaction with air in an atomizing zone external of the device into a spray of fine particles projected toward the article; a thin elongated charging electrode member having one end facing toward the article, said end being located farther from said liquid orifice than the zone of atomization and closer to the longitudinal axis of said spray pattern than to the liquid orifice, at least said end of said electrode being Within the pattern of the spray; a source of high potential having one terminal connected to said charging electrode; and means connected to the other terminal of said high potential source for creating an electrostatic depositing field to the article and a highly ionized zone adjacent said electrode end, said means including said article and a counter-electrode spaced appreciably from the atomizing zone, the potential being capable of providing an electrostatic field between the charging electrode and the counter-electrode which has an average gradient of at least in excess of 5,000 volts per inch to provide sufiicient ionization in said highly ionized zone to be capable of charging a spray to microcoulomb per gram, the spray-traversed region being free of any particle-collecting element.
10. In an electrostatic spray coating system wherein grounded articles are coated by having paint spray particles electrostatically attracted toward and deposited thereon While still in liquid state, means for atomizing and charging said particles comprising: an atomizing device having orifice forming means of insulating material for mechanically atomizing the paint emitted from the orifice into an expanding spray of fine particles projected toward the article; a source of high potential having one terminal connected to a paint stream flowing to and through said orifice; means connected to the other terminal of said high potential source for creating an electrostatic depositing field to the article and a single highly ionized zone Within the spray and in a region thereof where the spray has not expanded appreciably, said means including said article as a cooperating electrode, the potential being sufiicient to provide an electrostatic field extending to the article which has an average gradient in excess of 5,000 volts per inch to provide sufficient ionization in said highly ionized zone to highly charge the liquid particles, the spray-traversed region being free of any particle-collecting element.
11. In a spray gun for use in an electrostatic spraypainting system, a barrel of insulating material having a chamber adjacent its front end, a resistor disposed in said barrel and having a terminal spaced from said chamber, said gun having a paint-emitting orifice located in front of said chamber and communicating therewith, electroconductive means connected at its rear end to said resistor terminal, extending into said chamber through a wall thereof and projecting from the front end of said barrel, the front end of said means constituting a spray-charging electrode, said chamber having a paint inlet port offset laterally from said means, and means for connecting a second terminal of said resistor to a high-voltage source.
12. An electrostatic spray-painting device, comprising a spray-gun body having a front portion formed substantially entirely of insulating material, means at the front end of said body for atomizing liquid coating material by interaction with air and projecting the atomized material forwardly as a spray, a spray-charging discharge electrode carried by said body and having a tip located forwardly of said means, and a source of high voltage having one terminal connected to ground and the other terminal connected to said electrode for creating therefrom an electrostatic field to create a highly ionized atmospheric zone surrounding said electrode tip, said means being constructed and arranged so as to direct said spray through said zone before it expands appreciably and so that substantially all the spray particles passing the electrode tip do so at a point which is closer to the tip than to any object of particle-attracting potential.
13. In a spray gun for use in an electrostatic spraypainting system, a barrel of insulating material having at its front end a paint-emitting orifice and a chamber communicating with said orifice and in line therewith, a resistor carried by said gun and located outside said chamber, electroconductive means connected at its rear end to a terminal of said resistor, extending into said chamber References Cited by the Examiner UNITED STATES PATENTS 2,302,289 11/42 Bramston-Cook 118-629 X 2,636,471 4/53 Starkey 117-93 X 2,710,773 6/55 Sedlacsik.
2,766,064 10/56 Schweitzer 118-629 X 2,826,451 3 5 8 Sedlacsik.
2,890,388 6/59 Croskey et al.
3,051,394 8/62 Sedl-acsik 239-15 X RICHARD D. NEVIUS, Primary Examiner.