US 3296015 A
Description (OCR text may contain errors)
1967 J. W.JUVINALL ETAL 3,
METHOD AND APPARATUS FOR ELECTROSTATIC DEPOSITION 0F COATING MATERIALS Filed Jan. 6, 1964 2 Sheets-Sheet 1 m B m m 2% N 3 JAMEs W. JWNALL E'RHARD Kou-k 3* JOHN F MQQASLN ATTORNEYS 1957 .1. W.JUVINALL ETAL 3,296,015
METHOD AND APPARATUS FOR ELECTROSTATIC DEPOSITION OF COATING MATERIALS I Filed Jan. 6, 1964 2 Sheets-Sheet z I CURVE-A y W Z I g u; 400 CURVE-B l.o I
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RHARD K004 F. MQCAsUN A TORNEYS United States Patent Ofifice 3,296,615 Patented Jan. 3, 1967 Ohio Filed Jan. 6, 1964, Ser. No. 335,941 7 Claims. (Cl. 11793.44)
This invention relates to a method and apparatus for the deposition of coating materials under the influence of an electrostatic field, and has for its primary object to provide a method and apparatus in which the coating material is initially mechanically dispersed or atomized in such a manner that a prolonged residence time in an intense electrostatic field may be obtained. As a result, the dispersed or atomized coating material particles bloom out into a continuous pattern of greatly increased dimensions having good distribution from edge to edge. It is also found that, due to the prolonged exposure to the ion bombardment in the electrostatic field, the electrostatic charge on the particles is substantially increased, leading to a higher deposition efiiciency than would be otherwise expected.
In the normal operation of previously known air atomizing spray guns for liquid coating materials, the path of the spray particles between the gun and the work is very largely determined by the characteristics and path of the airstreams that are used to atomize and shape the fluid stream. Most conventional systems use central annular jets to cause the atomization of a liquid stream into a mist of finely divided particles, and use air horns or side jets to shape the spray to an elliptical cross section. Other conventional systems eliminate the air horns and utilize a so-called conical spray having a circular cross section that results from the action of a central annular jet of air on the stream of coating material that it surrounds. When an electrostatic field is added to either of these conventional systems (as by charging the gun to a high potential) the pattern of the spray remains substantially unchanged. If the spray is initially fanshaped it will remain so, while if it is initially conical this configuration will persist and the spray particles will remain largely within the confines of the pattern established or delineated by the airstream.
It has heretofore been proposed by W. H. Bennett and R. B. Taylor to impart an electrostatic charge to liquid spray particles by passing them through an intense electrostatic field established by a single highly char ed needle coaxially arranged with respect to the liquid orifice. (See US. Patent 2,491,889, Bennett et al. and abandoned application Serial No. 427,638, filed January 21, 1942, referred to therein.) In these Bennett guns atomization was obtained by the use of a concentric air orifice. In recently marketed guns in which the Bennett charging needle is employed air horns are used to shape the spray to the familiar fan form. It has been found that with the Bennett charging needle and either the conical spray form or the elliptical spray form about 75 percent of the charged spray will follow the path of the airstream between the spray gun and the work. Thus the great majority of the particles reach the work subject to the force imparted by the airstream.
It has long been taught that ideally the charged particles in an electrostatic system should reach the work without any substantial directing force remaining from the energy that caused their atomization. In this way, the deposition of the charged particles on the work would take place under the influence of the electrostatic field alone, and there would be no remaining air force to cause them to rebound from the work surface or to pass by the work and become lost. However, while the deposition efficiency of such an electrostatic painting system will exceed that of a non-electrostatic system, the overall effectiveness of the electrostatic system in which coating material is atomized, dispersed and deposited entirely by electrostatic force suffers markedly because of the inability to deliver coating particles into or behind recesses. This has often required that a complex part be touched up with a conventional spray gun having no electrostatic charging. The effectiveness of the conventional spray gun to deliver coating material into holes or depressions or similar recessed areas has long been recognized.
It has been found that in contrast to known air-atomizing electrostatic guns in which over 75 percent of the spray remains Within the airstream and only about 25 percent is outside of the airstream, the present invention will deliver well over percent of the charged spray through paths that are outside of and not influenced by the force of the atomizing airstream so that the spray reaches the work under the influence of the electrostatic field alone, even though the atomization is entirely due to the action of the atomizing airstream. This is in marked contrast to other electrostatic spray guns in which the Bennett charging needle is used and which deliver about 25 percent or less of the spray through paths outside of the path of the airstream and which exhibit only a small increase in pattern size under voltage conditions. By this difference in operation, the deposition efficiency of the electrostatic spray gun is substantially improved, and the ability of the gun to deposit coating material into holes and recesses in the work approaches that of the conventional, uncharged spray gun.
It is desirable in any air atomizing coating system that the consumption of air under pressure he kept to a minimum, and particularly so in an electrostatic system because once the air has performed its function of breaking the stream of coating material into satisfactory small particles, it has a tendency to interfere with the orderly deposition of the charged particles. The present invention so utilizes the atomizing air that finer atomization and greater dispersion take place with lower air volumes and, further, this is accomplished in the region of highest field intensity. It has been discovered that if a limited volume of air is given an intense swirling motion prior to the time that it engages and disrupts the stream of coating material, very complete atomization will take place. Further, if this atomization occurs around a Bennett charging needle from which an intense electrostatic field is radiating, the swirling motion imparted to the air substantially increases the length of the path traversed by a particle and this, coupled with the reduced velocity resulting from the reduced air flow significantly increases its residence time in the highly ionized zone. The transfer of charging ions into the coating material particles is thus substantially increased and the particles carry a much higher charge than is the usual case when the direction of the atomizing airstream is substantially parallel with respect to the direction of flow of the coating material particles. The increased charge per particle together with the lower particle velocity result in a greatly increased pattern size. Under a given and constant set of test conditions in which the spacing between the spray gun and the target, fluid flow, air pressure, fluid viscosity and other variables were held constant, it was found that the pattern diameter increased by more than three times when a standard charging voltage was applied to the charging electrode as compared to the diameter of the pattern at zero voltage. Making the same test without the swirling motion of the atomizing air showed only a small increase in pattern diameter between a charged and uncharged spray.
It has been found that the increased efficiency and superior distribution of the coating material resulting from the present invention are brought about by certain well defined relationships between the parts. These relationships will be fully discussed in describing a preferred embodiment of the invention with reference to the accompanying drawings, in which:
FIGURE 1 is a somewhat enlarged horizontal sectional view of a spray gun embodying the present invention;
FIG. 2 is a fragmentary, enlarged sectional view of the forward, cap portion of the spray gun;
FIG. 3 is a front elevational view of the spray gun shown in FIG. 2 with the outer air cap element removed;
FIG. 4 is a diagrammatic view indicating the distribution of spray particles with relation to the emitted airstream and total spray outline of the gun under charging conditions;
FIG. 5 is a diagrammatic view similar to FIG. 4 indicating the particle distribution when no charge is applied;
FIG. 6 is a diagrammatic view of the static pattern of a spray deposited as shown in FIG. 4;
FIG. '7 is a similar diagrammatic view of the static pattern of-a spray deposited as shown in FIG. 5;
- FIG. 8 is a graph representing the distribution of liquid coating material reaching a flat target from the spray gun, with and without a charging voltage, and the distribution of the airstream emanating from the spray gun;
FIG. 9 is a front elevational view of a spray gun embodying a modification of the present invention; and
FIG. 10 is a fragmentary, somewhat enlarged sectional view taken on line 1010 of FIG. 9.
In its mechanical aspects, the present invention comprises an apparatus for electrostatically coating articles maintained at (preferably) ground potential including in combination, a charging electrode, coupled to means to set up an intense electrostatic field between the charging electrode and the work to be coated, the intensity of the field in the region immediately around the charging electrode substantially exceeding the ionizing or coronaforming potential of still air at atmospheric pressure in the absence of coating material. In the combination the coating material is discharged from an orifice so related to the charging electrode that all of the discharged particles pass into and through the intense electrostatic field, the material being disrupted by a rapidly moving stream of air of limited volume. The present invention includes in the combination a means to direct the impinging air against the stream of coating material particles not only forwardly but also tangentially of the initial direction of flow of the stream to tend to impart a rotational or whirling movement to the dispersed particles in the region of high ion concentration. Thus, the path taken by a particle through said region is lengthened, and the electrostatic charge on the average particle is increased.
In known air atomizing systems, the airstream impinges on the coating material stream with substantially equal and opposite radial force components, either from air horns or from a forwardly directed annular airstream or both. The air flow requirements in such systems for a given degree of atomization are high by reason of the mutual cancelling effect of the inwardly directed radial force components. In the present invention the swirling motion of the atomizing air, or at least the impingement of the atomizing air on the coating material stream in a tangential direction, results in better atomization at lower air flow and thus a reduced overall velocity of the coating material particles. This permits the particles to remain within the region of very high ion concentration for a prolonged period and, further, imparts a direction of movement to the particles that causes them to cross the paths normally taken by ions flowing from the region of high ion concentration to the articles to be coated. This crosswise flow direction increases the probability of collision between the paint particles and the charging ions and thus results in a higher charge per particle. The efliciency of the spray gun is improved over known air atomizing guns employing the Bennett charging needle, and the ability of the spray gun to coat articles having complicated and recessed surfaces is very much enhanced over a conventional system employing electrostatic atomization.
In its method aspects, the present invention includes a method for electrostatically coating an article, comprising the steps of establishing and maintaining an electrostatic fie'ld varying in intensity from a region of corona at one area to a field of greatly reduced intensity at the article to be coated, discharging a stream of coating material particles into or adjacent the region of corona, and discharging into disrupting contact with said stream of discharged coating material particles within the region of very high ion concentration an airstream having an air flow rate only high enough to cause a predetermined fine atomization or dispersion of the coating material, and being effective to cause a general flow of air and disrupted particles toward the article to be coated. The airstream is so directed as to impinge forwardly and tangentially on the stream of coating material whereby the residence time of the particles formed therefrom within the region of high ion concentration is high. The electrical charge on the particles is thereby increased and the deposition efliciency of the system is improved.
Referring to the drawings and particularly to FIG. 1, the present invention is there shown in conjunction with an automatic spray gun to atomize and deliver liquid coating materials into an intense electrostatic field. Because air atomization is used to break up the liquid stream int-o droplets, the gun operates very satisfactorily with water base paints and similar materials that atomize difficultly or not at all under the influence of electrostatic forces alone.
The form of the spray gun shown in the drawings includes a gun body 10, the disclosure of which is somewhat diagrammatic. Air is supplied to the gun body through a coupling 11 to a passage 12 for on-off operation and from a coupling 13 to a second interior passage 14 for atomization purposes as hereinafter described. Coating material is supplied to a passage 15 leading to a coating-material issuance orifice 16 at the front of the gun.
The flow of coating material in the passage 15 is controlled by a fluid needle valve 17 cooperating with a valve seat 18 and extending rearwardly through .a suitable packing 19 to a fluid needle operating mechanism including a piston 20. The fluid needle operating piston 20 reciprocates in a cylinder 21 in the gun body and is urged forwardly by a spring 22. Cylinder 21 is supplied with operating air from passage 12, the piston 20 being forced rearwardly in the cylinder 21 when air pressure is applied. Automatic spray gun operating mechanisms of this type are well known in the art.
The piston 20 has a lost motion connection with the fluid needle 17, said connection comprising a head 23 on the needle and an actuating shoulder 24 on the piston. A heavy needle spring 25 urges the head 23 against the mirrors shoulder 24 and thus urges the needle 17 against its seat 18 and, when the needle is seated, the piston 20 may move away from the head 23 under the influence of its biasing spring 22. An adjustable stop member 26 is threaded into the rear of the 'gun body in the path of the piston 20, and the piston can thus carry the fluid needle 17 rearwardly only until it abuts the inner end of the adjustment member 26. The quantity of coating material issuing from the orifice 16 can thus be readily controlled. A locknut 27 is provided to maintain the fluid needle adjustment.
In the preferred form of the invention, the fluid needle 17 carries a charging electrode 28 extending :axially from the front thereof, to a point beyond the face of the gun. The electrode referably comprises a wire from about 0.010" to 0.020" in diameter, protruding from the front of the gun about one-fourth inch when the needle 17 is in the open position. The gun body 10 is connected to an appropriate source of high voltage D.C. so that an intense electrostatic field is created at the tip of the charging electrode 28. The electrode configuration is such that when a suitable charging voltage is impressed thereon, the field gradient will exceed the ionizing potential of still air and a region of corona will form around the electrode, at least prior to the introduction of coating material from the orifice 16. The coating material from orifice 16 thus issues into the intensely charged, or corona, region adjacent the front of the spray gun. This intensely charged region may be created by any suitable electrode configuration, such as a needle attached to the gun body and extending forwardly to a point adjacent the coating material stream but the centrally disposed needle-like wire electrode is preferred.
At the front of the gun body 10, a spray cap comprising an inner member 29 is threaded into the gun body. It is in this member that the fluid needle valve seat 18 and the coating material discharge orifice 16 are formed. An air cap 30 surrounds the inner member 29 and is held in assembled position by a gland nut 31 having an inturned flange 32 in engagement with an outwardly extending flange 33 on the member 29, the :gland nut being threaded over a threaded portion 34 on the front of the gun body 10.
Air to the interior of the air cap is taken from passage 14 around the exterior of the inner member 29 and through a series of openings 37 in a flange enlargement 38 to a forward air chamber 39. From the chamber 39 the air is passed over an angularly or spirally grooved flange 40 shown in section in FIG. 1 and in elevation in FIG. 2. The number, size and angular disposition of grooves 41 on the flange 40 will be hereinafter discussed. The effect of the passage of the atomizing air through grooves 41 is to impart a very rapid swirling motion to the air and this motion is hereinafter sometimes referred to as a spiral air flow, and the passages or grooves 41 are sometimes called spiral passages. The passages themselves may be formed by simply milling straight angular slots in the flange 40 or they may be actual spirals. At its front face, the air cap 30 is provided with an opening forming an air discharge annulus 42 in relation to a conical tip 43 on the inner fluid member. The relationship between the walls forming the air discharge annulus 42 has been investigated, and it has been found that it is preferable that these walls be parallel and form a conical passage. The angle of the cone will be discussed hereinafter. Air from the air discharge annulus 42 impinges immediately against a stream of coating material particles discharged from orifice 16; disrupting the stream into a mass of finely divided spray particles. This impingement takes place in the region of very high electrostatic field intensity around the charging needle 28 as will be more fully described.
FIGURE 2 of the drawings shows the preferred relationship between the air and fluid passages, and the present invention can best be described with reference to this figure. The variables have been investigated under the 6 following standard conditions, variation of each parameter being made with the others held constant.
,Width of air annulus 42 "inches" 0.016 Air annulus angle a i degrees 15 Fluid tip protrusion 12' inches 0.015 Diameter of fluid passage (orifice 16) -do 0.09 Angle b of passages 41 degrees 45 Air passage cross-sectional area sq. in-.. .0025 Air flow s.c.f.m 2.6 Diameter of liquid orifice 16 inches 0.085 Diameter of electrode 28 :do 0.020 Electrode protrusion do 0.250 Charging voltage kv l20 Gun-article spacing inches '15 Paint flow grams/min" 150 Paint viscosity Zahn No. 2 cup sec" 21 Paint conductivity ohm-centimeters 10 Exhaust booth air velocity f.p.m
To determine whether a change in any of the above listed parameters improved or degraded the operation of the spray 'gun the transfer efliciency of the gun was used as the principal determining factor, although fineness of atomization and coating distribution were also weighed in evaluating the results. Except for measurements of pattern diameter and film thickness, which require a flat target, the standardized target comprised a series of metal tubes one inch in diameter and spaced on three inch centers, with aluminum foil being wrapped around the cylindrical elements to form a readily renewable surface. The vertically disposed tubular elements were located 22 inches from the face of a standard spray booth and the exhaust air velocity was maintained at about 80 feet per minute.
Air velocities and the air distribution or air profile were measured with a hot wire anemometer. Particle velocities were determined photographically with a Polaroid camera and an Edgerton double flash electronic light source. From the known interval between flashes and the magnification factor of the camera, the velocity could be computed by measuring the distance between successively exposed particles on the photograph and dividing by the time interval.
Because of the importance of the coating material distribution in the deposited pattern, the pattern was obtained by exposing a stationary flat sheet target to the spray for a sufficient time to produce a continuous film without sagging. The static pattern diameter, discussed hereinafter, was the average of a horizontal and vertical measurement including about half of the fringe at the outside of the pattern. The film thickness distribution hereinafter reported was obtained by placing a strip of thin metal stock along a diameter of the expected static pattern, exposing it to the spray and, after baking the strip, measuring the thickness of the film at one inch intervals.
Investigation of the effect of the protrusion of the electrode 28 indicated that a one-fourth inch electrode protrusion was about optimum. It is desired to keep the elect-rode short to prevent bending and damage by physical contact. The tests indicated that transfer efliciency reached its peak at about one-fourth inch protrusion and fell off as the electrode was made longer. As the electrode protrusion was decreased to less than optimum the efiiciency of the gun decreased and, further, the front face of the air cap 30 tended to become coated with paint. At zero protrusion this coating was severe.
To determine the effect of varying the width of the air annulus 42, the atomizing air flow was varied to main tain a constant exit air velocity of 67.6 10 f.p.m. (Mach 1) while different annulus widths between 0.010 and 0.025 were tried. The performance of the gun, represented by an artificial coeflicient equaling the product of the average particle size of the atomized paint and the air flow rate improved as the annulus width decreased.
- It appears that coating of the air cap can be eliminated by a suflicient reduction in the annulus size at a given air flow rate, and that cap coating is largely a function of air velocity immediately adjacent the front of the cap and that a sufl-iciently high air velocity is required to keep the air cap clean. The best overall results were obtained with an air annulus of 0.016" width.
A study of variations in the angle a (FIG. 2) made by the air annulus With the axis of the gun indicates that the 15 annulus angle is preferable to 30 or 45 because of better atomization and a lesser tendency no coat the cap. It was also observed that the static pattern was generally more uniform at the 0 and annulus angles, although there was virtually no change in deposition efficiency as the annulus angle varied from 0 to 15. Somewhat better atomization occurs at the 15 angle, however, so this angle is preferred.
It was found preferable to protrude the fluid tip 43 slightly beyond the plane of the front face of the air cap 30 to reduce the tendency of the cap to become coated in use. This fluid tip protrusion need be only about 0.015". This minor protrusion appeared to make no difference in the pattern and transfer efliciency, and was settled upon merely for the reduction in the coating propensities of the cap 30. It appears probable that a variation in the geometry of the cap 30 might produce a similar result with zero protrusion of the fluid tip.
The present invention is based largely on the discovery that the charge on a particle of coating material can be significantly increased by increasing its residence time in an area of intense ion concentration, and that the time taken by a particle of coating material to traverse the highly charged area around the charging electrode can be increased, and its charge thereby increased, by reducing its velocity and by forming the particles from the stream of coating material by a forwardly and tangentially directed airstream or streams. The tangential force component of the atomizing air tends to impart a whirling motion to the particles, as heretofore noted. As the particle charge is increased the deposition efliciency is likewise increased. One manifestation of the increased particle charge is that the pattern broadens under an otherwise equal set of conditions from a cone having a base diameter of less than 6 inches when no charge is imposed on the particles to a cone having a base diameter of nearly 24 inches under charging conditions at the working voltage. Without the swirling motion of the atomizing air there is no perceptible change in pattern size from no voltage to full voltage.
FIGURE 8 shows at curve A the distribution of the airstream issuing from the annular orifice 42 with an air flow of 2.6 s.c.f.m. The left hand ordinate represents air velocity in feet per minute, while the abscissa represents distance in inches on each side of the center line of the airstream. No voltage was impressed on the electrode 28 during this measurement. At the center line, and for a distance of about 1 inch on each side the air velocity was measured at about 600 f.p.m. and dropped to essentially zero at about 4 inches on either side of the center line. This curve will be hereinafter referred to as the air profile. Every air atomizing spray gun has a characteristic curve of this nature that is easily plotted and compared with similar curves from other guns.
Curve B of FIGURE 8 shows the distribution of paint projected from the spray gun at zero voltage, the construction of the gun being as shown in FIGS. 1 and 2, and air flow remaining the same at 2.6 s.c.f.m. Again the abscissa represents distance on each side of the center line of the gun while the right hand ordinate represents paint film thickness in mils. It will be noted that, in general, the paint film thickness curve follows the contour of the air velocity curve and that only a small amount of paint is deposited in any region beyond the effective air current or air profile. -In these measurements as well as all others herein referred to, the flat target against which the paint was sprayed was disposed at right angles to the axis of the gun. Comparison of various apparatuses must be made with this disposition of the target, therefore.
Curve C in FIG. 8 shows the distribution of paint when a charging voltage is impressed on the electrode 28. The charging voltage in this instance, was kv. It will be seen that whereas only a minor quantity of the paint strayed from the airstream when no voltage was impressed on the electrode 28, more than 75 percent of the paint, at the target, is now outside the contour of the airstream when the charge is applied. This indicates that most of the paint is reaching the target under the influence of the electrostatic forces and without any significant applied force from the atomizing airstream. Only a minor portion of the paint is being deposited by the combined forces of the electrostatic field and the airstream within the air profile or the contour lines of the airstream itself. This minor portion, however, is very important because, being largely air influenced, it will penetrate into inaccessible areas on a complex workpiece that cannot be otherwise reached.
The ability of the spray particles to respond to the field in the manner indicated, by which at least 40 percent, and under most desirable operating conditions, 75 percent or more, of the coating material appears in an area outside of the pattern of the atomizing airstream is due to the lower particle velocity and the higher charge per particle, which in turn is due to the prolonged residence time in the field made possible by the reduced particle velocity and the motion imparted to the particles by the tangential component of the atomizi-ng air. When the angularly grooved flange 40 is omitted, or when the grooves 41 are made co-planar with the axis of the spray gun, the passage of the particles through the region of highest intensity is so rapid, because of the increased air flow required for a given fineness of atomization that the particles are insufliciently charged. It is believed that the mechanism of charging in the instance of the present invention is primarily by ion bombardment Within the very high intensity field region and that time is a very important variable in the process. The effective time in which the particles are subjected to ion bombardment is, of course, inversely proportional to particle velocity and is directly proportional to the extent of the zone of effective ionization. If the high intensity field region is increased or if the particle velocity is reduced the charge on the particles will be increased. The dimensions of the region of highest effective charge are fixed by the charging voltage and the contour of the electrode and surrounding field-influencing elements. Increasing the charging voltage beyond l20 kv. does not seem to increase the particle charge with the geometry of the gun shown in the drawings. Therefore, the time variable remains as the one that can be most readily influenced by modifications within the spray gun itself. By imparting a whirling motion to the atomizing air, reduced air flows can be used to achieve the required degree of atomization. The resulting reduced velocity increases the residence time of the particles in the intense electrostatic field and therefore the exposure of the particle to ions in the field is increased.
When the airstream is not given any whirling movement prior to impingement on the liquid stream the radial components of the forces of the air on opposite sides of the stream tend to cancel out so that a greater air flow is required to obtain a satisfactory degree of atomization and the atomized particles are restricted to a narrow region in the zone of atomization and charging. On the other hand, when the airstream is caused to impinge on the liquid stream with very substantial tangential components of force, the cancellation of atomizing air forces is greatly minimized and satisfactory atomization is obtained at lower air flows and the particles move freely 9 away from each other in the electrostatic field, with much lower forces remaining from the atomizing air. In the appended claims the atomization by the tangentially directed airstream is described as atomization by substantially unopposed, tangential components of force.
FIGURES 4 and 5 are diagrammatic representations of the spray issuing from the spray gun of the present invention, constructed from photographs of the spray. FIG- URE 4 shows the configuration of the spray under normal charging conditions and FIGURE 5 shows the configuration of the spray without charging, but all other conditions remaining the same. In both figures and in the photographs from which these views were constructed, the center portion of the spray following the outline of the atomizing airstream is clearly evident. The density of this portion, however, varies markedly from the uncharged to the charged condition. When the electrode 28 is at zero potential only about 20 to 30 percent of the particles in the spray are outside of the air profile or that region delineated by the airstream contour. When the preferred charging voltage is imposed, 70 or 80 percent of the spray particles will be found outside of this region. It will also be noted from a comparison of FIGURES 4 and 5 that whereas the contour is almost completely conical when the electrode is not charged, the contour blooms out into a curvilinear outline under charging conditions.
The patterns made by the sprays shown in side elevation in FIGS. 4 and 5 are themselves shown in FIGS. 6 and 7. These figures of the drawings were made from photographs showing the deposition of paint on a fiat target after the spray had been continued long enough to deposit a quite heavy coat without sagging or running. It will be seen that without any charge on the needle 28 the coating material particles are concentrated in the center area in a spot less than 6" in diameter at spacing between the gun and the target (FIG. 7). When an effective charging voltage, in this instance l kv., was placed on the needle 28 the diameter of the pattern in which the paint was deposited showed a very dramatic increase in size to nearly 24" (FIG. 6). In FIG. 7 only about 30 percent of the paint particles are outside the air profile, while in FIG. 6 about 75 percent are found outside this area.
This removal of the coating material from the air profile varies with the charging voltage and with the air flow, other factors being constant.
With a gun-target spacing of 15 inches, the first 30 kv. of charging voltage appears to have little effect on the coating material particles and therefore the pattern size is not greatly changed. Above this voltage, however, the pattern begins to enlarge rapidly as more and more of the coating material particles are removed from the air profile. The most drastic rate of increase appears at lower air flows. At '60 kv. charging voltage and 2.6 s.c.f.m. air flow, 70 percent of the particles are outside the air profile compared to 30 percent at no voltage. At 90 kv. nearly 80 percent of the particles have been so removed.
As the air flow is increased to 4.3 s.c.f.m. and then to 6.0 s.c.f.m. more of the particles appear to be largely airinfluenced and there is a less pronounced increase in the particle dispersion. However, even with the higher air flows substantially more than 40 percent of the particles will be outside the air profile at a charging voltage of between -60 kv. and --90 kv. at 15 inch gun-target spacing.
The particle dispersion with relation to the air profile will also change somewhat with the spacing between the spray gun and the target as would be expected because of the change in field gradient and because as the spacing is reduced the air turbulence caused by the presence of the fiat target tends to increase the number of particles that appear normally outside the air profile so that the zero point of the curve is higher. At no charge and a 10" spacing between the spray gun and the target, how- 10 ever, there are still less than 40 percent of the particles outside the air profile. As the target spacing is increased from 15" to 21" the particle removal is less, as expected, and only about 20 percent of the particles are outside the air profile at zero voltage while'at the preferred voltage of 120 kv. percent are removed.
That the characteristic of the invention described above :as it respects the removal of a great majority of the spray particles from the air profile is unique may be demonstrated by comparison with known electrostatic spray guns. For example, it has been found that a known spray gun, using the Bennett charging needle has a fan-shaped air profile from which about 20 percent of the paint particles are removed at zero voltage and 15" gun-target spacing. The imposition of full charging voltage of kv. increases the removal of paint particles from the air profile by less than 4 percent, so that even under charging conditions less than 25 percent of the paint particles are removed from the continual directive influence of the atomizing and shaping airstreams.
The effect of voltage variations on the efficiency of the spray gun was investigated, with all other parameters held constant. The maximum efiiciency appears, with the geometry of the gun shown in the drawings, and a coating material flow of grams per minute to be reached at about -120 kv. when the gun was spaced about 15" from the target. Reducing the voltage to 90 kv. reduced the efficiency by about 5 percent. Reducing the voltage to -60 kv. resulted in a decrease in efficiency of about 18 percent from the efficiency obtainable With the optimum charging voltage. However, reducing the voltage to 30 kv. made a drastic change in the efficiency, bringing about a reduction of nearly 67 percent. In each instance, the same determination was made with a positive charging voltage and it was found that the system was somewhat less efficient, as would be expected.
The quantity of atomizing air necessary for optimum operation of the spray gun according to the present inven tion will vary with changes in a number of parameters. The present invention is characterized by the ability of the rotary air flow to atomize or break up a stream of coating material particles at a substantially lower air fiow rate than other known air atomizing devices. As noted in the Table of Conditions set forth above, an air flow of only 2.6 s.c.f.m. satisfactorily atomizes and disperses a coating material column flowing at a rate of 150 grams per minute. With a higher rate of material fiow, more air is required, and the required air flow increases essentially linearly with paint flow from 1.6 s.c.f.m. for 50 grams per minute to 4.5 s.c.f.m. for 300 grams per minute. For 400 grams per minute, the air flow requirement increases somewhat more than linearly and becomes about 7.6 s.c.f.m.
The effect of the internal diameter of the fluid passage terminating in orifice 16 has been investigated at different atomizing air flow rates to determine the effect on atomization.- At each of three air flow rates, 2.2 s.c.f.m., 2.6 s.c.f.m. and 3.3 s.c.f.m., a substantially improved atomization occurred as the internal diameter was increased up to approximately 0.09 inch. A further increase to 0.10 inch produced a less consistent effect on atomization. The sensitivity of performance to the fiuid passage diameter that is apparent with the whirling motion of the atomizing airstream appears to result from the fact that atomization is accomplished at a much lower air flow.
Spiral passage angles (12) varying from 0 to 75, and total passage areas from 0.010 to 0.056 square inches at the spiral exit have been investigated and the resulting atomization and pattern diameter curves plotted at the various points. A saturation condition is reached at a calculated tangential air velocity of about 10,000 f.p.m. for both performance criteria. The investigation indicates that in general neither the passage angle nor the total passage area alone will determine the performance, but rather the combination of the two as reflected by the tangential air velocity. The investigation further revealed that no further improvement results (other things being equal) when the tangential air velocity is increased beyond approximately 10,000 feet per minute, but that undesirable effects such as coating of the air cap may occur if this velocity becomes too great. The substitution of helical air passages for straight spiral or angularly directed passages 41 was investigated and no difierence in performance was detected.
In the form of the spray gun shown in FIGS. 1 and 2, the whirlingair flow is produced by the grooves 41 in the flange 40 through which grooves the air passes on its way to the air orifice 42. A modification of the means to produce the rotary air flow is shown in FIGS. 9 and 10 and comprises external jet forming means consisting of a plurality of connected tubular air discharge elements 50 applied to or formed in the face of the air cap 30. Air to these tubular discharge elements may be supplied from the air channel 14 through any suitable conduit (not shown). Each of the tubular elements 50 discharges air axially of itself and tangentially of the stream of coating material issuing from orifice 16. The tangential airstreams may extend slightly forwardly as indicated in FIG. 10 by extending the face of the air cap 30 at an angle less than 90 from the axis of the stream of coating material particles. While the air flow in the jets from the tubular elements 50 is preferably at about 2.6 s.c.f.m., a minor atomizing airstream is discharged from a central annular air orifice 52. The central atomizing airstream need be only about 1.2 s.c.f.m. The charging needle 28 again is disposed centrally of the fluid orifice 16. Tests indicate that with this modification the particle charge on the coating material is again such that combined with the reduced overall velocity which is made possible by the rotational or whirling atomizing air movement the pattern increases about three times in diameter between the uncharged and charged condition as described in connection with the form of the device shown in FIGS. 1 and 2.
While air has been mentioned throughout the specificatron as the atomizing fluid, it should be expressly understood that other known atomizing gases such as steam or nitrogen may be used in place of compressed air in some modifications of the invention.
The invention has been disclosed in conjunction with a specific form and disposition of the parts, and of apparatus to carry out the new method. However, it should be expressly understood that numerous modifications and changes may be made without departing from the scope of the appended claims.
What we claim is:
I 1. In a method of coating an article with liquid coatng material from an electrostatically charged air atom- 12mg spray gun in which liquid coating material is discharged around a highly charged needle electrode and atomized by a surrounding airstream, the improvement which comprises directing the airstream to impinge against the stream of liquid coating material with a substantially unopposed tangential component of force to cause the liquid coating material to atomize in the volume immediately around the axis defined by the needle electrode and adjacent the tip thereof.
2. An apparatus for electrostatically coating an article with a liquid coating material comprising, means including a forwardly projecting centrally disposed needle electrode to establish and maintain an electrostatic field having a sufliciently high intensity to cause corona at a region spaced from the article to be coated, means to discharge a stream of liquid coating material into an unconfined atmosphere and around said needle electrode into the high intensity region of said field, means to disrupt said stream of coating material by impinging against the liquid stream a stream of gas having substantially unopposed tangential components of atomizing force to create a mass of finely divided highly charged liquid spray particles, said spray particles thereafter moving to the article for deposition thereon.
3. An apparatus for electrostatically depositing coating material on an article comprising, a spray gun body having a passage for coating material terminating in a discharge orifice and having a generally axially extending passage for air under pressure terminating in an air orifice surrounding said discharge orifice, said air passage converging in the area immediately adjacent said discharge orifice, air directing means interposed in said air passage to impart a whirling movement to air passing therethrough, and adapted to cause air issuing from said air orifice to impinge on coating material issuing from said coating material orifice with a substantial tangential component of velocity to disrupt said coating material into a mass of finely divided spray particles having a relatively low velocity in an axial direction, and charging means comprising a needle electrode protruding forwardly int-o said mass of finely divided spray particles to establish and maintain within the mass of finely divided spray particles a high intensity electrostatic field to impart a high electrostatic charge to said spray particles, said air orifice, said air directing means and said charging means being so constructed and arranged with relation to said coating material discharge orifice that the velocity of said airstream, its direction, and the particle charge are such that at least 40 percent of said spray particles are removed from the effective influence of said airstream and the continual directive influence of said airstream in passing to the article to be coated.
4. The apparatus defined in claim 3 in which the angle of convergence is about 15.
5. The apparatus defined in claim 3 in which said air orifice, said air directing means and said charging means are so constructed and arranged with relation to said coating material discharge orifice that the velocity of said airstream, its direction and the particle charge are such that at least a majority of said spray particles are removed from the effective influence of said airstream and the continual directive influence of said airstream in passing to the article to be coated.
6. The apparatus defined in claim 3 in which said air orifice, said air directing means and said charging means are so constructed and arranged with relation to said coating material discharge orifice that the velocity of said airstream, its direction and the particle charge are such that at least 70 percent of said spray particles are removed from the effective influence of said airstream and the continual directive influence of said airstream in passing to the article to be coated.
7. An apparatus for electrostatically depositing liquid coating material on an article comprising, a spray gun body having a passage for liquid coating material terminating in a liquid discharge orifice and having a generally axially extending passage for air under pressure terminating in an air orifice surrounding said liquid discharge orifice, said air passage in the region of said air orifice being convergent to the axis of said gun body, air directing means interposed in said air passage, said air directing means including a pluralty of peripherally spaced passages disposed at an acute angle to the longitudinal axis of said air passage to impart a whirling movement to air passing through said air passage and adapted to cause air issuing from said air orifice to impinge on liquid issuing from said liquid orifice with a substantial tangential component of velocity to disrupt said liquid immediately into a mass of finely divided spray particles having a relatively low velocity in an axial direction, and a centrally disposed charging needle extending through said liquid discharge orifice and projecting forwardly from the front of said gun body to establish and maintain within the mass of finely divided spray particles a high intensity electrostatic field to impart a high electrostatic charge to said spray particles, said air orifice, said air directing means and said charging needle being so constructed and arranged with References Cited by the Examiner UNITED STATES PATENTS Gustafsson et a1. 239298 Bennett et a1. 6511 Bodle et al. 11793.4 Schweitzer.
Sedlacsik 117930 X Ransbury 9/1961 Sedlacsik 239-15 10/1962 Walberg 239l5 10/1962 Verba et a1. 239-15 10/196'2 Nakaya 239-15 X 2/1965 Juvinall et al. 2393 X 5/1965 Reindl et a1 239l5 2/1966 Fraser 239-45 FOREIGN PATENTS 9/ 1942 Germany.
ALFRED L. LEAVITT, Primary Examiner.
RICHARD D. NEVIUS, Examiner.
42 15 A. GOLIAN, Assistant Examiner.