US3048498A - Electrostatic spray coating system - Google Patents

Description

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BELL EDGE KV Aug. 7, 1962 J. W. JUVINALL ETAL ELECTROSTATIC SPRAY COATING SYSTEM Filed Jan. 8, 1959 4 Sheets-Sheet 1 JAMES W. JUVINALL JAMES C. MARSH INVENTORS Aug- 7, 1962 J. w. JUVINALL ETAL 3,048,498

ELEcTRosTATIc SPRAY comme SYSTEM 4 Sheets-Sheet 2 Filed Jan. 8, 1959 INVENTORS JUVINALL a2ac-KW A fameys JAMES W. BY JAMES C Aug. 7, 1962 J. W. JUVINALL ETAL ELECTROSTATIC SPRAY COATING SYSTEM Filed Jan. 8, 1959 4 Sheets-Sheet 3 A H'ar y Aug. 7, 1962 Filed Jan. 8, 1959 J. W. JUVINALL EVAL ELECTROSTATIC SPRAY COATING vSYSTEM /lOa/ 4 Sheets-Sheet 4 Dc 00m/r INVENT OR.

JAMES W JUV/NALI. JAMES G MARSH Arfom y United States Patent 3,948,493 ELECTRGSTATIC SERAY CQATING SYSTEM James W. Juvinall and .lames C. Marsh, Indianapolis,

Ind., assignors to Ransburg Electro-Coating Corp., Indianapolis, Ind., a corporation of Indiana Filed lian. 8, 1959, Ser. No. 785,754

27 Claims. (Cl. 117--93) This invention relates to spray coating with4 a liquid coating such as paint, and more particularly to spray coating systems wherein electrostatic forces are utilized to aid in the deposition of spray particles on the article to be coated.

This application is a continuation-in-part of our application Serial No. 572,752 filed March 20, 1956 and now abandoned.

It is the general object of the present invention to produce new and improved electrostatic spray coating apparatus and methods.

Still another object of the invention is to produce an electrostatic spray painting system in which the average potential gradient of the electrostatic depositing eld may be maintained at predetermined desired values in spite of varying distances between the article being coated and the charging electrode, between which article and electrode the depositing field extends.

Yet another object is `to maintain the average potential gradient of an electrostatic atomizing field at or about the optimum atomizing gra-dient throughout substantial variations in the spacing between the atomizing zone and the other field electrode.

A further object of the present invention yis to provide an electrostatic spray painting system which includes a charging electrode for creating an electrostatic depositing field extending from the electrode to the article being coated and which -is incapable of producing dangerous disruptive discharges, even when the charging electrode is closely approached by the article being coated or by personnel.

Electrostatic spray coating systems have been successfully used in commerce in several `different forms, for example, systems have been successfully employed which utilize spraying or atomizing devices employing air as the atomizing medium. Such systems normally include an independent electrode in the form of a .grid of wires which is maintained at high potential, thereby establishing an electrostatic depositing field extending between such electrode and atleast one of the articles being coated as the other electrode. In such systems, the conventional air spray gun projects the atomized particles into the electrostatic field, with the result that the atomized particles become electrostatically charged with respect to the article and thereby are caused to be deposited thereon. As an example of this type of electrostatic spray coating system, reference may be had 4to United States Letters Patent No. 2,3 34,648.

Another system of electrostatic spray .painting widely used commercially obviates the necessity of air as an atomizing agent, one form of this system effecting atomization by electrostatic forces. In this form, the atomizing device is maintained at high electrical potential, and thus serves as the charging electrode for electro- 3,048,498 Patented Aug. 7, 1962 ICC , application of Edwin M. Ransburg, Serial No. 771,505,

statically charging the spray particles. =In systems where filed November 3, 1958 and now Patent No. 2,893,894, a continuation of application Serial No. 143,994, filed February 13, 1950V and now abandoned, or to James W. Iuvinall Patents No. 2,764,125 and No. 2,764,712, both issued September 25, 1956. Other forms of this system may also be used, such as, for instance, one where the coating material is atomized centrifugally or by the application of pressure to the liquid. In this type of system, the atomizer itself may constitute a charging electrode, or the particles so atomized may be projected into an electrostatic depositing field maintained between a separate charging electrode and the article.

It will therefore be seen lthat various -means for effecting atomization may be utilized in electrostatic spray coating systems, and that in such various systems, the charging electrode may be either the atomizer itself or a separate charging electrode.

Whereas devices of the type referred to readily lend themselves to commercial usage and are capable of greatly reducing the cost of spray painting through the avoidance of coating material losse-s and the reduction in other costs, including labor of coating, electrostatic spray coating systems have suffered certain 'limitations in usage which have prevented wider adoption and usage thereof. One of the limitations referred to is the fact that proper electrostatic atomization and efficient deposition of spray coatings have required the employment of an electrostatic field established by relatively high effective voltages, as of the order of one hundred thousand volts, which have many critical and unexpected characteristics, and resent diiiiculties relative to safety both as to fire and as to shock hazard to personnel.

In the type of system employing electrostatic atomization, it has been found that there is an upper as well as a lower limit on the voltages that should be employed, an unexpected characteristic of high voltage fields and electrostatic atomization thereby. Where electrostatic forces are employed to effect the atomization of the coating, it is important to maintain the potential gradient of the electrostatic field extending from the atomizing device to the article within predetermined limits, which limits define the range of the optimum atomization of the atomizing device. Thus, while it had been thought that fineness of atomization and the quality of the 'paint finish produced would be improved by increasing the average potential gradient of the field to the maximum consistent with safety, it has now been discovered that the quality of atomization does not continue indefinitely to improve with increases in the average potential gradient, but rather that there is, for any given set of other conditions, a rather definite potential gradient at which the quality of atomizat-ion is optimum, and that the potential gradient resulting in atomization of optimum quality is either increased or decreased appreciably, the .atomization Will suffer. It has been found that if, for any given set of conditions, the average potential gradient is progressivelyincreased from a low value, the cusps into which the field deforms the edge of the film of coating material on the atomizer, become more closely spaced and atomization becomes finer, i.e., improves in quality. During this initial increase in potential gradient, the cusps continue to be equally spaced and stable, but eventually a condition is reached wherein further increases in field gradient cause the cusps to become irregular and unstable. Specifically, the cusps seem to join together in some cases and to flail about 4in others, perhaps under the influence of increasing mutual electrostatic repulsion; an-d instead of the cusps breaking up at their tips uniformly and stably into spray particles of appropriate size, many objectionable large .particles are formed.

Because of the foregoing, it has been necessary to andasse maintain relatively definite and tiXed spatial relations between the electrostatic atomizing device and the article being coated, in order that the potential gradient may be kept Within desired limits. Any condition which resulted in changes of the atomiZer-article spacing caused corresponding changes in the potential gradient, and resulted in deterioration of atomization if the changes in potential gradient exceeded the limits described.

In the type of system which does not employ an electrostatic eld for effecting the atomization of the coating, but rather relies upon mechanical means for atomization, such as compressed air or liquid pressure, it is found that the higher the negative voltage on the charging electrode, the higher the coating efficiency. Thus, it is advisable to employ voltages on the charging electrode as high as 150 kv., or even more. As in the case of the electrostatic atomizer, however, the voltages actually employed are dictated at least in part by the ability to avoid situations where the electrode and the article spacing cannot be maintained within quite definite predetermined limits. y

As many of the spray coating materials employed are inilammable, and in some cases highly so, it has at all times been necessary to provide a minimum spacing between the electrodes in order to avoid the likelihood of sparks or arcs of suliicient intensity to initiate a fire or explosion. These considerations have dictated the necessity of confining the use of the various systems now employed in commercial practice to installations wherein the charging electrode is maintained in a fixed position with relation to the articles to be coated. As it is customary in commercial installations to have the articles suitably mounted on a traveling conveyor so that they are successively moved to pass the location where one or more atomizers are provided, it has been the practice to station the charging electrode in fixed relation to the path of movement of the articles. In this way the necessary distance may be maintained with sutiicient accuracy to assure the avoidance of disruptive discharges, such as might cause a lire or explosion, provided the electrode is spaced far enough from said Ipath to prevent diiculty. Suiciently more than the minimum sparking distance must be allowed to provide for anticipated variations in the spacing, as might result from a bent hanger, or swaying of the articles. In those instances where mechanical atomization is employed and a separate charging electrode is provided, it has been the practice to space the charging electrode from the article a distance which is twice the sparking distance. Maintaining this distance results, of course, in a decrease in efficiency of the operation, as the average potential gradient is necessarily reduced. This allowance of additional spacing for safety purposes, however, has been dictated by the practical necessities which result in variations in electrode-article spacing during normal operation of a system of this nature. For instance, many objects carried on conveyors of the type referred to are subject to some movement due to swaying, or may lie in a different plane due to a bent hanger, as mentioned above, which causes the articles to. move first closer and then farther away from the charging electrodes. In other instances, articles of irregular shape may be rotated while being sprayed, and due to their irregular shapes and the rotation, certain portions of their surfaces are disposed appreciably nearer to the charging electrode than other portions of the surfaces at other times during the spray coating.

Even in cases where the coating material employed is such as not to create a danger of fire or explosion, the necessity of avoiding 'disruptive discharges from the charging electrodes is still present. Close approach of personnel to the charging electrode will result in a severe and unpleasant shock, even though the current dow is maintained below lethal levels. Thus, it is also important to avoid disruptive discharges or this type, of such i intensity as to be severely or distressingly shocking, for the protection and comfort of personnel working in the area.

From the foregoing, it will be realized that the systems described have required the utilization of means for maintaining within predetermined limits the spacing between the electrodes creating the electrostatic teld, i.e., the spacing between the charging electrode and the article. This requirement has mitigated against the provision of an unrestrained hand-held spray device in an electrostatic coating system. This is apparent because an operator using-such a device would undoubtedly change the spacing between the electrodes, and thus change the potential gradient of the electrostatic lield.

lt has now been discovered that systems of the type referred to can be provided in which the variations in spacing between the electrodes which might occur in use, including the variation in spacing that would be effected through the use of a hand-held atomizing device, can be automatically compensated for to an extent which makes it possible to maintain the potential gradient of the held at values consistent with high depositing ethciencies (and with high atomizing eiciencies, in systems where electrostatic atomization is employed) throughout y a range of spacings extending from the greatest down to the least employed in spraying and which also makes it possible to insure that any gap-bridging discharge occurring at or below such least distance will be of unobjectionable intensity.

Two characteristics may be built into the system and may be employed conjointly to bring about the results referred to immediately above. The tirst characteristic is the use of an impedance oi' predetermined magnitude in the electrical system between the source of high voltage and the air gap between the charging electrode and the article. lf the impedance employed is sufficiently high with relation to the potential employed and the air gap commonly present, variations in the air gap which would normally have a direct and major iniluence on the potential gradient will result in voltage variations such as will satisfactorily compensate for such changes in spacing. Thus changes in potential gradient resulting from changes in spacing are greatly minimized and a predetermined desired held gradient can be maintained in spite of wide changes in the distance from the article to the charging electrode. "Furthermore, this compensating action can be established to such a degree to assure that at all times variations in the air gap between the electrodes will not result in undesirably low lield gradients.

The second characteristic which may be employed is the provision at the location of the air gap of a charging electrode or electrode system of low effective capacity. When we state herein that an electrode system has a low effective capacity we mean that the electrode system has such low true capacitance, such poor conductivity of the material of which it is formed, or such shape (particularly as to sharpness or bluntness of its configuration) that, in an electrostatic coating system of the character with which we are here concerned, the energy contributed to a disruptive discharge between such electrode system and an opposed blunt electrode by the electricity stored in the electrode system is insuticient to render such discharge objectionable. We have found that if some portion of the charging electrode or of the system between the charging electrode and the resistance previously referred to is of highly conductive material, such as metallic wire or the like, such can be tolerated, provided that it is of such limited area that it has no substantial capacitance. In other words, if Ahighly conductive materials are to be used adjacent the air gap, their actual capacitance must be so low as to be within tolerable limits. if, on the other hand, their capacitance is greater than the tolerable amount, they must be of such low conductivity that the energy thereof cannot be discharged in such a manner as to produce anl objectionable disruptive discharge.

In earlier commercial systems of the types previously mentioned, it has been the practice to equip the high voltage source with an effective impedance of about megohms in the power pack itself and in series with the hot terminal. This impedance was quite ineffective to prevent a current flow, in the event an article or other object too closely approached the electrode, which would give an objectionable disruptive discharge. In addition, the capacitances and conductances of the electrodes used were such as to permit an objectionable disruptive discharge between the electrode and the article upon a close approach of the article thereto.

While there are patents in various iields where high voltage is used which, like the impedances embodied in the high voltage sources described above, incorporate resistances to prevent lethally heavy shocks or severe arcing over upon the close approach of a person or object to the discharge electrode, none of such patents is in any way concerned with a system for automatically maintaining within predetermined limits the average potential gradient between the discharge electrode and the article in spite of wide Variations in the distance or air gap existing between the article and the electrode and in the presence of an essentially constant high voltage applied by the high voltage source, and which system, when cou-pled with an electrode having a capacity or conductivity insuiiicient to release a heavy charge (i.e., a suitable effective capacity), provides an electrostatic spray coating system incapable of producing objectionable disruptive discharges even when closely approached by the article being coated, or by an operator. For examples of the patents referred to above, reference may be had to the following United States Letters Patent: Chapman 878,272; Wintermute 1,913,784 and 1,968,330; Bennett 2,295,152; and Melton 2,187,624. Moreover, efforts have heretofore been made to solve such problems in the electrostatic spray-coating iield, reference being made to U.S. Patents 2,509,277 and 2,526,763, British Patent 688,788, and German application K 3942; but none of these did succeed in solving the above-mentioned problems.

As previously mentioned, atomizing devices embodying features of the present invention may be constructed in a manner which will maintain a predetermined range of potential gradient between an electrostatic atomizing head and an article, regardless of changes in the space therebetween, and wherein dangerous and objectionable discharges may be prevented, even -where the high potential eiectrode is accidentally brought into contact with the operator or with the article being coated. By reason of the foregoing, it is possible to produce a hand-held or manually manipulatable electrostatic atomizing device which is capable of the high eiciency performance of the electrostatic atomizing devices previously mentioned, yet may be used with perfect safety as a hand-held atomizer, and to materially improve systems utilizing a ixedly mounted atomizer.

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

FIGURE 1 is a perspective view of a system for electrostatically atomizing and depositing paint on the articles to be coated;

FlGURE 2 is a longitudinal sectional view of the atomizing device shown in FIGURE 1;

FIGURE 3 is a fragmentary View, partly in section, showing in more detail the means for rotating the head of the atomizing device and for supplying liquid `coating material thereto for atomization;

FIGURE 4 is a chart graphically illustrating, by appropriate curves, certain voltage and distance relationships;

FIGURE 5 is a longitudinal sectional view of another form of atomizing device adapted to be held in the hand and used in a system of the general character shown in FIGURE 1, and also having a rotating atomizing member;

FIGURE 6 is a longitudinal sectional view of still another form of atomizing device particularly adapted to be held in the hand, atomization in this case being elected by mechanical forces, high hydrostatic pressure of the paint supply in the form shown;

FGURE 7 is an elevational view of another system embodying our invention, atomization in this case being by a conventional air gun;

FIGURE 8 is a partial elevational View at rightanglesto that of FIGURE 7 and along the line 8-8 thereof; `FIGURE 9 is a schematic view of yet another system embodying our invention; and

FIGURE 10 is a circuit diagram of the power supply illustrated schematically in FIGURE 9.

Referring now to the particular embodiment of the invention illustrated in FIGURES l to 3, it will be seen that a succession of articles 10 are moved through a coating zone, the articles being here illustrated as garnish moldings for the window openings of automobiles, an article ditlicult to paint eiiiciently with conventional systern utiliZinghand-held air spray guns. The articles are here shown as being moved along to the right of the drawing as illustrated, by hangers 11 in turn connected to a chain (not shown) on a conveyor track 12 of conventional overhead conveyor means, as illustrated. This movement is without rotation and the articles are shown as being painted on one side only (except for such Wraparound as may occur' due to the electrostatic action), and it will be understood that in practice another man would normaily be stationed on the other side of the line at a second coating station to provide a coating of the desired lm thickness on the other side of the articles.

The atomizing device identified in general as 14 and to be more fully described in detail hereafter, includes a lonlg handle portion 15 which its outer housing of polvethylene or other non-conducting material with good high voltage insulating characteristics, and a rotating bell-like head 16 from the edge of which the liquid coating material is electrostatically atomized. The charged spray particles of paint or the like are then attracted to and deposited upon the articles under the influence of electrostatic forces, the articles normally being grounded and the hot terminal of the power pack 17 being connected to the atomizing edge in a manner hereinafter more fully described, and including a high resistance in the connection and close to the atomizing head 16.

In the system illustrated (and as may be best seen by also referring to FIGURE 3) a supply tank 18 supplies paint through a tube 19 to a pump 2t), preferably of a positive displacement Variable speed type driven by its own motor. The output of the pump is delivered through a paint supply conduit 20a tc the opening in a hollow shaft of an electric motor 22, this paint supply conduit continuing on up through a flexible conduit arrangement indicated in general as k23 leadnig to the hand-held atomizing device, the paint eventually issuing through a central opening in the bell-like rotating atomizer member 16 to fbe formed in a thin film on the inner surface of this member and to have the spray particles atomized from such thin film at the edge of the rotating member, as morel fully described and claimed in the E. M. Ransburg Patent No. 2,893,894. As is more fully described in such patent, the high potential electrostatic field existing between the atomizing edge and the articles draws the iilm edge into closely adjacent cusps with the spray particles or droplets issuing from the ends of these cusps during atomization. Except for the hand-held atomizirrg device and the high resistance connection, the system so far described is of a standard spaanse drive a hollow exible metallic shaft 2li surrounding a paint tube 21, the outer sheath of the conduit assembly being a conduit 25 of polyethylene or other material having good high voltage insulating properties. High voltage, of at least 40,000 volts and preferably 70,000 volts or more, is supplied from the high voltage pack 17 (generally from the negative terminal thereof) through the lead 26 to the motor ZZ, the liexible metallic drive tube 24 conducting the high voltage to the atorniZing device. The paint supply tank 18, the pump unit 2.0 comprising the pump and its motor, and the rotator motor 22, are all mounted on a support member 27 car-fied on legs 28 of ceramic or other rigid material of good high voltage insulating characteristics; and the pump motor and rotator motor are supplied with drive current (as conventional 60 cycle ll() volt A.C.) through isolating transformers `with proper high Voltage insulation so that the whole arrangement is maintained at high voltage with respect to ground. While not illustrated, these parts would normally all be enclosed in an insulating housing to protect against accidental contact of a person with any part at high voltage; and not only the input of the high voltage supply tuiit but also the isolating transformer inputs would have conventional on-off switches therein, so that the operator may terminate coating operation at any time merely `by moving to the off position a master switch, or individual switches provided in each input.

Referring now more particularly to PGURE 2, the atomizing device is shown in more detail. The rotating bell-like atomizing member 16 is of nylon, polyethylene or similar material of good high voltage insulating characteristics. lt has an axial opening 16a, an adjacent portion 16h which flares sharply outwardly from the axial opening, and a rim portion 1de extending forwardly more nearly axially with an inner surface about 15 off from the axis, this portion being tapered to a relatively sharp forward atomizing edge 16d. The inner surface of the rotating member, from the axial opening clear to the atomizing edge, is covered with a coating 1de of a material which has high chemical resistance to the constituents of the paint or other liquid coating material being used, high mechanical resistance to abrasion by the material owing thereover, and predetermined high elec trical resistance characteristics. A specific alkyd resin enamel-like coating material with finely `divided carbon particles therein has proved very satisfactory for this resistive layer and is the subject of William D. Gauthier Patent No. 3,021,077, wherein it is more specifically described.

For purposes of this application, it is sutlicient to state r that the material should have suitable chemical and mechanical resistance, yand be of high electrical resistance while still permitting a controlled ow of very small currents therethrough. The resistance with which we are here concerned is what may be termed a point-to-point resistance, in that the factor of importance is the resistance to current ovv from a given point on the surface to another surface'point, as at the atomizing edge. While the device illustrated achieves its desired resistance characteristics by using a body material for the bell which is substantially completely non-conducting (as nylon) and a surface coating which has some slight conductivity, it will be understood that the rotating member could be formed of a single homogeneous material provided it had the desired pointtopoint resistance characteristics and suitable mechanical strength. With a rotating bell member about 4 inches in diameter, a suitable size for a hand atomizing device, it has been found that the resistance from the apex (at the axial opening) to the rim or atomizing edge (when the same is placed in contact with a metal plate entirely around the extent of the rim edge) should be at least of the order of l0 megohms,`

and preferably of the order of 100 megohms. Under these circumstances the resistance per square (as for example per a one centimeter square) would be of the same order of magnitude as the above-mentioned apex-to-edge resistance, but it is more convenient commercially to determine the resistance from the apex to the edge. lt will also be understood that if rotating atomizing devices of larger diameter are used, it may be desirable to materially `raise the resistance.

The bell member 16 is mounted on a rotating sha-ft assembly 29, as by rubber O-rings. This shaft assembly comprises a commercially available high value resistor consisting of a hollow ceramic tube 29a of about l2 inches in length, with an outer coating 2919 of high resistance material. A coatingon the right-hand tip of the ceramic tube provides an electrical connection between the end of the resistance material 29h and the inner or apex portion of the high resistance coating 16e on the inner surface of the bell member. The entire resistor is held within a rigid carrier member 29C of phenolic `material or the like which rotates on bearing v surfaces provided by an outer insulating sheath 30,

which in turn lies within outer housing member 31 of polyethylene.

The outer sheath member 25 of the flexible conduit assembly 23 extends into the outer housing member 31, as may be seen at the left of the drawing, the whole assembly being held in the hand atomizing device by the cooperation of the cap 32 with an enlarged portion 33 molded in the sheath tube 25. The hollow flexible metallic shaft 2,4 within the tube 25 is mechanically coupled, through a splined assembly 34, to the rotating shaft assemblyV 29 carrying the bell member 16. Appropriate liquid seals in assembly 34 insure that all of the coating material flowing from the end of paint tube 2'1 is transferred into hollow shaft assembly 29. Paint supplied through the tube 21 passes through the hollow shaft and spreads out on the inside of the bell during rotation thereof to continuously supply the atomizing edge with paint at a rate determined by the operation of the positive displacement pump, a representative rate of supply being l0() cc. per minute.

The circuit connection from the hot terminal of the power supplyis completed through the hollow flexible metal shaft (which is, however, sheathed from accidental Contact by the outer polyethylene tube), and any current reaching the atomizing' edge must pass through the resistance not only of the high resistance inner coating 16e on the bell, but also through the extended resistance layer '2911, this being here shown as of sufcient extent to prevent any possibility of arcing around the outside thereof. It will be understood that suitable insulation can prevent the arcing regardless of the actual extent of the resistor. In parallel with these fixed highV resistance values is a current path through the paint, from the point where it leaves the flexible metallic shaft end to the atomizing edge. While a very high conductive paint, in the embodiment here being described, would thus nullify the value of the high resistance means just described, the resistance of the paint column in a passageway of the size illustrated (which would normally be only 1,@ or i/l, of an inch in diameter) is such that the resistance through the paint is at least several hundred megohms and may be many thousands of megohms with the lacquers and synthetic enamels normally used in commercial spray painting. It is preferable to have the total effective resistance from the conductive sheath to the atomizing edge at least several megohms per kilovolt applied by the high voltage source, wtih normal commercial operations preferably having at least 300 megohms and normally 1000 megohms resistance or more. Most commercially used coating materials provide a shunt resistance considerably higher than this, so that if the iixed resistances are arranged to provide a resistance at or about this value, the total working re. ,sistance will normally be approximately the same value.

Even if paint is quite non-conducting for all practical purposes, as is frequently the case, the fixed resistances 9 will provide the necessary voltage at the edge of the bell for atomizing and charging the spray particles.

Again referring particularly to FIGURE l, the operator doing the spraying with the hand atomizing device would normally follow around the metal framework comprising the garnish molding, speaking in a very general sense, and should normally keep the atomizing edge of the bell member 6 or 8 inches away from the metal of the object, for example, if a 100,000 volt pack is being used. However, hand operation or in fact any operation wherein there are appreciable changes in air gap distance between the charging electrode and `an article, introduces a number of problems in a-n electrostatic coating system; Variation in gap distance, if the voltage between the electrode and the article remain the same, would obviously result in Very considerable variations in the average potential gradient. That is, if the bell and shaft were both of metal connected to the 100,000 volt power pack, the average potential gradient at a l2- inch spacing would be only about 8000 volts per inch, whereas the average potential gradient at a S-inch gap spacing would be 20,000 volts per inch. An electrostatic coa-ting system of the kind just described, with atomization being effected electrostatically, achieves very` high paint deposition eliiciencies and is highly desirable commercially. However, it has now been found that there are certain very definite optimum values of the average potential gradient, and that these do not remain the same regardless of spacing but vary in accordance with the gap distance. A gradient at the film edge which is too low does not atomize particles of the desired fineness for a quality coating; but improvement in atomization'increases with the gradient only up to a certain point, and thereafter deteriorates rather than further improving.

We believe too high a voltage has certain field effects on the cusps causing at least some of them to merge and A ive undesirably large particles, and in any event to flail or whip around in a manner which also results in much poorer uniformity in the size of the spray particles. Whatever the full explanations may be, however, we have found that there is a definite optimum average potential gradient for electrostatic atomization, and that this does not remain constant with changes in gap spacing. For example, with a 4-inch metal bell rotating at about 1000 r.p.m. and supplied with 100 cc. per minute of a synthetic enamel, the optimum avera-ge potential gradient for electrostatic atomization at l2 inches is about 7500 volts per inch; at 9 inches is about 9000 volts per inch; at 6 inches is about 11,700 volts per inch; and at 3 inches is about 17,000 volts per inch. We have found, as may be best seen from reference to FIGURE 4, suitable selection of the control resistance keeps the voltage at the atomizing edge of the bell in close conformity with the optimum atomizing voltage throughout wide variations in `gap distance, as from 3 inches to l2 inches. This results in maintaining the quality of the finish on the article being sprayed through appreciable variations in gap distance, in a manner heretofore impossible.

Referring now more particularly to FIGURE 4, curves are shown illustrative of various conditions in electrostatic coating systems, both with and without the utilization of the present invention. The curve identified as 53 illustrates the gap break-down at various target spacings, where the target is a rgrounded article of fiat sheet metal, and the atomizing bell has a 4-inch diameter atomizing edge with a radius of only about .005 inch, and connected to the negative terminal of a power pack having its positive terminal grounded. It will be seen that the `gap breaks down a-t about 2 inches with 60 kilovolts, and at 3 inches with 90 kilovolts. The curve identified as 59a illustrates the voltage existing between the bell edge and the target at various spacings with atomizers and power packs of the conventional type heretofore used by the assignee of this `application wherein the only appreciable resistance is a l0 rnegohm limiting resistor built into the power pack. With kilovolts applied to the bell edge at a l2-inch spacing from the target, the curve remains substantially flat as the spacing is reduced, with a very slight drop in voltage in the last inch or two before the break-down distance is reached, this being at about 25/s inches a-nd there still being almost 80 kilovolts at the bell edge.

The optimum .atomizing voltage for a 4inch bell as described above, being rotated at 900 r.p.m. and supplied with cc. per minute of a conventional commercial synthetic enamel, is indicated by the dotted line curve identied as 59h. In order to obtain optimum atomization in an electrostatic system of the kind heretofore described, the voltage existing across the gap between the atomizing edge and the target should follow this curve 59h as closely as possible. It will be apparent that the curve 59a of the conventional system is at the optimum atomizing voltage only at the l2-inch spacing and that material reductions in this spacingresult in voltages greatly in excess of the optimum with attendant deterioration of the desired quality of atomization.

However, if the control resistance used in conjunction with a 100 kilovolt pack is 1500 megohms, the resultant curve (identified as 59e) will be seen to follow the optimum atomization curve 59k in a very satisfactory manner. That is, the curve 59e is never more than a very few percent away from the optimum atomization voltage curve 59h at any point, and intersects the gap breakdown line 58 when the spacing is less than l inch and when the voltage between the atomizing edge and the target has dropped below 30,000 volts. However, the curve is such that it not only closely follows the optimum atomization curve S919 but also provides in excess of 40,000 volts across the gap at all gap spacings of 2 inches or more, so that satisfactory atomization and deposition take place at spacings from l2 inches down to as little as 2 inches. On the other hand, if the control resistance is of too high value, as for example, 50,000 megohrns with a 10,0 kilovolt pack, the resultant curve (identified as 59d) will be seen to be so low at normal working spacings illustrated on the chart that satisfactory atomization and deposition is not obtained.

In order to achieve the desired automatic maintenance of theair gapvoltage drop at or near the optimum atomization voltage in an electrostatic coating system of the character described and with present commercial power packs of 100 kv., the control impedance should be chosen to `absorb only a small percentage (as about 10%) of the applied voltage at the greatest gap space which is expected to be encountered in normal operation, but to` absorb at least as much voltage as the air gap resistance at the lesser air gap spacings which may be encountered in operation. Again referring to the curve identified as 59e in FIGURE 4, it will be seen that the drop across the control resistance is about l5 kilovolts at the l2-inc'n spacing, but is `approximately the same as the drop across the air gap when this spacing has been reduced to 2% or 3 inches. It will be understood that this will be true with a relatively sharp edged device of the character described, since the space current increases very materially as the gap spacing is reduced yand thus enables the control resistance to absorb the desired amount of voltage drop thereacross even though the air gap resistance decreases to an effective resistance of the same order as that of the control resistance. Moreover, it will be apparent from the chart that when the atomizing edge approaches as close as l inch to the target or article surface, in the neighborhood of 70% of the voltage drop exists across the control resistance with only the relatively small remainder existing at the air gap.

While higher source voltages and higher control impedance values are preferable within certain limitations, the cost of a power pack increases much faster than the increase in potential available from it, so that it is at present commercially desirable to limit the maximum voltage to be used to about 150 kilovolts. Under these conditions the control resistance should be limited to the order of 15,000 megohms as a maximum in order to retain desirable atomization voltage variations and satisfactory deposition. it is to be understood, however, that where expense is not an important factor, power packs can be built even today which provide voltages about double those mentioned above as being commercially desirable; and that improvements in power supply technology may well result in the ability to build even higher voltage packs for general commercial use. However, our experiments have indicated that while the control impedance should be at least several megohms per kilovolt, and preferably of the order of megohms per kilovolt or slightly more, it is undesirable to utilize more than about 100 megohms per kilovolt; .and this relationship. obtains throughout all voltage ranges which we believe likely to be used for electrostatic spray coating.

Another important factor in the utilization of high voltages for electrostatic spray coating is the matter of safety, both from the standpoint of ignition of the combustible mixture which exists when paint solvents volatilize in air, and the matter of shock to the operator. Certain solvents provide a more combustible mixture than others, one of the very commonly used paint solvents, xylene, providing a relatively quite ignitable mixture as it volatilizes into the air during spray painting. We have found that even if the charging electrode is brought up undesirably close to the object, to the point where sparking occurs, as at a one-half inch spacing with control resistances of the value mentioned above, this sparking is so weak, of such low average intensity of energy level under conditions where there is a low enough effective capacity beyond the control resistance, that even more ignitable mixtures than those normally encountered in a spray operation will not be ignited. Moreover, while a low resistance of the order of one-tenth of a megohm per applied kilovolt in systems of the type under diseussion will prevent lethal shocks .and death of the operator if the charging edge should be accidentally touched, suitable low effective capacities with control resistances of the values described results in such a low shock level to the operator as not to even be distressing in the event the charged edge should be accidentally touched or approached too closely.

Even though .a control resistance of 1000 or 2000 megohms is used, we have found that any high value of effective capacity beyond the control resistance (i.e., between it and the charging electrode portion) will result in a spark, upon close approach to an article or operator, which will ignite a highly ignitable mixture .and which will give such a distressing shock to the operator that few men will continue working with the system Where they are exposed to occasional accidental shocks of this type. We have found by experiment that with control resistances of the values described the effective capacity ahead of such resistance must be extremely low. A 4- inch metal bell member would have a capacitance several times the safe value or more, and well into the area where ignition and distressing shock would result even if a control resistance of many thousands of megohms were used; and thus the presence of a large object with its resultant geometric capacitance ahead of the control resistance presented another problem from the standpoint of safety.

We have overcome this difficulty by the use of what might be termed very high distributed resistance in the discharge electrode, so that a spark originating at any one point ydoes not provide a discharge of an intensity greater than that which can be tolerated. That is, even though the 4-inch bell member may have an actual capacitance (as measured to a plate spaced one-half inch from its charging electrode edge portion) of about 10 micro-l microfarads, the fact that the bell member has its main body portion of completely non-conducting material, with the special coating on the inner surface providing a very high point-to-point resistance, prevents the surface charge at one point on the bell from substantially instantaneously moving across to and being available for discharge at another point which is brought too close to the object or operator.

A conventional air spray gun or a conventional metal bell member of the type heretofore used in electrostatic spray coating would in both cases have such high capacitance and such good conduction of all the stored electricity to a point Where a spark `originates that neither would besafe, either as to ignition or distressing shock, even if control resistances of the values heretofore considered were used between them and the high voltage source. By making any metal small enough, and by mak.-

ing a large body with a high geometric capacitance of such high distributed resistance that the quantity of electricity stored therein is not instantaneously available at a sparking point this danger is obviated. ln achieving effective capacity safety, it will be understood that it is the energy level at a sparking point which we have found to be important, and that physically large devices which have true geometric capacitance beyond those which could be tolerated can be rendered safe in conjunction with our control resistances by having a. high distributed resistance therein or on the surface of' a relatively completely nonconducting body member. lt is also to be understood that there is some relationship between the value of the control Iresistance and the amount of capacity which can be tolerated; but we have found that with control resistances of the character described, as, for example, in the order of 1,000 rnegohms with kilovolts applied from the negative terminal of a pack having its positive terminal grounded, the energy level of a disruptive discharge, to va grounded polished metal sp ere of about one centimeter radius from an electrode ahead of the control resistance, should preferably not exceed that from a polished metal sphere having a radius of about one centimeter replacing the electrode in the same system, and in any event should not exceed that from a sphere having a radius of about three centimeters.

We have found that a satisfactory and reproducible measurement of the amount of effective capacity which can ybe tolerated can be obtained by using polished metal spheres of different diameters as the electrode beyond the resistance. Such a sphere has a capacitance to an infinite ground around it which varies as its radius and which is about l micromicrofarad per centimeter of radius. Moreover, the capacitance at a spacing of even a very few inches from. a grounded `object (such as a metal plate) is very close to the theoretical capacitance to an infinite ground. For example, a polished metal sphere having a radius of l centimeter has a capacitance to innite ground of 1.1 micromicrofarad; and even when a flat grounded metal plate is brougnt within 1/i inch of it, its capacitance as measured to the plate is only about 1.4 micromiorofared. Accordingly, a polished metal sphere mounted on the end of the resistance element being used can be brought closer and closer to a grounded metal member (as the sphere of one centimeter radius mentioned above) or to the end of the linger of an operator, until a disruptive discharge takes place, and the energy level of such discharge is very reproducible with a igiven ball, resistor and applied voltage. F or example, a polished steel sphere of 1 centimeter radius mounted on the end of 1000 megohm resistor provides an unobjectionable discharge at any applied voltage below and up to 100 kv. The discharge from such a combination does not provide a disagreeable shock tothe person and has too low an energy level to ignite mixtures of the most readily ignitable nature, such as a saturated mixture of hexane and air, at 3 F. and at atmospheric pressure.

Higher resistor values (as for example 4000 megohms) lower applied voltage (as 50 kv.), and somewhat less ignitable solvents commonly used in painting, as toluene or xylene, permit the effective capacity to be about that of a sphere having a radius of 3 centimete-rs without objectionable shock and without danger of ignition of the most combustible vapor-air mixture possible in the spraying zone, as a saturated mixture of toluene and airl at 62 F. or xylene and air at ll5 F. It will be understood that the use of a polished metal sphere is for comparative or test purposes only, and that such would not be used as a charging electrode in the normal electrostatic spray painting system. The energy level o-f the discharge from a polished metal sphere of l `centimeter radiusenergized through a 1000 niegohm resistor, however, is about the same as that of a in-ch wire having a diameter .of .O50 inch similarly energized. Such a wire may be used Very satisfactorily as a charging electrode when atomization is separately effected, as will be hereinafter more fully described, or may be considered analogous to the atomizing edge of a 4-inch diameter non-metallic bell with highly distributed resistance, as heretofore described.

While the power pack provides a high voltage supply which is commercially termed D.C., it wil-l be understood that this is not pure direct current in the absolute sense. In commercial practice the power pack has its input connected to a conventional 60 cycle alternating current supply, and the high voltage output is rectified and partially filtered to provide a unidirectional field-creating voltage; but there is, of course, still an appreciable amount of ripple Accordingly, while we prefer to luse a pure resistance as the control means, any impedance which achieves the desired control action may be utilized. Also, while the control action in the embodi ment just described is a result of the total effective resistance provided by two parallel resistances (one being the resistor and the other being the resistance of the paint column), it is to be understood that the desired control impedance can be achieved in a number of ways. For example, it may be achieved by the use of carbon, metallic oxide, or other commercial resistors; by equipment components made of or `coated with materials having the desired resistance characteristics; by the resistance of the paint column or film; by the inter-element impedance of a high voltage vacuum tube; or by combinations of the above or other suitable impedances.

Referring now more particularly to the embodiment of the invention shown in FIGURE 5, a modified form of a hand-held atomizing device is identified in general as 40. In this device three separate connections are made to the rear of the device, although for convenience the three s eparate conduits 41, 42 and 43 are brought together into a single flexible conduit assembly preferably covered with a grounded flexible metallic outer sheath 44. The bell member 45 is mounted upon a rotatable shaft 46 connected to a flexible drive shaft in the conduit 42. In this embodiment of the invention, the bell member 45 is again of non-conducting material, such as nylon, with a surface layer 47 of predetermined resistances; but in this case the resistance layer 47 is on the exterior of the bell member while paint flows on the inner surface, contact between the paint and resistive layer occurring only at the atomizing edge 45a. High voltage is brought in through the electrical conduit or 1ead.41, passes through two fixed resistors 48 and 49 which may be, by example, of 500 megohms each, with the two providing a total extent of about l2 inches. The forward end of resistor 49 is electrically connected to a very small metal leaf spring 50` which makes rubbing contact with a portion of the resistive coating 47 to transfer current thereto.

This embodiment of the invention includes a liquid control valve in the atomizing device itself, the actuating member for the valve being the trigger element 51 pivoted at 52 on a handle 53. When the parts are in the position illustrated a valve member 54 closes off the liquid supply conduit 43, being urged to closed position by a spring 55. When the lower end of the trigger element 51 is pulled toward the handle 53, however, its engagement with the Valve actuating member 56 lifts the valve 54 off its seat, and paint can then flow through the passageway 57 to the hollow shaft 46, and thus through the axial opening in the bell member to spread out in a film on the inner surface thereof and move to the atomizing edge as a result of the rotation of the bell.

In this embodiment of the invention the flexible shaft and the paint supply conduit may be grounded, since they are isolated from the high voltage electrical supply, and thus the rotating motor and the paint supply means may be merely set on the floor or be otherwise suitably supported, but without the need of any high voltage insulation. Moreover, since there is a control valve in the atomizing device, no pump need be used for the paint, but instead the paint can be supplied if desired from a simple pressure container (generally termed a pressure pot) having air under pressure (as at 30 pounds per square inch) over the paint. Moreover, the handle and trigger can be of metal and grounded by being electrically connected to the grounded metal sheath 44. This results in a situation where any failure of insulation could not result in shock to the operator.

While this embodiment has a series or control resistance analogous to that of the first described embodiment, it is to be noted that in this form in FIGURE 5 the resistance of the paint column extending from the valve chamber to the atomizing edge is in parallel with the air gap resistance between the edge of the bell and the article being coated, rather than being in parallel with the resistors 48 and 49. This results from the fact that the paint column is grounded at the valve portion of the device, so that it is in effect connected to the ,same terminal of the power pack as the article, the bell edge potential and the average potential gradient of the field, duringvariations in gap spacing between the bell and the article being thus controlled in part by the series resistors 4S and 49, and in part by the parallel impedance provided by the paint column. Where the paint has a resistance characteristic such that passageway diameters and lengths will fall within reasonable and practical values to achieve desired resistances, better control of the charging electrode portion voltage may under some circumstances be obtained. It is also to be understood that if the paint is kept completely insulated from ground and at high potential in this form, or is of such high resistance as to be substantially completely non-conductive, similarl control may be obtained by utilization of another impedance between the charging electrode (here the edge portion of the bell) and ground, in conjunction with the series resistance provided by the resistors 48 and 49.

In connection with the electrostatic atomization effected in both of the embodiments heretofore described, we have found that the presence of the high series resistance improves atomization by maintaining the atomizing edge at or near optimum atomizing voltage as the gap spacing is varied. It has heretofore been thought that at least one fifth of a milliampere of current was necessary for suitable electrostatic atomization, but We have found that we can not only retain good atomization and charging of the spray particles, but even improve these, with currents of the order of onetwentieth of this, or even less, as about l0 microamperes. v This discovery has .enabled us to use resistances of several thousand megohms in conjunction with the now generally commercially used power pack of kv.

Referring now more particularly to FIGURE 6, an embodiment is illustrated which retains the advantages of deposition efficiency and safetyfof the forms heretofore described, but which effects atomization by hydrostatic pressure, rather than electrostatically. The atomizing device shown in this figure is again of a type adapted to be held in the hand, being generally pistol-like with a grounded metal handle portion 60 and a barrel-like portion 6l about l0 inchesl in length made of non-conducting material suitable for high voltage purposes, as polyethylene. Paint is supplied through an insulating high pressure hose 62 made of a material such as nylon leading from a high pressure vessel not shown.

Paint is delivered through hose 62 to a hydrostatic atomizing nozzle 63. ln order to minimize wear as the paint passes through the very small oriiice of the nozzle, the nozzle should be made of some suitable wear resistant material such as a sapphire, stainless steel, or tungsten carbide. lf the nozzle is of conducting material such as stainless steel, it must be kept small enough to avoid an objectionable discharge because of its capacitance. That is, it should be small enough that the energy level of a discharge therefrom preferably should not exceed that which would be available, under the test conditions previously described, from a metal sphere of about one centimeter radius, nor in any event from a sphere of about 3 centimeters radius. Nozzle 63 is sealed against the end of hose 62 and supported in barrel 61 by a packing gland assembly `64 comprising a solvent resistant packing 64a which is compressed against the tube and nozzle by threaded nylon members 64b and 64C.

In order to secure pressure atomization of a neness suitable for quality coatings, the paint in this case would be subjected to a pressure of several hundred pounds or more per square inch in the pressure Vessel or supply chamber. The outlet orifice of nozzle 63 would preferably be elongated in cross-section, with its narrowest dimension only a few thousandths of an inch7 and its longer dimension 2 or 3 times as great. Such an atomizing nozzle discharges the paint into a fan-like iilm of suitable characteristics to achieve the desired atomization. in the embodiment illustrated, the paint supply would be be mounted on an insulated table or the like, as in the embodiment illustrated in FiGURE l, and would have the hot terminal of the high voltage connected thereto.

To provide the necessary insulation for hose 62 carrying paint at high voltage, an insulating conduit 61a extends from the insulating barrel 61 to the paint supply. Members 6l and 61a may be fabricated integrally, or, if welded together as shown, the weld must be capable of providing insulation between the high voltage paint hose 62 and grounded handle 60. A grounded sheath 60a such as a braided metal may be provided over conduit 61a to insure all exposed surfaces of the gun up to barrel 61 are at ground.

The paint column itself in hose 62 provides the high resistance conductor for the high voltage applied to nozzle 63. The control impedance provided by the resistance of the paint column not only provides safety against objectionable discharges but also achieves the highly advantageous control action on average potential gradient variation during gap variations in accordance with our invention as previously described. Coating materials and their solvents possess widely Varying degrees of electrical conductivity and inorder to control the total resistance of the paint column within tube 62 a metal wire 65 may be inserted in tube 62l extending from the high pressure supply to such point within tube 62 such as will give the length of paint column required to provide the desired sole control resistance between the high voltage source and nozzle 63.

The supply of paint to the nozzle may be controlled by a trigger 66 mounted on the handle. In the embodiment shown, operation of trigger 66 depresses a spring 67 and makes electrical contact between the terminals or" a switch 68 which through wires ,`69 control the operation of a valve controlling the supply of paint through tube '62. This valve is preferably of the three-way type which provides a connection between the high pressure vessel and nozzle 63 when contact is made in switch 68 and which liti vents tube 62 to the atmosphere when the contact is broken.

important control and safety aspects of this invention can also be embodied in systems wherein the depositing eld extends from the article to an opposite electrode which is not a part of lthe atomizing device, and a representative system of this type is shown Ain FIGURES 7 and 8. Four small electrode wires 71a-d are arranged horizontally in a single plane with their ends one foot apart at what may be considered the corners of a square, as may best be seen in FIG-URE 8. These field electrodes are supplied with current through control resistance means 72a-d, these resistances having values of the character heretofore described and being of sufficient length that there is no danger of arcing around from the electrode wire 71 to the lead 73 from the high voltage supply pack 74. A conventional air spray gun 75 is illustrated as mounted on a support pedestal 76, the gun and pedestal being at ground potential and uninsulated, although it is to be understood that the spray gun can be hand-held and freely movable. Paint from a supply tank 77 is delivered through the hose 78 when compressed air is supplied to the gun through the hose 79. The axis of the spray gun intersects the center of the square formed by the tips of the electrode wire 71, but to one side of the plane of the electrode wires, as for example, 6 inches therefrom. On the opposite side of the plane of the electrodes is an article 80 here illustrated as a breadbox, carried by a conveyor spindle 81 with rotating means including the wheel element S2 bearing against a rotating rail 83.

It will be understood that a succession of such articles are moved through the coating zone by the conveyor, and their rotation during the coating causes the corners to be much closer to the electrode wires than the center of the wall panels. It is also to be understood that a slightly bent or loose spindle or an article improperly positioned on the spindle would, of course, result in even greater variations in the distance between the spray charging electrodes 7l and the article being coated. In the past this has been taken care of from a safety standpoint by utilizing a much greater electrodeto-article spacing than would be `desirable from the standpoint of deposition e'iciency. Where the closest approach of an article portion anticipated to occur in normal operation is double the sparking distance for the voltage used, it is obvious that the average potential gradient Will be at about the minimum of the desirable working range when such closest portion is presented; and that when the center portions of the walls of the bread-box illustrated are being presented toward the spray gun, or if the article wobbles somewhat further away as a result of a bent spindle, the field gradients will be substantially below the desired value. This results in the Vair blast blowing away more spray particles than would be the case if the average field gradient were at all times at a desirably high level. While electrostatic effects have advantage with average potential gradients of about 5,000 volts per inch at the six to twelve inch spacings being discussed, average gradients of well over 10,000 volts per inch are desirable for maximum deposition eiciencies in Va system of this character.

`ln the system illustrated, the side walls of the breadbox, when presented directly toward the spray gun by a straight spindle having the article properly positioned thereon, may be located only 6 inches from the electrode wires 71 although a 100 kv. power pack is being used, With a resistor 72 absorbing about 10% of the voltage, about 90,000 volts would exist on the electrode wires 71 and the average potential gradient of the field to the article under the conditions just described would thus be about 15,000 volts per inch. Even if a bent spindle or any improper positioning or swaying of the article results in the distance increasing a couple of inches, the average potential gradient is still in excess of 10,000 volts per inch. Yet as the article rotates and presents a corner directly toward the spray gun, or if some other factor 17 causes it to corne closer, reduction in the gap distance between the electrode Wires and the closest article portion (as for example to 3 inches) still permits an operative and highly efiicient field to exist, the resistor absorbing sufficient voltage drop in the system to prevent any disruptive discharge down to a very close spacing, as one-'half inch or one inch, and prevent it from being objectionable even then.

We have found it desirable to have the electrodes of appreciable length, extending preferably at least several inches from the resistors, a very satisfactory embodiment of this invention having lbeen constructed with each electrode wire being 6 inches long. The highly ionizing effect of a small wire (as ten thousandths of an inch in diameter) repels the paint and not only charges lthe spray but also creates conditions which keep the spray from painting back onto the remotely located sheaths or covers of the resistance elements, which is unsatisfactory. Moreover, the sharp configuration of such a ne wire, with attendant higher space currents across the air gap than would occur from a polished sphere or other blunt electrode, has a desirable action in connection with the control effect of the resistor, and in minimizing the elective capacity of a given electrode size. The conditions as to capacitance must still be observed, however, and the size of the metal electrode should be kept small enough that the discharge therefrom should preferably not exceed that from a metal sphere of one centimeter radius, or in any event should not exceed that from a metal sphere of three centimeters radius.

in :addition to using electrode wires having a length of at least several inches, but not exceeding a suitable capacitance, we find that where a plurality of electrode wires are used and supplied with current through independent control impedances, the electrodes should be spaced at least several inches apart. If the electrodes are too close together an operator or article can touch several of them at once and this would result in an objectionable discharge since the capacities and impedances would all be effectively in parallel under such conditions. With the use of a system of the character embodying the above discussed factors, however, safety consider-ations are retained while achieving high average field intensities despite appreciable vari-ations in gap distance, with resultant improved deposition eiiiciency. in a system constructed in accordance with the Idisclosure of FIGURES 7 and 8, for example, used in the spray coating of met-al broom handles 1 inch in diameter and spaced on 3 inch centers, deposition efficiency with such -a system was about 10% better than with the same `electrode and spr-ay gun arrangement without the control impedances `and with the path of article movement then being l-2 inches from the electrode rather than 6 inches in View of the necessity of being double the possible arcing distance.

In a system utilizing charging electrodes separate from the atomizing device, as illustrated in FIGURES 7 and 8, laverage gradients which are in excess of 10,000 volts per inch, and preferably 15,000 volts per inch or more, are desirable for maximum `deposition eiiiciency at spacings of six to twelve inches, for example. In the absence of a control resistance the minimum gap space between the electrode and :article would normally be required to :be about 12 inches with la 100 kv. pack, with the average potential gradient then being a little less than 10,000 volts per inch and dropping rapidly as the gap is increased by rotation of the article, or swaying of the Iarticle away from the electrode. We have found that very much improved deposition efficiencies can be achieved, particularly upon articles presenting a substantial amount of open space relative to the metal surface lbeing coated, by not only utilizing control resistance means but also by materially raising the power pack voltage beyond that which would otherwise normally be used, and have obtained very desirable results by use of a 160 kv. power pack with .appropriate control resistances. For example, if a 5,000 megohm control resistance is used with a 160 kv. power pack, the spacing arrangement can be such that if the minimum spacing encountered is 4 inches, the average potential gradient at that spacing would be about 18,000 volts; and yet at a l2-inch spacing the gradient would still be about 10,000 volts per inch. This is less than a 2. to 1 change of average gradient for a 3 to l change in gap spacing; and enables the average gradient to normally be at or about 15,000 volt per inch, with the gradient at the large l2-inch spacing still being about 10,000 volts per inch.

In general, and speaking of all of the embodiments of our invention heretofore described, it will be understood that it is preferable to utilize both a suitable control impedance and a low effective electrode capacity. The geometry and surface area of atomizing devices of the character heretofore generally used commercially may provide yan objectionable disruptive discharge even when suitable control impedances areused in series therewith. We have found that large area atomizing devices, such as latomizing bells or hand guns effecting mechanical atomization can have their effective capacities reduced fto Ia suitable value by being made substantially entirely of non-conductive material with a high distributed resistance. Where it is desirable to use a metal electrode, making it not only of small capacitance and preferably relatively sharp, as by using a wire electrode of small diameter, achieves the desired low effective capacity. Moreover the automatic contro-l action of the impedance element as shown is completely reliable in operation, without any complex electronic circuits or equipment subject to failure in operation.

The optimum value of the control resistors varies with the amount of actual or effective impedance present in the remainder of the electrostatic system, and particularly in the power supply. Even though the actual specific or ohmic value of the resistance in the power supply may be relatively small, the power supply can provide a large effective portion of the impedance in the system by virtue of its voltage regulation characteristics. Accordingly, it will be understood that the ohmic value of the resistors illustrated and heretofore described as examples of our invention will desirab-ly be made larger or smaller depending upon the total effective impedance of the electrostatic system, including the effective impedance ofthe power supply itself, being smaller when the effective impedance of the power supply is larger.

Referring now more particularly to the embodiment of our invention illustrated in FIGURES 9 and l0, we

again utilize a control impedance system in connection with atomizing and charging means of low effective capacity. In this form of our invention the effective system impedance still is very high, with a substantial amount of resistance still immediately adjacent the charging electrode, but with the power supply providing an effective portion of the system impedance.

Referring first to the schematic diagram shown in FIGURE 9, the atomizing and charging device 1011 is represented as a rotating bell having a very high resistance to reduce its effective capacity a-t least to below that of a sphere of polished metal of 3 cm. radius, and preferably of only l cm. radius. This atomizing and charging device may in all respects be analogous to the device 16 illustrated in FIGURES 1 and 2, and therefore it will not be more fully described here. The atomizer 101 is `adapted to be suitably spaced from each of a succession of articles conveyed through the coating zone, one such grounded article being identified here as 102. A. particle charging and depositing electrostatic field is created between the atolnizer 101 and the article 102 by the utilization of a power supply and suitable connections including a high resistance 103 of substantial value immediately adjacent and connected to the atomizer 101.

The various modifications of our invention illustrated in FIGURES l to 8 and heretofore described in this specification have been described as energized by a conansa/tee ventional commercial power pack of goed quality, a type of pack which has good regulation characteristics and maintains its output voltage close to its rated value despite substantial increases in load current. Moreover, all practical commercial power supplies known to be used lin electrostatic spray coating before the invention illustrated herein in FIGURES 9 and l() maintain sufficient volta-ge regulation (even Ithough relatively poor), that they were not sufiiciently self-regulating for optimum electrostatic atomization, or fully safe against fire or shock hazard, in Ithe absence of additional high resistance of the order of the minimums heretofore described in this application, even Iwhen utilizing very low effective capacity of the charging means cooperating with the article to produce the particle charging and depositing electrostatic field. The power supply shown herein, however, produces suicient effective system impedance, sufiicient drop of output voltage upon increase `of load current between the charging electrode and the article being coated, that the resistance external to the pack can be lower than that used in connection with a well regulated voltage supply, and can be approximately the same in megohms, or at least of the same order as, -t-he kilovolts being ap*- plied to the charging electrode at any given instantaneous operating condition.

While a power packof the ladder type has been known to us, we have known that such a pack will not in and of itself, without the substantial control impedances of the character heretofore discussed, effect adequate control of voltage during variations of spacing between the particle charging means and the article. However, we have found that if such a power pack is associated 4with an input network of a particular type as hereinafter more fully described, that satisfactorily effective `impedances for the coating system can be provided by the use of a resistance externalfto the pack, and preferably closely adjacent to the charging electrode, of a rvalue of at least about l megohm per kilovolt of potential applied to the field creating electrode at the lowest range of spacings expected during normal operation, and in any `event at least 40 megohms; provided, of course, that the charging electrode (the atomizing bell as illustrated in FIG- URE 9) has a low effective capacity of the small values called for throughout this application.

The ladder-type power pack identified in general as 104e: is supplied with alternating current through an input network identified in general as `ttt-tb, a network which materially increases the Vreduction of D.C. output voltage upon increases in load current. In fact, in one particular power supply of this character which we have built, the ladder type power pack itself, when supplied with an input of 110 volt, 60` -cycle alternating current (but without any input network), provided an output voltage of about 95 kilovolts at 20 microamperes of output current, 60` kilovolts at 0 microamperes, 53 kilovolts at 60 microamperes, 48 kilovolts at 70 microamperes, and 45 kilovolts at 80 microamperes. It will be noted that while there was an initial substantial drop in voltage upon an increase of output or load current from to 50 microamperes, that thereafter the voltage reduction decreased markedly and the curve of voltage reduction flattened out appreciably (and the voltage remained at a relatively high value) at output currents indicating a relatively close approach of the charging electrode to the article being coated. On the other hand, the same power pack when supplied through an input network of the character illustrated here, with 'an input voltage of about 110 volts of 60 cycle A.C., ex-

hibited a markedly greater reduction in output voltage at the higher levels of output current. A-t 20 microamperes output current (fully satisfactory for electrostatic atomization) the output voltage -w'as 85 kilovolts, which would be satisfactory for article-electrode spacings of 8 or 10 inches; at 50 microamperes, however, the output voltage was only 43 kilovolts; at 60 microamperes it was 30 kilovolts; at 70 microamperes the output voltage was 23 kilo- 2t) volts; and at microamperes it was 18 kilovolts. Moreover, tby the time the output current had reached 84 microamperes, in the power supply now being described, it dropped in output voltage to l0 kilovolts, whereas the ladder network alone (without the input network) was still 43 kilovolts.

It will -thus be apparent that a large portion of the effective system impedance (the electrical changes in voltage across the air gap whichtake place upon variations in the gap spacings or distance) can thus reside to a substantial extent in the power supply and need be supplemented only by a high resistance between it and the charging electrode, preferably near the electrode to minimize the effect of capacity present in the connecting lead or cable, which has a value in megohms approximately equal to the kilovolts applied to the field-creating electrode at the lower of the spacings expected to be encountered in normal operation, as 3 or 4 inches. The total system impedance, including this high resistance external to the power supply, under such circumstances is suflicient to provide optimum electrostatic atomizing voltages during variations in spacings between the electrostatic atomizer and the article being coated, and safety from fire and shock under any conditions of electrostatic coating when this impedance is combined with adequately low effective electrode capacity.

To describe the circuit in more detail, it will be seen that one leg or side of the ladder power supply network comprises (in the circuit illustrated in FIGURE l0) four capacities ltSa and lliSd, inclusive, connected in series between ground and the direct current output terminal. The other leg of the ladder network similarly comprises four capacities connected in series, here identified as 166:1 to 106d, inclusive, the capacities in this case being connected between one terminal of a transformer secondary 107b (having its other terminal connected to the other or first described leg of the ladder network), and the other end being connected to rectier 168k. Intermediate the two legs are a plurality of alternately oppositely connected rectifiers ltlSa to 198k, inclusive, these rectiiiers and their connections forming what are sometimes termed the crossconnections between the legs of the latter network. In operation of a network of this type, when a voltage is first developed across transformer secondary 1071) it results in charging condenser 106a. When the polarity reverses in the transformer secondary voltage impressed upon condenser a by the secondary is supplemented by voltage and current delivered from the condenser 10611 through rectifier 10811. As the process continues up the ladder, an output voltage is developed which is many times multiplied over the maximum A.C. voltage developed in the transformer secondary ltl'lb. However, as mentioned heretofore, such a network is not in and of itself suiiicient to provide proper control to achieve optimum electrostatic atomizing voltage during variations `in spacing from an article being coated, nor -to provide adequate safety against fire and shock hazard.

As mentioned hereinabove, we have found that more I of the effective impedance of the entire coating systems circuit can be provided 4in the power supply by feeding the input of the ladder network from a reactive bridge, as illustrated in the lower part of FIGURE 10. This bridge input network has two opposite corners 10961 and 109b supplied with the alternating current input, as conventional 60 cycle 11() volt power; and the other two corners of the bridge, here identified as l10n and 110b, are connected to and provide voltage to the primary 107er to energize the utilized capacitors 112e and 11211 of a 6.75 mfd. capacity; and as inductances used coils which had a D.C. resistance of 90 ohms and an inductance which varied from about 9 henries with a 115 volt input and no load on the output of the bridgenetwork to about 35 henries at short circuit on the output. In the particular power supply we are describing this coil was provided by winding 1160 turns of No. 34 wire in twelve equal l-ayers on the center leg of a closed (El) core with a 3A stack of 5/8 wide laminations of good grade of magnetic iron, sometimes termed dynamo grade.

We have found that it is not desirable to have a balanced bridge for the input network. Instead, we prefer a bridge relationship wherein the output voltage does not drop markedly until a predetermined current has been reached and then drops off as rapidly as possible thereafter. We have found that this is best effected by using capacitors with a reactance much less than that of the other components of the bridge, in the case given above the capacitors having a reactance of only about //l@ of that ofthe inductances at no output load and with a 60 cycle input.

It will be understood that a reactive input system for energizing the ladder-type power pack can be provided by suitable arrangements other than the capacitance-inductance bridge illustrated and just described. For example, a limited current transformer of the type illustrated in FIGURE 3 of Sola Patent 2,212,198 (with a characteristic curve of output voltage relative to load current as illustrated in FIGURE 7 of that patent) is very satisfactory when suitably designated for the load currents ydesired in the electrostatic coating system utilizing. the output of the ladder-type pack. Y

In spite of the very greatly increased voltage dropping characteristic of such a power supply upon reduction in spacing between the electrode and article and attendant increase in output current, we have found that satisfactory operating conditions cannot be provided by such a power supply alone. We find it of extreme importance that the electrode system be made of very low effective capacity, in any event as low as that which would be provided by a polished steel ball of 3 centimeter radius at the same point in the system, and preferably not above the effective capacity which would be provided by a polished steel ball of l centimeter radius; and that the system include a high resistance in the connection between the power pack output and the charging or field-creating electrode, a resistance of at least 40 megohms. While the impedance throughout the entire field-creating circuit effective upon the voltage across the gap between the electrode and article at low 'spacings is of considerable importance, we v find that use of an electrode system of low effective capacity, and an associated resistor in the circuit having a high resistance value and preferably closely adjacent the charging electrode, are of absolute necessity in achieving the operation desired by us.

While we have shown and described certain embodiments of our invention, it is to be understood that it is capable of many modifications. Changes, therefore, in the construction and arrangement may -be made without departing from the spirit and scope of the invention as disclosed in the appended claims.

We claim:

1. Apparatus for spray coating a plurality of articles, comprising an atomizing device adapted to be held in the hand and having at one end thereof a rotating member having an atomizing and ionizing charging portion with an extent of at least several inches, said member comprising high resistance material and the physical dimensions and resistivity of the member being such as to make it of low effective capacity, not greater than that of a metal sphere of a radius of about three centimeters; conveyor means for translating articles substantially at ground potential successively along a predetermined path through a coating zone where atomized particles are deposited thereon, the distance between said atomizing device and the articles being subject to variation which could in a conventional electrostatic spray coating systemcause an objectionable change in potential gradient; voltage supply means for creating a high unidirectional potential difference of at least forty thousand volts and of the order of at least five thousand volts per inch average gradient between said charging portion and the closest article portion to create a depositing field, the connection between the voltage supply means and said charging portion being completed throu-gh a high resistance immediately adjacent said charging portion of the electrode and including said high resistance material, and having a resistance of at least the order of several hundred megohms and at least several megohms per kilo- Volt of the voltage source, but not in excess of about one hundred megohms per kilovolt, whereby changes in average potential gradient resulting from variations in the distance between the articles and atomizing device is materially reduced and objectionable discharges are pre- Vented; a handle portion on the atomizing device beyond arcing distance from said charging portion; and means for supplying a controlled iow of liquid coat-ing material having some conductivity but relatively high resistance to said atomizing device for spreading out in a thin annular film on said rotating member for electrostatic atomization therefrom into the depositing field for deposition of spray particles on the article while still in liquid state, the voltage supplied from said source being substantially in excess of the voltage -which would be optimum for said electrostatic atomization in relation to the distance between the atomizing portion and an article at at least the lesser distances in the absence of said high resistance and its effect on changes in the average potential gradient.

2. Apparatus of the character claimed in claim l, wherein the resistance of said liquid coating material is electrically in parallel 'with said high resistance.

3. Apparatus of the character claimed in claim l, wherein s-aid atomizing device includes flow control means having a control element adjacent said handle portion, said control element and iiow control means being grounded whereby the resistance of said liquid coating material is electrically in parallel with the space between the atomizing portion and the grounded article being coated.

4. Apparatus for spray coating an article, comprising an atomizing device having an atomizing zone adjacent one end thereof adapted to be held in the hand and comprised substantially of non-conductive material, Said device -having -a very low effective capacity, not greater than that iof a metal sphere of a radius of about three centimeters, the distance between said device and the article being subject to variations which could in a conventional electrostatic spray coating system cause an objectionable change in potential gradient; voltage supply means for creating a high unidirectional potential difference of at least forty thousand volts` and at least sever-al thousand volts per inch average gradient between said atomizing device and the closest article portion to create a depositing eld, the connection between the voltage supply means and the atomizing zone of said device being completed through high resistance means having at least a substantial portion thereof immediately adjacent said zone yand having a resistance of at least several megohms per kilovolt of the voltage source; and means for supplying a controlled flow of liquid coating material to said atomizing device for atomization therefrom into `the depositing field for deposition of spray particles on the article while still in liquid state, the voltage supplied from said source being in excess of the voltage which could be used at at least the lesser distances between the atomizing device and article in the absence of said high resistance means and its effect on changes in the average potential gradient.

5. Apparatus of l the character claimed in claim 4, wherein said atomizing device has a handle portion beyond arcing distance from the atomizing zone, and wherein atomization is effected by fluid pressure.

6. Apparatus for spray coating a plurality of articles,

comprising charging electrode means having a plurality of separate ionizing portions several inches apart and each with an appreciable extent and each having an effective capacity not greater than that of a metal sphere of a radius of about one centimeter; conveyor means for translating conducting articles at ground potential successively along a predetermined path passing adjacent but at least several inches from said charging electrode means, the distance between said electrode means and the articles being subject to variations which could in a conventional electrostatic spray coating system cause an objectionable change in potential gradient; voltage supply means for creating `a high unidirectional potential difference of at least forty ,thousand volts and at least several thousand volts per inch average gradient between said ionizing portions and the closest article portion to create a depositing held, the connection between the voltage supply means and each said ionizing portion being separately completed through high resistance means immediately adjacent each said charging portion and having a resistance of at least several megohms per kilovolt of the voltage source; and means for supplying a controlled flow of liquid coating material and for atomizing it into the depositing ield for deposition of spray particles on the article while still in liquid state, the voltage supplied from said source being in excess of the voltage which could be used in the absence of said high resistance means and its effect on changes in the average potential gradient.

7. Apparatus for electrostatically depositing liquid coating material on an article, comprising a source of unidirectional high potential of at least forty thousand volts, an ionizing charging electrode electrically connected to said source, means for supporting the article to be coated substantially at ground potential, means connecting said article in circuit with said high potential source to establish an electrostatic iield between said charging electrode and said article, a source of liquid coating material, and means for atomizing said `liquid coating material into said electrostatic held, the distance between said charging electrode and the article being subject to variations which could in a conventional electrostatic spray coating system cause an objectionable change in potential gradient, said charging electrode being composed o such material and so dimensioned that its effective capacity is not greater than that of a metal sphere of a radius of about three centimeters, said means connecting said current source to said charging electrode including resistance means having at least a substantial portion thereof immediately adjacent the charging electrode and of the order of ten megohms per kilovolt applied by said potential source to minimize variations in the potential gradient of said electrostatic field that would otherwise occur as a result of variations in said distance, whereby at all distances objectionable discharge of electrical energy of a character to initiate a re or shock to the operator is avoided.

8. Apparatus for electrostatically depositing liquid coating material on an article, comprising a source of unidirectional high potential; an ionizing charging electrode electrically connected to said source; means for supporting the article to be coated; means connecting said article in circuit with said high potential source to establish an electrostatic held between said charging electrode and said article; a source of liquid coating material; and means for atomizing said liquid coating material into said electrostatic field, Ithe distance between said charging electrode and the article being subject to variations which could in a conventional electrostatic spray coating system cause an objectionable change in potential gradient, said charging electrode being composed of such material and so dimensioned that its effective capacity is not greater than that of a metal sphere of a radius of about one centimeter, said means connecting said high potential source to said charging electrode including resistance means having at least a substantial portion thereof immediately adjacent the charging electrode and of magnitude sutlicient to minimize variations in the potential gradient of said electrostatic iield that would otherwise occur as a result of variations in said distance, whereby at all distances objectionable discharge o electrical energy of a character to initiate a tire or shock to the operator is avoided.

9. Apparatus for electrostatically depositing liquid coating material on an article, comprising a source of unidirectional high potential; a charging electrode electrically connected to said source; means for supporting the article to be coated; means connecting said article in circuit with said high potential source to establish an electrostatic field between said charging electrode and said article; a source of liquid coating material; and means for atomizing said liquid coating material into said electrostatic eld, the distance between said charging electrode and the article being subject to variations which could in a conventional electrostatic spray coating system cause an objectionable change in potential gradient,`

said means connecting said high potential source to said charging electrode including resistance means immediately adjacent the charging electrode, the physical dimensions and electrical conductivity of the charging electrode, the total electrical resistance of said circuit between any discharge point on said charging electrode and said source of high potential and the magnitude of said high potential being such that for any distance, the total flow of current between the electrode and article, resulting from the potential applied to said circuit and the discharge of the capacitance of said charging electrode, has -a value and duration less than that which will initiate a disruptive discharge of an energy level so high as to be objectionable.

l0. Apparatus for lelectrostatically depositing liquid coating material on an article, comprising a source of unidirectional high potential; a charging electrode electrically connected to said source; means for supporting the article to be coated; means connecting said article in circuit -with said high potential source to establish an electrostatic field between said charging electrode and said article; a source of liquid coating material; and means for atomizing said liquid coating material into said electrostatic field, the distance between said charging electrode and the article being subject to variations which could lin a conventional electrostatic spray coating system cause an objectionable change in potential gradient, said means connecting said current source to said charging electrode including resistance means immediately adjacent the charging electrode and of at least several megohms per kilovolt of the high potential source, the physical dimensions and electrical conductivity of the charging electrode being such that the effective capacity is not greater than that of a metal sphere of a radius of about one centimeter, the total electrical resistance of said circuit between any discharge point on said charging electrode and said source of high potential and the magnitude of said potential being such that for any distance, the total flow of current between the electrode and article, resulting from the potential applied to said circuit and the discharge of the capacitance of said charging electrode, has a value and duration less than that which will initiate a disruptive discharge of an energy level so high as to be objectionable.

l1. A method of electrostatically spray coating articles in an electrostatic depositing field created by a unidirectional high potential between a charging electrode and another ,electrode substantially at ground potential and comprising at least one of the articles and wherein the distance between said charging electrode and said other electrode being subject to variations which could in a conventional electrostatic spray coating system cause an objectionable change in potential gradient, comprising providing a distributed resistance in a rotating atomizing 25 device suiiicient to limit the effective capacity to not in excess of that of a metal sphere of a radius of about three centimeters, flowing a stream of liquid coating material having some conductivity but relatively high resistance to the atomizing device, spreading it out in a thin annular film thereon, atomizing spray particles into said field from the edge of said film whereby said particles re charged to affect their deposition on the articles while still in liquid state, providing lresistance means including said distributed resistance `from the edge of the film tothe high potential source of at least several megohms per kilovoit, and supplying a voltage substantially in excess f the voltage which would otherwise be optimum in relation to the distance between electrodes at at least the lesser distances during operation.

l2. A method of electrostatically spray coating articles in an electrostatic field created by a unidirectional high potential between a charging electrode and another electrode comprising at least one of the articles and wherein the distance between said charging electrode and said other velectrode being subject to variations which could in a conventional `electrostatic spray coating system cause an objectionable change in potential gradient, comprising iiowing la stream of liquid coating material to an atomizing device, atomizing spray particles into said vield, whereby `said particles are charged to aiect their deposition on the articles, and automatically continuously changing the voltage between the electrodes as a direct function of changes in the distance therebetween to materially minimize variations in the average field gradient which would otherwise accompany such changes in distance, while maintaining the energy level of 4a discharge from the charging electrode below .that which would be provided by a metal sphere of a radius of about three centimeters in the same electrostatic system.

13. A method of electrostatically spray coating articles ina high potential electrostatic depositing iield or" at least forty thousand volts between a charging electrode and another electrode comprising at least one of the articles and wherein the distance between said charging electrode and said other electrode being subject to variations which could in a conventional electrostatic spray coating system cause `an objectionable change in potential gradient, comprising flowing a stream of liquid coating material to an atomizing device, atomizing spray particles into said eld therefrom, whereby said particles are charged to affect their deposition on the articles, providing a connection to the charging electrode having a high resistance of at least several hundred rnegohms and having an extent of at least several inches immediately adjacent the electrode, and applying to the kend of `said resistance connection farthest from the charging electrode a unidirectional potential higher than the potential which would cause sparking at the minimum gap distance between electrodes which would be encountered in normal use of the spray coating system, the charging electrode having an effective capacity not greater than that of a metal sphere of a radius of about one centimeter.

i4. ln an electrostatic coating system wherein liquid' coating material is atomized in response to a unidirectional voltage gradient between an article and an atomizing electrode and the atomized coating material is, moved toward the article in response to said voltage gradient, wherein a value of said gradient having an optimum atomizing ctiect exists for each spacing between said article and `said electrode, and means for automatically adjusting said gradient for optimizing atomizing eflect in accordance with variations in spacing.

15. Apparatus for electrostatically depositing vliquid coating material on an article, comprising, a source of unidirectional high potential; a charging electrode electrically connected to said source; means for supporting the article to be coated; means connecting said article in circuit with said high potential source to establish an electrostatic field between said charging electrode and said article, said circuit including high resistance means; a source of liquid coating material; and means for atomizing said liquid coating material into said electrostatic field, the distance between said charging electrode and the article being subject to variations which could in a conventional electrostatic spray coating system cause an objectionable change in potential gradient, said charging electrode being composed of such material and so dimensioned that its effective capacity is not greater than that of a metal sphere of a radius of about three centimeters, whereby at all distances objectionable discharge of electrical energy of a character to initiate a re or shock to the operator is avoided.

16. Apparatus of the character claimed in claim l5, wherein said effective capacity is not greater than that of a metal sphere of a radius of about one centimeter.

17. Apparatus for electrostatically depositing liquid coating material on an article, comprising, a source of unidirectional high potential; a charging electrode electrically connected to said source; means for supporting the article to be coated; means connecting said article in circuit with said high potential source to establish an electrostatic field between said charging electrode and said article; and means for atomizing said liquid coating material into said electrostatic field, the distance between said charging :electrode and the article being subject to variations which could in a conventional electrostatic spray coating system cause an objectionable change in potential gradient and said charging electrode vbeingcomposed ot such material and so dimensioned that its efiective capacity is not greater 'than that of a metal sphere ot a radius of about three centimeters, said effective capacity and the potential applied thereto being such that at all distances between the charging electrode and article objectionable discharge of electrical energy of a character to initiate a fire or shock to the operator is avoided.

18. Apparatus of the character claimed in claim 17, wherein said effective capacity is not greater than that of a metal sphere of a radius of about one centimeter.

19. A method of electrostatically spray coating articles in an electrostatic field created by a unidirectional high potential between a charging electrode and another electrode comprising at least one of the articles and wherein the distance between said charging electrode and said other electrode being subject to Ivariations which could in a conventional electrostatic spray coating system cause an objectionable change in potential gradient, comprising flowing a stream of liquid coating material to an atomizing device and atomizing spray particles into said field, whereby said particles `are charged to affect their deposition on the articles, and automatically continuously changing the voltage between the electrodes, by means including a high resistance, by and as a function of changes in space current across said distance to materially minimize variations in the average iield gradient which would otherwise accompany such variation in distance, while maintaining the energy level of a discharge from the charging electrode below that which wouldbe provided by a metal sphere of a radius of about three centimeters in the same electrostatic system.

20. In an electrostatic system for spray coating an article `w'herein an atomizing and charging device adapted to be held in the hand has a high potential electrostatic eld between it and the article and spray particles are electrostatically attracted toward the article and deposited thereon Wh-ile still in liquid state, an atomizing and charging device comprising: a portion on said device and adapted to face toward the article, said portion being comprised substantially entirely of non-conductive material and having a very low effective capacity, not greater than that of a metal sphere of a radius of about three centimeters, the distance between said device and the article being subject to variations which could in a conventional electrostatic spray coating system cause an objectionable change in potential gradient; means adapted to be conspaanse 2? nected to a voltage supply for creating a high unidirectional potential difference between said portion and the Vclosest article portion to create a depositing eld, the potential creating means including high resistance means having at least a substantial part thereof adjacent said portion and having a resistance of at least several megohms per kilovolt of the voltage applied to the device; and means for supplying a controlled ilow of liquid coating material to said yatomizing device for atomization therefrom into the depositing held, the voltage supply from the voltage source being in excess of the voltage which could be used at at least the lesser distances between the atomizing device and the article in the'absence of said high resistance means and its effect on changes in the average potential gradient.

2l. Apparatus of the character claimed in claim 210, wherein the portion adapted to face toward the article comprises a rotating member of very high resistivity.

22. In an electrostatic system for spray coating an article wherein an atomizing and charging device adapted to be held in the hand has a high potential electrostatic field between it and the article `and spray particles are electrostatically attracted toward the article and deposited thereon while still in liquid state, the atomizing and charging device comprising: a portion on said device and adapted to face toward the article, said portion being comprised substantially of non-conductive material and having a very low erective capacity, not greater than that of a metal sphere of a radius of yabout one centimeter, the distance between said device and the article being subject to variations which could in a conventional electrostatic spray coating system cause an objectionable change in potential gradient; means adapted to be connected to a voltage supply for creating a high unidirectional potential difference between said portion `and the closest article portion to create a depositing eld, the potential creating means including high resistance means having a megohm resistance of at least the same order as the kilovolts of the voltage `'applied to the device; and means for supplying a controlled flow of liquid coating material to said atomizing device for atomization therefrom into the depositing held, the voltage supplied from the voltage source being in excess of the voltage which could be used at at least the lesser distances between the atomizing device and the larticle in the absence of said high resistance means and its elect on changes -in the average potential gradient.

23. Apparatus for electrostatically depositing liquid coating material on an article, comprising, a source of unidirectional high potential; a charging electrode electrically connected to said source; grounded means for supporting the article to be coated; means connecting said article in circuit with said high potential source to establish an electrostatic iield between said charging electrode and said article, .said circuit having a high effective impedance and including high resistance means of at least one megohm per kilovolt adjacent said charging electrode; a source of liquid coating material; and means for atomizing said liquid coating material into said electrostatic held, the distance between said charging electrode and the article being subject to variations which could in a conventional electrostatic spray coating system cause an objectionable change in potential gradient, said high effective impedance minimizing vari-ations in average potential gradient of said field during relative movement between said article and electrode, said charging electrode being composed of such material and so dimensioned that its eective capacity is not greater than that of a metal sphere of a radius of about three centimeters, whereby at all distances between the charging electrode and article objectionable discharge of electrical energy of a character to initiate ya lire or shock to the operator is avoided.

24. Apparatus for electrostatieally depositing liquid coating material on an ar-ticle, comprising, a source of unidirectional high potential; a charging electrode elec- 2d trically connected to said source; grounded means for supporting the article to be coated; means connecting said article in circuit with said high potential source to establish an electrostatic ield between said charging electrode and said article, said circuit having a high effective impedance and including high resistance means of at least 4() megohms adjacent said charging electrode; a source of liquid coating material; and means for atomizing said liquid coating material into said electrostatic field, the distance between said charging electrode and the article being subject to variations which could in a conventional electrostatic spray coating system cause an objectionable change in potential gradient, said high effective impedance minimizing variations in average potential gradient of said ield during relative movement between said article and electrode and being provided at least in part within the source of high potential, said charging electrode being composed of such material and so dimensioned that its effective capacity is not greater than that of a metal sphere of a radius of about one centimeter, whereby at all distances between the charging electrode and article objectionable discharge of electrical energy of a character to initiate a tire or shock to the operator is avoided.

25. Apparatus for electrostatically depositing liquid coating material on an article, comprising, a source of unidirectional high potential; Ia charging electrode electnically connected to said source; grounded means for supporting the article to be coated; means connecting said article in circuit with said high potential source to establish an electrostatic field between said charging electrode and said article, said circuit having a high effective impedance and including high resistance means of at least 40 megohms adjacent said charging electrode; a source of liquid coating material; and means for atomizing said liquid coating material into said electrostatic iield, the distance between said charging electrode and the article being subject to variations which could in la conventional electrostatic spray coating system cause an objectionable change in potential gradient, said high effective impedance minimizing variations in average potential gradient of said field `during relative movement between said article and electrode, said high potential source dropping its output voltage markedly upon increases in output current, said charging electrode being composed of such material and so dimensioned that its effective capacity is not greater than that of a metal sphere of a radius of about one centimeter, whereby at all distances between the charging electrode 'and `article objectionable discharge of electrical energy of a character to initiate a re or shock to the operator is avoided.

26. Apparatus for electrostatically depositing liquid coating material on an article, comprising, a source of unidirectional high potential comprising a ladder-type voltage-multiplying rectifier circuit supplied with alternating current through Ia reactive input system; a. charging electrode electrically connected to said source; grounded means for supporting the article to be coated; means connecting said article in circuit with said high potential source to establish an electrostatic iield between said charging electrode and said article, said circuit having a high elective impedance and including high resistance means of at least 40 megohms adjacent said charging electrode; a source of liquid coating material; yand means for atomizing said liquid coating material into said electrostatic field, the distance between said charging electrode and the article being subject to variations Vwhich could in a conventional electrostatic spray coating system cause an objectionable change in potential gradient, said high effective impedance minimizing variations in average potential gradient of said iield during relative movement between said article and electrode, said charging electrode being composed of such material and so dirnensioned that its effective capacity is not greater than that of a metal sphere of a radius of about three centimeters, whereby at `all distances between the charging electrode and article 29 objectionable discharge of electrical energy of a character to initiate a fire or shock yto the operator is avoided.

27. Apparatus for electrostatically depositing liquid coating material on an article, comprising, a source of unidirectional high potential comprising 'a ladder-type voltage-multiplying rectifier ycircuit supplied with alternating current through an inductance capacitance bridge input network; a c'harging electrode electrically connected to said source; grounded means for supporting the article to be coated; means connecting said `article in circuit with said high potential source to establish an electrostatic field between said charging electrode and said article, said circuit havinga high effective impedance and including high resistance means adjacent said charging electrode; a source of liquid coating material; and means for atomizing said liquid coating material into said electrostatic eld, the distance between said charging electrode and the article being subject to variations which could in a conventional electrostatic spray coating system cause an objectionable change in potential gradient, said high resistance means having a megohm resistance of at least the same order as the kilovolts of -Voltage applied to the charging electrode at the lesser distances to be encountered during said relative movement, said high effective impedance minimizing Variations in average potential gradient of said eld dur-ing relative movement between said article and electrode, said charging electrode being composed of such material and so dimensioned that its effective capacity is not greater than. that of a metal sphere of a radius of about one centimeter, whereby at all distances between the charging electrode and article objectionable discharge of electrical energy of a character' to initiate a tire or shock to the operator is avoided.

References Cited in the tile of this patent UNITED STATES PATENTS 1,553,364 yChubb Sept. 15, 1925 1,974,328 Bouwers Sept. 18, 1934 1,992,908 Cockcroft Feb. 26, 1935 2,334,648 Ransburg et al. Nov. 16, 1943 2,415,116 Stiefel Feb. 4, 1947 2,509,277 Ransburg etal May 30, 1950 2,526,763 Miller Oct. 24, 1950 2,754,225 Gfeller July l0, 1956 2,764,125 Iuvinall Sept. 25, 1956 2,764,712 Juvinall Sept.k 25, 1956 2,878,143 Juvinall Mar. 17, 1959 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,048,498 August 7, 1962 James W. Juvinall et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 53, after "that" insert if line 58, after "cusps" insert a comma; line 70, for "objectionable" read objectionably column line 38, for "which" read with lin@ 58, for "leadnig" read leading column l1, line 34, for "of energy read or energy column 16, line 65, for "used," read used. column l', line 19, for "the "sharp configuration" read the "sharp" configuration line 30, for "cenlimeters' radius" read centimeters radius 4column 2l, line 30, for "designated" read designed Signed and sealed this 19th day of March 1963.

(SEAL) Attest:

Es'ron G. JOHNSON DAVID L. LADD Attesting Officer Commissioner of Patents

Claims (1)

19. A METHOD OF ELECTROSTATICALLY SPRAY COATING ARTICLES IN AN ELECTROSTATIC FIELD CREATED BY A UNIDIRECTIONAL HIGH POTENTIAL BETWEEN A CHARGING ELECTRODE AND ANOTHER ELECTRODE COMPRISING AT LEAST ONE OF ATHE ARTICLES AND WHEREIN THE DISTANCE BETWEEN SAID CHARGING ELCTRODE AND SAID OTHER ELECTRODE BEING SUBJECTED TO VARIATIONS WHICH COULD IN A CONVENTIONAL ELECTROSTATIC SPRAY COATING SYSTEM CAUSE AN OBJECTIONABLE CHANGE IN POTENTIAL GRADIENT, COMPRISING FLOWING A STREAM OF LIQUID COATING MATERIAL TO AN ATOMIZING DEVICE AND ATOMIZING SPRAY PARTICLES INTO SAID FIELD, WHEREBY SAID PARTICLES ARE CHARGED TO AFFECT THEIR DEPOSITION ON THE ARTICLES, AND AUTOMATICALLY CONTINUOUSLY CHANGING THE VOLTAGE BETWEEN THE ELECTRODES, BY MEANS INCLUDING A HIGH RESISTANCE, BY AND AS A FUNCTION OF CHANGES IN SPACE CURRENT ACROSS SAID DISTANCE TO MATERIALLY MINIMIZE VARIATIONS IN THE AVERAGE FIELD GRADIENT WHICH WOULD OTHERWISE ACCOMPANY SUCH VARIATION IN DISTANCE, WHILE MAINTAINING THE ENERGY LEVEL OF A DISCHARGE FROM THE CHARGING ELECTRODE BELOW THAT WHICH WOULD BE PROVIDED BY A METAL SPHERE OF A RADIUS OF ABOUT THREE CENTIMETERS IN THE SAME ELECTROSTATIC SYSTEM.

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