US2809902A - Method and apparatus for electrostatically coating articles - Google Patents

Description

Oct. 15, 1957 E. M. RANSBURG 2,809,902

METHOD AND APPARATUS F OR ELECTRSTATICALLY COATING ARTICLES Filed Aug. 9, 1951 3%. d /Q S N. N mm mm m om Attorney United States Patent st() METHGDtAND APPARATUS -FORELECTR- s STATICALLY COATDIG ARTICLES :,EdwinsM. Ransburg,.flndianapolis,Ind., assignor to Ransburg Electro-Coating ,Corp., Indianapolis, Y Ind., Aa cor- .poraton of Indiana .f Application August.f9, 1951,'Serial' No; 241,038

- .'SIClaims. (Cl. 117-,-93)

' v`.itself` issuingffroma. zsmalltorice); and, .more recently atomizers Whichuseelectrostatic forces-.fortproviding a -spray of liquid material,frorrrrtheaatomizing zone.

The use of pressure atomizing'devices to direct-a spray yof-liquid coating.material,generally.in,the direction of articles or surfaces .to -be,coated, with reliance -onthis general directionof movementofcoating material particles to cause at least .some of .them toldeposit uponia surfacebeing coated, V-is vof. course,` decades-old. On anythingv except rselativ'elydargeV surfaces;however,l.suchl` a spray coating arrangement is. inefcient; vand commercial operations using pressure atomizingdevices for production coating haveilost large percentages ofthe coating material in over-spray and have frequentlyhad to. resort ytowaterwashed spray booths or other undesirable and `expensive expedients to trap oversprayed'liquid coating material and reduce contamination ofthe air.

In recent years a form of spray coating knownfas electrostatic coating has been developed Where the spray issuing' from an atomizing-.device is given annelectric charge at the time it leaves thespraying device or shortly thereafter and wdeposition f the coating materialfparticles is, primarily the result of electrostatic. forces attracting the particles to 'the article `being coated. l.This is accomplished. by creating an electrostatic. rfield which includes 'the article being coated andthe charged-.coating material particles movingI generally toward-such article. "Coating systems of the electrostatic type, especiallyfthose using electrostatic zatomization, havecproved very satisfactory and extremely ecient commercially in coating various types of articles (even small articles or articles with large open spaces, as automobile steering wheels) Vwhich were considered extremely difcult to spray-coat f before the advent of electrostatic coating.

One'of the most eflicient electrostatic atomizing heads heretofore developed,"however,v comprises `a head with a circular 'atomizing edge, preferably'the edge of a bell- :shaped member which is rotated to improve the uniformity `oftdelivery` of liquidfcoating material inl'a thin lilm to the atomizingfedge,-' antelectro'staticatomizer of this type fbeing'disclosed and claimedy in my copending application, "Serial "No. I143,994,2iled Februaryd, 1950. `However, i the natural pattern delivered f from an l electrostatic uatorrrizingfiheadLv of' ;this"type is x a` .doughnut` Vorannular rbandfhavingfa centralportionfwhich iszcompletely or relaon other, portions. ,Y atomizer with..a,pattern size at'v least equal -to or slightly p greater than the maximum dimension of the article transj verse toilits line. of movement entails some loss by Way-of over-spray 4even with the presence of electrostatic field `forces for` assisting depositionand, in addition, produces ice tively' void ofcoating material. .--Referenceltona natural .pattern,l -or for lbrevity merely the pattern of the` atomizingy head,- is here'v intended-to mean the `pattern vwhich thev sprayingdevice would- -provideon a` relativelyvery -large sheet ofcondu'cting` material arranged-normal to :the

general line of movement of the =spray #particles and-at sa ldistance correspondingfto `the-,spacingof the head =from 1 an article to Ybe coated, with-the. respective. electric potentials being the. same .as those which would` be used with Y thearticles.

rWhere an` electrostaticatomizing. headof the type just referred` to..` is `used the width.. of the annular band and Uthe size, ofthevcentral void otthepatternare determined, wat least to. somek extent,by tsuch. -factors as .the 7diameter of the head,-the:gspacing betweenthehead and sheet, .the

voltage ditferentialbeingused, rtheviscosity androther characteristics of the coating material,y etc.; but the-commercially practical ,spacings .of at least several .mches ,(and usually at least.` 6. or `8tinches),feven for small circular electrostatic atomizing headscreate an annular pattern 1in which the `diameter ofthe central void may be as Vmuch as several inches. .Under these conditions a small incre- .ment of .area ofrarticler surface. passing along a diameter of the patternreceives a lesser amount of :coating than doesa similar .incrementlof area movingy parallel to the 'diameter' but along a line4 further out which does netpass through the centralvvoid fthespattern. With a single such` head usedalone, therefore, the coating thickness on an article having dimensions transverse to its line `of movement of the same `order as that of the outer diameter of the-annularpatternbandhas been irregular, with an unnecessary.thickness 0f coatinghaving tothe deposited on certainportions in order to secure-sulcient thickness On. the other hand, use of, pressure a coating frequentlyinon-uniform in thickness.

The;present' invention` provides a ycombination of a circular electrostaticatomizing head and a supplementary .atomizinghead in such a way that the/extremelyhigh ve'iliciencies associated with Aelectrostatic coating methods employing electrostatic atomization (of the order of 99% or even more) are retained, but with supplementary atomization employed to till in what would otherwise be the pattern void .and provide substantially uniform distribution of coating material within the limits of the tpattern with thev advantage of improving uniformity of coating thickness While retaining the high efciencies characteriz- Y ing electrostatic atomization.

wThe foregoing` and other features `and advantages-of "this inventiontvvill be apparent from the following. specification andtheidrawings, in which:

Figure 1 is a longitudinal sectional view of an atomizing unit providing both electrostatic and pressure atomizal tion in alsuitable relationship.

Figure-'2 is `a` diagrammatic view of coating apparatus incorporating" the atomizing'device shown inFigure 1.

rFigure Sais a fragmentary sectional view' of an alterna- Ytive combinedlapparatus wherein thepressure atomization of the` liquid' coating mater'ialV is :elfecte'ddirectly' by lthe f pressure of the liquidmaterialfitself; and p TFigure 4 isffragmentaryl sectional View" of still another alternative Aapparatus *wherein two independent electrostatic atomizersfarecombined in `a suitable relationship.

While three different forms of combined atomizing units are illustrated and will be described as useable for accomplishing the present invention, it will be understood that these are representative embodiments only; and it is to be understood that other embodiments may be utilized without departing from the contemplated scope of the present invention, and that no limitations are to be implied from such specific description as shall now be provided.

Referring now more especially to the particular embodiment of the invention illustrated in Figure 1, it will be seen that there are two coaxial atomizing heads of different types, the electrostatic atomizing head being identified in general as and the pressure atomizing head being identified in general as 11.

The inner pressure atomizing head is here shown as comprising an outer generally cylindrical member 12 and a concentric tubular member 13 therewithin. Liquid coating material is supplied within the tube'13 to issue from the small orifice 13a at the end of this tube; and air at suitable pressure is supplied through the annular passageway provided between the tubes 12 and 13 to issue through suitable orices 13b, as illustrated, surrounding the liquid orifice 13a, the liquid coating material thus being air atomized and sprayed out as a generally circular spray.

The electrostatic atomizing arrangement comprises a tubular portion 14 concentric with and slightly spaced from the tubular member 12, providing an annular passageway through which liquid coating material may flow in a manner hereafter more fully described; and a bell-shaped or generally conical member 15 terminating in a circular atomizing edge 15a which may, for eX- ample, be 6 inches in diameter. The bell-shaped member 15 is preferably rotated during operation of the electrostatic atomizing unit, and this is here shown as effected through meshed gears 16 and 17, the former fixed on tube 14 and the latter being rotated by a shaft 18 of electrically insulating material and enclosed within a support column or tube 19 also of electrically insulating material. In a manner indicated diagrammatically in Figure 2 and more fully described in the earlier mentioned copending application, the atomizing heads are both electrically charged to a high potential wit respect to ground (as 100,000 volts) by a connection to a suitable power pack 46; and articles 47 are normally carried on a grounded conveyor 48 so that the desired electrostatic field would be set up between the atomizing heads and the articles themselves (if of conducting material), or conveyor support electrodes therewithin. The electrostatic field thus established, at a suitable spacing of the atomizing edge 15a from the articles being coated (as S or l0 inches) causes electrostatic atomization of liquid coating material from the edge 15a, resulting in an annular or doughnut-shaped spray pattern.

Since the heads are coaxial and normally both operative during coating, the resultant natural pattern at the coating plane is a single circular pattern without any void, with a distribution of coating material particles throughout the entire circle, the coating material particles atomized by the pressure atomizing head 11 lilling in what would otherwise have been a void in the pattern of the electrostatic atomizing head. It will be understood that the heads are preferably so adjusted and operated that the periphery of the circular pattern of the pressure spray approximately coincides with the inner edge of the annular pattern of the electrostatically atomized spray. It may also be desirable to provide a somewhat less concentrated or thinner spray from the pressure atomizing head 11 than is provided by the electrostatic atomizing arrangement 10 in order to prevent undue thickness of coating of increments `of surface area moving along or very close to the diameter of the combined patterns. Bearing in mind that the surface to be coated will be moving through the spray, it will be obvious that the distribution of the paint parallel to path of surface movement is immaterial, and that the pattern of the pressure spray need not be circular to produce a deposited coating of satisfactorily uniform thickness.

Again referring more particularly to Figure l, it will be seen that the column 19 carries a housing or support member 20 which supports the tubular member 14 for rotation, as by the pair of ball bearing assemblies illustrated. An end member 21 carried by the member 20 provides a liquid coating material inlet 22 communieating with the annular passageway between the members 12 and 14 to supply liquid coating material to the inner surface of the bell-shaped member 15. An appropriate fluid-tight seal is provided between the rotating member 14 and the stationary member 21, this being here shown as comprising chevron packing members 23 held in sealing relation by a spring 24 bearing against a member carried by the stationary end member 21.

The outer member 12 of the stationary pressure atomizing head is provided with a flange 12a bolted to the stationary end member 21, as illustrated, by the studs 25, a gasket 26 providing a suitable fluid-tight seal. The left hand end of this tubular member 12 (as the parts are viewed in the drawing) is attached in such a manner as to be sealed to and to mechanically support a passageway member 27, this being here shown as having a tapered seat held on a tapered shoulder on the tubular member 12 by a screw cap 28, the uid seal being provided by an O-ring 29. The support member 20 is also provided with an extending arm portion 30 also connected to and serving to act as a support for the member 27, as by the connection provided by the pin 31.

The passageway member 27 provides another liquid coating material passageway 32 communicating with the interior of the tube 13, it being understood that the liquid material passageways 22 and 32 may be supplied with the liquid coating material through connection to the same conduit, or through separate conduits. In each case coating material would be suitably fed from some reservoir, as by a pump; and if separate control is desired, as is preferable, separate pumps and conduits may be used so that the regulation of supply of coating material to the two atomizing heads may be independently adjusted.

The passageway member 27 also provides a cylinder 34 for purposes to be hereinafter more fully described, and an air passageway 35 communicating with the annular passageway between the tubular members 12 and 13 to provide the atomizing air used in the particular pressure atomizing head illustrated. This air passageway 35 would, of course, be supplied with air under pressure from a suitable source, as a conventional air pump and pressure tank, with appropriate means for regulating the pressure.

Mounted within the cylinder 34 and urged toward the position illustrated by a spring 36 is a piston identified in general as 37. Passing through the center of this piston and through the center of the tubular member 13 is a control valve or needle member 38 adapted to close the liquid orifice 13a, being urged to orifice-closing position by a spring 39 here illustrated as in a cap member 40 and bearing against a shoulder element 38a on a pin` As will be readily apparent, if air under pressure is admitted to the passageway 41 to the right (as the parts are illustrated) of the piston 37 the piston will move to the left against the spring 39 and force the needle valve member 38 to the left, insuring the passageway 13a being open as long as air pressure is present in the passageway 41. Accordingly, connection of passageways 35 and 41 to the same conduit supplying air under pressure results in opening of the orific 13a upon a supply of air and automatic closing thereof when air pressure is turned off. This be admitted to "the passages 35 and .41 andsuitableadvjustments in its pressure (accompanied.withadjustments inlthe rate of liquid supply tothe passage'SZQ'ifdesired) .toadjust thesize andden'sity of the pattern provide'dl by Ithe pressure atomi'zing head 11. The .needlemernber` 38 would, ofcourse, be suitably sealedv in any w'a'll openings through which it passes, this being here illustrated as ef-n fected by the gland packing arrangements 43, 44 and 45.

While the particular pressure atomizing head illustrated as a part of the apparatus shown in Figure 1 utilizes air pressure in conjunction with liquid fiow, a similar solid circular spray pattern of the type obtained from this head may also be satisfactorily obtained from a pressure atomizing head utilizing no air but achieving its atomization directly by forcing the liquid coating material under considerable pressure through a fine orifice. Such an alternative arrangement is illustrated in Figure 3. In this case an electrostatic atomizing head 50, which may be similar to that illustrated more completely in Figure 1, is combined witha pressure atomizing head 51 comprising the single tube illustrated, with a small end orifice Sie. Liquid coating material delivered through the center of the tube 51 would issue through the fine orifice 51a in a spray of particles which, because of the electrostatic field forces involved, would be attracted by and travel toward the coating Zone. In utilizing such a pressure atomizing head there would, of course, be no need of an air supply passageway; and the automatically operate-d needle valve arrangement might be omitted to provide a simpler atomizing unit wherein feed of liquid coating material to the pressure atomization head might be manually stopped when desired.

Still another embodiment of my invention is illustrated in Figure 4. In place of the fluid pressure atomizing heads shown in Figures l and 3 a similar solid circular spray pattern of the type obtained from these heads is in the modification of Figure 4 obtained from a single orifice electro-static atomizinglhead. More specically, this apparatus comprises two electrostatic .atomizing heads designated generally as 60 and 61 arranged coaxially and maintained at a high electrical potential by a suitable voltage source so that a field exists adjacent each head of sufficient strength to atomize liquid coating material. Head 60 may be similar to ythose illustrated in Figures l and 3 and is provided with discharge edge 60a from which liquid is electrostatically atomized to form a generally annular pattern. Head 61 comprises a relatively small diameter tube (on the order of 1/16 I. D. and 3/16" O, D.), one end of which terminates in theform of a small orifice 61a substantially and preferably in the same plane as thatof the discharge edge 60a. Liquid coating material delivered to the orific 61a in any suitable manner such as by gravity feed or by a pump is electrostati- ,cally atomized in the form of a spray which substantially fills the void in the annular pattern which issues from head 60. By this apparatus the use of pressures for ob- -taining atomizatio-n is completely avoided and a completely electrical atomizing apparatus is provided for obtaining supplies of coating material which may be utilized in obtaining coatings on various surfaces of improved uniformity. g

The combination of two atomizing heads, respectively producing an annular and a solid pattern enables advantage to be taken of all Vthe desirable features of electrostatic atomization, particularly its extremely high coating efficiency, and at the same time permits a single atomizing unit to provide a more uniform coating thickness than has heretofore been possible in an electrostatic coating system utilizing a single spray unit. Another advantage of my invention is that where the spray heads are arranged coaxially the outer annular spray has an electrostatic confining action ou the inner spray which re- `suits 1sinsaffdenserspraytlthanthas?Ibeerrpreviously: possible Y i Whereas -tthefV apparatus -ofv'l-i'gure 1 -shows the shaft 18 txand support-columnl19-was being;.madeof-insulating maf teria] so ithat-f the latomi'zinghead iassembly cancbe held .at `yhigh rpoten'tialgfitI is to 'be understood thatv these yparts canhbefof#conductingffmaterial if other means. are pro- 44-vided 4forelectricallyinsulatingfthehead from its Ysurrouridings.

While I have shown and describedvcertainE embodiments of my 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.

I claim:

l.' A method for electrostatically coating an article, comprising providing a spray of coating material in a pattern which in -a plane substantially normal to the general line of movement of `spray particles, is an annular band; providing a second spray of coating material in a different pattern which supplements the first pattern to provide a substantially uniform distribution of coating material particles in said plane; and establishing an electrical potential difference between said spray particles and the article to `be coated.

2. A method for electrostatically coating an article, comprising providing a spray of coating material in a pattern which, in a plane su'bstantialy normal to the general line of movement of spray particles, is an annular band; and providing a second spray of coating material in a circular pattern having its periphery approximately coinciding with the inner edge of said annular pattern band at the coating plane to provide a substantially uniform distribution of coating material particles at such plane.

3. A method of coating an article, comprising forming first and second streams of liquid coating material, shaping said first stream into an annular film, electrostatically atomizing the coating material from the leading edge of said film to form an anular spray having a central void, passing the article through such annular spray along a diameter thereof whereby at least a portion of the surface of. the article will traverse said void, atomizing said second -stream to form a second spray and directing said second spray to impinge on that portion of the article surface which traverses said void.

4. A method for electrostatically coating an article,

' comprising providing a spray of liquid coating material having a pattern which in a plane generally normal to the line of movement of spray particles has a central portion substantially devoid of coating material particles, providing a second spray of liquid coating material in a pattern which supplements the first pattern to provide a substantially uniform distribution of coating material particles in said plane, and establishing an electrical potential difference between said spraying particles and the article to be coated.

5. Apparatus for electrostatically coating an article comprising: a first spraying means having a rotatable sharply defined annular atomizing edge positioned about a central axis, means for rotating said edge -about said axis, a conveyor for moving an article to be coated along a path spaced from said first spraying means and transverse to and passing through said axis, means for feeding liquid coating material to the annular edge of said first spraying means for atomizing and projecting toward the article path a hollow spray of finely divided particles in a pattern which in a plane at the article path and normal to said axis is an annular band having a central area substantially concentric with said axis and substantially devoid of spray particles, means including a source of high voltage connected to said atomizing edge and said article for establishing between the spray particles projected from the first spraying means and the article to be coated a particle-depositing electrostatic eld, and a second spraying means including an air spray gun having a discharge orifice located coincident with said axis and within said annular atomizing edge for providing a spray of liquid coating material particles in a pattern which at said plane has a substantially circular pattern the periphery of which approximately coincides With the inner edge of said annular band pattern, said atomizing edge being located radially outward beyond the spray from said second spraying means.

References Cited in the le of this patent UNITED STATES PATENTS Hopkins et a1. June 7, 1932 Collins May 30, 1933 Darrah May l5, 1934 Penny Sept. 5, 1944 Ransburg July 3, 1951 Crouse Sept. 18, 1951

Claims (1)

1. A METHOD FOR ELECTROSTATICALLY COATING AN ARTICLE, COMPRISING PROVIDING A SPRAY OF CAOTING MATERIAL IN A PATTERN WHICH IN A PLANE SUBSTANTIALLY NORMAL TO THE GENERAL LINE OF MOVEMENT OF SPRAY PARTICLES, IS AN ANNULAR BAND; PROVIDING A SECOND SPRAY OF COATING MATERIAL IN A DDIFFERENT PATTERN WHICH SUPPLEMENTS THE FIRST PATTERN TO PROVIDE A SUBSTANTIALLY UNIFORM DISTRIBUTION OF COATING MATERIAL PARTICLES IN SAID PLANE; AND ESTABLISHING AN ELEC-

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