US2658472A - Electrostatic coating apparatus - Google Patents

Description

Nov. l0,v 1953 E. M. RANSBURG ELECTROSTATIC COATING APPARATUS Filed Oct. 29, 1948 3 Sheets-Sheet 1 Fig. 3

INVENTOR.

EDWIN M RANSB URG BY g ATTORNEY E. M. RANSBURG ELECTROSTATIC COATING APPARATUS Nov. 1.0, 1953 3 Sheets-Sheet 2 Filed Oct. 29. 1948 INVENTOR.

E DW| N M. RANSBURG BY TTORNEY Nov. 10, 1953 E. M. RANSBURG ELECTROSTATIC COATING APPARATUS 5 Sheets-Sheet 3 FiledOct. 29. 1948 INVENTOR.

EDWIN M. RAN SBURG BY lvnwzfbo AT ORNEY Patented Nov. 10, 1953 UNITED STATES PATENT OFFICE ELECTROSTATIC COATING APPARATUS Edwin M. Ransburg, Indianapolis, Ind, assignor to Ransburg Electra-Coating Corp., Indianapolis, Ind., a corporation of Indiana Application October 29, 1948, Serial No. 57,260

in which the liquid coating material is both electrostatically atomized and electrostatically precipitated or deposited upon the article to be coated. Coating methods and apparatus embodying the electrostatic atomization and electrostatic deposition of liquid coating materials are shown in the prior application for U. S. Letters Patent, Serial No. 556,390, filed September 29, 1944, by Edwin M. Ransburg and William A. Starkey.

In apparatus employed for applying a liquid coating to discrete articles, whether or not the coating is electrostatically deposited, it is common to move the articles through a coating zone in which the coating is applied to them. Similar practice is followed in the coating of sheet material, the coating being applied as the sheet moves progressively through a coating zone. Satisfactory coating of articles and sheets can be obtained by prior processes embodying both electrostatic atomization and electrostatic deposition of the coating material when the dimensions of the article or articles being coated in a direction transverse to the path of article-movement through the coating zone is not too great; but some difficulty has been experienced in the past in electrostatically atomizing and precipitating liquid coating materials on surfaces of considerable extent transversely of the path of articlemovement through the coating 'zone. In the prior application above referred to, it was proposed to atomize the coating material from an elongated head having an atomizing surface or surfaces to which the coating material was fed through a series of restricted orifices. One disadvantage of such an atomizing head arises from the difiiculty of maintaining the rates of flow through the orifices co-ordinated in the desired manner, with the result that the spray pattern produced was not of uniform density throughout its extent.

It is an object of this invention to overcome the disadvantages heretofore attendant upon the electrostatic atomization of coating material from an extended atomizing head. More particularly, it is an object to facilitate the attainment of a uniform distribution of coating material to an elongated, atomizing head, whereby to increase uniformity in density in the resultant spray.

In carrying out the invention, the coating material is atom d om n exten d edge .of an atomizing head and is fed to such edge by a nozzle which is moved relatively to the edge and parallel thereto to distribute the coating material therealong in a uniform fashion. Where the edge from which atomization takes place is straight, the nozzle is reciprocated along it; and provisions may be embodied in the apparatus for preventing the change of nozzle velocity at and near the ends of the stroke from interfering with uniform distribution of the coating material along the edge. where the atomizing head is disposed vertically or at any substantial inclination to the horizontal, it may be provided with means for directing coating material discharged from the nozzle toward the atomizing edge of the head as such coating material flows generally downwardly under the influence of gravity. Other features of the invention will be apparent from the specification and claims.

In the accompanying drawing, Fig. 1 is a fragmental isometric view illustrating a coating apparatus; Fig. 2 is an isometric view, on a somewhat larger scale, illustrating the atomizing head, a reciprocating coating-material nozzle, and means for reciprocating such nozzle; Fig. 3 is a fragmental isometric View of the head of Fig.

2 on a still larger scale; Fig. 4 is an elevation of a modified form of head; Fig. 5 is a fragmental elevation, in partial section, illustrating a means which may be employed to counteract the effect of velocity changes at the ends of the stroke of a reciprocating coating-material nozzle; Fig. 6 is an elevation of a modified nozzle-reciprocating mechanism; Fig.7 is an axial section through a circular atomizing head embodying my invention; and Fig. 8 is an isometric View, in partial section, illustrating a modified form of circular atomizing head.

In Fig. 1 I have illustrated an arrangement of apparatus suitable for coating discrete cylindrical objects It. Such apparatus embodies a conveyor II carrying an endless series of vertically disposed, rotatable spindles l2 each of which is adapted to support one of the articles l0. Each spindle l2 has rigid with it a wheel I3 adapted to engage a stationary rail I4 as the spindle moves through the coating zone, whereby to rotate the spindle and the article ID it carries. Disposed at one side of the path of article-movement through the coating zone is a vertically positioned atomizing head [6 from which the liquid coating material is electrostatically atomized. To effect atomization of the coating material and electrostatic deposition thereof on the articles [0 as they pass the head IS, an electrostatic field of appropriate strength is maintained between the head and.

the articles. For this purpose, the articles may be grounded through the conveyor while the atomizing head is connected to one terminal of a high-voltage source H the other terminal of which is grounded.

The atomizing head 16, which is shown in greater detail in Figs. 2 and 3, comprises a vertical support 20 to one side of which there is affixed in any convenient manner an elongated member 2| which is preferably of electrically conducting material and connected to the ungrounded terminal of the high-voltage source IT. The member 2| is shown as shaped in cross-section to present toward the articles H) a relatively sharp edge 22. An average potential gradient of the order of 10,000 volts per inch is maintained between the member 2| and the articles l moving past it, and as a result liquid coating material feed to such edge will be electrostatically atomized and electrostatically precipitated on to the articles In, as more fully set forth and described in the prior application above referred to. -In order to secure a spray pattern of uniform density, it is important that the coating material be distributed uniformly to all points along the edge 22; and it is to apparatus for accomplishing that purpose that this invention is especially directed.

The coating-material distributing means shown in Fig. 2 comprises a rod 25 mounted for vertical reciprocation in appropriate guide means 26 and carrying at its upper end a short length of tube or pipe 21 disposed transversely of the rod. One end of the tube 21 is connected, as through a flexible hose 28, with a source of coating ma-- posed close to the edge 22 ofthe discharge head but spaced rearwardly a short distance from such edge so as to be electrically shielded thereby.

The means employed to reciprocate the rod 25 and nozzle 29 may take various forms. In Fig. 2, such means is shown as comprising an air cylinder 3| having a piston (not shown) mounted on the lower' end of the rod 25. A fourway valve 32, operated by solenoids 33-, controls the admission and discharge of air or other fluid under pressure alternately to opposite ends of the cylinder 3|. Conveniently, the valve 32 is arranged to be automatically reversed when the rod 25 reaches the end of its stroke in either direction. To this end, the solenoids 33 may be respectively controlled by limit switches 34 positioned to be engaged by the tube 21 to reverse the valve as the rod 25 reaches each end of its stroke.

In operation of the apparatus shown in Figs. 1 and 2 the articles to be coated are moved past the head l6 and coating material is supplied through the flexible hose 28 to the nozzle 29 while the rod 25 reciprocates to distribute such coating material along the discharge edge 22 of the head. Coating material applied to the member 2| in rear of the discharge edge 22 spreads to the discharge edge, and is electrostatically atomized and precipitated upon the articles passing the head l8 by the action of the electrostatic field which exists between those articles and the head. The rate at which coating material is supplied to the nozzle 29 is controlled so as to insure that the member 22 will not receive more coating material than can be atomized from its discharge edge. The rateof reciprocation ,of the rod 25 and the nozzle 29 can be controlled by controlling the rate at which fluid under pressure is supplied to the cylinder 3|, such rate of fluid-supply being adjusted as necessary to maintain a supply of coating material for electrostatic atomization at all points along the desired extent of the edge 22.

Generally speaking, it is advisable that the length of the discharge edge 22 approximate the parallel dimension of the surface or surfaces to be coated, although the length of the discharge edge mayin .many instances be somewhat less than the parallel dimension of such surface or surfaces without causing undue lack of uniformity in the coated products. In most instances, Where the article or articles to be coated have Surfaces,.such asthe upper and lower surfaces of the articleslO, which are presented edgewise to the discharge head, it will be advisable to employ a member 2| having a discharge edge 22 longer than would be required for the application of a satisfactory coating to a surface or surfaces extending parallel to the head.

In many instances, it will be advisable to utilize substantially less than the full length of the discharge edge 22 forthe electrostatic atomization of the coating material, forend portions of such edge to which coating material is not supplied serve to reduce spreading of the spray pattern in a direction parallel to the discharge edge. To prevent a concentration of field strength at the ends of the discharge edge 22, the head It may include end pieces 36 of conducting material which are secured to the support 20 in any convenient manner and extend outwardly therefrom to the respective endsof the edge 22. Immediately adjacent the ends of the edge 22, the end pieces 36 havea cross section corresponding to that of the member 2|, but outwardly beyond the ends of the edge 22, the exposed edges of the end pieces 36 are rounded off as indicated at 36'.

Wherethe head I6 is so disposed that the edge 22 is vertical or inclined to the horizontal at such angle that the coatingmaterial discharged from the nozzle 29 does nottend to flow unaided to the discharge edge under the influence of gravity, the member 2| may be provided adjacent the edge 22 with flow directing elements designed to divert towardthe edge 22 any coating material which flows downwardly over the surface of the member 2| generallyparallel to that edge. Such flow-directing elements may take the form of .wires 38 which are secured, as by soldering, to the face .of the member 2| in inclined positions. The elements 38, if used, desirably terminate in rear of the edge 22, as will be clear from Fig. 3, in order to avoid interference with the uniform distribution of field strength along such edge.

In a vertically disposed head provided with flow directing elements 38, a reasonably satisfactory distribution of coating material along the edge 22 may sometimes be obtained without reciprocating the nozzle 29. For example, if the nozzle 29 were held in the fixed position indicated in Fig. 3, it would discharge on to the adjacent face of the member 2| a stream of coating material 39; and as such stream of coating material ran down the face of the member 22 under the influence of gravity, portions of it would be successively diverted by the members 38 to form branch streams 39 which the members 38 direct to the discharge edge 22. Obviously, the extent of the discharge edge 22 which can be satisfactorily supplied with ,coating material in this manner from a single discharge orificeis limited; but there are situations in which this expedient could be utilized to supply coating material to an edge 22 of limited extent.

The relatively simple arrangement illustrated in Fig. 2 for distributing coating material along the member 2| in rear of the discharge edge 22 has a disadvantage due to the rapid velocity changes which occur at the ends of the stroke of the nozzle 29. To overcome this disadvantage, the arrangement illustrated in Fig. 4 may be employed, As there indicated, the member 2| is provided adjacent the path of the reciprocating nozzle 29 with pockets 49 into which such nozzle discharges when near the ends of its stroke. Any

inequality of coating-material distribution occurring as a result of severe accelerations at the ends of the nozzle-stroke take place when the nozzle is opposite one or the other of such pockets. The pockets are formed to trap coating material discharged into them and to prevent it from escaping on to the side surface of the member 2I where it might find its way to the discharge edge 22. To prevent an accumulation of coating material in the pockets, they may be provided with drain passages 41, and such drain passages may communicate with a drain conduit 42 through which the coating material may be conducted for reclamation.

Adverse effects of extreme accelerations on the discharge of coating material at the ends of the stroke of the nozzle 29 might be eliminated by employing a nozzle-reciprocating means which would not result in the existence of those extreme accelerations. In such a case, however, if the rate of flow through the nozzle were maintained constant, the end portions of the edge 22 would receive proportionately more coating material than would the mid-portion of such edge. In Fig. 5, there is illustrated an arrangement by which accelerations at the ends of the nozzle stroke may be reduced without causing the deposit of excessive quantities of coating material at the ends of the nozzle stroke. In that arrangement, the pipe 2'I is provided intermediate its length with a valve 42 formed to provide an orifice 43 through which the coating material flows in its passage from the flexible tube 28 to the nozzle 29." The orifice 53 is arranged with its axis parallel to the path of reciprocation and receives a double-ended needle valve 44 which is reciprocably mounted in the valve body 42 and the opposite ends of which project outwardly beyond the valve body. Springs 45 acting oppositely between the valve body and the needle valve tend to maintain the needle valve in a normal central position in which its smallerdiameter portion is disposed within the orifice 43. The support 29 carries abutments 4'! which are disposed adjacent the ends of the stroke of the tube 21 in position to engage the ends of the needle valve 44. Conveniently, the abutments 41 are adjustable and are so positioned that as the nozzle 29 begins to decelerate near the end of its stroke in either direction the needle valve 44 will engage the adjacent abutment 41 and will be moved thereby to cause the tapered portion of the needle valve 44 to enter the orifice 43 and reduce the rate of flow of coating material to the nozzle as, If desired, the parts can be so arranged that at the end of the stroke the supply of material to the nozzle 29 is completely shut off but ordinarily this will not be necessary.

Means for reciprocating the nozzle 29 in a manner which will not involve extreme accelerations at the end of the stroke may take the form shown in Fig. 6. Such mechanism comprises a cardioid cam 59 having opposite peripheral portions 5I "er substantially constant spiral pitch and smoothly curvedconnecting portions 52 and'53 joiningadjacent ends of the portions 5I. The cam 50 is mounted for rotation on an axis perpendicular to the rod 25, and such rod i provided with a cam following roller 54 which engages the cardioid peripheral surface of the cam. During the mid portion of the stroke of the rod25, the roller 54 is engaged by one-or the other of the uniform-pitch cam portions 5|, and the nozzle moves with uniform velocityQ As the rod 25 and nozzle 29 near the end of the stroke, the roller 54 is engaged by one of the connecting cam portions 52 and 53, and the direction of movement of the rod and nozzle is gently reversed.

-In the form of the invention shown in Fig. 7, the discharge head is of circular form to create a circular pattern in the atomized coating mate- 'is located rearwardly of the discharge edge I22 and closed by a cap I25. The cap rigidly supports one or more discharge nozzles I29 which communicate with the interior of the tube I24 and extend forwardly and outwardly to discharge against the innerface of the sleeve I2I a short distance in rear of the discharge edge I22. The pipe I24, which may be rotatably supported from the sleeve I2I through anti-friction bearings I26, extends rearwardly'beyond the tube and into a fitting I21 the hollow interior of which communicates through a hose I39 with a source of coating material. The fitting I2! is provided with a stuffing box I3I to prevent the escape of coating material around the tube I24; and between the sleeve I2I and fitting I2! the tube I24 is provided with a pulley I32 adapted to receive a belt I33 by which the tube and nozzles I29 are rotated.

In use, the head of Fig. '7 is supported opposite a surface to be coated, the tube I24 and nozzle I29 are rotated, and liquid coating material is supplied to the fitting I21 from which it flows through the tube I24 to the nozzles I29. The rate at which fluid is discharged from the nozzles I29 is regulated so as not to exceed the rate at which it can be electrostatically atomized from the edge I22.

The modification of the invention illustrated in Fig. 8 is similar to that shown in Fig. 7 except that relative movement between the circular head and the nozzles is obtained by rotating the head while the nozzles remain stationary. Aswill be clear, the co ating material is supplied to the head through a stationary pipe or tube 224 which extend into the head and rotatably supports a sleeve 22I. The sleeve 22I" has an exterior annular groove adapted to receive a belt 232 by means of which it may be rotated. Within the sleeve 221 the tube 224 communicates with an annular series of discharge-nozzles 229 adapted to discharge immediately in rear of a discharge edge 222'on the sleeve 22L Coating material is supplied to the head through the tube 224 and is discharged through the nozzles 229 at a rate no greater than that at which it can be atomized from the discharge edge 222.- As the sleeve ZZI rotates while the nozzles 229 remain stationary, the coating material discharged from the nozzles is evenly distributed along the edge 222. Ifthe the discharge edge 222 for electrostatic atomization therefrom. It may be noted that in using a rotating discharge element the structure of Fig. 8 embodies an invention broadly claimed in my patent application Serial No. 57,259, now abandoned, of even filing date herewith and my application Serial No. 143,994, filed February 13, 1950.

In both of Figs. 7 and 8, it is contemplated that the hollow sleeve, orat least that portion of it on which the discharge edge is provided, will be made of electrically conducting material and that such conducting material and the article or articles being coated will be associated with opposite terminals of a high-voltage source capable of producing at the discharge edge a fieldstrength great enough to produce electrostatic atomization of the coating material. As previously indicated, it is preferred to employ a highvoltage source capable of producing between the atomizing head and the article or articles being coated an electrostatic field having an average potentialgradient of the order of 10,000 volts perinch. In most cases itwill be convenient to ground the articles being coated and insulate the discharge head; but this arrangement may be reversed if desired. Where the head is insulated and charged and an electrically conductive coating material is being used, the source of such coating material should also be insulated from ground, as more fully set forth in my aforesaid patent-applications Serial No. 57,259, now abandoned, and Serial No. 143,994. Where the coating material being employed is electrically conductive in character, it is not essential that the member from which atomization takes place also be conductive; for the coating material on the discharge edge could be connected to the high-voltage source otherwise than through the discharge member.

In allmodifications shown the discharge edge of the atomizing head is shown as smooth and continuous. This is not an essential, however; as,

if desired, such discharge edge might be serrated.

What is claimed is:

1. In an electrostatic atomizing head for use in the electrostatic deposition of liquid coating material, a discharge member having an extended edge, and means for distributing liquid coating material to said edge for electrostatic atomization therefrom, said means comprising a discharge nozzle disposed to discharge on the discharge member adjacent said edge, and mechanism for reciprocating said nozzle over a path generally parallel to and spaced from said edge.

2. In an electrostatic atomizing head for use in the electrostaic deposition of liquid coating material, a discharge member having an extended annular discharge edge, and means for distributing liquid coating material to said edge for electrostatic atomization therefrom, said means comprising a discharge nozzle mounted for rotation about the axis of the annular discharge edge and disposed to discharge directly on to the discharge member adjacent said edge, and mechanism for rotating said nozzle about its axis.

3. The invention set forth in claim 1 with the addition that said discharge member is of electricallyconducting material, the edge of the discharge member continuing outwardly beyond each end of the stroke of the reciprocating nozzle.

4. The invention set forth in claim 1 with the addition that said discharge member is of electrically conducting material.

5. The invention set forth in claim 1 withthe addition that said discharge member is provided with means for guiding to said edge liquid coating material discharge by said nozzle.

6. The invention set forth inclaim 1 with the addition of means for trapping and diverting from said edge coating material discharged from the reciprocating nozzle near the ends of its stroke.

7. The invention set forth in claim 1 with the addition of means operative when the reciprocating nozzle is near the ends of its stroke for reducing the rate at which the coating material is discharged.

8. In an electrostatic atomizing head for use in the electrostatic deposition of liquid coating material, a discharge member havingan extended edge, and means for distributin liquidcoatin material to said edge for electrostatic atomization therefrom, said means comprising a plurality of discharge nozzles spaced from each other along said edge and disposed to discharge coating material directly on to the discharge .member adjacent the edge, and means for producing relative movement of said nozzles and discharge member in a direction parallel to said edge.

9. In electrostatic coating apparatus, a movable discharge nozzle, means forsupplyingliquid coating material to said nozzle, means for movin said nozzle over a predetermined path, a discharge member extending generally parallel to and adjacent said path for receiving coating material discharged from the nozzle, a support for supporting anarticle to Ice-coated in spaced relation to said member, and meanslincludinga high voltage source for maintaining between an article on said support and the coating material on said member an electrostatic field capable of atomizing such material and precipitating it on the article.

10. The invention set forth in claim 9 with the addition that said nozzle reciprocates over said predetermined path, and means operative when the nozzlenears the ends of said ath for reducing the rate at which the coating material is discharged.

11. In an electrostatic atomizing head for use in the electrostatic deposition of liquid coating material, a discharge member having an extended edge, means including a movable flow directing element for supplying a stream of liquid coating material to said member for distribution along said edge and electrostatic atomization therefrom, and means for moving said element to cause said stream to move along said member in a direction generally parallel to said edge.

12. In an electrostatic atomizer, a discharge member extended in a direction transverse to that in which atomized particles are to be projected from it, movable flow-directing means for supplying liquid sequentially to points distributed along such discharge member in the direction of its extent, and means including a high-voltage source for maintaining adjacent the liquid on said member an electrical potential gradient of sufiicient strength to electrostatically atomize and project finely divided liquid particles from such member.

13. In an electrostatic coating apparatus'in which particles of liquid coating material are atomized from an extended circular edge of a discharge member and are precipitated under the influence of an electrostaticfield ontoan 9 article spaced from said edge, a nozzle, means for supplying liquid coating material to said nozzle, said discharge member being disposed to receive the liquid coating material discharged from the nozzle, said edge having an extent many times the diameter of the nozzle, and means for producing relative movement of said nozzle and discharge member over a predetermined circular path parallel with but smaller in diameter than said edge to distribute the coating material on said discharge member for substantially niform flow to said edge.

14. In electrostatic coating apparatus, a support for an article to be coated, a discharge member having an extended edge presented toward and in spaced relation to an article on said support, a nozzle, means for supplying liquid coating material to said nozzle for discharge therefrom as a jet stream, an extended surface adjacent the discharge edge and disposed to receive directly the jet stream discharged from the nozzle, said surface having an extent many times the diameter of the jet stream, means for producing relative movement of said jet stream and said surface over a predetermined path along which said member extends and for a distance which is also many times the diameter of the jet stream to distribute the coating material substantially uniformly on said surface for flow to said discharge edge for atomization therefrom, means including a high voltage source for creating between the article on said support and the atomized coating material particles an electrostatic field for electrostatically precipitating the particles on the article, said discharge edge being sufficiently spaced from the article on said support to permit the particles atomized therefrom to be widely dispersed by the action of the electrostatic field during their movement to the article.

EDWIN M. RANSBURG.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,022,956 Lengerke et al Apr. 9, 1912 1,330,218 Reiber Feb. 10, 1920 1,855,869 Pugh Apr. 26, 1932 2,070,972 Lindenblad Feb. 16, 1937 2,451,288 Huebner Oct. 12, 1948 2,562,358 Huebner July 31, 1951

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