US3398892A

US3398892A - Electrostatic coating apparatus

Info

Publication number: US3398892A
Application number: US589749A
Authority: US
Inventors: Fave Richard L La; Norbert M Zupan
Original assignee: Ransburg Corp
Current assignee: Ransburg Corp
Priority date: 1966-10-26
Filing date: 1966-10-26
Publication date: 1968-08-27
Anticipated expiration: 1985-08-27
Also published as: GB1202598A; DE1652425A1

Description

7, 1968 R. 1.. LA FAVE ETA!- ELECTROSTATIC COATING APPARATUS 2 Sheets-Sheet 1 Filed Oct. 26, 1966 INVENTOR.

RICHARD L. LA FAVE NORBERT M. ZUPAN Aug. 27, 1968 R. L. LA FAVE ETAL 3,398,892

ELECTROSTAT IC COAT ING APPARATUS 2 Sheets-Sheet 2 Filed Oct. 26. 1966 INVENTOR.

RICHARD L. LA FAVE NORBERT M. ZUPAN United States 3,393,892 ELECTROSaTATHI COATTNG APPARATUS Richard L. La Fave and Norbert M. Zupan, Indianapolis,

Ind., assignors to Ranshurg Electra-Coating Corp Indianapolis, Ind, a corporation of Indiana Filed Oct. 26, 1966, Ser. No. 589,749 9 Claims. (Cl. 239-15) ABSTRACT OF THE DTSCLGSURE An atomizer for use in an electrostatic spray coating system comprises an elongated body having a flow-guiding surface which receives liquid coating material from an elongated slot and guides it for fiow as a film to a discharge edge from which it is electrostatically atomized. The slot is supplied from an elongated feed chamber containing longitudinally adjustable closures which regulate the effective length of the slot. The atomizer may include means for preliminarily wetting the slot walls and flowguiding surface with a solvent to promote the formation of the liquid material into a uniform film.

This invention relates to electrostatic spray coating apparatus of the general type shown in US. Letters Patent No. 2,893,893 to W. W. Crouse, wherein the coating material flows as a thin film to an extended edge of a charged atomizer and is electrostatically atomized from such edge. The invention is primarily concerned with features which contribute to the uniformity of distribution of the liquid coating material along the atomizer-edge and with features which permit a straight-edged atomizer to project sprays having widths that can be varied up to the full effective length of the atomizer.

A preferred form of apparatus embodying the invention comprises an elongated body having a downwardly inclined surface leading to a horizontally extending discharge edge. The upper edge of such surface is defined by one wall of a longitudinal slot communicating with a feed chamber which extends from one end of the body to the other, parallel to the discharge edge. A second and similarly extending chamber, or manifold, communicates with the feed chamber through a plurality of longitudinally spaced ports each of which is controlled by a valve. Within the feed chamber there are disposed two plugs or closures which can be adjusted along the chamber and the spacing between which determines the effective length of the chamber and slot and thereby the width of the spray discharged from the atomizer. Extending along the atomizer in rear of the aforesaid slot is a pipe provided with a series of longitudinally spaced discharge openings adapted to discharge a solvent onto the aforesaid downwardly inclined surface. An insulated support is provided for the atomizer, such support being connected to the atomizer by means which permits the atomizer-edge to be accurately leveled and the atomizer as a whole to be rotated about a horizontal axis to vary the inclination of the downwardly sloping surface.

When the atomizer body, positioned with its discharge edge spaced from a grounded article to be coated, is connected to a high-voltage source and the manifold is connected to a source of liquid coating material, the coating material will flow into the manifold and thence through opened ports into the feed chamber between the plugs therein, the coating material then emerges from the feed chamber through the slot and fiows downwardly as an exposed film over the downwardly inclined surface to the discharge edge, where it will be electrostatically atomized by the electrostatic field existing between the atomizer and the article. The charged coating-material par- 3,398,892 Patented Aug. 27, 1968 ticles thus formed will be deposited on the article by the electrostatic forces of the field.

Other features of the invention will become apparent from the following more detailed description and from the accompanying drawings, in which:

FIG. 1 is a view, partially diagrammatical in character, of a complete spraying apparatus showing the atomizer in end elevation;

FIG. 2 is a rear elevation of the atomizer and its supporting means;

FIG. 3 is a fragmental rear elevation of the atomizer on an enlarged scale, certain parts being broken away to appear in cross-section;

FIG. 4 is a vertical section on the line 44 of FIG. 3;

FIG. 5 is a lan view showing a device suitable for use in locating and securing in place the plugs which determine the effective length of the feed chamber; and

FIG. 6 is an end elevation of the atomizer body showing a preferred method of forming the paint-emitting slot.

In FIGS. 1 and 2 we have shown an elongated, horizontally disposed atomizer, designated in its entirety by the reference numeral 10, arranged to discharge electrostatically atomized liquid coating material on to the vertically moving stretch of a continuous web 11. The atomizer 10 shown comprises a body 12 of metal or other conductive material connected through a lead 13 to the ungrounded terminal of a high-voltage source 14, the other terminal of which is grounded. Liquid coating material from a source 15 delivered to the interior of the atomizer body 12 through a conduit 16, preferably by means of a variable speed, positive-displacement pump 17. Coating material from within the atomizer emerges, in the manner hereinafter described, along the upper edge of a plane film-supporting face 18 provided on the body 12, such face sloping forwardly and downwardly to a discharge edge 1?. The web 11 is effectively grounded, with the result that there exists between it and coating material on the discharge edge 19 an electrostatic field which serves to atomize finely divided, electrically charged particles from such material and to deposit such particles on the web 11.

An insulating support for the atomizer 10 may comprise a plurality of tubular posts 21 within each of which an adjusting screw 22 is rotatably mounted in fixed axial location. Each of the screws 22 carries a nut 23 projecting outwardly through a vertical slot 24 in the wall of the post and is prevented from rotating by engagement with the sides of such slot. At the location of each post 21 the atomizer body 12 is provided on its rear face with a U- shaped bracket 25 which receives a tongue 26 rigid with the associated nut 23 and pivotally connected to said bracket by a horizontal pivot pin 27. Preferably, the several pins 27 are rigidly secured in their respective tongues 26 and the brackets 25 are provided with set-screws 28 (FIG. 3) which, when tightened against the pins, hold the atomizer in fixed position about the common axis of the pins. Each of the screws 22 projects upwardly beyond the end of its post 21, where it is provided with a knob 29 by which it can be rotated to control the vertical position of the associated nut 23. The number of posts 21 may vary, depending on the length of the atomizer.

The construction of the atomizer 10 is best illustrated in FIGS. 3 and 4. As there shown, the atomizer body 12 is formed of a long metal body of generally trapezoidal cross-section having two

parallel bores

31 and 32 extending longitudinally through it from end to end. The bore 31, hereinafter referred to as the feed chamber, is located between the discharge edge 19 and the other bore 32, hereinafter referred to as the manifold. The two bores intercomrnunicate through a series of ports 33 which are spaced apart longitudinally of the body 12 and each of which is provided with an independently operated shutoff valve 34 screw-threadedly mounted in a boss 35 on the rear face of the body and provided at its rear end with a knob 36 by which it can be rotated to open or close its associated port 33. The ends of the manifold 32 are closed by pipe plugs 37 at least one of which is provided with an appropriate fitting 38 through which the manifold is connected to the coating-material supply conduit 16.

Extending along and defining the upper edge of the face 18 on the atomizer body 12 is the front wall of a slot 40 through which coating material emerges from the chamber 31 to flow by gravity as a thin film over the face 18 to the discharge edge 19. The effective length of the chamber 31 and slot 40, and hence the width of the liquid film on the face 18, is controlled by two plugs arranged for longitudinal adjustment in the chamber. As such plugs may be identical in construction, it will be necessary to describe but one of them, shown at the left in FIG. 3. As there shown, each plug comprises a hollow cylindrical body 41 having a sliding fit in the chamber 31 and provided at its inner end with a rabbet groove receiving an O-ring seal 42. A screw 43 mounted in the body 41 has a head which engages the seal 42 to compress it into sealing engagement with the wall of the chamber 31. Exteriorly, the plug body 41 has a longitudinal groove in which is fixed a baffle 44, preferably of Teflon or like material, that occludes the slot 40. Desirably, the inner end of said baifiie extends over, and is notched to receive, the seal 42 to prevent the seal from being distorted into the slot when the screw 43 is tightened.

For the purpose of positioning each of the plugs in the chamber 31 andoperating the screw 43, we may use a tool of the type shown in FIG. 5. Such tool comprises a long rod 46 having at one end a knob 47 and at the other end a bit 48 adapted to be received in a slot 49 in the end of the shank of the screw 43 and having a width greater than the diameter of the rod. At its outer end, the plugbody 41 has an internal flange 51 provided with a central opening 52 (FIG. 4) of a diameter to pass the rod 46 and having two diametrically opposite grooves through which the bit 48 may pass when properly oriented. To change the position of a plug, the tool of FIG. is inserted, bit-end first, into the chamber 31 from the end of the atomizer-body 12 until the bit 48 engages the end of the plug. The tool is then rotated to bring the bit into the plane of the grooves of opening 51 and moved inwardly, rotating it again if necessary, to bring the bit into the screw-slot 49, whereupon the screw may be loosened to relieve the O-ring seal and free the plug for movement along the chamber 31. Inward movement of the plug is effected by pressure applied to the screw 43 through the tool, outward movement by pulling the tool outwardly while the bit 48 is oriented to engage the inner surface of the flange 51. The inner and outer ends of the opening 52 may be chamfered to facilitate passage of the bit 48. When the plug is remote from the end of a long atomizer body, insertion of the bit through the opening 52 can be facilitated by the use of a bit-centering pilot 53 slidable on the rod 46 and adapted to be secured at any desired position therealong by a set-screw 54.

In an electrostatic atomizer, the quality of atomization obtained can be affected by the rate of liquid supply, increasing rates of supply tending to coarsen the atomization. In stationary atomizers, such as that herein shown, the character of atomization required for the production of even relatively poor finishes cannot be obtained if the rate of coating-material supply is much in excess of about cc. per minute per inch of effective discharge edge. Where the slot is coextensive with the discharge edge and of limited extent in the direction of flow through it, it is not practical, at such a low rate of supply, to control that rate as a function of slot width. This is so because, even at extremely low hydraulic pressures, a slot limiting flow therefore employ a slot 40 which is too wide to have any flow-regulating efiect and control the rate of coating-material supply by regulating the speed of the pump 17. In such an arrangement, the front edge of the slot serves in effect as a weir, and in order to provide uniform distribution of the coating material along the discharge edge 19 of the atomizer it is therefore necessary that such slotedge be accurately straight. For slot-widths of the range we find it convenient to use, it has been found impractical to form the slot as a saw-kerf with a thin milling cutter, as the inherent flexibility of such a cutter causes the slot to depart from accurate straightness, while tooth-marks on the slot walls would result in undesirable local irregularities. We therefore prefer to form only the front wall of the slot in the atomizer-body 12 and to employ as a rear wall the opposed, narrow front face of a bar or cover 55 which extends longitudinally of the body 12 and is secured thereto by screws 59. A preferred manner of forming the front slot-wall, shown in FIG. 6, involves the use of a rigid milling cutter 56, one edge portion of which cuts through the material of the atomizer body into the chamber 31 creating a generally V-shaped, longitudinal groove having two plane side surfaces 57 and 58 which are at a substantial angle to each other and both of which are parallel to the axis of chamber 31. The surface 57, which constitutes the front wall of the slot 40, lies in a plane at least generally radial of the chamber 31. Its intersection with the face 18 forms the weir over which the coating material spills to flow down that face to the discharge edge 19. The surface 58 lies in a plane spaced from the axis of the chamber 31 by a distance slightly less than the radius of such chamber. The cutter 56 is indicated in FIG. 6 as a cylindrical cutter, providing a angle between the

surfaces

57 and 58; but if a different angular disposition of those surfaces is desired, a frusto-conical cutter can be used. The milling operation performed by the cutter 56 leaves a sharp edge at the intersection of the surface 58 with the wall of the chamber 31 and provides a gap between that edge and the surface 57. The width of that gap should be somewhat greater than the intended width of the slot 40 in order to prevent contact of the sharp edge with the baffles 44; but otherwise the gap should be as narrow as practicable in order that it will not inhibit the sealing action of the O-ring 42.

We have found that when the atomizer as so far described is first placed in operation, the coating material tends to emerge from the slot 40 only at more or less isolated locations spaced alon the slot and to flow down the face 18 in separated films rather than as a continuous sheet and that material later emerging tends to follow the same paths. We have found that the persistence of such a condition can be prevented by temporarily reducing the viscosity of the liquid on the face 18, a result readily accomplished by supplying to coating material emerging from the slot and to unwetted portions of the face 18 a solvent or thinner which is compatible with the coating material. For that purpose, the atomizer may comprise a longitudinally extending tube 63 disposed in rear of the slot 49, connected to a solvent-source 64 through a valved conduit 65 (FIG. 3), and provided with a series of longitudinally spaced openings 66 directed to discharge solvent generally toward the slot and the film-supporting face 18 of the atomizer body.

For the purpose of emptying the

bores

31 and 32 after completion of a painting operation, the atomizer body 12 may be provided at the ends of such bores with drain openings, conveniently screw-threaded openings receiving removable closure screws 60.

In Setting up the apparatus for a coating operation, the atomizer 10 is adjusted to the desired position about the axis of the pins 27 and secured in such position by tightening the screws 28. If the spray from the atomizer is to be projected generally horizontally, as is the case in the arrangement shown in FIG. 1, the face 18 of the atomizer body 12 is desirably disposed to slope downwardly and forwardly to the discharge edge 19 at an angle to the horizontal of about 45 or somewhat less; but in any event, the atomizer should be so disposed that the lower surface of the body 12 has at least a slight downward slope toward the edge 19 in order to prevent the coating material from flowing around the edge and rearwardly on such lower surface. If the spray is to be projected in a general downward direction, the slope of the face 18 may be increased even to a point where the face is vertical. By adjustment of the screws 22 in the posts 21, the atomizer may be brought to the desired vertical position and its discharge edge 19 leveled. The plugs which serve as end closures for and determine the effective length of the feed chamber 31 are set to provide a spray of the desired width and location along the atomizer, the valves 34 between the plugs are all opened, and the valves 34 outwardly beyond the plugs are all closed.

With the atomizer so positioned, the pump 17 is placed in operation at a speed such as to supply coating material at the desired rate; and as coating material begins to emerge from the slot 40 solvent is supplied to the tube 63, and the supply of solvent is continued until the coating material has spread on the face 18 into a film which is continuous between the bafiies 44, whereupon the supply of solvent may be terminated. Thereafter, with the atomizer charged, coating material supplied to it emerges through the slot 40 between the bafiles 44, flows downwardly as a thin film across the wetted face 18 of the atomizer body, and is electrostatically atomized and deposited as described above and in the aforesaid Crouse patent.

An accurate indication as to the horizontal disposition of the discharge edge 19 is provided by the appearance of the coating material being atomized at such edge. The electrostatic field at such edge distorts the coating-material film into a series of spaced cusps the tips of which repeatedly break off to form the charged spray particles. Assuming substantial uniformity of field strength along the edge 19, the appearance at one portion of such edge of cusps having a wider spacing and larger diameter than the other cusps indicates that such edge portion is too low, a condition which can be corrected by appropriate adjustment of one or more of the screws 22.

The number of posts 21 used to support the atomizer will depend upon the length, rigidity, and inherent straightness of the atomizer body. The three posts shown in FIG. 2 were found advisable for an atomizer 52" in length designed to produce a spray having a width of 48" at the discharge edge. As shown in FIG. 2, the posts 21 are supported from their lower ends. However, remov- =ability of the pivot pins 27 permits the posts to be inverted and supported from overhead without inverting the atomizer.

While we have shown an atomizer as arranged to spray coating material generally horizontally on to a vertically moving, continuous web, it will be apparent that, by swinging the atomizer on the pins 27, it can be arranged to spray downwardly on to a web which is moving in a general horizontal path. It will also be apparent that the continuous web can be replaced by a conveyorized series of discrete articles. The path and form of the spray projected from the discharge edge 19 will be determined largely by the pattern of the lines of force of the field existing between that edge and the article or articles being coated, and it is therefore possible, for example, for the atomizer, oriented as in FIG. 1, to spray downwardly on to an article or articles passing below it. However, it is usually desirable that the cross-section of the atomizer body point generally in the direction of spray projection, as in that manner the spray will be more compact and the spray particles will have shorter distances to travel before being deposited.

We claim as our invention:

1. An electrostatic atomizer for use in an electrostatic spray coating system, said atomizer comprising an elongated body having a plane face terminating in an extended discharge edge from which liquid coating material is to be electrostatically atomized, said body being provided with an elongated feed chamber and, communicating with said chamber, an elongated slot through which liquid'coating material from said chamber can emerge to flow as a thin film over said face to the discharge edge, a closure for each end of said feed chamber and slot, at least one of said closures being adjustable longitudinally of the body to establish the effective length of the chamber and slot, and means for conveying liquid coating material into said chamber between said closures.

2. An electrostatic atomizer according to claim 1 wherein said last named means comprises an elongated passage extending parallel to said chamber and communicating therewith through a plurality of longitudinally spaced ports, each of said ports having an associated valve by means of which it can be opened or closed.

3. An electrostatic atomizer according to claim 1 wherein each adjustable closure comprises a generally cylindrical member freely slidable in said chamber and having a radially projecting bafile received in said slot to occlude it, an elastic annular seal carried by said body and distortable by axial compression into sealing engagement with the wall of said chamber, and releasable means on said cylindrical member for so distorting the seal.

4. An electrostatic atomizer according to claim 3 wherein said bafile overlies said sealing member to prevent its distortion into the slot.

5. An electrostatic atomizer according to claim 3 wherein said seal-distorting means comprises -a screw mounted in said member and having an axially disposed threaded shank provided at its end with a slot for the re ception of a tool-bit by which the screw can be rotated to release or distort the seal, said member having a central passage through which the tool-bit can be inserted into engagement with said slot, said member also having in said passage an abutment engageable by the tool-bit upon withdrawing movement thereof.

6. An electrostatic atomizer according to claim 1, wherein both said closures are adjustable longitudinally of said body.

7. An electrostatic atomizer for use in an electrostatic spray coating system, said atomizer comprising an elongated body having a plane face bounded on one side by a discharge edge from which liquid coating material is to be electrostatically atomized, said body being provided with an elongated cylindrical feed chamber extending parallel to said discharge edge and with a longitudinal external groove having first and second side surfaces disposed at a substantial angle to each other and intersecting the wall of the feed chamber along parallel lines defining a narrow gap, said first side surface intersecting the plane face of said body on a line parallel to said discharge edge, and a cover removably secured to said body and overlying the second side surface of said groove, said cover having a side extending parallel and in spaced relation to said first groove surface and defining therewith an elongated slot through which coating material supplied to said feed chamber can emerge to flow over said bodyface as a thin film to said discharge edge.

8. An electrostatic atomizer according to claim 7 wherein said feed-chamber is provided with a longitudinally adjustable end closure having a baffie projecting into said slot.

9. In combination with an electrostatic atomizer for use in an electrostatic spray coating system, said atomizer having a flow-guiding face terminating in an extended discharge edge from which liquid coating material is to be atomized, said atomizer also having, spaced from said discharge edge, an elongated slot through which liquid coating material supplied to the atomizer emerges to flow as a thin film over said face to said discharge edge, means mounted on said atomizer for discharging a liquid References Cited UNITED STATES PATENTS 7/1920 Diebold 239-13 8 a v Miller 239-15 Ransburg et a1 239-15 Crouse 239-15 Heuschkel 239-15 Marvin 239-15 EVERETT W. KIRBY, Primary Examiner.