US3512502A

US3512502A - Electrostatic coating apparatus

Info

Publication number: US3512502A
Application number: US588428A
Authority: US
Inventors: Edward W Drum
Original assignee: Ransburg Corp
Current assignee: Ransburg Corp
Priority date: 1966-10-21
Filing date: 1966-10-21
Publication date: 1970-05-19
Anticipated expiration: 1987-05-19
Also published as: BE705420A; NL7201962A; SE324129B; CH476522A; NL6712993A; SE363983B; DE1652423A1; GB1210257A; GB1210258A; DE1652423B2; NL134990C

Description

y 19, 7 E. w. DRUM 3,512,502

ELECTROSTATIC COATING APPARATUS Filed Oct. 21. 1966 34 I6 /27 29a 29b 35 29 INVENTOR.

EDWARD W. DRUM United States Patent Int. Cl. B05!) 5/04 US. Cl. 118624 Claims ABSTRACT OF TIE DISCLOSURE A spraying electrode that forms an annular spray pattern can be made to produce a smaller pattern and a more uniform coating by providing the spraying electrode with a small inner electrode and a plastic member having a forward edge extending forwardly of the inner electrode adjacent the spraying electrode. A uniform coating of greater extent can be obtained by a plurality of closely spaced spraying electrodes, each provided with an inner electrode and a plastic member between the inner electrode and the spraying electrode, without significantly increasing the spray particle size. Further increases in the uniformity and extent of the coating of such a plurality of atomizers may be obtained by oscillating the atomizer.

This invention is directed to method and apparatus for electrostatic spray coating and particularly to method and apparatus employing electrically charged sprayers producing an annular spray pattern and including spray pattern control means to achieve uniform deposited coating.

Atomizers possessing an electrically charged annular atomizing edge are well known in the art of electrostatic coating. Such atomizers produce a spray of finely divided charged liquid particles in an annular or doughnutshaped spray pattern having a central portion substantially devoid of particles. If coating material is projected perpendicularly from such an atomizer onto an extended fiat surface moving transversely of the direction of projection of the spray, the deposited coating will form a band having a width equal to the outer diameter of the projected spray patterns. The thickness of the deposited coating will be relatively thin at the center line of the band and increase to a maximum thickness between the center, and each outer edge of the pattern. Thus, the thickness of the coating deposited on a transversely moving extended flat surface from such an atomizer will have a double-humped distribution.

An extended uniform deposited coating can be achieved by arranging a plurality of such atomizers so that the deposited bands are blended to produce a wider band which is more uniform. One such method is disclosed in US. Pat. 2,839,425 issued to James W. Juvinall on June 17, 1958. Some limitations are encountered in using such a method however, because if the atomizers are placed too closely together their relative proximity to each other reduces the electrostatic field gradient adjacent each atomizer and can result in unsatisfactory atomization.

Another method of obtaining a uniform coating from atomizers producing an annular spray pattern is to improve the uniformity of the deposited coating from the individual atomizer.

US. Pat. 2,926,106 to W. D. Gauthier discloses that by positioning an electrode within the expanding annular spray pattern and maintaining it at an electrical potential between that of the atomizing device and the potential of the article, it is possible to divert a substantial number of spray particles into the central void of the annular pattern from the spray initially surrounding the ice central void and, thus, increases the thickness of the deposited band from the atomizer at its center and reduce the thickness of the humps lying on either side of the center. Use of the inner electrode disclosed however, does not significantly decrease the distance between the humps of the deposited coating pattern.

US. Pat. 2,989,241 to D. H. Badger discloses an improved apparatus for effecting a uniform distribution of paint deposited from a rotating annular atomizer. The invention disclosed in Pat. 2,989,241 resulted from the discovery that the probing action of an inner electrode is most effective when an inwardly directed component of the electrostatic field is produced adjacent the atomizing edge where the field intensity is high. In addition to improving the uniformity of the deposited coating, the diameter of the annular pattern and the distance between the two humps is reduced because the inwardly directed component of the electrosatic field opposes the centrifugal forces imposed on the coating material due to the rotation of the atomizer.

None of the prior disclosed methods of achieving a uniform deposited pattern of paint from an annular charged spray pattern are completely satisfactory in automatic spray coating equipment in which the electrodes are made of highly conductive rnaterials such. as metal. The configuration disclosed in US. Pat. 2,839,425 does not permit flexibility in automatically coating parts of different sizes. When the configuration disclosed in US. Pat. 2,989,241 is incorporated into an automatic spraying apparatus, erratic operation prevents full utilization of the capability of the disc-like inner electrode with small highly conductive spraying electrodes. In many instances paint is attracted to the concavely-dished disc electrode in such quantity that it is thrown from the concavely-dished disc in large globs and deposited onto the work, destroying the quality of the finish. To be commercially feasible, automatic equipment must run for an extended period of time without interruption for cleaning. A further disadvantage of this configuration is intermittent sparking which occurs when the spraying elec trode and inner electrode are highly conductive and the edge of the inner electrode is located closely adjacent the atomizing edge to obtain effective probing. Sparking, of course, cannot be tolerated in a painting operation because of the fire hazard.

In accordance with my invention a uniform deposited coating is achieved with a sprayer comprising a spraying electrode producing an annular spray pattern by using an electrode within the annular pattern which is maintained at a potential intermediate the potential of the spraying electrode and the article being coated and by positioning around the inner electrode a plastic member having an edge which extends forwardly of the inner electrode. Also, a plurality of atomizers may be closely spaced to achieve a more extensive uniform coating. One or more sprayers may be oscillated to improve the uniformity of the deposited coating even further I use the words forward and forwardly to refer to the axial direction of movement of coating material when apparatus is in use.

With my invention I am able to achieve a substantial control of the deposited pattern and effect a reduction in r the distance between the humps of the deposited coating pattern. Because I am able to use a small inner electrode, sparking between the spraying electrode and the inner electrode is not a problem. Use of the inner electrode and the plastic member as described is particularly desirable in an automatic installation in which a plurality of such atomizers are arranged so that their deposited patterns combine on the article to produce a uniform coating. Where a plurality of atomizers are required, the atomizers may be positioned closely adjacent each other without significantly disturbing the quality of atomizaas tion from the atomizers. The field gradient adjacent the atomizing edge of each atomizer is not substantially reduced by the proximity of the similarly charged atomizing edge adjacent it but is maintained by the electrostatic field control action of the inner electrode and the plastic member. Individual sprayers of such a group may be operated to coat articles of varied size without affecting the uniformity of the coating and the efliciency of utilization of coating material is enhanced by the smaller deposited pattern achieved with my invention.

An extended deposited coating of improved uniformity can be achieved by oscillating spraying electrodes rapidly over a distance about equal to one-half distance between the two humps. The improvement in uniformity of the deposited coating increases with the rapidity of oscillation of the spraying electrode.

My invention will be more fully understood from the following detailed description in the attached drawings in which:

FIG. 1 is an isometric view of a plurality of my atomizers mounted on a reciprocator for use in coating refrigerator cabinets.

FIG. 2 is a side elevational view of an atomizer and the pattern control members partially broken away.

Illustrating a manner in which my invention may be used, FIG. 1 shows an electrostatic coating unit in which three atomizers producing annular spray patterns are arranged to coat the refrigerator housing. Refrigerator cabinets are carried past the spraying apparatus by grounded conveyor 11 at a distance of 12-18 inches. The spraying electrodes 12 are carried on the roating shaft of motors 13 which are positioned on support 14. Polyphase A.C. power is supplied to motors 13 through conductor 15 from a polyphase transformer (not shown) in high voltage supply 16. In accordance with established practice, the secondary coils of the polyphase transformer are insulated to permit the coils to be charged to high voltage with respect to the primary coils and to ground. A multi-conductor high voltage cable connects the secondary coils of the polyphase transformer with the motor windings and motor frame so that polyphase power is delivered to motors 13 as well as the high DC. voltage. The spraying electrodes 12 are normally charged to 70,000 to 90,000 volts with respect to articles 10 but higher voltages such as 100,000 volts may be used. Support 14 is insulated from ground by rod 17 of insulating material carried by the reciprocator 18. Coating material is supplied to spraying electrode 12 from a source 19 by means of pump unit 20. Pump 20 is comprised of a positive displacement gear-type pumps 21 to supply coating material through hoses 22 and feed tubes 23 to each spraying electrode 12. Pumps 21 are driven from power unit 24 through a drive train 25 comprised of insulating material sutficient to withstand the high voltages used on the spraying electrode.

Details of the atomizer are shown in FIG. 2. Spraying electrode 12 is carried by hub 26 on rotating shaft 27 of motor 13. Within spraying electrode 12 an inner electrode 28 is carried by plastic member 29. Hub 26, spraying electrode 12, and electrode 28 are made of a metal, such as steel. Plastic member 29 is typically made of nylon, but other insulating materials may be used. Electrode 28 is maintained at a potential between the potential of atomizer 12 and the article 10 by resistor 30 which is grounded through an insulated high voltage cable 31. Current flowing from spraying electrode 12 to electrode 28 and passing through resistor 30 raises electrode 28 above ground potential. Resistor 30 is carried within insulating tube 32 which has sufficient insulation strength to isolate the resistor from the high voltage to which motor 13 is charged. Insulating tube 32 may be made of polyethylene or another suitable insulating material.

Spraying electrode 12 comprises a cylindrical rim having a sharpened atomizing edge at its forward end with a diameter of 4% inches. The interior surface of Spraying electrode 12 is angled slightly with respect to the axis of rotation so that liquid coating material deposited on the interior surface of the spraying electrode adjacent its rear through feed tube 23 will be urged by centrifugal force in the form of a thin film to the sharpened forward edge from which it will be atomized under the influence of.the electrostatic field. Spraying electrode 12 is carried upon hub 26 by a series of fasteners 33 which concentrically space spraying electrode 12 from hub 26. Hub 26 slips over rotating motor shaft 27 and is held fixed to shaft 27 by set screws 34. Hub 26 also carries a plastic member 29 which, in turn, carries conductive electrode 28 and electrically insulates it from hub 26 and spraying electrode 12. Electrode 28 is connected to ground through spring 35, resistor 30, spring 36, and conductor 31. Current flow from spraying electrode 12 to electrode 28 through resistor 30 to ground will maintain electrode 28 at a potential between the potential of spraying electrode 12 and ground.

Electrode 28 is a pin having a diameter of about inch which has been turned to a point on its forwardmost edge. Electrode 28 has been pointed to prevent further the accumulation of particles. It is believed that particles approaching the vicinity of electrode 28 will be recharged with a polarity such that they are repelled from the electrode. Although I have shown specifically an inner electrode in the shape of a pin which tapered to a sharp point at its forwardmost edge, a small inner electrode with an annular edge which tapers to a forwardly projecting knifeedge may also be used. However, I have found it best to use the smallest possible inner electrode in avoiding the erratic results which occur when large inner electrodes are used. An additional advantage in using a small inner electrode is the avoidance of sparking between spraying electrode and inner electrode.

Plastic member 29 with insulating tube 32 electrically insulates resistor 30, conductor 31, and electrode 28 from the motor shaft 27, hub 26, and spraying electrode 12 all of which are charged to high voltage. Plastic member 29 has a tubular section 29a extending rearwardly within hub 26 a distance in excess of the sparking distance for the potential difference between spraying electrode 12 and electrode 28. The length of tubular section 2911 prevents sparking along the surface of plastic element 29 or tube 32 or through the air gap between electrode 28 and the interior of hub 26. When the atomizer is charged to 70,000 volts and a resistance of 10,000 megohms is incorporated in resistor 30, a tubular portion having a length of about 4 inches is satisfactory. In addition to the tubular portion 29a extending rearwardly within the hub, plastic member 29 includes an annular web which extends to adjacent the interior surface of spraying electrode 12. At the circumference of the annular web is a forwardly extending rim portion 29b which forms a sharp forwardmost edge. Electrode 28 is carried on the axis of rotation within recess 29c at the center of the web portion of plastic member 29.

Plastic member 29 and electrode 28 coact to produce an electrical force at the atomizing edge 12a of spraying electrode 12 acting on the coating material which is directed toward the axis of rotation and in opposition to the centrifugal force imposed on the coating material by the rotation of the spraying electrode 12. The electrostatic field at atomizing edge 12a is very intense. Plastic member 29 and electrode 28 by the distortion of field at this point create a strong inwardly directed force acting on the coating material and thus may exert great control over the annular pattern produced by spraying electrode 12. Surprisingly, plastic member 29 has a great influence in the control of the annular pattern produced by spraying electrode 12. By properly shaping the plastic member with respect to spraying electrode and the inner electrode, pattern control can be achieved and with a very small inner electrode. The plastic member should have an annular portion between the spraying electrode and inner electrode which projects forwardly of the inner electrode. Good results have been obtained when the forwardly projecting portion is tapered to a sharp edge which lies approximately on the conical plane between the forwardmost edge of the inner electrode and the forwardmost edge of spraying electrode. With the embodiment shown in FIG. 2, very satisfactory results are obtained when the forwardmost point of electrode 28 is of an inch behind the atomizing edge 12a and the forwardmost edge of rim portion 2911 has a diameter of an inch smaller than that of the atomizing edge 12a and the plane of the forwardmost edge 2% is located & of an inch behind the plane of the atomizing edge 12a. With such an atomizer, it is possible to operate without collection of the coating material on the inner electrode and still achieve good pattern control.

In operation coating material is supplied from container 19 to the interior surface of annular spraying electrodes 12 through feed tube 23. Rotation of the spraying electrodes 12 forms the liquid coating material into a thin liquid film which is delivered to the edged the spraying electrode and atomized under the influence of the electrostatic field, producing an annular spray pattern. As the refrigerator cabinet is transported past the spraying electrodes 12, the paint is deposited in three bands on cabinet 10. To obtain a uniform coating on cabinet 10, the three bands are blended so that the deposited patterns of the individual atomizer combine into a uniform film. With my invention the deposited pattern of each spraying electrode 12 may be controlled to permit blending to be more easily effected, and the spraying electrodes do not have to be varied in size to achieve a uniform coating. My invention also permits greater freedom in the location of the atomizers with respect to each other. When a plurality of atomizers which do not include an inner electrode are closely spaced, the proximity of the charged spraying electrodes with respect to each other reduces the gradient of the electrostatic field at their atomizing edges, and produces a determinable increase in the size of the spray particles. Use of my invention permits spraying electrodes to be closely spaced without increasing the spray particle size. It is to be understood that when I use the term closely spaced, I mean spacing at such a distance that an increase in spray spot size may be determined if the inner electrode of the invention, maintained at a voltage intermediate the spraying electrode and the article to be coated, is not used. One method of determining particle size is to measure the diameter of spots formed by spray particles impinging on an appropriate target under controlled conditions. Conditions which have been used in practice involve passing a 4" by 6" fiat target through the spray longitudinally of its longest dimension with one face in a plane perpendicular to the spray axis and at a distance of about 12" from the spray source with sufiicient rapidity that the exposed face of the target is substantially free of overlapping spots. The average diameter of the ten largest spots on the target is used as the determination of spray spot size.

As one example of the benefit of my invention, an atomizer of the embodiment shown in FIG. 2 and having an atomizing edge with a diameter of 4% inches was arranged on a line between two atomizers having atomizing edges of 4 inches in diameter. The outermost atomizers were closely spaced to the central atomizer, each with a distance of 12 inches between its axis of rotation and the axis of rotation of the central atomizer. In one test all three atomizers were electrically charged to 70,000 volts. Lilly Varnish 565 Enamel, reduced to a viscosity of second Zahn 2 with SC 100 solvent, was supplied to the central atomizer which was being rotated at 900 rpm. at a rate of 70 cubic centimeters per minute. The spray spot size when measured as previously described was .011 inch. Another test was conducted under the same operating conditions, however in the second test plastic member 29, electrode 28, resistor (which was 10,000

megohms) and high voltage cable 31 were removed from the atomizer. The spray spot size from the spraying electrode 12 alone was .017 inch. A spray spot size of .015 is considered the maximum spray spot size acceptable for a high quality coating such as that required for refrigerator cabinets.

The atomizers shown in FIG. 1 may be operated while stationary or may be oscillated by reciprocator 18 to obtain a more uniform coating.

With atomizers using spraying electrodes but no inner electrode and plastic member, the atomizers must be widely spaced to prevent deterioration of the quality of atomization. Because of the large and widely spaced humps of the unmodified deposited pattern of such atomizers, achievement of a uniform coating requires a wide oscillation of the spraying electrode. To eliminate the humps of the deposited pattern of an atomizer with an annular atomizing edge having a 4 inch diameter which is rotating at 900 rpm, it is necessary to oscillate the atomizer through an 8 inch stroke. The width between the points on the edges of the deposited pattern which are /2 of the maximum thickness of the paint in the band (hereinafter referred to as the deposited pattern width) is about 18 inches. With my invention, uniform deposited patterns are obtained by oscillating an atomizer of the type shown in FIG. 2 through a 4 inch stroke, and the deposited pattern width is about 12 inches. When I have referred to oscillation of the atomizer, I have meant that the atomizer is moved so that its axis of rotation describes a sine wave with a transversely moving article to be coated. A reciprocator producing such movement may be made simply by mounting the atomizer on a support which is moved only in a vertical direction by a rotating arm. An example of such a mechanism is referred to commonly as a scotch yoke. Oscillation of a sprayer with means to control its deposited pattern is also useful where dry powdered coating materials are sprayed from the spraying electrode. A further advantage of this invention where a plurality of atomizers are used is that the closely spaced atomizers reduce the mass of the apparatus reciprocated.

Because of the higher concentration of coating material at the center of the deposited pattern and reduced band width of the deposited pattern, more efficient utilization of the coating material is achieved with my invention as well as more uniform coating. Use of the means to control the deposited pattern, such as plastic member 29 and electrode 28, permit a significantly lower voltage on the spraying electrode and a corresponding reduction in the spacing between the spraying electrode and the article which contributes to the reduction in size of the deposited pattern from the sprayer. The smaller and more concentrated deposited pattern resulting from my invention permits greater correspondence between the location of the sprayer and the edge of the article being coated and a reduction in coating material overspray. Corresponding reductions in overspray are obtained from the increased accuracy with which the sprayers may be operated in correspondence with the edges of the articles being coated to prevent spray particles from being projected into the voids between articles.

In practicing my invention the atomizers may be arranged vertically rather than at an angle with respect to the floor as shown in FIG. 1.

I claim:

1. An apparatus for electrostatically spray coating an article comprising a spraying electrode producing an annular spray pattern, means to rotate the spraying electrode, means to supply coating material to the spraying electrode, means to maintain a high voltage difference between the spraying electrode and the article to effect deposition of coating material on the article, means to control the pattern comprising an inner electrode maintained at a potential between the potential of the spraying electrode and the potential of the article, and a plastic member between said inner electrode and the spraying electrode having a forward edge extending forwardly of said inner electrode adjacent the spraying electrode.

2. Apparatus as set forth in claim 1, wherein the forward edge of the plastic member lies within a fraction of an inch of the spraying electrode and is formed into a sharp edge.

3. The apparatus as set forth in claim 2 wherein the forward edge of said plastic member is located about on a conical surface extending between the forward edge of said spraying electrode and the forward edge of said inner electrode.

4. Apparatus as set forth in claim 1 wherein said plastic member includes a web between its forward edge and said electrode, said web including a recess in which said inner electrode is carried.

5. Apparatus as set forth in claim 1 wherein said spraying electrode is a cylindrical rim having a sharpened forward atomizing edge and is carried by a hub adapted to engage a shaft on said means to rotate the spraying electrode, said plastic member including a tubular portion extending rearwardly within said hub to prevent sparking between said inner electrode and said hub.

6. Apparatus as set forth in claim 5 wherein a plastic tube extends within the tubular portion of said plastic member, said plastic tube carrying a resistor connected at its forward end with said inner electrode and grounded at its rearward end to maintain the potential of the electrode between the potential of the spraying electrode and ground potential.

7. Apparatus as set forth in claim 1 wherein said inner electrode is a pin being formed at its forwardmost end into a sharp point and being carried by said plastic member on the axis of rotation.

8. Apparatus for electrostatically spray coating an article, comprising a plurality of closely spaced spraying electrodes, means to rotate said spraying electrodes, means to supply coating material to said spraying electrodes,

means to charge said spraying electrodes to high voltage with respect to the article to be coated, means to oscillate said plurality of spraying electrodes, said spraying electrodes each including an inner electrode maintained at a potential between the potential of the spraying electrode and the potential of the article to be coated, and a plastic member between said inner electrode and the spraying electrode, said plastic member having a forward edge extending forwardly of said inner electrode adjacent the spraying electrode.

9. Apparatus as set forth in claim 8 wherein the plastic member includes a web between its forward edge and said electrode, said web including a recess in which said inner electrode is carried.

10. Apparatus as set forth in claim 8 wherein said inner electrode is a sharpened pin carried on the axis of rotation.

References Cited UNITED STATES PATENTS 2,559,225 7/1951 Ransburg 11793.42 3,402,697 9/ 1968 Kock 1l793.44 X 2,736,671 2/ 1956 Ransburg et al 11793.42 2,926,106 2/1960 Gauthier 11793.44 2,989,241 6/1961 Badger 117-93.42 X 3,155,539 11/1964 Juvinall 118-626 X 3,169,883 2/1965 Juvinall 11793.42

OTHER REFERENCES Ransburg Electro-Coating Corp.: Ransburg No. 2

Process, 1961, page 9.

ALFRED L. LEAVI'IT, Primary Examiner J. H. NEWSOME, Assistant Examiner US. Cl. X.R.

1l7-93.4; ll8-626; 23915