US 3186864 A
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June 1, 1965 w. s. PETTIGREW ETAL 3,186,864
METHOD FOR ELETROSTAT IC PAINTING Filed Feb. 19, 1962 VIBRATOR POWER 8 U PPLY VIBRATOR Q) (7 1kg ATTORNEY United States Patent 3,186,864 METHOD FGR ELEQTRGSTATIE PAH' ITING William S. Pettigrew, Fleasant Ridge, and Eugene W.
Christen, Birmingham, Mich, assignors to General Meters Corporation, Detroit, Mich, a corporation of Delaware Filed Feb. 19, 1962, Ser. No. 174,072 4 Claims. (Cl. 117-9342) This invention relates to electrostatic coating and particularly to a method and apparatus utilizing an endless belt as an atomizing means.
It has been previously known in the art of electrostatic coating to atomize a liquid material and to subject the atomized material to an electrical field which terminates on a workpiece for charging the material and de positing it on the workpiece. Various methods have been devised to atomize the material such as conventional air spray guns and rapidly spinning discs. The air spray guns have the advantage of throwing the atomized liquid in a desired direction, but inherently reduce the effectiveness of the electrostatic field by introducing strong air currents which tend to blow a large portion of the atomized material past the workpiece thereby wasting the paint. The spinning discs, on the other hand, atomize the coating material by centrifugal force and cause relatively little wind. An important disadvantage of the discs, however, is that the coating material comes off in all directions with respect to the axis of the disc or, in other words, the spray is not directional. This type of atomizer is not practical for use in the most common type of painting application; that is, where the material to be painted moves on a straight line conveyor past the painting equipment.
The aim of the present invention is to provide a centrifugal atomizer which forms a spray pattern which is directional in nature. The invention is carried out by providing an endless belt passing at high speed around a roller and means for applying coating material to the belt whereby the material will be atomized by centrifugal force as the belt passes around the roller. The atomized material will be thrown into an electrostatic field which assists in carrying the atomized material to the workpiece.
The above and other advantages of the invention will become more apparent from the following specification taken in conjunction with the accompanying drawings in which like numerals refer to like parts and in which:
FIGURE 1 is an elevational view of an electrostatic coating apparatus according to the invention;
FIGURE 2 is a plan view of the apparatus of FIG- URE 1;
FIGURE 3 is a schematic illustration of a coating apparatus according to another embodiment of the invention; and
FIGURE 4 is a schematic illustration of a coating apparatus according to still another embodiment of the invention.
FIGURES 1 and 2 show painting apparatus comprising an endless electrically conductive belt passing around a pair of horizontally spaced rollers 12 and 14. The rollers are supported by a frame 16 having a pedestal portion 18 connected to a hydraulic reciprocating mechanism 19 on the floor. The roller 12 is driven by a motor 20 while the roller 14 is an idler. A paint supply means 22 is mounted on the frame 16 above the belt 10 and has a plurality of feed orifices 24 arranged to apply paint to the upper surface of the belt 10. A vibrator 25 mounted on the frame 16 and connected to an air supply by a hose 26 agitates the apparatus so as to aid in attaining a uniform distribution of paint over the belt 10.
A charging ring 26 is mounted on an outwardly extending portion 28 of the frame 16 and defines a loop in front 3,185,864 Patented June 1, 1965 "ice of the roller 14. The portion 28 is electrically insulated from the frame 16 by an insulator 30 of any suitable type. A power supply of, say, 100,000 volts has one terminal connected to ground and the other terminal connected via conductor 32 to the charging ring 26 and also connected through a resistor 34 to the frame 16. Hence, the charging ring will have a potential of about kilovolts and the remainder of the atomizing apparatus will have a lower potential depending upon the size of the resistor 34. Preferably the resistor should be selected to maintain the potential on the belt 10 at about 80,000 volts. However, the device will be operative even if the resistoris eliminated.
A straight line conveyor 40 of conventional structure is arranged to carry workpieces to be painted past the painting apparatus at a distance of about 20 inches from the roller 14. The conveyor, and hence, the workpiece is grounded thereby establishing an electrostatic field between the workpiece and the charging ring 26.
In operation, the belt motor 20 will be energized to cause the belt to travel at a high speed around the idler roller 14. The paint is applied to the upper surface of the belt by the paint supply device 22 at a rate sufiicient to form a thin film on the belt. When the paint film reaches the idler roller 14, it will be thrown off the belt by centrifugal force and will be projected through the charging ring 26. While the paint is on the charged belt 10, and further as it passes through the charging ring 26, it will become electrically charged so that the electrostatic field between the charging ring 26 and the workpiece 42 will assist in transporting the paint particles to the workpiece for deposition thereon. The belt 10 should travel at a speed sufficient to atomize the paint into fine particles. However, this speed will depend upon the viscosity of the paint used as well as the radius of the idler roller 14.
Another embodiment of the invention, as schematically depicted in FIGURE 3, is the same as that of FIGURES 1 and 2 except that there is no high voltage connection to the belt 10 and the belt need not be electrically conductive. In this device the paint is atomized and projected through the charging ring 26 as before, but will be charged solely by the effect of the charging ring, and the atomized paint will then be deposited on the workpiece 42 by an electrostatic field extending between the charging ring and the workpiece.
The embodiment of the invention shown in FIGURE 4 is likewise similar to that of FIGURES l and 2 except that the charging ring is omitted and the full 100,000 volts of the power supply are connected to the conductive belt 10. Then the electrostatic field will extend from the belt 10 to the workpiece 42 and the atomization will take place in the same electrostatic field which deposits the atomized material on the workpiece.
In each of the embodiments of the invention described, the atomizing apparatus is mounted on a linear actuator so that the entire apparatus can be reciprocated vertically and the paint spray can be caused to vertically scan the area through which the workpieces pass. Alternatively, the atomizing apparatus can be made to oscillate about a horizontal axis to produce the same vertical scanning effect. In addition, support member 18 may be made to be rotatable by any suitable means, not shown, so that the atomizer can be aimed toward the workpieces at various' angles or can be made to continuously sweep the work area, if desired.
It will thus be seen that this invention provides a centrifugal atomizer which produces a directional spray of coating material without the deleterious effect of the air stream which accompanies a conventional air atomizer.
It will be apprecaited that considerable deviation from the specific embodiments shown herein may be made with- C9 in the spirit of the invention. Accordingly, it is intended that the scope of the invention be limited only by the following claims.
1. A method of electrostatically coating a workpiece comprising the steps of applying, coating material to an endless belt, centrifugally atomizing the coating material bypassing the belt through an arc at high velocity, and electrostatically depositing the atomized coating material on the workpiece.
2. A method of electrostatically painting a workpiece comprising the steps of applying'paint to an endless belt, centrifugally atomizing the coating material by passing the belt around a roller at high velocity, and electrostatically depositing the atomized paint on the workpiece.
3. A method of electrostatically coating a workpiece comprising the steps of applying coating material to an endless belt, centrifugally atomizing the coating material by passing the belt around a roller at high velocity, passing the atomized material through a charging ring, and applying a high potential between the charging ring and the workpiece for electrostatically depositing the atomized coating material on the workpiece.
4. A method of electrostatically coating a workpiece comprising the steps of applying coating material to an endless belt, centrifugally atomizing the coating material by passing the belt around a roller at a high velocity, and applying a high potential between the belt and the workpiece for electrostatically depositing the atomized coating material on the workpiece.
References (Jited by the Examiner UNITED STATES PATENTS 2,307,698 1/43 Melton et al. 7
2,681,725 6/54 Earnshaw 2392l8 2,687,803 8/54 Johnson 209127.1 2,723,646 11/55 Ransburg 117--93.42 2,967,33 1' 1/61 Kaspar 118*629 X 3,059,613 10/62 Nakaya 118624 20 RICHARD D. NEViUS, Primary Examiner.
JOSEPH REBOLD, Examiner.