Electrostatic coating device

Oct. 23, 1962 eizo nakaya 3,059,613

ELECTROSTATIC COATING DEVICE Filed Aug. 18. 1959 3 _Sheet₃-Sheet 1

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E1ZO NAKAYA 3,059,613

ELECTROSTATIC COATING DEVICE

Sheets-Sheet 2

Oct. 23, 1962

Filed Aug. 18. 1959

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Oct. 23, 1962

ΕΙΖΟ NAKAYA

ELECTROSTATIC COATING DEVICE

3,059,613

Filed Aug. 18, 1959

Sheets-Sheet 3

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United States Patent Office _Patenled

3,059,613 ELECTROSTATIC COATING DEVICE

Eizo Nakaya, 384 Shimura-Motohasunuma-machi, Itabashi-ku, Tokyo, Japan

Filed Aug. 18, 1959, Ser. No. 834,505

Claims priority, application Japan Aug. 27, 1958

Claims. (CI. 118—624)

The present invention relates to coating devices, and more particularly to electrostatic coating devices.

The electrostatic coating device now being widely used comprises a frusto-conical rotary cup or atomizing head driven at a constant speed by an electric motor and means to supply a liquid coating material to the interior of said atomizing head to eject the coating material beyond the outer edge of the atomizing head by centrifugal force. Said outer edge is formed into a sharp knife edge and a high direct current voltage is impressed between said atomizing head and the work or an article to be coated so as to electrostatically atomize the liquid coating material ejected from the outer edge of the atomizing head and to electrostatically convey and deposit the atomized coating material onto the work under the influence of the electrostatic force.

While this device has been recognized to have superior characteristics as compared to a conventional hand spray gun or a so-called grid type electrostatic coating device, it is known that the shape of the coating film or pattern formed on the work is generally annular or doughnut shaped, so that, as in conventional practice, when the work is fed across the area in front of the atomizing head, the thickness of the applied coating film varies in the vertical direction and the overlap of several patterns produced by several atomizing heads is not uniform when large articles such as refrigerator cabinets or steel cabinets are to be coated. Moreover, in such a device as described above, the force for conveying the atomized coating material to the work is only the electrostatic force. Accordingly, if the coating device is used to coat an irregular article, for instance, a casing of an electric motor having cooling fins on its surface, the coating material can not enter into the space or indentation between fins due to a concentration of the lines of force of the electric field at the outer edge of the fins. In addition, since rotating bodies are used, imbalance conditions during operation caused by bearing wear or other causes would result in an irregular configuration of patterns and hence nonuniform thickness of the coating film. Furthermore, it is well known in the art that atomizing of a liquid coating material containing pigment of low electric insulating resistance is difficult when the coating material is atomized by the electrostatic force alone.

Accordingly, it is the principal object of my invention to provide an improved coating device which does not include any rotating part, can form patterns of coating film of uniform thickness and also can form a uniform coating film on the surface of the articles having an irregular configuration, indentations or recesses.

A further object of my invention is to provide an improved electrostatic coating apparatus wherein atomizing of the liquid coating material is effected by a compressed gas of relatively low pressure and the atomized coating material is conveyed to the article to be coated by the combined action of the compressed air and electrostatic force.

Still another object of my invention is to provide an improved atomizing head for electrostatic coating which can uniformly eject and atomize the coating material through its annular nozzle or gap.

A still further object of my invention is to provide an electrostatic coating device capable of coating small articles having relatively small surface such as a coarse wire net or electric fan guard without appreciable loss of coating material.

According to one feature of this invention I provide an atomizing head comprising a narrow annular coating material ejecting gap or nozzle which is arranged obliquely with respect to the axis of the nozzle and a narrow annular air ejecting nozzle arranged close to said coating material ejecting nozzle in concentric relation therewith, and means to supply air to said air ejecting nozzle to eject air therethrough in a thin funnel shape at a relatively low speed, say from about 0.5 to 1.5 kilograms per square centimetre to eject and atomize the coating material by the pressure of the air. Inside said coating material ejecting nozzle, I provide an annular coating material distributing groove and a coating material supplying port which opens into the upper portion of said groove. The liquid coating material supplied to said groove through said port flows down along said groove during which time the coating material is completely atomized by the action of the air under pressure which is ejected from the air ejecting nozzle. Thus, the coating material is ejected uniformly throughout the periphery of the annular coating material ejecting nozzle. The coating material distributing groove is positioned inside said coating material ejecting nozzle and I provide an annular pocket on the back surface of the front wall defining said nozzle to aid uniform atomizing action of the coating material, said annular pocket having a cross section of a semi-circle.

According to another feature of this invention, I provide an annular discharge ring made of an electric conducting material around said atomizing head. One pole, usually the negative pole, of a high voltage direct current source is connected to the annular ring, while the other side, or the positive pole, is connected to the article to be coated. The liquid coating material atomized as described above is conveyed and then deposited on said article by the combined action of the electrostatic force and the pressure air. If the article to be coated is small or has small surface area such as a wire net, a substantial amount of the atomized coating material would pass the article or would pass through the space between the parts of the article without being deposited on the surface of the article. In a coating device in which the coating material is atomized by the action of compressed air or other mechanical force such as centrifugal force, the degree of loss of the coating material would increase with the intensity of the mechanical force. According to this invention, however, the pressure of the compressed air utilized to eject and atomize the coating material is relatively low, as pointed out above, so that the kinetic energy imparted to the atomized coating material by the compressed air is far less than that in the conventional spray gun. Therefore, the amount of the atomized coating material which does not move along the lines of force of the electrostatic field and which passes beyond the artide to be coated is relatively small. In order to minimize said loss of the coating material, I place a suitable counter electrode at the back of the article to be coated and impress thereon a potential of the same or different magnitude as said annular ring but of the same polarity. With such an arrangement, particles of the coating material usually having a minus charge would be repelled back to the article having a plus charge by the action of said counter electrode.

According to still another feature of this invention, I impress upon the atomizing head a potential of the same polarity as that of the discharge ring, but of a magnitude insufficient to atomize the coating material by itself so as to prevent movement of the atomized coating material toward the supporting stand.

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the con3,059,613 ³ .

eluding portion of the specification. The invention, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

FIG. 1 is a side elevation of an atomizing head embodying this invention;

FIG. 2 is an enlarged longitudinal cross sectional view of the head of FIG. 1;

FIG. 3 is a similar view to FIG. 2 but shows the construction of the ejecting nozzles in more detail;

FIG. 4 is a cross sectional view of the atomizing head along the line IV—IV of FIG. 2;

FIG. 5 is a diagrammatic view of an arrangement of an atomizing head, an article to be coated, and a counter electrode;

FIG. 6 is a cross sectional view of the atomizing head shown in FIG. 2 along the line VI—VI thereof; and

FIG. 7 is a side view of a modified air supplying nozzle.

Throughout the drawings, like parts are indicated by the same reference numbers.

Referring now to the accompanying drawings, more particularly to FIGS. 1 through 4, the main body of a nozzle or atomizing head, generally identified by numeral 10, comprises a composite pipe for passing the liquid coating material having a substantially horizontal longitudinal axis and consisting of three pipe sections 11, 12 and 13 which are connected in telescopic relation to provide an axial passage 14 for the coating material and outer cylindrical jackets 15 and 16 secured on the outside of the section 11. Between the jacket 16 and the section 11, there is provided an axial air passage 17, said jacket and section being separated by a plurality of radial ribs 18 integral with the jacket. These radial ribs also assure easy mounting of the jacket. Between the extension 11α of the section 11 and the section 12, there is a second air passage 20 which is in communication with the first air passage 17 through openings 19 provided through the wall of the section Ila. The outer end 21 of the section 12 is flared outwardly like a shallow funnel, and a cap 22 is secured to the outer end of the section 13, for instance by a screw, to form a narrow annular coating material ejecting nozzle or gap 23. The outer end of the jacket 15 is similarly flared outwardly to provide a similar narrow annular air ejecting nozzle or gap 24 behind and close to the gap 23. As best shown in FIG. 2, these gaps are at an angle to the longitudinal axis of the nozzle. Although the magnitude of this angle depends upon various factors such as the distance between the nozzle and the article to be coated, voltage used, size of the pattern to be formed on the article, viscosity of the coating material used and the like, it was found by experiment that it is advantageous to make this angle to about 85° to 65° with respect to the axis of the nozzle. I have found that it is advantageous to make the widths of the ejecting nozzle 23 and 24 to about 0.04 to 0.20 millimetre when an air pressure of 0.3 to 1.5 kilograms per square centimetre is used and preferably to about 0.06 millimetre at an air pressure of 1.2 kilograms per square centimetres.

As shown in FIGS. 2 and 3, an annular distributing groove 25 is provided inside the gap 23 and concentric with the passage 14 for uniformly distributing the liquid coating material to the entire periphery of the coating material ejecting nozzle 23. This groove is connected with the passage 14 by means in the form of an opening 26 provided in the upper part of the pipe section 13 and a cylindrical portion of the cap 22. With the construction as described above, the liquid coating material is supplied to the upper portion of the annular distributing groove 25 and then flows down along each half of the groove as indicated by arrows in FIG. 6. During this downward flow, the coating material is drawn out through the nozzle 23 by the partial vacuum created by a funnel shaped thin air stream ejected through the nozzle 24. In the present atomizing head, it is not necessary to supply the liquid coating material to the nozzle 23 under any 5 appreciable pressure. Usually a reservoir for the coating material is placed at a distance of about 1 or 2 metres above the atomizing head, thus permitting the coating material to flow to the nozzle 23 under a static head. For some applications, however, a suitable pump 10 may be used for this purpose. If the coating material were supplied directly to all points of the annular distributing groove 25 from the passage 14, the pattern formed on the work would not be uniform since the coating material has a tendency to be collected in the 15 lower portion of the distributing groove 25 or the nozzle 23 under the action of gravity, resulting in thicker film in the lower portion of the pattern. However, by supplying the coating material to only the upper portion of the distributing groove according to this invention, the coat20 ing material is drawn out successively while flowing down along opposite half section of the distributing groove 25. By suitably proportioning the viscosity and rate of flow of the coating material, dimensions of the opening 26, groove 25 and nozzle 23, and pressure of the air, it is 25 possible to make the amount of flow of the coating material substantially equal at all points around the periphery of the nozzle 23.

I have found that the distribution of the liquid coating material ejected from the ejecting nozzle or gap 23 can 30 be made more uniform by providing an annular pocket 25α of the cup-shaped cross section on the back of the cap 22, as shown in FIG. 3. This pocket acts as a temporary reservoir for the coating material and assures a continuous supply thereof to the nozzle 23.

When using the above atomizing head in an electrostatic coating apparatus, a suitable negative potential, say about 80,000 volts may be directly impressed on the head. However, the flaring of the atomized coating material by the above atomizing head is much wider than in 40 the conventional spray gun. Accordingly, as shown in

FIG. 5, I prefer to provide around the head discharge ring 27 made of a suitable electric conducting material, such as copper, and having a substantial diameter, say about 30 centimeters, and to connect the work 28 to the ⁴⁵ positive pole of the high voltage source of direct current, not shown in the drawing, which is usually grounded for safety reasons.

FIG. 5 shows a general arrangement of an electrostatic coating apparatus embodying this invention, wherein there ⁰⁸ is provided a counter electrode 29 at the rear of the work or article 28 to be coated. This electrode may be in the form of a metal ring, as shown, or a wire net or a metal plate and a potential of the same polarity and the same _rr or different magnitude as the atomizing head is im^M pressed upon the counter electrode. As shown in FIG. 5, the discharge ring 27 and the atomizing head 10 is supported by an insulating cylinder 30, which in turn is adjustably supported by a suitable supporting stand 31, usually made of metal and grounded. Pressure air is ⁶⁸ supplied to the insulating cylinder 30 from a suitable air compressor (not shown) or a pressure air line in the factory via a conduit 45 and thence to an inlet connector 32 (FIG. 2) under the control of a suitable valve, not shown, controlled by an adjusting handle 33. While ⁶⁵ the liquid coating material is supplied from a reservoir not shown, but positioned above the atomizing head at a distance of about 1 or 2 metres thereform through a pipe 35 to the central passage 14 under the control of a γθ suitable valve, not shown, controlled by an adjusting handle 36. For cleaning the nozzle 23, a suitable cleaning agent, such as thinner, may be supplied from a container 34.

In operation, a single or several atomizing heads are 75 used depending upon the size and configuration of the

8,059,613

Work. Usually the work is conveyed, while rotating, across the atomizing head by a suitable overhead conveyor, not shown. At first, the coating material is ejected and atomized by the action of pressure air in a funnel shape or conical plane which makes an angle of about 65° to 85° with respect to the longitudinal axis of the head. The atomized particles of the liquid coating material are converged toward the work by the electrostatic force existing between the work and the head or discharge ring, and are then deposited upon the work to form a film which is subsequently baked as is well known in the art. As has been pointed out above, in the conventional electrostatic coating apparatus wherein atomizing of the liquid coating material is effected solely by the electrostatic force, uniform coating was not obtained in work having any irregular configuration or indentations. According to this invention, however, since an appropriate kinetic energy is supplied to the particles of the liquid coating material by the pressure air, the coating material is deposited in a uniform film on work having any irregular configuration or indentations. Moreover, with the prior electrostatic coating apparatus utilizing a rotary cup-shaped spray head of conducting material, the pattern of the coating material formed on the work is generally annular or doughnut shaped, so that the thickness of the coating film formed inevitably varies in the vertical direction and the overlap of the several patterns produced by several atomizing heads disposed in a vertical plane with a suitable space therebetween is not uniform where large objects such as refrigerator cabinets, switchboards and the like are to be coated. According to this invention, however, since the atomized coating material is ejected at a relatively wide angle and then conveyed to the work by the combined action of the pressure air and electrostatic force, the pattern of the coating material formed on the work is uniform and there is no portion in the pattern where only a thin coating or no coating material is applied. I have found that the thickness of the coating film applied to the surface of the work can be made more uniform when a whirling motion is imparted to the air ejected from the nozzle 24. This may be conveniently carried out by providing a spiral path in the passage 20 for the air adjacent said air ejecting nozzle. In the embodiment shown in FIG. 7 two rows of spiral splines 50 are provided on the outer surface of the section 12. By properly selecting the number of the splines or their angle of inclination, any desired amount of whirling motion can be imparted to the air ejected from the air ejecting nozzle of FIG. 3.

The shape of the pattern produced by my improved atomizing head is generally circular and the diameter and the density of the pattern can be easily controlled by factors such as the pressure of the air, viscosity of the coating material, size and angle of the nozzle for ejecting air and coating material, distance between the head and the work, and magnitude of the impressed direct current voltage. If the article to be coated is small or has a small surface such as a wire net, a portion, of the atomized coating material would pass through the article owing to its kinetic energy, resulting in a substantial loss of the coating material. To minimize said loss, the counter electrode 29 is used, as shown in FIG. 5, said electrode acting to repel the negatively charged particles of the atomized coating material back to the work which is charged positively. By this arrangement, loss of coating material can be reduced substantially.

Although the coating material is ejected from the atomizing head toward the work, a portion of the atomized coating material has a tendency to move along the lines of force established between the discharge ring 27 maintained at a negative potential and the supporting stand 31 which is grounded, i.e., connected to the positive pole of the source through ground, thus causing some loss of the coating material. In order to prevent such movement of the atomized coating material toward the supporting stand, according to this invention the atomizing head 10 is maintained at a potential of the same polarity as the discharge ring 27, but of somewhat 5 lower magnitude. For instance, when a direct current potential of minus 80,000 volts is impressed upon the discharge ring 27, a potential of minus 30,000 to 50,000 volts impressed on the atomizing head is found suitable for this purpose. This value of the potential is suf10 ficient to prevent backward movement of the atomized coating material, but insufficient to atomize the coating material by the electrostatic force. This low voltage may be supplied from a separate source of direct current voltage, but it is advantageous to connect the atomizing head 13 10 with the discharge ring 27 or the negative pole of the source through a suitable current limiting resistor 40 or a semi-conductor 40 as diagrammatically shown in FIG. 5.

While I have explained my invention by describing 20 particular embodiments thereof, it will be apparent that improvements and modifications may be made without departing from the true spirit and scope of my invention as defined in the appended claims. It will also be understood that various numerical values shown hereinabove 25 are given only for the purpose of illustration.