US3460764A - Coating apparatus - Google Patents

Description

Aug. 12, 1969 N. R. WALLIS COATING APPARATUS Filed Aug. 9, 1965 IN VEN TOR Na. PLLDOLPH WALLIS United States Patent M US. Cl. 239-422 1 Claim ABSTRACT OF THE DISCLOSURE Electrostatic spray coating apparatus for use in the open air and provided with two rings of air jets. The inner ring of air jets produces a convergent air stream controlling the natural divergence of the paint particles, and the outer ring of jets produces a protective air stream designed to eliminate the effect of the wind on the atomised particles.

This invention relates to apparatus for applying liquid or powdered coating materials to articles. The term article" is used herein to apply to articles of manufacture, but also extends to structures, such as buildings, bridges, railings, fences, lamp-post, and the like. The invention is particularly applicable to electrostatic spray apparatus, but is also useful in connection with other types of spray coating apparatus.

It is an ob ect of the invention to provide apparatus and methods for applying coating materials to articles which are situated in a location liable to be subjected to atmospheric conditions likely to interfere with the satisfactory application of the material to the articles. Thus, it is a particular object of the invention to provide coating apparatus suitable for use in the open air.

If spray coating apparatus of any kind is used in the open air, it is found that, if there is a wind blowing with a component perpendicular to the line joining the apparatus to the article, coating material is liable to be blown out of the path to the article. This not only results in a waste of coating material, but may also increase labour costs by making necessary the subsequent removal of coating material from articles on which it is not required. The present invention solves this problem by the provision of a protective curtain of fluid, such as compressed air, around the atomised material thus substantially preventing the wind from having any effect on the direction of travel of the coating material.

From one aspect the invention consists in apparatus for applying coating material to an article including means for atomising the coating material, a first shroud at least partly surrounding said atomising means, a second shroud at least partly surrounding said first shroud, and means for directing first and second hollow streams of fluid towards said article from said first and second shrouds respectively. Preferably, said first and second hollow streams are annular in cross-section and co-axial.

From another aspect the invention consists in apparatus for applying coating material to an article including a generally cylindrical atomiser head, first and second generally cylindrical shrouds co-axial with, and of greater diameter than, said head, and means in each of said shrouds for producing a hollow stream of fluid substantially co-axial with said head.

From yet another aspect the invention consists in apparatus for applying coating material to an article, wherein atomised coating material is directed towards said article within two co-axial streams of fluid flowing towards said article.

The term fluid is used herein to include compressed air, a gas (preferably an inert gas), a liquid, for example,

3,460,764 Patented Aug. 12, 1969 ice a solvent for the coating material, and mixtures of gases and liquids.

Preferably each of said streams of fluids is formed by a plurality of elementary jets of fluid and preferably at least some of the said elementary jets of fluid are convergent. The coating material may be atomised by means of a conventional spray nozzle or by means of a rotating atomising head, which may, for example, be in the form of a cup from the rim of which the material is atomised. An electrostatic field may be maintained between the nozzle of the cup and the article to be coated.

The inner hollow stream of fluid serves to control the flow of the atomised coating material towards the article to be coated, whereas the outer hollow stream serves as a screen to prevent disturbances from the surrounding atmosphere from reaching the atomised coating material.

According to a subsidiary feature of the present invention, the fluid in the outer stream is heated. This feature is particularly useful if the apparatus is to be used in an atmosphere having a high moisture content. Further, arrangements may be provided to enable the outer stream to be used without the atomisation of coating material. Thus, when the article to be coated is wet, a stream of heated fluid (in this case, heated air or heated gas) may be used to dry the article before the coating material is applied. This arrangement is particularly useful in the painting of articles located in the open air.

When apparatus in accordance with the present invention is used in the open air, it will normally be carried in a mechanically-propelled vehicle which will carry an air compresser for producing the two streams, of fluid, a container for the coating material, and in the case of electrostatic spraying apparatus means for producing the high voltage required. In this case, it is advantageous to use the exhaust heat from the engine of the vehicle and/ or from the engine driving the compressor, if this is separate from the vehicle engine, to heat the fluid in the outer stream. In fact, under certain circumstances, and providing suitable filters can be supplied, it may be possible to use the exhaust gas from the engine as the fluid for the outer stream. However, in most instances, it will normally be preferable to provide a heat exchanger in the exhaust system of the engine and to pass the compressed air for the outer shroud through this heat exchanger.

The jets constituting the inner and outer streams may be arranged to issue from a double shroud. The compressed air or other fluid may be supplied to the outer ring of holes at approximately the same pressures as the fluid to the inner ring of holes, but in view of the larger diameter the volume of fluid supplied to the outer ring will normally be greater.

The apparatus for performing the invention will now be described with reference to the accompanying diagrammatic drawings in which:

FIGURE 1 is an overall side view of a manual spray gun in accordance with the invention;

FIGURE 2 is a sectional view of the head of the gun as illustrated in FIGURE 1;

FIGURE 3 is an end view of the head of the gun illustrated in FIGURES 1 and 2;

FIGURES 4 and 5 are fragmentary sectional views of parts of the head illustrated in FIGURES 2 and 3 looking in the direction of the arrows AA.

Referring now to FIGURE 1, it will be seen that the gun includes a barrel 21 in the form of a tube of insulating material. Surrounding the barrel 21 is a clamp 22, by means of which a handle 23 is secured to the barrel. The handle 23 includes a trigger 24 controlling a micro-switch 25, to which is connected a cable 26, which extends to the back of the gun.

The head of the gun comprises a shroud 27, and an atomising nozzle assembly 28. The shroud consists of insulating material, and is effective to produce two annular co-axial streams of air. Air is supplied under pressure to the shroud by means of air lines 29 and 129, which fit on to air connectors 30 and 130. Paint and compressed air are supplied to the atomising nozzle assembly 28 by means of a paint line 31, and an air line 34. As can be seen in FIGURE 2, the paint line 31 fits on a paint connector 32 screwed into the rear of the atomising nozzle assembly 28. The paint travels along a metal tube 60 which is open at its end remote from the paint line.

The metal tube 60 is surrounded by an air chamber 64, to which air is supplied from the air line 34, through a connector 65. The air chamber 64 is tapered at the front of the gun to form a small annular passage 66 surrounding the tube 60. Air issuing through the passage 66 atomises the paint as it leaves the tube 60. The atomised paint particles are charged due to the fact that an electrostatic field is maintained between the tube 60 and the article to be coated. For this purpose the tube is connected to a high-voltage supply, by means of a terminal arrangement 33 and a high-voltage supply line 36. A resistor may be incorporated in the line 36, ro the line itself may have a high resistance. If the main body of the atomising nozzle assembly 28 is made of metal, the connector 33 may merely be threaded into the rear of the assembly. If, however, the main body of the assembly consists of insulating material, a conductive path is provided between the connector 33 and the connector 32.

The shroud 27 consists primarily of three parts 46, 47 and 147. The two parts 47 and 147 are secured to the part 46 by means of a plurality of nylon screws 48. The rear part 46 includes two threaded holes into which the air connectors 30 and 130 are screwed. The inner end of each of these two air connectors includes a pair of holes which communicate with an axial drilling along the length of the connector. This arrangement enables air fed to connector 30 to pass into an annular channel 50 cut in the front face of the portion 46. Similarly air fed to the connector 130 passes into an annular channel 150 concentric with the channel 50.

The front portions 47 and 147 are each generally in the form of a shallow truncated cone. The front portion 47 includes a generally annular passage 52, which extends toward the front face of the truncated cone, in which a plurality of very fine holes, such as those indicated at 53 and 54, are drilled. Similarly the front portion 147 includes a generally annular passage 152 communicating with holes such as those illustrated at 153 and 154. Each set of holes is located on the circumference of a circle whose centre lies on the axis of the tube 60. Each hole extends through an appreciable thickness of the insulating material of which the respective front portion is made so that it provides a passage the length of which is considerably greater than its diameter, and which thus acts as a minute air jet.

The angles made by the axes of successive jets with the axes of the tube 60 vary in a pre-determined sequence around the circumference of each of the circles on which the holes are located. In one particular example this sequence is 8,11, 8, 5, 8,11, and so on, and in this particular example there are eighty-eight holes in the outer ring.

The chanel 52 is provided with a shoulder 57, on which is located a ring 58, a part of which is illustrated more clearly in FIGURE 5. A number of holes 59 are drilled in the ring 58, and in the particular example in which there are eighty-eight air jets 53 there may be thirty-six holes 59, Similarly the channel 152 is provided with a shoulder 157 on which is located a ring 158, having a number of holes 159. In each case the total area of the holes in the ring 58 (158) is slightly greater than the total area of the holes in the front of the respective shroud. This arrangement enables a back pressure to be maintained in each of the cavities formed between the rings 58 and 158, and the forward ends of the channelS 52 and 152 respectively.

The front portion 47 is also provided with a further shoulder 60, on which is located a further ring 61, a part of which is illustrated more clearly in FIGURE 4. This ring is provided with a number of holes 62, and in the particular example referred to there are eight such holes. Again the total area of the holes '62 is made slightly greater than the total area of the holes 59, so that a back pressure is maintained in the cavity between the two rings 58 and 61.

Similarly the front portion 147 is provided with a shoulder 160, on which is located a ring 161 provided with a number of holes 162. The total area of the holes 162 is made greater than the total area of the holes 159.

When the gun is in use, it is connected to the remainder of the apparatus by means of a multiple cable, which includes the three air lines, the paint line, the highvoltage cable, and the trigger control cable. The trigger control cable 26 connects the micro-switch 25 to control equipment which is mounted on a high-voltage generator. The control equipment is connected to a bank of four remote control valves. The first of these valves connects the paint line 31 to a supply of paint in a pressurised container. The second valve connects the air line 34 for atomising the paint to a source of compressed air. The third valve connects the air line 129 for the inner shroud to the same source of compressed air, while the fourth valve connects the air line 29 for the outer shroud to a source of compressed air, which is preferably heated. The arrangement of the control circuit is such that, when the trigger on the gun is pressed, air is supplied under pressure to the two shrouds, and to the air chamber 64. Thereafter the paint valve is opened, so that paint is supplied under pressure to the tube 60, and the high voltage is applied to the cable 36. When the trigger 24 is released, the paint valve is closed first and the electrostatic generator is disconnected. Thereafter the three air valves are closed. Preferably, however, the air valves are arranged so that they do not close completely under these conditions, but continue to supply a small quantity of air to the shrouds and to the air chamber.

In use the gun is held with the head at a short distance from the article to be coated. Paint particles issue from the atomising nozzle and are charged by the electrostatic field. Consequently they are attracted to the article to be coated since it is the nearest object at earth potential.

In the absence of air from the air jets in the shrouds, the actual trajectory of each particle of paint would be determined by its direction and velocity of motion on leaving the nozzle, by its mass, by its electrical charge, by the configuration of the electrostatic field along its trajectory, and by the effect of the air issuing from the annular orifice 66. These various forces would produce a divergent stream of particles, and the principal effect of the convergent stream of air issuing from the inner shroud is to control the paint particles to counteract their divergence. This stream of air has a further function in that it directs any uncharged particles of paint towards the article to be coated.

The purpose of the outer ring of air jets on the other hand is primarily to protect the stream of atmoised particles from external effects, such as that of the wind.

What I claim as my invention and desire to secure by Letters Patent of the United States is:

1. Apparatus for applying coating material to an article including:

a barrel in the form of a tube of insulating material,

a handle secured to the barrel intermediate the ends thereof, and

a head secured to the barrel at the forward end thereof,

said head comprising means for atomising the coating material and a shroud surrounding said atomising means and arranged coaxially therewith,

5 6' said shroud consisting of a rear portion and two front said first stream is surrounded by a second stream portions of insulating material, of air consisting of a plurality of elementary jets of said rear portion being provided with two air conair at least some of which are convergent.

nectors and with two concentric annular channels each in communication with one of said connectors, 5 References Cited each of said two front portions being generally in the UNITED STATES PATENTS form of a shallow truncated cone and includmg an annular passage which extends towards the front face 2,554,829 5/1951 leprllng 239-424 XR of the truncated cone and which is in communica- 2,569,251 9/1951 'Nle'burg 239-424 XR tion with a respective one of said annular channels, 10 2,804,337 8/1957 Marantz 239428 XR and 2,843,425 7/1958 Paasche 239424 XR respective concentric rings of holes being drilled in 3,169,882 2/1965 I'm/man et 118*6Z7 XR the front faces of said truncated cones with the axes 3,232,540 2/1966 Cassanmagnago' 239-424 XR of said drillings inclined to the axis of said atomising 3,268,171 8/1966 Walberg means at angles less than 15, whereby, when air under pressure is supplied to said 15 PETER FELDMAN Pn'mary Exammer connectors, the atomised coating material issuing U S cl XR from said atomising means is surrounded by a first stream of air consisting of a plurality of elementary 118-629; 23915, 424, 428, 433

jets of air at least some of which are convergent and

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