US3342418A

US3342418A - Coating apparatus

Info

Publication number: US3342418A
Application number: US303563A
Authority: US
Inventors: Wallis Neil Rudolph
Original assignee: Individual
Current assignee: Individual
Priority date: 1962-11-15
Filing date: 1963-08-21
Publication date: 1967-09-19
Anticipated expiration: 1984-09-19
Also published as: SE314015B; ES293557A1; NL300523A; CH439026A; DE1427677B2; GB1024892A; FI42807B; DE1427677A1; BE639903A; DE1427677C3; NL125216C

Description

Sept. 19, 1967 R. WALLIS comma APPARATUS Filed Aug. 21, 1963 2 Sheets-Sheet 1 //VI/-'NTOH N5; L RHDOL'PH WALLIS ad W Afro/M5215 Sept. 19, 1967 R. WALLIS comm; APPARATUS Filed Aug. 21,1963

2 Sheets-Sheet 2 //Vl /VTO/Q NEIL EMDOLPH k/AL L is ,JAJL

United States Patent 3,342,418 COATING APPARATUS Neil Rudolph Wallis, Cariad, Goring-on-Thames, England Filed Aug. 21, 1963, Ser. No. 303,563 Claims priority, application Great Britain, Nov. 15, 1962, 43,218/ 62 2 Claims. (Cl. 239296) This invention relates to methods and apparatus for applying liquid or powdered coating material to articles of manufacture. The invention is particularly applicable to electro-static spray apparatus but is not necessarily limited thereto.

The pattern produced by electro-spray apparatus on a fiat surface is normally in the form of an annulus having a central void. For many purposes and in particular when coating large flat surfaces such annular spray patterns are disadvantageous because if linear motion is produced between the spray apparatus and the article being coated a high proportion of the coating material will be deposited in parallel bands, there being a space between the bands having a relatively thin coating.

It is an object of the present invention to provide improvements in the uniformity of the thickness of coating produced by spray apparatus.

The invention consists in a method of spraying coating material to an article wherein the said material is ejected in finely divided form into a stream of fluid formed by a large number of elementary jets of air.

From another aspect, the invention consists in apparatus for applying coating material to an article comprising means for ejecting said material in finely divided form and means for forming a large number of elementary jets of fluid around said ejected material.

Preferably the material is atomised by means of a centrifugal atomiser such as a radial disc or cup and accordingly is ejected with a radial component of velocity. Preferably, the elementary jets of fluid are arranged in a circular or elliptical pattern around the atomiser and are directed generally in the direction of the article to be coated. However, the axes of at least some of these jets should converge towards the axis of rotation of the atomiser. If all the jets are arranged to be convergent it is desirable for the angles of convergence to vary in a predetermined sequence around the circumference of the circle and it has been found that a satisfactory sequence is 5, 8,11, 8, 5, 8, 11 and so on.

It has been found that the converging stream of com pressed fluid surrounding the atomised coating material can be made to fill in the central void in the annular pattern to a very substantial degree and it has also been found that spray apparatus which has been designed for use with an electro-static field between the centrifugal atomising head and the article to be coated can in fact be used without the application of the electro-static field. In this case, the spray pattern produced is substantially in the form of the circular portion which is normally absent from the centre of the annular pattern when the apparatus is used with the electrostatic field but without the application of the air jets.

It has been found that the invention is applicable to the spraying of powdered materials as well as to the spraying of liquid coating materials and that by suitably disposing the elementary fluid jets around the spray produced by the electro-static spraying apparatus it is possible to modify the pattern produced on a flat surface so that when linear motion is produced between the surface and the spraying apparatus a more uniform coating is produced than is possible with any other known electro-static spraying apparatus. Whether powdered or liquid material is employed, it will often be advantageous to use 3,42,4ld Patented Sept. 19, 1967 a non-combustible gas such as nitrogen in the elemental jets; in other cases, a liquid solvent may be used to advantage.

Apparatus in accordance with the invention may include for example an atomising cup surrounded by a shroud having a large number of very small forwardly facing holes drilled in a front surface thereof. The holes connect with an annular passage in the shroud which in turn connects through a number of baflies for even distribution of the fluid to a supply of fluid under pressure. The diameter of the circle on the circumference on which the holes are located is slightly greater than the maximum diameter of the rotary cup and the forward edge of the cup projects in front of the shroud by a distance substantially equal to the radial distance between the edge of the cup and the circumference of said circle.

The electrode may be stationary and the disc rotated by means of a pneumatic motor. Separate air supplies are provided for driving the motor and for feeding the shroud because it is desirable that a small amount of air should flow through the holes at all times in order to prevent them from becoming blocked. The line of maximum field intensity may be arranged to be either in the forward edge of the shroud or in the forward edge of the cup but in either case it is preferred that the speed of rotation of the cup should be such that atomisation of the coating material is largely produced by centrifugal action.

It is to be understood that in operation the majority of the coating material which is ejected from the atomiser will be charged as it enters the electro-static field and will accordingly be directed towards the article to be coated. Particles which are not charged will travel along a spiral path towards the fluid stream and as soon as the stream impinges on them they will receive a component of velocity in the direction of the article to be coated and the majority of them will arrive at the article.

Experiments have shown that the void in the annular pattern is substantially filled in by the use of air jets and it is thought the uncharged particles may contribute partly to this but it is clear that charged particles are also deflected from their normal course by the converging jets of air.

One method of 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 view of the front end of the gun illustrated in FIGURE 1 on a larger scale partly in section;

FIGURE 3 is a sectional view of the head of the gun illustrated in FIGURES l and 2;

FIGURES 4, 5 and 6 are fragmentary views of parts of the head illustrated in FIGURE 3 looking in the direction of the arrows xx; and

FIGURE 7 is a perspective view of a complete paint spraying installation in accordance with the invention.

Referring now to FIGURE 1, it will be seen that the gun includes a barrel 21 in the form of an insulating tube. 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 a rotatable cup 28. Both the cup and the shroud consist of insulating material, but it is desirable that conductive or semi-conductive material should be painted on or incorporated in the insulating material to give the shroud and the cup at very high electrical resistance. Air is supplied under pressure to the shroud by means of an air line 29 which fits on to an air connector 30. Further, paint is supplied to the rotating cup under pressure by means of a paint line 31 which fits on to a paint connector 32.

As can be seen in FIGURE 2, the cup 28 is rotated by means of an air motor 33 having a driving spindle 35. Air is supplied under pressure to the air motor 33 by means of an air line 34 which extends through the barrel of the gun. The air motor 33 also serves as the electrical connection between the high voltage supply and the cup 28. The high voltage supply is provided by means of a cable 36 which is connected to the air motor 33 through a resistor 37 and a spring 38. The resistor 37 is mounted in a housing 39.

It will be seen from FIGURE 2 that a nylon block 49 is secured in the end of the barrel 21 by means of a screw 41. This block includes a tapped hole in which the paint connector 32 is screwed and a drilling 42 which connects the hole for the paint connector 32 to a further hole into which a paint nozzle 43 is threaded. The paint nozzle is secured in position by means of a nut 44 and the drilling 42 is closed by means of a screw 45.

The head of the gun is shown in more detail in FIGURE 3 and it will be seen that the shroud 27 consists primarily of two

parts

46 and 47 which are secured to one another by means of a plurality of nylon screws 48. The rear part 46 includes a threaded hole into which the air connector 30 is screwed. The inner end of the air connector includes a pair of holes of which one is visible at 49 and which communicate with an axial drilling along the length of the connector. This arrangement enables air fed to the connector 30 to pass into an annular channel 50 cut in the front face of the portion 46. The rear portion 46 is also provided with an elongated slot 51 through which the spindle 35 and the nozzle 43 can pass.

The front portion 47 is generally in the form of a shallow truncated cone and includes a generally annular passage 52. This passage extends towards the front face of the truncated cone in which a plurality of very fine holes, such as 53 and 54, are drilled. The

holes

53 and 54 are located on the circumference of a circle whose centre lies on the axis of rotation of the cup 28 and a part of this ring of holes, such as 53, is shown in FIGURE 4. Each hole extends through an appreciable thickness of the insulating material from which the front portion 47 is made and acts as a minute air jet. The axis of the air jet 53 is represented by the dashed line 55 and the axis of the air jet 54 is represented by the dashed line 56. The angles made by the axes of successive jets with the axis of rotation of the cup 28 vary in a predetermined sequence around the circumference of the circle 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 88 holes made with a drill No. 80 (.0135 of an inch).

The channel 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 88 air jets there may be thirty-six holes 59. The total area of the thirty-six holes 59 is slightly greater than the total area of the 88 holes, such as 53 and 54. This arrangement enables a back pressure to be maintained in the cavity formed between the ring 58 and the forward end of the channel 52. The front portion 47 is also provided with a further shoulder 60 and on this shoulder is located a further ring 61. A part of this ring is illustrated more clearly in FIGURE 6. It will be seen that this ring is provided with a number of holes 62 and in the particular example referred to there are eight holes. Again, the total area of the eight holes 62 is made greater than the total area of the thirty-six holes 59 so that a back pressure is maintained in the cavity between the two rings.

The cup 28, which is made of ebonite, includes a metal spindle insert 63 in which the end of the shaft 35 is screwed. The cup includes a flat disc 64 which is joined to an outer cup-shaped portion 65 by means of three webs 66. The mouth of the cup is tapered to a comparatively sharp edge 67 which is located about of an inch forward of the front of the shroud 27. Further the radial distance between the edge 67 and the circumference of the circle on which the holes, such as 53 and 54 are located, is also approximately Z of an inch.

The installation illustrated in FIGURE 7 includes a manual gun 2 similar to that described with reference to FIGURES l to 6 which is used in a spraying booth 15. The article to be coated is exemplified by a chair 1 which is earthed by means of a metal hook 16 which connects the chair to an earthed rail 17. The gun is connected to the remainder of the apparatus by means of a multiple cable 18 which includes an air line 3 for the motor, a paint line 4, a high voltage cable 5, a trigger control cable 6 and an air line 29 for the air supply to the shroud.

The trigger control cable 6 connects the micro-switch 25 to control equipment 18 which is mounted on a high voltage radio frequency generator 7, the container for which is connected to earth. The control equipment and the high voltage generator are connected to the mains supply by means of a mains cable 15. The control equipment is connected by means of an interconnecting cable 9 to a bank of three remote controlled valves 19, 20 and 8. The valve 19 connects the paint line 4 through a further paint line 11 to a supply of paint in a container 13. The valve 20 connects the air line 3 for the motor through a line 70, a combined moisture separator, automatic lubricator, and atomiser control 19 and a further air line 12 to a main air supply inlet 14. The valve 8 connects the air line 29 for the shroud through an air line 71 to a junction in the air line 70.

The air supply inlet 14 is also connected through a reducing valve 72 to the interior of the paint container 13 so that paint is supplied to the line 11 under pressure. A meter 73 is provided to indicate the pressure in the container.

The arrangement of the control circuit 18 is such that when the trigger on the gun is pressed, air is supplied under pressure to the shroud and to the motor to cause the cup 28 to revolve. When the cup is revolving at a sufficient speed the paint valve 19 is opened so that paint is supplied under pressure to the nozzle 43. When the trigger 24 is released the paint valve 19 is closed first and thereafter the air valve 20 is closed to stop the motor. The air valve 8 is also de-energised, but this valve is arranged so that it does not close completely under these conditions, but always allows a small quantity of air to be supplied under pressure to the shroud. This ensures that the minute holes in the shroud are always kept clear.

In use, the gun is held with the head at a short distance from the article to be coated and paint is supplied to the rear surface of the disc 64. It is thrown off this surface by centrifugal force and forms a film on the interior surface of the walls of the cup 65. Centrifugal force causes the paint to travel towards the tapered edge 67 and the paint is atomised from this edge. The paint is charged by the electro-static field and is consequently attracted towards the article to be coated, since this is the nearest object at earth potential. In the absence of the air from the air jets in the shroud the actual trajectory of each particle of paint would be determined by its velocity and direction of motion when leaving the cup, by its mass, by its electrical charge and by the configuration of the electro-static field along its trajectory. If the article to be coated is a fiat object extending substantially perpendicularly to the axis of the gun, the pattern produced by the spray would be an annulus having a central void. In the presence of the stream of air, on the other hand, since the jets converge at varying angles on to the axis of the gun a number of particles of paint which would normally arrive at the surface to form part of the annular pattern, will be attracted inwards to fill in the central void. The result is that it is possible by correct disposition of the jets of air to replace the annular or doughnut pattern normally associated with electrostatic spraying apparatus by a uniform circular pattern.

The jets of air have another function in that they prevent the few uncharged particles of paint which would normally travel radially out from the cup from escaping and direct them towards the article to be coated. It has been found that the air jets are also useful in that they reduce the flash-off time required between spraying and baking by drying off some of the solvent used in the coating material.

Normally, the air motor will be run at between 6,000 and 10,000 revolutions per minute and the air will be supplied to the shroud at a pressure of between 40 and 50 lbs. per square inch. It is to be understood that references throughout the specification to the use of air are intended to include the use of an inert gas in place of air. Finally, the spray gun assembly and the auxiliary equipment may be protected against fire in the manner disclosed in my co-pending German patent application No. W 34,012, and in my co-pending British patent application No. 8,122/62.

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 rotating head for atomizing the coating material, a shroud surrounding said atomizer head and radially spaced therefrom, an uninterrupted circular pattern of a large number of very small jet outlets in the for ward end of said shroud, said jet outlets directing streams of fluid from said shroud toward said article, said jets converging forwardly toward the axis of rotation of said head and forming angles of convergence therewith, and wherein the angle of convergence of adjacent jets in the circular pattern varies in a predetermined sequence around the circumference of the circle, said jets being of substantially same size and under pressure, and wherein the rotating head is cup-like and the material is atomized from the rim thereof, said rim being forwardly of said jet outlets, and wherein means are provided for introducing material to be atomized into the interior of said head, whereby the large number of converging jets of compressed fiuid contact the spiralling atomized particles of coating material, and tend to bend their direction inwardly to aid in producing a coating of uniform thickness.

2. Apparatus according to claim 1, wherein the rotating head is of circular cross-section and rotates about its axis of symmetry, wherein the elementary jets are arranged to issue from the circumference of a circle on the shroud which is concentric with the head, and wherein said circle is of larger diameter than the exterior of said head and is spaced axially behind the forward rim of said head so that the radial distance between said head and said circle is between one half and twice as great as said axial spacing.

References Cited UNITED STATES PATENTS 1,782,309 11/1930 Ludwig 239200 X 1,861,475 6/1932 Hopkins et 211.

2,070,696 2/1937 Tracy 239296 X 2,271,779 2/1942 Peeps 239296 2,438,471 3/1948 Ball 117-104 X 2,646,314 7/1953 Peeps 239296 2,786,716 3/1957 Peeps.

2,894,691 7/1959 Sedlacsik 117-93.44 X 3,046,177 7/1962 Hankins 239-290 X 3,048,498 8/1962 Juvinall et al 11793.4 3,057,558 10/1962 Verba et al 118-627 X 3,144,209 8/1964 Grifl'iths 239-15 ALFRED L. LEAVITT, Primary Examiner. MURRAY KATZ, Examiner. A. GOLIAN, Assistant Examiner.

Claims

1. APPARATUS FOR APPLYING COATING MATERIAL TO AN ARTICLE INCLUDING A ROTATING HAD FOR ATOMIZING THE COATING MATERIAL, A SHROUD SURROUNDING SAID ATOMIZER HEAD AND RADIALLY SPACED THEREFRTOM, AN UNINTERRUPER CIRCULAR PATTERN OF A LARGE NUMBER OF VERY SMALL JET OUTLETS IN THE FORWARD END OF SAID SHROUD, SAID JET OUTLETS DIRECTING STREAMS OF FLUID FROM SAID SHROUD TOWARD SAID ARTICLE, SAID JETS CONVERGING FORWARDLY TOWARD THE AXIS OF ROTATION OF SAID HEAD AND FORMING ANGLES OF CONVERGENCE THEREWITH, AND WHEREIN THE ANGLE OF CONVERGENCE OF ADJACENT JETS IN THE CIRCULAR PATTERN VARIES IN A PREDETERMINED SEQUENCE AROUND THE CIRCUMFERENCE OF THE CIRCLE, SAID JETS BEING OF SUB-