Device For The Electrostatic Application Of Protective Coatings With Synthetic Powders By The Use Of Spray Guns

Abstract

A material coating system to apply charged materials to objects to be coated by electrostatic spray guns. The spray gun itself has a double cone-shaped deflecting member in its barrel and an adjustable collar on its barrel to change the spray pattern. An internal electrode in the material conveying tube charges the material and a wound wire ground connection attached to the gun prevents an accumulation of charge at the gun. A material storage bin with multiple venturis feeds material to the gun by suction via its conveying tube. In addition, a polarity reversing circuit allows the power source to apply different charges to sprayed materials.

Description

The invention involves a device for the electrostatic application of protective or decorative coatings with synthetic powders by the use of spray guns. It is known to be desirable to apply a synthetic coating to the surfaces of those raw materials that are not durable or long-lasting; for example, to protect them against corrosion, abrasion, and to provide electrical insulation. In electrostatic powder coating, the initial electrically neutral (uncharged) character of the powder particles is changed in that the particles receive an electrostatic charge.

While these particles are being sprayed from the gun, which also usually houses the ionizing element, the particles are being attracted to the grounded object (object being sprayed) because of their charge finally reaching the object.

The particles are deposited on the object and are subsequently "baked out" or cured to produce a protective coating.

The electrostatic charge can be applied in the following ways:

1. Through polarization, in which most electrical charge centers in the molecules are displaced or dislocated producing an electrical dipole.

2. Through ionization in which at least one electron is removed or added to the synthetic material particle.

While the polarization usually occurs through friction of the powder-air mixture on the walls of the conveyance tubes, but can also occur through friction between the powder particles themselves; the ionization of the particles is caused either by the particles coming in contact with a high voltage metal electrode or by passing through a high voltage field. Upon being ionized, the particles acquire the polarity of charge, thus the newly arriving particles are repelled by those already deposited on the object. With polarization, however, the electrical dipole is aligned in the direction of the grounded object, so that the newly arriving powder is attracted both by the grounded objects and by the particles already deposited on the object.

Electrostatic powder coating developed from electrostatic paint spraying, in which the high voltage electrode, which effects the ionization, is in the muzzle of the spray gun. Since with fluids there is, practically speaking, no friction, no value was placed on polarization charging. It could be demonstrated, however, that for the forming of industrially useful coatings, both charging methods are simultaneously essential.

The essential task of the invention then is to have a device which allows both of the charging methods to be realized simultaneously. The invention accomplishes this so: the flexible insulating hose carrying a powder-air mixture, which leads to the muzzle of the gun where there is a fixed-position body of revolution, has inside it a thin metal wire which acts as a high voltage electrode to form a strong corona effect, and this wire is connected to a high voltage generator, while the outer surface of the cable has a spiraling ground wire around it to introduce a high voltage gradient. The separation of the high voltage electrode from the head of the spray gun, the "safety distance," is important. At the muzzle of the gun, a cone-shaped open body of revolution is provided for the radial deflection of the exiting powder-air mixture.

The flexible insulating hose or conduit is directed through the handle of the gun and is housed within the conveyor pipe, which leads to the vicinity of the muzzle, and the conveyor pipe is housed within an outer sliding pipe while the end of the neighboring outlet in relation to the inner surface of the cone of the body of revolution forms the "counter cone." The supply bin has several parallel-mounted venturis connected to its outlet. These parallel-mounted venturis have a common suction chamber. The outlet of the supply bin incorporates a continuously variable damper.

The supply bin is tilted in the direction of the exit. Several component groups consisting of metering dampers, venturi tubes, and spray guns can be added to the supply bin. The high voltage generator chopper has a bridged capacitor and works directly through a cascade circuit which is connected directly to the high voltage transformer. The capacitor bridge and the high voltage transformer operate at or near a resonant frequency.

The polarity of the cascade output can be changed through polarity reversal of the switching element; it can also be changed by reversing the cascade rectifier or by transposing the cascade rectifier; and also by interchanging the unconnected open rectifier leads. For interchanging the open ends of the cascade rectifier, an adjustable switching bar can be provided. Finally, the polarity of the cascade output can be changed by sliding an additional, block-mounted rectifier.

An embodiment of the previously described invention is shown in the accompanying drawings.

FIG. 1 is a schematic representation and partial section of the muzzle part of the spray gun showing the deflector cone, powder-air mixture conveyor pipe and the high voltage electrode.

FIG. 2 is a longitudinal cross-sectional view of the muzzle part of the spray gun showing the deflector cone in an extended position so as to produce a more diffuse powder spray pattern.

FIG. 3 is a vertical cross-sectional view of the supply bin and the multiple venturi.

FIG. 4 is a vertical cross-sectional view of the supply bin shown in FIG. 3 after it has been rotated 90.degree. about the vertical axis.

FIGS. 5, 6 and 7 show the high voltage generator wiring diagrams as well as the modification required for the changing of polarity.

Referring now specifically to FIG. 1 embodiment, the electrostatic spray gun assembly has a spray gun for spraying powder-air mixtures with the tube 8 containing the muzzle part of the gun connected to the handle 7. The powder air mixture 1 is conveyed through an insulated flexible conduit or hose 2 to the spray gun, which is constructed from high resistive materials, making the gun shock and spark proof. The insulated hose 2 houses a thin electrically conductive high voltage metal electrode 3 which produces a corona discharge within the tube itself. This high voltage electrode 3 extends only to within about 30-40 mm of the powder-air exit passage at the front of the gun 5. This 30-40 mm distance 20 is termed the "safety distance."

The relative motion between the selected powder 1 and the large inside surface of the insulated hose 2 produces the required friction for polarization, while the high voltage field produced between the wire shaped high voltage electrode 3 and the ground wire 4 which is spirally wound around the insulated hose 2, produces the ionization. This spirally shaped arrangement guarantees an even and repeatable high voltage field through which the powder particles can pass. The charge of the individual powder particles is greatly increased over that obtained by employing devices in which only one of the charging methods is used.

Positioned at the exit of the spray gun is a generally double cone-shaped body 16 which serves to deflect the emerging powder-air mixture in a radial direction as shown in FIG. 2. The end of the flexible insulated hose 2, which is housed within the gun, is surrounded by two concentrically positioned pipes 8 and 9, which are constructed of a nonconductive, hard plastic. The inner pipe or barrel 8 of the gun is firmly attached to the gun handle 7 while the outer pipe or collar 9 can slide along the inner pipe 8. The outer pipe 9 can be positioned by axial movement over the cone-shaped bell mouthed opening at the muzzle end of the gun so as to make it possible for the powder-air mixture to be changed variably from a (beamed) narrow ray of powder as shown in FIG. 1 to a rather diffuse cloud of powder as shown in FIG. 2. Hence the emerging powder pattern can be quickly shaped, without interruption of the spraying process, to the most desirable pattern for spraying an object of a given shape.

The collar 9 encircles substantially the entire longitudinal portion of the barrel 8 of the gun. By moving the leading edge of the collar forward of the common base of the two conical shaped members that form the deflector member 16, the coating material is variably deflected in a generally forward direction. Deflector member 16, it is noted, resembles two conical members of different sizes attached at a common base side with the larger cone being forward of the smaller cone and having its apex at the most forward position of the deflector. An internal chamfer or groove at the leading edge of the collar 9 acts as an additional guide to direct material in a forward direction when the collar's leading edge is moved forward to the juncture of the common base of deflector 16. When the collar's leading edge is rearward of the common base attachment, coating material is deflected in a radial direction. A lug or other stop members may be placed on the barrel to limit the forward and rearward movement of collar 9. As for example, lug 19 rigidly attached to barrel 8 that slides in a closed slot in collar 9 (FIG. 1).

The supply bin 12 is set at an angle, so shown in FIG. 3, as to insure a continuous flow of powder to the chamber 17 of the venturi injector system shown in FIG. 4. The powder-air mixture in the suction chamber 17 is then forced with great velocity in the direction of the arrows 15 into the flexible insulated hose 2. Coating material storage bin 12 stores and dispenses material to conduit 2. This main storage bin 12 has an opening at its lowest side, a large fluid inlet (FIG. 4) with smaller venturis 14 connected to this opening, and an outlet conduit axially longitudinally aligned with the fluid inlet that has a diameter or cross section area greater than the areas of all of the venturi tubes combined.

The continuously variable metering gate or valve 18 shown in FIG. 3 controls the flow of powder to the suction chamber 17 to be throttled or even stopped.

The quantity of powder entrained into the suction chamber 17 depends directly upon the air velocity and flowrate emitted from the venturi, and hence the operating line pressure. For practical as well as economic reasons, line pressures should not exceed the ambient pressure by more than 75-84 psi. With these pressure drop limitations, the venturi tube conveyance method will produce maximum powder flow rates of 30 kg/hour. The maximum powder flow rate can be increased, without increasing line pressure, by employing several parallel mounted venturis 14 which are mounted in a single suction chamber 17 as shown in FIG. 4.

Additional metering gates 18 and venturi tubes 14 can be affixed to the same supply bin 12 so as to permit more than one spray gun to be operated from a single supply bin 12.

The HV generator of this invention works according to the well-known "chopper principle," in which low voltage direct current (8-10 volts) from the line rectifier is magnetically or mechanically "chopped" or rapidly switched. This "chopped" DC voltage is passed through a voltage-adjusting potentiometer to the primary side of the HV transformer, while the secondary side of the HV transformer is attached to a one-stage or multi-stage cascade multiplier 29, 30, 31, 32, consisting of capacitors 23, 24 and rectifier 25, 26. The cascade also rectifies the produced high voltage. The "chopper" 21 is capacitor bridged 33, with primary transformer winding and capacitor resonant frequency is approximately equal to the switching frequency of the "chopper" so that the HV transformer and the "chopper" operate at a resonant frequency, since in this manner the required high voltage can be obtained. Between the output point 29 of the cascade and the gun cable 3, a high ohm-value resistor is located.

There are powders which produce better operation with positive-polarity high voltage and others which operate better with negative-polarity high voltage. Therefore, the devices, according to the invention, have the capability to use both polarities. While known devices require two independent, correspondingly switched HV generators, according to this invention both polarities can be obtained by using only one HV generator and one cascade, in which the cascade has reversible polarity, and specifically:

1. by reversing the cascade rectifiers 25, 26 (FIG. 5);

2. by changing the unconnected ends 29, 30 of the cascade rectifier 25, 26 by moving a switching bar 34 and contact 35 (FIG. 6);

3. by location change of the cascade rectifier 25, 26 (FIG. 5);

4. by moving 36 the block mounted supplementary rectifier 37 (FIG. 7).

The foregoing specification accordingly describes an embodiment of the present invention and would suggest some of the ways of practicing it to those skilled in the art. Such a description should not be construed as limiting the invention as only the foregoing claims can do this.

Claims

Now, therefore, what I claim is:

1. A spray gun for applying coating material comprising: a gun barrel having a material conveying passage therethrough, said passage having an exit at the front end of said barrel; a collar having a leading edge encircling a portion of said barrel and movable in the longitudinal direction of said barrel with the leading edge in the forward most position; and a material deflector mounted within the front end of said barrel, said deflector being shaped to generally resemble two conical members attached at a common base side and the forward most conical member having a larger base side at the side of attachment and an apex at its forward most point, whereby coating material is deflected in a radial direction by the deflector when the leading edge of the collar is rearward of the common base attachment and deflected in a generally forward direction by the collar as its leading edge is moved forward of the common base attachment.

2. The spray gun of claim 1 wherein said collar has an internal chamfer near its leading edge to further assist in directing material from the gun barrel.

3. The spray gun of claim 1 wherein said collar extends substantially the entire longitudinal length of said barrel and has means for limiting the rearward and forward movement of said collar.

4. An electrostatic spray gun assembling comprising: an electrostatic spray gun for applying coating material and having an opening therethrough to convey material into said gun at an entrance and to discharge the material at the front end of said gun, said gun having a body at least a portion of which is electrically conductive; an elongated flexible material conveying electrically insulating conduit connectable to said spray gun opening at its entrance; a thin electrically conductive electrode wire mounted within said conduit along its longitudinal extent, said wire being connectable to a high potential source at one end and terminating at its other end within said gun near the front end of said opening; and a second conductive wire wound around the external surface of said material conveying conduit and electrically connected to ground potential at one end and to the conductive part of the gun body at the second end, whereby said second wire acts as a ground connection for the conductive part of the gun body.

5. The assembly of claim 4 wherein said gun body has a handle; and said conductive portion of the gun body being part of said handle.

6. The assembly of claim 4 including a high potential source, said source comprising: a transformer, a capacitor-bridged chopper circuit connected to the primary winding of said transformer, and a cascade multiplier circuit being connected to the second winding of said transformer and also being connected to said electrically conductive electrode wire.

7. The assembly of claim 6 wherein the resonant frequency of the circuit formed by said transformer primary winding and bridging capacitor is approximately equal to the operating switching frequency of said chopper.

8. The assembly of claim 6 wherein said cascade multiplier circuit has at least two rectifiers, and means for reversing the polarity of the output from the cascade multiplier circuit by reversing said two rectifiers.

9. The assembly of claim 8 wherein said means for reversing said two rectifiers comprises a switch to exchange the leads to said rectifiers.

10. The assembly of claim 6 wherein said means for reversing said two rectifiers comprises three rectifiers only two of which are active circuit elements at any one time.