US3590318A

US3590318A - Powder coating apparatus producing a flat powder spray

Info

Publication number: US3590318A
Application number: US878984A
Authority: US
Inventors: Richard O Probst; David D Harrold
Original assignee: Ransburg Corp
Current assignee: Ransburg Corp
Priority date: 1969-12-08
Filing date: 1969-12-08
Publication date: 1971-06-29
Anticipated expiration: 1988-06-29

Abstract

A nozzle for a powder spray gun having an elongated orifice from which a gas suspension of the powder is discharged in a flat, fanlike spray. The nozzle includes means adjacent the orifice inside the nozzle to force convergence of the suspension at the orifice and means in the flow of the suspension rearwardly from orifice cooperating with the means forcing convergence of the suspension to promote uniformity of powder distribution in the spray. The powder spray gun may include a suitable electrode adapted for connection to a high-voltage source to charge and promote the deposition of the powder particles in the spray.

Description

United States Patent FLAT POWDER SPRAY 22 Claims, 5 Drawing Figs.

[52] U.S.Cl 317/3,

[51] Int. Cl .1 1805b 1/04,

[50] Field of Search ..239/590, 15

[56] References Cited UNITED STATES PATENTS 398,327 2/1889 De Vars 317/3 X Primary Examiner- Lee T Hix Attorneys-Merrill N. John, Harry E. Downer, David H.

Badger and Verne A. Trask.

ABSTRACT: A nozzle for a powder spray gun having an elongated orifice from which a gas suspension of the powder is discharged in a flat, fanlike spray. The nozzle includes means adjacent the orifice inside the nozzle to force convergence of the suspension at the orifice and means in the flow of the suspension rearwardly from orifice cooperating with the means forcing convergence of the suspension to promote uniformity of powder distribution in the spray. The powder spray gun may include a suitable electrode adapted for connection to a high-voltage source to charge and promote the deposition ofthe powder particles in the spray.

1 POWDER Lo. SUPPLY L PATENTEDJUNZSISYI sum 10F 2 1 l l I U l l I l f n-w uh.

IN VENT( )R.

DAVID D HARROLD RICHARD O. PROBST PATENTEUJUNZSIQYI 3,590,318

SHEET 2 OF 2 INVIENIYJR. DAVlD 0. HARROLD BY RICHARD o. PROBST POWDER COATING APPARATUS PRODUCING A FLAT POWDER SPRAY This is a continuation of application Ser. No. 579,233, filed Sept. 14, 1966, now abandoned.

This invention relates to apparatus for use in spraying powders to form coatings on the surfaces of articles and is primarily concerned with, although not limited to, apparatus in which electrostatic forces aid in the formation of the desired coating. It is known to form a suspension of powder in air or other gas and to discharge the suspension through a nozzle to produce a spray directed toward the surface to be coated, and it is also known to create between the nozzle and the surface being coated an electrostatic field-promoting deposition of the powder on such surface. So far as I am aware, nozzles heretofore used in the application of powder coatings have produced a generally conical spray which, when directed against an extended flat surface, deposits the powder in a circular or annular pattern. In spray coating, whether with liquids or powders, sprays which form circular or annular deposited patterns are in general less desirable than flat or fan-shaped sprays which form elongated, relative narrow patterns.

When fan-shaped sprays are used in the coating of article surfaces, the usual practice involves moving either the spray gun or the article in such a manner that the spray lies in a plane transverse to the direction of relative movement. To avoid the production of streaks in the coating, it is therefore desirable that the distribution of spray particles across the width of the spray be reasonably uniform. In the spraying of liquid-coating materials under high hydraulic pressure, fanshaped sprays possessing satisfactorily uniform distribution of the spray particles have been produced by nozzles having a supply passage which terminates in a concave, hemispherical surface intersected by a groove of V-shaped in cross section extending diametrically across the end of the nozzle to form an elongated discharge orifice. However, when a nozzle of that type, appropriately changed in dimensions, is used to discharge a gaseous powder suspension it is found that the powder is heavily concentrated in the middle portion of the spray and is not uniformly distributed across the major portion of the sprays width, as is the case with liquids.

Our invention springs from the discovery that the distribution of powder in a spray discharged from an elongated nozzle opening can be controlled by providing in the passage leading to such opening a transverse pin or similar obstruction disposed parallel to the longest dimension of the opening and a short distance rearwardly from the opening; and a nozzle in accordance with the invention therefore embodies such an obstruction. Preferably, the orifice is formed by the intersection of a hemispherical surface at the end of the nozzle passage and a V-shaped slot extending across the end of the nozzle. When the nozzle is to be used in an electrostatic system, it may incorporate a spray-charging electrode desirably in the form of a semiconductive coating applied to a nozzle body of insulating material and terminating in an annular edge surrounding but spaced laterally outwardly from the discharge orifice. Accord ing to one feature of the invention, such an electrode may be supplemented with an auxiliary electrode, which can take various forms.

Further features of the invention will become apparent from the following more detailed description and from the accom panying drawing, in which:

FIG. 1 is an illustration, partially diagrammatic in character, of an electrostatic powder-coatin g apparatus including a spray gun, parts of which are broken away and shown in vertical section;

FIG. 2 is an end elevation of the nozzle embodied in the gun of FIG. 1;

FIG. 3 is a plan view of the nozzle;

FIG. 4 is an axial section through a nozzle provided with a platelike auxiliary electrode disposed in the nozzle orifice; and

FIG. 5 is a plan view, in panial section, showing a nozzle provided with a different form of auxiliary electrode.

fitting 17 to a supply conduit 18, conveniently a flexible 0 plastic tube, leading from a means 19 for delivering a suspension of the powder in a gaseous medium. Details of the means 19 are not illustrated, as various forms of such means are well known.

The tube 13 houses a resistor 21 the rear end of which is electricallyconnected to a flexible cable 22 secured to the rear plug by an appropriate fitting 23 and extending therefrom to the ungrounded terminal of a high-voltage source or power pack 24 the other terminal of which is grounded. Received in the front end of the'tube 13 is a plug 26 supporting a brush 27 which is connected to the front end of the resistor 21 and makes electrical contact with the nozzle 16.

' The nozzle 16 shown is in the form of a cylindrical body, preferably of insulating material, having a rearwardly opening, axially disposed recess 30 adapted to receive the front end of the tube 12. The front end of the nozzle has an elongated outlet orifice 31, desirably of a generally lenticular form when viewed axially of the nozzle, as in'FIG. 2. Such an opening may conveniently be formed by providing the recess 30 with a generally hemispherical forward end surface 32 intersected by a groove 33 of V-shaped cross section extending diametrically across the front end of the nozzle. While the size and proportions of the orifice 31 may vary, a suitable orifice has a length approximately equal to the diameter of the hemispherical surface 32 and a width of approximately one-sixth that length. Outwardly beyond the extreme forward end of the recess 30, the walls of the groove 33 may be, and preferably are, parallel.

As previously indicated, when a gaseous suspension of powder is discharged from an orifice such as that described, it will be found that the powder is largely concentrated close to the center of the discharged fan-shaped spray. To obtain a more uniform distribution of powder laterally of the spray, the nozzle 16 is provided with a pin 35 extending across the recess 30 a short distance rearwardly from the orifice 31 Generally speaking, the concentration of powder near the center of the spray will be reduced as the diameter of the pin 35 is increased; and it is therefore possible, by employing a pin of appropriate diameter, to effect almost any desired lateral distribution of powder within the spray.

In most spray-coating operations, whether the sprayed material is a liquid or a powder, the desideratum is the production of a coating of uniform thickness. Where the surface to be coated is large enough to require a plurality of passes of the spray, the mostdcsirable spray is one of fan shape having a relatively wide central portion within which the distribution of spray particles is reasonably uniform and side portions in which the number of particles per unit of spray width progressively decreases toward the edges of the spray. A nozzle which we have found to be capable of producing such a spray was of the form illustrated in the drawings and above described and had the following dimensions:

Diameter of nozzle body 1 Diameter of hemispherical surface 32 9/16" Length oforifice 31 9/16" Width oforifice 31 0.093" Diameter of pin 35 1/8 Axial spacing ofcentcr of pin from pole ofsurface 32 0.42"

were obtained at spacings between 0.4l and 0.48". Pins 35 of other than circular cross section have been used and have given satisfactory results; but pins having flat upstream faces are not preferred because some powders have a tendency to build up on such faces.

in the gun shown in FIGS. 1-3, electrical charges are imparted to the sprayed particles through the medium of an electrode in the form of semiconductive coating 37 of low effective capacity applied to the cylindrical outer surface of the nozzle 16 and connected to the front end of the resistor 21 through the flexible brush 27, which engages the coating 37 when the nozzle is in place. As explained in US. Letters Patent No. 3,048,498 granted. Aug. 7, 1962 on the application of J .W. .luvinall, et al., the use of the resistor 21 and reduction in the effective electrical capacity of conductive parts, including the electrode connected to the high-voltage source preferably glass-filled Teflon or similar resin characterized by relatively high lubricity. Such a'material, as compared with resins otherwise suitable, has relatively good wearing properties and little tendency to acquire built-up coatings of the powder. If the gun is to discharge relatively abrasive material,

such as tale for example, the pin 35, and even the body of the nozzle may be made of a ceramic to increase resistance to wear. No advantage, by way of charging spray particles, flows from using a pin of conductive material and connecting it to the high-voltage source.

In the gun shown in FIGS. l3, the forward edge of the coating 37 serves as a discharge electrode, the spray particles acquire their electrical charges by bombardment with atmospheric ions generated adjacent that edge and projected therefrom toward the grounded article being coated, and the charged particles are attracted to such article by electrostatic forces inknown manner. We have found that the efficiency of deposition, or the ratio of powder deposited to powder sprayed, attainable with a nozzlesuch as is shown in FIGS. 1- 3 and above described is relatively constant over the range of spraying distances (about 3 inches to about Qinches) normally maintained in practice between the gun and the article being coated. We have further found that the efficiency of deposition can be measurably increased by providing the nozzle 16 with certain forms of auxiliary electrodes now to be described.

In the arrangement shown in FIG. 4, the auxiliary electrode 40 is a flat piece of thin sheet metal seated centrally in the groove 33 (See FIG. 3), projecting forwardly therefrom, and desirably terminating in a curved front edge as shown. Such an electrode may conveniently be held in place by two rearwardly projecting, integral fingers 41 which snugly receive the nozzle body between them and engage the coating 37. Stainless steel 0.002" in thickness is a suitable material for the electrode 40. Such an electrode has been found to effect a significant increase in deposition efficiencies, particularly at spraying distances in the range from about inches to about inches, but has a disadvantage in that it possesses a relatively high effective electrical capacity and tends to acquire a surface coating when certain powders are sprayed.

The auxiliary electrode 43 shown in FIG. 5 is in the form of a disc of insulating material mounted on the nozzle 16 and having a forwardly projecting rim disposed to be approximately coplanar with the base of the groove 33. A semiconductive coating 44 contacting the coating 37 on the nozzle body extends over the rear face of the electrode to the peripheral edge thereof. Such an electrode 3 V4 in diameter mounted on a nozzle 1'' in diameter significantly increased deposition efficiencies, particularly at spraying distances from about 4" to about 11''. Electrodes 43-44 of smaller diameter also increased deposition efficiencies, but to a lesser degree.

An auxiliary electrode projecting forwardly from the front face of the noule beside the discharged spray has not been found efiective to increase deposition efficiency significantly. The same is true of two such electrodes located on opposite sides of the spray, and also true of a fine wire electrode supported on a conductive pin 35 and projecting forwardly through and beyond the orifice 31.

The various auxiliary electrodes above referred to were employed on nozzles 16 of insulating material provided with the pin 35 and a semiconductive coating 37 extending to the periphery of the front nozzle face. Nozzles so coated and used without auxiliary electrodes produced deposition efficiencies as high as, and in some cases higher than, uncoated nozzles of insulating material provided with electrodes of other forms. Extending the semiconductive coating over the walls of the groove 33 did not increase the deposition efficiency. At shorter spraying distances, a nozzle 16 of the type shown and above described but formed of metal produced a deposition efficiency slightly higher than that obtained with a similar nozzle fonned of insulating material and provided with the semiconductive coating 37; but at spraying distances greater than about 7", the latter nozzle was significantly superior.

The comparative deposition efficiency results referred to above were all obtained with a spray gun of the form shown in FIG. 1 in which the output voltage of the power pack 24 was kv. and the resistance in the output circuit, including the resistances of the coating 37, the resistor 21, and a second resistor (not shown) in the power pack, was about 450 megohms. Those values for resistance and applied voltage are within the range conventionally employed in using electrostatic spray guns.

We claim as our invention:

1. In a powder-coating apparatus having a spray-discharging nozzle provided with an elongated outlet orifice, a supply passage leading to such orifice, and means for delivering to such passage a gaseous suspension of powder to be discharged from the orifice as a fanlike spray; means for controlling the lateral distribution of powder within the spray, said means including a pin extending across said passage transversely to the flow therein in a position substantially coplanar with said oritice and spaced a short distance rearwardly therefrom.

2. Powder-coating apparatus as set forth in claim 1 wherein said orifice is of a generally lenticular shape as viewed axially of said passage.

3. Powder-coating apparatus as set forth in claim 2 wherein said orifice is defined by the intersection of a generally hemispherical surface at the end of the passage with a groove of V- shaped cross section extending across the front face of the nozzle.

4. Powder-coating apparatus as set forth in claim 2 wherein said orifice has a maximum width approximately one-sixth its length and said pin has a diameter greater than the maximum orifice-width.

5. Powder-coating apparatus as set forth in claim 2 wherein said orifice has a maximum width approximately one-sixth its length.

6. Powder-coating apparatus as set forth in claim 1 wherein said pin is of circular cross section and of uniform diameter throughout the diameter of said passage.

7. Powder-coating apparatus as set forth in claim 1 wherein said pin is of circular cross section.

8. Powder-coating apparatus as set forth in claim 1 wherein the nozzle is formed of electricallyinsulating material and provided with a discharge electrode surrounding said orifice an connected to a source of high voltage.

9. Powder-coating apparatus as set forth in claim 8 wherein the discharge electrode is the forward edge of a semiconductive coating applied to the sides of the nozzle, said coating being connected to the high-voltage source through a substantial resistance.

l0. Powder-coating apparatus as set forth in claim 8 with the addition of an annular auxiliary electrode which is of larger diameter than the nozzle and surrounds it.

11. Powder-coating apparatus as set forth in claim 8 with the addition of a thin, platelike auxiliary electrode located in said orifice and projecting forwardly therefrom.

12. A spray discharging nozzle for. discharging a suspension of coating material particles in a gaseous medium provided with an elongated outlet orifice, said orifice being formed at the intersection of a bore having converging walls inside of the nozzle and a notch on the outside of the nozzle, and means within the bore spaced rearwardly from the orifice for controlling the lateral distribution of the powder discharged from the nozzle, said means having a surface extending transversely of the bore to intercept and deflect laterally a portion of the flowing suspension.

13. A nozzle as set forth in claim 12 wherein the converging walls of the interior bore are spherical at the front of the bore and said outside notch is V-shaped.

14. A nozzle as set forth in claim 12 wherein said means for controlling the lateral distribution of the powder discharged from the nozzle is a pin extending transversely of the bore and spaced a short distance rearwardly from the orifice.

15. In a powder spraying apparatus, anelongated conduit for delivering a suspension of powder in a gaseous medium, a nozzle having an elongated outlet orifice at the end of said conduit, said nozzle including means adjacent the orifice inside of the noule to force a convergence of powder and gaseous medium at the orifice, and means in the flow of the suspension and spaced a short distance rearwardly from the orifice cooperating with said other means to control the lateral distribution of powder discharged from the nozzle.

16. In a powder spraying apparatus, a nozzle for spraying powder suspended in a gaseous medium, the nozzle comprising,

a body,

a passage in the body adapted to be connected to means for delivering the powder suspended in the gaseous medium to the passage, the passage terminating in a forward end surface to force a convergence of the powder and the gaseous medium,

an outlet orifice in the body communicating with the passage, the outlet orifice providing an opening adapted to emit the powder and the gaseous medium, and

means in the passage assisting in the distribution of the powder within the gaseous medium whereby the spray emitted through the outlet orifice includes a relatively wide central portion wherein the distribution of powder is reasonably uniform.

17. A powder spraying nozzle comprising a body having an elongated orifice at the front of the body and interior walls defining a passageway therethrough for transmitting a flowing suspension of powder in gas to the orifice, means spaced rearwardly of the orifice and acting within the body to deflect the flowing suspension,

the walls within said passageway converging forwardly of said means and adjacent the elongated orifice and cooperating with said means to project the deflected flowing suspension through the elongated orifice as an expanding fan-shaped powder spray with a uniform powder distribution.

18. A powder nozzle to spray a flowing suspension of powder and gas as an expanding fanlike spray with uniform powder distribution comprising a body having interior walls defining a passageway for the flowing suspension and an elongated orifice in the forward portion of the body adjacent the forward portion of the passageway to emit the spray, the walls of the passageway having a converging portion adjacent the elongated orifice, and i means spaced rearwardly of the orifice and a converging portion of the passageway and acting within the passageway to deflect the flowing suspension and in cooperation with the conver ing portion of the walls to distribute the powder uniformfiy within the expanding fanlike spray.

19. A spray nozzle adapted to be connected to a gaseous suspension of powder, the spray nozzle comprising an elongated opening, a passage leading to the opening and adapted to be connected to the gaseous suspension of powder, the gaseous suspension of powder to be discharged from the opening as a fanlike spray, means in the form of a surface in the passage and spaced a short distance from the opening and extending into the passage to intercept and deflect a portion of the gaseous suspension of powder and cooperating with the opening to control the distribution of the powder in the spray.

20. The spray nozzle of claim 19, wherein the surface is substantially coplanar with the longest dimension of the opening.

21. The spray nozzle of claim 20, wherein the surface is part of a pin.

22. A spray discharging nozzle for discharging a suspension of coating material particles in a gaseous medium provided with an elongated orifice, said orifice being formed at the intersection of a bore having converging walls inside of the nozzle and a notch on the outside of the nozzle, a pin extending transversely of the bore and spaced a short distance rearwardly from the orifice to intercept and effect laterally a portion of the flowing suspension, the pin being substantially parallel to the length dimension of the orifice and with the orifice controlling the lateral distribution of the powder discharged from the nozzle.

Claims

1. In a powder-coating apparatus having a spray-discharging nozzle provided with an elongated outlet orifice, a supply passage leading to such orifice, and means for delivering to such passage a gaseous suspension of powder to be discharged from the orifice as a fanlike spray; means for controlling the lateral distribution of powder within the spray, said means including a pin extending across said passage transversely to the flow therein in a position substantially coplanar with said orifice and spaced a short distance rearwardly therefrom.

3. Powder-coating apparatus as set forth in claim 2 wherein said orifice is defined by the intersection of a generally hemispherical surface at the end of the passage with a groove of V-shaped cross section extending across the front face of the nozzle.

8. Powder-coating apparatus as set forth in claim 1 wherein the nozzle is formed of electrically insulating material and provided with a discharge electrode surrounding said orifice and connected to a source of high voltage.

10. Powder-coating apparatus as set forth in claim 8 with the addition of an annular auxiliary electrode which is of larger diameter than the nozzle and surrounds it.

12. A spray discharging nozzle for discharging a suspension of coating material particles in a gaseous medium provided with an elongated outlet orifice, said orifice being formed at the intersection of a bore having converging walls inside of the nozzle and a notch on the outside of the nozzle, and means within the bore spaced rearwardly from the orifice for controlling the lateral distribution of the powder discharged from the nozzle, said means having a surface extending transversely of the bore to intercept and deflect laterally a portion of the flowing suspension.

15. In a powder spraying apparatus, an elongated conduit for delivering a suspension of powder in a gaseous medium, a nozzle having an elongated outlet orifice at the end of said conduit, said nozzle including means adjacent the orifice inside of the nozzle to force a convergence of powder and gaseous medium at the orifice, and means in the flow of the suspension and spaced a short distance rearwardly from tHe orifice cooperating with said other means to control the lateral distribution of powder discharged from the nozzle.

16. In a powder spraying apparatus, a nozzle for spraying powder suspended in a gaseous medium, the nozzle comprising, a body, a passage in the body adapted to be connected to means for delivering the powder suspended in the gaseous medium to the passage, the passage terminating in a forward end surface to force a convergence of the powder and the gaseous medium, an outlet orifice in the body communicating with the passage, the outlet orifice providing an opening adapted to emit the powder and the gaseous medium, and means in the passage assisting in the distribution of the powder within the gaseous medium whereby the spray emitted through the outlet orifice includes a relatively wide central portion wherein the distribution of powder is reasonably uniform.

17. A powder spraying nozzle comprising a body having an elongated orifice at the front of the body and interior walls defining a passageway therethrough for transmitting a flowing suspension of powder in gas to the orifice, means spaced rearwardly of the orifice and acting within the body to deflect the flowing suspension, the walls within said passageway converging forwardly of said means and adjacent the elongated orifice and cooperating with said means to project the deflected flowing suspension through the elongated orifice as an expanding fan-shaped powder spray with a uniform powder distribution.

18. A powder nozzle to spray a flowing suspension of powder and gas as an expanding fanlike spray with uniform powder distribution comprising a body having interior walls defining a passageway for the flowing suspension and an elongated orifice in the forward portion of the body adjacent the forward portion of the passageway to emit the spray, the walls of the passageway having a converging portion adjacent the elongated orifice, and means spaced rearwardly of the orifice and a converging portion of the passageway and acting within the passageway to deflect the flowing suspension and in cooperation with the converging portion of the walls to distribute the powder uniformly within the expanding fanlike spray.

21. The spray nozzle of claim 20, wherein the surface is part of a pin.