US3411715A - Centrifugal electrostatic spraying head - Google Patents

Description

Nov. 19, 1968 N. R. WALLIS CENTRIFUGAIQ"ELECTROSTATIC SPRAYING HEAD 4 Sheets-Sheet 1 Filed April 2, 1965 Wvg/vra@ NEM puozp WALL/s N0V 19, 1968 N. R. WALLIS i 3,411,715

CENTRIFUGAL'ELECTROSTATIG SPRAYING HED Filed April 2, 1965 4 Sheets-Sheet 2 /N VEN T012 NE /L @abo/DH MALL/s JLM.. AW/J azz Nov. 19, 1968 l N. R.wA| |s 3,411,715 v CENTRIFUGAL ELECTROSTATIC SPRAYING HEAD Filed April a, 1965 4 sheets-sheet 3 Wea/mz NE1L- ?uOoL/Jf/ WALL/5 Ncv. 19, 1968 N. R. WALLIS 3,411,715

CENTRIFUGAL ELECTROSTATIC SPRAYING HEAD Filed April 2, 1965 4 Sheets-Sheet 4 United States Patent O 3,411,715 CENTRIFUGAL ELECTROSTATIC SPRAYING HEAD Neil Rudolph Wallis, Cariad, Goring-on-Thames,

Oxfordshire, England Filed Apr. 2, 1965, Ser. No. 445,157 13 Claims. (Cl. 118-626) ABSTRACT OF THE DISCLOSURE Apparatus lfor coating articles with two-component coating materials including a rotating atomizing head preferably consisting of inner and outer coaxial caps each h'aving circular `discharge edges, with the edge of the inner This invention relates to methods and apparatus for coating articles with two-component coating materials.

Two-component coating materials are now used for certain purposes, one component being, for example, an epoxy or a polyurethane resin and the other component being a hardener. When the two components are mixed, the lmixture has a very short pot life and, accordingly, it is in many cases impossible to apply these materials to articles by conventional methods or with conventional apparatus.

Methods of coating articles with a single-component liquid coating material are known in which the liquid coating material is ejected in finely `divided form from a rotating atomising head and is deposited on the article to be coated under the influence of an electrostatic field.

It is an object of the invention to provide methods and apparatus by means of which two-component coating materials may be applied to -articles under the influence of an electrostatic eld.

From one aspect the invention consists in a method of coating an article with a two-component coating material, wherein the two components of the coating material are fed separately to two individual nozzles from which they are supplied to a rotating atomising head, and wherein said material is ejected in finely divided form rfrom said rotating head and is deposited on said article under the inuence of an electrostatic eld.

From another aspect the invention consists in apparatus for coating articles with two-component coating materials, including means for supplying the two components of the coating material separately through two individual nozzles to a rotating atomising head and means for maintaining an electrostatic eld `between said rotating head and the article to be coated.

The two individual nozzles may be coaxial or their axes may be separated. If the two nozzles are coaxial, they preferably comprise an inner nozzle of circular cross-section to which the hardener is fed and an outer nozzle of annular cross-section to which the resin is fed. The coaxial nozzles may be located immediately behind a rotating disc on which the ltwo components =mix and are thrown outwardly by centrifugal force.

When the two nozzles are separated, their axes are preferably parallel and their outlets co-planar. To assist in the mixing of the two components, the rotating atomising head preferably consists substantially of two coaxial cups,

3,411,715 Patented Nov. 19, 1968 FPice the hardener being supplied to the inner cup from the forward edge of which it is ejected in the |direction of the forward edge of the outer cup to which the resin is supplied.

Methods of performing the invention will now be described with reference to the `accompanying diagrammatic drawings in which:

FIGURE l is =a side view, partly in section of an electrostatic spray gun in accordance with the invention;

FIGURE 2 is an enlarged view of the atomising head of the spray gun illustrated in FIGURE l;

FIGURES 3 and 4 are enlarged views of the coaxial nozzles shown in the head illustrated in FIGURE 2;

FIGURE 5 is a diagrammatic representation of a complete electrostatic spraying system in accordance with the invention;

FIGURES 6, 7 and 8 are respectively a side sectionalv View, a front elevation and a perspective view of a double cup rotating atomising head for use with two spaced nozzles in `an embodiment of the invention;

FIGURE 9 is a side sectional view of an atomisin-g head in accordance with a third embodiment of the invention incorporating a triple cup member;

FIGURE 10 is a sectional view on the line 10-10 of FIGURE 9, looking in the direction of the arrows;

FIGURE 11 is a side sectional View of a cup member in accordance with a further embodiment of the invention; and

FIGURE 12 is a side sectional view of yet another embodiment of the invention.

Referring now to FIGURE l, 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 rotatable cup 28 consisting of insulating material. The two components of the coating material are supplied under pressure to the rotating cup by means of a pair of paint lines 29 and 31.

The cup 28 is rotated by means of an air motor located in the barrel 21 and air is supplied to the air motor by vmeans of an air line 3-4 which extends through the barrel of the gun. The air motor also serves as the electrical connection lbetween the high voltage supply and the cup 28. The high voltage supply is provided by means of `an electrolytic conductor 36.

Secured in the end of the barrel 21 are two nylon blocks 39 and 40 which are visible more clearly in FIGURE 2 of the drawings. The block 39 includes a tapped hole in which is secured a composite feed means 32 to which the paint lines 29 and 31 are connected.

The composite feed means 32 is shown in more detail in FIGURES 3 and 4 from which it will be seen that it comprises an inner nozzle 27 which is in communication with the pipeline 29 Vand an outer nozzle 30 which is in communication with the pipeline 31. Located within the nozzle 27 is a tapered needle 33. The needle 33 is movable axially with respect to the nozzle 27 and in FIG- URE 3 it is shown in its forwardmost position in which it completely closes the nozzle 27, whereas in FIGURE 4 it is shown in its rearwardmost position in which the nozzle 27 is completely clear. Further, the nozzle 27 is movable axially with respect to the nozzle 30 and again in FIGURE 3 this nozzle is shown in its forwardmost position in which it serves to close the nozzle 30, whereas in FIGURE 4 it is shown in its rearwardmost position in which the nozzle is completely clear. Means are provided for adjusting the axial positions of the needle 37 and the nozzle 27 and these are represented in FIGURE 2 by the knurled nuts 34 and 35 respectively.

The cup 28 includes a metal spindle insert 63 in which the end of the driving motor shaft is secured. The cup also 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.

The installation illustrated in FIGURE 5 includes a manual gun 2 similar to that described with reference to FIGURES l to 4 which is used in a spraying booth. 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 34 for the motor, two paint lines 29 and 31, a high voltage cable 36 and a trigger control cable 26.

The trigger control cable 26 connects the micro-switch to control equipment 3 which is mounted on a highvoltage radio-frequency generator 7, the container of which is connected to earth. The control equipment 3 and the high-voltage generator are connected to the main supply by means of a mains cable 15. The control equipment is connected by means of an inter-connecting cable 9 to a bank of three remote-controlled valves 19, 20 and 8. The valve 19 connects the paint line 31 through a further line 11 to a supply of resin in a container 13. The valve 20 connects the air line 34 for the motor through a line 70, a combined moisture separator automatic lubricator and atomiser control 10 and a further air line 12 to a main air supply inlet 14 through container 13. The valve 8 connects the paint line 29 through a further line 71 to a supply of hardener in a container 4.

The air supply inlet 14 is also connected through reducing Valves 72 and 73 to the interiors of the containers 13 and 4, so that the resin is supplied to the line 11 under pressure and the hardener is supplied to the line 71 under pressure. Meters 74 and 75 are provided to indicate the pressures in the containers.

The arrangement of the control circuit 3 is such that when the trigger in the gun is pressed, air is supplied under pressure to the motor to cause the cup 28 to revolve, and when the cup is revolving at a sucient speed, the paint valves 19 and 8 are opened so that the resin and hardener are supplied under pressure to the nozzles 30 and 27 respectively. When the trigger 24 is released, the hardener valve 8 is closed first. After a short delay the resin valve 19 is closed, and thereafter the air valve 20 is closed to stop the motor.

In use, the gun is held with the head at a short distance from the article to be coated and the knurled nuts 34 and 35 are adjusted to give the required proportions of resin and hardener. When the trigger 24 is pressed, the cup 28 revolves and resin and hardener mix in the outlet of the nozzle 30 as shown in FIGURE 4. The mixed resin and hardener are supplied to the rear surface of the disc 64 and the mixed material is thrown off this surface by centrifugal force and forms a film on the interior surface of the walls of the cup 65. The mixed material travels towards the tapered edge 67 and is atomised from this edge. The material is charged by the electrostatic field and is consequently attracted towards the article to be coated since this is the nearest object at earth potential.

As the two components of the coating material do not mix until they are leaving the outlet of the nozzle 30, material cannot harden and block these nozzles. Further, the closing of the hardener valve 8 Ibefore the resin valve 19 ensures that the outlet of the nozzle 30 is rinsed by pure resin when the equipment is switched off. Further, the even longer delay in stopping the motor ensures that this pure resin washes the cup 28 to prevent the resin from setting on the cup.

An advantage of the embodiment of the invention illustrated in FIGURES l to 5, is that it can be applied to an existing gun designed for spraying normal one-component materials. However, it may not always be desirable to utilise the comparatively complicated coaxial nozzle assembly of this embodiment and in such case the embodiment illustrated in FIGURES 6 to 8 may, for example, be preferred.

In this embodiment, two separate nozzles 81 and 82 are used in place of the single composite nozzle assembly 32 and the cup 28 is replaced by a double cup 83. This cup includes an inner flat disc 84 attached to a boss 85 in which the end of the drive shaft is screwed. The flat disc 84 is joined to an inner cup-sh-aped portion 86 by means of three webs 87, 88 and 89. The cup-shaped portion 86 is integral with a flat annular portion 90 which is joined to an outer cup-shaped portion 91 by means of three webs 92, 93 and 94. The mouth of the outer cupshaped portion is tapered to a comparatively sharp edge 95 whith is located forward of the front edge of the inner cup-shaped portion 86.

The open end of the nozzle 81 is located a short distance behind the disc 84 and the open end of the nozzle 82 is located a short distance behind the annular portion 90. In use, resin is supplied to the nozzle 82 and the hardener to the nozzle 81. The hardener impinges on the rear surface of the disc 84 and is thrown off this surface by centrifugal force to form a film on the interior surface of the walls of the inner cup-shaped portion 86. Centrifugal forces cause the hardener to travel towards the forward edge of the inner cup-shaped portion and the hardener is ejected radially from this edge. Similarly, the resin is supplied to the rear surface of the annular portion and is caused to form a film on the interior surface of the walls of the outer cup 91. The ejected hardener mixes with the resin on the inner surface of the outer cap and the composite material is atomised from the forward edge 95. The composite material is charged by the electrostatic eld and is consequently attracted towards the article to be coated.

The remaining parts of the gun may be similar to those illustrated in FIGURE l and the assembly may be arranged as shown in FIGURE 5.

A modification of the embodiment of the invention illustrated in FIGURES 6, 7 and `8 is shown in FIGURE 9. In this case a triple cup is provided in place of the double cup 83 to ensure that the two components are completely mixed before being atomised. In other respects, the arrangement may be substantially as illustrated in FIGURES 6, 7 and 8. Since no material has to be supplied from the back of the cup assembly to the inner wall of the outermost cup, the outermost cup 96 may be connected directly to the cup 91 by a continuous annulus 97.

The arrangement of the nozzles and passages in the nylon block 40 for this embodiment is illustrated in FIGURE 10.

FIGURE ll illustrates a modification of the arrangement illustrated in FIGURE 6 in which the outer cup 91 is in the form of a removable ring. This ring may be, for example, a press fit on the outer wall 98 of the cup assembly, a shoulder 99 being provided on this outer wall to locate the ring. The ring may be disposable and may be removed and discarded at the end of each spraying period. A new ring may then be fitted before spraying is recommenced and in this case any difficulties resulting from setting of the resin on the outer cup are avoided. The embodiment illustrated in FIGURE 9 may also be modified by making both the outer cups 91 and 96 and the connecting annulus 97 in the form of a single disposable ring-shaped member removably fitted on to a shoulder in the manner illustrated in FIGURE l1.

FIGURE l2 illustrates a further modification of the arrangement illustrated in FIGURE 6 in which the cupshaped rotatable member is replaced by a disc-shaped rotatable member. As explained in more detail in these applications, the particular shape chosen is intended to ensure that atomisation occurs substantially wholly rnechanically and not under the influence of the electrostatic iield. For this purpose, the rotatable member is constructed without sharp edges on its outer surface so as to ensure that the discharge current which normally exists between the rotating member and the article is reduced to a minimum.

In the embodiment illustrated, the rotating member is in the form of a disc 101 consisting of insulating material. The central portion of the surface of the disc facing the object to be coated is at and the outer portion of this surface is rounded to avoid sharp corners. The inner surface of the disc is dished to provide a circular trough 108. Located in the circular trough 108 is a metallic insert 109 which is secured to the insulated cup 101 by means of three screws such as the one illustrated at 110. Projecting from the centre of the insert 109 is a spindle 102 provided with a threaded bore by means of which the rotatable member is connected to the shaft of the driving motor. The surface of the insert from which the spindle projects is dished to produce two annular troughs 103 and 104 separated by a raised lip 111. The outer wall of the trough 104 slopes outwardly towards the rear of the insert as shown at 113. Concentric with the disc 101 and spaced a short distance from it on the same side as the spindle 102 is an annular member 105 0f the same outer diameter and general shape as the disc 101. The annular member 105 is fixed to the nylon block 39 by means of three screws one of which is illustrated at 106 and is spaced therefrom by means of spacers such as the one illustrated at f7.

Located in the gap between the disc 101 and the annular member 105 is a sharp edge 112 which is electrically screened from the electrostatic eld by the two members 101 and 105, the peripheral edges of which are formed as a portion of the surface an oblate spheroid. The two nozzles 81 and 82 project through the stationary member 105 to within a short distance of the dished surface of the insert 109.

In use, the disc 101 is rotated at a speed of, for example, 10,000 revolutions per minute and the hardener is supplied through the nozzle 81 to the trough 103. The hardener is caused by centrifugal forces to travel up the lip 111 from which it is atomised by contact with the air. Similarly, resin is supplied through the nozzle S2 to the trough 104 and is caused by centrifugal forces to form a film on the sloping portion 113. The hardener leaving the lip 111 also travels towards the sloping portion 113 where it mixes with the resin. The composite material then travels towards the lip 112 from which it is atomised by contact with the air. Due to the screening effect of the two portions 101 and 105 the electrostatic field which is maintained between the rotating member and the article is substantially ineffective at the gap between the two members in which the lip 112 is located.

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

1. Apparatus for coating articles with two-component coating materials, comprising a rotating atomizing head, iirst and second nozzles, means for supplying one of the two components of the coating material to said first nozzle and for supplying the other of said components to said second nozzle, and means for maintaining an electrostatic eld between said atomizing head and the article to be coated, wherein said atomizing head includes a first part so positioned in relation to said first nozzle that a ilm of said one component is formed on said first part, said iirst part being bounded by a circular edge to which said iirst component travels and from which it is ejected by centrifugal force, and wherein said atomizing head further comprises a second part so located with respect to said circular edge and to said second nozzle that the said other component of the coating material from the second nozzle and the first component from the said circular edge mix thereon to form a composite film, 'said second part being bounded by a further circular edge to which the composite material travels and from which it is ejected by centrifugal force.

2. Apparatus as claimed in claim 1, wherein the first and second parts of the rotating atomizing head are constituted by two co-axial cups, the circular edge of the nner cup being rearwardly located with respect to the circular edge of the outer cup.

3. Apparatus as claimed in claim 2, wherein the atomising head includes a central flat disc attached to an inner cup-shaped portion by means of a plurality of webs and wherein the inner cup-shaped portion is integral with a flat annular portion joined to an outer cup-shaped portion by means of a further plurality of webs.

4. Apparatus as claimed in claim 3, wherein the mouth of the outer cup-shaped portion is tapered to a sharp edge which is located forward of the front edge of the inner cup-shaped portion.

5. Apparatus as claimed in claim 4, wherein the open end of one of the nozzles is located a short distance behind said central disc and wherein the open end of the other nozzle is located a short distance behind said fiat annular portion.

6. Apparatus as claimed in claim 5, wherein the atomising head further includes an outermost cup-shaped portion co-axial with said inner and outer cup-shaped portions, said outermost cup-shaped portion being connected directly to the outer cup-shaped portion by a continuous annulus.

7. Apparatus as claimed in claim 6, wherein the outer cup-shaped portion, the continuous annulus and the outermost cup-shaped portion are in the form of a removable ring.

8. Apparatus as claimed in claim 3, wherein the outer cup-shaped portion is in the form of a removable ring.

9. Apparatus as claimed in claim 2, wherein the rotating atomising head includes a disc of insulating material, the outer surface of which is rounded to avoid sharp edges and the inner surface of which is dished to provide a circular trough, and a metallic insert located in said circular trough, the inner surface of said metallic insert being shaped to provide two annular troughs separated by a raised lip.

10. Apparatus as claimed in claim 9, wherein the open ends of the two nozzles are respectively located in the two annular troughs in the metallic insert.

11. Apparatus as claimed in claim 10, wherein there is provided behind the rotating atomising head an annular member of insulating material.

12. Apparatus as claimed in claim 11, wherein a sharp edge is located in the gap between the insulating disc and the insulating annular member.

13. Apparatus as claimed in claim 12 wherein the peripheral edges of the insulating disc and the insulating annular member are formed as a portion of the surface of an oblate spheroid.

References Cited UNITED STATES PATENTS 2,975,756 3/1961 Reindl et al. 239-15 X 3,008,808 11/1961 Hodges 117-1055 X 3,011,472 12/1961 Kent et al 117-104 X 3,066,874 12/1962 Becker 117-1055 X 3,133,702 5/1964 Stelchek 117-104 X 3,144,209 8/ 1964 Griths 239-15 3,147,137 9/1964 Glass et al. 1l7-93.4

FOREIGN PATENTS 978,763 12/ 1964 Great Britain.

ALFRED L. LEAVITT, Primary Examiner. I. H. NEWSOME, Assistant Examiner.

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