GB1578877A - Electrostatic coating equipment - Google Patents

Description

(54) ELECTROSTATIC COATING EQUIPMENT (71) We, VOLSTATIC COATINGS LIMITED, a British Company of 57-67, Stirling Road, Acton, London, W3 8DJ do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the fok lowing statement: The invention relates to spraying booths for coating objects by high voltage electrostatic deposition of coating powder. The voltages currently used are in the range of 15 kv. to 150 kv., and the powder, which commonly comprises epoxy, polyester or nylon resin, has a specific resistance of the order of 1013 to 1015 ohms.

In operation of these booths, there is a tendency for some of the powder to be precipitated on the booth interior instead of being deposited on the object to be coated.

Such precipitation not only lowers efficiency of a coating operation but also leads to delay when a change of powder colour is required, otherwise the precipitated powder of one colour could be disturbed and deposited on the object together with the powder of a different colour. The spraying operation therefore has to be stopped and the booth interior scoured clean manually using a jet of compressed air, or suction to remove powder of one colour before spraying can be resumed, using powder of another colour.

An object of the invention is to minimise such precipitation and delay.

Accordingly the invention resides in a spraying booth, for coating an object by electrostatic deposition of coating powder, comprising a tubular enclosure having spaced therealong inlet and outlet means for gaseous medium, usually air, flow control means for directing the medium from at least part of the inlet means so that the medium scours the booth interior surface, injecting means for injecting the powder into the booth adjacent at least part of the inlet to form a cloud of the powder in the medium, means for causing movement of the medium and of the powder borne thereby towards the outlet means, and means for charging the powder to a high electrostatic voltage in relation to the object.

The tubular interior surface of the booth extends between the inlet and the outlet means and may have an electrically insulating surface for enclosing the object for exposure to electrostatically charged powder borne by the medium on its way towards the outlet means. The interior surface of the booth may therefore serve as at least part of the electrostatically charging means.

The booth preferably also includes a baffle which extends longitudinally of the booth interior at least between the inlet and outlet means, is spaced angularly from at least part of the inlet means, and is shaped to deflect the medium from the interior surface of the booth inwardly towards the longitudinal axis of the booth. The baffle is preferably a metal plate which is electrically isolated from the tubular enclosure, but may be of material of high electrical insulating properties, e.g. nylon, polythene, pvc or ptfe. In either case, such a baffle can serve as the sole or as an auxiliary means of charging the powder to high voltage, the baffle acquiring a charge by tribo-friction with the gaseous medium and the coating powder borne by the medium.

The means for charging the powder may additionally comprise an electrode shaped for concentrating an electrostatic field and arranged in the means for injecting the powder into the booth. Alternatively, or in addition to such an electrode, a plurality of electrodes shaped for concentrating the field may be spaced between the inlet and outlet and may provide sharp or serrated edges or points for concentrating the field; or the electrode means may be a wire extending between the inlet and the outlet.

The means for causing the movement of the medium and the powder borne thereby preferably comprise an extractor arranged to produce a pressure differential in the medium between the inlet and the outlet therefor.

A plant incorporating a powder spraying booth constructed and equipped according to the present invention is hereinafter described, by way of example, with reference to the drawings, accompanying the provisional specification, in which: Figure 1 is a general plan of the plant; and Figures 2 and 3 are side and end elevations, respectively, on a larger scale than Figure 1, of the booth.

Refering to Figure 1, the plant comprises a loading station 10, powder spraying booth 11 and ancillary equipment which is hereinafter described, a tunnel oven 12 and an unloading station 13 which are connected as a closed circuit by an endless conveyor 14 for objects 15 (Figure 2) to be coated.

Referring to Figures 2 and 3, the booth 11 comprises a tubular enclosure 1 la which is a hollow cylinder extending on a horizontal axis, but may be of any convenient flow cross-section. The cylindrical part of the booth is formed of, or has a lining of, a material of high insulation properties e.g.

nylon, polythene, pvc or ptfe. Each end is closed by an end plate 16 except for a vertically elongate opening 17 through which the conveyor 14 and the objects 15 suspended from it can pass.

The openings 17, at the left and right ends of the booth, as viewed in Figure 2, admit air drawn in during working by an extractor. The extractor conveniently comprises trunking 18 (Figures 2 and 3), having a mouth 19 at an upper end protruding into the cylindrical part of the booth adjacent the right-hand opening 17 so that the mouth 19 is directed away from the latter, and a cyclone separator 20 supplied at the lower end of the trunking 18. The left and right end openings 17 both admit air at ambient pressure, but the enclosure 1 lea is also formed with a pair of circular air inlets 21 supplied by a blower (not shown) and these inlets 21 are of special significance. The mouth 19 of the trunking 18 is the air outlet of the booth.

The inlets 21 are spaced in a longitudinal plane of the booth and although shown arranged in the upper segment of the booth, they may be lower or lowerable. Air blown in from each of the inlets 21 impinges on the central zone of a first deflector plate 22 which is conveniently rectangular in plan, curved as shown in Figure 3 to conform with the booth interior, and spaced from it. Thus the blown air is directed in a generally radial sense of each inlet 21 and is also directed according to the curvature of the booth interior. The blown air therefore tends to scour the booth interior 360 around the inlet 21 and also tends to circulate the air in opposite senses around the longitudinal axis of the booth.

Such scouring needs to be effective particularly at the bottom of the booth where some powder might otherwise settle. This factor can be taken into consideration when positioning the inlets 21.

A baffle, provided by a second deflector plate 23, is angularly spaced from the inlets 21, in the manner indicated in Figure 3 for example, to intercept circulating air. The second deflector plate 23 extends longitudinally of the booth for a substantial portion of the length of the booth, for example as indicated in Figure 2, and also curves inwardly, for example as in Figure 3, from the cylindrical interior surface of the booth to approximately the central vertical plane of the booth, particularly to a location between the conveyor 14 and the objects 15.

A pair of spray guns 24 is inserted in corresponding longitudinally spaced port holes of the booth. As shown in Figure 3, the guns 24 extend in the central horizontal plane of the booth. These guns 24 merely deliver the powder in spray form without imparting an electrostatic charge to the powder, or they may be electrostatic and impart a charge. In either case there may also be an array of sharp electrodes 25 spaced along the interior of the booth 11 as shown. The electrodes 25 may be electrically interconnected as in Figure 2 or connected otherwise for charging and producing a suitable electric field.

The spray guns 25 are supplied by a fluidiser 26 for spraying powder through lines 27 (Figure 1). Pressure built up in the fluidiser during operation is vented through a line 28 which communicates with any convenient part of the booth where spray powder, becoming airborne in the fluidiser 26 and vented through the line 28, will become electrostatically charged and possibly deposited on the objects 15.

In operation, the blower and the cyclone separator 20 are activated, the latter causing air to be drawn in at both ends of the booth through the openings 17 and into the trunking 18 through the mouth 19. The mouth 19 is also adequate to take air forced in through the inlets 21. The objects 15 are loaded on the conveyor at station 10 and conveyed in succession through the right-hand end opening 17 of the booth and out through the left end. The fluidiser 26 is activated to supply coating powder to the guns 24 from which the powder is sprayed as a cloud into the air in the booth, and the objects are maintained at earth potential.

The movement of the air, and of the powder borne by the air, in relation to the second deflector plate 23 and to the cylindrical insulating surface of the booth interior, will itself charge the air, powder, deflector and booth interior electrostatically. The powder, being of high electric resistance, tends to hold its charge whilst aerially suspended. Some of the powder will be attracted to the objects 15 and coat them.

However, a high voltage field can be established in the booth in various other ways, which may be employed singly or in combination. Thus such a field can be established by operation of the spray guns 24 if of the electrostatic type, and/or by employing and charging the electrodes 25 to a high voltage, the spray guns 24, if electrostatic, being charged, possibly to the same order of voltage but not necessarily, as the electrodes 25 if employed with them, and as the voltage on the second deflector plate 23 and the insulating surface of the booth interior.

Some of the powder will be attracted directly to the objects 15 and some of the powder which is still airborne may at first go beyond the objects 15 as overspray because of inadequate attraction charge or of even opposite charge upon it, but eventually may be influenced by the charge on the deflector plates 22 or on the cylindrical insulative surface of the booth or on the electrodes 25 and become attracted to the objects 15. The possibility that the powder will be attracted to the objects is thereby enhanced with consequential improvement in coating and in utilisation of the airborne powder in its movement along the duct towards the mouth 19. Thus, there is advantage in employing a long booth, one of say 10 feet being found to be more efficient than one of 5 feet. Ideally all the air arriving at the mouth 19 has given up all the powder originally borne by it.

However, air extracted by the mouth 19 is drawn into the cyclone separator 20 where residual powder in the indrawn air is separated for recycling through the booth.

Throughout the operation of the booth, the supply of blown air to the inlets 21 is kept up. The first deflector plates 22 ensure that the air forced beween them and the booth surface continually scours the surface 360" around the respective inlets 21 and minimises or prevents deposition of powder on them. This enables a quick changeover to be made from spraying with powder of one colour to spraying with powder of another colour without having to delay to manually scour the booth extensively, or at all, using a compressed air or suction cleaner.

Furthermore, the air which is circulated in opposite senses around the longitudinal axis of the booth by the first deflector plates 22 is intercepted by the second deflector plate 23.

The air circulating counterclockwise with respect to Figure 3 is directed inwardly and downwardly on to the objects 15, thereby tending to carry the cloud of powder on to them. The air circulating clockwise tends to be directed back and down also, in the general direction of the objects.

Objects in the booth are conveyed through the oven 12. The oven operates at a temperature, typically in the range 1800 to 2500C, to cure the resinous component of the spray powder. From the oven 12, the objects are conveyed to the unloading station 13.

The order of high voltage employed is in the range already mentioned. It may depend, we have found, on the shape of the object to be coated, particularly on the depth or complexity of re-entrant surfaces.

WHAT WE CLAIM IS: 1. A spraying booth, for coating an object by electrostatic deposition of coating powder, comprising a tubular enclosure having spaced therealong inlet and outlet means for a gaseous medium, flow control means for directing the medium from at least part of the inlet means so that the medium scours the booth interior surface, injecting means for injecting powder into the booth adjacent at least part of the inlet means to form a cloud of the powder in the medium, means for causing movement of the medium and of the powder borne thereby towards the outlet means, and electrostatically charging means for charging the powder to a high electrostatic voltage in relation to the object.

2. A spraying booth, acording to Claim 1, in which the tubular interior surface of the booth extends between the inlet and the outlet means and has an electrically insulating surface for enclosing the object for exposure to electrostatically charged powder borne by the medium on its way to the outlet means.

3. A spraying booth, according to Claim 1 or Claim 2, in which the inlet means comprise at least one inlet aperture formed in the tubular enclosure and the flow control means comprise a first deflector plate extending radially inwardly of the or each aperture, transversely of the axis of said aperture.

4. A spraying booth, according to any preceding claim, comprising a baffle which extends longitudinally of the booth at least between the inlet and outlet means, is angularly spaced from at least part of the inlet means, and is shaped to deflect the medium from the interior surface of the booth inwardly towards the longitudinal axis of the booth.

5. A spraying booth, according to Claim 4, in which the baffle comprises a metal plate which is electrically isolated from the tubular enclosure.

6. A spraying booth, according to Claim 4 in which the baffle comprises material of high electrical insulating properties.

7. A spraying booth, according to Claim 5 or Claim 6, in which the baffle serves as at least part of the electrostatically charging means.

8. A spraying booth, for coating an object by electrostatic deposition of coating powder, substantially as hereinbefore described with reference to and as illustrated in the drawings accompanying the provisional specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. but not necessarily, as the electrodes 25 if employed with them, and as the voltage on the second deflector plate 23 and the insulating surface of the booth interior. Some of the powder will be attracted directly to the objects 15 and some of the powder which is still airborne may at first go beyond the objects 15 as overspray because of inadequate attraction charge or of even opposite charge upon it, but eventually may be influenced by the charge on the deflector plates 22 or on the cylindrical insulative surface of the booth or on the electrodes 25 and become attracted to the objects 15. The possibility that the powder will be attracted to the objects is thereby enhanced with consequential improvement in coating and in utilisation of the airborne powder in its movement along the duct towards the mouth 19. Thus, there is advantage in employing a long booth, one of say 10 feet being found to be more efficient than one of 5 feet. Ideally all the air arriving at the mouth 19 has given up all the powder originally borne by it. However, air extracted by the mouth 19 is drawn into the cyclone separator 20 where residual powder in the indrawn air is separated for recycling through the booth. Throughout the operation of the booth, the supply of blown air to the inlets 21 is kept up. The first deflector plates 22 ensure that the air forced beween them and the booth surface continually scours the surface 360" around the respective inlets 21 and minimises or prevents deposition of powder on them. This enables a quick changeover to be made from spraying with powder of one colour to spraying with powder of another colour without having to delay to manually scour the booth extensively, or at all, using a compressed air or suction cleaner. Furthermore, the air which is circulated in opposite senses around the longitudinal axis of the booth by the first deflector plates 22 is intercepted by the second deflector plate 23. The air circulating counterclockwise with respect to Figure 3 is directed inwardly and downwardly on to the objects 15, thereby tending to carry the cloud of powder on to them. The air circulating clockwise tends to be directed back and down also, in the general direction of the objects. Objects in the booth are conveyed through the oven 12. The oven operates at a temperature, typically in the range 1800 to 2500C, to cure the resinous component of the spray powder. From the oven 12, the objects are conveyed to the unloading station 13. The order of high voltage employed is in the range already mentioned. It may depend, we have found, on the shape of the object to be coated, particularly on the depth or complexity of re-entrant surfaces. WHAT WE CLAIM IS:

1. A spraying booth, for coating an object by electrostatic deposition of coating powder, comprising a tubular enclosure having spaced therealong inlet and outlet means for a gaseous medium, flow control means for directing the medium from at least part of the inlet means so that the medium scours the booth interior surface, injecting means for injecting powder into the booth adjacent at least part of the inlet means to form a cloud of the powder in the medium, means for causing movement of the medium and of the powder borne thereby towards the outlet means, and electrostatically charging means for charging the powder to a high electrostatic voltage in relation to the object.

2. A spraying booth, acording to Claim 1, in which the tubular interior surface of the booth extends between the inlet and the outlet means and has an electrically insulating surface for enclosing the object for exposure to electrostatically charged powder borne by the medium on its way to the outlet means.

3. A spraying booth, according to Claim 1 or Claim 2, in which the inlet means comprise at least one inlet aperture formed in the tubular enclosure and the flow control means comprise a first deflector plate extending radially inwardly of the or each aperture, transversely of the axis of said aperture.

4. A spraying booth, according to any preceding claim, comprising a baffle which extends longitudinally of the booth at least between the inlet and outlet means, is angularly spaced from at least part of the inlet means, and is shaped to deflect the medium from the interior surface of the booth inwardly towards the longitudinal axis of the booth.

5. A spraying booth, according to Claim 4, in which the baffle comprises a metal plate which is electrically isolated from the tubular enclosure.

6. A spraying booth, according to Claim 4 in which the baffle comprises material of high electrical insulating properties.

7. A spraying booth, according to Claim 5 or Claim 6, in which the baffle serves as at least part of the electrostatically charging means.

8. A spraying booth, for coating an object by electrostatic deposition of coating powder, substantially as hereinbefore described with reference to and as illustrated in the drawings accompanying the provisional specification.