GB2138324A - Electrostatic spraying - Google Patents

Abstract

A method and apparatus for electrostatic spraying of liquid or powder wherein during spraying the spray pattern is varied by varying the mass flow of the shroud air of the sprayhead to render the coating produced more even. The variation may be cyclic and be produced by alternatively connecting a high 21 and low 23 pressure air supply. There may be a pair of spray heads 11a, 11b having the shrouds supplied out of phase. <IMAGE>

Description

SPECIFICATION Electrostatic spraying This invention relates to a method of, and apparatus for, electrostatic spraying, particularly electrostatic spraying of liquid paint materials and electrostatic spraying of dry powder materials.

Irrespective of the material being sprayed the basic principle of operation is the same in that spray particles, whether liquid paint droplets or powder particles have a high voltage electrostatic charge applied thereto so that subsequently the particles are attracted to an electrically earthed target, the target being the workpiece to be sprayed.

In the case of liquid paint spraying it is usual to apply the electrostatic charge to the paint at the sprayhead or in the paint supply line adjacent the head. Atomization of the liquid paint at the sprayhead is achieved by feeding the paint onto the surface of a disc or bell which rotates at high speed. The paint flows to the edge of the disc or bell from where it is thrown radially outwardly by centrifugal action in the form of small liquid paint droplets which constitute a paint droplet cloud. It is believed that the atomization of the paint into the droplet cloud is assisted by the electrostatic attraction between the charged paint and the earthed target. In such an arrangement the conveying of the atomized paint cloud to the target is almost exclusively as a result of the electrostatic attraction of the charged droplets to the target.However, in order to confine the spray cloud and to guide it generally towards the target there is provided an air shroud which confines the paint droplet cloud. The air shroud is produced by compressed air issuing from the sprayhead by way of a circular row of air jets or outlets, or an annular outlet or the like.

In the case of dry powder spraying it is usual to convey the powder in an airstream from a fluidized bed, where the powder mass is fluidized by an air flow through the bed, to an air powered sprayhead at which the powder particles receive their electrostatic charge. The air powered sprayhead uses a low pressure compressed air supply to convey the powder through a nozzle or nozzles to form a cloud of particles, the cloud migrating to the target by the combined action of electrostatic attraction, and the flow of low pressure air through the nozzle or nozzles. As with the liquid paint spraying arrangement described above a series of jets or an annulus or the like, of the head directs compressed air to form an air shroud confining the cloud. It is also known in powder spraying to provide what may be called an internal shroud in the powder cloud.A core or internal shroud of compressed air issues into the cloud from a centrally disposed annulus or ring of jets on the head and shapes and directs the cloud from within rather than confining the cloud.

The cloud of paint droplets generated by the rotating disc or bell is annular in cross-section and thus sprays a circular annular region of the target. In order to achieve homogeneous coverage of the target it is usual to employ several sprayheads so arranged that their spray patterns overlap. Alternatively, or more often in addition, the sprayhead or heads are oscillated relative to the target and possibly also the target is moved relative to the sprayhead or heads. Similar problems arise in dry powder spraying where the area of the target which is sprayed by a single head is again annular, although not necessarily circular. The air powered sprayhead can produce a circular annulus but can also produce an eliptical annulus.It will be recognised that the need to use a large number of sprayheads often with the addition of the need to oscillate the heads and possibly also to move the target results in complex and expensive spraying installations, and it is an object of the present invention to provide an electrostatic spraying method and apparatus wherein these problems are minimized.

A method of electrostatic spraying in accordance with the present invention includes the step of varying the mass flow of the shroud air between predetermined limits during the spraying operation.

It will be recognised that the mass flow of the shroud air determines the spread of the droplet or particle cloud, and thus determines the outer diameter of the sprayed annulus. Thus a high mass flow of shroud air gives rise to a small diameter sprayed annulus whereas a low mass flow of shroud air permits a larger diameter sprayed annulus except in the case of an internal shroud or core where the conditions are reversed.

Preferably the mass flow of shroud air is varied in a predetermined repeating cycle during the spraying operation.

Desirably the mass flow of shroud air is varied by varying the pressure of the shroud air supply to the sprayhead.

Conveniently a second sprayhead is operated in conjunction with the first mentioned sprayhead.

Desirably the mass flow of shroud air of the second sprayhead is also varied between predetermined limits during the spraying operation.

Preferably the cycle of variation of the mass flow of shroud air of the second spray head is similar to but out of phase with the cycle of the first sprayhead.

Electrostatic spraying apparatus according to the invention includes a sprayhead, means for supplying air under pressure to define an air shroud for the droplet or particle cloud generated by the sprayhead, and means for varying the mass flow of the shroud air during the spraying operation.

Preferably said means for varying the mass flow of shroud air comprises means whereby the pressure of the shroud air supplied to the head is varied.

Preferably said means for varying the mass flow of shroud air causes a repeated cyclic variation in shroud air mass flow during the spraying operation.

Preferably said means for varying the pressure of the shroud air supply switches the supply between high and low pressure supplies in a repeated, predetermined sequence during the spraying operation.

In the accompanying drawings: Figure 1 is a diagrammatic representation of a single sprayhead, liquid paint, electrostatic spraying apparatus in accordance with one example of the present invention; Figure 2 illustrates the spray pattern of the apparatus of Figure 1 when subject to a higher shroud air mass flow; Figure 3 is a diagrammatic representation of a twin sprayhead apparatus; and Figure 4 is a view similar to Figure 3 of a modification.

Referring first to Figures 1 and 2, of the drawings, the electrostatic spraying apparatus includes a sprayhead 11 which receives liquid paint from a reservoir by way of a paint supply line 12. The head 11 also receives compressed air by way of a supply line 13. The head includes a conical bell 14 which is rotated about its axis 15. Liquid paint from the supply 12 is fed to the surface of the bell 14 and flows along the surface of the bell to the outer edge 16 from which it is thrown under centrifugal action to form a cloud of paint droplets. In the absence of any external influence the paint droplet cloud would be planar and at right angles to the axis 15.However, either in the supply line 12, or within the head 11, the liquid paint receives a high voltage electrostatic charge so that the droplets in the paint cloud are highly charged and are thus electrostatically attracted to an earthed target 17 positioned in front of the sprayhead. The target 17 is of course the workpiece which is to be sprayed. Thus the cloud of paint droplets leaving the edge 16 of the bell 14 migrates towards the workpiece 17 by virtue of the electrostatic attraction. In order to confine the cloud migrating towards the workpiece 17 the fixed part of the head 11 is provided with a plurality of air jets 18 which are presented towards the target 17, and which are arranged in a circlar row concentric with the axis of the bell 14.Air from the supply 13 issues from the jets 18 and defines an air shroud containing the cloud of paint droplets and also generally guiding the paint cloud towards the workpiece 17. As an alternative to a circular row of jets the air shroud could issue from an annular orifice of the head. Various modes of achieving an air shroud are known and the exact construction of the head in this respect is not critical to the invention.

The foregoing description is a description of a conventional liquid paint electrostatic spraying apparatus, and it will be understood that there are a number of known variants of such apparatus. For example, the form ofthe bell 14 may be altered to suit different applications, and in some instances it may be preferred to use a rotating disc rather than a bell. Moreover, the manner in which paint is supplied to the rotating component, and the manner in which the paint receives its electrostatic charge may vary from application to application.

In each instance however compressed air forms an air shroud containing the paint cloud.

In a conventional powder spraying installation (not shown) a supply line connects a sprayhead to a fluidized bed. The mass of powder particles is fluidized in the fluidized bed by means of air passing through the bed, and an air flow carries the fluidized supply of particles along the supply line to the sprayhead. The sprayhead has one or more nozzles through which the powder particles issue to form a powder cloud. The cloud of powder particles may be considered as extremely similar to the cloud of liquid paint droplets in that during passage through the head or earlier, or as the particles issue from the head, the particles are given an electrostatic charge of high voltage which causes the particles to be attracted towards the workpiece which is electrically earthed.Again the head has a series of jets oran annulus orthe like through which compressed air issues to define an air shroud confining the particle cloud. As an alternative the powder could be supplied by a mechanical feed to the sprayhead. As a further alternative instead of an external air shroud the powder sprayhead can generate an internal air shroud or core. Thus the head is provided with jets or an annulus or the like through which compressed air issues into the cloud rather than around the cloud. This internal core or shroud is effective in shaping the cloud and more particularly in directing the cloud towards the target.

Primarily in the liquid paint spraying apparatus described above, but also in the dry powder spraying apparatus the cloud which is produced is annular, and thus the area sprayed, assuming that the head and target do not move relative to one another, is an annulus. In the case of the liquid paint spraying apparatus where the paint is atomized by a rotating member then the sprayed annulus on the workpiece is a circular annulus. In the case of powder spraying where an airflow carries the powder through a nozzle then again generally the annulus sprayed will be a circular annulus. However, it is possible to control the nozzle such that the annulus sprayed is eliptical.

Thus each of the spraying apparatus mentioned above sprays an annular region of the workpiece, and in order to provide homogeneous spraying of the workpiece it has previously been proposed to use a plurality of sprayheads working in conjunction with one another, their spray patterns overlapping. Moreover, in many applications it has been found necessary also to oscillate the heads relative to the workpiece, and in some instances it is also necessary to move the workpiece relative to the heads in order to achieve a homogeneous spraying of a large area of the workpiece.

In the apparatus illustrated in the Figure 1, homogeneous spraying of the workpiece 17 is achieved in a simple and convenient manner without the necessity for relative movement between the sprayhead and the workpiece.

The shroud air supply line 13 is connected to a changeover valve 19 which in turn is connected to a high pressure supply line 21 and a low pressure supply line 22. For example the pressure in the line 21 may be 4 Bar whereas the pressure in the line 22 may be 2 Bar. The changeover valve 19 is controlled by any convenient timing system, for example a mechanically driven cam, or an electronically controlled electromagnetic device to switch the valve 19 between its two alternative positions in a predetermined sequence. When the valve 19 couples the line 21 to the supply line 13 then the mass flow of shroud air from the jets 18 is relatively high and thus the droplet cloud issuing from the head 11 is constrained to a relatively narrow configuration (as shown in Figure 2).Thus the annulus sprayed by the cloud of narrow configuration is of relatively small diameter, and may be sufficiently small as to cease to be an annulus, and to become a circular area. This will occur as a result of migration of the droplets within the cloud to cover the central region. By comparison, when the line 13 is connected to the line 22 then the mass flow of shroud air from the jets 18 is relatively low and the paint droplet cloud is correspondingly broader (as shown in Figure 1). Thus the annular area of the workpiece 17 which is sprayed is of relatively large diameter.

The timing mechanism controlling the valve 19 varies the mass flow of the shroud air issuing from the jets 18 in a repeating cycle of high-low, high-low throughoutthe spraying operation, and thus the configuration of the cloud of paint droplets, and correspondingiy the diameter of the annular area of the workpiece 17 which is sprayed varies in the same repeated cyclic manner. In this way the single sprayhead can achieve a homogeneous spray coating on the workpiece 17 over a relatively large area without movement of the sprayhead and workpiece relative to one another.

Although not shown in the drawing, it will be recognised that an identical principle can be utilized with dry powder spraying wherein again the annular pattern sprayed can be varied by varying the mass flow of the shroud air to achieve homogeneous spraying of a predetermined area without relative movement of the sprayhead and workpiece. It will be understood that where an internal shroud or core is used then high mass flow of shroud or core air gives rise to wide spray pattern whereas low mass flow will give rise to a narrower spray pattern.

It will be understood that the mechanism for varying the mass flow of shroud air during the spraying operation can take a wide variety of different forms. It is convenient to vary the mass flow by varying the pressure of the supply, rather than by varying the size ofthe jets 18 or the annulus since this would necessitate some cyclically operable mechanism within the head 11. However, this possibility, while not preferred, is not to be excluded since although it is convenient to vary the pressure when using conventional sprayheads it may in the future become desirable to construct new forms of sprayhead in which a mechanism is incorporated for varying the size of the orifice, that is to say the annulus or the equivalent of the jets 18 so that the shroud air mass flow can be varied without altering the supply pressure.

In the arrangement illustrated in Figure 1, two supply lines 21, 22 of different pressure are necessary. It is to be understood that where only a single pressure supply is available then the changeover valve 19 or its equivalent could be fed from the single supply for the high pressure line, and, in its other position could prevent the supply of shroud air or could be fed from a branch of the single supply through a pressure regulator for the low pressure line. However more variable arrangements are possible utilizing switching between a number of different pressure sources to reduce the size of the change in pressure at each step.

Indeed it would be possible, and perhaps desirable, to provide a pressure control arrangement which alters the shroud air supply pressure steplessley between predetermined limits.

In Figure 1 it can be seen that the connection through the valve 19 between the line 13 and the low-pressure line 22 is established giving rise to the large diameter spray cloud of the low mass flow situation. Similarly, in Figure 2 the connection between the high pressure line 21 is made giving rise to the small diameter cloud of the high mass flow situation.

Throughout this specification reference is made to compressed air particularly in relation to the medium for producing the shroud. It is to be understood however that should an inert gas be required for any particular application then references to compressed air are to be construed as references to the appropriate gas.

It will be recognised that each of the alternatives disclosed above could utilize two or more sprayheads operating in conjunction to spray simultaneously the same workpiece. Each of the heads could have its shroud air mass flow similarly controlled to achieve maximum coverage from each sprayhead without relative movement between the sprayheads and the workpiece.

Where a pair of sprayheads are utilized in conjunction with one another it may be convenient to operate their shroud air mass flow controls out of phase. Thus when the first head is operating at low mass flow the other head will be operating at high mass flow and vice versa. Two heads could be controlled in this manner from a single changeover valve, the changeover valve switching the high pressure and low pressure supply lines alternately between two output lines, one for each sprayhead. Should more than two sprayheads be operated simultaneously then an appropriate phasing of the mass flow controls can be determined if necessary by trial and error. It will be understood that many variants are possible depending upon the spraying application in question.

Thus a pair or more of heads could be operated with their shroud air mass flow varied, as described above, in phase or out of phase. One or more heads may have its shroud air mass flow constant while another or others have their shroud air mass flow varied.

Figure 3 iliustrates an arrangement wherein a pair of sprayheads 1 la, 1 ib are supplied with paint and shroud air from common supplies. Each sprayhead is supplied with shroud air from a high pressure line 22 or a low pressure line 21 by way of a respective electrically operated changeover valve 1 9a, 1 9b. The valves 19a, 19b can be operated in unison such that when valve 19a connects line 21 to sprayhead 1 la, line 22 is connected via valve 19b to sprayhead 1 lib. Moreover, the valves can be controlled such that each sprayhead is connected to each pressure line for equal periods of time. However, in a practical embodiment the two sprayheads are used to spray the horizontal or substantially horizontal surfaces of vehicle bodies on a production line.The heads are positioned side by side on opposite sides respective of the centreline of the track of production line and can be moved vertically to compensate for different vertical heights of surface to be sprayed. When a conventional spraying system is used to spray the roof section of a vehicle body, two particular problems arise. In order to avoid wastage of paint by spraying into the windscreen and rear window apertures of the body it is usual to commence the spraying of the roof section when substantially the whole of the spray pattern overlies the roof and to cease spraying as the rear edge of the roof passes beneath the rearmost edge of the spray pattern.Such operation although saving paint, gives rise to a thinner paint coating adjacent the front and rear edges of the roof section and is known as an "incomplete pass" because the whole of the spray pattern has not passed over the whole of the length of the roof section.

Furthermore, the positioning of a pair of sprayheads to obtain optimum coating of the remainder of the roof section often gives rise to thicker coating along the central region of the roof section where the patterns of the two sprayheads tend to overlap. In order, in the known arrangements, to try to minimise these problems it is usual to traverse the sprayheads across the roof section at right angles to the direction of movement of the car body. While such operation minimises the problem of a thicker central band, it does little to avoid the thinner coating of the front and rear edge regions.

In the apparatus of Figure 3, the pulsing of the shroud air, giving rise to alternating broadening and narrowing of the spray pattern produces an even coating of the roof section without the need to traverse the sprayheads. The exact relationship between the pulsing of the shroud air of the two adjacent sprayheads will be determined by the nature of the surfaces to be sprayed. For example a regular pulsing with each head having equal periods (e.g. 0.5 seconds) of high and low pressure and the two guns being 1800 out of phase may be suitable for many applications. However when spraying certain car body roof sections with primer paint it has been found that the following operating parameters give very good results in terms of the evenness of the coating thickness over the whole roof section.The two sprayheads are mounted at either side of the centreline of the car body track which is moving at about 14 feet/minute. The high pressure line 21 supplies air at 2.0 BAR and the low pressure line 22 supplies air at 0.2 BAR. Each sprayhead 1 la, 1 ib is such that when supplied with shroud air at 0.2 BAR its spray pattern is a circular annulus of approximately 1 metre diameter, whereas when supplied with shroud air at 2.0 BAR its pattern is a circular annulus of approximately 0.5 metre diameter. The two sprayheads each have a low pressure phase which is longer than the subsequent high pressure phase and the two sprayheads are out of phase as indicated in the following table showing the high and low pressure operating times for each sprayhead.

Time Shroud Air Pressure (Secs) L.H. Gun 77a R.H. Gun 11b .5 High Low .2 Low Low .5 Low High .2 Low Low .5 High Low .2 Low Low etc etc. etc.

Figure 4 shows an arrangement similar to Figure 3 but usable where only a single pressure supply is available. Thus the sprayheads 1 1a, 1 1b receive paint from a common supply line 12 and can receive shroud air under pressure from a common pressure line 21. Flow of airto each sprayhead is controlled by a respective valve 19a, 1 9b acting as an on-off valve. Thus, when a valve 19 is open its respective sprayhead receives shroud air under pressure from the line 21 and thus has a narrow spray pattern. However, when a valve 19 is closed its respective sprayhead receives no shroud air and so has a broader spray pattern.

It will be recognised that the control valves of the arrangement described above can be mechanically controlled or can be electromagnetically operated and can be underthe control of a microprocessor or the like which may also control other parameters of the spraying operation.

The other variants and modifications described with reference to Figures 1 and 2 are also applicable to the arrangements described with reference to Figures 3 and 4.

It will be recognised that spraying apparatus as described above has significant advantages over the prior art apparatus in that whereas in the prior art in order to achieve even coating it was necessary either to oscillate or otherwise move the sprayhead orto utilize multiple sprayheads, the arrangements described above achieve a more even coating by varying the mass flow of the shroud air, a solution to the problem which is considerably simpler and less expensive than moving the sprayhead or using multiple heads.

Moreover varying the mass flow of shroud air overcomes the problem of "banding" found in the prior art where multiple sprayheads are used. In the prior art bands ofthicker coating are produced where spray patterns of the multiple heads overlap, and bands of thinner coating occur in other regions. The usual method, in the prior art, to overcome "banding" is to oscillate the spray heads but this of course necessitates expensive and sometimes complex mechanisms use of which is obviated by varying the mass flow of shroud air as disclosed above.

Claims (14)

1. A method of electrostatic spraying including the step of varying the mass flow of the shroud air of the sprayhead between predetermined limits during the spraying operation.

2. A method as claimed in claim 1 wherein the mass flow of shroud air is varied in a predetermined repeating cycle during the spraying operation.

3. A method as claimed in claim 1 or claim 2, wherein the mass flow of shroud air is varied by varying the pressure of the shroud air supply to the sprayhead.

4. A method as claimed in any one of claims 1 to 3, wherein a second sprayhead is operated in conjunction with the first mentioned sprayhead.

5. A method as claimed in claim 4, wherein the mass flow of shroud air of the second sprayhead is also varied between predetermined limits during the spraying operation.

6. A method as claimed in claim 5, wherein the cycle of variation of the mass flow of shroud air of the second sprayhead is similar to but out of phase with the cycle of the first sprayhead.

7. Electrostatic spraying apparatus including a sprayhead, means for supplying air under pressure to define an air shroud for the droplet or particle cloud generated by the sprayhead, and means for varying the mass flow of the shroud air during the spraying operation.

8. Apparatus as claimed in claim 7, wherein said means for varying the mass flow of shroud air causes a repeated cyclic variation in shroud air mass flow during the spraying operation.

9. Apparatus as claimed in claim 7 or claim 8, wherein said means for varying the mass flow of shroud air comprises means whereby the pressure of the shroud air supplied to the head is varied.

10. Apparatus as claimed in claim 9, wherein said means for varying the pressure of the shroud air supply switches the supply between high and low pressure supplies in a repeated, predetermined sequence during the spraying operation.

11. Apparatus as claimed in claim 9, wherein said means for varying the pressure of the shroud air supply alternately connects the supply to and disconnects the supply from a high pressure supply.

12. Apparatus as claimed in claim 10 or claim 11, wherein the variation is stepless.

13. A method of electrostatic spraying substantially as hereinbefore described with reference to the accompanying drawings.

14. Apparatus for electrostatic spraying substantially as hereinbefore described with reference to the accompanying drawings.