WO1995008396A1

WO1995008396A1 - Method, applicator and apparatus for electrostatic coating

Info

Publication number: WO1995008396A1
Application number: PCT/GB1994/002102
Authority: WO
Inventors: John Brown Buchanan
Original assignee: John Brown Buchanan
Priority date: 1993-09-24
Filing date: 1994-09-26
Publication date: 1995-03-30
Also published as: GB9319706D0; AU7703494A

Abstract

The invention provides a method of, an applicator for, and apparatus for utilising an applicator for electrostatically coating an article with a thin coating of liquid, comprising creating an electrostatic field between an article to be coated and a very fine annular or longitudinally extending gap which is stable, of fixed width and of uniform cross section and supplying liquid to said very fine stable gap, whereby said liquid in passing through said stable gap is atomised to produce a spray of fine particles which is applied to the article to form a uniform coating of liquid thereon for example, for use in the aluminium, steel and automotive industries.

Description

METHOD. APPLICATOR AND APPARATUS FOR ELECTROSTATIC COATING

The present invention relates to a method of, and an applicator and apparatus utilising an applicator for, the electrostatic coating of articles with a fine atomising spray of oils, lubricants, chemicals and like liquids (hereinafter generically referred to as "liquids") in more particularly, but not exclusively, the aluminium, steel and automotive industries.

It is customary in industry for articles of aluminium and, in particular, steel to be coated with a coating of oil following manufacture to prevent against the corrosion of the article. Prior to the development of electrostatic coating, the conventional method for applying oil to, for example, steel sheet was to dip a coil of the sheet steel into an oil bath.

This method not only over applied oil to the steel coil, leading to wasteful usage of an expensive lubricant, it also created health and safety hazards for the employees of the manufacturer. Employees working in a manufacturing plant ran the risk of slipping in patches of oil as it drained from, for example, coils of metal following dipping, and were in danger of injuring themselves in a fall leading to loss of expertise to and insurance claims against the manufacturer and the breaching of government work safety guidelines.

Electrostatic coating was first introduced in the 1970's and is now a well-established process for applying a coating of a liquid such as paint, oil, lubricants and chemicals to an article. Electrostatic technology has also been applied to the field of agrochemical liquid spraying.

During electrostatic coating the liquid to be applied is supplied to, and fed from, an electrostatic coating applicator which is maintained at a high voltage relative lo the article to be coated. The voltage difference between the applicator and the article generates an electrostatic field which breaks up the liquid dispensed from the applicator, resulting in a fine atomising spray of charged liquid particles. Electrostatic coating applicators commonly comprise two members forming a gap therebetween for dispensing the liquid. At least one of the members at the dispensing edge is sharpened to a knife-like edge in order to provide a more concentrated electric field when a potential difference is applied between the dispensing edge and the article to be coated, and therefore a higher degree of atomization of the liquid.

Adjustments to the width of the gap is said to vary the flow of the coating liquid to be atomised. For example, increasing the gap width, thereby increasing the flow of coaling liquid lo the dispensing edge, leads to an increase in the charged particle size and therefore the thickness of the coating delivered to the article. Nariation of operating conditions such as fluctuations in temperature can lead to alterations in particle size and, therefore, the thickness of liquid coaling. Temperature fluctuations can therciore be accommodated by adjustment lo the gap width.

One known design of applicator features two straight blades defining a gap in the form of an elongate slot, see for example WO 84/01524 to Sale Tilney Technology Pic. The major drawbacks of this design are that width control of the gap, and hence the ability to produce a uniform slot for a uniform supply of a liquid to be sprayed, is very difficult, and change over to use incompatible lubricants is also laborious.

Generally the slot produced between the blades is designed to spread the coating liquid and is therefore difficult to width control. Additionally the blades are anywhere up lo a maximum of two metres long. If the blades are not minutely adjusted to provide a uniform slot width (which may be between 1 to 250 μm), the slot is distorted (a common form of slot width inaccuracy is a wave like distortion of the slot along its length). Therefore problems may arise relating to an increased flow of coating liquid corresponding to the greatest width along the slot's length: the liquid will always take the path of least resistance. These problems include spitting, dripping or streaming at the widest points of the slot where the coating liquid flows at a rate faster than the apparatus can atomise. Although methods have been proposed to recycle un-atomised liquid which drips from the applicator, reuse of the liquid can produce blockage problems and generally any excess is dumped at the expense of the manufacturer. Industry often requires the application of different liquids to an article, often at different stages in their production. Hence it is advantageous for the changeover of coating liquids to be effected as quickly, efficiently and as cost effectively as possible. Thus, the longer the downtime of an electrostatic coating apparatus, for example due to prolonged flushing out of lubricant, the higher the cost to the manufacturer.

Flushing out the long blade design apparatus has been found not to be 100% efficient and large amounts of flushing agent have to be used when attempting a liquid changeover, therefore being both expensive and a possible ecological hazard to dispose of safely. If a changeover is being attempted between immiscible liquids, it is essential that all of the previous liquid is flushed from the system as contaminants can cause blockages and lead to striping of the spray as discussed below.

Because each blade is precision ground, they are extremely expensive to produce, and hence replace. Moreover, as each blade is so long (up to a maximum of approximately two meters) it is cumbersome to install. Should a leading edge of a blade be damaged there is a risk of blockages and clogging occurring at for example nicks in the length of the blade which is particularly acute when thixotropic coating liquids are in use. Blockages leading to "striping" in a spray of liquid particles being applied to an article is potentially catastrophic. If a dry stripe develops in, for example, a protective oil coating applied to a steel strip, huge insurance claims may be incurred by the steel manufacturer for supplying corroded and faulty goods.

Another known design features an outer member and an inner member defining an annular gap which tapers in a direction towards a dispensing edge of the applicator. Examples of this design are disclosed in US Patent No. 2860599 to James H. Rice or US Patent No. 2893893 to William W. Crouse.

Referring to US Patent No. 2860599 lo Rice, this discloses an applicator of circular cross-section in which adjustment of the liquid spray pattern is provided by axial movement of the inner member relative lo the outer member. The means for axial adjustment of the inner member comprises a control rod onto which the inner member is threadedly mounted, the control rod communicating with a recess in which the inner member is located, via an axial bore into which the control rod is loosely located. Axial adjustment of the inner member relative to the outer member is for the purpose of adjusting the axial length of the annular gap within the applicator to vary the diameter of the annular spray.

Referring to the US Patent No. 2893893 to Crouse, the applicator is also of circular cross-section and the radial width of the annular gap between the inner member relative to the outer member is adjusted to vary the thickness of the liquid coating. The adjustment is performed by means of a two part inner member comprising a stem and a regulating member in the form of a plug of circular cross section. The leading edge of the plug defines an annular gap with the leading edge of the outer member being given a slight outward taper and the stem is screw-threadedly received within the body of the outer member and in turn screw-threadedly mounted on a rod. The plug is slidably located in a recess in the stem and is mounted on the rod for axial adjustment within the recess and hence relative to the outer member.

The Rice and Crouse embodiments share the same problem in that in employing screw- threaded mountings of the inner member of the applicator they are subject to off-setting forces resulting in the inner member not being accurately concentrically retained with respect lo the outer member. Therefore, the annular gap between the outer and inner members is not stable and uniform, which results in a non-uniform atomising spray and, therefore, in non-uniform coatings. The same disadvantages apply to US Patent No. 2923272 to R.J. Emmert.

European Patent application nos. 123538 and 243031 relate to applicators for agrochemicals in which the chemicals are fed through concentrically arranged, axially extending passages for the purpose of adjusting the liquid flow rate, but neither of these applicators would produce uniform coatings.

Non-uniform coatings are a particular problem in industries where a very thin coating of liquid is to be applied to metal articles. In the aluminium and steel manufacturing industries a thin coating of oil is applied to metal strip for the purposes of lubrication and corrosion protection before the metal strip is spirally wound into a roll. Heavy oiling on one side of the strip relative to the other can create problems during the winding of the metal sheet causing non-uniform winding and even damage, deformation or pitting of the metal due to the forces produced in winding the sheet metal compressing a non- uniformly applied incompressible liquid.

Non-uniform coating of lubricants is a particular problem in the automotive industry where compression of excess oil creates problems in the pressing of automotive body panels resulting in an improper shape of panel being pressed.

The main object of the present invention is to provide a method of, an applicator for, and apparatus for utilising an applicator for, the electrostatic coating of articles with a fine atomising spray of liquid particles of substantially equal size, a substantially equal charge density thereby to provide equal repulsion between particles so as to produce an evenly distributed spray of particles for use in, for example, the aluminium, steel and automotive industries in which the aforesaid disadvantages are minimised or at least substantially eliminated.

In accordance with one aspect of the invention there is provided a method of electrostatically coating an article with a thin coating of liquid comprising creating an electrostatic field between an article lo be coated and a very fine annular gap which is stable, of fixed radial width and of uniform cross-section and supplying liquid to said very fine annular gap, whereby said liquid in passing through said gap is atomised to produce a spray of fine particles which is applied to the article to form a uniform coating of liquid thereon.

In order to provide liquid spray particles of substantially uniform size and substantially uniform charge, the supply of liquid to the stable annular gap is interrupted in such manner that the liquid is supplied to said stable annular gap circumfercntially at a substantially uniform rate. Put another way, the liquid is more evenly distributed to the stable annular gap than would otherwise be the case.

In order to ensure that the article is covered by a uniform coating, e.g. in the case of a metal strip, throughout its width in one application, the liquid is advantageously supplied to a multiplicity of annular gaps which are so arranged with respect to one another that intersection of the annular sprays from each gap occurs.

However, this intersection of annular sprays does not lead to non-uniform coating as might be expected. This is because identically charged particles within the intersecting annular sprays repel each other thereby eliminating the intersection and leading to a uniform distribution of liquid particles and hence a uniform coating of liquid on the surface of the article.

According to another aspect of the invention, a method of electrostatically coating an article with a thin coating of liquid comprises causing relative movement between an article and a multiplicity of stable annular gaps and causing intersecting annular sprays to pass from said gaps to said article in such manner as to provide a uniform coating of liquid on the article.

Preferably the weight of the liquid coating on the surface of the article is between 5mgm/M² to 5gm/M², thus the apparatus is able to apply ultra-light coating weights.

From another aspect, the invention consists in a method of electrostatically coating an article with a thin coating of liquid comprising creating an electrostatic field between an article to be coated and a stable annular or longitudinal gap having a fixed radial width of 25 to 300μm, preferably 25 to 250μm, and of uniform cross-section and supplying liquid to said gap, whereby said liquid in passing through said gap is atomised to produce a spray of fine particles which is applied to the article to form a uniform coating of liquid thereon.

The invention further comprises a method of electrostatically coating an article with a thin coating of liquid, comprising the steps of advancing liquid coating material from a supply source along a supporting surface defining a first chamber to a first passage leading to a second chamber, progressing the liquid through a second passage leading from the second chamber to an atomization zone to provide a thin edge of liquid coating material which is spaced from the article, the liquid coating material being uniformly distributed along the entire extent of said edge, creating between the article and said edge an electrical potential which produces an electrostatic field of sufficient strength to atomise said edge into a spray of fine particles.

In a further aspect of the invention, there is provided an electrostatic coating applicator comprising a stationary outer member and a stationary inner member forming a gap therebetween for dispensing a coating material, and passage means for supplying said coaling material to said gap, said outer and inner members having precisely concentric walls defining a very fine annular flow-restrictive gap for said coating material.

It has been found to be an advantage for the outer and inner members of the applicator to be made of dissimilar materials, for example, the inner member to be of an electrically non-conductive material such as a plastics material, and the outer member to be conductive. This creates a resistive system of greatly reduced or no capacitance. Use of dissimilar materials allows the electric field produced by an applicator to be concentrated leading to less interference between the intersecting electric fields of each applicator in an array, and hence their respective atomised particle sprays. The result is a more uniform spray and a more even coating of a liquid applied to an article.

By means of the method and applicator of the invention it is possible to achieve an extremely uniform electrostatic field, and consequently a consistently uniform atomising spray of liquid producing a substantially uniform coating of liquid on the article. The equally charged particles repel one another and will therefore spread evenly and uniformly in the electrostatic field created between the applicator and the article ensuring even distribution. The result is a very high transfer efficiency and a total and uniform coating of liquid. Moreover, elimination of over oiling avoids the need for liquid recirculation thereby obviating liquid contamination problems which are problems of prior applicator designs.

The ability to provide a concentrated electric field from each respective applicator in an array allows the electrostatic coating apparatus to be used in spray cabins of much smaller and more compact size. Since accurate width control is possible, cleanliness is also ensured.

The applicators can be used in any attitude, any environment and are ideal for one sided applications. As will be appreciated the applicators are maintenance free because there are no moving parts and access to the applicators for replacement is simply and easily attained.

Another advantage of applicators constructed in accordance with the invention is that they allow both higher and lower fluid application levels than available with other apparatus hitherto known to the applicants. The apparatus according to the invention lends itself lo electronic high voltage generation.

Preferably, the annular gap has a radial width of 25 to 250 μm and, advantageously has an axial length from a liquid entry end to a liquid discharge end to be 5 mm. Such dimensions have been found to produce an atomizing spray of very fine particles, much finer that has hitherto been possible with known electrostatic coating applicators.

The free discharge end of the applicator comprising the inner and outer members is advantageously made flat, for example as by machining, with the stable annular gap opening onto this flat surface and being surrounded by a flat rim formed by the outer member. Therefore the edges of the stable annular gap are protected against damage which increases the useful life of the applicator.

If it is desired to vary the rate of liquid discharge from the applicator and thus the size of atomised spray particles, it is a simple matter to remove the inner member and replace it by another which provides an annular gap with a radial width which is greater or less than that of the original.

With a view to providing the annular gap with the requisite stability, the inner and outer members may be provided with precisely inter-engaging surfaces, which may be formed as by machining, having a close tolerance therebetween. In one embodiment such inter- engaging surfaces are cylindrical and the tolerance is so close as to provide a press fit between the cylindrical inter-engaging surfaces whereby to fix the two members together to form the stable annular gap.

In another embodiment such inter-engaging surfaces are provided by a projection engaging with a sliding fit in a cylindrical recess with the free end of the projection abutting the closed end of the recess and being fixed together by a fastening member to form the stable annular gap.

The cooperating cylindrical surfaces providing a snap fit of the first mentioned embodiment and the cooperating abutting surfaces of the second mentioned embodiment constitute seatings which enable the stability of the annular gap to be maintained in combination with the fastening member in the second embodiment.

In another aspect of the invention, the inner chamber is provided with a spreader means to evenly distribute the advancing liquid coating material as it progresses through the first passage into the second chamber. Advantageously, the spreader should be frusloconical.

It has been found to be beneficial to provide a means for precisely controlling the rate of supply of the coating liquid to the first chamber of the applicator. Such means may be constituted by a disc having a precisely dimensioned hole therein, for example the disc may be of metal and the hole formed by drilling.

In order to facilitate the uniformity of circumferential distribution of the liquid in the annular gap, means downstream of the annular gap are provided for interrupting the supply of liquid to the annular gap. This means can conveniently be provided by a circular spreader integral with the inner member, providing an annular passage with a cylindrical surface of the outer member, the annular passage having a radial width which is greater than that of the annular gap. The annular passage is also stable by virtue of being part of the fixed inner member. Preferably the surface of the spreader is frustoconical.

The effect of the spreader is to provide two chambers defined between the inner member and the outer member, the first chamber being in communication with a supply of liquid having an inlet which is offset with respect to, or in alignment with the axis of the inner member. The second chamber is conveniently defined between frustoconical surfaces of the inner member. The annular passage may have a radial width of from about 250 to 400 /«m.

It has been found to be beneficial for the forward surface of the first chamber to be frustoconical. Further, or as an alternative, it is advantageous for the rearward and/or forward surface of the second chamber to be frustoconical.

Surprisingly, applicants have found that the advantages attributable to an annular gap can also be obtained with a longitudinally extending gap. This is achieved by keeping the longitudinal extent of the longitudinal gap substantially the same as the circumferential extent of the annular gap, i.e. around 100mm, or such that the longitudinal extent of the longitudinal gap does not substantially exceed lOϋmm.

Accordingly, in modifications of any of the methods and applicators defined hereinabove, the stable annular gap is replaced by a stable longitudinal extending gap.

The invention further consists in an electrostatic coating apparatus incorporating any of the applicators defined hereinabove.

In a preferred embodiment, a plurality of such applicators may be arranged in equi- spaced relationship. For example, the applicators may be arranged in an linear array or in two linear arrays which are arranged at half pitch with respect to one another of which the latter is particularly advantageous.

Whatever the form of the array, the applicators provide at least one atomizing spray curtain.

In each instance the applicators may be arranged to be:

selectively shut off; and/or

selectively connected to a voltage source for effecting electrostatic spraying; and/or

selectively connected to voltage sources of different output voltages, lo vary the spray pattern for controlling the area of the atomising spray delivered.

It should be noted that an array of applicators constructed according to the invention can be produced at a significantly lower cost than the known blade design applicators.

It has been found that the electrostatic coating apparatus constructed in accordance with the invention can be used in both a vertical or horizontal plane and all angles through 360° therebetween.

In order that the invention may be readily understood some embodiments in accordance therewith will now be described, by way of- example only, with reference to the accompanying drawings, in which:

Figure 1 is a longitudinal view, partly in section, of an electrostatic coating applicator embodying the present invention;

Figure 2 is a plan view of an array of electrostatic coating applicators of the kind illustrated in Figure 1 as employed in electrostatic coating apparatus according to the invention; Figure 3 is a longitudinal section of another embodiment;

Figure 4 is an end view looking in the direction of IN in Fig. 3;

Figure 5 is a perspective view from one end of an apparatus for electrostatically coating articles with a fine atomising spray of a liquid and including a plurality of applicators;

Figure 6 is a detailed view of the apparatus of Fig. 5 showing the applicators in use;

Figure 7 is a detailed view of the apparatus of Fig. 5 being used to carry out a test;

Figure 8 is a plan view of a plurality of applicators arranged in a linear array and of the kind illustrated in Figure 9; and

Figure 9 is a longitudinal section of a part of another embodiment of applicator.

Referring initially lo Figure 1, this illustrates an electrostatic coating applicator 10 comprising a stationary electrically conductive outer member 12 and a stationary electrically conductive inner member 14. Alternatively, cither the inner or outer member may be made of electrically non-conductive material.

As shown, the outer member 12 is cylindrical having a highly precisely machined inner bore 16. The outer cylindrical wall 18 of the member 12 terminates in an inclined surface 20, which meets the inner cylindrical wall 22 forming the bore 16 in a circular edge 24.

The inner member 14 has a highly precisely machined outer surface 26, which is a snug press-fit in the bore 16 to hold the two parts firmly together. The inner part 14 terminates in a highly precisely machined cylindrical wall 28, which is of reduced diameter relative to the surface 26, in order to define with the outer member 12, a narrow annular gap 30. The radial width of the annular gap 30 are advantageously selected to be approximately 1 to 250 μm, preferably 25 to 250 μm, and approximately 5 mm in the axial direction. Between the surface 26 and the wall 28, the inner member 15 has a further reduced section 32 forming an annular chamber 34 for the collection of a coating liquid such as oil. Access to the chamber 34 is by way of an axial passage 36 and a cross passage 38 within the member 14. A frustoconical section 40 connects the section 32 to the wall 28 so that the flow path for the liquid is increasingly restricted towards the annular gap 30.

In use, liquid is supplied to the chamber 34 by way of the passages 36 and 38 and gradually fills the chamber and enters the gap 30. The precise machining of the walls 22 and 28 means that the gap 30 is extremely consistent and the liquid is uniformly distributed around the gap.

The gap 30 acts as a flow restriction resisting the flow of liquid and the coating liquid is supplied to the gap 30 substantially without imparting kinetic energy to the liquid.

When a high voltage is applied between the outer member 12 and an earthed article to be coated, liquid at the outer edge of the gap 30 breaks up into fine particles as an atomising spray. Because of the fact that the inner member 14 and the outer member 12 are precisely formed and in fixed relation to each other, the sides are parallel, a uniform annular gap 30 is achieved, i.e. does not change with variations in operating parameters voltage used, temperature, liquids used etc., and a uniform flow rate is achieved and a precisely determined atomising spray results. The shape of the spray can be altered by adjusting the voltage.

Alternatively, the shape of the spray can be altered by replacing the inner part 14 with a similar part having a wall 28 of different radius. Another possibility is to replace the part 14 with a similar part of different material.

Figure 2 shows how a plurality of identical electrostatic coating applicators 10 can be arranged in a bank or array 100 for providing an atomising spray curtain. The overall radial width of the curtain and the spray pattern may respectively be adjusted quite simply to suite different articles merely by means (not shown) effective to shut off the flow of coating liquid to applicators 10 at the outer edge of the array; and means (also not shown) for selectively connecting the array 100 to the same voltage source for electrostatic spraying or to a plurality of such voltage sources with different voltage outputs.

The applicator 40 shown in Fig. 3 differs from the applicator 10 in Fig. 1 in that the inner member 48 is provided with a projection 52 having an outer cylindrical surface 44 which is slidably engaged within a recess 56 in the outer member 42. The free end 58 of the projection 52 abuts against the closed end 60 of the recess 56, and is secured therein with an axially extending fixing screw 62 thereby ensuring that the annular gap 54 is stable. Coating liquid supply is by a passage 64 which is offset with respect to the axis 66 of the outer member and communicates with a first chamber 68 defined by the wall 46.

A disc 63 having a precisely dimensioned hole 65 therein is provided in a bore 67 in communication with the passage 64 for precisely controlling the rate of supply of the liquid coating material to the applicator.

The inner member is provided with an integral spreader member 70 which projects radially outwards to define with the wall 46 an annular passage 72 which has a greater radial width than that of the annular gap 54. The spreader member provides a second annular chamber 74 which communicates with the annular gap 54. The rearward surface 76 and the forward surface 78 defining the second annular chamber are frustoconical. The forward surface of the first annular chamber 80 defined by the spreader integral with the inner member is also frustoconical.

As will be appreciated from Fig. 4, the discharge end 82 of the applicator 40 is flat and the annular gap opens onto the flat surface 82 between a central portion 84 presented by the inner member 48 and a flat outer rim 86 formed by the outer member 42.

Referring to Fig. 5, this shows a frame 88 carrying two support members 90 on which are mounted carriers 91 for a multiplicity of applicators arranged in a form of an array at half pitch. The applicators of the upper and lower arrays 91 direct atomised liquid sprays 92 downwards and upwards to coat the respective sides of, for example a strip, with the coating liquid with the atomised liquid sprays 92 in Fig. 6.

The atomised liquid sprays 92 as shown in Fig. 6 are intersecting as previously described.

Fig. 7 shows a test strip 93 and collecting tubes 94 with containing coating liquid up lo the levels 95 in each tube 94 having been discharged from the respective applicators 40. As can be seen from the levels 95, the amounts of liquids collected from each applicator 40 in the tube 94 assigned to it is substantially the same in the respective tubes 94.

The applicator 41 in Fig. 9 differs from the applicator 40 in Fig. 3 in that a longitudinally extending stable gap 54A defined between two members, described herein for convenience as an inner member 48A and an outer member 42A and having substantially the same length as the circumferential length of the annular gap 54. A longitudinally extending spreader member 70A defines a passage 72A which has a greater width than that of the linear gap 54 A and allows the communication of coating liquid introduced into the applicator by passage 64 A and 64B. Passages 64A and 64B are located on either side of the linear spreader to facilitate a more uniform supply of coating liquid to the linear gap 54A. The forward surface 80 A and rearward surface 76A of the spreader member are frustoconical.

Figure 8 shows how a plurality of applicators 41 arranged having longitudinally extending gaps 54A can be arranged in a bank or array 102 for providing an atomising spray curtain. Conveniently, the juxtaposed inner members 48A and outer members 48A are common to all the longitudinal extending gaps 54A. In the arrangement shown in Fig. 8, there are two linear arrays arranged at half pitch with respect to one another with the lower array being shown in chain lines. The inner and outer members 48A and 42A may be held together with their opposing precisely machined surfaces in abutment by any appropriate means. Electrostatic coating applicators and apparatus according to the invention can produce an atomising spray formed of very fine particles of uniform particle size. The results are consistent and reliable, waste virtually no liquid, cause no contamination problems and the invention is suitable for the application of very fine liquid coatings.

(a) The applicators and apparatus described will provide real benefits for the application of liquid in the steel and aluminium and automotive industries, in that it will allow atomised sprays having not only uniform particle sizes but also atomised sprays having particle sizes which are much smaller or larger.

(b) also more accurate radial width control for continuous substrates is also achieved.

The benefits derived from a much smaller particle size will be an extension of the lower range of the hitherto known apparatus and accurate deposition at coating weights which are presently only achievable by the introduction of kinetic energy to assist the atomization process.

In the case of atomised sprays of larger particle size, this will give an extension of the coating weight range at the higher deposit end, thus allowing the apparatus lo be used for a wider range of applications.

With respect to (b) above, the industries have become much more sophisticated in recent years, and the variety of oils and chemicals used determines that, for different applications on the same mill or press, substantially different and incompatible liquids have to be applied. In these cases, it is not possible to use re-circulation systems, as is the practice at present, and one needs to increase the overall transfer efficiency to a level such that any excess can be dumped.

The prior art electrostatic coating applicator were used for dispensing materials with coating weights of the order of 50 times greater than those with which this invention is dealing by virtue of its ultra-fine stable annular gap. Al though specific embodiments have been described, it should be appreciated that various modifications can be made without departing from the scope of the invention as defined in the appended claims. For example instead of the applicators 40 being disposed substantially vertically they can be disposed in any other attitude appropriate for coating an article.

Claims

1. A method of electrostatically coating an article with a thin coating of liquid, comprising creating an electrostatic field between an article to be coated and a very fine annular or longitudinally extending gap which is stable, of fixed width and of uniform cross-section and supplying liquid to said very fine annular gap, whereby said liquid in passing through said gap is atomised to produce a spray of fine particles which is applied to the article to form a uniform coating of liquid thereon.

2. A method according to claim 1, said stable gap has a width of from about 25 to 300μm and preferably from about 25 to 250μm.

3. A method according to claim 1 or 2, wherein said stable gap has a circumferential or longitudinal extent of no greater than about 100mm.

4. A method according to any one of claims 1 to 3, wherein the supply of liquid to said stable gap is interrupted in such manner that the liquid is supplied to said stable gap circumferentially or longitudinally at a substantially uniform rate.

5. A method according to any one of claims 1 to 4, and further comprising supplying the liquid to a multiplicity of stable gaps to provide a multiplicity of sprays and causing adjacent ones of said sprays to intersect with one another.

6. A method according to claim 5, comprising causing relative movement between the article and the multiplicity of stable gaps.

7. A method according to any previous claim, wherein the weight of the liquid coating on the surface of the article is between 5mgm/M² to 5gm/M².

8. An electrostatic coating applicator comprising a stationary outer member and a stationary inner member forming a stable very fine gap therebetween for dispensing a coating liquid, and passage means for supplying said coating liquid to said very fine gap, said outer and inner members having precisely concentric or longitudinal walls defining a very fine annular flow-restrictive gap for said coating liquid.

9. An electrostatic coating applicator according to claim 8, and including a spreader means for interrupting supply of liquid to said stable gap in such manner that the liquid is supplied to said stable gap circumferentially or longitudinally at a substantially uniform rate.

10. An electrostatic coating applicator according claim 9, wherein the surface of the spreader means is frustoconical.

11. An electrostatic coating applicator according to claim 9 or 10, wherein the spreader means defines a gap with an inner wall of the outer member and having a width of from about 250 to 400 μm.

12. An electrostatic coating applicator according any of claims 8 to 11, wherein said stable gap has a width in the range approximately 25 to 300 μm and preferably of from about 25 to 250 μm.

13. An electrostatic coating applicator according to any one of claims 8 to 12, wherein said stable gap has a circumferential or longitudinal extent of about no greater than 100mm.

14. An electrostatic coating applicator according lo any of claims 8 to 13, wherein said stable gap has a length between entry to discharge ends of approximately 5 mm.

15. An electrostatic coating applicator according to any of claims 8 to 14, wherein the inner and outer members are of different materials.

16. An electrostatic coating applicator according to claim 15, wherein the inner member is of an electrically non-conductive material and the outer member is of an electrically conductive material.

17. An electrostatic coating applicator according to claim 16, wherein the inner member is of a conducting material and the outer member is of a non-conductive material.

18. An electrostatic coating applicator according to any one of claims 8 lo 17, wherein the inner and outer members have precisely inter-engaging surfaces.

19. An electrostatic coating applicator according to claim 18, wherein the outer and inner members have concentric walls, providing a said stable gap which is annular.

20. An electrostatic coating applicator according claim 19, wherein the inner member is a sliding fit in a recess of the outer member or is a sliding press fit in the outer member.

21. An electrostatic coating applicator according to any of claims 8 to 20 and including means for precisely controlling the rate of supply of the coating liquid to the applicator.

22. An electrostatic coating applicator according to any one of claims 8 to 21, wherein at least one of the outer and inner members is replaceable to alter the width of said stable gap for adjusting the electrostatic field and thereby the shape of an atomising spray delivered by the applicator.

23. An electrostatic coating applicator according to any one of claims any of claims 8 to 22, wherein at least one of the outer and inner members is replaceable by a corresponding member formed from a different material for adjusting the electrostatic field and thereby the shape of an atomising spray delivered by the applicator.

24. An electrostatic coating apparatus comprising at least one electrostatic coating applicator as claimed in any of claims 8 to 23, and means for maintaining the applicators at a high voltage relative to an article to be coated, for atomising a coating liquid and producing an atomising spray from the or each applicator.

25. An electrostatic coating apparatus according to claim 24, wherein a plurality of said applicators are juxtaposed, and the voltage is adjustable for changing the electrostatic field and thereby the shape of the atomising sprays delivered by the applicators, and/or the voltage applied to individual applicators is adjustable and/or is unequal by virtue of being connected to different voltage sources of unequal output.

26. An electrostatic coating apparatus according to claim 24 or 25 and comprising a plurality of said applicators arranged in an array for delivering a curtain of atomising spray.

27. An electrostatic coating apparatus according to claim 26, wherein the applicators are arranged in at least one linear array.

28. An electrostatic coating apparatus according to claim 27, wherein the applicators are arranged in two linear arrays which are at half pitch with respect to one another.

29. An electrostatic coating apparatus according to any of claims 24 or 28 and further comprising means for selectively controlling the flow of coating liquid to the applicators to control the dimensions of the atomising spray.

30. A method of electrostatically coating an article with a thin coating of liquid according lo claim 1 and substantially as hereinbefore described.

31. A method of electrostatically coating an article with a thin coating of liquid substantially as hereinbefore described with reference to Figs. 1 and 2 or Figs. 3 to 7 or Figs. 8 and 9 of the accompanying drawings.

32. An electrostatic coating applicator substantially as hereinbefore described with reference to, and as illustrated in Fig. 1, Fig 3 or Fig. 9 of the accompanying drawings.

33. An electrostatic coating apparatus substantially as hereinbefore described with reference to, and as illustrated in, Figs 1 and 2, Figs. 3 to 7 or Figs. 8 and 9 of the accompanying drawings.