BLADES FOR ELECTROSTATIC COATING, APPARATUSES INCORPORATING SUCH BLADES AND PROCESSES USING SUCH BLADES

Abstract: Electrostatic coating blade (11) for coating objects with a liquid. The blade consisting of two halves (12a, and 12b) held together by bolts (15). A central passage (14) is provided that receives liquid from a reservoir (not shown) and a narrow channel (16) can conduct liquid to a discharge edge (17). The blade can be used either horizontally (as is customary with electrostatic coating blades) or vertically. The blades (11) are particularly useful when vertically disposed to coat oil onto the edge of coils of sheet metal.

Description: BLADES FOR ELECTROSTATIC COATING, APPARATUSES INCORPORATING SUCH BLADES AND PROCESSES USING SUCH BLADES TECHNICAL FIELD The present invention relates to blades for use in electrostatically coating articles with liquid, coating apparatuses incorporating one or more such blades, and processes for electrostatically coating articles, especially for coating coils of metal strip with oil. THE STATE OF THE ART Electrostatic coating is a well-established process of covering an article with a coating of a liquid, e.g. paint or oil. The liquid is fed from a blade which is maintained at a high voltage relative to the article to be coated. The electrostatic field resulting from this voltage difference draws the liquid towards the article and also breaks the liquid up into fine particles or jets. Electrostatic coating has been used for applying paint and also for applying oil to sheet steel to inhibit rusting. Suitable blades are, for example, described in British Patent Specification Nos.l,193,252; 1,202,598; 1,281,512 and 1,303,625. All these blades are designed to work horizontally, i.e. the edge of the coating blade lies in a horizontal plane and different blades are required for spraying liquid upwardly onto an object and for spraying downwardly onto an object. Also, if it is desired to alter the amount of liquid being applied, there is a response time of several seconds between changing the rate of supplying liquid to the blade and the onset of the different amount of liquid leaving the coating blade. Furthermore, there is a minimum amount of liquid that can be applied using these blades if a satisfactorily uniform coating is to be obtained and it is sometimes desirable to obtain a thinner, but still uniform, coating than such a minimum application rate will allow. We are aware of only one blade that has been capable of operating in a plane other than the horizontal. This blade, which was available on a limited scale in the 1930's, was tilted at an angle of about 45 to the horizontal and was used to apply paint electrostat¬ ically onto articles. As far as we are aware, this blade was not capable of being operated either completely horizontally or completely vertically, i.e. with the edge applying paint lying in a horizontal or in a vertical plane. The blade, which is shown in schematic, cross-sectional perspective in Figure A, has two slanted wedge-shaped elements (1) with the apices of the elements being spaced apart to define a narrow gap (2) between them. A plate (3) is located between the elements (1) to close the top of the space between the elements (1) thereby forming a channel (4) which is closed except for the gap (2). The wedge-shaped elements (1) and the top plate (3) are held in position by two end blocks. In operation, the paint is fed into the channel (4), and a voltage difference between the blade and the object to be sprayed is established which draws the paint from the channel (4) through the slot (2) onto the object. The width of the slot can be adjusted by slid¬ ing each element (1) in a direction within its plane shown by the arrows (5). If this blade had been used vertically, an uneven covering of paint would ha^ resulted because the amount of paint sprayed from the bottom of the blade would be much greater than the amount sprayed from the top of the blade, indeed it is open to doubt whether any paint at all would be sprayed from the top of the blade. After the final rolling step in the manufacture of sheet steel, oil is applied to the faces of the sheet to stop it corroding during storage. The oil is applied by electrostatic coating using horizontal electrostatic coating blades located above and below the sheet after it leaves the rolling mill. Steel strip is usually stored as coils and corrosion attacks the stored coils by way of the edges of the strip on the side faces of the coils. To counteract this problem, the side faces of the coils are also sprayed with oil with a hand gun or a spray aerosol. Such a process is messy, wasteful on oil and time-consuming. DISCLOSURE OF THE INVENTION According to the present invention, there is provided an electrostatic coating blade comprising a central passageway and a discharge edge and the invention is characterised in that one or more flow-resistive channels extend between the passageway and the discharge edge for feeding fluid from the passageway to the discharge edge. OMPI Preferably, the resistance to fluid flow in the or each channel is achieved by making the channels narrow, i.e. having a width of between 0.007 and 0.07 cms, preferably 0.01-0.04 cm and/or relatively long, e.g. having a length in the range of from 0.1 cms to 5 cms, preferably 0.2 cms to 4 cms. As used in this specification, the length of a channel is the distance between the orifice of the channel into the passageway and the orifice of the channel at the discharge edge,and the width of a channel is the distance substantially at right angles both to the length of the channel and the length of the discharge edge. It will be appreciated that the longer the channel, the wider it can be and the smaller the width of the channel, the shorter it can be to provide the same flow- resistance. Thus, it is preferred that the ratio of the length of the or each channel to its width is in the range of from 5:1 to 500:1. Preferably, the length to width ratio of the channel is from 10:1 to 250:1, most preferably from 12:1 to 200:1. Although it is within the scope of the present invention for the blade to have a plurality of channels leading from the central passageway to the discharge edge, it is preferred that there is only a single such channel and that that channel extends along substantially the whole length of the blade. It is a substantial advantage of the blade of the present invention that it can be used to spray liquid from above and below an object and can also be used both horizontally and non-horizontally, e.g. in an upright mode and preferably substantially vertically. Even when vertical, fluid can be drawn off the whole length of the blade. The blade of the present invention, whether used horizontal¬ ly or vertically responds almost instantly to a change in the amount of liquid fed to the blade to alter the amount of liquid sprayed by the blade. Also, a blade in accordance with the present invention can be made that sprays liquid at a very low rate but still in a uniform coating. A blade used horizontally need not have such long and/ or such narrow passage(s) as a blade used vertically. Thus, for example, a blade used vertically may have a channel width of 0.02 cms and a channel length of 3.2 cms, whereas a horizontal blade could have a channel with the same width but a length of only 0.4 cms. The preferred channel dimensions for a horizontal blade are: width 0.01 to 0.03 cms and length: 0.2 to 1.0 cms, and for a vertical blade are: width 0.01 to 0.03 cms and length 2 to 4 cms. It is not necessary for the or each channel to be of uniform width throughout its length, e.g. it could taper and in the case of non-uniform channels, it is their mean widths that should preferably lie within the ranges given above for the channel widths. The blade, in a particularly simple and hence advant¬ ageous form, may comprise two blade halves between which the or each flow resistive channel is formed, the two blade halves being held together, e.g. by bolts, and a shim is located between the blade parts to space them apart. By using shims of differing thicknesses, the width of the channels can be altered. The discharge edge is provided at the apex of two converging surfaces and we have found it particularly advantageous if the converging surfaces are each concave. According to a further aspect of the present invention, there is provided a process of electrostatically spraying objects with a liquid using an electrostatic coating blade of the type defined immediately above. The blade may be disposed either horizontally or non- horizontally e.g. generally upright, especially vertically. In one advantageous embodiment of this process, the liquid being sprayed is oil and the objects are coils of sheet metal and in particular the end faces of such coils. This has the substantial advantage of reducing the amount of oil needed to protect the edge of the coil from corrosion, it is less time-consuming, less labour-intensive and is cleaner than presently used methods described above. When coating objects that are intermittently supplied to an electrostatic coating apparatus the amount of liquid used and the general cleanliness of the apparatus can be improved by shutting off the high voltage supply to the blades of the apparatus when no object is in a position to be coated. However, the metering pumps supplying liquid to the blade(s) should preferably be continuously operated. Thus, the process of the present invention preferably includes providing a line of supply of electrical power to the blade at a coating station, providing a line of supply of coating liquid to the blade of the coating station, the liquid line or the passageway in the blade communicating with a by- ^\JRE OMPI pass line via a valve, the process being further characterised in that the lines of supply are open and .the bypass valve is closed when a workpiece is in or proximate to the coating station and the electrical power supply line is closed,, the liquid supplyline is open and the bypass valve is open when a workpiece is not in, or not proximate to, the coating station. The liquid passing through the bypass line when the blade is not being used to coat a workpiece in the coating station can then be returned to the liquid reservoir for further use and hence is not wasted. The length and width of the or each channel in the blade is advantageously so set that, when the bypass valve is open, substantially no fluid flows through the channel. It will, of course, be appreciated that the width and length of the channel(s) at which, under these conditions, no flow therethrough occurs depends on several factors including the viscosity of the fluid used. The switching on and off of the high voltage and the opening and closing of the bypass orifices is preferably achieved automatically on the sensing of an object (workpiece) fed to an electrostatic coating station. The sensing is preferably also automatic. According to the present invention there is also provid- ed a process of electrostatically spraying oil onto a coil of sheet metal in which the oil is sprayed by a non- horizontal electrostatic coating blade. According to the present invention, there is also provided an apparatus for electrostatically coating objects, especially for coating coils of sheet metal with oil, which apparatus comprises one or more electrostatic coating blade(s) of the type described above. Preferably, the apparatus includes a line of supply of electrical power to the blade of a coating station, a line of supply of coating liquid to the blade, the liquid line or the passageway in the or each blade communicating with a bypass line by way of a bypass valve. BRIEF DESCRIPTION OF THE DRAI-JINGS There will now be described by way of example only, two coating blades, an apparatus and a process in accordance with the present invention with reference to the accompanying drawings, in which:- Figure 1 is a longitudinal side elevational view of one blade along the line I-I of Figure 3; Figure 2 is a bottom plan view of the blade shown in Figure 1; Figure 3 is a transverse cross-sectional view of the blade taken along the plane indicated by the line II-II in Figure 1; Figure 4 is a second transverse cross-sectional view of the blade taken along the plane indicated by the line III-III of Figure 1; Figure 5 is a third transverse cross-sectional view of the blade taken along the pLane indicated by the line IV- IV of Figure 1; Figure 6 is a transverse cross sectional view of a second form of the blade; and Figure 7 is a schematic view of the apparatus of the present invention. OMPI THE PREFERRED EMBODIMENTS Referring to Figures 1 to 5, there is shown a blade 11 primarily for use vertically, which is made of aluminium and formed in two halves 12a and 12b that are clamped together with bolts 13. A central passageway 14 runs the length of the blade. A shim 15 is located between the two clamped halves 12a_ and 12b to open up a very narrow channel 16 leading from the central passageway 14 to the discharge edge 17 of the blade. The width of the channel 16, which is generally between 0.007 and 0.07 cm, e.g. about 0.02 cms, can be altered by using shim(s) of differing thickness(es). The length of the channel is about 3.2 cms. Fine adjustment of the width of the channel 6 can be effected using several pairs of tensioning devices 18 and 19 (see Figures 3 and 4). Device 18 (see Figure 3) has a threaded bolt 20, the thread of which engages a blind threaded hole in part 12b. Thus, by turning the head of bolt 20, the two halves 12a and 12b can be fore- ed together, thereby decreasing the width of the channel 16. Device 19 (see Figure 3) works in a similar but opposite sense. It consists of a threaded bolt 22 that passes through an internally threaded hole 22a in half 12b and the end of the bolt abuts a seat 23 in half 12a.. By turning the head of bolt 22, the two halves 12 a and 12b are forced apart. As can be seen from Figure 1, several pairs of tensioning devices 18 and 19 are provided along the length of the blade 11 and they are primarily used for fine adjustment to ensure that the width of the channel 16 is uniform all the way along the length of the blade. OMPI The tensioning devices 18 and 19 can be omitted for simplic¬ ity if so desired. The liquid to be sprayed is fed into the central passageway 14 from metering pumps (shown in Figure 7 and described below) through four equidistantly spaced inlet ports 24. A bypass orifice 25 is also provided in direct communication with the central passageway, the function of this orifice is described below. Electrical connection to the blade is made through connector 26. The blade shown in Figure 6 is identical with that shown in Figures 1 to 5 (and identical reference numbers have been used for corresponding parts) except that (a) the tensioning devices 18 and 19 have been omitted, (b) the surfaces 21 leading up to the discharge edge are concave and (c) the channel 16',whose length is indicated by arrow 16" as 0.3 cms , is very much shorter than the channel 16 of the blade shown in Figures 1 to 5. It should also be explained that the channel 16' is shown with a width that is greater than that which is used in practice, which is approximately 0.02 cms. This blade is intended to be used horizontally. Referring to Figure 7, there is shown an apparatus incorporating two vertical blades 11, which are identical to those shown in Figures 1 to 5. Metering pumps 27 supply liquid from a reservoir 18 through an inlet port 24 into the central passageway 14 of each blade. Each blade is connected by way of connectors 26 to a high voltage switch 29, a high voltage transformer 30 and a control panel 31 which is supplied by the mains. A fluid supply line 32 connects the bypass orifice 25 of each blade to the reservoir 28. This line is norm¬ ally kept closed by a valve e.g. a solenoid valve 33, but it can be opened by suitably activating the valve. Each blade 1 is mounted on a carriage 34 so that the blades can be moved towards and away from each other according to the width of the object to be coated. These objects, in Figure 7 coils 35, 36, of sheet steel, are transported, typically on an earthed conveyor belt (not shown), between the two blades in the direction of arrow 50. Each carriage 24 includes a catchment tray 36 and a sump tank 37 to catch liquid that falls from the blades 1. Liquid in the sump tanks 37 can be returned to the reservoir 28 by pumps 39 via suitable filter units 40. The apparatus shown in Figure 7 is described in conn¬ ection with coating the edge faces of the coils 35, 36 with oil to prevent corrosion. However, it will be understood that the apparatus can be used to coat other objects and to spray liquids other than oil, e.g. paint. Inoperation, the coils 35, 36 are fed intermittently at a steady rate along the conveyor belt. As a coil approaches the apparatus, it activates three light barriers 41, 42 and 43, each consisting of a pair of co¬ operating sensors. Light barriers 41 and 42 are adjustable in position, as indicated by the double-headed arrows 51, 52 and are effective to sense the width of the coil and move the carriages 34 accordingly. Light barrier 43 activates the high voltage switch 29 to supply high voltage to the blades 11. Preferably, the switch is activated to supply high voltage only after a time delay that is sufficiently to allow the coils 35, 36 to travel into the space between the blades 11. When voltage issupplied to the blades, the voltage difference between the blades and the earthed coil 35 draws oil from the discharge edge 17 of each blade and breaks the oil into small droplets that are attracted to the edge faces of the coil. Oil is supplied to the dis¬ charge edge 17 of each blade 1 by way of the central passageway 14 and the channel 6. A further light barrier 44 in the form of a light emitter and a spaced apart light sensor is provided on the downstream side of the blades 11 and this barrier 44 is broken by the coil 36, as it leaves the space between the blades. The barrier 44 is linked to the high voltage switch 29 so that when the barrier is re-established after the coil has left the space, the high voltage switch is turned off (unless it has already been activated by a further coil approaching sensed by light barrier 43). At the same time as switching the high voltage switch 29 off, the solenoid valve 33 is opened and oil in the passageway 14 of each blade is then returned via bypass orifice 15 along bypass line 32 to the reservoir 28. The length and width of the channel 16 in each blade should be such that, when the solenoid valve 33 is open, the resistance to oil flow in the channel 16 is sufficiently large that n oil passes therethrough. With the arrangement described above, the oil metering pumps operate continuously but no oil seeps out of the blade, which is advantageous not only in terms of cleanliness, but also because it does not waste oil. Should any oil seep out, it is collected in the catchment trays 36 and sump tanks 37 from where it is returned to the reservoir 28. Oϊv-FI

Claims: CLAIMS 1. An electrostatic coating blade (11) comprising a central passageway (14) and a discharge edge (17) characterised in that one or more flow-resistive channels (16) extend between the passageway and the discharge 5 edge for feeding liquid from the passageway to the discharge edge. 2. A blade as claimed in claim 1, characterised in that the or each channel (16) has a mean width in the range of from 0.007 to 0.07 cms. 103. A blade as claimed in claim 2, characterised in that the or each channel (16) has a mean width in the range of from 0.01 to 0.4 cms. 4. A blade as claimed in claim 1, characterised in that the or each channel (16) has a length in the range of 15 from 0.1 to to 5 cms. 5. A blade as claimed in claim 4, characterised in that the channel (16) has a length in the range of from 0.2 to 4 cms. 6. A blade as claimed in claim 1, characterised in that 20 the ratio of the length to the mean width of the or each channel (16) is in the range of from 5 ;1 to 500 :1. 7. A blade as claimed in claim 6, characterised in that said ratio is in the range of from 10:1 to 250:1. 8. A blade as claimed in claim 1, characterised in that 25 a single channel (16) is provided extending along sub¬ stantially the whole length of the blade. IRE -15- 9. A blade as claimed in claim 1, characterised in that it comprises two blade halves (12a, 12b) that are held together and a shim (15) located between the blade halves to separate them, wherein the channel or channels (16) is or are formed between the two blade halves. 10. A blade as claimed in claim 1, characterised in that the discharge edge (17) is formed at the apex of two external converging surfaces, which surfaces are concave. 11. A process of spraying objects (35) with a liquid using an electrostatic coating blade (11), characterised in that the blade (11) is as claimed in claim 1. 12. A process as claimed in claim 11, characterised in that the blade (11) is non-horizontally disposed. 13. A process as claimed in claim 11, characterised in that the objects are coils of sheet metal (35) and the liquid is oil. 14. A process as claimed in claim 12 or 13, characterised in that it incLudes providing a line of supply of electrical power to the blade (11) of a coating station, providing a line of supply of coating liquid to the blade of the coating station, the liquid line or the passageway in the blade communicating with the bypass line (32) via a valve (25), the process being further characterised in that the lines of supply are open and the bypass valve (25) is closed when a work- piece (36) is in or proximate to the coating station and the electrical power supply line is closed, the liquid supply line is open and the bypass valve (25) is open when a workpiece (36) is not in, or not prox¬ imate to, the coating station. 15. A process as claimed in claim 14, characterised in that the or each channel (16) in the blade (11) is so dimensioned that no liquid flows through the respective channel(s) when the bypass valve (25) is open. 16. An apparatus for electrostatically coating objects, which apparatus comprises an electrostatic coating blade arranged at a coating station characterised in that the blade is as claimed in claim 1. 17. An apparatus as claimed in claim 16, characterised in that the blade is non-horizontally disposed. 18. An apparatus as claimed in claim 17, characterised in that it includes a line of supply of electrical power to the blade of a coating station, a line of supply of coating liquid to the blade (11), the liquid line or the passageway in the or each blade communicating with a bypass line (32) by way of a bypass valve (33). 19. A process of spraying oil on a coil of sheet metal, characterised in that the oil is sprayed by a non-horizontally disposed electrostatic coating blade.

Inventors: OTTERWAY PAUL HOWARD, SMYTHE KENNETH ROBERT, OTTERWAY, PAUL, HOWARD, SMYTHE, KENNETH, ROBERT
Applicants: SALE TILNEY TECHNOLOGY PLC
Application year: 1983
Main classification: B05B5
IPC codes: B05B1, B05B5, F16N7