ELECTROSTATIC ROTARY ATOMIZING LIQUID SPRAY COATING APPARATUS

Abstract: Spray coating apparatus comprising a support body (10) of insulative material and a rotary atomiser cup (20) having an inner surface (40) over which liquid coating may flow towards an atomising edge when the cup is rotated. Means are provided to charge the liquid as it flows out towards the atomising edge. The charging means extends through the atomiser cup (20) between the inner and outer surfaces thereof, and has an inner portion (46, 546, 646) which is contacted by the liquid coating. At least one electrical conductor (212) is mounted in close proximity to the outer portion (102, 502, 802) of the charging means, in order to transfer electrostatic energy to the charging means when the conductor is energised.

Description: This invention relates to electrostatic spray coating and more particularly to electrostatic liquid spray coating apparatus utilising rotary atomization. Electrostatic spray coating apparatus incorporating rotary atomizers have been available for many years. Typically a conductive cup or disc maintained at high voltage is rotated at very high speed causing liquid coating material fed, to the central part of the cup or disc to migrate outwardly over the cup or disc surface under centrifugal force, eventually leaving the cup and disc at the outer edge thereof where it becomes atomised. Because the atomising edge of the cup or disc is sharp, the high voltage applied to the conductive cup or disc causes ionisation of the air in the region of the atomising liquid coating particles in a manner well known in the field of electrostatic spray coating. Over the years the hazards associated with the use of the conductive atomising cup and discs maintained at high voltage, which take the form of personnel shock and ignition when combustible coatings are employed, have become well publicised. In brief, the hazards exist by virtue of the fact that substantial electrical energy is stored in capacitive form by a conductive cup or disc maintained at high voltage which can rapidly discharge if inadvertently grounded or brought near a grounded object. To minimise these hazards various solutions have been proposed. For example, it has been proposed to make the atomised cup or disc of insulative material except for a conductive skin or layer which is provided on the surface of the atomising member to conduct high voltage to the atomising edge for the purpose of creating ionisation thereat. Another proposal involves making the atomising cup or disc of resistive material. These and other proposals are contained in U.S. Patents: Gauthier 2,926,106, Gauthier 2,989,241, Schotland 2,955,565, Juvinall 3,009,441, Sedlacsik 3,010,428, Gauthier 3,021,077, Juvinall et al 3,048,498, Point 3,063,642, Point et al 3,072,341, Gauthier 3,083,121, Gauthier 3,128,045, Point 3,178,114, Felici et al 3,279,429, Scharfenberger et al 3,826,425, Point 3,075,706, and Robisch et al PCT International Publication No. WO 85/01455. The foregoing proposals have not been entirely satisfactory for various reasons, one of which is that the resulting transfer efficiency of the spray apparatus has not been sufficient to satisfy those desiring high coating transfer efficiencies in the range of 90% and above. By coating transfer efficiency is meant the percentage or proportion of coating material emitted from the spray device which actually coats the desired articles. European Patent Application No. 0243043 describes an electrostatic atomizing liquid spray coating apparatus with a rotary atomizer head. The atomizing head has a circular ring-shaped charging electrode on a first surface thereof, over which liquid coating flows outwardly, connected to a circular electrical current-conducting element on a second surface, separated from the first by the atomizing edge of the head. The ring-shaped charging electrode is connected to the electrical current-conducting element by a series of pins which extend through the walls of the atomizer head. Electrical energy is transferred to the electrical current-conducting element by a plurality of circumferentially-spaced electrical conductors provided on the forward section of the apparatus body such that the free ends thereof are positioned in closely spaced proximity to the electrical current-conducting element. The amount of conductive material in the rotating atomizer head is kept relatively low whilst the transfer efficiency is high due to the arrangement of the plural electrodes. Electrostatic rotary atomising liquid spray coating apparatus in accordance with the invention comprises a support body of insulative material, a rotary atomizer in the form of a cup of insulative material, having an inner surface over which liquid coating can flow outwardly to an atomising edge thereof when the cup is rotated about its axis of rotation, means being provided for charging the liquid coating, as it flows outwards over the inner surface, which extends through the atomizer cup between the inner and outer surfaces, the charging means having an outer portion and an inner portion which is contacted by the liquid coating material and comprises a circular ring-shaped charging electrode mounted on the inner surface of the cup, at least one electrical conductor mounted on the support body, the or each conductor having a free end located in close proximity to the outer portion of the charging means for transferring electrostatic energy to the charging means when the conductor is energised from a high voltage source, characterised in that the charging means further comprises a plurality of posts extending through the atomizer cup and electrically connected to the ring on the inner surface, each of the posts having ends proximate to the outer surface forming the outer portion of the charging means. The stationary electrode or electrodes facilitate transfer of electrostatic energy to the charging means when the stationary conductor is energised from a high voltage source, enabling contact charging of liquid coating supplied to the inner surface when the coating flows, under centrifugal force, outwardly over the inner surface in contact with the inner portion of the charging means toward the atomising edge. By minimising the amount of conductive material incorporated in the rotating atomizer, electrical energy stored in capacitive form by the atomizer is kept within safe limits while providing high transfer efficiency. The safety of the spray apparatus may be further enhanced by fabricating the charging means extending through the atomizer, including its inner portion, its associated outer portion and the connecting portion therebetween form semiconductive material. The inner portion of the charging means is in the form of a circular ring-shaped charging electrode mounted on the inner surface of the atomizer encircling the axis of rotation. A plurality of posts extend through the atomizer connecting the ring with the outer surface, the outer ends of the posts forming the outer portion of the charging means. For the purpose of still further improving the safety of the spray apparatus, the free ends of the posts which transfer electrical energy to the charging means on, the atomizer, as well as the outer portion of the charging means on the outer surface of the cup, are preferably protected from damage and inadvertent contact by being located substantially within a recess in which the atomizer cup rotates. To facilitate mounting of the spray apparatus to a post or the like, a mounting bracket is suitably provided of desired design from which several spaced parallel columns project in a forward direction which, at their forward end, mount the support body of the spray apparatus housing the various valves and the drive assembly for the rotary atomising element. In a preferred form, one of the columns is hollow for housing an electrostatic energy-conducting core for transporting high voltage electrostatic energy between a remote high voltage source and the stationary electrode which is located in close proximity to the conducting element on the rotating atomizer which is connected to the charging electrode. The hollow column may also encase a gun resistor which is in series with the stationary conductor. The invention will now be described by way of example with reference to the accompanying drawings in which: Figures 1 - 11 depict the prior art spray apparatus disclosed in EP-A-0243043. Figure 1 is a side elevational view, partially in cross section, of the rotary atomising liquid spray coating device. Figure 2 is a side elevational view, in cross section, of the front section of the rotary atomising liquid spray coating device depicted in Figure 1. Figure 3 is a cross-sectional view along line 3-3 of Figure 2. Figure 4 is a cross-sectional view along line 4-4 of Figure 3 showing the flow passages and valving for solvent for cleansing the exterior of the atomising cup. Figure 5 is a cross sectional view along line 5-5 of Figure 3 showing a portion of the path for the air for shaping the atomised liquid spray coating pattern. Figure 6 is a cross-sectional view along line 6-6 of Figure 1. Figure 7 is a cross-sectional view along line 7-7 of Figure 1 showing the general relationship of the valves for the liquid coating material and the solvent for cleansing the interior and exterior of the rotary liquid atomising cup. Figure 8 is a cross-sectional view along line 8-8 of Figure 3 showing the flow passages and valving for solvent for cleansing the interior of the rotary atomising cup. Figure 9 is a cross-sectional view along line 9-9 of Figure 1 showing the rear body section of the spray device, support columns, and various air and solvent hoses. Figure 10 is a front view of an alternative embodiment of a rotary atomising spray coating apparatus according to EP-A-0243043. Figure 11 is a partial cross-sectional view taken on line 11-11 of Figure 10. Figure 12 is a cross-sectional view similar in format to Fig. 11 of a rotary atomizer in accordance with the invention. Figure 13 is a view similar to Fig. 12 illustrating a further alternative embodiment of the charging means in accordance with the invention. With reference to Figures 1 and 2, a rotary atomising liquid spray coating device has a support body 10 having a front or forward section 12 and a rear section 14 between which is positioned an intermediate section 16. The body sections 12, 14 and 16 are generally cylindrical in shape. The diameter of the forward and rear body sections 12 and 14 are substantially the same. The diameter of the intermediate body section 16 is substantially less than that of the body sections 12 and 14, defining therebetween an annular cavity 18 within which can be located and mounted various valves for controlling the flow of liquid coating material and solvent for cleansing the interior and exterior of the rotary atomising cup described hereafter. A rotary atomising cup 20 extends forwardly from the front surface 22 of the forward body section 12. Removably secured to the front surface 22 of the forward section 12 of the body 10 in any suitable manner, such as by bolts, threaded engagement, or the like, is an annular ring 24. The ring 24 includes a circular air passage or manifold 26 formed in the rear surface thereof from which extend forwardly a plurality of circularly arranged air ports 28 for establishing a circular array of air jets for shaping the atomised liquid coating spray pattern 29 formed at the forward edge of rim 42 of the atomising cup 20. As noted, extending forwardly from the forward section 12 of the body 10 is the rotary atomising cup 20. Cup 20 is drivingly mounted on a shaft 23 for rotation about its axis. The cup drive shaft 23 extends through a bore 12b in forward body section 12 and an air or ball bearing 25 of a conventional commercially available type located within a suitably configured bearing cavity or bore 27 in intermediate body section 16. Shaft 23 is driven at its rear (left as viewed in Figure 2) by a rotary actuator 31, such as an air-driven turbine, also of a conventional commercially available type which is located behind the bearing 25 in a turbine cavity or bore 31a in rear body section 14. A liquid coating control valve 33 mounted to the rear surface of the flange-defining portion of the forward section 12 of the body 10 controls the flow of liquid coating material to a coating nozzle 30 via a passage 32 formed in the forward section 12 of the body 10. Liquid coating under slight pressure exits nozzle 30 and enters an annular cavity 34 formed in the rear section of the cup 20. Under centrifugal force due to the rotation of cup 20 by drive shaft 23, the liquid coating material in the annular cavity 34 passes radially outwards and forwards through a series of coating passages 36 in radial cup wall 20c to a forward cup cavity 38. Once in the forward cup cavity 38 the liquid coating moves radially and forwardly along a first surface defined by interior cup wall 40 toward the forward atomising edge 42 of the cup 20 whereat it is atomised under centrifugal force to form the atomised spray pattern 29. A flat circular ring-shaped charging electrode 46, which is embedded in the interior wall 40 and is connected to a conventional high voltage electrostatic supply (not shown) in a manner to be described, charges the liquid coating material by contact as it passes thereover in its movement from passages 36 in wall 20c to the forward atomising edge 42 of the cup whereat the liquid is centrifugally atomised to form spray pattern 29. The way in which the support body 10 is mounted is described in detail in European Patent Application No. 0243043, Figures 1 to 11 of which are identical with accompanying Figures 1 to 11. The mounting system includes mounting columns 62, 64 and 66, one of which 62 at its rear or left end has a reduced diameter portion 62c which passes through a suitable bore in plate 52, extending rearwardly of surface 55 thereof. A nut 62d threadedly engages the column ,end portion 62c to secure column 62 to plate 52. The support column 62 at its forward end passes through a suitably provided bore 70 in the rear section 14 of body 10 and extends forwardly to the rear wall 12a of the forward body section 12. The forwardmost portion 62a of the column 62 is of reduced diameter and threaded such that it will threadably engage a suitable threaded bore 72 formed in the rear surface 12a of the forward body section 12. The column 62 is provided with an axial bore 62b within which is positioned a high voltage insulated cable 74 connected at its rearward end to a high voltage electrostatic supply (not shown). The cable 74 at its forward end 74a connects to a gun resistor 76. An electrical conductor 78 extends between the forward end of the gun resistor for energising the electrode 46 in a manner to be described in more detail hereafter. As shown in Figure 1, a dump valve 80 mounted to the forward wall 57 of the plate 52 connects to the liquid coating valve 33 via a flexible conduit 82 and to a waste receptacle 86 via a conduit 88. Dump valve 80 diverts cleansing solvent from coating valve 33 during colour change operations in a manner well known in the art. Mounted to the rear surface 12a of the flange-defining portion of forward body section 12, in addition to the coating control valve 33, are solvent valves 90 and 92 which control the flow of solvent, to the exterior and interior of the rotary atomising cup, respectively, as shown in Figures 3, 4, 7, and 8. Valves 90 and 92 are located in the annular cavity 18. The rotary atomising cup 20, as best seen in Figure 2, includes a frusto-conical tubular section 20a and a hub 20b which are interconnected by radial wall 20c which collectively define the rear annular cavity 34 and the forward cavity 38. The cross section of the tubular section 20 increases along the axis thereof in the direction of the atomising edge 42. The hub 20b is provided with a tapered bore 20f which snugly engages a similarly tapered portion 23a of the drive shaft 23. The forward end 23b of the drive shaft 23 is threaded for threadedly receiving a retaining nut 100 which locks the hub 20b of the cup 20 in place on the drive shaft 23. embedded in the outer surface 20d of the frusto-conical section 20a of cup 20, in the embodiment illustrated in Fig.2, is a circular current-conducting flat ring element 102, preferably of semiconductive material. Ring element 102 is electrically connected to the flat electrode 46, which is also preferably fabricated of semiconductive material, via a series of conducting means in the form of pins 104, preferably made of semiconductive material, seated in suitably provided bores in the cup section 20a. The opposite ends of pins 104 are in electrical contact with the confronting surfaces of the ring 102 and electrode 46. The cup 20 is made of insulative material, as is the nut 100, shaft 23, bearing 25, annular ring 24, body 10, rotary actuator 31, valves 33, 80, 90 and 92, and associated fluid conduits, mounting bracket 50, and mounting columns 62, 64, and 66 for the purpose of minimising the storage of electrical energy in capacitive form in the spray coating device. A preferred type of insulating material for the cup 20 is PEEK (polyetheretherketone) available from I.C.I. of America, and for the remaining insulative elements is polyester, available from Erta Incorporated, Malvern, Pennsylvania. Surrounding the bracket 50 and body 10, as well as the various valves, is a tubular housing, as best shown in Figure 1, for enclosing the various operating components of the spray device. The housing is preferably fabricated of insulative material. The liquid coating valve 33, which may be of any conventional type, preferably includes a valve body 120 having a stepped diameter bore 122. Located in the forward end of the bore 122 is a valve seat insert mount 124 having a bore 126 within which is positioned a valve seat insert 128 having an axial passage 128a. This passage is normally blocked by a ball valve element 130 formed at the forward end of a reciprocal rod 132 which is normally forwardly biased by a spring-biased air-operated piston 134, 135 secured to the rear end 132a of the shaft 132. An air chamber 136 is connected to a source of pressurised air via a passage 138 in the wall of the rear portion of the valve body 120. When pressurised air is admitted into the chamber 136 via passage 138 under control of means not shown, the piston 134 is urged rearwardly (leftwardly) to unseat the ball valve element 130 relative to the seat of the seat insert 128, so connecting passage 128a with a liquid coating chamber 142. Chamber 142 communicates with a source of pressurised liquid coating (not shown) via a passage 144 formed in the wall of the valve body 120 which is connected to a coating supply conduit 145. Thus, when pressurised air is admitted into cavity 136 via passage 138 urging the piston 134 backwards and unseating the valve ball element 130, pressurised liquid coating in chamber 142 passes through passageway 128a into the passageway 32 of the forward body section 12. It then exits under pressure from the nozzle 30 into the rear cavity 34 of the rotary cup 20. In a manner described heretofore, the liquid coating material in rear cavity 34 flows through passages 36, along interior wall 40 of the forward cavity 38 and over flat ring electrode 46 whereat the coating material is electrostatically charged. Eventually the charged electrostatic coating is atomised at the forward edge 42 of the cup 20 to form spray pattern 29. Air cavity 136 and coating cavity 142 are separated by suitable seals 150 which permit axial reciprocation of the rod 132. The cavity 142 of the valve 33 connects via passage 152 formed in the wall of the valve body 120 to the conduit 82, ultimately being passed to a waste receptacle 86 via the dump valve 80 and the conduit 88. The dump valve 80 is substantially identical to the valve 33, except it has, in addition to a single inlet passage, only one outlet passage for the flow of liquid coating material. The dump valve 80, like the valve 33, is air-operated and for this purpose has a controlled source of pressurised air (not shown) connected to it via an air hose 80a. Shaping of the atomised liquid coating spray pattern 29 emanating from the forward edge 42 of the rotary atomising cup 20, as previously noted, is provided by a circular air passage 26 formed in the annular ring 24 which feeds a plurality of circularly arranged axially extending ports 28 which establish forwardly projecting air jets. To provide pressurised air to the circular air passage 26 formed in annular ring 24, the forward body section 12 is provided with a passage 160 which at its forward end communicates with the circular air passage 26 and at its rearward end connects to a suitable source of pressurised air (not shown) via a hose 162. Control means, also not shown, regulate the flow of air in the hose 162 in a conventional manner. When pressurised air is provided to the hose 162, air is emitted under pressure from the circularly arranged ports 28 in a forwardly direction, shaping the electrostatically charged atomised liquid coating particle spray pattern 29, as desired. The way in which the colour of the liquid coating material being sprayed is changed is described in detail in European Patent Application No. 0243043. High voltage electrostatic energy is coupled from the electrode 78 at the output of the gun resistor 76 to the semiconductive ring 102 (and ultimately to the semiconductive electrode 46 via the semiconductive pins 104) via a path which includes an electrically conductive spring contact 200 located in the forward end of the bore 72 formed in the forward body section 12, an electrical conductor 202 snugly fitting in a bore formed in the forward body section, an electrode ring 204 embedded in an annular ring 24, and several parallel circuit paths connected between the ring conductor 204 and the semiconductive ring 102. The series circuit paths between rings 204 and 102 include a resistor 210 disposed between an electrical conductor 212 (which is connected between the resistor 210 and the ring 204) and a conductor 214 extending from the forward end of the resistor 210 towards and in close proximity to the ring 102. An insulative sheath 216 threaded at its inner or rear end into a bore in the annular ring 24 encases the resistor 210, conductor 212, and conductor 214, with conductor 214 projecting from the forward end of the sheath. Insulative sheaths 218 and 220, identical to sheath 216, are mounted in circumferentially spaced relation around the annual ring 24 120° on either side of the sheath 216. The sheaths 218 and 220 contain resistors 218a (Figure 3) and 220a which are identical to resistor 210. Resistor 218a is connected between an outer electrical conductor 218b, which extends from the forward end of its associated sheath towards and in close proximity to the ring 102, and an electrical conductor 218c which is connected to the conductive ring 204, for transmitting electrostatic voltage to the resistor 218a. The forwardly projecting ends of the electrical conductors 214, 218b, and 220b are spaced very slightly from the exterior surface of the semiconductive ring 102 such that when high voltage is transmitted thereto via the insulated cable 74, gun resistor 76, conductor 78, spring 200, conductor 202, ring conductor 204, and conductor/resistor pairs 210/212, 218a/218c, and 220a/220c, electrostatic energy is transmitted across the gap to the semiconductive ring 102 and ultimately to the ring electrode 46 via pins 104 for contact charging of liquid coating material which flows radially outwards and forwards along inner wall 40 over the surface of the semiconductive electrode 46. It has been discovered that the coating transfer efficiency is enhanced by the use of three circumferentially-spaced conductors 212, 218c and 220c in comparison to that achieved when only a single conductor is used. Thus, plural conductors provide improved results and are clearly preferred where high transfer efficiency is desired. Gun resistor 76 can have a resistance which varies depending upon the operating range of the electrostatic power supply which energizes the cable 74. Preferably, for electrostatic supplies operating in the range of 50kV - 125 kV the gun resistor has a resistance of 76 megohms. The resistors 210, 218a, and 220a can also have varying resistances, although preferably each resistor has a resistance of approximately 12 megohms. The insulated cable 74 may take a variety of forms, although the preferred cable is one in which the conductive core 74b is fabricated of silicon carbide fibre in accordance with U.S. Patent 4,576,827, assigned to the assignee of the present application. The semiconductive ring 102, pins 104, and electrode 46 are also preferably fabricated of polyphenylene sulfide (PPS), available from Phillips 66, although other semiconductive materials may be used. In addition, and although not preferred, the ring 102, pins 104, and/or electrode 46 can be fabricated of conductive material. However, when fabricated of conductive material, the capability of the rotating atomizing cup 20 to capacitively store electrical energy is increased over that which exists when the ring 102, pins 104, and electrode 46 are fabricated of semiconductive material. If desired, the conductive elements 78, 200, 202, 204, 212, 214, 218b and 218c, and 220b and 220c can be fabricated of semiconductive material rather than conductive material. Accordingly, and for the purpose of minimising the electrical energy stored capacitively in the spray device of this invention, all elements of the spray device are preferably fabricated of insulative material, except for those which are fabricated of semiconductive and/or electrically conductive material for the purpose of transporting electrostatic energy at high voltage from a remote source (not shown) to the coating charging electrode 46 in the rotary atomising cup 20. The rotating atomising cup 20 has been described as being frusto-conical in shape. It will be appreciated that other shapes can be utilised. The valves 33, 80, 90 and 92 are generally constructed in accordance with the teachings of U.S. Patent 3,870,233, assigned to the Applicant. An alternative embodiment is shown in Figs. 10 and 11. Except for the differences to be described, the alternative embodiment is substantially the same as the first embodiment discussed above, with like parts having been assigned like reference numerals. The alternative embodiment of the prior art rotary atomising liquid spray coating device is seen to include a support body 10 having a front or forward section 12. As with the first embodiment, an annular cavity 18 is located rearwardly of the forward section 12. Within cavity 18 are located, as will be described in more detail hereafter, various valves for controlling the flow of liquid coating material and solvent for cleansing the interior and exterior of the rotary atomising cup 20. The atomising cup 20 extends forwardly from the front surface 22 of the forward body section 12. Removably secured to the front surface 22 of the forward section 12 of the body 10 in any suitable manner, such as by bolts, threaded engagement, or the like, is a cap 400 having a generally convex outer face 402 and a centrally disposed, inwardly tapering recess 404 inside which at least a portion of the atomising cup 20 may be located. Cap 400 includes a base 406 having a generally circular air passage or manifold 26 formed therein. A gasket 408 having suitably sized and positioned apertures is interposed between cap 400 and the front surface 22 of the forward section 12 of body 10 to provide a suitable seal for air and solvent passages, to be described later, which communicate between forward section 12 and cap 400. Similar to the annular ring 24 of the first embodiment, cap 400 includes a plurality of circularly arranged air ports 28 for establishing a circular array of air jets surrounding rotary atomising cup 20 for shaping the atomised liquid coating spray pattern 29 formed at the forward edge or rim 42 of the atomising cup 20 and projecting it toward a workpiece to be coated in the manner previously described. As noted, extending forwardly from the forward section 12 of the body 10 is the rotary atomising cup 20. Cup 20 is drivingly mounted for rotation on a shaft 23 of a rotatory actuator (not shown). The cup drive shaft 23 extends through a bore 12b in forward body section 12. As in the first embodiment, a liquid coating control valve 33 is mounted to the rear surface of the forward section 12 and controls the flow of liquid coating material to the coating nozzle 30. Liquid coating under slight pressure exits nozzle 30, enters cup 20 and passes therethrough as previously described with reference to the first embodiment. Mounted within cavity 18 and on the rear surface of the forward body section 12, in addition to the coating control valve 33, is a single solvent valve 412, in lieu of the solvent valves 90, 92 of the first embodiment. Valve 412 controls the flow of solvent, in a manner described in detail in European patent Application No. 0243043, to both the interior and exterior of the rotary atomising cup 20. The diameter of frusto-conical rotary atomising cup 20 increases along the axis of the cup in the direction of the atomising edge 42. Embedded in the outer surface 20d of the frusto-conical cup 20 is a circular current-conducting flat ring element 102, preferably of semi-conductive material. Ring element 102 is recessed substantially entirely within the recess 404 in which cup 20 is disposed thereby decreasing the likelihood that personnel or objects can contact element 102, creating a shock hazard. As with the first embodiment, ring element 102 is electrically connected to the flat charging electrode located on the interior surface of cup 20 in the manner previously described. A housing 416 is used to enclose all the operating components and the various conduits for coating material solvent and waste as well as the high voltage electrical cable are preferably routed rearwardly through appropriate apertures (not shown) in the rear mounting bracket rather than through the side walls as shown in Fig. 1. This locates the conduits and cable as far as possible from the spray pattern 29 emanating from the edge of atomising cup 20 to help prevent the accumulation of coating material on them. It also provides a sleek, alternative uncluttered appearance. The path for conducting high voltage electrostatic energy from gun resistor 76 to the charging electrode 102 embedded in the exterior wall 20d of atomising cup 20 according to this alternative embodiment will now be described in further detail. An annular conductor 430, which substantially encircles cap 400, is disposed in an annular stepped groove 432, cut in the base or rear face 434 of cap 400. Conductor 430 is captured within groove 432 by an insulating ring 436 which is sealed in the larger step of groove 432 using a suitable adhesive sealant such as an epoxy. The conductor 430 is connected by soldering, brazing or other suitable means to a conductive disk 438, which is preferably of brass or other electrically conductive corrosion resistant material. Disk 438 nests within a recess 440 of an electrically insulating bushing 442 which, in turn nests partially inside the front end 62a of the support column 62 which houses gun resistor 76. The opposite end of bushing 442 nests in a pocket in the ring 436. Bushing 442 includes an axial bore 444 which receives a cylindrical projecting portion 446 of column 62. Column end 62a and projection 446 include a bore 448 which communicates with gun resistor 76. Received within bore 448 is the hollow tubular body portion 450 of electrically conductive spring contact assembly 452. Body portion 448 contains a spring 454 which is compressively biased by a plunger 456 having a head 458 which abuts disk 438 as the base of body portion 450 abuts gun resistor 76 thereby providing good electrical contact between gun resistor 76 and disk 438 which is in turn connected to annular conductor 430. Electrostatic energy is transferred from conductor 430 to charging electrode 102 by way of three charging resistors 210, of identical nominal resistance connected electrically in parallel between charging electrode 102 and conductor 430. The charging resistors 210 are physically mounted within cap 400 in evenly circumferentially spaced relation to one another. Resistors 210 all fit snugly within bores 460 which communicate with conductor 430, and which are disposed within the recess 404 of cap 400 wherein atomizer cup 20 is located. Bores 460 each intersect recess 404 at a location opposite the ring element 102 of atomizer cup 20 so that the free ends 462 of the charging resistors act as electrodes which terminate in closely spaced proximity to semi-conductive ring element 102. By embedding charging resistors 210 within cap 400 substantial protections against their being damaged or misaligned due to accidental impact is achieved. Also, since the electrode leads 462 are located within recess 404, they are less likely to be contacted by personnel or objects thereby reducing the risk of electrical shock or mechanical damage. The opposite leads 464 of the charging resistors 210 pass through reduced diameter portions of bores 460 which intersect groove 432, at which point leads 464 are connected to conductor 430 by soldering or other suitable means. Thus, high voltage electrostatic energy is transmitted by way of high voltage cable 74 as previously described to gun resistor 76. It is then carried to conductor 430 by way of spring contact 452 and disk 438. From conductor 430, electrostatic energy is carried to charging electrode 102 of atomising cup 20 by way of the three charging resistors 210 connected electrically in parallel between conductor 430 and the gap between the electrodes or free ends 462 of said resistors and the ring element 102 on the outside of atomising cup 20. Electrostatic energy is then transmitted across the gap between each electrode 462 and the semi-conductive ring element 102. From ring element 102, the electrostatic energy is utilised in the manner of the first embodiment to impart a charge to the coating material. The resistances of gun resistor 76 and charging resistors 210 are selected as previously described. As with the embodiments previously described, and for the purpose of minimising the electrical energy stored capacitively in the spray device, all elements of the spray device are preferably fabricated of insulative material, except for those which are fabricated of semi-conductive and/or electrically conductive material for the purpose of transporting electrostatic energy at high voltage from a remote source (not shown) to the coating charging electrode 102 in the rotary atomising cup 20. The alternative embodiment of the prior art rotary atomising liquid spray system includes several features which help to project the spray pattern 29 forwardly toward the work piece to be coated and avoid the accumulation of"coating material on the sprayer itself thereby increasing transfer efficiency and decreasing fouling of the sprayer. One such feature, namely the provision of a plurality of air ports 28 for establishing an array of forwardly directed air jets surrounding the atomising cup 20, for shaping and projecting spray pattern 29 toward the workpiece to be coated, has already been described. Further, the sprayer of this embodiment also preferably includes at least one of the additional features which will now be described. Atomizer cup 20 is surrounded by electrostatic repulsion means which preferably takes the form of a substantially continuous conductive, or more preferably, semi-conductive ring 470. Ring 470 is embedded in a groove 472 cut in the outer face 402 of cap 400 so as to lie substantially flush therewith and not to interfere significantly with its contour for reasons which will later become apparent. Ring 470 is directly connected to conductor 430 by way of a conductive pin 474 so that ring 470 is energised with a high voltage charge of the same polarity as the charge carried by the coating droplets. This helps to pump the migration of spray pattern away from the spray apparatus and toward the workpiece to be coated. The cap 400 is provided with a curved, aerodynamically contoured outer face 402 as shown. The forward portion of cap 400 defines a circular dome having a contoured outer face 403 and a central recess 404 in which frustoconical atomising cup 20 is recessed. For the purpose of avoiding reverse air flow eddys, the degree to which cup 20 is recessed within cap 400 is not believed to be critical. In fact, recess 404 may be eliminated so that outer face 402 lies substantially entirely behind cup 20. However, so that conductive ring 102 and electrodes 462 may be protected as previously described cup 20 is preferably recessed within cap 400 for approximately one-half to two-thirds of its overall length. Recess 404 tapers inwardly at a slightly greater rate than the wall of cup 20 so that the gap between cup 20 and recess 404 is slightly narrower at its base than at its mouth. The transition edge between tapered recess 404 and curved outer face 402 is not sharp but rather is provided with a generous radius as shown in the drawings. This feature will become further apparent in light of its theory of operation which is believed to be as follows. As atomizer cup 20 rotates at an angular speed sufficient to atomise coating material, usually in the range of 10,000 to 40,000 R.P.M., its atomising edge 42, which is a larger diameter than its base 480, rotates at a greater surface speed than its base. Since the air surrounding cup 20 will tend to move with the surface of the cup 20 due to drag, there will be a pressure gradient along the outside wall 20d of cup 20 tending to cause a flow of air along the outside wall 20d in a direction generally parallel to wall 20d and oriented from base 480 toward edge 42. Since the aforementioned air flow would tend to partially evacuate the region near the base of the cup, it is believed that a make up air flow takes place along outer face 20d inwardly toward the base 480 of cup 20 along the wall of recess 404. The shape of cap 400, particularly the shape of its outer face 402 is selected such that under conditions of normal operation, the flow of make-up air across its surface will be in a substantially laminar flow regime. This is believed to help avoid the generation of eddy currents in the vicinity of cup 20 which would otherwise tend to draw coating material back towards the spray apparatus rather than permit it to be directed toward the workpiece as desired. Apparatus in accordance with the present invention is shown in Figures. 12 and 13. Except for the differences described, these embodiments are substantially the same as those discussed above, with like parts assigned like reference numerals. The charging means heretofore described with rings 46 and 102, respectively, forming the inner and outer portions thereof with pins 104 forming the connection therebetween is formed differently in Figs. 12 and 13. Referring to Fig. 12, the outer portion of charging means 501 of cup 20, rather than being in the form of the circular flat ring element 102 (Fig. 1), is formed by the outer ends 502, proximate the outer surface 20d of the cup 20, of a plurality of discrete posts 504 circumferentially spaced about the axis of the atomiser cup 20, preferably at equiangular increments. The pins 504, and their ends 502, are preferably eight or more in number, and preferably, sixteen. As such, the ends 502 of the posts 504 function in the same way as the circular ring element 102 of Figs. 1-11. The inner portion of the charging means 501 is in the form of a ring 546 configured and positioned in the same manner as the ring 46 of Figs. 1-11. Referring to Fig. 13, the charging means 601 rather than in the form of a circular ring 46 as in the embodiments above, is in the form of a plurality of discrete circumferentially spaced charging electrodes 604 extending between the inner and outer surfaces of the atomiser 20, and preferably spaced at equal angular increments about the atomiser's axis of rotation. The inner portion of the charging means 601 is formed of the inner ends 646 of the electrodes 604 which are proximate to the inner surface of the atomiser 20. The outer portion of the charging means is formed of the outer ends 602 of the electrodes 604 which are proximate the outer surface of the atomiser 20. Suitably, at least eight electrodes 604 are provided, and preferably about sixteen. The electrodes inner ends 646 function in the same manner and nearly as effectively in charging the liquid as the charging electrode ring 46 in the other embodiments, but hold less residual charge with improved safety.

Claims: Electrostatic rotary atomising liquid spray coating apparatus comprising a support body of insulative material, a rotary atomizer in the form of a cup of insulative material, having an inner surface over which liquid coating can flow outwardly to an atomising edge thereof when the cup is rotated about its axis of rotation, means being provided for charging the liquid coating, as it flows outwards over the inner surface, which extends through the atomizer cup between the inner and outer surfaces, the charging means having an outer portion and an inner portion which is contacted by the liquid coating material and comprises a circular ring-shaped charging electrode mounted on the inner surface of the cup, at least one electrical conductor mounted on the support body, the or each conductor having a free end located in close proximity to the outer portion of the charging means for transferring electrostatic energy to the charging means when the conductor is energised from a high voltage source, characterised in that the charging means further comprises a plurality of posts (504) extending through the atomizer cup and electrically connected to the ring (546) on the inner surface, each of the posts (504) having ends (502) proximate to the outer surface (20a) forming the outer portion of the charging means. Spray apparatus as claimed in Claim 1 characterised in that the charging means (546, 504) is formed at least in part of semi-conductive material. Spray apparatus as claimed in Claim 1 or Claim 2 wherein a plurality of circumferentially-spaced electrical conductors (462) is provided. Spray apparatus as claimed in any preceding claim characterised in that at least eight posts are provided. Spray apparatus as claimed in any preceding claim wherein the ends (502) of the posts (504) are disposed substantially within a gap defined between the atomizer cup (20) and the support body (10) whereby the ends (502) are afforded substantial protection against inadvertent contact.

Inventors: KNOBBE, ALAN J., MURPHY, JAMES C., SHARPLESS, JOHN, SHUSTER, DONALD E., WACKER, ROBERT L.
Applicants: NORDSON CORPORATION
Application year: 1990
Main classification: B05B5
IPC codes: B05B7, B05B5