Improvements in and relating to dispensing conductive coating materials

Abstract: An apparatus for transferring electrically conductive coating materials, such as water-based paint, from at least one source to one or more coating dispensers or spray guns (12) for discharge onto a substrate. Additionally, a rapid and efficient colour change capability is provided for the entire system which permits different coloured coating materials to be dispensed from the apparatus herein with minimum downtime of the coating operation.

Description: This invention relates to a method for dispensing electrically conductive coating materials from one or more dispensers wherein the source of supply of the conductive coating material is electrostatically isolated from a high voltage electrostatic powder supply. The application of coating materials using electrostatic spraying techniques has been practiced in industry for many years. In these applications, the coating material is discharged in atomized form and an electrostatic charge is imparted to the atomized particles which are then directed toward a substrate maintained at a different potential to establish an electrostatic attraction for the charged atomized particles. In the past, coating materials of the solvent-based variety, such as varnishes, lacquers, enamels and the like, were the primary materials employed in electrostatic coating applications. The problem with such coating materials is that they create an atmosphere which is both explosive and toxic. The explosive nature of the environment presents a safety hazard should a spark inadvertently be generated, such as by accidentally grounding the nozzle of the spray gun, which can ignite the solvent in the atmosphere causing an explosion. The toxic nature of the workplace atmosphere created by solvent coating materials can be a health hazard should an employee inhale solvent vapors. As a result of the problems with solvent-based coatings, the recent trend has been to switch to water-based coatings which reduce the problems of explosiveness and toxicity. Unfortunately, this switch from electrostatically spraying solvent-based coatings to those of the water-based type has sharply increased the risk of electrical shock, which risk was relatively minor with solvent-based coatings. The risk of electrical shock is occasioned in the use of water-based coatings due to their extreme electrical conductivity, with resistivities of such water-based coatings often falling within the range of 100 to 100,000 ohm centimeters. This is in contrast to resistivities of 200,000 to 100,000,000 ohm centimeters for moderately electrically conductive coatings such as metallic paint, and resistivities exceeding 100,000,000 ohm centimeters for solvent-based lacquers, varnishes, enamels and the like. The relative resistivity of the coating material is critical to the potential electrical shock which may arise during an electrostatic coating operation. With coating materials which are either not electrically conductive or only moderately electrically conductive, the column of coating material which extends from the charging electrode at the tip of the coating dispenser through the hoses leading back to the supply tank has sufficient electrical resistance to prevent any significant electrostatic charging of the material in the supply tank or the tank itself. However, when coating material is highly electrically conductive, as are water-based coatings, the resistance of the coating column in the supply hose is very low. As a result, a high voltage charging electrode located in the vicinity of the nozzle of the coating dispenser electrostatically charges not only the coating particles, but the coating material in the hose, the coating material in the supply tank and the supply tank itself. Under these circumstances, operating personnel inadvertently coming into contact with an exposed supply tank, or a charged hose, or any other charged part of the system, risk serious electrical shock unless such equipment is grounded to draw off the electricity. If the equipment is indeed grounded at any point, however, the electrostatics will not function because the high voltage charge would be conducted away from the coating dispenser electrode to the grounded point as well. One of the methods and apparatus for reducing the electrical shock problem is disclosed, for example, in U.S. Patent No. 4,313,475 to Wiggins. In apparatus of this type, a "voltage block" system is employed wherein an electrostatically conductive coating material is first transmitted from a grounded primary coating supply into a transfer vessel which is electrically isolated from one or more electrostatic coating dispensers. After being filled with coating material, the transfer vessel is first disconnected from the primary coating supply and then connected to an inventory tank, which, in turn, is connected to the coating dispensers. The coating material is transmitted from the transfer vessel into the inventory tank, with the transfer vessel disconnected from the primary coating supply, to fill the inventory tank with coating material for subsequent transfer to the coating dispensers. After the inventory tank is filled, the transfer vessel is disconnected from the inventory tank and connected back to the primary coating supply to receive another quantity of coating material so that the coating operation can proceed essentially continuously. Another "voltage block" system for transferring electrically conductive coating materials is disclosed in U.S. Patent No. 5,078,168, which is owned by the assignee of this invention. In this system, first and second shuttle devices are selectively connected to two large reservoir, piston pumps. The first shuttle device is movable between a transfer position, and a spaced, neutral position, relative to a filling station which is connected to a source of electrically conductive coating material. At the filling station, the first shuttle is operative to transfer coating material from the source into the reservoir of the first pump. In the neutral position, the first shuttle is electrically isolated, i.e., physically spaced, from the filling station. The second shuttle device is movable between a transfer position wherein it interconnects the first piston pump with the second piston pump, and a neutral position wherein the two pumps are electrically isolated from one another and the second piston pump supplies coating material to the dispensers. Movement of the shuttles is controlled to maintain one of the shuttles in a neutral position while the other is at the transfer position so that there is never a completed electrical path between the source of electrically conductive coating material and the electrostatically charged dispenser. One problem with apparatus of the type disclosed in US Patent Nos. 4,313,475 and 5,078,168 involves the pressure available to discharge the coating material from either the transfer vessel of the apparatus disclosed in US4313495 or the second reservoir disclosed in US5078168. For example, in US5078168, each of the first and second reservoir pumps includes a piston which is movable in one direction in response to the application of air pressure thereagainst to discharge coating material from the reservoir, and is movable in the opposite direction as new coating material is added to the reservoir. In order to permit filling of the reservoir of the second pump with coating material supplied from the first pump, the air pressure applied to the piston in the second pump must be reduced compared to that of the first pump, otherwise the piston within the second pump would not move and allow the reservoir therein to be filled. Because of this reduced pressure level within the second pump, the coating material is discharged therefrom at a relatively low pressure level. As a result, a comparatively few coating dispensers can be supplied with coating material, and the spray pattern emitted from such dispensers is not always stable. Another problem with voltage block systems of the type described above, and particularly the apparatus disclosed in US Patent No. 5,078,168, is a relatively wide pressure fluctuation in the coating material discharge from the second pump to the coating dispensers. When the reservoir of the second pump is filled and coating material is discharged by its piston moving in a downward direction toward the base of the reservoir, the fluid pressure output from the second pump is less than the air pressure at which the piston is forced downwardly because the seal friction with which the piston seals against the side walls of the pump reservoir opposes downward motion of the piston. This produces a comparatively low fluid discharge pressure, significantly lower than the air pressure, with the attendant disadvantages noted above. On the other hand, a higher fluid discharge pressure, e.g. higher than the air pressure, is output from the second pump when it is filled with coating material from the first pump. This is because the fluid pressure of the coating material introduced at the base of the second pump, on the bottom side of the piston, must overcome both the air pressure acting on the opposite or top side of the piston and the seal friction of the piston seals against the sidewall of the piston reservoir. Since the air pressure in the system remains constant, the fluid pressure fluctuates depending on whether the piston within the second pump is moving upwardly or downwardly. Accordingly, a potentially large pressure fluctuation can occur at the discharge side of the second pump depending upon whether or not the second pump is undergoing a fill cycle or a discharge cycle when coating material is discharged therefrom to the coating dispensers. Such pressure fluctuation limits the number of dispensers which can be supplied by the second pump, and/or adversely affects the spray pattern obtained from such dispensers. Another problem with apparatus of the type disclosed in US Patent Nos. 4,313,475 and 5,078,168 is that an appreciable pressure drop is produced when water, solvent and/or air is used to flush the system of paint of one color in preparation for the use of another colored paint. This pressure drop occurs because, as noted above, all of the hoses and transfer containers or pumps are interconnected in series with one another from the point at which the source of coating material is introduced into the system to the point at which the coating material is discharged to the coating dispensers. For example, in the system of US Patent No. 5,078,168, the coating material, flushing liquid and/or air must first enter the lines interconnecting the first shuttle to the first pump, travel through the line interconnecting the first pump to the second pump and then pass through the lines interconnecting the second pump to the coating dispenser. By the time the flushing fluid or coating material reaches the downstream portions of this flow path, a pressure drop has occurred which lessens the effectiveness with which the air or liquid can remove the coating material remaining in the system. While both of the systems disclosed in US Patent Nos. 4,313,475 and 5,078,168 are adapted for use with color changers connected to sources of different color paint, neither system is capable of effecting a color change rapidly in a production environment. Both of these systems provide an essentially "series" flow path between the source(s) of coating material and the dispensers. That is, the coating material is first transmitted from the source to the transfer vessel of the Wiggins apparatus, or to the first reservoir pump of the Konieczynski apparatus, and then delivered through lines to either the inventory tank or second reservoir pump for subsequent supply to the dispensers. In order to effect a color change in either system, a flushing liquid such as water must be introduced at the beginning of this flow path, i.e., where the coating material is introduced, and then pass through each line and element of the system in sequence, one after the other, to remove the old paint. In applications such as the coating of automobiles and/or other assembly line-type painting operations, such a relatively long "downtime" between color changes is unacceptable. It is therefore among the objectives of this invention to provide a method for dispensing electrically conductive coating materials, such as water-based paint, which protects against the transmission of an electrostatic charge between a high voltage electrostatic power supply and one or more primary coating supplies, which is capable of supplying a large number of coating dispensers, which avoids pressure fluctuations during operation, and which produces a consistent, acceptable spray pattern of coating material on a substrate. The invention provides a method of transmitting electrically conductive coating material to at least one electrostatic coating dispenser, comprising supplying coating material to a first pump reservoir from a first transfer unit which is connected to the source of coating material and to the coating dispenser, the transfer unit maintaining the first pump reservoir electrically isolated from the coating dispenser while coating material is supplied to the first pump reservoir, supplying a second pump reservoir with coating material from a second transfer unit which is connected to the source of coating material and to the coating dispenser, the transfer unit maintaining the second pump reservoir means electrically isolated from the coating dispenser while coating material is supplied to the second pump reservoir, transferring coating material from one of the first and second pump reservoirs to the coating dispenser, while maintaining said one pump reservoir electrically isolated from the source, and switching the flow of coating material transferred to the coating dispenser to the other of the first and second pump reservoirs when said one pump reservoir is depleted of coating material. In the method of transferring electrically conductive coating materials, such as water-based paint, from the at least one source to the one or more coating dispensers or spray guns for discharge onto a substrate, the electrically conductive coating material is transmitted from two "parallel" flow paths, each having a large reservoir pump, to means such as a common valve which switches flow to the coating dispensers from one flow path to the other. Each parallel flow path provides a voltage block, i.e., an air gap, between one or more sources of coating material and the electrostatically charged spray guns. This voltage block ensures that there is never an electrical path between the source of conductive coating material and the charged coating material during a coating operation. One aspect of this invention is the concept of replacing the "series" flow path arrangement found in the prior art with at least two "parallel" flow paths, each connected between one or more sources of coating material and the coating dispensers. The parallel flow path system of this invention eliminates the long, difficult-to-clean series flow paths employed in prior art systems of, the type described above. In a preferred form, each flow path comprises a voltage block construction which includes a transfer device having a filling station connected to the source(s) of coating material, a discharge station spaced from the filling station and a shuttle movable between and releasably coupled to the filling station and discharge station. Upon movement of the shuttle to the filling station of the transfer unit, the shuttle is effective to transfer coating material from the source into the reservoir of a piston pump associated with such flow path. When the reservoir of the piston pump is filled, the shuttle moves and is coupled to the discharge station wherein a connection is made allowing the coating material to be transferred from the pump through the discharge station of the transfer unit and into a "sync" valve connected to the dispensers. This sync valve is common to both flow paths and is effective to switch the flow of coating material to the dispensers from one flow path to the other. The operation of the system is synchronized such that when the pump of one flow path is supplying coating material to the dispensers, the pump of the other flow path is receiving coating material from the source. A voltage block is continuously maintained between the source and charged dispensers, and the dispensers can be essentially continuously supplied with coating material from one or the other of the parallel flow paths. Because each of the parallel flow paths are essentially separate from one another, the coating material is transmitted along a relatively short distance to the dispensers thus making cleaning of such flow path relatively fast and efficient compared to prior art systems. Additionally, because a separate pump is associated with each flow path, a higher pressure is available to transmit coating material to the dispensers than is obtained with prior art systems, thus enabling (1) more dispensers to be supplied with coating material at the same pressure, or (2) a higher flow of material to be transmitted to the dispensers, or (3) longer transfer lines to be used between the pumps and dispensers. Further, the essentially direct supply of coating material from a separate pump associated with each flow path to the coating dispensers substantially eliminates pressure fluctuations present in other voltage block systems. As a result, an improved spray pattern is obtained from the dispensers associated with the system of this invention. Another advantage of employing parallel flow paths, each with a separate pump, is that pump wear and/or seal failure is substantially reduced compared to other voltage block systems for the same flow volume. In the apparatus disclosed in US5078168, for example, the second reservoir pump would be required to stroke twice as often as each individual pump associated with the two flow paths of this system to deliver the same quantity of coating material to the dispensers. Additionally, the shuttles associated with both the first and second reservoir pumps of the apparatus are required to operate twice as often as the shuttle of each parallel flow path herein. As a result, a significant reduction in wear of the pumps and shuttles of this system is obtained compared to prior voltage block apparatus such as disclosed in Patent No. 5,078,168. The invention will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 is a schematic view of a parallel flow system for transmitting electrically conductive coating material in accordance with the invention; Fig. 1A is a partial cross sectional view of the common, sync valve shown in Fig. 1; Fig. 2 is a schematic depiction of the portion of the system of Fig. 1 which operates during normal operating conditions; Fig. 3 is a schematic depiction of an alternative embodiment of apparatus in accordance with the invention; and Fig. 4 is a schematic, block diagram of the apparatus illustrated in Fig. 3 in which three apparatus are shown in parallel, each connected to a source of different coloured paint. Referring initially to Fig. 1, the parallel flow system 10 in accordance with this invention includes structure for delivering electrically conductive coating material to one or more spray guns or rotary atomizers 12 while maintaining a "voltage block" or air gap between the source(s) of coating material and such spray guns 12. Preferably, the spray devices 12. are spray guns of the type sold by Nordson Corporation, of Westlake, Ohio, the assignee of this invention, under Model No. AN-9, or rotary atomizers sold by Nordson Corporation under Model No. RA-12. In order to facilitate understanding of the invention, the system 10 depicted in Fig. 1 will first be described The structure and operation of flow system 11, illustrated in Figs. 3 and 4, is then described. With particular reference to Figs. 1A and 2, that portion of the system 10 which is required to supply coating material to the spray guns 12 during normal operation is illustrated. The "normal operating" portion of system 10 comprises two essentially identical, parallel flow paths each comprising a transfer unit 14, a piston pump 16 and a valving system for operating the transfer unit 14 and piston pump 16. The parallel flow paths employ a common four-way valve and a common "sync" valve 20, both of which are described in detail below. As viewed in Fig. 2, one of the parallel flow paths is located on the lefthand side of the sheet in relation to the common sync valve 20, whereas the other, parallel flow path is located on the righthand side of the sheet therefrom. For purposes of the present discussion, the flow path on the lefthand side of the sheet of Fig. 2 is described in detail, it being understood that the structure and operation of the other flow path is identical. Reference numbers utilized to describe structure on the lefthand side of Fig. 2 are employed to denote the same structure on the righthand side thereof with the addition of a "prime." The transfer unit 14 comprises a filling station 22, a discharge station 24 and a shuttle 26 movable between the filling and discharge stations 22, 24. The filling station 22 is provided with male and female coupling elements 28, 30 which mate with male and female coupling elements 28, 30 carried by the shuttle 26. Preferably, these coupling elements 28, 30 are of the type disclosed in U.S. Patent No. 5,078,168 to Konieczynski et al, owned by the assignee of this invention, the disclosure of which is incorporated by reference in it entirety herein. As depicted in Fig. 2, electrically conductive coating material is supplied through a paint supply line 32 to the male coupling element 28 at the filling station 22 from a "paint kitchen" 34. This paint kitchen 34 includes appropriate paint pumps, water flushing pumps and a color changer (not shown), the detailed disclosure of which forms no part of this invention and is therefore not discussed herein. A color changer of the type such as disclosed in U.S. Patent No. 4,657,047 to Kolibas, owned by the assignee of this invention, is utilized in the paint kitchen 34 which supplies different colors for discharge by the spray guns 12. The female coupling element 30 of the filling station 22 is connected by a return line 36 to the paint kitchen 34. The shuttle 26 is movable into coupling engagement with the filling station 22 such that the female coupling element 30 at the top of shuttle 26 mates with the male coupling 28 of the filling station 22, and the male coupling 28 of shuttle 26 mates with the female coupling element 30 of filling station 22. The female coupling element 30 of shuttle 26 is connected by a transfer line 38 to the inlet side of the piston pump 16 which is preferably of the type disclosed in U.S. Patent 5,078,168. This piston pump 16 includes a large reservoir (not shown) and a piston rod 40 which extends outwardly from the pump interior. The outlet side of piston pump 16 is connected by a second transfer line 42 to the shuttle 26 in position to transmit coating material to the male coupling element 28 at the top of the shuttle 26 and a male coupling element 28 at the bottom thereof. This male coupling element 28 at the base of shuttle 26 is matable with a female coupling element 30 carried by the discharge station 24 of transfer unit 14. A discharge line 44 interconnects the female coupling element 30 at the discharge station 24 with one side of the sync valve 20 which is described below. The outlet of the sync valve 20 is connected to a circulation line 45 which is described in more detail below in connection with a discussion of Fig. 3. In turn, the circulation line 45 is intersected by a gun supply line 46 which leads to a number of separate gun shuttles 48 each connected to one of the spray guns 12. The gun shuttles 48 each comprise a discharge station 50 having male and female coupling elements 28, 30, and a filling station 52 having mating, male and female coupling elements 28, 30. The filling station 52 is mounted to a linear actuator 54 having a cylinder 56 and a reciprocating piston 58 which is connected to the filling station 52. In response to operation of actuator 54, the filling station 52 is moved into and out of engagement with the discharge station 50 such that the coupling elements thereof mate with one another. The actuators 54 of gun shuttles 48 are controlled by a control system 55 (Fig. 1) described in detail in U.S. Patent Application Serial No. 07/766,796, filed September 27, 1991, entitled "Apparatus For Dispensing Conductive Coating Material" which is owned by the assignee of this invention and the disclosure of which is incorporated by reference in its entirety herein. The detailed structure and operation of such control system forms no part of this invention and thus is not described herein, except it is noted that movement of the filling station 52 occurs when a dispenser 12 is actuated, such as by depressing the trigger. It should be understood that the gun shuttles 48 and control system 55 are employed only with manually operated dispenser 12. In applications utilizing automatic dispensers, a controller (not shown) associated with the paint kitchen 34 is effective to turn the dispensers 12 on and off and the supply line 46 is connected directly to each dispenser 12. The operation of transfer unit 14, piston pump 16 and sync valve 20 is controlled by a series of air-operated valves which are responsive to the quantity of coating material within the piston pump 16, as described below. Referring to the top portion of Fig. 2, pressurized air is supplied from an air source 60 through a primary air supply line 62 to an upper limit valve 64 via tap line 65, a lower limit valve 66 via tap line 67 and a common, four-way valve 68 via tap line 69. Preferably, the valves 64, 66 and 68 are of the type made by Clippard Laboratory, Inc. of Cincinnati, Ohio under Model Nos. MJV-3, MJVO-3 and MJV-4D, respectively. The upper limit valve 64 is connected by a pilot line 70 to the left side of a four-way valve 72 as depicted in Fig. 2, which, in turn, is supplied with pressurized air from a tap line 74 connected to the primary supply line 62. Valve 72 is the same type of valve as valve 68. The lower limit valve 66 is connected by a pilot line 76 to the left side of the four-way valve 68, and by a separate pilot line 78 to the opposite, righthand side of four-way valve 72. The four-way valve 72 controls the operation of a linear actuator 80 associated with the transfer unit 14. This linear actuator 80 includes a cylinder 82 having a piston 84 connected to the shuttle 26 of transfer unit 14. In response to operation of the actuator 80, the piston 84 moves the shuttle 26 between a discharge position coupled to the discharge station 24 as shown on the lefthand side of Fig. 2, and a pump filling position coupled to filling station 22 such as shown on the righthand side of Fig. 2 wherein shuttle 26' and filling station 22' are coupled to one another. In order to control operation of linear actuator 80, the four-way valve 72 is connected to a line 86 which intersects an operating line 88 extending between the top portion of linear actuator 80 and the piston pump 16. The four-way valve 72 is also connected by a pilot line 90 to the bottom of linear actuator 80, for purposes to become apparent below. With reference to the center portion of Fig. 2, the four-way valve 68 is connected by a first pilot line 94 to the lefthand side of sync valve 20, and a second pilot line 96 extends from the four-way valve 68 to the opposite, righthand side of sync valve 20. As noted above, the four-way valve 68 is common to both of the parallel flow paths herein, and, hence, the opposite or righthand side of four-way valve 68 is connected by pilot line 76' from the lower limit valve 66'. Operation of the parallel flow paths as depicted in Fig. 2 is predicated upon the concept of first supplying coating material to the spray guns 12 from the piston pump 16 associated with one flow path, and then supplying coating material from the piston pump 16' associated with the other flow path. While the piston pump 16 is discharging coating material to the spray guns 12, the piston pump 16' is being filled with fresh paint from the paint kitchen 34. By the time the piston pump 16 is empty, the other piston pump 16' has been completely filled and can be operated to supply paint to the spray guns 12 via the sync valve 20. The body of sync valve 20 is formed of metal or other electrically conductive material which is connected to a high voltage electrostatic source 21 by an electrical line 23. In the course of passage through the sync valve 20, the electrically conductive coating material receives an electrostatic charge and said charged coating material is then supplied via lines 45 and 46 to the dispensers 12. Regardless of which piston pump 16 or 16' supplies coating material to the spray guns 12, an air gap or voltage block is continuously maintained between the paint kitchen 34 and spray guns 12 to avoid the transmission of a high voltage electrostatic charge via the coating material therebetween. For purposes of the present discussion, assume piston pump 16 has already been "primed" or filled with coating material at the outset of operation of system 10. In such instance, the piston rod 40 associated with piston pump 16 is in an uppermost, raised position relative to the upper and lower limit valves 64, 66 because the reservoir of piston pump 16 is filled. In the course of moving to such uppermost position, the piston rod 40 trips the switch 98 associated with upper limit valve 64 thus permitting pilot air to flow through the upper limit valve 64 and pilot line 70 to the four-way valve 72. In turn, the spool of four-way valve 72 shifts to the position shown in Fig. 2 wherein a flow of air from branch line 74 is permitted to pass through the four-way valve 72 into the line 86. The pressurized air enters operating line 88 where it flows upwardly as depicted in Fig. 2 to pilot the linear actuator 80, and downwardly to force the piston of piston pump 16 toward the bottom of its reservoir. In response to the receipt of pilot air from line 88, the piston 84 of linear actuator 80 moves the shuttle 26 downwardly into mating engagement with the discharge station 24 of transfer unit 14. As a result, the second transfer line 42 extending between the piston pump 16 and shuttle 26 is interconnected via the filling station 22 with the discharge line 44 connected to sync valve 20. As the piston within piston pump 16 is forced downwardly under the influence of the air flow from line 88, the coating material therein is forced from the piston pump 16 along the flow path defined by second transfer line 42, shuttle 26, discharge station 24 and discharge line 44 to the sync valve 20. As described below in connection with a discussion of Fig. 1A, the sync valve 20 is operative to receive coating material from either of the piston pumps 16 or 16' and deliver such coating material via circulation line 45 and gun supply line 46 to the gun shuttles 48 associated with each spray gun 12. As noted above, the operation of such gun shuttles 48 is controlled by a separate control system which is fully described in U.S. Patent Application Serial No. 07/766,796. Under normal operating circumstances, the filling station 52 of each gun shuttle 48 is interconnected with the discharge station 50 thereof in response to activation of the associated spray gun 12, such as by pulling the trigger of a mutually operated gun. When the discharge and filling stations 50, 52 are coupled with one another, the flow of coating material from the sync valve 20, circulation line 45 and gun supply line 46 passes through such gun shuttles 48 to each activated spray gun 12 which deposits the coating material onto the target substrate. In the event any one or all of the spray guns 12 are deactivated, the discharge and filling stations 50 and 52 of the respective gun shuttle 48 disconnect from one another thus halting the flow of coating material into spray guns 12. As mentioned above, while one of the piston pumps 16 or 16' provides coating material to sync valve 20, the other piston pump is being filled with coating material. The pump filling operation proceeds as follows. After a period of time, the coating material within the reservoir of piston pump 15 becomes depleted and its piston rod 40 gradually moves downwardly within the pump reservoir. Upon reaching a predetermined lowermost position, the piston rod 40 releases the switch 100 associated with the lower limit valve 66. This closes lower limit valve 66 and permits the flow of pilot air through pilot line 76 to one side of the common four-way valve 68, and through second pilot line 78 to the righthand side of four-way valve 72. Such flow of pilot air initiates two operations within the system 10, which proceed at different speeds. First, the pilot air flowing through pilot line 76 shifts the position of the spool within four-way valve 68 so that operating air from primary supply line 62 and tap line 69 can flow through the common four-way valve 68 into the second pilot line 96. As described in more detail below, the pilot air from second pilot. line 96 causes the side of sync valve 20 connected to discharge line 44' to immediately open while the discharge line 44, which had been transmitting coating material from pump 16, is allowed to close. Coating material is then supplied from the piston pump 16' in the same manner as described above in connection with piston pump 16. Lagging behind this operation of sync valve 20 is the movement of shuttle 26 created by the pilot air flowing through pilot line 78. As noted above, pilot line 78 is connected to the side of four-way valve 72 opposite the pilot line 70 associated with upper limit switch 64. The pilot air from pilot line 78 shifts the spool within four-way valve 72 so that operating air from branch line 74 flows through the four-way valve 72 into the pilot line 90 connected to the bottom of the linear actuator 80 associated with transfer unit 14. This pilot air causes the piston 84 of linear actuator 80 to extend and move the shuttle 26 upwardly into mating engagement with the filling station 22, i.e., in the position of shuttle 26' shown on the righthand side of Fig. 2. With the shuttle 26 in this position, coating material from the paint kitchen 34 is supplied through paint supply line 32 and filling station 22 to the transfer line 38 connected to piston pump 16. The piston pump 16 therefore receives fresh paint from the paint kitchen 34 and its piston rod 40 begins to move upwardly as discussed below. The spray guns 12 can be provided with an essentially continuous supply of coating material because of the cooperation of the separate, parallel flow paths on the left and righthand sides of Fig. 2 which are both connected to the sync valve 20. With reference to Fig. 1A, the construction of the sync valve 20 makes possible a shift of supply of coating material from one piston pump 16 to the other piston pump 16' without any interruption in the flow of coating material to the spray gun 12. The sync valve 20 consists of a pair of air-open, spring-return ball valves 101 and 101' each having a valve body 102, 102', respectively. The valves 101, 101' are connected to a central mounting block 103 formed with a throughbore 104 which is intersected by an outlet 105 connected to the circulation line 45. The valves 101, 101' which form sync valve 20 are structurally and functionally identical, and therefore only the valve 101 is described in detail and with the same reference numbers being used with the addition of a "prime" to denote the structure of valve 101'. As viewed on the lefthand side of Fig. 1A, the valve body 102 of valve 101 is formed with a bore 110 which intersects an inlet port 112 connected to the discharge line 44 associated with piston pump 16. This bore 110 receives a rod 114 connected at one end to a piston 116 and at the opposite end to a collar 118 which mounts a ball 120. The piston 116 is movable within a chamber 122 formed in a two-piece end cap 124 mounted to one end of the valve body 102 by screws 126 which extend through the valve body 102 into the central mounting block 103. An air passage 128 is formed in the valve body 102 and end cap 124 which transfers pilot air from the first pilot line 94 against one side of the piston 116. Preferably, a spring 130 extends between the end cap 124 and the collar 118 to urge the ball 120 against the seat 132 of an insert 134 which is threadedly received within one end of the throughbore 104 of central mounting block 103 and rests against a flange 135 formed therein. Coating material from the discharge line 44 is introduced through the inlet port 112 into the bore 110 where it flows to the ball 120. In response to the supply of pilot air via line 94, the piston 116 is moved to the left as viewed in Fig. 1A which unseats the ball 120 from seat 132 thus allowing flow of coating material into the throughbore 104 of valve body 102 and out its outlet 105 into circulation line 45. The operation of sync valve 20 is controlled by the common, four-way valve 68 such that flow of coating material from only one of the piston pumps 16 or 16' is permitted at any given time, except for a brief period during which flow of the coating material shifts from an empty piston pump 16 or 16' to the other pump. As mentioned above, air valves 64, 66 and 72 control the operation of the linear actuator 80 associated with the transfer unit 14. When the piston pump 16 is nearly empty and lower limit valve 66 is tripped, four-way valve 72 is piloted to permit an air flow to the bottom of linear actuator 80 as described above. This causes the shuttle 26 to disengage the discharge station 24 of transfer unit 14 and move toward the filling station 22. But the operation of lower limit valve 66, four-way valve 72 and actuator 80 is slower than that of the four-way valve 68 and sync valve 20. Before the shuttle 26 can disengage the discharge station 24, the sync valve 20 has already shifted position, i.e., pilot air has been supplied via line 76 to the common four-way valve 68 which, in turn, allows air flow through second pilot line 96 to the sync valve 20. This immediately causes the ball 120' to move away from its seat 132' and thus initiate the flow of coating material into the throughbore 104 of sync valve 20 from the piston pump 16'. Such movement of the ball 120' occurs before the shuttle 26 can disengage from the discharge station 24 and before ball 120 completely seals against seat 132. As a result, as ball 120' is withdrawing and ball 120 is closing, the piston pump 16 continues to supply at least some coating material through the discharge line 44 connected to the lefthand side of sync valve 20 so that there is always coating material flowing through the throughbore 104 of sync valve 20. Once the shuttle 26 completely disengages discharge station 24 and the spring 130 forces the ball 120 against seat 132, ball 120' will be completely withdrawn permitting flow of coating material from only the piston pump 16'. At the same time, the shuttle 26 is moved to the filling station 22 of transfer unit 14 to begin the filling operation of piston pump 16 as described below. Under normal operating conditions, the transfer unit 14 and transfer unit 14', together with their associated piston pumps 16 and 16', undergo a sequential filling and discharge operation so that an essentially continuous supply of coating material is provided to the spray guns 12. Dependent on the position of piston rod 40 associated with each piston pump 16 and 16', the shuttles 26 and 26' are positioned to either supply coating material to their respective piston pumps 16, 16' or permit the discharge of coating material therefrom. It should be understood that while the shuttles 26 and 26' are shown in Fig. 2 at opposite positions, such shuttles 26, 26' operate completely independently of one another. Accordingly, both of the shuttles 26 and 26' could be in the down or discharge position at the same time in the event, for example, the piston pump 16 has not yet been emptied of coating material before piston pump 16' becomes completely filled. As noted above, operation of the sync valve 20 is controlled by the common four-way valve 68, which, in turn, is piloted in response to actuation of the lower limit valves 66 and 66'. These lower limit valves 66 and 66' do not supply pilot air except when the piston rod 40 or 40' of their associated pumps 16, 16' reach a predetermined, "empty" condition. Once that happens, then the transfer operation of the supply of coating material from one pump 16 or 16' to the other can proceed. As described above, the operation of system 10 under normal conditions involves the supply of coating material to the spray guns 12 alternately from the piston pump 16 in one parallel flow path, and then from the piston pump 16' in the other parallel flow path. But when operation of the spray guns 12 is terminated for a relatively long period of time, such as during a lunch break or if the coating production line is otherwise temporarily shut down, the coating material could remain stationary within the system 10. This can present problems with coating materials such as paint wherein the pigments, sediment and other solids can settle out if allowed to stagnate and remain stationary. In order to avoid this problem, the system 10 may be provided with a "circulation " mode wherein the coating material can be constantly circulated through the system while the spray guns 12 are not operated. This is described in detail in parent EP-A-0593 238. A number of different cleaning or flushing steps can be performed simultaneously to clear virtually all elements of the system 10 at the same time and thus reduce the overall downtime associated with a colour change operation. Again this is described in detail in parent EP-A-0593 238. With reference to Figs. 3, 3A and 4, a voltage block system 300 is depicted which is essentially a simplified embodiment of the system 10 shown in Figs. 1, 1A and 2 and discussed in detail above. System 300 incorporates a dedicated paint source 302 of a single color which is connected via lines 32 and 36 to the transfer units 14, 14'. The structure and operation of transfer units 14, 14' is identical to that described above. But, because system 300 employs a single, dedicated paint source 302, structure for performing a color change operation, and for cleaning or flushing the system 10, is eliminated in system 300. Additionally, in this embodiment, the sync valve 20 is directly connected by a line 304 to one or more dispensers 12. The coating material transmitted from sync valve 20 through line 304 is electrostatically charged by the power supply 21 connected to sync valve 20 by line 23 in the same manner described above in connection with Figs. 1-7. Preferably, the system 300 is used primarily with automatic spray guns or rotary atomizers rather than manual, hand-held guns. The embodiments of Figs. 3 and 3A also include structure for circulating the coating material back to the paint source 302 to maintain the coating material moving when the dispensers 12 are not operating. In Fig. 3, the circulation shuttle 138, four-way valve 166, door valves 275, 277 and check valve 287 described above in connection with Figs. 1, 1A and 2 are employed with the addition of a second check valve 290 having an input connected by a line 291 to check valve 287 and a output connected by a line 292 to the pilot of four-way valve 166. Additionally, a first connector line 293 is connected between the filling station 140 of shuttle 138 and paint supply line 32, and, a second connector line 294 is connected between the discharge station 146 of shuttle 138 and return line 36. In response to opening of either safety lock door valve 275 or 277, pilot air is supplied through check valve 287, line 291 and second check valve 290 to the pilot of four-way valve 166. As described above, when piloted, the four-way valve 166 causes the filling station 140 of shuttle 138 to couple with its discharge station 146 thus providing a flow path from line 304, through first connector line 293 to the shuttle 138, and then through second connector line 294 to the paint source 302 via return line 36. The coating material essentially bypasses the dispensers 12 and is transmitted along such flow path, to and from the source 302, while the remainder of the system 300 operates as if coating material was being supplied to the dispensers 12. In the alternative embodiment shown in Fig. 3A, the same circulation structure is illustrated as in Fig. 3, with the addition of a solenoid valve 295 connected by an electrical line 296 to a controller 299 and by an air line 297 to the air supply line 62. Tile controller 299 is a standard programmable control, such as a personal computer, which is also operatively connected to the dispensers 12 in a manner not shown. The solenoid valve 295, in turn, is connected by a line 298 to the second check valve 290. The purpose of solenoid valve 295 is to provide for circulation of the coating material depending upon whether the dispensers 12 are operating or not. For example, when automatic dispensers 12 are employed, the controller 299 is effective to turn the dispensers 12 on and off as required. At the same time controller 299 turns the dispensers 12 off, a signal is sent via line 296 to the solenoid valve 295 which is activated to allow pilot air from line 297 to pass therethrough and enter line 298 to second check valve 290. This air flow pilots the four-way valve 166, which, as explained above, causes the filling station 140 of circulation shuttle 138 to couple with discharge station 146 and circulate the coating material to and from the paint source 302. Accordingly, the Fig. 3A embodiment provides. essentially the same circulation of coating material through the system 300 as Fig. 3, except in Fig. 3A such circulation is initiated by closing of dispensers 12. With particular. reference to Fig. 4, the system 300 of Fig. 3 (or Fig. 3A) is shown in a configuration to permit different colored coating materials to be supplied to one or more dispensers 12. As schematically represented in Fig. 4, three separate sources of different color paint 302A, 302B, and 302C supply coating material to three separate systems 300A, 300B, and 300C, respectively. Each of these systems 300A, 300B, 300C are identical in structure and function to the system 300 depicted in Figs. 3 or 3A. Each separate system 300A, 300B, 300C is connected by a separate feed line 306A, 306B, 306C to a color changer 308 of the type disclosed in U.S. Patent No. 4,657,047 to Kolibas, owned by the assignee of this invention. As discussed in detail in that patent, the color changer 308 is effective to supply a selected color via a line 310 to the dispensers 12. Because each individual system 300A, 300B, 300C supplies a single color, no flushing or other cleaning is needed in between color changes except for the color changer 308, line 310 and dispensers 12. Such flushing operation can be easily and rapidly performed as described in Patent No. 4,657,047, thereby substantially limiting downtime between color changes. The embodiments of this invention depicted in Figs. 3, 3A and 4 therefore provide simplified alternatives to the Figs. 1, 1A and 2 embodiment, and are particularly useful in high volume applications employing automatic spray guns.

Claims: A method of transmitting electrically conductive coating material to at least one electrostatic coating dispenser (12), comprising supplying coating material to a first pump reservoir from a first transfer unit (14) which is connected to the source (34,302) of coating material and to the coating dispenser (12), the transfer unit (14) maintaining the first pump reservoir electrically isolated from the coating dispenser (12) while coating material is supplied to the first pump reservoir, supplying a second pump reservoir with coating material from a second transfer unit (14) which is connected to the source (34,302) of coating material and to the coating dispenser (12), the transfer unit (14) maintaining the second pump reservoir means electrically isolated from the coating dispenser (12) while coating material is supplied to the second pump reservoir, transferring coating material from one of the first and second pump reservoirs to the coating dispenser, while maintaining said one pump reservoir electrically isolated from the source (34,302), and switching the flow of coating material transferred to the coating dispenser (12) to the other of the first and second pump reservoirs when said one pump reservoir is depleted of coating material. A method as claimed in Claim 1 further comprising producing a signal representative of a depleted condition of said one of the first and second pump reservoirs, and switching the flow of coating material transferred to the coating dispenser (12) to the other of the first and second pump reservoirs in response to production of the signal. A method as claimed in Claim 2 in which the step of transferring coating material comprises transmitting coating material to the coating dispenser by movement of a piston within one of the first and second pump reservoirs, and wherein the step of producing a signal comprises sensing the movement of the piston within the reservoir and generating a signal when the piston reaches a predetermined position. A method as claimed in any preceding claim wherein the first transfer unit (14) is movable, wherein supplying the first pump reservoir comprises directing coating material from the source (34,302) through the first transfer unit (14) in a filling position to the first pump reservoir while utilizing the movable first transfer unit (14) in the filling position for both blocking flow of the conductive material from the first pump reservoir to the dispenser (12) and creating a voltage block therebetween, and wherein the transfer from the first pump reservoir to the dispenser (12 comprises directing the coating material from the first pump reservoir through the movable first transfer unit (14) in a discharge position to the dispenser (12) while utilizing the first transfer unit (14) in the discharge position for both blocking flow of the conductive material from the source (34,302) to the first pump reservoir and creating a voltage block therebetween. A method as claimed in Claim 4, wherein the second transfer unit (14') is movable, wherein supplying the second pump reservoir comprises directing coating material from the source (34,302) through the second movable transfer unit (14') in a filling position to the second pump reservoir while utilizing the second movable transfer unit (14') in the filling position for both blocking flow of the conductive material from the second pump reservoir to the dispenser (12) and creating a voltage block therebetween, wherein the transfer from the second pump reservoir to the dispenser (12) comprises directing the coating material from the second pump reservoir through the second movable transfer unit (14') in a discharge position to the dispenser (12) while utilizing the second transfer unit (14') in the discharge position for both blocking flow of the conductive material from the source (34,302) to the second pump reservoir and creating a voltage block therebetween, and wherein that flow of coating material to said dispenser (12) is shifted from the first pump reservoir and the second pump reservoir by controlling the operation of the movable transfer units (14,14')

Inventors: KONIECZYNSKI RONALD D, KONIECZYNSKI, RONALD D.
Applicants: NORDSON CORP, NORDSON CORPORATION
Application year: 1993
Main classification: B05B5
IPC codes: B05B12, B05B5