Patent Application
20050129872
The invention relates to the electrostatic spray coating of articles generally, and is particularly suited for the spray painting of automotive vehicles with water based paint.
In the electrostatic application of paint in the automotive finishing industry, paint may be delivered to a robotically maneuverable atomizer applicator from a plurality of sources, each source providing a different color paint. During application, a high voltage is imposed on the paint, which imparts positive charges on the atomized paint droplets, which are then uniformly attracted to grounded articles being coated, all in known assembly-line fashion. Water based paint is, generally, electrically conductive. Conductivity of the paint composition can create critical safety concerns and hazards in electrostatic operations, wherein the applicator itself must be maintained at a high voltage.
Concomitantly, recently enacted environmental constraints are exerting pressure on the automotive industry to reduce the amount of volatile organic compounds (VOCs) being released into the environment, the majority of which is produced by paint operations. To reduce VOC emissions, painting facilities have gradually been converting from solvent paint carrier systems to water based systems. Water based paint, although much lower in VOC content, creates another set of problems resulting from the electrostatic charges placed on the paint as it leaves the spray atomizer. The charge can travel back down through the electrically conductive paint, acting as a circuit, to ground, thereby presenting a safety concern. The solution is to completely isolate the charged conductive paint being atomized from ground during the painting process.
Several attempts have been undertaken to isolate the high voltage required for electrostatic painting from ground when using conductive, water based paint. One such system to provide galvanic isolation is disclosed in U.S. Pat. No. 4,785,760. This patent describes an electrostatic system for spraying conductive paint wherein a quantity of paint required to paint an object such as a vehicle is stored in a storage tank carried by a multi-axis robot. The spray applicator may be of the rotational, bell cup variety or the pneumatic or hydrostatic spray gun type. The sprayer, carried by the robot, is supplied during painting from a storage tank also mounted on the robot arm and connected to the sprayer. The high voltage generator itself is controllable, that is, its output voltage may be reduced to zero at any time and then re-established virtually instantaneously using conventional controls. See, e.g., the '760 patent at col. 5, line 16 et seq. The high voltage is reduced to zero before a color change cycle is initiated, the storage tank is filled at a local dispensing port, and the voltage is re-established after filling as spraying begins again, resulting in isolation of the high voltage during spraying from the various distribution circuits that are all electrically grounded, because there is no conductive paint conduit connecting them during the spraying operation. ('760, col. 6, lines 57-65).
U.S. Pat. No. 5,310,120 discusses the '760 patent and discloses an alternative storage tank for an electrically conductive liquid coating product. This patent discloses a procedure wherein the coating product, at a high voltage, is carried within a storage tank defined by a substantially cylindrical cavity formed in an insulative material body inside of which is a piston forming a mobile wall separating a coating product chamber from an actuation chamber filled with an electrically insulative actuation fluid. During spraying, because the tank is separated and isolated from the grounded robot carrying it, the electrostatic charge placed on the paint within the storage tank will not track back to ground.
In one further known operation for providing such isolation, an intermediate storage tank is filled with sufficient paint required for one application. Each color change requires that the intermediate tank and the conduits leading to it be cleaned, for example as disclosed in French patent No. 2,572,662. Galvanic isolation is re-established after filling the intermediate tank by draining and drying a sufficient length of conduit upstream of the intermediate storage tank and then commencing painting. This method, however, is said to require a “prohibitive length of time” on each color, and therefore to be “not practical”. See, e.g., U.S. Pat. No. 4,785,760, at col. 3, line 25 et seq.
As a point of reference, the '760 patent suggests providing the necessary galvanic isolation by a method involving robotically picking up one small storage tank 22, spraying its contents, and then “hanging it up” locally and getting another one. See, e.g., '760 at col. 7, line 62 et seq.
While such apparatus and procedures may isolate the charged spray paint, they are generally inefficient. Time is of the essence on the paint finish line, an entire vehicle being painted typically in 2-3 minutes. Travel time in these operations in manipulating storage tanks robotically around a paint room is costly. The more time that is spent in emptying, cleaning and color changing, the more costly is the process. It is therefore beneficial to have a system that does not require transporting storage tanks with affixed applicators by means of robot arms to and from paint distributing docking systems, and which can be directly connected to paint supply tanks, all while maintaining complete voltage isolation from ground of the paint being sprayed.
The present invention provides such a system.
Apparatus and a method are provided for isolating an electrostatic sprayer from an electrically grounded coating product distribution circuit connected thereto. The apparatus includes an electrostatic sprayer carried by a maneuverable robot arm, the sprayer capable of spraying an electrically conductive coating product such as water-based paint onto a workpiece passing in adjacent proximity thereby, on command. The coating product is supplied from a source of supply through at least one distribution circuit connected to the sprayer. The apparatus includes therein, carried by the robot arm, an electrically insulative storage tank for the coating product in valved fluid communication with the sprayer. The insulated storage tank is connected to and positioned downstream in the distribution circuit from a length of electrically insulative supply conduit. The length of supply conduit is connected to the distribution circuit and is also carried by the robot arm. The length of supply conduit includes a cleaning mechanism for cleaning a portion, including all, of the length of supply conduit, in situ, after filling of the storage tank with coating product and before spraying. In this way, substantially all of the conductive coating product is removed from the portion of supply conduit, thereby completely isolating the sprayer electrically from the distribution circuit. The storage tank and supply conduit are preferably formed within a unitary housing made of a non-conductive plastic such as polyacetal resin.
In a preferred embodiment, containment and storage of the coating product prior to spraying are effected within a deformable membrane housed within the storage tank. Spraying is effected by a metering pump positioned downstream of the storage tank and upstream from the sprayer, and preferably the pump is a gear pump.
The membrane can be made of an elastomer, and is preferably a fluoroelastomer such as a fluorinated ethylene propylene (FEP) elastomer or a perfluoroalkyl (PFA) elastomer.
In alternate embodiments, containment and storage of the coating product can be effected within the chamber of a piston-and-cylinder assembly housed within the storage tank prior to spraying or within a balloon-like chamber housed within the storage tank prior to spraying.
Preferably the supply conduit is tubular and the mechanism for cleaning the supply conduit includes a plunger positioned within the conduit and adapted to reciprocally and repeatedly traverse the length of the conduit. The plunger is preferably made of a fluoroelastomer such as FEP.
The apparatus includes driving means for driving the plunger reciprocally back-and-forth through the length of said conduit on command, and the driving means may be air under pressure controlled by valving. In this embodiment, the apparatus has a valve-controlled source of compressed air connected thereto.
To clean the system, the apparatus includes a valve-controlled source of solvent connected within the distribution circuit, and a preferred solvent is deionized water. The apparatus includes a high voltage generator, preferably carried within the apparatus of the invention, the generator being supplied with low voltage via an isolated connector from an external voltage source.
The apparatus may be connected to a plurality of coating product distribution circuits, these circuits optionally distributing coatings of different colors as desired.
A facility for coating a plurality of workpieces simultaneously is contemplated, the installation including a plurality of the apparatus as aforesaid connected to a plurality of coating product distribution circuits. The apparatus is especially suited for coating automotive vehicles.
A process according to the invention is also provided for electrostatically spraying an electrically conductive coating onto a work-piece. The process includes spraying a conductive coating such as water-based paint onto a workpiece passing in adjacent proximity thereto using an electrostatic sprayer carried by a maneuverable robot arm. The batch-operation spraying begins after first supplying the coating to the sprayer system from a source of supply through at least one grounded distribution circuit connected to the sprayer, wherein the distribution circuit includes therein, carried by the robot arm, an electrically insulative storage tank for the coating composition in valved fluid communication with the sprayer and being connected to and positioned downstream from a length of electrically insulative supply conduit. The length of supply conduit is also carried by the robot arm. The process includes cleaning a portion, including all, of the length of the supply conduit, in situ, after filling of the storage tank with coating product and before spraying, thereby removing substantially all of the conductive coating product from the portion of supply conduit within the distribution circuit. This results in isolation of the sprayer electrically from the distribution circuit before actual spraying.
This process of cleaning the supply conduit, using a plunger fitted therein adapted to reciprocally traverse the length of the conduit, effectively wipes it clean of coating product and galvanically isolates the sprayer from the distribution circuit.
In the accompanying drawings:
Electrostatic spray applicators, as discussed hereinabove, are widely used for spray coating of substrates such as automotive vehicles. Sprayers are typically mounted on and maneuvered by programmable robots in automated production lines. Hydrostatic or pneumatic spray gun applicators or rotary bell cup applicators are typically used to uniformly paint automobiles carried by conveyor to and through a paint booth or area. The time required to actually paint a vehicle typically can range from a few to several minutes. Successive vehicles often must be painted different colors, requiring multiple changing of paint colors at a particular painting station as the production progresses.
Paint supply tanks are generally located remotely from a local painting station, and paint is supplied via distribution lines from these remote storage tanks. In addition, in electrostatic spraying operations using water based, i.e. electrically conductive, paint compositions, which operations involve a high voltage source in implementing uniform coatings, it is imperative that the highly charged spray applicator be galvanically isolated from the grounded paint supply.
In stark contrast to prior techniques for providing such isolation, the present invention provides apparatus and a method for filling, cleaning and electrically isolating a paint storage tank and its associated applicator, in situ, all mounted on the end of a robot controlled arm, and all while maintaining continuous connections to a plurality of paint distribution sources, as needed. According to the invention, there are no multiple connect and disconnect operations of a plurality of storage tanks, or passing of same by robotic arms around a paint room operation, as occurs with certain prior art procedures.
The invention, concisely, provides apparatus and a method for electrically isolating, in situ, a length of the paint feed line leading to a storage compartment for paint to be sprayed onto a workpiece, after filling of the compartment, wherein the compartment and its associated spray applicator are carried by, and maneuvered by, a robot arm.
A detailed description of the invention and preferred embodiments is best provided with reference to the accompanying drawings wherein
One embodiment of the storage apparatus 10 and rotating bell cup spray applicator assembly 20 of the invention is shown in greater detail in the cross-sectional view of
Controlled low voltage (0 to 21 volts, d.c.) is supplied from cable 62. The internal cascade unit 66 steps the voltage up to as much as 100,000 volts. The high voltage is then transmitted through the manifold 26 to the turbine (air bearing motor) 76. The charge is placed on the bell cup assembly by a series of conductive fiber brushes (not shown) which touch the rotating shaft 75 within the turbine 76.
The bell cup body 23 is conventionally affixed to an electrically conductive turbine shaft 75 within the turbine assembly 76. The rotating shaft 75 of the compressed air turbine 76 drives the rotating bell cup assembly, including the body 23 and deflector 69, which expels the atomized paint from the applicator assembly.
Upstream from the applicator 20 is the storage chamber 10, which includes its auxiliary fluid circuitry, all according to the invention. The chamber assembly 10 has affixed to it the applicator assembly 20 by means of quick disconnect nut 12. The chamber 10 is affixed to the robot side base plate 30 by means of quick disconnect nut 16. The storage chamber apparatus 10 in the embodiment shown in
The storage tank housing 14, the sleeve 74 and the central support housing 15 are all made of an electrically insulative plastic material, preferably a polyacetal resin sold under the trademark DELRIN®. The bladder 70 is preferably of an elastomer, preferably a synthetic fluoroelastomer. Especially preferred is a membrane of fluorinated ethylene propylene (FEP) elastomer.
Formed within the central housing 15 is the paint fill line 80 preferably extending from an upstream valved (V1) inlet 50 through the central housing 15 to a valved (V2) outlet line 82. Upon command, paint enters through the paint fill line 80 from supply conduit 50, proceeds through line 82 to fill the bladder 70, at which point the supply is shut off.
From this stage of operation on, with the filled bladder 70 intact, the paint fill line 80 may be cleaned of all conductive paint, and dried, all as described below, thereby electrically isolating the stored paint in the bladder 70 and the applicator assembly 20 from the rest of apparatus. After such cleaning, and with the opening of valve V3, the painting operation proceeds, preferably by means of a metered gear pump (not seen) drawing paint from the storage bladder 70 and expelling it outwardly through the rotary bell cup applicator 20, all in complete electrical isolation from ground.
Before describing in detail the sequence of operative steps of filling the storage bladder 70, cleaning the fill line 80 to galvanically isolate the system, painting, flushing and refilling before a second painting operation, reference is re-directed to
In
The schematic diagrams of
At the beginning of a paint cycle, with the entire system clean, with reference to
When painting is desired, referring to
With reference to
It will be appreciated that alternative valving schemes may be employed to provide the fill-flush-dry operation according to the invention. The above is one example of such sequence.
By successively and alternatively opening and closing V4 and V5 to air, with V2 opened to waste and V4 opened to exhaust, as needed, and V1, V3 and V6 closed, all as illustrated in
Painting begins at
After the storage bladder is completely flushed with solvent, air may be introduced into the system to dry the entire system, including the bladder. The paint fill cycle may then be repeated, with reference back to
While the invention has been disclosed herein in connection with certain embodiments and detailed descriptions, it will be clear to one skilled in the art that modifications or variations of such details can be made without deviating from the gist of this invention, and such modifications or variations are considered to be within the scope of the claims hereinbelow.
