ROTARY ELECTROSTATIC DUSTING APPARATUS AND METHOD

Abstract: A technique for uniformly depositing particles, especially phosphor particles, onto a display panel and an apparatus for carrying out this technique are set forth in this application. This technique relies on both electrostatically and pneumatically passing charged particles to the display panel so that a uniform coating takes place both on the flat surfaces and on the curved surfaces.

Description: ROTARY ELECTROSTATIC DUSTING APPARATUS AND METHOD The present invention is directed to a novel technique and apparatus dusting particles, such as phosphor particles, onto inner screen surface of display devices, e.g. of cathode ray tubes, such as used in television. More particularly, the present invention provides a technique and apparatus for depositing the particles, e.g. phosphor particles onto such inner display screens by rotating a discharge spray unit in both electrostatic and pneumatic fields so that an improved overall uniformity phosphor particle deposits occur. A significant problem in display systems using cathode ray tubes, especially in colour television systems, involve the provision of a uniform layer of phosphors on the inner face of the display screen. In particular, problems of uneven coverage at edges and corners of the display screen have traditionally occurred. While various attempts at achieving uniform displays screens have been tried in the past, see European Patent Application No. 0 647 959, for example, these prior attempts have failed to achieve the results necessary in the field. The latter reference is directed at xerox-type devices, using a panel instead of paper and phosphors instead of ink. This type of device also uses a Tribo-charging system which is insufficient because significant drift occurs. Also, such a device requires a separate layer of conductive material and exposure through a mask before applying the phosphor. The present invention is directed to a technique of more uniformly depositing particles, e.g. phosphor particles, onto a display screen, as well as an apparatus for carrying out this technique in which both electrostatic and pneumatic fields are used. The technique according to the present invention comprises the steps of fixing a display panel onto a spray camber having a surface the display panel, establishing a potential difference between the display panel and the surface facing the display panel to form an electric field between the display panel and the said surface, providing an air flow 2 comprising charged particles into the spray chamber via at least one outlet of an electrostatic spray gun at a velocity sufficient to fill the spray chamber beneath the display panel and at a direction approximately parallel to the display panel, and electrostatically depositing, by means of the established electric field, the charged phosphor particles onto the display panel while pneumatically depositing the charged phosphor particles in the direction of the comer radii of the display panel and rotating the at least one outlet of the electrostatic spray gun about an axis transverse to the display panel. The combination of the electrostatic field between the display panel and the surface facing the display panel and the pneumatic deposition results in a more uniform deposition of the particles, especially in and near the comers and the edges of the display panel. Preferably the display panel is grounded. Although a fixed potential, not being ground, could be used, grounding the display panel reduces the risk of arcing. In a preferred embodiment air passing from the spray gun is ionized to charge the surface facing the display panel to establish a potential of said surface sufficient to form the electric field. Other charging methods can be used, but the preferred method is simple, and the risk of arcing is small. Preferably the surface facing the display panel is made of an electrically conductive material, preferably of metal. A conductive surface results in a more uniform layer and increased transfer efficiency. Preferably the electrostatic spray gun has more than one outlet through which the charged phosphor particles are passed. Although one outlet can be used, using more than one outlet results in a more uniform deposition of the phosphor layer. At the end of the deposition in embodiments of the method according to the invention the feeding particles from the spray gun is stopped as the rotation nears a final position while maintaining the flow of air, turning off the flow of air when the final position is reached, and after a delay removing the coated panel from the spray chamber. Maintaining the air flow for some time after the feeding of phosphor particles is stopped cleans the spray gun from particles, reducing the risk that the outlets become plugged up. Removing the panel after a delay gives the particles time to settle, thus reducing the occurrence of phosphor dust around the spray chamber when the display panel removed. The electro-static gun can be fixed in the spray chamber, or alternatively can be inserted into the spray chamber at the beginning of each deposition process. Preferably the particles are charged in the electrostatic gun. The distance between the point where the particles are charges and the point where the particles enter the spray chamber is then short. The longer said distance the larger the change that said particle are discharged. The technique of the present invention may be carried out by forming a negative voltage of -20 to -40 kV on the metal plate, while maintaining the display panel at ground potential. The charged particles may be passed into the spray chamber through spray nozzle openings of the spray gun at velocities upto about 170 feet per second (fps). In a preferred embodiment of the present invention, the outlets of the nozzle openings of the spray gun are rotated 360° about its axis from a start position to a stop position, and then the rotation is reversed back to the start position. However, upon the use of a sliding electrical connection to the spray gun, for example, then continuous rotation may be carried out. In the maintaining of the flow of air into the spray chamber after stopping the feeding of particles from the spray gun, a cleaning of the system is achieved before beginning the deposition onto the inner surface of a new display panel in a continuous operation. In this respect, at least 300 coated panels per hour may be manufactured and achieved by using this technique of the present invention. In a particular benefit of the present invention, die simultaneous electrostatic deposition of the charged phosphor particles onto the inner surface of the display panel while pneumatically depositing the charged particles in the direction of the comer radii of the display panel achieves a significant improvement over the prior art types of phosphor formation on display panels or windows because a much more uniform deposition of the charged phosphor particles has occurred by the present invention. The present invention is, however, also advantageously applicable for deposition other chargeable particles, for instance for deposition (colour) filter coatings. The present invention further is directed to an apparatus for accomplishing this deposition of phosphors onto the inner surface of a display window. The apparatus according to the present invention comprises the structure of a spray chamber having an open side and an oppositely disposed surface, preferably a metal plate, an electrostatic spray gun disposed through the opposing surface directed towards the open face, a means for disposing a display window to close the open side, a means for establishing a potential difference between the display window and the opposing surface, a means for passing charged phosphor particles in ionized air into the chamber approximately parallel to the display window at a velocity sufficient to fill the chamber beneath the display window, a means for rotating the means for passing the charged phosphor particles about an axis from a first position t a second stop position, a means for stopping the charged particles phosphor particles from entering the spray chamber when when nearing the second stop position, and a means for stopping the air flow from the spray gun at the second stop position. Preferably the apparatus also comprises energizing means for ionizing particles in the spray chamber by corona charging of surrounding surfaces of the spray chamber with the opposing surface being at a voltage sufficient to form a, preferably substantially uniform, repulsive electric field to the charged phosphor particles. In a particular feature of the present apparatus, the display window is provided at ground potential, while a very high negative voltage on the metal plate forms structure for electrostatically depositing the charged phosphor particles onto the inner surface of the display window. Further in this structure, the air flow with the charged particles from the spray nozzles is in such a manner as to cause a pneumatic spray of the charged phosphor particles into the comer radii of the inner surface of the display window. In this manner, a significant uniform coating of the inner surface of the display window occurs according to the present invention. In the present invention, the structure of the spray gun involves the passing of phosphor particles mixed in an air stream past a high voltage electrode of the spray gun to form electrical charge on the phosphor particles. Atomizing air mixes with the phosphor particle-air flow to further pass the charged phosphor particles into the spray chamber in a direction parallel to both the metal plate and the display window. The flow of the highly charged phosphor particles in air from the openings of the spray nozzles is parallel to me metal plate and the display window so as to strike the sides of the spray chamber and be pneumatically directed upwardly toward the radii at the comers of the display window. In this manner, the simultaneous electrostatic deposition and the pneumatic deposition of the charged phosphor particles onto the inner display window surface creates a very uniform deposition according to the present invention. The energizing structure for ionizing the phosphor particles in the spray chamber by corona charging involves the use of the high voltage electrode in the electrostatic spray gun, which electrode is at about 80 kV. The phosphor particles pass out of the spray nozzle at a high velocity of about 170 fps in order to fill the space of the spray chamber with the charged phosphor particles in ionized air so as to electrostatically deposit on the display window. In one embodiment of the present invention, the means for rotating the electrostatic spray gun first rotates the spray gun for at least about 360° about the axis of rotation from a first position, and then reverses the rotation back to the first position. However, according to the present invention, the spray gun may be continuously rotated about its axis of rotation beyond the about 360° position from the first position until a second stop position is reached where a uniform coating of all parts of the inner surface of the display window has occurred, including both the flat inner surfaces and the curved inner surfaces of the display window. The charged phosphor particles are stopped from the spray nozzles when the rotation is about 30° to 90° from the stop position. This may occur in a time of about one second from the stop position and enables the air flow, flowing into the spray chamber, to virtually clear the atmosphere in the chamber of the charged phosphor particles so that very little subsequent cleaning of the spray chamber is necessary. In this manner a high number of panel members may be coated on inner surfaces in a given period of time, such as, for example, about 300 pieces per hour. The single drawing figure in this application shows the structure of the presently claimed apparatus for carrying out the technique of the present invention. The drawing figure is schematic and not to scale in order to show the technique of the operation of the apparatus. The arrangement and operation of the present invention may be seen by reference to the single drawing figure. In this device, a spray chamber 1 is generally shown having a rectangular or oblong shaped bottom of a metal plate 2 with side walls 13 surrounding the metal plate 2. The spray chamber 1 is closed by attaching the panel or window 4 to be interiorly coated onto the side walls 13. Centrally located through the metal plate 2 is a spray gun arrangement 3, which may be Nordson Verga type spray gun. A high voltage (about 80 kV) electrode 5, activated by a voltage supply 6, extends in the spray gun 3 to a location just below the spray nozzle 7. The spray nozzle 7 has spray openings located around the circumference of the spray gun at its upper end. The phosphors to be coated onto the inner surface of the panel 4 pass into the spray gun 3 through the tube 9, together with a mixture of air both from the tube 9 and separately through the tube 10. This phosphor particle/air mixture passes by the high voltage electrode 5 to be ionized before passing into the spray chamber 1. The ionized phosphor and air particles fill the spray chamber 1 and are both electrostatically passed to the panel 4 and pneumatically passed to the inner comers of the panel 4, as may be seen by the arrows in the drawing figure. The metal plate 2 is at a very high negative voltage, ie. -20 to -40 kV, while the panel 4 is grounded, thus establishing a very high electrostatic field on the phosphor ions in the direction of the panel. The air stream out of the nozzles 7 is at a sufficiently high velocity, ie. 170 feet per second, to cause the phosphor particles initially coming out of the nozzle to strike the sidewalls 13 and be pneumatically deflected upwardly toward the comers of the panel 4. In this manner, the phosphor particles strike the inner comer faces of the panel 4 and coat them thoroughly. The high electric fields in the chamber cause the ionized phosphor particles to stick completely over the inner surface to complete the coating. To further uniformly coat the inner surface of the panel 4, the spray gun or the spray nozzle is rotated by way of a drive gear 8. In this manner the ionized phosphor particles and ionized air molecules continually fill the spray chamber in a uniform cloud so that all surfaces of the inner surface of the panel 4 may be uniformly coated. The ionized particles are at a potential of about -50 kV to be attracted to the grounded potential of the panel 4. The flow of the phosphor particles and air from the spray nozzles 7 is parallel to both the surface of the metal plate 2 and the panel 4. This parallel flow enables the phosphor particles to be pneumatically directed upwardly into the comers of the panel 4. Also, since the flow of particles is not directly at the panel, non-uniform powder delivery does not show up directly on the panel. In operation of the present invention the panel 4 is initially coated with a standard photoresist to aid in the subsequent attachment of the phosphor particles. Such photoresist may be a water soluble polymer, such as polyvinylalcohol dissolved in water. This photosensitive coating becomes electrically conductive in the electrostatic field at ground potential, aiding in the attraction and sticking of the ionized phosphor particles to the panel. Thereafter, power is turned on to the spray gun to a high voltage, ie. 80 kV, and this high voltage ionizes the air around the electrode 5. The ionized air begins to charge all of the nearby surfaces, including the metal plate 2 at the bottom of the spray chamber 1. The high negative field, ie. about -20 to -40 kV, on the metal plate 2 forms a uniform repulsive electric field to help direct the ionized phosphor particles to the panel 4. This operation is done in a clean environment for the protection of dirt, lint or other particles from being ionized in the spray chamber. Next, the spray gun 3, or the spray nozzle 7, begins to rotate and air at 7 to 12 cubic feet per minute and phosphor particles at 1 to 3 grams per second are fed through the gun past the high voltage electrode 5. The air and phosphor particles are then highly charged and ionized and shot out of me spray nozzles 7 at a high velocity, ie. 170 feet per minute. The charged particles and air stream strike the side walls and bottom of the spray chamber, slow down, and spread out, moving to fill the chamber and the area under the panel 4 in an ionized cloud. The pneumatic air flow pattern cause me particles to strike the side walls and be deflected upwardly into the internal comer radii of the panel at the areas 11, while the electrostatic fields between the metal plate 2 and the panel 4 move the phosphor particles to the grounded inner surface having the wetted photoresist to be stuck thereon. During this electrostatic deposition, the ionized phosphor particles are both being repelled by the metal plate 2 and attracted to the grounded panel 4. As this occurs, the spray gun, or the spray nozzle, is rotated in one direction. This rotation is for at least 360° and beyond to form a continuous cloud of charged particles being attracted or pneumatically pushed to the panel 4. In one process, me rotation is continuous and the particles are sprayed into the chamber until about 30° to about 90° from a stop position, depending on the type of phosphor particles and photoresist being used. At that point the phosphor particles are stopped from being fed into the spray gun and the air flow is continued to clean residual particles out of the feed hose 9 and to circulate the remaining particles in the spray chamber until they are attracted to the panel 4. When the spray gun or spray nozzles reach a stop position, the air flow is turned off. Subsequently, the ground contacts 12 are removed from the panel 4. Then the panel is removed from the spray chamber and spray gun. A next panel is then attached, and the process starts over again. Some 300 panel per hour can be coated this way, far exceeding conventional processes. Alternatively, the rotation of the spray nozzles, during the spraying of the charged particles, may be for only about 360° and then the rotation stopped. Thereafter, the rotation is reversed back to the start position under the same stop conditions as described above for stopping the feeding of phosphor particles and air flow. The invention has been exemplified by means of the above given examples. Many variations are possible. The display window can be for instance a display window of a cathode ray tube. The invention is, however, not restricted to such a type of display window, a display window of for instance a plasma display apparams (PDP) can also be provided with phosphor using the method of the invention and/or the apparatus according to the invention. The term "air" is not to be interpreted narrowly, but simply to indicated a carrier gas, although, it is remarked, air in the common sense of the word, is preferably used as the carrier gas, being inexpensive and easily available. In summary the invention relates to a technique for depositing particles, especially bot not exclusively phosphor particles, onto a display panel and an apparatus for carrying out this technique. The technique relies on both electrostatically and pneumatically passing charged particles to the display panel so that a coating with improved uniformity takes place both on flat surfaces and on curved surfaces.

Claims: CLAIMS: 1. A method for depositing particles onto a display surface comprising the steps of: a) fixing a display panel onto a spray chamber having a surface facing the display panel b) establishing a potential difference between the display panel and the surface facing the display panel to form an electric field between the display panel and the said surface c) providing an air flow comprising charged particles into the spray chamber via at least one outlet of an electrostatic spray gun at a velocity sufficient to fill the spray chamber beneath the display panel and at a direction approximately parallel to the display panel d) electrostatically depositing, by means of the established electric field, the charged particles onto the display panel while simultaneously pneumatically depositing the charged particles in the direction of the comer radii of the display panel and rotating the at least one outlet of the electrostatic spray gun about an axis transverse to the display panel. 2. A method as claimed in claim 1 characterized in that the display panel is grounded. 3. A method as claimed in claim 1 or 2, characterized in that air passing from the spray gun is ionized to charge the surface facing the display panel to establish a potential of said surface sufficient to form the electric field. 4. A method as claimed in 1, 2 or 3, characterized in that the surface facing the display panel is made of an electrically conductive material, preferably of metal. 5. A method as claimed in claim 1, 2, 3 or 4, characterized in that the electrostatic spray gun has outlets through which the charged particles are passed. 6. A method as claimed in claim 1, 2, 3, 4 or 5, characterized in that the feeding of particles from the spray gun is stopped as the rotation nears a final position while mamtaining the flow of air, rning off the flow of air when the final position is reached, and after a delay removing the coated panel from the spray chamber. 7. A mediod as claimed in any of the preceding claims, characterized in mat said charged particles are passed into said spray chamber at velocities of up to about 170 feet per second (55 m/sec). 8. A method as claimed in any of the preceding claims, characterized in that me at least one is or the outlets are continuously rotated at least 360° and beyond a stop position. 9. A method according to claim 8, characterized in that feeding said charged particles is stopped at about 30° to 90° from said stop position. 10. A method according to any of the preceding claims, characterized in that the at least one outlet is or the outlets are rotated at least 360° in one direction, stopped and reversed in rotation back to the start position. 11. A method as claimed in any of the preceding claims, characterized in that the particles are phosphor particles. 12. An apparatus for depositing particles onto a display surface comprising: a) a spray chamber having an open side and an oppositely disposed surface, b) an electrostatic spray gun disposed dirough the opposing surface directed towards the open face, c) a means for disposing a display window to close the open side, d) a means for establishing a potential difference between the display window and the opposing surface e) a means for passing charged particles in ionized air into the chamber approximately parallel to the display window at a velocity sufficient to fill the chamber beneam the display window, f) a means for rotating the means for passing me charged particles about an axis from a first position to a second stop position. g) a means for stopping the charged particles from entering the spray chamber when nearing the second stop position, and h) a means for stopping the air flow from the spray gun at the second stop position. 13. An apparams as claimed in claim 11, characterized in that the apparams comprises an energizing means for ionizing particles in the spray chamber by corona charging of surrounding surfaces of the spray chamber with the opposing surface being at a voltage sufficient to form a repulsive electric field to particles. 14. An apparatus as claimed in claim 11 or 12, characterized in mat the means for establishing a potential difference establish, in operation, a potential of -20 to -40 kV at the opposing surface and a ground potential at the display window. 15. An apparams as claimed in claim 11, 12, 13, characterized in that the apparams comprises a means for pneumatically spraying charged particles.

Inventors: GIELINCKI GARY S, STACHELHAUS GUSTAV A, GIELINCKI, GARY, S., STACHELHAUS, GUSTAV, A.
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1996
Main classification: B05B5
IPC codes: B05B5, B05B13, H01J9