Abstract:
An electrostatic rotary atomizer apparatus includes a housing encasing a turbine assembly, a shaping air assembly disposed radially outwardly from the turbine assembly for fluid communication with a shaping air supply, and a charge ring mounted to the housing and adapted to be connected to an electrical power supply. The charge ring has a plurality of electrodes within the housing. Ions emitted by the charge ring are directed through shaping air passages to intersect paint droplets leaving a rotating bell cup.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of the co-pending U.S. patent application Ser. No. 11/417,368 filed May 4, 2006, which is a continuation of International application no. PCT/US2006/037068 filed Nov. 4, 2004. 
   This application claims the benefit of U.S. Provisional Application Nos. 60/517,767, filed Nov. 6, 2003, and 60/583,078, filed Jun. 25, 2004. 

   BACKGROUND OF THE INVENTION 
   The present invention relates generally to a rotary atomizer for use with paint. More particularly, the invention pertains to an electrostatic rotary atomizer with indirect particle charging for use with water based conductive paints. 
   It is common practice to charge the paint droplets emitted from a rotary atomizer without directly charging the bell cup. These atomizers are often referred to as indirect or external charge systems and usually entail externally mounted electrodes. Externally charged systems have predominately been used for painting exterior surfaces of automobile bodies but are rarely used for painting the more intricate interior areas of the body. The limitation is primarily due to the size and geometry of the electrode ring. 
   Recent developments have proposed integrating the electrodes into the outside surface of the bell housing, however some shortcomings still exist. First, the electrodes can be easily contaminated because they attract paint particles and are placed in areas on the outside of the bell in proximity to the atomized cloud of paint particles. Second, the complex geometry of the electrodes does not facilitate easy cleaning and the electrodes are not compatible with automatic cleaning systems (smooth geometry best facilitates automatic cleaning). Third, externally mounted electrodes can be moved in the vicinity of a grounded part, or a portion of the automation which may become partially grounded. The electrode system is then susceptible to discharges, high current draw, or imbalanced charging. 
   For these reasons, paint application equipment suppliers have moved to direct charge systems, especially for interior waterborne bell painting systems. These direct charging systems generally require loading a volume of paint sufficient to paint the intended part and then isolating the stored volume from ground potential. These types of paint isolation systems are also called voltage blocking devices. Unfortunately, voltage blocking systems suffer from a longer color change time, higher color change waste and have a more complex fluid delivery system due to loading and isolation requirements. 
   Placing the electrodes inside of the bell housing resolves many of the shortcomings of the indirect external electrode charging and direct charging isolation systems. More specifically, fitting the electrodes in the shaping air supply chamber within the atomizer body will keep the electrodes clean and away from external ground points; furthermore the particle charging will be more uniform and balanced amongst the electrodes. When used in conjunction with water based (conductive) paints, relatively high transfer efficiency can be achieved without requiring external electrodes or a complex voltage blocking system. Many of the benefits of a simple fluid delivery system associated with direct charge solvent based systems can be incorporated into the internal inductive charge system. 
   An additional benefit can be claimed when using solvent-based paints in addition to water-based paints. In a particular implementation of the ionized air charging method, the charging ring can be insulated. Many of the metallic components are grounded minimizing the atomizer&#39;s capacitance. Having minimized capacitance and by hiding the electrodes, the high voltage is less likely to discharge to the grounded part. This particular charging implementation could offer improvements to automatic painting systems whether water based or solvent based paints are used. 
   SUMMARY OF THE INVENTION 
   The present invention concerns an electrostatic rotary atomizer apparatus consisting of a housing encasing a turbine assembly, a shaping air assembly disposed radially outward from the turbine assembly and in fluid communication with a shaping air supply, and a charge ring connected to an electrical power supply and to the shaping air assembly. The charge ring has a plurality of electrodes extending therefrom, the electrodes being operable to charge the paint droplets released from the rotating bell cup. 
   The benefits of the internal inductive charging method and system, according to the present invention, are particularly useful for (a) painting systems used to spray the interior compartments of automobiles where externally mounted electrodes prohibit suitable articulation of the applicator relative to the surface to be painted; (b) painting systems used to spray the interior compartments of automobiles where the color change time of a direct charge system requires application rates higher than desired or additional equipment is required to make up for the loss in painting efficiency; (c) painting systems utilizing conductive paints where no color change time is allotted, such as waterborne fascia painting systems or automotive body exterior systems utilizing batch painting methods; (d) painting systems using a combination of conductive and non-conductive paints, such as plastic part manufacturers supplying several customers requiring different paint technologies; and (e) automotive body exterior systems where there is a transition period when converting from solvent based paints to the more environmentally friendly water based paints. 

   
     DESCRIPTION OF THE DRAWINGS 
     The above, as well as other advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in light of the accompanying drawings in which: 
       FIG. 1  is a cross-sectional view of a rotary atomizer in accordance with the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The entire disclosures of U.S. Provisional Application No. 60/517,767, filed Nov. 6, 2003, U.S. Provisional Application No. 60/583,078, filed Jun. 25, 2004, and U.S. patent application Ser. No. 11/417,368, filed May 4, 2006 are incorporated herein by reference. 
   Referring now to  FIG. 1 , a rotary atomizer in accordance with the present invention is indicated generally at  200 . The atomizer  200  includes a generally tubular housing  200   a . A turbine assembly  201  having a turbine stator  204  is disposed in the housing  200   a  and a turbine rotor  202  is rotatably disposed in the housing  200   a . The turbine stator  204  and the turbine rotor  202  are grounded. A fluid injector  206  is in fluid communication with a fluid supply, such as a paint source  207 , and a seal air distribution ring  208  is in fluid communication with a supply of compressed air  205   a , discussed in more detail below. A bell cup distributor  210  and an encircling bell cup  212  having an internal edge  214  are each disposed on a free end of the turbine rotor  202 . The bell cup  212  can be constructed of conductive materials, non-conductive materials, or a combination of conductive and insulating materials. A thin insulating coating may be applied to portions of a metal bell cup to improve the charging system. 
   The atomizer  200  also includes a shaping air assembly  216  disposed radially outward of the turbine assembly  201 . The shaping air assembly  216  includes a seal air inlet  218  in fluid communication with the supply of compressed air  205   a  and a shaping air inlet  220  in fluid communication with a supply of compressed air  205   b . The seal air inlet  218  extends to a seal air passage  222  for the turbine assembly  201 . The shaping air inlet  220  extends to a shaping air manifold and ionizing air nozzle assembly  224   a , which further extends through a passageway  224   b  to a plurality of shaping air control nozzles  226  spaced about an outlet end of the housing  200   a . Seal air D exits the seal air distribution ring  208  and passes between an outer surface of the bell cup  212  and an inner diameter of the housing  200   a . A charge ring  228  has a plurality of electrodes  230  extending radially within the passageway  224   b . The charge ring  228  is connected to a supply of high voltage electrical power (not shown) for charging the electrodes. The electrodes  230  are preferably needle-like in cross section and extend into the shaping air passageway  224   b  and, when subjected to high voltage potential, ions break free into the air passageway  224   b . The ions pass through the passageway  224   b  and exit the shaping air control nozzles  226  during operation of the atomizer  200 . 
   A relatively high velocity air is directed through the shaping air control nozzles  226 . The shaping air control nozzles  226  direct the ionized air adjacent to the bell cup  212 . The turbine rotor  202  and the turbine stator  204  rotate the bell cup  212  and the bell cup distributor  210  at sufficient velocity to atomize the paint. The fluid injector  206  directs the paint supply stream into the center of the bell cup distributor  210 . Streams of shaping air flowing in a direction C and seal air in the direction D, along with the geometrical position of the air control orifices, guide the ion stream direction. The forward direction of the air flow makes it difficult for atomized paint droplets to travel rearward and accumulate on the electrodes. Controlling the distance between the electrodes  230  and the grounded elements of the rotary atomizer optimizes the ion generation. Further optimization is achieved by setting the proper high voltage level of the power supply connected to the charge ring  228 , providing the proper number of electrodes  230 , controlling the geometry of the electrodes  230 , and by adjusting the volume and the velocity of the air directed through the shaping or seal air passageways. Insulating material such as a non-conductive labyrinth can be added between the electrodes  230  and the bell cup  212  to optimize the electrostatic field. Various shaping air assemblies  216  can be provided depending on the application method, i.e. shape air flow requirement. Also, the electrodes  230  can be located in the seal air passage  222 . 
   The atomizer  200  is insulated such that electrostatic discharge or electrostatic erosion does not occur. If necessary, a non-conductive turbine rotor  202  and stator  204  can be used in conjunction with a separate grounding device (not shown) to reduce erosion or to provide suitable potential between the bell cup and the electrodes. 
   The atomizer  200  can operate in a reliable manner with similar maintenance intervals as other automotive painting class rotary atomizers. A solvent and air mixture can be injected into the seal air passage  222  and/or the portions of the shaping air passageway  224   b  to clean the parts of the bell cup  212  and/or the portions of the shaping air assembly  216  that would come in contact with the solvent and air mixture. 
   In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiment. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.