Abstract:
A powder spraying gun generates a desired pattern of electrostatically charged particles for coating a workpiece without rotating parts or particle deflectors. The powder pattern is generated with a funnel-shaped output in conjunction with air introduced into a powder charging chamber of the gun in a tangential swirling motion. The swirling air is additionally used to purge agglomerated powder particles from the charging electrodes in the charging chamber. The charging chamber surface is fashioned from material exhibiting low friction or high resistance to powder impact fusion.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates generally to spray guns for charging and distributing powders, such as electrostatically-charged powder paint particles, for deposition on the surface of a workpiece.  
           [0002]    Conventional powder applicators are based on exterior electrostatic charging of a dispersed particle cloud as partly described in U.S. Pat. No. 5,711,489. This patent also describes means for improving the particle dispersion by a rotating airstream in the interior of the gun, as well as temperature and humidity control of the powder feeding airstream. Other conventional powder applicators are based on rotating bell cup principles such as described in U.S. Pat. No. 5,353,995.  
           [0003]    U.S. Pat. No. 6,254,684 describes an internally charged powder spraying applicator wherein the powder is pre-charged in the interior charging chamber of the gun. The process of interior charging requires interior high voltage electrodes and at least one ground electrode. The &#39;684 patent discloses a first design in which a round powder cloud pattern is produced by means of a round conical deflector and a second approach wherein a flat spray pattern is generated by means of a slotted nozzle. Generation of a rounded powder cloud is important in cases where a robot or some other reciprocating machine is used to move an applicator around or inside of the painted workpiece object. The cloud generator in the &#39;684 patent has some disadvantages regarding contamination of the deflector by paint particles which leads to coating defects on the workpiece due to dripping of powder agglomerates on the surface of the workpiece. Generation of a flat spray pattern is less subject to contamination and is more widely used for flat workpiece surfaces. However, a flat pattern is more difficult to use for curved workpiece surfaces and for robotic applications, in that this design approach requires more robot arm reorientations when programming robot strokes for effecting desired surface covering.  
           [0004]    U.S. Pat. No. 6,053,420 discloses a conical powder dispersing unit based on a tangential air/powder mixture flow which provides a round powder cloud spray pattern, yet avoids use of a deflector in the direction of the powder flow. While this approach provided an improvement to U.S. Pat. No. 5,711,489, it has nevertheless been limited to cone sizes of 50 to 170 mm. diameter which is rather large for robotic applications. Additionally, at this size, the powder cloud becomes rather “soft” in order to be moved by a robot arm. The approach disclosed in the &#39;420 patent additionally anticipated a direct feeding from a fluidized powder bed feeder in a dense powder flow directly through a relatively small orifice.  
           [0005]    German Published Patent Application No. 19614193 describes the combination of interior or exterior powder charging combined with exterior tangential swirl flow which is intended to produce a softer rotating round pattern powder cloud while avoiding use of deflectors in the powder stream.  
           [0006]    There is seen, therefore, to be a need in the art for a powder applicator with the capability for utilizing shaping air rather than deflectors, yet have the capability to maintain the powder/air mixture in a more intense motion.  
         SUMMARY OF THE INVENTION  
         [0007]    Accordingly, in one aspect of the invention, a powder spraying gun for electrostatic powder coating application includes a gun body having an interior powder charging chamber defining a surface extending along an axis of the gun body and fashioned from a material exhibiting resistance to powder impact fusion. A powder supply input in fluid communication with the powder charging chamber at a first end thereof and extending at an angle to the axis of the gun different from 90° supplies the powder particles to the charging chamber. An output chamber having a funnel-shaped output is in fluid communication with the powder charging chamber at a second end thereof. A ground electrode extends into the first end of the powder charging chamber and a plurality of interior charging electrodes radially extend into the powder charging chamber upstream of the output chamber.  
           [0008]    In another aspect of the invention, a powder spraying gun for electrostatic powder coating application includes a gun body having an interior powder charging chamber defining a surface extending along an axis of the gun body. A powder supply input in fluid communication with the powder charging chamber at a first end thereof and extending at an angle to the axis of the gun different from 90° supplies the powder particles to the charging chamber. An output chamber having a funnel-shaped output is in fluid communication with the powder charging chamber at a second end thereof. A ground electrode extends into the first end of the powder charging chamber and a plurality of interior charging electrodes radially extend into the powder charging chamber upstream of the output chamber. A compressed air inlet is adapted for coupling a source of compressed air to a plurality of air conduits each tangentially opening at the surface of the powder charging chamber between pairs of the plurality of interior charging electrodes and for introducing air in a swirling pattern into the charging chamber for imparting a swirling motion to powder particles and for purging powder particles adhering to exposed surfaces of the interior charging electrodes.  
           [0009]    In yet another aspect of the invention, a powder spraying gun for electrostatic powder coating application includes a gun body having an interior powder charging chamber defining a surface extending along an axis of the gun body and fashioned from a material exhibiting resistance to powder impact fusion. A powder supply input in fluid communication with the powder charging chamber at a first end thereof and extending at an angle to the axis of the gun different from 90° supplies the powder particles to the charging chamber. An output chamber having a funnel-shaped output is in fluid communication with the powder charging chamber at a second end thereof. A ground electrode extends into the first end of the powder charging chamber. A plurality of exterior charging electrodes extend through the funnel-shaped output. A compressed air inlet is adapted for coupling a source of compressed air to a plurality of air conduits each tangentially opening at the surface of the powder charging chamber for introducing air in a swirling pattern into the charging chamber for imparting a swirling motion to powder particles. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0010]    The objects and features of the invention will become apparent from a reading of a detailed description, taken in conjunction with the drawing, in which:  
         [0011]    [0011]FIG. 1 is a perspective view of a powder spray gun arranged in accordance with the principles of the invention;  
         [0012]    [0012]FIG. 2 is a longitudinal cross-sectional view of FIG. 1; and  
         [0013]    [0013]FIG. 3 is a radial cross-sectional view of the spray gun of FIG. 2 taken in the vicinity of the interior charging electrodes of the gun. 
     
    
     DETAILED DESCRIPTION  
       [0014]    With the arrangement to be described below, a powder paint applicator will use internal pre-charging of the powder in a chamber having a diameter substantially reduced over that of the prior art in order to maintain the powder/air mixture in a more intense motion.  
         [0015]    With reference to FIGS.  1 - 3 , a powder spraying gun  100  for electrostatic powder coating application has an elongate gun body  106  extending along a longitudinal axis towards an output chamber comprised of a swirl bell cup  104  held in a cup retainer  102 . A powder/air feed mixture from a powder supply enters the gun body at inlet  108 .  
         [0016]    As seen more clearly from FIG. 2, gun  100  has its applicator housing  106  enclosing both a high voltage cascade  206  and a powder charging chamber  202  which provides a chamber surface  205  defined principally by a removable insert  204  fashioned from a low friction material which is resistant to powder impact fusion. Examples of such a suitable material are commercially available plastics.  
         [0017]    A first inlet end of powder charging chamber  202  is in fluid communication with powder/air mixture supply conduit  108 . Input  108  has a longitudinal axis which intersects the longitudinal axis of chamber  202  at an angle other than 90°, preferably at an angle on the order of 75°.  
         [0018]    The inlet end of chamber  202  is also in fluid communication, via an aperture  227 , with a ground electrode  224  which extends substantially along the longitudinal axis of chamber  202  from a first end of gun body  106  at a ground electrode purge air inlet  220  to an electrode tip adjacent aperture  227 . Electrode  224  comprises a hollow tube-type arrangement which enables introduction of purge air at inlet  220  to flow along the interior of the tube portion of the electrode  224  to at least one purge air aperture  226  located in the cylindrical surface of the electrode and exiting the aperture so as to purge powder particles adhering to the head of electrode  224 . Purge air entering the charging chamber  202  at aperture  227  assists in propelling powder particles entering at input  108  along the axis of the chamber  202 .  
         [0019]    Additionally located at the first end of gun body or housing  106  is a swirl air inlet  222  adapted to be coupled to a source of compressed air for direction into the gun body to a point around the circumference of the charging chamber  202  in the vicinity of interior charging electrodes  302   a - f  (FIG. 3). This compressed air conduit extending from air inlet  222  of FIG. 2 is shown in FIG. 3 as  308 . From  308 , the air is directed through a gap between insert  204  and the gun body through a plurality of air slots  309   a - f  formed in electrode mounting ring  304 , which is fashioned from electrically conductive plastic. Air slots  309   a - f  direct the compressed air into a groove  307  formed on the interior surface of ring  304 . Groove  307 , in turn, causes the air to enter air conduits  306   a - f  which causes the air to be tangentially directed into charging chamber  202 . The plurality of tangential conduits is equal in number to the plurality of interior needle charging electrodes  302   a - f.  In the example shown in FIG. 3, there are six needle electrodes and six tangential air conduits.  
         [0020]    The interior needle electrodes  302  radially enter chamber  202  via conductive plastic mounting ring  304 .  
         [0021]    An important feature of air conduits  306   a - f  is the simultaneous dual function of same to (a) impart the desired swirling motion to the powder particles as they enter swirl bell cup  104  and (b) provide a purging air source for cleaning the portions of the needle electrodes  302  exposed to the interior of charging chamber  202 .  
         [0022]    Conductive plastic ring  304  is coupled via a high voltage conductor  208  to the high voltage cascade (or DC-to-DC voltage converter)  206  which is adapted to be coupled to a source of potential at the first end of body  106 , as best shown in FIG. 2.  
         [0023]    Swirl bell cup or output chamber  104 , in conjunction with cup retainer  102  provides an output frusto-conical wall which forms a funnel-shaped outlet forming an angle of preferably on the order of about 120° to about 180°. The funnel-shaped outlet has a diameter preferably in the range of about 25 mm. to about 70 mm.  
         [0024]    The funnel-shaped output wall is formed by a radially inward portion  105   a  contributed by the swirl cup  102  and by a radially outward portion  105   b  provided by cup retainer  102 . Hence, by switching between various sized and/or angled cup retainers, the overall dimension and/or shape of the funnel-shaped output can be varied to generate a variety of powder patterns at the gun output.  
         [0025]    Charging chamber  202  has a longitudinal length preferably on the order of about 70 to about 150 mm., while the diameter of chamber  202  lies between about 13 mm. and 20 mm., with a preferred diametrical range of on the order of 15 mm. to 17 mm.  
         [0026]    In addition to or, optionally in place of, the interior charging electrodes  302   a - f,  a plurality of exterior charging needle electrodes  214  extend from a conductive plastic ring  210  surrounding chamber  202  and then through the swirl bell cup  104  to a point exterior of the funnel-shaped outlet. This arrangement is best shown in FIG. 2. The exterior charging electrodes  214  provide for electrostatic field control of the emerging powder cloud relative to a workpiece to be coated.  
         [0027]    In operation, a powder/air mixture enters charging chamber  202  via inlet  108 , wherein via ground electrode  224  and charging electrode needles  302 , the powder is electrostatically charged while simultaneously set in motion in a swirl-type pattern due to the injected air via tangential ducts  306 . Powder movement is also assisted in a longitudinal direction by the compressed air entering ground electrode purge air inlet  220  and exiting at hole(s)  226  at the head of ground electrode  224  in the vicinity of input  108 . As the powder moves toward the outlet end of the gun chamber, the swirling air effects a desired spray pattern which is defined by controlling the ratio of the longitudinal air flow with that of the swirl pattern. The invention further contemplates varying the tangential component of air flow for generating different shapes of spray patterns and different residence times of the powder particles, thus improving charging efficiency of the resultant cloud, the width of the spray pattern and the powder transfer efficiency.  
         [0028]    With the gun arrangement as shown and described above, more uniform electrostatic coating is effected due to improved powder dispersion. Additionally, more efficient continuous cleaning of the interior charging electrodes via the tangential air entry ports improves the efficiency of the internal charging of the powder coating material.  
         [0029]    The invention has been described with respect to an exemplary embodiment and the details of same are to be taken for the sake of example only. The scope and spirit of the invention are as set forth in appropriately interpreted claims.