Abstract:
A rotary atomizer applies particulate paints with good color matching by reducing paint droplet size deviation and then optimizing the other paint spraying parameters. Paint droplet size parameters are reduced by using a bell cup having reduced flow deviations, including an overflow surface having a generally constant angle between a deflector and an atomizing edge.

Description:
This application claims priority to U.S. Provisional Patent Application Ser. No. 60/079,565, filed Mar. 27, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to rotary atomizers and more particularly to a rotary atomizer having improved performance for particulate paints. 
     Currently, many paints are applied by rotary atomizers to work pieces, such as automobile bodies. Rotary atomizers include a rotating bell cup having a generally conical overflow surface between a radially inward central axial opening and a radially outward atomizing edge. At or near the atomizing edge, the angle of the overflow surface relative to the axis of the bell cup decreases sharply to form a lip adjacent the atomizing edge. The purpose of this lip is to generally direct the atomized paint more axially forward and reduce radial scatter. The known atomizer bell cups further include a deflector, also of generally rotational symmetry, disposed in front of the central axial opening. Paint entering the bell cup through the central axial opening contacts the rear surface of the deflector and is disbursed radially outwardly towards the overflow surface. 
     In the known atomizer bell cups, the paint follows a tortuous, turbulent path from the nozzle to the atomizing edge. As a result, the paint flow to the atomizing edge is turbulent and fluctuates cyclically. As a result, paint from the atomizer is atomized to a wide variety of paint droplet sizes. The paint droplets can vary by up to 100 microns or more. 
     Current rotary atomizers are unable to obtain good color matching applying paints with particulates, such as mica. Generally, the mica comprise particles on the order of 3 microns by 200 microns. When this paint is applied by rotary atomizers, the mica particles are oriented generally perpendicular to the application surface. As a result, the paint has a different tint or color than intended, i.e. with the mica particles laying flat. In order to correct this problem, a second coat of the paint is typically applied with air atomized spray guns rather than rotary atomizers. This second coat provides the proper color; however, air atomized spray guns have a low transfer efficiency (approximately 50%) compared to rotary atomizers (approximately 80%). The air atomized spray guns therefore increase the amount of paint lost, increasing the cost of the paint process and cause environmental concerns regarding the disposal of the lost paint. 
     SUMMARY OF THE INVENTION 
     The present invention provides a rotary atomizer which provides improved color matching. Generally, the improved atomizer provides a more uniformed paint droplet size, which in turn facilitates control of the particulates in order to assure proper orientation of the particulates and obtain good color matching. 
     The rotary atomizer bell cup according to the present invention provides several inventive features directed toward reducing deviation in paint droplet size. First, the bell cup includes a generally conical overflow surface having a generally constant flow angle between a deflector and the atomizing edge. Further, the exposed surface area of the overflow surface is increased by decreasing the size of the deflector relative to previous bell cups in order to cause evaporation of solvent from the paint from the overflow surface. The diameter of the atomizing edge is also increased, thereby reducing the thickness of the paint film at the atomizing edge. The bell cup is designed to reduce flow deviations of the paint as it travels from the axial opening to the spray edge in order to provide laminar flow of the paint across the overflow surface and the atomizing edge. 
     The bell cup is made hollow in order to reduce the weight of the bell cup. A rear cover is secured to the rear of the bell cup body, enclosing an annular cavity. 
    
    
     BRIEF 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 the light of the accompanying scale drawings in which: 
     FIG. 1 is a scale drawing of the atomizer of the present invention; 
     FIG. 2 is a scale drawing in cross section of the atomizer of FIG. 1; 
     FIG. 3 is a scale drawing front view of the bell cup of FIG. 2; 
     FIG. 4 is a scale enlarged view of the deflector of FIG. 2; 
     FIG. 5 is a scale cross-sectional view of an alternate bell cup; 
     FIG. 6 is an enlarged scale view of the deflector in the bell cup of FIG. 5; 
     FIG. 7 is a scale bottom view of the bell cup of FIG. 5; and 
     FIG. 8 illustrates one possible layout for applying a base coat with the atomizer of FIG.  1  and the bell cup of FIG. 2 or  5 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a rotary atomizer  20  and a bell cup  22  according to the present invention. The atomizer includes a shaping air ring  23  which preferably includes  30  nozzles generally parallel to the axis of the atomizer. The shaping air ring  23  supplies shaping air, preferably at 100 liters per minute. With the reduced number of holes from the known shaping air ring (typically  40 ), this produces increased turbulence by the shaping air. 
     The bell cup  22  is shown in more detail in FIGS. 2-3. Bell cup  22  includes a central axial opening  24  at the base of the bell cup  22 . The central axial opening  24  includes a coaxial passageway onto a front surface  26  of the bell cup  22 . The front surface  26  of the bell cup  22  includes a central flat portion  28  generally perpendicular to the axis of the bell cup  22  and a generally conical overflow surface  30  from the perpendicular portion  28  to a spray edge  32 . Between the perpendicular surface  28  and the spray edge  32 , the overflow surface  30  has a smooth continuous surface of a constant flow angle α relative to the annular spray edge  32 , preferably 5-40 degrees, more preferably 26-30 degrees and most preferably 28.25 degrees. The diameter of the annular spray edge  32  is preferably 63-75 mm, and most preferably 64.6 millimeters. 
     An annular hub  33  extends rearwardly from the bell cup  22  and includes an externally threaded portion  34 . A frustoconical rear cover  35  is threaded onto the threaded portion  34  of the annular hub  33  and welded or glued to the rear of the bell cup  22  behind the spray edge  32 . As a result, the body of the bell cup  22  behind the overflow surface  26  is hollow, reducing the weight of the bell cup  22 . A concentric inner hub  36  extends rearwardly from the bell cup  22  and is externally threaded for mounting to the atomizer  20 . Other means for attaching the bell cup  22  to the atomizer  20  can also be utilized. The spray edge  32  forms a sharp edge between the overflow surface  30  and a small bevel  38  leading to the outer rear surface of the bell cup  22 . 
     If the atomizer  20  is to be used to apply basecoat, the bell cup  22  preferably comprises a titanium alloy, preferably Ti-6A1-4V. If the atomizer  20  is to be used to apply clear coat or primer, the bell cup  22  is preferably Aluminum, most preferably 6A1-4V, 6A1-25N-4Zr-2MO. If the bell cup  22  is titanium, the rear cover  35  is preferably welded to the rear of the bell cup  22  behind the spray edge  32 . If Aluminum is used, the rear cover  35  is preferably glued to the rear of the bell cup  22  behind the spray edge  32 . Small serrations may be formed on the surface  26  at the spray edge  32  for clearcoat spraying. These serrations are well known and utilized in the art. 
     Positioned in front of the central axial opening  24  is a deflector  40  which includes a rear surface  42  generally parallel to the perpendicular surface  28  of the bell cup  22  and a rear conical surface  44  which is preferably parallel to the overflow surface  30  of the bell cup  22 . The deflector  40  is preferably approximately 22.3 millimeters in diameter, and preferably approximately ⅓ of the diameter of the spray edge  32 . More particularly, the diameter of the deflector is less than  40  percent, and most preferably approximately 34.5 percent the diameter of the spray edge  32 . 
     The deflector  40  is shown in more detail in FIG. 4. A passageway  50  leads from the rear surface  42  to a front surface  52  of the deflector  40  and includes four tubular passageways  54  (two shown) leading from the rear surface  42 . The deflector  40  is retained on the bell cup  22  with a plurality, preferably  3 , press fit, barbed connectors  56  having spacers  58  preferably 0.7 millimeters wide. 
     The improved bell cup  22  provides a reduced deviation in particle size, which in turn facilitates control of the particulates. In other words, if the size of the atomized paint particles from the spray edge  32  is known, the shaping air velocity, turbulence and RPM of the bell cup  22  and paint flow can be adjusted to ensure that the particles are forced to lay flat on the painted surface by the shaping air from the shaping air ring  23 . With a reduced deviation in particle size, these parameters can be optimized for a greater percentage of the paint droplets, thereby providing better color matching. 
     The reduced deviation in particle size is a result of several inventive aspects of the bell cup  22  and deflector  40 . First, the larger annular surface  30  causes more of the solvent (such as water) to evaporate before reaching the spray edge  32 . The large diameter spray edge  32  provides a thin film of paint at the spray edge  32 . The reduced ratio of the deflector disk  40  to the spray edge  32  provides a more constant, laminar flow across the overflow surface  30  to the spray edge  32 . Because the conical surface  30  is continuous and smooth from the deflector  40  to the spray edge  32  and has a constant angle α, the paint flow rate to the spray edge is constant (i.e. does not oscillate). As a result, better control over paint particle size is achieved. Further, as can be seen in FIG. 2, the bell cup  22  of the present invention provides only three flow deviations between the central axial opening  24  and spray edge  32 , thus providing a constant, substantially laminar paint flow at the spray edge  32  and therefore a reduced deviation in particle size. 
     FIGS. 5 through 7 disclose an alternative embodiment of a bell cup  100  having a deflector  110 . This bell cup  100  provides only two flow deviations between the central axial opening  112  and the spray edge  132 . The conical portion  130  of the overflow surface extends directly from the central axial opening  112  to the spray edge  132 . Thus, the overflow surface  126  does not include a perpendicular portion (like perpendicular portion  28  of FIG.  2 ). This further improves the laminar flow of the paint and reduces further the particle size deviation. The deflector  110  includes a generally conical rear surface  144  which extends to a generally rounded central rear surface  142 , thus reducing the flow deviation for the paint. A passageway  150  leads through the deflector  110  and includes four diverging tubular passageways  151 . Alternatively, the passageways  151  may converge. The bell cup  100  can also be mounted on atomizer  20  of FIG. 1 in place of bell cup  22 . 
     FIGS. 1-7 are scale drawings. 
     FIG. 8 illustrates one potential layout of a paint spray zone  170  for applying a basecoat to a vehicle body  152  utilizing the atomizer  20  of the present invention shown in FIGS. 1-7. The vehicle body  152  travels in the direction  154  through the zone  170  while atomizers  20  apply basecoat paint. The zone  170  is a two-pass, thirteen-bell zone which would apply basecoat with good color matching with the efficiency of rotary atomizers. In known systems, the basecoat would be applied by nine rotary atomizers and six air atomizers. The length of the zone  170  could be reduced to approximately thirty feet, compared to forty-five feet for the known basecoat zones. In the zone  170 , an overhead machine  156  includes two atomizers  20  and applies a first coat to the center of the horizontal surfaces. A pair of side machines  158  preferably each oscillate an atomizer  20  the full length of the doors of the vehicle  152  on the first pass. A pair of side machines  160  each include a pair of vertically and horizontally offset atomizers each mounted on arms  161 . A first arm  161   a  provides three axes of motion to contour the pillars and paint the edge of the hood and trunk. The second arm  161   b  is fixed with pivot and horizontal capp. to process the rocker. A pair of side machines  162  provide a second pass on the doors of the vehicle  152 . A second overhead machine  164  includes three atomizers  20  to provide a second pass on the horizontal surfaces. 
     An example will be given utilizing the inventive atomizer  20  of FIGS. 1-4 in the arrangement of FIG. 8 to spray BASF Prairie Tan Metallic Solvent based paint M6818A in a two-pass bell basecoat application with the following parameters: bell cup  22  rotation: 60,000 RPM; fluid flow: 200 cc/min on a first pass and 75 cc/min on a second pass; shaping air: 200 L/min on the first pass and 50 L/min on the second pass. Preferably, any resonant frequencies of the atomizer bearing are avoided. The atomizer  20  produces reduced droplet size deviation, typically 80% of the droplets will be within an 8-50 μm size deviation. With reduced size deviation, the other parameters can be adjusted to ensure that the mica particles lie flat, thereby providing good color matching. Most preferably, the particle size deviation is reduced below 30 μm. The atomizer  20  produces improved color matching over previous bell zones. The colorimetry data for the example is: ΔL&lt;2.0, Δ&lt;1.0 and ΔB&lt;1.0. By providing good color matching with rotary atomizers rather than air atomizers, efficiency is greatly improved. 
     More generally, the bell speed rotation is preferably between 60,000 and 80,000 RPM. Also, the fluid flow of paint preferably does not exceed 250 ml/min. 
     In accordance with the provisions of the patent statutes and jurisprudence, exemplary configurations described above are considered to represent a preferred embodiment of the invention. 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.