Abstract:
An apparatus and method for forming and controlling a pattern for spraying surfaces with a fluid uses a rotary atomizer spray head having an air shaping ring with shaping air nozzles inclined in a direction of rotation of a bell cup to direct the air onto the cup surface near the cup edge. The air shape ring optimizes the shape air control to create a stable, focused pattern that minimizes robot speed while maintaining high transfer efficiency. Nozzles extending parallel to the axis of rotation of the bell cup can be provided. Selection of the shaping air flow rate produces broad, collapsed and tubular spraying patterns.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to an apparatus and method for painting surfaces and, in particular, to an apparatus and method for forming and controlling a pattern for spraying surfaces with a fluid using a rotary atomizer spray head.  
         [0002]     Improvements in painting automobile bodies and component parts continue to advance. In the area of painting exterior surfaces, robots with rotary atomizers are now being used in place of less flexible bell machines. Robots offer more flexibility and new approaches are offered to reduce paint consumption and improve film build uniformity. Robots have long been used for painting interior compartments of car bodies, including the engine compartment, door rings, and trunk compartment. Robots are now being outfitted with rotary atomizers in place of spray guns to further reduce paint usage and improve coverage. With these advances, the pattern control of the rotary atomizer is being adapted to optimize film build uniformity and finish quality while reducing paint consumption.  
         [0003]     Exterior Optimization  
         [0004]     The use of robots outfitted with rotary atomizers continues to gain popularity for painting the exterior surfaces of automobile bodies. As the trend becomes a standard for the industry, refinements in the application method continue to develop. One area of particular importance is the spray pattern geometry and painting methods described in U.S. Pat. No. 6,703,079. The velocity and direction of the shaping air imparted to the outside edge of the bell cup is the main influence on the spray pattern geometry. Higher velocities result in a smaller and more defined pattern.  
         [0005]     Prior art apparatuses include the use of less flexible manipulators where the amount of shaping air is maintained at a low level. Consequently, the single applicator cast a large pattern covering a large surface area. The broadly cast pattern used in prior art application methods has a relatively low particle speed and largely relies on the electrostatic effect to carry the atomized droplets to the grounded surface of the car body. The thickness of the deposited paint film is susceptible to surface irregularities and the dynamics of spray booth air flow. Protrusions in the surface or the edges of the panels attract more paint due to the electrostatic effect. The slow moving particles are influenced by spray booth downdraft, which affects the paint cloud resulting in poor surface uniformity.  
         [0006]     The broadly cast pattern is also inefficient when painting smaller panels as a large portion of the paint droplets are sprayed beyond the desired target area. This paint is deposited on parts of the car that do not require the decorative media; for example, the inside surfaces of the car or the underside surfaces of the car.  
         [0007]     The use of a focused pattern as opposed to a broadly cast pattern can attain improved surface finish uniformity while maintaining a relatively high transfer efficiency. The increased flexibility of the robotic method permits the application of the focused pattern of charged paint droplets to be moved across the multi contoured exterior surface. In this manner, the paint is directed more specifically to the areas needed. The higher shape air setting produces a slightly higher particle velocity that minimizes the uniformity issues associated with electrostatic attraction and spray booth airflow effects. Optimization of the shape air control can create a stable focused pattern that minimizes robot speed while obtaining high transfer efficiency. The diameter of the air holes, the spacing or number of holes, the distance from the bell cup edge and the geometry of the external surface of the bell cup edge are the main factors for controlling the pattern shape.  
         [0008]     In past practices, the shape air holes were mainly straight having either a flow vector perpendicular to the bell cup edge directed into the paint stream and not on the cup surface, or originating behind the cup with the air directed along the majority of the cup&#39;s exterior.  
         [0009]     Shape air rings have been developed with air holes angled counter to the bell cup rotation. Optimization of the shape air ring design with holes pointing against the bell cup rotation produces dual pattern types. Lower airflow velocities produce the broadly cast pattern (also called soft pattern) while higher velocities produce the focused pattern (also called vortex pattern).  
         [0010]     Past experimentation was conducted to change the method in which the second coat of metallic base coat paints was applied. Previously the second coat was applied with a spray gun to achieve the desired alignment of metal flakes, particularly with the flakes aligned parallel to the surface. Having a significantly higher transfer efficiency (TE), it was desirable to use a rotary applicator to perform the same task.  
         [0011]     Modifications to the bell cup and use of air nozzles inclined against the rotational direction of the bell cup produced desired results with a significantly improved transfer efficiency (TE) when compared to a spray gun. The process of painting with nozzles inclined against the rotation of the bell cup is well known and used extensively in the industry today. Although this method provided suitable results at lower bell cup rotational speeds, the pattern would collapse into a narrow and unstable pattern at higher rotational speeds. At higher flow rates and with very viscous materials it is necessary to seek other solutions in order to achieve the desired color and surface finish required.  
         [0012]     Interior Optimization  
         [0013]     While the stable focused pattern is desirable for exterior applications, it is sometimes necessary to further collapse the atomized paint into a very narrow tubular spray pattern in order to deposit the paint into narrow and complex surfaces such as the interior door ring. This very narrow pattern is undesirable for exterior surfaces because it could lead to striping or very high robot movement but it is very desirable to get paint into the door hinge area. For this application, it is necessary to achieve the tubular pattern at lower bell speeds in order to have high transfer efficiency. The straight hole arrangement, with the nozzles in close proximity to the bell cup edge seems to be the most effective approach to develop the very concentrated pattern geometry.  
         [0014]     With the straight shaping air hole alignment, several prior art approaches exist to create narrow pattern widths necessary for interior cut in applications. The approaches consist of: 1) high volume shaping air with holes directing air flow off the bell cup edge; 2) shaping air holes located significantly rearward of the bell cup edge with a high volume of air traveling along the length of the cup; and 3) small diameter bell cups that create narrow pattern widths. In all three approaches, the high volume of shaping air is necessary to collapse the pattern.  
         [0015]     Each of the aforementioned approaches has drawbacks. With the shaping air holes directing air into the paint stream, not landing the shaping air on the bell cup edge, the high velocity air can pierce the paint pattern. Poor uniformity can occur at higher flow rates. More air is needed causing a venturi effect near the nozzles and a small portion of the paint droplets can get drawn into a circulating pattern causing secondary atomization. Shaping air located at the extreme rear of the cup requires significantly more air flow to achieve the necessary velocity to collapse the pattern into a sufficiently tight pattern for interior cut ins. Lastly, a smaller diameter bell cup must be operated at a higher bell speed, proportional to diameter, to achieve the same amount of atomization as a larger bell cup. A higher amount of shaping air is required to assist in atomization. In all cases higher shaping air velocity causes lower transfer efficiencies; moreover, higher velocities causes re-circulation leading to poorer atomization and over spray accumulation on the applicator. It is desirable to have a nozzle that uses the minimum amount of air to attain the tubular effect needed for interior cut-in type applications.  
       SUMMARY OF THE INVENTION  
       [0016]     The present invention concerns an apparatus and method for a rotary atomizer with improved pattern control operation for both exterior and interior applications. While a single nozzle can be designed to produce the acceptable performance to cover both applications, it is unlikely that a single nozzle can offer optimized benefits of a dual ring device.  
         [0017]     The apparatus and method utilize both straight and inclined air nozzles relative to the bell cup edge with the air directed onto the cup surface near the cup edge. This provides benefits of improved pattern control.  
         [0018]     The present invention optimizes the shape air control to create a stable, focused pattern for exterior painting that minimizes robot speed while maintaining high transfer efficiency. The invention offers improved transfer efficiency and quality performance compared to prior art nozzles by directing the shape air in the direction of rotation of the bell cup. While the straight hole approach is not novel, combining a ring of straight holes with a secondary ring of holes inclined in the direction of bell cup rotation is a new approach to achieving the benefits of both application methods with the same applicator.  
         [0019]     An optimum pattern and transfer efficiency is reached for each desired application, broad, focused, or tubular, by the particular combination of hole size, inclination angle, distance from bell cup edge, and geometry of the bell cup edge. The new air shape ring of the present invention is highly efficient with respect to air consumption. Consequently, desired pattern control is achieved at relatively low shape air velocities. The broad pattern is generally achieved with the inclined holes in the 50-180 slpm (standardized liters per minute) and the collapsed pattern is achieved in the 240-400 slpm range of shaping air flow. The straight hole arrangement can achieve the tubular pattern with 200-400 slpm.  
         [0020]     The result is significant for exterior applications as both spraying methods, broad and focused, can be achieved merely by adjusting the shaping air flow rate, and the shape air direction relative to the bell cup rotation. In addition, a second ring of straight holes can be added to develop the tubular shaped spray pattern needed for interior applications. The air flow of the two rings could have separate flow control circuits. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0021]     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 drawings in which:  
         [0022]      FIG. 1  is a side elevation view in partial cross section of a bell atomizer spray head a shape air ring according to the present invention;  
         [0023]      FIG. 2  is top view of a preferred embodiment of the shape air ring according to the present invention;  
         [0024]      FIG. 3  is a side view of the apparatus shown in  FIG. 2 ;  
         [0025]      FIG. 4  is an edge view of the apparatus shown in  FIG. 2 ;  
         [0026]      FIG. 5  is an enlarged view of the shape air ring edge shown in  FIG. 4 ;  
         [0027]      FIG. 6  is an enlarged view of a first preferred embodiment of the nozzle shown in  FIG. 3 ;  
         [0028]      FIG. 7  is an enlarged view of a second preferred embodiment of the nozzle shown in  FIG. 3 ;  
         [0029]      FIG. 8  is an enlarged view of a third preferred embodiment of the nozzle shown in  FIG. 3 ;  
         [0030]      FIG. 9  is a schematic representation of a preferred method according to the present invention;  
         [0031]      FIG. 10  is a schematic representation of the resulting pattern width and film thickness according to the present invention;  
         [0032]      FIGS. 11   a  and  11   b  are prior art schematic representations of typical shaping air flow; and  
         [0033]      FIGS. 12 and 13  are prior art schematic representations of the resulting pattern width and film thickness of the typical shaping air flow shown in  FIGS. 11   a  and  11   b , respectively. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0034]     Referring now to  FIG. 1 , a bell atomizer spray head is indicated generally at  20  adapted to be mounted at the end of a robot arm. The bell atomizer spray head  20  includes a generally cylindrical outer cover, shroud or housing  22  that encloses a drive motor  23  such as a magnetic air bearing turbine. The turbine  23  drives in rotation a generally frustroconical atomizing bell cup  24  positioned in an open end of the cover  22 . The atomizing bell cup  24  is supplied with paint through a central opening connected to a fluid injector  25  that extends through the turbine  23 . When the atomizing bell cup  24  is rotated by the turbine  23  and paint is supplied through the injector  25  through a supply line  27 , a fluid stream (not shown) enters the center of the bell cup  24  and covers an interior surface flowing to an outer edge  26  where the paint is released into the surrounding air in atomized form.  
         [0035]     The spray head  20  is connected to a robot wrist  28  through which the supply line  27  extends. The robot wrist  28  may be angled, as shown, or it may be a straight connector (not shown.). The robot wrist  28  is typically attached to a robot arm (not shown). The supply line  27  can be connected to a paint supply, such as a canister (not shown) carried by the robot arm. Alternatively, the supply line  27  is connected to a remote manifold (not shown) connected to storage tanks of a single type of or different color paints.  
         [0036]     Attached to a forward end of the cover  22  is a generally tubular shaping air assembly  29  that terminates adjacent an outer surface of the bell cup  24  near the outer edge  26  thereof. A plurality of air passages  30  are formed in the assembly  29  each having at one end a hole or slot outlet  31  facing the outer surface of the bell  24  and directed generally toward the edge  26 . The shaping air passages  30  are connected to a shaping air supply line  32  that extends through the robot wrist  28  to a shaping air supply (not shown) providing pressured air. The shaping air exiting the outlets  31  passes through a shaping air ring  34 , directing the atomized paint in a desired pattern toward the object to be painted. The shaping air ring  34  is secured at one end  36  to the housing  22  and the opposite end  38  extends toward the annular outer edge  26  of the bell cup  24 . The shaping air ring  34  is preferably located at a point rearward of the bell cup annular outer edge  26 .  
         [0037]     With reference to  FIGS. 2-8 , the shaping air ring  34  of the present invention is there shown and includes an annular hollow shaping air ring  40  having at least one nozzle  42  with a hole  44  extending from an inlet to an outlet for directing shaping air from the housing  22  through the nozzle  42  toward the annular outer edge  26  of the bell cup  24 . The nozzle  42  is preferably positioned adjacent an exterior surface  46  and an outer edge  48  ( FIG. 1 ) of the bell cup  24 .  
         [0038]     The hollow air shaping ring  40  is preferably formed to fit within the slot outlets  31  provided about the air passages  30  of the tubular shaping air assembly  29 . The hollow air shaping ring  40  is provided with a square slot  50  and an angled slot  52  for slip fitting about slot outlets  31 . The outer edge  54  of the ring  40  is larger than the inner edge  56  of the ring  40  to provide a tight fit about the outwardly angled edge  58  ( FIG. 1 ) of the tubular shaping air assembly  29 . At least one air passageway  60  is provided along the inner edge  56  for receiving shaping air from shaping air passages  30  connected to the shaping air supply line  32 . The air passageway  60  directs the shaping air about the hollow air shaping ring  40  and through the nozzle  42 . The nozzle  42  is preferably one of a plurality of a set number of nozzles  42  spaced in a set pattern along the hollow air shaping ring  40 . Air passageways  60  correspond to this set pattern and act as a manifold  61  ( FIG. 9 ) for supplying air to the set number of nozzles  42 .  
         [0039]     With reference to  FIGS. 6-9 , in a first preferred embodiment, the nozzle  42  is the hole  44  inclined in the rotation direction of the bell cup  24 . Alternatively, the nozzle  42  may preferably be a hole  46  extending perpendicular to the rotation direction of the bell cup  24 . Other additional embodiments include alternating angled  44  and perpendicular  46  nozzles, or two separate and distinct hollow air shaping rings  40 , where a first annular ring  40 ′ is secured at one end to the housing  22  and the opposite end extends toward the bell cup  24 . This first ring  40 ′ includes nozzles  42  extending in one direction, perpendicular or angled while a second ring  40 ″, secured to the interior of the first ring  40 ′, includes nozzles  42  extending in the opposing direction set by the first ring  40 ′. Therefore, if the first ring  40 ′ includes nozzles  42  with the holes  46  extending perpendicular to the rotation of the bell cup  24 , then the second ring  40 ″ preferably includes nozzles  42  with the holes  44  extending at an angle to the rotation direction of the bell cup  24 . With all of the embodiments, the angle of the nozzle  42  may be in either direction of incline from the horizontal exterior surface  62  of the hollow air shaping ring  40  regardless of the direction of rotation of the bell cup  24 .  
         [0040]      FIG. 9  illustrates a method for switching between the shaping air formed by first ring  40 ′ and second ring  40 ″. The air supply  27  is provided through the shaping air supply line  32  to valves  64 ,  66 . Valve  64  supplies the shaping air to the first air ring  40 ′ via its manifold  61 . Valve  66  supplies the shaping air to the second air ring  40 ″ via its manifold  61 . In this way, one or both rings  40 ′,  40 ″ may be activated to form and control the desired spray pattern.  
         [0041]     The preferred method for forming and controlling a pattern for spraying surfaces with a fluid using a rotary atomizer spray head  20  of the present invention is to provide a shaping air ring  40  with at least one nozzle  42  in the shaping air ring assembly of the rotary atomizer spray head. The nozzle  42  is preferably positioned adjacent the exterior surface and the outer edge  26  of the bell cup  24  of the rotary atomizer spray head. To optimize the pattern width for the surface to be sprayed, the outer diameter of the annual outer edge  26  of the bell cup  24  is adjusted along with the outer diameter of the nozzle  42  in the air shaping ring  40 .  
         [0042]     In a preferred embodiment, the shaping air ring  40  is located at a point rearward from the bell cup edge  26  of 2 mm. In a second preferred embodiment the shaping air ring  40  is located at a point rearward from the bell cup edge  26  of 20 mm. Depending on the preferred pattern, the shaping air ring  40  is preferably located at a point rearward from the bell cup edge  26  anywhere in the range of 2 to 20 mm with the nozzle hole diameters ranging from 0.4 to 1.0 mm and the bell cup diameter ranging between 40 mm to 120 mm.  
         [0043]     Determining alignment of the nozzle  42  relative to the horizontal edge  62  and the rotation of the bell cup  24  is necessary for optimum surface finish uniformity relative to the type of surface to be painted—whether an interior surface generally, or an edge surface, such as an automotive door edge specifically. In a preferred embodiment, the alignment of the nozzle may be perpendicular to the horizontal edge and rotation of the bell cup. Alternatively, the nozzle may be angled from the horizontal edge in either direction. In still another preferred embodiment, the shaping air ring  40  may include both perpendicular and angled nozzles. Additionally, two air rings may be provided,  40 ′,  40 ″, each ring having opposite nozzle shapes, providing alternate use of an angled or perpendicular air shaping or simultaneous use of both air shaping flows.  
         [0044]     In a preferred embodiment, the number of nozzles forming a set about the air shaping ring of the present invention is within a range of 30 to 120 per ring with a preferred shaping air rate between 50 to 1000 slpm.  
         [0045]     With reference to  FIGS. 10-13 , shaping air flow and resulting pattern width of fluid relative to film thickness are there shown. As previously discussed above, prior art shaping air flow shape air holes were mainly straight having a flow vector perpendicular to the bell cup edge directed into the paint stream and not on the cup surface, or originating behind the cup with the air directed along the majority of the cup&#39;s exterior as shown in  FIGS. 11   a  and  11   b.    
         [0046]     The progression of shaping air velocity for prior art straight nozzle alignment is shown in  FIG. 12 . This prior art shaping air flow results in the film build geometry shown. Low shaping air (SA) produces a wide pattern with some concavity in the center. As the shape air is increased an optimum broad pattern is attained. A further increase to the shape air velocity collapses the pattern and produces a center-weighted pattern. Too much paint in the center of the pattern is not optimal. Even narrow overlapping will produce a nonuniform film build. Moreover, tight overlap requires a high robot speed that has an adverse effect on the pattern stability.  
         [0047]     The progression of shaping air velocity for prior art angled nozzle alignment is shown in  FIG. 13 . This prior art shaping results in this method produced a higher transfer efficiency than the prior straight nozzle alignment but can only be used with a broadly cast pattern.  
         [0048]     The present invention of shaping air velocity for both straight and angled nozzle alignment is shown in  FIG. 10 . Placing the shaping air nozzles in close proximity and perpendicular to the bell cup edge such that the air impacts the cup near the edge and travels along the cup surface to produce less turbulence and circulation at the forward portion of the rotary atomizer. With the ability to optimize the desired pattern, a more stable collapsed pattern with a wider flat top results. The transfer efficiency of the collapsed pattern is nearly the same as the softer broadly cast pattern, while successfully combating the adverse effects of spray booth down draft, varying part position, fatty edges, and complex surface geometry. Additionally, the geometry of the collapsed pattern does not change over a wide range of shaping air flow, fluid flow, and bell rotational speed settings. This is ideal for reciprocating robot type painting methods where a change in process settings did not change the pattern width and the overlap remains constant. Advantageously, the robot path trajectory did not need to be changed over a wide window of process settings.  
         [0049]     Empirical testing of numerous combinations of hole size, spacing, number of holes, inclination angle, distance rearward and outward from bell cup edge revealed that a tighter pattern could be achieved at lower bell speeds with a large diameter cup. As the pattern width is optimized for the interior surfaces, fluid flow rates could be significantly decreased. In lab testing a comparison of a prior art application and this invention was conducted. A fluid flow rate decrease of ˜20% was realized. This optimal pattern width can be reproduced successfully for improved surface finish uniformity while maintaining high transfer efficiency of a rotary atomizer by adjusting the hole diameter of the nozzle, the angle of the nozzle to the bell cup rotation, the location of the nozzle to the bell cup, the number of nozzles, single or multiple array of nozzles, and the bell cup diameter and rotation result in significantly lower fluid flow rates than prior art applications.  
         [0050]     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.