Abstract:
A rotary electrostatic atomizer uses shaping air and pattern control air. Shaping airflow is supplied from shaping air holes aligned along outer one of concentric circles that are concentric with the rotation axis of the bell cup and located behind the front end of the bell cup. Pattern control airflow is supplied from pattern control air holes aligned along inner one of the concentric circles. Both the shaping air flow and the pattern control airflow are expelled in circumferentially twisted directions substantially with an equal twist angle opposite from the rotating direction of the bell cup. The shaping airflow passes a circular line near to and radially outwardly apart from the outer perimeter of the bell cup. The pattern control airflow intersects the shaping airflow from radially inside at the position near to and radially outwardly apart from the outer perimeter of the bell cup. Thereby, the pattern control airflow gives the shaping airflow a radially outward force to enhance the centrifugal force of the shaping air and enlarge the coating pattern regulated by the shaping airflow.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a rotary electrostatic coating device and coating pattern control method. 
       BACKGROUND 
       [0002]    Electrostatic coating techniques involve electrically depositing atomized paint onto a piece to be coated (workpiece) by means of electrostatic force, and rotary electrostatic coating devices provided with a rotary head are known as devices for achieving this, wherein a typical example of the rotary head is a cup-shaped bell cup. Electrostatic coating devices of this type are employed for powdered paint, insulating liquid paint (e.g. oil-based paint), and conductive liquid paint (e.g. metallic paint or water-based paint), and there are also rotary electrostatic coating devices which are known of the type in which a high voltage is applied to the rotary head, and of the type in which a high voltage is applied to an external electrode which is remote from the rotary head in the outwardly radial direction. 
         [0003]    Rotary electrostatic coating devices employ shaping air in order to direct paint onto the piece to be coated, and the coating pattern is dictated by means of this shaping air. As disclosed in the prior art section of Patent Document 1, the shaping air flows out from shaping air holes which are positioned to the rear of the bell cup, and it is then directed to the outer peripheral edge of the bell cup, and this has been the practice in the past. The shaping air which flows out from the shaping air holes strikes the outer peripheral edge of the back face of the bell cup, and therefore the flow speed thereof is reduced. This means that the flow of shaping air which has already passed by the bell cup is drawn radially inward because of the negative pressure in front of the bell cup which is produced by the flow of shaping air, and as a result the diameter of the coating pattern tends to be reduced. 
         [0004]    It is better for the minute paint particles in metallic paint to strike the surface of a workpiece at high speed, something which is known in the art. However, as the flow speed of the shaping air increases, so the negative pressure in front of the bell cup increases, as a result of which there are problems in that the diameter of the coating pattern becomes smaller. The response to this problem in Patent Document 1 concerns the orientation of the shaping air which flows out from the shaping air holes, and that document proposes setting the orientation of the shaping air holes such that the torsion direction thereof is oriented about the axis of rotation of the bell cup. The shaping air forms a helical swirling flow due to the shaping air holes whereof the torsion direction has been oriented in this way, and the diameter of the coating pattern can be increased by the centrifugal force of this swirling flow. 
         [0005]    Patent Document 2 proposes an improvement on the inventions of Patent Document 1. That is to say, the inventions of Patent Document 1 resolve the problems when the flow speed of the shaping air is increased for metallic coating, but Patent Document 2 focuses on problems such as excess spraying leading to paint loss when the diameter of the coating pattern is constant, for example when a narrow area such as an automobile pillar is being coated, and proposes an idea to improve on this situation. 
         [0006]    The inventions proposed in Patent Document 2 are based on setting the orientation of the shaping air, which was proposed in Patent Document 1, in other words on setting the orientation of the shaping air holes in a torsion direction about the axis of rotation of the bell cup, and in said document, control air holes are provided further outward in the radial direction than the shaping air holes, and pattern control air which flows out from these control air holes strikes the shaping air at the outer peripheral edge of the bell cup, then the amount of outflow of pattern control air is changed, whereby the coating pattern width is controlled. In this instance, the control air holes which are positioned radially outward of the shaping air holes have a zero torsion angle about the axis of rotation of the bell cup, and they are inclined toward the axis of rotation of the bell cup. In other words, the shaping air holes have a torsion angle about the axis of rotation of the bell cup, and they are directed at the outer peripheral edge of the back face of the bell cup. In contrast to this, the control air holes have a zero torsion angle about the axis of rotation of the bell cup. The control air is inclined toward the axis of rotation of the bell cup, and therefore it merges with the shaping air at the outer peripheral edge of the bell cup. 
         [0007]    When the outflow of pattern control air is stopped, the centrifugal force of the shaping air which swirls helically overcomes the suction force due to the negative pressure in front of the bell cup, whereby a coating pattern of relatively large diameter is formed. On the other hand, when the pattern control air is made to flow out, this pattern control air has a zero torsion angle, and therefore the pattern control air merges with the shaping air, whereby the torsion angle of the shaping air is substantially reduced, as a result of which the swirling force of the shaping air which swirls helically is weakened. Accordingly, the centrifugal force of the shaping air is relatively small, and therefore the effect of the negative pressure in front of the bell cup is weakened, and the diameter of the coating pattern is reduced. 
         [0008]    As described above, Patent Document 2 relates to metallic coating, and it proposes reducing the diameter of the coating pattern by reducing the torsion angle of the shaping airflow which swirls helically, using control air which merges with the shaping air. 
         [0009]    Patent Document 3 offers another proposal relating to variable control of the diameter of the coating pattern. The proposal of Patent Document 3 is similar to Patent Document 2 in that control air holes are provided radially outward of the shaping air holes, but the inventions of Patent Document 3 differ from those of Patent Document 2 firstly in that the control air is made to swirl helically, and they differ from Patent Document 2 secondly in that the pattern control air is parallel to the flow of shaping air (the control air and the shaping air do not merge). 
         [0010]    To be more specific, in the inventions of Patent Document 3, a flow of pattern control air which swirls helically at a torsion angle which is different from that of the shaping air is generated radially to the outer periphery of the flow of shaping air which swirls helically, and then the amount of outflow of this pattern control air is controlled to cause the general torsion angle of the shaping air to change. Changing the diameter of the coating pattern by changing the general torsion angle of the shaping air is as described for Patent Document 2. 
         [0011]    Patent Document 1: Japanese Unexamined Patent Application Publication H3-101858 
         [0012]    Patent Document 2: Japanese Unexamined Patent Application Publication H7-24367 
         [0013]    Patent Document 3: Japanese Unexamined Patent Application Publication H8-84941 
       SUMMARY 
     Issues to be Resolved 
       [0014]    One aim of the present invention is to provide a rotary electrostatic coating device and a coating pattern control method, in which the diameter of the coating pattern produced by the rotary electrostatic coating device can be varied. 
         [0015]    A further aim of the present invention is to provide a rotary electrostatic coating device and a coating pattern control method, in which the diameter of the coating pattern can be varied by different means from those of the inventions disclosed in Patent Documents 2 and 3. 
       Means of Resolving the Problems 
       [0016]    According to a first aspect of the present invention, the abovementioned technical issue is resolved by providing a rotary electrostatic coating device comprising: 
         [0000]    a rotary head which causes paint to be discharged radially outward,
 
a plurality of shaping air holes which are arranged at intervals on a first circle which is positioned further back than an outer peripheral part of said rotary head and which has the axis of said rotary head at its center, said shaping air holes directing paint discharged radially outward from the outer peripheral edge of the abovementioned rotary head toward a piece to be coated so as to produce a coating pattern, by means of a shaping airflow which flows out from said shaping air holes,
 
a plurality of control air holes which are arranged at intervals on a second circle which has a smaller diameter than said first circle and is positioned to the rear of the outer peripheral part of said rotary head and concentric with the abovementioned first circle, and
 
first control means for controlling the flow rate of pattern control air which flows out from said control air holes; and
 
the abovementioned shaping air holes and the abovementioned control air holes are oriented in substantially the same torsion angle direction, opposite to the direction of rotation of the abovementioned rotary head;
 
the axis of the abovementioned shaping airflow passes through a position close to and radially outward from the outer peripheral edge of the abovementioned rotary head; and
 
the axis of the abovementioned control airflow intersects the axis of the abovementioned shaping air at a position which is close to and radially outward from the outer peripheral edge of the abovementioned rotary head.
 
         [0017]    Furthermore, according to a second aspect of the present invention, the abovementioned technical issue is resolved by providing a method of controlling a coating pattern in which provision is made for a plurality of shaping air holes which are arranged at intervals on a first circle to the rear of a rotary head with the axis of rotation of said rotary head at the center, and which are oriented in a torsion angle direction opposite to the direction of rotation of the abovementioned rotary head, said shaping air holes directing paint discharged radially outward from the abovementioned rotary head toward a piece to be coated so as to produce a coating pattern, by means of shaping air which flows out from said plurality of shaping air holes; 
         [0000]    provision is made for control air holes which are arranged at intervals on a second circle having a smaller diameter than the first circle to the rear of the rotary head and concentric with said first circle, and which are oriented in the same torsion angle direction as the abovementioned shaping air holes;
 
said method comprising:
 
a paint discharge step in which paint is discharged radially outward from the abovementioned rotary head;
 
a coating pattern production step, in which the abovementioned shaping air flows out from the abovementioned shaping air holes to produce the abovementioned coating pattern; and
 
a coating pattern control step in which pattern control air which flows out from the abovementioned control air holes is made to intersect the abovementioned shaping airflow at a position close to and radially outward from the outer peripheral edge of the abovementioned rotary head, changing the diameter of the abovementioned coating pattern.
 
         [0018]    According to the present invention, pattern control airflow with the same torsion angle direction as the shaping airflow is made to merge from the inner peripheral side of the shaping airflow, and the merging position thereof lies at a position close to and radially outward from the outer peripheral edge of the rotary head, and therefore force in the outward radial direction can be imparted to the shaping airflow by the pattern control airflow. Accordingly, the centrifugal force of the shaping airflow can be intensified by the pattern control air without substantially changing the torsion angle of the shaping airflow which swirls helically, and this allows the diameter of the coating pattern which is dictated by the shaping airflow to be enlarged. 
         [0019]    The abovementioned aim and further aim of the present invention, and operational effects thereof, will become clear from the detailed description of exemplary embodiments which will be given below. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0020]      FIG. 1  is a basic structural diagram of the rotary electrostatic coating device of Exemplary Embodiment 1; 
           [0021]      FIG. 2  is a front view of the plane surface occupied by the outer peripheral edge of the bell cup seen from the front of the bell cup; 
           [0022]      FIG. 3  illustrates the position where the shaping airflow and the pattern control airflow merge; 
           [0023]      FIG. 4  is a side view illustrating the orientation of the shaping air holes and the control air holes; 
           [0024]      FIG. 5  is an oblique view of the rotary electrostatic coating device seen obliquely from the front; 
           [0025]      FIG. 6  is a basic structural diagram of the rotary electrostatic coating device of Exemplary Embodiment 2; and 
           [0026]      FIG. 7  is an enlarged view of the main parts showing the stepped structure of the air ring included in Exemplary Embodiment 2. 
       
    
    
     KEY TO SYMBOLS 
       [0000]    
       
         L axis of rotation of bell cup 
         A direction of rotation of bell cup 
         C 1  first circle (shaping air holes) 
         C 2  second circle (control air holes) 
         Fs shaping airflow 
         Fp pattern control airflow 
         θ torsion angle 
           10  rotary electrostatic coating device 
           11  device main body 
           12  air motor 
           13  bell cup 
           13   a  outer peripheral edge of bell cup 
           13   b  inner peripheral surface of bell cup 
           14  air ring 
           14   a  outer peripheral part of air ring 
           14   b  inner peripheral part of air ring 
           17  shaping air holes 
           18  control air holes 
           22  thread-like paint 
           23  minute paint particles 
           25  coating pattern 
           27  merging position 
       
     
       DETAILED DESCRIPTION 
       [0049]    Preferred exemplary embodiments of the present invention will be described below based on the appended figures. 
       Exemplary Embodiment 1 (FIGS.  1 - 5 ) 
       [0050]    Looking at  FIG. 1 , a rotary electrostatic coating device  10  which is depicted comprises a cup-shaped rotary head, i.e. a bell cup  13  which is rotated by an air motor installed in a device main body  11 , in the same way as a conventional device. Paint is supplied to a central portion of the bell cup  13 , and the paint moves radially outward along the inner surface of the bell cup  13 , after which it is discharged from an outer peripheral edge  13   a  of the bell cup  13 . In the figures, L denotes the axis of rotation of the bell cup  13 , and the arrow A denotes the direction of rotation of the bell cup  13 , as described above. 
         [0051]    The device has an air ring  14  lying further to the rear than the outer peripheral part of the bell cup  13 .  FIG. 2  is a front view of the bell cup  13 . Looking at  FIG. 2 , two annular spaces, namely first and second annular spaces  15 ,  16  are formed in the air ring  14 . Shaping air holes  17  and control air hole  18  are then arranged at the end face of the air ring  14 , on first and second concentric circles C 1 , C 2 . That is to say, a plurality of shaping air holes  17  are arranged at equal intervals on the first circle C 1  of relatively large diameter, and pressurized air is supplied to these shaping air holes  17  through the first annular space  15 . Meanwhile, a plurality of control air holes  18  are arranged at equal intervals on the second circle C 2  of relatively small diameter, and pressurized air is supplied to these control air holes  18  through the second annular space  16 . 
         [0052]    There are the same number of shaping air holes  17  as there are control air holes  18 , and one shaping air hole  17  and the corresponding control air hole  18  are positioned on a line which radiates from the axis of rotation L of the bell cup  13 . 
         [0053]    The reference symbol  20  in  FIG. 1  denotes a high voltage generator, and a high voltage DC which is generated by the high voltage generator  20  is supplied to the bell cup  13  via a case of an air motor  12 . An electric field is then generated between the bell cup  13  to which high voltage has been applied and the piece to be coated (workpiece). 
         [0054]      FIG. 3  is a front view of the bell cup  13 . Looking at  FIG. 3 , rotation of the bell cup  13  causes paint to spread radially outward along the inner peripheral surface of the bell cup  13 , the paint then extending thread-like from the outer peripheral edge  13   a  of the bell cup  13 , after which the thread-like paint  22  breaks up close to the outer peripheral edge of the bell cup  13 , becoming atomized particles  23 , and also being ionized. 
         [0055]    The paint particles  23  are directed forward, in other words toward the piece to be coated, by a shaping airflow Fs which flows out from the shaping air holes  17 . A coating pattern  25  ( FIG. 1 ) is dictated by the shaping airflow. Looking at  FIGS. 2 and 4 , the shaping air holes  17  are oriented in a torsion angle θ direction opposite to the direction of rotation A of the bell cup  13 . By means of this, the shaping airflow Fs which swirls helically can be generated in the same way as in Patent Documents 1 to 3, and the direction of swirling thereof is opposite to the direction of rotation of the bell cup  13 . The particles of paint can be atomized by the shaping airflow Fs which swirls helically in the opposite direction to the direction of rotation A of the bell cup  13 . In this exemplary embodiment, the shaping airflow Fs which flows out from the shaping air holes  17  is parallel to the axis of rotation L of the bell cup  13 , when seen from the side, as is clear from  FIG. 4 . 
         [0056]    Turning now to a description of the control air holes  18  which are positioned on the second circle C 2  of smaller diameter than the first circle C 1  ( FIG. 2 ) where the plurality of shaping air holes  17  are positioned, these control air holes  18  are also directed in a direction opposite to the direction of rotation A of the bell cup  13 , at substantially the same angle as the torsion angle θ of the shaping air holes  17  described above. 
         [0057]    Furthermore, these control air holes  18  are directed obliquely outward, when seen from the side, as is clear from  FIG. 4 , and by means of this the pattern control airflow Fp which flows out from the control air holes  18  merges with the shaping airflow Fs. 
         [0058]    The shaping airflow Fs and pattern control airflow Fp will be described in detail. The shaping airflow Fs and pattern control airflow Fp are both swirling flows which swirl helically in the opposite direction to the direction of rotation A of the bell cup  13 . The torsion angles of the shaping airflow Fs and pattern control airflow Fp are substantially the same (the torsion angles θ are substantially the same). 
         [0059]    Furthermore, setting the shaping airflow Fs which flows out from one shaping air hole  17  so that it merges with the pattern control airflow Fp which flows out from a corresponding control air hole  18  adjacent to this one shaping air hole  17  is as described above, but the point of merger lies close to the outer peripheral edge  13   a  of the bell cup  13  but away from the outer peripheral edge  13   a , on a plane occupied by the outer peripheral edge  13   a  of the bell cup  13 , and to be specific this is preferably 2-3 mm. 
         [0060]      FIG. 3  illustrates the merging position of the shaping airflow Fs which flows out from all of the shaping air holes  17  and the pattern control airflow Fp which flows out from the control air holes  18  which are arranged on radiating lines that correspond to each of the shaping air holes  17 . First of all,  FIG. 3  is a view in which the plane surface occupied by the outer peripheral edge  13   a  of the bell cup  13  is seen from the front of the bell cup  13 . In  FIG. 3 , the merging position of the shaping airflow Fs and the pattern control airflow Fp is shown by the reference symbol  27 . 
         [0061]    This merging position  27  is a position which is 2-3 mm radially outward from the outer peripheral edge  13   a  of the bell cup  13 . Specifically, this merging position  27  is set in relation to the paint which is discharged radially outward from the bell cup  13 . To describe this point, the fact that the paint extends in a thread-like form  22  from the outer peripheral edge  13   a  of the bell cup  13 , and then that the thread-like paint  22  breaks up and becomes minute paint particles  23  is as described above, but the merging position  27  is set at the tip end of the thread-like paint  22  or at a position immediately following where the minute paint particles  23  separate. The length of the thread-like paint  22  cannot of course be uniformly defined by the rotation speed of the bell cup  13  or the type of paint being used, or by similar factors, but this position can be said to be at the tip end of the thread-like paint  22  or a position immediately following where the minute paint particles  23  separate in most examples of application, provided that it is a position which is 2-3 mm radially outward from the outer peripheral edge  13   a  of the bell cup  13 . 
         [0062]    Referring to  FIG. 1 , the pressurized air source for the shaping airflow Fs and the pressurized air source for the pattern control airflow Fp is a shared source, and first and second flow control valves  32 ,  33  are placed along a first duct  30  which passes through the first annular space  15  (shaping air) of the air ring  14 , and a second duct  31  which passes through the second annular space  16  (pattern control air), respectively. The first and second flow control valves  32 ,  33  are controlled by means of a controller  35 . 
         [0063]    The diameter of the coating pattern which is related to the area on the piece to be coated (workpiece) is specifically achieved by controlling the second flow control valve  33  (control air flow rate). The first flow control valve  32  (shaping air flow rate) may also be controlled, of course. To describe a typical example in specific terms, the second flow control valve  33  is opened for an area where the surface to be coated is relatively large, and the pattern control airflow Fp flows out from the control air holes  18 . The pattern control airflow Fp merges with the shaping airflow Fs, whereby an outward radial force is applied to the shaping airflow Fs by the pattern control airflow Fp without any substantial effect on the torsion angle θ of the shaping airflow Fs, and the centrifugal force of the shaping airflow Fs which swirls helically is intensified by this force. Accordingly, the diameter of the coating pattern  25  can be enlarged by causing the pattern control airflow Fs to flow out from the control air holes  18 . 
         [0064]    On the other hand, the second flow control valve  33  is closed for an area where the surface to be coated is relatively small, and the outflow of the pattern control airflow Fp from the control air holes  18  is stopped. Accordingly, the coating pattern  25  of the electrostatic coating device  10  is dictated by the shaping airflow Fs which swirls helically. In other words, the coating pattern  25  is smaller than in the case where the pattern control airflow Fp is made to flow out. 
         [0065]    Furthermore, a description has been given in the exemplary embodiment described above of a typical example of control in which the pattern control airflow Fp is switched ON/OFF, but it goes without saying that multistage control or linear variable control may be employed for the pattern control airflow Fp. 
       Exemplary Embodiment 2 (FIGS.  6 ,  7 ) 
       [0066]    Exemplary Embodiment 2 is a variant example of Exemplary Embodiment 1. In Exemplary Embodiment 1, as regards the air ring  14 , the shaping air holes  17  and the control air holes  18  which are positioned radially further inward than said shaping air holes  17  open out in a common plane ( FIG. 1 ), but the end face of the air ring  14  may comprise a stepped face, and, as shown in the enlarged view of  FIG. 7 , an outer peripheral part  14   a  where the shaping air holes  17  are positioned may project further forward than an inner peripheral part  14   b  where the control air holes  18  are positioned, with the distance between the shaping air holes  17  and the outer peripheral edge  13   a  of the bell cup  13  being shortened. The height (Δh) of the stepped part between the outer peripheral part  14   a  and the inner peripheral part  14   b  of the air ring  14  is 2-3 mm. In other words, in Exemplary Embodiment 2, the end where the shaping air holes  17  open is positioned 2-3 mm forward of the end where the control air holes  18  open. 
         [0067]    In this way, the impact speed of the paint particles on the piece to be coated can be increased by bringing the end where the shaping air holes  17  open closer to the outer peripheral edge  13   a  of the bell cup  13 , that is to say, by bringing this end closer to the piece to be coated. It was confirmed with trial products in particular that this was effective in improving the coating quality of metallic coating. 
         [0068]    Exemplary embodiments have been described above, but, as an example of coating pattern control, control may be effected so that the diameter of the coating pattern  25  can be increased and/or decreased by combining the control of the first and second flow control valves  32 ,  33 . For example, the diameter of the coating pattern can be increased by opening the second flow control valve  33  wide (pattern control airflow Fp: large), while at the same time narrowing the first flow control valve to weaken the shaping airflow Fs. In this way, the diameter of the coating pattern  25  can be changed linearly by combining control of the first and second flow control valves  32 ,  33 , using control relating to increasing and decreasing the diameter of the coating pattern  25 . 
         [0069]    Furthermore, in the exemplary embodiments, the amount of paint supplied to the bell cup  13  is the same, regardless of the flow control of the pattern control airflow Fp, but the amount of paint which is supplied to the bell cup  13  may be controlled so that the amount of paint corresponds to the diameter of the coating pattern which is produced in correspondence with the flow control of the pattern control airflow Fp. It should be noted that examples of control in which the amount of paint is constant regardless of the flow control of the pattern control airflow Fp are not suitable for metallic coating in which the color is affected by the relationship between the diameter of the coating pattern and the amount of paint. Accordingly, if a control example is used in which the amount of paint is constant regardless of the ON/OFF state of the pattern control airflow Fp, paint other than metallic paint should be used. In other words, in the case of metallic coating, control of the amount of paint and control of the shaping air should be included, rather than limiting control to only the control air. 
         [0070]    Furthermore, in the exemplary embodiments, the shaping airflow Fs was parallel to the axis of rotation L of the bell cup  13 , when seen from the side, but it may be somewhat inclined, and the shaping airflow Fs may be inclined in a direction approaching the axis of rotation L, or conversely the shaping airflow Fs may be inclined in a direction moving away from the axis of rotation L. 
         [0071]    Exemplary embodiments of the rotary electrostatic coating device  10  in which a high voltage is applied to the bell cup  13  have been described above as examples of the present invention, but it goes without saying that the present invention can also be applied in the same way to rotary electrostatic coating devices provided with external electrodes which are used for conductive paint such as water-based paints.