Patent Publication Number: US-2006019036-A1

Title: Method and apparatus for delivering and applying an electrically conductive paint

Description:
FIELD OF THE INVENTION  
      This invention relates to a method and apparatus for delivering and applying an electrically conductive paint by an electrically charged paint applicator which electrically charges the paint at a high voltage received from a color changer at ground potential and which electrically isolates the color changer from the applicator.  
     BACKGROUND OF THE INVENTION  
      The automotive industry, for example, now proposes to replace solvent based paint with water based paint to reduce environmental concerns and pollution abatement apparatus. However, water based paint is highly conductive and in a typical mass production application, the paint is applied with an electrostatic rotary atomizer which electrically charges the paint to improve transfer efficiency. Further, such applications require rapid switching from one color paint to another using a color changer. A color changer is simply a valve system having a number of ports each connected to a source of a different color paint and generally also includes a port connected to a source of solvent, which is typically deionized water for water based paints. The color changer must be at ground potential and the color changer therefore must be electrically isolated from the applicator at high electrical potential.  
      The prior art has proposed various methods and apparatus for delivering and applying conductive liquid coating materials, such as water based paint, from a source at ground potential, such as a color changer, with an electrically charged paint applicator, such as an electrostatic rotary atomizer, which electrically isolates the source of liquid coating at ground potential from the electrically charged paint applicator, sometimes referred to as a “voltage block.” However, such voltage block devices are often complex, expensive, subject to failure and limited in application. The prior art has also proposed electrically isolating a predetermined quantity or volume of electrically conductive paint between a front and rear pig in the delivery line between the color changer and the applicator which are moved through the delivery line by pneumatic pressure. This method is also, however, limited in application and requires a delivery line of sufficient length such that the delivery line is longer than the distance between the front and rear pigs. There is therefore a need for a simple, reliable method and apparatus for conveying and applying a conductive paint from a color changer at ground potential to an electrically charged paint applicator, such as an electrostatic rotary atomizer, which avoids the problems associated with the prior art as set forth hereinbelow.  
     SUMMARY OF THE INVENTION  
      As set forth above, the method and apparatus of this invention is adapted to apply an electrically conductive paint, such as a water based paint, to a substrate with an electrically charged paint applicator, such as an electrostatic rotary atomizer, from a source of paint at ground potential, particularly, but not exclusively a color changer. As used herein, the term “paint” includes water based paints and generally any liquid coating, particularly including coatings for protective or decorative purposes. The method and apparatus of this invention is, however, particularly directed to the application of an electrically conductive paint which is applied to a substrate with an electrically charged paint applicator, such as an electrostatic rotary atomizer, and wherein the source of the electrically conductive paint is at ground potential. As used herein, the term “color changer,” includes any valving system adapted to deliver different paints, as this term is defined above.  
      The method of applying an electrically conductive paint to a substrate of this invention is particularly adapted for mass production applications, wherein the substrate is located in a confined paint spray booth and the electrically charged paint applicator is located in the paint spray booth. In a typical application, the paint applicator may be a robotic paint applicator typically mounted on rails which move with the substrate, such as an automotive body. However, the paint applicator may be an electrically charged paint spray gun or overhead or side mounted electrostatic rotary atomizers of the type presently used in mass production applications including, but not limited to the automotive industry. As set forth above, the apparatus further includes a color changer, as defined above, at ground potential and the electrically conductive paint is delivered through a delivery line, such as a plastic tube, preferably having a friction resistant inner layer and an intermediate dielectric layer, such as polyethylene, to prevent arcing.  
      The method of this invention then includes delivering the electrically conductive paint from the color changer at ground potential, then electrically isolating the color changer from both the delivery line and the electrically conductive paint. Finally, the method of this invention includes delivering the electrically conductive paint through the delivery line to the electrically charged paint applicator, thereby charging the conductive paint and applying the paint to a substrate while the color changer is electrically isolated from the paint in the delivery line. In a preferred embodiment, the conductive paint in the delivery line is a continuous stream which is electrically charged by the paint applicator.  
      One preferred embodiment of the method of this invention includes delivering the electrically conductive paint from the color changer to a paint cannister through a second delivery line between the color changer and the paint cannister, then electrically isolating the color changer from the paint cannister and finally delivering the electrically conductive paint from the paint cannister to the electrically charged paint applicator through the first delivery line described above. In one preferred embodiment, the second delivery line between the color changer and the paint cannister includes a pig or pigging element driving paint in the second delivery line from the color changer to the paint cannister, wherein the pigging element is driven by a nonconductive fluid, such as air, thereby electrically isolating the color changer from the paint cannister. In a preferred embodiment of the method of this invention, the paint cannister further includes a piston, wherein the method of this invention includes retracting the piston in the paint cannister to receive the conductive paint from the color changer, then extending the piston to drive the conductive paint through the first delivery line to the electrically charged paint applicator while the paint cannister is electrically isolated from the color changer.  
      In a preferred embodiment of the apparatus of this invention, the piston of the paint cannister is driven by a servomotor providing very accurate dosing of paint. The first delivery -line between the paint cannister and the applicator may also include a pigging element which is driven by the conductive paint from the paint cannister to a pig station adjacent the applicator. The pigging element is then driven by pneumatic pressure from the applicator to adjacent the paint cannister, thereby returning paint in the first delivery line to the paint cannister and electrically isolating the applicator from the paint cannister. The paint returned to the paint cannister can then be returned to the color changer by again extending the piston of the paint cannister.  
      A preferred embodiment of the apparatus for delivering and applying an electrically conductive paint of this invention thus includes a paint spray booth, an electrically charged paint applicator located within the paint spray booth and a color changer at ground potential preferably located outside the paint spray booth, which may be conventional as known by those skilled in this art. The apparatus of this invention further includes at least one paint cannister connected to the color changer by the second delivery line described above having a pig or pigging element movable between the color changer and the paint cannister to deliver paint in the second delivery line to the paint cannister and electrically isolate the color changer from the paint cannister following delivery of paint to the paint cannister as described above. The apparatus of this invention also includes a first delivery line from the paint cannister to the electrically charged paint applicator. In a preferred embodiment, the apparatus further includes a pig or pigging element in the first delivery line which is pushed by the conductive paint from the paint cannister to the paint applicator. Paint remaining in the first delivery line may then be driven to the paint cannister by the pigging element in the first delivery line by a nonconductive fluid, such as pressurized air or pneumatic pressure, wherein the piston in the paint cannister is retracted to receive the paint and the piston in then extended to return the paint to the color changer. As set forth above, the piston of the paint cannister is preferably driven by a servomotor or the like providing a very accurate metered volume or dose of paint to the paint applicator, which can be varied for each application of paint depending upon the application. In one preferred embodiment of the apparatus of this invention, the color changer and at least one paint cannister are located outside the paint booth in an isolation cabinet preferably formed of an electrically insulated or dielectric material.  
      Other advantages and meritorious features of the method and apparatus for delivering and applying an electrically conductive paint of this invention will be more fully understood from the following description of the preferred embodiments, the appended claims and the drawings, a brief description of which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a side elevation of a paint spray booth illustrating one embodiment of an exterior isolation cabinet;  
       FIG. 2  is an enlarged view of the isolation cabinet shown in  FIG. 1 ;  
       FIG. 3  is a side partially cross-sectioned view of one embodiment of a paint dosing cylinder or cannister illustrated in  FIGS. 1 and 2 ; and  
       FIGS. 4A  to  4 H are schematic illustrations of the method of delivering and applying an electrically conductive paint of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      As set forth above, the method and apparatus for delivering and applying an electrically conductive paint (as defined above) of this invention may be utilized in mass production applications, such as illustrated in  FIG. 1 , wherein electrically conductive paint is applied to a substrate in an enclosed clean environment which includes one or several paint applicators as described below. The paint spray booth  20  is defined by an enclosure wall  22 , which may include windows  24  for viewing the paint spray booth and at least one paint applicator, such as the robot paint applicator  26  illustrated. As will be understood, a conventional paint applicator is generally mounted on a rail (not shown) which moves with the substrate, such as an automotive body, to apply electrically conductive paint through an applicator at high voltage, such as the electrostatic rotary atomizer  27  illustrated in  FIG. 1 . In a preferred embodiment of the method and apparatus of this invention, the electrically conductive paint is electrostatically charged to a high voltage as it is received through the paint applicator  27  as described further hereinbelow. In one preferred embodiment of the apparatus of this invention, the paint cannisters  34  and color changer  30  are located outside the paint spray booth  20  in an electrically insulated isolation cabinet or voltage block module  28  as shown in  FIG. 1  and  FIG. 2  is an enlarged view of the voltage block module  28  as now described.  
      The voltage block module or electrical insulation cabinet  28  shown in  FIG. 2  includes a color changer  30  which, as described above, includes a plurality of valve modules  32  each connected to a source of different colored electrically conductive paint (not shown). This embodiment of the voltage block module  28  further includes two paint dosing devices or paint cannisters  34  each including a pig station  36  connected by supply or delivery lines  38  to color changer  30  and the applicator  26  shown in  FIG. 1  and described further below in regard to  FIGS. 4A  to  4 H. In one preferred embodiment, the paint cannisters  34  each include a piston  56  as described below with respect to  FIG. 3  having a servomotor drive  40 , such that the cannisters  34  each provide an accurate volume or “dose” of paint to the applicator  27  shown in  FIG. 1 . In a preferred embodiment, the voltage block module is defined by an electrically insulated cabinet frame  42 , preferably formed of a nonconductive or electrically insulating polymer, such as Delrin®, an acetal polymer available from DuPont, or other suitable nonconductive polymers. As described further in a co-pending application assigned to the assignee of this application, the voltage block module  28  includes resistors  44 , such as 9 G ohm resistors, solvent cannisters  46  and a hood or enclosure  48  which receives the lines from the paint cannisters  34  to the applicator (not shown). The disclosed embodiment of the electrical isolation cabinet or voltage block module  28  further includes a grounding switch cabinet  50  which includes various electrical components, including a high voltage cascade or voltage generator  52  and a grounding switch  54 . However, in view of the fact that the specific components of the electrical isolation cabinet or voltage block module  28  does not form a part of this invention, except for the color changer and paint dosing cylinders  34 , no further explanation is necessary for complete understanding of the invention as claimed.  
       FIG. 3  illustrates one preferred embodiment of the paint cannister or paint dosing cylinder  34  shown in  FIG. 2 . The disclosed embodiment of the paint dosing cylinder  34  may correspond to the device disclosed in German Patent Application No. DE 10233633 and includes a piston  56  connected to a piston guide rod  58 , which is connected to a bearing  60  and a gear drive  62  connected to a servomotor drive  40 . The piston  56  reciprocates in a piston cylinder or housing  64  which is preferably formed of a ceramic material providing electrical isolation for receipt of charged conductive paint as described below. The piston  56  may further include a guide rod  66  as shown. As described above, a preferred embodiment of the paint piston dosing device  34  includes a pig station  36  having ports  68  for receipt of the delivery lines  38  shown in  FIG. 2 . The components of the apparatus illustrated in FIGS.  1  to  3  will be more fully understood from the following description of the method of delivering and applying an electrically conductive paint shown in  FIGS. 4A  to  4 H described below.  
       FIGS. 4A  to  4 H illustrate schematically the apparatus described above, including the paint applicator  27  at high voltage, a color changer  30  at ground potential, at least one paint canister or paint dosing cylinder  34  including a piston  56 , reciprocable in a piston cylinder or housing  64  having a piston rod  58  preferably connected to a servomotor drive  40  shown in  FIG. 3  and a pig station  36  at one end of the piston dosing device  34 . The housing  64  of the piston dosing device  34  includes an inlet  70  and an outlet  72  in fluid communication with the first delivery line  74  between the piston dosing device  34  and the applicator  27  and the inlet  70  is connected by a second delivery line  76  to the color changer  30 . The disclosed apparatus further includes a second pig station  78  adjacent the applicator  27  which is connected to the first delivery line  74  by a line  80 . The color changer also includes a third pig station  82  adjacent or located within the color changer  30 . As shown in  FIG. 4A , the first and second pig stations  36  and  78 , respectively, are preferably in the form of trunk lines of the first delivery line  74 , such that paint or other fluid can flow from the outlet  72  of the paint cannister or piston dosing device  34  to the applicator  27  through line  80  without flowing around the pig or pigging element  84 . Similarly, the third pig station  86  is preferably a trunk line of the second delivery line  76 , such that paint or other fluid can flow from the color changer  30  through the second delivery line  76  without flowing past the pig or pigging element  86  in the third pig station  82 . However, as described further below, the inlet  70  of the paint cannister or piston dosing device  34  may be configured to receive paint around the second pigging element  86 . Further, each of the pig stations  36 ,  78  and  82  include a source of nonconductive fluid under pressure  88 ,  90  and  92 , respectively, which are controlled by valves  94 ,  96  and  98 , respectively. Having described one preferred embodiment of the apparatus of this invention, the method of delivering and applying an electrically conductive paint of this invention may now be described with reference to  FIGS. 4A  to  4 G as described below.  
      As set forth above, the color changer  30  comprises a series of valves each having a port connected to a separate source of liquid paint under pressure (not shown) by lines  100 A,  100 B and  100 C. In a typical application, the lines  100 A,  100 B and  100 C are connected to separate sources of liquid paint having different colors for application to a substrate by applicator  27 , such as automotive bodies received through the paint spray booth  20  illustrated in  FIG. 1 . As set forth above, the color changer  30  must be at ground potential because the color changer is connected by lines  100 A,  100 B and  100 C to sources of liquid paint. Further, the applicator  27  is at a high voltage or high electrical potential and applies a high electrical charge to the paint as it is received through the applicator  27  and the substrate (not shown) is generally at ground potential to improve transfer efficiency. Thus, paint is received through one of the lines  100 A,  100 B or  100 C.  
      In  FIG. 4B , paint “P” is received through line  100 A to the color changer  30  and the paint is then delivered under pressure through the second delivery line  76  to the paint cannister  34 . As set forth above, the piston guide rod  58  may be connected to a servomotor drive  40  illustrated in  FIGS. 2 and 3 , which withdraws the piston  56  to receive a predetermined volume or “dose” of paint in cylinder  64  of the paint piston dosing device  34  as shown in  FIG. 4B . As shown, the first pigging element  84  is then located in the first pig station  36  and the second pig or pigging element  86  is then located in the third pig station  82  and the pigging elements do not block the flow of paint. The valve  98  in the third pig station  82  is then opened, driving nonconductive fluid, such as air “A,” from the source  92  against the second pigging element  86 , which drives the second pigging element through the second delivery line  76  to the paint dosing device  34 , driving conductive paint P-remaining in the second delivery line  76  to the paint cannister or paint dosing device  34  as shown in  FIG. 4C , thereby electrically isolating or creating a voltage block “B” between the color changer  30  and the paint cannister  34  as illustrated by the arrows in  FIGS. 4C  to  4 E. As the second pigging element  86  is moved to the paint cannister  34 , the piston  56  is simultaneously withdrawn to fully charge the paint cannister or dosing device  34  as shown in  FIG. 4C . As will be understood, however, the piston  56  may be controlled to deliver any predetermined volume or dose of paint for delivery to the applicator  27 , as now described.  
      While the paint cannister or dosing device  34  is electrically isolated from the color changer  30 , as shown by “B”, the piston  56  is then reversed, driving paint P through the first delivery line  74  and line  80  to the applicator  27 , thereby electrically charging the paint and applying the paint through the applicator  27  to a substrate (not shown) in the paint spray booth  20  shown in  FIG. 1 . In this embodiment, the valve  94  to the source of nonconductive fluid  88  is opened to drive the first pigging element  84  ahead of the paint P as the paint is driven from the paint cannister  34  to the applicator  27 , thus receiving the first pigging element  84  in the second pig station  78  as shown in  FIG. 4D . Alternatively, the pigging element  84  may remain in the first pig station  36  and pneumatic pressure may then be utilized from source  88  to drive paint remaining in the first delivery line  74  to the paint applicator  27 . In the embodiment disclosed, upon completion of the first paint cycle, the valve  96  is open to the source of nonconductive fluid  90 , driving the first pigging element  84  from the second pig station  78  adjacent the applicator  27  to the first pig station  36  adjacent the paint dosing device  34 , thereby returning paint P remaining in the first delivery line  74  to the paint dosing device  34  as shown in  FIG. 4E . The paint applicator  27  is thus electrically isolated from the paint cannister forming a voltage block “B” as shown in  FIG. 4F . The piston  56  in the paint dosing device  34  is simultaneously retracted to receive the paint P delivered from the first delivery line  74  by the first pigging element  84 . The piston  56  of the paint dosing device  34  is then extended again, driving paint from the paint cannister or piston dosing device  34  to the color changer  30  and through line  100 A to the original source of conductive paint as shown in  FIG. 4F . Finally, the second pigging element  86  is driven through the second delivery line  76  from adjacent the paint cannister or piston dosing device  34  to the color changer, returning the paint in the second delivery line  76  to the color changer  30  and from the color changer to the source of liquid paint through line  100 A by driving the second pigging element  86  through the second delivery line  76  as shown in  FIGS. 4G and 4H .  
      As will be understood from the above description, the method of this invention includes delivering electrically conductive paint P from the color changer  30  and then electrically isolating the color changer  30  from the electrically conductive paint P, then delivering the electrically conductive paint through the first delivery line  74  to the electrically charged paint applicator  27 , thereby electrically charging the electrically conductive paint. In one preferred embodiment, the conductive paint is a continuous stream to the applicator  27  in the delivery line  74  as shown, for example, in  FIGS. 4D and 4E . In a preferred embodiment of the method of this invention, the electrically conductive paint is first delivered to a paint cannister, such as the piston dosing device  34  as shown in  FIG. 4C  and the color changer  30  is then electrically isolated from the piston dosing device  34  as shown in  FIG. 4C . In the disclosed embodiment, the color changer  30  is electrically isolated from the piston dosing device  34  by driving the second pigging element  86  through the second delivery line  76 , forming a voltage block B as shown in  FIG. 4C . As will be understood by those skilled in this art, however, the color changer  30  may be electrically isolated from the piston dosing device  34  by pneumatic pressure, although a pigging element  86  is shown is preferred. The paint P is then driven by the piston  56  to the electrically charged applicator  27  through line  74  as shown in  FIG. 4D  while the color changer  30  is electrically isolated from the electrically conductive paint and the piston dosing device  34 .  
      As will be understood by those skilled in this art, various modifications may be made to the method and apparatus for delivering and applying an electrically conductive paint of this invention within the purview of the appended claims. For example, the piston dosing device or paint cannister  34  may be eliminated, provided the color changer is electrically isolated from the first delivery line  74  and the paint P contained therein prior to delivery paint to the electrically charged applicator  27 . Although a piston dosing device of the general type is preferred, the first delivery line  74  may be of any length, including a coil, providing sufficient length to provide a voltage block as by utilizing a second pigging element  86  as described above. Further, the first pigging element  84  may be utilized to deliver paint remaining in the first delivery line  74  to the paint applicator  27 , rather than returning the paint to the color changer  30  as described above. Having described preferred embodiments of the method and apparatus for delivering and applying electrically conductive paint of this invention, the invention is now claimed as follows.