Patent Publication Number: US-7584771-B2

Title: Method and apparatus for delivering paint to an applicator and flushing same

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for delivering paint from a source of liquid paint to an applicator and flushing the delivery line and the applicator with solvent using pigging technology. 
     BACKGROUND OF THE INVENTION 
     The prior art discloses numerous paint delivery and application systems using pigging technology. Such systems may be used to apply seriatim paints of different colors, for example, to a plurality of substrates, such as vehicle bodies moving on a conveyor through a paint booth. As used herein, the term “paint,” includes solvent or water based paints used to paint or coat a substrate and generally any coating which may be applied to a substrate by an applicator including but not limited to a rotary spray atomizer. Depending on its function in the coating delivery system, pigs or pigging elements are variously referred to in the prior art as shuttles, separating elements, terminating pistons, plugs, etc. Generally, a pigging element is utilized in the prior art paint delivery systems to push paint from a source of paint, such as a color changer, through a delivery line to an applicator to apply paint to a substrate and the pigging element may also be utilized to return paint to the source of paint. As will be understood by those skilled in this art, a typical mass production application such as an automotive application, requires the application of different paints utilizing the same paint application system thus requiring purging of the delivery line and the applicator with solvent between applications. Typically, this requires a separate solvent delivery line to the applicator. The paint application system may include a first pig station adjacent the color changer and a second pig station adjacent the applicator. Paint is then delivered under pressure from the color changer to the delivery line and the pigging element is driven through the supply line generally by pneumatic pressure to push the paint in the supply line to the applicator and the pigging element is then received in the second pig station adjacent the applicator. The pigging element may also be utilized to return paint remaining in the supply line to the color changer by driving the pigging element from the second pigging station adjacent the applicator to the first pigging station adjacent the color changer generally under pneumatic pressure. 
     Finally, the prior art also includes the use of two or more pigging elements in the supply line which are utilized to separate a predetermined volume of paint or to provide an electrostatic barrier between volumes of paint which are being transmitted through the supply line particularly where the color change is at ground potential and the applicator is an electrostatic paint applicator at a greater electrical potential. 
     However, the prior art does not disclose a method or apparatus which eliminates the requirement for a separate solvent supply line to purge the applicator and the supply line and which may be utilized to return paint to the paint source. 
     SUMMARY OF THE INVENTION 
     The apparatus for conveying paint from a paint source to an applicator of this invention includes a paint supply line between the paint source and the applicator delivering paint under pressure from the paint source to the applicator, a first pig station adjacent the paint source and a second pig station adjacent or within the applicator, which may be an atomizer, a first and second pig or pigging element movable in the supply line between the first pig station and the second pig station and a source of liquid solvent connected to the first and/or to the second pig station adapted to deliver solvent under pressure between the first and second pigs, whereby movement of the first and second pigs between the first and second stations may deliver solvent to the applicator or the paint source, thereby flushing the applicator, the supply line, and/or the source. In the preferred embodiment, the source of liquid paint is a cannister receiving paint from a source of liquid paint, such as a color changer, having a piston, whereby movement of the piston supplies paint under pressure through the supply line to the applicator. The paint cannister includes an outlet communicating with the first pig station, whereby movement of the piston drives paint against an adjacent pigging element, thereby driving the first and second pigging elements with solvent therebetween from the first pig station to the second pig station first delivering solvent through the paint supply line to the applicator, purging the paint supply line and the applicator, and then delivering paint through the supply line to the applicator to apply paint to a substrate. 
     In a preferred embodiment, the apparatus further includes a source of liquid solvent under pressure connected either to the supply line adjacent the second pig station or connected to the second pig station which delivers solvent under pressure between the first and second pigs and the first and second pigs are driven under pressure, preferably pneumatic pressure, from the second pig station to the first pig station. The solvent between the first and second pigging elements thereby purges the supply line and the paint cannister prior to receiving a second paint. This embodiment may also be utilized to return paint to the paint source which, in the disclosed embodiment, is a paint cannister having a piston as described above and the paint may then be delivered by the paint cannister to the color changer. 
     The method of delivering solvent and paint from a source of liquid paint to an applicator through a delivery line having first and second pigs or pigging elements of this invention thus includes locating the first and second pigs adjacent the source of liquid paint, delivering solvent under pressure between the first and second pigs, thereby spacing the first and second pigs with solvent, and driving paint under pressure against the pigging element adjacent the paint source, thereby driving the first and second pigs with the solvent therebetween to the applicator, whereby the solvent is received through the delivery line and the applicator followed by paint to apply paint to a substrate. Where the source of liquid paint is a cannister receiving liquid paint having a piston, as described above, the method of this invention includes driving the piston in the cannister, thereby driving paint against the adjacent pigging element and driving the paint and solvent to the applicator, as described. In a preferred embodiment of the method of this invention, the method further includes delivering solvent under pressure between the first and second pigging elements when the first and second pigging elements are located adjacent the applicator and then driving the first and second pigging elements with solvent therebetween to the source of liquid paint, thereby flushing the delivery line and the paint cannister. A preferred embodiment of the method of this invention further includes driving the first and second pigs from the applicator to the source of liquid paint, thereby driving paint remaining in the supply line to the paint source. 
     In the preferred embodiment of the apparatus and method of this invention, the pigs or pigging elements each include a body portion having a diameter adjacent its opposed ends substantially equal to or greater than the internal diameter of the delivery line. In one preferred embodiment, the opposed end portions of the pigging elements each include a generally frustoconical resilient skirt portion which tapers outwardly from the body portion having a diameter equal to or greater than the internal diameter of the tubular delivery line providing improved wiping or cleaning of the delivery line during the purging cycles. In a preferred embodiment of this invention, the pigging elements further include projecting end portions on the opposed ends of the pigging elements, such that introduction of solvent under pressure will space the pigging elements as described above. That is, introducing solvent under pressure into the delivery line between the pigging elements will be received by the skirt portions of the opposed pigs, separating the pigs with solvent. The opposed end portions are preferably cylindrical having a flat end portion which abuts the adjacent pigging element such that the solvent will space the pigging elements as described even where the delivery line is flexible. In a preferred embodiment, the delivery line is tubular having a friction resistant inner layer, such as a fluorinated hydrocarbon, preferably a clear or translucent fluorinated polymer, such as a perfluoralkoxy resin, an intermediate dielectric material, such as polyethylene, to prevent arcing, and a thin outer layer of a hard plastic to prevent damage to the delivery line formed of a polyamid, for example. The delivery line, depending upon its diameter, is flexible and translucent, permitting visualization of the movement of the pigging elements, paint and solvent through the delivery line. 
     Other advantages and meritorious features of the paint delivery system and method 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 partially cross-sectioned view of a paint delivery line of the apparatus for delivering paint of this invention including a pigging element; 
         FIG. 2  is a cross-sectional end view of  FIG. 1  in the direction of view arrows  2 - 2 ; 
         FIG. 3  is an exploded view of the pigging element shown in  FIG. 1  and the paint delivery line; 
         FIGS. 4 and 5  are a partially cross-sectioned view of the paint delivery line with two pigs illustrating one step of the method of delivering paint and solvent of this invention; 
         FIGS. 6A to 6J  illustrate the method and apparatus for delivering paint to an applicator and flushing of this invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1 to 3  illustrate an embodiment of a pig or pigging element  20  and delivery line  22  which may be utilized in the method and apparatus of this invention as described below. The pigs or pigging elements  20  each include a body portion  24  including generally frustoconical skirt portions  26  on opposed ends of the body portion  24  which angle outwardly from the body portion  24  at an acute angle having annular diamond-shaped rib portions  28  adjacent the ends of the skirt portions  26 . In the preferred embodiment, the pig  20  is formed of a resilient polymer, such as polyethylene, and the frustoconical skirt portions  26  have a major diameter which is generally equal to or preferably greater than an internal diameter of the delivery line  22 , such that the skirt portions  26  thoroughly scrape the internal surface of the delivery line  22  as the pigging element  20  traverses the delivery line as described below. The preferred embodiment of the pig  20  further includes projecting end portions  30  which project beyond the skirt portions  26  as best shown in  FIG. 1  as further described below. In this embodiment, the end portions  30  are cylindrical each having a flat end face  32  which abuts the end face of an adjacent pig  20  as described below in regard to  FIGS. 4 and 5 . The annular frustoconical skirt portions  26  and the end portions  34  further define annular cavities  34  at the opposed ends of the body portion  26  which are generally wedge-shaped promoting movement of the pig when pressure is received in the wedge-shaped cavities  34  as described below. The preferred embodiment of the pig further includes a sensor element  36  which, in the disclosed embodiment, is a magnet. The sensor element  36  permits accurate determination of the location of the pigging element  20  in the delivery line  22  as described further below. 
     A preferred embodiment of the delivery line  22  includes an inner layer  38  preferably formed of a friction resistant polymer, for example, a fluorinated polymer, such as a perfluoralkoxy resin, an intermediate layer formed of a dielectric polymer, such as polyethylene to prevent arcing and an outer layer  42  which, in the preferred embodiment, is relatively thin and forms a protective outer shell of the delivery line  22  which may be formed of a polyamid, for example. The layers of the delivery line  22  may be co-extruded, for example, forming an intimate bond between the layers of the delivery line  22  and preventing separation during use. In a preferred embodiment, the delivery line  22  is relatively flexible and clear or translucent to permit visualization of the movement of the pigging element  20 , paint and solvent through the delivery line during use, as described below. As will be understood, however, the method and apparatus of this invention are not limited to the disclosed pigging element  20  or delivery line  22  except as specifically claimed hereinbelow. 
       FIGS. 4 and 5  illustrate a component of the apparatus and a step of the method of this invention, wherein the delivery line  22  includes two pigs or pigging elements  20 A and  20 B each including a body portion  24  having generally frustoconical skirt portions  26  and abutting or nearly abutting cylindrical end portions  30  which extend beyond the generally frustoconical skirt portions  26  of the body portion  24  as described above in regard to  FIGS. 1 to 3 . The end faces  32  of the end portions  30  are initially abutting as shown in  FIG. 4  and generally aligned with a port  44  connected to a source of solvent under pressure  46 . An electrically controlled valve  48  is in the line  50  connected to the port  44 . The valve  48  is connected to a controller  52  by line  54  and the controller is connected to sensors  56 A and  56 B by lines  58 A and  58 B. The sensors  56 A and  56 B may be any conventional sensor elements, such as a Hall-effect sensor which senses the location of the magnet  36  shown in  FIGS. 1 and 2 . When the sensors  56 A and  56 B sense the location of the pigs  20 A and  20 B in substantially abutting relation opposite the sensor port  44  as shown in  FIG. 4 , the valve  48  is opened by the controller  52 , delivering a predetermined volume of solvent  60  from the source of solvent  46  under pressure between the generally frustoconical skirt portions  26 , which separates the pigs  20 A and  20 B with a predetermined volume of solvent  60  as shown in  FIG. 5 . The pressure of the solvent  60  is received in the annular cavities  34  of the adjacent pigs  20 A and  20 B, separating the pigs with solvent  60  as shown in  FIG. 5 . As described below, a preferred embodiment of the apparatus and method for delivering paint to an applicator of this invention includes two sources of solvent  46 A and  46 B which separate the pigs or pigging elements  20 A and  20 B as now described. 
       FIGS. 6A to 6J  illustrate an embodiment of the apparatus for delivering paint and solvent to an applicator  62  and method of this invention, which may be a conventional electrostatic rotary atomizer or other suitable paint applicator known to this art. As will be understood by those skilled in this art, the applicator  62  is conventionally maintained at a high electrical potential and the source of paint, such as a color changer (not shown), is normally maintained at ground potential. In the preferred embodiment, the apparatus includes a paint cannister  64  having an inlet  66  connected to a source of liquid paint, such as a color changer (not shown), and an outlet  68  communicating with the delivery line  22  described above. The paint cannister includes a piston  70  having a piston head  72  and a shank or piston rod  74  driven by a servomotor or the like (not shown), such that the piston  70  delivers a predetermined volume or “dose” of paint through the delivery line  22  to the applicator  62  as described below. Thus, the cannisters  64  may also be characterized as paint “dosing cylinders.” The pigging elements  20 A and  20 B are initially located in a first pig station  76  which, in the preferred embodiment, is a trunk line of the delivery line  22 , such that paint may be delivered directly to the delivery line  22  without flowing around the pigging elements  20 A and  20 B. The first step in the method of this invention is to locate the pigging elements  20 A and  20 B in the first pig station  76  adjacent the dosing cylinder  64 , such that the projecting cylindrical end portions  30  abut or nearly abut and are located opposite the solvent delivery line  50  connected to a source of solvent under pressure  46 A. As will be understood, the first pig station or trunk line  76  includes a valve  48 , a controller  52  and sensors  56 A and  56 B as described above with regard to  FIGS. 4 and 5 , the details of which are not shown in  FIGS. 6A to 6J . The second step in the method of this invention is to separate the pigging elements  20 A and  20 B with solvent  60  as shown in  FIG. 6B  and described above with regard to  FIG. 5 . 
     The pigging elements  20 A and  20 B are then located in the delivery line  22  adjacent or opposite the outlet  68  of the cannister or dosing cylinder  64  as shown in  FIG. 6C  by any suitable means, such as by delivering air under pressure into the first pig station  76  from a source of pneumatic pressure  78 . Liquid paint is then delivered under pressure to the cannister or dosing cylinder  64  through inlet  66  from a source of liquid paint under pressure, such as a color changer (not shown) as shown in  FIG. 6C . As set forth above, the shank or piston rod  74  is withdrawn by the servomotor (not shown) a predetermined distance as shown by arrow  82  in  FIG. 6C  to receive a predetermined volume or dose of paint  80  in the dosing cylinder  64 . The direction of travel of the piston  70  is then reversed as shown by arrow  84  in  FIG. 6D , which drives paint from the cannister or dosing cylinder  64  through outlet  68  against an adjacent pigging element  20 A with solvent  60  between the pigging elements  20 A and  20 B as shown in  FIG. 6D . The paint then drives the pigging elements  20 A and  20 B through the delivery line  22  toward the applicator  62  as shown in  FIG. 6E , wherein the second pigging element  20 B is received in a second pig station  86 , which is also preferably a trunk line of the delivery line  22 . The solvent  60  is then delivered through line  88  to the applicator  62  as shown in  FIG. 6E , thereby flushing the applicator  62 . 
     As will be understood from the description of the method and apparatus of this invention thus far, the solvent  60  also flushes the delivery line  22  upstream of the paint  80 . The first pigging element  20 A is then received in the second pigging station  86  and paint is delivered through line  88  to the applicator  62  as shown in  FIG. 6F . As the paint  80  is delivered to the applicator  62  by the piston  70  of the paint dosing cylinder  64 , the first and second pigging elements  20 A and  20 B are located in the trunk line or second pigging station  86  with the projecting end portions  30  in abutting or nearly abutting relation opposite the solvent delivery line  50  of a second source of solvent  46 B as shown in  FIG. 6F , such that the pigging elements  20 A and  20 B may be separated with solvent  60  therebetween as shown in  FIG. 6G  and described above with regard to  FIGS. 5 and 6B . As will be understood, the second pig station or trunk line  86  may be identical to the arrangement shown in  FIGS. 4 and 5  and includes a valve  48 , controller  52 , and sensor elements  56 A and  56 B as shown in  FIG. 5 . As shown in  FIG. 6G , the solvent  60  may be introduced between the first and second pigging elements  20 A and  20 B as the paint  80  is delivered to the applicator  62  to reduce cycle time, but may also be introduced following delivery of paint to the applicator. The first and second pigging elements  20 A and  20 B are then delivered to the delivery line  22  ( FIG. 6H ) preferably by delivery of a nonconductive fluid from a source of nonconductive fluid, such as air  90 , which electrically isolates the paint cannister or dosing device  64  from the electrostatic applicator  62  during return of paint  80  remaining in the delivery line  22  to the ultimate source of paint, such as a color changer (not shown) as now described. 
     As shown in  FIG. 6I , the pneumatic pressure from the source of pneumatic pressure  90  drives the first and second pigging elements  20 A and  20 B through the delivery line  22  to the first pig station  76  adjacent the paint cannister or dosing device  64 . As the solvent  60  between the first and second pigging elements  20 A and  20 B is received through the delivery line  22 , the solvent washes or purges the delivery line of paint. Further, as described above, the pigging elements  20 A and  20 B having generally frustoconical end portions  26  (see  FIGS. 1 to 3 ) effectively wipe and purge the delivery line  22  particularly in combination with the solvent. The paint  80  is simultaneously delivered to the paint cannister or dosing device  64  and the piston  70  is retracted to receive the paint remaining in the delivery line  22  as shown by the arrow in  6 I. Finally, the piston is again extended to deliver paint received by the dosing device  64  as shown in  FIG. 6J  through the inlet  66  to the ultimate source of paint, such as a color changer (not shown). As described in a co-pending U.S and EP. patent application, the paint delivery system may include two paint dosing devices  64  operating in tandem each having a delivery line connected to the applicator  62  and an inlet connected to the ultimate source of liquid paint, such as a paint cannister (not shown) to reduce cycle time. 
     The method of delivering solvent and paint from a source of liquid paint through a delivery line to an applicator of this invention will be understood from the description of  FIGS. 6A to 6J , above. Briefly, the method of this invention includes locating first and second pigs or pigging elements  20 A and  20 B adjacent the source of liquid paint which, in the disclosed embodiment, is the paint cannister or dosing device  64 . The method then includes delivering solvent  60  under pressure between the first and second pigs  20 A and  20 B, thereby spacing the first and second pigs with solvent  60 . The method of this invention then includes driving paint  80  under pressure against an adjacent one of the first and second pigs  20 A adjacent the source of liquid paint, thereby driving the first and second pigs with solvent therebetween through the delivery line  22  to the applicator  62 , flushing the applicator and then driving paint  80  through the applicator  62  to apply paint to a substrate (not shown). In the preferred embodiment of the method of this invention, the source of liquid paint is a paint cannister or dosing device  64  having an outlet  68  communicating with the delivery line  22  and a piston  70  movable within the cannister, wherein the method of this invention includes driving the piston  70  in the cannister  64  to drive paint  80  under pressure against an adjacent one of the first and second pigs  20 A and  20 B, thereby first delivering solvent  60  to the applicator  62  and flushing the applicator, and then delivering paint  80  to the applicator  62  to apply paint to a substrate (not shown). 
     In a preferred embodiment of the method of this invention, as described above, the method of this invention further includes delivering solvent  60  under pressure between the first and second pigs  20 A and  20 B when the first and second pigs are located adjacent or within the applicator  62 , then driving the first and second pigs from the applicator to the source of liquid paint, thereby flushing the delivery line  22  with solvent. Where the source of liquid paint is a piston dosing device  64 , as described above, the first and second pigging elements  20 A and  20 B also deliver paint  80  remaining in the delivery line  24  to the piston dosing device  64  and the piston  70  is then extended to drive the paint  80  to the ultimate source of paint, such as a color changer (not shown). 
     As will be understood by those skilled in this art,  FIGS. 6A to 6J  are schematic and various modifications may be made to the apparatus and method of this invention within the purview of the appended claims. For example, the first pigging station  76  may include a solvent dump (not shown) which receives solvent following purging of the delivery line  22 . Alternatively, a valve may be provided in the outlet  68  of the dosing device (not shown), which releases solvent through the outlet  68  following delivery of paint across the face of the piston head  72 , thereby flushing the piston head, and a solvent dump may be provided in the inlet line  66 . Further, the inlet line  66  may include a valve (not shown) which is closed when the piston  70  is extended as shown in  FIG. 6D , preventing return of paint  80  to the color changer. Alternatively, as described in a co-pending application, the line  66  may also include a pigging element (not shown) which drives paint from the color changer (not shown) to the piston dosing device  64  and serves as a valve preventing return of paint to the color changer when the piston is extended as shown in  FIG. 6D . 
     Also the step of delivering solvent e.g. at the second pig station  86  as described with reference to  FIGS. 4 and 5  may be modified. In particular, when introducing the solvent between the two pigs  20 A and  20 B, the solvent may only move one of the pigs which is pushed into the line whereas the other pig remains at its initial position within the pig station until the complete amount of solvent is introduced. 
     There are several further possibilities of modifying the method as described above. For example, it may not be necessary to deliver solvent into the supply line  22  at the first pig station  76  for delivery thereof to the applicator  62 . It may be sufficient to deliver solvent only from the second pig station  86  into the line  22  and to push it to the first station  76 , from where the solvent may be returned to the second station  86  or may be delivered at least partly into the cannister  64 . 
     An important further feature is the possibility of using a dosing cannister such as cannister  64  for dosing the amount of liquid solvent to be delivered into a supply line connected to the cannister, or into any other element of the system such as the cannister, itself. The essential feature of a method according to that possibility comprises delivering the solvent into said element while simultaneously retracting the piston of the canister, thereby dosing or metering the amount of delivered solvent by the retraction of the piston driven by the dosing drive means of the canister such as, for example, a servo motor. The retraction is controlled by program control means of the system. This feature makes it possible to first define a predetermined amount of solvent needed for a given purpose (usually cleaning), and then to deliver precisely the predetermined amount, thereby avoiding an unnecessary excess or waste of solvent, in contrast to systems where the amount of solvent delivered e.g. into a supply line is determined by valve switching times. An additional advantage of this feature is the fact that the solvent may be delivered with relatively and in some cases extremely low pressure. 
     When for example solvent is delivered from the source  46 B into the line  22  following delivery of paint  80  to the applicator  62  as explained above with reference to  FIG. 6G , the piston  70  of dosing cannister  64  may simultaneously be retracted, thereby dosing an initial amount of paint  80  introduced into the cannister and consequently the amount of solvent delivered on the side of pig  20 A opposite to the paint  80 . The delivery of solvent is stopped by the program control means when the piston has moved a predetermined distance. The solvent can then be pushed through line  22  between the two pigs  20 A and  20 B as explained above with reference to  FIGS. 6H and 6I . 
     When the solvent  60  has reached the position as shown in  FIG. 6I , it is possible to first extend the piston  70  to remove the paint  80  from the cannister through its inlet  66 , whereupon at least a part of said solvent  60  may be delivered into the cannister by simultaneously retracting the piston, thereby dosing or metering the amount of delivered solvent. It is also possible to push the solvent  60  as shown in  FIG. 6I  or a part thereof between the two pigs  20 A and  20 B back to the second pig station adjacent or within the applicator  62 . 
     The feature of dosing solvent delivered to an element of a paint conveying system by piston retraction may also be implemented at other locations of the system, for example between a color changer and the dosing cannister. 
     As set forth above, the apparatus and method of this invention is not limited to the improved pigging element  20  illustrated in  FIGS. 1 to 3  which is also the subject of the above-referenced German patent application. Further, a conventional delivery line may be utilized with the apparatus and method of this invention. The term “paint,” as used herein is generic to any liquid coating which may be applied by an applicator. The preferred solvent will depend upon the application. For example, where the paint is a water based paint, the solvent may be deionized water. Although the preferred embodiment of the apparatus of this invention includes a paint cannister having a piston or piston dosing device as described, the source of liquid paint may also be the color changer. Finally, the delivery line may include more than two pigging elements, wherein the additional pigging elements are utilized to separate other fluids delivered through the delivery line. Having described preferred embodiments of the apparatus and method of this invention, the invention is now claimed as set forth below.