Patent Publication Number: US-6217654-B1

Title: Method and equipment for powder spray coating

Description:
TECHNICAL FIELD 
     The invention concerns a method and equipment for powder-spray coating of objects 
     BACKGROUND ART 
     Powder-coating equipment is known from the European patent application EP 0,718,043 A1 wherein a conveying airflow generates a partial vacuum in a vacuum zone of an injector and thereby sucks powder out of a container and moves it into a powder line. Downstream of the vacuum zone, supplemental air is supplied through a supplemental-air intake to the flow of conveying air and powder in order to implement a predetermined flow of total air. 
     European patent 0,412,289 B1 discloses electrostatic powder coating equipment having an injector fed with conveying and with supplemental air. The supplemental air can be conveyed as control air into the injector vacuum zone. 
     German Offenlegungsschrift DE 44 19 987 A1 discloses an injector/conveyor system to move coating powder, having a suction pipe connected to the injector vacuum zone and running downward into a powder container, whereby powder is sucked from this container into the vacuum zone. Compensating air is fed to the suction pipe end located remote from the vacuum zone to compensate for fluctuations in the suction amplitude. As shown by U.S. Pat. No. 3,746,254, the powder container can also be mounted above the injector vacuum zone, instead of underneath, whereby the powder suction aperture is located above the vacuum zone. 
     European patent application EP 0,769,327 A1 discloses powder coating equipment wherein the injector is located at the lower end of a pipe which is immersible into a powder container in order to convey powder from it. 
     The present invention encompasses all the above embodiments and is not restricted to any particular embodiment. 
     It is known with respect to powder coating equipment to cleanse the powder line of deposited powder particles from time to time, usually with a hose and spray apparatus, This procedure is required to preclude unchecked discharge of deposited, old powder particles which would interfere with coating the surface of an object. Ordinarily, compressed air is connected to the injector to flush the powder hose and the spray apparatus by being blown through the injector and the powder hose and spray apparatus. This known flushing procedure incurs a drawback in that the flushing air not only flows to the spray apparatus, but also divides inside the injector into other lines communicating with it. The known systems suffer from a further drawback in that the powder coating operation must be stopped when cleansing with flushing air and that later the conveying air and any supplemental air must be readjusted to the desired pressure levels and rates of conveyed powder. While powder deposits in the powder hose can be markedly reduced by using a hose made of an appropriate material, nevertheless, when dealing with great hose lengths, the quantity of deposited powder particles remains comparatively high. 
     The object of the invention is to achieve very high quality coating of a large series of objects without interfering with or interrupting this mass production on account of cleansing procedures or adjustments of the spray coating equipment. Illustratively, the invention enables mass-coating of automobile bodies with a surface quality as good as presently offered by liquid enamel. Accordingly, the object of the present invention is to place the highest requirements on powder coating quality regarding object adhesion, sealing of the powder coating after being baked onto the object, extreme uniformity and smoothness in the coating. The related method and equipment shall offer very high reliability in permanent operation. 
     SUMMARY OF THE INVENTION 
     A method for powder spray-coating objects utilizes pneumatic conveyance of powder from an injector to a spraying system having a spray coating zone through which objects are consecutively conveyed in an automated system. Power conveyance is interrupted between the consecutive objects as object gaps enter the spray coating zone while the conveying air flow is maintained by injection of controlling air into a vacuum zone of the injector which reduces the injector vacuum to a degree causing interruption of powder conveyance. Flushing compressed air may be injected between the injector and the spray device of the spraying system to automatically flush powder to maintain system integrity. Equipment for powder spray coating of objects utilizing the above method is also disclosed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 schematically depicts powder coating equipment of the invention (not to scale), and 
     FIG. 2 shows part of a further embodiment of powder coating equipment of the invention in which the omitted part is identical with the pertinent part of FIG.  1 . 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     The powder spray coating equipment shown in FIG. 1 contains a spraying system  2  comprising a spraying device  4  to spray coating powder on objects  6 , for instance automobile bodies, that are continuously conveyed along a conveyorized system  8  in the direction of an arrow  10 , transversely to and at the front side of the spraying device  4 , whereby these objects enter the spray coating zone of the spraying device. The objects  6  are separated from one another by a slight distance or gap  12 . The spraying device  4  may be a spray nozzle as shown in FIG. 1 or a rotating unit, for instance a spray bell. The spraying device  4  is fitted with one or more high-voltage electrodes  14  to electrostatically charge the powder. The electrode  14  may project from a support tube  16  from which air passes over the electrode  14  to prevent powder particles from being deposited thereon. The high voltage for the electrode  14  may be generated by a high-voltage source mounted inside or outside the spraying system  2 . The spraying device  4  is located at the downstream end of a powder duct  18  of the spraying system  2  of which the upstream opening  20  communicates through a powder line  22  with the powder outlet  24  of an injector  26 . A conveying-air intake  28  is axially opposite the powder outlet  24  and is connected by a conveying-air line  30  containing a pressure regulator  32  and by a manifold  34  to a pressure source  36 . In the vicinity of the conveying-air intake  28 , an injector duct  38  comprises a vacuum zone  40 , hereafter called vacuum zone  40 , having a widened diameter and a reduced diameter powder-outlet duct  42 . A powder suction tube  46  runs from the vacuum zone  40  through a powder-intake aperture  44  into a powder container  48 , so that the conveying-air flow of the conveying-air line  30  causes a partial pressure, hereafter vacuum, in the vacuum zone  40 , to suck powder out of the powder container  48  and convey it through the powder line  22  to the spraying  5  system  2  which sprays the powder in the form of a spray cloud or spray cone onto the objects  6 . 
     A controlling-air intake  50  communicates with the vacuum zone  40  of the injector  26  and is connected by a controlling-air line  52  containing a pressure regulator  54  and by the manifold  34  to the compressed-air source  36 . By introducing controlling air into the vacuum zone  40 , the vacuum therein can be reduced to an extent, or eliminated, such that the powder conveyance from the container  48  is interrupted when the conveying air of the conveying-air line  32  flows at a constant rate through the injector  26 . 
     A supplemental-air intake  56  communicates with the powder-outlet duct  42  and is connected by a supplemental-air line  58  optionally containing a pressure regulator  60  and by the manifold  34  to the pressure source  36 . Together with the conveying air, the supplemental air provides a total air flow which prevents powder-flow pulses and substantial powder deposits in the powder line  22 . The total air flow might also be controlled using the controlling air from the controlling-air line  52  instead of the supplemental air of the supplemental-air line  58 , however, such a selection would be disadvantageous because the controlling air affects the vacuum effect in the vacuum zone  40  and hence the conveyor effect of the conveying air of the conveying-air line  30  and thereby the conveying air also would have to be changed if controlling air were used to compensate for changes in air flow in the powder line  22 . For that reason, the invention provides that the controlling air of the controlling-air line  52  is used only to negate the vacuum in the vacuum zone  40  in order to thereby shut down the conveyance of powder rather than only reduce it. During this stoppage of powder conveyance, the invention provides that, while an object gap  12  is present in front of the spray device  4 , the conveying air of the conveying-air line  30  and the supplemental air of the supplemental-air line  58  are fed unchanged to the injector  26  and to the powder line  22 . Powder conveyance is turned ON again once the next object  6  is in front of the spray device  4  by shutting OFF the controlling air of the controlling-air line  52 . The turning ON and OFF steps depend on conveyance rates and size of the objects  6  and is automated by a processor-controlled electronic control system  62 . The following advantages are achieved: when turning OFF powder conveyance, the residual powder is still being moved out of the powder/air path ( 38 ,  42 ,  22 ,  2 ) and is sprayed by the spray device  4 . As a result, when the powder conveyance is again turned ON, a powder impact is prevented and hence also coating defects on the object  6  to be coated next. Preferably, the control system  62  is programmed in such manner that powder conveyance shall not be turned OFF at the object gap  12 , but shortly before the end of the object  6  that was just coated, whereby all the powder still present in the powder path shall still be deposited on this object without introducing coating defects. This procedure offers the further advantage of reducing powder loss, namely, powder not adhering to the objects  6 . 
     The suction tube  26  is fitted with a compensating-air intake  66  at its lower end segment  64  away from the injector  26  to move compressed air acting as compensating air into the suction tube  46  in order to compensate for or to damp any pressure fluctuations. The compensating-air intake  66  is connected by a compensating-air line  68  optionally containing a pressure regulator  70  to the manifold  34  of the compressed-air source  36 . The compensating air is supplied at constant pressure and constant flow both during powder conveyance and during pauses therein. 
     To keep the spraying system  2  and optionally also the powder line  22  clean, a flushing-air device  72  comprising at least one flushing-air intake  73  is provided. The flushing-air intake  73  is preferably situated at a site wherein the flow impedance to this flushing air in the air/powder path subtended by the powder line  22 , the spraying system  2  and the injector  26  will be smaller in the downstream direction as far as through the spraying system  2  than in the upstream direction through the injector  26 . As a result, the flushing air flows only toward the spray device  4  and then towards the objects  6 , not in the opposite direction to the injector  26 . Therefore the flushing-air intake  73  is located between the half length of the powder line  22  and the spray device  4 , and in a modified embodiment, several such sites or flushing-air intakes may also be provided. The more appropriate the material of the powder line  22 —which ordinarily is a hose—to preclude powder deposits inside it, the farther the flushing-air intake  73  can be shifted downstream toward the spray device  4 . The powder path in the spraying system  2  may consist of a similar material to reduce deposits of powder particles. The spray device  4 , even when of the same material, must be cleansed inside with flushing air because the powder particles not only slide along the inside surfaces of the spray device  4 , but also impinge transversely on these inside surfaces. Accordingly, preferred embodiments provide that the at least one flushing-air intake  73  be mounted at a flushing-air supply site located between the half length of the powder line  22  and the intake  20  preferably at about 75% downstream from the upstream end of the powder line  22  or directly at the powder intake  20  of the spraying system  2 , or, according to FIG. 2 of the drawings, inside the spraying system  2  at a short upstream distance from the spray device  4  in order that the flushing air shall impinge free of throttling on the inside surfaces of the spray device  4  and shall flush away any powder particles. 
     In the embodiment of FIG. 1, the powder line  22  is a hose divided into an upward hose segment  22 - 1  of more than half-length and a short hose segment  22 - 2  substantially shorter than half the hose length. The two hose segments  22 - 1  and  22 - 2  communicate with each other through the flushing-air intake  73  which comprises an annular slit nozzle  74 . The flushing air flows from the slit nozzle  74  through the downstream hose segment  22 - 2  and then through the spraying system  2 , none of this flushing air flowing in the opposite direction to the injector  26 . 
     The flushing air is shut OFF during powder conveyance and is turned ON in the absence of powder conveyance either at each object gap  12  or only at one of several object gaps  12  in front of the spray device  4 . In a preferred embodiment of the invention, the flushing air is not supplied as a permanent flow of compressed air, but rather in the form of at least one, preferably at least two, three or four of brief, consecutive pulses of compressed air. To generate these impulsive compressed-air pulses, the flushing-air intake  73  is connected, through a compressed-air line  76  containing, in sequence of flow, a pressure regulator  78 , a pressure gauge  80  and a reversing valve  82 , to the compressed-air source  36  or its manifold  34 . Preferably, the reversing valve  82  is an electromagnetic or pneumatic two/two-way valve controlled by the electronic control  62 . The flushing-air intake  73  is fitted with a backflow stop  84  which may be a filter to filtrate powder particles and/or a check valve opening only in the direction of flow of the flushing air toward the flushing-air intake  73  but which will close when the fluid pressure is in the opposite direction. The compressed air used as flushing air is fed to the flushing-air intake  73  preferably at a rate of 15 to 40 Nm 3 /h and a pressure up to a maximum of 6 bars, preferably up to 5 bars. The pressure of the flushing air is correspondingly adjustable at its regulator  78 . 
     The flushing air can be turned ON either when all air hookups of the injector  26  are shut OFF or, in the preferred implementation, while all air hookups or at least the conveying air of the injector  26  remain ON. As a result and in this preferred implementation, while an object  6  to be coated is opposite the spray device  4 , conveying air from the conveying-air line  30 , supplemental air from the supplemental-air line  58  and compensating air from the compensating-air line  68  will flow through the spray device  4 ; when, however, an object gap  12  is opposite the spray device  4 , and the powder conveyance is shut OFF due to the controlling-air supply from the controlling-air line  52 , then the supplemental air of the supplemental-air line  58 , the compensating air of the compensating-air line  68 , and, during the cleansing phase, the flushing air from the flushing-air intake  73 , will flow through the spray device  4 . In a special mode of implementation, the flushing air already is turned ON when an end segment of an object  6  to be coated still remains in the spray zone of the spray device  4  in order that all the powder for the coating procedure be used for coating. The powder coating on the object  6  thereby remains unaffected in thickness or other features when the powder conveyance also is turned OFF shortly ahead of the end of the object  6  that was just coated on account of turning ON the controlling air of the controlling-air line  52  and being moved into the vacuum zone  40 . The electronic control unit  62  assures time coordination of the diverse kinds of air. 
     In the embodiment of FIG. 1 the flushing-air intake  73  comprises at least one annular slit nozzle  74  enclosing the air/powder path to preclude supplying to this path excessive flushing air and without need for especially high pressure. In the embodiment of FIG. 2 the flushing-air intake  73  instead comprises a plurality of annular boreholes  75  issuing into the air/powder path. This embodiment achieves an effect similar to that obtained with an annular slit nozzle, however at a somewhat larger flow impedance. Preferably, both the annular slit nozzle  74  of FIG.  1  and the flushing-air boreholes  75  of FIG. 2 are arranged obliquely at an angle of about 45° relative to the direction of the air/powder flow.