Abstract:
An electrostatic fluidized bed powder coating apparatus including powder coating structure, such as a coating enclosure, hood or booth, and a powder fluidizing bed operatively associated with an electrostatic charging device. An enclosed powder accumulator is provided for collecting excess powder from the powder coating structure. A vacuum pump communicates between the powder coating structure and the powder accumulator and is operable by a source of compressed air for forming and controlling a cloud of powder emanating from the fluidizing bed and for transferring excess powder from the powder coating structure to the powder accumulator. In the preferred embodiment, the accumulator includes a cyclone housing. A powder reclaim feeder is disposed below and in communication with the cyclone housing and further communicates with a new powder feeder. A powder conveyor, in the preferred form of a rotating auger, transfers powder from the new powder feeder into the powder reclaim feeder and ultimately transfers the mixed powder into the powder coating structure.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to powder handling systems and methods and, more specifically, to electrostatic fluidized bed powder coating systems and methods. 
     BACKGROUND OF THE INVENTION 
     Powder must be handled and transferred in a wide variety of systems. For example, powder coating technology has generally evolved over several years into several different coating techniques performed with various types of coating systems. With each technique and apparatus, a powder, such as a resinous polymer or paint, is initially adhered to an electrically conductive object or substrate. This initial coating typically involves electrically grounding the object or substrate and electrostatically charging the powder particles such that the electrostatic attraction causes the powder to adhere to the part or substrate in a uniform thickness. This initial powder coating is then cured using heat or other techniques, such as infrared or ultraviolet light, to fully adhere the coating to the part or substrate. 
     Conventional techniques for adhering the powder particles to an object before curing have included three general types. Two of these coating techniques involve the use of fluidized powder beds. In the first of these techniques, the part is heated and then dipped into a fluidized bed of powder particles. The particles partially coalesce or tackify and thereby stick to the part. The second technique involves electrostatically charging the powder particles such that they emanate in a cloud from a fluidized powder bed. When an electrically conductive, grounded part is placed with this emanating cloud of electrostatically charged powder particles, the charged particles will be attracted to the outer surfaces of the part. The grounded part may be manually placed within a powder coating structure containing the electrostatically charged powder cloud or may be on a conveyor system or automatic feed system which moves parts or the substrate continuously into and out of the coating structure. A third general technique for powder coating also involves electrostatic charging of powder particles, however, this technique utilizes a spray gun. An electrostatic spray gun generally emits powder particles while electrostatically charging those particles by utilizing a charged electrode at the gun nozzle. Guns also exist which emit a stream of powder particles that are charged by a process commonly referred to as tribo charging. In spray coating techniques, the parts to be coated are again electrically grounded and are typically contained within a coating structure or hood having a vacuum system which collects excess sprayed powder. The three general techniques described above may also have many variations. 
     Each of the above described powder coating techniques has different advantages and limitations. Powder handling in general is also a problematic area in that it often involves a variety of powder filtering, transfer and containment challenges. Powder coating and process equipment generally is connected to powder collection equipment for collecting airborne powder which has not adhered to the part or substrate during the initial coating process. This equipment may also be referred to as powder reclaim equipment and has been free standing structure relative to the powder processing or coating equipment. This results in increased use of floor space and higher associated costs. For example, in a typical electrostatic fluidized bed coating system, excess powder is reclaimed from powder coating enclosure or structure with vacuum applied by a collector including a blower. Within the collector, and upstream of the blower, the powder is the trapped within one or more filters while air exits the collector. Periodically, the filters are internally pulsed with positively pressurized air to disengage the powder from the filter. The powder then may drop into a reclaim hopper located below the powder collector. The reclaimed powder is then transferred manually or by a conveyorized system to the powder processing equipment, such as the powder coating structure or enclosure. 
     Powder coating equipment, such as described above, also has drawbacks in terms of the ability to adjust the vacuum being applied to the powder coating structure or enclosure. The blower used in the powder collector portion of the system draws a specific amount of air usually designated in cubic feet per minute. One or more conduits may be connected between the blower and the powder coating structure or enclosure and, for adjustment purposes, slide gates have been connected within these conduits to selectively block the air and powder flow. In this way, air and powder being drawn out of the powder coating structure or enclosure may be increased or decreased depending on the position of the slide gate. While some operators have been known to mark the slide gate position at a desired location, this has not been a generally acceptable or precise manner of adjustment. Moreover, the use of a blower assembly in combination with a pulsable filter within a collector is rather cumbersome. In addition, as the filter or filters become clogged with powder, there can be an undesirable change in the level of collection vacuum applied to the powder coating structure or enclosure. This can adversely affect the powder coating process. 
     To address problems such as these in this general area of powder handling and coating technology, it would be desirable to provide a powder handling and/or processing or coating system which may be automated, compact and more portable, and more easily and precisely adjustable in accordance with the specific application needs. 
     SUMMARY OF THE INVENTION 
     The present invention provides powder handling and coating apparatus and methods achieving advantages to address the problems mentioned above as well as other powder coating and handling problems. For example, the invention can provide an automated powder coating system which is relatively compact as compared to prior systems. Powder may be conveniently added to supply a closed loop powder handling system of the invention. The system can also automatically mix reclaimed powder and new or so-called virgin powder prior to conveying the mixture into coating structure associated with the system. Also the system eliminates the need for the primary powder filters typically contained in the powder collection loop and therefore eliminates the change in collection vacuum associated with such filters. Also, powder color and/or powder type may be more easily changed due to the elimination of filters in the powder collection loop and the more compact system configuration. 
     In fulfillment of these and other advantages, and in accordance with one aspect of the invention, an electrostatic fluidized bed powder coating apparatus is provided which may include typical powder coating structure, powder fluidizing bed structure and an electrostatic charging device disposed to charge the powder such that it emanates from the fluidizing bed. The powder coating structure may be an enclosure which substantially fully encloses a product, part or substrate during a coating operation or may be a structure which has one or more openings to allow automated or manual introduction of such products, parts or substrates. In accordance with this aspect of the invention an enclosed powder accumulator collects excess powder from the powder coating structure during the powder coating operation. In accordance with the invention, a vacuum pump communicates between the powder coating structure and the powder accumulator and is operated by a source of compressed air capable of precise regulation. Due to the use of a vacuum pump in this way, primary powder filters and associated pulse valves are not necessary. This eliminates the significant drawbacks of blower and filter systems as generally used with fluidized bed systems in the past. The vacuum pump precisely controls the negative pressure in the powder coating structure to ensure full, uniform coating. The vacuum pump can also immediately transfer excess powder from the powder coating structure to the powder accumulator without the need for repeated filter pulsing operations. 
     A pressure regulator may be advantageously connected to the vacuum pump and, more specifically, to the compressed air being introduced into the vacuum pump. As mentioned above, this pressure regulation precisely controls the collection vacuum being applied to the coating structure. In one desirable embodiment, a plurality of vacuum pumps may be connected with a plurality of conduits leading from different locations of the powder coating structure to the powder accumulator. For example, a powder coating area may be a central area within the powder coating structure and the powder coating structure may further include a pair of powder drag out areas. These powder drag out areas are preferably connected with at least one additional source of vacuum, such as additional vacuum pumps as described above, to transfer powder from the drag out areas to the powder accumulator. 
     As a further advantage of this invention, the accumulator is preferably a cyclone housing including an air and powder inlet through which air and excess powder are received from the vacuum pump associated with the coating structure. A powder reclaim chamber or, more specifically, feeder is located below the air and powder inlet of the cyclone housing and an air vent is disposed above the air and powder inlet. Thus, air entering through the inlet exits the cyclone housing through the air vent while excess powder loses energy due to the cyclonic flow pattern and drops into the powder reclaim feeder. 
     Also in accordance with the invention, a powder conveyor is connected between the powder reclaim feeder and the powder coating area for transferring the excess or reclaimed powder back into the powder coating area. Even more desirable is a construction in which a new powder feeder is mounted adjacent to the powder reclaim feeder and a conveyor, preferably in the form of a motorized, rotatable auger, extends from the new powder feeder through the powder reclaim feeder, and into the powder coating area. Finally, the reclaimed powder is preferably transferred into the powder coating area at a rate faster than new or so-called virgin powder is transferred into the powder reclaim feeder. This helps prevent powder in the reclaim feeder from reaching a level that interferes with the operation of the cyclone housing. These many features and additional features of the inventive apparatus may be combined in various manners to achieve one or more advantages of the invention. 
     The invention further contemplates methods of forming and controlling a cloud of powder, such as during a powder coating process. Such methods can include electrostatically charging and fluidizing a bed of powder to initiate the formation of a cloud of powder, applying negative pressure to the cloud of powder using a powder transfer device operable by a source of compressed air, and regulating the compressed air to control the negative pressure. As generally discussed with respect to the apparatus described above, these methods can also include transferring excess or reclaimed powder through a cyclone housing and into a powder reclaim feeder, introducing new powder into the powder reclaim feeder to produce a mixture of new and excess powder, and transferring the mixture of new and excess powder into the powder coating area. In general, the methods can include various steps performed in accordance with the operation of systems embodying the inventive concepts. 
    
    
     Additional advantages and objectives of the invention will become more readily apparent to those of ordinary skill in the art upon review of a detailed description of one preferred embodiment of the invention, taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an electrostatic fluidized bed powder coating apparatus constructed in accordance with one embodiment of the invention; 
     FIG. 2 is a side elevational view of the apparatus shown in FIG. 1, and partially fragmented to show certain internal components and operation; 
     FIG. 3 is an enlarged view of the conveyor and powder feeding structure shown in FIG. 2; 
     FIG. 4 is an enlarged view showing the operation of the cyclone housing of FIG. 2; and 
     FIG. 5 is a schematic representation of the control system associated with the apparatus of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring generally to FIGS. 1 and 2, an electrostatic fluidized bed coating apparatus  10  is shown constructed in accordance with one preferred embodiment of the invention. It will be appreciated that the invention may take on many other forms as well. Apparatus  10  specifically includes powder coating structure  12  operatively connected with both a fluidized bed  14  and an electrostatic charging unit  16 . In this embodiment of the invention, powder coating structure  12  is almost fully enclosed and adapted to coat a wire  18  in a continuous manner. Other substrates, including discrete parts and products, may be coated instead. Fluidized bed  14  preferably includes a porous plate through which pressurized air is directed and electrostatic charging unit  16  is preferably an electrode including many wire bristles. Fluidized bed  14  and electrostatic charging unit  16  may be constructed generally as disclosed in U.S. Pat. No. 94,606,928, assigned to the assignee of this invention and the disclosure of which is hereby incorporated by reference. As discussed in U.S. Pat. No. 4,606,928, but not shown in the present drawings, such electrostatic coaters may also utilize a vortex generating device for uniformly coating substrates, such as wires and the like. Many different types of electrostatic coating structures may be used in place of structure  12  shown in FIGS. 1 and 2. These, for example, may include structures having one or more larger openings for allowing larger parts or objects to be inserted into the coater and coated electrostatically, or may include hood structures having completely open side portions. It will be understood that the electrostatic charging unit  16  may also take on many forms depending on the application needs and preferences of the operator. 
     As further shown in FIGS. 1 and 2, a powder accumulator  20  receives powder via conduits  22 ,  24 ,  26  communicating with the inside of powder coating structure  12 . A vacuum source or powder transfer device  28 , to be described below with respect to one advantageous form, draws air and powder from the inside of powder coating structure  12 . Ports  30 ,  32 ,  34  lead from vacuum source  28  to a cyclone housing  36  of accumulator  20 . These ports  30 ,  32 ,  34  communicate in a tangential fashion with the inside of cyclone housing  36  to induce a cyclonic flow pattern. A number of spare ports  38  may be included on cyclone housing  36  for use in different applications. When not in use, these ports  38  may be plugged. Cyclone housing  36  is preferably a Nordson Model 237-615 accumulator obtainable from Nordson Corporation in Westlake, Ohio. An air vent  42  is connected to an upper portion of cyclone housing  36  above ports  30 ,  32 ,  34  and includes a compressed air inlet  44  to receive a small amount of compressed air, such as on the order of 5-15 psig. Air vent  42  leads to a filter box  46  having a filter  48  which captures the generally small amount of powder traveling through vent  42 . Filter  48  may be removably affixed in place with knobs  49  to allow for cleaning or replacement. 
     As generally shown in FIG. 2, air and powder will be drawn into cyclone housing  36  with a cyclonic flow pattern. As is known, such as disclosed in U.S. Pat. No. 4,710,286, this flow pattern will cause the powder to lose energy and drop out of cyclone housing  36  while air will be separated from the powder and drawn upward through vent  42 . This upward suction of air is assisted in the present invention by the injection of compressed air, as described above, through inlet  44 . As also generally shown in FIGS. 1 and 2, apparatus  10  may be supported with suitable frame structure  50  generally associated with the accumulator and filter box portions of apparatus  10  and frame structure  50   a  generally associated with the powder coating structure  12  of apparatus  10 . Apparatus  10  may further be manufactured in a more portable manner and include lockable, height adjustable casters or wheels  52 . 
     Referring mainly to FIG. 2, cyclone housing  36  is mounted above and communicates with a powder reclaim feeder  60 . While cyclone housing  36  and powder reclaim feeder  60  are described as part of accumulator  20 , it will be appreciated that accumulator may take other forms when practicing various aspects of the invention. A new or virgin powder feeder  62  is mounted adjacent powder reclaim feeder  60  and includes a removable lid  64  for allowing feeder  60  to be loaded with new powder  65 . Powder reclaim feeder  60  receives excess powder  67  being drawn out of powder coating structure  12 . Respective aligned openings  66 ,  68  are disposed in powder reclaim feeder  60  and new powder feeder  62  to allow transfer of new powder  65  from new powder feeder  62  into powder reclaim feeder  60 . During this transfer, new powder is mixed with reclaimed or excess powder  67 . It should be noted that the term “new” as used throughout with respect to powder  65  and powder feeder  62  is not meant in any limiting fashion. For example, powder  65  could also be composed of reclaimed or recycled powder which is periodically transferred or loaded into feeder  62 . The powder transfer between new powder feeder  62  and powder reclaim feeder  60  may be accomplished in several manners, however, the preferred manner is with a conveyor  70  taking the form of a motorized, rotatable auger. Auger  70  is rotated with a conventional motor  72  and gear box  74  suitably connected to rotate auger  70 . Auger  70  further extends through a pipe or conduit  76  connected to an outlet portion of powder reclaim feeder  60  using fasteners  77 . As shown in FIG. 2, the outlet of pipe  76  leads to the interior of powder coating structure  12  such that powder drops onto fluidized bed  14 . 
     As will be discussed further below, and still referring to FIG. 2, powder reclaim feeder  60  and new powder feeder  62  include respective proximity sensors  78 ,  80  for sensing the levels of powder  67  and  65  within powder reclaim feeder  60  and new powder feeder  62 . For example, proximity sensor  78  can determine when a level of powder  67  is too high such that it may interfere with the operation of cyclone housing  36 . A signal from sensor  78  may then be used to prompt the operator to take action or direct the operation of motor  72  to turn auger  70 . On the other hand, proximity sensor  80  may be used to determine when the level of powder  65  within new powder feeder  62  is too low and then indicate that additional powder needs to be added by the user. As further shown in FIGS. 1 and 2, a plurality of control boxes  82 ,  84 ,  86  may be provided for housing the various electrical and pneumatic controls used to operate apparatus  10  as will be discussed below with reference to FIG.  5 . 
     Referring back to FIG. 1, in this exemplary embodiment powder coating structure  12  is an enclosure including a central area  90  in which the coating operation takes place and two powder drag out areas  92 ,  94 . As shown with respect to drag out area  92 , an internal wall  96  substantially separates central area  90  from drag out area  92 . Although not shown in the drawing, the same structure exists as between central area  90  and drag out area  94 . Appropriately sized openings, including openings  98 ,  100 ,  102 , are contained in powder coating structure  12  for allowing the continuous travel of wire  18  or any other desired part or parts to pass into and out of coating structure  12 . Coating structure  12  includes an upper lid  104  which may be hinged and locked in a substantially airtight fashion using latches  106 ,  108 . Thus, it will be appreciated from a review of FIGS. 1 and 2 that positively pressurized air forced through fluidized bed  14  and powder bed  110 , coupled with the vacuum being drawn through conduit  24  will cause a powder cloud  112  (FIG. 2) to form within central area  90 . This powder cloud  112  will be electrostatically attracted to the grounded wire  18 . At the same time, vacuum drawn through conduits  22  and  26  connected to respective drag out areas  92 ,  94  will draw smaller amounts of powder from central area  90  into the respective drag out areas  92 ,  94  and finally into accumulator. 
     Referring briefly to FIG. 3, one preferred auger conveyor  70  includes first and second sections  120 ,  122  respectively disposed within new powder feeder  62  and powder reclaim feeder  60 . First section  120  is connected to an output of gear box  74  with a coupling  121  and may be an auger portion having a smaller pitch and/or a smaller diameter than second section  122 . This will ensure that for a given revolution of auger  70 , more powder is transferred out of powder reclaim feeder  60  than out of new powder feeder  62 . This helps ensure that the level of excess or reclaimed powder  67  contained in powder reclaim feeder  60  does not reach a height which may interfere with the operation of cyclone housing  36 . In the preferred embodiment, first section  120  has a one inch outer diameter and a {fraction (15/16)} inch pitch while section  122  has a 1{fraction (7/16)} outer diameter and a 1⅛ inch pitch. Also, since powder reclaim feeder  60  is disposed closer to powder coating structure  12 , excess or reclaimed powder  67  will be the first powder to be transferred into powder coating structure  12 . As conveyor  70  leads from new powder feeder  62  into powder reclaim feeder  60 , mixing of the two powders  65 ,  67  will take place within powder reclaim feeder  60  prior to its transfer into powder coating structure  12 . 
     Referring now to FIG. 4, the vacuum source  28  in this exemplary embodiment advantageously comprises a plurality of vacuum pumps  130 ,  132 ,  134 . Although various types of powder transfer devices may be used to advantage in applying certain principles of this invention, it is preferred that vacuum pumps, powder pumps or air amplifiers be used which may be operated through the use of compressed air inputs. Thus, FIG. 4 shows respective inlets  136 ,  138 ,  140  that direct compressed air into pumps  130 ,  132 ,  134  to draw powder and air from left to right, as shown in FIG. 4, through conduits  22 ,  24 ,  26  and into cyclone housing  36  via ports  30 ,  32 ,  34 . Pumps  130 ,  132 ,  134  may be suitably mounted within a housing  142 . This type of pump is also sometimes referred to as an air amplifier and may be obtained as Model DF5-6 pumps from Vaccon Company, Inc. located in Medfield, Mass. 
     A control system is schematically shown in FIG. 5 for controlling the operation of pneumatic and electrical components associated with apparatus  10 . Generally speaking, control system  150  includes a portion  152  dedicated to controlling the powder collection and air venting aspects of accumulator  20 . Another control portion  154  is dedicated to pneumatic control associated with powder coating structure  12 . Finally, a conventional programmable controller  156  is provided and may, for example, process signals from proximity sensors  78 ,  80  to control the operation of motor  72  as well as perform other control functions as will be apparent to those of ordinary skill from a review of this disclosure. In this regard, for example, proximity sensor  78  may send a signal indicating a high level of powder in powder reclaim feeder  60 . Controller  156  may be programmed to shut down apparatus  10  if this condition exists or may be programmed to start motor  72  to transfer additional powder from powder reclaim feeder  60  into powder coating structure  12 . Proximity sensor  80  may send a signal to controller  156  if a low level of powder is indicated in new powder feeder  62 . This may, for example, activate an indicator light or sound generator prompting the operator to add powder. In this case, controller  156  may also be programmed to shut down apparatus  10  until additional powder is added to new powder feeder  62 . Proximity sensors  78  and  80  may each be capacitive type proximity sensors, such as Model #KIE2015BOA/LS100AK, available from Efector in Exton, Pa. 
     Control portion  152  includes an electrically operated two-way solenoid valve  158  connected to a source of compressed air  160  for delivering compressed air preferably at 80-100 psig and 70 cfm to a plurality of pressure regulators  162 ,  164 ,  166 ,  168 ,  170  each having respective gauges  162   a ,  164   a ,  166   a ,  168   a ,  170   a  for displaying the regulated pressure. It will be understood that other conventional control valve set ups may be used as well. Regulator and gauge  162 ,  162   a  are respectively connected to an air line  171  leading to air input  44 . This air pressure is preferably maintained at about 5-15 psig. Regulators and associated gauges  162 ,  162   a ,  164 ,  164   a ,  166 ,  166   a , are respectively connected to air input lines  136 ,  138 ,  140  associated with the operation of vacuum pump  130 ,  132 ,  134  as shown best in FIG.  4 . Air pressure within input lines  136 ,  140  is preferably set to 10-30 psig. This pressure may be adjusted depending on the application and coating requirements. Regulator and associated pressure gauge  170 ,  170   a  may be provided in control system  150  as a spare. 
     In accordance with another aspect of the invention, a differential pressure gauge  172  may be operatively connected to the interior of powder reclaim feeder  60  (FIG.  2 ). Specifically, this differential pressure gauge may be a Minihelic II differential pressure gauge, Series and Model 2-5000 with a sensing range of 0-1.0 inches water column obtainable from Dwyer in Willow Grove, Pa. Pressure gauge  172  is used to indicate the pressure inside powder reclaim feeder  60 . When the air pressure leading to air vent input  44  is increased, the reading on differential pressure gauge  172  will also increase thereby indicating a growing negative pressure inside powder reclaim feeder  60 . It is desirable to keep the pressure inside powder reclaim feeder  60  as neutral as possible. Preferably, a reading of 0.05-0.3 inches water column is maintained on gauge  172 . For the preferred apparatus  10 , the above-mentioned air pressure of 5-15 psig maintained this target pressure inside powder reclaim feeder  60 . Thus, when the system is operating in the proper range, new or virgin powder may be added to the new powder feeder  62  without experiencing airborne powder therein due to undesirable positive pressure in the system. This also prevents undesirable positive pressure from reaching coating structure  12  (FIG.  2 ), for example, through pipe  76 . 
     Control portion  154  may also include a solenoid air valve  174  also connected to compressed air supply  160  for selectively supplying compressed or positively pressurized air to one or more pressure regulators. In the specific system shown, pressure regulators  176 ,  178 ,  180  are shown. Each of these regulators are associated with a respective pressure gauge  176   a ,  178   a ,  180   a  showing the output air pressure. Regulator  178  is further connected to a flow meter  182  in a conventional manner to regulate the flow of positively pressurized air to fluidized bed  14 . Regulators  176 ,  180  connected to gauges  176   a ,  180   a  and air lines  184 ,  186  are specifically dedicated to a vortex option as generally discussed in the above incorporated U.S. Pat. No. 4,606,928 for the specific application of coating wires and the like. A photohelic level sensor  188  is connected to coating structure  12  and, specifically, to coating area  90 . As is known in the art, this sensor can operate in conjunction with controller  156  to cause powder to be conveyed into coating structure  12  as needed. In the present case, controller  156  would activate motor  72  to turn auger  70  (FIG. 2) as previously described. 
     The structure and operation of the preferred embodiments of this invention should be understood in accordance with the foregoing description. While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the Applicants to restrict or in any way limit the scope of the appended claims to such detail. As a general example, the various features of the apparatus described herein in detail may be combined or substituted in various manners. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods as shown and described.