Abstract:
An apparatus and method for the aerosolization of powders is disclosed. The invention includes a chamber in which a cloud of aerosolized powder is formed, the cloud having a relatively even distribution of powder particles. A Bernoulli tube may be used to extract powder from the powder cloud when the powder cloud reaches its equilibrium height. Adjustment of the air flow rate into the Bernoulli tube may be used to control the flow rate of powder out of the disclosed device.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to a system for aerosolizing powders for use in powder spray painting and other powder spray applications. In particular, the present invention is directed to a system for generating an air stream of aerosolized fine powder by forming a turbulent powder cloud from which powder is extracted, which results in a powder spray to a workpiece having a constant powder mass deposition rate. 
     A steady flow rate of unagglomerated powder particles from the powder spray device is essential to the formation of a smooth, uniform-thickness powder layer on the substrate to be painted in powder spray paint applications. Conventionally, fluidized beds or vibrating troughs have been used to feed powder into the powder spray system. For example, U.S. Pat. No. 5,745,954 to Shutic discloses a powder painting system including a fluidized powder bed. In the disclosed device, powder is fed into a powder feed hopper and falls to the fluidized bed at the bottom of the hopper. The fluidized bed is maintained through the use of pressurized air inlets in the floor of the hopper and rotating baffles. Suction tubes at the top of the hopper extract powder from the fluidized bed. 
     U.S. Pat. No. 5,654,042 to Watanabe et al. also discloses a powder coating system utilizing a fluidized bed. To improve performance of the device, Watanabe et al. also discloses the use of low-pressure gas pulses directed counter to the normal flow direction of the powder out of the hopper. These low-pressure gas pulses create microvibrations within the powder intake to alleviate adherence and cohesion among powder particles at the pump inlet. 
     Other prior art devices have attempted to solve the problem of uniform powder flow without fluidized beds. U.S. Pat. No. 5,752,788 to Crum discloses a powder spray device using a vaned impeller to distribute powder before it is delivered to a powder spray gun. The device includes a control system to adjust the rate at which powder is metered to the powder spray gun so that the mass of powder exiting to the powder spray gun remains relatively constant. 
     In addition, U.S. Pat. No. 4,116,367 to Kataoka et al. discloses an apparatus for supplying powder to a continuous metal casting mold. In this device, powder is fed down into a hopper with compressed air openings along its sides. The powder falls from the hopper through a hole in the bottom, and enters an intake section where the powder is pushed upward by air nozzles. The powder is then sent by a screw to a horizontal air nozzle, which pushes the powder toward the spray nozzle device. 
     None of these devices is completely satisfactory in operation for steady delivery of powders, and this problem remains as one of the primary difficulties in powder spray painting. In particular, these devices are unable to create a flow of powder in which powder is for the most part separated into individual particles prior to delivery of the powder to the powder sprayer. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the limitations of the prior art by creating a uniform cloud of powder from which powder is extracted and delivered to a spray gun or other aerosolized powder application. The powder is originally fed into a powder reservoir of the disclosed device. Compressed air is forced into the powder, thereby agitating the powder and creating a cloud of aerosolized powder that rises upwards on the air stream. 
     The powder passes through a hole in the top of the powder reservoir and enters a chamber connected to the powder reservoir. The chamber is shaped roughly as a funnel, with the diameter of the chamber generally increasing with height. As the aerosolized powder rises in the chamber, the air flow speed, and hence the upward air drag force on the powder particles, decreases due to the increasing diameter of the chamber. At some point, the air drag force on a powder particle becomes equal to the particle weight, and a cloud of powder collects in the chamber in the vicinity of this equilibrium height. It is believed that the formation of a powder cloud in the chamber is enhanced by the Bernoulli effect because the air pressure in the chamber increases with height as the air flow speed decreases. The resulting powder cloud is approximately stationary, internally turbulent, and has an approximately uniform space and time averaged mass density. 
     Aerosolized powder is extracted from the powder cloud by a Bernoulli tube which passes horizontally through the chamber at the equilibrium height of the cloud. Tests have shown that aerosolized powder consisting of single powder particles can be extracted from the device at a uniform rate. The rate of powder extraction can be varied simply by varying the air flow rate through the Bernoulli tube. 
     It is therefore an object of the present invention to provide a system for the aerosolization of powders using a turbulent powder cloud. 
     It is a further object of the present invention to provide a system for the aerosolization of powders that produces a uniform distribution of powder when the powder is distributed onto a substrate. 
     It is a still further object of the present invention to provide a system for the aerosolization of powders wherein single powder particles can be extracted from the system at a uniform rate. 
     Further objects and advantages of the present invention will be apparent from a consideration of the following detailed description of the preferred embodiments in conjunction with the appended drawing as briefly described following. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a cut-away elevational view of a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a preferred embodiment of the present invention is shown. A powder reservoir is defined by cap  1  and cup  2 . In the preferred embodiment, cap  1  and cup  2  are shaped roughly as funnels, and are joined at their large ends and mounted with their symmetry axes oriented vertically. A plug  3  closes the lower end of cup  2 ; in an alternative embodiment, cup  2  may have no hole in its lower end in which case plug  3  is not required. Cup  2  may be partially filled with glass beads (not shown) to facilitate the agitation of dry powder loaded therein. Above cap  1  is chamber  4 , which is connected to cap  1  by flexible coupling  5 . Chamber  4  is mounted with its small end down and with its vertical symmetry axis aligned with cap  1  and cup  2 . The diameter of chamber  4  increases with height, creating a roughly funnel-like shape. 
     In an alternative embodiment, the powder reservoir can be formed of a single piece of material. In another alternative embodiment, the entire powder reservoir and chamber assembly can be formed of a single piece of material. Also, although chamber  4 , cap  1 , and cup  2  in the preferred embodiment are made of an inexpensive plastic material, such as polyethylene, these parts could be made of conducting material. The use of a conductive material might serve to suppress tribocharging of powder particles due to agitation of powder in cap  1  and cup  2 . 
     Electromagnetic vibrator  6  has upper and lower parts separated by elastic spacer  7 . Vibrator  6  is powered by variac  8 , and can be a mechanical vibrator of any conventional design. AC line plug  9  directs power to variac  8 . Vibrator bracket  10  connects cup  2  to vibrator  6  so that the application of power to variac  8  will activate vibrator  6 , which will in turn agitate cup  2 . The lower part of vibrator  6  is fastened to support post  11  by means of bracket  12 . 
     Compressed air source  13  delivers compressed air to lower valve  14 , which controls the flow of air into lower nozzle  15  which is directed downward into cup  2 . Support rod  16 , secured in spacer ring  17 , holds lower nozzle  15  firmly in place within cup  2 . Baffle  18  is mounted above lower nozzle  15  within cap  1 . Compressed air source  13  also directs air to upper valve  19 . Upper valve  19  controls the flow of air into upper nozzle  20 , which is directed horizontally into chamber  4 . Opposite upper nozzle  20  in chamber  4  is flare  21 , which receives the flow of air from upper nozzle  20 . Upper nozzle  20  and flare  21  are supported by mounting brackets  22 . In an alternative embodiment, lower nozzle  15  and upper nozzle  20  may be fed by separate compressed air sources. Also, any compressed gas could be substituted for air in the present invention. 
     Within chamber  4  is screen  23 , which is supported by mounting ring  24 . The mesh size of screen  23  will depend upon the particular powder used. At the top of chamber  4  is filter  25 . Plenum  26  lies above filter  25 , and is connected to a suction vent (not shown) that maintains a slight vacuum across filter  25 . Additional mechanical support for the system is provided by support post  27 , bracket  28 , support post  29  and bracket  30 . Plastic bolts and screws (not shown) are used to join parts of the system. 
     The operation of a preferred embodiment of the invention may now be described. Powder A is loaded into cup  2  simply by pouring it into the top of chamber  4  with filter  25  removed and lower valve  14  and upper valve  19  closed. In an alternative embodiment, a standpipe port could be added to cap  1  for more convenient powder loading. After powder A is loaded into cup  2 , vibrator  6  is activated by applying power from AC line plug  9 . Vibrator  6  agitates cup  2 , thereby causing powder in cap  1  and cup  2  to settle continuously to the bottom of cup  2  during operation. 
     To create powder cloud B, first lower valve  14  is opened such that compressed air from compressed air source  13  is forced through lower nozzle  15  and into powder A in cup  2 . The air stream from nozzle  15  agitates the central portion of powder A in cup  2  such that powder is projected violently against the walls of cap  1 . Baffle  18  prevents powder from being projected directly into chamber  4 , but the air stream must flow eventually out the upper end of cap  1  around baffle  18 . Thus aerosolized powder is carried by the air flow stream upward through the bottom end of chamber  4  from cap  1 . 
     As the aerosolized powder rises in chamber  4 , the air drag force on entrained particles decreases as the air flow speed decreases. At some height, depending on the particle size and density, the air drag force on a given particle becomes equal to the particle weight. This stagnation results in the formation of powder cloud B, which is approximately stationary in chamber  4 . It is believed that the formation of powder cloud B in an approximately stationary location is aided by the Bernoulli effect because the air pressure in chamber  4  increases with height as the air flow speed decreases. 
     The combination of the Bernoulli effect and the equilibrium between the particle weight and air drag force results in the formation and persistence of powder cloud B in chamber  4 . With no powder being extracted, the average spatial density of particles in powder cloud B reaches an equilibrium state wherein more powder is prevented from entering the cloud from below because of the “weight” of the collective cloud particles exerted through collisions on particles rising below. In this state, an equilibrium exists between entry of additional particles from below and collection of particles from powder cloud B on the wall of chamber  4 . Powder collected on the wall of chamber  4  trickles back down the wall, with this flow process being aided by the vibrations of chamber  4  due to its mechanical connection with vibrating cap  1  and cup  2 . This powder reenters cup  2  or is reaerosolized in chamber  4 . Also, screen  23  blocks any large particle agglomerates that may have entered chamber  4  around baffle  18 . The agglomerates fall back into cup  2 . 
     To extract aerosolized powder from powder cloud B in chamber  4 , upper valve  19  is opened. Compressed air from compressed air source  13  then flows through upper valve  19  and then through upper nozzle  20 , across a short air gap in chamber  4 , and passes then to flare  21 . Together, upper nozzle  20  and flare  21  form a Bernoulli tube passing horizontally through chamber  4  at the central height of powder cloud B. This Bernoulli tube causes the extraction of powder from powder cloud B through flare  21  and in the direction of arrow C. The rate of powder extraction from powder cloud B is controlled simply by varying the air flow rate to upper nozzle  20  with upper valve  19 . When powder is being extracted from powder cloud B by flare  21 , the corresponding reduction in the “weight” of powder cloud B allows the extracted powder to be replaced by powder particles rising from cup  2  so that the spatial density of particles in powder cloud B remains approximately the same. Thus it is possible to extract powder from powder cloud B at a uniform rate because of the self-regulating behavior of the powder cloud density. 
     Although most powder rising in chamber  4  from cup  2  collects in powder cloud B, turbulence causes some powder particles to continue rising to the top of chamber  4 , especially the smaller, less dense particles. This relatively small amount of powder is removed from the air flow by filter  25  as the exhaust air flows out of the system with low speed into plenum  26  and is carried away by a weak suction vent at the top of the device (not shown). 
     One common problem in powder spray painting systems is that powder left in connecting hoses and other conduits at operation shutdown can contribute to uneven powder flow when the system is next activated. This problem may be avoided with the preferred embodiment of the present invention by first stopping air flow into cup  2  using lower valve  14 , and then allowing the device to clean itself due to the air stream from upper nozzle  20 . Upper valve  19  may be closed after an appropriate cleaning period. The device will then be clean immediately at the next startup. 
     I have verified the results described above by qualitative observations of the present invention in operation, and by powder deposition tests done with the invention used in conjunction with a powder sprayer. The powder deposition test results show that steady powder streams, relatively free of agglomerates, are routinely delivered to workpiece surfaces by the preferred embodiment of the present invention. The steadiness of the powder delivery rate is evidenced by the steadiness of the current carried to the workpiece by the generated charged powder stream. The largely agglomerate-free nature of the powder stream is verified by the study of photomicrographs of surfaces momentarily exposed to the powder stream. 
     It should be understood that the theories of operation provided herein may be incomplete or inaccurate without limiting the results described and the invention claimed below. The present invention has been described with reference to certain preferred and alternative embodiments which are intended to be exemplary only and not limiting to the full scope of the present invention as set forth in the appended claims.