Electrostatic powder coating apparatus

An apparatus for applying powder coating material to a substrate is disclosed. The apparatus comprises: (A) an electrostatic spray gun for dispensing a mixture of electrostatically charged powder coating particles and a gas (e.g., air) inert to said particles; (B) a shroud which is attached to said gun; (C) a mixing chamber disposed in the rearward section of said shroud and into which said gun dispenses said mixture; (D) a forward section of said shroud having an opening adapted to be disposed in a position relative to the substrate such that powder coating particles issuing from the opening are attracted to said substrate; (E) at least two slotted openings formed by pairs of baffle plates connecting the mixing chamber with the forward section such that the pressurized mixture from said mixing chamber may pass through said slotted openings, into said forward section and through the opening therein toward the substrate to be coated; (F) field electrodes in electrical connection with said gun and being disposed in the vicinity of said slotted openings so as to create an electrostatic field in the vicinity of the opening of said forward section; and (G) a vacuum intake port disposed in the vicinity of the opening of said forward section between the slotted openings and being adapted such that eddies of said pressurized mixture flowing from the slotted openings in the direction of said port are created and such that said powder not attracted to said substrate is collected at said port.

BACKGROUND OF THE INVENTION 
Powder coating compositions for use in the coating or painting of the 
surfaces of objects are extremely desirable. In typical use, the powder 
coating compositions are electrostatically applied to the object to be 
coated. The coated object is then heated, which results in the cure of the 
powder coating material and in its being bonded to the object. The primary 
advantage of powder coating compositions over liquid coating systems is 
that, in the case of the former, no volatile solvent or other liquid 
vehicle is given off during the curing or drying of the coating. This is 
particularly significant in large scale commercial coating operations in 
which it is desirable and necessary to minimize. 
Various powder coating compositions are known in the art. Typically, these 
materials are comprised of minute particles of resinous compounds. These 
resinous materials are finely pulverized to produce powder particle sizes 
ranging, for example, from a few microns to perhaps 50 microns. 
It has been prior art practice to apply these finely pulverized powder 
coating materials to an object to be coated by use of electrostatic spray 
techniques. In a common electrostatic spray installation, the object to be 
coated is suspended within and moving through a spray booth where it is 
electrically grounded. An operator standing outside the spray booth holds 
an electrostatic spray gun to which a high voltage of appropriate polarity 
is applied. This voltage may be on the order of tens of kilovolts. A 
mixture of compressed air and the powder coating material is guided 
through the electrostatic spray gun and is discharged into the booth 
through an opening provided therefor. A cloud of air-suspended powder is 
produced within the booth and a small portion of this powder is 
electrostatically attracted to the surface of the object to be coated. A 
vacuum source draws the remainder of the air and powder coating mixture 
from the booth through hoppers in the floor of the booth. This booth 
exhaust mixture is conveyed to a dust collector where the unused powder 
coating material is removed from the air. The collected powder coating 
material may be recycled if desired when a single color is being applied 
in the booth. However, when powders of different colors are applied within 
a short time span without completely cleaning the booth and hoppers, the 
collected powder is a mixture of the colors and must be discarded. 
In the case of large objects to be coated, it is possible to construct a 
large booth wherein the operator actually stands within the booth during 
the application of the powder coating material. However, it may be 
appreciated that the operator must be protected from the danger of powder 
inhalation, and the subatmospheric or vacuum pressure source for removing 
the air-suspended powder coating material from the booth must be of very 
large capacity. 
The prior art systems described above have a number of disadvantages, 
several of which are: 
1. They do not allow precise, definite control of the spray pattern and, 
thus, are not ideally suited to form a controlled design or pattern on the 
object being coated. 
2. Where a small booth is used and the operator stands outside, only small 
objects may be coated. 
3. Frequent changes of color are difficult because the booth and any vacuum 
exhaust from the booth must be thoroughly cleaned with each change of 
color if the unused powder is to be recycled. Failure to do this could 
result in a mixture of the various colors used. This problem is 
particularly acute in the case of a large booth where frequent color 
changes are even more difficult because of the booth size, a thorough 
cleaning of the booth being both time consuming and difficult. In those 
installations where it is necessary to frequently change colors, e.g., 
painting of automobile bodies with top coat materials, it is not practical 
to clean the system between colors and, therefore, it is impossible to 
recycle excess powder. As mentioned above, the excess powder is generally 
collected in hoppers in the floor of the booth and drawn by vacuum to bag 
houses. Obviously, when frequent changes of color are affected, the powder 
being drawn to the bag house contains a mixture of pigments. Thus, in such 
an operation, the excess powder must be discarded. This is not only 
wasteful, but also creates a disposal problem. 
4. Because of the size of most spray booths relative to the size of the 
article to be coated and the open nature of the booth, any return system, 
to be effective, must be inordinately large. This, of course, is 
inefficient and raises the expense of the installation. 
5. Because of the nature of most prior art spray booth installations, i.e., 
the object to be coated moving in an unconfined manner through the center 
of the booth on a conveyer, there are serious overspray problems resulting 
in contamination of the booth and a concomitant wast of powder. 
6. Because of the somewhat open nature of the interior of most booths, 
stray drafts tend to affect the dust clouds causing possible variations in 
application density and a waste of coating material. 
Ser. No. 443,555 filed Feb. 19, 1974 and now abandoned as a 
Continutation-in-Part of Ser. No. 212,294, and now abandoned, both filed 
by the present applicant discloses an apparatus and method which overcomes 
many of the deficiencies discussed above. The apparatus disclosed in that 
application permits frequent color changes allows more complete recovery 
and recycling of each color of powder employed, is adaptable for coating 
large as well as small objects, maximizes use of a vacuum return system, 
substantially reduces overspray and draft problems, and allows precise, 
definite control of a spray pattern. The process which achieves those 
results and which was claimed in that application comprises passing a 
powder mixture into a shroud having an opening of determined configuration 
positioned adjacent the grounded object to be coated so that the mixture 
is confined to a volume bounded in part by the object to be coated. This 
permits the pressurized mixture dispensed by the electrostatic spray gun 
to expand within the shroud, filling it with the mixture. A portion of the 
charged particles are electrostatically attracted to the object to form a 
coating on it in the configuration of the shroud opening. By employing a 
shroud with a configured opening it is possible using that method to apply 
a sharply defined pattern of paint, an accomplishment which theretofore 
had not been possible in the application of powders and possible in the 
application of liquid paints only with the use of masking. After 
application of the coating to the article, the remainder of the expanded 
mixture in accordance with that process is drawn with a subatmospheric 
pressure source into a chamber positioned around or surrounding the 
shroud. This chamber is wholly external of the interior of the shroud and 
after the powder enters the chamber, the mixture flows toward the 
subatmospheric pressure source. 
While the above discussed method and the apparatus for carrying it out 
overcomes many of the deficiencies of the prior art powder coating 
techniques and apparatus, it is still not ideally suited for application 
of powder coatings to substrates, particularly those of a curved contour. 
In fact, a serious disadvantage of the method is its marginal performance 
in coating sharply curved surfaces due to its dependence on a flat surface 
for forming a sufficient draught of air capable of causing atmospheric air 
to move unadhered powder to the powder collector. Also, that process, 
although a great improvement over prior art techniques, still demonstrates 
some deficiencies with respect to powder return for recycling. For maximum 
results with that method, it is necessary to space the shroud of the 
apparatus a maximum of 1/4 inch from the surface to be coated in order to 
effect efficient return of the powder. Also, without a flat surface in 
place, the device used in that process is not capable of recovering powder 
completely. Still further, the method and device of the former application 
does not demonstrate optimal efficiency insofar as application of the 
powder to the substrate is concerned. In fact, adhesion of desposited 
powder with that technique is weaker than desirable. Finally, the device 
and method of the prior application, although an improvement over the 
prior art, still requires more care in cleaning than is desirable to 
change colors rapidly. 
It is the object of this invention to overcome each of the above 
deficiencies by providing an apparatus in which: (1) powder over-spray is 
eliminated nearly completely due to a unique method of vacuuming 
electrostatically unadhered powder; (2) controllable auxiliary air, fed 
from a manifold to a mixing chamber, maybe used to assist in purging for 
color change; (3) application efficiency of virtually 100% is achieved due 
to recycling of unadhered powder; (4) precise spray patterns are achieved, 
controlled by the configuration of the applicator opening; (5) the device 
is adaptable for automation; and (6) the device demonstrates an 
outstanding ability to coat curved surfaces. 
SUMMARY OF THE INVENTION 
The above object is achieved by the apparatus of the subject invention 
which generally comprises: 
A. means for dispensing from an orifice a pressurized mixture of powder 
coating particles bearing an electrostatic charge and a gas which is inert 
to said particles; 
B. a shroud having (i) a rearward boundary which is attached to and 
surrounds the orifice such that said mixture may be dispensed into said 
shroud, and (ii) a forward section having an opening adapted to be 
disposed in a position relative to said substrate such that powder 
particles issuing from said opening are attracted to said substrate; 
C. a mixing chamber disposed within said shroud between said forward 
section and said rearward boundary and being adapted to receive said 
pressurized mixture from said orifice; 
D. at least two slotted openings connecting said mixing chamber with said 
forward section such that said pressurized mixture may pass from said 
chamber through said slotted opening into said forward section and through 
said opening in said forward section toward the substrate; 
E. field electrode means in electrical connection with said means for 
dispensing said pressurized mixture and disposed in the vicinity of said 
slotted openings, said field electrode means being adapted to create an 
electrostatic field such that said charged particles are repelled 
therefrom in the direction of said substrate; and 
F. an intake port which is disposed in the vicinity of said opening in said 
forward section between said slotted openings and which is connected to a 
source of subatmospheric pressure such that eddies of said pressurized 
mixture flowing from said slotted opening in the direction of said port 
are created and said powder not attracted to said substrate is collected 
at said port. 
Means for dispensing a pressurized mixture of powder coating materials and 
a gas, such as air, which is inert to said particles from an orifice are 
well known. For example, it is well known that electrostatic deposition of 
powder paint particles can take place by (1) fixed, applied electrostatic 
field lines of force; or (2) applied voltage between the charged powder 
and the object to be painted. For example, an electrical potential 
difference may be applied between the object to be coated and the 
electrode of a powder coating spray gun. The mixture is then guided past 
the spray gun electrode to thereby electrostatically charge particles of 
the powder coating material in the mixture. 
As mentioned above, the mixing chamber of the device is disposed within the 
shroud between the rearward boundary and the forward section thereof. The 
pressurized mixture issuing from the orifice of the spray gun is directed 
into this mixing chamber. If desired, the device may be provided with a 
means for supplying additional pressurized gas to said mixing chamber to 
thereby control the pressure under which the mixture passes from the 
mixing chamber through said slotted openings to said forward section. 
Desirably this means for applying additional pressurized gas comprises a 
manifold which is adapted to supply the pressurized gas to the mixing 
chamber in two or more places. The positioning of these orifices of the 
manifold can be optimized such that the manifold not only supplies 
additional pressurized gas so as to control the pressure of the mixture 
issuing from the mixing chamber, but also serves to aid in cleaning the 
mixing chamber and the slotted openings of the device when it is desired 
to change colors. This feature of the invention makes the device of this 
application very efficient for use in operations where frequent color 
changes are required. 
The slotted openings through which the pressurized mixture passes from the 
mixing chamber into the forward section of the shroud interior may be 
formed by pairs of baffle plates which converge toward each other and 
create a passageway of decreasing cross section in the direction of the 
flow of the mixture from the mixing chamber to the forward section. 
Generally, the slotted openings formed by the converging baffle plates 
will be directed slightly inward toward the intake port which is disposed 
centrally of the slotted openings. Preferably, the device comprises two of 
said slotted openings formed by pairs of baffle plates and the slotted 
openings extend from the top to the bottom of the shroud and are 
substantially parallel to each other. Also, it is preferred that the 
slotted openings formed by the baffle plates and the intake port be 
moveable relative to each other so as to adapt the device for easy control 
when applying powder material to various shaped surfaces and to maximize 
the collection of excess powder by the vacuum system. Also, in the 
preferred embodiment of the invention the slotted openings formed by the 
pairs of baffle plates extend beyond the leading edge of the forward 
section of the shroud. This aspect of the invention also affords greater 
control of the application of the powder to the substrate. 
The field electrode means may comprise conductive strips which are 
electrically connected to the electrode of the spray gun and which are 
disposed in the vicinity of the slotted openings, preferably being located 
on the interior surface of exterior baffle plate of each of said pairs. As 
mentioned previously the electrode strips are employed to create an 
electrostatic field in the vicinity of said forward section of said shroud 
near said substrate to be coated. The result is that charged particles 
(which bear the same charge as said field) are repelled from the 
electrostatic field in the direction of the substrate. This gives a much 
greater efficiency of application than was possible with the device of 
Ser. No. 443,555. 
In the preferred embodiment of the apparatus, the source of the 
subatmospheric pressure is connected to said intake port by a conduit 
which passes from said port through the interior of said shroud. This 
allows the apparatus to be rather compact, making it adaptable if desired 
to be used as a single hand held unit. 
By employing a shroud having a configured opening in the forward section 
thereof it is possible to apply a sharply defined pattern of powder paint, 
much as was the case with the apparatus disclosed in the above discussed 
application. However, with the device of this application it is possible 
to achieve an even sharper pattern with more complete deposition of powder 
coating material. By moving the shroud with its configured opening over 
the surface of the article to be coated, a continuous pattern of desired 
configuration, e.g., a stripe or a plurality of stripes if several units 
are used simultaneously, may be applied. Alternatively, the object to be 
coated could be moved so as to achieve a continuous pattern as desired. 
As was the case with the device disclosed in the above discussed 
application, the apparatus of this invention is not only adapted for 
applying coatings to a pattern defined by the shape of the shroud opening, 
but it is likewise adaptable for painting the entire surface of any part 
of any article to be coated. This may be accomplished merely by shaping 
the contouring and the opening of the forward section of the shroud 
employed in the apparatus such that the powder is electrostatically 
applied to all surfaces on which a coating is desired. Thus, the apparatus 
may be adapted to paint either an entire car body or a part thereof, such 
as a hood, door, etc. It is important to note here that the apparatus of 
this invention is much more efficient for painting curved surfaces such as 
would be found on the above mentioned substrates than was the previously 
discussed apparatus. 
As mentioned above, the intake port which is connected to the 
subatmospheric pressure source, is located centrally of the slotted 
openings through which the pressurized mixture is forced in operation of 
the apparatus. This is in contrast to the vacuum pick-up of the previously 
disclosed apparatus wherein the subatmospheric pressure was applied around 
the exterior of the shroud. The result of the revised disposition of the 
vacuum intake port is, in combination with the use of slotted openings for 
applying the powder coating material, a much greater efficiency in the 
pick up of the excess powder not adhered to the substrate. As was the case 
in the previous device, as the excess powder enters the intake port, the 
mixture flows toward the subatmospheric pressure source. Preferably, the 
mixture thus removed from the chamber is conducted through a dust 
collector where the unused powder coating material is collected for 
recycling through the system. For example, the pressurized mixture 
discharged from the spray gun may be expanded within the mixing chamber, 
forced through the slotted opening formed by the baffle plates into the 
forward section of the shroud and through the opening thereof where the 
particles are electrostatically attracted to the object to be coated. 
It is a significant feature of the apparatus of this invention that by 
disposing the vacuum intake port between the slotted openings through 
which the mixture is expelled, it is possible to create eddies of said 
pressurized mixture flowing from said slotted openings towards said intake 
port. These eddies assist in the application of the powder in an efficient 
manner to the substrate and also in the collection of the excess powder 
not attracted to the substrate. The above discussed field electrodes or 
strips also assist in the formation of the eddy currents inasmuch as they 
cause the powder particles coming from the slotted openings to be forced 
away from the slotted openings toward the substrate. Also, the slating in 
the formation of eddies flowing in a direction toward the intake port. 
By collecting any excess powder directly at the site of the application, it 
is possible to recycle a given color of powder without contaminating or 
mixing pigments. Furthermore, the apparatus of the invention makes it 
practical to apply various colors in sequence without mixing pigments. 
This could be accomplished preferably by employing a closed color loop, 
i.e., separate powder source, feed hose, shroud, vacuum source and 
collecting bag house, for each color. Thus, a production facility could 
include a number of such closed loops corresponding to the number of 
colors desired. Preferably, the shroud for applying the various colors 
would be set up in a clean, dust-free atmosphere such as a booth. However, 
it is pointed out that such a booth would serve the sole purpose of 
providing a clean, dust-free environment for the article being coated. 
Since each shroud would operate as a self-contained unit for applying and 
recycling powder, any booth employed would not serve the purpose of 
confining the powder and collecting the excess as was the case with the 
above-discussed prior art systems. As an alternative to the use of 
separate closed loops for various colors, it may be practical in 
production facilities to merely change the hoses connected to the shroud 
for feeding and collecting powders.

DETAILED DESCRIPTION OF THE INVENTION 
With reference to the drawings in which the numerals used correspond to the 
same parts in each figure, an electrostatic powder coating apparatus 
comprises an electrostatic spray gun assembly generally shown at 4 and a 
shroud-coater assembly generally shown at 36. The coating apparatus is 
shown in FIGS. 1 and 2 positioned adjacent to grounded object 30, such as 
a metal panel, on which the powder coating material is to be deposited. 
A suitable commercially available electrostatic spray gun 4 is manufactured 
by the Ashdee Corporation of Evansville, Indiana. This spray gun 4 
includes a body portion 6 having a central bore 8 therethrough. A powder 
inlet fitting, not shown, is located in one end of the bore 8 and a hose, 
not shown, is connected to it. At the other end of the bore 8, an 
electrode indicated at 10 is provided. This electrode includes a charging 
rod 12 and a charging tube 14. 
Electrically connected in series with the electrode 10 are a conductive 
connector rod 16, a conductive high-voltage connector 18, a resistor 20, 
and a conductive high-voltage spring 22. A conductive element inserted 
into an electrical connector, not shown, permits connection to the 
electrostatic spray gun 4 of one terminal 24 of a source of electrical 
energy, such as a direct-current high-voltage power supply 26. Another 
terminal 28 of the source of electrical energy is connected to the object 
30. Typically, terminal 28 and necessarily object 30 are electrically 
grounded as indicated in FIGS. 1 and 2. Thus, a large electrical potential 
difference may be established between electrode 10 of gun 4 and object 30. 
Spray gun 4 has a nozzle portion 32 which includes adjustment cone 34 
supported by charging rod 12 of electrode 10. 
The shroud-coater assembly generally shown at 36 in FIG. 1 includes mixing 
compartment or chamber 38, vacuum intake port 50, vacuum intake conduit 40 
running from said intake port to a source of subatmospheric pressure, 
strip electrodes 42, baffle plates 44 and auxliary air mainofold 46. 
In operation of the device, direct current power supply 26 supplies 
variable direct current at from 8,000 to 80,000 volts. The powder paint is 
fluidized in a conventional fluidizer and introduced at the gun barrel 
bore. Triggering of the gun causes fluidized powder to move under air 
pressure through the bore and simultaneously switches high voltage to the 
electrode. As mentioned above, the gun, power supply, fluidizer and powder 
transport components are commercially available items. Still further, in 
operation, the apparatus functions as follows: work piece 30 to which the 
powder is to be applied, is brought into the vicinity of the coater face, 
approximately to within 1 inch of baffle plates 44. When the trigger of 
the gun is activated, a powder-air mixture is propelled under air pressure 
past electrode 10 into mixing chamber 38. The powder is thus charged 
positively by electrode 10 before being agitated and uniformly dispersed 
in chamber 38. By properly adjusting auxiliary air pressure in manifold 
46, the thoroughly mixed powder-air mixture is blown through openings 48 
formed by baffle plates 44 into the vicinity of the grounded target work 
piece 30. By adjusting the subatmospheric pressure within 40, the powder 
air-mixture can be made to form eddies (shown by arrows), which enter 
opening 40 in two streams, and are returned to the original fluidizer 
source. However, with the grounded work piece in close proximity, a field 
of electrostatic force lines is formed between the work piece and 
electrode strips 42, which are connected by wires 52 to charging rod 12. 
Being of the same polarity as the charged powder, the electrostatic field 
causes some of the powder within the eddies to be repelled from electrode 
strips 42 and attracted toward grounded work piece 30. That portion of the 
powder not attracted to work piece 30 is drawn into port 40 and 
subsequently returned to the powder fluidizer from whence it originated. 
Several of the aforementioned advantages of this apparatus over the 
apparatus of Ser. No. 443,555 discussed above will be better understood by 
still further reference to the drawings. It will be noted by reference to 
FIG. 2 that the powder-air mixture blown through openings 48 formed by 
baffle plates 44, is returned continuously without loss to vacuum port 50. 
This condition is maintained even when target surface 30 is not near the 
base of the applicator. Formation of these eddy currents is enhanced by 
the action of the auxilary air brought into the mixing chamber 38 through 
manifold 46. The powder paint pattern thus formed is completely 
independent of the target surface. As is mentioned above, this is not the 
situation in the case of the device of Ser. No. 443,555 which depends on a 
flat surface. As also mentioned above, a serious disadvantage of the prior 
art device was its marginal performace in coating sharply curved surface 
due to its dependence on a flat surface for forming a sufficient draught 
of air capable of causing atmospheric air to move unadhered powder to the 
powder collector. The apparatus of the present invention, being 
independent of such a surface and in addition relying on the static 
electric field produced by strip electrodes 42, is capable of following a 
curved surface while applying powder effectively. 
FIG. 3 further defines placement and location of the field electrode strips 
42. It will be noted from both FIGS. 2 and 3 that the two strips have been 
placed on the inboard side of the outer baffle plates 44. In this 
placement, maximum opportunity is afforded powder to take on an 
electrostatic charge of the same polarity as electrode 10, to which the 
strips are connected by wires 52. Locating the strips so that their edges 
are normal to and within approximately 2 inches of the work piece creates 
an optimum electric field, further enhancing electrostatic attraction of 
powder particles to the work piece. 
Manifold 46, visible in FIGS. 1, 2 and 3, accounts for an interesting 
operational feature of the device. Its primary intended purpose is to 
assist in propelling the powder-air mixture from the mixing chamber 38 
through openings 48. It has been found, however, that by judicially 
locating openings in manifold 46, air jets could be so directed as to keep 
powder from accumulating on the walls and bottom panel of the coating 
chamber, thus contributing to self-cleaning, or purging of mixing chamber 
38. Control over the size of the eddies has already been mentioned. An air 
regulator valve and air pressure gauge, not shown, connected to the line 
leading to the manifold give this control. 
As will be noted from FIG. 1 the intake port 40 can be moved from left to 
right, so that the port opening 40 can be brought closer to or farther 
from the port openings 48 of the baffle plates. Conversely, the openings 
48 of the baffle plates, can be moved in relation to the opening 40, thus 
affording control of the formation of the powder eddies. It will also be 
noted from FIGS. 1 and 2 that the baffle plates 44 in the preferred 
embodiment of the device extend beyond the leading edge of the 
shroud-coater assembly 36. This also affords control in depositing powder 
in a controlled fashion. 
As discussed above, the apparatus of the invention is particularly 
advantageous in the electrostatic application of powder coating materials 
to large objects, such as automotive vehicle bodies, where frequent color 
changes are necessary and patterned application may be desired. 
Preferably, as noted above, separate electrostatic spraying apparatus 
could be provided for each color to be applied. Alternatively, where 
practical, the change could be made by simply disconnecting the tube from 
the electrostatic spray gun and removing subatmospheric pressure source 
connections followed by a brief cleaning of the apparatus by means of, for 
example, a blast of compressed air through the air manifold 46.