Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED POWDER COATING SYSTEM FOR DIFFICULT TO HANDLE
POWDERS
FIELD OF THE INVENTION
This invention relates to an improved method and
apparatus for the gas transport of powder, and more
particularly, to methods and apparatus which assure
delivery of uniform and consistent quantities of powder
per unit time to a spray gun for application to the
surface of an article to be coated.
RACRGROUND OF THE INVENTION
In a typical powder coating apparatus, powder is
maintained in a fluidized state within a hopper having a
fluidized bed, and is transported from the hopper to a
pump chamber of a powder pump, whereupon an ejector nozzle
in the pump directs a high pressure gas stream toward an
outlet of the chamber and along an exit tube connected to
the chamber. As a result, the powder is conveyed through
the exit tube to a spray gun which sprays the powder
toward the surface of an article to be coated. The
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ejector nozzle must be operated at sufficiently high
pressure to transport the particles to the end of the exit
tube and through the spray gun. During this pumping '
operation, the high pressure of the ej ector noz z 1e creates
a relatively low pressure at the intake to the pump
chamber, thereby drawing powder particles from the powder
container and into the chamber for ejection therefrom
along the exit tube.
One typical application for an apparatus of this
type involves coating the surface of an aluminum joint
with powdered solder flux prior to steps of heating and
melting the solder flux to weld the connecting aluminum
elements and form the joint. The reliability of the
connection depends upon the uniformity of the flux
coating.
In these and other applications wherein powder
particles are gas-transported to coat a surface, including
applications which involve electrostatic charging of the
particles, problems may result due to adherence and
accumulation of the powder particles at the inlet of the
pump from the hopper. This occurs with powders that have
the property of relatively easy coherence, easy adherence
due to viscosity, relatively low slipperiness, or powders
which simply do not have good flow characteristics and
A
tend to agglomerate. Powdered solder flux is such a
powder. With these types of powders, even powder flow
WO 94113405 PCT/US93/12391
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from the hopper to the spray gun is difficult to achieve,
and therefore, the amount of powder transported per unit
' time fluctuates. As a result, it is difficult when
spraying these powders to achieve a uniformly thick
coating of powder on the article and to consistently apply
a uniform coating from one article to the next in a
production line situation. This is possibly due to
changes in the flow of the powder resulting from a
reduction in the cross sectional area at the inlet to the
pump or because of the influence of static electricity
among the particles due to friction which encourages
agglomeration of the powder particles.
It is therefore one objective of the invention
to achieve greater uniformity in powder transport per unit
time by minimizing or reducing accumulation and adherence
of the particles during transport from the powder hopper
to the article to be coated.
In other applications for powder coating via gas
transport, it is often necessary to intermittently turn
the powder pump on and off. One example for the need of
this type of operation involves coating articles carried
on a conveyor, wherein it is desirable to spray coat the
articles at a coating station on the conveyor, and then
turn the pump off until the conveyor moves the next
article to the coating station. For applications which
require ON/OFF operation of a powder coating apparatus, it
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is desirable to achieve precise control of the powder flow
to effectively turn the apparatus off and on at the
desired times. Otherwise, powder is wasted. It is also
desirable to spray the same quantity of powder on each
article.
In the past, pinch valves made of rubber tubes
have been used to control intermittent powder flow at a
powder hopper. Rubberized pinch valves are pneumatically
operated and are relatively simple and inexpensive.
However, during closing, these valves have a tendency to
close upon some powder particles. Eventually this causes
gaps between the opposing rubber portions and produces air
leaks. These leaks allow some powder to move to the
downstream side of the pinch valve. Because there is no
way of knowing how much powder has reached the downstream
side of the pinch valve, the amount of powder ejected
during each ON/OFF cycle may vary. This will produce
nonuniformity in coating an article. Additionally, after
a certain number of switching operations, the rubber of
the pinch valve becomes fatigued and it deteriorates to a
point where it is impossible to use. Again, as this
occurs, the effectiveness of the valve becomes
questionable. As varying quantities of powder reach the
downstream side, the apparatus will eject varying
quantities of powder during each ON/OFF cycle.
It is another objective of this invention to '
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more precisely control the ON/OFF switching operation of
a powder pump, thereby to assure delivery of uniform
powder quantities during each ON/OFF cycle of operation.
It is still another objective of the invention
to simultaneously achieve uniform powder ejection per unit
time during an ON cycle and to effectively stop and start
powder ejection during switching operation between ON/OFF
and OFF/ON, respectively.
It is still another objective of the invention
to minimize the adverse effects of powder cohesion,
adherence, agglomeration and friction during flow from a
fluidized powder hopper to an outlet end of a spray gun,
thereby to achieve improved uniformity in powder delivery
to an article to be coated and to produce a more stable
coating thereon.
SUMMARY OF THE INVENTION
The above-stated objectives related to improved
uniformity in powder delivery during operation are
achieved by directing low pressure gas pulses toward the
fluidized powder hopper, counter to the normal flow
direction of the powder out of the hopper, thereby to
create microvibrations within the powder intake portion of
the ejector pump body and eliminate adherence and cohesion
among powder particles at the pump inlet. These low
pressure gas pulses are produced during the ON cycle of
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the pump.
The above-stated objectives related to improved
control over the ON/OFF switching operation for a powder
pump are achieved by directing a reverse flow of gas
through the powder pump inlet toward the fluidized hopper
and counter to the normal flow direction of the powder, at
a pressure sufficient to prevent particles from entering
the pump inlet during the OFF cycle . For both the low
pressure reverse direction pulses provided during the ON
cycle, and higher pressure reverse air flow provided
during the OFF cycle, the nozzle or nozzles which create
these air flows are oriented such that the axes of these
spray nozzles do not intersect the axis of the ejector
nozzle, and the flow paths of the air flows produced by
these nozzles do not cross with the flow path of the
ejector nozzle air flow.
According to a first preferred embodiment of the
invention, a powder ejection apparatus includes a
fluidized powder container, a pump body with a pump inlet
in communication with the powder container and an outlet
for directing the powder to an article to be coated, an
ejector nozzle for directing high pressure air through the
pump body toward the outlet and a reverse pulse nozzle
directed at the pump inlet. Preferably, the nozzles are
oriented such that their spray paths do not intersect. '
When the ejector nozzle is operated at a
2~.~~~~4
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relatively high pressure, powder is transported from the
pump body through the outlet. Low pressure at the powder
~ intake portion created by the ej ector noz z 1e causes powder
to flow therethrough from the powder hopper for subsequent
ejection. While the ejector nozzle operates, the reverse
pulsing nozzle also operates at a relatively low pressure,
and is pulsed off and on intermittently at high frequency
to cause microvibrations of powder particles within the
powder intake portion. This prevents adherence and
accumulation of powder particles to the side walls of the
powder intake portion, and thereby produces a uniformity
in powder flow through the apparatus per unit time. As a
result, more uniform coating of an article may be
achieved.
According to a second preferred embodiment of
the invention, more precise control of the switching
operation between OFF and ON, and vice versa, is achieved
via the use of a reverse flow nozzle adapted to spray
powder back into the powder hopper when the ejector nozzle
is off. When the ejector nozzle switches back on, the
reverse flow nozzle turns off. Each time the ejector
nozzle is turned on, the powder must traverse the entire
flow path from the powder hopper to the outlet of the
powder ejection apparatus, because the reverse flow nozzle
had previously blown all the powder within the hopper
outlet back into the powder hopper. This assures
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consistency in powder delivery during the ON cycle of the
apparatus.
This embodiment of the invention represents an
improvement over prior pinch valves located between the
hopper and the pump, which are subject to wear over a
period of time and inconsistent performance, due to
leakage of powder therethrough. With this embodiment, no
powder resides in the pump body prior to turning the
ejection nozzle back on.
In one variation of the second preferred
embodiment of the invention, the ejector nozzle and
reverse flow nozzle are mounted to separate bodies which
are interconnected via a connector line. In another
variation, the ejector nozzle and the reverse flow nozzle
are mounted to the same body and oriented such that their
spray paths do not intersect.
A third preferred embodiment of the invention
provides the advantages of both the first and second
embodiments by utilizing a single nozzle to provide
reverse pulsing during the pump ON cycle and reverse flow
to switch the apparatus OFF at the end of the ON cycle.
This nozzle is fed by two separate fluid lines, each with
a separate solenoid valve. Operation of the valves is
controlled by an electrical controller which also controls
operation of the flow of fluidizing air for the powder
hopper, ejector air for the ejector nozzle and transport
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air injected by a transport nozzle located downstream of
the ejector nozzle. Powder delivered by this embodiment
of the powder ejection apparatus is carried by an air
stream which comprises the confluence of the air from the
ejector nozzle and the air from the transport nozzle.
With this embodiment, precise switching' is
achieved to more effectively turn the apparatus OFF and ON
when desired, and vice versa, and in a manner which
assures consistent delivery of powder at the beginning of
the next ON cycle. Additionally, due to the use of
pulsing operation during powder ejection, uniformity in
powder delivery is achieved during the ON cycle.
Preferably, the controller also controls delivery of
powder to the hopper, as needed, and controls rotation of
a stirring blade within the hopper to minimize channeling
of powder, or the formation of chimneys of air in the
powder within the hopper, an effect which may result from
vertical air jets from the fluidizing plate being
undisturbed or uninterrupted for an extended period of
time. With the third embodiment, the powder intake
portion extends through the bottom of the powder hopper,
and the pump body for the powder ejection apparatus is
connected directly to the bottom of the powder hopper.
According to a fourth preferred embodiment of
the invention, precise powder flow control is achieved
during pumping and during switching in a matter similar to
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the third preferred embodiment. Additionally, the fourth
embodiment uses a reduced diameter stirring paddle located
directly above the powder intake portion of the apparatus. .
The powder intake, or inlet, extends through the bottom of
the powder hopper and into a pump body located therebelow.
As with the third embodiment, in the fourth embodiment
powder is pumped via operation of an ejector nozzle
directed toward an outlet line. Use of a transport air
nozzle is optional. During the off cycle, reverse air
flow is used to prevent powder particles from flowing
downwardly from the powder container into the powder inlet
of the apparatus. Additionally, the outlet line extends
horizontally from the pump body, without any upward turns,
and connects to a gun at its outlet end, which is directed
vertically. With this construction, powder does not fall
back into the pump body due to gravitational forces during
the off cycle. Additionally, with this construction
interaction among the powder particles which produces
accumulation, adherence, agglomeration and/or static
electricity from friction is reduced, thereby enhancing
the uniformity of powder delivery to an article to be
coated and producing a more stable coating.
Optimally, the spray gun utilized in this fourth
embodiment, and the other embodiments as well, has a
straight, unobstructed flow path with an external charging
electrode to prevent powder particles from agglomerating
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within the spray gun.
These and other features of the invention will
' be more readily understood in view of the following
detailed description and the drawings.
$RIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a vertical cross sectional view of
a powder ejection apparatus constructed in accordance with
a first preferred embodiment of the invention.
Figure 2 is a cross sectional view taken along
lines 2-2 of Fig. 1.
Figure 3 is a graph which depicts powder
discharge rate for the powder ejection apparatus shown in
Figs. 1 and 2.
Figure 4 is a vertical cross sectional view of
a powder ejection apparatus constructed in accordance with
a second preferred embodiment of the invention.
Figure 5 is a graph which illustrates operation
of the ejector nozzle and the reverse flow nozzle of the
powder ejection apparatus shown in Fig.4.
Figure 6 is a vertical cross section, similar to
Fig.4, of a variation of the second preferred embodiment
of the invention.
Figure 7 is a graph which illustrates operation
of the ejection nozzle and the reverse flow nozzle of the
powder ejection apparatus shown in Fig. 6.
Figure 8 is a cross sectional schematic view of
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a powder coating system constructed in accordance with a
third preferred embodiment of the invention.
Figure 9 is a graph which illustrates the '
operation of the powder coating system shown in Fig. 8.
Figure 10 is a cross sectional schematic view of
a powder coating system constructed in accordance with a
fourth preferred embodiment of the invention.
Figure 10A shows the powder coating gun of
Figure 10 in more detail.
Figure 11 is a cross sectional view taken along
lines 11-11 of Fig. 10.
Figure 12 is a graph which illustrates the
ON/OFF sequence for components of the powder coating
system shown in Fig. 10.
ETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Figures 1-3 relate to a first preferred
embodiment of the invention. More particularly, Fig. 1
shows a powder ejection apparatus 10 which includes a pump
body 12. The pump body 12 receives fluidized powder from
a powder hopper 14 and pumps the powder through an
ejection tube 17 connected to the pump body 12.
Directional arrow 18 shows the direction of flow of the
powder. The tube 17 may be of any desired length to
facilitate directing the powder flow to a spray gun. To
enter pump body 12 from the power hopper 14, the powder
enters a powder inlet 20. This powder flow is caused by
21~Q~'~
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an ejector nozzle 22 mounted to pump body 12 and directed
along the same axis as tube 17. An air supply tube 24 is
' connected to ejector nozzle 22, and as shown by
directional arrow 25, supplies air at a relatively high
pressure to ejector nozzle 22 to pump the powder outwardly
through tube 17. As ejector nozzle 22 directs the high
pressure air stream toward tube 17, a low pressure is
created at the powder inlet portion 20, thereby causing
the powder to move therethrough from the hopper 14 and
into the pump body 12 to be pumped through tube 17.
Because of the.relatively small cross sectional
dimension of inlet 20 relative to powder hopper 14, powder
has a tendency to adhere and accumulate within the inlet
enroute to pump body 12, particularly if the powder has
15 the property of high coherence, high adherence due to
viscosity, low slipperiness, a tendency to agglomerate or
low flow characteristics. If left unchecked, this powder
accumulation will narrow the powder inlet 20 and reduce
the total volume of powder supplied to the pump body 12
20 and to tube 17 per unit time, thereby adversely affecting
the uniformity of the powder coating on the article being
coated. This reduction in discharge quantity per unit
time is shown in Fig. 3, designated by reference numeral
32.
To solve this problem, a pulsing air flow is
directed from a pulsing nozzle 28 through the inlet 20 in
...
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a direction counter to that of the normal powder flow
inlet 20. Preferably, as shown in Fig. 2, the nozzle 28
and the nozzle 22 are oriented such that their axes and
flow paths do not intersect. Also, it is preferred that
this reverse flow be sprayed at a pressure lower than the
pressure of the air sprayed from nozzle 22. This pulsing
air flow from nozzle 28 produces microvibrations in the
powder within the inlet 20, which prevents or reduces
adherence and accumulation of the powder along the
sidewalls thereof. As a result, the quantity of powder
transferred from the hopper 14 through the pump body 12
and outwardly through tube 17 does not f luctuate with time
during operation of the powder ejection apparatus 10.
Therefore, the powder ejection apparatus 10 can be
operated continuously to produce uniformity in the volume
of powder ejected per unit time, thereby assuring
consistent powder flow to a spray gun for spraying onto a
surface to be coated and producing a higher quality
coating.
EBAMPLE NO. 1
Applicant tested powder ejection apparatus 10
using the following parameters:
Powder used - a fluoride powder flux;
Air pressure setting for the ejector nozzle 22 - .
4 kg/cmZ;
Air pressure setting for nozzle 28 - 2 kg/cmz, '
and the air was continuously pulsed to spray for a
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duration of about 20 to 100 milliseconds and then to stop
for a duration of about 40 to 200 milliseconds.
' Under these conditions, the powder ejection
apparatus 10 ejected the fluoride powder in a relatively
uniform quantity over the entire period of operation, as
shown in Fig. 3 by reference numeral 34, thereby improving
the powder flow uniformity.
While Figs. 1 and 2 show nozzle 28 above and to
the side of ejector nozzle 22, it is to be understood that
nozzle 28 could be located in any one of a number of
different positions, so long as the oppositely directed
pulsing air flows through inlet 20 and into powder hopper
14. The primary consideration is that the two gas flows
from nozzles 22 and 28 should not cross. Also, the powder
hopper 14 may be a fluidizing tank for supporting the
powder in a fluidized state, i.e., a fluidized bed.
Figs. 4-7 relate to a second preferred
embodiment of the invention. More particularly, Fig. 4
shows a powder ejection apparatus and powder coating
system 110 which also provides uniformity in volume and
consistency for a powder coating, but in a slightly
different manner. The apparatus 110 receives fluidized
powder from a powder hopper 114 and pumps the powder to a
spray nozzle 116 located at the end of an outlet tube 117.
The powder storage chamber 114 includes a fluiding plate
115 located adjacent the bottom thereof through which air
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is directed upwardly, as shown by directional arrows 119,
to fluidize the powder within hopper 114. Similar to the
first embodiment, pressurized air is sprayed from an ,
ejector nozzle 122 aimed along the axis of the tube 117.
However, in this second embodiment, instead of a single
pump body 12 , the powder ej ection apparatus 110 has a 'pump
body which includes a separate intake portion 112a and an
ejector portion 112b in fluid communication via a
connector 112c. The intake portion 112a is in fluid
communication with the powder hopper 114, thereby
providing a flow path for fluidized powder from the powder
hopper 114 to the ejector portion 112b. More
particularly, intake portion 112a includes powder inlet
portion 120 through which the powder must flow from powder
hopper 114 enroute to the ejector portion 112b. A reverse
flow nozzle 128 mounts to intake portion 112a, and is
directed toward powder inlet 120 to spray air through the
powder inlet 120 and into the powder hopper 114, counter
to the normal flow direction of the powder.
Ejector nozzle 122 and reverse flow nozzle 128
are operatively connected to a pressurized air source 125,
via fluid lines 124 and 130, respectively. Fluid lines
124 and 130 each include an in line gas regulator,
designated by reference numerals 134 and 140,
respectively. Additionally, fluid lines 124 and 130
include solenoid valves 135 and 138, respectively, which
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are electrically connected with an electric controller
142. The controller 142 controls the timing sequences of
the operation for the ejector nozzle 122 and the reverse
flow nozzle 128 to alternately actuate the solenoid valves
13 5 and 13 8 to spray air from noz z les 12 2 and 12 8 , thereby
alternating between drawing powder into powder ejection
apparatus 110 from hopper 114 and blowing powder away from
apparatus 110 into hopper 114.
Thus, compared to the first embodiment, which
produced uniformity in powder quantity per unit time
during ejection, this embodiment achieves uniformity in
ejection quantity per ON/OFF cycle. This. is due to the
uniformity in conditions during initiation of the "ON"
portion of the ON/OFF cycle of operation. At initiation,
no powder resides in the inlet portion 120, unlike prior
systems which relied upon pinch valves and were
susceptible to deterioration with age and inadvertent
powder flow. Preferably, the controller 140 is
programmable to select the desired operating sequence.
During operation, when the ejector nozzle 122
sprays air through ejector portion 112b, reduced pressure
at inlet 120 and in connector tube 112c causes the
fluidized powder to flow from the powder hopper 114
through powder intake portion 120 and ejector portion 112b
to nozzle 116. This produces a spray pattern 118 of
powder coating material from nozzle 116. After the nozzle
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122 turns off, reverse flow nozzle 128 is turned on to
spray air toward hopper 114, and in the opposite direction
of the normal powder ejection flow path of the fluidized .
powder. This spray from nozzle 128 blows the powder out
of the inlet portion 120 into the chamber 114. When
nozzle 128 is turned off and nozzle 122 is turned on, the
powder must move entirely from the chamber 114 to inlet
120, since no powder was already in the inlet 120 at the
commencement of spraying by nozzle 122. By cooperatively
pulsing the ejector nozzle 122 and the reverse flow nozzle
128, the same quantity of powder can be ejected during
each cycle of "ON/OF" sequences, thereby assuring
uniformity in powder ejection during coating operations
which require switching and eliminating prior uniformity
problems caused by inconsistent spray volumes especially
during initiation of the "ON" portion of the cycle. This
second embodiment of the invention is particularly
suitable for spray coating articles carried a conveyor,
due to the need for ON/OFF cycling at a coating station.
Fig. 5 shows the ON/OFF times for ejector nozzle
122 and reverse flow nozzle 128, as designated by
reference numerals 143 and 144, respectively. Reference
146 shows the duration of "ON" time for reverse flow air
nozzle 128 and reference numeral 147 shows the duration of
time thereafter until ejector nozzle 122 is turned back to
"on" to commence powder pumping. Thus, no powder is
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pumped during the time period represented by the sum of
146 and 147.
' With the reverse flow of air sprayed by nozzle
128 through powder inlet portion 120, this invention
eliminates the need to insert a pinch valve, or possibly
a mechanical type valve, between powder hopper 114 and the
inlet of ejector portion 112b for the purpose of starting
and stopping powder flow.
Fig. 6 shows a variation of the second preferred
embodiment of the invention. According to this variation,
the powder ejection apparatus 110 includes a single piece
pump body 112 to which the ejector nozzle 122 and the
reverse flow nozzle 128 are mounted. The ejector nozzle
122 is aimed to spray air outwardly through line 117, and
I5 the reverse flow nozzle 128 is aimed to spray air opposite
the normal flow of the fluidized powder, through powder
inlet 120 and toward powder hopper 114.
Fig. 7 shows timing control for switching the
operation of ejector nozzle 122 and the reverse flow
nozzle 128 for the apparatus 110 shown in Fig. 6. Because
of the proximity of the nozzles 122 and 128, the switching
spray pulses between ON/OFF can be more closely timed to
provide a shorter ON/OFF cycle for the apparatus 110.
Again, as with the apparatus shown in Fig. 4, the
apparatus of Fig. 6 provides effective stopping and
starting of each powder pumping cycle, thereby assuring
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uniformity in the volume of powder pumped per cycle.
Figs. 8 and 9 depict a third preferred
embodiment of the invention which combines features from
the first and the second embodiments. More specifically,
Fig. 8 shows a powder coating system 210 for applying
electrostatically charged particles to an article 209 to
be coated. The apparatus 210 conveys the particles
through an outlet conduit 217 to a spray nozzle 216 which
produces a spray pattern designated by reference numeral
218. To electrostatically charge the particles, the
apparatus 210 includes ~a corona electrode 208 mounted
adjacent the outlet 216. The corona electrode 208 is
connected to a power supply (now shown) in electrical
controller 248 via an electrical cable 206.
The system 210 further includes a pump body 212
which receives powder from a powder hopper 214 and then
conveys the powder to outlet conduit 217. The hopper 214
includes a fluidizing plate 215, located at a bottom end
thereof above a fluidizing air plenum 221. An air inlet
250 supplies pressurized air to the plenum 221, and a
resulting upward flow of the pressurized air through the
plate 215 causes the powder to be fluidized. A powder
supply container 251 supplies powder to the hopper 214 in
response to a level sensor (now shown) in a manner known
in the industry. The hopper 214 also includes a rotatable
stirring blade 254 which is driven by a motor 255. The '
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stirring blade 254 reduces channeling of the powder in the
hopper 214, as described above.
Air to the inlet nozzle 250 is supplied via a
line 258 connected to a pressurized air supply source and
controller, designated generally 240b, and the system 210
precisely controls the flow of air to inlet 250 via a
solenoid valve 260, which is electrically controlled by an
electrical controller 240a. The electrical controller
240a and the air supply source and controller 240b
cooperatively interact to control all operations of the
powder system 210. Preferably, each of the controllers
240a and 240b includes a microprocessor to facilitate
programmable control.
The pump body 212 is connected to the bottom of
the hopper 214 and includes a powder inlet 220 which
communicates with the hopper 214. A reverse flow nozzle
228 mounts to pump body 212 for directing a stream of air
through the powder inlet 220 and toward the hopper 214,
opposite the normal flow direction of powder during pump
operation. An ejector nozzle 222 also mounts to the pump
body 212, and ejector nozzle 222 delivers a high pressure
air stream through the pump body 212 along the axis of
outlet line 217. Additionally, pump body 212 includes a
gas transport nozzle 270 which supplies pressurized gas to
an annular region 271 within the pump body 212 and then
toward outlet line 217 via channels 272, which are also
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formed within the pump body 212. Thus, the transport
nozzle 270 supplies pressurized transport air downstream
of the ejector air supplied by the ejector nozzle 222. As '
a result, powder pumped from pump body 212 and into line
217 is carried by an air stream consisting of the
confluence of the ejector air and the transport air.
Air transport nozzle 270 is supplied with
pressurized air via a fluid line 273 connected to the
controller 240b, and air flow is controlled via a solenoid
valve 274, which is electrically connected to controller
240a. Similarly, ejector nozzle 222 is connected to
controller 240b via a fluid line 224, and ejector air flow
is controlled via a solenoid valve 235 which is
electrically connected to electrical controller 240a.
In this embodiment, the reverse flow nozzle 228
includes two separate fluid supply lines 230a and 230b
connected to controller 240b, through which air flow is
controlled by solenoid valves 238a and 238b, respectively.
Downstream of the solenoid valves 238a and 238b, the lines
230a and 230b merge to form a single air supply line 230
for nozzle 228. The use of two separate supply lines 230a
and 230b and two separate solenoid valves 238a and 238b
allows the apparatus 210 to use a single nozzle 228 for
providing reverse pulsing during powder pumping, as in the
first embodiment, and reverse flow to stop powder from
entering pump inlet 22o when the pump 15 is idle, as in
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the second embodiment.
In operation, at start-up, solenoid valves 235,
274 and 238b are closed. Solenoid valves 260 and 238a are
opened. This produces fluidizing air flow into fluidizing
plenum 221 via hopper inlet 250 to fluidize particles in
the hopper 214. Additionally, the reverse flow through
nozzle 228 prevents powder from entering the powder inlet
220. The pressure of the air supply sprayed from nozzle
228 may initially need to be adjusted to accomplish this
objective by means of a regulator (now shown).
Additionally, the stirring blade 254 is rotated by means
of motor 255. Solenoid valve 260 remains open so that
powder is continuously fluidized during the operation of
system 210, and likewise blades 254 are at all time
continuously rotating wherever the system 210 is in
operation. To spray powder, valves 235 and 274 are
opened, and solenoid valve 238a is closed. This causes
nozzle 222 and nozzle 270 to spray ejector air and
transport air, respectively, thereby causing powder to be
pumped from hopper 214 through pump body 212 to outlet
line 217.
At the same time, solenoid valve 238b is opened
and closed intermittently (pulsed) based on an electrical
signal from the controller 240a. Preferably, during this
intermittent opening and closing of valve 238b, the air
sprayed into pump body 212 from nozzle 228 is adjusted to
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a pressure lower than the pressure of the pressurized air
sprayed from the ejector nozzle 222. Valve 238b continues
to cycle for so long as solenoid valves 235 and 274 are
open. With the ejector nozzle 222 spraying air, due to
opening of solenoid valve 235, powder from the hopper 214
is drawn into pump body 212 through powder inlet 220 via
venturi operation. Additionally, due to the opening of
solenoid valve 274, transport nozzle 270 supplies
pressurized transport air to the pump body 212 downstream
of ejector nozzle 222. As a result, powder particles are
pumped through line 217 and are sprayed outwardly from
nozzle 216, whereupon corona electrode 208, powdered by
cable 206 via power supply and controller 204a,
electrostatically charges the particles to render them
electrostatically attracted to the article 209 to be
coated. Article 209 is typically electrically grounded by
the conveyor.
To temporarily stop the flow of powder from the
nozzle 216, valves, 235, 274 and 238b are closed, and at
the same time, valve 238a is opened. This causes air flow
from nozzle 228 to go from the intermittent pulsing
operation at relatively low pressure to reverse flow
operation wherein an air stream is sprayed at sufficient
pressure to force the powder particles in the powder inlet
220 back into the hopper 214. This operation of the
solenoid valves is shown more clearly in Fig. 9, with
WO 94/13405 PCT/US93/12391
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reference numeral 280 indicating a temporary stoppage of
powder flow.
With this powder coating system 210, a very
consistent powder coating may be applied to an article
209, with a uniformity of powder thickness and high
quality assured, due to improved control of the volume of
powder delivered per unit time during the "ON" portion of
the ON/OFF cycle of operation and the ability to
temporarily halt flow of the powder from the hopper 214
during switching between "ON" and "OFF°°. This assures
uniformity in conditions each time the powder pump of
system 210 is switched "ON".
FXAMPhE 2
A high quality coating was achieved with powder
coating system 210 using the following parameters:
Powder - fluoride powder flux;
Rotation frequency of stirring blade - 60 rpm;
Pressurized air for fluidization inlet 250 - 2kg/cmz;
Pressurized air for ejector nozzle 222 - 4 kg/cm2;
Pulsed air flow for nozzle 228/solenoid valve 238b - 2
kg/cma with pulsed "ON" time of about 20-100 milliseconds
and "OFF" time of about 40-200 milliseconds;
Reverse flow air for nozzle 228/solenoid valve 238a - 3
kg/cma;
Pressurized air for transport Nozzle 270 - 2-3 kg/cm2.
Figs. 10-12 relate to a fourth preferred
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embodiment of the invention, which varies somewhat from
the third preferred embodiment, but still provides reverse
pulsing during the pumping of powder and reverse flow when
the pump is idle. Fig. 10 shows a powder coating system
310 which sprays powder in a spray pattern 318 onto an
article to be coated 309. If desired, the article 309 may
be carried on a conveyor 301 and/or coated within an
environmentally controlled enclosure 302 wherein
oversprayed powder would be collected and possibly
returned to hopper 314. The apparatus 310 conveys
fluidized powder from hopper 314, through a pump body 312,
through an outlet line 317 to an electrostatic spray gun
316, such as a Model NPE-4AH, available from Nordson
Corporation, Amherst, Ohio, and shown in U.S. Patent No.
4,630,777. The hopper 314 includes a fluidizing plate 315
above an air plenum 321. Air supplied into the plenum 321
via inlet 350 passes upwardly through the plate 315 to
fluidize the powder particles within hopper 314. A paddle
354 is mounted within hopper 314 and rotated via motor 355
to uniformly mix the powder particles.
Particles are drawn from the hopper 314 into the
pump body 312 via venturi action caused by operation of an
ejector nozzle 322 mounted to the pump body 312 and
directed at outlet line 317. To get to the pump body 312,
the particles move through a powder inlet 320, which is
2~504'~~
WO 94113405 PCT/US93f12391
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preferably located directly below rotating paddle 354. As
shown in Fig. 11, pump body 312 further includes a pulse
nozzle 328 directed at the hopper 314 and along powder
inlet 320, and a flow nozzle 329 which is also directed at
the hopper 314 along the powder inlet 320. Thus, this
fourth preferred embodiment of the invention differs from
the third preferred embodiment in that separate nozzles,
i.e., nozzles 328 and 329, are used for the separate
functions of reverse pulsing inlet 320 during pumping, and
reverse flow to prevent the particles from entering inlet
320 when the pump is idle, respectively. This is in
contrast to the single nozzle 228 which was used with two
separate solenoid valves 238a and 238b and fluid lines
230a and 230b in the third preferred embodiment for these
same two functions.
Fig. 10 also clearly shows outlet line 317
extending horizontally from pump body 312, without any
horizontal or vertical bends, except for the downward bend
before gun 316, and is located either level with or
entirely below the outlet of pump body 312. Also, spray
gun 316 is oriented vertically and located below the
outlet of pump body 312. This structure eliminates the
possibility of powder particles returning to the pump
body 312 under gravitational forces when the ejector
nozzle 322 is switched off. The gun 316 is also oriented
vertically to further reduce the possibility of collisions
and/or coherence of the powder particles during flow from
WO 94/13405 PCT/LTS93/12391
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pump body 312 to the spray nozzle of gun 316. In total,
the flow path makes only one turn between pump body 312
and the article 309.
Fig. 11 shows the relative positions of the
nozzles 328 and 329 with respect to nozzle 322. Use of
the two separate nozzles, 328 and 329, for the separate
functions of pulsing and stoppage, respectively, produces
more precision in these controls and further allows the
flows emanating from these nozzles to be oriented in a
manner which does not interfere with the spray from nozzle
322.
Fig. 12 shows the ON/OFF timing operation of the
sprays from nozzles 322, 328 and 329. Air flow from these
nozzles is controlled in the same manner as described with
respect to the third preferred embodiment.
Fig. 10A shows the spray gun 316 in greater detail.
Spray gun 316 has a powder flow conduit 370 having an
inlet 351 at one end and a spray nozzle 352 at the other
end. Preferably, spray nozzle 352 is a slot nozzle having
a .12 inch wide slot available from Nordson Corporation,
Amherst, Ohio as part number 117,158. Conduit 370 is
completely unobstructed up to nozzle 352 which defines
spray pattern 318. This is facilitated by the use of a
charging electrode 360 which is completely external to
conduit 370 and nozzle 352. With this spray gun design
there are no places within the gun for the powder
i
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particles to agglomerate, and this facilitates the
consistent, uniform application of powder to articles by
means of the system 310 of this fourth embodiment.
In view of the above detailed description of four
5 preferred embodiments, it will be understood that
variations will occur in employing the principles of this
invention, depending upon materials and conditions, as
will be understood to those of ordinary skill in the art.
