Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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- ELECTROSTATIC POWDlE~ COATING IJTILI_ING
MULTIPLE SPRA'l STREAMS WIT PULSED ELECTROSTATIC FIELD
~. ~NC SPR~- PA`1~RN~
: Field of th~ Invention
This invention relatPs to electrostatic
powder coating. More part:i~laxly, this inventlon
` relates to improvmPnts in el~ctrostatic powder ~oating
which utiliæe a plurality of spray streams.
Backqround of the Inven~.ion
According to conventional powder coating
methods and apparatus, a pressuriz~d mixture of gas and
3 powder particles is electrostatically char~ed and
" e~ected or sprayed outwardly from a gun in the
direction of an object to be coated. The particle~
entrained in the gas-powder mixture may be charged
., 1~
while ins~de the gun via an applied electrostatic field
~;! or by frictional charging, iOe. triboelectric, or
~Y~ outside the gun via an electrostatic field produced by
an external electrode. During electrostatic powder
: ~ coating, the charged powder particles in the ~ixture
-
repel one another as they travel toward the object to
be coated. During flight9 the lower electrical
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~ -2 ~ ~ 7 0 ~ ~ C~
potential of the object to b~ coated electros~atically
attracts the particles.
To achieve uniformity in coating With a
conventional coating gun, it is common to locate a
trumpet-shaped deflector in front of the nozzle of the
gun. The deflector diffuses or spreads the flow path
of the pressurized mixture so that the pswder will
cover a broader surface area~ While this method works
reasonably well in xough coating an ob~ect, it su~fer~
from a several limitations. Primarily, due to the
relatively broad cross-sectional area, the sprayed
mixture produces a thick air cushion~ This air cushion
obstructs the flow of f2110wing particlPs and causes a
substantial number of the ~ubsequently sprayed
particles to rebound away from the coating surfaceO As
:
a result, powder coating by this method takes
`. additional time to ensure complete coating and
significant amounts of powder are lost due to
, .,
::l rebounding.
~. .
Conventional powder coating meth~ds and
apparatus also su~fer from limitations in coating
i~
uneven or irregular surfaces, such as the inside
surface of a cylindrical container or a me~allic pipe.
~: '.1 .
~:i When using an external electrode to establish an
!j ~'` electrostatic field between the gun and the o~ject, the
. ~
i : strongest lines of electrostatic force will be located
!
3~ J~
along a direct line of sight path -to the nearest
portion~ of the object, and electrostatic lin~s of
force direc-ted toward the rPcessed portions will be
significantly weaker. For instance, with respect to a
container, the lines of electrostatic force will be
strongest betwee.n the gun and the top o~ the in~ide
surface of the container and weakest between the gun
and the bottom of the container.
When po~der coating an object or work piece
with multiple projection~ andJor hollows, i.e. an
uneven surf~ce, the charged particles initi.ally take
the path of the strongest electrostatic ield lines.
.,
Thereafter, the parti~les tend to continue along this
~: ~ same path, ~entually arcumulating on discre~e areas o~
- ~
th~ object which are nearest the gun, or on projections
which ar~ closer to the gun than surrounding areas. As
s a result, the object is not uniformly coated. More
;~ specifically, workp.ieces or objects with projections
: .~
and/or hollows are coated very unevenly. In many
case~, it is common that the tops or edges o~ the
projections will be coated heavily, the bottoms of
~ i :
; hollows will be coated thinly and the corners of
: hollows will hardly be coated at all. The tendency of
the particles to accumulate in discrete ar~as rather
than uniformly over the entire surface is referred to
. :
~ as the Faraday cage ef~ect.
, .
; -4~ J $ ~3
Another probl~m associated with powder
coating relate~ to insufficient charging of the powder
particles mixture when ~rictional chaxging is used. To
increase the charg1ng rate when using ~rictional
charging, it is necessary to increas~ the contact area
of the mixture. ~owever, this results in a longer and
more complex flow path. ~s a result, if one wishes to
change colors, it takes a longer time to change o~er to
a di~erent color powderO
:
With an electrostatic field produoed ~y an
external electrode~ charging efficiency m~y sometim~
be incr~ased by increasing the ~ield skrength.
However, this may increase the adverse ~ffects produced
by the Faraday cage effect. Additionally, particles
having a higher charge retain their charge after
deposition, thereby repulsing subssquent particles r nd
inhibiting the application of a second coating.
Another problem associated with powder
coating relates to changes in the volume of powder
particles sprayed per unit time. Typically, the amount
i. ~,
~ of powder sprayed per unit time is changed by
. .
; ~ increasing or decrea~ing pump pressure. Howev~r,
; : pressure variations produce changes in ejection speed
rom the spray~ gun, which may result in different
coating phenomena, such as sort landing or continuous
` :J~ : jet stream collision. In soft landing, coating i5
5
affected via electrostatic at-traction i~nd coi~ting
weight increases where fur-ther coating is unnecessary,
due to the Faraday cag~ effect. Places where further
coating is necessary are insufficiently coated. W:ith
jPt stream collision, reboundin~ is so violent that
even deposited powder particles may be blown off the
object.
Another limitation of conventional pG~der
coating methods and apparatus relates to coatinc~
relatively small objects. If a lower pressure ~..s us2d~
e~ 1.5 Kg/cm2 or lower, to a~oommodate the lower
required volume, the spray pii~ttern becomes unstable,
resulting in uneven coa~ing.
~;. It is an objective 3f this invention to. s
improve uniformity in the powder coating of unevell/ or
irregularly shaped objects, such as the inside surface
of a container.
:~ 3
It is another o~jective of the invention to
reduce the amount of deflection or rebounding which
~ !
occurs during powder coating.
It is still another objective of ~he
invention to achieve higher efficiency in the charging
:: ~,'1 :
~ of particles entrained in an gas-powder mixture used in
. ~ : :
electrostatic powder coating, without adversPly
~ affecting the application of subsequent coatings.
:~ It is still another objective o~ the
.. :
l~ ~i
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, .~,. ::, ,.. ~ . ;.,.. ~ , ... .. . . . . . ... . . ..
~ ~ r
invention to minimize the adverse coatiny results
produced by the Faraday cage ef~ct, partic~larly whe~
utili~ing a powder co~ing apparatus equipp~d ~i~h a~
external electrode.
It is yet another objective of the inventi~n
to impr~ve the efficiency of powder coaking relatively
.` small objects or with relatively small volumes.
The methods and apparatus of this inYentlon
achieve the above-stated objecti~es by splitting a
pressurized mixture of gas and powder partlcles into a
~: plurali~y of separate spray streams. Splitting ~he
:1 mixture into a plurality of fine spray skreams reduc2s
:.~ re~oundiny effects and enables an operator to powder
~ Goat relati~ely large or small sur~ace areas with
:i
.i multiple flow paths arranged in a selecta~le pattern
~;~
~ which is dictated by the shape of the object to be
;~ coated.
.'' In general applications, particles entraine~
.~
~ within the multiple spray streams are electrostatically
; 1 charged by multiple external electrod~s, with at least
one external electrode associated with each of the
spray streams. The use of multiple fine spray strehms
in conjunction with multiple external electrodes
i provides eficient charging of powder particles in th~
.,
~ - separate spray streams with minimum air cushion.
:~1 Additionally, to further minimize air cushion
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and/or deflection, and to facilitate coating with
rel~tiv~ly smaller volumes, the inv~tioll contemplates
spraying the gas-powder mixture from th~ gun in a
pulsed manner. Pulsing the powder particles Erom the
gun produces more precise control over the amount of
: powder sprayed per unit time.
According to ano~her aspe t o~ the inven~ion,
~ the DC electrical power supplied to the external
: electrodes to produce the electrostatic fields i-or
; charging p~rticles may be pul~ed between an "off~' and
an 'Don" condition duri.ng spraying to perioclically pulse
the. electrostatic fields established between the
~ external electrodes and the object to be c3ated~
:') Pulsing of the electrostatic ~ields reduces ~he Faraday
i cage effect because when the -field is '-o~ , particles
"~, will flow unimpeded by the field along fliyht paths
produced by aerodynamic forces only. ~his produces
more uniform coating of areas such as container bottoms
~1 and recesses of irregular surfaces.
~;
~` : For general powder coating application~l this
invention contemplates pulsing the electrostati.c ~ield
in conjunction with pulsing of the powder flow.
vj
:Addit}onally, these two fea~ures may be ~urther
combined with the use of multiple spray streams and
multiple external electrodes.
The methods and apparatus of this invention
~'~
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are particularly suitabl~ for powder coating th~ inside
surfaces of c~ntainers. More particularly, the
combination of multiple spray streams i~nd powder
charging inside the gun reduces the Faraday cage e~f2ct
and allows uniform coating of the inside of ia can. The
invention further contemplates a number o~ additional
features related to electrostatic charging of particles
- inside the gunt such as utilization of multiple
charging chambPrs, either connected in series or in
parallel.
This invention contemplates several other
approaches to powder coating the inside sur:Eaces of
containers. More particularly, -this invention also
contemplates the combination of multiple spray streams
and multiple external electrod~s/ along with pulsing of
the electros-tatic fields established by the electrodes.
Additlonally, this i.nvention furthPr cont~mpla~es
~i pulsing the powder sprayed fxom the spray gun, either
:~ with a single spray stream or multiple spray streams.
As yet another approach to coating the inside surface
of a container, the invention contemplates pulsing the
electrostatic field in combination with pulsing the
powder ejections. These two ~eatures may be further
~! ; combined with the use of multiple spray streams and
:j external electrodes.
I
Structurally, the invention utilizes a multi
'~,','1~
-.9 ~
hole nozzle or a multi tube spray hole assPmbly to
split the pressurlzed gas-powder mixture into a
plurality o fine spray streams. If a multi-hole
nozzle is used, the nozzle connects to a forward, open
end of the gun, ~nd the holes of the multi~hole nozzl~
define the openings for the internal chamber of the
spray gun through which the powder is sprzyed. I~ a
multi~tube spray hole assembly is used, a manift.~ld of
the assembly connPcts to the forward end of the gun.
The manifold holds multiple tubes in place at the
forward end of the gun, in fluid communicatiorl with the
chamber. Opposite ends of the tubes are held in a
desired arrangement by a holder. With the multi tube
spray hole a~sembly, the tubes define the chamber
openings.
,
~i With either the multi-hole nozzle or the
; ~ multi-tube spray hole assembly, the chamber openings
,,
may be arranged in any desired pattern. The
arrangement of the chamber openings d termines the ~low
paths that the multiple spray streams will traverse
when ejected from the gun. The openings may be aligned
linearly, arranged around the circumference of a
circle, or arranged in mul~iple, parallel rows,
preferably with each row being staggered with respect
. ~
~1 : to the adjacent rows. The openings may also be tilted~
~ or angled, to produce an angled flow path.
.,':~"1
' ', : ~ . .. .. . :~ ' ' '
' '' . ' '
f~
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Depend.ing upon the shape and the sizP o the
object tn be coated, the chamber ope~ings may be
arranged 50 as to direct the flow paths in a desired
manner. This feature of the inventi.on is partic-llarly
important when powder coating objects with multiple
hollo~s, such as radiat~on fins, transformers or
radiators. With objects of thi~ type, it is usually
preferable to use six to twenty -four separate sp:ray
streams. In most instances, it is also prefera~l.e for
the inner diameters of the chamber openings, i.e. the
tu~es of multi-tube spray hole assembly or the holes of
the multi-hole nozzle, be in the range o~ about 2-B mm~
Wh~n using multiple chamber openings, it i~ preferable
that the chamber openings be spaced apart about 80-~800
, mm. The assembly tubes and the no~zle are preerably
.'f of plastic, such as flouroresins, nylon or
: polypropylene, depending on the polarity of the
charging or the coe~ficient of ~rict.ion~
i~i, One particular arrangement of the chamber
`. :J
, openings which provides ~enefits in coating irregular
sur~aces involves arranging the chamber openings around
the circumference of a circle and orienting the chamber
.,., I
openings obliquely with respect to the circle. With
this approach, if the chamber openings are directed
outwardly, mul~iple spiraling streams are produced
which are particularly suitable for coating of th~
~ ~ 7 ~ ~ ~ . J
inside surfaces of a container ox pipe. Alternately,
the chamber openinys may he directed inwardly, ~oward a
center axis through the circle, to provide circularly
arranged flow paths which initially will converge to a
point and then diverye outwardly. Depending upan the
distance b~twePn -the ~nd of the gun and -the object to
be coated, this arrangement can be used to coat either
large or small surface areasO
With any of these arrangements of th~ rhamber
openings, the pressurized gas~powder mixture may be
electrostatically charged insi~e the gun, outside the
gun by one or more external electrQdes, or by a
cambination of internal and external chargingO The
particular commercial applica-tion for the invention
"
! will determinP the ~os~ suitable manner of charc3ing
.`J powder particles entrained in the mixture.
~;~ When one or more ex~ernal electrades are usa~
to charye powder particles, it is sometimes desirabl~
,
: to pulse the electrostatic field produc~d by each
.~: electrode between an "on" and an 110~ condition. This
pulsing of electrostatic field is achieved by a pulsa
controller which provides salectable interruption of
~ .,
~ : the electrical connection between each external
:
electrode and a DC power supply. Pulsing the
electrastatic field reduces the Faraday cage effect
when external electrodes are used. Pulsing of the
~ '1
~1~ ~
electrostatic fleld is dis~losed in applicant~s
Japanese Kokai Publication No. 01 [1989] ll J 669
published January 17, 1989, w~ich is expressly
in~orporated by ref~rence herein in its en~irety~
As mentioned pre~iously, another aspect of
~he invention rela~es ~o pulsing of the powder
particles from the ~un. Pulsing of the powder
particles is achiQved by usiny a powder pulse
controller connected to thP ejector to eject the
mixture from the gun according to a desirPd wave. form.
By cycling between an "on~l and an. "of~l~
condition at a desired pressure, usually a un:iform
pres~ure, the powder particles are sprayed ou-t of the
end of the ~un in consecutive pulses~ and the air
cushion is reduced. Pulsing o~ the powder particle5
reduces deflect.ion or reboundiny of o~ the objec~ to
be coated. Because pulsing enables a unifor~ pre~sure
to be maintained during sprayin~, the controller
enables the duration and ampli-tude of the pulses to be
carefully controlled and the amount of powder particles
, 1 .
~.1 sprayed per unit time will remain relatively uniform~
'~'',ZI
even when coating small ob3ects or with relati~ely
smaller volumes. Pulsing o~ powder is disclose~ in
. ~:
applicant's Japanese Kokai Publication No. 62 [1987]
11,574, published January 20, 1987, which is expr~ssly
, ~ ~ incorporated by reference herein in its entiretyO
-13-
.
According to an~the~ aspect of the invention,
~ the holes o~ the mul~i-hole nozzle or thei tubes of the
i multi~tube spray hole assembly may be further equipped
; wi~h a small-scale nozzl~i which has either multiple
- smaller holPs or a single elongated slit. A small-
scale nozzle of this type provides further separation
of the mixture into even filler spray s~reams~
Depending upon -the -type of powder particles
:~: used, and the size, shape and composi-tion of the object
. to be coated, the above-described ~eatures may ~se used
-, in various com~inations to achieve uniform powder
coating. These and other features of the invention
will be more readily understood in view o~ the
.~ following detailed description and the drawi.ngs.
~ Brief Description of the Drawin~s
;~ Fig. 1 is a schemati~ which depicts, in
~:~ longitudinal cross-section, an elec^trostatic powder
.3 ~ spray coating apparatus in accordance with one
;l embodiment of the inventionO
: : Fig. 2 is a transvexse view taken ~long lines
2-2 of Fi~. 1.
Fig. 3 and FigO 4 are transverse cross-
sectional views taken along lines 3 3 and lines 4-4 of
:1 ~: : :
Fig. 1, respectively, which d~pict four separate spray
streams of gas-powder mixture as they progress toward
~ ~ an ar~icle to be coated.
:,.,-1 : :
~ r~
Fiy. 5 is a schematic which depicts, in
longitudinal cross-section, a second embodiment of th~
electros~atic powder spray coating apparatus depicted
in Fig~ 1.
; Fig. 6 is a transversP cross sectional view
taken along lines 6-6 o-E Fig. 5, which depicts a
coating pattern produced by the four spray stream~ of
gas-powder mixture shown in FigO So
Fig. 7 is a transverse view, similar to Fig.
2, which depicts a third embodiment of the
electrostatic powder spray coating apparatus depicted
.in Fig. 1, wherein the apparatus is equipped with a
multi-hole nozzle having a central electrode and holes
: i
arranged on a circumference of a circle.
Figs. 8l 9 and lO depict -the spray pattern
produced by the nozzle depicted in Fig. 7 at
progressively further distances from the nozzle, as the
; ..,
:: spray streams progress toward an article to b~ coa-ted,
Fig. 11 is a longitudinal schematic view,
s.imilar to Fig. 5, which depicts a fourth embodiment of
: the electrostatic powder spray coating apparatus of
this inven~ion, wherein the apparatus includes a multi-
tube spray hole assembly.
: ~ ~
~ : Fig. 12 is a transverse view taken along
:~ ~ : :
~: :: lines 12-12 of Fig~ 11.
t~ Figs. 13 and 14 are transverse cros ir~
-15-
secti~nal views ~a~en alony lines 13 13 and 14~1~ vf
Fig. 11, respectively, which depick a plurality of
spray streams of gas~powder mix~ure produced by the
multi-tube assembly depicted in Fig. 11, as the spray
streams progress toward an object to be coated.
Fig. 15 is an enlarged, transversP schematic
vi~w which depi~-ts another alternaitive embodiment of a
powder spray çoating apparatus according to the
invention~ wherein a wire is u~ed to create an
electrostatic particle charging ~iPldo
Fig. 16 is an enlarged, -transverse view,
similar to Fig. 15, which depicts a variation o~ the
embodiment shown in ~ig~ ~5 in that the wire is
insula-ted along its length except for a plurality of
spaiced, uncovered regions.
FigO 17 is an enlarged, transverse Vi2W,
similar to Figs. 15 and ~6, Which depicts another
variation of the embodiment shown in Fig. 15 in that
. .
`, the wire has angled slits which expose a plurality of. spaced, uncovered regions which face toward the product
being coated.
: Fig. 18 is a transverse view taken along
:~, lines 18-18 of Fig. 17.
:
Fig. 19 is a longitudinal schematic viewl
similar to Fig. 11, which depicts a ~iî th embodiment of
the inVention, another variation of the multi-tube
:
-16-
assembly.
;~ Flg. 20 is a transverse cross-sectional view
taken along lines 20-20 of Fig. 19, which depicts two
spray patterns formed by the multi-tube assembly shown
in Fig. 19.
Fig. 21 is a longitudinal schematic, similar
to Figs. 11 and 19, which depicts a sixth em~odiment of
the invention, yet another variation of the multi-tube
assembly~
Fig. 22 is a transverse view taken along
lines 22~22 of FigD 21.
~, Fig. 23 depicts a spray pattern formed by the
multi-tube assembly shown in Figs. 21 and 22.
Fig. 24 is a perspective view which depicts
one application of the invention wherein the tubes of
the multi~tube assembly are aligned linearly.
Fig. 25 is an elevational, or side, view of
the Fig. 24 application taken in the direction
.l indicated b~ arrow 25 in Fig. 24.
'I Fig. 25a depicts two graphs which illustrat~
another aspect of the inventionl pulsing the
: electrostatic field during the spraying of powder
particles.
~J~
; Fig. 26 is a longitudinal schematic which
depicts a saventh Pmbodiment of a invention, still
another variation of a multi-hole nozzle, wherein the
~, ~ ;7 ~
~17--
mlll.ti-hole nozzle h~s spray holes ~rxanged on -the
circumf~rerlce of a c.ircle, forme~ obliquely ancl
directed outwardly with respect to the center line of
the circle.
Fig. 27 is a transver~;e view taken along
lines 27~27 of Fig. ~6.
Fig. 28 depicts, in perspective vi~w; a
powder spray f low and depcssition patt~rn p:rodllced by
- the nozzlP shown in Figs. 26 ~nd 27.
Fig. 29 is a longitudinal cross~sectic3nal
schematic which depicts another application of the
.~ invention, namely the spray coating of -the in~ide
sur~ace of a can using the nozzl2 depicted in FigsO 26
and 27.
~: Fig. 30 is a transverse cross-sec~ional view
taken along lines 30-30 of Fig~ 29.
Fig. 31 depicts, in perspective view, a spray
, flow and deposition pattern formed when the multi-hole
:
~: nozzle depicted in Figs. 26 and 27 is varied so that
the spray holes are still oriented circ~mferPntially
.?~ ~ : and formed obliquely, but directed inwardly with
'tj~ respect to the center line of the circle~
: Fig. 32 depicts the spray pattern produced by
the multi-hole nozzle shown in Fig. 31 with the ~iew
aken along line 32-32 in Fig~ 31.
Fig. 33 is an enlarged/ longitudinal
:: ~
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schematic vi2w which depicts an eighth embodi.ment of
the .inv~ntion, still another variatlon of the multi~
tube assembly, wherein the tubes are arranged on the
circumference of a circle, formed obliquely and
directed inwardly with respect to the center lin of
the circle.
Fig. 34 is a transverse view taken ~long
lines 34-34 of Fig. 33.
FigO 35 is a perspective view o~ a sma~
scale, multi-hole noz21e that may be attached to a
.i spxay hole of either the multi-hole nozzle or th~
multi-tube assemblyO
-'t FigO 36 is a perspective view of a small-
scale, slit no~zle that may be attached to a spray hole
of the multi-hole nozzle or the multi-tube assembly.
Fig. 37 depicts ~n electrostatic powder spray
coating apparatus according to the invention, wherein
.!
~, the apparatus is equipped with a pulse generator ~or
pulsing the flow of the gas~particle mixture from the
I gun.
;.,~ Fi~. 38 depicts pulse waveforms which may ~e
used to control the operation of the electrostatic
powder spray coating apparatus depicted in Fig ~ 3 7 .
"-~
Fig~ 39 is a longitudinal cross~sectional
!~
schematic which depicts an electrostatic powder spray
.. ~ : coating apparatus according to the invention, wherein
i:
- i. : . : . ". . . : :, . ~ . . .: .: . . . .
19~ 3
,
particles entrained in the gas~particle mixture are
electro~tatici~lly charged inside the ~hamber of ~he
gun.
Fig~ 40 is a longitudinal cross-sectional
schematic, similar to F'ig. 39, which depicts another
variation of electrostatic powder spray coating
apparatus according to the invention, wher~in particles
are electrostatically charged inside the gun.
Fig. 41 is a longitudinal c~oss-sectional
sch~miti~ whi~h depicts an electr~static powder spriy
coating apparatus ~ccording to the invention wh~.~ein
. the apparatus includes tw~ chambers connected in
:
series.
Fig. 42 is a longitudinal cross-sectionial
. ~
schematic, similar to Fig. 41, which depicts another
'~ variation of the embodimen-t shown in Fig. 41.
~,j Detailed Description of the Drawln~s
:i
;l Fig. 1 shows an electrostatic powder spray
:~, coating apparatus 10 in accordance with on~ embodiment
of the invention. The apparatus 10 includes a power
supply hopper 12 where powder particles are mixed with
~:~ air to entrain the particles therein. An ejector, or
pump, 14 transports the gas-powder mixture from the
: tank 12 through a transfer tube 16 and into a gun body
18. An air compressor 20 drives the pump ~4 and
maintains a sufficiently high pressure to entrain
powder particles suspended in air from the hopper 12d
At the gun body 18, the mixture exit~ tu~e 16 and ~lows
into a chamber 22. From the cham~er 22, the mixture
exits a plurality of chamber op~nings, designated
generally by reference numexal ~4, formed within a
multi-hole noæzle 25 at a forward end of the gun 18.
.:
During operation, the gas-powder mixture is
sprayed olit of the chamber 22 via the chamber openings
.. 24 to create a plurality of distinct, fine spray
streams which traverse flow paths, designated
. .. .
. generally by reference numeral 26, whil~ they progress
toward a surface 28 of an article 30 to be co.~ted.
, :'
During spray coating, particles entrained
.~ within the gas-powder mixture are electrostatically
charged so that they will be attracted to the low~r
~ electrostatic potential o~ the surface 2$, indicated on
,,.",~!, Fig. 1 as a ground potential. Fig. 1 shows a high
~ .
,~1 voltage generator 32 which supplies a DC voltage for
producing one or more electrostatic fields for
electrostatically charging the particles in the gas-
, powder mixture. An electrically conductiYe cable 33
connects voltage source 32 to a plurality of
`.3 ~ : electrodes, designated generally by reference numeral
34, which project outwar~ly from a forward end of the
: gun 18. Each of the electrodes is associated with a
; r~spective chamber opening 24 so as to maximize thQ
~J~ O~ 3
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electros-tatic charging of particles in the gas-powder
mixture which traverse ~he respective flow pa~h 260
In all embodiments of the invention which
- utilize on2 or more external electrodes, it is desir~d
- ; that the electrodes be associated in their ele~tric
- circuits with a resistance (not shown) in the range of
:~ about 105-103 ohms, thereby to prevent sparking when the
gun 18 is close to the object 30 to be coated. In one
pre~erred embodiment, the ext~rnal electrodes are made
of silicon carbide and have a resistivity of about 10
n cm~
:
-.- If desi~ed, to ~urther incr~ase the
efficiency o~ electrostatically charging particles
entrained in ~he gas-powder mixture, the cable 33 may
:, also supply a high voltage to a DC electrode 36 located
~:~ inside the chamber 22 of the gun 1~. In coopera~ion
:l with an internal yround terminal 37, the electro~e 36
l sets up an electrostatic field insid~ the gun 18 to
::, produce charged ions which alectrosta-tically charge
!
particles entrained within the gas-powdar mixture
during flow through chamber 220 Because of the
relatively high pressure and ~low rate of the gas-
powder mixture while in the chamber 22, most of the
charged particles entrained therein move past the
~;-.. .
grounded terminal 37 and are sprayed out of the no~zle
25, although some charged par~icles may be attracted -to
, . . .
~22-
and deposited onto the grounded terminal 37~
Fig. 2 shows a ~ransverse, cro5s~sectional
view of a front or spraying end of the no~zle 250
According to this embodiment, the multi-hole nozzle 25
includes chamb~r openings 24a, 24b, 24c, and 24d which
produce spray streams that traverse flow paths 26~,
26b, 26c and 26d, respectively, and which ~re
electrostatically charged by external electrodes 34a,
34b, 34c, 34d, and 34e. Thus, in this embodimPrlt, each
~low path 26 generatPd by a respective chamber opening
24 extends between a pair of the external elec~rodes
34. This maximizes the number of par~i~les in th~ gas-
powder mixturP which are eleGtrostatically chaxged
during sprayingO
Fig. 3 shows, in cross~sectional view, the
distinct flow paths 26a, 26b, 26c and 26d traversed by
spray streams formed by chamber openings 24a, 24b, 24c
and 24d, respectively, while enroute towaxd surface 280
Fig. 4 shows a compos.ite spray pattern 38
which is formed on the surface 28 by the four separate
spray streams depicted in Fig. 3. As shown by Fig~ 4,
the ~low paths 26 traversed by the separate flow
streams become enlarged and merge together while
enroute toward surface 28. The final spray pattern 38
produced on surface 28 will depend upon the distance
between the front end of the gun 18 and surface 28.
, ~ ~
-23-
~- Fig. 5 shows a second ~mbodiment of the
inventioll, which is a varia tion of the electroskatic
powdex spray coatincJ apparatus 10 depicted in Fig. 1.
: According to this variation, he multi-hole no~zle 25
is replaced by a multi-hol~ nozzle 125 which has
tapered, or converging, chamber openings 12 4 ~ These
~:~ chamber openings ~24 produce four distinct, but
. .,
relatively close spray streams which tra~erse flow
p~ths 126 to produce a spray pattern 138, as shown in
Fig. 6, which is narrower and denser than thie spray
pattern 3~ dep.ic~ed in Fig. 4.
Fig. 7 depicts a third embodimenk of the
invPntion, which is another vairiation of the multi hol~
nozzles 25, ~25 used in the first two embodimentsi~
More particularly, the apparatus lO is ~quipped with a
multi-hole nozæle 225 which has a plurality of chamber
openin~s 224 arranged on the circumference of a circl20
With thiS nozzle 225, a single external electro~e 23~
, ~1
is located in the middle of the chamber openings 224.
Fig. 8 shows a plurality of circularly
. - 'i .
~:.................... arranged spray streams produced by the multi-hole
-. i~
nozzle 225, sihor~ly after ejection from the gun 18.
Because the cham~er openings 224 are arranged around a
: circle, the flow pathis 226 traversed by thP spray
` streams are also arranged in ai circular pa~tern~ FigO
9 shows the same spray streams as those depicted in
', ~ ' ' ' ~ . . ' ' ' . ' ' . ~ . , ' .
5~
--24--
FigO 8, but further away from the end of the gun 18.
n Fig. 9, the spray streams have merged to form a
single, annularly shaped flow path~
Fig. lo depîcts the same spray streams
:: depicted in Fig. 8 and Fig. 9, but after deposition
.; onto the surface 2iB. The deposited spray streams form
-; a disc-shaped pattern 238. Compa~ed to Fig. 9, Fig. 10
shows that the particles from the gas-powder mixture
.
`.~ have flowed toward the center so as to eliminate the
: central opening shown in FigsO 8 and 9.
Fig. 11 shows a further embodiment of the
invention, wherein the apparatus lo is equi.pped with a
; multi-tube spray hole assembly 325 ins~ead o~ the
~, multi-hole nozzles 25, 125 and 225 depicted in Fig. 1,
~:~ Fig. 5 and Fig~ 7, respectively. I~he multi-tube spray
;~ hole assembly 325 includes a manifold 327 connected tQ
~ the front end o~ the gun 18, a plurality of tubes 329
::~ connected to the mani~old 327 so as to be in fluid
:, i
~ communication with the chamber 22 and a holder 331
:~ which retains the forward ends of the tubes 32g .in a
predetermined arrangement. LiXe the holes in ~hose
embodiments of the invention which utilize a multi-hole
,, . :
~ nozzle, the tubes in the embodiments which utilize a
., ..s ~
mul ti-tube spray hole assembly define the chamber
openings.
An electrically conductive cable 333 has a
.
~; ~ :
.~ ."
,
... . ... ~ . ~ . . . . . .. . . .
-25-
first end which connects to a DC vol~age source ~no~
shown), and a second end which conn~cts ~o a plurali~y
of external electrodes, diesignated generally 3340 The
electrodes 334 exkend f orwardly from the holder 331.
Like the multi hole nozæle 25 or 225, the multi-hole
tube spray hole assembly 325 splits ~he gas-powder
mixture into a plurality of fine spray r~treams which
traverse a plurality of f~low paths 326 toward the
surface 280 In this embodiment, there are six :Elow
pa~hs designated 326a, 326b, ...32~f (see Fig. :1.3~, an~
particles entrained within the spray ctreams tr~aversing
the~e flow paths are electrostatically charged ~y
elPctrodes 336a, 336b, 336cv..~36f, respectively n See
Fig. l2.
.,j
Fig. l2 shows a front view of the holder 331~
. ~ with the forward ends of the tubes 329 aligned linearly
. ...
to locate the chamber openings 324a, 324b...rJ,324f in a
line.
Fig. 13 shows a cross-sertional view of the
, ~ "~
~ spray streams formed by multi-tube spray hole assembly
, ~, .
r ~ i 325~ shortly after ejection. At this distanc~, th~
i~ 1
: spray streams are separate and distinct and ta~e the
form of six linearly aligned discs. Fig. 14 ~hows the
same spray stxeams sometime thereafter, at a distance
where the spray streams have merged to form a single,
elongated spray pattern 338.
'~J ~
26~
Figs. 15, 16 arld 17 show variations o~ the
invention wherein the chamber openings 24 of a multl-
hole nozæle or multi-tube nozzle are aligned linearly
and electrostatic charging of particles entrained in
the gas-powder mixture is achieved via a singl~ wire ~2
which serves as the electrode. The wire 42 is parallel
with the chamber openings 2 ~, either directly in front
of and aligned with the openings, or offset to one
side. It is to be understood that this single-wir~
electrod~ variation of the inv~ntion is equally
suitable for either the multi-hole nozzle emhodimerl1;s
or the multi-tube spray hole assembly embodiments of
the invention. The objec:t to be coated (not shown) is
:1
placed on the other side of wire 42 from chamber
openings 2 4 .
Fig. 15 shows wire electrode 42 exposed along
its entire length, with insulative covering 43 is
removed at the ends of the wire ~ 2 beyond the chamber
: openings 24. Alternatively, the insulative covering 43
,~
~'~!i may extend along the length of the wire ~2, except :Eor
a plurality of spaced, selected regions ~4 which
correspon~l to the respective chamber openings 24 ~ where
the wire 4 2 is ~xposed .
Ac:cording to another alternative, shown in
Fig. 17 and Fig. 18, the insulative covering 43 is
~, ,
removed from a V shaped region ~7 at the bottom of wire
~: ~J: : ~ ~
~27 ~ 3
42 in Figs. 17 and 18, oppcsite the object bei~g
coated, to expose the bottom portion 44 thereof which
serves as an electrode.
Fig. 19 show~ a fifth embodiment of the
invention, another mult.i-tube spray hole assembly 425.
The multi-tuhe assembly ~25 includes a manifold 4~7,
which is identical to the manifold 327; a plurality of
tubes 429 with first ends which communicat~ with the
chamber 22 and opposite ends whi~h define a plu:rality
of chamber openings 424. These oppo~ite ends o: the
tubes 429 are retained within a holder 431, which
orients the chamber openings 424 in linear alignment,
but in two distinct groups of three.
In use, the multi-tube spray hole assembly
425 produces two separate groups of spray stream~, with
three spray streams included in each group. As a
result, as shown in Fig. 20~ the assembly 425 produces
a spray pattern 438 which includes an upper region 438a
formed by spray streams which traverse flow paths 426a,
426b, and 426c and a lower region 438b which is formed
by the ~pray streams which traverse flow paths 426d,
426e, and 426f.~ A conductive cable 433 connects from a
power suppIy (not shown~ to a plurality of external
electrode~ 434 which extend forwardly from the holder
431. The cable 433 and the external electrodes 434 are
dentical to the cable 333 and the external electrodes
-2~-
334 depicted in Fig. llo
Fig. 21 shows a sixth e~bodiment of the
invention, another multi~tube spray hole assembly 525
The multi tube spray hole assembly 525 includes a
manifold 527, a plurality of tub~s 529 with first. ends
connected to thP manifold 5Z7 and second e~ds r~t~insd
in a predetermined configuration in a holder 531. An
electrically conductive cable 533 connected to cl power
supply (not shown) extends along holder 531 and
co~nects to a plurality o-E ex~ernal electrodes,
designated generally 534, which extend forwardly from
the holder 531.
As in the first, second, fourth and fif-th
embodiments, it is preferable in the sixth embodiment
to have at least one external electrode 534 associated
with each of the chamber openings 524. As shown in
~ Z
Fig. 22, the ~orward ends of the tubes 529 are oriented
~, such that clilamber openings 524a are aligned in a first
row which is parallel to second row chamber openincJs
524b, with the openings of the f.irst and second rows
: being staggered with respect to each other~ Chamber
openings 5~4c form a third row which is parallel to the
first two rows and staggered with respect to ~he second
row, but aligned with the first row.
: Fig. 23 shows a spray pattern 538 formed ~y
the multi-tube spray hole assembly 525, with three
~ ~) 5~
-29
:
distinct coati~g lines 538a,, 538b and 538c of coating
which correspond to thf~ first, second and third rows of
chamber o,penings 524a, 52~b and 5~4ct rPspec~ive:ly~
~: Fi~g. 24 illustrates one applic~ation o~ he
i
electrostatic spray coating apparatus 10 of the
~ inventi~,n. Fig. 24 ~hows the multi-tube spray hole
`' a~sembly 325 used to spray coat an ok,3ect 50 which
ir1,cludes a botto~r,, horizontal mem,ber 51 and a pl.urality
of parallel~ vertical mem~ers 52 which define bottom
.
:' parallel isurfaces 53 and side walls 54, rei;pectively.
i. Because o:E the line~ar orientation of the :Erollt ends o:f
the tubes 329 within the holder 331, khe spray stream~;
may be directed toward the object 50 so as to c~at the
:;,
side walls 54 and th~i- bottom 53 of one slot ~t a tim~
as shown ln both Figs~ 24 and 25. If desired, the
:~ : assembly 325 may be extended downwardly within the
, ~
~ slots during spray coating.
Fig. 25 shows a pulse conkroller 339
: ~ ~ ` connected to the cable 333 which supplies DC electrical
~.
power to the electrodes 334. The pulce controller 339
provides switching to connect and disconnect DC power
to the electrodes 334 according to a desired sequence~
This pulses the electrostatic ~ields produced by the
electrodes 334 between an "on" and an 'loff'l conditi~n.
Fig. 25a illustrates one method of pulsing
the electrostatic fields during spraying. With the
1: :
~ ` il : :
6~ 3
--~o--
spray gun continuously spraying, as shown by the upper
wa~eform, the pulse controller 339 cycl~s the
electrostatic f.ield every 60 milliseconds~ with the
field turned "on" for 20 milliseconds and then l-off"
for 40 millisecondsa Pul~ing of the electrostatic
field is shown by the lower waveform.
It is to be und2rstood that the relative
duratio~s of the "on" and '~offa9 time, along with the
duration of the entire cycle, may be varied according
to any desired sequence. It is also to be understood
that pulsing of the electrostatic field may ~e used in
combination with the other powder co~ting features
disclosed in this application, such as pulsing of the
powder flow~ to provide uniform coating of uneven
surfaces, such as the inside surface of a containerO
it Fi~. 26 shows another multi-hole nozzle 625
in a~cordanciP with a seventh embodiment of the
invention. The multi-hole nozzle 625 would fi~ on the
end of gun 18 in Fig. 1, for example, like nozzle 25,
and includes a plurality of chamber openings 624 which
are arranged on the circumference of a circle which is
~1
coaxial with the longitudinal axi.s of gun 18, with the
openings 624 formed obliqu~ly and directed ou-twardly
with respect to a center axis 628 of the multi~hole
: nozzle 625. Fig. 27 more clearly shows the orientation
and configuration of the cham~er openings 624, along
;. .~
2~ 3
-31-
with an external elec-trode 63~ aligned along axis 628,
or the at the middle of the circle defined by the
chamber open.ings 6240
Fig. 28 shows the multi-hole nozzle ~25 in
use, with a plural.ity o spray streams emanating from
chamber openings 624 and traversing 10w paths 6260
~he spray streams which emanate -from multi-hole nozzle
625 form a spray pattern 638 which includes six
distinct disc-shaped regions arranged around the
cir~umference of a great circle.
FigO 29 shows one application ~or wh.ich the
apparatus 10 of this invention is well suited~
particularly the seventh embodiment of the inventiQn,
which utiliæes the multi-hole nozzlP 625 dep.icted in
Figs. 26 and 27. This application involves powder
spray coating of the interior sur~ace of a hollow
container 55. Because of the orien-ta~ion of the
chamber openings 624, the ~low paths 626 of the spray
stream are angled. When sprayed inside the container
55, or any other hollow~ cylindrically shaped object,
such as a pipe, the ~pray streams deflect of~ the
inside surface 58 of the container 55 and continue
alony ~low paths 626 which twist and descend, resulting
~, ,,
~J ~ in a reduced air cushion inside the can and producing
!:
more uniform coating o~ the inside sur~ace 5$. FigO 29
~: ~ and Fig n 30 include directional arrows 59 which show
'I : :
the tw.isting or spiralling eff~ct caused when the spra~
streams deflect off of the inside surfaca 58 of the
container 55 and progre~is towa~d the opposite or closed
end thereof~ ~eduction of the air cushion inside the
can which tends to preverlt adequate powdier coa ting
material from enteriny the can, can be fur~her ac:hieved
by pulsing the pump 14 of Fig. 1 asi is discussed later
in connection with Figs. 37 and 38. M~reov~er,
. -.
reduction of the ~araday cage effect within container
55 will be achieved by pulsing the power supply for
, ~
electrodfe 634 in a mann~r similar to tha~ describ~d
~, wit~ respect to Figs. 25 and 25a.
`~ Fig. 31 depict~ an eighth embodiment of the
invention, wherein a multi-hole nozzle 725 includes a
.' plurality o:f chamber openinys 724 which are arrange~d on
the circumference of a circle and formed obliquely like
~( those of multi-hole nozzl~ 625, but which are dir~cted
'1
"1 inwardly toward a center line through ~he circle. With
this multi-hole nozzle 725, the flow paths 737
traversed by the spray streams converge toward the
. ~
center line 737 ~nd then diverge outwardly thexefrom~
as shown in Fig. 31.
Fig. 32 shows a spray pattern 738 ~ormed by
multi hole nozzle 725. The spray pattern 73~ includes
: : : : six distinct region~ arranged around the circumference
~~;.. 3~ o~ a great circle, with the regions being slightly
~ ~ :
'~ ~'7 ~ ?3
-33-
ovaled and elongated radially with respect to ~he
circle. The patter~ formed by the spray s-treams
produced by multi-hole nozzle 725 .is sometimes referred
to as a Japanese hand drum. With this flow pat.h 726
arrangemen~, by adjusting the distance between the end
of the multi-hole nozzle 725 and an object to be
coated, the surface ~rea of coating can range from very
small to very large.
Fig. 33 shows a ninth ~mbodiment of the
invention, a multi~kube ~p~ay hole assembly 825 s~hich
is design~d to produce the same spray p~ttern as multi-
hole nozzle 625. The multi-tube spray hole assembly
825 includes a ~manifold 827, a plurality of tubes 829
which extend from the manifold 8~7 and are ret~ined by
a holder 831 in a pred~termined arrangementO As ~hown
most clearly in Fig~ 34/ the chamber openings 824
form~d by th~ tubes 829 are arranged on the
circumference of a circle, oriented obliquely and
directed inwardly with respect to the center line
throuyh the circle. If desired, a single electrode
(not shown~ may exte~d forwardly from holder 831.
Fig. 35 depicts a small scale, multi-hole
nozzle 60 which connects to one tube 29 of one of the
multi-tube as~emblies, Fig. 11 for example, so as to be
in fluid communication with the respective cham~er
opening 324, for example. The small ~cal~, multi-hole
~ ~7 ~
-3~-
noz~le 60 includes a frustoconically shaped, outwardly
flared passage 62 which termin~tes in a plurality of
small scale holes 6~ arranged around the ci~cu~ference
of a c.ircle 6~. The use of small scale holes 64
produces even smaller spray streams, thereby increasing
control over the directivity of the spray streams used
in powder coating.
As shown in Fig. 36, a small scale, slit
nozzle 70 may be attach2d to the forward end of a tube
29 of a multi-tube assembly, Fig. 11 for example, so as
to be in ~luid c~mmuni~ation with the chamber op~niny
324, ~or example. The small scale, slit nozzle 70
includes an elongated diver~ing hollow portion 72 which
terminates in an elongated slit 74.
While this small scale, multi-hol~ nozzle 60
and the small scale slit nozzle 70 are shown attached
to the end of a tuhe 29 of a multi-tube assembly/ it is
also to be understood that this principle could be
applied to the embodiments of the invention which
relate to a multi-hole nozzle 25~
Figs. 37 and 38 illustrate another aspect uf
the invention, that of spraying the gas-powder mixture
from the gun la in a pulsin~ manner. According to this
aspeot of the invention, a pulse generator 76 i~
electrically connected via conductive lines 77 to a
solenoid valve SV which controls air flow ~rom a
o
-35~
compressor 20 ~o powder pump 14 to cau5e the gAs~powder
mixture to flow from hopper 1.2 through the gun 18 and
outwardly therefrom in a series of pulses. The
operation of an elactrostatic powder coating apparatus
in a pulsing manner is des~ribed in Japanese Rokai No.
62 [1987] 11,574, which was mentioned previously in
this application.
Fig. 38 shows two example waveforms 7$ and 79
which may be used to control pulsing o~ the spra.y
stre.ami~i outwardly from the gun 18. As .in~icatec~. in the
above-identifiecl Japanes~ publication, by selecting the
number of pulses per unit time, the amplitude of th~
pulses and the duration of th pulses r the amount of
powder sprayed outwardly from ~he giun 18 may be readily
adiusted and precisely controlledO Perhaps most
importantly, the pressure of t,he air pressure to pump
14 can be increased to assure constant ejecition volume
and rate per unit tim~. This assures better uniformity
in coating. Moreover, pulsing the spray streams also
facilitates spray coating where a thin coating
thicXness is desirable.
.
:: : Figs. 39-42 show-electrostatic charginy oE
~ particles in the gas-powder mixture while inside the
:~ gun 18 of Fig. l. Fi~. 1 shows that internal charging
: may be used to supplement external charging via
~ external electrodes 34~ Alternatively, internal
f2 ~ a 3
--36~
charging may be the sole means for ele~trostatically
charging particles en~raine~ in ~h2 gas~powder mix~ure.
Internal charg.ing is particularly advantagevus in
coating the inside surfaces of metal container~, where
the use of external electrodes te~ds to produce a
Faraday cage effect, as explained in the background.
Fig. 39 d~pict5 an intPrnal electrode 80
charged by a power supply ~not shown~ and grouncl.ed
terminals 81 to produce an ~lectrostatic field i.nside
~he gun 18. When electrostatically charging particle~
o~ the gas powde.r mixture inside th2 gun, i.t is
important to prevent adherence of the powder particle~
to either the electrode 80 or the grounded terminals
81. Fig. 39 ~hows comprèssed air inlet ~2 which
co~municates with a conduit B3 via a port 84 in the
gun. The conduit 83 surrounds electrode 80 and blows
air around the electrode 80 to pr~vent charqed parti.cle
accumulation thereon. Another air inlet 85 supplies
pressurized air into a hollow annulus 86 which
circumscribes the outside of the gun~ Pre~isurized air
from the iannulus 86 flows radially inwardly into the
gun via a plurality of ports 87 spaced around the
circum~erence of the gun. Air flows directed radially
inwardly ~rom the ports 87 discourage the accumulation
of charged particles on the ground terminals ~1.
Fig. 40 shows an alternative structure for
~' ~
2~ 0~3
-37-
this same aipect of the in~ent.ion. Inlet 85 is aligned
with a single port 8~ in the outer wall of the gun.
The port 89 communicates with an annular hollow space
88 which cir~umscribes the gun along its internal
surface. The annular hollow space 89 is formed by a
tubular, conductive sinter 90 which is sonnected to an
electrical ground (no~ shown). The porosity of the
sinter 90 permits outflow into the gun of air supplied
to the inlet 85, there~y discouraging particle
accumulation thereon.
As indicated above, electrostatic charging
inside the gun may be used alone or in conjunction with
external charging. When used alone, it may be
de~irable to enhan~e or maximize the number of chirged
particles in the gas-powder mixt-lre by using multiple
charging chambers. For example, Fig. 41 shows an
alternative embodiment of the invention which includes
two charging chambers 22a and 22b connected in series
with electrodes 80a and 80b and grounded terminals 81a
and 81b located therein, respectively~ A multi-hole
nozzle 25 is located at the downstream end of thP
.
second chamber 22
Alternatively, the multiple chambers 22 may
:~ ke connected in parallel, so as to eliminate pressure
loss necessitated by a series connection. The use of
~: parallel connected chambers while not increasing the
' ` ' ~ , . . ~ . ,": !.'. , ` ,'. ' . , ~ ! .; .
-38-
charge on the powder does provide an increase in ~he
flow rate of powder sprayed from the gun.
Fig. 42 shows another variation of the
in~ention involving multipl~ ch~rging chambers.
According to this embodiment, an upstream chamber 22a
utilizes frictional charging, rather than an applied DC
electrostatic f ield . Fx ictional or triboelectric
charging occurs by routing the powd2r particles through
a tortuously configured plastic or Teflon condu.it 92
which preferably contacts the inside sur~ace of the
chamber 22, which is in turn connected to a gro~md
terminal 93. The powder particles becomP cha~ ged
triboelectrically by multiple frictional contacts with
the conduit 92. With this embodiment, i~ is important
to make sur~ th~t the second charging chamber 226
charges powder with the same polarity as tribocharging
chamber 22a.
From the above disclosure of the general
principles of the present invention and the preceding
detailed description o~ the pre~erred embodim~nts,
those skilled in the art will readily comprehend the
various modifications to which the present invention is
susceptihle. Therefore, we desire to be limited only
by the scope of the following claims and equivalents
thereo~.
.
.~
