CA2070063A1 - Electrostatic powder coating utilizing multiple spray streams - Google Patents

Abstract

ABSTRACT OF THE INVENTION

ELECTROSTATIC POWDER COATING UTILIZING
MULTIPLE SPRAY STREAMS WITH PULSED ELECTROSTATIC FIELD
AND SPRAY PATTERNS
An electrostatic powder coating gun splits a pressurized gas-powder mixture into a plurality of separate electrostatically charged spray streams to achieve a high degree of directionality and reduce deflection. The powder particles in the mixture may be electrostatically charged while inside one or more chambers in the gun, either by an applied DC
electrostatic field or by friction. The flow paths traversed by the spray streams may be arranged and/or oriented so as to advantageously powder coat a particular surface configuration, such as the inside of a container. Alternatively, electrostatic charging of powder particles may occur via a plurality of electrodes located external to the gun, with at least one external electrode associated with each spray stream. To further reduce deflection, the mixture may be sprayed from the gun in a pulsed manner.
Additionally, the electrostatic fields created by the external electrodes may be pulsed between an "off" and "on" condition during spraying.

Description

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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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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.
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~: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 !

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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 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 .. :
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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 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.
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~` : 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~

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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.
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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~
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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

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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.

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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~

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 :

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.
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; 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 .
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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

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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.

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~- 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 ', ~ ' ' ' ~ . . ' ' ' . ' ' . ~ . , ' .

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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 .
~; ~ :
.~ ."
, ... . ... ~ . ~ . . . . . .. . . .

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~

:

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 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 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 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 ' ` ' ~ , . . ~ . ,": !.'. , ` ,'. ' . , ~ ! .; .

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~.

.
.~

Claims (35)

1. A method for electrostatic powder coating comprising the steps of:
spraying a pressurized mixture of gas and powder particles from a spray gun in a plurality of separate spray streams; and electrostatically charging said powder particles entrained in said spray streams via a plurality of electrodes located external to the gun.

2. The method of claim 1 and further comprising the step of:
causing said plurality of spray streams to be sprayed in a pulsed manner.

3. The method of claim 1 wherein at least one external electrode is associated with each of said spray streams.

4. The method of claim 1 wherein each said electrode produces an electrostatic field when operatively connected to a power supply and said charging step further comprises:
causing said electrostatic fields to pulse during said spraying step.

5. The method of claim 4 and further comprising the step of performing said causing step according to a predetermined timing sequence, thereby to produce a desired coating effect.

6. A method for electrostatic powder coating comprising the steps of:
spraying a pressurized mixture of gas and powder particles outwardly from a gun in a series of pulses;
electrostatically charging said particles entrained in said mixture during said spraying step via an electrostatic field established by an electrode operatively connected to a DC power supply; and causing said electrostatic field to pulse during said spraying step.

7. The method of claim 6 wherein said charging step is performed via a plurality of electrodes located external to the gun and said spraying step further comprises:
splitting said mixture into a plurality of separate spray streams.

8. A method for electrostatic powder coating of the inside surface of a container comprising the steps of:
spraying a pressurized mixture of gas and powder particles outwardly from a spray gun in a plurality of separate spray streams; and electrostatically charging, within an internal chamber of the gun, said particles entrained in said mixture, thereby to produce a plurality of electrostatically charged spray streams for uniformly coating the inside surface of the container.

9. A method for electrostatic powder coating of the inside surface of a container comprising the steps of:
spraying a pressurized mixture of gas and powder particles outwardly from a spray gun in a plurality of separate spray streams;
electrostatically charging said particles entrained in said spray streams via a plurality of electrodes located external to the gun; and periodically pulsing the electrostatic fields associated with each of said electrodes during said spraying step.

10. A method for electrostatic powder coating of the inside surface of a container comprising the steps of:
spraying a pressurized mixture of gas and powder particles outwardly from a spray gun toward said surface along a flow path; and causing said mixture to be sprayed toward said surface in a pulsed manner, thereby to minimize deflection inside the container and more uniformly coat said surface.

11. The method of claim 10 wherein said spraying step further comprises:
splitting said mixture into a plurality of separate spray streams which traverse a plurality of corresponding flow paths.

12. A method for electrostatic powder coating of the inside surface of a container comprising the steps of:
spraying a pressurized mixture of gas and powder particles outwardly from a spray gun;
electrostatically charging, during said spraying step, said particles entrained in said mixture with an electrode, said electrode producing an electrostatic field when operatively connected to a power supply; and pulsing said electrostatic field during said spraying step.

13. The method of claim 12 wherein said charging step is achieved with a plurality of electrodes located external to said gun, and said spraying step further comprises:
splitting said mixture into a plurality of spray streams which traverse a plurality of corresponding flow paths.

14. A method for electrostatic powder coating of the inside surface of a container comprising the steps of:
spraying a pressurized mixture of gas and powder particles outwardly from a spray gun in a plurality of separate spray streams; and electrostatically charging said particles entrained in said mixture to produce a plurality of electrostatically charged spray streams, wherein said spray streams are arranged around the circumference of a circle which is coaxial with a forward end of the gun and said spray streams traverse flow paths which are oriented obliquely with respect to the circle and directed outwardly from a center axis through the middle of the circle, thereby to uniformly coat the inside surface of the container.

15. The method of claim 14 and further comprising the step of:
causing said spray streams to be sprayed from the gun in a pulsed manner.

16. An electrostatic powder coating apparatus comprising:
a gun having at least one internal chamber and the chamber having a plurality of chamber openings which open to atmosphere;
means for introducing a pressurized mixture of gas and powder particles into the chamber and spraying said mixture outwardly from said gun through said openings in a plurality of separate spray streams;
and means for electrostatically charging said particles entrained in said spray streams, said charging means including a plurality of electrodes located external to said gun.

17. The apparatus of claim 16 wherein said gun further comprises:
a multi-hole nozzle having a plurality of holes which define the plurality of chamber openings.

18. The apparatus of claim 16 wherein said gun further comprises:
a multi-tube spray hole assembly including a plurality of tubes which define the plurality of chamber openings.

19. The apparatus of claim 16 wherein at least one electrode is associated with each of said spray streams.

20. The apparatus of claim 16 wherein said electrodes are of silicon carbide and have a resistivity of about 106 ohms cm.

21. The apparatus of claim 16 wherein said plurality of chamber openings are aligned linearly and said means for charging further comprises:
a wire located outside of the gun and oriented parallel with said aligned chamber openings the wire serving as an electrode.

22. The apparatus of claim 21 and further comprising:
a layer of insulation partially covering said wire and leaving at least one selected portion uncovered, said at least one selected portion serving as an electrode.

23. The apparatus of claim 16 wherein the chamber openings are arranged in a plurality of parallel, staggered rows.

24. The apparatus of claim 16 and further comprising:
means for causing said mixture to be sprayed outwardly from the gun in pulses.

25. The apparatus of claim 16 and further comprising:
means for operatively connecting a power supply to said electrodes to produce electrostatic fields for electrostatically charging said particles entrained within said spray streams; and means for pulsing said electrostatic fields during spraying.

26. An electrostatic powder coating apparatus comprising:
a gun having at least one internal chamber and the chamber having at least one chamber opening which opens to atmosphere;
means for introducing a pressurized mixture of gas and powder particles into the chamber and spraying said mixture outwardly from said gun through said chamber opening;
means for electrostatically charging said particles entrained in said mixture by means of an electrode which produces an electrostatic field when operatively connected to a power supply;
means for causing said mixture to be sprayed from said gun in a pulsed manner; and means for pulsing said electrostatic field while said mixture is being sprayed.

27. The apparatus of claim 26 wherein said charging means further comprises:
a plurality of electrodes external to the gun, and said means for introducing and spraying further comprises:
means for splitting said mixture into a plurality of separate spray streams, wherein each of said streams is associated with at least one of said external electrodes.

28. An electrostatic powder coating apparatus comprising:
a gun having at least one internal chamber and the chamber having a plurality of chamber openings which open to atmosphere, said chamber openings being arranged around the circumference of a circle which is coaxial with the gun;
means for introducing a pressurized mixture of gas and powder particles into the chamber and spraying said mixture outwardly from said gun through said openings in a plurality of separate spray streams;
and means for electrostatically charging said particles entrained in said spray streams, said charging means including an electrode external to the gun.

29. The apparatus of claim 28 wherein said chamber openings are oriented obliquely and directed outwardly with respect to a center line through the circle.

30. The apparatus of claim 28 wherein the openings are oriented obliquely and directed inwardly with respect to a center line through the circle.

31. The apparatus of claim 28 and further comprising:
means for causing said mixture to be sprayed from the gun in pulses.

32. An electrostatic powder coating apparatus comprising:
a gun having at least two internal chambers, at least one of the chambers having at least two chamber openings which open to atmosphere;

means for introducing a pressurized mixture of gas and powder particles into said chambers and spraying said mixture outwardly from said gun through said at least two openings; and means for electrostatically charging said particles entrained in said mixture, said charging means located in each of said internal chambers.

33. The apparatus of claim 32 wherein said at least two chambers are connected in series.

34. The apparatus of claim 32 wherein said charging means further comprises:
an electrode located in at least one of said chambers and a frictional charging means located in at least one of said chambers.

35. The apparatus of claim 32 and further comprising:
means for causing the mixture to be sprayed from the gun in a pulsed manner.