Note: Descriptions are shown in the official language in which they were submitted.
3~3
IMPROVED P~RTICLE SPRAY GUN
This invention relates to particle spray equipment and
more particularly to an improved particle spray gun for
electrostatically applying coating particles to an article to
be coated.
Backqround of the Invention
Coating applied electrostatically to an object to be
coated can be either in the form of electrostatically charged
solid particles, i.e., powder, or electrostatically charged
liquid particles which have been atomized using a variety of
well known
LCM~jj
.
~4q~
techniques or principlesp in~ludiny air impin~ement
atomization, airless or hydrostatic pres~ure atomiza-
~ion, and/or electrostatic atomiza~ion. This in~ntion
iB useful wi~h both liquid and powder spray coating
applications.
In the application of solid ~artic~laie
coatings, such a~ p~wde~ed resins~ i~ ind~s*rial
~inishing applications, the particulat~ or pOW~
~ommonly conveyed to a spray device, often terme~ a
~gun", by air under pressure and is then sprayed ~rom
an opening in ~he forward end, or nozzle, of -~he yun
in the farm of a ~owde~-entrained ~ir ~tream which is
projected along a path from the gun toward the o~ject
ko be coated. In the ~rocess o~ spraying the coating
1~ particles frDm the gun, an electrical charge is
preferahly imparted to the particles by an electxoae
maintained at a high voltage which is mounted to the
gun n~]e ~roximat~ tD the ~ath of the powder c~ating
stream. The charged paxticles are ~hen electr~stati-
- 20 cally attracted toward the object to be coated which
is heId at electrical ground potential, enhanring the
efficiency with which charyed particle~ sprayed ~rom
the gun are deposited:on the ~arget article. ~f~er
the article is coated,-it is generally conveyed
,~5 through an oven where the powder coating material is
heated and fused onto the surface o~ the article to
permanently bond it thereto.
~l2S~
~ lectrostatic powder spray gun~ typically
include a mechanical p~wder deflector moun*ed a* the
~ozzl~ end of the gun. In one ~re~erred form t~e
deflector is in the shape of a cone and is disposed
axially in the flow path of the pDwder b~ing sprayed
from the g~n, defle~ting the powder into ~ onical
~pray patt~rn. ~ha~ is~ th~ deflector is Impacted by
the powa~r coating material ~ein~ sprayed f~om the gu~
in the nozzle region and dire~ts th~ pDwder radially
outwardly to form ~ conical spray patternO
Electros~atic liquid spray gun syst~ms
cus'omlarily include a sourc~ of pressurized liquid
which conveys the liquid coating to the gun via a hDse
where it is ~mitted from ~he noz~l~ in a stream o~
1~ atomized particles. Atomization can ~e produ~d by
impingement of the liquid~stream with ai~ in ~he
region of the nozzle~ which is ~nown as air atomiza-
tion. Plternative.y, the liqui~ coating c2n be ~.ighl~
pressllrized such that upon e~it f~om *he n~z~le
atomization results, which is termed hydrostatic or
airless atomization. In still other systems, th
liquid is ~ubjected to electrostatic fo~ces which
effectively atomi2e the liquid.
One of the o~jecti~es in the design ~f ~n
2~ electrostatic spray gun, either liquid or powder, is
to maximize the~efficiency with which charged coating
particles sprayed from ~he gun are deposited on the
- article ~eing coated. This is called the "transfer
3~
~4--
efficiency". It is generally believed ~y thos~
skilled in ~he ar* tha* transfer efficiency an ~2
increa~ed by increasing the charge vn the particles
and/or by increasing the strength o~ the elec*rostatic
i field between ~he gun and ~he aLrticle ~eing coated.
Accordingly, i* has ~een an objectiY~ of
this invention ~o co~struct an electrostati~ spray gun
which will ~oth increase the charge vn the particl~s
1~ and the strength o the electrostatic field between
gun and article ~eing coated, and there~y ~rouide
improved powder coating transfer efficiencyO This
o~jective has been accomplished by pro~i~i~g a particle
spray device, which has an opening from which a stream
1~ of particles i9 sprayea in a forward path toward an
object to ~e electrostatically coated, with a multi-
point electrode comprising a subs~antial num~er of
closel~- sp2ced electro~e elemen*s locatea ~oxima~
the opening through which the particle stream pas~es.
2~ Energization o~ the multi-point electrode from a high
voltage electrostatic supply results in the creation
of A plurality of corona charging points proximate the
particle stream, there~y enhancing coating transfer
e f ~iC iency.
~ In one preferred embodime~t of a powder
spray gun, a def lector is provided in the nozzle
powder stream path which is constructed of electrical-
~zs~
ly nonconductive material, and which has a~ a rear
surfac~ upon which the forwardly directea pDwder
~tream Impinge~ ana as a ~esult ther~of b~c~
deflected intD the desired s~rel~m configuration, ~ a
front surface facing the forward d-rectio~, nd c3 a
subs~antial number o~ electrode elements circumferen-
tially spac~d aro~nd the perimetex o~ t~e defle~tor
which are ~o~nected tD ~ high vDltage source via
associated resisti~e paths incorporatea in the de1ec-
lD tor. These electrode elements collectively ~unctionas a multi-pDint electrode tD provide a pl~rality of
corona charging points when the electrode is Pnergi2,ed.
In the one preferred embodiment descTibed
above, the multi-pDint electrode is in the form of a
1~ fibrous resistive sheet constructed from a mateIial
such ~s ~ilicor carbi~e, which is iDcorporated in the
deflector between the front and rear surfaces thereo~
to define as its periphe.^~, which is proximate to the
~eflector periph~ry7 a large number of raaially
: 20 ~rranged electrode elements which establish a plurali~y
of corona charging points past which the de~lected
powder stream passes to be electrostatically cha~ged
as the powder particles are s~rayed from the gun~ In
- this embodiment, in which the substantial number of
2~ radially arranged electrode elements ~ircum~erentially
spaced around the de~lector periphery ~unction tD
establish a plurality of corona charging points, the .
~2S9L~
resistive sheet located radially inwardly o~ the
electrode elements fune~ion~ as resis ive ~aths
; ~ incorporated in the deflector through which the
electrode elements are energized from a suitable high
voltage source~ While a silicon carbide material is
preferred f~r use as the resis~ive sheet in this
:- embodimen~l o*her ~ rous resistive material~ may also
be suitable.
An important adva~tage o~ the foregoing
embodiment of this inventi~n is that the peripheral
edge of the silicon car~ide sheet includes the en~s of
the many silicon car~ide fibers fonming the resistive
sheet and these fiber ends form a multitude of radially
arranged electrode elements which ~stablis~ ~ ~lurality
of corona charging points which charge the ~owder
particles-^as t~ley are sprayed. This deflecto~ struc
ture is believed to both increase the charye trans-
~erred to the powder particles, and to increase the
str~ngth of the electrosta~ic field between the gun
2~ and the workpiece, to enhance coating transfer effi-
ciency. Another advantage is that the deflector
stxucture, particularly the resistive paths and ~lural
circum~erentially-arranged electr~de el~ment~, is
~relatively inexpensiYe, easy to manufacture, and
2~ durable. It i~ also readily replacea~le shoul~ such
become necessary. These attributes ~nha~ce the
attxacti~eness of the g~n from a commercial standpoint.
3V
--7--
In the foregoing form o~ the i~vention, the
silicon carhide sheet has a cen~rally dispo~ed ~ig~
vol-tage texminal region remo~ fro~ *he edge thsreo~
for establishing an electrically resis~lve curren~
flow path through th~ sheet between the ~entral
terminal region whereat high voltage i~ ~up~ d and
the peripheral ~dge w~e~at co~Dna ~harginy ~i the
powder particles occurs from *h,e many silicon c~rbid~
fiber ends. This resistive path cDnsti~ute~ a re~a-
tively large xesistor and functions to minimizeignition h~zards due to inadvertent discharge of
electrical energy capacitively s-~ored in the s~ray
coating system of which the gun is a major ~omponent.
In another ~referred embodiment of this
1~ invention, the circumferentially spacea electro~e
elements aroun~ the ~eflec*o} perimeter are in the
~orm of discrete, fixed electrodes in the f~r~. of
electrically conductive needl~s or wires which project
radially outwardly from the perimeter of the d~fl~c~or~
- 20 -~ach of the discrete electrDde~ is connected tD 2 high
voltage source via a discrete resistor:embodied in the
deflector. If desired, the radially disposed elec-
trodes can be made fl~sh with the deflector peri~hery
in lieu of projecting outwardly th~rerom, there~y
reducing the likelihood of electrode dEmage.
In accordance with a still further, and also
preferred, embodiment of the invention, the deflector
: is provided with a.relatively narrow silicon carbide
-- ~54~3V
ribbon or t~lread, which ~unction as cir~umf~entially
arrang~d electrode el~ment~i ~ia disc~eet resist~ls
embodied in the deflec~or ~hic~ are radially ~i~pose~
and cir~ferentially spaced within the defl~ctor~
I~ *he em~odiments of the i~vention utili~ing
discreet xesistor~ embodied in the defl~ctor to
interconnect ~he high voltage ~ource and ~he ~irc~m-
~erentially ~paced el~trode elements on *he ~fle~to~
p~riphery, the resistors function tv minimize ignîtion
hazards au~ to inadvertent elec~rical energy dis~
charges, thereby enhancing the safety oE the gun.
In accordance with a further aspect of the
invention which can ~e advantageously incorporated
into each of the f~reyoing emb~dIments, the nozzle
1~ lvcate~ at the forward end o~ t~e nonconduc~i~e ~un
arrel is prvvided with an electrosta~ic shield. The
shield is disposed outwardly and rearwardly of the
pe~imeter of *he deflect~r whereat the corona c~arging
points ar~ located which ~lectrost~$i~ally charg~ the
70 aeflected powder stream as it p~sses through the
'' annular opening ~etween the nozzle and the conically-
shape~ ae~lector which is axially disposed in the
powder flow path. In a ~l~f~r~ed form, tne ~le~t~o-
static shield is formed ~y flaring the end of ~he
noz~le in the Iegion surrounding the forwara end of
the conical deflector, particularly the perimet~
thereof, from which extend the corona charging points.
In practice, the electrostatic shield has ~een ~ollnd
~S~3~
to significantly improve the transfer efficiency when
compared to a similarly-constructed spray device which does
not have the electrostatic shield.
By way of background, and as an aid to understanding
how the electrostatic shield of this invention enhances
transfer efficiency, in a typical electrostatic spray gun of
the type having an electrically-grounded handle or mounting
member, the corona zone proximate the periphery of the
deflector is approximately midway between the grounded gun
handle or mounting member which is located rearwardly thereof
and the electrically-grounded object being coated which is
located forwardly thereof. By way of example, the distance
between the grounded object being coated and the corona
charging zone is approximately ten inches, which is
approximately the same as the distance between the corona
zone at the gun nozzl.e and the rearwardly-located
electrically-grounded gun handle or mounting member. Without
the electrostatic shielding outboard and behind the corona
charging zone proximate the periphery of the deflector, the
electrically-charged coating particles issuing from the gun
nozzle are as close to the grounded article being coated as
is the grounded gun handle or mounting member, with the
result that some charged particles are electrostatically
attracted to the grounded gun handle or mounting member,
impairing the efficiency of the coating transfer process.
LCM:mls
.~
~5~3~
In addition, because the gun handle, or
moun-ting hardward, provides an attraction to some of
the charged particles, a corona current: path is set
up be-tween the deflector and the grouncle~ handle which
causes the available elec-trical energy for charging at
the deflector to be reduced by parasitic discharge.
The reduction in available charging energy at the
deflector, results in a corresponding reduc-tion in
-transfer efficiency. Therefore, by inclusion of the
electrostatic shield of this invention, the effect of
the electrically-ground gun handle or mount in terms
of attracting elec-trostatically-charged particles and
of providin~ a parasitic curre~t leakage path is
subs-tantially reduced, with the result that -transfer
efficiency is significantly increased. This is a
substantial improvement in transfer efficiency in
comparison to -the result if the electrostatic shield-
ing in the nozzle surrounding the deflector periphery
is ommitted.
The electrosta-tic shield can be used advantageously
with the guns, manual or automatic, which are designed
-to spray coating par-ticles of ei-ther the atomized liquid
or powder type.
In accordance with still other embodiments
of the invention, the multi~point electrode is in the
form of a disc with a sawtooth perimeter. The entire
disc may be fabricated of resistive, semiconductive or
conductive material. Al-ternatively~ the disc may be
, -~
-- 10 --
1~S~3~
of a comp~site construc~ion with an inner ci~cular
section, ana an out~r annular ~ection with ~eeth at
~he peripheryO The inn~r and~Dr ~uter sec~ions ~ay ~e
conductive, resistive, or semic:onductiYe solid sheet,
fi~rous or mesh material~ In lieu of the inner circu-
lar section, a series of electrical wires connected to
~he ~nnular section may ~e used to tr ns~ort high
voltage to the toothea periphery thereof
In ano~her form, the multi-point electrode
10 may be a disc-shaped mesh o~ conductive, semiconduc-
tive, Dr resistiv~ wire, or nonc~nductive wir4 havinq
a ~l~d~ing of conductive, semiconductive, ol resisti~e
materialO
In another embodiment, use~ul in a powder
gun ha~ing a deflector, the deflectDr is fabricated o~
..injection molded ~aterial containing.silicon carbide
or other resistive fi.bers, particularly a* the per-
.imeter thereof, which functiDn as multi-point elec-
trodes. ~he deflecto~ may also include semicondllctive,
2D resistive~ or conductive material t~ tran~port ~he
high voltage to the ~ilicon carbide ~ibers at the
deflector perImeter~ Instead of silicon carbide
fibers at the peliphery of.*he deflector, ~ multi -
- point electrode could be provided by mounting a large
.25 number of electrodes in ~he deflector ~erimeter to
function as multiple elec*rodesO
:In any of the-aforementioned embodiments, it
is desirable ~o pro~ide ~esis~ance s~fficiently close
3{~
to the multiple electrodes and in sufficient amount to
avold unsafe elekrical discharges should electrical
energy capacitively stored in the gun suddenly become
discharged by -the approach of a grounded article to
the multipoint electrode.
The multi-point electrode aspect of this
invention, while described in connection with a powder
gun having a deflector, is also useful in atomized
liquid spray devices. In such devices the multi-point
electrode is mounted in the nozzle region proximate
the path of atomized liquid particles being emit-ted
from the nozzle toward -the article -to be coated in
much the sa~e manner that the multi-point electrode :is
mounted in the deflector of a powder gun pro~imate the
path oE the emitted powder particles.
These and other ojectives, advantages, and
fe~-tur~sof-the invention will become more readily
apparent from a de-tailed description of the invention
taken in conjunction with the drawings in which:
Figure 1 is a side elevational view, partly
in cross section, depicting the principal components
of one embodimen-t of an electrostatic powder spray gun
incorporating the invention;
Figure 2 is an enlarged side elevational
view in cross section, showing the forward end of the
powder gun of Figure 1, including the nozzle~ deflector,
and powder-charging electrode.
- 12 -
jrc~
~5~3~
-13-
Figure 3 is a cross sPction 1 ~i~w along
line 3~3 of FiguIe 1.
Figure 4 is a front e:le~i~nal view, o~ the
nozzle of the gun of Figure 1, depictin~ the deflec~or
partially cut-away to show the xesistive fibrous
~heet~
Figur~ ~ is B ~ont eLevational view of
deflectr:r 7 incorporating radially outwardly ~rojec~in~
elec~rodes and discre~e resisiors, o~ another embodi
men* o~ the invention.
Figure 6 is a sid~ elevational view of the
de~lector of Figur~ ~.
Figure 7 is a front elevational view of a
deflector, incorporating a silicon carbide ribbon or
1~ thread in the rim thereo~ ana discrete xesistors, o~ a
. still u~t~er embodiment of the in~ention.
Figure 8 is a side elevational view of the
deflectDr o~ ~igur~ 7.
Figure 9 is a ~ront elevational view of a
multi-point electrode in the form of a sawtooth-edgea
disc o~ uniform construction throughout.
Figure 10 is a front elevational view of a
mu~ti-point ele~trode in tAe fDxm of a s~wtooth-edge~
disc o~ EOmpOsite construction~
Fig~re ll is a front ~levational view of a
multi-point electrode in the form of a composite disc
~avlng an outer annular ia~ric~ mesh or screen section
and an inner solid ci~cular section.
~54~3~
14
Figure 12 is a front perspective view of the barrel of
a spray device having a circular spray pattern, which uses a
multi-point electrode to charge the coating particles.
Figure 13 is a front perspective view of the barrel of
a spray device having a flat spray pattern, which uses a multi-
point electrode to charge the coating particlesO
With reference to the figures, one preferred form of
electrostatic spray gun incorporating the present invention is
depicted. In the preferred embodiment of the spray device 10
is in the form of a gun having an electrically grounded
conductive handl~ ll and a nonconductive or insulative barrel
12 which at its forward end terminates in a flared nozzle 14
having a central flared opening 15 from which projects a
combined powder deflector and electrode charging assembly 16.
Except for the assembly 16, the preferred embodiment of the
spray gun can be constructed in accordance with the teachings
of United States Patent No. 4,634,058 entitled "Improved Powder
Spray Gun", in the name of Thomas E. Hollstein, David E.
O'Ryan, and Joseph C. Waryu, assigned to the assignee of the
present application.
LCM:jj
~2~ 3~3
~ he ~arrel 12 includes an intern~l p~wder
entry chamber 17 which at its rearward end com~unicates
with a powder-entrained ~ess~ri.zed ai r s~pply hos~
13a via a port 13 in the barrel wall~ ~he int~rnal
powaer entry chamber 17 at its fvrwaId end communicat~s
with the nDzzl~ openi~g lS via a ta~ered bore 19 ~nd
intermediate cham~er 21. A nonconductive mo~nting
stub 22 for th~ de~lect~r an~ electrode assem~ly 1
extends axially ana Eorwardly ~rom a nonc~nductive
spiaer 2~ locate~ within ~he in$ermediate cha~er 21.
Extending axially and reaYwardly from the ~pi~er 2~ is
an electrically insulated conductive path ~9 incDrpor-
ating a conductor 76 Ito ~e descri~ed) which extend~
through a stepped diameter bore 30a ~nd 30b wh~re i~
1~ makes an appropriate conne~tion with an insulated high
~oltage supply ca~le 26 which passes ~hrough the
handle 11 exiting the butt thereof at 24 where it
connects to a remote high voltag~ ele~trostatic power
supply Inot shown).
The hanale 11 is provided with a m~vable
trigger 34 which when activated supplies pressurized
powder-entrained air to the pGwder entry cham~ex 17
. . Yia hose 13a~ Trigger 34 ~lso en2rgizes the remcte
high voltage supply to provide high vol~age electro-
2~ static power to an electrical conauctor 7D (l~ter
described) which is axially di~posed within the powder
deflector:.16. ~he conductor 7D is connected to the
high voltaye supply by high voltage cable 24, 26 and
-16
the electrically insulat~d conductive ~ath ~9 whic~
passes through the mounting stu~ 22 and s~ider 2~.
The powder-en*rained air passes ~naer pres~ure from
the entxy cham~er 17 successively through *he tap~red
bore l9 and intermediate chamber 21 to the ~lared
nozzle opening 15 wherea~ it is divert~d into a
conical path and electros~atically charged ~y ~e
electrode, to be de~cribed, incorporated in *he ~der
deflector and electrode charging assembly 1~. The
lD powder exits the nozzle opening in a generally conical
pattern of elec*rostatically charged parti~les for
impingement upon an electrically grounded articl4 (DDt
shown) to be coated~
r~he powder deflector and elec rode ~harging
assem~ly 167 considered in more detail in ~Dnnection
with Fig. 2, is ~enerally conical in shap~ ~a~ing a
circular flat front surface 40 and a conical rear
~: surface 42. ~ront surface 40 ~ould also be conv x Dl
concave, i~ desired. A ~esistive sheet electrDde i~
the ~orllZ of a circular wafer or: disc 4~ is lc~cated iD
a boundary region between the frDnt and rear surfaces
40 and 42. The edge 46 of the resisti~e elec*rode
sheet or disc 44 is ~re~erably flush:with the edges
- 40' and 42' of the frcnt and rear surfaces 40 and 42.
: 25 In a preferred form of the invention, the powder
deflector and electrode charging assembly 16 is a
composite:or sandwich assembly which in~ludes the
intermediate resistive electrode disc:-44, a circular
~S~L~3~
insulating disc 40a having a diameter equal to that of the
resistive electrode di~c 44, and a conical insulating section
42a the rearward surface of which constitutes the powder
deflecting conical surface 42. The conical section 42a,
resistive electrode disc 44, and disc 40a can be permanently
assembled to form an integral unit utilizing commercially
available adhesives. Alternativaly, the resistive sheet could
be molded into the deflector.
In a preferred form of the invention the resistive
electrode disc 44 is fabricated of nonwoven silicon carbide
fabric embodying randomly oriented silicon carbide fibers or
filaments in a resin matrix. The silicon carbide fibers or
filaments from which the fabric is made have the physical and
electrical characteristics of Nicalon fiber of the general type
disclosed in United States Patent No. 4,100,233 and
commercially available from Nippon Carbon Co., Ltd., Tokyo,
Japan, and Dow Corning, Midland, Michigan. In a preferred
embodiment the silicon carbide fibers are heat treated to
provide a specific resistivity of 1 X 103 ohm-cm., and a fiber
diameter in the approximate range oE 10-15 microns. The
following publications of Nippon Carbon Co., Ltd., Tokyo,
Japan, available from Dow Corning, Midland, Michigan, contain
information on Nicalon fibers:
LCM:jj
,~'' ' .~ .
~5q~330
Nicalon Silicon Carbide Fiber, 12 payes;
Price Listing Effective 1-1-84, Nicalon
Silicon Carbide Fiber Products Distributed
by Dow Corning Corporation, 2 pages; and
Industrialization of Silicon Carbide Fiber
and Its Applications, by Jun-Ishi Tanaka,
Executive Director, Nippon Carbon Co., Ltd.,
11 pages.
Nicalon con-tinuous silicon carbide fiber, in
one commercially available form, is physically charac-
terized as follows:
Filament Diameter: 10-15 microns,
Cross Section: round
Density: 0.093 pounds/inch3 ~2.55 g/cm3),
Tensile Strength: 360-470 ksi (250-300 kg/mm2),
Tensile Modulus : 26-2g x 103 ksi
(18-20 x 103 kq/mm2), and
Coefficient of Thermal Expansion
(parallel to Fiber): 3.1 x 10 6/oC.
The specific resistivity of Nicalon silicon
carbide fiber which is uniform -throughou-t the fiber
and independent of fiber flexure, can be varied by
heat treating the fiber at differen-t temperatures
subsequent to spinning. The variation in specific
resistivity as a function of heat treating temperature
can vary by a factor of approximately 104 from approxi-
mately 102 ohm-cm to 106 ohm-cm.
18 -
~ ~ .
~ZS~6?3~
The Nicalon continuous silicon carbide
fibers can be formed into woven abric, as well as
nonwoven Eabric of random fiber orientation. In
addi-tion, the resistive silicon carbide disc 44
can be fabricated of resin impregnated Nicalon
fabric composite, glass Nicalon fabric composite
and/or Nicalon fibers in a ceramic ma-trix.
The insulative fron-t disc 40a and insulative
conical deflector 42a can be fabricated of a variety
of nonconductive materials including glass-filled
Teflon plastic, Delrin plastic, and the like.
~he d~flector/electrode assembly 16 is
mounted to the s-tub 22 by the axial engagement of a
reduced diameter sec-tion 22a at the forward end of
the mountlng stub 22 and a blind hole or bore 64 formed
in the rear central portion of -the conical deflector 42a.
The bore 64 and reduced diameter end 22a of the stub 22
are dimensioned -to provide a snug sliding fit therebetween.
As noted previously, electrostatic energy is trans-
mitted from a remote power supply (not shown) to the
resis-tive charging disc 44 via the cable 24, 26
and -the electrically insulated resistorized conductive
path 29. Conductive path 29 includes an electrical
conductor (or electrode) 70 which projec-ts axially
from the end of the mounting stub 22 into a sui-tably
provided axial passage in -the conical deflector
section 42a to establish electrical contac-t wi-th the
~ 19 -
;, .
~rc~
~Sg~l~3~
resis-tive disc 44. The conductor 70 is connected to
the electrically conductive core of the cable 26 via a
resistor 75 and electrical conductor 76 which consti-
tute further elements of conduc-tive path 29, and which
are in elec-trical series circuit arrangement between
the conductor 70 and the conductive core of the high
voltage cable 26.
In operation, when the trigger 34 is ac-ti-
vated, powder-entrained pressurized air is introduced
into -the internal powder en-try chamber 17 via the hose
13a whereupon it flows through the -tapered bore 19
into the intermediate chamber 21 where is passes
through the spider 25 and impinges on the rear surEace
42 of the conical deflector 42a which causes -the path
of -the powder to deflect and form a conical path as it
exits the flared opening 15 of the nozzle 14 toward
the ar-ticle or -target substrate to be coated (not shown).
Activation of the trigger 34 also energizes a remote
power supply (not shown) -to cause high vol-tage electro-
static energy to be supplied to the resistive charging
disc 44 via the electrical pa-th previously described.
With the resistive charging disc 44 main-tained a-t a high
electros-tatic voltaqe, such as 90 Kv, a corona discharge is
produced at the multitude of resistive material fiber ends
46a located around the perime-ter 46 of the resistive charging
disc ~4,causing electrostatic charge to be imparted -to the
stream of powder as it exits the flared opening 15 of
20 -
nozzle 14 subsequent to deflec-tion by the rear conical
deflecting surface 42.
Experience has shown that higher coating
transfer efficiencies canbe achieved with the electro-
static spray coating gun of this invention. In practice,
the number of corona points,as well as their precise
loca-tion around the periphery 46 of -the resistive
charging disc 44, is somewhat variable. At no load
voltages of 90 Kv with a charging disc 44 having a diameter
oE approximately 1 lJ2 inches and a thickness of approxi-
mately 0.65 mm, anywhere between three and eight corona
points have been observed to simul-taneouly occur at
peripheral locations which are con-tinuously changing on a
more or less random basis.
Con-tributing in a material manner to -transfer
efficiency enhancement provided by the preferred
embodimen-t depicted in Figures 1-4, as well as the
other embodimen-ts herein described in more detail
hereafter, is the flared configuration of -the nozzle
14 relative to the corona charging zone located proximate
the edge 46 of the resistive electrode sheet 44. More
particularly, -the nonconductive, flared outer portion
of the nozzle 14, which is located outwardly and rear-
wardly of the corona charging zone proximate perimeter
46, functions as an electrostatic shield which effectively
shields electros-tatically-charged coating particles at the
exit end of the nozzle from the electrically grounded handle
11, reducing the tendency of a parasi-tic leakage current
~r~
~2S~3~
to be set up between the deflector and the handle 11.
Were -the shielding omitted, the grounded hand 11
would tend to electrostatically at-tract the charged
coating particles, set-ting up an undesirable leakage
current, and thereby reducing the charging energy
available at the deflector and the trans-fer efficiency.
This is particularly -true in view of the fact tha-t -the
grounded handle is typically located at approximately
the same dis-tance from the corona charging zone,
albeit rearwardly thereof, as the object being coated
which is electrically grounded and located forwardly
of the gun nozzle. Tests have shown that removel of
the portion of the flared nozzle 14 located radially
beyond the perimeter 46 of -the deElector, which in
turn eliminates the electrostatic shielding between
the deflec-tor perimeter and the electrically-grounded
handle 11, significantly reduces the -transfer efficiency.
While in the embodiment shown in Figures
1-4, the forward extrimity or lip 14a of the nozzle 14
is located slightly rearwardly rela-tive to -the edge 46
of the resistive electrode sheet 44 r the position of
-the lip 14a relative to the electrode shee-t edge 46
can be varied, such as by locating -the flared nozzle
mouth or lip 14a radially opposite the electrode sheet
edge 46a or forwardly thereof (lef-twardly as viewed in
Figure 2). Regardless of the exact location of flared
nozzle mouth or lip 14a relative to the edge 46 oE the
-22 -
~;~S4Q3~
resistive sheet 44, a-t least a portion of the noncon-
ductive flared nozzle 14a must be located radially
outwardly and rearwardly of the corona charging zone
proximate edge 46 of resistive sheet 44 such that
electrostatic shielding is provided between the
electrostatic charging corona zone and -the electri-
cally-grounded handle ll.
In the preferred embodimen-t, the electro~
s-tatic shield is described in connection with its use
in a powder gun. As noted, it can also be used -to
advantage in a liquid coa-tin~ gun wherein charged
atomized paint par-ticles are pro~ima-te -the gun nozzle.
Because of the resistive nature oE the
charging disc 44, the electrostatic spray gun oE this
invention has been found to prevent igni-tion when
subjected to standard ignition tests performed by
Nordson Corporation, assignee of -the present applica-
tion. In practice, -the disc 44 provides a resistance
of l.0 Megohn - 1.5 Me~ohn when measured between the
center which contacts conductor 70 and the periphery 46.
The composite or sandwich construction o~
the combined powder deflector and electrode charging
assembly 16 is extremely durable and inexpensive, and
yet is very effective both as a deflector and as an
electrostatic charging electrode configuration.
If desired, the charging disc can be mounted
on the fron-t surface 40, such that it faces forward
- 23 -
-i rr~
3~
and ls exposed, ra-ther than be sandwiched between
members ~IOa and 42a. However, the sandwich construc-
tion is preferred.
In the embodimen-t of Figures 1-4, as
described, the deflector 16 is principally fabricated
of insulative sections 40a and 42a. If desired, -the
deflector could be fabricated of resistive or semicon-
ductive material, or possibly even conductive material,
providing the multi-point electrode is located at the
periphery thereof. With such a construction, suitable
resistance is preferably provided in series with the
multi-point electrode to avoid unsafe elec-trical
discharges of electrical energy stored in the gun
should the multi-point electrode be acc:identally
grounded.
In accordance with the embodiment depicted
in Figures 5 and 6, only the deflector assembly of
which is shown, the nonconductive deflector 100 is
seen to have the same general overall shape as the
deflector of the embodimen-t of Figures 1-4. More
particularly, the deflector 100 has a rear surface 102
against which the par-ticle-entrained air stream is
directed in a generally axial (horizontally as viewed
in Figure 6) direction as it exits from the nozzle of
the gun in a forward (leftwardly as viewed in Figure 6~
direction. The deflector 100 also includes a generally
circular flat front surface 104, which if desired
could be either concave or convex. Embodied
- 24 -
,. ~ .
-irc~
-25~ 3~
in the deflector 100 and pr~j~cting radially outwardl~
from the periphery 106 theIe~f in a dir~cti~n ~rans-
verse to the deflected path of th~ powder s~ream ~re
plurality of electrode elements :ID8~ for e~ample~ in
the form of electrically con~uct:ive wires o~ needles~
The electxode elements 108; o which there ~re six
shown in the preferred embodime~t o~ Figures ~ and 6
although a lesser or greater ~umber can ~e used~ are
circumferentially spaced at substantially equal
intervals around the peri~hery 106 of the ~e~lector.
Resistive circuit paths in ~he ~Drm of discrete
radially disposed resistors 112 interconnecting each
of the elect.rodes 108 to a rentral, axially disposea
electrical conductor 110 which c~nnects to a r~mote
hiyh voltage source (not shown). The resistors 112,
which are incorporated in the ~ody-of the.deflector
between front and rear sur~aces 104 and 102, have a
resistance, in the presently preferre2 embodiment, of,
for example, 10 Megohms, alth~g~ ot~er ~esistance
values may be used, if desired. In accoraance with
variant of the embodiment actuaIly shown in Figures ~
and 6, the radially projecting electrodes 108 co~ld be
made flush with the perimet~r lOS ~f the deflector
100, thereby avoiding the possibility o~ aamage *~ the
electrodes.
In accordance with a still f~rther preferred
embodiment of the invention depicted in:Fig~res 7 and
8, of which only the deflecto~ assembly is shown, an
~5~30
electrostatic spray gun is provided in which the
nonconductive deflector 130 is seen to have the same
overall configuration as the deflector 100 shown in
the embodiment of Figures 5 and ~O More particularly,
deflector 130 includes a front surface 134, a rear
surface 132, and a perimeter 136. Like the deflector
shown in Figures 5 and 6, the deflector 130 shown in
Figures 7 and 8 incorporates in its body a plurali-ty
of resistive circuit paths in -the form of radially
disposed discrete resis-tors 142 which at their inner
end have leads 142b which are connected in common to
an axially disposed electrical conductor 144 which in
-turn is connected to a remote high voltage source (not
shown). The radially outward ends o~ resistors 142
have leads 142a which terminate in a circumferential
groove 148 formed in the periphery 136 of -the deflector
130. Located in the groove 148 is a circumferentially-
disposed silicon carbide thread or narrow ribbon 150.
The radially outboard ends of resis-tor leads 142a are
electrically connected with their respectively proxi-
mately located segments 130a of -the silicon carbi~e
thread 150. If desiredr a resistive material other
than silicon carbide can be used for the peripheral.ly
located ribbon or thread 150.
In -the embodiment of Figures 5 and 6, corona
charging takes place at the radially outboard ends of
the electrodes 108 pas-t which the powder passes on its
path -toward the object to be coated. In -the variant
` - 26 -
jrc:l:
J3~
of the embodiment shown in Figures 5 and 6, wherein
the electrodes 108 are flush wi-th -the perimeter 106 of
the deflector 100, corona occurs at the point where
the electrode joins the periphery 106 of the deflec-tor.
In the embodiment shown in Figures 7 and ~,
wherein a silicon carbide thread or ribbon used,
corona occurs at random locations around the surface
of the -thread 150. If the thread 150 is fabricated of
intertwined fibers of short length relative to the
circumference of the deflector perimeter 106, corona
willm~st probably occur where the fibers terminate
since -the ends thereof 150a (see Figure 8) ~unction as
electrodes -to form corona charging points. If the
silicon carbide thread does not contain short lengths
of fiber wi-th plural randomly located ends, corona
will occur at randomly located points around the
periphery of the silicon carbide -thread 150, the
location of which points will change more or less
continuously.
In the embodiment of Figures 7 and 8, the
thread 150 in deflec-tor groove 148 is efEectively a
continuous circular electrode comprised of six arcuate
elec-trode elements or segments which are interconnec-ted
end--to-end. The continuous circular electrode 130
functions in a manner analogous to -that of the periph-
ery 46 of the disc-shaped resistive .sheet ~4 of
Figures 1-4 which, in effect, a-t its periphery is also
a continuous circular electrode comprising plural
.~ - 27 -
jrc~ s
L~ f3
28-
peripheral arcuate electrode elements or seyments
connected end-to-end.
Instead of the silicon carbide resistive
fabric 44 shown in Figures 1-4, the multi-point
electrode can take the form of a sawtooth edge 200 on
the periphery of a disc 202, as shown in Figure 9.
The disc may be fabricated of the same material
throughout, such as a resistive, semiconductive, or
conductive material. Alternatively, the disc 202' may
be a composite having an annular outer section 203
with teeth 200' at the periphery, and an inner circular
section 205, as shown in Figure 10. The inner section
205 and/or the outer section 203 may be resistive,
semiconductive, or conductive.
Alternatively, disc 44, instead of being
entirely of silicon carbide fabric, or other resistive
material, as shown in Figuxe 4, could be of composite
construction ac shown in Figure 11. More particularly,
the resistive fabric 210 could be annular shaped, with
the remainder of the disc 211 comprising an inner
circular disc 212 of resistive, conductive, or semicon-
ductive solid sheeting.
Also, instead of constructing the multi~point
electrode of resistive fabric, as shown in Figure 4,
such as silicon carbide fabric, the electrode could be
constructed of screen or mesh, with the strands
thereof being resistive, conductive, or semiconductive
, .
~S~3C~
-29-
wire or nonconductive wire clad with re~istive,
conductive, or semiconductive material.
Figure 12 depicts an insulative gun barrel
230 having a longitudinal circular cross-sectional
bore 231 ~erminating in an opening 232 in face 233
from which is emitted coating pa:rticles. Located
coaxially within the bore 231 is an insulative column
234, at the outer end of which a multi-point electrode
235 is mounted. Electrode 235 may alternati~ely be
constructed like any of the electrode configurations
or structures shown in Figures 4-11. In the E'ig. 12
embodiment, like Figs. 4-11, electrode 235 has a
peripheral edge 235' which includes multiple electrodes
projecting therefxom. The electrode 235 connects to a
source of electrostatic voltage via an electrical
conductor (not shown) located with column 234. The
device of Figure 12 provides a circular spray pattern.
Figure 13 depicts an insulative barrel 240
having an upper rectangular cross-sectional longitudi-
nal bore 241 and a lower rectangular cross-sectional
longitudinal bore 242 separated by an insulative
longitudinal column 243. Mounted on the outer end of
column 243 is an electrode 244 having an upper multi-
point electrode edge 244 and a lower multi-point
electrode edge 245 for charging coating particles
emitted from upper and lower bores 243 and 242,
respectivel~. Electrode 244 is constructed similarly
to electrode 235 of Figure 12. The electrode 244
~25~3~
-30-
connects to a high voltage supply via an el~ctTical
conductor (not shown) within column 243. Th~ ~m~odi-
ment of Figure 13 provides a flat fan-shaped spray
pattern.
The embodiments of Figures 5-13, li~e the
embodiment of Figures 1-4, provide improved ~ransfer
efficiency due to the multi-point electrode ro~figura-
tion, and constitutes electrode assemblies which ~r~
inexpensive and simple in construction.
From the above disclosure of the geneIal
pxinciples of the present invention and the ~receding
detailed description of the preferred embodimients
thereof, those skilled in the art will readily compre-
hend the vari.ous modifications to which the p~esen*
invention is sus- ceptible. Therefore, I de~ire to ~e
limited only by the scope of the following claims and
equivalents thereof:
