U.S. patent number 3,698,635 [Application Number 05/117,494] was granted by the patent office on 1972-10-17 for spray charging device.
This patent grant is currently assigned to Ransburg Electro-Coating Corp.. Invention is credited to James E. Sickles.
United States Patent |
3,698,635 |
Sickles |
October 17, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
SPRAY CHARGING DEVICE
Abstract
An electrostatic spray charging device wherein means in a
passageway or a barrel of such device directs a jet of air between
a charged electrode in the passageway and a plurality of liquid
termini at an orifice within the passageway in a manner as to
direct particles atomized from the plurality of termini generally
away from the charged electrode and the side walls of the
passageway. The charged electrode and the liquid termini are
adapted to be connected to an electrical source thereby creating a
potential difference between the charged electrode and the liquid
termini. The potential difference therebetween provides a high
intensity electric field at the liquid termini whereby particles
atomized from the individual liquid termini carry a high
charge-to-mass ratio. Preferably, the jet of air passing between
the charged electrode and the liquid termini assists in atomizing
liquid particles from the liquid termini. The charged liquid
particles are attracted to and deposited upon the surface of an
object or article at a particle attracting potential.
Inventors: |
Sickles; James E.
(Indianapolis, IN) |
Assignee: |
Ransburg Electro-Coating Corp.
(Indianapolis, IN)
|
Family
ID: |
22373248 |
Appl.
No.: |
05/117,494 |
Filed: |
February 22, 1971 |
Current U.S.
Class: |
239/3;
239/706 |
Current CPC
Class: |
B05B
5/043 (20130101); B05B 5/03 (20130101) |
Current International
Class: |
B05B
5/043 (20060101); B05B 5/025 (20060101); B05B
5/03 (20060101); B05b 005/02 () |
Field of
Search: |
;239/3,15 ;317/3
;117/17,93.4 ;118/621 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Wood, Jr.; M. Henson
Assistant Examiner: Grant; Edwin D.
Claims
I claim:
1. A low voltage electrostatic spray gun comprising
a barrel including a passageway terminating in an opening,
the barrel including dielectric means for forming liquid into a
stream, the stream having a plurality of termini in the passageway
and spaced rearwardly from the opening, the liquid termini
providing liquid particles,
electrode means confined within the passageway rearwardly of the
opening and in close proximity with the liquid termini, the
electrode means and the stream of liquid adapted to be connected to
means for creating a potential difference between the electrode
means and the liquid termini, the potential difference therebetween
providing an electric field having a high potential gradient at the
termini, the dielectric means assisting in concentrating the
electric field at the liquid termini so as to electrostatically
charge the liquid particles, and
means in the barrel for directing a jet of air along the passageway
to carry the charged liquid particles formed from the liquid
termini away from the side walls of the passageway and the
electrode means.
2. The low voltage electrostatic spray gun of claim 1, wherein the
jet of air assists in atomizing particles from the liquid
termini.
3. The low voltage spray gun of claim 1, wherein the dielectric
means forms the liquid into a stream having an axis that is
substantially parallel to the axis of the electrode means.
4. The low voltage device of claim 1 wherein the dielectric means
for forming the stream includes a tube and a cap spaced from the
open end of the tube to provide an orifice from which the stream of
liquid is emitted at an angle to the electrode, the cap having a
radial extent slightly greater than the radial extent of the
tube.
5. The low voltage device of claim 4, wherein the portion of the
passageway in front and extending away from the cap has an
expanding volume.
6. The low voltage spray gun of claim 1, wherein the electrode
means is annular and the dielectric means forms the liquid into a
stream having an axis substantially concentric with the axis of the
annular electrode.
7. A low voltage electrostatic spray device comprising
a barrel having an opening,
a passageway in the barrel terminating in an opening near the
opening of the barrel,
means in the barrel providing a liquid stream having termini from
which liquid particles are formed in the passageway rearwardly of
the opening of the passageway, the means providing the stream
includes a cap spaced from an open end of a tube to form an orifice
from which the stream of liquid is emitted, the cap having a radial
extent different from the radial extent of the tube,
electrode means in the passageway and rearwardly of the opening of
the passageway and in close proximity to the liquid termini,
means connected to the electrode means and the liquid stream to
establish an electric field having a high gradient at the termini
of the liquid, the electric field being concentrated at the liquid
termini, and
means directing a jet of air along the passageway between the
electrode means and the liquid termini for projecting
electrostatically charged liquid particles generally away from the
side walls of the passageway and out of the opening of the
passageway and the opening of the barrel.
8. A low voltage electrostatic device as claimed in claim 7,
wherein the means providing the liquid stream is fabricated from
dielectric material to assist in concentrating the electric field
at the liquid termini.
9. A low voltage electrostatic device as claimed in claim 8,
wherein the voltage applied to the electrode means is of such a
value to prevent corona discharge.
10. A method of forming a cloud of electrostatically charged liquid
coating material particles comprising the steps of
providing a liquid stream of coating material having a plurality of
liquid termini in a passageway and spaced rearwardly from an end of
the passageway,
creating a potential difference between an electrode means in the
passageway and spaced rearwardly from the opening and the liquid
termini, the potential difference therebetween providing an
electric field having a high potential gradient at the liquid
termini,
forming charged liquid particles from the liquid termini having a
polarity different from the polarity of the electrode means,
and
directing a jet of air along the passageway to carry the charged
liquid coating material particles formed from the liquid termini
away from the side walls of the passageway to form a cloud of
electrostatically charged liquid particles.
11. The method of claim 10, wherein the liquid is paint having a
resistivity of about 15 megohm-centimeters or less.
12. The method of claim 10, wherein the stream having a plurality
of liquid termini is formed on a dielectric means.
13. The low voltage device of claim 1, wherein the dielectric means
for forming the stream includes a tube and a cap spaced from the
open end of the tube to provide an orifice from which the stream of
liquid is emitted.
14. The low voltage device of claim 13, wherein the cap has a
radial extent different from the radial extent of the tube.
15. A low voltage electrostatic spray gun comprising
a barrel having an opening,
a passageway in the barrel terminating in an opening near the
opening of the barrel,
the barrel including dielectric means for forming liquid into a
stream, the stream having a plurality of termini in the passageway
and spaced rearwardly from the opening of the passageway, the
liquid termini providing liquid particles,
electrode means confined within the passageway rearwardly of the
opening of the passageway and in close proximity with the liquid
termini, the electrode means and the stream of liquid adapted to be
connected to means for creating a potential difference between the
electrode means and the liquid termini, the potential difference
therebetween providing an electric field having a high potential
gradient at the termini, the dielectric means assisting in
concentrating the electric field at the liquid termini so as to
electrostatically charge the liquid particles, and
means in the barrel for directing a jet of air along the passageway
to carry the charged liquid particles formed from the liquid
termini away from the side walls of the passageway and electrode
means and out of the opening of the passageway and the opening of
the barrel.
Description
This invention generally relates to a low voltage electrostatic
spray device that provides a spray of electrostatically charged
particles, and more particularly, to a low voltage electrostatic
spray device wherein liquid particles are formed and
electrostatically charged in a passageway of the spray device and
are directed, by a jet of air, away from the side walls of the
passageway and an electrode cooperatively associated with the side
walls of the passageway.
The electrostatic spray device of the present invention is intended
primarily for use in the electrostatic application of liquid
particles to the surfaces of objects or articles of a particle
attracting potential to thereby provide such surfaces with a layer
of the liquid particles, and, preferably with a substantially
continuous film of coating of the liquid particles.
Several systems are commercially available which employ
electrostatic principles to aid in the deposition of liquid
particles upon an article to be coated. These systems differ from
each other in several respects including the means and methods used
to atomize and electrostatically charge the atomized liquid
particles.
One electrostatic spray system known as the "Ransburg No. 1
Process" uses a conventional metal air atomizing spray gun to form
uncharged liquid particles. A separate grid of highly charged wires
is employed to impart to the atomized liquid particles an
electrostatic charge. In this system an electric field of high
intensity exists between the grid of charged wires and the article
surface to be coated. The electric field creates an abundance of
atmospheric ions of the same electrical polarity as the grid. The
atmospheric ions are directed toward the article surface at ground
potential. The liquid particles are formed by the spray gun and
projected into the ion cloud where they are electrostatically
charged by ion bombardment. The charged liquid particles are
attracted to and deposited upon the surface of the article in
response to the interaction of the charge carried by the liquid
particles, the electric field and the grounded surface of the
article.
There are a number of other systems which are similar to the
Ransburg No. 1 Process in that electrostatic charging of the liquid
particle is accomplished by ion bombardment. The systems differ
from one another in respect to the location of the ionizing
electrode relative to the liquid particle forming means and to the
surface of the article to be coated.
In another system, "the Ransburg No. 2 System," a high voltage
deposition and charging electric field is established between the
article surface to be coated and an exposed body of liquid material
formed on an atomizer having an extremely thin edge portion. The
exposed body of liquid material is formed so as to be coextensive
with the extended thin edge portion of the atomizer. The electric
field has a high gradient at the extended edge of the liquid. The
electric field forms the extended edge of the liquid into a number
of fluid termini or cusps which extend outwardly. Each of these
termini are highly charged and, therefore, a strong electrical
repulsion exists between the main liquid body and the liquid at the
termini. Conversely, a strong electrical attraction exists between
the liquid at the termini and the surface of the article at a
particle attracting potential. Therefore, a small portion of the
liquid at each of the termini is aided in escaping the forces of
surface tension of the liquid. The small portion of liquid is
pulled into the electric field as a highly charged particle to be
deposited on the surface of the article at a particle attracting
potential.
In still another system for electrostatically charging liquid
particles, a spray gun is constructed of substantially dielectric
material and uses a wire-like electrode projecting from a liquid
orifice. The electrode charges air atomized liquid particles by
bombardment with atmospheric ions created at the tip of electrode.
The atmospheric ions are concentrated in the immediate vicinity of
the tip of the electrode. Ion bombardment of the atomized liquid
particles more completely charges the liquid particles. The
wire-like electrode also provides an electrostatic field extending
from its tip to the surface of the object or article to be coated.
The electrostatic field assists in promoting the deposition of the
charged liquid particles upon the surface of the object or article.
To provide the necessary atmospheric ions, the wire-like electrode
is connected to a direct voltage source capable of supplying a no
load output voltage of up to about 65,000 volts at a short circuit
current at the electrode of up to about 225 microamperes. It is
apparent that voltages of such magnitude create safety problems.
Furthermore, the power supply necessary to provide such high
voltages is bulky and expensive. In the event the electrostatic
spray device is to be manipulated by hand, the voltage cable
necessary to supply such high voltages to the device tends to be
stiff and heavy thus inhibiting free manipulation of the spray
device and causing operator fatigue.
Each of the above electrostatic devices and systems has attained a
high degree of technological and commercial success. Each of the
devices uses a voltage of high magnitude, each device differs from
the other devices in the magnitude of the charge imparted to the
liquid particles, each device differs from the other devices in the
types of liquid materials each is capable of spraying efficiently,
and each device differs from the other devices' general size,
weight and adaptability to a specific use.
It is, therefore, a desideratum to have an electrostatic spraying
device that will spray a wide variety of different types of liquid
materials without regard to physical and electrical characteristics
of such liquid materials; that will be light in weight and readily
maneuverable; that will not subject the operator to harmful shock
or discharge to grounded surfaces during operation; and that will
form particles of such size as will exhibit a high charge relative
to the mass of the particles so that electrostatic deposition will
occur at relatively high efficiency. Generally, liquid particles
carrying a high charge-to-mass ratio are more readily attracted to
and deposited on a surface at particle attracting potential than
are liquid particles having a low charge-to-mass ratio. In most
instances, the composition of the liquid material and, therefore,
its density, is determined by intended use of the liquid material.
For a particle of given size and density maximizing the
charge-to-mass ratio of the particle is a question of maximizing
the charge carried by the particle. It is highly desirable,
therefore, to provide a spray device possessing the capability of
producing highly charged particles while having the other desired
characteristics.
The present invention provides a relatively low voltage
electrostatic spray device capable of imparting very high
electrostatic charge to atomized liquid articles. The spray device
possesses a nonionizing electrode located within and spaced at a
distance from the end of a passageway formed within a barrel.
Within the passageway and in close proximity to the electrode is
located the termini of a liquid supply. Connected to the termini of
the liquid supply and the electrode is a means for establishing a
high intensity electric field therebetween. Preferably, the side
walls of the passageway except for the electrode portion are
fabricated from a dielectric material to thereby concentrate the
electric field at the terminus of the liquid supply. The electrode
and the side walls of the passageway are substantially continuously
swept by a jet of air which carries the particles out of the
passageway. Sweeping the electrode and side walls of the passageway
with a jet of air significantly reduces the number of liquid
particles which would otherwise accumulate on the electrode and
side walls of the passageway. If liquid particles were allowed to
accumulate to any significant degree on either the electrode or the
side walls of the passageway, undesirably large liquid droplets may
be formed from such an accumulation. The presence of undesirably
large droplets in a spray of charged liquid particles tends to
impair the finish usually desired on the surface of an object or
article and will likewise lower the charge-to-mass ratio created on
such particles. In addition, locating the electrode in close
proximity to the termini of the liquid supply and at a potential
different from the potential of the electrode and forming the side
walls of the passageway from a dielectric material permits the use
of a low voltage source to create a high intensity field at the
termini formed at the surface of the stream of liquid.
The electrical gradient, as high as is practicable, maintained
between the electrode and the termini of the stream of liquid in
the passageway, creates an electrostatic field of high intensity at
each termini. Liquid particles formed from the liquid termini
possess a high charge and are projected from the spray device by
the jet of air and are attracted to the surfaces of objects or
articles maintained at a particle attracting potential. Preferably,
the jet of air also aids in the atomization of particles from the
liquid termini in the passageway, the liquid termini being within a
few ten thousandths of an inch or less of the electrode. The spray
device has no exposed or external metallic electrode thereby
reducing the possibility of an operator contacting the metallic
electrode or the electrode engaging the surfaces of articles or
objects adapted to receive the charged liquid particles. The
internal location of the electrode within the spray device
increases the safety associated therewith.
The present invention also provides an electrostatic spray device
which employs direct current input voltages to the electrode of
about 10,000 volts or less and preferably about 7,000 volts or less
and low value input currents of about 20 microamperes or less and
preferably about 10 to 5 microamperes or less. The use of a low
value input voltage and current to the spray device permits a
significant reduction in the size and weight of the power supply
used to provide the field intensity necessary to appropriately
charge the liquid particles. It is contemplated the physical size
of the power supply will be such as to permit it to be located
either within the handle of the electrostatic spray device or on
the person of the operator, if desired. Locating the power supply
within the spray device or on the person of the operator
significantly increases the portability, ease of manipulation and
applicability of such a device. It is to be understood, however,
that if operating conditions and/or regulations so require, the
power supply of the spray device may be located remotely from the
site at which spraying is to be undertaken.
The appended drawings are intended to illustrate several spray
devices embodying the concepts of the present invention constructed
to function in the most advantageous modes presently devised for
the practical application of the principles involved in the
hereinafter described invention.
In the drawings:
FIG. 1 is a diagrammatic illustration of an electrostatic spray
device embodying the concepts of the present invention wherein the
power supply is preferably incorporated within the spray
device;
FIG. 2 is an enlarged partial cross sectional side view of the
front end of a barrel intended to be used with the spray device
illustrated in FIG. 1;
FIG. 3 is an enlarged side view showing the spray device in FIG. 1
including a spray-surrounding electrode; and
FIG. 4 is an enlarged partial cross sectional side view of the
front end of a barrel illustrating another form of a spray device
embodying the concepts of the present invention.
Referring now to FIG. 1 of the drawing, an electrostatic spray
device or gun incorporating the concepts of the present invention
is indicated by the reference numeral 10. The spray gun 10 includes
tubular means or barrel 11, handle 12 extending from the barrel at
an angle thereto, and trigger 17. Handle 12 may contain a suitable
battery-operated, direct current power supply (not shown) capable
of supplying up to about 10,000 volts to a metallic electrode
located within the barrel 11 of spray gun 10.
Conduits 13 and 14 project from the lower extremity of the handle
12 of spray gun 10. Conduit 13 connects the gun to air source 15
capable of supplying air in suitable volumes to spray gun 10 for
sweeping charged particles of liquid from the gun. Conduit 14
connects spray gun 10 to reservoir 16 containing a liquid material
to be atomized, charged and deposited on the surface of an object
or article. The liquid contained within reservoir 16 may be, for
example, a functional liquid capable of being atomized such as
paint, varnish, lacquer, emulsions or the like diluted, if
necessary, with a suitably conductive solvent or admixture of
solvents that are chemically compatible with the liquid to be
sprayed. Solvents, such as ketones, alcohols, ethers and the like
are preferred. However, it is to be understood that not all the
solvents in the above chemical groups are chemically compatible
with all the named liquid materials, but all groups of the listed
solvents include specific solvents within each group that are
compatible with, for example, paint.
Trigger 17, pivotally carried by barrel 11 of spray gun 10,
regulates the supply of air and liquid to the gun, and the voltage
supplied to the metallic electrode in any suitable, known manner.
Since this regulation mechanism may be of any known suitable form,
of which many are conventionally used, it has been omitted from the
drawing in the interest of a clearer showing of the inventive
portion of the spray gun 10. Preferably, the exposed external
components of the spray gun such as handle 12 and trigger 17,
fabricated from conductive materials such as metal, are
electrically grounded or earthed.
Referring now to FIG. 2, an enlarged partial cross sectional side
view of the front end of a barrel 11 incorporating the concepts of
the present invention is shown. Barrel 11 includes a substantially
tubular sheath or housing 18 having an integral, inwardly extending
radial flange 19 at its forwardmost extremity. Flange 19 provides
opening 20 through which a spray of electrostatically charged
liquid particles (not shown) is projected. Housing 18 is fabricated
from any suitable conductive material such as metal or the like or
from any dielectric material such as acetal resin and the like. In
the event the housing is fabricated from a conductive material, it
is, preferably, earthed or grounded.
Outer tube 25 and inner tube 26 are located within barrel housing
18 and are components of 11. Preferably, tube 25 and tube 26 are
concentric and both are fabricated from any suitable dielectric
material capable of withstanding the stresses associated with the
highest voltages provided by the power supply without an
accompanying breakdown or rupture of the material. A suitable
insulative material for tubes 25 and 26 is acetal resin, epoxy,
glass filled epoxy, glass filled nylon and the like.
Tube 26 includes at its forward extremity, an axially movable
"bullet-shaped" cap 27. Preferably, cap 27 is fabricated from a
dielectric material such as acetal resin. The rear surface 29 of
cap 27 is substantially parallel to and spaced from extremity 32 of
tube 26. Surface 29 and extremity 32 are substantially
perpendicular to the exposed surface of metallic electrode 31.
Extremity 32 of tube 26 and rear surface 29 of cap 27 cooperate so
as to provide an annular discharge orifice 30 through which a
stream of liquid issues. The stream of liquid is caused to flow to
the region near the peripheral annular edge of rear surface 29 of
cap 27 by the application of sufficient pressure to the liquid in
reservoir 16. Air moving through annular passageway 21 formed by
the structural cooperation relationship existing between tube 25
and tube 26 aids in forming the liquid into numerous liquid termini
near the radial extent of rear surface 29 of cap 27 and carries the
particles atomized from the termini forwardly through opening
23.
Preferably, the surface 29 of cap 27 is axially movable with
respect to extremity 32 to thereby provide an orifice 30 with a
variable annular opening to thereby assist in regulating the flow
of liquid from the liquid reservoir 16 to the liquid termini. As
illustrated in FIG. 2, the rear surface 29 of cap 27 has associated
therewith a diameter of greater extent than the diameter of
adjacent tube 26. Operation of the gun with such dimensional
relationships causes the periphery of the rear surface 29 of the
cap 27 to provide an annular edge 28 near which the liquid termini
are formed.
It has been observed that the degree to which the rear surface 29
of cap 27 radially extends beyond the extremity 32 of tube 26 is
also important to the efficient charging of the liquid particles.
The optimum difference between such dimensions depends on, among
other things, the contribution of the jet of air to the atomization
process and the resistivity of the liquid to be atomized and
charged. For example, liquid resistivities in the range of about
1.6 megohm-centimeters to about 13 megohm-centimeters experience
more charging when rear surface 29 of cap 27 has a radial extent
about 5 to 8 percent greater than the radial extent of extremity 32
of tube 26. Increasing the difference in radial extent of rear
surface 29 over the radial extent of extremity 32 to about 10
percent or more does not appear to harmfully affect the atomization
characteristics of the liquid; however, the charging of the liquid
particles is considerably reduced. It is to be understood that the
differences in radial extent vary as the resistivity of the liquid
varies from the liquid resistivities given above. For example, if
the resistivity of the liquid is about 1.5 megohm-centimeters or
less, optimum charging of the liquid particles occurs when the
radial extent of rear surface 29 of cap 27 and the radial extent of
extremity 32 of tube 26 are substantially equal.
Another important dimensional relationship is the distance between
edge 28 of cap 27 and electrode 31. The optimum distance for good
charging of the liquid particles is about 0.010 to 0.025 of an inch
(0.025-0.063 cm) and, preferably, about 0.015 to 0.020 of an inch
(0.038-0.051 cm). Increasing this distance beyond about 0.025 of an
inch (0.063 cm) significantly reduces the charge carried by the
liquid particles projected from barrel 11.
Annular electrode 31 having extended axial length is embedded in
tube 25 as shown in FIG. 2. The leading edge of electrode 31 is
adjacent but removed rearwardly from opening 23 of tube 25.
Electrode 31 may be coupled through current limiting resistor 41 of
suitable ohmic resistance to direct current power supply 22.
The ohmic value of the current limiting resistor varies depending
on the magnitude of the voltage appearing at the output terminal of
the power supply. For example, a direct current voltage source
providing an output of about 4,000 volts requires a current
limiting resistor having an ohmic value of about 50 to 60 megohms
to limit the short circuit current to about 70 to 80 microamperes
whereas a source providing an output voltage of about 10,000 volts
requires a current limiting resistor having an ohmic value of about
125 to 140 megohms to limit the short circuit current to about 70
to 80 microamperes. Power supply 22 may be located within spray gun
10 or on the person of the operator of the spray gun.
Alternatively, the resistor 41 may be removed and electrode 31 may
be directly connected to power supply 22. The voltage supplied to
electrode 31 by power supply 22 creates a high intensity electric
field extending between electrode 31 and the terminus of the liquid
present at annular orifice 30 adjacent to annular edge 28 of
bullet-like cap 27.
The configuration of cap 27 is important. The configuration of cap
27 should be such as not to inhibit or impede the projection of
liquid particles from the spray gun 10 to the surface of the
article at a particle attracting potential. The configuration of
the bullet-like cap 27 allows the air-liquid particle mixture to
expand into an area of increasing volume thereby assisting in
directing the liquid particles away from electrode 31 and the side
walls of tube 25. Sufficient turbulence and reduction in pressure
appears near the side walls of tube 25 to discourage the vast
majority of particles from accumulating thereon. The reduction in
air pressure near the side walls of tube 25 draws most of the
particles, if any, deposited thereon from the side walls prior to
any harmful accumulation thereof.
The distance between the electrode 31 and edge 28 of cap 27 is a
fraction of an inch. Preferably, the distance between electrode 31
and edge 28 is in the order of about 0.020 of an inch (0.051 cm) or
less. Preferably, the distance between electrode 31 and the
exterior surface of tube 26 is on the order of 0.030 of an inch
(0.076 cm). An electrostatic field of sufficient strength to
provide the liquid particles with a high charge-to-mass ratio
extends to the termini of the liquid provided by stream near edge
28. The electrostatic field is provided by, preferably, a direct
current voltage of about 10,000 volts or less, and preferably,
about 3,000 to 7,000 volts being applied to electrode 31 and the
termini of the stream of liquid being grounded through head 24
connected to the column of liquid in the hollow interior 34 of tube
26. The average linear voltage gradient associated with a distance
0.020 of an inch (0.051 cm) between electrode 31 at about 4,000
volts and the termini of the liquid at about ground potential is
about 200,000 volts per inch (80,000 volts per cm). It is seen that
liquid particles formed from the liquid termini near edge 28 are
formed in a region of very high electric field strength. Since the
material forming the orifice 30 is dielectric, the electric field
lines tend to be concentrated at the conductive liquid termini
which produces very efficient charging of the liquid particles. The
materials forming the orifice 30 should be non-conductive,
otherwise the charging efficiency of the device is substantially
reduced.
The highly charged liquid particles projected through opening 20
from the opening 23 and are attracted to surfaces (not shown) of
articles or objects maintained at a particle attracting potential,
preferably, ground or earth potential.
The foregoing and the following dimensions and parameters are given
to illustrate the operation of the barrel shown in FIG. 2 with the
spray gun shown in FIG. 1. Such dimensions and parameters are not
given by way of limitation, but for purposes of illustration only.
It should be noted that the dimensions of the components comprising
the barrel 11 may vary over a considerable range with respect to
one another.
Opening 20, provided by flange 19, has a diameter of about 0.750 of
an inch (1.90 cm). Opening 23 formed in tube 25 is spaced
rearwardly about 0.350 of an inch (0.90 cm) from opening 20.
Opening 23 has a diameter of about 0.460 of an inch (1.17 cm).
Bullet-shaped cap 27 has a diameter of about 0.42 of an inch (1.07
cm) at edge 28 and a taper of about 30.degree.. The axial length of
cap 27 from rear surface 29 to the forwardmost tip is about 0.360
of an inch (0.90 cm). Tube 26 has an external diameter of about
0.400 of an inch (1.02 cm) and has its extremity 32 spaced about
0.015 of an inch (0.038 cm) from the rear surface 29 of cap 27.
Thus it is seen that orifice 30 formed by the cooperative
relationship between rear surface 29 and extremity 32 has axial
width of about 0.015 of an inch (0.038 cm). The axial width of
orifice 30 may be varied, as desired, by turning the threaded end
33 of cap 27 into or out of hub 35 cooperatively associated with
tube 26.
A test fluid consisting essentially of about 1,500 milliliters of
boiled linseed oil, about 900 milliliters of VM & P (Varnish
Makers & Painters) naptha, about 900 milliliters of n-butyl
alcohol, and about 350 milliliters of methyl alcohol provides
approximately 1 gallon (3.8 liters) of sprayable liquid having a
resistivity of about 6.5 megohm-centimeters. The test fluid is
caused to flow from orifice 30 at flow rates of from about 150 to
540 milliliters per minute under gauge pressures of from about 15
to 20 pounds per square inch (2.0-2.4 atmospheres absolute).
The direct current input voltages supplied to electrode 31 are from
about 4,000 to 7,000 volts and, preferably, about 4,000 volts. The
input air pressure to the spray device 10 is about 35 pounds per
square inch (3.4 atmospheres) at a flow rate of about 7 cubic feet
per minute (200 liters per minute). The input current flowing from
the ground terminal of the voltage source to ground or earth ranges
from about 7.8 microamperes at a test fluid flow rate of about 150
milliliters per minute to about 14.0 microamperes at a test fluid
flow rate of about 540 milliliters per minute.
The current which flows between the surface being coated with
charged liquid particles and ground or earth ranges from about 8.5
microamperes at a test fluid flow rate of about 150 milliliters per
minute to about 32 microamperes at a test fluid flow rate of about
540 milliliters per minute.
As a result of being formed in a region of concentrated high field
strength, the liquid particles projected from the barrel of the
spray gun carry a high charge-to-mass ratio; the charge carried by
the particles is opposite in polarity to the polarity of the
electrode 31. It is seen that a cloud of charged liquid particles
is provided in the space between the front of the spray gun 10 and
the surface at a particle attracting potential. Such a cloud
creates a space charge effect which assists in causing the
deposition of those charged particles which are near the surface to
be coated.
It should be understood that the size of individual liquid
particles is determined by several factors including, the flow rate
of liquid and air to the gun, the constituents of the liquid and
the like. The average particle size should be in the range of about
125 microns or less, and preferably, about 50 microns or less for
quality paint finishes.
The barrel 11 illustrated in FIG. 2 may be modified so as to
include suitable means for dividing or splitting the jet of air
into two separate and distinct air flows; one of the air flows to
sweep across the surface of electrode 31 to provide the cleaning
action required and the other air flow capable of being adjusted so
that it can provide more or less contribution to the atomization of
particles from the termini of the stream of liquid at edge 28 of
rear surface 29 of cap 27.
The charged spray particles projected into the space between the
spray device 10 and the surface of an article at a spray attracting
potential carry a polarity opposite the polarity of electrode 31
and exert a space charge effect thereby creating an electrostatic
field extending to the surface of the article that assists in the
electrostatic deposition of the charged particles in close
proximity to the surface of such an article. If desired, the
electrostatic depositing effect can be enhanced by providing the
spray device with a spray-surrounding electrode 40 as shown in FIG.
3, which may be maintained at a deposition potential relative to
the surface of the article at ground or earth potential. Such a
spray surrounding electrode 40 functions so as to minimize the
tendency of the spray of charged coating material particles to
expand upon emerging from the opening of the spray device thereby
facilitating control over the resultant spray pattern on the
surface of the article. In addition, the spray surrounding
electrode 40 functions to shield the electrode 31 from the high
potential charged cloud of liquid particles having a polarity
opposite from the polarity of the electrode 31. The spray repelling
electrode is fabricated from any suitable metallic material such as
steel or the like, or from dielectric material where the surface
charge is induced thereon by the spray cloud.
Referring now to FIG. 4 of the drawing, another embodiment of the
present invention is illustrated. Barrel 11 includes a
substantially tubular sheath or housing 18. Housing 18 has a radial
flange 19 at its forwardmost extremity. Flange 19 forms opening 20
through which a spray (not shown) of electrostatically charged
liquid particles is projected from spray gun 10.
Substantially concentric outer tube 51 and inner tube 52 are
located within housing 18 and are components of barrel 11.
Preferably, tube 51 and 52 are coaxial with housing 18 and are
fabricated from any suitable dielectric material capable of
withstanding the stresses associated with high voltages without an
accompanying breakdown or rupture of the material. Suitable
dielectric materials include acetal resin, epoxy, glass filled
epoxy and glass filled nylon. Tube 51 includes an integral,
inwardly projecting flange 58 of determined extent that provides a
substantially circular particle emitting orifice or opening 53.
Tube 52 includes at its extremity a reduced external diameter 60.
The reduced external diameter 60 of tube 52 and the flange 58 of
tube 51 cooperate to provide annular opening 54 from which a jet of
air flows. Annular opening 54 communicates with the air source 15
through annular passageway 62 provided by the structural
cooperation between tube 51 and 52. Tube 52 includes passageway 55
which terminates in orifice 56. A stream of liquid under low
pressure issues from orifice 56 during operation of the spray gun
10. Preferably, the coating material flowing through passageway 55
of tube 52 is electrically grounded or earthed through head 63 in
contact with the liquid in passageway 55 and spaced rearwardly a
short distance from orifice 56.
An annular metallic electrode 57 of extended axial length is
imbedded in tube 51 and is spaced rearwardly of opening 53 as shown
in FIG. 4. Electrode 57 may be fabricated from any suitable
conductive material such as alloys of copper. Preferably, electrode
57 is coupled through a current limiting resistor 61 of suitable
ohmic value to power supply 22 in the spray gun 10 or on the person
of the operator. It should be understood that electrode 57 is
positioned within tube 51 and, hence, barrel 11 so as not to come
in contact with any article to be coated and not to engage with an
operator exercising usual operating caution, therefore, the current
limiting resistor may be eliminated. However, good safety practice
may dictate the use of a current limiting resistor of suitable
ohmic value coupled in series between electrode 57 and the output
terminal of power supply 22.
The voltage supplied to the annular electrode 57 by the power
supply 22 through current limiting resistor 61 creates a high
intensity electrostatic field extending across the annular air
discharge opening 54 to the grounded or earthed terminus of liquid
at orifice 56.
Supplying compressed air to passageway 62 at flow rates of up to 7
cubic feet per minute (200 liters per minute) and at gauge
pressures of up to about 50 pounds per square inch (4.4 atmospheres
absolute) and causing liquid to flow from orifice 56 at a flow rate
of up to about 280 milliliters per minute under a gauge pressure of
up to about 5 pounds per square inch (1.3 atmospheres absolute)
projects a spray of liquid particles having a high electrostatic
charge from opening 23. The jet of air flowing in annular
passageway 62 and between the annular electrode 57 and portion 60
of tube 52 causes the stream of liquid whose normal cross section
is that of orifice 56 to expand and be formed into discrete termini
upon which the field is concentrated. In addition, this air stream
is used to direct the liquid particles away from the inner wall of
the tube 51 and electrode 57. The jet of air contributes to the
atomization of the liquid particles from the termini of the liquid
issuing from orifice 56. It should be noted that the jet of air
flows through an enlarged area 64 and then through a reduced area
65 prior to flowing from annular opening 54. This allows the jet of
air to be accelerated in velocity just prior to expanding into the
area adjacent the aperture 26 thereby insuring that a high velocity
stream of air is between the outer edge of orifice 56 and electrode
57.
The diameter of orifice 56 of tube 52 and the distance between the
electrode 57 and exterior surface 60 of tube 52 are each a small
fraction of an inch (centimeter). For example, the separation
between electrode 57 and the exterior surface 60 of tube 52 is on
the order of about 0.035 of an inch (0.090 cm). The diameter of
orifice 56 is on the order of 0.060 of an inch (0.150 cm). The
axial extent of electrode 57 is on the order of 0.250 of an inch
(0.640 cm).
Electrode 57 is connected to a direct current voltage source
supplying about 10,000 volts or less, and, preferably, about 3,000
to 7,000 volts at its output terminal. Assuming a separation of
0.050 of an inch (0.177 cm) between the outer edge of the member
surrounding orifice 56 and electrode 57 and a voltage of about
4,000 volts impressed on electrode 57 the average voltage gradient
across the annular opening 54 is about 73,000 volts per inch
(29,000 volts per centimeter). It is seen that a high intensity
electrostatic field is provided to the liquid termini. As a result
of being formed in a region of high field strength, the liquid
particles emitted from the spray gun 10 bear or carry a high
charge-to-mass ratio and carry a polarity opposite to the polarity
of electrode 57.
The foregoing and the following dimensions and parameters are given
to illustrate the operation of the barrel shown in FIG. 4 with the
spray gun shown in FIG. 1. Such dimensions and parameters are not
given in the way of limitation, but for purposes of illustration
only. The dimensions of the components of the barrel may vary over
a considerable range with respect to one another.
Opening 53 has a diameter of about 0.17 of an inch (0.43 cm).
Orifice 56 is spaced rearwardly about 0.14 of an inch (0.36 cm)
from the forwardmost extent of electrode 57 and about 0.18 of an
inch (0.46 cm) from the forwardmost extent of opening 53. Orifice
56 has a diameter of about 0.060 of an inch (0.152 cm). Annular
orifice 54 has a radial extent of about 0.035 of an inch (0.089
cm).
A test fluid consisting essentially of about 1,500 milliliters of
boiled linseed oil, about 900 milliliters of VM & P (Varnish
Makers and Painters) naptha, about 900 milliliters of n-butyl
alcohol, about 300 milliliters of methyl alcohol and about 30
milliliters of diethylamine provides about 1 gallon of the fluid.
The resistivity of the test fluid is about 1.6 megohm-centimeters.
The test fluid is caused to flow from the orifice 56 at a flow rate
of about 50 to 280 milliliters per minute at a gauge pressure of
about 1 to 5 pounds per square inch (1.07 to 1.34 atmospheres
absolute).
The input air gauge pressure of the jet of air to the barrel 11 is
about 30 pounds per square inch (3.0 atmospheres absolute) at a
flow rate of about 3 cubic feet per minute (85 liters per minute).
The direct current input voltage supplied to electrode 57 is about
4,000 volts. The input current appearing in the circuit from the
ground terminal of the voltage source to ground or earth ranges
from about 3.4 microamperes at a test fluid flow rate of about 50
milliliters per minute to about 8.2 microamperes at a test fluid
flow rate of about 280 milliliters per minute. The liquid particle
current or output current appearing between the surface on which
liquid particles are being deposited and ground or earth ranges
from about 2.8 microamperes at a test fluid flow rate of about 50
milliliters per minute to about 7.6 microamperes at a test fluid
flow rate of 280 milliliters per minute.
Increasing the air pressure supplied to the barrel shown in FIG. 4
to about 50 psi gauge (4.4 atmospheres absolute) results in an
input current flowing from the ground terminal of the voltage
source to ground or earth in the range of about 3 microamperes at a
test fluid flow rate of about 50 milliliters per minute to about
3.6 microamperes at a test fluid flow rate of about 280 milliliters
per minute. The liquid particles current or output current
appearing between the surface on which liquid particles are being
deposited and ground or earth ranges from about 2.5 microamperes at
a test fluid flow rate of about 50 milliliters per minute to about
13.5 micromaperes at a test fluid flow rate of about 280
milliliters per minute.
While the invention is illustrated and described using several
embodiments, it is to be understood that modifications and
variations may be effected without departing from the scope and
concepts of the invention.
* * * * *