U.S. patent number 3,698,636 [Application Number 05/136,900] was granted by the patent office on 1972-10-17 for device for the electrostatic application of protective coatings with synthetic powders by the use of spray guns.
This patent grant is currently assigned to Graco Inc.. Invention is credited to Imre Szasz.
United States Patent |
3,698,636 |
Szasz |
October 17, 1972 |
DEVICE FOR THE ELECTROSTATIC APPLICATION OF PROTECTIVE COATINGS
WITH SYNTHETIC POWDERS BY THE USE OF SPRAY GUNS
Abstract
A material coating system to apply charged materials to objects
to be coated by electrostatic spray guns. The spray gun itself has
a double cone-shaped deflecting member in its barrel and an
adjustable collar on its barrel to change the spray pattern. An
internal electrode in the material conveying tube charges the
material and a wound wire ground connection attached to the gun
prevents an accumulation of charge at the gun. A material storage
bin with multiple venturis feeds material to the gun by suction via
its conveying tube. In addition, a polarity reversing circuit
allows the power source to apply different charges to sprayed
materials.
Inventors: |
Szasz; Imre (St. Gallen,
CH) |
Assignee: |
Graco Inc. (Minneapolis,
MN)
|
Family
ID: |
5770343 |
Appl.
No.: |
05/136,900 |
Filed: |
April 23, 1971 |
Foreign Application Priority Data
|
|
|
|
|
May 6, 1970 [DT] |
|
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P 20 22 088.6 |
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Current U.S.
Class: |
239/697;
239/518 |
Current CPC
Class: |
B05B
5/10 (20130101) |
Current International
Class: |
B05B
5/10 (20060101); B05B 5/08 (20060101); B05b
005/00 () |
Field of
Search: |
;239/15,3,518,515 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wood, Jr.; M. Henson
Assistant Examiner: Culp, Jr.; Thomas C.
Claims
Now, therefore, what I claim is:
1. A spray gun for applying coating material comprising: a gun
barrel having a material conveying passage therethrough, said
passage having an exit at the front end of said barrel; a collar
having a leading edge encircling a portion of said barrel and
movable in the longitudinal direction of said barrel with the
leading edge in the forward most position; and a material deflector
mounted within the front end of said barrel, said deflector being
shaped to generally resemble two conical members attached at a
common base side and the forward most conical member having a
larger base side at the side of attachment and an apex at its
forward most point, whereby coating material is deflected in a
radial direction by the deflector when the leading edge of the
collar is rearward of the common base attachment and deflected in a
generally forward direction by the collar as its leading edge is
moved forward of the common base attachment.
2. The spray gun of claim 1 wherein said collar has an internal
chamfer near its leading edge to further assist in directing
material from the gun barrel.
3. The spray gun of claim 1 wherein said collar extends
substantially the entire longitudinal length of said barrel and has
means for limiting the rearward and forward movement of said
collar.
4. An electrostatic spray gun assembling comprising: an
electrostatic spray gun for applying coating material and having an
opening therethrough to convey material into said gun at an
entrance and to discharge the material at the front end of said
gun, said gun having a body at least a portion of which is
electrically conductive; an elongated flexible material conveying
electrically insulating conduit connectable to said spray gun
opening at its entrance; a thin electrically conductive electrode
wire mounted within said conduit along its longitudinal extent,
said wire being connectable to a high potential source at one end
and terminating at its other end within said gun near the front end
of said opening; and a second conductive wire wound around the
external surface of said material conveying conduit and
electrically connected to ground potential at one end and to the
conductive part of the gun body at the second end, whereby said
second wire acts as a ground connection for the conductive part of
the gun body.
5. The assembly of claim 4 wherein said gun body has a handle; and
said conductive portion of the gun body being part of said
handle.
6. The assembly of claim 4 including a high potential source, said
source comprising: a transformer, a capacitor-bridged chopper
circuit connected to the primary winding of said transformer, and a
cascade multiplier circuit being connected to the second winding of
said transformer and also being connected to said electrically
conductive electrode wire.
7. The assembly of claim 6 wherein the resonant frequency of the
circuit formed by said transformer primary winding and bridging
capacitor is approximately equal to the operating switching
frequency of said chopper.
8. The assembly of claim 6 wherein said cascade multiplier circuit
has at least two rectifiers, and means for reversing the polarity
of the output from the cascade multiplier circuit by reversing said
two rectifiers.
9. The assembly of claim 8 wherein said means for reversing said
two rectifiers comprises a switch to exchange the leads to said
rectifiers.
10. The assembly of claim 6 wherein said means for reversing said
two rectifiers comprises three rectifiers only two of which are
active circuit elements at any one time.
Description
The invention involves a device for the electrostatic application
of protective or decorative coatings with synthetic powders by the
use of spray guns. It is known to be desirable to apply a synthetic
coating to the surfaces of those raw materials that are not durable
or long-lasting; for example, to protect them against corrosion,
abrasion, and to provide electrical insulation. In electrostatic
powder coating, the initial electrically neutral (uncharged)
character of the powder particles is changed in that the particles
receive an electrostatic charge.
While these particles are being sprayed from the gun, which also
usually houses the ionizing element, the particles are being
attracted to the grounded object (object being sprayed) because of
their charge finally reaching the object.
The particles are deposited on the object and are subsequently
"baked out" or cured to produce a protective coating.
The electrostatic charge can be applied in the following ways:
1. Through polarization, in which most electrical charge centers in
the molecules are displaced or dislocated producing an electrical
dipole.
2. Through ionization in which at least one electron is removed or
added to the synthetic material particle.
While the polarization usually occurs through friction of the
powder-air mixture on the walls of the conveyance tubes, but can
also occur through friction between the powder particles
themselves; the ionization of the particles is caused either by the
particles coming in contact with a high voltage metal electrode or
by passing through a high voltage field. Upon being ionized, the
particles acquire the polarity of charge, thus the newly arriving
particles are repelled by those already deposited on the object.
With polarization, however, the electrical dipole is aligned in the
direction of the grounded object, so that the newly arriving powder
is attracted both by the grounded objects and by the particles
already deposited on the object.
Electrostatic powder coating developed from electrostatic paint
spraying, in which the high voltage electrode, which effects the
ionization, is in the muzzle of the spray gun. Since with fluids
there is, practically speaking, no friction, no value was placed on
polarization charging. It could be demonstrated, however, that for
the forming of industrially useful coatings, both charging methods
are simultaneously essential.
The essential task of the invention then is to have a device which
allows both of the charging methods to be realized simultaneously.
The invention accomplishes this so: the flexible insulating hose
carrying a powder-air mixture, which leads to the muzzle of the gun
where there is a fixed-position body of revolution, has inside it a
thin metal wire which acts as a high voltage electrode to form a
strong corona effect, and this wire is connected to a high voltage
generator, while the outer surface of the cable has a spiraling
ground wire around it to introduce a high voltage gradient. The
separation of the high voltage electrode from the head of the spray
gun, the "safety distance," is important. At the muzzle of the gun,
a cone-shaped open body of revolution is provided for the radial
deflection of the exiting powder-air mixture.
The flexible insulating hose or conduit is directed through the
handle of the gun and is housed within the conveyor pipe, which
leads to the vicinity of the muzzle, and the conveyor pipe is
housed within an outer sliding pipe while the end of the
neighboring outlet in relation to the inner surface of the cone of
the body of revolution forms the "counter cone." The supply bin has
several parallel-mounted venturis connected to its outlet. These
parallel-mounted venturis have a common suction chamber. The outlet
of the supply bin incorporates a continuously variable damper.
The supply bin is tilted in the direction of the exit. Several
component groups consisting of metering dampers, venturi tubes, and
spray guns can be added to the supply bin. The high voltage
generator chopper has a bridged capacitor and works directly
through a cascade circuit which is connected directly to the high
voltage transformer. The capacitor bridge and the high voltage
transformer operate at or near a resonant frequency.
The polarity of the cascade output can be changed through polarity
reversal of the switching element; it can also be changed by
reversing the cascade rectifier or by transposing the cascade
rectifier; and also by interchanging the unconnected open rectifier
leads. For interchanging the open ends of the cascade rectifier, an
adjustable switching bar can be provided. Finally, the polarity of
the cascade output can be changed by sliding an additional,
block-mounted rectifier.
An embodiment of the previously described invention is shown in the
accompanying drawings.
FIG. 1 is a schematic representation and partial section of the
muzzle part of the spray gun showing the deflector cone, powder-air
mixture conveyor pipe and the high voltage electrode.
FIG. 2 is a longitudinal cross-sectional view of the muzzle part of
the spray gun showing the deflector cone in an extended position so
as to produce a more diffuse powder spray pattern.
FIG. 3 is a vertical cross-sectional view of the supply bin and the
multiple venturi.
FIG. 4 is a vertical cross-sectional view of the supply bin shown
in FIG. 3 after it has been rotated 90.degree. about the vertical
axis.
FIGS. 5, 6 and 7 show the high voltage generator wiring diagrams as
well as the modification required for the changing of polarity.
Referring now specifically to FIG. 1 embodiment, the electrostatic
spray gun assembly has a spray gun for spraying powder-air mixtures
with the tube 8 containing the muzzle part of the gun connected to
the handle 7. The powder air mixture 1 is conveyed through an
insulated flexible conduit or hose 2 to the spray gun, which is
constructed from high resistive materials, making the gun shock and
spark proof. The insulated hose 2 houses a thin electrically
conductive high voltage metal electrode 3 which produces a corona
discharge within the tube itself. This high voltage electrode 3
extends only to within about 30-40 mm of the powder-air exit
passage at the front of the gun 5. This 30-40 mm distance 20 is
termed the "safety distance."
The relative motion between the selected powder 1 and the large
inside surface of the insulated hose 2 produces the required
friction for polarization, while the high voltage field produced
between the wire shaped high voltage electrode 3 and the ground
wire 4 which is spirally wound around the insulated hose 2,
produces the ionization. This spirally shaped arrangement
guarantees an even and repeatable high voltage field through which
the powder particles can pass. The charge of the individual powder
particles is greatly increased over that obtained by employing
devices in which only one of the charging methods is used.
Positioned at the exit of the spray gun is a generally double
cone-shaped body 16 which serves to deflect the emerging powder-air
mixture in a radial direction as shown in FIG. 2. The end of the
flexible insulated hose 2, which is housed within the gun, is
surrounded by two concentrically positioned pipes 8 and 9, which
are constructed of a nonconductive, hard plastic. The inner pipe or
barrel 8 of the gun is firmly attached to the gun handle 7 while
the outer pipe or collar 9 can slide along the inner pipe 8. The
outer pipe 9 can be positioned by axial movement over the
cone-shaped bell mouthed opening at the muzzle end of the gun so as
to make it possible for the powder-air mixture to be changed
variably from a (beamed) narrow ray of powder as shown in FIG. 1 to
a rather diffuse cloud of powder as shown in FIG. 2. Hence the
emerging powder pattern can be quickly shaped, without interruption
of the spraying process, to the most desirable pattern for spraying
an object of a given shape.
The collar 9 encircles substantially the entire longitudinal
portion of the barrel 8 of the gun. By moving the leading edge of
the collar forward of the common base of the two conical shaped
members that form the deflector member 16, the coating material is
variably deflected in a generally forward direction. Deflector
member 16, it is noted, resembles two conical members of different
sizes attached at a common base side with the larger cone being
forward of the smaller cone and having its apex at the most forward
position of the deflector. An internal chamfer or groove at the
leading edge of the collar 9 acts as an additional guide to direct
material in a forward direction when the collar's leading edge is
moved forward to the juncture of the common base of deflector 16.
When the collar's leading edge is rearward of the common base
attachment, coating material is deflected in a radial direction. A
lug or other stop members may be placed on the barrel to limit the
forward and rearward movement of collar 9. As for example, lug 19
rigidly attached to barrel 8 that slides in a closed slot in collar
9 (FIG. 1).
The supply bin 12 is set at an angle, so shown in FIG. 3, as to
insure a continuous flow of powder to the chamber 17 of the venturi
injector system shown in FIG. 4. The powder-air mixture in the
suction chamber 17 is then forced with great velocity in the
direction of the arrows 15 into the flexible insulated hose 2.
Coating material storage bin 12 stores and dispenses material to
conduit 2. This main storage bin 12 has an opening at its lowest
side, a large fluid inlet (FIG. 4) with smaller venturis 14
connected to this opening, and an outlet conduit axially
longitudinally aligned with the fluid inlet that has a diameter or
cross section area greater than the areas of all of the venturi
tubes combined.
The continuously variable metering gate or valve 18 shown in FIG. 3
controls the flow of powder to the suction chamber 17 to be
throttled or even stopped.
The quantity of powder entrained into the suction chamber 17
depends directly upon the air velocity and flowrate emitted from
the venturi, and hence the operating line pressure. For practical
as well as economic reasons, line pressures should not exceed the
ambient pressure by more than 75-84 psi. With these pressure drop
limitations, the venturi tube conveyance method will produce
maximum powder flow rates of 30 kg/hour. The maximum powder flow
rate can be increased, without increasing line pressure, by
employing several parallel mounted venturis 14 which are mounted in
a single suction chamber 17 as shown in FIG. 4.
Additional metering gates 18 and venturi tubes 14 can be affixed to
the same supply bin 12 so as to permit more than one spray gun to
be operated from a single supply bin 12.
The HV generator of this invention works according to the
well-known "chopper principle," in which low voltage direct current
(8-10 volts) from the line rectifier is magnetically or
mechanically "chopped" or rapidly switched. This "chopped" DC
voltage is passed through a voltage-adjusting potentiometer to the
primary side of the HV transformer, while the secondary side of the
HV transformer is attached to a one-stage or multi-stage cascade
multiplier 29, 30, 31, 32, consisting of capacitors 23, 24 and
rectifier 25, 26. The cascade also rectifies the produced high
voltage. The "chopper" 21 is capacitor bridged 33, with primary
transformer winding and capacitor resonant frequency is
approximately equal to the switching frequency of the "chopper" so
that the HV transformer and the "chopper" operate at a resonant
frequency, since in this manner the required high voltage can be
obtained. Between the output point 29 of the cascade and the gun
cable 3, a high ohm-value resistor is located.
There are powders which produce better operation with
positive-polarity high voltage and others which operate better with
negative-polarity high voltage. Therefore, the devices, according
to the invention, have the capability to use both polarities. While
known devices require two independent, correspondingly switched HV
generators, according to this invention both polarities can be
obtained by using only one HV generator and one cascade, in which
the cascade has reversible polarity, and specifically:
1. by reversing the cascade rectifiers 25, 26 (FIG. 5);
2. by changing the unconnected ends 29, 30 of the cascade rectifier
25, 26 by moving a switching bar 34 and contact 35 (FIG. 6);
3. by location change of the cascade rectifier 25, 26 (FIG. 5);
4. by moving 36 the block mounted supplementary rectifier 37 (FIG.
7).
The foregoing specification accordingly describes an embodiment of
the present invention and would suggest some of the ways of
practicing it to those skilled in the art. Such a description
should not be construed as limiting the invention as only the
foregoing claims can do this.
* * * * *