U.S. patent number 3,635,400 [Application Number 05/023,227] was granted by the patent office on 1972-01-18 for paint spraying method and apparatus.
This patent grant is currently assigned to Nordson Corporation. Invention is credited to Eric T. Nord, Samuel R. Rosen, Peter W. Runstadler, Jr., Don R. Scarbrough, Burton J. Vilagi.
United States Patent |
3,635,400 |
Nord , et al. |
January 18, 1972 |
PAINT SPRAYING METHOD AND APPARATUS
Abstract
A method and apparatus for spraying paint and similar coatings.
In one form the paint is sprayed at low pressure from a flat fan
nozzle. A flat fan of air impinges upon the fan of paint at an
angle of from 30.degree.-70.degree.. The air atomizes the paint to
form a soft spray with low-forward velocity. In another form of the
method, two fans of paint are projected toward one another at an
angle and an air fan is directed along the bisector toward the zone
of intersection. The air fan atomizes both streams of paint. An
electrode can be placed in the air stream to ionize the air stream
which in turn charges the paint particles. The width of the paint
spray pattern is varied by varying the included angle of the air
fan. A spray gun is disclosed having two tubular paint nozzles for
projecting two fan-shaped fans of paint. An air nozzle is mounted
between the paint nozzles for projecting a fan of air against the
paint fans to atomize the paint. The air nozzle is adjustable to
provide air fans of different angles for varying the width of the
paint spray pattern. An electrode is disposed in the air nozzle for
ionizing the air stream. This electrode is energized through a
resistor and flexible cable. The cable is spring-urged toward the
resistor so that when the resistor is removed, the cable shifts
causing a safety switch to open deenergizing the gun. When a nozzle
mounting member is removed, the cable and surrounding tube are
shifted forwardly to automatically close a valve to seal off the
air and electrical conduits. Removable caps are provided on the
ends of the tubular paint nozzles for facilitating cleaning of the
nozzles.
Inventors: |
Nord; Eric T. (Oberlin, OH),
Rosen; Samuel R. (Lorain, OH), Scarbrough; Don R.
(Elyria, OH), Vilagi; Burton J. (Amherst, OH),
Runstadler, Jr.; Peter W. (Hanover, NH) |
Assignee: |
Nordson Corporation (Amherst,
OH)
|
Family
ID: |
21813828 |
Appl.
No.: |
05/023,227 |
Filed: |
March 27, 1970 |
Current U.S.
Class: |
239/706; 239/422;
239/705 |
Current CPC
Class: |
B05B
5/03 (20130101); B05B 7/0815 (20130101); B05B
5/035 (20130101) |
Current International
Class: |
B05B
5/025 (20060101); B05B 7/02 (20060101); B05B
7/08 (20060101); B05B 5/035 (20060101); B05B
5/03 (20060101); B05b 005/00 (); F23d 011/28 () |
Field of
Search: |
;239/422,3,15,543,544 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: King; Lloyd L.
Claims
Having described our invention, we claim:
1. A paint spray gun, said gun comprising,
a handle portion,
a barrel portion extending forwardly of said handle portion, said
handle portion being hollow and having an air passageway
therethrough,
an air conduit within said barrel portion in communication with the
air passageway in said handle,
a paint conduit in said barrel portion,
a paint nozzle in communication with said paint conduit,
an air nozzle in communication with said air conduit,
an electrode for causing said paint particles to become
electrostatically charged,
means for applying a potential to said electrode,
said means comprising a cable, said cable entering said gun through
said handle and including a bent portion being disposed within said
stock and a forwardly extending portion disposed within said
barrel, a resistor in abutment with the end of said cable and
extending forwardly thereof, said resistor providing an electrical
interconnection between said cable and said electrode, spring means
urging said cable toward said resistor, a switch effective to
control the voltage applied to said cable, said switch being
disposed for actuation to deenergize said cable when said cable is
shifted forwardly under the influence of said spring upon removal
of said resistor.
2. The paint spray gun of claim 1 in which a sleeve is slidably
disposed in said air conduit and said resistor and said cable are
disposed within said sleeve, a mounting member is disposed on the
end of said barrel for supporting said air nozzle, said paint
nozzle and said electrode, said mounting member carrying means for
restraining forward movement of said sleeve, whereby when said
mounting member is removed said cable moves forwardly and said
switch is actuated to deenergize said cable.
3. The paint spray gun of claim 2 in which said barrel is
configurated to form a seat and the forward end of said sleeve is
configurated to form a plug, whereby when said mounting member is
removed said sleeve moves forwardly and said plug engages said seat
to seal said air conduit.
4. The spray gun of claim 1 in which said switch is disposed within
the air passageway in the handle, said air conduit constitutes an
explosive proof chamber.
5. The spray gun of claim 1 further comprising a second switch for
controlling energization of said cable,
a trigger for actuating said second switch, said first named switch
and second switch being disposed within said air passageway, said
air passageway constituting an explosive proof chamber.
6. A paint spray gun for use in a system including a source of
compressed air and a source of paint,
said gun having a barrel portion,
an air conduit through said barrel portion, means for connecting
said air conduit to said source of compressed air,
a paint conduit through said barrel portion, means for connecting
said paint conduit to said source of paint,
a flat fan air nozzle in fluid connection with said air
conduit,
a flat fan-type paint nozzle,
means supporting said air nozzle and said paint nozzle on said
barrel,
said air nozzle and said paint nozzle being in fluid communication
with said air conduit and said paint conduit respectively,
said air nozzle being effective to project a flat fan of air
forwardly of said gun,
said paint nozzle being effective to project flat fan of paint
forwardly of said gun which intersects said flat fan of air in a
zone spaced forwardly of said gun,
means for ionizing said air in an area rearwardly of said zone in
which said air fan impinges upon said paint fan,
whereby the air fan is effective to atomize said paint and to
charge said paint to form a charged paint spray.
7. The paint spray gun of claim 6 in which said means for ionizing
said air comprises an electrode disposed within said air
nozzle.
8. The paint spray gun of claim 6 in which the paint nozzle is
disposed so that the flat fan of paint intersects the stream of air
at an angle of from 30.degree. to 70.degree..
9. The paint spray gun of claim 6 in which the air and paint
nozzles are disposed so that the length of air travel of the air
fan from the nozzle to the zone of air-paint impingement is from
one-sixth to three-fourths the length of travel of the paint fan
from the paint nozzle to the zone of air-paint impingement.
10. The paint spray gun of claim 6 in which the air and paint
nozzles are disposed so that the air fan travels a distance of
approximately one-eighth to three-eighths inch from the air nozzle
to the point of air-paint impingement.
11. The paint spray gun of claim 6 in which the air and paint
nozzles are disposed so that the paint fan travels from
approximately three-eighths to three-fourths inch from the paint
nozzle to the zone of air-paint impingement.
12. The paint spray gun of claim 6 in which said air nozzle
comprises a housing having a cylindrical chamber formed therein and
an elongated exit slot and an entrance opening communicating with
said chamber, a shiftable plug disposed within said chamber, said
shiftable plug including a plurality of nozzle slots formed
therein, each said nozzle slot being effective to limit a fan of
air having a different included angle, a central bore communicating
with said nozzle slots and an inlet opening communicating with said
bore, said plug being shiftable to selectively align any of said
exit slots with the slot in said housing, and an electrode extends
inwardly through the entrance opening in said housing and the inlet
opening of said plug, said electrode being disposed in alignment
with the exit slot of said chamber and the nozzle slot of said plug
in registry therewith.
13. A paint spray gun for use in a system including a source of
compressed air and a source of paint,
said gun having a barrel portion,
an air conduit through said barrel portion, means for connecting
said air conduit to said source of compressed air,
a paint conduit through said barrel portion, means for connecting
said paint conduit to said source of paint,
a flat fan air nozzle in fluid connection with said air
conduit,
a pair of flat fan-type paint nozzles,
means supporting said air nozzle and said paint nozzles on said
barrel,
said air nozzle and said paint nozzles being in fluid communication
with said air conduit and said paint conduit respectively,
said air nozzle being effective to project a flat fan of air
forwardly of said gun,
said paint nozzles being effective to project flat fans of paint
forwardly of said gun which intersect said flat fan of air in a
zone spaced forwardly of said gun,
means for ionizing said air in an area rearwardly of said zone in
which said air fan impinges upon said paint fans,
whereby the air fan is effective to atomize said paint and to
charge said paint to form a charged paint spray.
14. The paint spray gun of claim 13 in which the paint nozzles are
disposed so that the flat fans of paint intersect the stream of air
at an angle of from 30.degree. to 70.degree..
15. The paint spray gun of claim 14 in which the air and paint
nozzles are disposed so that the air fan is disposed substantially
on the bisector of the angle between said paint fans.
16. The paint spray gun of claim 13 in which the air and paint
nozzles are disposed so that the length of air travel of the air
fan from the nozzle to the zone of air-paint impingement is from
one-sixth to three-fourths the length of travel of the paint fans
from the paint nozzles to the zone of air-paint impingement.
17. The paint spray gun of claim 13 in which the air and paint
nozzles are disposed so that the air fan travels a distance of
approximately one-eighth to three-eighths inch from the air nozzle
to the point of air-paint impingement.
18. The paint spray gun of claim 13 in which the air and paint
nozzles are disposed so that each paint fan travels from
approximately three-eighths to three-fourths inch from the paint
nozzle to the zone of air-paint impingement.
19. The paint spray gun of claim 13 in which said means for
ionizing said air comprises an electrode disposed within said air
nozzle.
20. The paint spray gun of claim 13 in which said air nozzle
comprises a housing having a cylindrical chamber formed therein and
an elongated exit slot and an entrance opening communicating with
said chamber, a shiftable plug disposed within said chamber, said
shiftable plug including a plurality of nozzle slots formed
therein, each said nozzle slot being effective to limit a fan of
air having a different included angle, a central bore communicating
with said nozzle slots and an inlet opening communicating with said
bore, said plug being shiftable to selectively align any of said
exit slots with the slot in said housing, and an electrode extends
inwardly through the entrance opening in said housing and the inlet
opening of said plug, said electrode being disposed in alignment
with the exit slot of said chamber and the nozzle slot of said plug
in registry therewith.
Description
BACKGROUND OF THE INVENTION
This invention relates to methods and apparatus of spraying paints,
lacquers and similar coating materials and is particularly directed
to a novel method and apparatus for effecting air atomization of
the paint and for electrostatically charging the paint if desired
so that the electrostatic field forces can be utilized to increase
the effectiveness of paint deposition.
In the past there have evolved two distinct types of paint spraying
equipment. The first type can be characterized as "airless"
spraying equipment. In an airless type of spraying apparatus, a
paint stream is forced through an orifice under a relatively high
pressure, for example, a pressure of the order of 300-1,000 pounds.
As the paint is propelled through the small orifice it is broken
up, or atomized, into very fine droplets. The paint spray formed
from the gun moves at a relatively low velocity toward the article
to be painted. In many cases the paint spraying operation is
carried out in the presence of a high-voltage electrostatic field
in which the work to be coated is kept at, or close to, ground
potential while the atomized paint particles are charged to a
relatively high potential. These particles are then urged toward
the work by the forces of the electrostatic field.
This type of system has the recognized advantage of providing a
very high deposition efficiency, i.e., a large portion of the paint
spray emitted from the gun is effectively deposited on the article
to be coated. On the other hand, in some installations the airless
spray-type apparatus has certain inherent shortcomings. One such
objectionable characteristic is that the system requires a paint
supply system operated at a relatively high pressure. As a result,
it is not feasible to disconnect a gun from one high-pressure paint
line and quickly connect it to another high-pressure paint line
when, for example, it is desired to change the color of coating
being sprayed.
The second general type of spray equipment does not require a
high-pressure paint source. This second type of spray system, which
is known as "air spray" equipment, relies upon a stream of air to
break up the paint into particle size suitable for spraying. In
conventional air spray equipment, the paint is extruded from a
nozzle in a generally rodlike form and is subjected to a
high-pressure blast of air. In a typical air spray installation,
the air pressure at the gun is under a pressure of approximately
the order of 75 pounds per square inch. The air is used in large
quantities; for example, it is common practice to utilize an
airflow of 14 or 15 standard cubic feet of air per minute to carry
out atomization.
Air spray systems of this type do present certain advantages in
that they can be utilized to atomize particularly difficult types
of paint and can more readily be provided with quick disconnect
couplings to the paint line since the paint line is maintained at a
relatively low pressure of, for example, 50 p.s.i. At the same
time, however, prior art paint spray guns are subject to several
disadvantages.
In the first place, the large quantity of high-velocity air used to
atomize the paint together with the airflow it induces causes a
substantial portion of the paint spray to be carried past the
workpiece and wasted even when an electrostatic charge is applied
to the paint. Moreover, the rapidly moving air has a high kinetic
energy which causes it to bounce back or rebound from the surface
being coated carrying with it entrained paint particles.
Consequently, an appreciable portion of the paint which is directed
toward the workpiece is wasted due to rebound.
Another inherent disadvantage of conventional air spray equipment
is the problem of ventilation. Specifically, because of the high
volume of air emitted by each spray gun, and in many installations
there are several guns operating simultaneously, a ventilation
system must be provided having a high capacity for capture of paint
particles entrained in the large mass of high-speed air, for
example, capture velocities of 150 feet per minute.
Also, because of paint entrained in the large amount of overspray
and rebound, water curtains or other types of filters must be
provided for preventing sizable quantities of paint from escaping
and polluting the atmosphere.
SUMMARY OF THE INVENTION
The principal object of the present invention is to provide a novel
method and apparatus for spraying paint in which relatively small
quantities of air at a relatively low pressure are utilized to
atomize the paint in a very effective manner so that the resultant
paint spray is a soft, finely divided spray not unlike that which
could heretofore be produced only by an airless gun.
At the same time, the present method and apparatus are effective to
retain the advantages inherent in any air-type of spray gun, i.e.,
the ability to atomize various difficult types of coating material
and the facile interconnectability to various paint lines so that
the spray gun can be converted quickly from spraying one color or
type of coating to another.
More particularly, the present invention is predicated in part upon
the concept of spraying paint by emitting a thin, flat fan-shaped
stream of paint from one nozzle and a fan-shaped stream of air from
a second nozzle. The paint fan in most cases is initially in the
form of a continuous sheet. However, the fan can be in the form of
a thin, substantially planar discontinuous spray; for example, one
which is already partially atomized. The fan-shaped stream of paint
and air impinge upon one another at a substantial angle of from
approximately 30.degree.-70.degree.. The air thus exerts an optimum
shearing force upon the paint fan and breaks the paint down into
small particles of a low mean particle size with substantially no
particles of an objectionably large size.
This invention is further predicated in part upon the empirical
discovery and determination of various physical relationships and
values of certain parameters which make it possible to obtain an
atomization suitable for fine finishing purposes utilizing as one
starting component a low-pressure fan of paint. A flat stream of
paint from any presently known form of fan nozzle operated at low
pressure is inherently in a very difficult form to atomize into
small particles with the uniformity required for producing
satisfactory painted surfaces.
The difficulties involved are due to the fact that the paint stream
is not a uniformly thin flat sheet. Rather, the sheet in cross
section is somewhat like a dumbbell with a thin center web and two
enlarged portions, one at each edge. These enlarged portions are in
fact longitudinal beads or "streamers." These two streamers are
several times the thickness of the central web portion and tend to
remain integral and resist atomization even when the rest of the
sheet is broken up into particles. It will readily be appreciated
that no matter how finely most of the paint is atomized, if the
streamers are not broken up into particles of generally the same
fineness, the quality of the applied coating will suffer and may
well become totally unacceptable. The present paint spray method is
effective to cause effective breakup of all parts of the paint
stream, including the streamers, into small particles while using
only a relatively small quantity of air.
In accordance with the present invention, the air stream is
projected under a relatively low pressure, for example, of the
order of 9 to 45 pounds per square inch at the gun, while the paint
is sprayed under a low pressure of, for example from 30-80 pounds
per square inch. This method of atomization is so effective that
the quantity of air required to deposit a relatively large quantity
of paint, for example, 22-25 fluid ounces per minute of a typical
baking enamel, is approximately 7-8 standard cubic feet per minute.
As a result of this small volume of low-pressure air and the fine
atomization of the paint, the resultant spray is in the nature of a
soft spray, or slowly moving fog, having a low forward
velocity.
One of the principal advantages of this method of spraying is that
it results in a substantially higher deposition efficiency than
conventional air spraying methods. There is substantially less
paint waste due to overspray and rebound which is minimized or
practically eliminated.
Another advantage of the present spray method is that the
relatively small quantity of air involved and the smaller quantity
of paint entrapped in the exhausted air greatly simplifies the
problem of ventilation and prevention of atmospheric
contamination.
Another object of the present invention is to provide a novel
method for readily changing the width of the paint spray pattern.
This aspect of the present invention is predicated upon our
determination that the width of the paint spray can be controlled
by changing the included angle of the air fan without in any way
changing the paint fan.
In accordance with the present invention, the air fan is
selectively emitted from one of a series of nozzle openings
constructed so that the nozzle openings not only simultaneously
produce air fans of different included angles, but also produce air
fans which behave much as though they are emitted from a point
pressure source, the distance from the nozzle opening of which is
varied in an inverse relationship to the angular size of the nozzle
opening. By virtue of this relationship, the height of the air fan
along its line of impingement with the paint stream is maintained
substantially constant so that all portions of the paint stream are
effectively atomized with a minimum air usage.
In addition to the method concepts disclosed above, the present
invention also comprehends a preferred spray painting method
utilizing in part the method described. More particularly, in the
preferred method a fan of air is in effect "enveloped" between two
fans of paint. The fans of paint are directed toward each other
with the air fan bisecting the angle so that it impinges upon both
paint streams in substantially the same zone. This method results
in an even better atomization than the single spray method,
apparently due to the fact that in order to escape, a major portion
of the air must pass through one or the other paint streams or
through the paint particles being separated from the streams. Thus,
the kinetic energy of the air is more effectively utilized to shear
the paint and cause its breakup into particles of a small mean
particle size.
The present two-paint stream method of spraying is also
particularly advantageous when employed in conjunction with the
present novel method of electrostatic charging. More particularly,
in accordance with the present charging method, the paint is not
charged directly or by an electrode in close proximity to the paint
stream. Rather, it is our concept to ionize the air fan prior to
its impingement upon the paint streams by passing the air fan
through a corona discharge surrounding an electrode at a high
potential. As explained above, the air fan then impinges upon the
paint streams and causes the atomization of the paint streams.
The ions present in this air stream have a high mobility and are
subjected to the forces of the surrounding electrostatic field.
Specifically, these ions, which are charged and exist in a field of
high potential, move toward the paint streams and fog of paint
droplets existing in regions of a lower potential. The ions attach
themselves to the paint particles which are of substantially lower
mobility. These charged paint particles are propelled forward by
the air stream and under the additional influence of the
electrostatic field forces are brought into contact with the
surface to be coated which is normally maintained at ground
potential.
We have determined that the two-paint stream method results in
substantially more effective charging of paint particles than the
one-spray method and believe that this is due to the fact that the
ions are to a large degree surrounded by the paint streams and
particles and do not have a free path to one side of the paint as
is the case when only a single paint spray is utilized.
The present method of charging paint is not only advantageous
because of its high efficiency, but is further advantageous because
the entire paint supply system, including the tube supplying paint
to the gun, the paint pump and the paint reservoir, or tank, remain
at or near ground potential.
In contrast, in prior art electrostatic spraying systems,
particularly when a conductive paint or coating, such as one of the
metallic-containing finishes utilized in the automobile industry,
is used, the entire paint system became charged to the same order
of potential as the electrode, e.g., 75,000 volts. This
necessitated careful insulation of the entire paint system
including the paint reservoir. In the event of insulation failure,
for example in the paint supply tube, the high potential present
could, and not infrequently did, result in fires, electrical shock
hazards, and other operational difficulties. All of these problems
are completely eliminated by the present charging method in which
no charge is applied directly to the paint either by an electrode
in contact with the paint or by an electrode in close proximity to
the paint stream.
In addition to the method aspects of the present invention, the
invention is directed to the provision of a novel spray gun for
carrying out the method. More particularly, the present spray gun
includes a handle, a forwardly extending barrel and a nozzle
assembly mounted at the forward end of the barrel.
The nozzle assembly comprises two tubular paint nozzles effective
to direct thin flat fan-shaped paint sheets toward the axis of the
gun, and an air nozzle mounted between the paint nozzles and
effective to direct a flat fan-shaped stream of air along the axis
of the gun toward the zone of intersection of the two paint fans.
In accordance with the present invention the air nozzle and paint
nozzles are located so that the distance of the air nozzle from the
zone of air-paint impingement is only a fraction of the distance of
the paint nozzles from the zone of air-paint impingement. As a
result, the velocity of the air issuing from the nozzle is
attenuated only minimally prior to its impingement with the paint
fans. Each of the paint fans and air fan meet at a substantial
angle of the order of 30.degree. to 70.degree.. As a result, the
air fan exerts an optimum shearing force on the paint fans and
breaks up both fans simultaneously into a spray of particles of
small mean particle size free from inordinately large
particles.
One of the advantages of the present gun is that it is quite
compact and maneuverable. Despite the fact that the gun utilizes
three separate nozzles it is as small and easy to handle as
previous spray guns of the air or airless type.
It is another objective of the present invention to provide a spray
gun incorporating paint charging means of a substantially increased
efficiency. In fact, the present electrostatic charging system is
so efficient that when operated at 50,000 volts or even less, it
provides the same deposition efficiency as is attained using a
conventional electrostatic charging mechanism operating at a
potential of 75,000 volts.
More particularly, in accordance with the present invention the gun
includes an electrode mounted within the center air nozzle in
alignment with the nozzle discharge opening. This electrode is
effective to establish a corona through which the air passes prior
to its discharge from the nozzle. As the air passes through the
corona the gas molecules are ionized and dust particles charged.
These charged particles move under the electrostatic field forces
toward the paint which is at a low potential and are subsequently
deposited on the low-mobility paint particles.
As indicated above, one of the principal advantages of this
invention is that the paint stream itself does not become charged
so that there is at most a negligible charge buildup in any portion
of the paint supply system.
Another objective of the present invention is to provide a charging
circuit which is substantially safer to use than the prior art.
More particularly, in many conventional prior art charging systems
the charging electrode projects forwardly from the end of the spray
gun. This high-potential electrode is thus in an exposed position
in which it can be touched accidentally by a workman or can be
shorted by accidental contact with a grounded conductor. In
contrast, the electrode of the present invention is buried within
the air nozzle where it is effectively shielded from any contact
with either the person using the gun or a grounded surface.
The present gun also embodies a second important safety feature
which automatically prevents the application of a high potential to
the gun in the event that the current-limiting safety resistor is
left out. More particularly, it is common practice to provide a
resistor in series with the electrode and the cable connecting the
power pack to the gun. When a spray gun is overhauled, the resistor
is often removed and at times a workman may neglect to replace it
when reassembling the gun. When such a resistor is omitted the full
potential from the power pack can be capacitively discharged across
the void and applied to the gun electrode. In such a case, if a gun
is brought too close to a grounded article, a spark may occur which
might cause an ignition of the paint material or cause a painful
electrical shock to the operator.
In accordance with the present invention, this is prevented by a
novel safety switch arrangement in which the main power is
automatically cut off whenever the resistor is removed from the
gun.
Another objective of the present invention is to provide a
construction in which the air and electrical conduits are
automatically sealed off when the gun is disassembled by removing
the nozzle assembly. In accordance with the present invention, this
is accomplished by providing a valve seat at the forward end of the
air and electrical conduit in the gun barrel and by providing a
shiftable sleeve having a valve-closing plug at its forward end.
When the gun is disassembled, this sleeve is spring-urged to the
valve closing position to seal off the entrance to the air and
electrical conduit. In so moving, it also cuts off the power
supply.
A still further objective of the present invention is to provide a
novel form of air nozzle for selectively varying the air spray to
change the size of the paint spray pattern.
In a preferred form of nozzle, the angle of air spray is changed by
selectively emitting the air through one of a series of openings in
a nozzle plug. The plug is provided with internal arcuate surfaces
of different radii adjacent to each nozzle opening. We have made
the empirical discovery and determination that air projected from
such a nozzle appears to have somewhat the same properties as air
emitted from a point source which is displaced from the nozzle
opening a distance correlated with the radius of the internal
nozzle surface.
To obtain a fan spray having a smaller included angle, the plug is
shifted to present an opening in which the radius of this internal
surface is greater so that a smaller segment of its periphery is
cut away. This results in an air fan of smaller included angle and
at the same time results in a shifting of the point source away
from the nozzle opening. The net effect, as far as matching the
width of the paint fans, is thus similar to shifting the nozzle
away from the paint film. As a result, although its angle is
smaller, the height of the air fan along the line of impingement of
the paint film remains unchanged. Consequently, neither a large
portion of the air stream is wasted by passing outside the confines
of the paint film, nor are portions of the film improperly atomized
due to the fact that they are not impinged upon by the air stream.
At the same time the distance the air has to travel to reach the
zone of impingement is minimized.
These and other objects and advantages of our invention will be
more readily apparent from a consideration of the following
detailed description of the drawings illustrating the principles
involved in the present method of paint spraying and a preferred
form of apparatus for carrying out the method.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic view of a paint spray system embodying the
present invention.
FIG. 2 is a semidiagrammatic elevational view of a paint spray
issuing under relatively low pressure from a fan nozzle.
FIG. 3 is a cross-sectional view of the paint spray taken along
line 3--3 of FIG. 2.
FIG. 3A is a diagrammatic view plotting the preferred distribution
of mass flow rate from an air nozzle used to atomize a paint spray
film.
FIG. 4 is an enlarged top plan view of a portion of the paint spray
film shown in FIG. 2 illustrating the manner in which the paint
spray film decreases in thickness as it travels away from the
nozzle.
FIG. 5 is a diagrammatic view showing the cooperative relationship
between a paint spray nozzle and an air spray nozzle in carrying
out the present paint spray method.
FIG. 6 is a semidiagrammatic view taken along line 6--6 of FIG.
5.
FIG. 7 is a diagrammatic view showing the relationship of two paint
nozzles and an air nozzle to carry out a preferred method of paint
spraying in accordance with the present invention.
FIG. 8 is a view taken along line 8--8 of FIG. 7.
FIG. 9 is a longitudinal sectional view of a preferred form of
spray gun constructed in accordance with the present invention.
FIG. 10 is an enlarged cross-sectional view taken along line 10--10
of FIG. 9 and with portions rotated 90.degree. for purposes of
clarity.
FIG. 11 is a view taken along line 11--11 of FIG. 10.
FIG. 11A is a front elevational view of the air nozzle plug.
FIG. 12 is a cross-sectional view taken along line 12--12 of FIG.
10.
FIG. 13 is a cross-sectional view taken along line 13--13 of FIG.
10.
FIG. 14 is a cross-sectional view taken along line 14--14 of FIG.
10.
FIG. 15 is a semidiagrammatic view showing the relationship of the
air nozzle and paint film to a target.
FIG. 16 is a diagrammatic view similar to FIG. 15 showing the
manner in which the air nozzle opening and spacing from the paint
film are varied to decrease the width of paint spray.
FIG. 17 is a schematic circuit diagram of the safety circuit.
SYSTEM
FIG. 1 shows a paint spray system 10 for spraying paints and other
coatings in accordance with this invention. As is well recognized
in the art, coating systems of the general type shown are utilized
to apply coatings to many different types of products, such as
automobile parts, furniture, containers, and the like. The coating
materials utilized include not only paints, but also enamels,
lacquers, stains, varnishes, emulsions, waxes, adhesives, and the
like. In the following description the word "paint" will be used in
a very generic sense to encompass all of these various types of
coating materials.
In almost all instances, it is desirable that the paint or other
finish be applied in a smooth, even coating with the particles
deposited on the workpiece being of a small and relatively uniform
size. Paint systems for carrying out the present process include a
spray gun 11 which is supplied with paint by means of a pump 12
which feeds paint under pressure to the gun from a paint supply
tank 13 through a paint tube 14. (Alternatively pump 12 can be
eliminated and tank 13 pressurized). The gun is also supplied with
air under pressure from a conventional compressor or other air
pressure source. The air is applied to the gun through an air tube
15.
As is well known in the art, when spraying some types of products,
for example, the interiors of deep tubular structures, it is
desirable to operate the system as a straight air spray system
without applying any electrostatic field. However, in spraying
other types of articles, it is desirable to charge the paint
particles so that the deposition of the particles is aided by the
presence of a high-electrostatic field. In such instances, the
article being sprayed is normally maintained at ground potential so
that the electrostatically charged paint particles move toward the
article under the influence of the electrostatic field forces and a
high percentage of them are effectively deposited on the article to
be painted.
The present system can be operated either as a straight air spray
gun or as an electrostatic air spray gun. In the event that the gun
11 is to be operated as an electrostatic unit, it is connected to a
power pack, or source of DC voltage, 16. The power pack is
connected to the gun through an electrical cable 17.
Gun 11 may either be a hand-held gun or can be an automatic gun
mounted on a suitable support normally positioned adjacent to a
conveyor line by means of which the articles to be sprayed are
moved past the gun. In either case, and particularly in the case of
hand-held guns, it is desirable that the gun itself be compact and
maneuverable. With either form of gun the paint particles are
atomized by a low-pressure stream of air and are projected
forwardly from the gun in a soft spray of fine particles. A typical
spray pattern of the present system travels only about one-half of
the travel of a conventional air spray gun operated under the same
conditions.
PAINT SPRAYING METHOD
In order to understand various ramifications of the present paint
spraying process, it is desirable to consider the nature of a
fan-shaped paint spray film as it is sprayed from a nozzle under
relatively low air pressure. The pattern of a typical paint film 18
is illustrated in FIG. 2. The film is being emitted from a nozzle
of the flat fan spray-type. Such nozzles are conventionally
utilized in airless spray guns and one form is disclosed in Bede
U.S. Pat. No. 2,754,228. Another form of fan spray nozzle is
disclosed in detail below. The paint film shown in FIG. 2 is being
sprayed under a relatively low pressure, for example, a pressure of
the order of 40-50 p.s.i. as opposed to the normal pressure used in
airless spray guns of the order of 300-1,000 p.s.i.
As shown in FIGS. 2-4, the paint film is discharged from the
orifice in a generally fan-shaped pattern with the upper and lower
edges of the film diverging. The angle of divergence of these upper
and lower edges gradually decreases. The film is not uniform in
cross section. Rather, for a short distance as it leaves the nozzle
20, the film includes a central, relatively uniform web 21 and two
edge enlargements, or streamers 22 and 23. These beadlike streamers
run along the upper and lower edges of the fan and are
substantially thicker than the central web portion 21.
When low hydraulic pressures are used, such a film of viscous
coating material remains relatively stable for a distance of
perhaps one to several inches after it leaves the nozzle. However,
at the end of this stable region the film begins to form transverse
ripples, or waves, 24. As the sheet becomes more, more and more
unstable, the waves 24 eventually disintegrate into ligaments and
droplets 25. However, the streamers persist and strongly resist
disintegration.
In addition to these characteristics, the thickness of the film
decreases progressively as the film leaves the nozzle. This
decrease in film thickness is generally inversely proportional to
the distance from the nozzle so that the most rapid diminution of
sheet thickness occurs in the first fraction of an inch (e.g.,
one-quarter inch) of sheet travel from the nozzle and from that
point on the rate of diminution is substantially smaller.
The broadest aspect of the present paint spraying method is
illustrated in FIGS. 5 and 6. Essentially, we have determined that
very fine paint atomization characterized by low mean particle size
and freedom from random large particles can be obtained by spraying
a fan-shaped film of paint 27 under relatively low pressure, for
example, a pressure of the order of 30-80 p.s.i., from a nozzle 28
and impinging this fan-shaped paint spray with a fan-shaped stream
of air 30 emitted under a very low pressure of the order of 9-45
p.s.i. by an air nozzle 31. When the air stream impinges upon the
paint fan, it exerts a strong shearing force on the film and
effectively atomizes all portions of the paint stream, including
the streamers 32 and 33. The paint stream is deflected and
continues outwardly as a soft spray pattern, or fog, 34 of suitably
atomized paint particles.
In accordance with the present method, the paint is struck by a
relatively small volume of air at a relatively high velocity. We
have empirically determined that optimum atomization of particles
for paint spraying is obtained when the angle x between the paint
spray film and the air spray is made between approximately
30.degree. and 70.degree.. The line of impingement 35 between the
air stream and paint stream is spaced far enough from the nozzle so
that the paint stream has undergone a greater portion of the
attenuation of its thickness. At the same time, the line of
impingement 35 against a stream of the type shown in FIG. 2 should
occur within the region in which the paint film is substantially
planar, i.e., before the transverse waves 24 would commence in the
absence of an air stream.
Next, the length L.sub.A of the path of travel from the air nozzle
31 to the line of impingement 35 should be only a fraction of the
corresponding length L.sub.P of the paint film from the paint
nozzle 28 to the line of impingement 35. We have empirically
determined that the preferred range of ratio of L.sub.A /L.sub.P is
from one-sixth to three-fourths. In one preferred embodiment, the
actual length of paint film L.sub.P is approximately three-eighths
to three-fourths inch, while the actual length of air film L.sub.A
is approximately one-eighth to three-eighths inch. The air stream
is slightly wider (from edge-to-edge) along the line of impingement
35 of the paint film than is the paint film. This facilitates
breaking up and effective atomization of the streamers 32 and 33
which is essential to obtaining a good quality paint spray.
To further improve the atomization of the streamers, the air spray
itself is made nonuniform with the air stream having higher mass
flow rates adjacent its edges, in the areas of the stream which
impinge upon streamers 22 and 23. This is shown diagrammatically in
FIG. 3A in which the mass flow rate of the air is shown as being
substantially greater in the areas 36 and 37 in which the air
stream impinges upon the streamers 22 and 23 than in the central
area 38 of the air stream which impinges upon the thin web portion
21 of the paint film.
In practice, this nonuniform air stream can be obtained by emitting
air from a thin slot which is not of rectangular configuration, but
rather is wider adjacent to the ends. In many cases, however, the
use of such a specially configurated nozzle is not necessary since
we have found that the use of a flat fan nozzle made with a
rectangular gash gives a sufficiently heavy edged airflow to
atomize the heavy edges of the paint sheets.
In carrying out the present process, paint can be applied at a high
rate without adversely affecting the highly effective atomization
achieved. Moreover, this fine and uniform atomization is achieved
while utilizing only a relatively low amount of air in relation to
the quantity of paint. For example, on one typical operation an
acrylic enamel having a viscosity of 22 seconds as measured by a
Zahn No. 2 cup at room temperature was sprayed at a rate of 22
fluid ounces per minute. The paint pressure was approximately 40
p.s.i. and the air pressure was 20 p.s.i. at the gun. The airflow
was at the rate of 8 standard cubic feet per minute so that the
ratio of air-to-paint was 0.36 standard cubic feet of air per fluid
ounce of paint sprayed per minute. Conventional air spray guns
utilize an air-to-paint ratio approximately twice this large and an
air pressure of approximately 75 p.s.i. Consequently, the total
energy of the air utilized in the present process is only a small
fraction of that required in a conventional air gun. This same
relatively low ratio of air volume-to-paint is utilized in the
present method through the entire range of viscosities of coating
materials which are normally applied using the present method.
For example, a very viscous vinyl coating having a viscosity of 40
seconds as measured on a Zahn No. 2 cup at room temperature was
deposited at the rate of 17.2 fluid ounces per minute. The paint
pressure was 45 p.s.i. and the air pressure was 26 p.s.i. at the
gun. The volume of air employed was 9.5 standard cubic feet per
minute. Thus, the ratio of air to ounces of paint was 0.55 standard
cubic feet of air per minute per fluid ounce of paint per
minute.
As another example, a stain having a very low viscosity of 16.5
seconds as measured on a Zahn No. 2 cup at room temperature was
applied at a rate of 24 fluid ounces per minute. Again, the paint
pressure was 45 p.s.i. and the air pressure was 9 p.s.i. at the
gun. The amount of air employed to atomize this stain was 4.5
standard cubic feet per minute or approximately 0.18 standard cubic
feet of air per minute per fluid ounce paint per minute. In each of
the above two examples, the air utilized by a conventional spray
gun was approximately twice the amount utilized in the present
spray gun.
An even more advantageous version of the present paint spraying
process is disclosed in FIGS. 7 and 8. In this preferred method of
spraying paint, two substantially identical fan-shaped sprays of
paint 40 and 41 are emitted from fan spray nozzles 42 and 43. A
single fan of air 44 is emitted from an air nozzle 45. The fan of
air 44 is disposed so that it bisects the angle formed by the two
planar fans of paint 40 and 41. The two fan streams of paint
converge and are impinged by the air stream 44 in an area indicated
generally at 47. Both paint streams are broken up into a fine mist,
or fog, of generally uniform paint particles as indicated in FIG.
8. In fact, the atomization is even more efficient than in the
embodiment disclosed in FIGS. 5 and 6 since the air stream is in
effect trapped between the two fluid streams and its energy is more
effectively utilized in breaking up the paint into minute
droplets.
The same parameters described above in connection with the method
illustrated in FIGS. 5 and 6 are also applicable to the method
disclosed in FIGS. 7 and 8. More particularly, the angles Y and Y'
between the paint and air streams are in the range of from
approximately 30.degree. to approximately 70.degree.. The paint
pressure is from 30 to 80 p.s.i., while the air pressure is from 9
to 45 p.s.i. at the gun. The ratio of the length of air travel
L.sub.A from the nozzle to the zone of impingement compared to the
length of paint travel from the nozzle to the zone of impingement
is approximately in the range of one-sixth to three-fourths. Also,
the actual length of paint travel L.sub.P is of the range of
approximately three-eighths to three-fourths inch, while the actual
length of air film travel L.sub.A is approximately one-eighth to
three-eighths inch. Furthermore, the ratio of volume of air to
quantity of paint applied is the same as that described above. This
ratio is in the range of from 0.2 to 0.6 standard cubic feet of air
per fluid ounce of paint.
The process described above can also be employed to effect a
simultaneous atomizing, mixing and spraying of two different
liquids for coating. More particularly, it has been determined that
one type of liquid can be sprayed from one nozzle and a different
type of liquid can be sprayed from the second nozzle with the
result that the coating deposited on the work is of a uniform blend
of the two coating liquids.
The process of atomization described thus far can be employed by
itself to cause effective application of paint or a similar coating
to a product. However, the atomization process can also be utilized
in connection with a paint charging step to electrostatically spray
paint. In accordance with the present invention, this charging
operation is effected by utilizing the atomizing air as a charge
carrier for transferring charges to the paint particles.
As is shown in FIG. 7, an electrode 50 is disposed in the air
stream preferably within the air nozzle 45. This electrode is
connected to a source of high DC potential, for example, a
potential of the order of 50 kv. It will, of course, be understood
that the potential applied to the electrode 50 can be greater or
less than this figure, depending upon such variables as the
characteristics, e.g., resistivity of the paint being applied,
nature of the article being coated, distance of the gun from the
article, etc.
The potential applied to electrode 50 causes a corona discharge to
be formed surrounding the tip of the electrode. The air stream
passes through this corona on its path through the nozzle and as a
result large numbers of ions are formed in the stream. These ions,
i.e., ionized gas molecules have relatively high mobility and move
primarily under the influence of the electrostatic field toward the
coating material at a relatively low potential. In practice, the
ions thus move toward the paint which is at a low potential and
ultimately attach themselves to the atomized paint particles.
The two paint stream method illustrated in FIGS. 7 and 8 results in
an improved charging efficiency over the one paint stream method
shown in FIG. 5. One possible explanation for this is that the
paint streams envelope the air stream, i.e., are disposed on both
sides of it. Consequently, the ions must pass through a paint
stream or a fog of paint particles in order to reach a ground
surface. In this travel there is a high likelihood that they will
attach themselves to one of the paint particles. In contrast, in
the one paint stream method of FIG. 5, the ions can escape to a
grounded surface without passing either through a paint stream or a
fog of particles, e.g., as shown in FIG. 5 the charged ions could
be attracted to a grounded surface and move off to the right
without ever contacting the paint.
In any event, empirical tests have shown that the two paint fan
electrostatic paint spraying method of FIGS. 7 and 8 as compared to
the one paint fan method of FIG. 5 (even when the air stream of the
one paint fan method is ionized).
Another important facet of the present spraying method involves the
control of the width of the spray pattern. More particularly, it
will be appreciated that articles being sprayed vary in physical
dimensions so that in order to minimize overspray, it is often
desirable to alter the size of the paint spray pattern produced by
the gun. The novel manner in which this is accomplished in the
present invention is diagrammatically shown in FIGS. 15 and 16.
FIG. 15 shows a paint spray directed against a target 55. The
actual pattern 56 of spray has a width W which in actual practice
would be slightly longer than the target 55. We have empirically
determined that the width of the spray pattern can be varied by
changing the included angle S of the air stream without changing
the width of the paint stream.
As shown in FIG. 15, a fan-shaped sheet of air 57 is emitted from a
nozzle 58. The air impinges upon a fan-shaped film of paint 60. It
will be appreciated that this film of paint may be a single film
which is projected at an angle to the air fan as shown in FIGS. 5
and 6, or may in fact be constituted by two paint films as shown in
FIGS. 7 and 8.
In either event, the air is projected from the nozzle in a
fan-shaped stream which is shown as having upper and lower edges 61
and 62 including an angle S such that the air stream just
intercepts the entire section of the film against which the air
stream impinges. Again, it will be appreciated that in actual
practice the angle S would be slightly greater than that shown so
that the air stream passes slightly above and below the film. (This
exact relationship is not shown, however, to simplify the present
explanation.)
In practice, the spacing between the paint film 60 and the target
55 remains substantially constant. Accordingly, when it is desired
to spray a smaller target 55', as shown in FIG. 16, it is desirable
to produce a paint pattern 56' having a smaller width W' without
requiring shifting of the gun. In accordance with the present
invention, this change in the spray pattern is accomplished by
projecting the air fan at a smaller angle S'. It will be
appreciated that if the angle of the air fan in FIG. 15 were
reduced with no further change, the air stream would intersect only
a portion of the paint film 60. This would result in substantially
unatomized, or coarsely atomized, paint and, hence, a defective
finish.
In accordance with the present method, this difficulty is avoided
and the paint pattern width is varied without affecting the uniform
atomization of all of the paint. Specifically, the size of the
paint pattern is altered by making two changes in the air fan. In
the first place, the included angle of the air stream is reduced
from the angle S to the angle S'. In the second place, the internal
configuration of the nozzle opening is changed so that the apparent
focal point, or pressure source point, P of the air stream is
shifted away from the nozzle opening and away from the paint film
to a more remote point as indicated by P'.
More particularly, as shown in FIG. 15, it is apparent that the
nozzle opening is formed by the intersection of two segments of
circular arcs, an external arc 63 and an internal arc 64. In this
example, the arcs are disposed relative to one another so that the
nozzle opening encompasses approximately 180.degree. of the inner
arc 64. This nozzle functions as though the air emanates from a
point source P disposed relatively close to the nozzle opening.
The changed nozzle opening, as shown in FIG. 16, is also formed by
the intersection of outer arc 63' and inner arc 64'. In this case,
the radius of the outer arc 63' is the same as that of arc 63.
However, the radius of the inner arc 64' is appreciably greater
than the arc 64. As a result, the nozzle opening intersects a much
smaller portion of the inner arc 64'. This nozzle functions as
though the air emanates from a point pressure source P' spaced an
appreciable distance from the nozzle opening and from film sheet
60'.
As a result of these concomitant changes, i.e., lessening the
included angle of the air fan while at the same time shifting the
apparent focal point or apparent point source of the air stream
away from the nozzle and paint film, the height of the air fan at
its point of intersection with the film remains constant.
Consequently, by changing the air fan in this manner without
changing any other aspect of the operation of the gun, for example,
the paint pressure or paint sheet size, the paint pattern can be
compacted or enlarged. A preferred form of nozzle construction for
effecting these changes in a simple manner is illustrated in FIGS.
10-14, and the details of construction of this nozzle are explained
below.
SPRAY GUN
The details of construction of a preferred form of spray gun for
carrying out the present method of paint spraying are shown in
FIGS. 9-14. As there shown, the spray gun 11 comprises a hollow
handle portion 66, a barrel or extension portion 67 formed of a
suitable insulating material and mounted forwardly of the handle
and a nozzle assembly 68 mounted on the forward end of barrel
67.
The nozzle assembly 68 includes two paint nozzles 70 and 71
effective to project flat fan-shaped sprays of paint toward the
center line of the gun as indicated by dotted lines 72 and 73. The
gun further includes an air nozzle 74 effective to project a
fan-shaped stream of air forwardly along the axis of the gun toward
the area of intersection of the paint streams as indicated by line
75. Additionally, the gun includes an electrostatic charging
circuit including a needle electrode 76 for charging the atomizing
air and ultimately causing a charge to be applied to the atomized
paint particles.
Operation of the gun is controlled by a single trigger 77 which is
effective to control flow of paint through a paint valve 78, flow
of air through air valve 80, and application of electric potential
to electrode 76 through actuation of switches 81 and 82. The
trigger functions to first open air valve 80, then open paint valve
78 and finally close switch 81. When the trigger is released this
sequence is reversed.
More particularly, handle 66 is formed of a suitable conductive
material, such as aluminum or the like. The handle is hollow and is
configurated to form a hand grip portion 83, a forwardly extending
stock portion 84 and a suspension hook portion 85. A transverse
flange is formed at the forward end of the stock for abutment with
a mating flange on the barrel. The barrel and stock flanges are
bolted together in a conventional manner. Hand grip portion 83 is
provided adjacent its lower end with an opening 86 for receiving
the combined air and electrical input fitting 87.
Air is introduced to the gun through an air tube 15 which surrounds
electrical cable 17, there being an annular space between the tube
wall and cable for passage of air from the compressor to the inlet
fitting of the gun. The hollow interior of the gun is sealed at the
cable entrance in any suitable manner, such as by means of an
O-ring 90 compressed between a shoulder in the gun housing and a
threaded cap member 91. Air passes through an inlet passageway 92
formed in the handle and enters a valve chamber 93 of air valve
80.
This valve includes a reciprocating stem member 94 having an
extension 95 disposed for abutment with trigger member 77. An
intermediate portion of the stem carries an O-ring 96 or similar
packing member disposed between spaced shoulder 97. The inner end
of the stem carries a tapered plug 98 adapted to engage a tapered
seat 100 to seal off the air flow. When the trigger is depressed,
plug 98 is shifted against the force of spring 99 from engagement
with seat 100 and air is free to flow into internal chamber 101 in
the gun handle.
This chamber communicates with a longitudinal bore 102 in the stock
and a continuation of this bore 103 formed in the barrel member.
This chamber surrounds a sleeve 104 which is formed of insulating
material and is supported in mounting spiders 105 and 106. The
forward end of bore 103 communicates with a longitudinal bore 107
formed in nozzle support member 108 which carries the paint and air
nozzles. Nozzle support member 108 is preferably formed of an
insulating material such as "Delrin." Support member 108 is mounted
on the front end of barrel member 67 by means of a threaded cap 109
which is coupled to the mounting member by a retaining ring 119
fitted in opposed grooves in the cap and mounting member. Cap 109
engages threaded extension 129 on the barrel.
Support member 108 is provided with a circular plug-receiving
opening 110. This opening 110 houses a generally cylindrical air
nozzle plug 111 which is preferably made of a plastic material,
e.g., Delrin. Nozzle plug 111 includes an annular peripheral wall
112 having an arcuate inlet opening 113 which communicates with a
vertical central chamber 114. The lower end of the plug is closed
by a solid circular wall 115, while the top of the plug is closed
by a cap 116 which is press fit or otherwise secured to the top of
the plug. The cap seals off the upper end of chamber 114 and
includes an outwardly extending peripheral flange 117 having a
transverse slot 118 for rotating the cap and plug member.
The peripheral wall of the plug member opposite entrance slot 113
is provided with three parallel spaced, rectangular openings 120,
121 and 122. The plug can be rotated to selectively align any one
of these openings with front opening 123 in member 108. The
selection of the particular nozzle opening 120, 121 and 122
disposed in registry with front opening 123 of the stationary
support member 108 provides the means for varying the air stream to
vary the paint spray pattern in the manner shown diagrammatically
in FIGS. 15 and 16.
More particularly, opening 120 is effective to provide the widest
angle of air spray fan and, hence, the widest paint spray pattern.
Similarly, opening 121 is effective to provide the narrowest air
spray fan and, hence, the narrowest paint spray pattern. Nozzle
opening 122 is effective to provide an air spray fan and paint
spray pattern of intermediate size.
It will be noted that nozzle opening 120 is formed by a
semicylindrical internal cut 124 formed in peripheral wall 112, the
axis of cut 124 being horizontal. This cut is intersected by a
segmental disclike cut 125 oriented in a vertical plane and
extending inwardly from the outer periphery of wall 112 internally
along a radius of the plug member.
In one preferred embodiment of the plug member, the disclike cut
125 has a 0.375 -inch radius with a center located 0.178 inch
outwardly from the periphery of the plug wall. Internal cut 124 has
a 7/64 -inch radius located inwardly 0.169 inch from the periphery
of the plug. This nozzle opening has been found effective to emit a
fan-shaped stream of air having an included angle of approximately
94.degree.. Hydraulically, this nozzle opening functions generally
as though there were a point source of gas pressure located at
point P1 approximately one-fourth inch inwardly from the periphery
of the plug member.
Slot 121 is formed by an internal semicylindrical cut 126 formed in
peripheral wall 112 along a horizontal axis. Cut 126 is intersected
by a horizontal disclike cut 127. Disclike cut 127 extends radially
inwardly from the outer periphery of wall 112. In one preferred
embodiment of plug, the disclike cut 127 has a 0.375-inch radius
with the center located 0.219 inch outwardly from the periphery of
the plug wall.
Internal cut 126 has a radius of 0.203 inch with the center of the
cut being located 0.263 inch from the periphery of the plug. This
nozzle opening has been found to emit a fan-shaped stream of air
having an included angle of approximately 36.degree.. This nozzle
functions generally as though the air emitted from it emanated from
a point source P3 disposed substantially further from the
peripheral wall than point P1 (e.g., approximately four times as
far).
In order to maintain the volume of air emitted from nozzle openings
120 and 121 substantially equal, the width of nozzle opening 121 is
made slightly narrower than the width of nozzle opening 120 so that
the cross-sectional area of the opening is constant. Thus, in one
preferred embodiment, the width of nozzle opening 120 is made equal
to 0.074 inch, while the width of nozzle opening 121 is made equal
to 0.065 inch.
The third, or intermediate size, nozzle opening 122 is similarly
formed, i.e., this nozzle opening is formed from a segment of a
cylindrically internal cut 128 intersected by a vertical disclike
cut 130, disclike cut 130 extending inwardly along a radius from
the outer peripheral wall 112. In one preferred embodiment, the cut
130 has a 0.375-inch radius with a center located 0.168 inch from
the periphery of the plug. Inner cut 128 has a radius of 0.156 inch
located 0.216 inch from the periphery of the disc. This nozzle
opening emits a fan-shaped stream of air having an included angle
of approximately 64.degree.. Air is emitted from this nozzle in
generally the same manner as though the air flowed from a point
source P2 located of the order of twice the distance from the
periphery of the plug as point P1. In this preferred embodiment the
pressure of the air applied to the gun is relatively low, for
example, of the order of 9-45 p.s.i. at the gun. The exit velocity
of the air is approximately sonic. The distance, in the preferred
gun, of air travel L.sub.A to the point of impingement of the air
and paint streams is approximately three-sixteenths inch.
Paint is supplied to the gun from pump 12 through tube 14. This
tube is coupled to the gun through fitting 131 which communicates
with a longitudinal paint passage 132 formed in extension member
67. Passage 132 communicates with the seat area 133 of paint valve
78. This seat area is engaged by a spherical head 134 of valve stem
member 135. This valve stem member reciprocates longitudinally to
shift the head 134 toward and away from the seat member and is
spring-urged toward the seat by means of a compression spring
136.
The valve is adapted to be opened by pressure on trigger 77. When
the trigger is pivoted toward the handle member about pivot pin 137
which supports the upper end of the trigger, the trigger engages a
shoulder 138 secured to stem 135, forcing the stem rearwardly to
open the paint valve. With paint valve 78 open, paint is free to
flow through longitudinal conduit 140 into annular passageway 141
and from this passageway into axial angulated bores 142 and 143
formed in nozzle support member 108. These bores communicate with
paint nozzle assemblies 70 and 71.
More particularly, bores 142 and 143 are aligned with inlets 144
and 145 in cylindrical members 146 and 147. Each of these cylinders
is formed with an internal passageway 148 and a seat 150 for a
check valve formed by ball member 151. Each of the balls 151 is
spring-urged against the seat by means of a spring 152 bearing
against a thrust washer 153 mounted within a chamber 154 in
cylindrical members 146, 147. Each of the chambers opens into an
axial passageway 155 communicating with an axial bore 156 in
tubular nozzle member 157. Each of the nozzle members is preferably
formed of a suitable plastic material, such as Delrin.
The fluid discharge passageway in each of the nozzles is formed by
a V-shaped cut 158 extending inwardly from a periphery of the wall
to approximately the centerline of the tube. Each of the V-shaped
cuts 158 includes a rearward wall 160 and a forward wall 161. As
shown in FIG. 10, each nozzle 157 is angulated, or tilted, slightly
outwardly relative to the axis of passageway 155. This tilting
enables the nozzle to direct the paint stream at the desired angle
relative to the air fan.
More particularly, each nozzle 157 is effective to emit a thin
substantially planar fan-shaped stream of paint at an angle of
approximately 60.degree. to the axis of bore 155 which is parallel
to the axis of the gun. Consequently, this fan-shaped paint stream
is impinged by the fan-shaped air stream at approximately the same
angle, i.e., at approximately 60.degree.. In this preferred
embodiment, the length of paint travel L.sub.P from the nozzle to
the zone of impingement of the paint and air streams is
approximately five-eighths inch.
In accordance with the present invention, the ends of axial bores
156 in each of the nozzle tubes 157 is closed by a spring-loaded
plug 159. These plugs are secured to angulated spring members 162
in any suitable manner; for example, as shown in FIG. 10, plug 159
passes through a small bore in a portion of arm 162 and has an
overturned head 163 effective to clamp the plug to the arm. Both
plugs 159 and spring members 162 are preferably formed of a
suitable plastic material. Arms 162 include a transverse section
164 and a foot portion 165 which is mounted upon the support member
108 in any suitable manner, such as by means of bolts 166.
Each of the spring members 162 also includes an overturned flange
portion 167 by means of which the spring arm can be pulled
outwardly to disengage the plug from the seat formed in the end of
the associated tube member 157. In normal operation, the spring
force is sufficient to maintain the plug in sealing engagement with
the end of the tube 157. However, in the event that the nozzle
should become clogged, the passage can readily be cleared by
grasping flange portion 163 to pull the spring arm and plug
outwardly, opening the end of the tube. Trigger 77 is then
depressed to cause paint to be discharged from the end of the tube
removing whatever impediment had accumulated in the tube
section.
The electrical input to the gun is preferably through a
high-resistance cable 17. The details of construction of one
preferred form of cable are disclosed in Rosen U.S. Pat. No.
3,348,186. The remote end of this cable is connected to a power
pack 16 effective to develop a high DC potential, for example,
50,000 volts. Cable 17 enters the gun through the interior of
fitting 87 and passes upwardly through a hollow section 170 of the
handle. The cable is flexible and is bent rearwardly around
shoulder 171 at the junction of the hand grip portion 83 and the
stock portion 84 toward the rear wall 172 of the stock. The cable
is then bent forwardly and is inserted into insulating sleeve 104,
the cable being loosely received in the sleeve so that it is free
to move relative thereto. The forward end of the cable is provided
with a contact button 173 disposed for mating contact with a
similar contact button 174 provided on the rear end of cylindrical
resistor 175.
This resistor is preferably sealed in the end of sleeve 104 and has
an electrical resistance of the order of 75 megohms. The forward
end of resistor 175 is in electrical contact with a bolt 176
passing inwardly through a cap member 177 enclosing the forward end
of sleeve 104.
The head 180 of bolt 176 is engaged by a contact spring 178 which
is compressed between the head of the bolt and a head 181 formed on
electrode 76. Electrode 76 is in the form of an elongated pin
supported in axial alignment with the air nozzle by means of a
cruciform insulating member 182. This cruciform insulating member
includes a central bore 183 into which the pin is press fit. The
cruciform member is rigidly mounted in the central opening 107 in
support member 108.
Electrode 76 extends forwardly through entrance opening 113 in the
air nozzle member and extends forwardly to a position just inside
the peripheral wall 112 carrying the three nozzle openings 120,
121, and 122. In normal operation, this electrode is charged to a
suitable potential of the order, for example, of 40,000 volts. A
corona is established surrounding the tip of the electrode. The
stream of air flowing through the nozzle surrounds the electrode
and passes through the corona prior to the time it is discharged
from the nozzle opening. As the air passes through the corona gas
molecules are ionized and dust particles are charged. These ions
and charged particles move forwardly under the influence of the
electrostatic field forces toward a lower potential region of the
field constituted by the paint films and atomized particles. There
the charges attach themselves to the paint particles to form the
charged paint spray.
One of the advantages of this paint charging device is that the
paint supply in conduit 14 does not become charged so that the
entire paint supply system, including tube 14, pump 12 and the
paint supply tank 13, remain at or very near ground potential. In
fact, in the embodiment shown the paint supply system is grounded
through needle 135 and handle member 66 which is conventionally
grounded in use. This greatly simplifies the problem of insulating
the paint supply system and eliminates the dangers common in many
prior art systems due to charge accumulation in the paint supply
system.
In addition to the elements described above, the electrical
charging circuit includes a safety circuit for preventing
application of power to the cable in the event that resistor 175 is
removed from the gun and the gun reassembled without replacing the
resistor.
More particularly, as is best shown in FIG. 9, the cable 17 forms a
rearwardly extending resilient bend 190 adjacent to rear wall 172
of the stock. An actuating arm 191 is pivoted about pin 192 in the
stock portion of the gun and is interposed between cable bend 190
and rear wall 172. This arm includes a forwardly extending link
portion 189 which is attached to one end of tension spring 193, the
other end of the spring being secured to a stationary bracket
194.
Bracket 194 carries snap-action microswitches 81 and 82.
Microswitch 82 is normally closed, while microswitch 81 is normally
open. A spring arm 195 is disposed for engagement with actuating
plunger 196 of microswitch 82. This spring arm is adapted to be
shifted with link 189, for example, by engagement with a shoulder
197 carried by the link.
So long as the cable 17 is positioned rearwardly as shown in FIG.
9, plunger 196 is not depressed and switch 82 remains closed. In
normal operation of the gun, the cable is held in this position
against forward movement by the presence of resistor 175. However,
if resistor 175 and sleeve 104 are not present in the gun, spring
193 acting upon arm 191 forces the cable forwardly within conduit
103. This forward movement of arm 191 causes arm 195 to open switch
82.
As is shown in FIG. 17, switch 82 is connected in series with
switch 81 and a coil 198 of relay 200. Switch 81 is normally open
switch adapted to be closed upon actuation of trigger 77. More
particularly, microswitch 81 includes an actuating plunger 201
adapted to be engaged by a spring arm 202. This arm in turn is
positioned for contact with a head 203 formed on the continuation
of stem member 135.
When trigger 77 is depressed, the stem and its extension are
shifted rearwardly so that head 203 bends spring arm 202, closing
microswitch 81. When both microswitch 81 and 82 are closed, relay
200 is energized to complete a circuit to power pack 16. This power
pack is then effective to apply a potential to cable 17 and, hence,
to electrode 76.
The safety circuit just described is also effective to prevent
application of power to the cable when the nozzle assembly is
removed from the gun. More particularly, as is shown in FIG. 9,
tube 104 is supported in the surrounding bores in the stock member
and barrel by means of low friction spiders 105 and 106. The tube
is normally retained in its retracted position shown in FIG. 9 by
the abutment of spider 182 with the end of cap member 177. However,
when the nozzle mounting member 108 is removed by unthreading cap
109 from the threaded extension 129 on the barrel member, the
restraint on forward movement of the sleeve is removed. The sleeve
is then shifted forwardly under the influence of spring 193 acting
on the cable member.
As the cable member is shifted to the left, arm 191 is shifted
forwardly causing arm 195 to open switch 82 preventing energization
of the power pack and thereby preventing application of potential
to cable 17. At the same time, forward movement of sleeve 104 is
effective to provide a second function in that it automatically
seals the air chamber and end of the resistor or electrical conduit
from contamination. More particularly, as best shown in FIGS. 9 and
10, the forward end of cap 177 is tapered to form a seat 204. This
seat is effective to engage a mating tapered seat 205 formed in the
barrel 67 when sleeve 104 is shifted forwardly, i.e., to the left
in FIG. 10.
When the nozzle is reassembled, spider 182 engages cap 177 forcing
the cap away from the seat 205 to reopen the air conduit. At the
same time, the cable 17 is shifted rearwardly so that bend 190
causes arm 191 to pivot, thereby reclosing switch 82 to permit
reenergization of the power pack and cable 17 when trigger 77 is
depressed.
The construction of the gun as described above also incorporates
two additional safety features. In the first place, switches 81 and
82 and the other electrical contacts such as those between the
cable and resistor occur in an enclosed or pressurized
explosiveproof chamber. Moreover, this chamber is continuously
purged to remove any accumulated gases by the flow of the air
utilized to atomize the paint.
From the foregoing disclosure of the general principles of the
present invention and the above description of a preferred
embodiment, those skilled in the art will readily comprehend
various modifications to which this invention is susceptible.
For example, while the air fan and paint fan streams have been
described as being substantially planar, it is contemplated that
the paint fans may be slightly arcuate, i.e., in the form of thin
bowed sheets. Moreover, if a gun is to be utilized for atomizing,
mixing and spraying two different liquids by spraying one liquid
from one nozzle and another liquid from the other nozzle, it is
apparent that a separate supply line must be provided for each
nozzle. This can readily be accomplished by essentially duplicating
the liquid supply system shown while eliminating the conduit which
now interconnects the two nozzles. Also while air has been
described as the atomizing gas, it will be appreciated that in some
particular installations other gases could be employed.
Accordingly, the term "air" as used in the following claims should
be interpreted as including other gases as well.
Accordingly, we desire to be limited only by the scope of the
following claims in which it is to be understood that the term
"paint" is used in the generic sense to cover finish coatings of
the type described above and in which the term "gun" is used
generically to cover either a hand spray gun or a mechanically
supported control spray device.
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