U.S. patent number 3,672,569 [Application Number 05/055,539] was granted by the patent office on 1972-06-27 for electrostatic spray gun nozzle and air cap.
This patent grant is currently assigned to Graco Inc.. Invention is credited to Louis J. Bentsen, Robert G. Smead.
United States Patent |
3,672,569 |
Smead , et al. |
June 27, 1972 |
ELECTROSTATIC SPRAY GUN NOZZLE AND AIR CAP
Abstract
In an electrostatic spray gun, the nozzle serves as the
electrostatic charging electrode with a recessed forwardly facing
surface that joins a first sharp annular edge adjacent the coating
material ejected from the nozzle and a second annular sharp edge
adjacent the ejected atomizing air. An air cap surrounding the
nozzle provides an annular parallel air ejection passage.
Preferably, the sharp edges of the nozzle protrude slightly
forwardly of the termination of the air ejection passage formed by
the air cap.
Inventors: |
Smead; Robert G. (St. Charles,
IL), Bentsen; Louis J. (Arlington Heights, IL) |
Assignee: |
Graco Inc. (Minneapolis,
MN)
|
Family
ID: |
21998526 |
Appl.
No.: |
05/055,539 |
Filed: |
July 16, 1970 |
Current U.S.
Class: |
239/705;
239/704 |
Current CPC
Class: |
B05B
5/03 (20130101) |
Current International
Class: |
B05B
5/025 (20060101); B05B 5/03 (20060101); B05b
005/02 () |
Field of
Search: |
;239/3,15 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Wood, Jr.; M. Henson
Assistant Examiner: Mar; Michael Y.
Claims
We claim:
1. An electrostatic spray gun comprising:
an electrically conductive nozzle having a center line through
which coating material is ejected, said nozzle having an internal
orifice surface symmetrically positioned around the nozzle center
line and terminating at a first plane perpendicular to said center
line, an outer surface symmetrically positioned around the nozzle
center line and terminating substantially at said perpendicular
plane, and a recessed forward facing surface joining said internal
orifice surface and joining said outer surface to form inner and
outer sharp edges therebetween, and
an air cap through which gas is ejected having an internal orifice
surface surrounding said nozzle and substantially parallel to said
center line, said surface terminating in a second plane
perpendicular to said center line.
2. An electrostatic spray gun as specified in claim 1, wherein said
nozzle outer sharp edge protrudes forwardly of said second
plane.
3. An electrostatic spray gun as specified in claim 1, wherein said
nozzle inner sharp edge protrudes forwardly of said second
plane.
4. An electrostatic spray gun as specific in claim 1, wherein said
first plane is positioned at least three thousands of an inch
forwardly of said second plane.
5. An electrostatic spray gun as specified in claim 1, wherein said
first plane is positioned not more than ten thousands of an inch
forwardly of said second plane.
6. An electrostatic spray gun as specified in claim 1, wherein said
air cap is composed of electrically non-conductive material.
7. An electrostatic spray gun comprising:
an electrically conductive nozzle having a center line through
which coating material is ejected, said nozzle having an internal
orifice surface symmetrically positioned around the nozzle center
line and terminating at a first ionizing means, an outer surface
symmetrically positioned around the nozzle center line and
terminating at a second ionizing means, and a recessed forward
facing surface formed by joining said internal and external
surfaces, and
an air cap through which gas is ejected having an internal orifice
surrounding said nozzle and substantially parallel to said center
line, said surface terminating in a plane perpendicular to said
center line.
8. The electrostatic spray gun of claim 7, wherein the first and
second ionizing means are sharp edges.
9. The electrostatic spray gun of claim 8, wherein both of the
sharp-edged ionizing means protrude forward of the air cap.
10. The electrostatic spray gun of claim 8, wherein the air cap is
composed of electrically non-conductive material.
11. The electrostatic spray gun of claim 9, wherein the protruding
sharp edges extend at least three-thousandths of an inch forward of
the air cap.
12. The electrostatic spray gun of claim 9, wherein the protruding
sharp edges extend not more than ten-thousandths of an inch forward
of the air cap.
Description
This invention relates to electrostatic coating systems and more
particularly to nozzles for ejecting coating material wherein the
nozzle structure also serves as the electrostatic ionizing
electrode.
In the past entire metal spray guns have been charged as the
ionizing electrode in electrostatic deposition coating systems.
Many other electrostatic deposition coating systems utilized
separate sharp pointed ionizing electrodes which were either
physically removed from the spray guns or were mounted on the spray
gun structure. In air atomizing electrostatic deposition systems it
has been preferable to have the nozzle for ejecting coating
material also serve as the charging electrode. Such constructions
are disclosed in U.S. Pat. No. 3,056,557 issued Oct. 2, 1962, U.S.
Pat. No. 3,251,551 issued May 17, 1966 and U.S. Pat. No. 3,268,171
issued Aug. 23, 1966. The first of these patents disclosed a nozzle
having a right angle sharp edge between the internal orifice
surface and a flat forwardly facing surface and a further sharp
edge between the forward facing surface and an outer surface of the
nozzle. The last two patents disclosed a sharp edge formed at the
termination of the inner orifice surface by an acute angle with an
outer nozzle surface. A co-pending U.S. application Ser. No.
665,104 filed Sept. 1, 1967, now U.S. Pat. No. 3,641,971 discloses
a sharp edge orifice electrode wherein the sharp edge is not in
direct contact with coating material as it is ejected from the
orifice. In conventional non-electrostatic coating systems
utilizing air atomization, it has been found desirable to eject air
from an annular passage which is parallel to a coating material
ejection passage surrounded by the air passage. The sudden
expansion of the air upon ejection from the passage running
parallel to the coating material ejection passage efficiently
atomizes the ejected coating material. The present invention
provides for an ionizing sharp edge adjacent the flow of ejected
coating material and a second sharp edge immediately adjacent the
ejected and rapidly expanding atomizing air. These two sharp
annular edges are provided by recessing the forwardly facing
surface of the nozzle between the termination of the coating
material ejection passage and the termination of the air ejection
passage.
It is therefore an object of the present invention to provide new
and improved nozzles and air caps for electrostatic coating
systems.
A principal object of the present invention is to provide a nozzle
for an electrostatic spray gun which has a sharp edge formed at the
terminus of the coating material ejection passage and a second
sharp edge formed at the terminus of the air ejection passage with
both sharp edges lying substantially in the same plane which is
perpendicular to a center line of the nozzle.
A further object of the present invention is to provide an air cap
for the nozzle described in the previous paragraph wherein air is
ejected in a direction substantially parallel to the direction of
flow of the ejected coating material and the terminus of that
passage determined by the position of the air cap is in a second
plane perpendicular to the center line of the nozzle with the sharp
edges of the nozzle protruding slightly forwardly of the second
plane.
Further objects and advantages will become apparent from the
following detailed description taken in connection with the
accompanying drawings, in which:
FIG. 1 is a sectional elevational view of a preferred embodiment of
the invention;
FIG. 2 is an enlarged sectional elevational view of the forward
portion of the preferred embodiment illustrated in FIG. 1;
FIG. 3 is a further enlarged portion of the nozzle of the preferred
embodiments of the invention illustrated in FIGS. 1 and 2; and
FIG. 4 is an enlarged sectional drawing of a modified form of the
nozzle portion illustrated in FIG. 3.
Although this invention is subsistable of embodiment of many
different forms there is shown in the drawings and will herein be
described in detail, embodiments of the invention with the
understanding that the present disclosures are to be considered as
exemplifications of the principles of the invention and are not
intended to limit the invention to the embodiments illustrated. The
scope of the invention will be pointed out in the appended
claims.
Referring now to FIGS. 1 and 2, a spray gun 10 comprises a metallic
rear housing 11 and a barrel portion of nylon insulating material,
generally indicated at 12, which comprises a first barrel section
20 threaded to a second barrel section 21 and an air cap 22 secured
to the second barrel section 21 by a threaded annular retainer 23.
The barrel section 20 is secured to section 11 by an annular
fitting 24 to which an annular retainer 25 is secured by threads,
and an annular cylinder 26 which is threaded onto section 20 and
has an annular edge which contacts an annular edge of the retainer
25. A stainless steel valve 27 closes a passage 28 through the
barrel portion 12 and the rear housing 11 when it engages an
annular tapering valve seat 29 in a fluid tip 30. The valve 27 is
connected to a stainless steel operating shaft 31. Shaft 31 is
connected to an adjustable needle plunger (not shown) which has a
spring urging it towards the forward portion of the spray gun.
Coating material from a source (not shown) enters an aperture 32
that connects with the passage 28 containing the shaft 31 in order
that coating material may flow around the shaft to the fluid tip
30. Thus, the coating material is conducted directly around the
shafts which operate the valve in the front of the gun to the valve
27 within the fluid tip 30. The fluid tip 30 is secured between the
air cap 22 and a fluid tip housing 33. Sealing rings 34, 35 and 36
prevent fluid from the passage 28 from leaking past the fluid tip
housing 33. The fluid tip 30 and the fluid tip housing 33 are
constructed of steel. A high voltage electric lead (not shown) is
connected to metallic rear housing 11. The air hose (not shown) is
connected to rear housing 11 to supply air to the passages
generally indicated at 37 and 38. The passage 37 supplies air to
two air horns 39 and 40 which form part of the air cap 22, and the
passage 38 supplies air to an annular air passage 41 in the fluid
tip 30.
A nozzle 50 which forms the forward portion of the fluid tip 30
forms an electrode.
The length of the overall barrel 12 is normally of the order of 18
inches which prevents any substantial electrostatic field from
existing between the charged rear housing 11 and a grounded
product. The strongest electrostatic field exists between nozzle 50
toward a grounded product.
A pilot bushing 42 aids in completing the passage 28 across the
junction barrel sections 20 and 21 and a cylinder adapter 43 aids
in completing this passage across the junction of the rear housing
11 and the barrel section 20.
Referring now to the enlarged nozzle and air cap sections of the
embodiment illustrated in FIGS. 2 and 3, the nozzle 50 has a center
line 51. An internal orifice surface 52 is symmetrically positioned
around the orifice center line 51. The internal orifice 52
terminates forwardly at a plane 53 perpendicular to the center line
51. An outer surface 54 of the nozzle 50 is symmetrically
positioned above the nozzle center line 51 and terminates at the
plane 53. A recessed forward facing surface 55 joins the internal
orifice surface 52 and the nozzle outer surface 54 to form two
sharp annular edges 56 and 57. In the embodiment illustrated in
FIGS. 1-3, the recessed forward facing surface 55 is a V-shaped
groove.
The nozzle 50 which is a portion of fluid tip 30, housing 33 and
shaft 31, being constructed of electrically conductive material,
electrically connect the source of electrical potential to the
sharp edges 56 and 57 to provide the ionizing electrode.
Atomizing gas, which is in most instances air, is ejected by an
annular passage 58 formed by the nozzle outer surface 54 and an
internal orifice surface 59 of the air cap 22. The air cap internal
orifice surface 59 surrounds the nozzle 50 and is parallel to the
center line 51. It terminates at a second plane 60 perpendicular to
the center line 51.
The air cap 22 may be made of electrically conductive material, but
it is preferably constructed of electrically non-conductive
material to make the nozzle 50 essentially an ionizing point source
as is more fully disclosed in the U.S. Pat. No. 3,251,551 issued
May 17, 1960.
The sharp edges 56 and 57 protrude slightly forward of the end of
the air (gas) passage 58 at plane 60. This protrusion of the sharp
edges 56 and 57 at plane 53 beyond plane 60 should be at least
0.003 inch. The sharp edges could be in the same plane 60 as the
termination of the passage 58 which is the point of rapid expansion
of the ejected air, but if they do not project at least 0.003 inch
the pressure of the expanding air creates a back pressure against
the pressure of the coating material being ejected from the nozzle
50.
The sharp edges 56 and 57 are preferably located in the plane 53
rather than having one slightly project forwardly of the other in
order to provide a recessed forwardly facing surface 55 which is
equivalent to a flat surface for atomization. The expanding air
expands across the forward facing surface 55 in essentially the
same manner as if the surface was not recessed. Such a spacing is
necessary to allow for efficient expansion of the air before the
molecules of expanding air impinge upon the molecules of the solid
stream of ejected coating material from the nozzle 50. However,
from an ionizing standpoint the expanding air passes over the sharp
ionizing edge 57 while the fluid being ejected passes over the
sharp ionizing edge 56. Although it is not absolutely necessary
that both sharp edges terminate in the exact same plane, it is
preferable that both project forwardly of the plane 60.
The edges 56 and 57 may project substantially beyond the terminus
of the air passage 58 at plane 60 and provide an efficiently
operating spray gun. However, it has been found to be desirable to
limit the distance between the sharp edges 56 and 57 to 0.010 inch
beyond the terminus of the air passage 58 at plane 60. Thus the
most preferred range of projection of the edges 56 and 57, that is,
the spacing between plane 53 and plane 60 is between three
thousands and ten thousands of an inch. However, as aforementioned,
the edges 56 and 57 may be flush with the air cap 22 or may
protrude far beyond ten thousands of an inch without departing from
the basic principle of the present invention.
Referring now to FIG. 4 a portion of a fluid tip, 30a is
illustrated which has the exact construction of the fluid tip 30
except that a forward facing surface 100 is concave rather than
being V-shaped. The corresponding elements of the fluid tip 30a in
FIG. 4 have the same numerals applied as are used in FIGS. 1-3 with
the addition of a subscript a. The fluid tip 30a with its nozzle
50a is completely interchangeable in the spray gun 10 with the
fluid tip 30 having its nozzle 50. The V-shaped groove 55 is
simpler to produce from a manufacturing standpoint, but the concave
shaped groove 100, has the advantage of producing sharper edges 56a
and 57a than the corresponding sharp edges 56 and 57.
* * * * *