U.S. patent number RE31,590 [Application Number 06/250,095] was granted by the patent office on 1984-05-29 for atomization in electrostatic coating.
This patent grant is currently assigned to Ransburg Japan, Ltd.. Invention is credited to Michio Mitsui.
United States Patent |
RE31,590 |
Mitsui |
May 29, 1984 |
**Please see images for:
( Certificate of Correction ) ** |
Atomization in electrostatic coating
Abstract
A method of atomizing liquid paint using a rotating atomizing
device and electrostatically coating an article with a smooth
homogeneous film of paint and without the generation of foam or
other surface irregularities on the article being coated, wherein
an electrostatic field is established between the peripheral edge
of the rotating atomizing device and the article to be coated and
the liquid paint flows toward the edge of the atomizing device as a
continuous thin film, which film is formed into a circumferential
series of branch flows of narrow width flowing in the peripheral
direction of the atomizing edge, and the liquid paint is atomized
from the series of branch flows as they are projected beyond the
edge of the atomizing device. The rotary atomizing device may be in
the form of a bell or disk and includes a plurality of shallow
grooves near its periphery preferably extending radially and of
increasing depth in the direction of paint flow and terminating at
the discharge edge.
Inventors: |
Mitsui; Michio (Yokohama,
JP) |
Assignee: |
Ransburg Japan, Ltd. (Tokyo,
JP)
|
Family
ID: |
26347865 |
Appl.
No.: |
06/250,095 |
Filed: |
April 1, 1981 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
872066 |
Jan 25, 1978 |
04148932 |
Apr 10, 1979 |
|
|
Foreign Application Priority Data
|
|
|
|
|
Feb 7, 1977 [JP] |
|
|
52/12286 |
May 31, 1977 [JP] |
|
|
52/63872 |
|
Current U.S.
Class: |
427/484; 239/703;
118/626 |
Current CPC
Class: |
B05B
5/0407 (20130101) |
Current International
Class: |
B05B
5/04 (20060101); B05D 001/04 (); B05B 005/04 () |
Field of
Search: |
;427/31 ;118/626
;239/3,700,703,224,223 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
165258 |
|
Dec 1904 |
|
DE2 |
|
P2095 |
|
0000 |
|
DE |
|
884326 |
|
Jul 1953 |
|
DE |
|
32362 |
|
Dec 1955 |
|
DE |
|
973478 |
|
Mar 1960 |
|
DE |
|
975380 |
|
Nov 1961 |
|
DE |
|
1652390 |
|
Jul 1970 |
|
DE |
|
982327 |
|
Jan 1951 |
|
FR |
|
326665 |
|
Dec 1957 |
|
CH |
|
429275 |
|
May 1935 |
|
GB |
|
665655 |
|
Jan 1952 |
|
GB |
|
710920 |
|
Jun 1954 |
|
GB |
|
846181 |
|
Aug 1960 |
|
GB |
|
1198946 |
|
Jul 1970 |
|
GB |
|
1515511 |
|
Jun 1978 |
|
GB |
|
Other References
Gebhardt "Protection Against Corrosion by Metal Finishing" from
Oberflachenbehandlung als Korrosionsschutz edited by N. Ibl et al,
Forster-Verlag AG Zurich, 1967, pp. 297-303. .
Adler et al, "Chem. Eng. Prog.", vol. 47, No. 10, pp. 515-522, Oct.
1951; vol. 47, No. 12, pp. 601-608, Dec. 1951..
|
Primary Examiner: Newsome; John H.
Attorney, Agent or Firm: Willian, Brinks, Olds, Hofer,
Gilson & Lione, Ltd.
Claims
We claim:
1. A method of atomizing a liquid paint using a rotating atomizing
device and electrostatically coating an article with a smooth
homogeneous film of liquid paint, wherein an electrostatic field is
established between the peripheral edge of the rotating atomizing
device and the article to be coated and the liquid paint flows
toward the edge of the atomizing device as a continuous thin film,
characterized in substantially reducing the thickness of the paint
film as it reaches the peripheral edge by flowing the film over a
series of circumferentially spaced recessed grooves which run
substantially in the direction of paint flow and terminate at the
discharge edge, where said grooves extend into the peripheral edge
of the device, and atomizing the liquid paint film as it is
projected beyond the peripheral edge.
2. A method of atomizing liquid paint according to claim 1 wherein
the thickness of the paint film reaching the peripheral edge does
not exceed 100 microns.
3. A method of atomizing a liquid paint using a rotating atomizing
device and electrostatically coating an article with a smooth
homogeneous film of liquid paint, wherein an electrostatic field is
established between the peripheral edge of the rotating atomizing
device and the article to be coated and the liquid paint flows
toward the edge of the atomizing device as a continuous thin film,
characterized by providing adjacent the peripheral edge
circumferentially spaced, recessed grooves which extend into the
peripheral edge of the device to form the thin flowing film of
liquid paint into a series of branch flows of narrow width flowing
in the peripheral direction of the edge of the atomizing device,
and atomizing the liquid paint from the series of branch flows as
the paint is projected beyond the edge of the atomizing device.
4. A method of atomizing liquid paint according to claim 3 wherein
the thickness of the branch flows reaching the peripheral edge does
not exceed 100 microns. .Iadd. 5. A method of atomizing a liquid
and electrostatically spray coating the surface of an article with
the atomized liquid comprising:
feeding liquid at a controlled rate from a source to an atomizing
means having a surface effective during rotation of said atomizing
means for supporting a film of liquid to be atomized, and a
circular discharge edge adjacent the liquid film support
surface;
rotating said atomizing means such that the liquid fed to said
atomizing means is formed into a substantially uniform film of
liquid on the liquid film support surface;
flowing the film of liquid through a plurality of grooves in the
liquid film support surface aligned generally in the direction of
liquid flow and extending into the discharge edge such that a
series of independent streams of the liquid are formed by said
grooves, said streams being uniformly spaced circumferentially
adjacent the discharge edge, and terminating in strands of liquid
that extend beyond the discharge edge and produce a spray of finely
divided discrete particles; and
establishing between said discharge edge and the article an
electric field of sufficient strength to draw the particles away
from the atomizing means
toward the article. .Iaddend. .Iadd. 6. A method of atomizing a
liquid and electrostatically spray coating the surface of an
article with the atomized liquid comprising:
feeding liquid at a controlled rate from a source to an atomizing
means having a surface effective during rotation of said atomizing
means for supporting a film of liquid to be atomized, and a
circular discharge edge adjacent the liquid film support-surface
having a flat surface generally perpendicular to said liquid film
support surface;
rotating said atomizing means such that the liquid fed to said
atomizing means is formed into a substantially uniform film of
liquid on the liquid film support surface;
flowing the film of liquid through a series of grooves in the
liquid film support surface aligned generally in the direction of
liquid flow, terminating at the discharge edge, and opening into
the perpendicular surface, such that a series of independent
streams of the liquid are formed by said grooves, said streams
being uniformly spaced circumferentially adjacent the discharge
edge, and producing a spray of finely divided discrete particles;
and
establishing between said discharge edge and the article an
electric field of sufficient strength to draw the particles away
from the atomizing means toward the article. .Iaddend. .Iadd. 7.
The method of claims 5 or 6 wherein said atomizing means is a
generally bell-shaped atomizing device having a diameter of between
about 4 and about 10 centimeters, the discharge edge has a
thickness at between about 0.2 and about 1.0 millimeters, each of
said grooves has a maximum depth of between about 0.1 and about 0.4
millimeters, each of said grooves has a length of between about 1.0
and about 10 millimeters, and said grooves have a pitch between
about 0.2 and about 1.0 millimeters. .Iaddend..Iadd. 8. The method
of claims 5 or 6 wherein said atomizing means is a generally
disk-shaped atomizing device having a diameter of between about 10
and about 64 centimeters, the discharge edge has a thickness of
between about 0.2 and about 4 millimeters, each of said grooves has
a maximum depth of between about 0.1 and about 3 millimeters, each
of said grooves has a length of between about 1.0 and about 15
millimeters, and said grooves have a pitch of between about 0.2 and
about 3 millimeters. .Iaddend. .Iadd. 9. The method of claim 6,
wherein said atomizing means has a diameter of between about 4 and
about 64 centimeters, the discharge edge has a thickness of between
about 0.2 and about 4 millimeters, said grooves have a pitch of
between about 0.2 and about 3 millimeters, and each of said grooves
has a maximum depth of between about 0.1 and about 3 millimeters
and a length of between about 1.0 and about 15 millimeters.
.Iaddend. .Iadd. 10. A method of atomizing a liquid and
electrostatically spray coating the surface of an article with the
atomized liquid comprising:
feeding liquid at a controlled rate from a source to an atomizing
means havng a surface effective during rotation of said atomizing
means for supporting a film of liquid to be atomized, and a
circular discharge edge adjacent the liquid film support
surface;
rotating said atomizing means such that the liquid fed to said
atomizing means is formed into a substantially uniform film of
liquid on the liquid film support surface;
flowing the film of liquid through a plurality of grooves in the
liquid film support surface aligned generally in the direction of
liquid flow and having increasing depth in the direction of liquid
flow towards the discharge edge such that a series of independent
streams of the liquid are formed by said grooves, said streams
being uniformly spaced circumferentially adjacent the discharge
edge, and terminating in strands of liquid that extend beyond the
discharge edge and produce a spray of finely divided discrete
particles; and
establishing between said discharge edge and the article an
electric field of sufficient strength to draw the particles away
from the atomizing means
toward the article. .Iaddend..Iadd. 11. A method of atomizing a
liquid and electrostatically spray coating the surface of an
article with the atomized liquid comprising:
feeding liquid at a controlled rate from a source to an atomizing
means having a surface effective during rotation of said atomizing
means for supporting a film of liquid to be atomized, and a
circular discharge end adjacent the liquid film support surface
having a surface inclined with respect to said liquid film support
surface;
rotating said atomizing means such that the liquid fed to said
atomizing means is formed into a film of liquid on the liquid film
support surface;
flowing the film of liquid through a series of grooves in the
liquid film support surface extending into the discharge end
surface, such that a series of independent streams of the liquid
are formed by said grooves, said streams being spaced along the
discharge end surface at positions corresponding to the positions
of said grooves and producing a spray of finely divided discrete
particles; and
establishing between said discharge end and the article an electric
field of sufficient strength to draw the particles away from the
atomizing means toward the article. .Iaddend..Iadd. 12. A method of
atomizing a liquid and electrostatically spray coating the surface
of an article with the atomized liquid comprising:
feeding liquid at a controlled rate from a source to an atomizing
means having a surface effective during rotation of said atomizing
means for supporting a film of liquid to be atomized, and a
circular discharge end adjacent the liquid film support
surface;
rotating said atomizing means such that the liquid fed to said
atomizing means is formed into a film of liquid on the liquid film
support surface;
flowing the film of liquid through a plurality of grooves in the
liquid film support surface extending into the discharge end such
that a series of independent streams of the liquid are formed by
said grooves, said streams being spaced along the discharge end at
positions corresponding to the positions of said grooves, and
terminating in strands of liquid that extend beyond the discharge
end and produce a spray of finely divided discrete particles;
and
establishing between said discharge end and the article an electric
field of sufficient strength to draw the particles away from the
atomizing means
toward the article. .Iaddend..Iadd. 13. A method of atomizing a
liquid and electrostatically spray coating the surface of an
article with the atomized liquid comprising:
feeding liquid at a controlled rate from a source to an atomizing
means having a surface effective during rotation of said atomizing
means for supporting a film of liquid to be atomized, and a
circular discharge end adjacent the liquid film support surface
having a surface inclined with respect to said liquid film support
surface;
rotating said atomizing means such that the liquid fed to said
atomizing means is formed into a film of liquid on the liquid film
support surface;
flowing the film of liquid through a series of grooves in the
liquid film support surface extending into the discharge end
surface, such that a series of independent streams of the liquid
are formed by said grooves, said streams being spaced along the
discharge end surface at positions corresponding to the positions
of said grooves, and producing a spray of finely divided discrete
particles, each of said grooves producing one of said liquid
streams; and
establishing between said discharge end and the article an electric
field of sufficient strength to draw the particles away from the
atomizing means toward the article. .Iaddend.
Description
This invention relates to a method of electrostatically atomizing a
liquid paint and performing the electrostatic coating of an article
to be coated by the use of a rotary atomizing device, especially a
rotary atomizing device rotated at a high speed, said method
preventing the formation of foam on the paint film applied to the
article, whereby a high quality coating is obtained. The present
invention utilizes bell and disk type rotary atomizing devices for
electrostatic coating.
There has been an increasing trend in recent years toward the use
of liquid paints having a small solvent content and a relatively
high viscosity for the purpose of preventing environmental
pollution. To satisfactorily atomize a liquid paint of a relatively
high viscosity using a rotary atomizing device, however, it is
often necessary to rotate the rotary atomizing device at a
considerably higher rotational speed.
In atomizing a liquid paint using a rotary atomizing device, the
degree of atomization of the paint is generally in inverse
proportion to the thickness of the film of the liquid paint that is
led in the state of a thin film to the circular discharge edge
along the surface of the rotary atomizing device. On the other
hand, film thickness is proportional to the quantity of the paint
discharged and inversely proportional to the product of the
rotational frequency of the rotary atomizing device and the radius
of the circular discharge edge.
For this reason, when use is made of a compact rotary atomizing
device in which either the radius of the device or that of the
circular discharge edge is reduced so as to reduce the size and
weight of the device, it is necessary to significantly increase the
rotational frequency of the device during the atomization of even a
liquid paint of a relatively low viscosity in order to obtain
satisfactory atomization of the liquid paint, or to reduce the
thickness of the liquid paint film supplied to the circular
discharge edge.
However, when the rotational frequency of the rotary atomizing
device exceeds 4000 rpm during the electrostatic coating, a large
number of bubbles may form on the surface of the paint film applied
to the article being coated, depending upon the kind of the liquid
paint used, the discharge quantity of the paint per unit time, and
so forth. The bubbles deteriorate the quality of the resulting
coating, and excessive foaming can completely spoil the coated
article itself.
It is therefore an object of the present invention to provide a
method of electrostatic coating using a rotary atomizing device
which prevents the occurrence of foam or other imperfections on a
paint film applied to the surface of an article so as to provide a
high-quality coating, irrespective of the rotational frequency of
the rotary atomizing device, the kind of the liquid paint used, the
discharge quantity of the paint per unit time, and the like. It is
another object of the present invention to utilize bell type and
disk type rotary atomizing devices which prevent the development of
foam on the paint film and enable electrostatic coating to be
performed in a satisfactory manner.
The invention may best be understood by referring to the following
description and accompanying drawings.
In the drawings:
FIG. 1 illustrates a cusp formation and configuration adjacent the
circular discharge edge of a rotary atomizing device;
FIG. 2 illustrates a method of electrostatic coating and a rotary
atomizing device for practicing the method;
FIG. 3 is a sectional side view illustrating an embodiment of a
rotary atomizing device;
FIG. 4 is a sectional side view illustrating an embodiment of a
rotary atomizing device;
FIGS. 5, 6 and 7a-c are fragmentary sectional side elevational
views illustrating various construction details of rotary atomizing
devices;
FIGS. 8a-d are fragmentary end elevational views illustrating
various construction details of rotary atomizing devices; and,
FIG. 9 is a graph illustrating distribution of atomized paint
droplet diameters according to the present method and apparatus and
according to a prior art method and apparatus.
Various factors have been pointed out as the causes of foaming on
paint films. The inventors of this invention have assumed that the
important factors are the physical conditions of the liquid paint
when it is being led to the circular discharge edge along the
surface of the rapidly rotating rotary atomizing device, and when
it is discharged from the discharge edge and atomized. On the basis
of this assumption and in order to clarify the factors involved in
foaming, the inventors have taken a number of stroboscopic pictures
of the state of the liquid paint on the surface of the rotary
atomizing device and the conditions under which the liquid paint is
discharged and atomized.
As a result, the present inventors have discovered that when the
electrostatic atomization of the paint is normally carried out by
the rotary atomizing device, the liquid paint flows toward the
circular discharge edge having a knife edge-shaped section to the
outside in an axial direction (in the case of the bell type device)
or in the radial direction (in the case of the disk type device),
thereupon forming a number of so-called "cusps" (liquid strands).
Due to the action of the electrostatic field generated by high DC
voltage applied between the discharge edge and the article for
coating, atomization is attained by a small amount of the liquid
paint at the tip of each cusp being separated and removed and
formed into a fine droplet.
However, under the condition where the rotary atomizing device is
rotated at a high speed and a number of air bubbles form on the
paint film applied to the surface of the article, atomization of
the paint by the release of minute paint droplets from the tip of
each of a large number of cusps formed along the whole periphery of
the circular discharge edge is not attained. On the contrary, the
inventors have discovered as shown in FIG. 1, there is formed a
liquid film 3 composed of a number of irregular triangles that have
a considerable width and extend from the periphery beyond the
entire circumference of the circular discharge edge 2 of the rotary
atomizing device 1 towards the flared forward portion to the
outside or towards the flared outward portion. The outer periphery
4 of this liquid film 3 is extremely unstable and interacts with
the ambient air due to the high speed rotation of the rotary
atomizing device.
While the film 3 is thus turned over and twisted and draws in the
air due to the interaction, it is acted upon by the electrostatic
field whereby its outer periphery 4 is torn off and aggregates in
spherical form, thus forming a number of paint droplets 5 each of
which entraps trace amounts of air. It has been found by the
inventors that these air-entrapping paint droplets 5 are admixed
and released together with ordinary paint droplets 6.
It is therefore believed that the development of foam on the paint
film on the surface of the article coated by electrostatic coating
using a rapidly rotating rotary atomizing device is primarily
caused by the fact that a number of air-entrapping paint droplets 5
are attracted to the article to be coated by the action of the
electrostatic field, attach to the surface of the article and form
the paint film with entrapped air.
In order to prevent the formation of the air-entrapping paint
droplets arising from the torn outer periphery of the irregular
triangular liquid film, the inventors have experimented with a bell
type rotary atomizing device having a number of triangular
protuberances along the circumference of the circular discharge
edge, as disclosed in Japanese Patent Publication No. 1266/1961. It
was found that when the paint has a relatively low viscosity and is
discharged in small quantities, a substantially triangularly shaped
liquid film is supported by each triangular protuberance.
Accordingly, the outer periphery of each liquid film forms a cusp
from the apex of the triangular protuberance or from the outer
periphery along two sides thereof and atomization of paint is
effected from the tip of the cusp.
However, when the viscosity of the liquid paint and the quantity
discharged exceed certain critical values (e.g., a discharge rate
of about 200 cc/min. at a viscosity of 30 sec/Zahn cup No. 2 and a
discharge rate of about 300 cc/min. at a viscosity of 25 sec/Zahn
cup No. 2), it has been found that the liquid films span adjacent
pairs of the triangular protuberances, and the outer periphery of
each liquid film is turned over or twisted due to its interaction
with the electrostatic field and forms air-entrapping paint
droplets which result in the development of air bubbles or foam on
the liquid paint film on the article being coated.
Moreover, it has been confirmed that since the above-described bell
type rotary atomizing device having a number of the triangular
protuberances provided over the entire discharge edge has a number
of apexes where there is a high concentration of an electric field,
the potential gradient increases to a dangerous extent so that the
device cannot be safely used.
Accordingly, the inventors have carried out intensive research in
quest of a method of preventing the formation of the
above-mentioned irregular liquid films on the circular discharge
edge of a rapidly rotated rotary atomizing device to eliminate the
development of foam on the deposited paint film. As a result, the
inventors have perfected a method of atomizing liquid paint using
an electrostatically charged rotary atomizing device in which the
liquid paint led in the form of a thin continuous film along one
surface of the rotary atomizing device, for example, the internal
surface of a bell shaped atomizer or one surface of a disk shaped
atomizer, is formed into a multiplicity of narrow branching streams
separated from one another in the circumferential direction of the
rotary atomizer 1 as schematically illustrated in FIG. 2.
When the liquid paint supplied in this manner to the entire
circumference of the discharge edge 2 in the form of a multiplicity
of narrow film-like branching streams reaches the discharge edge 2,
it does not form a liquid film extending beyond the discharge edge
towards the flared forward portion to the outside or towards the
flared outside portion as shown in FIG. 1, but forms cusps 7 in the
form of fine strands corresponding to each of the film-like
branched streams which extend beyond the discharge end 2. The tip
of each cusp is atomized and released as a fine droplet 6 which has
not entrapped air. The droplet is then drawn by electrostatic force
to coat the article. Thus, it is possible to prevent the
development of foam on the paint film applied to the surface of the
article.
According to the present invention, the continuous thin film along
one surface of the rotary atomizing device may be formed into a
number of narrow film-like branching streams 6 by a variety of
means. One very effective means is to provide a number of shallow
grooves, e.g., thin triangular grooves 8 as illustrated, on the
surface to which the liquid paint is led in the state of a thin
film, that is, on the circumferential wall surface of the internal
cavity of the bell type atomizer or on one surface of the disk type
atomizer, whereby the grooves 8 reaching the discharge edge, extend
substantially in the same direction as the advancing direction of
the flow of the liquid paint, i.e., substantially in the axial
direction for the bell atomizer and substantially in the radial
direction for the disk atomizer.
In a rotary atomizing device rotated at speeds ranging from 4,000
to 16,000 rpm, the thickness of the liquid paint flowing along the
surface of the device is generally on the order of about several
tens of microns but does not exceed 100 microns when the discharge
rate ranges from about 50 to 500 cc/min. By forming each of the
grooves 8 to a depth of from 0.2 to 0.4 mm, the flowing film of
liquid paint is divided into film-like branching flows mutually
spaced in the circumferential direction by said grooves. A length
of from 1.5 to about 4 mm is usually sufficient for each
groove.
FIG. 3 is a sectional side view showing one embodiment of a bell
type rotary atomizing device produced in accordance with our
invention. The rotary atomizing device comprises a boss 12 fitted
to the forward end of a rotary shaft 11 of a rotary driving device
(not shown) capable of high speed rotation at from about 10,000 to
16,000 rpm, such as a pneumatic motor, a disk 13 coaxially coupled
to the forward edge of the boss, a cylinder 14 coaxially and
rearwardly extending from the circumference of the disk 13, a hub
member 16 secured to the rotary shaft 11 by a clamping nut 15, and
a bell type paint atomizing member 20 which includes an open
internal cavity 17 having a circular section and a circular
discharge edge 18 having a knife edge-like forward end. The
atomizing member 20 is coaxially fitted to the outside of the
cylinder 14 of the hub 16 and secured thereto by a lock nut 19.
The liquid paint from a suitable supply source (not shown) through
a supply pipe 21 into the gap between the boss 12 of the hub 16 and
the cylinder 14 is supplied, dve to the high speed rotation of the
device; to the rear end portion of the internal cavity 17 through a
plurality of paint apertures 22 provided at the forward end portion
of the cylinder 14, and led as thin film having a thickness of
about 0.1 mm along the cicumferential wall 23 of the internal
cavity.
Along the forward portion of internal cavity 17 are formed a number
of grooves 8 each having a length of about 1.5 mm and a maximum
depth of about 0.2 to 0.3 mm as the grooves reach the discharge
edge 18. These grooves 8 may be formed by knurling using a knurling
tool.
The grooves 8 divide the paint film as described above so that at
the discharge edge 18 the paint is atomized by the action of the
electrostatic field generated by a high DC voltage, e.g. from about
80 to 120 KV, impressed between the discharge edge 18 and an
article to be coated (not shown) and electrostatically deposited
onto the surface of the article.
When the rotary atomizing device has the above-described
construction, having a circular discharge edge with a diameter of
7.3 cm, and operated at a high speed, say at 16,000 rpm, using a
liquid paint having a high viscosity of 30 seconds on a Zahn cup
No. 2 and a paint discharge rate from about 150 cc/min. to about
500 cc/min., the development of foam is completely prevented on the
paint film and a high-quality coating is obtained.
In order to ascertain the effect of the grooves 8 on the dark
current, experiments to measure the dark currents were made on a
bell type rotary atomizing device according to the present
invention and also the prior art. The term "dark current" means the
total current flow, usually expressed in microamperes, which is
expended by the high voltage painting system. The device used for
our experiment had the construction of FIG. 3, including a large
number of grooves having a length of about 1.5 mm and a maximum
depth of about 0.2 to 0.3 mm. The device of the prior art also used
for the experiment has the same shape and size as those of the
device shown in FIG. 3 but was not provided with the grooves 8.
When liquid paint is atomized into minute droplets and sprayed onto
an article, the quality of the paint film or coating on the article
depends largely upon the maximum and average diameters of the
atomized paint droplets. Large maximum diameter droplets lower the
quality of the coating film according to the following empirically
accepted relationship between maximum particle diameter and paint
film quality:
______________________________________ Maximum particle diameter
Quality of paint film ______________________________________
100-200 microns (.mu.) Excellent 200-300 microns Good 300-450
microns Rather poor over 450 microns Poor
______________________________________
To form a paint film of excellent quality it is necessary that the
atomized paint have small maximum and average droplet diameters.
However, atomized paint containing a large amount of droplets of
extremely small diameters is not particularly good because the
solvent for the paint evaporates quickly from droplets of extremely
small diameter as they move toward the article to be coated. As a
result, the substantially solidified resin and pigment causes a
reduction in paint film quality. It is instead desirable that the
maximum droplet diameter of the atomized paint be adjusted to a
small value, for example, a value in the above-mentioned range of
100 to 200.mu., and that the diameters of most all the droplets be
adjusted to similar values.
In using conventional rotary atomizing devices for electrostatic
coating, the diameters of the atomized paint droplets may vary to a
great extent depending upon various factors such as the kind of
resin used, the kind of solvent, the kind of pigment, the viscosity
of paint at the time of use, the electrical resistance and the
discharge rate thereof, the diameter and rotational speed of the
atomizing device, and the value of the DC voltage applied between
the rotary atomizer and article to be coated.
In the case of water based paint and the so-called high-solids
paint having a low volatile content which have come to be used in
large quantities in recent years for the prevention of
environmental pollution, it is often difficult or impossible to
obtain atomized paint droplets having the desirable diameters. Even
in the case of the ordinary synthetic paints of various types used
in many industrial fields, it is sometimes impossible to obtain
atomized paint droplets having the desirable diameters.
The diameters of droplets of liquid paint atomized by a rotary
atomizing device used for electrostatic coating are determined by
the number and thickness of the cusps (liquid threads) formed at
the discharge edge of the atomizing device. The paint droplet
diameter is large when the number of cusps is small and cusp
thickness large, and the paint droplets have small diameters when
the number of cusps is large and cusp thickness small. In general,
the thickness of the cusps is influenced by the thickness of the
paint film at the discharge edge, as expressed by the following
formula: ##EQU1##
To more readily achieve the desired maximum and average diameters
of the paint particles we have found that the rotary atomizing
device, rather than possessing the more conventional sharp or
rounded forward edge, should have its forward or discharge end
possess a narrow uniform width generally perpendicular to the
surface over which the paint flows. A multiplicity of shallow
grooves of gradually increasing depth should be provided along the
inner peripheral surface over which the paint flows. By use of the
foregoing construction, alternative forms of which are shown in
FIGS. 4, 5, 6, 7 and 8, the length of the inner peripheral surface
of the discharge end of the rotary atomizing device is remarkably
increased as compared with conventional rotary devices.
Consequently the circumference of the paint film as it is supplied
to the discharge end of the atomizing device is greatly increased
and the thickness of the paint film is thereby reduced
considerably. As a result the number of cusps formed increases and
the diameter of these cusps becomes smaller. Accordingly, atomized
paint droplets having a small maximum diameter and a narrow
distribution of droplet diameters are discharged in a stable
condition from the entire circumference of the circular end with a
resulting improvement in the quality of the paint film deposited on
the article.
The dark current was measured for each of these two devices, by
using a plate-like opposed electrode and a needle-like opposed
electrode of 0.7 mm diameter respectively, and varying the distance
D between the device and the grounded electrode and also the DC
voltage V to be impressed on the device, in which the quantity of
the discharged paint is zero (where the dark current is larger than
in the state of the paint being discharged).
The results are illustrated in the Table below. This confirms that
the increase in the dark current due to the provision of the
grooves is extremely small and therefore does not pose any
operational hazard.
______________________________________ Experimental Results of Dark
Current Measurement Voltage V -90 KV -120 KV Current Out Prior Out
Prior Electrode Distance D Invention Art Invention Art
______________________________________ Plate 20 cm 210 .mu.A 200
.mu.A 440 .mu.A 420 .mu.A Electrode 25 cm 170 .mu.A 160 .mu.A 320
.mu.A 310 .mu.A 30 cm 120 .mu.A 120 .mu.A 280 .mu.A 270 .mu.A
Needle 20 cm 250 .mu.A 230 .mu.A 700 .mu.A 700 .mu.A Electrode 25
cm 170 .mu.A 160 .mu.A 420 .mu.A 420 .mu.A 30 cm 120 .mu.A 120
.mu.A 320 .mu.A 310 .mu.A
______________________________________
FIG. 4 is a side elevational view in cross section of a small
rotary atomizing device constructed according to the present
invention. This device comprises a hub member 36 including a boss
32 fitted on the front portion of a rotary shaft 31 of a rotary
driving means (not shown) such as an air motor rotatable at high
speed, for example, 10,000 to 18,000 rpm, a disk portion 33
coaxially connected to the front end of boss portion 32, and a
cylindrical portion 34 coaxially extended from the peripheral
portion of the disk portion 33, which hub member 36 is fixedly
mounted on rotary shaft 31 with a nut 35; and a small diameter
paint atomizing bell 39 having a circular cross section and
provided with a cavity 37 the front end of which is opened and a
circular discharge end 38 surrounding the opening of cavity 37.
Bell 39 is connected to hub 36 by coaxially securing the rear end
portion of the bell 39 on the outer surface of cylindrical portion
34 of hub 36 by a set-screw 40. A liquid paint supplied from a
suitable paint supply source (not shown) into an annular chamber
42, which is defined by boss 32 and cylindrical portion 34 of hub
36, through a paint feed pipe 41 flows, by high-speed rotation
driven by rotary shaft 31, into the rear end portion of cavity 37
in bell 39 through a plurality of apertures 43 provided in the wall
of cylindrical portion 34 and directed along the inner surface 44
of cavity 37 to the discharge end 38 in the form of thin film the
thickness of which is usually less than about 0.1 mm. The paint
film thus directed to discharge end 38 is atomized by the
electrostatic field created between discharge end 38 and an article
(not shown) to be coated by a high DC voltage of, for example,
between 80 and 120 KV applied between bell 39 and the article by a
suitable high DC voltage source (not shown), and the resulting
atomized paint is electrostatically deposited onto the surface of
the article.
The circular discharge end 38 has a narrow end surface 45 of
uniform width substantially at right angles to the peripheral or
front end portion of inner surface 44 defining cavity 37 shown in
FIG. 5. The front portion of inner surface 44 is provided with a
multiplicity of grooves 46 extending in the direction of the flow
of liquid paint along the inner surface 44, and these grooves 46
are close to one another with the distances between the center
lines thereof being substantially the same, the outer ends of the
grooves 46 being open at discharged end surface 45. The grooves 46
may be of an optional elongated shape in plan but are preferably of
such a shape that the width and depth are gradually increased from
the inner end to the outer end thereof, for example, an elongated
V-shape (refer to FIG. 7a), an elongated U-shape (refer to FIG. 7b)
and an elongated V-shape having a curved or arc-shaped central line
(refer to FIG. 7c). The grooves 46 may be of shapes in cross
section as may be understood from FIGS. 8a, 8b, 8c and 8d, such as
a shape of V (refer to FIGS. 8a and 8c), a shape of U (refer to
FIG. 8b) or a trapezoidal shape (refer to FIG. 8d). The grooves 46
may be made so that their depth is unvaried but they are preferably
made so their depth is gradually increased from their inner to
outer end.
FIG. 6 is an enlarged side view in cross section of the peripheral
portion of a paint atomization and discharge disk 47, constructed
according to the present invention. In this device, the circular
discharge end is also so formed that it has a narrow end surface 45
of uniform width which is at right angles to the inner surface 48
of disk 47 or the surface along which a liquid paint flows toward
the discharge end. The peripheral portion of inner surface 48 is
provided with a multiplicity of grooves 46 extending substantially
in the radial direction and closely spaced at regular intervals
with the outer ends thereof opened at end surface 45.
The following are examples of rotary atomizing devices which
achieve the objects of the present invention, with numerical values
for the width b of the end surface 45 of the circular discharge
end, depth d of the outer end portion of grooves 46 opened at end
surface 45, pitch P or distance between the central lines of
grooves 46, and length l of grooves 46.
EXAMPLE I
A small paint atomization bell having a diameter of 4 to 10 cm:
Width b of end surface of discharge end: 0.2-1.0 mm
Depth d of outer end portion of grooves: 0.1-0.4 mm
Pitch P of grooves: 0.2-1.0 mm
Length l of grooves: 1.0-10 mm
EXAMPLE II
A bell-shaped or disk-type paint atomization device having a
diameter of 10 to 64 cm:
Width b of end surface of discharge end: 0.2-4 mm
Depth d of outer end portion of grooves: 0.1-3 mm
Pitch P of grooves: 0.2-3 mm
Length l of grooves: 1.0-15 mm
In the above examples, the thickness of paint film supplied to
discharge end along the inner surface of paint atomization and
discharge member is usually several tens of microns but does not
exceed 100 microns.
Experiments were conducted using a rotary atomizing bell 39 as
shown in FIG. 4 having a diameter of about 7.3 cm (27/8 in.),
having a discharge end 38 and end surface 45 of a width b of 1.0
mm. The grooves 46 were shaped in plane and cross section as shown
in FIGS. 7a and 8a with a depth d of 0.1 to 0.4 mm, a pitch P of
1.0 mm and a length l of 5 mm. A DC voltage of 90 KV was applied
between discharge end 38 and the article to be coated and the
revolutions of bell 39 were varied from 7000 to 18,000 rpm. The
results showed that, when various kinds of paint having viscosities
of 20.degree. C. of from 15 to 50 seconds on a Zahn cup No. 2, are
subjected to atomization at paint discharge rates of from 50 to 700
cc/minute, fine atomized paint droplets having a maximum diameter
of less than 200.mu. and a narrow distribution of diameters or a
substantially uniform diameter are obtained.
Curve I shown in FIG. 9 shows the distribution of atomized paint
droplets obtained by using the bell 39 referred to above rotating
at 16,000 rpm, and using paint having a viscosity at 20.degree. C.
of 25 seconds on a Zahn cup No. 2 at a paint discharge rate of 450
cc/minute. Curve I shows an average droplet diameter of about
100.mu. and a variation in droplet diameters of about 20.mu..
Curve II shows an average droplet diameter of about 150.mu. and a
variation in droplet diameters of about 60.mu. which represents the
distribution of diameters of atomized paint droplets obtained under
the same conditions as mentioned above except that a conventional
rotary atomizing bell is used of the same diameter as mentioned
above, but which has an annular knife edge-like discharge end and
no grooves in the inner peripheral surface of the bell. By
comparing Curve I with Curve II, it is readily seen that the
present invention produces an excellent improvement compared with a
conventional rotary atomizing device.
The above are the explanations about a specific embodiment of the
present invention but the present invention is not limied to the
above embodiment. The present invention includes, of course,
various kinds of changes and modifications which are within the
spirit thereof.
* * * * *