Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
'7~1
Background of the Invention
The present invention relates to shroud air
generating structures for spray coating apparatus of
the type which comprises a centrifugal spra~er member
or head to which a rapid rotation is imparted, and in
particular to a shroud air generating structure which
provides a very even and uniform volumetric distribution
of air across the rotary head to aid in movement of
spray particles toward articles to be coated.
Electrostatic spray coating systems have been
successfully used in commerce in several different forms.
For example, systems have been used which utilize
spraying or atomizing devices employing hydraulic forces
or air as the atomizing mèdium. In another type of
system, to which the present invention relates, atomization
is accomplished by means o a centrifugal sprayer member
or head to which a rapid rotation is imparted. The
head has a surface against which a stream or jet of
liquid to be sprayed is directed, the liquid on striking
the revolving surface progressing radially outwardly
thereover in a thin film under centrifugal force toward
a sharp annular peripheral edge of the head, where~t it
is divlded into fine particles so that it leaves the
periphery in the form of a spray. For electrostatic
deposition of coating material the rotàry head is made
o a conductive material and connected to a high d.c.
potential, so that the spray particles on moving past
the peripheral edge of the head are charged to a high
electrostatic poten~ial. The resulting ionized or
electrostatically charged cloud of particles is then
attracted to and settles on the surfaces of articles
,. , ~ .
--2--
,
11'7'7;~41
,
or ware to be coated, which àre usually maintained at
ground potential.
Spray heads of such electrostatic coating apparatus
have heretofore been rotated at relatively low speeds,
e.g., on the order of 3,000 to 6,000 rpm. However,
recent developments contemplate rotating the heads at
considerably greater speeds on th order of 30,000 to
60,000 rpm, which enhances both the atomization and
flow rate of coa~ing material sprayed from the head.
Obviously, at such high speeds of rotation spray
particles leaving the peripheral edge of the head are
flung at significant speeds generally radially of the
head and parallel to ware to be coated, so that if
electrostatic forces alone were relied upon to move
the particles to the ware a substantial portion of
the particles would not reach the ware. Consequently,
the art also contemplates generating an annular shroud
o air for movement around, across and forwardly of
the rotary head to aid in moving electrostatically
~harged spray particles toward the ware.
To generate the shroud air, there is conventionally
provided an annular manifold behind and around the
rotary head. The manifold has a plurality of outlet
passages arranged in an annular array, and air introduced
into the manifold exits through the passages to form
a shroud of air around the head. In an attempt to
obtain a uniform layer or distribution of shroud air~
thereby to enhance uniformity of deposition of coating
material on ware, the outlet passages are closely spaced
and of relatively small diameter. Unfortunately, despite
the close spacings and small diameters of the passages,
the shroud air pattern i5 nevertheless nonuniform in
that is is comprised of a plurality of discrete jets
or streams of air. Also, in consequent of the small
passage diameters, the same are prone to clogging by
spray particles or contaminants in the air supply,
which results in further nonuniformities in the shroud
air pattern and the need to frequently clear the
passages. For high volume production lines, this often
yields a product output which is less than desirable
or optimal.
Objects of the Invention
An object of the present invention is to provide
an improved structure for generating shroud air for a
rotary head electrostatic spray coating system.
Another object of the invention is to provide a
structure which generates a very unifoxm distribution
of shroud air.
A further obiect of the invention is to provide
such a shroud air generating structure which is not
subject to blocking or clogging by spray particles or
contaminants in the air supply.
Summa ~ e Invention
In accordance with the present invention, there is
provided in combination with a rotary head electxostatic
spray coating system, of the type to which a rapid
ro$ation is imparted to a rotary spray head and a jet
of coating material to be sprayed directed thereagainst
for discharge from a peripheral edge thereof in a spray,
a structure for generating a uniform and annular layer
or curtain of shroud air for movement around, across
and foxwardly of the rotary head to aid eleotxostatic
forces in moving spray particles toward articles to be
coated. In one facet, said shroud air generating structure
4~
comprises a manifold having an air inlet for receiving
air under pressure and a plurality of air outlets in
an annular array for providing therethrough the curtain
of shroud air. Said outlets have unequal spacings
therebetween, and the spacings are such as to pro~ide
a substantially uniform volumetric flow rate of air
around the arcuate extent of said outlets despite
variations in air pressure throughout the manifold.
In another facet, said shroud air generating structure
comprises a manifold having an air inlet for receiving
air under pressure and an air outlet, and in this
case said air outlet is a continuous annular opening,
behind the rotary head, for providing therethrough
the annular curtain of shroud air.
In a preferred practice of the invention, the :
foregoing features are combined in a single structure,
whi~h comprises an annular channel in a body portion -
of the spray coating system behind the rotary head
and coaxial tberewith. Said channel has an open
side toward the head and an air inlet thereto, and a
wall extends across said open side of said channel
and has a plurality of passages formed therethrough.
Said passages are in an annular array, and have
relatively large diameters and spacings therebetween
which progressively decrease with increasing distances
from said air inlet. An annular cover mounts on the
body portion of the spray coating system coaxial with
said annular channel, and has a radial flange which
extends across and spaced from said wall to form
therewith an annular chamber having a continuous
annular opening coaxial with and behind the rotary
head. In operation of said structure, uneven air
11'7'7;~
pressures develop throughout said channel upon intro-
duction of air therein, and the uneven spacings of
said passages cause a substantially uniform pressure
and.volumetric distribution of air to be developed
within said chamber. Consequently, air discharged
through said annular opening orms a very uniform
annular layer or curtain of shroud air for movement
around, across and forwardly of the rotary head to aid
electrostatic forces in moving spray particles toward
articles to be coated.
The foregoing and other objects, advantages and
- features of the invention will become apparent upon
a consideration of the following detailed description,
when taken in conjunction with the accompanying drawings.
Brlef Description of the Drawings
Fig. 1 is a perspective view of a portion of a
rotary head electrostatic spray coating system, which
includes a shroud air generating structure embodying
the teachings of the present invention;
Fig. 2 is a cross sectional elevation view taken
substantially along the lines 2-2 of Fig. 1, and
illustrates the forward portion of the rotary head
electrostatic spray coating system and the arrangement
of the shroud air generating structure thereon;
. Fig. 3 is a front elevation view, partly in
cross section, taken substantially along the lines 3-3
of Fig. 2, and shows the arrangement of air outlet
passages in an air distribution manifold of the shroud
air structure, and -
Fig. 4 is a c~oss sectional, fragmentary, sideelevation view ~aken substantially along the lines
4-4 of Fig. 3, illustrating the particular arrangement
of a cover of the shroud air structure, which provides
~17'~41
a continuous annular outlet for a very uniformly
distributed curtain of shroud air.
Detailed Description
Referring now to the drawings, there is shown in
Figs. 1 and 2 and indicated generally at 10 a rotary
head electrostatic spray coating system which includes
a shroud air generating structure, indicated generally
at 12, configured in accordance with the teachings of
.. . . ;
the present invention. The spray coating apparatus
includes a rotatable, bell-shaped head 14 of conductive
material, mounted on an output shaft 16 of a high speed
air driven turbine 18. The turbine rotates the head
at a high rate of speed on the order of 3Q,000 to 60,000
rpm, and is supported on an end of an arm 20 of
insulating material. An opposite end o~ the arm is
connected to any suitable mechanism (not shown) for
moving the apparatus to positions pe~mitting proper
deposition of coating material on articles, for example
on articles moved past the apparatus on a conveyor.
In operation of the spray coating apparatus 10,
coating material in a passage 22 passes through an
orifice 24 and is directed in a stream or jet onto
an interior surface 26 of a center portion 28 o the
head 14. Because of rotation of the head, upon
striking the surface the coating material progresses
radially outwardly through a passage 30 and across
an inner wall 32 of the head in a thin film under
centrifugal force toward a sharp, circular, peripheral
edge 34 o the head, whereat it is divided as it
leaves the head into fine particles in the fo~m of a
spray~ A high d.c. voltage on the order of 12Q,000
volts is connected with the head; so that spray
l~t~7~41. ,
particles on moving past the peripheral edge a~e charged
to a high electrostatic potential for attraction to
articles to be coated, which are maintained at a
different and usually ground potential. To this end,
the insulating material support arm 20 isolates peripheral
equipment from high voltages present at the spray head
of the system.
--Although Lmprovements are obtained in coating .
material deposition by virtue of electrostatic attraction
of the.spray particles to the ware, if electrostatic .
attraction were the sole means for moving spray particles
to the ware significant portions of the particles would .
not reach the ware. This may be appreciated if it is
considered .that at the ver~ high speed of rotation of
the spray head 14, spra~ particles discharged from the
- peripheral edge thereof travel at significant velocities
in directions generally radially of the head and
parallel to the ~rticles, since articles to be coated
are usually positioned forwardly of the head along the
axis of rotation thereof~ Thus, the direction of
movement of the spray particles must be changed by
up to 30 and more for the particles to reach the ware,
which can hardly be accomplished with electrostatic
forces alone. Con~equently, additional means is
provided for aiding movement of spray particles toward
the ware.
To aid movement of spray particles toware ware
to be coated, the art contemplates generating a shroud
or curtain of air for movement over, around and
forwardly of the head and against the particles as
they leave the peripheral edge of the head. Heretofore,
should air genera~ing structures have included an
117'~;~41
annular manifold behind the head for receiving air
under pressure, and a plurality of shroud air outlet
passages in an annular array around the manifold.
In an attempt to ob~ain a uniform layer or distribution
of shroud air, thereby to enhance uniformity of
movement of coating material toward and deposition on
articles, the outlet passages are closely spaced and
of relatively small diameter. ~owever, despite the
close spacings and small diameters of the passages,
the shroud air pattern is nevertheless nonuniform in
that is is comprised of a plurality of discrete stxeams
or iets of air. Also, because of the small diameter
of the passages, the same are subject to blockage by
spray particles or contaminants in the air supply,
which further impairs the uniformity of the shroud air
pattern and therefore the uniformity of dispersion of
spray particles toward articles to be coated, with the
result thatprior apparatus of the general type must
be frequently shut down to clear the passages.
To overcome the disadvantages of prior shroud air
generating structures, and with reference also to
Figs. 3 and 4, in accordance with the teachings of
the present invention the improved shroud air generating
structure 12 includes an annular trough, openingl channel
or manifold 36 formed in a forward face of a body
portion 38 of the spray coating apparatus. The manifold
is of sufficient diameter to extend beyond the rearward
end of the rotary head 14, and a supply (not shown) of
air under pressure is connected with the manifold through
an air inlet passage 40. An annular wall or air diffuser
ring 42 closes a forward side of the manifold, preferably
_g_ -
~7'~;~41
by being press fit therein, and a plurality of 6 air
outlet passages 44 are formed through the diffuser
ring in an annular array.
The shroud air flows through the passages 44,
and in accordance with one facet of the invention
the passages are of relatively large diameter and
unevenly spaced around the diffuser ring 42, the spacing
being such as to provide a generally uniform volumetric
distri~ution of` air discharged through the passages -~
around the arcuate extent of the ring. In this connection,
it is understood that with a single air inlet to the
manifold 36 from the air passage 40, movement of air
through the outlet passages results in une~ual air
pressures within the manifold, such that the pressure
of air adjacent the inlet passage is greater than at
positions remote from the inlet passage. Consequently,
if the outlet passages were equally spaced around the
diffuser ring, greater volumetric discharges of air
would occur in proximity with the inlet passage than
elsewhere, and would decrease with increasing distances
from the inlet passage. However, by providing unequal
spacings between the outlet passages, such that the
passages remote from the inlet passage are spaced more
closely together than are the passages in proximity
with the inlet passage, and so that a progressively
decreasing spacing between passages occurs rom a
position adjacent the inlet passage to positions remote
therefrom, a very volumetrically uniform discharge of
air is obtained around the entire arcuate periphery o~
the di ffuser ring. At the same tLme, the relatively
large diameters of the outlet passages prevents the
same from becoming clogged by contaminants in the
-10- ,
1 1'7'~
air supply, and fewer outlet passages are requfired
for a given volume flow of air.
For the structure th~s far described, it is
apparent that the air patt~rn would be improved over
tha~ provided by prior structures, in that there
would be a uniform volumetric outlet of air around
the arcuate extent of the annular array of outlet
passages, but tha~ the air pattern would nevertheless
be comprised of a plurality of discrete streams of -~
air. Accoxdingly, to generate a very uniform and
continuous annular layer or curtain of shroud air
for movement across and forwardly of the rotary head
14, in accordance with a further feature of the
invention an annular shroud air cover~46 is threaded
onto thP forward end of the body portion 38. The
diffuser ring 42 is generally L-shaped, and a forwardly
.extending leg thereof defines an outer, annular,
peripheral surface 4~. The cover has a radially
inwardly extending flange or wall 50, and when the
cover is on the ~ody portion a radially extending
annular chamber 52 is formed be~ween the wall and
portions of the forward faces of the body portion and
diffuser ring, and an annular and axially extending
shroud air outlet passage 54 is defined between an
innermost end or edge of the wall and the peripheral
surface 48 of the diffuser ring. Also, when the cover
is mounted on the body por~ion an annular seal is
formed between the cover and body portion along a
sealing surface 56 of the cover, thereby preventing
unwanted leakage of shroud air from the chamber 52~
11~7'~
It may now be appreciated that the invention
provides an improved structure for generating a very
uniform annular layer or curtain of shroud air around,
across and forwardly of the rotary head 14. To this
end, the uneven spacings between the outlet passages
44 in the diffuser ring 42 provide within the
chamber 52 a very uniform volumetric and pressure
distribution of air, despite variations in the pressure .
.,.. . , :~
of air within the manifold 36. At the same time, the
annular outlet passage 54 is continuously open around
its circumference, so that with the uniform volumetric
.and pressure distri~ution of air within the chamber,
the shroud alr pattern issuing through the passage
is very uniform and is not comprised of discrete
streams of air. Consequently, the shroud air pattern
imparts a very uniform movement of spray particles
toward ware to be coated, so that significant
improvements are obtained in deposition of coating
material on the ware. In addition, because of their
relatively large diameters, not only are fewer passages
44 required for a given volume flow of air, which
decreases manufacturing costs of the structure, but
the passages are also not prone to blockage by
contaminants in the air supply. Further, since the
wall 50 of the cover extends across the passages, the
passages are also shielded against deposition therein
o~ spray particles.
While embodiments o the invention have been
described in detail, it is understood that various
modifications and other embodiments thereof may be
deviced by one skilled in the art without departing
fxom the sp~rit and scope of the invention, as
defined in the appended claims.
-12- J .
