Note: Descriptions are shown in the official language in which they were submitted.
WO 94/22589 PCT/US94/03828
2159216
METHOD AND APPARATUS FOR COATING THREE DIMENSIONAL ARTICLES
Field of the Invention
This invention relates to an improved method and apparatus for
powder coating three dimensional articles having surface irregularities
which vary in angle, curvature, and/or surface area.
Background of the Invention
One common method and apparatus for coating three
dimensional articles involves spray coating powder particles onto the
external surface of the articles as they are conveyed by a conveyor
past a spray gun. Typically, a sensor detects when an article carried
by the conveyor moves into the spray pattern of the gun and signals
the apparatus to spray a preset uniform quantity of powder particles
to coat the article. Thus, to coat a plurality of articles, the apparatus
intermittently sprays a preset, uniform quantity of coating material
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onto each article as the articles pass the gun.
This manner of coating three dimensional articles is generally
acceptable if all of the articles have a flat coating surface, or a
surface which is parallel to the conveyor and perpendicular to the
orientation of the gun.
However, with three dimensional articles which have surface
irregularities such as cut-out regions, angled or curved surfaces,
protrusions, indentations, or bent edges, etc., these surface
irregularities make it difficult to uniformly coat the entire external
surface of the article. One reason for this difficulty relates to the
angular orientation of the surface irregularities with respect to the
direction of the gun. The more the surface varies from an orientation
perpendicular to the spray direction, the more difficult it becomes to
adequately coat the surface. For curved or angled surfaces difficulty
in coating occurs in part because an angled or curved surface has a
greater density of surface area than a flat surface. This means that
as the conveyor moves the three dimensional articles past the spray
gun, the surface area per unit time which passes the gun is greater
for angled or curved surfaces than for flat surfaces. Additionally,
some surface irregularities are actually cut-out regions, which require
no coating at all. Continued operation of a spray gun as a cut-out
region passes by represents a waste of coating material.
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Thus, as the topography of the three dimensional article varies,
it becomes more difficult to uniformly coat the entire surface area,
particularly for conveyors commonly used in the industry which
convey such articles past the spray gun at a relatively constant
speed.
One way to assure that the entire surface is coated is to
operate the spray gun at a sufficiently high pressure to discharge a
quantity of coating material which is greater than that which is
actually necessary to coat the surface, with the pressure being
determined by the portion of the surface which is most difficult to
coat. This assures some coating on the most steeply angled or
curved surfaces. However, a coating applied in this manner is
generally not uniform due to the surface irregularities. This manner
of coating also results in a tremendous amount of wasted energy and
coating material.
It is an objective of this invention to improve uniformity in
coating three dimensional articles with surface irregularities.
It is another objective of this invention to adequately coat the
irregular surface areas of a three dimensional article while minimizing
the amount of wasted coating material.
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Summary of the Invention
The above-stated objectives are achieved by a method and
apparatus which control the discharge rate of coating material
according to the irregularity and/or area of the surface of a three
dimensional article as it is conveyed past a spray gun. For powder
spray coating applications, the discharge rate is controlled by
regulating the air pressure input to the powder pump. This controls
the volume of powder mixed into the conveying air stream moving
through the pump and the rate at which the mixed powder-air stream
is discharged from a spray gun toward the three dimensional article.
For flat surfaces, a standard reference discharge rate is used. For
steeply curved surface portions, the discharge rate increases
commensurately to assure adequate coating of the increased surface
density which moves past the gun per unit time. For cut-out regions,
discharge of the powder is temporarily stopped to reduce waste.
Depending upon the dimensions of the articles to be coated,
more than one spray gun may be necessary. Each spray gun is
adapted to coat along a topographical strip, or channel, of the
external surface of the article. The discharge rate for a gun
dedicated to a particular channel is predetermined to correspond to
the particular irregularities of the surface portions of that channel.
Each gun is controlled independently, so that each channel of the
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three dimensional article is coated according to a predetermined
discharge sequence which corresponds uniquely to the configuration
of the surface portions thereof.
By varying the discharge rate in accordance with curvature
and/or surface area of the surface portions as they pass in front of a
gun along a conveying path, this invention assures uniform coating of
all surface portions of the article, regardless of surface irregularities.
Additionally, because the discharge rate is lowered for flat surfaces,
and discharge is discontinued entirely for cut-out regions, this
invention reduces the amount of coating material which is wasted
during the coating of three dimensional articles.
According to a preferred embodiment of the invention, a
method and apparatus for coating three dimensional articles includes
a conveyor, a spray gun, a powder pump, a powder hopper, a
pressurized air source, a master controller, and electro-pneumatic air
regulator, a position sensor for articles on the conveyor and a speed
sensor for the conveyor. The conveyor carries three dimensional
articles in spaced relation along the conveying path, which is oriented
perpendicular to the discharge path of the spray gun. Stated another
way, the discharge path of the spray gun intersects the conveying
path at a 90° angle. The powder pump conveys a mixed stream of
pressurized air and powder particles to the gun via a transport hose.
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The powder hopper, preferably a fluidized bed, supplies powder
particles to the powder pump. The pressuried air source supplies
pressurized air via a supply tube to the powder pump. The electro-
pneumatic air regulator is connected in the supply tube between the
pressurized air source and the powder pump, and the electro-
pneumatic air regulator regulates the flow rate of pressurized air
supplied to the powder pump. Because the amount of powder
particles drawn into the powder pump is directly proportional to the
flow rate therethrough, this also controls the discharge rate from the
gun.
The controller operatively connects to the electro-pneumatic air
regulator and controls operation thereof according to a predetermined
discharge sequence, the sequence initially determined by an operator
to uniformly coat a topographical channel of the three dimensional
article as the various surface portions thereof pass in front of the
gun. A position sensor senses movement of an article by the
conveyor into the discharge path of the gun, and thereby activates
the controller to initiate the predetermined coating sequence. The
conveyor speed sensor operatively connects to the controller and
signals to the controller the speed of the conveyor, thereby to
correlate the predetermined discharge sequence with the actual
speed of the conveyor. Stated another way, the speed sensor serves
WO 94122589 215 9 2 l 6 PCT/US94/03828
as a feedback device to the controller to assure that the
predetermined coating sequence actually matches the topography of
the surface portions of the channel as the article is transported in
front of the gun.
An air pressure sensor may be located in the supply line
between the electro-pneumatic air regulator and the pump, thereby to
sense and provide an indication of the air pressure in the line. If
desired, this air pressure can be calculated to determine, and provide
a display of, the discharge rate from the gun.
Depending upon the transverse dimension of the articles to be
coated and/or the variations in surface topography for the articles to
be coated, one or more additional surface channels may be
designated. This will necessitate the use of one or more additional
guns, along with the corresponding additional powder supply
apparatus. This additional powder supply apparatus functions in the
same manner as described above, and discharge from each gun is
independently controlled by the master controller. However, only a
single air supply source is necessary, with a control valve located
downstream thereof and operatively connected to the master
controller, thereby to turn "off" or "on" all pressurized air flowing into
the apparatus.
These and other features of the invention will be more readily
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_g_
understood in view of the following detailed description and the
drawings.
Brief Description of the Drawings
Fig. 1 is a schematic of a powder coating apparatus for coating
three dimensional articles, in accordance with a preferred
embodiment of the invention.
Fig. 2 is an enlarged perspective view of a portion of the
apparatus shown in Fig. 1, showing the orientation of the spray guns
with respect to a three dimensional article during coating thereof.
Fig. 3A is a plan view of one spray coating gun during coating
of a first channel of the three dimensional article shown in Fig. 2.
Fig. 3B is a graph which illustrates the surface area to be
coated for the surface portions of the first channel shown in Fig. 3A.
Fig. 3C is a graph which illustrates the quantity of coating
material discharged as successive surface portions of the first
channel move past the gun.
Fig. 4A is a plan view of a second spray coating gun during
coating of a second channel of three dimensional article shown in Fig.
2.
Fig. 4B is a graph which illustrates the surface area to be
coated for the surface portions of the second channel shown in Fig.
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_g_
4A.
Fig. 4C is a graph which illustrates the quantity of coating
material discharged as successive surface portions of the second
channel move past the gun.
Detailed Description of the Drawings
Fig. 1 schematically shows an improved spray coating
apparatus 10 for powder coating a three dimensional article 12 in
accordance with a preferred embodiment of the invention. While the
figures show an apparatus 10 particularly suitable for powder
coating, the invention is not limited thereby and is also applicable to
other types of coating and coating materials. More specifically, the
apparatus 10 coats a first external surface 13 of three dimensional
articles 12 carried by a conveyor 14. The conveyor 14 moves the
articles 12 along a conveying path and past a spray coating gun 16.
The gun 16 is adapted to spray coating material along a discharge
path which intersects the conveying path of the conveyor 14 at 90°.
This discharge path is defined by a conical-shaped pattern 18 which
is formed during operation of the gun 16 under high pressure
discharge conditions.
As shown in Fig. 1, the first external surface 13 of article 12
includes first and second widths, or topographic channels 19 and 21,
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respectively. As shown in Fig. 1 , channel 19 is located above
channel 21, though this invention is not limited to the use of a
horizontal conveying path and a horizontal discharge path, and the
channels may reside next to each other in the X, Y or Z planes. The
number of channels depends upon the transverse dimension of the
first external surface 13 with respect to the direction of the conveyor
14. As shown in Fig. 1 , the conveyor 14 moves the articles 12
along a conveying path which comes out of the page, and thus the
transverse dimension of external surface 13 is along the vertical, or
y-axis. Another factor which plays a role in determining the number
of channels, in addition to the transverse dimension, is the surface
variation of the first external surface 13. For instance, channel 21
has a cut-out region while channel 19 does not..
In addition to the gun 16 for discharging coating material in a
conical-shaped pattern 18 toward the first channel 19 of surface 13,
Fig. 1 also shows a second gun 16a for discharging coating material
in a conical-shaped pattern 18a toward the second channel 21 . As
described above, depending upon the number of channels and the
surface configuration of first external surface 13, one or more
additional guns may be added as necessary. The structural elements
which supply coating material to second gun 16a are identical to the
structural elements which supply first gun 16, and reference
WO 94/22589 215 9 216 PCT/US94/03828
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numerals for these elements are identical, but have an "a" appended
thereto. To simplify the explanation of Fig. 1 , only the elements
associated with first gun 16 will be described.
A supply hose 22 conveys a mixed stream of pressurized air
and powder paint coating material to the gun 16. A powder pump
24 creates this mixture of pressurized air and coating material. The
pump 24 is mounted to the top of a powder hopper 26 which
maintains a fluidized bed of powder coating material. Pump 24
includes a venturi pumping chamber under negative pressure which is
connected by suction tube 28 to the fluidized bed of powder in
hopper 26 to draw powder into pump 24.
A pressurized air supply tube 30 defines a flow path for
conveying pressurized air from a pressurized air source 32 to the
pump 24. This pressurized air creates the negative pressure
condition in the venturi pumping chamber of pump 24 which draws
powder from hopper 26 into the pump. The supply of pressurized air
from pressurized air source 32 is turned "off" and "on" via a solenoid
valve 34 which is controlled by a master controller 36. The master
controller 36 is preferably programmable and includes a central
processing unit.
An electro-pneumatic air regulator 38 is installed in the flow
path defined by supply tube 30, between the pressurized air source
CA 02159216 2003-12-04
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32 and the pump 24. Regulator 38 is preferably a voltage to pressure regulator
manufactured
by Nordson Corporation of Amherst, Ohio under Part No. 113,626. An electrical
signal is
provided to regulator 38 via a line 40 from controller 36 indicating the air
pressure to be
provided at the output of regulator 38. Regulator 38 is also described in
applicant's U.S.
Patent No. 5,957,393. Operation of this electro-pneumatic air regulator 38
regulates the flow
of pressurized air along the tube 30, which in turn regulates the amount of
negative pressure in
the venturi pumping chamber of pump 24 and the flow rate of the mixed powder-
air stream
from hopper 26 along supply tube 22, and the discharge rate of powder coating
material from
gun 16. Supply tube 30 also includes an air pressure sensor 42 which is
operatively connected
to the master controller 36, and the master controller 36 includes or is
operatively connected
to a display 41, such as an LED or LCD, for displaying the air pressure in the
tube 30.
The master controller 36 also connects to position sensors 46a and 46b, which
may be a light beam detector, to sense movement of the article 12 into the
discharge path of
the gun 16. The signal generated by the sensors 46a and 46b may then be used
to actuate the
master controller 36 to initiate coating.
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The master controller 36 is programmed to control the
operation of the regulator 38 (and regulator 38a) according to a
predetermined coating sequence. This coating sequence may involve
increasing or decreasing the supply of pressurized air to the pump 24
from regulator 38, thereby to increase or decrease the discharge rate
of coating material from the gun 16 in accordance with the particular
surface configuration or topography of the channel 19 land channel
21 ). For instance, if the surface channel 19 includes flat portions
and angled portions or curved portions, i.e. portions not parallel to
the conveying path, the surface area of the non-parallel portions
which pass by the gun 16 per unit time will be greater than the
surface area of the flat portions which pass by the gun 16 per unit
time, assuming the conveyor 14 moves at a constant speed. As
described in more detail below with respect to Fig. 2, these uneven
or non-parallel portions therefore require a higher discharge rate, with
the discharge rate being commensurate with the slope of a tangent
line to the surface.
If desired, the speed of the conveyor 14 may be fed back to
the master controller 36. This may be done by fixing a rotatable
spool 48 in contact with the conveyor 14 so that the spool 48
rotates upon movement of the conveyor 14. An axle 50 connects to
the spool 48 and supports a disc 52 which rotates therewith. The
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disc 52 is coded via punched out regions at a diameter which
corresponds to a location of light beam sensors 54a and 54b. As the
disc 52 rotates, the passage or obstruction of the light beam
between sensors 54a and 54b indicates to the master controller 36
the speed of the conveyor 14. In its simplest form, with uniformly
spaced punch-out regions in the disc 52, this structure may be used
simply to indicate to the master controller 36 the speed of the
conveyor 14 and whether the conveyor 14 has stopped or started,
via sensing at the controller 36 the rate of receipt of the "obstructed"
and "unobstructed" signals. If the conveyor 14 always runs at the
same speed to coat the same articles 12, this speed sensing
structure may not be necessary, because the predetermined coating
sequences can be correlated to the channel or channels of the article
12 in relation to that constant speed. However, this added degree of
control is preferable because of possible fluctuations in the speed of
the conveyor 14 and/or the desire to operate the conveyor 14 at
different speeds for coating different articles 12.
If desired, the coding on the disc 52 may be specific to a
particular article 12, and arranged such that one complete rotation of
disc 52 corresponds to movement of the conveyor 14 from the
leading edge of one article 12 to the leading edge of the next
succeeding article 12. Each surface portion to be coated can then be
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correlated to an arcuate section of the disc 52. The spacing of the
cut out regions could then dictate the discharge rate. Thus, the
invention contemplates added levels of feedback control, if desired.
Fig. 2 shows the first external surface 13 of article 12 in
greater detail. More particularly, Fig. 2 shows the topographic
surface details of first upper channel 19 and second lower channel
21. The surfaces of channels 19 and 21 are coated by material
discharged from gun 16 and 16a, respectively, as the article 12
moves along the conveying path in a direction designated by
reference numeral 56. Channel 19 includes multiple surface portions,
designated 19a, 19b, 19c, 19d and 19e. Surface portion 19a is
oriented parallel to the discharge path of the gun 16 and
perpendicular to the conveying path of the conveyor 14. Surface
portion 19b is parallel to the conveying path and perpendicular to the
discharge path. Surface portion 19c is curved, and a tangent line to
this curve is almost parallel with the discharge path adjacent portion
19b, but becomes almost perpendicular to the discharge path as the
surface portion 19c flattens, adjacent the flat or parallel surface
portion 19d. Surface portion 19e is oriented parallel with the
discharge path and perpendicular to the conveying path. Surface
portions 21 a, 21 b, 21 c and 21 d are similar to surface portions 19a,
19b, 19c and 19d, respectively. However, channel 21 also includes
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a cut-out region 21 a which does not require discharge of any coating
material, followed by a flat surface portion 21 f and a perpendicular
surface portion 21 g.
As shown in Fig. 2, if article 12 is moved past guns 16 and
16a at a constant rate of speed, a greater surface area of the article
12 passes the guns 16 and 16a per unit time during passage of those
portions which are non-parallel to the conveying path i.e. such as
portions 19c and 21 c. Also, the greater the curvature, or angle of
the surface portions with respect to the conveying path, the greater
the amount of surface area which passes the guns 16 and 16a per
unit time. Thus, to uniformly coat surface portions 19b and 19c with
a layer of coating material of uniform thickness, and to minimize
waste of coating material, more coating material must be discharged
as portion 19c passes gun 16 than when portion 19b passes gun 16,
assuming the conveyor 14 operates at constant speed.
Subsequently, as surface portion 19c levels off toward surface
portion 19d, the needed volume of coating material decreases. The
effect is similar for channel 21, but channel 21 also includes cut-out
21 e, which requires no coating material to be discharged.
To accomplish the desired increases and decreases in quantity
of coating material discharged, as dictated by the surface
configuration of the channels 19 and 21, the master controller 36
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controls the flow rate of pressurized air along tube 30 by means of
regulator 38. For a portion which is flat, such as 19b and 21 b, the
standard reference for surface area passing gun 16 per unit time is
1.0 and the standard reference for quantity discharged is also 1Ø
When a curved portion such as 19c moves in front of the gun 16,
(adjacent portion 19b) the initial surface area which passes gun 16
per unit time is about 4.4 times the reference value region. As the
angle of surface portion 19c decreases to about 30°, (adjacent
portion 19d) the region which passes gun 16 per unit time is about
1.15 times the standard reference value.
Thus, the apparatus 10 opens up or closes down the flow
passage in regulator 38, under the control of controller 36, to
increase or decrease, respectively, the discharge rate from guns 16
and 16a relative to the surface area of the respective channel 19 or
21. Thus, for example, the flow passage through regulator 38
would be opened wider during the coating of portion 19c, than during
the coating of portion 19b. This results in the most efficient use of
the coating material, since excess coating material is not discharged
onto flat portions, and additional material is discharged on curved
portions to accomodate the additional surface area defined by the
surface topography. Additionally, coating material is saved because
the apparatus 10 does not discharge coating material toward cut-out
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-18-
regions, such as portion 21e.
For portions which are substantially parallel to
the discharge path, such as portions 19a, 21a, 19e, and
21g, it is extremely difficult to uniformly coat the
exposed surface area, due to the angular orientation of
the surface with respect to the guns 16 and 16a.
Therefore, it is desirable to electrostatically charge
the powder particles to promote attraction toward these
surfaces and uniform coverage thereof. As shown in
l0 Fig. 1, electrostatic charging of the powder particles
may occur via use of corona charging electrodes 37 and
37a external to the guns 16 and 16a, respectively,
though it is preferable to electrostatically charge the
powder particles while in the apparatus 10, either via
an internal corona electrode or an internal charging
system such as a triboelectric friction charging
system. Moreover, this type of electrostatic powder
coating gun is preferably also used for coating the
other portions of the article 12 as well.
Fig. 3A shows topographic channel 19 in plan view,
and particularly surface portions 19a, 19b, 19c, 19d
and.l9e.. Fig. 3H includes a curve 58 which graphically
illustrates the surface. area of channel 19 which passes
is frost.. of gun, lb .~iag move . o~ . ~htt , coa~ra~or
14. For instance, the surface area reprea~tted-tor.
portions 19a and 19d are equal to the standard
refersr:ca -valnew l:Q. . Tha surface: ~~ =~ea-
for the slightly:inclined re~gion:ot portiow 19c
A~~!Ef~:DcD SHEET
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is 1 .15 times the reference value, while the surface area for the
steeper region of portion 19c increases from 1 .15 to 4.4 of the
reference value. For curved portions such as 19c, Fig. 3B also
reflects the slope of a tangent line to the surface.
Fig. 3C graphically illustrates the quantity of coating material
which should be discharged according to the invention as surface
portions 19a, 19b, 19c, 19d and 19e pass in front of gun 16. This
quantity is represented by curve 60. The shaded region located
above curve 60, and designated by reference numeral 62, represents
the amount of coating material that is saved by using this invention,
since without this invention it would otherwise be necessary to
discharge at a rate sufficient to cover the steepest region of surface
portion 19c. The other option of course, though equally undesirable,
would be to discharge at a rate insufficient to adequately coat the
steepest region of surface portion 19c.
Figs. 4A, 4B and 4C correspond to Figs. 3A, 3B and 3C,
respectively, but relate to coverage of second topographic channel
21. Curve 64 in Fig. 4B shows the surface area for portions 21 a,
21 b, 21 c, 21 d, 21 e, 21 f and 21 g of channel 21, and curve 66 in Fig.
4C graphically shows the quantity of coating material discharged as
these surface portions pass the gun 16a. Also, the reference
numeral 68 designates the amount of coating material saved with
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this invention by varying the discharge rate.
Compared to Figs. 3A, 3B and 3C, Figs. 4A, 4B and 4C differ only in respect
to the cut-out portion 21e, which does not require any coating material. The
second discharge
path 18a and the second gun 16a are necessary for coating article 12 because
of the different
surface configurations, or surface topography, represented by the cut-out
region 21e. In some
cases, as explained previously, additional or fewer guns may be needed, but
the number of
guns necessary will be determined by the transverse dimension of the article
12 and the
number of topographic variations in the surface orientation of the article 12.
The invention
requires one gun, and therefore one topographic channel, for each variation
and surface
orientation across the transverse dimension of the article 12.
While the preferred embodiment of the invention has been described, it is to
be
understood that modifications may be made to the preferred embodiment without
departing
from the scope of the invention. Accordingly, applicant wishes to be bound
only by the
claimed appended hereto.
