WO1994022589A1

WO1994022589A1 - Method and apparatus for coating three dimensional articles

Info

Publication number: WO1994022589A1
Application number: PCT/US1994/003828
Authority: WO
Inventors: Tatsuo Otani; Hidetaka Tsukamoto; Makoto Sekiguchi
Original assignee: Nordson Corporation
Priority date: 1993-04-07
Filing date: 1994-04-07
Publication date: 1994-10-13
Also published as: EP0693973A1; DE69429163D1; JP3489035B2; JPH07880A; EP0693973B1; AU6628294A; CA2159216A1; DE69429163T2; EP0693973A4; CA2159216C

Abstract

A method and apparatus (10) for powder coating three dimensional articles (12) carried on a conveyor (14) includes a controller (36) for controlling the rate at which coating material is discharged from a gun (16), so that the rate is proportional to the surface area of the surface portions of the article (12) as they pass the gun (16). Some surface portions topographically have a greater surface area which must be covered, while others are flat or have cut-out regions, requiring less coverage or no coverage at all. In powder coating operations, the discharge rate is controlled via automatic or programmed control of an electro-pneumatic air regulator (42) which regulates the supply of pressurized air to a powder pump (24) connected to the spray gun (16). The apparatus (10) assures uniformity in coating for a three dimensional article (12) with multiple surface portions of varying topography such as curves, angles, cut outs, etc., while minimizing the amount of wasted coating material.

Description

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 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. Thus, as the topography of the three dimensional article varies,

it becomes more difficult to uniformiy 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. Summarv 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 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. 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 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 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. 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 1 2 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 1 2 carried by a conveyor 14. The conveyor 14 moves the

articles 12 along a conveying path and past a spray coating gun 1 6.

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 1 8 which

is formed during operation of the gun 1 6 under high pressure

discharge conditions.

As shown in Fig. 1 , the first external surface 1 3 of article 12

includes first and second widths, or topographic channels 1 9 and 21 , respectively. As shown in Fig. 1 , channel 1 9 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 1 3 with respect to the direction of the conveyor

14. As shown in Fig. 1 , the conveyor 14 moves the articles 1 2

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 1 9 does not..

In addition to the gun 1 6 for discharging coating material in a

conical-shaped pattern 18 toward the first channel 19 of surface 1 3,

Fig. 1 also shows a second gun 1 6a 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 1 3, one or more

additional guns may be added as necessary. The structural elements

which supply coating material to second gun 1 6a are identical to the

structural elements which supply first gun 1 6, and reference 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 1 6 will be described .

A supply hose 22 conveys a mixed stream of pressurized air

and powder paint coating material to the gun 1 6. 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 fiuidized bed of powder coating material. Pump 24

includes a venturi pumping chamber under negative pressure which is

connected by suction tube 28 to the fiuidized 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 32 and the pump 24. Regulator 38 is preferably a voltage to pressure regulator manufactured by Nordson Corporation of Amherst,

Ohio under Part No. 1 13,626. 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 copending U.S. Application Serial Number 08/206,597, filed March 3, 1994 which is hereby

incorporated by reference in its entirety. 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 a display, 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. 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 (and 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 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 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 1 2 in

greater detail. More particularly, Fig. 2 shows the topographic

surface details of first upper channel 1 9 and second lower channel

21 . The surfaces of channels 19 and 21 are coated by material

discharged from gun 1 6 and 1 6a, respectively, as the article 1 2

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 1 9e. Surface portion 1 9a is

oriented parallel to the discharge path of the gun 1 6 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 1 9c 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 1 9c flattens, adjacent the flat or parallel surface

portion 19d. Surface portion 1 9e 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 a cut-out region 21 e 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 1 2 is moved past guns 1 6 and

16a at a constant rate of speed, a greater surface area of the article

12 passes the guns 1 6 and 1 6a per unit time during passage of those

portions which are non-parallel to the conveying path i.e. such as

portions 1 9c 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 1 6 and 1 6a per unit time. Thus, to uniformly coat surface portions 1 9b and 1 9c with a layer of coating material of uniform thickness, and to minimize

waste of coating material, more coating material must be discharged

as portion 1 9c passes gun 1 6 than when portion 19b passes gun 1 6,

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 controls the flow rate of pressurized air along tube 30 by means of

regulator 38. For a portion which is flat, such as 1 9b and 21 b, the

standard reference for surface area passing gun 1 6 per unit time is

1 .0 and the standard reference for quantity discharged is also 1 .0.

When a curved portion such as 1 9c moves in front of the gun 1 6,

(adjacent portion 19b) the initial surface area which passes gun 1 6

per unit time is about 4.4 times the reference value region. As the

angle of surface portion 1 9c decreases to about 30°, (adjacent

portion 19d) the region which passes gun 1 6 per unit time is about

1 .1 5 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 1 6

and 1 6a 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 1 9c, 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 regions, such as portion 21 e.

For portions which are substantially parallel to the discharge

path, such as portions 19a, 21 a, 19e, and 21 g, it is extremely

difficult to uniformly coat the exposed surface area, due to the

angular orientation of the surface with respect to the guns 1 6 and

1 6a. Therefore, it is desirable to electrostatically charge the powder

particles to promote attraction toward these surfaces and uniform

coverage thereof. Electrostatic charging of the powder particles may

occur via use of a corona charging electrode external to the guns 1 6 and 1 6a, 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 1 2 as well.

Fig. 3A shows topographic channel 1 9 in plan view, and

particularly surface portions 19a, 19b, 1 9c, 19d and 19e. Fig. 3B

includes a curve 58 which graphically illustrates the surface area of

channel 19 which passes in front of gun 1 6 during movement of the

conveyor 14. For instance, the surface area represented for portions

19a and 19d are equal to the standard reference value 1 .0. The

surface area represented for the slightly inclined region of portion 19c is 1 .1 5 times the reference value, while the surface area for the steeper region of portion 1 9c increases from 1 .1 5 to 4.4 of the

reference value. For curved portions such as 1 9c, Fig. 3B also

reflects the slope of a tangent line to the surface.

Fig. 3C graphically illustrates the quantity of coating material

which she ld be discharged according to the invention as surface

portions 19a, 19b, 19c, 19d and 1 9e pass in front of gun 1 6. 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 1 6a. Also, the reference

numeral 68 designates the amount of coating material saved with 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 21 e, which does not require

any coating material. The second discharge path 1 8a and the second

gun 1 6a are necessary for coating article 1 2 because of the different

surface configurations, or surface topography, represented by the

cut-out region 21 e. 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 1 2 and the number of topographic variations in the surface

orientation of the article 1 2. The invention requires one gun, and therefore one topographic channel, for each variation and surface

orientation across the transverse dimension of the article 1 2.

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 claims appended hereto.

We Claim:

Claims

1. An apparatus for powder coating a three dimensional article having an external surface of at least two varying topographical

surface portions, comprising: a gun aimed in a discharge direction; means for conveying, relative to the gun, the article along a conveying path to successively convey the surface portions past the

J gun; a powder pump operatively connected to the gun;

means for supplying fiuidized powder particles to the pump; means for supplying pressurized air to the pump via a flow path, the pump adapted to combine the pressurized air and powder particles in a mixed stream and to convey the powder-air stream

along a transport path to the gun for discharge therefrom toward the surface; flow control means located in the flow path and adapted to regulate the flow of pressurized air therethrough, thereby to regulate the flow rate of the mixed powder-air stream conveyed to the gun and discharged therefrom; and

a controller operatively connected to the flow control means, the controller adapted to control the flow control means to control the discharge rate of the stream from the gun according to a predetermined discharge sequence which corresponds to the topography of the surface portions as the portions pass in front of the gun, thereby to promote uniform coating of the surface portions

and reduced waste of powder particles during coating of the article.

2. The apparatus of claim 1 and further comprising: a sensor operatively connected to the controller and adapted to

sense the article as it moves in front of the gun and to signal the

controller to initiate the predetermined discharge sequence.

3. The apparatus of claim 1 and further comprising: speed sensing means operatively connected to the controller and adapted to determine the rate of speed of the conveying means, thereby to correlate the predetermined discharge sequence with the rate at which the surface portions pass in front of the gun.

4. The apparatus of claim 1 and further comprising:

an air pressure sensor located in the flow path between the flow control means and the pump; and

display means operatively connected to the air pressure sensor for indicating the air pressure in the flow path.

5. The apparatus of claim 1 and further comprising:

means for electrostatically charging the particles in the stream.

6. The apparatus of claim 5 wherein the electrostatic charging

means is located in the transport path.

7. The apparatus of claim 1 wherein the flow control means

comprises an electro-pneumatic regulator which receives an electrical

signal from the controller and in response varies the output pressure

of air supplied from the regulator.

8. The apparatus of claim 1 wherein the gun remains in a fixed

position and the conveying means comprises a conveyor which carries the article past the gun.

9. The apparatus of claim 1 and further comprising:

a second gun adapted to discharge pressurized air and powder

particles in a second stream toward successively conveyed surface

portions of the article, the second gun and the first gun arranged

along a line perpendicular to the conveying path and the discharge direction.

10. A coating apparatus comprising: a conveyor for conveying a three dimensional article along a

conveying path, the article having an external surface with a plurality

of surface portions of varying topography at least one of which

portions is not parallel to the conveying path; a gun aimed along a discharge path which intersects the conveying path; means for supplying a coating material to the gun in a

pressurized condition for discharge therefrom to coat the surface portions as the article is moved through the discharge path via the conveyor; and a controller operatively connected to the supplying means and adapted to variably control the discharge rate of the coating material according to a predetermined discharge sequence so that the discharge rate for each surface portion is commensurate with the surface area thereof, thereby to uniformly coat the surface portions and minimize waste of coating material.

11. The apparatus of claim 10 and further comprising: a sensor operatively connected to the controller and adapted to sense the article as it moves in front of the gun and to signal the controller to initiate the predetermined discharge sequence.

12. The apparatus of claim 10 and further comprising:

speed sensing means connected to the controller and adapted

to determine the rate of speed of the conveying means, thereby to

correlate the predetermined discharge sequence with the rate at

which the surface portions pass in front of the gun.

13. The apparatus of claim 10 and further comprising:

air pressure sensing means operatively connected to the

supplying means and adapted to sense and display a parameter

proportional to the discharge rate.

14. The apparatus of claim 10 and further comprising:

means for electrostatically charging the particles in the stream.

15. The apparatus of claim 10 wherein the gun remains in a fixed

position and the conveying means comprises a conveyor which

carries the article past the gun.

1 6. The apparatus of claim 10 and further comprising:

a second gun adapted to discharge coating material along a

second discharge path which also intersects the conveying path as

the article moves along the conveying path, the second gun and the

first gun arranged along a line perpendicular to the conveying path

and the discharge paths.

17. A method of coating comprising the steps of: moving a three dimensional article to be coated along a conveying path relative to a spray gun, the gun aimed along a discharge path generally perpendicular to and intersecting the

conveying path, the article having an external surface to be coated with a plurality of surface portions having varying surface topography and surface area; discharging a coating material from the gun along the

discharge path to successively coat the surface portions as the article

moves along the discharge path; and controlling the discharge rate of the coating material according to a predetermined coating sequence, the rate being dependent upon the surface topography and the surface area of the portion located in the discharge path, thereby to uniformly coat the surface portions while minimizing the amount of wasted coating material.

18. The method of claim 17 wherein the coating material is powder and further comprising the step of:

electrostatically charging the discharged particles.

19. The method of claim 18 wherein the charging occurs inside the gun.

20. The method of claim 17 and further comprising the step of: sensing the article as it moves into the discharge path, thereby

to actuate the controller to initiate the predetermined coating

sequence.

21. The method of claim 17 and further comprising the step of: measuring the speed at which the article is moved through the

discharge path and along the conveying path; and

inputting the measured rate to the controller to control the predetermined coating sequence.