GB2119287A

GB2119287A - Paint spraying apparatus

Info

Publication number: GB2119287A
Application number: GB08308919A
Authority: GB
Inventors: Erhard Kock; James Joseph Gimple; Raymond Joseph Bihn
Original assignee: Champion Spark Plug Co
Current assignee: Federal Mogul Ignition Co
Priority date: 1982-05-03
Filing date: 1983-03-31
Publication date: 1983-11-16
Also published as: IT1163284B; IT8320885A0; DE3310983A1; CA1188935A; AU553607B2; BR8302206A; MX158574A; GB8308919D0; FR2525927A1; GB2119287B; BE896532A; FR2525927B1; AU1289783A; IT8320885A1; JPS58202060A

Abstract

A hand-held spray gun has a body (11) which defines a central air-powder passageway (29,30,31). The air- powder mixture is deflected outwardly by a body of generally frusto-conical shape. A shaping air passageway (43) in the body (11) defines a path (P) which intersects the air-powder mixture. The shaping air pressure is regulated to adjust the impact force of the shaping air striking the air-powder mixture. A central electrode (16) electrostatically charges the powder. <IMAGE>

Description

SPECIFICATION Method and apparatus for powder painting This invention relates to a method and apparatus for powder coating and more specifically to a method and apparatus for electrostatic powder painting.

United States Patent No. 4,228,961 discloses an electrostatic powder painting head in which clean gas from an outer chamber is fed to the surface of a strip electrode. A monopolar corona discharge current flows between the strip electrode and the needle electrode. The current charges the air-powder mixture moving through the central powder ejection port. The clean gas on the periphery is used to keep the strip electrode clean or powder free thereby ensuring a proper corona discharge current. The clean air is not used to regulate or shape the air-powder mixture.

United States Patent No. 4,235,381 discloses an electrostatic powder gun having an auxiliary compressed gas supply. The gas is passed to the nozzle and ionized so that upon being discharged at the tip of the apparatus it assists in placing the final charge on the powder.

It is preferable in powder painting to provide means for regulating the area of the paint particle stream, which in turn regulates the ultimate pattern size on the work piece or target. Various prior art apparatus and method have been used in powder painting. Prior art powder apparatus having means to shape the pattern size of the ejected powder included both mechanical types and vortex air types. One mechanical type prior art shaping means included an exterior adjusting sleeve which was normally cylindrical and was reciprocated into and out of the powder pattern. The sleeve deflected the powder particles into a forward direction. Often erosion of the sleeve and fusion of the powder on the interior surface of the sleeve became a major problem.

Another type of prior art mechanical shaping device included the placing of a powder deflector in the center of the powder-air mixture and attaching a rod to the deflector. The rod normally exited the rear of the powder gun. Pattern adjustment was accomplished by moving the deflector closer to or further away from the nozzle opening.

A prior art vortex air shaping means is shown in Figure 5 of the present drawings. A similar type vortex air structure is disclosed in United States Patent No. 3,940,061. Referring to Figure 5, the prior art spray gun A has a central air-powder passageway B, a centrally disposed diffuser C and a nozzle D. The nozzle D defines a plurality of vortex air passages E which are in communication with a clean air chamber F. As the air-powder mixture, in the Figure 5 gun, moves around the diffuser C, it is engaged by compressed vortex air from the passages E. The vortex air strikes the air-powder mixture tangentially and causes the air-powder stream to assume a whirling movement. In this type of prior art structure, as the force of the tangential vortex air increases, the down stream area is increased.When the vortex air force is reduced, the down stream area of the air-powder mixture is reduced in cross sectional area.

Patent No. 3,964,683 discloses another prior art electrostatic spray apparatus for the deposition of powdered coating material. This patent is directed to a pneumatic safety interlock feature.

The present invention is a powder spray gun apparatus which is normally a high voltage electrostatic type powder spray gun. The spray gun includes a remotely controlled shaping feature, whereby the pattern size of the air-powder mixture leaving the central region of the gun nozzle may be adjusted or regulated.

In a preferred form of the invention, a flared diffuser is positioned within the nozzle assembly of a powder gun. A powder gun, according to the invention, may be either a manually operated gun or an automatic gun.

The flared diffuser turns the air-powder mixture sharply outwardly.

A shaping air annulus which is in communication with a clean compressed air supply is defined by the nozzle assembly. The shaping annulus is in the shape of a cylinder having its major axis parallel and normally coincidental with the longitudinal axis of the central air-powder passageway of the gun.

Compressed air which is discharged from the shaping air annulus impacts the air-powder mixture with sufficient force to shape the downstream pattern size. According to the present invention, increasing the air pressure of the air discharged from the shaping air annulus increases the impact force and reduces the cross sectional area and the resulting pattern area of the air-powder mixture. Decreasing the air pressure and impact force increases the resulting pattern area. It has been found that this novel method results in greater transfer efficiencies particularly when large patterns are required.

Accordingly, it is an object of the present invention to provide a powder painting method and apparatus resulting in greater transfer efficiencies.

Another object of the invention is to provide an electrostatic powder coating apparatus having a remotely controlled air-powder shaping means.

Another object of the present invention is to provide an electrostatic powder apparatus in which mechanical movable components are not used to vary the powder pattern size.

Another object of the present invention is to provide an improved electrostatic powder apparatus in which shaping air is utilized during the less frequent times when a reduced pattern size is an operational requirement.

Other objects and advantages of the invention will become apparent from the following detailed description, with reference being made to the accompanying drawings.

Figure 1 is an elevational view of an electrostatic powder apparatus for the deposition of a powder material on a workpiece in accordance with a preferred embodiment of the invention.

Figure 2 is an exploded view showing individual major components of the apparatus of Figure 1; Figure 3 is a fragmentary cross-sectional view of the apparatus of Figure 1, shown on an enlarged scale; Figure 4 is a cross-sectional view taken along line 4-4 of Figure 3; and Figure 5 is a fragmentary, cross-sectional view, similar to Figure 3, and showing a prior art electrostatic handgun having a vortex air shaping device.

Referring to Figures 1 and 2 a handgun 10 for the electrostatic deposition of a powdered coating material, such as paint, is shown. It is understood that the handgun 10 is only one embodiment of the present invention. The present invention may be used in automatic painting apparatus rather than in the manual handgun shown.

The handgun 10 includes a gun body 11 and a barrel 12 having a front section 13. Referring to Figures 2 and 3, an electrode holder 15, mounting an electrode 16 is received by the front section 13. A nozzle assembly 18 includes an inner nozzle member 19 and an outer nozzle member 20 which are press fitted together.

The nozzle assembly 18 defines a cylindrical recess 22 which receives a diffuser 23. As best shown in Figure 4, the diffuser 23 includes legs 24 which engage the cylindrical recess 22 in a seated position. During normal operation, the diffuser 23 does not move longitudinally with respect to the longitudinal axis 27 of the barrel 12. The diffuser 23 includes a flared end 28. Referring to Figure 3, the handgun 10 includes a cylindrical powder tube 29 which mates with a cylindrical recess 30 which is defined by the electrode holder 15. The recess 30 discharges into a powder passageway 31 which is defined by the nozzle assembly 18. During operation, an air powder mixture is pumped through the aligned passageways defined by the tube 29, the cylindrical recess 30 and the passageway 31.As the air powder mixture moves along the longitudinal axis 27, the diffuser 23 and its flared end 28 deflect the air powder mixture sharply angularly outwardly into a frusto-conical shape.

The gun 10 includes a power supply circuit, generally indicated as 33 and having a current limiting resistor 34. The power supply circuit 33 includes biasing springs 35 and 36 which are in electrical communication with the electrode 16.

The present handgun 10 includes safety valving means generally indicated by the reference number 40.

The safety valving means 40 senses air pressure at the face 41. The safety valving means 40 shuts off both the powder supply and the electrical power as soon as the nozzle assembly 18 is loosened or removed from the gun 10.

Referring to Figure 3, a compressed air chamber 42 is defined by the front section 13 of the handgun. The clear air supplied to this chamber normally ranges between 0 psig and 40 psig. The nozzle assembly 18 defines an annular shaping air cavity 43 which discharges through a shaping air annulus 44. The air annulus 44 is preferably between .005 inches wide and .025 inches wide, with the preferable width in the present embodiment being .015 inches. The shaping air annulus 44 extends in a direction parallel to the longitudinal axis 27. When shaping air is discharged through the air annulus 44 it is discharged along a cylindrical path which has its major axis coinciding with the longitudinal axis 27. Referring to Figure 3, the shaping air path P intersects the frusto-conical air-powder path S.A plane T which is perpendicular to both the longitudinal axis 27 and the path P intersects the frusto-conical air-powder path S and defines an acute angle R, which in the present embodiment is 23". However, the angle R may vary, normally falling between 15 and 30 , with a preferable range between 20 and 25".

A plurality of circumferentially spaced holes 46 are defined by the inner member 19 of the nozzle assembly 18 and are in communication with the shaping air cavity 43. The other end of the holes 46 are in communication with an annular air chamber 47 which in turn communicates with the compressed air chamber 42 through passageways, one of which is indicated by dashed lines at 48. Therefore, the air cavity 43 is in full communication with the compressed air chamber 42 through the holes 46, the annular chamber 47 and the passageway 48.

When using an automatic system, the air pressure within the compressed air chamber 42 and consequently the shaping air cavity 43 is regulated at a remote location. It is not necessary to manually adjust a rod or manually adjust the longitudinal positioning of a deflector mechanism to shape the air-powder mixture being discharged from the gun.

In the present embodiment, the air pressure in the chamber 42 and cavity 43 is adjusted between 0 psig and 40 psig. As the air-powder mixture passes through the powder tube 29 and is deflected outwardly in the frusto-conical shape along the path S, the shaping air interesects the frusto-conical path S and impacts the air-powder mixture. The impact force drives the air-powder mixture into patterns of a smaller area as the air pressure is increased. If the air pressure is decreased, the downstream pattern size increases with the maximum pattern size being present when the air pressure reaches 0 psig. It has been found that the transfer efficiencies, particuiarly in the larger pattern sizes is greatly improved by the present method and apparatus.

Transfer efficiency is the amount of powder placed upon the workpiece compared to the amount of powder passing through the spray apparatus. The following test results, while not represented as covering all prior art apparatus nor covering all parameters of operation, indicate that at the largest pattern size, where many industrial powder painting operations occurthe apparatus, according to the present invention (shown in Figure 3 and identified as "DeVilbiss FL.") has a significantly higher transfer efficiency.

Maximum Transfer Effective Flow Test Gun Efficiency Pattern Rate DeVilbissFL. 91% 19 in. 200 gr./min.

DeVilbissStandard 81% 15.5 in. 200gr./min.

EHP-504 N 84% 19 in. 200 gr./min.

G 78% 16 in. 200 gr./min.

The DeVilbiss Standard EHP-504 apparatus is the prior art apparatus, which uses vortex air for shaping the pattern size. This apparatus is shown in Figure 4.

The "N" nut uses replaceable diffusers having different diameters to adjust pattern size.

The "G" unit uses a diffuser which is movable along the longitudinal axis of the nozzle to vary the pattern size.

During operation, the electrode 16, in the present embodiment electrostatically charges the particles being discharged from the gun 10.

Claims

1. A method of powder painting using powder particles comprising the steps of: moving an air-powder mixture along a longitudinal axis, deflecting the air-powder mixture angularly outwardly into a generally frusto-conical shape, and intersecting the frusto-conical air-powder mixture with a cylindrical path for a shaping air stream, such cylindrical path having a major axis coinciding with such longitudinal axis, said shaping air steam having sufficient force to impact the air powder mixture and shape the outer periphery of said mixture, whereby the downstream pattern size is regulated to a predetermined size.

2. A method according to claim 1, including adjusting the force of the shaping air steam as it intersects the air-powder mixture to vary the downstream pattern of the air-powder mixture.

3. A method according to claim 2, including the step of electrostatically charging the powder particles.

4. A method according to claim 3, wherein a plane which is perpendicular to such longitudinal axis and to such cylindrical path intersects said frusto-conical shaped air-powder mixture thereby defining an acute angle.

5. A method according to claim 3, wherein the pressure of the shaping air steam is variable between 0 and 40 psig.

6. A method according to claim 4, wherein said acute angle is between 20 and 25 .

7. An electrostatic powder painting apparatus comprising, in combination, a body assembly defining a longitudinally extending air-powder passage having a longitudinal centerline, a diffuser assembly positioned along such centerline adjacent the discharge end of said air-powder passageway, said diffuser assembly defining an angular path in communication with said air-powder passageway, said body assembly defining a cylindrical clean air passageway, which discharges air in an air path parallel to such centerline, such air path intersecting such angular path, whereby said clean air may impact said air-powder mixture moving along said angular path to shape such mixture to a predetermined size, and means for electrostatically charging said powder.