US3870232A

US3870232A - Nozzle for projecting powdered solid products

Info

Publication number: US3870232A
Authority: US
Inventors: Adrien Lacchia
Original assignee: Air Industrie SA
Current assignee: Air Industrie SA
Priority date: 1972-05-26
Filing date: 1973-04-30
Publication date: 1975-03-11
Anticipated expiration: 1992-03-11
Also published as: GB1413394A; IT984188B; JPS4942742A; JPS529219B2; DE2325989A1; CH570207A5

Abstract

The invention relates to a nozzle for projecting powdered solid products intended for coating objects, of the kind comprising a device for shaping the incident mixture consisting of said powdered solid and its conveyor gas, into an outlet jet of substantially flat form, said nozzle essentially comprising a plurality of separate pipes passing at least partly through the nozzle in the direction of emission of the jet, and intended to be completely traversed by a specific portion of said jet, each of said separate pipes being provided with a suction device for said incident mixture, said suction devices being of the Venturi type, each comprising at least one conduit for injecting a flow of auxiliary gas into said pipes. Especially advantageous applications of the invention are found in the field of electrostatic powdercoating apparatus.

Description

United States Patet [1 1 Lacchia 1 Mar. 11,1975

[54] NOZZLE FOR PROJECTING POWDERED SOLID PRODUCTS Adrien Lacchia, Meylan, France [30] Foreign Application Priority Data May 26, 1972 France 72.19635 Feb. 1, 1973 France 73.04028 [52] U.S. CI 239/15, 239/318, 239/433, 239/566 [51] Int. Cl B05b 1/04, BOSb 5/02, B05b 7/04 [58] Field of Search 239/310, 311, 318, 398, 239/429, 430, 433, 434, 434.5, 548, 552,

2,961 0/1885 Great Britain 239/566 Primary E.\'aminerRobert S, Ward, Jr. Attorney, Agent, or FirmBrisebois & Kruger [57] ABSTRACT The invention relates to a nozzle for projecting powdered solid products intended for coating-objects, of the kind comprising a device for shaping the incident mixture consisting of said powdered solid and its conveyor gas, into an outlet jet of substantially flat form, said nozzle essentially comprising a plurality of separate pipes passing at least partly through the nozzle in the direction of emission of the jet, and intended to be completely traversed by a specific portion of said jet, each of said separate pipes being provided with a suction device for said incident mixture, said suction devices being of the Venturi type, each comprising at least one conduit for injecting a flow of auxiliary gas into said pipes.

Especially advantageous applications of the invention are found in the field of electrostatic powdercoating apparatus.

8 Claims, 10 Drawing Figures Great Britain 239/398 FATENTEB NARI 1 I975 SHEET Q 0F 5 NOZZLE FOR PROJECTING POWDERED SOLID PRODUCTS The present invention relates to a projection nozzle for powdered products which can be used in all projection apparatus, electrostatic or other, for powdered solid products such as PVC, polyethylene, epoxy resin, polyamides, enamel, talcum powder, etc., these powdered solid products being intended for coating objects.

The invention is concerned with a projection nozzle for powder products of the type specified above, in which the jet of powder mixed with its conveyor gas is discharged from the nozzle in the form of a flat jet shaped like a fan. The invention is also concerned with an electrostatic projection nozzle of this kind.

' In accordance with French Patent No. 1,302,415, the known devices of this kind comprise a flat fan-shaped nozzle in which the jet of powder, mixed with its conveyor gas, is projected in a thin uniform layer. Nozzles of this kind are not however entirely satisfactory in operation.

In fact, the flat fan-shaped jet obtained is surrounded on both sides by a cloud of powder particles due to the turbulence created by the expansion to free air of the jet of gas passing out of the nozzle.

These vortices which surround the jet slow-down and deflect a large percentage of the powder particles and it is then no longer possible to speak of a true flat jet, but rather of a cloud of ill-defined shape.

Now, and especially in the case of certain applications of electrostatic powdering, it is essential to be able to obtain a flat jet having a well-defined shape and a high-speed of projection of the particles. It is in fact known that in the case of electrostatic powdering, if their speed is sufficiently low, the particles of powder follow the lines of the electrostatic field created between the projection nozzle and the object to be coated. In the case where the parts to be coated do not have cavities or recesses, this phenomenon is of particular advantage since it prevents the loss of particles of powder. It is then advantageous to reduce thespeed of the jet to the maximum, for example by using a jet of secondary turbulent air, in accordance with French Patent No. 1,340,629.

However, in the case of parts having cavities or recesses, in which the lines of force of the electrostatic field can obviously not re-close, it is on the contrary essential to give the jet of powder a fairly high speed in order that a sufficient number of powder particles may overcome the forces due to the electrostatic field becoming re-closed on the projecting parts of the object and in consequence may ensure the coating of the said cavities or recesses.

The charged particles of powder having penetrated into these cavities or recesses then become fixed on the wall by virtue of the attraction due to the electrical image forces and to inter-molecular forces.

A nozzle with a round jet could obviously be employed, but its use would however be difficult in the case where the surface to be coated is fairly large and where only a short time is available in order to ensure its coating, which is the case in the field of industrial powdering.

On the other hand, a flat jet gives more rapid and furthermore more uniform powdering of the cavities and especially of those of elongated shape. In addition, it should be noted that when such flat jet nozzles are utilized in such manner that the jet is vertical in its largest dimension, and does not give a uniform distribution of powder in the jet.

In fact, due to the effect of gravity, the density of powder per cubic centimetre is much greater in the lower part of the fan formed by the jet than in its upper part. In consequence, the coating of the object is also irregular, which is particularly undesirable. On the other hand, the jet in the shape of a fan is incurvcd to-- wards the bottom, so that the use of the nozzle becomes difficult.

Another known powdering nozzle device with a flat jet consists of a hemispherical nozzle, at the downstream extremity of which is provided a horizontal slot cutting the hemisphere and passing thereinto in the shape of a fan.

This device has a number of disadvantages. On the one hand, the said slot has been given a small dimension with respect to the radius of the hemisphere, in order to obtain a sufficiently fine jet having an outlet angle which is not too large in order to ensure the coating of objects of all shapes. In consequence of the small dimensions of this slot, the flow-rate is limited.

In actual fact, the supply of powder projection devices at the present time is effected by means of a Venturi system, and the existence of this low conductance at the extremity of the nozzle, due to the small dimensions of the slot, results in a limitation of the flow-rate of conveying air, and therefore in operation with a limited flow-rate of powder. On the other hand, it has been found that the distribution of the product at the outlet of this type of nozzle is bad, the powder density at the centre of the jet being much too large at high flowrates.

The device according to the invention makes it possible to overcome these drawbacks. In this device in fact, the flow-rate of powder can be made variable over a very wide range, the cloud surrounding the jet in nozzles known at the present time being eliminated; the distribution of powder in the jet is well defined, and it is possible to construct nozzles according to the invention in such manner that the jet has any desired width, either very large or very small.

The device is characterized in that the projection nozzle is constructed in such manner that its front portion comprises a number of separate openings, each of these openings forming the larger base of a tube in the shape of a truncated cone, each of these tubes terminating towards the rear portion of the nozzle in a separate powder injection suction device of the Venturi type, by means of which a definite portion of the mixture of powder and conveyor gas is tapped off by individual suction and is injected into the said pipe. The said circular openings are preferably placed in alignment in order that the jets discharged from each of these openings at a correct angle become mixed so as to form a flat jet.

The invention will be better understood by means of the description which follows below, given by way of example of construction of two electrostatic projection nozzles for powder products, reference beingmade to the accompanying drawings, in which:

FIGS. la and 1b are perspective views of an electrostatic projection nozzle for powder products in accordance with the invention;

FIG. 2 is a front view of the said nozzle;

FIG. 3 is a longitudinal section of a view looking on the side of the said nozzle;

FIG. 4 is a longitudinal section ofa view looking from above on the said nozzle;

FIGS. 5a and 5b are perspective views of a second electrostatic projection nozzle for products in powdered form, according to the invention;

FIG. 6 is a front view of the said nozzle;

FIG. 7 is a longitudinal section of a view looking on the side of the said nozzle;

FIG. 8 is a view in cross-section projected at the level of the charge electrodes of the said nozzle.

The electrostatic powdering nozzle shown diagrammatically in FIGS. 1 to 4 comprises a nozzle body 1 constituted by an electrically-insulated material such for example as that sold commercially under the trademark DELRIN or TEFLON, and screwed on the extremity 2 of an electrostatic powdering apparatus, such as a manual or automatic powdering gun described in U.S. Pat. Nos. 3,248,606 and 3,682,384 and sold by the applicants under the respective Trade- Marks STAJETYand MEGASTAJET, and also comprises a rear inlet ring 3 made of an electricallyconducting and preferably resistive material which forms the inlet orifice for the mixture of powder and its conveyor gas, and which is brought up to the high electric voltage necessary to create the electrostatic field and to charge the particles of powder, this directcurrent high-tension being generally in the neighbourhood of 70 kV. Two wire electrodes 4 and 5, electrically-connected to the ring 3, extend towards the front up to the level of the mouth 6 of the nozzle.

The Applicants have found it advantageous to use an inlet ring 3 of Teflon charged with 60 percent of bronze, a material with a low coefficient of friction and high electrical resistance, the electrical resistance of the ring 3 then having a value of several megohms.

The electrodes 4 and 5 are each constituted for example by a length of piano wire of 0.4 mm. in diameter, the whole of the length except for the front portion being embedded in an insulating material such as Araldite. The front face of each nozzle is provided, according to the invention, with a number of

separate orifices

7, 8, 9, 10, 11 and 12, through which the powder is ejected in the direction of the object to be coated.

In practice, the electrode 4 ensures the charging of the powder particles discharged from the

orifices

7, 8 and 9, while the electrode 5 ensures the charge of the powder particles proceeding from the orifices 10, I1 and 12. Each of the

orifices

7, 8, 9, 10, 11 and 12 forms the termination of

separate tubes

13,14, 15, 16,17 and 18 which are preferably of frusto-conical section flaring outwards towards the front portion of the nozzle.

The mixture of the powder and it conveyor gas is distributed over each of the pipes l3, 14, 15, 16, I7 and 18. According to the invention, this distribution is controlled in intensity by individual suction devices constituted, for each ofthe pipes 13 to 18, by one or a plurality of inlets I9, 20 of auxiliary gas under pressure, terminating in an oblique manner towards the front in the corresponding tube, so as to create a suction in the said tube by a system of the Venturi type. For this purpose, the auxiliary inlets l9 and 20 are connected to an auxiliary gas inlet such as the inlet which is normally intended, in the STAJET and MEGASTAJET spraying guns for the injection of turbulent secondary air. In the case of the STAJET or MEGASTAJET gun, it is in practice only necessary to remove the vortex nozzle with which it is normally fitted, and then to screw on instead of this latter the nozzle according to the invention.

The quantity of powder sucked-in by a given pipe is a function of the diameter of the orifice of the pipe and also of the diameter of the auxiliary inlets l9 and 20. During the construction of the nozzle, it is thus possible to regulate the flow-rate of the jet of air discharged from each of the pipes 13 to 18, in such manner that the disadvantages of the flat jet nozzles of the prior art no longer exit.

In order to improve the formation of the flat jet, it is possible further to modify the nozzle in various ways. For example, the orifices 7 to 12 may be set back from the mouth 6, which latter is then constituted by a uniform slot which ensures a better mixture of the separate jets in order to form the flat jet.

The optimum apex angle of each of the truncated cones was found by experiment to be 7, satisfactory operation being however ensured when the value of this angle is comprised between 0 and l4.

The axes of the pipes 13 to 18 are so arranged that they form as a whole a fan opening out towards the front of the nozzle, the angle existing between the axes of the

end pipes

13 and 18 determining in practice the outlet angle of the jet of powder.

FIGS. 5 to 8 show diagrammatically an electrostatic projection nozzle for powder products according to the invention, similar to that shown diagrammatically in FIGS. 1 to 4 and described above, but of an improved type.

The electrostatic projection nozzle for powder products previously described comprises at least one charge electrode located outside the jet and level with the front mouth of the nozzle. Electrodes of this type are each constituted, as has been seen, by a glow-discharge point of small radius. It has unfortunately been found that, due to the glow-discharge produced by such a point, the latter was brought up to a temperature sufficient to melt the particles which become accidentally deposited on it.

It is known that epoxy powders become soft and melt at a temperature of about 40C. In the case of points brought to a high voltage varying from 60 to kV, as is customary for electrostatic projection apparatus for powder products, the temperature to which these points are heated due to the discharge may easily exceed this latter value of 40C. It is then found that there is melting of the particles of powder on the point and the formation of a real insulating shell, which, after several hours operation, may attain up to a centimetre in thickness.

Due to this insulating ball which then caps the point, the current falls, the field and the charge are also reduced and the amount of deposit is diminished to inacceptable values. In addition, a danger of electrical breakdown exists across the shell formed, and since the voltage is a maximum at the instant preceding the breakdown, the whole of the current possible is thus established, so that a prohibitive spark is obtained which may result in danger of fire.

In order to overcome this disadvantage of the discharge points employed in the field of electrostatic powdering, it has already been proposed, in the case of an apparatus intended to produce a deposit of powder, to surround the discharge points with an auxiliary jet of air which thus ensures that no particles of powder can become deposited on the points. With a device of this kind, there existed however the complicated and costly necessity of providing a supply of auxiliary air especially intended for that use.

The nozzle in accordance with FIGS. 5 to 8, makes it possible, in the case of an electrostatic projection nozzle for powder products according to the invention, to prevent the points utilized as charge electrodes from being covered with powder, without requiring any additional supply of secondary air. In this case there is utilized the supply of auxiliary gas intended for the various suction devices of the Venturi type in the different pipes provided on a nozzle according to the invention, to create, in a manner known per se, a jet of auxiliary gas around needles constituting the charge electrodes, thereby preventing these needles from becoming covered with powder tending to melt on them and in consequence to form an insulating shell.

The electrostatic powdering nozzle shown diagrammatically in the said FIGS. 5 to 8, thus comprises, in the same way as the nozzle previously described and shown diagrammatically in FIGS. 1 to 4, a nozzle body 21 made of an electrically-insulating material such as DELRIN of TEFLON and fixed on the extremity 22 of an electrostatic powdering device, such as a manual or automatic spraying gun sold by the Applicants under the respective Trade-Marks STAJET or MEGASTA- JET, together with a rear inlet ring 23 made of an electrically-conducted and preferably resistive material, forming the inlet orifice for the mixture of powder and its conveyor gas, and which is brought up to the high-tension voltage.

Two

electrodes

24 and 25 constituted by points of small radius are electrically connected to the ring 23 and extend towards the front up to the level of the mouth 26 of the nozzle. The

electrodes

24 and 25 are for example constituted, as previously, each by a length of piano wire of 0.4 mm. in diameter, the whole of which with the exception of the front portion is embedded in an insulating material.

The extremity of each of these

points

24 and 25 is surrounded by a sheath of air coming from the supply of secondary air provided, as previously, in order to create a suction device of the Venturi type in each of the separate frusto-conical pipes forming the interior of the nozzle.

It happens in fact that the relatively-high air pressure utilized for these separate Venturi systems is perfectly suitable for ensuring the continuous cleaning of the charge electrodes. In order to do this, the body 21 of the nozzle has been pierced with three channels, 28, 29 and 210. The channel 210 is intended to surround the

corresponding charge electrode

24 or 25, the channel 29 and the channel 28 are intended to provide a communication between the channel 210 and the supply of secondary air to the projection gun. A plug 211 prevents communication between the channel 29 and the exterior.

The powdering nozzles according to the invention may be utilized on all apparatus intended for the coating of objects by projection of powdered solid products. They find a particularly advantageous industrial application in the field of electrostatic powdering for coating objects.

What I claim is:

l. A spray nozzle for projecting solid particles carried by a conveyor gas from a spray gun of the type equipped with means for supplying secondary air, said nozzle being adapted to shape the projected particles and said conveyor gas into a single fan-shaped outlet spray, said nozzle comprising:

a nozzle body defining a plurality of separate ducts passing through said body in the direction of said spray, each separate duct having an inlet and a spray outlet and adapted to be completely traversed by a portion of said particles, said plurality of ducts being positioned to form beyond said outlets a spray in'which said particles are uniformly distributed, and

a suction device of the venturi type associated with each of said separate ducts, each suction device comprising a constriction at the inlet of the separate duct with which it is associated and at least one injection conduit positioned for discharging auxiliary gas received from sid secondary air supply means into said associated duct downstream of said constriction.

2. A nozzle as claimed in claim 1 in which each of said separate ducts has the shape of a truncated cone flared toward the outlet end of said duct.

3. A nozzle as claimed in claim 2 in which the apex angle of said truncated cone lies between 0 and 14 degrees.

4. A nozzle as claimed in claim 2 in which the apex angle of said truncated cone is substantially equal to 7 degrees.

5. A nozzle as claimed in claim 1 in which the axes of said separate ducts lie in a single plane.

6. A nozzle as claimed in claim 1 which comprises at least one charge electrode positioned to impart an electrical charge to said particles but located outside the region occupied by said spray and level with the outlet of said nozzle.

7. A nozzle as claimed in claim 6 in which each charge electrode defines a point of small radius and is housed in a hollow tube positioned to be connected to said secondary air supply means.

8. A nozzle as claimed in claim 7 in which said hollow tube is connected to said secondary air supply means through a duct in the body of said nozzle.

Claims

1. A spray nozzle for projecting solid particles carried by a conveyor gas from a spray gun of the type equipped with means for supplying secondary air, said nozzle being adapted to shape the projected particles and said conveyor gas into a single fanshaped outlet spray, said nozzle comprising: a nozzle body defining a plurality of separate ducts passing through said body in the direction of said spray, each separate duct having an inlet and a spray outlet and adapted to be completely traversed by a portion of said particles, said plurality of ducts being positioned to form beyond said outlets a spray in which said particles are uniformly distributed, and a suction device of the venturi type associated with each of said separate ducts, each suction device comprising a constriction at the inlet of the separate duct with which it is associated and at least one injection conduit positioned for discharging auxiliary gas received from sid secondary air supply means into said associated duct downstream of said constriction.

1. A spray nozzle for projecting solid particles carried by a conveyor gas from a spray gun of the type equipped with means for supplying secondary air, said nozzle being adapted to shape the projected particles and said conveyor gas into a single fan-shaped outlet spray, said nozzle comprising: a nozzle body defining a plurality of separate ducts passing through said body in the direction of said spray, each separate duct having an inlet and a spray outlet and adapted to be completely traversed by a portion of said particles, said plurality of ducts being positioned to form beyond said outlets a spray in which said particles are uniformly distributed, and a suction device of the venturi type associated with each of said separate ducts, each suction device comprising a constriction at the inlet of the separate duct with which it is associated and at least one injection conduit positioned for discharging auxiliary gas received from sid secondary air supply means into said associated duct downstream of said constriction.