CA1088298A - Process and apparatus for the electrostatic deposition of powder to a grounded metallic substrate - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Process and device are provided for applying a vitreous enamel to metallic objects; more specifically, a method and a device for the application of vitreous enamel in dry powder form to metallic substrates by the projection of electrostatically charged powdered porcelain enamel with the aid of at least one detached supplementary electrostatic field emitter for re-directing stray charged particles towards the metallic object, the emitter comprising a rod member placed at the focal point of a metallic concave reflector.

Description

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At the present time, the application of vitreous, porcelain enamel in dry powder form is carried out by pro~ecting a cloud of the dry, powdered enamel from a slight distance, generally about 200 mm., from the objects to be enamelled which are principally objects of sheet steel .
In order to improve uniformity of deposition, an electrostatic field is created in the cloud of powdered material, so that the particles receive a negative electrostatic charge, while the conducting objects to be coated are grounded, and thus have a relatlvepos~ive p~n- ;.
tial. There ensues an attraction of the negatively charged particles by the conducting objects to be coated, which assume a positive potential with respect to said particles .
As for electrostatic powder application, in the currently employed process, various means are used; ~-~
generally by aerating the powder~ with projection being ensured by one or several spray gun which function by compressed air .
The electrostatic field is created by a conti- .
nuous current generator which charges the electrodes negatively in order to create the electrostatic field~
Generally, the field can be regulated between 60,000 and 100,000 volts, and the electrodes are positioned at the place from which the powder is projected, that is about 2.00 mm from the objectsO More specifically, when the powder is projected by a pneumatic spray gun, the electrode is placed at the center of the spray gun's nozzle .
The application of vitreous enamel by the techni-que in general use at the present time, and which has .2. ~`~

1~88298 just been described, allows the deposit of a layer of dry material which has an average thickness of 0.20 mm~
on the surface of metallic objects, and which, after firing or fusing, gives an average coating thickness of 0.14 mm.
It will be noted that, in spite of the use of an electrostatic ~ield which directs the particles of powdered enamel to the conducting object to be coated, only part of the cloud of particles e~fectively reaches the object; the deposit efficiency being in the range of 30 to 40~ (this is the ratio between the quantity of material actually deposited on the object, and the quantity of material pro~ected. The portion (70 to 60%) of enamel which is not deposited is recovered by means of different kinds of apparatus, mainly cyclone sepa-rators and filters, with the recovered powder being recycled toward tne spray gun, for purposes of reuti-lization. While it is true that the loss of powdered material is relatively limited, due to this recycling, nevertheless, the efficiency of the various kinds of apparatus used for recovery, such as cyclone separators and filters, is not 100%; moreover the recycled powder is somewhat contaminated during the recovery and recy-cling. Thus it would be advantageous to increase the ef~iciency of the projection, and this is being done at the present time by various means, notably :
- improvement of the devices for projecting powder~
- the arrangement and struct-ure of the electrodes which will electrostatically charge the particles, - improvement of the enamel powders by increasing their capacity to receive an electrostatic charge O

Despite all these efforts, the efficiency is rare-~ ` '' ,"' '' `,' ' ~

ly greater than 40%.
This invention consists of reinforcing the electrostatic fieldcreated by the electrode or electrodes placed in immediate contact with the enamel powder at the time of projection by means of an intense directional electrostatic field, capable of directing the suspended particles in the air -~
toward the objects, with this intense directional electrostatic fieLt being produced principally by field emitters, each of which has the form of the antenna of a television set placed at the focal point of a parabo~ic reflector, and which emit an intense electrostatic field, with these emitters being given the same charge as the electrode or electrodes.
In accordance with the present invention there is provided in a device for applying coating particles to a metallic, electrically grounded --object comprising means for supporting the object, said device having a first means for projecting the particles towards said object, said first means in-cluding means for electrostatically charging said particles at a high poten-tial and initially projecting same towards said metallic object, the improve-ment of at least one supplementary electrostatic field emitter having an int - ; -tense directional electrostatic field for re-directing said charged initially directed particles towards said object, said emitter being composed of a rod ~ ;
member placed at the foca~ point of a metallic concave reflector and being mechanically and physically disassociated from said first means.
Thus one succeeds in implementing, in a very intense way, the phenomenon called wrap-around, which allows the particles that do not come directly in front of the frontal part of the object to be attracted by it and to be deposited on the lateral or upper or lower surfaces, and even to go around the object completely and~ in some cases, to be deposited on the far surface.
This wrap-around phenomenon is very important since it allows the -powder to be deposited on all parts of a metallic object and not only on the parts that directly face the projecting apparatus.
It is the application of this phenomenon that permits the conception of simple and automated devices for coating objects that have edges or sur-4~?
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1~88298 faces at right angles to the principal surface to be coated. F~rmerly,devices capable of complicated maneuvers were necessary to project the powder directly onto such secondary surfaces residing in a different plane, or it :~
was necessary `~
.

' - 4a -~088298 to forego automation for coating these surfaces, and pro~ect the powder with the help of a manually operated projection apparatus O
The appropriately situated, supplementary field emitters ensure that the contours of the cloud are thrown in the direction of the ob~ect to be coated, and thus ensure greater efficiency in the deposit of the particles on the ob~ects and a better coverage of the edges, and rlght angle surfaces, fully utilizing the wrap-around phenomenon .
Moreover, the fact that they have the same poten-tial as the primary electrodes avoids disturbances and i.
turbulence phenomena on the cloud .
In any case, the invention will be understood :~:
easily with the help of the description that ~ollows, as well as the attached drawings, which are, of course, given primarily for guidance O
Figure 1 portrays, in lateral view, a device pro-vided with the improvements of the invention, for ap-plying a powdered, dry vitreous enamel to metallic ob;ects .
Figure 2 is a top view of the same device .
According to the invention, and particularly ac-cording to the embodiment which should be given prefe- -rence, for example, for the purpose of realizing a device ~or applying a dry, powdered vitreous enamel to metallic ob~ects, one should proceed as follows, or in an a~alogous manner .
First, in accordance with the well-known technique, a metallic object 1, such as a steel sheet, is electri-cally grounded as at 2; ob~ect 1 is supported by means of cables 4, for example, suspended from an upper rail 3, 1C~88zg8 electrically connected to electrical ground 2.
Metallic object 1 can be composed of an object of any form and dimensions, as well as a plate, and may even have bent edges, due to ~he improvements of the invention.
It need not be steel, but can also be cast-iron and other metals, particularly aluminum and copper.
The powder is projected on to object 1, after having been aerated by means of a spray gun 6, which works hy compressed air, and which has in its center an electrode 7, charged with a high potential, which can be between 60,000 and 100,000 volts, continuous current, negative polarity, by means of a high tension generator 8, to which it is connected by a conductor 9. Spray guns of this kind are well known to the art. As for the powder, it can be composed of any suitable vitreous enamel powder. For example, a powder of the kind described in United States Patent No 3,928,668 issued December 23, 1975 "Electrostatic Deposition of Dr~ Ceramic Powders" may be used, which powder has been pre-coated with an organopolysiloxane.
One can also use an enamel having the following camposition in parts by weight :
SiQ2 3Q to 50 R2 ~ lQ to 30 Na20 3 to 15 K2Q 5 to 10 Ti02 10 to 25 P2Q5 Q to 10 F2 to lQ
These two types of enamel are ~n no way limiting.
Qne envisions, in the well-known manner, of course, means of shifting spray gun 6 with respect to plate 1.

108829~3 Thus~ in Figure 1 are illustrated, in unbroken and broken lines, respectively, the two extreme positions (with regard to height of spray gun 6, namely, posi- ~-tions 6a and 6b. In Figure 2, 6c represents a cloud of powder in suspension in the air and pro~ected by spray gun 6 m e system that has ~ust been described is of the well-known kind .
In accordance with the invention, two supplemen-tary electrostatic field emitters 10 are foreseen on each side of spray gun 6. These two emitters are parabolic an d are directed toward ob~ect 1 . Each emitter 10 is advantageously composed of a television set antenna 11 placed at the focal point of a parabo-lic reflector 12, and emits an intense electrostatic field. A conductor 13 puts each emitter 10 (mounted on a support 14 by means of an insulating bar 15 born by a rod 16) at the same potentlal as electrode 7 or ;~ spray gun 6. As a result, the emitters 10 are brought A to the same potential as electrode 7, a potential comprised between 60,000 and 10,000 volts, continuous current, negative polarity .
Other supplementary field emitters can be placed at will under and over ob~ect 1, depending on the direction one is trying to give to the powder cloud 6c, particularly if ob~ect 1 includes lower and/or upper edges .
The method and the device of the ~nvention permit, a~ong others, application to porcelain enamelled house-hold articl~s such as cookware, parts of electric ovens, washing machines, and refrigeratorsO They are ~lso applicable to enamelled pannels for building façades.

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While part o~ a television antenna provides a convenient means of achieving supplementary field emitter member 11, it is obvious that the design and dimensions of member 11 may be varied extensively to accommodate varying processing requirements, and any variety of branched or straight rod-like members will serve equally well .
It is to be understood, of course, that the particular embodiment illustrated in ~igures 1 and

2, and the kinds of ob~ects to be enamelled, as well as the enamel composition disclosed~are merely exempla-ry and may be modlfied extensively ~without departing from the princlple of this invention.
Although the preferred embodiment illustrated above discloses a parabolic reflector, it ls to be understood that this particular shape is but exemplary and, depending upon the parameters of a given coating operation, the reflector may assume any one of myriad shapes, so long as it is of a generally concave nature and is capable of reflecting and directing the field emitted from member 11 .

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Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a device for applying coating particles to a metallic, electrically grounded object comprising means for supporting the object, said device having a first means for projecting the particles towards said object, said first means including means for electrostatically charging said particles at a high potential and initially projecting same towards said metallic object, the improvement of at least one supplementary electrostatic field emitter having an intense directional electrostatic field for re-directing said charged initially directed particles towards said object, said emitter being composed of a rod member placed at the focal point of a metallic concave reflector and being mechanically and physically disassociated from said first means.

2. The device of claim 1 wherein said particles are powdered, vitreous enamel.

3. The device of claim 1 wherein said concave reflector is parabolic.

4. The device of claim 1, wherein said emitter is chargeable to the same potential as the means for electrostatically charging said powder.

5. The device of claim 1 wherein said high potential is negative.