CA1086491A - Method and device for applying a porcelain enamel to metallic objects - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An improved device for applying vitreous enamel to metallic objects; more specifically a method and a device are provided for applying vitreous enamel in dry powder form on grounded metallic substrates by projection of electrostatically charged, pulverized, powdered porcelain enamel frit in combination with at least one substantially portable, detached side deflector as a "non-source" supplement to the initial source of powdered porcelain enamel.

Description

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; At the present time, vitreous enamel in dry powder form is applied to metallic objects by projecting a cloud of dry glaze powder from a slight distance, generally on the order of 200 mm, from the objects to be enamelled which are usually of sheet steel.
In order to improve deposition on the object, an elec-trostatic field is created in the cloud, so that the par-ticles of powder carry an electrostatic charge. In general, the particles of powder receive a negative electrostatic charge, while the conducting objects to be enamelled are ; grounded, and thus have a relative positive potential.
; There ensues an attraCtiOn of the negatively charged par-ticles of enamel by the conducting pieces to be enamelled, which have a positive potential with respect to them.
As for the projection itself, in the current technique, various means are used, generally aeration of this powder, with pro~ection being enSured by one or several compressed air spray guns.
i~The elèctrostatic field is created by a continuous current generator which charges the electrodes negatively and creates the electrostatic field. Generally the fi~ld :
i;can be regulated between 60,000 and 100,000 volts, and the electrodes are placed at the very place from which the projection originates, that is, 200 mm from the objects. More specifically, when the powder is projected from a pneumatic spray gun, the electrode is placed at the center of the spray gun's nozzle.
Application of vitreous enamel by the technique gene-;rally used at the present time, and which has just been ~30 described, allows a layer of dry enamel to be deposited, , :' ~
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, which has an average thickness of 0.20 mm, and which, after firing, gives an after fire layer of enamel that has an average thickness of 0.14 mm.
It will be observed that, in spite of the application - 5 of an electrostatic field which directs the particles of ~` pulverized enamel to the conducting object to be ename~ed, only a part of the cloud of particles effectively reaches the object. The deposit efficiency of the enamel is in the range of 30 to 40% (this is the ratio of the quantity of enamel deposit~d on the object and the quantity of enamel projected)~ The portion of enamel (70 to 60%) which is not deposited is recovered by means of various kinds of apparatus, principally cyclone separators and filters, : with the recovered powder being recycled, for purposes of reutilization, towards the spray gun. While, due to this recycling, losses of enamel powder are relatively limited, the fact remains that the efflciency of the : recovery apparatus, such as cyclones and filters, is not 100%; moreover, the recycled powder is somewhat contami-, 20 nated during recovery and recycling. Thus, it would be advantageous to increase the efficiency of the projection, and this is currently realized by various means, notably:
- improvement of the devices which project enamel powders, - the arrangement and structure of the electrodes which will electrostatically charge the particles of enamel, - improvement of the enamel powders to increase their capacity to receive an electrostatic charge.
Despite all of this, the efficiency achieved rarely exceeds ~0%.
Another drawback to the methods and devices currently in use for applying enamel in powder form is the phenome-non of counter-emission, which results from the electrical - 5 resistivity of the particles of enamel, due to which the electrical charges, which are held to the particles of enamel by the elec-trostatic ~ield, are kept after deposi~
tion on the conductor object to be enamelled, for some time. This phonomenon is ad~antageous since it increases the adherence of the particles to the obj~ct; however, when the charges have completely dissipated, the adherence ~` is lost. Meanwhile, the accumulation of particles of ena-mel on the surface of the object;, that is, the increase ~ in thickness of the layer, increases the electrostatic ,r~,15 field between the negatively charged exterior of the layer !, and the positive ob;ect. When the value of the electrosta-: .
i tic field in this condenser reaches a certain limit, gene-rally corresponding to a thickness of 0.15 to 0.25 mm of powder (according to the powder's electrical resistivity), ~ .
~ strains occur throughout the layer. These s~ains show up ` as pin holes coming to the surface of the ena~el powder on the surface of the conductor ob;ect.
The lines of the field going from the electrodes to ` the object converge on these holes, which become the center of emission of ions of the opposite sign from those formed by the electrode; of the spray gun. This is called counter emission. Irregular depositions of powder result from it, ; principally depressions of the coating at the site of these pin holes which show up as depressions in the surface of the enamel after firing.

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Similarly, the counter-emission fields created around these pin holes affect the distribution of the enamel and cause localized irregularities in the film thickness, which, after firing, show up as defects, thus creating an enamel defect characteristic of this procedure, called counter-emission defect.
Beyond localized defects, the appearance of counter-emission `~ weakens the electrostatic field, and consequently, the new projected powders will receive a charge too weak to have a sufficient power of adhesion.
The deposition does not continue. The maximum thickness of 0.15 to 0.25 mm has been reached.
It can thus be seen that the currently used technique presents at least three disadvantages, namely, a relatively low efficiency of projection (in the range of 30 to 40%), and a maximum thickness of the layer ~0.15 to 0.25 mm), which may be insufficient for ensuring excellent protection, and finally, an irregular enamelled surface, with defects.
The invention at hand has the objective of reducing the afore-:~ mentioned drawbacks.
According to one aspect of the present invention there is provided a device for applying coating particles to a metallic object comprising support ~ 20 means for supporting the object, a first means for projecting the particles -`` toward said object, said first means including means for electrostatically charging said particles at a high potential and means for initially projecting same towards said metallic object, at least one insulating element between the object and ground for insulating said object with respect to ground, and at least one supplementary electrostatic field emitter having an intense directional electrostatic field for re-directing said charged initially directed particles toward said object, said emitter being composed of a rod member placed at the focal point of a concave reflector.
This is fundamentally opposed to the theory accepted until now to explain the deposit of enamel on conductor objects, and it is also contrary .~
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to practice, since all the devices that have as their purpose the application of vitreous enamel in dry powder form by the electrostatic method use the direct application of the accepted theory, . .
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and deposit the negatively charged enamel to the objects which are grounded~
Implementation of the method and the devices of the : invention offers the following advantages :
1) The phenomenon of counter-emission is completely suppressed, with all that arises from it; that is, surface defects and limited thickness due to counter-emission.
A new maximum layer appears, this time due to the for-ce of attraction of the layer of powder with respect to ; 10 the weight of this layer. But the thickness of this new maximum layer is greater that that which one is trying to deposit, and does not pose any problem.

2) The layer of powder which is deposited is at least as adherent before firing as the one which results from the customary process. Being insula-ted, the ob~ects acqui-re an electrostatic charge. In fact, experience shows that these insulated objects act as asel~harging condenser~,whih was not the case when the objects were grounded to the positive pole, and ensured that the charges were permanntly dissipated~ This acquired charge is discharged to g~eat extend at the time of the subsequent transfer of the ob-jects form their insulated support on metallic supports grounded to the conveyor of the firing furnace. Only the charges of the particles remain, with these charges being retained for a period of time which is a function of the ,~
electrical resistivity of the powder~ The layer of powder which is already deposited does not undergo this phenome-; non of discharge, and is not changed or altered.

; The firing of the enamel is carried out as it has been ^ 30 in the past, when the objects were grounded at the time .~:

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of projection of the enamel. The result is an equally adherent layer of enamel after firing.

3) The deposit efficiency of the powder with respect to the powder projected is increased, compared to an identical installation working with the same powder, but in which the objects to be enamelled are grounded, and in which the parabolic electrostatic emitters are not in place.
~ ) The method and the device of the invention allow more freedom in the choice of the type of powder and its resistivity, since the counter-emission phenomenon is suppressed, hence the possibility of improving effi-ciency still more.
Preferably, the emitters ~, 9~
are brought to the same potential as the electrode or elec-trodes.
Thus a very intense implementation of the phenomenon called wrap~around is achie~ed, which permits the parti-cles of enamel which do not arrive directly in front of the frontal part of the object to still be attracted by it and to be deposited on its lateral or upper or-lower sides, and even to go around the piece completely, and . .
in some cases, to be deposited on the far side.
This wrap~around phenomenon is very important, since it permits the deposition of enamel powder cn all sides of a metallic object, and not only on the parts facing the enamel projection apparatus directly.
It is the application of this phenomenon which allows one to conceive of simple and automated deYices for apply-ing enamel to objects that have edges, since previously, devices capable of complicated maneuvers were necessary to project the enamel directly in front of said edges, ; or one had to fore~o automation for these parts and pro-ject the enamel using the same method, but, in this case with the help of a hand held projection apparatus.
The appropriately situated field emitters ensure that the contours of the cloud are thrown in the direction of the objects and thus ensure a greater efficiency in the `` 25 deposit of the particleson the objects and a better cove-ring o~ the edges, that is, improvement of the wrap-:
around phenomenon.
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Moreover, the fact that they are at the same potential as the electrodes eliminates disturbances and turbulence .;.
~ 30 phenomena on the cloud of glaze.

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The invention will be easily understood with the help ; of the following description, as well as the appended dra-wings, both of which are given, of cour~ primarily for guidance.
Figure ~ portrays, in lateral view9 a device for apply-ing a vitreous enamel to metallic objects, with this device having the two arrangements mentioned above.
Figure 2 is a top view of the same device.
According to the invention, and more specifically, according to those embodiments which should be given pre-ference, such as, for exemple, intending to realize a device for applying a vitreous enamel to metallic objects, one should proceed as follows or in an analogous manner.
First, in accordance with the ~sential characteristic of the invention, metallic object 1, such as a sheet of steel, is insulated electrically from ground 2; it is for example, supported from an upper rail 3 by means of cable ~ with the interposing of insulating elements 5 between ; object 1 and the part of the cables4 which are electrically grounded.
- Metallic object 1 can be composed not only o~ a plate, but also by an object of any form a~ dimensions, even in cluding one with folded edges la, when the second arran-gement mentioned above is implemented, namely~ the imple-; 25 mentation of electrostatic field emitters. It can be not " only of steel, but also of cast-iron and other metals5 particularly aluminum and copper.
The enamel powder is projected onto object 1 after having been aerated by means of a compressed air spray gun 6, which contains, at its center, an electrode 7, .

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brought to a high potential, which can ~all between 60,000 and 100,000 volts, continuous current, negative polarity, by means of a high tension generator 8. Spray guns of this kind are well known in the art. As for the enamel powder, it can be composed of any appropriate inorganic vitreous - enamel powder. For example, a powder of the type described in UOS. Patent n 3,928,668, for "Electrostatic Deposi-tion of Dry Ceramic Powders", that is, a ceramic powder coated with an organopolysiloxane in the solid state can be used.
An enamel having the following composition in parts by weight can also be used :

SiO2 30 to 50 ` 15 B203 10 to 30 Na20 3 to 15 K20 5 to 10 TiO2 10 to 25 P205 0 to 10 0 to 10 These two types of powder are in no way limiting.
Of course, means of shifting spray gun 6 with respect :.
; to plate 1 in the customary manner are envisioned. Thus, ~-in Figure 1, the two extreme positions (with regard to height) of spray gun 6, namely positions 6a and 6b, are indicated by an unborken line and a broken line respecti-vely. In Figure 2, 6c represents the cloud of enamel pow-;~
der in suspension in the air and projected by spray gun 6.

The system which has just been described and which differs from the usual devices because of the existence )' , , g ;L~
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of insulating elements 5, which effect the electrical insulation between ground and the object to be enamelled 1, implements the first arrangement of - the invention.
This arrangement can be combined to advantage with another one, namely that which is described in applicants copending patent application serial No. 275,622, filed on April 5, 1977, and which consists of two electro-static field emitters 10 on either side of projection spray gun 6. These two emitters are parabolic and are directed towards the object.
; Each emitter 10 is comprised, to advantage, of a television set antenna 11 placed at the focal point of a parabolic reflector 12 and emits an intense electrostatic field. A conductor 13 brings each emitter 10 ~mounted on a support 1~ by means of an insulating bar 15 borneby a rod 16) at the same potential as electrode 7 of spray gun 6. Consequently, the emitters 10, like electrode 7, are brought to a potential which falls between 60,000 and L00,000 volts, continuous current, negative polarity.
Other field emitters can be placed at will below and above object ~; 1, depending on the direction one wants to impose on powder cloud 6c, particularly if object 1 involves upper and/or lower rims.
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The procedures and the devices of the invention allow such household goods with a vitreous enamel as pots to be made, among others.
They allow the coating of plates of enamelled sheet iron for electric or gas ovens, washing machinesg and refrigerators. They can also be used to make enamelled panels for building facades.
It is to be understood that the particular embodiment illustrated in Figures 1 and ~, on the one hand, and on .~
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the other, the kinds of pieces to be enamelled, and the enamel powder, and the applications envisioned~ are only offered as examples, and they can be modified while still coming within the framework of the invention. Moreover, given that object 1 is insulated, electrode 7 and emi-tters 10, where they exist, can be brought to a high positive potential instead of a high negative potential.
While part of a television antenna provides a conve-nient means of achieving supplementaty field emitter member 11, it is obvious that the design and dimensions of member 11 may be varied extensively to accomodate varying processing requirements, and any variety of bran-ched or straight rod-like members will serve equally well.
Although the preferred embodiment illustrated above discloses a parabolic reflector, it is to be understood that this particular shape is but examplary and, depen-ding upon 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 o~

reflecting an directing the field emitted from member 11 , . 11 , .

Claims (5)

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

1. A device for applying coating particles to a metallic object comprising support means for supporting the object, a first means for projecting the particles toward said object, said first means including means for electrostatically charging said particles at a high potential and means for initially projecting same towards said metallic object, at least one insulating element between the object and ground for insulating said object with respect to ground, and at least one supplementary electrostatic field emitter having an intense directional electrostatic field for re-directing said charged initially directed particles toward said object, said emitter being composed of a rod member placed at the focal point of a concave reflector.

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.