Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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
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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-
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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),
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~ 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
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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
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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
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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-
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around phenomenon.
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Moreover, the fact that they are at the same potential
as the electrodes eliminates disturbances and turbulence
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~ 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
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; 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-
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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
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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
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