Note: Descriptions are shown in the official language in which they were submitted.
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The 1nvention relates to a powder spray apparatus substantiallycomprising a booth with a reservoir in which powder is dispersed
in air, a powder feed tube, a spray gun in which the powder is
electrically charged and sprayed toward the work-piece, an
exhaust system, a system with a fan and a filtering device to
return the powder.
Electrostatically powder~spraying of organic powders is a method
which has generally been applied for the last 10 years. Apparatus
for said method are described in the patents, namely Nordson,
Gema (USP 3 918 641), Sames (F.P. 1 337 069) and the ~utch
principal patent no. 82 716 of Metallgesellschaft.
It is the object of the method according to the above-mentioned
patents to achieve, by automatically returning the powder, which
is not deposited on the work-piece to be coated, to the reservoir,
a saving in use of powder and labour expenses.
Still another advantage is that air pollution is strongly opposed.
An apparatus according to the known methods consists of a hopper
in which the powder is dispersed in air, a powder feed tube, a
spray gun in which the powder is electrically charged and sprayed
toward the work-piece a booth in which the spray gun and the
workpiece are provided, an exhaust system a-t the bottom side of
said booth, a transport system to return the powder-air mixture
to the recuperation installation, a recovery system consisting of
a cyclone and a filtering device to separate the powder from the
air and of a transport system to return the recuperated powder
to the hopper again.
Electrostatically spraying enamel suspensions is also a method
which has been applied for a long time. In said method the enamel
together with clay, water, settling salt and inorganic pigments
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is milled and the obtained suspension is sprayed onto the work-
piece. A disadvantage of said method i5, however, that the
material, which does not arrive on the work-piece deposites on
the ]bottom of the spray booth and in the air filtering
installation. Said material must be manually collected at the end
of a working day, or during the change of color, it has to be
milled again with clay, water and settling salts and after being
sieved it has to be added to a fresh milling. So, aut~matically
recuperation is impossible with said method.
A method, analogous to the above described method for organic
powders will therefore also provide considerable savings and
advantages for inorganic powders. Electrostatically spraying dry
ground enamel powders appeared, however, to be impossible.
Because of the considerably lower electrical resistance, when
comparing with organic powders (organic powder 1 x 10 ohm/cm
enamel powder 1 x 10 ohm/cm), it was not possible to maintain
the charge on the p4wder particles, because of which the powder
adherence was insufficient.
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only when coating the enamel particle~s with an organic coating
(Dutch patent application 75 05337) it was possible to increase
the electrical resistance such that the adherence of the enamel
powder remained sufficient up to within the enamel furnace. As
apparatus for said method an apparatus was used, which is known
per se for the organic powder spraying operation, in which only
at the location where high air velocities are developed, as for
instance in the venturies of the powder pump and in the spray
gun, the usual soft materials like P.T.F.E. were replaced by more
wear resistant material like for instance tool steel or ceramic
material. The latter was necessary because the considerably
higher hardness of enamel particles (hardness 6 according to the
Mohs-scale) with respect to organic powder particles (hardness 3
according to the Mohs-scale) caused too much wear. In practice
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said apparatus for powder spraying inorganic powder appeared to
involve considerable disadvantages.
Because of the above-mentioned considerable hardness of the
enamel powder particles an unacceptable wear in the recovery
installations, especially in the exhaust system, transport system,
and cyclone is developed. Moreover, the powder is polluted by
the abraded metal particles, causing a strong undesired change
of color of the enamel surface after being passed through the
furnace.
Another disadvantage of abraising the enamel particles against
the walls of the recovery installation is further that the
organic coating, which has a thickness of only 0.001~ is damaged,
because of which the electric resistance and consequently the
powder adherence is strongly decreased.
The method applied in the usual "wet" enamel techniques r
according to which the enamel is milled together with the
inorganic pigment, neither seems to be transferable to these
installations.
When the enamel and the inorganic pigment are dry milled and then
sprayed in a device suitable for organic powder spray operations
it appears that a portion of the inorganic pigment disappeared
from the recirculated powder and that the remaining portion of
the pigment is no longer homogeneously distributed. These losses
and demixtures can be explained on the basis of the differences
in particle sizes and specific gravity of enamel and pigment
particles.
Enamel: specific gravity + 2.60 - particle size 95~ between
5-70~.
Pigment: dependent on the type of pigment - specific gravity
varying from 4.0 IO 7.0~ - particle size 100~5~.
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The collection efficiency of the cyclones in the usual recovery
installations for organic powders having a particle si~e of 95~
between 20-40~ is not sufficient for the less coarse inorganic r
powder systems.
It is the object of the present invention to provide an apparatus
and a method for electrostatically powder spraying inorganic
powder systems, in which all the above-mentioned disadvantages,
which occur during the use of the methods and apparatus for
organic powder spraying, which are known up to now, are avoided.
It is an other object of the invention to provide an apparatus
and a method having the possibility to change quickly of color
without the danger of pollution and without extensive cleaning
operations.
The apparatus according to the present invention is embodied
such that the powder recirculation takes place completely within
the powder spray module.
The filtering devices and the reservoir are preferably simply
exchangeable as the cleaning of said parts takes more time than
the cleaning of the remaining parts of the module. So, said
property of the invention reduces the number of necessary
modules.
The exhaust air is blown back through the filtering devices via
air seals substantially such that an inwardly directed jet is
developed. The advantage hereof is that the air condition in
the module as for instance the air moisture contents is kept
constant. The relative humidity of the air influences the
depositing velocity and the adherence of the enamel powder on
the work-piece.
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Another advantage of such an apparatus is that an air treatment
installation for the ambient air is not necessary and can be left
out.
When, as is preferably the case, the guide of the spray guns be
arranged in the module itself. only narrow slot openings are
necessary to guide the work-piece through the module.
Such a module can be applied most efficiently in a method which
is characterized in that the articles to be sprayed are moved
along the spray guns by means of a conveyor.
The apparatus will be further elucidated on the basis of the
enclosed drawing. In the drawing:
fig. 1 shows a schematic top view of the interior of a module
according to the inventioni and
fig. 2 shows a cross section according to the line II-II of
figure 1.
The bottom of the powder spray booth 1 consists of a reservoir 13
in which the powder is dispersed with air and via a powder feed
tube 7 and the spray gun 2, in which the powder is electro-
statically charged, it is sprayed toward the work-piece 3. The
powder, which is not deposited on the work-piece, will, for the
greater part, fall back into the reservoir 13 under the influence
of gravity.
In order to take care of the fact that the powder, which does not
fall back directly, can leave the mo-dule via the entrance and
exit openings 6, a slight under-pressure is provided in the
module by exhausting air by the fans 5 via the filtering elements
4. The powder collected by the filter is periodically blown in
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the opposite direction of the filters by means of a pressure blow.
The air sucked by the fans 5 is partially blo~ back into the module
via the slots at both sides of the entrance and exit openings 6, by which an
inwardly directed air jet is developed taking care of the fact, that no
powder particles, which float in the module itself can escape via the openings.
The guide 8 along which the guns can be moved up and down, is
arranged within the module, so that no openings for guns andpowder supply
tubes, which can not be easily sealed, are created, and the escape of powder
along said track is prevented.
The dispersion ofpowderin the air is effected by a pump 11 and
results in a so-called fluidized bed ofpowderin the reservoir.
By the features as described above the quantity of air which has to
be exhausted via the filtering elements 4 is so small that in comparison with
the known apparatus both the number of filters and the charge per filter can
be considerably reduced.
For organic powders a rather high air displacement must be maintained
to reduce the danger of explosion.
Because of said low charge per filter the alternately periodically
cleaning by means of a blow a pressurised air and returning the air via the
slots the recuperation efficiency is also very high for very fine powders, as
is usual in inorganic enamel pigment systems.
The apparatus according to the invention provides good advantages
over the above-described apparatus for the organic powder spraying operation.
Because the enamel and pigment particles can no longer contact the
metal walls of the transport and recuperation systems at a
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high velocity, the mechanic wear of the recuperation system is
prevented. The reduction in quality of the enamel powder by
pollution and the loss of powder adherence is avoided in said
apparatus as well.
Because of the simple and cheap construction it is possible in
case of changing color, to quickly change the module by simply
uncoupling fans, the motion mechanism and the pressurised air
conducts. A module destined for another color is driven in and
connected, after which the production is immediately continued
without the time-devouring cleaning of the module and
recuperation system.
The above described modules seem to be most suitable for
proauction increase and for the application of "2-coat -
one fire" by placing the modules one behind the other.
