Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Translation
PCT/DE93/00339
WO 93/242~2
METHOD AND DEVICE FOR POWDER~COATING
The invention relates to a method and a device ~ --
for powder coating electrically conductive workpieces by
means of a coating powder, in particular by means of a
powdered paint.
From practice, methods and apparatus for powder
coating have been known for years. Basically, powder
coating is to be assigned to the field of surface
treatment, the process consisting of applying in an
environmentally protective manner a coating to the surface
o~ a workpiece by thermal action. Contrary to thc
conventional painting with solvent-containing paints, in
this method a coating powder or powdered paint is applied ~
to a surface to be coated, the coating powder being melted ;~ ~ ;
by thermal action until a homogeneous coating is generated ;~
on the surface.
A basic problem with powder coating consists, ~ i`
however, in the absolutely necessary adherence of the
coating powder to the surface to be coated. This
necessity o~ adherence before the heat treatment that
produces the coating, is achieved by the electrostatic
powder coating process,iwhich is likewise known alreaqy
~rom state o~ the art and based on the fact that particles ;
having an opposite electric charge attract one another.
Consequently, suitable for powder coating are exclusively .
conductive or thermally stable solids. Until now, it has
been common practice to coat in this manner household
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appliances, garden furniture, automobile accessories,
fixtures, wire articles, sections, façade elements, etc.
The method known until now is carried out in such a
manner that a dry coating powder is filled into a supply
container, fluidized or stirred, and fed by means of ~ -
compressed air to a spray gun. In the spray gun, a low -
voltage of about 10 volts is converted into high voltages
by the cascade principle, and one or several electrodes
charge the powder with 60 to 100 kilovolts during
spraying. An electric field is formed between the gun and
the grounded workpiece. The powder particles follow its
field lines, and as a result of the residual charge, they
are caused to adhere to the article to be coated. Thus-
treated workpieces may be transported manually or
automatically to a subsequent dryer, in which the coating
powder predominantly consisting of plastic is fused and
cured at temperatures of up to 200C to form a smooth
film. When enamel powder is used as coating powder, the
temperatures will have to be around 800C.
Coating powders include primarily thermoplastic ~ -
and thermosetting plastics, when a kind of painted coating
iq to be produced. Basically however, coatings with such ~ ;~
powders are possible, the melting point of which is below ~ ~;
the melting point of the workpiece to be coated. In the ~;
past, in particular polyester resin powder and
polyurethane powder, as well as epoxy powder have proven
to be useful. -
However, common to all methods of powder coating
is the problem thatl thé coating powders used for coating
must be applied somehow to the workpiece to be coated.
The therefor necessary measures or the methods being used
to this end, require in most cases costly apparatus, or
lead to the use of so-called adhesive agents, which either
impede the coating itself, or are critical with respect to
the environment.
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Furthermore, the known methods and apparatus for
powder coating are problematic in that it is always
necessary to heat the workpieces to be coated in a
furnace, a chamber, or the like, so that the coating
powder adhering to the surface is able to melt to a
homogeneous coating. Accordingly, the temperature acts
upon the workpiece from the outside, the latter exhibiting
an inwardly directed temperature gradient depending on the
duration of heating. Consequently, the application of the
known method of powder coating to often large-sized
workpieces, for example, the coating of long pipes or the
like, requires extra large industrial furnaces, which are
per se highly uneconomical based on their high energy
requirements.
Finally, with the known methods, it is extremely ;~
difficult to effectively coat the pipes on their inside
surface, inasmuch as the heat being effective from the
outside penetrates only unevenly to the inside wall. At
most, a very long warmup phase for heating the entire ~';
workpiece could be of help in this instance, which is -~
however extremely cost-intensive. ;~
It is therefore the object of the invention to
describe both an apparatus and a method for powder
coating, which allow to apply the coating powder used for
the coating with simple means to the workpiece to be
coated, and further permit the coating to proceed without
problems even on the inner side of a hollow body, such as
a pipe or the like.
~ The méthod of'the present invention, by which
electrically conductive workpieces are coated by means of ;~
a coating powder, in particular a powdered paint, and
which solves the above-described problem, is characterized
by the steps as defined in claim 1. Accordingly, the
method of the present invention proceeds by the following
steps: first, the workpiece to be coated, which is
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cleaned, if necessary, and/or surface-treated, is ;~
inductively heated to the processing or melting or
softening temperature of the coating powder.
Subsequently, i.e. immediately after heating, the coating
powder is applied to the inductively heated workpiece by
means of a spraying device. Based on the already heated
condition of the workpiece, no further adhesive agents are
needed, since immediately upon impacting on the surface of ~ -
the workpiece, the coating powder is melted and, thus,
adheres to the workpiece. The temperature, which is to be
obtained on the surface of the workpiece by the inductive
heating, is adjusted such that the coating powder is not
only fused, but forms a smooth or homogeneous surface
based on the developing surface tension.
Accordingly, it has been recognized by this
invention that it is possible to solve quasi in one
operation, on the one hand, the problem of causing the x -
coating powder to adhere to the workpiece to be coated
and, on the other hand, the problem of a heating directed
to the workpiece to be coated from the outside, in ''''~
particular with respect to otherwise normal, large -~
industrial furnaces, in that the workpiece is inductively
heated already before the actual coating and sprayed with
the coating powder, in its heated condition, immediately
after the heating. In the manner of the present
invention, it is not necessary to employ either a costly
method or expensive apparatus for causing the coating
powder to adhere to the workpiece, or an expensive
industrial furnace for the subsequent fusing of the
applied coating powder. Rather, the above-described, ;~
independent steps of adhesion and fusion occur in quasi
one operation , namely by inductive heating and subsequent
coating, there occurring only an insignificant delay in - i~
time between the heating and coating. Both quasi merged
operations take place at one location, so that a time-
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consuming and, therefore, cost-intensive transportation
from one working station to another is eliminated.
As regards the actual coating process, it is of
special advantage, when the spraying of the coating powder
is started after a preceding heating phase by the inductor
and, then, continues with the heating and the spraying.
To complete the coating process, the inductor is ~-
deactivated, and the remaining area is fully sprayed.
If the workpiece to be coated is an elongate
part, such as, for example, a shaft or even a pipe, it
will be possible to put this workpiece into rotation about
its longitudinal axis during the inductive heating and
spraying based on its rotational symmetric configuration,
preferably by means of a clamping device. As a result, it
would be possible to arrange, in this instance, the ;~
inductor and sprayiAg device stationarily, so that the
elongate part or the tube rotates in the effective range
of the inductor and the spraying device for purposes of -~
obtaining an all around homogeneous coating. .
Furthermore, it will be of advantage, when ; ;
during the inductive heating and spraying, the workpiece
is movahle relative to the inductor and the spraying
device, or the inductor and the spraying device are
movable relative to the workpiece. This relative movement
could occur in the axial direction of the workpiece, so
that the rotational movement of the workpiece that is
produced by the clamping device, and the referenced axial ~;
movement of either the workpiece or the inductor could be
supplemented with the spraying device to the extent that`
the workpiece, for example the pipe, can be effectively ~ .
coated as a whole, i.e. over its entire length. ~ ,
Specifically, in the instance of pipes that are
to be coated, it will be of advantage, when the inductive
heating of the pipe wall, in particular via the frequency ~
adjustment of the inductor current, is laid out such that ~ `
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a complete heating occurs throughout the pipe wall to the
inside wall or inside surface, so as to be able to spray
or coat the pipe likewise from the inside by means of a
further spraying device. Such a development of the method
in accordance with the invention, would have the great
advantage that the tube is heated zonally from the outside
by way of induction, this heating extending right to the
interior of the pipe. A spraying device subsequent to the
inductor would allow to apply the coating powder outside
as well as inside, when nozzles are arranged both outside
and inside of the pipe, the coating powder adhering to the
respective surfaces immediately upon its impact thereon.
Furthermore, it would be conceivable that the
pipe is heated both from outside and inside, preferably ;~
simultaneously, by an independent inductor for each side,
and sprayed or coated by a corresponding spraying device.
The inductor entering into the tube would have to be
constructed in the form of a probe, preferably in the
shape of a bar, the two inductors being in a position to
operate completely independently of one another and also
at different places. A partial coating of the pipe both
from outside and inside is easily possible.
In a particularly advantageous manner, the
coating thickness to be obtained can also be adjusted via
khe ~eed rate of the inductor and the spraying device.
Furthermore, it is possible to include the rotational -~
speed of the workpiece as well as the heating device of
the inductor, so as to influence the thickness of the
coating ;
The surface already sprayed with the coating ~ ;
powder could, in a further advantageous manner, be heated
again by the inductor, so that also the coating powder
having partially been fused, at least until now, is
allowed to smooth on the surface based on its specific-
surface tension in the molten or softened state. Such a
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reheating could be understood to be an aftertreatment,
provided that only an adherence of the coating powder was
achieved in the first passage. In any event, a further
heating would allow to adjust a defined surface roughness ~ ~
or smoothness of the coating to be obtained. ~ ;
When spraying the coating powder to the
inductively heated workpiece, it will always be of
advantage to direct the powder to the workpiece not as a
jet or a point, but to direct the powder to the workpiece ~i,
rather in the form of a spraying mist, i.e., with a
certain scattering width. In so doing, it cannot be
avoided that substantially more powder particles are
sprayed in direction of the workpiece than the workpiece ;,~
i8 able to receive. To prevent contaminations and, thus,
adverse effects on the environment by the coating powder,
and to avoid enormous powder losses and, thus, to reduce !',
the costs, it will be highly advantageous, when the ~;
sprayed powder particles not yet fused to the workpiece - ~ -~
are removed by a special suction device. The sucked-off
pcwder particles could then be supplied again to the ` ~
spraying device for further spraying, in that same would ~ ~ -
be returned by the suction device, first, into a
collection or supply container, whence they would be
supplied again to the spraying device.
A further object of the present invention is an
apparatus in accordance therewith, which realizes the here ~ -
underlying, apparatus-related object by the features of ~ ~ ;
claim 12. Accordingly, a device of powder coating
electrically conductive workpieces by meanls of a coating
powder, in particular a powdered paint, preferably for use
of the above described method of this invention, is
characterized in that it comprises at least one inductor
for inductively heating ~he workpiece and at least one
spraying device for spraying the heated workpiece with the
coating powder. Likewise, in the instance of this ;
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apparatus, it has been recognized in accordance with the
invention that the combined use of an inductor and a
spraying device makes up for all disadvantages, which have
occurred until now with known apparatus and methods for
powder coating.
Preferably, the inductor is connected to an ~
inverter and biased by alternating current of a variable ~ -
frequency. Preferably, the frequency of the inductor is
infinitely variable, so as to influence the penetration `~ -~
depth of the eddy currents induced by the inductor in the
workpiece. Likewise however, it is easily possible to
predetermine individual steps for the adjustment of -~
different frequencies.
If the workpiece to be coated is an elongate
part, preferably a pipe, the inductor could be designed
such that it surrounds the workpiece at least in part at a ~ ~ ~A'~
preferably adjustable distance. In this arrangement, the ` ~ ~
inductor should be laid out such that it biases already a ~ ~ri
relatively large area of the workpiece without a relative
movement between the workpiece and the inductor. In a
particularly advantageous manner, the inductor could be
made semicircular. A semicircular configuration is ~ -
conceivable, at least when the workpiece to be coated is =~
tubular, and it is possible to move the inductor from a
free side over the pipe. Finally, the inductor could also
be constructed, more or less, in the form of pliers, it ;~
being again possible to make the respective plier parts
movable, so as to be able to apply the inductor halves
around the workpiecé to be coated without influencing~it,
and to close the halves so as to form a quasi circular
inductor.
For a reliable hold of the workpiece to be
coated, it will be o~ further advantage, when a clamping
device for the workpiece is provided, which has preferably -`
two holding ~ixtures. Especially suitable for holding are
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fixtures or carriers serving to secure the workpiece,
inasmuch as the latter can be held on both sides, for
example, on its front ends. ~ -
In a further advantageous manner, the clamping
device or the holding fixtures of the clamping device are
rotatably driven, so that the clamped workpiece can be put `~
into rotation. A rotational movement of the workpiece to
be coated has the great advantage that same can be heated `
all around and, thus, can be coated without moving the
inductor and already, even when the latter is, for
example, constructed semicircular.
For a homogeneous coating of the entire surface
of the workpiece, the clamping device could be movable in
the axial direction of the workpiece, so that as a ~ ~;
supplement to the rotational movement of the workpiece,
same can be heated both over its entire circumference and ;~
over its entire length, and be coated thereafter. The
arrangement of the clamping device and, thus, the clamping
of the workpiece could be realized such that the workpiece ~ ~ ;
can be clamped in a horizontal orientation. Likewise
conceivable would be a vertically extending clamping of
the workpiece, it being then necessary to adapt the
inductor each time to the respective orientation or ;~
¢lamping of the workpiece, both in its arrangement and in ;;~
its mobility.
Likewise, it would be conceivable to have the
inductor move linearly along the workpiece, so that the
olamping device would have to perform only a rotational
movement for the rotation of the workpiece. It would even
be possible that both the inductor and the workpiece move
in axial direction, so that both the inductor and the
clamping device would have to traverse each only half the ~ -
dlstance. Thus, the maximum traverse of the inductor and
the clamping device wouid be reduced effectively. ~
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Both the movement of the inductor and the linear
movement of the clamping device could be variable with
respect to speed, so as to be able to likewise influence ~ -
the coating process in an effective manner. - --
If the workpiece to be coated is a pipe, it ~ ;~
would be possible to provide a further spraying device,
which can be moved into the pipe, preferably by means of a
crossbeam and a lance or cable, so as to spray or coat the
inside wall of the pipe. However, in this instance, it
would be necessary to ensure that the inductive heating of
the pipe from the outside is laid out such that the eddy
currents generating the heat possess an adequate depth of
penetration into the pipe, so that likewise the inside
~urface of the pipe undergoes indeed an adequate heating.
The second spraying device could effect the coating inside `~
the pipe, parallel to the first spraying device operating
on the outside, both spraying devices operating at about
the same level and advancing at about the same speed along
the pipe. A rotation of the pipe occurring at the same
time would ensure an all around coating.
As an alternative to the above-described
possibility of layout, it would also be possible to
provide an additional inductor, which would be adapted for ' '~
entry into the pipe, preferably by means of a crossbeam
and a lance or cable, so as to heat the inside wall of the
pipe. This second inductor could supplement the heating
performance of the inductor operating on the outside, so
as to homogeneously heat the wall as a whole.
!' With respect to the arrangement of the spraying
device, it will be of advantage, when same follows the ~` -
inductor, so that when viewed in the direction of advance
of the inductor, it is always possible to spray, behind
the inductor, the inductively heated area of the workpiece
with the coating powder, so that same is caused, as a
result of heating the workpiece, to thus adhere or fuse
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and homogeneously distribute thereon. In this instance,
the spraying device could be fixedly combined with the
inductor, i.e., it could be mounted, for example, together
with the inductor on one frame. Likewise, this
arrangement would allow to spray the coating powder always
on the inductively heated area, which follows the -~
inductor, when viewed in direction of its advance, the
movement of the inductor resulting always in an identical ~-;
movement of the spraying device, so that the possibly
adjustable and, thus, always predetermined spacing between ;~
the inductor and spraying device does not change at least
during the coating.
In particular with respect to protecting the
environment or preventing contaminations, as well as with ~ -
respect to a smallest possible consumption of coating
powder, it will be of further advantage, when the spraying
device is combined with a suction device, which is used to ;~
remove by suction powder particles that have not fused to
the surface of the workpiece. The suction device could be -~
provided with a substantially funnel-shaped hood, the
latter extending at least partially around the spraying ;~
devlce, i.e., it could be formed or curved at least
slightly around the workpiece. Quite conceivable are
suction hoods, which are adapted to the respective shapes
o~ the workpieces to be coated, so as to ensure an
absolutely effective suction or catching of excessive
particles of the coating powder.
The suction device could transport the removed
powder into a collection bin. Likewise, the suction !
device could be operatively connected with a supply
container of the spraying device, so as to permit to ; ;
supply the sucked-off coating powder again directly to the ~;~
spraying device.
Finally, and specifically with respect to the ~
geometry of the workpiece and in view of preventing the -
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removal of large quantities of coating powder by suction,
it would be possible to adapt the nozzle of the spraying
device each time to the workpiece to be coated. To this
end, it would be necessary that the nozzle of the spraying
device be exchangeable, it being again possible to adjust
the coating thickness to be achieved via different nozzle
shapes and feed rates of the spraying device or the
inductor.
There exist various possibilities of perfecting
and further developing the teaching of the present
invention. To this end reference may be made on the one
hand to the claims dependent on claims 1 and 12, on the - - -
other hand to the following description of an embodiment
of the invention with reference to the drawing. In
conjunction with the detailed description of the preferred
embodiment of the invention with reference to the drawing,
also generally preferred embodiments and further
developments of the teaching are explained.
In the drawing, the only Figure is a schematic
block diagram, partially cut, of an embodiment of the
device in accordance with the invention for powder coating
electrically conductive workpieces by means of a coating
powder, the embodiment comprising one inductor and two `;~ ;;
spraying devices.
~he only Figure illustrates an embodiment of a -~
device in accordance with the invention for powder coating ~
electrically conductive workpieces, the workpiece being a ~ 4
pipe 1. Used in this embodiment as coating powder is a
powdered paint not shown in the Figure, which serves to
apply a coat o~ paint to the surface of pipe 1. ;~
In a manner in accordance with the invention, an
inductor 2 is used for inductively heating pipe 1. A
spraying device 3 sprays the heated workpiece or pipe 1 :~
with the coating powder.
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As is further indicated in the only Figure, the -~
inductor 2 is connected to an inverter 4, which allows to
bias inductor 2 with an alternating current of a variable
frequency. The frequency adjustment of inductor 2 serves
to influence the penetration depth of the eddy currents
induced on tube 1 by inductor 2 and, preferably, is ~ ~
infinitely variable. ~ ;
The only Figure also indicates that the inductor ---
2 surrounds pipe 1 in part. The spacing between pipe 1 ~ ;~
and inductor 2 is adjustable. Furthermore, the inductor 2 -
is constructed more or less semicircular, a circular ~ ;
configuration as well as a design in the form of pliers
with two inductor parts being easily possible.
To hold pipe 1, a clamping device 5 is provided -
which keeps pipe 1 between two holding fixtures 6, 7. The ~-
clamping device 5 or holding fixtures 6, 7 of clamping ;~
device 5 are rotatably driven, so that pipe 1 secured
therein can be rotated by clamping device 5. Both
clamping device 5 and inductor 2 can be adapted for
movement in the axial direction of pipe 1, so as to be
able to effectively coat pipe 1 over is entire length.
In the here selected embodiment, a clamping
device 5 is arranged such that the workpiece or pipe 1
extends vertically in its clamped condition. The inductor
2 is here linearly movable along the workpiece or pipe 1. -~
The movement of inductor 2 occurs at a variable or ;~
ad;ustable speed.
Also indicated in the only Figure is that a
~urther spraying device 8 is added to spraying device 3, ~ ;
which serves to spray or coat the inside wall of pipe 1. ~ ~
This spraying device 8 is adapted for entry by means of a ~ ;
crossbeam 9 and a lance 10 into pipe 1 through upper --
holding fixture 6.
The spraying devices 3, 8 follow inductor 2,
when viewed in the direction of advance of inductor 2, so
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as to be able to apply the coating powder to the ~ `
inductively heated area of pipe 1, which always follows
lnductor 2, when viewed in direction of its advance.
Furthermore, the spraying device 3 is fixedly combined
with inductor 2 on a common frame or holder 11, so that
spraying device 3 is urged to follow inductor 2.
Associated to each spraying device 3, 8 is a ~- :
suction device 12, 13, which serves to remove powder
particles that are not fused to the surface of the ~.
workpiece or pipe 1. These suction devices 12, 13 are
provided each per se with a substantially funnel-shaped
suction hood 14, 15. The suction hood 14, 15 is directed ~ .
toward tube 1, thus, totally surrounding the nozzle of .
spraying device 3, 8 on the side opposite to the spraying
device.
Arranged on the lower end of tube 1 is a further
suction device 20, which is operative as a whole on the
opening of tube 1. This suction device is associated to :::
the lower holding fixture 7 or forms an integral part
thereof.
Finally, it can be noted from the only Figure `~
that the suction devices 12, 13, 20 are operatively :~
connected via compressed-air hoses 16, 17, 21 with a
supply container 18 of spraying device 3, 8, so that the
removed coating powder can be supplied again to spraying ~ .
devices 3, 8. Compressed air made available by a pressure
tank 19 allows to supply the coating powder, via further .
compressed-air hoses 22, 23, again to spraying devices 3, .:~
8. I :
The above-described embodiment serves only for a
better understanding of the teaching of this invention, .
without however limiting same thereto.
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