CA2136866A1

CA2136866A1 - Method and device for powder-coating

Info

Publication number: CA2136866A1
Application number: CA002136866A
Authority: CA
Inventors: Werner Harnisch; Horst Rettenmaier
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-05-29
Filing date: 1993-04-16
Publication date: 1993-12-09
Also published as: CN1043906C; EP0642391B1; KR0127775B1; MX9303176A; EP0642391A1; KR950701844A; DE59306892D1; ATE155052T1; DE4217754C2; WO1993024242A1; ES2105238T3; CN1083127A; TW239084B; AU3887693A; JPH07505337A; DE4217754A1

Abstract

METHOD AND DEVICE FOR POWDER-COATING

ABSTRACT

Proposed is a method of powder-coating electrically conducting workpieces (1) with a coating powder, in particular a paint coating powder. The method comprises the following steps: first, the optionally cleaned and/or surface-treated workpiece (1) is heated inductively by means of an inductor (2) to the processing temperature or melting or softening point of the coating powder. The coating powder is then sprayed on to the inductively heated workpiece (1) by means of a spraying device (3, 8) so that the coating powder is at least fused on to the inductively heated workpiece (1) by the heat of the workpiece (1).

Description

21~866 Translation 2~2 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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21~5~66 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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- ~136~66 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-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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21~B865 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 .

- 213686~

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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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21~686~ ~ ~

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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Claims

Translation The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows.
A M E N D E D C L A I M S

1. Method of powder coating electrically conductive workpieces (1) by means of a coating powder, in particular a powdered paint, comprising the steps of heating the optionally cleaned and/or surface treated workpiece (1) to the processing or melting or softening temperature of the coating powder, and spraying the coating powder onto the inductively heated workpiece (1) by means of a spraying device (3), and during the induction heating, the workpiece (1) being movable relative to the inductor (2) and the spraying device (3), the inductor (2) and spraying device (3) being movable relative to the workpiece (1), characterized in that when viewed in the direction of advance, the inductively heated area of the workpiece (1), which extends directly behind the inductor (2), is sprayed with the coating powder, so that the coating occurs time delayed after the heating outside the range of induction.

2. Method as in claim 1, characterized in that the spraying of the coating powder starts after a preceding heating phase by the inductor (2), and continues thereafter synchronously with the heating.

3. Method as in claim 1 or 2, the workpiece (1) to be coated being an elongate part, preferably a pipe, characterized in that during the inductive heating and spraying, the workpiece (1) is put into rotation about its longitudinal axis, preferably by means of a clamping device (5).

4. Method as in claims 1-3, characterized in that the relative movement occurs in the axial direction of the workpiece (1).

5. Method as in one of claims 1-4, the workpiece being a pipe (1), characterized in that the inductive heating of the pipe wall, in particular by the frequency of the inductor current, is laid out such that a heating occurs throughout right to the inside wall, and that the pipe (1) can also be sprayed or coated from inside by means of a further spraying device (8).

6. Method as in one of claims 1-5, characterized in that the pipe (1) can be heated both from the outside and from the inside, preferably simultaneously, respectively by one inductor (2), and can be sprayed or coated by a spraying device (3, 8).

7. Method as in one of claims 1-6, characterized in that the coating thickness to be obtained is controlled by the feed rate of inductor (2) and spraying device (3, 8), and/or by the rotational speed of the workpiece (1), and/or by the rate of heating.

8. Method as in one of claims 1-7, characterized in that the surface of the workpiece (1) having already been sprayed with coating powder, is again heated by the inductor (2), so that the at least partially molten coating powder is able to smooth on the surface based on its specific surface tension in the molten or softened state.

9. Method as in one of claims 1-8, characterized in that the sprayed powder particles not adhering to the workpiece (1) are removed by suction via a suction device (12, 13).

10. Method as in claim 9, characterized in that the powder particles having been removed by suction are again supplied to the spraying device (3, 8) for further spraying.

11. Apparatus for powder coating electrically conductive workpieces (1) by means of a coating powder, in particular a powdered paint, the apparatus comprising one inductor (2) for inductively heating the workpiece (1) and at least one spraying device (3) for spraying the heated workpiece (1) with the coating powder, and during the heating and spraying, the workpiece (1) being movable relative to the inductor (2) and the spraying device (3), or the inductor (2) and the spraying device (3) being movable relative to the workpiece (1), characterized in that the spraying device (3, 8) follows the inductor (2), when viewed in the direction of advance, and that the inductively heated area of the workpiece (1) extending behind the inductor, can be sprayed with the coating powder, so that the coating occurs time delayed after the heating outside of the range of induction.

12, Apparatus as in claim 11, characterized in that the inductor (2) is connected to an inverter (4), and can be biased with alternating current of variable frequency.

13. Apparatus as in claim 12, characterized in that the frequency of inductor (2) for influencing the depth of penetration of the eddy currents induced in workpiece (1) by the inductor (2) is preferably infinitely variable.

14. Apparatus as in one of claims 11-13, the workpiece (1) being made elongate, preferably in the form of a pipe, characterized in that the inductor (2) surrounds the workpiece (1) at least in part at a preferably adjustable distance.

15. Apparatus as in claim 14, characterized in that the inductor (2) is made approximately semicircular.

16. Apparatus as in claim 14, characterized in that the inductor (2) surrounds the pipe (1) circularly and is constructed bipartite, if need be.

17. Apparatus as in claim 15, characterized in that the inductor (2) is constructed more or less in the form of pliers.

18. Apparatus as in one of claims 11-17, characterized in that a clamping device (5) having preferably two holding fixtures (6, 7) is provided for workpiece (1).

19. Apparatus as in claim 18, characterized in that the clamping device (5) or the holding fixtures (6, 7) of clamping device (5) is/are rotatably driven, so that the clamped workpiece (1) can be put into rotation.

20. Apparatus as in claim 18 or 19, characterized in that the clamping device (5) is movable in the axial direction of workpiece (1).

21. Apparatus as in one of claims 18-20, characterized in that the clamping device (5) is arranged such that the workpiece (1) can be clamped in a horizontal orientation.

22. Apparatus as in one of claims 18-20, characterized in that the clamping device (5) is arranged such that the workpiece (1) can be clamped in a vertical orientation.

23. Apparatus as in one of claims 11-21, characterized in that the inductor (2) is linearly movable along the workpiece (1).

24. Apparatus as in claim 22, characterized in that the movement of inductor (2) occurs at a variable speed.

25. Apparatus as in one of claims 11-23, the workpiece (1) being a pipe, characterized in that a further spraying device (8) is provided, which can be moved into pipe (1), preferably by means of a crossbeam (9) and a lance (10) or a cable, so as to spray or coat the inside wall of pipe (1).

26. Apparatus as in one of claims 11-24, the workpiece being a pipe (1), characterized in that a further inductor is provided, which can be moved into the pipe (1), preferably by means of a crossbeam (9) and a lance (10) or a cable, so as to heat the inside wall of pipe (1).

27. Apparatus as in one of claims 11-26, characterized in that the spraying device (3, 8) is combined with a suction device (12, 13), which serves to remove by suction powder particles that have not fused to the surface of workpiece (1).

28. Apparatus as in claim 11-27, characterized in that the suction device (12, 13) is provided with a substantially funnel-shaped suction hood (14, 15).

29. Apparatus as in claim 27, characterized in that the suction hood (14, 15) extends at least in part around the spraying device (3, 8).

30. Apparatus as in claim 24, and optionally one of claims 25-28, characterized in that at the lower end of the pipe a further suction device is provided, which is operative as a whole on the opening of pipe (1).

31. Apparatus as in claim 29, characterized in that the suction device (20) is associated to the lower holding fixture (7).

32. Apparatus as in claim 29, characterized in that the suction device (20) is an integral part of the lower holding fixture (7).

33. Apparatus as in one of claims 27-31, characterized in that the suction devices (12, 13, 20) are operatively connected with a supply container (18) of spraying device (3, 8), so that the removed coating powder can be supplied again to the spraying device (3, 8).

34. Apparatus as in one of claims 11-32, characterized in that the nozzle of spraying device (3, 8) is exchangeable, and that the coating thickness to be obtained is adjustable via the nozzle shape and feed rate of spraying device (3, 8) or inductor (2).