PROCESS FOR COATING THE SURFACE OF WORKPIECES

Abstract

In a process for coating the surface of workpieces, a coating agent is applied to the workpiece and then cured in an electromagnetic alternating field. In order to enable a high-quality surface coating even with standard coating agents, in particular with liquid coatings, despite the short duration of the process, first the volatile components of the coating agent are expelled in an electromagnetic alternating field having a first frequency spectrum. For the purpose of crosslinking and/or curing the remaining coating agent components, the surface of the workpiece is then heated in an electromagnetic alternating field having a second frequency spectrum, the frequency range of which is below the first frequency spectrum.

Claims

1. A process for coating the surface of workpieces, said process comprising: applying a coating agent to the workpiece and thereafter curing the workpiece in an electromagnetic alternating field; wherein volatile components of the coating agent are first expelled in a first electromagnetic alternating field having a first frequency spectrum, and wherein the surface of the workpiece is then heated in a second electromagnetic alternating field having a second frequency spectrum having a frequency range that is below the first frequency spectrum so as to crosslink and/or cure coating agent components remaining after the volatile components are expelled.

2. The process according to claim 1, wherein the first frequency spectrum is in a range of 1 to 3 GHz.

3. The process according to claim 1, wherein the frequency range of the second frequency spectrum is 35 to 400 kHz.

4. The process according to claim 1, wherein the workpiece is exposed to the first electromagnetic alternating field with the first frequency spectrum for a first period of time that is longer than a second period of time to which the workpiece is exposed to the second electromagnetic alternating field with the second frequency spectrum.

5. The process according to claim 1, wherein the workpiece is exposed to the first electromagnetic alternating field with the first frequency spectrum for 10 to 20 minutes and to the second electromagnetic alternating field with the second frequency spectrum for 5 to 10 minutes.

6. The process according to claim 1, wherein the electromagnetic alternating fields are applied with large-area emitters that are supported so as to be displaceable in at most one spatial direction and with emitters displaceable in at least two spatial directions for areas of the workpiece that are difficult to access with the large area emitters.

7. The process according to claim 1, wherein the coating agent or a curing agent applied prior to curing comprises inductively or dielectrically heatable particles, and an alternating magnetic field is applied to the heatable particles so as to cure the coating agent.

8. The process according to claim 7, wherein the dielectrically or inductively heatable particles are nanoparticles.

9. The process according to claim 1, wherein the process further comprises placing the workpiece a fluid-impermeable, electromagnetically permeable capsule, to which the coating agent is supplied and withdrawing an excess of the coating agent from the capsule, and then applying an electromagnetic alternating field to the capsule so as to cure the coating agent.

10. The process according to claim 9, wherein a curing agent comprising inductively or dielectrically heatable particles is supplied to the capsule before curing.

11. The process according to claim 9, wherein the capsule is rotated about a horizontal axis of rotation after the coating agent and/or the curing agent is being supplied.

12. The process according to claim 10, wherein the capsule is rotated about a horizontal axis of rotation after the coating agent and/or the curing agent is being supplied.

13. The process according to claim 2, wherein the frequency range of the second frequency spectrum is 35 to 400 kHz.

14. The process according to claim 2, wherein the workpiece is exposed to the first electromagnetic alternating field with the first frequency spectrum for a first period of time that is longer than a second period of time to which the workpiece is exposed to the second electromagnetic alternating field with the second frequency spectrum.

15. The process according to claim 3, wherein the workpiece is exposed to the first electromagnetic alternating field with the first frequency spectrum for a first period of time that is longer than a second period of time to which the workpiece is exposed to the second electromagnetic alternating field with the second frequency spectrum.

16. The process according to claim 13, wherein the workpiece is exposed to the first electromagnetic alternating field with the first frequency spectrum for a first period of time that is longer than a second period of time to which the workpiece is exposed to the second electromagnetic alternating field with the second frequency spectrum.

17. The process according to claim 2, wherein the workpiece is exposed to the first electromagnetic alternating field with the first frequency spectrum for 10 to 20 minutes and to the second electromagnetic alternating field with the second frequency spectrum for 5 to 10 minutes.

18. The process according to claim 2, wherein the coating agent or a curing agent applied prior to curing comprises inductively or dielectrically heatable particles, and an alternating magnetic field is applied to the heatable particles so as to cure the coating agent.

19. The process according to claim 3, wherein the coating agent or a curing agent applied prior to curing comprises inductively or dielectrically heatable particles, and an alternating magnetic field is applied to the heatable particles so as to cure the coating agent.

20. The process according to claim 13, wherein the coating agent or a curing agent applied prior to curing comprises inductively or dielectrically heatable particles, and an alternating magnetic field is applied to the heatable particles so as to cure the coating agent.

Description

BRIEF DESCRIPTION OF THE INVENTION

[0019] In the drawing, the subject matter of the invention is shown by way of example, wherein:

[0020] FIG. 1 shows a schematic side view of a production line for carrying out the process according to the invention in accordance with a first embodiment,

[0021] FIG. 2 shows a schematic side view of a production line equipped with electromagnetically permeable capsules for carrying out the process according to the invention in accordance with a second embodiment, and

[0022] FIG. 3 shows a schematic side view of a production line for carrying out the process according to the invention in accordance with a third embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] As can be seen in FIG. 1, the process according to the invention can be applied in an electrophoretic deposition process known from the prior art, for example a cathodic dip coating. For this purpose, the workpiece 1 is arranged on a positioning frame 2 and is immersed through a paint bath 3 by a positioning drive (not shown). It is understood that the paint bath 3 is filled with an electrically conductive paint as a coating agent and various additives known from the prior art. If a DC voltage is now applied between the workpiece 1 acting as a cathode and the anode 4 arranged in the paint bath 3, the paint precipitates on the workpiece 1 and remains there. To cure or crosslink, the workpiece 1 is passed through an emitter 5 which generates an electromagnetic alternating field.

[0024] With the aid of the electromagnetic alternating field generated by the emitter 5 with a first frequency spectrum, the volatile components, for example water or other volatile solvents, are first expelled from the solvent applied to the workpiece 1. Accordingly, at this first frequency spectrum, the coating agent is predominantly excited by the alternating field, which entails low-energy loss expulsion of the volatile components. Subsequently, the workpiece 1 is subjected to an alternating field with a second frequency spectrum. Since the frequency range of the second frequency spectrum is below the first frequency spectrum, only the surface of the workpiece 1 itself is heated and maintained at a desired temperature. As a result, the thermal energy is also transferred to the remaining coating agent components by thermal conduction and heat transfer, causing them to crosslink and/or cure.

[0025] As a first frequency spectrum, a range of 1-3 GHz has proven to be particularly suitable for driving out the volatile components from the applied coating agent.

[0026] The second frequency spectrum may be in the range of 35-400 kHz, since it has been found that the energy of this alternating electromagnetic field is high enough to heat the surface of the workpiece 1, but not to change its microstructure.

[0027] In the case of common car bodies as workpieces, the best results with regard to a high-quality and yet energy-saving surface coating have been obtained when the workpiece 1 is exposed to the electromagnetic alternating field with the first frequency spectrum for 10-20 minutes and to the electromagnetic alternating field with the second frequency spectrum for 5-10 minutes. In principle, it can be stated that tests in which the workpiece 1 has been exposed to the electromagnetic alternating field with the first frequency spectrum for longer than to the electromagnetic alternating field with the second frequency spectrum have tended to result in better surface coatings.

[0028] FIG. 2 shows a further embodiment of the surface coating process according to the invention. For this purpose, the workpiece 1, which is not shown for reasons of clarity, is arranged in an electromagnetically permeable capsule 6. The capsule 6 thus forms a sealed reaction chamber which can be filled or emptied via supply units 7a, 7b, 7c. If the surface coating is, for example, an electrophoretic deposition process, a first supply unit 7a can apply a cleaning agent 8 to the interior of the capsule for removing grease or paint residues adhering to the workpiece 1. After the cleaning agent 8 has been removed by the supply unit 7a, the capsule 6 is uncoupled and conveyed with the aid of a positioning drive 9 of a positioning frame 2 to a further supply unit 7b, which fills the capsule interior, for example, with an electrolyte 10 for producing a conversion layer on the workpiece 1 and then empties it again. A third supply unit 7c can supply electrically conductive liquid paint 11 to the capsule interior for coating the workpiece. A DC voltage field is now applied between the workpiece 1 connected as a cathode, for example, and an anode mounted in the capsule 6, as a result of which the paint particles on the workpiece 1 precipitate. It probably need not be mentioned further that the workpiece 1 can also be connected as an anode. In this case, a cathode must be arranged in the capsule 6. In a final process step, the applied coating is crosslinked by passing the capsule 6 with the workpiece 1 arranged in it through the electromagnetic alternating field of an emitter 5.

[0029] As further shown in FIG. 2, the capsule 6 can be rotated about a horizontal axis of rotation at the supply units 7b for sufficient distribution of the coating agents applied. It is understood that the production line can be designed in such a way that the capsule 6 can also be rotated at other positions.

[0030] The different filling levels of the cleaning agent 8, the electrolyte 10 and the liquid coating 11, indicated by dashed lines, show the different process steps in time during filling and emptying of the capsule contents.

[0031] The capsules 6 can be hermetically sealed and are designed in two parts, which favors easy loading of the capsules 6 with a workpiece 1.

[0032] FIG. 3 shows possible embodiments of the emitters 5 for application of the electromagnetic alternating fields. In order that the process according to the invention can also be applied to large workpieces 1 and moreover to existing production lines, the electromagnetic alternating fields can be applied with large-area emitters 12 which can be displaced in at most one spatial direction. Due to the displacement in only one spatial direction, no complex control devices are necessary, whereby production lines can be upgraded with the process according to the invention in a cost-effective manner. The alternating field with a first frequency spectrum for driving out the volatile components can be applied via a first large-area emitter 12a, and the alternating field with a second frequency spectrum for crosslinking and/or curing the remaining coating agent components can be applied via a second large-area emitter 12b. The large-area emitter 12 can, for example, comprise several emitters 5. It is also conceivable that a large-area emitter 12c that cannot be moved in any spatial direction is also provided. To ensure that even complex geometries can be surface-coated in a process-safe manner, areas of the workpiece 1 that are difficult to access can be subjected to an alternating electromagnetic field generated by emitters 5 that can be displaced in at least two spatial directions. These emitters 5 can be displaced by robot arms 13, for example.