Process of powder coating of workpieces
BACKGROUND OF THE INVENTION 
     This invention is directed to a process of powder coating of workpieces in which initially a workpiece is coated with powder so that the powder adheres to the workpiece and subsequently the powder is baked in an oven. 
     The complete, homogeneous (single-color) coating of workpieces with color powder for instance in electrostatic powder coating plants with subsequent baking of the color powder which adheres electrostatically to the workpiece, has been known (DE 3429580 A1, GB 1465146). When it was desired to form a multi-colored pattern on a workpiece the workpiece was covered with an appropriate mask, the respective color powder was applied to the exposed surface area of the workpiece to be coated with said color and subsequently baked by placing the entire workpiece in the baking oven. These steps were then repeated for every further color with corresponding covering masks and color powders until the desired pattern had been fully applied. Apart from the considerable time and energy requirements of said process it also exhibits the particular drawback that the quality of the coating is deteriorated by the multiplicity of baking steps. 
     EP 0,480,828 and EP 0,481,869 disclose processes of powder coating of workpieces in which a mixture of powder and a neutral gas is injected via a nozzle into a laser beam so that the powder either impinges on a workpiece together with the laser beam or initially passes through the laser beam and thereafter impinges on the workpiece. During this process the powder is melted by the thermal radiation of the laser, and the melted powder is deposited on the workpiece. Alternatively, the powder may also be applied to the workpiece prior to the laser treatment. In said coating process the velocity of the powder stream has to be relatively low so that the laser beam can reliably melt or fuse the powder particles. The relatively high temperatures produced in the process require the use of the neutral gas. Since the laser beam has a limited width such a coating process requires not only much technical effort but is also slow and only suited for relatively small areas. 
     This invention is based on the objective of providing a process of powder coating of workpieces in which locally exactly defined surface areas of a workpiece can be coated in a simple way. It is a secondary objective of this invention to provide a process in which multi-colored two-dimensional bodies such as images or patterns can be applied onto a workpiece by powder coating without having to subject the workpiece to a plurality of baking operations. 
     The specified objective is achieved in accordance with the invention by a process wherein powder adhering to the workpiece after coating and prior to baking is irradiated with electromagnetic radiation within a defined surface portion such that the powder present within the defined surface portion will gelate and stick to the workpiece. The color powder which adheres to the workpiece outside of the defined surface portion is removed subsequent to the electromagnetic irradiation step. In the process of the invention the workpiece is initially surface-coated with powder in the usual way, for instance in an electrostatic powder coating plant. Thereupon a desired defined surface portion of the workpiece is irradiated with electromagnetic radiation such as IR-light or UV-light so as to transfer energy to the powder and to initiate crosslinking so that the powder adhering to the workpiece will gelate in said defined surface portion and stick to the workpiece. The color powder adhering to the workpiece outside of the defined surface portion is removed such as by blowing. Subsequently, the color powder is baked in a baking oven. The process according to the invention eliminates the making and using of masks for desired patterns, while a two-dimensional pattern can easily and rapidly be applied to a workpiece surface. 
     For applying a multi-colored pattern or image to the workpiece surface, the workpiece is successively coated with a respective one of the color powders and irradiated with electromagnetic radiation within the defined surface portion allotted to the respective color. Thereafter any excess color powder outside of the associated surface portion is blown off, and the steps of coating, irradiating and blowing are repeated for each additional color of the workpiece until the desired pattern or image has been formed thereon. Subsequently, the entire color powder is baked in a single operation. With the method of the present invention it is therefore possible to apply a multicolored pattern or image in its entirety to the workpiece, the powder respectively gelling in the individual portions due to the electromagnetic irradiation, and thereafter to bake the pattern or image in a single operation so that the initially coated portions will not deteriorate due to multi-baking of the color powder. 
     Gelling here means first of all fusing of the powder. To this end, external energy must be transferred to the powder because an endothermal operation will take place. By the use of a laser, for example, the emission wavelength of the source of radiation can be adjusted accurately to the absorption wavelength of the powder so as to enable a highly effective energy transfer. As soon as the powder has changed to the liquid phase, so-called crosslinking will take place, i.e., three-dimensional molecular networks will be formed which finally constitute the homogeneous powder film which is what a powder coat actually is. 
     In addition to sources of IR-radiation also sources of UV-radiation are suitable for gelling or partial crosslinking of the powder. The important factor when selecting the source of radiation and the applied electromagnetic radiation is that the transferred energy should be able to initiate crosslinking in the powder or fusion of the powder. 
     The source of IR-radiation may, for example, be an IR-laser or a &#34;Nernst stylus&#34; which sweeps or &#34;scans&#34; the desired, defined surface portion or directly projects the desired pattern on the workpiece. 
     When an IR-laser is used as the source of IR-radiation it may be coupled directly with a computer for driving the laser or laser scanner so as to transfer a digitised image directly onto the workpiece. 
     As the source of radiation for partially gelling or partially crosslinking the powder, one may also make use of a source of UV-radiation of the kind used in stereolithography for producing three-dimensional workpiece patterns from liquid synthetic plastics. The chemical crosslinking which occurs in said process is very similar to the powder crosslinking process. 
     In view of presently feasible technologies and with suitable color powder there is provided electromagnetic irradiation to cause the color powder to gelate, the surface being heated for about 5 seconds at approximately 160° C. The source of radiation is not limited to a particular spectral range. Rather, it is essential that the radiation emitted by the source be adjusted to the absorption occurring within the powder which will initiate crosslinking in the powder. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A, 1B, 1C, 1D, 1E and 1F are schematic plan views which illustrate examples of individual steps of the coating process in which a two-color pattern is applied onto a workpiece. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1A illustrates a first layer of color powder 12 is two-dimensionally applied to the surface of a workpiece 10 (only a portion of which is shown in FIGS. 1A to 1F) by a known electrostatic coating method. Due to the potential difference between the grounded workpiece 10 and the color powder, which is charged by a high-voltage generator and suitable electrode means (not illustrated), the color powder 12 adheres to the workpiece surface. As illustrated in FIG. 1B, immediately after application of the color powder 12 the surface of the workpiece 10 is irradiated by a suitable source of radiation 20, for instance an IR-laser, within a defined surface portion 14 so that the powder will form a gel due to the energy transferred by the radiation. Consequently the powder in the irradiated surface portion 14 will not only adhere electrostatically to the workpiece but will also stick to the workpiece 10 due to the gelling or caking process. Subsequently, as indicated in FIG. 1C, the workpiece is subjected to a flow of air in order to blow those powder particles which adhere only electrostatically to the workpiece while not sticking thereto, off the surface of the workpiece 10 externally of the defined surface portion 14 . Therefore the color powder pattern 14 remaining on the workpiece will correspond to the infrared image projected onto the workpiece. 
     As illustrated in FIG. 1D, in color order to apply a two-color pattern, a second color layer 16 is subsequently applied onto the surface of the workpiece 10 and will also electrostatically adhere to the workpiece 10. As illustrated in FIG. 1E, the same source of radiation 20 then projects the surface portion 18 corresponding to the second color on the surface of the workpiece 10 so that the color powder 16 in said portion will form a gel and stick to the workpiece. Subsequently, and replace &#34;color&#34; with color the color powder particles externally of the second surface portion 18 are blown off so that only the first color powder 12 within the portion 14 and the second color powder 16 within the portion 18, which stick to the workpiece surface, will remain on said surface. 
     Hence, with the process of the present invention an appropriate focused source of IR- or UV-radiation 20 will not uniformly irradiate the entire color powder-coated workpiece but rather a particular pattern within a defined surface portion 14, 18 is projected onto the workpiece so that energy is transferred to the powder in said portion to initiate crosslinking whereby the powder forms a gel and sticks to the workpiece. Of course, various kinds of patterns with any number of colors can be applied onto the workpiece. 
     As soon as all color layers have been applied onto the workpiece with the intended shape so as to obtain a desired image or pattern, the workpiece with the gelled color powder thereon is baked in a single operation in a known baking oven (not illustrated). 
     The radiation sources for irradiating a respective defined surface portion of the powder so as to cause gelling or partial crosslinking of the powder may be those whose emission wavelength can be matched with the absorption wavelength of the used powders. The source may, for instance, be a Nernst stylus with a suitable optical system or an IR-laser whose beam scans the workpiece in line-by-line fashion. 
     Therefore an IR- or a UV-image may be projected entirely onto the workpiece or may scan across the desired surface portion of the workpiece. When a laser scanner including a suitable control means is chosen as the source of IR-radiation, the scanner may be coupled to a computer so that digitised images can be transferred direct from the computer onto the workpiece. 
     It is advantageous to heat the surface portion during irradiation to a maximum of about 160° C. for a period of about 2 to 5 seconds. 
     With today&#39;s technologies for electromagnetic sources of radiation and with the known color powders it is intended to heat each surface portion to be coated for a period of 5  seconds to c.160° about. Upon further development of such technologies it would, however, be conceivable that a shorter irradiation period at a lower temperature will suffice to cause suitable color powders to gelate so that they will stick to the workpiece. 
     Other types of coating in which the process of the present invention can be used are, among others, coating of textile materials (patterns on webs of fabric), coating of wood and coating of ceramic materials (tiles, decoration and china goods). To this end the source of radiation must cause complete crosslinking of the powder so that subsequent baking in an oven is no longer required and the coated materials will not be subjected to unnecessarily high temperatures. 
     It is possible with the process according to the invention to form different-colored patterns in a relatively simple and easy way. Particularly, the manufacture and use of different masks for different patterns can be omitted because the source of radiation permits all kinds of shapes and patterns to be projected or &#34;scanned&#34; onto the workpiece. In a particularly advantageous embodiment the IR- or UV-image is applied to the workpiece by a computer-controlled laser scanner whereby an extremely flexible coating process is provided which permits the application of various colors and patterns without troublesome adaptations to different kinds of workpieces. 
     Although the present invention has been described with reference to a specific embodiment, those with skill in the art will recognize that changes may be made thereto without departing from the scope and spirit of the invention as set forth in the appended claims.