Abstract:
The subject matter of the invention is a method for laser exposure of a screen ( 11 ), whereas only one portion of the screen ( 11 ) is used for motive exposure ( 16 ). Providing a method for laser exposure of a screen of the type mentioned herein above, in which soiling of the screen printing apparatus is prevented without high costs is achieved by also exposing a margin region ( 18 ) of the screen ( 11 ) located outside the motive exposure area ( 16 ).

Description:
[0001]     This application claims Priority from German Applications No. DE 10 2005 062 217.8-51 filed on Dec. 24, 2005 and DE 20 2005 020 198.7 filed on Dec. 24, 2005  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a method according to the preamble of claim  1  and to an apparatus according to the preamble of claim  8 .  
         [0004]     In screen printing with laser-exposed screens, the actual image, the so-called motive exposure area, is usually disposed in the center of the screen. For this purpose, the region of concern is covered with a light-sensitive emulsion and then a negative of the image wanted is projected thereon. In the exposed areas, the emulsion binds durably with the screen and remains there. The rest of the emulsion is then removed.  
         [0005]     The screen is dimensioned so as to be significantly larger than the actual motive exposure area so that images of different sizes can be applied and that sufficient space is provided for ink application during subsequent screen printing. It happens that the ink inadvertently passes through the screen in the margin region lying outside of the motive exposure area, thus soiling the entire screen printing apparatus. To prevent this from happening, the unexposed margin region is tediously covered, in a separate step with an ink impermeable screen filler preventing ink from passing in the margin region.  
         [0006]     2. Description of the Prior Art  
         [0007]     It is known from the document GB 1 517 291 A to provide the screen with a light-sensitive adhesive, the region outside of motive exposure area also being exposed to light in order to fix said adhesive to the screen.  
         [0008]     It is known from the document EP 0 246 741 A2 to provide the entire screen with a photosensitive emulsion that cures after exposure except in the region of the useful exposure area.  
         [0009]     In the two printed documents mentioned, the screen is exposed to conventional light sources, not to laser light. There is the risk that the actual motive exposure area may be affected by scattered light. Moreover, such an exposure is very expensive both in terms of cost and time.  
       BRIEF SUMMARY OF THE INVENTION  
       [0010]     In view thereof, the object of the present invention is to provide a method and an apparatus for laser exposure of a screen of the type mentioned herein above in which the margins are exposed much faster and at much lower cost in order to avoid soiling of the screen printing apparatus.  
         [0011]     As a technical solution to this object, the invention proposes a method having the features of claim  1  and an apparatus having the features of claim  8 . Advantageous developed implementations of this method and of this apparatus will become apparent in the respective dependent claims.  
         [0012]     A method implemented according to this technical teaching and an apparatus implemented according to this technical teaching have the advantage that, by also exposing the margin region of the screen, this part of the screen outside of the motive exposure area is also provided with an ink impermeable coating by exposing the emulsion applied thereon, so that the entire screen is filled. As a result, no ink will be allowed to pass through the screen outside of the motive exposure area so that inadvertent soiling of the system is avoided. Consequently, there is no longer any need to separately apply a screen filler, which brings a significant cost saving.  
         [0013]     The exposure of the margins is thereby performed by means of a light bar equipped with a number of diodes, whereas motive exposure is performed using a prior art laser exposer. The advantage thereof is that the much cheaper light bar exposes the margins whilst the very expensive laser exposer is used for motive exposure only. Accordingly, the margins are exposed at very low cost.  
         [0014]     It has been found advantageous to expose this margin region so completely, meaning with sufficient intensity, that the emulsion applied in the margin region forms an ink impermeable coating in the fully exposed condition.  
         [0015]     Another advantage is that the light bar has a much higher light output than the laser exposer so that the margins can be exposed much faster. Accordingly, the processing of one single screen can be performed in far less time, which lowers the machine and processing costs.  
         [0016]     In an advantageous developed implementation, the diodes can be activated individually. This makes it possible to fully expose discrete portions of the margin region without exposing other portions of the screen.  
         [0017]     In a particularly preferred embodiment, the light bar and the laser exposer take turns in exposing the screen. The advantage thereof is that motive exposure and margin exposure can be performed in one step.  
         [0018]     In an advantageous embodiment, the light bar accommodates a number of UV LEDs, diodes or other light sources. Between 10 and 200, ideally 54, such LEDs are preferably utilized in order for the screen to be exposed across its entire width. These very cheap LEDs, diodes or other light sources can be individually replaced in case of failure, which keeps the operating and repair costs low. The number of UV LEDs, diodes or other light sources depends on the desired angle of reflection, the light efficiency and the luminous power needed to cure the screen.  
         [0019]     In a particularly preferred embodiment, the light bar is surrounded by a light impermeable housing that is light permeable toward the screen. This prevents the light bar from emitting scattered light which might damage a possibly already performed motive exposure. Scattered light is in particular minimized by the fact that the housing is brought as close as possible to the screen, it being preferred that  
         [0020]     To increase the luminous power and the homogeneity, it is advantageous to configure the inner side of the housing to be reflective.  
         [0021]     In dimensioning the housing, it has been found advantageous to configure the permeable opening in such a manner that an outlet port of between 10° and 15°, preferably of 12° be formed. This allows achieving sufficient exposure without unnecessary scattered light. At this angle, the advance speed may also be adjusted so as to be quite high, this allowing to reduce significantly the processing time.  
         [0022]     Further advantages of the method of the invention and of the apparatus of the invention will become apparent in the appended drawing and in the following description of embodiments thereof. Likewise, the invention lies in each and every novel feature or combination of features mentioned above or described herein after. The embodiments discussed herein are merely exemplary in nature and are not intended to limit the scope of the invention in any manner. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0023]      FIG. 1   a - d  is a schematic illustration of a first embodiment of margin exposure and of motive exposure of a screen at different instants in time;  
         [0024]      FIG. 2   a - d  is a schematic illustration of a second embodiment of margin exposure and motive exposure of a screen at different instants in time;  
         [0025]      FIG. 3   a - d  is a schematic illustration of a third embodiment of margin exposure and motive exposure of a screen at different instants in time;  
         [0026]      FIG. 4  is a schematically illustrated, sectional side view of a fourth embodiment of an apparatus of the invention;  
         [0027]      FIG. 5  is a schematically illustrated, sectional side view of a fifth embodiment of an apparatus of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     The  FIGS. 1   a  through  1   d  schematically illustrate a screen  11  prepared with a light sensitive emulsion and held in a frame  10  and across which a laser exposer  12  and a light bar  13  can move. The laser exposer  12  and the light bar  13  are commonly mounted on a movable bridge  14 . The light bar  13  is equipped with a number of diodes  15  that are individually activatable. The laser exposer  12  passing across the screen  11  performs motive exposure  16  in the center of the screen  11 , whereas the margin region  18  of the screen  11 , which is located outside of the motive exposure area  16 , also receives exposure from the light bar  13 . More specifically:  
         [0029]     The bridge  14 , together with the laser exposer  12  and the light bar  13  passes across the screen  11  twice (back and forth). The bridge  14  starts at a first edge  20  of the screen  11  and moves towards a second edge  22  of the screen  11 , opposite the first edge. While the bridge  14  moves from the first edge  20  to the beginning of the motive exposure area  16 , the light bar  13  is switched on and the laser exposer  12  is switched off, as can be seen from  FIG. 1   a . In this phase, the bridge  14  can move at very high speed since the diodes have sufficient power to sufficiently expose the margin region  18 , even if the bridge  14  moves at high speed. Once the edge of the motive exposure area  16  has been reached, the light bar  13  is switched off, the laser exposer  12  is switched on and the speed of the bridge  14  is decelerated for the laser exposer  12  to be able to perform the motive exposure in the known way, as can be seen in  FIG. 1   b . Upon completion of the motive exposure  16 , the laser exposer  12  is switched off again and the light bar  13  switched on in order to fully expose the area of the margin region  18  located between the motive exposure area  16  and a second edge  22  of the screen, as can be seen in  FIG. 1   c . Again, the bridge  14  moves at high speed.  
         [0030]     On its way back, meaning on the second passage over the screen  11 , the areas of the margin region  18  between the useful exposure area  16  and the side edges  24  of the screen  12  are fully exposed. Only those diodes  15  are switched on that are arranged above the margin region  18  whilst the diodes disposed above the motive exposure area  16  remain switched off in order to avoid further exposure of the motive exposure area  16 , which is not desired.  
         [0031]     Next, such an exposed screen  11  is freed in an actually known manner from the non hardened parts of the emulsion so that that portion of the screen is rendered ink impermeable through which no ink should pass later. Since the margin region  18  is now also exposed, this margin region  18  is automatically also provided with the hardened emulsion so that this no longer requires a separate step and that the screen printing apparatus is prevented from being soiled through ink inadvertently passing through the margin region  18 .  
         [0032]     In the transition area between the motive exposure area  16  and the margin region  18 , there is formed what is referred to as a light rim  26  that is exposed both through motive exposure  16  and through margin region exposure  18 .  
         [0033]     It is understood that the margin region is exposed so strongly that it is finally covered with an ink impermeable coating.  
         [0034]     The second embodiment illustrated in the  FIGS. 2   a  through  2   d  only differs from the first embodiment illustrated in the  FIGS. 1   a  through  1   d  only by the fact that, once motive exposure  16  is complete, both the light bar  13  and the laser exposer  12  remain switched off and the bridge  14  is quickly moved to the second edge  22  without performing another exposure. The light bar  13  is only activated on the way back, is then exposing the margin region  18  while the bridge  14  is moved back from the second edge  22  to the first edge  20 , as can be seen from  FIG. 2   c.    
         [0035]     The third embodiment illustrated in the  FIGS. 3   a  through  3   d  only differs from the first embodiment illustrated in the  FIGS. 1   a  through  1   d  in that, during motive exposure  16  through the laser exposer  12 , some diodes  15  of the light bar  13  are activated as well in order to fully expose the margin region  18  between the motive exposure area  16  and the side edges  24 , as can be seen in  FIG. 3   b . Since during motive exposure  16  the bridge  14  moves at a slower pace than during mere margin exposure  18 , the diodes  15  only need to be operated at reduced power in this phase in order to achieve sufficient exposure.  
         [0036]     Since in this embodiment both the margin exposure  18  and the motive exposure  16  are completed at the first passage of the screen  11 , the bridge  14  may be moved back at higher speed, as can be seen from  FIG. 3   d.    
         [0037]     The fourth embodiment shown in  FIG. 4  of an apparatus of the invention for exposing a screen for use in screen printing comprises a light bar  112  that is movable across a screen  110  to be exposed. This light bar  112  includes a housing  114  and LEDs  116  that are housed side-by-side along a virtual line  154  in the housing  114  and that are individually activatable.  
         [0038]     The housing  114  is light impermeable on three sides and has an approximately 1 mm wide slot  120  on its side turned toward the screen  110 . The inner side of the housing  114  is configured to be reflective so that the light emitted by the LEDs is almost completely directed toward the screen  110 .  
         [0039]     The light bar  112  is mounted directly beside a laser exposer  122  and is moved back and forth across the screen  110  together therewith.  
         [0040]     The fifth embodiment illustrated in  FIG. 5  only differs from the embodiment illustrated in  FIG. 4  by the fact that the housing  134  is configured to conically taper on its side turned toward the screen  130 .  
         [0041]     In another embodiment that has not been illustrated herein, the light bar is solidly mounted and the screen is accordingly moved back and forth underneath the light bar.  
       List of Numerals  
       [0000]    
       
         
           
               10  frame  
               11  screen  
               12  laser exposer  
               13  light bar  
               14  bridge  
               15  diodes  
               16  useful exposure (area)  
               18  margin region  
               20  first edge of the screen  
               22  second edge of the screen  
               24  side edge  
               26  light rim  
               110 ,  130  screen  
               112  light bar  
               114 ,  134  housing  
               116  LED  
               120  slot  
               122  laser exposer