Abstract:
The invention relates to the development of lithographic printing plates and comprises the application of a thin film of developer solution to each plate in a controlled manner using a jetting printhead which scans the plate to apply the developer solution as the plate is conveyed under the jetting printhead. Sensors and control means activate and deactivate the scanning jetting printhead in response to the presence or absence of a plate. The developer is allowed to dwell on the plate as it is conveyed across a support structure for a sufficient time to allow for percolation into and/or dissolution of the soluble areas of the coating and is then washed off and sent to waste.

Description:
The present invention is directed to a method and apparatus for applying a uniform film of fluid to a flat surface being conveyed through a workstation and specifically for applying a uniform film of developer to lithographic printing plates in a developing station. The invention is particularly directed to the use of a jetting printhead for uniformly distributing the fluid over the workpiece. The present invention is applicable to various planar workpieces and various types of workstations but is particularly applicable to lithographic printing plates which have been imaged and require the application of a developer to remove the areas of the coating on the plate which have been rendered soluble by the imaging process. Although the invention has a broader application, it will be described with particular reference to lithographic printing plate development. 
     One method for the development of the imaged plates entails the application of a thin film or layer of developing solution to the imaged plate surface of each imaged plate to be developed. This thin film of developer solution is allowed to dwell on the plate for a time sufficient to complete the development and then rinsed from the plate. Because only a thin film of developer solution is applied to each plate, any variation of any part of the surface of the plate from being substantially flat and horizontal and any variation in the thickness of the film of developer and any variation in the dwell time of the developer on different areas of the plate can result in the improper development of the coating. Also, the coating of individual, discrete plates requires the ability to precisely initiate the coating process and precisely terminate the coating process on individual plates delivered at irregular intervals. The developer fluid must be applied in the correct amount uniformly distributed across the width and length of the plate, with minimal waste. 
     Typical imaging methods include exposure to radiation and writing by ink jet. As is well known in the art, the imaging process renders the coating soluble in the imaged areas of a positive-working plate and renders the coating insoluble in the imaged areas of a negative-working plate. In either case, it is the coating which has been rendered soluble or the coating which has remained soluble that is removed. The particular compositions of the developer solutions for these different types of printing plates are well known. For example, many of the printing plates currently in use are positive-working plates and have coatings that contain alkali-soluble resins, specifically phenolic or acrylic resins. These coatings usually contain dissolution inhibitors that render them insoluble in the alkaline developers. The imaging process reverses this dissolution inhibition and the coating then becomes soluble in the areas subjected to the imaging radiation. 
     If the developer is applied to the surface in excess and the excess metered off by some device and subsequently recovered, the recovered excess has been exposed to the atmosphere, and as such is subject to degradation. It is well documented in the art that atmospheric carbon dioxide rapidly reduces the alkalinity of aqueous alkaline developers of the type very commonly used in the processing of positive-working plates. Thus the recovered developer that is being reused will not have the same alkalinity as fresh, new developer. This recovered developer is in fact therefore reused in a way that with each application of developer to the plate, some fraction of the developer metered off and recovered will have been removed in a previous cycle of development. Thus the repeated exposure to carbon dioxide and resultant degradation will further alter the effective alkalinity of the developer. Further, if the developer is applied some distance ahead of the point where the excess is removed, some degree of development is certain to take place and it may very well be uneven. 
     With positive-working plates, the difference between the solubility of the imaged and non-imaged areas of the coating is generally less than the difference in solubility for negative-working plates. For that reason, the development process is more critical for positive-working plates. Also, the development mechanism for positive-working plates is a percolation process and a quiescent film of developer solution is critical. Any relative movement between the developer and the surface of the plate must be minimized or eliminated. Furthermore, the film of developer must be uniform with no bubbles. For these reasons, it is critical how the developer is applied to the plate. 
     SUMMARY OF THE INVENTION 
     The invention is specifically directed to the development of lithographic printing plates and comprises a novel system and method for applying the thin film of developer solution to each plate in a controlled manner using a jetting printhead (commonly referred to as an inkjet printer) as a coating device operating in a continuous mode as the plate is conveyed through the developer station. The developer solution is allowed to dwell on the plate as it is conveyed across a horizontal support structure for a sufficient time to allow for percolation into and/or dissolution of the soluble areas of the coating. The invention permits and includes the processing of plates passing side-by-side through the developer and at different intervals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a general diagrammatic sketch of a coating apparatus, specifically a lithographic printing plate developer, incorporating the novel method and apparatus for the present invention for applying a uniform film of fluid to a moving flat surface. 
     FIG. 2 is a diagrammatic plan view of the conveyor showing the processing of two plate lines at different intervals. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The diagrammatic drawing illustrates the general arrangement for practicing the invention illustrating the equipment and method for developing an imaged lithographic printing plate. The developer apparatus comprises a substantially horizontal support structure which is preferably a platen  12  which may be any flat, horizontal surface composed of materials which will be unaffected by the particular developer solution to be used. In the context of the present invention and as used herein, the terms substantially flat and or substantially horizontal are defined as deviating from flat and/or horizontal only to the degree that the developer solution applied to the plate does not flow over or off of the surface of the plate. That is, the developer solution will remain as a film on the plate and have a thickness that produces uniform development over the entire area of the plate. The printing plate  14 , which has been exposed and thus imaged, is carried across the platen  12  by means of a conveyor which comprises the conveyor drive rollers  16  and  18  and a continuous flexible conveyor belt  20 . The conveyor belt  20  is composed of a material which will be unaffected by the developer solution, such as stainless steel or a polymer material. The printing plate  14  is fed by the feed rollers  22  and  24  onto the feed platform  26  which directs the printing plate onto the conveyor belt  20  for transport across the platen  12 . After processing, the printing plate is guided by the discharge platform  28  into a pair of discharge rollers  30  and  32 . Although the flat platen is the preferred support structure, other supports can be employed for the conveyor belt. For example, the support structure could be a series of rollers which have a small diameter and are closely spaced such that they provide adequate support to maintain a flat plate. Also, although the drawing depicts a conveyor belt for conveying the plate across the support structure, other conveying means could be employed. Merely as one example, the plate can initially be conveyed across a support structure such as a platen by the feed rollers for the plate and it can then be further conveyed the remaining distance directly by small driven rollers. 
     In order to properly develop an imaged plate in accordance with the present invention, it is essential that the thin film of developer solution be substantially uniformly distributed over the entire upper, imaged surface of the plate as it is being conveyed across the platen. This requires that the plate on the conveyor be substantially flat and substantially horizontal or level and begins with having a substantially flat, horizontal support structure and, therefore, a substantially flat horizontal conveyor belt. Since the printing plates are very thin and flexible, surface tension is used to hold the plate firmly in position and flat on the conveyor belt. For example, this can be done by providing a film of water between the plate and the conveyor belt. 
     In the present invention, a jetting printhead represented at  34  is used for metering and feeding the developer solution to the plate to control the thickness and assure the uniformity of the developer solution on the plate. The jetting printhead  34  is mounted on the track  36  for movement back and forth across the plate in a raster or scan pattern as is well known in the inkjet printer technology. The developer solution in the jetting printhead  34  is replenished from the reservoir  38  through the flexible tube  40 . The plate sensor  42 , such as a photoelectric sensor, detects the leading and trailing ends of the plate  14 . This sensor  42  is connnected to the jetting printhead control unit  44  which is connected to and operates the jetting printhead. Since the conveyor speed is known, the plate sensor  42  can initiate operation of the jetting printhead at exactly the proper time to begin the coating right at the leading end of the plate. Likewise, the sensor can terminate operation when the trailing end has been coated. 
     In contrast to the known techniques where an excess of fluid is applied and subsequently metered off the plate, the present invention delivers precisely the required volume of developer at precisely the rate required to obtain a uniform film of developer on the plate in the amount needed to process the plate with very little excess. The delivery of the developer is commenced at the beginning of the plate and continues at the appropriate rate until the end of the plate where it is stopped. Since there is little if any excess developer applied, the problems with developer degradation that arise from the recirculation of excess developer are eliminated. 
     The printing plate which has been coated with the developer solution continues to travel across the platen. The length and speed of travel is selected such that the developer solution will have completed the development process by the time the printing plate reaches the discharge end of the platen. A typical development time is 20 to 60 seconds. At this point, rinse water from the supply  65  is sprayed onto the plate through the spray nozzles  66  and  68 . Located below the conveyor structure is a collection pan  70  which collects all of the liquid run off from the printing plate including the spent developer solution and rinse water now containing the portion of the coating which has been dissolved away. The developer solution which is rinsed from the plate is collected at  72  and sent to waste. It can be seen that there is always only fresh developer solution being applied to the plates and that there is only a small quantity of developer solution applied to each plate. It has been discovered that the consumption of developer solution can be reduced by as much as 50% when compared to a conventional printing plate development processor. 
     The present invention is also particularly suited to the development of lithographic printing plates in a plurality of lines or lanes in a single developing station with a single conveyor system. This aspect of the system is shown in FIG. 2 which illustrates a portion of the conveyor belt  20  moving in the direction of the arrow  46 . Located on the conveyor belt  20  are two plates  14  (it could be more than two) in side-by-side relationship. The two plates can be directly side-by-side or they can be staggered as may be desired or convenient as shown in this FIG.  2 . In effect, each plate has its own “lane” on the conveyor belt. Extending across the conveyor belt is the track  36  on which is mounted the jetting printhead  34  for movement back and forth across both plates. A plate sensor  42  is located in each lane for detecting the leading and trailing ends of the plates. These sensors are connected to the control unit  44  whereby the jetting printhead can be activated to move and activated to dispense the developer solution only in a lane containing a plate. In other words, the jetting printhead will dispense developer during the travel across a plate in one lane but will then shut off during the travel across the empty lane. As a further refinement, an optical scanner  48  mounted on a track  50  may be used to detect the side edges of the plates. With this scanner  48  connected to the control unit  44 , the jetting printhead can be activated to dispense the developer only onto the plates. The dispensing of developer would then be deactivated between plates and past the outside edges of the plates. 
     A specific example of jetting printhead which could be employed in the present invention is the Spectra Nova 256/80 series of printhead from Spectra, Inc. of Lebanon, New Hampshire. These printheads have the following characteristics: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Image height 
                 2.8 inches 
               
               
                   
                 Number of orifices 
                 256 
               
               
                   
                 Drop rate per orifice 
                 20,000/sec. 
               
               
                   
                 Drop volume 
                 75 picoliters (pl) 
               
               
                   
                   
               
             
          
         
       
     
     To process plates at the rate of 48 inches per minute, the time for each swipe of the printhead would be:            2.8                 inches   ×   60                   sec./min.         48                   inches/min.         =     3.5                 seconds                            
     Thus there would be: 
     3.5 sec.×20,000 drops/sec.−70,000 drops/orifice/swipe 
     For 256 orifices, there would be: 
     256×70,000 drops×75 pi=1.34 ml=0.082 in 3    
     For a 14 inch wide plate, the area covered in one swipe is 39.2 in 2 . Therefore, the film thickness is:            0.082                   in   3         39.2                   in   2         =     0.0021                 in                 or                 2.1                 mils                            
     A feature of the invention is that the fluid is applied uniformly over the plate at the required thickness. This contrasts sharply with prior art arrangements where the fluid is applied to the plate some distance ahead of a device which spreads the developer uniformly over the plate. A further feature is that the fluid is applied in the exact amount required to form a uniform film of the desired thickness without the need to remove and recycle excess fluid.