Abstract:
A roller used in handling a media sheet or plate has a non smooth surface. In some embodiments the surface comprises a braided sheath. The sheath may comprise a material such as polyethylene.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of the filing date of U.S. application No. 60/369,863 filed on Apr. 5, 2002. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to the field of imaging laser sensitive media. Specifically, the invention relates to methods and apparatus for loading and unloading media in the presence of debris.  
         BACKGROUND OF THE INVENTION  
         [0003]    In certain laser imaging systems a laser sensitive media is mounted onto the surface of an imaging cylinder and an image is imparted onto the media using a focused write laser of relatively high power. FIG. 1 shows a prior art external drum imaging system  1  having an imaging head  2  directing a laser beam or beams  18  toward a media  4 . Media  4  is rotated on a drum  5  while imaging head  2  is translated along the drum by a leadscrew  6 , thus scanning or writing a series of bands or a helical pattern around the drum.  
           [0004]    In other laser imaging systems a media is held on a flatbed platen and relative motion is generated in two dimensions between the media and the imaging beam. Such imaging systems are used in devices for imaging many different kinds of media including lithographic plates, flexographic plates, screens for screen printing, gravure cylinders as well as layers for flat panel displays, printed circuit boards and the like. Such imaging systems could be incorporated directly on a printing press for imaging plates in-situ. Such systems are well known in the art and will not be discussed further in this application.  
           [0005]    During imaging, the interaction of the laser and the media causes a physical and/or chemical change to the imaged areas of the media. In the process of imaging, matter may be expelled from the laser sensitive media. The expulsion of matter from the media is referred to as ablation. The matter expelled may consist of solids, liquids, gases, and plasma. The matter is sometimes called smoke or particulate debris. Ablative media are imaged by dislodging or evaporating material from a layer of the media to form an image. While ablative media by nature produce ablation debris, media traditionally regarded as non-ablative also produce fumes and/or particle debris, particularly when imaged by high power lasers, albeit in smaller quantities. A certain amount of ablated debris remains on the surface of the imaged media after imaging.  
           [0006]    When loading and unloading media onto the media-bearing surface, a roller is commonly employed to guide the media on or off of the surface. The roller may also help to hold the media against the surface. Typically, the roller is covered with a soft, relatively smooth material, which will not damage the laser-sensitive emulsion of the media. FIG. 2 shows an existing external drum imaging system during the media loading process. While one end of the media  4  is secured to drum  5  with a clamp  10 , a roller  15  is brought into contact with the surface of the media  4 . Roller  15  forces media  4  against the drum&#39;s surface while the drum rotates  12  to load the media onto the drum&#39;s surface. The other end of the media can then be clamped with a second set of clamps  20  to fully secure the media onto the drum, and then the roller  15  can be released. The same system can be employed in unloading the media, by reversing the direction of rotation  17  of the drum. Roller  15  is brought into contact with the now laser-imaged media  4  near the clamps  20  on one end of the media, and then the clamps  20  are released. The drum  5  is rotated  17  in the opposite direction from the loading process until the roller is near the other set of clamps  10 . The roller  15  is then released, along with the clamps  10 , so that the plate can be removed.  
           [0007]    If there is remnant debris on the media&#39;s imaged surface, a portion of this debris is transferred onto roller  15  during the unloading process. Additionally, after repeated loading and unloading cycles, debris tends to further accumulate on roller  15 . Debris on roller  15  can transfer onto the media&#39;s surface during the load cycle. This debris can then mask the laser&#39;s exposure of the media&#39;s surface, thus causing imaging artefacts.  
           [0008]    One way to address the problem of debris accumulation on the roller is by periodic cleaning of the roller. This can be done either by the machine operator or by some sort of automated roller cleaner. Having the machine operator clean the roller is perceived as an annoyance, and results in machine down time. Thus, there is a need for increasing the interval between cleanings or to eliminate them altogether. One example of an automated roller cleaner is in the Trendsetter™ Spectrum™ device sold by Creo Inc of Burnaby, British Columbia, Canada. This machine, which images proofing and other media, is highly sensitive to regular dust normally found in an office environment. The roller cleaner comprises a separate sticky roller, with a consumable, tear-off sticky coating. The sticky roller contacts and rolls against the media-contacting roller, removing a good portion of the dust and debris. This solution requires periodic replacing of the sticky material by the machine operator. The sticky material is a consumable cost to the machine owner. Also, this solution requires that the architecture of the machine&#39;s frame be designed to specifically accommodate this sticky roller. This may involve a significant up-front cost. It may not be feasible to upgrade existing equipment to include such a sticky roller. There is thus a need for a solution that does not require substantial machine alterations, and preferably involves less maintenance.  
           [0009]    The cleaning interval for rollers depends in part on the material from which the rollers are made. The cleaning material can be increased by makin gthe surface of the rollers from a properly seleted material. However, the surface of the media being imaged is coated in a laser-sensitive emulsion. This emulsion is often sensitive to having anything contact it. Therefore the material of the roller as well as the pressure the roller exerts on the media are important design considerations. This poses an additional complication in that the roller material should not damage the emulsion surface, in addition to not being prone to debris accumulation. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    In drawings which illustrate non-limiting embodiments of the invention:  
         [0011]    [0011]FIG. 1 shows a prior art external drum imaging system;  
         [0012]    [0012]FIG. 2 shows a prior art media loading and unloading system; and,  
         [0013]    [0013]FIG. 3 shows a roller according to one embodiment of this invention and  
         [0014]    [0014]FIG. 3A is an expanded view of a portion of the roller of FIG. 3. 
     
    
     DETAILED DESCRIPTION  
       [0015]    Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these details. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense.  
         [0016]    This invention provides rollers surfaced with a roller material that contacts the media surface. The rollers have substantially non-smooth roller surfaces. The surface of the rollers may comprise braided sheaths.  
         [0017]    The roller shown in FIG. 3 includes a sheath  25  (also referred to as a sleeve) that is fitted over an existing media-surface-contacting roller  15  (the full length of the roller is not shown in FIG. 3). Sheath  25  is made of a woven material  27 . A commercially available woven material that works well is polyester expandable sleeving, commonly used as a sheath for abrasion resistance for wires, cables and tubing. An example of one material that has been found to be suitable was purchased from Electro Sonic Inc. of Vancouver, Canada and is described as “Expandable Braided Polyester Sleeving”, part number GRP-120-1-3/4-50B.  
         [0018]    This solution has the added advantages of being very low cost and easy to install or upgrade on existing rollers. The braided sheath may comprise a number of interwoven strands. Each strand may comprise a number of fibers. For example, the braided polyester sleeve described above has strands which are each composed of 4 small fibers of polyester. Each strand is around 0.25 mm in diameter.  
         [0019]    Sheath  24  may be made from any suitable material or combination of materials and may be braided in a braiding patter. Instead of a braided sheath, roller  15  may be covered in an alternative roller surface that has a substantially non-smooth roller surface. For example, a solid roller may be provided which has its surface etched in a pattern which gives the roller surface some relief.  
         [0020]    A braided sheath used to cover a roller may be made of a much harder material than a smooth surfaced roller without damaging the emulsion surface. Test results show that in the same machine set-up, using a media known to be sensitive to roller markings, the use of a roller covered with a braided polyester sleeve eliminates the roller markings when directly compared against a commonly-used soft material roller, despite the fact that polyester is a harder material than the softer roller.  
       EXAMPLE  
       [0021]    In a controlled experiment, a media known to leave substantial amounts of debris on the media&#39;s surface after imaging, was loaded, imaged and unloaded many times. The regular smooth-surfaced, soft roller visibly accumulated debris, as expected. Debris was seen to redeposit onto subsequently loaded plates. When the same roller was covered with a braided polyester sheath as described above, no debris could be seen to redeposit onto any subsequently loaded plate.  
         [0022]    The use of such a sleeve or surface can be advantageous in many loading and unloading systems or other media-handling systems used in the graphic arts industry where a roller is required to contact a media surface to guide it onto, or off of an imaging platen or drum (internal or external drum). Another common use of rollers is in the conveyance of a media from a cassette or similar storage to an imaging engine for loading. Where there is a chance that debris can accumulate and impair the imaging process the use of a roller according to the invention can be advantageous. It is also common to employ a pair of nip rollers for advancing media in a loading operation. The roller of this invention may be advantageously employed on such nip rollers. One or both of the rollers could be sheathed or constructed according to the invention, thus avoiding the contamination of the media by debris from the machine environment.  
         [0023]    There have thus been outlined significant features of the invention in order that it may be better understood, and in order that the present contribution to the art may be better appreciated. Those skilled in the art will appreciate that the conception on which this disclosure is based may readily be utilized as a basis for the design of other apparatus and methods for carrying out the invention. It is most important, therefore, that this disclosure be regarded as including such equivalent apparatus and methods as do not depart from the spirit and scope of the invention.