Abstract:
An external drum platesetter, including: two or more drums aligned along a same rotational axis for supporting one or more printing plates; one or more rails positioned parallel to the rotational axis of the drums; a single imaging head mounted on and moveable along the one or more rails, where the imaging head includes a laser source or ink jets for transferring an image to the one or more printing plates; one or more motors for rotating the drums together or individually; and a computer for controlling operation of the platesetter in accordance with instructions stored or transferred to the computer.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention, in the field of pre-press and industrial printing, is generally directed towards saving time and money when making printing plates, and is more specifically directed towards increasing throughput in a machine for making printing plates, e.g. a platesetter. 
         [0002]    A platesetter is used to transfer an image to a printing plate which, in turn, is mounted on a commercial printing press, such as those made by Heidelberger Druckmaschinen AG of Germany. The image from the printing plate is then transferred on the press, for example by offset printing, to a final product such as a newspaper, magazine, poster, packaging, etc. 
         [0003]    The process of transferring text and/or graphic information from electronic form to visual form on a printing plate medium is called imaging. In an external drum platesetter, a medium is mounted onto the external surface of the drum, then the drum is rotated while an imaging beam traverses longitudinally along the length of the drum and an image is transferred to the plate. The information is typically transferred by a laser beam or beams. The medium may be a printing plate that is sensitive to a specific wavelength, such as a photosensitive substrate which is sensitive to visible light, a plate sensitive to ultraviolet light, or a plate which requires high power for thermal fusion or ablation of particles on the plate. The frequency and intensity of the laser beam will thus vary according to the requirements for imaging the printing plate. 
         [0004]    Two key considerations in the manufacture and development of an external drum platesetter are (1) minimizing or lowering the costs to build and operate the machine, and (2) increasing throughput of printing plates, in other words, increasing the number of printing plates that are imaged on the platesetter over a given period of time. These are ongoing problems in the printing industry. 
         [0005]    Throughput of printing plates can be increased for example by (1) increasing the speed of rotation of the drum, (2) increasing the power of the laser source, (3) increasing the number of imaging beams being used, (4) and minimizing the down time, defined as the time necessary to stop the drum from rotating, remove the imaged printing plate, install a new blank printing plate to be imaged, resetting all hardware and software to default positions prior to imaging, and bringing the drum up to speed again for imaging. Each of the above factors are limited whereby at a certain point, no further improvement in throughput can be attained. 
         [0006]    A common procedure for installing, mounting and removing plates on a platesetter is disclosed in U.S. Pat. No. 6,412,413 issued Jul. 2, 2002 to Tice et al. which describes a media handling system including clamps for securing a printing plate on an external drum platesetter. Also, U.S. Pat. No. 6,794,626 issued Sep. 21, 2004 to Kiermeier et al. discloses a method and system for verifying correct mounting of a printing plate on an external drum imaging machine. Both of these patents are herein incorporated by reference in their entirety for background information only. 
         [0007]    It is an object of the present invention to overcome the above identified and related problems in the prior art by increasing throughput of printing plates in an external drum platesetter. 
       SUMMARY OF THE INVENTION 
       [0008]    An external drum platesetter, including: two or more drums aligned along a same rotational axis for supporting one or more printing plates; one or more rails positioned parallel to the rotational axis of the drums; a single imaging head mounted on and moveable along the one or more rails, where the imaging head includes a laser source or ink jets for transferring an image to the one or more printing plates; one or more motors for rotating the drums together or individually; and a computer for controlling operation of the platesetter in accordance with instructions stored or transferred to the computer. 
     
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0009]      FIG. 1  (not drawn to scale) is a perspective view of portions of an external drum platesetter in accordance with the principles of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0010]    In a first preferred embodiment or first operational mode,  FIG. 1  illustrates a portion of an external drum platesetter  100  used for imaging printing plates. Initially, one or more printing plates  130  are mounted on an outside surface of a first external drum  110  for imaging. Also one or more printing plates  140  are mounted on an outside surface of a second external drum  120 . The plates  130  and  140  are mounted and secured in any known fashion, such as with edge clamps as described in U.S. Pat. No. 6,412,413. The two drums  110  and  120  are both aligned along a common rotational axis  190  which also represents a drive shaft or shafts for rotating the drums via one or more motors  170 . The mounting surface for mounting printing plates on each drum is registered or calibrated with respect to the mounting surface of the other drum. The drums are separated by a gap A so that each drum can be independently operable. 
         [0011]    The platesetter  100  includes a single imaging head  150  which includes a laser source and optics (not shown) for directing and focusing one or more laser beams  200  onto the printing plates for imaging. The imaging head  150  is moveable along one or more rails  160  in a linear direction B. In preparation for imaging printing plates mounted on the first drum  110  or the second drum  120 , the imaging head is moved respectively to a first or second predetermined default position C or D. A computer or controller  180  is used to control the overall operation of the platesetter  100 . 
         [0012]    In the first embodiment, both drums have plates mounted thereon and are brought up to operational speed for imaging at the same time. Thereafter the drums are alternately stopped so that, either automatically or by an operator, the imaged plate or plates can be removed and new plates can be installed and secured for imaging. Then the drum is again brought up to operational speed and the second drum is stopped for removing imaged plates and installing new plates. The cycle continues so that one plate is always rotating for imaging while the other drum is in the mode of loading or unloading a plate. 
         [0013]    In a second preferred embodiment or second operational mode, an operator will mount a first plate or plates  130  onto the first drum  110 . The imaging head  150  is initially moved to a first predetermined default position C in preparation of imaging to the first printing plates  130  mounted on the first drum  110 . 
         [0014]    Software within the computer  180  will control the overall operation of the specific print jobs. For example, this software could be resident within the memory of the computer, it could be read from another source, it could be input via a compact disk or memory card, or it could be received by way of operator input on a keyboard (not shown). 
         [0015]    The imaging head  150  is dependent upon the type of printing plates to be imaged. If the plates require high power, such as thermal ablation plates or thermal fused plates, then the laser source must be a high powered source (e.g. having a 830 nm wavelength). On the other hand if the plates are sensitive to ultraviolet light, then the laser source within the imaging head must provide one or more laser beams consisting of ultraviolet light (e.g. having a 410 nm wavelength). Other imaging heads could also be used on the platesetter, such as a laser head providing a laser beam of any known wavelength, or an ink jet printing head for ink jet transfer of an image. 
         [0016]    Once the first plate or plates  130  are mounted on the first drum  110 , then the motor and drive system  170  will start rotating the first drum  110  until it reaches a first predetermined operational speed for imaging. The imaging head  150  will thereafter begin transferring the desired image via the laser beam or beams  200  (or via ink jet) to the first printing plate or plates  130  on the first drum  110 . 
         [0017]    While the first drum is being brought up to speed by the drive system and motor  170  and the first plates  130  on the first drum  110  are being imaged, the operator can mount a second plate or plates  140  onto the second drum  120 . Once the imaging of the first plates  130  on the first drum  110  is finished and the first drum  110  is being slowed to a stop, then the second drum  120  can be brought up to a second predetermined operational speed by the drive system and motor  170  for imaging. 
         [0018]    The drive system and motor  170  are operational so that both the first and second drums  110 ,  120  can be rotated together at a same operational speed, or they can be operated separately and independently and at different operational speed if desired. The first and second predetermined operational speeds may vary according to the specific requirements of the particular print jobs on queue in the computer  180 . The two drums can be physically locked together by mechanical means if desired. 
         [0019]    In preparation for transferring an image onto the second plate or plates  140  of drum  120 , the imaging head  150  will move along the rail or rails  160  along linear direction B and across the distance coincident with gap A to a second predetermined default position D. While the imaged first printing plate or plates  130  are being removed from the first drum  110  by the operator, then the second printing plate or plates  140  are being imaged by the imaging head  150  on the second drum  120 . 
         [0020]    Once the imaging of the second printing plate or plates  130  is complete, then the imaging head  150  will move along the rail  160  in the direction B and traverse the distance coincident with gap A along the rail or rails  160  to again move to the first predetermined default position C in preparation for imaging again to plates mounted on the first drum  110 . The second drum  120  will be slowed to a stop to allow the operator to access and remove the second printing plate or plates  140  from second drum  120 , while the first drum  110  is brought up to the first predetermined operational speed by the drive system and motor  170  in preparation for imaging to plates mounted thereon. 
         [0021]    This cycle continues so that printing plates are nearly constantly being imaged. The idle or down time of the imaging head  150  is drastically reduced from the prior art teachings, and the throughput or number of printing plates imaged by the platesetter is significantly increased from the platesetters of the prior art. 
         [0022]    In the manufacture of external drum platesetters, the imaging head is one of the most costly components. Thus, according to the present invention, a single costly imaging head is utilized for maximum use in contrast to a conventional prior art system whereby the imaging head is idle during the period of time that is necessary to stop the drum, unload the plates, re-install new plates, and bring the drum back up to imaging speed. 
         [0023]    Many different embodiments of the external drum platesetter shown in  FIG. 1  are included in the invention as claimed. For instance, the number of drums aligned along a common rotational axis within a single machine is not limited to two. 
         [0024]    The motor and drive system for mechanically rotating the drums includes any known drive system currently available, as long as the drums can optionally be independently rotated and stopped. Optionally the multiple drums can be connected to turn together, for example, by inserting a pin across the gap A to connect the drums, or locking the drive shaft. In this way, an operator can mount a large printing plate that would not fit on a single drum. 
         [0025]    The exact timing between starting, stopping and operating each drum is not critical so long as the overall gist of the invention is maintained to allow that one portion of the platesetter to nearly always in the mode of transferring an image to printing plates, while another portion of the platesetter is in the mode of plate transfer to or from the drum. 
         [0026]    While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various form changes in and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.