Abstract:
A method of adjusting a machine ( 1 ) for printing plate elements ( 12 ) equipped with at least one rotary impression cylinder ( 27 ) includes the steps of measuring a speed of the element ( 12, 13 ) passing through the machine ( 1 ), generating an operating signal ( 34 ) as a function of the measured speed and a tangential speed of the cylinder ( 27 ), and adjusting an operating speed of the machine ( 1 ) as a function of the signal generated so that the operating speed of the machine ( 1 ) is such that the speed of the element ( 12, 13 ) is substantially equal to the tangential speed of the cylinder ( 27 ).

Description:
[0001]    The present invention concerns a method for automatically adjusting a machine for printing plate elements. The invention relates to an adjustment arrangement intended for a machine for printing plate elements. The invention also concerns a printing machine having an adjustment arrangement. 
         [0002]    A printing machine is used in the packaging industry to print plate elements, such as sheets of paper or cardboard. The machine includes a plurality of successive stations. A first station located most upstream is an infeed station successively introducing the sheets one after the other from the bottom of a stack. The infeed station feeds a plurality of printing stations in the form of one or more printing units placed one after the other. Each of the printing units prints one color. A delivery station that collects the printed sheets is provided at the end of the machine. 
         [0003]    In the case of printing sheets of cardboard, more particularly corrugated cardboard, the technology most frequently employed is flexographic printing. A flexo machine includes one or more printing units as a function of the number of colors required. A printing unit notably includes a plate cylinder around which is wrapped and on which is tensioned a flexible plate. This plate prints the sheet after it has been coated with ink using a screened cylinder known as the anilox cylinder and an inking device. The sheet passes between the plate cylinder and an pressure roller. The printing is effected on the bottom of these sheets, so the sheets are transported from the top. A plate cylinder prints one or more patterns using the same color during each of its rotations. 
         [0004]    The plates are produced with a screen that consists of a rectangular array of larger or smaller dots. This screen is used to transform a half-tone original, for example a photo, into a printable dotted image. Screens with round dots are most frequently used in flexographic printing. The dots enable better reproduction of details and a better transition between tones. 
         [0005]    To obtain a final image of good quality on the printed sheet, it is notably necessary for all the patterns in different colors to be superposed exactly. It is also necessary that the screen dots are not deformed, for example from a round shape to an oval shape. 
       PRIOR ART 
       [0006]    The sheets are transported by a vacuum system using a belt or flat belts or steel rollers driven to move the sheets longitudinally from one printing unit to another in the upstream to downstream direction, from the infeed station to the delivery station. To obtain printing of optimum quality, the basic principle is that the sheets are transported at the most regular speed possible. 
         [0007]    However, the belt(s) wear and stretch and the sheet transport speed is no longer regular from one job to another. The performance of the machine is degraded. Maintenance of the transport system takes time and involves stopping production completely. In the case of roller conveyors, it is the non-homogeneous nature of the sheets to be printed or being printed that causes speed variations. Ovalization or deformation of the printed screen dots can be seen on the printed sheets. 
       SUMMARY OF THE INVENTION 
       [0008]    A main objective of the present invention consists in developing a method enabling adjustment of a machine for printing plate elements. A second objective is to improve and to maintain constant the printing quality of a printing machine. A third objective is to reduce the adjustment time or the down times and the number of plate elements printed to adjust a printing machine. A fourth objective is to employ a method for automatic, simple and rapid adjustment of a machine thanks to a specific arrangement. A fifth objective is to provide an adjustment method enabling the disadvantages of the prior art methods and arrangements to be avoided. A further objective is for the operator to succeed in adjusting a machine before starting production printing. 
         [0009]    A method in accordance with one aspect of the present invention is for adjusting a machine for printing plate elements equipped with at least one rotary impression cylinder. The adjustment method includes the steps of:
       measuring a speed of the plate element passing through the machine for printing plate elements,   generating an operating signal for the machine for printing plate elements as a function of the measured speed of the plate element and a tangential speed of the rotary impression cylinder or cylinders, and   adjusting an operating speed of the machine for printing plate elements as a function of the operating signal generated,   so that the operating speed of the machine for printing plate elements is such that the speed of the plate element is substantially equal to the tangential speed of the rotary impression cylinder or cylinders.       
 
         [0014]    In other words, the real speed of the machine is regulated to avoid a speed difference between the speed of the plate element and the tangential speed of the plate carried by the impression cylinder, which causes printing of poor quality because the screen dots are deformed. With this adjustment, the speed of the transported plate element is perfectly synchronized with the tangential speed of the rotary impression cylinder and therefore with the tangential speed of the plate fixed to the rotary impression cylinder. With this adjustment, the tangential speed of the rotary impression cylinder and therefore the tangential speed of the plate fixed to the rotary impression cylinder is perfectly synchronized with the speed of the transported plate element. 
         [0015]    Moreover, such a method enables easy starting of a job in the machine because all the colors are in the same row and at the same place on the plate element. 
         [0016]    In accordance with another aspect of the invention, an adjustment arrangement for a machine for printing plate elements is intended to implement the method. In accordance with a further aspect of the invention, an arrangement for adjusting a machine for printing plate elements equipped with at least one rotary impression cylinder includes:
       detector means adapted to detect the passage of a plate element and to send a detection signal, and   a calculation and control unit connected to the detector means, receiving the signal from the detector means, and able to calculate the speed of the element passing through the machine for printing plate elements and to generate automatically an operating signal for the printing machine such that the operating speed of the machine for printing plate elements is such that the speed of the plate element is substantially equal to the tangential speed of the rotary impression cylinder or cylinders.       
 
         [0019]    In accordance with a further aspect of the invention, a printing machine including at least one printing unit provided with a rotary impression cylinder is characterized in that it includes an adjustment arrangement having one or more of the technical features described hereinafter and claimed. 
         [0020]    Throughout the description, the board or sheet element is defined by way of non-exhaustive example as being made from a material such as paper, flat cardboard, corrugated cardboard, laminated corrugated cardboard, flexible plastic, for example polyethylene (PE), polyethylene terephthalate (PET), bi-oriented polypropylene (BOPP) or other polymers, or other materials suitable for printing. 
         [0021]    The longitudinal direction is defined with reference to the direction of movement of the plate element in the machine, along its longitudinal median axis. The upstream and downstream directions are defined with reference to the direction of movement of the element in the longitudinal direction of the printing machine as a whole. The front edge of the element is defined with reference to the direction of movement in the longitudinal direction in the printing machine as a whole. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The invention will be clearly understood and its various advantages and features will emerge more clearly from the following description of the nonlimiting embodiment with reference to the appended diagrammatic drawing in which the single FIGURE represents a lateral general view of a printing machine including an adjustment arrangement in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0023]    As the FIGURE shows, a printing machine  1 , such as a flexo printing machine, includes:
       a frame  2 ,   a feeder or infeed station  3 ,   one or more printing units, in this case five successive printing units  4 ,  6 ,  7 ,  8  and  9  in line printing five different colors, for example black, blue, red, green and yellow, and   a machine outfeed or delivery station  11  (shown in dashed outline).       
 
         [0028]    The infeed station  3  receives a stack of plate elements, for example in the form of unprinted sheets  12  of corrugated cardboard and sends them one after the other into the first printing unit  4 . At the outfeed of the machine  1  the delivery station  11  then recovers the printed sheets  13 . 
         [0029]    The sheets  12  and  13  pass from one printing unit to another (arrow F), from the infeed station  3  to the delivery station  11  in the longitudinal direction. The sheets  12  and  13  are transported by drive means in the form of series of top drive rollers  14 . The rollers  14  are driven by a motor  15 , its motor drives and the necessary gears and transmission devices (see for example the document EP 0.363.662). The sheets  12  and  13  are pressed against the rollers  14  by a series of vacuum suction arrangements  16  disposed between the printing units  4 ,  6 ,  7 ,  8  and  9 . 
         [0030]    A printing unit may include an inking device  17  with a lower ink reservoir  18 , a pump  19 , means  21  for circulating the ink and a squeegee chamber  22  (last two printing units  8  and  9 ). A printing unit may also include an inking device  17  with a lower ink reservoir  18 , a pump  19 , means  21  for circulating the ink and an inking roller dipping into an ink pan  23  (first three printing units  4  to  7 ). The inking device  17  inks an anilox cylinder  24 . 
         [0031]    The anilox cylinder  24  coats with ink the plate  26  mounted on the exterior surface of a printing plate cylinder  27 . The plate cylinder  27  is driven in rotation (arrow R) by rotating means such as a motor, its motor drives and the necessary gears and transmission devices, which enables adjustment of the angular position and/or the speed of the plate  26  relative to the sheet  12 . 
         [0032]    In accordance with a first embodiment of the method in accordance with the invention the operating speed of the machine  1  is adjusted using the drive means  14  and  15 . The new speed at which the sheet  12  passes through is therefore synchronized with the tangential speed of the plate or plates  26  fixed to the respective plate cylinder or cylinders  27 . This process is effected thanks to an arrangement for adjusting the speed of the drive means  14  and  15  in the machine  1 . 
         [0033]    In accordance with a second embodiment (not shown), the operating speed of the machine  1  is adjusted using means for rotating the plate cylinder or cylinders  27 . The new tangential speed of the plate or plates  26  fixed to the respective plate cylinder or cylinders  27  and the tangential speed of the plate cylinder or cylinders  27  are therefore synchronized with the speed at which the sheet  12  passes through. This process is effected using an arrangement for adjusting the speed of the means for rotating the plate cylinder or cylinders  27  in the machine  1 . 
         [0034]    The adjustment arrangement includes detector means adapted to detect the passage of the sheets  12  and  13 . In a first embodiment, the detector means are formed by a sensor  28  for sensing the front edge of the printed sheet  13 . This sensor  28  is advantageously installed downstream of the last plate cylinder  27  corresponding to the output of the last printing unit  9 . For accurate calculation of the speed, this sensor  28  is installed as far downstream as possible from the infeed station  3  and just upstream of the delivery station  11 . The detector means, i.e. the sensor  28 , send a detection signal  29  as soon as the printed sheet  13  passes it. 
         [0035]    In accordance with a second embodiment, the detector means are preferably formed by two sensors of the front edge of the non-printed sheet  12  and the printed sheet  13 . The first of these two front edge sensors is similar to the sensor  28  of the first embodiment. 
         [0036]    The second of these front edge sensors is formed by a sensor  31  for sensing the front edge of the sheet  12 . This second sensor  31  is installed upstream of the first plate cylinder  27 , corresponding to the input of the first printing unit  4 . For accurate calculation of the speed, this sensor  31  is installed as far upstream as possible from the delivery station  11  and just downstream of the infeed station  3 . The detector means, i.e. the second sensor  31 , send a detection signal  32  as soon as the non-printed sheet  12  passes it. 
         [0037]    The adjustment arrangement further includes a calculation and control unit  33 . In the first embodiment, the unit  33  is connected to the detector means, i.e. to a sensor  28  or to the two sensors  28  and  31 , and also to the means  15  for driving the rollers  14 . In the second embodiment the unit  33  is connected to the detector means, i.e. to a sensor  28  or to the two sensors  28  and  31 , and also to the means for rotating the plate cylinder or cylinders  27 . The unit  33  receives the signal from the detector means  28  or  28  and  31 . 
         [0038]    The unit  33  is able to calculate the speed of the non-printed sheet  12  and of the printed sheet  13  passing through the machine  1  and is able to generate an adjustment signal for operation of the machine  1 . In the first embodiment, the unit  33  is consequently able to generate automatically an adjustment signal  34  for the drive means  15 . The adjustment signal  34  makes it possible to equalize the speed of the sheet  12  or  13  with the tangential speed of the plate or plates  26  on its respective plate cylinder or cylinders  27 . In the second embodiment the unit  33  is consequently able to generate automatically an adjustment signal for the means for rotating the plate cylinder or cylinders  27 . The adjustment signal  34  makes it possible to equalize the tangential speed of the plate or plates  26  on its respective plate cylinder or cylinders  27  with the speed of the sheet  12  or  13 . 
         [0039]    In the first and second embodiments the adjustment signal  34  makes it possible to equalize the speed of the drive means  15  with the tangential speed of the plate cylinder or cylinders  27 . 
         [0040]    The method of adjusting the machine  1  includes a plurality of successive steps following on from the steps of printing the pattern or patterns on the sheets  12 . 
         [0041]    In a first step, the speed of the non-printed sheet  12  and the printed sheet  13  passing through the machine  1  (arrow F) is measured. This step consisting in measuring the speed of the sheet  12  or  13  passing through the machine  1  (arrow F) may be implemented by calculation determining the time for the sheet  12  or  13  to pass between a machine input, i.e. the infeed station  3 , and a machine outfeed, i.e. the delivery station  11 . 
         [0042]    This step consisting of measuring the speed of the sheet  12  or  13  passing through the machine  1  (arrow F) may be implemented by calculation determining the time for the sheet  12  or  13  to pass upstream and downstream of the cylinder or cylinders  27 . 
         [0043]    This step consisting in measuring the speed of the sheet  12  or  13  passing through the machine  1  (arrow F) may also be implemented by sending and receiving waves, for example light waves or radar waves, reflected by the sheet  12  or  13 . 
         [0044]    In accordance with the first embodiment, to determine the start of the passage time, a signal indicating departure of the sheet  12  is sent by the infeed station  3  of the machine  1  or by the machine  1  itself. This departure signal is similar to a departure pulse. To determine the end of the passage time, the passage of the sheet  12  is detected downstream of the plate cylinder or cylinders  27  of the respective printing unit or groups  4 ,  6 ,  7 ,  8  and  9 , thanks to the sensor  28  which generates its detection signal  29 . The real speed of the sheet  12  or  13  is calculated knowing the distance between the infeed station  3  and the sensor  28 . 
         [0045]    In accordance with the second embodiment, to determine the start of the passage time the passage of the sheet  12  is detected upstream of the plate cylinder or cylinder  27  of the printing unit or groups  4 ,  6 ,  7 ,  8  and  9  thanks to the second sensor  31  which generates its detection signal  32 . To determine the end of the passage time the passage of the sheet  12  is detected downstream of the plate cylinder or cylinders  27  of the printing unit or groups  4 ,  6 ,  7 ,  8  and  9  thanks to the first sensor  28  which generates its detection signal  29 . The real speed of the sheet  12  or  13  is calculated knowing the distance between the plate cylinder or cylinders  27  or between the two sensors  28  and  31 . 
         [0046]    In a second step of the method an adjustment signal  34  is generated by the unit  33  and sent to the drive means  15 . This signal  34  is a function of the passage time F measured for the non-printed sheet  12  and the printed sheet  13 , i.e. the calculated speed, and the rotation speed of the plate cylinder or cylinders  27 . The real speed calculated for the non-printed sheet  12  and the printed sheet  13  is compared with the tangential speed of the plate cylinder or cylinders  27 . 
         [0047]    In a third step of the first embodiment of the method the speed of the drive means  15  is adjusted as a function of the adjustment signal  34  generated. 
         [0048]    If the real speed calculated for the non-printed sheet  12  or for the printed sheet  13  is greater than the tangential speed of the plate or plates  26  on the respective plate cylinder or cylinders  27  the adjustment signal  34  generated takes a value corresponding to a deceleration of the drive means  15 . If the real speed calculated for the non-printed sheet  12  and the printed sheet  13  is less than the tangential speed of the plate or plates  26  on the respective plate cylinder or cylinders  27  the adjustment signal  34  generated takes a value corresponding to an acceleration of the drive means  15 . If the real speed calculated for the non-printed sheet  12  and the printed sheet  13  is equal to the rotation speed of the plate or plates  26  on the respective plate cylinder or cylinders  27  the adjustment signal  34  generated takes a null value. 
         [0049]    The adjusted new speed for the drive means  15  is such that the new speed of passage of the sheet  12  or  13  is equal to the tangential speed of the plate or plates  26  on its plate cylinder or cylinders  27 . 
         [0050]    In a third step of the second embodiment of the method the speed of the means for rotating the plate cylinder or cylinders  27  is adjusted as a function of the adjustment signal generated. 
         [0051]    If the real speed calculated for the non-printed sheet  12  and the printed sheet  13  is greater than the tangential speed of the plate cylinder or cylinders  27  the adjustment signal generated takes a value corresponding to an acceleration of the means for rotating the plate cylinder or cylinders  27 . If the real speed calculated for the non-printed sheet  12  and the printed sheet  13  is less than the tangential speed of the plate cylinder or cylinders  27  the adjustment signal generated takes a value corresponding to a deceleration of the means for rotating the plate cylinder or cylinders  27 . If the real speed calculated for the non-printed sheet  12  and the printed sheet  13  is equal to the rotation speed of the plate cylinder or cylinders  27  the adjustment signal generated takes a null value. 
         [0052]    The adjusted new tangential speed for the means for rotating the plate cylinder or cylinders  27  is such that the new tangential speed of the plate or plates  26  on its plate cylinder or cylinders  27  is equal to the speed of passage calculated for the non-printed sheet  12 . 
         [0053]    The present invention is not limited to the embodiments described and shown. Numerous modifications may be made without departing from the scope of the invention defined by the set of claims. Sensors may be installed after each printing unit  4 ,  6 ,  7  and  8  and their information enables adjustment of the speed at which the printed sheets  13  are transported.