Abstract:
A method for registering at least two colors for a machine for printing plate elements equipped with at least two printing units: printing marks of different colors in spaced pairs on edges of the plate elements; detecting position errors between different marks of possibly the same color and comparing the position with respective theoretical relative positions; adjusting operation of the printing units to have both marks of a color overlie.

Description:
[0001]    The present invention relates to a method for automatically registering colors in a machine for printing plate elements. The invention also relates to a color registration arrangement for a machine for printing plate elements. The invention further relates to a printing machine having a color registration arrangement. 
         [0002]    A printing machine is used in the packaging industry to print plate elements, such as sheets of paper or cardboard. The machine comprises a plurality of successive stations. A first station situated at the downstream end is an infeed station for inserting sheets successively from the bottom of a stack. The infeed station feeds a plurality of print stations in the form of a printing unit or a succession of printing units. Each printing unit prints one color. At the end of the machine there is provided a delivery station that collects the printed sheets. 
         [0003]    In the case of printing sheets of cardboard, the technology employed most frequently is flexographic printing. A printing unit includes a plate cylinder around which is wrapped and on which is tensioned a flexible printing plate. This printing plate prints the sheet after it has been coated with ink by means of a screened roll called an anilox roll and an inker device. The sheet passes between the plate cylinder and a pressure roller. The sheets are transported from one group to another from above by means of a train of rollers and a suction system, which leaves the lower surfaces of the sheets totally free. Given the position of the plate cylinder and the transport means, printing is effected from below the sheets. 
         [0004]    Each plate cylinder prints one or more patterns using a different color during each rotation. To obtain a finished image of good quality, it is necessary in particular that all the different color patterns are exactly overlapped. High quality registration is achieved by an operation for registering the colors, optimizing the adjustment of the relative position of the various printing plates on the plate cylinders. 
       PRIOR ART  
       [0005]    In existing printing machines, the register of these printings is checked by means of register marks printed by each printing roll in the area reserved for quality control signs, i.e. generally in the margin of the sheet. These marks enable determination of the register error of each color relative to a reference color. This operation, known as color registration, is carried out on each change of job. 
         [0006]    Numerous known reading systems and vision systems enable detection and reading of these marks printed on the sheets passing in front of a light source. To compensate shifting, the operator inputs a manual correction that is applied to the lateral and/or angular position of the plate cylinders of the corresponding printing units. 
         [0007]    A first phenomenon that has to be corrected is a longitudinal shifting of the printing plate on its plate cylinder. A second phenomenon that has to be corrected is a transverse shifting of the printing plate on its plate cylinder. A third phenomenon that has to be corrected is a skewing of the printing plate on its plate cylinder, which is characterized by an askew printing on the sheet. These phenomena may arise in the event of an error of the operator when mounting the printing plate, for example. They may equally occur in the case of an error in the design of the printing plate. 
         [0008]    However, such a registration phase necessitates the printing of successive sheets before starting production printing. Each sheet printed is analyzed by the operator, who then adjusts the colors relative to each other by repositioning the printing plate and adjusting the speed of the plate cylinder. On each change of job, this phase takes time during which the machine is not in production. This phase is costly in terms of the ink consumed and the number of sheets used and then rejected after examination by the operator. 
         [0009]    The document EP-1.693.199 describes a method of correcting a printing length comprising a step of printing an element with a plurality of printing units and a step of detecting register marks. The positions of the various register marks are determined relative to the edge of the element and the printing length is corrected automatically when the position of a mark deviates relative to the edge. 
         [0010]    However, the accuracy of registration correction with such a method depends on detecting the edge of the printed element. If the quality is not high, especially in terms of accurate cutting of the edge, then the correction is insufficient. Moreover, correction accuracy depends on the distance between the edge and the printed register mark. The mark is printed in an area of the element outside the main printed image and constituting waste. Nevertheless, this distance is reduced by the presence of the image situated primarily at the center of the element. With an area of the element outside the image having a small surface, the distance is small and consequently the accuracy is low. 
         [0011]    The document U.S. Pat. No. 6,553,906 describes a method of detecting a register deviation on a printed element. One step consists in detecting at least one printed registration mark that is associated with one color relative to at least one other printed registration mark that is associated with another color. The marks are detected with a densitometer by virtue of their superimposition. Register errors are detected if the marks are shifted relative to each other in only one direction. 
         [0012]    This type of detection method necessitates a specific detection device. Moreover, detecting different register errors necessitates printing the marks with a plurality of different orientations. Printing all these marks requires space in an area of the element outside the main printed image constituting waste. 
       STATEMENT OF THE INVENTION  
       [0013]    A principal objective of the present invention consists in developing a method enabling registration of a plurality of colors in a machine for printing plate elements. A second objective is to enable calibration of a printing machine by a simple and fast automatic method. A third objective is to reduce the time to adjust a printing machine and the number of plate elements printed for that adjustment. A fourth objective is to implement a method for registering colors thanks to a specific arrangement. A fifth objective is to avoid the drawbacks of the prior art methods and arrangements. A further objective is to succeed in adjusting a machine by simplifying the work of the operator before starting production printing. 
         [0014]    A method for registering at least two colors for a machine for printing plate elements equipped with at least two printing units, the method comprising the steps of: 
         [0015]    printing on a plate element a first mark of a first color using a first printing unit, 
         [0016]    printing on the plate element a second mark at a distance from the first mark of the first color using the first printing unit, 
         [0017]    printing on the plate element a third mark of a second color using a second printing unit, 
         [0018]    printing on the plate element a fourth mark at a distance from the third mark of the second color using the second printing unit, 
         [0019]    detecting on the printed plate element the first and third marks and the second and fourth marks. 
         [0020]    The method according to the invention is characterized in that it comprises the steps of: 
         [0021]    determining first position deviations between the first and third printed marks, 
         [0022]    comparing the first position deviations with a known position deviation between the first and third marks, 
         [0023]    determining second position deviations between the second and fourth printed marks, 
         [0024]    comparing the second position deviations with a known theoretical position deviation between the second and fourth marks, 
         [0025]    generating an adjustment signal for the second printing unit as a function of the first and second deviations so determined, 
         [0026]    adjusting the second printing unit as a function of the signal generated, 
         [0000]    so as to register the second color relative to the first color. 
         [0027]    Throughout the description, the plate or sheet element is defined, by way of non-comprehensive example, as being in a material such as paper, plate cardboard, corrugated cardboard, corrugated fiberboard, flexible plastic, for example polyethylene (PE), polyethylene terephthalate (PET), bi-oriented polypropylene (BOPP), or other polymers, or other materials having an aptitude for being printed. 
         [0028]    The longitudinal and lateral directions are defined with reference to the direction of movement of the plate element along a median longitudinal axis in the machine. The upstream and downstream directions are defined with reference to the direction of movement of the element in the longitudinal direction in the printing machine as a whole. The front, rear and lateral edges of the element are defined with reference to the direction of movement in the longitudinal direction in the printing machine as a whole. 
         [0029]    In other words, register errors are corrected by printing two series of marks of different colors at two different places spaced from each other on a single plate element and by providing a step of detecting and comparing these marks for deviations. In order to enable calculation of printed mark position deviations, theoretical predefined positions of the series of marks are known. 
         [0030]    The first mark and the second mark serve as reference marks for a single color. The deviations in both dimensions, i.e. the lateral, longitudinal and oblique deviations between the first and second marks of the first color and the third and fourth marks of the second color are calculated. This determines the registration deviations to be corrected by comparison with the theoretical deviations known in advance. Only two series of marks with each of the colors are necessary, taking up little room on the area of the element outside the principal printed image. 
         [0031]    The first and second printing units respectively printing the first and second colors are defined as being two different printing units chosen by the operator from those that are available in the printing machine. The same steps of this method are valid for all the other marks with all the other colors by comparison with the first and second marks of the first color. The method finds an application in flexographic or offset printing. 
         [0032]    In another aspect of the invention, an arrangement for registering at least two colors for a plate element printing machine is adapted to use the method. In another aspect of the invention, an arrangement registers at least two colors for a plate element printing machine equipped with at least two printing units. The arrangement comprises: 
         [0033]    at least one vision system installed downstream of the printing units and adapted to detect on a plate element a first mark printed with a first color, a third mark printed with a second color, a second mark printed with the first color, a fourth mark printed with the second color, and to generate a detection signal; and 
         [0034]    a calculation and control unit connected to the vision system and to the printing unit, receiving the signal from the vision system and adapted to determine position deviations between the first and third marks and between the second and fourth marks, to compare them to known theoretical deviations and to generate automatically an adjustment signal for the printing unit to register the second color relative to the first color. 
         [0035]    According to a further aspect of the invention, a printing machine comprising at least two printing units is characterized in that it comprises an arrangement for registering at least two colors having one or more of the technical features described hereinafter and claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    The invention will be better understood and its various advantageous and different features will emerge more clearly from the following description of a nonlimiting embodiment of the invention given with reference to the appended diagrammatic drawings, in which: 
           [0037]      FIG. 1  is a synoptic lateral view of a printing machine comprising a registration arrangement of the invention; 
           [0038]      FIG. 2  is a view of a printed plate element carrying printing marks in accordance with a first embodiment; 
           [0039]      FIG. 3  is a view of a printed plate element carrying printing marks in accordance with a second embodiment; and 
           [0040]      FIGS. 4 to 7  are partial views of a printed plate element carrying printing marks in accordance with a third embodiment. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0041]    As  FIG. 1  shows, a printing machine  1 , for example a flexo printing machine, comprises: 
         [0042]    a frame  2 , 
         [0043]    an infeed station  3 , 
         [0044]    five successive printing units  4 ,  6 ,  7 ,  8  and  9  disposed in line to print five different colors, for example black, blue, red, green and yellow, and 
         [0045]    a delivery station  11  (represented in dashed outline). 
         [0046]    The infeed station  3  receives a stack of plate elements, for example in the form of sheets  12  of corrugated board, free of any printing, and feeds them successively into the first printing unit  4 . The delivery station  11  recovers the printed sheets  13 . The sheets  12  and  13  circulate from one printing unit to another thanks to trains of upper drive rollers  14  and a series of aspiration means  16 . 
         [0047]    Each printing unit  4 ,  6 ,  7 ,  8  and  9  comprises an inking device  17  with a bottom ink tank  18 , a pump  19 , ink circulation means  21  and a chambered doctor blade  22  or an inking roller semi-submerged in an ink pan  23 . The inking device enables inking an anilox roll  24 . The anilox roll  24  coats with ink a printing plate  26  mounted on a plate cylinder  27 . 
         [0048]    The printing plate  26  is adjustable on the plate cylinder  27 , for example by means of a device such as that described in the document EP-0.972.639, which enables adjustment of position relative to the sheet  12 . The plate cylinder  27  is motorized, enabling adjustment of the angular position and/or speed of the printing plate  26  relative to the sheet  12 . 
         [0049]    In the method of the invention, the registration of the five colors may be adjusted firstly by moving the printing plate  26  to reposition it differently on the plate cylinder  27 . This registration of the five colors may equally be adjusted by angularly shifting the plate cylinder  27  to advance or retard it. 
         [0050]    The method comprises a plurality of successive steps, including steps of printing register marks on the printed sheets  13  and steps of detecting these marks. The marks are provided from the outset by being included on the printing plates  26  of all the printing units  4 ,  6 ,  7 ,  8  and  9 . The method is implemented by a color registration arrangement. 
         [0051]    The arrangement comprises a digital video camera type vision system  28  for detecting the register marks. The vision system  28  is installed at the exit from the final printing unit  9 , ahead of the delivery station  11 . 
         [0052]    In a first step, a first mark  29  is printed on the sheet  13  by the first printing unit  4 . This first mark  29  has a first color, in this case black. In a first preferred embodiment (see  FIG. 2 ), the first mark  29  is printed in the vicinity of a first edge, in this case the front edge  31  of the sheet  13 . This first color of the mark  29  serves as a reference. 
         [0053]    In a second step, a second mark  32  identical to the first mark  29  is printed on the sheet  13 , again by the first printing unit  4 . This second mark  32  is therefore black. The second mark  32  is at a distance from the first mark  29 . In this first embodiment, the second mark  32  is printed in the vicinity of a second edge, in this case the rear edge  33  of the sheet  13 . The black second mark  32  also serves as a reference. 
         [0054]    In the first embodiment ( FIG. 2 ) and in a second embodiment ( FIG. 3 ) the choice has been made to give the first mark  29  and the second mark  32  strictly identical shapes, in this case a calibrated grid shape. The grid is defined as a cross-ruled array with a plurality of boxes with predetermined dimensions. The registration of the second color (blue) and the subsequent other colors (red, green and yellow) is adjusted relative to the first color (black). The first mark  29  on the front edge  31  and the second mark  32  on the rear edge  33  are printed in addition to the black pattern of the finished image  34  nearer the middle of the sheet  13 . 
         [0055]    In a third step, a third mark  35  is printed on the sheet  13  by the second printing unit  6 . This third mark  35  has a second color, in this case blue. In this first embodiment, the third mark  35  is printed in the vicinity of the front edge  31 . The third mark  35  is printed in the vicinity of the first mark  29 . An existing theoretical relative position between the first mark  29  and the third mark  35  is known, being provided from the outset when producing the printing plate  26 . 
         [0056]    In a fourth step, a fourth mark  36  is printed on the sheet  13  by the second printing unit  6 . This fourth mark  36  is therefore blue. The fourth mark  36  is at a distance from the third mark  35 . In this first embodiment, the fourth mark  36  is printed in the vicinity of the rear edge  33 . The fourth mark  36  is printed in the vicinity of the second mark  32 . An existing theoretical relative position between the second mark  32  and the fourth mark  36  is known, being provided from the outset when producing the printing plate  26 . 
         [0057]    In the first embodiment ( FIG. 2 ) and in the second embodiment ( FIG. 3 ) the choice has been made to give the third mark  35  and the fourth mark  36  strictly identical shapes, in this case a square shape. When the first and second colors are in register, the square of the third mark  35  coincides perfectly with the grid of the first mark  29  and the square of the fourth mark  36  coincides perfectly with the grid of the second mark  32 , being disposed at the center of the corresponding box. The third mark  35  on the front edge  31  and the fourth mark  36  on the rear edge  33  are printed in addition to the blue pattern of the finished image  34  nearer the middle of the sheet  13 . 
         [0058]    In a fifth step, the vision system  28  detects the first mark  29  and the third mark  35  in the vicinity of the front edge  31  and the second mark  32  and the fourth mark  36  in the vicinity of the rear edge  33 . 
         [0059]    The marks  29 ,  32 ,  35  and  36  are printed in the vicinity of substantially the same longitudinal line, which in this case is a median longitudinal line L of the sheet  13 . Given that the marks  29 ,  32 ,  33  and  35  are printed at the front and rear edges  31  and  33  and centered relative to the sheet  13 , the vision system  28  is installed on the median longitudinal axis of the machine  1 . A single system  28  is needed for scanning a small area, which enables reduction of costs. 
         [0060]    In a sixth step, position deviations between the first mark  29  and the third mark  35  in the vicinity of the front edge  31  are determined. These position deviations are compared with the known theoretical relative position of the first mark  29  and the third mark  35 . Position deviations between the second mark  32  and the fourth mark  36  in the vicinity of the rear edge  33  are determined. These position deviations are compared with the known theoretical relative position of the second mark  32  and the fourth mark  36 . 
         [0061]    In a seventh step, an adjustment signal  37  for the second printing unit  6  is automatically generated as a function of the deviations determined and compared in the sixth step. 
         [0062]    In an eighth step, the second printing unit  6  is adjusted as a function of the generated adjustment signal  37 . The blue color printed by the second printing unit  6  is registered with the black color printed by the first printing unit  4 . 
         [0063]    The color registration arrangement comprises a calculation and control unit  38  implementing the sixth and seventh steps of the method. The unit  38  is electrically connected to the vision system  28  and to the printing units  4 ,  6 ,  7 ,  8  and  9 . The calculation and control unit  38  receives a detection signal  39  from the vision system  28  and consequently generates the adjustment signal  37  automatically. 
         [0064]    A first type of adjustment signal  37  comprises a signal for adjusting the position of the printing plate  26  on the plate cylinder  27  of the second printing unit  4 . A second type of adjustment signal  37  comprises a signal for adjusting the speed and the angular position of the plate cylinder  27  of the second printing unit  4 . 
         [0065]    These steps of the method for registering the first color with the second color are applied in an analogous manner to register the first color with the third color (red) printed by the third printing unit  7 , the fourth color (green) printed by the fourth printing unit  8  and the fifth color (yellow) printed by the fifth printing unit  9 . 
         [0066]    In the second embodiment (see  FIG. 3 ), the first mark  41  and the third mark  42  are printed in the vicinity of a first edge  43  that is one of the lateral edges of the sheet  13 . The second mark  44  and the fourth mark  46  are printed in the vicinity of a second edge  47  that is the other of the lateral edges of the sheet  13 . These marks  41 ,  42 ,  44  and  46  are also in the vicinity of the front edge  31  of the sheet  13 . In a manner analogous to that of the first embodiment ( FIG. 2 ), the choice has been made to give the first mark  29  and the second mark  32  strictly identical calibrated grid shapes. The choice has been made to give the third mark  35  and the fourth mark  36  strictly identical square shapes. 
         [0067]    Given that the marks  41 ,  42 ,  44  and  46  are printed at the lateral edges  43  and  47  of the sheet  13 , two vision systems  28  are installed one on each side of the passage of the sheets  13  at the lateral edges  43  and  47 . 
         [0068]    In a third embodiment (see  FIGS. 4 to 7 ), and in a manner analogous to that of the first embodiment, the first mark  29  is printed in the vicinity of the front edge  31  of the sheet  13 . This first color mark  29  serves as a reference. The second mark  32  is printed in the vicinity of the rear edge  33  of the sheet  13 . This second mark  32  also serves as a reference. The third mark  35  is printed in the vicinity of the front edge  31 . The third mark  35  is printed in the vicinity of the first mark  29 . The fourth mark  36  is printed in the vicinity of the rear edge  33 . The fourth mark  36  is printed in the vicinity of the second mark  32 . 
         [0069]    An existing theoretical relative position between the first mark  29  and the third mark  35  is known, being provided from the outset when producing the printing plate  26  (see the dashed lines in  FIGS. 4 to 7 ). An existing theoretical relative position between the second mark  32  and the fourth mark  36  is known, being provided from the outset when producing the printing plate  26  (see dashed lines in  FIGS. 4 to 7 ). 
         [0070]    In this third embodiment, the choice has been made to give all the marks  29 ,  32 ,  35  and  36  strictly identical shapes, in this case a round shape. The marks  29 ,  32 ,  35  and are printed in the vicinity of substantially the same longitudinal line, which in this case is the median longitudinal line L of the sheet  13 . The third mark  35  and the fourth mark  36  are on the median longitudinal line L. 
         [0071]    As may be seen in  FIG. 4 , the marks  29 ,  32 ,  35  and  36  are detected as being at their reference position. There is no lateral, longitudinal or oblique error. No registration correction is necessary. 
         [0072]    As may be seen in  FIG. 5 , there is no lateral error and no oblique error. A longitudinal error is detected, the third mark  35  and the fourth mark  36  both being shifted toward the rear edge  33  of the sheet  13 . The arrangement registers the second color by correcting the angular position of the plate cylinder  27 . 
         [0073]    As may be seen in  FIG. 6 , there is no longitudinal error and no oblique error. A lateral error is detected, the third mark  35  and the fourth mark  36  being both shifted toward one of the lateral edges of the sheet  13 . The arrangement registers the second color by correcting the lateral position of the printing plate  26  on its plate cylinder  27 . 
         [0074]    As may be seen in  FIG. 7 , there is no lateral error and no longitudinal error. An oblique error is detected, only the third mark  35  being shifted toward one of the lateral edges of the sheet  13 . The arrangement registers the second color by correcting the oblique position of the printing plate  26  on its plate cylinder  27 . 
         [0075]    All cases of combinations of lateral errors, longitudinal errors and oblique errors for all colors may be corrected using the method and the arrangement of the invention. 
         [0076]    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 claims.