Abstract:
A relief print master is created with a printhead that jets droplets of a polymerizable liquid on a cylindrical sleeve. The droplets follow a spiral path on the cylindrical sleeve. In a multiple printhead unit, there are different spiral paths associated with the constituting printheads in the multiple printhead unit. The distance between the spiral paths is not even. Preferred embodiments of the current invention provide a method and a system so that the distance between these spiral paths becomes even. The system can also be used for the creation of other types of print masters, such as offset print plates.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a 371 National Stage Application of PCT/EP2011/054177, filed Mar. 21, 2011. This application claims the benefit of U.S. Provisional Application No. 61/318,812, filed Mar. 30, 2010, which is incorporated by reference herein in its entirety. In addition, this application claims the benefit of European Application No. 10158421.7, filed Mar. 30, 2010, which is also incorporated by reference herein in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention deals with the field of creating print masters, and more specifically with digital methods and systems for creating a flexographic print master on a drum with a fluid depositing printhead. 
         [0004]    2. Description of the Related Art 
         [0005]    The invention reduces a problem that may result when a printhead unit is used that uses more than one nozzle row. 
         [0006]    In flexographic printing or flexography a flexible cylindrical relief print master is used for transferring a fast drying ink from an anilox roller to a printable substrate. The print master can be a flexible plate that is mounted on a cylinder, or it can be a cylindrical sleeve. 
         [0007]    The raised portions of the relief print master define the image features that are to be printed. 
         [0008]    Because the flexographic print master has elastic properties, the process is particularly suitable for printing on a wide range of printable substrates including for example, corrugated fiberboard, plastic films, or even metal sheets. 
         [0009]    A traditional method for creating a print master uses a light sensitive polymerizable sheet that is exposed by a UV radiation source through a negative film or a negative mask layer (“LAMS”-system) that defines the image features. Under the influence of the UV radiation, the sheet will polymerize underneath the transparent portions of the film. The remaining portions are removed, and what remains is a positive relief print plate. 
         [0010]    In the published applications EP-A1-2199066 and EP-A1-2199065, both assigned to Agfa Graphics NV and having a priority date of 2008-12-19, a digital solution is presented for creating a relief print master using a fluid droplet depositing printhead. 
         [0011]    The published application EP-A1-2199065 teaches that a relief print master can be digitally represented by a stack of two-dimensional layers and discloses a method for calculating these two-dimensional layers. 
         [0012]    The published application EP-A1-2199066 teaches a method for spatially diffusing nozzle related artifacts in the three dimensions of the stack of two-dimensional layers. 
         [0013]    Both published applications also teach a composition of a fluid that can be used for printing a relief print master, and a method and apparatus for printing such a relief print master. 
         [0014]      FIG. 1  shows a preferred embodiment of such an apparatus  100 .  140  is a rotating drum that is driven by a motor  110 . A printhead  150  moves in a slow scan direction Y parallel with the axis of the drum at a linear velocity that is coupled to the rotational speed X of the drum. The printhead jets droplets of a polymerizable fluid onto a removable sleeve  130  that is mounted on the drum  140 . These droplets are gradually cured by a curing source  160  that moves along with the printhead and provides local curing. When the relief print master  130  has been printed, the curing source  170  provides an optional and final curing step that determines the final physical characteristics of the relief print master  120 . 
         [0015]    An example of a printhead is shown in  FIG. 3 . The printhead  300  has nozzles  310  that are arranged on a single axis  320  and that have a periodic nozzle pitch  330 . 
         [0016]      FIG. 2  demonstrates that, as the printhead moves from left to right in the direction Y, droplets  250  are jetted onto the sleeve  240 , whereby the “leading” part  211  of the printhead  210  prints droplets that belong to a lower layer  220 , whereas the “trailing” part  212  of the printhead  210  prints droplets of an upper layer  230 . 
         [0017]    Because in the apparatus in  FIGS. 1 and 2  the linear velocity of the printhead in the direction Y is directly coupled with the rotational speed X of the cylindrical sleeve  130 ,  240 , each nozzle of the printhead jets fluid along a spiral path on the rotating drum. This is illustrated in  FIG. 5 , where it is shown that fluid droplets ejected by nozzle  1  describe a spiral path  520  that has a pitch  510 . 
         [0018]    In  FIG. 5 , the pitch  510  of the spiral path  520  was selected to be exactly double the length of the nozzle pitch  530  of the printhead  540 . The effect of this is that all the droplets of nozzles  1 ,  3 ,  5  having an odd index number fall on the first spiral path  520 , whereas the droplets ejected by nozzles  2 ,  4 ,  6  having an even index number fall on the second spiral path  550 . Both spiral paths  520   550  are interlaced and spaced at an even distance  560  that corresponds with the nozzle pitch  530 . 
         [0019]    The lowest value of the nozzle pitch  330  in  FIG. 3  is constrained by technical limitations in the production of a printhead. One solution to overcome this constraint is to use a multiple printhead unit. 
         [0020]    The concept of a multiple printhead unit is explained by means of  FIG. 4 . As the figure shows, two printheads  401  and  402  are mounted back to back to form a multiple printhead unit  400 . By staggering the position of the nozzles  410  on the axis  420  of head  401  and the nozzles  411  on axis  421  of printhead  402  over a distance of half a nozzle pitch, the effective nozzle pitch  431  of the back to back head is half the nozzle pitch of each printhead  401 ,  402  and the effective printing resolution is doubled. 
         [0021]    The use of a multiple printhead unit in an apparatus as shown in  FIG. 1  or  FIG. 2  for the purpose of printing a relief print master introduces an unexpected and undesirable side effect. 
         [0022]      FIG. 6 . shows a first spiral path  610  on which fluid droplets from the nozzles having an odd index number  1 ,  3  and  5  land and a second spiral path  611  on which the fluid droplets of the nozzles having an even index number  2 ,  4  and  6  land. 
         [0023]    The nozzles with an odd index number are located on a first axis  620  and the nozzles having an even index number are located on a second axis  621 , parallel with the first axis  620 . 
         [0024]    Because these two axes  620  and  621  of the nozzle rows in the multiple printhead unit are not congruent, the spiral paths  610  and  611  are not evenly spaced with regard to each other. For example, in  FIG. 6  the distance  640  is different from the distance  641 . 
         [0025]    The uneven spacing of the spiral paths  610  and  611  causes an uneven distribution of the fluid droplets along the Y direction when they are jetted onto the sleeve and this negatively affects the quality of the print master that is printed. 
       SUMMARY OF THE INVENTION 
       [0026]    In view of the problems described above, preferred embodiments of the current invention to improve the evenness of the distribution of fluid droplets that are jetted onto a drum to create a relief print master using a back to back printhead unit or—more in general—a printhead unit that comprises multiple printheads. 
         [0027]    The preferred embodiments of the current invention are realized by a system and a method as described below. 
         [0028]    By slightly shifting the nozzle rows in a multiple printhead unit with regard to each other, the distance between the interlaced spiral paths can be adjusted so that they become evenly spaced. 
         [0029]    The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  shows a preferred embodiment of an apparatus for printing a relief print master on a sleeve. 
           [0031]      FIG. 2  shows a different view of a preferred embodiment of an apparatus for printing a relief print master on a sleeve. 
           [0032]      FIG. 3  shows a printhead with a single row of nozzles. 
           [0033]      FIG. 4  shows a multiple printhead unit with two rows of nozzles. 
           [0034]      FIG. 5  shows two spiral paths on which the fluid droplets ejected by the nozzles of a printhead as in  FIG. 3  land. 
           [0035]      FIG. 6  shows two spiral paths on which the fluid droplets land that are ejected by the nozzles of a printhead as in  FIG. 4 . 
           [0036]      FIG. 7  describes in detail the geometrical interactions between the movements of the printhead and the cylindrical sleeve, and the distance between the spiral paths when the axis of the printhead is parallel with the axis of the cylindrical sleeve. 
           [0037]      FIG. 8  describes in detail the geometrical interactions between the movements of the printhead and the cylindrical sleeve, and the distance between the spiral paths when the nozzle rows of the printhead are shifted with regard to each other. 
           [0038]      FIG. 9  shows a preferred embodiment according to the current invention in which the nozzle rows are shifted with regard to each other. 
           [0039]      FIG. 10  shows a printhead unit that comprises not two but three printheads. 
           [0040]      FIG. 11  describes in detail the geometrical interactions between the movements of the printhead and the cylindrical sleeve, and the distance between the spiral paths when the axis of the printhead is parallel with the axis of the cylindrical sleeve for the case that a printhead unit is used that comprises three printheads. 
           [0041]      FIG. 12  describes in detail the geometrical interactions between the movements of the printhead and the cylindrical sleeve, and the distance between the spiral paths when the nozzle rows of the printhead are shifted with regard to each other for the case that a printhead unit is used that comprises three printheads. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0042]    Referring to  FIG. 6 ,  600  is a rotating sleeve or support that has a diameter  601  represented by the variable SleeveDiameter. 
         [0043]    The circumference of the sleeve is represented by the variable SleeveCircumference and has a value equal to: 
         [0000]      SleeveCircumference= PI *SleeveDiameter 
         [0044]    The sleeve rotates in a X direction at a frequency that is represented by the variable NumberofRevolutionsperSecond. The time of one revolution is represented by the variable RevolutionPeriod. It is equal to: 
         [0000]      RevolutionPeriod=1/NumberofRevolutionsperSecond. 
         [0045]    The circumferential speed of the sleeve has a value CircumferentialSpeed. It is equal to: 
         [0000]      CircumferentialSpeed=SleeveCircumference*NumberofRevolutionsperSecond 
         [0046]    The direction and magnitude of the circumferential speed defines a first speed vector  670  that is tangential to the cylindrical sleeve and perpendicular to its axis. 
         [0047]    The distance between two neighboring nozzles in a single printhead is the nozzle pitch  631  and is represented by a variable P. 
         [0048]    In the multiple printhead unit as shown in  FIG. 6 , two printheads are positioned in such a way that the nozzles in the printheads are interlaced. In a prior art technique, the nozzles on a second row  621  of nozzles in a second printhead are shifted over a distance P/2 ( 630  in  FIG. 6 ) with regard to the nozzles on a first row  620  of nozzles in a first printhead. The resulting two printhead unit has a nozzle pitch  630  that is half the nozzle pitch  631  of the constituting printheads. This means that the resulting multiple printhead unit has an intrinsic resolution that is double of the resolution of the constituting printheads. 
         [0049]    The movement of the printhead is linked to the rotation of the sleeve by a mechanical coupling (for example by a worm and gear) or by an electronic gear (electronically coupled servomotors). During a single revolution of the sleeve, the printhead moves over a distance  650  that is represented by a variable PrintheadPitch. The value of this distance should be an integer multiple, represented by a variable IntegerMultiplier of the distance between two neighboring nozzles: 
         [0000]      PrintheadPitch=IntegerMultiplier* P/ 2 
         [0050]    The speed at which the printhead moves in the Y direction is represented by the variable PrintheadSpeed. Its value is equal to: 
         [0000]      PrintheadSpeed=PrintheadPitch/RevolutionPeriod 
         [0051]    The speed and magnitude of the printhead defines a second speed vector  671 . 
         [0052]    The sum of the two speed vectors  670  and  671  corresponds with the speed vector  672 . This speed vector is tangential to the spiral path on which liquid droplets are jetted. The angle α between the speed vector  672  and the first speed vector  670  is expressed by: 
         [0000]      tan(α)=PrintheadSpeed/CircumferentialSpeed
 
         [0000]      α= a  tan(PrintheadSpeed/CircumferentialSpeed)
 
         [0053]    The distance  660  between two nozzle rows  620  and  621  in  FIG. 6  is represented by the variable D. 
         [0054]    Unlike in  FIG. 5 , the two spiral paths  610 ,  611  in  FIG. 6  are not evenly spaced along the Y direction. More specifically, the distance  640  in  FIG. 6  is shorter than the distance  641 . This is a result of the distance  660  between the two nozzle rows  620 ,  621 . 
         [0055]      FIG. 7  shows a detail of  FIG. 6  that is used for geometrically describing the difference between the distance  640  and the distance  641  in  FIG. 6 . 
         [0056]    It is assumed that the length of the distance D is negligible with regard to the length of the Circumference. In that case the cylindrical surface of the sleeve can be approximated by a plane so that conventional (two-dimensional) trigonometry can be used to describe the geometrical relationships between the different variables. 
         [0057]    In  FIG. 7 :
       the distance P corresponds with the nozzle pitch  631  in  FIG. 6 ;   the distance D corresponds with the distance  660  between two nozzle rows in  FIG. 6 ;   the distance A corresponds with the distance  640  between two spiral paths in  FIG. 6 ;   the distance E corresponds with the distance  641  between two spiral paths in  FIG. 6 .       
 
         [0062]    The distance dY corresponds with the amount that the distance A is shorter than the distance P/2 (half the nozzle pitch), and the amount that the distance E is longer than the distance P/2. This is mathematically expressed as follows: 
         [0000]        A=P/ 2 −dY    
         [0000]        E=P/ 2 +dY    
         [0000]        A+E= 2 *P/ 2 
         [0063]    The value of dY can be directly expressed as a function of the angle α and the nozzle row distance D: 
         [0000]      tan(α)= dY/D  
 
         [0000]        dY=D *tan(α)
 
         [0000]      And hence: 
         [0000]        A=P/ 2 −D *tan(α)
 
         [0000]    The above expression teaches that: 
         [0000]        A=P/ 2 
         [0000]    when at least one of the following two conditions is met:
       1. D=0 (this is the situation that is shown in  FIG. 5 )   2. α=0 (this situation is only approximated when the PrintheadPitch is very small with respect to the CircumferentialSpeed, which is the case in many practical situations)       
 
         [0066]    The above expression also teaches that dY becomes larger when the distance D between the nozzle rows increases or when the ratio (tan(α)) of the PrintheadSpeed over the CircumferentialSpeed increases. 
         [0067]    We will now describe by means of  FIG. 8  that it is possible to reduce dY, or even to make equal to zero and hence to make: 
         [0000]        A=E=P/ 2 
         [0000]    without setting α=0 or setting D=0, but instead by shifting one of the nozzle rows in the multiple printhead unit with regard to the other nozzle row over a specific distance S. 
         [0068]    In  FIG. 8 , the value of A is expressed as: 
         [0000]        A=P/ 2 −dY+S    
         [0000]        A=P/ 2 −D *tan(α)+ S  
 
         [0069]    If the following value for S is selected: 
         [0000]        S=D *tan(α)
 
         [0000]    then it is obtained that: 
         [0000]        A=E=P/ 2 
         [0070]    In other words, by shifting one of the rows of nozzles over a distance S that is equal to D*tan(α), it is obtained that these interlaced paths are equidistant at a distance equal to P/2. 
         [0071]      FIG. 9  gives a further illustration of a preferred embodiment of the invention. By shifting the two rows of nozzles with regard to each other, it is possible to equalize the distance  910  between the spiral paths  950  and  951  and to make them equal to the P/2. 
         [0072]    The above description provides an exemplary preferred embodiment of the current invention on which a number of variations exist. 
         [0073]    In the first place it is not always required that the shifting S of a nozzle row is exactly equal to D*tan(α). It was already demonstrated by means of  FIG. 7  that if the distance D between the nozzle rows is small compared to the circumference of the cylindrical sleeve, that the deviation dY is small compared to the distance P of the nozzle pitch. In that case a shift S of the row of nozzles by an amount that is less than D*tan(α) provides already a sufficient improvement of the evenness of the distances A and E between the spiral paths. In general, a shift of r*D*tan(α) in which r is a parameter that has a value of approximately one will already improve the evenness of the distances A and E. 
         [0074]    Preferably: 
         [0000]        S=r*D *tan(α)
 
         [0000]      in which: 
         [0000]      0.1 ≦r≦ 1.9 
         [0000]    Even more preferably: 
         [0000]      0.5 ≦r≦ 1.05 
         [0000]    Even more preferably: 
         [0000]      0.9 ≦r≦ 1.1 
         [0000]    And most preferably: 
         [0000]      0.99 ≦r≦ 1.01 
         [0075]    In the second place, preferred embodiments of the invention are not limited to a combined head that uses only two rows of nozzles. The number of rows of nozzles can, in principle, be any integer number N (such as 2, 3, 4 or more). 
         [0076]    An example of a system that uses three rows of nozzles is shown in  FIG. 10 . A first printhead has a first row of nozzles  1021 , a second printhead has a second row of nozzles  1022  and a third printhead has a third row of nozzles  1023 . 
         [0077]    A more general preferred embodiment of a printhead unit has N nozzle rows having index numbers  1 ,  2 ,  3 ,  4  . . . N. The index numbers of the nozzle rows do not necessarily correspond with the order that the nozzle rows are physically mounted. 
         [0078]    The distance in the X dimension between the first nozzle row  1021  and the second nozzle row  1022  has a value D[1][2], whereas the distance in the X dimension between the first nozzle row  1021  and the third nozzle row  1023  is D[1][3]. 
         [0079]    In a more general preferred embodiment the distance between a first nozzle row having an index number i and a second nozzle row having an index number j is equal to D[i] [j] and can be obtained by subtracting the value of an X coordinate of the first nozzle row with index number i from the value of an X coordinate of the second nozzle row having index number j. 
         [0080]    Each individual printhead in  FIG. 10  has a pitch P. In a prior art system, the second row of nozzles  1022  is shifted over a distance P/3 in the Y dimension with regard to the first nozzle row  1021  and the third nozzle row  1023  is shifted over a distance 2*P/3 in the Y dimension with regard to the first nozzle row  1021 . 
         [0081]    In a perfectly equivalent preferred embodiment the second nozzle row  1022  is shifted over a distance 2*P/3 and the third nozzle row  1023  over a distance P/3 in the Y dimension with regard to the first nozzle row  1021 . 
         [0082]    In yet another equivalent preferred embodiment, a row of nozzles is shifted in the Y dimension over an additional distance that corresponds with an arbitrary multiple of the pitch P. For example: the second row of nozzles  1022  could be shifted additionally over a distance of 2*P so that the total shift becomes 2*P+2*P/3, and the third row of nozzles over an additional distance of 5*P so that the total shift becomes 5P+1*P/3. 
         [0083]    Of the essence is that shifting the nozzle rows  1021 ,  1022  and  1023  by a multiple of P/3 is done in a way that the resulting nozzle pitch of the printhead unit in the part where the nozzle rows of the constituting printheads overlap is P/3. 
         [0084]    In the case that a printhead unit comprises N printheads, the nozzle rows are shifted in the Y dimension with regard to a first nozzle row over distances m*P/N that are integer multiples of P/N so that the pitch of the resulting printhead unit becomes equal to P/N. 
         [0085]      FIG. 11  demonstrates the effect of the distance D[1][2] on the distance A[1][2] in the Y dimension between a first spiral path  1111  on which droplets are ejected by nozzle belonging to nozzle row  1021  and a second spiral path  1112  on which droplets are ejected by a second nozzle row  1022 . This distance A[1][2] is equal to: 
         [0000]        A[ 1][2 ]=P/ 3 −dY[ 1][2] 
         [0000]        dY[ 1][2 ]=D[ 1][2]*tan(α)
 
         [0000]        A[ 1][2 ]=P/ 3 −D[ 1][2]*tan(α)
 
         [0086]    Similarly  FIG. 11  demonstrates the effect of the distance D[1][3] on the distance A[1][3] in the Y dimension between a first spiral path  1111  on which droplets are ejected by nozzles belonging to nozzle row  1021  and a third spiral path  1113  on which droplets are ejected by nozzles belonging to a third nozzle row  1023 . This distance A[1][3] is equal to: 
         [0000]        A[ 1][3]=2 *P/ 3 −dY[ 1][3] 
         [0000]        dY[ 1][3 ]=D[ 1][3]*tan(α)
 
         [0000]        A[ 1][3]=2 *P/ 3 −D[ 1][3]*tan(α)
 
         [0087]    In a general prior art preferred embodiment with N printheads, a distance A[i][j] between a first spiral path on which droplets are ejected by nozzles belonging to a first nozzle row having an index number i and a second spiral path on which droplets are ejected by nozzles belonging to a nozzle row having an index number j, whereby D[i][j] refers to the distance in the X direction between the nozzle rows having index numbers i and j meets the equation: 
         [0000]        A[i][j]=m*P/N−D[i ]*tan(α)
 
         [0000]    in which m is an integer. 
         [0088]      FIG. 12  shows how a preferred embodiment of the current invention can be advantageously used for equalizing the distances between three different spiral paths. 
         [0089]    In  FIG. 12  the nozzle row  1022  is shifted over a distance P/3+D[1][2]*tan(α) in the Y dimension with regard to the nozzle row  1021 . As a result, the distance between the spiral paths  1111  and  1112  is now equal to P/3. 
         [0090]    Similarly, the nozzle row  1023  is shifted over a distance 2*P/3+D[1][3]*tan(α) in the Y dimension with regard to the nozzle row  1021 . As a result, the distance between the spiral paths  1111  and  1113  is now equal 2*P/3. 
         [0091]    The effect of the present preferred embodiment of the invention is that the distances between two neighboring spiral paths are always equal to P/3. In other words, the spiral paths are equally spaced with regard to each other in the Y dimension. 
         [0092]    In the general case of a printhead unit that includes N printheads, according to a preferred embodiment of the invention, a second nozzle row having an index number j is shifted with regard to a first nozzle row having an index number i in the Y dimension over a distance S that meets the following equation: 
         [0000]        S=m*P/N+D[i][j ]*tan(α)
 
         [0000]    whereby D[i] [j] refers to the distance between the first nozzle row having an index number i and the a second nozzle row having an index number j, and whereby m refers to an integer number. 
         [0093]    Whereas preferred embodiments of the invention have been described in the context of an apparatus for creating a flexographic print master using a printhead that comprises fluid ejecting nozzles, it can just as well be used for other external drum based recording systems that use parallel rows of marking elements. 
         [0094]    A first example of an alternative recording system is a laser imaging system that uses a laser head with rows of laser elements as marking elements. 
         [0095]    A second example of an alternative recording system uses a spatial light modulator with rows of light valves as marking elements. Examples of spatial light modulators are liquid crystal devices or grating light valves. 
         [0096]    A third example of an alternative recording system uses rows of digital mirror devices. 
         [0097]    All these systems can be used for creating a print master. For example, a laser based marking system, a light valve marking system or a digital mirror device marking system can be used to expose an offset print master precursor. 
         [0098]    Using the preferred embodiment shown in  FIGS. 1 and 2  that was earlier explained, the present preferred embodiment of the invention is advantageously used for creating a relief print master. 
         [0099]    A relief print master can also be obtained for example by using one of the following preferred embodiments. 
         [0100]    In a first preferred embodiment an imaging system according to the current invention is used for selectively exposing a mask layer that is on top of a flexible, photopolymerizable layer. The exposed areas of the mask layer harden out, constitute a mask and after UV flood exposure and processing define the features of the print master that are in relief. The unexposed areas are removed during processing and define the recessed portions of the relief print master. 
         [0101]    In a second preferred embodiment, the imaging system according to a preferred embodiment of the current invention selectively exposes a flexible, elastomeric layer, whereby material is directly removed from the flexible layer upon impingement, and the recessed portions of the relief print master are formed. In this case the unexposed areas of the flexible layer define the relief features of the print master. 
         [0102]    In a third preferred embodiment an imaging system according to a preferred embodiment of the current invention is used for selectively exposing a mask layer that is on top of a flexible, photopolymerizable layer. The exposed areas of the mask layer are partially removed as a result of ablation. As a result a mask is constituted and after UV flood exposure and processing the exposed areas are removed and define the recessed portions of the print master. The unexposed areas define the features of the print master that are in relief. 
         [0103]    While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.