Abstract:
A relief print master is created by a printhead that jets droplets of a polymerisable 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 different constituting printheads. The distance between these spiral paths is not even in a prior art system. By rotating the printhead under a specific angle, the distance between these spiral paths becomes even. The invention can also be used for the creation of other types of print plates, such as for example offset print plates.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 371 National Stage Application of PCT/EP2011/063549, filed Aug. 5, 2011. This application claims the benefit of U.S. Provisional Application No. 61/375,248, filed Aug. 20, 2010, which is incorporated by reference herein in its entirety. In addition, this application claims the benefit of European Application No. 10173533.0, filed Aug. 20, 2010, which is also incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention deals with the field of creating print masters, and more specifically with digital methods and systems for creating a digital flexographic print master on a drum by a fluid depositing printhead. 
     The invention reduces a problem that may result when a printhead unit is used that comprises more than one nozzle row. 
     2. Description of the Related Art 
     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. 
     The raised portions of the relief print master define the image features that are to be printed. 
     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. 
     A traditional method for creating a print master uses a light sensitive polymerisable 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 printing plate. 
     In the unpublished applications EP08172281.1 and EP08172280.3, 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. 
     The application EP08172280.3 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. 
     The application EP08172281.1 teaches a method for spatially diffusing nozzle related artifacts in the three dimensions of the stack of two-dimensional layers. 
     Both 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. 
       FIG. 1  shows an embodiment of such an apparatus  100 .  140  is a rotating drum that is driven by a motor  110 . A printhead  160  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 polymerisable fluid onto a removable sleeve  130  that is mounted on the drum  140 . These droplets are gradually cured by a curing source  150  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 . 
     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 . The orifices of the nozzles are located in a nozzle plate that is substantially planar. 
       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 . 
     Because in the apparatus in  FIGS. 1 and 2  the linear velocity of the printhead in the direction Y is locked 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 . 
     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 . 
     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. 
     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 to form a multiple printhead unit  400 . The nozzle rows  420  and  421  are substantially parallel. 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 multiple printhead unit is half the nozzle pitch of each constituting printhead  401 ,  402  and the effective printing resolution is doubled. 
     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. 
       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. 
     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 . 
     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 . 
     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 
     In order to overcome the problems described above, preferred embodiments of the current invention improve the evenness of the distribution of the spiral paths on which the fluid droplets are jetted by a printhead unit that comprises multiple printheads. 
     Preferred embodiments of the current invention are realized by a system and a method as described below. 
     By rotating the multiple printhead unit in the plane that is perpendicular with the jetting direction of the nozzles, the unevenness of the distances between the interlaced spiral paths can be reduced or even eliminated. 
     Various preferred embodiments are also described below. 
     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 
         FIG. 1  shows an embodiment of an apparatus for printing a relief print master on a sleeve. 
         FIG. 2  shows a different view of an embodiment of an apparatus for printing a relief print master on a sleeve. 
         FIG. 3  shows a printhead with a single row of nozzles. 
         FIG. 4  shows a multiple printhead unit with two rows of nozzles. 
         FIG. 5  shows two spiral paths on which the fluid droplets ejected by the nozzles of a printhead as in  FIG. 3  land. 
         FIG. 6  shows two spiral paths on which the fluid droplets land that are ejected by the nozzles of a multiple printhead unit as the one shown in  FIG. 4 . 
         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 nozzle rows of the printhead are parallel with the axis of the cylindrical sleeve. 
         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 rotated in a plane that is orthogonal to the jetting direction of the nozzles. 
         FIG. 9  shows a preferred embodiment according to the current invention in which the nozzle rows are rotated so that the distances between the spiral paths on which the nozzles eject droplets becomes more even. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 6  a rotating sleeve  600  or support that has a diameter  601  is represented by the variable SleeveDiameter. 
     The circumference of the sleeve is represented by the variable SleeveCircumference and has a value equal to:
 
SleeveCircumference= PI *SleeveDiameter
 
     The sleeve rotates in the X direction at a frequency that is represented by the variable NumberofRevolutionsperSecond. The direction and magnitude of this rotation with regard to the printhead defines a first speed vector  670  that is tangential to the cylindrical sleeve and perpendicular to its central axis. 
     The time of one revolution is represented by the variable RevolutionPeriod. It is equal to:
 
RevolutionPeriod=1/NumberofRevolutionsperSecond.
 
     The circumferential speed of the sleeve has a value represented by the variable CircumferentialSpeed and is equal to:
 
CircumferentialSpeed=SleeveCircumference*NumberofRevolutionsperSecond
 
     The distance between two adjacent nozzles along the Y-dimension in the multiple printhead unit in  FIG. 6  is the nozzle pitch  630  and is represented by a variable P. 
     The movement of the printhead in the Y direction is locked to the rotation of the sleeve by a mechanical coupling (for example 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  650  should be an integer multiple of the nozzle pitch  630  and this multiple is represented by a variable IntegerMultiplier:
 
PrintheadPitch=IntegerMultiplier* P  
 
     In  FIG. 6  the value of IntegerMultiplier is equal to 2. 
     The speed at which the printhead moves in the Y direction is represented by the variable PrintheadSpeed. Its value is equal to:
 
PrintheadSpeed=PrintheadPitch/RevolutionPeriod
 
     The speed and magnitude of the printhead defines a second speed vector  671 . 
     The sum of the first speed vector  670  and the second speed vector  671  defines a third speed vector  672 . This speed vector  672  is tangential to the spiral path on which the liquid droplets are jetted. The angle α between the first speed vector  670  and the sum  672  of the first and second speed vectors is expressed by the following formulas:
 
tan(α)=PrintheadSpeed/CircumferentialSpeed
 
α= a  tan(PrintheadSpeed/CircumferentialSpeed)
 
     The distance  660  between the two nozzle rows  620  and  621  in  FIG. 6  is represented by the variable D. 
     Unlike in the case shown in  FIG. 5  where a printhead has only one row of nozzles, the two spiral paths  610 ,  611  in  FIG. 6  on which droplets land that are ejected from two different nozzle rows are not evenly spaced along the Y direction. More specifically, the distance  640  in  FIG. 6  is shorter than the distance  641 . This effect is the result of the distance D  660  between the two nozzle rows  620 ,  621 . 
       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 . 
     In the analysis that follows, 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 locally approximated by a plane so that conventional (two-dimensional) trigonometry can be used to describe the geometrical relationships between the different variables. 
     In  FIG. 7 :
         the distance P corresponds with the nozzle pitch  630  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 B corresponds with the distance  641  between two spiral paths in  FIG. 6 .       

     The distance dY corresponds with the amount that the distance A is shorter than the nozzle pitch P, and the amount that the distance B is longer than the distance P. This is mathematically expressed as follows:
 
 A=P−dY  
 
 B=P+dY  
 
 A+B= 2 *P  
 
     The value of dY can be directly expressed as a function the angle α and the nozzle row distance D:
 
tan(α)= dY/D  
 
 dY=D *tan(α)
 
And hence:
 
 A=P−D *tan(α)
 
     The above expression teaches that:
 
 A=P  
 
under the following two conditions:
         1. D=0 (this is essentially 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)       

     The above expression also teaches that dY becomes larger when the distance D between the nozzle rows increases or when the ratio of the PrintheadSpeed over the CircumferentialSpeed increases. 
     We will now describe by means of  FIG. 8  that it is possible to reduce dY, or even to make dY equal to zero and hence to make:
 
 A=B=P  
 
without setting α=0 or setting D=0, but instead by rotating the printhead in a plane that is orthogonal to the jetting direction of the nozzles and under a specific angle β. Such a plane is parallel with the
 
     In  FIG. 8 , the following expression is derived for dY:
 
tan(α−β)= dY/D  
 
 dY=D*a  tan(α−β)
 
By setting:
 
β=α
 
it is obtained that:
 
 A=P=B  
 
     In other words, by rotating the printhead over an angle β in a plane that is orthogonal to the jetting direction of the nozzles, whereby the angle β is equal to the angle α, it is obtained that these interlaced paths become equidistant and become spaced at a distance equal to the nozzle pitch. 
       FIG. 9  gives a further illustration of a preferred embodiment of the current invention. By rotating the printhead under an angle β in the plane defined by the two nozzle rows, whereby the angle β corresponds with the angle α, it is possible to equalize the distance  960  between the spiral paths  950  and  951  and to make them equal to the nozzle pitch  940 . 
     The above description provides an exemplary preferred embodiment of the current invention on which a number of variations exist. 
     In the first place it is not required that the value of IntegerMultiplier is equal to 2 as in  FIG. 5 ,  6  or  9 . In principle any integer number N can be used such as 2, 3, 4 or more. From the above explanation it should be clear to a person skilled in the art that a value of N for the variable IntegerMultiplier will also result in N interleaved spiral paths. 
     In the second place it is not always required that the angle α and angle β are exactly equal to each other. 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 rotation β of the printhead that is less than a provides already a sufficient improvement of the evenness of the distances A and B between the spiral paths. 
     Preferably:
 
|α−β|&lt;0.5*|α|
 
Even more preferably
 
|α−β|&lt;0.1*|α|
 
And even more preferably:
 
|α−β|&lt;0.01*|α|
 
     In the third place, preferred embodiments of the invention are not limited to a multiple printhead unit that comprises only two rows of nozzles. The number of rows of nozzles can, in principle, be any integer number M (such as 2, 3, 4 or more). In the case that more than two nozzle rows are present, the rotation of each one of the constituting printheads takes preferably place in a plane that is orthogonal to the direction in which the droplets are ejected by each printhead. 
     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. 
     A first example of an alternative recording system is a laser imaging system that uses a laserhead with rows of laser elements as marking elements. 
     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 digital micro mirror devices, grating light valves and liquid crystal devices. 
     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 micro mirror device marking system can be used to expose an offset print master precursor. 
     Preferred embodiments of the invention are advantageously used for creating a relief print master by building up the relief layer by layer using a system such as the one that is shown in  FIG. 1  or  FIG. 2 . A relief print master, however, can also be obtained for example using one of the following preferred embodiments. 
     In a first preferred embodiment an imaging system according to the current invention is used for imagewise exposing a mask so that that it comprises transparent and non-transparent portions. The mask is than put on top of a flexible, photopolymerizable layer and exposed by a curing source. The areas that exposed through transparent portions of the mask harden out and define the features of the print master that are in relief. The unexposed areas are removed and define the recessed portions of the relief print master. 
     In a second preferred embodiment, the imaging system according to a preferred embodiment of the current invention selectively exposes a flexible, elastomeric layer, whereby the energy of the exposure directly removes material from the flexible layer upon impingement. In this case the unexposed areas of the flexible layer define the relief features of the print master. 
     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.