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 flexographic print master on a drum by means of a fluid depositing printhead.

The invention reduces a problem that may result when a printhead unit is used that uses more than one nozzle row.

BACKGROUND OF THE INVENTION

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 print 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In the unpublished applications EP08172281.1 and EP08172280.3, both assigned to Agfa Graphics NV and having a priority date of 2008-12-19, a digital solution is presented for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 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 polymerisable 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.

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.

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 21 1 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 FIG. 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.

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 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 4 1 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.

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 6 1 , 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 64 .

The uneven spacing of the spiral paths 610 and 6 1 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.

The object of the current invention is 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.

SUMMARY OF THE INVENTION

The invention is realized by means of a system and a method as described in the claims.

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.

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 printhead as 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 axis of the printhead is 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 shifted with regard to each other.

FIG. 9 shows an embodiment according to the current invention in which the nozzle rows are shifted with regard to each other.

FIG. 10 shows a printhead unit that comprises not two but three printheads.

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.

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.

DESCRIPTION

Referring to FIG. 6, 600 is a rotating sleeve or support that has a diameter 601 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 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:

RevolutionPeriod = 1 / NumberofRevolutionsperSecond.

The circumferential speed of the sleeve has a value CircumferentialSpeed. It is equal to: CircumferentialSpeed =

SleeveCircumference * NumberofRevolutionsperSecond

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.

The distance between two neighboring nozzles in a single printhead is the nozzle pitch 631 and is represented by a variable P.

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.

The movement of the printhead is locked to the rotation of the sleeve by means of a mechanical coupling (for example by means of a worm and gear) or by means of 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:

PrintheadPitch =lntegerMultiplier * P / 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 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 a between the speed vector 672 and the first speed vector 670 is expressed by: tan(ct) = PrintheadSpeed / CircumferentialSpeed

a = a tan (PrintheadSpeed / CircumferentialSpeed)

The distance 660 between two nozzle rows 620 and 621 in FIG. 6 is represented by the variable D. Unlike in FIG. 5, the two spiral paths 610, 61 1 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.

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.

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.

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.

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:

A = P/2 - dY

E = P/2 + dY

A + E = 2 * P/2

The value of dY can be directly expressed as a function of the angle a and the nozzle row distance D: tan(a) = dY/D

dY = D * tan(a)

And hence: A = P/2 - D * tan( )

The above expression teaches that:

A = P/2 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 Pri nth ead Pitch 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 (tan(cc)) 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 equal to zero and hence to make:

A = E = P/2 without setting = 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.

In FIG. 8, the value of A is expressed as:

A = P/2 - dY + S

A = P/2 - D * tan(a) + S

If the following value for S is selected: S = D * tan(ot)

then it is obtained that:

A = E = P/2

In other words, by shifting one of the rows of nozzles over a distance S that is equal to D * tan (a), it is obtained that these interlaced paths are equidistant at a distance equal to P/2.

FIG. 9 gives a further illustration 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.

The above description provides an exemplary embodiment of the current invention on which a number of variations exist.

In the first place it is not always required that the shifting S of a nozzle row is exactly equal to D * tan(ot). 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(a) 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(a) in which r is a parameter that has a value of approximately one will already improve the evenness of the distances A and E

Preferably:

S = r* D * tan(ct)

in which:

0.1 < r < 1.9

Even more preferably:

0.5 < r < 1.05

Even more preferably:

0.9 < r < 1.1

And most preferably:

0.99 < r < 1.01

In the second place, the invention is 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).

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.

A more general 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.

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].

In a more general 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.

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.

In a perfectly equivalent 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.

In yet another equivalent 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.

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.

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.

FIG. 1 1 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 11 12 on which droplets are ejected by a second nozzle row 1022. This distance A[1][2] is equal to:

A[1][2] = P/3 - dY[1][2]

dY[1][2] = D[1][2]*tan(a)

A[1][2] = P/3 - D[1][2]*tan(a)

Similarly FIG. 1 demonstrates the effect of the distance D[1][3] on the distance A[1][3] in the Y dimension between a first spiral path 1 111 on which droplets are ejected by nozzles belonging to nozzle row 1021 and a third spiral path 11 13 on which droplets are ejected by nozzles belonging to a third nozzle row 1023. This distance A[1][3] is equal to:

A[1][3] = 2*P/3 - dY[1][3]

dY[1][3] = D[1][3]*tan(a)

A[1][3] = 2*P/3 - D[1][3]*tan(a)

In a general prior art 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: A[i][j] = m*P/N - D[i]*tan(<x)

in which m is an integer.

FIG. 12 shows how the current invention can be advantageously used for equalizing the distances between three different spiral paths.

In FIG. 12 the nozzle row 1022 is shifted over a distance P/3 + D[1][2]*tan(cc) in the Y dimension with regard to the nozzle row 1021. As a result, the distance between the spiral paths 11 1 and 1 112 is now equal to P/3.

Similarly, the nozzle row 1023 is shifted over a distance 2 ^* P/3 +

D[1][3]*tan(a) in the Y dimension with regard to the nozzle row 1021. As a result, the distance between the spiral paths 11 11 and 11 13 is now equal 2*P/3.

The effect 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.

In the general case of a printhead unit that includes N printheads, according to 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:

S = m*P/N + D[i][j]*tan(a)

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.

Whereas the invention has 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 liquid crystal devices or grating light valves.

A third example of an alternative recording system uses rows of digital mirror 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 mirror device marking system can be used to expose an offset print master precursor.

Using the embodiment shown in FIG. 1 and 2 that was earlier explained, the invention is advantageously used for creating a relief print master.

A relief print master can also be obtained for example by using one of the following embodiments.

In a first 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.

In a second embodiment, the imaging system according to 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.

In a third 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 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.