EP4554795A1

EP4554795A1 - A method for aligning a print pattern on a print medium and a printing device

Info

Publication number: EP4554795A1
Application number: EP23738017.5A
Authority: EP
Inventors: Davide ROSSELLO
Original assignee: Bobst Italia SpA
Current assignee: Bobst Italia SpA
Priority date: 2022-07-11
Filing date: 2023-07-04
Publication date: 2025-05-21
Anticipated expiration: 2043-07-04
Also published as: JP2025522151A; CN119562895A; KR20250027829A; TWI881379B; WO2024012923A1; EP4554795B1; TW202411080A

Abstract

A method for aligning a print pattern on a print medium (30) in a printing device (10) with at least a first printing unit (16) and a second printing unit (18), wherein each printing unit (16, 18) comprises a printing roller (24, 25) and wherein the second printing unit (18) comprises an optical detection unit (26), wherein the print 5 medium (30) comprises a first lateral edge region (35) and a second lateral edge region (37) opposed to the first lateral edge region (35) and travels along a longitudinal direction from the first printing unit (16) to the second printing unit (18). One method step comprises printing a first register mark (38) on the first lateral edge region (35) and a second register mark (40) on the second lateral edge region10 (37) being opposed to the first lateral edge region (35) of the print medium (30) by a first printing unit (16). A further method step comprises determining the horizontal and vertical position of the first register mark (38) at the first lateral edge region (35) using the detection unit (26) and adjusting a vertical and/or horizontal register based on the position of the first register mark (38). Another method step 15 comprises, after adjusting the vertical and/or horizontal register based on the position of the first register mark (38), printing a third register mark (46) on the first lateral edge region (35) and a fourth register mark (48) on the second lateral edge region (37) of the print medium (30) by the second printing unit (18). The maximum width of the second and fourth register marks (40, 48) on the second lateral edge 20 region (37) is smaller than the maximum width of the first and third register marks (38, 46) on the first lateral edge region (35). Moreover, a printing device (10) is provided.

Description

A method for aligning a print pattern on a print medium and a printing device

The invention refers to a method for aligning several print patterns on a print medium in a printing device and to a printing device with at least two printing units. In particular, the invention refers to a rotogravure press, a flexographic press, or an offset press.

Printing devices usually have several printing units for printing different colour planes. A print medium passes the different printing units subsequently.

The alignment of the colour planes has to be very accurate. In particular, a colour misalignment that exceeds 50 microns affects the print quality in a noticeable way. Thus, there is a need to control the register of the printing units with regard to horizontal and vertical alignment as well as with regard to skewing.

It is known to print register marks in the lateral edge regions of a printing medium which facilitate an alignment control. However, the area with the register marks does not contribute to the finalized print and has to be cut at the end of the printing process.

It is thus am object of the present invention to enable a sufficient alignment of a print pattern on a print medium in a printing device while reducing the generated waste.

This object is achieved by a method for aligning a print pattern on a print medium in a printing device with at least a first printing unit and a second printing unit, wherein each printing unit comprises a printing roller and wherein the second printing unit comprises an optical detection unit, wherein the print medium comprises a first lateral edge region and a second lateral edge region opposed to the first lateral edge region and travels along a longitudinal direction from the first printing unit to the second printing unit. One method step comprises printing a first register mark on the first lateral edge region and a second register mark on the second lateral edge with the first printing unit. Further method steps comprise determining the horizontal and vertical position of the first register mark at the first lateral edge region using the detection unit and adjusting a vertical and/or horizontal register based on the position of the first register mark. Another method step comprises after adjusting the vertical and/or horizontal register based on the position of the first register mark, printing a third register mark on the first lateral edge region and a fourth register mark on the second lateral edge region of the print medium by the second printing unit. The maximum width of the second and forth register marks on the second lateral edge region is smaller than the maximum width of the first and third register marks on the first lateral edge region.

Due to the width of register marks on the second lateral edge region being smaller than the width of the register marks on the first lateral edge region it is possible to gain space on the print medium and thus reduce waste. In particular, the width of the register marks on the second lateral edge region may me reduced because the horizontal register is adjusted based on the position of the first register mark printed by the first printing unit on the first lateral edge region such that the horizontal position of the register marks printed by the second printing unit is already well defined. Thus, it is made sure that the register mark printed by the second printing unit on the second lateral edge region crosses a sensitive area of the optical detection unit despite its reduced size. However, the inventive method still allows a highly accurate alignment.

The width of the register mark on the second lateral edge region is for example between one third and two third of the width of the register mark on the first lateral edge region.

The vertical register can be adjusted by temporarily varying the rotation speed of the printing roller of the second printing unit.

The horizontal register can be adjusted by shifting the printing roller of the second printing unit along its rotation axis, in particular with a dedicated shifting device.

According to one aspect, the register marks on the first lateral edge region are two-dimensional register marks and the register marks on the second lateral edge regions are one- or two-dimensional register marks. A two-dimensional register mark is a mark that allows to measure two positions, in particular a horizontal and vertical position of the mark using a single point sensor, for example an optical photocell. A one-dimensional register mark is a mark that allows to measure only one position, in particular a vertical position. For example, the register marks on the first lateral edge region have a horizontal edge and an edge tilted to the horizontal edge. The horizontal edge allows detecting the vertical position of the register mark while the tilted edge allows measuring the horizontal position. Due to the tilted edge, the detected length of the register mark is different depending on the horizontal position of the register mark.

For example, a trigger code is printed on the first lateral edge region by the first printing unit, wherein the first register mark printed by the first printing unit on the first lateral edge region is printed after the trigger code with respect to the travel direction of the print medium. The trigger code allows measuring the marks without knowing a priori where the marks are located longitudinally. In particular, the trigger code is used to set a zero position of the register system.

Preferably, a trigger code is printed on the second lateral edge region by the first printing unit, the second register mark printed by the first printing unit on the second lateral edge region being printed after the trigger code with respect to the travel direction of the print medium. In addition to the advantages already mentioned with respect to the trigger code on the first lateral edge region, the trigger code on the second lateral edge region allows placing the register mark on the second lateral edge region at an arbitrary vertical location compared to the register mark on the first lateral edge region. Thus, the printing pattern of the register marks is more flexible.

A vertical distance of the third reference mark printed by the second printing unit on the first lateral edge region to a reference point may be measured and a vertical distance of the fourth reference mark printed by the second printing unit on the second lateral edge region to a reference point is measured and based on the measured distances a skewing between the first and the second printing unit is detected. This allows to detect a skewing misalignment in a simple manner.

The reference point can be a trigger code, a register mark printed by the first printing unit or another mark with a defined distance to the third register mark respectively the fourth register mark.

According to one embodiment, a vertical distance between the register marks printed by the first and the second printing unit on the first lateral edge region is measured and a vertical distance between the register marks printed by the first and the second printing unit on the second lateral edge region is measured, and the vertical distances between the register marks may be compared to detect a skewing between the first and the second printing unit. In particular, skewing can be detected based on the comparison of the two measured vertical distances. If the two detected vertical distances are equal, no skewing is present, but if the two measured vertical distances differ from each other, the printing units have to be adjusted with respect to skewing.

For example, the degree of skewing between the first printing unit and the second printing unit is determined and the skewing is adjusted by pivoting a skewing roller located between the printing roller of the first printing unit and the printing roller of the second printing unit. Thereby, a skewing misalignment is corrected.

A horizontal position of the second register mark printed by the first printing unit on the second lateral edge region may be measured and the horizontal register may be again adjusted based on the position of the second register mark on the second lateral edge region. Thereby, the alignment of the colour planes may be even more accurate. In particular, the horizontal adjustment may by improved by averaging the measurement of the first and second register mark.

The object of the invention is further solved by a printing device configured to perform the inventive method and the skewing adjustment with at least two printing units being arranged subsequent to each other, wherein each printing unit comprises a printing roller and a skewing roller and the second printing unit being arranged subsequent to the first printing unit comprises an optical detection unit, the optical detection unit preferably comprising a first and a second photocell being arranged downstream of the printing roller such that the first photocell can detect the register marks in the first lateral edge region of the print medium and the second photocell can detect the register marks in the second lateral edge region of the print medium.

The second photocell preferably has a sensitive area smaller in its width than the first photocell. The second photocell may be of the same type and model of the first photocell but oriented differently. The rotation axis of the skewing rollers is for example moveable on one side and fixed on the opposing side. Thereby, a skewing parameter may be adjusted.

As an alternative implementation, the skewing rollers may be moveable on both sides.

Further features and advantages become apparent from the following description and from the enclosed Figures. In the Figures:

Figure 1 schematically shows an inventive printing device,

Figure 2 shows a printing unit of the printing device of Figure 1 ,

Figure 3 shows a printing pattern,

Figure 4 shows a register mark, and

Figure 5 shows a sensor path.

Figure 1 schematically shows a printing device 10.

The printing device comprises an unwinder 12, a preheating unit 14, a first printing unit 16 and a second printing unit 18 being arranged subsequent to the first printing unit 16. However, the printing device 10 may comprise more than two printing units.

For example, the printing device 10 is a heptachrome printing device with seven printing units.

Please note that the first printing unit 16 is not necessarily the first printing unit of the printing device. There might be a printing unit before the first printing unit 16 that does not require a precise register. Also, the second printing unit 18 may be any printing unit arranged subsequent to the first printing unit, but not necessarily contiguous to the first printing unit 16.

After each printing unit 16, 18 a dryer 20 is arranged. Each printing unit 16, 18 comprises a skewing roller 21 , 22 and a printing roller 24, 25.

The second printing unit 18, which is also visualized in Figure 2, furthermore comprises an optical detection unit 26 that is arranged downstream of the printing roller 25.

The detection unit 26 may comprise two photocells 28, 29.

The photocells 28, 29 are for example monochromatic photocells.

As an alternative, the optical detection unit 26 may be made of two cameras 28, 29.

A print medium 30 runs in the printing device 10 along a travel direction indicated by arrows 32.

In order to ensure a high printing quality, the printing units 16, 18 have to be aligned with a high accuracy in a horizontal and vertical direction as well as with respect to skewing.

The vertical direction corresponds to the travel direction of the print medium 30.

The horizontal direction is transverse to the travel direction of the print medium 30.

To adjust a horizontal misalignment of a printing unit 16, 18 the printing roller 24, 25 is moved along its rotation axis.

To adjust a vertical misalignment of a printing unit 16, 18, the angular position of the printing roller 24, 25 is adjusted by adjusting the phase of the printing roller before printing, i.e. before putting the printing roller 24, 25 in contact with the print medium 30. This can be done by temporarily varying the rotation speed of the printing roller 24.

Skewing is corrected by means of pivoting the skewing roller 21 , 22, for example by moving one side of the skewing roller 21 , 22 along a direction designated by arrows 23 in Figure 2 while keeping the other side fixed.

A method for detecting a misalignment of different colour planes is described with respect to Figures 3 to 5. A colour plane is the pattern printed by a single printing unit 16, 18.

At first, a trigger code 34 is printed on a first lateral edge 35 region of the print medium 30 by the first printing unit 16. In the Figures, the first lateral edge region 35 is on the left side of the print medium 30.

Also, a trigger code 36 is printed by the first printing unit 16 on a second lateral edge region 37 being opposed to the first lateral edge region 35 of the print medium 30, which is on the right side of the print medium 30 in the Figures.

The trigger codes 34, 36 are codes that have a distinct signature along a vertical direction and comprise marks having horizontal edges.

The trigger codes 34, 36 on the first and second lateral edge region 35, 37 are positioned with a vertical distance with respect to each other with respect to a travel direction 32 of the print medium 30.

In the Figures, marks printed by one printing unit are indicated by the same shading. In reality, the marks printed by one printing unit have the same colour.

The first printing unit 16 also prints a first register mark 38 on the first lateral edge region 35 and a second register mark 40 on the second lateral edge region 37.

The first and second register marks 38, 40 are printed subsequently to the trigger codes 34, 36. Thus, the register marks 38, 40 are positioned with a vertical distance with respect to the trigger codes 34, 36.

The distance between the first and second register marks 38, 40 and the respective trigger code 34, 36 is defined.

Figure 4 shows the first register mark 38 printed on the first lateral edge region 35 in an enlarged view.

The first register mark 38 has a horizontal edge 42 and an edge 44 tilted to the horizontal edge 42. In the depicted embodiment, the first register mark 38 is a right angled triangle.

Also, a sensor path of the respective photocell 28 is visualized in Figure 4 by means of a dashed line. The width of the first register mark 38 printed on the first lateral edge region 35 is such that it covers the maximum transversal error of the position of the print medium 30.

For example, the first register mark 38 has a width w of 6 mm.

In the embodiment, the second register mark 40 on the second lateral edge region 37 has the same shape as the first register mark 38 on the first lateral edge region 35.

Both of the first and second register marks 38, 40 are two-dimensional register marks.

However, the second register mark 40 on the second lateral edge region 35 may also be a one-dimensional register mark, for example a square or a rectangle.

When the print medium 30 travels along the travel direction and the section with the trigger codes 34, 36 enters the second printing unit 18, the trigger codes 34, 36 are read by the detection unit 26 while the second printing unit 18 is not printing, that means the printing roller 24 is not in contact with the print medium 30.

By reading the trigger codes 34, 36, an alignment process is started.

After reading the trigger code 34, the detection unit 26 detects the first register mark 38 on the first lateral edge region 35.

When the first register mark 38 travels along the respective photocell 28, the vertical position of the first register mark 38 can be determined by detecting the position of the horizontal edge 42.

The horizontal position of the first register mark 38 can be determined by detecting the length I of the first register mark 38, as indicated in Figure 4. The length I detected is specific to a horizontal position of the first register mark 38 due to the edge 44 being tilted to the horizontal edge 42. Preferably, the edge 44 is tilted to the horizontal edge 42 by an angle between 40° and 50°, in particular 45°. An angle in this range ensures a high precision.

After determining the horizontal and vertical position of the first register mark 38 on the first lateral edge region 35, a vertical and/or horizontal register is adjusted based on the position of the first register mark 38. After adjusting the vertical and/or horizontal register based on the position of the first register mark 38, a third register mark 46 is printed on the first lateral edge region 35 and a fourth register mark 48 is printed on the second lateral edge 37 region of the print medium 30 by the second printing unit 18.

The shape of the third and fourth register marks 46, 48 printed by the second printing unit 18 corresponds to the shape of the first and second register marks 38, 40 printed by the first printing unit 16. However, different shapes are also possible.

Due to the previous adjustment, the horizontal and vertical position of the third and fourth register marks 46, 48 printed by the second printing unit 18 is already well-defined.

Because of the well-defined position, the monochromatic photocells can determine which printing unit created a register mark based on its position.

In the next step, a vertical distance vi between the first register mark 38 and the third register mark 46 printed by the first and the second printing unit 16, 18 on the first lateral edge region 35 is measured and a vertical distance V2 between the second register mark 40 and the fourth register mark 48 printed by the first and the second printing unit 16, 18 on the second lateral edge region 37 is measured. The vertical distances vi and V2 between the register marks are compared to detect a skewing between the first and the second printing unit 16, 18.

In particular, if the distances vi and V2 differ from each other, skewing correction is necessary.

The degree of skewing between the first printing unit 16 and the second printing unit 18 is determined and the skewing is adjusted by pivoting the skewing roller 22 located between the printing roller 24 of the first printing 16 unit and a printing roller 25 of the second printing unit 18.

In an alternative embodiment, the vertical distance of the register marks 46, 48 can be measured with respect to the trigger codes 34, 36, respectively, or with respect to other reference points in order to detect skewing.

In an optional method step, a horizontal and/or vertical position of the second register mark 40 printed by the first printing unit 16 on the second lateral edge region 37 is measured and the vertical and/or horizontal register is again adjusted based on the position of the second register mark 40 on the second lateral edge region 37. Thereby, the alignment of the colour planes can be even more accurate.

The method steps being described with respect to the first printing unit 16 and the second printing unit 18 can be repeated for subsequent printing units of the printing device.

The maximum width of the second and fourth register marks 40, 48 on the second lateral edge region 37 is smaller than the maximum width of the first and third register marks 38, 46 on the first lateral edge region 35, in particular between one third and two third of the width of the first and third register marks 38, 46 on the first lateral edge region 35.

For example, the maximum width of the register marks 40, 48 on the second lateral edge region 37 is 4 mm when using the same photocells 28, 29 with the same orientation on both sides 35,37 of the medium. In another example, the maximum width of the register marks 40, 48 on the second lateral edge region 37 is 2 mm when using a photocell 29 with a narrower sensitive area along the transversal direction.

The smaller width is possible because after the first alignment, the maximum transversal error is reduced. Also, the vertical position, but not necessarily the horizontal position of the register marks 40, 48 on the second lateral edge region 37 is measured. This also contributes to a smaller possible width of the register marks 40, 48 on the second lateral edge region 37.

Thereby, the waste generated in the printing process is reduced.

In particular, the lateral edge regions 35, 37 with the register mark do not contribute to a finished print and are cut after the printing process.

Figure 5 visualizes a sensor path of the photocells 28, 29 of the detection unit 26, in particular a path of the sensitive area of the photocells 28, 29.

The sensitive area of the photocells 28, 29 is rectangular.

For example, the sensitive area of the photocells is 2 mm x 1 mm.

Figure 5 shows that the second photocell 29 has a smaller width than the first photocell 28. For example, the photocells 28, 29 are identical, but the second photocell 29 is turned with respect to the first photocell 28 around 90°. In particular, the photocell 28 is used in a horizontal orientation on the larger register marks 38, 46 on the first lateral edge region and the second photocell 29 is used in a vertical orientation on the smaller marks 40, 48 on the second lateral edge region.

In another example, the photocells 28, 29 are different, wherein the second photocell 29 has a smaller sensitive area compared to the first photocell 28.

Due to a transversal error of the position of the print medium 30, the width of the register marks 38, 46 one the first side 35 of the medium 30 added to the width of the sensitive area of the photocell 28 has to be at least slightly larger than the initial transversal position error tolerances, thereby ensuring that the mark crosses the sensitive area of the photocell 28. The width of the register marks 38, 46, is preferably at least as large as the width of the sensitive area of the photocell 28 to ensure that the register marks 38, 46 cross the sensitive area of photocell 28 completely. Thus, the width of register marks 38,46 are preferably slightly larger than the width of the sensitive area of photocell 28. The same holds for the second side 37 of the medium, i.e., for the register marks 40, 48, and photocell 29. But since the transversal error tolerances on the second side 37 are smaller, because applied after an initial transversal alignment based on the marks on the first side 35 of the medium, the size of the marks on the second side 37 of the medium can be made smaller.

Due to the horizontal position of the second and fourth register marks 40, 48 being already set according to the position of the first and third register marks 38, 46 on the first lateral edge region 35, the error in the horizontal position is in the range of 1 mm.

Claims

1. A method for aligning a print pattern on a print medium (30) in a printing device (10) with at least a first printing unit (16) and a second printing unit (18), wherein each printing unit (16, 18) comprises a printing roller (24, 25) and wherein the second printing unit (18) comprises an optical detection unit (26), wherein the print medium (30) comprises a first lateral edge region (35) and a second lateral edge region (37) opposed to the first lateral edge region (35) and travels along a longitudinal direction from the first printing unit (16) to the second printing unit (18), the method comprising printing a first register mark (38) on the first lateral edge region (35) and a second register mark (40) on the second lateral edge region (37) with the first printing unit (16), printing a third register mark (46) on the first lateral edge region (35) and a fourth register mark (48) on the second lateral edge region (37) of the print medium (30) by the second printing unit (18), determining the horizontal and vertical position of the first register mark (38) at the first lateral edge region (35) using the detection unit (26), determining the horizontal position of the third register mark (46) at the first lateral edge region (35) using the detection unit (26), adjusting a horizontal register of the second printing unit (18) based on the relative horizontal position of the first register mark (38) and the third register mark (46),

- wherein the maximum width of the second and forth register marks (40, 48) on the second lateral edge (37) region is smaller than the maximum width of the first and third register marks (38, 46) on the first lateral edge region.

2. The method according to claim 1 , wherein the method comprises determining the vertical position of the first register mark (38) at the first lateral edge region (35) using the detection unit (26), determining the vertical position of the third register mark (46) at the first lateral edge region (35) using the detection unit (26), adjusting a vertical register of the second printing unit (18) based on the relative vertical position of the first register mark (38) and the third register mark (46).

3. The method according to claim 1 or 2, wherein the register marks (38, 46) on the first lateral edge region (35) are two-dimensional register marks and the register marks (40, 48) on the second lateral edge regions (37) are one- or two- dimensional register marks.

4. The method according to any of the previous claims, wherein the register marks (38, 46) on the first lateral edge region (35) have a horizontal edge (42) and an edge (44) tilted to the horizontal edge (42).

5. The method according to any of the previous claims, wherein a trigger code (34) is printed on the first lateral edge region (35) by the first printing unit (16), wherein the first register mark (38) printed by the first printing unit (16) on the first lateral edge region (35) is printed after the trigger code (34) with respect to the travel direction of the print medium (30).

6. The method according to any of the previous claims, wherein a trigger code (36) is printed on the second lateral edge region (37) by the first printing unit (16), the second register mark (40) printed by the first printing unit (18) on the second lateral edge region (37) being printed after the trigger code (36) with respect to the travel direction of the print medium (30).

7. The method according to any of the previous claims, wherein a vertical distance of the third reference mark (46) printed by the second printing unit (18) on the first lateral edge region (35) to a reference point is measured and a vertical distance of the fourth reference mark (48) printed by the second printing unit (18) on the second lateral edge region (37) to a reference point is measured, and based on the measured distances a skewing between the first and the second printing unit (16, 18) is detected when said measured distances differ.

8. The method according to claim 7, wherein a vertical distance (vi) between the register marks (38, 46) printed by the first and the second printing unit (16, 18) on the first lateral edge region (35) is measured and a vertical distance (V2) between the register marks (40, 48) printed by the first and the second printing unit (16, 18) on the second lateral edge region (37) is measured, and wherein the vertical distances (vi , V2) between the register marks (38, 40, 46, 48) are compared to detect a skewing between the first and the second printing unit (16, 18).

9. The method according to claim 7 or 8, wherein the degree of skewing between the first printing unit (16) and the second printing unit (18) is determined and the skewing is adjusted by pivoting a skewing roller (22) located between the printing roller (24) of the first printing (16) unit and the printing roller (25) of the second printing unit (18).

10. The method according to any of the previous claims, wherein a horizontal position of the second register mark (40) and of the fourth register mark (48) on the second lateral edge region (37) is measured and the horizontal register is again adjusted based on the position of the second register mark (40) and of the fourth register mark (48) on the second lateral edge region (37).

11. A printing device (10) configured to perform the method according to any of the previous claims and in addition according to claim 8, with at least two printing units (16, 18) being arranged subsequent to each other, wherein each printing unit (16, 18) comprises a printing roller (24, 25) and a skewing roller (21 , 22) and the second printing unit (18) being arranged subsequent to the first printing unit (16) comprises an optical detection unit (26), the optical detection unit (26) comprising a first and a second photocell (28, 29) being arranged downstream of the printing roller (25) such that the first photocell (28) can detect the register marks (38, 46) in the first lateral edge region (35) of the print medium (30) and the second photocell (29) can detect the register marks (40, 48) in the second lateral edge region (37) of the print medium (30).

12. The printing device according to claim 11 , wherein the second photocell (29) has a smaller width than the first photocell (28).