Printing press operating procedures

Abstract

A method for operating a printing press comprises a portrait mode and a landscape mode. In both modes, sheets are printed in front and back and turned over in a perfecting unit (7) between print runs. Furthermore, in both modes, the position of a sheet edge relative to another sheet edge of the respective sheet is determined by means of at least one detection device (8), and the sheet is aligned for back printing based on this determination in an alignment unit (5). Front and back printing is carried out in a printing station (3), preferably by inkjet.

Description

[0001] The present invention relates to a method for operating a printing press in different modes.

[0002] Such methods are used for printing sheets with variable values ​​from print job to print job.

[0003] For example, US 8,308,158 B2 describes a method for operating a printing press in which, in a first mode ("single-sided printing"), a first sheet is printed on one side of the sheet, and in a second mode ("double-sided printing"), a second sheet is printed on both sides. This document also states that the invention described therein is applicable both when a sheet is transported with its long edge as the leading edge and when a sheet is transported with its short edge as the leading edge.

[0004] EP 2 064 140 B1, which describes a device for aligning sheets in a printing press, constitutes further prior art.

[0005] The object of the invention is to provide a further method for operating a printing press.

[0006] The problem is solved by a method for operating a printing press in different modes, characterized in that in a first mode a first sheet is transported in portrait format along a transport plane in a transport direction, a front side print is applied to the front of the first sheet, the first sheet is turned over in a turning device, and a back side print is applied to the reverse of the first sheet, wherein between the turning and the back side print the first sheet is aligned by means of a first pair of rollers and a second pair of rollers, wherein between the front side print and the alignment the position of a first sheet edge relative to a second sheet edge of the first sheet is determined by means of a pair of photoelectric sensors of a detection device, and wherein the first sheet edge and the second sheet edge of the first sheet each run transversely relative to the transport direction.that in a second mode, a second sheet is transported in landscape format along the transport plane in the transport direction, a front side print is applied to one side of the second sheet, the second sheet is turned over in the turning device, a reverse side print is applied to one side of the second sheet, wherein between the turning and the reverse side print, the second sheet is aligned by means of the first pair of rollers and the second pair of rollers, wherein between the front side print and the alignment, the position of a first edge of the second sheet relative to a second edge of the second sheet is determined by means of the reflective photoelectric sensor pair of the detection device, and wherein the first edge and the second edge of the second sheet each run transversely relative to the transport direction.

[0007] An advantage of the method according to the invention is that it also allows sheets which deviate considerably from the ideal rectangular shape within the limits of their manufacturing tolerances to be aligned in such a way that impairments to the subsequent processing of these sheets, such as angular errors between the printed image and the cut image, are avoided.

[0008] The following training courses are available individually or in any combination: When aligning the first arc depending on the result of determining the position of the first arc edge relative to the second arc edge of the second arc, a first motor and a second motor can be controlled by a controller, and the first motor can drive the first pair of rollers and the second motor can drive the second pair of rollers.

[0009] Between the printing on the front of the first sheet and the alignment of the first sheet, a further determination of the position of the first sheet edge relative to the second sheet edge of the first sheet can be carried out by means of another pair of reflective photoelectric sensors of another detection device.

[0010] A computer can calculate an average of the results of determining the position of the first arc edge relative to the second arc edge of the first arc using the pair of reflecting photoelectric sensors and the results of determining the position of the first arc edge relative to the second arc edge of the first arc using the other pair of reflecting photoelectric sensors.

[0011] When aligning the first arc depending on the mean value, a first motor and a second motor can be controlled by a controller, and the first motor can drive the first pair of rollers and the second motor can drive the second pair of rollers.

[0012] When aligning the second arc depending on the result of determining the position of the first arc edge relative to the second arc edge of the second arc, a first motor and a second motor can be controlled by a controller, and the first motor can drive the first pair of rollers and the second motor can drive the second pair of rollers.

[0013] Between the printing on the front of the second sheet and the alignment of the second sheet, a further determination of the position of the first sheet edge relative to the second sheet edge of the second sheet can be carried out by means of another pair of reflective photoelectric sensors of another detection device.

[0014] A computer can calculate an average of the results of determining the position of the first arc edge relative to the second arc edge of the second arc using the pair of reflecting photoelectric sensors and the results of determining the position of the first arc edge relative to the second arc edge of the second arc using the other pair of reflecting photoelectric sensors.

[0015] When aligning the second arc depending on the mean value, a first motor and a second motor can be controlled by a controller, and the first motor can drive the first pair of rollers and the second motor can drive the second pair of rollers.

[0016] The front side of the first sheet and the back side of the first sheet can each be printed using inkjet printing, and the front side of the second sheet and the back side of the second sheet can each be printed using inkjet printing.

[0017] The turning device can be a turning bag.

[0018] Further information can also be obtained from the following description of an exemplary embodiment and the associated drawing.

[0019] It shows: Fig. 1. A printing press in schematic representation, Fig. 2a a sheet alignment device of the printing press made of Fig. 1 in side view showing a first sheet position detection in a landscape format print mode, Fig. 2b one with Fig. 2a corresponding top view, Fig. 3 the bow alignment device from the Fig. 2a and Fig. 2b in top view showing a second sheet position detection in the landscape format print mode, Fig. 4 the bow alignment device from the Fig. 2a and Fig. 2b in top view showing an arc orientation in the landscape format print mode, Fig. 5 the sheet alignment device of the printing press Fig. 1 Top view showing sheet position detection in a portrait format print mode, Fig. 6 the sheet alignment device of the printing press Fig. 1 Top view showing an arc alignment in portrait format print mode, Fig. 7 a sheet measuring device of the printing press Fig. 1 Top view showing a first sheet position detection at a sheet leading edge in a landscape format reverse printing mode, Fig. 8 the arc surveying device from Fig. 7 showing a second sheet position detection at the sheet leading edge in the landscape format reverse printing mode, Fig. 9 the arc surveying device from Fig. 7 showing a first sheet position detection at the sheet trailing edge in the landscape format reverse printing mode, Fig. 10 the arc surveying device from Fig. 7 showing a second sheet position detection at the sheet trailing edge in the landscape format reverse printing mode, Fig. 11 the sheet alignment device of the printing press Fig. 1. Top view showing a sheet orientation in landscape format reverse printing mode, and Fig. 12 the sheet alignment device of the printing press Fig. 1 Top view showing a sheet orientation in a portrait-format reverse printing mode,

[0020] In Fig. Figure 1 shows a digital printing machine for printing sheets using a non-contact printing process (NIP - Non-Impact Printing), in particular inkjet. The printing machine comprises a sheet inlet 1, e.g., a sheet feeder, and a sheet outlet 2, e.g., a sheet delivery unit, which is indicated schematically in the drawing by arrows. The printing machine further comprises a printing station 3, in which several printheads and, preferably, one or more dryers for drying the printed images on the sheet are arranged; these are not shown in the drawing.

[0021] The sheets travel along a transport path 4 in the printing press, in which an alignment device 5 is located. The alignment device 5 aligns the sheets correctly for subsequent printing in the printing station 3. After printing one side of the respective sheet (front printing), it can be directed by a switching switch 6 either to the sheet exit 2 or to a turning device 7, in particular a turning pocket. In the latter case, the sheet is turned in the turning device 7 and then passes through the printing station 3 again, where the reverse side of the sheet is then printed (back printing).

[0022] Several detection devices 8 for detecting arc edges are preferably arranged along the transport path 4. The detection devices 8 are differentiated with respect to the number of rollers in Fig. Figure 1 is simplified. A first detection device 8 is arranged in a section of the transport path 4 located between the pressure station 3 and the switching switch 6. A second detection device 8 is arranged in a section of the transport path 4 located between the switching switch 6 and the reversing device 7. A third detection device 8 is arranged in a section of the transport path 4 located between the reversing device and the alignment device 5. All three detection devices 8 can be present, or any two of them. The detection devices 8 send signals to a computer 9, which belongs to or is connected to a controller 10, which controls a first motor 11 and a second motor 12 (see Figure 1). Fig. 2b) controls the alignment device 5. In the computer 9, an average is calculated from the values ​​measured by the detection devices 8. A cost-effective variant is also possible in which only one of the detection devices 8 is present and no average is calculated.

[0023] In the Fig. 2a and Fig. Figure 2b shows the alignment device 5 in more detail. A section of the transport path 4 forms a transport plane 13 in which the alignment device 5 is arranged. The sheets are transported one after the other in the transport direction 14 along the transport plane 13 in a transport gap between guide plates 15 by means of roller pairs 16. In the top view according to Fig. For the sake of clarity, the upper rollers of the roller pairs 16 and the upper plates of the guide plates 15 are not shown in 2b.

[0024] The alignment device 5 comprises a first pair of rollers 17 and a second pair of rollers 18. A drive roller 19 of the first pair of rollers 17 and a drive roller 20 of the second pair of rollers 18 are rotatably mounted coaxially in a carriage 21. The two drive rollers 19, 20 each operate in conjunction with a different counter-pressure roller 22. During alignment, the sheet 25 is clamped between the lower drive rollers 19, 20 and the two upper counter-pressure rollers 22. The drive roller 19 is driven rotaryally by the first motor 11, and the drive roller 20 by the second motor 12. Both motors 11, 12 are electric motors.

[0025] If the sheet 25 is tilted relative to the transport direction 14, a rotation of the sheet 25 about an imaginary vertical axis orthogonal to the transport plane 13 is required, a tilt correction of the sheet 25. During the tilt correction, the two drive rollers 19, 20 are rotated asynchronously with each other, at different speeds, in order to effect said rotation of the clamped sheet 25.

[0026] When the two drive rollers 19, 20 rotate synchronously, the sheet 25 is advanced in the transport direction 13 without sheet rotation, provided no tilt correction is required.

[0027] To generate an adjustment movement 23 (see Fig. 4) The carriage 21, together with the drive rollers 19, 20, is equipped with a drive 24. The adjustment movement 23 serves to laterally align the arc 25 and is parallel to the transport plane 13 and orthogonal to the transport direction 14.

[0028] The two counter-pressure rollers 22, of which in Fig. 2a The rear rollers, which are concealed by the front rollers, are rotatably mounted in a carrier 26 and can be positioned against and disengaged from the two drive rollers 19, 20 by a lifting movement 27 of the carrier 26. The carrier 26 is rotatably mounted, so that the lifting movement 27 is a pivoting movement. The periodic lifting movement 27, which occurs in the transport cycle of the sheets, is driven by a drive mechanism not shown in the drawing. This drive mechanism could be a pneumatic working cylinder, a linear actuator, a lifting magnet, or a motorized cam drive.

[0029] The alignment device 5 further comprises a first reflective photoelectric sensor 29 and a second reflective photoelectric sensor 30, which are embedded in the transport plane 13. The two reflective photoelectric sensors 29, 30 lie on a common alignment line that runs orthogonally to the transport direction 14. The two reflective photoelectric sensors 29, 30 are arranged between the two pairs of rollers 17, 18 and the printing station 3 and detect an edge of the sheet running transversely to the transport direction 14 when the respective sheet is clamped in the pairs of rollers 16, 17.

[0030] The alignment device 5 also comprises a first line sensor 31 and a second line sensor 32, which are preferably CCD line sensors. The two line sensors 31, 32 are embedded in the transport plane 13 and lie on a common alignment line that is parallel to the transport direction 14. The line of each line sensor 31, 32 runs in a direction orthogonal to the transport direction 14. The two line sensors 31, 32 detect, as shown in Fig. 3 symbolically indicated by means of a cross, a side edge of the sheet when the respective sheet is clamped in the roller pairs 16, 17.

[0031] The at least one detection device 8 comprises a pair of photoreceptors 33, which are identical in construction to the first photoreceptor 29 and the second photoreceptor 30. If, according to the functionally preferred variant, several detection devices 8 are present, then each detection device 8 comprises such a pair of photoreceptors 33, which is advantageous with regard to the aforementioned averaging.

[0032] The detection of the arc edge (front edge or rear edge) running transversely to the transport direction 14 by the reflecting light sensors 29, 30 of the alignment device 5 and by the reflecting light sensors of the reflecting light sensor pair 33 of the respective detection device 8 are symbolically indicated in the drawing by means of crosses.

[0033] When operating the depicted printing press in a first mode according to the Fig. 5, Fig. 6 and Fig. Twelve large sheets, whose longer of the two edge lengths is longer than the maximum format width that can be processed by the printing press, run through the printing press in portrait orientation. This allows for the largest possible printing area with compact machine dimensions.

[0034] When operating the printing press in a second mode according to the Fig. Steps 2a to 4 and 7 to 11 feed small sheets, whose longer of the two edge lengths is shorter than the maximum format width that can be processed by the printing press, through the printing press in landscape format. This increases the sheet throughput and thus the productivity of the machine.

[0035] The sheets are aligned along their long edge in both modes. Whether the sheets 25 of the respective print job run through the machine in portrait or landscape orientation has the following effects on the sheet alignment: For a print job with the first sheet 34 in portrait orientation, the alignment occurs along the side edge 28 of the first sheet 34, and for a print job with the second sheet 35 in landscape orientation, the alignment occurs along the leading edge 36 of the second sheet 35. This is explained in detail below with reference to the individual figures: The Fig. Figures 2a to 4 show the orientation of a second sheet 35 in the second mode for the front printing taking place in the printing station 3, on a front side 37 of the second sheet 35 which points upwards or towards the printheads during front printing.

[0036] In Fig. 2a and Fig. Figure 2b shows that the second arc 35 runs with its leading edge 36 over the first reflective light sensor 29, which is thereby activated and sends a first signal to the computer 9.

[0037] Fig. Figure 3 shows that the second arc 35, with its leading edge 36, passes over the second photoelectric sensor 30 shortly thereafter, which is thereby activated and sends a second signal to the computer 9. The computer 9 calculates the position and inclination of the leading edge 36 of the second arc 35 from the time difference between the first and second signals, i.e., the time offset between the triggering of the first photoelectric sensor 29 and the triggering of the second photoelectric sensor 30. Simultaneously with the triggering of the second photoelectric sensor 30, the two line sensors 31 and 32 each perform a measurement and signal the measured values ​​to the computer 9, which then calculates the position and inclination of the side edge 28 of the second arc 35.

[0038] In Fig. Figure 4 shows that the alignment device 5 has aligned the second sheet 35 into the correct sheet orientation for printing. To achieve this, the control unit 10, based on calculated information about the position and inclination of the leading edge 36 and the side edge 28, actuated the first motor 11 and the second motor 12 such that the drive rollers 19, 20 connected to the motors 11, 12 pivoted the second sheet 25 into a position in which the leading edge 36 is oriented orthogonally to the transport direction 14. Furthermore, based on information calculated by the computer 9, the control unit 10 actuated the drive 24 such that the drive 24 laterally aligned the carriage 21, including the drive rollers 19, 20 and the second sheet 25 clamped by the latter, with reference to a defined, single side edge point 38, by means of the adjustment movement 23.This side edge point 38 is the same point at which the measurement is taken by one of the line sensors 31, 32 - here the second line sensor 32, cf. . Fig. 3 - has been completed.

[0039] The Fig. 5 and Fig. Figure 6 shows the orientation of a first sheet 34 in the first mode for the front printing taking place in the printing station 3 towards a front side 39 of the first sheet 34 which points upwards or towards the print heads during front printing.

[0040] Fig. Figure 5 shows that in portrait format the measurement process with the reflective light sensors 29, 30 and the line sensors 31, 32 of the alignment device 5 is carried out in the same way as in landscape format.

[0041] Out of Fig. Figure 6 shows that the first sheet 34 is then aligned by means of the roller pairs 17, 18 so that its side edge 40 is parallel to the transport direction 14. The first sheet 34 is aligned at a defined, single leading edge point 41, namely at the leading edge point 41 where one of the two photoelectric sensors 29, 30 – here the second photoelectric sensor 30, cf. – is located. Fig. 5 - was triggered by a leading edge 42 of the first arc 34.

[0042] In the Fig. Figures 7 to 10 illustrate, using the second sheet 35 in landscape format as an example, the sheet measurement performed by one of the detection devices 8 for the sheet alignment performed by the alignment device 5 for the reverse side printing. Here, the position of a sheet trailing edge 43 – which can also be referred to as the first sheet edge 43 – is not measured absolutely, but relative to the position of a sheet leading edge 44 – which can also be referred to as the second sheet edge 44. This results in the advantage that, for all sheet formats that can be processed in the machine (minimum format, maximum format, and any intermediate formats), trailing edge measurement is possible without the use of comparatively expensive line sensors.

[0043] The detection device 8 has a first sensor 45 and a second sensor 46, which are designed as cost-effective reflective photoelectric sensors and together form the aforementioned reflective photoelectric sensor pair 33.

[0044] If the detection device 8 used for sheet measurement for the reverse side printing is one of the two detection devices 8 arranged in the transport path 4 upstream of the turning device 7, then the sheet leading edge 44 is the leading edge 36 that runs ahead during front printing. However, if the detection device 8 arranged downstream of the turning device 7 is used for sheet measurement and the second sheet 35 is turned over in the turning device 7, then the sheet leading edge 44 is the trailing edge of the second sheet 35 that runs behind during front printing.

[0045] Fig. Figure 7 shows that the first sensor 45 is covered by the arc leading edge 44, thereby triggering a signal, and Fig. Figure 8 shows that the second sensor 46 is subsequently triggered by the leading edge 44 of the arc, which is indicated graphically by cross symbols. Based on the signals from the two optical sensors 45 and 46, the computer 9 connected to them determines the position and inclination of the leading edge 44 of the arc.

[0046] Fig. Figure 9 shows that the first sensor 45 is released by the trailing edge 43 of the arc, thereby triggering a signal, and Fig. Figure 10 shows that the second sensor 46 is subsequently triggered by the trailing edge 43 of the arc.

[0047] From the values ​​available in the computer 9 for the position and inclination of the bow leading edge 44, for the position and inclination of the bow trailing edge 43 and for the transport speed of drive rollers 47 of the detection device 8, the computer 9 calculates the distance between the bow leading edge 44 and the bow trailing edge 43 and the angle between the bow leading edge 44 and the bow trailing edge 43.

[0048] The measured values ​​obtained during the arc measurement can deviate from the actual distance and angle dimensions due to various influencing factors, e.g., roundness or diameter deviations of the drive rollers 47. To compensate for these measurement errors, repeated arc measurement using several detection devices 8 with subsequent averaging in the computer 9 is advantageous.

[0049] The measurement of the first sheet 34 in portrait format for its reverse-print alignment is carried out in the first mode in the same way as previously described using the example of the measurement of the second sheet 35 in landscape format in the second mode and therefore does not need to be described separately.

[0050] In Fig. Figure 11 shows how the second sheet 35 in landscape format, after being measured by the detection device 8 and turned over by the turning device 7, is aligned by the alignment device 5 based on the measured values ​​obtained during the measurement for the reverse side printing, such that a current trailing edge 48 (corresponding to the former leading edge 36 in the front side printing) of the second sheet 35 is oriented orthogonally to the transport direction 14. At its side edge 28, the second sheet 35 is aligned at the same side edge point 38 as during its alignment for front side printing (see Figure 11). Fig. 4) aligned. Then, in printing station 3, the reverse printing takes place on a back side 52 of the second sheet 35, which points upwards after the sheet turning or during the reverse printing.

[0051] In Fig. Figure 12 shows how the first sheet 34 in portrait format, after being measured by the detection device 8 and turned over by the turning device 7, is aligned for the reverse printing by the alignment device 5 based on the measured values ​​obtained during the measurement, such that the side edge 40 of the first sheet 34 is oriented parallel to the transport direction 14 for the reverse printing. At its current trailing edge 49 (corresponding to the former leading edge 42 in the front printing stage), the first sheet 34 is aligned at the same edge point 50 (corresponding to the leading edge point 41 in the front printing stage) as during its alignment for the front printing stage (see Figure 12). Fig.6) aligned. Then, in printing station 3, the reverse printing takes place on a back side 51 of the first sheet 34, which points upwards after the sheet turning or during the reverse printing. Reference symbol list 1 curved inlet 2 curved exit 3 printing stations 4 Transport route 5 Alignment device 6 Switching points 7 Reversing device 8 Detection device 9 computers 10 Control 11 first engine 12 second engine 13 Transport level 14. Transport direction 15 Circuit board 16 pairs of rollers 17 first pair of rollers (of the alignment device 5) 18 second pair of rollers (of the alignment device 5) 19 Drive pulley (of the first pair of pulleys 17) 20 drive rollers (of the second pair of rollers 18) 21 sleds (of the alignment device 5) 22 Counter-pressure roller 23 Adjustment movement 24 drive 25 sheets 26 carriers 27 Lifting movement 28 side edge 29 first reflective light sensor (of the alignment device 5) 30 second reflective light sensor (of the alignment device 5) 31 first line sensor (of the alignment device 5) 32 second line sensor (of the alignment device 5) 33 pairs of reflective photoelectric sensors (of the detection device 8) 34 first sheet (portrait format sheet) 35 second sheets (landscape format sheets) 36 Front edge (for landscape format printing) 37 Front side (of the second sheet 35) 38 Side edge point 39 Front side (of the first sheet 34) 40 side edge 41 Leading edge point 42 Front edge (for portrait format printing) 43 First arc edge / arc trailing edge (during arc measurement) 44 second arc edge / arc leading edge (in arc measurement) 45 first sensor (of the reflective photoelectric sensor pair 33) 46 second sensor (of the reflective photoelectric sensor pair 33) 47 Drive roller (of the detection device 8) 48 Rear edge (for landscape format reprint) 49 Trailing edge (for portrait-format reverse printing) 50 edge point 51 Back

Claims

[1] Methods for operating a printing press in different modes, characterized by , that in a first mode - a first sheet (34) is transported in portrait format along a transport plane (13) in a transport direction (14), - a printing on a front side (39) of the first sheet (34) is carried out, - a turning of the first arc (34) takes place in a turning device (7), - a reverse print is made on the back side (51) of the first sheet (34), wherein between the turning and the counter-pressure an alignment of the first sheet (34) takes place by means of a first pair of rollers (17) and a second pair of rollers (18), wherein between the printing and the alignment, the position of a first sheet edge relative to a second sheet edge of the first sheet (34) is determined by means of a pair of reflecting photoelectric sensors (33) of a detection device (8), and wherein the first sheet edge and the second sheet edge of the first sheet (34) each run transversely relative to the transport direction (14), that in a second mode - a second sheet (35) in landscape format is transported along the transport plane (13) in the transport direction (14), - a printing on a front side (37) of the second sheet (35) is carried out, - a turning of the second arc (35) in the turning device (7) takes place, - a reverse print is made on the back side (52) of the second sheet (35), wherein between the turning and the back printing an alignment of the second sheet (35) takes place by means of the first pair of rollers (17) and the second pair of rollers (18), wherein between the printing and the alignment, the position of a first sheet edge (43) relative to a second sheet edge (44) of the second sheet (35) is determined by means of the reflective photoelectric sensor pair (33) of the detection device (8), and wherein the first arc edge (43) and the second arc edge (44) of the second arc (35) each run transversely relative to the transport direction (14). [2] Method according to claim 1, characterized by, that when aligning the first arc (34) depending on a result of determining the position of the first arc edge relative to the second arc edge of the first arc (43) by a control (10) a first motor (11) and a second motor (12) are controlled, the first motor (11) drives the first pair of rollers (17) and the second motor (12) drives the second pair of rollers (18). [3] Method according to claim 1, characterized by , that between the printing on the front side (39) of the first sheet (34) and the alignment of the first sheet (34) a further determination of the position of the first sheet edge relative to the second sheet edge of the first sheet (34) is carried out by means of a further pair of reflecting light sensors of a further detection device (8). [4] Method according to claim 3, characterized by, that a mean value is formed from a result of determining the position of the first arc edge relative to the second arc edge of the first arc (34) using the pair of reflecting photosensors (33) and a result of determining the position of the first arc edge relative to the second arc edge of the first arc (34) using the further pair of reflecting photosensors by a computer (9). [5] Method according to claim 4, characterized by , that when aligning the first arc (34) depending on the mean value by a control (10) a first motor (11) and a second motor (12) are controlled, the first motor (11) drives the first pair of rollers (17) and the second motor (12) drives the second pair of rollers (18). [6] Method according to claim 1, characterized by, that when aligning the second arc (35) depending on a result of determining the position of the first arc edge (43) relative to the second arc edge (44) of the second arc (35) by a control (10) a first motor (11) and a second motor (12) are controlled, the first motor (11) drives the first pair of rollers (17) and the second motor (12) drives the second pair of rollers (18). [7] Method according to claim 1, characterized by , that between the printing on the front side (37) of the second sheet (35) and the alignment of the second sheet (35) a further determination of the position of the first sheet edge (43) relative to the second sheet edge (44) of the second sheet (35) is carried out by means of a further pair of reflecting light sensors of a further detection device (8). [8] Method according to claim 7, characterized by, that a mean value is formed from a result of determining the position of the first arc edge (43) relative to the second arc edge (44) of the second arc (35) using the pair of reflecting photosensors (33) and a result of determining the position of the first arc edge (43) relative to the second arc edge (44) of the second arc (35) using the further pair of reflecting photosensors by a computer (9). [9] Method according to claim 8, characterized by , that when aligning the second arc (35) depending on the mean value by a control (10) a first motor (11) and a second motor (12) are controlled, the first motor (11) drives the first pair of rollers (179) and the second motor (12) drives the second pair of rollers (18). [10] Method according to any one of claims 1 to 9, characterized by , that the printing on the front (39) of the first sheet (34) and the printing on the back (51) of the first sheet (34) are each done by inkjet printing, and that the printing on the front (37) of the second sheet (35) and the printing on the back (52) of the second sheet (35) are each carried out by inkjet printing.

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