Bow alignment procedure

Abstract

In a method for aligning a sheet (1) in a digital printing machine, the sheet (1) is pivoted by means of coaxially arranged and asynchronously driven alignment rollers (3,4) with a first movement (11) about a pivot axis (8) oriented in a z-direction (z), with a second movement (12) transported forward in an x-direction (x) and with a third movement (13) laterally displaced in a y-direction (y), so that as a result of a superposition of the three mentioned movements (11, 12, 13) of the sheet (1) the pivot axis (8) lies constantly in the center of gravity (9) of the sheet (1) during the entire sheet alignment.

Description

[0001] The present invention relates to a method for aligning a sheet in a digital printing machine, wherein the sheet is moved by means of coaxially arranged and asynchronously driven alignment rollers.

[0002] Such a method can be carried out with the device of EP 1 110 888 B1.

[0003] The object of the invention is to provide a further sheet alignment method.

[0004] The problem is solved by a method having the features of claim 1.

[0005] An advantage of the method according to the invention is that it can also be used to align sheets with a large format length without risk of damage.

[0006] Further developments arise from the following description of an embodiment and the associated drawing.

[0007] It shows: Fig. 1 the side view of a section of a digital printing machine and Fig. 2 the top view of the section from Fig. 1.

[0008] In a digital printing press, the respective sheet 1 is printed in a printing station 2, e.g., using an inkjet printer. To ensure correct registration, the diagonal register (sheet skew correction) is first adjusted using alignment rollers 3 and 4, and then the lateral register (print image lateral position correction) is adjusted using printing station 3.

[0009] During skew correction, the sheet 1 is between the alignment rollers 3, 4 and cooperating counter pressure rollers 5, which are Fig. 2 are not shown, and the alignment rollers 3, 4 are driven by different motors 6, 7 at different speeds. As a result, the sheet 1 is rotated with a first movement 11 about a pivot axis 8, which lies at a center of gravity 9 of the sheet 1. The pivot axis 8 is oriented in a z-direction (vertical). At the same time, the sheet 1 is transported by the alignment rollers 3, 4 with a second movement 12 in an x-direction x (horizontal) to the printing station 2 and moved transversely thereto with a third movement 13 in a y-direction y (horizontal).

[0010] The counter pressure rollers 5 are positioned against the alignment rollers 3, 4 with the sheet 1 clamped between them, as symbolized in the drawing with an arrow 14, and are counter pressure rollers 15, which are Fig.2 are not shown, are parked by drive rollers 16, as symbolized by arrows 17. The counterpressure rollers 15 and drive rollers 16 belong to upstream transport roller pairs.

[0011] When the sheet 1 is moved in the y-direction y, a drive 18 moves a carriage 19 in which the alignment rollers 3, 4 are rotatable and axially aligned with one another, in the y-direction y.

[0012] Line sensors 20 are provided for detecting a side edge 10 of the sheet 1. The line sensors 20 signal the sheet position to a computer-aided control system, and the control system then controls the motors 5, 6 and the drive 18.

[0013] The control of the motors 5, 6 and the drive 9 by the controller is carried out in such a way that this leads to an overlay of the first, second and third movements 11, 12, 13 such that from the beginning to the end of the alignment of the side edge 10 parallel with the x-direction x, the imaginary pivot axis 8 is always held in the central center of gravity 9 of the sheet 1. In this case, the sheet 1 and the pivot axis 8 "wander" from a starting position designated by the reference symbols "1a" or "8a" via an intermediate position designated "1b" or "8b" into the end position of the sheet skew correction designated "1c" or "8c".

[0014] One advantage is that the sheet 1 does not wrinkle, buckle, or become similarly deformed during its alignment movement because the forces and moments acting on the sheet 1 are minimized by keeping the pivot axis 8 at the center of gravity 9. A resulting additional advantage is that the pressure forces of the counterpressure rollers 5 can be reduced, thus reliably preventing marks on the sheet 1 caused by these rollers.

[0015] Furthermore, the alignment method described herein has the advantage that it is particularly suitable for printing long sheets 1 with a high ratio of side edge length to leading edge length—so-called banner printing. With such sheets 1, the risk of deformation would be high without the use of this method.

[0016] The sheet alignment according to the invention works reliably even at high printing speeds.

[0017] Since a movement component required for holding the pivot axis 8 at the center of gravity 9 is the third movement 13 of the sheet 1, this results in a lateral register error, the size of which is known to the above-mentioned control system through the sheet position measurement with the line sensors 20.

[0018] This lateral offset of sheet 1 relative to the print head(s) in print station 2 is compensated for by the controller controlling print station 2 during printing so that the print image is printed on sheet 1 offset by exactly the same lateral offset in the y-direction. If print station 2 has inkjet nozzles, these are controlled accordingly to compensate for the required lateral offset. List of reference symbols 1, 1a, 1b, 1c sheets 2 printing stations 3 Alignment roller 4 aligner roller 5 Counterpressure roller 6 Engine 7 Engine 8, 8a, 8b, 8c swivel axis 9 Focus 10 Side edge 11 Movement 12 Movement 13 Movement 14 Arrow 15 Counterpressure roller 16 drive roller 17 Arrow 18 Drive 19 sleds 20 line sensors x x-direction y y-direction z z-direction QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] EP 1 110 888 B1

[0002]

Claims

[1] Method for aligning a sheet (1) in a digital printing machine, wherein the sheet (1) is pivoted by means of coaxially arranged and asynchronously driven alignment rollers (3, 4) with a first movement (11) about a pivot axis (8) oriented in a z-direction (z), transported forwards in an x-direction (x) with a second movement (12) and displaced laterally in a y-direction (y) with a third movement (13), so that as a result of a superposition of the three aforementioned movements (11, 12, 13) of the sheet (1), the pivot axis (8) lies constantly in the center of gravity (9) of the sheet (1) during the entire sheet alignment. [2] Method according to claim 1, characterized bythat in a printing station (2) the sheet (1) is printed with a print image and in the process a lateral offset of the sheet (1) relative to the printing station (2) resulting from the third movement (13) of the sheet (1) in the y-direction (y) is compensated by printing the print image on the sheet (1) in register by the same lateral offset in the y-direction (y).

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