PRINTING MECHANISM FOR A FLEXOGRAPHIC PRINTING PRESS AND METHOD FOR ITS OPERATION

Abstract

A printing mechanism (10) has a plate cylinder (12) that supports a printing plate (18). A printing plate reference field (100) has a lowest reference field surface (101) lower than the printing plate (18) and a highest reference field surface (105) higher the printing plate in the printing motif region. A control unit can vary a distance between an impression cylinder (28) and the plate cylinder (12) for pressing a printing substrate against the printing plate (18) and can very a distance between an inking roller (20) and the printing plate (18). A first sensor (34) connected to the control unit determines a quality of a printed image of the printing plate reference field (100) on the printing substrate (30) and a second sensor (36) connected to the control unit determines a quality of a negative image of the printing plate reference field (100) on the inking roller (20).

Claims

1. A printing mechanism (10) for a flexographic printing press, comprising a plate cylinder (12) which supports a printing plate (18) with a printing motif region and a printing plate reference field (100) having a plurality of reference field surfaces (101-105) of different heights, wherein at least one lowest reference field surface (101) has a lower height than the printing plate (18) in the printing motif region, and at least one highest reference field surface (105) has a greater height than that of the printing plate in the printing motif region, an impression cylinder (28), the distance from which to the plate cylinder (12) can be varied, controlled by a control unit, for the purpose of pressing a printing substrate against the printing plate (18), an inking roller (20), the distance from which to the printing plate (18) can be varied, controlled by the control unit, the surface of which can be wetted with ink from an attached ink reservoir (22) a first sensor (34) connected to the control unit for determining a quality of a printed image of the printing plate reference field (100) on the printing substrate (30) and a second sensor (36) connected to the control unit for determining a quality of a negative image of the printing plate reference field (100) on the inking roller (20).

2. The printing mechanism (10) of claim 1, wherein the printing plate (18) is fixed on a flexible printing plate support (16) designed as a continuous belt that is tensioned between the plate cylinder (12) and a tensioning cylinder (14) that can be displaced perpendicular to the plate cylinder (12).

3. The printing mechanism (10) of claim 1, wherein upon startup of the printing mechanism, the control unit is set to first vary a positioning pressure (26) of the inking roller (20) until the negative image of the printing plate reference field (100) attains a pre-set quality level, then varies a positioning pressure (32) of the impression cylinder (28) until the printed image of the printing plate reference field (100) attains a pre-set quality level.

4. The printing mechanism (10) of claim 1, wherein the control unit is set to monitor, by means of the first sensor (34), the quality of the printed image of the printing plate reference field (100) on the printing substrate (30) and, by means of the second sensor (36), to monitor the quality of the negative image of the printing plate reference field (100) on the inking roller (20), and if only the quality detected by means of the first sensor (34) deviates from a pre-set quality level, to vary only a positioning pressure (32) of the impression cylinder (28) until the pre-set quality level of the printed image of the printing plate reference field (100) is attained, but if the quality detected by means of the second sensor (36) deviates from a pre-set quality level, to vary first a positioning pressure (26) of the inking roller (20) until the pre-set quality level of the negative image of the printing plate reference field (100) is attained, and then to vary the positioning pressure of the impression cylinder until the pre-set quality level of the printed image of the printing plate reference field (100) is achieved

5. The printing mechanism (10) of claim 1, wherein the first sensor (34) is synchronized with a transport movement of the printing substrate (30).

6. The printing mechanism (10) of claim 5, wherein the first sensor (34) is synchronized with a transport movement of the printing plate (18).

7. The printing mechanism (10) of claim 1, wherein the second sensor (36) is synchronized with a rotational movement of the inking roller (20).

8. A method for actuating the printing mechanism (10) of claim 1, comprising monitoring the quality of the printed image of the printing plate reference field (100) on the printing substrate (30) by the first sensor (34) and monitoring the quality of the negative image of the printing plate reference field (100) on the inking roller (20) by the second sensor (36), whereby if only the quality detected by means of the first sensor (34) deviates from a pre-set quality level, only a positioning pressure (32) of the impression cylinder (28) is varied until the pre-set quality level of the printed image of the printing plate reference field (100) is attained, and if the quality detected by means of the second sensor (36) deviates from a pre-set quality level, first a positioning pressure (26) of the inking roller (20) is varied until the pre-set quality level of the negative image of the printing plate reference field (100) is attained, and then the positioning pressure (32) of the impression cylinder (28) is varied until the pre-set quality level of the printed image of the printing plate reference field (100) is attained.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIG. 1 a highly schematic depiction of a flexographic printing press in cross-section.

[0038] FIG. 2 an exemplary depiction of a printing plate reference field.

[0039] FIG. 3 images of the printing plate reference field from FIG. 2 resulting from different positioning pressures, namely the printed image on the printing substrate (S1) and its negative image on the inking roller (S2).

DETAILED DESCRIPTION

[0040] Identical reference numbers in the Figures refer to identical or analogous elements.

[0041] FIG. 1 is a highly schematic depiction of a cross-section through a printing mechanism 10 of a flexographic printing machine employing belt technology. A central element of the printing mechanism 10 is the plate cylinder 12. It is arranged essentially parallel to a tensioning cylinder 14 at a distance from it. A flexible printing plate support 16 is slung over both. The printing plate support 16 is designed as a continuous belt and bears the printing plate, which is a relief made of elastic material fixed on the printing plate support 16. The tensioning cylinder 14 is displaceable in the direction of its perpendicular distance to the plate cylinder 12 in order to tension the printing plate support 16.

[0042] To the left of the plate cylinder in FIG. 1, a rotating inking roller 20, which is connected to an ink reservoir 22, is arranged in essentially parallel orientation to the plate cylinder 12. When the inking roller 20 rotates, its surface is wetted with the ink. The thus inked inking roller 20 is displaced against the plate cylinder 12 with an adjustable positioning pressure 26 and thereby pressed against the printing plate 18 that runs between the plate cylinder 12 and inking roller 20. In the process, ink is transferred from the surface of the inking roller 20 to the printing plate 18.

[0043] Above the plate cylinder 12 in FIG. 1, an impression cylinder 28 is arranged in essentially parallel orientation to the plate cylinder 12. A roll-shaped printing substrate 30, e.g., a paper roll 30, is slung around it, whereby the printing substrate 30 passes through the nip between the plate cylinder 12 and the impression cylinder 28. To press the printing substrate against the printing plate 18 that runs around the plate cylinder 12, the plate cylinder 12 is displaceable against the impression cylinder 28 with an adjustable positioning pressure 32. The printing substrate 30 is thereby pressed against the inked printing plate 18, which results in ink being transferred from the printing plate 18 to the printing substrate 30, i.e., the actual printing process.

[0044] Within the scope of the invention, it is essential that the printing plate 18 have a printing plate reference field 100, an embodiment of which is schematically shown in FIG. 2 as an example. The printing plate reference field 100 has a plurality of surfaces 101-105 of various heights, which are preferably connected to one another. With regard to the shown embodiment, the reference field surfaces 101-105 have different shapes in order to better differentiate them in a top-down view. In the example shown, the reference field surface 101 has the lowest height and is in the shape of a square. The reference field surface 102 has the second-lowest height and is shaped as a circle inscribed within the square. The next-highest reference field surface 103 is shaped as a triangle inscribed within the circle. The second-highest reference field surface 104 has the shape of an oval inscribed within the triangle. The reference field surface 105, with the greatest height, has the shape of a rectangle inscribed within the oval. The heights of the reference field surfaces 101-105 are chosen such that at least the lowest reference field surface 101 has a lower height than the printing plate 18 in the region of the actual printing motif and that at least the highest reference field surface has a greater height than the printing plate in the region of the actual printing motif. During the printing process described above, the reference field 100 as well as the printing motif region of the printing plate 18 are inked by the inking roller and leave a printed image on the printing substrate 30.

[0045] As shown in FIG. 1, a first sensor 34 is arranged on the printing substrate 30, the signal from which sensor is synchronized with the transport speed of the printing substrate 30, the sensor being set to detect the printed image of the printing plate reference field on the printing substrate 30. Furthermore, a second sensor 36 is arranged on the surface of the inking roller 20, the signal from which sensor is preferably synchronized with the rotation movement of the inking roller 20, the sensor being set to detect the negative image which is left by the printing plate reference field 100, when rolling past the inking roller 20, in the ink film on the latter's surface.

[0046] FIG. 3 is a schematic depiction of possible images to be detected by the first sensor 34 (line S1) and the second sensor 36 (line S2). In reference to FIG. 3, a possible, automated process for adjusting the positioning pressures of the inking roller and impression cylinder 28 is described. For the purposes of simplicity, it is assumed in this explanation that the sensors 34 and 36 are imaging sensors and an evaluation of image data is performed within the scope of actuation. One skilled in the art will recognize, however, that line sensors or sensors without spatial resolution, such as simple photo diodes, can also be used to detect a pre-set quality of the images.

[0047] Line S2 in FIG. 3 shows different shapes of the negative images that can be left by the printing plate reference field 100 in the ink film on the surface of the inking roller 20, as a direct function of the positioning pressure 26. In FIG. 3, the positioning pressure decreases from left to right. At high positioning pressure, the entire printing plate reference field 100, up to and including its lowest reference field 101, is dipped into the ink film such that the outer outline of the negative image is square-shapedwhich corresponds to the shape of the reference field surface 101. The outlines of the remaining reference field surfaces are shown as dotted in line 2 of FIG. 3 since, depending on the thickness and viscosity of the ink film as well as the quality of image detection, they can remain detectable by the second sensor 36. With a slightly reduced positioning pressure 26 of the inking roller 20, the printing plate reference field 100 is only dipped up to and including its circular surface 102 and accordingly leaves a negative image with a circular outline. At a still further reduced positioning pressure 26 of the inking roller 20, the printing plate reference field 100 is only dipped into the ink film up to and including its triangular reference field surface 103 and leaves a negative image with a triangular outline. The situation is analogous at further reduced positioning pressure 26, whereby negative images with oval or rectangular outlines are produced, and are detected by the second sensor 36. For the purpose of the exemplary example being explained, it is assumed that the positioning pressure of the inking roller 20 required to produce an optimal printed image of the actual printing motif is that pressure at which the printing plate reference field is dipped into the ink film up to and including its triangular reference field surface 103. The corresponding negative image is therefore shown in bold in line S2 of FIG. 3. This adjustment value of the positioning pressure 25 can easily be automatically identified and applied by a control unit through evaluation of the sensor signal from the second sensor 36. To do so, the control unit varies the positioning pressure 26, especially by horizontally displacing the inking roller 20, according to pre-determined rules, until the negative image shown in bold in line 2 of FIG. 3 is produced.

[0048] In a next step, the optimal positioning pressure 32 of the impression cylinder 28 can then be sought and adjusted, in particular through vertical displacement of the plate cylinder 12. The optimal positioning pressure 32 is given when precisely the inked regions of the printing plate 18 also leave a printed image on the printing substrate 30. Higher positioning pressure results in excessive deformation of the elastic printing plate relief; lower positioning pressure results in incomplete ink transfer onto the printing substrate. The latter would be the case in the example being explained if the inked reference field surface 103 did not leave a printed image on the printing substrate 30, but rather only one or both of the higher reference field surfaces 104, 105 were to do so. These possibilities are shown in line 1 of FIG. 3, whereby this depiction is a schematic reproduction of the printed image detected by means of the first sensor 34. The printed image that belongs to the correct positioning pressure 32 is shown in bold in line S1 of FIG. 3. Automated variation of the positioning pressure 32 of the impression cylinder 28 until this printed image results can be easily implemented by one skilled in the art by referring to the technical teaching explained here.

[0049] However, one skilled in the art will realize that the same printed image would also be produced at an excessively high positioning pressure 32, since in that case one or both of the lower-lying reference field surfaces 101, 102 would be pressed against the printing substrate 30; yet without inking of these surfaces 101, 102, no ink transfer would be possible. For implementation of an automated positioning pressure adjustment functionality it is therefore expedient first to set a positioning pressure 32 that is too low, that will only result in printing of a reference field surface 104, 105 that is higher than the lowest inked reference field surface 103, and then to increase the positioning pressure 32 until the printed image on the printing substrate 30 corresponds to the lowest inked reference field surface 103. In the example explained above, this would mean that the positioning pressure 32 is initially set such that a printed image with a rectangular or oval outline is shown. Afterwards, the positioning pressure 32 is increased sufficiently until a printed image with a triangular outline is produced.

[0050] Of course, other strategies are also conceivable with regard to implementing automated positioning pressure adjustment. For example, a positioning pressure 26 of the inking roller 20 could intentionally be first set too high, in order to find the optimum positioning pressure 32 of the impression cylinder 28.

[0051] Of course, the embodiments discussed in the special description and shown in the figures are only illustrative exemplary embodiments of the present invention. This disclosure gives one skilled in the art a broad spectrum of possible variations. In particular, the shape and complexity of the printing plate reference field 100 could be varied across a large scope. The specific sensor technology chosen for the first and second sensor 34, 36 is also only limited in terms of optical sensitivity; however, it is in no way limited with regard to a certain resolution capacity. Furthermore, the specific manner of producing the positioning pressures, in particular the choice of the element that is displaceable relative to the machine frame, is not relevant to the present invention. Ultimately one skilled in the art can also rely on a large amount of corresponding knowledge from the field of control technology with regard to the specific choice of optimization strategies for adjustment of the positioning pressures 26, 32.

LIST OF REFERENCE NUMBERS

[0052] 10 Printing mechanism [0053] 12 Plate cylinder [0054] 14 Tensioning cylinder [0055] 16 Printing plate support [0056] 18 Printing plate [0057] 20 Inking roller [0058] 22 Ink reservoir [0059] 26 Positioning pressure of 20, pressure arrow [0060] 28 Impression cylinder [0061] 30 Printing substrate [0062] 32 Positioning pressure of 28, pressure arrow [0063] 34 First sensor [0064] 36 Second sensor [0065] 100 Printing plate reference field [0066] 101 Reference field surface of 100 [0067] 103 Reference field surface of 100 [0068] 104 Reference field surface of 100 [0069] 105 Reference field surface of 100