EP3102415A1

EP3102415A1 - Printing mechanism for a flexographic printing press, and a method for operating same

Info

Publication number: EP3102415A1
Application number: EP15702150.2A
Authority: EP
Inventors: Wolfgang Becker
Original assignee: CONPRINTA & Co KG GmbH
Current assignee: CONPRINTA & Co KG GmbH
Priority date: 2014-02-05
Filing date: 2015-01-16
Publication date: 2016-12-14
Also published as: WO2015117806A1; US20170165956A1; DE102014101433A1; DE102014101433B4

Abstract

The invention relates to a printing mechanism (10) for a flexographic printing press, comprising - a plate cylinder (12) which supports a printing plate (18) having a printing motif region and a printing plate reference field (100) that has a plurality of reference field surfaces (101-105) of different heights, at least one lowest reference field surface (101) being a lower height than that of the printing plate (18) in the printing motif region, and at least one highest reference field surface (105) being a greater height than that of the printing plate in the printing motif region, - an impression cylinder (28) whose distance to the plate cylinder (12) can be varied, controlled by a control unit, for the purpose of pressing a printing substrate (30) onto the printing plate (18), - an inking roller (20) whose distance to the printing plate (18) can be varied, controlled by the control unit, and whose surface can be wetted with ink from an attached ink reservoir (22), for the purpose of inking said plate, - a first sensor (34) connected to the control unit for determining a quality of a printing 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 said inking roller (20). The invention also relates to a method for operating same.

Description

Printing unit for a flexographic printing machine and method for its operation

description

Field of the invention

The invention relates to a printing unit for a flexographic printing press, comprising

a plate cylinder carrying a cliché with a print motif area and a cliché reference field having a plurality of reference field areas of different height, at least one lowest reference field area (101) having a lower height than the cliche (18) in FIG

Print area and at least one highest reference field area (105) has a greater height than the cliché in the print area,

a counter-pressure cylinder whose distance from the plate cylinder is controllable controlled by a control unit for the purpose of pressing a printing material onto the printing plate,

a dyeing roller whose distance from the printing plate is controlled by the control unit for the purpose of coloring it and whose surface is wettable with ink from a connected ink reservoir,

- A connected to the control unit, the first sensor for detecting a quality of a printed image of the cliché reference field on the substrate.

State of the art

Such printing units for flexographic printing presses are described in German Patent Application DE 10 2013 010 763.6, unpublished on the priority date of the present patent application. The art of flexographic printing has long been known to those skilled in the art. Corresponding flexographic printing machines typically consist of several printing units arranged serially one behind the other, which are successively passed through by a printing material. Each printing unit leaves a printed image on the substrate, whereby different printing units are typically assigned different inks. For monochrome prints, it is also possible that the flexographic printing press comprises only a single printing unit.

Essential components of a printing unit of a flexographic printing press are the

Plate cylinder, the impression cylinder and the dyeing roller. The plate cylinder carries the cliché of an elastic material. In the so-called sleeve variant, the cliché is fully fixed on the plate cylinder. In the so-called belt variant, the cliché is fixed on a flexible, an endless belt forming cliché carrier, between the plate cylinder and a perpendicular to this movable, in

Essentially aligned parallel clamping cylinder is tensioned. The present invention is applicable to both variants of flexographic printing units.

The counter-pressure cylinder, which is aligned substantially parallel to the plate cylinder, serves for pressing the printing material, typically a paper web, onto the printing plate so that ink can be transferred from the inked printing plate to the printing material. For this purpose, the impression cylinder and the plate cylinder are movable relative to each other, so that substrate and plate in the nip between the impression cylinder and the plate cylinder with that of the control unit

given contact pressure are pressed against each other. There are both known variants in which the counter-pressure cylinder stored stationary in a machine frame and the plate cylinder relative to the machine frame on the impression cylinder to linearly movable, as well as variants in which the impression cylinder is linearly movable relative to the machine frame. The present invention is applicable to both variants, although the former is normally preferred.

For coloring the plate a dyeing roller, typically designed as an anilox roller, is provided. Their surface can be wetted in a reproducible manner with ink from a connected color reservoir. For transferring the color from the dyeing roller to the plate, the dyeing roller and the printing plate are movable relative to one another, wherein here also a predetermined by the control unit Anstelldruck is to be generated. The contact pressure of dyeing roller and cliché is therefore typically in the range of substantially parallel to the dyeing roller aligned plate cylinder, so that the cliché between the plate cylinder and dyeing roller is pressed with the controlled contact pressure. In the case of the belt technology, it is also conceivable, although not normally preferred, the dyeing roller in the region of

To arrange clamping cylinder and to make these two elements movable relative to each other. There are both known variants in which the dyeing roller mounted stationary in a machine frame and the plate cylinder relative to the machine frame linear to the dyeing roller to be moved, as well as variants in which conversely the dyeing roller is linearly movable relative to the machine frame. The present invention is applicable to both variants, although the latter is normally preferred. The same applies to the contact pressure of the dyeing roller in the region of the clamping cylinder.

Compliance with correct contact pressures is essential for optimum printing results. If the contact pressure of the dyeing roller is too high, the elastic cliché becomes

Dyeing process too heavily squeezed, so that deeper cliché regions are colored, which can lead to pressure marks on unwanted areas on the substrate. If the contact pressure of the dyeing roller, however, is too low, there is no sufficient coloring of the cliché, so that to be printed areas of the

Substrate remain unstained. If the contact pressure of the impression cylinder is too great, the elastic cliché deforms too much during the printing process, so that fine contours are smeared. If the contact pressure of the impression cylinder is too low, the substrate may be insufficiently colored.

In the aforementioned patent application, it is therefore proposed to provide the cliché, preferably outside the actual printing area, with a cliché reference field which has a plurality of reference field areas

having different heights. For example, the cliché reference field may be in the form of a stepped pyramid. The cliché reference field is used together with the rest of the cliche, i. especially together with its print motif area, colored and leaves on the substrate a characteristic print image.

In particular, the correct setting of the dyeing roller setting pressure and the impression cylinder setting pressure leads to a defined printed image of the cliché Reference field to which only reference surfaces of selected heights contribute. A quality of the printing image of the cliché reference field on the printing substrate can be monitored by means of an optical sensor and gives the worker at the printing press valuable feedback regarding the correct setting of the setting pressures. The term used here of the "quality" of the reference field printed image detected by the sensor is not to be understood here in a limiting sense as its "goodness". Rather, any property of the reference field print image can be determined by the choice of

Anstelldrücke depends on how the sensor detected quality used. In particular, the particular choice of quality detected will depend on the particular design of the cliché reference field. For example, an outline shape or the area of a printed area, the occurrence of an edge or another property of the printed image can be used as the quality to be detected by the sensor.

A disadvantage of the printing method described is that both of the above

Anstelldrücke that can be independently controlled by the control unit, together contribute to the formation of the reference field pressure image. If the quality detected by the sensor does not correspond to the specifications expected with the correct setting pressure setting, the operator or known automatic systems can not easily decide which of the adjustable setting pressures is set incorrectly. It must therefore be tried by varying both Anstelldrücke so long until the

Reference field print image meets the specifications. How fast this correct contact pressure constellation is found depends to a great extent on the individual experience of the operator or the algorithms of the automatic, iterative system. Due to the high running speeds of modern printing machines, delays in the seconds range already cause high losses of printing material and thus high costs can.

From DE 10 201 1 086 047 A1 a flexographic printing press is known in which the dyeing roller is observed over its entire width by means of an optical sensor in order to monitor the negative image of the print motif area of the cliché with respect to its quality.

From DE 10 2007 028 327 A1 a flexographic printing machine is known, whose

Printing units in the edge area of the substrate one for the actual Print motif representative Messtreifen print. For this purpose, the respective cliché has a measuring strip printing element, comprising pressure elements of equal height to one another and to the printing motif area of the cliché, which are separated by lower non-printing elements. The quality of the printed measuring strips has the same dependency on a mismatching of the setting pressures, such as the quality of the actual printing motif itself.

DE 20 2012 000 246 U1 and DE 10 2008 025 1 14 A1 generally disclose the automated setting of setting pressures of a flexographic printing press on the basis of optical monitoring of the printed image.

DE 20 2006 020 066 U1 discloses a similar system which, however, has color-specific sensors for monitoring the printed image.

task

It is the object of the present invention to provide an improved printing unit for a flexographic printing press and a method for the control thereof, which enable an automated adjustment of an optimal contact pressure constellation.

Presentation of the invention

This object is achieved in conjunction with the other features of claim 1, characterized in that the printing unit according to the invention further comprises a second sensor connected to the control unit for detecting a quality of a negative image of the cliché reference field on the dyeing roller.

An inventive method for controlling such a printing unit is characterized in that by means of the first sensor, the quality of the printed image of the cliché reference field on the substrate and by means of the second sensor, the quality of the negative image of the cliché reference field is monitored on the dyeing roller, said - In case of deviation only the detected by the first sensor quality of a given degree of quality only one contact pressure of the impression cylinder is varied until the predetermined degree of quality of the printed image of the cliché reference field is achieved, and

- When deviation of the detected by means of the second sensor quality of a given level of quality first Anstelldruck the dyeing roller is varied until the predetermined degree of quality of the negative image of the cliché reference field is achieved, and then the Anstelldruck the impression cylinder is varied until the predetermined quality level of the printed image of the cliché reference field is achieved.

Preferred embodiments are subject of the dependent claims.

By means of the second sensor can thus be monitored independently of the Anstelldruck the impression cylinder, the contact pressure of the dyeing roller. The negative image of the cliché reference field shows only the correct setting of the Anstelldrucks

Reference field areas above a predetermined limit, but this completely. If the setting pressure of the dyeing roller is too high, the negative image will also

Reference field surfaces of lesser height shown. On the other hand, if the setting pressure of the dyeing roller is too low, not all intended reference field areas in the negative image are displayed on the coloring roller. The negative image on the dyeing roller is due to the fact that color is transferred from the color layer on the surface of the dyeing roller to the cliché reference field. Accordingly, the layer thickness of the color layer on the surface of the dyeing roller changes. If the quality of the negative image detected by the second sensor does not correspond to the specifications, the contact pressure of the dyeing roller can be varied independently, whereby the required direction of the variation also results directly from the negative image. After correct setting of the

Anstelldrucks the dyeing roller, a deviation of the quality of the printed image of the cliché reference field on the substrate only from the setting of the

Depend on the set pressure on the impression cylinder. This can thus also be set independently, which also results in the required direction of the variation directly from the printed image. The invention thus an independent adjustment of Anstelldrücke after simple rules is possible, which can be readily implemented by appropriate software implementation in an automation of the adjustment process and thus become independent of the individual experience of a worker. In addition, the setting can also be done at the actual working speed of the printing press when using suitable fast sensors. In the case of a visual inspection by a worker, the adjustment process can only take place at a significantly reduced speed, due to the limited viewing speed of even the most experienced worker. By contrast, the correct set pressure values themselves depend on the running speed of the machine.

Conveniently, the control unit is set up when starting the printing initially a Anstelldruck the dyeing roller to vary until a predetermined degree of quality of the negative image of the cliché reference image is achieved, and then to vary a Anstelldruck the impression cylinder until a predetermined degree of quality of the printed image of the cliché reference field is achieved. This thus represents an inventive printing unit with automated initial adjustment of Anstelldrücke.

Alternatively or additionally, however, a result-oriented Anstelldruck- monitoring during operation of the printing unit based on the invention carried out. For this purpose, it is provided that the control unit is set up to monitor the quality of the print image of the cliché reference field on the printing substrate and by means of the second sensor the quality of the negative image of the cliché reference field on the dyeing roller by means of the first sensor, and

- In case of deviation only the quality detected by means of the first sensor from a given quality level to vary only one Anstelldruck the impression cylinder until the predetermined quality level of the printed image of the cliché reference field is achieved, but

- In deviation of the detected by means of the second sensor quality of a given degree of quality first to vary a contact pressure of the dyeing roller until the predetermined degree of quality of the negative image of the cliché reference field is achieved, and then the Anstelldruck of To vary counterpressure cylinder until the specified quality level of the printed image of the cliché reference field is achieved.

In other words, the printed image and the negative image of the cliché reference field are continuously monitored during the entire printing process.

Deviations from the specifications can be unambiguously attributed to a mismatch of one or the other or both setting pressures by looking at the qualities detected by the sensors. Accordingly, the control unit can make a targeted readjustment. This can be done even with low quality deviations, so that rejects are avoided.

The sensors can be designed in different ways. It is conceivable that

Use of imaging sensors. Due to the increased data volume, however, such sensors are limited in their speed and the evaluation of their data requires considerable computing time, which is not always with the

Working speeds of modern printing machines are compatible. As an alternative to the image sensor, a line sensor arranged perpendicular to the transport direction of the printing substrate or parallel to the axis of rotation of the dyeing roller can be used. Even the use of a non-spatially resolving sensor, e.g. a simple photodiode, which is set to a characteristic feature of the print or negative image, can be used as a sensor in the present invention.

In any case, it is advantageous if the first sensor is synchronized with the transport movement of the printing material. This will not be done by a favorably

Synchronization with the rotational movement of the plate cylinder, but achieved by a synchronization with the transport movement of the plate. The latter can in the case of a working on the belt principle printing machine of the

However, the speed of rotation of the plate cylinder differ (this problem does not occur in printing machines according to the sleeve principle), but in any case equal to the transport speed of the substrate and can be easily detected as a machine parameter.

For the second sensor is a synchronization with the rotation of the

Dyeing roller cheap. In the cases of a line detector or a photodiode described above, a synchronization is particularly important, even if sync-free cases are conceivable. In the case of an image sensor, synchronization is also advantageous,

for example, to create a still image that is accessible to visual inspection by a worker.

Further features and advantages of the invention will become apparent from the following specific description and the drawings.

Brief description of the drawings

Show it:

Figure 1: a highly schematic representation of a flexographic printing in the

Cross-section,

FIG. 2 shows an exemplary representation of a cliché reference field,

FIG. 3 shows images of the cliché resulting from different setting pressures;

Reference field of Figure 2, namely its printed image on the

Substrate (S1) and its negative image on the staining roller (S2).

Description of preferred embodiments

Like reference numerals in the figures indicate like or analogous elements.

Figure 1 shows a highly schematic representation of a cross section through a

Printing unit 10 of a belt-fed flexographic printing machine. A central element of the printing unit 10 is the plate cylinder 12. Dier is arranged substantially parallel to a spaced clamping cylinder 14. Both are of a flexible

Cliché carrier 16 entwined. The plate carrier 16 is formed as an endless belt and carries the cliché, which as a relief of an elastic material on the

Cliché carrier 16 is fixed. The clamping cylinder 14 can be used to tighten the Cliché carrier 16 in the direction of its vertical distance to the plate cylinder 12 method.

In Figure 1 left of the plate cylinder is a substantially parallel to

Plate cylinder 12 aligned, rotating dyeing roller 20 which is in communication with a paint reservoir 22. Upon rotation of the dyeing roller 20 is their

Surface wetted with the paint. The dyeing roller 20 dyed in this way is moved with an adjustable contact pressure 26 against the plate cylinder 12 and pressed against the cliché 18 running between the plate cylinder 12 and the dyeing roller 20. In this case, color is transferred from the surface of the dyeing roller 20 to the plate 18.

In Figure 1 above the plate cylinder 12 is a substantially parallel to

Plate cylinder 12 aligned impression cylinder 28 is arranged. This is in sections with a web-shaped printing material 30, e.g. a paper web 30 wrapped around, wherein the printing material 30 passes through the printing gap between the plate cylinder 12 and the impression cylinder 28. For pressing the printing material to the plate cylinder 12 rotating cliché 18 of the plate cylinder 12, with an adjustable Anstelldruck 32 against the impression cylinder 28 movable. In this case, the printing material 30 is pressed against the inked cliché 18, wherein there is a transfer of ink from the plate 18 to the substrate 30, i. to the actual

Printing process, comes.

It is essential within the scope of the invention that the cliché 18 has a cliché reference field 100, for which an embodiment is shown schematically by way of example in FIG. The cliché reference field 100 has a plurality of preferably contiguous areas 101-105 of different heights. In the embodiment shown, the reference field areas 101 - 105 have different shapes for better distinctness in plan view. In the example shown, the reference field surface 101 is formed with the lowest height as a square. The second-lowest-height reference array area 102 is formed as a circle written lower in the square. The next higher reference field area 103 has the shape of a triangle lower than the circle. The second highest

Reference pad 104 has the shape of an oval lowercase to the triangle. The reference field area 105 having the greatest height has the shape of a rectangle written lower in the oval. The heights of the reference field areas 101-105 are selected so that at least the lowest reference field area 101 has a lower height than the cliche 18 in the area of the actual print motif and the at least the highest reference field area has a greater height than the cliche in the area of the actual print motif. In the context of the printing process explained above, the reference field 100, as well as the print motif area of the plate 18, is dyed by the dyeing roller 20 and leaves a printed image on the printing substrate 30.

As shown in FIG. 1, a first sensor 34 aligned with the printing substrate 30 is provided, the sensor signal of which is synchronized with the transport speed of the printing material 30 and which is set up on the printing substrate 30 to record the printed image of the cliché reference field. Further, a second sensor 36 aligned with the surface of the dyeing roller 20 is provided, the sensor signal of which is preferably synchronized with the rotational movement of the dyeing roller 20 and which is adapted to detect the negative image which causes the cliché reference field 100 to roll on the coloring roller 20 in the dyeing roller 20 Color film on the surface leaves.

FIG. 3 shows a schematic representation of possible images for detection by the first sensor 34 (line S1) and the second sensor 36 (line S2). At the same time, a possible, automated setting method of the setting pressures of the dyeing roller and impression cylinder 28 will be explained with reference to FIG. In the context of this explanation, for the sake of simplicity, it is assumed that the sensors 34 and 36 are imaging sensors and, as part of the control, a

Evaluation of image data is performed. However, those skilled in the art will recognize that line or non-spatially resolving sensors, for example, simple photodiodes, can be used to detect a given quality of the images.

Line S2 of Figure 3 shows different shapes of the negative image, which the cliché reference field 100 in the ink film on the surface of the dyeing roller 20 in

Depend on dependence of Anstelldruck 26 can leave. In Figure 3 takes while the

Adjustment pressure from left to right. At high contact pressure, the entire

Cliché reference field 100 is dipped into its color film down to its lowest reference field area 101 so that the outer contour of the negative image is square - according to the shape of the reference field surface 101. The outlines of the rest

Reference field surfaces are shown by dashed lines in line 2 of Figure 3, since they depending on the thickness and viscosity of the ink film and the quality of the image detection by the second

Sensor 36 can remain visible. At a slightly reduced Anstelldruck 26 of the

Dyeing roller 20 dips the cliché reference field 100 only to its circular surface 102 in the color film and leaves a corresponding negative image with a circular outline. With further reduced contact pressure 26 of the dyeing roller 20, the cliché reference field 100 is immersed in the color film only up to its triangular reference field area 103 and leaves a negative image with a triangular outline. The same applies to a further reduction of the Anstelldrucks 26, wherein negative images are generated with an oval or rectangular outline and detected by the second sensor 36. In the context of the illustrated exemplary embodiment, it is assumed that the contact pressure of the dyeing roller 20 required for generating an optimum print image of the actual print motif is the one in which the cliché reference field is immersed in the color film up to its triangular reference field surface 103. The corresponding negative image is therefore shown in bold in line S2 of FIG. This adjustment of the contact pressure 26 can be automatically detected and adjusted by evaluation of the sensor signal of the second sensor 36 easily by a control device. In this case, the control device varies the contact pressure 26, in particular by horizontal displacement of the dyeing roller 20, according to predetermined rules until the negative image shown in line 2 of Figure 3 sets.

In a next step, then the optimal Anstelldruck 32 of

Evidence counterpressure cylinder 28 and, in particular by vertical displacement of the plate cylinder 12, can be adjusted. The optimal Anstelldruck 32 is present when exactly the colored areas of the plate 18 and a printed image on the

Substrate 30 left. Higher setting pressure leads to excessive deformation of the elastic plate relief; lower setting pressure leads to an imperfect ink transfer to the substrate. The latter would be the case in the illustrated example, if not the dyed reference field surface 103, but only one or both of the higher reference field surfaces 104, 105 would leave a printed image on the substrate 30. These possibilities are shown in line S1 of FIG. 3, these being

Representation of a schematic representation of the means of the first sensor 34th

recorded print image is. The "right" Anstelldruck 32 associated printed image is shown in bold in line S1 of FIG. An automated variation of the Anstelldrucks 32 of the impression cylinder 28 until the setting of this print image can be readily realized by the expert in view of the technical teaching explained here.

However, the person skilled in the art will recognize that the same print image would also be set at a set pressure 32 that was selected too high, since then one or both of the lower lying reference field surfaces 101, 102 would be pressed onto the printing material 30, but in the case of lack of coloring of these surfaces 101 , 102 not one

Color transfer could come. In the realization of an automated

Adjustment pressure setting, it will therefore be useful, initially one too small

Adjusting 32 set, which only leads to the impression of a higher than the deepest colored reference field surface 103 reference field surface 104, 105, and then increase it until the printed image on the substrate 30 of the deepest colored reference field area 103 corresponds. In the example explained above, this would mean that the setting pressure 32 is initially adjusted so that a printed image with a rectangular or oval outline is displayed. Thereafter, the Anstelldruck 32 just increased so far until a printed image results in a triangular outline.

Of course, other strategies are conceivable in the realization of an automated Anstelldruck- setting. For example, an excessively large setting pressure 26 of the coloring roll 20 could initially be set in a targeted manner in order to find the optimum for the setting pressure 32 of the impression cylinder 28.

Of course, the embodiments discussed in the specific description and shown in the figures represent only illustrative embodiments of the present invention. Those skilled in the art will appreciate the broad scope of the disclosure herein

Range of variation possibilities given to the hand. In particular, the shape and complexity of the cliche reference field 100 can be varied widely. The specifically selected sensor technology for the first and second sensors 30, 36 is only with regard to optical sensitivity, but in no way to any

Resolution limited. Next is the concrete way of generating the

Anstelldrücke, in particular the choice of each movable relative to the machine frame element, for the present invention is not relevant. Finally, can the expert also with regard to the special choice of optimization strategies for the adjustment of Anstelldrücke 26, 32 fall back on a large fund of knowledge corresponding to the control technology.

LIST OF REFERENCE NUMBERS

10 printing units

12 plate cylinders

14 clamping cylinders

16 cliché carriers

18 cliché

20 dyeing roller

22 color bath

26 contact pressure of 20, pressure arrow

28 impression cylinders

30 printing material

32 contact pressure of 28, pressure arrow

34 first sensor

36 second sensor

100 cliché reference field

101 reference field area of 100

103 reference field area of 100

104 reference field area of 100

105 reference field area of 100

Claims

claims

Printing unit (10) for a flexographic printing machine, comprising

a plate cylinder (12) carrying a cliché (18) having a print subject area and a cliché reference field (100) having a plurality of reference field areas (101-105) of different height, at least one lowest reference field area (101) having a smaller height as the cliché (18) in the print motif area and at least one highest

Reference field area (105) a greater height than the cliché in the

Printmotive area has,

- A counter-pressure cylinder (28) whose distance from the plate cylinder (12) for the purpose of a contact pressure of a printing material (30) to the plate (18) is controlled controlled by a control unit,

- A dyeing roller (20) whose distance from the plate (18) for the purpose of coloring controlled by the control unit variable and whose surface is wettable with ink from a connected paint reservoir (22),

a first sensor (34) connected to the control unit for detecting a quality of a printed image of the cliché reference field (100) on the

Substrate (30) and

- A connected to the control unit, second sensor (36) for detecting a quality of a negative image of the cliché reference field (100) on the dyeing roller (20).

Printing unit (10) according to claim 1,

characterized,

that the cliché (18) on a flexible, forming an endless belt

Cliched carrier (16) is fixed, which is stretched between the plate cylinder (12) and a perpendicular to this movable clamping cylinder (14).

3. Printing unit (10) according to one of the preceding claims,

characterized,

in that the control unit is set up to initially vary an adjusting pressure (26) of the dyeing roller (20) until a predetermined quality level of the negative image of the printing plate reference frame (100) is achieved, and then a setting pressure (32) of the Reverse impression cylinder (28) until a predetermined degree of quality of the printed image of the cliché reference field (100) is achieved.

4. Printing unit (10) according to one of the preceding claims,

characterized,

the control unit is set up by means of the first sensor (34) the quality of the printed image of the cliché reference field (100) on the printing material (30) and by means of the second sensor (36) the quality of the negative image of the cliché reference field (100) to monitor the dyeing roller (20), and

if only the quality detected by means of the first sensor (34) deviates from a predetermined quality level, only one set pressure (32) of the

Counterpressure cylinder (28) to vary until the predetermined degree of quality of the printed image of the cliché reference field (100) is achieved, but

- In deviation of the detected by means of the second sensor (36) quality of a given degree of quality first a Anstelldruck (26) of the dyeing roller (20) to vary until the predetermined degree of quality of the negative image of the cliché reference field (100) is achieved, and then to vary the contact pressure (32) of the impression cylinder (18) until the predetermined quality level of the printed image of the cliché reference field (100) is achieved.

5. Printing unit (10) according to one of the preceding claims,

characterized,

the first sensor (34) is synchronized with a transport movement of the printing material (30).

6. Printing unit (10) according to claim 5,

characterized,

that the first sensor (34) is synchronized with a transport movement of the plate (18).

7. Printing unit (10) according to one of the preceding claims,

characterized,

in that the second sensor (36) is synchronized with a rotational movement of the coloring roller (20).

8. A method for controlling a printing unit (10) according to claim 1,

characterized,

in that by means of the first sensor (34) the quality of the printed image of the cliché reference field (100) on the printing material (30) and by means of the second

Sensors (36) the quality of the negative image of the cliché reference field (100) on the coloring roller (20) is monitored, wherein

if only the quality detected by means of the first sensor (34) is different from a given quality level, only one set pressure (32) of the

Counterpressure cylinder (28) is varied until the predetermined

Quality level of the printed image of the cliché reference field (100) is achieved, and

in the case of a deviation of the quality detected by means of the second sensor (36) from a predetermined quality level, first an adjusting pressure (26) of the

Dyeing roller (20) is varied until the predetermined degree of quality of the negative image of the cliché reference field (100) is achieved, and then the Anstelldruck (32) of the impression cylinder (28) is varied until the predetermined degree of quality of the printed image of the plate Reference field (100) is achieved.