Method for detecting undesired ink application on substrate sheet in offset printing press

Abstract

The invention relates to a method for detecting an undesired ink application (set-off) on a substrate sheet in an offset printing press, wherein a measurement image and a reference image are captured by means of at least one camera and a computer-assisted comparison is carried out. The image is acquired in a region of the substrate sheet that is not in contact with the ink.

Description

TECHNICAL FIELD

[0001] The present invention relates to a computer-aided method for detecting undesired ink application on a substrate sheet in a sheet-fed offset printing press. BACKGROUND

[0002] Offset printing is one of the most common printing methods in today's printing industry. In principle, ink is transferred from an ink device to a printing cylinder which carries a developed printing plate. In addition, a dampening device next to the printing plate cylinder also regulates the application of dampening medium on the printing plate. The printing plate only receives ink from the ink device in predetermined areas, thereby transferring the desired printed image to an adjacent blanket cylinder, which in turn transfers the printed image to a substrate in a printing nip together with a counter pressure cylinder. After drying, a printed product is obtained. Such a printing press usually has at least four offset printing devices, usually for cyan, magenta, yellow and black (CMYK). Special colors and varnish applied by additional printing devices and varnish devices also belong to this.

[0003] In offset printing, undesired ink application often occurs as a printing technical problem. This includes the transfer of ink to the substrate in places where the printing plate and blanket should not actually transfer ink. This undesired ink application is also referred to as smearing. There are many reasons for this. For example, if there is too little water on the printing plate, the hydrophilic areas of the printing plate are undesirably dampened by the ink, resulting in undesired ink application. Causes of too little water on the printing plate include incorrect dampening device settings, roll wear, incorrect plate maker settings, too high machine temperature or too high printing room temperature.

[0004] The smearing phenomenon usually starts at the beginning of a sheet and can occur on one or both sides of the substrate sheet. In most cases, smearing does not occur on all offset printing devices at the same time, but is limited to a single printing device. The occurrence of smearing is often unpredictable and can quickly intensify, so that the substrate sheet soon becomes unusable. So-called edge smearing is almost always tolerable within small boundaries, but if the smearing is too severe, the sheet cannot be sold anymore and has to be discarded as waste. An early signal that smearing is imminent is often a light ink mist, especially at the edges of the sheet. This ink mist differs from the background of the substrate sheet by only a few color values, and for some substrate sheets, such as natural paper, the difference can even be smaller.

[0005] In the prior art, undesired ink application is detected in the form of smearing, in principle by means of a color measuring device, and compared to a reference value.

[0006] DE 102 08 041 430 Al describes a method for checking the plausibility of determined measurement values in the printing process of a printing press. The method measures the fading of the surface of the substrate and compares it with a reference value. Measurement values deviating from the reference value are assessed as ink blots or smears.

[0007] The methods of the prior art all have disadvantages. In some cases, theoretical values, for example data from the pre-press stage, are used as reference values. However, this does not take into account the actual situation of the substrate sheet, for example non-ideal or non-uniform colorings, for example with natural paper. As a result, this can lead to problems with the early detection of smears. In other methods, the reference point is detected in a region of the substrate sheet which itself is in principle also affected by smears, for example in the printed area of a color measurement bar. In this case, too, problems can arise with the reliable early detection of smears. SUMMARY

[0008] It has now been found that smears can be reliably identified in that, in the camera image matching process, the following region on the substrate sheet is used as the reference image, which, from the point of view of the printing, cannot be affected by smears because it does not come into contact with the ink at all in the offset printing device. Here, it is preferably a region which does not come into contact with the blanket at all during printing, nor with the following region of the blanket, which, due to the ink-repellent imaging of the associated printing plate, should not actually have any ink in the corresponding region.

[0009] The present application therefore relates to a method for detecting undesired ink application on a substrate sheet in an offset printing press by means of a computer, in which method the substrate sheet is introduced into the offset printing press, printed in at least one offset printing device by means of at least one printing plate and by means of at least one blanket, at least partially photographed in a measurement region by means of at least one camera to obtain a measurement image, and partially photographed in a reference region by means of the at least one camera to obtain a reference image, wherein the computer compares the measurement image and the reference image with one another in order to detect undesired ink application on the substrate sheet, wherein the reference region is located in a region of the substrate sheet which does not come into contact with the ink when the substrate sheet is printed in the at least one offset printing device.

[0010] The undesired ink application is preferably a smear. A smear occurs in the offset printing process if ink is transferred where it is not required to be transferred by the printing plate and the blanket, which is a problem in printing technology. Smear phenomena usually first occur in the edge regions of the substrate sheet.

[0011] In principle, all offset printing plates described in the prior art are suitable for use as printing plates in the method according to the present application, including self-developing offset printing plates.

[0012] According to the present application, the substrate sheet is at least partially imaged by at least one camera in a measurement area to obtain a measurement image. Correspondingly, the measurement image can comprise the entire substrate sheet or only a part thereof. In a preferred embodiment, the printed substrate sheet is at least partially imaged in a measurement area which is in the area of the printed substrate sheet which is in contact with the blanket, but which does not comprise the actual printed image of the plate. In other words, in this preferred embodiment, the unprinted area of the substrate sheet is imaged as the measurement area in which the blanket is in contact with the substrate sheet, but no ink is intended to be applied due to the imaging of the plate assigned to the blanket. In a particularly preferred embodiment, the measurement area is the unprinted area of the substrate sheet which is in the area of the color measurement bar, preferably directly adjacent to the color measurement bar, in particular at a distance of less than 10 mm or less than 5 mm from the color measurement bar, or in an equally preferred embodiment, in the unprinted area of the color measurement bar itself. In another preferred embodiment, the printed substrate sheet is at least partially imaged in a measurement area which is outside the actual printed image of the plate transferred by the blanket. This measurement area is preferably located at the beginning of the substrate sheet, i.e. in the area which is in contact with the blanket but has not yet been printed with the printed image. Thereby, edge smearing can be identified particularly easily.

[0013] According to the present application, the substrate sheet is at least partially imaged by at least one camera in a measurement area to obtain a measurement image. Correspondingly, the measurement image can comprise the entire substrate sheet or only a part thereof. In a preferred embodiment, the printed substrate sheet is at least partially imaged in a measurement area which is in the area of the printed substrate sheet which is in contact with the blanket, but which does not comprise the actual printed image of the plate. In other words, in this preferred embodiment, the unprinted area of the substrate sheet is imaged as the measurement area in which the blanket is in contact with the substrate sheet, but no ink is intended to be applied due to the imaging of the plate assigned to the blanket. In a particularly preferred embodiment, the measurement area is the unprinted area of the substrate sheet which is in the area of the color measurement bar, preferably directly adjacent to the color measurement bar, in particular at a distance of less than 10 mm or less than 5 mm from the color measurement bar, or in an equally preferred embodiment, in the unprinted area of the color measurement bar itself. In another preferred embodiment, the printed substrate sheet is at least partially imaged in a measurement area which is outside the actual printed image of the plate transferred by the blanket. This measurement area is preferably located at the beginning of the substrate sheet, i.e. in the area which is in contact with the blanket but has not yet been printed with the printed image. Thereby, edge smearing can be identified particularly easily.

[0014] According to the application, the substrate sheet for the measurement image is at least partially imaged by the at least one camera in a measurement region. Accordingly, the substrate sheet can be imaged completely or only partially by the at least one camera. In a preferred embodiment of the application, the measurement region imaged by the at least one camera is located on the substrate sheet in the region of contact with the blanket. Here, the substrate sheet can be detected completely or at least in all regions of contact with the blanket by the at least one camera, and when the measurement image and the reference image are compared by the computer, only the partial region of the digital measurement image is used for the computer-aided comparison, i.e. preferably the region of the printing plate image in which no ink is intended to be applied to the substrate sheet by the blanket.

[0015] According to the application, the reference region for creating the reference image and for comparison by the computer with the measurement image is located on the substrate sheet in a region of the substrate sheet which is not in contact with ink when printed in the at least one offset printing device. In other words, the substrate sheet is fed sequentially into offset printing devices, for example four offset printing devices for the four colors CMYK, and printed in each offset printing device by means of an offset printing plate by a blanket in the respective color if a respective color portion is present on the printing image. The reference region on the substrate sheet is in a region of the substrate sheet which is not in contact with ink in the printing technology. Preferably, this is a region which is not in contact with any blanket used in the offset printing device, but even not in contact with a region of the blanket in which no ink is intended to be transferred at all according to the printing plate image. The reference region is therefore preferably outside the region covered by the blanket. In a preferred embodiment, the reference region is therefore a region on the substrate sheet which is not in contact with the blanket during printing. In a particularly preferred embodiment of the application, the reference region is a region on the substrate sheet which is not in contact with the blanket during printing and which is located between the gripper and the region of the substrate sheet which is in contact with the blanket during printing. In a particularly preferred embodiment, the reference region is a region on the substrate sheet which is not in contact with the blanket during printing and which is located between the gripper and the region of the substrate sheet which is in contact with the blanket during printing and which is substantially wave-free against the printing press cylinder when the reference image is imaged. Since the substrate sheet is substantially wave-free against the printing press cylinder when the reference image is imaged, an unwanted ink application can be detected better since the camera image is less disturbed.

[0016] According to the application, the computer compares the measurement image and the reference image with each other in order to detect undesired ink application on the substrate. The computer compares the digital measurement image and the digital reference image with each other. In a preferred embodiment, the computer compares the measurement image and the reference image column by column. In another preferred embodiment, the computer compares the measurement image and the reference image in terms of grey values or color values, in particular color values. When comparing in terms of color values, it is possible in particular to correspond to the offset printing device in which the offset occurred. For example, if the computer detects that the cyan color exceeds a specified tolerance value on the basis of the measurement image, it is clear that the offset printing device for the cyan color is offset. Thus, it is possible in particular to eliminate the offset particularly effectively.

[0017] In a preferred embodiment, the method is carried out continuously and the computer compares the measurement images and the reference images of a plurality of substrate sheets printed in succession with each other, preferably averaged. In a particularly preferred embodiment, the computer divides the measurement image into different color zones by means of a grid. In a particularly preferred embodiment, the computer averages the color values in the different color zones over a plurality of substrate sheets. In a particular embodiment, the computer displays the color zones and the colors in which the undesired ink application occurred to the printer operator when an undesired ink application is detected. For example, the optical display can take place on a screen of the printer control panel.

[0018] It is also possible to determine a plurality of reference regions on one substrate sheet. In this way, it is possible to average a plurality of reference regions in order to compensate for local errors and inaccuracies, such as paper defects, fluff, bumps or the like. It is also possible to determine a plurality of reference regions on one substrate sheet and to average the plurality of reference regions over a plurality of substrate sheets which are printed in succession and detected by means of the at least one camera. In principle, this makes it possible to obtain a better reference image. Thus, it is possible not only to average in terms of position, but also in terms of time, in order to obtain a better detection result. This is particularly advantageous for substrates having a thin grammage, which have a light-transmissive printed image.

[0019] In a preferred embodiment, the computer detects the undesired ink application on the substrate sheet by comparing the measured image with the reference image as follows: The taken digital measured image is gridded and thereby divided into rows and columns. The computer averages the grey values or color values of all rows and columns. Here, the column averaging is preferably in the form of a moving average. The taken digital reference image is also gridded and thereby divided into rows and columns. In this case, the computer averages the grey values or preferably color values of all rows and columns. Here, the column averaging is preferably also in the form of a moving average. In the evaluation, a difference signal is preferably calculated column by column as a result of the subtraction of the measured image from the reference image. If the difference signal is higher than a threshold value, preferably higher than five grey values, it is evaluated as evidence of smearing. Depending on the standard deviation of the difference signal, the computer can determine at which threshold value it should be classified as a fault. Here, the computer can take into account that the threshold value depends on various properties of the substrate, such as the paper structure, the paper roughness or the paper color.

[0020] In another preferred embodiment, the computer changes at least one parameter of the at least one offset printing device when an undesired ink application is detected in order to correct the undesired ink application. In a particularly preferred embodiment, the computer changes at least one parameter of the dampening device and / or the ink device in the at least one offset printing device in order to correct the undesired ink application. In a particularly preferred embodiment, the computer changes at least the amount of ink applied to the printing plate per time and / or the amount of dampening medium in the at least one offset printing device in order to correct the undesired ink application. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 The structure of an offset printing device is schematically shown.

[0022] Figure 2 A preferred embodiment of the method according to the application in an offset printing press is schematically shown.

[0023] Figure 3 A flow chart of a preferred embodiment of the method according to the application is schematically shown.

[0024] Figure 4 A photo showing a part of a substrate sheet in front of a black background.

[0025] Figure 5 A difference signal along the full width of a substrate sheet is shown. DETAILED DESCRIPTION

[0026] Figure 1The structure of the flexographic printing device 4 is schematically shown. The printing plate 5 is tensioned on a plate cylinder. An ink device 13 and a dampening device 14 are located near the plate cylinder. The printing plate 5 is in contact with a blanket placed on a blanket cylinder 11. The blanket cylinder 11 and a counter pressure cylinder 12 form a printing nip in which the substrate sheet 1 is printed with a printed image.

[0027] Figure 2 A preferred embodiment of the method according to the application in a flexographic printing press 2 is schematically shown. The printed substrate sheet 1 comes from the flexographic printing device 4. The substrate sheet 1 has a printed image 9 and a color measurement strip 10. A measurement image is taken in the measurement area 6 by a camera 7 present in the flexographic printing press 2, which measurement area is located in the color measurement strip 10. In addition, a reference image is also taken in the reference area 8. The measurement image and the reference image are then compared by the computer 3. If the deviation is outside the tolerance range, feedback is sent to the flexographic printing device 4, whereby for example the amount of dampening medium is increased.

[0028] Figure 3 A flowchart of a preferred embodiment of the method according to the application is schematically shown. In the first method step 15, flexographic printing is performed using a flexographic printing press. In the subsequent method step 16, a reference image is captured with a camera. In the subsequent method step 17, a measurement image is captured with a camera. In the subsequent method step 18, the measurement image is compared with the reference image by a computer. If a parameter correction 19 is required, the parameters of the flexographic printing press are corrected, for example the amount of dampening medium is increased. If necessary, the faulty print is ejected (not shown). If no correction of the parameters 20 is required, a perfect print 21 is obtained.

[0029] Figure 4 A photograph is shown on which a part of the substrate sheet 1 is in front of a black background. The reference area 8 outside the printable area and the measurement area 6 at the start of the printable area are defined as lines, respectively, which are shown in excerpt and subsequently framed. A smudge 22 has occurred in the measurement area 6. The lines of the reference area 8 are only a few millimeters higher than the sheet gripper (not shown).

[0030] Figure 5 The difference signal along the full width of the substrate sheet 1 is shown. In the left-hand area of the substrate sheet 1, the threshold value 5 is exceeded (see Figure 5 lower inset) because a smudge has occurred there. The sheet gripper is also partly shown in the figure (black bar at the lower edge of the figure).

[0031] List of reference signs

[0032] 1 substrate sheet

[0033] 2 flexographic printing press

[0034] 3 computer

[0035] 4 offset printing device

[0036] 5 printing plate

[0037] 6 measurement area

[0038] 7 camera

[0039] 8 reference area

[0040] 9 printed image

[0041] 10 color measurement bar

[0042] 11 blanket cylinder

[0043] 12 counter pressure cylinder

[0044] 13 ink device

[0045] 14 dampening device

[0046] 15 offset printing using an offset printing press

[0047] 16 capturing a reference image with a camera

[0048] 17 capturing a measurement image with a camera

[0049] 18 comparing the measurement image and the reference image by a computer

[0050] 19 need for parameter correction

[0051] 20 no need for parameter correction

[0052] 21 printed product

[0053] 22 smearing

Claims

1. A method for detecting undesired ink application on a substrate sheet (1) in an offset printing machine (2) by means of a computer (3), wherein the substrate sheet (1) is introduced into the offset printing machine (2), printed in at least one offset printing unit (4) by means of at least one printing plate (5) and at least one blanket, the printed substrate sheet (1) is at least partially imaged in a measuring area (6) by means of at least one camera (7) to obtain a measuring image and the printed substrate sheet (1) is partially imaged in a reference area (8) by means of the at least one camera (7) to obtain a reference image, wherein the computer (3) compares the measuring image and the reference image with each other to detect undesired ink application on the substrate sheet (1), characterized in that the reference area (8) is located in an area of the substrate sheet (1) which is not in contact with ink when printed in the at least one offset printing unit (4).

2. The method of claim 1, wherein, the reference area (8) is an area on the substrate sheet (1) which is not in contact with the at least one blanket during printing.

3. The method of claim 2, wherein, the reference area (8) is an area on the substrate sheet (1) which is located between the gripper and the area on the substrate sheet (1) which is in contact with the at least one blanket during printing.

4. The method according to one of the preceding claims, wherein, the reference area (8) is an area on the substrate sheet (1) which lies flat on the printing cylinder without waves when the reference image is imaged.

5. The method according to one of the preceding claims, wherein, the undesired ink application is smearing.

6. The method according to one of the preceding claims, wherein, the measuring area (6) is an unprinted area on the substrate sheet (1) which is located in the area of the color measurement bar (10).

7. The method of claim 6, wherein, the measuring area (6) is an unprinted area on the substrate sheet (1) which is located directly adjacent to the color measurement bar (10) or in the color measurement bar (10) itself.

8. The method according to one of the preceding claims, wherein, the method is carried out continuously and the computer (3) compares the measuring images and the reference images of a plurality of sequentially printed substrate sheets (1) with each other.

9. The method of claim 8, wherein, the computer (3) divides the measuring image into different color zones by means of a grid.

10. The method of claim 9, wherein, the computer (3) averages the color values over a plurality of substrate sheets (1) within the different color zones.

11. The method according to one of claims 8 to 10, wherein the computer (3) displays to a printer operator the color zone and the color in which the undesired ink application has occurred when the undesired ink application is detected.

12. The method according to one of the preceding claims, wherein, the computer (3) changes at least one parameter of the at least one offset printing unit (4) to correct the undesired ink application when the undesired ink application is detected.

13. The method of claim 12, wherein, the computer (3) changes at least one parameter of the at least one offset printing unit (4) to correct the undesired ink application when the undesired ink application is detected.

14. The method of claim 13, wherein, the computer (3) changes at least one parameter of the at least one offset printing unit (4) to correct the undesired ink application when the undesired ink application is detected.

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