METHOD FOR AUTOMATIC PASSING CORRECTION IN A PRINTING MACHINE

Abstract

The present invention relates to a method for automatic registration correction in a printing press with at least one image acquisition system for registration control, which can be used particularly advantageously when using circular measuring marks and large registration deviations. For this purpose, two digital correction methods are carried out in parallel in a computer and, depending on the extent of the registration deviation, a coarse or a fine correction method is sent to the printing press.

Description

TECHNICAL AREA

[0001] The present disclosure relates to a method for automatic registration correction in a printing press. In particular, the present disclosure relates to a computer-aided method for determining and correcting registration misalignment, wherein circular measuring marks are used for the precise determination of the misalignments. TECHNICAL BACKGROUND

[0002] In the field of printing technology, it is standard practice for printing presses to perform precise registration and alignment corrections to achieve high print quality. These corrections are crucial to ensure that the various color separations of a printed image are perfectly aligned, without any visible shifts or blurring. Common registration correction systems often employ manual or semi-automatic methods, requiring an operator to monitor the press and make adjustments. However, these methods are time-consuming and prone to human error, which can lead to inconsistent print quality. Automated systems based on optical sensors or cameras are also in use, but they often reach their limits in terms of accuracy and speed of correction.

[0003] Common systems typically involve the use of registration marks printed on the substrate to detect misalignments. These marks are detected by sensors, and the data is used to calculate corrections. However, a frequent problem with these systems is their limited accuracy; furthermore, the algorithms used to calculate the corrections are often unable to react quickly enough to changes in the printing process, which can lead to delays and waste.

[0004] Another problem with existing systems is their dependence on the quality of the printed registration marks. Contamination or uneven printing conditions can impair the detection and evaluation of the marks, leading to erroneous corrections. Furthermore, many systems require complex sensor calibration to achieve accurate results, increasing setup time and limiting the flexibility of the printing press. Integrating such systems into existing printing presses can also be challenging, especially if the machines are designed for high printing speeds.

[0005] A particularly troublesome problem arises with camera-based registration correction systems when circular registration marks are very close together or even overlap, resulting in a large registration error. This is especially common with very small marks, which are often used due to their small footprint on the printing sheet. Existing methods exhibit weaknesses in this situation, which can lead to inadequate correction of the registration error. Despite considerable progress in automated print correction, there remains a need for systems that offer high accuracy and an acceptable response time, particularly when dealing with significant misalignment of circular registration marks, in camera-based correction systems.

[0006] DE 10 2019 211 758 A1 describes a method for automated registration and register measurement in a printing press using a computer and circular measuring marks.

[0007] DE 10 2018 211 922 A1 describes a method for the automated image sensor calibration of printheads in an inkjet printing machine, in which test print patterns are printed from circular discs and digitally evaluated using a computer.

[0008] Therefore, one of the technical problems underlying the present invention is to provide a method that at least partially overcomes the disadvantages of known systems. SUMMARY OF THE INVENTION

[0009] The object of the present invention is to provide a method for automatic registration correction in a printing press that enables more precise and efficient correction, thereby improving print quality, shortening production time, and reducing material consumption, thus overcoming one or more disadvantages of known methods. The method should be particularly suitable for use in camera-based correction systems with circular measuring marks.

[0010] The method according to the invention is suitable for correcting registration errors quickly and accurately. By theoretically executing a coarse and a fine correction method in parallel on the computer and sending the more suitable method to the printing press for registration correction as needed, time and waste are saved. If necessary, a coarse correction method is applied before the finer correction method, which may fail in cases of very large registration deviations, so that the fine correction method can ultimately be advantageously used for precise registration correction.

[0011] Accordingly, a first aspect of the present invention relates to a method for register correction in a printing press using a computer, wherein at least one test pattern comprising several color separations is printed by the printing press onto a substrate, and the at least one test pattern is captured by at least one image acquisition system in the form of a digital test pattern image, the digital test pattern image is evaluated by the computer with regard to a register misalignment, and the computer corrects any existing register misalignment on the basis of this evaluation by adjusting the printing press, wherein each color separation comprises at least one circular measuring mark with a diameter determined before printing onto the substrate, the computer determines the center position of each circular measuring mark from the digital test pattern image and thus calculates the register misalignment, which is characterized in that one i) prior to adjusting the printing press, a first digital correction procedure and a second digital correction procedure are carried out separately in the computer during the evaluation of the digital test pattern image, wherein the first digital correction procedure calculates the registration error with higher accuracy than the second digital correction procedure, ii) the computer decides whether the first digital correction method substantially corrects the passport offset or not, and ii.1) in the event that the first digital correction method substantially corrects the registration error completely, the computer adjusts the printing press to correct the registration error based on the first digital correction method, and ii.2) in the event that the first digital correction method does not substantially correct the registration error, the computer adjusts the printing press to correct the registration error based on the second digital correction method, at least one further test pattern comprising several color separations is printed on a substrate by the printing press, at least one further test pattern is captured by the at least one image acquisition system in the form of a further digital test pattern image, the further digital test pattern image is evaluated by the computer with regard to a registration error, this evaluation comprising the first digital correction method, and the computer then adjusts the printing press to correct the registration error based on the first digital correction method.

[0012] In a preferred embodiment, the method according to the invention is carried out continuously during a print job in the printing press. It is particularly preferred that the method according to the invention is carried out with respect to each sheet of the printed material in the printing press. This ensures that each sheet is checked for correct registration, and if a deviation is detected, the subsequent sheets are corrected. However, it is also possible to use the method according to the invention to check, for example, only every second or third sheet of printed material for registration error.

[0013] In a further preferred embodiment, the inventive method is carried out during a printing job in the printing press only at the request of the printing press operator. In this preferred embodiment, the operator is preferably warned by acoustic or visual indication that a registration error has been detected. Various registration error detection methods can be used for this purpose; for example, a purely visual inspection of the substrate by the operator is also possible. The operator then has the opportunity to carry out the inventive method and thus correct the registration error.

[0014] In a further preferred embodiment, the registration error is continuously detected and calculated in the printing press during a print job according to the method of the invention. However, the adjustment of the printing press to correct the registration error is only carried out at the request of the printing press operator, preferably after an acoustic or visual indication triggered by the computer. This makes it possible to continuously monitor the registration error, but to perform a correction only when necessary.

[0015] In a further preferred embodiment, the computer determines the center position of each circular measurement mark from the digital test pattern image in both the first and second digital correction methods and uses this information to calculate the pass / fail offset. In a particularly preferred embodiment, the computer determines the center position of each circular measurement mark from the digital test pattern image by determining the center of mass of each circular measurement mark. In this context, the term "center of mass" refers to the geometric center of the circular measurement marks, which is determined by analyzing the distribution of pixel intensities in the overall digital image. This method utilizes the fact that the circular measurement marks have a uniform color distribution, which allows the center of mass to be precisely located.The advantage of this method lies in its robustness and efficiency, as it enables a fast and reliable determination of the position of the measurement marks, even if the print quality or image acquisition conditions vary. The method for determining the centers of mass of such measurement marks is described, for example, in DE 10 2019 211 758 A1 cited above. In a particularly preferred embodiment, the computer determines the center position of each circular measurement mark from the digital test pattern image in the first digital correction method using a subpixel-accurate, model-based parameter adjustment procedure. This can be done according to a method such as that described, for example, in DE 10 2018 211 922 A1 cited above. This method uses a model-based approach in which a mathematical model of the image of a circular disk is fitted to the actually captured image.The model's parameters, including the center position of the circular disk, are determined very precisely. This modeling method has the advantage of also taking the noise from the image sensor into account.

[0016] In another preferred embodiment, the computer decides in step ii) that the first digital correction method substantially corrects the register offset if the deviation of the center positions of the circular measuring marks from the specified target value is below a tolerance value. If the tolerance value is exceeded, the first, more accurate digital correction method is not used to correct the register offset. In this case, the second, less accurate digital correction method is used instead to coarsely correct the register until the deviation of the center positions again falls below the tolerance value. Then the first, more accurate correction method is applied again to finely correct the register.

[0017] In a further preferred embodiment, the at least one image acquisition system is calibrated using calibration points printed on the substrate before the digital test pattern image is captured. This allows the image acquisition system to be calibrated initially or at regular intervals, or to be recalibrated.

[0018] In another preferred embodiment, the at least one image acquisition system is at least one inline camera of the printing press. For example, it can be an image acquisition system with two inline cameras, each inline camera capturing a different side of the double-sided printed substrate. In this case, each inline camera can provide a separate digital test pattern image. It is also possible to use an image acquisition system with four inline cameras, with two inline cameras capturing the front of the substrate and two inline cameras capturing the back of the substrate. Each inline camera can cover a different half of the substrate sheet.

[0019] In a further preferred embodiment, the circular measurement marks on the substrate are arranged such that they are completely captured by the at least one image acquisition system and represented in a single digital test pattern image. In a particularly preferred embodiment, the at least one image acquisition system essentially only captures the area in which the circular measurement marks are represented and transmits only this area as a digital test pattern image to the computer for evaluation.

[0020] In a further preferred embodiment, the circular measuring marks for each color separation are selected from filled circular disks with a predetermined diameter and unfilled circular disks in the form of circular rings with a predetermined diameter. Such measuring marks are described, for example, in DE 10 2018 211 922 A1 cited above.

[0021] In another preferred embodiment, the circular measuring marks are arranged in the pressure control strip on the substrate.

[0022] Another aspect of the invention relates to a printing press configured for carrying out the method according to the invention. In a preferred embodiment, the printing press is selected from offset printing presses, flexographic printing presses, and inkjet printing presses. FIGURE DESCRIPTION Fig. Figure 1 schematically shows the setup of a printing press in which a preferred embodiment of the method according to the invention is carried out. A sheet of substrate (4) is fed into the printing press (1), which is an offset printing press, and printed. An image acquisition system (5), which is a camera inside the printing press, captures the test pattern (3) on the substrate (4) in the form of a digital test pattern image and sends it to the computer (2). The computer (2) executes the first digital correction process and the second digital correction process separately in memory and decides that the first digital correction process is not promising due to a significant registration error. Therefore, the computer (2) sends only the second digital correction process to the printing press (1) for execution.After the second digital correction procedure has been executed in the printing press (1) to correct the registration error, test samples (3) are printed again, and the first and second digital correction procedures are theoretically performed again in the computer (2). The computer (2) now recognizes that the first digital correction procedure is promising and therefore sends only the first digital correction procedure to the printing press (1) to correct the registration error. Fig. Figure 2 schematically shows a test pattern (3) with the ideal positions for the circular measuring marks (6) to be printed, indicated by six circles (not shown). Fig. Figure 3 schematically shows a test pattern (3) with three printed circular measurement marks (6). The deviations of the three circular measurement marks (6) from the ideal positions are comparatively small. The other three circular measurement marks shown are printed using a reference color separation and serve as reference points for calibrating the image acquisition system. Fig.Figure 4 schematically shows a test pattern (3) with three printed circular measurement marks (6). The deviations of the three circular measurement marks (6) from the ideal positions are comparatively large and even lead to a partial overlap of the circular measurement marks (6), see the checkered measurement mark and the hatched measurement mark in the middle column. The other three circular measurement marks shown are printed using a reference color separation and serve as reference points for calibrating the image acquisition system. When this test pattern (3) occurs, the two-stage method for registration correction according to the invention can be applied particularly advantageously. REFERENCE MARK LIST 1 printing press 2 computers 3 test samples 4. Printing material 5 Image capture system 6 circular measuring marks

Claims

A method for register correction in a printing press (1) using a computer (2), wherein at least one test pattern (3) comprising several color separations is printed by the printing press (1) onto a substrate (4) and the at least one test pattern (3) is captured by at least one image acquisition system (5) in the form of a digital test pattern image, the digital test pattern image is evaluated by the computer (2) with regard to a register misalignment and the computer (2) corrects any existing register misalignment on the basis of this evaluation by adjusting the printing press (1), wherein each color separation comprises at least one circular measuring mark (6) with a diameter determined before printing onto the substrate (4), the computer (2) determines the center position of each circular measuring mark (6) from the digital test pattern image and thus calculates the register misalignment, characterized in thatthat (i) before adjusting the printing press (1), during the evaluation of the digital test pattern image, a first digital correction procedure and a second digital correction procedure are carried out separately in the computer (2), wherein the first digital correction procedure calculates the registration error with higher accuracy than the second digital correction procedure, (ii) the computer (2) decides whether the first digital correction procedure completely corrects the registration error or not, and (ii.1) in the case that the first digital correction procedure completely corrects the registration error, the computer (2) adjusts the printing press (1) to correct the registration error based on the first digital correction procedure, and (ii.2) in the case that the first digital correction procedure does not completely correct the registration error,The computer (2) adjusts the printing press (1) to correct the registration error based on the second digital correction method; at least one further test pattern (3) comprising several color separations is printed by the printing press (1) onto a substrate (4); at least one further test pattern (3) is captured by the at least one image acquisition system (5) in the form of a further digital test pattern image; the further digital test pattern image is subjected to an evaluation by the computer (2) with regard to registration error, this evaluation comprising the first digital correction method; and the computer (2) then adjusts the printing press (1) to correct the registration error based on the first digital correction method. Method according to claim 1, wherein the method is carried out continuously during a printing job in the printing machine (1). Method according to claim 1, wherein the method is carried out during a printing job in the printing machine (1) only at the request of the operator of the printing machine (1). Method according to claim 1, wherein during a printing job in the printing machine (1) the registration error is continuously calculated and the adjustment of the printing machine (1) to correct the registration error is carried out only on request of the operator of the printing machine (1). Method according to one of the preceding claims, wherein the computer (2) determines the center position of each circular measuring mark (6) from the digital test pattern image in the first digital correction method and in the second digital correction method, and thus calculates the passer offset. Method according to one of the preceding claims, wherein the computer (2) determines the center position of each circular measuring mark (6) from the digital test pattern image by determining the center of mass of each circular measuring mark (6). Method according to one of the preceding claims, wherein the computer (2) determines the center position of each circular measuring mark (6) in the first digital correction method from the digital test pattern image by means of a subpixel-accurate, model-based parameter adjustment method. Method according to one of the preceding claims, wherein the computer (2) in step ii) decides that the first digital correction method completely corrects the passer offset if the deviation of the center positions of the circular measuring marks (6) from the specified target value falls below a tolerance value. Method according to one of the preceding claims, wherein, prior to recording the digital test pattern image, a calibration of the at least one image acquisition system (5) is performed using calibration points printed on the substrate (4). Method according to one of the preceding claims, wherein the at least one image acquisition system (5) is at least one inline camera of the printing machine (1). Method according to one of the preceding claims, wherein the circular measuring marks (6) are arranged on the substrate (4) such that they are completely captured by the at least one image acquisition system (5) and depicted in a single digital test pattern image. Method according to one of the preceding claims, wherein the circular measuring marks (6) for each color separation are selected from filled circular discs with a predetermined diameter and unfilled circular discs in the form of circular rings with a predetermined diameter. Method according to one of the preceding claims, wherein the circular measuring marks (6) are arranged in the pressure control strip on the substrate (4). Printing press, equipped for carrying out the method according to any one of claims 1 to 13.

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