Dampening solution control in a printing press

Abstract

Method for controlling the amount of dampening solution in the printing unit of a printing press using a computer and a measuring device, comprising the following process steps: - Adjusting the amount of dampening solution in the printing unit based on the data of the current print job, - Reduction of the dampening agent quantity towards the lubrication limit (LS) by the computer, - Acquisition of color measurement values ​​by the measuring device in at least one area with solid tone at low area coverage (FD) and at least one area with halftone at high area coverage (FD) on a substrate (1) of the current print job, - The computer stops reducing the amount of dampening solution if the dot gain of the halftone increases significantly in the high coverage area (FD) and the density of the solid tone increases significantly in the low coverage area (FD).

Description

[0001] The present invention relates to a method for controlling the amount of dampening solution in the printing unit of a printing press using a computer and a measuring device.

[0002] To achieve the correct color balance in the printing unit of offset printing presses, the dampening solution and printing ink must be precisely metered. The dampening solution should be metered so that as little dampening solution as possible is used, but the printing ink is not yet at the smearing limit. Currently, no functional sensors are known that can precisely measure the dampening solution content in the printing press. Measuring halftone values ​​in the printed image or in color measurement strips on the substrate using colorimeters has the disadvantage that the halftone value only changes once the smearing limit has already been reached, making smearing impossible to prevent.

[0003] European patent application EP 1 477 314 A1 discloses a method for controlling the dampening solution supply in offset printing presses. This method involves determining the ink density in specific patterns with different ink coverages. The ink coverages are calculated, which are then assigned corresponding amounts of dampening solution. In a second calculation step, coefficients are calculated that relate to the ink emulsion that matches the measured control patches. In a third step, the dampening solution quantity is then regulated by taking the ink coverage and the aforementioned coefficients into account when calculating the dampening solution quantity.

[0004] A method for adjusting the dampening system in an offset printing press is also known from patent application DE 10 2006 029 618 A1. This method uses a test form, from the impression of which the optimal dampening solution application rate and precise adjustment of the dampening system in the printing unit of an offset printing press can be derived. The test form has a mirror-image arrangement of two large-area screen gradients that extend essentially across the entire printing plate. The mirror plane runs perpendicular to the printing direction, and the screen gradients decrease up to the mirror plane and increase again from there towards the opposite edge. Here, too, screen fields with different area coverages are arranged. The dampening solution quantity is then adjusted by specifically changing the fountain roller speed.

[0005] The published patent application DE 10 2008 061 599 A1 relates to a method for adjusting a dampening system of a printing press, which has a fountain roller and a metering roller attached to it. This method uses a control system for the offset device and pressure device in the dampening system. Additionally, a measuring device for measuring the ink density and dot gain, as well as a device for evaluating the measurement, are used. The dot gain and ink density are measured in all measuring fields, and the fountain roller speed is subsequently reduced until the dot gain has increased above a predetermined limit in at least one of the measuring areas.The control system can also automatically adjust the smear limit, because if the measured dot gain has exceeded a threshold value, the speed of the fountain roller is increased until the dot gain has fallen below the threshold value again and the printed sheet is thus smear-free.

[0006] A method for controlling the ink / fountain solution balance in a rotary offset printing press is known from published patent application DE 100 58 550 A1. The ink quantity target value is changed in a closed control loop, with the ink coverage or ink density on the printing form or on the printing substrate being determined densitometrically or colorimetrically using a sensor. The ink quantity is increased or decreased to a desired ink quantity target value for the print job being processed, and a dampening solution target value is achieved in the same way by increasing or decreasing the dampening solution quantity target value. The supplied dampening solution quantity is controlled in the closed control loop depending on the dampening solution quantity target value. The dampening solution quantity target value can be increased or decreased simultaneously with the ink quantity increasing or decreasing to the desired ink quantity target value.

[0007] Color measuring devices are known from patents US 7,884,926 B2, US 7,894,065 B2 and US 7,515,267 B2.

[0008] However, the solutions mentioned have not proven successful in practice, as they do not allow for precise and sensitive dampening solution control due to the sluggish response of the screen zone values.

[0009] It is an object of the present invention to provide a method for controlling the amount of dampening solution in the printing unit of a printing press, which method allows a reliable and automatic control of the amount of dampening solution in the printing unit of the printing press.

[0010] This object is achieved according to the invention by patent claim 1. Advantageous embodiments of the inventions can be found in the dependent claims and the drawings. In the method according to the invention, printing substrates produced in the printing press are measured with a colorimeter and the color measurement values ​​are then fed to a computer which controls the dosage of the dampening solution in the printing unit of the printing press. In a first step, the correct dampening solution quantities for each printing unit are calculated based on the data of the current print job. This calculation takes into account the characteristics of the printing press stored in the computer, as well as the properties of the printing inks and printing substrates used. The print template for the current print job also plays a major role, in particular the coloring of the print image, which can contain more or fewer halftones or solid tones.However, the preset calculated for the current print job does not correspond to the optimal dampening solution setting in practice due to the failure to take into account boundary conditions such as temperature and humidity, and thus represents only a starting point. After the press starts up, the computer initially reduces the dampening solution quantity, starting from the calculated dampening solution quantity or an increased dampening solution quantity above the calculated dampening solution quantity, towards the so-called smear limit. The smear limit marks the point at which the dampening solution quantity is so low that the ink begins to smear on the substrates, thus producing unusable substrates.

[0011] During the reduction process of the dampening solution quantity towards the smear limit, color measurements are recorded by the colorimeter in certain predetermined areas on the printing substrate. On the one hand, measurements are taken in at least one area with a solid tone at low area coverage and in at least one area with a halftone at high area coverage in the printed image or color measuring strip. The color measurements determined in this way are then evaluated in the computer. The reduction in the dampening solution quantity is stopped by the computer when the dot gain of the halftone increases significantly in the area of ​​high area coverage and the density of the solid tone increases significantly in the area of ​​low area coverage. This criterion ensures that printing takes place with the lowest possible amount of dampening solution, but beyond the smear limit, which corresponds to the optimal setting of the dampening solution quantity in the production run.The area with high area coverage is chosen because there is a high dampening solution requirement in this area. Thus, reducing the dampening solution leads to a rapid increase in halftone tones and thus to a quickly measurable reaction on the substrate. The disadvantage of the area with the highest possible area coverage is that solid ink densities hardly change and thus cannot be measured precisely. For this reason, measurements are also taken in an area with low area coverage and a correspondingly low dampening solution requirement. Here, the halftones only increase slowly, but the solid ink density increases quickly, so that a corresponding reaction on the substrate can be quickly determined when the dampening solution is reduced.The optimal criterion is thus derived from the combined evaluation of the two areas, since the overall sensitivity is greatest here and thus the desired optimal setting of the dampening solution quantity can be best recorded.

[0012] In a first embodiment of the invention, the measuring device records color measurement values ​​in at least two screen tonal ranges on the printing substrate. In this case, color measurement values ​​are recorded in different screen tonal ranges, such as a 50% and a 75% screen tone. Advantageously, at least one screen tone has a tonal value density of 70 to 90%. The 75% screen tone, in particular, has proven to be a particularly advantageous measurement range. In these ranges, a particularly significant increase in the tonal value of the screen tone can be observed at high area coverage, so that the control can react particularly sensitively here.

[0013] In a further embodiment of the invention, when a print job changes, the amount of dampening solution required for the upcoming print job is calculated by the computer and initially increased slightly above the calculated value at the start of the print job. This increase ensures that the process always starts on the safe side, beyond the optimal dampening solution setting, when reducing the amount of dampening solution. Since every control system has a certain dead time and the printing press is generally a sluggish control system, the slight increase ensures that the first measured values ​​from the colorimeter are still recorded at a setting where the optimal dampening solution setting has not yet been undercut to the point of approaching the smear limit.

[0014] Advantageously, the computer also checks a print template for suitable color areas for the color measurement processes, determines the suitable color measurement areas in the printed image or color measurement fields in a print control strip, and carries out the measurements in these determined areas. In this case, the printing press operator does not have to select the appropriate areas for the colorimeter's measurements; instead, this is done automatically by the computer. The computer analyzes the digital print template, which is provided to it, for example, by prepress, for suitable color measurement areas in the printed image and in the color measurement strip. In particular, the computer determines the areas with solid tones with low area coverage and the areas with halftone tones with high area coverage.The areas thus determined are then transferred to the colorimeter, which then calculates the corresponding color measurement values ​​in the automatically determined areas and sends them to the computer. This ensures that the printer does not accidentally select the wrong measurement areas on the printing press and subsequently adjust the dampening solution dosage incorrectly.

[0015] It is also intended that the computer reduces the dampening solution quantity gradually, preferably in 3% steps, and that the areas are measured by the measuring device during the reduction process. The gradual reduction enables the effect of the dampening solution reduction to be reflected in the colored areas on the printing substrates, and that these areas can then be reliably recorded by the color measuring device. If the dampening solution quantity were to be reduced continuously, the dampening solution control could easily become unstable, since the dampening solution quantity would already have been reduced again by the time the printing substrates are recorded by the color measuring device. In particular, if the printing substrates are recorded on a color measuring device located outside the printing press, a corresponding period of time in which the dampening solution quantity does not change is required in order to achieve stable dampening solution quantity control.However, if the dampening solution reduction steps are too small, the dampening solution control process takes a very long time. If the step size is too large, the control cannot operate with sufficient precision, and the optimal dampening solution quantity may be undershot. A 3% reduction in the dampening solution quantity has proven to be the preferred step size.

[0016] It is particularly advantageous if the printing substrates are measured using a colorimeter on the printing press, as this allows the relevant areas to be continuously recorded at short intervals for evaluation based on the criteria stored in the computer. In this way, even small changes in the dampening solution quantity that affect the inking on the printing substrate can be detected by the colorimeter and transmitted to the computer for dampening solution control. This enables particularly fast and precise control, as the fulfillment of the criteria stored in the computer for setting the optimal dampening solution quantity above the smear limit can be recorded with particular precision.

[0017] In a further embodiment of the invention, the computer compares different color measurement patches with different dampening solution requirements in a plausibility check. By measuring multiple color measurement patches, for example, in a print control strip, but also in the printed image, it is possible to prevent individual outliers in the color measurement values ​​from preventing the criteria from being met too quickly or too early, thus leading to incorrect dampening solution levels.

[0018] Furthermore, a halftone patch is provided in the 25% halftone. This 25% halftone is also referred to as the highlight tone and shows another way of checking the dampening solution quantity setting process. Color measurements are recorded by the colorimeter in the 25% halftone, and the dampening solution reduction is only carried out until the color measurements drop. Only if the color measurements do not drop in the highlight tone during dampening solution reduction can it be assumed that the optimal dampening solution quantity has not yet been undercut. This provides additional security that the dampening solution quantity setting does not fall below the optimal dampening solution quantity, even if the print image has particularly difficult coloring and the corresponding areas that are checked in the computer to meet the criteria are not present in an appropriate manner.This prevents the dampening solution from being reduced too much and the optimal dampening solution quantity from being undercut, even with difficult print templates.

[0019] The present invention is described and explained in more detail below with reference to several figures. They show: Fig. 1 a printed image on a substrate with an area of ​​high area coverage and an area of ​​low area coverage, Fig. 2 the effect of dampening solution reduction in the printed image on the tonal value at an 84% screen tone and Fig. 3 the effect of dampening solution reduction on the color density in the solid tone.

[0020] In Fig. 1 shows a print sheet 1 which was printed in a printing press according to a print template with eight print copies. However, the eight copies of the print image 3 are colored differently. The four copies on the left have color coverage, while the copies on the right have low area coverage. This is because the print template for the left-hand copies in print image 3 contains dark motifs, while the right-hand copies in print image 3 contain light motifs. Print control strips 2 are attached around all of the copies of the print image 3 and contain color measurement patches that can be detected by a colorimeter. These print control strips 2 contain different color measurement patches with solid tones and halftones.

[0021] In Fig. 2 shows the relationship between the dosage of the dampening solution and the dot gain. Fig. Figure 2 illustrates the relationship using an 84% screen value as an example. The smear limit SG is located at a dampening solution dosage of approximately 38% and must not be exceeded during printing, as otherwise smearing effects will occur. The optimal setting of the dampening solution OK is located at Fig. 2 at approximately 56%. Two curves are shown, one for the left part of the image in Fig. 1 with a high area coverage FD and once for the right part of the image in Fig. 1 with a low dot area coverage (FD). The optimal setting of the dampening solution quantity (OK) is achieved when the dot gain of the 84% halftone tone increases significantly in the high dot area coverage (FD). Therefore, the curve with the high dot area coverage (FD) is relevant for this criterion, as only here are the effects on the 84% halftone tone in print image 3 quickly apparent. Fig. 2 shows that with an 84% halftone with high area coverage FD the dot gain increases significantly.

[0022] In Fig. Figure 3 shows the ink density plotted against the dampening solution quantity setting (Damp Poti), with measurements taken in solids. The relationship between high and low area coverage (FD) is also plotted here. It can be seen that at high area coverages (FD), the ink density (DV) in solids barely changes, so this cannot be used as a control parameter. However, at low area coverages (FD), it can be seen that, approaching from the right toward the optimal dampening solution quantity setting (OK), the ink density (DV) of the solid increases quickly and early.

[0023] If one now applies the criterion according to the invention and evaluates the curve of the high area coverage FD for the 84% halftone and the curve for the low area coverage FD for the solid tone, one arrives at the setting of 56% for the optimal dampening solution dosage OK. Fig. 2 and Fig. 3 shows that this is significantly away from the smear limit SG. The approach to the optimal dampening solution dosage OK is achieved by the control computer of the printing press first calculating the optimal dampening solution dosage based on the job data such as substrate, printing ink, and characteristics of the printing press. In addition, the dampening solution dosage is increased compared to this calculated value, so that the computer always doses the dampening solution quantity to the right of the optimal dampening solution dosage OK in the Fig. 2 and Fig. 3 starts. The amount of dampening solution is now reduced in small 3% steps and measured with a colorimeter on the substrates 1 in the corresponding areas until the tonal value curve of the 84% screen tone for high area coverage FD in Fig. 2 a significant increase occurs and in Fig. 3 for the solid ink density (DV) in the low area coverage curve (FD), a rapid or early increase also occurs. At this point, the dampening solution reduction is stopped, and the optimal dampening solution quantity (OK) is set in the respective printing unit of the press. List of reference symbols 1 printed sheet 2 print control strips 3 Print image FD area coverage DV color density gradient SG lubrication limit OK correct dampening solution setting

Claims

[1] Method for controlling the amount of dampening solution in the printing unit of a printing press using a computer and a measuring device, comprising the following method steps: - Setting the dampening solution quantity in the printing unit based on the data of the current print job, - Reduction of the dampening solution quantity towards the smear limit (SG) by the computer, - Recording of colour measurement values ​​by the measuring device in at least one area with solid tone at low area coverage (FD) and at least one area with halftone at high area coverage (FD) on a printing substrate (1) of the current print job, - Stop the reduction of the amount of dampening solution by the computer if the dot gain of the halftone increases significantly in the area of ​​high area coverage (FD) and the density of the solid tone increases significantly in the area of ​​low area coverage (FD). [2] Method according to claim 1, characterized bythat color measurement values ​​in at least two halftone areas on the printing material (1) are recorded by the measuring device. [3] Method according to one of the preceding claims, characterized by that at least one halftone has a tonal density between 70 and 90%. [4] Method according to one of the preceding claims, characterized by that when the print job changes, the amount of dampening solution required for the next print job is calculated by the computer and is initially increased slightly above the calculated value at the start of the next print job. [5] Method according to one of the preceding claims, characterized by that the computer checks a print template for suitable color areas for the color measuring processes, determines the suitable color measuring areas in the print image (3) or color measuring fields in a print control strip (2) and carries out the measuring processes in these determined areas. [6] Method according to one of the preceding claims, characterized by that the computer gradually reduces the amount of dampening solution and that during this reduction process the areas are measured by the measuring device. [7] Method according to one of the preceding claims, characterized by that the computer compares different color measurement fields with different dampening solution requirements in a plausibility check. [8] Method according to one of claims 2 to 7, characterized by that a grid field in 25% screen tone is provided. [9] Method according to claim 8, characterized by that color measurements are recorded by the measuring device in 25% halftone and the dampening solution reduction is only carried out until the color measurements drop. [10] Method according to one of the preceding claims, characterized by that the color measurement values ​​are recorded using the measuring device in the printing press during printing operation.

Images