Method for optimizing the inking of printed sheets in offset printing

Abstract

The invention relates to a method for optimising the colouring of printing material sheets in an offset printing unit of a sheet-fed printing press, in which the colouring is continuously monitored by determining the colouring values ​​using a colour measuring device and the printing supply is controlled and, if necessary, adjusted on the basis of an evaluation of the colouring values ​​by a computer.

Description

[0001] The invention relates to a method for optimizing the coloring of printing sheets in an offset printing unit of a sheet-fed printing press and a sheet-fed printing press for offset printing, which is set up for the method according to the invention.

[0002] When printing with sheet-fed offset presses, it is desirable to maintain consistent print quality on the printed sheet to minimize waste. Print quality in offset presses is typically monitored by an optical image inspection system using a camera. Another method for quality control is measurement using colorimeters. Such devices can be used offline as handheld units or within the printing press as inline colorimeters.

[0003] During printing, a problem can arise where the pressure between the blanket cylinder and impression cylinder is insufficient. This results in poor print quality, where, for example, the substrate is not evenly inked in solid areas, but instead exhibits microscopically small spots with little or no ink. This leads to an undesirable reduction in color. The reasons for this include variations in the actual substrate thickness, the blanket height (i.e., the position of the blanket over the Schmitz rings), the blanket's positioning, and the compressibility of the substrate. To counteract these problems, the prior art typically involves increasing the ink layer thickness by using more ink, but this can lead to problems. Such problems can include increased stenciling, emulsification, fogging, or similar issues.

[0004] DE 10 2007 063 852 B3 describes a method for preparing and controlling print jobs in a sheet-fed printing press. In this method, an optical scanning unit captures quality measurements of printed sheets, which are then evaluated by a computer. Based on these measurements, automatic correction settings are made on the printing press. These corrections can also include inking defects, which are corrected by changing the ink settings. Incremental changes to the ink settings and the evaluation of inking values ​​using a mathematical model for optimizing inking settings are not described.

[0005] EP 2 087 996 B1 describes a method for transferring a transfer layer from a carrier film using a transfer gap, whereby control variables such as pressure application and contact force can be varied.

[0006] CN 103 687 727 A describes a device and a method for controlling the coloring in a printing process in which substrates are passed through a printing gap and the colorings of the substrates are detected by means of an optical sensor.

[0007] The object of the present invention is to provide a method for optimizing the coloring of printed sheets in sheet-fed offset printing presses. This results in a larger number of printed sheets with satisfactory, uniform coloring. The described method also makes it possible to reduce waste.

[0008] This task is solved by a process in which the color values ​​on the printed material sheet are continuously monitored by a color measuring device and, based on the measured color values, the printing quantity is adjusted after a computer-aided evaluation so that the color remains in the optimal range.

[0009] Accordingly, the invention relates to a method for optimizing the coloring of substrate sheets in an offset printing unit of a sheet-fed printing press, comprising a blanket cylinder and an impression cylinder, wherein the blanket cylinder and the impression cylinder form a printing gap, in which a substrate sheet serving as a reference sheet is inserted into the printing gap, printed in the printing gap, after printing the color value on the substrate sheet serving as a reference sheet is determined in a printed area using a colorimeter and, if necessary, the ink setting in the offset printing unit is changed, a substrate sheet following the substrate sheet serving as a reference sheet is inserted into the printing gap, printed in the printing gap, and the color value of the following substrate sheet is determined using the colorimeter.wherein a computer determines and sets the optimal value of the printing additive based on the determined color value of the reference sheet used as a reference sheet and the determined color value of the subsequent sheet of printed material, wherein the determined color values ​​of the color measuring device depend on the layer thickness of the ink on the sheet of printed material.

[0010] According to the invention, the coloring of the printed material sheets is optimized by monitoring their coloring through the determination of the color values ​​by the color measuring device and, based on an evaluation by the computer, controlling and, if necessary, adjusting the printing process.

[0011] In a preferred embodiment, the method is carried out continuously during the printing of sheets of substrate. In a particularly preferred embodiment, the method is carried out continuously by treating each subsequent sheet of substrate as a new reference sheet in the printing nip during each printing operation. In another preferred embodiment, instead of continuously measuring each individual sheet of substrate in the offset printing press, one or more sheets are omitted. In this preferred embodiment, the color value is determined using the colorimeter, for example, only on every second, third, or fourth sheet of substrate. However, measuring the color value using the colorimeter on every single sheet of substrate is particularly preferred.

[0012] There are no particular restrictions regarding the materials used for the substrate sheets. All substrates commonly used in sheet-fed offset printing presses can be used. Paper sheets, cardboard sheets, or film sheets are preferred as substrate sheets for the process according to the invention.

[0013] According to the invention, changing the printing pressure involves changing the distance between the blanket cylinder and the impression cylinder in the offset printing unit. This change in printing pressure is generally achieved electromechanically by actuators, controlled by a computer.

[0014] In a preferred embodiment, the computer evaluates the determined color values ​​as a function of the corresponding pressure settings using a mathematical model, determines the optimal pressure setting by evaluating the mathematical model, and adjusts the pressure setting accordingly. Thus, the optimal pressure setting is determined by mathematical analysis based on the color values ​​and pressure setting values.

[0015] Optimal pressure setting is defined as a pressure setting at which the measured color value does not increase further, or only to a negligible extent, with a further increase in pressure setting, and remains below a predetermined limit value for the pressure setting, which is determined by the technical design of the printing unit. The pressure setting is considered optimal when the color on the substrate increases only slightly with a further increase in pressure setting and decreases significantly with a further decrease in pressure setting.

[0016] In a preferred embodiment, the computer evaluates the determined color values ​​as a function of the corresponding pressure settings using a mathematical model, wherein the mathematical model is a curve and the optimal pressure setting is the mathematical minimum of the curve. In a particularly preferred embodiment, the mathematical model is a curve, wherein the curve describes the change in color values ​​as a function of the pressure setting. In a most particularly preferred embodiment, the curve is a regression curve. In a special embodiment, the mathematical model is a curve that describes the change in color values ​​as a function of the pressure setting, wherein the curve is a regression curve and wherein the optimal value of the pressure setting is the mathematical minimum of the regression curve.In a very specific embodiment, the computer increases the optimal pressure setting value obtained by the mathematical model by a value in the range of 0.01 to 0.03 mm, preferably by a value in the range of 0.015 to 0.025 mm, and sets the pressure setting to this slightly increased value above the theoretically determined optimal value. This serves to reduce the sensitivity.

[0017] In another preferred embodiment, the computer changes the pressure setting in equal increments when a change is necessary. In a particularly preferred embodiment, the computer changes the pressure setting in equal increments in the range of 0.005 to 0.025 mm. In a most preferred embodiment, the computer increases the pressure setting in equal increments in the range of 0.005 to 0.025 mm. In a special embodiment, the computer first increases the pressure setting in equal increments in the range of 0.005 to 0.025 mm and then decreases the pressure setting in equal increments in the range of 0.005 to 0.025 mm.

[0018] In another preferred embodiment, the colorimeter is an inline colorimeter of the sheet-fed printing press. Unlike a handheld device, the inline colorimeter is permanently installed in the sheet-fed printing press. It is preferably located after the last offset printing unit but before the delivery of the sheet-fed printing press.

[0019] In another preferred embodiment, the color value on the substrate sheet serving as a reference sheet is determined using a colorimeter in a print control strip. Print control strips for substrates are generally known to those skilled in the art. The print control strip is preferably located outside the actual printed image.

[0020] According to the invention, the color values ​​determined by the colorimeter depend on the layer thickness of the ink on the printed sheet. Colorimeters for printing presses are generally known to those skilled in the art. Spectrophotometers or densitometers are preferred as colorimeters for the method according to the invention. Spectrophotometers measure the reflectance values ​​of the ink at different wavelengths on the printed sheet, while densitometers quantitatively determine the ink density on the printed sheet.

[0021] Another object of the invention is a sheet-fed printing press for offset printing, configured to carry out the method according to the invention. A printing press according to the invention preferably comprises at least one offset printing unit and an inline color measuring device.

[0022] Figure 1Figure 1 schematically illustrates the process according to the invention. A reference sheet (6) was printed in the printing nip (5), which is formed by a blanket cylinder (3) and an impression cylinder (4) in the offset printing unit (1) of the sheet-fed printing press (2). The color value on the sheet-fed sheet (6) was measured using a colorimeter (7) and transmitted to a computer (11). Based on the measured color values ​​and the printing parameters, the computer (11) determined the optimal printing parameters. Based on the color value, the computer (11) then changed the printing parameters in the offset printing unit (1) if necessary. In a second step, a subsequent sheet-fed sheet (10) was printed in the printing nip (5) with the modified printing parameters. The two sheets-fed sheets (6) and (10) were then ejected from the sheet-fed printing press (2).The process is then repeated for further subsequent sheets of printing material in the sheet-fed printing machine (2).

[0023] Figure 2 Figure 1 shows a diagram plotting the pressure adjustment against the change in the measured color values. The small dots are the measurement points determined with the colorimeter (7), and the dotted curve is a regression curve fitted to these measurement points. The minimum of the curve, i.e., the optimal value of the pressure adjustment, is represented by the large dot. Reference symbol list

[0024] 1 Offset printing unit 2 Sheet-fed printing press 3 Rubber cylinder 4 Indent cylinder 5 Printing gap 6 Substrate sheet serving as reference sheet 7 Colorimeter 10 Following substrate sheet 11 Computer

Claims

1. Method for optimising the inking of substrate sheets in an offset printing unit (1) of a sheet-fed printing press (2), which comprises a blanket cylinder (3) and an impression cylinder (4), the blanket cylinder (3) and the impression cylinder (4) forming a printing nip (5), in which a substrate sheet (6) serving as a reference sheet is introduced into the printing nip (5) and printed in the printing nip (5), after printing, the colouring value on the sheet of substrate (6) serving as the reference sheet is determined in a printed area by means of a colour measuring device (7) and, if necessary, the impression pressure adjustment in the offset printing unit (1) is changed, a sheet of substrate (10) following the sheet of substrate (6) serving as the reference sheet is introduced into the printing nip (5), this sheet is printed in the printing nip (5) and the colouring value of the following sheet of substrate (10) is determined by means of the colour measuring device (7), characterized in that a computer (11) determines and sets the optimum value of the impression pressure adjustment on the basis of the determined colouring value of the substrate sheet (6) serving as reference sheet and the determined colouring value of the subsequent substrate sheet (10), the determined colouring values of the colour measuring device (7) being dependent on the layer thickness of the ink on the substrate sheet.

2. Method according to claim 1, wherein the method is carried out continuously during the printing of substrate sheets.

3. Method according to claim 2, wherein the method is carried out continuously by treating each subsequent substrate sheet (10) as a new substrate sheet (6) serving as a reference sheet in the printing nip (5) during each further printing operation.

4. Method according to one of the preceding claims, wherein the change in the impression pressure adjustment is a change in the distance between the blanket cylinder (3) and the impression cylinder (4) in the offset printing unit (1).

5. Method according to one of the preceding claims, wherein the computer (11) evaluates the determined colouring values as a function of the associated impression pressure adjustments by means of a mathematical model and determines and sets the optimum value of the impression pressure adjustment by evaluating the mathematical model.

6. Method according to claim 5, wherein the mathematical model is a curve and the optimum value of the impression pressure adjustment determined by the computer (11) is the mathematical minimum of the curve.

7. Method according to claim 6, wherein the impression pressure adjustment set by the computer (11) is increased by a value in the range from 0.015 to 0.025 mm compared to the determined optimum value of the impression pressure adjustment.

8. Method according to any one of claims 5 to 7, wherein the curve describes the change in the colouring values as a function of the impression pressure adjustment.

9. Method according to one of the claims 5 to 8, wherein the curve is a regression curve.

10. Method according to one of the preceding claims, wherein the computer (11) changes the impression pressure in equal steps when the impression pressure adjustment is changed.

11. Method according to claim 10, wherein the computer (11) changes the impression pressure adjustment in equal steps in the range from 0.005 to 0.025 mm when the impression pressure adjustment is changed.

12. Method according to one of the preceding claims, wherein the colour measuring device (7) is an inline colour measuring device of the sheet-fed printing press (2).

13. Method according to one of the preceding claims, wherein the colouring value on the substrate sheet (6) serving as reference sheet in a print control strip is determined by means of the colour measuring device (7).

14. Sheet-fed printing press (2) for offset printing, arranged for carrying out the method according to one of claims 1 to 13.

Images