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 inking of printing substrate sheets in an offset printing unit of a sheet-fed printing press and to a sheet-fed printing press for offset printing which is equipped for the method according to the invention.

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

[0003] During printing, the problem can arise that the pressure between the blanket cylinder and the impression cylinder is insufficient. This then manifests itself in a poor print where, for example, the substrate is not completely covered with ink in solid areas, but rather has microscopically small spots with little or no ink. This then results in an undesirable reduction in inking. The reasons for this can include deviations in the actual substrate thickness, the winding heights (i.e. the position of the blanket over the bearer rings), the positioning of the blanket and the compressibility of the substrate. To counteract these problems, the prior art usually increases the ink film thickness by using more printing ink, but this can be associated with problems. Such problems can include increased stencilling, emulsification, fogging or the like.

[0004] DE 10 2007 063 852 B3 describes a method for preparing and controlling print jobs in a sheet-fed printing press. Quality measurements from printed sheets are recorded using an optical acquisition unit, evaluated by a computer, and automatically corrected on the basis of these measurements. These can also include inking defects, which are corrected by changing the print setting. An incremental change in the print setting and an evaluation of inking values ​​using a mathematical model to optimize inking settings are not described.

[0005] The object of the present invention is to provide a method for optimizing the coloring of printing substrate sheets in sheet-fed offset printing presses. A larger number of printed printing substrate sheets with satisfactory flat coloring are obtained. The described method makes it possible to reduce the resulting waste.

[0006] This task is solved by a process in which the inking values ​​on the printing substrate sheets are continuously monitored by a colorimeter and, based on the measured inking values, the printing adjustment is adjusted after a computer-assisted evaluation so that the inking remains in the optimal range.

[0007] Accordingly, the invention relates to a method for optimising the inking of printing substrate sheets in an offset printing unit of a sheet-fed printing press, which comprises a blanket cylinder and an impression cylinder, wherein the blanket cylinder and the impression cylinder form a printing nip, in which a printing substrate sheet serving as a reference sheet is introduced into the printing nip, this is printed in the printing nip, after printing, the inking value on the printing substrate sheet serving as a reference sheet is determined in a printed area using a colour measuring device and, if necessary, the printing adjustment in the offset printing unit is changed, a printing substrate sheet following the printing substrate sheet serving as the reference sheet is introduced into the printing nip, this is printed in the printing nip and the inking value of the following printing substrate sheet is determined using the colour measuring device,whereby a computer determines and sets the optimal value of the print provision on the basis of the determined inking value of the printing material sheet serving as the reference sheet and the determined inking value of the subsequent printing material sheet.

[0008] According to the invention, the coloring of the printing substrate sheets is optimized by monitoring their coloring by determining the coloring values ​​using the color measuring device and controlling and, if necessary, adjusting the printing setting on the basis of an evaluation by the computer.

[0009] In a preferred embodiment, the method is carried out continuously during printing of substrate sheets. In a particularly preferred embodiment, the method is carried out continuously by treating each subsequent substrate sheet in the printing nip as a new substrate sheet serving as a reference sheet during each further printing operation. In a further preferred embodiment, instead of continuously measuring each individual substrate sheet in the offset printing press, one or more substrate sheets are omitted. In this preferred embodiment, the coloration value is accordingly determined by means of the colorimeter, for example, only on every second, every third or every fourth substrate sheet. However, it is particularly preferred to measure the coloration value by means of the colorimeter on each individual substrate sheet.

[0010] There are no particular restrictions regarding the materials used for the printing substrate sheets. All substrates commonly used in sheet-fed offset printing presses can be used. Paper sheets, cardboard sheets, or foil sheets are preferred as printing substrates for the process according to the invention.

[0011] According to the invention, the change in the printing pressure is achieved by changing the distance between the blanket cylinder and the impression cylinder in the offset printing unit. The change in the printing pressure is usually achieved electromechanically by actuators controlled by the computer.

[0012] In a preferred embodiment, the computer evaluates the determined coloring values ​​as a function of the associated printing settings using a mathematical model, determines the optimal printing setting by evaluating the mathematical model, and optimally adjusts the printing setting accordingly.

[0013] Thus, the optimal pressure setting is determined by mathematical analysis based on the coloring values ​​and pressure setting values.

[0014] An optimal print quality is defined as a print quality where the determined ink level does not increase further, or only increases to a negligible extent, with a further increase in the print quality and lies below a predetermined limit for the print quality value, which is determined by the technical design of the printing unit. The print quality is considered to be optimal if the ink level on the printing substrate increases only slightly with a further increase in the print quality and decreases significantly with a further decrease in the print quality.

[0015] In a preferred embodiment, the computer evaluates the determined inking values ​​as a function of the associated printing settings using a mathematical model, wherein the mathematical model is a curve and the optimal printing 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 the inking values ​​as a function of the printing setting. In a very particularly preferred embodiment, the curve is a regression curve. In a special embodiment, the mathematical model is a curve which describes the change in the inking values ​​as a function of the printing setting, wherein the curve is a regression curve and wherein the optimal value of the printing setting is the mathematical minimum of the regression curve.In a very special embodiment, the computer increases the optimal pressure adjustment value obtained from 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 adjustment to this slightly increased value above the theoretically determined optimal value. This serves to reduce sensitivity.

[0016] In a further preferred embodiment, the computer changes the pressure supply in equal steps whenever it is necessary to change the pressure supply. In a particularly preferred embodiment, the computer changes the pressure supply in equal steps in the range from 0.005 to 0.025 mm. In a very particularly preferred embodiment, the computer increases the pressure supply in equal steps in the range from 0.005 to 0.025 mm. In a special embodiment, the computer initially increases the pressure supply in equal steps in the range from 0.005 to 0.025 mm and then decreases the pressure supply in equal steps in the range from 0.005 to 0.025 mm.

[0017] In another preferred embodiment, the color measuring device is an inline color measuring device of the sheet-fed printing press. Unlike a handheld measuring device, the inline color measuring device 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.

[0018] In another preferred embodiment, the colorimeter determines the color value on the substrate sheet serving as the reference sheet in a print control strip. Print control strips for substrates are known in principle to those skilled in the art. The print control strip is preferably located outside the actual print image.

[0019] In a further preferred embodiment, the colorimetric values ​​determined by the colorimeter depend on the ink layer thickness on the printing substrate. Colorimeters for printing presses are known in principle to those skilled in the art. Spectrophotometers or densitometers are preferred colorimeters for the method according to the invention. Spectrophotometers measure the reflectance values ​​of the ink at different wavelengths on the printing substrate, while densitometers quantitatively determine the ink density on the printing substrate.

[0020] The invention further relates to a sheet-fed offset printing press configured to implement the method according to the invention. A printing press according to the invention preferably comprises at least one offset printing unit and an inline colorimeter. Figure 1shows a schematic of the process according to the invention. A printing substrate sheet (6) serving as a reference sheet 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). Using a colorimeter (7), the inking value (8) on the printing substrate sheet (6) was measured and passed on to a computer (11). The computer (11) determined the optimal printing nip (9) based on the measured inking values ​​(8) and the printing settings (9). Based on the inking value (8), the computer (11) then changed the printing setting (9) in the offset printing unit (1) if necessary. In a second step, a subsequent printing substrate sheet (10) is printed with the changed printing setting (9) in the printing nip (5). The two printing substrate sheets (6) and (10) are then discharged from the sheet-fed printing press (2).The process is then repeated for further subsequent printing sheets in the sheet-fed printing press (2). Figure 2 shows a diagram in which the pressure adjustment (9) is plotted 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 the measurement points. The minimum of the curve, i.e., the optimal value of the pressure adjustment (9), is represented by the large dot. List of reference symbols

[0021] 1Offset printing unit 2Sheet-fed printing press 3Blanket cylinder 4Impression cylinder 5Printing nip 6Reference substrate sheet 7Colorimeter 8Ink value 9Printing supply 10Subsequent substrate sheet 11Computer

Claims

1. A method for optimizing the inking of printing material sheets in an offset printing unit (1) of a sheet-fed printing press (2) comprising a blanket cylinder (3) and an impression cylinder (4), wherein the blanket cylinder (3) and the impression cylinder (4) form a printing nip (5), in which a printing material sheet (6) serving as a reference sheet is introduced into the printing nip (5), this is printed in the printing nip (5), after printing, the inking value (8) on the printing material sheet (6) serving as a reference sheet is determined in a printed area using a colorimeter (7) and, if necessary, the printing adjustment (9) in the offset printing unit (1) is changed, a printing material sheet (10) following the printing material sheet (6) serving as the reference sheet is introduced into the printing nip (5), this is printed in the printing nip (5), and the inking value (8) of the following printing material sheet (10) is determined using the colorimeter (7), characterized by thata computer (11) determines and sets the optimal value of the print provision (9) on the basis of the determined inking value (8) of the printing material sheet (6) serving as the reference sheet and the determined inking value (8) of the subsequent printing material sheet (10).

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

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

4. Method according to one of the preceding claims, wherein the change in the pressure setting (9) is the 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 coloring values ​​(8) as a function of the associated printing adjustments (9) by means of a mathematical model and determines and sets the optimal value of the printing adjustment (9) by evaluating the mathematical model.

6. The method according to claim 5, wherein the mathematical model is a curve and the optimal value of the pressure supply (9) determined by the computer (11) is the mathematical minimum of the curve.

7. The method according to claim 6, wherein the pressure setting (9) set by the computer (11) is increased by a value in the range of 0.015 to 0.025 mm compared to the determined optimal value of the pressure setting (9).

8. Method according to one of claims 5 to 7, wherein the curve describes the change in the coloring values ​​(8) as a function of the pressure setting (9).

9. The method according to any one of 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 pressure setting (9) in equal steps.

11. The method according to claim 10, wherein the computer (11) changes the pressure setting (9) in equal steps in the range of 0.005 to 0.025 mm.

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

13. Method according to one of the preceding claims, wherein the color value (8) on the printing material sheet (6) serving as a reference sheet is determined in a print control strip by means of the color measuring device (7).

14. Method according to one of the preceding claims, wherein the determined color values ​​(8) of the color measuring device (7) are dependent on the layer thickness of the ink on the printing material sheet.

15. Sheet-fed printing machine (2) for offset printing, arranged to carry out the method according to one of claims 1 to 14.

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