Method for optimizing coloration of printed sheets in offset printing

Abstract

The invention relates to a method for optimizing the coloration of a printing sheet in an offset printing unit of a sheet printing press, in which the coloration of the printing sheet is continuously monitored by determining a coloration value using a color photometer and the pressure feed is controlled and optionally adjusted on the basis of the evaluation of the coloration value by a computer.

Description

Technical Field

[0001] The invention relates to a method for optimizing the coloring of printed sheets in an offset printing unit of a sheet-fed printing press and to a sheet-fed printing press equipped for this method. Background Art

[0002] When printing on sheet-fed presses, it's important to achieve the most consistent print quality possible across the entire sheet, minimizing waste. To this end, print quality in offset presses is often monitored using optical image detection systems equipped with cameras. Another quality control method is colorimetry. These devices can be used as offline handheld devices or as inline (or online) colorimeters within the press.

[0003] During the printing process, problems may arise with insufficient pressure between the blanket cylinder and the impression cylinder. This can lead to poor print output, for example, solid areas of the substrate are not completely covered with ink, but instead have tiny spots with little or no ink. This can result in an undesirable reduction in coloring. The causes of this can be deviations in the actual thickness of the printed sheet, the height (i.e. orientation) of the blanket on the pillow ring, the positioning of the blanket, and the compressibility of the printed sheet. To solve these problems, the prior art usually increases the ink layer thickness by increasing the amount of ink, but this can cause some problems. These problems can be, for example, more severe stenciling, emulsification, atomization or similar problems.

[0004] DE 10 2007 063 852 B3 describes a method for preparing and controlling print jobs in a sheet-fed printing press. Quality measurements of the printed sheets are acquired by an optical inspection unit, evaluated by a computer, and based on these, correction settings are automatically made on the printing press. This can also include correcting coloring problems by varying the pressure feed. However, incremental changes in the pressure feed and optimization of coloring settings by analyzing coloring values ​​using mathematical models are not described. Summary of the Invention

[0005] The object of the present invention is to provide a method by which the coloring of printed sheets can be optimized in sheet-length offset printing presses. This allows for a greater number of printed sheets with a satisfactory and uniform coloring to be obtained. The described method also reduces the generation of waste.

[0006] This object is achieved by a method in which the coloring values ​​on the printed sheet are continuously monitored by a colorimeter and the pressure feed is adjusted based on the measured coloring values ​​after computer-assisted evaluation, so that the coloring remains within an optimal range.

[0007] Therefore, the present invention relates to a method for optimizing the coloring of a printing sheet in an offset printing unit of a sheet-forming printing press, the offset printing unit comprising a rubber blanket cylinder and an impression cylinder, wherein the rubber blanket cylinder and the impression cylinder form a printing gap, in which method a printing sheet serving as a reference sheet is introduced into the printing gap, the printing sheet is printed in the printing gap, after printing, the coloring value in the printed area on the printing sheet serving as the reference sheet is measured by a colorimeter and the pressure feed in the offset printing press is changed if necessary, then a subsequent printing sheet following the printing sheet serving as the reference sheet is introduced into the printing gap, the subsequent printing sheet is printed in the printing gap and the coloring value of the subsequent printing sheet is determined by a colorimeter, wherein a computer determines and sets (or sets) an optimal value for the pressure feed based on the determined coloring value of the printing sheet serving as the reference sheet and the determined coloring value of the subsequent printing sheet.

[0008] According to the invention, the coloring of the printed sheet is optimized by monitoring the coloring by determining the coloring values ​​using a colorimeter and controlling and, if necessary, adjusting the pressure feed based on a computer evaluation.

[0009] In a preferred embodiment, the method is carried out continuously while printing the printing sheet. In a particularly preferred embodiment, the method is carried out continuously in such a way that each subsequent printing sheet is treated as a new printing sheet used as a reference sheet in each further printing process in the printing gap. In a further preferred embodiment, not each individual printing sheet is measured continuously in the offset printing press, but one or more printing sheets are skipped. Accordingly, in this preferred embodiment, the color value is determined by means of a colorimeter, for example, only for every second, every third, or every fourth printing sheet. However, it is particularly preferred to measure the color value of each printing sheet by means of a colorimeter.

[0010] There are no particular restrictions on the material of the printing sheet. All printing materials commonly used in sheet-fed printing presses can be used. Preferably, the printing sheet used in the method of the present invention is a paper sheet, a cardboard sheet or a film sheet.

[0011] According to the invention, the change in the pressure feed involves a change in the distance between the blanket cylinder and the impression cylinder in the offset printing unit. The change in the pressure feed is usually performed electromechanically by means of an actuator (under computer control).

[0012] In a preferred embodiment, the computer evaluates the functional relationship between the obtained coloring value and the relevant pressure feed through a mathematical model, obtains the optimal pressure feed through analysis of the mathematical model, and sets the optimal pressure feed accordingly.

[0013] Therefore, the optimal pressure feed is obtained through mathematical analysis based on the coloring value and the pressure feed value.

[0014] Here, optimal feed pressure is understood to mean a feed pressure at which, with further increases in the feed pressure, the determined color value does not increase further or increases only negligibly and falls below a predefined threshold value for the feed pressure value predetermined by the technical design of the printing unit. Thus, if the coloring on the printing material increases only slightly with further increases in the feed pressure and decreases significantly with further reductions in the feed pressure, then this feed pressure is considered optimal.

[0015] In a preferred embodiment, the computer evaluates the relationship between the obtained coloring value and the related pressure feed through a mathematical model, wherein the mathematical model is a curve and the mathematical minimum of the curve is the optimal pressure feed. In a particularly preferred embodiment, the mathematical model is a curve, wherein the curve describes the relationship between the change in coloring value and the pressure feed. In a very particularly preferred embodiment, the curve is a regression curve. In a specific embodiment, the mathematical model is a curve describing the relationship between the change in coloring value and the pressure feed, wherein the curve is a regression curve and the minimum of the regression curve is the optimal pressure feed value. In a very specific embodiment, the computer increases the optimal pressure feed value obtained by the mathematical model by a value in the range of 0.01 to 0.03 mm, preferably a value in the range of 0.015 to 0.025 mm, and sets the pressure feed to a value slightly higher than the theoretical optimal value. This is used to reduce sensitivity.

[0016] In another preferred embodiment, the computer changes the pressure feed in equal steps when a change is required. In a particularly preferred embodiment, the computer changes the pressure feed in equal steps within a range of 0.005 to 0.025 mm. In a very particularly preferred embodiment, the computer increases the pressure feed in equal steps within a range of 0.005 to 0.025 mm. In a specific embodiment, the computer first increases the pressure feed in equal steps within a range of 0.005 to 0.025 mm and then decreases the pressure feed in equal steps within a range of 0.005 to 0.025 mm.

[0017] In another preferred embodiment, the colorimeter is an inline colorimeter for a sheet-fed printing press. Unlike handheld measuring devices, an inline colorimeter is permanently installed in the 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 color value of the printed sheet used as a reference sheet is determined by a colorimeter from the print control strip of the reference sheet. Print control strips for printed materials are generally known to those skilled in the art. These print control strips are typically located outside the actual printed image.

[0019] In another preferred embodiment, the color value determined by the colorimeter depends on the thickness of the ink layer on the printed sheet. Colorimeters for printing presses are generally known to those skilled in the art. The colorimeter used in the method of the present invention is preferably a spectrophotometer or a densitometer. A spectrophotometer measures the reflectance of ink at different wavelengths on the printed sheet, while a densitometer quantitatively measures the density of the ink on the printed sheet.

[0020] Another subject matter of the present invention is a sheet-fed printing press for offset printing, which is configured to carry out the method according to the invention. The printing press according to the invention preferably comprises at least one offset printing unit and an inline colorimeter. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A schematic flow chart showing the method of the present invention is shown;

[0022] Figure 2 Graph showing the measured change in coloration value as a function of the pressure feed 9. DETAILED DESCRIPTION

[0023] Figure 1 A schematic flow chart of the method of the present invention is shown. A printing sheet 6 serving as a reference sheet is printed in the press gap 5 formed by the blanket cylinder 3 and the impression cylinder 4 in the offset printing unit 1 of a sheet-fed printing press 2. Color values ​​8 on the printing sheet 6 are measured using a colorimeter 7 and transmitted to a computer 11. Based on the measured color values ​​8 and the pressure feed 9, the computer 11 determines the optimal pressure feed 9. Based on the color values ​​8, the computer 11 then changes the pressure feed 9 in the offset printing unit 1, if necessary. In a second step, a subsequent printing sheet 10 is printed in the press gap 5 using the changed pressure feed 9. The two printing sheets 6 and 10 are then discharged from the sheet-fed printing press 2. The method is then repeated for further subsequent printing sheets in the sheet-fed printing press 2.

[0024] Figure 2 This graph shows the relationship between changes in measured color values ​​and pressure feed 9. Small dots represent measurement points obtained using the colorimeter 7, and the dashed line represents a regression curve fitted to the measurement points. The minimum value of the curve, i.e., the optimal value of pressure feed 9, is represented by a large dot.

[0025] Reference Signs List

[0026] 1 offset printing unit

[0027] 2-sheet printing press

[0028] 3 blanket rollers

[0029] 4 Impression Cylinder

[0030] 5. Printing gap

[0031] 6 Sheets used as reference sheets

[0032] 7. Colorimeter

[0033] 8 shading values

[0034] 9 Pressure feed

[0035] 10 subsequent printing sheets

[0036] 11. Computer.

Claims

1. A method for optimizing the coloring of a printed sheet in an offset printing unit (1) of a sheet-fed printing press (2), the offset printing unit comprising a blanket cylinder (3) and an impression cylinder (4), wherein the blanket cylinder (3) and the impression cylinder (4) form a printing gap (5), wherein a printing sheet (6) serving as a reference sheet is introduced into the printing gap (5), the printing sheet is printed in the printing gap (5), after printing, a coloring value (8) is determined on the printing sheet (6) serving as the reference sheet in the printing region by means of a colorimeter (7), and the pressure feed (9) in the offset printing unit (1) is optionally changed, a subsequent printing sheet (10) following the printing sheet (6) serving as the reference sheet is introduced into the printing gap (5), the subsequent printing sheet is printed in the printing gap (5), and a coloring value (8) of the subsequent printing sheet (10) is determined by means of the colorimeter (7), It is characterized by: The computer (11) determines and sets the optimal value of the pressure feed (9) based on the determined coloring value (8) of the printing sheet (6) used as a reference sheet and the determined coloring value (8) of the subsequent printing sheet (10).

2. The method according to claim 1, wherein The method is carried out continuously while the printed sheets are being printed.

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

4. The method according to claim 1, wherein: The change in the pressure feed (9) is a change in the distance between the blanket cylinder (3) and the impression cylinder (4) in the offset printing unit (1).

5. The method according to any one of the preceding claims, wherein: The computer (11) evaluates the relationship between the determined color value (8) and the associated pressure feed (9) through a mathematical model and determines and sets an optimal value for the pressure feed (9) by analyzing the mathematical model.

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

7. The method according to claim 6, wherein: The pressure feed (9) set by the computer (11) is higher than the obtained optimal value of the pressure feed (9) by a value within a range of 0.015 to 0.025 mm.

8. The method according to any one of claims 5 to 7, wherein: The curve describes the dependence of the coloration value (8) on the pressure feed (9).

9. The method according to any one of claims 5 to 8, wherein: The curve is a regression curve.

10. The method according to any one of the preceding claims, wherein When the pressure feed (9) changes, the computer (11) changes the pressure feed in steps of the same size.

11. The method according to claim 10, wherein: When the pressure feed (9) is changed, the computer (11) changes the pressure feed in steps of the same size in the range of 0.005 to 0.025 mm.

12. The method according to any one of the preceding claims, wherein The colorimeter (7) is an online colorimeter of the sheet printing press (2).

13. The method according to any one of the preceding claims, wherein A colorimeter (7) is used to determine a color value (8) on a printed sheet (6) serving as a reference sheet in a printing control strip.

14. The method according to any one of the preceding claims, wherein The color value (8) determined by the colorimeter (7) depends on the thickness of the ink layer on the printed sheet.

15. A sheet-fed printing press (2) for offset printing, which is configured to carry out the method according to one of the preceding claims 1 to 14.

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