Method for the automated definition of color test values

Abstract

A method sets the inking in a printing press with multiple ink zones in an inking unit of a printing unit. The method includes: printing a test form onto a printing substrate with an intentional pre-defined color deviation from the intended target color; correcting the previously set ink setting and the defined set color deviation with regard to the respective target value in the wet condition of the printing substrate by a color measuring device; and taking color measurements of the dried printing substrate using the color measuring device. The measured color values established in the dry state and the original of the test form in a computer are compared. The ink setting that belongs to the ink zone with the smallest deviation from the intended target color is used as the target ink setting for the printing process.

Claims

1. A method for adjusting ink settings in a printing press having a number of ink zones in an inking unit of a printing unit, which method comprises the steps of: printing a test form onto a printing substrate with a preset color deviation from an intended target color in at least one ink zone in the inking unit of the printing unit and with at least one other ink zone in the inking unit of the printing unit without any preset color deviation from the intended target color; correcting a previously set ink setting and the preset color deviation with respect to a respective target value in a wet condition of the printing substrate by means of a color measuring device; taking color measurements of a dried printing substrate by means of the color measuring device; comparing measured color values established in a dry state with an original of a test form in a computer; and selecting an ink setting of the ink zone that has a smallest deviation from the intended target color as a target ink setting for the printing process.

2. The method according to claim 1, which further comprises measuring the printing substrate by the color measuring device outside the printing press.

3. The method according to claim 1, which further comprises measuring the printing substrate by the color measuring device inside the printing press.

4. The method according to claim 1, which further comprises calculating and setting defined color deviations by the computer.

5. The method according to claim 4, which further comprises modifying target color values in the ink zones in steps in the computer to set the defined color deviations.

6. The method according to claim 4, wherein the number of ink zones to which the defined color deviations are applied is selectable by means of the computer.

7. The method according to claim 6, wherein an initial ink zone that is without defined color deviations is selectable by the computer.

8. The method according to claim 7, which further comprises invariably storing the number of the ink zones with the defined color deviations and the initial ink zone without defined color deviation in the computer.

9. The method according to claim 7, wherein the initial ink zone without the defined color deviations is in a center of the ink zones in the inking unit of the printing unit and wherein on one side of the initial ink zone, the defined color deviations are positive color deviations and on the other side of the initial ink zone the color deviations are negative color deviations.

10. The method according to claim 4, wherein the defined color deviations in the ink zones progress in two-percent steps.

11. The method according to claim 1, wherein once a wet printing substrate has been compensated for, storing a set of actual color values of the ink zones in a file in the computer.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) Referring now to the single FIGURE of the drawing in detail, the FIGURE illustrates an implementation of a method of the invention in a sheet-fed rotary lithographic offset printing press 101. It is to be understood that the method may be used in other lithographic offset printing presses. The sheet-fed printing press 101 in the FIGURE has four printing units 12 of essentially identical construction. Each printing unit 12 includes an inking unit 16 that is supplied with ink via an ink fountain 11 and evenly distributes the ink across the rollers in the inking unit 16 before applying it to a plate cylinder 13 carrying the printing form. The plate cylinder 13 in turn transfers the ink to the blanket cylinder 14. The blanket cylinder 14 and an impression cylinder 15 form a printing nip of the printing unit 12. Sheet-shaped printing substrates 20 are passed through the printing nip in each printing unit 12 to receive the respective color separation. In the printing press 101 in the FIGURE, four color separations may in this way be applied on top of each other in a straight printing mode. Alternatively, two color separations may be applied in the perfecting mode using reversing drums 150 provided between the printing units 12 to turn the sheets 20 from the front side to the back so that they may be printed on both sides. The printing substrates 20 are removed from a feeder stack 131 in the feeder 130 and transferred to the first printing unit 12 by a sheet feeding device 120. Once the sheets 20 have been printed and exited the last printing unit 12, they are transferred to the delivery 140 and deposited on a delivery stack 141.

(2) The printing press 101 is controlled by a press computer 160, which has a screen 170 for displaying the current condition of the machine. The press computer 160 is connected to a power supply 6 of the press 101 and controls all drives and actuators of the printing press 101. The press computer 160 is further connected to an in-line measurement device 24 provided downstream of the last printing unit 12 and to an external color measuring device 21. The external color measuring device 21 consists of a sheet deposit 23 for receiving a printed sheet and a movable measuring bar scanning the printed sheet 20. To generate color measurements, the measuring devices 21, 24 either scan the entire surface of the produced sheets 20 or at least a color measurement strip 22 printed on the side. As the in-line measurement device 24 is arranged immediately downstream of the last printing unit 12, the sheets 20 are still wet when they are scanned by the in-line measurement device 24, which thus provides only current actual color values in the wet state, whereas the external color measuring device 21 may also measure dry printing substrates 20.

(3) In accordance with the method of the invention, a test form is printed with different inking levels in the press 101. For this purpose, corresponding printing plates for the printing unit 12 are exposed by a plate exposure unit and mounted to the printing press 101. Once a number of printing substrates 20 have been produced using these printing forms and the corresponding ink settings in the respective printing unit 12, the wet printing substrates 20 are measured either in the press 101 using the in-line color measuring device 24 or outside the press 101 using the external color measuring device 21. The printing substrates 20 are produced in accordance with steps stored in the computer 160, i.e. under-inking and over-inking occurs in two-percent steps every two ink zones to the left or right of the central two ink zones. Thus two ink zones have the same inking level in the color measurement strip 22 on the printing substrate 20. The target color values defined in this way are then compensated by the control computer 160 with the aid of the wet-state actual color values measured by the color measuring devices 21, 24. As soon as the printing press 101 is in the corrected state, a sample sheet 20 is taken and the actual ink zone color values are stored by the control computer 160 in terms of the respective ink zone and color and, if desired, including additional data such as the job title, date, time of the measurement, color separation, number of the ink zone etc.

(4) When the printed sheet 20 that has been corrected in the wet state has dried, a dry-state measurement is taken by the external color measuring device 21. If the in-line color measuring device 24 and the external color measuring device 21 have been calibrated relative to each other prior to the measurement, this may be done even if the printed sheet has been measured in the wet state by the in-line color measuring device 24. Like the measured actual wet-state color values, the actual color values measured in the dry state are likewise stored in a corresponding way, ink zone by ink zone. Subsequently, the press computer 160 carries out an automated comparison in which the measured dry-state color values and the measured wet-state color values are compared, factoring in process parameters such as dot gain, gray balance, slurring, doubling etc. Then the measured actual dry-state value is selected that has the smallest deviation from the target dry-state color value, i.e. from the measured color value of the original. Subsequently, that ink setting of the gradual over-inking or under-inking that corresponds to this established measured dry-state color value is selected as the correct setting by the control computer 160.

(5) The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention: 6 power supply 11 ink fountain 12 printing unit 13 plate cylinder 14 blanket cylinder 15 impression cylinder 16 inking unit 20 sheet 21 external color measuring device 22 color measurement strip 23 sheet deposit 24 in-line color measuring device 101 printing press 120 sheet feeding device 130 feeder 131 feeder stack 140 delivery 141 delivery stack 150 reversing drum 160 press computer 170 screen