Method of compensating for printing substrate deformation in multicolor duplex printing

Abstract

A method for compensating for printing substrate deformation in a multicolor duplex printing operation by way of a computer. The substrate deformation that arises when individual color separations are printed onto the first and second sides of the printing substrate in the printing units is established and compensated for in the course of an automated register correction process. The computer calculates a forecast of the substrate deformation on a basis of the established first side deformation parameters of the printing substrate. When further color separations are printed onto the second side the forecast is used as an input parameter for a compensation of the substrate deformation of the second side.

Claims

1. A method for compensating for printing substrate deformation in a multicolor duplex printing operation with a plurality of printing units in a printing machine, the method comprising: printing individual color separations onto a first side of a printing substrate; in a course of an automated register correction process, measuring register marks printed in a previous printing unit with at least one image sensor in a respectively following printing unit to establish deformation parameters of the first side of the printing substrate; compensating for the substrate deformations based on the established deformation parameters of the first side of the printing substrate; calculating with a computer a substrate deformation of a last printing unit that prints on the first side based on the established substrate deformation values of the previous printing units; and calculating with a computer a forecast for a substrate deformation that will occur when further color separations are printed onto the second side, including a parameter representing the deformation of the last printing unit on the first side, and using the forecast as an input parameter for a compensation of the substrate deformation of the second side.

2. The method according to claim 1, wherein the step of calculating of the substrate deformation of the last printing unit printing on the first side by means of the computer is attained by extrapolating the substrate deformation that has been established for the previous printing units.

3. The method according to claim 2, which comprises, for extrapolating, converting the register deviations caused by the established substrate deformation of the previous printing units to a standardized area coverage, and setting off the standardized area coverage against actual area coverage values of the color separation in the last printing unit.

4. The method according to claim 1, which comprises, in a calculation of the input parameter for the compensation of the substrate deformation of the second side, factoring in a type of reversal of the printing substrate including reversal along a length and reversal along a width of the printing substrate.

5. The method according to claim 1, which comprises, when the printing substrate is a pre-treated substrate, calculating any substrate deformation that is caused by a pre-treatment and using as an input parameter for calculating the substrate deformation of the first side in the course of the printing operation.

6. The method according to claim 5, wherein the pre-treatment of the printing substrate is at least one of a hot-foil stamping operation and an application of a foil to the printing substrate.

7. The method according to claim 1, which comprises printing in an offset printing process and determining the substrate deformation as a substrate expansion.

8. The method according to claim 1, which comprises printing in an inkjet printing process and determining the substrate deformation as a substrate shrinkage.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 is a schematic representation of the fan-out phenomenon;

(2) FIG. 2 illustrates the result of a local register deviation; and

(3) FIG. 3 is a schematic flow chart of the method according to the invention.

(4) In the drawings, mutually corresponding elements have the same reference symbols.

DETAILED DESCRIPTION OF THE INVENTION

(5) Referring now to the figures of the drawing in detail and first, particularly, to FIGS. 1 and 2 thereof, register deviations 4 that occur in a printing operation and need to be compensated for are mainly caused by paper deformations. They are schematically shown in FIG. 2 in the form of a register cross 3 that is used to measure the accuracy of the placement of the different color separations relative to one another. The example shown in FIG. 2 clearly shows the deviations 4 of the individual color separations that occur when there are local register deviations. The fact that these local register inaccuracies may be caused by process-inherent deformations of the printing materials is shown in FIG. 1, which illustrates an example of how paper may expand due to the fan-out phenomenon. FIG. 1 clearly shows the deforming forces 2 that occur in a direction perpendicular to the printing direction and the deforming forces 1 that occur along the printing direction. However, since the deformation forces influence one another, as a result, there are deformations in all directions 1 and 2. These deformations 1 and 2 need to be factored in to obtain high-quality prints. For this purpose, a prior art method for paper expansion compensation for simplex printing operations may be carried out by a control unit 8.

(6) FIG. 3 schematically illustrates a preferred exemplary embodiment of the method of the invention for compensating for paper deformations that occur in duplex printing. The preferred embodiment will be explained in more detail below based on an example of a multicolor sheet-fed lithographic offset printing operation. In this case, the printed printing substrate is a print sheet 6. Naturally, the first step is to complete the printing operation on the first side. In this process, a number of printing units print the print image data in the form of individual color separations coming from the print preparation department 5. The expansion of a printed print sheet 6 occurs when the sheet 6 leaves the printing unit. The result is a register deviation 4 between two units as a measure of the expansion of the sheet caused by the previous printing unit. By way of example, the following section assumes a standard color sequence B, C, M, Y for a four-color printing operation.

(7) B=black is the color of reference. An example of a set of measurements (in mm) is

(8) TABLE-US-00001 register deviation in the printing unit circumferential direction C 0.1 M 0.2 Y 0.3

(9) The black printing unit has thus elongated the paper by 0.1 mm, the cyan printing unit by 0.2 mm, etc.

(10) The longitudinal expansion caused by Y cannot be determined by register measurements because Y is the last color separation. However, printing experiments have shown that the printing units behave approximately alike. To minimize constructional tolerances, an average of the previous units may be assumed as the longitudinal expansion 1 of the last printing unit, which results from the following formulas:
l(n)=l(n)l(n1)
establishes the contribution by an individual unit n.

(11) $\ln =\frac{1}{n-1}\underset{0}{\overset{n-1}{.Math.}}l\left(n\right)$
is an estimated expansion in the last printing unit.

(12) $\mathrm{Lges}=\frac{1}{n-1}\underset{0}{\overset{n-1}{.Math.}}l\left(n\right)+\underset{0}{\overset{n-1}{.Math.}}l\left(n\right)$
is the total expansion 1 of the sheet 6.

(13) This analogously applies to the expansion in a lateral direction 2: Bges

(14) In this way, the offset relative to an undeformed sheet may be determined for every point 4 whose register deviation is known:

(15) $P\left(\begin{array}{c}x1\\ y1\end{array}\right)=P\left(\begin{array}{c}x0\\ y0\end{array}\right)+\left(\begin{array}{c}B\mathrm{ges}\\ \mathrm{Lges}\end{array}\right)$

(16) Then, depending on the type of the printing operation, the image on the second side needs to be pre-distorted by a starting value 7. In this context, the type of the printing operation above all refers to the way in which the sheet 6 is turned.

(17) Depending on the printing system, the sheet 6 may be turned along its length or along its width. When the sheet is turned along its width, the value of the left-hand upper corner of the first side needs to be assumed as the pre-distortion for the upper right-hand corner of the image. When the sheet is turned along the length, the lower right-hand corner needs to be adapted in a corresponding way.

(18) Thus the transformation rule for any point

(19) $P\left(\begin{array}{c}\mathrm{xB}\\ \mathrm{yB}\end{array}\right)$
on the second side of the sheet is

(20) $P\left(\begin{array}{c}\mathrm{xB}\\ \mathrm{yB}\end{array}\right)=P\left(\begin{array}{c}\mathrm{xB}\\ \mathrm{yB}\end{array}\right)+\left(\begin{array}{c}B\mathrm{ges}\\ \mathrm{Lges}\end{array}\right)*\left(\begin{array}{c}0\\ -1\end{array}\right)\mathrm{turning}\mathrm{along}\mathrm{the}\mathrm{width}$$P\left(\begin{array}{c}\mathrm{xB}\\ \mathrm{yB}\end{array}\right)=P\left(\begin{array}{c}\mathrm{xB}\\ \mathrm{yB}\end{array}\right)+\left(\begin{array}{c}B\mathrm{ges}\\ \mathrm{Lges}\end{array}\right)*\left(\begin{array}{c}-1\\ 0\end{array}\right)\mathrm{turning}\mathrm{along}\mathrm{the}\mathrm{length}$

(21) This model may be used to establish the starting parameters 7 for a compensation of printing substrate deformations that occur when the second side of the sheet 6 is printed on. The substrate deformations 1, 2 that have occurred when the first side received the print may thus be taken into consideration for a compensation process when the second side receives a print. The invention thus represents a significant improvement in terms of the efficiency of the compensation for duplex printing.

(22) The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention: 1 substrate expansion along the printing direction 2 substrate expansion perpendicular to the printing direction 3 register cross 4 register cross in the case of deviated color separations 5 image data from the print preparation department 6 printed printing substrate 7 starting parameters for second side substrate expansion 8 computer