EP0490093A1 - Method for controlling inking of printed products - Google Patents

Abstract

The invention relates to a method for controlling inking of printed products which are produced on a printing machine operating by autotypy, in particular a sheet-fed offset printing machine. Also printed on the print sheet is a print control strip, which contains a measuring field of specific halftone value for each ink to be printed in each ink metering zone and, additionally, a full-tone measuring field in at least one ink metering zone. If the inking is to be tuned to a prescribed desired area coverage (FDSoll), the Murray-Davies formula is used to calculate desired halftone ink densities (DRN1Soll, DRM1Soll) from the full-tone ink densities (DVN1, DVM1) measured on the full-tone measuring fields. If more than one full-tone measuring field is present in the print control strip, it is possible, on the one hand, to average over these desired halftone ink densities (DRN1Soll, DRM1Soll) calculated in this way, the mean value then being used as desired halftone ink density for all ink metering zones to be controlled. On the other hand, it is possible if more than one full-tone measuring field is present in the pressure control strip to average over the measured full-tone densities (DV), and then to use the Murray-Davies formula to calculate a new desired halftone ink density value. Thus, in each control operation new desired halftone ink densities are calculated, and the ink metering zones are controlled accordingly.

Description

The invention relates to a method for controlling the coloring of printed products according to the preamble of the first claim.

From the field of sheetfed offset printing, it is known to regulate the coloration of printed products either according to solid color density or screen color density. For this purpose, a print control strip is printed on the sheet, which contains either halftone or full-tone measuring fields for each color metering zone. Target color densities are specified for these measuring fields. During printing, sheets are drawn and the color densities of these measuring fields are determined by means of appropriate devices, whereupon the ink metering zones are automatically regulated until the measured color densities and the predetermined target color densities are the same within a predetermined tolerance range. However, it is not possible to transfer target color densities (screen tone / full tone) from a template that was produced under different printing conditions. The reason for this is that ink density values are meaningful only for an ink-substrate combination.

DE 34 40 706 A1 discloses a method for controlling the coloring of printed products, in which the area coverage achieved in printing is compared to a target area coverage (template) and target color densities are formed from this comparison (the difference). The actual area coverage for a color is determined densitometrically using at least one pair of measuring fields in the print control strip, consisting of a halftone and full-tone measuring field. Using the formula of Murray-Davies then determines the actual area coverage from the color density of the solid-tone and that of the halftone measuring field. New target color densities for screen tone and full tone are then formed from the target-actual area coverage deviation. The ink metering zones are then regulated in accordance with these new target ink densities. The print control strip contains halftone measuring fields in each color metering zone. By means of this method, which is described in more detail in the document mentioned at the outset, nominal values, namely the nominal values of the area coverings, can be adopted directly by measurement from an OK sheet (pressure) produced by another printing method.

A disadvantage of the above-described method according to the document cited at the outset, however, is that a linear dependency between the target-actual differences to be determined of the solid or halftone color densities and the target-actual difference of the area coverage is assumed. The corresponding conversion factors must be determined empirically and known very precisely. An error with which these factors are known does not affect the control accuracy - if the target-actual difference of the area coverings becomes zero, then the target-actual differences of the corresponding color densities also become zero - however, how Explicitly stated in this document, imprecise knowledge of the conversion factors can cause over- or under-regulation. Especially in the set-up phase, in which the color scheme is matched to a target color, several control steps are necessary, in which a corresponding amount of waste is incurred. Additional control steps are then caused by the over- or under-regulation, caused by the imprecise knowledge of the conversion factors.

The object of the present invention is therefore to provide a method for controlling the coloring of printed products, by means of which the setting up of the ink guide to a desired area coverage is achieved with a minimum of control steps.

This object is achieved by the characterizing features of the first claim. Advantageous embodiments of the method according to the invention result from the subclaims in connection with the description.

The invention can be used in the entire area of autotypically operating printing presses, that is to say it can be used in sheet-fed and web offset printing presses, sheet-fed and web presses in gravure printing, etc.

The method according to the invention is now explained in more detail below in connection with FIG. 1, FIG. 1 showing the relationship between area coverage and full tone / screen color density. The color guide is set up on a multi-color sheetfed offset printing machine according to a predetermined target area coverage FD _target for each color. A print control strip is also printed on each print sheet, which has measurement fields with a screen tone value of, for example, 80% for all colors in each color metering zone. In addition, this print control strip also has a full tone measuring field for each color in at least one color metering zone. The color densities are measured with a traversing densitometer, as known from DE-AS 27 28 738. An example of the print control strip described is the 6-color CCI print control strip from MAN Roland Druckmaschinen AG.

At the start of the setup, no target color densities - here screen color densities - are specified. There is only a target area coverage FD _target for each color. The ink metering elements in each inking unit are preset by known methods, for example by values which resulted in a printing plate scan. Printing is then started and, after establishing a steady state of coloring, a first sheet is drawn and measured. Control processes for a color, for example black, are also described. The same procedure is followed for the other colors.

The first measurement showed, for example, the actual full-tone color densities DV _N1 and DV _M1 on the full-tone measuring fields in the color metering zones N, M (FIG. 1). 1, as can be seen, is modeled on the Fogra network table, in which the curves of the area coverings are plotted according to the Murray-Davies formula in the solid color density (DV) - halftone color density (DR) plane. Actual area coverage FD _N1 , FD _{M1 is achieved} in points N1, M1 with the halftone color densities associated with the solid color densities DV _N1 , DV _M1 . In the example according to FIG. 1, the actual area coverage FD _N1 achieved in ink metering zone N is smaller, and the actual area coverage FD _N1 achieved in ink metering zone M is greater than the predetermined target area coverage FD _Soll .

${\text{DR}}_{\text{N1 Soll}}{\text{= - lg (1 - FD}}_{\text{Soll}}{}^{\text{·}}{\text{(1 - 10}}^{\text{⁻ DV}}\text{M1))}$

wobei hier für das Papierweiß bzw. den reinen Bedruckstoff eine Farbdichte von Null angenommen wird. Der allgemeine Ausdruck für diese Formel ergibt sich, wenn die jeweils rechts neben der Klammer stehende "1" durch 10^-DP ersetzt wird, wobei DP die Farbdichte des Papierweiß bzw. des Bedruckstoffes ist.
Für alle Farbdosierzonen wird nun eine Soll-Rastertonfarbdichte gewählt, welche das aritmethische Mittel aus DR_{N1 Soll} und DR_M1Soll ist. Entsprechend dieser Soll-Rastertonfarbdichte werden somit alle Farbdosierzonen geregelt.By means of the solid color densities DV _N1 , DV _M1 measured in the color metering zones N and M and the predetermined target area coverage FD _target , _target values for the screen color densities DR _{N1 target} , DR _{M1 target are} formed using the Murray-Davies formula. In terms of formulas, the following applies to these target screen color densities:

${\text{DR}}_{\text{N1 target}}{\text{= - lg (1 - FD}}_{\text{Should}}{}^{\text{·}}{\text{(1-10}}^{\text{⁻ DV}}\text{N1)) or}$

${\text{DR}}_{\text{N1 target}}{\text{= - lg (1 - FD}}_{\text{Should}}{}^{\text{·}}{\text{(1-10}}^{\text{⁻ DV}}\text{M1))}$

whereby a color density of zero is assumed for the paper white or the pure printing material. The general expression for this formula is obtained when the "1" to the right of the parenthesis is replaced by 10 ^-DP , where DP is the color density of the paper ^white or the substrate.
A target raster color density is now selected for all color metering zones, which is the arithmetic mean of DR _{N1 target} and DR _{M1 target} . All ink metering zones are thus regulated in accordance with this target raster tone color density.

If, as in the previous explanatory example in the print control strip, there are not only two additional full-tone measurement fields, but generally n, then n full-tone color densities DV and the specified target area coverage FD _Soll n are used to form n target screen-tone color densities using the Murray-Davies formula. It is then arithmetically averaged over these n target raster tone color densities. The arithmetic mean value obtained then serves as the target screen tone color density for all ink metering zones, which are regulated accordingly.

The first regulation when setting up will generally not lead directly to the target value of the area coverage FD _target , but only approximate it. A second measurement on the full tone measuring fields - again in the ink metering zones N and M - will lead to points N2, M2 in the diagram according to FIG. 1. The area coverage now achieved in the ink metering zones N, M has approximated the target area coverage FD _target . In a second control step, the procedure is now the same as before. From the spot color densities now measured and the predetermined target area coverage FD _target , target raster color tone densities are again calculated using the Murray-Davies formula. These are again averaged and the mean value obtained in this way is the target screen color density for the other color metering zones.

The method according to the invention is therefore used iteratively, with new set-tone color densities being determined in each control step from the measured solid color densities and the predetermined target area coverage. With the method according to the invention the greatest possible approximation of the area coverage achieved in printing to the target area coverage FD _{Soll is} achieved, since an increase / decrease in the halftone color densities brings about an increase / decrease in the corresponding solid color densities by appropriate adjustment of color metering elements.

As an alternative to the procedure described above, according to which from general n spot color densities in the corresponding Color dosing zones are first calculated n target screen color densities using the Murray-Davies formula and then an arithmetic mean is determined from these n target screen color densities can also be arithmetically averaged over the n measured solid color densities. A new target screen color density value is calculated from the mean value of the solid color density obtained in this way, by specifying the target area coverage FD _target and using the Murray-Davies formula. This target screen tone color density value then applies as the target specification for all ink metering zones. These are regulated accordingly. In this alternative, the Murray-Davies formula is used only once for n full-tone measuring fields in the print control strip.

However, the method according to the invention described so far cannot only be used when setting up the color guide. After the target coloring (target area coverage) has been reached, production continues. Sheets are also obtained and measured at regular intervals in production printing. As long as the area coverage determined on this arch does not deviate too much from the target area coverage, the color guidance is not changed. However, if a larger deviation is determined, then how new target screen color densities are determined from the measured solid color densities in the setup phase is regulated accordingly.

Claims (3)

Method for controlling the coloring of printed products which are produced on an autotypically working printing machine, in particular sheetfed offset printing machine, wherein a print control strip printed on each printed product has screen tone measuring fields for each color in each color metering zone with a specific screen tone value and additionally a full tone measuring field for each color in at least one color metering zone , these measuring fields are scanned densitometrically, the color metering elements in the color metering zones are adjusted according to a target-actual comparison of the screen tone color densities, the target values for the screen tone color densities being selected such that the area coverage achieved in the print is approximated to a predetermined target area coverage,
characterized by
that the target screen color density DR _{Soll is} calculated after each measurement process from the solid color densities DV of the full tone measuring field (s) and from the specified target area coverage FD _Soll using the Murray-Davies formula:

${\text{DR}}_{\text{Should}}{\text{= - lg (1 - FD}}_{\text{Should}}{}^{\text{·}}{\text{(1-10}}^{\text{-DV}}\text{)).}$

Method according to claim 1,
characterized by
that n target screen color densities are calculated for n full tone measuring fields present in the print control strip, that the arithmetic mean value is calculated from these n target screen color densities and this is then specified as the target screen tone color density for the control of all color metering zones. Method according to claim 1,
characterized by
that the arithmetic mean of the n measured full-tone color densities is calculated for n full-tone measuring fields present in the print control strip and that a target screen-tone color density is calculated from this mean value, which is specified as a common target screen-tone color density for the control of all color metering zones.

Images