EP0282446B1 - Method for continuously controlling inking in an intaglio or flexographic process, and corresponding printing machine - Google Patents

Description

The invention relates to a method for regulating the production of the coloring in gravure or flexographic printing according to the preamble of claim 1.

In gravure printing and also in flexo printing, the color concentration (relative composition of the printing inks from color concentrate, waste and solvent) is one of the most important parameters that must be checked. It has a decisive influence on the color of the process and the color and tonal quality of the printed product. The quality is still assessed practically exclusively by the eye and with the help of hand densitometers.

For decades there has been no lack of attempts to control the rotogravure machine directly. However, they could not prevail in practice, even though the problem itself is much simpler (only longitudinal color fluctuations) than with the color control for offset printing already installed today (additional transverse color fluctuations).

Known solutions (e.g. DE-B-24 10 753) have the disadvantage that they are based on the control of the single-color strength (in remission or density) of individual color patterns, which do not allow a satisfactory price-performance ratio with the already high quality standard of rotogravure printing and also with special completely fail critical colors in terms of accuracy.

Various methods for controlling the printing process based on colorimetric measurements are specified in EP-A-0 255 586, which has a higher priority, although flexo printing, high printing and gravure printing are also mentioned in very general terms are, this publication deals primarily with offset printing. How and in which way, due to the measured color deviations, should be interfered with in the printing process in gravure printing is not apparent from this document.

It is an object of the present invention to provide a method which is particularly simple in terms of printing technology and which permits fast and highly precise regulation of the coloring in gravure or flexographic printing.

The method according to the invention is described in independent claim 1. Preferred configurations result from the dependent claims.

According to the main idea of the invention, the color control is carried out on the basis of a single (i.e. gray) control field containing all the (colored) printing inks involved, which is not measured and evaluated densitometrically, as is usually the case, but colorimetrically. So you regulate the colorimetric consistency of the gray field and essentially don't care how the individual printing inks behave. This is a "rule philosophy" that deviates from most of the usual procedures. In these known methods, each color has been controlled individually and, in addition, different tones (high tones, midtones) have been evaluated for the measurement. Mixed tones have only been used secondarily in order to obtain a certain amount of correction information if the single color control fails. The method according to the invention, on the other hand, is limited to a single point of the printing characteristic curves and considers the course of the individual colors (in particular the individual full tones) to be of secondary importance. According to the prevailing doctrine, this procedure would not work in practice.

From EP-A-89016 it is known to set a flexographic printing machine using a single gray control field. There, the gray field is first evaluated not colorimetrically, but densitometrically, and secondly, it is not the coloring, but the contact pressure of the plate cylinder that is regulated. This is an entirely different problem.

CH-A-649 842 and FR-A-2 594 131 are also known from the prior art. CH-A-649 842 shows a press control based on densitometric measurements. FR-A-2 594 131 discloses a transfer printing machine with a computer controlled mixing system for providing the correct printing inks. None of these publications deals with the special control of the coloring on flexographic or gravure printing machines on a colorimetric basis, as defined by claim 1.

• Fig. 1 eine schematische Gesamtdarstellung der erfindungsrelevanten Teile einer erfindungsgemässen Druckmaschine und
• Fig. 2 ein Flußschema zur Erläuterung des erfindungsgemässen Verfahrens.

The invention is explained in more detail below with reference to the drawing. Show it:

• Fig. 1 is a schematic overall representation of the parts of a printing press according to the invention and
• 2 shows a flow diagram to explain the method according to the invention.

Only one ink fountain 3 and one impression cylinder 2 of the actual gravure printing machine are shown in FIG. 1. It goes without saying that these and other parts, not shown, are present several times according to the number of different printing inks (for example three). The finished printed sheet 1, which has a printed image 5 and a printed control measurement field 4, which is also explained in more detail below, runs past a photoelectric measuring head 6 and is guided by a deflection roller 6a. The measuring head 6 is connected to an electronic, computer-controlled processing device 100, which cooperates with a monitor 20, an input keyboard 21, a log printer 22 and a synchronization device 25 and controls a metering control 12. This acts on valves 17-19 in supply lines 14-16 for color concentrates F _i , blends V _i and solvents T _i (i represents all the printing inks involved) and controls the compositions of the printing inks in the individual ink fountains 3 according to a (parametrically adjustable ) Setpoint generator 13 and correction variables calculated by the processing device 100. The synchronization device 25, which can be, for example, a clock and angle encoder or a sensor for any synchronization marks that are also printed, synchronizes the printing cylinder 2 with the processing device 100 and ensures that the measuring head 6 is activated exactly at the moment in which the control field 4 passes under it.

So far, the printing press shown corresponds essentially to the prior art, so that a more detailed explanation is unnecessary. The differences relevant to the invention concern the Special type of control field 4 used, its measurement and the evaluation and processing of the measured values for the correction variables already mentioned for the dosing control 12. These differences are explained in more detail below.

The control measuring field 4 comprises a print of the three participating printing inks cyan, magenta and yellow, the ratio of which is selected so that an approximate gray with a density of approximately 0.5 results. The size of the control field is typically around 4-10 mm square. It essentially depends on the necessary light output and the speed of the printing sheet.

In addition to the advantages of controlling using a single gray field already mentioned, there is also an advantage in that only a small space is required for the single measuring field and it can therefore be accommodated anywhere on the printed sheet without difficulty. This is in stark contrast to most previous methods, which all require a large number of measuring fields. In addition, only a relatively small amount of data is processed.

The measuring head 6 is designed as a spectral measuring head which detects the remissions of the gray field 4 over the entire range of the visible spectrum in e.g. 35 discrete wavelengths recorded (e.g. every 10 nm). Spectral reflectance measuring heads of this type are known and therefore require no further explanation.

The processing device 100 detects as the most important stages or functional units (all of which are of course advantageously implemented in software) a standard color value calculator 7, a color coordinate calculator 8, a target value memory 10 for predetermined color coordination target values, a difference generator 9, a parameter memory 24 and a correction calculator 11. Constant values and parameters, as is generally the case, are either stored during programming or entered via keyboard 21. Color coordinates Setpoints F₀ * can either be entered using the keyboard or, as is generally the case, read in and saved by measuring a reference control measuring field.

The standard color value calculator 7 calculates the standard color values N (X, Y, Z) according to the formulas of CIE 1931 (Commission Internationale de L'Eclairage) from, for example, 35 individual spectral reflectance values R (possibly averaged over a number of printed sheets). From these values, the color coordinate computer 8 then calculates the three color coordinates F * (L *, a *, b *) of the L *, a *, b * color space of the CIE (or a corresponding other equally spaced color space). This color space is homogeneous in terms of sensation and is particularly well suited for the present purposes. The color coordinates F * of the scanned control measuring field 4 can be compared with the corresponding target color coordinates F₀ * (entered or read in from the reference measuring field), the difference vector ΔF * = F * - F₀ * being formed, the components of which ΔL *, Δa * and Δb * are. From this difference vector ΔF *, which represents the colorimetric deviation of the measured control field 4 from a reference field or the corresponding target color coordinates, three correction vectors ΔM _i are now calculated in the correction computer 11, which represent the necessary changes in the compositions of the individual printing colors by the Correct the color deviation of the control measuring field (of the sheets subsequently printed). The index i stands for the individual inks (cyan, yellow, magenta). The vectorial representation is chosen because the color concentrate, the blend and the solvent are influenced. The actual dosing is carried out by the dosing control 12, which of course also takes into account preset (recipe) basic values (vectors M _oi ) in addition to the correction vectors ΔM _i and ensures that the dosing corrections carried out are not only carried out once, but rather that the target vectors are updated accordingly . (The new target vectors are the sum of the last valid target vectors and the correction vectors). The practical implementation of the metering control 12 (for example analogous to CH-PS 622 632) is clear to the person skilled in the art and therefore requires no further explanation.

In FIG. 2, they are individual calculation steps which lead from the difference vector ΔF * representing the color locus deviation of the control measuring field 4 to the three correction vectors ΔM _i , in the form of a flow chart.

The difference vector ΔF * has the three components ΔL *, Δa * and Δb *. ΔL * expresses the deviation in brightness, Δa * and Δb * the chromatic deviation.

berechnet (27). Danach folgt ein betragsmässiger Vergleich von Helligkeitsabweichung ΔL* und Betrag ΔC* der chromatischen Abweichung und eine Verzweigung je nach dem, wie der Vergleich ausgeht. Ist die Helligkeitsabweichung nicht kleiner als die chromatische Abweichung, wird der mit 28 bezeichnete Berechnungspfad eingeschlagen, andernfalls der Pfad 29.In a first step, the amount ΔC * of the chromatic deviation is first
according to

calculated (27). This is followed by a comparison of the magnitude of the brightness deviation ΔL * and the amount ΔC * of the chromatic deviation and a branching depending on how the comparison is based. If the deviation in brightness is not less than the chromatic deviation, the calculation path designated 28 is taken, otherwise path 29.

A further decision is first made in path 28: if the brightness deviation is not negative (the control measuring field is too bright), the further calculation is carried out according to path 30, otherwise according to path 31.

If the print is too light (path 30), the control is carried out primarily via the color concentrates, by multiplying the brightness deviation ΔL * by a constant vector ff to be explained and yielding the concentrate correction vector ΔF = ΔL * · ff. If the pressure is too dark (path 31), the control is primarily carried out via the blend, whereby the brightness deviation ΔL * with the constant corresponds Vector vf is multiplied so as to give the waste correction vector ΔV = ΔL * · vf. In both cases, the solvent correction vector ΔT = ΔF · (tf) or ΔV · (tv) is formed from the concentrate correction vector or the waste correction vector by multiplication with corresponding diagonal matrices (tf) or (tv). The purpose of this correction is to avoid jumps in viscosity when adding color or blends, which are usually more viscous.

The solvent correction vector .DELTA.T and the concentrate correction vector .DELTA.F and the waste correction vector .DELTA.V determine the currently required correction of the compositions of the printing inks, i.e. the amount of concentrate, blend and solvent (e.g. toluene) to be added at the moment to achieve the required composition correction. Now, however, this new composition must also be retained (until a possible new correction is made), which means that the dosing recipe (relative proportions of the color components) must also be adjusted accordingly. For this purpose, the concentrate correction vector ΔF is multiplied by a diagonal matrix (pf) (path 30) or the blend correction vector ΔV by a diagonal matrix (pv) (path 31) to give the recipe correction vector Δf, which is then combined with the other correction vectors of the metering control 12 are supplied and processed by the latter in the sense explained.

The constant vectors ff and vf are each three-component, whereby one component is assigned to one of the three participating (colored) printing inks. The vector ff indicates the color effect in the existing color composition, ie how many volume units (eg liters) of color concentrate must be added to the existing concentrate-blend-solvent mixture in order to change ΔL * by one unit. Accordingly, the vector vf indicates the blending effect, that is, the amount of blending to be added for the change in unit ΔL * (for example liters). The components of these vectors are empirical values and must (when entering of the machine) can be determined empirically. They depend, among other things, on the circulating volume, tank size, concentration of color concentrates, etching depth, level of emptying, printing speed, etc. Practical values for the components of ff and vf are (5.1 / 3.2 / 1.1) and (2 , 5 / 0.9 / 1.8) for the colors cyan, magenta and yellow.

The diagonal matrices (tf) and (tv) each have 3 rows and 3 columns. Their diagonal elements indicate the amount of solvent (toluene) to be added per unit amount of color concentrate or blend in order to keep the overall viscosity (to some extent) constant. Practical values for the diagonal elements of the two matrices are, for example, (0.4 / 0.3 / 0.5) for (tf) and 0.9 / 0.4 / 0.6) for (tv) in the order cyan, magenta and yellow.

The diagonal matrices (pf) and (pv) indicate the percentage by which the concentration of the color concentrate (amount of color concentrate based on the sum of the amounts of color concentrate and blend) changes when a unit amount (e.g. 1 liter) of color concentrate or blend in the total amount in circulation is added. Here, of course, primarily the tank size or the total amount of paint in circulation is included. In addition, (pf) and (pv) obviously satisfy the relationship (pv) = 1 - (pf). Practical values for the diagonal elements of (pf) and (pv) are (0.4 / 0.5 / 0.3) and (0.6 / 0.5 / 0.7), for example.

im Windelbereich α_C ≦ α ≦ α_M die Matrix

und im Winkelbereich α_M ≦ α ≦ α_y die Matrix

gelten.If the measured color locus deviation is essentially a chromatic deviation (path 29), the direction α of the color deviation is first determined according to α = arctan (Δb * / Δa *). Then based on the angle α = α + 180 ° (the direction opposite to α) one of three correction matrices (r) is selected which is required for the further calculation. If α _Y (∼100 °), α _C (∼ 215 °) and α _M (∼ 330 °) are the directions (angles) (stored in parameter memory 24) of the primary color axes for yellow, cyan and magenta, then for (r ) in the angular range α _Y ≦ α ≦ α _C the matrix

in the diaper area α _C ≦ α ≦ α _M the matrix

and in the angular range α _M ≦ α ≦ α _y the matrix

be valid.

The correction matrices (r) assume that each color deviation can be corrected by changing only two (of the three involved) colors, and indicate the percentage changes (ie based on the unit of the amount ΔC *) for the two colors concerned. Which two colors are used is determined from the direction α of the color deviation according to the above selection scheme. (These are the two colors with the direction between their primary color axes α falls.)

The amount ΔC * with the (due to α selected) correction matrix (r) multiplied and then calculated in exactly the same way as in the case of ΔL * ≧ ΔC *, but with the product ΔC * · (r) instead of ΔL *. Here too, paths 45 and 46 are split depending on whether ΔC * ≧ φ or ΔC * <φ was. The end result is then neither a concentrate correction vector ΔF or a blend correction vector ΔV, a solvent correction vector ΔT and a formulation correction vector Δf, all of these correction vectors now only influencing two colors, the third component being zero or (in practice) at all Does not exist.

The advantage of the described method lies in the unbundling, which is increasingly difficult to achieve with color correction questions. In addition, the process is clear and practical.

Claims (5)

1. A process for the control of inking in continuous rotogravure or flexographic printing, whereby simultaneously printed control fields in the form of grey fields (4) containing all the colour printing inks involved are measured colourimetrically, the measuring results (F*) are compared with corresponding set values (F $\genfrac{}{}{0ex}{}{\text{*}}{\text{o}}$

) in a sensitometrically homogenous system, in particular the Lab colour space according to CIE, and the relative compositions of the printing inks of concentrate (F_i), diluent (V_i) and solvent (T_i) being adjusted depending on the comparison result (ΔF*), characterised in that the comparison of measuring results (F*) and set values (F $\genfrac{}{}{0ex}{}{\text{*}}{\text{o}}$

) takes place in a colour space which permits determination of brightness (L*) and colour (C*) independently, that one proceeds differently in calculating the values necessary for the adjustment of the printing ink compositions depending on whether the brightness deviation (ΔL*) of the grey field (4) is greater or smaller than the colour deviation (ΔC*), the calculation in the first case being made on the basis of the brightness deviation and in the second case on the basis of the colour deviation, and that the calculation takes place differently depending on whether the brightness deviation (ΔL*) determined is positive or negative, whereby in one case substantially only the ink concentrate (F_i) is acted on and in the other case substantially only the diluent (V_i) is acted on.

2. A process according to Claim 1, characterised in that the grey fields have a density of approximately 0.5.

3. A process according to one of Claims 1-2, characterised in that the short-term adjustment of the printing ink composition is carried out by the direct and immediate addition of ink concentrate (F_i) and diluent (V_i) and solvent (T_i).

4. A process according to one of Claims 1-3, characterised in that the long-term adjustment of the printing ink composition is carried out substantially only by variation of the relative proportions of concentrate (F_i) and diluent (V_i).

5. A process according to one of Claims 1-4, characterised in that substantially only a single grey field (4) is provided per printed product.

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