EP0089016A1 - Method of automatically adjusting the colours printed by flexographic printing machines in a four-colour printing process - Google Patents

Abstract

In four-color printing with flexographic printing presses with printing units equipped with printing plate and impression cylinders, the printed printing colors yellow, magenta, cyan and black must be set individually in order to obtain a print image that is as true to the original as possible. In order to be able to automatically set the printed colors correctly without changing the color tone and / or the color strength of the individual printing inks, the color setpoint is determined in a color triangle used for color comparison, the corners of which are printed by yellow (Y) magenta (M) and cyan ( C) are determined, the color location is determined for a raster test field printed out by all prints, in which each of the printing inks with its own color portion is involved, for example, between 40 and 60%. To determine the actual value of the color, the actually present portions of the printing inks in the test field are measured on the basis of a densitometric measurement and the color location of the actual value in the color triangle is determined from the measurement. From the comparison of the color words, the deviations of the actual values of the color components from the nominal values of the color components of each color are determined. According to the deviations, the center distance of the plate cylinder of the respective color from the impression cylinder is changed until the actual and the target value in the color triangle match.

Description

The invention relates to a method for the automatic setting of the colors printed by flexographic printing presses with printing plate cylinders with printing plate and impression cylinders for four-color printing with the printing colors yellow, magenta, cyan and black.

Even if the printing inks are correctly adjusted according to color and strength of color, there can still be color shifts in flexographic printing due to the fact that the individual plate cylinders are positioned differently against the associated impression cylinders, so that the individual grid points of the flexible clichés are widened on the basis of the pressure corresponding to the respective center distance of the plate and impression cylinder and cover an area corresponding to the respective pressure on the web to be printed. The conditions are such that the halftone dots are widened with a decreasing center distance and thus increasing contact pressure and cover an increasingly larger area of the material to be printed. This changes the percentage of the color printed on the printed surface, so that the color tone changes accordingly.

The object of the invention is therefore to provide a method by which the printed colors can be automatically adjusted correctly without changing the color tone and / or the color strength of the individual printing inks.

According to the invention, this object is achieved in that, in order to determine the color setpoint in a color triangle serving to compare the colors, the corners of which are defined by the printing inks yellow, magenta and cyan, the color location for a screen test field printed out by all printing units, in which each of the printing inks has its own Color portion is involved between about 40 and 60%, it is determined that to determine the actual value of the color based on a densitometric measurement, the actually present portions of the printing inks are measured in the test field and from the measurement of the color location of the actual value in the color triangle It is determined that from the comparison of the color words, the deviations of the actual values of the color components from the nominal values of the color components of each color are determined and that, according to the deviations, the center distance of the plate cylinder of the respective color from the impression cylinder is changed until the actual and the target - Match the value in the color triangle.

Before setting the printed colors using the method according to the invention, the correct color tone and the correct color strength of the individual printing inks are set, for example, using the method described in DE-OS 30 07 421. For this purpose, each color box color is measured in a known manner with a densitometer with blue, red and green filters, so that the proportions of the complementary colors in the color box color that correspond to the original result from this measurement. The remission curve with which the measured colors can be clearly determined can be determined from the densitometer measurements by approximate integration.

A triangle of a polar coordinate system is formed from the color box colors, in the present case yellow, magenta and cyan, the zero point of which is determined by the achromatic point. From this achromatic point, vectors can be drawn to the degrees of mixing that delimit the triangle, so that the angle of the vector indicates the miscible hue. The length of the vector is a measure of the degree of chromaticity, which can be determined by adding black. The Farbpolygonzug or the color triangle indicates the respective color plane, wherein the color strength is defined arben plane through the length of the perpendicular to the bottom _F, the coordinates of zero is white. The coordinate zero point of the top color level is black. In the specified system, the color intensity is determined by the length of the vector, the color tone by the vector angle and the color intensity by the length of the solder to determine the measured color and compared with a predetermined target value. Depending on the deviation from the target value, color concentrate or waste is added in the manner specified in DE-OS 30 07 421.

As soon as the ink fountain colors have been set according to the templates forming the target values, according to the invention the printed colors are set according to the procedure. For this purpose, the color location for the color setpoint is determined in a color triangle, the corners of which are defined by the printing inks yellow, magenta and cyan, which must be shown by a screen test field printed out by all printing units. This color setpoint is expediently set in the color triangle in such a way that each of the printing inks with a color proportion of between about 40 and 60% is involved. The proportion of all printing inks should be large enough to be able to measure sufficiently large values.

The actual value of the color in the actually printed raster test field is determined in a known manner by densitometric measurements with red, green and blue filters, so that the color location of the actual value in the previously defined color triangle can be determined from the measurement. By comparing the color words, the deviations of the actual values of the color components from the nominal values of the color components of each color can be determined. In accordance with the deviations, the center distance of the plate cylinder of the respective color from the impression cylinder is then changed in such a way that the actual value and the target value in the color triangle are brought into agreement.

According to a preferred embodiment of the invention it is provided that the measured actual value of the color can be converted to the setpoint value by rotating rays around the magenta corner point and the cyan corner point of the triangle, starting from the corners in the color triangle Center of gravity, which corresponds to the black point, lines are drawn and the quadrilaterals, which are defined by the triangle lines starting from the corners and the lines which extend beyond the center of gravity, are completely covered by the color determined by the associated triangle corner point, that of for each line of the square within the color triangle assigned to a color, which is formed by the line extended beyond the center of gravity, a decreasing color wedge of the color of the square closed by this line to the opposite. overlying corner point is formed in the triangle, which is determined with this line as the base through a triangle side and the connecting line between the center of gravity and the corner point, and that the extent of the axis distance change of the plate cylinder from the impression cylinder is determined by the angles that the two beams each sweep from the actual value to the target value. According to the above-mentioned method, values can be determined and calculated which can be processed advantageously by microcomputers, so that the color setting according to the invention can be controlled by microcomputers.

According to a further advantageous embodiment, it is provided that a section of the printed image is selected as the raster test field which corresponds to a selected target value with a color distribution that is as uniform as possible. Such suitable raster test fields can be found in all printed images, so that it is not necessary to provide a separate raster test field, but instead the densitometric measurements for determining the actual value can be carried out directly in the printed image.

According to the method according to the invention, the corner points of the triangle need not be formed by the basic printing inks, they can also consist of other colors.

The invention is explained below with reference to the drawing, in the single figure of which the color triangle is shown.

The color tone achieved on the material to be printed is determined by the coloristic properties of the printing ink and its concentration. Experience has shown that the color tone for highly qualified four-color halftone prints must be in very specific areas. The corresponding hue values and the color strength as well as the degree of color are determined for each of the printing inks yellow, magenta and cyan.

The color tone is defined here as the angle of rotation around the gray axis of the color circle, the color strength as total density and the degree of color as the distance of the color tone vector from the black center of the color circle.

• 
  Durch Eichung kann bestimmt werden, welche Zugaben bestimmter Druckfarben welche Änderung der Farbkennzahlen bewirken, z. B.:
  • Bei Gelb: Farbtonzahl größer als der Sollwert = Farbton zu rötlich. Zugabe: Gelb, grünlich 1 Einheit entspricht x Gram oder Dosierpompenhuben. Farbtonzahl kleiner als der Sollwert = Farbton zu grünlich. Zugabe: Gelb, rötlich 1 Einheit entspricht x Gramn oder Dosierpumpenhüben.
  • Bei Magenta: Farbtonzahl größer als der Sollwert = Farbton zu bläulich. Zugabe: Rot 1 Einheit entspricht x Gram oder Dosierpunpenhüben. Farbtonzahl kleiner als der Sollwert = Farbton zu rötlich. Zugabe: Magenta, bläulich 1 Einheit entspricht x Gramm oder Dosierpumpenhüben.
  • Bei Cyan: Farbtonzahl größer als der Sollwert = Farbton zu grün. Zugabe: Blau, rötlich 1 Einheit entspricht x Gramm oder Dosierpumpenhüben. Farbtonzahl kleiner als der Sollwert = Farbton zu rot. Zugabe: Blau, grünlich 1 Einheit entspricht x Gram oder Dosierpumpenhüben.

The numerical values can be determined from the optical densities R, G and B, which are measured behind the red, green and blue filters of the densitometer, using the following formulas:

• 
  Calibration can be used to determine which additions of certain printing inks cause which change in color indicators, e.g. B .:
  • For yellow: hue number greater than the target value = hue too reddish. Addition: yellow, greenish 1 unit corresponds to x gram or dosing pompen. Hue number less than the target value = hue too greenish. Addition: Yellow, reddish 1 unit corresponds to x grams or dosing pump strokes.
  • For magenta: hue number greater than the target value = hue too bluish. Addition: Red 1 unit corresponds to x grams or dosing pump strokes. Color number less than the target value = color too reddish. Addition: magenta, bluish 1 unit corresponds to x grams or dosing pump strokes.
  • For cyan: hue number greater than the target value = hue too green. Addition: Blue, reddish 1 unit corresponds to x grams or dosing pump strokes. Color number less than the target value = color too red. Addition: blue, greenish 1 unit corresponds to x grams or dosing pump strokes.

The color strength value depends on the color concentration. The upward deviation from the target value indicates a higher concentration, which can be compensated for by adding blends. If the color strength is too low, it is corrected by adding concentrate.

If the concentration of the printing ink is set correctly, the color strength measured in the full-area test field depends on the ink layer thickness. This is determined in the inking unit by the inclusion of the anilox roller on the forme cylinder, so that it is possible to adjust the center distance via the corresponding servomotors after the measured color strength. ,

For four-color printing, it is also necessary to adhere to a certain characteristic curve, ie the position of the forme cylinder on the impression cylinder is of particular importance. Due to the elasticity of the flexographic printing plate, an increase in the contact pressure leads to an enlargement of the halftone dot in the print and thus to an increase in the percentage area coverage of the printed image by the halftone dots. However, this must not significantly exceed or fall below the values specified by the reproduction. Otherwise Otherwise undesirable color shifts would occur in the printed image.

The density values of the printed colors are measured on a test field printed in three colors in the basic colors with known screen coverings and the actual screen coverings are calculated. The control commands for the servomotors, which set the center distance of the plate and counter-pressure cylinders, result from the difference between the actual and the setpoints.

The type of determination of the control commands for the servomotors is explained in more detail below using the color triangle Y-M-C. The color triangle shown is created by printing raster wedges in three colors. The corner colors of the triangle are yellow due to the three basic colors. (Y), magenta (M) and cyan (C). The focus of this triangle S is the full-surface overprinting of the primary colors.

At the point G, money and cyan, at the point R yellow and magenta and at the point V magenta and cyan are printed over the entire surface, so that G corresponds to the color green, R to the color red and V to the color violet. In the rectangle I, which is defined by the corner points Y, R, S and G, yellow is printed over the entire surface, cyan in the direction C and magenta in the direction R as an increasing grid wedge.

In square II, which is defined by the points R, M, V and S, magenta is printed over the entire surface and money and cyan are printed with a correspondingly increasing screen wedge. Finally, in the rectangle III, which is defined by the points S, V, C and G, cyan is printed over the entire surface and the other two colors are printed with decreasing or increasing grid wedges.

From the color triangle it can be seen that, for example in the square I, the cyan raster portion is a function of the angle of rotation α around the magenta point M and the magenta raster portion is a function of the angle of rotation β about the cyan point C. The situation in the other fields is similar. Each point in the color triangle is thus clearly defined by two angles of rays that can be rotated around the magenta and cyan points. The corresponding grid coverage can be calculated if the functions are determined experimentally. The coordinate system rc shown in the drawing is the same as that used to determine the dimensions for the hue, the color strength and the chromaticity. The grid coverage proportions can thus be determined from the optical densities.

• Das Rasterprüffeld hat folgende, am Klischee gemessene Zusammensetzung: Gelb 48%, Magenta 50%, Cyan 50%.

An example of a measurement is given below:

• The grid test field has the following composition measured on the cliché: yellow 48%, magenta 50%, cyan 50%.

However, the composition and measurement result: yellow 55%, magenta 52%, cyan 45%.

Assessment: contact pressure in the inking unit yellow too high! Contact pressure in the magenta inking unit within the tolerance! Contact pressure in the cyan inking unit too low!

The differences between the target and actual values are measured numbers for the number of pulses for the servomotors.

In practice, the process is carried out as follows: The densitometers are mounted in the printing press in such a way that a number of measuring marks can be scanned during the run. To determine the setting of the ink fountain colors, yellow, magenta and cyan are printed out over the entire surface and measured.

To set the colors printed in the grid, the densitometer is used to measure the actual value using the grid test field, in which the colors yellow, magenta and cyan are each approximately 40 to 60%. The densities measured by the densitometer behind the three colored filters are then fed to a process computer.

In the set-up phase, the actual values according to hue, color strength and degree of color are measured for the inking units according to the method described in DE-OS 30 07 421 and the corresponding values are obtained from the comparison with the specified target values. Corrections made. ,

The corresponding control commands for the servomotors for the adjustment of the plate cylinder to the impression cylinder are determined from the target / actual value comparison of the grid area covers determined from the grid test field. During the set-up phase, the target values are approximated until the desired print image is achieved after visual assessment. Then the actual values achieved are determined as target values for the production run. The specified target numbers are continuously monitored automatically during the printing phase. The results can be displayed on a screen. The readjustment can take place automatically as in the setup phase.

Claims (4)

1. A method for automatically setting the flexographic printing presses provided with printing and printing cylinders for four-color printing with the printing inks yellow, magenta, cyan and black colors, characterized in that for determining the color setpoint in a color triangle serving the color comparison, the corners are determined by the printing inks yellow (Y), magenta (M) and cyan (C), the color location is determined for a raster test field printed out by all printing units, in which each of the printing inks with its own color share is between approximately 40 and 60% that to determine the actual value of the color based on a densitometric measurement actually present of the printing inks in the _P rüffeld are measured and the actual value is determined in the color triangle of the measurement of the color coordinates, that the deviations of the actual values of the color components of the target values of the color components of each color are determined from the comparison of the color points, and that in accordance with the the respective color is changed to the impression cylinder of the deviations of the center distance _K lischeezylinders until the actual and the desired value in the color triangle agree. 2. The method according to claim 1, characterized in that the measured actual value of the color can thereby be converted to the desired value by rotating rays around the magenta point (M) and the cyan point (C) that in the color triangle ( Y, M, C) draw lines from the corners through the center of gravity (S), which corresponds to the black point, and the quadrilaterals, through the triangle lines starting from the corners (Y, M, C) and over the center of gravity (S ) extended lines are defined, are completely covered by the color determined by the corresponding triangle corner point (Y. M, C), that of each line assigned to a color (R - S, S - V, S - G) of the quadrilaterals within the Color triangle, which is formed by one of the lines extended beyond the center of gravity (S), a decreasing color wedge of the color of the rectangle closed by this line to the opposite corner point (Y, M, C) in the triangle formed by this line as a base through a triangle side (Y - R, R - M, M - V, V - C, C _- G, G - Y, Y - R) and the connecting lines between the center of gravity (S) and the corner points (Y, M, C) and that the measure of the center distance change is determined by the angles which the two beams pass from the actual value to the desired value. 3. The method according to claim 1 or 2, characterized in that the point assigned to the target value is determined in the immediate vicinity of the center of gravity (S). 4. The method according to any one of claims 1 to 3, characterized in that a section of the printed image is selected as the raster test field, which corresponds to a selected target value with a color distribution that is as uniform as possible.

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