EP0089016B1 - Method of automatically adjusting the colours printed by flexographic printing machines in a four-colour printing process - Google Patents

Description

The invention relates to a method for the automatic setting of the colors printed by flexographic printing presses with printing plate cylinders 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 the color and strength of the color, flexographic printing can still cause color shifts due to the fact that the individual plate cylinders are positioned differently against the associated impression cylinder, so that due to the respective center distance of the plate and impression cylinder, the pressure is appropriate the individual grid points of the flexible clichés are broadened 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 each 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 component is between about 40 and 60% involved, it is determined that to determine the actual value of the color based on a densitometric measurement, the actually present proportions of the printing inks are measured in the test field and from the measurement 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 the center distance of the plate cylinder of the respective color from the impression cylinder is changed according to the deviations until the actual and the target -Value in the color triangle match.

Before the printed colors are adjusted using the method according to the invention, the correct hue and the correct color strength of the individual printing inks are set, for example, using the method described in DE-A-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 this measurement gives the parts of the complementary colors in the color box color which correspond to the original. 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 color polygon or the color triangle indicates the respective color level, whereby the color strength is defined by the length of the plumb line on the lowest color level, whose coordinate zero point 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 hue by the vector angle and the color intensity by the length of the solder and is compared with a predetermined target value in order to determine the measured color. Depending on the deviation from the target value, color concentrate or waste is added in the manner specified in DE-A-30 07 421.

As soon as the ink fountain colors have been set in accordance with the templates forming the target values, the printed colors are set using the method according to the invention. 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 are brought into agreement in the color triangle.

According to a preferred embodiment of the invention it is provided that the measured actual value of the color by rotating rays around the magenta corner point and the cyan corner point of the Triangles can be converted to the setpoint that lines are drawn in the color triangle starting from the corners through the center of gravity that corresponds to the black point, and the quadrilaterals that are defined by the triangle lines starting from the corners and the lines that extend beyond the center of gravity , are completely covered by the color determined by the associated triangular corner point, that from each line of the square within the color triangle assigned to a color, which is formed by the line extending beyond the center of gravity, a decreasing color wedge corresponds to the color of the square closed by this line the opposite 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 Rays from each n Cover 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.

• Farbton = arctg (c/r) x 180/π
  • c=R/(R+G+B)-1/3 r=G/(R+G+B)-1/3
• Farbstärke = (R + G + B) x 10
• Buntgrad = (c² + r²)^1/2 x 100

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:

• Hue = arctg (c / r) x 180 / π
  • c = R / (R + G + B) -1/3 r = G / (R + G + B) -1/3
• Color strength = (R + G + B) x 10
• Color grade = (c ² + r ² ) ^1/2 x 100

• Bei Gelb :
  • Farbtonzahl größer als der Sollwert = Farbton zu rötlich.
  • Zugabe : Gelb, grünlich
  • 1 Einheit entspricht x Gramm oder Dosierpumpenhüben.
  • Farbtonzahl kleiner als der Sollwert = Farbton zu grünlich.
  • Zugabe : Gelb, rötlich
  • 1 Einheit entspricht x Gramm oder Dosierpumpenhüben.
• Bei Magenta :
  • Farbtonzahl größer als der Sollwert = Farbton zu bläulich.
  • Zugabe : Rot
  • 1 Einheit entspricht x Gramm oder Dosierpumpenhüben.
  • 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 Gramm oder Dosierpumpenhüben.

Calibration can be used to determine which additions of certain printing inks cause which change in color indicators, e.g. B.:

• With yellow:
  • Color number greater than the target value = color too reddish.
  • Encore: yellow, greenish
  • 1 unit corresponds to x grams or dosing pump strokes.
  • Hue number less than the target value = hue too greenish.
  • Encore: yellow, reddish
  • 1 unit corresponds to x grams or dosing pump strokes.
• For magenta:
  • Hue number greater than the target value = hue too bluish.
  • Encore: red
  • 1 unit corresponds to x grams or dosing pump strokes.
  • Color number less than the target value = color too reddish.
  • Encore: magenta, bluish
  • 1 unit corresponds to x grams or dosing pump strokes.
• With cyan:
  • Color number greater than the target value = color too green.
  • Encore: blue, reddish
  • 1 unit corresponds to x grams or dosing pump strokes.
  • Color number less than the target value = color too red.
  • Encore: 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 printing characteristic, i. H. that 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 increase in 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 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 points of the triangle are formed by the three primary colors yellow (Y), magenta (M) and cyan (C). The focus of this triangle S is the full overlay of the primary colors.

At point G, yellow and cyan, at point R, yellow and magenta and at 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 the square 11, which is defined by the points R, M, V and S, magenta is printed over the entire surface and yellow and cyan with a correspondingly increasing screen wedge. Finally, in the square 111, 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 1 the cyan raster portion is a function of the angle of rotation a about 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, which 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 %.
• Aus der Messung und Berechnung ergibt sich aber die Zusammensetzung : Gelb 55 %, Magenta 52 %, Cyan-45 %.

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 from the following: yellow 55%, magenta 52%, cyan 45%.

• Anpreßdruck im Farbwerk Gelb zu hoch !
• Anpreßdruck im Farbwerk Magenta innerhalb der Toleranz !
• Anpreßdruck im Farbwerk Cyan zu niedrig !

Evaluation :

• 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 der Druckmaschine werden die Densitometer so angebracht, daß eine Reihe von Meßmarken während des Laufs abgetastet werden können. Zur Bestimmung der Einstellung der Farbkastenfarben werden Gelb, Magenta und Cyan vollflächig ausgedruckt und gemessen.

In practice, the procedure 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 out in the grid, the densitometer is used to measure the grid test field, in which the colors yellow, magenta and cyan are each present to about 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 according to the method described in DE-A-3007421 for Farbwerke measured the actual values according to hue, color strength and color level and the corresponding corrections are made from the comparison with the specified target values.

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 process for automatically adjusting the colours printed by flexographic printing machine comprising printing mechanisms having plate and impression cylinders for four-colour printing in the colours yellow, magenta, cyan and black, characterized in that the desired colour value in a colour triangle which serves for a colour comparison and has corners defined by the colours yellow (Y), magenta (M), and cyan (C) is determined in that the colour location is determined for a screen test field which has been printed by all printing mechanisms and in which the contribution of each of the colours is between about 40 and 60 %, that the actual colour value is determined in that the actual contributions of the colours in the test field are measured by a densitometric measurement and the colour location of the actual value in the colour triangle is derived from the measurement, that the colour locations are compared to determine the deviations of the actual values of the colour contributions of each colour, and that the distance between the plate cylinder for the respective colour from the impression cylinder is changed in dependence on the deviations until the actual and desired colour values in the colour triangle agree.

2. A process accordance with claim 1, characterized in that the measured actual colour value is adapted to be converted to the desired value in that rays are rotated about the magenta point (M) and the cyan point (C) in such a manner that lines are drawn in the colour triangle (Y, M, C) from the corners through the centre of gravity (S) corresponding to the black point, and the quadrangles defined by the lines extending in the triangle lines the corners (Y, M, C) and by the lines which are extended beyond the center of gravity (S) are covered in their entire areas by the colour which is determined by the associated corner (Y, M, C) of the triangle, that each line (R-S, S-V, S-G), which defines one of the quadrangles within the colour triangle and is associated with a colour and consists of one of the lines that have been extended beyond the centre of gravity (S), is used to define a colour wedge of the colour of the quadrangle which is closed by that line, which colour wedge tapers to the opposite corner (Y, M, C) in the triangle which is defined by a base consisting of said line, by a side of the triangle (Y-R, R-M, M-V, V-C, C-G, G-Y) and by the lines connecting the centre of gravity and the corners (Y, M, C) and that the extent to which the distance between the axes is changed is determined by the angles which are subtended by the two rays from the actual value to the desired value.

3. A process according to claim 1 or 2, characterized in that the point associated with the desired value is fixed in close proximity to the centre of gravity (S).

4. A process according to any of claims 1 to 3, characterized in that the screen test field is selected to consist of a portion of the printed picture, which portion corresponds to a selected desired value and in which the colour distribution is as uniform as possible.

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