EP1279502B1 - Method for controlling gravure printing - Google Patents

Description

The present invention relates to a method according to the preamble of claim 1.

In gravure printing, the color concentration (relative composition of the inks of color concentrate, blend and solvent) is one of the most important parameters to control. It decisively influences the coloring of the process as well as the color and tonal quality of the printed product. The color concentration in turn depends on the nature of the substrate to be printed on which a particular color impression is to be produced. Today, quality is assessed almost exclusively by eye and with the help of densitometers.

The quality inspection is done on the basis of control strips or control blocks, which each consist of an array of control panels.

The arrangement of several control fields are called control strips or control blocks. Under a control strip is understood according to DIN 16527 Part 1, March 1993, a one-dimensional sequence of control fields. Similarly, a control block is a two-dimensional array of control panels. There is a control strip printed in monochrome or intended for monochrome printing, as well as control blocks printed in multiple colors or intended for multicolored printing. Control strips intended solely for the control, regulation or regulation of printing are specifically called print control strips, those which are exclusively used to monitor the copy are specifically called copy control strips, while strips which can serve both purposes are called control strips only.

According to DIN 16527 Part 1, control field is understood to mean an area on which one or more image elements are arranged for testing purposes; In accordance with DIN 16 527 Part 1, the term control field refers to a test surface which is transferred to a flat support material (eg a film) in a directly visible form, regardless of whether the information was previously in analogue (eg film ) or digital form (eg magnetic memory, image plate) was stored.

Control strips and control blocks are used for visual or metrological control of copying and printing operations, eg for proofing or printing on printing machines according to the principles of planographic printing, gravure printing, high pressure or throughprint.

Another application of such control means is the control of the pressure during production. The first application can be carried out after a visual as well as a metrological evaluation of one or more control fields, the latter application only after metrological evaluation. For this purpose, according to the prior art densitometers or colorimeters, such. Spectrophotometer used. Both devices capture the optical energy reflected from a printed control panel with a spot.

Control fields provided for metrological and / or visual evaluation consist of either a full-tone field, i. a full-surface printed or full-surface printing area or grid, i. a surface that is printed or printed for halftone dots or lines. Both solid tones and raster tones can be monochrome or multicolored. On the substrate to be printed a control panel is called multicolored, if it is created by the overprinting of several printing inks. Overprinting is the name for a print in which printing inks of different colors are applied on the same surface. As printing inks are understood here pigment layers, which are transmitted for example as suspensions by a printing press on the substrate to be printed.

Inks whose pigments have the primary colors used for multicolor printing, e.g. Black (K), Cyan (C), Magenta (M), Yellow (Y) (Yellow) in four-color printing, are called scale printing inks.

A first category of control panels includes the multi-color control panels for visual assessment or measurement, such as e.g. the solid-color fields of the secondary colors blue, green, red, the tertiary color black, which are produced by the overprint of the colored primary colors (CMY), and the gray balance fields.

According to DIN 16527 Part 1, the gray balance fields are multicolored grids in which the color densities of the involved printing inks are in such a numerical ratio that a print produced under undisturbed, average printing conditions appears un-colored, ie neutral gray to the eye. From a minimum viewing distance, unbranded printed gray balance fields can not be distinguished from grids printed only with the achromatic ink black (K).

Another category of control fields are the monochrome control fields of the scale printing inks (CMYK) as well as, if necessary, the special inks. These control fields make it possible to control the optical density and the color location of said scale printing inks or special inks, e.g. by means of densitometer.

The geometry of the control fields is usually such that their shape is adapted to the conventional shape of the measurement spots of densitometers and other reflectometers. They are therefore rectangular in the case of measuring instruments with an elliptical measuring spot and square, circular or hexagonal in the case of circular measuring spots.

The prior art discloses numerous sophisticated methods of checking printed results using the aforementioned control strips. However, these methods mainly relate to offset printing methods as well as to the printing of paper, which are subdivided into classes according to specific guidelines.

In the field of packaging, however, one is confronted with a variety of packaging materials with a variety of surfaces to be printed on. Such surfaces may e.g. be made of paper, plastic, metal or ceramic coatings.

However, the different substrate surfaces affect the ink transfer from the printing plate to the substrate in a crucial manner.

In the gravure printing process, the printing areas are recessed in the printing form compared to the non-printing ends. The ink to be applied lies in the printing wells, the so-called sieve cups. The sieve cups are mutually delimited by so-called webs or web surfaces, which are elevated and accordingly carry no printing ink. In halftone images, the grid-shaped arranged sieve cups have a stepped size. During the printing process, the ink in the print zone is transferred from the screen cups to the substrate to be printed.

It is known that when printing raster images in gravure printing, the color elements from a certain tone, also called color coverage or color coverage value, tend to flow on the substrate to be printed and flood the substrate surface, which sets a higher tone than that set is provided. The sudden increase in the tone value caused by the flooding of the printing surface is not to be confused with the so-called dot gain. Although the dot gain indicates how large the area coverage on the printed substrate compared to the target value, for example, based on a template, it is based however, on the increase in size of the color elements on the printed substrate relative to the original and can occur at all surface coverage without sudden changes.

Considering a raster graphic with a continuous increase of the tone values according to the set value, a so-called color jump occurs at a certain tone value, i. an abrupt increase in the degree of coverage occurs. The color jump is due to the aforementioned flooding of the screen area, i. The applied color elements begin to flow into each other and connect to each other, whereby the spaces created by the webs or web surfaces are filled with printing ink.

If the grid pattern consists, for example, of dots, the dot jobs begin to fuse with each other beyond a certain areal density, and color bridges are formed between the dots, whereby the color gaps are completely covered by an abrupt increase in the areal coverage with printing ink. A further increase of the tonal values above the flooding area consequently only affects the color density by increasing the layer thickness of the color.

The predetermined tone values converted to the printing form, called setpoints in this text, are therefore difficult to achieve in the region of such a color jump. It is therefore desirable to detect and locate this color jump in order to influence the printing process accordingly. Since the occurrence of the color jump is highly dependent on the nature of the substrate to be printed and the printing ink as well as on various process parameters, the problem of the color jump can only be addressed individually for each printing process.

Control strips or control blocks of the prior art also have the disadvantage that the verification of the optical density (color density) of the individual primary colors is possible only by measuring the monochrome control fields (full-tone field) of the primary colors by means of densitometers. Therefore, the ink densities can not be continuously monitored by means of the aforementioned measuring method for reasons of effort in the print run. However, it is desirable to monitor the color densities of the primary colors continuously with as little effort as possible, so that in case of discrepancies for the purpose of error detection, the monochrome control fields of the primary colors (CMY) can be measured immediately.

Out DE-A 43 38 976 and US-A-4,310,248 Methods are known for controlling and / or controlling the multicolor printing of a gravure printing process with a control strip or control block containing an array of control fields.

The invention is based on the object, with regard to the most varied of materials or substrates to be printed, the arrangement of control fields of a control strip or control block for a gravure printing process, which should allow the detection of a color jump in Tonwertbereich. In a continuation of the task, an arrangement of control fields is to be proposed, which allows the control of the density values of the primary colors.

According to the invention, the object is achieved by a method having the features of claim 1.

The target value of a tone value depends on the effective grid engraved on the printing cylinder.

The means for displaying the nominal values of the tonal values in the grid preferably comprise a tonal scale assigned to the grid at the lateral longitudinal edge of the color-gradient control field.

The course of the nominal values of the surface coverages is preferably linearly increasing or decreasing. The grid preferably has a dot grid.

The course of the nominal values of the tonal values in the grid is preferably reproduced for the range from 0% to 100%. Since the critical tone value at which said color jump occurs can in each case lie in a similar tonal range for a class of substrates to be printed, for such a class of substrates, only a specific section from the tonal range of 0-100% can be used. be printed as a control field.

The reproduction of the course of the nominal values of the tonal values in the grid can therefore be limited in particular to the tonal range of 50% to 100% and advantageously to the range of 70% -100%.

The color gradient control field allows the color jump in the grid to be continuously determined visually or with a measuring instrument, and to be assigned to a specific tone value (nominal value) on the basis of the tonal scale listed on the side. In the knowledge of this critical tone value, for example, in the print preparation, a tonal range in the artwork can be purposefully avoided.

The tone value desired value determined with the method according to the invention, in which the color jump occurs, can be taken over into an image processing process for carrying out corresponding corrections.

1. a) die ersten Kontrollteilfelder bunte Graubalancefelder aus dem Übereinanderdruck der Primärfarben Cyan (C), Magenta (M) und Yellow (Y) in abgestuften Tonwerten von 0-100% sind und
2. b) die zweiten Kontrollteilfelder unbunte Rasterfelder von Schwarz (K) in abgestuften Tonwerten von 0-100% sind,

und die Tonwerte des ersten und zweiten Kontrollteilfeldes eines Mehrbereich-Kontrollfeldes so gewählt sind, dass das Mehrbereich-Kontrollfeld als einheitliche Tonfläche erscheint und die beiden Kontrollteilfelder eines Mehrbereich-Kontrollfeldes unmittelbar und einander anstossend benachbart sind.The control strip or control block further includes in a preferred embodiment, multi-area control fields, each with a first and second control subfield, wherein

1. a) the first control subfields are colorful gray balance fields from the overprinting of the primary colors cyan (C), magenta (M) and yellow (Y) in graduated tonal values of 0-100% and
2. b) the second control subfields are achromatic grids of black (K) in graduated tonal values of 0-100%,

and the tonal values of the first and second control subfields of a multigrade control field are selected such that the multigrade control field appears as a uniform tone surface and the two control subfields of a multirange control field are immediately adjacent and abutting one another.

The monitoring of the color densities of the primary colors (CMY) thus takes place via gray balance control fields, whereby these are not measured densitometrically but visually checked for constancy by comparison with the neighboring achromatic black grids. So you monitor the constancy of the gray balance fields and not how the individual primary inks (CMY) behave for themselves.

The first and second control subfields of a multi-range control field preferably have matching color values, i. a consistent color impression, on.

The multigrade control fields essentially represent the structure of the gray axis of the primary colors CMY on the one hand and of black (K) on the other hand.

The multi-area control panels are preferably in two parts. They are also preferably arranged adjacent to one another in the order of increasing or decreasing tone values, preferably directly and abutting one another, the gray balance fields lying adjacent to each other on one axis and the achromatic black grids adjacent to each other on one axis.

The multigrade control panel is preferably rectangular or square. However, it may also be circular or elliptical, or a polygonal, e.g. hexagonal, have structure.

The division of the multigrade control field into a first and second control subfield can be done in any way. The division should, however, be such that a direct visual comparison of the first control subfield with the second subfield control field is possible.
The multigrade control field can, for example, be subdivided into a core area and edge area as subfields, wherein the core area is preferably dimensioned such that it can be dimensioned with a colorimeter. In the core area, for example, the gray balance fields and preferably the achromatic black grids can be arranged.

The multigrade control field is preferably in a rectangular or square shape and is subdivided into two rectangular control subfields of equal size.

In a preferred embodiment, the control strip or control block also contains an array of control fields in the form of single-color grids of the primary colors CMY with, preferably uniform, graduated tonal values of 0-100%. That for each primary color a control field is created for a certain number of tonal values in the range of 0-100%.

The multicolor control panels and / or the primary color (CMY) single-color grids preferably each contain a multi-range control field or control field as a solid field (100%).

The aforementioned monochrome control fields of the primary colors (CMY), as well as the gray balance fields and thus also the achromatic black grids are advantageously created for the tonal values 5%, 20%, 40%, 60%, 80% and 100%, with 100% of a solid field equivalent.

The control fields of the individual primary colors (CMY) are preferably arranged in the order of the increasing or decreasing areal coverage, preferably directly and adjoining one another.

The gray axis created from black (K) is the reference for the CMY gray axis (gray balance fields) with regard to color density at certain tonal values (taking into account the dot gain). If there are marked differences from the visual comparison of the two control subfields, ie if the gray balance fields contain a color cast, then a discrepancy in the color densities of the primary colors (CMY) can be derived from this. This inconsistency can be due to a disturbed ink acceptance of one or more printing inks (CMY), for example.

When such inconsistencies occur, in a next step, the color densities can be measured by means of densitometer from the full-tone control fields (100%) of the primary colors CMY and compared with the nominal values.

The arrangement of control fields thus allows the continuous (visual) monitoring of the color densities of the primary colors CMY during the Auflagedrukkes, without having to permanently measure the color densities of the primary colors CMY individually.

Through the continuous review of the gray balance fields, a timely readjustment of the color cylinder or the ink supply can be ensured.

The substrate to be printed is preferably a film, in particular a flexible packaging film. The packaging film may be transparent, translucent opaque and / or stained or colored through. The packaging films may contain colored, reflective or metallized surface layers and have optically colored, reflective and / or metallizing surfaces.

The printable packaging films can be monofilms or film composites. The packaging films may contain or consist of layers or films of plastic, such as polyolefins, polyesters, polypropylenes, polyamides or of metals, such as aluminum, iron, steel or alloys thereof, or of pulps, such as paper, glassine or cellophane.

The surfaces to be printed can, for example, from one of the aforementioned papers, plastics or metals. Furthermore, the surface to be printed can be a ceramic coating.

The thickness of the packaging material may e.g. from 5 to 1000 .mu.m, in particular from 7 to 200 microns.

The color densities of the individual primary colors (CMY) of the gray balance fields are preferably adapted to the optical and / or mechanical properties of the substrate to be printed. The color densities of the primary colors (CMY) must therefore be determined beforehand to create the gray balance fields.

In a first step, a pressure of a test form, e.g. an IT 8.7 / 3 test form according to ISO 300. The pressure is applied to the same substrate class as the subsequent print run. Subsequently, the test form is measured by means of spectrophotometer. Based on the measured test form, a so-called ICC input profile (International Color Consortium) is created. This means that the measured device values are converted to the device-independent L * a * b color space.

Based on the measured test form, the L * a * b values of the achromatic black gray values are subsequently determined. Then, on the basis of the ICC input profile, the gray values are calculated from CMY for the corresponding achromatic black gray values. The values obtained are used as a basis for the following edition print.

Fig. 1:

Kontrollfelder der Primärfarben (CMYK) mit integrierten Graubalancefeldern;

Fig. 2:

Farbverlauf-Kontrollfeld mit stetigem Verlauf des Tonwertes von 0-100%;

Fig. 3:

Mehrbereich-Kontrollfelder mit Kontrollteilfelder der Primärfarbe unbunt Schwarz (K) und der Graubalance.

In the following the invention will be described by way of example and with reference to the accompanying drawings. Show it:

Fig. 1:

Primary color control fields (CMYK) with integrated gray balance fields;

Fig. 2:

Gradient control field with continuous tone value from 0-100%;

3:

Multi-range control fields with control subfields of primary color uncolored black (K) and gray balance.

Fig. 1 shows a particularly advantageous arrangement of control fields of a control strip. The control fields denoted by the reference numerals 1 to 4 represent monochrome grids of the primary colors CMYK with different tonal values. The reference numerals 1 relate to control fields of the primary color cyan (C), the reference numerals 2 to control fields of the primary color magenta (M), the reference signs 3 to control fields the primary color yellow (Y) and the reference numeral 4 on control subfields of the primary color achromatic black (K). The letters a to f stand for different tonal values, where a stands for a tone value of 100%, b for 80%, c for 60%, d for 40%, e for 20% and f for 5%. The reference numeral 2e therefore refers to the number 2 and the letter e as a control field of the primary color magenta (M) with a coverage of 20%. The control fields described are adjacent and abutting each other.

The control subfields 4 of the primary color black in this embodiment correspond to half of a multi-area control field 6, which corresponds in size to the control fields 1, 2, 3 of the primary colors (CMY). The second control subfield is occupied by the colorful gray balance fields 5. The control subfields are arranged so that there is always a gray balance field 5 opposite an achromatic black screen 4 with the same color impression. For the sake of clarity, only the reference number of the control field 6f formed from the partial control fields 4f and 5f is shown by way of example in FIG.

This arrangement allows the visual control of the gray balance fields in direct comparison to the achromatic black grids which represent the reference values. If the printing process is correct and the color reproduction is undisturbed, the two opposing control panels should each leave the same color impression. However, if the gray balance fields deviate from the achromatic black grids, the color density of the primary colors in the full-tone control fields 1a, 2a, 3a must be measured by densitometer in order to determine any discrepancies in the color densities.

Of course, any other gradation of the tonal values can be made. Furthermore, the gradation of the tonal values can be varied with additional control fields or by omitting control fields. Moreover, the size and the type of subdivision of the two-part control fields can vary as desired. However, it has proved to be advantageous that the multigrade control fields correspond to the size of the control fields of the primary colors (CMY).

FIG. 3 shows the multi-area control fields 6 with the control partial fields 4 and 5 according to the description of FIG. 1.

FIG. 2 shows a color gradient control field 10 with a strip-type grid 11. The strip-type grid 11 reproduces the steady-state linear progression of the tone values from 0 to 100% according to nominal values. For reasons of space Fig. 2 represents only a section of such a color gradient control panel 10.

Laterally of the strip-shaped grid 11, a value scale 12 with the associated nominal values of the tonal values is arranged in percent. The gradation or resolution of the scale of values can be arbitrary. In the present embodiment, it has proven to be advantageous to list tone values in increments of 3% in each case.

The arrangement allows to detect the color jump in the strip-shaped grid 11 visually or with a measuring device and to assign the color jump to the corresponding tonal value on the basis of the associated tonal values listed on the side.

The control strip or control block may also include other control fields, such as secondary red, green, blue primary color (CMY), black (K), achromatic 50% halftone fields with different halftones, such as dot and line grids , a Siemens star for checking the resolution and / or a register field for checking the register shift.

Claims (3)

1. Method for detecting the colour jump for checking and/or control of single-colour or multicolour printing in an intaglio printing process by means of a control strip or control block containing an arrangement of control fields,
   characterised in that
   the control strip or control block contains a colour variation control field (10) in the form of a strip-shaped raster field (11) of a primary colour, wherein the colour variation control field (10) over its longitudinal extension reproduces a steadily rising or falling variation of tone values according to desired values and means are associated with the strip-shaped raster field (11) for displaying the desired values of the tone value (12) and the tone value desired value corresponding to the degree of area coverage on occurrence of flooding of the raster field (11) is determined via the means of display.
2. Method according to claim 1, characterised in that the means of display of the tone value desired values (12) comprise a tone value scale assigned to the raster field (11) on a lateral longitudinal edge of the colour variation control field (10).
3. Method according to claim 1 or 2, characterised in that the tone value desired value (12) determined is taken over into an image processing process for appropriate corrections to be made.

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