EP1310088A1

EP1310088A1 - Method for engraving printing cylinders

Info

Publication number: EP1310088A1
Application number: EP00982988A
Authority: EP
Inventors: Ernst-Rudolf-Gottfried Weidlich
Original assignee: Hell Gravure Systems GmbH and Co KG
Current assignee: Hell Gravure Systems GmbH and Co KG
Priority date: 1999-10-19
Filing date: 2000-10-12
Publication date: 2003-05-14
Also published as: DE10050792A1; US7085018B1; WO2001031911A1

Abstract

The invention relates to a method for engraving printing cylinders in an electronic engraving machine. According to said method, at least two gravure sections which lie adjacent to one another along the direction of the axis are engraved using a respective engraving organ. The electric properties of the engraving organ are adjusted using a standard calibration function in such a way, that during the operation of the engraving organ by means of signal values S belonging to characteristic predetermined pressure densities Dpred, cups are engraved with geometric values that have been predetermined by the standard calibration function. In order to standardise the pressure densities in the individual gravure sections, deviations between the predetermined pressure densities Dpred and the actual pressure densities Dact are determined and a corrected section calibration is derived for each gravure section. The corrected section calibrations are created by determining corrected geometric values or corrected signal values S from the deviations. The corrected section calibrations are used to continually adapt the calibration automatically to the changing characteristics of the engraving organs and the gravure sections.

Description

Process for engraving printing cylinders

The invention relates to the field of electronic reproduction technology and relates to a method for engraving printing cylinders in an electronic engraving machine, in which at least two engraving strands lying next to one another in the axial direction are each engraved with one engraving member on a printing cylinder.

From DE-C-25 087 34 an electronic engraving machine for engraving printing cylinders by means of an engraving member is already known. The engraving element with an engraving stylus controlled by an engraving control signal as a cutting tool, for example in the form of a diamond, moves in the axial direction along a rotating printing cylinder. The engraving stylus cuts a series of cups arranged in a printing grid into the outer surface of the printing cylinder. The engraving control signal is formed in an engraving amplifier by superimposing a periodic raster signal, also called vibration, with image signal values which represent the print densities to be reproduced between "light" and "depth". While the raster signal causes an oscillating stroke movement of the engraving stylus to engrave the cells arranged in the printing raster, the image signal values determine the depths of cut of the engraved cells in accordance with the tonal values to be reproduced.

For magazine printing, a large number of axially adjacent, strip-shaped cylinder areas, called engraving strands, often have to be engraved on a printing cylinder or on the printing cylinders of a color set, which are engraved in succession in an engraving machine or simultaneously in several engraving machines, with one engraving element each , For example, the different print pages of a print job are produced in the individual engraving strands. The engraving control signals for the individual engraving organs are generated in separate electronic units, called engraving channels.

A prerequisite for good reproduction quality is that the engraved print densities in the individual engraving strands match, ie that a called strand equality is achieved. Even if the individual engraving channels are electrically balanced, the engraving styluses often have a different degree of wear. The result is that wells with different geometrical dimensions or volumes are engraved in the individual engraving strands, as a result of which disruptive differences in printing density occur in the engraving strands. Worn engraving styluses also produce cells with a rougher inner surface, which changes the ink acceptance behavior in the printing press and thus the printing density. Different printing densities in the engraving lines can also be attributed to influences in the printing machine, e.g. if the contact pressure between the printing cylinder and the impression cylinder varies in the axial direction or if the doctor blade with which the excess printing ink is wiped does not lie equally close to the printing cylinder.

In order to achieve the same print densities in the engraving lines, engraving styluses with the same possible degree of wear are selected for the engraving. As a precaution, the engraving styluses are exchanged for new engraving styluses even after a certain number of operating hours, which is relatively complex and expensive.

Even if new engraving styluses are used, density differences can soon arise in the individual engraving lines due to the differently sized engraved areas and the associated different speed of wear of the engraving stylus. Different engraving properties such as the hardness of the material and the cutting behavior of the engraving stylus in the material, the material generally being copper, can arise, for example, due to an uneven galvanization of the impression cylinder.

The differences in printing density in the engraving strands can occur with respect to a printing density value or with respect to a printing density range and can be of different sizes for each engraving strand. In addition, the engraving properties on the circumference of the printing cylinder can change, so that differences in printing density can also occur within an engraving strand. In order to equalize such differences in printing density, the engraved printing cylinder is in practice chemically post-treated in a time-consuming and labor-intensive work process, particularly when the quality of the printed products printed with the cylinder is high.

The invention is therefore based on the object of improving a method for engraving printing cylinders in an electronic engraving machine, in which at least two engraving strands lying next to one another in the axial direction, each with an associated engraving member, are engraved on a printing cylinder in such a way that disturbing differences in printing density in the engraving strands automatically be compensated.

This object is solved by the features of claim 1. Advantageous further developments are specified in the subclaims. The invention is described below with reference to Figures 1 to 9. Show it:

1 shows the relationship between signal value and pressure density for three characteristic target density values, FIG. 2 shows the relationship between the target pressure densities and the corresponding transverse diagonals (standard calibration function) corresponding to them, FIG. 3 shows the residual deviations of the actual pressure densities for three characteristic

Tonal values and for the individual engraving strands, FIG. 4 the residual deviations between actual printing densities and target printing densities in an engraving strand,

5 the correction function ΔQ = f (Q) for an engraving strand, FIG. 6 the corrected calibration function for an engraving strand, FIG. 7 the correction of the signal values,

8 shows the assignment between signal values and corrected signal values, and FIG. 9 shows the change in the geometric values over time.

According to the prior art, for the calibration of the printing density D, target printing densities D _so n are used for characteristic tonal values of a test wedge to be engraved specified in each engraving strand of a printing cylinder. The test wedge comprises, for example, three characteristic tonal values with the target print densities D _so n = (0.25; 0.5; 0.8). These include the signal values S = (161; 80; 1) with which the engraving system is controlled. 1 shows the relationship between the signal values S and the printing densities D. A test wedge with the specified signal values S is engraved in a engraving cylinder in each engraving strand. The test wedges can be engraved on the printing cylinder separately or simultaneously with the engraving of the actual printing form in cylinder areas lying outside the printing form. Areas of production engraving can also be used for calibration if they contain the characteristic tonal values. After the engraving of the impression cylinder, the engraved impression cylinder is printed on in a printing machine. In the proof, the actual pressure densities Dj _S t of the test wedges engraved in the individual engraving strands are measured with a suitable density measuring device and the deviations from the target pressure densities D _so n are determined (FIG. 1). These deviations are compensated for by a suitable calibration of the transfer function of the engraving systems in the individual engraving lines, for example by setting the signal amplification and the starting point of the amplification of the engraving systems.

Since, for reasons of effort, it is not always possible to make new proofs during the setting of the engraving systems in order to determine the actual pressure densities Dj _S t that have already been reached, the actual pressure densities Dj _{St are} derived from the measurement of geometric values of the engraved test wedge cells. The specified nominal pressure densities D _so n correspond to nominal geometry values which define the desired shape and size of the cells to be engraved. Geometric values can be the longitudinal diagonals, the transverse diagonals, the areas or the volumes of the wells, depending on which measuring method is used to measure the engraved well sizes. The transverse diagonals of the cells are preferably used, since they are easy to measure. Fig. 2 shows the relationship between the target pressure densities D _SO ιι and their corresponding transverse diagonals Q _so n of the wells. With the standard The calibration function corresponds to the characteristic target pressure densities D _so n = (0.25; 0.5; 0.8) and the target transverse diagonals Q _so n = (30μm; 100μm; 170μm).

The target print densities D _so n are converted into the corresponding signal values for controlling the engraving amplifiers assigned to the individual engraving strands, in which the engraving control signals for controlling the engraving stylus of the engraving members are generated. The actual geometry values of the wells are measured in each engraved test wedge of an engraving strand. The measurement of the geometry values can take place with the aid of a measuring microscope or in a video image recorded by a video camera. The engraving systems of the individual engraving strands are set in such a way that the actual geometry values reach the target geometry values which correspond to the predetermined target printing densities.

For this calibration according to the prior art, a standard calibration function between the target pressure densities D _SO ιι and, for example, the corresponding transverse diagonals Q _so n is used (FIG. 2), which was determined for a new engraving stylus and by averaging was determined using several engraved test wedges and pressure tests. The consequence of this is that the influences explained at the beginning, from engraving strand to engraving strand, such as the degree of wear of the engraving stylus, ink acceptance behavior, material hardness, cutting behavior, etc., are not taken into account in the calibration. Therefore, even after calibration, the actual print densities D _ιst in the individual engraving strands can still deviate from the target print densities D _so n. 3 shows these residual deviations of the actual print densities Dj _St for the three characteristic tonal values and for the individual engraving strands.

According to the method according to the invention, correction values are derived from the residual deviations for the individual engraving strands, which are included in the calibration of the next engraving of a printing cylinder with the same engraving system in the respective engraving strand, so that the calibration takes into account the strand-specific influences and differences, and thus the Desired print densities in all engraving lines can be achieved more reliably and more precisely. The method is explained below using the example of engraving strand No. 1. FIG. 4 shows the target pressure densities D _so n and the actual pressure densities D, _s t achieved in this engraving line according to the standard calibration as a function of the transverse diagonal Q. FIG. 4 shows the residual deviations between target pressure densities Dsoii and actual print densities D, _st have been drawn in a greatly exaggerated manner in order to be able to clearly illustrate the method according to the invention. For the value D _so n = 0.5 of the target print density, a transverse diagonal of Q _so n = 100 μm is set after the standard calibration (point A). The actual pressure density D, _s t thus achieved is higher by the residual deviation ΔD (point B). Corresponding deviations result for the other characteristic tonal values for which the transverse diagonals Q _SO ιι = 30 μm or Q _so n = 170 μm had been set for the engraving. If one connects the points of the actual pressure densities D, st reached for the three characteristic tonal values in this diagram, the dashed curve of the actual pressure densities is obtained. According to the curve of the actual pressure densities, the target pressure density value D _SO ιι = 0.5 is reached at point C, ie with a transverse diagonal that deviates by ΔQ = - 16 μm. So if you set the corrected diagonal Q orr = 100 μm + ΔQ = 84 μm for the target pressure density value D _so n = 0.5 for the next engraving in this engraving line, the target pressure density value D _so n is achieved exactly or at least very much much more precisely. Following the same consideration, correction values ΔQ for the different values of the transverse diagonals can be derived from the comparison of the curves for the target pressure density D _s0 n and actual pressure density D, _st . This finally results in an individual correction function ΔQ = f (Q) for each engraving strand, which is shown in FIG. 5 for the illustrated example. This correction function can also be included in the standard calibration function according to FIG. 2, as a result of which a strand calibration function is obtained which is used for this engraving strand during the next production engraving (FIG. 6).

After one or more production engravings of a printing cylinder with the same engraving systems in the individual engraving strands or also in At certain regular time intervals, the actual pressure densities D _{actual obtained are again determined} by means of the wells engraved in the test wedges and any remaining deviations from the target pressure densities D _so n (FIG. 3). From this, an improved strand calibration function is calculated in the manner described above (FIG. 6), which is then used in the subsequent production engravings. A new strand calibration is expediently calculated when the residual deviations between the target pressure densities D _so n and the actual pressure densities D, _st have exceeded a predetermined tolerance limit. The calibration of the engraving strands thus takes place in a process of "self-learning", in which the settings of the engraving amplifiers in the individual engraving channels are continuously optimally adapted to the changing technical boundary conditions, such as different degrees of wear of the engraving stylus used.

The calibration method according to the invention for adjusting the density of the engraving strands was explained using the example of setting the engraving channels by means of the transverse diagonals of the engraved cells. The method can be carried out in the same way if a different geometry value of the engraved test wedge cells is used instead of the transverse diagonals, for example the longitudinal diagonal, the area or the volume of the cells. For this purpose, analogous to the relationship of FIG. 2, a standard calibration function is used which relates the geometry value used to the target pressure densities D _so n. After setting the target geometry values in accordance with this standard calibration function, the actual pressure densities D, _{st of} the engraved test wedges are measured and individual corrections of the geometry value used are derived for the individual engraving strands in order to set up a strand calibration function (analogous to Fig 4 and 5).

An improved accuracy of the calibration method according to the invention can be achieved if the geometry value used is set not only for three characteristic target pressure densities but for additional intermediate levels, for example for tonal values in a gradation of 10% between light and depth.

In another embodiment of the calibration method according to the invention, instead of the geometry value used, the signal values S with which the engraving channels are controlled are individually corrected for each individual strand. This is illustrated in FIG. 7, where the relationship between the signal values S and the print densities D is shown (cf. FIG. 1). The target pressure densities D _so n and the actual pressure densities Dj _St achieved in a specific engraving line after the standard calibration are plotted as a function of the control signal values S. For the value D _so n = 0.5 of the target pressure density, a signal value S = 80 was used for control (point E). The actual pressure density thus achieved is higher by the residual deviation ΔD (point F). Corresponding deviations result for the other characteristic tonal values, for which the signal values S = 1 or S = 161 were used for the control for the engraving. If one connects the points of the actual pressure densities Djst reached for the three characteristic tonal values in this diagram, the dashed curve of the actual pressure densities is obtained. According to the curve of the actual pressure densities, the target pressure density value D _{is reached} n = 0.5 at point G, ie with a signal value deviating by ΔS = 15. If one uses the corrected signal value S _k orr = 80 + ΔS = 95 for the target pressure density value D _so n = 0.5 during the next engraving in this engraving line, the target pressure density value D _so n is achieved exactly or at least very much more accurate. Following the same consideration, correction values ΔS for all signal values S can be derived from the comparison of the curves for the target pressure density D _S0 n and the actual pressure density D _ist . This finally results in an individual correction function S _k orr = g (S) for the signal values for each engraving strand, which is shown in FIG. 8 for the illustrated example. This correction function can be implemented, for example, by a table _memory in each engraving channel, with which a corrected signal _value S _k0 rr is assigned to each input signal value S. In this embodiment of the calibration method according to the invention, care should be taken to ensure that the actual pressure density Dj _S t for the smallest Signal values S have higher or equal values than the target pressure density D _so n, since otherwise negative signal values would have to be generated by the calibration. This condition can be ensured, for example, by increased pigmentation of the printing ink, the pigmentation having to be increased to such an extent that the above condition is met in all strands.

In a further embodiment of the calibration method according to the invention, the temporal change in the residual deviations between the actual printing density Djst and the target printing density D _so n in the individual engraving lines is additionally taken into account in order to make a prediction about the expected residual deviations and thus about the expected changes in the To make strand calibration function. One reason for the change over time is the progressive wear of the engraving stylus with the age or frequency of use of the engraving stylus. A different degree of wear on the engraving stylus can be due to the fact that previously differently large areas were engraved in the engraving strands and / or the engraving strands have different engraving properties, which can be attributed, for example, to an uneven galvanizing of the printing cylinder.

9 shows for a strand the time dependency of the transverse diagonals Q to be set in the strand calibration according to the method according to the invention for the three characteristic tone values. It is assumed that the transverse diagonals Q to be set have already been redetermined for two adaptation periods of the strand calibration, each at time interval T. From the slope that the curves then reached at time 2T, a prediction can be made for the change in the transverse diagonals Q to be set for the next engravings (dashed part of the curves) until the time 3T the exact values again from the measurement of the actual -Density Dj _S can be determined. This extrapolation means that the measurement and adjustment of the string calibration need not be carried out as often. Instead of depending on the time, the change in the geometry values relevant for the calibration can also be applied, for example, depending on the frequency of use of the engraving stylus in order to derive a prediction of the setting values for the next engravings. The frequency of use can be measured, for example, by adding up the cumulative number of engraved cells in a counter that is present in each engraving channel. Alternatively, this number can also be determined and stored in the control software.

In an advantageous embodiment of the invention, the measurements of the actual print densities and the set geometry values are carried out by automatic measuring devices. Furthermore, it is advantageous to store and manage the measured values and the determined setting values for the individual strand calibrations as well as the time-dependent dependencies and development trends in a central computer, so that the density adjustment between the individual engraving strands takes place automatically and also over a longer period Time is automatically adapted to the changing technical conditions.

Claims

claims

1. Method for the engraving of printing cylinders in an electronic engraving machine, in which - at least two engraving strands lying next to one another in the axial direction are each engraved on an impression cylinder, each with an associated engraving element,

- to control the engraving elements, signal values S are generated which represent the desired printing densities D _SO ιι to be achieved, - engrave the engraving elements controlled with the signal values S into the pressure cylinder, the geometry _{values of which} represent actual pressure densities D _ιs t,

- A standard calibration function is specified, which describes the relationship between the geometric values and the target pressure densities D _so n, and

- The electrical properties of the engraving elements are calibrated so that when the engraving elements are actuated with signal values S _so n belonging to characteristic target pressure densities D, wells are engraved with the geometric values specified by the standard calibration function, characterized in that the pressure densities in the individual engraving strands

- wells are engraved in the engraving strands with which the predetermined characteristic target printing densities D _so n are to be achieved,

- after printing, the actual print densities D, _st achieved are determined by measurement,

- A corrected strand calibration is derived from the deviations between the target _printing densities D _so n and the actual _printing densities D _ιst for the respective engraving strand, and

- The assigned corrected strand calibration is applied in each engraving strand.

2. The method according to claim 1, characterized in that the corrected strand calibration is generated by _so n corrected geometry values are determined in the standard calibration function for the target printing densities D.

3. The method according to claim 1 or 2, characterized in that the geometric values of a cell are the transverse diagonal, the longitudinal diagonal, the cell area or the cell volume.

4. The method according to claim 1, characterized in that the corrected strand calibration is generated by correcting S to the signal values

Signal values S orr can be determined.

5. The method according to any one of claims 1 to 4, characterized in that improved corrected strand calibrations are determined again when the deviations between the target pressure densities D _so n and the actual

Pressure densities exceed a tolerance limit.

6. The method according to any one of claims 1 to 4, characterized in that improved corrected strand calibrations are determined again after a predetermined time interval T, after a predetermined number of engraved impression cylinders or after a predetermined frequency of use of the engraving member.

7. The method according to any one of claims 1 to 6, characterized in that improved corrected strand calibrations are determined by extrapolation from the change over time in the corrected geometric values or the corrected signal values S determined.

8. The method according to any one of claims 1 to 7, characterized in that the measurement of the actual pressure densities Dj _S t and the geometry values are carried out by automatic measuring devices.

, Method according to one of claims 1 to 8, characterized in that the measured values and the determined corrected string calibration functions and the time dependencies are stored and managed in a central computer.

10. The method according to any one of claims 1 to 9, characterized in that the corrected strand calibrations are continuously automatically adapted to the changing properties of the engraving members and the engraving strands.