Method for operating a printing installation, printing installation, and computer program product

Abstract

The invention relates to a method for operating a printing installation (2) which has a first printer (4) and a second printer (6), wherein a web (10) runs through the printing installation (2) at a web speed, wherein by means of the first printer (4) a first printed image (8) is printed onto the web (10) and subsequently by means of the second printer (6) a second printed image (12) is printed over the first printed image (8), wherein the first printed image (8) is longitudinally distorted between the first printer (4) and the second printer (6), wherein a first sensor (18) is allocated to the first printer (4), said first sensor generating a first sensor signal (G1) which is dependent on the web speed of the web (10) in the first printer (4), wherein a second sensor (22) is allocated to the second printer (6), said second sensor generating a second sensor signal (G1) which is dependent on the web speed of the web (10) in the second printer (6), wherein based on the two sensor signals (G1, G2) a scaling parameter (P) is determined which is used to compensate the longitudinal distortion.

Description

[0001] Description Method for operating a printing system, printing system and computer program product

[0002] The invention relates to a method for operating a printing system, such a printing system and a computer program product in connection therewith.

[0003] In a printing system, a web, such as a paper web, is printed with a print. The print is usually composed of several consecutively printed images. This is also referred to as "overprinting." For example, a colored print image is selectively overprinted with a varnish. With this type of overprinting, the most accurate alignment (registration) of the various print images relative to one another is desirable.

[0004] The various print images are each printed with a separate printer. Between the various printers, the web travels a certain path during which the web changes its dimensions due to drying and / or moistening, namely its length in the longitudinal direction (longitudinal distortion) and its width in the transverse direction (transverse distortion). Accordingly, a print image printed with one printer may be distorted and then not correctly overprinted with a subsequent print image on the next printer downstream. The problem is further exacerbated when material parameters (e.g., grammage) and / or operating parameters (e.g., web speed) change dynamically.

[0005] Against this background, one object of the invention is to improve the overprinting of a first print image with a second print image. The overprinting should be as precise as possible. In particular, dynamic compensation of the distortion experienced by the first print image until it is overprinted with the second print image should also be possible. For this purpose, an improved method for operating a printing system is to be specified. Furthermore, a corresponding printing system and a computer program product are to be specified.

[0006] The object is achieved according to the invention by a method having the features according to claim 1, by a printing system having the features according to claim 13 and by a computer program product having the features according to claim 14. Advantageous embodiments, further developments and variants are the subject of the dependent claims. The statements in connection with the method also apply mutatis mutandis to the printing system and the computer program product and vice versa. If steps of the method are stated implicitly or explicitly below, advantageous embodiments for the printing system arise from the fact that it is designed to carry out one or more of these steps. For this purpose, the printing system has in particular a correspondingly designed control unit.

[0007] The method is used to operate a printing system that has a first printer and a second printer. During operation, a web runs through the printing system at a specific web speed. The web speed may vary along the printing system. The first printer prints a first print image onto the web, and the second printer then overprints the first print image with a second print image (subsequent print image). This is referred to as "overprinting"; a fixed relationship (target position) between the two print images relative to one another is specified. The two print images together form a print.

[0008] Preferably, both printers are digital printers. Preferably, the first printer is a digital multi-color printer, in particular a CMYK inkjet printer, and the second printer is a digital varnish printer. Such an embodiment is assumed below, without loss of generality. The multi-color printer prints several different colors, while the varnish printer prints a varnish, in particular a transparent varnish. In both printers, printing is carried out in a position-selective (digital) manner, i.e. in the varnish printer in particular, not just a simple full-surface application takes place, but the varnish is only printed selectively, in particular where at least one color has already been printed previously by the multi-color printer. The actual printing of the respective print image in the respective printer takes place at a so-called printing point (first printing point of the first printer and second printing point of the second printer).The printing point is the location in the printer where the ink / varnish is applied to the web to create the print image. The printing point is therefore not a location on the web, but rather a fixed point within the printing system through which the web is conveyed.

[0009] The web is, in particular, a paper web. Both printers are arranged inline, meaning the web is fed seamlessly from the first printer to the second printer and optionally undergoes additional processing between the two printers. The web generally has a longitudinal direction, which corresponds to the conveying direction of the web and is also referred to as the "machine direction." The two printers are, in particular, fundamentally independent and / or separate machines. The two printers are, in particular, spatially separated from one another, specifically spaced apart longitudinally.

[0010] The first printed image undergoes longitudinal distortion between the first printer and the second printer, i.e. distortion in the longitudinal direction. This also results in an actual position which deviates from the target position. Typically, the printed image also undergoes transverse distortion, which is usually greater than the longitudinal distortion. In the present case, however, the focus is on the longitudinal distortion, which can be up to 1 mm / m, for example. Distortion is generally understood to mean shrinkage or stretching. In the following, longitudinal shrinkage is assumed without loss of generality. In an expedient embodiment, the longitudinal distortion results in particular from the fact that at least one drying and / or moistening unit is arranged between the first printer and the second printer, with which the web (and thus also the first printed image) is dried and / or moistened.A first sensor is assigned to the first printer, which generates a first sensor signal that is a measure of the web speed of the web in the first printer. Similarly, a second sensor is assigned to the second printer, which generates a second sensor signal that is a measure of the web speed of the web in the first printer. "Assigned" preferably means that the respective sensor is part of the respective printer. Each printer therefore has a sensor that measures a parameter that is directly or indirectly dependent on the web speed in order to output a corresponding sensor signal depending on the parameter.

[0011] optically by means of a laser sensor or a camera, or mechanically or in some other way. A particularly advantageous embodiment is one in which a first roller which guides the web is assigned to the first printer, the first sensor being a first sensor which measures a rotation of the first roller and in the process generating the first sensor signal, and in which, analogously, a second roller which guides the web is assigned to the second printer, the second sensor being a second sensor which measures a rotation of the second roller and in the process generating the second sensor signal. In this embodiment, the web speed is thus measured by means of sensors on rollers for guiding the web. A respective roller is therefore also referred to as a measuring point. The sensor is expediently a rotary sensor, e.g. an encoder.However, the exact method used for the measurement and the measuring principle used are of secondary importance. What is more important is that the sensor effectively measures the web speed, i.e. the speed of the web in the longitudinal direction, in the respective printer. In the aforementioned configuration with sensors, the web speed at the respective roller is measured by measuring the rotation of the respective roller. The sensor signal is therefore a measure of the web speed at the respective roller. In the following, without loss of generality, the aforementioned configuration with sensors on rollers is assumed; the two sensor signals are then also referred to as sensor signals. However, the explanations also apply analogously to any other measuring principle with which two sensor signals are generated as a respective measure of the web speed at two different measuring points in the printing system.

[0012] Based on the two sensor signals, a scaling parameter (also known as a scaling factor) is determined, or more precisely, a value of the scaling parameter at a given point in time. The scaling parameter is then used to compensate for the longitudinal distortion, i.e. to reduce it at least partially or completely. The scaling parameter is determined, for example, as the difference or ratio of the two sensor signals or derived from it, for example, using a table or a function. There are various ways of compensating for the longitudinal distortion using the scaling parameter, some of which are explained in detail below. What is more important is that in this case the two sensor signals are used to correct the longitudinal distortion instead of, for example, markings on the track. The scaling parameter is actually also a measure of the longitudinal distortion, so this is also measured.

[0013] A key idea of ​​the invention is in particular the use of sensors, preferably encoders, in the printing system to correct (compensate) the longitudinal distortion experienced by the first print image on its way to the second printer. This path is usually long and lies, for example, in the range of 10 m to 100 m. Without correction of the longitudinal distortion, the second print image would be printed longer or shorter than the first print image, resulting in a suboptimal overlap. For example, with a longitudinal distortion of 0.5 mm / m (= 0.05%), with a 5 m long print and with optimal overlap, there would be an offset of 2.5 mm at the beginning of the two print images and at the end of their length. In the present case, it has now been recognized that the longitudinal distortion also results in a difference in the web speed in the first printer and the second printer, i.e. the web speed varies along the printing system.In the case of longitudinal shrinkage, the web speed in the second printer is lower than in the first printer. The web speed can be measured using an appropriate sensor (e.g. a sensor on one of the rollers of the printing system). As a result, longitudinal distortion control is achieved in particular based on two sensor signals at two different measuring points. The respective sensor is expediently located as close to the respective printing point, i.e. is at least assigned to the respective printer. The closer the respective measuring point is to the respective printing point, the more accurately the longitudinal distortion can be compensated. In principle, however, it is initially sufficient if there is no or only slight longitudinal distortion between the measuring point and the respective printing point and if, in particular, the web is neither dried nor moistened there.

[0014] In particular, the two rollers are each also driven, i.e. they each have their own drive. The two drives are independent of one another. The respective sensor in the form of a transmitter is then located, for example, on the drive of the corresponding roller or on a central or rotary axis of the roller. Optionally, both drives and thus also the web speed on a respective roller are controlled by a web tension control. In this case, the printing system has a measuring roller between each two driven rollers, with which the web tension is measured in order to then suitably control one of the driven rollers depending on the result. However, the invention can also be used in a printing system without web tension control.

[0015] Preferably, the first sensor and the second sensor are each coupled (also: “connected”) to the web without slippage. The respective sensor signal is therefore a particularly accurate measure of the web speed. Especially with two rollers and two encoders, the web is guided slip-free by the first roller and the second roller. This improves the accuracy of the compensation of the longitudinal distortion, as it is now ensured that the web speed is measured as precisely as possible. It is particularly expedient if the web is guided slip-free overall between the printing point and the respective measuring point. A slip-free coupling of the encoder to the web can be achieved, for example, by the roller having a suitable coating. Alternatively or additionally, the roller can be wrapped around the web in such a way that it is self-locking.For this purpose, a wrap of more than 180°, in particular of at least 270°, is particularly suitable.

[0016] A preferred embodiment is one in which the first roller is a printing cylinder of the first printer and / or the second roller is a printing cylinder of the second printer. The printing cylinder is the printer roller over which the web is guided when the print image is printed. The measuring point and the printing point are then identical, which results in maximum accuracy when compensating for longitudinal distortion. Alternatively, however, it is also possible to use a different roller instead of the printing cylinder, which is expediently located inside the printer and, in any case along the web, as close as possible to the printing cylinder and thus as close as possible to the printing point. A suitable roller, for example, is one which defines a wrap around the printing cylinder by the web.Such a roller is positioned immediately before and after the printing cylinder to guide the web upstream of the printing cylinder and remove it downstream, thereby determining the wrap (defined by an angle) of the printing cylinder. In general, however, any roller that is positioned as close as possible to the printing point along the web and is coupled to the web up to the printing point with as little slippage as possible is suitable.

[0017] Each of the printers preferably has a number of print nozzles for printing the respective print image. The print nozzles are arranged, in particular, in an arc around the printing cylinder of the respective printer. “A number of” is understood to mean “one or more”; typically, a respective printer has a large number (approximately 10 to 500) of print nozzles. The print nozzles are then controlled such that they print the respective print image line by line. For this purpose, the print nozzles are activated (“fired”) at a specific cycle that depends on the web speed, in order to always be able to print the respective print image with the same length at different web speeds. Therefore, a sensor, in particular a transmitter, is typically already present on the respective printing cylinder, with which the timing of the print nozzles is then controlled accordingly.The sensor signal from this sensor therefore determines at what times the print nozzles are activated to create a single dot of the print image. This sensor is now advantageously also used to compensate for the longitudinal distortion, as described, so that no additional hardware is required for this purpose and existing components of the printing system are used to implement the invention. In a correspondingly suitable embodiment, the first sensor signal is then used to control the timing of print nozzles for printing the first print image, and the second sensor signal is used to control the timing of print nozzles for printing the second print image.

[0018] For timing the print nozzles, a precision sensor is preferably used, e.g., with an accuracy in the range of 1 / 10 mm to 1 / 100 mm. However, such high accuracy is not required for the compensation of longitudinal distortion described here; a lower accuracy is sufficient.

[0019] Advantageously, the scaling parameter modifies a control signal that controls a number of print nozzles (particularly the print nozzles already mentioned above) of one of the two printers. As a result, the corresponding print image is printed with a longitudinal distortion, particularly corresponding to the longitudinal distortion of the first print image, ultimately resulting in optimal overprinting. The control signal controls the print nozzles in a time-dependent manner so that they print the print image line by line on the passing web. The control signal thus determines the timing of the print nozzles described above.

[0020] On the one hand, the control signal is particularly dependent on the sensor signal, which specifies the timing, i.e. when a particular line is printed, and adapts it to the web speed. On the other hand, the control signal is also particularly dependent on print data (image data) which define the print image, i.e. which print nozzle is activated in a given line of the print image in order to then print a corresponding pixel. The control signal (first control signal) for the print nozzles of the first printer is therefore formed from the first sensor signal and the print data for the first print image. Analogously, the control signal (second control signal) for the print nozzles of the second printer is formed from the second sensor signal and the print data for the second print image. It is also conceivable for only one of the two printers to have print nozzles, since it is ultimately sufficient if one of the two print images is scaled.

[0021] In order to modify the control signal, the scaling parameter can be applied at one or more different points. In a first suitable embodiment, the control signal is generated based on one of the two sensor signals, which is scaled using the scaling parameter. The scaling parameter is therefore applied to one of the sensor signals, e.g. the second sensor signal, in order to scale this and thus ultimately also the control signal. This has the advantage that the print data can remain unchanged. Expediently, however, the sensor signal is only scaled after the scaling parameter has been determined on its basis. In a second suitable embodiment, at least one of the print images is defined by print data (as already mentioned), which is then scaled using the scaling parameter and by means of which the control signal is then generated.The scaling parameter is thus applied to the print data, leaving the sensor signal unaffected. The current longitudinal distortion is thus immediately recognizable from the print data, which is also displayed in real time, for example. The first and second embodiments can also be combined, e.g., by only partially scaling the sensor signal and the print data in order to optimally compensate for the longitudinal distortion in combination. A configuration in which the sensor signal and the print data are retained unchanged, and only the control signal formed from them is scaled with the scaling parameter, is also conceivable and suitable.

[0022] In this case, the scaling parameter is used to adjust the lengths of the two print images to each other in order to compensate for the longitudinal distortion. However, any fixed longitudinal offset, i.e. a constant shift in the longitudinal direction by a fixed value along the web, cannot be corrected in this way. For this purpose, a mark is expediently used, which is printed, for example, with the first printer and is used by the second printer as a start signal (trigger signal) for printing the second print image. The sensors, especially in a configuration as encoders, are too imprecise to generate a start signal, especially because of the long path for the web between the two printers. The mark only ensures a reasonably optimal overprint at the beginning of the print. The scaling parameter then prevents a progressive divergence of the two print images along the print, i.e.This prevents an increasing longitudinal offset across the print. The mark itself is typically too small to detect longitudinal distortion, but the detection and corresponding compensation of transverse distortion are more likely with the mark, since transverse distortion is typically a factor of 3 to 10 greater than longitudinal distortion.

[0023] Suitably, a pre-scaling is specified for one of the two print images, preferably the first print image, depending on at least one material parameter of the web and / or at least one operating parameter of the printing system, so that this print image is printed in a pre-scaled manner. The idea behind this is that the required value for the scaling parameter can be roughly estimated in advance depending on the material parameters and / or operating parameters and, based on this, a dynamic modification of the scaling parameter depending on the two sensor signals is then significantly easier. The pre-scaling is therefore a rough presetting (default value) for subsequent fine control. The pre-scaling is determined depending on one or more material parameters and / or one or more operating parameters, e.g. using a database in which corresponding empirical values ​​or test results are stored.Suitable material parameters include the paper type or grammage. Suitable operating parameters include the web speed or the temperature and / or humidity to which the web is exposed. The scaling parameter is expediently determined repeatedly, in particular continuously, so that the longitudinal distortion is dynamically compensated during operation. This allows for a response, in particular, to rapid changes during operation of the printing system, which may result, for example, from temperature and / or humidity control, which exposes the web to varying temperature and / or humidity between the two printers. The system also responds accordingly to dynamic changes in web speed, e.g. due to web tension control or when changing jobs.Overall, a longitudinal distortion control, in particular a longitudinal shrinkage control, is realized in which the longitudinal distortion that occurs between the two printers is measured and compensated by suitable control of the first printer and / or the second printer.

[0024] A printing system according to the invention is designed to carry out a method as described above and, for this purpose, has, in particular, a correspondingly designed control unit. In particular, the two sensors and the print nozzles (if present) are connected to the control unit. In particular, the print data for the two print images are also transmitted to the control unit, which then derives corresponding control signals for the print nozzles from these print data and the sensor signals. The control unit also determines, in particular, the scaling parameter and, preferably, uses this to modify the control signal.

[0025] The computer program product according to the invention has instructions which cause the printing system to carry out a method as described above.

[0026] In the following, exemplary embodiments of the invention are explained in more detail with reference to a drawing. In each case, the following schematically show:

[0027] Fig. 1 a method,

[0028] Fig. 2 a printing system, Fig. 3a a first print image at a first printing point,

[0029] Fig. 3b the first print image from Fig. 3a at a second printing point,

[0030] Fig. 3c shows the first print image from Fig. 3b and a second print image at the second printing point according to Fig. 3b,

[0031] Fig. 4 shows part of the printing system from Fig. 2.

[0032] Fig. 1 shows an embodiment of a method for operating a printing system 2. An exemplary printing system 2 with which the method is carried out is shown in Fig. 2. The printing system 2 has a first printer 4 and a second printer 6. A web 10 runs at a web speed through the printing system 2. In a first step S1, the first printer 4 prints a first print image 8 onto the web 10 (here a paper web) and then, in a second step S2, the second printer 6 overprints the first print image 8 with a second print image 12 (subsequent print image). This is referred to as "overprinting" and a fixed relationship (target position) of the two print images 8, 12 relative to one another is specified. The two print images 8, 12 together form a print.

[0033] In the illustrated embodiment, both printers 4, 6 are digital printers. Specifically, the first printer 4 is a digital multi-color printer, here a CMYK inkjet printer, and the second printer 6 is a digital varnish printer for spot varnishing. The multi-color printer prints several different colors, while the varnish printer prints a transparent varnish. In both printers 4, 6, position-selective (digital) printing takes place. The actual printing of the respective print image 8, 12 in the respective printer 4, 6 takes place at a so-called printing point D1, D2 (first printing point D1 of the first printer 4 and second printing point D2 of the second printer 6). The printing point D1, D2 is the point in the printer 4, 6 at which the ink / varnish is applied to the web 10 to produce the print image 8, 12. Both printers 4, 6 are arranged inline, i.e.The web 10 is fed uninterrupted from the first printer 4 to the second printer 6 and optionally undergoes additional processing between the two printers 4 and 6. The web 10 generally has a longitudinal direction L, which corresponds to a conveying direction of the web 10 and is also referred to as the "machine direction."

[0034] The first printed image 8 undergoes longitudinal distortion, i.e. distortion in the longitudinal direction L, between the first printer 4 and the second printer 6, i.e. between the first step S1 and the second step S2. This is shown as an example in Figs. 3a and 3b, where Fig. 3a shows the first printed image 8 at the first printing point D1 and Fig. 3b shows the longitudinally distorted first printed image 8 at the second printing point D2 (solid line) in comparison to the first printed image 8 at the first printing point D1 (dashed line). If the second printed image 12 were to be printed at the second printing point D2 to fit the first printed image 8 at the first printing point D1, namely according to the dashed line in Fig. 3b, this would result in incorrect coverage. In the example shown in Figs. 3a, 3b, the first printed image 8 undergoes longitudinal shrinkage, and the second printed image 12 would be printed too long without correction. This is exemplified in Fig.3c, which shows both printed images 8, 12 superimposed. In the illustrated embodiment, the longitudinal distortion results from the fact that at least one drying and / or moistening unit 14 is arranged between the first printer 4 and the second printer 6, with which the web 10 (and thus also the first printed image 8) is dried and / or moistened.

[0035] Typically, the printed image 8 also experiences transverse distortion (not shown), which is usually greater than the longitudinal distortion. However, transverse distortion is not the subject of this paper and will therefore not be considered further here.

[0036] In the exemplary embodiment shown here, the first printer 4 is assigned a first roller 16, which guides the web 10, as well as a first sensor 18, here a sensor, which measures a rotation of the first roller 16 and generates a first sensor signal G1 (first sensor signal). Similarly, the second printer 6 is assigned a second roller 20, which guides the web 10, as well as a second sensor 22, here also a sensor, which measures a rotation of the second roller 20 and generates a second sensor signal G2 (second sensor signal). Instead of measuring the rotation of the rollers 16, 20 with sensors 18, 22 designed as sensors, other measuring concepts are also conceivable and suitable. It is particularly important that the sensors 18, 22 each generate a sensor signal G1, G2, which is a measure of the web speed of the web 10 in the respective printer 4, 6. The rotation, e.g. measured as speed, of the respective roller 16, 20, is such a measure.

[0037] The measurements are carried out continuously and parallel to steps S1 and S2. The respective roller 16, 20 is in this case part of the respective printer 4, 6. Each printer 4, 6 therefore has a roller 16, 20 whose rotation is measured by a respective sensor 18, 22 in order to output a corresponding sensor signal G1, G2 as a function thereof. A respective roller 16, 20 is therefore also referred to as a measuring point. The sensor 18, 22 is, for example, a rotary encoder. The two sensors 18, 22 then effectively also measure a web speed, i.e. a speed of the web 10 in the longitudinal direction L, at the respective roller 18, 22. The respective sensor signal G1, G2 is thus a measure of the web speed at the respective roller 16, 20.

[0038] Based on the two sensor signals G1, G2, a scaling parameter P (also known as a scaling factor) is now determined in a third step S3, more precisely a value of the scaling parameter P at a given point in time. The scaling parameter P is then used in a fourth step S4 to compensate for the longitudinal distortion, i.e. to reduce it at least partially or completely. An exemplary embodiment of this is shown in Fig. 4, and this and variants thereof are explained in more detail below. In the exemplary embodiment shown, the scaling parameter P is determined as the difference A between the two sensor signals G1, G2 or derived therefrom using a table or a function (not explicitly shown). The scaling parameter P is actually also a measure of the longitudinal distortion, so that this is effectively also measured in the present case.In the present case, the sensors 18, 22 of the printing system 2 are therefore used to correct (compensate) the longitudinal distortion which the first printed image 8 experiences on its way to the second printer 6. This path is usually long and lies, for example, in the range of 10 m to 100 m. The longitudinal distortion results in a difference in the web speed in the first printer 4 and the second printer 6. In the event of longitudinal shrinkage, the web speed in the second printer 6 is lower than in the first printer 4, which leads to a result as shown in Figs. 3a to 3c. The web speed is measured with a corresponding sensor 18, 22 on one of the rollers 16, 20 of the printing system 2. Based on two sensor signals G1, G2 at two different measuring points, a longitudinal distortion control is then implemented. The respective sensor 18, 22 is located as close as possible to the respective printing point D1, D2, and is therefore at least assigned to the respective printer 4, 6.The closer the respective measuring point (roller 16, 20) is to the respective printing point D1, D2, the more accurately the longitudinal distortion can be compensated. In principle, however, it is initially sufficient if there is little or no longitudinal distortion between the measuring point and the respective printing point D1, D2, and if, in any case, neither drying nor moistening of the web 10 takes place there.

[0039] In the exemplary embodiment of Fig. 2, the first roller 16 is a printing cylinder of the first printer 4 and the second roller 20 is a printing cylinder of the second printer 6. The measuring point and the printing point D1, D2 are then identical. Alternatively, however, it is also possible to use a different roller instead of the printing cylinder, which is expediently located inside the printer 4, 6 and in any case along the web 10 as close as possible to the printing cylinder and thus as close as possible to the printing point D1, D2. A suitable example is a roller 24 which defines a wrap around the printing cylinder by the web 10. Such a roller 24 is arranged in Fig. 2 immediately before and after the printing cylinder in order to guide the web 10 upstream of the printing cylinder to the latter and to remove it again downstream of the printing cylinder and thereby define the wrap around the printing cylinder.In general, however, any roller is suitable which is located as close as possible to the printing point D1, D2 along the path 10 and which is also coupled to the path 10 up to the printing point D1, D2 with as little slippage as possible.

[0040] In this case, the two rollers 16, 20 are each also driven, i.e., they each have their own drive (not shown). The two drives are independent of each other. The respective sensor 18, 22 is then located, for example, on the drive of the corresponding roller 16, 20 or on a central or rotational axis A of the roller 16, 20. Optionally, both drives, and thus also the web speed at a respective roller 16, 20, are controlled by a web tension control (not shown).

[0041] In this case, the first sensor 18 and the second sensor 22 are each coupled (also: "connected") to the web 10 in a slip-free manner. The web 10 is therefore guided by the two rollers 16, 20 in a slip-free manner. A slip-free coupling of the sensor 18, 22 to the web 20 is achieved, for example, by providing the roller 16, 20 with a suitable coating. In the embodiment of Fig. 2, for slip-free guidance, the roller 16, 20 is wrapped around the web 10 in such a way that a self-locking effect is achieved.

[0042] Each of the printers 4, 6 shown here has a number of print nozzles 26, 28 for printing the respective print image 8, 12. "A number of" is understood to mean "one or more." The print nozzles 26, 28 are then controlled such that they print the respective print image 8, 12 line by line. For this purpose, the print nozzles 26, 28 are activated ("fired") at a specific cycle, which depends on the web speed, in order to always be able to print the respective print image 8, 12 with the same length at different web speeds. Therefore, a sensor 18, 22 (special encoder) is already present on the respective printing cylinder, with which the timing of the print nozzles 26, 28 is then controlled accordingly. The sensor signal G1, G2 of this sensor 18, 22 therefore determines at which times the print nozzles 26, 28 are activated in order to produce a single dot of the print image 8, 12.This sensor 18, 22 is now also used to compensate for the longitudinal distortion as described. The first sensor signal G1 then controls the timing of (first) print nozzles 26 for printing the first print image 8, and the second sensor signal G2 controls the timing of (second) print nozzles 28 for printing the second print image 12.

[0043] In this case, the scaling parameter P modifies at least one of two control signals S1, S2, which are used to control the print nozzles 26, 28 of the two printers 4, 6. As a result, the corresponding print image 8, 12 is printed with a longitudinal distortion corresponding to the longitudinal distortion of the first print image 8, ultimately resulting in optimal overprinting. The control signals S1, S2 control the print nozzles 26, 28 in a time-dependent manner such that they print the respective print image 8, 12 line by line on the passing web 10. The control signals S1, S2 thus determine the timing of the print nozzles 26, 28.

[0044] As also shown in Fig. 4, the respective control signal S1, S2 is on the one hand dependent on one of the sensor signals G1, G2, which specifies the timing, i.e. when a respective line is printed. On the other hand, the respective control signal S1, S2 is also dependent on print data X1, X2 (image data) which define the respective print image 8, 12, i.e. which print nozzle 26, 28 is activated in a given line of the print image 8, 12 in order to then print a corresponding pixel. The control signal S1 (first control signal) for the print nozzles 26 of the first printer 4 is therefore formed from the first sensor signal G1 and from the print data X1 for the first print image 8. Analogously, the control signal S2 (second control signal) for the print nozzles 28 of the second printer 6 is formed from the second sensor signal G2 and from the print data X2 for the second print image 12.It is also conceivable that only one of the two printers 4, 6 has print nozzles 26, 28, since it is ultimately sufficient if one of the two print images 8, 12 is scaled.

[0045] In order to modify the control signal S1, S2, the scaling parameter P can be applied at one or more different points. Four possible points are indicated by dashed lines in Fig. 4. Exactly one of these is used here, but in principle a combination is also possible. In a first embodiment, the control signal S1, S2 is generated based on one of the two sensor signals G1, G2, which is scaled with the scaling parameter P. The scaling parameter P is therefore applied to one of the sensor signals G1, G2 in order to scale this and thus ultimately also the control signal S1, S2 derived from it. The print data X1, X2 can remain unchanged. In a second embodiment, the print data X1, X2, which define the print images 8, 12, are scaled with the scaling parameter P and the respective control signal S1, S2 is then generated using this scaled print data X1, X2.It is sufficient if the scaling parameter P is applied either to the print data X1 or to the print data X2. The sensor signal G1, G2 can thus remain unaffected. A configuration not explicitly shown is also conceivable in which the sensor signal G1, G2 and the print data X1, X2 are retained unchanged, and only the control signal S1, S2 formed from these is scaled with the scaling parameter P.

[0046] In this case, the lengths (measured in the longitudinal direction L) of the two printed images 8, 12 are adjusted to one another using the scaling parameter P in order to compensate for the longitudinal distortion. However, any fixed longitudinal offset, i.e. a constant shift in the longitudinal direction L by a value fixed along the web 10, cannot be corrected in this way. For this purpose, a mark 30 is used in the present case, which is printed by the first printer 4 and is used by the second printer 6 as a start signal for printing the second printed image 12. An exemplary mark 30 is shown in Figs. 3a to 3c. By means of the mark 30, a reasonably optimal overprint is ensured only at the beginning of the print. The scaling parameter P then prevents a progressive divergence of the two printed images 8, 12 along the print, i.e. an increasing longitudinal offset across the print, so to speak.

[0047] In the present case, a pre-scaling 32 is optionally specified for one of the two print images 8, 12 depending on at least one material parameter of the web 10 and / or at least one operating parameter of the printing system 2, so that this print image 8, 12 is printed in a pre-scaled manner. This is based on the consideration that the required value for the scaling parameter P can be roughly estimated in advance depending on the material parameters and / or operating parameters and, based on this, a dynamic modification of the scaling parameter P depending on the two sensor signals G1, G2 is then significantly easier. The pre-scaling 32 is therefore a rough presetting (default value) for subsequent fine control. The pre-scaling 32 is determined depending on one or more material parameters and / or one or more operating parameters, e.g. using a database in which corresponding empirical values ​​or test results are stored.Examples of material parameters are paper type or grammage of the paper (ie the web 10), examples of operating parameters are web speed or temperature and / or humidity to which the web 10 is exposed.

[0048] In this case, the scaling parameter P is determined repeatedly, specifically continuously, so that the longitudinal distortion is dynamically compensated during operation. This reacts to rapid changes during operation of the printing system 2, which result, for example, from temperature and / or humidity control, by means of which the web 10 between the two printers 4, 6 is subjected to varying temperature and / or humidity. A corresponding reaction is also taken to a dynamic change in the web speed, e.g. due to web tension control or when changing jobs. Overall, therefore, a longitudinal distortion control is implemented in which the longitudinal distortion resulting between the two printers 4, 6 is measured and compensated by suitable control of the first printer 4 and / or the second printer 6.

[0049] To carry out the method as described above, the printing system 2 has a correspondingly designed control unit 34, which is shown in Fig. 4. The two sensors 18, 22 and the print nozzles 26, 28 are connected to the control unit 34. The print data X1, X2 for the two print images 8, 12 are also transmitted to the control unit 34, which then derives corresponding control signals S1, S2 for the print nozzles 26, 28 from these print data X1, X2 and the sensor signals G1, G2. The control unit 34 also determines the scaling parameter P and uses this to modify one or both control signals S1, S2.

[0050] List of reference symbols

[0051] 2 printing system

[0052] 4 first printer (CMYK)

[0053] 6 second printer (varnish)

[0054] 8 first print image

[0055] 10 webs (paper web)

[0056] 12 second print image

[0057] 14 Drying and / or humidifying unit

[0058] 16 first roller

[0059] 18 first sensor (first encoder)

[0060] 20 second roller

[0061] 22 second sensor (second encoder)

[0062] 24 rollers

[0063] 26 (first) printing nozzles

[0064] 28 (second) pressure nozzles

[0065] 30 mark

[0066] 32 Prescaling

[0067] 34 Control unit

[0068] A axis of rotation

[0069] D1 first pressure point

[0070] D2 second pressure point

[0071] G1 first sensor signal (first encoder signal)

[0072] G2 second sensor signal (second encoder signal)

[0073] L longitudinal direction

[0074] P scaling parameters

[0075] 51 first step (print first image)

[0076] 52 second step (print second image)

[0077] X1 (first) print data (for the first print image)

[0078] X2 (second) print data (for the second print image)

[0079] A Difference (of the sensor signals)

Claims

Claims 1 . Method for operating a printing system (2) which has a first printer (4) and a second printer (6), a. wherein a web (10) runs through the printing system (2) at a web speed, b. wherein a first print image (8) is printed onto the web (10) using the first printer (4) and the first print image (8) is subsequently overprinted with a second print image (12) using the second printer (6), c. wherein the first print image (8) undergoes longitudinal distortion between the first printer (4) and the second printer (6), d. wherein a first sensor (18) is assigned to the first printer (4), which generates a first sensor signal (G1) which is a measure of the web speed of the web (10) in the first printer (4), e. wherein the second printer (6) is assigned a second sensor (22) which generates a second sensor signal (G1) which is a measure of the web speed of the web (10) in the second printer (6), f.wherein a scaling parameter (P) is determined on the basis of the two sensor signals (G1, G2), with which the longitudinal distortion is compensated.

2. Method according to claim 1, wherein the first printer (4) is assigned a first roller (16) which guides the web (10), wherein the first sensor (18) is a first encoder which measures a rotation of the first roller (16) and in the process generates the first sensor signal (G1), wherein the second printer (6) is assigned a second roller (20) which guides the web (10), wherein the second sensor (22) is a second encoder which measures a rotation of the second roller (20) and in the process generates the second sensor signal (G2).

3. The method according to claim 2, wherein the first sensor (16) and the second sensor (20) are each coupled to the web (10) in a slip-free manner.

4. The method according to claim 2 or 3, wherein the first roller (16) is a printing cylinder of the first printer (4) and / or the second roller (20) is a printing cylinder of the second printer (6).

5. Method according to one of claims 1 to 4, wherein the first sensor signal (G1) controls a timing of printing nozzles (26) for printing the first print image (8) and the second sensor signal (G2) controls a timing of printing nozzles (28) for printing the second print image (12).

6. Method according to one of claims 1 to 5, wherein the scaling parameter (P) is used to modify a control signal (S1, S2) with which a number of print nozzles (26, 28) of one of the two printers (4, 6) are controlled.

7. The method according to claim 6, wherein the control signal (S1, S2) is generated from one of the two sensor signals (G1, G2), which is scaled with the scaling parameter (P).

8. The method according to claim 6, wherein at least one of the print images (8, 12) is defined by print data (X1, X2) which are scaled with the scaling parameter (P) and by means of which the control signal (S1, S2) is generated.

9. Method according to one of claims 1 to 8, wherein depending on at least one material parameter of the web (10) and / or at least one operating parameter of the printing system (2) for a pre-scaling (32) is specified for one of the two print images (8, 12) so that this print image (8, 12) is printed in a pre-scaled manner.

10. The method according to any one of claims 1 to 9, wherein the scaling parameter (P) is determined recurringly, in particular continuously, so that the longitudinal distortion is dynamically compensated during operation.

11. Method according to one of claims 1 to 10, wherein at least one drying and / or moistening unit (14) is arranged between the first printer (4) and the second printer (6), with which the web (10) is dried and / or moistened.

12. Method according to one of claims 1 to 11, wherein the first printer (4) is a digital multi-color printer, in particular a CMYK inkjet printer, and wherein the second printer (6) is a digital varnish printer.

13. Printing system (2) which is designed to carry out a method according to one of claims 1 to 12.

14. A computer program product comprising instructions causing the printing system (2) according to claim 13 to execute the method according to any one of claims 1 to 12.

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