Method for operating a printing device, printing device and composite consisting of a printing device of this kind and a collator

Abstract

A method for operating a printing device (2) is provided, the printing device (2) having a printing unit (8) for printing on a web (10) according to a predetermined job using predetermined job data, the web (10) being conveyed to pass through the printing device (2) at a predetermined web speed, the printing device (2) having at least one dryer (18, 20) with an adjustable drying capacity for drying the web (10) and thereby causing shrinkage in the web (10), the printing device (2) having at least one humidifier (22) with an adjustable humidifying capacity for humidifying the web (10) and thereby adjusting the moisture content of the web (10), the actual shrinkage and actual moisture content of the web (10) being determined and target shrinkage and target moisture content being set, and the shrinkage and the moisture content being closed-loop controlled to the target shrinkage and the target moisture content by adjusting the drying capacity and the humidifying capacity depending on the actual shrinkage and the actual moisture content. Furthermore, a printing device (2) and a composite consisting of a printing device (2) of this kind and a collator (4) are provided.

Description

Technical Field

[0001] The present invention relates to a method for operating a printing device, and to a device comprising such a printing device, optionally combined with a collator.

[0002] The printing device is used to print a printing image, briefly a printed image, onto a web. The printed image, and thus the web, are usually dried by a dryer (for example a hot air dryer and / or an IR dryer), whereby the printed image and the web shrink, and accordingly moisture is necessarily removed from the web.

[0003] During operation, a problem arises with the change in the web speed when the web is transported through the printing device. If the web speed is different, the time for which the web is exposed to the dryer will accordingly be different, so that the shrinkage of the web, as well as the moisture of the web, will vary depending on the web speed. This is disadvantageous especially with regard to the dimensional accuracy of the printed image and the maintenance of a specific moisture content, for example for further processing.

[0004] This problem can basically be avoided by simply operating the printing device at a constant web speed. This is particularly conceivable in a stand-alone operation or a roll-to-roll operation in which the web is unwound in front of the printing device and wound up behind the printing device. However, in an in-line operation of a complex comprising a printing device and at least one other device, i.e. when the web speed is constant, the performance of the complex may in some cases be limited. Therefore, it is desirable to keep the shrinkage and moisture of the web at the outlet side of the printing device as constant as possible, even though the web speed is variable.

[0005] Against this background, the object of the present invention is to achieve as flexible an operation as possible of the printing device. For this purpose, a corresponding method for operating the printing device should be provided. Furthermore, a corresponding printing device, as well as a complex comprising a printing device and a collator should be provided.

[0006] According to the present invention, this problem is solved by a method having the features described in claim 1 and a composite comprising a printing device or a printing device of that kind and a collator having the features described in claim 13. Advantageous configurations, developments and variants are the subject of the dependent claims. The explanations given in connection with the method apply mutatis mutandis to the printing device and the composite, and vice versa. In the following, insofar as the steps of the method are described, advantageous configurations for the printing device and the composite result from the fact that the printing device and the composite are designed to carry out one or more of those steps. For this purpose, the printing device or the composite has, in particular, a control unit which is correspondingly designed.

[0007] The core idea of the present invention is, in particular, a closed-loop control of the dimensional stability of digital printed products, taking into account the water content of the paper, and more precisely, a closed-loop control of both the shrinkage and the moisture of the web printed in the printing device each time.

[0008] The method according to the present invention is used to operate a printing device. This operation is, in particular, an in-line operation, i.e., an operation of a printing device which forms a composite in combination with at least one other device, but does not necessarily have to be an in-line operation. The printing device has a printing unit for printing on a web according to a predetermined job using predetermined job data defining a job (also referred to as a "print job"). For this purpose, the printing unit has, in particular, one or more printing heads. The printing unit is also referred to as a printing chamber. In particular, the printing device is a digital printing device and carries out printing using an inkjet method.

[0009] The job data of a job is set, in particular, using a higher-level control unit, for example, directly by this higher-level control unit itself or by an operator who inputs the job data to the higher-level control unit via an interface (also referred to as visualization). The higher-level control unit is part of the printing device or is formed separately from the printing device. The job data includes, in particular, the production mode in which the printing device should operate and web data. The production mode defines, in particular, specific print quality, the amount of ink used for printing, whether a primer should be applied before printing, whether a varnish or overprint varnish should be applied after printing, etc., or combinations thereof. The web data includes, in particular, paper type, basis weight of the web, etc., or combinations thereof. Configuration data (also referred to as a recipe) is derived, in particular, from the job data, and this configuration data appropriately includes corresponding machine data for settings that depend on the job of the printing device. Here, the importance of deriving configuration data (lower level) from job data (higher level) is low and is carried out based on, for example, characteristic curves, tables, and / or appropriate models obtained experimentally. Those characteristic curves, tables, and / or appropriate models are stored, for example, in the higher-level control unit. Subsequently, the configuration data is transferred to the machine control unit of the printing device in order to perform corresponding adjustment and control of the printing device.

[0010] The web is generally conveyed so as to pass through the printing apparatus at a predetermined web speed in the conveying direction. A configuration in which the web is supplied to a collator after printing is particularly suitable. The collator is designed to produce cardboard sheets, and here, the web printed by the printing apparatus is used, and the cardboard sheet is formed by combining the web with other webs. The web speed is set particularly by the collator and changes over time. The web speed typically changes when the job is changed, but can also change within the same job. Therefore, since the present invention described herein is particularly suitable for a composite body composed of a printing apparatus and a collator, hereinafter, without limiting generality, it is premised on that composite body.

[0011] Hereinafter, the terms "upstream side" and "downstream side" are used to characterize the relative positions of two components with respect to the conveying direction. Component A disposed on the upstream side of component B is disposed in the direction opposite to the conveying direction relative to component B. That is, the web first passes through component A and then through component B. Correspondingly, the opposite is true for the "downstream side".

[0012] The printing apparatus has at least one dryer having an adjustable drying capacity for drying the web. By drying, the web, and thus the printed image, shrinks, and in particular, the moisture content of the web also decreases. The dryer is basically disposed on the downstream side of the printing unit. The dryer is also referred to as a printed image dryer. The dryer is particularly an IR dryer (i.e., an infrared dryer) or a hot air dryer, and when a plurality of dryers are provided, a combination consisting of at least one IR dryer and one hot air dryer is advantageously used for each. In one suitable configuration, a plurality of IR dryers are first disposed on the downstream side of the printing unit, and a plurality of hot air dryers are further disposed on the downstream side thereof. The drying capacity is represented, for example, as the temperature (e.g., in units of °C) when the web passes through the dryer, but other units are equally suitable.

[0013] The printing apparatus further has at least one humidifier having an adjustable humidifying capacity for humidifying the web, thereby adjusting the moisture content of the web (i.e., the moisture content of the paper), and in particular for adjusting swelling (i.e., negative shrinkage, and thus generally shrinkage of the web). During humidification, typically, the web swells and thus the printed image is enlarged. The humidifier is arranged, inter alia, downstream of the dryer and thus compensates for moisture loss due to drying. Thus, the humidifier is also referred to as a rehumidifier. In one suitable configuration, the humidifier has at least one spray bar that extends in a direction transverse to the transport direction, in particular over the entire width of the web, and sprays water onto the web during operation. The humidifying capacity is expressed, for example, as the amount of water applied to the web per unit area (e.g., in ml / m 2 ), but other units are equally suitable.

[0014] Within the framework of the operation of the method, the actual shrinkage and actual moisture content of the web are determined, and the target shrinkage and target moisture content are set. The shrinkage and moisture content are closed-loop controlled to the target shrinkage and target moisture content by adjusting the drying capacity and humidifying capacity depending on the actual shrinkage and actual moisture content. In other words, by adapting the drying capacity and humidifying capacity, during operation, the shrinkage and humidity are adapted, in particular continuously adapted.

[0015] The target shrinkage and target moisture content (closed-loop control parameters) are, in particular, part of the configuration data and are thus determined based on the job data or are already included in the job data itself. The target shrinkage and target moisture content are job-specific and are thus appropriately changed when the job changes (corresponding to a job change). Here, for the same or similar jobs, correspondingly the same or similar job data and configuration data are used. Preferably, a higher-level control unit sets the target shrinkage and target moisture content job-specifically and transmits them to the machine control unit, which uses them to perform closed-loop control of the shrinkage and moisture content and, for this purpose, performs open-loop control of the dryer and humidifier.

[0016] To control shrinkage in a closed loop, in particular the actual shrinkage is determined, which represents how much the web has shrunk after drying. The determination of the actual shrinkage is carried out in particular inline. For this purpose, a corresponding sensor is used, which is arranged downstream of the printing unit, in particular downstream of the dryer and the humidifier, in order to obtain as accurate a value as possible for the shrinkage.

[0017] In a preferred configuration, the web width (in the direction perpendicular to the transport direction) of the web is measured downstream of the dryer and compared with the web width upstream of the printing unit, thereby determining the actual shrinkage of the web. Alternatively or additionally, the actual shrinkage of the web is determined by optically measuring the shrinkage of the printed image marks on the web downstream of the dryer. For example, the above-mentioned sensor is a web width sensor or, alternatively, an optical camera system (especially as part of a printing device), which optically detects the printed image and the printed image marks included in the printed image, and calculates the deviation between the target shrinkage and the actual shrinkage from the job data. The optical measurement using the camera system has the advantage that the shrinkage of the printed image can be directly determined compared to the web width measurement. In particular, when a very large amount of ink is applied to one side of the web and correspondingly strong swelling occurs, different shrinkages are assumed to occur across the width of the web. If the shrinkage is only indirectly determined by the web width measurement, these deviations across the width cannot be quantified. The actual shrinkage is, in fact, the transverse shrinkage of the web, at least when measuring the web width. Since the longitudinal shrinkage of the web is usually less than the transverse shrinkage, only the transverse shrinkage needs to be used in the closed-loop control to achieve the dimensional accuracy of the printed image in both the transverse and longitudinal directions. In particular, the web width upstream of the printing unit is also measured using a web width sensor upstream of the printing unit. Each web width sensor determines the web width, for example, using web edge identification or a light grid. The two web widths are compared, for example, by taking the ratio of the difference between their web widths to the web width upstream of the printing unit. This ratio represents the shrinkage in percentage units based on the web width upstream of the printing unit. If another device follows the printing unit, preferably, in this other device, the printed image is checked using another sensor. For example, the printed image includes a QR code, and its dimensions are checked by the subsequent device. Preferably, for this purpose, the distance between a plurality of QR codes in the printed images arranged side by side across the web width is measured. Alternatively or additionally, the QR code is preferably used as the above-mentioned printed mark.

[0018] In particular, the actual shrinkage is determined at the end of the printing device, for example, just before the winder for the web, or rather outside the printing device, for example, along the transport section or in a subsequent device. That is, the actual shrinkage is determined when the web is no longer processed by the printing device, and especially before, during, or after the web is delivered to a subsequent device. The dryer, or more precisely the dryer capacity, is closed-loop controlled using a shrinkage control device based on the target shrinkage as a reference parameter and the actual shrinkage as a closed-loop control parameter. In this way, the shrinkage occurring in the printing device is closed-loop controlled to the target shrinkage. In particular, the shrinkage control device is part of the printing device, especially the machine control unit, and is realized especially using a programmable logic controller (PLC).

[0019] In an advantageous configuration, the actual shrinkage is typically determined, except when splicing is performed, using the first sensor that is placed at the end of the printing device as described above on the downstream side of the printing unit. When splicing is performed, the actual shrinkage is determined using a second sensor that is placed upstream of the first sensor and thus closer to the printing unit. This advantageously shortens the web travel distance from the printing unit to the sensor to 1 / 5 to 1 / 20, for example, from 250 meters to just 20 meters. When splicing is performed, a new web with different characteristics in some cases and thus different job data is joined to the end of the web currently in use. In this case, advantageously, a faster response of the closed-loop control is obtained by shortening the web travel distance as much as possible for the closed-loop control of shrinkage. At this time, the closed-loop control no longer takes into account the entire web processing on the downstream side of the printing unit. After a predetermined time has elapsed or after a predetermined length of the web has passed, the first sensor is used again. If necessary, an offset is further taken into account. For example, the first sensor is placed on the downstream side of the printing unit, upstream of the front stage of the varnish dryer and upstream of two of the three spray bars there. Due to shrinkage by drying and swelling by the spray bars, the dimensions of the web change by a factor x depending on the setting, and in some cases with a certain tolerance. Based on empirical values, for the shorter control section caused by the above-described arrangement configuration, further changes in shrinkage on the downstream side are estimated or assumed using the offset. That is, the actual shrinkage generally occurs as the sum of the offset that depends particularly on the downstream-side machine setting and the measured value for shrinkage. Therefore, the offset is generally used for correcting the actual shrinkage caused particularly by the relative arrangement of the sensor with respect to the components that affect shrinkage.

[0020] To control the moisture in a closed loop, similar to the shrinkage, the actual moisture representing how much moisture the web after humidification contains is determined. The determination of the actual moisture is carried out especially inline. For this purpose, a corresponding sensor is used, which is arranged downstream of the printing unit and also downstream of all humidifiers. The sensor is, in one suitable configuration, a microwave moisture sensor. The sensor is preferably calibrated for webs with different paper properties (paper type, basis weight, etc.), for example using respective characteristic curves. Similar to the measurement of the actual shrinkage, the actual moisture is also determined especially at the end of the printing device or rather outside the printing device. Subsequently, the humidifier, more precisely the humidifier capacity, is closed-loop controlled using a moisture control device based on the target moisture as a reference parameter and the actual moisture as a closed-loop control parameter. Basically, in order to produce at maximum capacity, it might be considered to simply operate the printing device at the maximum web speed. However, at least in inline operation, variable web speeds are required because, in other devices (especially collators), it typically frequently happens that the web speed changes abruptly. That is, for example, the web speed is reduced for a short time when, for example, various quality changes are made (such as when the sheet length is changed). In contrast, the web speed is reduced for a long time for certain qualities (such as seamless corrugated sheets at a lower web speed of, for example, 200 - 250 m / min). Also, by changing the web speed, usually, the residence time in the heating section and the stretching section (i.e., the Double Facer) is shortened or extended, thereby eliminating quality-related problems such as warping (bending of corrugated cardboard) and poor adhesion. In very general cases and when other problems occur, for example, when the paper quality is poor or when the storage section at the end of the device is full, the web speed may also be reduced. Regardless of the specific reason, jamming of the device always means that paper remains in the heating section and the stretching section, thereby becoming defective products. If the composite is operated at a constant web speed, when the web speed decelerates, it will cause overly large shrinkage and overly little moisture, resulting in corresponding defective products. Therefore, for various adjustable qualities, additionally, it is necessary to save alternative mechanical settings and printing image settings when the web speed is reduced, which increases complexity. Furthermore, a flexible operation, especially for open-loop control of the quality of corrugated sheets, becomes impossible.

[0021] Here, it has been found that controlling both shrinkage and moisture in a closed loop is particularly advantageous for the successful in-line operation of a printing device. Without such closed-loop control, shrinkage and moisture vary strongly depending on the regularly changing web speed of other devices, which speed is taken over by the printing device. The closed-loop control of shrinkage and moisture described here further enables changing the job (i.e., changing the job data) without stopping the printing device, i.e., during operation. The adaptation required to avoid changes in shrinkage and moisture due to changes in job data is automatically achieved by the closed-loop control. Thereby, the printing device can be used particularly flexibly. Without such particularly active closed-loop control, it would be necessary to stop the printing device each time the job is changed, because otherwise the quality of further web processing would be insufficiently considered.

[0022] As a result of experiments, it has been found that shrinkage and moisture strongly depend on the web speed (corresponding to the drying time). Different from roll-to-roll operation, in in-line operation it is usually not possible to maintain a constant web speed. As a result, with a constant drying capacity and humidifying capacity, it is not always possible to achieve a constant shrinkage or sufficient moisture. Variations in shrinkage and moisture, i.e., variations in the quality of the printed web, usually cause problems with further processing and quality in subsequent devices, such as a folder gluer. For example, if the moisture is excessively low, as a result, poor adhesion or warping may occur. Excessively large or small printed images cause problems with further processing of the web, especially the corrugated sheet manufactured from the web. That is, shrinkage changing during operation usually causes problems at the slitter scorer and problems at the cut-off of the folder gluer, because they have to readjust the length and width changes that occur in some cases. As a result, incorrect cutting frequently occurs, whereby the printed image on the finished corrugated sheet is no longer correctly positioned.

[0023] Within the inspection framework, the actual shrinkage and actual moisture are recorded over a long period of time, and from the measurement values obtained at that time, the relationships among job data (paper type, basis weight, production mode (i.e., using varnish / not using varnish, using primer / not using primer)), web speed, drying capacity, and humidifying capacity are derived. Based on this, closed-loop control of drying and humidifying based only on moisture or only on shrinkage is basically possible, but it has been found that only unsatisfactory results can always be obtained. Therefore, by combining the two characteristic quantities of "shrinkage" and "moisture" and performing closed-loop control, both a dimensionally stable print image and sufficient moisture are ensured over a wide range of different job data and over a large range of web speeds.

[0024] Preferably, the drying capacity and the humidifying capacity are closed-loop controlled such that the shrinkage becomes constant and the moisture is maintained at or above the minimum moisture. This is maintained within the same job (i.e., for a given job data), even when the web speed changes, particularly in an in-line operation, and also when the job data changes. Here, a constant shrinkage and sufficient moisture are achieved by the closed-loop control of the drying capacity and the humidifying capacity. Furthermore, it is advantageous if the fluctuations in shrinkage and moisture are as small as possible. That is, it is advantageous for the dynamics of the two closed-loop control parameters to be as small as possible. In particular, the shrinkage is closed-loop controlled such that the actual shrinkage falls within an acceptable range near the target shrinkage (e.g., ±0.5% to ±1% of the target shrinkage, or ±0.05% of the full width of the web, or ±1 to 2 mm in the direction of the full width). If it deviates from the acceptable range, preferably, a warning is output. The acceptable range is set, in particular, as part of the job data and depends, for example, on the accuracy required for the printed image using a higher-level control unit. Similarly, as an option and additionally if necessary, an offset for the target shrinkage is set and taken into account. In particular, the moisture is closed-loop controlled such that the actual moisture corresponds at least to a minimum moisture of, for example, 5%. The minimum moisture is set, in particular, by a higher-level control unit, similar to the acceptable range. If the minimum moisture is undershot, preferably, a warning is output or the humidifying capacity is increased, particularly up to the maximum value depending on the paper type and basis weight, and thereafter, if the humidifying capacity cannot be increased further, in particular, a warning is output. This is based, in particular, on the consideration that when the moisture is excessively high, i.e., when excessive humidification is carried out, it becomes impossible to wind up the web in good quality. This can usually be visually recognized by the diamond pattern / waves in the wound web.

[0025] Preferably, the closed-loop control of shrinkage is limited by a minimum value for the drying capacity, so that the minimum allowable dimensions during drying are guaranteed. Thus, the drying capacity is not closed-loop controlled to arbitrarily decrease greatly to reduce shrinkage. Instead, the reduction of the drying capacity is limited to the minimum capacity by the minimum value. The minimum value preferably depends on the web speed and also, in particular, on the amount of ink applied and, in some cases, on the job data. If the amount of ink is small, usually only slight drying is required, whereas if the amount of ink is large, more drying is required. Furthermore, the drying of the ink on coated paper always takes longer than on uncoated paper. In particular, for any web speed, the closed-loop control is adjusted so that sufficient drying is ensured up to a certain amount of ink applied. When coating is performed using a very small amount of ink, in some cases, overly strong drying is performed, but this is tolerated. However, advantageously, differentiation is made with respect to the amount of ink applied, in particular to save energy by avoiding unnecessary drying. The relationship between the web speed and the minimum value is stored, for example, as a characteristic curve.

[0026] Particularly before implementing the method described herein, when creating new job data, for example, empirical values or experimental results are used for the target shrinkage of interest, or alternatively, in a defined proofing process, the target shrinkage is determined. Subsequently, this target shrinkage is preferably used as an initial value for iterative optimization, which, in a suitable configuration, is part of the method and thus performed during operation, or alternatively, also performed outside of operation.

[0027] In addition to the printing unit, dryer, and humidifier, which are the components described above, the printing apparatus has one or more other components that are particularly used to implement various production modes and / or further support drying and / or humidification.

[0028] That is, in a suitable configuration, the printing apparatus has a primer application device for applying a primer to the web. Since the primer application device is particularly arranged upstream of the printing unit, the primer is applied before printing. The primer represents a base for subsequent printed images. By means of the primer application device, the printing apparatus has a production mode using the primer (application) and a production mode not using the primer (application). Downstream of the primer application device and upstream of the printing unit, the printing apparatus has at least one dryer, also referred to as a primer dryer, for drying the primer. This dryer operates particularly only in jobs involving primer application. The dryer is preferably a hot air dryer. In a preferred configuration, one or more (e.g., two) humidifiers are additionally arranged downstream of the primer dryer.

[0029] Alternatively or additionally, the printing apparatus has a sizing application device, particularly for applying sizing (i.e., for sizing) to the entire surface of the web. Since the sizing application device is particularly arranged downstream of the printing unit, the sizing is applied after printing, and accordingly, on top of the printed image. The sizing represents a surface treatment. By means of the sizing application device, the printing apparatus has a production mode involving sizing application and a production mode not involving sizing application. Downstream of the sizing application device and upstream of the printing unit, the printing apparatus has at least one dryer, also referred to as a sizing dryer, particularly for drying the sizing. This dryer operates particularly only in jobs involving sizing application. The sizing dryer is preferably a hot air dryer. Preferably, a humidifier is arranged upstream of the sizing application device. Alternatively or additionally, preferably, a humidifier is arranged downstream of the sizing dryer.

[0030] Alternatively or additionally, the printing apparatus has a varnish printing unit, in particular a digital printing unit, for overprinting varnish on the web (i.e., for overprint varnish, which is a so-called "digital varnish"). The varnish printing unit is arranged, in particular, downstream of the printing unit, so that the varnish is printed after printing and thus on top of the printed image. In particular, the varnish printing unit is also arranged downstream as long as a varnish application device is provided. The varnish represents a surface finish. By means of the varnish printing unit, the printing apparatus has a production mode using overprint varnish and a production mode without using overprint varnish. Downstream of the varnish printing unit, the printing apparatus has at least one dryer, also referred to as an overprint varnish dryer, in particular for drying the varnish. This dryer operates only in jobs using overprint varnish. In particular, as with the printing unit, a plurality of IR dryers and a plurality of hot air dryers are arranged. Preferably, a humidifier is arranged downstream of the varnish printing unit and, in particular, downstream of the overprint varnish dryer.

[0031] One or more of the above-mentioned humidifiers are used, in particular, for the closed-loop control of the humidity as described at the beginning and are correspondingly designed, for example, as spray bars.

[0032] The primer dryer, the varnish dryer, and the overprint varnish dryer are not particularly used during closed-loop control and operate or stop independently of the closed-loop control according to the required production mode. However, as long as one or more of these dryers are operating, shrinkage increases and moisture decreases, but this is advantageously automatically compensated by the closed-loop control. However, basically, it is also possible to use one or more of these dryers during closed-loop control of shrinkage, and in particular, it is advantageous for expanding the control range regarding shrinkage and moisture that can be accessed by the closed-loop control. In one advantageous configuration, one or more of the aforementioned dryers are appropriately incorporated into the closed-loop control. For example, here, the overprint varnish dryer is used as a booster to perform readjustment within the apparatus. Alternatively or additionally, the primer dryer has both a hot air dryer and an IR dryer, and the hot air dryer provides the basic load regarding the drying capacity, and the IR dryer is incorporated into the closed-loop so as to operate only during control peaks, that is, only when there is a temporary demand for increasing the drying capacity.

[0033] The above-described web width sensor for measuring the web width in the upstream stage of the printing unit is preferably disposed downstream of the primer coating device, and more particularly, downstream of the primer dryer. Downstream of the printing unit, the web width is measured at one or more measurement locations, preferably using one web width sensor at each location as described above. A first suitable measurement location is located downstream of the dryer and upstream of the varnish coating device in the downstream stage of the printing unit. A second suitable measurement location is located between the varnish coating device and the varnish printing unit, and more particularly, downstream of the varnish dryer. A third suitable measurement location is located at the end of the printing unit, and is located downstream of the varnish printing unit and more particularly downstream of the overprint varnish dryer as long as the varnish printing unit and the overprint varnish dryer are provided. The first and second measurement locations are particularly suitable for the arrangement of the second sensor described above for identifying the actual shrinkage when splicing is performed. In contrast, the third measurement location is particularly suitable for the arrangement of the first sensor described above where the actual shrinkage is determined standardly.

[0034] Target shrinkage is optimally defined (i.e., adjusted) especially when the drying capacity of the dryer and the humidifying capacity of the humidifier, which are most required to dry the ink, are adjusted by closed-loop control at the maximum web speed (e.g., 300 m / min). This most required drying capacity and humidifying capacity do not necessarily correspond to 100% of the available capacity each time. Rather, preferably, due to the shrinkage differences inherent in the paper, there is an upper closed-loop control buffer, whereby the difference can be compensated and adjusted by over-drying. The same applies to the closed-loop control for each of those individual dryers and / or humidifiers when multiple dryers and / or multiple humidifiers are used. Shifting the optimal settings may, in exceptional cases, bring advantages when combined with restrictions as required (in some cases, a narrower web speed range or a job change cannot be achieved without stopping, etc.). For example, as in the case already described above, any job change can be carried out (within certain physical limits). However, at some point, it becomes impossible to turn another dryer / humidifier off or on anymore. However, when the job is restricted and only a part of all basically conceivable jobs is covered, for example, when a job using a primer cannot be performed, when there are only a few jobs using additional coatings, and when there are a very large number of jobs using varnish, the target shrinkage for jobs not using additional coatings is preferably adjusted more greatly. This certainly has the drawback of increased energy consumption in this particular job, but it has the advantage that more drying can be blocked, whereby the heated varnish dryer can be compensated and adjusted. That is, a larger process window is obtained. Similarly, during other job changes, optimization is preferably also carried out regarding which types of jobs are frequently performed. When more drying / less drying is required to achieve the target shrinkage at the maximum web speed, this drying is weakened / strengthened for future jobs.This is preferably done automatically by a higher-level control unit, and in this sense, this higher-level control unit learns by itself. For a new job (e.g., a new paper type, basis weight), it is appropriately started using an empirical approximation for the target shrinkage. For example, for a new paper type, there is already 100 g / m². 2 and 200 g / m² 2 and if there are respective target shrinkages for each basis weight, based on them, for example, for 150 g / m² 2 a first value for the target shrinkage is assumed and, for example, simply interpolated.

[0035] A very difficult problem regarding closed-loop control is the rapid change in web speed. In particular, since actual shrinkage is first identified at the end of the printing device or, rather, outside the printing device, the rapid change in web speed when the travel distance of the web between the printing unit and the sensor for identifying actual shrinkage is long is a very difficult problem. Therefore, preferably, for a plurality of intervals of web speed, starting values for the drying capacity and the humidifying capacity are each stored (e.g., as part of job data), and when the web speed changes during the transition from the first interval to the second interval, first, the starting value for the second interval is adjusted, and subsequently, shrinkage and moisture are closed-loop controlled based thereon. That is, when the web speed is changed so as to correspond to another interval having a different starting value, in order to be able to respond better without having to wait for closed-loop control, those starting values are adjusted. The starting values at the start of a job are, among other things, similarly iteratively improved with respect to existing job data. If the job remains the same, the starting value always corresponds, in particular, to the starting value last used at that web speed (e.g., determined as a moving average from the last x productions). If the quality within the job remains the same, the last setting is temporarily buffered appropriately depending on the web speed or interval. Subsequently, when multiple changes in web speed are made within a short period of time, it is restarted using the last value regarding that web speed range or interval, and an empirical average value is not used. This is based on the consideration that different lots of the same type of paper exhibit somewhat different behaviors, particularly with regard to shrinkage, within a certain framework. This avoids having to perform an initial adaptation each time the web speed is changed.

[0036] Preferably, as in the case of web speed, the method is implemented when, for example, at the time of a corresponding job change, there is a large change in the target shrinkage. For this purpose, in a suitable configuration, when the target shrinkage changes by an amount exceeding a predetermined limit value, the drying capacity and the humidifying capacity are each changed by a respective predetermined value only once (i.e., first when the target shrinkage changes), and subsequently, based thereon, the shrinkage and the moisture are closed-loop controlled. In this way, the closed-loop control is supported first when adapting to a sudden new target shrinkage. When the limit value is exceeded (i.e., when the change is very small), that kind of support is not necessary, and the change is completely compensated and adjusted by the closed-loop control. The limit value is typically selected to be a large value, for example, 0.2% of the total width of the web. That is, the target shrinkage changes by at least 0.2% of the total width of the web. The limit value can be defined as an absolute value, but it can also be defined, for example, relatively with respect to the total width of the web or the target shrinkage itself.

[0037] Advantageously, in order to allow for a certain amount of shrinkage and avoid the need for compensation adjustment by closed-loop control, the correction factor is already set using the job data, which also reduces the load on the closed-loop control by printing the printed image larger than the target size. This is especially based on the recognition that shrinkage actually cannot be fully compensated physically because shrinkage always includes a reversible part and an irreversible part (keratinization of paper fibers during drying). Therefore, the printed image is intentionally enlarged and printed, and then shrinks to the target size due to shrinkage in the printing apparatus. Subsequently, the closed-loop control performs compensation adjustment only for dynamic changes in web speed and / or paper-specific differences during shrinkage, or for at least most of such dynamic changes and / or differences. For example, the width of the paper machine is from 9m to 12mm, and the web produced thereby is divided into multiple rolls. In the roll from the center of the web, the paper fibers are compensated more in the web direction than in the edge rolls. In subsequent processing within the apparatus, shrinkage and swelling of different magnitudes occur in each web depending on the fiber orientation. Humidification releases the residual stress from paper manufacturing. Depending on the value of this so-called set residual stress, the strength of web swelling during humidification varies. This difference can be up to approximately ±0.15% at most. The printing unit prints the printed image enlarged appropriately according to the correction factor. The correction factor ultimately aims to shift the start condition of the closed-loop control corresponding to the job. The correction factor is, for example, determined in advance by experiments and / or optimized by iterative adaptation during operation. The correction factor depends especially on other job data, such as paper type, basis weight, production mode (using / not using primer, sizing, overprint sizing). The correction factor should be used or adapted especially when the allowable range regarding shrinkage or the minimum moisture cannot be maintained for a given job data. This is especially advantageous when the amount of ink applied to the printed image is very large, or for jobs using primer, sizing or overprint sizing. In this case, since one or more additional dryers are operating, the correspondingly changing shrinkage can be taken into account.This is appropriately done using a correction factor for the job using the corresponding job data.

[0038] When changing the job while the current job is still being processed, since the job data for the new job is always known, it is possible and advantageous to use the job data of the new job for future closed-loop control of the dryer and / or humidifier. In particular, when the new job involves a quality change and, unlike the current job, requires additional coatings, especially primers, varnishes and / or overprint varnishes, preferably the corresponding dryer for that coating, i.e., a primer dryer, a varnish dryer or an overprint varnish dryer, is already preheated during the current job (and preferably, the disturbing effects of additional dryers are also automatically compensated by changes in dryer capacity and humidifier capacity), so that at the start of the new job, the required drying capacity can be fully or at least mostly utilized. During the current job, the increased shrinkage and reduced moisture thereby are automatically compensated by closed-loop control. In particular, when a predetermined temperature, for example, exceeds 80°C, the closed-loop control range for shrinkage and / or moisture is expanded. For example, when varnish drying is additionally active, advantageously, ink drying is weakened more than usual. Here, although it is certain that the printed image may no longer be completely dried by ink drying alone, the active varnish drying continues to post-dry the printed image appropriately, so complete drying is ensured. In contrast, in the case of a primer dryer, it is exactly the opposite. Since shrinkage already occurs before printing, the printed image can no longer shrink as strongly, so more drying is required here to achieve the same target shrinkage. However, such additional drying of that magnitude is blocked below a predetermined temperature (for example, the aforementioned 80°C). Thereby, the tolerance and the minimum moisture are maintained without being particularly affected. Advantageously, with regard to the change time, jobs with a large tolerance or paper types with a large grammage (small shrinkage difference) are preferred.

[0039] In one advantageous configuration, the dryer is a hot air dryer, and the printing apparatus further includes an IR dryer for drying the web in addition to this hot air dryer. In other words, to control shrinkage in a closed loop, the drying capacity adjusted by the closed loop control is provided by the combination of the IR dryer and the hot air dryer. The IR dryer is advantageously faster in reaction time than the hot air dryer, but has lower efficiency during operation. In particular, when it is required to improve the drying capacity by closed loop control, for example, when the job data changes correspondingly during a job change, the IR dryer operates alternatively during the heating phase of the hot air dryer to meet that requirement. For example, when the hot air dryer drops below a specific temperature and it is necessary to raise the temperature until the hot air dryer is heated, the IR dryer is first used alternatively. The above-described configuration is particularly advantageous, for example, when a slow web speed is set for the current job and, based on that, the closed loop control is performed to reduce the drying capacity in order to reduce shrinkage and increase moisture. When the web speed increases, the IR dryer first compensates for the reduced drying capacity of the hot air dryer until the drying capacity reaches the required drying capacity again.

[0040] In many cases, advantageously, pre-drying of the web can achieve an expansion of the possible web speed range before printing has even started. Thus, the printing apparatus, in a suitable configuration, has a pre-dryer for pre-drying the web before printing. As a dryer, the primer dryer already described is particularly suitable, and this primer dryer may operate for pre-drying as required, although no primer is applied. Nevertheless, in order to avoid heating and cooling times, preferably the pre-dryer is operating permanently. Pre-drying results in particular in pre-shrinking of the web. Any paper type exhibits a maximum shrinkage where the web contains hardly any water, depending on basis weight etc. Preferably, this maximum shrinkage is achieved since no further shrinkage can occur in the web. Subsequently, in this state of maximum shrinkage, the web is printed, and the drying time, which varies with the web speed, no longer affects the shrinkage. In this case, subsequently, sufficient moisture is again brought to the web by humidification, and the swelling thereby caused is controlled and in particular still guaranteed to be maintained constantly. Pre-drying is particularly suitable when heat can be utilized cost-effectively.

[0041] Hereinafter, embodiments of the present invention will be described in more detail based on the drawings.

Brief Description of the Drawings

[0042]

Figure 1

Figure 2

[0043] In FIG. 1, a printing apparatus 2 and a collator 4, which together form a composite 6, are schematically shown. FIG. 2 shows a detailed embodiment of the printing apparatus 2, which is here a digital printing apparatus for printing using the inkjet method. The printing apparatus 2 in FIG. 2 is shown in a roll-to-roll operation, but is however also suitable for an in-line operation, in particular for use in a composite 6 as shown in FIG. 1.

[0044] The printing device 2 has a printing unit 8 for performing printing on the web 10 according to a predetermined job that uses predetermined job data defining the job. For this purpose, the printing unit 8 has one or more print heads 12. The printing device 2 is shown in an in-line operation in FIG. 1, whereas it is shown in a roll-to-roll operation in FIG. 2.

[0045] The job data for each job is set using the host control unit 14. For example, it is set directly by the host control unit 14 itself, or is set by an operator who inputs the job data to the host control unit 14 via an interface, for example. The host control unit 14 is formed separately from the printing device 2 in FIG. 1. However, in an alternative form not shown, it is part of the printing device 2. The job data includes the production mode in which the printing device 2 is to operate and web data. The production mode defines, for example, a specific print quality, the amount of ink used for printing, whether a primer should be applied before printing, whether a varnish or overprint varnish should be applied after printing, etc., or a combination thereof. The web data includes, for example, the paper type, the basis weight of the web, etc., or a combination thereof. Configuration data is derived from the job data, and this configuration data includes corresponding machine data for adjustments depending on the job of the printing device 2 and is transferred to a machine control unit (not explicitly shown) of the printing device 2.

[0046] Web 10 is generally conveyed to pass through the printing apparatus 2 at a predetermined web speed in the conveyance direction, and in FIG. 1, it is supplied to the collator 4 after printing. The collator 4 is designed to manufacture the cardboard sheet 16, and here, the web 10 printed by the printing apparatus 2 is used, and the cardboard sheet 16 is formed by combining the web 10 with other webs. In the illustrated embodiment, the web speed is set by the collator 4 and changes over time. Instead of the collator 4, in an alternative configuration, another apparatus for further processing the printed web 10 is arranged.

[0047] Hereinafter, the terms "upstream side" and "downstream side" are used to characterize the relative positions of two components with respect to the conveyance direction.

[0048] The printing apparatus 2 has at least one dryer 18, 20 with an adjustable drying capacity for drying the web 10. By drying, the web 10, and thus the printed image, shrinks. The dryers 18, 20 are basically arranged on the downstream side of the printing unit 8 and are also referred to as printed image dryers. In the embodiment of FIG. 2, a total of six dryers 18, 20, namely four IR dryers 18 and two hot air dryers 20, are used.

[0049] The printing apparatus 2 further has at least one humidifier 22 with an adjustable humidifying capacity for humidifying the web 10 and thereby adjusting the moisture content of the web 10 (i.e., the moisture content of the paper). During humidification, the web 10 swells, and thus the printed image is enlarged. In the embodiment of FIG. 2, three such humidifiers 22 are shown. In FIG. 2, all the humidifiers 22 are arranged on the downstream side of the dryers 18, 20 and are thus also referred to as re - humidifiers. Each humidifier 22 has, for example, at least one spray bar, and this spray bar extends in a direction transverse to the conveyance direction, particularly over the entire width of the web 10 (i.e., in FIG. 2, it extends perpendicular to the plane of the paper) and sprays water onto the web during operation.

[0050] Here, within the framework of the operation of the printing apparatus 2, for the web 10, the actual shrinkage and the actual moisture are specified, and the target shrinkage and the target moisture are set. The shrinkage and the moisture are closed-loop controlled to the target shrinkage and the target moisture by adjusting the drying capacity and the humidifying capacity depending on the actual shrinkage and the actual moisture. In other words, by adapting the drying capacity and the humidifying capacity, the shrinkage and the moisture are adapted during operation.

[0051] The target shrinkage and the target moisture (closed-loop control parameters) are part of the configuration data and are thus determined based on the job data or are already included in the job data itself. The target shrinkage and the target moisture are job-specific and are thus appropriately changed when the job changes. Here, the upper control unit 14 sets the target shrinkage and the target moisture specific to the job and transmits them to the machine control unit, and this machine control unit uses them to perform closed-loop control of the shrinkage and the moisture, and for this purpose, open-loop controls the dryers 18, 20 and the humidifier 22.

[0052] The actual shrinkage represents how much the web 10 has shrunk after drying. The specification of the actual shrinkage is performed inline using corresponding sensors, here a plurality of web width sensors 24, 26, 28 arranged downstream of the printing unit 8. In the embodiment of FIG. 2, the web width of the web 10 (in the direction perpendicular to the conveying direction) is measured downstream of the dryers 18, 20 and compared with the web width in the front stage of the printing unit 8, whereby the actual shrinkage is specified. Thus, actually, the actual shrinkage is the shrinkage in the transverse direction. In FIG. 2, the web width in the front stage of the printing unit 8 is measured, for example, similarly using the web width sensor 30 upstream of the printing unit 8. The web width in the front stage and the web width in the rear stage of the printing unit 8 are both compared, for example, by taking the ratio of the difference between those web widths and the web width in the front stage of the printing unit 8.

[0053] Alternatively or additionally, the actual shrinkage of the web 10 is determined by optically measuring the shrinkage of the printed image marks on the web 10 downstream of the dryers 18, 20. In FIG. 2, for this purpose, optical camera systems 52, 54 are shown, which optically detect the printed image and the printed image marks contained therein, and calculate the deviation between the target shrinkage and the actual shrinkage from the job data. In FIG. 2, two possible positions of the camera systems 52, 54 are shown, and they are installed at only one of them or both simultaneously. As shown in FIG. 2, by using the two camera systems 52, 54, inspection of the printed image downstream of the printing unit 8 and inspection of the overprint varnish image downstream of the varnish printing unit 46 are also realized. The camera system 52 (assembly position 1) located further upstream realizes a very short control section, for example, when splicing is performed. In contrast, the camera system 54 (assembly position 2) located further downstream preferably operates during the closed-loop control operation (and also during normal operation). This is because this position is very close to the winder 32, and thus the finished printed image is best represented.

[0054] Here, typically, the actual shrinkage is determined using the web width sensor 28 at the end of the printing device 2 immediately before the winder 32, or outside the printing device 2 in the case of FIG. 1, for example, along the transport section 34 or within the collator 4. That is, generally, the actual shrinkage is determined when the web 10 is not further processed by the printing device 2. Using the shrinkage control device, the drying capacity of the dryers 18, 20 is closed-loop controlled using the target shrinkage as a reference parameter and the actual shrinkage as a closed-loop control parameter.

[0055] As already mentioned, in the illustrated embodiment, the actual shrinkage is identified using the web width sensor 28 which is arranged as far downstream as possible from the printing unit 8. In contrast, when splicing takes place, the actual shrinkage is located upstream of the web width sensor 28 and is thus identified using one of the web width sensors 24, 26 which are arranged closer to the printing unit 8. This shortens the web travel distance from the printing unit 8 to the web width sensors 24, 26, 28.

[0056] To control the moisture in a closed loop, similar to the shrinkage, the actual moisture representing how much moisture the web 10 contains after humidification is identified. Here, the identification of the actual moisture is likewise carried out inline. For this purpose, a corresponding sensor, here the moisture sensor 36, is used, which is arranged downstream of the printing unit 8 and also downstream of all humidifiers 22. Similar to the measurement of the actual shrinkage, the actual moisture is also identified at the end of the printing device 2 or, rather, outside the printing device 2. Subsequently, using a moisture control device, the humidifying capacity of the humidifiers 22 is closed-loop controlled using the target moisture as a reference parameter and the actual moisture as a closed-loop control parameter.

[0057] Here, the drying ability and the humidifying ability are controlled so that the shrinkage is maintained constant and the moisture is maintained above the minimum moisture. This is maintained within the same job (i.e., with fixed job data) as well as with variable job data, at an in-line operation and variable web speed. The shrinkage is closed-loop controlled such that the actual shrinkage is near the target shrinkage within an allowable range of, for example, ±0.5 to 1% of the target shrinkage. The moisture is closed-loop controlled such that the actual moisture corresponds to at least a minimum moisture of, for example, 5%. The allowable range and the minimum moisture are set using, for example, the upper control unit 14. When deviating from the allowable range or falling below the minimum moisture, a warning is output each time. The humidifying ability is increased up to a maximum value depending on the paper type and basis weight before a warning is output. Here, since the closed-loop control of the shrinkage is limited by the minimum value for the dryer ability, this dryer ability is guaranteed to have a minimum drying without arbitrarily decreasing significantly. The minimum value depends on the web speed, paper type, and ink application amount.

[0058] In addition to the printing unit 8, dryers 18, 20, and humidifier 22 which are the components described above, the printing apparatus 2 in FIG. 2 is used to realize various production modes and / or has one or more other components that further support drying and / or humidifying.

[0059] That is, the printing apparatus 2 illustrated in FIG. 2 has a primer application device 38 for applying a primer to the web 10. Since the primer application device 38 is disposed upstream of the printing unit 8, the primer is applied before printing. By the primer application device 38, the printing apparatus 2 has a production mode using a primer (application) and a production mode not using a primer (application). Downstream of the primer application device 38 and upstream of the printing unit 8, the printing apparatus 2 has a dryer 40 for drying the primer, here a hot air dryer also referred to as a primer dryer. This dryer mainly operates in jobs including primer application.

[0060] Additionally, the printing apparatus 2 in FIG. 2 has a sizing application device 42 for applying sizing to the web 10 (i.e., for sizing drawing). Since the sizing application device 42 is disposed on the downstream side of the printing unit 8, sizing is applied after printing and accordingly on top of the printed image. By means of the sizing application device 42, the printing apparatus 2 has a production mode including sizing application and a production mode not including sizing application. On the downstream side of the sizing application device 42 and on the upstream side of the printing unit 8, the printing apparatus 2 has a dryer 44 for drying the sizing, here a hot air dryer also referred to as a sizing dryer. This dryer 44 operates only in jobs involving sizing application. Here, one of the humidifiers 22 is disposed on the upstream side of the sizing application unit 42, and the other of the humidifiers 22 is disposed on the downstream side of the sizing dryer 44.

[0061] Additionally, the printing apparatus 2 in FIG. 2 has a sizing printing unit 46 for printing sizing on the web 10, here a digital printing unit for overprinting sizing. Since the sizing printing unit 46 is disposed on the downstream side of the printing unit 8, sizing is overprinted after printing and thus on top of the printed image. Also, the sizing printing unit 46 is disposed on the downstream side of the sizing application unit 42. By means of the sizing printing unit 46, the printing apparatus 2 has a production mode using overprint sizing and a production mode not using overprint sizing. On the downstream side of the sizing printing unit 46, the printing apparatus 2, similar to the printing unit 8, has a plurality of dryers 48, 50 for drying the sizing, namely, two IR dryers 48 and two hot air dryers 50, each also referred to as an overprint sizing dryer. These dryers 48, 50 operate only in jobs using overprint sizing. On the downstream side of the sizing printing unit 46 and the overprint sizing dryers 48, 50, one of the humidifiers 22 is disposed.

[0062] The primer dryer 40, the varnish dryer 44, the overprint varnish dryers 48, 50 are not necessarily used during closed-loop control, and operate or stop independently of the closed-loop control according to the required production mode. However, as long as one or more of these dryers 40, 44, 48, 50 are operating, the shrinkage changes and the moisture content decreases, but this is automatically compensated and adjusted by the closed-loop control. When the primer dryer 40 is operated, if the ink drying setting is the same, the subsequent print image shrinkage decreases due to pre-drying.

[0063] The above-described web width sensor 30 for measuring the web width in the front stage of the printing unit 8 is disposed downstream of the primer coating device 38 and also downstream of the primer dryer 40. On the downstream side of the printing unit 8, the web width is measured at a plurality of measurement positions using one of the web width sensors 24, 26, 28 as already described. The first measurement position is located downstream of the dryers 18, 20 and upstream of the varnish coating device 42. The second measurement position is located between the varnish coating device 42 and the varnish printing unit 46 and downstream of the varnish dryer 44. The third measurement position is located at the end of the printing device 2, downstream of the varnish printing unit 46 and downstream of the overprint varnish dryers 48, 50. The web width sensors 24, 26 are disposed at the first measurement position or the second measurement position to identify the actual shrinkage when splicing is performed. In this case, the web width sensor 28 is disposed at the third measurement position to standardly identify the actual shrinkage.

[0064] A rapid change in web speed is a very difficult problem for closed-loop control. Therefore, in the illustrated embodiment, for a plurality of intervals of the web speed, starting values for the drying capacity and the humidifying capacity are each stored (e.g., as part of job data), and when the web speed changes during the transition from the first interval to the second interval, first, the starting value for the second interval is adjusted, and subsequently, shrinkage and moisture are closed-loop controlled based thereon. That is, when the web speed is changed to correspond to another interval having a different starting value, in order to be able to respond better without having to wait for closed-loop control, those starting values are adjusted.

[0065] Here, as in the case of the web speed, for example, when there is a large change in the target shrinkage at the time of a corresponding job change, this method is implemented. For this purpose, when the target shrinkage changes by only a value exceeding a predetermined limit value, the drying capacity and the humidifying capacity are each changed by a respective predetermined value only once (i.e., first when the target shrinkage changes), and subsequently, shrinkage and moisture are closed-loop controlled based thereon. In this way, closed-loop control is supported first when adapting to a sudden new target shrinkage.

[0066] Additionally, here, in order to allow for a certain amount of shrinkage by printing the printed image larger than the target dimensions and to avoid the need for compensation adjustment by closed-loop control, the correction factor is already set using the job data, thereby also reducing the load on the closed-loop control. Therefore, the printed image is intentionally enlarged and printed, and subsequently shrinks to the target size due to shrinkage in the printing apparatus 2. Subsequently, the closed-loop control performs compensation adjustment only for dynamic changes such as, for example, web speed or differences in the base paper, or for at least most of such dynamic changes. The printing unit 8 prints the printed image by appropriately enlarging it according to the correction factor, so that finally, depending on the job, the start conditions for the closed-loop control are adapted to correspond to that job. The correction factor depends on, for example, other job data, such as paper type, basis weight, production mode (using / not using primer, varnish, overprint varnish).

[0067] When the job is changed while the current job is still being processed, usually, since the job data for the new job is known here, the job data for the new job is used for the forward-looking open-loop control of the dryers 18, 20 and the humidifier 22. In particular, when the new job involves a quality change and requires additional primer, varnish and / or overprint varnish different from the current job, the primer dryer 40, the varnish dryer 44 or the overprint varnish dryers 48, 50 are preheated in advance during the current job accordingly, so that at the start of the new job, the required drying capacity is prepared. The shrinkage and moisture during the current job, which change thereby, are automatically compensated and adjusted by the closed-loop control.

[0068] Here, a combination of an IR dryer 18 and a hot air dryer 20 is utilized, whereby, for example, when it is required to improve the drying capacity by closed-loop control during a corresponding job change, the IR dryer 18 operates alternatively during the heating phase of the heating dryer 20 to meet that requirement. The above configuration is applied, for example, when a slow web speed is set for the current job and, based on that, closed-loop control is performed to reduce shrinkage and increase moisture content by reducing the drying capacity. When the web speed increases, the IR dryer 18 first compensates for the reduced drying capacity of the hot air dryer 20 until the drying capacity reaches the required drying capacity again.

[0069] In many cases, advantageously, by pre-drying the web 10, it is possible to achieve an expansion of the possible web speed range before printing has even started. Accordingly, the printing apparatus 2, in a suitable configuration, has a pre-dryer 40 for pre-drying the web 10 before printing. In FIG. 2, as the pre-dryer 40, the primer dryer 40 already exemplified above is used.

Description of Reference Numerals

[0070] 2 Printing apparatus 4 Collator 6 Composite 8 Printing unit 10 Web 12 Print head 14 Higher-level control unit 16 Corrugated sheet 18 Dryer, printed image dryer, IR dryer 20 Dryer, printed image dryer, hot air dryer 22 Humidifier 24 Web width sensor (at the first measurement location on the downstream side of the printing unit) 26 Web width sensor (at the second measurement location on the downstream side of the printing unit) 28 Web width sensor (at the third measurement location on the downstream side of the printing unit) 30 Web width sensor (on the upstream side of the printing unit) 32 Rewinder 34 Conveying section 36 Humidity sensor 38 Primer coating device 40 Dryer, hot air dryer, primer dryer 42 Varnish coating device 44 Dryer, hot air dryer, varnish dryer 46 Varnish printing unit 48 Dryer, IR dryer, overprint varnish dryer 50 Dryer, hot air dryer, overprint varnish printing dryer 52 Camera system 54 Camera system

Claims

1. A method for operating a printing device (2), in particular for in-line operation, wherein: - the printing device (2) has a printing unit (8) for printing on a web (10) according to a predetermined job using predetermined job data; - the web (10) is conveyed so as to pass through the printing device (2) at a predetermined web speed; - the printing device (2) has at least one dryer (18, 20) with an adjustable drying capacity for drying the web (10), thereby causing shrinkage in the web (10); - the printing device (2) has at least one humidifier (22) with an adjustable humidifying capacity for humidifying the web (10), thereby adjusting the moisture content of the web (10); - for the web (10), the actual shrinkage and the actual moisture content are determined, and the target shrinkage and the target moisture content are set; - the shrinkage and the moisture content are closed-loop controlled to the target shrinkage and the target moisture content by adjusting the drying capacity and the humidifying capacity depending on the actual shrinkage and the actual moisture content.

2. The web (10) is supplied to a collator (4) after printing, and the web speed is set by the collator (4) and changes over time. The method according to claim 1.

3. The actual shrinkage is determined by measuring the web width of the web (10) downstream of the dryer (18, 20) and comparing it with the web width in front of the printing unit (8), or by optically measuring the shrinkage of the printed image marks on the web (10). The method according to any one of claims 1 or 2.

4. The actual shrinkage is determined using a first sensor (28) disposed downstream of the printing unit (8), at the end of the printing apparatus (2) or outside the printing apparatus (2), except when splicing is performed. When splicing is performed, the actual shrinkage is determined using second sensors (24, 26) disposed upstream of the first sensor (28). The method according to any one of claims 1 to 3.

5. The shrinkage is closed-loop controlled such that the actual shrinkage falls within an allowable range near the target shrinkage. The moisture is closed-loop controlled such that the actual moisture corresponds to at least the minimum moisture. The method according to any one of claims 1 to 4.

6. The closed-loop control of the shrinkage is limited by a minimum value for the drying capacity, thereby ensuring a minimum allowable dimension during drying. The method according to any one of claims 1 to 5.

7. Start values for the drying capacity and the humidifying capacity are stored respectively for a plurality of intervals of the web speed. When the web speed changes during the transition from a first interval to a second interval, first, the start value for the second interval is adjusted, and subsequently, based thereon, the shrinkage and the moisture are closed-loop controlled. The method according to any one of claims 1 to 6.

8. When the target shrinkage changes by a value exceeding a predetermined limit value, the drying capacity and the humidifying capacity are each changed by a respective predetermined value only once, and subsequently, based thereon, the shrinkage and the moisture are closed-loop controlled. The method according to any one of claims 1 to 7.

9. When the job is changed from the current job to a new job and the new job requires additional coating, the dryers (40, 44, 48, 50) for the coating are preheated already during the current job. The method according to any one of claims 1 to 8.

10. The dryer (20) is a hot air dryer, and the printing apparatus (2) has an IR dryer (18) for drying the web (10) in addition to the hot air dryer (20). When it is required to improve the drying capacity by the closed-loop control, in order to meet the requirement, the IR dryer (18) operates alternatively during the heating phase of the dryer (20). The method according to any one of claims 1 to 9.

11. The printing apparatus (2) has a pre-dryer (40) for pre-drying the web (10) before the printing. The method according to any one of claims 1 to 10.

12. By printing the printed image larger than the target dimension to allow a certain degree of shrinkage and not require compensation adjustment by the closed-loop control, the load of the closed-loop control is reduced by setting a correction coefficient using the job data already. The method according to any one of claims 1 to 11.

13. A printing apparatus (2) designed to carry out the method according to any one of claims 1 to 12, or a composite body comprising the printing apparatus (2) and a collator (4).

Images