System having a digital printer, and method for printing a paper web

Abstract

The invention specifies a system (2) which comprises a digital printer (6), with a plurality of print heads (8, 10, 12), each for printing a paper web (4), wherein a first one of the print heads (8, 10, 12) is a colour print head (10), for printing the paper web (4) with a coloured ink, wherein a varnish printing function is integrated into the digital printer (6), by a second one of the print heads (8, 10, 12) being a varnish print head (12), for printing the paper web (4) with a varnish, wherein the varnish print head (12) is arranged downstream of the colour print head (10) along the paper web (4), for overprinting the coloured ink with the varnish. The invention also specifies a method for printing a paper web (4).

Description

[0001] Description System with a digital printer and method for printing a paper web

[0002] The invention relates to a system having a digital printer and a method for printing a paper web, in particular for corrugated cardboard.

[0003] A digital printer is also known as an inkjet printer. Printing on a paper web with a digital printer is contactless, meaning the print head of the digital printer does not touch the paper web during printing. Instead, the ink printed by the print head is projected toward the paper web. Accordingly, a digital printer does not require a physical printing form, allowing for a high degree of freedom in the design of the printed image. Furthermore, even small jobs can be completed cost-effectively, eliminating the need to retool the system.

[0004] Reference is made to US 2014 029 284 3A1 , US 2015 0 029 259 A1 , US 2005 0 140 763 A1 , US 9 004 642 B2, EP 1 362 704 A1 , EP 2 982 510 B1 , EP 1 555 133 B1 , US 6 655 797 B2, US 2006 0 158 481 A1 and US 2020 148 906 A1.

[0005] Specifically, US 2005 0 140 763 A1 describes a method for inkjet printing in which a first dye ink and a first fixing ink are applied successively to substantially the same location on a substrate such that the first fixing ink is applied over the first dye ink within a time interval of less than about 125 ms.

[0006] Against this background, one object of the invention is to improve the printing of a paper web in a system with a digital printer. In particular, the system should be as compact as possible. Accordingly, an improved system and an improved method are to be provided.

[0007] The object is achieved according to the invention by a system having the features according to claim 1 and by a method having the features according to claim 15. Advantageous embodiments, refinements, and variants are the subject of the dependent claims. The statements relating to the system also apply mutatis mutandis to the method, and vice versa. If steps of the method are described implicitly or explicitly below, advantageous embodiments for the system result from the fact that it is designed to carry out one or more of these steps.

[0008] The system has a digital printer, which is also referred to as an inkjet printer. The digital printer has several print heads, each for printing on a paper web. The paper web is generally conveyed in one conveying direction (also web travel direction) and at one web speed through the system and the digital printer. The print heads are arranged one behind the other in the conveying direction and not next to each other. The paper web is provided in particular as a so-called continuous web, i.e. printing is not carried out on just individual pages or sheets. The paper web is processed into individual pages or sheets, for example, outside the system and in any case only after printing.Accordingly, the system is preferably used to produce a so-called preprint. Preprint here means printing before the paper web is further processed, especially before the paper web is used as a cover or liner for a corrugated board web. Printing on the finished corrugated board or corrugated board web, on the other hand, is referred to as post-print.

[0009] For printing, the print heads are lowered towards the paper web as required and then eject ink in this direction depending on the corresponding image data in order to create a print on the paper web. This takes place without contact, i.e. the print heads do not touch the paper web during printing and also not at any other time. An individual print head is also referred to as a print row or print bar. The digital printer is preferably a so-called single-pass system. The print heads are preferably mounted in pairs on so-called carriages, i.e. the digital printer has one or more carriages, each with exactly two print heads mounted. Alternatively, more than two print heads are mounted on one carriage. If several print heads are mounted together on one carriage, the print heads are arranged one behind the other, particularly in the conveying direction, and not next to each other.Each print head may be composed of several sub-units in the transverse direction. Accordingly, the digital printer in particular, but not necessarily, has an even number of print heads. The carriages are used in particular for the mechanical mounting of the print heads and for moving them (in particular sideways, e.g. for maintenance and / or lowering and raising) relative to the paper web. Accordingly, the print heads are mechanically coupled to one another in pairs by the carriages; to service an individual print head, the entire carriage with both print heads must be removed and moved, for example, sideways to the conveying direction. However, control for printing is preferably possible individually, so that the print heads are then independent of one another, at least with regard to printing.

[0010] A first of the print heads is a color print head for printing the paper web with a colored ink. In other words, the digital printer has a first print head that is configured to print in color. The colored ink is, in particular, a water-based ink that contains corresponding color pigments. Examples of colors and colored inks are cyan (C), magenta (M), yellow (Y), and black (K), but other colors are also possible. Preferably, the digital printer even has multiple print heads, each of which is designed as a color print head, in particular for printing the paper web with a plurality of different colored inks of a color system. In a suitable embodiment, the digital printer has at least four color print heads for printing the paper web with a corresponding number of different colored inks, in particular color inks of a CMYK color system.According to the invention, a varnish printing function is additionally integrated into the digital printer, in that a second of the print heads is a varnish print head for printing the paper web with a varnish. In other words, the digital printer has a second print head which is set up to print varnish. The varnish is also referred to as digital varnish because it is now printed using a digital printer. Just like the color ink, the varnish can also be printed selectively, i.e. it is not necessarily printed over the entire surface. The varnish is in particular transparent. The varnish expediently serves to realize an additional function, in particular as mechanical and / or thermal protection for the ink underneath the varnish and / or the paper web itself. The varnish expediently produces a gloss of maximum 55 GU (“gloss unit”, in particular according to DIN EN ISO 2813, whereby the measurement is taken at an angle of 60°).

[0011] The first and second print heads are preferably of the same design and differ primarily in the printed material, namely the ink and the varnish. The print heads are designed separately from each other, meaning that printing with ink and varnish is not done with the same print head, but with different print heads. The digital printer thus has at least two print heads.

[0012] The varnish print head is positioned along the paper web downstream of the color print head for overprinting the colored ink with varnish. When printing on the paper web, the colored ink is printed first, followed by the varnish. In particular, the varnish is printed at least partially at the same location on the paper web as the colored ink, so that the latter is overprinted with the varnish. In particular, the varnish is printed directly onto the colored ink, meaning that no other material is present or printed between the colored ink and the varnish.

[0013] The method according to the invention is used for printing a paper web using a system comprising a digital printer with multiple print heads, each for printing on the paper web. Analogous to the above, at least one of the print heads is a color print head, by means of which the paper web is printed with a colored ink. A varnish printing function is integrated into the digital printer—as also already described—in that at least one of the print heads is a varnish print head, by means of which the paper web is printed with a varnish. The varnish print head is arranged along the paper web downstream of the color print head, and the colored ink is overprinted with the varnish.

[0014] When printing with a digital printer, i.e., digital printing, multiple color inks are typically applied to create a printed image. Color inks include the aforementioned cyan, magenta, yellow, and black, as well as, or alternatively, spot colors such as orange, green, violet, or even white. White, in particular, is conveniently printed as a preprint and to suppress the subsequent other color inks.

[0015] The printed image with the printed inks is dried appropriately after printing. For this purpose, the system features one or more active dryers through which the paper web passes. Examples of active dryers include IR radiators and hot-air dryers.

[0016] After the print image has dried, it is generally advantageous to apply an additional layer of varnish to protect the print image. In principle, it is possible to print the paper web with two separate printing units: a digital printer (color printer) for the color inks and a further printer (varnish printer) for the varnish. This additional printer can also be a digital printer or, alternatively, an analog printer with which an analog varnish is applied, e.g. via a transfer roller. A dryer is then arranged between the color printer and the varnish printer to dry the print image before it is overprinted with varnish. After the varnish has been printed, further drying takes place using another dryer downstream of the varnish printer. The disadvantage of this is that the resulting system is very long due to the two printers and two dryers.The path the paper web has to travel between the two printers also makes it difficult to print the color ink and varnish precisely (in register). In addition, the system is very energy- and cost-intensive due to the two dryers. Another problem is the shrinkage that the paper web experiences on the way between the two printers, particularly due to the dryer located there. This shrinkage has to be corrected at great expense, e.g. by means of additionally printed marks and their evaluation and, if necessary, real-time adjustment of the digital print data. In addition, the repeated drying of the paper web, in particular and typically additional cooling and remoistening, is very time-consuming and energy-intensive and can have a negative impact on the properties of the paper web. Any defects in the paper web that may arise on the way between the color printer and the varnish printer are also problematic.In some cases, it may be necessary to lift the print heads in the coating printer, temporarily suspending the coating application and resulting in uncoated areas on the paper web. Alternatively, the coating application may continue, with the risk of possible damage to the coating printer.

[0017] A key idea of ​​the present invention is a system design that is as compact and preferably also energy-saving as possible. A particularly compact design is achieved if - as described above - the varnish application is integrated into the digital printer for printing with colored ink, i.e. the varnish application is combined with a color application in a single digital printer. Accordingly, the system described here only has a single digital printer, with which both colored ink and varnish are printed. This eliminates the need for an additional varnish printer, as the digital printer serves simultaneously as a color printer and a varnish printer. Accordingly, the overall length of the system is significantly reduced. The system described here advantageously has a total length of no more than 35 m. In contrast, the overall length of a system with two printers would typically be around 50 m.In all of this, it is particularly accepted that the arrangement of a dryer between ink application and varnish application is now no longer possible or at least is only possible to a greatly reduced extent, ie in particular that the paper web between two print heads is heated by a dryer by a maximum of 10°C or the dryer has a correspondingly low area performance or only a correspondingly small amount of water is removed from the paper web.

[0018] The color print head and the varnish print head are preferably adjacent to one another and spaced apart by a maximum of 1 m as measured along the paper web. This results in a particularly compact design. “Adjacent” is understood in particular to mean that the color print head and the varnish print head follow one another without another print head being arranged in between. Expediently, the first and the second print head and in particular all of the print heads of the digital printer are accommodated together in a single housing of the digital printer, so that a particularly high degree of integration is achieved. A distance between two adjacent print heads as measured along the paper web can also be less than the specified distance of 1 m. The distance between the first and the last print head or from the first of several color print heads to the varnish print head is preferably a maximum of 10 m.

[0019] Alternatively or additionally, the first and second print heads, and in particular all of the print heads of the digital printer, are advantageously arranged such that the paper web is conveyed past the print heads within a dwell time in the range of 0.2 s to 2 s. In other words, a given point on the paper web passes all of the print heads within 0.2 s to 2 s. This also results in a particularly compact design. The specified range for the dwell time arises in particular at a web speed of the paper web through the system in the range of 100 m / min to 1000 m / min. The dwell time between two adjacent print heads can be significantly shorter than the specified 0.2 s. However, the dwell time between the first of several color print heads and the varnish print head is preferably at least 150 ms.A further advantage is that, due to the now very short path between the ink application and the varnish application, the register is maintained particularly well, meaning that the ink and varnish are printed on top of each other with particularly high precision. Shrinkage or swelling between the ink application and the varnish application need not be taken into account; such shrinkage would be negligible at best.

[0020] Advantageously, the system is also designed in such a way that no active drying of the paper web, and thus also of the printed image, takes place between the color print head and the varnish print head of the digital printer. A corresponding dryer, e.g. IR radiator or hot air dryer, is dispensed with. Passive drying also takes place, in particular at most to the extent that the printed image dries on the path between the first and second print heads. However, due to the small distance, this effect is so small that the resulting passive drying is negligible. Accordingly, the color ink is then expediently overprinted with the varnish while the color ink is still wet. This means in particular that the color ink introduces a certain proportion of water into the paper web and that at least 95%, preferably at least 98%, of this water content is still retained when printing with the varnish.This is also called “wet-on-wet” printing or “wet-on-wet printing”.

[0021] Especially with a view to wet-on-wet printing, the ink and varnish are suitably matched in terms of their surface tension. The varnish expediently has a lower surface tension than the ink. The varnish thus enables greater spreading and better cross-linking and / or bonding of the generated print dots. Due to the lower surface tension of the varnish, a smaller contact angle with the substrate is formed. The lower the surface tension, the larger the area covered by a drop of varnish under otherwise identical conditions (larger dot diameter of the individual drop). The decrease in the contact angle is also faster, meaning the droplet's enlargement rate is higher.In an advantageous embodiment, the varnish has a surface tension of at most 25 mN / m, preferably in the range from 20 mN / m to 22 mN / m, and / or the color ink has a surface tension of more than 25 mN / m.

[0022] However, the compact design, and especially the short dwell time and / or the short distance between the color print head and the varnish print head, is problematic in that it prevents sufficient drying of the color ink before varnish application. The wet varnish mixes with the wet, not yet immobilized color ink, which can impair the quality of the print image. However, this disadvantage is accepted in this case and advantageously at least mitigated by one or more measures as described below.

[0023] According to a first suitable measure, the paper web is selected such that the ink is absorbed as quickly as possible. A suitable paper web for this purpose is made of uncoated paper.

[0024] A second suitable measure is to fix the color ink to the paper web, e.g. by means of a chemical reaction, preferably by pretreating the paper web with a primer (also called undercoat).

[0025] Accordingly, a primer printing function is expediently integrated into the digital printer, analogous to the varnish printing function, in that at least one of the print heads (i.e. in particular a third print head) is a primer print head for printing the paper web with a primer. This is also referred to as digital primer printing. Preferably, the above and following statements regarding the relationship (distance, dwell time, spatial arrangement, etc.) of the color print head relative to the varnish print head apply analogously to the primer print head relative to the color print head. In particular, the primer is then printed upstream of the color ink and overprinted with the color ink while the primer is still wet. In this regard, the statements regarding wet-on-wet printing of color ink and varnish apply analogously, so that overall wet-on-wet-on-wet printing of primer, color ink, and varnish is realized.Alternatively or additionally, the system includes a primer application unit for applying a primer to the paper web, with the primer application unit located upstream of the digital printer. This is also referred to as analog primer printing.

[0026] Fixation using a primer as described is particularly useful for a paper web made from coated paper. Coated papers are papers that have a coating (coating), in particular with a thickness in the range of 1 μm to 30 μm. This coating contains, in particular, one or more inorganic materials such as CaCO3, kaolin, TiO2, Al2O3 or other inorganic materials that contribute to improving the whiteness, paper smoothness, printability or one or more other functional properties of the paper web. Optionally, the coating contains a binder to increase strength and / or other process additives. The primer can be applied digitally or analogically.

[0027] For a coated paper web, intermediate drying of the primer using a suitable dryer is advisable between the primer application and the ink application. For a uncoated paper web, such intermediate drying is only optional. With a digital primer application, intermediate drying is not necessary.

[0028] The primer generally represents a precoating. Alternatively or in addition to the primer, printing with a white ink is also advantageous as a precoating. The above statements regarding the primer also apply analogously to printing the paper web with a white ink before printing with one or more other colored inks. Any precoating, whether primer or white ink, expediently increases the C value (chroma) and / or the L value (brightness) of the subsequently printed colors by at least 10% compared to printing on the paper web without such a precoating.

[0029] According to a third suitable measure, the paper web is made of a special paper which intrinsically fixes the color ink on the paper web.

[0030] The paper web is preferably guided along a curved path in the area of ​​the print heads, e.g. by means of several conveyor rollers, rods and / or guide rollers which are arranged along an imaginary arc and which each produce a spatial change in the direction of the paper web. The corresponding multiple changes in direction then result in the aforementioned arc. The paper web is then supported by the conveyor rollers, rods and / or guide rollers and runs between them, in particular straight and then overall in a curved shape. All conveyor rollers act in particular only on one side of the paper web, namely the non-printed side of the paper web. Accordingly, printing by the print heads then takes place on the other, opposite side.

[0031] A curved guide for the paper web is also possible using a printing cylinder, also known as a central cylinder. In this case, the paper web actually follows a continuously curved path. In an advantageous embodiment, the digital printer then has a printing cylinder to guide the paper web, and the print heads are arranged around the printing cylinder for printing the paper web on the printing cylinder. The paper web is wrapped around the printing cylinder, in particular around at least a quarter or a half of the circumference of the printing cylinder, and is then both supported and guided by the printing cylinder. The print heads are thus arranged along an imaginary arc around the printing cylinder, whereby this imaginary arc is then expediently part of an imaginary circle which is concentric with the printing cylinder.The print heads span an angle (also angular range) that is suitably in the range of 90° to 180°, preferably 120° to 180°, depending on the number of print heads and their spacing. The digital printer preferably has 6 to 18 print heads. Suitably, the print heads or at least the carriages are arranged equidistantly (i.e., at the same distance or at the same angle) from one another. The printing cylinder expediently has a diameter in the range of 1.6 m to 2 m.

[0032] The curved guide advantageously prevents the paper web from buckling at its edges, reduces or prevents wrinkles in the center of the paper web, and generally ensures particularly stable web travel. In the case of a central cylinder, the contact and friction between the paper web and the central cylinder also advantageously reduces swelling of the paper web when moisture is introduced during printing.

[0033] Even regardless of the use of a printing cylinder, it is advantageous if the print heads are arranged together along an imaginary arc. This imaginary arc is not necessarily a circular arc.

[0034] In a preferred embodiment, the color print head and the varnish print head are arranged at an angle of at least 10° relative to each other, particularly when these two print heads are also adjacent to each other, i.e., no further print head is arranged between them. Generally speaking, any two adjacent print heads are arranged at an angle of at least 10° relative to each other. For a digital printer with 6 to 18 print heads, this results in the aforementioned angle of 90° to 180° for the print heads as a whole.

[0035] The system expediently has a dryer for drying the paper web and the system is further designed such that at least one printed side of the paper web is conveyed without contact from the color print head to the dryer. The printed side is the side on which color ink and / or varnish is printed using the print heads. Preferably, however, the printed side of the paper web is conveyed without contact at least from the color print head to the varnish print head. Such contact-free conveying is particularly possible if the paper web is guided by conveyor rollers or a printing cylinder as described above. No machine element comes into contact with the printed side of the paper web, especially not in any unprinted areas of the generally printed side.

[0036] Another advantageous configuration is one in which the digital printer has at least two varnish print heads that can be controlled independently of one another. This makes it possible to remove one of the varnish print heads from the digital printer during operation, e.g., for maintenance, while the other varnish print head continues to apply varnish. This enables what is known as "print while clean" operation. The same applies analogously to the color print head and, if applicable, also to the primer print head. Preferably, all print heads are provided with corresponding redundancy.

[0037] In a suitable embodiment, the digital printer has at least two carriages, each of which houses two print heads. The two carriages are redundant with respect to the color inks and / or varnishes that can be printed with the print heads of the two carriages. For example, for a color print head with the colors cyan and magenta, this means that two carriages are present, each with a color print head for cyan and magenta, so that removal during continued operation is also possible.

[0038] Below are some suitable digital printer configurations. Each letter represents a printhead and indicates its color ink (C: cyan, M: magenta, Y: yellow, O: orange, G: green, Vio: violet, K: black, W: white, and generally F: color ink), varnish (V), or primer (P). The expression (EMPTY) indicates that no printhead is used at this location. Two printheads are grouped together in a carriage; the carriages are separated by hyphens (-). The paper path passes through the printheads from left to right.

[0039] 1. PP - FF - FF - FF - FF - W

[0040] 2. PP - WW - FF - FF - FF - FF - W 3. PW-YC-YC-MK-MK- W

[0041] 4. PW-WY-YM-MC-CK-W

[0042] 5. PW - WY - YM - MC - CK - K(EMPTY) - W

[0043] 6. PP-WW-YY-MM-OO-GG-KK-W

[0044] 7. PP-YY-MM-OO-GG-VioVio-KK-W

[0045] Other variants are also suitable. If a primer (P) is used, the corresponding primer printhead is positioned upstream of any color printheads. A varnish printhead (V), on the other hand, is positioned downstream of any color printheads. To achieve the redundancy described above, it is advantageous to mount two varnish printheads in different carriages, so that, if necessary, one of the varnish printheads is positioned upstream of a single color printhead.

[0046] The system preferably has a dryer with an inlet for the paper web, and the inlet of the dryer is arranged a maximum of 5 m downstream of the varnish print head, measured along the paper web. This dryer is preferably also the only dryer in the entire system (although there may be another dryer specifically for the primer if this is printed outside the digital printer). The close arrangement of the dryer relative to the varnish print head and thus relative to the digital printer as a whole makes the system particularly compact. The dryer in particular has one or more IR emitters or one or more hot air dryers, or a combination thereof. Advantageously, the paper web changes direction on its way from the digital printer to the dryer. In a suitable embodiment, the paper web is deflected by at least 5° on its way from the varnish print head to the drying unit.However, the printed side of the paper web preferably remains untouched.

[0047] The system suitably comprises a nozzle inspection system that analyzes the print image downstream of the varnish print head, and generally, in particular, downstream of the digital printer, in order to detect a malfunction of one or more nozzles of the print heads. Malfunctions of both the color print head and the varnish print head, and possibly other print heads, are detected using the same nozzle inspection unit, i.e., the system comprises only a single nozzle inspection system for all print heads. Preferably, the nozzle inspection system examines the dried print image, i.e., the nozzle inspection system is also suitably arranged downstream of the previously described dryer. This yields a more accurate result than analyzing the still-wet print image.

[0048] 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:

[0049] Fig. 1 a system,

[0050] Fig. 2 shows a digital printer for the system from Fig. 1 ,

[0051] Fig. 3 a variant for the digital printer from Fig. 2,

[0052] Fig. 4 shows another variant for the digital printer from Fig. 2.

[0053] Fig. 1 shows a side view of a system 2 for printing a paper web 4. The system 2 has a digital printer 6, two exemplary embodiments of which are also shown in a side view in Figs. 2 to 4. The digital printer 6 has a plurality of print heads 8, 10, 12, each for printing the paper web 4, which is generally conveyed in a conveying direction F and at a web speed through the system 2 and the digital printer 6. The paper web 4 is provided in the present case as a so-called endless web, i.e. printing is not carried out on just individual pages or sheets. For this purpose, the system 2 has a splicer 14 with two unwinders for paper rolls, so that a new paper roll is added to the end of a paper roll. The paper web 4 is made up into individual pages or sheets, for example, outside the system 2 and in any case only after printing.Accordingly, System 2 produces a so-called preprint. Preprint here means that printing takes place before the paper web is further processed, in particular before the paper web is used as a cover or liner for a corrugated board web. Printing on the finished corrugated board or corrugated board web, on the other hand, is referred to as post-print.

[0054] For printing, the print heads 8, 10, 12 are lowered towards the paper web 4 as required and then eject ink in this direction depending on the corresponding image data in order to create a print on the paper web 4. This takes place without contact, i.e. the print heads 8, 10, 12 do not touch the paper web 4 during printing and also not at any other time. In the present case, the print heads 8, 10, 12 are mounted in pairs on so-called carriages 16, i.e. the digital printer 6 has one or more carriages 16, on each of which exactly two print heads 8, 10, 12 are mounted. The carriages 16 serve to mechanically hold the print heads 8, 10, 12 and to move them. Accordingly, the print heads 8, 10, 12 are mechanically coupled to one another in pairs by the carriages 16.

[0055] A first of the print heads 8, 10, 12 is a color print head 10 for printing the paper web 4 with a color ink, i.e., a first print head 10 is configured for color printing. In the exemplary embodiments shown here, the digital printer 6 even has several print heads 10, each configured as a color print head 10, for printing the paper web 4 with several different color inks of a color system.

[0056] Additionally, a varnish printing function is integrated into the digital printer 6, in which a second of the print heads 8, 10, 12 is a varnish print head 12 for printing the paper web 4 with a varnish, i.e., a second print head 12 is configured for printing varnish. The varnish is also referred to as digital varnish, as it is now printed using a digital printer 6. Just like the color ink, the varnish can also be printed selectively, i.e., it is not necessarily printed over the entire surface.

[0057] The first and second print heads 10, 12, as well as all other print heads 8, 10, 12, are designed identically and differ primarily in the printed material: the color ink, the varnish, and, in this case, an optional primer. The print heads 8, 10, 12 are designed separately from each other.

[0058] The varnish print head 12 is arranged along the paper web 4 downstream of the color print head 10 for overprinting the colored ink with varnish. When printing on the paper web 4, the colored ink is printed first, followed by the varnish. The varnish is printed at least partially at the same location on the paper web 4 as the colored ink, so that the latter is overprinted with the varnish. The varnish is printed directly onto the colored ink.

[0059] When printing with a digital printer 6, i.e. during digital printing, typically and also in the embodiments shown here, a number of color inks are applied to create a print image. Color inks include, for example, the already mentioned cyan, magenta, yellow and black, alternatively or additionally also special colors such as orange, green, violet or even white. White in particular is expediently printed as a pre-print and to suppress the other color inks that then follow. If a primer is used, the corresponding primer print head 8 is arranged upstream of any color print heads 10. A varnish print head 12, on the other hand, is arranged downstream of any color print heads 10. To achieve redundancy, for example, two varnish print heads 12 are mounted in different carriages 16, so that if necessary one of the varnish print heads 12 is arranged upstream of an individual one of the color print heads 10.

[0060] 2 and 3, the digital printer 6 has two varnish print heads 12, four color print heads 10, and one optional primer print head 8, which can be controlled independently of one another. In the embodiment of Fig. 4, the digital printer 6 has two varnish print heads 12, ten color print heads 10, and two optional primer print heads 8, wherein the print heads 8, 10, 12 are mounted in pairs on seven carriages 16, respectively. The two varnish print heads 12 are mounted on different carriages 16, so that one varnish print head 12 is available for maintenance or can be replaced, while the other varnish print head 12 continues to print varnish. The printed image is dried after printing; for this purpose, the system has one or more active dryers 18 through which the paper web 4 passes. Examples of active dryers 18 are IR radiators and hot air dryers.

[0061] The system 2 shown here has a particularly compact design. This is achieved by integrating the varnish application into the digital printer 6 for printing with colored ink, i.e., the varnish application is combined with the color application in a single digital printer 6. Accordingly, the system 2 described here has only a single digital printer 6, with which both colored ink and varnish are printed. The system 2 shown here therefore has a total length of no more than 35 m.

[0062] In the present case, one of the color print heads 10, namely the last color print head 10, is adjacent to the varnish print head 23 and spaced apart by a maximum of 1 m as measured along the paper web 4, i.e. a distance A1 along the paper web is a maximum of 1 m. No further print head is arranged between this color print head 10 and the varnish print head 12. All print heads 8, 10, 12 are housed together in a single housing of the digital printer 6. A distance A1 between two adjacent print heads measured along the paper web 4 can also be less than the specified distance A1 of 1 m. A distance A2 between the first and the last print head 8, 10, 12 or from the first of several color print heads 10 to the varnish print head 12 is a maximum of 10 m.

[0063] In addition, in the present case, all print heads 8, 10, 12 of the digital printer 6 are arranged such that the paper web 4 is conveyed past the print heads 8, 10, 12 at a web speed in the range of 100 m / min to 1000 m / min within a dwell time in the range of 0.2 s to 2 s. The dwell time between two adjacent print heads 8, 10, 12 can be significantly shorter than the specified 0.2 s. The dwell time between the first of several color print heads 10 and the varnish print head 12 is at least 150 ms. The system 2 shown here is also designed such that no active drying of the paper web 4 takes place between the color print heads 10 and the varnish print head 12 of the digital printer 6. Passive drying also occurs at most to the extent that the print image dries on the path between the first and second print heads 10, 12. However, this effect is negligible.Accordingly, the colored ink is then overprinted with the varnish while the colored ink is still wet. This is also known as "wet-on-wet" or "wet-on-wet" printing.

[0064] Especially with a view to wet-on-wet printing, the ink and varnish are matched to each other in terms of their surface tension.

[0065] For example, the varnish has a lower surface tension than the colored ink.

[0066] Sufficient drying of the ink prior to varnish application is no longer possible. The wet varnish mixes with the wet, not yet immobilized ink. However, this is accepted in this case and at least mitigated by one or more optional measures as described below.

[0067] According to a first measure, the paper web 4 is made of uncoated paper so that the color ink is absorbed as quickly as possible.

[0068] A second measure is to fix the colored ink to the paper web 4, e.g., by pretreating the paper web 4 with a primer. Accordingly, a primer printing function is then integrated into the digital printer 6, analogous to the varnish printing function, in that at least one of the print heads 8, 10, 12 is a primer print head 8 (third print head) for printing the paper web 4 with a primer. The above and following statements regarding the relationship of the color print head 10 relative to the varnish print head 12 apply analogously to the primer print head 8 relative to the color print head 10. The primer is printed upstream of the colored ink and overprinted with the colored ink while the primer is still wet. In this regard, the statements regarding wet-on-wet printing of colored ink and varnish apply analogously, so that overall wet-on-wet-on-wet printing of primer, colored ink, and varnish is realized. Alternatively or additionally, the system 2 has, for example, as shown in Fig.1 shows a primer application unit 20 for applying a primer to the paper web 4, wherein the primer application unit 20 is arranged upstream of the digital printer 6. Fixation by means of a primer as described is particularly expedient for a paper web 4 made of coated paper.

[0069] For a paper web 4 made of coated paper, intermediate drying of the primer takes place between the primer application and the ink application using a suitable dryer 22. For a paper web 4 made of uncoated paper, such intermediate drying is merely optional. With a digital primer application, intermediate drying can be omitted.

[0070] According to a third measure, the paper web 4 is made of a special paper which intrinsically fixes the color ink on the paper web 4.

[0071] As can be seen from Figs. 2 and 4, the paper web 4 is guided along an arcuate path in the area of ​​the print heads 8, 10, 12. In Fig. 2, several conveyor rollers 24 are arranged along an imaginary arc for this purpose. The paper web 4 is then supported by the conveyor rollers 24 and runs straight between them and then in an overall arc. All conveyor rollers 24 act on only one side of the paper web 4, namely the non-printed side of the paper web 4. Accordingly, printing by the print heads 8, 10, 12 then takes place on the other, opposite side.

[0072] A curved guidance of the paper web 4 is also possible by means of a printing cylinder 26, e.g. as shown in Figs. 3 and 4. In this case, the paper web 4 actually follows a continuously curved course. The digital printer 6 accordingly has a printing cylinder 26 for guiding the paper web 4, and the print heads 8, 10, 12 are arranged around the printing cylinder 26 for printing the paper web 4 on the printing cylinder 26. The paper web 4 is wrapped around the printing cylinder 26, in this case around at least a quarter of a circumference of the printing cylinder 26, and is then both supported and guided by the latter. The print heads 8, 10, 12 are thus arranged along an imaginary arc around the printing cylinder 26, wherein this imaginary arc is then part of an imaginary circle which is concentric with the printing cylinder 26. The print heads 8, 10, 12 span an angle Wg of 90° to 180°, e.g.120°, depending on the number of print heads 8, 10, 12 and their distance A1 from each other.

[0073] The print heads 8, 10, 12, or at least the carriages 16, are arranged equidistant from one another. In the exemplary embodiments shown here, the print heads 8, 10, 12 are arranged at an angle W1 of at least 10° relative to one another.

[0074] The system 2 further comprises a dryer 18 for drying the paper web 4 and is designed such that at least one printed side of the paper web 4 is conveyed contact-free from the color print head 10 to the dryer 18. The printed side is the side on which color ink and / or varnish is printed using the print heads 10, 12. Thus, the printed side of the paper web 4 is also conveyed contact-free from the color print head 10 to the varnish print head 12. No machine element engages the printed side of the paper web 4.

[0075] The dryer 18 has an inlet 28 for the paper web 4, and the inlet 28 of the dryer 18 is measured along the paper web 4 at most

[0076] 5 m downstream of the varnish print head 12. On the way from the digital printer

[0077] 6 to the dryer 18, the paper web 4 also undergoes a change of direction W2.

[0078] System 2 also includes an optional nozzle inspection system 30, which analyzes the print image downstream of the varnish print head 12 and generally downstream of the digital printer 6 to detect a malfunction of one or more nozzles of the print heads 8, 10, 12. The same nozzle inspection unit 30 detects malfunctions of both the color print head 10 and the varnish print head 12, and possibly other print heads 8, 10, 12, i.e., system 2 has only a single nozzle inspection system 30 for all print heads 8, 10, 12.

[0079] List of reference symbols

[0080] 2 Appendix

[0081] 4 paper web

[0082] 6 digital printers

[0083] 8 Printhead (primer, primer printhead, third printhead)

[0084] 10 Print head (color ink, color print head, first print head)

[0085] 12 Print head (varnish, varnish print head, second print head)

[0086] 14 Splicers

[0087] 16 Carriage

[0088] 18 dryers

[0089] 20 primer application units

[0090] 22 T dryer (for primer)

[0091] 24 conveyor roller

[0092] 26 printing cylinders

[0093] 28 Entrance (of the dryer)

[0094] 30 nozzle inspection system

[0095] A1 , A2 distance

[0096] F Conveying direction

[0097] W1 angle

[0098] W2 Change of direction

[0099] Wg angle

Claims

Claims 1. System (2), a. which has a digital printer (6) with a plurality of print heads (8, 10, 12), each for printing a paper web (4), b. wherein a first of the print heads (8, 10, 12) is a color print head (10) for printing the paper web (4) with a colored ink, c. wherein a varnish printing function is integrated into the digital printer (6) in that a second of the print heads (8, 10, 12) is a varnish print head (12) for printing the paper web (4) with a varnish, d. wherein the varnish print head (12) is arranged along the paper web (4) downstream of the color print head (10) for overprinting the colored ink with the varnish.

2. System (2) according to claim 1, wherein the color print head (10) and the varnish print head (12) are adjacent to each other and are spaced apart from each other by at most 1 m as measured along the paper web (4).

3. System (2) according to claim 1 or 2, wherein the print heads (8, 10, 12) are arranged such that the paper web (4) is conveyed past the print heads (8, 10, 12) within a dwell time in the range of 0.2 s to 2 s.

4. System (2) according to one of claims 1 to 3, wherein the varnish has a lower surface tension than the color ink.

5. System (2) according to one of claims 1 to 4, wherein the print heads (8, 10, 12) are arranged together along an imaginary arc.

6. Plant (2) according to one of claims 1 to 5, wherein the digital printer (6) has a printing cylinder (26) for guiding the paper web (4), wherein the print heads (8, 10, 12) are arranged around the printing cylinder (26) for printing the paper web (4) on the printing cylinder (26).

7. System (2) according to one of claims 1 to 6, wherein the color print head (10) and the varnish print head (12) are arranged at an angle (W1) of at least 10° relative to each other.

8. System (2) according to one of claims 1 to 7, wherein it is designed such that no active drying of the paper web (4) takes place between the color print head (10) and the varnish print head (12) of the digital printer (6).

9. Plant (2) according to one of claims 1 to 8, wherein it has a dryer (18) for the paper web (4) and is designed such that at least one printed side of the paper web (4) is conveyed contact-free from the color print head (10) to the dryer (18).

10. System (2) according to one of claims 1 to 9, wherein the digital printer (6) has at least two varnish print heads (12) which can be controlled independently of one another.

11. System (2) according to one of claims 1 to 10, wherein the digital printer (6) has at least two carriages (16), on each of which two of the print heads (8, 10, 12) are arranged, wherein the two carriages (16) are redundant with respect to one another with regard to the color inks and / or varnishes which can be printed with the print heads (8, 10, 12) of the two carriages (16).

12. Plant (2) according to one of claims 1 to 11, wherein a primer printing function is integrated into the digital printer (6), in that at least one of the print heads (8, 10, 12) is a primer print head (8) for printing the paper web (4) with a primer, wherein the primer print head (8) is arranged upstream of the color print head (10).

13. Plant (2) according to one of claims 1 to 12, wherein it comprises a primer application unit (20) for applying a primer to the paper web (4), wherein the primer application unit (20) is arranged upstream of the digital printer (6).

14. Plant (2) according to one of claims 1 to 13, wherein it comprises a dryer (18) with an inlet (28) for the paper web (4), wherein the inlet (28) of the dryer (18) is arranged along the paper web (4) at most 5 m downstream of the varnish printing head (12).

15. Method for printing a paper web by means of a system (2) which has a digital printer (6) with a plurality of print heads (8, 10, 12), each for printing the paper web (4), a. wherein at least one of the print heads (8, 10, 12) is a color print head (10) by means of which the paper web (4) is printed with a colored ink, b. wherein a varnish printing function is integrated into the digital printer (6) in that at least one of the print heads (8, 10, 12) is a varnish print head (12) by means of which the paper web (4) is printed with a varnish, c. wherein the varnish print head (12) is arranged along the paper web (4) downstream of the color print head (10) and the colored ink is overprinted with the varnish.

16. Method according to claim 15, where the color ink is overprinted with the varnish while the color ink is still wet.

17. The method of claim 16, wherein a primer is printed upstream of the colored ink, which primer is overprinted with the colored ink while the primer is still wet.

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