Method for printing with an inkjet printing machine in an inert gas atmosphere

Abstract

A printing process using an inkjet printing machine is described in which the UV ink is cured by irradiation with UV radiation in the presence of an inert gas-enriched atmosphere. Conventional UV LED pinning modules can be used as the UV radiation source, and their radiation is sufficient for complete curing under these conditions, thus eliminating the need for an additional curing step with a UV dryer.

Description

[0001] The invention relates to a method for printing with an inkjet printing machine, the use of a UV LED lamp in an inkjet printing machine for curing UV ink, and an inkjet printing machine configured for the method according to the invention.

[0002] Various printing processes are used in the printing industry. One important printing process is inkjet printing, in which inkjet ink is applied to a substrate via print nozzles. Inkjet inks used for this purpose are typically divided into water-based and UV inks. After printing, water-based inks are usually cured thermally using hot air and infrared light. UV inks are typically cured or hardened by exposure to UV light or electron beam radiation.

[0003] In inkjet printing presses, several colors such as cyan, magenta, yellow, and black (CMYK) are typically printed sequentially onto the substrate to create multi-colored images. These different colors are applied one after the other at several printing stations, each of which can consist of one or more printheads. In inkjet printing with UV ink, it has proven advantageous to cure the UV ink with UV light after each printing station, a process also known as pinning in the prior art, and then, after all desired colors have been applied, to perform a final curing with high-intensity UV radiation. This results in a finished printed product that no longer smudges and can be used as intended.

[0004] Pinning prevents the UV ink from running on the substrate as it passes through the inkjet printing machine. Pinning only partially cures or hardens the UV ink. High-intensity UV radiation is not required for this; for example, LED-based UV lamps can be used. Final curing, on the other hand, typically uses high-intensity UV light sources such as high-energy mercury vapor lamps or other UV emitters.

[0005] The design of inkjet printing presses with intermediate curing (pinning) and final curing systems has proven to be partially disadvantageous. For example, different light sources must be kept within the printing press, which can be problematic in terms of maintenance. The equipment used for final curing also requires considerable space within the press. Furthermore, the use of mercury vapor lamps is increasingly undesirable from an environmental perspective. The energy consumption for final curing is also a disadvantage.

[0006] Therefore, there is a need for alternative inkjet printing processes that, in particular, avoid the need for final curing of the UV ink in the inkjet printing machine.

[0007] Surprisingly, it has now been found that UV inks can be cured so completely with UV radiation from the pinning light sources that no separate final curing is required if the irradiation is carried out in an inert gas-enriched atmosphere. The inert gas-enriched atmosphere can also be understood as meaning that the predominant gas atmosphere, usually air, has been depleted of oxygen because the oxygen content of the air has been displaced by the inert gas. It is assumed that the inventive method works successfully because oxygen has a disruptive influence on the curing reaction, and its presence is significantly reduced under the conditions of the inert gas-enriched atmosphere during the curing step.Thus, the radiation from UV LED light sources, which are commonly used for pinning, can ideally be used for complete curing when the radiation from these light sources is applied under an inert gas atmosphere.

[0008] The curing of UV inks under an inert gas atmosphere has been described in principle in the prior art. EP 1 473 341 A1 discloses an inkjet printing process in which UV ink is cured under a nitrogen atmosphere. Curing with UV LED lamps in pinning mode is not described therein. EP 3 795 934 A1 discloses the UV curing of UV ink under an inert gas atmosphere to reduce the migration of low-molecular-weight ink components. A single-stage UV curing process under an inert gas atmosphere is not described therein.

[0009] Accordingly, the present invention relates to a method for printing with an inkjet printing machine having at least two printing stations, wherein a substrate is successively printed with UV ink in each of the at least two printing stations and after each of the at least two printing stations a curing step is carried out by irradiating the UV ink printed onto the substrate by the respective printing station with UV radiation from at least one UV light source, wherein in at least one curing step the irradiation of the UV ink printed onto the substrate with UV radiation from the at least one UV light source takes place in a gas atmosphere enriched with inert gas.

[0010] The inventive method offers several advantages. The use of a final dryer is eliminated, saving space and maintenance. The use of mercury vapor lamps, which are increasingly undesirable from an environmental perspective, is also eliminated. The energy consumption of the inkjet printing press can be reduced by omitting the final dryer. Less heat is generated in the inkjet printing press, eliminating the need for complex heat dissipation. Despite this reduction, printed products of high optical quality are still obtained, and they do not smudge when used as intended.

[0011] In a preferred embodiment of the method according to the invention, the UV ink printed on the substrate is irradiated with UV radiation from at least one UV light source in the gas atmosphere enriched with inert gas at each curing step.Accordingly, in this preferred embodiment, the method according to the invention relates to a method for printing with an inkjet printing machine with at least two printing stations, wherein a substrate is successively printed with UV ink in each of the at least two printing stations and after each of the at least two printing stations a curing step is carried out by irradiating the UV ink printed onto the substrate by the respective printing station with UV radiation from at least one UV light source, wherein in each curing step the irradiation of the UV ink printed onto the substrate with UV radiation from the at least one UV light source takes place in a gas atmosphere enriched with inert gas.

[0012] In a further preferred embodiment, each curing step in the inert gas-enriched atmosphere leads to a substantially complete curing of the UV ink printed onto the substrate by the respective printing station. After substantially complete curing, the UV ink no longer rubs off or smudges.

[0013] In another preferred embodiment, the UV ink, whose curing step takes place in an inert gas-enriched atmosphere, is a UV ink that exhibits a strong tendency to spread. The phenomenon of spreading is known in principle to those skilled in the art. Spreading causes the ink droplets to undesirably spread across the substrate, so that the desired printed image, which is composed of the individual ink droplets, can have optical defects. For example, the printed image may then appear blurred or indistinct to the viewer. In the prior art, the UV ink is therefore generally pre-cured, i.e., partially cured, to prevent spreading.At the end of the printing process, once the UV inks of all colors have been applied one after the other, the UV curing process takes place in the inkjet printing machine using high-energy UV dryers to achieve an essentially complete curing of the UV inks.

[0014] The UV inks suitable for the process according to the invention are not particularly limited. In principle, all UV inks described in the prior art that can be cured by irradiation with UV radiation are suitable for the process according to the invention. In the process according to the invention, inks containing one or more photoinitiators are preferably used. Such UV inks are generally known to those skilled in the art. They are liquid, reactive, UV-curable compositions with one or more colorants that can be ejected onto substrates such as paper, cardboard, or film in inkjet printing presses through the printhead nozzles. Irradiation with UV light or electron beam radiation can initiate a curing reaction in the UV ink, which involves a reaction of the unsaturated monomers it contains.Such UV inks typically contain one or more photoinitiators that trigger the curing reactions. As an alternative to UV inks, thermally drying inks are known to those skilled in the art; these contain larger proportions of water or other solvents and dry essentially by evaporation of the solvent during treatment with infrared radiation and / or hot air.

[0015] In a further preferred embodiment, the at least one UV light source is selected from UV LED lamps. UV LED lamps are generally known to those skilled in the art. They are radiation sources based on light-emitting diodes (LEDs) that exhibit a comparatively high proportion of radiation in the ultraviolet (UV) wavelength range, i.e., less than 400 nm, in their emitted radiation spectrum.

[0016] In a further preferred embodiment, the at least one UV light source is operated in pinning mode. This means that the at least one UV light source is operated in a mode commonly used in the prior art for pinning UV ink in inkjet printing presses. This is distinct from operating a UV light source in UV end dryers of inkjet printing presses, where very high radiation densities are generated.

[0017] In another preferred embodiment, the at least one UV light source is selected from UV LED pinning modules. UV LED pinning modules are known in principle to those skilled in the art and are commercially available. UV LED pinning modules comprise several UV light-emitting diodes (LEDs) that emit a comparatively high proportion of radiation in the ultraviolet (UV) wavelength range when an electrical voltage is applied. In the prior art, UV LED pinning modules are used in inkjet printing machines to cure (pin) the printed UV inks. Because curing or pinning requires only comparatively low radiation densities, UV LED pinning modules are not designed to achieve particularly high radiation densities such as those of UV end dryers.

[0018] In another preferred embodiment, the duration of each hardening step in the gas atmosphere enriched with inert gas is in the range of 1 to 1000 ms, preferably in the range of 5 to 500 ms.

[0019] In a further preferred embodiment, the irradiation energy of the at least one UV light source during the curing step in the inert gas enriched gas atmosphere is in the range of 10 to 400 mJ / cm², preferably in the range of 20 to 300 mJ / cm².

[0020] In principle, all gases and gas mixtures that are inert under the conditions of the respective curing step are suitable as inert gases in the process according to the invention. In a preferred embodiment, the inert gas is nitrogen, argon, or mixtures thereof. In a particularly preferred embodiment, the inert gas is nitrogen.

[0021] In a further preferred embodiment, the gas atmosphere enriched with inert gas comprises a proportion of the inert gas, preferably nitrogen, of at least 80 vol.% or particularly preferably at least 90 vol.%, based on the total gas atmosphere enriched with inert gas.

[0022] In a preferred embodiment, an inert gas atmosphere is created only in the areas downstream of the printing stations where the curing steps take place. This allows the amount of inert gas, preferably nitrogen, used to be reduced. In another preferred embodiment, the inert gas atmosphere is generated in larger, contiguous areas inside the inkjet printing machine. In this case, the printing and curing steps can also take place in the same inert gas atmosphere. The inert gas atmosphere is preferably generated by displacing the prevailing gas atmosphere, usually air. In a preferred embodiment, the inert gas flows continuously from a reservoir, preferably a gas cylinder, into a partially enclosed cavity and displaces the prevailing gas atmosphere, usually air.The excess inert gas continuously escapes the partially enclosed cavity to the outside. Due to the overpressure of the inert gas, no air can flow into the partially enclosed cavity. In a further preferred embodiment, the inert gas-enriched atmosphere is provided in the form of a so-called gas curtain in the process according to the invention. In this case, there is essentially no spatial shielding of the inflowing inert gas by structural measures; rather, the inert gas flows essentially unrestricted onto the substrate during the curing step. In this preferred embodiment, the inert gas is preferably allowed to flow continuously at elevated pressure in the ambient atmosphere, usually air, onto the target location on the substrate.This procedure also allows the oxygen from the ambient air to be displaced at the desired target location on the printing material, thus creating the gas atmosphere enriched with inert gas required according to the invention.

[0023] Therefore, the inert gas-enriched atmosphere can, on the one hand, be created locally on the substrate in those areas where the curing steps take place. On the other hand, an inert gas-enriched atmosphere can also be created throughout the entire interior of the inkjet printing machine, thereby including the areas where the curing steps take place.

[0024] In a further preferred embodiment, UV ink of a different color is used in each of the at least two printing stations. In a particularly preferred embodiment, the UV inks are selected from cyan, magenta, yellow, and black (CMYK).

[0025] In a further preferred embodiment, the UV ink used in the inventive method comprises one or more photoinitiators. Photoinitiators for UV inks are generally known to those skilled in the art and are commercially available. These decompose upon irradiation with UV light into reactive compounds that initiate the curing reaction of the UV ink.

[0026] In a further preferred embodiment, the substrate is selected from paper, cardboard, and film. In another preferred embodiment, the substrate is selected from sheet-shaped and web-shaped substrates. It is also possible to print substrates in the form of objects such as helmets, footballs, or the like using the method according to the invention, provided that the inkjet printing machine used is designed for printing such objects.

[0027] In a further preferred embodiment, no additional final hardening by at least one final dryer is provided after the last of the at least two printing stations. In a particularly preferred embodiment, no final dryer is provided, which is selected from final dryers based on mercury vapor lamps and UV LED final dryers.

[0028] The present invention also relates to the use of at least one UV LED lamp in an inkjet printing machine for curing at least one UV ink on a substrate in the presence of an inert gas enriched gas atmosphere, wherein the at least one UV LED lamp is operated in pinning mode.

[0029] The present invention also relates to an inkjet printing machine configured for the method according to the invention. In a preferred embodiment, the inkjet printing machine according to the invention has several printing stations, with a curing station provided downstream of each printing station. In this curing station, a substrate can be cured in the presence of an inert gas-enriched gas atmosphere by irradiation with UV light from a UV light source, preferably UV light from one or more UV LED pinning modules. In a further preferred embodiment, no final dryer is provided downstream of the last printing station in the inkjet printing machine according to the invention.

[0030] Figure 1Figure 2 schematically shows a preferred embodiment of the method according to the invention. A substrate (3) is provided and fed into the inkjet printing machine (1). At the first printing station (2), which consists of several adjacent inkjet printheads, each with numerous print nozzles, UV ink (4) of the color cyan is ejected from the print nozzles onto the substrate (3). After printing with the UV ink (4), the first curing step takes place, in which the UV ink (4) just printed is irradiated with UV light from a UV light source (5), here several UV LED pinning modules, in an inert gas-enriched gas atmosphere (6), here a nitrogen atmosphere generated by supplying gaseous nitrogen from a gas cylinder. The UV LED pinning modules are operated in pinning mode, i.e., they emit comparatively low radiant energy.After the first curing step, the printed substrate (3) is transferred to the next printing station (2), which in turn consists of several inkjet printheads, each with numerous print nozzles. There, the substrate (3) is printed with magenta UV ink (4). The second curing step follows, in which the UV ink (4), now magenta, is cured again in the presence of an inert gas atmosphere (6) by irradiation with a UV light source (5) in the form of several UV LED pinning modules. After the second curing step, the UV ink (4) on the substrate (3) is completely cured. Additional final drying or curing using high-energy UV radiation is not required, as the UV ink (4) used is already so completely cured by the curing steps in the inert gas enriched atmospheres (6) that the UV ink (4) no longer runs or smears.The finished printed substrate (3) can accordingly be discharged from the inkjet printing machine (1) and stacked directly without color transfer, used commercially as intended or transferred to a further processing machine such as a folding machine. Reference symbol list

[0031] 1 Inkjet printing machine 2 Printing station 3 Substrate 4 UV ink 5 UV light source 6 Inert gas enriched atmosphere

Claims

1. Method for printing with an inkjet printing machine (1) having at least two printing stations (2), wherein a substrate (3) is successively printed with UV ink (4) in each of the at least two printing stations (2) and after each of the at least two printing stations (2) a curing step is carried out by irradiating the UV ink (4) printed onto the substrate (3) by the respective printing station (2) with UV radiation from at least one UV light source (5), characterized by that in at least one curing step the UV ink (4) printed on the substrate (3) is irradiated with UV radiation from the at least one UV light source (5) in a gas atmosphere (6) enriched with inert gas.

2. Method according to claim 1, wherein the irradiation of the UV ink (4) printed on the substrate (3) with UV radiation from the at least one UV light source (5) in the gas atmosphere (6) enriched with inert gas takes place at each curing step.

3. Method according to one of the preceding claims, wherein each curing step in the inert gas enriched gas atmosphere (6) leads to a substantially complete curing of the UV ink (4) printed onto the substrate (3) by the respective printing station (2).

4. Method according to one of the preceding claims, wherein the UV ink (4), the curing step of which takes place in the gas atmosphere (6) enriched with inert gas, is a UV ink (4) which has a strong tendency to spread out.

5. Method according to one of the preceding claims, wherein the at least one UV light source (5) is selected from UV LED lamps.

6. Method according to one of the preceding claims, wherein the at least one UV light source (5) is operated in pinning mode.

7. Method according to one of the preceding claims, wherein the at least one UV light source (5) is selected from UV LED pinning modules.

8. Method according to any of the preceding claims, wherein the duration of each hardening step in the inert gas enriched atmosphere (6) is in the range of 1 to 1000 ms.

9. Method according to one of the preceding claims, wherein the irradiation energy of the at least one UV light source (5) during the curing step in the inert gas enriched gas atmosphere (6) is in the range of 10 to 400 mJ / cm² 2 lies.

10. Method according to any of the preceding claims, wherein the inert gas of the inert gas enriched atmosphere (6) is nitrogen, argon or mixtures thereof.

11. Method according to one of the preceding claims, wherein the inert gas-enriched gas atmosphere (6) comprises a proportion of the inert gas of at least 80 vol.%, based on the total inert gas-enriched gas atmosphere (6).

12. Method according to one of the preceding claims, wherein UV ink (4) of a different color is used in each of the at least two printing stations (2).

13. Method according to any of the preceding claims, wherein the UV ink (4) comprises one or more photoinitiators.

14. Method according to any of the preceding claims, wherein the substrate (3) is selected from paper, cardboard and film.

15. Method according to one of the preceding claims, wherein no additional final hardening by at least one final dryer is provided after the last of the at least two pressure stations (2).

16. Use of at least one UV LED lamp in an inkjet printing machine (1) for curing at least one UV ink (4) on a substrate (3) in the presence of an inert gas enriched gas atmosphere (6), wherein the at least one UV LED lamp is operated in pinning mode.

17. Inkjet printing machine (1), configured for the method according to any one of claims 1 to 15.

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