Method for hardening a coating in a printing press by means of a differently controlled gas discharge lamp

Abstract

The invention relates to a method for curing a radiation-curable coating in a printing press, wherein the same or identical light source is used for initial curing and final curing, the light source being operated in two different operating modes, which are suitable for initial curing on the one hand and for final curing on the other hand. Preferred light sources are electrical gas discharge lamps, in particular xenon flash lamps.

Description

Technical Field

[0001] The invention relates to a method for curing a coating in a printing press, a printing press itself and to the use of a gas discharge lamp in a printing press for the partial and complete curing of radiation-curable coatings. Background Art

[0002] In order to produce printed products in a printing press, a substrate such as paper, cardboard or foil can be printed with various printing media. The printing media are usually printing inks, inks, varnishes or combinations thereof. After the printing media has been applied, the printed substrate must be dried in the printing press.

[0003] A common group of printed media is dried by removing the solvent (usually water). Infrared dryers and hot air dryers are usually used in printing presses.

[0004] Another group of common printing media is dried under the action of UV radiation or electron radiation. These printing media often involve components based on acrylates or other unsaturated compounds, which partially polymerize under the action of UV light or electron radiation with the participation of one or more photoinitiators and thus harden.

[0005] The curing process of UV-curable coatings in a printing press is usually carried out in two stages. In a first step, only the radiation-curable coating (eg UV ink, UV varnish or UV printing ink) is initially cured. This initial curing Also called pinning. For this purpose, the radiation-curable coating is usually irradiated with relatively low doses of relatively long-wave UV radiation shortly after it has been applied to the printing substrate. This fixes the printed image sufficiently so that it can be transported further through the machine without undesired changes and printed with other colored print media. For the initial curing, LED emitters are usually used in the prior art.

[0006] In a second step, usually at the end of the printing process, after all colors have been printed and, if necessary, the printed substrate has been finished with the aid of UV varnish, final curing is carried out. In the prior art, mercury vapor lamps are usually used for final curing, which emit, in particular, high-intensity short-wave UV light. This completely or almost completely ends the curing reaction in the initially cured radiation-curable coating. The result is a hardened printed product (e.g. printed packaging, advertising posters or brochures) which can be used as intended without sticking or fading.

[0007] Document DE 2019270 A1 describes a device for curing a layer applied to a body, wherein a xenon pulsed electron tube is used as a light source. A two-stage curing method in a printing press is not disclosed here.

[0008] In the printing industry, there is a need for hardening solutions that are cost-effective and require little space. These hardening solutions should include as few operating components as possible and should be simpler to cool and maintain. In addition, the use of mercury vapor lamps should be reduced from an environmental point of view.

[0009] Surprisingly, a gas discharge lamp, such as a xenon flash lamp, can be controlled in two different operating modes and in each case emits radiation which is suitable in particular for the initial and final curing of radiation-curable coatings in a printing press. Summary of the invention

[0010] The invention has many advantages. For initial curing and final curing, the same type of lamp can be used, so that instead of two separate control devices a single control device can be used. This makes the supply of spare parts and maintenance simpler. If the radiation incident on the printing substrate is controlled by another component (for example a shield or the like), initial curing and final curing can be carried out with the same lamp, which can be controlled in the first or second operating mode as required. This can also further optimize the space requirement and cooling in the printing press. It is no longer necessary to use mercury vapor lamps, which are environmentally problematic.

[0011] Accordingly, the invention relates to a method for curing a radiation-curable coating in a printing press, wherein a printing substrate is provided, the printing substrate having a radiation-curable coating,

[0012] i) partially curing the radiation-curable coating at a first location in the printing press using a first radiation; and

[0013] ii) completely curing a partially cured radiation-curable coating by means of a second radiation at a second position in the printing press different from the first position; characterised in that the same electric radiation source is used for the first radiation as for the second radiation, wherein the electric radiation source is operated in a first electrical operating mode for the first radiation and the electric radiation source is operated in a second electrical operating mode for the second radiation, wherein the radiation spectrum of the electric radiation source in the first electrical operating mode is different from the radiation spectrum in the second electrical operating mode.

[0014] In principle, radiation-curable coatings are all UV-curable compositions that can be processed in a printer, in particular in an inkjet printer, an offset printer, and a screen printer. Preferred radiation-curable coatings are UV-curable compositions selected from the group consisting of UV-curable inks, UV-curable inks, UV-curable varnishes, and mixtures thereof.

[0015] In a preferred embodiment, the electric radiation source is at least one gas discharge lamp, in particular at least one xenon flash lamp or krypton flash lamp. In a particularly preferred embodiment, it is a xenon flash lamp. Xenon flash lamps suitable for the invention are commercially available, for example, from Fa. Heraeus Noblelight GmbH in Hanau, Germany.

[0016] In another preferred embodiment, the first electric radiation source and the second electric radiation source are consistent. Here, one of the above-mentioned electric radiation sources is preferably used as the electric radiation source. In a particularly preferred embodiment, the partially cured position is shielded from the radiation of the electric radiation source during complete curing, and the fully cured position is shielded from the radiation of the electric radiation source during partial curing. Preferably, the radiation incident on the printing substrate is controlled by a component (such as a shield) that reversibly blocks radiation, wherein a shutter / shield is opened or closed accordingly. In another preferred embodiment, the spatial arrangement of the electric radiation source in the printing press is changed during partial curing relative to the complete curing, for example, by moving the electric radiation source on a track or by rotating the electric radiation source to change the position of the incident radiation. It is thus possible to perform initial curing and final curing with the same electric radiation source, wherein the electric radiation source is controlled in the first or second operating mode as required.

[0017] In another preferred embodiment, the radiation spectrum in the second electrical operating mode is shifted to a shorter wavelength than the radiation spectrum in the first electrical operating mode. The radiation spectrum in the first electrical operating mode has a longer wavelength and is particularly suitable for initial curing of UV-curable coatings (such as UV inks, UV inks or UV varnishes). The radiation spectrum in the second electrical operating mode has a shorter wavelength and is particularly suitable for final curing of UV-curable coatings (such as UV inks, UV inks or UV varnishes).

[0018] In a further preferred embodiment, the wavelength width of the electric radiation source in the second electrical operating mode is different from the wavelength width of the electric radiation source in the first electrical operating mode.

[0019] In general, the voltage of the gas discharge lamp (preferably a xenon flash lamp) determines the power of the electric radiation source and thus the penetration depth into the UV-curable coating. The flash duration or pulse width determines the radiation dosage (i.e. the energy used for curing). The control parameters can be varied depending on the length and diameter of the gas discharge lamp (preferably a xenon flash lamp) used.

[0020] In a preferred embodiment, the first electrical operating mode and the second electrical operating mode are distinguished from each other by at least one parameter selected from the group consisting of: voltage intensity, current intensity and current density. A particularly preferred parameter is current density. In the case of preferred electrical radiation sources, in particular gas discharge lamps, such as xenon flash lamps and krypton flash lamps, the current density is in particular a function of the applied voltage intensity and the flash duration.

[0021] In a particular embodiment, a xenon flash lamp is used, wherein during initial curing the xenon flash lamp is operated at a voltage of 200 V, with one flash occurring with a flash duration of 5 milliseconds, and during final curing the xenon flash lamp is operated at a voltage of 355 V, with ten flashes occurring with a flash duration of 1 millisecond each.

[0022] In a preferred embodiment, the electric radiation source is operated in a pulsed manner, i.e., one or more flashes with the same or different flash durations. Particularly preferably, the electric radiation source is operated in a first electrical operating mode with a voltage and flash duration that differ from the voltage and flash duration in a second electrical operating mode.

[0023] It can also be based on direction, angle, and surface coverage. The position and angle of one or more electric radiation sources can be changed based on other parameters related to the substrate. Such changes can also be the result of optical or other measurements of the finished printed product, or can be related to data introduced into the printing process (such as digital printing image data, substrate data, or parameters of the UV printing medium).

[0024] The subject matter of the invention also relates to a printing press which has the same electrical radiation source for a) partial curing and b) complete curing of radiation-curable coatings, wherein the electrical radiation source is at least one gas discharge lamp. Preferred gas discharge lamps are xenon flash lamps and krypton flash lamps.

[0025] In a preferred embodiment, the printing press according to the invention has the same source of electrical radiation for a) partial curing and b) complete curing. Such a printing press preferably has a component that reversibly blocks the radiation (e.g. a shield), has the possibility of locally moving the radiation source (e.g. on rails) or has a mechanism that can rotate the source of electrical radiation in order to change the position of the incident radiation, or a combination of these measures.

[0026] The subject matter of the invention also relates to the use of at least one gas discharge lamp, preferably a xenon flash lamp, for a) partial curing and b) complete curing of radiation-curable coatings in a printing press. In a preferred embodiment, the same gas discharge lamp, preferably a xenon flash lamp, is used for a) partial curing and b) complete curing in a printing press. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Schematically illustrates the process of the method according to the present invention;

[0028] Figure 2 Schematically illustrating a method according to the invention with two separate identical sources of electrical radiation in an inkjet printer;

[0029] Figure 3 Schematically illustrating the method according to the invention with a single source of electrical radiation in an inkjet printer;

[0030] Figure 4 The wavelength spectra of the same electrical radiation source are shown in simplified form in a first operating mode for initial curing and in a second operating mode for final curing. DETAILED DESCRIPTION

[0031] Figure 1 The process of the method according to the invention is schematically shown. For example, a printing substrate (1) is converted into a printing substrate (1a) coated with a UV-curable coating by printing in a printing press. The coated printing substrate (1a) is initially cured by using an electric radiation source emitting UV radiation in a first electrical operating mode of the radiation source, thereby obtaining an initially cured printing substrate (1b). The initially cured printing substrate (1b) is finally cured by using the same electric radiation source as used for the initial curing in a second electrical operating mode of the radiation source. The finished printed product (1d) is obtained at the output of the printing press.

[0032] Figure 2The method according to the invention with two separate identical electric radiation sources in an inkjet printer is schematically shown. A printing substrate (1) is introduced into the printer (4). A UV-curable ink is applied to the printing substrate (1) by means of an inkjet print head (2), so that a coated printing substrate (1a) is obtained. By irradiating with the electric radiation source (3a) in a first electrical operating mode, an initially cured printing substrate (1b) is obtained. By irradiating with the same electric radiation source (3b) in a second electrical operating mode, a finally cured printing substrate (1c) is obtained. The finished printed product (1d) is obtained at the output of the printer.

[0033] Figure 3 The method according to the invention with a single electric radiation source in an inkjet printer is schematically shown. A printing substrate (1) is introduced into a printer (4). A UV-curable ink is applied to the printing substrate (1) by means of an inkjet print head (2), so that a coated printing substrate (1a) is obtained. By irradiating with the electric radiation source (3c) in a first electrical operating mode, an initially cured printed product (1b) is obtained. By irradiating with the same electric radiation source (3c) in a second electrical operating mode, a finally cured printing substrate (1c) is obtained. A finished printed product (1d) is obtained at the output of the printer. During the initial curing and final curing, a shield (5) is respectively opened or closed so that only the desired areas on the printing substrate (1b) or (1c) are irradiated.

[0034] Figure 4 The wavelength spectra of the same electric radiation source are shown in simplified form in a first operating mode (6) for initial curing and in a second operating mode (7) for final curing.

[0035] Reference numerals list

[0036] 1 Printing substrate

[0037] 1a Coated substrate

[0038] 1b Initial hardening of the substrate

[0039] 1c The printed substrate after final hardening

[0040] 1d printing products

[0041] 2 Inkjet print heads

[0042] 3a Electrical radiation source in the first electrical operating mode

[0043] 3b Electrical radiation source in the second electrical operating mode

[0044] 3c Electric radiation source

[0045] 4 Printing press

[0046] 5. Shielding

[0047] 6 Wavelength spectrum in the first electrical operation mode

[0048] 7 Wavelength spectrum in the second electrical operation mode

Claims

1. A method for curing a radiation-curable coating in a printing press (4), wherein: Providing a printing substrate (1) having a radiation-curable coating, i) partially curing the radiation-curable coating at a first location in a printing press (4) using a first radiation exposure; as well as ii) fully curing the partially cured radiation-curable coating at a second location in the printing press (4) different from the first location using a second radiation exposure; It is characterized in that For the first irradiation, the same electric radiation source (3) is used as for the second irradiation, wherein the electric radiation source (3) is operated in a first electric operating mode for the first irradiation and the electric radiation source (3) is operated in a second electric operating mode for the second irradiation, The radiation spectrum of the electric radiation source (3) in the first electric operation mode is different from the radiation spectrum in the second electric operation mode.

2. The method according to claim 1, in, The source of electrical radiation (3) is at least one gas discharge lamp.

3. The method according to claim 1, in, The source of electrical radiation (3) is at least one xenon flash lamp or a krypton flash lamp.

4. The method according to any one of claims 1 to 3, in, The electrical radiation source in the first electrical operating mode and the electrical radiation source in the second electrical operating mode are identical.

5. The method according to claim 4, in, During the partial curing in step i), the second location is shielded from the radiation of the electric radiation source (3), and during the complete curing in step ii), the first location is shielded from the radiation of the electric radiation source (3).

6. The method according to any one of claims 1 to 3, in, The radiation spectrum of the electric radiation source (3) in the second electrical operation mode is shifted to shorter wavelengths compared to the radiation spectrum of the electric radiation source (3) in the first electrical operation mode.

7. The method according to any one of claims 1 to 3, in, The wavelength width of the electric radiation source (3) in the second electric operation mode is different from the wavelength width of the electric radiation source (3) in the first electric operation mode.

8. The method according to any one of claims 1 to 3, in, A first electrical operating mode of the electric radiation source (3) is distinguished from a second electrical operating mode of the electric radiation source (3) by at least one parameter selected from the group consisting of voltage intensity, current intensity and current density.

9. The method according to any one of claims 1 to 3, in, The electrical radiation source (3) is operated in a first electrical operating mode with a voltage and a flash duration which are different from the voltage and the flash duration in a second electrical operating mode.

10. A printing press (4) comprising at least one electric radiation source, the at least one electric radiation source comprising at least one gas discharge lamp, the at least one electric radiation source being arranged to: a) partially curing the radiation-curable coating at a first location in a printing press (4) using a first radiation; and b) fully curing the partially cured radiation-curable coating at a second location in the printing press (4) different from the first location using a second radiation exposure; in, For the first irradiation, the same electric radiation source (3) is used as for the second irradiation, wherein the electric radiation source (3) is operated in a first electric operating mode for the first irradiation and the electric radiation source (3) is operated in a second electric operating mode for the second irradiation, The radiation spectrum of the electric radiation source (3) in the first electric operation mode is different from the radiation spectrum in the second electric operation mode.

11. The printing press (4) according to claim 10, For a) partial curing and b) complete curing, the printing press has the same source of electrical radiation (3).

12. Use of at least one gas discharge lamp for a) partial hardening and b) complete hardening of a radiation-curable coating in a printing press (4) according to claim 10 or 11.

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