Method for forming a varnish-coated image

Abstract

The printing method comprises the steps of: forming an image by applying a marking material (I) on the surface of a substrate (14); subjecting the image to a fixing treatment; and applying a layer (V) of an ionically stabilized varnish on the surface of the image, wherein when the image is formed, a layer of a liquid solvent (S) is formed on at least a part of the surface of the image, whereby ions move from the marking material (I) into the solvent (S) and are then absorbed into the substrate (14) together with a part of the solvent before the varnish is applied.

Description

Technical Field

[0001] The present invention relates to - a step of forming an image by applying a marking material on the surface of a substrate; - a step of subjecting the image to a fixing process; - a step of applying a layer of an ionically stabilized varnish on the surface of the image and relates to a printing method.

Background Art

[0002] This type of method is disclosed in WO2021201873A1. The main purpose of overcoating the printed image with a varnish layer is to improve the gloss of the image.

[0003] EP3415334A1 discloses an inkjet printing method in which the fixing process includes a step of spraying super-heated steam onto the substrate.

[0004] WO2015191305A1 states in the prior art discussion that a varnish layer can also be used to make the printed image less susceptible to smudging or offsetting. However, this document proposes reducing the susceptibility to smudging or offsetting by subjecting the printed image to a steam fixing process rather than overcoating the printed image with a varnish layer.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] An object of the present invention is to provide a method of the type described in the opening paragraph, with improved control of the gloss of an image.

Means for Solving the Problems

[0007] To achieve this object, in the method according to the present invention, when an image is formed, a layer of a liquid solvent is formed on at least a part of the surface of the image, whereby ions move from the marking material into the solvent and then are absorbed into the substrate together with a part of the solvent before the varnish is applied.

[0008] When an ionically stabilized varnish is used to obtain a high gloss of an image, the deterioration of the gloss of the image can be caused by the movement of ions from the marking material into the varnish. For example, when the varnish is anionically stabilized, a high gloss is obtained because the anions in the varnish keep the varnish in a fluid state for a time period long enough for the varnish to form a smooth surface. Thus, if the marking material contains cations, these cations can move into the varnish and cause premature destabilization of the varnish, and as a result, the flow of the varnish is impaired before a smooth surface is formed.

[0009] In the method according to the present invention, the layer of the liquid solvent formed on the surface of the image moves ions that can destabilize the varnish from the marking material into the solvent. Then, at least a part of the solvent in which the ions are dissolved is absorbed into the substrate. In this way, the concentration of harmful ions in the marking material can be reduced before the varnish is applied.

[0010] More specific optional features of the present invention are set forth in the dependent claims.

[0011] In one embodiment, a pretreatment liquid, e.g., a primer, can be applied to the surface of the substrate before the printing step to which the marking material is applied. The pretreatment liquid helps to destabilize the marking material applied, for example, in the form of liquid ink, and may contain, for example, Mg ++ cations, or other divalent cations. This has the advantage that the sensitivity of the ink to color bleeding is reduced, and as a result, the image quality can be improved. However, the use of a primer may have an undesirable side effect that the cations that have migrated from the primer into the ink may remain in the ink and then further migrate into the varnish, where they can cause an undesirable destabilization of the varnish. However, in the method according to the invention, this side effect is suppressed by absorbing the residual ions into the substrate before the varnish is applied. In this way, the choice of available primers and the allowable content of cations in the primer can be increased without impairing the gloss of the image.

[0012] The varnish may be an aqueous varnish, and the solvent film in which the ions from the marking material will dissolve may be, for example, a thin water film applied by a heat-setting treatment with superheated steam (SHS).

[0013] The varnish layer can be applied using an anilox roller. In that case, the absorption of at least some of the ions into the substrate also reduces the contamination of the anilox roller caused by the destabilized varnish.

[0014] In another embodiment, the varnish can be applied using an inkjet nozzle.

[0015] Embodiment examples are described hereinafter in connection with the drawings.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Mode for Carrying Out the Invention

[0017] The printing system shown in FIG. 1 includes a conveyor 10 that constitutes a conveyance path 12 for a sheet-like printing substrate 14. A primer application unit 16, an inkjet printing engine 18, a fixing unit 20, and a varnish applicator 22 are arranged in this order in the conveyance direction along the conveyance path 12.

[0018] The primer application unit 16 is constituted by, for example, an inkjet printing head, and is configured to apply a uniform or non-uniform layer of a liquid primer onto at least a part of the surface of each substrate 14. The primer may be any known and commercially available primer suitable as a pretreatment liquid for printing. Typically, the primer contains one or more salts containing divalent cations such as Mg ++ and the like.

[0019] The printing engine 18 includes a multi-color inkjet printing head assembly suitable for forming a printed image on the surface of the substrate 14 or rather on the surface of the primer layer by ejecting ink droplets onto the substrate. The inks of different colors may be, for example, aqueous inks each containing a dispersion of a coloring pigment. As long as the ink on the substrate is still in a liquid state, the cations from the primer layer can dissolve into the ink and cause the desired destabilization of the ink, thereby reducing the fluidity or mobility of the ink, and thus suppressing the bleeding of one color ink into the adjacent area of the image carrying different color inks.

[0020] The immobilization unit 20 comprises a high-temperature gas nozzle array 20a on the upstream side, (optionally) a low-temperature gas nozzle array 20b, and an SHS nozzle array 20c on the downstream side. The high-temperature gas nozzle array 20a is configured to blow a high-temperature gas stream onto the surface of the substrate 14 in order to cure the liquid ink and thus immobilize the printed image. The high-temperature gas may comprise hot air at a temperature of 120 °C at a pressure of 20.4 hPa. The low-temperature gas nozzle array 20ab is configured to blow a low-temperature gas stream onto the surface of the substrate 14 to cool the substrate. The SHS nozzle array 20c is configured to blow a superheated steam (SHS) stream onto the substrate. The superheated steam (water vapor) may have a temperature of 120 °C, a pressure of 0.4 hPa, and an oxygen content of 13%. Using different SHS conditions (temperature, oxygen content) and / or variations in the substrate temperature, the amount of water on the surface of the substrate can be controlled by the dew point. Optionally, the immobilization unit may be configured to vary the ratio of hot air and steam ejected onto the substrate. In one embodiment, the immobilization unit may be configured to apply a superheated steam stream uniformly across the entire surface of the substrate. In another embodiment, the SHS nozzle unit may comprise an array of controllable nozzles such that the processing conditions (exposure time and / or SHS flow rate) can be locally varied so that different portions of the printed image are subjected to different treatments.

[0021] When the superheated steam stream impinges on the surface of the substrate 14 having a lower temperature after leaving the low-temperature gas nozzle array 20b, the superheated steam condenses on the surface of the substrate and thus forms a thin film of liquid water, for example having a thickness of 4 μm, on the surface of the printed image. At the same time, the substrate 14 is heated to a high temperature.

[0022] While the heated substrate travels from the immobilization unit 20 to the varnish applicator 22, the water contained in the liquid ink as well as the water applied by the immobilization unit 20 evaporates, and as a result, the ink is cured and the image is immobilized on the substrate. As long as the water film still exists on the surface of the substrate, the cations from the primer solution that have migrated into the ink can further migrate into the water film. In other words, an excessive amount of salt that may be present in or on the ink layer dissolves into the water film.

[0023] When the substrate moves towards the varnish applicator 22, it should be understood that only a part of the water in the film evaporates, while another part of the water is absorbed into the substrate. The amount of water absorbed into the substrate depends on the absorption rate of the substrate material, and this absorption rate increases when the substrate is made of, for example, paper. Due to this absorption process, a substantial part of the cations dissolved in the water film is absorbed into the substrate, and as a result, the amount of cations remaining on the surface of the cured image is significantly reduced.

[0024] In the example shown, the varnish applicator 22 comprises an anilox roller configured to apply a film of an anionically stabilized aqueous varnish onto the surface of the cured image. Since the cations remaining on the surface of the image tend to move into the varnish and destabilize the varnish, the fluidity and mobility of the liquid varnish are reduced to some extent, thereby reducing the ability of the varnish to form a uniform surface layer before the varnish hardens. As a result, the gloss of the image is somewhat reduced due to the premature destabilization of the varnish. However, since the amount of cations present on the surface of the image has been reduced by absorbing most of the cations into the substrate, the deterioration of the gloss of the printed image can be kept within acceptable limits even if the primer applied by the applicator unit 16 has a high concentration of Mg salts in order to suppress color bleeding and improve the quality of the printed color image. In this way, it is possible to obtain a printed image having both high image quality and high gloss.

[0025] 2 is a cross-sectional view of one of the substrates 14 at the time of leaving the fixing unit 20. A primer layer P and an ink layer I are formed on the surface of the substrate, and superheated steam applied in the fixing process causes the temporary formation of a layer of solvent (S) (water) on the ink layer. The arrows represent the migration of cations from the primer layer P into the ink layer I and further into the solvent S. Part of the solvent with ions dissolved therein is absorbed into the substrate 14, and another part of the solvent evaporates, as represented by the "wavy" arrow. The varnish layer V to be applied later is shown by a dotted line.

[0026] Figure 3 shows the relationship between the amount of Mg in the primer and the amount of Mg in the ink for different parameters of the immobilization process. ++ 3 is a bar graph illustrating the reduction in gloss caused by the presence of ions. Bar 24 in FIG. 3 corresponds to a primer coverage of 3 g / m2 of substrate 14. 2 For the case where the primer coverage was 2 g / m 2 For the case where the primer coverage was 1 g / m 2 The horizontal axis in Figure 3 represents three different settings for the fixation process, with the percentage of exposure time to superheated steam being 0%, 12.5%, and 25% of the total treatment time in the fixation unit 20, respectively.

[0027] It can be seen that the deterioration of the gloss of the printed image is highest at high primer coverage. This is because at high primer coverage, there is a large amount of Mg that can migrate into the varnish. ++ This is explained by the fact that ions are present and cause a gloss reduction. Moreover, it can be seen in Figure 3 that the gloss reduction is effectively suppressed when the percentage of superheated steam exposure is increased from 0% to 12.5% ​​and further to 25%.

[0028] Figure 4 shows three curves 30, 32, and 34 showing the reduction of glossiness of a printed image according to primer coverage for three different settings of the immobilization process. Curve 30 relates to the case where only hot air was applied for a time period of 3 seconds. Curve 32 relates to the case where hot air was applied for 2.25 seconds and superheated steam was applied for 0.75 seconds. In both cases, the reduction of glossiness of the image is relatively high and the amount of primer coverage has little effect on the glossiness.

[0029] Curve 34 relates to the case where the immobilization process consisted of exposure to 100% superheated steam and was applied for a period of 1 second. When no primer is used at all, there is a slight or even adverse effect on the glossiness, but the reduction of glossiness is significantly suppressed when the primer coverage is larger. The improvement is particularly significant for primer coverage of 2 g / m 2 or more.

Claims

1. A printing method, - The step of forming an image by applying a marking material (I) to the surface of a substrate (14), - A step of subjecting the image to a fixation process, - The step of applying an ionically stabilized varnish layer (V) onto the surface of the image. Includes, A method characterized in that, once an image is formed, a layer of liquid solvent (S) is formed on at least a portion of the surface of the image, thereby causing ions to move from the marking material (I) into the solvent (S) and then be absorbed into the substrate (14) along with a portion of the solvent before the varnish is applied.

2. The method according to claim 1, wherein the varnish is an anionically stabilized varnish.

3. The method according to claim 1 or 2, wherein the pretreatment solution (P) is applied to the surface of the substrate (14) before the marking material (I) is applied.

4. The method according to claim 3, wherein the pretreatment solution (P) comprises at least one salt having a divalent cation.

5. The method according to claim 1 or 2, wherein varnish (V) is an aqueous varnish.

6. The method according to claim 1 or 2, wherein the solvent (S) is water.

7. The method according to claim 6, wherein the fixation process includes exposing the surface of the printed image to an atmosphere containing water vapor.

8. The method according to claim 7, wherein the fixation process includes the step of blowing a high-temperature gas onto the surface of the printed image, and at least a portion of the high-temperature gas is composed of superheated steam.

9. The method according to claim 1 or 2, wherein an anilox roller is used to apply varnish (V) to the surface of an image.

10. A printing system comprising a primer application unit (16), a printing engine (18), a fixing unit (20), and a varnish applicator (22), arranged in the following order along a transport path (12) for a printed substrate (14), wherein the fixing unit (20) is configured to blow superheated steam onto the surface of the substrate (14).

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