EP3615346B1 - Method for printing non-absorbent substrates with a water-based ink - Google Patents

Description

The invention relates to a method for printing non-absorbent substrates with a water-based ink according to the preamble of claim 1.

through the U.S. 9,376,582 B1 a method for printing non-absorbent substrates with a water-based ink is known, in which ink drops are applied to the respective substrate, in which the ink is applied to the substrate in question with at least one inkjet print head, in which substrates are used that are in air at a temperature of 20°C each have a surface tension of less than 45 mN/m, at which the surface tension of the substrates to be printed is increased to a value of at least 45 mN/m before the ink drops are applied, at which point the ink drops only are then applied to the respective substrate having the increased surface tension, an ink having a water content of at least 70% being used.

through the U.S. 9,573,349 B1 a method for printing non-absorbent substrates with a water-based ink is known, in which ink drops are applied to the respective substrate, in which substrates are used, each of which has a surface tension of less than 45 mN/m, in which prior to the application of the ink drops the surface tension of the substrates to be printed is increased to a value of at least 45 mN/m, at which point the ink droplets are only then applied to the respective substrate having the increased surface tension.

US 2012/0026264 A1 reveals that a printed substrate can be printed by the action of heat, e.g. B. to dry by infrared radiation, for which Drying temperature range between 40 ° C and 100 ° C and for the exposure time a range between 0.2 s and 10 s is suggested.

CH 703 704 A1 discloses a printing device for printing flat structures in the form of tin plates with at least one printing ink, the surface tension of this flat structure being smaller than the surface tension of the printing ink, comprising at least one printing unit that can be activated and a final drying system downstream of this at least one printing unit that can be activated for drying the at least one activatable printing unit on the fabric applied printing ink, wherein upstream of the first printing unit that can be activated in the process sequence there is at least one irradiation device, each with at least one electromagnetic radiation source for irradiating and increasing the surface tension of the fabric before it is fed to the first printing unit that can be activated, and wherein the radiation source is an elongated UV gas discharge tube or an elongated one Infrared heater is.

DE 10 2012 017 538 A1 discloses a method for treating the surface of objects with fluids, in which the fluid is applied using a first tool that can be moved under computer control and then further treated with the second tool that can be moved under computer control, with the time delay between the application of the Fluid is adapted to the object and the subsequent further treatment of the spreading behavior of the fluid.

The invention is based on the object of creating a method for printing non-absorbent substrates with a water-based ink and a method for operating a digital printing machine with at least one inkjet print head, with which non-absorbent substrates can be printed with a print image of high print quality, in particular, despite the use of a water-based ink be printed while avoiding spreading of the ink drops.

The object is achieved according to the invention by a method having the features of claim 1. The dependent claims relate to advantageous refinements and/or developments of the solution found.

The advantages that can be achieved with the invention are, in particular, that knowledge of the properties of the surface of the substrates to be printed that are actually present before printing and that influence wetting is not required, but that the substrates to be printed are preferably immediately before their Printing can be brought into a state of good, ie at least partial to very good, ie complete wettability, while at the same time possible negative effects on the printed substrates are avoided by this measure, or at least limited.

Another advantage is a very flat print with a low haptic ink build-up, i. H. a low relief formation, and a pressure with a high degree of gloss, whereby z. B. in the packaging market frequently asked requirements can be met in a simple manner. Moreover, substrates printed in this way can be formed without disruptive abrasion in the forming machine. This is also a good prerequisite for overcoating and a small and therefore environmentally friendly application of ink with water as an environmentally friendly solvent.

There is e.g. B.in packaging printing, the need to print non-absorbent substrates in an industrial printing process with at least one water-based ink. These substrates are z. B to preferably consist of a solid especially flat substrates, z. B. to substrates each made of a metallic material, in particular metal panels, z. B. from a steel sheet or from a tinplate or from an i. i.e. R. surface-oxidized aluminum material. Alternatively, non-absorbent substrates made of, in particular, pre-lacquered wood, of a plastic or of a composite material can also be considered, provided these substrates can withstand hot air contact for a maximum of one second, e.g. B. dimensionally stable. The substrates are preferably in the form of plates or slabs; B. but also be formed in web form or each as a round body.

In particular, metallic substrates are z. B. with a primer, z. B. with a white ground, and / or pre-coated with a varnish. These substrates have z. B. an oiled or greased surface or treated with an anti-corrosion agent. The surface of the substrates to be printed is in particular hydrophobic educated. For printing on these substrates, a z. B. pigment-based, especially soluble ink with a water content of at least 70%, especially at least 80% is used. If the substrate in question is intended to be further processed into packaging for food, an ink without photoinitiators is preferably used. To carry out the present invention z. B. substrates with a non-polar surface and to print on this surface, a polar ink is used.

The respective ink is applied to the respective substrate in the form of ink drops, in particular through the use of at least one inkjet print head that ejects the ink drops. B. during a monodirectional movement (single pass method) or during a bidirectional movement (multi pass method) of the print head and / or the substrate in question are applied to the same. The ink drops z. B. in a screen printing process z. B. each applied to the respective substrate with a dot density of at least 360 dpi, preferably 600 dpi. In order to generate a predetermined print image, the ink drops are usually applied in a grid having a plurality of positions in a precisely positioned manner on the relevant substrate. It can be provided that ink drops of variable size and/or that ink drops with different hues and/or that ink drops with different brightness intensities are respectively applied to the respective substrate. For a multicolored printed image structure, color points from a basic color set, e.g. B. CMYK used.

In order to be able to print the substrates successfully, the surface tension of the substrates in question must be at least as high as the surface tension of the ink used. Only in this way can at least partial wetting of the respective surface of the relevant substrates with the intended ink take place. Otherwise they will pull onto the non-absorbent surface of the substrates in question applied ink drops together and wetting is not possible. If at all, if the surface tension of the substrates in question is insufficient, the result is at least an unclear, in particular blurred or pale or color-untrue and therefore unusable printed image. The surface tension of the relevant substrates and the surface tension of the ink used are usually temperature-dependent. Other environmental conditions such. B. the humidity or certain surface properties of the substrates in question, such as their roughness and / or their pretreatment and / or their degree of contamination affect the wettability of the respective surface of these substrates.

The surface tension of solid substrates can e.g. B. be determined with commercially available test inks. The surface tension of a water-based ink can e.g. B. using the ring method by Lecomte De Noüy or using the plate method by Wilhelmy or using the ironing method by Lenard with a tensiometer or by the capillary effect or approximately by a comparative measurement with test inks.

At room temperature, e.g. B. in the surrounding air with a temperature of 20 ° C can be present for metallic substrates in solid form, ie in particular for sheet metal z. B. assume the following values for their surface tension: untreated steel 29mN/m aluminum (oxide) 33-35mN/m tinned steel approx. 35 mN/m phosphated steel 43-46mN/m

Oily or greased surfaces have a surface tension of around 30 mN/m.

The present invention assumes that substrates are used which are in air with a temperature of 20°C a surface tension z. B. less than 50 mN / m, preferably less than 45 mN / m. In any case, at the same room temperature of z. B. 20°C the surface tension of the substrates intended for printing is lower than the surface tension of the water contained as a solvent in the ink and usually also lower than the surface tension of the ink used for printing itself.

At standard pressure (SATP), i.e. at 0.1 MPa corresponding to 1 bar and at a temperature of 20°C, water has a surface tension of 72.74 mN/m and at a temperature of 50°C a surface tension of 67.95 mN/m m.

A method is now proposed in which z. in the machine executing the printing process, e.g. B. printing machine, in particular a printing formless working digital printing machine having at least one inkjet print head, but in any case before the application of the ink drops the surface tension of the substrates to be printed z. B. by at least 10%, preferably by more than 40% in each case to a value of at least 45 mN / m, in particular at least 50 mN / m, preferably to a value of over 70 mN / m, in which only after this Increasing the surface tension, the ink drops are applied to the respective substrate having the increased surface tension and in which the applied ink drops are each caused by heat input and/or heat exposure, e.g. B. by hot air, in particular at a temperature of at least 100°C, preferably at least 150°C, in particular at a temperature in a range from 200°C to 450°C and/or by infrared radiation, each with an exposure time of at most 1 second , preferably heated for less than 0.2 seconds and thereby dried. Through a z. B. when setting up a new print job to be made adjustment or setting of the heat input and / or heat exposure, z. B. the temperature of the hot air and / or the volume of this hot air z. B. the substrate used and / or the ink used can Quality of the printed image, in particular the image effect can be easily optimized.

In order to improve the wettability of non-absorbent surfaces intended for printing with a water-based ink, in particular metallic surfaces, the surface tension of the substrates in question can be increased, e.g. B. by phosphating these substrates and / or by applying a suitable coating, z. B. be increased by applying a white base and / or a special coating. Alternatively or additionally, the surface tension of the substrates in question can be increased by pretreating them with a corona method or with a plasma method or with a chemical method or by flame treatment or by UV irradiation or with other methods of surface activation. When increasing the surface tension of the substrates to be printed, a value is sought and set that not only reaches the minimum value for the preferably complete wettability of the surface of the substrates in question, but i. i.e. R. clearly exceeds.

The surface tension of the substrates in question, which is increased beyond the minimum value for wettability, leads to a mostly strong spreading of the ink drops applied to their surface, ie to an undesired spreading of these ink drops on the surface of the substrates in question. The spreading of the ink droplets results in a very rapidly changing color property in the printed image and/or also leads to an unwanted flow of the ink, e.g. B. into micro-recesses of the substrate in question. Another effect when using ink drops with different color tones is that these ink drops bleed, which can be caused by e.g. B. by a merging of different color dots and thus leads to completely undesirable color effects in the printed image, not only in color, but also in the sharpness of contours of printed structured surfaces such. B. in a font or on an image edge.

It is therefore proposed according to the invention that directly, d. H. within just a single second, preferably within 0.5 seconds after the printing of the substrates by a shock-like (over)drying of the ink drops to stop or even largely prevent them from spreading. This is done by introducing heat into the substrate in question and/or in the applied ink drops and/or by the substrate in question and/or the applied ink drops being exposed to heat, with the exposure time to this heat being e.g. B. is less than a single second, preferably less than 0.2 seconds. For example, this particularly very brief heat input and/or the corresponding heat exposure takes place in that z. B. hot air, in particular hot air with a temperature of at least 100 ° C, preferably of at least 150 ° C, in particular with a temperature in the range of 200 ° C to 450 ° C is blown by means of a dryer. This shock-like drying stops spreading, but due to the short exposure time it does not damage either the ink or the substrate in question. As an alternative or in addition to hot air, infrared radiation generated by a dryer, in particular near-infrared radiation, can also be used. Due to the massive effect of heat in each case, the water contained in the ink as a solvent in the ink drops applied to the substrate in question is almost abruptly, i. H. very quickly, if necessary, except for an i. i.e. R. low residual moisture of z. B. less than 25% of the original water content is advantageously evaporated while maintaining the respective dot size desired for the relevant ink droplets without damaging the substrate, which is heat-resistant at least up to the temperature of the hot air or radiation used. In the case of a binder-based ink, the at least one binder gels as a result of the massive heat effect exerted by hot air and/or radiation, which likewise prevents or stops the ink droplets from spreading on the substrate in question.

Claims (15)

1. Method for printing non-absorbent substrates with a water-based ink, in which method in each case ink droplets are applied to the respective substrate, in which method the ink is applied to the relevant substrate by way of at least one inkjet print head, in which method substrates are used which in each case have a surface tension of less than 45 mN/m in air at a temperature of 20°C, in which method, prior to the application of the ink droplets, the surface tension of the substrates to be printed is increased in each case to a value of at least 45 mN/m, in which method the ink droplets are only afterwards applied to the respective substrate which has the increased surface tension, characterized in that the applied ink droplets are dried by way of an input of heat and/or an exposure to heat, in each case with a duration of action of less than 0.2 seconds, within only a single second after the printing of the relevant substrate, wherein an ink which has a water content of at least 70% is used, wherein the water contained as solvent in the ink is evaporated from the ink droplets applied to the relevant substrate, by way of the input of heat and/or the exposure to heat, in each case to a residual moisture content of less than 25% of the original water content.
2. Method according to claim 1, characterized in that the ink droplets are applied to the relevant substrate during a monodirectional movement (single-pass method) or during a bidirectional movement (multi-pass method) of the print head and/or of the relevant substrate.
3. Method according to claim 1 or 2, characterized in that the input of heat and/or the exposure to heat is effected in each case by hot air at a temperature of at least 100°C and/or by an infrared radiation.
4. Method according to claim 1 or 2 or 3, characterized in that the input of heat and/or the exposure to heat is adjusted or set when setting up a new print job, wherein the temperature of the hot air and/or a volume of said hot air is set and/or adjusted in each case as a function of the substrate used and/or the ink used.
5. Method according to claim 1 or 2 or 3 or 4, characterized in that flat or web-format substrates or substrates formed in each case as a round body are used, and/or substrates which are precoated with a primer or a paint are used.
6. Method according to claim 1 or 2 or 3 or 4 or 5, characterized in that the surface tension of the relevant substrates is increased by phosphating said substrates and/or by applying a white basecoat.
7. Method according to claim 1 or 2 or 3 or 4 or 5 or 6, characterized in that the surface tension of the relevant substrates is increased by pretreating said substrates using a corona process or a plasma process or a chemical process, or by way of a flame treatment or UV irradiation.
8. Method according to claim 1 or 2 or 3 or 4 or 5 or 6 or 7, characterized in that a binder contained in the ink is gelled by the input of heat and/or the exposure to heat.
9. Method according to claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8, characterized in that substrates which are made in each case of a metal material or of a wood or of a plastic or of a composite material are used, and/or in that substrates which have a hydrophobic surface are used.
10. Method according to claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9, characterized in that an ink which contains no photoinitiators is used.
11. Method according to claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10, characterized in that substrates which have a non-polar surface are used, and a polar ink is used to print said surface.
12. Method according to claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10 or 11, characterized in that the ink droplets are applied to the respective substrate in each case in a dot density of at least 360 dpi, and/or in that ink droplets of variable size are applied to the respective substrate.
13. Method according to claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10 or 11 or 12, characterized in that the ink droplets are applied to the relevant substrate at precise positions within a grid comprising multiple positions.
14. Method according to claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10 or 11 or 12 or 13, characterized in that ink droplets in different colours and/or in different brightness intensities are applied to the respective substrate.
15. Method according to claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9 or 10 or 11 or 12 or 13 or 14, characterized in that it is carried out in a digital printing press.