Method for printing the surface of an object

Abstract

The invention relates to a method for printing a decoration on the surface of an object using a digital printing system, which has a transfer roller and an ink application unit for each color to be printed, and the method comprises the following steps: - creating separation data for each color to be printed, wherein the separation data includes printing data for the respective color, - applying ink to the transfer rollers of each color according to the separation data of the corresponding color, - transferring at least a part of the ink applied to the transfer rollers to the surface of the object to be printed.

Description

[0001] The invention relates to a method for printing a decoration on the surface of an object using a digital printing system and to a digital printing system.

[0002] Such processes are known as indirect printing or indirect digital printing. The digital printing system is used to apply the respective printing ink not directly to the object to be printed, but to a transfer roller, which is brought into mechanical contact with the surface to be printed and thereby transfers the applied ink to the surface of the object. Such a process is known, for example, from EP 2 636 531 A1. In this process, the respective printing inks are applied to a transfer roller in the digital printing system by means of ink applicators used to apply different colored printing inks. The transfer roller then transfers the ink to the surface to be printed.

[0003] In many cases, the transfer roller is coated with an elastic material, such as a rubber-elastic coating. Alternatively, the entire transfer roller can be made of rubber or a rubber-elastic compound. This is particularly advantageous if the surface to be printed has a structure, such as a relief. These are also referred to as profiled surfaces, which can be printed in this way.

[0004] The object whose surface is printed using such a process is, for example, a wood-based panel, in particular an MDF, HDF, or USB panel. These are used, for example, in the production of panels that can be used as floor panels, wall panels, or ceiling panels. Such elements are also used as furniture panels. Preferably, the top surface is sanded and then finished with the decorative coating.

[0005] From EP 2 636 531 A1, it is known to apply several printing inks, each with its own ink applicator, whereby the different printing inks are applied to a single transfer roller. This must be done sequentially by the different ink applicators used, each with its own printhead. The transfer roller then transfers all the printing inks simultaneously to the surface to be printed, which is considered particularly advantageous.

[0006] The invention is based on the objective of improving such an indirect printing process.

[0007] The invention solves the stated problem, wherein the digital printing system has a transfer roller and an ink application unit for each color to be printed, for applying ink to the respective transfer roller, wherein the method comprises the following steps: Creating separation data for each color to be printed, wherein the separation data includes printing data for the respective color, applying ink to the transfer roller of each color according to the separation data of the corresponding color, transferring at least part of the ink applied to the transfer roller to the surface of the object to be printed.

[0008] Unlike the prior art, different colored printing inks are not applied to the same transfer roller. Instead, the colors are applied individually to transfer rollers, which then transfer each individual printing ink separately to the surface of the object to be printed.

[0009] The separation data contains printing data for a single color. Separate separation data is created for each color used in the decoration that contains printing ink and is to be applied to the surface to be printed. This data includes information such as where and how much printing ink of a specific color should be applied.

[0010] Preferably, the ink application unit has a plurality of printheads for each color, extending across the entire printing width. Each printhead then preferably applies ink of a single color. Preferably, the printheads of one color are arranged in at least two rows in the feed direction. In a particularly preferred embodiment, separation data is generated for each row of printheads. For example, if the ink application unit has two rows of printheads for each of the four printing colors, there are a total of eight rows of printheads. Eight separation data sets are then preferably generated and used during printing.If printheads are used in several rows for one color, the individual printheads are preferably arranged offset from each other, so that the individual printheads are not positioned directly behind one another in the feed direction, but are also offset transversely to the feed direction.

[0011] The inventive method eliminates the possibility of different colored printing inks being applied to a single transfer roller, thus preventing smearing caused, for example, by the inks running into each other and resulting in uncontrolled mixing. Instead, the respective transfer rollers are used with color-pure ink, i.e., with only one color. Each ink application unit is configured to apply ink of a single color to a separate transfer roller, from which the ink is then applied to the surface of the object to be printed. This necessarily means that the different colored inks are applied to the surface of the object to be printed sequentially.

[0012] Preferably, the various transfer rollers are designed differently. They can differ, for example, in the material and / or the physical properties of the surface of the transfer rollers. Different printing inks of different colors can place different demands on the transfer rollers, such as different absorbencies, temperatures, or other properties. The inventive method provides a separate transfer roller for each printing ink, thus meeting these requirements. Furthermore, since separation data is available for each individual color present in the design, corrections can be made particularly easily if, for example, the color values ​​of a color need to be adjusted to achieve and ensure the desired quality of the printed design.

[0013] Preferably, the digital printing system includes at least 2, and particularly preferably at least 4, transfer rollers, the number depending on the number of colors used, i.e., the number of different printing inks. Traditionally, 4 different colors are used, usually black, yellow, red, and cyan, so in this case, 4 transfer rollers are used.

[0014] Preferably, the print data includes information about the location and quantity of ink of each color to be applied to the respective transfer roller.

[0015] In a particularly preferred embodiment, each transfer roller has a cleaning unit that removes residual printing inks remaining on the roller after it has transferred the inks to the surface to be printed. This cleaning unit includes, for example, a scraper or a doctor blade and is preferably arranged such that the surface of the transfer roller comes into contact with the cleaning unit after it has contacted the surface of the object to be printed and before it passes the ink application unit again. This cleaning unit makes it possible to exceed the natural repeat length, i.e., the circumferential length of the transfer roller, which would otherwise limit the possible decoration length.A repeat length, according to which the printed decoration must be repeated and which is well known as a problem in roller printing, has thus become irrelevant.

[0016] Advantageously, the separation data is calculated from a digital representation of the design by means of an electrical control system, in particular an electronic data processing unit. Preferably, the electrical control system is part of the digital printing system. In this case, the digital representation of the design is provided to the digital printing system, for example, by being stored as a file in an electronic data storage device to which the electrical control system has access. The electrical control system then calculates the respective separation data for the printing inks to be used from this digital representation and then controls the corresponding ink application units so that, according to the separation data for each color, the respective printing ink is applied to the transfer roller.

[0017] Preferably, the application of ink to at least one transfer roller involves transferring the ink from the ink applicator to a printing cylinder and from there to the transfer roller. Particularly preferably, each ink is first transferred to a printing cylinder and from there to the respective transfer roller, preferably using a separate printing cylinder for each color. This is especially advantageous when the temperature difference between the ink and the ink applicator used for that ink, on the one hand, and the surface of the object to be printed, on the other hand, is particularly large.To prevent condensation on the printhead of the ink application unit, it is advantageous to achieve the lowest possible temperature of the ink and the ambient air at the printhead. Therefore, a greater distance to the potentially warm or even hot object whose surface is to be printed is beneficial. Preferably, the surface of the printing cylinder is non-elastic. Preferably, the printing cylinder is chrome-plated.

[0018] In a preferred embodiment, a primer is applied to the surface to be printed before the ink is transferred. For this purpose, a primer application unit is preferably used, to which a drying unit is preferably assigned. The primer is applied by the primer application unit to the surface of the object to be printed, which is then fed to the drying unit where the applied primer is dried. The primer application unit and drying unit are preferably arranged upstream of the ink application unit. Preferably, an amount of at least 2 g / m², more preferably at least 4 g / m², more preferably at least 5 g / m² and at most 30 g / m², more preferably at most 20 g / m², and more preferably at most 10 g / m², of primer is applied.The primer, also known as a base coat, prevents the ink from penetrating the surface of the object to be printed and / or any previously applied colored primer, especially white. It prepares the substrate, i.e., the surface of the object to be printed, uniformly for digital printing and compensates for irregularities and inconsistencies, such as in the absorbency of the surface.

[0019] Preferably, the primer is colored, and in particular not white. This is especially advantageous when the design to be printed includes a color that must be applied over the entire surface or at least almost the entire surface, and this color preferably corresponds to the color of the colored primer. This allows for savings on potentially expensive printing ink. The primer application unit is particularly preferably controlled by the same electrical control system that also calculates the separation data and controls the ink application unit. In this way, for example, the amount of primer applied to the surface to be printed can be adjusted to the amount of printing ink required for the design. Particularly preferably, the amount of primer applied to the surface to be printed can be adjusted with spatial resolution, so that more primer is applied to some areas of the surface than to others.Preferably, this depends on the amount of ink to be applied to the surface for decoration.

[0020] The invention solves the problem posed by a digital printing system for printing on the surface of an object, comprising at least one ink application unit, past which an object to be printed is guided in a feed direction, and which has a plurality of printheads arranged in at least two rows offset from one another in the feed direction. Preferably, the digital printing system is configured to carry out a method described herein. It therefore has several ink application units and several transfer rollers, wherein the ink application units are configured to apply printing ink of individual colors to the respective transfer rollers according to separation data. The object to be printed, which is guided past the printheads of the ink application units, is in this case the respective transfer roller from which the respective printing ink is transferred to the component to be printed.Alternatively, a digital printing system according to the invention can also be designed without transfer rollers and be designed for so-called direct printing. In this case, the object to be printed, which is guided past the at least one ink application unit in the feed direction, is the component to be printed.

[0021] Preferably, the at least two rows of printheads extend in a direction perpendicular to the feed direction, which can also be referred to as the transverse direction. Particularly preferably, the at least two rows have a distance of at least 2 cm, more preferably at least 3 cm, more preferably at least 5 cm and at most 20 cm, more preferably at most 15 cm, more preferably at most 10 cm, from each other.

[0022] By using at least two rows of printheads, it is possible to double the print resolution in the print length, i.e., along the feed direction, while the resolution in the print width, i.e., along the direction of each row, remains the same. Naturally, the higher the feed speed at which the object to be printed moves through the digital printing system, the lower the resolution in the feed direction becomes. Using two rows of printheads doubles the resolution in the feed direction compared to using only one row. Furthermore, it is possible to create smaller print dots, which further improves the resolution and also increases print quality. For example, twice as many smaller print dots can be created, which also leads to higher printing speeds.Preferably, the printheads have a resolution of 1200 dpi (dots per inch) in the print width and a resolution of at least 1200 dpi in the print length when the printing system speed, i.e., the feed rate at which the object to be printed is moved past the ink application unit, is 80 m / min. Preferably, the feed rate is at least 20 m / min and at most 135 m / min.

[0023] Advantageously, the printheads are arranged such that the ink leaves the printheads at an angle of less than 10°, preferably less than 5°, and most preferably 0°, to the direction of gravity. This positioning and orientation makes it possible to place the printheads as far away as possible from the object to be printed, which is particularly advantageous when the object has a significantly higher temperature than the printheads and the ink. This also reduces emissions and outgassing from the object being printed, as well as the risk of condensation on the respective printhead.

[0024] If the ink application unit has two or more rows of printheads for one color, these are preferably arranged symmetrically to the direction of gravity, for example a first row at an angle of 5° to the direction of gravity and a second row at an angle of -5° to the direction of gravity.

[0025] Each individual printhead is moved close to the transfer roller to apply the ink, until the distance is, for example, less than 2 mm, preferably 1 mm.

[0026] Preferably, the digital printing system of the type described herein is suitable for carrying out one of the processes described herein, wherein the digital printing system has a transfer roller and an ink application unit for each color to be printed and has the electrical control system already described, preferably an electronic data processing unit, which is configured to carry out the process. In a particularly preferred embodiment, at least one transfer roller is elastically designed. Preferably, all transfer rollers are elastically designed.

[0027] Preferably, the electrical control system is set up to create separate separation data for each row of printheads and to use this data when applying the ink to the transfer rollers.

[0028] Preferably, after printing on a transfer roller and transferring the ink to the surface of the object to be printed, any ink adhering to the transfer roller is removed, for example, by means of a squeegee, so that the transfer roller can be reprinted. In this way, as already explained, the transfer roller can be printed regardless of the design length. For example, if the transfer roller has a circumference of 1400 mm, significantly longer designs, such as those with a design length of 5000 mm, can still be produced.

[0029] In a first specific embodiment, wood, stone, and fantasy decors were printed onto flat HDF cores measuring 280 cm x 207 cm using a 4-color CRYK (cyan, red, yellow, black) digital printing system with a printing width of 210 cm. The complete system comprises a plate feeder, a sanding unit, and a digital printing unit according to an embodiment of the invention described herein. This unit includes a white primer application device with drying and a primer application unit with drying, both of which are located upstream of the ink application units of the digital printing system. The digital printing system is designed for indirect digital printing, with each of the four CRYK colors having its own rubber transfer roller and two rows of printheads.The complete system also includes a drying unit for the printing ink applied in indirect digital printing, a protective resin unit with drying, a plate cooling device, and a plate stacking system. For example, a brown walnut wood decor is produced with an ink application rate of 4 g / m². By using a primer of 4 g / m² before applying the printing inks, this decor can be produced cost-effectively and with good quality. Tests have shown that this decor does not achieve the same color intensity without a primer and with an ink application rate of 8 g / m² as with the aforementioned combination. Without a primer, the decor could not have been produced in this form and at this cost, as the ink pigments penetrate too deeply into the core of the board, regardless of the application rate.

[0030] In a second specific embodiment, wood, stone, and fantasy decors are printed onto flat HDF cores measuring 280 cm x 207 cm using a 4-color CRYK digital printing system with a printing width of 210 cm. The complete system comprises a plate feeder, a sanding unit, and a digital printing unit according to an embodiment of the invention described herein. This digital printing unit includes a white primer application unit with drying and a primer application unit with drying, both of which are located upstream of the ink application units of the digital printing system. The digital printing system is designed for indirect digital printing, with each of the four CRYK colors having its own rubber transfer roller and two rows of printheads. The complete system also includes a drying unit for the printing ink applied in indirect digital printing, a protective resin unit with drying, a plate cooling device, and a plate stacking unit.A light oak decor is produced with an ink application rate of 2 g / m². By using a primer prior to digital printing at 4 g / m², this decor can be produced cost-effectively while maintaining good quality. Tests have shown that without a primer, this decor requires approximately 5 g / m² of ink to achieve the same color intensity as with the aforementioned combination. Without a primer, the decor requires 3 g / m² more ink than with a primer. Since the ink is roughly three times as expensive as the primer, the cost advantage is enormous.

[0031] In a third specific embodiment, wood, stone, and fantasy decors are printed on 280 cm x 207 cm HDF cores using a 4-color CRYK digital printing system with a printing width of 210 cm. The complete system comprises a plate feeder, a sanding unit, and a digital printing unit according to an embodiment of the invention described herein. This unit includes a white primer application device with drying and a primer application unit with drying, both of which are located upstream of the ink application units of the digital printing system. The digital printing system is designed for indirect digital printing, with each of the four CRYK colors having its own rubber transfer roller and two rows of printheads. The complete system also includes a drying unit for the printing ink applied in indirect digital printing, a protective resin unit with drying, a plate cooling device, and a plate stacking unit.

[0032] A stone-effect pattern is produced at a printing speed of 135 m / min using printheads in a dual row, with a resolution of 1200 dpi x 1200 dpi. An identical pattern was also printed for testing with a single printhead row, resulting in a resolution of 1200 dpi x 600 dpi. The results showed that significantly larger dot sizes were required to compensate for the reduced color intensity achieved with only one printhead row at the same speed. The print quality deteriorated due to the lower resolution and was more prone to banding in the printing direction because of the larger dot sizes.

[0033] In a fourth specific embodiment, wood, stone, and fantasy decors were printed on 280 cm x 207 cm HDF cores using a 4-color CRYK digital printing system with a printing width of 210 cm. The complete system comprises a plate feeder, a sanding unit, and a digital printing unit according to an embodiment of the invention described herein. This unit includes a white primer application device with drying and a primer application unit with drying, both of which are located upstream of the ink application units of the digital printing system. The digital printing system is designed for indirect digital printing, with each of the four CRYK colors having its own rubber transfer roller and two rows of printheads. The complete system also includes a drying unit for the printing ink applied in indirect digital printing, a protective resin unit with drying, a plate cooling device, and a plate stacking unit.

[0034] A wood decor with a design length of 5600 mm is produced. The transfer rollers of the printing unit have a circumference of 1400 mm. By removing any remaining ink from the rubber roller with a squeegee after transferring the design to the flat cores, reprinting with the digital printing unit is possible. The rubber transfer roller is printed four times in its entirety with different designs before the decor repeats. As a result of this processing, 40 different planks are produced for this floor decor, which has a plank length of 1400 mm and a width of 200 mm. Typically, in analog rotary printing, approximately 10 planks are produced, with the number limited by the circumference of the engraved printing cylinders.

Claims

1. A method for printing a decoration on the surface of an object using a digital printing system, which has a transfer roller and an ink application unit for each color to be printed, for applying ink to the respective transfer roller, the method comprising the following steps: - creating separation data for each color to be printed, wherein the separation data includes printing data for the respective color, - applying ink to the transfer rollers of each color according to the separation data of the corresponding color, - transferring at least part of the ink applied to the transfer rollers to the surface of the object to be printed.

2. Method according to claim 1, characterized by the fact that The print data includes information about the location and quantity of ink of each color to be applied to the transfer roller.

3. Method according to claim 1 or 2, characterized by the fact thatThe separation data are calculated from a digital representation of the decor by means of an electrical control, in particular an electronic data processing device.

4. Method according to any of the foregoing claims, characterized by the fact that The application of ink to at least one transfer roller involves the ink being transferred from the ink application unit to a printing cylinder and from there to the transfer roller.

5. Method according to claim 4, characterized by the fact that Each ink is first transferred to a printing cylinder and from there to the respective transfer roller, preferably using a separate printing cylinder for each color.

6. Method according to any of the foregoing claims, characterized by the fact that A primer is applied to the surface to be printed before the ink is transferred.

7. Digital printing system for printing on the surface of an object, wherein the digital printing system has at least one ink application unit past which a component to be printed is passed in a feed direction and which has a plurality of printheads which are arranged in at least two rows offset from each other in the feed direction.

8. Digital printing system according to claim 7, characterized by the fact that at least two rows extend in one direction perpendicular to the feed direction.

9. Digital printing system according to claim 7 or 8, characterized by the fact that the at least two rows have a distance of at least 2 cm, preferably at least 3 cm, particularly preferably at least 5 cm and at most 20 cm, preferably at most 15 cm, particularly preferably at most 10 cm.

10. Digital printing system according to one of claims 7 to 9, characterized by the fact thatthe printheads are arranged such that ink leaves the printheads at an angle of less than 10°, preferably less than 5°, particularly preferably 0° to the direction of gravity.

11. Digital printing system according to one of the claims for carrying out a method according to one of the preceding claims, comprising a transfer roller and an ink application unit for each color to be printed and having an electrical control, preferably an electronic data processing device, which is set up to carry out the method.

12. Digital printing system according to claim 11, characterized by the fact that The electrical control system is set up to create separate separation data for each row of printheads and to use it when applying the ink to the transfer rollers.

13. Digital printing system according to claim 11, characterized by the fact that at least one transmission roller, preferably all transmission rollers, are elastic.

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