Method and device for printing plate-shaped workpieces

Abstract

The invention relates to a method for printing a plate-shaped workpiece by means of a digital printing device having at least one inking device for applying a printing ink, the printing ink having an ink temperature when applied to the workpiece, the workpiece reaches a predetermined workpiece temperature in a temperature control device before being conveyed to the inking device, the predetermined workpiece temperature depending on the ink temperature.

Description

Technical Field

[0001] This invention relates to a method for printing plate-shaped workpieces using a digital printing apparatus having at least one inking device for applying printing ink, wherein the printing ink has an ink temperature when applied to the workpiece. The invention also relates to an apparatus for carrying out this method. Background Technology

[0002] Printing on sheet-like workpieces is common in many applications of existing technologies and is used, for example, in the manufacture of floor paneling, furniture components, or work panels. Here, wood trim, stone trim, or creative designs are printed on a sheet-like core material, which may contain, for example, wood-based panels. The digital printing equipment used here typically includes a primer component to apply a white primer to the core material to be printed in multiple steps, if necessary. In this method, each individually applied primer layer is dried online without stopping the board on its conveyor belt. This requires a significant amount of heat energy, which is introduced into the method and thus into the digital printing equipment. The core material to be printed is heated to temperatures above 70°C and is often conveyed to the inking unit at, for example, 55°C. This has several disadvantages. Due to the hot plates, which are typically equipped with multiple layers of primer, a large amount of liquid and moisture is introduced into the digital printing equipment. Furthermore, it is known, for example, by EP 2 927 003 A1, that the temperature of the workpiece to be printed has a significant impact on the printing result. Temperature affects the viscosity of the applied ink droplets and thus the visual impression of the resulting decoration, such as the chroma and sharpness of the produced image.

[0003] To at least ensure the reproducibility of the printing results, EP 2 927 003 A1 proposes heating or cooling the surface to be printed to a temperature within a predetermined range. For printing on paper, another challenge exists because moisture introduced by a heated workpiece significantly affects the surface properties of the paper, such as waviness, compared to solid workpieces, such as wood panels. Therefore, as known from EP 2 206 605 B2, different climatic zones with varying room air humidity are created, and different parts of the actual printing method are carried out in these zones. This results in high equipment costs and high energy demands, thus complicating and increasing the cost of the method. Summary of the Invention

[0004] Therefore, the object of the present invention is to provide a method and an apparatus that can at least mitigate, and in the best case overcome, the disadvantages of the prior art.

[0005] This invention achieves its proposed objective by using the method described in the preamble of claim 1, unique in that the workpiece is brought to a predetermined workpiece temperature in a temperature control device before being conveyed to the inking apparatus, wherein the predetermined workpiece temperature depends on the ink temperature. This invention is based on the understanding that there is no absolute workpiece temperature that always provides optimal printing results, independent of other parameters. Instead, the predetermined workpiece temperature depends on the temperature of the ink used. Generally, it has been found that higher ink temperatures also result in higher predetermined workpiece temperatures.

[0006] The digital printing equipment used preferably includes an electrical controller, such as an electronic data processing device, configured to process sensor data, for example, from a temperature sensor. This temperature sensor can be part of the digital printing equipment and is used, for example, to determine the temperature of the printing ink used. The measured value detected by the sensor is transmitted to the electrical controller, which thereby calculates a predetermined workpiece temperature. At least one additional temperature sensor, also part of the digital printing equipment, is used to determine the current workpiece temperature, i.e., the actual value of the workpiece temperature. This measured value is also transmitted to the electrical controller, which compares the current workpiece temperature thus known with the predetermined workpiece temperature and sets up a temperature control device to adjust the current workpiece temperature to match the predetermined workpiece temperature.

[0007] The temperature control device preferably has at least one heating element to increase the current workpiece temperature and / or at least one cooling element to decrease the current workpiece temperature. The electrical controller is configured to control these elements based on sensor measurements.

[0008] Preferably, the ink temperature and the predetermined workpiece temperature have a difference of up to 10°C, preferably up to 5°C, and particularly preferably up to 2°C. Preferably, the ink temperature is at least 20°C, preferably at least 25°C, particularly preferably at least 30°C and at most 40°C, preferably at most 38°C, and particularly preferably at most 35°C.

[0009] Due to the small temperature difference between the ink temperature and the predetermined workpiece temperature, only a small amount of condensation forms in the area of ​​the printhead of the at least one inking device when the printing ink is applied to the plate-shaped workpiece inside the at least one inking device. This is advantageous because condensation on the printhead can affect sensitive electronic devices on the one hand, and adversely affect the droplet formation process of the printing ink on the printhead on the other hand.

[0010] Preferably, the digital printing equipment has multiple inking devices for applying printing inks of different colors, wherein the ink temperature difference between adjacent inking devices is at most 1.5°C, preferably at most 1.3°C, and particularly preferably at most 1.0°C. Preferably, the predetermined workpiece temperature depends on the ink temperature of the first inking device or the average of all ink temperatures.

[0011] Advantageously, at least one primer is applied to the workpiece before it is conveyed to the temperature control device. Preferably, the workpiece is dried after the application of the at least one primer and before it is conveyed to the temperature control device. If the primer is applied in multiple separate layers, it is advantageous to dry the workpiece after each separate layer of primer is applied. Regardless of the number of individual primer layers applied, drying the workpiece before it is conveyed to the temperature control device reduces the amount of moisture introduced into the device.

[0012] Preferably, the workpiece is cooled to a predetermined workpiece temperature in a temperature control device.

[0013] Preferably, the predetermined workpiece temperature is a temperature range within which the current workpiece temperature must be to allow the workpiece to be conveyed to the inking device. The predetermined workpiece temperature range covers, for example, up to 5°C, preferably up to 3°C, and particularly preferably up to 1°C. Alternatively, the predetermined workpiece temperature can be understood as a temperature range, or it can be a precise value, in which case the workpiece is conveyed to the inking device if the difference between the current workpiece temperature and the predetermined workpiece temperature is less than a predetermined limit value. This limit value is, for example, 2.5°C, preferably 1.5°C, and particularly preferably 0.5°C. In this case, if the difference between the current workpiece temperature and the predetermined workpiece temperature is less than the predetermined limit value, the current workpiece temperature is considered to have "reached the predetermined workpiece temperature."

[0014] Preferably, when at least one parameter measured by at least one sensor is outside a predetermined range, the workpiece is sorted before being conveyed to at least one inking device, wherein the workpiece is then preferably post-processed to influence the at least one parameter such that the at least one parameter is within a predetermined range after post-processing. If the parameter is the humidity of the workpiece to be printed, the workpiece is sorted, for example, if the humidity is too high to obtain a sufficiently good printed image. The post-processing then includes drying the workpiece to be printed. If the parameter is the waviness of the surface of the workpiece to be printed, the workpiece is sorted, for example, if the waviness is too high. The post-processing then includes smoothing the surface.

[0015] Furthermore, the present invention achieves the proposed objective by means of an apparatus for implementing the method of the above type, wherein the apparatus has a digital printing device, a temperature control device, and an electrical controller, the digital printing device having at least one inking device, and the digital printing device being configured to implement the method described above.

[0016] Preferably, the device has at least one coating device for primer and / or the digital printing device has multiple inking devices.

[0017] In a preferred embodiment, the apparatus has at least one dryer arranged in the process direction behind at least one inking device of the digital printing apparatus. Particularly preferably, the apparatus is equipped with a dryer for each inking device and / or for each primer coating device, wherein each dryer is arranged in the process direction behind the corresponding coating device. This means that a primer or ink is first applied to the workpiece to be printed in the coating device and then dried by the dryer.

[0018] Preferably, the temperature control device has at least one temperature sensor for determining the current workpiece temperature, wherein the sensor is configured to transmit sensor data to an electrical controller.

[0019] Preferably, the temperature control device has at least one temperature control element, preferably at least one heating element and / or at least one cooling element, through which the current workpiece temperature can be affected.

[0020] The electrical controller is configured to monitor the current workpiece temperature of the surface of the workpiece to be printed. This is particularly low because the current workpiece temperature is transmitted from at least one temperature sensor to the electrical controller at regular time intervals, and the electrical controller compares the current workpiece temperature with a predetermined workpiece temperature.

[0021] Preferably, the at least one temperature sensor is configured to determine the current workpiece temperature in a position-resolved manner. Preferably, for this purpose, there are multiple temperature sensors, for example, arranged across the width of the workpiece, which measure the current workpiece temperature in a position-resolved manner and transmit the measurement data to the electrical controller.

[0022] Preferably, the temperature regulating element is configured to affect the current workpiece temperature in a position-resolved manner. Advantageously, the temperature regulating element has multiple heating elements and / or multiple cooling elements, each configured to affect the current workpiece temperature only in one region of the surface.

[0023] Preferably, a predetermined workpiece temperature is present inside the temperature control device, wherein the electrical controller is configured to guide the workpiece from the temperature control device to at least one inking device only when the difference between the current workpiece temperature and the predetermined workpiece temperature is less than a predetermined limit value or when the current workpiece temperature is within a range limited by the predetermined workpiece temperature.

[0024] Preferably, the apparatus includes a sorting device to sort the workpieces before they are conveyed to at least one inking device. Preferably, the sorting device is configured to then convey the workpieces to another station where post-processing can be performed.

[0025] Particularly preferably, the device includes a receiving device, for example, as part of a temperature regulating device, which allows the device to allow a single workpiece to remain in the temperature regulating device for a period of time until a predetermined workpiece temperature is reached. This can be achieved, for example, by a transport loop in the form of a bucket elevator. The included workpieces move cyclically along a closed conveyor track in the transport loop. At each cycle, there is the possibility of guiding or removing the corresponding workpiece from the temperature regulating device or the receiving device and conveying it to at least one inking device. Here, when the current workpiece temperature corresponds to the predetermined workpiece temperature, the workpiece is conveyed to the inking device. Otherwise, the workpiece remains in the transport loop until the current workpiece temperature corresponds to the predetermined workpiece temperature.

[0026] Furthermore, such a receiving device or transport loop preferably has the function of influencing the loading or pushing of workpieces into the inking device, and can be used as a workpiece buffer when a later device in the process direction, such as the inking device, does not have the expected workpiece production capacity due to functional interference. In this case, the electrical controller of the device is preferably configured to retain the workpiece in the receiving device or transport loop for a period of time until the interference is eliminated and the workpiece can be transported to the inking device or other devices again.

[0027] Preferably, the sheet-like workpiece has a core material or is composed of a core material. The core material is preferably a wood-based board, such as MDF, HDF, OSB, particleboard, or plywood. The use of boards composed of a mixture of wood particles and plastic is also a preferred embodiment of the core material. This relates to, for example, WPC (wood-plastic composite) boards. The core material can also be entirely composed of plastic, for example, in the form of plastic sheets or SPC (solid polymer core) boards. Boards provided under the name "Corepel" (see www.corepel.com) and boards having, for example, an inner core constructed of wood fibers surrounded by resin, preferably a high-performance resin, can also advantageously be used as core materials. The thickness of the core material is preferably at least 3 mm.

[0028] Embodiments of the invention are further described with specific examples. For laminate flooring manufacturers, wood trim, stone trim, and creative trim are printed on high-density wood fiberboard (HDF) using digital printing. The boards are 2800 mm long, 2070 mm wide, and have a thickness of 6 mm to 12 mm. The entire apparatus has a feed rate of 80 m / min with a gap of approximately 1500 mm between two successive workpieces. The entire apparatus includes an automated board loading device and all the components necessary for the method. The surface temperature of the boards can reach above 70°C through the primer unit and its drying unit. The apparatus has a receiving device in the form of a bucket elevator, which is positioned in front of the inking unit in the process direction and has a board receiving section. In this exemplary embodiment, the receiving device is part of a temperature control device and has the possibility of cooling the boards contained therein, and can also be individually or collectively heated by an irradiator emitting near-infrared (NIR) radiation. The apparatus receives all pre-primed and / or pre-primed boards from the equipment. The surface temperature of each board is measured and monitored at nine points. These points are located at the front, middle, and rear in the longitudinal direction of the board, and at the left, middle, and right in the transverse (width) direction. In this embodiment, the average value of these measurement points is used as the key criterion. The ink temperature is 32°C (tolerance + / - 1°C), which is approximately equal to the printhead temperature. The temperature control device is controlled by an electronic controller to adjust the current workpiece temperature of the board to a predetermined workpiece temperature before it is introduced into the ink dispenser, i.e., cooling or heating the board before introduction.

[0029] A separate temperature monitoring device (also with nine measuring points) located immediately in front of at least one inking unit shows an average plate temperature of 32.8°C. The current workpiece temperature of this plate in the bucket elevator, just before it is output to the inking unit, is 30.8°C, resulting in a 2°C reduction in temperature tolerance. The temperature monitoring and regulation device of the electrical controller detects and accounts for this 2°C increase in surface temperature from the output of the receiving device to the area immediately in front of the inking unit. Due to environmental influences, the temperature differences between the ink temperature, the plate temperature in the bucket elevator, and the plate temperature immediately in front of the inking unit vary; these differences are continuously adjusted and maintained within a given tolerance. Through precise monitoring and regulation of the plate surface temperature, no condensation occurs in the printing area due to strong temperature differences. Furthermore, the pre-primed plate has time to evaporate in the bucket elevator. Because the material temperature is nearly uniform within the printing area of ​​the inking unit, no malfunctions or technical downtime due to plate overheating are prevented. Attached Figure Description

[0030] Embodiments of the invention are further illustrated with the aid of the accompanying drawings. The drawings show: Figure 1 A schematic flowchart illustrating a method according to an embodiment of the present invention is shown. Detailed Implementation

[0031] Figure 1 The diagram schematically illustrates a possible flow of a method for printing plate-shaped workpieces. First, the workpieces 2, provided or supplied in the form of stacks 4, are separated and guided to the apparatus for performing the method. The computer 6, schematically shown, is displayed at almost all the stations traversed during the method, and it illustrates an electrical controller applicable to the control of each station, preferably also applicable to the control of the entire apparatus, and is used therein.

[0032] After the workpiece 2 is separated, it is conveyed to a sander 8, where the surface of the workpiece 2 to be printed is sanded to achieve better printability. This step is advantageous, however, it is not necessary for this method. The separated and sanded workpiece 2 is then preheated in a heating device 10 and conveyed to a first coating unit 12, where a clear primer is applied. The applied layer is then dried in a subsequent dryer 14. The workpiece 2 is then passed through multiple coating units 16 with interconnected dryers 14, where a layer of white primer is applied in the illustrated embodiment. Depending on the need, more or fewer coating units may be operated.

[0033] After the primer is applied, the workpiece 2, in the illustrated embodiment, is conveyed to another coating apparatus 18, in which the primer is applied to the surface to be printed. A dryer 14 is also connected to this coating apparatus 18.

[0034] The workpieces prepared in this manner are conveyed to a temperature control device 20, which, in the illustrated embodiment, has a receiving device 22. This receiving device is designed as a surrounding transport loop in which the individual workpieces are transported in a circular manner. The temperature control device 20 has at least one heating element 24 and at least one cooling element 26, which in... Figure 1 The diagram is shown schematically only. Preferably, there are multiple heating elements 24 and multiple cooling elements 26, and more preferably, the multiple heating elements 24 and multiple cooling elements 26 affect the individual workpieces 2 and thus can change the temperature. The workpieces 2 can leave the temperature control device 20 in two different ways. If the workpieces 2 are not suitable to be conveyed to another process, they can be sorted by the sorting device 28. Otherwise, they are conveyed after the temperature control device 30 to at least one inking device 32, which in the illustrated embodiment is connected to the dryer 14.

[0035] Then, in the illustrated embodiment, a coating layer is applied to the printed surface in the coating apparatus 16, and the coating layer is dried by the connected dryer 14 before the workpiece 2 is cooled to its final temperature in the cooling apparatus 34 and then supplied as a stack 4.

[0036] List of reference numerals 2 workpieces 4 stacking 6 Computers 8 Grinding Machines 10 heaters 12 First Coating Device 14 Dryer 16 Coating Device 18 Coating Device 20 Temperature control device 22 receiving devices 24 heating elements 26 cooling elements 28-classification device 30 Temperature control device 32 Inking Device 34. Cooling device.

Claims

1. A method for printing a plate-shaped workpiece using a digital printing apparatus, the digital printing apparatus comprising at least one inking device for applying printing ink, wherein the printing ink has an ink temperature when applied to the workpiece, characterized in that, The workpiece reaches a predetermined workpiece temperature in a temperature control device before being conveyed to the inking device, the predetermined workpiece temperature depending on the ink temperature.

2. The method according to claim 1, characterized in that, The ink temperature and the predetermined workpiece temperature have a difference of up to 10°C, preferably up to 5°C, and particularly preferably up to 2°C.

3. The method according to claim 1 or 2, characterized in that, The ink temperature is at least 20°C, preferably at least 25°C, particularly preferably at least 30°C, and at most 40°C, preferably at most 38°C, and particularly preferably at most 35°C.

4. The method according to any one of the preceding claims, characterized in that, The digital printing equipment has multiple inking devices for applying printing inks of different colors, wherein the ink temperature difference between adjacent inking devices is at most 1.5°C, preferably at most 1.3°C, and particularly preferably at most 1.0°C.

5. The method according to claim 4, characterized in that, The predetermined workpiece temperature depends on the ink temperature of the first inking device or the average of all ink temperatures.

6. The method according to any one of the preceding claims, characterized in that, Before the workpiece is conveyed to the temperature control device, at least one primer is applied to the workpiece, wherein, preferably, the workpiece is dried after the application of the at least one primer and before the workpiece is conveyed to the temperature control device.

7. The method according to any one of the preceding claims, characterized in that, The workpiece is cooled to the predetermined workpiece temperature in the temperature control device.

8. The method according to any one of the preceding claims, characterized in that, If at least one parameter measured by at least one sensor is not within a predetermined value range, the workpiece is classified before it is conveyed to the at least one inking device, wherein the workpiece is then preferably post-processed to affect the at least one parameter so that the workpiece is within the predetermined value range after the post-processing.

9. An apparatus for carrying out the method according to any one of the preceding claims, characterized in that, The apparatus includes a digital printing device, a temperature control device, and an electrical controller. The digital printing device has at least one inking device and is configured to implement the method according to any one of the preceding claims.

10. The apparatus according to claim 9, characterized in that, The device has at least one applicator for primer and / or the digital printing equipment has multiple inking devices.

11. The apparatus according to claim 9 or 10, characterized in that, The device has at least one dryer arranged along the process direction behind at least one inking device of the digital printing equipment.

12. The apparatus according to claim 11, characterized in that, The apparatus is equipped with a dryer for each inking device and / or each coating device for primer, wherein each dryer is arranged in the process direction after the corresponding coating device.

13. The apparatus according to any one of claims 9 to 12, characterized in that, The temperature control device has at least one temperature sensor for determining the current workpiece temperature, the temperature sensor being configured to transmit sensor data to the electrical controller.

14. The apparatus according to any one of claims 9 to 13, characterized in that, The temperature control device has at least one temperature control element, preferably at least one heating element and / or at least one cooling element, which can affect the current workpiece temperature of the workpiece.

15. The apparatus according to claim 14, characterized in that, At least one temperature control element is configured to affect the current workpiece temperature in a position-resolved manner.

16. The apparatus according to any one of claims 9 to 15, characterized in that, The predetermined workpiece temperature is present inside the temperature control device, and the electrical controller is configured to guide the workpiece from the temperature control device to the at least one inking device only when the difference between the current workpiece temperature and the predetermined workpiece temperature is less than a predetermined limit value.

17. The apparatus according to any one of claims 9 to 16, characterized in that, The apparatus has a sorting device for sorting the workpiece before it is conveyed to the at least one inking device.

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