Method and device for printing a plate-shaped workpiece

Abstract

The invention relates to a method for printing a plate-shaped workpiece using a digital printing system, which has at least one ink application unit for applying printing ink, wherein the printing ink has an ink temperature when it is applied to the workpiece, wherein the workpiece is brought to a predetermined workpiece temperature in a temperature control device before it is fed to the ink application unit, wherein the predetermined workpiece temperature depends on the ink temperature.

Description

[0001] The invention relates to a method for printing a plate-shaped workpiece using a digital printing system that has at least one inking unit for applying printing ink, wherein the printing ink has an ink temperature when applied to the workpiece. The invention also relates to a device for carrying out such a method.

[0002] Printing on a panel-shaped workpiece is now state of the art for many applications and is used, for example, in the manufacture of floor panels, furniture components, or worktops. Wood, stone, or fantasy decors are printed onto panel-shaped cores, which may contain a wood-based panel. The digital printing systems used for this process often include primer components to apply a white primer to the panel cores to be printed, possibly in several steps. Each individual primer layer is dried in-line, without stopping the panels on their conveyor belts. This requires a lot of heat energy, which is introduced into the process and thus into the digital printing system. The cores to be printed reach temperatures of over 70°C and are often fed to the ink application unit at a temperature of, for example, 55°C. This has several disadvantages.The warm plates, which are often coated with multiple layers of primer, introduce a large amount of liquid and moisture into the digital printing system. Furthermore, it is known from EP 2 927 003 A1, for example, that the temperature of the workpiece to be printed has a significant influence on the print result. It affects the viscosity of the applied ink droplets and thus the visual impression of the resulting decoration, for example, the color saturation and sharpness of the resulting image.

[0003] In order to at least ensure the reproducibility of the print result, EP 2 927 003 A1 proposes heating or cooling the surface to be printed to a temperature within a predetermined value range. Further difficulties arise when printing on paper, as the moisture introduced by the warm workpiece influences the surface quality, for example the waviness, of printing paper to a much greater extent than would be the case with a solid workpiece, such as a wood-based panel. Therefore, EP 2 206 605 B2 discloses creating different climate zones with different room humidity levels in which different parts of the actual printing process take place. This results in a high level of equipment complexity and high energy consumption, making the process complicated and cost-intensive.

[0004] The invention is therefore based on the object of proposing a method and a device with which the disadvantages of the prior art are at least mitigated and, at best, overcome.

[0005] The invention solves the stated problem by a method according to the preamble of claim 1, which is characterized in that the workpiece is brought to a predetermined workpiece temperature in a temperature control device before being fed to the ink application unit, wherein the predetermined workpiece temperature depends on the ink temperature. The invention is based on the finding that there is no absolute workpiece temperature that always delivers the optimal printing result, regardless of other parameters. Rather, the predetermined workpiece temperature depends on the temperature of the ink used. In general, it has been shown that a higher ink temperature also results in a higher predetermined workpiece temperature.

[0006] The digital printing system used preferably has an electrical control system, for example, an electronic data processing device configured to process sensor data, for example from temperature sensors. Such a temperature sensor, which can also be part of the digital printing system, is used, for example, to determine the temperature of the printing ink used. The measured values ​​recorded by the sensor are transferred to the electrical control system, which calculates a predetermined workpiece temperature from them. At least one further temperature sensor, which can also be part of the digital printing system, is used to determine the current workpiece temperature, i.e., the actual value of the workpiece temperature.This measured value is also transferred to the electrical control system, which compares the current workpiece temperature thus determined with the predetermined workpiece temperature and is set up to control the tempering device in such a way that the current workpiece temperature is adjusted to 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 reduce the current workpiece temperature. The electrical control system is configured to control these elements depending on the measured values ​​of the sensors.

[0008] Preferably, the ink temperature and the predetermined workpiece temperature have a difference of at most 10°C, preferably at most 5°C, particularly preferably at most 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, particularly preferably at most 35°C.

[0009] Due to the small temperature difference between the ink temperature and the predetermined workpiece temperature, only minimal condensation occurs in the area of ​​the print heads of the at least one inking unit when the printing ink is applied to the plate-shaped workpiece within the at least one inking unit. This is advantageous because condensation on print heads can, on the one hand, affect the sensitive electronics and, on the other hand, negatively impact the process of ink droplet formation on the print head.

[0010] Preferably, the digital printing system comprises a plurality of inking units for applying printing ink of different colors, wherein the ink temperatures of adjacent inking units differ by a maximum of 1.5 °C, preferably a maximum of 1.3 °C, particularly preferably a maximum of 1.0 °C. Preferably, the predetermined workpiece temperature depends on the ink temperature of the first inking unit or on the average of all ink temperatures.

[0011] Advantageously, at least one primer is applied to the workpiece before the workpiece is fed to the tempering device. Preferably, the workpiece is dried after the application of the at least one primer before it is fed to the tempering device. If the primer is applied in several individual layers, it is advantageous to dry the workpiece after each individual layer of primer has been applied. Regardless of how many individual layers the primer is applied in, drying the workpiece before feeding it to the tempering device reduces the amount of moisture introduced into the tempering device.

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

[0013] Preferably, the predetermined workpiece temperature is a temperature range within which the current workpiece temperature must lie so that the workpiece can be fed to the inking unit. The temperature range of the predetermined workpiece temperature extends, for example, over a maximum of 5 °C, preferably over a maximum of 3 °C, particularly preferably over a maximum of 1 °C. Alternatively, rather than understanding the predetermined workpiece temperature as a temperature range, the predetermined workpiece temperature can also be an exact value, in which case the workpiece is fed to the inking unit when 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, particularly preferably 0.5 °C.In this case, the current workpiece temperature is considered to be "brought to the predetermined workpiece temperature" if the difference between the current workpiece temperature and the predetermined workpiece temperature is less than the predetermined limit.

[0014] Preferably, the workpiece is sorted out before it is fed to the at least one inking unit if at least one parameter measured by at least one sensor does not lie within a predetermined value range, wherein the workpiece is then preferably subjected to post-treatment in order to influence the at least one parameter such that it lies within the predetermined value range after post-treatment. If the parameter is the moisture content of the workpiece to be printed, the workpiece is sorted out, for example, if the moisture content is too high to achieve a sufficiently good print image. The post-treatment 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 out, for example, if the waviness is too high. The post-treatment then includes smoothing the surface.

[0015] The invention also achieves the stated object by a device for carrying out a method of the type described here, wherein the device comprises a digital printing system with at least one ink application unit, a temperature control device and an electrical control system which is configured to carry out a method described here.

[0016] Preferably, the device has at least one application unit for a primer and / or the digital printing system has several ink application units.

[0017] In a preferred embodiment, the device has at least one dryer, which is arranged downstream of at least one inking unit of the digital printing system in the process calculation. Particularly preferably, the device has a dryer for each inking unit and / or for each primer application unit, with each dryer being arranged downstream of the respective application unit in the process direction. This means that the workpiece to be printed is first exposed to primer or ink in the application unit and then dried by the dryer.

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

[0019] The tempering device preferably has at least one tempering element, preferably at least one heating element and / or at least one cooling element, by means of which the current workpiece temperature of the workpiece can be influenced.

[0020] The electrical control system is configured to monitor the current workpiece temperature of the surface of the workpiece to be printed. This means, in particular, that the current workpiece temperature is transmitted from the at least one temperature sensor to the electrical control system at regular intervals, which compares this current workpiece temperature with the predetermined workpiece temperature.

[0021] Preferably, the at least one temperature sensor is configured to determine the current workpiece temperature with spatial resolution. Preferably, several temperature sensors are provided for this purpose, which are arranged, for example, across the width of the workpiece and measure the current workpiece temperature with spatial resolution and transmit the measurement data to the electrical control system.

[0022] Preferably, the temperature control element is configured to influence the current workpiece temperature in a spatially resolved manner. For this purpose, it is advantageous if the temperature control element comprises several heating elements and / or several cooling elements, each of which is configured to influence the current workpiece temperature only in one area of ​​the surface.

[0023] Preferably, the predetermined workpiece temperature prevails within the tempering device, wherein the electrical control is configured to guide the workpiece from the tempering device to the at least one paint application unit only when a difference between the current workpiece temperature and the predetermined workpiece temperature is smaller than the predetermined limit value or the current workpiece temperature is within the range defined by the predetermined workpiece temperature.

[0024] Preferably, the device has a sorting device to sort out a workpiece before it is fed to the at least one paint application unit.

[0025] Preferably, this sorting device is designed to then feed the workpiece to a further work station where further treatment can take place.

[0026] Particularly preferably, the device, for example as part of the temperature control device, has a collecting device that enables the device to allow individual workpieces to remain in the temperature control device until the predetermined workpiece temperature is reached. This can be achieved, for example, by a transport loop, for example in the form of a paternoster. The workpieces contained move in a circular manner in this transport loop on a closed transport path. During each circulation, it is possible to guide the respective workpiece out of the temperature control device or the collecting device or to remove it and feed it to the at least one paint application unit. The workpiece is fed to the paint application unit when the current workpiece temperature corresponds to the predetermined workpiece temperature.Otherwise, the workpiece remains in the transport loop until the current workpiece temperature corresponds to the predetermined workpiece temperature.

[0027] Such a receiving device or transport loop preferably also has the task of influencing the loading or workpiece insertion to the inking unit and can be used as a workpiece buffer if a device located later in the process direction, for example, the inking unit, does not achieve the expected throughput of workpieces due to a malfunction. In this case, the electrical control of the device is preferably configured to leave the workpieces in the receiving device or transport loop until the malfunction is resolved and workpieces can be fed back to the inking unit or the further device.

[0028] Preferably, the plate-shaped workpiece has or consists of a core. The core is preferably a wood-based panel, for example, MDF, HDF, OSB, chipboard, or plywood. Panels that use a mixture of wood particles and plastic are also preferred core designs. This applies, for example, to WPC (wood plastic composite) panels. The core can also consist entirely of a plastic, for example, in the form of plastic panels or SPC (solid polymer core) panels. Panels that are offered, for example, under the name "Corepel" (see www.corepel.com ) and which, for example, have an inner core made of wood fibers surrounded by resins, preferably high-performance resins, can be advantageous as a core. The core is preferably at least 3 mm thick.

[0029] An embodiment of the invention is described in more detail using a specific example. A manufacturer of laminate flooring boards uses digital printing to print wood, stone, and fantasy decors on high-density fiberboard (HDF). The boards are 2800 mm long, 2070 mm wide, and have a thickness of 6 mm to 12 mm. The entire system has a feed speed of 80 m / min with a gap between two consecutive workpieces of approximately 1500 mm. The entire device includes an automated board feeder and all elements necessary for the process. Due to the priming device and its drying units, the boards reach surface temperatures of over 70°C. The device has a collecting device in the form of a paternoster, which is arranged upstream of the paint application unit in the process direction and can accommodate 100 boards.In this embodiment, the collecting device is part of the temperature control system and has the option of cooling the boards contained therein as well as heating them individually or collectively using radiators that emit radiation in the near infrared (NIR) range. All pre-primed and / or primed boards are collected from the system. The surface temperature of each individual board is measured and monitored at nine points. These are positioned lengthwise along the board at the front, center, and back, as well as crosswise (widthwise) on the left, center, and right. The mean value of these measuring points is decisive and evaluated in this embodiment. The ink temperature, and thus approximately the temperature of the print heads, is 32°C (tolerance + / -1°C).By means of the electrical control, the tempering device is controlled in such a way that the current workpiece temperature of the plates is adjusted to this temperature, which is used as the predetermined workpiece temperature, directly before they are fed to the ink application unit by cooling or heating them before they are fed.

[0030] Further temperature monitoring (also with 9 measuring points) directly in front of at least one inking unit shows an average value of 32.8°C for one plate. The current workpiece temperature of this plate in the paternoster was 30.8°C directly before being output to the inking unit, thus resulting in a reduced temperature tolerance of 2°C. The temperature monitoring and control of the electrical control system determined that the surface temperature warmed up again by 2°C from the output from the collecting device to immediately in front of the inking unit and took this into account accordingly. The temperature differences between the ink temperature, the plate temperature in the paternoster, and the plate temperature immediately in front of the inking unit vary due to environmental influences and are preferably continuously adjusted and maintained within specified tolerances.Thanks to precise monitoring and control of the plate surface temperature, no condensation occurs in the printing area due to significant temperature differences on the materials. Furthermore, the pre-primed plates were given time to evaporate in the paternoster. The virtually identical material temperatures in the printing area of ​​the inking unit prevent malfunctions or technical failures due to overheating from hot plates.

[0031] An embodiment of the present invention is explained in more detail with the aid of the accompanying drawing. It shows Figure 1 - the schematic flow of a method according to an embodiment of the present invention.

[0032] Figure 1schematically shows a possible sequence of a method for printing a plate-shaped workpiece. First, the workpieces 2, which are delivered or provided in the form of a stack 4, are separated and fed to the device for carrying out the method. The schematically illustrated computer 6, which is shown at almost all stations passed through during the process, represents the electrical control system, which is suitable and used to control the respective station and preferably the entire device.

[0033] After the workpieces 2 have been separated, they are fed to a grinder 8, in which the surface of the workpiece 2 to be printed on later is sanded to improve printability. This step is advantageous but not necessary for the process. The separated and sanded workpieces 2 are then preheated in a heater 10 before being fed to a first application unit 12, in which they are provided with a transparent primer coat. The applied layer is dried in the subsequent dryer 14. The workpieces 2 then pass through several application units 16, each with a subsequent dryer 14, in each of which, in the embodiment shown, a layer of a white primer is applied. Depending on requirements, more or fewer application units can be passed through here.

[0034] After the primer has been applied, the workpieces 2 in the illustrated embodiment are fed to another application unit 18, where a primer is applied to the surface to be printed. This application unit 18 is also followed by a dryer 14.

[0035] The workpieces prepared in this way are fed to a tempering device 20, which in the illustrated embodiment has a collecting device 22. This is designed as a circulating transport loop in which the individual workpieces are conveyed in a circulating manner. The tempering device 20 has at least one heating element 24 and at least one cooling element 26, which are arranged in Figure 1are only shown schematically. Preferably, a plurality of heating elements 24 and a plurality of cooling elements 26 are present, which particularly preferably influence individual workpieces 2 and can thus change the temperature. The workpieces 2 can leave the temperature control device 20 in two different ways. If the workpieces 2 are not suitable for further processing, they can be sorted out via the sorting device 28. Otherwise, after a temperature check 30, they are fed to at least one inking unit 32, which in the illustrated embodiment is followed by a dryer 14.

[0036] In the embodiment shown, a cover layer is then applied to the surface thus printed in an application unit 16, which is dried with the subsequent dryer 14 before the workpiece 2 is cooled to the final temperature in the cooling unit 34 and then made available again as a stack 4. List of reference symbols

[0037] 2Workpiece 4Stack 6Computer 8Grinder 10Heater 12First coating unit 14Dryer 16Applicator 18Applicator 20Temperature control device 22Collector 24Heating element 26Cooling element 28Sorting device 30Temperature control 32Ink application unit 34Cooling

Claims

1. A method for printing a plate-shaped workpiece by means of a digital printing system having at least one inking unit for applying printing ink, wherein the printing ink has an ink temperature when it is applied to the workpiece, characterized in that the workpiece is brought to a predetermined workpiece temperature in a tempering device before it is fed to the ink application unit, wherein the predetermined workpiece temperature depends on the ink temperature.

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

3. 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, particularly preferably at most 35°C.

4. Method according to one of the preceding claims, characterized in that the digital printing system has a plurality of inking units for applying printing ink of different colors, wherein the ink temperatures of adjacent inking units differ by a maximum of 1.5 °C, preferably a maximum of 1.3 °C, particularly preferably a maximum of 1.0 °C.

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

6. Method according to one of the preceding claims, characterized in that at least one primer is applied to the workpiece before the workpiece is fed to the tempering device, wherein preferably the workpiece is dried after the application of the at least one primer before it is fed to the tempering device.

7. Method according to one of the preceding claims, characterized in thatthe workpiece is cooled in the tempering device to the predetermined workpiece temperature.

8. Method according to one of the preceding claims, characterized in that the workpiece is sorted out before it is fed to the at least one inking unit if at least one parameter measured by at least one sensor is not within a predetermined value range, wherein the workpiece is then preferably subjected to a post-treatment in order to influence the at least one parameter such that it lies within the predetermined value range after the post-treatment.

9. Apparatus for carrying out a method according to one of the preceding claims, characterized in that the device comprises a digital printing system with at least one ink application unit, a temperature control device and an electrical control system which is configured to carry out a method according to one of the preceding claims.

10. Device according to claim 9, characterized in thatthe device has at least one application unit for a primer and / or the digital printing system has several ink application units.

11. Device according to claim 9 or 10, characterized in that the device has at least one dryer which is arranged in the process direction behind at least one ink application unit of the digital printing system.

12. Device according to claim 11, characterized in that the device comprises a dryer for each paint application unit and / or for each primer application unit, wherein each dryer is arranged downstream of the respective application unit in the process direction.

13. Device according to one of claims 9 to 12, characterized in that the tempering device has at least one temperature sensor for determining a current workpiece temperature of the workpiece, which is configured to transmit sensor data to the electrical control.

14. Device according to one of claims 9 to 13, characterized in that the tempering device has at least one tempering element, preferably at least one heating element and / or at least one cooling element, by means of which the current workpiece temperature of the workpiece can be influenced.

15. Device according to claim 14, characterized in that at least one tempering element is configured to influence the current workpiece temperature of the workpiece in a spatially resolved manner.

16. Device according to one of claims 9 to 15, characterized in that the predetermined workpiece temperature prevails within the tempering device and the electrical control is configured to guide the workpiece from the tempering device to the at least one paint application unit only when a difference between the current workpiece temperature and the predetermined workpiece temperature is smaller than a predetermined limit value.

17. Device according to one of claims 9 to 16, characterized in thatthe device has a sorting device for sorting out a workpiece before it is fed to the at least one paint application unit.

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