METHOD AND DEVICE FOR PRINTING ON A PLATE-SHAPED WORKPIECE

Description

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

[0002] Such a process is known, for example, from US 2021 / 046782 A1, in which two layers of printing ink are applied and later pressed together.

[0003] JP 2008 194827 A deals with a printing ink that is particularly resistant to fading and weathering.

[0004] Printing on sheet-shaped workpieces is now standard practice for many applications and is used, for example, in the production of floor panels, furniture components, or worktops. Wood, stone, or fantasy decors are printed onto sheet-shaped cores, which typically contain a wood-based panel. The digital printing systems used often include priming components to apply a white primer to the cores to be printed, sometimes in several steps. Each individual primer layer is dried inline during the process, without stopping the panels on their conveyor belts. This requires a significant amount of heat energy, which is fed 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 inking unit at a temperature of, for example, 55 °C. This has several disadvantages.The warm printing plates, often coated with several layers of primer, introduce a large amount of liquid and moisture into the digital printing system. Furthermore, it is known, for example from EP 2 927 003 A1, that the temperature of the workpiece to be printed has a significant influence on the printing result. It affects the viscosity of the applied ink droplets and thus the optical appearance of the resulting design, such as color saturation and the sharpness of the image.

[0005] To ensure at least the reproducibility of the printing result, EP 2 927 003 A1 proposes heating or cooling the surface to be printed to a temperature within a predetermined range. Printing on paper presents further difficulties, as the moisture introduced by the warm workpiece affects the surface properties, such as waviness, of printing paper much more significantly than would be the case with a solid workpiece, such as a wood-based panel. EP 2 206 605 B2 therefore discloses the possibility of creating different climate zones with varying levels of humidity, in which different parts of the actual printing process take place. This results in significant equipment costs and high energy consumption, making the process complicated and expensive.

[0006] The invention is therefore based on the objective of proposing a method and a device with which the disadvantages of the prior art are at least mitigated, or at best overcome.

[0007] 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 unit before being fed to the ink application unit, wherein the predetermined workpiece temperature depends on the ink temperature. The invention is based on the understanding that there is no absolute workpiece temperature that always delivers the optimal printing result, independent of other parameters. Rather, the predetermined workpiece temperature depends on the temperature of the ink used. It has generally been shown that a higher ink temperature also results in a higher predetermined workpiece temperature.

[0008] The digital printing system used preferably has an electrical control unit, for example, an electronic data processing unit, which is configured to process sensor data, such as 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. The measured value acquired by the sensor is transmitted to the electrical control unit, which calculates a predetermined workpiece temperature. 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 passed 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 temperature control device so that the current workpiece temperature is adjusted to the predetermined workpiece temperature.

[0009] 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 based on the sensor readings.

[0010] Preferably, the ink temperature and the predetermined workpiece temperature differ by no more than 10 °C, preferably by no more than 5 °C, and particularly preferably by no more than 2 °C. Preferably, the ink temperature is at least 20 °C, preferably at least 25 °C, particularly preferably at least 30 °C and no more than 40 °C, preferably no more than 38 °C, and particularly preferably no more than 35 °C.

[0011] Due to the small temperature difference between the ink temperature and the predetermined workpiece temperature, only minimal condensation forms in the area of ​​the printheads of the at least one ink application unit when the printing ink is applied to the flat workpiece. This is advantageous because condensation on printheads can affect the sensitive electronics and also negatively impacts the ink droplet formation process on the printhead.

[0012] Preferably, the digital printing system has several ink application units for applying printing ink of different colors, wherein the ink temperatures of adjacent ink application units differ by 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 ink application unit or on the average of all ink temperatures.

[0013] Advantageously, at least one primer coat is applied to the workpiece before it is fed into the temperature control unit. Preferably, the workpiece is dried after the application of the primer coat before being fed into the temperature control unit. If the primer is applied in several individual layers, it is advantageous to dry the workpiece after the application of each individual layer of primer. Regardless of the number of individual layers of primer applied, drying the workpiece before feeding it into the temperature control unit reduces the amount of moisture introduced into the unit.

[0014] Preferably, the workpiece is cooled to the predetermined workpiece temperature in the temperature control device.

[0015] Preferably, the predetermined workpiece temperature is a temperature range within which the current workpiece temperature must lie in order for the workpiece to be fed to the inking unit. The temperature range of the predetermined workpiece temperature extends, for example, to a maximum of 5 °C, preferably to a maximum of 3 °C, and particularly preferably to a maximum of 1 °C. Alternatively, instead of understanding the predetermined workpiece temperature as a temperature range, the predetermined workpiece temperature can also be a precise 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, and 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 value.

[0016] Preferably, the workpiece is rejected before being fed to the at least one inking unit if at least one parameter measured by at least one sensor is not within a predetermined range. The workpiece is then preferably subjected to post-treatment to adjust the parameter so that it falls within the predetermined range after the post-treatment. For example, if the parameter is the moisture content of the workpiece to be printed, the workpiece is rejected if the moisture content is too high to achieve a sufficiently good print image. The post-treatment then includes drying the workpiece. If the parameter is the waviness of the workpiece's surface to be printed, the workpiece is rejected if the waviness is too high. The post-treatment then includes smoothing the surface.

[0017] The invention also solves the stated problem by means of a device for carrying out a method of the type described herein, 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 herein.

[0018] Preferably the device has at least one application unit for a primer and / or the digital printing system has several color application units.

[0019] In a preferred embodiment, the device has at least one dryer arranged downstream of at least one ink application unit of the digital printing system. Particularly preferably, the device has one dryer for each ink application unit and / or for each primer application unit, with each dryer arranged downstream of the respective application unit in the process direction. This means that the workpiece to be printed is first coated with primer or ink in the application unit and then dried by the dryer.

[0020] 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 the electrical control system.

[0021] 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, by which the current workpiece temperature can be influenced.

[0022] The electrical control system is designed to monitor the current temperature of the workpiece surface to be printed. Specifically, this means that at regular intervals, the current workpiece temperature is transmitted from at least one temperature sensor to the electrical control system, which then compares this current workpiece temperature with the predetermined workpiece temperature.

[0023] Preferably, 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.

[0024] Preferably, the temperature control element is configured to influence the current workpiece temperature with spatial resolution. For this purpose, it is advantageous if the temperature control element has several heating elements and / or several cooling elements, each configured to influence the current workpiece temperature only in a specific area of ​​the surface.

[0025] Preferably, the predetermined workpiece temperature prevails within the temperature control device, wherein the electrical control is configured to direct the workpiece from the temperature control device to the at least one paint application unit only when a difference between the current workpiece temperature and the predetermined workpiece temperature is less than the predetermined limit value or the current workpiece temperature is within the range defined by the predetermined workpiece temperature.

[0026] Preferably, the device has a sorting unit to remove workpieces before they are fed to the at least one paint application unit. Preferably, this sorting unit is configured to then feed the workpiece to a further workstation where post-processing can take place.

[0027] Preferably, the device, for example as part of the temperature control unit, includes a holding device that allows individual workpieces to remain in the temperature control unit until the predetermined workpiece temperature is reached. This can be achieved, for example, by a transport loop, such as a paternoster lift. The workpieces move cyclically within this transport loop on a closed conveyor track. On each revolution, the respective workpiece can be removed from the temperature control unit or the holding device and fed to the at least one inking unit. The workpiece is fed to the inking unit when its current temperature matches the predetermined workpiece temperature.Otherwise, the workpiece remains in the transport loop until the current workpiece temperature matches the predetermined workpiece temperature.

[0028] Such a collection device or transport loop preferably also serves to influence the feeding or workpiece insertion to the inking unit and can be used as a workpiece buffer if a downstream unit, for example the inking unit, does not have the expected throughput of workpieces due to a malfunction. In this case, the electrical control of the device is preferably configured to keep the workpieces in the collection device or transport loop until the malfunction is resolved and workpieces can again be fed to the inking unit or the downstream unit.

[0029] Preferably, the plate-shaped workpiece has or consists of a core. The core is preferably a wood-based panel, for example, MDF, HDF, OSB, particleboard, or plywood. Panels using a mixture of wood particles and plastic are also preferred core configurations. This applies, for example, to WPC (wood-plastic composite) panels. The core can also consist entirely of plastic, for example, in the form of plastic sheets or SPC (solid polymer core) panels. Panels offered under the name "Corepel," for example, are also suitable (see [reference]). 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.

[0030] An embodiment of the invention is described in more detail using a specific example. At a manufacturer of laminate flooring planks, wood, stone, and fantasy decors are digitally printed onto high-density fiberboard (HDF). The planks are 2800 mm long, 2070 mm wide, and 6 mm to 12 mm thick. The entire system 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 plank feeder and all elements necessary for the process. The priming unit and its drying units allow the planks to reach surface temperatures of over 70°C. The apparatus features a paternoster collection system, arranged in the process direction upstream of the coating unit, which can hold 100 planks.In this embodiment, the collection device is part of the temperature control unit and can cool the plates it contains, as well as heat them individually or together using near-infrared (NIR) emitters. All pre-primed and / or primed plates are removed from the system. The surface temperature of each individual plate is measured and monitored at nine points. These points are positioned longitudinally along the plate at the front, middle, and rear, and transversely (across the width) at the left, middle, and right. The average temperature of these measurement points is decisive and evaluated in this embodiment. The ink temperature, and thus approximately the temperature of the printheads, is 32°C (tolerance + / -1°C).The electrical control system controls the temperature control unit 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 being fed to the ink application unit, by cooling or heating them before feeding them into the system.

[0031] A further temperature monitoring system (also with 9 measuring points) directly in front of at least one inking unit shows an average temperature of 32.8°C per plate. The actual workpiece temperature of this plate in the paternoster was 30.8°C immediately before being discharged to the inking unit, thus representing a reduced temperature tolerance of 2°C. The temperature monitoring and control system of the electrical control unit detected that the surface temperature rises again by 2°C from the point of discharge from the collection device to immediately before the inking unit and has taken this into account accordingly. The temperature differences between the ink temperature, the plate temperature in the paternoster, and the plate temperature immediately before the inking unit vary due to environmental influences and are preferably continuously adjusted and maintained within predefined tolerances.Precise monitoring and control of plate surface temperature prevents condensation from occurring in the printing area due to significant temperature differences between materials. Furthermore, the pre-primed plates had time to off-gas in the paternoster lift. The nearly identical material temperatures in the printing area of ​​the inking unit prevent malfunctions or technical failures caused by overheating from hot plates.

[0032] An embodiment of the present invention is explained in more detail with reference to the accompanying drawing. It shows Figure 1 - the schematic sequence of a method according to an embodiment of the present invention.

[0033] Figure 1Figure 1 schematically illustrates a possible process for printing on 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 process. The schematically depicted computer 6, which is shown at almost all stations along the process, represents the electrical control system suitable for and used to control the respective station and preferably the entire device.

[0034] After the workpieces 2 are separated, they are fed to a grinder 8, where the surface of the workpiece 2 to be printed is roughened to improve printability. This step is advantageous but not essential for the process. The separated and roughened workpieces 2 are then preheated in a heater 10 before being fed to a first coating unit 12, where they are coated with a transparent primer. The applied layer is then dried in the subsequent dryer 14. The workpieces 2 then pass through several coating units 16, each with a subsequent dryer 14, where, in the illustrated embodiment, a layer of white primer is applied in each unit. Depending on the requirements, more or fewer coating units can be used.

[0035] After the primer has been applied, the workpieces 2 in the illustrated embodiment are fed to a further coating unit 18, in which a primer is applied to the surface to be printed. A dryer 14 also follows this coating unit 18.

[0036] The prepared workpieces are fed to a temperature control unit 20, which in the illustrated embodiment includes a collection device 22. This is designed as a circulating transport loop in which the individual workpieces are conveyed continuously. The temperature control unit 20 has at least one heating element 24 and at least one cooling element 26, which are arranged in Figure 1The diagram is only schematically represented. Preferably, several heating elements 24 and several cooling elements 26 are provided, which particularly preferably influence individual workpieces 2 and thus change the temperature. The workpieces 2 can leave the temperature control unit 20 in two different ways. If the workpieces 2 are not suitable for further processing, they can be sorted out by the sorting unit 28. Otherwise, after a temperature check 30, they are fed to the at least one paint application unit 32, which, in the illustrated embodiment, is followed by a dryer 14.

[0037] In the illustrated embodiment, a top layer is then applied to the printed surface 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. Reference symbol list

[0038] 2 Workpiece 4 Stack 6 Computer 8 Grinder 10 Heater 12 First application unit 14 Dryer 16 Application unit 18 Application unit 20 Temperature control unit 22 Collection unit 24 Heating element 26 Cooling element 28 Sorting unit 30 Temperature control 32 Paint application unit 34 Cooling

Claims

1. A method for printing onto a planar workpiece (2) by means of a digital printing facility comprising at least one ink application unit (32) for applying printing ink, wherein the printing ink has an ink temperature when applied to the workpiece (2), characterised in that the workpiece (2) is brought to a predetermined workpiece temperature in a temperature regulation device (20) before being fed to the ink application unit (32), the predetermined workpiece temperature depending on the ink temperature.

2. The method according to claim 1, characterised in that 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.

3. The method according to claim 1 or 2, characterised 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. The method according to one of the preceding claims, characterised in that the digital printing facility comprises multiple ink application units (32) for applying printing ink in different colours, wherein the ink temperatures of adjacent ink application units (32) differ by at most 1.5°C, preferably at most 1.3°C, particularly preferably at most 1.0°C.

5. The method according to claim 4, characterised in that the predetermined workpiece temperature depends on the ink temperature of the first ink application unit (12) or on the average of all ink temperatures.

6. The method according to one of the preceding claims, characterised in that at least one primer coating is applied to the workpiece (2) before the workpiece (2) is fed to the temperature regulation device (20), wherein the workpiece (2) is preferably dried following application of the at least one primer coating before it is fed to the temperature regulation device (20).

7. The method according to one of the preceding claims, characterised in that the workpiece (2) is cooled in the temperature regulation device (20) to the predetermined workpiece temperature.

8. The method according to one of the preceding claims, characterised in that the workpiece (2) is screened out before being fed to the at least one ink application unit (32) if at least one parameter measured by at least one sensor does not lie within a predetermined range of values, wherein the workpiece (2) is then preferably subjected to a post-treatment in order to influence the at least one parameter in such a way that it lies within the predetermined range of values following post-treatment.

9. A device for carrying out a method according to one of the preceding claims, characterised in that the device comprises a digital printing facility with at least one ink application unit (32), a temperature regulation device (20) and an electric control unit, which is configured to carry out a method according to one of the preceding claims.

10. The device according to claim 9, characterised in that the device comprises at least one application unit (16, 18) for a primer coating and / or the digital printing device comprises multiple ink application units.

11. The device according to claim 9 or 10, characterised in that the device comprises at least one dryer (14), which is arranged downstream of at least one ink application unit (32) of the digital printing facility in the process direction.

12. The device according to claim 11, characterised in that the device comprises a dryer (14) for each ink application unit (32) and / or for each application unit (16, 18) for a primer coating, wherein each dryer (14) is arranged downstream of the respective application unit (16, 18) in the process direction.

13. The device according to one of the claims 9 to 12, characterised in that the temperature regulation device (20) comprises at least one temperature sensor for determining a current workpiece temperature of the workpiece (2), which is configured to transmit sensor data to the electric control unit.

14. The device according to one of the claims 9 to 13, characterised in that the temperature regulation device (20) comprises at least one temperature regulation element, preferably at least one heating element (24) and / or at least one cooling element (26), by means of which the current workpiece temperature of the workpiece (2) can be influenced.

15. The device according to claim 14, characterised in that at least one temperature regulation element is configured to influence the current workpiece temperature of the workpiece (2) in a spatially resolved manner.

16. The device according to one of the claims 9 to 15, characterised in that, within the temperature regulation device (20), the predetermined workpiece temperature prevails and the electric control unit is configured to only guide the workpiece (2) from the temperature regulation device (20) to the at least one ink application unit (32) when a difference between the current workpiece temperature and the predetermined workpiece temperature is smaller than a predetermined limit value.

17. The device according to one of the claims 9 to 16, characterised in that the device comprises a sorting device (28) for screening out a workpiece (2) before it is fed to the at least one ink application unit (32).