Inkjet printer for monitoring a chamber for airtightness according to the preamble the content of which is set out in US 2012 / 0285708 A1

Abstract

Inkjet printing device with a printer housing (11) in which a transport unit for transporting a web-shaped recording medium (10), a printing unit (14) with at least one printhead for printing on the recording medium (10), and a drying device (15) for drying and fixing the ink applied to the recording medium (10) are arranged, - wherein the drying facility (15) -- a dryer housing (23) with openings (24, 25) through which the recording medium (10) is transported through the drying device (15), at least one heating / cooling module (30, 31) for temperature control of the drying device (15), -- at least one air supply module (27) for supplying tempered air into the dryer housing (23), and -- at least one exhaust air module (28) for removing operating exhaust air from the dryer housing (23), - wherein a first temperature sensor (36, 36', 36'') is arranged outside the dryer housing (23) near an opening (24, 25) of the dryer housing (23), which measures the temperature at this point, and - wherein a device control unit (40) is connected to the first temperature sensor (36, 36', 36") for evaluating measured temperature values, wherein a measured temperature value is compared with at least one reference value and an error signal is generated if a threshold value is exceeded, characterized in that - the opening (24, 25) has a tunnel-shaped collar (41) in the area of ​​which two temperature sensors (36', 36'') are arranged one behind the other in the direction of flow at a predetermined distance from each other.

Description

[0001] The invention relates to an inkjet printing device for monitoring a chamber for airtightness, wherein the chamber is connected to another chamber via at least one opening and wherein both chambers have different temperature values ​​during operation.

[0002] Inkjet printers can be used for printing in one or more colors onto a web-like recording medium. The design of such inkjet printers is well known. In particular, high-speed digital inkjet printers are known in which ink is ejected onto the moving recording medium by nozzles of a printhead. The applied ink is dried in a drying or fixing unit, so that the ink is firmly bonded to the recording medium, thus printing the finished image onto the recording medium.

[0003] Inkjet printing typically uses liquid ink, which usually consists of several components. The main component is often water, to which various binders and solvents, such as alcoholic or aromatic compounds like glycol, are added for functional purposes, along with dyes. These binders and solvents often have high boiling points in the range of 200 °C to 300 °C. After the liquid ink is applied to the substrate, it is typically dried by applying heat to fix the printed image onto the substrate.

[0004] During the drying process, a large portion of the water, binders, and solvents evaporates, releasing undesirable substances such as volatile organic compounds (VOCs and VVOCs). Only a small amount of particles is released during drying. However, the emissions of these substances are far below legal limits. Nevertheless, care must be taken when operating high-performance printers to ensure that excessive amounts of pollutants or hazardous substances are not released into the environment.

[0005] Current pressure systems do not explicitly monitor whether emissions from the pressure vessel are escaping directly into the surrounding environment. After the initial installation of a pressure vessel, a setup procedure is performed, during which any escaping mists or vapors are to be detected visually during operation. However, current systems cannot detect drift in the functional parameters relevant to leak tightness (e.g., contamination / obstruction of flow cross-sections) and in the physical properties of gases (density changes due to varying temperatures and different installation altitudes).

[0006] The settings are always set "well" within the safe range to generally "safety margin" for as many malfunctions, changes, and tolerances as possible. As the machine parameters are pushed to their limits, the usable safety margins decrease. For example, the drying unit is heated more as a precaution, which negatively impacts the energy efficiency and thermal performance of the printing system. Conventionally, it is also ensured that more air is extracted from the drying unit than is added. This creates a negative pressure within the drying unit, so that the air is essentially only removed via an exhaust module, possibly with the addition of a purified exhaust air. The aim is to minimize the amount of exhaust air entering the printer housing or its immediate surroundings, thus preventing people from being endangered by harmful emissions.

[0007] Similarly, existing sensors can perform extensive tests and calculations to potentially draw conclusions about existing leaks. However, this method cannot reliably detect actual leaks.

[0008] As an alternative, supplementary sensor technology, "turbidity sensors" similar to the operating principle of a smoke detector are also possible. However, when used in a printing system, these are subject to significant contamination from paper dust and potentially operating fluids such as ink mist. This sensor technology would therefore require constant cleaning, for example in high-performance printers, which would entail technical effort or lead to a significantly reduced productivity.

[0009] German patent application DE 10 2018 117 699 A1 discloses an inkjet printer with air-suspended drying, in which an ink-printed recording medium is dried by hot air. In the dryer housing, the supply and exhaust air are controlled in such a way as to create a slight negative pressure within the dryer housing, which is intended to prevent unwanted leakage of exhaust air.

[0010] Document US 2019 / 0 283 463 A1 discloses a drying unit for an inkjet printer.

[0011] The publication EP 0 373 932 A2 discloses a thermal fixing unit with gas purification agents for use in an image recording medium. The gas purification device captures gaseous components that are released from a developer sheet when it is heated.

[0012] Document US 2020 / 0 019 102 A1 discloses an image-generating device comprising an image-generating unit, an image-heating device, a filter, a display unit and a control unit.

[0013] Document US 2018 / 0 266 763 A1 discloses a heating device comprising several heating devices for heating a heating object, several air nozzles for blowing air onto the heating object, and a blower for supplying air to the several air nozzles.

[0014] The invention is based on the problem of creating an inkjet printer for monitoring a chamber for airtightness, which ensures that no operating exhaust air enters the adjacent chamber during operation through functionally relevant openings of a chamber whose internal temperature has a significant temperature difference to an adjacent chamber.

[0015] This problem is solved by an inkjet printer having the features of claim 1.

[0016] An inkjet printing device according to the invention comprises a first chamber (dryer housing) whose airtightness is to be measured. An adjacent chamber (printer housing) comprises a transport unit for transporting a web-shaped recording medium, a printing unit with at least one printhead for printing on the recording medium, and the dryer housing in which the ink applied to the recording medium is dried and fixed. The two chambers are connected by at least one functionally relevant opening for airflow. The recording medium is transported through the drying unit via these openings.

[0017] Inside the dryer housing, at least one heating module for temperature control of the drying module, at least one air intake module for supplying tempered air into the dryer housing, and at least one exhaust air module for removing air from the dryer housing are arranged. A first temperature sensor is located outside the dryer housing near an opening in the dryer housing. The temperature sensor measures the (air) temperature at this location. The measured temperature value is sent to a device control unit connected to the temperature sensor, where it is evaluated. The measured temperature value is compared with at least one stored reference value, and an error signal is generated if a difference threshold is exceeded. Furthermore, the opening includes a tunnel-shaped collar in which two temperature sensors are arranged one behind the other in the direction of airflow, at a predetermined distance from each other.

[0018] This allows the system to detect whether air is entering the cooler interior of the printer housing through the functionally relevant openings of the dryer housing, as the temperature in the area of ​​the opening has changed. This situation must be avoided and, if necessary, remedied by countermeasures, since it can be assumed that potentially hazardous substances have also entered the printer housing.

[0019] This solution has the advantage of being simple in design and significantly reducing manufacturing costs due to the use of only a few components, such as temperature sensors. It also saves considerable installation space by eliminating the need for otherwise complex components. Temperature measurement allows for the rapid detection of whether typically hot solvents are diffusing into areas where they should not be.

[0020] A method for monitoring the airtightness of a chamber between two chambers located at different air temperatures and connected by openings comprises the following steps. First, a temperature value is measured in the chamber being monitored near an opening to the second chamber. The measured temperature value is compared to a reference value. An error signal is generated if the difference exceeds a threshold indicating that air has unintentionally entered the first chamber from the second chamber through openings in the system. This error signal also allows for immediate countermeasures to be taken to restore the airtightness of the chamber.

[0021] It is therefore advantageous if the first temperature sensor is a thermocouple positioned in the printer housing near the opening to the dryer housing, so that it measures the temperature of the leaking air flowing out of the dryer housing through this opening. This has the advantage of quickly detecting any unwanted airflow, since during operation the two interior spaces of the printer housing and the dryer housing are at significantly different temperatures, as the dryer housing is considerably warmer due to the hot air drying process.

[0022] Additionally, a second temperature sensor can be located inside the dryer housing and a third temperature sensor inside the printer housing. These temperature sensors measure temperature values ​​that can be used as reference values. This allows for easy verification of the temperature difference between the two interiors and their respective temperature levels. Based on the measured temperature, the source of the leakage air can be identified, and the temperature difference compared to a reference value indicates the strength of the leakage airflow. Therefore, the temperature difference or gradient of the leakage air flow can quickly provide information about the flow of exhaust air from the dryer housing.

[0023] If the opening has a tunnel-shaped collar, two temperature sensors can be positioned in the collar area, approximately one behind the other in the direction of airflow, at a predetermined distance from each other, thus within the path of leaking air. This allows the direction and intensity of the leaking airflow to be detected.

[0024] It is advantageous to position only one temperature sensor near one of several openings in the dryer housing to detect the temperature change of the air flowing between the interiors of the dryer housing and the printer housing. Advantageously, the opening through which the most leakage air can flow and which is most likely to flow in an undesired direction is selected. This way, only a single temperature sensor is required if its readings also provide the reference value (previously measured temperature values).

[0025] As a precaution, a temperature sensor can also be placed near each opening of the dryer housing to detect temperature changes in the air flowing between the interiors of the dryer housing and the printer housing. This ensures that all unwanted air currents through all openings are detected and can be analyzed accordingly.

[0026] Exemplary embodiments of the invention are explained in more detail below with reference to schematic drawings. These show: Fig. 1. A view of an inkjet printer used for printing on a web-shaped recording medium, Fig. 2 an excerpt from a block diagram of a known inkjet printer with the control of supply and exhaust air of a dryer housing 23, Fig. 3 an excerpt from a block diagram of an inkjet printer according to the invention for monitoring a dryer housing 23 for airtightness in a sealed dryer housing 23, Fig. 4 an excerpt from a block diagram of the inkjet printer for monitoring a dryer housing 23 for airtightness in the case of a leaking dryer housing 23, Fig. 5 an excerpt from a block diagram of a further embodiment of an inkjet printer for monitoring a dryer housing 23 for airtightness in the case of a leaking dryer housing 23, and Fig. 6. A flowchart of a procedure for monitoring a chamber for airtightness.

[0027] In the following figures, functionally identical or equivalent elements are marked with the same reference symbols.

[0028] In Fig. Figure 1 shows an inkjet printer for printing on a web-shaped recording medium 10. The inkjet printer has a chamber (referred to here as the printer housing 11) through which the recording medium 10 to be printed is transported. The recording medium 10 is drawn into the inkjet printer via a feeder 12 in the transport direction (see arrows on the recording medium 10). A rotating frame 13 controls the lateral orientation of the recording medium 10 during transport through the inkjet printer. A printing unit 14 with a plurality of ink printheads prints on one side of the recording medium 10. In an adjacent drying unit 15 or fuser unit, the ink, and thus the printed image, is dried and fixed to the recording medium 10. The recording medium 10 is then ejected from the inkjet printer via a ejector 16.The recording medium 10 can then be turned over by a turning unit (not shown) so that the reverse side can be printed in another inkjet printer. Depending on requirements, the printed recording medium 10 can also be fed to a post-processing unit (not shown) where it is further processed, for example, by being cut to size.

[0029] For multicolor printing, several printing bars with at least one printhead are arranged in the printing unit 14, with each printing bar capable of printing a different color or liquid. Fig. Figure 1 shows the printing unit 14 with six printing bars (four of which are for the primary colors YMCK).

[0030] The printer housing 11 has an opening designated as a feed slot 20, through which the recording medium 10 is drawn into the inkjet printer. Furthermore, an opening designated as a release slot 21 is provided, from which the recording medium 10 is transported out of the inkjet printer.

[0031] Air nozzles (so-called air knives) or rollers can also be arranged in the area of ​​the openings to reduce leakage. This can also reduce the airflow of a laminar boundary layer into and out of the openings when a rapidly transported recording medium 10 is moved.

[0032] The drying device 15 has a dryer housing 23, which also has openings, such as an inlet slot 24 and an outlet slot 25. These are only as large as necessary to prevent damage to the recording medium 10 at the inlet slot 24 or outlet slot 25 and to prevent excessive air leakage through these openings. The dryer housing 23 can be divided into several sub-chambers 26, 26' in which the recording medium 10 is heated to dry the ink and later (in Fig. 1. The rightmost, last sub-chamber 26') can be subjected to cold for cooling the recording medium 10. Sub-chambers 26 and 26' are also provided with openings / slots for transporting the recording medium 10.

[0033] The drying unit 15 is designed here as a so-called air-suspended dryer, in which hot air is blown onto the recording medium 10 from both sides via air nozzles to dry the ink. The last sub-chamber 26' in the transport direction can, for example, have a cooling system that cools the recording medium 10 by means of air nozzles so that the printed image cannot be damaged during further transport, even upon contact with transport rollers. Thus, during operation of the inkjet printer, the temperature inside the printer housing 11 is significantly lower (e.g., 25–35 °C) than in the heated dryer housing 23 (e.g., 100–200 °C). A possible exception is sub-chamber 26' if it is designed as a cooling chamber within the dryer housing 23, in which case the temperature is typically similar to or slightly higher than in the printer housing 11.

[0034] Heated or cooled air is supplied to the air nozzles via a supply air module 27 (hereinafter referred to as supply air), and moist and warm air (hereinafter referred to as exhaust air or operating exhaust air) is discharged to the outside via an exhaust air module 28 and an exhaust air duct (not shown in detail), as shown in the following Fig. 2, Fig. 3, Fig. 4 to Fig. 5 is shown.

[0035] In Fig. Figure 2 shows an excerpt from a block diagram of a known prior art inkjet printer, in which the tightness of a dryer housing 23 is regulated via the quantities of supply air (air supply) and exhaust air (air removal) via a supply air module 27 and exhaust air module 28, respectively.

[0036] A conventional inkjet printer has a drying unit 15, the outer walls of which form a largely enclosed chamber (i.e., a dryer housing 23). A heater 30, which generates heat for drying the ink, and a cooling unit 31, which generates cold for cooling the recording medium 10, supply tempered air to the dryer housing 23.

[0037] Here, an air-suspension drying process is depicted, in which heated air is blown through air nozzles onto the recording medium 10. Such air-suspension dryers are already known, for example from DE 10 2018 117 699 A1 (= US 2020 / 023634 A1), the disclosure content of which is hereby explicitly incorporated into the present disclosure.

[0038] Such a drying unit 15 requires a constant supply of fresh air, which can be preheated by the heater 30 or cooled by the cooling unit 31 to obtain tempered supply air. Drying the ink produces a large amount of water vapor and other vapors, for example, from binders and solvents contained in the ink. These vapors must be discharged to the outside to prevent a potentially unacceptable concentration of these vapors in the operating air within the dryer housing 23 and to ensure more effective drying of the printed images. The exhaust air must be cleaned frequently before being released to the outside (the exhaust air cleaning process is not shown here).

[0039] In such a drying device 15, it is important to prevent the operating exhaust air generated during printing from the dryer housing 23 from entering the printer housing 11 and from there into the environment of the inkjet printer. Instead, it should only be released to the outside via the designated exhaust air module 28, often after further treatment by cleaning and filtering devices. Since the dryer housing 23 has at least one inlet slot 24 and one outlet slot 25 for the recording medium 10, operating exhaust air can be released through these openings as leakage air 32 (indicated by the arc-shaped arrows in the Fig. 2, Fig. 3, Fig. 4 to Fig. (5 symbolized) from the dryer housing 23 into the printer housing 11. To prevent this, conventional inkjet printers are designed so that slightly more (operating) exhaust air is always extracted from the dryer housing 23 via an extraction port 29 than enters via the supply air. This creates a slight negative pressure in the dryer housing 23, resulting in a slight airflow (leakage air 32) from the printer housing 11 into the dryer housing 23 via the inlet slot 24 and the outlet slot 25, i.e., through the openings. As long as leakage air 32 flows into the dryer housing 23, the dryer housing 23 is assumed to be sealed. However, if operating exhaust air, as leakage air 32, unintentionally enters the printer housing 11 from the dryer housing 23, the dryer housing 23 is defined as leaking.

[0040] The supply and exhaust air are therefore adjusted so that more exhaust air is always discharged to the outside than supply air is drawn in. This creates a desired negative pressure in the dryer housing 23, through which leakage air 32 from the printer housing 11 is drawn into the dryer housing 23. The difference between supply and exhaust air is estimated in advance based on empirical evaluation, ensuring that the dryer housing 23 is always "airtight" when new and that the negative pressure is set based on empirical data to compensate for machine tolerances, leaks, aging processes such as changes in the delivery rate of the supply and exhaust air fans, temperature fluctuations, and changes in air density.

[0041] Typically, the vacuum in the dryer housing 23 is measured and monitored during operation. The vacuum in the dryer housing 23 is also set on-site and checked at regular intervals, being readjusted if necessary. This serves, among other things, to adjust the height at which the unit is located.

[0042] Optionally, in such a known inkjet printer, the temperature in the dryer housing 23 can be measured by a temperature sensor (hereinafter referred to as the internal sensor 33) and by a temperature sensor in the printer housing 11 (hereinafter referred to as the external sensor 34, and the air in the printer housing 11 as the outside air). These temperature sensors are positioned away from the housing walls and far enough away from openings so that they can measure the average temperature in the respective housing reliably and without interference.

[0043] However, machine-specific changes that could lead to overpressure in the dryer housing 23 are not detected despite temperature measurements, so that the dryer housing 23 would then be “leaking”, as operating exhaust air could flow into the printer housing 11 as leakage air 32.

[0044] In Fig. Figure 3 shows a part of an inkjet printer according to the invention for monitoring a chamber (here, a dryer housing 23) for airtightness. After the inkjet printer is switched on, the internal and external temperatures are approximately the same. During the heating of the drying unit (or before the start of drying), the internal temperature in the dryer housing 23 rises to the operating temperature of approximately 100°C to 200°C. The exhaust air must now be set higher than the supply air, causing leakage air 32 to flow into the dryer housing 23 in accordance with the pressure difference. The dryer housing 23 is thus airtight, since the operating exhaust air is extracted via the exhaust air module 28.

[0045] To monitor the tightness during further operation, at least one temperature sensor (hereinafter referred to as leakage air sensor 36) is arranged in the flow area of ​​possible leakage air 32 through at least one functionally relevant opening of the dryer housing 23. Fig. 3 The leakage air sensor 36 is positioned near the inlet slot 24 to the dryer housing 23 such that it is exposed to any leakage air 32 flowing through the inlet area (regardless of the flow direction of the leakage air 32). The leakage air sensor 36 is located outside the dryer housing 23.

[0046] The leakage air sensor 36 measures the temperature of the leakage air 32 flowing past it. Normally, the leakage air 32 flows inwards due to the negative pressure, so the measured temperature corresponds approximately to that inside the printer housing 11. Due to unexpected temperature fluctuations or any malfunctions or defects, the negative pressure may disappear, causing leakage air 32 to flow outwards. This allows hot exhaust air to escape to the outside and flow past the leakage air sensor 36. The sensor now measures a higher temperature value. The measured air temperature is compared to a reference value. If the temperature difference between the two exceeds a predefined or set threshold, an error signal is generated, as it is assumed that too much exhaust air is escaping to the outside.

[0047] The temperature of the air in the dryer housing 23 is largely known during operation and is typically in the range of 100 °C to approximately 220 °C, depending on the drying process. This temperature can also be optionally measured with an internal sensor 33. The temperature of the air in the printer housing 11 is typically in the range of 20 °C to 35 °C, depending on the ambient temperature and the thermal insulation of the dryer housing 23. This temperature can optionally be measured with an external sensor 34.

[0048] The temperature of the leakage air 32 is measured by the leakage air sensor 36 in the immediate vicinity, on the outside and in the flow area of ​​the functionally relevant opening (here the inlet slot 24), so that it can be verified whether the dryer housing 23 is sealed and the leakage air 32 continues to flow from the outside to the inside.

[0049] The outside temperature measured by the outside sensor 34 can be used as a reference value. A previously measured temperature value from the leakage air sensor 36 can also be used as a reference value. Furthermore, previously measured temperature values ​​from the leakage air sensor 36 can be used as a reference value.

[0050] The temperature difference must still be within a predefined tolerance threshold, which means that it can be assumed that no or only very little leakage air 32 from the dryer housing 23 enters the printer housing 11, or conversely, that leakage air 32 flows into the dryer housing 23. In this state, the dryer housing 23 is still considered to be sealed.

[0051] After starting up in pressure mode, the temperature of the leakage air 32 is constantly monitored and compared with a reference value.

[0052] If hot exhaust air from the operation now flows to the outside as leakage air 32 (as is the case in Fig. (as shown in Figure 4), the leakage air sensor 36 measures a higher temperature than before. If the temperature difference between the measured temperature and the reference value becomes too large (threshold is exceeded), this is detected immediately. A leaking dryer housing 23 is then assumed, and a warning should be issued and countermeasures initiated.

[0053] The initial reference value is the ambient temperature inside the printer housing 11, which is either measured directly or stored as a previous measurement. During operation, the reference value can be derived from the external sensor 34 or from previously measured temperature values ​​from the leakage air sensor 36 and adjusted accordingly. Multiple consecutively recorded measurements can also be used and compared. All temperature sensors (including the leakage air sensor 36) are connected to a device control unit 40, which processes the measured values ​​and calculates and stores reference values.

[0054] If the internal and external temperatures are known, the amount of leakage air 32 can be determined from the temperature difference between the measured temperature of the leakage air 32 and the external temperature, or from the temperature increase. As soon as it is detected that the dryer housing 23 is leaking and leakage air 32 is flowing to the outside, the device control unit 40 generates an alarm or takes countermeasures to prevent any operating exhaust air, in this case leakage air 32, containing potentially hazardous substances, vapors, or gases, from entering the printer housing 11 and flowing to the outside.

[0055] A temperature gradient can also be determined, indicating how quickly the measured temperature values ​​of the leakage air 32 change. If the gradient exceeds a threshold value, it can no longer be assumed that the dryer housing 23 is sealed, as the condition is changing rapidly and countermeasures must be taken quickly.

[0056] A commercially available thermocouple, for example, can be used as a temperature sensor (leakage air sensor 36) and is positioned in the flow path of the leakage air 32. The greater the temperature difference between the internal temperature of the air in the dryer housing 23 and the external temperature of the air in the printer housing 11, the greater the effect on the air temperature of the leakage air 32 when hot exhaust air flows outwards in the leakage air 32. A decrease or increase in air temperature also occurs in the direction of flow of the leakage air 32 as soon as warmer or cooler air masses mix.

[0057] A thermocouple has the advantage of being able to detect temperature differences caused by escaping hot gases very quickly. As soon as large temperature differences are measured compared to a reference value, an error signal can be generated in response, triggering a warning or activating other appropriate safety circuits to counteract the "leak." A fast control loop can also be established, which, by appropriately controlling the supply air, exhaust air, and / or the internal temperature, ensures a constant negative pressure inside the dryer housing 23, thus preventing the escape of exhaust air into the outside of the printer housing 11. If countermeasures are no longer sufficient to seal the dryer housing 23, printing should be stopped and service called.

[0058] In Fig. Figure 4 illustrates the situation where operating exhaust air escapes undesirably through the openings (inlet and outlet slots 24, 25) from the dryer housing 23 into the printer housing 11 (i.e., to the outside) via the leakage air vent 32. In this case, the dryer housing 23 is leaking.

[0059] The internal temperature is significantly higher than the external temperature during operation. If hot exhaust air from the system now flows to the outside as leakage air 32 via the leakage air sensor 36, the measured air temperature rises. This creates a temperature difference compared to previously measured temperature values ​​or stored reference values. Depending on the rate of increase (gradient) or the magnitude of the temperature difference compared to the last measured temperature value within a predefined time period or to another reference value, the dynamics of the temperature increase can even be detected.

[0060] In Fig. Figure 5 shows a further embodiment for detecting the tightness of the dryer housing 23. Here, at least one opening (here the inlet slot 24) has a tunnel-shaped collar 41. The collar 41 extends outwards from the dryer housing 23 with a predetermined length. The recording medium 10 is transported through this tunnel-shaped opening. Along the collar 41, in the direction of transport, an outer and an inner leakage air sensor 36', 36'' are arranged at a predetermined distance from each other in the region of the opening. In the basic state without leakage air 32 or with leakage air 32 to the inside, the outer leakage air sensor 36' measures approximately the outside temperature, and the inner leakage air sensor 36'' measures a slightly higher temperature due to its proximity to the significantly warmer air inside the dryer housing 23.

[0061] If the temperature values ​​measured by the inner leakage sensor 36" and the outer leakage sensor 36' are approximately the same, it can be assumed that the leakage is directed inwards or is very slight. The dryer housing 23 is detected as sealed. No further action is required.

[0062] If, however, hot leakage air 32 flows outwards into the printer housing 11, then hot operating exhaust air first flows past the inner leakage air sensor 36" and only then past the outer leakage air sensor 36'. The air temperature therefore rises first at the inner leakage air sensor 36" and then, somewhat later, at the outer leakage air sensor 36'. As soon as the measured temperature difference increases significantly within a short time, the interior begins to leak. An error signal is generated, which then allows countermeasures to be taken, such as increasing the amount of exhaust air and / or reducing the amount of supply air and / or changing the temperature of the dryer housing 23.

[0063] These countermeasures can be continued until the interior is sealed again, when the measured temperature value returns to the reference value or the measured temperature value approaches the outside temperature again.

[0064] With two leakage air sensors 36' and 36'' arranged in series, the direction of temperature change can be detected. A gradient of temperature change can also be detected, i.e., how quickly and in which direction the temperature of the leaking air changes within a period of time.

[0065] Preferably, only a single leakage air sensor 36 is located at a single opening (preferably the inlet slot 24). The largest opening of the dryer housing 23 is specifically chosen, as this is where most of the leakage air 32 is likely to flow. A temperature change is most likely to be detected there, allowing for a rapid response to the leak with appropriate countermeasures.

[0066] The air temperature outside the dryer housing 23 is measured at a single point. It is then assumed that conditions are largely the same at all other openings. Only if a hot airflow (flowing, warmer leakage air 32 from the dryer housing 23) is present do the temperature values ​​measured by the leakage air sensors 36, 36', 36'' change.

[0067] At least one leakage air sensor 36 can also be arranged near several or each opening of the dryer housing 23 to detect the temperature change of the air flowing between the interiors of the dryer housing 23 and the printer housing 11 (as shown in Fig. 4 with the second leakage air sensor 36 near the outlet slot 25 is shown). All measured temperature values ​​are individually evaluated by the device control unit 40 for each opening and assessed accordingly to determine whether the dryer housing 23 is sealed or leaking.

[0068] As a countermeasure, after triggering an error signal, the device control unit 40 can control the exhaust air volume and / or supply air volume and / or the internal temperature in order to reduce the unwanted leakage air 32 to the outside and reverse the flow so that it flows back into the dryer housing 23. This sealed condition is detected based on temperature values ​​measured again by the leakage air sensors 36, 36', 36''.

[0069] Due to the error signal, a visual or audible alarm can also be triggered to warn people in the vicinity or to summon service personnel so that the cause of the error can be rectified.

[0070] In Fig. Figure 6 shows a flowchart for a procedure for monitoring a chamber for airtightness. It is assumed that there are generally two chambers (in the case of an inkjet printer, the dryer housing 23 and the printer housing 11) connected to each other by at least one system-related opening. The interiors of the two chambers 11 and 23 have significantly different air temperatures. The aim is to determine whether either chamber 11 or 23 is airtight or leaking. If a leak is detected, appropriate countermeasures should be taken to restore the chamber's airtightness.

[0071] According to the invention, this is achieved in step S1 by measuring the air temperature of the leakage air 32 in the area of ​​an opening, provided that both chambers 11, 23 have significantly different chamber temperatures. The leakage air 32 can flow from one chamber to the other through the openings. At least one temperature value is measured with the temperature sensor (leakage air sensor 36, 36', 36"). Subsequently, in step S2, the measured temperature value is compared with a reference value. As long as the measured temperature value is lower than the reference value, the air temperature is measured again after a short pause and compared with a reference value.

[0072] If, however, the measured temperature exceeds a threshold, an error signal is generated in step S3. This error signal indicates an undesirably high leakage airflow. Subsequently, in step S4, the chambers can optionally be brought back into equilibrium by controlling the supply and / or exhaust air volumes into and out of the respective chambers and / or by adjusting the chamber temperature, so that the measured temperature is again below the threshold. The chamber is then sealed again. This is verified by temperature measurements from the temperature sensor. Thus, a leak can be corrected so that the inner chamber (here 23) is sealed again.

[0073] A leak is a hole, opening, or defect in a housing through which gases (leakage air 32) can unintentionally enter or escape. A leak can lead to malfunctions or health hazards if it is not detected and repaired as quickly as possible.

[0074] In an inkjet printer, the leak consists of system-related slots 20, 21, 24, 25 through which a recording medium 10 is transported.

[0075] These openings are required for functional reasons, but should be kept as small as possible to prevent excessive leakage air 32 from flowing through them. The openings must also be large enough to ensure that the recording medium 10 is not damaged during transport (i.e., the openings are designed with a safety margin around the area of ​​the recording medium 10).

[0076] Temperature sensors are electrical or electronic components that provide an electrical signal as a measure of temperature. An example of a temperature sensor is a thermocouple, which consists of a pair of metallic conductors made of different materials, connected at one end and suitable for temperature measurement due to the thermoelectric effect. Thermocouples can measure air temperature very accurately and quickly. Fig. 2 - 5 are the locations of the temperature sensors represented by thick dots, which are connected via wires to the respective measuring electronics (with the symbol T for temperature).

[0077] Of course, other suitable temperature sensors, such as a thermal air mass meter, can also be used. This would have the advantage that, in addition to the temperature of the leaking air 32, the humidity and the mass flow rate of the leaking air 32 (molar mass per unit time) could also be measured and evaluated.

[0078] The temperature sensors are positioned near the openings so that they can measure the temperature of the leakage air 32 flowing through the opening. The leakage air sensor 36 is located in the area and near the opening, but outside the chamber being monitored. A prerequisite for this is a large temperature difference between the inside and outside temperatures, so that by measuring the temperature of the leakage air 32 and comparing it to a reference value, sufficiently reliable conclusions can be drawn about the tightness of the chambers.

[0079] If the local flow conditions of the leakage air 32 are known, the location of the leakage air sensors 36, 36', 36'' can be precisely selected to measure the temperature in the area of ​​strongest leakage air flow. Typically, this is the opening that allows the greatest leakage air leakage, and is located in a central area between the edges of the slots 24, 25 and the recording medium 10 (i.e., also outside the influence of the laminar boundary airflow, which is always present on the surface of a rapidly transported recording medium 10).

[0080] In the inkjet printer, the inlet slot 24 is the best location for the leakage air sensor 36, as the temperature difference between the internal and external temperatures is greatest there. This is because, during operation with the dryer housing 23 sealed, the external temperature in the area of ​​the leakage air sensor 36 is approximately 30 °C (± 5 °C), while the internal temperature in the adjacent dryer housing 23 is between 100 °C and 200 °C.

[0081] The outlet slot 25 is less suitable as a location for the leakage air sensor 36. This is because the recording medium 10 is cooled in the space directly in front of the outlet slot 25 on the outlet side of the dryer housing 23. The temperature in this space (approximately 30 °C to 60 °C) is close to the outside temperature. Therefore, the temperature difference between the inside and outside temperatures is very small. Consequently, it is more difficult to make a statement about the tightness based solely on the measured temperature of the leakage air 32. It also makes less sense to measure the leakage air 32 there, as only a few vaporous solvents are present in this space due to the cooler temperatures.

[0082] Fresh air is supplied to the interior of the dryer housing 23 from the outside, and "used," contaminated, or high-water-vapor exhaust air is extracted. The quantities of supply and exhaust air can be controlled or regulated via the supply air module 27 and the exhaust air module 28, for example, depending on the temperature measured by the leakage air sensor 36 compared to a reference value, i.e., depending on whether the interior is airtight or leaking.

[0083] The associated supply and exhaust air modules 27 and 28 can be equipped with heat recovery for energy saving. Furthermore, the supply and exhaust air modules 27 and 28 can be equipped with various air filter elements and exhaust gas purification to ensure that pollutant emissions remain within legal limits. This allows the exhaust air to be filtered and purified before being released into the open air via chimneys or other exhaust ducts.

[0084] In the context of the inkjet printer, the term "outside" refers to the printer housing 11. Thus, the outside temperature is the air temperature inside the printer housing 11. When leakage air 32 flows outwards, it enters the space effectively enclosed by the printer housing 11 (this corresponds to the interior of the printer housing 11 excluding the interior of the dryer housing 23). Therefore, the outer leakage air sensor 36' is located closer to the interior of the printer housing 11 than the inner leakage air sensor 36''.

[0085] The term "inside" refers to the dryer housing 23. Thus, the internal temperature is the air temperature inside the dryer housing 23. When leakage air 32 flows inwards, it enters the space effectively enclosed by the dryer housing 23 (this corresponds to the interior of the dryer housing 23). The internal leakage air sensor 36" is located closer to the interior of the dryer housing 23 than the external leakage air sensor 36'.

[0086] The term "operating exhaust air" refers to the air inside the dryer housing 23. The operating exhaust air contains the substances that evaporate during the drying of the printed image, such as water vapor and, if applicable, solvents – all substances that are necessarily contained in the ink.

[0087] Volatile organic compounds (VOCs) may be present in some inks as necessary components. It is undesirable for such VOCs to enter areas where people are present in large quantities, as some can be hazardous to health.

[0088] The term "tight" means that no or only a small amount of leakage air 32 (including operating exhaust air) flows to the outside through the openings. During operation, a small amount of leakage air 32 flows into the dryer housing 23 if it is tight. However, as soon as a significant amount of leakage air 32 flows to the outside through the opening, the dryer housing 23 is said to be "leaking." This is because no (or very little) operating exhaust air should enter the printer housing 11.

[0089] The invention relies on measured air temperatures and temperature differences to determine the airtightness of a chamber. Changes in air density are not taken into account, but could be considered with suitable sensors.

[0090] The measured temperature value is compared with at least one reference value. One or more previously measured temperatures can serve as the reference value. Multiple previously measured temperatures can also be processed using mathematical methods, such as averaging, to disregard singular measurement errors / outliers. Similarly, the external temperature inside the printer housing 11 and / or the internal temperature can serve as reference values.

[0091] When evaluating the temperature difference, the rate of change of the temperature values ​​and / or the gradient (temperature rise / fall) can also be taken into account. A complete series of measurements consisting of numerous readings can also be continuously evaluated. The temperature can, for example, be measured every second. However, detecting leaks only makes sense during printing. This is because it is primarily during printing (while the printed images are drying) that potentially unwanted and possibly harmful solvents and substances evaporate. After extended periods of inactivity, the temperatures inside the inkjet printer are largely uniform, and leakage air plays only a minor role.

[0092] The method also works if the inner chamber is a cold storage room and the outer chamber (ambient space) is significantly warmer. For example, in a cold storage room, this method could be used to monitor whether too much warm air enters the chamber when goods to be cooled are introduced through an opening (airlock or door). A leakage sensor 36 could then be placed near the opening in the cold storage room to measure the temperature of the leaking air 32. The detected temperature difference can then be used to determine whether the cold storage room is airtight or not. The cold storage room should not heat up unnecessarily. If a leak is detected, countermeasures can be taken, such as lowering the temperature in the cold storage room or creating a slight positive pressure via the supply air so that less leaking air 32 enters from the outside when the airlock is open.

[0093] The invention was explained in more detail using an inkjet printer as an example. Of course, the invention can also be used with other printing devices that use dry or liquid toner and where the printed image is dried by heat in a drying housing. Reference symbol list 10 recording media 11 Printer housings 12 Move-in 13 rotating frames 14 printing units 15 Drying equipment 16 deduction 20 inlet slots 21 trigger slots 23 dryer housings 24 inlet slots 25 outlet slots 26, 26' Sub-chamber of the dryer housing 27 Supply air module 28 Exhaust air module 29 Extraction 30 Heating 31 Cooling 32 Leakage air 33 Indoor sensor 34 External sensor 36 Leakage air sensor 36' external leakage air sensor 36" internal leakage air sensor 40 Device control unit 41 collars S1, S2, S3, S4 Procedure step

Claims

[1] Inkjet printing device with a printer housing (11) in which a transport unit for transporting a web-shaped recording medium (10), a printing unit (14) with at least one printhead for printing on the recording medium (10), and a drying device (15) for drying and fixing the ink applied to the recording medium (10) are arranged, - wherein the drying facility (15) -- a dryer housing (23) with openings (24, 25) through which the recording medium (10) is transported through the drying device (15), at least one heating / cooling module (30, 31) for temperature control of the drying device (15), -- at least one air supply module (27) for supplying tempered air into the dryer housing (23), and -- at least one exhaust air module (28) for removing operating exhaust air from the dryer housing (23), - wherein a first temperature sensor (36, 36', 36'') is arranged outside the dryer housing (23) near an opening (24, 25) of the dryer housing (23), which measures the temperature at this point, and - wherein a device control unit (40) is connected to the first temperature sensor (36, 36', 36") for evaluating measured temperature values, wherein a measured temperature value is compared with at least one reference value and an error signal is generated if a threshold value is exceeded, characterized by , that - the opening (24, 25) has a tunnel-shaped collar (41) in the area of ​​which two temperature sensors (36', 36'') are arranged one behind the other in the direction of flow at a predetermined distance from each other. [2] Inkjet printing device according to claim 1, characterized by, that the first temperature sensor (36, 36', 36'') is a thermocouple arranged in the printer housing (11) in the area of ​​the opening (24, 25) to the dryer housing (23) in such a way that it measures the air temperature which may flow out of the dryer housing (23) through the opening (24, 25). [3] Inkjet printing device according to any one of the preceding claims, characterized by , that a second temperature sensor (33) is arranged inside the dryer housing (23) which measures the internal temperature there, and a third temperature sensor (34) is arranged inside the printer housing (11) outside the dryer housing (23) which measures the external temperature there. [4] Inkjet printing device according to any one of the preceding claims, characterized by, that a temperature sensor (36) is arranged near one of several openings (24, 25) of the dryer housing (23) to detect the temperature change of the air flowing between the interiors of the dryer housing (23) and the printer housing (11). [5] Inkjet printing device according to one of claims 1-3, characterized by , that a temperature sensor (36) is arranged near each opening (24, 25) of the dryer housing (23) in order to detect the temperature change of the air flowing between the interior spaces of the dryer housing (23) and the printer housing (11).

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