Patent Publication Number: US-2021187953-A1

Title: System And Method To Attenuate The Drying Of Aqueous Inks In A Printhead

Description:
TECHNICAL FIELD 
     This disclosure relates generally to devices that produce ink images on media, and more particularly, to devices that eject fast-drying ink from inkjets to form ink images. 
     BACKGROUND 
     Inkjet imaging devices eject liquid ink from printheads to form images on an image receiving surface. The printheads include a plurality of inkjets that are arranged in some type of array. Each inkjet has a thermal or piezoelectric actuator that is coupled to a printhead controller. The printhead controller generates firing signals that correspond to digital data for images. The actuators in the inkjets of the printheads respond to the firing signals by expanding into an ink chamber to eject ink drops through the inkjet nozzles onto an image receiving member and form an ink image that corresponds to the digital image used to generate the firing signals. 
     Some inkjet imaging devices use inks that change from a low viscosity state to a high viscosity state relatively quickly. Aqueous inks are such inks and they can dry out quickly in inkjets that are not operated relative frequently even during printing operations. Additionally, some aqueous ink colors are more susceptible to drying than other ink colors. One way of addressing this problem is to fire inkjets that are not being used to form a portion of the ink image so ink continues to move through the inkjets and does not dry. Firing unused inkjets, however, without adversely impacting the quality of the ink image is difficult as intricate schemes are necessary to spread the extraneous ink over the ink image to camouflage the ink from the eye of a human observer. Being able to maintain the viscosity level of aqueous inks in inkjets so they do not dry during printing operations without resorting to occasional firing of the inkjets would be beneficial. 
     SUMMARY 
     A method of inkjet printer operation conditions the print zone environment so aqueous ink at the nozzles of a printhead maintains its low viscosity state and does not dry. The method includes moving media with a media transport past a plurality of printheads, operating the printheads to form ink images on the media as the media moves past the plurality of printheads, heating water in reservoir of a humidifying chamber to a predetermined temperature range, moving air into the humidifying chamber through an air inlet to humidify the air, discharging humidified air from the humidifying chamber through an air discharge, and directing the discharged humidified air into a space between a faceplate of at least one of the printheads and a path for media passing by the faceplate of the at least one printhead. 
     An environmental conditioner conditions air in a print zone of an inkjet printer so aqueous ink at the nozzles of a printhead maintains its low viscosity state and does not dry. The environmental conditioner includes a humidifying chamber having a reservoir configured to contain a volume of water, a heater configured to heat the water in the reservoir to a predetermined temperature range, an air inlet to move air into the humidifying chamber, an air discharge configured to remove humidified air from the humidifying chamber and direct the humidified air into a space between a faceplate of a printhead and a path for media passing by the faceplate of the printhead. 
     An inkjet printer is configured with a device that conditions the print zone environment of the printer so aqueous ink at the nozzles of a printhead maintains its low viscosity state and does not dry. The printer includes a plurality of printheads, a media transport configured to move media past the plurality of printheads so the printheads form ink images on the media as the media moves past the plurality of printheads, and an environmental conditioner having a humidifying chamber having a reservoir configured to contain a volume of water, a heater configured to heat the water in the water reservoir to a predetermined temperature range, an air inlet to move air into the humidifying chamber, an air discharge configured to remove humidified air from the humidifying chamber and direct the humidified air into a space between a faceplate of a printhead and a path for media passing by the faceplate of the printhead. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and other features of a system and method that conditions the print zone environment of an inkjet printer so aqueous ink at the nozzles of a printhead maintains its low viscosity state and does not dry are explained in the following description, taken in connection with the accompanying drawings. 
         FIG. 1  is a schematic drawing of an aqueous inkjet printer that prints ink images directly to a web of media and that conditions the print zone environment in the printer so aqueous ink at the nozzles of the printheads maintains its low viscosity state and does not dry. 
         FIG. 2A ,  FIG. 2B , and  FIG. 2C  are schematic diagrams of different embodiments of an environmental conditioner for the printer of  FIG. 1  that conditions the print zone environment in the printer so aqueous ink at the nozzles of the printheads maintains its low viscosity state and does not dry. 
         FIG. 3  depicts an alternative configuration of the conditioners shown in  FIG. 2A, 2B, and 2C  that are positioned at an entrance of a print zone in the printer of  FIG. 1 . 
         FIG. 4  is a graph of temperature and humidity levels that is used to identify the absolute maximum humidity and the minimum absolute humidity for the printer of  FIG. 1  that are used to identify the maximum and minimum temperatures for the operation of the conditioners shown in  FIG. 2A ,  FIG. 2B , and  FIG. 2C . 
         FIG. 5  is a flow diagram of a process used to operate the printer of  FIG. 1  that conditions the print zone environment in the printer so aqueous ink at the nozzles of the printheads maintains its low viscosity state and does not dry. 
     
    
    
     DETAILED DESCRIPTION 
     For a general understanding of the environment for the system and method disclosed herein as well as the details for the system and method, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate like elements. As used herein, the word “printer” encompasses any apparatus that produces ink images on media, such as a digital copier, bookmaking machine, facsimile machine, a multi-function machine, or the like. As used herein, the term “process direction” refers to a direction of travel of an image receiving surface, such as an imaging drum or print media, and the term “cross-process direction” is a direction that is substantially perpendicular to the process direction in the plane of the image receiving surface. Also, the description presented below is directed to a system for conditioning the air within the print zone of an inkjet printer to reduce evaporation of aqueous ink at the nozzles of the inkjets in the printer. As used in this document, the term “environment conditioning” means treating the ambient air in a print zone of a printer so aqueous ink at the nozzles of the printheads maintains its low viscosity state and does not dry. The reader should also appreciate that the principles set forth in this description are applicable to similar imaging devices that generate images with pixels of other marking materials. 
       FIG. 1  illustrates a high-speed aqueous ink image producing machine or printer  10  in which a controller  80  has been configured to operate the print zone environment conditioner  60  so the aqueous ink at the nozzles of the printheads  34 A,  34 B,  34 C, and  34 D maintain a low viscosity state during printing jobs. As illustrated, the printer  10  is a printer that directly forms an ink image on a surface of a web W of media pulled through the printer  10  by the controller  80  operating one of the actuators  40  that is operatively connected to the shaft  42  to rotate the shaft and the take up roll  46  mounted about the shaft. In one embodiment, each printhead module has only one printhead that has a width that corresponds to a width of the widest media in the cross-process direction that can be printed by the printer. In other embodiments, the printhead modules have a plurality of printheads with each printhead having a width that is less than a width of the widest media in the cross-process direction that the printer can print. In these modules, the printheads are arranged in an array of staggered printheads that enables media wider than a single printhead to be printed. Additionally, the printheads can also be interlaced so the density of the drops ejected by the printheads in the cross-process direction can be greater than the smallest spacing between the inkjets in a printhead in the cross-process direction. 
     The aqueous ink delivery subsystem  20  has at least one ink reservoir containing one color of aqueous ink. Since the illustrated printer  10  is a multicolor image producing machine, the ink delivery system  20  includes four (4) ink reservoirs, representing four (4) different colors CYMK (cyan, yellow, magenta, black) of aqueous inks. Each ink reservoir is connected to the printhead or printheads in a printhead module to supply ink to the printheads in the module. Pressure sources and vents of a purge system  24  are also operatively connected between the ink reservoirs and the printheads within the printhead modules to perform manifold and inkjet purges. Additionally, although not shown in  FIG. 1 , each printhead in a printhead module is connected to a corresponding waste ink tank with a valve to collect ink produced by manifold and inkjet purge operations. The printhead modules  34 A- 34 D can include associated electronics for operation of the one or more printheads by the controller  80  although those connections are not shown to simplify the figure. Although the printer  10  includes four printhead modules  34 A- 34 D, each of which has two arrays of printheads, alternative configurations include a different number of printhead modules or arrays within a module. The controller  80  also operates the print zone environmental conditioner  60  to preserve the low viscosity of the ink in the nozzles of the printheads in the printhead modules as described more fully below. 
     After an ink image is printed on the web W, the image passes under an image dryer  30 . The image dryer  30  can include an infrared heater, a heated air blower, air returns, or combinations of these components to heat the ink image and at least partially fix an image to the web. An infrared heater applies infrared heat to the printed image on the surface of the web to evaporate water or solvent in the ink. The heated air blower directs heated air over the ink to supplement the evaporation of the water or solvent from the ink. The air is then collected and evacuated by air returns to reduce the interference of the air flow with other components in the printer. 
     As further shown, the media web W is unwound from a roll of media  38  as needed by the controller  80  operating one or more actuators  40  to rotate the shaft  42  on which the take up roll  46  is placed to pull the web from the media roll  38  as it rotates with the shaft  36 . When the web is completely printed, the take-up roll can be removed from the shaft  42 . Alternatively, the printed web can be directed to other processing stations (not shown) that perform tasks such as cutting, collating, binding, and stapling the media. 
     Operation and control of the various subsystems, components and functions of the machine or printer  10  are performed with the aid of a controller or electronic subsystem (ESS)  80 . The ESS or controller  80  is operably connected to the components of the ink delivery system  20 , the purge system  24 , the printhead modules  34 A- 34 D (and thus the printheads), the actuators  40 , the heater  30 , and the print zone environmental conditioner  60 . The ESS or controller  80 , for example, is a self-contained, dedicated mini-computer having a central processor unit (CPU) with electronic data storage, and a display or user interface (UI)  50 . The ESS or controller  80 , for example, includes a sensor input and control circuit as well as a pixel placement and control circuit. In addition, the CPU reads, captures, prepares and manages the image data flow between image input sources, such as a scanning system or an online or a work station connection, and the printhead modules  34 A- 34 D. As such, the ESS or controller  80  is the main multi-tasking processor for operating and controlling all of the other machine subsystems and functions, including the printing process. 
     The controller  80  can be implemented with general or specialized programmable processors that execute programmed instructions. The instructions and data required to perform the programmed functions can be stored in memory associated with the processors or controllers. The processors, their memories, and interface circuitry configure the controllers to perform the operations described below. These components can be provided on a printed circuit card or provided as a circuit in an application specific integrated circuit (ASIC). Each of the circuits can be implemented with a separate processor or multiple circuits can be implemented on the same processor. Alternatively, the circuits can be implemented with discrete components or circuits provided in very large scale integrated (VLSI) circuits. Also, the circuits described herein can be implemented with a combination of processors, ASICs, discrete components, or VLSI circuits. 
     In operation, image data for an image to be produced are sent to the controller  80  from either a scanning system or an online or work station connection for processing and generation of the printhead control signals output to the printhead modules  34 A- 34 D. Additionally, the controller  80  determines and accepts related subsystem and component controls, for example, from operator inputs via the user interface  50 , and accordingly executes such controls. As a result, aqueous ink for appropriate colors are delivered to the printhead modules  34 A- 34 D. Additionally, pixel placement control is exercised relative to the surface of the web to form ink images corresponding to the image data, and the media can be wound on the take-up roll or otherwise processed. 
     Using like numbers for like components, a print zone environmental conditioner  60  that can attenuate the evaporation of quickly drying inks from printheads is shown in  FIG. 2A ,  FIG. 2B , and  FIG. 2C .  FIG. 4  depicts a flow diagram for the process  400  that operates the environmental conditioner  60  to cover the faceplates of the printheads with humidified air having a high relative humidity at an elevated temperature to preserve the viscosity of the ink in the nozzles of the printheads at a viscosity adequate to attenuate drying of the ink at the nozzles. In the discussion below, a reference to the process  400  performing a function or action refers to the operation of a controller, such as controller  80 , to execute stored program instructions to perform the function or action in association with other components in the printer. The process  400  is described as being performed with the print zone environmental conditioner  60  in the printer  10  of  FIG. 1  for illustrative purposes. 
     Three different embodiments of a print zone environmental conditioner  60 ,  60 ′, and  60 ″ that reduces the evaporation of aqueous ink during periods of inkjet inactivity are shown in  FIG. 2A, 2B, and 2C , respectively. Each of the conditioners includes a humidifying chamber  204 , a heater  208 , an ambient air inlet  212 , and a humidified air discharge  216 . In general, ambient air enters the humidifying chamber  204  through the ambient air input  212  and, after being heated to raise the moisture capacity of the air and having the heated air humidified to an appropriate humidity level, the treated air is discharged into the print zone between the printheads and the media moving through the print zone in the process direction. The conditioners  60  differ in the configuration and types of elements used to treat the air in the humidifying chamber. In  FIG. 2A , the humidifying chamber  204  includes a water reservoir  220  that is configured to hold a predetermined volume of water. A water inlet to water reservoir  220  is fluidically coupled by conduit  228  to a water source  232  and controller  80  is operatively connected to a valve  236  inserted in the conduit  228 . A fluid level sensor  240  in the water reservoir  220  generates a signal that indicates the water level in the reservoir  220 . The controller  80  receives the signal from the sensor  240  and is configured to operate the valve  236  so water enters the reservoir  220  when the signal from the sensor  240  indicates the water level in the reservoir is at a predetermined minimum. The controller  80  is also configured to operate the valve  236  to close the valve when the signal from the sensor  240  indicates the water level in the reservoir is at predetermined maximum. The ambient air inlet  212  is connected to a manifold  244  having perforated holes  248 . The incoming ambient air is urged by a pressure source  252 , such as a fan or pump, at a sufficient pressure to urge the air into the manifold  244  and out through the perorated holes in the manifold without water entering the manifold. A temperature sensor  256  is positioned in the reservoir  220  at a position below the predetermined minimum level in the reservoir and the sensor  256  generates a signal indicative of the temperature of the water in the reservoir  220 . The heater  208  is also operatively connected to the controller  80  and the controller  80  is configured to operate the heater to heat the water in the reservoir when the signal from the sensor  256  indicates the temperature of the water is below a predetermined minimum temperature. When the signal from the sensor  256  indicates the water in the reservoir  220  is at or above a predetermined maximum temperature, the controller  80  is configured to deactivate the heater  208 . As explained below, the maximum temperature of the water is equal to or slightly greater than the temperature of the printheads to prevent the humidified air from condensing on the faceplates of the printheads and the minimum temperature is greater than the temperature of the incoming ambient to increase the water carrying capacity of the air. Thus, the air exiting the perforated holes in the manifold  244  percolates through the heated water of the reservoir  220  so the air is heated to increase its water absorbing capacity and so the heated air becomes humified before being discharged from the humidifying chamber  204  through the air discharge  216 . 
     In the embodiment of  FIG. 2B , the water inlet  224  of the conditioner is connected to the water source  232  through a conduit  228  as described previously and the water level is monitored with a level sensor  240  so the controller can operate the valve  236  to maintain the water level at or above a predetermined minimum level as described above with regard to  FIG. 2A , although those components have been left out of the figure to simplify the view. Likewise, a temperature sensor  256  is monitored by the controller  80  so the controller operates the heater  208  in a manner to keep the water in the water reservoir  220  of the humidifying chamber  204  at a temperature at or above a predetermined temperature. Again, these components are not shown in  FIG. 2B  to simplify the view. In the embodiment of  FIG. 2B , the air inlet  212  is positioned above the maximum water level of the water in the reservoir  220 . An ultrasonic atomizer  260  is operatively connected to a power source (not shown) so the ultrasonic atomizer  260  vibrates the heated water to produce a moisture mist in the humidifying chamber  204 . The heated water heats the incoming air to increase the moisture capacity of the air, which absorbs the moisture mist before exiting the humidifying chamber  204  through air discharge  216 . 
     In the embodiment of  FIG. 2C , the humidifying chamber  204  is provided with a water reservoir  220  that is provided with water from a water source  232  through a water inlet  224  and conduit  228  and the level of the water in the reservoir  220  is regulated by the controller  80  using a water level sensor  240  and a valve  236  as described previously with reference to  FIG. 2A , although not shown in the figure to simplify the view. The controller  80  operates the heater  208  to heat the water in the reservoir  220  and the temperature of the water in the reservoir is regulated by the controller using the signal from the temperature sensor  256  as previously described as well. The air space over the heated water in the reservoir  220  is filled with wicking fabric  268 , such as woven fabrics made of cellulose fibers (cotton for example), synthetic fibers and other materials, especially woven fabrics that have been treated for enhanced hydrophilicity. The wicking fabric  268  absorbs heated moisture produced from the heated water in the reservoir  220 . The incoming air inlet  212  is positioned to direct air into this wicking fabric  268  at a pressure sufficient to push the air through the wicking fabric  268  to the discharge  216 . As the air travels through the wicking fabric  268 , it is heated to increase its moisture capacity and this heated air absorbs moisture circulating through the wicking fabric to increase the relative humidity of the air. 
     In some embodiments of these three conditioner configurations, another pressure source  272  (shown in  FIG. 1 ), which can be another fan or pump, is operated by the controller  80  to pull the heated and humidified air from the humidified air discharge  216  and direct the humidified air into a tube or manifold  276  ( FIG. 1 ) that extends along the length of the print zone and is parallel to the print zone. This tube or manifold  276  has perforated holes at an edge of the print zone between the printheads and the web W so the humidified air exits the tube or manifold and enters the print zone between the faceplates of the printheads and the web W moving through the print zone. The pressure source  272  is configured to maintain a velocity of this discharged air below a predetermined velocity that would disturb the flight paths of ink drops ejected from the nozzles of the inkjets toward the media moving through the print zone. The humidified air in the print zone attenuates the likelihood of the ink in the nozzles of the printheads drying in the nozzles. In other embodiments of the three configurations of conditioner  60 , the conditioner  60  is located over the media immediately prior to the media entering the print zone as shown in  FIG. 3 . In these embodiments, the humidified air discharge  216  is configured to direct the heated and humidified air toward the media as the media enters the print zone. Thus, the humidified air is carried into the print zone by the media to attenuate the likelihood of the ink in the nozzles of the printheads drying in the nozzles. 
     The temperature requirements for the discharged heated air are discussed with reference to the curves depicted in  FIG. 4 . The heated and humidified air must be at a relative humidity level that is less than the dew point of the printheads in the print zone, otherwise, the heated and humidified air condenses on the faceplates and possibly drips onto the media. Curve  404  in  FIG. 4  depicts the absolute humidity of air as the temperature of a printhead increases. As used in this document, the term “100% relative humidity” means the dew point of air at a particular temperature as illustrated by the curve  404 . Curves  408 ,  412 , and  416  represent the 80%, 50%, and 20% relative humidity levels, respectively, over the range of temperatures shown in the graph. The temperature range and relative humidity range shown in  FIG. 4  are for a printhead that is operated at an exemplary temperature of 37 degrees C. 
     To determine the operating parameters for the print zone environmental conditioner  60 , the following procedure is performed using the graph of  FIG. 4 . The maximum absolute humidity is determined by the dew point at the maximum printhead temperature, which is point B on the graph. The minimum absolute humidity is dependent upon the type of printhead being used and the ink being ejected and is determined empirically with experiments and this value intersects the moisture saturation curve (100% relative humidity) at point A. The temperature at point B is the maximum operating temperature T max  of the conditioner and it ensures the humidity does not exceed the dew point at point B. Because the air cannot reach 100% relative humidity and because potential dilution of the humidified air with ambient air in or around the print zone before the humidified air reaches a printhead face plate, T max  can be slightly higher than the temperature at point B. The temperature at point A is the lower limit of the minimum operating temperature T min  for the conditioner. At this minimum temperature, 100% relative humidity without any loss of moisture barely provides enough moisture for keeping the ink in the nozzles adequately hydrated. Practically, T min  should be significantly higher than the temperature at point A. The ranges of the operating temperature and relative humidity are illustrated by the space enclosed by the triangular-like shape ABC. If the controller controls the operating temperature of the conditioner precisely, then the optimal operating temperature of the conditioner can be set to be slightly below the temperature at point B. At this temperature, a significant range of relative humidity percentages can be tolerated and still provide enough moisture to keep the ink in the inkjets adequately hydrated. Compared to ambient air having a relative humidity of 50%, the conditioner  60  provides up to five times more moisture to the printhead faceplates at 37 degrees C. during printing without a significant risk of water condensation on the faceplates of the printheads. 
       FIG. 5  depicts a flow diagram for a process  500  that operates the conditioner  60  to supply heated and humidified air to the faceplates of the printheads in the print zone of a printer during printing operations. In the discussion below, a reference to the process  500  performing a function or action refers to the operation of a controller, such as controller  80 , to execute stored program instructions to perform the function or action in association with other components in the printer. The process  500  is described as being performed for a print zone environmental conditioner  60  in the printer  10  of  FIG. 1  for illustrative purposes. 
     The process  500  begins with the maximum and minimum absolute humidity being identified and the maximum and minimum water temperatures are identified from these values (block  504 ). Prior to printing operations commencing, the controller fills the water reservoir  220  to an appropriate level and regulates the temperature of the water to be within the operational range (block  508 ). As the printing operation begins, ambient air is moved into the humidifying chamber (block  512 ) where it absorbs moisture and heat before being discharged into the print zone (block  516 ). As the printing operation continues, the controller continues the process by regulating the water level in the water reservoir as explained above and as well as the temperature of the water as explained above (block  520 ). When the printing operations stop (block  524 ), the flow of water to the humidifying chamber is disabled and the heater is deactivated (block  528 ) until another print job is detected (block  532 ). 
     It will be appreciated that variants of the above-disclosed and other features, and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art, which are also intended to be encompassed by the following claims.