Patent Publication Number: US-9427992-B2

Title: System and method for lead edge release coating for improved media stripping in an aqueous inkjet printer

Description:
TECHNICAL FIELD 
     This disclosure relates generally to indirect inkjet printers, and, in particular, to surface preparation for inkjet printing. 
     BACKGROUND 
     In general, inkjet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or image forming surface. An aqueous inkjet printer employs water-based or solvent-based inks in which pigments or other colorants are suspended or in solution. Once the aqueous ink is ejected onto an image receiving surface by a printhead, the water or solvent is evaporated to stabilize the ink image on the image receiving surface. When aqueous ink is ejected directly onto media, the aqueous ink tends to soak into the media when it is porous, such as paper, and change the physical properties of the media. To address this issue, indirect printers have been developed that eject a skin layer onto the blanket mounted to a drum or endless belt, and an ink layer onto the skin layer. The ink is dried on the skin layer and then transferred to media. Such a printer avoids the changes in media properties that occur in response to media contact with the water or solvents in aqueous ink. Indirect printers also reduce the effect of variations in other media properties that arise from the use of widely disparate types of paper and films used to hold the final ink images. 
     In these indirect printers, media stripping may be a problem. Both the skin and the ink layer must adhere to the media so that the image is transferred to the media. However, high adhesion also makes stripping the media and ink image from the blanket difficult, particularly when relatively thin media are used. Air knives have been proposed to aid in media stripping in the past. Insufficient media lead edge separation from the blanket usually prevents an air knife from successfully stripping the media from the blanket. Stripper fingers have also been proposed for this purpose, but they must be pressed into the blanket with high pressures in order to strip the lead edge of the media from the blanket. Such high pressures increase the wear and reduce the life of the blanket. Other ways of successfully stripping media from a blanket in an inkjet printer without adversely affecting the blanket surface would be beneficial. 
     SUMMARY 
     A printer has been developed that facilitates media lead edge separation from a blanket in an inkjet printer to enable removal of the media from the blanket without decreasing the life of the blanket. The printer includes at least one printhead configured to eject liquid ink, a rotating member being positioned to rotate past the at least one printhead to enable the at least one printhead to eject liquid ink and form an ink image, a first applicator positioned with reference to the rotating member to apply a first material to a surface of the rotating member to enable the ink image to be formed on a surface of the first material on the surface of the rotating member by the at least one printhead, a second applicator position with reference to the rotating member to apply a second material on a portion of the first material after the ink image has been formed on the surface of the first material, a transfer roller configured to move into and out of engagement with the rotating member to form a nip selectively with the rotating member, and a controller operatively connected to the printhead, the first applicator, the second applicator, and the transfer roller, the controller being configured to operate the first applicator to apply the first material to the rotating member, to operate the second applicator to apply the second material to the portion of the first material on the surface of the rotating member, and to move the transfer roller into engagement with the rotating member to form the nip with the rotating member as the second material on the portion of the first material approaches the nip and as media approaches the nip to enable a leading edge of the media to engage the second material on the portion of the first material in the nip and facilitate separation of the leading edge of the media from the first material on the surface of the rotating member after the media exits the nip. 
     A method of operating a printer has been developed that facilitates media lead edge separation from a blanket in an inkjet printer to enable removal of the media from the blanket without decreasing the life of the blanket. The method includes operating with a controller a first applicator positioned with reference to a rotating member to apply a first material to a surface of the rotating member, operating with the controller inkjets in at least one printhead positioned with reference to the rotating member to eject ink on a surface of the first material to form an ink image, operating with the controller a second applicator positioned with reference to the rotating member to apply a second material on a portion of the surface of the first material after the ink image has been formed on the surface of the first material, and operating with the controller a transfer roller to move into and out of engagement with the rotating member to form a nip selectively with the rotating member as the second material on the portion of the surface of the first material approaches the nip and as media approaches the nip to enable a leading edge of the media to engage the second material on the portion of the surface of the first material in the nip and facilitate separation of the leading edge of the media from the first material on the surface of the rotating member after the media exits the nip. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial schematic of an aqueous indirect inject printer that produces images on sheet media. 
         FIG. 2  is a schematic diagram of a device that uses contact to apply coating material to an imaging surface. 
         FIG. 3  is a schematic diagram of a device that ejects drops of coating material onto an imaging surface. 
         FIG. 4  is a perspective view of a media sheet with lead edge release coating from a coating applicator. 
         FIG. 5  is a perspective view of another media sheet with lead edge release coating from a coating applicator. 
         FIG. 6  is a perspective view of another media sheet with lead edge release coating from a coating applicator. 
         FIG. 7  is a flow diagram of a method of operating a printer that uses a lead edge release coating on an imaging surface. 
     
    
    
     DETAILED DESCRIPTION 
     For a general understanding of the present embodiments, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate like elements. As used herein, the terms “printer,” “printing device,” or “imaging device” generally refer to a device that produces an image with one or more colorants on print media and may encompass any such apparatus, such as a digital copier, bookmaking machine, facsimile machine, multi-function machine, or the like, which generates printed images for any purpose. Image data generally include information in electronic form which are rendered and used to operate the inkjet ejectors to form an ink image on the print media. These data can include text, graphics, pictures, and the like. The operation of producing images with colorants on print media, for example, graphics, text, photographs, and the like, is generally referred to herein as printing or marking. As used in this document, the term “aqueous ink” includes liquid inks in which colorant is in solution with water and/or one or more solvents. 
     The term “printhead” as used herein refers to a component in the printer that is configured with inkjet ejectors to eject ink drops onto an image receiving surface. A typical printhead includes a plurality of inkjet ejectors that eject ink drops of one or more ink colors onto the image receiving surface in response to firing signals that operate actuators in the inkjet ejectors. The inkjets are arranged in an array of one or more rows and columns. In some embodiments, the inkjets are arranged in staggered diagonal rows across a face of the printhead. Various printer embodiments include one or more printheads that form ink images on an image receiving surface. Some printer embodiments include a plurality of printheads arranged in a print zone. An image receiving surface, such as a print medium or the surface of an intermediate member that carries an ink image, moves past the printheads in a process direction through the print zone. The inkjets in the printheads eject ink drops in rows in a cross-process direction, which is perpendicular to the process direction across the image receiving surface. 
       FIG. 1  illustrates a high-speed aqueous ink image producing machine or printer  10 . Although the description of the system and method for lead edge release coating for improved media stripping is directed to an aqueous inkjet printer, the reader should appreciate that the system and method can be used in other liquid inkjet printers. Additionally, while printer  10  is described below as printing to sheet media, the reader should understand that subsystem that facilitates the stripping of media from the blanket on which an ink image is printed can be used in a printer that passes media as a web pulled from a supply roll through the transfer nip and rewound on a take-up roller for later processing. As illustrated, the printer  10  is an indirect printer that forms an aqueous ink image on a surface of a blanket  21  mounted about a receiving member  12  and then transfers the ink image to media  49  passing through a nip  18  formed with the blanket  21  and transfix roller  19 . The printer  10  includes a frame  11  that supports directly or indirectly operating subsystems and components, which are described below. The receiving member  12  is shown in the form of a drum, but can also be configured as a supported endless belt. The receiving member  12  has an outer blanket  21  mounted about the circumference of the member  12 . The blanket moves in direction  16  as the receiving member  12  rotates. Transfix roller  19  is rotatable in direction  17  and is loaded against the surface of blanket  21  to form the transfix nip  18 , within which ink images formed on the surface of blanket  21  are transfixed onto media sheet  49 . 
     The blanket is formed of a material having a relatively low surface energy to facilitate transfer of the ink image from the surface of the blanket  21  to the media sheet  49  in the nip  18 . Such materials include silicones, fluro-silicones, Viton, and the like. A surface maintenance unit (SMU)  192  removes residual ink left on the surface of the blanket  21  after the ink images are transferred to the media sheet  49 . The SMU  192  has a cleaning blade that is selectively moved to remove un-transferred ink pixels from the surface of the blanket  21 . The SMU  192  is operatively connected to a controller  80 , described in more detail below, to enable the controller to operate the cleaning blade selectively. In some embodiments, instead of a cleaning blade, SMU  192  has an air knife to clean the surface of the blanket selectively. In other embodiments, SMU  192  instead uses stripper fingers. Any known stripping mechanism may be implemented into SMU  192  to clean the blanket from residuals. 
     The low energy surface of the blanket does not aid in the formation of good quality ink images because such surfaces do not spread ink drops as well as high energy surfaces. Consequently, a first coating applicator  110  applies a coating to the blanket surface. The coating helps aid in wetting the surface of the blanket, inducing solids to precipitate out of the liquid ink, providing a solid matrix for the colorant in the ink, and aiding in the release of the ink image from the blanket. Such coatings include surfactants, starches, and the like. 
     The first coating applicator  110  includes a reservoir (not shown) with a fixed volume of coating material and a donor roller, which can be smooth or porous and is rotatably mounted in the reservoir for contact with the coating material. The donor roller can be an elastomeric roller or a rigid roller such as an anilox roller. The coating material is applied to the surface of the blanket  21  to form a thin layer on the blanket surface. The first coating applicator  110  is operatively connected to a controller  80 , described in more detail below, to enable the controller to move the donor roller selectively. 
     As shown in  FIG. 2 , the first coating applicator  110  can include a donor roller applicator  304 . The roller applicator  304  can be partially immersed in a reservoir  308  of the coating material to enable the roller to pick up the coating material and apply it to the surface of the blanket  21 . Although the applicator  100  has been described as having a donor roller and reservoir, in another embodiment shown in  FIG. 3 , the applicator instead includes an applicator head  320  having a plurality of nozzles  328  through which the coating material is ejected in a mist to form a discontinuous film of very small drops onto the blanket surface. The size of the drops would be much smaller than the size of ink drops ejected by the printheads  34 A to  34 D, which are described in more detail below. The drops can contain compounds that induce solids in the ink to precipitate out of solution. A discontinuous film can be advantageously used with blanket surfaces having very low surface energy since liquid films, such as those produced by a roller applicator, tend to break up on low surface energy materials. If a liquid coating film breaks up then some ink drops land on the coating while other ink drops land directly on the blanket. Consequently, the applicator head is configured to produce a significant number of coating drops on the blanket for every ink drop and to distribute the drops evenly. If too few drops are ejected, the ink drops do not interact with an adequate number of drops. If too many drops are ejected, then the drops agglomerate into larger pools that may affect the uniformity of the printed surface. When a discontinuous film of the coating is used on the imaging surface, the “thickness” of the coating refers to an average thickness of the coating drops on the imaging surface. 
     Printer  10  also includes a dryer  120  configured to dry the first coating applied to the blanket by the first coating applicator  110 . The controller  80  operates a fan and heater within the dryer to control the amount of airflow and heat of the air directed by the dryer  120  towards the first coating on the blanket. 
     The high-speed aqueous ink printer  10  also includes a plurality of printhead modules, also known as print box units,  34 A through  34 D. Each printhead module  34 A- 34 D effectively extends across the width of the blanket and ejects ink drops onto the surface of the blanket  21 . A printhead module can include a single printhead or a plurality of printheads configured in a staggered arrangement. Each printhead module is operatively connected to a frame (not shown) and aligned to eject the ink drops to form an ink image on the coating on the blanket surface. The printhead modules  34 A- 34 D can include associated electronics, ink reservoirs, and ink conduits to supply ink to the one or more printheads. As is generally familiar, each of the one or more printheads in a printhead module can eject a single color of ink. In other embodiments, the printheads can be configured to eject two or more colors of ink. For example, printheads in modules  34 A and  34 B can eject cyan and magenta ink, while printheads in modules  34 C and  34 D can eject yellow and black ink. The printheads in the illustrated modules are arranged in two arrays that are offset, or staggered, with respect to one another to increase the resolution of each color separation printed by a module. Such an arrangement enables printing at twice the resolution of a printing system only having a single array of printheads that eject only one color of ink. 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. 
     As shown in  FIG. 1 , printer  10  also includes a second coating applicator  140 . The second coating applicator  140  includes one or more ejectors (not shown) and is operatively connected to controller  80  to enable the controller to operate the ejectors to eject a lead edge release coating on the lead edge of the image surface of the blanket  21 , as described in more detail below. The lead edge release coating is a release film fluid that, when dried, produces a low adhesion film on the surface of the blanket  21 . In one embodiment, the release film fluid is a solution of polyvinyl acetate and water. Other release film fluids can be used. Such fluids are capable of application by ejection, drying, and having low adhesion to media and are capable of being removed easily from the blanket with a cleaning solution. The film enables the ink image to transfer to media  49  when the roller  19  is loaded against the surface of the blanket  21  to form transfix nip  18  and then allows the lead edge of the media to separate from the blanket  21  without an outside agency intervening between the two surfaces as the lead edge exits the nip. 
     Printer  10  includes a dryer  130  configured to dry the lead edge release coating applied by the coating applicator  140 . The image dryer  130  includes an infrared heater, a heated air source, and air returns (not shown). The infrared heater applies infrared heat to the printed image and the lead edge release coating on the surface of the blanket  21  to evaporate water in the coating as well as water and solvent in the ink. The heated air source 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 and to reduce the interference of the air flow with other components in the printing area. 
     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 SMU  192 , the first coating applicator  110 , the dryer  120 , the printhead modules  34 A- 34 D, the second coating applicator  140 , the dryer  130 , receiving member  12 , and transfix roller  19 . The ESS or controller  80 , for example, is a self-contained, dedicated mini-computer having a central processor unit (CPU) with electronic storage, and a display or user interface (UI). 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 discussed below. 
     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, controller  80  sends signals to the first applicator  110  to eject a first coating to the surface of the blanket  21 . As discussed above, the coating helps aid in wetting the surface of the blanket, inducing solids to precipitate out of the liquid ink, providing a solid matrix for the colorant in the ink, and aiding in the release of the ink image from the blanket. The controller  80  also send signals to the dryer  120  to control airflow from dryer  120  towards the blanket  21 , to dry the first coating on the surface of the blanket  21 . Image data for an image to be produced are sent to the controller  80  from either a scanning system or via the online or work station  90  for processing and generation of the printhead control signals output to the printhead modules  34 A- 34 D. Additionally, the controller  80  determines and/or accepts related subsystem and component controls, for example, from operator inputs via the user interface, 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 blanket surface to form ink images corresponding to the image data, and the media, which can be in the form of media sheets  49 , and handled by a recording media transport system for timed delivery to the nip  18 . In the nip  18 , the ink image is transferred from the blanket and coating  21  to the media substrate within the transfix nip  18 . 
     In some printing operations, a single ink image can cover the entire surface of the blanket  21  (single pitch) or a plurality of ink images can be deposited on the blanket  21  (multi-pitch). In a multi-pitch printing architecture, the surface of the image receiving member can be partitioned into multiple segments, each segment including a full page image in a document zone (i.e., a single pitch) and inter-document zones that separate multiple pitches formed on the blanket  21 . For example, a two pitch image receiving member includes two document zones that are separated by two inter-document zones around the circumference of the blanket  21 . Likewise, for example, a four pitch image receiving member includes four document zones, each corresponding to an ink image formed on a single media sheet, during a pass or revolution of the blanket  21 . 
     Once an ink image or images have been formed on the first coating on the surface of the blanket under by the printheads under control of the controller  80 , the controller  80  causes the second coating applicator  140  to eject release film fluid at the lead edge of the print. The release film fluid has low adhesive strength, which enables the lead edge of media to be more easily stripped from the blanket following image transfer to the media in the nip. In most cases, the lead edge of the media separates from the blanket as it exits the nip without the need of a blade or stripper fingers that may damage or prematurely wear the blanket over time. The location and amount of release film fluid ejected to the blanket by the second coating applicator  140  may be determined by the controller  80  based on the several factors, including the stiffness and/or thickness of media  49  to which the ink image is transferred and the image data. 
       FIGS. 4-6  show strips of release film on media  49 , originating from the release film fluid ejected onto blanket  21  and transferred to media  49 . In a first operation, controller  80  determines the proper minimum width of a strip of release film fluid to be ejected by coating applicator  140  that results in self-stripping of the ink to the media  49  based on the thickness or stiffness of media  49  used in a printing operation measured from the leading edge of the media to the start of the printed image. Generally, stiff media may self-strip and need no release film, or only a strip of release film having a relatively small width. Thinner, less stiff media may require a strip of release film having a relatively large width in order to ensure that the ledge edge self-strips and is transferred properly to the media  49 . Width as meant in this context refers to the distance between the leading edge that extends towards the printed ink image.  FIG. 4  shows three minimum widths  160 ,  162 ,  164  of strips of release film applied by the applicator  140  measured from the leading edge of the media in a direction towards the image content, each width  160 ,  162 ,  164  associated with a media  49  having a different stiffness or thickness. Width  160  is the smallest of the three widths  160 ,  162 ,  164 , and is the minimum width for the thickest or stiffest media  49  of the three strips shown. Width  164  is the largest and is the minimum coating width for media having the least thickness or stiffness. Width  162  is between the width  160  and the width  164 , and represents the minimum width of release film that should be applied to media having a thickness or stiffness value between the media associated with widths  160  and  164 . 
     Controller  80  is also programmed to take into consideration image data when determining the width of the strip of release film ejected by coating applicator. In a second operation, controller  80  is configured to determine the maximum thickness of the width of the strip to be applied by the coating applicator  140  based on image data, measured as the distance from the leading edge of the media receiving the image to the leading edge of the ink image. Application of the film from the leading edge of the media  49  in excess of the minimum width up to the image content increases the ability of the media to strip from the blanket  21 , but application of the film over the image content would cover the image content, and prevent its transfer to the media. As shown in  FIG. 5 , the minimum width  170  is shown based on the stiffness or thickness of media  49 , as discussed above. In addition, the maximum width  172  is shown in excess of the minimum width and up to a lead edge of the image content. In such a case, applying the release film beyond the minimum width  170  to the maximum width  172  at the beginning of the image content would assist in stripping without covering up image content. 
     As shown in  FIG. 6 , the minimum width  180  determined by controller  80  based on the stiffness or thickness of media  49  exceeds the beginning of the image content at least for part of the image data. Application of the release film over the image content would cover the image content and prevent proper transfer to the media  49 , so the controller  80  instead causes release film to form around and conform to the leading edge of the image content, as indicated by the variable widths  182  and  184 . In this case, the release film is optimized for image shape to help with stripping at the corners of the media, which enables stripping in the image area as well. Controlling the controller  80  to conform to the image as shown in  FIG. 6  is not limited to situations where the minimum width of the release film based on the stiffness or thickness of the media exceeds the beginning of the image content. Ejecting the release film fluid to conform to the image is equally applicable to other printing scenarios, including that shown in  FIG. 5 , where the minimum width  170  does not exceed the image content. That is to say the controller  80  can cause applicator  140  to eject release film to conform around the lead edge of the image content shown in  FIG. 5  as well. 
     Printer  10  transfers and fixes the image or images from the blanket surface to media in the printer  10  with controller  80  operating actuators to drive one or more of the rollers to move the media sheet  49  to a position adjacent the transfix roller  19  and then through the transfix nip  18  between the transfix roller  19  and the blanket  21 . The transfix roller  19  applies pressure against the back side of the recording media  49  in order to press the front side of the recording media  49  against the blanket  21  and the receiving member  12 . The pressure produced by the transfix roller  19  on the back side of the heated media sheet  49  facilitates the transfixing (transfer and fusing) of the image from the receiving member  12  onto the media sheet  49 . 
     The rotation or rolling of both the receiving member  12  and transfix roller  19  not only transfixes the images onto the media sheet  49 , but also assists in transporting the media sheet  49  through the nip. The receiving member  12  continues to rotate to continue the transfix process for the images previously applied to the coating and blanket  21 . 
     A method of printer operation that monitors the application of a coating to a rotating surface is shown in  FIG. 7 . In the description of the method, a statement that the process is performing some function refers to a processor or controller executing programmed instructions stored in a memory operatively connected to the processor or controller to operate one or more printer components to perform the function. In the process, a first coating material is applied to the surface of the rotating member (block  904 ). The coating material can be applied either by a contact applicator, such as a roller, or by a liquid drop or dry particle applicator as described above. After the first coating is applied, firing signals are delivered to the printheads to eject aqueous ink onto the layer of first coating material on the surface of the rotating member to form an aqueous ink image on the layer of first coating material on the surface of the rotating member surface (block  908 ). After the aqueous ink image is formed, a second coating material is applied to the layer of first coating material on the surface of the rotating member after the aqueous ink image is formed on the surface of the rotating member by the at least one printhead (block  912 ). A transfer roller moves into engagement with the rotating member to form a nip with the rotating member as the second material on the portion of the surface of the first material approaches the nip and as media approaches the nip to enable a leading edge of the media to engage the second material on the portion of the surface of the first material in the nip and facilitate separation of the leading edge of the media from the first material on the surface of the rotating member after the media exits the nip (block  916 ). 
     It will be appreciated that variations of the above-disclosed apparatus 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.