Patent Publication Number: US-10768554-B2

Title: Maintenance program for liquid electro-photographic printing processes

Description:
BACKGROUND 
     Liquid electro-photographic (LEP) printing, sometimes also referred to as liquid electrostatic printing, uses liquid toner to form images on paper, foil, or another print medium. The liquid toner, which is also referred to as electro-ink, includes charged pigmented particles, and a printing process is controlled on the basis of electrophoresis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description refers to the drawings in which like reference numerals refer to like parts throughout the various views, unless otherwise specified: 
         FIG. 1  is a schematic cross-sectional view of an LEP printing system in production mode, according to an example; 
         FIG. 2  illustrates a printing phenomenon which is to be mitigated by switching the system from production mode to maintenance mode; 
         FIG. 3  is a schematic cross-sectional view of the LEP printing system in maintenance mode, according to an example; 
         FIG. 4  is a flow-chart of a process of operating the LEP printing system, according to an example; and 
         FIG. 5  is a flow-chart of a process of determining a condition upon which a LEP printing system is to switch from production mode to maintenance mode, according to an example. 
     
    
    
     DETAILED DESCRIPTION 
     A LEP printing process may involve selectively charging/discharging areas on a photoconductor, also referred to as photo imaging plate, PIP, to produce a latent electrostatic image. For example, the PIP may be uniformly charged and selectively exposed to light to dissipate the charge accumulated on the exposed areas of the photoconductor in a pixel pattern. The resulting latent image on the photoconductor may then be developed by applying a thin layer of charged particles to the photoconductor. 
     The charged particles may adhere to the discharged areas on the photoconductor (discharged area development DAD) or to the charged areas on the photoconductor (charged area development CAD), depending on the charge of the particles and the charge accumulated on the PIP surface. The image on the PIP formed by the charged pigmented particles adhering to the PIP may then be transferred to a charged and heated intermediate transfer member, ITM, which fuses the charged particles and transfers the image to the print medium. 
       FIG. 1  is a schematic cross-sectional view of an LEP printing system  10  in production mode, according to an example. The system  10  may comprise a binary ink developer, BID, assembly  12  which, during operation, feeds charged pigmented particles which may be suspended in a non-conductive carrier liquid, e.g., an iso-paraffinic imaging oil, from an ink reservoir  14  to a PIP  16 . The carrier fluid may be a fluid in which polymers, particles, colorant, charge directors and other additives can be dispersed to form a liquid electrostatic ink or electrophotographic ink. While  FIG. 1  shows a single BID assembly  12 , the system  10  may also comprise a plurality of BID assemblies  12 , e.g., one per process color, which in turn feed charged pigmented particles from respective ink reservoirs  14  to the PIP  16 . 
     When the BID assembly  12  feeds charged pigmented particles to the PIP  16 , a developer roller  18  of the BID assembly  12  may be in mechanical contact with the PIP  16 . When the BID assembly  12  is to be disengaged from the PIP  16 , the developer roller  18  or the whole BID assembly  12  may be moved away from the PIP  16  to a standby position. As shown in  FIG. 3 , the developer roller  18  may have lost mechanical contact to the PIP  16  when in the standby position. Nevertheless, the developer roller  18  may continue to rotate in the standby position, unless the system  10  is halted. 
     Continuing with the example shown in  FIG. 1 , the developer roller  18  may, during operation, receive charged pigmented particles from the ink reservoir  14  via an ink supply path  20 . In order to form a dense particle layer on the developer roller  18 , the BID assembly  12  may comprise an electrode arrangement  22 , including a main electrode and the back electrode, defining the ink supply path  20 , wherein ink from the ink supply path  20  passes through a gap between the electrode arrangement  22  and the developer roller  18 , at the end of the electrode arrangement  22  facing the developer roller  18 . The gap may have a width perpendicular to a flow direction of the ink, wherein the width may span a nominal printing width of the system  10  and substantially correspond to the width of the developer roller  18  (in the direction of its rotational axis). 
     If the electrode arrangement  22  is charged to a different voltage than the developer roller  18 , an electric field may be generated which may be directed in a radial direction towards the developer roller  18 . The electric field may draw the charged pigmented particles to the surface of the developer roller  18 . Furthermore, the BID assembly  12  may comprise a squeegee roller  24 . The squeegee roller  24  may exert mechanical and electrostatic forces onto the charged pigmented particles adhering to the surface of the developer roller  18  when the particles pass the nip between the squeegee roller  24  and the developer roller  18 . The squeegee roller  24  may be charged to a different voltage than the developer roller  18  to increase a density of the charged pigmented particles layer on the developer roller  18  by exerting also the electrostatic forces. 
     After transferring charged pigmented particles onto the surface of the PIP  16 , the remaining charged pigmented particles may be removed from the developer roller  18  by a cleaning device  26 . The cleaning device  26  may comprise a cleaner roller which may be electrically charged and remove the remaining charged pigmented particles from the developer roller  18 . A wiper blade and a sponge roller may be used to remove the charged pigmented particles from the cleaner roller and remix the removed charged pigmented particles with carrier liquid fed from the ink supply path  20 . 
     The PIP  16  may comprise a thin film of photoconductive material wrapped around a cylindrical surface of a rotating drum. In another example, a photoconductive film may be provided on a belt or platen which is movable relative to the BID assembly  12 . During operation, electrostatic charge may be uniformly applied to the surface of the photoconductive material, as the photoconductive material passes by a charging station  28 . To charge the surface of the photoconductive material, the charging station  28  may, for example, comprise a scorotron, a charge roller, or another charging device. 
     The uniformly charged surface of the photoconductive material may be guided past a discharging station  30  which is capable of selectively discharging the surface. The discharging station  30  may, for example, selectively expose the surface of the photoconductive material to light. As a result, the charge on the exposed areas may dissipate, which allows to produce a “digital” pattern which may comprise discharged areas adjacent to charged areas. The discharged areas and charged areas may correspond to a pixel pattern of an image to be printed. 
     For instance, the surface of the photoconductive material may be selectively discharged by a laser enabled to scan over the surface, or another suitable photo imaging device. Hence, the surface of the photoconductive material passing by the discharging station  30  may be divided into charged and discharged areas, wherein a voltage difference between the charged and the discharged areas may, for example, be more than 200 V, more than 400 V, or more than 600 V, or in the range of 200 V to 1000 V. 
     The latent image on the PIP  16 , formed on the surface areas having passed the discharging station  30 , may then be developed by transferring charged pigmented particles onto the PIP  16 . In the case of DAD, the charged pigmented particles may adhere to the discharged areas of the PIP  16  while being repelled from the charged areas of the PIP  16 . In the case of CAD, the charged pigmented particles may adhere to the charged areas of PIP  16  while being repelled from the discharged areas of the PIP  16 . 
     In either case, a pattern of charged pigmented particles in a layer of uniform particle concentration may be selectively formed on areas on a surface of the PIP  16 . The residual charge may then be removed from the PIP  16 , e.g., by passing the surface of the PIP  16  past a discharging device  32  e.g., an LED lamp, which exposes the PIP  16  surface to light. While the discharging station  30  may selectively discharge the surface of the PIP  16 , the discharging device  32  may expose to light the whole surface of the PIP  16  passing by the discharging device  32 . 
     The charged pigmented particles may then be transferred to the ITM  34 , wherein the BID  12  is moved away from the PIP  16  during transfer. As shown in  FIG. 1 , the ITM  34  may, for example, comprise a chargeable blanket wrapped around a rotating drum. In another example, the chargeable blanket may be provided on a belt or platen which is movable relative to the PIP  16 . The surface of the blanket may be formed of a layer of silicone rubber comprising, for example, polydimethylsiloxane (PDMS). The blanket may be heated to fuse the charged pigmented particles adhering to the ITM  34 . The resulting pigmented layer may be transferred from the ITM  34  to a print medium or print target  36 . 
     The print medium  36 , which may be paper, foil, or any other medium, may be delivered to the system  10  as a continuous web, e.g., dispensed from a roll, or as individual sheets and pass through a nip between the ITM  34  and a pressure roller  38 . The pressure roller  38 , which is also referred to as an impression cylinder (IMP), may press the print medium  36  in the nip against the pigmented layer on the ITM  34  surface, such that the pigmented layer may be cooled down and stick to the print medium  36 . 
     After transferring the pigmented layer onto the ITM  34 , ink residue may be removed from surface areas of the PIP  16  which pass by a cleaning station  40 . For example, the cleaning station  40  may comprise a cleaning roller and a wiper blade. After passing the cleaning station  40 , a uniform electrostatic charge may be re-applied to the PIP  16  surface passing by the charging station  28 , to start a new cycle. In each cycle, a process color may be printed by transferring charged pigmented particles of the respective process color onto the PIP  16 . 
     If an image is printed from more than a single process color, multiple pigmented layers having different colors may be transferred one after the other to the ITM  34 . The ITM  34  may collect the pigmented layers and transfer the full image onto the print medium  36 , or the color layers may be transferred one after the other onto the print medium  36 . In the first case, the pressure roller  38  may be engaged with the ITM  34  after the pigmented layers are collected on the ITM  34 . 
     The operation of the system  10  may be controlled by a control unit  42 . The control unit  42  may comprise machine-readable instructions stored on a machine-readable medium  44  and a processor  46 . The processor  46  may be communicatively connected to a controller which controls a movement of the PIP  16  and the ITM  34 . For instance, the controller may control an electric motor mechanically connected to a drive shaft of the PIP  16  and/or the ITM  34  to rotate the PIP  16  and the ITM  34  at the same surface speed. 
     The processor  46  may also be communicatively connected to a controller which controls a movement of the pressure roller  38 . For example, the controller may control an electric motor mechanically connected to a drive shaft of the pressure roller  38  to rotate the pressure roller  38  at a same surface speed as the ITM  34 . The controller may also control a linear drive which allows engaging/disengaging the pressure roller  38  and the ITM  34 . 
     The processor  46  may also be communicatively connected to a controller which controls the operation of the BID assembly  12 . For instance, the controller may control an electric motor mechanically connected to a drive shaft of the developer roller  18  to rotate the developer roller  18  at a same surface speed as the PIP  16 . The controller may also control a linear drive which allows engaging/disengaging the developer roller  18  and the PIP  16 . 
     The control unit  42  may also comprise a human-machine interface  48  allowing a user to start and halt the system  10 , as well as manually initiating or scheduling a maintenance program during which the system  10  operates in a maintenance mode, wherein the scheduled maintenance program may be automatically initiated if a specific condition is met. For example, the condition may effect that the maintenance program is to be initiated after a predefined number of printed images, or if a maintenance score meets a threshold. 
     The maintenance score may be a function of the number of printed images as well as other factors derived from analyzing the printed images. For example, printing an image which is evenly black may allow for lowering a maintenance frequency as compared to an image having multiple colors and small text, both, in terms of having to cope with uneven wear due to printing an uneven image, as well as meeting a desired print quality. 
     The control unit  42  may also comprise a monitoring interface which allows feeding the control unit  42  with parameters or even pictures of printed images. The control unit  42  may analyze the provided data and strive to detect print quality issues. If print quality issues are detected, a maintenance program may be initiated. For instance, the control unit may select a maintenance program to mitigate a detected print quality defect. 
     As some print quality defects may be more easily detected in some images than others, the control unit  42  may also print a (single) test image or a small number (e.g., less than 100 or less than 10) of different test images. The test image may be used to verify a print defect detected in a printed image of a regular printing task, or to reveal up-coming print defects before they become detectable in printed images of a regular printing task, which would otherwise possibly have to be discarded. 
     Thus, in addition to controlling the production mode, the control unit  42  may schedule maintenance operations which are to be carried out in the maintenance mode, wherein the maintenance operations may be directed at avoiding or mitigating print defects as discussed in more detail with reference to  FIGS. 2 and 3 . 
       FIG. 2  illustrates circumstances under which a print defect may come up, as well as the printing defect itself.  FIG. 2  shows an image  50  to be printed as well as the printed image  52 . Notably, the printed image  52  comprises an area  54  of small (or even zero) pigment density and an area  56  of high pigment density. In this regard, it is to be noted that the area  56  of high pigment density may relate to pigments of a same color as well as pigments of different colors. 
     If, after repeated printing cycles of the same image  50  on the same area of the blanket, a new printing task with another image is printed such as, for example, a gray image  58 , a negative dot gain (NDG) “ghost memory” of the previously printed image  52  may be observed in the printed images  60  of the new printing task. A NDG “ghost memory” may manifest itself in subsequent printings by producing ghost images with decreased optical density or dot size and brighter visual appearance, as compared to the background, depending on the image  50  which caused the NDG “ghost memory”. 
     Without being bound to any theory, a possible explanation of the generation of the NDG “ghost memory” might be that, after a large number of printing cycles of the same image, hidden charged and uncharged spots remain below the surface of the ITM, or there is some other physical or chemical effect within the ITM surface layer caused by the preceding printing cycles. The repetitive printing of the same image  50  also may affect the transfer of charged pigmented particles between the PIP  16  and the ITM  34 , e.g because of absorption of carrier liquid by the blanket, which may be uneven over the blanket surface. For instance, the printed image  52  having an area  54  of small (or even zero) pigment density and an area  56  of high pigment density may lead to an uneven swelling of the blanket. 
     To overcome or mitigate the NDG “ghost memory” effect and related printing defects, the control unit  42  may initiate a maintenance program. The maintenance program may comprise driving or moving the PIP  16  and the ITM  34  while applying a voltage difference between the PIP  16  and surface portions of the ITM  34 , as illustrated in  FIG. 3 . During maintenance, selective discharge of the PIP  16  may be disabled or halted. The maintenance program may also comprise disabling or halting particle transfer to the PIP  16 . Hence, the BID assembly  12  may be disengaged from the PIP  16 . Further, the cleaning station  40  may be disengaged from the PIP  16 . Similarly, the pressure roller  38  may be disengaged from the ITM  34 . When the pressure roller  38  is disengaged from the ITM  34 , the pressure roller  38  may be halted. Also, delivering the print medium  36  to the system  10  may be halted. 
     The voltage difference between the PIP  16  and the blanket of the ITM  34  may be larger than a nominal operation voltage difference. For instance, the voltage difference between the PIP  16  and the blanket of the ITM  34  may be about 3 to about 4 times a nominal operation voltage difference. For example, the voltage difference may be above 1950V with a voltage of about −1000 V to about −1200 V being applied to the PIP  16  and a bias voltage of about +950 V being applied to the blanket of the ITM  34 . These values, which are indicated for illustration purposes, are assumed for a configuration where, during printing, the PIP  16  is discharged after moving away from the BID assembly  12  and before contacting the ITM  34  and is at a low voltage, e.g. below 60 V, or close to zero voltage, and a nominal bias voltage of the ITM  34  is about +550 V, resulting in a nominal printing operation voltage difference of about 550 V to 600 V, for example. 
     Accordingly, the charging station  28  may be operated at a higher charging level as compared to a nominal charging level applied during productive printing, i.e., when the system  10  is in production mode as described with reference to  FIG. 1 , while the operation of the discharging station  30  and the discharging device  32  may be disabled or halted. Also, the bias voltage of the blanket of the ITM  34  may be increased as compared to a nominal bias voltage applied during productive printing. Hence, the voltage difference between the PIP  16  and the blanket of the ITM  34  may be higher during maintenance mode, than any voltage differences between surface portions of the PIP  16  and the blanket of the ITM  34  during production mode. For example, the PIP  16  and the ITM  34  may be charged to a maximum admissible voltage value. 
     Applying the high voltage to the blanket of the ITM  34  may reduce or eliminate the NDG “ghost memory” effect and related printing defects. The high voltage may be applied during a null cycle, e.g. at a substantially normal process speed of the ITM  34  without image transfer. 
     The maintenance program may be carried out for a predetermined time, e.g., about 0.5 to 2.5 or 0.5 to 3 minutes, or for about 1 minute, while the exact time amount may vary and be chosen on the basis of such factors as severity of a printing defect and/or blanket age. The maintenance program may further comprise a predetermined number of full cycles (rotations) of the PIP  16  and the ITM  34 . The maintenance program may be repeated in regular or irregular intervals. For example, the maintenance program may be initiated every fixed number of prints, such as after every 1000 prints or integer multiples thereof, such as 2000 prints, 3000 prints, 4000 prints, etc. or every 10,000 prints, 11,000 prints, 12,000 prints, etc. The maintenance program may be initiated as a matter of precaution before a printing defect is observed or after detecting that a printing defect occurred. A printing defect can be detected during a productive printing operation or by printing a test image. 
     During the maintenance program, the surface speed of the PIP  16  and the ITM  34  may be substantially equal to a nominal printing speed. Yet, the surface speed of the PIP  16  and the ITM  34  may also be substantially lower than a nominal printing speed. For instance, the surface speed of the PIP  16  and the ITM  34  may be below half or below one tenth of the nominal printing speed. 
     Also, a test image or a production image may be printed at the end of the maintenance program and the maintenance program may be resumed based on an analysis of said image. For example, the maintenance program may be resumed if the printing defect persists, or if the printing quality is found to be not yet acceptable. Also, a voltage difference between the PIP  16  and the blanket of the ITM  34  may be increased between successive initiations of the maintenance program, or on demand, if the maintenance program is resumed (which may be performed in case that a printing defect persists). 
     The voltage difference between the PIP  16  and the blanket of the ITM  34  may be kept below a voltage threshold at which any chemical, electrical, or physical effect or a combination of such effects may adversely affect blanket characteristics of the ITM  34 . 
     In yet another example, the discharging station  30  may be operated to establish a latent image which is substantially inverse to the latent image that caused the printing defect. Accordingly, the higher voltage difference may be applied to the surface portions of the blanket which exhibit the NDG “ghost memory” effect. In this case, the transfer of charged particles to the PIP  16  may be resumed and the inverse image may be printed. The printed inverse images may be classified as “maintenance prints” and exhibit a print quality below a print quality achieved during normal production mode, if the voltage difference between the PIP  16  and the blanket of the ITM  34  is increased above the nominal voltage difference. 
       FIG. 4  is a flow-chart of a process of operating the LEP printing system  10 , according to an example. The process comprises at  62  by transferring, during the production mode, layers of charged pigmented particles form the PIP  16  to the ITM  34 . At  64 , the process applies, during maintenance mode, a voltage difference between the PIP  16  and the blanket of the ITM  34 . 
       FIG. 5  is a flow-chart of a process of determining the condition upon which the system  10  is to switch from production mode to maintenance mode, according to an example. At  66 , the process comprises detecting a predetermined number of prints of a same image or a memory effect of the blanket. At  68 , the process switches to the maintenance mode.