Patent Publication Number: US-9405237-B2

Title: Transfer station for a liquid toner printing system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This patent application claims the benefit of German Patent Application No. 102014117493.3, filed Nov. 28, 2014, which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     The exemplary embodiments described herein generally relate to a digital printer for printing to a recording medium with toner particles that are applied with the aid of a liquid developer, including high-capacity printing systems for printing to web-shaped or sheet-shaped recording media. 
     Given such digital printers, a latent charge image of a charge image carrier is inked by means of electrophoresis, with the aid of a liquid developer. The toner image that is created in such a manner is indirectly transferred to the recording medium via a transfer element (i.e. via a transfer station). The print image is subsequently fixed on the recording medium. In the transfer step, an electrical field is used in order to transfer the toner image to the recording medium. 
     In numerous use cases, in particular in printing of packaging, recording media with a significant thickness (for example paper grammages of up to 450 g/m 2 ) are used. Given use of such thick recording media, the degree of transfer efficiency—and therefore the print quality—typically decreases. 
     SUMMARY 
     It is an object to provide a printing system having an efficiently and reliably increased transfer efficiency of the transfer station of the printing system, where the transfer station transfers a toner image to a recording medium by means of electrophoresis. 
     In a method or a transfer station for a digital printer, liquid developer with toner particles can be directed towards a first side of a recording medium and onto the first side of the recording medium at a transfer point. A transfer roller can be configured to direct the liquid developer towards the recording medium. The toner particles can have a first polarity and the recording medium can move past the transfer roller along a transport direction. A layer of charged particles can be applied from a second side of the recording medium by a surface charging station. The surface charging station can be arranged before the transfer point in the transport direction. The second side of the recording medium can be opposite the first side. The charged particles can have a second polarity that is opposite the first polarity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the embodiments of the present disclosure and, together with the description, further serve to explain the principles of the embodiments and to enable a person skilled in the pertinent art to make and use the embodiments. 
         FIG. 1  illustrates a printer according to an exemplary embodiment of the present disclosure. 
         FIG. 2  illustrates a schematic design of a print head of the printer of  FIG. 1 . 
         FIG. 3  illustrates a transfer station according to an exemplary embodiment of the present disclosure. 
         FIG. 4  illustrates a method for increasing the toner transfer efficiency of a transfer station according to an exemplary embodiment of the present disclosure. 
     
    
    
     The exemplary embodiments of the present disclosure will be described with reference to the accompanying drawings. 
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the embodiments of the present disclosure. However, it will be apparent to those skilled in the art that the embodiments, including structures, systems, and methods, may be practiced without these specific details. The description and representation herein are the common means used by those experienced or skilled in the art to most effectively convey the substance of their work to others skilled in the art. In other instances, well-known methods, procedures, components, and circuitry have not been described in detail to avoid unnecessarily obscuring embodiments of the disclosure. 
     According to an exemplary embodiment, a transfer station for a digital printer is described. The transfer station comprises a transfer roller that is configured to direct liquid developer with toner particles towards a first side of a recording medium and—at a transfer point—onto the first side of the recording medium. The toner particles have a first polarity. The recording medium moves past the transfer roller along a transport direction. The transfer station additionally comprises a surface charging station arranged before the transfer point (in the transport direction), which surface charging station is configured to apply a (charged) layer of charged particles from a second side of the recording medium. The second side is opposite the first side. Furthermore, the charged particles have a second polarity that is opposite the first polarity. 
     According to an additional aspect, a print group is described that comprises the transfer station described in this document. Moreover, a digital printer is described that comprises at least one such print group. 
     According to an additional aspect, a method is described for increasing the toner transfer efficiency of a transfer station. The method includes the application of a (charged) layer of charged particles from a second side of a recording medium. The charged particles have a second polarity. Furthermore, the method includes the transfer of toner particles from a liquid developer onto a first side of the recording medium that is opposite the second side. The toner particles have a first polarity that is opposite the second polarity. 
     According to  FIG. 1 , a digital printer  10  for printing to a recording medium  20  has one or more print groups  11   a - 11   d  and  12   a - 12   d  that print a toner image (print image  20 ′; see  FIG. 2 ) onto the recording medium  20 . As shown, a web-shaped recording medium  20  as a recording medium  20  is unspooled from a roll  21  with the aid of a take-off  22  and is supplied to the first print group  11   a . The print image  20 ′ is fixed on the recording medium  20  in a fixer  30 . The recording medium  20  may subsequently be taken up on a roll  28  with the aid of a take-up  27 . Such a configuration is also known as a roll-to-roll printer. 
     In the configuration shown in  FIG. 1 , the web-shaped recording medium  20  is printed to in full color on the front side with four print groups  11   a  through  11   d , and on the back side with four print groups  12   a  through  12   d  (what is known as a 4/4 configuration). For this, the recording medium  20  is unwound from the roll  21  by the take-off  22  and supplied to the first print group  11   a  via an optional conditioning group  23 . In the conditioning group  23 , the recording medium  20  may be pre-treated or coated with a suitable substance. Wax, or chemically equivalent substances, may preferably be used as a coating substance (also known as a primer). 
     The coating substance may be applied over the entire area or only at the points of the recording medium  20  that are to be printed to later, in order to prepare the recording medium  20  for printing and/or in order to affect the absorption behavior of the recording medium  20  upon application of the print image  20 ′. It is therefore prevented that the toner particles or the carrier fluid that are applied later do not penetrate too far into the recording medium  20 , but rather remain essentially at the surface (whereby the color quality and image quality are improved). 
     The recording medium  20  is subsequently supplied first, in order, to the first print groups  11   a  through  11   d , in which only the front side is printed to. Each print group  11   a - 11   d  typically prints to the recording medium  20  in a different color, or also with a different toner material (for example MICR toner which can be read electromagnetically). 
     After printing to the front side, the recording medium  20  may be turned in a turner  24  and be supplied to additional print groups  12   a - 12   d  for printing to the back side. In the region of the turner  24 , an additional conditioning group (not shown in  FIG. 1 ) may be arranged via which the recording medium  20  is prepared for the printing to the back side, for example an intermediate fixer (described in this document) to fix (at least partially fix) the previously printed front-side print image. It is thus prevented that the front-side print image is mechanically damaged upon further transport through the subsequent print groups. 
     In order to achieve a full-color printing, at least four colors (and therefore at least four print groups  11 ,  12 ) are required, and in fact the primary colors YMCK (Yellow, Magenta, Cyan and Black), for example. Still more print groups  11 ,  12  with special colors (for example customer-specific colors or additional primary colors in order to expand the printable color space) may also be used. 
     Arranged after the print group  12   d  is a register  25  via which registration marks—which are printed on the recording medium  20  independently of the print image  20 ′ (in particular outside of the print image  20 ′)—are evaluated. The transversal and longitudinal registration (the primary color dots that form a color point should be arranged atop one another or spatially very close to one another; this is also known as color registration or four-color registration) and the registration (front side and back side must spatially coincide precisely) can therefore be adjusted so that a qualitatively good print image  20 ′ is achieved. 
     Arranged after the register  25  is the fixer  30  via which the print image  20 ′ is fixed on the recording medium  20 . Given electrophoretic digital printers, a thermal dryer is preferably used that largely evaporates the carrier fluid so that only the toner particles still remain on the recording medium  20 . Thus occurs under the effect of heat. The toner particles may thereby also be fused onto the recording medium  20  insofar as they have a material (resin, for example) that can melt as a result of the effect of heat. 
     Arranged after the fixer  30  is a puller  26  that pulls the recording medium  20  through all print groups  11   a - 12   d  and the fixer  30  without an additional drive being arranged in this region. The danger that the as of yet unfixed print image  20 ′ could be smeared would exist due to a friction drive for the recording medium  20 . 
     The puller  26  feeds the recording medium  20  to the take-up  27 , which rolls up the printed recording medium  20 . 
     Centrally arranged in the print groups  11 ,  12  and the fixer  30  are all supply devices for the digital printer  10 , such as climate controller  40  (e.g., air-conditioner), power supply  50 , controller  60 , fluid management system  70  (such as fluid controller  71  and reservoirs  72  of the different fluids). In particular, pure carrier fluid, highly-concentrated liquid developer (high proportion of toner particles in relation to carrier fluid) and serum (liquid developer plus charge control substances) are required as fluids in order to supply the digital printer  10 , as well as waste containers for fluids to be disposed of or containers for cleaning fluid. In one or more exemplary embodiments, the climate controller  40 , power supply  50 , controller  60 , and/or fluid management system  70  can include processor circuitry configured to perform their corresponding functions. 
     The digital printer  10 , with its structurally identical print groups  11 ,  12 , is of modular design. The print groups  11 ,  12  do not differ mechanically, but rather only due to the liquid developer (toner color or toner type) used therein. 
     The principle design of a print group  11 ,  12  is shown in  FIG. 2 . Such a print group is based on the electrophotographic principle, in which a photoelectric image carrier is inked with charged toner particles with the aid of a liquid developer, and the image that is created in such a manner is transferred to the recording medium  20 . 
     The print group  11 ,  12  is essentially comprised of an electrophotography station  100 , a developer station  110  and a transfer station  120 . 
     The core of the electrophotography station  100  is a photoelectric image carrier that has a photoelectric layer (what is known as a photoconductor) on its surface. The photoconductor here is designed as a roller (photoconductor roller  101 ) and has a hard surface. The photoconductor roller  101  rotates past the various elements to generate a print image  20 ′ (rotation in the arrow direction). 
     The photoconductor  101  is initially cleaned of all contaminants. For this, an erasure light  102  is present that erases charges that still remain on the surface of the photoconductor  101 . The erasure light can be calibrated (is locally adjustable) in order to achieve a homogeneous light distribution. The surface may therefore be pre-treated uniformly. 
     After the erasure light  102 , a cleaner  103  mechanically cleans off the photoconductor  101  in order to remove toner particles that are possibly still present on the surface of the photoconductor  101 , possible dirt particles and remaining carrier fluid. The cleaned-off carrier fluid is supplied to a collection container  105 . The collected carrier fluid and toner particles are prepared (possibly filtered) and fed—depending on color—to a corresponding liquid color reservoir, i.e. to one of the storage containers  72  (see arrow  105 ′). 
     The cleaner  103  preferably has a blade  104  that rests on the surface shell of the photoconductor roller  101  at an acute angle (approximately 10° to 80° relative to the outflow surface) in order to mechanically clean off the surface. The blade  104  may move back and forth, transversal to the rotation direction of the photoconductor roller  101 , in order to optimally clean the surface shell along the entire axial length with as little wear as possible. 
     The photoconductor  101  is subsequently charged by a charger  106  to a predetermined electrostatic potential. For this, multiple corotrons (in particular glass shell corotrons) are preferably present. The corotrons are comprised of at least one wire  106 ′ at which a high electrical voltage is present. The air around the wire  106 ′ is ionized by the voltage. A shield  106 ″ is present as a counter-electrode. The corotrons are additionally flushed with fresh air that is supplied via special air channels (air feed channel  106  for aeration and exhaust channel  108  for ventilation) between the shields (see also the air flow arrows in  FIG. 2 ). The supplied air is then uniformly ionized at the wire  106 ′. A homogeneous, uniform charging of the adjacent surface of the photoconductor  101  is thereby achieved. The uniform charging is further improved with dry and heated air. Air is discharged via the exhaust channels  108 . Ozone that is possibly created may likewise be drawn away via the exhaust channels  108 . 
     The corotrons can be cascaded, meaning that then two or more wires  106 ′ are present per shield  106 ″ given the same shielding voltage. The current that flows across the shield  106 ″ is adjustable, and the charge of the photoconductor  101  is thereby controllable. The corotrons may be fed with currents of different strengths in order to achieve a uniform and sufficiently high charge at the photoconductor  101 . 
     Arranged after the charger  106  is a character generator  109  that, via optical radiation, discharges the photoconductor  101  per pixel depending on the desired print image  20 ′. A latent image is thereby created that is later inked with toner particles (the inked image corresponds to the print image  20 ′). A light emitting diode (LED) character generator  109  is preferably used in which an LED line with many individual LEDs is arranged stationary over the entire axial length of the photoconductor roller  101 . The number of LEDs and the size of the optical mapping points on the photoconductor  101  determine (among other things) the resolution of the print image  20 ′ (e.g., resolution of 600×600 dpi). The LEDs may be controlled with individual timing and with regard to their radiation power. To generate raster points (comprised of multiple image points or pixels), multi-level methods may thus be applied or image points may be chronologically delayed in order to electro-optically implement corrections, for example given an incorrect color registration or register. 
     The character generator  109  has a control logic that must be cooled due to the plurality of LEDs and their radiation power. The character generator  109  is preferably liquid-cooled. The LEDs may be activated in groups (multiple LEDs combined into a group) or separately from one another. 
     The latent image generated by the character generator  109  is inked with toner particles by the developer station  110 . The developer station  110  has for this a rotating developer roller  111  that directs a layer of liquid developer towards the photoconductor  101  (the functionality of the developer station  110  will be explained in detail further below). Since the surface of the photoconductor roller  101  is relatively hard, the surface of the developer roller  111  is relatively soft, and if the two are pressed against one another a thin, high nip (a gap between the rollers) is created in which the charged toner particles migrate electrophoretically from the developer roller  111  onto the photoconductor  101  at the image points, due to an electrical field. No toner transfers to the photoconductor at the non-image points. The nip filled with liquid developer has a height (width of the gap) that is dependent on the mutual pressure of the two rollers  101 ,  111  and the viscosity of the liquid developer. The height of the nip is typically in a range of greater than approximately 2 μm up to approximately 20 μm (the values may also change depending on viscosity of the liquid developer). The length of the nip is approximately a few millimeters. 
     The inked image rotates with the photoconductor roller  111  up to a first transfer point at which the inked image is essentially completely transferred to a transfer roller  121 . At the first transfer point (nip between photoconductor roller  101  and transfer roller  121 ), the transfer roller  121  moves in the same direction as the photoconductor  101  and preferably at an identical speed. After the transfer of the print image  20 ′ to the transfer roller  121 , the print image  20 ′ (toner particles) may optionally be recharged or charged with the original polarity by means of a charger  129  (a corotron, for example) in order to be able to subsequently better transfer the toner particles to the recording medium  20 . 
     The recording medium  20  travels through between the transfer roller  121  and a counter-pressure roller  126 , in the transport direction  20 ″. The contact region (nip) represents a second transfer point at which the toner image is transferred to the recording medium  20 . In the second transfer region, the transfer roller  121  moves in the same direction as the recording medium  20 . The counter-pressure roller  126  also rotates in this direction in the region of the nip. The velocities of the transfer roller  121 , the counter-pressure roller  126  and the recording medium  20  are matched to one another at the transfer point and are preferably identical so that the print image  20 ′ is not smeared. At the second transfer point, the print image  20 ′ may be electrophoretically transferred onto the recording medium  20  due to an electrical field between the transfer roller  121  and the counter-pressure roller  126 . Moreover, the counter-pressure roller  126  typically presses against the relatively soft transfer roller  121  with a large mechanical force, whereby the toner particles may also remain stuck to the recording medium  20  due to the adhesion. 
     Since the surface of the transfer roller  121  is relatively soft and the surface of the counter-pressure roller  126  is relatively hard, upon rolling a nip is created in which the toner transfer occurs. Unevenness of the recording medium  20  may therefore be compensated so that the recording medium  20  may be printed to without gaps. Such a nip is also well suited in order to print to thicker or more uneven recording media  20 , for example as is the case given printing of packaging. 
     The print image  20 ′ should in fact transfer completely to the recording medium  20 ; nevertheless, a few toner particles may undesirably remain on the transfer roller  121 . A portion of the carrier fluid always remains on the transfer roller  121  as a result of the wetting. The toner particles that are possibly still present should be nearly completely removed by a second cleaner  122  following the second transfer point. The carrier fluid still located on the transfer roller  121  may also be completely removed from the transfer roller  121 , or removed up to a predetermined layer thickness so that, after the cleaner  122  and before the first transfer point from the photoconductor roller  101  to the transfer roller  121 , the same conditions prevail due to a clean surface or a defined layer thickness with liquid developer on the surface of the transfer roller  121 . 
     This cleaner  122  is preferably designed as a wet chamber with a cleaning brush  123  and a cleaning roller  124 . In the region of the brush  123 , cleaning fluid (for example, carrier fluid or a separate cleaning fluid may be used) is supplied via a cleaning fluid feed  123 ′. The cleaning brush  123  rotates in the cleaning fluid and thereby “brushes” the surface of the transfer roller  121 . The toner adhering to the surface is thereby loosened. 
     The cleaning roller  124  is at an electrical potential that is opposite the charge of the toner particles. As a result of this, the electrically charged toner is removed from the transfer roller  121  by the cleaning roller  124 . Since the cleaning roller  124  contacts the transfer roller  121 , it also removes carrier fluid (together with the supplied cleaning fluid) remaining on said transfer roller  121 . A conditioning element  125  is arranged at the outflow from the wet chamber. As shown, a retention plate that is arranged at an obtuse angle (for instance between 100° and 175° between plate  125  and outflow surface) relative to the transfer roller  121  may be used as a conditioning element  125 , whereby residues of fluid on the surface of the roller are nearly completely kept back in the wet chamber and supplied to the cleaning roller  124  for removal via a cleaning fluid discharge  124 ′ to a cleaning fluid reservoir (at the reservoirs  72 ) (not shown). 
     Instead of the retention plate  125 , a doser (not shown) may also be arranged there that, for example, has one or more dosing rollers. The dosing rollers have a predetermined clearance from the transfer roller  121  and remove so much carrier fluid that a predetermined layer thickness appears after the dosing roller as a result of the squeezing. The surface of the transfer roller  121  is then not completely cleaned off; carrier fluid of a predetermined layer thickness remains over the entire surface. Removed carrier fluid is directed back via the cleaning roller  124  to the cleaning fluid reservoir. 
     The cleaning roller  124  itself is kept mechanically clean by a blade (not shown). Fluid that is cleaned off, inclusive of toner particles, is captured for all colors by a central capture container, cleaned, and supplied to the central cleaning fluid reservoir for reuse. 
     The counter-pressure roller  126  is likewise cleaned by a cleaner  127 . A blade, a brush and/or a roller may remove contaminants (paper dust, toner particle residues, liquid developer etc.) from the counter-pressure roller  126 . The cleaned fluid is collected in a collection container  128  and provided again to the printing process (possibly after cleaning) via a fluid discharge  128 ′. 
     In the print groups  11  that print to the front side of the recording medium  20 , the counter-pressure roller  126  does not press against the unprinted side (thus the side that is still dry) of the recording medium  20 . 
     Nevertheless, dust/paper particles or other contaminating particles may already be located on the dry side, which particles are then removed by the counter-pressure roller  126 . For this, the counter-pressure roller  126  may be wider than the recording medium  20 . As a result of this, contaminants outside of the print area may also be cleaned off well. 
     In the print groups  12  that print to the back side of the recording medium  20 , the counter-pressure roller  126  presses directly on the as of yet unfixed, damp print image  20 ′ of the front side. So that the print image  20 ′ is not removed by the counter-pressure roller  126 , the surface of the counter-pressure roller  126  may have anti-adhesion properties with regard to toner particles and also with regard to the carrier fluid on the recording medium  20 . 
     The developer station  110  inks the latent print image  20 ′ with a predetermined toner. For this, the developer roller  111  supplies toner particles to the photoconductor. In order to ink the developer roller  111  itself with a layer over its entire surface, liquid developer is initially supplied at a predetermined concentration from a mixing container (not shown; within the fluid controller  71 ) via a fluid feed  112 ′ to a reservoir chamber  112 . From this reservoir chamber  112 , the liquid developer is supplied in abundance to a pre-chamber  113  (a type of pan that is open at the top). An electrode segment  114  is arranged towards the developer roller  111 , which electrode segment  114  forms a gap between itself and said developer roller  111 . 
     The developer roller  111  rotates through the pre-chamber  113  that is open at the top and thereby carries liquid developer along in the gap. Excess liquid developer flows out from the pre-chamber  113  back to the reservoir chamber  112 . 
     Due to the electrical field (formed by the electrical potentials) between the electrode segment  114  and the developer roller  111 , the liquid developer in the gap is divided up into two regions, and in fact into: a layer region in proximity to the developer roller  111 , in which layer region the toner particles concentrate (concentrated liquid developer); and a second region in proximity to the electrode segment  114 , which is low in toner particles (very low concentration liquid developer). 
     The layer of the liquid developer is subsequently transported further to a dosing roller  115 . The dosing roller  115  squeezes out the upper layer of the liquid developer so that afterward a defined layer thickness of liquid developer—of approximately 5 μm thickness—remains on the developer roller  111 . Since the toner particles are essentially located near the surface of the developer roller  111 , in the carrier fluid, the outwardly situated carrier fluid is essentially squeezed out or retained and ultimately is returned back to a collection container  119 , but not to the reservoir chamber  112 . 
     As a result of this, it is predominantly highly concentrated liquid developer that is conveyed through the nip between dosing roller  115  and developer roller  111 . A uniformly thick layer of liquid developer is thus created, with approximately 40 percent by mass toner particles and approximately 60 percent by mass carrier fluid after the dosing roller  115  (the mass ratios may also fluctuate more or less depending on the printing process requirements). This uniform layer of liquid developer is transported in the nip between the developer roller  111  and the photoconductor roller  101 . There the image points of the latent image are then electrophoretically inked with toner particles, while no toner transfers to the photoconductor in the area of non-image points. Sufficient carrier fluid is absolutely necessary for electrophoresis. The fluid layer divides approximately in the middle after the nip as a result of wetting, such that one portion of the layer remains adhered to the surface of the photoconductor roller  101  and the other portion (essentially carrier fluid for image points and toner particles and carrier fluid for non-image points) remains on the developer roller  111 . 
     So that the developer roller  111  may again be coated with liquid developer under the same conditions and uniformly, remaining toner particles (these essentially represent the negative, untransferred print image) and liquid developer are electrostatically and mechanically removed by a cleaning roller  117 . The cleaning roller  117  itself is cleaned by a blade  118 . The cleaned-off liquid developer is supplied to the collection container  119  for reuse, to which collection container  119  the liquid developer cleaned off from the dosing roller  115  (by means of a blade  116 , for example) and the liquid developer cleaned off from the photoconductor roller  101  (by means of the blade  104 ) are also supplied. 
     The liquid developer collected in the collection container  119  is supplied to the mixing container via the fluid discharge  119 ′. Fresh liquid developer and pure carrier fluid are also supplied to the mixing container as needed. Sufficient fluid at the desired concentration (predetermined ratio of toner particles to carrier fluid) must always be present in the mixing container. The concentration in the mixing container is continuously measured and regulated accordingly depending on the supply of the quantity of the cleaned-off liquid developer and its concentration as well as the quantity and concentration of fresh liquid developer or carrier fluid. 
     For this, extremely concentrated liquid developer, pure carrier fluid, serum (carrier fluid and charge control substances in order to control the charge of the toner particles) as well as cleaned-off liquid developer may be supplied separately from the corresponding reservoirs  72  to this mixing container. 
     The photoconductor  101  may preferably be designed in the form of a roller or as a continuous belt. An amorphous silicon may thereby be used as a photoconductor material, or an organic photoconductor material (also known as OPC) may be used. 
     Instead of a photoconductor  101 , other image carriers (such as magnetic, ionizable etc. image carriers) may also be used that do not operate according to the photoelectric principle but rather on which latent images according to other principles are impressed electrically, magnetically or otherwise, which latent images are then inked and ultimately are transferred to the recording medium  20 . 
     LED rows, or also lasers with corresponding scan mechanism, may be used as a character generator  109 . 
     The transfer element  121  may similarly be designed as a roller or as a continuous belt. The transfer element  121  may also be omitted. The print image  20 ′ is then transferred directly from the photoconductor roller  101  to the recording medium  20 . 
     What is to be understood by the term “electrophoresis” is the migration of the charged toner particles in the carrier fluid as a result of the action of an electrical field. In each transfer of toner particles, the corresponding toner particles essentially transfer completely to another element. After the two elements come into contact, the fluid film is split approximately in half as a result of the wetting of the participating elements, such that approximately one half remains adhered to the first element and the remaining portion remains adhered to the other element. The print image  20 ′ is transferred and, in the next part, is then transported further in order to in turn allow an electrophoretic migration of the toner particles in the next transfer region. 
     The digital printer  10  may have one or more print groups  11  for printing to the front side and, if applicable, one or more print groups  12  for printing to the back side. The print groups  11 ,  12  may be arranged in a line, an L-shape or a U-shape. 
     Instead of the take-up  27 , post-processing devices (not shown)—such as cutters, folders, stackers etc.—may also be arranged after the puller  26  in order to bring the recording medium  20  into the final form. For example, the recording medium  20  could be processed to such an extent that a finished book is created in the end. The post-processing devices may likewise be arranged in a row or at an angle. 
     As was previously described as a preferred exemplary embodiment, the digital printer  10  may be operated as a roll-to-roll printer. It is also possible to cut the recording medium  20  into sheets at the end and to subsequently stack the sheets or suitably process them further (roll-to-sheet printer). It is likewise possible to feed a sheet-shaped recording medium  20  to the digital printer  10  and to stack the sheets or process them further at the end (sheet to sheet printer). 
     If only the front side of the recording medium  20  is printed to, at least one print group  11  with one color is required (simplex printing). The at least one print group  11  may also be designated as a simplex print group. If the back side is also printed to, at least one print group  12  is also required for the back side (duplex printing). The at least one print group  12  may also be designated as a duplex print group. Depending on the desired print image  20 ′ on the front side and back side, the printer configuration includes a corresponding number of print groups for front side and back side, wherein each print group  11 ,  12  is always designed for only one color or one type of toner. 
     The maximum number of print groups  11 ,  12  is only technically dependent on the maximum mechanical tensile load of the recording medium  20  and the free draw length. Typically, arbitrary configurations from a 1/0 configuration (only one print group for the front side to be printed to) up to a 6/6 configuration (in which six print groups may respectively be present for front side and back side of the recording medium  20 ) are possible. The preferred embodiment (configuration) is shown in  FIG. 1  (a 4/4 configuration), with which the full-color printing for front side and back side with the four primary colors is accomplished. The order of the print groups  11 ,  12  in a four-color printing preferably goes from a print group  11 ,  12  that prints light (yellow) to a print group  11 ,  12  that prints dark; for example, the recording medium  20  is printed to from light to dark in the color order Y-C-M-K. 
     The recording medium  20  may be manufactured from paper, paperboard, cardboard, metal, plastic and/or other suitable printer materials. 
     As presented above, the present document deals in particular with the technical object of printing to relatively thick recording media  20  reliably and cost-effectively. For the electrophoretic transfer of toner image from the transfer roller  121  onto the recording medium  20 , an electrical field is typically applied between transfer roller  121  and counter-pressure roller  126 , which electrical field draws the (for example positively charged) toner particles in the direction of the recording medium  20 . However, the effective active electrical field is reduced with increasing thickness of the recording medium  20 , whereby the transfer efficiency declines and the print quality decreases. 
     One possibility to compensate for this is to increase the electrical voltage set between the cores of the rollers  121 ,  126  in order to achieve the same effective active electrical field for transfer of the toner in comparison to a thin recording medium  20 . However, the increase of the voltage is typically subject to limitations, for example due to the dielectric strength of the recording medium  20 , due to the dielectric strength of an elastomer layer of the transfer roller  121 , and/or due to the dielectric strength of the ball bearings of the rollers  121 ,  126 . An increase of the voltage is thus possibly only within a limited scope, and thus only for a limited thickness of the recording medium  20 . 
     Alternatively or additionally, the transfer efficiency may be improved by an increase of the positively charged toner particles. However, such an increase typically leads to print quality artifacts such as the blurring of raster points. 
     Alternatively or additionally, the effective active electrical field may be increased, and therefore the transfer to the recording medium  20  may be increased, via a pre-treating of the recording medium  20 . In particular, the recording medium  20  may be pre-treated with a corotron which charges the top side of the recording medium  20  with the polarity matching the toner system (for example negatively given positive toner particles) before the (second) transfer point. 
     Given a single-pass multicolor system (for example given the digital printer  10 ), the surface treatment is only suitable to charge the recording medium  20  once depending on the charge of the toner particles. In a digital printer  10  with a plurality of print groups  11 , the surface already has an applied toner layer in a second and all further print groups  11   b ,  11   c ,  11   d , which applied toner layer affects the properties of the surface of the recording medium  20 . A defined charging of the surface of the recording medium  20  for the second and for additional print groups  11   b ,  11   c ,  11   d  by means of the corotron is therefore for the most part not possible at all, or is possibly only with a complicated regulation. Additionally, due to the charging of the corotron at the polarity opposite that of the toner charge, the danger exists that already-deposited toner from upstream print group  11   a  is discharged or has its charge reversed and, in immediately following print group  11   b , is transferred from the recording medium  20  back onto the transfer roller  121  of said following print group  11   b.    
     An additional disadvantage of the corotron charging of the top side of the recording medium  20  is the normally unknown interaction of the charge applied to the surface of the recording medium  20  with the liquid developer. If the charge of the surface is soluble in the liquid developer and more mobile than the toner particles, the charge flows away quickly, meaning that the effective active electrical field decreases relatively quickly upon passage through the nip between transfer roller  121  and counter-pressure roller  161 . The transfer efficiency may thus be degraded by the surface treatment. 
     Therefore, it is proposed to treat the underside of the recording medium  20  by means of a corotron before the electrically assisted transfer point from the transfer roller  121  onto the recording medium  20 . A treatment of the underside of the recording medium  20  may in particular take place immediately before the transfer point (for example immediately before the nip between transfer roller  121  and recording medium  20 ). Via the treatment of the underside of the recording medium  20 , ions are applied onto the underside of said recording medium  20 , wherein the ions have a polarity that is opposite the polarity of the toner particles in the liquid developer. Via the treatment of the underside of the recording medium  20 , the effective field in the liquid developer is thus increased, whereby the transfer efficiency is in turn increased. 
     The charge applied by the corotron may possibly flow away across the counter-pressure roller  126  and/or through the recording medium  20 . The applied charge thereby flows away slower the greater the resistances of the recording medium  20  and/or of the counter-pressure roller  126 . Via a suitable design of the recording medium  20  and/or of the counter-pressure roller  126  (for example given use of a dielectric layer), it may thus be achieved that the effective electrical field in the liquid developer is remained for a sufficient length of time in order to provide a sufficient transfer efficiency. 
       FIG. 3  shows example components of a transfer station  120  for a digital printer  10 . In particular,  FIG. 3  shows the transfer roller  121  of a transfer station  120 . As has already been presented above, the transfer roller  121  is configured to direct liquid developer  301  with toner particles towards a first side of a recording medium  20  and to transfer them onto the first side of the recording medium  20  at a transfer point (for example at the nip). For example, the first side of the recording medium  20  may be the top side of the recording medium  20 . The toner particles have a first polarity (for example a positive polarity). 
     The recording medium  20  is moved past the transfer roller  121  along a transport direction  20 ″. The transfer roller  121  thereby typically moves with a corresponding rotation velocity so that respective different segments of the transfer roller  121  and of the first side of the recording medium  20  meet at the transfer point. The toner particles are typically arranged on the transfer roller  121  according to a print image  20 ′ to be printed on the recording medium  20 . The print image  20 ′ is thus created little by little on the first side of the recording medium  20  via the movement of the recording medium  20  and of the transfer roller  121 . 
     The transfer station  120  additionally comprises a surface charging station  310  that is arranged before the transfer point in the transport direction  20 ″. The surface charging station  310  can be configured to generate and/or apply a (charged) layer  314  of charged particles from a second side of the recording medium  20  or to introduce the charged particles into said recording medium  20 . In other words: the surface charging station  310  may apply charged particles from the second side of the recording medium  20  onto said recording medium  20 . These charged particles then form a charged layer  314  that may also penetrate or partially penetrate from the second side of the recording medium  20  into the inside of said recording medium  20 . The second side is thereby opposite the first side. For example, the second side is the underside of the recording medium  20 , and the first side is the (to-be-printed) top side of the recording medium  20 . In an exemplary embodiment, the surface charging station  310  can include processor circuitry configured to generate and/or apply the charged layer  314  of charged particles from the second side of the recording medium  20 . In an exemplary embodiment, additionally or alternatively to the processor circuitry of the surface charging station  310 , the controller  60  can be configured to generate and/or apply the charged layer  314  of charged particles from the second side of the recording medium  20 . 
     The charged particles (and therefore the charged layer  314 ) applied onto the second side of the recording medium  20  have a second polarity that is opposite the first polarity of the toner particles. For example, the second polarity may be a negative polarity that is opposite a first, positive polarity of the toner particles. 
     Via the layer  314  of charged particles, a charged layer  314  may thus be provided that attracts the oppositely charged toner particles from the transfer roller  121  towards the first side (in the direction of the second side) of the recording medium  20 . In other words: via the charged layer  314  of charged particles, an electrical field is generated that attracts the oppositely charged toner particles towards the first side of the recording medium  20 . The transfer efficiency of the transfer station  120  may thus be reliably increased. Due to the fact that the charged layer  314  with the second polarity is located on the second side of or inside (near the second side) the recording medium  20 , an unregulated discharging of the toner particles and/or an unregulated interaction with the liquid developer cannot occur. 
     The surface charging station  310  may comprise one or more corotrons  312  that are configured to generate a corona or a plasma  313  on or in front of (for example in an environment in front of) the second side of the recording medium  20 . 
     In particular, air may be ionized by the one or more corotrons  312 . 
     The ions of the second polarity may then be applied as a charged layer  314  of charged particles onto the second side of the recording medium  20 . For this purpose, the surface charging station  310  may comprise a counter-electrode  311  that is arranged on or in front of (for example in an environment in front of) the first side of the recording medium  20 , and that is configured to attract ions from the corona or the plasma  313  with the second polarity towards the recording medium  20  in order to apply the charged layer  314  of charged particles from the second side of the recording medium  20 . The counter-electrode  311  may thereby possibly contact the surface of the first side of the recording medium  20 , for example in order to produce an optimally large force on the ions. On the other hand, an air gap may be provided between the counter-electrode  311  and the surface of the first side of the recording medium  20 , for example in order to preserve a print image that is already present on the first side of the recording medium  20 . 
     The penetration of the ions with the second polarity into the inside of the recording medium  20  may be regulated via a field strength of the electrical field generated by the counter-electrode  311 . In particular, the electrical field generated by the counter-electrode may be such that the ions are not (substantially) pulled through the recording medium  20  to the first side of said recording medium  20 . The electrical field generated by the counter-electrode may be such that a distribution of charged particles across the cross section of the recording medium  20 , from the second side to the first side, results that decreases towards the first side (and is substantially zero at the first side). In an exemplary embodiment, the processor circuitry of the surface charging station  310  can be configured to determine the field strength of the electrical field generated by the counter electrode  311  to regulate the penetration of the ions. In an exemplary embodiment, additionally or alternatively to the processor circuitry of the surface charging station  310 , the controller  60  can be configured to determine the field strength of the electrical field generated by the counter electrode  311 . 
     The counter-electrode  311  typically has a clearance from the first side of the recording medium  20 . The clearance thereby encompasses at least a height of a print image  20 ′ that is already located on the first side of the recording medium  20 . It may thus be ensured that the counter-electrode  311  is arranged sufficiently close to the first side of the recording medium  20  in order to generate a sufficient electrical field so that the ions with the second polarity are introduced into the recording medium  20  from the second side of said recording medium  20 . On the other hand, an already applied print image  20 ′ is protected from damage by the clearance. 
     The surface charging station  310  can be configured to adapt an energy and/or a spatial distribution of the corona or of the plasma  313  depending on one or more parameters. In an exemplary embodiment, the processor circuitry of the surface charging station  310  can be configured to adapt the energy and/or spatial distribution of the corona or of the plasma  313  depending on the one or more parameters. In an exemplary embodiment, additionally or alternatively to the processor circuitry of the surface charging station  310 , the controller  60  can be configured to adapt the energy and/or spatial distribution of the corona or of the plasma  313  depending on the one or more parameters. The one or more parameters may include one or more of: the humidity in the environment in front of the second side of the recording medium  20 ; the moisture of the recording medium  20 ; the thickness of the recording medium  20 ; and/or the material of the recording medium  20 . By taking into account one or more parameters in the generation of the charged layer  314 , it may be ensured that—even given changing conditions—an electrical field is generated by the charged layer  314  generated by the surface charging station  310  such that the toner particles reliably transfer with a uniformly high transfer efficiency from the transfer roller  121  onto the first side of the recording medium  20 . 
     In an exemplary embodiment, the surface charging station  310  can be configured such that a charged layer  314  is generated over the entire region of the recording medium  20  that is to be printed, including over the entire print width. For this purpose, the surface charging station  310  may comprise a plurality of corotrons  312  that are arranged over the width of the recording medium  20 , transversal to the transport direction  20 ″. 
     The transfer station may additionally comprise a counter-pressure roller  126  that is configured to press the recording medium  20  onto the transfer roller  121  at the transfer point. For this purpose, the counter-pressure roller  126  is in contact with the second side of the recording medium  20 . The counter-pressure roller  126  may have a dielectric outer layer. A flow of the charged particles of the charged layer  314  away across the counter-pressure roller  126  may thus be avoided, or at least slowed. 
     The transfer station may additionally include a voltage source that is configured to generate an electrical field  302  between the transfer roller  121  and the counter-pressure roller  126 , which has the effect that the toner particles are drawn onto the first side of the recording medium  20 . The transfer efficiency may be further increased (even given thick recording media  20 ) via the generation of an additional electrical field  302  which assists the electrical field generated by the charged layer  314 . 
       FIG. 4  shows a workflow diagram of an example method  400  to increase the toner transfer efficiency of a transfer station  1120 . The method  400  includes the application  401  of a charged layer  314  of charged particles (ions, for example) from a second side (the underside, for example) of a recording medium  20 . The charged particles—and therefore the layer  314 —thereby have a second polarity (a negative polarity, for example). The method  400  additionally includes the transfer  402  of toner particles from a liquid developer  301  onto a first side (a top side, for example) of the recording medium  20  that is opposite the second side. The toner particles have a first polarity (a positive polarity, for example) that is opposite the second polarity. 
     The transfer station  120  described in this document, and the corresponding method  400 , enable an increase of the transfer efficiency at the transfer point of the transfer station  120  given a reduced voltage difference between transfer roller  121  and counter-pressure roller  126 . The danger of a breakdown may thus be reduced. Furthermore, an increased adhesion of already deposited toner layers from upstream print groups  11  is achieved via the applied charged layer  314 . This prevents a return transfer of color separations that have already been transfer-printed. The color separations remain at uniform quality, although they pass additional transfer points of additional print groups  11 . Moreover, an interaction of the applied charged particles with the liquid developer may be reduced or entirely suppressed via the application of a charged layer  314  on the side of the recording medium  20  that is situated opposite the side to be printed. Overall, additional degrees of freedom in the selection of recording media  20  to be printed (in particular with regard to the thickness of the recording medium  20  that is to be printed to and/or with regard to the grammage of the recording medium  20  that is to be printed to) result via the transfer station  120  described in this document. 
     CONCLUSION 
     The aforementioned description of the specific embodiments will so fully reveal the general nature of the disclosure that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, and without departing from the general concept of the present disclosure. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance. 
     References in the specification to “one embodiment,” “an embodiment,” “an exemplary embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     The exemplary embodiments described herein are provided for illustrative purposes, and are not limiting. Other exemplary embodiments are possible, and modifications may be made to the exemplary embodiments. Therefore, the specification is not meant to limit the disclosure. Rather, the scope of the disclosure is defined only in accordance with the following claims and their equivalents. 
     Embodiments may be implemented in hardware (e.g., circuits), firmware, software, or any combination thereof. Embodiments may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact results from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. Further, any of the implementation variations may be carried out by a general purpose computer. 
     For the purposes of this discussion, the term “processor circuitry” shall be understood to be circuit(s), processor(s), logic, or a combination thereof. For example, a circuit can include an analog circuit, a digital circuit, state machine logic, other structural electronic hardware, or a combination thereof. A processor can include a microprocessor, a digital signal processor (DSP), or other hardware processor. In one or more exemplary embodiments, the processor can include a memory, and the processor can be “hard-coded” with instructions to perform corresponding function(s) according to embodiments described herein. In these examples, the hard-coded instructions can be stored on the memory. Alternatively or additionally, the processor can access an internal and/or external memory to retrieve instructions stored in the internal and/or external memory, which when executed by the processor, perform the corresponding function(s) associated with the processor, and/or one or more functions and/or operations related to the operation of a component having the processor included therein. 
     In one or more of the exemplary embodiments described herein, the memory can be any well-known volatile and/or non-volatile memory, including, for example, read-only memory (ROM), random access memory (RAM), flash memory, a magnetic storage media, an optical disc, erasable programmable read only memory (EPROM), and programmable read only memory (PROM). The memory can be non-removable, removable, or a combination of both. 
     REFERENCE LIST 
     A reference list is provided below.
       10  digital printer     11 ,  11   a - 11   d  print group (front side)     12 ,  12   a - 12   d  print group (back side)     20  recording medium     20 ′ print image (toner)     20 ″ transport direction of the recording medium     21  roll (input)     22  take-off     23  conditioning group     24  turner     25  register     26  puller     27  take-up     28  roll (output)     30  fixer     40  climate controller     50  power supply     60  controller     70  fluid management system     71  fluid controller     72  storage reservoir     100  electrophotography station     101  photoconductor, photoconductor roller     102  erasure light     103  cleaner (photoconductor)     104  blade (photoconductor)     105  collection container (photoconductor)     106  charger (corotron)     106 ′ wire     106 ″ shield     107  feed channel (aeration)     108  exhaust channel (ventilation)     109  character generator     110  developer station     111  developer roller     112  storage chamber     112 ′ fluid feed     113  pre-chamber     114  electrode segment     115  dosing roller (developer roller)     116  blade (dosing roller)     117  cleaning roller (developer roller)     118  blade (cleaning roller of the developer roller)     119  collection container (liquid developer)     119 ′ fluid discharge     120  transfer station     121  transfer roller     122  cleaner (wet chamber)     123  cleaning brush (wet chamber)     123 ′ cleaning fluid feed     124  cleaning roller (wet chamber)     124 ′ cleaning fluid discharge     125  blade     126  counter-pressure roller     127  cleaner (counter-pressure roller)     128  collection container (counter-pressure roller)     128 ′ fluid discharge     129  charger (corotron at transfer roller)     301  liquid developer with charged toner particles     302  electrical field     310  surface charger     311  counter-electrode     312  corotron     313  corona or plasma     314  Layer with charged particles     400  method to increase the toner transfer efficiency     401 ,  402  method steps