Abstract:
An apparatus and method for reducing contamination of an image transfer surface in an image transfer device includes a charging device for charging the image transfer surface. An airflow control system ventilates the charging device and restricts airflow adjacent the image transfer surface.

Description:
BACKGROUND OF THE INVENTION 
   The present invention generally relates to image transfer technology and, more particularly, to an apparatus and method for reducing contamination of image transfer surfaces of image transfer devices during the printing process, and an image transfer device having the apparatus. 
   As used herein, the term “image transfer device” generally refers to all types of devices used for creating and/or transferring an image in a liquid electrophotographic process, including laser printers, copiers, facsimiles, and the like. 
   In a liquid electrophotographic (LEP) printer, the surface of a photoconducting material (i.e., a photoreceptor) is charged to a substantially uniform potential so as to sensitize the surface. An electrostatic latent image is created on the surface of the photoconducting material by selectively exposing areas of the photoconductor surface to a light image of the original document being reproduced. A difference in electrostatic charge density is created between the areas on the photoconductor surface exposed and unexposed to light. In LEP, the photoconductor surface is initially charged to approximately ±1000 Volts, with the exposed photoconductor surface discharged to approximately ±50 Volts. 
   The electrostatic latent image on the photoconductor surface is developed into a visible image using developer liquid, which is a mixture of solid electrostatic toners or pigments dispersed in a carrier liquid serving as a solvent (referred to herein as “imaging oil”). The carrier liquid is usually insulative. The toners are selectively attracted to the photoconductor surface either exposed or unexposed to light, depending on the relative electrostatic charges of the photoconductor surface, development electrode, and toner. The photoconductor surface may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles. For LEP printers, the preferred embodiment is that the photoconductor surface and toner have the same polarity. 
   A sheet of paper or other medium is passed close to the photoconductor surface, which may be in the form of a rotating drum or a continuous belt, transferring the toner from the photoconductor surface onto the paper in the pattern of the image developed on the photoconductor surface. The transfer of the toner may be an electrostatic transfer, as when the sheet has an electric charge opposite that of the toner, or may be a heat transfer, as when a heated transfer roller is used, or a combination of electrostatic and heat transfer. In some printer embodiments, the toner may first be transferred from the photoconductor surface to an intermediate transfer medium, and then from the intermediate transfer medium to a sheet of paper. 
   Charging of the photoconductor surface may be accomplished by an ionization device. Several types of ionization devices are known, such as a corotron (a corona wire having a DC voltage and an electrostatic shield), a dicorotron (a glass covered corona wire with AC voltage, and electrostatic shield with DC voltage, and an insulating housing), a scorotron (a corotron with an added biased conducting grid), a discorotron (a dicorotron with an added biased conducting strip), a pin scorotron (a corona pin array housing a high voltage and a biased conducting grid), or a charge roller. Each of these ionization devices generate ozone (O 3 ), and nitric oxides (NO x ), which if present in sufficient quantities, must be vented and filtered from the image transfer device. 
   An active flow of air through the image transfer device may be provided to ventilate and filter ozone and/or nitric oxides from the image transfer device. Although an active airflow through the image transfer device is sometimes required or desired for ventilation, airflow through or past the photoconductor surface is problematic in long term use of the photoconductor surface. In particular, active airflow is problematic because the airflow evaporates the submicron oil layer on the photoconductor surface and entrains oil vapors present above the oil layer, thereby effectively thinning the oil layer. The remaining oil layer includes residual materials such as charge directors and other dissolved ink components that have high molecular weight and do not easily evaporate. The thinned oil layer provides reduced buffering of the molecules of residual material against ion bombardment, UV exposure and ozone penetration. Therefore, the residual materials in the oil are more likely to react and polymerize on the photoconductor surface. Additionally, the dissolved residual material in the thinned oil layer is much closer to or beyond its solubility limit. This increases the chance for dissolved residual materials to drop out of solution and polymerize on the photoconductor surface. 
   The contaminating film of polymerized material on the photoconductor surface eliminates the ability to either form latent images of small dots on the photoconductor surface, or transfer small dots from the photoconductor surface to paper. As contamination of the photoconductor increases over time, the print quality of subsequently printed images is reduced, and the useful life of the photoconductor surface is shortened. The contamination problem is often referred to as old photoconductor syndrome (OPS). 
   Representations of prior art embodiments of charging apparatuses using ionization-type charging devices and having ventilation systems are schematically illustrated in  FIGS. 2A–2B . In the charging apparatus  30  of  FIG. 2A , an active ventilating airflow in the direction of arrows  71  is established by a suitable vacuum system  72 . Fresh air is drawn into the chamber  96  containing the charging device (i.e., corona wire  90  and grid  92 ) from outside the charging apparatus housing  80 , and passes through a small gap  73  (created by positioning pins  86 ) between the housing  80  and the photoconductor surface  22 , and then through conductive grid  92 . The ozone generated near the corona wire  90  is drawn through an opening  74  at the end of chamber  96  opposite photoconductor surface  22 , and then to a filter system  75 . Due to the airflow between the housing  80  and the photoconductor surface  22 , the submicron oil layer on the photoconductor surface  22  evaporates such that the oil layer is thinned, and some oil vapor becomes entrained in the airflow. 
   Problems caused by the illustrated airflow include contamination of the charging device (both corona wire  90  and grid  92 ), and contamination of the photoconductor surface  22 . The charging device and interior housing walls become contaminated as the oil vapor entrained in the airflow reacts with the ozone, energetic ions and UV light to polymerize, and then coats the corona wire  90 , conductive grid  92  and housing walls with sticky material. The efficiency of the coated corona wire  90  is immediately reduced. Further, the contamination forces frequent cleaning and/or replacement of the corona wire  90 , conductive grid  92  and housing. The photoconductor surface  22  becomes contaminated as the residual material in the thinned oil layer reacts with the ozone, energetic ions and UV light to polymerize on the photoconductor surface  22 , or drops out of solution and polymerizes on the photoconductor surface  22 , as described above. 
   In the charging apparatus of  FIG. 2B , an active ventilating airflow in the direction of arrows  76  is established by a suitable vacuum system  72 . Fresh air is drawn into the chamber  96  containing the charging device (i.e., corona wire  90  and conductive grid  92 ) from a plenum  77  at the end of chamber  96  opposite photoconductor surface  22 . The airflow moves through opening  74 , past corona wire  90  and toward photoconductor surface  22 . After the flow of air moves through the conductive grid  92  and small gap  73 , the air is drawn out at one or more outlets  78  adjacent the photoconductor surface  22 , and then to filter system  75 . The ozone generated near the corona wire  90  is thereby forcibly moved through the conductive grid  92  and against the photoconductor surface  22 . 
   As the airflow passes through the small gap  73  between the housing  80  and the photoconductor surface  22 , the submicron oil layer on the photoconductor surface  22  evaporates such that the oil layer is thinned, and some oil vapor becomes entrained in the airflow. The photoconductor surface  22  becomes contaminated as the residual material in the thinned oil layer reacts with the ozone, energetic ions and UV light to polymerize on the photoconductor surface  22 , or drops out of solution and polymerizes on the photoconductor surface  22 , as described above. The rate of residual material polymerization on the photoconductor surface  22  is further increased as ozone is actively pulled toward the photoconductor surface  22  by the airflow path, thereby increasing the chemical exposure of the oil layer on the photoconductor surface  22 . 
   During the process of charging the photoconductor surface, it is desirable that the photoconductor surface is free of residual materials from previous printing cycles, such as toner, charge directors and other dissolved materials in the imaging oil. However, effectively cleaning the photoconductor surface of all residual materials is very difficult, and some amount of residual material inevitably remains on the photoconductor surface. Therefore, there is a need for an apparatus or method to lessen or eliminate polymerization of the residual materials and the resulting filming of the photoconductor surface. 
   SUMMARY OF THE INVENTION 
   The invention described herein provides an apparatus and method for reducing contamination of an image transfer surface in an image transfer device. In one embodiment, the apparatus includes at least one charging device for charging the image transfer surface. An airflow control system is configured to ventilate the charging device and direct airflow in a direction substantially parallel to and spaced apart from the image transfer surface. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of an exemplary image transfer device, showing a liquid electrophotographic printer for use with a charging apparatus having an airflow control system according to one embodiment of the invention. 
       FIGS. 2A–2B  are schematic cross-sectional views of embodiments of prior art charging apparatuses. 
       FIG. 3  is a schematic cross-sectional view of one embodiment of a charging apparatus having a single charging device and an airflow control system according to the invention. 
       FIG. 4A  is a schematic cross-sectional view of one embodiment of a charging apparatus having more than one charging device and an airflow control system according to the invention. 
       FIG. 4B  is a schematic illustration of an alternate airflow control system in the charging apparatus of  FIG. 4A . 
       FIG. 5  is an exemplary graph illustrating the improved photoconductor aging achieved using the charging apparatus with an airflow control system of  FIG. 3 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
   An exemplary image transfer device having an image transfer surface, specifically an LEP printer  10  having a photoconductor surface  22 , is schematically shown in  FIG. 1 . Although, for purpose of clarity, embodiments according to the invention are illustrated herein with respect to an LEP printer having a photoconductor surface, the invention is understood to be applicable and useful with other embodiments of image transfer surfaces and image transfer devices. As illustrated, the LEP printer  10  includes a printer housing  12  having installed therein a photoconductor drum  20  having the photoconductor surface  22 . Photoconductor drum  20  is rotatably mounted within printer housing  12  and rotates in the direction of arrow  24 . Several additional printer components surround the photoconductor drum  20 , including a charging apparatus  30 , an exposure device  40 , a development device  50 , an image transfer apparatus  60 , and a cleaning apparatus  70 . 
   The charging apparatus  30  charges the photoconductor surface  22  on the drum  20  to a predetermined electric potential (typically ±500 to 1000 V). In some embodiments, as shown in  FIG. 1 , more than one charging apparatus  30  is provided adjacent the photoconductor surface  22  for incrementally increasing the electric potential of the surface  22 . In other embodiments, only a single charging apparatus  30  is provided. In addition, referring to  FIGS. 3 and 4 , each charging apparatus  30  may contain a single charging device  88  for charging the photoconductor surface  22  to the desired electric potential in a single step ( FIG. 3 ), or multiple charging devices  88  for charging the photoconductor surface  22  to the desired electric potential in a series of incremental steps ( FIG. 4A ). The number of charging apparatus  30  and charging devices  88  will be affected by factors including the process speed of surface  22  and the desired electric potential of the surface  22 . 
   In one embodiment, charging apparatus  30  utilizes an ionization-type charging device  88 . Referring to  FIG. 3 , during operation of the charging device  88 , an electric potential sufficient to ionize air molecules within the chamber  96  is provided to the corona wire  90 . For example, in one embodiment a potential of approximately −6000 Volts is provided to the corona wire  90 . Forming what is referred to as a corona current, the ionized air molecules are drawn to the fully or partially discharged photoconductor surface  22  through the associated conductive grid  92 . The grid  92  is biased to the desired potential of the photoconductor surface  22 , for example approximately −1000 Volts. When charging of photoconductor surface begins, the photoconductor surface  22  is at an electric potential lower than the desired potential, and the corona current flows past the grid  92  to the surface  22 . When the photoconductor surface  22  reaches the same potential as the grid  92  (i.e., the desired potential), the corona current to the surface  22  ceases. The grid  92  thus acts to control the final charge of the photoconductor surface  22 . 
   The exposure device  40  forms an electrostatic latent image on the photoconductor surface  22  by scanning a light beam (such as a laser) according to the image to be printed onto the photoconductor surface  22 . The electrostatic latent image is due to a difference in the surface potential between the exposed and unexposed portion of the photoconductor surface  22 . The exposure device  40  exposes images on photoconductor surface  22  corresponding to various colors, for example, yellow (Y), magenta (M), cyan (C) and black (K), respectively. 
   The development device  50  supplies development liquid, which is a mixture of solid toner and imaging oil (such as Isopar), to the photoconductor surface  22  to adhere the toner to the portion of the photoconductor surface  22  where the electrostatic latent image is formed, thereby forming a visible toner image on the photoconductor surface  22 . The development device  50  may supply various colors of toner corresponding to the color images exposed by the exposure device  40 . 
   The image transfer apparatus  60  includes an intermediate transfer drum  62  in contact with the photoconductor surface  22 , and a fixation or impression drum  64  in contact with the transfer drum  62 . As the transfer drum  62  is brought into contact with the photoconductor surface  22 , the image is transferred from the photoconductor surface  22  to the transfer drum  62 . A printing sheet  66  is fed between the transfer drum  62  and the impression drum  64  to transfer the image from the transfer drum  62  to the printing sheet  66 . The impression drum  64  fuses the toner image to the printing sheet  66  by the application of heat and pressure. 
   The cleaning apparatus  70  cleans the photoconductor surface  22  of some of the residual material using a cleaning fluid before the photoconductor surface  22  is used for printing subsequent images. In one embodiment according to the invention, the cleaning fluid is imaging oil as used by the development device  50 . As the photoconductor surface  22  moves past the cleaning apparatus  70 , a submicron layer of oil having residual material therein remains on the photoconductor surface  22 . 
   Although not shown in  FIG. 1 , the liquid electrophotographic printer  10  further includes a printing sheet feeding device for supplying printing sheets  66  to image transfer apparatus  60 , and a printing sheet ejection device for ejecting printed sheets from the printer  10 . 
   As described above, airflow against the photoconductor surface  22  causes the submicron oil layer on the photoconductor surface  22  to evaporate, such that the oil layer is thinned, and some oil vapor becomes entrained in the airflow. The photoconductor surface  22  then becomes contaminated as the residual material in the thinned oil layer reacts with the ozone, energetic ions and UV light to polymerize on the photoconductor surface  22 , or drops out of solution and polymerizes on the photoconductor surface  22 , as described above. 
   One embodiment of a charging apparatus  30  having an airflow control system according to the invention that reduces contamination of the photoconductor surface  22  is schematically illustrated in  FIG. 3 . Charging apparatus  30  includes a housing  80  having a first end  82  and a second end  84 . First end  82  of housing  80  is configured for positioning adjacent photoconductor surface  22  without contacting surface  22 . It is preferred to avoid contact with photoconductor surface  22 , such as with wipers or seals, so as to avoid mechanical thinning of the submicron oil layer. Mechanical thinning of the oil layer results in problems similar to those encountered when the oil layer is thinned by evaporation. Specifically, the thinned oil layer provides reduced buffering of the molecules of residual material against ion bombardment, UV exposure and ozone penetration. Therefore, the residual materials in the thinned oil layer are more likely to react and polymerize on the photoconductor surface  22 . In addition to mechanically thinning the oil layer, wipers or seals pressed against the photoconductor surface  22  also act to remove oil vapor normally present above the oil layer as the photoconductor surface  22  moves past the wiper or seal. The removal of the oil vapor decreases the partial vapor pressure of the oil immediately adjacent the oil layer, and thereby further increases the rate of evaporation of the oil layer. As best seen in  FIGS. 1 and 3 , the housing  80  of the charging apparatus  30  may be positioned adjacent the photoconductor surface  22  without touching the surface  22  by a bridge assembly  85  that is connected to the printer housing  12 , and also by positioning pins  86  that hold housing  80  away from photoconductor surface  22 . 
   Referring again to  FIG. 3 , at least one charging device  88  is positioned within chamber  96  of housing  80 , adjacent first end  82  of housing  80 , such that the at least one charging device  88  is arranged adjacent photoconductor surface  22 . Photoconductor surface  22  moves in the direction generally indicated by arrow  24 . The charging device  88  is characterized by corona producing wire  90  and associated electrically conductive screen or grid  92  disposed between the corona wire  90  and the photoconductor surface  22  to be charged. The corona producing wire  90  comprises an elongated wire extending across the photoconductor surface  22 . In preferred embodiments, corona wire  90  is positioned in the range of 4 to 15 mm from photoconductor surface  22 , while conductive grid  92  is positioned approximately 1 mm or less from the photoconductor surface  22 . In some embodiments, excess lengths of the corona wire  90  may be provided on a bobbin or other suitable supply device (not shown), such that the corona wire  90  can be periodically refreshed. Additionally, as illustrated in  FIG. 3  by alternate corona wires  90 ′, more than one corona wire can optionally be provided in chamber  96 . Although, for purposes of clarity, the charging device  88  of charging apparatus  30  is illustrated herein as a scorotron, the invention is understood to be applicable and useful with other types of charging devices, particularly ionization-type charging devices used in image transfer devices, such as corotrons, dicorotrons, and discorotrons. 
   In the charging apparatus of  FIG. 3 , the airflow control system establishes an active ventilating airflow that protects the oil layer on the photoconductor surface from evaporative thinning. As seen in  FIG. 3 , the airflow control system directs air through chamber  96  in the direction of arrows  98  by a suitable vacuum system  72  providing a volume airflow in the range of 0.1 to 30 liters/second, depending upon the ventilation requirements of the particular imaging application. An air inlet  100  and air outlet  102  are provided in opposite side walls  104  of the chamber  96 , such that air flows through chamber  96  from the air inlet  100  to the air outlet  102  in a direction substantially parallel to and spaced apart from the photoconductor surface  22  and the conductive grid  92 , and then on to a filter system  75 , without being directed toward or against the photoconductor surface  22 . The air inlet  100  and air outlet  102  are preferably positioned in the sidewalls  104  of chamber  96  such that the airflow is directed over corona wire  90 , and further such that airflow between the photoconductor surface  22  and the conductive grid  92  is restricted or eliminated. Air inlet  100  and air outlet  102  are positioned at least as far from photoconductor surface  22  as conductive grid  92  is positioned from photoconductor surface  22  (e.g., at least 1 mm). Preferably, air inlet  100  and air outlet  102  are positioned from photoconductor surface  22  by approximately the same distance as corona wire  90  is positioned from photoconductor surface  22  (e.g., in the range of 4 to 15 mm). In a preferred embodiment, airflow  98  moves in the same direction as the photoconductor surface  22 , so as to reduce or minimize the creation of eddy currents at the air/oil boundary. In one embodiment, the volume of airflow  98 , the size of air inlet  100  and the size of air outlet  102  are selected such that the speed of airflow  98  between inlet  100  and outlet  102  approximates the speed of photoconductor surface  22  past the charging apparatus  30 . That is, the relative difference between the speed of airflow  98  and the speed of photoconductor surface  22  is preferably minimized. In this manner, evaporative thinning of the submicron oil layer on the photoconductor surface  22  is reduced or eliminated. In addition, because ozone is not actively moved toward the photoconductor surface  22 , the chemical exposure of the oil layer on the photoconductor surface  22  is reduced or eliminated. The reduction or elimination of evaporative thinning and chemical exposure of the oil layer on the photoconductor surface  22  reduces the amount and rate of polymerization of residual material in the oil layer, and thereby reduces filming of the photoconductor surface  22 . 
   In  FIG. 3 , air inlet  100  and air outlet  102  of chamber  96  are illustrated as being connected to plenums  110 ,  112 , respectively, that are integrated into the housing  80 . In turn, plenums  110 ,  112  are in fluid communication with the fresh air source and vacuum system  72 , respectively. However, the plenums  110 ,  112 , of the airflow control system do not need to be integrated into the housing  80 , and may be eliminated in alternate embodiments. For example, inlet  100  and outlet  102  may be directly connected to the fresh air supply and vacuum system  72  without the use of plenums  110 ,  112 . 
   In other embodiments according to the invention, more than one charging device  88  is provided in the housing  80 , with the airflow control system providing each charging device  88  with its own ventilating airflow. In  FIGS. 4A and 4B , the illustrated charging apparatus  30  includes two discrete charging devices  88   a  and  88   b  each positioned adjacent first end  82  of housing  80 , such that the charging devices  88   a ,  88   b  are arranged adjacent the photoconductor surface  22 . Photoconductor surface  22  moves in the direction generally indicated by arrow  24 . As discussed with respect to the embodiment of  FIG. 3 , first end  82  of housing  80  is configured for positioning adjacent photoconductor surface  22  without contacting surface  22 . Each charging device  88   a ,  88   b  is characterized by a corona producing wire  90   a ,  90   b , respectively, and an associated electrically conductive screen or grid  92   a ,  92   b  disposed between the associated corona wire  90   a ,  90   b  and the surface  22  to be charged. The charging devices  88   a ,  88   b  operate as discrete charging devices within a single housing  80 , and are positioned within different chambers  96   a ,  96   b , respectively, of the housing  80 . In other embodiments according to the invention, additional charging devices  88  may be provided in the housing  80 . As described above with respect to the embodiment of  FIG. 3 , the corona producing wires  90   a ,  90   b  are positioned in the range of 4 to 15 mm from photoconductor surface  22 , while conductive grids  92   a ,  92   b  are positioned approximately 1 mm or less from the photoconductor surface  22 . 
   In the charging apparatus of  FIG. 4A , the airflow control system establishes an active ventilating airflow through each chamber  96   a ,  96   b  that protects the oil layer on the photoconductor surface  22  from evaporative thinning. As seen in  FIG. 4A , the airflow control system directs air through chambers  96   a ,  96   b  in the direction of arrows  120  by a suitable vacuum system  72  providing a volume airflow in the range of 0.1 to 30 liters/second, depending upon the ventilation requirements of the particular imaging application. An air inlet  122  and air outlet  124  are provided in opposite side walls  104  of each of the chambers  96   a ,  96   b , respectively, such that air flows through chambers  96   a ,  96   b  from the air inlet  122  to the air outlet  124  in a direction substantially parallel to and spaced apart from the photoconductor surface  22  and the conductive grids  92   a ,  92   b , and then on to a filter system  75  without being directed toward or against the photoconductor surface  22 . In the embodiment illustrated in  FIG. 4A , a common air inlet  122  is provided from the common wall  126  dividing chambers  96   a ,  96   b , and separate air outlets  124  are provided for each chamber  96   a ,  96   b . In an alternate embodiment, the airflow direction can be reversed from that illustrated in  FIG. 4A , such that common air inlet  122  becomes an air outlet, and the air outlets  124  become air inlets. In yet another alternate embodiment, separate air inlets and outlets can be provided for each chamber. 
   The air inlet  122  and air outlets  124  are preferably positioned in the sidewalls  104  of chambers  96   a ,  96   b  such that the airflow is directed substantially parallel to and spaced apart from the photoconductor surface  22 , over corona wires  90   a ,  90   b , and further such that airflow between the photoconductor surface  22  and the conductive grids  92   a ,  92   b  is restricted or eliminated. Air inlet  122  and air outlets  124  are positioned at least as far from photoconductor surface  22  as conductive grids  92   a ,  92   b  are positioned from photoconductor surface  22  (e.g., at least 1 mm). Preferably, air inlet  122  and air outlets  124  are positioned from photoconductor surface  22  by approximately the same distance as corona wires  90   a ,  90   b  are positioned from photoconductor surface  22  (e.g., in the range of 4 to 15 mm). In this manner, evaporative thinning of the submicron oil layer on the photoconductor surface  22  is reduced or eliminated. In addition, because ozone is not actively moved toward the photoconductor surface  22 , the chemical exposure of the oil layer on the photoconductor surface  22  is reduced or eliminated. The reduction or elimination of evaporative thinning and chemical exposure of the oil layer on the photoconductor surface  22  reduces the amount and rate of polymerization of residual material in the oil layer, and thereby reduces filming of the photoconductor surface  22 .  FIG. 4B  illustrates a variation of the airflow control system in the charging apparatus of  FIG. 4A . 
   In  FIG. 4B , the airflow control system directs air through chambers  96   a ,  96   b  in the direction of arrow  120 , such that air flows through chambers  96   a ,  96   b  from the air inlet  132 , through opening  133  in common wall  126  to the air outlet  134  in a direction substantially parallel to, spaced apart from, and in the same direction as the photoconductor surface  22 , and then on to filter system  75  without being directed toward or against the photoconductor surface  22 . Air inlet  132  and air outlet  134  are positioned at least as far from photoconductor surface  22  as conductive grids  92   a ,  92   b  are positioned from photoconductor surface  22  (e.g., at least 1 mm). Preferably, air inlet  132  and air outlet  134  are positioned from photoconductor surface  22  by approximately the same distance as corona wires  90   a ,  90   b  are positioned from photoconductor surface  22  (e.g., in the range of 4 to 15 mm). In a preferred embodiment, vacuum system  72  creates volume airflow in the range of 0.1 to 30 liters/second, depending upon the ventilation requirements of the particular imaging application. Preferably, the volume of the airflow, the size of air inlet  132 , opening  133  and air outlet  134  are selected such that the speed of the airflow between inlet  132  and outlet  134  approximates the speed of photoconductor surface  22 . That is, the relative difference between the speed of the air and the speed of photoconductor surface  22  is preferably minimized. 
   EXAMPLE 
   A liquid electrophotographic (LEP) printer was operated with a charging apparatus having an airflow control system like that illustrated in  FIG. 2A  for 100,000 printing cycles at 10% and 20% grayscale, and the dot area was measured at periodic intervals. Dot area is the estimated ink coverage of a tint patch, and is typically derived using an optical densitometer. The LEP printer was also operated for 100,000 printing cycles at 10% and 20% grayscale with a charging apparatus  30  having an improved airflow pattern like that illustrated in  FIG. 3 , and the dot area was measured at periodic intervals. The change in dot area for the prior art airflow pattern of  FIG. 2A  and the improved airflow pattern of  FIG. 3  is illustrated in the graph of  FIG. 5 , where lines  150  and  152  indicate the prior art airflow pattern at 10% and 20% grayscale, respectively, and lines  154  and  156  indicate the improved airflow pattern at 10% and 20% grayscale, respectively. A decrease in dot area is indicative of filming of the photoconductor surface. Examining  FIG. 5 , it can be seen that the improved airflow pattern results in a much slower decrease in dot area for both 10% and 20% grayscale when compared to the prior art airflow pattern. The dip occurring in each of lines  150 ,  152 ,  154 ,  156  at approximately 45,000 printing cycles coincides with replacement of the intermediate transfer roller  62 . 
   As described herein, the liquid electrophotograpic printer with the charging apparatus  30  having an airflow control system with improved airflow according to the present invention reduces the amount and rate of accumulation of residual materials and contaminants on the photoconductor surface  22  during operation of the LEP printer. Thus, the rate of deterioration of print quality is decreased and the life span of the photoconductor surface  22  is increased. 
   Although specific embodiments have been illustrated and described herein for purposes of description of the preferred embodiment, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. Those with skill in the mechanical, electro-mechanical, and electrical arts will readily appreciate that the present invention may be implemented in a very wide variety of embodiments. This application is intended to cover any adaptations or variations of the preferred embodiments discussed herein. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.