Patent Publication Number: US-7711298-B2

Title: Methods and devices to transfer toner in an image forming device to control charge buildup on a toner image

Description:
BACKGROUND 
   The present application relates generally to electrophotographic image forming devices, and in particular to a toner transfer apparatus to control charge buildup in a toner image as the toner image passes through one or more image transfer stations. 
   Electrophotographic image forming devices, such as laser printers, facsimile machines, copiers, all-in-one devices, etc, are well known in the art. Color electrophotographic image forming devices may form a plurality of latent electrostatic images, develop each color plane image with toner particles, and ultimately transfer the color plane images to a media sheet and then fuse them to the media sheet using heat and pressure. Color electrophotographic image forming devices may be divided into two types by considering how toner is transferred to the media sheet. In a direct to media (DTM) type image forming device, the developed toner image of each color plane is successively transferred directly to the media sheet. In an intermediate transfer mechanism (ITM) type image forming device, the developed toner image of each color plane is successively transferred to an intermediate transfer mechanism, such as a belt, and then the full-color image is transferred to a media sheet at a secondary transfer location. 
   One known problem that particularly affects ITM type image forming devices is charge buildup on the developed toner on the ITM as the toner passes successively through high-voltage image transfer stations. Toner which has passed through multiple image transfer stations may be at a different charge than toner which has not passed through any additional image transfer stations. When the toner image is transferred to the media sheet at the secondary transfer location, the toner that is less charged may transfer at a lower voltage than more highly charged toner. In order to transfer the entire toner image, a voltage high enough to affect the transfer of the most highly charged toner is used. High transfer voltages may create a phenomenon known as Paschen breakdown. In Paschen breakdown, toner particles reverse polarity and their placement becomes unpredictable. The toner particles may even backtransfer from the media sheet to the ITM. Backtransfer detrimentally impacts image quality. 
   SUMMARY 
   The present application is directed to methods and devices to transfer toner in an image forming device to control charge buildup on a toner image as the toner image passes through one or more transfer nips. Charge buildup may be reduced by laterally offsetting a transfer roller from a photoconductor drum. The transfer roller may be constructed of an essentially non-compressible conductive material. AC current may be used to generate an electrical field between the photoconductor drum and the transfer roller. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of an image forming device according to one embodiment. 
       FIG. 2  is schematic diagram of a prior art image transfer station. 
       FIG. 3  is a cross-sectional view of a prior art transfer roller. 
       FIG. 4  is a schematic diagram of a photoconductor drum and a transfer roller according to one embodiment. 
       FIG. 5A  is a perspective view of a prior art arrangement of a photoconductor drum and a transfer roller. 
       FIG. 5B  is a perspective view of a photoconductor drum and a transfer roller according to one embodiment. 
       FIG. 6A  is a graphical representation of an AC current without a DC offset according to one embodiment. 
       FIG. 6B  is a graphical representation of an AC current with a DC offset according to one embodiment. 
       FIG. 7  is a graphical representation of toner charge buildup after passing under downstream nips according to one embodiment. 
   

   DETAILED DESCRIPTION 
   The present application is directed to methods and devices to transfer toner in an image forming device to control charge buildup on a toner image as the toner image passes through one or more transfer nips. Each transfer nip is comprised of a photoconductor drum and a transfer roller positioned on opposite sides of an intermediate transfer member. In one embodiment, the transfer roller is offset from the photoconductor drum such that the point where the photoconductor drum contacts the intermediate transfer member is laterally offset from the point where the transfer roller contacts the intermediate transfer member. AC current may be used to generate an electrical field between the photoconductor drum and the transfer roller. 
   To understand the workings and context of the present application,  FIG. 1  depicts a representative image forming device, indicated generally by the numeral  10 . The image forming device  10  comprises a main media sheet stack  16 . Within the image forming device body  12 , the image forming device  10  may include a plurality of removable image formation cartridges  26 , each with a similar construction but distinguished by the toner color contained therein. In one embodiment, the image forming device  10  includes a black cartridge (K), a magenta cartridge (M), a cyan cartridge (C), and a yellow cartridge (Y). Each cartridge  26  forms an individual monocolor image that is combined in layered fashion with images from the other cartridges  26  to create the final multi-colored image. The image forming device  10  may further include an intermediate transfer mechanism (ITM)  24 , one or more imaging devices  29 , and a fuser  45 . A controller  50  may oversee operation of the image forming device  10 . 
   The operation of the image forming device  10  is conventionally known. Upon command from the controller  50 , the media sheet  15  is “picked,” or selected, from either the primary media stack  16  by a pick roller  17  and conveyed into a media feed path  21  or introduced through a manual input  20  into the media feed path  21 . Regardless of its source, the media sheet  15  is transported to drive rollers  18 , and then to a secondary transfer location  22  to receive a toner image from the ITM  24 . In this embodiment, ITM  24  is an endless belt that rotates in the direction indicated by arrow R around a series of rollers adjacent to photoconductor drums  14  of the respective image formation cartridges  26 . Toner is deposited from each photoconductor drum  14  as needed to create a full color image on the ITM belt  24 . The deposited toner is transferred from the ITM belt  24  to the media sheet  15  at the secondary transfer location  22 . The media sheet  15  and attached toner next travel through a fuser  45  having a pair of rollers and a heating element that heats and fuses the toner to the media sheet  15 . The media sheet  15  with fused image is then transported out of the printer body  12  for receipt by a user. Alternatively, the media sheet  15  is moved through a duplex path  13  for receiving an image on a second side. 
   The image forming device  10  may include one or more power supplies, indicated generally by reference number  70  in  FIG. 1 . The power supply  70  may provide the voltage necessary to electronically bias the photoconductor drums  14  to receive toner. The power supply  70  may also provide voltage to electrically bias charging units  31 , developer rollers  32 , and transfer rollers  34  as described in more detail below. The power supply  70  may include more than one power supply  70 , and may include at least one AC power supply  70  and/or at least one DC power supply  70 . 
     FIG. 2  is a schematic diagram illustrating an exemplary prior art image transfer station  30 . Each image transfer station  30  may include the photoconductor drum  14 , the charging unit  31 , the developer roller  32 , the transfer roller  34 , and a cleaning blade  35 . The photoconductor drum  14  is a cylindrically shaped roller and illustrated in this embodiment as a drum. However, it will be apparent to those skilled in the art that the photoconductor drum  14  may comprise any appropriate structure. The charging unit  31  charges the surface of the photoconductor drum  14  to a generally uniform negative potential, such as approximately −1000 volts (V). A laser beam  60  from the imaging device  29  (see  FIG. 1 ) selectively discharges areas on the photoconductor drum  14  to form a latent image on the surface of the photoconductor drum  14 . The areas of the photoconductor drum  14  illuminated by the laser beam  60  are discharged, resulting in a potential of approximately −200V. The transfer roller  34  is charged to an appropriate positive potential, such as +1600 V. The potential of the transfer roller  34  may vary depending on the type and age of the ITM belt  24 , the electrical or other property of the toner being applied to the ITM belt  24 , environmental conditions, and other factors. 
   As illustrated in  FIG. 2 , the photoconductor drum  14  is disposed on one side of the ITM belt  24 , and the transfer roller  34  is disposed directly opposed to the photoconductor drum  14  on an opposite side of the ITM belt  24  such that the ITM belt  24  is pressed between the photoconductor drum  14  and the transfer roller  34 . A transfer nip  46  is formed where the photoconductor roller  14  and the transfer roller  34  contact the ITM belt  24 . At the transfer nip  46 , the transfer roller  34  urges the ITM belt  24  into contact with the photoconductor roller  14  to facilitate transfer of the toner onto the ITM belt  24 . 
   The developer roller  32  transports negatively-charged toner to the surface of the photoconductor drum  14 , to develop the latent image on the photoconductor drum  14 . The developer roller  32  core is held more negatively charged that the discharged areas of the photoconductor drum  14 . The toner is attracted to the most positive surface, i.e., the area discharged by the laser beam  60  and is repelled by more-negatively charged areas of the photoconductor drum  14  (i.e. those not optically discharged). As the photoconductor drum  14  rotates, a positive voltage field produced by the transfer roller  34  attracts and transfers the toner adhering to the discharged areas on the surface of the photoconductor drum  14  to the ITM belt  24 . Any remaining toner on the photoconductor drum  14  is then removed by the cleaning blade  35 . The toner thus may experience a relative potential difference of 400 V between the developer roller  32  and the photoconductor drum  14 , and a potential difference of 1800 V between the photoconductor drum  14  and the transfer roller  34 . 
     FIG. 3  illustrates a cross-sectional view of the prior art transfer roller  34 . The transfer roller  34  may be comprised of a resilient (e.g., foam or rubber) outer surface  40  disposed around a conductive axial shaft  41 . The transfer roller  34  is able to produce the positive voltage field due to the high resistivity of the outer surface  34  relative to the shaft  41 , ITM belt  24 , and photoconductor drum  14 . 
   The image transfer process is complex and is sensitive to many inputs. The operating environment (temperature, humidity, and the like), ITM belt  24  properties, photoconductor drum  14  characteristics, toner formulation, and other factors all influence image quality. All of these inputs may directly impact the electrical potential across toner transfer boundaries in an image transfer station  30 . In particular, the resistivity of the toner gives rise to the toner collecting charge as it progresses through downstream image transfer stations  30 . 
   In order to reduce toner charge buildup, one embodiment of the present application as illustrated in  FIG. 4  includes the transfer roller  34  comprised of the conductive axial shaft  41  without the resilient outer surface  40  of the prior art transfer roller  34 . In one embodiment, the transfer roller  34  is constructed of an essentially non-compressible conductive material. In one embodiment, the transfer roller  34  includes a uniform cross-sectional composition. 
   With the resilient outer surface  40  absent, the ITM belt  24  now controls the resistivity of an electrical path from the transfer roller  34  to the photoconductor drum  14 . If the positioning of the photoconductor drum  14  and the transfer roller  34  in this embodiment was the same as that illustrated in  FIG. 2  (i.e., directly opposed to one another), then the electrical path between the photoconductor drum  14  and the transfer roller  34  may pass through a relatively small volume of the ITM belt  24 . Consequently, the electrical path may have less resistivity than the resilient outer surface  40  of the prior art transfer roller  34 . This is illustrated by the shaded section  24 A of the ITM belt  24  in  FIG. 5A . Section  24 A is the section of the ITM belt  24  that the electrical current may pass through in the electrical path between the transfer roller  34  and the photoconductor drum  14 . Because the section  24 A of the ITM belt  24  is narrow, the transfer voltage required to transfer the toner from the photoconductor drum  14  to the ITM belt  24  may primarily be a function of a surface resistivity value of the ITM belt  24 . 
   In the embodiment of  FIG. 4 , however, the transfer roller  34  is laterally offset from the photoconductor drum  14  such that the transfer roller  34  is not directly opposed to the photoconductor drum  14 . The lateral offset is designated by L in  FIG. 4 . The lateral offset L is defined as the lateral distance in the direction of travel of the ITM belt  24  between the point where the photoconductor drum  14  contacts the ITM belt  24  and the point where the transfer roller  34  contacts the ITM belt  24 . Stated another way, the lateral offset L is the lateral distance between a line passing through a center point of the photoconductor drum  14  and orthogonal to the ITM belt  24  (broken line A in  FIG. 4 ) and a line passing through a center point of the transfer roller  34  and orthogonal to the ITM belt  24  (broken line B in  FIG. 4 ). 
     FIG. 4  further illustrates the degree of lateral offset L between the transfer roller  34  and the photoconductor drum  14 . The lateral offset L may be sufficient to position the transfer roller  34  apart from the photoconductor drum  14  such that the point where the transfer roller  34  contacts the ITM belt  24  (the point where broken line B intersects the ITM belt  24 ) is further downstream than a most downstream point P 3  of the ITM belt  24  in contact with the photoconductor drum  14 . The lateral offset L may be further illustrated by drawing a line between a center point P 1  of the photoconductor roller  14  and a center point P 2  of the transfer roller  34  (broken line C in  FIG. 4 ). Line C intersects the ITM belt  24  at point P 4 . Point P 4  is further downstream than the most downstream point P 3  of the ITM belt  24  in contact with the photoconductor drum  14 . Because of the lateral offset L, the ITM belt  24  is not pressed between the photoconductor drum  14  and the transfer roller  34 . 
   The transfer roller  34  may be laterally offset from the photoconductor drum  14  in either an upstream or downstream direction. All of the transfer rollers  34  may be offset in the same direction (either all upstream or all downstream), or the transfer rollers  34  may have a mixture of offsets. For example, the first transfer roller  34  may be offset downstream from the first photoconductor drum  14 , and the remaining transfer rollers  34  offset upstream for the photoconductor drums  14 . When the transfer roller  34  is offset downstream from the photoconductor drum  14  as illustrated in  FIG. 4 , the ITM belt  24  contacts the photoconductor drum  14  prior to contacting the transfer roller  34 . In the upstream offset configuration (effectively reversing the direction of travel of the ITM belt  24  from that illustrated in  FIG. 4 ), the ITM belt  24  contacts the transfer roller  34  prior to contacting the photoconductor drum  14 . 
   In one embodiment, the lateral offset L is 20 mm. As illustrated in  FIG. 5B , the electrical path now has a larger section  24 B of the ITM belt  24  to pass through. The transfer voltage may now be a function of both the surface resistivity and the volume of the ITM belt  24  the electrical path passes through (i.e., a surface resistivity of the ITM belt  24 ). Section  24 B may provide greater resistivity than section  24 A of the ITM belt  24 , resulting in a higher transfer voltage. 
   The prior art transfer roller  34  illustrated in  FIG. 3  may not allow the use of AC current for the transfer voltage. The resilient outer surface  40 , due to its resistivity, causes a time delay along a current path from the conductive axial shaft  41  through the resilient outer surface  40 . This time delay may tend to damp out higher frequency oscillations of the AC current. 
   In one embodiment of the present application, AC current may be used for the transfer voltage. There may be less time delay in the current path through section  24 B of the ITM belt  24 , resulting in little or no damping of the higher oscillations of the AC current. AC current is desirable for toner transfer because it enhances the transfer operation. The oscillating nature of the AC current first loosens some of the toner particles from the photoconductor drum  14 . As the voltage of the AC current begins to reverse, loose toner particles are drawn back to the photoconductor drum  14  and collide with toner particles remaining on the photoconductor drum  14 . The collisions provide a mechanical force to loosen the toner particles, resulting in a lower voltage potential to affect transfer of the toner to the ITM belt  24 . 
   In one embodiment, the AC current includes a DC offset. The DC offset provides the electrical bias necessary to carry the toner from the photoconductor drum  14  to the ITM belt  24 .  FIG. 6A  illustrates a graphical representation of an AC current with no DC offset. Without the offset, the effective bias voltage seen by the toner over a period of time may be zero. Consequently, there may be little or no toner transfer to the ITM belt  24  even though the AC current mechanically loosened the toner on the photoconductor drum  14 . In contrast,  FIG. 6B  graphically illustrates an AC current with a DC offset indicated as V o . In this embodiment, the oscillations of the AC current help to loosen the mechanical bonds of the toner particles on the photoconductive drum  14 , and the DC offset provides the electrical bias to transfer the toner to the ITM belt  24 . While  FIGS. 6A and 6B  illustrate the waveform of the AC current as a sine wave, it would be apparent to one skilled in the art that other waveforms may be used with the present application. For example, the waveform of one embodiment could include a square wave with a duty cycle varied, or the duty cycle may be offset to the square wave. 
     FIG. 6B  illustrates one embodiment where the DC offset V o  is greater than the amplitude of the AC current. In other embodiments, the DC offset V o  may be less than the amplitude, or even equal to the amplitude. The amount of both the amplitude of the AC current and the DC offset V o  may be adjusted to minimize print defects. 
   The magnitude of the DC offset V o  may be less than the voltage needed for the transfer operation of the prior art image transfer station  30  illustrated in  FIG. 2 . The lower DC voltage results in less charge buildup in the toner image on the ITM belt  24  as the toner image passes through upstream image transfer stations  30 . In addition, the AC current has little effect on toner charge buildup. The effect on toner charge buildup of one embodiment of the present application is illustrated in  FIG. 7 , wherein the units of graphs AC 1  and DC 2  are Q/A, and the units of graphs AC 2  and DC 1  are Q/m. The desired charge on the toner entering the secondary transfer location  22  for the image forming device represented in  FIG. 7  is about −45 uC/g. Toner transfer using AC current with a DC offset (graphs AC 1  and AC 2 ) shows only a slight charge buildup and then a bounce back close to the desired value after the third transfer nip. However, toner transfer using only DC current (graphs DC 1  and DC 2 ) shows a larger charge buildup and, even after the bounce back after the third transfer nip, is nearly twice the desired value. 
   Embodiments of the present application lend themselves to a wide range of AC current amplitudes and frequencies. In one embodiment, the frequency ranges from about 100 Hz to about 2 kHz. In one embodiment, the frequency is 500 Hz. The amplitude (voltage) may vary with the surface resistivity of the ITM belt  24 . In one embodiment, the amplitude varies directly with surface resistivity, such that lower resistivities may require a lower voltage and higher resistivities may require higher voltages. In one embodiment, the amplitude ranges from about 100 V peak-to-peak to about 2500 V peak-to-peak. In one embodiment, the amplitude ranges from about 500 V peak-to-peak to about 1200 V peak-to-peak. In one embodiment where a DC offset is used, the AC voltage is about 700 V peak-to-peak and the DC offset is about 300 V. In one embodiment, the AC voltage is about 500 V peak-to-peak and the DC offset is 500 V. In other embodiments, the amplitude ranges from 100 percent AC voltage to 100 percent DC voltage. 
   In addition to the lateral offset L between the photoconductor drum  14  and the transfer roller  34 , there may also be a height offset H as illustrated in  FIG. 4 . The height offset H is defined as the vertical distance (e.g., generally orthogonal to the direction of the lateral offset L or the direction of travel of the ITM belt  24 ) measured between the point on the photoconductor drum  14  in contact with the ITM belt  24  and the point on the transfer roller  34  in contact with the ITM belt  24 . More specifically, each contact point defines a plane within the ITM belt  24 , these planes being parallel to one another. The height offset H is the distance separating the planes. The height offset H maintains contact between the ITM belt  24  and the photoconductor drum  14  and forms the transfer nip  46 . The transfer nip  46  promotes adequate toner transfer to the ITM belt  24 . In addition, the height offset H maintains continuous contact between the ITM belt  24  and the transfer roller  34  which helps prevent electrical arcing between ITM belt  24  and the transfer roller  34 . 
   The transfer nip  46  may be formed by slightly changing a direction of travel of the ITM belt  24  at the points where the ITM belt  24  contacts the photoconductor drum  14  and the transfer roller  34 . As illustrated in  FIG. 4 , the ITM belt  24  is in a generally horizontal orientation prior to the photoconductor drum  14 . At the point of contact with the photoconductor drum  14 , the direction of travel is altered slightly toward vertical, thus forming the transfer nip  46 . The ITM belt  24  changes direction again at the transfer roller  34 . The directional change may be opposite the change at the photoconductor drum  14  and returns the ITM belt  24  to an essentially horizontal orientation. 
   In one embodiment, the lateral offset L is adjustable. Varying the lateral offset L varies the volume of the section  24 B of the ITM belt  24  that the current passes through between the transfer roller  34  and the photoconductor drum  14 . The variable lateral offset L allows a wider range of transfer voltages to be used than with a fixed lateral offset L. For example, the ITM belt  24  may be constructed of a material with a high surface resistivity, and a high transfer voltage may be desirable to assure adequate toner transfer. 
     FIGS. 1 ,  4 ,  5 A, and  5 B each illustrate the image forming device  10  as having a horizontal architecture. It would be readily apparent to one skilled in the art that the embodiments of the present application may be used with image forming devices  10  utilizing a vertical architecture with equal effect. 
   Spatially relative terms such as “under”, “below”, “lower”, “over”, “upper”, and the like, are used for ease of description to explain the positioning of one element relative to a second element. These terms are intended to encompass different orientations of the device in addition to different orientations than those depicted in the figures. Further, terms such as “first”, “second”, and the like, are also used to describe various elements, regions, sections, etc. and are also not intended to be limiting. Like terms refer to like elements throughout the description. 
   As used herein, the terms “having”, “containing”, “including”, “comprising”, and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise. 
   The present invention may be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.