Abstract:
An image forming apparatus according to the present invention is provided with a plurality of image bearing members, an intermediate transfer member, a plurality of first transfer devices, a second transfer device, and a charge amount adjustment section. The image bearing members carry images of respective colors. The intermediate transfer member is rotatably disposed at a position opposed to the image bearing members. The first transfer devices form a full-color toner image on the intermediate transfer member by transferring color toner images formed on the image bearing members to the intermediate transfer member. The second transfer device transfers the full-color toner image formed on the intermediate transfer member to a recording sheet. The charge amount adjustment section adjusts the charge amount of toner particles constituting the full-color toner image so as to reduce non-uniformity in a charge amount of toner particles of the full-color toner image on the intermediate transfer member per unit area.

Description:
CROSS REFERENCE 
   This Nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2004-288207 in Japan on Sep. 30, 2004, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to an electrophotographic image forming apparatus for forming a color image in an intermediate transfer system, and a transferring method applied to that image forming apparatus. 
   One method for forming a color image in electrophotographic image forming apparatuses is an intermediate transfer system. The intermediate transfer system is a system in which a yellow toner image, a magenta toner image, a cyan toner image, and a black toner image are transferred to an intermediate transfer member (first transfer), and then these toner images attached to the surface of the intermediate transfer member are transferred all at once to a recording sheet (second transfer). 
   However, in an intermediate transfer system it is difficult to set the electric field strength during the second transfer. This is because an appropriate range of the electric field strength in the second transfer varies in accordance with the charge amount of toner particles attached to the intermediate transfer member per unit area. In addition, the charge amount of toner particles per unit area is affected by the thickness of a toner image formed on the intermediate transfer member, and thus the appropriate range of the electric field strength in the second transfer is sometimes different from position to position even within the same toner image. 
   When performing the second transfer outside the appropriate range of the transfer electric field strength, toner particles to be transferred to a recording sheet in the second transfer are more likely to remain on the side of the intermediate transfer member. When toner remains on the intermediate transfer member, a desired density cannot be attained in a monochrome image. Further, in the case of a color image, the ratio at which color toner images are mixed changes, and thus the color balance of a reproduced full-color toner image is degraded. 
   In order to address this problem, JP H08-292661A has disclosed a configuration in which a photoreceptor drum is used as a second transfer device, so as to satisfactorily perform second transfer when forming a color image in the intermediate transfer system. In this configuration, a latent image potential based on the image data is formed on the circumferential face of the photoreceptor drum as the second transfer device when toner images are transferred all at once from the intermediate transfer member to paper. It is described that the configuration enables a plurality of kinds of electric fields in the second transfer each having an appropriate strength to be set for the portions of the toner image, and thus it is possible to satisfactorily perform the second transfer regardless of the non-uniformity in the charge amount of toner particles attached to the intermediate transfer member per unit area. 
   However, the invention relating to JP H08-292661A cannot be applied when the photoreceptor drum is not used as the second transfer device, and thus the types of image forming apparatuses to which the invention can be applied are limited. 
   It is an object of the present invention to provide an image forming apparatus and a transferring method with which the second transfer strength in the color image forming process in an intermediate transfer system can be set easily with a simple configuration. 
   SUMMARY OF THE INVENTION 
   The image forming apparatus according to the present invention is provided with a plurality of image bearing members, an intermediate transfer member, a plurality of first transfer devices, a second transfer device, and a charge amount adjustment section. The image bearing members carry images with colors that are mutually different. The intermediate transfer member is rotatably disposed at a position opposed to the image bearing members. The first transfer devices form a full-color toner image on the intermediate transfer member by transferring color toner images formed on the image bearing members to the intermediate transfer member. The second transfer device transfers the full-color toner image formed on the intermediate transfer member to a recording sheet. The charge amount adjustment section adjusts the charge amount of toner particles constituting the full-color toner image so as to reduce the non-uniformity in the charge amount of the toner particles of the full-color toner image on the intermediate transfer member per unit area. 
   The color toner images formed on the image bearing members are transferred via the intermediate transfer member to a recording sheet. At that time, a full-color toner image is formed on the intermediate transfer member by placing the color toner images one on the top of another on the intermediate transfer member. In the full-color toner image formed on the intermediate transfer member, the charge amount of toner particles per unit area is large in thick portions, and the charge amount of toner particles per unit area is small in thin portions. 
   The charge amount adjustment section narrows the range of non-uniformity in the charge amount of toner particles of a full-color toner image per unit area. This is because the charge amount of toner particles constituting a full-color toner image on the intermediate transfer member affects an appropriate range of the electric field strength in the second transfer when the full-color toner image is transferred from the intermediate transfer member to the recording sheet. When the range of the non-uniformity in the charge amount of toner particles of the full-color toner image per unit area is narrowed by the charge amount adjustment section, the range of the electric field strength appropriate for the entire range of the full-color toner image in the second transfer widens . As a result, the second transfer voltage value and the second transfer current value in the image forming process in the intermediate transfer system can be set easily. 
   As a representative example of the charge amount adjustment section, a charger is conceivable that has the same polarity as the polarity of toner particles constituting a full-color toner image, and that charges the toner particles of the full-color toner image on the intermediate transfer member. Further, the amount of color toner images attached to the image bearing members may be adjusted or the charge amount of toner particles constituting the color toner images may be adjusted by adjusting the stirring speed of developer or the development bias in the developing device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a view showing a structural overview of an image forming apparatus of the present invention. 
       FIG. 2  is a block diagram showing a structural overview of the image forming apparatus of the present invention. 
       FIG. 3  is a diagram showing the relationship between the charge amounts of toner and appropriate values of the electric field strength during the second transfer. 
       FIG. 4  is a diagram showing the relationship between the charge amounts of toner and appropriate values of the electric field strength in the second transfer. 
       FIG. 5  is a diagram showing the relationship between the second transfer current value and the remaining toner density. 
       FIG. 6  is a diagram showing the relationship between the second transfer current value and the remaining toner density. 
       FIG. 7  is a diagram showing the relationship between the second transfer current value and the remaining toner density. 
       FIG. 8  is a diagram showing the relationship between the charge amount difference and the easiness of setting the second transfer current. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An image forming apparatus  100  shown in  FIG. 1  forms a multi-color or single-color image on paper based on input image data. The image forming apparatus  100  is provided with image forming portions  10 A to  10 D, an exposure unit  20 , an intermediate transfer belt  11 , first transfer rollers  13 A to  13 D, a second transfer roller  14 , a fixing device  15 , paper transport paths  81  to  83 , a paper feed cassette  16 , a manual paper feed tray  17 , and a paper receiving tray  18 . 
   The image forming portions  10 A to  10 D form images based on image data respectively corresponding to the colors black (K), cyan (C), magenta (M), and yellow (Y). The image forming portions  10 A to  10 D are arranged along the direction in which the intermediate transfer belt  11  rotates, indicated by the arrow Z. The image forming portion  10 A is provided with a photoreceptor drum  101 A, a charge roller  103 A, a developing unit  102 A, a transfer roller  13 A, and a cleaning unit  104 A. The image forming portion  10 B is provided with a photoreceptor drum  101 B, a charge roller  103 B, a developing unit  102 B, a transfer roller  13 B, and a cleaning unit  104 B. The image forming portion  10 C is provided with a photoreceptor drum  10 C, a charge roller  103 C, a developing unit  102 C, a transfer roller  13 C, and a cleaning unit  104 C. The image forming portion  10 D is provided with a photoreceptor drum  101 D, a charge roller  103 D, a developing unit  102 D, a transfer roller  13 D, and a cleaning unit  104 D. Herein, the image forming portions  10 A to  10 D have the same basic configuration, and thus mainly the configuration of the image forming portion  10 A is described, and an explanation of the image forming portions  10 B to  10 D is omitted. 
   The charge roller  103 A is a contact charger that charges the circumferential face of the photoreceptor drum  101 A uniformly to a predetermined potential. It is also possible to use contact charging devices using charge brushes or non-contact charging devices using chargers, instead of the charge roller  103 A. 
   The exposure unit  20  is provided with a polygon mirror  4 , reflection mirrors, and a semiconductor laser (not shown), and emits a plurality of laser beams modulated based on black (K), cyan (C), magenta (M), and yellow (Y) color image data onto the photoreceptor drums  101 A to  101 D, respectively. Thus, latent electrostatic images with the colors black (K), cyan (C), magenta (M), and yellow (Y) are respectively formed on the photoreceptor drums  101 A to  101 D. 
   The developing unit  102 A supplies toner particles to the photoreceptor drum  101 A on which the latent image is formed to form a toner image on the photoreceptor drum  101 A. The developing unit  102 A stores black toner particles and forms a black toner image on the photoreceptor drum  101 A. Further, the developing units  102 B to  102 D store cyan, magenta, and yellow toner particles. The cleaning unit  104 A removes and recovers toner remaining on the circumferential face of the photoreceptor drum  101 A after development and image transfer. 
   The intermediate transfer belt  11  is disposed above the photoreceptor drums  101 A to  101 D. The intermediate transfer belt  11  is stretched around a driving roller  11 A and a driven roller  11 B, and rotates in the direction of the arrow Z. The outer circumferential face of the intermediate transfer belt  11  is opposed to the circumferential faces of the photoreceptor drums  101 A to  101 D. 
   The first transfer rollers  13 A to  13 D are arranged at positions that are opposed to the photoreceptor drums  101 A to  101 D having the intermediate transfer belt  11  therebetween. The first transfer rollers  13 A to  13 D have a configuration in which the circumferential face of a shaft made of a metal with a diameter of 8 to 10 mm is coated with a conductive elastic material. In this embodiment, stainless steel is used as the shafts of the first transfer rollers  13 A to  13 D, and ethylene propylene rubber (EPDM) is used as the elastic material on the circumferential faces. However, it is possible to use urethane foam as the elastic material on the circumferential faces, instead of EPDM. 
   When a first transfer bias with a polarity opposite to that of the toner is applied to the first transfer rollers  13 A to  13 D, the toner images carried on the circumferential faces of the photoreceptor drums  101 A to  101 D are transferred to the intermediate transfer belt  11 . In this embodiment, the electric field strength during the first transfer is controlled by a constant voltage control. When color toner images are transferred from the photoreceptor drums  101 A to  101 D to the intermediate transfer belt  11 , a full-color toner image is formed on the outer circumferential face of the intermediate transfer belt  11 . It is usually understood that a full-color toner image includes all of a black toner image, a cyan toner image, a magenta toner image, and a yellow toner image, but in the present invention, any toner image is taken as a full-color toner image as long as it includes at least one of a black toner image, a cyan toner image, a magenta toner image, and a yellow toner image. In a case in which a image forming process is performed based only on a subset of the colors black (K), cyan (C), magenta (M), and yellow (Y), a toner image is formed only on a subset of the photoreceptor drums, among the four photoreceptor drums  101 A to  101 D, corresponding to the colors of input image data. For example, during monochrome image formation, a toner image is formed only on the photoreceptor drum  101 A, and only a black toner image is transferred to the outer circumferential face of the intermediate transfer belt  11 . 
   The second transfer roller  14  is pressed at a predetermined nip pressure against the outer circumferential face of the intermediate transfer belt  11 . The full-color toner image transferred to the outer circumferential face of the intermediate transfer belt  11  is transported to the position of the second transfer roller  14  by the rotation of the intermediate transfer belt  11 . While paper fed from the paper feed cassette  16  or the manual paper feed tray  17  passes a position between the second transfer roller  14  and the intermediate transfer belt  11 , a second transfer bias with polarity opposite to that of the toner is applied to the second transfer roller  14 . 
   A charger  30  and a cleaning unit  12  are arranged around the intermediate transfer belt  11 . The charger  30  is disposed such that it is opposed to the intermediate transfer belt  11  at a position between the second transfer roller  14  and the first transfer roller  13 A. The cleaning unit  12  recovers toner particles remaining on the intermediate transfer belt  11 . 
   The fixing device  15  is provided with a heating roller  15 A and a pressing roller  15 B, and fixes a toner image transferred to the paper, onto the paper with heat and pressure. The paper receiving tray  18  holds paper discharged from the image forming apparatus  100  by paper discharge rollers  18 A. 
   The paper transport path  81  extends from the paper feed cassette  16 , via a position between the second transfer roller  14  and the intermediate transfer belt  11 , to the paper discharge rollers  18 A. Pick-up rollers  16 A for feeding paper in the paper feed cassette  16  onto the paper transport path  81  one by one, transport rollers  91  for transporting the fed paper upward, and registration rollers  19  for guiding the transported paper to a position between the second transfer roller  14  and the intermediate transfer belt  11  at a predetermined timing are arranged along the paper transport path  81 . 
   The paper transport path  82  extends from the manual paper feed tray  17  to a junction with the paper transport path  81 . Pick-up rollers  17 A are arranged in the most upstream portion of the paper transport path  82 . The paper transport path  83  guides the paper that has passed through the fixing device  15  again to the position of the registration rollers  19 . 
   The paper discharge rollers  18 A are freely rotatable in both the forward and reverse directions. The paper discharge rollers  18 A are driven in the forward direction to discharge paper to the paper receiving tray  18  during simplex image formation in which an image is formed on one side of paper, and during the second side image formation of duplex image formation in which an image is formed on both sides of paper. On the other hand, during the first side image formation of duplex image formation, the paper discharge rollers  18 A are driven in the forward direction until the rear edge of the paper passes through the fixing device  15 , and are then driven in the reverse direction to guide the paper onto the paper transport path  83  in a state where the rear edge of the paper is held by the paper discharge rollers  18 A. 
   In the image forming apparatus  100 , the first transfer rollers  13 B to  13 D are parted from the intermediate transfer belt  11 , and only the first transfer roller  13 A is in contact with the intermediate transfer belt  11  during monochrome image formation. On the other hand, all of the first transfer rollers  13 A to  13 D are in contact with the intermediate transfer belt  11  when monochrome image formation is not being performed. 
     FIG. 2  is a block diagram showing a structural overview of the image forming apparatus  100 . The image forming apparatus  100  is provided with a CPU  50 . An interface portion  53 , the image forming portions  10 A to  10 D, an electric power circuit  60 , a paper feed and transport control portion  70 , a RAM  51 , a ROM  52 , the charger  30 , the first transfer rollers  13 A to  13 D, the second transfer roller  14 , a sensor group  40 , and the exposure unit  20  are connected to the CPU  50 . The interface portion  53  is connected to a network, and receives image data input through the network. The electric power circuit  60  supplies electric power to the portions of the image forming apparatus  100 . For example, the electric power circuit  60  supplies set electric power to the first transfer rollers  13 A to  13 D and the second transfer roller  14 , based on a command from the CPU  50 . The paper feed and transport control portion  70  controls a paper feed operation and a paper transport operation in the image forming apparatus  100 , based on the command from the CPU  50 . The RAM  51  is a volatile memory for temporarily storing, for example, image data. The ROM  52  stores a program necessary for the operation of the image forming apparatus  100 . 
   The charger  30  is a scorotron pin array charger with the same polarity as the toner. The charger  30  has a grid for controlling charged particles passing through. The grid is attached to the opening face of the charger  30 , and is used for letting the charge potential converge on a predetermined value. In this embodiment, the potential of the grid on the charger  30  is set to approximately −150 V. 
   The sensor group  40  detects information necessary for controlling the image forming apparatus  100 . In this embodiment, the thickness of a full-color toner image on the intermediate transfer belt  11  is detected using the sensor group  40 . 
     FIG. 3  is diagram showing the relationship between the charge amounts of toner and appropriate values of the electric field strength during the second transfer. In  FIG. 3 , the electric field strengths optimal for a single-layered portion, a double-layered portion, and a triple-layered portion in the second transfer are respectively shown by α, β, and γ. Further, the amounts by which the layer thickness changes in accordance with the change of the gradation between the single-layered portion, the double-layered portion, and the triple-layered portion are respectively shown by A 1 , A 2 , and A 3 , and appropriate ranges of the electric field strengths in the second transfer of the single-layered portion, the double-layered portion, and the triple-layered portion are shown by rectangle forms. Herein, the single-layered portion refers to a portion in which toner particles with a single color in a full-color toner image are layered, the double-layered portion refers to a portion in which toner particles with two colors in a full-color toner image are layered, and the triple-layered portion refers to a portion in which toner particles with three or more colors in a full-color toner image are layered. 
   A case is considered in which the electric field strength in the second transfer is set to α. In this case, the set electric field strength α in the second transfer is smaller than the optimal electric field strength β during the second transfer of the double-layered portion by X 1 , and is smaller than the optimal electric field strength γ during the second transfer of the triple-layered portion by X 2 . As a result, a problem may occur in which a part of a full-color toner image remains on the intermediate transfer belt  11  during the second transfer of the double-layered portion or the triple-layered portion. 
   On the other hand, a case is considered in which the electric field strength during the second transfer is set to γ. In this case, the set electric field strength γ during the second transfer is larger than the optimal electric field strength α during the second transfer of the single-layered portion by Y 1 , and is larger than the optimal electric field strength β during the second transfer of the double-layered portion by Y 2 . As a result, a problem may occur in which toner particles scatter during the second transfer of the single-layered portion or the double-layered portion. 
   These problems occur because the thickness of the full-color toner image on the intermediate transfer belt  11  is not uniform. The charge amount of toner particles in a full-color toner image per unit area usually increases in proportion to the thickness of the full-color toner image at that position. Accordingly, the charge amount of toner particles per unit area becomes non-uniform even within one full-color toner image, and an appropriate value of the electric field strength during the second transfer is different from position to position even within one full-color toner image. 
   In this embodiment, toner particles constituting a full-color toner image on the intermediate transfer belt  11  are charged with the charger  30 . Thus, the non-uniformity in the charge amount of the toner particles in one full-color toner image per unit area is reduced, so that the range in which the appropriate ranges of the electric field strengths in the second transfer of the single-layered portion, the double-layered portion, and the triple-layered portion are overlapped becomes wider as shown in  FIG. 4 . As a result, the electric field strength that can be applied as appropriate to all of the single-layered portion, the double-layered portion, and the triple-layered portion in the second transfer can be set easily. 
     FIG. 5  shows the relationship between the second transfer current and the remaining toner density relating to the single-layered portion when the charger  30  is not used. FIG. shows an example in which the optimal transfer current of the single-layered portion having a charge amount of −0.010 μC/cm 2  is 18 μA to 26 μA. 
     FIG. 6  shows the relationship between the second transfer current and the remaining toner density relating to the triple-layered portion when the charger  30  is not used.  FIG. 6  shows an example in which the optimal transfer current of the triple-layered portion having a charge amount of −0.036 μC/cm 2  is 28 μA to 35 μA. In the examples shown in  FIGS. 5 and 6 , the difference between the charge amounts of the single-layered portion and the triple-layered portion is 0.026 μC/cm 2 , and there is no transfer current value that is appropriate for both the single-layered portion and the triple-layered portion. 
     FIG. 7  shows the relationship between the second transfer current and the remaining toner density relating to the single-layered portion and the triple-layered portion when the charger  30  is used.  FIG. 7  shows an example in which the optimal transfer current is 16 μA to 28 μA for both of the single-layered portion having a charge amount of −0.008 μC/cm 2  and the triple-layered portion having a charge amount of −0.024 μC/cm 2 . In the example shown in  FIG. 7 , the difference between the charge amounts of the single-layered portion and the triple-layered portion is 0.016 μC/cm 2 , and the range of the transfer current value appropriate for both the single-layered portion and the triple-layered portion is wider than the case in  FIGS. 5 and 6 . 
   From the results shown in  FIGS. 5 to 7 , it is clear that setting the second transfer current for the second transfer roller  14  becomes easier as the range of non-uniformity in the charge amount of toner on the intermediate transfer belt  11  per unit area becomes smaller. Furthermore, an investigation of the relationship between the charge amount difference and the easiness of setting the second transfer current obtained the results shown in  FIG. 8 . 
   The applicant has investigated the size of the second transfer current range that allows transfer to be satisfactorily performed, within the range in which non-uniformity in the charge amount of toner on the intermediate transfer belt  11  per unit area is equal to or less than 0.030 μC/cm 2 . As a result, it was discovered that when the range of non-uniformity in the charge amount of toner on the intermediate transfer belt  11  per unit area is smaller than 0.025 μC/cm 2 , the second transfer current range that allows transfer to be satisfactorily performed is widened to the extent that the second transfer can be satisfactorily performed. 
   In this embodiment, toner is charged using the charger  30  with the same polarity as the toner before the second transfer is performed with the second transfer roller  14 . For example, when the apparent charge potential of the single-layered portion is −50V and the apparent charge potential of the multi-layered portion is −150 V, the charger  30  maybe activated so that all toner particles of the full-color toner image on the intermediate transfer belt  11  are charged to the potential of the multi-layered toner image. 
   In this embodiment, the CPU  50  activates the charger  30  when the CPU  50  determines, based on the results of detecting toner image thickness with the sensor group  40 , that the range of non-uniformity in the charge amount of the full-color toner image on the intermediate transfer belt  11  per unit area is 0.025 μC/cm 2  or more. In addition, the CPU  50  activates the charger  30  when using toner in which the charge amount difference between the toner charge amounts formed on the photoreceptor drums  101 A to  101 D is 0.003 μC/cm 2  or more. 
   According to the aforementioned embodiment, the range of the electric field strength appropriate for all of the single-layered portion, the double-layered portion, and the triple-layered portion during the second transfer widens, and thus defects during the second transfer occur less even when the types of paper or the use environment varies to some extent. As a result, the color balance of a full-color toner image is not likely to be disturbed even when the color image is formed at a high speed with the tandem image forming apparatus  100 . 
   Furthermore, in the aforementioned embodiment, the charger  30  is used for setting the range of non-uniformity in the charge amount of the full-color toner image per unit area to be smaller than 0.025 μC/cm 2 , but the following methods also can be used for setting the range of non-uniformity in the charge amount of the full-color toner image per unit area to be smaller than 0.025 μC/cm 2 , without the use of the charger  30 . 
   As another method for setting the range of non-uniformity in the charge amount of the full-color toner image per unit area to be smaller than 0.025 μC/cm 2 , a method is conceivable in which the development conditions are adjusted. For example, the development conditions are set such that the amount of toner supplied to the photoreceptor drums  101 A to  101 D is smaller than 0.4 mg/cm 2  and the charge amount of the toner particles at that time is smaller than −20 μC/g, so that the absolute value of the charge amount of the toner particles on the photoreceptor drums  101 A to  101 D per unit area becomes smaller than 0.008 μC/cm 2 . It is experimentally known that when the absolute value of the charge amount of the toner particles on the photoreceptor drums  101 A to  101 D per unit area is smaller than 0.008 μC/cm 2 , the range of non-uniformity in the charge amount of the full-color toner image per unit area is smaller than 0.025 μC/cm 2  Examples of development condition adjustment include adjustment of the development bias, adjustment of the toner stirring speed, and adjustment of the contact pressure between the development roller and the blade. 
   Furthermore, when the first transfer pressure, that is, the contact pressure between the photoreceptor drums  101 A to  101 D and the transfer rollers  13 A to  13 D, is set to be in a range of 1 g/mm 2  to 5 g/mm 2  in the image forming apparatus  100 , toner particles can be prevented from scattering before the second transfer. In the image forming apparatus  100 , the region in which the photoreceptor drums  101 A to  101 D and the intermediate transfer belt  11  are in contact with each other is widened by horizontally shifting the axes of the transfer rollers  13 A to  13 D with respect to the axes of the photoreceptor drums  101 A to  101 D, in order to reduce the contact pressure between the photoreceptor drums  101 A to  101 D and the intermediate transfer belt  11 . 
   It should be noted that when the contact pressure between the photoreceptor drums  101 A to  101 D and the transfer rollers  13 A to  13 D is less than an appropriate pressure, transferred toner particles are dispersedly layered on the intermediate transfer belt  11 . Thus, when toner particles with another color are transferred to the intermediate transfer belt  11 , the toner image that has already been formed on the intermediate transfer belt  11  may be disturbed. On the other hand, when the contact pressure between the photoreceptor drums  101 A to  101 D and the transfer rollers  13 A to  13 D is larger than an appropriate pressure, layered toner particles are firmly fixed, and thus the toner particles tend to remain on the intermediate transfer belt  11  during the second transfer. 
   It is also possible to use a non-contact roller instead of the charger  30  in the aforementioned embodiment. However, it is important to bring the non-contact roller sufficiently close to the intermediate transfer belt  11  and to mirror finish the circumferential face of the non-contact roller, so as to narrow the range of non-uniformity in the charge amount of the full-color toner image per unit area with the non-contact roller. 
   Finally, the embodiments described above are to be considered in all respects as illustrative and not limiting. The scope of the invention is indicated by the appended claims rather than by the foregoing embodiments. Furthermore, all changes which come within the meaning and range of equivalency of the claims are intended to be embraced in the scope of the invention.