Patent Publication Number: US-7221883-B2

Title: Electrostatic image transfer device using intermediate transfer belt having simplified image transfer voltage requirements

Description:
CROSS REFERENCE 
   This Nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2003-435394 filed in Japan on Dec. 26, 2003, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a transfer device for use in an electrophotographic image forming apparatus in which a toner image as formed on an image carrier is firstly transferred to an endless intermediate transfer belt and the toner image is secondly transferred from the intermediate transfer belt to a record medium such as a sheet of paper (hereinafter referred to merely as a sheet). The present invention relates in particular to a transfer device which controls transfer power to be supplied in first and second transfer operations. 
   Japanese Patent Application Laid-Open No. H10-039651 discloses a tandem-type full-color image forming apparatus having a semiconductive endless belt and a plurality of (e.g. four) image forming sections. The endless belt is installed rotatably, and the image forming sections each provided for forming a developed image of corresponding color are aligned along an outer circumference of the endless belt. This arrangement allows a full-color image to be formed in at least one full rotation of the endless belt. 
   There is also known a tandem-type full-color image forming apparatus using an intermediate transfer method. In the image forming apparatus, developed images for respective colors formed on photoreceptor drums as image carriers in respective image forming sections are accumulated on an outer circumferential surface of an endless belt (an intermediate transfer belt) and then transferred to a sheet, to form a full-color image. 
   More specifically, toner images are formed on the image carriers in the respective image forming sections, based on image data for the respective colors obtained by color separation from an original image. The toner images are firstly transferred from the image carriers to the intermediate transfer belt to be accumulated, or first transfer operations are performed. Then, the accumulation of toner images is secondly transferred from the intermediate transfer belt to the sheet, or a second transfer operation is performed. 
   Accordingly, the formation of a full-color image involves the first transfer operations performed in a plurality of, for example four, first transfer regions, and the second transfer operation performed in a second transfer region other than the first transfer regions. While following a loop travel path, the intermediate transfer belt passes through the first transfer regions and the second transfer region, in the order. 
   In the image forming apparatus using the intermediate transfer method, transfer power in full-color image formation is supplied to the intermediate transfer belt in the first transfer regions and in the second transfer region. The transfer power supplied to one of the transfer regions has undesirable effects on another transfer region positioned downstream thereof through the intermediate transfer belt, thereby preventing a predetermined transfer power from being supplied to the transfer region positioned downstream. 
   This is particularly true in case of the second transfer region being positioned immediately downstream of the first transfer region provided most downstream with respect to a traveling direction of the intermediate transfer belt with an aim to downsize the apparatus and achieve high-speed image formation. As a result, a toner image on the intermediate transfer belt cannot be transferred properly to a sheet. 
   Since a black toner image is generally transferred to the intermediate transfer belt in the first transfer region positioned most downstream with respect to the traveling direction, the transfer power supplied to the first transfer region interferes with the transfer operation in the second transfer region, in monochromatic image formation as well. 
   There have been proposed solutions to the foregoing problem, such as arrangement of first and second transfer regions at a longer distance from each other or use of an intermediate transfer belt with a higher resistance. 
   However, such arrangement of the first and second transfer regions causes an increase in size, and a decrease in image formation speed, of an image forming apparatus. Also, the intermediate transfer belt with a higher resistance requires a discharging device for each of the transfer regions, resulting in an increase in size and in manufacturing costs of the apparatus. 
   A feature of the present invention is to offer a transfer device that controls transfer power supply so that transfer power is timely supplied to the intermediate transfer belt in first and second transfer regions. Simple timing control allows the transfer device to avoid the undesirable effects of transfer power supplied to the respective transfer regions on the transfer operations performed in the other transfer regions. The transfer device thereby allows uniform transfer operations to be performed in the respective transfer regions and therefore constant high-quality image formation to be achieved, without an increase in size, or a decrease in image formation speed, of the image forming apparatus. 
   SUMMARY OF THE INVENTION 
   A transfer device of the present invention includes an endless intermediate transfer belt following a loop path; an image carrier for a toner image to be formed in an electrophotographic method; and a control section for controlling first and second transfer operations performed in one full rotation on the loop path of the intermediate transfer belt by supplying a predetermined level of transfer power to each of one or more first transfer regions where the toner image is transferred from the image carrier to the intermediate transfer belt and to a second transfer region where the toner image is transferred from the intermediate transfer belt to a record medium. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view illustrating a construction of an image forming apparatus including a transfer device according to a first embodiment of the present invention; 
       FIG. 2  is a block diagram illustrating a construction of the transfer device according to the first embodiment; 
       FIG. 3  is a flowchart illustrating processing steps performed in full-color image formation by a control circuit of the transfer device; 
       FIG. 4  is a block diagram illustrating a construction of a transfer device according to a second embodiment of the present invention; and 
       FIG. 5  is a flowchart illustrating processing steps performed in full-color image formation by a control circuit of the transfer device according to the second embodiment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a cross-sectional view illustrating a construction of an image forming apparatus including a transfer device according to a first embodiment of the present invention. An image forming apparatus  100  forms a multi-color or monochromatic image on a record medium such as a sheet of paper (hereinafter referred to merely as a sheet) based on image data transmitted externally. The image forming apparatus  100  has an exposure unit E, four photoreceptor drums (image carriers of the present invention)  101 A to  101 D, four developing units  102 A to  102 D, four charging rollers  103 A to  103 D, four cleaning units  104 A to  104 D, an intermediate transfer belt  11 , four first transfer rollers  13 A to  13 D, a second transfer roller  14 , a fusing device  15 , sheet transport paths P 1 , P 2 , and P 3 , a sheet feed cassette  16 , a manual sheet feed tray  17 , and a sheet catch tray  18 . 
   The transfer device of the present invention includes the intermediate transfer belt  11 , the first transfer rollers  13 , and the second transfer roller  14 . 
   The image forming apparatus  100  forms an image based on image data obtained by color separation from an original color image. The image data correspond to four colors, i.e. the three subtractive primary colors—yellow (Y), magenta (M), and cyan (C)—and black (K), respectively. There are four image forming sections PA to PD provided correspondingly to the four colors. The photoreceptor drums  101 A to  101 D, the developing units  102 A to  102 D, the charging rollers  103 A to  103 D, the first transfer rollers  13 A to  13 D, and the cleaning units  14 A to  14 D are provided, one each in each of the four image forming sections PA to PD. The image forming sections PA to PD are aligned in a direction in which the intermediate transfer belt  11  travels (or a sub scanning direction). 
   The charging rollers  103 A to  103 D are contact-type chargers provided for charging respective outer circumferential surfaces of the photoreceptor drums  101 A to  101 D uniformly so that the surfaces have a predetermined potential. The charging rollers  103 A to  103 D are replaceable with a contact-type charger using a charging brush or with a noncontact-type charging device. The exposure unit E has a not-shown semiconductor laser, a polygon mirror  4 , and reflecting mirrors  8 . The exposure unit E shines laser beams modulated depending on the image data for the four colors of black, cyan, magenta, and yellow on the photoreceptor drums  101 A to  101 D, respectively. Latent images corresponding to the four colors are thus formed on the photoreceptor drums  101 A to  101 D, respectively. 
   The developing units  102 A to  102 D feed the respective surfaces of the photoreceptor drums  101 A to  101 D carrying the latent images with toners, so that the latent images are developed into toner images. More specifically, the developing units  102 A to  102 D store therein black, cyan, magenta, and yellow toners, respectively, and develop the latent images formed on the photoreceptor drums  101 A to  101 D into black, cyan, magenta, and yellow toner images, respectively. The cleaning units  104 A to  104 D remove and collect residual toners on the respective surfaces of the photoreceptor drums  101 A to  101 D after developing and transferring operations. 
   Arranged above the photoreceptor drums  101 A to  101 D, the intermediate transfer belt  11  is stretched over a drive roller  11 A and a driven roller  11 B to form a loop traveling path. As the intermediate transfer belt  11  travels, an outer circumferential surface thereof faces the photoreceptor drum  101 D, the photoreceptor drum  101 C, the photoreceptor drum  101 B, and the photoreceptor drum  101 A, in the order. The first transfer rollers  13 A to  13 D are positioned to face the photoreceptor drums  101 A to  101 D, respectively, through the intermediate transfer belt  11 . First transfer regions of the present invention include the first transfer rollers  13 A to  13 D and the photoreceptor drums  101 A to  101 D, respectively. In the respective first transfer regions, a toner image is transferred from the drums  101 A to  101 D to the intermediate transfer belt  11 . 
   The intermediate transfer belt  11  is an endless belt formed with a film of 100 μm to 150 μm thickness. The intermediate transfer belt  11  has a resistance of 1×10 11  to 1×10 13  Ω·cm. A lower resistance causes power leakage from the intermediate transfer belt  11 , thereby preventing a sufficient level of transfer power for the first transfer operations from being maintained. A higher resistance requires a discharging device for discharging the intermediate transfer belt  11  each time after the belt  11  passes through the respective first transfer regions. 
   To the first transfer rollers  13 A to  13 D, a first transfer bias (or transfer power of the present invention) is applied at a constant voltage for transferring of the toner images as carried on the photoreceptor drums  101 A to  101 D onto the intermediate transfer belt  11 . The first transfer bias is opposite in polarity to the charge of the toners. The toner images for the respective colors are thus transferred sequentially and accumulated on the outer circumferential surface of the intermediate transfer belt  11  to form a full-color toner image. 
   When image data for only some of the four colors are input, latent image(s) and toner image(s) are formed only on some of the photoreceptor drums  101 A to  101 D, depending on the input color image data. In monochromatic image formation, for example, a latent image and a toner image are formed only on the photoreceptor drum  101 A corresponding to the color black. Accordingly, only a black toner image is transferred to the outer circumferential surface of the intermediate transfer belt  11 . 
   Each of the first transfer rollers  13 A to  13 D includes a metal (e.g. stainless steel) shaft of 8 to 10 mm diameter. A surface of the metal shaft is coated with conductive elastic material (e.g. EPDM or urethane foam), through which a high voltage is uniformly applied to the intermediate transfer belt  11 . The first transfer rollers  13 A to  13 D are replaceable with brush-type transfer members. 
   In addition, the first transfer rollers  13 A to  13 D are biased toward the photoreceptor drums  101 A to  101 D, respectively, in a direction other than respective normal directions of the photoreceptor drums  101 A to  101 D. 
   The rotation of the intermediate transfer belt  11  feeds the full-color or monochromatic toner image as transferred to the outer circumferential surface of the belt  11  to a position where the belt  11  faces the second transfer roller  14  (i.e. a second transfer region of the present invention). In image formation, the second transfer roller  14  is pressed at a predetermined nip pressure against the outer circumferential surface of the intermediate transfer belt  11  where a reverse, inner circumferential surface of the belt  11  is in contact with the drive roller  11 A. A high voltage opposite in polarity to the charge of the toners is applied to a sheet as fed from the sheet feed cassette  16  or from the manual sheet feed tray  17  as the sheet passes between the second transfer roller  14  and the intermediate transfer belt  11 . The full-color or monochromatic toner image is thus transferred from the outer circumferential surface of the intermediate transfer belt  11  to a surface of the sheet. 
   To maintain the predetermined nip pressure, either one of the second transfer roller  14  and the drive roller  11 A is a roller of hard material (i.e. metal), and the other is an elastic roller of soft material (i.e. elastic rubber or resin foam). 
   In some instances, some of the toners are not transferred to the sheet and remain on the intermediate transfer belt  11 . The residual toners are collected by a cleaning unit  12  to avoid mixture of toners of different colors in subsequent image formation. 
   The sheet with the full-color or monochromatic toner image transferred thereto is led into the fusing device  15  and passes between a heat roller  15 A and a pressure roller  15 B to be heated and pressed. The toner image is thus firmly fixed to the surface of the sheet. The sheet with the fixed toner image is then ejected onto the sheet catch tray  18  by sheet eject rollers  18 A. 
   The image forming apparatus  100  has the sheet transport path P 1  leading approximately vertically from the sheet feed cassette  16 , through a gap between the second transfer roller  14  and the intermediate transfer belt  11  and through the fusing device  15 , to the sheet catch tray  18 . Arranged along the sheet transport path P 1  are a pick-up roller  16 A, transport rollers R, registration rollers  19 , and the sheet eject rollers  18 A. The pick-up roller  16 A feeds sheets as stored in the sheet feed cassette  16 , sheet by sheet, into the sheet transport path P 1 . The transport rollers R transport a fed sheet upward. The registration rollers  19  lead the sheet between the second transfer roller  14  and the intermediate transfer belt  11  at a predetermined timing. The sheet eject rollers  18 A eject the sheet onto the sheet catch tray  18 . 
   The image forming apparatus  100  also has the sheet transport path P 2  leading from the manual sheet feed tray  17  to the registration rollers  19 . A pick-up roller  17 A and transport rollers R are arranged along the sheet transport path P 2 . Also provided is the sheet transport path P 3  leading from the sheet eject rollers  18 A to upstream of the registration rollers  19  on the sheet transport path P 1 . 
   The sheet eject rollers  18 A are rotatable in forward and backward directions. In single-side image formation, and in image formation on a second side of a sheet in double-side image formation, the sheet eject rollers  18 A are rotated in the forward direction, so that the sheet is ejected onto the sheet catch tray  18 . In image formation on a first side of the sheet in the double-side image formation, the sheet eject rollers  18 A are first rotated in the forward direction until a tail end of the sheet passes through the fusing device  15 . Then, with the tail end nipped therebetween, the eject rollers  18 A are rotated in the backward direction to feed the sheet into the sheet transport path P 3 . Thus, in the double-side image formation, the sheet having an image formed on the first side thereof is fed into the sheet transport path P 1 , the tail end first, with the second side facing the side of the drive roller A. 
   The registration rollers  19  feed a sheet as fed either from the sheet feed cassette  16  or the manual sheet feed tray  17 , or through the sheet transport path P 3 , between the second transfer roller  14  and the intermediate transfer belt  11  in synchronized timing with the rotation of the intermediate transfer belt  11 . 
   At the time the photoreceptor drums  101 A to  101 D and the intermediate transfer belt  11  start rotating, the registration rollers  19  have their own rotation stopped. A sheet as fed or transported before the intermediate transfer belt  11  initiates rotating is stopped, with a leading end thereof in contact with the registration rollers  19 . 
   Then, as the leading end of the sheet and a leading end of the toner image formed on the intermediate transfer belt  11  meet each other at the contact position of the second transfer roller  14  and the intermediate transfer belt  11 , the registration rollers  19  initiate rotating. 
   In the image forming apparatus as illustrated in  FIG. 1 , the first transfer rollers  13 A to  13 D included in the respective first transfer regions are provided along a lower portion of the loop traveling path of the intermediate transfer belt  11 . The image forming sections PA to PD including the rollers  13 A to  13 D are arranged in proximity to each other. The second transfer roller  14  is positioned immediately downstream of the first transfer roller  13 A that is arranged most downstream with respect to a traveling direction of the intermediate transfer belt  11 . 
   This positioning is aimed at achieving high-speed image formation as well as at downsizing the image forming apparatus in which a toner image is secondly transferred from the intermediate transfer belt  11  to a sheet as transported approximately vertically. The high-speed image formation is allowed by reducing time taken from initiation of first transfer process by the first transfer roller  13 D positioned most upstream, to completion of second transfer process by the second transfer roller  14 . 
   Consequently, transfer power supplied to the first transfer rollers  13 A to  13 D and the second transfer roller  14 , respectively, are likely to interfere with each other through the intermediate transfer belt  11 . 
   In the full color image formation involving toner image formation performed in all of the image forming sections PA to PD, the intermediate transfer belt  11  is pressed by all of the first transfer rollers  13 A to  13 D against the photoreceptor drums  101 A to  101 D, respectively. In the monochromatic image formation involving toner image formation performed only in the image forming section PA, the intermediate transfer belt  11  is pressed by only the first transfer roller  13 A against the photoreceptor drum  101 A. 
     FIG. 2  is a block diagram illustrating a construction of the transfer device according to the first embodiment. A transfer device  200  of the present invention includes a motor drive circuit  201 , a first transfer power supply circuit  202 , a second transfer power supply circuit  203 , and a control circuit  204 . The control circuit  204  is connected to a control section  110  of the image forming apparatus  100 . Upon receipt of input data from the control section  110 , the control circuit  204  outputs, according to a predetermined program, driving data for a motor M, and data on transfer power to be supplied to the first transfer rollers  13 A to  13 D and to the second transfer roller  14 A (hereinafter referred to merely as the transfer power data), to the motor drive circuit  201 , the first transfer power supply circuit  202 , and the second transfer power supply circuit  203 , respectively. 
   According to the driving data output from the control circuit  204 , the motor drive circuit  201  drives the motor M provided for rotating the drive roller  11 A. According to the transfer power data output from the control circuit  204 , the first transfer power supply circuit  202  supplies transfer power to each of the first transfer rollers  13 A to  13 D. According to the transfer power data output from the control circuit  204 , the second transfer power supply circuit  203  supplies transfer power to the second transfer roller  14 . 
   In the first transfer operations performed by the first transfer rollers  13 A to  13 D, a constant-voltage control allows stable supply of transfer power to the transfer rollers  13 A to  13 D. This is because a toner image is transferred to the intermediate transfer belt  11  that is relatively electrically stable. In the second transfer operation performed by the second transfer roller  14 , in contrast, a constant-current control is required for stable supply of transfer power to the transfer roller  14 . This is because the toner image is transferred to a sheet with electrical properties varying depending on the type, thickness, and moisture content thereof. 
   Thus, the first transfer power supply circuit  202  supplies a predetermined level of transfer power to each of the first transfer rollers  13 A to  13 D at a constant voltage. The second transfer power supply circuit  203  supplies a predetermined level of transfer power to the second transfer roller  14  at a constant current. 
     FIG. 3  is a flowchart illustrating processing steps performed in the full-color image formation by the control circuit of the transfer device. The control circuit  204  awaits input of operation initiation data from the control section  110  (step S 1 ). The operation initiation data is used for specifying the timing of initiating an image forming operation. Upon input of the operation initiation data, the control circuit  204  outputs the driving data for the motor M to the motor drive circuit  201 , thereby causing the intermediate transfer belt  11  to initiate traveling on the travel path (step S 2 ). 
   Then, the control circuit  204  awaits input of transfer initiation data from the control section  110  (step S 3 ). The transfer initiation data is used for specifying the timing of initiating a first transfer operation of a toner image formed on the photoreceptor drum  101 D being transferred to the intermediate transfer belt  11  by the first transfer roller  13 D in a first transfer region provided in the image forming section PD which is positioned most upstream with respect to the traveling direction of the belt  11 . 
   Upon input of the transfer initiation data, the control circuit  204  turns on a timer T for measuring a predetermined time period (step S 4 ). Then, the circuit  204  outputs the transfer power data to the first transfer power supply circuit  202  and to the second transfer power supply circuit  203  in order to initiate supplying transfer power to the first transfer rollers  13 A to  13 D and to the second transfer roller  14  (steps S 5  and S 6 ). 
   The predetermined time period to be measured by the timer T is time taken for the intermediate transfer belt  11  to travel a distance, plus a sheet length, from the first transfer roller  13 A to the second transfer roller  14 . More specifically, the timer T measures time elapsed from the initiation of the first transfer operation to the completion of the second transfer operation, in an image formation process performed on a sheet. 
   The control circuit  204  then waits until the timer T has measured the time elapsed (step S 7 ). Next, the circuit  204  determines whether process completion data for indicating the completion of the image forming process is input from the control section  110  (step S 8 ). When the image forming apparatus  100  has no subsequent image data to be processed and the process completion data is input from the control section  110 , the control circuit  204  stops the first transfer circuit  202  and the second transfer circuit  203  from supplying the transfer power (steps S 9  and S 10 ), and stops the motor drive circuit  201  from driving the motor M (step S 11 ). 
   As described above, the control circuit  204  initiates supplying the transfer power to the first transfer rollers  13 A to  13 D and to the second transfer roller  14  when the first transfer operation by the first transfer roller  13 D is initiated. Then, the circuit  204  stops supplying the transfer power to the first transfer rollers  13 A to  13 D and to the second transfer roller  14  when the second transfer operation by the second transfer roller  14  is completed. 
   Accordingly, the transfer power supplied to the first transfer rollers  13 A to  13 D and the second transfer roller, respectively, is free from fluctuation during a period from the initiation of the first transfer operation by the roller  13 D to the completion of the second transfer operation, even if the transfer power as supplied interfere with each other through the intermediate transfer belt  11 . The first transfer operations and the second transfer operation are thus performed in a stable manner. 
   In the monochromatic image formation involving image formation performed only in the image forming section PA, the transfer power is supplied only to the first transfer roller  13 A and not to the first transfer rollers  13 B to  13 D. In step S 3 , thus, the control circuit  204  awaits input of transfer initiation data for specifying the timing of initiating a first transfer operation of a toner image formed on the photoreceptor drum  101 A being transferred to the intermediate transfer belt  11  by the first transfer roller  13 A. In steps S 5  and S 9 , respectively, the circuit  204  initiates, and stops, supplying transfer power only to the first transfer roller  13 A. 
   Alternatively, in step S 3 , the control circuit  204  awaits input from the control section  110  of data for specifying the timing of initiating a developing operation in the image forming section in which the first transfer operation is first to be performed (i.e., the image forming section PD in the full-color image formation or the image forming section PA in the monochromatic image formation). The alternative allows earlier initiation of supplying the transfer power before the first transfer operation is first performed, thereby ensuring that the first transfer operation is performed at an appropriate level of transfer power, even if it takes some time for the transfer power to reach a predetermined level after the initiation of supply thereof. 
   Also, an appropriate level of transfer power to be supplied to the first transfer rollers  13 B to  13 D and to the second transfer roller  14  varies depending on environmental conditions such as temperature or humidity. Therefore, an absolute level of transfer power to be supplied is modulated according to the result of detection by a not-shown environmental sensor. 
   Additionally, in consecutive image formation where a single job involves a plurality of sheets undergoing consecutive image formation processes, the first transfer power supply circuit  202  and the second transfer supply circuit  203  continue to supply transfer power during a period from the initiation of first transfer operation to a first sheet in the image forming section most upstream with respect to the traveling direction of the intermediate transfer belt  11  to the passage through the second transfer region of a tail end of a last sheet. 
     FIG. 4  is a block diagram illustrating a construction of a transfer device  200  according to a second embodiment of the present invention. The transfer device  200  is provided with a photoreceptor power supply circuit  205  for applying a predetermined level of voltage to the photoreceptor drums  101 A to  101 D, as well as the components as illustrated in  FIG. 2 . The voltage applied from the circuit  205  to the drums  101 A to  101 D has such polarity and value as to prevent extra toners from being attracted to the photoreceptor drums  101 A to  101 D. More specifically, the circuit  205  applies a high voltage having the same polarity as the toners to a conductive base material of the photoreceptor drums  101 A to  101 D. 
     FIG. 5  is a flowchart illustrating part of processing steps performed by the control circuit of the transfer device  200  according to the second embodiment. In addition to the processing steps as illustrated in  FIG. 3 , the control circuit  204  of the transfer device  200  follows processing steps (steps S 21  to S 28 , and S 29 ) of applying a predetermined level of voltage to the photoreceptor drums  101 A to  101 D, respectively, through the photoreceptor power supply circuit  205  within a period from the completion of developing operation in each of the image forming sections PA to PD to the completion of the second transfer operation. The completion of developing operation in each of the sections PA to PD is determined by measuring a predetermined time period from the moment the operation initiation data is input in step S 1 . 
   When the first transfer rollers in the image forming sections which have completed the first transfer operation have a continued supply of transfer power, the application of voltage thus prevents extra toners from being transferred from the photoreceptor drums  101 A to  101 D to the intermediate transfer belt  11 . Toners are thus prevented from being consumed wastefully or from contaminating the interior of the image forming apparatus. 
   In the monochromatic image formation, the control circuit  204  applies the predetermined level of voltage through the photoreceptor power supply circuit  205  to only the photoreceptor drum  101 A in the image forming section PA. 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.