Patent Publication Number: US-10761471-B2

Title: Image forming apparatus that removes residual charges on image carrier

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2018-157156, filed on Aug. 24, 2018, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to an image forming apparatus. 
    
    
     
       BACKGROUND 
       An image forming apparatus develops an electrostatic latent image that has been formed on a photoconductive drum by using toner. After a toner image is transferred onto an intermediate transfer belt, the charge on the photoconductive drum is removed. If a light used in removing the photoconductive drum charge leaks to an upstream side of a toner image transfer position, image defects may result. 
       An image forming apparatus that can suppress image defects is desirable. 
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically illustrating a configuration of an image forming apparatus according to an embodiment. 
         FIG. 2  is a diagram schematically illustrating a configuration of an image forming section. 
         FIG. 3  is a diagram illustrating a charge removing position. 
         FIG. 4  is a diagram illustrating a charge removing effect at the charge removing position. 
         FIG. 5  is a diagram illustrating an effective light receiving width of charge removing light. 
         FIG. 6  is a diagram illustrating comparison of width dimensions among respective portions of the image forming section. 
     
    
    
     DETAILED DESCRIPTION 
     According to an embodiment, an image forming apparatus includes an image carrier, a transfer member, and a light source. The image carrier has a photoconductive layer on a surface of the image carrier. The transfer member is configured to cause a toner image formed on the surface of the image carrier to be transferred to a transferee member at a transfer position of the image carrier. The light source is configured to emit light toward the surface of the image carrier after transfer of the toner image to remove residual charges on the photoconductive layer. An effective light receiving width of the image carrier in a width direction thereof is less than a width of the transfer member. 
     Hereinafter, an image forming apparatus of an example embodiment is described with reference to the accompanying drawings. 
     In the present application, an X direction, a Y direction, and a Z direction are defined for explanatory convenience. The Y direction is a width direction parallel to a rotational axis of the photoconductive drum. The Z direction is a generally vertical direction. The X direction is a generally horizontal direction and is orthogonal to the Y and Z directions. 
       FIG. 1  is a diagram schematically illustrating a configuration of an image forming apparatus according to the embodiment. The image forming apparatus  1  includes a scanner section  12 , an image processing section  14 , a sheet feed section  16 , a registration roller  18 , a printing section  20 , a fixing section  52  and a sheet discharge section  58 . 
     The scanner section  12  scans an image that has been formed on a sheet. For example, the scanner section  12  scans an image on a sheet to generate image data of three component colors including red (R), green (G), and blue (B). The scanner section  12  outputs the generated image data to the image processing section  14 . 
     The image processing section  14  converts the image data to color signals for respective colors. For example, the image processing section  14  converts the image data to color signals for four colors including yellow (Y), magenta (M), cyan (C), and black (K). The image processing section  14  controls an exposure section  26  based on the color signals for respective colors. 
     The printing section  20  forms an output image (hereinafter, referred to as a toner image) with toner (developer) based on the image data received from the image processing section  14 . The printing section  20  transfers the toner image onto a surface of a sheet. The details of the printing section  20  are described below. 
     The sheet feed section  16  supplies sheets to the registration roller  18  one by one in accordance with a timing at which the printing section  20  forms the toner image. 
     The registration roller  18  aligns a position of a leading end of the sheet in a conveyance direction of the sheet by adjusting the sheet at a nip. The registration roller  18  conveys the sheet in accordance with a timing at which the printing section  20  transfers the toner image onto the sheet. 
     The fixing section  52  applies heat and pressure to the sheets to fix the transferred toner image to each sheet. The fixing section  52  discharges the sheets to the sheet discharge section  58 . 
     The sheet discharge section  58  is used to store the discharged sheet(s). For example, the sheet discharge section  58  is a sheet discharge tray. 
     The printing section  20  includes an image forming section  30 , an intermediate transfer belt  22 , and a transfer section  28 . 
     The image forming section  30  has a photoconductive drum  31 . The image forming section  30  forms a toner image corresponding to the image data on the photoconductive drum  31 . Along the intermediate transfer belt  22 , a plurality of image forming sections  30 Y,  30 M,  30 C, and  30 K is arranged. The plurality of the image forming sections  30 Y,  30 M,  30 C, and  30 K forms toner images with yellow toner, magenta toner, cyan toner, and black toner, respectively. Each image forming section  30  first transfers the toner image from the photoconductive drum  31  to the intermediate transfer belt  22 . Details of the image forming section  30  are described below. 
     The toner image on the surface of the photoconductive drum  31  is first transferred onto the intermediate transfer belt  22 . 
     The transfer section  28  transfers the toner image from the intermediate transfer belt  22  to the surface of the sheet at a secondary transfer position. 
       FIG. 2  is a diagram schematically illustrating a configuration of an image forming section  30 . The image forming section  30  includes the photoconductive drum  31  (also referred to as an image carrier  31 ), a charging member  32 , an exposure section  26 , a developing mechanism  34 , a transfer member  36 , a charge removing member  38 , and a cleaning mechanism  40 . The components of the image forming section  30  are provided in plane symmetrical manner with respect to an XZ plane passing through the center of the photoconductive drum  31  in the Y direction. 
     The photoconductive drum  31  carries an electrostatic latent image. The photoconductive drum  31  has a cylindrical shape. The photoconductive drum  31  can rotate around a rotation axis  31   c . A photoconductive layer  31   f  is formed on an outer peripheral surface of the photoconductive drum  31 . The photoconductive layer  31   f  is an optical semiconductor material or the like. In this context, the optical semiconductor material has a property of retaining static electricity in a dark (unexposed) place (i.e., a place at which no light has been applied) and releasing the static electricity in portions irradiated with light. In other words, in the photoconductive layer  31   f , the static electrical charge is removed in those portions irradiated with light. 
     The charging member  32  performs a charging process to charge the surface of the photoconductive drum  31 . The charging member  32  has a discharge source such as a wire or a needle. If a high voltage is applied to the discharge source, the discharge source performs corona discharge. In this way, a charge moves to the surface of the photoconductive drum  31 . For example, the charging member  32  charges the surface of the photoconductive drum  31  with negative charge. In other examples, the charging member  32  may charge the surface of the photoconductive drum  31  with a roller, a brush, or the like. 
     The exposure section  26  performs an exposure process including scanning and exposing the surface of the photoconductive drum  31  based on the image data received from the image processing section  14 . The exposure section  26  has a scanning optical system. The scanning optical system includes a light source, a polygon mirror, and the like. For example, the light source is a laser light source or an LED (Light Emitting Diode) light source. The polygon mirror reflects the light emitted from the light source while rotating. In this way, the exposure section  26  scans and exposes the surface of the photoconductive drum  31 . The negative charge in the exposed portions of the surface of the photoconductive drum  31  is removed. In this way, an electrostatic latent image based on the image data is formed on the surface of the photoconductive drum  31 . 
     The developing mechanism  34  performs a developing process of attaching the toner to the photoconductive drum  31  to develop the electrostatic latent image. The developing mechanism  34  contains toner and carrier. The developing mechanism  34  has a developing member  35  for feeding the toner to the photoconductive drum  31 . A longitudinal direction of the developing member  35  is parallel to the Y direction. The developing member  35  has a width (third width) W 35  in the Y direction (refer to  FIG. 6 ). For example, the developing member  35  is a developing roller having a cylindrical sleeve and a magnet roller arranged at the inner side of the sleeve. The sleeve rotates around the magnet roller in a state in which the toner and the carrier adhere to the outer peripheral surface of the sleeve. The toner is negatively charged. In a portion of the photoconductive drum  31  where the latent electrostatic image is formed, the negative charge is removed. The other portion of the photoconductive drum  31  still has the negative charge. Therefore, the toner is attached only to the portion of the photoconductive drum  31  where the latent electrostatic image is formed by electrostatic force. The electrostatic latent image on the photoconductive drum  31  is developed with the toner to form a toner image on the surface of the photoconductive drum  31 . In the photoconductive drum  31 , a position at which a distance to the developing member  35  is shortest is a developing position  35   p  in a circumferential direction of the photoconductive drum  31  (hereinafter, simply referred to as a developing position). The developing member  35  performs the developing process at the developing position  35   p.    
     The transfer member  36  performs a transfer process for transferring the toner image on the photoconductive drum  31  onto the intermediate transfer belt  22 . The intermediate transfer belt  22  is an example of a transferee member. A longitudinal direction of the transfer member  36  is parallel to the Y direction. The transfer member  36  has a width (second width) W 36  in the Y direction (refer to  FIG. 6 ). For example, the transfer member  36  is a transfer roller. The transfer member  36  is arranged to face the photoconductive drum  31  across the intermediate transfer belt  22 . The transfer member  36  presses the intermediate transfer belt  22  against the photoconductive drum  31 . The transfer member  36  causes the intermediate transfer belt  22  to contact the photoconductive drum  31 . The transfer member  36  applies a high voltage to charge the intermediate transfer belt  22  with a positive charge. The toner negatively charged is attached to the intermediate transfer belt  22  by the electrostatic force. Thereby, the toner image on the photoconductive drum  31  is transferred onto the intermediate transfer belt  22 . A contact position of the intermediate transfer belt  22  with the photoconductive drum  31  is a transfer position  36   p  in the circumferential direction of the photoconductive drum  31 . The transfer member  36  performs the transfer process at the transfer position  36   p.    
     The charge removing member  38  performs a charge removing process for removing the charge remaining on the surface of the photoconductive drum  31 . A longitudinal direction of the charge removing member  38  is parallel to the Y direction. The charge removing member  38  has a width W 38  in the Y direction (refer to  FIG. 6 ). For example, the charge removing member  38  include a plurality of LED light sources arranged in the Y direction. The charge removing member  38  emits light towards the photoconductive layer  31   f  on the surface of the photoconductive drum  31 . The charge removing member  38  lowers an electrical resistance of the photoconductive layer  31   f  to remove the charge on the surface by neutralization. The charge remaining on the surface of the photoconductive drum  31  is removed through the light emitted from the charge removing member (hereinafter, referred to as charge removing light). The charge removing member  38  keeps a surface potential of the photoconductive layer  31   f  constant before the charging process. In this way, the surface of the photoconductive drum  31  is uniformly charged in the charging process. A position at which the charge removing light is incident on the photoconductive drum  31  is a charge removing position  38   p  in the circumferential direction of the photoconductive drum  31  (hereinafter, simply referred to as a charge removing position). The charge removing member  38  performs the charge removing process at the charge removing position  38   p.    
     The cleaning mechanism  40  performs a cleaning process for removing the toner remaining on the surface of the photoconductive drum  31 . The cleaning mechanism  40  has a blade  42  as a cleaning member, a cover  44 , an auger  46 , and a sealing member  45 . 
     The blade  42  scrapes the toner remaining on the surface of the photoconductive drum  31  to remove the toner. The blade  42  is made of a rubber material or the like. A leading end of the blade  42  abuts against the surface of the photoconductive drum  31 . The position where the blade  42  abuts against the photoconductive drum  31  is referred to as a cleaning position  42   p  in the circumferential direction of the photoconductive drum  31 . The blade  42  performs a cleaning process at the cleaning position  42   p  of the photoconductive drum  31 . 
     The cover  44  prevents the scraped toner from scattering. 
     The auger  46  recovers the scraped toner and conveys it to a waste toner container. 
     The sealing member  45  seals a space between the cover  44  and the photoconductive drum  31 . The sealing member  45  prevents the scraped toner from flowing out from the space between the cover  44  and the photoconductive drum  31 . The sealing member  45  is made of a polymer film or urethane foam. The leading end of the sealing member  45  contacts the surface of the photoconductive drum  31 . The position where the sealing member  45  contacts the photoconductive drum  31  is a sealing position  45   p  in the circumferential direction of the photoconductive drum  31 . The sealing member  45  prevents the toner removed by the blade  42  from scattering at the sealing position  45   p.    
     The image forming section  30  repeatedly performs the charging process, the exposure process, the developing process, the transfer process, the charge removing process, and the cleaning process described above in this order. In this way, the image forming section  30  forms a toner image on the surface of the photoconductive drum  31 . The image forming section  30  transfers the toner image on the photoconductive drum  31  onto the intermediate transfer belt  22 . 
     The charge removing position and the charge removing effect are described. 
       FIG. 3  is a diagram illustrating the charge removing position. 
     For example, A to E shown in  FIG. 3  are considered as charge removing positions  38   p . The charge removing positions A and B are positions on an upstream side in a rotation direction of the photoconductive drum  31  (hereinafter, simply referred to as an upstream side) with respect to the cleaning position  42   p . The charge removing positions A and B are between the transfer position  36   p  and the cleaning position  42   p . The charge removing positions C to E are positions on a downstream side in the rotation direction of the photoconductive drum  31  (hereinafter, simply referred to as a downstream side) with respect to the cleaning position  42   p.    
     The charge removing effect by the charge removing member  38  is evaluated by the charge remaining on the surface of the photoconductive drum  31  after charge removing. The photoconductive drum  31  contacts the intermediate transfer belt  22  which is positively charged in the transfer process. Therefore, the surface of the photoconductive drum  31  after the transfer process can be affected by this positive charge. As a result, it may become difficult to negatively charge the surface of the photoconductive drum  31  in the subsequent charging process. Therefore, in the charge removing process between the transfer process and the charging process, it is desirable to remove the positive charge as well as any negative charge. In other words, it is desirable that an amount of the positive charge remaining on the surface of the photoconductive drum  31  after charge removal processing is small. It is desirable that a positive potential of the surface of the photoconductive drum  31  after charge removing is small. The electric potential of the surface of the photoconductive drum  31  after charge removal is measured at a position Mp on the upstream side of the developing member  35 . For a state in which the charging member  32  and the exposure section  26  have been removed, the potential after charge removing is measured. 
       FIG. 4  is a diagram illustrating the charge removing effect. 
     As shown in  FIG. 4 , in the case of the charge removing positions E and D, the potential is the positive potential, and the positive charge remains. In the case of the charge removing position C, the potential is almost zero, and the positive charge has been removed. In the case of the charge removing positions B and A, the potential is the negative potential. In other words, the positive charge has been removed, and the surface of the photoconductive drum  31  has been furthermore negatively charged. According to this result, it can be understood that the charge removing effect becomes higher as the charge removing position  38   p  is positioned further on the upstream side. It is considered in this context that the surface charge is naturally neutralized as time goes by after charge removal processing. 
     Based on the result of  FIG. 4 , the charge removing position  38   p  is set in a first area A 1  (see  FIG. 2 ) between the transfer position  36   p  and the cleaning position  42   p . In this way, the charge remaining on the surface of the photoconductive drum  31  can be sufficiently removed. Before the charging process, the surface potential of the photoconductive layer  31   f  is thus kept constant. 
     The charge removing member  38  is arranged on the outer side of the cleaning mechanism  40 . In the case in which the charge removing position  38   p  is set between the sealing position  45   p  and the cleaning position  42   p , it is difficult for the charge removing light to be incident at the particular charge removing position  38   p  from the charge removing member  38 . Therefore, it is desirable that the charge removing position  38   p  is arranged in a second area A 2  (see  FIG. 2 ) between the transfer position  36   p  and the sealing position  45   p . Thereby, even when the charge removing member  38  is arranged on the outer side of the cleaning mechanism  40 , the charge removing light from the charge removing member  38  can be incident at the charge removing position  38   p.    
     A wraparound action of the charge removing light is described. 
     The charge removing light emitted from the charge removing member  38  spreads while travelling. Therefore, a part of the charge removing light may act on a third area A 3  (see  FIG. 2 ) between the developing position  35   p  and the transfer position  36   p . However, in the third area A 3 , on the surface of the photoconductive drum  31 , there is an image forming portion on which a toner image is formed. In the third area A 3 , a non-image forming portion other than the image forming portion is negatively charged. If the charge removing light acts on this non-image forming portion, the negative charge thereof is removed. Thus, the toner might jump out from the image forming portion to the non-image forming portion. As a result, image defects may occur. 
     As described above, the charge removing position  38   p  is arranged in the first area A 1 . The charge removing position  38   p  in this case is thus potentially close to the third area A 3 . Therefore, the charge removing light tends to act on the third area A 3 . The transfer member  36  causes the intermediate transfer belt  22  to contact the photoconductive drum  31 . There is no gap between the intermediate transfer belt  22  and the photoconductive drum  31  at the inner side of the transfer member  36  in the Y direction. On the outer side of the transfer member  36  in the Y direction, a gap is formed between the intermediate transfer belt  22  and the photoconductive drum  31 . The charge removing light penetrates this gap, travels around the outer side of the transfer member  36  in the Y direction, and acts on the third area A 3 . As a result, image defects occur in an end area of an image forming area in the Y direction. 
       FIG. 5  is a diagram illustrating an effective light receiving width for the charge removing light. The horizontal axis in  FIG. 5  is a position in the Y direction at the charge removing position  38   p  in the circumferential direction of the photoconductive drum  31 . CL in  FIG. 5  refers to a center of the photoconductive drum  31  in the Y direction (width direction). A vertical axis in  FIG. 5  is a light receiving amount (%) at each position in the Y direction when the maximum light receiving amount of the charge removing light at the charge removing position  38   p  is 100%. 
     The charge removing member  38  is formed in plane symmetry manner with respect to an XZ plane including the center CL. The width of the charge removing member  38  in the Y direction is W 38 . At the charge removing position  38   p , in an area corresponding to the width W 38  of the charge removing member  38 , the light receiving amount of the charge removing light reaches the maximum light receiving amount (100%). The charge removing light emitted from the charge removing member  38  travels while spreading. At the charge removing position  38   p , in an area on the outer side of the charge removing member  38  in the Y direction, the light receiving amount is smaller than the maximum light receiving amount. The light receiving amount decreases further towards the outer side of the charge removing member  38  in the Y direction. As a result of the experiment, it is found that the image is not adversely affected in the area where the light receiving amount is less than 80% of the maximum light receiving amount. It is considered that if the light receiving amount is less than 80% of the maximum light receiving amount, the charge removing effect is small and the toner does not jump out. In contrast, in an area in which the light receiving amount is 80% or more of the maximum light receiving amount, a sufficient charge removing effect can be achieved. At the charge removing position  38   p , the width at which the light receiving amount of charge removing light is 80% or more of the maximum light receiving amount is referred to as an effective light receiving width (first width) EW 38 . The effective light receiving width EW 38  is larger than the width W 38  of the charge removing member  38 . The charge removing member  38  removes the charge of the photoconductive drum  31  at the effective light receiving width EW 38  at the charge removing position  38   p.    
       FIG. 6  is a diagram illustrating comparison of the width dimensions of respective portions of the image forming section. CL in  FIG. 6  is the center of the photoconductive drum  31  in the Y direction (width direction). 
     The width (second width) W 36  of the transfer member  36  in the Y direction is larger than the effective light receiving width EW 38 . In this way, the charge removing light emitted to the inner side of the effective light receiving width EW 38  (hereinafter, referred to as inner charge removing light) is blocked by the intermediate transfer belt  22  pressed towards the photoconductive drum  31  by the transfer member  36 . The inner charge removing light does not act on the third area A 3  of the photoconductive drum  31 . Therefore, the charge in the third area A 3  is not removed by the inner charge removing light, and the toner does not jump out. On the other hand, the charge removing light emitted to the outer side of the effective light receiving width EW 38  (hereinafter, referred to as an outer charge removing light) penetrates the gap between the intermediate transfer belt  22  and the photoconductive drum  31  on the outer side of the transfer member  36  in the Y direction. The outer charge removing light may act on the third area A 3  of the photoconductive drum  31 . Even in this case, only the outer charge removing light having a small charge removing effect acts on the third area A 3 . Therefore, the charge in the third area A 3  is not removed by the outer charge removing light, and the toner does not jump out. Thus, the occurrence of image defects can be suppressed. 
     As shown in  FIG. 6 , the width (third width) W 35  of the developing member  35  in the Y direction is smaller than both the width W 36  of the transfer member  36  and the effective light receiving width EW 38 . In this way, a range in which the toner adheres to the photoconductive drum  31  is restricted. Therefore, image defects can be suppressed. 
     As described in detail above, the image forming apparatus  1  of the embodiment has the photoconductive drum  31 , the charge removing member  38 , and the transfer member  36 . The photoconductive drum  31  has the photoconductive layer  31   f  in which the charge of a light irradiated area is removed on the surface. The charge removing member  38  irradiates the surface of the photoconductive drum  31  with the light at the charge removing position  38   p  of the photoconductive drum  31 . The charge removing member  38  removes the charge of the photoconductive drum  31  in the Y direction at the effective light receiving width EW 38 . The transfer member  36  has the width W 36  in the Y direction. The transfer member  36  transfers the toner attached to the photoconductive drum  31  onto the intermediate transfer belt  22  at the transfer position  36   p  of the photoconductive drum  31 . The width W 36  of the transfer member  36  is larger than the effective light receiving width EW 38 . 
     According to such a configuration, the inner charge removing light emitted to the inner side of the effective light receiving width EW 38  is blocked by the intermediate transfer belt  22  pressed towards the photoconductive drum  31  by the transfer member  36 . The inner charge removing light does not act on the third area A 3  on the upstream side of the transfer member  36 . Therefore, the charge in the third area A 3  is not removed by the inner charge removing light, and the toner does not jump out. On the other hand, the outer charge removing light emitted to the outer side of the effective light receiving width EW 38  penetrates the gap between the intermediate transfer belt  22  and the photoconductive drum  31  on the outer side of the transfer member  36 . The outer charge removing light may act on the third area A 3  of the photoconductive drum  31 . Even in this case, only the outer charge removing light having the small charge removing effect acts on the third area A 3 . Therefore, the charge in the third area A 3  is not removed by the outer charge removing light, and the toner does not jump out. Thus, the occurrence of image defects can be suppressed. 
     The effective light receiving width EW 38  is a width at which the light receiving amount of the surface of the photoconductive drum  31  irradiated by the charge removing member  38  is 80% or more of the maximum light receiving amount. 
     According to this configuration, the charge removing effect of the outer charge removing light emitted to the outer side of the effective light receiving width EW 38  is small. Therefore, the charge in the third area A 3  of the photoconductive drum  31  is not removed by the outer charge removing light. Therefore, the occurrence of image defects can be suppressed. 
     The image forming apparatus  1  has the blade  42  that removes the toner remaining on the photoconductive drum  31  at the cleaning position  42   p  of the photoconductive drum  31 . The charge removing position  38   p  is set between the transfer position  36   p  and the cleaning position  42   p.    
     According to such a configuration, a sufficient charge removing effect can be achieved before the charging process. However, since the charge removing position  38   p  is close to the third area A 3  of the photoconductive drum  31 , the charge removing light tends to act on the third area A 3 . Even in this case, the occurrence of the image defects can be suppressed as described above. 
     The developing member  35  has the width W 35  in the Y direction. 
     The developing member  35  attaches the toner to the photoconductive drum  31  for developing at the developing position  35   p  of the photoconductive drum  31 . The width W 35  of the developing member  35  is smaller than the effective light receiving width EW 38 . 
     According to such a configuration, the range in which the toner adheres to the photoconductive drum  31  is restricted. Therefore, the occurrence of image defects can be suppressed. 
     The sealing member  45  prevents the toner removed by the blade  42  from scattering at the sealing position  45   p  between the transfer position  36   p  and the cleaning position  42   p  of the photoconductive drum  31 . The charge removing position  38   p  is set between the transfer position  36   p  and the sealing position  45   p.    
     According to such a configuration, even when the charge removing member  38  is arranged on the outer side of the cleaning mechanism  40 , the charge removing light can be incident at the charge removing position  38   p  from the charge removing member  38 . 
     According to at least one embodiment described above, the transfer member  36  has the width W 36  that is larger than the effective light receiving width EW 38  of the charge removing light. Thereby, the occurrence of image defects can be suppressed. 
     While certain embodiments of the present invention have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the invention. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the invention. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.