Patent Publication Number: US-10761452-B2

Title: Developing device including a resin regulating blade

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to a developing device including a resin regulating blade. 
     Description of the Related Art 
     A developing device described in Japanese Patent Laid-Open No. 2015-34929 includes a resin developer regulating member molded from resin, and a resin developing frame member molded from resin. 
     The developing device includes a developing frame member, a rotatable developer carrier configured to carry a developer to develop an electrostatic latent image formed on an image bearing member, and a regulating blade as a developer regulating member configured to regulate the amount of developer carried on the developer carrier. The regulating blade is, across a direction parallel to a rotational axis of the developer carrier, arranged facing the developer carrier through a predetermined gap (hereinafter referred to as a “SB gap”) from the developer carrier. The SB gap is the minimum distance between the developer carrier and the regulating blade. The size of the SB gap is adjusted such that the amount of developer conveyed to a development area where the developer carrier faces the image bearing member is adjusted. 
     In association with an increase in the width of a sheet on which an image is formed, the length of the area (the maximum image area of the developing frame member) of the developing frame member corresponding to the maximum image area of an image area where an image can be formed on the image bearing member is increased in the direction parallel to the rotational axis of the developer carrier. 
     In a case where the developing frame member is molded from resin by injection molding, a gate portion as an inlet through which the resin flows into the molded article through a gate when the molten resin is poured into the molded article through the gate is provided at the developing frame member as the resin molded article. When a developing frame member with a great length in a longitudinal direction is molded from resin, a distance for circulating the molten resin is long, and therefore, the gate portion is typically provided in the maximum image area of the resin developing frame member such that the molten resin efficiently flows in the longitudinal direction of the developing frame member. 
     In a case where the developing frame member is molded from resin by injection molding, great molding pressure is on the gate portion when the molten resin flows into the gate portion through the gate, and therefore, residual stress is generated at the gate portion. The residual stress from the gate portion provided at the resin developing frame member is on the developing frame member over time, and deforms the resin developing frame member over time. As a result, in a state that the resin regulating blade is fixed to the resin developing frame member, the size of the SB gap might fluctuate over time due to the residual stress from the gate portion provided at the developing frame member. 
     SUMMARY OF THE INVENTION 
     An aspect of the present disclosure is to provide a developing device configured so that temporal fluctuation in the size of a SB gap due to residual stress from gate portions provided at a resin developing frame member can be reduced in a state that a resin regulating blade is fixed to the resin developing frame member. 
     Another aspect of the present disclosure is to provide a developing device including a developing rotary member configured to carry and convey a developer including toner and a carrier toward a position at which an electrostatic image formed on an image bearing member is developed; a resin regulating blade arranged facing the developing rotary member in a non-contact manner and configured to regulate the amount of the developer carried on the developing rotary member; a developing frame member at least including a first chamber where the developer is supplied to the developing rotary member, a second chamber divided from the first chamber by a partition wall, and an attachment portion for attachment of the regulating blade, the attachment portion being provided in the maximum image area of an image area of the image bearing member where an image can be formed on the image bearing member in a rotational axis direction of the developing rotary member; a first conveying screw arranged in the first chamber and configured to convey the developer of the first chamber in a first conveying direction; and a second conveying screw arranged in the second chamber and configured to convey the developer of the second chamber in a second conveying direction as the opposite direction of the first conveying direction. The regulating blade is fixed to the area of the attachment portion corresponding to the maximum image area of the image bearing member in the rotational axis direction of the developing rotary member in a state that the regulating blade is deflected such that a gap between the developing rotary member supported on the developing frame member and the regulating blade attached to the attachment portion falls within a predetermined range across the rotational axis direction of the developing rotary member. When the area of the developing frame member corresponding to the maximum image area of the image bearing member is, in a section perpendicular to a rotational axis of the developing rotary member, divided by a straight line passing through the center of rotation of the developing rotary member and the center of rotation of the image bearing member and a perpendicular line passing through the center of rotation of the first conveying screw with respect to the straight line, a gate portion is provided at a bottom portion of the developing frame member in a divided area of the developing frame member not provided with the attachment portion, and a gate portion is not provided at the bottom portion of the developing frame member in a divided area of the developing frame member provided with the attachment portion. 
     Further features of the present disclosure will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a configuration of an image forming device. 
         FIG. 2  is a perspective view of a configuration of a developing device. 
         FIG. 3  is a perspective view of the configuration of the developing device. 
         FIG. 4  is a sectional view of the configuration of the developing device. 
         FIG. 5  is a perspective view of a configuration of a resin doctor blade (a single member). 
         FIG. 6  is a perspective view of a configuration of a resin developing frame member (a single member). 
         FIG. 7  is a schematic view for describing stiffness of the resin doctor blade (the single member). 
         FIG. 8  is a schematic view for describing stiffness of the resin developing frame member (the single member). 
         FIG. 9  is a schematic view for describing straightness of the resin doctor blade (the single member). 
         FIG. 10  is a perspective view for describing deformation of the resin doctor blade due to a temperature change. 
         FIG. 11  is a sectional view for describing deformation of the resin doctor blade due to developer pressure. 
         FIG. 12  is a perspective view of a configuration of a developing device according to a first embodiment. 
         FIG. 13  is a sectional view of the configuration of the developing device according to the first embodiment. 
         FIG. 14  is a lower view of the configuration of the developing device according to the first embodiment. 
         FIG. 15  is a perspective view of a configuration of a developing device according to a comparative example. 
         FIG. 16  is a sectional view of the configuration of the developing device according to the comparative example. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the attached drawings. Note that the embodiments below are not intended to limit the present disclosure according to the scope of the claims, and all combinations of features described in a first embodiment are not necessarily essential for a solution according to the present disclosure. The present disclosure can be implemented for various use applications such as a printer, various printing machines, a copying machine, a FAX, and a multi-function machine. 
     [First Embodiment] 
     (Configuration of Image Forming Device) 
     First, a configuration of an image forming device  60  according to the first embodiment of the present disclosure will be described with reference to a sectional view of  FIG. 1 . As illustrated in  FIG. 1 , the image forming device  60  includes an endless intermediate transfer belt (ITB)  61  as an intermediate transfer member, and includes four image forming units  600  from an upstream side to a downstream side along a rotation direction (the direction of an arrow C of  FIG. 1 ) of the intermediate transfer belt  61 . Each image forming unit  600  is configured to form a toner image in a corresponding one of yellow (Y), magenta (M), cyan (C), and black (Bk). 
     Each image forming unit  600  includes a rotatable photosensitive drum  1  as an image bearing member. Moreover, each image forming unit  600  includes a charging roller  2  as a charging unit, a developing device  3  as a developing unit, a primary transfer roller  4  as a primary transfer unit, and a photosensitive drum cleaner  5  as a photosensitive drum cleaning unit, these components being arranged along a rotation direction of the photosensitive drum  1 . 
     Each developing device  3  is detachably attachable to the image forming device  60 . Each developing device  3  has a developer container  50  configured to store a two-component developer (hereinafter simply referred to as a “developer”) containing nonmagnetic toner (hereinafter simply referred to as “toner”) and a magnetic carrier. Each of toner cartridges each configured to store the toner in the colors of Y, M, C, and Bk is detachably attachable to the image forming device  60 . The toner in each color of Y, M, C, and Bk is supplied to a corresponding one of the developer containers  50  through a toner conveying path. Note that details of the developing device  3  will be described later with reference to  FIGS. 2 to 4 , and details of the developer container  50  will be described later with reference to  FIG. 5 . 
     The intermediate transfer belt  61  is stretched around a tension roller  6 , a follower roller  7   a , the primary transfer roller  4 , a follower roller  7   b , and an internal secondary transfer roller  66 , and is conveyed and driven in the direction of the arrow C of  FIG. 1 . The internal secondary transfer roller  66  also serves as a drive roller configured to drive the intermediate transfer belt  61 . In association with rotation of the internal secondary transfer roller  66 , the intermediate transfer belt  61  rotates in the direction of the arrow C of  FIG. 1 . 
     The intermediate transfer belt  61  is pressed by the primary transfer roller  4  from a back side of the intermediate transfer belt  61 . Moreover, the intermediate transfer belt  61  contacts the photosensitive drum  1  such that a primary transfer nip portion as a primary transfer portion is formed between the photosensitive drum  1  and the intermediate transfer belt  61 . 
     An intermediate transfer body cleaner  8  as a belt cleaning unit contacts a position facing the tension roller  6  through the intermediate transfer belt  61 . Moreover, an external secondary transfer roller  67  as a secondary transfer unit is arranged at a position facing the internal secondary transfer roller  66  through the intermediate transfer belt  61 . The intermediate transfer belt  61  is pinched between the internal secondary transfer roller  66  and the external secondary transfer roller  67 . Thus, a secondary transfer nip portion as a secondary transfer portion is formed between the external secondary transfer roller  67  and the intermediate transfer belt  61 . At the secondary transfer nip portion, the toner image adsorbs to a surface of a sheet S (e.g., paper or a film) by application of predetermined pressing force and a transfer bias (an electrostatic load bias). 
     The sheet S is stored with the sheet S being stacked in a sheet storage unit  62  (e.g., a sheet cassette or a feeding deck). A feeding unit  63  is configured to feed the sheet S according to image formation timing by means of, e.g., a friction separation system using a feeding roller etc. The sheet S sent out by the feeding unit  63  is conveyed to a registration roller  65  arranged in the middle of a conveyance path  64 . After skew correction or timing correction has been performed at the registration roller  65 , the sheet S is conveyed to the secondary transfer nip portion. The timing at which the sheet S arrives at the secondary transfer nip portion coincides with the timing at which the toner image arrives at the secondary transfer nip portion, and the secondary transfer is performed. 
     A fixing device  9  is arranged on the downstream side of the secondary transfer nip portion in a conveying direction of the sheet S. The fixing device  9  applies a predetermined pressure and a predetermined amount of heat to the sheet S conveyed to the fixing device  9 , and in this manner, the toner image is melted and fixed onto the surface of the sheet S. The sheet S on which the image has been fixed in this manner is directly discharged to a discharging tray  601  by forward rotation of a discharging roller  69 . 
     In the case of performing two-sided image formation, the discharging roller  69  is rotated backward after the sheet S has been conveyed by forward rotation of the discharging roller  69  until a trailing end of the sheet S passes through a switching member  602 . In this manner, the sheet S is conveyed to a two-sided printing conveyance path  603  with leading and trailing ends of the sheet S being switched. Thereafter, the sheet S is, according to subsequent image formation timing, again conveyed to the conveyance path  64  by a re-feeding roller  604 . 
     (Image Forming Process) 
     In image formation, the photosensitive drum  1  is rotatably driven by a motor. The charging roller  2  uniformly charges, in advance, a surface of the photosensitive drum  1  to be rotatably driven. An exposure device  68  forms, based on an image information signal input to the image forming device  60 , an electrostatic latent image on the surface of the photosensitive drum  1  charged by the charging roller  2 . The photosensitive drum  1  can form electrostatic latent images with multiple sizes. 
     The developing device  3  has a rotatable developing sleeve  70  as a developer carrier configured to carry the developer. The developing device  3  uses the developer carried on a surface of the developing sleeve  70  to develop the electrostatic latent image formed on the surface of the photosensitive drum  1 . In this manner, the toner adheres to an exposure portion on the surface of the photosensitive drum  1 , and is converted into a visible image. The transfer bias (the electrostatic load bias) is applied to the primary transfer roller  4 , and therefore, the toner image formed on the surface of the photosensitive drum  1  is transferred onto the intermediate transfer belt  61 . The toner (transfer residual toner) slightly remaining on the surface of the photosensitive drum  1  after primary transfer is collected by the photosensitive drum cleaner  5 , and is again provided for a subsequent image formation process. 
     The image formation processing for each color as parallel processing by the image forming units  600  for the colors of Y, M, C, and Bk is performed at such timing that the toner image is superimposed in a sequential order on the toner image of the upstream color primarily transferred onto the intermediate transfer belt  61 . As a result, the full-color toner image is formed on the intermediate transfer belt  61 , and is conveyed to the secondary transfer nip portion. The transfer bias is applied to the external secondary transfer roller  67 , and the toner image formed on the intermediate transfer belt  61  is transferred onto the sheet S conveyed to the secondary transfer nip portion. The toner (the transfer residual toner) slightly remaining on the intermediate transfer belt  61  after the sheet S has passed through the secondary transfer nip portion is collected by the intermediate transfer body cleaner  8 . The fixing device  9  fixes the toner image transferred onto the sheet S. The sheet S subjected to fixing by the fixing device  9  is discharged to the discharging tray  601 . 
     A series of image formation process as described above ends, and preparation for subsequent image formation operation is made. 
     (Configuration of Developing Device) 
     A typical configuration of the developing device  3  will be described with reference to a perspective view of  FIG. 2 , a perspective view of  FIG. 3 , and a sectional view of  FIG. 4 .  FIG. 4  is the sectional view of the developing device  3  in a section H of  FIG. 2 . 
     The developing device  3  includes the developer container  50  having a resin developing frame member (hereinafter simply referred to as a “developing frame member  30 ”) molded from resin and a resin cover frame member (hereinafter simply referred to as a “cover frame member  40 ”) formed separately from the developing frame member  30  and molded from resin.  FIGS. 2 and 4  illustrate a state in which the cover frame member  40  is attached to the developing frame member  30 , and  FIG. 3  illustrates a state in which the cover frame member  40  is not attached to the developing frame member  30 . Note that details of a configuration of the developing frame member  30  (a single member) will be described later with reference to  FIG. 6 . 
     At the developer container  50 , an opening is provided at a position corresponding to a development area where the developing sleeve  70  faces the photosensitive drum  1 . The developing sleeve  70  is rotatably arranged at the developer container  50  such that part of the developing sleeve  70  is exposed through the opening of the developer container  50 . A bearing  71  as a bearing member is provided at each end portion of the developing sleeve  70 . 
     The inside of the developer container  50  is, by a partition wall  38  extending in the vertical direction, divided into a development chamber  31  as a first chamber and a mixing chamber  32  as a second chamber. The development chamber  31  and the mixing chamber  32  are connected to each other at both ends in a longitudinal direction through two communication portions  39  of the partition wall  38 . Thus, the developer can be communicated between the development chamber  31  and the mixing chamber  32  through the communication portions  39 . The development chamber  31  and the mixing chamber  32  are arranged side by side in a horizontal direction. 
     In the developing sleeve  70 , a magnet roll as a magnetic field generating unit having multiple magnetic poles along a rotation direction of the developing sleeve  70  and configured to generate a magnetic field for carrying the developer on the surface of the developing sleeve  70  is arranged in a fixed manner. The developer in the development chamber  31  is pumped up due to influence of the magnetic field by the magnetic poles of the magnet roll, and is supplied to the developing sleeve  70 . Since the developer is supplied from the development chamber  31  to the developing sleeve  70  as described above, the development chamber  31  will be also referred to as a “supply chamber”. 
     In the development chamber  31 , a first conveying screw  33  as a conveying unit configured to mix and convey the developer in the development chamber  31  is arranged facing the developing sleeve  70 . The first conveying screw  33  includes a rotary shaft  33   a  as a rotatable shaft portion, and a spiral blade portion  33   b  as a developer conveying unit provided along the outer periphery of the rotary shaft  33   a . The first conveying screw  33  is rotatably supported on the developer container  50 . A bearing member is provided at each end portion of the rotary shaft  33   a.    
     Moreover, in the mixing chamber  32 , a second conveying screw  34  as a conveying unit configured to mix the developer in the mixing chamber  32  and convey the developer in a direction opposite to that of the first conveying screw  33  is arranged. The second conveying screw  34  includes a rotary shaft  34   a  as a rotatable shaft portion, and a spiral blade portion  34   b  as a developer conveying unit provided along the outer periphery of the rotary shaft  34   a . The second conveying screw  34  is rotatably supported on the developer container  50 . A bearing member is provided at each end portion of the rotary shaft  34   a . The first conveying screw  33  and the second conveying screw  34  are rotatably driven, and in this manner, a circulation path for circulating the developer is formed between the development chamber  31  and the mixing chamber  32  through the communication portions  39 . 
     In the developer container  50 , a regulating blade (hereinafter referred to as a “doctor blade  36 ”) as a developer regulating member configured to regulate the amount (also referred to as a “developer coating amount”) of developer carried on the surface of the developing sleeve  70  is attached facing the surface of the developing sleeve  70  in a non-contact manner. The doctor blade  36  has a coating amount regulating surface  36   r  as a regulating unit configured to regulate the amount of developer carried on the surface of the developing sleeve  70 . The doctor blade  36  is a resin doctor blade molded from resin. Note that a configuration of the doctor blade  36  (a single member) will be described later with reference to  FIG. 5 . 
     The doctor blade  36  is arranged facing the developing sleeve  70  with a predetermined gap (hereinafter referred to as a “SB gap G”) from the developing sleeve  70  across a longitudinal direction (i.e., a direction parallel to a rotational axis of the developing sleeve  70 ) of the developing sleeve  70 . In the present disclosure, the SB gap G indicates the minimum distance between the maximum image area of the developing sleeve  70  and the maximum image area of the doctor blade  36 . Note that the maximum image area of the developing sleeve  70  indicates the area of the developing sleeve  70  (the so-called maximum image area of the developing sleeve  70 ) corresponding to the maximum image area of an image area where an image can be formed on the surface of the photosensitive drum  1  in the direction parallel to the rotational axis of the developing sleeve  70 . Moreover, the maximum image area of the doctor blade  36  indicates the area of the doctor blade  36  (the so-called maximum image area of the doctor blade  36 ) corresponding to the maximum image area of the image area where the image can be formed on the surface of the photosensitive drum  1  in the direction parallel to the rotational axis of the developing sleeve  70 . In the first embodiment, the photosensitive drum  1  can form the electrostatic latent images with the multiple sizes, and therefore, the maximum image area indicates an image area corresponding to a largest one (e.g., an A3 size) of image areas with the multiple sizes formable on the photosensitive drum  1 . On the other hand, in a variation in which an electrostatic latent image only with a single size can be formed on the photosensitive drum  1 , the maximum image area is interpreted as an image area with the single size formable on the photosensitive drum  1 . 
     The doctor blade  36  is substantially arranged facing a peak position of a magnetic flux density of the magnetic poles of the magnet roll. The developer supplied to the developing sleeve  70  is influenced by the magnetic field by the magnetic poles of the magnet roll. Moreover, the developer regulated and scraped off by the doctor blade  36  tends to be accumulated at an upstream portion of the SB gap G. As a result, a developer sump is formed on the upstream side of the doctor blade  36  in the rotation direction of the developing sleeve  70 . Then, part of the developer in the developer sump is conveyed to pass through the SB gap G in association with rotation of the developing sleeve  70 . At this point, the layer thickness of the developer passing through the SB gap G is regulated by the coating amount regulating surface  36   r  of the doctor blade  36 . In this manner, a thin layer of the developer is formed on the surface of the developing sleeve  70 . 
     Then, a predetermined amount of developer carried on the surface of the developing sleeve  70  is conveyed to the development area in association with rotation of the developing sleeve  70 . Thus, the size of the SB gap G is adjusted such that the amount of developer conveyed to the development area is adjusted. In the first embodiment, a target size (a so-called target value of the SB gap G) of the SB gap G upon adjustment of the size of the SB gap G is set to about 300 μm. 
     The developer conveyed to the development area magnetically stands up in the development area to form a magnetic brush. This magnetic brush comes into contact with the photosensitive drum  1  to supply the toner in the developer to the photosensitive drum  1 . Then, the electrostatic latent image formed on the surface of the photosensitive drum  1  is developed as the toner image. The developer (hereinafter referred to as a developer after a developing step) on the surface of the developing sleeve  70  after the toner has been supplied to the photosensitive drum  1  through the development area is stripped off from the surface of the developing sleeve  70  by a repulsive magnetic field formed among the magnetic poles of the magnet roll with the same polarity. The developer after the developing step, which has been stripped off from the surface of the developing sleeve  70 , drops into the development chamber  31 , and is collected to the development chamber  31 . 
     As illustrated in  FIG. 4 , a developer guide unit  35  configured to guide the developer such that the developer is conveyed toward the SB gap G is provided at the developing frame member  30 . The developer guide unit  35  and the developing frame member  30  are configured integrally, and the developer guide unit  35  and the doctor blade  36  are configured separately. The developer guide unit  35  is formed in the developing frame member  30 , and is arranged on the upstream side of the coating amount regulating surface  36   r  of the doctor blade  36  in the rotation direction of the developing sleeve  70 . The flow of developer is stabilized by the developer guide unit  35 , and is adjusted such that a predetermined developer density is provided. In this manner, the weight of the developer at such a position that the coating amount regulating surface  36   r  of the doctor blade  36  is closest to the surface of the developing sleeve  70  can be defined. 
     Moreover, as illustrated in  FIG. 4 , the cover frame member  40  is formed separately from the developing frame member  30 , and is attached to the developing frame member  30 . Further, the cover frame member  40  covers part of the opening of the developing frame member  30  such that part of an outer peripheral surface of the developing sleeve  70  is covered across an entire area in the longitudinal direction of the developing sleeve  70 . In this state, the cover frame member  40  covers part of the opening of the developing frame member  30  such that the development area facing the photosensitive drum  1  of the developing sleeve  70  is exposed. In the first embodiment, the cover frame member  40  is fixed to the developing frame member  30  by ultrasonic bonding. However, the method for fixing the cover frame member  40  to the developing frame member  30  may be any method such as screw fastening, snap fitting, bonding, and welding. Note that regarding the cover frame member  40 , the cover frame member  40  may be formed from a single component (a resin molded article) as illustrated in  FIG. 4 , or may be formed from multiple components (resin molded articles). 
     (Configuration of Resin Doctor Blade) 
     The configuration of the doctor blade  36  (the single member) will be described with reference to a perspective view of  FIG. 5 . 
     During the image formation operation (development operation), the pressure (hereinafter referred to as “developer pressure”) of the developer generated from the flow of developer is on the doctor blade  36 . When the developer pressure is on the doctor blade  36  during the image formation operation, tendency shows that lower stiffness of the doctor blade  36  results in more deformation of the doctor blade  36  and more fluctuation in the size of the SB gap G. During the image formation operation, the developer pressure acts in a widthwise direction (the direction of an arrow M of  FIG. 5 ) of the doctor blade  36 . For this reason, for reducing fluctuation in the size of the SB gap G during the image formation operation, the stiffness of the doctor blade  36  in the widthwise direction thereof is preferably increased such that resistance against deformation of the doctor blade  36  in the widthwise direction thereof is provided. 
     As illustrated in  FIG. 5 , the shape of the doctor blade  36  is in a plate shape in the first embodiment, considering mass productivity and a cost. Moreover, as illustrated in  FIG. 5 , in the first embodiment, the sectional area of a side surface  36   t  of the doctor blade  36  is small, and a length t 2  of the doctor blade  36  in a thickness direction thereof is smaller than a length t 1  of the doctor blade  36  in the widthwise direction thereof. With this configuration, the doctor blade  36  (the single member) is configured easily deformable in the direction (the direction of the arrow M of  FIG. 5 ) perpendicular to a longitudinal direction (the direction of an arrow N of  FIG. 5 ) of the doctor blade  36 . Thus, in the first embodiment, for correcting straightness of the coating amount regulating surface  36   r , the doctor blade  36  is fixed to a blade attachment portion  41  of the developing frame member  30  with at least part of the doctor blade  36  being deflected in the direction of the arrow M of  FIG. 5 . Note that details of correction of the straightness of the doctor blade  36  will be described later with reference to  FIG. 9 . 
     (Configuration of Resin Developing Frame Member) 
     The configuration of the developing frame member  30  (the single member) will be described with reference to a perspective view of  FIG. 6 .  FIG. 6  illustrates a state in which the cover frame member  40  is not attached to the developing flame member  30 . 
     The developing frame member  30  has the development chamber  31  and the mixing chamber  32  divided from the development chamber  31  by the partition wall  38 . The partition wall  38  is molded from resin. The partition wall  38  may be configured separately from the developing frame member  30 , or may be configured integrally with the developing frame member  30 . 
     The developing frame member  30  has a sleeve support portion  42  configured to support the bearing  71  provided at each end portion of the developing sleeve  70  to rotatably support the developing sleeve  70 . Moreover, the developing frame member  30  has the blade attachment portion  41  formed integrally with a sleeve support portion  42  and provided for attachment of the doctor blade  36 .  FIG. 6  illustrates a virtual state in which the doctor blade  36  is floating above the blade attachment portion  41 . 
     In the first embodiment, in a state that the doctor blade  36  is attached to the blade attachment portion  41 , an adhesive A applied to a blade attachment surface  41   s  of the blade attachment portion  41  is hardened, and in this manner, the doctor blade  36  is fixed to the blade attachment portion  41 . 
     (Stiffness of Resin Doctor Blade) 
     The stiffness of the doctor blade  36  (the single member) will be described with reference to a schematic view of  FIG. 7 . The stiffness of the doctor blade  36  (the single member) is measured in a state that the doctor blade  36  is not fixed to the blade attachment portion  41  of the developing frame member  30 . 
     As illustrated in  FIG. 7 , a concentrated load F 1  is, in the widthwise direction of the doctor blade  36 , on a center portion  36   z  of the doctor blade  36  in the longitudinal direction thereof. In this state, the stiffness of the doctor blade  36  (the single member) is measured based on the deflection amount of the center portion  36   z  of the doctor blade  36  in the widthwise direction thereof. 
     Suppose that a concentrated load F 1  of 300 gf is, in the widthwise direction of the doctor blade  36 , on the center portion  36   z  of the doctor blade  36  in the longitudinal direction thereof. In this case, the deflection amount of the center portion  36   z  of the doctor blade  36  in the widthwise direction thereof is equal to or greater than 700 μm. Note that in this state, the deformation amount of the center portion  36   z  of the doctor blade  36  in section is equal to or less than 5 μm. 
     (Stiffness of Resin Developing Frame Member) 
     Stiffness of the developing frame member  30  (the single member) will be described with reference to a schematic view of  FIG. 8 . The stiffness of the developing frame member  30  (the single member) is measured in a state that the doctor blade  36  is not fixed to the blade attachment portion  41  of the developing frame member  30 . 
     As illustrated in  FIG. 8 , the concentrated load F 1  is, in a widthwise direction of the blade attachment portion  41 , on a center portion  41   z  of the blade attachment portion  41  in a longitudinal direction thereof. In this state, the stiffness of the developing frame member  30  (the single member) is measured based on the deflection amount of the center portion  41   z  of the blade attachment portion  41  in the widthwise direction thereof. 
     Suppose that a concentrated load F 1  of 300 gf is, in the widthwise direction of the blade attachment portion  41 , on the center portion  41   z  of the blade attachment portion  41  in the longitudinal direction thereof. In this case, the deflection amount of the center portion  41   z  of the blade attachment portion  41  in the widthwise direction thereof is equal to or less than 60 μm. 
     Suppose that the same level of concentrated load F 1  is on the center portion  36   z  of the doctor blade  36  and the center portion  41   z  of the blade attachment portion  41  of the developing frame member  30 . The deflection amount of the center portion  36   z  of the doctor blade  36  in this case is more than ten times as large as the deflection amount of the center portion  41   z  of the blade attachment portion  41 . Thus, the stiffness of the developing frame member  30  (the single member) is more than 10 times as large as the stiffness of the doctor blade  36  (the single member). Thus, in a state that the doctor blade  36  is attached to the blade attachment portion  41  of the developing frame member  30  and is fixed to the blade attachment portion  41  of the developing frame member  30 , the stiffness of the developing frame member  30  is dominant over the stiffness of the doctor blade  36 . In the case of fixing to the developing frame member  30  across the entirety of the maximum image area of the doctor blade  36 , the stiffness of the doctor blade  36  increases with the doctor blade  36  being fixed to the developing frame member  30  as compared to the case of fixing only both end portions of the doctor blade  36  in the longitudinal direction thereof. 
     Moreover, the level of the stiffness of the developing frame member  30  (the single member) is greater than the level of the stiffness of the cover frame member  40  (the single member). Thus, in a state that the cover frame member  40  is attached to the developing frame member  30  and is fixed to the developing frame member  30 , the stiffness of the developing frame member  30  is dominant over the stiffness of the cover frame member  40 . 
     (Correction of Straightness of Resin Doctor Blade) 
     In association with an increase in the width of the sheet S such as the A3 size being the width of the sheet S on which the image is formed, the length of the maximum image area of the image area where the image can be formed on the surface of the photosensitive drum  1  is increased in the direction parallel to the rotational axis of the developing sleeve  70 . Thus, in association with an increase in the width of the sheet S on which the image is formed, the length of the maximum image area of the doctor blade  36  is increased. In the case of molding a doctor blade with a great longitudinal length from resin, it is difficult to ensure straightness of a coating amount regulating surface of the resin doctor blade molded from resin. This is because in the case of molding the doctor blade with the great longitudinal length from resin, when the thermally-expanded resin thermally contracts, a portion where thermal contraction progresses and a portion where thermal contraction is delayed are easily formed depending on a location in the longitudinal direction at the doctor blade. 
     Thus, in the resin doctor blade, tendency shows that a greater length of the doctor blade in the longitudinal direction thereof results in, due to the straightness of the coating amount regulating surface of the doctor blade, a more variable SB gap in a longitudinal direction of a developer carrier. With variation in the SB gap in the longitudinal direction of the developer carrier, there might be variation in the amount of developer carried on a surface of the developer carrier in the longitudinal direction thereof. 
     For example, in a case where a resin doctor blade (hereinafter referred to as a “resin doctor blade corresponding to the A3 size) whose length in the longitudinal direction corresponds to the A3 size is manufactured with the accuracy of a typical resin molded article, the straightness of the coating amount regulating surface is about 300 μm to 500 μm. Even if the resin doctor blade corresponding to the A3 size is manufactured with high accuracy by means of a high-accuracy resin material, the straightness of the coating amount regulating surface is about 100 μm to 200 μm. 
     In the first embodiment, the size of the SB gap G is set to about 300 μm, and the tolerance of the SB gap G (i.e., a tolerance with respect to the target value of the SB gap G) is set to equal to or lower than ±10%. Thus, in the first embodiment, it means that an adjustment value of the SB gap G is 300 μm±30 μm, and a value acceptable as the tolerance of the SB gap G is up to 60 μm. Thus, even when the resin doctor blade corresponding to the A3 size is manufactured with the accuracy of the typical resin molded article or with high accuracy by means of the high-accuracy resin material, only the accuracy of the straightness of the coating amount regulating surface exceeds a range acceptable as the tolerance of the SB gap G. 
     In the developing device including the resin doctor blade, the SB gap G preferably falls within a predetermined range across the direction parallel to the rotational axis of the developer carrier regardless of the straightness of the coating amount regulating surface in a state that the doctor blade is fixed to an attachment portion of a developing frame member. Thus, in the first embodiment, even when a resin doctor blade with low straightness of a coating amount regulating surface is used, the straightness of the coating amount regulating surface is corrected such that the SB gap G falls within a predetermined range across the direction parallel to the rotational axis of the developing sleeve  70  in a state that the doctor blade is fixed to an attachment portion of a developing frame member. 
     The straightness of the coating amount regulating surface  36   r  of the doctor blade  36  will be described with reference to a schematic view of  FIG. 9 . The straightness of the coating amount regulating surface  36   r  is represented by an absolute value of a difference between the maximum value and the minimum value of the outer shape of the coating amount regulating surface  36   r  with reference to a predetermined spot of the coating amount regulating surface  36   r  in a longitudinal direction thereof. Suppose that a center portion of the coating amount regulating surface  36   r  in the longitudinal direction thereof is an origin of an orthogonal coordinate system, a predetermined straight line passing through the origin is an X-axis, and a straight line drawn at right angle to the X-axis from the origin is a Y-axis. In this orthogonal coordinate system, the straightness of the coating amount regulating surface  36   r  is represented by an absolute value of a difference between the maximum value and the minimum value of a Y-coordinate of the outer shape of the coating amount regulating surface  36   r.    
     As illustrated in  FIG. 9 , the resin doctor blade (the single member) is in such a shape that the center portion of the coating amount regulating surface  36   r  of the doctor blade  36  in the longitudinal direction thereof is greatly deflected. Thus, a difference in the position of a tip end portion  36   e  ( 36   e   1  to  36   e   5 ) of the doctor blade  36  illustrated in  FIG. 5  needs be reduced to correct the straightness of the coating amount regulating surface  36   r . Considering, e.g., an acceptable value of the tolerance of the SB gap G and the accuracy of attachment of the doctor blade  36  to the developing frame member  30 , the straightness of the coating amount regulating surface  36   r  of the doctor blade  36  needs to be corrected to equal to or less than 50 μm. Note that considering that the accuracy of straightness of a metal doctor blade by secondary cutting is equal to or less than 20 μm, the straightness of the coating amount regulating surface  36   r  of the resin doctor blade  36  is more preferably corrected to equal to or less than 20 μm. In the first embodiment, a set value for correction of the straightness of the coating amount regulating surface  36   r  of the doctor blade  36  is set to about 20 μm to 50 μm, considering a realistic mass production step. 
     Thus, in the first embodiment, force (also referred to as “straightness correction force”) for deflecting at least part of the maximum image area of the doctor blade  36  is provided to the doctor blade  36  to deflect at least part of the maximum image area of the doctor blade  36 . In this manner, the straightness of the coating amount regulating surface  36   r  of the doctor blade  36  is corrected to equal to or less than 50 μm. 
     In an example of  FIG. 9 , the straightness correction force is provided to the tip end portions  36   e   2 ,  36   e   3 , and  36   e   4  in the direction of arrow I of  FIG. 9  such that the outer shapes of the tip end portions  36   e   2 ,  36   e   3 , and  36   e   4  fit the reference outer shapes of the tip end portions  36   e   1  and  36   e   5  of the doctor blade  36 . As a result, the shape of the coating amount regulating surface  36   r  of the doctor blade  36  is corrected from a coating amount regulating surface  36   r   1  to a coating amount regulating surface  36   r   2 , and therefore, the straightness of the coating amount regulating surface  36   r  of the doctor blade  36  can be corrected to equal to or less than 50 μm. Note that in the example of  FIG. 9 , the reference upon fitting of the outer shape of the tip end portion  36   e  of the doctor blade  36  is the outer shapes of the tip end portions  36   e   1  and  36   e   5  (both end portions of the coating amount regulating surface  36   r  in the longitudinal direction thereof), but may be the outer shape of the tip end portion  36   e   3  (the center portion of the coating amount regulating surface  36   r  in the longitudinal direction thereof). In this case, the straightness correction force is provided to the doctor blade  36  such that the outer shapes of the tip end portions  36   e   1 ,  36   e   2 ,  36   e   4 , and  36   e   5  fit the reference outer shape of the tip end portion  36   e   3  of the doctor blade  36 . 
     As described above, for performing correction of the straightness of the doctor blade  36 , the stiffness of the doctor blade (the single member) needs to be decreased such that at least part of the maximum image area of the coating amount regulating surface  36   r  is deflected when the straightness correction force is provided to the doctor blade  36 . 
     (Method for Adjusting SB Gap) 
     Adjustment of the SB gap G is performed in such a manner that the position of the doctor blade  36  relative to the developing frame member  30  is moved such that the position of the doctor blade  36  attached to the blade attachment portion  41  relative to the developing sleeve  70  supported on the sleeve support portion  42  is adjusted. The doctor blade  36  whose maximum image area is at least partially deflected is, at a predetermined position of the blade attachment portion  41  determined by adjustment of the SB gap G, fixed with the adhesive A applied in advance across the entirety of the maximum image area of the blade attachment surface  41   s . Note that the maximum image area of the blade attachment surface  41   s  is the area of the blade attachment surface  41   s  corresponding to the maximum image area of the image area where the image can be formed on the surface of the photosensitive drum  1  in the direction parallel to the rotational axis of the developing sleeve  70 . In this case, the area deflected for correcting the straightness of the coating amount regulating surface  36   r  in the maximum image area of the doctor blade  36  is fixed to the blade attachment portion  41 . Note that when the area having received the force for deflecting at least part of the maximum image area of the doctor blade  36  is fixed to the blade attachment portion  41  with the adhesive A, no adhesive A may be applied to part of the blade attachment surface  41   s . Application of the adhesive A across the entirety of the maximum image area of the blade attachment surface  41   s  indicates that the following conditions are satisfied: the area, which is deflected for correcting the straightness of the coating amount regulating surface  36   r , of the area corresponding to the maximum image area of the doctor blade  36  is included, and the adhesive A is applied to 95% or more of the maximum image area of the blade attachment surface  41   s.    
     With this configuration, restoring of the area, which is deflected for correcting the straightness of the coating amount regulating surface  36   r , of the maximum image area of the doctor blade  36  from a deflected state to an original state before deflecting can be reduced. Thus, the doctor blade  36  is fixed to the blade attachment portion  41  with the straightness of the coating amount regulating surface  36   r  being corrected to equal to or less than 50 μm. 
     Note that in the first embodiment, the size of the SB gap G is measured (calculated) by a method described below. Note that measurement of the size of the SB gap G is performed in such a state in which the developing sleeve  70  is supported on the sleeve support portion  42  of the developing frame member  30 , the doctor blade  36  is attached to the blade attachment portion  41  of the developing frame member  30 , and the cover frame member  40  is fixed to the developing frame member  30 . 
     For measuring the size of the SB gap G, a light source (e.g., an LED array or a light guide) is inserted into the development chamber  31  across a longitudinal direction thereof. The light source inserted into the development chamber  31  irradiates the SB gap G with light from the inside of the development chamber  31 . Moreover, a camera configured to capture an image from a light beam emitted from the SB gap G to the outside of the developing frame member  30  is arranged at each of five spots corresponding to the tip end portions  36   e  ( 36   e   1  to  36   e   5 ) of the doctor blade  36 . 
     The cameras arranged at these five spots are each configured to capture the image from the light beam emitted from the SB gap G to the outside of the developing frame member  30  to measure the positions of the tip end portions  36   e  ( 36   e   1  to  36   e   5 ) of the doctor blade  36 . In this state, each camera reads a position at which the developing sleeve  70  is closest to the doctor blade  36  on the surface of the developing sleeve  70  and the tip end portion  36   e  ( 36   e   1  to  36   e   5 ) of the doctor blade  36 . Subsequently, a pixel value is, from image data generated by reading by the cameras, converted into a distance, and the size of the SB gap G is calculated. In a case where the calculated size of the SB gap G does not fall within a predetermined range, adjustment of the SB gap G is performed. Then, when the calculated size of the SB gap G falls within the predetermined range, a position at which the doctor blade  36  whose maximum image area is at least partially deflected is fixed to the blade attachment portion  41  of the developing frame member  30  is determined. 
     Note that in the first embodiment, it is, by a method described below, determined whether or not the SB gap G falls within the predetermined range across the direction parallel to the rotational axis of the developing sleeve  70 . First, the maximum image area of the doctor blade  36  is divided into four or more portions at equal intervals, and the SB gap G is measured at five or more spots in each divided portion (including both end portions and the center portion of the maximum image area of the doctor blade  36 ) of the doctor blade  36 . Then, from samples of a measurement value of the SB gap G measured at five spots or more, the maximum value of the SB gap G, the minimum value of the SB gap G, and the median value of the SB gap G are extracted. 
     In this case, an absolute value of a difference between the maximum value of the SB gap G and the median value of the SB gap G may be equal to or less than 10% of the median value of the SB gap G, and an absolute value of a difference between the minimum value of the SB gap G and the median value of the SB gap G may be equal to or less than 10% of the median value of the SB gap G. In this case, the tolerance of the SB gap G is taken as equal to or less than ±10%, and a condition where the SB gap G falls within the predetermined range across the direction parallel to the rotational axis of the developing sleeve  70  is taken as satisfied. For example, in a case where the samples of the measurement value of the SB gap G measured at five or more spots show that the median value of the SB gap G is 300 μm, the maximum value of the SB gap G may be equal to or less than 330 μm, and the minimum value of the SB gap G may be equal to or greater than 270 μm. That is, in this case, the adjustment value of the SB gap G is 300 μm±30 μm, and a value of up to 60 μm is acceptable as the tolerance of the SB gap G. 
     (Linear Expansion Coefficient) 
     Subsequently, deformation of the doctor blade  36  and the developing frame member  30  due to a change in a temperature due to heat generated during the image formation operation will be described with reference to a perspective view of  FIG. 10 . The heat generated during the development operation includes, for example, heat generated upon rotation of a rotary shaft and the bearings  71  of the developing sleeve  70 , heat generated upon rotation of the rotary shaft  33   a  of the first conveying screw  33  and the bearing members thereof and heat generated when the developer passes through the SB gap G. A temperature surrounding the developing device  3  is changed due to these types of heat generated during the image formation operation, and the temperatures of the doctor blade  36 , the developing frame member  30 , and the cover frame member  40  are also changed. 
     As illustrated in  FIG. 10 , the stretching amount of the doctor blade  36  due to a temperature change is H [μm], and the stretching amount of the blade attachment surface  41   s  of the blade attachment portion  41  of the developing frame member  30  due to a temperature change is I [μm]. Moreover, the linear expansion coefficient α 1  of resin forming the doctor blade  36  and the linear expansion coefficient α 2  of resin forming the developing frame member  30  are different from each other. In this case, the deformation amount due to a temperature change is different between the developing frame member  30  and the doctor blade  36  because of a difference in the linear expansion coefficient. For filling a difference between H [μm] and I [μm], the doctor blade  36  deforms in the direction of an arrow J of  FIG. 10 . Deformation of the doctor blade  36  in the direction of the arrow J of  FIG. 10  will be hereinafter referred to as “deformation in a warping direction of the doctor blade  36 ”. Deformation in the warping direction of the doctor blade  36  leads to fluctuation in the size of the SB gap G. Reduction in fluctuation in the size of the SB gap G due to heat relates to the linear expansion coefficient α 2  of resin forming the sleeve support portion  42  and the blade attachment portion  41  of the developing frame member  30  (the single member) and the linear expansion coefficient α 1  of resin forming the doctor blade  36  (the single member). That is, in a case where the linear expansion coefficient α 1  of resin forming the doctor blade  36  and the linear expansion coefficient α 2  of resin forming the developing frame member  30  are different from each other, the deformation amount due to a temperature change varies due to a difference in the linear expansion coefficient. 
     Typically, a resin material has a greater linear expansion coefficient than that of a metal material. In a case where the doctor blade  36  is made of resin, warp deformation occurs at the doctor blade  36  in association with a temperature change due to the heat generated during the image formation operation, and therefore, the center portion of the doctor blade  36  in the longitudinal direction thereof is easily deflected. As a result, in the developing device configured such that the resin doctor blade  36  is fixed to the resin developing frame member, the size of the SB gap G easily fluctuates in association with a temperature change during the image formation operation. 
     (Configuration of Developing Device According to First Embodiment) 
     In the first embodiment, at least part of the maximum image area of the doctor blade  36  is deflected to correct the straightness of the coating amount regulating surface  36   r  to equal to or less than 50 μm. Moreover, a method is employed, in which the doctor blade  36  whose maximum image area is at least partially deflected is, with the adhesive A, fixed to the blade attachment portion  41  of the developing frame member  30  across the entirety of the maximum image area of the doctor blade  36 . 
     In this case, when there is a great difference between the linear expansion coefficient α 2  of resin forming the developing frame member  30  and the linear expansion coefficient α 1  of resin forming the doctor blade  36 , the following problem is caused when a temperature change occurs. That is, when a temperature change occurs, the deformation amount (the extension amount) of the doctor blade  36  due to a temperature change and the deformation amount (the extension amount) of the developing frame member  30  due to a temperature change are different from each other. As a result, even when a position at which the doctor blade  36  is attached to the blade attachment surface  41   s  of the developing frame member  30  is determined, even if the SB gap G is adjusted with high accuracy, the size of the SB gap G might fluctuate due to a temperature change during the image formation operation. 
     In the first embodiment, the doctor blade  36  is fixed to the blade attachment surface  41   s  across the entirety of the maximum image area, and therefore, fluctuation in the size of the SB gap G due to a temperature change during the image formation operation needs to be reduced. For reducing variation in the amount of developer carried on the surface of the developing sleeve  70  in the longitudinal direction thereof, the fluctuation amount of the SB gap G due to heat typically needs to be reduced to equal to or less than ±20 μm. 
     The difference of the linear expansion coefficient α 2  of resin forming the developing frame member  30  having the sleeve support portion  42  and the blade attachment portion  41  from the linear expansion coefficient α 1  of resin forming the doctor blade  36  will be hereinafter referred to as a “linear expansion coefficient difference α 2 −α 1 ”. A change in the maximum deflection amount of the doctor blade  36  due to the linear expansion coefficient difference α 2 −α 1  will be described with reference to Table 1. In a state that the doctor blade  36  is fixed to the blade attachment portion  41  of the developing frame member  30  across the entirety of the maximum image area of the doctor blade  36 , measurement of the maximum deflection amount of the doctor blade  36  when a temperature change from a normal temperature (23° C.) to a high temperature (40° C.) is provided was performed. 
     The linear expansion coefficient of resin forming the developing frame member  30  having the sleeve support portion  42  and the blade attachment portion  41  is α 2  [m/° C.], and the linear expansion coefficient of resin forming the doctor blade  36  is α 1  [m/° C.]. Results from measurement of the maximum deflection amount of the doctor blade  36  with a change in a parameter of the linear expansion coefficient difference α 2 −α 1  are shown in Table 1. In Table 1, the maximum deflection amount is good (a white circle) in a case where an absolute value of the maximum deflection amount of the doctor blade  36  is equal to or less than 20 μm, and is poor (a cross mark) in a case where the absolute value of the maximum deflection amount of the doctor blade  36  is greater than 20 μm. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                 LINEAR EXPANSION COEFFICIENT 
               
               
                   
                 DIFFERENCE α2 − α1 [×10 −5  m/° C.] 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 0 
                 +0.20 
                 +0.40 
                 +0.50 
                 +0.54 
                 +0.55 
                 +0.56 
                 +0.57 
                 +0.60 
               
               
                   
               
               
                 MAXIMUM DEFLECTION AMOUNT OF 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 X 
                 X 
                 X 
               
               
                 DOCTOR BLADE 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 LINEAR EXPANSION COEFFICIENT 
               
               
                   
                 DIFFERENCE α2 − α1 [×10 −5  m/° C.] 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 0 
                 −0.20 
                 −0.40 
                 −0.44 
                 −0.45 
                 −0.46 
                 −0.47 
                 −0.50 
               
               
                   
               
               
                 MAXIMUM DEFLECTION AMOUNT OF 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 X 
                 X 
                 X 
               
               
                 DOCTOR BLADE 
               
               
                   
               
            
           
         
       
     
     As seen from Table 1, the linear expansion coefficient difference α 2 −α 1  needs to satisfy the following relational expression (Expression 1) for suppressing the fluctuation amount of the SB gap G due to heat to equal to or less than ±20 μm.
 
−0.45×10 −5 [m/° C. ]≤α2−α1≤0.55×10 −5 [m/° C.]  (Expression 1)
 
     Thus, the resin forming the developing frame member  30  and the resin forming the doctor blade  36  may be selected such that the linear expansion coefficient difference α 2 −α 1  is equal to or greater than −0.45×10 −5  [m/° C.] and equal to or less than 0.55×10 −5  [m/° C.]. Note that in a case where the same resin is selected as the resin forming the developing frame member  30  and the resin forming the doctor blade  36 , the linear expansion coefficient difference α 2 −α 1  is zero. 
     Note that when the adhesive A is applied to the doctor blade  36  or the developing frame member  30 , the linear expansion coefficient of the doctor blade  36  or the developing frame member  30  to which the adhesive A has been applied fluctuates. However, the volume of the adhesive A itself applied to the doctor blade  36  or the developing frame member  30  is extremely small, and is an ignorable level as influence on dimension fluctuation in a thickness direction of the adhesive A due to a temperature change. For this reason, when the adhesive A is applied to the doctor blade  36  or the developing frame member  30 , deformation in the warping direction of the doctor blade  36  due to fluctuation in the linear expansion coefficient difference α 2 −α 1  is at an ignorable level. 
     Similarly, the cover frame member  40  is fixed to the developing frame member  30 , and therefore, deformation in the warping direction of the cover frame member  40  leads to fluctuation in the size of the SB gap G when the deformation amount due to a temperature change is different between the developing frame member  30  and the cover frame member  40 . The linear expansion coefficient of resin forming the developing frame member  30  having the sleeve support portion  42  and the blade attachment portion  41  is α 2  [m/° C.], and the linear expansion coefficient of resin forming the cover frame member  40  is α 3  [m/° C.]. Moreover, a difference of the linear expansion coefficient α 3  of resin forming the cover frame member  40  from the linear expansion coefficient α 2  of resin forming the developing frame member  30  having the sleeve support portion  42  and the blade attachment portion  41  will be hereinafter referred to as a “linear expansion coefficient difference α 3 −α 2 ”. In this case, as in Table 1, the linear expansion coefficient difference α 3 −α 2  needs to satisfy the following relational expression (Expression 2).
 
−0.45×10 −5 [m/° C.]≤α3−α2≤0.55×10 −5 [m/° C.]  (Expression 2)
 
     Thus, the resin forming the developing frame member  30  and the resin forming the cover frame member  40  may be selected such that the linear expansion coefficient difference α 3 −α 2  is equal to or greater than −0.45×10 −5  [m/° C.] and equal to or less than 0.55×10 −5  [m/° C.]. Note that in a case where the same resin is selected as the resin forming the developing frame member  30  and the resin forming the cover frame member  40 , the linear expansion coefficient difference α 3 −α 2  is zero. 
     (Developer Pressure) 
     Subsequently, deformation of the doctor blade  36  due to application of the developer pressure generated from the flow of developer to the doctor blade  36  during the image formation operation will be described with reference to a sectional view of  FIG. 11 .  FIG. 11  is the sectional view of the developing device  3  in the section (the section H of  FIG. 2 ) perpendicular to the rotational axis of the developing sleeve  70 . Moreover,  FIG. 11  illustrates a configuration of the vicinity of the doctor blade  36  fixed to the blade attachment portion  41  of the developing frame member  30  with the adhesive A. 
     As illustrated in  FIG. 11 , a line connecting the position of the coating amount regulating surface  36   r  closest to the developing sleeve  70  of the doctor blade  36  and the center of rotation of the developing sleeve  70  is the X-axis. In this case, the doctor blade  36  has a long length in an X-axis direction, and has high stiffness in a section along the X-axis direction. Moreover, as illustrated in  FIG. 11 , the percentage of a sectional area T 1  of the doctor blade  36  with respect to a sectional area T 2  of a wall portion  30   a  of the developing frame member  30  positioned close to the developer guide unit  35  is small. 
     As described above, in the first embodiment, the stiffness of the developing frame member  30  (the single member) is more than 10 times as large as the stiffness of the doctor blade  36  (the single member). Thus, in a state that the doctor blade  36  is fixed to the blade attachment portion  41  of the developing frame member  30 , the stiffness of the developing frame member  30  is dominant over the doctor blade  36 . As a result, the displacement amount (the maximum deflection amount) of the coating amount regulating surface  36   r  of the doctor blade  36  when the doctor blade  36  receives the developer pressure during the image formation operation is substantially equivalent to the displacement amount (the maximum deflection amount) of the developing frame member  30 . 
     During the image formation operation, the developer pumped up from the first conveying screw  33  is conveyed onto the surface of the developing sleeve  70  through the developer guide unit  35 . Thereafter, when the layer thickness of the developer is defined to the size of the SB gap G by the doctor blade  36 , the doctor blade  36  also receives the developer pressure in various directions. As illustrated in  FIG. 11 , when a direction perpendicular to the X-axis direction (the direction of defining the SB gap G) is a Y-axis direction, the developer pressure in the Y-axis direction is perpendicular to the blade attachment surface  41   s  of the developing frame member  30 . That is, the developer pressure in the Y-axis direction is force in the direction of detaching the doctor blade  36  from the blade attachment surface  41   s . Thus, bonding force by the adhesive A needs to be sufficiently greater than the developer force in the Y-axis direction. For this reason, in the first embodiment, the bonding area and application thickness of the adhesive A on the blade attachment surface  41   s  are optimized considering the force of detaching the doctor blade  36  from the blade attachment surface  41   s  by the developer force and the bonding force of the adhesive A. 
     As described above, in the first embodiment, the resin doctor blade  36  is, with the adhesive A, fixed to the blade attachment portion  41  of the resin developing frame member  30  across the entirety of the maximum image area of the doctor blade  36 . Moreover, in the first embodiment, when the resin doctor blade  36  is fixed to the blade attachment portion  41  of the resin developing frame member  30 , the resin doctor blade  36  having low stiffness is used to perform correction of the straightness of the doctor blade  36  (the single member). Thus, in the developing device  3  configured such that the resin doctor blade  36  having low stiffness is fixed to the resin developing frame member  30 , the stiffness of the resin developing frame member  30  (the single member) needs to be increased to increase the stiffness of the doctor blade  36  fixed to the developing frame member  30 . This is because the stiffness of the doctor blade  36  fixed to the developing frame member  30  is increased such that fluctuation in the SB gap G during the image formation operation is reduced and that the SB gap G falls within the predetermined range during the image formation operation. 
     For increasing the stiffness of the resin developing frame member  30  (the single member), the basic thickness of the developing frame member  30  might be increased. However, in a resin molded article having a basic thickness greater than a predetermined value, when resin thermally expanded upon molding thermally contracts, the degree of causing a difference in the progress of thermal contraction between the inside and outside of the resin molded article is easily increased as compared to a resin molded article having a basic thickness equal to or lower than the predetermined value. In other words, the molding contraction ratio of a resin molded article having a thickness size greater than a predetermined value becomes more non-uniform as compared to a resin molded article having a thickness size equal to or less than the predetermined value. This is because resin thermally expanded upon molding is gradually cooled from the outside of the resin molded article as a portion contacting a mold toward the inside of the resin molded article as a portion not contacting the mold, and thermal contraction progresses. Thus, in a case where the basic thickness size of the resin molded article is greater than the predetermined value, a sink mark tends to be more easily caused at the resin molded article as compared to a case where the basic thickness size of the resin molded article is equal to or less than the predetermined value. 
     Moreover, a cooling time or a cycle time upon molding is increased in association with an increase in the thickness size of the resin molded article, and therefore, this is disadvantageous considering mass productivity. For this reason, the degree of increasing the basic thickness size of the developing frame member  30  for the purpose of increasing the stiffness of the resin developing frame member  30  (the single member) is limited. Thus, in the first embodiment, the basic thickness size of the developing frame member  30  is set to equal to or greater than 1.0 mm and equal to or less than 3.0 mm such that no disadvantage is caused considering mass productivity. Moreover, the basic thickness size of the developing frame member  30  is preferably typically uniform such that the molding contraction ratio is not non-uniform. 
     The length of the maximum image area of the developing frame member  30  is increased in association with an increase in the width of the sheet S such as the A3 size being the width of the sheet S on which the image is formed. Note that the maximum image area of the developing frame member  30  is the area of the developing frame member  30  corresponding to the maximum image area of the image area where the image can be formed on the surface of the photosensitive drum  1  in the direction parallel to the rotational axis of the developing sleeve  70 . 
     In a case where the developing frame member  30  is molded from resin by injection molding, gate portions  80  as inlets through which the resin flows into the molded article through gates when the molten resin is poured into the molded article through the gates are provided at the developing frame member  30  as the resin molded article. When the developing frame member  30  having a great length in the longitudinal direction is molded from resin, a distance for circulating the molten resin is long, and therefore, the gate portions  80  are typically provided at the maximum image area of the resin developing frame member  30  such that the molten resin efficiently flows in the longitudinal direction of the developing frame member  30 . Note that the gate portions  80  can be typically viewed as marks (so-called gate marks) fulfilling a role as the inlets through which the molten resin flows into the molded article through the gates when an outer appearance of the resin developing frame member  30  is viewed. 
     Moreover, in a case where the developing frame member  30  is molded from the resin by injection molding, great molding pressure is on the gate portions  80  when the molten resin flows into the gate portions  80  through the gates, and therefore, residual stress is generated at the gate portions  80 . The residual stress from the gate portions  80  provided at the resin developing frame member  30  is on the developing frame member  30  over time, and deforms the resin developing frame member  30  over time. As a result, in a state that the resin doctor blade  36  is fixed to the resin developing frame member  30 , the size of the SB gap G due to the residual stress from the gate portions  80  provided at the developing frame member  30  might fluctuate over time. 
     The residual stress on the developing frame member  30  has a component on the developing frame member  30  along a direction intersecting the rotational axis of the developing sleeve  70 . Note that the direction intersecting the rotational axis of the developing sleeve  70  includes not only a direction perpendicular to the rotational axis of the developing sleeve  70 , but also a direction at an angle (note that an acute angle) of greater than 5° and less than 90° with respect to the rotational axis of the developing sleeve  70 . When the component of the residual stress on the developing frame member  30  along the direction intersecting the rotational axis of the developing sleeve  70  is on the developing frame member  30  over time, the developing frame member  30  fixed to the doctor blade  36  is distorted along the direction intersecting the rotational axis of the developing sleeve  70 . Thus, this contributes to fluctuation in the size of the SB gap G due to the residual stress (the component of the residual stress on the developing frame member  30  along the direction intersecting the rotational axis of the developing sleeve  70 ) from the gate portions  80 . 
     As described above, it has been demanded to reduce fluctuation in the SB gap G during the image formation operation in a state that the resin doctor blade  36  having low stiffness is fixed to the resin developing frame member  30 . Thus, in a state that the resin doctor blade  36  having low stiffness is fixed to the resin developing frame member  30 , fluctuation in the size of the SB gap G in association with temporal application of the residual stress from the gate portions  80  provided at the maximum image area of the developing frame member  30  to the developing frame member  30  is preferably reduced. For this reason, the positions of the gate portions  80  at the maximum image area of the developing frame member  30  are designed such that fluctuation in the size of the SB gap G due to the residual stress from the gate portions  80  is reduced in a state that the resin doctor blade  36  having low stiffness is fixed to the resin developing frame member  30 . 
     In the first embodiment, the gate portions  80  are provided at the maximum image area of the developing frame member  30  such that fluctuation in the size of the SB gap G due to the residual stress from the gate portions  80  is reduced in a state that the resin doctor blade  36  having low stiffness is fixed to the resin developing frame member  30 . Details will be described below. 
     The configuration of the developing device according to the first embodiment will be described with reference to a perspective view of  FIG. 12 , a sectional view of  FIG. 13 , and a lower view of  FIG. 14 .  FIG. 12  illustrates the maximum image area of a developing frame member  310  provided at a developing device  300  according to the first embodiment.  FIG. 13  is the sectional view of the developing device  300  in a section H (the maximum image area of the developing frame member  310 ) of  FIG. 12 .  FIG. 14  is the lower view of the developing device  300  when the developing device  300  attached to the image forming device  60  is viewed from below in the vertical direction. In each of  FIGS. 12, 13, and 14 , the same numerals are used to represent the same configurations as those of  FIGS. 2, 3, and 4 . Differences of the configuration of the developing device  300  (the configuration of the developing frame member  310 ) from the configuration (the configuration of the developing frame member  30 ) of the developing device  3  described above with reference to each of  FIGS. 2, 3, and 4  will be mainly described. 
     In the first embodiment, the gate portions  80  are not provided at a bottom portion of the developing frame member  310  in the maximum image area in an area P of the developing frame member  310 , and are provided at the bottom portion of the developing frame member  310  in the maximum image area in an area Q of the developing frame member  310 , as illustrated in  FIGS. 13 and 14 . Note that the bottom portion of the developing frame member  310  described in the first embodiment is not limited to an outer wall portion (e.g., an outer wall portion positioned at a bottom portion of the partition wall  38 ) positioned on the lowermost side of the developing frame member  310  in the vertical direction when the developing sleeve  70  is at such a position that an electrostatic image formed on the photosensitive drum  1  is developed. Further, the bottom portion of the developing frame member  310  includes not only an outer wall portion positioned at a U-shaped bottom surface of the development chamber  31  and an outer wall portion positioned on a U-shaped bottom surface of the mixing chamber  32 , but also an outer wall portion positioned at a U-shaped side wall surface of the development chamber  31  and an outer wall portion positioned at a U-shaped side wall surface of the mixing chamber  32 . 
     When the developing device  300  is viewed in the section perpendicular to the rotational axis of the developing sleeve  70 , the developing frame member  310  is divided into multiple areas by a straight line L passing through the center of rotation of the developing sleeve  70  and a closest position N and a perpendicular line M passing through the center of rotation of the first conveying screw  33  with respect to the straight line L. Note that the closest position N is a position at which the developing sleeve  70  is closest to the photosensitive drum  1 . That is, as illustrated in  FIG. 13 , the straight line L is a straight line passing through the center of rotation of the developing sleeve  70  and the center of rotation of the photosensitive drum  1 . Of the multiple divided areas of the developing frame member  310 , the divided area of the developing frame member  310  provided with the blade attachment portion  41  is the area P of the developing frame member  310 . In other words, the area P of the developing frame member  310  is an area covering 0 degrees to 90 degrees on the upstream side of the closest position N in the rotation direction of the developing sleeve  70 . Of the multiple divided areas of the developing frame member  310  in this state, the divided area of the developing frame member  310  not provided with the blade attachment portion  41  is the area Q of the developing frame member  310 . In other words, the area Q of the developing frame member  310  is an area covering 90 degrees to 180 degrees on the upstream side of the closest position N in the rotation direction of the developing sleeve  70 . 
     As described above, in the first embodiment, the gate portions  80  are provided at the bottom portion of the developing frame member  310  in the maximum image area in the area Q of the developing frame member  310 , and the positions of the gate portions  80  at the maximum image area of the developing frame member  310  are sufficiently apart from the maximum image area of the blade attachment portion  41 . Thus, the degree of contribution of the residual stress from the gate portions  80  provided at the bottom portion of the developing frame member  310  in the maximum image area in the area Q of the developing frame member  310  to fluctuation in the size of the SB gap G is sufficiently small. On the other hand, in the first embodiment, the gate portions  80  are not provided at the bottom portion of the developing frame member  310  in the maximum image area in the area P of the developing frame member  310 . Thus, influence of contribution of the residual stress generated due to the gate portions  80  provided at the bottom portion of the developing frame member  310  in the maximum image area in the area P of the developing frame member  310  to fluctuation in the size of the SB gap G is not necessarily taken into consideration. 
     Thus, in the first embodiment, fluctuation in the size of the SB gap G in association with temporal application of the residual stress from the gate portions  80  to the developing frame member  310  in a state that the resin doctor blade  36  having low stiffness is fixed to the resin developing frame member  310  can be reduced. 
     (Comparative Example) 
     Subsequently, a configuration of a developing device  500  according to a comparative example will be described with reference to a sectional view of  FIG. 15  and a lower view of  FIG. 16 .  FIG. 15  is the sectional view of the developing device  500  in the section H (the maximum image area of a developing frame member  510 ) of  FIG. 12 .  FIG. 16  is the lower view of the developing device  500  when the developing device  500  attached to the image forming device  60  is viewed from below in the vertical direction. In each of  FIGS. 15 and 16 , the same numerals are used to represent the same configurations as those of  FIGS. 13 and 14 . An area P illustrated in  FIG. 15  is illustrated as the same area as the area P illustrated in  FIG. 13 . Moreover, an area Q illustrated in  FIG. 15  is illustrated as the same area as the area Q illustrated in  FIG. 13 . Differences of the configuration (the configuration of the developing frame member  510 ) of the developing device  500  according to the comparative example from the configuration (the configuration of the developing frame member  310 ) of the developing device  300  according to the first embodiment described above with reference to each of  FIGS. 13 and 14  will be mainly described. Note that subsequent description will be made with a definition of a bottom portion of the developing frame member  510  described in the comparative example being similar to that of the bottom portion of the developing frame member  310  described in the first embodiment. 
     In the comparative example, the gate portions  80  are not provided at the bottom portion of the developing frame member  510  in the maximum image area in the area Q of the developing frame member  510 , and are provided at the bottom portion of the developing frame member  510  in the maximum image area in the area P of the developing frame member  510 , as illustrated in  FIGS. 15 and 16 . As described above, in the comparative example, the gate portions  80  are provided at the bottom portion of the developing frame member  510  in the maximum image area in the area P of the developing frame member  510 , but the positions of the gate portions  80  in the maximum image area of the developing frame member  510  are relatively closer to the maximum image area of the blade attachment portion  41  than that in the first embodiment. Thus, the degree of contribution of the residual stress from the gate portions  80  provided at the bottom portion of the developing frame member  510  in the maximum image area in the area P of the developing frame member  510  to fluctuation in the size of the SB gap G is relatively greater than that in the first embodiment. Specifically, in a state that the resin doctor blade  36  having low stiffness is fixed to the resin developing frame member  510 , the degree of fluctuation in the size of the SB gap G due to the residual stress from the gate portions  80  provided at the bottom portion of the developing frame member  510  in the maximum image area in the area P of the developing frame member  510  tends to be increased. As a result, warp deformation of the doctor blade  36  occurs in the direction of an arrow J illustrated in  FIG. 16  in association with temporal application of the residual stress from the gate portions  80  provided at the bottom portion of the developing frame member  510  in the maximum image area in the area P of the developing frame member  510  to the developing frame member  510 . Then, the center portion of the doctor blade  36  in the longitudinal direction thereof is deflected, and the size of the SB gap G fluctuates. 
     On the other hand, in the first embodiment, the degree of contribution of the residual stress from the gate portions  80  provided at the bottom portion of the developing frame member  310  in the maximum image area in the area Q of the developing frame member  310  to fluctuation in the size of SB gap G is sufficiently small. Thus, in the first embodiment, no warp deformation occurs at the doctor blade  36  in the direction of the arrow J illustrated in  FIG. 16  in association with temporal application of the residual stress from the gate portions  80  provided at the bottom portion of the developing frame member  310  in the maximum image area in the area Q of the developing frame member  310  to the developing frame member  310 . 
     According to the first embodiment described above, the positions of the gate portions  80  are designed such that fluctuation in the size of the SB gap G due to the residual stress from the gate portions  80  is reduced in a state that the resin doctor blade  36  having low stiffness is fixed to the resin developing frame member  310 . Specifically, as illustrated in  FIGS. 13 and 14 , the gate portions  80  are not provided at the bottom portion of the developing frame member  310  in the maximum image area in the area P of the developing frame member  310 , and are provided at the bottom portion of the developing frame member  310  in the maximum image area in the area Q of the developing frame member  310 . With this configuration, fluctuation in the size of the SB gap G due to the residual stress from the gate portions  80  can be, in the first embodiment, reduced in a state that the resin doctor blade  36  having low stiffness is fixed to the resin developing frame member  310 . 
     Note that in the first embodiment, two gate portions  80  are provided with a spacing at the bottom portion of the developing frame member  310  in the maximum image area in the area Q of the developing frame member  310  as illustrated in  FIG. 14 . Since the multiple gate portions  80  are provided at the developing frame member  310  as described above, the amount of resin flowing into each gate portion  80  is dispersed proportional to the number of gate portions  80  provided at the developing frame member  310  when the molten resin flows into the gate portions  80  through the gates. Then, the molding pressure on each gate portion  80  is smaller in a case where the number of gate portions  80  provided at the developing frame member  310  is a multiple number than in a case where the number of gate portions  80  provided at the developing frame member  310  is only one. As a result, the residual stress generated from each gate portion  80  is smaller in a case where the number of gate portions  80  provided at the developing frame member  310  is a multiple number than in a case where the number of gate portions  80  provided at the developing frame member  310  is only one. 
     That is, the multiple gate portions  80  are provided at the bottom portion of the developing frame member  310  in the maximum image area in the area Q of the developing frame member  310 , and therefore, the influence of contribution of the residual stress from the gate portions  80  to fluctuation in the size of the SB gap G can be further reduced. Thus, it is advantageous because the influence of contribution of the residual stress from the gate portions  80  to fluctuation in the size of the SB gap G can be further reduced when the number of gate portions  80  provided at the bottom portion of the developing frame member  310  in the maximum image area in the area Q of the developing frame member  310  is not one but a multiple number. 
     Moreover, in the first embodiment, the gate portions  80  having a greater thickness than the basic thickness of the developing frame member  310  are provided at the bottom portion of the developing frame member  310  in the maximum image area in the area Q of the developing frame member  310  as illustrated in  FIG. 14 . When the molten resin flows into the gate portions  80  through the gates, the amount of resin flowing in per unit area of the gate portion  80  is dispersed proportional to the size of the sectional area of the gate portion  80  provided at the developing frame member  310 . Thus, the molding pressure on each gate portion  80  is smaller in a case where the sectional area of the gate portion  80  provided at the developing frame member  310  is greater than a predetermined value than in a case where the sectional area of the gate portion  80  provided at the developing frame member  310  is equal to or less than the predetermined value. As a result, the residual stress generated from each gate portion  80  is smaller in a case where the sectional area of the gate portion  80  provided at the developing frame member  310  is greater than the predetermined value than in a case where the sectional area of the gate portion  80  provided at the developing frame member  310  is equal to or less than the predetermined value. 
     That is, the gate portions  80  having a greater thickness than the basic thickness of the developing frame member  310  are provided at the bottom portion of the developing frame member  310  in the maximum image area in the area Q of the developing frame member  310  so that the influence of contribution of the residual stress from the gate portions  80  to fluctuation in the size of the SB gap G can be further reduced. Thus, it is advantageous because the thickness of each gate portion  80  provided at the bottom portion of the developing frame member  310  in the maximum image area in the area Q of the developing frame member  310  is greater than the basic thickness of the developing frame member  310  so that the influence of contribution of the residual stress from the gate portions  80  to fluctuation in the size of the SB gap G can be further reduced. 
     (Other Embodiments) 
     The present disclosure is not limited to the above-described embodiment. 
     Various modifications (including organic combinations of the embodiments) can be made based on the gist of the present disclosure, and are not excluded from the scope of the present disclosure. 
     In the present embodiment, the image forming device  60  configured to use the intermediate transfer belt  61  as the image bearing member as illustrated in  FIG. 1  has been described by way of example, but the present disclosure is not limited to above. The present disclosure is also applicable to an image forming device configured such that a recording medium sequentially directly comes into contact with a photosensitive drum  1  for performing transfer. In this case, the photosensitive drum  1  forms a rotatable image bearing member configured to carry a toner image. 
     Moreover, in the above-described embodiment, the developing device  3  configured such that the developing sleeve  70  rotates counterclockwise and the doctor blade  36  is arranged below the developing sleeve  70  as illustrated in  FIG. 2  has been described by way of example, but the present disclosure is not limited to above. The present disclosure is also applicable to a developing device  3  (a developing device  300 ) configured such that a developing sleeve  70  rotates clockwise and a doctor blade  36  is arranged above the developing sleeve  70 . 
     Further, in the above-described embodiment, the developing device  3  (the developing device  300 ) configured such that the development chamber  31  and the mixing chamber  32  are arranged side by side in the horizontal direction as illustrated in  FIG. 2  has been described by way of example, but the present disclosure is not limited to above. The present disclosure is also applicable to a developing device  300  configured such that a development chamber  31  and a mixing chamber  32  are arranged on one another in the direction of gravitational force. 
     In addition, in the above-described embodiment, the developing device  300  has been described as a single unit, but similar advantageous effects are obtained even in such a process cartridge form that the image forming units  600  (see  FIG. 1 ) including the developing device  3  is integrally unitized and is detachably attachable to the image forming device  60 . Further, as long as the image forming device  60  includes the developing device  300  or the process cartridge, the present disclosure is applicable regardless of a black-and-white machine or a color machine. 
     While the present disclosure has been described with reference to exemplary embodiments, it is to be understood that the disclosure is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2017-172337, filed Sep. 7, 2017, and No. 2018-146714, filed Aug. 3, 2018, which are hereby incorporated by reference herein in their entirety.