Patent Publication Number: US-2020301315-A1

Title: Developing device and image forming apparatus

Description:
FIELD 
     Embodiments described herein relate generally to a developing device, an image forming apparatus, and methods related thereto. 
     BACKGROUND 
     A developing device of an image forming apparatus stirs a developer containing a toner and a carrier, thereby triboelectrically charging the toner. The charged toner is adsorbed on a magnet roller and thereafter moves to a photoconductive drum. 
     In this manner, the developing device causes the toner contained in the developer to adhere to the photoconductive drum according to an electrostatic latent image. When the toner is consumed by development, the toner is supplied to the developing device. The developing device includes a stirring screw. The stirring screw stirs the developer and the supplied toner, and also circulates and conveys the developer in the developing device. 
     The stirring screw more preferably equalizes the charge amount of the toner and the distribution density of the toner. The stirring screw needs to uniformly feed a necessary amount of the developer to the magnet roller. The stirring screw needs to have excellent stirring performance and conveying performance. 
     For example, in order to improve the stirring performance of the stirring screw, a notch is known to be provided to the screw. However, when a notch is provided, the rigidity and uniformity of the stirring screw are deteriorated, and therefore, uneven development easily occurs. When the screw is not provided with a notch, the conveying performance becomes stable, however, the stirring performance is deteriorated. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view showing an overall configuration example of an image forming apparatus of an embodiment. 
         FIG. 2  is a cross-sectional view showing an example of a principal portion of a developing device of an embodiment. 
         FIG. 3  is an A-A cross-sectional view in  FIG. 2 . 
         FIG. 4  is a B-B cross-sectional view in  FIG. 2 . 
         FIG. 5  is a schematic perspective view showing an example of a first stirring screw of the developing device of the embodiment. 
         FIG. 6  is a schematic perspective view showing an example of the first stirring screw of the developing device of the embodiment. 
         FIGS. 7A and 7B  are cross-sectional views in  FIG. 5 . 
         FIG. 8  is a schematic cross-sectional view illustrating an act of the developing device of the embodiment. 
         FIG. 9  is a schematic perspective view showing an example of a first stirring screw of a first modification of the embodiment. 
         FIGS. 10A to 10D  are schematic cross-sectional views in  FIG. 9 . 
         FIG. 11  is a schematic cross-sectional view showing an example of a first stirring screw of a second modification of the embodiment. 
         FIG. 12  is a schematic cross-sectional view showing an example of a first stirring screw of a third modification of the embodiment. 
         FIG. 13  is a schematic cross-sectional view showing an example of a first stirring screw of a fourth modification of the embodiment. 
         FIG. 14  is a schematic cross-sectional view showing an example of a first stirring screw of a fifth modification of the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A developing device of an embodiment includes a developer storage unit, a magnet roller, a first stirring screw, and a second stirring screw. The developer storage unit includes a first storage chamber and a second storage chamber. The first storage chamber and the second storage chamber store a developer. In the first storage chamber and the second storage chamber, a first opening portion and a second opening portion are formed in both end portions in a longitudinal direction. The developer storage unit is configured to be made to be able to circulate and convey the developer through the first opening portion and the second opening portion. The magnet roller adsorbs the developer. The magnet roller is disposed extending in the longitudinal direction in an upper portion of the first storage chamber. The first stirring screw conveys the developer from the second opening portion to the first opening portion along the longitudinal direction. The first stirring screw includes a rotating shaft and a protrusive ridge. The rotating shaft is disposed along the longitudinal direction below the magnet roller inside the first storage chamber. The protrusive ridge swirls spirally around the rotating shaft. A plurality of notches are formed in part of the protrusive ridge. The plurality of notches are formed at positions where the phases in the circumferential direction of the rotating shaft at respective central positions in the plurality of notches are shifted in the axial direction. The second stirring screw is disposed parallel to the first stirring screw inside the second storage chamber. The second stirring screw conveys the developer from the first opening portion to the second opening portion along the longitudinal direction. 
     Embodiments 
     Hereinafter, a developing device and an image forming apparatus of embodiments will be described with reference to the drawings. 
       FIG. 1  is a schematic cross-sectional view showing an overall configuration example of an image forming apparatus of an embodiment. In the respective drawings hereinbelow, the same components are denoted by the same reference numerals unless otherwise specified. 
     As shown in  FIG. 1 , an image forming apparatus  100  of the embodiment includes a control panel  1 , a scanner unit  2 , a printer unit  3 , a sheet feed unit  4 , a conveying unit  5 , and a body control unit  6 . 
     Hereinafter, when referring to a relative position in the image forming apparatus  100 , X1 direction, X2 direction, Y1 direction, and Y2 direction shown in the drawings are used in some cases. The X1 direction is a direction directed from left to right when standing in front of the image forming apparatus  100  (on the front side of the sheet of  FIG. 1 ). The X2 direction is an opposite direction to the X1 direction. The Y1 direction is a direction directed from the rear face to the front face of the image forming apparatus  100 . The Y2 direction is an opposite direction to the Y1 direction. When the direction is irrespective of the X1 (Y1) direction or the X2 (Y2) direction or both directions are included, the direction is simply referred to as “X (Y) direction”. 
     The control panel  1  makes the image forming apparatus  100  act by being operated by an operator. 
     The scanner unit  2  reads the image information of an object to be copied as the contrast of light. The scanner unit  2  outputs the read image information to the printer unit  3 . 
     The printer unit  3  forms an image on a sheet S based on the image information from the scanner unit  2  or from the outside. 
     The printer unit  3  forms an output image (toner image) with a developer containing a toner. The printer unit  3  transfers the toner image onto the surface of the sheet S. The printer unit  3  applies heat and pressure to the toner image on the surface of the sheet S, thereby fixing the toner image to the sheet S. 
     The sheet feed unit  4  feeds the sheet S one by one to the printer unit  3  in accordance with the timing of forming the toner image by the printer unit  3 . 
     The sheet feed unit  4  includes a plurality of paper feed cassettes  20 A,  20 B, and  20 C. Each of the paper feed cassettes  20 A,  20 B, and  20 C can store the sheets S whose size and type are set in advance for each cassette in a stacked state. 
     The paper feed cassettes  20 A,  20 B, and  20 C can be attached to and detached from a body portion of the sheet feed unit  4 . The paper feed cassettes  20 A,  20 B, and  20 C are disposed in a stacked state in this order from the top to the bottom when being attached to the sheet feed unit  4 . 
     The sheet feed unit  4  includes pickup rollers  21 A,  21 B, and  21 C corresponding to the respective paper feed cassettes  20 A,  20 B, and  20 C. The pickup rollers  21 A,  21 B, and  21 C each pick up the sheet S loaded in the paper feed cassettes  20 A,  20 B, and  20 C, respectively, one by one. The pickup rollers  21 A,  21 B, and  21 C each pick up the uppermost sheet S in the loading direction among the loaded sheets S. The pickup rollers  21 A,  21 B, and  21 C convey the picked up sheet S to the conveying unit  5  toward the printer unit  3 . 
     The conveying unit  5  includes a conveying roller  23  and a resist roller  24 . The conveying unit  5  conveys the sheet S fed from the pickup rollers  21 A,  21 B, and  21 C to the resist roller  24 . The resist roller  24  conveys the sheet S in accordance with the timing of transferring the toner image to the sheet S by the printer unit  3 . 
     The conveying roller  23  abuts the front end in the conveying direction of the sheet S against a nip N of the resist roller  24 . The conveying roller  23  bends the sheet S so as to adjust the position of the front end of the sheet S in the conveying direction. 
     The resist roller  24  aligns the front end of the sheet S sent from the conveying roller  23  in the nip N. Further, the resist roller  24  conveys the sheet S toward the below-mentioned transfer unit  28 . 
     The printer unit  3  includes image forming units  25 Y,  25 M,  25 C, and  25 K, alight exposure unit  26 , an intermediate transfer belt  27 , a transfer unit  28 , a fixing device  29 , and a transfer belt cleaning unit  35 . 
     The image forming units  25 Y,  25 M,  25 C, and  25 K are disposed in this order in the X1 direction. 
     Each of the image forming units  25 Y,  25 M,  25 C, and  25 K forms a toner image to be transferred to the sheet S on the intermediate transfer belt  27 . 
     Each of the image forming units  25 Y,  25 M,  25 C, and  25 K has a photoconductive drum. The image forming units  25 Y,  25 M,  25 C, and  25 K form toner images of yellow, magenta, cyan, and black on the corresponding photoconductive drums  7 , respectively. 
     On the circumference of each of the photoconductive drums  7 , a charger, a developing device  8 , a transfer roller, a cleaning unit, and a charge neutralizer are disposed. The transfer roller is opposed to the photoconductive drum  7 . The intermediate transfer belt  27  is held between the transfer roller and the photoconductive drum  7 . The light exposure unit  26  is disposed below the charger and the developing device. 
     A detailed configuration of each of the developing devices  8  will be described later. 
     Toner cartridges  33 Y,  33 M,  33 C, and  33 K are disposed above the image forming units  25 Y,  25 M,  25 C, and  25 K, respectively. In the toner cartridges  33 Y,  33 M,  33 C, and  33 K, toners of yellow, magenta, cyan, and black are stored, respectively. 
     The toner cartridges  33 Y,  33 M,  33 C, and  33 K communicate with the developing devices  8  of the image forming units  25 Y,  25 M,  25 C, and  25 K through the below-mentioned toner supply tubes  34  (not shown in  FIG. 1 ). 
     In each of the toner cartridges  33 Y,  33 M,  33 C, and  33 K and the respective toner supply tubes  34 , a toner conveying mechanism (not shown) for sending the toner to the developing device  8  is provided. 
     The respective toners in the toner cartridges  33 Y,  33 M,  33 C, and  33 K are supplied to the respective developing devices  8  through the toner supply tubes  34  (not shown). 
     The light exposure unit  26  irradiates a laser beam onto the surface of each of the charged photoconductive drums  7 . The emission of the laser beam is controlled based on the image information. The light exposure unit  26  can also adopt a configuration in which LED light is irradiated in place of the laser beam. 
     The light exposure unit  26  is supplied with image information corresponding to each of yellow, magenta, cyan, and black. 
     The light exposure unit  26  forms electrostatic latent images based on the image information on the surfaces of the respective photoconductive drums  7 . 
     The intermediate transfer belt  27  is composed of an endless belt. Tension is applied to the intermediate transfer belt  27  by a plurality of rollers abutting against the inner circumferential face. The intermediate transfer belt  27  is stretched into a flat shape. The inner circumferential face of the intermediate transfer belt  27  comes into contact with a support roller  28   a  at a position in the X1 direction farthest in the stretching direction. The inner circumferential face of the intermediate transfer belt  27  comes into contact with a transfer belt roller  32  at a position in the X2 direction farthest in the stretching direction. 
     The support roller  28   a  forms part of the below-mentioned transfer unit  28 . The support roller  28   a  guides the intermediate transfer belt  27  at a secondary transfer position. 
     The transfer belt roller  32  guides the intermediate transfer belt  27  at a cleaning position. 
     On the lower face side in the drawing of the intermediate transfer belt  27 , the image forming units  25 Y,  25 M,  25 C, and  25 K excluding the transfer rollers are disposed in this order in the X1 direction. The image forming units  25 Y,  25 M,  25 C, and  25 K are disposed spaced apart from one another in a region between the transfer belt roller  32  and the support roller  28   a.    
     The respective developing devices  8  of the image forming units  25 Y,  25 M,  25 C, and  25 K store the developers containing the toners of yellow, magenta, cyan, and black, respectively. However, the developers  8  mutually have the same configuration except that the developers are different. 
     The respective developing devices  8  are disposed opposed in the X2 direction to the respective photoconductive drums  7  of the image forming units  25 Y,  25 M,  25 C, and  25 K. The respective developing devices  8  develop the electrostatic latent images formed on the photoconductive drums  7  which the respective developing devices  8  are opposed to. As a result, toner images are formed on the respective photoconductive drums  7 . 
     The respective transfer rollers of the image forming units  25 Y,  25 M,  25 C, and  25 K transfer (primarily transfer) the toner images on the surfaces of the respective photoconductive drums  7  onto the intermediate transfer belt  27 . 
     When the toner images reach primary transfer positions, a transfer bias is applied to the respective transfer rollers. 
     The respective cleaning units of the image forming units  25 Y,  25 M,  25 C, and  25 K remove the untransferred toner on the surfaces of the respective photoconductive drums  7  after the primary transfer by scraping or the like. 
     The respective charge neutralizers of the image forming units  25 Y,  25 M,  25 C, and  25 K irradiates light onto the surfaces of the respective photoconductive drums  7  after passing through the cleaning units. The respective charge neutralizers of the image forming units  25 Y,  25 M,  25 C, and  25 K neutralize the charge on the photoconductive drums  7  which the respective charge neutralizers are opposed to. 
     On the intermediate transfer belt  27 , the transfer unit  28  is disposed at a position adjacent to the image forming unit  25 K. 
     The transfer unit  28  includes the support roller  28   a  and a secondary transfer roller  28   b . The secondary transfer roller  28   b  and the support roller  28   a  hold the intermediate transfer belt  27  therebetween. The position where the secondary transfer roller  28   b  and the intermediate transfer belt  27  come into contact with each other is a secondary transfer position. 
     The transfer unit  28  transfers the charged toner image on the intermediate transfer belt  27  onto the surface of the sheet S at the secondary transfer position. The transfer unit  28  applies a transfer bias to the secondary transfer position. The transfer unit  28  transfers the toner image on the intermediate transfer belt  27  to the sheet S by the transfer bias. 
     The fixing device  29  applies heat and pressure to the sheet S. The fixing device  29  fixes the toner image transferred to the sheet S by the heat and the pressure. 
     The transfer belt cleaning unit  35  is opposed to the transfer belt roller  32 . The transfer belt cleaning unit  35  holds the intermediate transfer belt  27  therebetween. The transfer belt cleaning unit  35  scrapes the toner on the surface of the intermediate transfer belt  27 . The transfer belt cleaning unit  35  collects the scraped toner in a waste toner tank. 
     The printer unit  3  includes a reversing unit  30 . The reversing unit  30  reverses the sheet S discharged from the fixing device  29  by switchback. The reversing unit  30  conveys the reversed sheet S into a conveying guide on the upstream side of the resist roller  24  again. The reversing unit  30  reverses the sheet S for forming an image on the reverse side. 
     The body control unit  6  controls the respective device portions of the image forming apparatus  100 . The control performed by the body control unit  6  includes control of stirring and conveying the developer in the respective developing devices  8 . 
     Next, a detailed configuration of the developing device  8  will be described. 
       FIG. 2  is a cross-sectional view showing an example of a principal portion of the developing device of the embodiment.  FIG. 3  is an A-A cross-sectional view in  FIG. 2 .  FIG. 4  is a B-B cross-sectional view in  FIG. 2 .  FIGS. 5 and 6  are each a schematic perspective view showing an example of a first stirring screw of the developing device of the embodiment.  FIGS. 7A and 7B  are cross-sectional views in  FIG. 5 . In particular,  FIG. 7A  is a Ci-Ci cross-sectional view in  FIG. 5  (provided that i=2, 5, or 8). In particular,  FIG. 7B  is a Dj-Dj cross-sectional view in  FIG. 5  (provided that j=3, 6, or 9). 
     The developing device  8  performs development by a two-component development system. 
     As shown in  FIG. 2 , the developing device  8  includes a developer storage container  8   a , a magnet roller  9 , an upper cover  8   b , a left cover  8   c , a first stirring screw  10 , and a second stirring screw  11 . 
     The developer storage container  8   a  is a container which is long in the Y direction (longitudinal direction). The developer storage container  8   a  opens upward. The developer storage container  8   a  stores, for example, a developer  12   y ,  12   m ,  12   c , or  12   k.    
     The developer  12   y  ( 12   m ,  12   c , or  12   k ) is a mixture of a carrier composed of magnetic fine particles and a yellow (magenta, cyan, or black) toner. When the developer  12   y  ( 12   m ,  12   c , or  12   k ) is stirred, the toner is triboelectrically charged. The toner is adhered to the surface of the carrier. 
     In the following description, when the color of the toner does not particularly need to be distinguished, any of the developers  12   y ,  12   m ,  12   c , and  12   k  is denoted by “developer  12 ” for simplicity. 
     A first partition  8   d  is provided in a central portion in the X direction of the developer storage container  8   a.    
     The first partition  8   d  divides a space in the developer storage container  8   a  into two parts in the X direction. According to this, groove portions  8   f  and  8   g  are lined up in this order in the X2 direction inside the developer storage container  8   a . The cross section in the X direction of each of the groove portions  8   f  and  8   g  has a U-shaped form. 
     As shown in  FIG. 3 , the developer storage container  8   a  extends in the Y direction from a first end portion E 1  to a second end portion E 2  of the developing device  8 . Here, the first end portion E 1  is an end portion in the Y1 direction in the developing device  8 . The second end portion E 2  is an end portion in the Y2 direction in the developing device  8 . However, in  FIG. 3 , for ease of viewing, illustration of the developer  12  is omitted. 
     Each of the first partition  8   d  and the groove portions  8   f  and  8   g  extends in the Y direction. 
     A first notch portion  8   h  (a first opening portion or an inflow portion of a toner) is formed in an end portion in the Y1 direction of the first partition  8   d . The first notch portion  8   h  allows the groove portions  8   f  and  8   g  to communicate with each other. The developer  12   y  ( 12   m ,  12   c , or  12   k ) in the groove portion  8   f  can move to the groove portion  8   g  through the first notch portion  8   h.    
     A second notch portion  8   i  (a second opening portion) is formed in an end portion in the Y2 direction of the first partition  8   d . The second notch portion  8   i  allows the groove portions  8   f  and  8   g  to communicate with each other. The developer  12   y  ( 12   m ,  12   c , or  12   k ) in the groove portion  8   g  can move to the groove portion  8   f  through the second notch portion  8   i.    
     As shown in  FIG. 2 , the magnet roller  9 , the upper cover  8   b , and the left cover  8   c  are disposed above the developer storage container  8   a . The magnet roller  9 , the upper cover  8   b , and the left cover  8   c  are opposed to the opening on the upper side of the developer storage container  8   a  from above. 
     The magnet roller  9  supplies the developer  12  to the surface of the photoconductive drum  7 . Further, the magnet roller  9  develops the electrostatic latent image on the surface of the photoconductive drum  7 . The magnet roller  9  includes a cylindrical developing sleeve  9   a  and a magnet  9   b  disposed inside the developing sleeve  9   a . The magnet  9   b  is provided with a magnetic field distribution for performing drawing-up, napping, and nap-cutting of the developer  12 . 
     The magnet roller  9  has a wider developing width than the electrostatic latent image forming width of the photoconductive drum  7 . The roller width of the magnet roller  9  is shorter than that of the developer storage container  8   a.    
     The magnet roller  9  is disposed above the opening of the groove portion  8   f . The surface of the developing sleeve  9   a  and the surface of the photoconductive drum  7  are in proximity to each other. 
     The magnet roller  9  is rotationally driven counterclockwise in the drawing by a developing motor (not shown). The magnet roller  9  is rotated by the developing motor so as to obtain a developing linear speed to be determined according to the linear speed of the photoconductive drum  7 . 
     The upper cover  8   b  covers the surface of the magnet roller  9  excluding a portion coming close to the photoconductive drum  7  from above the groove portions  8   f  and  8   g.    
     The left cover  8   c  covers a portion which is not covered with the upper cover  8   b  in the X2 direction and the Y direction above the developer storage container  8   a  and the groove portion  8   g.    
     Between the upper cover  8   b  and the first partition  8   d , a second partition  8   e  is disposed over substantially the same length as that of the first partition  8   d.    
     The second partition  8   e  divides an internal space located above the developer storage container  8   a  and the first partition  8   d  and below the upper cover  8   b  and the left cover  8   c  into two parts in the X direction. Although an illustration is omitted, the second partition  8   e  closes the opening on the upper side of the first notch portion  8   h  and the second notch portion  8   i.    
     In the developing device  8 , the developer storage container  8   a , the upper cover  8   b , the left cover  8   c , the first partition  8   d , and the second partition  8   e  constitute a developer storage unit  8 Z. 
     The developer storage unit  8 Z has an internal space surrounded by the developer storage container  8   a , the upper cover  8   b , and the left cover  8   c . The internal space is divided into two parts in the X direction by the first partition  8   d  and the second partition  8   e . The developer storage unit  8 Z is composed of a first storage chamber  8   j  including the groove portion  8   f  and a second storage chamber  8   k  including the groove portion  8   g . The first storage chamber  8   j  and the second storage chamber  8   k  are lined up in this order in the X2 direction. 
     The first storage chamber  8   j  has a length capable of storing at least the magnet roller  9  in the Y direction. 
     The first storage chamber  8   j  is used for circulating the developer  12  between the groove portion  8   f  and the magnet roller  9 . When the developer  12  is drawn up from the groove portion  8   f  by the magnet roller  9  (see an upward white arrow in the drawing), the developer  12  moves counterclockwise in the drawing with the rotation of the magnet roller  9 . The developer  12  after completion of development moves to an upper part of the first storage chamber  8   j  with the rotation of the magnet roller  9 . When the magnet roller  9  rotates so as to be opposed to the second partition  8   e , the magnetic attraction of the developer  12  to the magnet roller  9  is released. The developer  12  drops on the groove portion  8   f  in the first storage chamber  8   j  by the own weight (see a downward white arrow in the drawing). 
     In this manner, in the first storage chamber  8   j , the developer  12  circulates in the vertical direction. The first storage chamber  8   j  is separated from the second storage chamber  8   k  by the second partition  8   e , and therefore, the developer  12  in the first storage chamber  8   j  is prevented from scattering to the second storage chamber  8   k.    
     The second storage chamber  8   k  has a larger volume than the entire volume of the developer  12  stored in the developer storage container  8   a . In an upper part of the groove portion  8   g  in the second storage chamber  8   k , in an initial state of the developing device  8 , an unused developer  12  is stored. The unused developer  12  is introduced into the developer storage container  8   a  by removing a seal (not shown) before starting to use the developing device  8 . 
     As shown by a two-dot chain line in  FIG. 3 , a toner supply port  8   m  (toner supply unit) is opened above the groove portion  8   f  near to the first end portion E 1 . The toner supply port  8   m  is provided on the groove portion  8   f  at a position opposed in the X direction to a central position of the first notch portion  8   h  in the Y direction. 
     As shown in  FIG. 4 , a toner transfer tube  8   p  is provided and erected in an inner edge portion of the toner supply port  8   m . The toner transfer tube  8   p  communicates with the toner supply port  8   m.    
     At an upper end of the toner transfer tube  8   p , a shutter  8   n  that opens and closes the opening of the toner transfer tube  8   p  is provided. 
     When the shutter  8   n  is opened, the toner supply tube  34  (not shown in  FIG. 4 ) can be connected to the upper end of the toner transfer tube  8   p . When the toner supply tube  34  is connected to the toner transfer tube  8   p , as shown by a two-dot chain line in  FIG. 3 , the toner supply tube  34  is located above the toner supply port  8   m.    
     The shutter  8   n  closes when the developing device  8  is detached from the image forming apparatus  100 . 
     When the toner supply tube  34  is connected to the toner transfer tube  8   p , the toner conveyed through the toner supply tube  34  is supplied to the developing device  8  through the toner transfer tube  8   p  and the toner supply port  8   m . Through the toner supply port  8   m , the conveyed toner is supplied onto the groove portion  8   f.    
     For example, the toner supply tube  34  is connected to the toner cartridge  33 Y ( 33 M,  33 C, or  33 K) shown in  FIG. 1 . Through this toner supply tube  34 , the toner of the same color as the toner contained in the developer  12   y  ( 12   m ,  12   c , or  12   k ) is supplied to the developer  12   y  ( 12   m ,  12   c , or  12   k ). 
     The toner supplied from the toner supply tube  34  drops on the groove portion  8   f  at a position opposed in the X direction to a central position of the first notch portion  8   h  in the Y direction. The toner after dropping on the groove portion  8   f  flows in the groove portion  8   g  from the groove portion  8   f  along with the developer  12  through the first notch portion  8   h.    
     The toner supply amount is controlled by controlling the act of the toner conveying mechanism by the body control unit  6 . For example, when the output of a toner density sensor (not shown) provided in the developing device  8  decreases, the body control unit  6  drives the toner conveying mechanism so as to compensate for the decreased amount. 
     As shown in  FIG. 2 , the first stirring screw  10  and the second stirring screw  11  are disposed inside the groove portions  8   f  and  8   g  of the developer storage container  8   a , respectively. 
     As shown in  FIG. 3 , each of the first stirring screw  10  and the second stirring screw  11  extends in the Y direction. 
     The first stirring screw  10  is disposed parallel to the magnet roller  9 . The first stirring screw  10  conveys the developer  12  in the groove portion  8   f  in the Y1 direction (first direction). 
     As shown in  FIGS. 5 and 6 , the first stirring screw  10  includes a rotating shaft  10   a , a first protrusive ridge  10   b  (protrusive ridge), and a second protrusive ridge  10   c  (protrusive ridge). 
     As shown in  FIG. 3 , the rotating shaft  10   a  extends straight in the Y direction. A first end portion e 1  and a second end portion e 2  of the rotating shaft  10   a  are supported rotatably by bearing portions provided in the developer storage container  8   a . Here, the first end portion e 1  is an end portion in the Y1 direction of the rotating shaft  10   a . The second end portion e 2  is an end portion in the Y2 direction of the rotating shaft  10   a.    
     The rotating shaft  10   a  can rotate around the central axis line O 11  (see  FIG. 2 ) of the rotating shaft  10   a.    
     A gear  10   n  is provided in the second end portion e 2  of the rotating shaft  10   a.    
     The gear  10   n  is connected to a motor (not shown) through a transmission mechanism (not shown). 
     The motor that drives the first stirring screw  10  may be a developing motor or a motor other than a developing motor. In this embodiment, as one example, the first stirring screw  10  is driven by a developing motor. The rotational speed of the first stirring screw  10  has a certain relationship determined according to a transmission gear ratio of the transmission mechanism with respect to the developing linear speed. 
     In this embodiment, the first stirring screw  10  is rotated counterclockwise in  FIG. 2  (left-handed when seen in the Y2 direction, see the arrow R 1  in  FIG. 2 ). 
     The first protrusive ridge  10   b  is formed in a spiral shape on an outer circumferential portion of the rotating shaft  10   a . For example, the first protrusive ridge  10   b  is formed along a helical line with a fixed lead angle. The turning direction of the first protrusive ridge  10   b  is, for example, a left-hand direction. 
     However, in the first protrusive ridge  10   b , a notch Nt is formed at a plurality of sites in the extending direction. The shape of the first protrusive ridge  10   b  excluding the notch Nt is not particularly limited as long as the developer  12  in the groove portion  8   f  can be conveyed in the Y2 direction according to the rotating direction of the rotating shaft  10   a.    
     A detailed shape and the like of the notch Nt will be described later. 
     The second protrusive ridge  10   c  is formed in a spiral shape on an outer circumferential portion of the rotating shaft  10   a  between the pitches of the first protrusive ridge  10   b . The second protrusive ridge  10   c  is formed in parallel to the first protrusive ridge  10   b  along a helical line with the same lead angle as that of the first protrusive ridge  10   b . The second protrusive ridge  10   c  is provided at a position equidistant from the first protrusive ridge  10   b  adjacent thereto in the Y direction. 
     As shown in  FIG. 2 , the second protrusive ridge  10   c  in this embodiment is formed at a position rotationally symmetrical by 180° to the first protrusive ridge  10   b  with respect to the central axis line O 10  in a cross section orthogonal to the central axis line O 10 . The cross-sectional shape of the second protrusive ridge  10   c  is the same as the cross-sectional shape of the first protrusive ridge  10   b  in a region excluding the notch Nt. 
     In the second protrusive ridge  10   c , the notch Nt may be formed in the same manner as in the first protrusive ridge  10   b . However, in this embodiment, as one example, the notch Nt is not formed in the second protrusive ridge  10   c.    
     In the example shown in  FIG. 5 , the first protrusive ridge  10   b  is a spiral screw which rotates ten times in the Y2 direction. The turning direction of the first protrusive ridge  10   b  is left-handed (counterclockwise) when seen in the Y2 direction. 
     Hereinafter, with respect to a unit screw obtained by dividing the first protrusive ridge  10   b  for each turning pitch, the expression of “the n-th portion Sn” (provided that n=1, . . . , or  10 , the same also applies hereinbelow) is used in some cases. For example, the first portion S 1  represents a unit screw which is the nearest to the Y1 direction side in the first protrusive ridge  10   b . When k is assumed to be an integer of 1 or more and 9 or less, the (k+1)-th portion S (k+1) is adjacent to the k-th portion Sk in the Y2 direction. 
     The n-th portion Sn starts at a point P (n−1) on the outer circumference of the first protrusive ridge  10   b  and ends at a point Pn on the outer circumference of the first protrusive ridge  10   b . The respective points Pn are aligned on a straight line parallel to the central axis line O 10 . 
     Hereinafter, the position on the outer circumference of the first protrusive ridge  10   b  is represented by a turning angle θ. For example, the positions of the points P 1 , P 2 , and P 3  are represented by θ=0°, θ=360°, and θ=720°, respectively. Further, the position on the outer circumferential line of the n-th portion Sn is represented by a phase ϕ (provided that 0°&lt;ϕ&lt;360°). The phase ϕ is represented by ϕ=θ−{360°×(n−1)}. 
     As shown in  FIG. 6 , on the outer circumference of the n-th portion Sn, an intermediate point between the point P(n−1) and the point Pn is referred to as “point Qn” in some cases. The phase ϕ of the point Qn is 180°. 
     In the following Table 1, the arrangement of the notches Nt is shown. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 phase ϕ 
               
               
                   
                 n 
                 area 
                 notch Nt 
                 (°) 
               
               
                   
                   
               
             
            
               
                   
                  1 
                 P0-Q1-P1 
                 without 
                 — 
               
               
                   
                  2 
                 P1-Q2-P2 
                 Nta 
                 315 
               
               
                   
                  3 
                 P2-Q3-P3 
                 Ntb 
                 135 
               
               
                   
                  4 
                 P3-Q4-P4 
                 without 
                 — 
               
               
                   
                  5 
                 P4-Q5-P5 
                 Nta 
                 315 
               
               
                   
                  6 
                 P5-Q6-P6 
                 Ntb 
                 135 
               
               
                   
                  7 
                 P6-Q7-P7 
                 without 
                 — 
               
               
                   
                  8 
                 P7-Q8-P8 
                 Nta 
                 315 
               
               
                   
                  9 
                 P8-Q9-P9 
                 Ntb 
                 135 
               
               
                   
                 10 
                 P9-Q10-P10 
                 without 
                 — 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 1, in this embodiment, the notch Nt is formed in each of the second portion S 2 , the third portion S 3 , the fifth portion S 5 , the sixth portion S 6 , the eighth portion S 8 , and the ninth portion S 9 . The notch Nt is composed of two types: a notch Nta (see  FIG. 5 ) and a notch Ntb (see  FIG. 6 ). 
     The notch Nta is formed at a position where the phase is 315° in the second portion S 2 , the fifth portion S 5 , and the eighth portion S 8 .  FIG. 7A  is a cross-sectional view orthogonal to the central axis line O 10  including the notch Nta. The symbol i in the drawing denotes 2, 5, or 8.  FIG. 7A  corresponds to a C2-C2 cross section, a C5-C5 cross section, or a C8-C8 cross section in  FIG. 5 . 
     As shown in  FIG. 7A , the phase of the notch Nta is a phase of a central axis line Aa of the notch Nta. 
     The notch Nta is a recessed portion composed of a bottom face  10   d , a first inner face  10   e , and a second inner face  10   f  on the first protrusive ridge  10   b.    
     The bottom face  10   d  is a curved face formed by intersecting a cylindrical face having a radius r with the central axis line O 10  as the center with the first protrusive ridge  10   b . Here, the radius r is larger than the radius Ra of the rotating shaft  10   a  and smaller than the outer radius Rb of the first protrusive ridge  10   b  (provided that Rb&gt;Ra). A specific magnitude of r is determined according to the balance between the stirring performance and the conveying performance for the developer  12  required for the first stirring screw  10 . 
     The first inner face  10   e  is a flat plane that includes the central axis line O 10  and is formed by intersecting a flat plane having a phase of 270° with the first protrusive ridge  10   b  up to the bottom face  10   d.    
     The second inner face  10   f  is a flat plane that includes the central axis line O 10  and is formed by intersecting a flat plane having a phase of 0° with the first protrusive ridge  10   b  up to the bottom face  10   d.    
     Therefore, the notch Nta has a shape in which the outer circumferential portion of the first protrusive ridge  10   b  is notched in a fan-like shape having a central angle of 90° when seen in the Y2 direction. 
     The depth of the notch Nta is a fixed value (Rb-r). 
     As shown in Table 1, the notch Ntb is formed at a position where the phase is 135° in the third portion S 3 , the sixth portion S 6 , and the ninth portion S 9 .  FIG. 7B  is a cross-sectional view orthogonal to the central axis line O 10  including the notch Ntb. The symbol j in the drawing denotes 3, 6, or 9.  FIG. 7B  corresponds to a D3-D3 cross section, a D6-D6 cross section, or a D9-D9 cross section in  FIG. 5 . 
     As shown in  FIG. 7B , the phase of the notch Ntb is a phase of a central axis line Ab of the notch Ntb. 
     The notch Ntb may have a different shape from the notch Nta. However, in this embodiment, as one example, the notch Ntb has the same shape as the notch Nta. 
     The notch Ntb is a recessed portion on the first protrusive ridge  10   b  in the same manner as the notch Nta. The notch Ntb includes a bottom face  10   g , a first inner face  10   h , and a second inner face  10   i  corresponding to the bottom face  10   d , the first inner face  10   e , and the second inner face  10   f  of the notch Nta, respectively. 
     The bottom face  10   g , the first inner face  10   h , and the second inner face  10   i  are the same faces as the bottom face  10   d , the first inner face  10   e , and the second inner face  10   f , respectively, except that the formed phase is different by 180°. 
     That is, the depth of the notch Ntb is equal to that of the notch Nta. The opening length of the notch Ntb in the extending direction of the first protrusive ridge  10   b  is the same as the opening length of the notch Nta. 
     The notch Nt in this embodiment is configured to have a repetitive pattern in which the notches Nta and Ntb are alternately lined up in this order in the axial direction of the first stirring screw  10  (Y direction). Therefore, the phases ϕ of the plurality of notches Nt are 315° and 135° which alternate in this order. A phase difference between the notch Nta and the notch Ntb adjacent to each other in the axial direction is 180° and constant. 
     Further, the plurality of notches Nt are configured such that a pair of notches Nta and Ntb is sandwiched among the first portion S 1 , the fourth portion S 4 , the seventh portion S 7 , and the tenth portion S 10  where the notch Nt is not formed. Therefore, the first protrusive ridge  10   b  has a repetitive pattern in which, provided that k=1, 4, or 7, a k-th portion Sk where the notch Nt is not formed, a (k+1)-th portion S(k+1) where the notch Nta is formed, and a (k+2)-th portion S(k+2) where the notch Ntb is formed are lined up. 
     Therefore, a distance between the notches Nta and Ntb sandwiching the k-th portion Sk therebetween is longer than a distance between the notches Nta and Ntb in the (k+1)-th portion S(k+1) and the (k+2)-th portion S(k+2). 
     In the first stirring screw  10 , the rotating shaft  10   a , the first protrusive ridge  10   b , and the second protrusive ridge  10   c  may be formed of the same material or different materials. For example, in this embodiment, the rotating shaft  10   a , the first protrusive ridge  10   b , and the second protrusive ridge  10   c  are integrally formed by resin molding. 
     As shown in  FIG. 3 , the second stirring screw  11  is adjacent to the first stirring screw  10  in the X2 direction with the first partition  8   d  interposed therebetween. The second stirring screw  11  is disposed parallel to the first stirring screw  10 . Therefore, the second stirring screw  11  is disposed parallel to the magnet roller  9  at a position farther apart from the magnet roller  9  than the first stirring screw  10 . 
     The second stirring screw  11  conveys the developer  12  in the groove portion  8   g  in the Y2 direction (second direction). 
     The second stirring screw  11  includes a rotating shaft  11   a  and a screw  11   b.    
     The rotating shaft  11   a  extends straight in the Y direction. Both end portions of the rotating shaft  11   a  are supported rotatably by bearing portions provided in the developer storage container  8   a.    
     The rotating shaft  11   a  can rotate around the central axis line O 11  (see  FIG. 2 ) of the rotating shaft  11   a.    
     A gear  11   c  is provided in the second end portion e 2  of the rotating shaft  11   a.    
     The gear  11   c  is connected to a motor (not shown) through a transmission mechanism (not shown). 
     The motor that drives the second stirring screw  11  may be a developing motor or a motor other than a developing motor. In this embodiment, as one example, the second stirring screw  11  is driven by a developing motor. The rotational speed of the second stirring screw  11  has a certain relationship determined according to a transmission gear ratio of the transmission mechanism with respect to the developing linear speed. 
     As shown in  FIG. 3 , the screw  11   b  is formed in a spiral shape on an outer circumferential portion of the rotating shaft  11   a . The shape of the screw  11   b  is not particularly limited as long as the developer  12  in the groove portion  8   g  can be conveyed in the Y2 direction according to the rotating direction of the rotating shaft  11   a . For example, in this embodiment, the screw  11   b  is a single threaded spiral screw. 
     In the second stirring screw  11 , the rotating shaft  11   a  and the screw  11   b  may be formed of the same material or different materials. For example, in this embodiment, the rotating shaft  11   a  and the screw  11   b  are integrally formed by resin molding. 
     Next, an act of the image forming apparatus  100  will be described while focusing on an action of the developing device  8 . 
     First, an act of image formation of the image forming apparatus  100  will be briefly described. 
     In the image forming apparatus  100  shown in  FIG. 1 , image formation is started by an operation of the control panel  1  or an external signal. Image information is sent to the printer unit  3  by reading an object to be copied by the scanner unit  2  or is externally sent to the printer unit  3 . 
     The printer unit  3  feeds the sheet S from the sheet feed unit  4  to the resist roller  24 . The sheet S to be fed from the sheet feed unit  4  is selected by the body control unit  6  based on an operation of the control panel  1  or an external signal. 
     When an operation input for image formation is performed from the control panel  1 , the body control unit  6  performs control for starting paper feed from a paper feed cassette and image formation. 
     The image forming units  25 Y,  25 M,  25 C, and  25 K form electrostatic latent images on the respective photoconductive drums  7  based on the image information according to the respective colors. The respective electrostatic latent images are developed by the developing devices  8 , respectively. Therefore, toner images corresponding to the electrostatic latent images are formed on the surfaces of the respective photoconductive drums  7 . 
     The respective toner images are primarily transferred to the intermediate transfer belt  27  by the respective transfer rollers. At this time, the transfer timing is appropriately shifted according to the arrangement positions of the image forming units  25 Y,  25 M,  25 C, and  25 K. Therefore, the respective toner images are sequentially overlapped with one another with the movement of the intermediate transfer belt  27  without causing a color shift and sent to the transfer unit  28 . 
     On the other hand, the sheet S is fed and sent from the resist roller  24  to the transfer unit  28 . The toner images after reaching the transfer unit  28  are secondarily transferred to the sheet S. The secondarily transferred toner images are fixed to the sheet S by the fixing device  29 . 
     On the intermediate transfer belt  27 , a transfer residual toner remains. The transfer residual toner is a toner which cannot be transferred onto the sheet S by the transfer unit  28 . The transfer residual toner is scraped off by the transfer belt cleaning unit  35 . The intermediate transfer belt  27  is cleaned so as to be made reusable. 
     Next, an act of the developing device  8  will be described while focusing on an act of stirring the developer  12 . 
       FIG. 8  is a schematic cross-sectional view in the axial direction illustrating an act of the developing device of the embodiment. 
     While performing image formation by the image forming apparatus  100 , in the developing device  8 , the first stirring screw  10  and the second stirring screw  11  are rotated by a motor. 
     The first stirring screw  10  conveys the developer  12  in the groove portion  8   f  in the Y1 direction while stirring the developer  12 . The developer  12  in the groove portion  8   f  is conveyed from the second notch portion  8   i  to the first notch portion  8   h.    
     The second stirring screw  11  conveys the developer  12  in the groove portion  8   g  in the Y2 direction while stirring the developer  12 . The developer  12  in the groove portion  8   g  is conveyed from the first notch portion  8   h  to the second notch portion  8   i.    
     The conveyance amounts of the first stirring screw  10  and the second stirring screw  11  are equal to each other. Therefore, the developer  12  after reaching the first notch portion  8   h  moves to the groove portion  8   g  through the first notch portion  8   h . The developer  12  after reaching the second notch portion  8   i  moves to the groove portion  8   f  through the second notch portion  8   i . In this manner, the developer  12  is circulated and conveyed in the developer storage container  8   a.    
     As a result, a substantially uniform flow occurs in the developer  12  in the groove portion  8   f  opposed to the magnet roller  9 . To the developer  12  that moves to the groove portion  8   f , the uniformly charged toner is adhered by the rotation of the second stirring screw  11 . 
     As shown in  FIG. 2 , the developer  12  in the vicinity of the magnet roller  9  is drawn up on the developing sleeve  9   a  by the magnetic force of the magnet  9   b . The developer  12  adsorbed on the surface of the developing sleeve  9   a  rotates along with the developing sleeve  9   a . The developer  12  forms a magnetic brush at a position opposed to the photoconductive drum  7  according to the magnetic force distribution of the magnet  9   b.    
     When a developing bias is applied to the toner in the magnetic brush by the body control unit  6 , the toner is electrostatically adsorbed on the electrostatic latent image on the photoconductive drum  7 . According to this, the electrostatic latent image on the photoconductive drum  7  is developed with the toner. 
     The developer  12  in which part of the toner is lost moves in the first storage chamber  8   j  with the rotation of the developing sleeve  9   a  and drops on the groove portion  8   f  from the developing sleeve  9   a  according to the magnetic force distribution of the magnet  9   b.    
     When image formation proceeds, the developer  12  in which the toner is reduced is mixed in the developer  12  conveyed by the first stirring screw  10 . The developers  12  having different toner adhesion amounts are moved in the Y1 direction and also stirred by the first stirring screw  10 . 
     In this manner, when image formation is started, the toner density of the developer  12  in the groove portion  8   f  decreases. 
     The body control unit  6  monitors the toner density by the toner density sensor (not shown). The body control unit  6  controls toner supply as needed. According to this, the toner is supplied from the toner cartridge through the toner supply tube  34  and the toner supply port  8   m.    
     The supplied toner moves from the groove portion  8   f  to the end portion in the Y1 direction of the groove portion  8   g  along with the developer  12  that moves to the groove portion  8   f  through the first notch portion  8   h.    
     The second stirring screw  11  conveys the developer  12  and the toner in the groove portion  8   g  in the Y2 direction while stirring the developer  12  and the toner. The toner is adsorbed on the carrier of the developer  12  by triboelectric charging during stirring. 
     The developer  12  is further conveyed in the Y2 direction while being stirred by the second stirring screw  11 . 
     In this manner, in the developer storage container  8   a , a flow of the developer  12  that circulates in the groove portions  8   f  and  8   g  is formed. 
     The developer  12  flowing in the groove portion  8   f  may not be stirred so vigorously as in the groove portion  8   g  because the developer  12  sufficiently stirred in the groove portion  8   g  flows therein. However, in the groove portion  8   f , the toner is consumed accompanying image formation. The developer  12  needs to be continuously stirred to such an extent that unevenness does not occur in the toner distribution in the developer  12 . 
     As shown in  FIG. 8 , in the groove portion  8   f , when the first stirring screw  10  rotates, the developer  12  is conveyed along a spiral groove between the first protrusive ridge  10   b  and the second protrusive ridge  10   c  adjacent to each other in the axial direction (Y direction) by the action of the first protrusive ridge  10   b  and the second protrusive ridge  10   c  (see white arrows in the drawing). The developer  12  as a whole is conveyed in the Y1 direction. 
     However, in this embodiment, a plurality of notches Nt are formed in the first protrusive ridge  10   b . In the first protrusive ridge  10   b  in which the notch Nt is formed, a conveying force in the turning direction of the first protrusive ridge  10   b  is decreased as compared with the first protrusive ridge  10   b  in which the notch Nt is not formed. However, a pressing force from the developer  12  on the upstream side in the conveying direction acts thereon, and therefore, part of the developer  12  in the vicinity of the notch Nt goes forward in the turning direction of the first stirring screw  10 , and the rest of the developer  12  passes through the notch Nt in the Y1 direction. 
     In this manner, slowing down of the speed of the developer  12 , branching of the flow of the developer  12 , and mixing in another flow path occur in the vicinity of the notch Nt. As a result, stirring of the developer  12  is promoted. 
     Accordingly, the notch Nt in the first stirring screw  10  promotes stirring by forming a linear speed difference in a tip portion in the protruding direction of the first protrusive ridge  10   b.    
     The conveying performance of the first protrusive ridge  10   b  in a region where the notch Nt is formed (hereinafter referred to as the “conveying performance in the notch Nt”) becomes higher as the height of the bottom face  10   d  or  10   g  is higher. On the other hand, the stirring performance of the first protrusive ridge  10   b  in a region where the notch Nt is formed (hereinafter referred to as the “stirring performance in the notch Nt”) becomes higher as the height of the bottom face  10   d  or  10   g  is lower. 
     The conveying performance in the notch Nt becomes higher as the length of the notch Nt in the extending direction of the first protrusive ridge  10   b  is shorter. On the other hand, the stirring performance in the notch Nt becomes higher as the length of the notch Nt in the extending direction of the first protrusive ridge  10   b  is longer. 
     On the other hand, the notch Nt deteriorates the flexural rigidity of the first stirring screw  10 . Therefore, the first stirring screw  10  easily bends accompanying the rotation of the first stirring screw  10 . When the first stirring screw  10  bends during rotation, the parallelism of the first stirring screw  10  with respect to the magnet roller  9  is reduced. Due to this, unevenness occurs in the adhesion amount of the developer  12  to the developing sleeve  9   a , and as a result, uneven development is likely to occur. 
     The notch Nt is preferably formed in consideration of the bending deformation of the first stirring screw  10 . The notch Nt is particularly preferably formed so that the bias in the circumferential direction of the first stirring screw  10  is decreased from the viewpoint that anisotropy in the flexural rigidity can be suppressed. 
     In this embodiment, the notches Nta and Ntb having a mutual phase difference of 180° are adjacent to each other in a length twice as long as the turning pitch in the axial direction. Adjacent thereto, the first protrusive ridge  10   b  in which the notch Nt is not formed extends in the length of the turning pitch. Further, in the first stirring screw  10 , such a repetitive pattern is repeated three times in the axial direction. 
     According to such a configuration, the bias in the circumferential direction is decreased in the plurality of notches Nt of the first stirring screw  10 . 
     Further, a region having high stirring performance and a region having high conveying performance alternate with each other in the axial direction, and therefore, the bias of the developer  12  due to stirring is dispersed in the axial direction. 
     In addition, in this embodiment, the notch Nt is not formed in the second protrusive ridge  10   c . Since the second protrusive ridge  10   c  having constant conveying performance is continuous in the axial direction, the uniformity of the conveying performance in the axial direction is improved. 
     As described above, according to the image forming apparatus  100  and the developing device  8  of this embodiment, since the first stirring screw  10  is included, the stirring performance for the developer  12  is improved. Accordingly, the developer  12  to be adhered to the magnet roller  9  is stably conveyed while being favorably stirred, and therefore, uneven development is reduced. 
     In particular, according to this embodiment, the first stirring screw  10  having favorable stirring performance and less bias in the flexural rigidity in the axial direction can be provided. Due to this, uneven development caused by bending deformation of the first stirring screw  10  can be suppressed. 
     According to at least one embodiment described above, the developing device and the image forming apparatus capable of improving the stirring performance for the developer can be provided. 
     Hereinafter, modifications of the above-mentioned embodiments will be described with reference to the drawings. 
     First Modification 
     A developing device of a first modification will be described. 
       FIG. 9  is a schematic perspective view showing an example of a first stirring screw of the first modification of the embodiment.  FIGS. 10A to 10D  are schematic cross-sectional views in  FIG. 9 .  FIG. 10A  is an E 1 -E 1  cross-sectional view in  FIG. 9 . Similarly,  FIG. 10B  is an E 2 -E 2  cross-sectional view,  FIG. 10C  is an E 3 -E 3  cross-sectional view, and  FIG. 10D  is an E 4 -E 4  cross-sectional view. 
     A developing device  8 A of the first modification shown in  FIGS. 1 and 2  includes a first stirring screw  10 A (see  FIG. 2 ) in place of the first stirring screw  10  of the developing device  8  of the embodiment. 
     The developing device  8 A can be used in the image forming apparatus  100  in place of the developing device  8 . 
     Hereinafter, different points from the embodiment will be mainly described. 
     As shown in  FIG. 9 , the first stirring screw  10 A is different from the first stirring screw  10  in the number of notches Nt and sites where the notches Nt are formed. 
     The notches Nt in the first stirring screw  10 A have a repetitive pattern different from the embodiment. According to this repetitive pattern, the first stirring screw  10 A is composed of a first portion U 1 , a second portion U 2 , and a third portion U 3 . 
     Hereinafter, arrangement of the notches Nt in the second portion U 2  will be described with reference to points p 0  to p 3  and points q 0  to q 3  shown in  FIG. 9 . Here, the points p 0  to p 3  are a four-point sequence having a phase of 0° of the first protrusive ridge  10   b  continuously arranged in the Y2 direction. The points q 0  to q 3  are a four-point sequence having a phase of 0° of the second protrusive ridge  10   c  continuously arranged in the Y2 direction. However, the point p 0  is a point adjacent to the point q 1  in the Y2 direction. 
     The second portion U 2  is composed of an area including the points p 0 , q 1 , p 1 , q 2 , p 2 , q 3 , and p 3 . 
     In the following Table 2, arrangement with respect to the second portion U 2  is shown. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 protrusive ridge 
                 area 
                 notch Nt 
                 phase ϕ (°) 
               
               
                   
               
             
            
               
                 first protrusive ridge 10b 
                 p0-p1 
                 without 
                 — 
               
               
                 second protrusive ridge 10c 
                 q0-q1 
                 Nt1 
                 315 
               
               
                 first protrusive ridge 10b 
                 p1-p2 
                 Nt2 
                  45 
               
               
                 second protrusive ridge 10c 
                 q1-q2 
                 Nt3 
                 135 
               
               
                 first protrusive ridge 10b 
                 p2-p3 
                 Nt4 
                 225 
               
               
                 second protrusive ridge 10c 
                 q2-q3 
                 without 
                 — 
               
               
                   
               
            
           
         
       
     
     As shown in Table 2, the notch Nt of the first modification is composed of four types: notches Nt 1 , Nt 2 , Nt 3 , and Nt 4 . However, in  FIG. 9 , the notch Nt 3  is not shown due to the projection direction. 
     The notches Nt 1 , Nt 2 , Nt 3 , and Nt 4  all have the same shape as the notch Nta of the embodiment. 
     As shown in Table 2, the notch Nt 1  is formed at a position where the phase is 315° in the area q 0 -q 1  of the second protrusive ridge  10   c .  FIG. 10A  is a cross-sectional view orthogonal to the central axis line O 10  including the notch Nt 1 . As shown in  FIG. 10A , the phase of the notch Nt 1  is a phase of a central axis line A 1  of the notch Nt 1  in the area q 0 -q 1 . 
     The notch Nt 2  is formed at a position where the phase is 45° in the area p 1 -p 2  of the first protrusive ridge  10   b .  FIG. 10B  is a cross-sectional view orthogonal to the central axis line O 10  including the notch Nt 2 . As shown in  FIG. 10B , the phase of the notch Nt 2  is a phase of a central axis line A 2  of the notch Nt 2  in the area p 1 -p 2 . 
     The notch Nt 3  is formed at a position where the phase is 135° in the area q 1 -q 2  of the second protrusive ridge  10   c .  FIG. 10C  is a cross-sectional view orthogonal to the central axis line O 10  including the notch Nt 3 . As shown in  FIG. 10C , the phase of the notch Nt 3  is a phase of a central axis line A 3  of the notch Nt 3  in the area q 1 -q 2 . 
     The notch Nt 4  is formed at a position where the phase is 225° in the area p 2 -p 3  of the first protrusive ridge  10   b .  FIG. 10D  is a cross-sectional view orthogonal to the central axis line O 10  including the notch Nt 4 . As shown in  FIG. 10D , the phase of the notch Nt 4  is a phase of a central axis line A 4  of the notch Nt 4  in the area p 2 -p 3 . 
     In order to form the above-mentioned four types of notches Nt in the second portion U 2 , the notch Nt is not formed in each of the area p 0 -p 1  and the area q 2 -q 3 . 
     The first portion U 1  has the same configuration as the second portion U 2  except that the notch Nt 1  is not formed due to the start position of the second protrusive ridge  10   c.    
     The third portion U 3  has the same configuration as the second portion U 2 . 
     In this manner, in the first stirring screw  10 A, the arrangement pattern of the notches Nt in the second portion U 2  is repeated 2 and 3/4 times in the axial direction. 
     In the first stirring screw  10 A, the mutual phase difference between the notches Nt adjacent to each other in the axial direction is 90°. 
     The arrangement of the notches Nt in the first protrusive ridge  10   b  of this modification is the same as the arrangement in the first protrusive ridge  10   b  of the embodiment. For example, the notches Nta and Ntb of the embodiment correspond to the notches Nt 2  and Nt 4  of this modification, respectively. 
     The arrangement of the notches Nt in the second protrusive ridge  10   c  of this modification is the same as the arrangement in the first protrusive ridge  10   b  except that the phase is different by 90°. 
     According to the developing device  8 A, the first stirring screw  10 A includes the plurality of notches Nt whose phases are shifted by 90° in the axial direction, and therefore, in the same manner as the developing device  8  of the embodiment, the stirring performance for the developer  12  is improved. According to this, uneven development is reduced. 
     In particular, in this modification, the number of notches Nt per unit length is increased as compared with the embodiment, and therefore, the stirring performance is further improved. 
     In this modification, an example in which the depth and the length of each notch Nt are the same as those of the notch Nt of the embodiment is described. However, the depth and the length of each notch Nt may be different from those of the notch Nt of the embodiment. For example, by shortening the length of the notch Nt or reducing the depth of the notch Nt, the flexural rigidity of the first stirring screw  10 A can be made equal to that of the first stirring screw  10 . 
     Second Modification 
     A developing device of a second modification will be described. 
       FIG. 11  is a schematic cross-sectional view showing an example of a first stirring screw of the second modification of the embodiment. 
     A developing device  8 B of the second modification shown in  FIGS. 1 and 2  includes a first stirring screw  10 B (see  FIG. 2 ) in place of the first stirring screw  10  of the developing device  8  of the embodiment. 
     As shown in  FIG. 11 , in the first stirring screw  10 B, a plurality of notches Ntc are formed in the first protrusive ridge  10   b  of the first stirring screw  10  of the embodiment as the notches Nt. In  FIG. 11 , as shown by the central axis line Ac, the notch Ntc formed at a position where the phase is 315° is shown. 
     The developing device  8 B can be used in the image forming apparatus  100  in place of the developing device  8 . 
     Hereinafter, different points from the embodiment will be mainly described. 
     The notch Ntc includes a bottom face  10   j  in place of the bottom face  10   d  of the notch Nta of the embodiment. The bottom face  10   j  is a curved face formed by intersecting a cylindrical face having a radius rc (provided that r&lt;rc&lt;Rb) with the central axis line O 10  as the center with the first protrusive ridge  10   b.    
     Therefore, the stirring performance of the notch Ntc is decreased as compared with that of the notch Nta. 
     In this modification, in order to compensate for the decrease in the stirring performance, the notch Ntc is formed in each n-th portion Sn. In the following Table 3, the arrangement of the notches Ntc is shown. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 n 
                 area 
                 notch Nt 
                 phase ϕ (°) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 1 
                 P0-Q1-P1 
                 Ntc 
                 315 
               
               
                 2 
                 P1-Q2-P2 
                 Ntc 
                 135 
               
               
                 3 
                 P2-Q3-P3 
                 Ntc 
                 315 
               
               
                 4 
                 P3-Q4-P4 
                 Ntc 
                 135 
               
               
                 5 
                 P4-Q5-P5 
                 Ntc 
                 315 
               
               
                 6 
                 P5-Q6-P6 
                 Ntc 
                 135 
               
               
                 7 
                 P6-Q7-P7 
                 Ntc 
                 315 
               
               
                 8 
                 P7-Q8-P8 
                 Ntc 
                 135 
               
               
                 9 
                 P8-Q9-P9 
                 Ntc 
                 315 
               
               
                 10 
                 P9-Q10-P10 
                 Ntc 
                 135 
               
               
                   
               
            
           
         
       
     
     As shown in Table 3, in this modification, the notch Nt is composed of the notch Ntc. The notch Ntc is formed at a position where the phase is 315° in an n-th portion Sn wherein n is an even number. The notch Ntc is formed at a position where the phase is 135° in an n-th portion Sn wherein n is an odd number. 
     Therefore, each notch Ntc is continuously disposed in the axial direction for each turning pitch. The phase differences between the notches Ntc adjacent to each other are all 180°. 
     According to such an arrangement of the notches Ntc, the uniformity of the stirring performance is improved throughout the axial direction of the first stirring screw  10 B. In this modification, an n-th portion Sn where the notch Ntc is not formed is not present, and therefore, the flexural rigidity may be deteriorated as compared with the embodiment. Due to this, in this modification, by reducing the depth of each notch Ntc, the flexural rigidity is improved. Accordingly, even if the stirring performance is improved, the same flexural rigidity as in the embodiment can be ensured. 
     According to the developing device  8 B, the first stirring screw  10 B includes the notches Ntc, and therefore, in the same manner as the developing device  8  of the embodiment, the stirring performance for the developer  12  is improved. Accordingly, uneven development is reduced. 
     This modification is an example of a case where the notch Nt may be provided in each n-th portion Sn. 
     Third Modification 
     A developing device of a third modification will be described. 
       FIG. 12  is a schematic cross-sectional view showing an example of a first stirring screw of the third modification of the embodiment. 
     A developing device  8 C of the third modification shown in  FIGS. 1 and 2  includes a first stirring screw  10 C (see  FIG. 2 ) in place of the first stirring screw  10  of the developing device  8  of the embodiment. 
     As shown in  FIG. 12 , in the first stirring screw  10 C, in place of the notches Nta and Ntb of the first stirring screw  10  of the embodiment, notches Ntd are formed at positions of the same phases as the respective notches Nta and Ntb. In  FIG. 12 , as shown by the central axis line Ad, the notch Ntd formed at a position where the phase is 315° is shown. 
     The developing device  8 C can be used in the image forming apparatus  100  in place of the developing device  8 . 
     Hereinafter, different points from the embodiment will be mainly described. 
     The notch Ntd is constituted by an inclined plane  10   k . The inclined plane  10   k  has a shape in which the first protrusive ridge  10   b  is cut off with a flat plane that is a flat plane parallel to the central axis line O 10  and is orthogonal to the diameter of the first stirring screw  10 C within a phase range of 90°. 
     Therefore, the depth of the notch Ntd gradually increases from both end portions in the extending direction of the first protrusive ridge  10   b  to an intersection with the central axis line Ac. 
     According to the developing device  8 C, the first stirring screw  10 C includes the notches Ntd, and therefore, in the same manner as the developing device  8  of the embodiment, the stirring performance for the developer  12  is improved. Accordingly, uneven development is reduced. 
     This modification is an example of a case where the depth of the notch Nt may gradually changes inside the notch Nt. 
     Fourth Modification 
     A developing device of a fourth modification will be described. 
       FIG. 13  is a schematic cross-sectional view showing an example of a first stirring screw of the fourth modification of the embodiment. 
     A developing device  8 D of the fourth modification shown in  FIGS. 1 and 2  includes a first stirring screw  10 D (see  FIG. 2 ) in place of the first stirring screw  10  of the developing device  8  of the embodiment. 
     As shown in  FIG. 13 , in the first stirring screw  10 D, in place of the notches Nta and Ntb of the first stirring screw  10  of the embodiment, notches Nte are formed at positions of the same phases as the respective notches Nta and Ntb. In  FIG. 13 , as shown by the central axis line Ae, the notch Nte formed at a position where the phase is 315° is shown. 
     The developing device  8 D can be used in the image forming apparatus  100  in place of the developing device  8 . 
     Hereinafter, different points from the embodiment will be mainly described. 
     The notch Nte is constituted by a curved face  10   m . The curved face  10   m  is a recessed portion in which the first protrusive ridge  10   b  is cut off with a cylindrical plane protruding to the inside in the radial direction within a phase range of 90°. 
     Therefore, the depth of the notch Nte gradually increases from both end portions in the extending direction of the first protrusive ridge  10   b  to an intersection with the central axis line Ae. 
     According to the developing device  8 D, the first stirring screw  10 D includes the notches Nte, and therefore, in the same manner as the developing device  8  of the embodiment, the stirring performance for the developer  12  is improved. Accordingly, uneven development is reduced. 
     This modification is an example of a case where the depth of the notch Nt may gradually changes inside the notch Nt. 
     Fifth Modification 
     A developing device of a fifth modification will be described. 
       FIG. 14  is a schematic cross-sectional view showing an example of a first stirring screw of the fifth modification of the embodiment. 
     A developing device  8 E of the fifth modification shown in  FIGS. 1 and 2  includes a first stirring screw  10 E (see  FIG. 2 ) in place of the first stirring screw  10  of the developing device  8  of the embodiment. 
     As shown in  FIG. 14 , in the first stirring screw  10 E, in place of the notches Nta and Ntb of the first stirring screw  10  of the embodiment, notches Ntf are formed at positions of the same phases as the respective notches Nta and Ntb. The phase of the arrangement position of the notch Ntf is defined by the position of the central axis line Af of the notch Ntf in the same manner as in the embodiment. 
     The developing device  8 E can be used in the image forming apparatus  100  in place of the developing device  8 . 
     Hereinafter, different points from the embodiment will be mainly described. 
     The notch Ntf is different from the notch Nta in the opening length thereof. For example, in the example shown in  FIG. 14 , the opening length of the notch Ntf is longer than the opening length of the notch Nta. For example, the length of the notch Ntf may be a length corresponding to a phase of 120°. However, the opening length of the notch Ntf may be shorter than the opening length of the notch Nta. 
     The notch Ntf has different stirring performance from the notch Nta depending on the difference in the opening length. 
     According to the developing device  8 E, the first stirring screw  10 E includes the notches Ntf, and therefore, in the same manner as the developing device  8  of the embodiment, the stirring performance for the developer  12  is improved. Accordingly, uneven development is reduced. 
     Hereinabove, modifications of various notches Nt are described in the first to fifth modifications. The notches Nt in the respective modifications can be used by being appropriately combined with the above-mentioned embodiments and modifications. 
     Further, hereinabove, an example of a case where one notch Nt is formed in the unit screw is described. However, one or more notches Nt may be formed in the unit screw. 
     Hereinabove, an example of a case where the phase difference between the notches Nt is 90° or 180° is described. However, the phase difference between the notches Nt is not limited to 90° or 180°, and may be, for example, 60°, 120°, or the like. 
     Other than in the operating examples, if any, or where otherwise indicated, all numbers, values and/or expressions referring to parameters, measurements, degrees, etc., used in the specification and claims are to be understood as modified in all instances by the term “about.” 
     While certain embodiments have been described these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms: furthermore various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and there equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.