Patent Publication Number: US-9851658-B2

Title: Developing device and image forming apparatus

Description:
This application is based on Japanese Patent Application No. 2014-234920 filed on Nov. 19, 2014, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a developing device employing so-called trickle development technology, and an image forming apparatus including the same. 
     2. Description of Related Art 
     Some electrophotographic image forming apparatuses include a trickle developing device which supplies a photoreceptor drum with a two-component developer, which contains carrier and toner, thereby developing an electrostatic latent image formed on a circumferential surface of the photoreceptor drum. In the trickle developing device, a developer container constantly discharges a small and fixed amount of developer, and is replenished with a developer, which includes toner mixed with a small amount of carrier, in accordance with the amount of toner remaining in the developer container. 
     An example of the trickle developing device is described in Japanese Patent Laid-Open Publication No. 2000-66500. In this developing device, the interior space of the developer container is divided by a partition into first and second developer compartments in which the developer is circulated. The first developer compartment and the second developer compartment respectively include a first conveyance member and a second conveyance member, which are disposed horizontally and have rotating blades in order to convey the developer. Moreover, the first conveyance member is positioned along a developing roller so as to face the developing roller. The second developer compartment has a developer discharge port provided in its top portion. In the second developer compartment, the force of the second conveyance member conveying the developer by the rotating blade is low in some sections positioned on the downstream side with respect to the discharge port. Accordingly, the developer accumulates in such sections, and the upper level of the developer becomes higher than in the other sections. 
     In Japanese Patent Laid-Open Publication No. 2000-66500, to reduce the conveyance force in the aforementioned sections, the rotating blade of the second conveyance member gradually decrease in diameter toward the downstream side. Accordingly, for example, in the case where the image forming apparatus is installed in an unlevel place, so that the developing device is slanted, some developer accumulating in such sections might flow out of the space between the second conveyance member and the developer container. In such a case, the amount of developer conveyed in the first developer compartment and the second developer compartment varies over time, so that the amount of developer ejected from the discharge port becomes more likely to vary, resulting in reduced image quality. 
     Furthermore, recent image forming apparatuses might be required to print out more pages per unit time (i.e., increase print speed), or perform printing at various speeds. However, if the speed of developer conveyance is increased or rendered variable in order to meet such requirements, the amount of developer ejected from the discharge port still becomes more likely to vary. 
     SUMMARY OF THE INVENTION 
     A developing device according to an embodiment of the present invention includes a developer carrying member, a first conveyance member, and a second conveyance member. The first conveyance member conveys a developer in a first direction through a first conveyance path and supplies the developer to the developer carrying member. The second conveyance member conveys the developer in a second direction through a second conveyance path included in a circulation path along with the first conveyance path. The second conveyance member has a helical second screw around a second shaft. The second conveyance path includes a developer replenishing port. The second conveyance path includes a developer discharge port provided in a top portion. The second screw has an approximately constant diameter in a second section being a portion extending a predetermined distance in the second direction from an end of the discharge port in the second direction when viewed in a top view. The second screw is adapted to be capable of conveying the developer at a lower speed in the second section than on an upstream side with respect to the second section. 
     In another embodiment of the present invention, an image forming apparatus includes a developing device of the above embodiment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view illustrating a general configuration of an image forming apparatus; 
         FIG. 2A  is a first schematic view illustrating a cross section of a developing device (without a developer) taken along zx plane in  FIG. 1 ; 
         FIG. 2B  is a second schematic view illustrating the cross section of the developing device (with a developer) taken along zx plane in  FIG. 1 ; 
         FIG. 3  is a schematic view illustrating a cross section of the developing device taken along xy plane in  FIG. 1 ; 
         FIG. 4  is a schematic view illustrating a cross section of the developing device taken along yz plane in  FIG. 1 ; 
         FIG. 5  is a schematic view illustrating the relationship between the cress-sectional area of a second screw and the cress-sectional area of a second conveyance path in a second section; 
         FIG. 6  is a schematic view illustrating the state of the developer, as shown in, for example,  FIG. 2B , in the second conveyance path; 
         FIG. 7  is a schematic view illustrating the configuration of a developing device according to a first modification; and 
         FIG. 8  is a schematic view illustrating the configuration of a developing device according to a second modification. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, a toner conveyance member according to an embodiment of the present invention, along with a fuser and an image forming apparatus, will be described in detail with reference to the drawings. 
     Section 1: Definitions 
     In some figures, the x-, y-, and z-axes are perpendicular to one another, and represent the right-left, front-back, and up-down direction of the image forming apparatus  1 . Moreover, some reference characters herein and also in the drawings are suffixed with the lowercase alphabet letter “a”, “b”, “c”, or “d”. The letters “a”, “b”, “c”, and “d” respectively denote yellow (Y), magenta (M), cyan (C), and black (K). For example, the term “photoreceptor drum  5   a ” refers to a photoreceptor drum for yellow. Moreover, any reference character which can be suffixed with such a letter but has no letter added thereto represents one or any of the colors. For example, the term “photoreceptor drum  5 ” refers to a photoreceptor drum for one or any of the colors Y, M, C, and K. 
     Section 2: Overall Configuration and Print Operation of Image Forming Apparatus 
     In  FIG. 1 , the image forming apparatus  1  is, for example, a copier, printer, or fax machine, or a multifunction machine provided with all or some of the functions, and is adapted to print a variety of types of images (e.g., full-color images) on sheets of paper using a tandem system with a well-known electrophotography technology. To this end, the image forming apparatus  1  typically includes imaging units  2 , an intermediate transfer belt  3 , and a secondary transfer roller  4 . 
     For example, the imaging units  2  for the aforementioned colors are arranged side by side so as to be approximately parallel to the z-axis, and include respective photoreceptor drums  5  for their corresponding colors. 
     Each photoreceptor drum  5  is in the shape of a column extending in the y-axis direction, and rotates, for example, in the direction of arrow α (referred to below as the “rotational direction α”). Arranged around the photoreceptor drum  5 , from upstream to downstream in the rotational direction α, are, at least, a charger  6 , a developing device  8 , and a primary transfer roller  9 . 
     The charger  6  uniformly charges the circumferential surface of the photoreceptor drum  5  while the photoreceptor drum  5  is rotating. 
     Provided below the photoreceptor drum  5  is an exposing device  7 . The exposing device  7  irradiates an exposure area of the photoreceptor drum  5 , which is immediately downstream from the charged area, with an optical beam B based on image data, thereby forming an electrostatic latent image in a corresponding color. 
     The developing device  8  is a so-called trickle developing device, and supplies a two-component developer for the corresponding color to a developing area R 3  (see, for example,  FIG. 2A ) of the photoreceptor drum  5 , which is immediately downstream from the exposure area, thereby forming a toner image in the corresponding color in the developing area R 3 . The two-component developer herein contains carrier and toner. In the present embodiment, the two-component developer will be referred to below simply as the developer. 
     The intermediate transfer belt  3  is a so-called endless belt, which is stretched between outer circumferential surfaces of at least two rollers arranged in the up-down direction and rotates, for example, in the direction of arrow β. The outer circumferential surface of the intermediate transfer belt  3  abuts the left end of each photoreceptor drum  5 . 
     The primary transfer roller  9  is positioned opposite to the photoreceptor drum  5  with the intermediate transfer belt  3  positioned therebetween, and presses the inner circumferential surface of the intermediate transfer belt  3  from left, thereby creating a primary transfer area R 1  between the photoreceptor drum  5  and the intermediate transfer belt  3 . The toner image formed on the photoreceptor drum  5  is transferred to the primary transfer area R 1  while the intermediate transfer belt  3  is rotating. 
     The secondary transfer roller  4  is positioned near the upper end of the intermediate transfer belt  3  so as to press the outer circumferential surface of the intermediate transfer belt  3 , thereby creating a secondary transfer area R 2  at the contact between the secondary transfer roller  4  and the intermediate transfer belt  3 . In the secondary transfer area R 2 , the image carried on the intermediate transfer belt  3  is transferred to a sheet of paper. The sheet of paper passes through a well-known fuser, and thereafter is ejected into a tray as a print. 
     Furthermore, the image forming apparatus  1  has cartridges (not shown) provided therein, and each cartridge contains a developer for its corresponding color. The cartridge replenishes its corresponding developing device  8  with the developer when the amount of toner remaining in the developing device  8  decreases to a reference value or lower. Moreover, the cartridge is detachable from the image forming apparatus  1 , and therefore, when the cartridge is completely (or nearly) emptied, the cartridge is replaceable with a new cartridge. 
     Section 3: Details of Configuration and Operation of Developing Device 
     Next, the configuration and the operation of the developing device  8  will be described in detail with reference to  FIGS. 2A to 5 . Note that for the sake of clarity,  FIG. 2A  shows no developer, and  FIG. 2B  shows the developer with dots. 
     The developing device  8  includes a developer container  81  and a partition  82 . The developer container  81  has a developer for its corresponding color stored therein. The developer container  81  extends in the y-axis direction along the photoreceptor drum  5  for the corresponding color, and has an opening at least in a position where the developer container  81  faces the developing area R 3 . 
     The partition  82  protrudes upward (in the z-axis direction) from the bottom of the developer container  81 , and extends in the y-axis direction. Accordingly, the developer container  81  is divided into a first conveyance path FP 1  and a second conveyance path FP 2 ; the first conveyance path FP 1  is closer to the photoreceptor drum  5  than is the second conveyance path FP 2 . Moreover, the conveyance paths FP 1  and FP 2  communicate with each other in two places whose positions in the y-axis direction are different (e.g., at both ends). This results in a developer circulation path within the developer container  81 . 
     The developing device  8  further includes a replenishing port  83 . The replenishing port  83  is an opening provided completely through a top portion  84  of the developer container  81  above the second conveyance path FP 2 . The replenishing port  83  herein is positioned approximately at the center of the top portion  84  in the y-axis direction. When the density of the toner in the developer container  81  becomes low, the cartridge for the corresponding color supplies a supplementary developer. The developer is introduced into the developer container  81  through the replenishing port  83 . Note that a well-known density sensor utilizing magnetic permeability detection is used to detect the toner density. 
     The developing device  8  further includes a cylindrical buffer portion  85  protruding upward from the top portion  84 . The buffer portion  85  is preferably provided on the negative side in the y-axis direction with respect to the replenishing port  83  (i.e., on the upstream side in the developer conveyance direction). More specifically, the buffer portion  85  includes a discharge port  86  provided at the top (i.e., the discharge port  86  is positioned higher than the top of the second conveyance path FP 2 ), and an opening  87  provided at the bottom. When viewed in a plan view from the positive side in the z-axis direction (referred to below as “in a top view”), it is preferable that the discharge port  86  and the opening  87  completely overlap with each other. Moreover, the discharge port  86  and the opening  87  are connected at their outer edges by at least one side surface  88 . Accordingly, the discharge port  86  communicates with the second conveyance path FP 2 . The space enclosed by the side surface  88  holds a portion of the developer in the second conveyance path FP 2 , and a portion of the developer held in the space enclosed by the side surface  88  overflows from the discharge port  86  as an excess developer. The excess developer flows out of the buffer portion  85  along a slanted surface of the discharge port  86  and is forced out of the developing device  8 . 
     The developing device  8  also includes a developing roller  89 , which is a typical example of a developer carrying member. The developing roller  89  has receiving poles and developing poles arranged in fixed positions within a rotatable sleeve. Moreover, the developing roller  89  is disposed so as to be approximately parallel to the photoreceptor drum  5  for the corresponding color, and proximal to the developing area R 3  for the corresponding color, and further, the developing roller  89  spans from one side to the other within the developer container  81  and is rotatable in the direction of arrow γ. The developing roller  89  receives the developer from a first conveyance member  810 , which will be described later, at the receiving pole, and carries the developer on its outer circumferential surface. Thereafter, the developing roller  89  rotates to convey the developer on the outer circumferential surface to the developing pole, and then supplies the developer to the developing area R 3  of the photoreceptor drum  5 . 
     The developing device  8  further includes the first conveyance member  810 . The first conveyance member  810  is disposed to the right of the developing roller  89  in the first conveyance path FP 1  so as to be approximately parallel to and proximal to the developing roller  89 , and further, the first conveyance member  810  spans from one side to the other in the first conveyance path FP 1  and is supported about an axis so as to be rotatable in the direction of arrow θ 1 . Through rotation, the first conveyance member  810  supplies the developer to the receiving pole of the developing roller  89 , and also conveys the developer in the first conveyance path FP 1  in a first direction (i.e., in the opposite direction to the y-axis direction) while stirring the developer. Once the developer is conveyed to the end of the first conveyance path FP 1  in the first direction (i.e., the starting end in the y-axis direction), the developer flows into the second conveyance path FP 2  through the communicating portion. Note that the directions in which the developer is carried in the conveyance paths FP 1  and FP 2  are indicated by hatched arrows in the figures. 
     Described next is a configuration example of the first conveyance member  810 . The first conveyance member  810  is made from, for example, a resin by injection forming or suchlike. The first conveyance member  810  at least includes a first shaft  811  and a first screw  812 . The first shaft  811  is a rotational shaft extending in the first direction, and rotates clockwise, as indicated by arrow θ 1 , when viewed in a plan view from the negative side in the y-axis direction. The first screw  812  is a bladed member winding around the outer circumferential surface of the first shaft  811  in the clockwise direction θ 1  (also referred to simply as the “direction θ 1 ”), and the bladed member continues to extend helically in the first direction, from the positive end to the negative end of the first shaft  811  in the y-axis direction. 
     The developing device  8  further includes a second conveyance member  813 . The second conveyance member  813  is disposed in the second conveyance path FP 2  so as to be approximately parallel to the first conveyance member  810  with the partition  82  positioned therebetween, and further, the second conveyance member  813  spans from one side to the other in the second conveyance path FP 2  and is supported about an axis so as to be rotatable in the direction of arrow θ 2 . Through rotation, the second conveyance member  813  conveys the developer in the second conveyance path FP 2  in a second direction while stirring the developer, so that the developer flows from the second conveyance path FP 2  into the first conveyance path FP 1  through the communicating portion. Note that the second direction herein is the opposite direction to the first direction. 
     The second conveyance member  813  has a similar configuration to the first conveyance member  810 . Specifically, the second conveyance member  813  includes at least a second shaft  814  and a second screw  815 . The second shaft  814  is a rotational shaft extending in the second direction and having approximately the same thickness across the entire length in the y-axis direction. Moreover, the second shaft  814  rotates counterclockwise, as indicated by arrow θ 2 , when viewed in a plan view from the negative side in the y-axis direction. The second screw  815  is a bladed member winding around the outer circumferential surface of the second shaft  814  in the counterclockwise direction θ 2  (also referred to simply as the “direction θ 2 ”), and the bladed member continues to extend helically in the second direction, from the negative end to the positive end of the second shaft  814  in the y-axis direction. 
     The second conveyance member  813  has a second section I 2  as defined in the following. The second section I 2 , when viewed in a top view, includes a portion I 21  and a portion I 22 ; the portion I 21  extends from an arbitrary position on the second screw  815  which falls within the boundary of the discharge port  86  to the end of the discharge port  86  in the second direction, and the portion I 22  extends a predetermined distance in the second direction from the end of the discharge port  86  in the second direction. Assuming here that the portion I 22  and the discharge port  86  respectively have lengths L 22  and L 86  in the second direction, the length L 22  is designed to be about a half of the length L 86 . Moreover, the second conveyance path FP 2  is designed to have approximately the same shape (preferably, circular) at least in the second section I 2  if the second conveyance path FP 2  is cut approximately parallel to the zx plane at any position in the y-axis direction. 
     Furthermore, the rest of the second screw  815 , excluding the second section I 2 , is defined herein as a first section I 1 . 
     Assuming here that the screw pitch of the first section I 1  and the screw pitch of the second section I 2  are a first pitch P 1  and a second pitch P 2 , respectively, the pitch P 1  is designed to be wider than the pitch P 2 . Also, assuming that the developer conveyance velocity is a first velocity v 1  in the first section I 1  and also a second velocity v 2  in the second section I 2 , the pitches P 1  and P 2 , and other design values, are determined such that the velocity v 2  is lower than the velocity v 1 . More preferably, the value of v 2 /v 1 , which is the ratio of the second velocity v 2  to the first velocity v 1 , falls within the range from 0.3 to 0.5. 
     Furthermore, herein, the diameter of the second screw  815  is approximately constant at least in the second section I 2 . Accordingly, the distance d between the top portion  84  and each blade edge of the second screw  815  in the second section I 2  is approximately constant. Here, the distance d is preferably designed to be as short as possible. More specifically, when viewed in a plan view in the second direction (i.e., the y-axis direction), as shown in  FIG. 5 , the second conveyance path FP 2  has an area S 1  in the second section I 2 , and the second screw  815  has an area S 2  within its outer edge in the second section I 2 , the value of (S 1 −S 2 )/S 2 , which is the ratio of the area (S 1 −S 2 ) to the area S 2 , is preferably designed to be less than 1. However, the second screw  815  and the second conveyance path FP 2  are only required to be designed such that the ratio (S 1 −S 2 )/S 2  falls within the range from 0.2 to 0.4. 
     Section 4: Actions and Effects of Developing Device 
     In the developing device  8  configured as above, the conveyance members  810  and  813  respectively rotate in the directions θ 1  and θ 2  during a print operation, so that the developer is circulated through a circulation path including the conveyance paths FP 1  and FP 2 . During the course of the circulation, the developer flows into the second conveyance path FP 2  and is conveyed at the first velocity v 1 , which is relatively high, to the second section I 2  of the second conveyance member  813 . In the second section I 2 , the developer is conveyed at the second velocity v 2 , which is low compared to the conveyance velocity on the upstream side with respect to the second section I 2 , and therefore, the surface of the developer is swollen and held in part within the buffer portion  85 , as shown in  FIG. 6 . Even during this period, the developer continues to flow into the second section I 2 , so that the developer in the buffer portion  85  overflows in part from the discharge port  86 , and is forced out of the developing device  8 . Note that when the amount of developer in the developer container  81  becomes low during the print operation, a supplementary developer is supplied, as described earlier. 
     Furthermore, the second section I 2  includes the portion I 22 , which, when viewed in a top view, extends a distance equivalent to the length L 22  in the second direction from the end of the discharge port  86  in the second direction. In the portion I 22  also, the screw pitch is the second pitch P 2 , which is relatively narrow. Accordingly, in the portion I 22  also, the developer is conveyed at the second velocity v 2 , which is low. More specifically, the ratio v 2 /v 1  is designed to fall within the range from 0.3 to 0.5, and the length L 22  is designed to be about a half of the length L 86 , as described above. The distance d between the blade edge of the second screw  815  and the top portion  84  is designed as short as possible. With the above configuration, it is rendered possible to hold the developer within the buffer portion  85  while filling the portion I 22  of the second conveyance path FP 2  with the developer, and therefore, even if the developing device  8  is slanted, or the developer conveyance velocity is high and/or variable, the developer held within the buffer portion  85  can be prevented from undesirably flowing into the portion I 22 . Accordingly, the amount of developer conveyed in the second conveyance path FP 2  on the downstream side with respect to the second section I 2  can always be approximately uniform, whereby the amount of developer conveyed in the circulation path can be inhibited from varying. Thus, naturally, the amount of developer ejected from the discharge port  86  can be inhibited from varying, whereby it is rendered possible to suppress image quality reduction. 
     Section 5: Supplementary  1   
     In the above preferred example, the second section I 2  includes the portions I 21  and I 22 . However, this is not limiting, and the second section I 2  may simply include the portion I 22 , which extends a predetermined distance in the second direction from the end of the discharge port  86  in the second direction. 
     Furthermore, in the above preferred example of holding the developer, the buffer portion  85  protrudes from the top portion  84 , such that the discharge port  86  is positioned higher than the top portion  84 . However, this is not limiting, and the discharge port  86  of the buffer portion  85  may be an opening provided completely through the top portion  84 . That is, the distance between the discharge port  86  and the opening  87  may be essentially equal to the thickness of the top portion  84 . 
     Section 6: First Modification 
     In the foregoing, the screw pitch of the first section I 1  and the screw pitch of the second section I 2  are respectively set to be the first pitch P 1  and the second pitch P 2 , so that the developer conveyance velocity varies between the first section I 1  and the second section I 2 . However, this is not limiting, and the developer conveyance velocity may be adjusted by setting both the diameter and the pitch of the second screw  815  to be equal between the first section I 1  and the second section I 2  while making the second shaft  814  thicker in the second section I 2  than in the first section I 1 , as shown in  FIG. 7 . 
     Section 7: Second Modification 
     The developer conveyance velocity can also be adjusted as shown in  FIG. 8 . Specifically, the screw pitch of the first section I 1  and the screw pitch of the second section I 2  are respectively designed to be the first pitch P 1  and the second pitch P 2 , as described above. The thickness of the second shaft  814  is designed to be equal between the first section I 1  and the second section I 2 . However, both the diameter of the second screw  815  and the diameter of the second conveyance path FP 2  may be designed to be smaller in the second section I 2  than in the first section I 1 . Note that it is preferable that the configuration shown in  FIG. 8  also satisfies the relationship between the areas S 1  and S 2  shown in  FIG. 5 . 
     Although the present invention has been described in connection with the preferred embodiment above, it is to be noted that various changes and modifications are possible to those who are skilled in the art. Such changes and modifications are to be understood as being within the scope of the invention.