Patent Publication Number: US-8995851-B2

Title: Developing device having agitation conveyance member with scraper for wiping toner sensor and image forming apparatus having the developing device

Description:
INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2013-049879 filed on Mar. 13, 2013, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to a developing device and an image forming apparatus including the developing device. More particularly, the disclosure relates to: a developing device including a toner detection sensor that detects a toner concentration or a remaining amount of toner in a developer container, and a scraper that cleans a detection surface of the toner detection sensor; and an image forming apparatus including the developing device. 
     In an image forming apparatus, an electrostatic latent image formed on an image carrier composed of a photosensitive member and the like is developed by a developing device so as to be visible as a toner image. As an example of such a developing device, there is a developing device that adopts a two-component developing method using a two-component developer. This type of developing device includes a developer container that contains a two-component developer composed of carrier and toner, a developing roller that supplies the developer to the image carrier, and an agitation conveyance member that conveys the developer in the developer container while agitating the developer, and supplies the developer to the developing roller. 
     In the developing device, the toner is consumed by the developing operation. Therefore, in order to replenish the toner by an amount consumed by the development, it is necessary to measure the toner concentration in the developer by a toner concentration detection sensor (toner detection sensor) provided in the developer container. 
     In order to accurately measure the toner concentration, it is necessary to suppress accumulation of the developer on a detection surface of the toner concentration detection sensor. Therefore, the agitation conveyance member is provided with a scraper for cleaning the detection surface of the toner concentration detection sensor. When the agitation conveyance member rotates, the scraper slides on the detection surface of the toner concentration detection sensor to clean the detection surface. When a nonwoven fabric is used as the scraper, the detection surface of the toner concentration detection sensor can be effectively cleaned. 
     It is noted that a developing device using a nonwoven fabric as a scraper for cleaning a detection surface of a toner concentration detection sensor has been known. 
     SUMMARY 
     A developing device according to an aspect of the present disclosure includes a developer container, an agitation conveyance member, a toner detection sensor, and a scraper. The developer container contains a developer including toner. The agitation conveyance member agitates and conveys the developer in the developer container. The toner detection sensor detects a toner concentration or a remaining amount of toner in the developer container. The scraper is provided in the agitation conveyance member, and cleans a detection surface of the toner detection sensor when the agitation conveyance member rotates. The agitation conveyance member is configured to be rotatable forward and reversely. The scraper includes a first member that comes into contact with the detection surface of the toner detection sensor when the agitation conveyance member rotates forward, and a second member that comes into contact with the detection surface of the toner detection sensor when the agitation conveyance member rotates reversely. The first member has a wear resistance higher than that of the second member, and a coefficient of friction between the second member and the detection surface is higher than a coefficient of friction between the first member and the detection surface. 
     An image forming apparatus according to another aspect of the present disclosure includes a developing device. The developing device includes a developer container, an agitation conveyance member, a toner detection sensor, and a scraper. The developer container contains a developer including toner. The agitation conveyance member agitates and conveys the developer in the developer container. The toner detection sensor detects a toner concentration or a remaining amount of toner in the developer container. The scraper is provided in the agitation conveyance member, and cleans a detection surface of the toner detection sensor when the agitation conveyance member rotates. The agitation conveyance member is configured to be rotatable forward and reversely. The scraper includes a first member that comes into contact with the detection surface of the toner detection sensor when the agitation conveyance member rotates forward, and a second member that comes into contact with the detection surface of the toner detection sensor when the agitation conveyance member rotates reversely. The first member has a wear resistance higher than that of the second member, and a coefficient of friction between the second member and the detection surface is higher than a coefficient of friction between the first member and the detection surface. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description with reference where appropriate to the accompanying drawings. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing an entire configuration of an image forming apparatus including a developing device according to an embodiment of the present disclosure. 
         FIG. 2  is a sectional side view showing a structure of the developing device according to the embodiment of the present disclosure. 
         FIG. 3  is a sectional plan view showing a structure of an agitation portion of the developing device according to the embodiment of the present disclosure. 
         FIG. 4  is a perspective view showing a structure of a first helical member of the developing device according to the embodiment of the present disclosure. 
         FIG. 5  is a perspective view showing the structure of the first helical member of the developing device according to the embodiment of the present disclosure. 
         FIG. 6  is a sectional side view showing a structure in the vicinity of the first helical member of the developing device according to the embodiment of the present disclosure. 
         FIG. 7  is an enlarged perspective view showing a structure of a scraper of the developing device according to the embodiment of the present disclosure. 
         FIG. 8  is an enlarged perspective view showing the structure of the scraper of the developing device according to the embodiment of the present disclosure. 
         FIG. 9  is a sectional side view showing the structure in the vicinity of the first helical member of the developing device according to the embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. 
     With reference to  FIGS. 1 to 9 , a structure of an image forming apparatus  1  according to the embodiment of the present disclosure will be described. The image forming apparatus  1  of the present embodiment is a tandem-type color printer. As rotatable photosensitive drums (image carriers)  11   a  to  11   d , for example, organic photoconductors (OPC) including organic photosensitive layers or amorphous silicon photosensitive members including amorphous silicon photosensitive layers are used. The photosensitive drums  11   a  to  11   d  are disposed corresponding to colors of magenta, cyan, yellow, and black, respectively. Developing devices  2   a  to  2   d , an exposure unit  12 , charging devices  13   a  to  13   d , and cleaning devices  14   a  to  14   d  are disposed around the photosensitive drums  11   a  to  11   d , respectively. 
     The developing devices  2   a  to  2   d  are disposed to the right of the photosensitive drums  11   a  to  11   d  so as to oppose the photosensitive drums  11   a  to  11   d , and supply toner to the photosensitive drums  11   a  to  11   d , respectively. The charging devices  13   a  to  13   d  are disposed upstream of the developing devices  2   a  to  2   d  in a photosensitive drum rotation direction so as to oppose the surfaces of the photosensitive drums  11   a  to  11   d , and uniformly charge the surfaces of the photosensitive drums  11   a  to  11   d , respectively. 
     The exposure unit  12  scans and exposes the photosensitive drums  11   a  to  11   d , based on image data, such as characters and pictures, input to an image input portion (not shown) via a personal computer or the like. The exposure unit  12  is disposed beneath the developing devices  2   a  to  2   d . The exposure unit  12  includes a laser light source and a polygon mirror, and further includes reflection mirrors and lenses corresponding to the respective photosensitive drums  11   a  to  11   d . Laser light emitted from the laser light source is applied, through the polygon mirror, the reflection mirrors, and the lenses, to the surfaces of the photosensitive drums  11   a  to  11   d  from the downstream side of the charging devices  13   a  to  13   d  in the photosensitive drum rotation direction. The applied laser light forms an electrostatic latent image on each of the surfaces of the photosensitive drums  11   a  to  11   d . The electrostatic latent image is developed into a toner image by each of the developing devices  2   a  to  2   d.    
     An intermediate transfer belt  17  that is an endless belt is extended on and between a tension roller  6 , a driving roller  25 , and a follower roller  27 . The driving roller  25  is driven to rotate by a motor (not shown), and the intermediate transfer belt  17  is driven to circulate by rotation of the driving roller  25 . 
     The photosensitive drums  11   a  to  11   d  are arranged under the intermediate transfer belt  17 , side by side along a conveyance direction (a direction indicated by an arrow in  FIG. 1 ) such that they are in contact with the intermediate transfer belt  17 . Primary transfer rollers  26   a  to  26   d  oppose the photosensitive drums  11   a  to  11   d  across the intermediate transfer belt  17 , respectively, and are pressed against the intermediate transfer belt  17  to form a primary transfer portion. In the primary transfer portion, the toner images of the photosensitive drums  11   a  to  11   d  are sequentially transferred to the intermediate transfer belt  17  at predetermined timings, according to rotation of the intermediate transfer belt  17 . Thus, on the surface of the intermediate transfer belt  17 , a full color toner image is formed in which the toner images of the four colors, i.e., magenta, cyan, yellow, and black, are superimposed. 
     A secondary transfer roller  34  opposes the driving roller  25  across the intermediate transfer belt  17 , and is pressed against the intermediate transfer belt  17  to form a secondary transfer portion. In the secondary transfer portion, the toner image on the surface of the intermediate transfer belt  17  is transferred to a paper sheet P. After the transfer, a belt cleaning device  31  removes residual toner on the intermediate transfer belt  17  to clean the intermediate transfer belt  17 . 
     In the lower portion of the image forming apparatus  1 , a sheet feed cassette  32  in which paper sheets P are stored is disposed. A stack tray  35  for manual sheet feeding is disposed to the right of the sheet feed cassette  32 . A first paper sheet conveyance path  33  through which a paper sheet P fed from the sheet feed cassette  32  is conveyed to the secondary transfer portion of the intermediate transfer belt  17 , is disposed to the left of the sheet feed cassette  32 . A second paper sheet conveyance path  36  through which a paper sheet fed from the stack tray  35  is conveyed to the secondary transfer portion, is disposed to the left of the stack tray  35 . Further, a fixing portion  18  that performs a fixing process for a paper sheet P on which an image is formed, and a third paper sheet conveyance path  39  through which the paper sheet on which the fixing process has been performed is conveyed to a paper sheet discharge portion  37 , are disposed in the upper left portion of the image forming apparatus  1 . 
     The sheet feed cassette  32  can be replenished with paper sheets when being pulled out from the main body of the image forming apparatus  1  (the near side in  FIG. 1 ). The stored paper sheets P are one by one fed to the first paper sheet conveyance path  33  side by a pickup roller  33   b  and a sheet feed roller pair  33   a.    
     The first paper sheet conveyance path  33  and the second paper sheet conveyance path  36  join together at a position before a registration roller pair  33   c , and the paper sheet P is conveyed to the secondary transfer portion such that a timing of the image forming operation at the intermediate transfer belt  17  and a timing of the sheet feeding operation are adjusted by the registration roller pair  33   c . Onto the paper sheet P conveyed to the secondary transfer portion, the full color toner image on the intermediate transfer belt  17  is secondarily transferred by the secondary transfer roller  34  to which a bias voltage is applied, and then the paper sheet P is conveyed to the fixing portion  18 . 
     The fixing portion  18  includes a fixing belt heated by a heater, a fixing roller inscribed to the fixing belt, a pressure roller disposed so as to be pressed against the fixing roller across the fixing belt, and the like, and performs a fixing process by heating and pressurizing the paper sheet P on which the toner image is transferred. After the toner image is fixed on the paper sheet P by the fixing portion  18 , the paper sheet P is, according to need, reversed in a fourth paper sheet conveyance path  40  and then a toner image is secondarily transferred also onto the other side of the paper sheet P by the secondary transfer roller  34  and fixed by the fixing portion  18 . The paper sheet P having the toner image fixed thereon is discharged through a third paper sheet conveyance path  39  to the paper sheet discharge portion  37  by a discharge roller pair  19 . 
     Next, the structure of the developing device  2   a  will be described in detail with reference to  FIG. 2 .  FIG. 2  shows the developing device  2   a  viewed from the back side of  FIG. 1 , and right and left of each of the components in the developing device  2   a  are reversed from those shown in  FIG. 1 . Hereinafter, the configuration and operation of the developing device  2   a  corresponding to the photosensitive drum  11   a  shown in  FIG. 1  will be described. Since the configurations and operations of the developing devices  2   b  to  2   d  are identical to those of the developing device  2   a , description thereof will be omitted. In addition, symbols “a” to “d” indicating the developing devices and the photosensitive drums corresponding to the respective colors will be omitted. 
     As shown in  FIG. 2 , the developing device  2  includes a developing roller (developer carrier)  20 , a magnetic roller  21 , a regulation blade  24 , an agitation conveyance member  42 , a developer container  22 , and the like. 
     The developer container  22  forms an outer frame of the developing device  2 , and a partition portion  22   b  thereof partitions a lower space in the developer container  22  into a first conveyance chamber  22   c  and a second conveyance chamber  22   d . A two-component developer composed of carrier and toner is contained in the first conveyance chamber  22   c  and the second conveyance chamber  22   d . The agitation conveyance member  42 , the magnetic roller  21 , and the developing roller  20  are rotatably supported by the developer container  22 . Further, the developer container  22  has an opening  22   a  that exposes the developing roller  20  toward the photosensitive drum  11 . 
     The developing roller  20  is disposed to the left of the photosensitive drum  11  so as to oppose the photosensitive drum  11  at a predetermined gap. A development region D in which the developing roller  20  supplies the toner to the photosensitive drum  11  is provided at a position where the developing roller  20  and the photosensitive drum  11  are closest to each other. The magnetic roller  21  is disposed diagonally left-downward of the developing roller  20  so as to oppose the developing roller  20  at a predetermined gap. The magnetic roller  21  supplies the toner to the developing roller  20  at a position where the magnetic roller  21  and the developing roller  20  closely oppose each other. The agitation conveyance member  42  is disposed substantially beneath the magnetic roller  21 . The regulation blade  24  is fixed to and held by the developer container  22  at a position diagonally right-downward of the magnetic roller  21 . 
     The agitation conveyance member  42  is composed of two helical members, i.e., a first helical member  43  and a second helical member  44 . The second helical member  44  is disposed under the magnetic roller  21  and inside the second conveyance chamber  22   d . The first helical member  43  is disposed to the left of the second helical member  44  and inside the first conveyance chamber  22   c.    
     The first and second helical members  43  and  44  agitate the developer to charge the toner in the developer at a predetermined level. Thereby, the toner is held by the carrier. Communication portions (an upstream-side communication portion  22   e  and a downstream-side communication portion  22   f  described later) are provided at both ends in a longitudinal direction (a direction perpendicular to the surface of the sheet of  FIG. 2 ) of the partition portion  22   b  that separates the first conveyance chamber  22   c  from the second conveyance chamber  22   d . When the first helical member  43  rotates, the charged developer is conveyed from one of the communication portions provided in the partition portion  22   b  to the second helical member  44 , and the developer circulates in the first conveyance chamber  22   c  and the second conveyance chamber  22   d . Then, the developer is supplied from the second helical member  44  to the magnetic roller  21 . 
     The magnetic roller  21  includes a roller shaft  21   a , a magnetic pole member M, and a nonmagnetic sleeve  21   b  formed of a nonmagnetic material. The magnetic roller  21  carries the developer supplied from the agitation conveyance member  42 , and supplies, to the developing roller  20 , only the toner out of the carried developer. The magnetic pole member M is composed of a plurality of cross-sectionally fan-shaped magnets having different polarities at the outer circumferential portions thereof. The magnetic pole member M is fixed to the roller shaft  21   a  by adhesion, for example. The roller shaft  21   a  is, inside the nonmagnetic sleeve  21   b , unrotatably supported by the developer container  22  such that a predetermined space is provided between the magnetic pole member M and the nonmagnetic sleeve  21   b . The nonmagnetic sleeve  21   b  is rotated by a drive mechanism (not shown) including a motor and a gear, in the same direction as the developing roller  20  (the counterclockwise direction in  FIG. 2 ), and a bias voltage resulting from superposing an AC voltage on a DC voltage is applied to the nonmagnetic sleeve  21   b . On the surface of the nonmagnetic sleeve  21   b , the charged developer is carried, in a form of a magnetic brush, by the magnetic force of the magnetic pole member M, and the magnetic brush is adjusted to a predetermined height by the regulation blade  24 . 
     When the nonmagnetic sleeve  21   b  rotates, the magnetic brush is conveyed while being carried on the surface of the nonmagnetic sleeve  21   b  by the magnetic pole member M. When the magnetic brush comes into contact with the developing roller  20 , only the toner in the magnetic brush is supplied to the developing roller  20  in accordance with the bias voltage applied to the nonmagnetic sleeve  21   b.    
     The developing roller  20  includes a stationary shaft  20   a , a magnetic pole member  20   b , a developing sleeve  20   c  that is formed of a nonmagnetic metal material in a cylindrical shape, and the like. 
     The stationary shaft  20   a  is unrotatably supported in the developer container  22 . The developing sleeve  20   c  is rotatably held by the stationary shaft  20   a . Further, the magnetic pole member  20   b  formed of a magnet is fixed to the stationary shaft  20   a  by adhesion or the like such that the magnetic pole member  20   b  opposes the magnetic roller  21  with a predetermined space provided between the developing sleeve  20   c  and the magnetic pole member  20   b . The developing sleeve  20   c  is rotated by a drive mechanism (not shown) including a motor and a gear in a direction (counterclockwise direction) indicated by an arrow in  FIG. 2 . Further, a development bias voltage resulting from superposing an AC voltage on a DC voltage is applied to the developing sleeve  20   c.    
     When the developing sleeve  20   c  to which the development bias voltage is applied rotates in the counterclockwise direction in  FIG. 2 , then, in the development region D, the toner carried on the surface of the developing sleeve  20   c  flies toward the photosensitive drum  11  due to a difference between the potential of the development bias voltage and the potentials of the exposed portions of the photosensitive drum  11 . The flying toner successively attaches to the exposed portions on the photosensitive drum  11  rotating in a direction (clockwise direction) indicated by an arrow in  FIG. 2 , and thereby the electrostatic latent image on the photosensitive drum  11  is developed. 
     Next, the agitation portion of the developing device will be described in detail. 
     In the developer container  22 , as shown in  FIG. 3 , the partition portion  22   b , the first conveyance chamber  22   c , the second conveyance chamber  22   d , the upstream-side communication portion  22   e , and the downstream-side communication portion  22   f  are provided as described above, and in addition, a developer replenishment port  22   g  is provided. The developer replenishment port  22   g  is an opening for replenishing the developer container  22  with new toner and carrier from a developer replenishment container (not shown) disposed above the developer container  22 . The developer replenishment port  22   g  is provided at an upper portion on the upstream side of the first conveyance chamber  22   c  (the left side in  FIG. 3 ). In the first conveyance chamber  22   c , the left side in  FIG. 3  is the upstream side, and the right side in  FIG. 3  is the downstream side. Further, in the second conveyance chamber  22   d , the right side in  FIG. 3  is the upstream side and the left side in  FIG. 3  is the downstream side. Accordingly, the communication portions are referred to as “upstream-side communication portion” and “downstream-side communication portion” with reference to the second transport chamber  22 . 
     The partition portion  22   b  extends in the longitudinal direction of the developer container  22  and partitions the developer container  22  into the first conveyance chamber  22   c  and the second conveyance chamber  22   d  so as to be parallel to each other. The upstream-side communication portion  22   e  and the downstream-side communication portion  22   f  are provided at one end and the other end of the partition portion  22   b  in the longitudinal direction thereof (an end in direction A1 and an end in direction A2), respectively. The upstream-side communication portion  22   e  connect the end portions, in the direction A1, of the first conveyance chamber  22   c  and the second conveyance chamber  22   d  to each other. The downstream-side communication portion  22   f  connects the end portions, in the direction A2, of the first conveyance chamber  22   c  and the second conveyance chamber  22   d  to each other. Thereby, the developer is allowed to circulate in the first conveyance chamber  22   c , the upstream-side communication portion  22   e , the second conveyance chamber  22   d , and the downstream-side communication portion  22   f.    
     The first helical member  43  has a rotation shaft  43   b , and a first helical blade (blade)  43   a  formed integrally with the rotation shaft  43   b . The first helical blade  43   a  has a helical shape winding around the rotation shaft  43   b  in its axial direction at a constant pitch. The rotation shaft  43   b  is rotatably supported by the developer container  22 . The first helical blade  43   a  conveys the developer in the first conveyance chamber  22   c  in the direction A1 while agitating the developer. 
     Further, as shown in  FIGS. 4 and 5 , the first helical member  43  has a plurality of ribs  43   c  that are formed integrally with the first helical blade  43   a  and the rotation shaft  43   b . The ribs  43   c  control the developer conveyance speed. It is noted that the first helical member  43  is driven to rotate by a motor (not shown), and is configured to be rotatable in a forward direction for a printing operation (when image formation is performed) and in a reverse direction for a cleaning operation (when image formation is not performed) by means of a nonwoven fabric  52   a  described later. 
     As shown in  FIG. 3 , the second helical member  44  has a rotation shaft  44   b , and a second helical blade  44   a  formed integrally with the rotation shaft  44   b . The second helical blade  44   a  has a helical shape winding around the rotation shaft  44   b  in its axial direction at the same pitch as the first helical blade  43   a . The second helical blade  44   a  is a blade facing in a direction opposite to the direction of (being in a phase opposite to the phase of) the first helical blade  43   a . The rotation shaft  44   b  is disposed in parallel with the rotation shaft  43   b , and is rotatably supported by the developer container  22 . The second helical blade  44   a  conveys the developer in the second conveyance chamber  22   d  in the direction A2 (a direction opposite to the direction A1) while agitating the developer to supply the developer to the developing roller  20 . 
     The first helical member  43  is formed of resin such as PS (polystyrene), ABS (acrylonitrile butadiene styrene copolymer), or PC (polycarbonate), and the first helical blade  43   a  and the rotation shaft  43   b  are integrally molded. Likewise, the second helical member  44  is also formed of resin such as PS, ABS, or PC, and the second helical blade  44   a  and the rotation shaft  44   b  are integrally molded. The rotation shafts  43   b  and  44   b  are formed of resin only, and have no metal rods as shaft cores. 
     As shown in  FIGS. 3 and 6 , in the first conveyance chamber  22   c , a toner concentration detection sensor (toner detection sensor)  51  is disposed near the upstream side of the upstream-side communication portion  22   e  in a developer conveyance direction (a direction indicated by an white arrow in  FIG. 3 ). 
     As an example of the toner concentration detection sensor  51 , a magnetic permeability sensor is used which detects a magnetic permeability of a developer in the developer container  22 . When the magnetic permeability of the developer is detected by the toner concentration detection sensor  51 , a voltage value corresponding to the detection result is output to a control portion (not shown). The control portion determines the toner concentration based on the output value from the toner concentration detection sensor  51 . 
     The output value from the sensor  51  varies according to the toner concentration. The higher the toner concentration, the higher the ratio of the toner to the magnetic carrier. Such an increase in the ratio of the toner that is not magnetically conductive results in a reduction in the output value. On the other hand, the lower the toner concentration, the lower the ratio of the toner to the carrier. Such an increase in the ratio of the carrier that is magnetically conductive results in an increase in the output value. 
     Further, as shown in  FIGS. 4 to 6 , the first helical member  43  has a scraper  52  disposed at a portion opposing the toner concentration detection sensor  51 . As shown in  FIGS. 7 and 8 , the scraper  52  is formed by bonding the nonwoven fabric (second member)  52   a  and a polyethylene sheet (first member)  52   b  which have the same shape, by using an adhesive layer (not shown). Thereby, a coefficient of friction between a detection surface  51   a  and the member (nonwoven fabric  52   a ) of the scraper  52  on the opposite side from the polyethylene sheet  52   b  can be easily made higher than a coefficient of friction between the detection surface  51   a  and the polyethylene sheet  52   b.    
     The nonwoven fabric  52   a  has a thickness of about 1 mm, and the polyethylene sheet  52   b  has a thickness of about 0.1 mm to about 0.2 mm. The coefficient of friction between the nonwoven fabric  52   a  and the detection surface  51   a  of the toner concentration detection sensor  51  is higher than the coefficient of friction between the polyethylene sheet  52   b  and the detection surface  51   a  of the toner concentration detection sensor  51 . In addition, the polyethylene sheet  52   b  is formed of so-called ultra-high molecular weight polyethylene having a molecular weight of about 1 million to about 7 million, and therefore, has a wear resistance higher than that of the nonwoven fabric  52   a . Thus, the wear resistance of the polyethylene sheet  52   b  can be easily made higher than the wear resistance of the nonwoven fabric  52   a.    
     An adhesive layer  53  such as a double-sided adhesive tape is bonded to the inner circumferential side of the nonwoven fabric  52   a  (a portion of the nonwoven fabric  52   a  on the rotation shaft  43   b  side when the nonwoven fabric  52   a  is bonded to the first helical member  43 ). Then, as shown in  FIG. 6 , the scraper  52  is bonded, by using the adhesive layer  53 , to a surface of the first helical blade  43   a , which surface faces the downstream side in the rotation direction when the first helical member  43  rotates forward. Thereby, when the first helical member  43  rotates forward, the detection surface  51   a  of the toner concentration detection sensor  51  can be easily cleaned by the polyethylene sheet  52   b . At this time, the scraper  52  is bonded to the first helical blade  43   a  such that the projection height of the scraper  52  from a tip (an outer circumferential surface, a lower surface in  FIG. 6 ) of the first helical blade  43   a  is larger than the distance between the tip of the first helical blade  43   a  and the detection surface  51   a  of the toner concentration detection sensor  51 . Therefore, the scraper  52 , with its tip portion being bent, comes into contact with the detection surface  51   a  of the toner concentration detection sensor  51 . 
     It is noted that, if the rotation shaft  43   b  is formed of resin only as in the present embodiment, the above-mentioned projection height of the scraper  52  is set to be larger so that the scraper  52  reliably slides on (comes into contact with) the detection surface  51   a  of the toner concentration detection sensor  51  even when the rotation shaft  43   b  is bent. 
     When the first helical member  43  rotates forward, the surface of the polyethylene sheet  52   b  (one surface of the scraper  52 ) slides on the detection surface  51   a  of the toner concentration detection sensor  51 . On the other hand, when the first helical member  43  rotates reversely, as shown in  FIG. 9 , the surface of the nonwoven fabric  52   a  (the other surface of the scraper  52 ) slides on the detection surface  51   a  of the toner concentration detection sensor  51 . In this way, the detection surface  51   a  of the toner concentration detection sensor  51  is rubbed and cleaned by either the polyethylene sheet  52   b  or the nonwoven fabric  52   a.    
     As for timing to cause the first helical member  43  to rotate reversely, the timing may be when each printing operation is ended or when the number of printed sheets reaches a predetermined number. Further, when the first helical member  43  is caused to rotate reversely, the second helical member  44  may also be caused to rotate reversely. 
     Conventionally, when nonwoven fabric is used as a scraper, the nonwoven fabric is worn out due to its sliding on a detection surface of a toner concentration detection sensor over a long period of time. Therefore, it is difficult to prevent accumulation of a developer on the detection surface of the toner concentration detection sensor over a long period of time. In the present embodiment, however, as described above, the scraper  52  includes the polyethylene sheet  52   b  that comes into contact with the detection surface  51   a  of the toner concentration detection sensor  51  when the first helical member  43  rotates forward, and the nonwoven fabric  52   a  that comes into contact with the detection surface  51   a  of the toner concentration detection sensor  51  when the first helical member  43  rotates reversely. The polyethylene sheet  52   b  has a wear resistance higher than that of the nonwoven fabric  52   a . Thereby, during forward rotation of the first helical member  43 , the scraper  52  is suppressed from being worn out due to its sliding on the detection surface  51   a  of the toner concentration detection sensor  51 , and therefore, accumulation of the developer on the detection surface  51   a  of the toner concentration detection sensor  51  can be prevented over a long period of time. Therefore, the toner concentration can be accurately detected by the toner concentration detection sensor  51  over a long period of time. 
     Further, the coefficient of friction between the nonwoven fabric  52   a  and the detection surface  51   a  is higher than the coefficient of friction between the polyethylene sheet  52   b  and the detection surface  51   a . That is, the nonwoven fabric  52   a  has a higher cleaning power against the detection surface  51   a  of the toner concentration detection sensor  51  than the polyethylene sheet  52   b . Therefore, by rotating the first helical member  43  reversely, the detection surface  51   a  of the toner concentration detection sensor  51  can be cleaned more effectively by the nonwoven fabric  52   a . Accordingly, it is possible to remove, by the nonwoven fabric  52   a , a thin layer of the developer that has been gradually accumulated on the detection surface  51   a  of the toner concentration detection sensor  51  and cannot be completely removed by the polyethylene sheet  52   b . Therefore, the toner concentration can be accurately detected by the toner concentration detection sensor  51  over a long period of time. 
     Further, the first helical member  43  rotates forward during image formation to agitate and convey the developer in the developer container  22 , and rotates reversely when image formation is not performed. Therefore, when image formation is not performed, the detection surface  51   a  can be cleaned by the nonwoven fabric  52   a  having the high cleaning power. 
     When the rotation shaft  43   b  is formed of resin, the rotation shaft  43   b  is likely to be bent due to a counterforce when agitating and conveying the developer. Therefore, when the scraper  52  is bonded to the first helical blade  43   a , the projection height thereof from the tip of the first helical blade  43   a  is set to be larger so that the scraper  52  can reliably slide on the detection surface  51   a  of the toner concentration detection sensor  51  even when the rotation shaft  43   b  is bent, which makes the scraper  52  more likely to be worn out. Accordingly, suppressing wear of the scraper  52  is more particularly effective when the rotation shaft  43   b  is formed of resin. 
     It is to be understood that the embodiments herein are illustrative and not restrictive, since the scope of this disclosure is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims. 
     For example, in the above embodiment, the present disclosure is not limited to the tandem-type color printer, and is applicable to various image forming apparatuses provided with a developing device including a toner detection sensor and a scraper, such as a digital or analog monochrome copy machine, a color copy machine, a facsimile, and the like. 
     In the above embodiment, the two-component developer composed of carrier and toner is used. However, the present disclosure is not limited thereto. A single-component developer composed of toner only may be used. In this case, a remaining-amount-of-toner detection sensor that detects a remaining amount of toner in the developer container may be used as a toner detection sensor. 
     In the above embodiment, the magnetic permeability sensor is used as a toner detection sensor. However, the present disclosure is not limited thereto. A toner detection sensor other than the magnetic permeability sensor, such as a piezoelectric sensor, may be used. 
     In the above embodiment, the first member is formed of ultra-high molecular weight polyethylene. However, the present disclosure is not limited thereto. The first member may be formed of polyethylene other than ultra-high molecular weight polyethylene. Alternatively, the first member may be formed of a material (e.g., resin) other than polyethylene. 
     In the above embodiment, the second member is formed of nonwoven fabric. However, the present disclosure is not limited thereto. The second member may be formed of a material other than nonwoven fabric. 
     In the above embodiment, the toner detection sensor is disposed in the first conveyance chamber, and the scraper is bonded to the first helical member. However, the toner detection sensor may be disposed in the second conveyance chamber, and the scraper may be bonded to the second helical member. 
     It is to be understood that the embodiments herein are illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims.