Patent Publication Number: US-7715777-B2

Title: Image forming apparatus forming a developed image using an image carrier

Description:
This application is based on and claims priority under 35USC 119 from Japanese Patent Application No. 2007-197407 filed Jul. 30, 2007. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus where a developed image is formed on an image carrier, transferred from the image carrier, and fixed onto a recording medium. 
   2. Description of the Related Art 
   Image forming apparatuses such as predominantly a printer and a copying machine are widely prevailed in recent years, and therefore, there become widely available techniques relating to various elements constituting such an image forming apparatus. In a type adopting an electrophotographic system among various types of image forming apparatuses, an image carrier is electrically charged by the use of a charger, and then, a printing pattern is usually formed by forming an electrostatic latent image different in potential from the surroundings on the charged image carrier. The electrostatic latent image such formed as described above is developed with a developer agent containing a toner therein, and transferred onto a recording medium. 
   Since a high voltage is applied to the charger which conducts the electric charging, substance such as ozone or nitrogen oxide is secondarily produced from air around the charger with the application of the high voltage in many cases. If an unnecessary substance such as a discharged product adheres onto the image carrier, the charging performance of the image carrier is liable to be degraded. A marked degradation of the charging performance blurs an image formed on the recording medium, thereby causing the deterioration of a quality of an image (so-called image blurring). 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above circumstances and provides an image forming apparatus, in which an unnecessary substance adhering onto an image carrier is removed, as required, so as to achieve favorable image formation. 
   An image forming apparatus according to the present invention includes: an image carrier; a charger that applies an electric charge to the image carrier; an image forming section that forms an electrostatic latent image on the image carrier and forms a developed image by developing the electrostatic latent image; a transferring-fixing section that transfers the developed image from the image carrier and fix the image onto a recording medium; a cleaning member that cleans an unnecessary substance adhering onto a surface of the image carrier by abutting against the surface of the image carrier, the unnecessary substance being caused by the application of the electric charge by the charger, the cleaning member being capable of moving between an abutment position where the cleaning member abuts against the surface of the image carrier and a separation position where the cleaning member is separated from the image carrier; and a cleaning member moving section that moves the cleaning member from the separation position to the abutment position in accordance with a surface resistance of the surface of the image carrier. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram illustrating the schematic configuration of a full-color image forming apparatus in an embodiment of an image forming apparatus according to the present invention; 
       FIG. 2  is a graph illustrating the relationship between a surface resistance of an image carrier and a degree of image blurring; 
       FIG. 3  is a diagram illustrating the schematic configuration of a surface resistance-measuring device for measuring a surface resistance of the image carrier illustrated in  FIG. 1 ; 
       FIGS. 4A and 4B  are diagrams illustrating a discharged product removing device illustrated in  FIG. 1 ; 
       FIG. 5  is a diagram illustrating the general configuration of an image forming apparatus provided with a contact type charger which serves as an electrode in the surface resistance measuring device; 
       FIG. 6  is a diagram illustrating the schematic configuration of the surface resistance-measuring device illustrated in  FIG. 5 ; 
       FIG. 7  is a diagram illustrating the general configuration of an image forming apparatus including simplified detecting device that detects the degree of adhesion of a discharged product; 
       FIG. 8  is a diagram illustrating the schematic configuration of a detecting device that detects a degree of adhesion of a discharged product and that is provided in an image forming apparatus  1000 ″ illustrated in  FIG. 7 ; 
       FIG. 9  is a graph illustrating the relationship between a current flowing into a base through a photosensitive layer from an electrode placed on the image carrier and the degree of the image blurring; and 
       FIGS. 10A and 10B  are diagrams illustrating a manner in which an abutment force of a cleaning blade is switched. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Explanation will be made below on exemplary embodiments according to the present invention. 
     FIG. 1  is a diagram illustrating the general configuration of an image forming apparatus in an exemplary embodiment according to the present invention. 
   An image forming apparatus  1000  is a monochromatic one-sided output printer which adopts an electrophotographic system. The image forming apparatus  1000  is provided with a laminated type image carrier  61  for the electrophotographic system, which is rotated in a direction indicated by an arrow A in  FIG. 1  during image formation, and a charger  63  which electrically charges the image carrier  61  by rotating in contact with the image carrier  61 , upon the application of an AC voltage superimposed on a predetermined DC voltage by a charged voltage applying section which is not illustrated. Furthermore, the image forming apparatus  1000  includes: an exposing section  7  which emits a laser beam to the image carrier  61 , so as to form, on the image carrier  61 , an electrostatic latent image different in potential from the surroundings; a developing device  64  which allows a toner to adhere to the electrostatic latent image and forms a developed image by development; a transferring roll  50  which transfers the developed image formed on the image carrier  61  to a sheet to be transported with the application of a transferring bias voltage; a fixing device  10  which fixes a transferred image to the sheet by applying heat and pressure to the image transferred onto the sheet; a cleaning device  62  (corresponding to one example of a removing member according to the present invention) which removes a toner (i.e., a residual toner) adhering to and remaining on the image carrier  61  by a cleaning blade abutting against the image carrier  61 , after the transfer of the developed image; a surface resistance-measuring device  66  which measures a surface resistance of the image carrier  61 ; a discharged product removing device  65  which removes a discharged product adhering onto the image carrier  61 ; and a CPU (Central Processing Unit)  4  which controls each of the component elements. 
   The image forming apparatus  1000  is further provided with a toner cartridge, not illustrated, which contains the toner therein and replenishes the toner in the developing device  64 . Sheets, onto which developed images are transferred, are stacked in a tray  1 . Upon instruction of image formation by a user, the sheet is transported from the tray  1 , and then, is transported onto the left in  FIG. 1  after the transfer of the developed image by the transferring roll  50 . In  FIG. 1 , a sheet transportation path at this time is depicted by a channel indicated by leftward arrows. The sheet is transported on the sheet transportation path to the fixing device  10 , in which the image transferred onto the sheet is fixed, and then, the sheet is output leftward. 
   In general, since a high voltage is applied to the charger at the time of the electric charging by the charger, the application of the high voltage frequently produces ozone from air around the charger, to secondarily produce substance such as oxide nitride. When such a discharged product adheres to the image carrier, the charging performance of the image carrier is liable to be deteriorated. A marked degradation of the charging performance blurs the image formed on the recording medium, thereby causing the degradation of the quality of the image (so-called image blurring). 
   Here, description will be given of a change in surface resistance of the image carrier which is attributable to the adhesion of the discharged product to the image carrier. 
     FIG. 2  is a graph illustrating the relationship between the surface resistance of the image carrier and the degree of the image blurring. 
   In  FIG. 2 , the degree (i.e., the level) of the image blurring, which is obtained by observing the image, is adopted as a variable on a lateral axis. The image blurring becomes more conspicuous rightward in a direction in  FIG. 2  along the lateral axis. In contrast, the common logarithm of the surface resistance of the image carrier is adopted as a variable on a vertical axis in  FIG. 2 .  FIG. 2  is the graph illustrating the result obtained from experiments for examining the relationship between the surface resistance of the image carrier and the degree (i.e., the level) of the image blurring.  FIG. 2  illustrates the relationship in which the surface resistance of the image carrier is decreased more as the level of the image blurring becomes higher. As a result, it is found from the graph that the surface resistance of the image carrier is decreased more as the amount of the discharged product adhered onto the image carrier becomes increased. 
   The image blurring concerned from the viewpoint of the image formation is so clearly observed as a deficiency of a quality of an image even by an ordinary user, as illustrated by a level G 0  or higher of the image blurring at a point P on the graph. The image blurring lower than the level G 0 , if any, can be seldom recognized as the deficiency of the quality of the image by the ordinary user. The surface resistance at the point P on the graph is ρ 0  [Ω], which is set as a threshold, so that the image blurring raises a problem in the case where the surface resistance of the image carrier becomes ρ 0  or smaller. 
   In view of this, the surface resistance of the image carrier  61  is detected in the image forming apparatus  1000  illustrated in  FIG. 1 . If it is detected that the surface resistance is ρ 0  or smaller, the discharged product is removed. Hereinafter, explanation will be first made on the detection of the surface resistance of the image carrier  61 , and subsequently, a description will be given of the discharged product removal. 
     FIG. 3  is a diagram illustrating the schematic configuration of the surface resistance-measuring device  66  for measuring the surface resistance of the image carrier illustrated in  FIG. 1 . 
   The surface resistance-measuring device  66  illustrated in  FIG. 1  includes three electrodes  660 ,  661  and  662  arranged along the surface of the image carrier  61 , and an ammeter  66   b  for measuring a current flowing in the electrode  660  at a center of the three electrodes in  FIG. 3 . Each of the three electrodes  660 ,  661  and  662  is a columnar electrode having a predetermined length extending in a direction of a rotary shaft of the image carrier  61 . The three electrodes  660 ,  661  and  662  are juxtaposed each other on the image carrier  61 , to be rotated following the rotation of the image carrier  61 . A circular cross section of each of the three electrodes is depicted in  FIG. 3 . 
   Each of the three columnar electrodes is configured such that a cylindrical surface of a conductive columnar base is covered with an elastic layer, which is made of mainly a rubber material and contains a conductive agent therein, although not illustrated in  FIG. 3 . Further on the elastic layer is laminated a surface layer made of a resin containing a conductive agent therein in order to enhance abrasion durability. Materials of the base are exemplified by iron, bronze, aluminum, stainless and a resin containing a conductive agent therein. Among them, stainless is preferable from the viewpoint of durability. Materials of the elastic layer are exemplified by rubbers such as a silicon rubber, urethane, polybutadiene, polyisobutylene and an ethylene-propylene-diene rubber (abbreviated as “an EPDM”). The conductive agent contained in the elastic layer is exemplified by metallic particles made of carbon black, zinc or iron, or metallic oxide such as zinc oxide or tin dioxide. Materials of the surface layer are exemplified by an acrylic resin, a polyamide resin, a polyurethane resin and a polyester resin, in which the conductive agent is dispersed. The thickness of the elastic layer is preferably 1 mm or more and 4 mm or less, and more preferably, 2 mm or more and 3 mm or less. In the meantime, the thickness of the surface layer is preferably 10 μn or more and 500 μm or less, and more preferably, 10 μm or more and 200 μm or less. 
   The surface resistance-measuring device  66  illustrated in  FIG. 3  is provided with a power source  66   a  for applying the same DC voltage between the center electrode  660  and the left electrode  661  and between the center electrode  660  and the right electrode  662  in  FIG. 3 . As for the power source  66   a,  the center electrode  660  is an anode whereas the left electrode  661  and the right electrode  662  are cathodes, wherein an anode side is grounded. The image carrier  61  in  FIG. 3  is configured such that a photosensitive layer  611  for generating and transporting an electric charge is laminated on a metallic photosensitive base  612 , which is grounded. With this configuration, the photosensitive base  612  is identical in potential to the center electrode  660  in  FIG. 3 . As a consequence, the current flowing into the center electrode  660  cannot flow into the photosensitive base  612  through the photosensitive layer  611 . The current flowing into the center electrode  660  is half divided into a current flowing in the left electrode  661  and a current flowing in the right electrode  662 , and thus, flows along the image carrier  61 . Here, a distance from the center electrode  660  to the left electrode  661  in  FIG. 3  is equal to a distance from the center electrode  660  to the right electrode  662  in  FIG. 3 . Consequently, an electric resistance at the surface of the image carrier  61  between the center electrode  660  and the left electrode  661  in  FIG. 3  is equal to an electric resistance at the surface of the image carrier  61  between the center electrode  660  and the right electrode  662  in  FIG. 3 . Assuming that the electric resistance is designated by R [Ω], the resistance R is obtained by the following equation:
 
 R=E /( I/ 2)   (1)
 
where E [V] denotes the magnitude of the power source  66   a  and I [A] expresses the magnitude of the current (i.e., the current flowing into the center electrode  660 ) measured by the ammeter  66   b.  
 
   Here, the reason why the current I as a denominator on a right side is divided by 2 is that the current flowing into the center electrode  660  is half divided into the current flowing in the left electrode  661  and the current flowing in the right electrode  662 , as described above. 
   Assuming that the surface resistance of the image carrier  61  is designated by ρ [Ω], the surface resistance ρ is determined by the following equation:
 
ρ= R ×( a/b )   (2)
 
where the resistance R is obtained by the equation (1), a [m] denotes the length of each of the three electrodes  660 ,  661  and  662  (the predetermined length extending in the direction of the rotary shaft of the image carrier  61 ), and b [m] designates the distance from the center electrode  660  to the left electrode  661  in  FIG. 3  (also the distance from the center electrode  660  to the right electrode  662 ).
 
   In combination of the equations (1) and (2), the surface resistance ρ is determined by the following equation:
 
ρ=2 E ×( a/b )/ I    (3)
 
   The CPU  4  instructs the power source  66   a  in the surface resistance-measuring device  66  in  FIG. 3  to apply the voltage between the electrodes immediately after the turning-on of the power source in the image forming apparatus  1000  and immediately after the completion of a series of image formation (i.e., jobs) instructed by the user, and then, acquires the current I measured by the ammeter  66   b  from the ammeter  66   b.  The CPU  4  determines the surface resistance ρ in accordance with the equation (2), and then, judges whether or not the resistance is the threshold ρ 0  or lower in reference to the graph in  FIG. 2 . If it is judged that the resistance ρ is not the threshold ρ 0  or lower, the CPU  4  leaves as it is. In contrast, if it is judged that the resistance ρ is the threshold ρ 0  or lower, the CPU  4  instructs the discharged product removing device  65  illustrated in  FIG. 1  to remove the discharged product, described below. 
     FIGS. 4A and 4B  are diagrams illustrating the discharged product removing device illustrated in  FIG. 1 . 
   The discharged product removing device  65  is adapted to slide on the image carrier  61  in abutment of a web  656  against the surface of the image carrier  61 . A web guide roll  657 , around which the web  656  is stretched, is of a type which can rotate on a movable roll rotary shaft  655   b  extending in a direction perpendicular to the drawing and freely moving within a plane of the drawing, and therefore, the web guide roll  657  is moved within the plane of the drawing according to the movement of the movable roll rotary shaft  655   b.  The movable roll rotary shaft  655   b  can be moved between a web abutment position, at which the web  656  abuts against the surface of the image carrier  61 , and a web separation position, at which the web is separated from the surface of the image carrier  61 .  FIG. 4A  illustrates the movable roll rotary shaft  655   b  at the web separation position: in contrast,  FIG. 4B  illustrates the movable roll rotary shaft  655   b  at the web abutment position. Referring to  FIGS. 4A and 4B , a description will be given below of a mechanism for moving the movable roll rotary shaft  655   b  between the web abutment position and the web separation position. 
   The web guide roll  657  is of a type which can be rotated on the movable roll rotary shaft  655   b  movable within the plane of the drawing. As illustrated in  FIG. 4A , one end of an L shape of an L-shaped fitting  655  and a spring  658  are connected to the movable roll rotary shaft  655   b.  A fixed shaft  655   a  extending in a vertical direction in  FIG. 4A  penetrates the L-shaped fitting  655  at a point bent into an L shape at the L-shaped fitting  655 . In this manner, the L-shaped fitting  655  can be rotated on the fixed shaft  655   a.  The other end of the L-shaped fitting  655  is connected to one end of a buffer spring  653 : in contrast, the other end of the buffer spring  653  is secured to a ferromagnetic shaft  651 . The metallic shaft  651  is such configured as to be moved leftward only at a predetermined position by a stopper member, not illustrated, although the metallic shaft  651  receives force exerting leftward in  FIG. 4A  from the buffer spring  653 .  FIG. 4A  illustrates the shaft  651  which is moved most leftward in  FIG. 4A . Here, the shaft  651  is partly inserted into a solenoid coil  652 , which can be applied with a voltage from a voltage applying section  654  where the CPU  4  controls the voltage application. When the web guide roll  657  is located at the web abutment position, as illustrated in  FIG. 4A , the solenoid coil  652  is applied with no voltage from the voltage applying section  654 . In this state, the image is formed, as illustrated in  FIG. 1 . The CPU  4  instructs the voltage applying section  654  to apply the voltage to the solenoid coil  652  in the case where it is judged that the surface resistance ρ is the threshold ρ 0  or lower. 
   Upon the application of the voltage from the voltage applying section  654  to the solenoid coil  652 , force is produced to pull the shaft  651  farther into the coil than the position illustrated in  FIG. 4A  by a magnetic field generated inside of the coil, so that the shaft  651  is moved in a direction indicated by an arrow B. When the shaft  651  is moved in the direction indicated by the arrow B, the end of the L-shaped fitting  655  connected to the buffer spring  653  is pulled rightward in  FIG. 4A  via the buffer spring  653 , and thus, the L-shaped fitting  655  is rotated on the fixed shaft  655   a  in a direction indicated by an arrow C. With the rotation, the movable roll rotary shaft  655   b  connected to the L-shaped fitting  655  is moved downward in  FIG. 4A  together with the web guide roll  657  and the web  656  while allowing the spring  658  to expand, and finally, the movable roll rotary shaft  655   b  reaches a web abutment position illustrated in  FIG. 4B . Here, in  FIG. 4B , the web  656  is pressed against the image carrier  61  by the web guide roll  657  by force greater than the abutment force of the cleaning blade of the cleaning device  62  illustrated in  FIG. 1  against the image carrier  61 . A web winding-up device, not illustrated, is driven on the basis of an instruction from the CPU  4  in a state illustrated in  FIG. 4B , and then, winds up the web  656 , which wipes the surface of the image carrier  61 , in a direction indicated by an arrow X. Here, when the web winding-up device is driven, the image carrier  61  also is driven to be rotated in the direction indicated by the arrow A in  FIG. 1 . The winding-up speed of the web  656  by the web winding-up device has been previously set in such a manner as to be different by predetermined percentage (e.g., ±0.5%) in proportion to the rotational speed of the image carrier  61 . The web  656  wipes off the surface of the image carrier  61 , thus satisfactorily removing the discharged product adhering onto the image carrier  61 . 
   Since the particle diameter of the discharged product adhering onto the image carrier is generally smaller than that of a residual toner, the discharged product is less removed compared with the residual toner. In view of this, when the discharged product adhering onto the image carrier is removed, the cleaning blade needs to slide on the image carrier in abutment by force greater than the abutment force for use in removing the toner remaining on the image carrier (i.e., the residual toner) by the cleaning blade. The abutment force is too small to satisfactorily remove the discharged product, thereby causing a possibility of a deficient image with image blurring. It may be construed that the discharged product can be satisfactorily removed by increasing the abutment force of the cleaning blade. Usually, the cleaning blade abuts against the surface of the image carrier all the time in order to remove the residual toner. Therefore, if the abutment force of the cleaning blade is very great, the image carrier may be possibly abraded in turn. As a result, it is not preferable from the viewpoint of the quality of the image that the cleaning blade for removing the residual toner should also remove the discharged product as it is. 
   The image forming apparatus  1000  illustrated in  FIG. 1  is provided with the discharged product removing device  65  adopting the system for wiping off the discharged product by causing the web  656  to abut against the surface of the image carrier independently of the cleaning device  62  for removing the residual toner. Consequently, the image forming apparatus  1000  can satisfactorily remove the discharged product by the abutment force suitable for the removal of the discharged product, thus avoiding any occurrence of the image blurring. Moreover, the discharged product is removed by causing the web  656  to abut against the surface of the image carrier  61  in the image forming apparatus  1000  only when so large quantity of discharged product as to raise the problem of the image blurring adheres onto the image carrier  61 , so that the image carrier can be avoided from being abraded due to the unnecessary slide on the surface of the image carrier. 
   If the web  656  is pressed against the surface of the image carrier all the time or at a timing after the completion of a job irrespective of the quantity of discharged product adhering onto the image carrier  61 , the web  656  may be smeared with the residual toner to degrade the removability of the discharged product by the discharged product removing device  65  at once in addition to the problem of the abrasion of the image carrier. Such a problem also can be solved in the image forming apparatus  1000 . As a consequence, the removability of the discharged product can be maintained for a long period of time in the discharged product removing device  65  in the image forming apparatus  1000 . 
   The web  656  having the predetermined length is wound up by the web winding-up device, thus completing the removal of the discharged product. Upon the completion of the removal, the CPU  4  instructs the voltage applying section  654  to stop the application of the voltage. As a result, the shaft  651  is moved leftward in  FIG. 4B , so that the L-shaped fitting is rotated on the fixed shaft  655   a  in a direction indicated by an arrow D, to be returned to the state illustrated in  FIG. 4A . 
   Although the discharged product removing system by the use of the web guide roll  657  having the winding-up type web  656  stretched therearound is adopted in the discharged product removing device  65 , a discharged product removing system where a cleaning roll is caused to abut against the surface of the image carrier  61  may be adopted in place of the web guide roll  657  having the winding-up type web  656  stretched therearound according to the present invention. In this case, it is preferable that the discharged product should be removed while the cleaning roll is rotated at a rotational speed different by predetermined percentage (e.g., ±0.5%) from that of the image carrier  61 . 
   Additionally, although the discharged product removing device  65  is disposed downstream of the surface resistance-measuring device  66  in the rotational direction of the image carrier  61  in the image forming apparatus  1000  illustrated in  FIG. 1 , the discharged product removing device may be disposed upstream of the surface resistance-measuring device in the rotational direction of the image carrier according to the present invention. 
   Although the surface resistance-measuring device  66  and the charger  63  are independent of each other in the image forming apparatus  1000 , a contact type charger may serve as an electrode for the surface resistance-measuring device according to the present invention. Hereinafter, a description will be given of an image forming apparatus, in which a contact type charger serves as an electrode for a surface resistance-measuring device. 
     FIG. 5  is a diagram illustrating the general configuration of an image forming apparatus provided with a contact type charger which serves as an electrode in a surface resistance measuring device. 
   In a configuration of an image forming apparatus  1000 ′ illustrated in  FIG. 5 , the same constituent elements as those in the image forming apparatus  1000  illustrated in  FIG. 1  are designated by the same reference numerals, and therefore, a duplicate explanation will be omitted below. 
   In the image forming apparatus  1000 ′ illustrated in  FIG. 5 , a contact type charger  63  serves as a device for electrically charging an image carrier  61 , and further, functions as one of three electrodes included in a surface resistance-measuring device  66 ′. Here, generating sources of a voltage to be supplied to the charger  63  illustrated in  FIG. 5  are switched in electrically charging the image carrier  61  and in measuring a surface resistance of the image carrier  61 , as described below. 
     FIG. 6  is a diagram illustrating the schematic configuration of the surface resistance-measuring device  66 ′ illustrated in  FIG. 5 . 
   In the configuration of the surface resistance-measuring device  66 ′ illustrated in  FIG. 6 , the same constituent elements as those in the surface resistance-measuring device  66  illustrated in  FIG. 3  are designated by the same reference numerals, and therefore, a duplicate explanation will be omitted below. The configuration of the surface resistance-measuring device  66 ′ illustrated in  FIG. 6  is different from that of the surface resistance-measuring device  66  illustrated in  FIG. 3  in that the center electrode  660  out of the three electrodes included in the surface resistance-measuring device  66  illustrated in  FIG. 3  is replaced with the contact type charger  63  in the surface resistance-measuring device  66 ′ illustrated in  FIG. 6  and that the &#39;surface resistance-measuring device  66 ′ is provided with a switch  663  for switching a generating source of a voltage to be supplied to the charger  63 . The switch  663  is turned on or off under the control by the CPU  4 . The CPU  4  controllably switches the switch  663 , so as to allow a charged voltage applying section  63   a  to apply an AC voltage, which is superimposed on a predetermined DC voltage, to the charger  63  when the image carrier  61  is electrically charged during the image formation: whereas a DC source  66   a  is caused to apply a DC voltage to the charger  63 , as also described in reference to  FIG. 3 , when the surface resistance of the image carrier  61  is measured. Here, the charger  63  illustrated in  FIG. 6  is formed into a columnar shape extending in a direction of a rotary shaft for the image carrier  61 , like the center electrode  660  illustrated in  FIG. 3 , and further, the length and position are identical to those of the center electrode  660  illustrated in  FIG. 3 . Thus, the surface resistance-measuring device  66 ′ illustrated in  FIG. 6  also measures the surface resistance of the image carrier  61  based on the equation (3) in the same manner as described in reference to  FIG. 3 . Therefore, the duplicate-explanation will be omitted below. 
   The configuration of the surface resistance-measuring device is devised such that the current (i.e., the current measured by the ammeter) flowing toward the image carrier does not flow into the image carrier but flows only at the surface of the image carrier in the image forming apparatuses  1000  and  1000 ′, as described above (see  FIG. 3 ). However, even if the configuration of the surface resistance-measuring device is not devised, an image forming apparatus may detect the degree of the adhesion of the discharged product onto the image carrier, giving priority to the simplification of the configuration of the apparatus according to the present invention. The current flows into the image carrier in the not-devised apparatus, with an attendant degradation of accuracy in comparison with the system illustrated in  FIG. 3 : in contrast, the configuration of the device can be simplified, with an attendant advantage of cost reduction or miniaturization. 
   Hereinafter, explanation will be made on an image forming apparatus including a simplified device which detects the degree of adhesion of a discharged product. 
     FIG. 7  is a diagram illustrating the general configuration of an image forming apparatus including a simplified device which detects the degree of adhesion of a discharged product. 
   In a configuration of an image forming apparatus  1000 ″ illustrated in  FIG. 7 , the same constituent elements as those in the image forming apparatus  1000 ′ illustrated in  FIG. 5  are designated by the same reference numerals, and therefore, a duplicate explanation will be omitted below. 
   In the image forming apparatus  1000 ″ illustrated in  FIG. 7 , a contact type charger  63  serves as a device for electrically charging an image carrier  61 , and further, functions as an electrode for measuring the magnitude of a current flowing toward the image carrier. At this point, the image forming apparatus  1000 ″ illustrated in  FIG. 7  is identical to the image forming apparatus  1000 ′ illustrated in  FIG. 5 . However, the electrode is only one, that is, the charger  63  in the image forming apparatus  1000 ″ illustrated in  FIG. 7 , unlike the image forming apparatus  1000 ′ illustrated in  FIG. 5 . 
     FIG. 8  is a diagram illustrating the schematic configuration of a detecting device for detecting a degree of adhesion of a discharged product, provided in the image forming apparatus  1000 ″ shown in  FIG. 7 . 
   In the configuration of a detecting device  66 ″ illustrated in  FIG. 8 , the same constituent elements as those in the detecting device  66 ′ illustrated in  FIG. 6  are designated by the same reference numerals. A charged voltage applying section  63   a  for applying an AC voltage, which is superimposed on a predetermined DC voltage, to the charger  63  in electrically charging the image carrier  61  in the detecting device  66 ″ illustrated in  FIG. 8  applies DC voltage for current measurement to the charger  63  in measuring the current flowing toward the image carrier. Here, a CPU  4  controls the charged voltage applying section  63   a.  The current flowing toward the image carrier  61  flows into a photosensitive base  612  through a photosensitive layer  611  in the detecting device  66 ″ illustrated in  FIG. 8 . Here, if a large quantity of discharged product adheres onto the image carrier  61 , the current is liable to flow along the surface of the image carrier  61 . Therefore, the current flowing in the image carrier  61  from the charger  63  flows into the photosensitive base  612  through the photosensitive layer  611  while being dispersed around the charger  63  in a lateral direction in  FIG. 8 . As a consequence, as the larger quantity of discharged product adheres onto the image carrier  61 , the larger quantity of current flows into the photosensitive base  612  through the photosensitive layer  611 . In other words, a passing cross-sectional area when the current flows the photosensitive layer  611  is enlarged, thereby decreasing an effective resistance of the photosensitive layer  611  acting as an electric resistance. Here, the magnitude of the current flowing into the photosensitive base  612  through the photosensitive layer  611  is measured by an ammeter  66   b.    
     FIG. 9  is a graph illustrating the relationship between the current flowing into the base through the photosensitive layer from the electrode placed on the image carrier and the degree of the image blurring. 
     FIG. 9  is a graph illustrating the relationship between the current and the degree (i.e., the level) of the image blurring when the degree (i.e., the level) of the image blurring is adopted as a variable on a lateral axis: in contrast, the magnitude of the current flowing into the base through the photosensitive layer from the electrode mounted on the carrier is adopted as a variable on a vertical axis.  FIG. 9  illustrates that the current flowing into the base through the photosensitive layer is increased more as the level of the image blurring becomes higher. As a result, it is found from the graph that the current flowing into the base through the photosensitive layer is increased more as the amount of the discharged product adhered onto the image carrier becomes increased. 
   The image blurring which is clearly observed as the deficiency of the quality of the image, as described in reference to  FIG. 2 , raises the problem from the viewpoint of the image formation when the image blurring becomes G 0  or higher at the point P on the graph. Therefore, the image blurring raises a problem in the case where the current becomes I 0  or higher at a point Q on the graph. 
   In view of this, the CPU  4  illustrated in  FIG. 8  judges whether or not the current measured by the ammeter  66   b  becomes a threshold I 0  or higher. If it is judged that the current measured by the ammeter  66   b  becomes the threshold I 0  or higher, the CPU  4  instructs the discharged product removing device  65  illustrated in  FIG. 7  (which is the same as that illustrated in  FIG. 4 ) to remove a discharged product. The removal of the discharged product is conducted in the same manner as described in reference to  FIG. 4 , and therefore, a duplicate explanation will be omitted below. 
   In other words, the detecting device illustrated in  FIG. 8  detects that the current flowing into the image carrier becomes I 0  or higher, thus indirectly detecting that the surface resistance of the image carrier becomes the threshold ρ 0  or smaller, as described above in reference to  FIG. 2 . Based on the detection result, the discharged product is removed. 
   The discharged product removing device  65  for wiping off the discharged product by the web is adopted in removing the discharged product in the image forming apparatuses  1000 ,  1000 ′ and  1000 ″. However, the cleaning device for removing the residual toner by the use of the cleaning blade also serves as the discharged product removing device according to the present invention. Therefore, there may be adopted a cleaning device for switching an abutment force in such a manner that the cleaning blade in removing the discharged product abuts against the image carrier with an abutment force stronger than that in removing the residual toner. 
   An image forming apparatus adopting such a cleaning device is identical in configuration to the image forming apparatus  1000  illustrated in  FIG. 1  except that there is no discharged product removing device  65  since the cleaning device for removing the residual toner serves as also the discharged product removing device. Explanation will be omitted on the same constituent elements, but will be made on the switch of the abutment force of the cleaning blade by the cleaning device. 
     FIGS. 10A and 10B  are diagrams illustrating a manner in which an abutment force of a cleaning blade is switched. 
     FIG. 10A  illustrates a state in which a cleaning blade  622  disposed in a cleaning device  62 ′ removes a residual toner. The cleaning blade  622  is a plate-like member which extends in a direction perpendicular to  FIGS. 10A and 10B  and is made of an urethane rubber, and further, is supported by a supporting member  621  at one plane and side surfaces of the plate.  FIGS. 10A and 10B  illustrate the cleaning blade  622 , as viewed sideways. The supporting member  621  receives a rotational drive force from a motor, not illustrated, and then, can be rotated on a rotary shaft  621   a  extending in the direction perpendicular to  FIGS. 10A and 10B . The cleaning blade  622  also is rotated together with the rotation of the supporting member  621 . The motor is controlled by a CPU (Central Processing Unit), not illustrated in  FIGS. 10A and 10B . The CPU controls the motor so as to rotate the cleaning blade  622  in a direction indicated by an arrow E in  FIG. 10A  in the case where a surface resistance measured by a surface resistance-measuring device is the threshold ρ 0  or smaller, as described above in reference to  FIG. 2 . 
   When the cleaning blade  622  is rotated in the direction indicated by the arrow E, an end of the cleaning blade  622  that abuts against the surface of an image carrier  61  (i.e., a right end in  FIG. 10A ) in a state illustrated in  FIG. 10A  is separated from the image carrier  61 , and then, the other end of the cleaning blade  622  in separation from the image carrier  61  (i.e., a left end in  FIG. 10A ) in the state illustrated in  FIG. 10B  abuts against the surface of the image carrier  61 .  FIG. 10B  illustrates a state after the end of the cleaning blade  622  that abuts against the surface of the image carrier  61  is switched in the above-described manner. A discharged product is removed in the state illustrated in  FIG. 10B . The CPU controls the drive force of the motor, not illustrated, in such a manner that an abutment force by the cleaning blade  622  against the surface of the image carrier  61  in removing the discharged product becomes larger than that in removing a residual toner. 
   The abutment end of the cleaning blade  622  abuts against the image carrier  61  in a direction (i.e., downward in  FIG. 10A ) opposite to a movement direction (i.e., a direction indicated by an arrow A in  FIG. 10A ) of the image carrier  61  in the state illustrated in  FIG. 10A . In contrast, the abutment end of the cleaning blade  622  abuts against the image carrier  61  in the same direction (i.e., upward in  FIG. 10B ) as the movement direction (i.e., the direction indicated by the arrow A in  FIG. 10A ) of the image carrier  61  in the state illustrated in  FIG. 10B . A tip of the cleaning blade  622  that abuts against the image carrier  61  slides on the image carrier  61  by hooking on the surface of the moving image carrier  61  in the state illustrated in  FIG. 10A , thus producing a more favorable effect in removing an unnecessary substance having a large particle diameter such as the residual toner adhering onto the image carrier  61  in comparison with that in the state illustrated in  FIG. 10B . However, a stick slip is liable to occur due to the elasticity of the cleaning blade  622  in the state illustrated in  FIG. 10B . As a consequence, an unnecessary substance having a small particle diameter such as the discharged product passes through the cleaning blade  622 , and therefore, the unnecessary substance having the small particle diameter cannot be satisfactorily removed in many cases. In contrast, the cleaning blade  622  hardly causes any stick slip so as to be suitable for the removal of the unnecessary substance having the small particle diameter such as the discharged product in the state illustrated in  FIG. 10B  in comparison with the state illustrated in  FIG. 10A . In view of this, the cleaning device  62 ′ performs the removal in the state illustrated in  FIG. 10A  in the case where the unnecessary substance to be removed is the residual toner, whereas the removal is performed in the state illustrated in  FIG. 10B  in the case where the unnecessary substance to be removed is the discharged product. Thus, both of the residual toner and the discharged product are satisfactorily removed. 
   Here, the abutment force of the cleaning blade  622  against the surface of the image carrier  61  at the time of the removal of the discharged product preferably should be 1.05 times or more and 1.20 times or less the abutment force at the time of the removal of the residual toner, and more preferably, 1.07 times or more and 1.15 times or less. Furthermore, in order to enhance the discharged product removing effect, an abrasive made of ceric oxide or the like may be dispersed in the tip of the cleaning blade that abuts against the surface of the image carrier  61  at the time of the removal of the discharged product. In this case, it is preferable to disperse the abrasive within a range from 10% or more to 20% or less by a volumetric content. 
   The removal of the discharged product is completed by predetermined times of rotations of the image carrier  61  in the state illustrated in  FIG. 10B . Upon the completion of the removal, the CPU controls the motor, so as to rotate the supporting member  621  in a direction indicated by an arrow F in  FIG. 10B . Consequently, the cleaning blade  622  is returned again to the abutment state in preparation for the removal of the residual toner, as illustrated in  FIG. 10A . 
   Hereinafter, a description will be given, on the basis of experiment results, of the discharged product which is removed by directly or indirectly detecting that the surface resistance of the image carrier becomes a predetermined threshold or less, thus actually avoiding any occurrence of the image blurring. 
   EXAMPLE 1 
   An image forming apparatus in Example 1 is configured in the same manner as the image forming apparatus  1000  illustrated in  FIG. 1 . In the image forming apparatus, an electrode formed by laminating elastic layers made of an epichlorohydrin rubber obtained by dispersing a conductive agent incorporating a quaternary ammonium salt or carbon black in a stainless base is used as each of three electrodes mounted on an image carrier. Moreover, the rotational speed of a web at the time of removal of a discharged product has a difference by about 0.5% from that of the image carrier. In the image forming apparatus, a common logarithm of a surface resistance as a threshold is 14 [log Ω] at the time of the removal of the discharged product, which is equivalent to 10 14  [Ω] of the surface resistance ρ 0  illustrated in  FIG. 2 . A job which sequentially outputs 50 sheets of a predetermined image is repeated 100 times by the use of the image forming apparatus in Example 1. In the image forming apparatus in Example 1, the surface resistance of the image carrier is detected upon the completion of each of the jobs. As a result, when the surface resistance became 10 14  [Ω] or less, the discharged product is removed by causing the web to abut against the surface of the image carrier. 
   An examination of the occurrence of image blurring on the image first output in each of the jobs revealed no occurrence of the image blurring which raised a problem from the viewpoint of the quality of the image. 
   EXAMPLE 2 
   An image forming apparatus in Example 2 is configured in the same manner as the image forming apparatus  1000 ″ illustrated in  FIG. 7 . In the image forming apparatus, the rotational speed of a web at the time of removal of a discharged product has a difference by about 0.5% from that of an image carrier. In the image forming apparatus, a current as a threshold is 0.4 [μA] at the time of the removal of the discharged product, which is equivalent to the current I 0  of 0.4 [μA] illustrated in  FIG. 2 . A job which sequentially outputs 50 sheets of a predetermined image is repeated 100 times in the same manner as in Example 1 by the use of the image forming apparatus in Example 2. In the image forming apparatus in Example 2, the current flowing into the image carrier is measured upon the completion of each of the jobs. As a result, when the current became 0.4 [μA] or higher, the discharged product is removed by causing the web to abut against the surface of the image carrier. 
   An examination of the occurrence of image blurring on the image first output in each of the jobs revealed no occurrence of the image blurring which raised a problem from the viewpoint of the quality of the image. 
   COMPARATIVE EXAMPLE 1 
   An image forming apparatus in Comparative Example 1 is configured in the same manner as the image forming apparatus  1000  illustrated in  FIG. 1  except that there are provided neither a device for measuring a surface resistance of an image carrier nor a discharged product removing device. In the image forming apparatus, a cleaning blade disposed in a cleaning device abuts against the surface of the image carrier all the time. A job which sequentially outputs 50 sheets of a predetermined image is repeated 100 times in the same manner as in Example 1 by the use of the image forming apparatus in Comparative Example 1. 
   As a result of an examination of the occurrence of image blurring on the image first output in each of the jobs, a deficiency of a quality of an image clearly regarded as image blurring is markedly observed particularly in the latter half of the 100 jobs. 
   The results of Example 1, Example 2 and Comparative Example 1 can conclude as follows: the degree of the adhesion of the discharged product onto the image carrier is examined upon the completion of the job, and then, the discharged product is wiped off in the case of the adhesion of the discharged product in a large quantity, so that the occurrence of the image blurring can be effectively avoided, like in Example 1 and Example 2. 
   Incidentally, although the image forming apparatuses, described above, are monochromatic one-sided output printers, the image forming apparatus according to the present invention may be applied to a monochromatic double-sided output printer, a full-color one- or double-sided output printer, or a facsimile.