Patent Publication Number: US-11650515-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an image forming apparatus. 
     Description of the Related Art 
     In an image forming apparatus, an image is formed by developing an electrostatic latent image formed on the surface of an image bearing member using developer on a developer bearing member. In this image formation, a configuration of a contact developing system, which develops the image in the state of the developer bearing member contacting the image bearing member, is known. For the developer bearing member having such a configuration, a developing roller, having an elastic layer on an outer peripheral surface of a shaft center which is rotary-driven, is used. 
     On the other hand, in order to downsize the image forming apparatus and to reduce cost by minimizing members, an image bearing member cleanerless image forming apparatus, which does not include a cleaning unit to remove and collect toner remaining on the image bearing member, is proposed. 
     In the case of the cleanerless type image forming apparatus, untransferred toner and toner causing fogging directly enter between the photosensitive drum and the charging roller, hence stress may be generated between the photosensitive drum and the charging roller, and melted toner may adhere to the photosensitive drum. If melted toner is adhered, exposure may be interrupted, and blank dots may be generated in the image. Further, if the melted portions are adhered continuously to the photosensitive drum in the circumferential direction, white streaks may appear in the image. 
     In the cleanerless system, the developing portion needs to collect the toner that was not transferred to form the image, however the toner that melted and adhered to the photosensitive drum cannot be removed and collected unless a strong force is applied in the developing portion. Normally the developing roller is covered with a toner layer, but if the surface layer of the developing roller is exposed from the toner layer, the photosensitive drum can be scraped more strongly, whereby toner on the photosensitive drum can be removed. 
     Japanese Patent No. 4669557 discloses an apparatus that includes a reset mode, in which the difference of the apply voltage between a developing roller and a toner supply roller is reduced than that during printing, so as to change the toner amount on the developing roller when an image is not formed. In the apparatus disclosed in Japanese Patent No. 4669557, the toner amount on the developing roller is decreased by reducing the apply voltage difference between the developer bearing member and the toner supply roller in the reset mode, so that such a problem as fogging is not generated when the toner amount on the developing roller is increased. Thereby the toner amount on the developing roller can be maintained at a constant level for a long period. 
     SUMMARY OF THE INVENTION 
     However, Japanese Patent No. 4669557 does not disclose the difference of the peripheral speed between the developing roller and the photosensitive drum, and an object thereof is not to scrape off the melted toner adhered to the photosensitive drum. Therefore if the method disclosed in Japanese Patent No. 4669557 is directly applied, the melted toner adhered to the photosensitive drum may not be sufficiently removed, or the photosensitive drum may be excessively rubbed by the developing roller, whereby the developer may deteriorate more quickly. 
     With the foregoing in view, it is an object of the present invention to prevent the adhesion of melted toner to the photosensitive drum of the image forming apparatus, so as to acquire good image quality. 
     The present invention provides an image forming apparatus comprising: 
     an image bearing member configured to be rotatable; 
     an exposure unit configured to expose a surface of the image bearing member to form an electrostatic latent image on the surface of the image bearing member; 
     a developing member configured to be rotatable, which contacts with the image bearing member to form a developing portion, and supplies developer to the surface of the image bearing member in the developing portion, and which rotates at a rotation speed different from a rotation speed of the image bearing member; 
     a developer supply member configured to supply developer to the surface of the developing member; 
     a developing voltage-applying unit configured to apply developing voltage to the developing member; 
     a supply voltage-applying unit configured to apply supply voltage to the developer supply member; and 
     a control unit configured to control the developing voltage-applying unit and the supply voltage-applying unit, wherein 
     the control unit controls execution of: an image forming operation in which a developer image is formed on the surface of the image bearing member by supplying the developer from the developing member to the electrostatic latent image formed on the surface of the image bearing member; and a cleaning operation, which is an operation other than the image forming operation and which removes a substance adhered to the surface of the image bearing member, using the developing member, 
     in the case of executing the cleaning operation, the control unit controls at least one of the developing voltage-applying unit and the supply voltage-applying unit so that the cleaning force to clean the substance adhered to the surface of the image bearing member using the developing member becomes stronger than that in the case of executing the image forming operation, and 
     in the case of executing the cleaning operation, the control unit controls at least one of the developing voltage-applying unit and the supply voltage-applying unit so that the supply amount of the developer supplied from the developer supply member to the developing member decreases compared with the case of executing the image forming operation. 
     The present invention also provides an image forming apparatus comprising: 
     an image bearing member configured to be rotatable; 
     an exposure unit configured to expose a surface of the image bearing member to form an electrostatic latent image on the surface bearing member; 
     a developing member configured to be rotatable, which contacts with the image bearing member to form a developing portion, and supplies developer to the surface of the image bearing member in the developing portion, and which rotates at a rotation speed different from a rotation speed of the image bearing member; 
     a developer supply member configured to supply developer to the surface of the developing member; 
     a developing voltage-applying unit configured to apply developing voltage to the developing member; 
     a supply voltage-applying unit configured to apply supply voltage to the developer supply member; and 
     a control unit configured to control the developing voltage-applying unit and the supply voltage-applying unit, wherein 
     the control unit controls to be capable of executing: an image forming operation in which a developer image is formed on the surface of the image bearing member by supplying the developer from the developing member to the electrostatic latent image formed on the surface of the image bearing member; and a cleaning operation, which is an operation other than the image forming operation and which removes a substance adhered to the surface of the image bearing member, using the developing member, 
     in the case of executing the cleaning operation, the control unit controls at least one of the developing voltage-applying unit and the supply voltage-applying unit so that the cleaning force to clean the substance adhered to the surface of the image bearing member using the developing member becomes stronger than that in the case of executing the image forming operation, and 
     the control unit is capable of changing the cleaning force in the cleaning operation. 
     According to the present invention, the adhesion of melted toner on the photosensitive drum of the image forming apparatus can be prevented, so as to acquire good image quality. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic configuration diagram depicting an example of an image forming apparatus of Example 1; 
         FIGS.  2 A to  2 C  are diagrams depicting an example of a developing roller used for the image forming apparatus of Example 1, and a scraping operation thereof; 
         FIG.  3    is a diagram indicating a melt adhesion scraping control that is used for the image forming apparatus of Example 1; and 
         FIG.  4    is a diagram depicting control blocks of the image forming apparatus of Example 1. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention will be described in detail using examples, with reference to the drawings. The dimensions, materials, shapes and relative positions of the components described in the embodiments may be changed appropriately in accordance with the configuration and various conditions of the apparatus to which the present invention is applied. In other words, these particulars are not intended to limit the scope of the invention to the following embodiments. 
     EXAMPLE 1 
     Overview of Image Forming Apparatus 
     A general configuration of an electrophotographic image forming apparatus (hereafter “image forming apparatus”) according to Example 1 of the present invention and an image forming operation thereof will be described with reference to  FIG.  1   .  FIG.  1    is a schematic cross-sectional view depicting a general configuration of the image forming apparatus  100  according to the embodiment of the present invention. 
     In the present embodiment, image forming stations (image forming units) for four colors (yellow, magenta, cyan and black) are disposed from left to right in  FIG.  1   . Each image forming stations has an electrophotographic image forming mechanism with identical configuration, except that the color of the developer (hereafter “toner”)  90  contained in the respective developing apparatus is different. In the following description, the suffixes Y (yellow), M (magenta), C (cyan) and K (black), which are attached to each reference sign to indicate the color of the element, are omitted unless distinction is especially needed. 
     As a major configuration, each image forming station includes a photosensitive drum  1  (image bearing member), a charging roller  2  (charging unit), a developing apparatus  4 , a primary transfer apparatus  51 , and the like. An exposure apparatus  3  may be shared by each image forming station, or may be disposed in each image forming station. In the present embodiment, the photosensitive drum  1 , the charging roller  2  and the developing apparatus  4  are integrated as a process cartridge  8 , which is configured to be detachable from the image forming apparatus main unit (portion of the image forming apparatus  100  when the process cartridge  8  is removed). The process cartridge of the present invention, however, may include at least the photosensitive drum  1  and the developing apparatus  4 , which are integrally detachable from the apparatus main unit. The developing apparatus  4  may be configured to be independently detachable from the apparatus main unit or from the process cartridge  8 . Further, the photosensitive drum  1  and the developing apparatus  4  may be fixed to the image forming apparatus main unit so that replacement by the user is unnecessary. 
     The photosensitive drum  1  is a rotatable cylindrical photosensitive member, and rotates around the shaft thereof in the arrow direction (counterclockwise in  FIG.  1   ). In the present embodiment, the outer peripheral surface thereof is rotary-driven at a 180 mm/sec rotation speed. For example, as indicated in  FIG.  4   , the photosensitive drum  1  is rotated by the rotary driving force of a motor  15 , which is controlled by a control unit  25 . The surface of the photosensitive drum  1  is uniformly charged by the charging roller  2 . In the present embodiment, the charging roller  2  is a conductive roller having a conductive rubber layer formed on a core metal. The charging roller  2  is contacted with the photosensitive drum  1  in parallel at a predetermined pressure, and rotates as the photosensitive drum  1  rotates. A charging voltage can be applied to the charging roller  2  from a power supply device  67  (power supply unit). In the present embodiment, the photosensitive drum  1  is charged by applying a −1150V DC voltage to the charging roller  2 , and the surface potential of the photosensitive drum  1  in this case is about −500V. 
     The exposure apparatus  3  (exposure unit) acquires an image signal from a control unit  65 , and the surface of the photosensitive drum  1  is scanned with a laser light in accordance with the image signal. Thereby an electrostatic latent image in accordance with the image signal is formed on the charged photosensitive drum  1 . The image signal may be acquired from an external information processor (not illustrated). For the control unit  65 , an information processor, such as a control circuit which includes arithmetic resources (e.g. processor, memory), can be used, for example. 
     The developing apparatus  4  supplies toner  90  to the electrostatic latent image on the photosensitive drum  1 , so that the electrostatic latent image becomes visible as a toner image (developer image). In the present embodiment, the developing apparatus  4  is a contact-developing type reversal-developing apparatus that contains toner  90  as a single component developer having a negative normal charging polarity (charging polarity for developing the electrostatic latent image). 
     The developing apparatus  4  includes a rotatable developing roller  42  (developing member), a toner supply roller  43  (developer supply member), and a regulating blade  44  (developer-regulating member). The toner supply roller  43  is an elastic sponge roller, in which a foam member is disposed on an outer periphery of the conductive core metal. The toner supply roller  43  is disposed so as to contact the developing roller  42  with a predetermined degree of pressure. The toner  90 , which is supplied by the toner supply roller  43  and is held by the developing roller  42 , has a layer thickness regulated by the regulating blade  44 , and is provided for development processing. Here the regulating blade  44  not only has the function to regulate the layer thickness of the toner  90  on the developing roller  42 , but also has a function of a developer charging unit that provides predetermined charges to the toner  90  on the developing roller  42 . 
     The power supply device  67  can function as: a developing voltage-applying unit  671  that applies developing voltage to the developing roller  42  included in the developing apparatus  4 ; a supply voltage-applying unit  672  that applies supply voltage to the toner supply roller  43 ; and a regulating voltage-applying unit that applies the regulating voltage to the regulating blade  44 .  FIG.  4    indicates an example of a control system of the power supply device  67  related to the control by the control unit  25 . For the power supply device  67 , a charging power supply for the charging roller and a developing power supply for the developing apparatus may be independently disposed. In this case, collectively the charging power supply and the developing power supply may be regarded as the power supply unit. Further, the developing power supply for the developing roller and the power supply for the toner supply roller may be independently disposed. In this case, collectively the charging power supply, the developing power supply and the power supply for the toner supply roller may be regarded as the power supply unit. The power supply device  67  may be used for applying power when image transfer is performed. A transfer power supply may be disposed independently from the power supply device  67 . The power supply device  67  of the present embodiment changes the voltage to be applied to each composing element in accordance with the control of the control unit  65 . 
     The developing roller  42  is rotary-driven in the arrow direction (clockwise in  FIG.  1   ), so that the surfaces of the developing roller  42  and the photosensitive drum  1  move in the same direction. For example, as indicated in  FIG.  4   , the developing roller  42  and the toner supply roller  43  are rotated by the rotary-driving force generated by a motor  16  that is controlled by the control unit  25 . The developing roller  42  can rotate at a rotating speed that is different from the photosensitive drum  1 . In the present embodiment, in order to acquire an appropriate image density, the developing roller  42  is rotary-driven such that the moving speed of the surface of the developing roller  42  is faster than the moving speed of the surface of the photosensitive drum  1 . The developing apparatus  4  is pushed toward the photosensitive drum  1  by an energizing means (not illustrated), and as a result, the developing roller  42  is pressed against the photosensitive drum  1 . Thereby the surface of the developing roller  42  is deformed, and a developing nip (developing portion) is formed, so as to perform stable development in a stable contact state. 
     As illustrated in  FIG.  2 A , the developing roller  42  is constituted of a base layer  422  and a surface layer  423 , which are formed on the outer periphery of the shaft center  421 . Further, as illustrated in  FIG.  2 B , the surface layer  423  has a structure where coarse particles  423   b  are dispersed in a surface layer bonding resin  423   a . Therefore a plurality of depressed portions and a plurality of protruded portions are formed on the surface of the surface layer  423  for transporting the toner. The ten-point average roughness RzJIS of the protruded portions is larger than the volume average particle size of the toner  90 , and in the case of the present embodiment, the volume average particle size of the toner is 7 μm, and RzJIS of the surface layer  423  is 10 μm. The range of the RzJIS of the surface layer of the developing roller is preferably about 8 μm to 30 μm. 
     To measure the ten-point average roughness RzJIS of the developing roller  42  of the present invention, a contact type surface roughness meter, SurfCoder SE3500 (made by Kosaka Laboratory), can be used, for example. The measurement conditions used here are: a cut-off value of 0.8 mm; a measurement length of 2.5 mm; and a feed speed of 0.1 mm/sec. For each developing roller, three arbitrary prints, which are disposed at different locations in the longitudinal direction, are measured, and the average of the acquired measurement values is regarded as the RzJIS of the developing roller  42 . 
     For the volume average particle size of the toner  90 , a measurement value measured by the following method can be used. For the measurement device, the Coulter Multi-Sizer IV (made by Beckman Coulter) can be used. For the electrolytes, a high grade sodium chloride dissolved in deionized water to about 1% by mass, such as ISOTON II (made by Beckman Coulter), can be used. For the measurement method, 0.5 ml alkylbenzene sulfonic acid is added to a 100 ml electrolyte solution as the dispersant, and 10 mg of measurement sample is added thereto. Then dispersing processing is performed for 1 minute on electrolytes, in which the measurement sample is suspended, the volume particle size distribution is measured using a 30 μm aperture of the measurement device, and the median value (D50) of the measurement results is regarded are the volume average particle size. 
     The toner image formed on the photosensitive drum  1  is electrostatically transferred to an intermediate transfer belt  53  by the primary transfer apparatus  51 , which is one of the transfer members. Each color of the toner image is sequentially transferred onto the intermediate transfer belt  53 , so as to be superimposed with each other, whereby a full color toner image is formed. The full color toner image is then transferred to a recording material (transferred member) by a secondary transfer apparatus  52 , which is another transfer member, and is different from the primary transfer apparatus  51 . Then the toner image on the recording material is pressed and heated by a fixing apparatus  6 , whereby the toner image is fixed to the recording material, and the recording material is discharged as an image formed product. 
     In the moving direction of the intermediate transfer belt  53 , a belt cleaning device  7  is disposed on the downstream side of the secondary transfer apparatus  52 , so as to remove and collect untransferred toner  90  remaining on the intermediate transfer belt  53 . 
     In the present embodiment, an image bearing member-cleanerless system is used, in which untransferred toner  90  remaining on the surface of the photosensitive drum  1  is collected by the developing apparatus  4 , without disposing a dedicated cleaner device for the photosensitive drum  1 . This means that there is no member that contacts with the photosensitive drum  1 , until the surface of the photosensitive drum  1 , which passed the position facing the primary transfer apparatus  51  (primary transfer position), reaches the contact position with the charging roller  2  (charging position). Thereby when the developing roller  42  of the developing apparatus  4  is contacted with the photosensitive drum  1 , untransferred toner  90  remaining on the photosensitive drum  1  after an image is formed, can be collected by the developing apparatus  4 . In the case of using such a cleanerless system, it is preferable to use a non-magnetic single component developer for the toner  90 . However, the present invention is not limited to the above mentioned configuration to implement the effect of the invention. 
     The image forming apparatus  100  includes a detection device (not illustrated) to detect a printing state and a printing environment. The control unit  65  can execute an image forming operation or a later mentioned scraping operation, which is an operation other than the image forming operation, in accordance with the detected printing state and printing environment. 
     Control 
     An operation of the image forming apparatus according to the present invention will be described. When an image is formed, −300V is applied to the developing roller  42 . On the other hand, −400V is applied to the regulating blade  44 , and −400V is also applied to the toner supply roller  43 . Since the charging polarity of the toner  90  of the present invention is a negative charging polarity, toner can be easily supplied from the toner supply roller  43  to the developing roller  42 . 
     In the photosensitive drum  1 , it is controlled such that the surface potential of an image printing portion where a toner image is formed is on the side of the normal charging polarity of the toner  90 , and the absolute value of the surface potential becomes lower than the applied voltage to the developing roller  42 . In a non-image printing portion where a toner image is not formed, on the other hand, the surface potential is controlled to the drum potential of −500V. Thereby in the image printing portion, toner, charged by the potential difference from the developing roller  42 , is developed. 
     The toner image developed on the photosensitive drum is transferred to the intermediate transfer belt  53  in a primary transfer portion formed by the primary transfer apparatus  51 , but toner with a low charging level or toner charged with an opposite polarity of normal charging polarity, is not transferred and enters between the charging roller  2  and the photosensitive drum  1 . Toner causing fogging also enters between the charging roller  2  and the photosensitive drum  1 . 
     As a result, stress may be applied to the toner between the charging roller  2  and the photosensitive drum  1 , and the toner may be deformed and adhere to the photosensitive drum  1 . In a portion of the photosensitive drum where toner adheres (indicated by the X sign), transferability is lower in the next image formation, which means that untransferred toner is even more easily generated, and grows as an adhered substance on the photosensitive drum. 
     On the portion of the photosensitive drum where melted toner adheres, a laser beam emitted from the exposure apparatus  3  is blocked, hence the surface potential of the photosensitive drum  1  does not reach a predetermined potential, and the image density drops in the solid printing portion. Especially if the melted toner is continuously adhered in the rotating direction of the photosensitive drum, white streaks are generated in the image. To prevent this, it is necessary to remove toner remaining on the photosensitive drum. 
     Melt Adhesion Scraping Operation 
     An operation to prevent the growth of the melt adhered substance on the drum according to the present embodiment will be described. Each voltage, in the image forming operation, is as follows. 
     Apply voltage V_DEV to developing roller  42 : −300V 
     Apply voltage V_S to the toner supply roller  43 : −400V 
     In other words, the absolute value of the developing voltage is smaller than the absolute value of the supply voltage in the image forming operation. Therefore the difference of the apply voltages between the toner supply roller  43  and the developing roller  42  is −100V. In the present embodiment, the normal charging polarity of the toner is negative, hence the potential difference V_DIFF that is generated as the driving force supplied from the toner supply roller  43  to the developing roller  42  is as follows. 
     Potential difference V_DIFF: +100V 
     When the image forming operation on the intermediate transfer belt  53  ends for each printing job, the control unit  65  changes the apply voltage V_S to the toner supply roller  43  from −400V, which is a value used for image formation, to −350V, and allows the toner supply roller  43  to rotate for a predetermined time. Thereby the potential difference V_DIFF changes from 100V to 50V, that is, the pressing force on the toner from the toner supply roller  43  to the developing roller  42  becomes weaker than the time of image formation, and the toner amount on the developing roller decreases compared to the time of image formation. In other words, the potential difference in the case of the scraping operation is smaller than in the case of the image forming operation. Hereafter, this control to decrease the potential difference V_DIFF to be smaller than the case of image formation, so that the developing roller  42  and the photosensitive drum  1  contact and rotate in this state, is referred to as the “scraping operation”. The “scraping operation” referred to here is given as a preferable example of the “cleaning operation” for cleaning the toner on the photosensitive drum  1 . 
     For the potential relationship, the apply voltage to the toner supply roller  43  may be set to −250V when the scraping is performed, so that the potential difference V_DIFF becomes −50V, and the toner can be returned from the developing roller  42  to the toner supply roller  43  side. When the image forming operation is performed, the absolute value of the developing voltage may be set to be smaller than the absolute value of the supply voltage, and when the scraping operation is performed, the potential difference of the developing voltage and the supply voltage may be set to zero. Or when the image forming operation is performed, the absolute value of the developing voltage may be set to the absolute value of the supply voltage or less, and when the scraping operation is performed, the absolute value of the developing voltage may be set to be larger than the absolute value of the supply voltage. 
     In the case where the normal charging polarity of the toner is positive, the apply voltage to the developing roller  42  may be set to +300V and the apply voltage to the toner supply roller  43  may be set to +400V when the image forming operation is performed, and the apply voltage to the toner supply roller  43  may be changed to +350V when the scraping operation is performed, for example. 
     The end of the image forming operation on the intermediate transfer belt  53  refers to the timing at which the rear end of the image at each image forming station is transferred to the intermediate transfer belt. In the present embodiment, the scraping operation is simultaneously started at all the image forming stations Y, M, C and K to simplify the control. For this reason, the timing of the change is a timing at which the rear end of the image at the image forming station K, which is located on the far downstream side, is transferred to the intermediate transfer belt  53 . However, the scraping operation may be started sequentially from the image forming station located upstream of which image forming operation ended. 
       FIG.  3    is a time chart in the case where the scraping operation of each image forming station is started simultaneously. The abscissa indicates the elapse of time. The ordinate indicates the absolute value of voltage, and indicates the apply voltage V_DEV to the developing roller  42 , the apply voltage V_S 1  to the toner supply roller  43  in image formation, and the apply voltage V_S 2  to the toner supply roller  43  in the scraping. 
     The image forming operation starts at timing t 1 , and the apply voltage to the toner supply roller  43  is set to V_S 1 . Then at the timings t 2 _ 1  to t 2 _ 4 , formation of the Y image, M image, C image and K image sequentially end. At timing t 2 _ 4 , the scraping operation simultaneously starts at each image forming station, and the apply voltage to the toner supply roller  43  is changed to V_ 2 . At timing t 3 , when a predetermined scraping time has elapsed, the scraping operation ends. Then the printing end operation starts, and the printing end operation ends at t 4 . 
     Here the meaning of the scraping operation will be described. In the case where the apply voltage V_S to the toner supply roller  43  in the image formation is on the supply side (in the case of apply voltage difference V_DIFF&gt;0), as in the case of the present embodiment, it becomes more difficult for the toner to be supplied to the developing roller  4  and the supply amount decreases if the apply voltage V_S to the toner supply roller  43  in the scraping operation becomes closer to the apply voltage of the developing roller  42  than to the apply voltage V_S in the image formation. As a result, the amount of toner on the developing roller decreases, and the depressed/protruded portions on the surface of the developing roller  42  directly contact the photosensitive drum  1  more easily. 
     For example, if the apply voltage V_S to the toner supply roller  43  is changed from −400V to −350V, the toner on the developing roller decreases, and a part of the surface layer of the developing roller is exposed from the toner coated on the developing roller  42  after the regulating blade  44  passes.  FIG.  2 B  indicates a state immediately before the portion (exposed portion P in  FIGS.  2 B and  2 C ), which is exposed from the toner coating the protruded portions of the surface layer of the developing roller, and the protruded portions scrape off the substance X that melted and adhered to the photosensitive drum.  FIG.  2 C  indicates a state immediately after the exposed portion P scraped most of the substance X that melted and adhered to the photosensitive drum. Here when the peripheral speed of the developing roller  42  is V 1  and the peripheral speed of the photosensitive drum  1  is V 2 , V 1  and V 2  have a peripheral speed difference, and is V 1 &gt;V 2  in the present embodiment. In this way, the substance that melted and adhered to the photosensitive drum can be scraped off if the portion exposed from the toner coating on the surface layer of the developing roller contacts with the photosensitive drum  1 , and rotates at a peripheral speed different from the peripheral speed of the photosensitive drum  1 . This operation of the developing roller  42  scraping and removing the toner  90  on the photosensitive drum  1  is referred to as the “scraping operation”. Hereafter, a capability of scraping the toner  90  on the photosensitive drum  1  in the scraping operation is referred to as the “scraping force”. The “scraping force” referred to here is given as a preferable example of the “cleaning force” when cleaning the toner on the photosensitive drum  1 . The control unit can increase the scraping force to increase the amount of the toner to be scraped, or decrease the scraping force to decrease the amount of the toner to be scraped, by controlling voltage to be applied to each composing element, for example, as described later. 
     In the present embodiment, the apply voltage V_S to the toner supply roller  43  in the scraping operation is changed to −350V for all the image forming stations, but the apply voltage V_S may be controlled for each image forming station. 
     Further, in the present embodiment, the apply voltage V_S to the toner supply roller, that is, −400V in the image formation, is changed to −350V in the scraping. However, the present invention is not limited to this, and it is sufficient if the exposed portions P are formed by performing control to decrease the potential difference V_DIFF. For example, the apply voltage V_DEV to the developing roller  42  may be changed while the apply voltage V_S to the toner supply roller  43  is fixed, or both the apply voltage V_S to the toner supply roller  43  and the apply voltage V_DEV to the developing roller  42  may be changed. 
     Then in the state where the apply voltage V_S to the toner supply roller  43  is lowered than in the image formation, the developing roller  42  and the photosensitive drum  1  are contacted and rotated to perform the scraping operation for a predetermined time, and then the developing roller  42  is separated from the photosensitive drum  1 , and the printing end operation is performed. 
     Details of Example 1 
     As indicated in Table 1, Examples 1-1 to 1-5 and Comparative Examples 1-1 to 1-3 are the results when a printing operation was performed for 6000 prints as follows, with combining various printing conditions, then formed solid black images were evaluated. The peripheral speed difference between the peripheral speed V 1  of the surface of the developing roller and the peripheral speed V 2  of the surface of the photosensitive drum is 70 mm/s. 
     Printing environment (room temperature): 15° C. to 30° C. 
     Number of prints per job: 1 print/2 prints 
     Apply voltage V_S to toner supply roller  43 : −400V/−350V 
     Apply voltage change time (scraping time after each job): 1.1 s to 6 s 
     For the image quality, the result of determining the degree of white streaks caused by melt adhesion is indicated in “Melt adhesion white streak result” in Table 1. Here ∘ indicates no problem, A indicates minor white streaks causing no practical problem, and x indicates white streaks causing a problem. The scraping force can be increased if the scraping time is extended, but extending the scraping time without limit accelerates the deterioration of toner in contact portions between the regulating blade  44  or the developing roller  42  and the photosensitive drum  1 , and generates unevenness in the solid black images in the latter half of the durability test. This result is indicated in “Development deterioration result” in Table 1. Here ∘ indicates no problem, and x indicates that development deterioration is observed. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                   
                   
                 Comparative 
                 Comparative 
                 Comparative 
               
               
                   
                 Example 
                 Example 
                 Example 
                 Example 
                 Example 
                 Example 
                 Example 
                 Example 
               
               
                   
                 1-1 
                 1-2 
                 1-3 
                 1-4 
                 1-5 
                 1-1 
                 1-2 
                 1-3 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Printing 
                 15° 
                 C. 
                 15° 
                 C. 
                 15° 
                 C. 
                 23° 
                 C. 
                 30° 
                 C. 
                 15° 
                 C. 
                 15° 
                 C. 
                 15° 
                 C. 
               
               
                 environment 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Number of prints 
                 1 
                 2 
                 2 
                 2 
                 2 
                 1 
                 2 
                 2 
               
               
                 per job 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Apply voltage to 
                 −350 
                 V 
                 −350 
                 V 
                 −350 
                 V 
                 −350 
                 V 
                 −400 
                 V 
                 −350 
                 V 
                 −400 
                 V 
                 −350 
                 V 
               
               
                 supply roller 
               
               
                 Apply voltage 
                 1.8 
                 s 
                 2.5 
                 s 
                 1.8 
                 s 
                 1.6 
                 s 
                 1.1 
                 s 
                 1.5 
                 s 
                 2.5 
                 s 
                 6 
                 s 
               
               
                 change time 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Melt adhesion 
                 ∘ 
                 ∘ 
                 Δ 
                 ∘ 
                 ∘ 
                 x 
                 x 
                 ∘ 
               
               
                 white streak 
               
               
                 result 
               
               
                 Development 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
               
               
                 deterioration 
               
               
                 result 
               
               
                   
               
            
           
         
       
     
     Description of Examples 1-1 to 1-3 
     In Example 1-1, a number of prints per job is 1, apply voltage V_S to the toner supply roller  43  is −350V, and scraping time is 1.8 seconds. In Example 1-2, a number of prints per job is 2, and scraping time is 2.5 seconds. In Example 1-3, a number of prints per job is 2, and scraping time is 1.8 seconds. The printing environment room temperature of the Examples 1-1 to 1-3 is 15° C. 
     According to the comparison of Example 1-1 and Example 1-3, minor blank dots are generated if a number of prints per job is high, even if scraping time is the same. This is because the interval between the scraping operations increases, and more melted and adhered substance accumulated. In the case where the number of prints per job is 2, the growth of the melted and adhered substance can be suppressed if the scraping time is increased to 2.5 seconds or more, as indicated in Example 1-2. 
     Description of Examples 1-4 and 1-5 
     Now a case where the specified printing environment is relatively high temperature will be considered. In Example 1-4, the apply voltage change time is set to 1.6 seconds when the specified printing environment room temperature is 23° C. In Example 1-5, the specified printing environment room temperature is 30° C. In the case of Example 1-5, when the scraping is performed after the image formation, constant rotation is maintained without changing the apply voltage V_S to the toner supply roller  43  from that at the image formation, and the scraping time is set to 1.1 seconds. 
     According to the comparison of Examples 1-3 to 1-5, the scraping time required for suppressing melt adhesion becomes long if the temperature in the printing environment becomes low. This is because in a low temperature environment, deformation followability of the charging roller is reduced and stronger stress is applied to the toner, which makes toner deformed and melt adhesion more easily occurs. In Example 1-5 in which temperature is especially high, on the other hand, white streaks can be suppressed with a short scraping time, even if the apply voltage V_S to the toner supply roller  43  is not changed. 
     Description of Comparative Example 1-1 
     Each comparative example will now be described. In Comparative Examples 1-1 to 1-3, the specified printing environment room temperature is 15° C. In Comparative Example 1-1, the scraping time is 1.5 seconds, and the rest of the conditions are the same as Example 1-1. A comparison of Example 1-1 and Comparative Example 1-1 indicates that the generation of melt adhesion can be suppressed if the scraping time is increased. This is because the distance in which the developing roller  42  and the photosensitive drum  1  rub each other increases in a state where the apply voltage difference V_DIFF between the developing roller  42  and the toner supply roller  43  is decreased, and the effect of scraping the melted substance adhered to the photosensitive drum can be increased. In order to scrape each portion of the photosensitive drum  1  in the circumferential direction, at least a time for the photosensitive drum  1  to make one rotation is required for the scraping time for one scraping operation. 
     Description of Comparative Example 1-2 
     In Comparative Example 1-2, a number of prints per job is 2, the apply voltage V_S to the toner supply roller  43  is the same as that in the image formation, and the scraping time is 2.5 seconds, which is the same as Example 1-2. Comparison of Example 1-2 and Comparative Example 1-2 indicates that white streaks, due to melt adhesion, are generated if the apply voltage V_S to the toner supply roller  43  in the scraping is not changed from that in the image formation. 
     Description of Comparative Example 1-3 
     In Comparative Example 1-3, a number of prints per job is 2, and the scraping time is 6 seconds. The scraping force is sufficient but the developing rotation time per job becomes long, hence the time when the toner is rubbed inside the developing portion and at the contact portion of the developing roller  42  and the photosensitive drum  1  becomes long. As a result, the toner deteriorates and unevenness is generated in the solid black image. 
     As described above, in a state where the photosensitive drum  1  and the developing roller  42  have a peripheral speed difference, the voltages to be applied to the developing roller  42  and the toner supply roller  43 , the scraping time (contact-rotation time) and other conditions, are set in accordance with the printing environment and the conditions of the printing job, then the scraping operation is performed, thereby the portions exposed from the toner coat on the surface of the developing roller can remove the melted substance that adhered to the surface of the photosensitive drum. 
     Example 2 
     Example 2 will be described. Description is omitted for portions that same as Example 1. In Example 2, when a number of prints per job is high, the apply voltage V_S to the toner supply roller  43  is changed in order to increase the scraping force. Specifically, when the scraping operation is performed, the potential difference V_DIFF of the apply voltage between the developing roller  42  and the toner supply roller  43  in the direction of pressing toner from the toner supply roller  43  to the developing roller  42  is decreased as a number of prints per job is higher. For example, when the potential difference V_DIFF in the image formation is +100V, V_DIFF is set to +50V if the number of prints per job is 2, and is set to 0V if the number of prints per job is 3. As the potential difference V_DIFF decreases, the toner amount on the developing roller decreases, and the depressed/protruded portions on the surface of the developing roller  42  are exposed even more, that means that the scraping force increases. 
     The test result of Example 2 is indicated in Table 2. In Example 2, the scraping time is a constant 1.8 seconds. The peripheral speed difference between the surface of the developing roller and the surface of the photosensitive drum is 70 mm/s. The specified printing environment room temperature is 15° C. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                 Com- parative 
               
               
                   
                 Example 
                 Example 
                 Example 
                 Example 
               
               
                   
                 2-1 
                 2-2 
                 2-3 
                 2-1 
               
               
                   
               
             
            
               
                 Number of prints per job 
                 2 
                 2 
                 3 
                 3 
               
               
                 Apply voltage to  supply roller 
                 −350 V 
                 −300 V 
                 −300 V 
                 −350 V 
               
               
                 Melt adhesion  white streak result 
                 Δ 
                 ◯ 
                 Δ 
                 X 
               
               
                   
               
            
           
         
       
     
     Details of Example 2 
     In Example 2-1, a number of prints per job is 2, the apply voltage to the toner supply roller  43  at the scraping is set to −350V, and the scraping operation is performed for 1.8 seconds after each job ends (same conditions as Example 1-3). In Example 2-2, the apply voltage to the toner supply roller in the scraping is set to −300V. In Example 2-3, a number of prints per job is 3, and the apply voltage in the scraping is −300V. 
     Details of Comparative Example 2 
     In Comparative Example 2-1, a number of prints per job is 3, and the apply voltage to the toner supply roller  43  in the scraping is set to −350V. Compared with Example 2-1, a number of prints per job is higher. 
     Comparison of Example 2 and Comparative Example 2 
     As a comparison of Example 2-1 and Example 2-2 indicates, a minor melt adhesion occurs to the image in the case of V_S=−350V (potential difference V_DIFF=+50V), but no melt adhesion occurs to the image in the case of V_S=−300V (potential difference V_DIFF=0V). By decreasing the potential difference V_DIFF generated between the developing roller  42  and the toner supply roller  43 , the scraping force can be further increased. 
     As the comparison of Example 2-1 and Example 2-3 indicates, if the apply voltage V_S to the toner supply roller  43  in the scraping is changed from −350V to −300V to increase the scraping force, the minor melt adhesion state can be maintained for the images even if the number of prints per job is increased from 2 to 3. 
     On the other hand, the comparison of Example 2-1 and Comparative Example 2-1 indicates that if the apply voltage V_S to the toner supply roller  43  in the scraping is not changed from −350V, the white streaks on the image due to melt adhesion worsen when the number of prints per job is increased from 2 to 3. 
     Example 3 
     Example 3 will be described. Description is omitted for portions the same as Examples 1 and 2. In Example 3, when a number of prints per job is high, the peripheral speed difference between the developing roller  42  and the surface of the photosensitive drum is increased to be higher than that in the image formation, and the surface distance is increased, in order to increase the scraping force. Specifically, as a number of prints per job is higher, the peripheral speed difference between the developing roller  42  and the photosensitive drum  1  in the scraping operation is increased. 
     The test result of Example 3 is indicated in Table 3. The scraping time is a constant 1.8 seconds, and the apply voltage V_S to the toner supply roller  43  at the scraping is always −350V. The specified printing environment room temperature is 15° C. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                  Exam- 
                  Exam- 
                  Exam- 
                 Com- parative 
               
               
                   
                 ple 
                 ple 
                 ple 
                 Example 
               
               
                   
                 3-1 
                 3-2 
                 3-3 
                 3-1 
               
               
                   
               
             
            
               
                 Number of prints per job 
                 2 
                 2 
                 3 
                 3 
               
               
                 Peripheral speed  difference between 
                 70  mm/s 
                 120  mm/s 
                 120  mm/s 
                 70  mm/s 
               
               
                 developing roller and  photosensitive drum 
                   
                   
                   
                   
               
               
                 Melt adhesion white  streak result 
                 Δ 
                 ◯ 
                 Δ 
                 X 
               
               
                   
               
            
           
         
       
     
     Details of Example 3 
     In Example 3-1, a number of prints per job is 2, and white streaks generated due to melt adhesion, when the developing roller  42  and the photosensitive drum  1  are rotated at the peripheral speed difference 70 mm/s (developing roller is rotating faster) in the scraping, are indicated. Here only minor white streaks, create no practical problem, were generated in the images. Example 3-2 indicates a result of the white streaks generated due to melt adhesion when the peripheral speed difference is 120 mm/s. In Example 3-2, no white streaks were generated. Example 3-3 indicates a result when the peripheral speed difference is 120 mm/s and a number of prints per job is 3. In Example 3-3, the white streaks generated due to melt adhesion were at a minor level. 
     Details of Comparative Example 3 
     In Comparative Example 3-1, the peripheral speed difference between the developing roller  42  and the photosensitive drum  1  is 70 mm/s, and a number of prints per job is 3 (same as Comparative Example 2-1). In the case of Comparative Example 3-1, white streaks generated due to melt adhesion were observed. 
     Comparison of Example 3 and Comparative Example 3 
     As a comparison of Example 3-1 and Example 3-2 indicates, the scraping force can be increased by increasing the peripheral speed difference. Further, as the comparison of Example 3-1 and Example 3-3 or the comparison of Example 3-1 and Comparative Example 3-1 indicates, as a number of prints per job increases, the peripheral speed difference needs to be increased to increase the scraping force. 
     Example 4 
     The present invention can be suitably executed by the control unit  65  setting appropriate operation conditions in accordance with the pre-conditions, such as the printing environment and the printing job setting, and operating each composing element of the image forming apparatus. For this, it is preferable to store a table or mathematical formulae in advance, in which pre-conditions (e.g. printing environment, printing job setting) and operation settings (e.g. apply voltage, scraping time, peripheral speed difference) are linked, in a memory or the like. The control unit  65  acquires printing job setting information based on the printing job instructed by the user, refers to the memory using the printing job setting as a key, and sets appropriate operation conditions. 
     It is more preferable that the image forming apparatus includes an environment information acquisition unit  69 . The control unit  65  refers to the memory using the printing environment information detected by the environment information acquisition unit  69  as a key, and sets appropriate operation conditions. The environment information acquisition unit  69  may include a thermometer. 
     As described above, according to each example of the present invention, such operation conditions as the voltages applied to the developing roller  42  and the toner supply roller  43  respectively, the scraping time (contact-rotation time) and the peripheral speed difference between the photosensitive drum  1  and the developing roller  42 , are set according to such pre-conditions (printing conditions) as the printing environment (room temperature) and the printing job setting (number of prints per job), and the scraping operation is performed based on the operation conditions. Thereby the exposed portions P are formed on the surface of the developing roller, and the melted substance that adhered to the surface of the photosensitive drum can be removed. Particularly in a cleanerless type image forming apparatus, which does not include a cleaning blade to remove the toner  90  remaining on the photosensitive drum  1 , executing the scraping operation described in the present embodiment is very effective to remove adhered substances. As long as the melted substance that adhered to the surface of the photosensitive drum  1  can be removed, combinations of the conditions to be set are not limited to each example described above, and may be set in any combination. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2021-069612, filed Apr. 16, 2021, which is hereby incorporated by reference wherein in its entirety.