Patent Publication Number: US-11048192-B1

Title: Image forming apparatus capable of suppressing occurrence of image defects in response to difference in carrier resistance and obtaining high image quality

Description:
INCORPORATION BY REFERENCE 
     This application is based on and claims the benefit of priority from Japanese Patent Application No. 2019-221147 filed on Dec. 6, 2019, the contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The present disclosure relates to an image forming apparatus. 
     In electrophotographic image forming apparatuses such as copiers, printers, and the like, apparatuses in which toner is adhered to an electrostatic latent image that is formed on the surface of an photosensitive drum as an image carrier and developed, whereby a toner image to be transferred later to paper is formed, are widely used. An example of a typical image forming apparatus capable of stably forming a high-quality image is disclosed. 
     A typical disclosed image forming apparatus uses a carrier having a specified particle size and saturation magnetization, a developer amount regulating member having rigidity and a magnetic property, and a developer carrier having a plurality of grooves extending in the width direction, and the normal direction magnetic flux density of the surface portion of the developer carrier on which the developer amount regulating member faces is set within a predetermined range. As a result, image unevenness or the like due to a change in the amount of developer on the developer carrier may be suppressed, and a high-quality image may be stably formed. 
     SUMMARY 
     In order to solve the problems described above, the image forming apparatus according to the present disclosure includes an image carrier, a charging unit, a developing unit, a developing power supply, a current detecting unit, and a control unit. The image carrier has a photosensitive layer and an electrostatic latent image being formed on a surface thereof. The charging unit charges the surface of the image carrier. The developing unit has a developer carrier that carries toner in a two-component developer that includes a toner and a magnetic carrier on the surface thereof, and forms a toner image by adhering the toner to the electrostatic latent image that is formed on the image carrier. The developing power supply applies a developing voltage obtained by superimposing an AC voltage on a DC voltage to the developer carrier. The current detecting unit detects a developing current that flows between the developer carrier and the image carrier when the developing voltage is applied to the developer carrier. The control unit controls the operation of the image carrier, the charging unit, the developing unit, and the developing power supply. The control unit causes the charging unit to charge the surface of the image carrier, and when the developing voltage is applied to the developer carrier by the developing power supply, derives a carrier resistance based on the developing current detected by the current detecting unit, and controls an AC amplitude of the AC voltage based on the carrier resistance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view illustrating a configuration of an image forming apparatus of an embodiment according to the present disclosure. 
         FIG. 2  is a block diagram illustrating a configuration of an image forming apparatus of an embodiment according to the present disclosure. 
         FIG. 3  is a cross-sectional view illustrating the periphery of an image forming unit of an image forming apparatus of an embodiment according to the present disclosure. 
         FIG. 4  is a graph illustrating the relationship between the carrier resistance and the developing current of an image forming apparatus of an embodiment according to the present disclosure. 
         FIG. 5  is a graph illustrating the relationship between the AC amplitude of the AC voltage and the image density during development by the image forming apparatus of an embodiment according to the present disclosure. 
         FIG. 6  is a graph illustrating the relationship between the AC amplitude of the AC voltage during development and the image density of each of an amorphous silicon photoconductor and an organic photoconductor. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments according to the present disclosure will be described with reference to the drawings. Note that the present disclosure is not limited to the following contents. 
       FIG. 1  is a schematic cross-sectional view illustrating a configuration of an image forming apparatus  1 .  FIG. 2  is a block diagram illustrating the configuration of the image forming apparatus  1 .  FIG. 3  is a cross-sectional view illustrating the periphery of an image forming unit  20  of the image forming apparatus  1 . An example of the image forming apparatus  1  of the present embodiment is a tandem color printer that transfers a toner image onto paper P using an intermediate transfer belt  31 . The image forming apparatus  1  may be a so-called multifunction machine having functions such as printing (printing), scanning (image reading), facsimile transmission and the like. 
     As illustrated in  FIGS. 1 and 2 , the image forming apparatus  1  includes a paper supply unit  3 , a paper conveying unit  4 , an exposing unit  5 , an image forming unit  20 , a transfer unit  30 , a fixing unit  6 , a paper discharge unit  7 , a control unit  8 , and a storage unit  9  that are provided in a main body  2 . 
     The paper supply unit  3  accommodates a plurality of sheets of paper P, and separates and feeds out the paper P one sheet at the time of printing. The paper conveying unit  4  conveys the paper P fed from the paper supply unit  3  to a secondary transfer unit  33  and the fixing unit  6 , and further discharges the paper P after fixing from a paper discharge port  4   a  to the paper discharge unit  7 . In a case of performing double-sided printing, the paper conveying unit  4  sorts the paper P after fixing on the first side to a reverse conveying unit  4   c  by a branch unit  4   b , and the paper P is again conveyed to the secondary transfer unit  33  and the fixing unit  6 . The exposing unit  5  irradiates the image forming unit  20  with a laser beam that is controlled based on the image data. 
     The image forming unit  20  is arranged below the intermediate transfer belt  31 . The image forming unit  20  includes an image forming unit  20 Y for yellow, an image forming unit  20 C for cyan, an image forming unit  20 M for magenta, and an image forming unit  20 B for black. These four image forming units  20  have the same basic configuration. Accordingly, in the following description, the identification symbols of “Y”, “C”, “M”, and “B” representing each color may be omitted unless it is particularly necessary to limit them. 
     The image forming unit  20  includes a photosensitive drum (image carrier)  21  that is rotatably supported so as to be able to rotate in a specific direction (clockwise in  FIGS. 1 and 3 ). The image forming unit  20  further includes a charging unit  40 , a developing unit  50 , and a drum cleaning unit  60  around the photosensitive drum  21  along the rotation direction thereof. Note that a primary transfer unit  32  is arranged between the developing unit  50  and the drum cleaning unit  60 . 
     The charging unit  40  charges the surface of the photosensitive drum  21  to a specific potential. Then, an electrostatic latent image of a document image is formed on the surface of the photosensitive drum  21  by the laser beam emitted from the exposing unit  5 . The developing unit  50  adheres toner to the electrostatic latent image and develops the toner to form a toner image. Each of the four image forming units  20  forms a toner image of a different color. 
     The transfer unit  30  includes an intermediate transfer belt  31 , a primary transfer unit  32 Y,  32 C,  32 M,  32 B, a secondary transfer unit  33 , and a belt cleaning unit  34 . The intermediate transfer belt  31  is arranged above the four image forming units  20 . The intermediate transfer belt  31  is an intermediate transfer body that is rotatably supported so as to rotate in a specific direction (counterclockwise in  FIG. 1 ) and on which toner images formed by each of the four image forming units  20  are sequentially superimposed and primarily transferred. The four image forming units  20  are arranged in a so-called tandem method, in a row from the upstream side to the downstream side in the rotation direction of the intermediate transfer belts  31 . 
     The primary transfer units  32 Y,  32 C,  32 M,  32 B are arranged above the image forming units  20 Y,  20 C,  20 M,  20 B of each color with the intermediate transfer belt  31  located therebetween. The secondary transfer unit  33  is arranged further on the upstream side in the paper conveying direction of the paper conveying unit  4  than the fixing unit  6 , and is arranged further on the downstream side in the rotation direction of the intermediate transfer belt  31  of the transfer unit  30  than the image forming units  20 Y  20 C,  20 M,  20 B of each color. A belt cleaning unit  34  is arranged further on the upstream side in the rotation direction of the intermediate transfer belt  31  than the image forming units  20 Y,  20 C,  20 M,  20 B of each color. 
     The toner image is primarily transferred to the outer peripheral surface of the intermediate transfer belt  31  by the primary transfer units  32 Y,  32 C,  32 M,  32 B of each color. Then, as the intermediate transfer belt  31  rotates, the toner images of the four image forming units  20  are continuously superimposed and transferred to the intermediate transfer belt  31  at a specific timing. As a result, a color toner image in which four color toner images of yellow, cyan, magenta, and black are superimposed is formed on the outer peripheral surface of the intermediate transfer belt  31 . The drum cleaning unit  60  cleans by removing toner and the like remaining on the surface of the photosensitive drum  21  after the primary transfer. 
     The color toner image on the outer peripheral surface of the intermediate transfer belt  31  is transferred to the paper P synchronously by the paper conveying unit  4  by a secondary transfer nip unit formed on the secondary transfer unit  33 . The belt cleaning unit  34  cleans by removing toner and the like remaining on the outer peripheral surface of the intermediate transfer belt  31  after the secondary transfer. 
     The fixing unit  6  heats and pressurizes the paper P on which the toner image is transferred to fix the toner image on the paper P. 
     The control unit  8  includes a CPU, an image processing unit, other electronic circuits and electronic components. The CPU controls the operation of each component provided in the image forming apparatus  1  based on the control program or data stored in the storage unit  9 , and performs processing related to the function of the image forming apparatus  1 . Each of the paper supply unit  3 , the paper conveying unit  4 , the exposing unit  5 , the image forming unit  20 , the transfer unit  30 , and the fixing unit  6  receives commands individually from the control unit  8  and performs printing on the paper P in conjunction with each other. Moreover, the control unit  8  is able to obtain an output value from a current detecting unit  13  described later. 
     The storage unit  9  is configured by combining a non-volatile storage device such as a program ROM (Read Only Memory), a data ROM, and the like and a volatile storage device such as a RAM (Random Access Memory). 
     Subsequently, the configuration of the image forming unit  20  and the surroundings thereof will be described with reference to  FIGS. 2 and 3 . Note that the image forming unit  20  of each color has the same basic structure, so the identification code representing each color is omitted. 
     The image forming unit  20  includes a photosensitive drum  21 , a charging unit  40 , a developing unit  50 , and a drum cleaning unit  60  illustrated in  FIGS. 2 and 3 . Furthermore, the image forming apparatus  1  includes a charging power supply  11 , a developing power supply  12 , and a current detecting unit  13 . 
     The photosensitive drum  21  is rotatably supported with the center axis thereof horizontal, and is rotated at a constant speed around the axis by a driving unit. The photosensitive drum  21  has a photosensitive layer made of an inorganic photosensitive body such as amorphous silicon (a-Si) or the like on the surface of a metal drum tube such as aluminum or the like. An electrostatic latent image is formed on the surface of the photosensitive drum  21 . 
     The charging unit  40  has, for example, a charging roller  41  and a charge cleaning roller  42 . 
     The charging roller  41  is rotatably supported with the center axis thereof being horizontal, and by coming into contact with the surface of the photosensitive drum  21 , the charging roller  41  rotates according to the rotation of the photosensitive drum  21 . The charging roller  41  has, for example, a conductive layer made of crosslinked rubber or the like including an ionic conductive material on the surface of a metal core. When a specific charging voltage is applied to the charging roller  41  that comes into contact with the surface of the photosensitive drum  21  so as to be driven and rotated, the surface of the photosensitive drum  21  is uniformly charged. The charge cleaning roller  42  comes into contact with the surface of the charging roller  41  and cleans the surface of the charging roller  41 . 
     The charging roller  41  is electrically connected to the charging power supply  11 . The charging power supply  11  has an AC constant voltage power supply and a DC constant voltage power supply. The AC constant voltage power supply outputs a sinusoidal AC voltage generated from a low-voltage DC voltage modulated in a pulse shape using a step-up transformer. The DC constant voltage power supply outputs a DC voltage obtained by rectifying a sinusoidal AC voltage generated from a low-voltage DC voltage modulated in a pulse shape using a step-up transformer. The charging power supply  11  generates a charging voltage obtained by superimposing the AC voltage (AC component) outputted from the AC constant voltage power supply onto the DC voltage (DC component) outputted from the DC constant voltage power supply, and applies the charging voltage to the charging roller  41 . 
     The developing unit  50  includes a developing container  51 , a first stirring and conveying member  52 , a second stirring and conveying member  53 , a developing roller (developer carrier)  54 , and a regulating member  55 . 
     The developing container  51  stores, for example, a two-component developer that includes toner and a magnetic carrier as a developer to be supplied from the developing unit  50  to the surface of the photosensitive drum  21 . The first stirring and conveying member  52  and the second stirring and conveying member  53  are arranged inside the developing container  51 . The first stirring and conveying member  52  and the second stirring and conveying member  53  are supported by the developing container  51  so as to be rotatable around an axis extending parallel to the photosensitive drum  21 . In addition, the first stirring and conveying member  52  and the second stirring and conveying member  53  rotates around the axis, whereby the developer is conveyed while being stirred in the direction of the rotation axis. The toner is circulated and charged inside the developing container  51 . 
     The developing roller  54  is supported by the developing container  51  so as to be rotatable around an axis extending parallel to the photosensitive drum  21 . The developing roller  54  has, for example, a cylindrical shaped developing sleeve that rotates counterclockwise in  FIG. 3 , and a developing roller-side magnetic pole that is fixed in the developing sleeve. The developing roller  54  carries toner to be adhered to the surface of the photosensitive drum  21  in a developing region facing the photosensitive drum  21 . 
     The developing roller  54  is electrically connected to the developing power supply  12 . The configuration and operation of the developing power supply  12  is the same as the configuration and operation of the charging power supply  11 . The developing power supply  12  generates a developing voltage obtained by superimposing the AC voltage (AC component) outputted from the AC constant voltage power supply onto the DC voltage (DC component) outputted from the DC constant voltage power supply, and applies the developing voltage to the developing roller  54 . 
     The regulating member  55  is arranged on the upstream side in the rotation direction of the developing roller  54  in the developing region where the developing roller  54  and the photosensitive drum  21  face each other. The regulating member  55  is arranged close to the developing roller  54  and so that there is a specific gap between the tip end thereof and the surface of the developing roller  54 . The regulating member  55  regulates the layer thickness of the developer passing through the gap between the tip end thereof and the surface of the developing roller  54 . 
     The developer is stirred, circulated and charged by the first stirring and conveying member  52  and the second stirring and conveying member  53  in the developing container  51 , and is carried on the surface of the developing roller  54 . The layer thickness of the developer supported on the surface of the developing roller  54  is regulated by the regulating member  55 . On the surface of the developing roller  54 , a magnetic brush composed of toner and a magnetic carrier is formed. When a specific developing voltage is applied to the developing roller  54 , the toner carried on the surface of the developing roller  54  flies to the surface of the photosensitive drum  21  in the developing region due to the potential difference between the surface potential of the photosensitive drum  21  and the developing roller  54 , and the electrostatic latent image on the surface of the photosensitive drum  21  is developed. 
     The drum cleaning unit  60  has a cleaning roller  61 , a cleaning blade  62 , and a recovery spiral  63 . 
     The cleaning roller  61  comes into contact with the surface of the photosensitive drum  21  at a specific pressure, and is rotated in a direction in which the contact region with the photosensitive drum  21  is moved in the same direction as the photosensitive drum  21  by the driving unit. The cleaning blade  62  comes into contact with the surface of the photosensitive drum  21  at a specific pressure. The cleaning roller  61  and the cleaning blade  62  clean by removing toner and the like remaining on the surface of the photosensitive drum  21  after the primary transfer. The recovery spiral  63  conveys the waste toner or the like removed from the surface of the photosensitive drum  21  to a waste toner recovery container provided outside the drum cleaning unit  60 . 
     The operations of the photosensitive drum  21 , the charging unit  40 , the developing unit  50 , the drum cleaning unit  60 , the charging power supply  11 , and the developing power supply  12  are controlled by the control unit  8 . 
     The current detecting unit  13  is able to detect the developing current flowing between the developing roller  54  and the photosensitive drum  21  when the developing voltage is applied to the developing roller  54 . 
     In developing a toner image using a two-component developer, the developing current is composed of a toner transfer current and a carrier current. The toner transfer current is a current that flows by the toner moving between the developing roller  54  and the photosensitive drum  21 . The toner transfer current has a correlation with the amount of toner that moves between the developing roller  54  and the photosensitive drum  21 , and increases as the amount of toner that moves increases. The carrier current is a current that flows in a state where development with toner is mostly not performed. 
     The direction in which the developing current flows is determined by the potential difference between the potential of the developing roller  54  and the surface potential of the photosensitive drum  21 . In other words, when the potential of the developing roller  54  is higher than the surface potential of the photosensitive drum  21 , the developing current flows from the developing roller  54  toward the photosensitive drum  21 . Then, when the potential of the developing roller  54  is lower than the surface potential of the photoconductor drum  21 , the developing current flows from the photosensitive drum  21  toward the developing roller  54 . 
       FIG. 4  is a graph illustrating the relationship between the carrier resistance and the developing current. The horizontal axis of the graph of  FIG. 4  indicates three levels (low, medium, and high) according to the magnitude of carrier resistance, and the vertical axis indicates the developing current. 
     The magnitude of the developing current is affected by the level of carrier resistance. According to  FIG. 4 , the higher the carrier resistance level, the smaller the developing current. As a result, the carrier resistance of the developing current may be derived by detecting the developing current by the current detecting unit  13  and converting from the current value of the detected developing current. For example, the control unit  8  is able to derive the carrier resistance based on the developing current detected by the current detecting unit  13  by storing a table or the like corresponding to the graph of  FIG. 4  in the storage unit  9  or the like in advance, and using that table. 
       FIG. 5  is a graph illustrating the relationship between the AC amplitude of the AC voltage and the image density during development. The horizontal axis of the graph of  FIG. 5  indicates the AC amplitude of the AC voltage during development, and the vertical axis indicates the image density.  FIG. 5  illustrates the relationship between the AC amplitude of the AC voltage during development and the image density when the carrier resistance level is high (broken line) and when low (solid line). 
     The AC amplitude AL 1  when the carrier resistance is at a high level and the AC amplitude AL 1  when the carrier resistance is at a low level indicate the image density saturation voltage, respectively. In a case where the AC amplitude of the AC voltage during development is less than the image density saturation voltage, image defects such as image unevenness in a solid image, for example, may occur. Therefore, the AC voltage during development needs to be set to an AC amplitude equal to or higher than the image density saturation voltage at which the image density is stable. 
     The AC amplitude AH 2  in a case where the carrier resistance is at a high level and the AC amplitude AL 2  when the carrier resistance is at a low level indicate the AC amplitude at which a leakage occurs on the surface of the photosensitive drum  21 , respectively. The AC voltage during development needs to be set to an AC amplitude at which leakage does not occur on the surface of the photosensitive drum  21 , but the AC amplitude varies depending on the level of the carrier resistance. 
     Therefore, for example, by storing a table or the like corresponding to the graph of  FIG. 5  in the storage unit  9  or the like in advance and using the table, the control unit  8  controls the AC amplitude of the AC voltage based on the carrier resistance. With this configuration, for example, even in a case where the carrier resistance of the developer changes with the passage of time, the developing conditions may be controlled based on the carrier resistance. As a result, it is possible to deal with the difference in carrier resistance and suppress the occurrence of image defects such as image unevenness in a solid image. Therefore, it is possible to obtain high image quality. 
     Then, according to  FIG. 5 , the control unit  8  increases the AC amplitude of the AC voltage as the carrier resistance increases. According to this configuration, the developing conditions may be changed according to the difference in carrier resistance. 
     Even more specifically, the control unit  8  sets the AC amplitude of the AC voltage during development to a range that is equal to or higher than the image density saturation voltage and does not cause leakage on the surface of the photosensitive drum  21 . For example, according to  FIG. 5 , in a case where the carrier resistance is at a high level, the AC amplitude of the AC voltage during development is set in a range of the AC amplitude AH 1  or more of the image density saturation voltage and less than the AC amplitude AH 2  at which leakage occurs on the surface of the photosensitive drum  21 . Moreover, according to  FIG. 5 , in a case where the carrier resistance is at a low level, the AC amplitude of the AC voltage during development is set in the range of the AC amplitude AL 1  or more of the image density saturation voltage and less than the AC amplitude AL 2  where leakage occurs on the surface of the photosensitive drum  21 . The range of AC amplitude of the AC voltage during development differs depending on the magnitude of the carrier resistance. With this configuration, regardless of the level of carrier resistance, it is possible, for example, to suppress the occurrence of image defects such as image unevenness in a solid image, and suppress the occurrence of leakage to the surface of the photosensitive drum  21 . 
     Then, the control unit  8  executes the detection of the developing current by the current detecting unit  13  by using the non-exposure region of the photosensitive drum  21  at the time of non-image formation. With this configuration, a white background region in which the toner does not fly is used during non-image formation, so the developing current does not include the toner transfer current but includes only the carrier current. Therefore, it is possible to improve the accuracy of deriving the carrier resistance based on the developing current detected by the current detecting unit  13 . 
       FIG. 6  is a graph illustrating the relationship between the AC amplitude and the image density of the AC voltage during development of each of the amorphous silicon photoconductor (a-Si) and the organic photoconductor (OPC). The horizontal axis of the graph of  FIG. 6  indicates the AC amplitude of the AC voltage during development, and the vertical axis indicates the image density. An example of the level of the carrier resistance in each photoconductor is illustrated. 
     According to  FIG. 6 , the range of Ao 1  or more and less than Ao 2  of the OPC photoconductor is wider than the range of Aa 1  or more and less than Aa 2  of the amorphous silicon photoconductor. Ao 1  is the AC amplitude of the image density saturation voltage of the AC voltage during development in the case of the OPC photoconductor. Ao 2  is the AC amplitude at which leakage occurs on the surface of the photosensitive drum. Aa 1  is the AC amplitude of the image density saturation voltage of the AC voltage during development. Aa 2  is the AC amplitude at which a leakage occurs on the surface of the photosensitive drum. Then, in the case of an OPC photoconductor, the range in which the AC amplitude of the AC voltage during development is equal to or higher than the image density saturation voltage and no leakage occurs on the surface of the photosensitive drum includes portions that overlap even when the carrier resistance of the developer is different. Therefore, in the case of an OPC photoconductor, when the AC amplitude of the AC voltage is set in the overlapping portion, it is not necessary to control the AC amplitude of the AC voltage based on the carrier resistance. 
     As described above, the photosensitive layer of the photosensitive drum  21  of the present embodiment is an amorphous silicon photosensitive layer. In the case of an amorphous silicon photoconductor, the range in which the AC amplitude of the AC voltage during development is equal to or higher than the image density saturation voltage and leakage does not occur on the surface of the photosensitive drum differs depending on the level of the carrier resistance. Therefore, in the case of an amorphous silicon photoconductor, it is necessary to control the AC amplitude of the AC voltage based on the carrier resistance. As a result, the occurrence of image defects can be suppressed, and high image quality may be obtained. 
     Although the embodiments of the present disclosure have been described above, the scope of the present disclosure is not limited to this, and it can be implemented with various modifications without departing from the gist of the disclosure. 
     For example, the table corresponding to  FIG. 4 , for example, for deriving the carrier resistance based on the developing current and the table corresponding to  FIG. 5 , for example, for controlling the AC amplitude of the AC voltage based on the carrier resistance may be replaced with calculation formulas or the like. It is desirable that these tables and calculation formulas are constructed based on carrier resistance information of the developer at the time of manufacture and information reflecting the relationship between the AC amplitude of the AC voltage and the image density. 
     Moreover, in the embodiment described above, the image forming apparatus  1  is a so-called tandem type image forming apparatus for color printing that sequentially superimposes and forms images of a plurality of colors. However, the image forming apparatus  1  is not limited to such a model, and may be an image forming apparatus for color printing or an image forming apparatus for monochrome printing that is not a tandem type. 
     In a typical technique, in a case where the AC amplitude of the AC voltage during development is insufficient, for example, in the development of a solid image, image unevenness (pitch unevenness) in which lights and shades are continuous in the circumferential direction of the photosensitive drum may occur. On the other hand, in the developing unit, in order to realize a stable developing operation, the developer is conveyed while stirring in the developing unit. As a result, when a two-component developer that includes a toner and a magnetic carrier is used, the carrier resistance of the developer may differ with the passage of time. Then, in developing a solid image, there is a problem in that the conditions for the occurrence of image unevenness differ depending on the magnitude of the carrier resistance. 
     In view of the situation described above, the object according to the present disclosure is to provide an image forming apparatus capable of suppressing the occurrence of image defects in response to a difference in carrier resistance and obtaining high image quality. 
     With the configuration according to the present disclosure, for example, the developing conditions may be controlled based on the carrier resistance even in a case where the carrier resistance of the developer changes with the passage of time. As a result, it is possible to deal with the difference in carrier resistance and suppress the occurrence of image defects such as image unevenness in a solid image. Therefore, it is possible to obtain high image quality. 
     The technique according to the present disclosure may be applied to an image forming apparatus.