JP2010117491A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent generation of top image density unevenness and development ghost in an image forming apparatus adopting a touchdown developing system. <P>SOLUTION: A control part 100 for the image forming apparatus sets the rotational speed of a developing roller 72 so that the density of a solid image formed by one rotation of the developing roller 72 is a predetermined first value or more based on the density of a solid image formed on an intermediate transfer member. Then, it sets the potential difference of the voltages to be applied on the developing roller 72 and a magnetic roller 73 by a voltage applying part 80 so that the density difference of the density of the solid image formed by one rotation of the developing roller 72 and the density of the solid image formed by two rotations of the developing roller 72 is a predetermined second value or less. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus using an electrophotographic system, and more particularly to an image forming apparatus that uses a two-component developer composed of a carrier and a toner and develops an electrostatic latent image by holding only the toner on the developing roller. About.

  As a developing method of an image forming apparatus using an electrophotographic method, a one-component developing method and a two-component developing method are known.

  In the one-component development method in which the developer is composed only of toner, a magnetic brush formed of carrier and toner is not formed. Therefore, the electrostatic latent image on the electrostatic latent image carrier (photoconductor) is not disturbed by the magnetic brush, and is suitable for higher image quality than the two-component development method. However, it is difficult for the one-component development method to stably maintain the toner charge amount over a long period of time compared to the two-component development method. Moreover, in the case of a color toner, since the toner is required to have transparency, the toner needs to be a non-magnetic toner, and it is further difficult to stably maintain the charge amount of the toner. For this reason, in a full-color image forming apparatus, a two-component development method using a carrier as a medium for charging and transporting toner is often employed.

  On the other hand, the two-component development method in which the developer is composed of toner and carrier is suitable for extending the life because a stable charge amount can be obtained over a long period of time. However, due to the influence of the magnetic brush, the two-component development method is disadvantageous in terms of image quality compared to the one-component development method.

  In recent years, attention has been paid to a touch-down development method having advantages of both a one-component development method and a two-component development method. In the touch-down development method, a two-component development method is adopted for the toner charging region to extend the life, and a one-component development method is adopted for the development region to improve the image quality.

  That is, in the touch-down development method, a two-component developer containing toner and carrier is carried on the surface of a magnetic roller that is a developer carrying member to form a magnetic brush, and only the toner is transferred from the magnetic brush to the toner carrying member. The toner layer is transferred to the surface of the developing roller, and a toner layer is formed. The toner is ejected from the toner layer onto the surface of the photoreceptor on which the electrostatic latent image is formed, and the electrostatic latent image is developed as a toner image. In particular, the use of the touch-down development method in a full-color image forming apparatus where high image quality and long life are important, provides the advantages of both the one-component development method and the two-component development method. Has an effect.

  However, the touch-down development method has a problem of unevenness in the front end image density in which the developability is lowered in the second round of the developing roller as compared with the front end portion of the image (see FIG. 4). When the developing roller needs to be rotated two or more times in order to supply toner to the developing region to the saturation region, the image density unevenness at the leading edge is detected when the toner is supplied only for one rotation of the developing roller. This occurs because a sufficient image density cannot be obtained. For example, when a solid image is printed from the leading edge to the trailing edge of the printing paper, and the solid image is continuously printed, the magnetic roller is used for the next development with the toner on the developing roller being used up. Therefore, toner is supplied to the developing roller. For this reason, development is performed in a state where toner is supplied to the developing roller for only one round. Therefore, the toner at the leading end is sufficiently supplied onto the developing roller, so that the necessary image density can be obtained, but the remaining portion Most of the image density is lower than the image density of the first round.

  For the same reason, in the touch-down development method, there is a difference between the amount of toner on the second developing roller in the portion where the solid image is printed and the amount of toner on the second developing roller in the portion where the solid image is not printed. Therefore, there is also a problem that a so-called development ghost (history phenomenon), in which an afterimage after the toner flies from the developing roller to the photosensitive member, appears also in the second round, is likely to occur (see FIG. 5).

  As a countermeasure to such a problem in the touch-down development method, for example, Patent Document 1 discloses an image forming apparatus that increases the density of a magnetic brush by using a low-magnetization carrier and increases the effect of removing the toner on the developing roller. Is disclosed. According to the image forming apparatus of Patent Document 1, since the toner is newly supplied to the developing roller after the toner on the developing roller is peeled off, the above-described problem that occurs because the toner density on the developing roller becomes non-uniform. Can be solved.

Further, Japanese Patent Application Laid-Open No. 2005-228867 discloses a developing device of an image forming apparatus that promotes replacement of toner on the developing roller by applying an AC voltage having an opposite phase to the AC component of the developing roller to the magnetic roller. . According to the developing device of Patent Document 2, since the toner on the developing roller is replaced, the above-described problem caused by uneven toner density on the developing roller can be solved.
JP 2003-295613 A JP-A-2005-242281

  However, in the method of Patent Document 1, when the density and uniformity of the magnetic brush are impaired due to carrier deterioration, the stripping performance is also lowered, so that it is difficult to eliminate the development ghost over a long period of time. Furthermore, in the method of Patent Document 2, complicated control is required to cope with changes in the level of occurrence of development ghosts and development leaks due to deterioration of the developer, so it is not easy to completely eliminate development ghosts. Absent.

  The present invention has been made in view of such conventional problems, and an object of the present invention is to prevent leading edge image density unevenness and development ghosts from occurring for a long period of time in an image forming apparatus employing a touch-down development method. And

  An image forming apparatus according to the present invention is an image forming apparatus that performs development by forming a magnetic brush with a two-component developer containing toner and a carrier, and an image carrier on which an electrostatic latent image is formed; A developing roller disposed opposite to the image carrier and carrying a developer toner on its surface; a magnetic roller carrying the two-component developer and supplying the toner to the developing roller; the developing roller; A developing device including a voltage applying unit that applies a bias voltage to the magnetic roller, a control unit that controls the bias voltage and the rotation speed of the developing roller, and the toner onto which the toner image has been transferred from the image carrier. An intermediate transfer member for transferring an image to a recording medium; a toner patch forming portion for forming a plurality of toner patches, which are solid images, on the image carrier or the intermediate transfer member; and the toner A density detection unit that detects an image density of the patch, and the developing device brings the two-component developer conveyed by the magnetic roller into contact with or close to the developing roller, so that the two-component developer in the two-component developer The toner is carried on the surface of the developing roller, and the voltage application unit applies the developing bias voltage to the developing roller, thereby causing the toner conveyed by the developing roller to fly to the surface of the image carrier, The electrostatic latent image previously formed on the surface of the image carrier is visualized as a toner image, and the control unit sets the developing roller to 1 based on the image density of the toner patch detected by the density detection unit. After setting the rotation speed of the developing roller and setting the rotation speed so that the image density of the toner patch formed by rotating is equal to or higher than a predetermined first value, the current density is set. An image density difference between the image density of the toner patch formed by rotating the roller once and the image density of the toner patch formed by rotating the developing roller twice is less than a predetermined second value. Thus, the potential difference between the voltage applied to the developing roller and the voltage applied to the toner supply roller is set.

  According to this configuration, the control unit has a predetermined first image density of the toner patch formed by rotating the developing roller once based on the image density of the toner patch detected by the density detection unit. The rotation speed of the developing roller is set so as to be equal to or greater than the value of. As a result, the toner necessary for forming a solid image can be supplied to the photosensitive member only by one rotation of the developing roller.

  Further, according to this configuration, the control unit is formed by setting the rotation speed and then rotating the developing roller once, and forming the toner patch density formed by rotating the developing roller twice. The potential difference between the voltage value applied to the developing roller and the voltage value applied to the toner supply roller is set so that the density difference between the toner patch and the toner patch is less than a predetermined second value. . Therefore, even when solid images are printed continuously, there is an effect that there is no density difference that can be visually recognized by the user between the solid image density printed earlier and the solid image density printed later.

  According to the present invention, due to the two effects described above, the occurrence of the leading edge image density unevenness and the development ghost, which are the problems of the conventional touchdown development method, can be eliminated, and a uniform image can be obtained over a long period of time.

  In the above configuration, the toner patch formation by the toner patch formation unit, the image density detection of the toner patch by the density detection unit, and the rotation speed and the potential difference of the developing roller by the control unit are set. The image density of the toner patch formed by rotating the developing roller until the image density of the toner patch formed by rotating the developing roller reaches the first value or more is increased. It is preferable that the process is repeated until the density difference between the image density of the toner patch formed by rotating the developing roller twice is equal to or less than the second value.

  According to this configuration, the rotation speed is set so that the image density of the toner patch formed by rotating the developing roller once is equal to or higher than a predetermined first value, and the developing roller is rotated once. The image density difference between the image density of the toner patch formed by rotating the developing roller and the image density of the toner patch formed by rotating the developing roller twice is less than the second value. The potential difference is set more reliably.

  Therefore, the occurrence of the leading edge image density unevenness and the development ghost, which are the problems of the conventional touch-down development method, can be solved more reliably.

  Furthermore, the first value is preferably set to 1.2 in Macbeth concentration. If the image has a Macbeth density of 1.2 or higher, the user can recognize that the image is a solid image. Therefore, by setting the first value to a Macbeth density of 1.2, when the user prints a solid image, a solid image having a sufficient density intended by the user can be obtained.

  Furthermore, the second value is preferably set to 0.1 in terms of Macbeth concentration. When comparing two solid images, if the density difference between them is Macbeth density of 0.1 or less, the user cannot discriminate the density difference between them. Therefore, by setting the second value to 0.1 as the Macbeth density, a solid image having a uniform density can be obtained.

  According to the image forming apparatus of the present invention, the occurrence of the leading edge image density unevenness and the development ghost, which are problems of the conventional touch-down development method, is eliminated, and a uniform image can be obtained over a long period of time. Therefore, the life of the developing device can be extended, and the monetary cost and time cost required for maintenance can be reduced.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

<Overall configuration of image forming apparatus>
FIG. 1 is a cross-sectional view schematically showing the overall configuration of an image forming apparatus 1 according to an embodiment of the present invention. The image forming apparatus 1 is a color printer, for example, and has a box-shaped device main body 1a as shown in FIG. In the apparatus main body 1a, an image forming unit that feeds a sheet P and an image forming unit that transfers a toner image based on image data or the like to the sheet P while conveying the sheet P fed from the sheet feeding unit 2. A fixing unit 4 that performs a fixing process for fixing the unfixed toner image transferred onto the paper P by the image forming unit 3 to the paper P is provided. Further, on the upper surface of the apparatus main body 1a, a paper discharge unit 5 for discharging the paper P subjected to the fixing process by the fixing unit 4 is provided.

  The paper feed unit 2 includes a paper feed cassette 21, a pickup roller 22, paper feed rollers 23, 24, 25, and a registration roller 26. The paper feed cassette 21 is provided so as to be detachable from the apparatus main body 1a, and stores the paper P of each size. The pickup roller 22 is provided at the upper left position of the paper feed cassette 21 shown in FIG. 1 and takes out the paper P stored in the paper feed cassette 21 one by one. The paper feed rollers 23, 24, and 25 send out the paper P taken out by the pickup roller 22 to the paper transport path. The registration roller 26 temporarily waits for the paper P sent to the paper transport path by the paper feed rollers 23, 24, and 25, and then supplies it to the image forming unit 3 at a predetermined timing.

  The paper feeding unit 2 further includes a manual feed tray (not shown) and a pickup roller 27 that are attached to the left side surface of the device main body 1a shown in FIG. The pickup roller 27 takes out the paper P placed on the manual feed tray. The paper P taken out by the pickup roller 27 is sent out to the paper conveyance path by the paper feeding rollers 23 and 25 and is supplied to the image forming unit 3 by the registration roller 26 at a predetermined timing.

  The image forming unit 3 includes an image forming unit 7 and an intermediate transfer belt 31 (intermediate transfer member) on which a toner image based on image data transmitted from a computer or the like is primarily transferred to the surface (contact surface) of the image forming unit 7. And a secondary transfer roller 32 for secondary transfer of the toner image on the intermediate transfer belt 31 onto the paper P fed from the paper feed cassette 21.

  The image forming unit 7 includes a black unit 7K, a yellow unit 7Y, a cyan unit 7C, and a magenta unit 7M which are sequentially arranged from the upstream side (right side in FIG. 1) to the downstream side. ing. In each of the units 7K, 7Y, 7C, and 7M, a photosensitive drum 37 as an image carrier is arranged at the center position so as to be rotatable in the direction of an arrow (clockwise). Around each photosensitive drum 37, a charger 39, an exposure device 38, a developing device 71, a cleaning device (not shown), a static eliminator, and the like are sequentially arranged from the upstream side in the rotation direction.

  The charger 39 uniformly charges the peripheral surface of the photosensitive drum 37 rotated in the direction of the arrow. Examples of the charger 39 include a non-contact discharge type corotron type and scorotron type charger, a contact type charging roller, a charging brush, and the like. The exposure device 38 is a so-called laser scanning unit, which irradiates the peripheral surface of the photosensitive drum 37 uniformly charged by the charger 39 with laser light based on the image data input from the image reading device or the like, and thereby the photosensitive member. An electrostatic latent image based on the image data is formed on the drum 37.

  The photoreceptor drum 37 is preferably an amorphous silicon (a-Si) photoreceptor. When the thickness of the a-Si photoconductor is reduced, the saturation charging potential is lowered and the withstand voltage leading to dielectric breakdown is lowered. However, the potential of the latent image portion (exposure portion, image forming portion) is as low as 20 V or less. The potential of the non-latent image portion (non-exposed portion, non-image forming portion) is about 350V. Furthermore, the charge density on the surface of the photoreceptor when a latent image is formed tends to improve, and the development performance tends to improve. This characteristic is particularly remarkable when the dielectric constant is about 10 or less for an a-Si photoreceptor having a dielectric constant of about 10 or less, more preferably 20 μm or less. For example, a photosensitive drum having a diameter of 30 mm is used as the photosensitive drum 37.

  When a positively charged organic photoconductor (OPC) is used as the photoconductor, the thickness of the photoconductive layer is set to 25 μm or more in order to reduce the residual potential to 100 V or less. It is particularly important to increase the amount added. In particular, OPC having a single-layer structure is advantageous because a charge generating material is added to the photosensitive layer, so that the change in sensitivity is small even when the photosensitive layer is reduced. Even in this case, setting the DC voltage value of the developing bias voltage to 400 V or lower, more preferably 300 V or lower, is preferable in terms of preventing a strong electric field from being applied to the toner.

  The developing device 71 supplies toner to the peripheral surface of the photosensitive drum 37 on which the electrostatic latent image is formed, thereby forming a toner image based on the image data. The toner image is primarily transferred to the intermediate transfer belt 31. The cleaning device cleans the toner remaining on the peripheral surface of the photosensitive drum 37 after the primary transfer of the toner image to the intermediate transfer belt 31 is completed. The static eliminator neutralizes the peripheral surface of the photosensitive drum 37 after the primary transfer is completed. The peripheral surface of the photosensitive drum 37 cleaned by the cleaning device and the charge eliminator goes to the charger for a new charging process, and a new primary transfer is performed.

  The intermediate transfer belt 31 is an endless belt-like rotating body, and includes a driving roller 33, a driven roller 34, a backup roller 35, and a surface (contact surface) side so as to abut on the circumferential surface of each photosensitive drum 37. It is spanned across a plurality of rollers such as the primary transfer roller 36. The intermediate transfer belt 31 is configured to rotate endlessly by the plurality of rollers in a state in which the intermediate transfer belt 31 is pressed against the photosensitive drum 37 by a primary transfer roller 36 disposed to face each photosensitive drum 37. The driving roller 33 is rotationally driven by a driving source such as a stepping motor, and gives a driving force for rotating the intermediate transfer belt 31 endlessly. The driven roller 34, the backup roller 35, and the primary transfer roller 36 are rotatably provided, and are driven to rotate with the endless rotation of the intermediate transfer belt 31 by the driving roller 33. These rollers 34, 35, 36 are driven to rotate via the intermediate transfer belt 31 in accordance with the main rotation of the drive roller 33 and support the intermediate transfer belt 31.

  The primary transfer roller 36 applies a primary transfer bias (a polarity opposite to the toner charging polarity) to the intermediate transfer belt 31. By doing so, the toner image formed on each photosensitive drum 37 circulates in the direction of the arrow (counterclockwise) by driving the driving roller 33 between each photosensitive drum 37 and the primary transfer roller 36. The intermediate transfer belt 31 is sequentially transferred (primary transfer) in an overcoated state.

  The secondary transfer roller 32 applies a secondary transfer bias having a polarity opposite to that of the toner image to the paper P. By doing so, the toner image primarily transferred onto the intermediate transfer belt 31 is secondarily transferred to the paper P between the secondary transfer roller 32 and the backup roller 35, whereby a color transfer image is transferred to the paper P. (Unfixed toner image) is transferred.

  The image forming unit 7 also functions as a toner patch forming unit by forming toner patches for each color of CMYK on the intermediate transfer belt 31. The image density of the toner patch is a position downstream of each of the image forming units 7K to 7M and facing the toner patch forming surface of the intermediate transfer belt 31 (in FIG. 1, on the left side of the magenta unit 7M and above the intermediate transfer belt 31). In other words, it is detected by an image density detection unit 90 provided at a position directly above the drive roller 33.

  The image density detection unit 90 is a sensor that detects the image density of the toner patch, and is provided for each color of CMYK. For example, when a reflection type density sensor is used as the sensor, the toner is obtained by comparing the reflected light from the surface of the intermediate transfer belt 31 on the background surface where the toner patch is not formed and the reflected light from the surface of the toner patch. Detect the image density of the patch. Based on the image density of the toner patch detected by the image density detection unit 90, the number of rotations of the developing roller and the developing bias voltage value in the developing device 71 that determines the toner density are set. The setting of the rotation number of the developing roller and the developing bias voltage value will be described in detail later. The toner patch is formed on the intermediate transfer belt 31 because it is necessary to detect the image density of the toner patch and perform the above setting before the primary transfer is completed and the secondary transfer is started. Because there is.

  The fixing unit 4 performs a fixing process on the transfer image transferred to the paper P by the image forming unit 3. The fixing unit 4 is disposed to face the heating roller 41 heated by the energization heating element and the heating roller 41. And a pressure roller 42 that presses and contacts the peripheral surface of the heating roller 41.

  The transfer image transferred to the paper P by the secondary transfer roller 32 in the image forming unit 3 is subjected to fixing processing by heating in the fixing unit 4 when the paper P passes between the heating roller 41 and the pressure roller 42. Applied to the paper P. The paper P on which the fixing process has been performed is discharged to the paper discharge unit 5. In the image forming apparatus 1 according to the present exemplary embodiment, the conveyance roller 6 is disposed between the fixing unit 4 and the paper discharge unit 5 at an appropriate position.

  The paper discharge unit 5 is formed by recessing the top of the apparatus main body 1a of the image forming apparatus 1, and a paper discharge tray 51 for receiving the discharged paper P is formed at the bottom of the concave portion. Yes.

<Configuration and operating principle of developing device>
Next, the configuration of the developing device 71 provided in the image forming apparatus 1 according to an embodiment of the present disclosure will be described. FIG. 2 is a cross-sectional view schematically showing the configuration of the developing device 71, and shows an enlarged peripheral portion of the developing device 71 provided in the image forming apparatus 1 shown in FIG.

  The developing device 71 includes a developing roller 72, a magnetic roller 73, a paddle mixer 74, a stirring mixer 75, a spike cutting blade 76, a partition plate 77, and a voltage applying unit 80.

  The developing roller 72 carries (conveys) toner on the surface thereof, and visualizes (develops) an electrostatic latent image formed in advance on the surface of the photosensitive drum 37 as a toner image. The magnetic roller 73 attracts the two-component developer by a magnet disposed therein to generate a magnetic brush, and supplies toner to the developing roller 72. For example, the developing roller 72 uses a roller having a diameter of 20 mm, and the magnetic roller 73 uses a roller having a diameter of 25 mm, for example.

  The paddle mixer 74 and the agitation mixer 75 have spiral blades, and agitate the toner while conveying the two-component developer in opposite directions to charge the toner. Further, the paddle mixer 74 supplies a two-component developer containing charged toner and a carrier to the magnetic roller 73. The ear cutting blade 76 regulates the thickness of the magnetic brush formed on the magnetic roller 73. The partition plate 77 is provided between the paddle mixer 74 and the stirring mixer 75 so that the two-component developer can freely pass outside the both end sides of the partition plate 77.

The carrier is, for example, magnetic particles of magnetite (Fe ₃ 0 ₄ ), and has two roles of toner collection and supply. In order to supply the toner necessary for development to the developing roller 72 by separating the toner firmly and electrostatically attached to the developing roller 72 with a magnetic brush formed on the surface of the magnetic roller 73 and collecting the toner. The volume resistivity is preferably 10 ⁶ to 10 ¹³ Ωcm. Further, in order to increase the surface area of the carrier and increase the number of contacts with the toner, it is preferable that the carrier is a small particle diameter carrier having an average particle diameter of 50 μm or less. As a carrier for a two-component developer satisfying such conditions, for example, a carrier having a volume resistivity of 10 ¹⁰ Ωcm, a saturation magnetization of 65 emu / g, and an average particle diameter of 45 μm is used.

  The voltage application unit 80 is a power source that applies a bias voltage to the developing roller 72 and the magnetic roller 73, a power source 80 a that applies a direct current (DC) bias to the magnetic roller 73, and a direct current (DC) bias that is applied to the developing roller 72. And a power source 80c for applying an alternating current (AC) bias to the developing roller 72.

  The control unit 100 controls the rotation speed of each roller and each mixer such as the developing roller 72 constituting the developing device 71 and the bias voltage applied by the voltage applying unit 80 to the developing roller 72 and the magnetic roller 73.

  The control unit 100 includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like, and controls the number of rotations of each roller such as the developing roller 72 and each mixer constituting the developing device 71. In addition, it controls the overall operation of the image forming apparatus including control of the bias voltage applied by the voltage application unit 80 to the developing roller 72 and the magnetic roller 73.

  FIG. 3 is a diagram for explaining development in the developing device 71. FIG. 3 is a schematic diagram for explaining the development of the developing device 71, and the positional relationship among the photosensitive drum 37, the developing roller 72, the magnetic roller 73, and the ear cutting blade 76 is different from that in FIG. The two-component developer 83 including the toner 81 and the carrier 82 charged by the paddle mixer 74 and the stirring mixer 75 is supplied to the magnetic roller 73. The two-component developer 83 supplied to the magnetic roller 73 is conveyed as a magnetic brush by a magnet inside the magnetic roller 73. Thereafter, the magnetic brush is moved by the rotation of the sleeve on the surface of the magnetic roller 73, and the thickness thereof is regulated when passing between the spike cutting blade 76 and the magnetic roller 73.

  A potential difference is generated between the developing roller 72 and the magnetic roller 73 due to the voltage applied by the voltage applying unit 80. Therefore, when the magnetic brush whose thickness is regulated moves to the vicinity of the developing roller 72, only the toner 81 charged by this potential difference moves to the developing roller 72. The toner 81 moved to the developing roller 72 becomes a uniform toner layer. A potential difference is also generated by the voltage application unit 80 between the photosensitive drum 37 and the developing roller 72. Due to this potential difference, the toner 81 further moves to the electrostatic latent image formed on the photosensitive drum 37 and development is performed (touch-down development).

<Generation mechanism of leading edge image density unevenness and development ghost>
Next, the generation mechanism of the leading edge image density unevenness will be described. FIG. 4 is a diagram illustrating an example of tip image unevenness. FIG. 4A is a diagram showing a solid image printed normally, and FIG. 4B shows a case where the leading edge image unevenness is added to the solid image printed later when the solid image is continuously printed. It is a figure which shows the state which occurred.

  In order to supply the toner to the developing roller 72 to the saturation region, it is assumed that the developing roller 72 needs to be rotated two or more times. In spite of such a situation, if the electrostatic latent image on the photosensitive drum 37 is developed in a state where the toner is supplied only for one rotation of the developing roller 72, a sufficient image density cannot be obtained. In such a case, unevenness in the tip image density occurs. For example, when a solid image is printed from the leading edge to the trailing edge of the printing paper, and the solid image is continuously printed, the magnetic roller is used for the next development with the toner on the developing roller being used up. The toner is supplied from 73 to the developing roller 72. For this reason, development is performed in a state where toner is supplied for only one turn on the developing roller 72, and therefore, the front end portion of the solid image printed later is necessary because toner is sufficiently supplied onto the developing roller 72. However, most of the remaining image density is lower than that of the first round.

  FIG. 5 is a diagram illustrating an example of a development ghost (history phenomenon). FIG. 5A is a diagram illustrating a normally printed image, and FIG. 5B corresponds to a position where the solid image is printed in an image printed immediately after the solid image is printed. It is a figure which shows the state which the development ghost produced in the part to carry out.

  When printing is performed immediately after the solid image is printed, the amount of toner on the second turn of the developing roller in the portion where the solid image is printed is printed for the same reason as the occurrence of unevenness in the leading edge image density. This is less than the toner amount on the second round of the developing roller at the unexposed portion. For this reason, a so-called development ghost in which an afterimage after the toner flies from the developing roller 72 to the photosensitive drum 37 also appears on the second round is likely to occur.

  The following two factors can be cited as the factors causing the leading edge image density unevenness and the development ghost (hereinafter collectively referred to as image density unevenness). That is, the first factor is a lack of toner collection capability from the developing roller 72 to the magnetic roller 73, and a second factor is a lack of toner supply capability from the magnetic roller 73 to the developing roller 72.

  First, the first factor will be described below. If the toner on the developing roller 72 is completely peeled off by the magnetic brush and new toner is always supplied to the developing roller 72, the amount of toner on the developing roller 72 will be the same regardless of the number of rotations of the developing roller 72. Since it becomes constant, image density unevenness does not occur. However, in the case of the touch-down development method, the above-described toner collection and toner supply are performed almost simultaneously via a magnetic brush existing between the magnetic roller 73 and the development roller 72. Therefore, it is extremely difficult to completely collect the toner on the developing roller 72 unless there is a device such as providing a separate toner collecting mechanism on the developing roller 72.

  Next, the second factor will be described below. If the toner amount on the developing roller 72 reaches the saturated toner amount by only one rotation of the developing roller 72, the toner amount on the developing roller 72 will be the saturated toner amount even if the developing roller 72 rotates two or more times. Remains. That is, the toner amount on the developing roller 72 is constant, and image density unevenness does not occur.

  The amount of saturated toner is determined by the potential difference ΔV between the DC voltages applied to the magnetic roller 73 and the developing roller 72. By reducing ΔV, the amount of saturated toner decreases. Therefore, if ΔV is reduced, the saturated toner amount is reached only by one rotation of the developing roller 72, so that the image density unevenness is eliminated. However, the saturated toner amount is not always a sufficient toner amount to satisfy the image density of the solid image. For example, when the charge amount of the toner is increased in a low temperature and low humidity environment, the amount of toner flying to the photosensitive drum 37 is decreased, and the image density of the solid image may not be sufficiently obtained.

  In order to prevent uneven image density, the saturated toner amount is sufficient to satisfy the image density of the solid image, and the necessary toner amount is supplied onto the photosensitive drum 37 only by one rotation of the developing roller. It is essential that Since the amount of toner supplied onto the photosensitive drum 37 is considered to correlate with the rotational speed of the developing roller 72, the rotational speed of the developing roller 72, the photosensitive drum 37, and the developing roller 72 in Examination Examples 1 and 2 are considered. The relationship with the above toner amount was examined.

<Examination example 1>
FIG. 6 is a diagram illustrating the results of Study Example 1 in which the relationship between the rotation speed of the developing roller 72 and the amount of toner on the photosensitive drum 37 and the developing roller 72 is examined. From the result of Study Example 1, the amount of toner on the developing roller 72 increases by slowing the rotation speed of the developing roller 72, but the amount of toner on the photosensitive drum 37, that is, the amount of toner to be developed, decreases. I understand. It can be considered that such a result is obtained because the rotation speed ratio of the developing roller 72 to the photosensitive drum 37 is lowered. Therefore, when the image density unevenness occurs, the amount of toner to be developed is insufficient. Therefore, it is only necessary to increase the rotation amount of the developing roller 72 and increase the amount of toner to be developed. Become.

  However, when the rotation speed of the developing roller 72 increases, the toner on the developing roller 72 is likely to be scattered. Further, due to the configuration of the developing device 71, the rotation of the developing roller 72 and the rotation of the magnetic roller 73, the stirring mixer 75, and the paddle mixer 74 are not independent. Therefore, when the rotation speed of the developing roller 72 is increased, the rotation speed of these rollers and the mixer is also increased, and the developer is vigorously stirred, thereby promoting the deterioration of the developer. Therefore, it is desirable to set the rotation speed of the developing roller 72 as low as possible. That is, it is only necessary to determine the minimum value of the rotation speed of the developing roller 72 in which the amount of toner developed on the photosensitive drum 37 with only one rotation of the developing roller becomes the amount of toner necessary to obtain the solid image density. Become.

<Examination example 2>
Incidentally, the toner collecting ability from the developing roller 72 to the magnetic roller 73 and the toner supplying ability from the magnetic roller 73 to the developing roller 72 depend on various factors in addition to the rotation speed of the developing roller 72. For example, the distance between the developing roller 72 and the magnetic roller 73, the layout of the magnetic poles of the magnetic roller 73, and the height of the ears of the magnetic brush formed on the magnetic roller 73 by the two-component developer, that is, the cutting blade 76 and the magnetic roller 73 Such as gap, carrier particle size and saturation magnetization. Also, the amount of toner supplied to the developing roller 72 is likely to fluctuate due to environmental changes such as toner concentration and temperature change in the two-component developer, and changes in toner charge amount due to carrier deterioration.

  Among the above elements, the temperature change, which is a representative element, was examined in Study Example 2. FIG. 7 is a diagram illustrating the results of Study Example 2 in which the relationship between the rotation speed of the developing roller 72 and the toner amount on the photosensitive drum 37 and the developing roller 72 is examined for each temperature.

  The result of Study Example 2 indicates that when the temperature is high, the amount of toner on the photosensitive drum 37 increases, that is, developability increases, and when the temperature is low, developability decreases. Accordingly, it is understood that when the temperature is high, the rotation speed of the developing roller 72 is sufficient at a low speed. Conversely, when the temperature is low, it is necessary to increase the rotation speed of the developing roller 72 to compensate for the reduced development amount. . In other words, since the environment such as temperature varies with each printing, the optimum rotation speed of the developing roller 72 also varies with each printing.

<Development roller rotation speed setting>
The setting of the rotation speed of the developing roller 72 will be described below. FIG. 8 is a flowchart showing a process for setting the rotation speed of the developing roller 72 in the image forming apparatus 1 according to the embodiment of the present disclosure.

  First, the control unit 100 sets the rotation speed of the developing roller 72 to a default value (step S1). This default value is stored in the ROM of the control unit 100, for example. Subsequently, the control unit 100 causes the developing roller 72 to make one rotation at the rotation speed of the default value, so that the toner remaining on the developing roller 72 is peeled off by the magnetic brush (step S2). As a result, the density of the solid image formed with the amount of toner for one rotation of the developing roller 72 is accurately detected, and the rotation speed of the developing roller 72 is accurately set.

  The control unit 100 performs toner layer formation on the surface of the developing roller 72 from which the toner on the surface has been peeled off by rotating the developing roller 72 only one turn (step S3). The controller 100 causes the developing device 71 to form the solid image 1 (toner patch) on the intermediate transfer belt 31 with the toner for one round (step S4).

  The density detector 90 detects the density of the solid image 1 formed on the intermediate transfer belt 31 (step S5). When the image density detected by the density detection unit 90 is 1.2 or higher, that is, when the image density is sufficiently secured (YES in step S6), the control unit 100 sets the default value to the rotation speed of the developing roller 72. (Step S8). The lower limit value of the density of the solid image is set to 1.2 because it is possible for the user to recognize the solid image as long as the density is equal to or higher than this density.

  On the other hand, if the image density of the solid image 1 detected by the density detection unit 90 is less than 1.2, that is, if the image density does not reach the target (NO in step S6), the process proceeds to step S7, and the control unit 100 develops. The rotational speed of the roller 72 is increased. Steps S2 to S7 are repeated until the image density of the solid image 1 becomes 1.2 or more.

<Setting of ΔV>
Next, the setting of the potential difference ΔV of the DC voltage applied to the developing roller 72 and the magnetic roller 73 will be described below. FIG. 9 is a flowchart showing ΔV setting processing in the image forming apparatus 1 according to the embodiment of the present disclosure.

  After setting the rotation speed of the developing roller 72, the control unit 100 continues to set ΔV. First, the control unit 100 sets ΔV to a default value (step S101). This default value is stored in the ROM of the control unit 100, for example, in the same manner as the default value of the rotation speed described above.

  Subsequent to step S101, the control unit 100 rotates the developing roller 72 only once to form the solid image 1 in the same manner as the speed setting of the developing roller 72 described above (steps S2 to S4). Thereafter, the control unit 100 rotates the developing roller 72 twice to form the solid image 2 and compares the densities of the two solid images, which is different from the speed setting of the developing roller 72 described above.

  Subsequent to step S4, the control unit 100 makes the developing roller 72 make one turn so that the toner remaining on the developing roller 72 after the solid image 1 is formed is peeled off by the magnetic brush (step S105). .

  Next, the controller 100 forms the toner layer on the surface of the developing roller 72 from which the toner on the surface has been peeled off by rotating the developing roller 72 twice (step S106). The control unit 100 causes the developing device 71 to form the solid image 2 on the intermediate transfer belt 31 with the toner for two rounds (step S107).

  The density detector 90 detects the density of the solid image 1 and the solid image 2 formed on the intermediate transfer belt 31 (step S108). If the density difference between the two solid images detected by the density detector 90 is 0.1 or less, that is, if the density difference between the two images is sufficiently small (YES in step S109), the control unit 100 sets the default value as ΔV. (Step S111). Note that the upper limit of the density difference is set to 0.1 because if the density difference is equal to or less than this density difference, the user cannot recognize the density difference, that is, the user cannot recognize the image density unevenness.

  On the other hand, if the density difference exceeds 0.1, that is, if image density unevenness has occurred (NO in step S111), the process proceeds to step S110, and the control unit 100 decreases ΔV. Steps S2 to S110 are repeated until the density difference becomes 0.1 or less.

  Although the rotation speed setting and the ΔV setting described above are performed for each printing, which is ideal, in this case, the printing efficiency is reduced and the toner consumption is increased. Practically, it is sufficient to carry out every 500 printings, for example, every time the developing device 71 reaches a certain driving time. However, when the printing environment changes drastically, or when it is left unattended for a long time, the image quality is likely to change, so the rotation speed setting and ΔV setting may be performed. preferable.

  Hereinafter, although the said setting is demonstrated further in detail, giving an Example and a comparative example, this invention is not limited only to the following Examples.

<Examples and Comparative Examples>
A modified machine of FS-C5016N (Kyocera Mita printer) was used as an image evaluation machine. As shown in FIG. 10, the applied bias to the developing roller is fixed at 100 V, the rotational speed of the photosensitive drum is fixed at 200 mm / sec, and the combination of the rotational speed of the developing roller and the applied bias to the magnetic roller is changed. Front end image density unevenness was evaluated by printing a sample image (front end image density unevenness evaluation pattern) in which a solid image as shown in FIG. The rotation speed of the developing roller is changed from 200 mm / sec to 300 mm / sec in increments of 50 mm / sec, and the applied bias to the magnetic roller is changed from 250 V to 350 V in increments of 50 V (see FIG. 10). ).

  In the printed tip image density unevenness evaluation pattern, the image density of the portion corresponding to the first round of the developing roller and the portion corresponding to the second round of the developing roller were measured using a Macbeth reflection densitometer (RD914). The image density was determined to be acceptable when the image density of the leading edge image density unevenness evaluation pattern was 1.2 or higher. Further, for the leading edge image density unevenness, a case where the difference in image density between the first and second rounds of the developing roller of the leading edge image density unevenness evaluation pattern is 0.1 or less was determined to be acceptable. The evaluation results are shown in FIG. In FIG. 10, “◯” indicates a pass determination and “×” indicates a fail determination.

  Examples 1 and 2 and Comparative Example 1 are examples in which the toner amount for one rotation of the developing roller is supplied, and Comparative Example 2 is an example in which only a toner amount less than one rotation of the developing roller is supplied, Comparative Example Reference numeral 3 denotes an example in which the toner amount for two rotations of the developing roller is supplied.

  As shown in the result of Comparative Example 3, when the toner was supplied to the developing roller over two rounds, the leading edge image density unevenness occurred. Conversely, from the results of Examples 1 and 2 and Comparative Example 1, when the saturated toner amount is reached in one rotation of the developing roller regardless of the rotation speed (200 mm / sec to 300 mm / sec) of the developing roller, the leading edge image density unevenness It was found that does not occur. However, from the result of Comparative Example 1, it was found that when the rotation speed of the developing roller is low (200 mm / sec), a necessary image density cannot be obtained.

  The condition of Comparative Example 2 is that the developing roller rotation speed is the median value (250 mm / sec) in this evaluation, but the bias applied to the magnetic roller is small (250 V, which is the minimum value in this evaluation). Therefore, under the conditions of Comparative Example 2, the potential difference ΔV between the developing roller and the magnetic roller having a positive correlation with the toner amount on the developing roller becomes too small (150V), and the toner on the developing roller decreases, and the required image density It is thought that the result was not obtained. That is, it is suggested that ΔV needs to be set according to the rotation speed of the developing roller.

  From the above results, in order to avoid the leading edge image density unevenness and ensure the image density within the range of the setting conditions of this evaluation, the rotation speed of the developing roller is 250 mm / sec or more, and the rotation of the developing roller It was found that the conditions of Examples 1 and 2 in which ΔV was set according to the speed were necessary. However, in consideration of suppression of toner scattering and reduction of stress on the developer, it can be said that Example 2 in which the rotation speed of the developing roller is as small as 250 mm / sec is preferable to Example 1.

  The image forming apparatus 1 according to the embodiment of the present invention has been described above. However, the present invention is not limited to this, and for example, the following modified embodiment can be taken.

  (1) In the above embodiment, a tandem type image forming apparatus has been described as an example of the image forming apparatus. However, the present invention is not limited to the tandem type, and can be applied to general image forming apparatuses using a touch-down development method. .

  (2) In the above embodiment, a color printer has been described as an example of the image forming apparatus. However, the present invention is not limited to the image forming apparatus being a color printer. The image forming apparatus may be, for example, a monochrome printer, a facsimile machine, a multifunction machine, or the like.

  (3) In the above embodiment, the photosensitive drum, which is a drum-shaped photosensitive member, is described as an example of the image carrier. However, the image carrier is not limited to this. The present invention can also be applied to a belt-shaped photoreceptor, a sheet-shaped photoreceptor, and the like.

  (4) In the above embodiment, the toner patch is formed on the intermediate transfer belt, but the toner patch may be formed on an image carrier such as a photosensitive drum.

1 is a cross-sectional view illustrating an outline of an overall configuration of an image forming apparatus according to an embodiment of the present invention. 1 is a cross-sectional view schematically illustrating a configuration of a developing device provided in an image forming apparatus according to an embodiment of the present disclosure. FIG. 4 is a diagram for explaining development in a developing device provided in the image forming apparatus according to the embodiment of the present disclosure. It is a figure which shows an example of a front-end | tip image nonuniformity, (A) is a figure which shows the solid image printed normally, (B) is printed later, when a solid image is printed continuously. It is a figure which shows the state which the front-end | tip image nonuniformity produced in the solid image. FIG. 8 is a diagram illustrating an example of a development ghost, where (A) illustrates a normally printed image, and (B) illustrates an image printed immediately after a solid image is printed. It is a figure which shows the state which the development ghost produced in the part corresponding to the printed position. It is a figure which shows the result of the examination example 1 which examined the relationship between the rotational speed of a developing roller, and the toner amount on a photoconductive drum and a developing roller. It is a figure which shows the result of the examination example 2 which examined according to temperature about the relationship between the rotational speed of a developing roller, and the toner amount on a photoconductive drum and a developing roller. 6 is a flowchart illustrating processing for setting a rotation speed of a developing roller in an image forming apparatus according to an embodiment of the present invention. 5 is a flowchart showing a setting process of a potential difference between DC voltages applied to a magnetic roller and a developing roller in an image forming apparatus according to an embodiment of the present invention. FIG. 10 is a tabular view showing a result of evaluating the leading edge image density unevenness by printing a sample image by changing the combination of the rotation speed of the developing roller and the bias applied to the magnetic roller.

Explanation of symbols

1 Image forming apparatus 31 Intermediate transfer belt (intermediate transfer member)
37 Photosensitive drum (image carrier)
71 Developing Device 72 Developing Roller 73 Magnetic Roller 74 Paddle Mixer 75 Agitation Mixer 76 Ear Cutting Blade 77 Partition Plate 80 Voltage Applying Unit 80a DC Power Supply 80b AC Power Supply 80c DC Power Supply 81 Toner 82 Carrier 83 Two-Component Developer 90 Image Density Detection Unit 100 Control Part

Claims (4)

An image forming apparatus that performs development by forming a magnetic brush with a two-component developer including toner and carrier,
An image carrier on which an electrostatic latent image is formed;
A developing roller disposed opposite to the image carrier and carrying a developer toner on its surface; a magnetic roller carrying the two-component developer and supplying the toner to the developing roller; and the developing roller; A developing device including a voltage applying unit that applies a bias voltage to the magnetic roller;
A control unit for controlling the bias voltage and the rotation speed of the developing roller;
An intermediate transfer body for transferring the toner image from the image carrier and transferring the transferred toner image to a recording medium;
A toner patch forming unit that forms a plurality of toner patches, which are solid images, on the image carrier or the intermediate transfer member;
A density detector for detecting the image density of the toner patch,
The developing device causes the two-component developer conveyed by the magnetic roller and the developing roller to contact or approach each other so that the toner in the two-component developer is carried on the surface of the developing roller, and the voltage application unit Applies the developing bias voltage to the developing roller to cause the toner transported by the developing roller to fly to the surface of the image carrier and to form an electrostatic latent image previously formed on the surface of the image carrier. Is visualized as a toner image,
The controller is based on the image density of the toner patch detected by the density detector.
A rotation speed of the developing roller is set so that an image density of the toner patch formed by rotating the developing roller once is equal to or higher than a predetermined first value;
After setting the rotation speed, the density of the toner patch formed by rotating the developing roller once and the density of the toner patch formed by rotating the developing roller twice An image forming apparatus that sets a potential difference between a voltage applied to the developing roller and a voltage applied to the toner supply roller so that the difference is equal to or less than a predetermined second value. Forming the toner patch by the toner patch forming unit;
Image density detection of the toner patch by the density detection unit;
Setting the rotational speed of the developing roller and the potential difference by the control unit,
The image density of the toner patch formed by rotating the developing roller until the image density of the toner patch formed by rotating the developing roller reaches the first value or more is increased. The image forming apparatus according to claim 1, wherein the image forming apparatus repeats until the density difference between the toner patch and the image density formed by rotating the developing roller twice is equal to or less than the second value.   The image forming apparatus according to claim 1, wherein the first value is a Macbeth density of 1.2.   The image forming apparatus according to claim 1, wherein the second value is 0.1 in Macbeth density.

Images