JP5693166B2 - Image forming apparatus - Google Patents

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic image forming apparatus, and more particularly to an image forming apparatus including a developing device that includes a temperature and humidity detecting unit and controls a developing bias in response to a detected change in temperature and humidity.

  Conventionally, in an image forming apparatus using an electrophotographic system, the surface of a drum-shaped electrophotographic photosensitive member (hereinafter referred to as “photosensitive drum”) as an image carrier is uniformly charged by a charging device. Then, the charged photosensitive drum is exposed according to image information by an exposure device, and an electrostatic latent image is formed on the photosensitive drum. The electrostatic latent image formed on the photosensitive drum is visualized with developer toner using a developing device. The visualized image (toner image) is transferred to a recording medium by a transfer device. Thereafter, the toner image transferred onto the recording medium is fused and fixed to the recording medium by heat and pressure by a fixing device. The toner remaining on the photosensitive drum after the transfer process is removed by a cleaning device to prepare for the next image forming process.

  Some developing devices use a two-component developer including non-magnetic toner particles (toner) and magnetic carrier particles (carrier) as a developer. In particular, color image forming apparatuses are widely used because the toner does not need to contain a magnetic material, and the color is good.

  For example, a developing device using a two-component developer is generally configured as shown in FIGS.

  That is, the developing device 4 includes a developing container 41 that stores a developer. The developing container 41 is divided into a developing chamber 41a that also serves as a developer transport path and a stirring chamber 41b that also serves as a developer transport path by a partition wall 41c extending in the vertical direction. In the developing chamber 41a and the agitating chamber 41b, an agitating and conveying screw 41d serving as a first developer conveying member and an agitating and conveying screw 41e serving as a second developer conveying member are disposed, respectively. In addition, at the end in the longitudinal direction of the partition wall 41c, delivery portions 41f and 41g are provided as communication portions that also serve as a developer conveyance path that allows the developer to pass between the developing chamber 41a and the stirring chamber 41b. The first and second agitating and conveying screws 41 d and 41 e convey the developer while agitating it and circulate it in the developing container 41. As shown in FIG. 4, a developing sleeve 44 as a developer carrying member is rotatably disposed at a position of the developing container 41 facing the photosensitive drum 1. The developing sleeve 44 incorporates a magnet roll 44a as a magnetic field generating means.

  The two-component developer that is conveyed while being agitated by the first and second agitating and conveying screws 41d and 41e and in which the toner adheres to the surface of the carrier due to frictional charging is developed by a magnetic field generated by the magnet roll 44a. 44 is attracted and carried on the surface. Then, the developer passes through the developing blade 42 by the rotation of the developing sleeve 44, is coated on the surface of the developing sleeve 44 with a thin layer, and is conveyed to a portion facing the photosensitive drum 1. At the facing portion, the developer forms chain-shaped magnetic spikes by the magnetic field generated by the magnet roll 44a. These magnetic spikes are close to or in contact with the photosensitive drum 1, and only the toner is transferred to the electrostatic latent image formed on the surface of the photosensitive drum 1 by the developing bias applied to the developing sleeve 44, and is static on the surface of the photosensitive drum 1. A toner image corresponding to the electrostatic latent image is formed.

  As described above, in the developing device 4 using the two-component developer used in the electrophotographic image forming apparatus, the toner and the carrier of the two-component developer contained in the developing device 4 are agitated and rubbed. Charge. Thereafter, the latent image on the photosensitive drum 1 is developed by supplying it to the photosensitive drum 1 serving as an electrostatic latent image bearing member by a developing sleeve 44 serving as a developer bearing member.

  In addition, the toner consumed in the developing device 4 measures and conveys and replenishes the amount of toner necessary for replenishment from a hopper or bottle as a toner storage means so as to maintain the ratio of the toner and the carrier in the two-component developer. The toner is replenished through toner replenishing means.

  As described above, in the development method using dry toner, the toner used for image formation is an aggregate of fine particles having different particle diameters. Therefore, selective development in which toner of a specific particle size is preferentially consumed in the development process. I know that there is sex. Therefore, selective developability may appear more conspicuously in a method in which an AC electric field is superimposed on a DC electric field. In particular, in recent years, there has been a demand for higher image quality such as improved resolution, and toner particle size has been reduced. As the particle size decreases, the particle size distribution of the powder tends to become broader. If the particle size distribution to be developed is biased due to this selective developability, the halftone image will vary in roughness and color, and image stability will be impaired, especially for long-term image forming operations.

  As described above, a method as disclosed in Patent Document 1 is disclosed with respect to image instability due to change in toner particle size distribution in the developing device due to selective developability and image quality degradation. The method disclosed in Patent Document 1 controls the development selectivity by controlling the peak-to-peak amplitude (Vpp) of the AC power supply for development according to the image printing ratio. This is a method for preventing selective development. Specifically, it is disclosed that the tendency of the toner particle size distribution to change is often seen when the image printing ratio is low, and is caused by continuous image formation with a small amount of toner replenishment. Therefore, when the image printing ratio is low, the particle size distribution changes on the small particle size side, so that Vpp is increased to allow the small particle size toner to fly.

  Patent Document 2 discloses a configuration in which Vpp is reduced when the relative humidity increases. However, this is a method for improving the sacrificial effect, which is caused by the electrostatic latent image being disturbed by the charge injection due to the increase in Vpp, and is ineffective for the problem to be solved by this case.

JP-A-2006-285201 US Pat. No. 7,532,830

  Here, as a condition for causing the selective development, which is the subject of the present invention, there is a case where it occurs due to a change in relative humidity due to standing durability. In other words, the relative humidity of the developer increases as the temperature decreases due to the temperature rising due to the operation of the image forming apparatus. For this reason, the toner adhesion force changes, and the particle size of the flying toner changes due to the developing bias. As a result, selective development occurs. Further, since the temperature inside the image forming apparatus is increased by increasing the development high voltage AC, the internal temperature immediately after the endurance rises, and the relative humidity change before and after being left tends to increase.

  Next, details of the problem will be described.

  In a high-temperature and high-humidity environment, if the temperature and humidity in the image forming apparatus changes, for example, at the end of the day and the morning of the next day, the development selectivity may vary. The development selectivity changes in the morning and is biased toward the coarse powder, resulting in image defects such as a change in color tone and a worsening of the halftone image.

  Due to the temperature rise in the image forming apparatus after image formation, the developer temperature rises and the relative humidity decreases, thereby increasing the charge amount of the developer and increasing the electrostatic adhesion to the carrier and the developing sleeve. Developability deteriorates. When image formation is completed and image formation is continued on the next day, the temperature in the image forming apparatus decreases, the relative humidity increases, and the charge amount of the developer decreases.

  At this time, although the average charge amount of the developer after being left is lowered, the large particle size toner has a large total charge amount, so that it easily receives a force due to an electric field, and therefore is easily developed. However, small particle size toners are difficult to develop due to their strong mirror power even when the surface charge is the same, and non-electrostatic adhesion such as liquid cross-linking force. Selectivity for development occurs. As a result, the halftone tone of the morning is worsened.

  For the above reason, the particle size distribution of the developer tends to gradually decrease toward the small particle size day by day, and as the durability progresses, the developability decreases, the scattering and the like deteriorate, and the image quality decreases.

  The present invention changes the flying bias, which is the amplitude of the developing AC bias toward the photosensitive drum in accordance with the amount of change in relative humidity, and improves the bias in the particle size distribution of the developer developed due to the temperature and humidity changes described above. High quality is obtained through this. Further, when Vpp is simply increased, there is a risk that temperature rise in the apparatus, ring marks, leaks, etc. may occur. On the other hand, when Vpp is extremely increased, a fine powder development state is caused, and the chargeability is improved, but coarse powder is liable to accumulate.

  In the method disclosed in Patent Document 1 described above, when an image with a high printing ratio is formed, the development Vpp is lowered, and on the contrary, coarse powder development is promoted, and the image quality such as roughness is deteriorated. There is drowning.

SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus that can suppress a change in development selectivity even when the in-machine environment changes greatly and can obtain a high-quality image with little roughness. It is.

The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides an image carrier on which an electrostatic latent image is formed, a developer carrier that carries a developer and uses the electrostatic latent image of the image carrier as a toner image, and the developer carrier. A developing bias applying means for applying a developing bias in which an AC bias is superimposed on a DC bias on the body; a control unit for controlling a duty ratio of the developing bias; and a humidity detecting means for detecting a relative humidity around the image forming apparatus. And when the control unit has passed a predetermined time or more after the image forming operation has been stopped , the control unit is based on the detection result of the humidity detection unit, and the control unit is relatively later than before the stop for the predetermined time or more. than when the relative humidity differential across the humidity is high and the predetermined time or more stop is in the first relative humidity difference also the relative humidity later than the previous predetermined time or more stop is high and the predetermined time Relative before and after the stop Towards the case of the second relative humidity difference degree difference is greater than the first relative humidity difference, the amplitude value on the side where the toner flies is increased from the developer carrying member of the AC bias to said image bearing member as the run mode to change the Duty ratio, does not execute the mode when the stop the image forming operation has not passed the predetermined time or more, is an image forming apparatus characterized by .

According to the present invention, also it is possible to suppress a change in developing selectivity when machine environment has changed significantly, it is possible to obtain a less high-quality image with roughness.

1 is a schematic configuration diagram of an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a partially enlarged schematic configuration diagram of the image forming apparatus in FIG. 1. It is a schematic top view of the developing device. FIG. 2 is a schematic sectional view of a developing device. 1 is a block diagram illustrating an embodiment of a control system that controls an image forming apparatus. FIG. 2 is a layout diagram of a temperature / humidity sensor and a temperature sensor in the image forming apparatus. It is a flowchart explaining temperature / humidity detection control. It is a flowchart explaining development AC bias control. FIG. 6 is a development AC bias waveform diagram illustrating a development bias duty ratio. It is a particle size distribution before and after leaving for a long time. FIG. 6 is a relationship diagram between development AC bias and particle size distribution.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

Example 1
[Overall Configuration and Operation of Image Forming Apparatus]
First, an overall configuration and operation of an embodiment of an image forming apparatus according to the present invention will be described with reference to FIGS. FIG. 1 is an overall schematic view of an electrophotographic image forming apparatus according to this embodiment, and FIG. 2 is an enlarged schematic view of a part of the image forming apparatus.

  Referring to FIGS. 1 and 2, the image forming apparatus 100 according to the present exemplary embodiment is a quadruple tandem type image forming apparatus. The image forming apparatus 100 includes first, second, third, and fourth image forming units PY, PM, PC, and PK that form yellow, magenta, cyan, and black images, respectively, as a plurality of image forming units. .

  In the image forming apparatus 100 according to the present exemplary embodiment, a document reading apparatus connected to an image forming apparatus main body (apparatus main body) or a host device such as a personal computer is connected to the apparatus main body so that communication is possible. Therefore, in accordance with image information from the host device, four color full-color images of yellow (Y), magenta (M), cyan (C), and black (K) are recorded on a recording material (recording paper, plastic) using an electrophotographic method. Sheet, cloth, etc.).

  In the image forming apparatus 100 of this embodiment, a belt-shaped intermediate transfer member (hereinafter referred to as “intermediate transfer belt”) 51 constituting the transfer device 5 moves in the direction of the arrow shown in the figure and passes through each image forming unit. In addition, the image of each color is superimposed on the intermediate transfer belt 51 in each image forming unit. The multiple toner images superimposed on the intermediate transfer belt 51 are transferred to the recording material S to obtain a recorded image.

  In this embodiment, the configuration of each image forming portion P is substantially the same except that the development colors are different. Accordingly, in the following, unless there is a particular distinction, the subscripts Y, M, C, and K given to the reference numerals to indicate that the element belongs to any one of the image forming units will be omitted, and a general description will be given. .

  In FIG. 2, the image forming portion P has a drum-shaped photosensitive member (photosensitive drum) 1 as an image carrier. On the outer periphery of the photosensitive drum 1, a charging device 2 as a charging unit, an exposure device 3 as an exposure unit, a developing device 4 as a developing unit, a transfer member 52 as a primary transfer unit, and a cleaning unit 7 as a cleaning unit are provided. It has been.

  The transfer device 5 has an intermediate transfer belt 51 as an intermediate transfer member. The intermediate transfer belt 51 is wound around a plurality of rollers 5a, 5b, and 5c and rotates (rotates) in the direction of the arrow shown in the drawing. A primary transfer member 52 is disposed at a position facing each photosensitive drum 1 via the intermediate transfer belt 51. A secondary transfer member 53 is provided at a position facing one roller 5b among the rollers around which the intermediate transfer belt 51 is wound.

  At the time of image formation, first, the surface of the rotating photosensitive drum 1 is uniformly charged by the charging roller 2 which is a charging device connected to the power source 21 (FIG. 4). Next, an electrostatic image is formed on the photosensitive drum 1 by performing scanning exposure on the surface of the charged photosensitive drum 1 according to an image information signal by an exposure device 3 which is a laser exposure optical system in this embodiment. The electrostatic image formed on the photosensitive drum 1 is visualized as a toner image by the developer toner using the developing device 4. The toner image formed on the photosensitive drum 1 is subjected to the action of the primary transfer bias applied to the primary transfer member 52 at the primary transfer portion (primary transfer nip) T1 where the intermediate transfer belt 51 and the photosensitive drum 1 abut. Transfer (primary transfer) is performed on the intermediate transfer belt 51. For example, when a four-color full-color image is formed, a toner image is transferred from each photosensitive drum 1 onto the intermediate transfer belt 51 sequentially from the first image forming unit PY, and a four-color toner image is formed on the intermediate transfer belt 51. A superimposed toner image is formed.

  On the other hand, the recording material S accommodated in the cassette 9 as the recording material accommodating portion is supplied toward the transfer device 5 by the recording material conveyance member 10 such as the pickup roller 10a, the conveyance rollers 10b and 10c, and the registration roller 10d. Is done. The recording material S is conveyed in synchronization with the toner image on the intermediate transfer belt 51 to a secondary transfer portion (nip portion) T2 where the intermediate transfer belt 51 and the secondary transfer member 53 abut. The multiple toner images on the intermediate transfer belt 51 are transferred onto the recording material S by the action of the secondary transfer bias applied to the secondary transfer member 53 in the secondary transfer portion T2.

  Thereafter, the recording material S separated from the intermediate transfer belt 51 is conveyed to the fixing device 6. The toner image transferred onto the recording material S is melted and mixed by being heated and pressurized by the fixing device 6 and is fixed onto the recording material S. Thereafter, the recording material S is discharged out of the apparatus.

  Deposits such as toner remaining on the photosensitive drum 1 after the primary transfer process are collected by the cleaning device 7. Thereby, the photosensitive drum 1 is prepared for the next image forming process. Further, deposits such as toner remaining on the intermediate transfer belt 51 after the secondary transfer step are removed by the intermediate transfer body cleaner 54.

  Note that the image forming apparatus 100 according to the present exemplary embodiment may form a single-color or multi-color image using a desired single-color or several-color image forming unit among four colors, such as a black single-color image. Is possible.

[Basic configuration of developing device]
Next, the developing device 4 will be further described with reference to FIGS.

  The developing device 4 includes a developing container 41 that stores a two-component developer including a nonmagnetic toner and a magnetic carrier. The developing container 41 is provided with a developing sleeve 44 as a developer carrying member, and a magnet roll (magnet) 44 a as a magnetic field generating means is fixedly disposed in the developing sleeve 44. The developing container 41 further includes a developing blade 42 as a developer regulating member that forms a thin layer of the developer on the surface of the developing sleeve 44, and first and second agitating and conveying the developer in the developing container 41. Developer transport members 41d and 41e are disposed.

  The interior of the developing container 41 is partitioned into a developing chamber (developer transport path) 41a and a stirring chamber (developer transport path) 41b by a partition wall 41c extending in the vertical direction. A first developer conveying member 41d is arranged in the developing chamber 41a, and a second developer conveying member 41e is arranged in the stirring chamber 41b. At both ends in the longitudinal direction of the partition wall 41c (left side and right side in FIG. 3), transfer portions (developer transport paths) 41f and 41g that allow the developer to pass between the developing chamber 41a and the stirring chamber 41b are provided. ing.

  In this embodiment, the first and second developer conveying members 41d and 41e are both screw-like members (hereinafter referred to as “first screw” and “second screw”, respectively). In other words, in the present embodiment, the first and second screws 41d and 41e are each formed by providing a spiral blade as a transport unit around the axis (rotation axis) of the magnetic body. In the present embodiment, in addition to the blades, the second screw 41e also has a stirring rib 41e1 protruding in the radial direction from the shaft and having a predetermined width in the developer transport direction. The rib 41e1 stirs the developer as the shaft rotates. The first screw 41d stirs and conveys the developer in the developing chamber 41a. The second screw 41e agitates and conveys the toner supplied from the replenishing port 41i and the developer already in the agitating chamber 41b under automatic toner replenishment control (ATR: Auto Toner Replenisher). Thus, the toner density is made uniform.

  The first and second screws 41 d and 41 e are arranged substantially in parallel along the rotation axis direction (developing width direction) of the developing sleeve 44. The first screw 41 d and the second screw 41 e transport the developer in the opposite directions along the rotation axis direction of the developing sleeve 44. Thus, the developer is circulated in the developing container 41 by the first and second screws 41d and 41e through the connecting portions 41f and 41g. In other words, the developer in the developing chamber 41a in which the toner is consumed in the developing process due to the conveying force of the first and second screws 41d and 41e and the toner density is reduced becomes one connecting portion 41f (on the left side of FIG. 3). ) To move into the stirring chamber 41b.

  A replenishing port 41i for replenishing toner is provided at the most upstream part of the agitating chamber 41b, and communicates with the toner hopper 8 serving as toner storage means. The replenishing port of the toner hopper 8 is provided with a replenishing screw (not shown) capable of conveying a certain amount of toner. Then, by the ATR control described above, the rotation speed is controlled according to the image ratio at the time of image formation, the inductance sensor as the toner density sensor 45, and the density detection result of the patch image, and the toner is supplied into the developing container 41. Then, the developer in the stirring chamber 41b, which has been replenished with toner and moved, moves to the developing chamber 41a via the other connecting portion 41g (the right side in FIG. 3). Further, the developing chamber 41 a of the developing device 4 is opened at a position corresponding to the developing area facing the photosensitive drum 1, and the developing sleeve 44 rotates so as to be partially exposed to the opening of the developing container 41. Arranged to be possible. In this embodiment, the developing sleeve 44 is made of a nonmagnetic material and rotates in the direction of the arrow shown in FIG. 4 during the developing operation. As described above, the magnet roll 44 a having a plurality of magnetic poles is fixed inside the developing sleeve 44 along the circumferential direction as a magnetic field generating means.

  The developer in the developing chamber 41a is supplied to the developing sleeve 44 by the first screw 41d, and a predetermined amount of the developer supplied to the developing sleeve 44 is carried on the developing sleeve 44 by the magnetic field generated by the magnet roll 44a. To form a developer reservoir. As the developing sleeve 44 rotates, the two-component developer on the developing sleeve 44 passes through the developer reservoir, the layer thickness is regulated by the developing blade 42, and the developer is conveyed to the developing region facing the photosensitive drum 1. Is done. In the developing area, the developer on the developing sleeve 44 spikes to form a magnetic spike. In this embodiment, the electrostatic image on the photosensitive drum 1 is developed as a toner image by bringing the magnetic spike into contact with the photosensitive drum 1 and supplying the developer toner to the photosensitive drum 1. Further, in order to improve the development efficiency, that is, the application rate of the toner to the latent image, the development sleeve 44 is usually supplied with a development bias in which a DC voltage and an AC voltage are superimposed from a development bias power source 49 as a development bias application unit. A voltage is applied. The developer on the developing sleeve 44 after supplying the toner to the photosensitive drum 1 returns to the developing chamber 41a when the developing sleeve 44 further rotates.

[Temperature and humidity detection and configuration of the invention]
FIG. 5 is a block diagram showing the control system of the present invention, and FIG. 6 shows the arrangement of the temperature / humidity sensor 83 and the temperature sensor 81 in this embodiment.

  The temperature sensors 81 (Y, M, C, K) are arranged in the developing devices 4 of the respective colors, detect the temperatures T (Y, M, C, K), and notify the control device 300 of them. Note that it is preferable that the temperature sensors 81 are disposed in the developing device 4 to measure the temperature of the two-component developer most accurately. However, if there is a restriction on the arrangement or there is a possibility of replacement of the developing device 4, it may be substituted by measuring the approximate temperature by contacting or near the outer wall of the developing device 4. As shown in FIG. 6, the image forming apparatus is provided with a temperature / humidity sensor 83 for measuring the atmosphere (temperature and humidity) inside and outside the image forming apparatus (that is, inside and around the apparatus). .

  The relative humidity inside the developing container 41 is calculated by the method described below based on a signal from a temperature / humidity sensor 83 attached to the image forming apparatus main body. In the present invention, there is no problem even if a temperature / humidity sensor is arranged as a temperature / humidity detection means of the developing container 41 and the relative humidity is directly detected and fed back to the replenishment control.

  7 and 8 are flowcharts for explaining in detail the operations characteristic of the present invention in accordance with the present embodiment. 7 is a flowchart for explaining temperature / humidity detection control, and FIG. 8 is a flowchart for explaining development AC bias control.

First, temperature / humidity detection control will be described with reference to FIG. In the temperature / humidity detection control, the printer control unit 300 acquires the temperature (° C.) and the relative humidity (%) detected by the temperature / humidity sensor 83 and calculates the absolute volume humidity (water vapor amount) e [g / m ³ ]. (S-1). In order to obtain the absolute volume humidity (water vapor amount) e [g / m ³ ] from the temperature and relative humidity, the image forming apparatus of the present invention is used in an environment of approximately 1 atm and a temperature of approximately 0 ° C. to 60 ° C. The calculation method is based on the Tetens approximation.
What is an approximation of Tetens?
E (t) = 6.11 × 10 ^(7.5 × ^{t / (t + 237.3))} ... Formula (1)
(E: saturated water vapor pressure, t: Celsius temperature)
The saturated water vapor pressure E at the temperature t ° C. is obtained. By multiplying the saturated water vapor pressure E by the value of the relative humidity RH1 (%), the volumetric absolute humidity (water vapor amount) e [g / m3] at the temperature and humidity is obtained.
Ep1 = E * RH1 / 100 ..... Formula (2)
e = 217 * Ep1 / T (3)

Next, the printer control unit 300 acquires the temperature measurement value T (developer temperature, ° C.) of the temperature sensor 81, and the CPU 301 obtains the water vapor pressure Ep2 of the measurement air from T (° C.). Then, the developing device humidity Rh [dv] (%) is obtained by dividing the saturated water vapor pressure E at that temperature by that value (S-2).
Ep2 = e * T / 217 ..... Formula (4)
RH [dv] = Ep2 / E * 100 (5)

  In this embodiment, when the image forming operation interval in the image forming apparatus 100 is long, the peak-to-peak of the AC component of the development high voltage AC is determined according to the result of the temperature and humidity detection described above. The duty ratio of the amplitude (Vpp) is changed.

Normally, the internal humidity of the image forming apparatus is large when the power supply is OFF and during sleep, because the control board, various drives, temperature control of the fixing unit, and the like are turned OFF, and the relative humidity change amount before and after that is large. That is, the relative humidity difference increases. Therefore, in this embodiment, the relative humidity before and after the power is turned off and before and after the sleep is measured, and the relative humidity difference ΔRH is calculated (S-3).
ΔRH = RH [a] −RH [b] (6)
RH [a]: Relative humidity before being left RH [b]: Relative humidity after being left

  The development bias control will be described with reference to FIG. In this embodiment, the criterion for determining the image forming apparatus to be left for a predetermined time is the long-term determination criterion for 2 hours after the last job is completed. (S-4). And when it is judged that it is 2 hours or more, the above-mentioned temperature / humidity detection control (S-1 to 3) described with reference to FIG. 7 is executed. In this temperature / humidity detection control, the calculation result of the relative humidity difference ΔRH obtained by the equation (6) is obtained. From this calculation result, the development high-voltage AC bias is changed according to the table in Table 1 (flight bias table with respect to the relative humidity change amount). That is, in the present embodiment, the amplitude duty ratio of the developing AC bias is determined so as to increase the amplitude value on the flying side of the developing bias according to the table of Table 1 (S-5) (S-6).

  At this time, since the image density changes immediately after the development high voltage AC bias is changed, a patch image is formed, the density is measured by the patch detection sensor 48, and the image density control is performed. That is, the printer control unit 300 controls the optimum values of the charging bias power source 21, the developing bias power source 49, and the laser light source 31 of the exposure apparatus 3, and performs control to adjust the image forming contrast to the optimum image density and gradation. (S-7).

  Here, the waveform of the developing bias in this embodiment will be described. As described above, the developing bias used in this embodiment is a bias in which an AC bias (AC bias) and a DC bias (DC bias) are superimposed.

  As shown in FIG. 9, the development AC bias has a pulse shape in which 12 kHz rectangular waves are alternately combined with two wavelengths and blanks for two wavelengths. The potential in the blank portion is the same as the DC bias (Vdc).

  In this embodiment, the amplitude (Vpp) of the development AC bias in the normal state is 1.3 kV, the amplitude Duty is 50%, the flying bias in the developing direction on the photosensitive drum 1 side, and the developing sleeve 44 side. Both withdrawal pullbacks are 0.65 kV. Then, when changing the duty ratio of the amplitude, that is, the ratio between the flying bias and the pulling back bias, the area formed by the frequency direction and the amplitude is set to be equal on the flying side and the pulling side.

  The operation of the image forming apparatus when using the above-described control in a high-temperature and high-humidity environment (30 ° C./80%) will be described in detail.

  First, a temperature / humidity sensor 83 for measuring the atmosphere in and around the apparatus detects a temperature / humidity of 30 ° C. and 79%, which is a substantially room temperature environment. When 3000 images with an image ratio of 5% were continuously output, the developer temperature sensor 81 of the developing device 4 showed 37 ° C. due to the temperature rise in the device. At this time, the printer control unit 300 calculated that the absolute water content was 33.53 [g] and the relative humidity in the developing device 4 was 53%.

  When the temperature of the inside of the image forming apparatus 100 is raised, the solid image formed on the photosensitive drum 1 is stopped and the particle size distribution of the toner developed on the photosensitive drum 1 is measured. 10 (a), which almost coincided with the particle size distribution of the toner in the developer carried on the developing sleeve 44. Then, after the copy operation on that day is finished, the power supply of the image forming apparatus main body is turned off, and the power supply is turned on the next morning to resume the copy operation.

  The next day, when the image forming apparatus was returned from the sleep state by pressing the power switch in the morning, the temperature and humidity in the image forming apparatus was 31 ° C. and 81%, which is the indoor environment. At this time, when the particle size distribution of the toner developed on the photosensitive drum 1 was measured, as shown in FIG. 10B, a tendency of coarse powder development was observed as compared with the toner on the developing sleeve 44.

  At this time, when the magnitude of the flying bias of the rectangular duty is changed while the amplitude Vpp of the developing AC bias is kept as it is, the particle size distribution is changed as shown in FIG. By increasing the flying bias, that is, by increasing the amplitude value on the flying side of the developing bias, the particle size distribution of the toner developed on the photosensitive drum 1 is a distribution closer to the particle size distribution existing on the developing sleeve 44. It became a state.

  Therefore, a halftone image having an image density ratio of about 40% was formed, and the level of 'smoothness' was evaluated. The evaluation of “Hatsuki” of the halftone image was as shown in Table 2 (effect on the Satsuki). In addition, the improvement in the particle size distribution described above has improved the level of 'smoothness' of the halftone image.

  However, when the image formation degree operation is executed in a state where the magnitude of the flying bias is increased from the normal image forming state, the particle size distribution of the developer flying by increasing the duty ratio is reversed as shown in FIG. Shift to fine powder side. For this reason, coarse powder continues to stay inside the developing device during normal image formation, so that the amount of coarse powder developed after being left alone in the morning gradually increases and the roughness becomes worse. Further, when the flying duty of the rectangular duty is larger than 70%, the electrostatic latent image is disturbed due to the charge injection from the carrier particles onto the photosensitive drum 1, and conversely, the 'grabbing' level is deteriorated. Accordingly, the duty ratio of the developing bias is such that the flying bias is 70% at the maximum.

  In addition, although the relative humidity in the image forming apparatus before being left varies depending on the operation state of the image forming apparatus, as shown in Table 1, the development AC bias depends on the relative humidity change amount, that is, the relative humidity difference ΔRH. The duty ratio (ratio of flying bias and pullback bias) is changed. As a result, the development selectivity closest to the toner particle size distribution on the developing sleeve 44 could be obtained.

  As described above, by using the above-described means, it is possible to reduce the “grazing” due to the development selectivity, and it is possible to provide a high-quality image forming apparatus.

  In other words, in this embodiment, by changing the duty ratio of the development AC bias according to the relative humidity, it becomes possible to maintain the development selectivity uniformly even when the in-machine environment changes greatly. An image forming apparatus with few high image quality can be provided. Also, by making the peak-to-peak amplitude (Vpp) of the AC component constant at all times, it is possible to reduce the load on the development high-voltage substrate and suppress heat generation, and the rise in the image forming apparatus. By suppressing the temperature, the generation amount of the relative humidity difference ΔRH can be reduced, and a high-quality image forming apparatus with less roughness can be provided.

Example 2
In this embodiment, the case where the above-described control is used will be described in detail in a durability test by an image forming operation under a high temperature and high humidity environment (30 ° C./80%) in which a relative humidity change tends to be large due to an increase in the temperature inside the apparatus.

  In electrophotographic image formation using a two-component developer, as the number of durable sheets progresses, the chargeability of the carrier and the toner deteriorate, and the developability tends to decrease. This is because the non-electrostatic adhesion force increases as the charge amount of the fine powder toner decreases, and the conveying force due to the electric field decreases. For this reason, when a large change in relative humidity occurs depending on the durability deterioration level, the developed toner tends to gradually shift to the coarse powder side.

  In this embodiment, the development bias is increased according to the durability deterioration level, that is, the amplitude value on the flying side of the development bias is increased, thereby improving the development selectivity and providing a higher quality image. A method is disclosed.

  In this embodiment, the deterioration state of the developer in the developing device is judged by the durable sheet number, and the amplitude value on the flying side of the developing bias is increased as the durable sheet number measured by the durable counter Z (see FIG. 5) increases. It was decided to increase.

  In this embodiment, an ACR (Auto Carrier Refresh) type developer replenishing method that requires higher durability is used. The ACR system includes carrier particles in the developer for replenishment, and by providing a developer discharge port in the developing device, the carrier particles present in the developing device are gradually replaced to reduce the charging ability of the carrier particles. An electrophotographic image forming apparatus is widely used as a method for extending the saturation life. When this ACR system is used, since the developer life itself is long, the number of times of experiencing a large relative humidity change upon standing is large, and the development selectivity tends to be further deteriorated.

Further, in this embodiment, a developer having a lifetime of 100 k is used as a specification, and at this time, when the average image ratio is 5%, the deterioration state of the charge amount and the coarse powder development level are largely as follows. It can be roughly divided into endurance history.
Initial to 20k, 20k to 40k, 40k to 60k, 60k and beyond (1k = 1000).

  Therefore, a table of relative humidity variation and development AC waveform was created according to these four durability histories, and development selectivity and roughness level were confirmed.

  Table 3 (Duration History, Flight Bias Table for Relative Humidity Change) shows a flight bias duty table for the durability history and relative humidity change.

  When the table of Table 3 of this embodiment is used, when the present invention is not used, and when image formation is performed by a method that does not depend on the durability history as in Embodiment 1, the level of roughness is shown in Table 4. (Effects against Satsuki). When image formation is performed under the control of the conventional example in which the flying bias is constant, the coarse powder development is performed every time a long-term standing such as at night during durability enters, and the roughness becomes NG level from about 40k sheets. Further, in the method of Example 1, the deterioration of the roughness is reduced as compared with the conventional example, but an appropriate developability can be obtained with respect to the change in the development selectivity with respect to the relative humidity change amount that changes with durability. Absent. Accordingly, the consumption of the fine powder component slightly increases due to long-term durability, so that the coarse powder gradually accumulates and the roughness gradually deteriorates. Here, in the method of this example, a result that the roughness changes at a good level up to 100k sheets was obtained.

  As described above, by using the above-mentioned means, it is possible to reduce the rust due to the development selectivity, and it is possible to provide a higher quality image forming apparatus through durability.

  In other words, according to the present embodiment, the relationship between the duty ratio of the development AC bias and the relative humidity change amount, that is, the relative humidity difference ΔRH is changed according to the deterioration state of the developer in the developing device, so that the It is possible to provide a high-quality image forming apparatus with less roughness.

  The endurance history for controlling the flying bias presented in the present embodiment can also be used for an image forming apparatus of a normal developing method that does not use the ACR method. It is preferable to change the durability history table according to POD, etc.

1 Photosensitive drum (image carrier)
2 Charging device (charging roller)
3 Exposure Device 4 Development Device 44 Development Sleeve (Developer Carrier)
44a Magnet roll 51 Intermediate transfer belt (intermediate transfer member)
81 Development device temperature sensor (temperature / humidity detection means)
83 Temperature / humidity sensor of image forming apparatus (temperature / humidity detection means)
100 Image forming apparatus

Claims (3)

An image carrier on which an electrostatic latent image is formed;
A developer carrier that carries a developer and uses an electrostatic latent image of the image carrier as a toner image;
A developing bias applying means for applying a developing bias in which an AC bias is superimposed on a DC bias to the developer carrier;
A control unit for controlling the duty ratio of the developing bias;
A humidity detecting means for detecting the relative humidity around the image forming apparatus,
The controller is
When a predetermined time or more has passed since the image forming operation was stopped , based on the detection result of the humidity detection means, the relative humidity is higher after the stop than the predetermined time and before the predetermined time. Relative humidity before and after the stop for more than the predetermined time is higher than before the stop for the predetermined time or more before the stop for more than the predetermined time than when the relative humidity difference before and after the stop is the first relative humidity difference. as the difference is more in the case of the second relative humidity difference has magnitude than the first relative humidity difference, the amplitude value on the side where the toner flies is increased from the developer carrying member of the AC bias to said image bearing member to, perform the mode for changing the Duty ratio,
The mode is not executed when the predetermined time or more has not elapsed since the image forming operation was stopped.
An image forming apparatus.   2. The image forming apparatus according to claim 1, wherein the amplitude (Vpp) of the AC component in the developing bias is substantially constant.   The image forming apparatus according to claim 1, wherein the control unit changes the duty ratio so that the amplitude value on the flying side of the developing bias increases as the number of durable sheets increases.

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