JP4825577B2 - Image forming apparatus, charging voltage applying method, and developing bias voltage applying method - Google Patents

Description

  The present invention relates to an image forming apparatus that exposes the surface of a photoconductor to form a latent image, visualizes the latent image with toner, and fixes the toner on recording paper. The present invention relates to a method for controlling a charging voltage or a developing bias voltage, which is an image forming apparatus that sometimes prevents fogging potential difference and prevents unnecessary toner adhesion and carrier rising.

In the case of the reversal development method, the electrophotographic image forming apparatus uniformly charges the photosensitive member to the positive charging potential Vd by the charging unit, exposes the charged region by the exposing unit, and exposes the exposed portion to the exposure unit. The potential is set to VL and an electrostatic latent image is formed. A developing sleeve provided with a positive bias voltage and a positively charged toner is applied to the electrostatic latent image, and the toner is attached to be visualized.
Here, the developing bias voltage applied to the developing sleeve carrying the toner and the carrier is set lower than the non-image forming portion potential of the photosensitive member and higher than the image forming portion potential. As a result, the positively charged toner is attracted to an image forming portion (that is, a potential portion lower than the developing bias potential) on the surface of the photosensitive member to form a toner image. At this time, since the potential of the non-image forming portion of the photoconductor is set higher than the developing bias voltage, toner adhesion is prevented. That is, an amount of toner proportional to the potential difference (Vb−VL) between the developing bias voltage Vb applied to the developing sleeve and the electrostatic latent image VL adheres to the surface of the photoreceptor.
A potential difference (Vb−VL) between the developing bias voltage Vb and the electrostatic latent image VL is referred to as a developing potential. As an example, the charging potential of the photosensitive member is −600V, and the developing bias voltage is −400V.

  In such an image forming apparatus, a scotron charger is used as a charger for uniformly charging the surface of the photoreceptor. The Scotron charger performs corona discharge with a discharge electrode stretched substantially parallel to the axial direction of the photosensitive member to supply electric charge to the surface of the photosensitive member, thereby giving a constant charging potential to the surface of the photosensitive member. The Scotron charger includes a grid electrode between the discharge electrode and the photosensitive member, and a DC voltage is applied to the grid electrode to control the charging voltage on the surface of the photosensitive member.

At the beginning of the rotation of the photoconductor when the image forming apparatus starts up, the surface potential of the photoconductor does not reach the specified charging voltage at the same time as the charging voltage power supply of the charger is turned on. Voltage. This is because the charger has a certain width in the direction of rotation of the photoconductor, so that at the initial stage of rotation of the photoconductor, the photoconductor is charged by the charger on the upstream side in the rotation direction of the charger and the rotation direction vulcanization side. This is because the receiving time is different and the charging voltage gradually rises. On the other hand, the developing bias voltage applied to the developing means has no time delay and becomes a prescribed developing bias voltage according to the control target.
For this reason, when the charge rising section of the photosensitive member passes the developing position, a fogging potential is generated regardless of the timing at which the developing bias voltage is turned on. As a result, unnecessary toner adhesion and carrier scattering (carrying up) Occurs, causing problems such as shortening the life of the photoconductor and degrading the image. Such a phenomenon also occurs at the fall.

  The generation of the above-described fog potential is more likely to occur when the thickness of the photosensitive member is reduced. That is, since the developing sleeve, the cleaning unit, and the recording paper come into contact with the surface of the photoconductor, the photoconductor film formed on the surface of the photoconductor is shaved and thinned as the image forming apparatus is used. The photoreceptor film is formed by laminating a charge generation layer and a charge transport layer on an aluminum tube, but the thickness of the charge transport layer is particularly reduced. Thus, when the photosensitive member film thickness decreases, the light attenuation characteristics of the photosensitive member fluctuate and the surface charge increases. For this reason, in a photoconductor with a reduced film thickness, the difference from the developing bias voltage becomes large, and a fog (ground fog) potential is likely to occur.

In order to solve this problem, when the surface of the photoreceptor is raised to a specified potential, Patent Document 1 first supplies a low voltage bias as a grid voltage of the charger to raise the surface of the photoreceptor to a predetermined low potential. Thereafter, a technique for supplying a high voltage is disclosed.
Further, Patent Document 2 controls the lowering of the voltage applied to the charger based on the accumulated film reduction operation time of the photosensitive member or (the potential increase due to the film reduction of the photosensitive member) − (the potential increase of the photosensitive member exposed portion). As a quantity, a technique for lowering the voltage applied to the charger by the control amount is disclosed.
Patent Document 3 discloses a technique for detecting the surface potential of a photoconductor and controlling the developing bias voltage at the time of rising and falling of the developing bias voltage based on the detection output.
JP 2000-214662 A JP 2005-195890 A JP 2003-270913 A

Patent Document 1 is a method for solving the conventional problem by switching the grid voltage of a charger from a low voltage to a high voltage at a predetermined timing. However, Patent Document 1 describes a change such as a film reduction of a photoconductor. It cannot respond sufficiently.
Patent Document 2 controls the applied voltage of the charger with a control amount of (potential increase due to film reduction of the photoconductor) − (potential increase of the exposed portion of the photoconductor). However, in Patent Document 2, a predetermined charging voltage is applied to the photosensitive member by a charger, and normal exposure is performed. At that time (potential increase due to film reduction of the photosensitive member)-(potential of the photosensitive member exposed portion) Is measured). Therefore, fogging potential and carrier scattering may occur during this measurement.
Patent Document 3 controls the developing bias voltage in accordance with the surface potential of the photoconductor. The developing bias voltage is gradually increased as the surface potential of the photoconductor gradually increases at the beginning of the rise. . For this reason, it takes time until the surface potential of the photosensitive member and the developing bias voltage sufficiently reach the specified voltage, and the first copy is delayed.

Further, when a photoconductor left for a long period of time is charged, the surface potential Vo ₁ of the photoconductor for the first rotation is lowered from the predetermined surface potential Vo, and the predetermined surface potential Vo is obtained after the second rotation. Occurs. This is because at the first charging after being left for a long period of time, the photoconductor traps the carrier due to light fatigue, and the carrier trapped by the charging is released to lower the surface potential. However, in the charge after the second rotation, the trapped carriers disappear and a predetermined surface potential Vo is obtained.
In order to avoid the occurrence of fog or the like associated with a decrease in surface potential due to light fatigue, the following treatment is performed. That is, after being left for a long time, the first rotation of the photosensitive member is charged with a grid voltage of -200 V, and the next rotation (second rotation) is set to a normal developing bias and a normal surface potential Vo is obtained to obtain a predetermined surface potential Vo. Charging is performed at a grid voltage of −700 V obtained by adding light fatigue correction (Vo ₁ correction) to the grid voltage of −600 V. Then, after the third rotation after the surface potential is stabilized, charging and image forming processing are performed with a normal developing bias and a normal grid voltage of −600V. As described above, the idling of two rotations entered at the beginning of the copy operation, which was an obstacle to shortening the first copy time.

  In view of the above-described problems, the present invention stabilizes the image density by ensuring a fog prevention potential difference at the start-up and shutdown of the image forming apparatus to prevent unnecessary toner adhesion and carrier rising. It is something to be made. Furthermore, the idling time for absorbing the decrease in surface potential due to light fatigue is shortened, and the first copy is shortened.

In order to solve the above problems, an image forming apparatus of the present invention includes a photoconductor, a charging unit that charges the photoconductor with a charging voltage (Vg), and an exposure unit that forms an electrostatic latent image on the charged photoconductor. A developing means for developing the electrostatic latent image into a toner image, a transfer means for transferring the toner image to a recording medium, and a photosensitive member surface potential measuring means, wherein the charging means has a first charging voltage (Vg ₁ ). And a voltage generation unit that generates a _second charging voltage (Vg ₂ ) higher than the first charging voltage, and a measurement result of the surface potential of the photosensitive member charged by the _first charging voltage (Vg ₁ ). The amount of decrease in surface potential (ΔV ₁ ) due to the liberation of carriers trapped in the photosensitive member when the photosensitive member is charged by the first charging voltage is calculated, and the second charging voltage (Vg ₂ ) is adjusted by the following equation: A charging voltage control unit,
Vg ₂ = Vg− (Vg / Vg ₁ ) · ΔV ₁
The photosensitive member is charged by the adjusted second charging voltage (Vg ₂ ), and after the carriers trapped in the photosensitive member disappear, the photosensitive member is charged by the charging voltage (Vg). This eliminates the generation of fogging potential when the charging potential is raised or lowered, prevents unnecessary toner adhesion and carrier rising, and stabilizes the image density. In addition, it is possible to shorten the first copy after being left for a long time.

In order to solve the above problems, the image forming apparatus of the present invention includes a photoconductor, a charging unit that charges the photoconductor with a charging voltage (Vg), and an exposure unit that forms an electrostatic latent image on the charged photoconductor. A developing bias generator, a developing unit that applies a developing bias voltage (Vb) from the developing bias generator to develop the electrostatic latent image into a toner image, a transfer unit that transfers the toner image to a recording medium, and a photosensitive member. A body surface potential measuring unit, and the charging unit includes a first charging voltage (Vg ₁ ) and a voltage generation unit that generates a _second charging voltage (Vg ₂ ) higher than the first charging voltage. The bias generation unit is configured to detect the surface potential of the photosensitive member charged by the _first charging voltage (Vg ₁ ), and the carrier trapped on the photosensitive member when the photosensitive member is charged by the first charging voltage. Of surface potential due to the release of Calculating a lower amount (ΔV _1), a developing bias voltage control unit for correcting the following equation a developing bias voltage (Vb _2),
Vb ₂ = Vb + (Vg / Vg ₁ ) · ΔV ₁
The corrected developing bias voltage (Vb ₂ ) is applied to the developing means, and after the carriers trapped on the photosensitive member disappear, the developing bias voltage (Vb) is applied to the developing means. This eliminates the generation of fogging potential when the charging potential is raised or lowered, prevents unnecessary toner adhesion and carrier rising, and stabilizes the image density. In addition, it is possible to shorten the first copy after being left for a long time.

Further, the present invention comprises the photosensitive member surface potential measuring means upstream of the developing means, and after the charged region of the photosensitive member charged with the second charging voltage passes through the photosensitive member surface potential measuring means , It is desirable to expose the photoreceptor by exposure means. Thereby, the measuring means is not contaminated by the scattered toner. In addition, the charged region can be detected earlier in the early stage of rotation of the photosensitive member, and the first copy can be further shortened.
In the present invention, it is preferable that the photosensitive member is exposed by the exposure unit after the charged region of the photosensitive member charged with the second charging voltage has been rotated once or more. Thus, the entire surface of the photosensitive member is uniformly charged and then exposed, so that a high-quality image can be obtained.
In the image forming apparatus of the present invention, it is desirable that the photoconductor surface potential measuring unit measures a plurality of locations on the surface of the photoconductor and obtains an average value thereof. As a result , it can be measured whether the entire surface of the photosensitive member is uniformly charged .

Further, the image forming apparatus of the present invention further includes a leaving time measuring unit that measures a leaving time after the end of the last final printing operation, and when the leaving time is equal to or greater than a predetermined value, the measurement by the photoconductor surface potential measuring unit is performed. It is desirable to adjust the second charging voltage based on the result, and perform exposure after the photosensitive member charged by the adjusted second charging voltage makes one rotation or more . As a result, the first copy operation can be started after the charge is uniformly applied to the entire surface of the photoconductor on which the carrier is trapped. Therefore, a copy without failure can be obtained. In this case, it is desirable to measure the surface of the photoconductor at a plurality of locations and obtain the average value. This makes it possible to make a copy after confirming that the entire surface of the photosensitive member has been uniformly charged.
Further, when the standing time is less than a predetermined value , the second charging voltage is adjusted based on the measurement result of the photosensitive member surface potential measuring means, and the photosensitive member is charged with the adjusted second charging voltage. It is desirable to expose . As a result, the first copy can be further shortened by effectively using the charged potential remaining on the photoconductor after the end of the final printing operation.

Furthermore, according to another aspect of the present invention, there is provided a charging voltage application method for an image forming apparatus, the photosensitive member, a charging unit for charging the photosensitive member with a charging voltage (Vg), and the charged photosensitive member electrostatically. an exposure means for forming a latent image, a developing means for developing the electrostatic latent image into a toner image, comprising a transfer unit that transfers the toner image to the recording medium, the photosensitive member surface potential measuring means, the first charging voltage ( a first charging step of charging the photosensitive member in vg _1), a measuring step of measuring the surface potential of the first charged by the charging step the the photosensitive member, on the basis of the measurement result, the first charging A step of calculating a reduction amount (ΔV ₁ ) of the surface potential due to liberation of carriers trapped in the photosensitive member when the photosensitive member is charged by the voltage, and a charging voltage for adjusting the second charging voltage (Vg ₂ ) by the following equation Control steps;
Vg ₂ = Vg− (Vg / Vg ₁ ) · ΔV ₁
Charging the photosensitive member with the adjusted second charging voltage (Vg ₂ ); and charging the photosensitive member with the charging voltage (Vg) after the carriers trapped in the photosensitive member disappear . This eliminates the generation of fogging potential when the charging potential is raised or lowered, prevents unnecessary toner adhesion and carrier rising, and stabilizes the image density. Also, the first copy can be shortened.

The present invention also relates to a method for applying a developing bias voltage of an image forming apparatus, comprising: a photosensitive member; a charging unit that charges the photosensitive member with a charging voltage (Vg); and an exposure that forms an electrostatic latent image on the charged photosensitive member. Means, a developing bias generator, a developing means for applying a developing bias voltage (Vb) from the developing bias generator to develop the electrostatic latent image into a toner image, a transfer means for transferring the toner image to a recording medium, A first charging step for charging the photosensitive member with a first charging voltage (Vg ₁ ) ; and a surface potential of the photosensitive member charged in the first charging step. A measurement step, a step of calculating a reduction amount (ΔV ₁ ) of the surface potential due to the liberation of carriers trapped in the photoconductor when the photoconductor is charged by the first charging voltage based on the measurement result, and a development bias Voltage A developing bias voltage control step of adjusting by the following formula Vb _2),
Vb ₂ = Vb + (Vg / Vg ₁ ) · ΔV ₁
A developing bias applying step of applying the adjusted developing bias voltage (Vb ₂ ) to the developing means , and a step of applying the developing bias voltage (Vb) after the carriers trapped on the photosensitive member disappear . This eliminates the generation of fogging potential when the charging potential is raised or lowered, prevents unnecessary toner adhesion and carrier rising, and stabilizes the image density. Also, the first copy can be shortened.

  According to the image forming apparatus of the present invention, the occurrence of fogging potential at the time of rising and falling of the charging potential is eliminated, unnecessary toner adhesion and carrier rising are prevented, and the image density is stabilized. In addition, it is possible to shorten the first copy after being left for a long time. Alternatively, the entire surface of the photoreceptor can be uniformly charged to obtain a high quality image.

The image forming apparatus of the present invention is applied to an image forming apparatus that forms an image by an electrophotographic process such as a copying machine, a printer apparatus, a facsimile apparatus, and a multifunction machine having these functions. It can also be applied to monochrome compatible machines.
FIG. 1 is a configuration diagram illustrating an image forming portion 100 of the image forming apparatus. The image forming apparatus includes a photosensitive member 1, a charging unit 2, an exposure unit 3, a developing unit 4, a transfer unit 5, a cleaning unit 6, a charge removing unit 7, and the like disposed on the outer periphery thereof, and further includes a fixing unit 8. Composed. The recording paper 9 is conveyed to the image forming portion 100 from a paper feeding device (not shown) by a conveyance path in accordance with the transfer timing of the transfer means 5. After the transfer, the toner is fixed to the recording paper 9 by the fixing means 8. Then, it is discharged out of the image forming apparatus. In FIG. 1, the recording paper 9 is shown as a long continuous paper for the sake of convenience, but a recording paper of a suitable paper size is conveyed by the conveying means.

The charging unit 2 is a device that uniformly charges the surface of the photoreceptor 1 to a predetermined potential (for example, −400 to −800 V, preferably −500 to −700 V). The charging unit 2 a and the charger 2 b are connected to each other. In addition, the surface potential of the photoreceptor 1 is controlled by the charging control unit 2a. A scorotron charger can be used as the charger. In addition, a contact-type charging device such as a roller type or a brush type can be used.
In FIG. 1, a scorotron charger is shown, a high voltage for discharge is applied to the discharge electrode in the charger 2b, and the power supply means 11 is connected to the grid electrode. Charge control unit 2a for applying a charging voltage to the grid electrode comprises a first grid bias voltages Vg ₁ and the second grid bias voltage Vg _2. A first grid bias voltage Vg1 second grid bias voltage Vg ₂ is at startup as will be described later, is switched at a predetermined timing during the fall.

The exposure unit 3 exposes the surface of the photosensitive member 1 uniformly charged by the charging unit 2 by scanning the light corresponding to the image information in the axial direction of the photosensitive member, thereby exposing the surface potential VL after the exposure, for example. The electrostatic latent image is formed at −75 V or less. Although the present invention can be applied to both the regular development system and the reverse development system, the latter system will be described here.
The exposure means 3 illuminates the original placed on the original reading table, reads the reflected light from the original through an optical system such as a mirror and a lens, and converts the read signal into a digital signal. Thereafter, image processing is performed to generate an image signal, and the light beam is PWM-modulated by the image signal, scanned in the axial direction of the photosensitive member 1, and sub-scanned in the rotational direction of the photosensitive member to expose the image region. In addition, an image signal acquired through a personal computer or the Internet may be used. An exposure laser, an exposure LED array, or an exposure lamp can be used as the exposure light source.
The exposure unit 3 includes, for example, a control unit that controls exposure energy, and can control the exposure power.

  The developing means 4 supplies toner to form a toner image on the surface of the photoreceptor 1 on which the electrostatic latent image is formed, in order to visualize the surface by a negative or positive development method. As a developing method, a dry two-component developing method is used. The developing means 4 is provided with a developing sleeve (a developing sleeve shown in FIG. 1) 4a that is in close proximity to the photosensitive member 1, and further provided with a stirrer that stirs the toner and the carrier in the developing unit, thereby reducing the toner to minus. Triboelectrically charged. In addition to this, the developing device includes a toner supply roller, a toner supply port, and a toner bottle for supply. Further, a developing bias is supplied from the power supply unit 11 to the developing sleeve 4a. Therefore, when the developing sleeve 4a holding the toner contacts or approaches the photoreceptor 1 on which the electrostatic latent image is formed, the toner on the surface of the developing sleeve 4a is statically caused by a potential difference between the photoreceptor 1 and the developing sleeve 4a. Transpose to an electrostatic latent image. As a result, the image is visualized.

The transfer unit 5 is disposed at a position facing the photoconductor 1 with the recording paper 9 interposed therebetween, and receives a power supply of, for example, plus 5 μA from the power supply unit 11. When the toner image on the surface of the photoconductor 1 approaches the transfer unit 5, the toner image adhering to the surface of the photoconductor 1 due to static electricity is transferred in the direction of the transfer unit 5 due to a potential difference between the photoconductor 1 and the transfer unit 5. At that time, the recording paper 9 is conveyed between the photosensitive member 1 and the transfer unit 5 at a timing when the toner image is transferred from the photosensitive member 1 to the transfer unit 5 by the registration roller. In this way, the toner image is transferred to the recording paper 9.
Here, the transfer unit 5 has been described as being directly transferred onto the recording paper 9, but such a method is convenient for forming a monochrome image. When forming a color image, the image forming portion 100 shown in FIG. 1 is prepared for each color of YMCK, and the toner images of each color of YMCK formed by the YMCK image forming portion 100 are sequentially superimposed and transferred onto the intermediate transfer belt. A method of forming a full-color toner image may be used.

The cleaning means 6 collects the toner remaining on the surface of the photoconductor 1 and is configured to contact a cleaning blade made of urethane rubber or the like formed in a plate shape on the surface of the photoconductor 1. Alternatively, the belt cleaning is arranged so as to contact the photoconductor.
The neutralizing unit 7 removes a residual potential on the photosensitive member, and a neutralizing lamp is used.
The fixing means 8 pressurizes and heats the recording paper to which the toner image is transferred, thereby melting and pressurizing the toner to fix it on the recording paper.
The power supply unit 11 includes, for example, a transformer, a rectifier, a converter, and the like, and supplies a discharge current and a grid bias to the charging unit 2 to supply a development current to the discharge current generation unit, the grid bias generation unit, and the development unit 4. A transfer current generator is provided to supply a transfer current to the developing bias generator and the transfer means 5.
In addition, a power source for supplying power to the heater of the heating roller of the fixing device 8, a power source for supplying power to the internal heater of the photosensitive member, and a power source for driving the driving roller on the photosensitive member and the conveyance path may be provided. Good.

The image forming apparatus of the present invention is provided with the following sensors in order to acquire information that affects the electrophotographic process.
For example, the surface potential meter 21 that measures the charged portion potential Vd, the exposed portion potential VL, the residual potential Vs, and other potentials of the photosensitive member 1 is provided between the charging unit 2 and the exposing unit 3 or between the exposing unit 3 and the developing unit 4. Are disposed close to the surface of the photoreceptor 1. In this embodiment, in order to shorten the first copy, the surface electrometer 21 is arranged between the charging unit 2 and the exposure unit 3 as shown in FIG. In order to measure the exposure portion potential VL, it may be provided on the downstream side of the exposure means 4. One surface electrometer 21 may be provided, but a plurality of surface electrometers may be arranged in the axial direction of the photosensitive member, and the measurement results may be averaged.

  In the first embodiment of the present invention, the surface electrometer 21 uses a detection output detected at the initial rotation of the photoreceptor 1. In the second embodiment, measurement is performed a plurality of times (for example, measurement is performed every time the photoconductor rotates 60 °) while the photoconductor 1 rotates once, and the measurement results are averaged to obtain a measurement result. In this way, by using a plurality of surface potentiometers 21 and measuring a plurality of times along with the rotation of the photoreceptor, the potential of the entire surface of the photoreceptor can be measured.

  As the surface potential meter 21, as shown in FIG. 2, a vibration capacitance type surface potential meter that measures the surface potential of the photoreceptor without contact is used. The vibration capacity type surface potential meter has a measurement electrode 31 disposed at a distance of 1.7 to 2.3 mm from the surface of the photoreceptor 1, and forks 32 and 33 disposed between the measurement electrode 31 and the photoreceptor 1; The piezoelectric element 34 for vibration and the piezoelectric element 35 for vibration detection are arranged as chopping means, and the surface potential is measured by measuring the charge induced in the measurement electrode 31 by the measuring means 36. The vibration capacity type surface potential meter may be a tuning fork type chopper device, a motor type chopper device, or a pendulum type chopper device.

A toner adhesion amount detection sensor 22 is disposed on the downstream side of the developing unit 4 and on the upstream side of the transfer unit 5. The toner adhesion amount detection sensor 22 irradiates the photoreceptor 1 with light from, for example, a light emitting element that emits infrared light or visible light, receives the reflected light by the light receiving element, and measures the toner adhesion amount based on the light reception amount. To do. A so-called photo-interrupter type photoelectric element is used.
In the intermediate transfer belt method, the toner adhesion amount on the intermediate transfer belt may be measured by a reflective optical sensor at a position downstream of the primary transfer.

A temperature sensor 23 and a humidity sensor 24 are provided to detect the temperature and humidity of the image forming unit. The temperature sensor 23 and the humidity sensor 24 may be any place that detects the atmosphere of the image forming unit, and is desirably in the vicinity of the photosensitive member 1.
The present invention further includes a cumulative copy number counter 25 and a cumulative rotation time meter 26 of the photosensitive member 1. The installation location of the copy number counter 25 is not particularly limited, and may be a position where the number of recording sheets conveyed can be counted. The rotation time meter 26 of the photosensitive member 1 may monitor the rotation of the rotating shaft of the photosensitive member or the roller driven by the rotation of the photosensitive member and measure the time. Instead of the rotation time meter 26, a cumulative number of rotations for counting the number of rotations of the photosensitive member may be used. In addition, a counter is provided that counts the number of photoconductor replacements when the photoconductor is replaced.

  Here, each potential relationship of the reversal development method in the electrophotographic process is as follows. First, when the photosensitive member 1 is uniformly charged to the surface potential Vo by the charging unit 2 and the charged portion is exposed by the exposure unit 3, the exposed portion becomes the exposed portion potential VL. Here, the difference (Vb−VL) between the developing bias voltage Vb and the exposed portion potential VL is referred to as a developing potential, and an amount of toner proportional to the developing potential adheres to the surface of the photoreceptor 1 and development is performed. It is. The toner adhering to the surface of the photoreceptor 1 is transferred to the recording paper 8, whereby an image is formed on the recording paper 8. Thus, the toner adhesion amount varies depending on the magnitude of the development potential. That is, the relationship between the surface potential of the photoreceptor 1, the toner charge amount, and the toner adhesion amount is that the toner adheres to the surface of the photoreceptor 1 in proportion to the development potential (Vb−VL). Inversely proportional to

In general, the image forming apparatus is negatively charged by frictional charging between the developer and the toner in the developing device. Therefore, when the developer is used for a long time, it tends to be difficult to be charged. Therefore, it is related to the degree of fatigue of the developer. Eventually, when the toner charge amount is low, the toner concentration is high, and when the toner charge amount is high, the reverse is true. The relationship between the humidity and the toner adhesion amount is such that the toner charge amount is high when the humidity is low and the toner charge amount is low when the humidity is high.
The photosensitive member 1 is an OPC photosensitive member, in which a charge generating layer is formed with a thickness of 1 μm and a charge transport layer is formed with a thickness of 30 μm on an aluminum tube having a thickness of 2 mm, a diameter of 60 mm, and a length of 340 mm. is there. Since the developing sleeve 4a, the cleaning means 6 and the recording paper are usually brought into contact with the surface of the photoconductor, the charge transport layer as the surface layer of the photoconductor 1 is scraped to reduce the film thickness and the exposure sensitivity changes with time. As a result, the light attenuation characteristics of the photosensitive member 1 fluctuate, and the optimum developing bias shifts. As a result, density variation occurs.

Further, the transfer is changed by the transfer current of the transfer unit 5, and the image density is also changed by the heating temperature and pressurization degree of the fixing unit 8.
As described above, the toner adhesion amount is determined by the photosensitive member charging potential, developing bias voltage, exposure energy, developing potential, toner charging amount, toner concentration, transfer current, temperature, humidity, and the like.

As shown in FIG. 3, the Scotron charger used in the present invention includes a discharge electrode 41 stretched substantially parallel to the axial direction of the photoreceptor 1, and the photoreceptor 1 between the discharge electrode 41 and the photoreceptor 1. The grid electrode 42 for controlling the potential and the shield case 43 supported so as to cover the discharge electrode 41 constitute a main part.
A high voltage is applied to the discharge electrode 41 by a DC power supply 44 having a constant current output of −500 μA, and at the same time, a predetermined bias voltage to be charged to the grid electrode 42 is applied from the DC power supply 45, thereby corona discharge on the surface of the photoreceptor 1. To supply a charge to give a constant charging potential to the surface of the photoreceptor. Here, the DC power supply 45 applied to the grid electrode 42 can supply the first grid bias voltage Vg ₁ and the second grid bias voltage Vg _2, and the two grid bias voltages are switched at a predetermined timing. It is done.

The first grid bias voltage Vg ₁ is desirably about −150 to −300 V in order to suppress the fluctuation range due to the decrease in the photoreceptor surface potential due to the liberation of trapped carriers. For example, the grid electrode 42 is −200 V. -200V is applied to. Here, in order to prevent fogging, the potential difference between the photoreceptor surface potential and the developing bias voltage, so-called fogging prevention potential difference (cleaning field), is set to 50 to 500V, preferably 100 to 300V. Since −200 V is applied to the grid electrode 42 at the initial rotation of the photoreceptor 1, the developing bias voltage is set to +50 V. As a result, the surface potential Vo of the photosensitive member including the potential drop is about −150 to −200 V, and 200 to 250 V is secured as the fog prevention potential difference (cleaning field), so that the fog is surely prevented. As a result, unnecessary toner adhesion is prevented and occurrence of carrier scattering is prevented.

Second grid bias voltage Vg ₂ is set to a voltage applied to normal charge voltage on the photosensitive member 1, to measure the surface potential of the photosensitive member 1 charged by the first grid bias voltages Vg ₁ as described below , Adjusted according to the measurement result. That is, it is -600 to -800V.
Switching between the first grid bias voltage Vg ₁ and the second grid bias voltage Vg ₂ is performed by a CPU provided in the image forming apparatus.
The contact charger described below is the same in the voltage and switching of the first grid bias voltage Vg ₁ and the second grid bias voltage Vg ₂ . However, the grid bias voltage is read as the charging bias voltage.

  In the contact charger, as shown in FIG. 4, the charging roller 51 is pressed against the surface of the photoconductor 1 by a biasing force (not shown) such as a spring at a pressure suitable for charging, and rotates following the rotation of the photoconductor 1. To do. The charging roller 51 is obtained by integrally processing a conductive rubber material 53 by molding on the outside of the core metal 52 except for both ends. A charging bias voltage is applied to the cored bar 52 from a charging power source 54 to charge the surface of the photoreceptor 1.

The charging roller 51 has a rubber elastic body 53 having a volume resistance of 10 ⁶ to 10 ⁸ Ωcm concentrically arranged around a metal shaft such as aluminum, and the surface thereof has a higher volume resistance of 10 ⁸ to 10 ^10. It is formed by coating with a surface protective layer 57 of Ωcm. As the rubber elastic body 53, the first type is an electronic conductive type in which conductive resistance such as carbon and metal powder is dispersed in an insulating organic material such as butadiene and EPDM to make the volume resistance a predetermined value. The second type is an organic material added with an ionic agent such as polar rubber or metal salt, surfactant, which is a high volume resistance organic material having a polar group with chemical activity in the molecular structure, such as urethane and hydrin. And the like.

FIG. 5 is a block diagram of the image forming apparatus of the present invention.
In FIG. 5, the charging unit 61 corresponds to the charging unit 2 shown in FIG. 1, and is connected to the bus line 62 via the charging control unit 2a. The output voltage of the charge control unit 2a is provided with a first grid bias voltage Vg _1, it is capable of switching the second grid bias voltage Vg _2. Further, the application of the high voltage Vh to the discharge electrode 41 is controlled. The exposure unit 63 corresponds to the exposure unit 3 in FIG. 1, and the exposure unit 63 is connected to the bus line 62 via the exposure power control unit 64. The developing unit 65 corresponds to the developing unit 4 in FIG. 1, applies a developing bias voltage to the developing sleeve 4a, and is connected to the bus line 62 via the developing bias controller 4b.

A CPU 66 is connected to the bus line 62, and the CPU 66 realizes image formation by sequentially controlling each component based on a process control sequence program 68 stored in the memory 67. These steps are executed to realize a charging voltage applying method or a developing bias voltage applying method of the image forming apparatus. The memory 67 further includes an image memory 69 and a surface potential data storage unit 70 when the first charging voltage is applied. The outputs of the surface potential meter 21, the toner adhesion amount sensor 22, the temperature sensor 23, and the humidity sensor 24 are shown as outputs of various sensors 71 and connected to the bus line 62. Further, the image forming apparatus of the present invention includes a calculator 72, a charging voltage adjusting unit 73, or a developing bias voltage adjusting unit 74.
The surface potential data storage unit 70, the calculator 72, the charging voltage adjusting unit 73, and the developing bias voltage adjusting unit 74 when the charging voltage is applied will be described later.

FIG. 6 is a time chart for explaining the first copy of the image forming apparatus of the present invention.
First, as shown in FIGS. 6A, 6B, and 6C, at the time t0, the drum motor of the photosensitive member 1 starts rotating, and at the same time, the static elimination lamp 7 is turned on. At the same time, a constant current output of −500 μA is applied from the DC power supply 44 to the discharge electrode 41 of the Scotron charger. Further, as shown in FIGS. 6D and 6E, the first grid bias voltage Vg ₁ (−200 V) is applied to the grid electrode 42. A developing bias Vb (+ 150V) is applied to the developing roller 4a.

Here, as shown in FIG. 3, the most upstream end of the Scotron charger is P ₁ and the most downstream end is P ₂ . Over time T1 from the time t0, the photosensitive member 1 is rotated, the photoconductor region S2 facing the downstream end P ₂ of the charger as shown in FIG. 6 (f) to a position with a surface potential sensor 21 Suppose that it rotates. That is, when the photosensitive body 1 advances a distance s from the position facing the charger 2 to a position where the surface potential sensor 21 is located, the surface potential is detected and starts to rise, and further, the time T2 passes and the most upstream end P _1. After reaching, the surface potential Vos (−170 V) detected by the surface potential sensor 21 is constant.
The period after reaching the most upstream end P ₁ is defined as a surface electrometer side period T3. Here, in order to prevent the influence of measurement system noise and the like, the surface potential sensor output is read 8 times at an interval of 10 ms and averaged to obtain an average surface potential Vos (−170 V).

Here, the amount of decrease ΔV _{1 in} the surface potential during charging by the first grid bias voltage Vg ₁ (−200 V) is:
ΔV ₁ = abs (Vg ₁ −Vos) = 30V
Can be calculated.
Therefore, the decrease amount ΔV ₂ of the surface potential at the normal grid bias voltage Vg (−600 V) is
ΔV ₂ = (Vg / Vg ₁ ) ・ ΔV ₁ = 90V
Can be estimated. As a result, the second grid bias voltage _{Vg 2 Vg 2 = Vg- △ V} 2 = -690V
Can be obtained as Therefore, the charging voltage is switched to the second grid bias voltage Vg ₂ (−690 V) at the timing of time t2.

As shown in FIG. 6G, when time T4 elapses from time t0 and time t1, the most downstream end P ₂ reaches the development position and the surface potential Vod (−170 V) of the photoreceptor 1 at the development position. Starts to rise and becomes constant. As described above, the surface potential Vod at the developing position starts to rise again at time t3 when the time T5 has elapsed from the time t2 when switching to the second grid bias voltage Vg ₂ (−690 V). At this time, the surface potential is charged by the second grid bias voltage (−690 V), but as described above, the surface potential is lowered (90 V) due to light fatigue, and the surface potential becomes −600 V.

As described above, since the developing bias Vb is maintained at +150 V while the charging voltage is changed, that is, during the period of time t0 to t1, a fog prevention potential difference of 250 V is secured. In addition, by switching the development bias Vb from ( +150 V) to (+50 V) in synchronization with the time T2 having elapsed and the surface potential at the development position becoming Vod (−170 V), the period from time t2 to t3 is also achieved. A fog prevention potential difference of 250 to 220 V can be secured.

Time T6 the photoreceptor rotation 1 from time t2, keeping the second grid bias voltage Vg ₂ (-690V), switches to 1 rotated time t4 regular grid bias voltage Vg (-600 V). Since the carrier trapped by the first charge is extinguished in the photoconductor, the normal surface potential Vo (−600 V) is obtained thereafter.
In synchronism with the charged surface by the second grid bias voltage Vg ₂ (−690 V) reaching the developing position, the surface potential Vod at the developing position starts increasing, and the surface potential Vod (−600 V) becomes constant, The developing bias Vb is switched from +50 V to −350 V, and the fog prevention potential difference is 220 to 250 V during the period from time t3 to t3 + T2, and the fog prevention potential difference is 250 V after time t3 + T2.
As shown in FIG. 6 (h), when time T2 + T5 has elapsed from time t4 and the most upstream end P ₁ ′ after one rotation has reached the exposure position, the laser diode LD is turned on according to the image data and static Create an electrostatic latent image.
As described above, in the first embodiment, it is possible to prevent the occurrence of fogging and carrier scattering, and it is possible to shorten the first copy time.

(Second Embodiment)
The first embodiment corrects the grid bias voltage to absorb the effect of light fatigue, whereas the second embodiment shown in FIG. 7 corrects the development bias voltage to absorb the effect of light fatigue. Therefore, description of operations similar to those of the first embodiment is omitted.

From time t0 to t2, the same operation as that of the first embodiment is performed, and the surface potential decrease amount ΔV ₂ (90 V) at the normal grid bias voltage Vg (−600 V) is estimated. Then, the second developing bias voltage Vb ₂ obtained by correcting the normal developing bias Vb (−350 V) with the decrease amount ΔV ₂ is expressed as Vb2 = Vb + ΔV _2.
= -260V
Is switched to the regular grid bias voltage Vg (−600 V) at the timing of time t2, and the regular grid bias voltage Vg is maintained after time t2.

At time t3 when time T5 has elapsed from time t2, the surface potential Vod at the development position starts to rise again, the surface potential decreases due to light fatigue of 90V, and the surface potential becomes −510V.
Further, after time t3 , the charged surface by the regular grid bias voltage Vg (−600 V) reaches the developing position, the surface potential Vod at the developing position starts to rise, and the surface potential Vod is kept constant at −510 V. Become. In synchronization with this, the developing bias Vb is switched from +50 V to the second developing bias Vb ₂ (−260 V).
In this manner, the fog prevention potential difference 220 to 250 V is secured during the period from time t3 to t3 + T2, and the fog prevention potential difference 250V is secured after time t3 + T2.

Thereafter, when time T6 elapses from time t2 and time t4 when the photoconductor rotates once, the carrier trapped by the first charge disappears, and the rising starts and the normal surface The potential is Vo (−600 V). In order to synchronize with this, when the time t4 + T4 is obtained by adding the movement time T4 to the development position, the normal development bias voltage Vb (−350 V) is switched.
Therefore, the fog potential (250 V) after time t3 is secured.
As described above, in the second embodiment, the occurrence of fogging and carrier scattering can be prevented, and the first copy time can be shortened.

(Embodiment 3)
In the time chart of the first embodiment shown in FIG. 6, except that the timing of image formation by the laser diode is changed to t5 ′ = t3 + T2 + T5, it is the same as in the first embodiment, the photofatigue is corrected, and the normal photoreceptor surface potential Vo is corrected. An electrostatic latent image is formed at (-600V), and the first copy time can be further shortened.

(Embodiment 4)
In the time chart of the second embodiment shown in FIG. 7, the timing of image formation by the laser diode is the same as that of the second embodiment except that t5 ′ = t3 + T2 + T5. An electrostatic latent image is formed at (-600V), and the first copy time can be further shortened.

1 is a configuration diagram of an image forming apparatus. A schematic diagram of a surface electrometer is shown. A schematic diagram of a scorotron charger is shown. A schematic diagram of a contact charger is shown. 1 shows a block diagram of an image forming apparatus. 2 shows a time chart of the first embodiment. The time chart figure of Embodiment 2 is shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Photoconductor 2 Charging means 3 Exposure means 4 Developing means 5 Transfer means 8 Fixing means 11 Power supply means 21 Surface potential meter 72 Calculator 73 Charging voltage control means 74 Developing bias voltage control means

Claims (9)

A photosensitive member, a charging means for charging the photosensitive member by the charging voltage (Vg), an exposure means for forming an electrostatic latent image on the charged photoreceptor, a developing unit for developing the electrostatic latent image into a toner image, toner A transfer means for transferring an image to a recording medium, and a photoreceptor surface potential measuring means;
The charging means includes a first charging voltage (Vg ₁ ) and a voltage generation unit that generates a _second charging voltage (Vg ₂ ) higher than the first charging voltage, and the first charging voltage (Vg ₁ ). Based on the measurement result of the surface potential of the charged photoconductor, the amount of decrease in surface potential (ΔV ₁ ) due to the liberation of carriers trapped in the photoconductor when the photoconductor is charged by the first charging voltage is calculated, A charging voltage control unit for adjusting the second charging voltage (Vg ₂ ) according to the following equation;
Vg ₂ = Vg− (Vg / Vg ₁ ) · ΔV ₁
The photosensitive member is charged with the adjusted second charging voltage (Vg ₂ ), and after the carriers trapped in the photosensitive member disappear, the photosensitive member is charged with the charging voltage (Vg). apparatus. A photosensitive member; a charging unit that charges the photosensitive member with a charging voltage (Vg) ; an exposure unit that forms an electrostatic latent image on the charged photosensitive member; a development bias generation unit; Vb) is applied, and developing means for developing the electrostatic latent image into a toner image, transfer means for transferring the toner image to a recording medium, and photoconductor surface potential measuring means,
The charging unit includes a voltage generation unit that generates a first charging voltage (Vg ₁ ) and a second charging voltage (Vg ₂ ) higher than the first charging voltage ,
The developing bias generator is trapped by the photosensitive member when the photosensitive member is charged with the first charging voltage based on the measurement result of the surface potential of the photosensitive member charged with the _first charging voltage (Vg ₁ ). A developing bias voltage control unit for calculating a reduction amount (ΔV ₁ ) of the surface potential due to the liberation of the carrier and correcting the developing bias voltage (Vb ₂ ) by the following equation;
Vb ₂ = Vb + (Vg / Vg ₁ ) · ΔV ₁
The corrected developing bias voltage (Vb ₂ ) is applied to the developing unit, and the developing bias voltage (Vb) is applied to the developing unit after the carriers trapped in the photoreceptor disappear. Forming equipment. The photoconductor surface potential measuring means is provided on the upstream side of the developing means, and after the charged area of the photoconductor charged with the second charging voltage passes through the photoconductor surface potential measuring means , the photoconductor is exposed by the exposure means. The image forming apparatus according to claim 1 , wherein the image forming apparatus is exposed.   4. The image formation according to claim 1, wherein the photosensitive member is exposed by an exposure unit after the charged region of the photosensitive member charged with the second charging voltage has rotated one revolution or more. 5. apparatus. 5. The image forming apparatus according to claim 1, wherein the photosensitive member surface potential measuring unit measures a plurality of locations on the surface of the photosensitive member and calculates an average value thereof. 6. A leaving time measuring means for measuring a leaving time after completion of the last final printing operation is further provided, and when the leaving time is a predetermined value or more, a second charging voltage is determined based on a measurement result of the photosensitive member surface potential measuring means. 3. The image forming apparatus according to claim 1 , wherein exposure is performed after the photosensitive member charged with the adjusted second charging voltage makes one or more rotations . It further includes a leaving time measuring means for measuring a leaving time after the last final printing operation, and when the leaving time is less than a predetermined value , the second charging voltage is determined based on the measurement result of the photoreceptor surface potential measuring means. The image forming apparatus according to claim 1 , wherein the photosensitive member is charged with the adjusted second charging voltage and then exposed . A photosensitive member; a charging unit that charges the photosensitive member with a charging voltage (Vg); an exposure unit that forms an electrostatic latent image on the charged photosensitive member; a developing unit that develops the electrostatic latent image into a toner image; A transfer means for transferring an image to a recording medium, and a photoreceptor surface potential measuring means;
A first charging step for charging the photosensitive member with a _first charging voltage (Vg ₁ ) ;
A measuring step for measuring the surface potential of the photoconductor charged in the first charging step;
Calculating a reduction amount (ΔV ₁ ) of the surface potential due to liberation of carriers trapped in the photoconductor when the photoconductor is charged by the first charging voltage based on the measurement result ;
A charging voltage control step for adjusting the second charging voltage (Vg ₂ ) according to the following equation;
Vg ₂ = Vg− (Vg / Vg ₁ ) · ΔV ₁
A second charging step for charging the photosensitive member with the adjusted second charging voltage (Vg ₂ ) ;
A charging voltage applying method for an image forming apparatus, comprising: a step of charging the photosensitive member with a charging voltage (Vg) after the carrier trapped on the photosensitive member disappears . A photosensitive member; a charging unit that charges the photosensitive member with a charging voltage (Vg); an exposure unit that forms an electrostatic latent image on the charged photosensitive member; a development bias generation unit; Vb) is applied, and developing means for developing the electrostatic latent image into a toner image, transfer means for transferring the toner image to a recording medium, and photoconductor surface potential measuring means,
A first charging step for charging the photosensitive member with a _first charging voltage (Vg ₁ ) ;
A measuring step for measuring the surface potential of the photoconductor charged in the first charging step;
Calculating a reduction amount (ΔV ₁ ) of the surface potential due to liberation of carriers trapped in the photoconductor when the photoconductor is charged by the first charging voltage based on the measurement result ;
A development bias voltage control step of adjusting the development bias voltage (Vb ₂ ) by the following equation;
Vb ₂ = Vb + (Vg / Vg ₁ ) · ΔV ₁
A developing bias applying step of applying the adjusted developing bias voltage (Vb ₂ ) to the developing means ;
A development bias voltage application method for an image forming apparatus, comprising: applying a development bias voltage (Vb) after the carriers trapped in the photoreceptor disappear .

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