JP2003241544A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus to which an optimal transfer bias can be set even when an electric charge per the unit area of toner on an image carrier is changed due to density control, etc. <P>SOLUTION: The image forming apparatus is provided with a transfer means for transferring a toner image formed on the image carrier to a matter to be transferred by applying the transfer bias, and has: a detection means for detecting a current value and a voltage value at the time while applying the transfer bias to the transfer means; and a transfer bias setting means for setting the transfer bias based on the current value and the voltage value detected by the detection means. The image forming device performs the constant voltage control of the transfer bias in the case of transferring the toner image formed on the image carrier. When controlling the electric charge per the unit area of the toner image on the image carrier to be increased or reduced, the transfer bias setting means resets the transfer bias after that. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine or a printer, and more specifically, it performs control to increase or decrease the amount of charge per unit area of a toner image on an image carrier, and thereafter. The present invention relates to an image forming apparatus that resets a transfer bias.

[0002]

2. Description of the Related Art An image forming apparatus using a conventional electrophotographic method will be described with reference to FIG. In FIG. 5, 1
Is a photosensitive drum, 2 is a primary charging means, 3 is an exposing means, 4 is a developing device, 5 is a transferring means, and 6 is a cleaner. After the photosensitive drum 1 is uniformly charged by the primary charging device 2, the exposure unit 3 exposes the photosensitive drum 1 to form an electrostatic latent image on the photosensitive drum 1. After that, the toner image is developed by the developing device 4,
The toner image on the photosensitive drum 1 is transferred to the recording material P by the transfer unit 5. The transfer residual toner remaining on the photosensitive drum 1 is collected by the cleaner 6.

In this structure, the amount of toner image formed on the photosensitive drum 1 per unit area is controlled by the developing contrast shown in FIG. Here, Vd is the surface potential of the photosensitive drum 1 charged by the primary charging means 2, Vl is the surface potential of the photosensitive drum 1 exposed by the exposing means 3, and Vdc is the developing device 4.
Is the developing bias applied to. When the developing contrast shown in FIG. 8 is large, more toner is developed on the surface of the photosensitive drum 1 in the developing section. Here, the developing contrast corresponds to the charge amount per unit area of the toner image. In addition, the amount of developed toner per unit area largely depends on the charge charge of the toner (the amount of charge per unit weight).

On the other hand, in the conventional image forming apparatus using the electrophotographic method, an ATVC (Active Transfer Voltage Co
A method has been proposed in which an electric current is applied to the transfer portion during non-image formation, which is called "ntrol", and the optimum transfer bias is set from the current-voltage value at this time.

In FIG. 5, the transfer means 5 is of a contact charging type using an elastic roller, and has conventionally been often used in an electrophotographic image forming apparatus because of advantages such as ozone-less and low cost. However, in the above elastic roller (hereinafter referred to as “transfer roller”), it is difficult to suppress resistance variation during manufacturing, and the resistance changes due to temperature / humidity change of atmospheric environment, deterioration of durability, and the like. For this reason, the transfer high-voltage power supply has a control means capable of constant current control and constant voltage control and a means for detecting the voltage and current at this time, and the transfer bias is controlled at constant current during pre-rotation before image formation. The optimum transfer voltage with respect to the charging potential of the drum 1 and the resistance value of the transfer roller 5 is detected, and when the image is transferred, constant voltage control is performed with the transfer voltage previously obtained. By doing so, it becomes possible to flow a necessary transfer current while performing constant voltage control.

The advantage of using the constant voltage control over the constant current control in the transfer control is that the current flowing in a portion other than the transfer material is smaller when the transfer material having a width smaller than that of the photosensitive drum or the transfer roller is transferred. However, it is possible to control by ignoring these influences, and it is also advantageous in terms of cost. Further, Japanese Patent Laid-Open No. 11-109689 discloses a method of controlling a transfer bias according to a change in voltage applied to a charging unit. Explaining with reference to FIG. 9,
By changing the charging conditions by changing the temperature and humidity of the atmosphere,
By setting the transfer voltage Vtr so that the transfer contrast with Vd is always constant even when Vd is changed, it is possible to prevent image defects such as scattering.

[0007]

In recent years, the demand for higher image quality has made it necessary to form higher density images. In this case, it is necessary to form more toner on the transfer material. is there. On the other hand, for a transfer material such as OHT that is used in a transparent manner, the amount of toner on the transfer material needs to be equal to or less than a certain amount in order to maintain transparency, and the amount of toner applied per unit area is controlled. The need is emerging.

Further, since the amount of toner developed on the photosensitive drum varies depending on the durability of the carrier in the developing device, the temperature and humidity of the ambient environment, etc., it is necessary to constantly increase or decrease the amount of toner.

However, when the amount of charge per unit area of the toner on the photosensitive drum changes due to the control of the toner amount, etc., the above method cannot obtain the optimum transfer condition. This is because the optimum transfer current corresponds to the charge amount per unit area of toner. For example,
When control is performed to increase the amount of charge per unit area of toner by increasing the development contrast, a larger transfer current is required, and therefore the transfer contrast also needs to be increased. If the transfer current is insufficient, an image such as weak voids in which transfer is not sufficiently performed, and conversely, if it is too strong, a defective image such as splattering occurs.

The present invention has been made in view of the above problems, and its object is to achieve optimum transfer even when the charge amount per unit area of toner on the image carrier changes due to density control or the like. An object is to provide an image forming apparatus capable of setting a bias.

[0011]

In order to achieve the above object, the present invention provides a transfer means for transferring a toner image formed on an image bearing member onto a transferred member by applying a transfer bias. A detection unit for detecting a current value and a voltage value at that time while applying the transfer bias to the transfer unit; and a transfer bias for setting the transfer bias based on the current value and the voltage value detected by the detection unit. When the toner image formed on the image carrier is transferred by the setting means, the amount of electric charge per unit area of the toner image on the image carrier is increased or decreased in the configuration in which the transfer bias is controlled to a constant voltage. When such control is performed, after that, the transfer bias is reset by the transfer bias setting unit.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings.

<First Embodiment> FIG. 1 schematically shows the vicinity of an image carrier of an image forming apparatus according to a first embodiment of the present invention.
This image forming apparatus includes, as an image carrier, a photosensitive drum 1 rotatably supported by a main body (not shown). The photosensitive drum 1 is a cylindrical electrophotographic photosensitive member having a conductive base 11 made of aluminum or the like and a photoconductive layer 12 formed on the outer periphery thereof as a basic configuration. The support shaft 13 is provided at the center thereof, and the support shaft 13 is rotationally driven in the direction of arrow R1 by a drive means (not shown).

A charging roller 2 as a primary charging device is arranged above the photosensitive drum 1. Charging roller 2
Is in contact with the surface of the photosensitive drum 1 and has a predetermined polarity,
It is uniformly and uniformly charged to a potential, and is configured as a roller as a whole. The charging roller 2 is composed of a conductive cored bar 21 arranged in the center, a low resistance conductive layer 22 and a medium resistance conductive layer 23 formed on the outer periphery thereof, and both ends of the cored bar 21 are bearing members (not shown). It is rotatably supported by and is arranged parallel to the photosensitive drum 1. The bearing members at the both ends are urged toward the photosensitive drum 1 by pressing means (not shown), whereby the charging roller 2 is pressed against the surface of the photosensitive drum 1 with a predetermined pressing force.

The charging roller 2 is indicated by an arrow R1 on the photosensitive drum 1.
Along with the rotation of the direction, it is driven to rotate in the direction of arrow R2. A bias voltage is applied to the charging roller 2 by a power source 24, so that the surface of the photosensitive drum 1 is uniformly and uniformly contact-charged.

An exposure unit 3 is arranged downstream of the charging roller 2 in the rotation direction of the photosensitive drum 1. The exposure means 3 emits a laser beam based on image information, for example.
It exposes the photosensitive drum 1 by scanning while FF / ON, and forms an electrostatic latent image according to image information. The developing device 4 arranged on the downstream side of the exposure unit 3 has a developing container 41 containing a two-component developer.
Developing sleeve 42 in the opening facing the photosensitive drum 1.
Is rotatably installed, and a magnet roller 4 for carrying a developer on the developing sleeve 42 is provided inside the developing sleeve 42.
3 is fixedly arranged so as not to rotate with respect to the rotation of the developing sleeve 42. A regulating blade 44 that regulates the developer carried on the developing sleeve 42 to form a thin developer layer is installed below the developing sleeve 42 of the developing container 41.

Further, a developing chamber 45 and an agitating chamber 46 which are partitioned are provided in the developing container 41, and a replenishing chamber 47 containing a toner for replenishment is provided above it. When the developer formed on the thin developer layer is transported to the developing area facing the photosensitive drum 1, the magnet roller 43 is conveyed.
The magnetic brush of the developing main pole located in the developing area causes the magnetic brush of the developer to be formed. The surface of the photosensitive drum 1 is rubbed with this magnetic brush and the developing sleeve 4
2, by applying a developing bias voltage from the power source 48, the toner adhering to the carrier forming the ears of the magnetic brush adheres to the exposed portion of the electrostatic latent image and is developed, and the toner on the photosensitive drum 1 is An image is formed.

A transfer roller 5 is disposed below the photosensitive drum 1 on the downstream side of the developing device 4. Transfer roller 5
Is a core metal 51 biased by a power source 54,
A cylindrical conductive layer 52 is formed on the outer peripheral surface of the conductive layer 52, and a surface layer 53 that covers the surface of the conductive layer 52. Both ends of the transfer roller 5 are urged toward the photosensitive drum 1 by pressing members such as springs (not shown), whereby the surface layer 53 of the transfer roller 5 is applied to the surface of the photosensitive drum 1 with a predetermined pressing force. A pressure contact is made, and a transfer nip portion is formed between the photosensitive drum 1 and the transfer roller 5. The recording material P conveyed in the direction of arrow K1 by a sheet feeding mechanism (not shown) is supplied to the transfer nip portion in synchronization with the rotation of the photosensitive drum 1. The recording material P is nipped and conveyed in the transfer nip portion N, and at the time of this passage, the recording material P is
A bias voltage having a polarity opposite to the polarity of the toner is applied by the power supply 54, whereby the toner image on the photosensitive drum 1 is transferred to the surface of the recording material P.

The recording material P, to which the toner image has been transferred, is separated from the photosensitive drum 1 and conveyed to an image fixing means (not shown) where the toner image is fixed and then a final copy of the apparatus main body. The photosensitive drum 1 is discharged to the outside, or again through the conveying means in the case of multiplex copying or double-sided copying.
And the image forming section.

The photosensitive drum 1 after the image transfer is cleaned by the cleaner 6
By this, adhered substances such as residual toner are removed. The cleaner 6 is composed of a cleaner blade 61 and a conveying screw 62. The cleaner blade 62 is brought into contact with the photosensitive drum 1 at a predetermined angle and pressure by a pressurizing means (not shown), and the surface of the photosensitive drum 1 is abutted. Collect the remaining toner, etc. The collected residual toner and the like are conveyed and discharged by the conveying screw 62.

Next, the method of controlling the optimum transfer bias will be described in detail.

First, FIG. 2 shows the relationship between the transfer current I and the transfer efficiency from the photosensitive drum 1 to the transfer material P when transferring the toner image. According to the study by the present inventor, as the transfer current I is increased, the transfer efficiency from the photosensitive drum 1 to the transfer material P becomes higher, and the transfer efficiency becomes maximum near the current Ib. Then, at the current Ib as a boundary, an image defect called "strikethrough" begins to occur, and the transfer efficiency starts to decrease. At this time, it is considered that discharge is occurring between the photosensitive drum 1 and the transfer material P. As described above, since the toner transfer efficiency is determined by the transfer current, the transfer voltage is determined by ATVC so that the desired transfer current Ib always flows even when the resistance value of the transfer roller changes due to durability.

Regarding ATVC, several methods are known. Here, a transfer high voltage power source capable of constant voltage control and a means for detecting voltage and current at this time are provided, and the photosensitive drum 1 is charged to a predetermined charging potential. While being charged, two-stage voltages V1 and V2 are applied while the transfer roller 5 makes one revolution, and respective transfer currents I1 and I2 at this time are obtained.
From these relationships, the optimum transfer current I for a given density is obtained.
The voltage Vb required to flow b is obtained by linear interpolation (FIG. 4), and when the image is transferred, the voltage Vb previously obtained is added to the voltage Vp shared by the transfer material to obtain a voltage (Vb + Vp). A method of performing voltage control is used. Further, in the present embodiment, the ATVC control is performed during the pre-multi revolution after the power is turned on. This is because the ATVC is a control that requires a relatively long time, and for example, if the control is started after the image formation is started, the time from the start of printing to the end of printing becomes long. Multi-rotation before turning on the power requires a long time to warm up the fixing unit (not shown),
ATVC can occur within these times.

Further, the image forming apparatus according to this embodiment is provided with a mechanism for adjusting the maximum image density. In order to adjust the image density, it is necessary to change the amount of toner image to be developed on the surface of the photosensitive drum 1 per unit area. Here, as the method, the development contrast shown in FIG. 8 is adjusted. Use the method. That is, when the development contrast is increased, the amount of charge per unit area of the toner image developed on the surface of the photosensitive drum 1 increases, and the amount of toner applied per unit area also increases accordingly, so that the image density increases. On the contrary, when the development contrast is reduced, the amount of charge per unit area of the toner image developed on the surface of the photosensitive drum 1 is reduced, so that the amount of toner applied per unit area is reduced and the image density is lowered.

Here, it is assumed that the user makes a setting to reduce the image density. By this setting, the development contrast is controlled to be small, and accordingly, as described above, the toner image developed on the surface of the photosensitive drum 1 is
The amount of charge per unit area is small.

As described above, the optimum transfer current depends on the amount of charge per unit area of the toner image on the photosensitive drum 1, so that the optimum transfer current required to transfer this toner image changes.

Therefore, in the present invention, the transfer bias is set again immediately after the control for changing the charge amount per unit area of the toner image on the image carrier is performed. As a result, the transfer bias suitable for the amount of electric charge per unit area of the toner image at that time is set, so that optimum transfer conditions are always obtained.

In the present embodiment, the ATVC performed during the pre-multi-rotation is set so that the optimum transfer current Ib1 for the developing contrast Vdc1 corresponding to the initial maximum image density flows. When the setting is made to be lower, the ATVC is performed again immediately after this, and the transfer bias is changed so that the transfer current Ib2 optimal for the development contrast Vdc at this time flows.

As described above, by controlling the transfer bias for each control for changing the charge amount per unit area, a stable image without transfer defects could be obtained.

<Second Embodiment> FIG. 3 schematically shows an image forming apparatus according to a second embodiment of the present invention. The image forming apparatus of the present invention is a full-color electrophotographic image forming apparatus having four photosensitive drums and using an intermediate transfer member. Hereinafter, the image forming apparatus of the present invention will be described in detail.

As shown in FIG. 3, the present image forming apparatus is a process as an image forming means including a charging means, an exposing means, a developing device, a cleaner and the like around a photosensitive drum which is a latent image carrier. The image on the photosensitive drum formed by each of the four process units is sequentially transferred onto the intermediate transfer body that moves adjacent to the photosensitive drum, and the image transferred onto the intermediate transfer body is Further, the second transfer portion is configured to be transferred to a recording material such as paper.

The details of the image forming apparatus according to this embodiment will be described below.

Process units Pa, Pb for forming images of respective colors of yellow, magenta, cyan and black,
The photosensitive drums 1a, 1b and 1 are respectively denoted by Pc and Pd.
c and 1d are arranged, and each photosensitive drum is rotatable in the arrow direction.

Further, each photosensitive drum 1a, 1b, 1c, 1
Around the d, charging means 2a, 2b, 2c, 2d, exposure means 3a, 3b, 3c, 3d, developing devices 4a, 4b, 4
c, 4d, and cleaners 6a, 6b, 6c, 6d
Are sequentially arranged along the rotation direction of the photosensitive drum. Since the configuration of the process unit is similar to that of the first embodiment, detailed description thereof will be omitted here. An intermediate transfer unit 5 is arranged below each photosensitive drum. The intermediate transfer unit 5 includes an intermediate transfer belt 51, transfer rollers 53a, 53b, 53c and 53d, secondary transfer rollers 56 and 57, and an intermediate transfer belt cleaner 5.
Have five.

In the printer as described above, the toner images of the respective colors formed on the photosensitive drums 1a, 1b, 1c and 1d are sequentially subjected to the transfer bias from the transfer rollers 53 which face each other with the intermediate transfer belt 51 interposed therebetween, and are sequentially transferred. Intermediate transfer belt 5
The image is transferred onto the first transfer unit 1 and is conveyed to the secondary transfer unit as the belt rotates.

On the other hand, by this time, the recording material P taken out from the paper feeding cassette 8 is supplied to the conveying roller 82 via the pickup roller 81, is further conveyed to the left side in FIG. The toner image is transferred onto the recording material P by the secondary transfer bias applied to the secondary rollers 56 and 57. The transfer residual toner and the like on the intermediate transfer belt 51 are removed and collected by the transfer belt cleaner 55.

The fixing device 7 comprises a fixing roller 71 which is rotatably arranged and a pressure roller 72 which rotates while being in pressure contact with the fixing roller 71. A heater 73 such as a halogen lamp is provided inside the fixing roller 71, and the temperature of the surface of the fixing roller 71 is adjusted by controlling the voltage to the heater 73. In this state, when the recording material is conveyed, the fixing roller 71 and the pressure roller 72 rotate at a constant speed, and when the recording material P passes between the fixing roller 71 and the pressure roller 72, the recording material P comes from both front and back surfaces. The unfixed toner image on the surface of the recording material is melted and fixed by being pressed and heated at a substantially constant pressure and temperature, and a full-color image is formed on the recording material P.

Further, not only the color image forming apparatus of the present embodiment, but also the color image forming apparatus is generally provided with a mechanism for adjusting the density of the output image, and in many cases, the output image density is automatically adjusted. It has appropriate control means. Particularly, in the image forming apparatus that outputs a full-color image as in the present embodiment, more accurate temperature control is required for each of yellow, magenta, cyan, and black in order to obtain a desired color balance. .

As shown in FIG. 6, an optical density sensor 90 as a detecting means used for the density control of the present embodiment is installed so as to face the intermediate transfer belt 1, and has a light emitting element 91 such as an LED, a photodiode, and the like. Of the light receiving element 92 and the holder 93, the infrared light from the light emitting element 91 is irradiated onto the detection image IM on the intermediate transfer belt 1, and the reflected light from the detection image IM is measured by the light receiving element 92 for detection. The density of the working image IM is measured. In the optical density sensor 90, when the normal line L is used as a reference so that the specularly reflected light from the detection image IM does not enter the light receiving element 92, the detection image I
Only the diffusely reflected light is measured by setting the irradiation angle of M to α = 45 ° and the light receiving angle of the reflected light from the detection image IM to 0 °.

The amount of infrared light sensed by the optical density sensor 90 is almost proportional or inversely proportional to the amount of adhered toner.
Generally, the amount of adhered toner and the density of the output image are not in a proportional relationship. However, since the amount of adhered toner and the density of the output image have a one-to-one correlation, the density of the output image can be estimated from the measurement value of the optical density sensor 90. The image density control of the image forming apparatus according to this embodiment will be described below.

Image density control for determining image forming conditions is performed for each color.

First, a detection image IM for determining image forming conditions is formed. The detection image IM is primarily transferred from the photosensitive drums 1a to 1d onto the intermediate transfer belt 51,
From the optical density measurement result of the detection image IM detected by the optical density sensor 90 installed facing the intermediate transfer belt 51, the image forming condition closest to the target optical density of the detection image IM (here, The development contrast) is estimated and the image forming condition is controlled.

Although the image density is influenced by environmental characteristics of the photosensitive drum and the developing carrier, variations in manufacturing, variations in laser exposure amount, etc., by performing the above density control for each predetermined number of sheets, It is possible to stabilize the effect of fluctuations in the above range to some extent.

Next, a method of setting the transfer bias for the primary transfer from the photosensitive drums 1a to 1d to the intermediate transfer belt 51 will be described.

Here, by the above-mentioned image density control,
It is assumed that the development contrast is increased in order to increase the lowered image density. In this case, as described in the first embodiment, the amount of charge per unit area of the toner image developed on the surfaces of the photosensitive drums 1a to 1d increases as the development contrast increases. Then, the optimum transfer current required to transfer this toner image also increases.

In this embodiment as well, the transfer bias optimum for the initial image density is set at the time of the pre-multi-rotation as in the case of the first embodiment. When the charge amount per unit area of the toner image is changed, the optimum bias is reset by performing ATVC again.

As described above, the present invention is also effective in an apparatus for adjusting the amount of toner image on the image carrier per unit area by the automatic image density control means. Although the method of setting the transfer bias in the primary transfer portion has been described in the present embodiment, the optimum transfer current changes when the charge amount per unit area of the toner image changes. Since the same applies to the secondary transfer portion onto the transfer material, the same control may be performed in the secondary transfer portion.

<Third Embodiment> A third embodiment of the present invention.
In the image forming apparatus having the same configuration as that of the second embodiment, the similar transfer control is adopted when the control using the analog development is performed.

The configuration and operation of the image forming apparatus according to the present embodiment are almost the same as those in the second embodiment, and the description thereof will be omitted here.

The image T / D ratio control method using analog development in the present embodiment will be described below with reference to FIG.

First, the surfaces of the photosensitive drums 1a to 1d are charged to a predetermined potential Vd by a charger. In this embodiment,
The charging rollers 2a to 2b are used as the charging device, and the surfaces of the photosensitive drums 1a to 1d are charged with a value close to the DC component of the charging bias applied to the charging roller. Then, the photosensitive drums 1a to 1d charged to the predetermined potential Vd.
A toner image is developed on the surface by applying a developing bias Vdc to the developing devices 4a to 4d. At this time, as shown in FIG. 7, the developing bias Vdc is applied with a value larger in the negative polarity than the charging potential Vd. However, in the present embodiment, the toner is negative toner, and the charging potential Vd at this time is The toner image is developed by the development contrast, which is the difference in the development bias Vdc.

Here, after charging the photosensitive drums 1a to 1d,
The reason why the general reversal development process of developing the exposed portion is not performed is because the potential Vl of the exposed portion is likely to change due to environmental changes and endurance changes of the photosensitive drum. This is to avoid the influence of the fluctuation of Vl when the difference is the bias Vdc. That is, the purpose of analog development is to purely grasp the development characteristics at a predetermined development contrast.

By the way, even when analog development is performed with the same development contrast, if the weight ratios of toner and carrier in the developing device are different, the density developed on the drum is generally different. Known to. When the ratio of toner is high, the developability is increased, and more toner is developed on the photosensitive drum with the same development contrast.

On the other hand, when the toner ratio is low, the amount of toner developed per unit area of the photosensitive drum surface is small. Here, the toner images analog-developed on the surfaces of the photosensitive drums 1a to 1d are primarily transferred onto the intermediate transfer belt 51, and the density thereof is detected by the optical density sensor 90 installed facing the intermediate transfer belt 51. Read. Therefore, the weight ratio of the toner and the carrier in the developing device can be estimated from this result.

Generally, when the weight ratio of the toner and the carrier is increased, the toner is insufficiently charged, and fogging is likely to occur. On the contrary, when the ratio of the toner becomes low, the toner is charged up, that is, the charge amount per unit weight of the toner increases, and the developing amount per unit area (amount on the drum) at a predetermined developing contrast decreases. As a result, the image density is lowered, and the image tends to be rough.

In the present embodiment, the weight ratio of the toner to the carrier in the developing device becomes lower than the optimum value and the toner is charged up, but the developing contrast is increased by the method of the second embodiment. Therefore, assuming that the weight ratio of toner and carrier is corrected to the optimum value when the density is adjusted to the optimum value, T /
A method of setting the D ratio control and the transfer bias will be described.

1. First, as described above, an image obtained by analog development is recorded on the intermediate transfer belt 51 by the optical density sensor 9
By reading 0, it is detected that the weight ratio of the toner and the carrier in the developing device is lower than the optimum value.

2. In response to this, the toner contained in the replenishing chamber 47 is replenished into the developing device in order to increase the weight ratio of the toner and the carrier. Then, since the chargeability of the toner is lowered, the charge amount per unit weight of the toner in the developing device is lower than that before the replenishment and approaches an appropriate value.

3. The development contrast is set to a high value in accordance with the case where the weight ratio of the toner and the carrier is low (the charge amount per unit weight of the toner is high), but the charge amount per unit weight of the toner decreases to an appropriate value. The density is kept constant by correcting the development contrast to a lower level in accordance with the above.

4. Since the charge amount per unit area of the toner image developed on the photosensitive drum decreases, the optimum transfer bias is reset.

Also in this embodiment, the first embodiment
By resetting the transfer bias in the same manner as described in 2, it is possible to obtain an optimum transferred image regardless of the fluctuations caused by the above control.

As described above, the present invention is also effective in an apparatus for adjusting the amount of charge per unit weight of toner.

In this embodiment, the case where the toner is replenished when the weight ratio of the toner is reduced has been described. On the contrary, when the weight ratio of the toner is increased, the replenishment amount of the toner is regulated. It is also possible to control transfer when performing control.

[0064]

As is apparent from the above description, according to the present invention, even when the control for increasing or decreasing the charge amount per unit area of the toner image on the image carrier is performed, the transfer is performed immediately after that. By resetting the bias, optimum transfer can be achieved without being affected by the above control.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an image forming apparatus according to a first embodiment of the present invention.

FIG. 2 is a diagram showing the relationship between transfer current I and transfer efficiency in the first embodiment of the present invention.

FIG. 3 is a schematic configuration diagram of an image forming apparatus according to a second embodiment of the present invention.

FIG. 4 is a diagram showing a relationship between a transfer voltage V and a transfer current I according to the first embodiment of the present invention.

FIG. 5 is a schematic configuration diagram of a conventional image forming apparatus.

FIG. 6 is a schematic diagram of an optical density sensor according to a second embodiment of the present invention.

FIG. 7 is a diagram showing a development contrast according to the third embodiment of the present invention.

FIG. 8 is a diagram showing a development contrast in the conventional and Embodiment 1 of the present invention.

FIG. 9 is a diagram showing a conventional transfer contrast.

[Explanation of symbols]

1 photosensitive drum 2 charger 3 exposure means 4 Developing device 5 Transfer means 6 cleaner

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 2H027 DA09 DA10 DE02 DE07 EA01                       EA02 EA03 EA04 EC03 EC06                       EC19                 2H077 AD02 AD06 AD35 AE06 DA10                       DA31 DA47 DA63 DB00 EA03                       GA13                 2H200 FA05 GA23 GA34 GA44 GA45                       GA47 GA60 GB12 GB22 GB25                       HA02 HA28 HB12 HB22 JA02                       JA29 JC03 JC20 PA03 PA29                       PB06 PB20

Claims (6)

[Claims] 1. A transfer device for transferring a toner image formed on an image bearing member onto a transfer-receiving member by applying a transfer bias, and while applying the transfer bias to the transfer device, Of the toner image formed on the image carrier having detection means for detecting the current value and voltage value of the toner image and transfer bias setting means for setting the transfer bias based on the current value and the voltage value detected by the detection means. In the case of controlling the transfer bias at a constant voltage, the charge amount per unit area of the toner image on the image carrier is increased or decreased. An image forming apparatus characterized in that the transfer bias is reset by. 2. A charging device for charging a photoconductor as an image carrier, an exposing means for exposing the photoconductor to form an electrostatic latent image, and a developing bias to apply a static charge on the photoconductor. A developing device that forms a toner image by attaching charged toner to the latent image is provided. By changing at least one condition of the charging device, the exposure unit, and the developing device, the amount of charge per unit area of the toner image is changed. The image forming apparatus according to claim 1, wherein the image forming apparatus controls to increase or decrease. 3. The image forming apparatus according to claim 1, wherein control is performed to increase or decrease the amount of toner per unit area of the toner image formed on the image carrier. 4. The image forming apparatus according to claim 1, wherein the charge amount per unit weight of the toner image formed on the image carrier is controlled to increase or decrease. 5. A toner image is provided by providing a developing device using a two-component toner having a toner and a carrier for transporting the toner to the vicinity of an image carrier in the developing device, and controlling a weight ratio of the toner and the carrier. Charge amount per unit weight of
The image forming apparatus according to claim 4, wherein control for increasing or decreasing is performed. 6. The image forming apparatus according to claim 1, further comprising density detecting means for detecting a toner density on the image bearing member or the transfer target member.