JP2003295532A - Apparatus and method for forming image - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an apparatus and a method for forming images by which image density is stabilized simply and precisely. <P>SOLUTION: A toner image consisting of a group of 1-dot lines of 1-ON and 1-OFF, e.g. is formed as a patch image for low density by each DC development bias condition while increasing the absolute value of a DC development bias Vavg step by step from the minimum value Vmin to the maximum value Vmax of a density saturation range while fixing exposure energy to reference energy E2 in a gradually approaching range. Then, from among patch images formed like this, the DC development bias when density is nearly coincident with a target density set in advance is obtained and the value is stored in a memory 127 as an optimal development condition. Further, exposure energy in this case is stored in the memory 127 as the optimal development condition. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to exposing a surface of a photoconductor to a light beam to form an electrostatic latent image on the surface of the photoconductor and applying a developing bias to a toner carrier carrying the toner. The present invention relates to an image forming apparatus and method for forming a toner image having a target density by visualizing the electrostatic latent image by moving toner from a carrier to the photoconductor.

[0002]

2. Description of the Related Art In this type of image forming apparatus, the image density may change due to fatigue of the photoconductor and toner, a change over time, and a change in temperature and humidity around the apparatus. Therefore, conventionally, many techniques have been proposed for stabilizing the image density by appropriately adjusting parameters (image forming conditions) such as charging bias, developing bias, and exposure energy. For example, in the invention disclosed in JP-A-10-239924, the image density is stabilized by appropriately adjusting the charging bias and the developing bias. That is, in this conventional technique, the reference patch image is formed on the photoconductor while changing the charging bias and the developing bias,
The image density of each reference patch is detected. Then, the optimum charging bias and the developing bias are determined as the optimum charging condition and the optimum developing condition, respectively, based on these detected values.

[0003]

However, in the above-mentioned prior art, the charging bias-development bias characteristic is checked in advance before the reference patch image is formed, and this characteristic is satisfied when the reference patch image is formed. Thus, the charging bias and the developing bias are set. As described above, in order to obtain the optimal charging bias and the optimal developing bias to stabilize the image density, it is necessary to obtain the charging bias-development bias characteristics for each image forming apparatus, which is a complicated task. have.

Further, the charging bias-development bias characteristics are not always constant and may change with time. If the characteristics change in this way, it becomes difficult to accurately calculate the optimum charging bias or developing bias. To solve this problem,
The charging bias-development bias characteristics may be updated as appropriate, but such an updating operation is troublesome and inferior in terms of maintainability.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an image forming apparatus and an image forming method capable of stabilizing image density easily and with high accuracy.

[0006]

SUMMARY OF THE INVENTION The present invention has an exposing means for exposing a surface of a charged photoreceptor to a light beam to form an electrostatic latent image, and a toner carrier for carrying toner on the surface. A developing unit that moves the toner to the surface of the photosensitive member according to the developing bias applied to the toner carrier to visualize the electrostatic latent image with the toner, and the developing bias is set to the optimum developing condition to set a predetermined target. The present invention relates to an image forming apparatus and an image forming method that include a control unit that forms a toner image of high density, and is configured as follows to achieve the above object. The term "asymptotic range" in the following configuration means, among the light attenuation characteristics of the photoconductor, when a photoconductor is irradiated with a light beam to form an electrostatic latent image corresponding to a high-density patch image, It means the range of energy density in which the surface potential of the photoconductor asymptotically approaches a predetermined potential as the energy density of the light beam increases, and the term "density saturation range" refers to the range in which the developing bias can be set. , A developing bias range in which the increase in the optical density with respect to the increase in the density of the toner attached to the electrostatic latent image corresponding to the high density patch image is almost saturated.

The image forming apparatus according to the present invention forms the toner image with the developing bias set to the optimum developing condition and the energy density of the light beam is set within the asymptotic range, and the toner image is formed if necessary. Prior to formation, a low-density patch image is formed with each bias value while the development bias is changed and set in multiple steps within the density saturation range, and an optimization process is performed to find optimum development conditions based on the image density. .

In the image forming method according to the present invention, the developing bias is set to the optimum developing condition, and if necessary, the toner image is formed in a state where the energy density of the light beam is set within the asymptotic range. The following steps (a) to (c), (a) step of forming a low density patch image with each bias value while changing the development bias in multiple steps within the density saturation range, (b) low density patch The step of detecting the image density of the image, (c) the step of obtaining the optimum developing condition based on the detected image density, and the developing bias necessary for forming the toner image of the target density are set as the optimum developing condition. Looking for.

In the invention (image forming apparatus and method) configured as described above, attention is paid to the image quality of a low density image such as a thin line image or a halftone image, that is, an image in which the area ratio of dots to the entire image is low, and the image is The optimum developing condition is determined based on the density of the low-density patch image actually formed under each bias condition while changing and setting the developing bias as the image forming condition that affects the density in multiple stages. Therefore, on the low density side, it is only necessary to change the developing bias while ensuring the predetermined image density, and it is possible to greatly simplify the processing compared to the conventional optimization processing technology. There is.

In the image forming apparatus and method in which the optimum developing condition of the developing bias is obtained by so-called patch processing, the developing bias is set to the optimum developing condition and the energy density of the light beam is set within the asymptotic range. Since the toner image is formed in the set state, it is of course possible to obtain good image quality in a low-density image, but it is also possible to obtain a high-density image, that is, a high dot area ratio to the entire image such as a solid image. It is possible to form a toner image with stable image quality. The reason is as follows.

When an electrostatic latent image of a high density image is formed with the energy density of the light beam set within the asymptotic range,
As shown in FIG. 3, which will be described later, the surface potential of the electrostatic latent image formed on the photoconductor by the irradiation of the light beam is substantially constant regardless of the energy density of the light beam. Therefore,
The image density of the high density image depends on the developing bias. That is, as in the case of the low-density image, the density of the toner forming the toner image also changes in the high-density image by changing the developing bias.

However, in the high-density image, the area of the electrostatic latent image to which the toner is to be applied is relatively large, and when the toner is attached over such a large area, the toner density is to some extent. If it is more than that, the image density will not increase much even if the toner density is further increased. In this way, there is a density saturation range in which the image density changes little with respect to the toner density. In the present invention, the developing bias is changed and set in the density saturation range to adjust the density of the low density image, and the optimum developing condition is obtained. Therefore, the optimum developing condition is naturally included in the density saturation range. Therefore, the density of the high density image does not largely deviate from the target density. By doing so, the image forming apparatus and method can stably form a toner image with good image quality in a wide density range from a low density image to a high density image.

Here, not only the optimum developing condition which is the optimum value of the developing bias but also the optimum value of the energy density of the light beam,
That is, the optimum exposure condition may be obtained, the developing bias may be set to the optimum developing condition, and the energy density of the light beam may be set to the optimum exposure condition to form a toner image having a predetermined target density. Image forming conditions (optimal development conditions and optimal exposure conditions) for obtaining the target density by doing this
Will be easier to find.

[0014]

1 is a diagram showing an embodiment of an image forming apparatus according to the present invention. 2 is a block diagram showing an electrical configuration of the image forming apparatus of FIG. This image forming apparatus forms a full-color image by superposing four color toners of yellow (Y), cyan (C), magenta (M), and black (K), or uses only black (K) toner. Is a device that forms a monochrome image. In this image forming apparatus, when an image signal is given to the main controller 11 of the control unit 1 from an external device such as a host computer, the main controller 1
The engine controller 12 controls each part of the engine section EG in response to a command from the controller 1 to form an image corresponding to the image signal on a sheet S such as a copy sheet, a transfer sheet, a sheet, and an OHP transparent sheet.

In this engine section EG, seven units are provided: (a) photoconductor unit 2; (b) yellow developing unit (hereinafter referred to as "Y developing unit") 3Y; (c) magenta developing unit ("M developing unit"). ) 3M; (d) cyan developing unit ("C developing unit") 3C; (e) black developing unit ("K developing unit") 3K; (f) intermediate transfer unit 4 and (g) fixing unit 5 are the main body of the apparatus. 6
It is removable with respect to. And all the units 2, 3Y, 3M, 3C, 3K, 4, 5 are the device main body 6
1, the photoconductor 21 of the photoconductor unit 2 rotates in the direction D1 of the arrow in FIG. 1 and is charged around the photoconductor 21 along the rotation direction D1. Part 22, developing units 3Y, 3M, 3C, 3K
Rotary developing section 3 and cleaning section 23
Are arranged respectively.

Seven units 2, 3Y, 3M, 3C, 3
Among K, 4, and 5, the photoconductor unit 2 contains the photoconductor 21, the charging unit 22, and the cleaning unit 23.
These can be integrally attached to and detached from the apparatus body 6. A charging bias is applied to the charging unit 22 from the charging bias generation unit 121 to uniformly charge the outer peripheral surface of the photoconductor 21. Further, the photoconductor unit 2 is provided with a cleaning unit 23 on the upstream side of the charging unit 22 in the rotation direction D1 of the photoconductor 21, and the toner remaining on the outer peripheral surface of the photoconductor 21 after the primary transfer. Scrape off. In this way, the surface of the photoconductor 21 is cleaned.

Photoreceptor 21 charged by charging section 22
The light beam L is emitted from the exposure unit 8 toward the outer peripheral surface of the. As shown in FIG. 2, the exposure unit 8 is electrically connected to the image signal switching unit 122, and the exposure power control unit 123 exposes the image signal according to the image signal given through the image signal switching unit 122. Control unit 8,
The light beam L is exposed on the photoconductor 21 to form an electrostatic latent image corresponding to the image signal on the photoconductor 21.

In the image forming apparatus constructed as above,
For example, based on a command from the CPU 124 of the engine controller 12, when the image signal switching unit 122 is in conduction with the patch creation module 125, the patch image signal output from the patch creation module 125 is sent to the exposure power control unit 123. Given a patch latent image is formed.
On the other hand, the image signal switching unit 122 is the main controller 11
When conducting with the CPU 111, the light beam L emits a light beam L in accordance with an image signal given from an external device such as a host computer via the interface 112.
An electrostatic latent image corresponding to the image signal is exposed on the photoconductor 2
1 is formed on.

In this embodiment, in particular, the photoconductor 21 having the light attenuation characteristic shown in FIG. 3A is adopted. Usually, the light attenuation characteristics are different between the case of forming a high-density image (for example, a high-density patch image) and the case of forming a low-density image (for example, a low-density patch image). That is, when the photoconductor 21 charged to the uniform surface potential Vd by the charging unit 22 is partially exposed by the light beam L,
The charge in that portion is neutralized to form an electrostatic latent image on the surface of the photoconductor 21, but in a high-density patch image such as a solid image, a relatively wide range of the surface of the photoconductor 21 is exposed. Therefore, the surface potential profile becomes a well type, and while the energy per unit area of the light beam L (hereinafter, simply referred to as “exposure energy”) is relatively small, the surface potential of the exposed portion, so-called bright area, increases as the exposure energy increases. The part potential gradually decreases. Then, when the residual potential is reduced to about the residual potential determined by the characteristics of the photoconductor 21, the surface potential of the exposed portion hardly changes even if the exposure energy is increased. As a result, the high-density patch image has a light attenuation characteristic as shown by the curve Ch in FIG. 3, and this light attenuation characteristic is such that the surface potential of the photoconductor 21 is a predetermined potential Vr as the exposure energy increases. Has an asymptotic range that is asymptotic to.

On the other hand, in a low-density patch image such as a line image, the exposed area is narrow, so that the surface potential has a sharp dip-shaped profile. Therefore, the change in the surface potential of the photoconductor 21 that is actually measured is determined by the change in the surface potential (bright portion potential) of the exposed portion and the change in the contrast ratio between the unexposed portion and the exposed portion. In particular, in this embodiment, the latter change is dominant. As a result, for the low-density patch image on the photoconductor 21, the straight line C in FIG.
The light attenuation characteristics are as shown in l.

Returning to FIG. 1, the description of the device configuration will be continued. The electrostatic latent image thus formed is toner-developed by the rotary developing section 3 corresponding to the "developing means" of the present invention.
In the rotary developing unit 3, a developing unit 3K for black, a developing unit 3C for cyan, a developing unit 3M for magenta, and a developing unit 3Y for yellow.
Is rotatably provided around the axis. The developing units 3Y, 3M, 3C, and 3K are moved and positioned at a plurality of predetermined positions, and the photosensitive member 2
1 is selectively positioned at the developing position. In FIG. 1, the black developing unit 3K is positioned at the developing position, and in this positioned state, the developing roller 31 provided in the developing unit 3K is arranged to face the photosensitive member 21 while being separated therefrom. Unit 3Y, 3
With respect to M and 3C as well, the developing roller 31 provided in each developing unit is arranged to face the photoconductor 21 by positioning the developing unit at the developing position, just like the developing unit 3K.

In the developing unit positioned at the developing position, the toner stored in the unit housing is carried to the developing position while being carried by the developing roller 31. As described above, in this embodiment, the developing roller 31 corresponds to the “toner carrier” of the invention. Although the following description will be made assuming that the toner is negatively charged, a toner that is positively charged can be used by appropriately changing the potential of each part of the apparatus.

The alternating voltage shown in FIG. 4 is applied to the developing roller 31 as a developing bias from the developing bias generator 126. As shown in FIG. 4, this developing bias is an alternating voltage having a waveform in which a rectangular wave voltage having an amplitude Vpp is superimposed on the DC component Vavg. Here, the amplitude Vpp (= | Vbmax-Vbmin |) as the AC component
Moreover, a rectangular wave-shaped AC component having a ratio of the peak potential period Tb on the potential Vmax side to one cycle Ta of the AC component, that is, a duty percentage (= (Tb / Ta) × 100%) of 50% is applied. By applying the developing bias having such a waveform, the flying amount of toner can be controlled by the amplitude Vpp, while the DC component V
It is possible to control the image density with avg.

Here, paying attention to the relationship between the developing bias and the optical density of the toner image, the following relationship exists.
That is, on the high density side of a solid image or the like, as shown in FIG. 5, the absolute value of the direct current component of the developing bias (hereinafter referred to as “direct current developing bias”) Vavg is relatively small, that is, the low contrast potential Vcont (see FIG. 3). (See), the toner adhesion amount increases in proportion to the increase in the DC developing bias (increase in the contrast potential Vcont), and as a result, the image density of the toner image increases. However, when the toner adhesion amount per unit area exceeds a certain value, the optical density does not change so much even if the DC developing bias Vavg is further increased, and the toner saturation density Ds is exhibited. As described above, the optical density is almost saturated in the developing bias range equal to or higher than the DC developing bias Vavg_s, and this range is referred to as a "density saturation range" in the present invention. In addition, this "solid image" includes not only an image in which dots are formed on the entire surface of the patch image, but even an image in which light and shade are partially present is substantially seen when viewed as the entire image. Also, an image in which toner adheres to the entire surface of the image is included. The latter substantially includes, for example, a solid image in which the surface potential of each portion of the electrostatic latent image corresponding to a patch image is within a range of 10 V or less. The latter also includes images in which the area ratio of dots to the entire patch image is about 80% or more.

The waveform of the alternating voltage as the developing bias is not limited to this, and for example, a sine wave or a triangular wave may be superimposed on the DC component. Also,
A waveform whose duty ratio is not 50% may be used.
In this case, as the DC component Vavg, a weighted average voltage, that is, a value obtained by averaging the instantaneous values of the voltage waveform whose amplitude changes with time over a certain time range and converting it to a DC voltage value can be used.

By applying an alternating voltage to the developing roller 31, an alternating electric field is formed between the developing roller 31 and the photosensitive member 21, and the toner in the unit housing selectively positioned at the developing position is transferred from the developing roller 31 to the photosensitive member. It flies to 21, and visualizes an electrostatic latent image. In this way, the toner image of the selected color is formed on the surface of the photoconductor 21.

The toner image developed by the developing section 3 as described above is primarily transferred onto the intermediate transfer belt 41 of the intermediate transfer unit 4 in the primary transfer region TR1. That is, the intermediate transfer unit 4 includes an intermediate transfer belt 41 that is wound around a plurality of rollers, and a drive unit (not shown) that rotationally drives the intermediate transfer belt 41, and when transferring a color image to the sheet S. In order to form a color image by superimposing the toner images of the respective colors formed on the photoconductor 21 on the intermediate transfer belt 41, a single color image is formed on the photoconductor 21 when transferred to the sheet S. Only the black toner image is transferred onto the intermediate transfer belt 41 to form a monochromatic image.

Further, in the image forming apparatus according to the present embodiment, the patch sensor PS is arranged so as to face one roller around which the intermediate transfer belt 41 is stretched in order to detect the density of the patch image.

The image thus formed on the intermediate transfer belt 41 is secondarily transferred onto the sheet S taken out from the cassette 9 in a predetermined secondary transfer region TR2. Then, the sheet S on which the toner image is transferred is introduced into the fixing unit 5 having a heater (not shown) built therein, and the toner is fixed on the sheet S by applying pressure while heating. The sheet S on which the image is thus formed is conveyed to a discharge tray portion provided on the upper surface of the apparatus body 6.

In FIG. 2, reference numeral 113 indicates an interface 11 from an external device such as a host computer.
2 is an image memory provided in the main controller 11 for storing an image given via
7 is a calculation program executed by the CPU 124,
Is a memory (storage unit) for storing the calculation result in, the control data for controlling the engine unit EG, and the like.

In the image forming apparatus constructed as described above, the surface potential of the electrostatic latent image can be made substantially constant by setting the exposure energy in the asymptotic range on the high density side, and the DC developing bias Vavg is used. When the electrostatic latent image is toner-developed by setting to a density saturation range equal to or higher than the DC developing bias Vavg_s, the fluctuation of the optical density of the toner image can be suppressed regardless of the value of the energy density of the light beam L. Therefore, in the image forming apparatus according to this embodiment,
By forming the toner image with the DC developing bias Vavg set within the density saturation range and the energy density of the light beam L set within the asymptotic range, the toner image is stably formed on the high density side. .

On the other hand, when the DC developing bias Vavg is set within the density saturation range and the energy density of the light beam L is set within the asymptotic range, as shown by the straight line Cl in FIG. The surface potential of the electrostatic latent image changes in proportion to the exposure energy, and the DC developing bias V
The optical density of the toner image changes proportionally according to avg.
Therefore, in this embodiment, in order to obtain an image of a predetermined target density on the low density side, a predetermined patch image is formed at an appropriate timing such as when the power is turned on, and the image forming condition is optimized based on the image density. The optimization process is performed. Specifically, the CPU 1 of the engine controller 12
24 executes a prestored program to perform the processing shown in FIG. 6 for each toner color.

FIG. 6 is a flow chart showing the processing for optimizing the developing bias in this image forming apparatus. First, the exposure energy is fixed to the reference energy E2 in the asymptotic range (step S11). Then, step S12
15 is executed to increase the absolute value of the DC developing bias Vavg from the minimum value Vmin to the maximum value Vmax of the density saturation range by one step at a time, and as a patch image for low density under each developing bias condition, for example, as shown in FIG. On 10 Off 1
A toner image PI composed of a dot line group is formed. By doing so, a plurality of low-density patch images having the densities corresponding to the respective developing bias conditions are formed on the intermediate transfer belt 41.

Here, the change setting range of the DC developing bias Vavg is arbitrary as long as it is within the density saturation range, but in this embodiment, in order to improve image quality, the minimum value Vmin to the maximum value Vmax. Is limited to. That is, when the DC developing bias Vavg becomes large, as shown in FIG.
The contrast potential Vcont shown in (a) becomes large. Therefore, if the DC developing bias Vavg is too large, the edge of the image becomes extremely high in density, which is known as an edge effect, or the toner adhesion amount becomes excessive, and toner is scattered in the transfer / fixing process in the subsequent stage. However, there is a problem that the image quality is deteriorated. On the other hand, DC developing bias Vavg
Is too small, the contrast potential Vcont becomes small and the force for adhering the toner to the photoconductor 21 becomes weak.
For example, as in the example of the rectangular pattern RI shown in FIG. 8A, a phenomenon occurs in which the toner on the left and right sides and the downstream side of the image when viewed from the moving direction of the photoconductor 21 is peeled off and the image is distorted. . As shown in FIG. 8B, the distortion amount D has a characteristic that it decreases as the contrast potential Vcont increases and gradually approaches a constant value.

Therefore, in order to form a stable toner image with good image quality, the contrast potential Vcont is low enough not to cause an edge effect or toner scattering, and the image distortion amount D falls within an allowable range. It is necessary that the value is moderately high. This range can be obtained in advance by evaluating the image quality while forming a toner image with various contrast potentials Vcont. In the experiments conducted by the inventors, the contrast potential Vcont is 50 in the image forming apparatus of this embodiment. .About.250V, and as the DC developing bias Vavg, about-(100 to 380) V was a preferable range.

When a plurality of low-density patch images are formed on the intermediate transfer belt 41 as described above, each patch image is moved by the patch sensor P as the intermediate transfer belt 41 moves.
The output signal from the sensor PS is read at the timing of moving to a position facing S, and the optical density of each patch image is obtained based on the signal (step S16).

The image density Dmin of the patch image formed with the DC developing bias Vmin is the target density D (op),
For example, whether or not the density is higher than OD = 0.22 is determined in step S1.
In step S18, it is determined whether the image density Dmax of the patch image formed with the DC developing bias Vmax is lower than the target density D (op). Here, FIG.
As shown in (a), when Dmin ≦ D (op) Dmax ≧ D (op), the process proceeds to step S19, where the density of the image densities of the low density patch images is set to a preset target density (OD = 0.22), the DC developing bias when it substantially matches is stored, and the value is stored in the memory 127 as the optimum developing condition. The exposure energy at that time is stored in the memory 127 as the optimum exposure condition.

On the other hand, as shown in FIG. 9B, the image density D
If min is higher than the target density D (op), step S
At 20, the exposure energy is lowered from the reference energy E2 by one step to set the exposure energy E1, and then the procedure returns to step S12 to repeat the optimization process. In addition, FIG.
When the image density Dmax is lower than the target density D (op) as shown in (c), the exposure energy is increased by one step from the reference energy E2 to set the exposure energy E3 in step S21, and then the process returns to step S12. And repeat the optimization process. Since the exposure energy E is changed and set according to the image density of the patch image in this way, the density control range can be widened and the image forming conditions (optimal development condition and optimum exposure condition) for obtaining the target density can be obtained. ) Is easier to obtain.

As described above, according to this embodiment, a low density patch image such as a fine line image or a halftone image is formed to optimize the image forming conditions (optimal developing condition and optimum exposing condition), and this image is obtained. Since the image is formed under the forming conditions, it is of course possible to obtain good image quality in a low-density image, but the optimum developing conditions and the optimum exposure conditions are in the density saturation range and the asymptotic range, respectively. For the same reason as described in detail in the section "Means for solving the problem", it is possible to form a toner image with stable image quality even on the high density side.

The parameter to be changed and set for adjusting the image density is basically only the DC developing bias Vavg applied to the developing roller 31, and the control is simple and the processing can be performed in a short time. Is.

Further, since the variable range of the DC developing bias Vavg is limited from the viewpoint of improving the image quality, it is possible to more stably form a toner image of a constant image quality.

Further, when the desired image density cannot be obtained only by adjusting the DC developing bias Vavg, the exposure energy is used as an auxiliary for the density adjustment, so that the density can be controlled over a wide range. is there. In other words,
In an apparatus that does not need to use the exposure energy as an auxiliary, the optimum exposure condition is fixed to the exposure energy existing in the asymptotic range, and only the DC development bias is changed and set within the density saturation range. A low-density patch image may be formed, and the optimum developing condition may be obtained based on the image density.

The present invention is not limited to the above-described embodiment, and various modifications other than those described above can be made without departing from the spirit of the present invention. For example, although the exposure energy is used supplementarily in the above embodiment, the exposure energy may be optimized as a main image forming condition like the DC developing bias. That is, the DC developing bias Vavg and the exposure energy E are changed and set in multiple steps in the density saturation range and the asymptotic range as image forming conditions, and a low density patch image is formed under each image forming condition, and based on the image density. The optimum developing condition and the optimum exposure condition may be obtained.

In the above embodiment, a solid image is used as the high density patch image and a line image consisting of a plurality of 1-dot lines spaced apart from each other is used as the low density patch image. The image that can be used is not limited to these and may be an image having another pattern. These should be appropriately changed according to the characteristics of the toner used and the sensitivity of the patch sensor PS. Further, the low-density side target density of each patch image is not limited to the above numerical value and may be changed appropriately.

In the above embodiment, the developing roller 31 is used.
The present invention is applied to an image forming apparatus of a so-called non-contact developing system in which the toner is moved to the photoconductor 21 by applying an alternating voltage as a developing bias to the electrostatic latent image to form a visible image. It goes without saying that the present invention can be applied to a device that applies a voltage as a developing bias and a device of a contact developing system.

In the above embodiment, the patch sensor P
S is configured to detect the density of the patch image on the intermediate transfer belt 41 immediately after the primary transfer from the photoconductor 21, but the position of the patch sensor PS may be another position, for example, The density of the patch image formed on the photoconductor 21 may be detected.

Further, although the above-mentioned embodiment is an apparatus capable of forming a color image by using toner of four colors, the present invention is also applicable to an apparatus which forms only a monochrome image, for example. Can be applied.

[0048]

As described above, according to the present invention, a low density patch image is formed with each bias value while the development bias is changed and set in multiple steps in the density saturation range, and the optimum density is obtained based on the image density. The developing condition is obtained, the developing bias is set to the optimum developing condition, and the toner image is formed with the energy density of the light beam set within the asymptotic range. Not only is it possible to obtain high image quality, but it is also possible to form a toner image with stable image quality for high-density images, and a toner image with good image quality over a wide density range from low-density images to high-density images. Can be easily formed with high precision and stability.

Further, according to the present invention, the development bias and the energy density of the light beam are set as image forming conditions in multiple steps in the density saturation range and the asymptotic range, and the low density patch image is formed under each image forming condition. After forming the image, the optimum developing condition and the optimum exposing condition are obtained based on the image density, the developing bias is set to the optimum developing condition, and the toner beam is formed with the energy density of the light beam set to the optimum exposing condition. With this configuration, the image density can be stabilized with higher accuracy.

[Brief description of drawings]

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

FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus of FIG.

FIG. 3 is a diagram showing light attenuation characteristics of a photoconductor used in the image forming apparatus of FIG. 1, and setting ranges of developing bias and exposure energy.

4 is a diagram showing electrical characteristics of an alternating voltage applied to a developing roller in the image forming apparatus of FIG.

FIG. 5 is a diagram showing a relationship between a developing bias and an image density of a toner image.

FIG. 6 is a flowchart showing an optimization process of this image forming apparatus.

FIG. 7 is a diagram showing an example of a low-density patch image used in this optimization processing.

FIG. 8 is a diagram exemplifying distortion of an image when a DC developing bias is inappropriate.

FIG. 9 is a diagram showing an operation of optimization processing.

[Explanation of symbols]

3 ... Rotary developing unit (developing means) 8 ... Exposure unit (exposure means) 12 ... Engine controller (control means) 21 ... Photoreceptor 31 ... Developing roller (toner carrier) 124 ... CPU (control means) E2 ... Standard energy L ... light beam

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Claims (9)

[Claims] 1. An exposure means for exposing a surface of a charged photoreceptor to a light beam to form an electrostatic latent image, and a toner carrier for carrying a toner on the surface thereof, which is applied to the toner carrier. Developing means for moving the toner to the surface of the photoconductor to visualize the electrostatic latent image with the toner according to the developing bias, and a toner image having a predetermined target density by setting the developing bias to the optimum developing condition. In an image forming apparatus including a control unit for forming an electrostatic latent image corresponding to a high-density patch image by irradiating the photoconductor with a light beam among the light attenuation characteristics of the photoconductor, A range of the energy density in which the surface potential of the photoconductor asymptotically approaches a predetermined potential as the energy density of the light beam increases is defined as an asymptotic range. Pack When the range of the development bias where the increase in the optical density with respect to the increase in the density of the toner adhering to the electrostatic latent image corresponding to the image is almost saturated is the density saturation range, the control means sets the development bias to the optimum development condition. In addition, a toner image is formed in a state where the energy density of the light beam is set within the asymptotic range, and if necessary, the developing bias is changed and set in multiple steps within the density saturation range prior to toner image formation. However, the image forming apparatus is characterized in that a low-density patch image is formed with each bias value, and an optimization process for obtaining an optimum developing condition is executed based on the image density. 2. The image forming apparatus according to claim 1, wherein the control unit can set the energy density of the light beam in addition to the developing bias in multiple steps within the asymptotic range. 3. The control means sets the energy density of the light beam to a reference energy existing in the asymptotic range, and changes the developing bias in multiple steps to set a low-density patch image at each bias value. To form
3. When the optimum developing condition cannot be obtained based on the image density, the energy density of the light beam is changed to a value different from the reference energy in the asymptotic range and the optimization process is executed again. The image forming apparatus described. 4. The image forming apparatus according to claim 3, wherein the control unit determines the change setting value of the energy density according to the image density of the low density patch image. 5. An exposure means for exposing a surface of a charged photoreceptor to a light beam to form an electrostatic latent image, and a toner carrier for carrying a toner on the surface thereof, which are applied to the toner carrier. Developing means for moving the toner to the surface of the photoconductor according to the developing bias to visualize the electrostatic latent image with the toner, and setting the developing bias to the optimum developing condition and setting the energy density of the light beam. An image forming apparatus comprising: a control unit configured to form a toner image having a predetermined target density by setting an optimum exposure condition. Among the light attenuation characteristics of the photoconductor, a light beam is applied to the photoconductor to produce a high density image. When forming an electrostatic latent image corresponding to a patch image, the range of the energy density in which the surface potential of the photoconductor asymptotically approaches a predetermined potential as the energy density of the light beam increases is referred to as an asymptotic range. In the settable range of the development bias, the range of the development bias in which the increase in the optical density with respect to the increase in the density of the toner attached to the electrostatic latent image corresponding to the high-density patch image is almost saturated is referred to as a density saturation range. At this time, the control means forms the toner image in a state where the developing bias is set to the optimum developing condition and the energy density of the light beam is set to the optimum exposure condition, and if necessary, prior to the toner image formation. While the development bias and the energy density of the light beam are set as image forming conditions in multiple steps in the density saturation range and the asymptotic range, a low density patch image is formed under each image forming condition, and the image density An image forming apparatus which executes an optimization process for obtaining an optimum developing condition and an optimum exposure condition based on the above. 6. The image forming apparatus according to claim 1, wherein the high-density patch image is a solid image. 7. The low-density patch image is composed of either a plurality of dots spaced apart from each other or a plurality of 1-dot lines spaced apart from each other. The image forming apparatus described. 8. A surface of a photoconductor is exposed to a light beam to form an electrostatic latent image on the surface thereof, and a developing bias is applied to a toner carrier carrying toner to allow the photoconductor to move from the toner carrier. An image forming method of forming a toner image having a predetermined target density by visualizing the electrostatic latent image by moving toner to a surface of the photoconductor, the method comprising: When the electrostatic latent image corresponding to the high density patch image is formed, the range of the energy density in which the surface potential of the photoconductor gradually approaches the predetermined potential as the energy density of the light beam increases. As the asymptotic range, in the range in which the developing bias can be set, the developing bias in which the increase in the optical density with respect to the increase in the density of the toner attached to the electrostatic latent image corresponding to the high-density patch image is almost saturated When the envelope is set to the density saturation range, the developing bias is set to the optimum developing condition, and if necessary, before the toner image is formed in the state where the energy density of the light beam is set to the asymptotic range. An image forming method characterized in that a developing bias necessary for forming a toner image having the target density is obtained as the optimum developing condition by performing the following steps (a) to (c). (a) A step of forming a low density patch image with each bias value while changing and setting the developing bias in multiple steps within the density saturation range. (b) A step of detecting the image density of the low-density patch image. (c) A step of obtaining the optimum developing condition based on the detected image density. 9. A surface of a photoconductor is exposed to a light beam to form an electrostatic latent image on the surface thereof, and a developing bias is applied to a toner carrier carrying toner to allow the photoconductor to move from the toner carrier. An image forming method of forming a toner image having a predetermined target density by visualizing the electrostatic latent image by moving toner to a surface of the photoconductor, the method comprising: When the electrostatic latent image corresponding to the high density patch image is formed, the range of the energy density in which the surface potential of the photoconductor gradually approaches the predetermined potential as the energy density of the light beam increases. As the asymptotic range, in the range in which the developing bias can be set, the developing bias in which the increase in the optical density with respect to the increase in the density of the toner attached to the electrostatic latent image corresponding to the high-density patch image is almost saturated When the envelope is set to the density saturation range, the developing bias is set to the optimum developing condition, and the toner image is formed under the condition that the energy density of the light beam is set to the optimum exposure condition. The steps (a) to (c) are performed to obtain the developing bias and the energy density of the light beam required to form the toner image having the target density as the optimum developing condition and the optimum exposing condition, respectively. Image forming method. (a) forming a low-density patch image under each image forming condition while changing the image forming condition including the developing bias and the energy density of the light beam in multiple steps in the density saturation range and the asymptotic range . (b) A step of detecting the image density of the low-density patch image. (c) A step of obtaining the optimum developing condition and the optimum exposure condition based on the detected image density.