JP5392200B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus, and more particularly to a technique for suppressing the occurrence of roughness in a density flat portion of an output image of the image forming apparatus.

  In an electrophotographic image forming apparatus, the surface of a photoconductor is uniformly charged by a charger with laser light, and then the charged photoconductor surface is scanned with laser light to form an electrostatic latent image. The electrostatic latent image is visualized by supplying toner from a developing unit. The toner image thus obtained is primarily transferred to an intermediate transfer member such as an intermediate transfer belt, and then secondarily transferred onto a recording sheet to form an image.

By the way, among the images formed in this way, in particular, in a portion where the density of image data in the highlight portion is flat (hereinafter referred to as “density flat portion”), fine density unevenness occurs and the surface is rough. (Hereinafter, this phenomenon is called “Gastsuki”).
It can be understood that the presence or absence of such roughness is mainly caused by non-uniform distribution of the charge amount of the toner in the developing device.

Factors that cause non-uniformity in the toner charge amount distribution include the following.
(A) New and old mixing ratio of developer There is a difference in the remaining amount of the toner post-processing material (external additive material) between the old and new toner, and as a result, a difference in fluidity and a difference in chargeability occur.
(B) Environmental History In general, toner has a high charge amount in a low humidity environment and low in a high humidity environment, but the history varies depending on the timing of toner replenishment, and this causes uneven charge amount.

(C) Toner Bulk If the image forming apparatus is left unattended for a long time, the toner bulk in the developing unit is reduced, and the toner is tightened compared to the case where the image forming operation is frequently performed. Thus, the fluidity is lowered and the stirring property is deteriorated.
Due to the uneven charge amount of the toner in the developing device caused by the above factors, a difference occurs in the amount of toner adhering to the dot portion of the photosensitive member exposed by the laser beam, and this causes roughness in the density flat portion of the output image. It is the cause.

  It is known that it is effective to increase the amount of laser light in the exposure scanning unit in order to suppress the occurrence of such roughness in the density flat part. By increasing the amount of laser light, the adhesion area of toner per dot increases, and even if there is some uneven charging of individual toner particles, a certain amount of adhesion can be secured for the entire dot. This is because the uneven density becomes inconspicuous.

JP-A-4-150472

Conventionally, however, the amount of laser light has been increased appropriately, causing problems in image reproducibility, particularly fine line reproducibility.
That is, if the toner adhesion area per dot increases due to an increase in the amount of laser light, this leads to an increase in the width of the fine line as it is, which causes a problem in reproducing the fine fine line portion and causes the quality of the reproduced image to deteriorate.

  The present invention has been made in view of the above circumstances, and in particular, provides an image forming apparatus capable of suppressing the occurrence of roughness as much as possible without impairing the reproducibility of the fine line portion of the image. It is aimed.

  In order to achieve the above object, an image forming apparatus according to an aspect of the present invention performs exposure scanning on a surface of a photoconductor with laser light emitted from an exposure unit based on image data subjected to pseudo gradation conversion processing by an image processing unit. An image forming apparatus that forms an electrostatic latent image, visualizes it with toner supplied from a developing device, and transfers the image to a transfer material to form an image. The amount of laser light emitted from an exposure unit is reduced. When a line image is formed with the laser light quantity control means to be controlled and image data that should have a specified line width, an error between the formed line width and the specified line width is within an allowable range. The determining means for determining the upper limit value and the lower limit value of the laser light amount in the exposure unit, and the surface of the photoreceptor with the first laser light amount that exceeds the lower limit value and is less than the upper limit value based on the image data to be the density flat portion Based on the electrostatic latent image drawn on A density fluctuation detecting means for forming a toner pattern and detecting a density fluctuation amount of the toner pattern, and the laser light quantity control means, when the density fluctuation amount detected by the density fluctuation detection means exceeds a predetermined value, The laser light amount is set to a second laser light amount that is increased toward the upper limit value from the first light amount.

As a result, the amount of laser light is not increased beyond the determined upper limit value, and the occurrence of roughness can be suppressed as much as possible while keeping the increase in line width within an allowable range.
Here, even when the laser light amount is set to the second light amount by the laser light amount control means, if the density fluctuation amount detected by the density fluctuation detection means exceeds a predetermined value, the toner in the developer is removed. A toner forced replacement means for forcibly discharging and supplying new toner may be provided.

Here, when the density fluctuation amount detected by the density fluctuation detection means exceeds a predetermined value even if the laser light quantity is set to the second light quantity by the laser light quantity control means, Forcibly stirring means for forcibly stirring the developer may be provided.
Here, the laser light quantity control means may set the laser light quantity to the first light quantity when the density fluctuation amount detected by the density fluctuation detection means is a predetermined value or less.

Further, it is desirable that the specified line width is 250 μm to 350 μm, and the allowable range of the error is within ± 20% with respect to the specified line width.
Here, it is preferable that the first laser light amount is a laser light amount when the line width is a central value within an allowable range of the error.
Here, the second laser light amount may be the upper limit value.

The image processing unit includes a gradation conversion curve generation unit that generates a gradation conversion curve so that a gradation value of input image data and a gradation value of an output image have a predetermined relationship; The image processing apparatus may further include pseudo gradation conversion means for performing pseudo gradation conversion processing on the input image data based on the gradation conversion curve thus input.

1 is a schematic diagram illustrating an overall configuration of a printer according to an embodiment of the present invention. It is a block diagram which shows the structure of the control part of the said printer. It is a flowchart which shows the control content of the image stabilization process in the said control part. It is a flowchart which shows the content of the developing potential determination process of step S2 of FIG. FIG. 6 is a diagram illustrating an example of a development characteristic pattern formed on an intermediate transfer belt in the development potential determination process. It is a figure for demonstrating the determination method of developing potential. It is a figure which shows the example of the pattern for line | wire width detection for determining the laser beam quantity which makes the error | error of the width | variety of a thin line within tolerance. It is a graph for determining the laser light quantity which makes the error of the width | variety of a thin line within an allowable range based on the detection value of the line width detection pattern. It is a figure which shows the example of the pattern for (gamma) correction at the time of calculating | requiring a gamma correction curve. (A) shows the output fluctuation of the density detection sensor of the highlight portion of the toner pattern when there is no roughness, and (b) shows the density detection sensor of the highlight portion of the toner pattern when the roughness is generated. It is a figure which shows an output fluctuation | variation. It is a figure for demonstrating the preparation method of (gamma) correction curve. It is a figure which shows the example of the dot pattern in the case of expressing gradation by the dither method.

Hereinafter, an embodiment of an image forming apparatus according to the present invention will be described by taking as an example a case where it is applied to a tandem type color digital printer (hereinafter simply referred to as “printer”).
(1) Overall Configuration of Printer FIG. 1 is a schematic diagram showing the overall configuration of the printer 1 according to the present embodiment.
The printer 1 forms an image on a recording sheet by a known electrophotographic method, and includes an image processing unit 10, a transfer unit 20, a paper feeding unit 30, a fixing unit 40, and a control unit 45, and a network. Color and monochrome printing is selectively executed based on a print job received from an external terminal device (not shown) via a LAN (for example, a LAN).

The image processing unit 10 includes image forming units 10Y to 10K corresponding to development colors of yellow (Y), magenta (M), cyan (C), and black (K).
The image forming unit 10Y includes a photosensitive drum 11, a charger 12, an exposure unit 13, a developing unit 14, a primary transfer roller 15, a cleaner 16, and the like disposed around the photosensitive drum 11.
The charger 12 charges the peripheral surface of the photosensitive drum 11 that rotates in the direction indicated by the arrow A.

The exposure unit 13 exposes and scans the charged photosensitive drum 11 with laser light emitted from a laser diode (LD), and forms an electrostatic latent image on the photosensitive drum 11.
The developing unit 14 contains a developer containing toner, and develops the electrostatic latent image on the photosensitive drum 11 with the toner, whereby a Y-color toner image is formed on the photosensitive drum 11. .
The developing method of the developing unit 14 may be either a one-component developing method using only toner or a two-component developing method using toner and carrier.

The primary transfer roller 15 transfers the Y color toner image on the photosensitive drum 11 onto the intermediate transfer belt 21 by electrostatic action. The cleaner 16 cleans residual toner remaining on the photosensitive drum 11Y after transfer. The other image forming units 10M to 10K have the same configuration as the image forming unit 10Y, and the reference numerals are omitted in FIG.
The transfer unit 20 includes an intermediate transfer belt 21 that is stretched around a drive roller 24 and a driven roller 25 and circulates in the direction of the arrow.

  When color printing (color mode) is executed, a corresponding color toner is formed on the photosensitive drum 11 for each of the image forming units 10Y to 10K, and each of the formed toner images is intermediately transferred. Transferred onto the belt 21. The image forming operations for the respective colors Y to K are executed while shifting the timing from the upstream side to the downstream side so that the toner images of the respective colors are transferred to the same position on the traveling intermediate transfer belt 21. The

The paper feeding unit 30 feeds the sheets S from the paper feeding cassette one by one in accordance with the above image forming timing, and conveys the sheets S toward the secondary transfer roller 22 via the conveyance path 31.
When the sheet S conveyed to the secondary transfer roller 22 passes between the secondary transfer roller 22 and the intermediate transfer belt 21, each color toner image formed on the intermediate transfer belt 21 is transferred to the secondary transfer roller 22. Secondary transfer is collectively performed on the sheet S by electrostatic action.

The sheet S after the toner images of the respective colors are secondarily transferred is conveyed to the fixing unit 40, where the toner on the surface is fused and fixed to the surface of the sheet S by being heated and pressurized in the fixing unit 40. Then, the paper is discharged onto the paper discharge tray 33 by the paper discharge roller 32.
In the above, the operation in the case of executing the color mode has been described. However, in the case of executing monochrome (for example, black) printing (monochrome mode), only the black image forming unit 10K is driven, and the same as described above. With this operation, black image formation (printing) is performed on the recording sheet S through the steps of charging, exposing, developing, transferring, and fixing the black color.

The toner or toner pattern that has not been transferred onto the recording sheet S on the intermediate transfer belt 21 is removed by a cleaning blade 26 disposed at a position facing the driven roller 25 across the intermediate transfer belt 21.
Further, a density detection sensor 23 made of, for example, a reflection type photoelectric sensor is disposed on the downstream side in the traveling direction of the intermediate transfer belt 21 in the image forming unit 10K. The density of the toner pattern formed on the belt 21 is detected.

The density detection sensor 23 includes a light emitting element that emits light of a specific wavelength and a light receiving element that receives regular reflection light or diffused light from the intermediate transfer belt 21 and outputs a voltage corresponding to the amount of light received. . Usually, a light emitting diode (LED) and a photodiode (PD) are used for the light emitting element and the light receiving element, respectively.
Above the intermediate transfer belt 21, an environmental sensor 27 for detecting the temperature and humidity in the apparatus is disposed. Based on the detection result, whether or not the image stabilization process is necessary is determined.

In addition, an operation panel 35 is arranged at a position on the front side and the upper side of the apparatus main body, which is easy for the user to operate. The operation panel 35 includes buttons for receiving various instructions from the user, a touch panel type liquid crystal display unit, and the like, and transmits the received instruction content to the control unit 45 or displays information indicating the status of the printer 1 on the liquid crystal display. To display.
The control unit 45 controls each unit based on the print job data received from the external terminal device via the network to execute a smooth print operation.

(2) Configuration of Control Unit 45 FIG. 2 is a block diagram showing the configuration of the control unit 45.
As shown in the figure, the control unit 45 includes a CPU 451, a communication I / F (interface) unit 452, an image memory 453, an image processing unit 454, a RAM 457, a ROM 458, an EEPROM 459, and the like.

The communication I / F unit 452 is a LAN card or a LAN board for connecting an external client terminal to a LAN, receives print job data transmitted from the client terminal via the LAN, and sends it to the CPU 451.
The image memory 453 stores the received print job image data for each RGB.

The image processing unit 454 includes a γ correction unit 455 and a dither processing unit 456. After the RGB image data is converted into gradation data of each development color, the γ correction unit 455 applies γ for the gradation data of each development color. A correction process is performed, and a conversion process for expressing the gradation data γ-corrected by the dither processing unit 456 by the dither method is performed.
This dither method is a well-known technique for expressing a gradation value of a specific pixel by a pseudo gradation expression by a set of a plurality of dots whose density is binarized or multi-valued.

The image data processed in this way is temporarily stored in the RAM 457 for each development color. The CPU 451 reads out the image data and controls the light amount of a laser diode (not shown) of each exposure unit 13 of the image processing unit 10 to form an electrostatic latent image on each of the photosensitive drums 11Y to 11K.
The image memory 453 is a volatile memory, and serves as a work area when the CPU 451 executes a program and a page memory for temporarily storing image processed image data.

The ROM 458 stores a program for controlling the operation of each unit in the printer 1, a program for image stabilization processing, and image data for printing various toner patterns formed at that time.
The EEPROM 459 is a recordable nonvolatile memory, and accumulates and stores the number of prints.

The CPU 451 reads out necessary programs from the ROM 458 based on the acquired image data and controls the operations of the image processing unit 10, the transfer unit 20, the paper feeding unit 30, and the fixing unit 40 in a unified manner while measuring the timing. The forming operation is executed smoothly, and image stabilization processing is executed based on the output of the density detection sensor 23.
(3) Image Stabilization Processing In the image stabilization processing in the present embodiment, mainly (a) development potential determination processing, (b) fine line reproducibility ensuring processing (LD light amount range determination processing), and (c) roughness level detection. Process, and (d) γ correction process.

Note that in the tandem type printer as in the present embodiment, as the contents of the image stabilization process, the color misregistration amount of the toner image formed by each of the image forming units 10Y to 10K is further detected, A so-called registration correction for correcting the color misregistration of each color is also included, but is omitted here.
FIG. 3 is a flowchart showing the control contents of the image stabilization process executed by the control unit 45, and is executed as a subroutine in a main flowchart (not shown) for controlling the overall operation of the printer 1.

<Determination of image stabilization processing timing>
First, it is determined whether or not it is time to execute the image stabilization process (step S1).
This timing determination is performed by determining whether or not a predetermined condition that requires execution of the image stabilization process (hereinafter referred to as “image stabilization process execution condition”) is satisfied.

  The content of the image stabilization processing execution condition itself is not particularly new and has been conventionally used in many image forming apparatuses. For example, (a) after the image forming apparatus is turned on, (b ) Whether a print job has been accepted, (c) Whether the power saving mode (sleep mode) has been canceled, (d) Whether the temperature and / or humidity fluctuations in the apparatus have exceeded predetermined threshold values, (E) Whether the number of prints since the previous execution of the image stabilization process has exceeded a predetermined number (for example, 1000 sheets), and the like. There is a tendency.

  Note that, for example, when the image stabilization processing execution conditions (d) and (e) described above are satisfied during execution of the print job, the immediate print job may be interrupted and the image stabilization processing may be executed. However, when the number of unprocessed sheets is smaller than a predetermined value, the print job may be continued as it is, and the image stabilization process may be executed after the print job ends. In particular, the former is desirable for color printing that requires reproduction with high image quality.

<Development density determination process>
If it is determined in step S1 that the image stabilization process is to be performed (step S1: YES), the flag F in the RAM 457 is set to “0” and the development potential determination process is executed (step S2, S3).
FIG. 4 is a flowchart showing the contents of this development potential determination subroutine.

  First, a solid pattern for detecting development characteristics (hereinafter referred to as “development characteristics”) based on predetermined gradation value data while varying the potential (development potential) of the voltage applied to the development roller of the development section 14 within a predetermined range. Pattern ”) is formed on the intermediate transfer belt 21 (step S21). Usually, as the gradation value data, data indicating the maximum density is employed. The range of the development potential is determined in advance by experiments or the like and stored in the ROM 458 within the range where development is possible even with the toner having the lowest charge amount that can be normally obtained. The laser light amount of the exposure unit 13 at this time is an initial value set as a standard value in the ROM 458 or a value set by the previous image stabilization process.

  FIG. 5 is a schematic view of the intermediate transfer belt 21 on which the development characteristic pattern is formed as viewed from the lower side of FIG. 1. Reference numeral 23 a indicated by a dotted circle is an intermediate position detected by the density detection sensor 23. A detection position on the transfer belt 21 is indicated. 21a is a pattern formed for detecting the home position (reference position). If the pattern is outside the image area, the intermediate transfer belt 21 is manufactured in advance. It may be marked with an appropriate paint or a sticker that is easy to reflect may be attached.

The intermediate transfer belt 21 moves in the direction of the white arrow Y in the figure, and the rotation position of the intermediate transfer belt 21 when the mark 21a is detected by the density detection sensor 23 becomes the home position.
When the developing unit 211 forms the development characteristic patterns 211, 212, 213,... At a predetermined timing after the detection of the mark 21a, the control unit 45 sets the development potentials to different potentials V1 to V4. The image processing unit 10 and the intermediate transfer unit 20 are controlled.

The development characteristic pattern group 210 formed on the intermediate transfer belt 21 is detected through the detection position 23a of the density detection sensor 23 as the intermediate transfer belt 21 rotates, and the detected value is acquired by the CPU 451. (Step S22).
Then, the correlation between the two is obtained from the development potential and the detected value when the patterns 211, 212, 213,... Of the development characteristic pattern group 210 are formed (step S23).

FIG. 6 is a graph showing an example of the detection result of the development characteristic pattern group 210 by the density detection sensor 23. The horizontal axis indicates the absolute value of the development potential when each pattern of the development characteristic pattern group 210 is formed, and the vertical axis indicates the detection value of the density detection sensor 23 for each pattern.
As shown in the figure, as the absolute value of the developing potential increases, the difference from the electrostatic latent image potential on the photosensitive drum 11 increases, and the toner tends to adhere to the photosensitive drum 11 and the pattern of the development characteristic is increased. Since the density increases and the sensor detection value decreases, the approximate straight line Ldev is obtained by linearly approximating these values by, for example, a known least square method, and the development potential and the detection value of the density detection sensor 23 are Can be obtained as a correlation.

Next, the development potential corresponding to the target detection value is acquired from the correlation (step S24).
That is, if the gradation data of the development characteristic pattern group 210 is correctly reflected in the toner image of the development characteristic pattern group 210 of the intermediate transfer belt 21, the value of the detection value of the density detection sensor 23 (target value). ) Is stored in the ROM 458 in advance, and the set value that should be the development potential for obtaining the target value can be obtained based on the approximate straight line Ldev in FIG. 6 (step S25).

The development potential determination process described above is performed for all of the image forming units 10Y to 10K, and is set in the RAM 457 or the EEPROM 459 as a development potential to be applied at the time of image formation executed in each image forming unit. The potential determination process is terminated and the process returns to the flowchart of FIG.
Then, in steps S4 to S7 in FIG. 3, a fine line reproducibility ensuring process is executed.

<Thin wire reproducibility ensuring process>
In this thin line reproducibility ensuring process, the amount of laser light is adjusted so that the width of the thin line converted from the reflection density of the line width detection pattern formed on the intermediate transfer belt 21 as a toner image does not exceed a predetermined allowable range. It is executed for the purpose of determining an appropriate range.
First, in step S4, a plurality of line width detection patterns are formed on the intermediate transfer belt 21 while varying the laser light amount (LD light amount).

FIG. 7 is a diagram showing an example of forming the line width detection pattern 220 in this case. With the LD light amounts L1 to L5, the line width detection patterns are formed in the order of KCMY.
In this example, as shown in the partial enlarged view 221 at the bottom of the figure, each pattern has three white pixels (pixels to which no toner is attached) in the main scanning direction and one black pixel (toner is attached). Pixel) to be formed, and is formed as a set of thin lines extending obliquely downward to the right by 45 °.

Then, the reflection density in each pattern is detected and converted into a line width (step S5), and the upper limit value of the tolerance of the line width error of each thin line and the median value of the LD light quantity that becomes the target line width are obtained. Obtain (step S6).
Hereinafter, a method for obtaining the upper limit value and the median value of the LD light amount will be specifically described.
In FIG. 8, the horizontal axis indicates the magnitude of the LD light quantity, and the vertical axis indicates the detection value of the density detection sensor 23. The curve Lld plots the detection value for each LD light quantity obtained in step S5 ( (Not shown) is a curve that approximates and is obtained based on a known curve approximation method.

  The detection value of the concentration detection sensor 23 with respect to the line width (300 μm in the present embodiment) as the center of the target and the upper and lower limits (± 50 μm in the present embodiment) allowable for the line width ( dmid, dmax, dmin) are obtained in advance and stored in the ROM 458. From this value and the value of the approximate curve Lld in FIG. 8, the median value Lmid of the LD light quantity corresponding to the target line width and the target The upper limit value Lmax of the LD light amount corresponding to the upper limit of the allowable range for the line width is obtained, and the median value Lmid and the upper limit value Lmax of the LD light amount are stored in the EEPROM 459, for example.

  In step S6, the allowable range of the target line width is ± 50 μm with respect to a 300 μm fine line as a reference in this embodiment, but this is only an example according to this embodiment, and is applicable. An appropriate value derived empirically depending on the resolution of the model to be used or whether it is monochrome or color may be applied. In general, it is considered that the reproducibility of the fine line can be ensured by allowing the line width to fall within an error of ± 20% with respect to the fine line of 250 μm to 350 μm.

Then, the current LD light quantity is set to the median value Lmid indicating the target line width (step S7).
The reason why the median value is set as the LD light quantity in this way is that the first target is to reproduce the width of the fine line as faithfully as possible as the image data.
<Gas level detection process>
Next, a solid patch is formed in the highlight portion, and processing for detecting the density fluctuation amount as a level indicating the degree of roughness (hereinafter referred to as “gray level”) is performed. In this embodiment, γ correction is performed. The highlighted portion of the pattern is also used as a solid patch for detecting the roughness level.

First, in step S8 in FIG. 3, a γ correction pattern is formed on the intermediate transfer belt 21 such that the LD light quantity becomes the median value Lmid set in step S7.
FIG. 9 shows an example of a γ correction pattern. Following the K γ correction pattern 301 </ b> K along the traveling direction Y of the intermediate transfer belt 21, the C, M, and Y γ correction patterns 301 </ b> C. To 301Y (301M, Y are not shown).

  The γ correction pattern 301K has a higher gradation value (for example, in a case where the image density is configured to be reproduced with 256 gradations from 0 to 255, 255, 254, 253,. (. 3, 2, 1, 0, etc.) Solid patches 3011K, 3012K, and 3013K having a predetermined width are continuously arranged. The same applies to the other γ correction patterns 301C to 301Y.

Then, the density of each solid patch is detected at a predetermined sampling interval, and the average value is stored in the RAM 457 as the density detection value of the solid patch (step S9).
).
On the other hand, among the solid patches of the pattern for γ correction, the solid patch of a specific highlight portion (solid patch formed based on gradation data of a density region in which the output image is easily noticeable) based on the sampling detection value, and the variation amount To get.

Here, the highlight portion is a density region up to 50 gradations among 0 to 255 gradations (see the portion surrounded by a circle in FIG. 11A).
In the present embodiment, the roughness level is obtained for the solid patch corresponding to the gradation data of 30 gradations. Each solid patch can be identified by, for example, the reference clock generated by the CPU 451 based on the number of counts after the mark 21a is detected.

FIGS. 10A and 10B show the sampling detection values of the solid patch of the highlight part. The difference between the maximum peak value and the minimum peak value among the detected values is acquired as the concentration fluctuation amount. However, in order to remove the influence of noise and reliably determine the roughness level, the average value of the differences between the maximum peak value and the minimum peak value may be used as the density fluctuation amount.
Moving to step S10, it is determined whether or not the acquired fluctuation amount is equal to or less than a predetermined value (step S11).

  As shown in FIG. 10B, when the density fluctuation amount is larger than a predetermined value (in this embodiment, the fluctuation value of the detection value of the density detection sensor 23 exceeds “0.4 V”. However, the value is not limited to this value, and an appropriate value that is empirically determined that the roughness is within the allowable range is determined in advance through experiments or the like.), The roughness level is determined to exceed the allowable value, If it is less than that, it is determined to be within the allowable range.

The predetermined value is obtained by experiments or the like before shipment and stored in the ROM 458.
If it is determined in step S11 that the fluctuation amount of the density detection value is equal to or smaller than the predetermined value, a γ correction curve is created based on the density detection value of the γ correction pattern obtained in step S9, and this is used. The data is stored in the EEPROM 459 (step S12), and the process returns to the main flowchart.

The γ correction curve is such that the density of the input gradation data and the output image has a desired correlation (generally a linear relationship, but can be changed by user designation). It is a curve for tone conversion.
FIGS. 11A and 11B show how the γ correction curve is created.
First, in FIG. 11A, the curve La shows the correlation of the detected density value of the output image with the input gradation data, and the straight line La shows the target value when these correlations are linear. Indicates.

For example, when the image density is configured to be reproduced with 256 gradations of 0 to 255 as in this example, the gradation data value of a specific solid patch is set to “100”. And
Then, if the density value obtained by detecting the formed solid patch by the density detection sensor 23 is “110”, the density value is “10” higher than the original density value. When the density value of the input signal value is “100”, the output signal is set to decrease by about 9.1% (10/110 = 0.0909...).

By executing this for each density toner pattern, a γ correction curve Lc shown in FIG. 11B is finally obtained.
Returning to FIG. 3, if “NO” in the step S11, it is determined whether or not the flag F = 0, and if so (step S13: YES), the LD light amount is obtained in the step S6. After setting the upper limit value (step S14) and setting the flag F to “1” indicating that the LD light quantity has been changed (step S15), the above steps S8 to S10 are repeated.

That is, a γ correction pattern is formed based on the electrostatic latent image drawn with the LD light amount of the upper limit Lmax (step S8), the density is detected (step S9), and the solid patch of a specific highlight portion is further detected. The variation amount of the detected density value is obtained (step S10).
If it is determined in step S11 that the value is equal to or less than the predetermined value, a γ correction curve is created based on the detected density value of the γ correction pattern obtained in step S9 and stored in the EEPROM 459 (step S12). Return to the main flowchart.

  On the other hand, if the non-uniformity in the toner charge amount distribution in the developing unit 14 is larger than expected, the roughness level may not be within the allowable range even if the LD light amount is set to the upper limit (step S11: NO). ). In this case, since F = 1 is already set in step S15, the determination in step S13 is “NO”, and the γ correction curve creation in step S12 is executed as it is.

  As a result, slight roughness in the highlight portion remains, but if the LD light quantity is increased further, the width of the fine line becomes so thick that it cannot be ignored, and the fine line is particularly emphasized in the reproduced image. In the embodiment, such processing is performed in order to give priority to the reproducibility of thin lines. Then, the image stabilization process ends, and the process returns to the main flowchart.

In the case of passing through step S14, it is considered that the overall density slightly increases as the LD light quantity increases. However, the density is adjusted to an appropriate density by receiving the gradation correction by the γ correction curve. is there.
For example, when specific gradation data is indicated by the dot pattern 613 in FIG.
By receiving the gradation correction, the dot pattern 611 or 612 is selected and adjusted to an appropriate density.

The image stabilization process is performed for all of the image forming units 10Y to 10K.
<Modification>
As described above, the present invention has been described based on the embodiment. However, the present invention is not limited to the above-described embodiment, and the following modifications may be considered.
(1) In the image stabilization processing in the above embodiment, in step S7 of FIG. 3, first, the LD light quantity is set to the central LD light quantity Lmid obtained in FIG. 8 so as to achieve the target line width. This is to reproduce the width of the fine line as faithfully as possible as image data. However, in step S7, it is not always necessary to strictly set the median value Lmid, and at least between the upper limit value Lmax and the lower limit value Lmin. It may be an appropriate value.

For example, when the apparatus environment is hot and humid, or when a large amount of originals with a low B / W ratio is printed and there is a lot of old toner in the developing unit 14, the fluidity is deteriorated. Since the probability of occurrence is high, an appropriate value larger than the median value Lmid and smaller than the upper limit value Lmax may be set as the LD light quantity.
(2) In the above embodiment, in order to detect the roughness level, first, a pattern for γ correction is formed, and the variation amount of the density of the solid patch in the highlight portion is acquired (FIG. 3). Steps S8 to S10), before forming the γ correction pattern, a solid patch dedicated to detecting the roughness level is formed, and the determination in step S11 is performed from the detected value, and a γ correction pattern is formed according to the result. Then, a γ correction curve may be created.

(3) In step S14 in FIG. 3, the LD light quantity is suddenly set to the upper limit value Lmax. However, in order to emphasize the fine line reproducibility more, it may be increased in multiple steps from the median value Lmid to the upper limit value Lmax. I do not care.
(4) In the above embodiment, the LD light amount is set to the upper limit value Lmax in step S14, and a γ correction pattern is formed in step S8. When it is determined that the value fluctuation amount is not less than or equal to the predetermined value, the γ correction pattern is inevitably created as it is (step S12). For example, after it is determined “NO” in step S13, the developing unit In order to correct the non-uniformity of the toner charge amount within 14, the predetermined pre-processing may be executed, and then the process may return to step S8 again and the subsequent steps may be repeated.

As this pre-processing, (a) forced discharge of toner, (b) forced stirring of toner, etc. can be considered.
The forced discharge of the toner (a) is achieved by, for example, forming a high-density solid image for a predetermined page. That is, a solid image is formed on the photosensitive drum 11 of the image forming unit that needs to be forcibly discharged without being fed, and transferred to the intermediate transfer belt 21.

At this time, the potential of the secondary transfer roller 22 is set to a negative high potential so that the toner of the solid image does not adhere to the secondary transfer roller 22. The solid image on the intermediate transfer belt 21 is removed by the cleaning blade 26.
By executing this for a preset page, the old toner in the developing unit 14 is forcibly consumed and new toner is replenished from a toner cartridge (not shown), so the toner charge distribution is greatly improved. Is done. Note that the configuration itself for detecting that the toner amount in the developing unit 14 has decreased and replenishing the toner from the toner cartridge to the developing unit 14 is conventionally known, and is not specifically described here.

  The forced stirring of the toner in (b) is realized by interrupting the image forming operation and driving the developing unit 14 for a predetermined time while being uniformly charged without forming an electrostatic latent image on the photosensitive drum. The Of course, during the image forming operation, since the toner in the developing unit 14 is circulated and supplied to the developing roller, a stirring operation is also performed at the same time, but this is performed when stirring alone is not sufficient.

This method is particularly effective when the toner in the developing section 14 is tightened due to being left for a long time, the bulk density of the toner is increased, and the fluidity is deteriorated. Since the fluidity of the toner is restored by forced stirring, toner mixing is improved, and an improvement in charge amount distribution can be expected.
However, in the case of (a), the toner is wasted, and in the case of (b), it takes time for the forced stirring. Therefore, as in the above embodiment, first, image stabilization is performed. It is desirable to give priority to the control of the LD light quantity as part of the processing.

(5) Although the example in which the developing device and the image forming apparatus according to the present invention are applied to a tandem color digital printer has been described, the scope of application of the present invention is not limited thereto.
For example, an intermediate transfer drum may be provided as an intermediate transfer body, and the intermediate transfer drum may be rotated four times so that CMYK toner images are sequentially superimposed and transferred onto the peripheral surface.
Further, the present invention can be applied to a copying machine, a facsimile machine, a multi-function peripheral (MFP), etc., regardless of color or monochrome image formation.

  Moreover, you may combine the content of the said embodiment and modification as much as possible.

  INDUSTRIAL APPLICABILITY The present invention is useful as a technique for suppressing the roughness of a density flat portion in an output image in an image forming apparatus.

DESCRIPTION OF SYMBOLS 1 Printer 10 Image process part 11 Photosensitive drum
12 Charger 13 Exposure section
DESCRIPTION OF SYMBOLS 14 Developing part 15 Temporary transfer roller 16 Cleaner 20 Intermediate transfer part 21 Intermediate transfer belt 22 Secondary transfer roller 23 Concentration detection sensor 24 Drive roller 25 Driven roller 26 Cleaning blade 27 Environmental sensor 30 Paper feed part 31 Conveyance path 32 Paper discharge roller 33 Output tray 35 Operation panel
40 Fixing Unit 45 Control Unit 451 CPU
452 Communication I / F unit 453 Image memory
454 Image processing unit 455 γ correction unit 456 Dither processing unit 457 RAM
458 ROM
459 EEPROM
S Recording sheet

Claims (8)

An electrostatic latent image is formed by exposing and scanning the surface of the photosensitive member with laser light emitted from the exposure unit based on the image data subjected to the pseudo gradation conversion processing in the image processing unit, and this is formed with toner supplied from the developing unit. An image forming apparatus that visualizes and transfers an image to a transfer material to form an image,
Laser light quantity control means for controlling the laser light quantity emitted from the exposure unit;
When a line image is formed with image data that should have a specified line width, an upper limit value of the laser light amount in the exposure unit is set so that an error between the formed line width and the specified line width is within an allowable range. And a determining means for determining a lower limit value;
A toner pattern is formed based on an electrostatic latent image drawn on the surface of the photoconductor with a first laser light amount that exceeds the lower limit and less than the upper limit based on image data to be a density flat portion, A concentration fluctuation detecting means for detecting a concentration fluctuation amount;
With
The laser light quantity control means includes
When the density fluctuation amount detected by the density fluctuation detection means exceeds a predetermined value, the laser light quantity is set to a second laser light quantity that is increased toward the upper limit value from the first light quantity. Image forming apparatus. Even if the laser light quantity control means sets the laser light quantity to the second light quantity, if the density fluctuation amount detected by the density fluctuation detection means exceeds a predetermined value, the toner in the developer is forcibly The image forming apparatus according to claim 1, further comprising a forced toner replacement unit that discharges and replenishes new toner. Even if the laser light quantity control means sets the laser light quantity to the second light quantity, if the density fluctuation amount detected by the density fluctuation detection means exceeds a predetermined value, the developer in the developer is forced The image forming apparatus according to claim 1, further comprising a forced stirring unit that stirs the head. The laser light quantity control means includes
4. The image forming apparatus according to claim 1, wherein the laser light quantity is set to a first light quantity when the density fluctuation amount detected by the density fluctuation detection unit is a predetermined value or less. 5. . 5. The specified line width is 250 μm to 350 μm, and the allowable range of the error is within ± 20% with respect to the specified line width. 6. Image forming apparatus. The image forming apparatus according to claim 1, wherein the first laser light amount is a laser light amount when the line width becomes a central value within an allowable range of the error. The image forming apparatus according to claim 1, wherein the second laser light quantity is the upper limit value. The image processing unit
Gradation conversion curve generation means for generating a gradation conversion curve so that the gradation value of the input image data and the gradation value of the output image have a predetermined relationship;
The image according to any one of claims 1 to 7, further comprising: pseudo gradation conversion means for performing pseudo gradation conversion processing on the input image data based on the generated gradation conversion curve. Forming equipment.