Electrophotography – Control of electrophotography process – Of plural processes
Reexamination Certificate
2002-10-22
2004-07-27
Grainger, Quana (Department: 2852)
Electrophotography
Control of electrophotography process
Of plural processes
C399S046000
Reexamination Certificate
active
06768878
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an image forming method and an image forming apparatus, both to form images electrophotographically on recording paper by controlling image forming conditions, and more particularly to image density control technology.
For a conventional image forming apparatus, such as copying machine or printer, that has a developing device constructed so that a latent image formed on its photoreceptor is developed using a two-component developing agent consisting of toner and carrier, an image density control method is employed in which the toner concentration in the developing agent is maintained by replenishing toner according to the amount of toner which has been consumed during the development.
The above-mentioned toner addition is controlled so as to be performed when a decrease in toner concentration is detected by, for example, detecting the resistance or permeability of the developing agent.
With this conventional method, however, there is a limit to supplying high-quality images stably over long periods of time, because detection errors are prone and partly because changes in the developing performance of the developing agent cannot be properly accommodated.
There is a method of controlling image forming conditions to avoid such inconveniences in image density control as described above. That is to say, this method forms a control patch on the photoreceptor, then detects the image density of the control patch by use of a density detection means, and controls image forming conditions using the detection signal sent from the density detection means.
Image density control using this method has the characteristic that since the image density of the image actually formed is constantly maintained, almost no control errors basically occur.
With regard to such image density control, the applicant for the present patent performed several proposals in Unexamined Japanese Application Patent Laid-Open Publications Nos. Hei 07-137346 and 2000-181155.
Under this image density control method, a control patch is formed on an image forming body such as a photoreceptor in accordance with image data of a reference density, then the image density of the formed control patch is detected using an image density detection means, and the quantity of electric charge, the exposure amount, the developing bias, the developing agent carrying velocity, the concentration of the toner in the developing agent, and other image forming conditions are controlled using the detection signal sent from the image density detection means.
Under prior art, a control patch is formed on an image forming body such as a photoreceptor in accordance with the image data relating to reference density, then the image density of the formed control patch is detected using an image density detection means, and the quantity of electric charge, the exposure amount, the developing bias, the developing agent carrying velocity, the toner concentration in the developing agent, and other image forming conditions are controlled using the detection signal sent from the image density detection means.
In this case, a patch that has been formed as an image of the required density by varying the developing bias and the charging potential is usually used as a reference control patch. However, since it is difficult with the above-mentioned control method to respond to the tendencies towards faster image formation and toner particle size reduction in recent years, the formation of a dither pattern and an error diffusion pattern for imagewise exposure based on reference input density image data, or of solid and non-solid reference patterns with densities adjusted by laser pulse width modulation has been proposed. Hereby, although, heretofore, the developing field has been reduced by reducing the developing bias voltage in order to obtain a patch image with almost the maximum density, a patch image that stably changes in density can be obtained using the method proposed above.
Even when a control patch is formed by imagewise exposure based on image data of a reference input density, since changes in the sensitivity of the photoreceptor, associated with changes in the temperature and humidity of the ambient environment, and the deterioration in the characteristics of the developing agent change the quality of the control patch formed, a control patch more stable in image density must be formed to provide optimal control of the image forming conditions.
In the meantime, although, as described above, the image density control method using a control patch is useful technology, it has become clear that this method poses problems associated particularly with the image formation in which the image forming rate is increased or toner is reduced in particle size, such as polymerized toner, is used.
For example, during a continuous image forming process, although a control patch is formed in both the leading image area and the following image area, it is difficult to make setting of the image forming conditions for the control patch follow the progress of the image forming process.
More specifically, although the conventional formation of a control patch has been reducing the developing bias voltage value and charging potential value during the normal image forming process, there have occurred the problems that the slow response speeds of the charging device and developing bias power supply have resulted in the control patch becoming unstable in density or the end portion of the normal image area being becoming uneven in density.
Also, although the conventional control patch is formed as an image of uniform density (generally called “solid image”), since the solid image is not stable against changes in image forming conditions and suffers changes in the density of the control patch due to time-varying changes in the developing performance of the developing agent, the conventional control method has the problem that although it basically is useful density control technology, it reduces the control accuracy of the image forming conditions for normal image formation.
Accordingly, there arises the problem that when the density detection means detects a decrease in the density of the control patch below the required value due to an extended time of use of the developing agent, since an excessive amount of toner will be added, image density will increase too significantly and the toner will scatter.
Such a discrepancy between the density of the control patch and that of the image actually formed is particularly significant in the case of using polymerized toner.
In order to solve the above new problems, the present inventors have improved the control of image forming conditions, based on the formation of a control patch of stable density, by dither-patterning the above-mentioned control patch.
However, even under the configuration that uses a control patch of stable density, since changes in the sensitivity of the image forming body according to ambient temperature and humidity or changes in the characteristics of the developing agent also change, although slightly, the density of the control patch, it has been found that density control technology still admits of improvement.
SUMMARY OF THE INVENTION
The first object of the present invention is to supply an image forming method, and an image forming apparatus, by which the optimal density of a control patch not affected by changes in the sensitivity of the image forming body or changes in the response characteristics of writing light according to the particular type of exposure means can be obtained and thus the formation of images with stable image density can be maintained over long periods of time.
The second object of the present invention is to supply an image forming method, and an image forming apparatus, by which a control patch of image density can be stably formed to control image forming conditions either during the warming-up time following the power-on sequence of the image forming apparatus or after the required number of images have been printe
Ishikawa Tetsuya
Kabashima Hirotaka
Komatsu Toru
Yamaguchi Toru
Grainger Quana
Konica Corporation
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