Patent Application
20080025741
In the accompanying drawings:
Embodiments of an image forming apparatus and an image forming method of the invention will be described with reference to the accompanying drawings.
In the scanner unit 2, a document is read, and image data of, for example, three primary colors of R, G and B are created. In the image processing unit 3, color conversion processing from the three primary colors of R, G and B to four print colors of K (black), C (cyan), M (magenta) and Y (yellow) is performed on the respective image data, and further, various image processings are performed.
The image-processed K signal, C signal, M signal and Y signal are inputted to the exposure units 9a, 9b, 9c and 9d through the picture quality maintaining control unit 4.
The process cartridges 6a, 6b, 6c and 6d correspond to the four colors for color printing, are constructed of the four process cartridges for the K signal, the C signal, the M signal and the Y signal, and are structured to be attachable/detachable to/from the image forming apparatus 1. The basic structures of the respective process cartridges 6a, 6b, 6c and 6d are the same although the colors of toners included in developing units 8a, 8b, 8c and 8d are different. Then, in the following description concerning the process cartridge, the suffixes of a, b, c and d attached to reference numerals will be omitted and the description will be made.
The process cartridge 6 includes a photoconductor 7, the developing unit 8, and a charging device 10. The surface of the photoconductor 7 is charged to a specified potential by the charging device 10, and an electrostatic latent image is formed on the surface by a light, for example, a laser light irradiated from the exposure unit 9. The electrostatic latent image is developed with a toner supplied from the developing unit 8, and the developed image corresponding to each toner color is formed on the surface of the photoconductor 7.
The developed image formed on the photoconductor 7 is superimposed and transferred onto the intermediate transfer belt 11 in the order of Y, M, C and K, and at the time point when it passes the photoconductor 7a for K, a full color toner image in which the four colors are combined is formed on the intermediate transfer belt 11.
The density (or reflectivity) of the toner image is detected by a sensor unit 12 and is supplied for the processing of a picture quality maintaining control described later.
The toner image on the intermediate transfer belt 11 is transferred in the recording sheet transfer unit 14 to the recording sheet supplied from the paper feed unit 13. The toner image transferred to the recording paper is fixed to the recording sheet in the fixing unit 15, and it is discharged to the outside from the paper discharge unit 16.
A picture quality maintaining control method of the image forming apparatus 1 constructed as described above will be described.
The photosensitive characteristic of the photoconductor 7 and the charging characteristic of the toner are changed according to the environment in which the image forming apparatus 1 is installed or the number of use years. Thus, a change occurs in the quality of an image formed by the image forming apparatus 1. Especially, there is a tendency that the reproducibility of a minute point is deteriorated. More specifically, when the number of use years becomes large, the diameter of the minute point formed becomes small and it blurs, or in some cases, it can not be reproduced (the image of the minute point becomes too small to be formed). Also in the case where the use environment such as temperature and humidity is significantly changed, similarly, there is also a case where the reproducibility of the minute point is deteriorated.
The picture quality maintaining control method of the image forming apparatus 1 according to the embodiment provides means and method in which even in the case where the environment such as temperature and humidity is changed or the use is made for a long period of time, the reproducibility of the minute point is stably maintained.
In the picture quality maintaining control according to the first embodiment, the picture quality of a minute point is maintained by changing an exposure resolution as one of exposure parameters.
First, at step ST1, a presently set exposure resolution, for example, an exposure resolution of 1200 dpi is used, and an image patch of a minute point pattern is printed.
The minute point pattern is created by the image patch creation unit 5, and the minute point pattern is supplied to the exposure unit 9 through the picture quality maintaining control unit 4. The exposure unit 9 forms an electrostatic latent image of the minute point pattern on the photoconductor 7. The electrostatic latent image is developed by the developing unit 8, and a toner developed image is formed on the surface of the photoconductor 7. The toner developed image on the surface of the photoconductor 7 is intermediately transferred onto the intermediate transfer belt 11.
At step ST2, the reflectivity of the minute point pattern formed on the intermediate transfer belt 11 is detected by the sensor unit 12, and the reflectivity is converted into density.
The density of the image patch of the minute point pattern in the case where the reproduction of the respective minute points is excellent is previously held as a reference density, and at step ST3, the reference density and the density detected at step ST2 are compared with each other.
When the detected density is within the range of the reference density (Yes at step ST4), advance is made to step ST7, and a normal image forming processing is performed without changing the set exposure resolution (in this case, 1200 dpi).
On the other hand, when the detected density is outside the range of the reference density (No at step ST4), an exposure resolution at which the minute point can be reproduced is determined, and the determined exposure resolution is set for the exposure unit 9 (step ST5, 6). Thereafter, the normal image forming processing is performed using the set exposure resolution (step ST7).
When the characteristics of the photoconductor 7 or the toner are changed due to the long time use or the change of the environment, as shown in
Then, by the foregoing picture quality maintaining control, the exposure resolution is changed from 1200 dpi and is reduced to, for example, 600 dpi. As a result, as exemplified in
Although a specific changing method of the resolution is not particularly limited, for example, there is a method of increasing the power of a laser light. By increasing the power of the laser light, the thickness of the laser beam can be enlarged equivalently, and the exposure resolution can be reduced. Besides, a method may be such that the exposure resolution is reduced by widening the pulse modulation width of laser light.
Here, the exposure pattern for forming the minute point means the dot number of dots (each being a dot formed by one laser beam) forming the minute point or the arrangement of dots.
Processing from step ST11 to step ST14 of
When the detected density is within the range of the reference density (Yes at step ST14), advance is made to step ST17, and the normal image forming processing is performed without changing the set exposure pattern.
On the other hand, the detected density is outside the range of the reference density (No at step ST14), an exposure pattern in which the minute point can be reproduced is determined, and the determined exposure pattern is set for the exposure unit 9 (step ST15, ST16). Thereafter, the normal image forming processing is performed using the set exposure pattern (step ST17).
When the characteristics of the photoconductor 7 or the toner are changed due to the long time use or the change of the environment, as shown in
Then, the exposure pattern is changed to an exposure pattern as exemplified in
The second embodiment is the embodiment in which the exposure pattern is changed to ensure the reproducibility of the minute point, and since the exposure pattern has the dot number or the dot array as its element, the degree of freedom is high, and various exposure patterns can be formed.
For example, the usage time has further passed, and in the case where the minute point is further decreased from the state shown in
Although the processing exemplified in the flowchart of
In this case, a counter to measure the usage time of the image forming apparatus 1, or a counter to count the number of print sheets is provided, and when the counter exceeds a specified value, the exposure pattern is changed from the first exposure pattern to another exposure pattern which is, for example, composed of more dots.
Tests No. 1 to 3 indicate the comparison results of the reproducibility of the minute point in a room temperature and normal humidity environment.
Test No. 1 indicates the print result of the case where the picture quality maintaining control is not performed. The reproducibility with respect to the number of print sheets in the tests was evaluated such that, at a resolution of 1200 dpi, whether or not an isolated point of 2×2 dots could be reproduced was observed with the naked eye under magnification. Evaluation was made at three levels: “A” indicates that it was excellently reproduced, “B” indicates that it blurred but was roughly discriminated, and “C” indicates that it could not be reproduced.
As a result, in the case of test No. 1 in which the picture quality maintaining control was not performed, a blur occurred at 20,000 (20 k) sheets, and the minute point could not be reproduced at 30,000 (30 k) sheets or more.
On the other hand, test No. 2 is the test of the case where the picture quality maintaining control method (correction by the resolution change) of the first embodiment was applied, and test No. 3 is the test of the case where the picture quality maintaining control method (correction by the exposure pattern change) of the second embodiment was applied. In both test No. 2 and test No. 3, it was confirmed that when the picture quality maintaining control was performed, even if 50,000 (50 k) sheets were printed, the reproducibility of the minute point was excellently maintained.
Test No. 4 to No. 6 indicate comparison results of the reproducibility of the minute point in a low temperature and low humidity environment.
Test No. 4 is the result when the picture quality maintaining control was not performed in the low temperature and low humidity environment (temperature of 10° C. and humidity of 20%), and after being left in the low temperature and low humidity environment for 8 hours, a blur occurred.
On the other hand, in test No. 5 of the case where the picture quality maintaining control method (correction by the resolution change) of the first embodiment was applied, and test No. 6 of the case where the picture quality maintaining control method (correction by the exposure pattern change) of the second embodiment was applied, it was confirmed that the reproducibility of the minute point was excellent even after being left for 8 hours.
From the results of the effect confirmation tests, it was confirmed that the reproducibility of the minute point became excellent by applying the picture quality maintaining control of the first and the second embodiments.
As described above, according to the image forming apparatus of this embodiment and the image forming method, even in the case where the environment such as temperature and humidity is changed or the use is made for a long period of time, the reproducibility of the minute point can be stably maintained.
It should be understood that the present invention is by no means restricted to the above-described embodiments; rather, in carrying out the invention, various alterations and modifications may be made with regard to the components without departing from the spirit and scope of the present invention. Further, various arrangements may be made within the scope of the present invention by arranging the components in various ways, or by omitting one or more of the components. Moreover, arrangements obtained by suitably combining the components of the above-described embodiments with components of other embodiments according to the present invention are also encompassed by the present invention.
