JP2000267370A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain halftone images whose densities are the same in the case of both low pixel density and high pixel density by compensating the failure of a latent image with a developing condition even though the same beam diameter is used. SOLUTION: This image forming device provided with an optical writing device 12 capable of performing printing by switching plural pixel densities and capable of using the same beam diameter of a semiconductor laser 12a as a latent image forming means regardless of the pixel density is provided with a function for adjusting the developing condition so that the densities of the halftone images become the nearly the same densities regardless of the pixel densities. As a concrete example, a function for selecting the direct current component of a developing bias to be charged on a developing roller 13a so that the densities of the halftone images become the nearly the same densities regardless of the pixel densities (600 dpi and 1200 dpi for example) is provided and the selection of the direct current component of the developing bias is set low in the case of the selection of the high pixel density and high in the case of the selection of the low pixel density.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile, and the like.

[0002]

2. Description of the Related Art Conventionally, an image carrier such as a photoreceptor is uniformly charged by a charging device, and then an optical latent image is formed on the image carrier by a latent image forming means using a semiconductor laser or the like to form an electrostatic latent image. Then, after the electrostatic latent image is developed with a developer of a developing device and visualized, the visualized image is transferred to a recording material such as recording paper by a transfer device, and a visible image transferred to the recording material is formed. 2. Description of the Related Art An electrophotographic image forming apparatus that forms an image by fixing with a fixing device is known, and is widely used as a copying machine, a printer, a facsimile, and the like.

Recently, the cost and size of the image forming apparatus using the electrophotographic method have been reduced, and the image forming unit having a smaller and simplified image forming unit has been put on the market. On the other hand, the print image quality has been improved, and a high pixel density such as 1200 dpi (dots per inch) has been adopted as the writing density. However, in the case of a printer, for example, printing at a high pixel density of 1200 dpi or the like has a drawback that the printing speed is reduced due to an increase in the amount of print data. Therefore, many printers adopt a method of performing printing by switching a plurality of pixel densities (for example, 1200 dpi, 600 dpi, etc.). Originally, when switching the pixel density, it is necessary to switch the beam diameter of the laser of the optical writing device as the latent image forming means. However, since the optical writing device of the method of switching the beam diameter is expensive, the cost is high. Some systems adopt a method of changing the pixel density without changing the beam diameter. When the pixel density is 1200 dpi, one theoretical dot is 21 μm. However, if an optical device capable of writing such minute dots is used, the optical writing device becomes very expensive. Therefore, it is desirable to write with relatively large dots even in the case of high density in terms of cost.

[0004]

In the image forming apparatus according to the present invention, for example, an optical system having a beam diameter in the main scanning direction of 72 μm and a beam diameter in the sub-scanning direction of 90 μm is used.
It is assumed that printing of dpi and 1200 dpi is performed, but if the beam system is not switched when the pixel density is switched in the case of a high pixel density, the area gradation of a binary image such as a halftone image is expressed. In such a case, there is a disadvantage that the density of the halftone image (halftone image) differs for each print pixel density. However, originally, even when the pixel density is changed, it is desirable that the image printed out from the same data has the same density. The reason why the halftone image becomes dark in this way is that it is ideally desirable to irradiate the laser beam only on the part to be printed, but since the laser beam has a Gaussian distribution, This is because there is a spread beyond the irradiated portion, and it is difficult to expose only the portion to be printed.

[0005] As assumed in the image forming apparatus according to the present invention, the beam diameter in the main scanning direction is 72 µm, which corresponds to a beam diameter three times or more the theoretical value of one dot, and the beam diameter in the sub-scanning direction is 90 µm. When a halftone image is printed at every other dot by the optical system described above, light will also hit a portion where printing is not to be performed due to the spread of the foot of the beam. As a result, the dots that are not to be printed are also developed,
A situation of black filling occurs. Further, when printing is performed with the beam diameter as described above, at 600 dpi, the overlap of the irradiation light at the foot of the beam is small, but at 1200 dpi, the overlap of the irradiation light at the foot of the beam is large, so that the overlapping portion increases. Development is performed on the overlapping portion. Therefore, when printing is performed with a beam diameter much larger than such an ideal value, especially in the case of 1200 dpi, a situation occurs in which the density is increased due to the large overlap of the beams.

Therefore, in the present invention, the development of the laser beam irradiation spread portion is avoided by changing the development conditions, whereby even if the same beam diameter is used, the problem of the latent image is not improved. Is corrected under development conditions to obtain a low pixel density (600 dpi) and a high pixel density (120 dpi).
An object of the present invention is to realize an image forming apparatus capable of obtaining a halftone image having the same density at both pixel densities of 0 dpi).

[0007]

According to one aspect of the present invention, there is provided an image forming apparatus capable of performing printing by switching a plurality of pixel densities. On the other hand, in an image forming apparatus provided with an optical writing device using the same beam diameter regardless of the pixel density, the density of the halftone image is substantially independent of the pixel density. This is a configuration having a function of adjusting development conditions so as to have the same density.

According to a second aspect of the present invention, in the image forming apparatus of the first aspect, the developing bias applied to the developer carrier is adjusted so that the density of the halftone image becomes substantially the same regardless of the pixel density. This configuration has a function of selecting a DC component. The invention according to claim 3 is based on claim 2
In the above-described image forming apparatus, the DC component of the developing bias is selected to be low when a high pixel density is selected and set high when a low pixel density is selected.

Further, the invention according to claim 4 is the invention according to claim 1.
In the image forming apparatus described above, a configuration is provided that has a function of selecting a frequency of an AC component of a developing bias applied to the developer carrier so that the density of the halftone image is substantially the same regardless of the pixel density. Things. According to a fifth aspect of the present invention, in the image forming apparatus of the fourth aspect, the frequency of the AC component of the developing bias is set to be high when a high pixel density is selected and set low when a low pixel density is selected. It is.

Further, the invention according to claim 6 is the invention according to claim 1.
The image forming apparatus according to the above, further comprising a function of selecting a peak value of an AC component of a developing bias applied to the developer carrier such that the density of the halftone image is substantially the same regardless of the pixel density. It was done. According to a seventh aspect of the present invention, in the image forming apparatus of the sixth aspect, the peak value of the AC component of the developing bias is selected to be low when a high pixel density is selected and set high when a low pixel density is selected. Things.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The construction and operation of an image forming apparatus according to the present invention will be described below in detail with reference to the drawings. FIG.
1 is a schematic configuration diagram of an image forming apparatus showing one embodiment of the present invention. The image forming apparatus shown in FIG. 1 includes a drum-shaped photoconductor 10 as an image carrier, a charging device 11 using a charging roller for uniformly charging the photoconductor 10, and a semiconductor modulated according to an image signal. A laser beam 12e from a laser 12a is irradiated onto the photoreceptor 10 via optical scanning means 12b such as a rotating polygon mirror, an imaging optical system 12c, a mirror 12d, and the like, and the photoreceptor is scanned in the main scanning direction (the axis of the Direction) to form an electrostatic latent image, a developing device 13 for supplying toner to the electrostatic latent image on the photoreceptor 10 to make it visible, and a paper supply unit (not shown). A transfer device 14 using a transfer roller for transferring a visible image (toner image) on a photoconductor onto a recording sheet S fed via a paper device 17 and a conveyance guide 18, and a toner image transferred onto the recording sheet S Fixing device 16 for fixing the image The toner remaining on the body 10 Sogiotosu has a configuration in which a cleaning device 15 constituted by a blade or the like. In the image forming apparatus of this embodiment, components other than the optical writing device 12, the transfer device 14, and the fixing device 16 are integrated cartridges (not shown).
It is put in.

First, the charging device 11 will be described in detail, but the following description is based on the conditions when the print pixel density is set to 600 dpi. The charging device 11 is a contact charging type charging device using an elastic rubber roller as a charging roller, and performs charging along with the photoconductor 10.
As a charging condition at the time of image formation, a sine-wave voltage of -600 V DC component, 1.8 to 2.6 KV peak-to-peak, and 1 KHz frequency, and a frequency of 1 kHz are applied to the charging roller by a power supply (not shown) to expose the photosensitive member. The body 10 is being charged.

Next, the developing device 13 will be described in detail. The developing device 13 includes a developing roller 1 as a developer carrier.
3a, a developing blade 13b as a thin layer regulating member,
A stirring member 13c for stirring toner near the developing roller;
The agitator 13d supplies toner to the developing roller. The developing roller 13a is composed of a cylindrical developing sleeve and a magnet arranged inside the developing roller, and carries and conveys a one-component magnetic toner (or a two-component developer composed of a non-magnetic toner and a magnetic carrier) by magnetic force. I have. The distance between the developing roller 13a and the photosensitive member 10 is regulated by a gap roller (not shown) (hereinafter, referred to as a developing roller 13a).
The developing roller 13a is connected to the photoconductor 10
Are rotated at a linear velocity ratio 1.2 times that of the above. Furthermore, the developing roller 13 is set by setting the core side of the photoreceptor 10 to ground potential.
a is connected to a developing bias application power supply 19;
At the time of development, a predetermined developing bias in which a DC voltage and an AC voltage are superimposed is applied to the developing roller 13a. The developing bias application power supply 19 is configured to be able to switch the setting of the frequency and peak value of the DC component and the AC component of the developing bias according to the developing conditions.

At the time of image formation, the charging device 11 causes -6
The photoreceptor 10 charged to 00V is exposed to a laser beam 12e by an optical writing device 12 to attenuate the image portion to -100V to form an electrostatic latent image. The toner is developed with the toner carried by the roller 13a.
Is applied as a developing bias, DC component _VDC is _VDC =
-400V, AC component V _AC peak value (peak-to-
V _AC = 1.8 KV at peak), frequency f = 1.8 K
Hz is applied to develop a rectangular wave. The print pixel density is configured to be switchable between 600 dpi and 1200 dpi.

Here, in the configuration of the first aspect, a problem that occurs when printing is performed without changing the beam diameter of the laser beam 12e irradiated on the photoreceptor 10 by the optical writing device 12 when the print pixel density is changed. The correction of the density difference of the halftone image due to the difference between the electrostatic latent images at 600 dpi and 1200 dpi is performed by selecting the developing conditions so that the density becomes the same, and the density of the halftone image is compared with that at 600 dpi and 1200 dpi.
The same density is set at the time of dpi. In this method, the difference between the electrostatic latent images due to the difference in pixel density is adjusted by changing the level of development for the latent image.

Next, the configuration of claim 2 relates to specific means of the system described in claim 1. That is, in the configuration of claim 2, the density difference of the halftone image due to the difference of the electrostatic latent image is corrected, and the developing roller 13a is controlled so that the density of the halftone image becomes the same regardless of the pixel density. The DC component _VDC of the developing bias to be applied is selected and corrected, and the density of the halftone image is set at 600 dpi and 1200 dpi.
The same density is set at the time of i. According to this method, when the density of the halftone image is 600 dpi and 12
It is set to be almost the same at 00 dpi.

Claim 3 relates to specific means of the structure of claim 2. As shown in FIG.
When the DC component _VDC of the developing bias at the time of 1200 dpi is the same as that at −400 V, a density difference occurs in the density of the halftone image. Therefore, as one embodiment of the present invention, as shown in FIG. 2, at 600 dpi, the DC component _VDC of the developing bias is set to _VDC = −400 V, and 1200 dpi
At the time of i, V _DC = −350 V is set.
Thus, at 600 dpi, the developing potential is -30.
At 0 V and 1200 dpi, the developing potential is -250.
V, and the halftone with a halftone dot area ratio of 25% at this time has substantially the same density.

Next, the structure of claim 4 relates to another specific means of the method described in claim 1. That is, the configuration of claim 4 is for correcting the density difference of the halftone image due to the difference of the electrostatic latent image, and means for making the density of the halftone image the same density regardless of the pixel density. The difference between the latent images at 600 dpi and 1200 dpi is corrected by utilizing the fact that the developing characteristic can be changed by changing the set value of the frequency f of the AC component of the developing bias applied to the developing roller 13a. The development conditions are determined. More specifically, as shown in FIG. 3, the characteristic that the developing characteristic of the image density ID with respect to the dot area ratio becomes looser when the frequency f of the AC component of the developing bias is increased, and that the rise becomes faster when the frequency f is lowered. Therefore, with respect to the developing conditions when forming an image of 600 dpi,
By increasing the frequency f of the AC component of the developing bias when forming an image at 1200 dpi, the density of the halftone image is set to be substantially the same at 600 dpi and 1200 dpi. By this method, the density of the halftone image is set so as to be substantially the same at the time of 600 dpi and at the time of 1200 dpi.

Claim 5 relates to specific means of the constitution of claim 4. Specifically, as shown in FIG.
At 00 dpi, the frequency f of the AC component of the developing bias is f
= 1.8 KHz, but 1200 dp
At the time of i, f = 2.4 KHz is set.
According to this method, the density of the halftone image is 600 dpi.
And 1200 dpi.

Next, the structure of claim 6 relates to still another specific means of the method described in claim 1. That is, the configuration of claim 6 is for correcting the density difference of the halftone image due to the difference of the electrostatic latent image, and means for making the density of the halftone image the same density regardless of the pixel density. as, by utilizing the ability to change the developing performance by changing the setting value of the peak value of the AC component V _AC of the developing bias applied to the developing roller 13a (peak-to-peak), at 600dpi at a 1200dpi The development conditions are determined so as to correct the difference between the latent images. More specifically, as shown in FIG. 4, when the peak value of the _AC component VAC of the developing bias is lowered, the developing characteristic of the image density ID with respect to the dot area ratio becomes gradual, and when the peak value is increased, the rising characteristic becomes faster. However, by utilizing this, the developing conditions at the time of forming an image of 600 dpi
By lowering the peak value of the _AC component VAC of the developing bias when forming an image at 1200 dpi, the density of the halftone image is set to be substantially the same at 600 dpi and 1200 dpi. With this method, when the density of the halftone image is 600 dpi and 1200 dpi
It is set to be almost the same at i time.

Claim 7 relates to specific means of the constitution of claim 6. Specifically, as shown in FIG.
Whereas 00dpi when the alternating current component V _AC peak value of the developing bias (peak-to-peak) is used V _AC = 1.8 kV, 1200 dpi at the V _AC = 1.5 KV
Is set to. By this method, it is possible to set the density of the halftone image to be substantially the same at the time of 600 dpi and at the time of 1200 dpi.

[0022]

As described above, the image forming apparatus according to the first aspect has the function of adjusting the developing conditions so that the density of the halftone image is substantially the same regardless of the pixel density. With this configuration, the halftone image density can be kept constant irrespective of the pixel density selection value.
A good high-quality image can always be obtained.

In the image forming apparatus according to the second aspect,
The configuration is provided with a function to select the DC component of the developing bias applied to the developer carrier so that the density of the halftone image is substantially the same regardless of the pixel density. Since the image density of the halftone (halftone) can be made constant, a good high-quality image can always be obtained.

In the image forming apparatus according to a third aspect,
In addition to the configuration of the second aspect, the selection of the DC component of the developing bias is set low when the high pixel density is selected and high when the low pixel density is selected. Can make the image density constant, so that a good high-quality image can always be obtained.

In the image forming apparatus according to a fourth aspect,
The configuration is provided with a function to select the frequency of the AC component of the developing bias applied to the developer carrier so that the density of the halftone image becomes substantially the same regardless of the pixel density. Since the image density of the halftone (halftone) can be made constant regardless of the image quality, a good high-quality image can always be obtained.

In the image forming apparatus according to a fifth aspect,
In addition to the configuration of the fourth aspect, the selection of the frequency of the AC component of the developing bias is set high when the high pixel density is selected and low when the low pixel density is selected. Tone) image density can be made constant, so that a good high-quality image can always be obtained.

In the image forming apparatus according to claim 6,
Since the configuration is provided with a function to select the peak value of the AC component of the developing bias applied to the developer carrier so that the density of the halftone image is substantially the same regardless of the pixel density, the pixel density is selected. Since the halftone (halftone) image density can be made constant regardless of the value, a good high-quality image can always be obtained.

In the image forming apparatus according to claim 7,
In addition to the configuration according to claim 6, the peak value of the AC component of the developing bias is selected to be low when the high pixel density is selected and set high when the low pixel density is selected. Since the (halftone) image density can be made constant, a good high-quality image can always be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an image forming apparatus showing one embodiment of the present invention.

FIG. 2 is an explanatory diagram of an embodiment of the present invention, and shows a relationship between a dot area ratio and an image density when a setting of a DC component of a developing bias is changed with respect to a pixel density; FIG.

FIGS. 3A and 3B are explanatory diagrams of the embodiments of FIGS. 4 and 5, wherein the halftone dot area ratio and the image density when the setting of the frequency of the AC component of the developing bias is changed with respect to the pixel density; FIG.

FIG. 4 is an explanatory diagram of the embodiment of claim 6, wherein the halftone dot area ratio and the image when the setting of the peak value of the AC component of the developing bias is changed with respect to the pixel density and when the setting is not changed; It is a figure which shows the relationship of density.

[Explanation of symbols]

 REFERENCE SIGNS LIST 10 photoconductor (image carrier) 11 charging device 12 optical writing device 12a semiconductor laser 12b optical scanning means 12c imaging optics 12d mirror 12e laser beam 13 developing device 13a developing roller (developer carrier) 13b developing blade 13c stirring member 13d Agitator 14 Transfer device 15 Cleaning device 16 Fixing device 17 Paper feed device 18 Transport guide 19 Power supply for applying developing bias S Recording paper

Claims (7)

[Claims] An image forming apparatus capable of performing printing by switching a plurality of pixel densities, wherein a beam diameter of a semiconductor laser as a latent image forming means is independent of the pixel density for a plurality of printable pixel densities. An image forming apparatus provided with an optical writing device using the same beam diameter, and having a function of adjusting development conditions so that the density of a halftone image is substantially the same regardless of the pixel density. Image forming apparatus. 2. The image forming apparatus according to claim 1, wherein a DC component of a developing bias applied to the developer carrier is selected such that the density of the halftone image is substantially the same regardless of the pixel density. An image forming apparatus comprising: 3. The image forming apparatus according to claim 2, wherein the DC component of the developing bias is set low when a high pixel density is selected and set high when a low pixel density is selected. 4. The image forming apparatus according to claim 1, wherein the frequency of the AC component of the developing bias applied to the developer carrier is selected such that the density of the halftone image becomes substantially the same regardless of the pixel density. An image forming apparatus comprising: 5. The image forming apparatus according to claim 4, wherein the frequency of the AC component of the developing bias is selected to be high when a high pixel density is selected and low when a low pixel density is selected. 6. The image forming apparatus according to claim 1, wherein the peak value of the AC component of the developing bias applied to the developer carrier is adjusted so that the density of the halftone image becomes substantially the same regardless of the pixel density. An image forming apparatus having a selection function. 7. The image forming apparatus according to claim 6, wherein the peak value of the AC component of the developing bias is selected to be low when a high pixel density is selected and set high when a low pixel density is selected. .