JPS60249166A - Method for adjusting image density of electrophotograph - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for optimizing the density of an image formed by electrophotography.

[Conventional technology]

Devices that apply electrophotography include electrophotographic copying machines, facsimile receivers, laser beam printers, and the like. As an example, FIG. 1 shows a process layout diagram of the main parts of a laser beam printer.

In the figure, a photosensitive drum 1 consisting of a photosensitive layer 2 and a substrate 3, which is an image carrier, forms an image while rotating in the direction of the arrow. - The photosensitive drum l uniformly charged by the charger 4 is exposed to a laser beam 5 modulated in accordance with an image signal, and an electrostatic latent image is formed on the drum l. Subsequently, the drum 1 undergoes a developing process by a developing device 6, and the latent image is visualized.

Thereafter, the developed image is transferred by a transfer charger 12 onto a transfer paper (omitted in this figure) that has been guided by a transfer guide 11. The transferred toner image is fixed on the transfer paper by a fixing device (not shown).

It is ejected from the aircraft and a hard copy is obtained. On the other hand, after the untransferred developed toner remaining on the drum l is cleaned by the cleaning device 13 and the residual charge is short-circuited and eliminated by irradiation with the pre-exposure light slO, the drum l is transferred to the first step, which is the primary charging process. sent to and used repeatedly.

An example of the formed image is shown in FIG. 2, and a developed image 19 is transferred onto 6 transfer paper xd. Laser beam 5 which is image exposure light
In the step of irradiating the image, an image scanning method is adopted in which a portion of the photosensitive drum corresponding to the developed image 19 is irradiated with a laser beam, but the background portion is not irradiated with the laser beam. This method is superior to the background scanning method in which the background is irradiated with the laser and only the portion corresponding to the visible image 19 is not irradiated with the laser, as this method does not leave scan marks on the background image and is superior in the reproducibility of the visible image. be.

FIG. 3 shows the behavior of the surface potential of the drum 1 during latent image formation. In this example, a phthalocyanine-based organic semiconductor is used as the photosensitive layer 2 of the photosensitive drum 1, and the −order charging is performed with negative polarity. - The surface potential obtained by the charger 4 is the difference between dark decay and bright decay caused by laser irradiation IV d -
It is converted to a latent image potential of about 550 V at V 11, that is, a contrast.

In the developing process, a magnetic component developer is used as the developing toner T. The toner T in the developing device 6 is charged by mutual friction and contact friction with the developing sleeve 9, doctor blade 7, etc. and sleeve 9 by doctor blade 7
The sheet is placed on the sheet with a uniform thickness and is conveyed as the magnetic roller 8 rotates. An AC power source 15 and a DC power source 16 are connected in series to the sleeve 9, which is a developing means, and a biased AC bias voltage is applied thereto. When the sleeve 9 rotates and comes closest to the drum L, the toner T jumps to the latent image area to be developed due to the electric attraction force due to the electric field between the potential of the sleeve 9 and the electrostatic latent image potential. . A portion corresponding to the bright area potential 7 of the latent image shown in FIG. 3 is developed, and so-called reversal development is performed. The toner is charged with a charging electrode (negative in this case) that develops a relatively high potential in the positive direction. FIG. 4 schematically shows how toner does not adhere to the negative potential portion of the dark potential Vd, which is the background, and negatively charged toner adheres to the laser irradiation area Vl near the ground potential.

Conventionally, in such a developing system, in order to adjust the density of a developed image, the voltage of the power supply 16 was changed between the bright area potential V and the dark area potential Vd, and the developing bias potential V was changed.

As shown in FIG. 4, when it is desired to increase the image density, the developing bias potential is changed from V to vI to make it easier for toner to fly from the developing sleeve 9 to the V1 portion on the drum. Conversely, if you want to lower the image density, change the developing bias potential from V to V.
Change to 2 to make toner less likely to fly.

However, this method has the following disadvantages because the difference between the dark potential Vd and the bright potential 7 of the electrostatic latent image is constant. When the developing bias potential is changed from V to 1, the difference IVd-V,l from the dark area potential Vd becomes smaller. As a result, a phenomenon in which toner adheres to the white background (dark area) of the image, so-called fog, occurs. When the developing bias potential is changed from V to V2, the potential difference I
V d −V , l increases. Therefore, some toner (in this example, + in this example, hereinafter referred to as "reverse polarity toner") whose charging electrode is different from the normal polarity toner (in this example, 11 or less, referred to as "standard polarity toner") is contained in the toner. It adheres to the white background area (dark area potential Vd), causing fogging, thick lines, and protrusion.

In order to avoid the above problem, it is also possible to adjust the image density by changing the bright area potential. With this method, it is easy to increase the bright area potential 1v and lower the density. However, due to the sensitivity limit of the photoreceptor and the output limit of an image exposure light source such as a laser, it is not necessarily easy to increase the difference IV l -Vl between the bright area and the developing bias potential in order to increase the density. In particular, increasing the amount of light for image exposure in order to lower Vl increases the memory of the photoreceptor (charges remain trapped in the photoreceptor) and accelerates the deterioration of the photoreceptor. At the same time, it has the disadvantage of shortening the life of the light source.

[Purpose of the invention]

It is an object of the present invention to provide an image density adjustment method for electrophotography which eliminates the drawbacks of such conventional methods and provides a particularly good image quality.

[Structure of the invention]

A first invention to achieve this object is to form an electrostatic latent image by exposing a charged image carrier, and then develop the electrostatic latent image in reverse using a developing means to which a bias voltage is applied to form a visible image. This image density adjustment method is characterized in that in an electrophotograph to be formed, the density of the electrophotographic image is adjusted by synchronously changing the charge amount of the charge and the voltage amount of the bias voltage.

The second invention is an electrophotographic method in which a charged image bearing member is exposed to light to form an electrostatic latent image, and the electrostatic latent image is reversely developed by a developing means to which a bias voltage is applied to form a visible image. The image density adjustment method is characterized in that the density of the developed image is adjusted by synchronously changing the charge amount of the charge, the voltage amount of the bias voltage, and the exposure amount of the image exposure.

〔Example〕

FIG. 5 shows the main part of the apparatus for carrying out the first invention, in which the grid 20 is attached to the blower 17. - When the blowing charge is made negative as in the conventional example, the grid bias is made a negative voltage. The bias voltage for the developing sleeve 9 and the bias voltage for the grid 20 can be adjusted by interlocking operations.

As shown in FIG. 6, a shaft 23 is shared, and the resistance values of the interlocking variable resistors 21 and 22 can be changed by rotating the shaft 23. The variable resistor 21 is used to adjust the output voltage of the developing bias power supply 25. A voltage output terminal DB of the power source 25 is connected to the developing sleeve 9. The variable resistor 22 is used to adjust the output voltage of the grid bias power supply 26. A voltage output terminal GB of power supply 26 is connected to grid 20 . The rest of the configuration is the same as the configuration shown in FIG. 1, so repeated explanation will be omitted.

In an apparatus with this configuration, when increasing the image density, the shaft 23 is rotated clockwise. Then, the negative output voltage from the developing bias power supply 25 and the negative output voltage from the υ grid bias power supply 26 increase. If the negative voltage of the grid bias increases, the −th order charging voltage becomes more negative. Therefore, the dark potential IV dl becomes high. Since the developing bias potential IVI has also become high, in FIG. 7, the initial state a changes to the state b, and the developing contrast voltage v
- Since the V text becomes larger, the image density becomes higher. vice versa,
To lower the image density, rotate the shaft 23 counterclockwise to reduce the developing bias voltage i! The negative output voltage of A25 and the grid bias voltage w, reduce the negative output voltage of 26. Then, the developing bias potential IVI and the dark area potential lVd
1 can be lowered. The initial state a changes to state C, and the development contrast voltage V-VX becomes smaller, resulting in a decrease in image density.

As a means for changing the primary charging voltage to change the dark area potential Vd and simultaneously changing the developing bias potential V, other suitable means other than the variable resistor may be used. - In changing the secondary charging voltage, other methods may be used instead of changing the grid bias voltage. For example, without providing a grid, - changing the amount of corona discharge of the blower.

According to experiments, in order to eliminate fog caused by standard polarity toner, the difference between the dark area potential and the development level (IV d -
It is necessary that Vl>200V. On the other hand, in order to eliminate fog due to toner of opposite polarity, the potential difference IV
It is necessary that d-Vl<300V.

Kononame, light potential V l =-150V dark potential V
d = -1150V to -750V (variable), development bias potential V = -400V to -500V (variable),
While maintaining the potential difference IV d - Vl = 250V,
The developing bias potential V and the dark area potential Vd were varied. As a result, there was no fog due to the standard polarity toner and no fog due to the reverse polarity toner, and the image density could be adjusted within a practically sufficient range. The state of adjustment is shown by the solid line in FIG. The slope of the change in the dark area potential Vd and the developing bias potential (2) is kept constant.

, Incidentally, the difference between the dark area potential and the developing bias potential IV d −
vl does not necessarily need to be maintained constant, and can be varied to adjust the density within a range where neither the fog caused by the standard polarity toner nor the fog caused by the reverse polarity toner occurs. For example, the 9th
As shown by the dotted line in the figure, the dark area potential Vd and the development bias potential V may have different slopes of change. Further, the changes in v and vd do not necessarily have to be linear.

Next, an embodiment of the second invention will be described.

When the dark area potential Vd is changed in the first invention, the bright area potential Vl is changed as shown in FIG.
will also change. The bright area potential Vl may vary greatly depending on the environment in which the photosensitive drum is used, the history of use, or the material of the photosensitive layer.

Therefore, in the second invention, the value of the bright area potential Vl is kept constant even if the dark area potential Vd and the developing bias potential (2) are changed.

To increase the image density, increase the negative voltage to a negative value to raise the dark area potential IV dl, increase the developing bias potential IVI, and at the same time increase the output of the laser driver (not shown). the exposure amount by the laser beam 5 is increased. As the amount of exposure increases, the attenuation of bright areas increases. On the other hand, when thinning the image, the developing bias IVI is lowered in conjunction with lowering the dark area potential IV dl, and at the same time, the image exposure amount is reduced, thereby reducing the attenuation of the bright area. Therefore, the bright area potential can be maintained at a constant potential. FIG. 8 shows changes in the dark area potential Vd, the bright area potential 7, and the developing bias potential 2 in such a case. a is the normal state.

When increasing the density, the dark area potential lVd1 and the developing bias potential IVI are increased while the bright area potential v remains unchanged, as shown in b. Conversely, when lowering the density, the bright area potential Vl remains unchanged as shown in C, and the dark area potential Vd - development bias potential ■
becomes lower.

If the bright area potential 7 is kept constant, the effect of changing the dark area potential Vd and the developing bias potential V at the same time as described in detail in the first invention becomes even greater.

In particular, since the difference between the bright area potential and the development bias potential can be changed, the development density can be changed reliably. It becomes possible to adjust the density more stably and over a wider range than the method of changing the two levels of the dark area potential Vd and the developing bias potential V at the same time in the first invention.

〔effect〕

As explained above, according to the image density adjustment method of the present invention, there is no fog in the formed image, and it becomes possible to adjust the image density to an appropriate level.

Note that the present invention is applicable not only to the illustrated laser beam printer but also to all other electrophotographic application devices.

[Brief explanation of drawings]

Figure 1 is a schematic diagram of a device that applies electrophotography, Figure 2 is a diagram of an image formed by the device, Figure 3 e Figure 4 is a diagram explaining the surface potential of a photoreceptor, Figures 5 and 6 The figure is a schematic diagram of a main part of an apparatus capable of carrying out the present invention, and FIGS. 7 to 9 are diagrams illustrating changes in dark area potential, bright area potential, and development bias potential of a photoreceptor. 1 is an image carrier, 4 and 17 are primary chargers, 5 is an image exposure light,
9 is a developing means, 20 is a charger grid, 21 and 22 are variable resistors, 25 is a developing bias power source, 26 is a grid bias power source, Vd is a dark area potential, Vl is a bright area potential, and ■ is a developing bias potential. Patent applicant Canon Co., Ltd. Figure 9; L-→Immersion 1 &

Claims (2)

[Claims] (1) In electrophotography, a charged image bearing member is exposed to light to form an electrostatic latent image, and the electrostatic latent image is reversely developed using a developing means to which a bias voltage is applied to form a visible image. Synchronously changing the amount of charge and the amount of voltage of the bias voltage,
An image density adjustment method characterized by adjusting the density of the visualized image. (2) In electrophotography, a charged image carrier is exposed to light to form an electrostatic latent image, and the electrostatic latent image is reversely developed using a developing means to which a bias voltage is applied to form a visible image. An image density adjustment method comprising adjusting the density of the microscopic image by synchronously changing the amount of charge, the amount of voltage of the bias voltage, and the amount of exposure of the imagewise exposure.