JPH0352631B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image density control method, and particularly to a method for controlling the density of a developed image in an electrophotographic device or an electrostatic recording device.

As is well known, in electrophotographic devices and electrostatic recording devices, an electrostatic latent image formed on an image carrier such as a photoreceptor or dielectric is developed with colored fine particles called toner. , the image is played. It is known that the density of the reproduced toner image depends on many factors, and typically depends on the toner density in the developer. The developer normally used in such image forming apparatuses is called a two-component developing agent and is composed of two components: toner and chiaria. The carrier triboelectrically charges the toner to a predetermined polarity. It has the role of attracting this and transporting it to the development position. At the development position, only toner is attracted to the electrostatic latent image, and the electrostatic latent image is developed. Therefore, by repeated development, the toner concentration in the developer decreases, and if development is performed using the developer with such a decreased concentration, the developed toner image will naturally have a low concentration. Therefore, a method has been developed to constantly appropriately control the toner concentration in the developer by measuring the density of the developed image, that is, the amount of toner adhering to the image area. An example of such a method is
No. 43-16199, JP-A-48-29448, JP-A-49-
No. 36341, JP-A-50-22642, JP-A-50-125745
It is disclosed in the publication number etc. In these conventional methods, a toner adhesion area for measurement is provided on the surface of the photoreceptor outside the normal copying area, and a predetermined charge is applied to this area to attract toner, which requires a large photoreceptor. There are drawbacks. In addition, an image on a standard density plate placed outside the normal copying area of the photoreceptor, for example on the back of the front edge of the document platen, is exposed to light in the same way as a normal original to form a latent image, and this is developed to form a latent image. Methods of measuring quantities are known. In this method, if the toner concentration in the developer is controlled based on the measurement results, undesirable results may occur. For example, when the amount of toner adhesion is low, even if new toner is replenished into the developer,
There is no problem if the cause is truly a decrease in the toner concentration in the developer, but if the cause is insufficient charging due to charging or deterioration of the photoreceptor, or overexposure due to voltage fluctuation, the toner concentration may decrease. If it becomes too high, problems such as background smearing and toner scattering may occur.

In this way, the density of the developed image, that is, the amount of toner adhesion, typically depends on the toner concentration in the developer, but it also depends on other factors such as the amount of charge, amount of exposure, photoreceptor characteristics, developer characteristics, and development. Various factors such as bias amount, toner charge amount, development gap, and development time are involved. Therefore, if toner is replenished immediately even though the amount of toner adhesion has decreased, the above-mentioned problems may occur. In addition, recently, even when using a one-component developer consisting only of toner, different types of toner may be mixed or additives may be mixed. There is a need to control the density of developed images, including controlling the composition of the components.

An object of the present invention is to provide a method for maintaining image density appropriately by detecting changes in image density mainly due to changes in development characteristics and controlling development characteristics based on the detection results. Development characteristics include all factors related to development, such as the toner concentration in the developer, the amount of charge on the toner, the amount of development bias, the development time, the development gap between the development sleeve and the photoreceptor, and the rotation speed of the development sleeve. This includes: The method according to the present invention involves charging the image carrier surface separately into a potential part of approximately zero V and a potential part having a polarity opposite to the charging potential used in the normal image forming process, and the development electrode is charged with this opposite charge. This method includes applying voltages of the same polarity to cause toner to adhere to a zero-potential portion of the image carrier surface, and measuring the amount of adhered toner. According to this invention, toner is attached to the surface of the image carrier using the most stable saturated residual potential near zero V, so image density changes due to changes in charging characteristics, exposure characteristics, etc. This makes it possible to quickly and accurately control image density by controlling only changes in development characteristics.

Hereinafter, the present invention will be explained with reference to the accompanying drawings. FIG. 1 shows an example of an electrophotographic apparatus to which the image density control method according to the present invention is applied. A charger 2, an exposure optical system 3, an erase lamp 4, a developing device 5, a light emitting element 6 for detecting the amount of toner adhering, and a light receiving element are arranged around the photoreceptor drum 1 along the counterclockwise rotation direction thereof. 7, transfer charger 8, fixing device 9, static elimination charger 10, static elimination lamp 11, cleaning device 1
2. A reverse charging charger 30 and the like are arranged.
The surface of the photoreceptor drum 1 is first charged by the charger 2.
When the photoreceptor is uniformly charged to a predetermined polarity, a light image of the original to be copied is irradiated onto the photoreceptor through the exposure optical system 3, and the charges on the photoreceptor are selectively erased. An electrostatic latent image corresponding to the image is formed. This electrostatic latent image is developed by being supplied with toner from the developing device 5. The developing device 5 includes a container 14 containing a two-component developer 13 made of toner and a magnetic carrier.
The toner is charged to a polarity opposite to that of the electrostatic latent image due to friction with the carrier. After both are transferred from the pumping roller 16 to the developing roller 17, only the toner is attracted to the electrostatic latent image at a position close to the photoreceptor, and development is performed. Developing roller 1 after development
The developer on the developing roller 17 is scraped off by the scraper 18 . In a normal copying cycle, the toner image obtained by development is superimposed on the transfer paper 23 fed from the paper cassette 19 by the paper feed roller 20 and transport rollers 21 and 22, and transferred to the transfer charger 8. The toner is transferred to the transfer paper 23 by being charged with a polarity opposite to that of the toner. After the transfer, the transfer paper 23 separated from the photoreceptor surface enters the fixing device 9 where the transferred toner image is fixed, and is discharged onto the tray 24. On the other hand, the photoreceptor after the transfer is subjected to removal of residual charges by a charger 10 and a lamp 11, and removal of residual toner by a cleaning device 12.

In order to perform image density control in this electrophotographic apparatus, first, the timing generation circuit 25
At the same time as turning on the concentration detection circuit 26, a signal is sent to the charging control circuit 27, so that power is supplied from the power supply 28 to the charging charger 2, and from the power supply 29 having the opposite polarity to the power supplying to the reverse charging charger 30. Switch to This reverse charging charger 30 is
As shown in FIG. 2, the corona wire 3
A shielding plate 31 is provided in front of 0b, so that a part of the corona discharge current is shielded. As a result, the surface of the photoreceptor 1 is charged separately into a potential part with a polarity opposite to that used in the normal image forming process and a part with almost zero potential that is shielded by the shielding plate 31. .

On the other hand, at the same time as this switching of the charging polarity, the bias power switching switch 32 for the developing roller 17 similarly switches between contacts 32a and 32 in response to a signal from the timing generation circuit 25.
can be switched to b. A bias power supply 33 is connected to the contact 32a side, and during the normal image forming process, the developing roller 17, which also serves as a developing electrode, is supplied with a potential slightly lower than the background of the electrostatic latent image formed on the photoreceptor 1. A bias potential similar to this, that is, a bias potential having a polarity opposite to that of the toner is applied. Thereby, background development on the photoreceptor 1 is suppressed, and background stains on copies are prevented. On the other hand, a bias power source 34 having the same polarity as the oppositely charged portion on the photosensitive pair, that is, the same polarity as the charging polarity of the toner, is connected to the contact 32b side. This causes the toner to adhere only to the zero potential area on the photoreceptor. At this time, the reverse bias potential is set so that the charged polarity of the toner is negative, and the potential of the oppositely charged portion on the photoconductor is set to -500V or more, preferably from -500V to -
For 800V, the range of -200 to -400V is most effective for selective deposition of toner on the photoreceptor and for density measurements.

The toner adhering to the zero potential portion on the photoreceptor in this way is then transferred to the light emitting element 6 and the light receiving element 7.
Detected by a detection device including. That is,
Light from a light emitting element 6 such as a lamp or a light emitting diode is reflected on the surface of the photoreceptor, and the reflected light is received by a light receiving element 7 such as a photodiode. The change in the amount of light received due to the toner adhesion at this time is taken out as an electrical signal, which is compared with an electrical signal indicating a reference adhesion amount in a circuit 26 to detect the magnitude of the toner adhesion amount. Based on the detection result at this time, for example, if the toner adhesion amount is less than the reference adhesion amount, the toner replenishment control circuit 35 is driven to remove a certain amount of toner in the toner replenishment device 36 attached to the developing device 5. The toner is replenished into the developer container 14 to increase the toner concentration in the developer 13. After doing this, detect the toner adhesion amount again, and if the toner adhesion amount does not increase even though toner has been replenished, it is determined that the cause is not a lack of toner in the developer. is possible,
Check and adjust other development characteristic factors, such as toner charge amount, development bias amount, development time, development gap, and development roller rotation speed.

In the above embodiment, the corona charger 30 for reversely charging the photoreceptor during image density control is specially prepared, but the charger 2 used in the normal image forming process may also be used.
That is, as shown in FIG. 3, a shielding plate 31' is inserted into the discharge opening of the charging charger 2' to partially cover it, and the power supplies 28 and 29 applied to the charging charger 2' are connected to the charging opening. The selection can be made by the control circuit 27'. and,
In a normal image forming process, the charging charger 2' is connected to the power source 28 and the shielding plate 31' is left open, and when controlling the image density, the charging charger 2'
is connected to the power supply 29, and the discharge opening of the charging charger 2' is partially blocked by the shielding plate 31'. As a result, a zero potential portion and a reverse potential portion are formed on the photoreceptor 1 during image density control.

As described above, according to the present invention, a surface potential of approximately zero V, which is less affected by charging characteristics and exposure characteristics, is partially formed on the surface of the photoreceptor and toner is attached thereto, so that changes in the amount of toner adhesion are prevented. It is possible to grasp this almost as a change in development characteristics, and also to prevent wasteful consumption of toner and perform faster and more accurate image density control.

Although this invention has been described above with reference to the specific embodiments shown in the drawings, this invention can be modified in various ways, and all modifications within the spirit of this invention as set forth in the claims are as follows: It is included in this invention.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing an example of an electrophotographic apparatus to which the image density control method according to the present invention is applied;
The figure is a schematic sectional view showing an example of a charger for reverse charging, and FIG. 3 is a schematic view showing only the main parts of another embodiment of the present invention. 1...Photosensitive drum, 2...Electrostatic charger,
17... Developing roller, 30... Reverse charging charger, 31... Shielding plate, 36... Toner replenishing device.

Claims (1)

[Claims] 1. A two-component developer consisting of toner and carrier is used, and a bias voltage with a polarity opposite to that of the toner is applied to a developing electrode to form charges on an image carrier with a polarity opposite to that of the toner. In an image density control method used in an image forming method including developing an electrostatic latent image formed by electrifying the toner separately, switching the voltage applied to the developing electrode to a voltage having the same polarity as the charging polarity of the toner, and adhering the toner to the partially formed almost zero-volt image carrier surface; Image density comprising: detecting the amount of the toner adhering to a zero volt potential portion partially formed on the surface of the image carrier; and controlling the toner density of the two-component developer based on the detection result. Control method. 2. The image density control method according to claim 1, wherein the divided charging is performed by a reverse charging corona charger that partially shields a corona discharge area. 3. The separate charging is performed by switching the polarity of the voltage applied to a charging corona charger used in a normal image forming process and shielding a part of the corona discharge area. image density control method.