JP2000305342A - Electrostatic charger and image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To decide an appropriate voltage to be applied to an electrostatic charging roller, in accordance with the change of an electrostatic capacity due to the shaving of a photoreceptor and the environment, etc., and in accordance with the ambient temperature, and to prevent the occurrence of surface staining and the reduction of image density. SOLUTION: Two kinds of voltage, V1 and V2 equal to or exceeding a discharge start voltage Vth are applied to the electrostatic charging roller, and then, flowing currents I1 and I2 are measured, and the discharge start voltage Vth is obtained from a voltage V when I=0 is established in an expression; I-I1=((I2-I1)/(V2-V1)}(V-V1). Besides, a temperature detecting means is arranged near the electrostatic charger, and the voltage Vc to be applied on the electrostatic charging roller is obtained in accordance with the detected temperature in order to obtain a desired photoreceptor surface potential Vd by using the inclination α of the photoreceptor surface potential Vd with respect to the applied voltage experimentally obtained beforehand, that is, by using Vc=Vth+Vd/α, and then, the Vc is applied on the electrostatic charging roller so as to obtain a fixed photoreceptor surface potential Vd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charging device and an image forming apparatus, and more particularly, to a charging member in which a charging member is brought into contact with or opposed to a member to be charged via a minute gap. A charging device for applying a DC voltage to discharge an object to be charged to charge the object to be charged (charging in this specification includes charge elimination), and an image bearing device for such a charging device The present invention relates to an image forming apparatus serving as a body charging unit.

[0002]

2. Description of the Related Art Conventionally, in an image forming apparatus such as an electrophotographic copying machine or a printer, a corona discharger has been widely used as a charging means for an image carrier such as a photoconductor or an electrostatic recording dielectric. However charging processing system using a corona discharger, a high voltage application needs charging efficiency is low, in addition the corona discharge products (О _3, NОX etc.) has a problem such as dirt generation and discharge wires.

[0003] In recent years, a contact charging device having features such as ozonelessness and low power consumption has attracted attention and has been put to practical use. In this contact charging device, a conductive charging member is brought into contact with a member to be charged such as a photoreceptor, and a voltage is applied to the charging member to cause the member to be discharged to discharge. The surface is charged to a predetermined potential. The charging member is disposed in a non-contact and opposed manner through a minute air gap (air gap) that may cause a discharge phenomenon between the charging member and the surface of the member without contact with the member. Even when the charging bias is applied, the charging process on the surface of the charged body can be executed in the same manner as in the case where the charging member is arranged in contact with the charged body.

However, in the contact charging device, the discharge starting voltage changes due to abrasion of the member to be charged over time, and changes in the capacitance of the member to be charged and the charging member due to the environment and the like. May change the charging potential of the member to be charged. In the case of an image forming apparatus, the charging potential deviates from a desired value initially set due to a change in a discharge starting voltage due to a change in capacitance due to abrasion of a photoreceptor as a member to be charged. May be disturbed.

In the charging device and the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 06-194933, the surface potential of the photosensitive member increases linearly with a slope of 1 with respect to the applied voltage after the discharge starting voltage. To obtain the required photoreceptor surface potential required for photography, it is sufficient to apply a voltage of photoreceptor surface potential + discharge start voltage to the charging roller. The discharge start voltage is determined by measuring the applied voltage, and the voltage to be applied to the charging member is determined based on the voltage.

[0006]

However, in an actual image forming apparatus, the photosensitive member surface potential with respect to the applied voltage may not be linear with a slope 1 depending on the material of the charging member and the linear speed of the image forming apparatus. The voltage applied to the charging roller to make the surface potential of the photoreceptor not equal to “photoreceptor surface potential + discharge starting voltage”. Therefore, Japanese Patent Application Laid-Open No. H06-1
When the voltage applied to the charging roller is determined as disclosed in Japanese Unexamined Patent Publication No. 94933, image quality defects such as background contamination and reduced density occur.

In some cases, the surface potential of the photosensitive member with respect to the applied voltage does not become linear with a slope of 1 depending on the temperature of the charging roller. It does not become “body surface potential + discharge starting voltage”, and the same problem as described above occurs.

Accordingly, the present invention provides a method for charging an image forming apparatus in addition to abrasion of a member to be charged and an image carrier over time and a change in capacitance of the member to be charged, an image carrier and a charging member due to an environment or the like. It is an object of the present invention to provide a charging device capable of preventing the occurrence of background contamination and a decrease in image density by being able to determine an appropriate voltage to be applied to a charging roller in accordance with the temperature in the vicinity of the device.

[0009]

According to a first aspect of the present invention, there is provided a charging device in which a charging member is brought into contact with a member to be charged or a charging member is minutely contacted with the member to be charged. In the charging device, which is disposed opposite to each other with a gap therebetween and applies a DC voltage to the charging member to discharge the member to be charged and charge the member to be charged, the charging member has a small current value. A constant current is supplied, and control means for obtaining the discharge start voltage by measuring a voltage applied by the constant current is provided, and a temperature detection means is provided in the vicinity,
The control means calculates an applied voltage suitable for the charging member according to a detected temperature using a relationship between a difference in surface potential of the object to be charged and an applied voltage based on a temperature difference obtained in advance. I do.

According to a second aspect of the present invention, there is provided an image forming apparatus for performing image formation by applying an image forming process including a step of charging an image carrier to achieve the above object. Then, a charging member is brought into contact with the image carrier or the image carrier and the charging member are disposed to face each other with a small gap therebetween, and a DC voltage is applied to the charging member to apply a DC voltage to the image carrier. A charging device that discharges the image bearing member to charge the image bearing member, and a control that applies a constant current of a minute current value to the charging member and measures a voltage applied by the constant current to obtain a discharge starting voltage. Means, and a temperature detecting means is provided near the charging device,
The control means calculates an applied voltage suitable for the charging member according to a detected temperature by using a relationship between a difference in surface potential of the image carrier and an applied voltage based on a temperature difference obtained in advance. I do.

[0011]

Embodiments and examples of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention.
The image forming apparatus of this example is a laser beam printer using a transfer type electrophotographic process.

In FIG. 1, reference numeral 1 denotes a photosensitive drum as an image carrier (charged body). The photosensitive drum 1 is, for example, a cylindrical OPC photosensitive member having a diameter of 30 mm, and has a thickness of 25 μm on a charge generating layer. And a charge transport layer (CT layer) is disposed in a clockwise direction X as shown by an arrow in FIG.
A predetermined process speed (peripheral speed: for example, 90 mm /
Seconds).

In FIG. 1, reference numeral 2 denotes a charging roller as a charging member brought into contact with the photosensitive drum 1;
Rotates following the rotation of the photosensitive drum 1, and a predetermined charging bias is applied from a voltage unit (HVT, power supply unit) 3 to bring the peripheral surface of the rotating photosensitive drum 1 to a predetermined polarity and potential. (In this example, negative charge).

An image-modulated laser beam L output from the laser beam scanner 4 is irradiated (scanning exposed) onto the charged surface of the photosensitive drum 1 charged in this way, and the potential of the exposed portion is attenuated and statically applied. An electrostatic latent image is formed. Then, when the latent image arrives at a developing site facing the developing device 5 with the rotation of the photosensitive drum 1, negatively charged toner is supplied from the developing device 5 to form a toner image by reversal development. A conductive transfer roller 6 is disposed in pressure contact with the photosensitive drum 1 on the downstream side of the developing device 5 in the rotation direction of the photosensitive drum 1, and a nip portion between the photosensitive drum 1 and the conductive transfer roller 6 forms a transfer portion. Is formed.

When the toner image formed on the surface of the photosensitive drum 1 reaches the transfer site as the photosensitive drum 1 rotates, the transfer material P is supplied from the guide 7 to the transfer site at the same time as the toner image. At the same time, a predetermined voltage is applied to the transfer roller 6 at a predetermined time by the voltage unit 3,
The toner image is transferred from the surface of the photosensitive drum 1 to the transfer material P. The transfer material P on which the toner image has been transferred at the transfer portion is conveyed to the fixing device 8, where the toner image is fixed, and discharged out of the apparatus. In the figure, reference numeral 10 denotes a control unit (CPU) which controls the power supply unit 3.

On the other hand, the transfer residual toner remaining on the surface of the photosensitive drum 1 is scraped off by a urethane counter blade (cleaning blade) 9, and the surface of the photosensitive drum 1 is cleaned to prepare for the next image formation.

The photosensitive drum 1 is slightly scraped by durable paper passing. The charging roller 2 has a two-layer configuration having a high resistance layer on the surface. This is the photosensitive drum 1
When a pinhole is formed in the charging roller 2, the charging current is concentrated on that portion, and the surface potential of the charging roller 2 is prevented from dropping to prevent poor charging of the horizontal streaks. The developing device 5 is 2
The electrostatic latent image on the peripheral surface of the photosensitive drum 1 is subjected to reversal development with a non-magnetic toner, and the exposed portion is visualized with the toner. For example, a voltage of 3 kV is applied to the transfer roller 6 to perform transfer.

The control of the voltage applied to the charging roller 2 will be described with reference to FIG. As described above, when a DC voltage is applied to the charging roller 2, charging starts when the applied voltage is equal to or higher than the discharge start voltage Vth, and thereafter, the surface potential of the photoconductor increases at the same rate as the increase in the applied voltage. . For this reason, when the environment and the scraping of the photoconductor are ignored, the charging roller 2 may be controlled by a voltage obtained by adding the discharge start voltage Vth to the target photoconductor surface potential Vd, as shown in FIG. In addition, when the environment is changed or the photosensitive drum 1 is scraped, the value of the photosensitive member surface potential Vd may change if the discharge start voltage Vth is changed and the control is performed with a constant voltage. become. That is, as shown in FIG.
After the endurance of the N environment and the initial stage of the L / L environment, there is a difference of 160 V between the photoconductor surface potentials Vd. If the constant voltage control is performed by estimating the discharge start voltage Vth to be 640 V assuming the initial state of the normal environment, in the L / L environment, the photoconductor surface potential Vd decreases to cause background contamination, and endurance occurs. Later, the photoconductor surface potential Vd is significantly increased, thereby increasing the charged potential VL and lowering the image density. In order to detect a change in the discharge start voltage Vth, a photoconductor surface potential measuring device may be provided in the printer body, but this increases the cost and requires additional hardware such as a power supply.

In the present embodiment, the voltage applied to the charging roller 2 and the charging current flowing thereby are detected, and the discharge start voltage Vth is predicted and controlled based on this relationship. Specifically, as shown in FIG. 3, two voltages V1 and V2 equal to or higher than the discharge start voltage Vth are applied to the charging roller 2, and currents I1 and I2 flowing therethrough are measured. At this time, the photosensitive drum 1
Since the relationship between the charging potential and the charging current cannot be clarified unless the potential is a fixed value, image exposure is performed and measurement is performed with the potential set to zero. In FIG. 3, the discharge starting voltage Vt
Since h is the point A indicating the start of discharge, the voltages V1 and V2
The currents I1 and I2 flowing at the time of application are measured, and a linear equation obtained by the measurement is obtained.

I-I1 = ｛(I2-I1) / (V2-V
1) The discharge start voltage Vth is obtained by calculating the voltage V when I = 0 of｝ (V−V1).

When the slope of the photosensitive member surface potential Vd with respect to the applied voltage is linear, the voltage Vc (Vc) is obtained by adding the desired photosensitive member surface potential Vd to the discharge starting voltage Vth obtained by the above equation (1). = Vth + Vd) to the charging roller 2
, A constant photoconductor surface potential Vd is obtained irrespective of scraping of the photoconductor 1 and fluctuations in the environment.

As described above, as shown in FIGS. 4 and 5, in an actual image forming apparatus, the surface potential of the photosensitive member with respect to the applied voltage may not be linear with a slope 1 depending on the temperature of the charging device. Therefore, when the image forming apparatus is used in a place where the temperature is high or low, or when the temperature difference is large between morning and noon in winter, the problem described in the related art may occur. Therefore, the temperature detecting means 11 (for example, a thermistor) is provided near the charging device.
And the slope α of the photoconductor surface potential Vd with respect to the applied voltage experimentally obtained in advance in accordance with the detected temperature.

## EQU2 ## The value is determined as α = ΔVd / ΔVc, and using this, the voltage Vc applied to the charging roller for obtaining the required photoconductor surface potential Vd is determined. Specifically, the applied voltage Vc is

Vc = Vth + Vd / α If there is a correlation with the temperature near the charging device, the temperature at an appropriate place can be detected and used.

An example in which an image is actually formed will be described. When the temperature in the vicinity of the charging device is 15 ° C.
9) In the L / L environment, the above-mentioned control was performed using the photoconductive drum from which the CT layer was cut to 15 μm. V1 =
The currents flowing when 1200 V and V2 = 1800 V were applied were 30 μA and 60 μA, respectively. During this measurement, image exposure was always performed, and the potential of the photoconductor before charging was set to 0V.

The current I1, which flows when the voltages V1, V2 are applied,
When the value of I2 is substituted into the above-described equation 1, the discharge starting voltage V
Since th = 600 V is obtained, a value obtained by multiplying this value by 1 / α to a necessary photoconductor surface potential Vd = 900 V, that is, 900 / 1.1 = 818 V is added, and a voltage of 1418 V is applied to the image forming voltage Vc. When the image was formed at this voltage, a good image could be obtained. When the surface potential Vd of the photoconductor at this time was measured, it was 890 V, and a value close to the predicted value was obtained.

On the other hand, when the inclination α of the photosensitive member surface potential Vd with respect to the applied voltage is set to 1 without using the present invention, the voltage 15 obtained by adding 900 V to the discharge starting voltage Vth = 600 V is used.
00V is applied to the charging roller 2, and the photoconductor surface potential Vd at this time has become 980V.
The potential of the image portion increased, and the image density became low (bright). That is, if the inclination α of the photosensitive member surface potential Vd with respect to the applied voltage is simply set to 1, the image may be deteriorated depending on the temperature in the vicinity of the charging device. However, in the present invention, this can be suppressed as described above.

[0025]

The charging device according to the first aspect is as described above. Therefore, in addition to the scraping of the charged body and the change in the capacitance of the charged body and the charging member due to the environment, etc. In addition, an appropriate voltage applied to the charging roller can be determined according to the temperature in the vicinity of the apparatus, and the charging potential of the member to be charged can be stabilized irrespective of the ambient temperature of the image forming apparatus.

Since the image forming apparatus according to the second aspect is as described above, it is possible to stabilize the charging potential of an image carrier such as a photosensitive member, which is a member to be charged, regardless of the ambient temperature. Thus, there is an effect that a good image can always be output stably.

[Brief description of the drawings]

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

FIG. 2 is a diagram illustrating a control example of a voltage applied to a charging roller.

FIG. 3 is a diagram showing the relationship between a voltage equal to or higher than a discharge starting voltage applied to a charging roller and a current flowing thereby.

FIG. 4 is a diagram illustrating a relationship between a slope of a photoconductor surface potential and an applied voltage.

FIG. 5 is a diagram illustrating an example in which the slope of the photoconductor surface potential with respect to an applied voltage changes with temperature.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 photoconductor drum 2 charging roller 3 voltage section 4 laser beam scanner 5 developing device 6 conductive transfer roller 7 guide 8 fixing device 9 counter blade (cleaning blade) 10 control section (CPU) 11 temperature detecting means L laser beam P transfer material Vth Discharge start voltage Vd Photoconductor surface potential

Claims (2)

[Claims] 1. A charging member is brought into contact with a member to be charged or a charging member and a member to be charged are disposed to face each other with a small gap therebetween. A DC voltage is applied to the charging member to apply a charge to the member to be charged. In the charging device for discharging the object to be charged by charging the object to be charged, a constant current having a minute current value is applied to the charging member, and a voltage applied by the constant current is measured to obtain a discharge starting voltage. Means, and a temperature detecting means is provided in the vicinity, and the control means uses the relationship between the gradient of the surface potential of the object to be charged and the applied voltage due to a previously determined temperature difference, and performs the charging in accordance with the detected temperature. A charging device for calculating an applied voltage suitable for a member. 2. An image forming apparatus for performing image formation by applying an image forming process including a step of charging an image carrier, wherein a charging member is brought into contact with said image carrier or said image carrier is contacted. A charging device that charges the image carrier by applying a DC voltage to the charging member to cause the image carrier to discharge by applying a DC voltage to the charging member; A constant current having a minute current value is passed through the member, and a control means for obtaining the discharge starting voltage by measuring a voltage applied by the constant current is provided. A temperature detecting means is provided near the charging device, and the control means is provided. But,
An image forming apparatus comprising: calculating an applied voltage suitable for the charging member according to a detected temperature by using a relationship between a difference in surface potential of the image carrier and an applied voltage based on a temperature difference obtained in advance.