JPH09179383A - Contact electrifying device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make the surface potential of a photoreceptor electrified in a specific charge by the simple control, even using a high resistance material for an electrifying roller whose resistance is fluctuated by the temp. and the humidity. SOLUTION: The electrifying roller 2 to electrify the surface of a drum 12 while holding it in contact with the photoreceptor 11, is composed of a roller shaft 2a, an elastic body 3 consisting of a columnar high resistive material, and a protective layer 3a on a surface thereof. Respective threshold value is perliminarily set for the temp. and humidity, and then a CPU 1 is allowed to indicate a voltage for as constant-voltage circuit 5 and a current value for the constant-current circuit 6, while allowed to make the constant-voltage applied on the electrifying roller 2, in selecting the constant-voltage circuit 5 by a changeover switch 7. When the temp. and the humidity detected by a sensor 9 severally becomes below the respective threshold value, the CPU 1 is allowed to command the changeover switch 7 so as to switch to the constant current circuit 6 for supplying the constant current output to the electrifying roller 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a contact charging device for charging a photoconductor of an image forming apparatus using an electrostatic latent image system.

[0002]

2. Description of the Related Art In an image forming apparatus such as a copying machine or a printer which uses a technique called an electrostatic latent image method or an electrophotographic method, a corotron or scorotron is used as a charger for charging the surface of a photoconductor prior to exposure. There are known ones using a discharge device such as the above, and a contact charging device for contacting a charging roller having elasticity with a photoconductor.

Recently, mainly due to environmental hygiene problems, instead of the discharge method in which a large amount of ozone is generated along with power loss,
The number of image forming apparatuses that employ a contact charging method that causes less power loss and less ozone generation is increasing.

In the charging roller used in such a contact charging device, a rubber elastic body having a volume resistance of, for example, 10 ⁶ to 10 ⁸ Ωcm is concentrically provided around a shaft made of metal such as aluminum. And the surface thereof is coated with a surface protective layer having a higher volume resistance, for example, in the range of 10 ⁸ to 10 ¹⁰ Ωcm, so that the charging becomes uniform.

Rubber elastic bodies are generally classified into the following two types. The first type is an electronic conductor in which electrically conductive powder such as carbon or metal powder is dispersed in an insulating organic material which is almost an insulator, such as butadiene and EPDM, so that the volume resistance is within a predetermined range. It is a type. The second type is polar rubber, which is a high volume resistance organic material having a polar group with chemical activity represented by urethane and hydrin in the molecular structure, or an ionic agent such as metal salt and surfactant. It is an ion conductive type of added organic material.

The electron-conducting type material, which belongs to the first type, has an advantage that the change in volume resistance due to environmental conditions is small, but it is very difficult to uniformly disperse it in the manufacturing process, so that there is a local variation in resistance. Unevenness cannot be avoided, and in some cases there may be a difference of one digit or more. for that reason,
Spotwise uneven charging occurs on the surface of the photoconductor, and the quality of the formed image is significantly reduced.

If the volume resistance is reduced to a level where the influence of the resistance variation can be ignored (the amount of the dispersion material is increased), there is a problem that the light leaks to a pinhole or the like of the photoconductor and locally destroys the photoconductor. As a countermeasure against this, a surface protective layer having a higher volume resistance is provided, but it is difficult to increase the electrical withstand voltage because the thickness of the protective layer cannot be made too thick. Therefore, delicate resistance control is required at the time of manufacture, and there are many defects and the cost increases.

Contrary to the electron conductive type material, the ion conductive type material belonging to the second type has less variation and unevenness in resistance at the time of manufacturing, so that resistance control is easy, there are few defects, and the cost is low. However, since the volume resistance fluctuates greatly depending on environmental conditions and may reach one digit or more in some cases, there is no partial density unevenness in the formed image, but there is a variation in the density of the entire image for each sheet. There is a problem that it is easy.

Therefore, for example, Japanese Patent Laid-Open No. 1-284876.
JP, JP-A-3-288174, JP, 4-6
As shown in each of Japanese Patent Publication No. 567, the contact charging member is provided with a heating means so as to have a predetermined control temperature, for example, 35 to 55 ° C.
There was a first suggestion to keep it.

Further, as disclosed in Japanese Patent Application Laid-Open No. 4-9883, a constant current is made to flow in a non-image forming area to detect the voltage at that time, and a constant voltage control is performed by the voltage detected when the image forming area is reached. There was a second proposal to do.

Furthermore, Japanese Patent Laid-Open No. 4-186381,
As disclosed in JP-A-4-316064, a third proposal in which a temperature sensor is provided on the contact charging member and the applied voltage or the AC voltage superimposed on the DC voltage is varied according to the detected temperature. There was also.

[0012]

The first proposal, however, maintains the contact charging member at a temperature higher than the ambient temperature even during standby, so that power loss cannot be avoided and the contact charging member is kept stationary during standby. If this is done, the upper surface and the lower surface will have different heat dissipation conditions, resulting in temperature unevenness, so they must be rotated at least at a low speed. Further, there is a problem that when the power is turned on, the temperature is kept waiting until it reaches a constant temperature.

Further, the second proposal is excellent in that the influence of the characteristic variation among the manufactured contact charging members is eliminated, but as described in the above publication, the partial characteristics of the contact charging member are described. In order to remove the unevenness and the influence of the history of the photoconductor, it is necessary to detect the voltage a plurality of times to remove the abnormal value and to remove the charge of the photoconductor in advance, and the operation is complicated.

The third proposal varies the applied voltage or the AC voltage according to the temperature of the contact charging member detected by the temperature sensor, and is therefore excellent in maintaining the surface potential of the photosensitive member with accuracy. Control complexity is inevitable.

Alternatively, the second and third proposals have no problem when the temperature changes slowly, but when the power is turned on or when the continuous use time is long, the ambient temperature changes rapidly, so that the temperature of each part changes. There is a problem that a temperature difference occurs due to a difference in heat capacity and the control cannot follow the temperature change, or conversely the correction is over. Further, all of the first to third proposals have a problem in that only the temperature is targeted and no consideration is given to the humidity that affects the resistance value like the temperature.

The present invention has been made in view of the above points. Even if a high resistance material whose volume resistance varies depending on environmental conditions is used for the charging roller, the influence of temperature and humidity can be removed by simple control. The purpose is to uniformly charge the surface potential of the photoconductor.

[0017]

SUMMARY OF THE INVENTION In order to achieve the above-mentioned object, the present invention brings a charging roller made of a high resistance material whose resistance value varies according to environmental conditions such as temperature and humidity into contact with a photosensitive member and its surface. In a contact charging device for charging
Constant voltage control means for applying a constant voltage to the charging roller, constant current control means for supplying a constant current to the charging roller, environmental condition detection means for detecting environmental conditions, and constant voltage control means according to the detection result by the means. And switching means for switching the constant current control means.

The high resistance material is a polar rubber containing a polar group, or a rubber to which an ionic material such as a metal salt or a surfactant is added.

In the above contact charging device, the environmental condition detecting means is means for detecting temperature and humidity, and the switching means detects that the temperature detected by the environmental condition detecting means is less than a preset temperature threshold value. It is preferable to have a function of switching from the constant voltage control means to the constant current control means when the humidity falls below a preset humidity threshold value.

Further, the constant current control means may be means for controlling so as to maintain the current value immediately before that when the constant voltage control means is switched by the switching means.

Alternatively, the switching means may have a function of selecting the constant voltage control means when the temperature detected by the environmental condition detecting means is equal to or higher than the temperature threshold value or the detected humidity is equal to or higher than the humidity threshold value. Very good.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be specifically described below with reference to the drawings. 4 and 5 belong to the second type of ion-conducting type material, which is a high resistance material used for the charging roller of the contact charging device according to the embodiment of the present invention, and has a polar group in the molecular structure. It is a diagram showing an example of the temperature resistance and humidity characteristics of the volume resistance of hydrin rubber, which is a type of rubber.

The volume resistance of hydrin rubber against temperature is
When the humidity is 55% RH and the temperature scale is plotted on the horizontal axis and the logarithmic scale of volume resistance is plotted on the vertical axis as shown in FIG. 4, a straight line descending to the right is obtained. That is, since the volume resistance (Ωcm) rises exponentially as the temperature (° C) decreases, it can be seen that the change in volume resistance increases as the temperature decreases.

Further, the volume resistance vs. humidity characteristics of hydrin rubber are as follows. When the temperature is 26 ° C., the horizontal axis shows the relative humidity scale and the vertical axis shows the logarithmic scale of volume resistance as shown in FIG. It can be seen that the volume resistance changes more rapidly and sharply than the temperature resistance characteristics shown in FIG. 4 because the curve is upward as () decreases.

The characteristics shown in FIGS. 4 and 5 are common not only to hydrin rubber but also to other ion conductive type elastic materials such as other polar rubbers or ionic agent added rubbers. Therefore, in order to take advantage of the features that are easy to manufacture and have less variation and unevenness in resistance, it is clear that considering only the temperature as in the various proposals described above.

FIGS. 6 to 9 show that such a change in volume resistance is caused by the surface potential of the photosensitive member, the voltage applied to the charging roller, and the charging current flowing from the charging power source to the ground via the charging roller, the photosensitive member, and the drum. It is a diagram which shows an example of the influence which each has. However, the material and thickness of the photoconductor and the size (length, diameter) of the charging roller and the drum are not changed.

FIG. 6 is a diagram showing an example of the effect of the change in volume resistance on the surface potential and charging current of the photoconductor in the case of constant applied voltage, that is, constant voltage control, and FIG. The surface potential (B) in the figure shows the characteristics of the charging current.

In the case of constant voltage control, as shown in FIG. 6, both the surface potential and the charging current do not change much while the volume resistance is low, but exceed a certain point (near 10 ⁷ Ωcm in FIG. 6). Then, as the volume resistance increases, the decrease becomes remarkable. In the constant voltage control method, the previous history of the photoconductor has almost no effect on the surface potential.

FIG. 7 is a diagram showing an example of the influence of the change in volume resistance on the surface potential and applied voltage of the photoconductor in the case of constant charging current, that is, constant current control, and FIG. The surface potential (B) in the figure shows the characteristics of the applied voltage.

In the case of the constant current control, as shown in FIG. 7A, the surface potential is constant and is not affected by the volume resistance, but as shown in FIG. Does not change so much while the volume resistance is low, starts to rise beyond a certain point (slightly left of 10 ⁷ Ωcm), and rises sharply as the volume resistance increases.

However, in the constant current control system, the surface potential is easily affected by the previous history of the photoconductor, and uneven charging is likely to occur.
Although the smaller the diameter of the drum 12 becomes, the more difficult it becomes, but it is necessary to sufficiently remove the charge by exposing the charge removing lamp or the like before charging to make the state of the photoconductor uniform.

FIG. 8 is a diagram showing an example of the surface potential vs. charging current characteristic of the photoconductor. As shown in FIG. 8, the surface potential and the charging current have a linear relationship with each other and are not affected by the volume resistance. This is also clear from the fact that the surface potential becomes constant regardless of the volume resistance during the constant current control shown in FIG. 7 (A).

FIG. 9 is a diagram showing an example of the surface potential vs. applied voltage characteristic of the photoconductor. In this case, as shown in FIG. 9, when the volume resistance is constant, the surface potential and the applied voltage have a linear relationship with each other, but when the volume resistance changes,
The slope of the straight line (the variation of the surface potential / the variation of the applied voltage) changes greatly around the upper limit value of the applied voltage (for example, near 580 V in FIG. 9) where the surface potential is 0.

From the examples shown in FIGS. 6 to 9, the volume resistance of the elastic body used for the charging roller is the resistance threshold value (for example, 10 ⁷ Ω).
It is found that the surface potential and the charging current in the case of the constant voltage control and the applied voltage in the case of the constant current control greatly change when the voltage exceeds a little (cm). On the other hand, from the examples shown in FIGS. 4 and 5, the volume resistance increases as the temperature or humidity decreases,
Especially, it has been found that the humidity rapidly increases in the low humidity range.

Therefore, the temperature and humidity among the environmental conditions are detected, the volume resistance is calculated from the detected data,
Ideally, the applied voltage or charging current should be controlled so that the surface potential does not change significantly even if the result exceeds the resistance threshold, but as a practical matter, the volume resistance is calculated from the temperature and humidity detection data. It's not impossible to do, but it requires a fairly complex formula.

Further, the humidity may be data detected in the vicinity of the charging roller 2, but as described above, the temperature data detected in the vicinity does not always correctly indicate the internal temperature of the elastic body of the charging roller. Absent. Further, if the surface potential of the photoconductor is within a certain range, it is possible to control the density of the formed image by changing the bias voltage applied to the developing roller, for example.

FIG. 1 is a diagram showing an example of the configuration of a contact charging device according to an embodiment of the present invention. The contact charging device shown in FIG. 1 has a CPU 1 for controlling the entire device, a charging roller 2, a charging power source 4 for supplying a current to the charging roller 2, and an environmental condition detecting means for detecting ambient temperature and humidity. Sensor 9 which is

The charging roller 2 comprises a roller shaft 2a of good conductivity made of aluminum or the like, a high resistance material whose volume resistance varies depending on environmental conditions, such as polar rubber or metal salt containing a chemically active polar group, and an interface. A cylindrical elastic body 3 made of an ion conductive type rubber such as a rubber to which an ionic material such as an activator is added, and a cylindrical elastic body 3 surrounding the roller shaft 2a, and a surface protective layer made of a material similar to the elastic body 3 and thinly covering the surface thereof. 3a
And the volume resistance of the elastic body 3 is 10 ⁶
To 10 ⁸ [Omega] cm, the volume resistivity of the surface protective layer 3a is set to higher than ^{10 8 ~10} 10 ^Ωcm.

The charging roller 2 elastically contacts the photoconductive photosensitive member 11 lightly coated on the surface of the drum 12 made of aluminum or the like connected to the ground, and follows when the drum 12 is rotating. While being electrically rotated, a current supplied from the charging power source 4 to the roller shaft 2a is sent to the photoconductor 11 via the elastic body 3 and the surface protection layer 3a, so that the surface of the photoconductor 11 is charged.

The charging power source 4 switches the output of the constant voltage circuit 5 which is the constant voltage control means, the constant current circuit 6 which is the constant current control means, and the constant voltage circuit 5 or the constant current circuit 6 to the charging roller 2. A changeover switch 7 which is a changeover means for supplying, and a photoconductor 11, which is supplied from the changeover switch 7 to the charging roller 2,
The current detector 8 detects a charging current flowing to the ground via the drum 12.

The constant voltage circuit 5 and the constant current circuit 6 are power supply circuits for outputting a high voltage DC of a constant voltage and a constant current according to an instruction from the CPU 1, respectively. The changeover switch 7 is C
The input is switched to the constant voltage circuit 5 or the constant current circuit 6 according to a command from the PU 1, but the constant voltage circuit 5 is selected under normal environmental conditions. The current detector 8 constantly outputs the detected value of the charging current, and the sensor 9 constantly outputs the detected ambient temperature and humidity to the CPU 1.

FIG. 2 is a flow chart showing an example of a switching instruction routine for instructing the changeover switch 7 of the control routine of the CPU 1 in a subroutine form. The output voltage of the constant voltage circuit 5 under normal environmental conditions is set as an initial voltage value in advance, and the temperature threshold and the humidity threshold of the environmental conditions are, for example, 20 ° C. and 4 respectively.
It is assumed to be 0% RH (relative humidity).

When the switching instruction subroutine shown in FIG. 2 is started, it is first judged in step 1 whether the temperature is lower than 20 ° C. If not, the routine jumps to step 6 and 2
If it is less than 0 ° C, the humidity is 40% RH in step 2.
If it is less than 40%, the process jumps to step 6, and if it is less than 40% RH, that is, if the temperature is less than 20 ° C. and the humidity is less than 40% RH, the process proceeds to step 3.

In step 3, it is judged whether or not the input of the changeover switch 7 is on the constant current circuit 6 side, and if it is the constant current side, the routine directly returns to the main routine. In step 4, the detected value of the charging current is input from the current detector 8 and the value is instructed to the constant current circuit 6. Then, in step 5, the changeover switch 7 is instructed to switch to the constant current circuit 6 side and the process returns.

Step 1 or Step 2 to Step 6
When jumping to, it is judged whether or not the input of the changeover switch 7 is on the constant voltage circuit 5 side, and if it is the constant voltage side, the routine directly returns to the main routine. After the preset initial voltage value is instructed to the constant voltage circuit 5, the changeover switch 7 is instructed in step 8 to switch to the constant voltage circuit 5 side and return.

FIG. 3 is a diagram showing an example of a control range of whether the voltage or current applied to the charging roller 2 by the charging power source 4 is constant voltage controlled or constant current controlled according to environmental conditions. If the temperature is equal to or higher than the temperature threshold value (20 ° C.) or the humidity is equal to or higher than the humidity threshold value (40% RH), the CPU 1 executes the switching instruction subroutine shown in FIG. Since it is within the range, the charging power source 4 applies a constant voltage power having a preset initial voltage value to the charging roller 2.

When the temperature is below the temperature threshold and the humidity is below the humidity threshold, it means that the charging current has entered the constant current control region as shown in FIG. The constant current power controlled to maintain the value of the charging current detected by the container 8 is supplied to the charging roller 2. When the environmental condition returns to the constant voltage control region again, the charging power source 4 applies constant voltage power having an initial voltage value to the charging roller 2.

As described above, the contact charging device according to the present invention uses, as the elastic body 3, a polar rubber having polar groups belonging to the ion conductive type in its molecular structure, or an ionic material such as a metal salt or a surfactant. By using the added organic material rubber, the charging roller 2 can be obtained at a low cost and free from local resistance variations and unevenness.
Therefore, it is possible to form a high-quality image without partial density unevenness.

Regarding the influence of the environmental conditions due to the use of the ion conductive type rubber, depending on the type of rubber used and the shape and size of the charging roller 2, for example, the surface potential as shown in FIG. In consideration of the resistance to volume resistance, the surface potential is within its allowable range by constant voltage control (range in which density can be corrected by the bias voltage of the developing roller).
First, the maximum allowable volume resistance that can be accommodated is set.

Next, set allowable maximum values, temperature characteristics and humidity characteristics of volume resistance as shown in FIGS. 4 and 5, and constant voltage / constant current control range as shown in FIG. , The temperature threshold value and the humidity threshold value are respectively set, and the C value during execution of the subroutine shown in FIG.
In response to an instruction from PU1, the charging power source 4 changes the charging roller 2
The constant voltage control or constant current control of the power applied to is performed.

As a result, in the range of environmental conditions in which the volume resistance is low and the influence on the surface potential of the photoconductor 11 is small, constant voltage control is performed in which the influence of the history of the photoconductor 11 does not appear.
On the other hand, when the environmental conditions are such that the volume resistance becomes high, the current is switched to the constant current control in which the surface potential is stable, so that the formed image does not have a temporal variation in density.

Therefore, there is no problem such as the problem in the second proposal already described, that is, the proposal of setting the applied voltage so that a constant charging current is obtained in the non-image forming area (including time), and the control is performed. It's much easier.

Moreover, when the constant voltage control is switched to the constant current control, the constant current control is performed so as to maintain the value of the charging current immediately before that, so that there is no change in the density of the formed image due to the switching of the control. On the contrary, when the constant current control is switched to the constant voltage control, the constant voltage control may be performed by the applied voltage immediately before, and in that case, the density change due to the control switching does not occur.

However, in the embodiment in which the routine is shown in FIG. 2, when switching to the constant voltage control, the constant voltage control is returned to the preset initial voltage value. Therefore, it cannot be said that there is a possibility that a density change will occur when switching, but in general, in the office room or the like in which the main unit of the contact charging device, for example, the image forming apparatus is installed, the difference between the temperature and the humidity is different from the outdoors. Have a certain correlation with each other and do not deviate significantly.

Therefore, when switching from the constant voltage control range to the constant current control range or vice versa, the points of cutting the threshold lines shown by broken lines in FIG. 3 are close to each other.
The charging current when switching to the constant voltage control with the initial voltage hardly changes, and the density change for that purpose is not a problem even if it happens.

Further, even when the constant current control is switched to the constant voltage control, if the constant voltage control is performed with the applied voltage immediately before the switching, as the environmental condition changes drastically and the number of times the threshold line is cut increases, Although the voltage value of the constant voltage control may change gradually, the embodiment shown in FIG. 2 also has an advantage that no voltage deviation occurs.

[0057]

As described above, in the contact charging device according to the present invention, even if a high resistance material, which has no local variation or unevenness of resistance but its volume resistance varies depending on environmental conditions, is used as the elastic body of the charging roller. The effects of temperature and humidity can be removed by simple control, and the surface potential of the photoconductor can be charged to a constant level.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a configuration of a contact charging device according to an embodiment of the present invention.

FIG. 2 is a flow chart showing an example of a switching instruction routine executed by a CPU shown in FIG. 1 in a subroutine form.

3 is a diagram showing an example of a control region in which the charging power source shown in FIG. 1 performs constant voltage control or constant current control by executing the routine shown in FIG.

4 is a diagram showing an example of a volume resistance vs. temperature characteristic of hydrin rubber used as an elastic body of the charging roller shown in FIG.

FIG. 5 is a diagram showing an example of a volume resistance vs. humidity characteristic of hydrin rubber.

FIG. 6 is a diagram showing an example of the influence of a change in volume resistance on the surface potential and charging current of a photoconductor in constant voltage control.

FIG. 7 is a diagram showing an example of the influence of a change in volume resistance on the surface potential and applied voltage of a photoconductor in constant current control.

FIG. 8 is a diagram showing an example of the surface potential-charging current characteristic of the photoconductor.

FIG. 9 is a diagram showing an example of surface potential vs. applied voltage characteristics of a photoconductor.

[Explanation of symbols]

 1: CPU 2: charging roller 3: elastic body 4: charging power source 5: constant voltage circuit (constant voltage control means) 6: constant current circuit (constant current control means) 7: changeover switch (switching means) 8: current detector 9: Sensor (environmental condition detection means) 11: Photoconductor 12: Drum

Claims (5)

[Claims] 1. A contact charging device for charging a surface of a charging roller using a high resistance material, the resistance value of which varies depending on environmental conditions such as temperature and humidity, to a surface of the photosensitive member, and a constant voltage is applied to the charging roller. Constant voltage control means for applying, constant current control means for supplying a constant current to the charging roller, environmental condition detection means for detecting the environmental conditions, and the constant voltage control means and the constant voltage control means according to the detection result by the means. A contact charging device comprising switching means for switching the current control means. 2. The contact charging device according to claim 1, wherein the high resistance material is a polar rubber containing a polar group, or a metal salt,
A contact charging device comprising a rubber to which an ionic material such as a surfactant is added. 3. The contact charging device according to claim 1, wherein the environmental condition detecting unit is a unit that detects temperature and humidity, and the switching unit detects the temperature detected by the environmental condition detecting unit in advance. A contact charging device having a function of switching from the constant voltage control means to the constant current control means when the detected humidity is lower than a preset temperature threshold and the detected humidity is lower than a preset humidity threshold. 4. The contact charging device according to claim 3, wherein the constant current control means controls the current value immediately before the constant current control means when the switching means switches the constant voltage control means. Is a contact charging device. 5. The contact charging device according to claim 3, wherein the switching unit has a temperature detected by the environmental condition detecting unit equal to or higher than the temperature threshold value or a detected humidity equal to or higher than the humidity threshold value. In the case, the contact charging device has a function of selecting the constant voltage control means.