JP2984396B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus, and more particularly to an image forming apparatus suitable for a case where a transfer member such as a transfer roller is in contact with the back side of a transfer material. The present invention relates to a forming apparatus.

2. Description of the Related Art A toner image is formed by a charged toner on a moving image carrier, and a contact-type transfer member such as a transfer roller that moves synchronously with the image carrier is pressed against the toner image so that a pressure contact nip portion between the two members is formed. The transfer position is set, a transfer material such as paper is inserted between the transfer positions, and a transfer bias having a polarity opposite to that of the toner is applied to the transfer member, and the toner image on the image carrier is transferred by the action of the electric field formed thereby. An image forming apparatus configured to transfer to a material has already been proposed.

[0003] Such a transfer means can hold the transfer material more reliably than in the case of using a well-known corona discharger, so that a transfer deviation at a transfer position hardly occurs and a transfer bias requires a relatively low pressure. Therefore, the device can be made compact and compact,
There are various advantages such as no generation of ozone, but on the other hand, when the width of the original is wider than the width of the transfer material or when printing a thick original such as a book, the outer part of the width of the transfer material is developed. As a result, the toner in that portion adheres to the transfer roller, and the toner may scatter and contaminate the respective portions near the transfer roller, or may cause back contamination of the subsequent transfer material.

To avoid such a situation, Japanese Patent Laid-Open No.
As disclosed in Japanese Patent Application Laid-Open Nos. 0-153642, JP-A-51-9840 and JP-A-1-292385, when a transfer member is not present at a transfer position, a bias for cleaning having the same polarity as toner is applied to a transfer material. A method has been proposed in which the toner applied to the transfer member is returned to the image carrier by applying the voltage.

[0005]

[Problems to be solved by the invention]
For example, the charging polarity of charging means for charging the image carrier to form an image on the image carrier, and the charging of the transfer charging means for transferring the image of the image carrier to a transfer material, such as an image forming apparatus using a regular developing system. When applied to the transfer member having the same polarity, the image carrier after cleaning when the transfer member is cleaned by applying a cleaning bias voltage having the same polarity as the toner to the transfer member, that is, a polarity opposite to the charging polarity of the charging means. In some cases was shifted to a polarity opposite to the charging polarity of the charging unit. In this way, when the potential of the image carrier becomes the polarity opposite to the charging polarity of the charging unit, it remains as a memory,
Next, even if the image carrier is charged by the charging means in order to form an image, the image carrier does not rise to a desired potential and the image density is reduced. Further, the potential of the image carrier after the transfer of the toner image to the transfer material at the transfer position is the same polarity as the charging polarity of the charging unit,
When a potential having the same polarity as the charging polarity and a potential having a polarity opposite to the charging polarity of the charging unit due to the cleaning of the toner of the transfer member as described above are present in the image carrier, both potential portions are changed to the next potential. If it is in the image area of the image carrier at the time of image formation, a difference appears in the potential of the image carrier after charging by the charging means, and there is a problem that density unevenness occurs particularly in a halftone image.

In particular, when a photoreceptor having an organic photoconductive layer on its surface is used as an image carrier, the above-mentioned image defects such as a decrease in image density and image unevenness have occurred remarkably.

On the other hand, it is conceivable to set a cleaning current smaller than an optimum value in order to reduce a potential difference between the image carrier before and after the transfer member by a cleaning bias applied to the transfer member. Could not cope with the scattering of paper and the stain on the back of the transfer material.

The above problem occurs not only when a transfer member such as a transfer roller is used but also when, for example, a voltage having a polarity opposite to the above-mentioned charge polarity is applied to a member which comes into contact with an image carrier. is there.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object of the present invention is to provide an image forming apparatus in which a transfer member is cleaned to prevent back stains on a transfer material. It is.

Another object of the present invention is to provide an image forming apparatus which forms an excellent image by preventing image defects such as image unevenness.

It is another object of the present invention to provide an image forming apparatus which charges an image carrier for forming an image and makes the potential of the image carrier which becomes an image area after charging uniform.

It is a further object of the present invention to provide an image forming apparatus capable of satisfactorily cleaning a member which comes into contact with an image carrier.

Further objects and features of the present invention will become apparent from the following detailed description when read in conjunction with the drawings.

According to the present invention, which achieves the above object, there is provided a photoconductor which moves endlessly, a charging means for charging the photoconductor, and an electrostatic latent image formed by exposing the photoconductor charged by the charging means to an image. Exposure means for forming an image; developing means for developing the electrostatic latent image on the photoreceptor with toner charged to a polarity opposite to the charging polarity of the photoreceptor; and electrostatically applying the toner image on the photoreceptor to the transfer material. And a cleaning bias application unit for applying a cleaning bias having the same polarity as the toner charging polarity to the transfer unit during non-transfer. At least a part of the image area is used as an image area, and an area to be an image area receiving the cleaning bias is charged in advance with a polarity opposite to the polarity of the cleaning bias. You.

[0015]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic side view of a main part of a two-image forming apparatus suitable for applying the present invention.

The surface of the negatively charged photoreceptor 2, which is a cylindrical image carrier having an axis perpendicular to the paper surface and rotating endlessly in the direction of arrow X, is made negative by a charging roller 4 as charging means. The charged surface is uniformly charged, and the charged surface is subjected to image exposure according to the image information by the exposure means 5 to form an electrostatic latent image. A positively charged toner charged to a polarity opposite to the charging polarity of the charging means is supplied from the developing device 6 to the electrostatic latent image to form a toner image, that is, the toner image is regularly developed and transferred to the photoconductor 2. The transfer roller 1 which is a member arrives at a transfer position formed by pressing.

At the same time, the transfer material P is guided to the transfer position by the transfer guide 3, and the back side of the transfer material is pressed against the transfer roller 1 and the transfer roller 1 has a polarity opposite to that of the toner during development. The toner image is transferred to the transfer material P by the action of the electric field applied by the transfer bias power supply 13 and thus formed. At this time, the switch of the power supply 13 is located on the solid line 13a side.

Thereafter, the transfer material P is neutralized by the action of the static elimination needle 8 and is conveyed to a fixing portion (not shown) via a conveyance path. A part of the residual toner which has not been transferred to the transfer material P during transfer is removed by the cleaner 7. Then, the photosensitive member 2 enters the next image forming step.

On the other hand, when a transfer bias having a polarity opposite to that of the toner at the time of development is applied to the transfer roller 1, the transfer roller 1, which is a contact member that comes into contact with the photosensitive member 2, is larger than the size of the transfer material P than the transfer member If the toner attaching area of No. 2 is large, toner adheres to the transfer roller 1. Therefore, when performing a continuous image forming operation on the photosensitive member 2 during the pre-rotation of the photosensitive member 2 before the image forming operation on the photosensitive member 2 is performed, the next time the transfer material P passes through the transfer position after the transfer material P has passed, the next A polarity opposite to that at the time of transfer to the transfer roller 1 during at least a part of the time when the transfer material P is not present at the transfer position, such as until the transfer material P reaches the transfer position;
That is, a cleaning bias having the same polarity as that of the toner during development is applied by the power supply 13. It is desirable that the cleaning bias be applied to the transfer roller 1 at least during the time when the transfer roller 1 rotates. As a result, the toner adhering to the transfer roller 1 is returned to the photoreceptor to prevent the transfer material from being stained and the toner from scattering into the apparatus. Note that the switch of the power supply 13 at the time of applying the cleaning bias is located on the dotted line 13b side.

In the illustrated apparatus, the photosensitive member 2 is an OPC photosensitive member having a negative charge polarity, and has a diameter of 60 mm and a process speed of 70 mm / sec. The transfer roller 1 has a diameter of 20 mm and a length of 230 mm. Has a foamed EPDM (terpolymer of ethylene propylene diene) layer dispersed with carbon, and has a volume resistivity of 10 ⁹ Ωcm when a voltage of 1 KV is applied to the transfer roller 1. Transfer roller 1
During the transfer, a constant voltage control of -4.0 KV is performed at the time of transfer, and a constant current control of +0.5 μA is performed at the time of cleaning.

The charging roller 4 has a diameter of 12 mm, has a carbon dispersed EPDM layer on a metal core, and has a volume resistivity of 5 × 10 ⁸ Ωcm when a voltage of 1 KV is applied to the roller 4.

Next, FIG. 9 shows the relationship between the volume resistivity of the transfer roller 1 as a transfer member and the voltage and current applied to the transfer roller 1.

FIG. 3 shows the range of the voltage and the current value at which the cleaning efficiency of the toner adhered to the transfer roller is the highest with respect to the change in the volume resistivity of the transfer roller. If there is a maximum effect at about + 50.0V but nearly +1 at 10 ¹⁰ [Omega] cm or more.
4KV or more is required.

Regarding the current, in this case, the cleaning efficiency is maximized at about +0.5 μA for all volume resistivity.

This is approximately 3 × when converted to a charge density.
At 10 ⁻⁹ C / cm ² , the charge density is substantially the same as the charge density of the toner when the dark portion potential portion of the photoconductor 2 charged by the charging roller 4 is developed without being exposed.

When the toner adhering to the above-mentioned dark portion potential portion of the photoconductor is transferred to the transfer roller, a cleaning efficiency for returning the toner on the transfer roller to the photoconductor is given by applying a charge to the transfer roller to cancel the charge. It is considered that when the charge is excessive, the toner on the transfer roller is charged to a polarity opposite to the charge polarity at the time of development, and the cleaning efficiency is reduced.

FIG. 10 shows the relationship between the actual current applied to the transfer roller and the cleaning efficiency of the transfer roller. That is, as described above, the cleaning efficiency is maximized when the current applied to the transfer roller is +0.5 μA.

FIG. 11 shows that the current applied to the transfer roller is +
After cleaning at 0.5 μA (downstream of transfer position)
And the photoreceptor potential before cleaning (upstream of the transfer position) before cleaning.

As described above, when the optimum cleaning current is set, the photosensitive member potential after cleaning shifts to the voltage polarity side applied to the transfer roller with respect to before cleaning.

When the regular development method is employed as in an analog copying machine, the polarity of the toner during development is
That is, the polarity of the charging means for charging the photoconductor for forming an image on the photoconductor is opposite to the polarity of the charging bias. Therefore, the polarity of the cleaning bias applied to the transfer roller is also opposite to the charging polarity.

Incidentally, if there is a difference in charging polarity in the potential of the photoconductor before charging the image area of the photosensitive member after charging by the charging means for forming an image, it depends on the charging ability of the charging means. However, a difference appears in the photoreceptor potential after charging, and this appears as density unevenness particularly in a halftone image.

FIG. 12 shows the dependence of the potential after charging and the potential before charging when the OPC photosensitive member having the negative charging polarity is charged by the charging roller 4 as charging means. If the previous photosensitive member potential is not on the side of the charging polarity (minus), the photosensitive member potential after charging is lower than the desired potential of -650 V, and density unevenness occurs. The charging roller 4 has a peak-to-peak voltage Vpp of 15
A sine wave of 00 V applies a DC component of -650 V.

In order to prevent such density unevenness, after the cleaning bias (plus) applied to the transfer roller 1 is switched to the transfer bias (minus) by the power supply 13, while the transfer bias is being applied to the transfer roller 1. The image forming operation is controlled so that the surface of the photoconductor 2 that has passed through the transfer position becomes the front end of the next image area. That is, the surface of the photoconductor 2 charged negatively by the transfer roller 1 is used as an image area when the next image is formed.

However, if such a control sequence is taken, the time for applying the cleaning bias to the transfer roller 1 is shortened. Further, at this time, if the cleaning bias is applied for a sufficient time, the leading edge of the image on the photoreceptor 2 must be always the same during continuous image formation. However, there is a disadvantage that the time required for the transfer material to reach a longer time is longer and the continuous image forming speed is lower.

Therefore, it is desirable to control the apparatus by a CPU so as to obtain a timing chart as shown in FIG.

As for the voltage applied to the charging roller 4, the AC component is 400 Hz, and the peak-to-peak voltage Vpp is 1500 V.
, And the DC component is switched between -650 V and -200 V.

As described above, the transfer roller 1 has -4.0 KV at the time of transfer, and +0.5 μA at the time of cleaning.
Constant current control.

FIG. 3 shows a state one rotation before the leading end of the image area (shaded area) of the photosensitive member 2 reaches the charging position after the charging by the charging roller 4. That is, this shows a state in which the history of the surface potential remains in the hatched portion of the photoreceptor 2 after the transfer. At this time, the voltage is applied to the charging roller 4. The DC component of the voltage is -200 V, and the surface potential of the hatched portion becomes -200 V when the photosensitive member 2 moves from the state shown in FIG. Therefore, even if the developing bias applied to the developing sleeve at the developing position is not switched between the image area and the non-image area, the photosensitive area having the potential of -200 V is not developed. When the hatched portion reaches the transfer position, a cleaning bias voltage is applied to the transfer roller 1 by the power supply 13 so that +0.5 μA flows, and the positively charged toner attached to the transfer roller 1 is transferred to the photoconductor 2. . At this time, since the photosensitive member 2 to which the positively charged toner is transferred has a negative potential (−200 V), cleaning can be performed satisfactorily. Due to the application of the cleaning bias, the surface potential of the hatched portion after passing through the transfer position becomes -180 V, and returns to the state of FIG. At this time, the DC component of the voltage applied to the charging roller 4 is switched to -650 V, and the hatched portion serving as the image area is charged to -650 V. Next, the shaded portion is developed after image exposure and becomes an image area. FIG. 4 shows a state in which the leading end A of the image area has reached the transfer position thereafter.

Immediately before the state as shown in FIG. 4, the voltage applied to the transfer roller becomes -4.0 KV.
The upper toner image is transferred to the transfer material P. Since the photoconductor surface potential after the transfer is negative, the charging roller 4
Is constant at -650 V as long as the DC bias applied to the charging roller 4 is -650 V, and a desired uniform post-charge potential can be maintained over the entire image area.

FIG. 2 shows the timing chart described above, where t ₁ is the time required for the photosensitive member 2 to move from the charging position to the transfer position, and t ₂ is the time required for the photosensitive member 2 to move from the transfer position to the charging position. T ₁ + t ₂ is the time required for the photoconductor 2 to make one rotation. In this embodiment,
An image in which the length of the image area in the moving direction of the photoconductor 2 is larger than the circumference of the photoconductor 2 for one rotation can be formed. In such a case, the timing chart of FIG. It is.

As shown in FIG. 2, in the present embodiment, the cleaning bias is applied to the transfer roller 1 even before one rotation (at the time of arrow B) at the start of the transfer where the transfer position is the leading end of the image area of the photosensitive member. Therefore, it is not necessary to switch the cleaning bias to the transfer bias at the time point B as in the above-described embodiment. Therefore, in this embodiment, the cleaning bias time can be made longer and the continuous image forming speed can be made faster than in this case. Further, according to the present embodiment, the photoconductor region to which the cleaning bias (the polarity opposite to the charging polarity of the photoconductor) is applied to the transfer roller 1 at the transfer position is charged in advance to the charging polarity of the photoconductor,
This prevents the potential of the photoconductor after the transfer roller 1 has been cleaned from being opposite to the charging polarity of the photoconductor.
Therefore, by performing charging for forming an image next, it can be charged to a desired potential, and the image density does not become uneven.

FIG. 5 is a schematic side view showing another embodiment. The cleaning blade 7a provided on the cleaner 7 is made of urethane rubber in which carbon is dispersed to adjust the volume resistance to 10 ⁸ Ωcm. As shown in the figure, the power supply 9 applies an alternating current of 400 Hz and a peak-to-peak voltage Vpp of 1300 V and a direct current of -200 V to the cleaning blade 7 a.

The construction and operation timing of the photosensitive member 2, the transfer roller 1 and the like are the same as those in the above embodiment, but the DC component to the charging roller 4 is fixed at -650 V, and the transfer roller 1 is cleaned at the transfer position. Until the area of the photoconductor 2 to which the bias is applied passes the cleaning blade 7a, the above-described voltage is applied to the blade. That is, even if the photosensitive member 2 is positively charged by the application of the cleaning bias to the transfer roller 1, the area passes through the cleaning blade 7 a to which a negative voltage is applied, and is negatively charged. Then, even if image formation is performed thereafter, density unevenness does not occur.

FIG. 6 is a schematic view of a main part showing another embodiment of the present invention. The construction and operation timing of the photosensitive member 2, the transfer roller 1, and the charging roller are the same as those in the above embodiment. is there.

A blank shutter 10 reflects and condenses light from a lamp 11 to irradiate the surface of the photosensitive member 2.

The DC component applied to the charging roller 4 is -650.
In this case, the area of the photosensitive member 2 to which the cleaning bias is applied to the transfer roller 1 is irradiated with light from the lamp 11 so that the potential after the blank exposure becomes -200 V. Therefore, the area of the photoconductor 2 that has passed through the transfer roller 1 to which the cleaning bias has been applied is negatively charged, and no density unevenness occurs during the subsequent image formation.

The configuration shown in FIGS. 5 and 6 is advantageous in terms of cost since there is no need to switch the charging bias.

FIG. 7 shows still another embodiment in which the present invention is applied to an apparatus having a precharger 12 between a developing position and a transfer position, and a charging roller as in the previous embodiment. 4, the area of the photoconductor 2 to which the cleaning bias is applied to the transfer roller 1 instead of charging the photoconductor 2 to -200 V is previously negatively charged (-200 V).
V).

Even with such a configuration, there is no need to switch the DC component of the charging voltage.

FIG. 8 is a side view of a principal part showing still another embodiment of the present invention. The photosensitive member 2 and the transfer roller 1 are the same as those in the above-mentioned case.

The charging means is a scotron changer 13 instead of the charging roller 4, and the bias applied to the grit 13g is switched in two stages to control the charging potential.

In the photoreceptor region where the transfer bias is applied to the transfer roller 1 after the charging, the grid voltage is set to -700 V.
The photoreceptor region charged to 650V and to which a cleaning bias is applied to the transfer roller 1 has a grid voltage of -250.
By switching to V, the photosensitive member is charged to -200 V, does not cause a positive memory of the photoconductor due to the cleaning bias, and does not cause density unevenness during subsequent image formation.

In the above embodiment, the transfer roller 1 is provided so as to be in pressure contact with the photosensitive member 2. However, the present invention is not limited to this, and a gap smaller than the thickness of the transfer material is formed between the photosensitive member 2 and the transfer roller 1. May be provided in between.

In the above embodiment, the transfer roller 1
Has been described, but the present invention is also applicable to other members that come into contact with the image carrier, such as a cleaning roller. That is, a bias having the same polarity as that of the toner during development is applied to the contact member to return the toner on the contact member to the image carrier.

[0056]

As described above, according to the present invention,
The transfer member can be cleaned satisfactorily.
It was possible to prevent the toner from scattering in the apparatus.

Further, according to the present invention, it was possible to obtain a good image without density unevenness without generating a memory in the image carrier.

[Brief description of the drawings]

FIG. 1 is a schematic side view of a main part showing a configuration of an image forming apparatus suitable for applying the present invention.

FIG. 2 is a diagram showing an operation sequence of the above.

FIG. 3 is an explanatory diagram showing an aspect of an image carrier surface potential.

FIG. 4 is an explanatory diagram showing an aspect of an image carrier surface potential.

FIG. 5 is a schematic side view of a main part of an image forming apparatus showing another embodiment of the present invention.

FIG. 6 is a schematic side view of a main part of an image forming apparatus showing another embodiment of the present invention.

FIG. 7 is a schematic side view of a main part of an image forming apparatus showing another embodiment of the present invention.

FIG. 8 is a schematic side view of a main part of an image forming apparatus showing another embodiment of the present invention.

FIG. 9 is a graph showing the relationship between the resistance of the transfer roller and the voltage and current at which a good cleaning action can be obtained.

FIG. 10 is a graph showing a relationship between a transfer roller current and a cleaning effect.

FIG. 11 is a graph showing the surface potential of an image carrier before and after transfer charging.

FIG. 12 is a graph showing the surface potential of an image carrier before and after transfer charging.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Transfer roller 2 Photoconductor 4 Charging roller

Claims (5)

(57) [Claims] An endless moving photosensitive member; a charging unit for charging the photosensitive member; an exposing unit for exposing the photosensitive member charged by the charging unit to form an electrostatic latent image; Developing means for developing the electrostatic latent image with toner charged to a polarity opposite to the charge polarity of the photoconductor, transfer means for electrostatically transferring the toner image on the photoconductor to a transfer material, and transfer means for non-transfer A cleaning bias applying unit for applying a cleaning bias having the same polarity as the charging polarity of the toner, wherein at least a part of the area on the photoconductor which has received the cleaning bias is used as an image area, An image forming apparatus, wherein an area to be an image area is charged in advance with a polarity opposite to a polarity of a cleaning bias. 2. The image forming apparatus according to claim 1, wherein a charging potential charged in advance with a polarity opposite to the cleaning bias is lower than a charging potential during image formation. 3. The image forming apparatus according to claim 1, wherein the charging unit has a contact charging member that contacts and charges the photoconductor. 4. The image forming apparatus according to claim 1, wherein the transfer unit includes a contact transfer member that contacts a photosensitive member. 5. The image forming apparatus according to claim 1, wherein the photoconductor has an organic photoconductor.